title:
Strongly Universal Quantum Turing Machines and Invariance of Kolmogorov
  Complexity

abstract:
  We show that there exists a universal quantum Turing machine (UQTM) that can
simulate every other QTM until the other QTM has halted and then halt itself
with probability one. This extends work by Bernstein and Vazirani who have
shown that there is a UQTM that can simulate every other QTM for an arbitrary,
but preassigned number of time steps. As a corollary to this result, we give a
rigorous proof that quantum Kolmogorov complexity as defined by Berthiaume et
al. is invariant, i.e. depends on the choice of the UQTM only up to an additive
constant. Our proof is based on a new mathematical framework for QTMs,
including a thorough analysis of their halting behaviour. We introduce the
notion of mutually orthogonal halting spaces and show that the information
encoded in an input qubit string can always be effectively decomposed into a
classical and a quantum part.


title:
Irreversibility in Collapse-Free Quantum Dynamics and the Second Law of
  Thermodynamics

abstract:
  Proposals to solve the problems of quantum measurement via non-linear
CPT-violating modifications of quantum dynamics are argued to provide a
possible fundamental explanation for the irreversibility of statistical
mechanics as well. The argument is expressed in terms of collapse-free
accounts. The reverse picture, in which statistical irreversibility generates
quantum irreversibility, is argued to be less satisfactory because it leaves
the Born probability rule unexplained.


title: Projective measurement in nuclear magnetic resonance

abstract:
  It is demonstrated that nuclear magnetic resonance experiments using
pseudopure spin states can give possible outcomes of projective quantum
measurement and probabilities of such outcomes. The physical system is a
cluster of six dipolar-coupled nuclear spins of benzene in a liquid-crystalline
matrix. For this system with the maximum total spin S=3, the results of
measuring $S_X$ are presented for the cases when the state of the system is one
of the eigenstates of $S_Z$.


title: Entanglement between qubits induced by a common environment with a gap

abstract:
  We study a system of two qubits interacting with a common environment,
described by a two-spin boson model. We demonstrate two competing roles of the
environment: inducing entanglement between the two qubits and making them
decoherent. For the environment of a single harmonic oscillator, if its
frequency is commensurate with the induced two-qubit coupling strength, the two
qubits could be maximally entangled and the environment could be separable. In
the case of the environment of a bosonic bath, the gap of its spectral density
function is essential to generate entanglement between two qubits at
equilibrium and for it to be used as a quantum data bus.


title: On the Paradoxical Book of Bell

abstract:
  This is an essay-review on a recently re-issued book of John Bell "Speakable
and Unspeakable in Quantum Mechanics". The discussion concentrates around the
Bell Theorem, its assumptions, consequences and frequent overinterpretations.


title: Quantum memory protocol in QED cavity based on Photon echo

abstract:
  A new protocol of the optical quantum memory based on the resonant
interactions of the multi atomic system with a cavity light mode is proposed.
The quantum memory is realized using a controllable inversion of the
inhomogeneous broadening of the resonant atomic transition and impact
interaction (on request) of additional short 2 - laser pulse resonant to an
adjacent atomic transition. We demonstrate that the quantum memory protocol is
effective for arbitrary storage time and can be used for new quantum
manipulations with transient entangled states in the field-atoms evolution. The
effect of the fast absorption and emission of the light field is predicted.


title: Optimal entanglement criterion for mixed quantum states

abstract:
  We develop a strong and computationally simple entanglement criterion. The
criterion is based on an elementary positive map Phi which operates on state
spaces with even dimension N >= 4. It is shown that Phi detects many entangled
states with positive partial transposition (PPT) and that it leads to a class
of optimal entanglement witnesses. This implies that there are no other
witnesses which can detect more entangled PPT states. The map Phi yields a
systematic method for the explicit construction of high-dimensional manifolds
of bound entangled states.


title:
Quantum Dissipation and Decoherence via Interaction with Low-Dimensional
  Chaos: a Feynman-Vernon Approach

abstract:
  We study the effects of dissipation and decoherence induced on a harmonic
oscillator by the coupling to a chaotic system with two degrees of freedom.
Using the Feynman-Vernon approach and treating the chaotic system
semiclassically we show that the effects of the low dimensional chaotic
environment are in many ways similar to those produced by thermal baths. The
classical correlation and response functions play important roles in both
classical and quantum formulations. Our results are qualitatively similar to
the high temperature regime of the Caldeira-Leggett model.


title: Room temperature coherent control of coupled single spins in solid

abstract:
  Coherent coupling between single quantum objects is at the heart of modern
quantum physics. When coupling is strong enough to prevail over decoherence, it
can be used for the engineering of correlated quantum states. Especially for
solid-state systems, control of quantum correlations has attracted widespread
attention because of applications in quantum computing. Such coherent coupling
has been demonstrated in a variety of systems at low temperature1, 2. Of all
quantum systems, spins are potentially the most important, because they offer
very long phase memories, sometimes even at room temperature. Although precise
control of spins is well established in conventional magnetic resonance3, 4,
existing techniques usually do not allow the readout of single spins because of
limited sensitivity. In this paper, we explore dipolar magnetic coupling
between two single defects in diamond (nitrogen-vacancy and nitrogen) using
optical readout of the single nitrogen-vacancy spin states. Long phase memory
combined with a defect separation of a few lattice spacings allow us to explore
the strong magnetic coupling regime. As the two-defect system was well-isolated
from other defects, the long phase memory times of the single spins was not
diminished, despite the fact that dipolar interactions are usually seen as
undesirable sources of decoherence. A coherent superposition of spin pair
quantum states was achieved. The dipolar coupling was used to transfer spin
polarisation from a nitrogen-vacancy centre spin to a nitrogen spin, with
optical pumping of a nitrogen-vacancy centre leading to efficient
initialisation. At the level anticrossing efficient nuclear spin polarisation
was achieved. Our results demonstrate an important step towards controlled spin
coupling and multi-particle entanglement in the solid state.


title:
An Argument for 4D Blockworld from a Geometric Interpretation of
  Non-relativistic Quantum Mechanics

abstract:
  We use a new distinctly "geometrical" interpretation of non-relativistic
quantum mechanics (NRQM) to argue for the fundamentality of the 4D blockworld
ontology. Our interpretation rests on two formal results: Kaiser, Bohr &
Ulfbeck and Anandan showed independently that the Heisenberg commutation
relations of NRQM follow from the relativity of simultaneity (RoS) per the
Poincare Lie algebra, and Bohr, Ulfbeck & Mottelson showed that the density
matrix for a particular NRQM experimental outcome may be obtained from the
spacetime symmetry group of the experimental configuration. Together these
formal results imply that contrary to accepted wisdom, NRQM, the measurement
problem and so-called quantum non-locality do not provide reasons to abandon
the 4D blockworld implication of RoS. After discussing the full philosophical
implications of these formal results, we motivate and derive the Born rule in
the context of our ontology of spacetime relations via Anandan. Finally, we
apply our explanatory and descriptive methodology to a particular experimental
set-up (the so-called "quantum liar experiment") and thereby show how the
blockworld view is not only consistent with NRQM, not only an implication of
our geometrical interpretation of NRQM, but it is necessary in a non-trivial
way for explaining quantum interference and "non-locality" from the spacetime
perspective.


title:
The Dynamics of Entanglement in the Adiabatic Search and Deutsch
  Algorithms

abstract:
  The goal of this paper is to study the effect of entanglement on the running
time of a quantum computation. Adiabatic quantum computation is suited to this
kind of study, since it allows us to explicitly calculate the time evolution of
the entanglement throughout the calculation. On the other hand however, the
adiabatic formalism makes it impossible to study the roles of entanglement and
fidelity separately, which means that results have to be interpreted carefully.
We study two algorithms: the search algorithm and the Deutsch-Jozsa algorithm.
We find some evidence that entanglement can be considered a resource in quantum
computation.


title: Invertible Quantum Operations and Perfect Encryption of Quantum States

abstract:
  In this note, we characterize the form of an invertible quantum operation,
i.e., a completely positive trace preserving linear transformation (a CPTP map)
whose inverse is also a CPTP map. The precise form of such maps becomes
important in contexts such as self-testing and encryption. We show that these
maps correspond to applying a unitary transformation to the state along with an
ancilla initialized to a fixed state, which may be mixed.
  The characterization of invertible quantum operations implies that one-way
schemes for encrypting quantum states using a classical key may be slightly
more general than the ``private quantum channels'' studied by Ambainis, Mosca,
Tapp and de Wolf (FOCS 2000). Nonetheless, we show that their results, most
notably a lower bound of 2n bits of key to encrypt n quantum bits, extend in a
straightforward manner to the general case.


title:
Design and Performance of an InGaAs-InP Single-Photon Avalanche Diode
  Detector

abstract:
  This paper describes the design, fabrication, and performance of
planar-geometry InGaAs-InP devices which were specifically developed for
single-photon detection at a wavelength of 1550 nm. General performance issues
such as dark count rate, single-photon detection efficiency, afterpulsing, and
jitter are described.


title:
Efficiency limits for linear optical processing of single photons and
  single-rail qubits

abstract:
  We analyze the problem of increasing the efficiency of single-photon sources
or single-rail photonic qubits via linear optical processing and destructive
conditional measurements. In contrast to previous work we allow for the use of
coherent states and do not limit to photon-counting measurements. We conjecture
that it is not possible to increase the efficiency, prove this conjecture for
several important special cases, and provide extensive numerical results for
the general case.


title:
Information Extraction from Decohering Systems via Indirect Continuous
  Measurement

abstract:
  In this article we discuss a general information extraction scheme to gain
knowledge of the state and the amount of decoherence based on indirect
continuous measurement. The purpose of this information extraction is to
determine the feedback in order to control decoherence based on an ``output
equation", described by a bilinear form. An interacting bilinear form of
control system is used instead of master equation to analyze the dynamic
properties of the system. The analysis of continuous measurement on a
decohering quantum system has not been extensively studied before.


title:
Bound-States of the Spinless Salpeter Equation for the PT-Symmetric
  Generalized Hulthen Potential by the Nikiforov-Uvarov Method

abstract:
  The one-dimensional spinless Salpeter equation has been solved for the
PT-symmetric generalized Hulth\'{e}n potential. The Nikiforov-Uvarov {NU)
method which is based on solving the second-order linear differential equations
by reduction to a generalized equation of hypergeometric type is used to obtain
exact energy eigenvalues and corresponding eigenfunctions. We have investigated
the positive and negative exact bound states of the s-states for different
types of complex generalized Hulthen potentials.


title: Gigahertz Clocked Quantum Key Distribution in Passive Optical Networks

abstract:
  Two multi-user approaches to fiber-based quantum key distribution systems
operating at gigahertz clock frequencies are presented, both compatible with
standard telecommunications fiber.


title:
New experimental limit on the Pauli Exclusion Principle violation by
  electrons

abstract:
  The Pauli Exclusion Principle (PEP) is one of the basic principles of modern
physics and, even if there are no compelling reasons to doubt its validity, it
is still debated today because an intuitive, elementary explanation is still
missing, and because of its unique stand among the basic symmetries of physics.
The present paper reports a new limit on the probability that PEP is violated
by electrons, in a search for a shifted K$_\alpha$ line in copper: the presence
of this line in the soft X-ray copper fluorescence would signal a transition to
a ground state already occupied by 2 electrons. The obtained value, ${1/2}
\beta^{2} \leq 4.5\times 10^{-28}$, improves the existing limit by almost two
orders of magnitude.


title:
Tavis-Cummings model and collective multi-qubit entanglement in trapped
  ions

abstract:
  We present a method of generating collective multi-qubit entanglement via
global addressing of an ion chain following the guidelines of the
Tavis-Cummings model, where several qubits are coupled to a collective motional
mode. We show that a wide family of Dicke states and irradiant states can be
generated by single global laser pulses, unitarily or helped with suitable
postselection techniques.


title: State tomography for two qubits using reduced densities

abstract:
  The optimal state determination (or tomography) is studied for a composite
system of two qubits when measurements can be performed on one of the qubits
and interactions of the two qubits can be implemented. The goal is to minimize
the number of interactions to be used. The algebraic method applied in the
paper leads to an extension of the concept of mutually unbiased measurements.


title: On SIC-POVMs in Prime Dimensions

abstract:
  The generalized Pauli group and its normalizer, the Clifford group, have a
rich mathematical structure which is relevant to the problem of constructing
symmetric informationally complete POVMs (SIC-POVMs). To date, almost every
known SIC-POVM fiducial vector is an eigenstate of a "canonical" unitary in the
Clifford group. I show that every canonical unitary in prime dimensions p > 3
lies in the same conjugacy class of the Clifford group and give a class
representative for all such dimensions. It follows that if even one such
SIC-POVM fiducial vector is an eigenvector of such a unitary, then all of them
are (for a given such dimension). I also conjecture that in all dimensions d,
the number of conjugacy classes is bounded above by 3 and depends only on d mod
9, and I support this claim with computer computations in all dimensions < 48.


title: Decoherence by engineered quantum baths

abstract:
  We introduce, and determine decoherence for, a wide class of non-trivial
quantum spin baths which embrace Ising, XY and Heisenberg universality classes
coupled to a two-level system. For the XY and Ising universality classes we
provide an exact expression for the decay of the loss of coherence beyond the
case of a central spin coupled uniformly to all the spins of the baths which
has been discussed so far in the literature. In the case of the Heisenberg spin
bath we study the decoherence by means of the time-dependent density matrix
renormalization group. We show how these baths can be engineered, by using
atoms in optical lattices.


title:
Quantum Key Distribution in a Multi-User Network at Gigahertz Clock
  rates

abstract:
  In recent years quantum information research has lead to the discovery of a
number of remarkable new paradigms for information processing and
communication. These developments include quantum cryptography schemes that
offer unconditionally secure information transport guaranteed by
quantum-mechanical laws. Such potentially disruptive security technologies
could be of high strategic and economic value in the future. Two major issues
confronting researchers in this field are the transmission range (typically
<100km) and the key exchange rate, which can be as low as a few bits per second
at long optical fiber distances. This paper describes further research of an
approach to significantly enhance the key exchange rate in an optical fiber
system at distances in the range of 1-20km. We will present results on a number
of application scenarios, including point-to-point links and multi-user
networks. Quantum key distribution systems have been developed, which use
standard telecommunications optical fiber, and which are capable of operating
at clock rates of up to 2GHz. They implement a polarization-encoded version of
the B92 protocol and employ vertical-cavity surface-emitting lasers with
emission wavelengths of 850 nm as weak coherent light sources, as well as
silicon single-photon avalanche diodes as the single photon detectors. The
point-to-point quantum key distribution system exhibited a quantum bit error
rate of 1.4%, and an estimated net bit rate greater than 100,000 bits- per
second for a 4.2 km transmission range.


title: 3.3 Gigahertz Clocked Quantum Key Distribution System

abstract:
  A fibre-based quantum key distribution system operating up to a clock
frequency of 3.3GHz is presented. The system demonstrates significantly
increased key exchange rate potential and operates at a wavelength of 850nm.


title: The Frobenius formalism in Galois quantum systems

abstract:
  Quantum systems in which the position and momentum take values in the ring
${\cal Z}_d$ and which are described with $d$-dimensional Hilbert space, are
considered. When $d$ is the power of a prime, the position and momentum take
values in the Galois field $GF(p^ \ell)$, the position-momentum phase space is
a finite geometry and the corresponding `Galois quantum systems' have stronger
properties. The study of these systems uses ideas from the subject of field
extension in the context of quantum mechanics. The Frobenius automorphism in
Galois fields leads to Frobenius subspaces and Frobenius transformations in
Galois quantum systems. Links between the Frobenius formalism and Riemann
surfaces, are discussed.


title: Atom interferometry

abstract:
  In this paper, we present a brief overview of atom interferometry. This field
of research has developed very rapidly since 1991. Atom and light wave
interferometers present some similarities but there are very important
differences in the tools used to manipulate these two types of waves. Moreover,
the sensitivity of atomic waves and light waves to their environment is very
different. Atom interferometry has already been used for a large variety of
studies: measurements of atomic properties and of inertial effects
(accelerations and rotations), new access to some fundamental constants,
observation of quantum decoherence, etc. We review the techniques used for a
coherent manipulation of atomic waves and the main applications of atom
interferometers.


title: Phase rigidity and avoided level crossings in the complex energy plane

abstract:
  We consider the effective Hamiltonian of an open quantum system, its
biorthogonal eigenfunctions $\phi_\lambda$ and define the value $r_\lambda =
(\phi_\lambda|\phi_\lambda)/<\phi_\lambda|\phi_\lambda>$ that characterizes the
phase rigidity of the eigenfunctions $\phi_\lambda$. In the scenario with
avoided level crossings, $r_\lambda$ varies between 1 and 0 due to the mutual
influence of neighboring resonances. The variation of $r_\lambda$ may be
considered as an internal property of an {\it open} quantum system. In the
literature, the phase rigidity $\rho$ of the scattering wave function
$\Psi^E_C$ is considered. Since $\Psi^E_C$ can be represented in the interior
of the system by the $\phi_\lambda$, the phase rigidity $\rho$ of the
$\Psi^E_C$ is related to the $r_\lambda$ and therefore also to the mutual
influence of neighboring resonances. As a consequence, the reduction of the
phase rigidity $\rho$ to values smaller than 1 should be considered, at least
partly, as an internal property of an open quantum system in the overlapping
regime. The relation to measurable values such as the transmission through a
quantum dot, follows from the fact that the transmission is, in any case,
resonant with respect to the effective Hamiltonian. We illustrate the relation
between phase rigidity $\rho$ and transmission numerically for small open
cavities.


title: General criterion for oblivious remote state preparation

abstract:
  A necessary and sufficient condition is given for general exact remote state
preparation (RSP) protocols to be oblivious, that is, no information about the
target state can be retrieved from the classical message. A novel criterion in
terms of commutation relations is also derived for the existence of
deterministic exact protocols in which Alice's measurement eigenstates are
related to each other by fixed linear operators similar to Bob's unitaries. For
non-maximally entangled resources, it provides an easy way to search for RSP
protocols. As an example, we show how to reduce the case of partially entangled
resources to that of maximally entangled ones, and we construct RSP protocols
exploiting the structure of the irreducible representations of Abelian groups.


title:
Scalable Generation of Graph-State Entanglement through Realistic Linear
  Optics

abstract:
  We propose a scheme for efficient construction of graph states using
realistic linear optics, imperfect photon source and single-photon detectors.
For any many-body entanglement represented by tree graph states, we prove that
the overall preparation and detection efficiency scales only polynomially with
the size of the graph, no matter how small the efficiencies for the photon
source and the detectors.


title:
Creation of macroscopic superpositions of flow states with Bose-Einstein
  condensates

abstract:
  We present a straightforward scheme for creating macroscopic superpositions
of different superfluid flow states of Bose-Einstein condensates trapped in
optical lattices. This scheme has the great advantage that all the techniques
required are achievable with current experiments. Furthermore, the relative
difficulty of creating cats scales favorably with the size of the cat. This
means that this scheme may be well-suited to creating superpositions involving
large numbers of particles. Such states may have interesting technological
applications such as making quantum-limited measurements of angular momentum.


title:
Squeezed Coherent States and a Semiclassical Propagator for the
  Schroedinger equation in Phase

abstract:
  We construct semiclassical solutions of the symplectically covariant
Schroedinger phase-space equation rigorously studied in a previous paper; we
use for this purpose an adaptation of Littlejohn's nearby-orbit method. We take
the opportunity to discuss in some detail the so fruitful notion of squeezed
coherent state and the action of the metaplectic group on these states.


title:
Violation of Bell's Inequality with Transverse Spatial Variables Using
  Fractional Fourier Transforms

abstract:
  This paper has been withdrawn due to a crucial theoretical and experimental
error.


title:
Relevance of the slowly-varying electron gas to atoms, molecules, and
  solids

abstract:
  Under a certain scaling, the electron densities of finite systems become both
large and slowly-varying, so that the gradient expansions of the density
functionals for the Kohn-Sham kinetic and exchange energies become
asymptotically exact to order $\nabla^2$. Neutral atoms of large $Z$ scale
similarly, but a cusp correction at the nucleus requires generalizing the
gradient expansion for exchange, producing the wrong gradient coefficient in
the slowly-varying limit. Meta-generalized gradient approximations (meta-GGA's)
recover both the slowly-varying and large-$Z$ limits. GGA correlation energies
of large-Z atoms are found to be accurate.


title: Entanglement in a Quantum Mixed-Spin Chain

abstract:
  The entanglement in a general mixed spin chain with arbitrary spin $S$ and
1/2 is investigated in the thermodynamical limit. The entanglement is witnessed
by the magnetic susceptibility which decides a characteristic temperature for
an entangled thermal state. The characteristic temperature is nearly
proportional to the interaction $J$ and the mixed spin $S$. The bound of
negativity is obtained on the basis of the magnetic susceptibility. It is found
that the macroscopic magnetic properties are affected by the quantum
entanglement in the real solids. Meanwhile, the entanglement can be
quantitatively evaluated by the thermodynamical observable.


title:
Improving the security of multiparty quantum secret splitting and
  quantum state sharing

abstract:
  A protocol for multiparty quantum secret splitting (MQSS) with an ordered $N$
Einstein-Podolsky-Rosen (EPR) pairs and Bell state measurements is recently
proposed by Deng {\rm et al.} [Phys. Lett. A 354(2006)190]. We analyzed the
security of the protocol and found that this protocol is secure for any other
eavesdropper except for the agent Bob who adopts intercept-and-resend attack.
Bob can obtain all the information of Alice's alone without being found. We
also propose an improved version of the MQSS protocol.


title:
Secure Quantum Secret Sharing Based on Reusable GHZ States as Secure
  Carriers

abstract:
  We show a potential eavesdropper can eavesdrop whole secret information when
the legitimate users use secure carrier to encode and decode classical
information repeatedly in the protocol [proposed in Bagherinezhad S and
Karimipour V 2003 Phys. Rev. A \textbf{67} 044302]. Then we present a revised
quantum secret sharing protocol by using Greenberger-Horne-Zeilinger state as
secure carrier. Our protocol can resist Eve's attack.


title: Manipulating time-bin qubits with fiber optics components

abstract:
  We propose two experimental schemes to implement arbitrary unitary single
qubit operations on single photons encoded in time-bin qubits. Both schemes
require fiber optics components that are available with current technology.


title:
Integrated Conditional Teleportation and Readout Circuit Based on a
  Photonic Crystal Single Chip

abstract:
  We demonstrate the design of an integrated conditional quantum teleportation
circuit and a readout circuit using a two-dimensional photonic crystal single
chip. Fabrication and testing of the proposed quantum circuit can be
accomplished with current or near future semiconductor process technology and
experimental techniques. The readout part of our device, which has potential
for independent use as an atomic interferometer, can also be used on its own or
integrated with other compatible optical circuits to achieve atomic state
detection. Further improvement of the device in terms of compactness and
robustness could be achieved by integrating it with sources and detectors in
the optical regime.


title: The Wavefunction of an Anyon

abstract:
  We consider a two-dimensional spin system in a honeycomb lattice
configuration that exhibits anyonic and fermionic excitations [Kitaev,
cond-mat/0506438]. The exact spectrum that corresponds to the translationally
invariant case of a vortex-lattice is derived from which the energy of a single
pair of vortices can be estimated. The anyonic properties of the vortices are
demonstrated and their generation and transportation manipulations are
explicitly given. A simple interference experiment with six spins is proposed
that can reveal the anyonic statistics of this model.


title: Time as a quantum observable

abstract:
  Some results are reviewed and developments are presented on the study of Time
in quantum mechanics as an observable, canonically conjugate to energy.
Operators for the observable Time are investigated in particle and photon
quantum theory. In particular, this paper deals with the hermitian (more
precisely, maximal hermitian, but non-selfadjoint) operator for Time which
appears: (i) for particles, in ordinary non-relativistic quantum mechanics; and
(ii) for photons, in first-quantization quantum electrodynamics.


title: High Energy Quantum Teleportation Using Neutral Koans

abstract:
  We describe a scheme of stochastic implementations of quantum teleportation
and entanglement swapping in terms of neutral kaons. In this scheme, the kaon
whose state is to be teleported collides with one of the two entangled kaons in
an Einstein-Podolsky-Rosen state. Subsequent detection of the outgoing
particles of the collision completes the two-qubit projection on Alice side.
There appear novel features, which connects quantum information science with
fundamental laws of particle physics.


title: Quantum Gate Design Metric

abstract:
  What is the time-optimal way of realizing quantum operations? Here, we show
how important instances of this problem can be related to the study of shortest
paths on the surface of a sphere under a special metric. Specifically, we
provide an efficient synthesis of a controlled NOT (CNOT) gate between qubits
coupled indirectly via Ising-type couplings to a third spin. Our implementation
of the CNOT gate is more than twice as fast as conventional approaches. The
pulse sequences for efficient manipulation of our coupled spin system are
obtained by explicit computation of geodesics on a sphere under the special
metric. These methods are also used for the efficient synthesis of indirect
couplings and of the Toffoli gate. We provide experimental realizations of the
presented methods on a linear three-spin chain with Ising couplings.


title:
Lower and Upper Bounds on CSL Parameters from Latent Image Formation and
  IGM Heating

abstract:
  We study lower and upper bounds on the parameters for stochastic state vector
reduction, focusing on the mass-proportional continuous spontaneous
localization (CSL) model. We show that the assumption that the state vector is
reduced whan a latent image is formed, in photography or etched track
detection, requires a CSL rate parameter $\lambda$ that is larger than
conventionally assumed by a factor of roughly $2 \times 10^{9\pm 2}$, for a
correlation length $r_C$ of $10^{-5}{\rm cm}$. We reanalyze existing upper
bounds on the reduction rate and conclude that all are compatible with such an
increase in $\lambda$. The best bounds that we have obtained come from a
consideration of heating of the intergalactic medium (IGM), which shows that
$\lambda$ can be at most $\sim 10^{8\pm 1}$ times as large as the standard CSL
value, again for $r_C=10^{-5} {\rm cm}$. (For both the lower and upper bounds,
quoted errors are not purely statistical errors, but rather are estimates
reflecting modeling uncertainties.) We discuss modifications in our analysis
corresponding to a larger value of $r_C$. With a substantially enlarged rate
parameter, CSL effects may be within range of experimental detection (or
refutation) with current technologies. ADDED NOTE: A careful calculation of
radiation from atomic systems in the CSL model (Adler and Ramazanoglu,
arXiv:0707.3134) has reinstated the bound obtained by Fu as the best upper
bound on lambda, with significant implications for CSL model phenomenology.


title: Best Fidelity Conditions for Three Party Quantum Teleportation

abstract:
  Using the entangled three qubit states classified by Acin et al. we find the
best fidelity conditions for quantum teleportation among three parties.


title: Improving the Fidelity of deletion

abstract:
  In this work we study the quantum deletion machine with two transformers and
show that the deletion machine with single transformer performs better than the
deletion machine with more than two transformers. We also observe that the
fidelity of deletion depends on the blank state used in the deleter and so for
different blank state the fidelity is different. Further, we study the
Pati-Braunsein deleter with transformer.


title:
Cavity-QED entangled photon source based on two truncated Rabi
  oscillations

abstract:
  We discuss a cavity-QED scheme to deterministically generate entangled
photons pairs by using a three-level atom successively coupled to two single
longitudinal mode high-Q cavities presenting polarization degeneracy. The first
cavity is prepared in a well defined Fock state with two photons with opposite
circular polarizations while the second cavity remains in the vacuum state. A
half-of-a-resonant Rabi oscillation in each cavity transfers one photon from
the first to the second cavity, leaving the photons entangled in their
polarization degree of freedom. The feasibility of this implementation and some
practical considerations are discussed for both, microwave and optical regimes.
In particular, Monte Carlo wave function simulations have been performed with
state-of-the-art parameter values to evaluate the success probability of the
cavity-QED source in producing entangled photon pairs as well as its
entanglement capability.


title: Quantum key distribution system clocked at 2 GHz

abstract:
  An improved quantum key distribution test system operating at clock rates of
up to 2GHz using a specially adapted commercially available silicon single
photon avalanche diode is presented. The use of improved detectors has improved
the fibre-based test system performance in terms of transmission distance and
quantum bit error rate.


title: Robust Quantum Algorithms with $\eps$-Biased Oracles

abstract:
  This paper considers the quantum query complexity of {\it $\eps$-biased
oracles} that return the correct value with probability only $1/2 + \eps$. In
particular, we show a quantum algorithm to compute $N$-bit OR functions with
$O(\sqrt{N}/{\eps})$ queries to $\eps$-biased oracles. This improves the known
upper bound of $O(\sqrt{N}/{\eps}^2)$ and matches the known lower bound; we
answer the conjecture raised by the paper by Iwama et al. affirmatively. We
also show a quantum algorithm to cope with the situation in which we have no
knowledge about the value of $\eps$. This contrasts with the corresponding
classical situation, where it is almost hopeless to achieve more than a
constant success probability without knowing the value of $\eps$.


title: Geometrical phase of thermal state in hydrogen atom

abstract:
  In this paper, the geometric phase of thermal state in hydrogen atom under
the effects of external magnetic field is considered. Especially the effects of
the temperature upon the geometric phase is discussed. Also we discuss the time
evolution of entanglement of the system. They show some similar behaviors.


title: Geometrical aspects of entanglement

abstract:
  We study geometrical aspects of entanglement, with the Hilbert--Schmidt norm
defining the metric on the set of density matrices. We focus first on the
simplest case of two two-level systems and show that a ``relativistic''
formulation leads to a complete analysis of the question of separability. Our
approach is based on Schmidt decomposition of density matrices for a composite
system and non-unitary transformations to a standard form. The positivity of
the density matrices is crucial for the method to work. A similar approach
works to some extent in higher dimensions, but is a less powerful tool. We
further present a numerical method for examining separability, and illustrate
the method by a numerical study of bound entanglement in a composite system of
two three-level systems.


title:
When the Left Hand and the Right Hand Cooperate Without Informing their
  Owner

abstract:
  A specific instantiation of classical correlation from entangled quantum
resources can be established at a distance through the use of local
measurements without classical communication. It is thereby possible to, e.g.,
allow distant entities to randomly combine forces on a course of action which
is an element of a predetermined set of possible actions. When this protocol is
used, no one knows which action has been chosen. The protocol is intrinsically
secure in the same sense as a one-time pad. The entities must have a shared
entanglement resource, be able to measure that resource in a predetermined
reference frame, and know what they must do to carry out all of the possible
actions.


title: Geometric phases, gauge symmetries and ray representation

abstract:
  The conventional formulation of the non-adiabatic (Aharonov-Anandan) phase is
based on the equivalence class $\{e^{i\alpha(t)}\psi(t,\vec{x})\}$ which is not
a symmetry of the Schr\"{o}dinger equation. This equivalence class when
understood as defining generalized rays in the Hilbert space is not generally
consistent with the superposition principle in interference and polarization
phenomena. The hidden local gauge symmetry, which arises from the arbitrariness
of the choice of coordinates in the functional space, is then proposed as a
basic gauge symmetry in the non-adiabatic phase. This re-formulation reproduces
all the successful aspects of the non-adiabatic phase in a manner manifestly
consistent with the conventional notion of rays and the superposition
principle. The hidden local symmetry is thus identified as the natural origin
of the gauge symmetry in both of the adiabatic and non-adiabatic phases in the
absence of gauge fields, and it allows a unified treatment of all the geometric
phases. The non-adiabatic phase may well be regarded as a special case of the
adiabatic phase in this re-formulation, contrary to the customary understanding
of the adiabatic phase as a special case of the non-adiabatic phase. Some
explicit examples of geometric phases are discussed to illustrate this
re-formulation.


title: Comparison of SDL and LMC measures of complexity: Atoms as a testbed

abstract:
  The "simple" measure of complexity of Shiner, Davison and Landsberg (SDL) and
the "statistical" one, according to Lopez-Ruiz, Mancini and Calbet (LMC), are
compared in atoms as functions of the atomic number Z. Shell effects i.e. local
minima at the closed shells atoms are observed, as well as certain qualitative
trends of SDL and LMC measueres of complexity. If we impose the condition that
SDL and LMC behave similarly as functions of Z, then we can conclude that
complexity increases with Z and for atoms the strength of disorder is zero and
order is four.


title: Long-lived mesoscopic entanglement outside the Lamb-Dicke regime

abstract:
  We create entangled states of the spin and motion of a single $^{40}$Ca$^+$
ion in a linear ion trap. The motional part consists of coherent states of
large separation and long coherence time. The states are created by driving the
motion using counterpropagating laser beams. We theoretically study and
experimentally observe the behaviour outside the Lamb-Dicke regime, where the
trajectory in phase space is modified and the coherent states become squeezed.
We directly observe the modification of the return time of the trajectory, and
infer the squeezing. The mesoscopic entanglement is observed up to $\Delta
\alpha = 5.1$ with coherence time 170 microseconds and mean phonon excitation
$\nbar = 16$.


title: Semiclassical Theory of Amplification and Lasing

abstract:
  In this article we present a systematic derivation of the Maxwell-Bloch
equations describing amplification and laser action in a ring cavity. We derive
the Maxwell-Bloch equations for a two-level medium and discuss their
applicability to standard three- and four-level systems. After discusing
amplification, we consider lasing and pay special attention to the obtention of
the laser equations in the uniform field approximation. Finally, the connection
of the laser equations with the Lorenz model is considered.


title: Generation of coherent terahertz pulses in Ruby at room temperature

abstract:
  We have shown that a coherently driven solid state medium can potentially
produce strong controllable short pulses of THz radiation. The high efficiency
of the technique is based on excitation of maximal THz coherence by applying
resonant optical pulses to the medium. The excited coherence in the medium is
connected to macroscopic polarization coupled to THz radiation. We have
performed detailed simulations by solving the coupled density matrix and
Maxwell equations. By using a simple $V$-type energy scheme for ruby, we have
demonstrated that the energy of generated THz pulses ranges from hundreds of
pico-Joules to nano-Joules at room temperature and micro-Joules at liquid
helium temperature, with pulse durations from picoseconds to tens of
nanoseconds. We have also suggested a coherent ruby source that lases on two
optical wavelengths and simultaneously generates THz radiation. We discussed
also possibilities of extension of the technique to different solid-state
materials.


title: Communicating over adversarial quantum channels using quantum list codes

abstract:
  We study quantum communication in the presence of adversarial noise. In this
setting, communicating with perfect fidelity requires using a quantum code of
bounded minimum distance, for which the best known rates are given by the
quantum Gilbert-Varshamov (QGV) bound. By asking only for arbitrarily high
fidelity and allowing the sender and reciever to use a secret key with length
logarithmic in the number of qubits sent, we achieve a dramatic improvement
over the QGV rates. In fact, we find protocols that achieve arbitrarily high
fidelity at noise levels for which perfect fidelity is impossible. To achieve
such communication rates, we introduce fully quantum list codes, which may be
of independent interest.


title:
Mathematical Theory of the Duality Computer in the Density Matrix
  Formalism

abstract:
  We give the mathematical theory of duality computer in the density matrix
formalism. This result complements the mathematical theory of duality computer
of Gudder in the pure state formalism.


title:
Entanglement Split:Comment on ``Quantum secret sharing based on reusable
  Greenberger-Horne-Zeilinger states as secure carriers'' [Phys. Rev. A 67,
  044302 (2003)]

abstract:
  In a recent paper [S. Bagherinezhad and V. Karimipour, Phys. Rev. A 67,
044302 (2003)], a quantum secret sharing protocol based on reusable GHZ states
was proposed. However, in this comment, it is shown that this protocol is
insecure because a cheater can gain all the secret bits before sharing, while
introducing one data bit error at most in the whole communication, which makes
the cheater avoid the detection by the communication parities.


title:
Local-field correction to the spontaneous decay rate of atoms embedded
  in bodies of finite size

abstract:
  The influence of the size and shape of a dispersing and absorbing dielectric
body on the local-field corrected spontaneous-decay of an excited atom embedded
in the body is studied on the basis of the real-cavity model. By means of a
Born expansion of the Green tensor of the system it is shown that to linear
order in the susceptibility of the body the decay rate exactly follows Tomas's
formula found for the special case of an atom at the center of a homogeneous
dielectric sphere [Phys. Rev. A 63, 053811 (2001)]. It is further shown that
for an atom situated at the interior of an arbitrary dielectric body this
formula remains valid beyond the linear order. The case of an atom embedded in
a weakly polarizable sphere is discussed in detail.


title: The Mutually Unbiased Bases Revisited

abstract:
  The study of Mutually Unbiased Bases continues to be developed vigorously,
and presents several challenges in the Quantum Information Theory. Two
orthonormal bases in $\mathbb C^d, B {and} B'$ are said mutually unbiased if
$\forall b\in B, b'\in B'$ the scalar product $b\cdot b'$ has modulus
$d^{-1/2}$. In particular this property has been introduced in order to allow
an optimization of the measurement-driven quantum evolution process of any
state $\psi \in \mathbb C^d$ when measured in the mutually unbiased bases
$B\_{j} {of} \mathbb C^d$. At present it is an open problem to find the maximal
umber of mutually Unbiased Bases when $d$ is not a power of a prime number.
\noindent In this article, we revisit the problem of finding Mutually Unbiased
Bases (MUB's) in any dimension $d$. The method is very elementary, using the
simple unitary matrices introduced by Schwinger in 1960, together with their
diagonalizations. The Vandermonde matrix based on the $d$-th roots of unity
plays a major role. This allows us to show the existence of a set of 3 MUB's in
any dimension, to give conditions for existence of more than 3 MUB's for $d$
even or odd number, and to recover the known result of existence of $d+1$ MUB's
for $d$ a prime number. Furthermore the construction of these MUB's is very
explicit. As a by-product, we recover results about Gauss Sums, known in number
theory, but which have apparently not been previously derived from MUB
properties.


title:
Perturbative Treatment of the Evolution Operator Associated with Raman
  Couplings

abstract:
  A novel perturbative treatment of the time evolution operator of a quantum
system is applied to the model describing a Raman-driven trapped ion in order
to obtain a suitable 'effective model'. It is shown that the associated
effective Hamiltonian describes the system dynamics up to a certain
transformation which may be interpreted as a 'dynamical dressing' of the
effective model.


title:
Optical implementations, oracle equivalence, and the Bernstein-Vazirani
  algorithm

abstract:
  We describe a new implementation of the Bernstein-Vazirani algorithm which
relies on the fact that the polarization states of classical light beams can be
cloned. We explore the possibility of computing with waves and discuss a
classical optical model capable of implementing any algorithm (on $n$ qubits)
that does not involve entanglement. The Bernstein-Vazirani algorithm (with a
suitably modified oracle), wherein a hidden $n$ bit vector is discovered by one
oracle query as against $n$ oracle queries required classically, belongs to
this category. In our scheme, the modified oracle is also capable of computing
$f(x)$ for a given $x$, which is not possible with earlier versions used in
recent NMR and optics implementations of the algorithm.


title:
Time-bin modulated polarization-entangled biphotons from cavity-enhanced
  down-conversion

abstract:
  We have generated a new type of biphoton state by cavity-enhanced
down-conversion in a type-II phase-matched, periodically-poled KTiOPO_4 (PPKTP)
crystal. By introducing a weak intracavity birefringence, the
polarization-entangled output was modulated between the singlet and triplet
states according to the arrival-time difference of the signal and idler
photons. This cavity-enhanced biphoton source is spectrally bright, yielding a
single-mode fiber-coupled coincidence rate of 0.7 pairs/s per mW of pump power
per MHz of down-conversion bandwidth. Its novel biphoton behavior may be
utilized in sensitive measurements of weak intracavity birefringence.


title: Homological Error Correction: Classical and Quantum Codes

abstract:
  We prove several theorems characterizing the existence of homological error
correction codes both classically and quantumly. Not every classical code is
homological, but we find a family of classical homological codes saturating the
Hamming bound. In the quantum case, we show that for non-orientable surfaces it
is impossible to construct homological codes based on qudits of dimension
$D>2$, while for orientable surfaces with boundaries it is possible to
construct them for arbitrary dimension $D$. We give a method to obtain planar
homological codes based on the construction of quantum codes on compact
surfaces without boundaries. We show how the original Shor's 9-qubit code can
be visualized as a homological quantum code. We study the problem of
constructing quantum codes with optimal encoding rate. In the particular case
of toric codes we construct an optimal family and give an explicit proof of its
optimality. For homological quantum codes on surfaces of arbitrary genus we
also construct a family of codes asymptotically attaining the maximum possible
encoding rate. We provide the tools of homology group theory for graphs
embedded on surfaces in a self-contained manner.


title: Quantum teleportation between light and matter

abstract:
  Quantum teleportation is an important ingredient in distributed quantum
networks, and can also serve as an elementary operation in quantum computers.
Teleportation was first demonstrated as a transfer of a quantum state of light
onto another light beam; later developments used optical relays and
demonstrated entanglement swapping for continuous variables. The teleportation
of a quantum state between two single material particles (trapped ions) has now
also been achieved. Here we demonstrate teleportation between objects of a
different nature - light and matter, which respectively represent 'flying' and
'stationary' media. A quantum state encoded in a light pulse is teleported onto
a macroscopic object (an atomic ensemble containing 10^12 caesium atoms).
Deterministic teleportation is achieved for sets of coherent states with mean
photon number (n) up to a few hundred. The fidelities are 0.58+-0.02 for n=20
and 0.60+-0.02 for n=5 - higher than any classical state transfer can possibly
achieve. Besides being of fundamental interest, teleportation using a
macroscopic atomic ensemble is relevant for the practical implementation of a
quantum repeater. An important factor for the implementation of quantum
networks is the teleportation distance between transmitter and receiver; this
is 0.5 metres in the present experiment. As our experiment uses propagating
light to achieve the entanglement of light and atoms required for
teleportation, the present approach should be scalable to longer distances.


title: Quantum Information and Entropy

abstract:
  Thermodynamic entropy is not an entirely satisfactory measure of information
of a quantum state. This entropy for an unknown pure state is zero, although
repeated measurements on copies of such a pure state do communicate
information. In view of this, we propose a new measure for the informational
entropy of a quantum state that includes information in the pure states and the
thermodynamic entropy. The origin of information is explained in terms of an
interplay between unitary and non-unitary evolution. Such complementarity is
also at the basis of the so-called interaction-free measurement.


title: On connection between purity and fidelity of quantum channels

abstract:
  Quantum channels depending on a number of classical control parameters are
considered. Assuming the stochastic fluctuations of the control parameters in
the small errors limit it is shown that the channel fidelity is equal to the
average value of the channel purities calculated in the cases of the control
errors presence and absence respectively. The result validity is demonstrated
on two particular examples, namely, one-qubit quantum gates implemented in the
framework of the ion trap setup and the single qubit anisotropic depolarizing
quantum channel.


title: Deterministic single photons via conditional quantum evolution

abstract:
  A source of deterministic single photons is proposed and demonstrated by the
application of a measurement-based feedback protocol to a heralded single
photon source consisting of an ensemble of cold rubidium atoms. Our source is
stationary and produces a photoelectric detection record with sub-Poissonian
statistics.


title:
Reply to Entanglement split: "Comment on quantum secret sharing based on
  reusable Greenbergr-Horne-Zeilinger states as secure carriers"

abstract:
  In a recent comment, it has been shown that in a quantum secret sharing
protocol proposed in [S. Bagherinezhad, V. Karimipour, Phys. Rev. {\bf A}, 67,
044302, (2003)], one of the receivers can cheat by splitting the entanglement
of the carrier and intercepting the secret, without being detected. In this
reply we show that a simple modification of the protocol prevents the receivers
from this kind of cheating.


title: Assessment of a quantum phase gate operation based on nonlinear optics

abstract:
  We analyze in detail the proposal for a two-qubit gate for travelling
single-photon qubits recently presented by C. Ottaviani \emph{et al}. [Phys.
Rev. A \textbf{73}, 010301(R) (2006)]. The scheme is based on an ensemble of
five-level atoms coupled to two quantum and two classical light fields. The two
quantum fields undergo cross-phase modulation induced by electromagnetically
induced transparency. The performance of this two-qubit quantum phase gate for
travelling single-photon qubits is thoroughly examined in the steady-state and
transient regimes, by means of a full quantum treatment of the system dynamics.
In the steady-state regime, we find a general trade-off between the size of the
conditional phase shift and the fidelity of the gate operation. However, this
trade-off can be bypassed in the transient regime, where a satisfactory gate
operation is found to be possible, significantly reducing the gate operation
time.


title: On the Generator of Lorentz Boost

abstract:
  Traditionally, the theory related to the spatial angular momentum has been
studied completely, while the investigation in the generator of Lorentz boost
is inadequate. In this paper we show that the generator of Lorentz boost has a
nontrivial physical significance, it endows a charged system with an electric
moment, and has an important significance for the electrical manipulations of
electron spin in spintronics. An alternative treatment and interpretation for
the traditional Darwin term and spin-orbit coupling are given.


title: First Quantized Theory of the Photon

abstract:
  In near-field optics and optical tunneling theory, photon wave mechanics,
i.e., the first quantized theory of the photon, allows us to address the
spatial field localization problem in a flexible manner which links smoothly to
classical electromagnetics. In this letter, photon wave mechanics is developed
in a rigorous and unified way, based on which field quantization is obtained in
a new way.


title: On Concurrence and Entanglement of Rank Two Channels

abstract:
  Concurrence and further entanglement quantifyers can be computed explicitly
for channels of rank two if representable by just two Kraus operators. Almost
all details are available for the subclass of rank two 1-qubit-channels. There
is a simple geometric picture beyond, explaining nicely the role of
anti-linearity.


title:
Existence of A Rigorous Density-Functional Theory for Open Electronic
  Systems

abstract:
  We prove that the electron density function of a real physical system can be
uniquely determined by its values on any finite subsystem. This establishes the
existence of a rigorous density-functional theory for any open electronic
system. By introducing a new density functional for dissipative interactions
between the reduced system and its environment, we subsequently develop a
time-dependent density-functional theory which depends in principle only on the
electron density of the reduced system.


title: Quantum to Classical Transition per the Relational Blockworld

abstract:
  The transition from the quantum realm to the classical realm is described in
the context of the Relational Blockworld (RBW) interpretation of
non-relativistic quantum mechanics. We first introduce RBW, discuss its
philosophical implications and provide an example of its explanatory
methodology via the so-called "quantum liar experiment." We then provide a
simple example of a quantum to classical transition in this context using a
gedanken twin-slit experiment. We conclude by speculating on the extrapolation
of RBW to quantum field theory, suggesting the need for a new principle of
physics based in spatiotemporal relationalism. Accordingly, RBW suggests a
novel approach to new physics which supplies a means for its falsification.


title: Two-way Protocol for Quantum Cryptography with Imperfect Devices

abstract:
  The security of a deterministic quantum scheme for communication, namely the
LM05 [1], is studied in presence of a lossy channel under the assumption of
imperfect generation and detection of single photons. It is shown that the
scheme allows for a rate of distillable secure bits higher than that pertaining
to BB84 [2]. We report on a first implementation of LM05 with weak pulses.


title: Entanglement in a Spin-$s$ Antiferromagnetic Heisenberg Chain

abstract:
  The entanglement in a general Heisenberg antiferromagnetic chain of arbitrary
spin-$s$ is investigated. The entanglement is witnessed by the thermal energy
which equals to the minimum energy of any separable state. There is a
characteristic temperature below that an entangled thermal state exists. The
characteristic temperature for thermal entanglement is increased with spin $s$.
When the total number of lattice is increased, the characteristic temperature
decreases and then approaches a constant. This effect shows that the thermal
entanglement can be detected in a real solid state system of larger number of
lattices for finite temperature. The comparison of negativity and entanglement
witness is obtained from the separability of the unentangled states. It is
found that the thermal energy provides a sufficient condition for the existence
of the thermal entanglement in a spin-$s$ antiferromagnetic Heisenberg chain.


title:
Entanlement-Assisted Classical Capacity of Quantum Channels with
  Correlated Noise

abstract:
  We calculate the entanglement-assisted classical capacity of symmetric and
asymmetric Pauli channels where two consecutive uses of the channels are
correlated. It is evident from our study that in the presence of memory, a
higher amount of classical information is transmitted over quantum channels if
there exists prior entanglement as compared to product and entangled state
coding.


title: Full time nonexponential decay in double-barrier quantum structures

abstract:
  We examine an analytical expression for the survival probability for the time
evolution of quantum decay to discuss a regime where quantum decay is
nonexponential at all times. We find that the interference between the
exponential and nonexponential terms of the survival amplitude modifies the
usual exponential decay regime in systems where the ratio of the resonance
energy to the decay width, is less than 0.3. We suggest that such regime could
be observed in semiconductor double-barrier resonant quantum structures with
appropriate parameters.


title: Is Weak Pseudo-Hermiticity Weaker than Pseudo-Hermiticity?

abstract:
  For a weakly pseudo-Hermitian linear operator, we give a spectral condition
that ensures its pseudo-Hermiticity. This condition is always satisfied
whenever the operator acts in a finite-dimensional Hilbert space. Hence weak
pseudo-Hermiticity and pseudo-Hermiticity are equivalent in finite-dimensions.
This equivalence extends to a much larger class of operators. Quantum systems
whose Hamiltonian is selected from among these operators correspond to
pseudo-Hermitian quantum systems possessing certain symmetries.


title: Cluster state quantum computing in optical fibers

abstract:
  A scheme for the implementation of the cluster state model of quantum
computing in optical fibers, which enables the feedforward feature, is
proposed. This scheme uses the time-bin encoding of qubits. Following
previously suggested methods of applying arbitrary one-qubit gates in optical
fibers, two different ways for the realization of fusion gate types I and II
for cluster production are proposed: a fully time-bin based encoding scheme and
a combination of time-bin and polarization based encoding scheme. Also the
methods of measurement in any desired bases for the purpose of the processing
of cluster state computing for both these encodings are explained.


title: Area law and vacuum reordering in harmonic networks

abstract:
  We review a number of ideas related to area law scaling of the geometric
entropy from the point of view of condensed matter, quantum field theory and
quantum information. An explicit computation in arbitrary dimensions of the
geometric entropy of the ground state of a discretized scalar free field theory
shows the expected area law result. In this case, area law scaling is a
manifestation of a deeper reordering of the vacuum produced by majorization
relations. Furthermore, the explicit control on all the eigenvalues of the
reduced density matrix allows for a verification of entropy loss along the
renormalization group trajectory driven by the mass term. A further result of
our computation shows that single-copy entanglement also obeys area law
scaling, majorization relations and decreases along renormalization group
flows.


title: Geometric approach to the discrete Wigner function

abstract:
  We analyze the Wigner function constructed on the basis of the discrete
rotation and displacement operators labeled with elements of the underlying
finite field. We separately discuss the case of odd and even characteristics
and analyze the algebraic origin of the non uniqueness of the representation of
the Wigner function. Explicit expressions for the Wigner kernel are given in
both cases.


title: Reaching 7Li BEC with a Mini-Trap

abstract:
  A novel mm-scale Ioffe-Pritchard trap is used to achieve Bose-Einstein
condensation in 7Li. The trap employs free-standing copper coils integrated
onto a direct-bond copper surface electrode structure. The trap achieves a
radial magnetic gradient of 420 G/cm, an axial oscillation frequency of 50 Hz
and a trap depth of 66 G with a 100 A drive current and 7 W total power
dissipation.


title:
General recursive solution for one dimensional quantum potentials: a
  simple tool for applied physics

abstract:
  A revision of the recursive method proposed by S.A. Shakir [Am. J.Phys.
\textbf{52}, 845 (1984)] to solve bound eigenvalues of the Schr\"odinger
equation is presented. Equations are further simplified and generalized for
computing wave functions of any given one-dimensional potential, providing
accurate solutions not only for bound states but also for scattering and
resonant states, as demonstrated here for a few examples.


title: An interpretation of quantum mechanics based on a waveguide analogy

abstract:
  Based on an observation that the basic mode of a common microwave waveguide
is a solution to the Klein-Gordon equation, quantum mechanics is modeled as the
wave-function propagated inside a waveguide. The guide width is determined by
the rest-energy and the potential energy. Moreover, a geometrical optics model
of the propagation illustrates how the width as a function of distance
determines the velocity and accelerations of the particle. The model entails
the disappearance of action-at-a-distance problem, and that interactions are
conveyed via the potential, i.e. by photons.


title: The one-way CNOT simulation

abstract:
  In this paper we present the complete simulation of the quantum logic CNOT
gate in the one-way model, that consists entirely of one-qubit measurements on
a particular class of entangled states.


title:
Experimental Test of Two-way Quantum Key Distribution in Presence of
  Controlled Noise

abstract:
  We describe the experimental test of a quantum key distribution performed
with a two-way protocol without using entanglement. An individual incoherent
eavesdropping is simulated and induces a variable amount of noise on the
communication channel. This allows a direct verification of the agreement
between theory and practice.


title: Coherent state quantization and phase operator

abstract:
  Phase operators are constructed using a Klauder-Berezin coherent state
quantization in finite Hilbert subspaces of the Hilbert space of Fourier
series. The study of infinite dimensional limits of mean values of some
observables phase leads towards a simpler convergence to the canonical
commutation relations.


title: Interference, reduced action, and trajectories

abstract:
  Instead of investigating the interference between two stationary, rectilinear
wave functions in a trajectory representation by examining the two rectilinear
wave functions individually, we examine a dichromatic wave function that is
synthesized from the two interfering wave functions. The physics of
interference is contained in the reduced action for the dichromatic wave
function. As this reduced action is a generator of the motion for the
dichromatic wave function, it determines the dichromatic wave function's
trajectory. The quantum effective mass renders insight into the behavior of the
trajectory. The trajectory in turn renders insight into quantum nonlocality.


title:
Welcher Weg? A trajectory representation of a quantum Young's
  diffraction experiment

abstract:
  The double slit problem is idealized by simplifying each slit by a point
source. A composite reduced action for the two correlated point sources is
developed. Contours of the reduced action, trajectories and loci of transit
times are developed in the region near the two point sources. The trajectory
through any point in Euclidian 3-space also passes simultaneously through both
point sources.


title: Efficient retrieval of a single excitation stored in an atomic ensemble

abstract:
  We report significant improvements in the retrieval efficiency of a single
excitation stored in an atomic ensemble and in the subsequent generation of
strongly correlated pairs of photons. A 50% probability to transform the stored
excitation into one photon in a well-defined spatio-temporal mode at the output
of the ensemble is demonstrated. These improvements are illustrated by the
generation of high-quality heralded single photons with a suppression of the
two-photon component below 1% of the value for a coherent state. A broad
characterization of our system is performed for different parameters in order
to provide input for the future design of realistic quantum networks.


title: Exact Solutions of a Non-Polynomially Nonlinear Schrodinger Equation

abstract:
  A nonlinear generalisation of Schrodinger's equation had previously been
obtained using information-theoretic arguments. The nonlinearities in that
equation were of a nonpolynomial form, equivalent to the occurence of
higher-derivative nonlinear terms at all orders. Here we construct some exact
solutions to that equation in 1+1 dimensions. On the half-line, the solutions
resemble exponentially damped Bloch waves even though no external periodic
potential is included: the periodicity is induced by the nonpolynomiality. The
solutions are nonperturbative as they do not reduce to solutions of the linear
theory in the limit that the nonlinearity parameter vanishes. An intriguing
feature of the solutions is their infinite degeneracy: for a given energy,
there exists a very large arbitrariness in the normalisable wavefunctions. We
also consider solutions to a q-deformed version of the nonlinear equation and
discuss a natural discretisation implied by the nonpolynomiality. Finally, we
contrast the properties of our solutions with other solutions of nonlinear
Schrodinger equations in the literature and suggest some possible applications
of our results in the domains of low-energy and high-energy physics.


title:
Preparation of a Single Photon in Time-bin Entangled States via Photon
  Parametric Interaction

abstract:
  A novel method for preparation of a single photon in temporally-delocalized
entangled modes is proposed and analyzed. We show that two single-photon pulses
propagating in a driven nonabsorbing medium with different group velocities are
temporally split under parametric interaction into well-separated pulses. In
consequence, the single-photon "time-bin-entangled" states are generated with a
programmable entanglement easily controlled by driving field intensity. The
experimental study of nonclassical features and nonlocality of generated states
by means of balanced homodyne tomography is discussed.


title:
Splitting times of doubly quantized vortices in dilute Bose-Einstein
  condensates

abstract:
  Recently, the splitting of a topologically created doubly quantized vortex
into two singly quantized vortices was experimentally investigated in dilute
atomic cigar-shaped Bose-Einstein condensates [Y. Shin et al., Phys. Rev. Lett.
93, 160406 (2004)]. In particular, the dependency of the splitting time on the
peak particle density was studied. We present results of theoretical
simulations which closely mimic the experimental set-up. Contrary to previous
theoretical studies, claiming that thermal excitations are the essential
mechanism in initiating the splitting, we show that the combination of
gravitational sag and time dependency of the trapping potential alone suffices
to split the doubly quantized vortex in time scales which are in good agreement
with the experiments. We also study the dynamics of the resulting singly
quantized vortices which typically intertwine--especially, a peculiar vortex
chain structure appears for certain parameter values.


title: Efficient Generation of Generic Entanglement

abstract:
  We find that generic entanglement is physical, in the sense that it can be
generated in polynomial time from two-qubit gates picked at random. We prove as
the main result that such a process generates the average entanglement of the
uniform (Haar) measure in at most $O(N^3)$ steps for $N$ qubits. This is
despite an exponentially growing number of such gates being necessary for
generating that measure fully on the state space. Numerics furthermore show a
variation cut-off allowing one to associate a specific time with the
achievement of the uniform measure entanglement distribution. Various
extensions of this work are discussed. The results are relevant to entanglement
theory and to protocols that assume generic entanglement can be achieved
efficiently.


title: Conclusive quantum state classification

abstract:
  In this paper, we have considered the problem of general conclusive quantum
state classification; the necessary and sufficient conditions for the existence
of conclusive classification strategies have also been presented. Moreover, we
have given the upper bound for the maximal success probability.


title: Squeezing frequency combs

abstract:
  We have developed the full theory of a synchronously pumped type I optical
parametric oscillator (SPOPO). We derive expressions for the oscillation
threshold and the characteristics of the generated mode-locked signal beam. We
calculate the output quantum fluctuations of the device, and find that, in the
degenerate case (coincident signal and idler set of frequencies), perfect
squeezing is obtained when one approaches threshold from below for a well
defined "super-mode', or frequency comb, consisting of a coherent linear
superposition of signal modes of different frequencies which are resonant in
the cavity.


title: Phase Space Quantum Mechanics - Direct

abstract:
  Conventional approach to quantum mechanics in phase space, (q,p), is to take
the operator based quantum mechanics of Schrodinger, or and equivalent, and
assign a c-number function in phase space to it. We propose to begin with a
higher level of abstraction, in which the independence and the symmetric role
of q and p is maintained throughout, and at once arrive at phase space state
functions. Upon reduction to the q- or p-space the proposed formalism gives the
conventional quantum mechanics, however, with a definite rule for ordering of
factors of non commuting observables. Further conceptual and practical merits
of the formalism are demonstrated throughout the text.


title: Determinate Values for Quantum Observables

abstract:
  This is a comment on J. A. Barrett's article ``The Preferred-Basis Problem
and the Quantum Mechanics of Everything'' in Brit. J. Phil. Sci. 56 (2005),
which concerns theories postulating that certain quantum observables have
determinate values, corresponding to additional (often called ``hidden'')
variables. I point out that it is far from clear, for most observables, what
such a postulate is supposed to mean, unless the postulated additional variable
is related to a clear ontology in space-time, such as particle world lines,
string world sheets, or fields.


title:
Comment on "Can Two-Photon Correlation of Chaotic Light be Considered as
  Correlation of Intensity Fluctuations ?"

abstract:
  A Comment on the Letter by G. Scarcelli, V. Berardi and Y. Shih, Phys. Rev.
Lett. 96, 063602 (2006).


title:
A Generalization of the Preceding Paper ``A Rabi Oscillation in Four and
  Five Level Systems"

abstract:
  In the preceding paper quant-ph/0312060 we considered a general model of an
atom with n energy levels interacting with n-1 external laser fields and
constructed a Rabi oscillation in the case of n =3, 4 and 5.
  In the paper we present a systematic method getting along with computer to
construct a Rabi oscillation in the general case.


title:
Boundary effects to the entanglement entropy and two-site entanglement
  of the spin-1 valence-bond solid

abstract:
  We investigate the von Neumann entropy of a block of subsystem for the
valence-bond solid (VBS) state with general open boundary conditions. We show
that the effect of the boundary on the von Neumann entropy decays exponentially
fast in the distance between the subsystem considered and the boundary sites.
  Further, we show that as the size of the subsystem increases, its von Neumann
entropy exponentially approaches the summation of the von Neumann entropies of
the two ends, the exponent being related to the size. In contrast to critical
systems, where boundary effects to the von Neumann entropy decay slowly, the
boundary effects in a VBS, a non-critical system, decay very quickly. We also
study the entanglement between two spins.
  Curiously, while the boundary operators decrease the von Neumann entropy of L
spins, they increase the entanglement between two spins.


title: Game Theory Formulated on Hilbert Space

abstract:
  We present a consistent formulation of quantum game theory that accommodates
all possible strategies in Hilbert space. The physical content of the quantum
strategy is revealed as a family of classical games representing altruistic
game play supplemented by quantum interferences. Crucial role of the
entanglement in quantum strategy is illustrated by an example of quantum game
representing the Bell's experiment.


title:
Detection of Gravitational Wave - An Application of Relativistic Quantum
  Information Theory

abstract:
  We show that a passing gravitational wave may influence the spin entropy and
spin negativity of a system of $N$ massive spin-1/2 particles, in a way that is
characteristic of the radiation. We establish the specific conditions under
which this effect may be nonzero. The change in spin entropy and negativity,
however, is extremely small. Here, we propose and show that this effect may be
amplified through entanglement swapping. Relativistic quantum information
theory may have a contribution towards the detection of gravitational wave.


title: A realist view on the treatment of identical particles

abstract:
  Some basic concepts concerning systems of identical particles are discussed
in the framework of a realist interpretation, where the wave function is the
quantum object and |psi(r)|^2 d^3r is the probability that the wave function
causes an effect about the point r. The topics discussed include the role of
Hilbert-space labels, wave-function variables and wave-function parameters, the
distinction between permutation and renaming, the reason for symmetrizing the
wave function, the reason for antisymmetric wave functions, the spin-statistics
theorem, the correction term -k lnN! in the entropy, the Boltzmann limit of the
Fermi and Bose cases, and the derivation of the Fermi and Bose distributions.


title: Minimum uncertainty measurements of angle and angular momentum

abstract:
  The uncertainty relations for angle and angular momentum are revisited. We
use the exponential of the angle instead of the angle itself and adopt
dispersion as a natural measure of resolution. We find states that minimize the
uncertainty product under the constraint of a given uncertainty in angle or in
angular momentum. These states are described in terms of Mathieu wave functions
and may be approximated by a von Mises distribution, which is the closest
analogous of the Gaussian on the unit circle. We report experimental results
using beam optics that confirm our predictions.


title: Topological Quantum Distillation

abstract:
  We construct a class of topological quantum codes to perform quantum
entanglement distillation. These codes implement the whole Clifford group of
unitary operations in a fully topological manner and without selective
addressing of qubits. This allows us to extend their application also to
quantum teleportation, dense coding and computation with magic states.


title: Decay by tunneling of Bosonic and Fermionic Tonks-Girardeau Gases

abstract:
  We study the tunneling dynamics of bosonic and fermionic Tonks-Girardeau
gases from a hard wall trap, in which one of the walls is substituted by a
delta potential. Using the Fermi-Bose map, the decay of the probability to
remain in the trap is studied as a function of both the number of particles and
the intensity of the end-cap delta laser. The fermionic gas is shown to be a
good candidate to study deviations of the non-exponential decay of the
single-particle type, whereas for the bosonic case a novel regime of
non-exponential decay appears due to the contributions of different resonances
of the trap.


title:
The rise and fall of quantum and classical correlations in open-system
  dynamics

abstract:
  Interacting quantum systems evolving from an uncorrelated composite initial
state generically develop quantum correlations -- entanglement. As a
consequence, a local description of interacting quantum system is impossible as
a rule. A unitarily evolving (isolated) quantum system generically develops
extensive entanglement: the magnitude of the generated entanglement will
increase without bounds with the effective Hilbert space dimension of the
system. It is conceivable, that coupling of the interacting subsystems to local
dephasing environments will restrict the generation of entanglement to such
extent, that the evolving composite system may be considered as approximately
disentangled. This conjecture is addressed in the context of some common models
of a bipartite system with linear and nonlinear interactions and local coupling
to dephasing environments. Analytical and numerical results obtained imply that
the conjecture is generally false. Open dynamics of the quantum correlations is
compared to the corresponding evolution of the classical correlations and a
qualitative difference is found.


title: Worldline algorithms for Casimir configurations

abstract:
  We present improved worldline numerical algorithms for high-precision
calculations of Casimir interaction energies induced by scalar-field
fluctuations with Dirichlet boundary conditions for various Casimir geometries.
Significant reduction of numerical cost is gained by exploiting the symmetries
of the worldline ensemble in combination with those of the configurations. This
facilitates high-precision calculations on standard PCs or small clusters. We
illustrate our strategies using the experimentally most relevant sphere-plate
and cylinder-plate configuration. We compute Casimir curvature effects for a
wide parameter range, revealing the tight validity bounds of the commonly used
proximity force approximation (PFA). We conclude that data analysis of future
experiments aiming at a precision of 0.1% must no longer be based on the PFA.
Revisiting the parallel-plate configuration, we find a mapping between the
D-dimensional Casimir energy and properties of a random-chain polymer ensemble.


title: Into the Fold: Searching for a Theory of Natural Inference

abstract:
  We introduce relationalism and discuss how it is useful for interpreting
probability theory and quantum mechanics. This paper is written in relatively
lay terms and presumes no prior knowledge of quantum theory.


title:
Procrustean entanglement concentration of continuous variable states of
  light

abstract:
  We propose a Procrustean entanglement concentration scheme for continuous
variable states inspired by the scheme proposed in Fiurasek et. al. Phys. Rev.
A 67, 022304, (2003). We show that the eight-port homodyne measurement of
Fiurasek et. al. Phys. Rev. A 67, 022304, (2003) can be replaced by a balanced
homodyne measurement with the advantage of providing a success criterion that
allows Alice and Bob to determine if entanglement concentration was achieved.
In addition, it facilitates a straightforward and feasible experimental
implementation.


title: A probabilistic approach to quantum mechanics based on tomograms

abstract:
  It is usually believed that a picture of Quantum Mechanics in terms of true
probabilities cannot be given due to the uncertainty relations. Here we discuss
a tomographic approach to quantum states that leads to a probability
representation of quantum states. This can be regarded as a classical-like
formulation of quantum mechanics which avoids the counterintuitive concepts of
wave function and density operator. The relevant concepts of quantum mechanics
are then reconsidered and the epistemological implications of such approach
discussed.


title: Statistical properties of the truncated state with random coefficients

abstract:
  In this Letter we introduce the truncated state with random coefficients
(TSRC). As the coefficients of the TSRC have in principle no algorithm to
produce them, our question is concerned about to what type of properties will
characterize the TSRC. A general method to engineer TSRC in the running-wave
domain is employed, which includes the errors due to the nonidealities of
detectors and photocounts.


title: Selective truncations of an optical state using projection synthesis

abstract:
  Selective truncation of Fock-state expansion of an optical field can be
achieved using projection synthesis. The process removes predetermined Fock
states from the input field by conditional measurement and teleportation. We
present a scheme based on multiport interferometry to perform projection
synthesis. This scheme can be used both as a generalized quantum scissors
device, which filters out Fock states with photon numbers higher than a
predetermined value, and also as a quantum punching device, which selectively
removes specific Fock states making holes in the Fock-state expansion of the
input field.


title: A quasi-Hermitian pseudopotential for higher partial wave scattering

abstract:
  We formulate a new quasi-Hermitian delta-shell pseudopotential for higher
partial wave scattering, and show that any such potential must have an
energy-dependent regularization. The quasi-Hermiticity of the Hamiltonian leads
to a complete set of biorthogonal wave functions that can be used as a basis to
expand and diagonalize other two-body Hamiltonians. We demonstrate this
procedure for the case of ultracold atoms in a polarization-gradient optical
lattice, interacting pairwise when two atoms are transported together from
separated lattice sites. Here the pseudopotential depends explicitly on the
trapping potential. Additionally, we calculate the location of trap-induced
resonances for higher partial waves, which occur when a molecular eigenstate is
shifted to resonance with a trap eigenstate. We verify the accuracy of the
pseudopotential approach using a toy model in which a square well acts as the
true interaction potential, and see excellent agreement.


title:
On the Relationship between Convex Bodies Related to Correlation
  Experiments with Dichotomic Observables

abstract:
  In this paper we explore further the connections between convex bodies
related to quantum correlation experiments with dichotomic variables and
related bodies studied in combinatorial optimization, especially cut polyhedra.
Such a relationship was established in Avis, Imai, Ito and Sasaki (2005 J.
Phys. A: Math. Gen. 38 10971-87) with respect to Bell inequalities. We show
that several well known bodies related to cut polyhedra are equivalent to
bodies such as those defined by Tsirelson (1993 Hadronic J. S. 8 329-45) to
represent hidden deterministic behaviors, quantum behaviors, and no-signalling
behaviors. Among other things, our results allow a unique representation of
these bodies, give a necessary condition for vertices of the no-signalling
polytope, and give a method for bounding the quantum violation of Bell
inequalities by means of a body that contains the set of quantum behaviors.
Optimization over this latter body may be performed efficiently by semidefinite
programming. In the second part of the paper we apply these results to the
study of classical correlation functions. We provide a complete list of tight
inequalities for the two party case with (m,n) dichotomic observables when
m=4,n=4 and when min{m,n}<=3, and give a new general family of correlation
inequalities.


title: Two-photon wave mechanics

abstract:
  The position-representation wave function for multi-photon states and its
equation of motion are introduced. A major strength of the theory is that it
describes the complete evolution (including polarization and entanglement) of
multi-photon states propagating through inhomogeneous media. As a demonstration
of the two-photon wave function's use, we show how two photons in an
orbital-angular-momentum entangled state decohere upon propagation through a
turbulent atmosphere.


title:
Using quantum oblivious transfer to cheat sensitive quantum bit
  commitment

abstract:
  It is well known that unconditionally secure bit commitment is impossible
even in the quantum world. In this paper a weak variant of quantum bit
commitment, introduced independently by Aharonov et al. [STOC, 2000] and Hardy
and Kent [Phys. Rev. Lett. 92 (2004)] is investigated. In this variant, the
parties require some nonzero probability of detecting a cheating, i.e. if Bob,
who commits a bit b to Alice, changes his mind during the revealing phase then
Alice detects the cheating with a positive probability (we call this property
binding); and if Alice gains information about the committed bit before the
revealing phase then Bob discovers this with positive probability (sealing). In
our paper we give quantum bit commitment scheme that is simultaneously binding
and sealing and we show that if a cheating gives epsilon advantage to a
malicious Alice then Bob can detect the cheating with a probability
Omega(epsilon^2). If Bob cheats then Alice's probability of detecting the
cheating is greater than some fixed constant lambda>0. This improves the
probabilities of cheating detections shown by Hardy and Kent and the scheme by
Aharonov et al. who presented a protocol that is either binding or sealing, but
not simultaneously both. To construct a cheat sensitive quantum bit commitment
scheme we use a protocol for a weak quantum one-out-of-two oblivious transfer.


title: On the geometry of four qubit invariants

abstract:
  The geometry of four-qubit entanglement is investigated. We replace some of
the polynomial invariants for four-qubits introduced recently by new ones of
direct geometrical meaning. It is shown that these invariants describe four
points, six lines and four planes in complex projective space ${\bf CP}^3$. For
the generic entanglement class of stochastic local operations and classical
communication they take a very simple form related to the elementary symmetric
polynomials in four complex variables. Moreover, their magnitudes are
entanglement monotones that fit nicely into the geometric set of $n$-qubit ones
related to Grassmannians of $l$-planes found recently. We also show that in
terms of these invariants the hyperdeterminant of order 24 in the four-qubit
amplitudes takes a more instructive form than the previously published
expressions available in the literature. Finally in order to understand two,
three and four-qubit entanglement in geometric terms we propose a unified
setting based on ${\bf CP}^3$ furnished with a fixed quadric.


title:
Phase-modulated shaping of narrowband type-I parametric down-converted
  photons

abstract:
  We present a general theoretical description of the temporal shaping of
narrowband noncollinear type-I down-converted photons using a spectral phase
filter with a symmetric phase distribution. By manipulating the spectral phase
of the signal or idler photon, we demonstrate control of the correlation time
and shape of the two-photon wave function with modulation frequency and
modulation depth of the phase distribution.


title: M-body Pure State Entanglement

abstract:
  The simple entanglement of N-body N-particle pure states is extended to the
more general M-body or M-body N-particle states where $N\neq M$. Some new
features of the M-body N-particle pure states are discussed. An application of
the measure to quantify quantum correlations in a Bose-Einstien condensate
model is demonstrated.


title: Multipartite continuous-variable entanglement

abstract:
  Necessary and sufficient observable conditions for the nonnegativity of all
partial transpositions of multi-mode quantum states are derived. The result is
a hierarchy of inequalities for minors in terms of moments of the given state.
Violations of any inequality is a sufficient condition for entanglement. Full
entanglement can be certified for a manifold of multi-mode quantum states. A
\textit{Mathematica} package is given for a systematic test of the hierarchy of
conditions.


title: Estimating entanglement of unknown states

abstract:
  The experimental determination of entanglement is a major goal in the quantum
information field. In general the knowledge of the state is required in order
to quantify its entanglement. Here we express a lower bound to the robustness
of entanglement of a state based only on the measurement of the energy
observable and on the calculation of a separability energy. This allows the
estimation of entanglement dismissing the knowledge of the state in question.


title:
Improved Gap Estimates for Simulating Quantum Circuits by Adiabatic
  Evolution

abstract:
  We use elementary variational arguments to prove, and improve on, gap
estimates which arise in simulating quantum circuits by adiabatic evolution.


title: Robust transmission of non-Gaussian entanglement over optical fibers

abstract:
  We show how the entanglement in a wide range of continuous variable
non-Gaussian states can be preserved against decoherence for long-range quantum
communication through an optical fiber. We apply protection via
decoherence-free subspaces and quantum dynamical decoupling to this end. The
latter is implemented by inserting phase shifters at regular intervals $\Delta
$ inside the fiber, where $\Delta $ is roughly the ratio of the speed of light
in the fiber to the bath high-frequency cutoff. Detailed estimates of relevant
parameters are provided using the boson-boson model of system-bath interaction
for silica fibers, and $\Delta $ is found to be on the order of a millimeter.


title:
Information-disturbance tradeoff in estimating a maximally entangled
  state

abstract:
  We derive the amount of information retrieved by a quantum measurement in
estimating an unknown maximally entangled state, along with the pertaining
disturbance on the state itself. The optimal tradeoff between information and
disturbance is obtained, and a corresponding optimal measurement is provided.


title: Second quantization method in the presence of bound states of particles

abstract:
  We develop an approximate second quantization method for describing the
many-particle systems in the presence of bound states of particles at low
energies (the kinetic energy of particles is small in comparison to the binding
energy of compound particles). In this approximation the compound and
elementary particles are considered on an equal basis. This means that creation
and annihilation operators of compound particles can be introduced. The
Hamiltonians, which specify the interactions between compound and elementary
particles and between compound particles themselves are found in terms of the
interaction amplitudes for elementary particles. The nonrelativistic quantum
electrodynamics is developed for systems containing both elementary and
compound particles. Some applications of this theory are considered.


title:
Pulse-induced acoustoelectric vibrations in surface-gated GaAs-based
  quantum devices

abstract:
  We present the results of a numerical investigation which show the excitation
of acoustoelectric modes of vibration in GaAs-based heterostructures due to
sharp nano-second electric-field pulses applied across surface gates. In
particular, we show that the pulses applied in quantum information processing
applications are capable of exciting acoustoelectric modes of vibration
including surface acoustic modes which propagate for distances greater than
conventional device dimensions. We show that the pulse-induced acoustoelectric
vibrations are capable of inducing significant undesired perturbations to the
evolution of quantum systems.


title: Optimal cloning of mixed Gaussian states

abstract:
  We construct the optimal 1 to 2 cloning transformation for the family of
displaced thermal equilibrium states of a harmonic oscillator, with a fixed and
known temperature. The transformation is Gaussian and it is optimal with
respect to the figure of merit based on the joint output state and norm
distance. The proof of the result is based on the equivalence between the
optimal cloning problem and that of optimal amplification of Gaussian states
which is then reduced to an optimization problem for diagonal states of a
quantum oscillator. A key concept in finding the optimum is that of stochastic
ordering which plays a similar role in the purely classical problem of Gaussian
cloning. The result is then extended to the case of n to m cloning of mixed
Gaussian states.


title:
Typicality vs. probability in trajectory-based formulations of quantum
  mechanics

abstract:
  Bohmian mechanics represents the universe as a set of paths with a
probability measure defined on it. The way in which a mathematical model of
this kind can explain the observed phenomena of the universe is examined in
general. It is shown that the explanation does not make use of the full
probability measure, but rather of a suitable set function deriving from it,
which defines relative typicality between single-time cylinder sets. Such a set
function can also be derived directly from the standard quantum formalism,
without the need of an underlying probability measure. The key concept for this
derivation is the {\it quantum typicality rule}, which can be considered as a
generalization of the Born rule. The result is a new formulation of quantum
mechanics, in which particles follow definite trajectories, but which is only
based on the standard formalism of quantum mechanics.


title:
Quantum Information Processing in Optical Lattices and Magnetic
  Microtraps

abstract:
  We review our experiments on quantum information processing with neutral
atoms in optical lattices and magnetic microtraps.
  Atoms in an optical lattice in the Mott insulator regime serve as a large
qubit register. A spin-dependent lattice is used to split and delocalize the
atomic wave functions in a controlled and coherent way over a defined number of
lattice sites. This is used to experimentally demonstrate a massively parallel
quantum gate array, which allows the creation of a highly entangled many-body
cluster state through coherent collisions between atoms on neighbouring lattice
sites.
  In magnetic microtraps on an atom chip, we demonstrate coherent manipulation
of atomic qubit states and measure coherence lifetimes exceeding one second at
micron-distance from the chip surface. We show that microwave near-fields on
the chip can be used to create state-dependent potentials for the
implementation of a quantum controlled phase gate with these robust qubit
states. For single atom detection and preparation, we have developed high
finesse fiber Fabry-Perot cavities and integrated them on the atom chip. We
present an experiment in which we detected a very small number of cold atoms
magnetically trapped in the cavity using the atom chip.


title: Universal role of correlation entropy in critical phenomena

abstract:
  In statistical physics, if we successively divide an equilibrium system into
two parts, we will face a situation that, within a certain length $\xi$, the
physics of a subsystem is no longer the same as the original system. Then the
extensive properties of the thermal entropy $S($AB$)= S($A$)+S($B$)$ is
violated. This observation motivates us to introduce the concept of correlation
entropy between two points, as measured by mutual information in the
information theory, to study the critical phenomena. A rigorous relation is
established to display some drastic features of the non-vanishing correlation
entropy of the subsystem formed by any two distant particles with long-range
correlation. This relation actually indicates the universal role of the
correlation entropy in understanding critical phenomena. We also verify these
analytical studies in terms of two well-studied models for both the thermal and
quantum phase transitions: two-dimensional Ising model and one-dimensional
transverse field Ising model. Therefore, the correlation entropy provides us
with a new physical intuition in critical phenomena from the point of view of
the information theory.


title: Quantum Teleportation in One-Dimensional Quantum Dots System

abstract:
  We present a model of quantum teleportation protocol based on one-dimensional
quantum dots system. Three quantum dots with three electrons are used to
perform teleportation, the unknown qubit is encoded using one electron spin on
quantum dot A, the other two dots B and C are coupled to form a mixed
space-spin entangled state. By choosing the Hamiltonian for the mixed
space-spin entangled system, we can filter the space (spin) entanglement to
obtain pure spin (space) entanglement and after a Bell measurement, the unknown
qubit is transfered to quantum dot B. Selecting an appropriate Hamiltonian for
the quantum gate allows the spin-based information to be transformed into a
charge-based information. The possibility of generalizing this model to
N-electrons is discussed.