[1]Raphael Joos

1]Institut für Halbleiteroptik und Funktionelle Grenzflächen (IHFG), Center
for Integrated Quantum Science and Technology (IQST) and SCoPE, University of
Stuttgart, Allmandring 3, Stuttgart, 70569, Germany

# Triggered telecom C-band single-photon source with high brightness, high
indistinguishability and sub-GHz spectral linewidth

<EMAIL_ADDRESS>Stephanie Bauer  Christian Rupp  Sascha
Kolatschek  Wolfgang Fischer  Cornelius Nawrath  Ponraj Vijayan  Robert Sittig
Michael Jetter  Simone L. Portalupi  Peter Michler  [

###### Abstract

Long-range, terrestrial quantum networks will require high brightness single-
photon sources emitting in the telecom C-band for maximum transmission rate.
Many applications additionally demand triggered operation with high
indistinguishability and narrow spectral linewidth. This would enable the
efficient implementation of photonic gate operations and photon storage in
quantum memories, as for instance required for a quantum repeater. Especially,
semiconductor quantum dots (QDs) have shown these properties in the near-
infrared regime. However, the simultaneous demonstration of all these
properties in the telecom C-band has been elusive. Here, we present a
coherently (incoherently) optically-pumped narrow-band
($0.8\text{\,}\mathrm{GHz}$) triggered single-photon source in the telecom
C-band. The source shows simultaneously high single-photon purity with
$g^{(2)}(0)=$0.026\text{\,}$$ ($g^{(2)}(0)=$0.014\text{\,}$$), high two-photon
interference visibility of $0.508\text{\,}$ ($0.664\text{\,}$) and high
application-ready rates of $0.75\text{\,}\mathrm{MHz}$
($1.45\text{\,}\mathrm{MHz}$) of polarized photons. The source is based on a
QD coupled to a circular Bragg grating cavity combined with spectral
filtering. Coherent (incoherent) operation is performed via the novel SUPER
scheme (phonon-assisted excitation).

A key building block of many prospective quantum technologies such as quantum
key distribution (QKD) [1, 2] or quantum metrology [3, 4] are highly efficient
sources of single photons. Semiconductor quantum dot (QD) based single-photon
sources are commonly viewed as a promising platform for those applications
especially due to their exceptional brightness [5, 6] in combination with a
low multi-photon probability. However, as many applications deal with the
distribution of information on a metropolitan or even larger scale, minimizing
the losses in a quantum link becomes essential. For this reason, sources with
emission directly in the telecom C-band ($1530\text{\,}\mathrm{nm}$ \-
$1565\text{\,}\mathrm{nm}$) become crucial, among other reasons, due to the
global loss minimum in standard glass fibers.
In this context, recent theoretical [7] and experimental studies [8, 9] on QDs
coupled to circular Bragg grating cavities (CBG) represent a major advance for
QD single-photon sources emitting in the telecom C-band. Apart from the
single-photon nature, the indistinguishability of emitted photons becomes a
key property for interference-based applications. This opens possibilities
towards implementations of such as quantum teleportation [10, 11] or the
generation of entangled photon states e.g. for enhanced resolution in quantum
metrology [12, 13, 14]. Finally, narrow linewidth of the emitted photons
becomes crucial for an interface with some other system, as for instance with
a quantum memory [15, 16]. All of the introduced criteria (triggered single-
photons, high brightness and indistinguishability, telecom C-band
compatibility and narrow linewidth) are required for the implementation of an
efficient single-photon quantum repeater [17], a pivotal element for long-
distance quantum communication.
Depending on an actual protocol for a quantum communication application, also
the coherence in the photon number basis becomes important [18, 1, 19]. Some
applications, as for instance twin-field QKD rely on coherence in the photon
number basis [20, 19]. On the other hand, the performance of other protocols,
e.g. BB84, is affected by this coherence since it opens additional
possibilities for attacks [1, 21].

In this work, we present a triggered, telecom C-band single-photon source
which combines high brightness and indistinguishability with sub-GHz
linewidth. In this context, values of $g^{(2)}(0)=$0.026(1)\text{\,}$$
($g^{(2)}(0)=$0.014(1)\text{\,}$$), an application-ready rate of
$0.75\text{\,}\mathrm{MHz}$ ($1.45\text{\,}\mathrm{MHz}$), a two-photon
interference visibility of $0.504(14)\text{\,}$ ($0.664(4)\text{\,}$) and a
linewidth of $0.79(1)\text{\,}\mathrm{GHz}$ ($0.80(1)\text{\,}\mathrm{GHz}$)
are simultaneously achieved under coherent (incoherent) pumping combined with
spectral filtering. To the best of our knowledge, a combination of these
characteristics in a single source has been elusive, paving the way towards
single-photon quantum repeater implementations.
We show the versatility of the source by operation under incoherent and
coherent pumping conditions with similar performance which makes it appealing
for a broad range of applications. For the coherent pumping conditions we use
the novel SUPER (Swing-Up of Quantum EmitteR population) scheme which was only
recently proposed [22] and realized for QDs emitting in the near-infrared
regime [23, 24]. This work denotes the first implementation of the SUPER
scheme with a telecom C-band QD whereas we use an alternative approach for the
generation of the synchronized pumping pulses.

## Experimental conditions for (coherent) operation

Figure 1: Schematic of the setup used for coherent pumping via the SUPER
scheme using two synchronized fiber lasers (FLs). The pulses of two
synchronized FLs are superimposed in an unbalanced Mach-Zehnder interferometer
(MZI). Comparing the arrival time of the pulses on a superconducting nanowire
single-photon detector (SNSPD) to the synchronization signal of FL1 with a
time-to-digital converter (TDC) provides the relative time delay between the
pulses which can be adjusted win the tunable delay in the MZI. The emission of
the QD-microcavity system pumped by the synchronized pulses is collected with
an objective and filtered with a series of volume Bragg grating (VBG) notch
(N) and bandpass (BP) filters. The single-photon stream is coupled into a
single-mode fiber via fiber coupler attached to a polarizing BS (PBS).
Additionally, the signal can be filtered with a narrow-band etalon.

The source is based on a QD-microcavity system consisting of an In(Ga)As QD
emitting in the telecom C-band coupled to a circular Bragg grating cavity. For
the measurements, the positively charged exciton state at
$1551\text{\,}\mathrm{nm}$ is employed which exhibits a lifetime of
$\sim$500\text{\,}\mathrm{ps}$$ owing to a Purcell enhancement of
$4.3(8)\text{\,}$ (see supplementary material section I). The emission of the
QD-microcavity system is highly polarized ($>$95\text{\,}\mathrm{\char
37\relax}$$) which we ascribe to the small ellipticity of the cavity and an
spatial offset of the QD from the cavity center resulting in an uneven
coupling to the two cavity polarization modes [6, 25].
Commonly, coherent operation of a QD single-photon source is performed via
resonance fluorescence with polarization-based filtering [26, 27]. This comes,
however, along with an intrinsic limitation of the achievable efficiency which
can only be reduced using specially tailored microcavities [6, 5]. In this
work, we employ coherent pumping via the recently proposed SUPER scheme [22].
There, the QD is pumped with two synchronized laser pulses which are both red-
detuned compared to QD transition enabling straight forward spectral
filtering. For a given detuning $\Delta_{1}<0$ of the first laser, maximum
population of the excited state can be reached for a detuning the second laser
of $|\Delta_{2}|>2\cdot|\Delta_{1}|$ [22, 28].
In previous implementations of the SUPER scheme, it has been shown that the
two pulses can be carved out of a single, broadband pulse of a laser with
femtosecond (fs) pulse length [23, 24] resulting in two spectrally detuned
picosecond (ps) pulses, as needed for the QD excitation. In this work, we take
an alternative route, superimposing independently generated pulses from two
separate fiber lasers (FLs) directly operating with pulse lengths in the ps-
regime. As the SUPER scheme operates at large detunings in the range of
several $\text{\,}\mathrm{m}\mathrm{e}\mathrm{V}$ and with comparably high
excitation powers, this approach circumvents possible power limitations due to
the intrinsically inefficient pulse-shaping and enables the use of ps-lasers
that are commonly employed in typical QD setups. On the other hand, the
possibility to synchronize the two laser sources with ps-precision is
required.
Fig. 1 shows a schematic of the employed setup for that purpose. At first, the
two FLs are precisely synchronized based on the arrival times of the pulses on
a superconducting nanowire single-photon detector (SNSPD). Alternatively, the
synchronization of the FLs can also be achieved by exploiting an intrinsic
effect of the excitation scheme when using standard glass fibers: Due to the
small non-linearity of optical fibers, a (weak) four-wave mixing (FWM) signal
is observed after the synchronization of the FLs. Vice versa, maximizing the
FWM signal after the fiber optimizes the pulse overlap due to the strong power
dependence of the process (see further information in supplementary material
section II).
The FWM signal is filtered before reaching the QD whereas the collected
single-photon emission is separated from the laser signal with spectral
filters (more information in supplementary material section III). Finally, the
emission is coupled into a single-mode fiber with an attached polarizing beam
splitter (PBS) exploiting the high degree of polarization of the source to
generate a polarized photon stream. Employing narrowband filtering with an
etalon enables operation conditions of the final source in the sub-GHz regime.

## Coherent pumping

Figure 2: Emission characteristics of the QD-microcavity system under coherent
SUPER excitation. a Emission spectrum under SUPER pumping and components of
the spectrum when excited with only one laser via the phonon sideband. b Scan
of detuning $\Delta$ and power $P$ of FL2 for
$\Delta_{1}=$4\text{\,}\mathrm{nm}$$ and $P_{1}=$350\text{\,}\mathrm{nW}$$
(repetition rate $f_{\text{rep}}=$76\text{\,}\mathrm{MHz}$$). Final operation
point of the SUPER scheme marked with green ellipse. c Measurement of the
photon autocorrelation $g^{(2)}(\tau)$ with $g^{(2)}(0)=$0.076(1)\text{\,}$$.
d High-resolution spectrum of the emission line revealing a doublet-peak with
single-peak linewidth of
$\Gamma_{\text{FWHM}}=$4.94(5)\text{\,}\mathrm{GHz}$$. e,f Photon
indistinguishability measurement yields a two-photon interference (TPI)
visibility $V_{\text{TPI}}=$0.104(14)\text{\,}$$. The orthogonal measurement
in panel e is offset horizontally for the sake of visual clarity.

In order to obtain efficient excitation in the SUPER scheme, it is necessary
to investigate the rich parameter space of the two laser pulses. Therefore, we
scan the detuning $\Delta$ to the QD transition and power $P$ of FL2 (no time
delay between the laser pulses) for a fixed set of parameters of the first
laser ($\Delta_{1}=$4\text{\,}\mathrm{nm}$$, $P_{1}=$350\text{\,}\mathrm{nW}$$
at a repetition rate of $f_{\textbf{rep}}=$76\text{\,}\mathrm{MHz}$$). An
exemplary spectrum is shown in Fig. 2a representing the emission at the
optimal operation conditions (obtained from the scan depicted in Fig 2b). A
single, narrow line at $1551\text{\,}\mathrm{nm}$ is observed when both lasers
are employed, as anticipated for a successful implementation of the SUPER
scheme. Interestingly, also with only one of the two lasers active, the
transition is still excited, though with strongly reduced intensity. This non-
zero emission is ascribed to pumping via the absorption of a phonon (see
supplementary material section IV). Nevertheless, both signals are more than
an order of magnitude lower than when using the actual SUPER pumping. The
result of the scan can be seen in Fig 2b where the color code corresponds to
the integrated intensity of the emission line corrected for the signal
component due to the phonon-induced pumping. The resonance around
$3\cdot|\Delta_{1}|>|\Delta_{2}|>2.5\cdot|\Delta_{1}|$ clearly shows the
fingerprint of the coherent-pumping in the SUPER scheme [24]. In order to
minimize the influence of the incoherent phonon-induced pumping, we choose the
final operation point for the measurements presented in the following at the
edge of the resonance at $\Delta_{2}=2.7\cdot\Delta_{1}$, $P_{2}=2\cdot P_{1}$
($10.75\text{\,}\mathrm{nm}$, $700\text{\,}\mathrm{nW}$ at
$f_{\text{rep}}=$76\text{\,}\mathrm{MHz}$$). All of the measurement discussed
in the following are carried out at
$f_{\text{rep}}=$152\text{\,}\mathrm{MHz}$$ (same detuning and power per pulse
as for the parameter scan).
For any application of a single-photon source, purity and brightness of the
emitted photon stream are decisive characteristics. To assess the former, we
carry out a measurement of the photon autocorrelation $g^{(2)}(\tau)$ in a
Hanbury-Brown and Twiss (HBT) interferometer, as displayed in Fig. 2c. By
comparing the integrated coincidences within one repetition period around zero
time delay (only corrected for detector dark counts) to the same quantity of a
peak in the Poissonian level yields a good single-photon purity with
$g^{(2)}(0)=$0.076(1)\text{\,}$$. Based on this value, the detected count rate
($CR_{\text{raw}}=$6.50\text{\,}\mathrm{MHz}$$) and the detection efficiency
($76.8\text{\,}\mathrm{\char 37\relax}$, a detailed description of the
efficiencies can be found in the supplementary material), an application-ready
rate of polarized single-photons of
$CR_{\text{end}}=$8.14\text{\,}\mathrm{MHz}$$ coupled into a single-mode
fiber, corresponding to an end-to-end efficiency of
$\eta_{\text{end}}=$5.36\text{\,}\mathrm{\char 37\relax}$$, is found. Until
now, application oriented research on QD-based single-photon sources in
telecom C-band has been limited to incoherent excitation schemes while
investigations on coherent pumping schemes have put the focus more on
fundamental research [29, 30, 31, 32]. However, in terms of application-ready
brightness, a rate of $500\text{\,}\mathrm{kHz}$ under continuous-wave
excitation has not yet been surpassed [32] underlining the significance of the
implementation in this work.
Next, the photon indistinguishability is investigated. For QDs under coherent
pumping this is mainly affected by spectral diffusion [31, 33] due to noise in
the magnetic and electric field in the QD vicinity [34, 33]. To probe the
influence of this broadening on the QD linewidth, a high-resolution spectrum
of the emission line is acquired via a scanning Fabry-Pérot interferometer, as
depicted in Fig. 2d. Interestingly, the scan reveals a doublet peak with a
splitting of $6.19\text{\,}\mathrm{GHz}$ whereas ’Peak 1’ has a relative area
of $39\text{\,}\mathrm{\char 37\relax}$ of ’Peak 2’. Such a finding can be
explained by jumps of the line by the energy of the splitting, e.g. due to a
fluctuating occupancy of a defect state in the vicinity of the QD [35, 36,
37]. Fitting this spectrum with a sum of two Voigt profiles with a common
width yields the introduced splitting and amplitudes as well as a single-peak
linewidth of $\Gamma_{\text{FWHM}}=$4.94(5)\text{\,}\mathrm{GHz}$$. As the
Gaussian component of the Voigt profile dominates, the Lorentzian component
has to be fixed to a value of $0.32\text{\,}\mathrm{GHz}$ corresponding to the
Fourier limit. The influence of the finite resolution of the FPI of
$70\text{\,}\mathrm{MHz}$ is considered via a convolution of the Voigt profile
with a Lorentzian with said width. However, the degree of indistinguishability
is only affected by the amount of broadening taking place on the time scale of
the photon separation [37], i.e. $6.6\text{\,}\mathrm{ns}$ for the
measurements presented here. This is probed by Hong-Ou-Mandel (HOM) type
measurements shown in Fig. 2e,f. Comparing the central peak areas (integration
over all coincidences in one full repetition period) of the measurement with
photons with parallel polarization interfering on the BS to the fully
distinguishable case (orthogonal polarization) yields a degree of
indistinguishability of $V_{\text{TPI}}=$0.104(14)\text{\,}$$. This value is
comparable to previously reported results on similar CBG structures emitting
in the telecom C-band [8, 9]. Integrating the QD layer into a gated structure,
thus stabilizing the local electrical field, is expected to decrease the line
broadening due to the spectral diffusion [33], consequently improving the
photon indistinguishability.

## Incoherent pumping

Figure 3: Incoherent, phonon-assisted pumping of the QD-microcavity system. a
Photon autocorrelation measurement yields $g^{(2)}(0)=$0.069(1)\text{\,}$$. b
High-resolution spectrum shows a linewidth of
$\Gamma_{\text{FWHM}}=$4.83(3)\text{\,}\mathrm{GHz}$$. c,d Photon
indistinguishability measurement resulting in
$V_{\text{FPI}}=$0.349(5)\text{\,}$$. The orthogonal measurement in panel c is
offset horizontally for the sake of visual clarity.

In the following, we operate the QD-microcavity system under incoherent
pumping conditions. As we focus in this paper on the high-brightness aspect,
primarily spectrally filterable pumping schemes are considered due to the, in
general, high intrinsic losses of polarization based filtering [6]. Here, we
choose LA pumping, an incoherent excitation scheme [38] with a blue-detuned
laser which can combine robustness, high brightness [39] and high
indistinguishability [40].
The measurements are performed at $\Delta=$-2\text{\,}\mathrm{nm}$$ and
$P=$700\text{\,}\mathrm{nW}$$ ($f_{\text{rep}}=$152\text{\,}\mathrm{MHz}$$).
The photon autocorrelation measurement under these conditions, depicted in
Fig. 3a, yields a value of $g^{(2)}(0)=$0.069(1)\text{\,}$$ and thus a single-
photon purity comparable to SUPER pumping. This is accompanied by brightness
measures of $CR_{\text{end}}=$9.25\text{\,}\mathrm{MHz}$$
($CR_{\text{raw}}=$7.36\text{\,}\mathrm{MHz}$$,
$\eta_{\text{end}}=$6.09\text{\,}\mathrm{\char 37\relax}$$). Thus, similar
performance under brightness aspects are achieved under coherent and
incoherent pumping. It has to be noted that for LA pumping a reduced blinking
behavior is found (QD is optically active in $94\text{\,}\mathrm{\char
37\relax}$ ($67\text{\,}\mathrm{\char 37\relax}$) of the time under LA
(SUPER)) pumping which contributes to the difference in the brightness
measures. The difference is likely related to an altered QD environment for
the two pumping schemes which can arise due to the different energies of the
pumping lasers.
This assumption is further supported by the FPI scan of the emission line in
Fig. 3b. Contrary to the doublet feature under SUPER pumping, only a single
peak with a linewidth of $\Gamma_{\text{FWHM}}=$4.83(3)\text{\,}\mathrm{GHz}$$
is observed. This indicates a stable charge configuration of the defect state
in the vicinity of the QD preventing the line jumps observed under SUPER
excitation. In turn, a more stable charge configuration is expected to reduce
blinking due to the suppression of tunneling of charge carriers from QD to
defect state [41]. Again, an integration of the QD layer into a gated
structure is expected to further diminish such detrimental effects of local
defects [35].
To finalize the comparison, we perform a HOM measurement under LA excitation,
as depicted in Fig. 3c,d which yields $V_{\text{TPI}}=$0.349(5)\text{\,}$$.
This denotes a clear improvement compared to the degree of
indistinguishability under SUPER pumping. As the (single-peak) FPI linewidths
of the different excitation schemes are similar, this indicates that spectral
diffusion (and line jumps) take place on a shorter time scale for the SUPER
than for LA. However, this degree of indistinguishability does not only
represent an improvement compared to the SUPER scheme but also to previously
reported results on InAs/GaAs QDs in the telecom C-band [31, 8] and a similar
performance as the currently highest reported value across all telecom C-band
QD sources [42].

## Narrow-band source with high indistinguishability

Figure 4: Source performance including narrow-band filtering. SUPER (LA)
pumping in the top (bottom) row. a,b High-resolution spectra showing
Lorentzian lineshapes with
$\Gamma_{\text{FWHM}}=$0.79(1)\text{\,}\mathrm{GHz}$$
($\Gamma_{\text{FWHM}}=$0.80(1)\text{\,}\mathrm{GHz}$$). c,d Photon
autocorrelation measurements yielding $g^{(2)}(0)=$0.026(1)\text{\,}$$
$(g^{(2)}(0)=$0.014(1)\text{\,}$)$. e-h Photon indistinguishability
measurements with $V_{\text{TPI}}=$0.508(18)\text{\,}$$
($V_{\text{TPI}}=$0.664(1)\text{\,}$$). The orthogonal measurements in the
panels e,g are offset horizontally for the sake of visual clarity.

The excellent brightness of the QD-microcavity system allows to employ
spectral filtering with a narrow-band etalon in order to further boost the
indistinguishability. The lineshape of the source is then governed by the
linewidth of the etalon and, consequently exhibits a Lorentzian profile with
$\Gamma_{\text{FWHM}}=$0.79(1)\text{\,}\mathrm{GHz}$$
($\Gamma_{\text{FWHM}}=$0.80(1)\text{\,}\mathrm{GHz}$$) under SUPER (LA)
pumping, as depicted in Fig. 4a(b). Such a narrow linewidth ($\sim$$2.5$ times
the Fourier limit) benefits any interface of the source with other systems, as
for instance required for quantum teleportation [10, 11]. This is the case
since remote interference does only depend on the measured linewidth and not
on the TPI of consecutive photons from one source [43]. Especially, narrowband
operation of the single-photon source in the sub-GHz regime enables efficient
interfacing with atomic memories [15, 16].
Furthermore, the filtering positively influences the single-photon purity of
the source resulting in values of $g^{(2)}(0)=$0.026(1)\text{\,}$$
($g^{(2)}(0)=$0.014(1)\text{\,}$$), depicted in Fig. 4c(d), under SUPER (LA).
Despite the additional losses due to the filtering, the final source exhibits
an appealing brightness with $CR_{\text{end}}=$0.75\text{\,}\mathrm{MHz}$$
($CR_{\text{raw}}=$0.58\text{\,}\mathrm{MHz}$$,
$\eta_{\text{end}}=$0.49\text{\,}\mathrm{\char 37\relax}$$) under SUPER
pumping. As this is partially limited due to the doublet feature in the
unfiltered spectrum, even higher rates are observed under LA excitation with
$CR_{\text{end}}=$1.45\text{\,}\mathrm{MHz}$$
($CR_{\text{raw}}=$1.12\text{\,}\mathrm{MHz}$$,
$\eta_{\text{end}}=$0.95\text{\,}\mathrm{\char 37\relax}$$).
Finally, the indistinguishability of the source, including the narrow-band
filtering, is probed via HOM measurements, as depicted in Fig. 4e-f. The
central peak of the parallel measurements is strongly suppressed for both
pumping schemes leading to values of $V_{\text{TPI}}=$0.508(18)\text{\,}$$
($V_{\text{TPI}}=$0.664(4)\text{\,}$$) for SUPER (LA). To the best of our
knowledge, both values represent the highest indistinguishabilities reported
so far for QD sources emitting in the telecom C-band. The small discrepancy
between the two values is due to different time scale of spectral diffusion
mechanisms for the two pumping schemes.

## Conclusion

In summary, we have shown the first triggered telecom C-band single-photon
source that combines high values of purity
($g^{(2)}(0)=$0.026(1)\text{\,}$$/$g^{(2)}(0)=$0.014(1)\text{\,}$$),
brightness ($0.75\text{\,}\mathrm{MHz}$/$1.45\text{\,}\mathrm{MHz}$
application-ready rate) and indistinguishability (two-photon interference
visibility of $0.504(14)\text{\,}$/$0.664(4)\text{\,}$) with narrow linewidth
($0.79(1)\text{\,}\mathrm{GHz}$/$0.80(1)\text{\,}\mathrm{GHz}$) under
coherent/incoherent operation. This constitutes an important step towards
prospective implementations of a single-photon quantum repeater where mature
sources combining these features are required. The system is based on an
In(Ga)As/GaAs QD coupled to a circular Bragg grating cavity combined with
efficient narrowband spectral filtering.
The source is operated under coherent as well as incoherent pumping conditions
with similar performance. This versatility in the operation conditions opens
the way for a broad range of applications benefitting from the presence or
absence of coherence in the photon number states. Coherent pumping is
implemented by the first realization of the SUPER scheme for QDs emitting in
the telecom C-band. In this way, emission with state-of-the-art brightness
(application-ready rate of $8.14\text{\,}\mathrm{MHz}$ without narrowband
filtering) under coherent pumping is achieved, whereas previous high
brightness QD sources in the telecom C-band were only operated under
incoherent pumping.

## Methods

For synchronization of the laser pulses in the SUPER scheme, we use two fiber
lasers (FL) from PriTel, namely the models FFL (FL1) and UOC (FL2), which are
close in their natural repetition rate. This facilitates to use the electrical
signal of FL1 as clock for the lasing operation of FL2 leading to synchronized
operation of the lasers. In order to set the relative time delay between the
pulses from the different lasers, a superconducting nanowire single-photon
detector (SNSPD, model Single Quantum Eos) and a time-to-digital-converter
(TDC, model Swabian instruments Time Tagger Ultra) are employed. The relative
time delay between both lasers is determined by comparing the arrival time of
each FLs pulses on the SNSPD to the electrical pulse of FL1 with the TDC. As
introduced in the main text, also maximization of a four-wave mixing signal
can be used as indicator for the overlap between the pulses. This time delay
is adjusted by tuning the optical path length of the pulses from FL1.
As both lasers operate at a repetition rate of $76\text{\,}\mathrm{MHz}$ an
unbalanced Mach-Zehnder interferometer is used to double the repetition rate.

Supplementary information Additional information to the presented work is
online available.

Acknowledgments The authors like to thank Julian Maisch and Michal Vyvlecka
for fruitful discussions and the solid-state quantum photonics group of the
IFW Dresden (in person of Caspar Hopfmann) as well as the institute of solid
state physics of the Leibniz Universität Hannover (in person of Fei Ding) for
technical support during the measurements.

Funding The authors gratefully acknowledge funding by the German Federal
Ministry of Education and Research (BMBF) via the project QR.X (No.16KISQ013)
and the European Union’s Horizon 2020 research and innovation program under
Grant Agreement No. 899814 (Qurope). Furthermore, this project (20FUN05
SEQUME) has received funding from the EMPIR programme co-financed by the
Participating States and from the European Union’s Horizon 2020 research and
innovation programme. This work was also funded by the Deutsche
Forschungsgemeinschaft (DFG, German Research Foundation) - 431314977/GRK2642
and the Quantum Technology BW via the project TelecomSPS.

Contributions R.J. performed the measurements with support of W.F. and C.N..
R.J. analyzed the data with the help of W.F.. S.B., C.R. and S.K. fabricated
the circular Bragg grating microcavities. P.V. and R.S. grew the sample with
supervision of M.J.. R.J. wrote the manuscript with the help of C.N., S.L.P
and P.M.. S.L.P. and P.M. supervised the project. All authors contributed to
scientific discussions.

Data availability Data supporting the findings presented in this work are
available from the authors upon reasonable request.

Competing interests The authors declare no competing interests.

## References

  *   * [1] Bozzio, M. _et al._ Enhancing quantum cryptography with quantum dot single-photon sources.  _npj Quantum Inf._ 8, 104 (2022).
  * [2] Xu, F., Ma, X., Zhang, Q., Lo, H.-K. & Pan, J.-W.  Secure quantum key distribution with realistic devices.  _Rev. Mod. Phys._ 92, 025002 (2020).
  * [3] Chu, X.-L., Götzinger, S. & Sandoghdar, V.  A single molecule as a high-fidelity photon gun for producing intensity-squeezed light.  _Nat. Photonics_ 11, 58–62 (2017).
  * [4] Georgieva, H. _et al._ Absolute calibration of a single-photon avalanche detector using a bright triggered single-photon source based on an InGaAs quantum dot.  _Opt. Express_ 29, 23500 (2021).
  * [5] Tomm, N. _et al._ A bright and fast source of coherent single photons.  _Nat. Nanotechnol._ 16, 399–403 (2021).
  * [6] Wang, H. _et al._ Towards optimal single-photon sources from polarized microcavities.  _Nat. Photonics_ 13, 770–775 (2019).
  * [7] Barbiero, A. _et al._ Design study for an efficient semiconductor quantum light source operating in the telecom C-band based on an electrically-driven circular Bragg grating.  _Opt. Express_ 30, 10919 (2022).
  * [8] Nawrath, C. _et al._ Bright Source of Purcell-Enhanced, Triggered, Single Photons in the Telecom C-Band.  _Adv. Quantum Technol._ 2300111, 1–6 (2023).
  * [9] Holewa, P. _et al._ Scalable quantum photonic devices emitting indistinguishable photons in the telecom C-band.  _arXiv:2304.02515_ (2023).
  * [10] Bouwmeester, D. _et al._ Experimental quantum teleportation.  _Nature_ 390, 575–579 (1997).
  * [11] Anderson, M. _et al._ Gigahertz-Clocked Teleportation of Time-Bin Qubits with a Quantum Dot in the Telecommunication $C$ Band.  _Phys. Rev. Appl._ 13, 054052 (2020).
  * [12] Müller, M. _et al._ Quantum-Dot Single-Photon Sources for Entanglement Enhanced Interferometry.  _Phys. Rev. Lett._ 118, 257402 (2017).
  * [13] Giovannetti, V., Lloyd, S. & Maccone, L.  Advances in quantum metrology.  _Nat. Photonics_ 5, 222–229 (2011).
  * [14] Slussarenko, S. _et al._ Unconditional violation of the shot-noise limit in photonic quantum metrology.  _Nat. Photonics_ 11, 700–703 (2017).
  * [15] Thomas, S., Sagona-Stophel, S., Schofield, Z., Walmsley, I. & Ledingham, P.  Single-Photon-Compatible Telecommunications-Band Quantum Memory in a Hot Atomic Gas.  _Phys. Rev. Appl._ 19, L031005 (2023).
  * [16] Thomas, S. E. _et al._ Deterministic Storage and Retrieval of Telecom Quantum Dot Photons Interfaced with an Atomic Quantum Memory.  _arXiv:2303:04166_ 1–13 (2023).
  * [17] Sangouard, N. _et al._ Long-distance entanglement distribution with single-photon sources.  _Phys. Rev. A_ 76, 050301 (2007).
  * [18] Loredo, J. C. _et al._ Generation of non-classical light in a photon-number superposition.  _Nat. Photonics_ 13, 803–808 (2019).
  * [19] Karli, Y. _et al._ Controlling the Photon Number Coherence of Solid-state Quantum Light Sources for Quantum Cryptography.  _arXiv:2305.20017_ 1–17 (2023).
  * [20] Lucamarini, M., Yuan, Z. L., Dynes, J. F. & Shields, A. J.  Overcoming the rate-distance limit of quantum key distribution without quantum repeaters.  _Nature_ 557, 400–403 (2018).
  * [21] Cao, Z., Zhang, Z., Lo, H.-K. & Ma, X.  Discrete-phase-randomized coherent state source and its application in quantum key distribution.  _New J. Phys._ 17, 053014 (2015).
  * [22] Bracht, T. K. _et al._ Swing-Up of Quantum Emitter Population Using Detuned Pulses.  _PRX Quantum_ 2, 040354 (2021).
  * [23] Karli, Y. _et al._ SUPER Scheme in Action: Experimental Demonstration of Red-Detuned Excitation of a Quantum Emitter.  _Nano Lett._ 22, 6567–6572 (2022).
  * [24] Boos, K. _et al._ Coherent Dynamics of the Swing-Up Excitation Technique.  _arXiv:2211.14289_ 1–11 (2022).
  * [25] Peniakov, G. _et al._ Polarized and Un-Polarized Emission from a Single Emitter in a Bullseye Resonator.  _arXiv:2308.06231_ (2023).
  * [26] Muller, A. _et al._ Resonance Fluorescence from a Coherently Driven Semiconductor Quantum Dot in a Cavity.  _Phys. Rev. Lett._ 99, 187402 (2007).
  * [27] Benelajla, M., Kammann, E., Urbaszek, B. & Karrai, K.  Physical Origins of Extreme Cross-Polarization Extinction in Confocal Microscopy.  _Phys. Rev. X_ 11, 021007 (2021).
  * [28] Bracht, T. K. _et al._ Dressed-state analysis of two-color excitation schemes.  _Phys. Rev. B_ 107, 035425 (2023).
  * [29] Nawrath, C. _et al._ Coherence and indistinguishability of highly pure single photons from non-resonantly and resonantly excited telecom C-band quantum dots.  _Appl. Phys. Lett._ 115, 023103 (2019).
  * [30] Anderson, M. _et al._ Coherence in single photon emission from droplet epitaxy and Stranski-Krastanov quantum dots in the telecom C-band.  _Appl. Phys. Lett._ 118 (2021).
  * [31] Nawrath, C. _et al._ Resonance fluorescence of single In(Ga)As quantum dots emitting in the telecom C-band.  _Appl. Phys. Lett._ 118, 244002 (2021).
  * [32] Wells, L. _et al._ Coherent light scattering from a telecom C-band quantum dot.  _arXiv:2205.07997_ 1–22 (2022).
  * [33] Kuhlmann, A. V. _et al._ Transform-limited single photons from a single quantum dot.  _Nat. Commun._ 6, 8204 (2015).
  * [34] Sallen, G. _et al._ Subnanosecond spectral diffusion measurement using photon correlation.  _Nat. Photonics_ 4, 696–699 (2010).
  * [35] Houel, J. _et al._ Probing Single-Charge Fluctuations at a GaAs/AlAs Interface Using Laser Spectroscopy on a Nearby InGaAs Quantum Dot.  _Phys. Rev. Lett._ 108, 107401 (2012).
  * [36] Hauck, M. _et al._ Locating environmental charge impurities with confluent laser spectroscopy of multiple quantum dots.  _Phys. Rev. B_ 90, 235306 (2014).
  * [37] Vural, H. _et al._ Characterization of spectral diffusion by slow-light photon-correlation spectroscopy.  _Phys. Rev. B_ 101, 161401 (2020).
  * [38] Vyvlecka, M. _et al._ Robust excitation of C-band quantum dots for quantum communication.  _Appl. Phys. Lett._ 123 (2023).
  * [39] Glässl, M., Barth, A. M. & Axt, V. M.  Proposed Robust and High-Fidelity Preparation of Excitons and Biexcitons in Semiconductor Quantum Dots Making Active Use of Phonons.  _Phys. Rev. Lett._ 110, 147401 (2013).
  * [40] Reindl, M. _et al._ Highly indistinguishable single photons from incoherently excited quantum dots.  _Phys. Rev. B_ 100, 155420 (2019).
  * [41] Yang, J. _et al._ Photoneutralization of charges in GaAs quantum dot based entangled photon emitters.  _Phys. Rev. B_ 105, 115301 (2022).
  * [42] Vajner, D. A. _et al._ On-demand Generation of Indistinguishable Photons in the Telecom C-Band using Quantum Dot Devices.  _arXiv:2306.08668_ 1–5 (2023).
  * [43] Kambs, B. & Becher, C.  Limitations on the indistinguishability of photons from remote solid state sources.  _New J. Phys._ 20, 115003 (2018).