title: Modematching an optical quantum memory

abstract:
  We analyse the off-resonant Raman interaction of a single broadband photon,
copropagating with a classical `control' pulse, with an atomic ensemble. It is
shown that the classical electrodynamical structure of the interaction
guarantees canonical evolution of the quantum mechanical field operators. This
allows the interaction to be decomposed as a beamsplitter transformation
between optical and material excitations on a mode-by-mode basis. A single,
dominant modefunction describes the dynamics for arbitrary control pulse
shapes.
  Complete transfer of the quantum state of the incident photon to a collective
dark state within the ensemble can be achieved by shaping the control pulse so
as to match the dominant mode to the temporal mode of the photon. Readout of
the material excitation, back to the optical field, is considered in the
context of the symmetry connecting the input and output modes. Finally, we show
that the transverse spatial structure of the interaction is characterised by
the same mode decomposition.


title: Entanglement of Dirac fields in non-inertial frames

abstract:
  We analyze the entanglement between two modes of a free Dirac field as seen
by two relatively accelerated parties. The entanglement is degraded by the
Unruh effect and asymptotically reaches a non-vanishing minimum value in the
infinite acceleration limit. This means that the state always remains entangled
to a degree and can be used in quantum information tasks, such as
teleportation, between parties in relative uniform acceleration. We analyze our
results from the point of view afforded by the phenomenon of entanglement
sharing and in terms of recent results in the area of multi-qubit
complementarity.


title:
One-way quantum key distribution: Simple upper bound on the secret key
  rate

abstract:
  We present a simple method to obtain an upper bound on the achievable secret
key rate in quantum key distribution (QKD) protocols that use only
unidirectional classical communication during the public-discussion phase. This
method is based on a necessary precondition for one-way secret key
distillation; the legitimate users need to prove that there exists no quantum
state having a symmetric extension that is compatible with the available
measurements results. The main advantage of the obtained upper bound is that it
can be formulated as a semidefinite program, which can be efficiently solved.
We illustrate our results by analysing two well-known qubit-based QKD
protocols: the four-state protocol and the six-state protocol. Recent results
by Renner et al., Phys. Rev. A 72, 012332 (2005), also show that the given
precondition is only necessary but not sufficient for unidirectional secret key
distillation.


title:
Witnessing effective entanglement in a continuous variable
  prepare&measure setup and application to a QKD scheme using postselection

abstract:
  We report an experimental demonstration of effective entanglement in a
prepare&measure type of quantum key distribution protocol. Coherent
polarization states and heterodyne measurement to characterize the transmitted
quantum states are used, thus enabling us to reconstruct directly their
Q-function. By evaluating the excess noise of the states, we experimentally
demonstrate that they fulfill a non-separability criterion previously presented
by Rigas et al. [J. Rigas, O. G\"uhne, N. L\"utkenhaus, Phys. Rev. A 73, 012341
(2006)]. For a restricted eavesdropping scenario we predict key rates using
postselection of the heterodyne measurement results.


title:
Non-Hermitian d-dimensional Hamiltonians with position dependent mass
  and their $\eta$-pseudo-Hermiticity generators

abstract:
  A class of non-Hermitian d-dimensional Hamiltonias with position dependent
mass and their $\eta$-pseudo-Hermiticity generators is presented. Illustrative
examples are given in 1D, 2D, and 3D for different position dependent mass
settings.


title: Deterministic entanglement and tomography of ion spin qubits

abstract:
  We have implemented a universal quantum logic gate between qubits stored in
the spin state of a pair of trapped calcium 40 ions. An initial product state
was driven to a maximally entangled state deterministically, with 83% fidelity.
We present a general approach to quantum state tomography which achieves good
robustness to experimental noise and drift, and use it to measure the spin
state of the ions. We find the entanglement of formation is 0.54.


title: Relations among quantum processes: bisimilarity and congruence

abstract:
  Full formal descriptions of algorithms making use of quantum principles must
take into account both quantum and classical computing components, as well as
communications between these components. Moreover, to model concurrent and
distributed quantum computations and quantum communication protocols,
communications over quantum channels which move qubits physically from one
place to another must also be taken into account.
  Inspired by classical process algebras, which provide a framework for
modeling cooperating computations, a process algebraic notation is defined.
This notation provides a homogeneous style to formal descriptions of concurrent
and distributed computations comprising both quantum and classical parts. Based
upon an operational semantics which makes sure that quantum objects, operations
and communications operate according to the postulates of quantum mechanics, an
equivalence is defined among process states considered as having the same
behavior. This equivalence is a probabilistic branching bisimulation. From this
relation, an equivalence on processes is defined. However, it is not a
congruence because it is not preserved by parallel composition.


title:
Generation of Entangled Two-Photons Binomial States in Two Spatially
  Separate Cavities

abstract:
  We propose a conditional scheme to generate entangled two-photons generalized
binomial states inside two separate single-mode high-Q cavities. This scheme
requires that the two cavities are initially prepared in entangled one-photon
generalized binomial states and exploits the passage of two appropriately
prepared two-level atoms one in each cavity. The measurement of the ground
state of both atoms is finally required when they exit the cavities. We also
give a brief evaluation of the experimental feasibility of the scheme.


title: On Conserved Current in Markovian Open Quantum Systems

abstract:
  We reexamine the markovian approximation of local current in open quantum
systems, discussed recently by Gebauer and Car. Our derivation is more
transparent, the proof of current conservation becomes explicit and easy.


title:
Observables have no value: a no-go theorem for position and momentum
  observables

abstract:
  A very simple illustration of the Bell-Kochen-Specker contradiction is
presented using continuous observables in infinite dimensional Hilbert space.
It is shown that the assumption of the \emph{existence} of putative values for
position and momentum observables for one single particle is incompatible with
quantum mechanics.


title: Accessible versus Holevo Information for a Binary Random Variable

abstract:
  The accessible information acc(E) of an ensemble E is the maximum mutual
information between a random variable encoded into quantum states, and the
probabilistic outcome of a quantum measurement of the encoding. Accessible
information is extremely difficult to characterize analytically; even bounds on
it are hard to place. The celebrated Holevo bound states that accessible
information cannot exceed chi(E), the quantum mutual information between the
random variable and its encoding. However, for general ensembles, the gap
between the acc(E) and chi(E) may be arbitrarily large.
  We consider the special case of a binary random variable, which often serves
as a stepping stone towards other results in information theory and
communication complexity. We give explicit lower bounds on the the accessible
information acc(E) of an ensemble E = {(p, rho_0), (1-p, rho_1)}, with 0 <= p
<= 1, as functions of p and chi(E). The bounds are incomparable in the sense
that they surpass each other in different parameter regimes.
  Our bounds arise by measuring the ensemble according to a complete orthogonal
measurement that preserves the fidelity of the states rho_0,rho_1. As an
intermediate step, therefore, we give new relations between the two quantities
acc(E), chi(E) and the fidelity B(rho_0,rho_1).


title: Polarization preserving quantum nondemolition photodetector

abstract:
  A polarization preserving quantum nondemolition photodetector is proposed
based on nonlinearities obtainable through quantum coherence effects. An atomic
level scheme is devised such that in the presence of strong linearly polarized
drive field a coherent weak probe field acquires a phase proportional to the
number of photons in the signal mode immaterial of its polarization state. It
is also shown that the unavoidable phase-kicks resulting due to the measurement
process are insensitive to the polarization state of the incoming signal
photon. It is envisioned that such a device would have tremendous applicability
in photonic quantum information proposals where quantum information in the
polarization qubit is to be protected.


title: Many-body quantum dynamics of polarisation squeezing in optical fibre

abstract:
  We report new experiments that test quantum dynamical predictions of
polarization squeezing for ultrashort photonic pulses in a birefringent fibre,
including all relevant dissipative effects. This exponentially complex
many-body problem is solved by means of a stochastic phase-space method. The
squeezing is calculated and compared to experimental data, resulting in
excellent quantitative agreement. From the simulations, we identify the
physical limits to quantum noise reduction in optical fibres. The research
represents a significant experimental test of first-principles time-domain
quantum dynamics in a one-dimensional interacting Bose gas coupled to
dissipative reservoirs.


title:
Probability density function characterization of multipartite
  entanglement

abstract:
  We propose a method to characterize and quantify multipartite entanglement
for pure states. The method hinges upon the study of the probability density
function of bipartite entanglement and is tested on an ensemble of qubits in a
variety of situations. This characterization is also compared to several
measures of multipartite entanglement.


title:
Decoherence induced by squeezing control errors in optical and ion trap
  holonomic quantum computations II

abstract:
  We study decoherence induced by stochastic squeezing control errors
considering the particular implementation of Hadamard gate on optical and ion
trap holonomic quantum computers. We analytically obtain both the purity of the
final state and the fidelity for Hadamard gate when the control noise is
modeled by Ornstein-Uhlenbeck stochastic process. We demonstrate the purity and
the fidelity oscillations depending on the choice of the initial superimposed
state. We derive a linear formulae connecting the gate fidelity and the purity
of the final state.


title: A (5,5) and (6,6) PPT edge state

abstract:
  Entangled states with a positive partial transpose (PPTES) have interest both
in quantum information and in the theory of positive maps. In $3\otimes 3$
there is a conjecture by Sanpera, Bru{\ss} and Lewenstein [PRA, 63, 050301]
that all PPTES have Schmidt number two (or equivalently that every 2-positive
map between $3\times 3$ matrices is decomposable). In order to prove or
disprove the conjecture it is sufficient to look at edge PPTES. Here the rank m
of the PPTES and the rank n of its partial transpose seem to play an important
role. Until recently all known examples of edge PPTES had ranks (4,4) or (6,7).
In a recent paper Ha and Kye [quant-ph/0509079] managed to find edge PPTES for
all ranks except (5,5) and (6,6). Here we complement their work and present
edge PPTES with those ranks.


title: Quantum homodyne tomography of a two-photon Fock state

abstract:
  We present a continuous-variable experimental analysis of a two-photon Fock
state of free-propagating light. This state is obtained from a pulsed
non-degenerate parametric amplifier, which produces two intensity-correlated
twin beams. Counting two photons in one beam projects the other beam in the
desired two-photon Fock state, which is analyzed by using a pulsed homodyne
detection. The Wigner function of the measured state is clearly negative. We
developed a detailed analytic model which allows a fast and efficient analysis
of the experimental results.


title:
Probabilistic Quantum Gates between Remote Atoms through Interference of
  Optical Frequency Qubits

abstract:
  We propose a scheme to perform probabilistic quantum gates on remote trapped
atom qubits through interference of optical frequency qubits. The method does
not require localization of the atoms to the Lamb-Dicke limit, and is not
sensitive to interferometer phase instabilities. Such probabilistic gates can
be used for scalable quantum computation.


title:
Entanglement enhanced classical capacity of quantum communication
  channels with correlated noise in arbitrary dimensions

abstract:
  We study the capacity of d-dimensional quantum channels with memory modeled
by correlated noise. We show that, in agreement with previous results on Pauli
qubit channels, there are situations where maximally entangled input states
achieve higher values of mutual information than product states. Moreover, a
strong dependence of this effect on the nature of the noise correlations as
well as on the parity of the space dimension is found. We conjecture that when
entanglement gives an advantage in terms of mutual information, maximally
entangled states saturate the channel capacity.


title: Quantum Circuit Simplification and Level Compaction

abstract:
  Quantum circuits are time dependent diagrams describing the process of
quantum computation. Usually, a quantum algorithm must be mapped into a quantum
circuit. Optimal synthesis of quantum circuits is intractable and heuristic
methods must be employed. With the use of heuristics, the optimality of
circuits is no longer guaranteed. In this paper, we consider a local
optimization technique based on templates to simplify and reduce the depth of
non-optimal quantum circuits. We present and analyze templates in the general
case, and provide particular details for the circuits composed of NOT, CNOT and
controlled-sqrt-of-NOT gates. We apply templates to optimize various common
circuits implementing multiple control Toffoli gates and quantum Boolean
arithmetic circuits. We also show how templates can be used to compact the
number of levels of a quantum circuit. The runtime of our implementation is
small while the reduction in number of quantum gates and number of levels is
significant.


title:
3D phase-matching conditions for the generation of entangled triplets by
  chi(2) interlinked interactions

abstract:
  An analytical calculation of the interaction geometry of two interlinked
second-order nonlinear processes fulfilling phase-matching conditions is
presented. The method is developed for type-I uniaxial crystals and gives the
positions on a screen beyond the crystal of the entangled triplets generated by
the interactions. The analytical results are compared to experiments realized
in the macroscopic regime. Preliminary tests to identify the triplets are also
performed based on intensity correlations.


title:
Atom-Photon Interactions with Respect to Quantum Computation:
  Three-Level Atom in Two-Mode Field

abstract:
  In the paper, analysis of a quantum optical system--three-level atom in a
quantum electromagnetic field is given. Evolution operators are constructed in
closed form.


title: Quantum computation with scattering matrices

abstract:
  We discuss possible applications of the 1-D direct and inverse scattering
problem to design of universal quantum gates for quantum computation. The
potentials generating some universal gates are described.


title: Suppression of Zeno effect for distant detectors

abstract:
  We describe the influence of continuous measurement in a decaying system and
the role of the distance from the detector to the initial location of the
system. The detector is modeled first by a step absorbing potential. For a
close and strong detector, the decay rate of the system is reduced; weaker
detectors do not modify the exponential decay rate but suppress the long-time
deviations above a coupling threshold. Nevertheless, these perturbing effects
of measurement disappear by increasing the distance between the initial state
and the detector, as well as by improving the efficiency of the detector.


title: Regimes of stability of accelerator modes

abstract:
  The phase diagram of a simple area-preserving map, which was motivated by the
quantum dynamics of cold atoms, is explored analytically and numerically.
Periodic orbits of a given winding ratio are found to exist within wedge-shaped
regions in the phase diagrams, which are analogous to the Arnol'd tongues which
have been extensively studied for a variety of dynamical systems, mostly
dissipative ones. A rich variety of bifurcations of various types are observed,
as well as period doubling cascades. Stability of periodic orbits is analyzed
in detail.


title:
Hawking-Unruh effect and the entanglement of two-mode squeezed states in
  Riemannian spacetime

abstract:
  We consider the system of free scalar field, which is assumed to be a
two-mode squeezed state from an inertial point of view. This setting allows the
use of entanglement measure for continuous variables, which can be applied to
discuss free and bound entanglement from the point of view from non-inertial
observer.


title: The mathematical basis for deterministic quantum mechanics

abstract:
  If there exists a classical, i.e. deterministic theory underlying quantum
mechanics, an explanation must be found of the fact that the Hamiltonian, which
is defined to be the operator that generates evolution in time, is bounded from
below. The mechanism that can produce exactly such a constraint is identified
in this paper. It is the fact that not all classical data are registered in the
quantum description. Large sets of values of these data are assumed to be
indistinguishable, forming equivalence classes. It is argued that this should
be attributed to information loss, such as what one might suspect to happen
during the formation and annihilation of virtual black holes.
  The nature of the equivalence classes is further elucidated, as it follows
from the positivity of the Hamiltonian. Our world is assumed to consist of a
very large number of subsystems that may be regarded as approximately
independent, or weakly interacting with one another. As long as two (or more)
sectors of our world are treated as being independent, they all must be
demanded to be restricted to positive energy states only. What follows from
these considerations is a unique definition of energy in the quantum system in
terms of the periodicity of the limit cycles of the deterministic model.


title:
Quantum cloning with nonlocal assistance: Complement of Jozsa's stronger
  no-cloning theorem

abstract:
  We investigate the cases where a set $S$ of states $\{\ket{\psi_i} \}$ cannot
be cloned by itself, but is clonable with the help of another system prepared
in state $\hat{\rho}_i$. When $S$ is pair-wise nonorthogonal, it is known that
one can generate the copy from $\hat{\rho}_i$ alone, with no interaction with
the original system. Here we show that a set containing orthogonal pairs
exhibits a property forming a striking contrast; For any such set, there is a
choice of $\hat{\rho}_i$ that enables cloning only when the two systems are
interacted in a purely quantum manner that is not achievable via classical
communication.


title: Universal resources for measurement-based quantum computation

abstract:
  We investigate which entanglement resources allow universal measurement-based
quantum computation via single-qubit operations. We find that any entanglement
feature exhibited by the 2D cluster state must also be present in any other
universal resource. We obtain a powerful criterion to assess universality of
graph states, by introducing an entanglement measure which necessarily grows
unboundedly with the system size for all universal resource states.
Furthermore, we prove that graph states associated with 2D lattices such as the
hexagonal and triangular lattice are universal, and obtain the first example of
a universal non-graph state.


title: Scaling of Berry's Phase Close to the Dicke Quantum Phase Transition

abstract:
  We discuss the thermodynamic and finite size scaling properties of the
geometric phase in the adiabatic Dicke model, describing the super-radiant
phase transition for an $N$ qubit register coupled to a slow oscillator mode.
We show that, in the thermodynamic limit, a non zero Berry phase is obtained
only if a path in parameter space is followed that encircles the critical
point. Furthermore, we investigate the precursors of this critical behavior for
a system with finite size and obtain the leading order in the 1/N expansion of
the Berry phase and its critical exponent.


title: New low frequency oscillations in quantum dusty plasmas

abstract:
  The existence of two new low-frequency electrostatic modes in quantum dusty
plasmas is pointed out. These modes can be useful to diagnose charged dust
impurities in micro-electro-mechanical systems.


title: Beyond i.i.d. in Quantum Information Theory

abstract:
  The information spectrum approach gives general formulae for optimal rates of
codes in many areas of information theory. In this paper the quantum spectral
divergence rates are defined and properties of the rates are derived. The
entropic rates, conditional entropic rates, and spectral mutual information
rates are then defined in terms of the spectral divergence rates. Properties
including subadditivity, chain rules, Araki-Lieb inequalities, and monotonicity
are then explored.


title: Time evolution of non-Hermitian Hamiltonian systems

abstract:
  We provide time-evolution operators, gauge transformations and a perturbative
treatment for non-Hermitian Hamiltonian systems, which are explicitly
time-dependent. We determine various new equivalence pairs for Hermitian and
non-Hermitian Hamiltonians, which are therefore pseudo-Hermitian and in
addition in some cases also invariant under PT-symmetry. In particular, for the
harmonic oscillator perturbed by a cubic non-Hermitian term, we evaluate
explicitly various transition amplitudes, for the situation when these systems
are exposed to a monochromatic linearly polarized electric field.


title:
Minimum communication cost of simulating quantum correlations is one
  cbit for all spin singlet states

abstract:
  This paper has been withdrawn due to a fatal error in the argument of the
simulation scheme.


title: Comb entanglement in quantum spin chains

abstract:
  Bipartite entanglement in the ground state of a chain of $N$ quantum spins
can be quantified either by computing pairwise concurrence or by dividing the
chain into two complementary subsystems. In the latter case the smaller
subsystem is usually a single spin or a block of adjacent spins and the
entanglement differentiates between critical and non-critical regimes. Here we
extend this approach by considering a more general setting: our smaller
subsystem $S_A$ consists of a {\it comb} of $L$ spins, spaced $p$ sites apart.
Our results are thus not restricted to a simple `area law', but contain
non-local information, parameterized by the spacing $p$. For the XX model we
calculate the von-Neumann entropy analytically when $N\to \infty$ and
investigate its dependence on $L$ and $p$. We find that an external magnetic
field induces an unexpected length scale for entanglement in this case.


title:
Properties of entangled photon pairs generated in one-dimensional
  nonlinear photonic-band-gap structures

abstract:
  We have developed a rigorous quantum model of spontaneous parametric
down-conversion in a nonlinear 1D photonic-band-gap structure based upon
expansion of the field into monochromatic plane waves. The model provides a
two-photon amplitude of a created photon pair. The spectra of the signal and
idler fields, their intensity profiles in the time domain, as well as the
coincidence-count interference pattern in a Hong-Ou-Mandel interferometer are
determined both for cw and pulsed pumping regimes in terms of the two-photon
amplitude. A broad range of parameters characterizing the emitted
down-converted fields can be used. As an example, a structure composed of 49
layers of GaN/AlN is analyzed as a suitable source of photon pairs having high
efficiency.


title: Quantum Reversibility and Echoes in Interacting Systems

abstract:
  In Echo experiments, imperfect time-reversal operations are performed on a
subset of the total number of degrees of freedom. To capture the physics of
these experiments, we introduce a partial fidelity, the Boltzmann echo, where
only part of the system's degrees of freedom can be time-reversed. We present a
semiclassical calculation of the Boltzmann echo. We show that, as the
time-reversal operation is performed more and more accurately, the decay rate
of the Boltzmann echo saturates at a value given by the decoherence rate of the
controlled degrees of freedom due to their coupling to uncontrolled ones. We
connect these results with NMR spin echo experiments.


title: Recurrence Tracking Microscope

abstract:
  In order to probe nanostructures on a surface we present a microscope based
on the quantum recurrence phenomena. A cloud of atoms bounces off an atomic
mirror connected to a cantilever and exhibits quantum recurrences. The times at
which the recurrences occur depend on the initial height of the bouncing atoms
above the atomic mirror, and vary following the structures on the surface under
investigation. The microscope has inherent advantages over existing techniques
of scanning tunneling microscope and atomic force microscope. Presently
available experimental technology makes it possible to develop the device in
the laboratory.


title: Entanglement dynamics of qubits in a common environment

abstract:
  We use the quantum jump approach to study the entanglement dynamics of a
quantum register, which is composed of two or three dipole-dipole coupled
two-level atoms, interacting with a common environment. Our investigation of
entanglement dynamics reflects that the environment has dual actions on the
entanglement of the qubits in the model. While the environment destroys the
entanglement induced by the coherent dipole-dipole interactions, it can produce
stable entanglement between the qubits prepared initially in a separable state.
The analysis shows that it is the entangled decoherence-free states contained
as components in the initial state that contribute to the stable entanglement.
Our study indicates how the environmental noise produces the entanglement and
exposes the interplay of environmental noise and coherent interactions of
qubits on the entanglement.


title: A search on the Nikiforov-Uvarov formalism

abstract:
  An alternative treatment is proposed for the calculations carried out within
the frame of Nikiforov-Uvarov method, which removes a drawback in the original
theory and by pass some difficulties in solving the Schrodinger equation. The
present procedure is illustrated with the example of orthogonal polynomials.
The relativistic extension of the formalism is discussed.


title: Vibration induced phase noise in Mach-Zehnder atom interferometers

abstract:
  The high inertial sensitivity of atom interferometers has been used to build
accelerometers and gyrometers but this sensitivity makes these interferometers
very sensitive to the laboratory seismic noise. This seismic noise induces a
phase noise which is large enough to reduce the fringe visibility in many
cases. We develop here a model calculation of this phase noise in the case of
Mach-Zehnder atom interferometers and we apply this model to our thermal
lithium interferometer. We are thus able to explain the observed dependence of
the fringe visibility with the diffraction order. The dynamical model developed
in the present paper should be very useful to further reduce this phase noise
in atom interferometers and this reduction should open the way to improved
interferometers.


title:
Quantum Fractals. Geometric modeling of quantum jumps with conformal
  maps

abstract:
  Positive matrices in SL(2,C) have a double physical interpretation; they can
be either considered as "fuzzy projections" of a spin 1/2 quantum system, or as
Lorentz boosts. In the present paper, concentrating on this second
interpretation, we follow the clues given by Pertti Lounesto and, using the
classical Clifford algebraic methods, interpret them as conformal maps of the
"heavenly sphere" S^2. The fuzziness parameter of the first interpretation
becomes the "boost velocity" in the second one. We discuss simple iterative
function systems of such maps, and show that they lead to self--similar fractal
patterns on S^2. The final section of this paper is devoted to an informal
discussion of the relations between these concepts and the problems in the
foundations of quantum theory, where the interplay between different kinds of
algebras and maps may enable us to describe not only the continuous evolution
of wave functions, but also quantum jumps and "events" that accompany these
jumps. Paper dedicated to the memory of Pertti Lounesto.


title:
Quantum Dynamical Effects as a Singular Perturbation for Observables in
  Open Quasi-Classical Nonlinear Mesoscopic Systems

abstract:
  We review our results on a mathematical dynamical theory for observables for
open many-body quantum nonlinear bosonic systems for a very general class of
Hamiltonians. We show that non-quadratic (nonlinear) terms in a Hamiltonian
provide a singular "quantum" perturbation for observables in some "mesoscopic"
region of parameters. In particular, quantum effects result in secular terms in
the dynamical evolution, that grow in time. We argue that even for open quantum
nonlinear systems in the deep quasi-classical region, these quantum effects can
survive after decoherence and relaxation processes take place. We demonstrate
that these quantum effects in open quantum systems can be observed, for
example, in the frequency Fourier spectrum of the dynamical observables, or in
the corresponding spectral density of noise. Estimates are presented for
Bose-Einstein condensates, low temperature mechanical resonators, and nonlinear
optical systems prepared in large amplitude coherent states. In particular, we
show that for Bose-Einstein condensate systems the characteristic time of
deviation of quantum dynamics for observables from the corresponding classical
dynamics coincides with the characteristic time-scale of the well-known quantum
nonlinear effect of phase diffusion.


title:
Postponement of dark-count effects in practical quantum key-distribution
  by two-way post-processing

abstract:
  The influence of imperfections on achievable secret-key generation rates of
quantum key distribution protocols is investigated. As examples of relevant
imperfections, we consider tagging of Alice's qubits and dark counts at Bob's
detectors, while we focus on a powerful eavesdropping strategy which takes full
advantage of tagged signals. It is demonstrated that error correction and
privacy amplification based on a combination of a two-way classical
communication protocol and asymmetric Calderbank-Shor-Steane codes may
significantly postpone the disastrous influence of dark counts. As a result,
the distances are increased considerably over which a secret key can be
distributed in optical fibres reliably. Results are presented for the
four-state, the six-state, and the decoy-state protocols.


title: Maximum Confidence Quantum Measurements

abstract:
  We consider the problem of discriminating between states of a specified set
with maximum confidence. For a set of linearly independent states unambiguous
discrimination is possible if we allow for the possibility of an inconclusive
result. For linearly dependent sets an analogous measurement is one which
allows us to be as confident as possible that when a given state is identified
on the basis of the measurement result, it is indeed the correct state.


title: Quantum Teleportation with Continuous Variables: a survey

abstract:
  Very recently we have witnessed a new development of quantum information, the
so-called continuous variable (CV) quantum information theory. Such a further
development has been mainly due to the experimental and theoretical advantages
offered by CV systems, i.e., quantum systems described by a set of observables,
like position and momentum, which have a continuous spectrum of eigenvalues.
According to this novel trend, quantum information protocols like quantum
teleportation have been suitably extended to the CV framework. Here, we briefly
review some mathematical tools relative to CV systems and we consequently
develop the concepts of quantum entanglement and teleportation in the CV
framework, by analogy with the qubit-based approach. Some connections between
teleportation fidelity and entanglement properties of the underlying quantum
channel are inspected. Next, we face the study of CV quantum teleportation
networks where more users share a multipartite state and an arbitrary pair of
them performs quantum teleportation. In this context, we show alternative
protocols and we investigate the optimal strategy that maximizes the
performance of the network.


title: Phase noise due to vibrations in Mach-Zehnder atom interferometers

abstract:
  Atom interferometers are very sensitive to accelerations and rotations. This
property, which has some very interesting applications, induces a deleterious
phase noise due to the seismic noise of the laboratory and this phase noise is
sufficiently large to reduce the fringe visibility in many experiments. We
develop a model calculation of this phase noise in the case of Mach-Zehnder
atom interferometers and we apply this model to our thermal lithium
interferometer. We are able to explain the observed phase noise which has been
detected through the rapid dependence of the fringe visibility with the
diffraction order. We think that the dynamical model developed in the present
paper should be very useful to reduce the vibration induced phase noise in atom
interferometers, making many new experiments feasible.


title: A new description of spin tunneling in magnetic molecules

abstract:
  A new approach is used that allows to describe the magnetic molecules main
properties in a direct and simple way. Results obtained for the Fe8 cluster
show good agreement with the experimental data.


title: Non-Markovian thermalization of entangled qubits

abstract:
  We study the decoherence properties of an entangled bipartite qubit system,
represented by two two-level atoms that are individually coupled to
non-Markovian reservoirs. This coupling ensures that the dynamical equations of
the atoms can be treated independently. The non-Markovian reservoirs are
described by a model which leads to an exact non-Markovian master equation of
the Nakajima-Zwanzig form [J. Salo, S. M. Barnett, and S. Stenholm, Optics
Commun. 259, 772 (2006)]. We consider the evolution of the entanglement of a
two-atom state that is initially completely entangled, quantified by its
concurrence. Collapses and revivals in the concurrence, induced by the memory
effects of the reservoir, govern the dynamics of the entangled quantum system.
These collapses and revivals in the concurrence are a strong manifestation of
the non-Markovian reservoir.


title: Thermal entanglement of spins in mean-field clusters

abstract:
  We determine thermal entanglement in mean field clusters of $N$ spin one-half
particles interacting via the anisotropic Heisenberg interaction, with and
without external magnetic field. For the $xxx$ cluster in the absence of
magnetic field we prove that only the N=2 ferromagnetic cluster shows
entanglement. An external magnetic field $B$ can only entangle $xxx$
anti-ferromagnetic clusters in certain regions of the $B-T$ plane. On the other
hand, the $xxz$ clusters of size $N>2$ are entangled only when the interaction
is ferromagnetic. Detailed dependence of the entanglement on various parameters
is investigated in each case.


title:
Excited state contribution to the Casimir-Polder force at finite
  temperature

abstract:
  Using the master equation we calculate the contribution of the excited state
of a two-level atom to its interacting potential with a perfectly conducting
wall at finite temperature. For low temperature, $\hbar \omega_0/k_B T = k_0
\lambda_T\gg 1$, where $\omega_0 = k_0 c$ is the transition frequency of the
atom and $\lambda_T$ is the thermal wavelength, we show that this contribution
is very small $(\propto e^{-k_0\lambda_T})$. In the opposite limit
$(k_0\lambda_T \ll 1)$, however, we show that the expression for the
interacting potential, for all relevant distance regimes, becomes exactly the
same as that for very short distances $(k_0 z \ll 1)$ and with the field in the
vacuum state.


title: Casimir-Polder forces from density matrix formalism

abstract:
  We use the density matrix formalism in order to calculate the energy level
shifts, in second order on interaction, of an atom in the presence of a
perfectly conducting wall in the dipole approximation. The thermal corrections
are also examined when $\hbar \omega_0/k_B T = k_0 \lambda_T \gg 1$, where
${$\omega_0=k_0 c$}$ is the dominant transition frequency of the atom and
$\lambda_T$ is the thermal length. When the distance $z$ between the atom and
the wall is larger than $\lambda_T$ we find the well known result obtained from
Lifshitz's formula, whose leading term is proportional to temperature and is
independent of $c$, $\hbar$ and $k_0$. In the short distance limit, when
$z\ll\lambda_T$, only very small corrections to the leading vacuum term occur.
We also show, for all distance regimes, that the main thermal corrections are
independent of $k_0$ (dispersion is not important) and dependent of $c$, which
means that there is not a non-retarded regime for the thermal contributions.


title:
A master equation approach for the interaction of an atom with a
  dielectric semi-infinite medium

abstract:
  We use the master equation approach to calculate the energy level shifts of
an atom in the presence of a general dielectric semi-infinite medium
characterized by a dielectric constant $\epsilon(\omega)$. Particularly, we
analyze the case of a non-dispersive medium for which we obtain a general
expression for the interaction as well as the asymptotic behaviors for $k_0 z
\ll 1$ (non-retarded regime) and $k_0 z \gg 1$ (retarded regime), where
$\omega_0 = k_0 c$ is the main transition frequency of the atom. The limiting
cases $\epsilon \simeq 1$ and $\epsilon \gg 1$ are discussed for both retarded
and non-retarded limits. For the retarded limit, we compute the non-additivity
contribution of van der Waals forces.


abstract: The security proof of the ping-pong protocol is wrong.


title:
An immunity against correlation attack on quantum stream cipher by Yuen
  2000 protocol

abstract:
  This paper presents the security analysis on the quantum stream cipher so
called Yuen-2000 protocol (or $\alpha\eta$ scheme) against the fast correlation
attack, the typical attack on stream ciphers. Although a very simple
experimental model of the quantum stream cipher without a random mapper may be
decrypted in the information theoretic sense by the fast correlation algorithm,
it is not a basic feature of Yuen 2000 protocol. In fact, we clarify that there
exists a randomization scheme which attains the perfect correlation immunity
against such attacks under an approximation. And in this scheme, the running
key correlation from the second randomization that determines the mapping
patterns is dismissed also by quantum noise. In such a case, any fast
correlation attack does not work on the quantum stream cipher.


title: Universal Approach to Optimal Photon Storage in Atomic Media

abstract:
  We present a universal physical picture for describing storage and retrieval
of photon wave packets in a Lambda-type atomic medium. This physical picture
encompasses a variety of different approaches to pulse storage ranging from
adiabatic reduction of the photon group velocity and pulse-propagation control
via off-resonant Raman fields to photon-echo based techniques. Furthermore, we
derive an optimal control strategy for storage and retrieval of a photon wave
packet of any given shape. All these approaches, when optimized, yield
identical maximum efficiencies, which only depend on the optical depth of the
medium.


title: Entropic information-disturbance tradeoff

abstract:
  We show the flaws found in the customary fidelity-based definitions of
disturbance in quantum measurements and evolutions. We introduce the "entropic
disturbance" D and show that it adequately measures the degree of disturbance,
intended essentially as an irreversible change in the state of the system. We
also find that it complies with an information-disturbance tradeoff, namely the
mutual information between the eigenvalues of the initial state and the
measurement results is less than or equal to D.


title: Tomography in abstract Hilbert spaces

abstract:
  The tomographic description of a quantum state is formulated in an abstract
infinite dimensional Hilbert space framework, the space of the Hilbert-Schmidt
linear operators, with trace formula as scalar product. Resolutions of the
unity, written in terms of over-complete sets of rank-one projectors and of
associated Gram-Schmidt operators taking into account their non-orthogonality,
are then used to reconstruct a quantum state from its tomograms. Examples of
well known tomographic descriptions illustrate the exposed theory.


title: Gain narrowing in few-atom systems

abstract:
  Using a density matrix approach, we study the simplest systems that display
both gain and feedback: clusters of 2 to 5 atoms, one of which is pumped. The
other atoms supply feedback through multiple scattering of light. We show that,
if the atoms are in each other's near-field, the system exhibits large gain
narrowing and spectral mode redistribution. The observed phenomena are more
pronounced if the feedback is enhanced. Our system is to our knowledge the
simplest exactly solvable microscopic system which shows the approach to laser
oscillation.


title:
Exact Solutions of the Schr\"{o}dinger Equation with position-dependent
  effective mass via general point canonical transformation

abstract:
  Exact solutions of the Schrodinger equation are obtained for the Rosen-Morse
and Scarf potentials with the position-dependent effective mass by appliying a
general point canonical transformation. The general form of the point canonical
transformation is introduced by using a free parameter. Two different forms of
mass distributions are used. A set of the energy eigenvalues of the bound
states and corresponding wave functions for target potentials are obtained as a
function of the free parameter.


title: Geometric phase in weak measurements

abstract:
  Pancharatnam's geometric phase is associated with the phase of a
complex-valued weak value arising in a certain type of weak measurement in pre-
and post-selected quantum ensembles. This makes it possible to test the
nontransitive nature of the relative phase in quantum mechanics, in the weak
measurement scenario.


title: Coherent imaging of a pure phase object with classical incoherent light

abstract:
  By using the ghost imaging technique, we experimentally demonstrate the
reconstruction of the diffraction pattern of a {\em pure phase} object by using
the classical correlation of incoherent thermal light split on a beam splitter.
The results once again underline that entanglement is not a necessary feature
of ghost imaging. The light we use is spatially highly incoherent with respect
to the object ($\approx 2 \mu$m speckle size) and is produced by a
pseudo-thermal source relying on the principle of near-field scattering. We
show that in these conditions no information on the phase object can be
retrieved by only measuring the light that passed through it, neither in a
direct measurement nor in a Hanbury Brown-Twiss (HBT) scheme. In general, we
show a remarkable complementarity between ghost imaging and the HBT scheme when
dealing with a phase object.


title: Does The Uncertainty Relation Determine The Quantum State?

abstract:
  The example of nonpositive trace-class Hermitian operator for which
Robertson-Schroedinger uncertainty relation is fulfilled is presented. The
partial scaling criterion of separability of multimode continuous variable
system is discussed in the context of using nonpositive maps of density
matrices.


title: Better Bell Inequality Violation by Collective Measurements

abstract:
  The standard Bell inequality experiments test for violation of local realism
by repeatedly making local measurements on individual copies of an entangled
quantum state. Here we investigate the possibility of increasing the violation
of a Bell inequality by making collective measurements. We show that
nonlocality of bipartite pure entangled states, quantified by their maximal
violation of the Bell-Clauser-Horne inequality, can always be enhanced by
collective measurements, even without communication between the parties. For
mixed states we also show that collective measurements can increase the
violation of Bell inequalities, although numerical evidence suggests that the
phenomenon is not common as it is for pure states.


title: Casimir interaction between excited media in electromagnetic field

abstract:
  We investigate the Casimir-Polder interaction between two atoms one of which
is excited. We show that the perturbation theory results in divergence of
integrals for the interaction between an excited atom and a media of dilute
gas. We considered the interaction between two atoms embedded in a dielectric
medium. The non-perturbative method used in this paper shows that the
interaction between the atoms is suppressed due to absorption of photons by the
medium. Now the integrals are divergent no more. Interaction between two media
of dilute gases is considered for the case of high temperatures.


title: Characterizing quantum phase transitions by single qubit operations

abstract:
  We introduce observable quantities, borrowing from concepts of quantum
information theory, for the characterization of quantum phase transitions in
spin systems. These observables are uniquely defined in terms of single spin
unitary operations. We define the energy gap between the ground state and the
state produced by the action of a single-qubit local gate. We show that this
static quantity involves only single-site expectations and two-point
correlation functions on the ground state. We then discuss a dynamical local
observable defined as the acceleration of quantum state evolution after
performing an instaneous single-qubit perturbation on the ground state. This
quantity involves three-point correlations as well. We show that both the
static and the dynamical observables detect and characterize completely quantum
critical points in a class of spin systems.


title: Interconnection Networks for Scalable Quantum Computers

abstract:
  We show that the problem of communication in a quantum computer reduces to
constructing reliable quantum channels by distributing high-fidelity EPR pairs.
We develop analytical models of the latency, bandwidth, error rate and resource
utilization of such channels, and show that 100s of qubits must be distributed
to accommodate a single data communication. Next, we show that a grid of
teleportation nodes forms a good substrate on which to distribute EPR pairs. We
also explore the control requirements for such a network. Finally, we propose a
specific routing architecture and simulate the communication patterns of the
Quantum Fourier Transform to demonstrate the impact of resource contention.


title: Tight informationally complete quantum measurements

abstract:
  We introduce a class of informationally complete positive-operator-valued
measures which are, in analogy with a tight frame, "as close as possible" to
orthonormal bases for the space of quantum states. These measures are
distinguished by an exceptionally simple state-reconstruction formula which
allows "painless" quantum state tomography. Complete sets of mutually unbiased
bases and symmetric informationally complete positive-operator-valued measures
are both members of this class, the latter being the unique minimal rank-one
members. Recast as ensembles of pure quantum states, the rank-one members are
in fact equivalent to weighted 2-designs in complex projective space. These
measures are shown to be optimal for quantum cloning and linear quantum state
tomography.


title:
Entanglement criteria based on local uncertainty relations are strictly
  stronger than the computable cross norm criterion

abstract:
  We show that any state which violates the computable cross norm (or
realignment) criterion for separability also violates the separability
criterion of the local uncertainty relations. The converse is not true. The
local uncertainty relations provide a straightforward construction of nonlinear
entanglement witnesses for the cross norm criterion.


title: Optimal Generation of Pulsed Entangled Photon Pairs

abstract:
  We experimentally investigate a double-pass parametric down-conversion scheme
for producing pulsed, polarization-entangled photon pairs with high visibility.
The amplitudes for creating photon pairs on each pass interfere to compensate
for distinguishing characteristics that normally degrade two-photon visibility.
The result is a high-flux source of polarization-entangled photon pulses that
does not require spectral filtering. We observe quantum interference visibility
of over 95% without the use of spectral filters for 200 femtosecond pulses, and
up to 98.1% with 5 nm bandwidth filters.


title: On the Role of Shared Entanglement

abstract:
  Despite the apparent similarity between shared randomness and shared
entanglement in the context of Communication Complexity, our understanding of
the latter is not as good as of the former. In particular, there is no known
"entanglement analogue" for the famous theorem by Newman, saying that the
number of shared random bits required for solving any communication problem can
be at most logarithmic in the input length (i.e., using more than O(log n)
shared random bits would not reduce the complexity of an optimal solution).
  In this paper we prove that the same is not true for entanglement. We
establish a wide range of tight (up to a polylogarithmic factor) entanglement
vs. communication tradeoffs for relational problems. The low end is: for any
t>2, reducing shared entanglement from log^t(n) to o(log^{t-2}(n)) qubits can
increase the communication required for solving a problem almost exponentially,
from O(log^t(n)) to \Omega(\sqrt n). The high end is: for any \eps>0, reducing
shared entanglement from n^{1-\eps}log(n) to o(n^{1-\eps}/log(n)) can increase
the required communication from O(n^{1-\eps}log(n)) to
\Omega(n^{1-\eps/2}/log(n)). The upper bounds are demonstrated via protocols
which are exact and work in the \e{simultaneous message passing model}, while
the lower bounds hold for \e{bounded-error protocols}, even in the more
powerful \e{model of 1-way communication}. Our protocols use shared EPR pairs
while the lower bounds apply to any sort of prior entanglement.
  We base the lower bounds on a strong direct product theorem for communication
complexity of a certain class of relational problems. We believe that the
theorem might have applications outside the scope of this work.


title: Dark periods and revivals of entanglement in a two qubit system

abstract:
  In a recent paper Yu and Eberly [Phys. Rev. Lett. {\bf 93}, 140404 (2004)]
have shown that two initially entangled and afterwards not interacting qubits
can become completely disentangled in a finite time. We study transient
entanglement between two qubits coupled collectively to a multimode vacuum
field and find an unusual feature that the irreversible spontaneous decay can
lead to a revival of the entanglement that has already been destroyed. The
results show that this feature is independent of the coherent dipole-dipole
interaction between the atoms but it depends critically on whether or not the
collective damping is present. We show that the ability of the system to
revival entanglement via spontaneous emission relies on the presence of very
different timescales for the evolution of the populations of the collective
states and coherence between them.


title: On the quantum master equation for fermions

abstract:
  A quantum master equation is obtained for identical fermions by including a
relaxation term in addition to the mean-field Hamiltonian. [Huang C F and Huang
K N 2004 Chinese J. Phys. ${\bf 42}$ 221; Gebauer R and Car R 2004 Phys. Rev. B
${\bf 70}$ 125324] It is proven in this paper that both the positivity and
Pauli's exclusion principle are preserved under this equation when there exists
an upper bound for the transition rate. Such an equation can be generalized to
model BCS-type quasiparticles, and is reduced to a Markoff master equation of
Lindblad form in the low-density limit with respect to particles or holes.


title: About mathematical apparatus of many body quantum dynamics

abstract:
  We discuss the possibility to modify many-body Hilbert quantum formalism that
is necessary for the representation of quantum systems dynamics. The notion of
effective classical algorithm and visualization of quantum dynamics play the
key role.


title: Quantum Versus Classical Proofs and Advice

abstract:
  This paper studies whether quantum proofs are more powerful than classical
proofs, or in complexity terms, whether QMA=QCMA. We prove three results about
this question. First, we give a "quantum oracle separation" between QMA and
QCMA. More concretely, we show that any quantum algorithm needs
$\Omega(\sqrt{2^n/(m+1)})$ queries to find an $n$-qubit "marked state"
$\lvert\psi\rangle$, even if given an $m$-bit classical description of
$\lvert\psi\rangle$ together with a quantum black box that recognizes
$\lvert\psi\rangle$. Second, we give an explicit QCMA protocol that nearly
achieves this lower bound. Third, we show that, in the one previously-known
case where quantum proofs seemed to provide an exponential advantage, classical
proofs are basically just as powerful. In particular, Watrous gave a QMA
protocol for verifying non-membership in finite groups. Under plausible
group-theoretic assumptions, we give a QCMA protocol for the same problem. Even
with no assumptions, our protocol makes only polynomially many queries to the
group oracle. We end with some conjectures about quantum versus classical
oracles, and about the possibility of a classical oracle separation between QMA
and QCMA.


title:
The analytical solution of the Schr\"{o}dinger equation in
  Born-Oppenheimer approximation for $H_2^+$ molecular ion

abstract:
  An analysis of the analytical solution of the Schr\"{o}dinger equation (which
is a second order differential equation) for $H_2^+$ shows that the second
linear independent solution of this equation is a square integrable function
and therefore the ground state total wave function is a linear combination of
two linear independent wave functions of different space symmetry: cylindrical
and spherical. The wave function of cylindrical symmetry is well known. It has
maxima at the positions of nuclei. The wave function of spherical symmetry and
the corresponding spherical electron distribution, which exists at $R\neq0$ and
locates at the middle of the bond, represents a quasiatom of electron density
of non-nuclear united atom. In the light of the new result the qualitative
behavior of the ground state wave function and the electron density of $H_2^+$
has been reinvestigated. It is shown analytically that a transformation of the
total molecular wave function with two maxima to that one with one maximum
passes through a flat wave function. The presented three-dimension figures of
the electron density visualize the spherical component of the total wave
function and its transformation with increasing internuclear separation.


title:
Dynamical Casimir Effect in a one-dimensional uniformly contracting
  cavity

abstract:
  We consider particle creation (the Dynamical Casimir effect) in a uniformly
contracting ideal one-dimensional cavity non-perturbatively. The exact
expression for the energy spectrum of created particles is obtained and its
dependence on parameters of the problem is discussed. Unexpectedly, the number
of created particles depends on the duration of the cavity contracting
non-monotonously. This is explained by quantum interference of the events of
particle creation which are taking place only at the moments of acceleration
and deceleration of a boundary, while stable particle states exist (and thus no
particles are created) at the time of contracting.


title:
Query complexity for searching multiple marked states from an unsorted
  database

abstract:
  An important and usual problem is to search all states we want from a
database with a large number of states. In such, recall is vital. Grover's
original quantum search algorithm has been generalized to the case of multiple
solutions, but no one has calculated the query complexity in this case. We will
use a generalized algorithm with higher precision to solve such a search
problem that we should find all marked states and show that the practical query
complexity increases with the number of marked states. In the end we will
introduce an algorithm for the problem on a ``duality computer'' and show its
advantage over other algorithms.


title: Fake-signal-and-cheating attack on quantum secret sharing

abstract:
  We present a fake-signal-and-cheating attack strategy for the dishonest agent
in quantum secret sharing (QSS) to steal the information of the other agents'
fully and freely. It is found that almost all the QSS protocols existing, such
as the two famous QSS protocols, the Hillery-Bu$\check{z}$ek-Berthiaume [Phys.
Rev. A \textbf{59}, 1829 (1999)] and the Karlsson-Koashi-Imoto [Phys. Rev. A
\textbf{59}, 162 (1999)], can be eavesdropped freely if the process for the
eavesdropping check is accomplished with the cooperation of the dishonest
agent. He can sends a fake signal to the other agents after intercepting the
original photons and storing them. His action can be hidden with entanglement
swapping and cheating when the photons are chosen as the samples for checking
eavesdropping. Finally, we present a possible improvement of these QSS
protocols' security with decoy photons.


title: (4,1)-Quantum Random Access Coding Does Not Exist

abstract:
  An (n,1,p)-Quantum Random Access (QRA) coding, introduced by Ambainis, Nayak,
Ta-shma and Vazirani in ACM Symp. on Theory of Computing 1999, is the following
communication system: The sender which has n-bit information encodes his/her
information into one qubit, which is sent to the receiver. The receiver can
recover any one bit of the original n bits correctly with probability at least
p, through a certain decoding process based on positive operator-valued
measures. Actually, Ambainis et al. shows the existence of a (2,1,0.85)-QRA
coding and also proves the impossibility of its classical counterpart. Chuang
immediately extends it to a (3,1,0.79)-QRA coding and whether or not a
(4,1,p)-QRA coding such that p > 1/2 exists has been open since then. This
paper gives a negative answer to this open question.


title: Fixed-point Quantum Search for Different Phase Shifts

abstract:
  Grover recently presented the fixed-point search algorithm. In this letter,
we study the fixed-point search algorithm obtained by replacing equal phase
shifts of $\pi /3$ by different phase shifts.


title:
Canonical Coset Parameterization and the Bures Metric of the Three-level
  Quantum Systems

abstract:
  An explicit parameterization for the state space of an $n$-level density
matrix is given. The parameterization is based on the canonical coset
decomposition of unitary matrices. We also compute, explicitly, the Bures
metric tensor over the state space of two- and three-level quantum systems.


title: Relational EPR

abstract:
  We study the EPR-type correlations from the perspective of the relational
interpretation of quantum mechanics. We argue that these correlations do not
entail any form of 'non-locality', when viewed in the context of this
interpretation. The abandonment of strict Einstein realism implied by the
relational stance permits to reconcile quantum mechanics, completeness,
(operationally defined) separability, and locality.


title:
Signatures of the Unruh effect from electrons accelerated by
  ultra-strong laser fields

abstract:
  We calculate the radiation resulting from the Unruh effect for strongly
accelerated electrons and show that the photons are created in pairs whose
polarizations are maximally entangled. Apart from the photon statistics, this
quantum radiation can further be discriminated from the classical (Larmor)
radiation via the different spectral and angular distributions. The signatures
of the Unruh effect become significant if the external electromagnetic field
accelerating the electrons is not too far below the Schwinger limit and might
be observable with future facilities. Finally, the corrections due to the
birefringent nature of the QED vacuum at such ultra-high fields are discussed.
PACS: 04.62.+v, 12.20.Fv, 41.60.-m, 42.25.Lc.


title: Classical and Quantum Chaos in Atom Optics

abstract:
  The interaction of an atom with an electromagnetic field is discussed in the
presence of a time periodic external modulating force. It is explained that a
control on atom by electromagnetic fields helps to design the quantum analog of
classical optical systems. In these atom optical systems chaos may appear at
the onset of external fields. The classical and quantum chaotic dynamics is
discussed, in particular in an atom optics Fermi accelerator. It is found that
the quantum dynamics exhibits dynamical localization and quantum recurrences.


title: Speed and entropy of an interacting continuous time quantum walk

abstract:
  We present some dynamic and entropic considerations about the evolution of a
continuous time quantum walk implementing the clock of an autonomous machine.
On a simple model, we study in quite explicit terms the Lindblad evolution of
the clocked subsystem, relating the evolution of its entropy to the spreading
of the wave packet of the clock. We explore possible ways of reducing the
generation of entropy in the clocked subsystem, as it amounts to a deficit in
the probability of finding the target state of the computation. We are thus
lead to examine the benefits of abandoning some classical prejudice about how a
clocking mechanism should operate.


title:
Bounds on the heat kernel of the Schroedinger operator in a random
  electromagnetic field

abstract:
  We obtain lower and upper bounds on the heat kernel and Green functions of
the Schroedinger operator in a random Gaussian magnetic field and a fixed
scalar potential. We apply stochastic Feynman-Kac representation, diamagnetic
upper bounds and the Jensen inequality for the lower bound. We show that if the
covariance of the electromagnetic (vector) potential is increasing at large
distances then the lower bound is decreasing exponentially fast for large
distances and a large time.


title:
Survival probability of surface excitations in a 2d lattice:
  non-Markovian effects and Survival Collapse

abstract:
  The evolution of a surface excitation in a two dimentional model is analyzed.
I) It starts quadratically up to a spreading time t_{S}. II) It follows an
exponential behavior governed by a self-consistent Fermi Golden Rule. III) At
longer times, the exponential is overrun by an inverse power law describing
return processes governed by quantum diffusion. At this last transition time
t_{R} a survival collapse becomes possible, bringing the survival probability
down by several orders of magnitude. We identify this strongly destructive
interference as an antiresonance in the time domain.


title:
Quantum Memory Hierarchies: Efficient Designs to Match Available
  Parallelism in Quantum Computing

abstract:
  The assumption of maximum parallelism support for the successful realization
of scalable quantum computers has led to homogeneous, ``sea-of-qubits''
architectures. The resulting architectures overcome the primary challenges of
reliability and scalability at the cost of physically unacceptable system area.
We find that by exploiting the natural serialization at both the application
and the physical microarchitecture level of a quantum computer, we can reduce
the area requirement while improving performance. In particular we present a
scalable quantum architecture design that employs specialization of the system
into memory and computational regions, each individually optimized to match
hardware support to the available parallelism. Through careful application and
system analysis, we find that our new architecture can yield up to a factor of
thirteen savings in area due to specialization. In addition, by providing a
memory hierarchy design for quantum computers, we can increase time performance
by a factor of eight. This result brings us closer to the realization of a
quantum processor that can solve meaningful problems.


title:
Kinetics and thermodynamics of electron transfer in Debye solvents: An
  analytical and nonperturbative reduced density matrix theory

abstract:
  A nonperturbative electron transfer rate theory is developed based on the
reduced density matrix dynamics, which can be evaluated readily for the Debye
solvent model without further approximation. Not only does it recover for
reaction rates the celebrated Marcus' inversion and Kramers' turnover
behaviors, the present theory also predicts for reaction thermodynamics, such
as equilibrium Gibbs free-energy and entropy, some interesting
solvent-dependent features that are calling for experimental verification.
Moreover, a continued fraction Green's function formalism is also constructed,
which can be used together with Dyson equation technique, for efficient
evaluation of nonperturbative reduced density matrix dynamics.


title: Brief History of Quantum Cryptography: A Personal Perspective

abstract:
  Quantum cryptography is the only approach to privacy ever proposed that
allows two parties (who do not share a long secret key ahead of time) to
communicate with provably perfect secrecy under the nose of an eavesdropper
endowed with unlimited computational power and whose technology is limited by
nothing but the fundamental laws of nature. This essay provides a personal
historical perspective on the field. For the sake of liveliness, the style is
purposely that of a spontaneous after-dinner speech.


title: Bound energy for the exponential-cosine-screened Coulomb potential

abstract:
  An alternative approximation scheme has been used in solving the Schrodinger
equation for the exponential-cosine-screened Coulomb potential. The bound state
energ\i es for various eigenstates and the corresponding wave functions are
obtained analytically up to the second perturbation term.


title: Thermal limitation of far-field matter-wave interference

abstract:
  We assess the effect of the heat radiation emitted by mesoscopic particles on
their ability to show interference in a double slit arrangement. The analysis
is based on a stationary, phase-space based description of matter wave
interference in the presence of momentum-exchange mediated decoherence.


title:
Weyl's symbols of Heisenberg operators of canonical coordinates and
  momenta as quantum characteristics

abstract:
  The knowledge of quantum phase flow induced under the Weyl's association rule
by the evolution of Heisenberg operators of canonical coordinates and momenta
allows to find the evolution of symbols of generic Heisenberg operators. The
quantum phase flow curves obey the quantum Hamilton's equations and play the
role of characteristics. At any fixed level of accuracy of semiclassical
expansion, quantum characteristics can be constructed by solving a coupled
system of first-order ordinary differential equations for quantum trajectories
and generalized Jacobi fields. Classical and quantum constraint systems are
discussed. The phase-space analytic geometry based on the star-product
operation can hardly be visualized. The statement "quantum trajectory belongs
to a constraint submanifold" can be changed e.g. to the opposite by a unitary
transformation. Some of relations among quantum objects in phase space are,
however, left invariant by unitary transformations and support partly geometric
relations of belonging and intersection. Quantum phase flow satisfies the
star-composition law and preserves hamiltonian and constraint star-functions.

categories: quant-ph cond-mat.stat-mech hep-th math-ph math.AP math.CA math.DS math.MP


title:
Evolution Law of Quantum Observables from Classical Hamiltonian in
  Non-Commutative Phase Space

abstract:
  The evolution equations of quantum observables are derived from the classical
Hamiltonian equations of motion with the only additional assumption that the
phase space is non-commutative. The demonstration of the quantum evolution laws
is quite general; it does not rely on any assumption on the operator nature of
x and p and is independent of the quantum mechanical formalism.


title:
Quantum adiabatic evolutions that can't be used to design efficient
  algorithms

abstract:
  Quantum adiabatic computation is a novel paradigm for the design of quantum
algorithms, which is usually used to find the minimum of a classical function.
In this paper, we show that if the initial hamiltonian of a quantum adiabatic
evolution with a interpolation path is too simple, the minimal gap between the
ground state and the first excited state of this quantum adiabatic evolution is
an inverse exponential distance. Thus quantum adiabatic evolutions of this kind
can't be used to design efficient quantum algorithms. Similarly, we show that a
quantum adiabatic evolution with a simple final hamiltonian also has a long
running time, which suggests that some functions can't be minimized efficiently
by any quantum adiabatic evolution with a interpolation path.


title: Bound states of a more general exponential screened Coulomb potential

abstract:
  An alternative approximation scheme has been used in solving the Schrodinger
equation to the more general case of exponential screened Coulomb potential,
V(r)=-(a/r)\[1+(1+br)e^{-2br}]. The bound state energies of the 1s, $2s, and
3s-states, together with the ground state wave function are obtained
analytically upto the second perturbation term.


title: The Free Will Theorem

abstract:
  On the basis of three physical axioms, we prove that if the choice of a
particular type of spin 1 experiment is not a function of the information
accessible to the experimenters, then its outcome is equally not a function of
the information accessible to the particles. We show that this result is
robust, and deduce that neither hidden variable theories nor mechanisms of the
GRW type for wave function collapse can be made relativistic. We also establish
the consistency of our axioms and discuss the philosophical implications.


title: Effects of Hawking radiation and Wigner rotation on fermion entanglement

abstract:
  In this work, we report that the Hawking radiation effect on fermions is
fundamentally different from the case of scalar particles. Intrinsic properties
of fermions (exclusion principle and spin) affect strongly the interaction of
fermions with both Hawking radiation and metric of the spacetime. In particular
we have found the following: first, while the fermion vacuum state seen by the
Rindler observer is an entangled state in which the right and left Rindler
wedge states appear in correlated pairs as in the case of the scalar particles,
the entanglement disappears in the excited state due to the exclusion
principle; second, the spin of the fermion experiences the Winger rotation
under a uniform acceleration; and third, the quantum information of fermions
encoded in spin (entangled state is composed of different spin states but with
the same mode function) is dissipated not by the Hawking radiation but by the
Wigner rotation as the pair approaches the event horizon.


title:
Controlled photon transfer between two individual nanoemitters via
  shared high-Q modes of a microsphere resonator

abstract:
  We realize controlled cavity-mediated photon transfer between two single
nanoparticles over a distance of several tens of micrometers. First, we show
how a single nanoscopic emitter attached to a near-field probe can be coupled
to high-Q whispering-gallery modes of a silica microsphere at will. Then we
demonstrate transfer of energy between this and a second nanoparticle deposited
on the sphere surface. We estimate the photon transfer efficiency to be about
six orders of magnitude higher than that via free space propagation at
comparable separations.


title: Aspects of quantum coherence in the optical Bloch equations

abstract:
  Aspects of coherence and decoherence are analyzed within the optical Bloch
equations. By rewriting the analytic solution in an alternate form, we are able
to emphasize a number of unusual features: (a) despite the Markovian nature of
the bath, coherence at long times can be retained; (b) the long-time asymptotic
degree of coherence in the system is intertwined with the asymptotic difference
in level populations; (c) the traditional population-relaxation and decoherence
times, $T_1$ and $T_2$, lose their meaning when the system is in the presence
of an external field, and are replaced by more general overall timescales; (d)
increasing the field strength, quantified by the Rabi frequency, $\Omega$,
increases the rate of decoherence rather than reducing it, as one might expect;
and (e) maximum asymptotic coherence is reached when the system parameters
satisfy $\Omega^2 = 1/(T_1 T_2)$.


title: Wave equations for determining energy-level gaps of quantum systems

abstract:
  An differential equation for wave functions is proposed, which is equivalent
to Schr\"{o}dinger's wave equation and can be used to determine energy-level
gaps of quantum systems. Contrary to Schr\"{o}dinger's wave equation, this
equation is on `bipartite' wave functions. It is shown that those `bipartite'
wave functions satisfy all the basic properties of Schr\"{o}dinger's wave
functions. Further, it is argued that `bipartite' wave functions can present a
mathematical expression of wave-particle duality. This provides an alternative
approach to the mathematical formalism of quantum mechanics.


title: Nonlinear optical Galton board

abstract:
  We generalize the concept of optical Galton board (OGB), first proposed by
Bouwmeester et al. {[}Phys. Rev. A \textbf{61}, 013410 (2000)], by introducing
the possibility of nonlinear self--phase modulation on the wavefunction during
the walker evolution. If the original Galton board illustrates classical
diffusion, the OGB, which can be understood as a grid of Landau--Zener
crossings, illustrates the influence of interference on diffusion, and is
closely connected with the quantum walk. Our nonlinear generalization of the
OGB shows new phenomena, the most striking of which is the formation of
non-dispersive pulses in the field distribution (soliton--like structures).
These exhibit a variety of dynamical behaviors, including ballistic motion,
dynamical localization, non--elastic collisions and chaotic behavior, in the
sense that the dynamics is very sensitive to the nonlinearity strength.


title:
A Polynomial-Time Algorithm for the Equivalence between Quantum
  Sequential Machines

abstract:
  Quantum sequential machines (QSMs) are a quantum version of stochastic
sequential machines (SSMs). Recently, we showed that two QSMs M_1 and M_2 with
n_1 and n_2 states, respectively, are equivalent iff they are
(n_1+n_2)^2--equivalent (Theoretical Computer Science 358 (2006) 65-74).
However, using this result to check the equivalence likely needs exponential
expected time. In this paper, we consider the time complexity of deciding the
equivalence between QSMs and related problems. The main results are as follows:
(1) We present a polynomial-time algorithm for deciding the equivalence between
QSMs, and, if two QSMs are not equivalent, this algorithm will produce an
input-output pair with length not more than (n_1+n_2)^2. (2) We improve the
bound for the equivalence between QSMs from (n_1+n_2)^2 to n_1^2+n_2^2-1, by
employing Moore and Crutchfield's method (Theoretical Computer Science
  237 (2000) 275-306). (3) We give that two MO-1QFAs with n_1 and n_2 states,
respectively, are equivalent iff they are (n_1+n_2)^2--equivalent, and further
obtain a polynomial-time algorithm for deciding the equivalence between two
MO-1QFAs. (4) We provide a counterexample showing that Koshiba's method to
solve the problem of deciding the equivalence between MM-1QFAs may be not
valid, and thus the problem is left open again.


title: Quantum computation with Kerr-nonlinear photonic crystals

abstract:
  In this paper, we consider a method for implementing a quantum logic gate
with photons whose wave function propagates in a one-dimensional Kerr-nonlinear
photonic crystal. The photonic crystal causes the incident photons to undergo
Bragg reflection by its periodic structure of dielectric materials and forms
the photonic band structure, namely, the light dispersion relation. This
dispersion relation reduces the group velocity of the wave function of the
photons, so that it enhances nonlinear interaction of the photons. (Because
variation of the group velocity against the wave vector is very steep, we have
to tune up the wavelength of injected photons precisely, however.) If the
photonic crystal includes layers of a Kerr medium, we can rotate the phase of
the wave function of the incident photons by a large angle efficiently. We show
that we can construct the nonlinear sign-shift (NS) gate proposed by Knill,
Laflamme, and Milburn (KLM) by this method. Thus, we can construct the
conditional sign-flip gate for two qubits, which is crucial for quantum
computation. Our NS gate works with probability unity in principle while KLM's
original one is a nondeterministic gate conditioned on the detection of an
auxiliary photon.


title:
On the Quantum-Classical Character of the Quantum Wavefunction of
  Material Particles

abstract:
  We show that the quantum wavefunction, interpreted as the probability density
of finding a single non-localized quantum particle, which evolves according to
classical laws of motion, is an intermediate description of a material quantum
particle between the quantum and classical realms. Accordingly, classical and
quantum mechanics should not be treated separately, a unified description in
terms of the Wigner distribution function being possible. Although defined on
classical phase space coordinates, the Wigner distribution function
accommodates the nonlocalization property of quantum systems, and leads to both
the Schrodinger equation for the quantum wavefunction and to the definition of
position and momentum operators.


title:
Relative information encoded in the degree of entanglement to
  discriminate bipartite states

abstract:
  It has been recently shown (Bartlett et al. 2003) that information encoded
into relative degrees of freedom enables communication without a common
reference frame using entangled bipartite states. In this case the relative
information stored in the two-qubit system is shared between the polarization
degrees of freedom and the degree of entanglement. In the present article a
specific state discrimination problem is envisioned where the degree of
entanglement carries the only relative parameter, so that certain maximally
entangled states are perfectly distinguishable, while discrimination of product
states is impossible.


title:
A Number Theoretic Interpolation Between Quantum and Classical
  Complexity Classes

abstract:
  We reveal a natural algebraic problem whose complexity appears to interpolate
between the well-known complexity classes BQP and NP:
  (*) Decide whether a univariate polynomial with exactly m monomial terms has
a p-adic rational root. In particular, we show that while (*) is doable in
quantum randomized polynomial time when m=2 (and no classical randomized
polynomial time algorithm is known), (*) is nearly NP-hard for general m: Under
a plausible hypothesis involving primes in arithmetic progression (implied by
the Generalized Riemann Hypothesis for certain cyclotomic fields), a randomized
polynomial time algorithm for (*) would imply the widely disbelieved inclusion
NP \subseteq BPP. This type of quantum/classical interpolation phenomenon
appears to new.


title:
Noise Threshold for a Fault-Tolerant Two-Dimensional Lattice
  Architecture

abstract:
  We consider a model of quantum computation in which the set of operations is
limited to nearest-neighbor interactions on a 2D lattice. We model movement of
qubits with noisy SWAP operations. For this architecture we design a
fault-tolerant coding scheme using the concatenated [[7,1,3]] Steane code. Our
scheme is potentially applicable to ion-trap and solid-state quantum
technologies. We calculate a lower bound on the noise threshold for our local
model using a detailed failure probability analysis. We obtain a threshold of
1.85 x 10^-5 for the local setting, where memory error rates are one-tenth of
the failure rates of gates, measurement, and preparation steps. For the
analogous nonlocal setting, we obtain a noise threshold of 3.61 x 10^-5. Our
results thus show that the additional SWAP operations required to move qubits
in the local model affect the noise threshold only moderately.


title: Sufficiency in quantum statistical inference. A survey with examples

abstract:
  This paper attempts to give an overview about sufficiency in the setting of
quantum statistics. The basic concepts are treated paralelly to the the measure
theoretic case. It turns out that several classical examples and results have a
non-commutative analogue. Some of the results are presented without proof (but
with exact references) and the presentation is intended to be self-contained.
The main examples discussed in the paper are related the Weyl algebra and to
the exponential family of states. The characterization of sufficiency in terms
of quantum Fisher information is a new result.


title:
Scattering and Bound State Green's Functions on a Plane via so(2,1) Lie
  Algebra

abstract:
  We calculate the Green's functions for the particle-vortex system, for two
anyons on a plane with and without a harmonic regulator and in a uniform
magnetic field. These Green's functions which describe scattering or bound
states (depending on the specific potential in each case) are obtained exactly
using an algebraic method related to the SO(2,1) Lie group. From these Green's
functions we obtain the corresponding wave functions and for the bound states
we also find the energy spectra.


title: Induced photon statistics in three-level lasers

abstract:
  The statistical properties of three-level lasing are investigated
theoretically. It is assumed that the three-level medium is coherently excited
by another laser with an arbitrary photon statistics. It is proved that, under
the specific conditions, the photon statistics of the three-level laser
duplicate the photon statistics of the exciting laser. We call this phenomenon
an induced photon statistics. We suggest to use this to analyze the statistical
properties of a laser involved into a feedback process. Applying this laser for
the coherent pump of a three-level laser, we can follow its photon statistics
by means of direct following the three-level generation. In accordance with [H.
M. Wiseman and G. J. Milburn, Phys. Rev. A, 49, 1350-1366 (1994)], we conclude
that the feedback in itself is unable to generate the non-classical
manifestation in the laser field.


title:
Decoy state quantum key distribution with two-way classical
  post-processing

abstract:
  Decoy states have recently been proposed as a useful method for substantially
improving the performance of quantum key distribution protocols when a coherent
state source is used. Previously, data post-processing schemes based on one-way
classical communications were considered for use with decoy states. In this
paper, we develop two data post-processing schemes for the decoy-state method
using two-way classical communications. Our numerical simulation (using
parameters from a specific QKD experiment as an example) results show that our
scheme is able to extend the maximal secure distance from 142km (using only
one-way classical communications with decoy states) to 181km. The second scheme
is able to achieve a 10% greater key generation rate in the whole regime of
distances.


title:
Series Solutions of the N-Dimensional Position-Dependent Mass
  Schrodinger Equation with a General Class of Potentials

abstract:
  The analytical solutions of the N-dimensional Schrodinger equation with
position-dependent mass for a general class of central potentials is obtained
via the series expansion method. The position-dependent mass is expanded in
series about origin. As a special case, the analytical bound-state series
solutions and the recursion relation of the linear-plus-Coulomb (Cornell)
potential with the decaying position-dependent mass m=m_{0}e^{-\lambda r} are
also found.


title:
Solution to the Mean King's problem with mutually unbiased bases for
  arbitrary levels

abstract:
  The Mean King's problem with mutually unbiased bases is reconsidered for
arbitrary d-level systems. Hayashi, Horibe and Hashimoto [Phys. Rev. A 71,
052331 (2005)] related the problem to the existence of a maximal set of d-1
mutually orthogonal Latin squares, in their restricted setting that allows only
measurements of projection-valued measures. However, we then cannot find a
solution to the problem when e.g., d=6 or d=10. In contrast to their result, we
show that the King's problem always has a solution for arbitrary levels if we
also allow positive operator-valued measures. In constructing the solution, we
use orthogonal arrays in combinatorial design theory.


title: Quantum criticality in a generalized Dicke model

abstract:
  We employ a generalized Dicke model to study theoretically the quantum
criticality of an extended two-level atomic ensemble interacting with a
single-mode quantized light field. Effective Hamiltonians are derived and
diagonalized to investigate numerically their eigenfrequencies for different
quantum phases in the system. Based on the analysis of the eigenfrequencies, an
intriguing quantum-phase transition from a normal phase to a superradiant phase
is revealed clearly, which is quite different from that observed with a
standard Dicke model.


title: Dynamics of entanglement of bosonic modes on symmetric graphs

abstract:
  We investigate the dynamics of an initially disentangled Gaussian state on a
general finite symmetric graph. As concrete examples we obtain properties of
this dynamics on mean field graphs of arbitrary sizes. In the same way that
chains can be used for transmitting entanglement by their natural dynamics,
these graphs can be used to store entanglement. We also consider two kinds of
regular polyhedron which show interesting features of entanglement sharing.


title: Thermal Entanglement in Ferrimagnetic Chains

abstract:
  A formula to evaluate the entanglement in an one-dimensional ferrimagnetic
system is derived. Based on the formula, we find that the thermal entanglement
in a small size spin-1/2 and spin-s ferrimagnetic chain is rather robust
against temperature, and the threshold temperature may be arbitrarily high when
s is sufficiently large. This intriguing result answers unambiguously a
fundamental question: ``can entanglement and quantum behavior in physical
systems survive at arbitrary high temperatures?"


title: Founding quantum theory on the basis of consciousness

abstract:
  In the present work, quantum theory is founded on the framework of
consciousness, in contrast to earlier suggestions that consciousness might be
understood starting from quantum theory. The notion of streams of
consciousness, usually restricted to conscious beings, is extended to the
notion of a Universal/Global stream of conscious flow of ordered events. The
streams of conscious events which we experience constitute sub-streams of the
Universal stream. Our postulated ontological character of consciousness also
consists of an operator which acts on a state of potential consciousness to
create or modify the likelihoods for later events to occur and become part of
the Universal conscious flow. A generalized process of measurement-perception
is introduced, where the operation of consciousness brings into existence, from
a state of potentiality, the event in consciousness. This is mathematically
represented by (a) an operator acting on the state of potential-consciousness
before an actual event arises in consciousness and (b) the reflecting of the
result of this operation back onto the state of potential-consciousness for
comparison in order for the event to arise in consciousness. Beginning from our
postulated ontology that consciousness is primary and from the most elementary
conscious contents, such as perception of periodic change and motion, quantum
theory follows naturally as the description of the conscious experience.


title: Voronoi Diagrams for Pure 1-qubit Quantum States

abstract:
  1-qubit quantum states form a space called the three-dimensional Bloch ball.
To compute Holevo capacity, Voronoi diagrams in the Bloch ball with respect to
the quantum divergence have been used as a powerful tool. These diagrams
basically treat mixed quantum states corresponding to points in the interior of
the Bloch ball. Due to the existence of logarithm in the quantum divergence,
the diagrams are not defined on pure quantum states corresponding to points on
the two-dimensional sphere. This paper first defines the Voronoi diagrams for
pure quantum states on the Bloch sphere by the Fubini-Study distance and the
Bures distance. We also introduce other Voronoi diagrams on the sphere obtained
by taking a limit of Voronoi diagrams for mixed quantum states by the quantum
divergences in the Bloch ball. These diagrams are shown to be equivalent to the
ordinary Voronoi diagram on the sphere.


title: On Quantum and Classical BCH Codes

abstract:
  Classical BCH codes that contain their (Euclidean or Hermitian) dual codes
can be used to construct quantum stabilizer codes; this correspondence studies
the properties of such codes. It is shown that a BCH code of length n can
contain its dual code only if its designed distance d=O(sqrt(n)), and the
converse is proved in the case of narrow-sense codes. Furthermore, the
dimension of narrow-sense BCH codes with small design distance is completely
determined, and - consequently - the bounds on their minimum distance are
improved. These results make it possible to determine the parameters of quantum
BCH codes in terms of their design parameters.


title:
A novel interferometer to beat the standard quantum limit using optical
  transverse modes in multimode waveguide

abstract:
  We propose a novel interferometer by using optical transverse modes in
multimode waveguide that can beat the standard quantum limit. In the scheme,
the classical simulation of $N$-partical quantum entangled states is generated
by using $N$ independent classical fields and linear optical elements. Similar
to the quantum-enhanced measurements, the classical simulation can also achieve
$\sqrt{N}$ enhancement over the precision of the measurement $N$ times for
independent fields. Due to only using classical fields and linear optical
elements, the scheme can be realized much more easily.


title: Projective invariant measures and approximation of quantum circuits

abstract:
  In this paper we introduce a projective invarinat measure on the special
unitary group. It is directly related to transition probabilities. It has some
interesting connection with convex geometry. Applications to approximation of
quantum circuits and entanglement are given.


title:
Estimating Information Gain in Measurements in Suboptimal Bases for
  Quantum State Tomography

abstract:
  It is known that mutually unbiased bases, whenever they exist, are optimal in
an information theoretic sense for the determination of unknown state of a
quantum ensemble. These bases may not exist in most dimensions and some
suboptimal choices have to be made. The present paper deals with estimates of
the information loss in suboptimal choice of bases. The information is
calculated directly in terms of transition probabilities. I give estimates for
the information content of measurement in some approximate MUBs proposed
recently.


title: $\eta$-weak-Pseudo-Hermiticity generators and exact solvability

abstract:
  Exact solvability of some non-Hermitian $\eta$-weak-pseudo-Hermitian
Hamiltonians is explored as a byproduct of $\eta$-weak-pseudo-Hermiticity
generators. A class of V_{eff}(x)=V(x)+iW(x) potentials is considered, where
the imaginary part W(x) is used as an $\eta$-weak-pseudo-Hermiticity generator
to obtain exactly solvable $\eta$-weak-pseudo-Hermitian Hamiltonian models.


title: Degenerate Quantum Codes for Pauli Channels

abstract:
  A striking feature of quantum error correcting codes is that they can
sometimes be used to correct more errors than they can uniquely identify. Such
degenerate codes have long been known, but have remained poorly understood. We
provide a heuristic for designing degenerate quantum codes for high noise
rates, which is applied to generate codes that can be used to communicate over
almost any Pauli channel at rates that are impossible for a nondegenerate code.
The gap between nondegenerate and degenerate code performance is quite large,
in contrast to the tiny magnitude of the only previous demonstration of this
effect. We also identify a channel for which none of our codes outperform the
best nondegenerate code and show that it is nevertheless quite unlike any
channel for which nondegenerate codes are known to be optimal.


title:
Relation between two measures of entanglement in spin-1/2 and spinless
  fermion quantum chain systems

abstract:
  The concepts of concurrence and mode concurrence are the measures of
entanglement for spin-1/2 and spinless fermion systems respectively. Based on
the Jordan-Wigner transformation, any spin-1/2 system is always associated with
a fermion system (called counterpart system). The comparison of concurrence and
mode concurrence can be made with the aid of the Marshall's sign rule for the
ground states of spin-1/2 $XXZ$ and spinless fermion chain systems. We observe
that there exists an inequality between concurrence and mode concurrence for
the ground states of the two corresponding systems. The spin-1/2 XY chain
system and its spinless fermion counterpart as a realistic example is discussed
to demonstrate the analytical results.


title:
Universal observable detecting all two-qubit entanglement and
  determinant based separability tests

abstract:
  We construct a single observable measurement of which mean value on four
copies of an {\it unknown} two-qubit state is sufficient for unambiguous
decision whether the state is separable or entangled. In other words, there
exists a universal collective entanglement witness detecting all two-qubit
entanglement. The test is directly linked to a function which characterizes to
some extent the entanglement quantitatively. This function is an entanglement
monotone under so--called local pure operations and classical communication
(pLOCC) which preserve local dimensions. Moreover it provides tight upper and
lower bounds for negativity and concurrence. Elementary quantum computing
device estimating unknown two-qubit entanglement is designed.


title: Observation of high-order quantum resonances in the kicked rotor

abstract:
  Quantum resonances in the kicked rotor are characterized by a dramatically
increased energy absorption rate, in stark contrast to the momentum
localization generally observed. These resonances occur when the scaled
Planck's constant hbar=(r/s)*4pi, for any integers r and s. However only the
hbar=r*2pi resonances are easily observable. We have observed high-order
quantum resonances (s>2) utilizing a sample of low temperature, non-condensed
atoms and a pulsed optical standing wave. Resonances are observed for
hbar=(r/16)*4pi r=2-6. Quantum numerical simulations suggest that our
observation of high-order resonances indicates a larger coherence length than
expected from an initially thermal atomic sample.


title: Quantum chaos in the spectrum of operators used in Shor's algorithm

abstract:
  We provide compelling evidence for the presence of quantum chaos in the
unitary part of Shor's factoring algorithm. In particular we analyze the
spectrum of this part after proper desymmetrization and show that the
fluctuations of the eigenangles as well as the distribution of the eigenvector
components follow the CUE ensemble of random matrices, of relevance to
quantized chaotic systems that violate time-reversal symmetry. However, as the
algorithm tracks the evolution of a single state, it is possible to employ
other operators, in particular it is possible that the generic quantum chaos
found above becomes of a nongeneric kind such as is found in the quantum cat
maps, and in toy models of the quantum bakers map.


title: Artificial Decoherence and its Suppression in NMR Quantum Computer

abstract:
  Liquid-state NMR quantum computer has demonstrated the possibility of quantum
computation and supported its development. Using NMR quantum computer
techniques, we observed phase decoherence under two kinds of artificial noise
fields; one a noise with a long period, and the other with shorter random
period. The first one models decoherence in a quantum channel while the second
one models transverse relaxation. We demonstrated that the bang-bang control
suppresses decoherence in both cases.