# Diamond Integrated Quantum Photonics: A Review

Prasoon K. Shandilya1, Sigurd Flågan1, Natalia C. Carvalho1, Elham Zohari2,
Vinaya K. Kavatamane1,
Joseph E. Losby1, and Paul E. Barclay1

Corresponding author<EMAIL_ADDRESS>1Institute for Quantum Science and
Technology, University of Calgary, Calgary, Alberta T2N 1N4, Canada
2Department of Physics, University of Alberta, Edmonton, Alberta T6G 2E1,
Canada

###### Abstract

Integrated quantum photonics devices in diamond have tremendous potential for
many quantum applications, including long-distance quantum communication,
quantum information processing, and quantum sensing. These devices benefit
from diamond’s combination of exceptional thermal, optical, and mechanical
properties. Its wide electronic bandgap makes diamond an ideal host for a
variety of optical active spin qubits that are key building blocks for quantum
technologies. In landmark experiments, diamond spin qubits have enabled
demonstrations of remote entanglement, memory-enhanced quantum communication,
and multi-qubit spin registers with fault-tolerant quantum error correction,
leading to the realization of multinode quantum networks. These advancements
put diamond at the forefront of solid-state material platforms for quantum
information processing. Recent developments in diamond nanofabrication
techniques provide a promising route to further scaling of these landmark
experiments towards real-life quantum technologies. In this paper, we focus on
the recent progress in creating integrated diamond quantum photonic devices,
with particular emphasis on spin-photon interfaces, cavity optomechanical
devices, and spin-phonon transduction. Finally, we discuss prospects and
remaining challenges for the use of diamond in scalable quantum technologies.

##  I Introduction

Quantum technology has come a long way since the Stern–Gerlach experiment was
conducted a century ago[2]. Developments in quantum theory have provided the
foundation for much of present-day technology, including, but not limited to,
lasers, atomic clocks, and techniques in medical imaging, and have the
potential to impact diverse fields spanning computing[3, 4], communication[5,
6], and sensing[7, 8, 9]. We are now at an exciting moment where early-stage
quantum computers have demonstrated quantum advantage[10, 11, 12, 13],
quantum-secured communication has been demonstrated over intercontinental
distances[14, 15], and quantum sensors have surpassed the sensitivity limits
defined by the laws of classical physics[16].

A cornerstone for the development of real-world quantum applications is the
resilient and scalable interconnection of different quantum systems. While
several proof-of-principle experiments have been conducted with trapped atoms
and ions[17, 18, 19, 20, 21, 22, 23, 24, 25], scaling to a larger quantum
system remains a challenge, in-part due to the complex laser trapping
techniques required. Solid-state systems offer the possibility to adapt
nanofabrication capabilities developed by the semiconductor industry to mass-
produce quantum devices[26]. Superconducting quantum circuits[27], gate-
defined quantum dots[28], and semiconductor spin qubits[29] all take advantage
of scalable nanofabrication to create quantum information processing devices.
Integrated photonic devices could play a key role in these efforts by
harnessing light’s ability to transmit quantum information, and are becoming
increasingly important as quantum technologies mature. Diamond has emerged as
a promising material for photonic quantum systems build around artificial
atoms formed by impurities in the diamond crystal[30]. These atomic-scale
impurities are optically active, allowing them to the controlled and connected
through photonic channels, and to be interfaced with other photonic quantum
technologies.

In recent decades, the development of photonic quantum technologies[31] has
expanded the quantum information ecosystem, particularly in the fields of
quantum communication and quantum sensing. Quantum communication has many
potential applications, including clock synchronization[32] and encrypted
communication[33, 34]. Central to these applications is a quantum photonic
network enabling the transmission and entanglement of quantum bits (qubits)
over long distances[35].

To realize a quantum network, remote quantum network nodes must be entangled
via quantum links. As illustrated in Fig. 1 (a), these network nodes are
small-scale quantum processors that combine robust storage of quantum
information with an interface to the quantum communication channels[36]. The
nodes must enable high-fidelity operations on several entangled qubits and
fault-tolerant multi-qubit protocols[37]. Furthermore, the nodes need to be
capable of storing quantum states utilizing ‘memory’ qubits for a longer time
than is required to generate asynchronous entanglement across numerous distant
nodes. An additional requirement for network nodes is the realization of a
fast and efficient interface between stationary qubits and ‘flying’ photonic
qubits. For long-distance communication, the flying qubits must have minimal
losses in the photonic quantum channel, thus limiting the use to telecom
photons that are compatible with pre-existing low-loss optical fiber
networks[38, 39, 35].

Optically active spin-qubits in diamond have successfully been utilized in
several proof-of-principle experiments targeted towards the integration of the
above mentioned components to realize a quantum network[40, 41, 42, 43, 44].
Advances in spin-photon entanglement[45] paved the way for photon-mediated
entanglement of remote qubits[46, 47, 48], culminating in the recent
demonstration of a three-node quantum network[49, 50]. While these
demonstrations have been fruitful, it is worth noting that parallel advances
have been made with other experimental platforms including trapped atoms and
ions[18, 51, 52, 53, 54, 55], superconducting resonators[56, 57], self-
assembled quantum dots[58, 59, 60], and defect-based qubits in other wide-
bandgap semiconductors and dielectrics[61, 62, 63, 64, 65, 66, 67, 68, 69, 70,
71].

Realizing an efficient interface between diamond qubits and photons at
telecommunication wavelengths for fiber-based quantum channels remains a
significant hurdle as diamond-based qubits do not intrinsically couple to
telecom photons. The optically active spin qubits found in diamond typically
emit photons in the visible range, thus requiring quantum frequency conversion
to telecom wavelengths[72] to mitigate absorption loss in fiber optic
links[35]. The need for optical frequency conversion, however, can be
circumvented by taking inspiration from the development of quantum
transducers[73] and utilizing the mechanical properties of diamond in
combination with device engineering[74]. Diamond is the stiffest material
known to man with Young’s modulus exceeding 1,200 GPa[75], facilitating the
fabrication of high-frequency mechanical resonators[76, 77]. These mechanical
resonators have the potential to be universal quantum transducers[78, 79],
capable of coupling a myriad of different quantum systems, including
connecting superconducting qubit resonators and optical photons[80, 81].
Photon-phonon and subsequent spin-phonon coupling in a mechanical resonator
constitute a promising route to realize a spin-photon interface natively
operating at telecom wavelengths[82, 74, 83]. Furthermore, the high mechanical
frequencies of these diamond resonators suppress the population of thermal
phonons, facilitating preparing of the mechanical resonator to the quantum
ground state.

Figure 1: Key motives for diamond integrated quantum photonics: (a) The
requirements of a quantum network node. A photonic platform in diamond is a
promising candidate to meet all the requirements. (b) The solid-state
structure of diamond allows for a variety of device designs utilizing photons,
phonons and applied RF / magnetic fields to manipulate qubits via en-masse
fabrication techniques. Color centers can be deterministically created through
implantation and annealing. (c) A quantum sensor based on the single electron
spin associated with the NV center. Sensing of the local spin environment or
magnetic fields, temperature, and electric / stain field has been demonstrated
using NV centers. Schematic structure of (d) the NV and (e) the group-IV
split-vacancy centers in diamond. The NV center has outstanding spin
properties, with long coherence times at elevated temperatures. The inversion
symmetry of the group-IV color centers leads to a larger Debye-Waller factor
and a vanishing permanent electric dipole moment, making these color centers
suitable for integration in nanophotonic devices.

Unleashing diamond’s full potential as a material platform for quantum
information processing is impaired by two preeminent challenges: deterministic
creation of highly coherent qubits and scalable device fabrication. A key
requirement for scaling beyond a few qubits is the precise fabrication of
arrays of identical qubits separated by a few nanometers, a level of precision
required to magnetically couple the electron spins to mediate multi-qubit
operations[84, 85, 86]. Significant progress has been achieved in ‘top-down’
qubit fabrication techniques, however it remains challenging to meet the
necessary requirements[87]. A promising approach to overcome this obstacle is
‘bottom-up’ atomically-precise fabrication that has been successfully
demonstrated in silicon[88]. Alternatively, proposals for coupling qubits via
a quantum optical bus require devices that combine deterministic creation of
qubits with quantum control via externally applied fields[89], such as
optical, stress, magnetic, and radio and microwave frequency electric fields,
see Fig. 1 (b). The engineering of these control mechanisms requires robust
device fabrication. Utmost care needs to be taken during device fabrication as
the qubits are intrinsically sensitive to their surrounding environment and
their properties can be degraded by their proximity to surfaces[90]. However,
by turning the problem around, their sensitivity to their environment (see
Fig. 1 (c)) can be exploited to realize quantum sensors[9] that offer
advantages over their classical counterparts. Diamond-based sensors are being
developed at a rapid pace and offers to fill the gap created by other
candidates in terms of resolution, portability, and cost.

In this article, we focus on recent developments in the field of diamond
integrated photonics and its impact on the quantum technology ecosystem. The
article is organized as follows: In Sec. II we introduce the physics and the
strengths and weaknesses of diamond colour centers. In Secs. III and IV, we
provide an overview of the recently developed field of diamond cavity
optomechanics and discuss the potential of photon-phonon coupling for quantum
technology in diamond. In Secs. V, VI, VII, and VIII, we review the current
state-of-the-art, discuss open questions, and provide a roadmap unravelling
the promising future for diamond nanofabrication, quantum photonic devices,
sensing and metrology, and qubit-photon interfaces.

##  II Color centers in diamond

Diamond, an inherently transparent material, is host to over 200 optically
active defect centers[91, 92]. These defects, known as color centers, can
occur naturally or be created in the lab (see Sec. V-C). Some color centers
combine atom-like optical transitions with long-lived electron spins,
resembling ions trapped in the diamond lattice[93]. Diamond’s large electronic
bandgap ($5.47\,\textrm{eV}$) and resulting wide transparency window and large
energy separation between colour center energy levels and the crystal’s
conduction and valance bands, combined with a low population of thermal
phonons (Debye temperature$\,\Theta\sim 2000\,\textrm{K}$[94, 75]) promotes
coherent single-photon emission and long spin coherence times, making diamond
an ideal host for solid-state qubits. Furthermore, the diamond lattice is
largely composed of nuclear spin-0 12C atoms (natural abundance $98.9\,\%$),
thus suppressing background magnetic noise. Synthetic growth of diamond using
isotopically purified starting material further reduces magnetic noise,
leading to prolonged spin coherence times[95]. The electron spin associated
with color centers may be utilized as a qubit that, when interfaced with
photons, can form part of a quantum network node. Furthermore, coupling to
nearby long-lived ${}^{13}\textrm{C}$ nuclear spins[96] can be harnessed to
build quantum memories[97] and multi-qubit spin registers[98]. Entanglement
swapping between the electron spin and the long-lived nuclear quantum
memory[99, 41], can free up the communication qubit, enabling high-fidelity
multi-qubit protocols[100]. For scalability, it is desirable to position the
memory qubit in close spatial proximity to the communication qubit; a
challenging task using 13C spins owing to the low natural abundance of only
$1.1\,\%$[101]. As an alternative, a quantum memory can be realized using the
nuclear spins intrinsic to the color center[101]. This latter approach carries
the advantage that the memory qubit is automatically located adjacent to the
communication qubit[102].

Being robust single-photon sources[103, 104], the nitrogen-vacancy (NV)[105]
and silicon-vacancy (SiV)[106] centers have over the years gained traction as
promising candidates for a variety of applications in photonic quantum
technologies[107, 30]. NV centers have been used in various proof-of-principle
experiments, including, but not limited to quantum sensing[108], quantum
teleportation[40], quantum error correction[109, 84, 110], demonstration of a
multi-qubit quantum processor with coherence times approaching a second for
the electron spin[111] and a minute for the nuclear spin[98], fault-tolerant
operations[37], and the realization of multinode quantum networks[49, 50]. On
the other hand, SiV centers integrated with nanophotonic structures with
exceptionally large cavity cooperativities have been used to demonstrate
memory-enhanced quantum communication[43]. In addition, novel color centers
based on the heavier group-IV atoms such as germanium (Ge), tin (Sn), and lead
(Pb), show promising potential for quantum applications[112]. However,
research on these defects is still in the early stages and will therefore not
be discussed extensively in this review.

###  II-A Defect Structure

The NV center, depicted schematically in Fig. 1 (d), consists of a
substitutional nitrogen atom and an adjacent lattice vacancy with the symmetry
axis along the $\langle 111\rangle$ crystal direction. The group-IV atoms, on
the other hand, are too large to occupy a carbon site. Instead, the impurity
atom takes up an interstitial position, flanked by a vacancy on either
side[113, 114]. This split-vacancy configuration, with the group-IV atom
positioned at the inversion point of the diamond lattice, is shown
schematically in Fig. 1 (e). The centrosymmetric configuration results in a
vanishing permanent electric dipole moment, rendering the group-IV color
centers insensitive to linear Stark shifts[115, 116]. The unpaired electrons
associated with the neutral charge states, NV0 and XV0, where X refers to one
of the group-IV atoms in Fig. 1 (e), can efficiently capture an electron from
the environment, forming the negative charge states, NV- and XV-,
respectively[117, 118]. The negative charge state is the most studied for both
classes of defects. However, it is worth mentioning that for the SiV center,
careful doping and surface treatment have stabilized the neutral charge state
SiV0[119, 120]. Similar results are yet to be reported for the heavier group-
IV defects. Therefore, in the context of this review, we will be referring to
the negative charge states, unless explicitly stated otherwise.

TABLE I: Overview of the Spin and Optical Properties of NV and SiV Color
Centers in Diamond

Color center |  $\lambda_{\text{zpl}}$ (nm) |  Radiative lifetime (ns) |  Quantum efficiency |  Debye–Waller factor |  GS $T_{2}$ (Hahn-echo) |  Spin-strain coupling (per strain) |  Orbital-strain coupling (per strain)
---|---|---|---|---|---|---|---
NV- | 637 | $\simeq 12^{1}$ |  $>$ 85$\,\%^{2}$ | 0.033 |  1.7 ms4 (RT, 69 mT) |  21.5 GHz5 (GS; RT) $\sim$290 GHz5 (ES; $<$ 10 K) |  $\sim$ 1 Hz5 (ES; $<$ 10 K)
SiV- | 7386 |  $\simeq 1.7-1.8$7 | 1-10$\,\%^{8}$ | 0.79 |  0.3 ms10 (0.1 K, 160 mT) | N /A † |  $\sim$ 1 PHz11 (GS; mK)

1[121], 2[122], 3[123], 4[124], 5[125], 6[118], 7[126, 127], 8[128, 127],
9[129], 10[130], 11[131].
†Intrinsic spin-strain coupling is absent in SiV-[132]. In certain regimes,
spin-orbit interaction results in a spin-strain susceptibility of $\sim
0.1\,\frac{\text{PHz}}{\text{strain}}$[131].

###  II-B Spin and Optical Properties

The NV center ground-state manifold is composed of an orbital singlet, spin-
triplet state ($S=1$)[133], where, for zero magnetic field, the
$m_{\textrm{s}}=0$ and the $m_{\textrm{s}}=\pm 1$ spin sub-levels are split by
$2.87\,\textrm{GHz}$ due to spin-spin interactions[105]. For the group-IV
split-vacancies, the ground-state manifold is an orbital-doublet state, where
spin-orbit interactions split the orbital branches by
$\Delta_{\textrm{GS}}$[118, 132]. As will be discussed below, the value of
$\Delta_{\textrm{GS}}$ directly determines the spin coherence times. A
comparison between the key spin and optical properties for the NV and SiV
center is summarized in Table I.

As a direct consequence of the difference in symmetry (Fig. 1 (e)-(f)) and
corresponding energy level structure, the NV and SiV centers exhibit different
spin and optical properties, in turn determining in which areas they find
applications. Due to the weak spin-orbit coupling[134] and the largely spin-
free diamond lattice[95], the NV center electron spin is only weakly coupled
to its environment, resulting in exceptionally long spin-lattice relaxation
times ($T_{1}$) and spin-coherence ($T_{2}$) at room temperature[75]. The long
spin coherence time makes the NV center a workhorse in quantum sensing
applications across a broad temperature range[135, 136]. The SiV center, on
the other hand, suffers from relatively short spin coherence times. In the
case of the SiV center, the spin coherence time is limited by phonon-assisted
population transfer between the two orbital branches (split by
$\Delta_{\textrm{GS}}\sim 48$ GHz), resulting in coherence times limited by
the orbital $T_{1}$ time[132, 137, 138]. Cooling down to millikelvin
temperatures suppresses the thermal phonon population, thereby increasing the
spin coherence time[130]. SiV coherence time-scales are typically in the range
of 100 ns at room temperature, extendable to a few ms at millikelvin
temperatures, thus limiting the potential sensing applications of SiV centers.

The observation of two-photon quantum interference from spatially separated
emitters[139, 140, 141], a prerequisite for remote entanglement protocols[46,
47], requires coherent emission of indistinguishable photons. For NV centers,
the photon flux is limited by the long radiative lifetime of
$12\,\textrm{ns}$. Furthermore, emission into the zero-phonon line (ZPL) at
$637\,\textrm{nm}$, quantified by the Debye-Waller (DW) factor, accounts for
only $\sim 3\,\%$ of the total emission[123]: the remaining $97\,\%$ is
accompanied by phonons, resulting in a broad phonon-sideband (PSB) extending
to $\sim 800\,\textrm{nm}$. The SiV center, on the other hand, exhibits a
comparatively short radiative lifetime of $1.8\,\textrm{ns}$[126, 127].
Moreover, $\sim 70\,\%$ of the emitted photons are directed along the ZPL at
$738\,\textrm{nm}$[118, 129]. However, the SiV center suffers from a low
quantum efficiency (QE) of only $\sim 0.1$[128, 127] compared to a near-unity
QE for the NV center[122].

In brief, the large difference in the DW factor between NV and SiV centers can
be attributed to the symmetry of the defect[134]. In the case of the NV
center, excitation from the ground-state (GS) to excited-state (ES), denoted
by ${}^{3}A_{2}$ and ${}^{3}E$, respectively[133], shifts the charge
distribution towards the N atom[142, 143]. This charge redistribution alters
the equilibrium position of the nuclei, resulting in a reduced overlap between
the ground- and excited vibronic states manifested by a large PSB[105, 134].
In comparison, the charge distribution between the ground
(${}^{2}E_{\textrm{g}}$) and excited state (${}^{2}E_{\textrm{u}}$) of the SiV
center remains similar; there is little change in the nuclear equilibrium
coordinates. Consequently, optical emission from the SiV center occurs largely
into the ZPL[134, 113].

For both families of color centers, the presence of a magnetic field lifts the
ground-state spin degeneracy: the spin sub-levels can be individually
addressed using externally applied microwave[144, 145, 130] or strain
fields[146, 147, 131], enabling the formation of a qubit. Furthermore, both
defects exhibit cycling optical transitions, enabling all-optical spin
control[148, 149] and single-shot spin-readout[150, 130] at cryogenic
temperatures.

##  III Coherent Photon-Phonon Coupling in Diamond: Cavity Optomechanics

Initial efforts to integrate diamond qubits with photonic devices have focused
on photon-spin coupling, as discussed in previous reviews[151, 152] and
addressed later in this review. Recently, efforts to harness spin-phonon
coupling in diamond for realizing both on-chip and long-distance quantum
coherent connections between diamond spin qubits have emerged[75, 153].
Central to these efforts is the field of cavity-optomechanics, which can
provide coherent photon-phonon coupling in integrated photonic devices. Cavity
optomechanics[154, 155], has enabled quantum technologies in fields including
sensing and precision measurement, studies of the quantum properties of
mechanical objects, quantum memories, and quantum transduction. It has been
investigated in a plethora of materials and designs, and has recently been
shown to be a viable and promising route to control and read out spin qubits
in diamond[74]. This latter demonstration harnesses the convenient frequency
matching between mechanical resonances of integrated devices and solid-state
qubit spin transitions.

Cavity optomechanical systems use two distinct mechanisms to create and
enhance the interaction between the optical field in a cavity and mechanical
resonators: moving mechanical boundaries[156] and photoelasticity[157]. The
mechanical displacement of vibrating cavity walls drives the moving boundary
effect, whereas the photoelastic effect is a bulk response that manifests as
strain-induced modulation of the refractive index. These light-matter
interactions are quantified through the single-photon optomechanical coupling
rate, $g_{0}$, calculated as the product of the optical frequency shift per
unit displacement and the mechanical zero-point fluctuation amplitude of a
mechanical resonator mode of interest. Experimental determination of $g_{0}$,
in general, is realized by the parametric fitting of well-known optomechanical
(OM) effects, such as the optical spring effect and the optomechanical (anti-)
damping. A direct measure of $g_{0}$ is possible by comparing the OM
transduction with an artificial phase-modulated tone, a method commonly used
and described by Gorodetsky et al.[158]. These measurements are corroborated
by numerical simulations utilizing perturbation theory and the finite element
method (FEM)[159].

To be used in quantum applications, optomechanical cavities must be designed
to achieve high $g_{0}$ while also overcoming optomechanical losses. This
requirement is characterized by the optomechanical cooperativity, given by:

$C_{\text{om}}=\frac{4\overline{n}_{\text{cav}}g_{0}^{2}}{\kappa\gamma_{\text{m}}}\,.$
(1)

Here, $\kappa$ and $\gamma_{\text{m}}$ are the energy decay rates of the
cavity’s optical and mechanical modes of interest, respectively, and
$\overline{n}_{\text{cav}}$ is the average intracavity photon number.
$C_{\text{om}}$ can be interpreted as the probability of coherent OM
interaction[160]. The condition $C_{\text{om}}>1$ has been realized in a
variety of cavity OM devices, including those fabricated from diamond[77, 76].
Moreover, quantum state transfer between light and mechanics that can generate
entanglement between photons and phonons demands devices with
$C_{\text{om}}>n_{\text{th}}+1$, where $n_{\text{th}}$ is the average thermal
phonon number of the mechanical resonator due to its thermal bath[161, 162].
Lowering $n_{\text{th}}$ either requires cooling resonators to cryogenic
temperatures, using high-frequency resonators, or a combination of both.
Another condition that is key to many applications is reaching the sideband-
resolved regime, that is, when the resonator’s mechanical mode frequency
exceeds $\kappa/2$, facilitating delayed optical back-action that can be
harnessed for optical amplification or cooling of mechanical resonance.

The above discussion only accounts for the real part of the optomechanical
coupling. In cases where the change in a cavity’s optical losses via
mechanical displacement is significant, $g_{0}$ will be defined as a complex
quantity with dispersive and dissipative contributions[159]. Furthermore, when
the device is operated under high optical power, dynamic and static
photothermal and thermoelastic effects may be relevant, which can
significantly modify the physics of the problem[163, 164, 165]. In this
review, we focus on the dispersive OM interaction; the above-mentioned effects
are neglected unless explicitly stated.

###  III-A Relevant material properties

A fortuitous combination of exceptional intrinsic material properties is
responsible for making diamond an ideal material for cavity optomechanics.
Advances in the synthetic growth of single-crystal diamond (SCD) combined with
the development of fabrication techniques to realize suspended structures from
high-quality SCD chips have accelerated the recent emergence of diamond
nanophotonic and nanomechanical devices for quantum technologies. Diamond’s
key material properties relevant to quantum photonics are outlined below.

#### Optics

The strong covalent bond of diamond hinders electrical conductivity, resulting
in a wide bandgap of 5.47 eV (225 nm). This large bandgap creates a broad
transparency window ranging from ultraviolet to radio frequencies, except for
a weak absorption window appearing between 2.6 and 6.2 $\mu$m due to multi-
phonon coupling[166]. Outstanding transparency, when combined with a high
refractive index of $\sim$ 2.4 and low dispersion, makes diamond a robust and
versatile medium to confine photons. Additionally, the large bandgap
suppresses multi-photon absorption, providing the ability to handle large
optical power inside a cavity with minimal nonlinear absorption and resulting
thermal effects, unlike other popular semiconductors such as silicon and
gallium arsenide[167].

While thermal nonlinear effects are mostly undesirable, optical nonlinearities
lead to interesting physics. Diamond’s lowest-order non-zero nonlinear
susceptibility, $\chi^{3}$, allows the study of phenomena such as Raman
scattering[168, 169, 170], the Kerr effect[171, 172], and four-wave
mixing[173]. Amid many prospects, the on-chip demonstration of diamond-based
microcombs[174], Raman lasers[175, 176], and single-photon frequency
conversion via a combination of parametric processes with color centers are
among the most promising[177].

Special attention must be paid to surface termination, which can introduce
additional losses when device dimensions reach the nanoscale. A high
concentration of impurities, such as color centers, may have an impact on both
optical and mechanical loss rates. Even though diamond is an optically
isotropic material due to its cubic lattice symmetry, the strain induced by
the presence of impurities can cause birefringence.

#### Mechanics

Diamond has exceptional mechanical properties. It has a high Young’s modulus
of 1220 GPa, four times larger than silicon, making it a stiff material with a
large speed of sound ($\sim$18,000 m / s). The large stiffness facilitates the
fabrication of high-frequency resonators by reducing reliance on small
geometry, ameliorating access to mechanical resonances resonant with a wide
range of spin qubit transition frequencies. Equally important, diamond is a
material with low intrinsic mechanical dissipation. In general, the
predominant geometry-independent dissipative channels for nanomechanical
semiconductor resonators near room temperature are thermoelastic damping and
phonon-phonon scattering[178]. Both these effects are temperature-dependent,
and therefore, their contribution is relatively small in diamond due to its
high thermal conductivity (2,200 W / mK, several times higher than copper) and
density (3,500 kg /m3), alongside its low thermal expansivity. At cryogenic
temperatures, scattering by defects such as dangling bonds, intrinsic or
extrinsic dopants, and lattice imperfections can dominate phonon losses[179].
Such interactions are commonly modelled as strain waves coupling to two-level
systems and might be the ultimate limit for achieving ultra high-Q
nanomechanical resonators[180].

Besides the lattice anisotropy introduced by defects, even undoped SCD is an
imperfect isotropic elastic material. Although the anisotropy level is often
disregarded without penalty, fine tailoring of diamond’s mechanical properties
is sometimes necessary. Huang et al.[181] have demonstrated that Young’s
modulus varies depending on the crystal orientation. Likewise, other
properties such as photo-elasticity are also influenced by the
crystallographic direction, as demonstrated in gallium arsenide[157]. These
variations can be accounted for by a thorough description of the diamond
elasticity tensor[182, 183].

###  III-B Experimental realizations of diamond optomechanical devices

Figure 2: Diamond optical and mechanical resonators: Simulation of (a)
microdisk cavity and (b) 1D optomechanical crystal (OMC). WGM: whispering
gallery mode; (R)BM: (radial) breathing mode; $|\mathbf{u}|$ and $E_{z(y)}$
stand for normalized mechanical displacement and electric field at the z (y)
direction, respectively. Reported quality factors: (c) optical and (d)
mechanical diamond resonators (experimental data). Nanobeams: 1[184] and
11[185]; 1D Photonic crystals (PhC): 2[186], 3[187], 5[188], 6[189], 7[190],
8[191], 9[192], 14[193], 18[194], 23[77] and 24[77]; 2D PhCs: 4[187], 15[195],
16[196], 17[197] and 25[198]; Cantilevers: 10[185], 12[179] and 13[199];
Microdisks: 19[76], 20[76], 21[200] and 22[201].

Efforts to utilize optical fields to manipulate and probe micro-and nano-scale
mechanical objects have led to numerous types of optomechanical devices[155].
Amongst these, microdisk resonators are one of the simplest optical cavity
designs and play an important role in many photonics and optomechanics
experiments. Microdisks support optical whispering gallery modes (WGM), in
which the electromagnetic field is confined by total internal reflection at
the circular boundary (Fig. 2 (a)). The mode volume of WGMs decreases with
decreasing microdisk radius, while optical radiation loss increases. An
optical quality factor of $Q_{\text{o}}\sim 10^{5}$ at telecommunication
wavelengths was achieved in 5$\,\mu\text{m}$ diameter SCD microdisks by
Mitchell et al.[200], limited by fabrication related surface roughness. At
visible wavelengths resonant with NV optical transitions,
$Q_{\text{o}}>10^{4}$ have been observed in SCD microdisks[76].

Optomechanical microdisks are supported by small pedestals that minimize
anchor losses, as shown in Fig. 2 (a). Optical WGMs couple most effectively to
mechanical radial breathing modes (RBM): resonances with mechanical frequency
$\Omega_{\text{m}}/2\pi\sim 2$ GHz, mechanical quality factor
$Q_{\text{m}}\sim 10,000$, and $g_{0}/2\pi\simeq 26$ kHz have been reported
for a diamond microdisk[76]. This device demonstrated $C_{\text{om}}\sim 3$ at
ambient conditions, enabling a number of coherent optomechanics experiments,
including reversible photon-phonon conversion via optomechanically-induced
transparency[202, 203], phonon-mediated wavelength conversion[204], and
optomechanical memory[203]. These demonstrations took advantage of the
multimode nature of the diamond microdisk; a topic that will be discussed in
Sec. IV.

The advantages of microdisk resonators go beyond their ease of fabrication and
effective OM interaction. Usually, resonators with small dimensions are
targeted due to their low effective modal mass, leading to a higher
$\Omega_{\text{m}}$ and larger $g_{0}$. Tuning of the device geometry modifies
the optical free spectral range (FSR) and can enable mechanical frequencies
ranging from kHz to GHz – such design flexibility is favorable for coupling to
spin qubits[153]. On the downside, mechanical quality factors are highly
dependent on the disk-to-substrate interconnection. While M. Mitchell et
al.[76] demonstrated the feasibility of fabricating a thin pedestal in diamond
microdisks, the patterning of a nanostructured phononic shield on the top of
the disk could provide a solution to further isolate clamping and reduce
anchor losses[205, 206].

Optomechanical crystals (OMC) are another device design used to simultaneously
confine optical and mechanical modes[207]. Their sophisticated engineering
allows for small optical mode volumes and high OM coupling rates compared to
other geometries. The basic principle involves introducing a geometric defect
in an artificially patterned lattice (Fig. 2 (b)) that generates periodicity
in the dielectric constant and elastic properties of a material. This in turn
modifies the dispersion of photonic and phononic modes of the structure. These
defects are generally implemented by alternating air and material regions, for
example by introducing air holes in a suspended film. The resulting photonic
and phononic band structures have regions with vanishing density of states.
Introducing defects that break the lattice’s translational symmetry allows the
formation of localized optical and mechanical resonances at frequencies within
the bandgaps of the respective optical and photonic band structures.
Additionally, these modes can simultaneously have low radiation and clamping
losses, respectively, by ensuring that the defect minimizes the coupling to
the lossy modes of the underlying periodic structure[208].

Diamond photonic crystal cavities (PCC) and nanomechanical resonators were
first explored independently. Photonic crystal optical cavities using
triangular nanobeam PCCs were demonstrated with a
$Q_{\text{o}}\,\sim\,1000$[188], soon followed by a $Q_{\text{o}}>10^{5}$ due
to advances in fabrication techniques[186]. Rectangular cross-section photonic
crystal cavities[189] better suited for integration into a larger photonic
integrated circuit have been demonstrated using quasi-isotropic undercutting
of SCD with $Q_{\text{o}}\sim 14,000$[192]. These rectangular cross-section
cavities minimize the mixing of the TE and TM optical modes which can be
responsible for optical loss in triangular cross-sections[188].

Photonic crystal cavities fabricated using heteroepitaxy of diamond on
silicon[187] or diamond membrane transfer[193] have $Q_{\text{o}}\sim 10^{4}$.
Two-dimensional PCCs, have been realized from diamond films bonded or
deposited over a different material substrate. They can currently reach
optical $Q$-factors of a few thousand[187, 195, 196, 197]. Recently, quasi-
isotropic etching[209] was used to realize 2D photonic crystals with
$Q_{\text{o}}>6\times 10^{3}$[198]. Several of the aforementioned devices have
been realized with cavities in the visible wavelength range, as shown in Fig.
2 (c).

The realization of SCD nanomechanical resonators has made steady progress in
recent years, as illustrated in Fig. 2 (d). Cantilevers with $\mu\text{m}$
dimensions suspended over a SiO2 substrate have been demonstrated with a
$Q_{\text{m}}\sim 3\times 10^{5}$ in vacuum[199], while Tao et al.[179]
reported that similar structures made from SCD film in a ‘quartz sandwich’ can
achieve $Q_{\text{m}}\sim 10^{6}$. These high-$Q_{\text{m}}$ cantilevers have
also been used to investigate the relevance of surface termination and defect
concentration. In Ref.[185] both singly- and doubly-clamped nanobeams showed
$Q_{\textrm{m}}>10^{4}$ in angle-etched SCD.

Khanaliloo et al.[184] demonstrated one-dimensional suspended waveguides in
SCD supporting flexural mechanical modes with $Q_{\text{m}}>7\times 10^{5}$.
Polycrystalline diamond nanobeams and H-resonators with $Q_{\text{m}}\sim
10^{4}$ have been connected to nanophotonic circuits[210, 195]. Cady et
al.[194] have fabricated diamond OMCs using a SCD-on-insulator platform with
integrated telecom waveguides with $Q_{\text{o}}>10^{4}$ and GHz mechanical
modes with $Q_{\text{m}}\sim 100$. Burek et al.[77] fabricated and measured a
1D OMC with a $Q_{\text{o}}>10^{5}$, $Q_{\text{m}}\times f_{\text{m}}\sim
10^{14}$ and $g_{0}/2\pi\sim 240$ kHz. These realizations demonstrate that
diamond cavity optomechanics has enormous potential. However, the fabrication
of smooth sidewalls and the precise etching of small periodic features in
diamond OMCs are currently major obstacles to further experimental progress,
as will be discussed in Sec. V.

##  IV Multimode Optomechanics

The majority of cavity optomechanics studies utilize systems with a single
optical mode coupled to one mechanical mode. As discussed, an asset of diamond
cavities is their ability to support optical modes across a wide transparency
window, thus facilitating the study of multimode optomechanics, where multiple
optical modes are coupled to the same mechanical mode (see Fig. 3 (a, left)).
Similarily, mechanical resonators typically support a spectrum of modes that
can couple to a single optical mode (see Fig. 3 (a, right)). Constraining the
physics to the ‘one-to-one’ model is often justified and dictated by the
experimental design. For example, cooling or heating in the resolved sideband
regime singles out a particular mechanical mode via the choice of laser
detuning[211].

Setups with multimode coupling have been realized in a wide range of
materials[212] and have a number of additional valuable features[213, 214,
215, 216, 217, 218]. For example, bulk acoustic modes in a macroscopic scale
quartz cavity have been used to demonstrate optomechanically induced
transparency and absorption[219]. Multiple mechanical modes coupled to an
optical mode have demonstrated a cascaded optical transparency scheme by
leveraging the parametric phonon–phonon coupling with an aluminium nitride
microwheel resonator[220], coherent optomechanical state swapping between two
spatially and frequency separated resonators with silicon nitride trampoline
resonators[221], and multimode phonon lasing with a 1D silicon optomechanical
crystal cavity[222]. Furthermore, exploring multimode systems in synchronized
OM oscillator arrays constitutes an interesting approach for sensing and
metrology[223].

Multimode cavity optomechanical devices also offer great potential to connect
disparate quantum technologies in a network, via phonon-mediated coherent
transduction of quantum information from visible or microwave photons to
telecommunication wavelengths. As such, the field of optical-to-microwave
transduction[224] is currently a highly active area of research[225].
Optomechanical systems with $C_{\text{om}}>1$ are capable of frequency
conversion between multi-wavelength cavities[226, 227, 204], eliminating the
need for material-dependent nonlinear optical processes. Cavity optomechanical
wavelength conversion coherently couples two optical cavity modes via their
independent optomechanical coupling to a common mechanical resonance[226,
204].

Figure 3: Multimode cavity optomechanical systems and applications: (a)
Schematic illustration of two different multimode optomechanical systems. (b)
Amplified wavelength conversion using an optomechanical device. Two optical
modes at different frequencies are illuminated by a strong red-detuned and
blue-detuned control beam, respectively. A signal injected at the resonance of
mode 1 is converted into a signal emerging from mode 2, as demonstrated using
diamond microdisk in Ref. [204].

In diamond, optomechanical cavities have been used to demonstrate wavelength
conversion between frequencies separated by over 4 THz with an internal
efficiency of 45 $\%$[204]. For wavelength up- and down-conversion, both
strong optical control fields are typically red-detuned by the mechanical
frequency from cavity modes resonant with each wavelength of interest.
However, blue detuning of one of the control fields can be harnessed for
optomechanically amplified wavelength conversion, as illustrated in Fig. 3
(b). Dynamics in both of these wavelength conversion schemes can be modelled
by a photon-phonon beamsplitter or squeezing Hamiltonians[154]. These systems
can, in principle, be utilized to convert the zero phonon line emissions from
NV and SiV centers in diamond to telecom photons for application in quantum
networks, provided the cavity supports high-$Q_{\text{o}}$ optical modes at
both the photon emission wavelength and the telecom wavelength of interest,
and that $C_{\text{om}}\geq 1$ can be achieved for each mode. However, these
advancements are currently limited due to modest $Q_{\text{o}}$ at visible
wavelengths. Advances in fabrication techniques and better cavity design can
overcome this problem, see Sec. V. Coupling higher intensity control fields to
the cavity at the visible wavelength in order to achieve larger
$\overline{n}_{\text{cav}}$ will also increase $C_{\text{om}}$.

Other multimode optomechanical phenomena with promising quantum applications
have been demonstrated in diamond devices enabled by double-optomechanically
induced transparency (DOMIT). Examples include all-optical two-color
switching[228] and optomechanicaly tunable pulse storage[203]. Recently,
optical modes were used to drive a broad mechanical mode spectrum of diamond
microdisks. In this work, higher-order mechanical modes with frequencies over
10 GHz were observed. Devices with one optical and multiple mechanical modes
could lead to many interesting studies, as already demonstrated in the
microwave domain[229], but have not yet been explored in SCD optomechanical
systems. Hybrid SCD cavity optomechanical devices integrated with
superconducting quantum devices could accelerate the realization of
transducers for connecting superconducting quantum computers to quantum
networks[78].

##  V Diamond Nanofabrication

Advances over the last decade in the fabrication of nanostructures from
single-crystal diamond (SCD) have accelerated the development of components
necessary for fully integrated quantum photonics platforms. These components,
engineered to enhance light-matter interactions, play a crucial role in the
fields of cavity optomechanics (see Sec. III and IV), nanophotonics (see Sec.
VI), and qubit-photon interfaces (see Sec. VIII). They include, but are not
limited to, single-photon sources based on color-centers[92], wave-guiding
structures[230], optical[231, 197] and mechanical resonators[76, 194], and
optical fiber-based / free-space couplers[190, 232, 233]. In this section, the
challenges of SCD nanostructuring are briefly introduced, followed by
discussions of top-down fabrication methods for bulk diamond, as well as
methods for creating color centers in these structures.

###  V-A Single-Crystal Diamond Fabrication Challenges

In addition to diamond’s ability to host color centres, its allure for
nanophotonic and nanomechanical applications derives from the advantageous
material properties of diamond introduced above (see Sec.III-A). The unit cell
of SCD consists of two diagonally inter-penetrating face-centered cubic
lattices where each carbon atom is purely covalent-bonded to its four nearest-
neighbours with tetrahedral symmetry. This atomic structure grants diamond its
unique mechanical, thermal, and optical properties, including the wide
transparency window and chemical inertness[166].

Although these properties make SCD an excellent material for integrated
photonic devices, they also provide significant challenges in nanostructuring.
Traditional top-down nanofabrication approaches rely on lithographic
patterning of masking material, and subsequent etching steps to define
photonic and mechanical components. The high chemical inertness of diamond
limits etching chemistries to processes involving oxygen plasma, while the
strong bonding nature of the carbon atomic lattice makes physical etching,
such as focused ion beam milling, difficult. Currently, high-quality SCD in
thin-film form, where a refractive index contrast confines light within the
film thickness, is not commercially available, although recent progress has
been made in hetero-epitaxially grown films on various substrates[234]. Though
polycrystalline thin-films are readily available, they might not be suitable
for hosting quantum emitters due to the presence of dopants and grain
boundaries[166, 235]. Several paths have been taken in order to overcome these
challenges, both in etch mask development and through the use of different SCD
precursor materials, ranging from thinned membranes to bulk substrates.

###  V-B Creating suspended structures in bulk diamond

High-quality, commercially available SCD in bulk form, grown on diamond
templates via chemical vapour deposition (CVD), is a popular precursor
material for diamond nanostructuring. Both optical and electronic grade forms
of CVD diamond, with the latter having less than 10 parts per billion in
residual nitrogen density, are available down to millimeter-scale substrate
form. The substrate surfaces are mechanically polished (a specialized
procedure that is often outsourced) to sub-nanometer RMS surface roughness.
Initial cleaning of the substrates typically uses a boiling piranha solution
(3:1 ratio of sulfuric acid and hydrogen peroxide) for approximately 15
minutes, followed by a rinse in water and drying with a nitrogen gun. These
steps can be supplemented with a combination of a hydrofluoric acid dip for 5
minutes and sonication in acetone and or methanol[236]. Optical microscopy
inspection of the substrate post-cleaning should yield a clean surface, absent
of graphitic or other residues, often evident as dark spots. If not, the
cleaning process should be repeated. In cases where the substrates have been
treated with high-temperature annealing or ion irradiation, the induced
graphitic or pyrolytic carbon residue can be etched in a tri-acid cleaning
solution (a boiling mixture of nitric, sulfuric, and perchloric acids), though
care should be taken with the associated risks in this potentially volatile
process.

Figure 4: Single-crystal diamond nanostructuring: (a) Process flow of
nanofabrication via quasi-isotropic etch undercut. The resist, which sits on
the top of the anti-charging layer, silicon nitride (Si3N4) hard mask, and
single-crystal diamond (SCD) stack is exposed using electron beam lithography
(EBL), such that the pattern can be transferred through the hard mask via
inductively coupled plasma reactive ion etch (ICPRIE). An oxygen ICPRIE
further defines the pattern into diamond. A sidewall protection hard mask
consisting of Si3N4 through plasma enhanced chemical vapour deposition (PECVD)
or alumina (Al2O3) by atomic layer deposition (ALD) is anisotropically etched,
removing the bottom surface. A quasi-isotropic oxygen etch undercuts the
device, releasing the optomechanical device. The residual hard mask is removed
using hydrofluoric acid (HF). Color centers can be implanted in the device
through methods such as localized ion irradiation, and vacancy diffusion can
be promoted through high temperature annealing. Representative devices
fabricated through this method are shown in (b) as a microdisk capable of
containing whispering gallery optical modes coupled to radial and wine glass-
like mechanical modes and in (c) as a nanomechanical beam with an integrated
1D photonic crystal cavity defined by air holes through the beam thickness
(inset). (d) An array of diamond nanopillars defined on a $\langle
111\rangle$-oriented SCD substrate substrate via ICPRIE etch using EBL-defined
cylindrical etch masks. Panel (d) is reprinted from E. Neu et al., Appl. Phys.
Lett. 104, 153108 (2014), with the permission of AIP Publishing.

Perhaps the most challenging aspect of diamond nanostructuring lies in
identifying a mask material with sufficiently high selectivity to oxygen-based
plasma etching, in order to withstand pattern transfer into the substrate.
Polymer resists commonly used in electron beam (or photo-) lithography are not
sufficient, and therefore an additional hard mask layer is required. A common
choice for a hard mask is silicon nitride (Si3N4) deposited by plasma-enhanced
chemical vapor deposition (PECVD) with a thickness of about 250 nm for
anisotropic etches into diamond of several micrometers[209, 192, 231, 200,
184]. The process flow is schematically outlined in Fig. 4 (a). After hard
mask deposition, a thin ($\sim$5 nm) conductive film is deposited onto the
substrate in order to provide a charge compensating layer during the electron
beam lithography (EBL). This is required due to the highly insulating nature
of diamond. There are several options for use as the anti-charging layer
including evaporated amorphous carbon, titanium, and niobium, with the latter
two providing secondary functionality as an adhesion layer between diamond and
the resist[237]. After spin-coating and baking the resist, patterning is
performed with EBL followed by resist development. In the case of positive-
tone resists (such as PMMA or ZEP 520A) where scission of polymer chains
occurs during development, cooling of the developer to around -10∘C will yield
improved pattern and line edge resolution[238]. The pattern in the developed
resist is transferred to the nitride layer using an inductively coupled plasma
reactive ion etch (ICPRIE) with C4F8 / SF6 chemistry, followed by an
anisotropic oxygen plasma etch to transfer the pattern to the diamond
substrate. Alternatively, negative-tone HSQ electron beam resists can be
patterned directly on the diamond. Patterned HSQ has an oxide-like nature that
is resistant to oxygen plasma etching[237].

Quasi-isotropic diamond undercut etching can then be used to create suspended
diamond devices. This technique, which was inspired by the silicon SCREAM
fabrication process[239], has been adopted in numerous studies[192, 240, 200,
241, 242]. It requires that the sidewalls of the diamond nanostructure to be
protected. A conformal layer of silicon nitride deposited using a similar
procedure as in the application of the initial hard mask, or an Al2O3 layer
created by atomic layer deposition[198], has been used for sidewall shielding.
The horizontal surfaces of the protective layer are removed preferentially
using an anisotropic ICPRIE etch, leaving the sidewalls covered while exposing
the diamond surfaces to be etched. The undercutting of the devices uses a zero
DC-bias quasi-isotropic oxygen etch[209] at elevated temperatures of 200-300∘C
to increase the vertical and horizontal etch rates, which are dependent on the
diamond crystal facet orientations in the bulk substrate. Finally, the
residual resist and the hard mask are removed in a hydrofluoric acid wet etch.
Representative devices fabricated using this process in the geometries of a
microdisk and one-dimensional photonic crystal cavity beam are shown in Figs.
4 (b) and (c), respectively.

Two alternative approaches for creating free-standing structures from bulk
single-crystal diamond are reactive ion beam angled etching (RIBAE)[243, 237]
and Faraday cage angled reactive ion etching[244]. In RIBAE, an incident beam
normal to the substrate surface defines the depth of the nanostructure through
a lithographically patterned mask. The substrate is then tilted, such that the
ion beam is at an acute angle to the substrate surface and etching occurs
under rotation, yielding suspended nanostructures with a triangular cross-
section. In Faraday cage angled etching, a Faraday cage surrounding the sample
is used to redirect the RF field driven ions in the plasma.

###  V-C Color Center Fabrication

A common feature for the color centers discussed in Sec. II is the presence of
an impurity atom combined with adjacent lattice vacancies. However, for many
quantum applications, a high-quality diamond with a low concentration of
impurities is desired to mitigate electric- and magnetic noise caused by
impurity ions. Both the impurity atom and the vacancies can be introduced in
various ways. For example, vacancies can be introduced by irradiation of high-
energy particles, such as electrons, neutrons, and ions, or via intense,
ultrafast laser pulses (see the last panel, Fig. 4 (a))[245, 246, 247, 248].
High-temperature annealing ($T\geq 800^{\circ}\,\text{C}$) provides sufficient
thermal activation energy for the vacancies to diffuse. During this diffusion
process, the vacancies can combine with impurity atoms, forming color centers.
The annealing is usually carried out in a high vacuum or in an inert
atmosphere to avoid the etching of the diamond surface and the formation of a
graphitic surface layer[249]. The bombardment of particles during irradiation
leads to crystal damage–a potential source of paramagnetic noise adversely
affecting the spin and optical properties of the color centers. In principle,
annealing mitigates these noise sources by repairing the crystal damage[250].

For the integration of color centers in devices such as photonic cavities[231,
123] and all-diamond scanning probes[251], precise control of the lateral and
vertical position of the color centers with respect to the relevant fields
involved is of paramount importance. In principle, this can be accomplished by
deterministically fabricating the devices around the color centers–a
challenging task with the current state-of-the-art fabrication techniques.
Another way to remedy this problem is to introduce impurity ions via
$\delta$-doping. This technique has been successfully demonstrated for
nitrogen-vacancy centers by introducing nitrogen gas during the CVD diamond
growth process[252, 253]. The depth of the doping layer is controlled by the
subsequent diamond overgrowth[252, 254]. The aforementioned irradiation and
thermal annealing techniques can then be used to form NV centers in
$\delta$-doped diamond.

Contrary to nitrogen, silicon is rarely found in natural diamond[253].
However, silicon doping is readily achievable during diamond growth, for
example by the incorporation of SiO2 or SiC into the growth chamber[126] or by
introducing silane (SiH4) gas during the growth process[255, 256]. Similarly,
the introduction of germane (GeH4) gas allows the formation of GeV
centers[257].

Alternatively, impurity ions can be incorporated into the diamond post-growth
via ion implantation. Here, the implantation aids the formation of vacancies,
which, during thermal annealing, can form color centers[248]. Ion implantation
has successfully demonstrated the creation of NV centers[258, 259], SiV
centers[260, 261], GeV centers[262], SnV centers[263, 264] and PbV
centers[265, 266]. However, the large size of the heavy group-IV atoms leads
to greater lattice damage and larger strain in the crystal after thermal
treatment[267]. However, shallow implantation and subsequent diamond
overgrowth[268] allow for the creation of deep group-IV defects while
maintaining a crystalline environment.

For ion-implanted samples, spatial positioning can, in principle, be achieved
by implanting through an AFM tip[269, 196], by the use of lithographically
defined masks[102, 270] or using focused ion beam (FIB)[271, 272].
Furthermore, the ion implantation energy can be adjusted according to the
desired target depth, allowing for the creation of color centers at a depth
ranging from a few to several tens of nanometers. In particular, the use of
FIB allows for high-precision implantation into pre-fabricated photonic
structures, such as photonic crystals[273] and nanobeam cavities[127, 42,
272]. However, the creation of deeper color centers requires the use of high-
energy ions. These ions lose energy in collision with electrons and atomic
nuclei in the lattice, thereby leading to a trail of lattice damage along the
trajectory[274]. Furthermore, the collision with nuclei causes deviation from
the designated path, thereby reducing the spatial accuracy.

Vacancy generation using tightly focused, ultrafast laser pulses constitutes a
promising alternative to ion implantation for the creation of NV centers deep
in the diamond[275, 276]. In this process, vacancies are formed as a
consequence of optical breakdown caused by tunnelling or multiphoton
absorption[277]. The highly nonlinear nature of this process confines the
lattice damage within the focal volume of the excitation laser[278]. Relying
on the natural occurrence of nitrogen in electronic grade diamond
($[\textrm{N}]<5\,\textrm{ppb}$), laser writing and subsequent annealing has
led to the formation of highly stable NV centers[275, 279, 280]. The creation
of stable SiV centers has been demonstrated by coating the diamond surface
with silicon nanoballs followed by fs laser illumination[281].

For quantum sensing applications (see Sec. VII), it is necessary to have the
probe spin in close proximity to the target spins[282, 283, 284]. The reason
is twofold: firstly, magnetic dipolar coupling of probe spin with the nearby
target spins scales as $1/r^{3}$, where $r$ is the probe-to-target spin
distance. Consequently, only $r$ in the nanometer range can provide sufficient
coupling strengths. Secondly, for imaging applications the spatial resolution
is determined by the minimum $r$, thus demanding close proximity of the probe
and target spins, which is even more critical since the targets often lie
exterior to the diamond surface. Clearly, only probe spins located a few
nanometers below the diamond surface are useful for such applications.
However, these so-called shallow spins are also extremely sensitive to various
unwanted noise sources present on the surface, thereby severely degrading
their spectral and spin properties[285, 286, 90]. In this regard, engineering
robust sensing qubits is an ongoing endeavour.

##  VI Quantum Photonic Devices

Efficient collection of photoluminescence (PL) is of paramount importance for
applications using color centers in diamond. However, the large refractive
index of diamond ($n=2.4$) leads to total internal reflection at the diamond-
air interface, limiting the detection efficiency to only a few $\%$ for color
centers in bulk diamond when using a high numerical aperture microscope[287].
To remedy this problem and improve the photon flux, a variety of different
micro- and nano-photonic devices have been fabricated on diamond[288].

###  VI-A Efficient Photon Extraction via Device Fabrication

A widely adopted approach for improving the collection efficiency of broadband
PL from color center is fabricating solid immersion lenses (SILs, see Fig. 5
(a))[287, 289]. PL emitted from a color center located in the focus of the SIL
will strike perpendicular to the diamond surface, thus suppressing total
internal reflection. SILs have been used to demonstrate enhanced detection
efficiency from NV centers[150, 290], SiV centers[126, 118] and GeV
centers[291, 292]. One key advantage of SILs is their relatively easy
fabrication using FIB milling around pre-characterized color centers located
deep in the diamond. This is of particular importance for experiments using NV
centers, where the proximity to surfaces often deteriorates their optical
coherence[90]. State-of-the-art demonstrations of remote entanglement of NV
centers[46, 47] utilized diamond SILs fabricated around naturally occurring NV
centers[274]. Diamond SILs provide a detection efficiency of $\sim
10\,\%$[293], which can be further improved by the application of an anti-
reflective coating[40].

Although diamond SILs suppress total internal reflection, the photon detection
efficiency is still limited by the non-directional emission of the color
center PL. Recently, it has been demonstrated that the detection efficiency
can be further increased by using layered dielectric optical antennas[294].
Such a device combines layers of different dielectric materials, where the
contrast in refractive index modifies the emission pattern, leading to
preferential photon emission into the high-index material[295]. In the context
of diamond photonics, a low-loss, broadband dielectric antenna has been
demonstrated using a diamond micromembrane ($n=2.4$) bonded to a macroscopic
GaP SIL ($n=3.3$) suspended in air ($n=1.0$)[296]. As a consequence of the
asymmetric refractive index profile, PL is predominantly emitted into the GaP.
The hemispherical shape of the SIL suppresses total internal refraction, and
can thus, in principle, provide a near-unity collection efficiency[297, 298].
An alternative approach constitutes the use of inverse design[232] to
fabricate photonic structures. Recently, this approach demonstrated a 14-fold
enhancement of broadband PL from NV centers located at a depth of $\sim$100
nm[233].

Nanophotonic waveguiding structures, such as nanowires[299, 300, 301] and
nanopillars (see Fig. 4 (d))[302, 303], offer an alternative method to improve
the collection efficiency of broadband PL. These devices act like antennas,
directing the emitted PL into a well-defined guided mode[151]. Detection
efficiency of $40\,\%$ has been demonstrated using top-down fabricated
nanowires on diamond with ingrown NV centers[299]. One particular advantage of
nanopillars is the possibility to embed single color centers close to the tip
($\sim 10\,\textrm{nm}$) by shallow ion implantation (see Section V-C) prior
to fabrication, while maintaining long spin coherence times[251, 304]. This is
of particular importance for applications in quantum sensing, where the
spatial resolution depends on the sensor to sample distance[108]. Tailoring
the pillar geometry can further increase the collection efficiency. Recently,
a collection efficiency of $57\,\%$ was demonstrated for an NV center located
at the focus of a truncated parabolic reflector diamond nanopillar[305].

###  VI-B Engineering of the Photonic Environment

The devices discussed so far enhance the collection efficiency of broadband
PL, which is particularly useful for applications in quantum sensing, where an
increased photon flux leads to improved sensitivity to magnetic fields[108].
However, quantum information applications require a high flux of coherent,
indistinguishable photons, and therefore modification of the optical
properties of the color centers is often desired. The rate of remote
entanglement in schemes relying on two-photon quantum interference is limited
by the detection rate of coherent photons. At the time of writing, the current
state-of-the-art experiment using NV centers achieved an entanglement rate of
$\sim 10\,\textrm{Hz}$[48, 49], limited by the long radiative lifetime
($\tau\simeq 12\,\textrm{ns}$), the aforementioned small branching into the
ZPL (Debye-Waller factor of $\sim 3\,\%$) and poor photon extraction
efficiency owing to total internal reflection. While the group-IV defect
centers possess more favorable optical properties in terms of a larger Debye-
Waller factor and shorter radiative lifetime, to date, no experiments
demonstrating remote entanglement have been conducted, though recently
indistinguishable photons from SnV color centers were reported[306].

Figure 5: Photonic devices for efficient light extraction: (a) SEM image of a
SIL fabricated using FIB milling. (b) Top panel: schematic of a hybrid
photonic platform based on GaP on diamond. Evanescent coupling between color
centers and optical resonators enhances the photon emission, which is further
waveguided and detected via grating out-couplers. Bottom panel: SEM image of
the hybrid photonic device. (c) Schematic of a diamond membrane embedded in an
open microcavity. The fully tunable platform allows for in situ tuning of both
the cavity resonant frequency and the position of the color center with
respect to the cavity mode. Panel (a) is reprinted from M. Jamali et al., Rev.
Sci. Instrum. 85, 123703 (2014) with permission of AIP Publishing. Panel (b)
is reprinted with permission from M. Gould et al., Phys. Rev. Applied 6,
011001 (2016), Copyright 2016 by the American Physical Society. Panel (c) is
reprinted with permission from D. Riedel et al., Phys. Rev. X 7, 031040 (2017)
licensed under the terms of the Creative Commons Attribution 4.0 license.

In principle, these shortcomings can be addressed by embedding color centers
in photonic resonators[307]. Resonant coupling of the ZPL to a single cavity
mode enhances the ZPL emission on two grounds. First, the cavity directs the
emission into a well-defined output mode, improving the photon collection
efficiency[308, 309]. Second, the emitter experiences a Purcell effect[310]
that enhances the spontaneous emission rate of transitions resonant with a
cavity mode. This can be used to increase the fraction of photons emitted into
the ZPL[123]. For an optical cavity with quality factor $Q$ and mode volume
$V$, the Purcell factor is given by

$\begin{split}F_{\textrm{P}}&=F_{\textrm{P}}^{\textrm{max}}\xi\frac{1}{1+4Q^{2}\left(\lambda_{\textrm{ZPL}}\,/\lambda_{\textrm{cav}}-1\right)^{2}}\,,\\\
F_{\textrm{P}}^{\textrm{max}}&=1+\frac{3}{4\pi^{2}}\frac{Q}{V}\left(\frac{\lambda}{n}\right)^{3}\,,\\\
\end{split}$ (2)

where
$\xi=\left(\frac{\absolutevalue{\boldsymbol{\mu}\cdot\mathbf{E}}}{\absolutevalue{\boldsymbol{\mu}}\cdot\absolutevalue{\mathbf{E}}}\right)^{2}$
describes the overlap between the dipole moment $\boldsymbol{\mu}$ and the
electric field $\mathbf{E}$. Note that $F_{\textrm{P}}$ is independent of any
emitter properties–$F_{\textrm{P}}$ is described solely by the characteristics
of the cavity[309]. The scaling $F_{\textrm{P}}\propto Q\,/V$ motivates the
use of high-quality resonators with minimal mode volume[152].

Coupling between color centers in diamond and photonic resonators has been
demonstrated on various different platforms[152], including, but not limited
to, photonic crystal cavities[311, 197, 187, 312, 313], nanobeam cavities[314,
315, 42, 316, 43, 241, 242], waveguides[317, 230, 318], microrings[231, 319,
320], hybrid optical devices[321, 322, 323, 324, 325, 326] and open
microcavities[327, 328, 123, 329, 330, 331, 332].

In photonic crystal cavities (PCC), the periodic change in refractive index
creates a photonic bandgap. Tailoring of the periodicity can confine light to
a mode volume of $\lesssim(\lambda\,/n)^{3}$. However, at the visible
wavelengths relevant for color centers in diamond, imperfection in the
fabrication of these devices limits the achievable $Q$-factor to $\sim
10^{3}-10^{4}$. The current state-of-the art PCC containing SiV centers
exhibits $Q=2\times 10^{4}$ and $V=0.5(\lambda\,/n)^{3}$[43]. Note that PCCs
with $Q>10^{5}$ have been demonstrated for $\lambda=1,550\,\textrm{nm}$[186].
As discussed in Section V-C, deterministic coupling of color centers to PCCs
can be achieved via FIB milling around pre-characterized color centers[312],
or by using a FIB to implant ions in fabricated structures. The latter
approach has been proven to be particularly successful for SiV centers[42].
Furthermore, red-shifting of the cavity resonance is possible via deposition
of $\textrm{N}_{2}$[42] or Ar[241] gas, thereby maximizing the spectral
overlap between the color canter ZPL and the cavity mode.

Coupling to WGMs in diamond microdisk- and ring resonators offer a monolithic
alliterative to PCCs. In general, WGMs offer a larger $Q$-factor, albeit at
the expense of a larger $V\sim 10(\lambda\,/n)^{3}$. In practice, due to
fabrication imperfection and surface roughness, the achievable $Q$-factors are
comparable to those of PCCs[231]. Nevertheless, due to advances in device
fabrication, a diamond microdisk with $Q=11,000$ has been demonstrated for
$\lambda=638\,\textrm{nm}$[76].

The monolithic photonic devices discussed above offer large $Q\,/V$ ratios at
the expense of invasive nanofabrication. While NV centers with close-to
lifetime limited optical linewidths have been reported in bulk[333, 249], NV
centers embedded in nanophotonic devices often suffer from poor optical
coherence and inhomogeneous broadening of the ZPL linewidth on the account of
a fluctuating charge environment caused by fabrication induced surface
damage[334, 90]. Therefore, increasing the defect free, crystalline
environment has proved to be beneficial[335].

Hybrid photonic platforms constitute an alternative to monolithic resonators
on the account of the possibility to use bulk diamond. These devices (see Fig.
5 (b)) combine single-crystal diamond with waveguides and resonators
fabricated from a high-index material[336, 337]. Evanescent coupling of NV
centers to waveguides[338, 322] and microcavites[321, 339] has been
demonstrated on various platforms based on GaP[323]. However, while red-tuning
of the cavity resonance are possible via gas deposition, one remaining
drawback of these hybrid devices is the difficulty in positioning the color
center close to the field maxima of the guided mode[322]. Furthermore,
evanescent coupling requires near-surface NV centers, which, as discussed
above, suffer from inhomogenous linewidth broadening[90]. Nevertheless, hybrid
photonic devices offer a viable route to integrate color centers in diamond
with large-scale photonic integrated circuits[340, 341]. The state-of-the-art
experiment demonstrated a 128-channel ‘quantum microchiplet’, a diamond
waveguide array containing highly-coherent SiV and GeV centers interfaced with
an integrated AlN photonic circuit[325].

In recent years, planar-concave open Fabry-Perot microcavities (see Fig. 5
(c)) have emerged as a compelling alternative to the monolithic and hybrid
optical resonators discussed above[123, 331, 332]. These cavities are formed
from a highly-reflective planar distributed Bragg-reflector (DBR), to which a
diamond membrane is bonded. A second highly-reflective DBR mirror, with
micron-sized concave indentations fabricated using FIB milling[342, 327] or
CO2 laser ablation[343, 344, 345], concludes the cavity. The Gaussian-shaped
indentations facilitate efficient coupling of the cavity mode to single-mode
external detection optics[346, 347]. The open microcavity offers the
possibility to incorporate micron-sized single-crystal diamond membranes[348],
thus preserving the optical coherence of NV centers[332], while maintaining a
large $Q\,/V$-ratio[309]. Furthermore, with the use of piezoelectric
nanopositioners, the open microcavity platform offers full in situ tunability
and control of both the cavity resonant frequency and the relative position of
the color center with respect to the cavity mode[123]. Resonant coupling of a
single NV center to an open microcavity has demonstrated enhancement of the
fraction of ZPL photons to $\sim 46\,\%$[123]. Finally, the incorporation of a
diamond membrane with a small thickness gradient has demonstrated full in situ
control of both the resonant frequency, and the relative frequency spacing of
adjacent cavity modes[349], thus providing a platform for tunable nonlinear
optics.

##  VII Sensing and Metrology

Quantum sensing and metrology involve the detection and measurement of
physical quantities using quantum properties such as coherence and
entanglement, with the goal of reaching fundamental limits in the
measurement[7, 9]. Varieties of such sensors have been developed in the past
several decades[9]. Among the solid-state platforms, spins in diamond have
emerged as versatile sensors for a variety of physical quantities, both
classical and quantum[350]. The NV center outperforms other emitters in
diamond primarily due to its excellent spin (e.g. long coherence times) and
optical (e.g. photostability, brightness) properties.

###  VII-A NV Center-Based Sensing

The NV center’s electron spin has proven to be an exquisite probe for
measuring several physical quantities, both external and internal to the host
diamond, like electric and magnetic fields[9, 351], temperature[352, 353],
pressure[354], strain[355], rotation[356], and charge[357] with high
sensitivity (see Fig. 6). Diamond based sensing has been intensely pursued and
is rapidly progressing owing to the potentially immediate commercial and
fundamental interests in physics, chemistry, biology and clinical
research[350, 9, 351].

One added advantage of the NV spin as a sensor is its atomic size, which
provides extremely high spatial resolution, outperforming any other sensors
developed to date[282]. Depending on the application in question, NV-based
sensors have been realized using ensembles or single NV centers in diamond
nano-, micro-, or bulk crystals[350, 351]. One area where the NV sensors have
made a profound impact is the realization of nanoscale nuclear magnetic
resonance (NMR) spectroscopy[358] and magnetic resonance imaging (MRI) at
ambient conditions and moderate bias magnetic fields[283, 284]. NMR using NV
centers has addressed the concerns of the low sensitivities associated with
traditional NMR methods[359] and the complexities associated with advanced
methods like magnetic resonance force microscopy (MRFM)[360]. Recent advances
in NV nano-NMR methods have placed NV centers at the forefront of molecular-
scale NMR with unprecedented spectroscopic[358] and structural
resolution[361].

For magnetic resonance based methods to work at the level of a single target
molecule or atom, the probe has to have sufficient sensitivity to individual
spins as well as possess fine spectral resolution. In this regard, NV centers
have shown to be sensitive to individual molecules and spins external to the
diamond surface. An NV spin, being an atomic scale probe, is capable of
measuring NMR signals from target spins in about $(\text{5}\,\textrm{nm})^{3}$
sample volumes[362, 363] with a sub-Angstrom spatial resolution[364, 361] at
ambient conditions. In comparison, traditional NMR requires nanoliter sample
volumes and the spatial resolution is limited to $\mu$m-scale[359], excluding
their application to the level of single live cells, for instance. Recent
advancements in NV sensing protocols have also allowed the achievement of
finer spectroscopic details of the target sample. These approaches include
utilizing long-lived nuclear spin lifetimes[365] and quantum heterodyne
methods[366, 367, 358] to achieve several orders of magnitude improvement in
spectral resolution, effectively bypassing the standard limits set by the
$T_{2}$ and $T_{1}$ times of NV spin. The demonstrated sensitivities are
sufficient to resolve subtle chemical signatures in picoliter sample
volumes[358]. In addition, NV centers have shown to be promising sensors in
condensed matter[368] and biomagnetism[350, 369], and considerable attempts
have been made in using NVs for detecting dark matter[370].

Figure 6: Diamond-based quantum sensing with spins as multi-modal sensors: (a)
Diamond NV centers for sensing biomagentic fields, like those from, for
example, neuronal action potential from an organism at ambient conditions. (b)
NV centers as temperature sensors with high sensitivity (mK /
$\sqrt{\text{Hz}}$) and spatial resolution[353]. (c) Realization of NV spins
as rotation sensor with an aim of miniaturized high precision quantum
gyroscope. The schematic is of an experiment that compares sensor performance
with a standard rotation sensor[356]. (d) All-diamond nanopillar housing a
single NV defect as the scanning probe for high spatial resolution magnetic
imaging. Left panel, green and red arrows indicate NV and target spins,
respectively. Right panel is the SEM image of the scanning probe. (e) NV
centers in diamond on top of an infinitely long magnon waveguide for magnon-
mediated spin-spin entanglement. Panel (a) is reprinted with permission from
J. F. Barry et al., PNAS, 113(49)14133-14138, 2016. Panel (b) is reprinted
with permission from P. Neumann et al., Nano Lett. 2013 13 (6), 2738-2742.
Copyright 2013 American Chemical Society. Panel (c) reprinted with permission
from V. V. Soshenko et al., Phys. Rev. Lett. 126, 197702 (2021), Copyright
2021 by the American Physical Society. Panel (d, left) is adapted with
permission from P. Maletinsky, Quantum Sensing Lab, University of Basel. Panel
(d, right) is reprinted from P. Appel et al., Rev. Sci. Instrum. 87, 063709
(2016), with the permission of AIP Publishing. Panel (e) is reprinted with
permission from M. Fukami et al., Phys. Rev. X Quantum 2, 040314 (2021)
licensed under the terms of the Creative Commons Attribution 4.0 license.

Magnetometers are ubiquitous in many areas of application. The current state-
of-the-art magnetometers (e.g. superconducting quantum interference devices or
SQUIDs and optically pumped magnetometers or OPM) suffer from non-ambient
operating conditions, modest spatial resolution, and limited dynamic range,
albeit offering the highest sensitivities. To this end, progress in NV-
magnetometers over the last decade has proven to be promising, however, some
key challenges remain to be addressed[350, 351]. The primary challenge is that
the sensitivities achievable with NV magnetometers are worse than both the
theoretical predictions and the competing techniques (SQUIDs and OPMs) by at
least three orders of magnitude[9, 351]. This is mainly due to inefficient NV
spin readout methods and limited coherence times. For a diamond sample with
$N$ number of NV sensors per volume ($N$=1 for a single NV center), the
magnetic field sensitivity, $\eta_{\text{B}}$, scales as[371, 372]

$\eta_{\text{B}}\propto\frac{1}{C^{\prime}}\frac{1}{\sqrt{N.T^{\prime}}}\,,$
(3)

where $C^{\prime}$ is the readout fidelity, and $T^{\prime}$ is the
interrogation time (here, the spin initialization and readout times are
neglected). $C^{\prime}$ is determined by the PL photon collection efficiency
and the contrast of the spin states, both of which are far below unity, thus
rendering $C^{\prime}\ll$1\. $\eta_{\text{B}}$ is limited primarily by the
photon shot noise since the readout is performed optically[351], and
ultimately by the quantum projection noise, $\eta_{\text{Bp}}$, so that
$\eta_{\text{B}}=\eta_{\text{Bp}}\,/C^{\prime}$[9]. Several approaches have
been explored to improve $C^{\prime}$ beyond the limits of standard optical
readout by employing non-PL based readout schemes. However, some key
challenges remain for all these methods in terms of optimizing the overhead
(spin initialization and readout) times, added experimental complexities, and
limited readout fidelities. As an alternative to PL detection schemes, cavity-
enhanced readout methods, based on the IR absorption by the NV center singlet
state[373] and interaction between NV and microwave photons in a
resonator[374] have been demonstrated. While these methods have substantially
enhanced the fidelity $C^{\prime}$ (and hence $\eta_{\text{B}}$) by achieving
near unity IR light collection efficiency[373] and unity contrast[374], the
overall $\eta_{\text{B}}$ is still short of reaching the $\eta_{\text{Bp}}$.
Further miniaturization of the sensor in these cavity-based systems for high
spatial resolution remains challenging due to their mm-lengthscale. In this
context, diamond microcavities could fill this gap given their sub-$\mu$m
size, and appreciable $Q\,/V$ ratio (see Sec.VI-B)[209], thus potentially
enabling stronger interaction between spins and cavity fields, leading to
better $\eta_{\text{B}}$. However, it is important to understand the charge
state dynamics of the NV centers under extreme optical powers in microcavities
as they strongly affect $C^{\prime}$.

The next critical factor influencing $\eta_{\text{B}}$ is $T^{\prime}$:
$\eta_{\text{B}}$ improves with longer $T^{\prime}$. The nature of the signal
that can be probed depends on the characteristic timescales of the sensor
spins: their inhomogeneous dephasing time ($T_{2}^{*}$) and spin coherence
time ($T_{2}$). Short-lived $T_{2}^{*}$ times allow for the detection of only
static or slowly varying target $B$-fields such as those in biological
samples[369] or condensed matter[368]. On the other hand, long-lived $T_{2}$
times can probe AC fields or fast oscillations like those from the spins
inside[375] and outside diamond crystal[362, 363]. Engineering qubits with
maximum $T_{2}^{*}$ and $T_{2}$ directly affects quantum sensing strategies
involving phase accumulation, and is therefore currently one of the major
material fabrication challenges.

The remaining parameter influencing $\eta_{\text{B}}$ is $N$. However,
arbitrarily increasing $N$ degrades the $T_{2}^{*}$ and $T_{2}$ times due to
enhanced dipolar interaction among the sensing spins (NV-NV) and with
substitutional nitrogen spins[376]. Another drawback of sensing using a large
$N$ is the compromise of a key advantage of the NV center spin: its atomic
spatial resolution. Currently, the best-reported $\eta_{\text{B}}$ are in the
range of $\upmu$T (for DC)[371] and nT (for AC)[95, 377] for a single NV, and
about pT (DC and AC) for an ensemble NV sensor[351, 378].

In summary, a crucial challenge to realize diamond as a robust quantum sensor
performing at its fundamental limits lies in achieving unity readout
fidelities and engineering high-quality, affordable, diamond samples with
prolonged coherence times. From a material engineering point of view, a
complete understanding and mitigation of the undesired surface-induced noise
sources are essential for sensing target signals. Ongoing investigations on
the novel color centers in diamond may overcome the challenges posed by NV
centers as a sensor.

###  VII-B Hybrid Diamond-Magnetic Sensors

A related area of research that could benefit from the properties of diamond
constitutes the development of nano-optomechanical torque sensors, which have
demonstrated enhanced sensitivities to mechanical torque[379, 380, 16]. They
naturally respond to the fundamental interaction of magnetic moments with an
external field–the magnetic torque–at a magnitude proportional to the total
magnetization[381] providing insight into the dynamics of nanomagnetic
structures.

The coupling of magnetic and mechanical systems has enabled observation of the
Barkhausen effect from nanoscale magnetic defects[382, 383] and spin
resonances in nanomagnets[384]. A longstanding goal for these devices is the
demonstration of strong coupling of magnetic torques to mechanical degrees of
freedom, granting coherent information transfer between the two systems[385].
A more ambitious target would be the realization of torque sensitivities on
the order of $\sim 10^{-29}{\rm Nm/\sqrt{Hz}}$, corresponding to the torque
generated by a single Bohr magneton under a field of 1 A / m. In some ways,
analogous to single-spin detection achieved nearly two decades ago using
magnetic force microscopy[386], the observation of pure magnetic torque
remains elusive.

To date, diamond-based nano-optomechanical sensors of magnetic torque have yet
to be realized. Their development would welcome increased torque sensitivities
based on diamond’s favourable mechanical and optical properties compared to
previously developed silicon-based sensors (see Secs. III and V). From a
device performance point of view, while it is possible to engineer good
mechanical devices in silicon at low temperatures[387], the performance of
these devices is, in general, limited by coupling to surface defects; a
problem that is difficult to circumvent[388]. However, diamond exhibits much
lower mechanical dissipation, and it is therefore expected that the mechanical
properties of diamond will surpass those of silicon. In addition, the
detection sensitivities can be further enhanced by operation at high optical
powers, thanks to diamond’s large optical bandgap and excellent thermal
properties[166]. The challenge of affixing magnetic material to the sensor
without detrimentally affecting its optomechanical coupling and the
suppression of unwanted noise remains.

Recent diamond quantum sensing efforts are directed toward creating hybrid
quantum technologies involving magnetic excitations. Magnons are quantized
spin waves in magnetic materials. NV center-based magnetic resonance has been
used to detect and image the stray fields generated by spin waves with
nanometer-scale spatial resolution. This has enabled time-domain imaging of
their coherent transport, dispersion, and interference[389] and has been shown
to have the sensitivity needed for imaging static magnetization in monolayer
van der Waals materials[390]. Magnons have also been proposed as mediators of
quantum information[391, 392, 393] and as a means for quantum control and
enhanced readout of spin qubits due to their strong local magnetic
fields[394]. The long coherence length of spin waves has enabled coherent
control of color centers over distances of 200$\,\mu$m[395]. An appealing
opportunity exists for long-range spin qubit entanglement schemes mediated by
magnons (a schematic is shown in Fig. 6 (e)), with predicted cooperativities
exceeding unity for NV-NV coupling at low temperature[396].

##  VIII Qubit-Photon Interface

As described at the outset of this review, the realization of quantum networks
requires the interconnection of remote network nodes[6]. Recently, two distant
superconducting qubits housed in separate cryostats were connected using a
cryogenically cooled microwave waveguide[397]. Room-temperature optical links
have been used to efficiently distribute quantum states over long distances at
room temperature, owing to the small interaction cross-section of photons with
the environment[35]. However, a major hurdle toward the realization of a
large-scale quantum network is the development of efficient interfaces between
stationary qubits and optical photons[160].

Coherent qubit-photon coupling is inherently limited by the weak light-matter
interaction[19]. However, as depicted in Fig. 7 (a), cavity quantum
electrodynamics (QED) provides a promising route to overcome this hurdle by
strong confinement of light inside optical resonators, thus enhancing the
qubit-photon interactions[398]. In principle, solid-state emitters embedded in
nanophotonic cavity QED devices offer scalable fabrication[399], on-chip
photonic routing[325], and electronic- and mechanical control[64, 400]: key
requirements for integrated network nodes in large-scale quantum networks.

For quantum network applications, the spin-photon interface has to operate in
a regime where a single photon coherently and reversibly couples to a spin
qubit. In general, the coupling between two different quantum systems is
characterized by the cooperativity parameter $C$, defined as[19]

$C=\frac{4g^{2}}{\gamma_{1}\gamma_{2}}\,,$ (4)

where $g$ is the coupling rate between the systems and $\gamma_{1,2}$ are the
energy decay rates of each system. The condition $C>1$ enables coherent
interaction between the two quantum systems, despite their internal
decoherence, and has been demonstrated for a variety of systems, including,
but not limited to, atoms[401], superconducting resonators[402],
molecules[403, 404], and semiconductor QDs[398]. However, the optical
coherence of solid-state emitters is strongly influenced by the host
material[405, 293]: interactions with the local environment leading to the
inhomogeneous broadening of the optical linewidth and consequently loss of
photon coherence. The effect of inhomogeneous broadening (with rate
$\gamma^{*}$) can be incorporated by introducing the coherence
cooperativity[160, 293], where $\gamma_{2}\rightarrow\gamma_{2}+\gamma^{*}$ in
Eq. 4[309].

Figure 7: Development of qubit-photon interfaces: Cartoons of different
canonical interfaces. (a) Qubit-photon interface based on cavity QED, (b)
spin-mechanical interface, and (c) cavity optomechanical qubit-photon
interface. Schematic of the experimental setup used for (d) a spin-photon
interface using a cavity QED system[43, 406], (e) a spin-mechanical interface
as a step towards cavity optomechanical spin-photon interfaces[146]. (f, left)
Diamond cavity optomechanical interface between spin and telecommunication
photons[74]. (f, right) Spin-photon interface based on optomechanical crystal
cavities with phononic shields can reach $C_{\textrm{sm}}>100$ and
$C_{\textrm{om}}>1$. These next-generation spin-photon interfaces can be
realized with current state-of-the-art devices. Panel (d) is reprinted with
permission from C.T. Nguyen et al., Phys. Rev. Lett. 123, 183602, Copyright
2019 by the American Physical Society. Panel (e) is adapted with permission
from P. Maletinsky, Quantum Sensing Lab, University of Basel.

Remote entanglement protocols relying on two-photon quantum interference
require a high flux of coherent indistinguishable photons. In the Barrett and
Kok protocol[407], successful entanglement events are heralded by the
detection of two independent ZPL photons. The overall success rate of this
protocol scales with $\frac{1}{2}\eta_{\textrm{ZPL}}^{2}$, where
$\eta_{\textrm{ZPL}}$ is the probability of detecting a ZPL photon and the
factor $\frac{1}{2}$ accounts for the need to detect two photons per
successful entanglement event. As discussed in Section VI,
$\eta_{\textrm{ZPL}}$ can be drastically enhanced by utilizing the Purcell
effect in cavity QED devices.

To date, all experiments demonstrating remote entanglement of color centers in
diamond[46, 47] have been conducted using native NV centers and SILs[274].
Reversible spin-photon coupling requires $C_{\textrm{sp}}>1$, a condition that
cannot be satisfied using SILs. In practice, NV centers embedded in cavities
typically suffer from inhomogeneous linewidth broadening[123], manifested by
compromised photon indistinguishability and $C_{\textrm{sp}}\ll 1$. While
recent results using native NV centers in nanopillars are promising[408], for
network nodes using NV centers, the long-term optical stability remains a
significant hurdle. On the other hand, nanoscale positioning of SiV centers in
nanocavities, depicted schematically in Fig. 7 (d), shows remarkable
properties; $C_{\textrm{sp}}>100$[43], enabling spin-state dependent photon
reflection and single-shot spin-state readout with fidelity $>99.9\,\%$,
combined with a fiber collection efficiency surpassing $90\,\%$[190] have been
demonstrated. However, at the time of writing, entanglement of remote SiV
centers, or any other group-IV defects, remains to be reported.

###  VIII-A Strategies for a Universal Qubit–Photon Interface

It is likely that future long-distance quantum networks will make use of
existing low-loss fiber infrastructure. However, to date, most cavity-spin
systems operate at wavelengths resonant with the spin optical transition,
usually in the visible range where fiber loss is high relative to
telecommunication wavelengths. Coherent, entanglement preserving quantum
frequency down-conversion from visible to telecommunication wavelengths is
therefore required to minimize losses in the fiber links[72, 409, 410, 411].
However, cavity QED systems with subsequent frequency down-conversion, while
promising, come with limitations. First and foremost, for this approach to
work, the qubit system must exhibit well-defined intrinsic spin-conserving
optical transitions, a feature absent for popular qubit platforms such as
gate-defined quantum dots and superconducting resonators. Second, differences
in the local environment leads to spectral wandering, rendering photon
emission from two cavities distinguishable. At the time of writing, cavity-
enhanced entanglement of remote color centers in diamond has yet to be
demonstrated.

An active area of research to tackle these challenges at a fundamental level
involves using mechanical systems. Mechanical degrees of freedom are central
to many quantum technologies[412], thanks to their ability to couple to a wide
range of fields–electrical, magnetic, optical, and gravitational–through
device engineering[413]. Nanomechanical systems can be naturally integrated
with solid-state quantum systems by on-chip fabrication, providing a universal
platform to interface a variety of solid-state qubit systems. For example,
phonons have been used to mediate quantum gates between trapped ions[414],
coherently connect superconducting qubits[415] and to manipulate quantum-dot
single-photon sources[416, 417]. The last decade has seen significant
experimental effort towards coupling between spin qubits and mechanical
motion[153].

Spins can be coupled to mechanics via induced strain in their host crystal or
a oscillating magnetic moment [418], and can be modeled using the canonical
system shown in Fig. 7 (b). In recent experiments, coupling between
piezoelectronically actuated mechanical motion and electronic spins has been
realized for color centers in bulk diamond and SiC[419, 420, 421, 422], in
hybrid nanowire[423, 424] and cantilever mechanical resonators (see Fig. 7
(e))[425, 426, 427, 146]. However, combining these spin-mechanical devices
with a coherent optical interface remains challenging owing to the weak
interaction between photons and mechanical motion.

Cavity optomechanical devices provide a platform to enhance the spin-phonon-
photon interaction by integrating mechanical resonators within optical
cavities (See Sec. III). In a cavity optomechanical system, resonant optical
recirculation extends the photon-phonon interaction time, and a parametric
enhancement of the optomechanical coupling rate,
$g_{\textrm{om}}=\sqrt{\overline{n}_{\text{cav}}}g_{0}$, proportional to the
intracavity photon number $\overline{n}_{\text{cav}}$ can be exploited. A
cavity optomechanical device can be designed to support optical modes in the
telecommunication wavelength range, while simultaneously supporting mechanical
modes resonant with the qubit spin transitions. To maximize the spin-phonon
and photon-phonon coupling rates, the devices can be engineered to minimize
mechanical and optical mode volumes, respectively[426]. Nanoscale cavity
optomechanical devices such as optomechanical crystals [428] and microdisks
[76] typically support mechanical modes in the gigahertz frequency regime,
which can be cooled to their quantum ground state [429, 211, 161] in a
dilution fridge.

The operating principle of a spin-optomechanical interface based on spin-
strain coupling is twofold, see Fig. 7 (c): radiation pressure from photons in
an optical mode coherently excites the vibrations of a mechanical mode. This
vibrational motion creates a microscopic stress field oscillating at the
mechanical resonance frequency, which can interact with embedded spin qubits.
The optomechanical interaction can be tuned for reversible photon-phonon
conversion, and can operate at any wavelength resonant with a low-loss mode of
the optical cavity. Recently, a proof-of-principle realization of a room-
temperature cavity optomechanical spin-photon interface was demonstrated using
a diamond microdisk resonator[74]. In this work, a photonic coherent state in
the 1,550 nm telecommunication wavelength band was used to manipulate the
electronic spin of a small ensemble of NV centers. Crucially, the resulting
spin-photon interface does not rely on the intrinsic optical transitions of
the color centers, thereby mitigating the aforementioned problems with
spectral stability[74]. Moreover, this approach is completely generic, and can
be applied to color centers in other host materials[430, 431, 432, 433, 434],
alongside providing a method to control optically inactive qubits[435, 436].

###  VIII-B Universal Optomechanical Spin Interface: Challenges and Solutions

As previously discussed, quantum network applications require operation in a
regime where a single photon coherently and reversibly couples to a spin
qubit. In optomechanical devices, coherent spin-phonon interaction can be
reached provided both the optomechanical- and the spin-phonon cooperativity
$C_{\text{om}}$ and $C_{\text{sm}}$, respectively, exceed the thermal phonon
number $n_{\textrm{th}}$. Cooling of the mechanical resonator to near the
mechanical ground state ensures $n_{\textrm{th}}\ll 1$ – the following
discussion will therefore ignore $n_{\textrm{th}}$. Assuming both the photon-
phonon and the spin-phonon couplings are always ‘on’, the transduction
efficiency $\eta_{\text{sp}}$ of such a two-interface system is given by [78]:

$\eta_{\text{sp}}=\frac{4C_{\text{sm}}C_{\text{om}}}{(1+C_{\text{sm}}+C_{\text{om}})^{2}}\,.$
(5)

A near-unity transduction efficiency is achievable in the limit
$C_{\text{sm}}=C_{\text{om}}\gg 1$[78]. However, recent proposals lift the
above restriction by employing temporal control of the coupling rates[81,
437].

The condition $C_{\text{om}}>1$ has routinely been demonstrated in diamond
optomechanical devices (see Sec. III). However, an outstanding technical
challenge is to perform coherent photon-phonon conversion in a device
cryogenically cooled to near its mechanical quantum ground state, without
heating due to optical absorption. Ground-state cooling, recently demonstrated
in silicon optomechanical quantum memories[161, 162], will be aided by
diamonds low nonlinear absorption and excellent thermal properties[166].

To date, realizing $C_{\text{sm}}>1$ has been hindered by the intrinsically
weak spin-phonon coupling for the NV center ground state[74]. However, in
principle, reaching the coherent transduction regime is possible using already
demonstrated diamond cavity optomechanical devices coupled to spin states with
higher stress-sensitivity. For example, using SiV centers in microdisk
resonators allows for $C_{\text{sm}}\sim 1$, owing to their $10^{5}$-fold
enhanced sensitivity to strain compared to the NV center ground state[153].
Embedding SiV centers in optomechanical crystals will further increase
$g_{\textrm{om}}$ on the account of smaller mechanical mode volume and a lower
damping rate. Upon doing so, the regime $C_{\text{sm}}>100$ can be reached
using already demonstrated spin qubits and devices[77, 194, 387, 74].
Incorporating phononic shields can drastically reduce $\gamma_{\textrm{m}}$,
potentially enabling $C_{\text{sm}}>10^{4}$, paving the way for deterministic
quantum state transfer between single telecom photons and single spins[74] and
the use of phonons as on-chip quantum information carriers[438, 439]. Working
with SiV centers necessitates operation at mK temperatures. However, the NV
center ground-state spins coupled via a phonon-assisted optical Raman process
can reach similar high cooperativities at 8 K[420], albeit stabilizing the
optical transition of NV centers in nanostructures presents a formidable
challenge.

##  IX Outlook: Enabling Quantum Networks with Diamond Integrated Photonics

Developments in diamond photonics have advanced tremendously over the years.
However, to realize a fully functional quantum network, and for related
quantum technologies to reach their maximum potential, there are numerous
obstacles to overcome. A roadmap highlighting imminent challenges and future
objectives in relation to quantum hardware elements is presented in Fig. 8.
The following discussion is centered around addressing these challenges within
the context of integrated diamond photonics.

###  IX-A Robust Qubits

For quantum networking protocols using NV centers [47, 41, 48, 49],
scalability is limited by the low flux of coherent photons. While, in
principle, resonant coupling to a cavity enhances the photon flux[123], NV
centers embedded in nanophotonic cavities typically suffer from inhomogeneous
linewidth broadening[231], compromising photon indistinguishability and
limiting the achievable entanglement rates, thus necessitating tuning and
stabilizing of the ZPL transition[341, 440]. On the contrary, environment-
insensitive color centers, such as SiV, perform excellently in nanophotonic
resonators[43]. However, the SiV center experience heating-induced spin
decoherence manifesting in slow and low-fidelity coherent microwave
control[316, 130]. These limitations motivate the investigation of novel color
centers.

As discussed briefly in Section II-A, careful control of defect concentration
and boron doping, stabilizes the neutral SiV center, SiV0[119]. With a $S=1$
ground-state, SiV0 combines the excellent spin properties of the NV center
($T_{2}\sim 900\,\textrm{ms}$ at 4 K[119]) with the favourable optical
properties of SiV-[441, 119]. Research on SiV0 is still in the early stages,
largely limited by the Fermi level pinning required to stabilize the charge
state[119].

TABLE II: Optical Properties of Group-IV Color Centers in Diamond

Color center |  $\lambda_{\text{zpl}}$ (nm) |  Radiative lifetime (ns) |  QE ($\%$) |  DW factor |  $\Delta_{\textrm{GS}}$ (GHz)
---|---|---|---|---|---
SiV- | 7381 |  $\simeq 1.7-1.8$2 |  $1-10$3 | 0.74 | 485
SiV0 | 9466 |  $\simeq 1.8$7 |  Not known | 0.98 | N /A
GeV- | 6029 |  $\simeq 1.4-6$10 | 12-2511 | 0.612 |  $\sim 160$13
SnV- | 62014 |  $\simeq 4.5-7$15 |  $\sim$ 8014 | 0.5716 | 85014
PbV- |  52017 / 55018 |  $\simeq$ $3-3.5$17,18 |  Not known |  Not known |  570017 / 400019

1[118], 2[126, 127], 3[127, 128], 4[129], 5[132], 6[119], 7[119], 8[119],
9[317], 10[262, 331], 11[331, 442], 12[317], 13[317, 291], 14[267], 15[443,
264], 16[264], 17[266], 18[265, 444], 19[445].

Figure 8: Roadmap to the realization of a large-scale quantum network with
diamond: The objectives have been broadly categorized into six categories,
each section discussed briefly in the text. Subjective difficulty of each task
/ category is represented by the color of the coins/cherries, with green being
the easiest and red being the hardest.

For the negatively charged group-IV color centers, the ground-state splitting
$\Delta_{\textrm{GS}}$ is found to increase with atomic number, owing to
stronger spin-orbit interactions[446]. As a consequence, phonon-assisted
population transfer between the orbital ground-states is suppressed, leading
to prolonged spin coherence times at elevated temperatures[443, 447, 112].
Table II summarizes and compares the key physical properties of group-IV color
centers. Note that research on the PbV center is still in the early stages and
key parameters remain to be determined with consistency[445].

Additionally, the reversible and coherent spin-photon coupling could, in
principle, enable on-chip entanglement routing, thus potentially improving the
communication rate[448]. Alternatively, coupling of spins to propagating
phonons has been proposed as a means to implement scalable, high-fidelity and
hybrid on-chip quantum networks[438, 439].

###  IX-B Nanofabrication

The performance of integrated quantum photonic systems relies fundamentally on
the engineering and nanofabrication of their key components. Significant
progress has been made in the nanostructuring of single-crystal diamond since
the demonstration of nanowire antennas[299] and microring resonators[321, 231]
over a decade ago to the recent realization of complex optomechanical devices
integrated as spin-photon interfaces and waveguides in large-scale
platforms[325]. The lack of wafer-scale single-crystal diamond, though, limits
the scalability of integrated photonic architectures and is a significant
impediment to the realization of scalable fabrication analogous to what is
achieved in the semiconductor industry.

A key challenge in diamond nanostructuring is the mitigation of surface
roughness that arises from the transfer of lithographically defined mask edge
defects and mask erosion during plasma etching, as well as imperfections
inherent to the diamond surface which have evolved from inhomogeneities on the
seed substrate during CVD growth[449]. This roughness can lead to degradation
of both optical and acoustic (both bulk- and phononic-mode) quality factors,
affecting the coupling between the two systems and hindering the transmission
of information over long distances required for quantum communications
applications. The fabrication of devices with dimensions required for smaller
optical mode volumes is also affected by the prevalence of defects. Efforts
for surface roughness abatement prior to nanofabrication processes have
included fine mechanical, chemical, and plasma-based smoothing[450], though
the mechanisms for the anisotropic wear along different crystal facets during
polishing are not yet well understood and therefore results can vary
significantly based on methods[451]. There is also a necessity for the
development of more robust, higher resolution electron beam resists with
improved adhesion, as well as the optimization of hard masks and diamond
plasma etching conditions.

The creation of color centers with long coherence times at the surface or
subsurface (within a few tens of nanometers) of diamond devices is an ongoing
endeavor. Lattice defects induced by ion implantation as well as those
inherent at the diamond surface create charge potentials that couple to the
dipole moment of the spin[452], leading to dephasing between the two charge
states and resulting in optical linewidth broadening compared to what is
exhibited in the bulk[90, 334]. Several approaches have been made to
understand and lessen the impacts of ion implantation, such as exposure
through lithographically defined masks to minimize scattering[453] and post
dosage annealing, though these methods have not been perfected. Surface and
subsurface damage removal has been accomplished through oxygen termination of
the diamond surface post-annealing, improving optical linewidths and spin
coherence by an order of magnitude[450].

###  IX-C Spin-Photon Interface

The realization of a quantum network hinges on the interconnection of remote
network nodes[6]. Cavity QED devices offer an ideal platform to interface
stationary spin qubits with photons on account of the enhanced light-matter
interactions[19]. These devices need to combine a large photon extraction
efficiency[43] with the capability of tuning the emission frequency via on-
chip application of electrical[46] or mechanical[454, 400] fields and
microwaves for spin control. In addition, efficient on-chip frequency down-
conversion from visible to telecommunication band is required to mitigate
losses in the photonic links. Heralded entanglement schemes rely on quantum
interference of indistinguishable photons. Quantum frequency down-conversion
provides a method for interfering photons from spectrally distinguishable
emitters, by detuning the pump laser to compensate for the spectral
difference[455]. Alternatively, coherent photon-phonon and spin-phonon
coupling in an optomechanical cavity provide a way to realize a spin-photon
interface operating directly at telecom wavelengths[74, 418, 83].

###  IX-D Quantum Processor

A universal quantum processor consists of numerous qubits with long coherence
times while supporting conditional and unconditional quantum gates[84]. The
realization of such a multi-qubit processor requires quantum gates to
individually address the memory qubits, without introducing cross-talk
affecting the coherence of the remaining spin-register[98]. Using dynamical-
decoupling sequences[456, 457, 111, 458] and selective coupling to 13C nuclear
spins, a ten-qubit spin register based on the NV center has successfully been
demonstrated[98]. While coupling to 13C nuclear spins has been demonstrated
for the SiV center[316, 43], extending these results to multi-qubit registers
remains challenging, owing to the spin-half nature of both the SiV and
${}^{\text{13}}$C spin systems that requires extended decoupling times[293,
316]. Realizing a multi-qubit register based on SiV or other group-IV color
centers requires experimental effort to mitigate the aforementioned problem
with heating-induced spin decoherence as a result of the applied microwave
pulses.

###  IX-E Hybrid Integrated Devices

Hybrid integrated devices combine the strengths of disparate quantum systems
to build complex quantum architectures. While individual monolithic diamond
spin-photon interfaces have been realized with excellent performance[190],
wafer-scale device fabrication remains elusive[325]. Interfacing diamond with
well-established photonic materials, such as AlN and GaP, provides a path
towards incorporating diamond in photonic integrated circuits (PIC) for on-
chip photon routing[325]. Furthermore, using materials with strong
$\chi^{2}$-nonlinearities enables on-chip frequency conversion[459, 460]. In
addition, frequency shifting of single photons using electro-optical
modulators paves the way for the entanglement of spectrally distinguishable
quantum emitters[461].

Finally, piezo-electric materials have been proposed as a phononic interface
between superconducting qubits and quantum memories based on color centers in
diamond[437]. In this transduction scheme, a microwave photon is converted to
a phonon via the piezoelectric effect, which further interacts with the
quantum memory via spin-phonon coupling[462]. Quantum interference of the
emitted photons thus enables entanglement of remote superconducting circuits.
An advantage of this quantum memory-based transduction scheme over direct
microwave-to-optical-photon conversion is a potential reduction in heating of
the cryostat on account of the lower laser power required, which might
preserve the coherence of the superconducting resonators[462].

###  IX-F Error Correction and Scalability

Quantum systems are inherently noisy: interactions with the environment lead
to decoherence, inevitably manifested by the emergence of errors[109].
Correcting errors is therefore a necessity. Quantum error correction protocols
have been demonstrated on spin registers based on NV centers in diamond[84,
109, 110, 463]. These protocols harness the weak coupling between the electron
spin and the surrounding nuclear spins. Recently, this development was pushed
one step further by the demonstration of fault-tolerant operations on a
diamond quantum processor[37]. While in the early stages, the successful
implementation of fault-tolerant operations has the potential to bring diamond
to the forefront of quantum information processing based on solid-state spins.

At the time of writing, the scalability of diamond as a platform for quantum
information processing is limited in part by challenges in nanofabrication and
large-scale growth of single-crystal diamond[464]. A tremendous experimental
effort is being invested to advance the state-of-the-art in diamond
fabrication[200, 465], and the aforementioned hybrid platforms provide a
promising route to realize scalable photonic platforms [325]. In parallel, new
techniques[466] are being pursued to increase the quality[467] and size[468,
234] of synthetic diamonds. However, despite this progress large-scale
synthetic growth of single-crystal diamond wafers remains elusive.

### Concluding Remarks

Research in the field of diamond integrated quantum photonics is vibrant and
fast-moving, owing to diamond’s unique combination of physical properties and
ability to host robust spin qubits. In this article, we have reviewed the
current state-of-the-art in the field, with particular emphasis on advances
and outstanding challenges in nanofabrication, cavity optomechanics, and the
development of qubit-photon interfaces. To conclude, we provided a road map
illuminating the path towards the realization of a universal quantum network.

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