title: Transition from antibunching to bunching in cavity QED

abstract:
  The photon statistics of the light emitted from an atomic ensemble into a
single field mode of an optical cavity is investigated as a function of the
number of atoms. The light is produced in a Raman transition driven by a pump
laser and the cavity vacuum [M.Hennrich et al., Phys. Rev. Lett. 85, 4672
(2000)], and a recycling laser is employed to repeat this process continuously.
  For weak driving, a smooth transition from antibunching to bunching is found
for about one intra-cavity atom. Remarkably, the bunching peak develops within
the antibunching dip. For saturated driving and a growing number of atoms, the
bunching amplitude decreases and the bunching duration increases, indicating
the onset of Raman lasing.


title:
A new general approximation scheme(NGAS) in quantum theory:application
  to the anharmonic- and double well oscillators

abstract:
  A new scheme of approximation in quantum theory is proposed which is
potentially applicable to arbtrary interacting systems. The method consists in
in approximating the original Hamiltonian by one corresponding to a suitable
exactly solvable system (with interaction) such that the "quantum average" of
both are equal, thus forcing self-consistency.The method transcends the
limitations of the variational method and the perturbation theory.The results
are systematically improvable by the construction of a improved perturbation
theory (IPT) which automatically satisfies the condition of convergence.
Uniformly accurate results are obtained for the case of the quartic-,sextic-
and octic anharmonic oscillators as well as the quartic and sextic double well
oscillators. The leading order results reproduce earlier results by different
methods using different input assumptions.The results for the sextic
oscillators agree well with exact prediction of supersymmetry. We also discuss
the stability and structure of the effective vacuum state of the approximation.

categories: quant-ph cond-mat.other hep-th math-ph math.MP


title:
Interference Phenomena in Spontaneous Emission from Driven Multilevel
  Atoms

abstract:
  We study spontaneous emission from an atom under the action of laser fields.
We consider two different energy level diagrams. The first one consists of two
levels resonantly driven by laser radiation where either of levels may decay to
a separate level. For such a system we show that the presence of the second
decay channel may deteriorate the destructive interference occurring in case of
one decay channel because of Autler-Townes effect. The second diagram
represents two two-level resonantly driven systems with the upper levels
decaying to a common level. For this diagram we show that there is no
interference between the two decay channels when the laser fields are described
in the Fock representation, while in case of definite-phase classical fields
such interference takes place and is partially or completely destructive or
constructive depending on the initial conditions and on the mutual orientation
of the spontaneous emission dipole moments.


title:
Exact solutions of the isoholonomic problem and the optimal control
  problem in holonomic quantum computation

abstract:
  The isoholonomic problem in a homogeneous bundle is formulated and solved
exactly. The problem takes a form of a boundary value problem of a variational
equation. The solution is applied to the optimal control problem in holonomic
quantum computer. We provide a prescription to construct an optimal controller
for an arbitrary unitary gate and apply it to a $ k $-dimensional unitary gate
which operates on an $ N $-dimensional Hilbert space with $ N \geq 2k $. Our
construction is applied to several important unitary gates such as the Hadamard
gate, the CNOT gate, and the two-qubit discrete Fourier transformation gate.
Controllers for these gates are explicitly constructed.


title: Stroboscopic quantum walks

abstract:
  Discrete time (coined) quantum walks are produced by the repeated application
of a constant unitary transformation to a quantum system. By recasting these
walks into the setting of periodic perturbations to an otherwise freely
evolving system we introduce the concept of a stroboscopic quantum walk.
Through numerical simulation, we establish the link between families of
stroboscopic walks and quantum resonances. These are observed in the nonlinear
systems of quantum chaos theory such as the delta-kicked rotator or the
delta-kicked accelerator.


title: Macroscopic Thermodynamical Witnesses of Quantum Entanglement

abstract:
  We show that macroscopic thermodynamical properties - such as functions of
internal energy and magnetization - can detect quantum entanglement in solids
at nonzero temperatures in the thermodynamical limit. We identify the parameter
regions (critical values of magnetic field and temperature) within which
entanglement is witnessed by these thermodynamical quantities.


title: Casimir Effects: An Optical Approach I. Foundations and Examples

abstract:
  We present the foundations of a new approach to the Casimir effect based on
classical ray optics. We show that a very useful approximation to the Casimir
force between arbitrarily shaped smooth conductors can be obtained from
knowledge of the paths of light rays that originate at points between these
bodies and close on themselves. Although an approximation, the optical method
is exact for flat bodies, and is surprisingly accurate and versatile. In this
paper we present a self-contained derivation of our approximation, discuss its
range of validity and possible improvements, and work out three examples in
detail. The results are in excellent agreement with recent precise numerical
analysis for the experimentally interesting configuration of a sphere opposite
an infinite plane.


title: Complete Correlation, Detection loophole and Bell's Theorem

abstract:
  Two new formulations of Bell's theorem are given here. First, we consider a
definite set of two entangled photons with only two polarization directions,
for which Bell's locality assumption is violated for the case of perfect
correlation. Then, using a different approach, we prove an efficient Bell-type
inequality which is violated by some quantum mechanical predictions,
independent of the efficiency factors.


title: Soliton Atom Laser with Quantum State Transfer Property

abstract:
  We propose a scheme to obtain soliton atom laser with nonclassical atoms
based on quantum state transfer process from light to matter waves in nonlinear
case, which may find novel applications in, e.g., an atom interferometer. The
dynamics of the atomic gray solitons and the accompanied frequency chirp effect
are discussed.


title:
Implementation of NMR quantum computation with para-hydrogen derived
  high purity quantum states

abstract:
  We demonstrate the first implementation of a quantum algorithm on a liquid
state nuclear magnetic resonance (NMR) quantum computer using almost pure
states. This was achieved using a two qubit device where the initial state is
an almost pure singlet nuclear spin state of a pair of 1H nuclei arising from a
chemical reaction involving para-hydrogen. We have implemented Deutsch's
algorithm for distinguishing between constant and balanced functions with a
single query.


title:
Bohmian transmission and reflection dwell times without trajectory
  sampling

abstract:
  Within the framework of Bohmian mechanics dwell times find a straightforward
formulation. The computation of associated probabilities and distributions
however needs the explicit knowledge of a relevant sample of trajectories and
therefore implies formidable numerical effort. Here a trajectory free
formulation for the average transmission and reflection dwell times within
static spatial intervals [a,b] is given for one-dimensional scattering
problems. This formulation reduces the computation time to less than 5% of the
computation time by means of trajectory sampling.


title: The hidden subgroup problem and permutation group theory

abstract:
  We employ concepts and tools from the theory of finite permutation groups in
order to analyse the Hidden Subgroup Problem via Quantum Fourier Sampling (QFS)
for the symmetric group. We show that under very general conditions both the
weak and the random-strong form (strong form with random choices of basis) of
QFS fail to provide any advantage over classical exhaustive search. In
particular we give a complete characterisation of polynomial size subgroups,
and of primitive subgroups, that can be distinguished from the identity
subgroup with the above methods. Furthermore, assuming a plausible group
theoretic conjecture for which we give supporting evidence, we show that weak
and random-strong QFS for the symmetric group have no advantage whatsoever over
classical search.


title: Multiphoton entanglement through a Bell multiport beam splitter

abstract:
  Multiphoton entanglement is an important resource for linear optics quantum
computing. Here we show that a wide range of highly entangled multiphoton
states, including W-states, can be prepared by interfering single photons
inside a Bell multiport beam splitter and using postselection. A successful
state preparation is indicated by the collection of one photon per output port.
An advantage of the Bell multiport beam splitter is that it redirects the
photons without changing their inner degrees of freedom. The described setup
can therefore be used to generate polarisation, time-bin and frequency
multiphoton entanglement, even when using only a single photon source.


title: Experimental Bell Inequality Violation with an Atom and a Photon

abstract:
  We report the measurement of a Bell inequality violation with a single atom
and a single photon prepared in a probabilistic entangled state. This is the
first demonstration of such a violation with particles of different species.
The entanglement characterization of this hybrid system may also be useful in
quantum information applications.


title: Entangling flux qubits with a bipolar dynamic inductance

abstract:
  We propose a scheme to implement variable coupling between two flux qubits
using the screening current response of a dc Superconducting QUantum
Interference Device (SQUID). The coupling strength is adjusted by the current
bias applied to the SQUID and can be varied continuously from positive to
negative values, allowing cancellation of the direct mutual inductance between
the qubits. We show that this variable coupling scheme permits efficient
realization of universal quantum logic. The same SQUID can be used to determine
the flux states of the qubits.


title: Temporal Dynamics of Photon Pairs Generated by an Atomic Ensemble

abstract:
  The time dependence of nonclassical correlations is investigated for two
fields (1,2) generated by an ensemble of cold Cesium atoms via the protocol of
Duan et al. [Nature Vol. 414, p. 413 (2001)]. The correlation function R(t1,t2)
for the ratio of cross to auto-correlations for the (1,2) fields at times
(t1,t2) is found to have a maximum value Rmax=292(+-)57, which significantly
violates the Cauchy-Schwarz inequality R<=1 for classical fields. Decoherence
of quantum correlations is observed over 175 ns, and is described by our model,
as is a new scheme to mitigate this effect.


title: Violation of time symmetry

abstract:
  Recently, [{arXiv:0810.3134}] is accepted and published. We would like to
study the relation between a local realistic theory and commutativity of
observables in a finite-dimensional space. We would like to conjecture that a
realistic theory of the Bell type (a local realistic theory) for events would
not imply the commutative algebraic structure into the set of all observables
if all experimental events would be reproduced by a local realistic theory. We
would like to suggest that a violation of Bell locality would be derived within
a realistic theory of the Kochen-Specker type within quantum events.


title: Generalized master equation with nonhermitian operators

abstract:
  By extending the mean-field Hamiltonian to include nonhermitian operators,
the master equations for fermions and bosons can be derived. The derived
equations reduce to the Markoff master equation in the low-density limit and to
the quasiclassical master equation for homogeneuos systems.


title:
Quantification and scaling of multipartite entanglement in continuous
  variable systems

abstract:
  We present a theoretical method to determine the multipartite entanglement
between different partitions of multimode, fully or partially symmetric
Gaussian states of continuous variable systems. For such states, we determine
the exact expression of the logarithmic negativity and show that it coincides
with that of equivalent two--mode Gaussian states. Exploiting this reduction,
we demonstrate the scaling of the multipartite entanglement with the number of
modes and its reliable experimental estimate by direct measurements of the
global and local purities.

categories: quant-ph cond-mat.other hep-th math-ph math.MP physics.optics


title: Incompleteness of trajectory-based interpretations of quantum mechanics

abstract:
  Trajectory-based approaches to quantum mechanics include the de Broglie-Bohm
interpretation and Nelson's stochastic interpretation. It is shown that the
usual route to establishing the validity of such interpretations, via a
decomposition of the Schroedinger equation into a continuity equation and a
modified Hamilton-Jacobi equation, fails for some quantum states. A very simple
example is provided by a quantum particle in a box, described by a wavefunction
initially uniform over the interior of the box. For this example there is no
corresponding continuity or modified Hamilton-Jacobi equation, and the
spacetime dependence of the wavefunction has a known fractal structure.
Examples with finite average energies are also constructed.


title: Stochastic Schrodinger equations as limit of discrete filtering

abstract:
  We consider an open model possessing a Markovian quantum stochastic limit and
derive the limit stochastic Schrodinger equations for the wave function
conditioned on indirect observations using only the von Neumann projection
postulate. We show that the diffusion (Gaussian) situation is universal as a
result of the central limit theorem with the quantum jump (Poissonian)
situation being an exceptional case. It is shown that, starting from the
correponding limiting open systems dynamics, the theory of quantum filtering
leads to the same equations, therefore establishing consistency of the quantum
stochastic approach for limiting Markovian models.


title: On the classical-quantum correspondence for the scattering dwell time

abstract:
  Using results from the theory of dynamical systems, we derive a general
expression for the classical average scattering dwell time, tau_av. Remarkably,
tau_av depends only on a ratio of phase space volumes. We further show that,
for a wide class of systems, the average classical dwell time is not in
correspondence with the energy average of the quantum Wigner time delay.


title: Quantum Dynamics of a Particle with a Spin-dependent Velocity

abstract:
  We study the dynamics of a particle in continuous time and space, the
displacement of which is governed by an internal degree of freedom (spin). In
one definite limit, the so-called quantum random walk is recovered but,
although quite simple, the model possesses a rich variety of dynamics and goes
far beyond this problem. Generally speaking, our framework can describe the
motion of an electron in a magnetic sea near the Fermi level when linearisation
of the dispersion law is possible, coupled to a transverse magnetic field.
Quite unexpected behaviours are obtained. In particular, we find that when the
initial wave packet is fully localized in space, the $J_{z}$ angular momentum
component is frozen; this is an interesting example of an observable which,
although it is not a constant of motion, has a constant expectation value. For
a non-completely localized wave packet, the effect still occurs although less
pronounced, and the spin keeps for ever memory of its initial state. Generally
speaking, as time goes on, the spatial density profile looks rather complex, as
a consequence of the competition between drift and precession, and displays
various shapes according to the ratio between the Larmor period and the
characteristic time of flight. The density profile gradually changes from a
multimodal quickly moving distribution when the scatttering rate is small, to a
unimodal standing but flattening distribution in the opposite cas case.


title:
Preparation of macroscopic quantum superposition states of a cavity
  field via coupling to a superconducting charge qubit

abstract:
  We propose how to generate macroscopic quantum superposition states using a
microwave cavity containing a superconducting charge qubit. Based on the
measurement of charge states, we show that the superpositions of two
macroscopically distinguishable coherent states of a single-mode cavity field
can be generated by a controllable interaction between a cavity field and a
charge qubit. After such superpositions of the cavity field are created, the
interaction can be switched off by the classical magnetic field, and there is
no information transfer between the cavity field and the charge qubit. We also
discuss the generation of the superpositions of two squeezed coherent states.


title: Photon-induced entanglement of distant mesoscopic SQUID rings

abstract:
  An experiment that involves two distant mesoscopic SQUID rings is studied.
The superconducting rings are irradiated with correlated photons, which are
produced by a single microwave source. Classically correlated (separable) and
quantum mechanically correlated (entangled) microwaves are considered, and
their effect on the Josephson currents is quantified. It is shown that the
currents tunnelling through the Josephson junctions in the distant rings, are
correlated.


title: Casimir force between eccentric cylinders

abstract:
  We consider the Casimir interaction between a cylinder and a hollow cylinder,
both conducting, with parallel axis and slightly different radii. The Casimir
force, which vanishes in the coaxial situation, is evaluated for both small and
large eccentricities using the proximity approximation. The cylindrical
configuration offers various experimental advantages with respect to the
parallel planes or the plane-sphere geometries, leading to favourable
conditions for the search of extra-gravitational forces in the micrometer range
and for the observation of finite temperature corrections.


title: Entanglement Witnesses in Spin Models

abstract:
  We construct entanglement witnesses using fundamental quantum operators of
spin models which contain two-particle interactions and posses a certain
symmetry. By choosing the Hamiltonian as such an operator, our method can be
used for detecting entanglement by energy measurement. We apply this method to
the cubic Heisenberg lattice model in a magnetic field, the XY model and other
familiar spin systems. Our method is used to obtain a temperature bound for
separable states for systems in thermal equilibrium. We also study the
Bose-Hubbard model and relate its energy minimum for separable states to the
minimum obtained from the Gutzwiller ansatz.


title: Stochastic Quantization of the Time-Dependent Harmonic Oscillator

abstract:
  We extend the stochastic quantization method recently developed by Haba and
Kleinert to non-autonomous mechanical systems, in the case of the
time-dependent harmonic oscillator. In comparison with the autonomous case, the
quantization procedure involves the solution of a nonlinear, auxiliary
equation.


title: Probabilities of failure for quantum error correction

abstract:
  We investigate the performance of a quantum error-correcting code when pushed
beyond its intended capacity to protect information against errors, presenting
formulae for the probability of failure when the errors affect more qudits than
that specified by the code's minimum distance. Such formulae provide a means to
rank different codes of the same minimum distance. We consider both error
detection and error correction, treating explicit examples in the case of
stabilizer codes constructed from qubits and encoding a single qubit.


title: Stability of global entanglement in thermal states of spin chains

abstract:
  We investigate the entanglement properties of a one dimensional chain of spin
qubits coupled via nearest neighbor interactions. The entanglement measure used
is the n-concurrence, which is distinct from other measures on spin chains such
as bipartite entanglement in that it can quantify "global" entanglement across
the spin chain. Specifically, it computes the overlap of a quantum state with
its time-reversed state. As such this measure is well suited to study ground
states of spin chain Hamiltonians that are intrinsically time reversal
symmetric. We study the robustness of n-concurrence of ground states when the
interaction is subject to a time reversal antisymmetric magnetic field
perturbation. The n-concurrence in the ground state of the isotropic XX model
is computed and it is shown that there is a critical magnetic field strength at
which the entanglement experiences a jump discontinuity from the maximum value
to zero. The n-concurrence for thermal mixed states is derived and a threshold
temperature is computed below which the system has non zero entanglement.


title:
Multiplicativity of maximal output purities of Gaussian channels under
  Gaussian inputs

abstract:
  We address the question of the multiplicativity of the maximal p-norm output
purities of bosonic Gaussian channels under Gaussian inputs. We focus on
general Gaussian channels resulting from the reduction of unitary dynamics in
larger Hilbert spaces. It is shown that the maximal output purity of tensor
products of single-mode channels under Gaussian inputs is multiplicative for
any p>1 for products of arbitrary identical channels as well as for a large
class of products of different channels. In the case of p=2 multiplicativity is
shown to be true for arbitrary products of generic channels acting on any
number of modes.


title: Simple quantum model for light depolarization

abstract:
  Depolarization of quantum fields is handled through a master equation of the
Lindblad type. The specific feature of the proposed model is that it couples
dispersively the field modes to a randomly distributed atomic reservoir, much
in the classical spirit of dealing with this problem. The depolarizing dynamics
resulting from this model is analyzed for relevant states.


title: Entanglement in a Valence-Bond-Solid State

abstract:
  We study entanglement in Valence-Bond-Solid state. It describes the ground
state of Affleck, Kennedy, Lieb and Tasaki quantum spin chain. The AKLT model
has a gap and open boundary conditions. We calculate an entropy of a subsystem
(continuous block of spins). It quantifies the entanglement of this block with
the rest of the ground state. We prove that the entanglement approaches a
constant value exponentially fast as the size of the subsystem increases.
Actually we proved that the density matrix of the continuous block of spins
depends only on the length of the block, but not on the total size of the chain
[distance to the ends also not essential]. We also study reduced density
matrices of two spins both in the bulk and on the boundary. We evaluated
concurrencies.


title: Lower entropy bounds and particle number fluctuations in a Fermi sea

abstract:
  We demonstrate, in an elementary manner, that given a partition of the single
particle Hilbert space into orthogonal subspaces, a Fermi sea may be factored
into pairs of entangled modes, similar to a BCS state. We derive expressions
for the entropy and for the particle number fluctuations of a subspace of a
fermi sea, at zero and finite temperatures, and relate these by a lower bound
on the entropy. As an application we investigate analytically and numerically
these quantities for electrons in the lowest Landau level of a quantum Hall
sample.


title: Exact norm-conserving stochastic time-dependent Hartree-Fock

abstract:
  We derive an exact single-body decomposition of the time-dependent
Schroedinger equation for N pairwise-interacting fermions. Each fermion obeys a
stochastic time-dependent norm-preserving wave equation. As a first test of the
method we calculate the low energy spectrum of Helium. An extension of the
method to bosons is outlined.


title: Optimal estimation of one parameter quantum channels

abstract:
  We explore the task of optimal quantum channel identification, and in
particular the estimation of a general one parameter quantum process. We derive
new characterizations of optimality and apply the results to several examples
including the qubit depolarizing channel and the harmonic oscillator damping
channel. We also discuss the geometry of the problem and illustrate the
usefulness of using entanglement in process estimation.


title: Aperiodic Quantum Random Walks

abstract:
  We generalize the quantum random walk protocol for a particle in a
one-dimensional chain, by using several types of biased quantum coins, arranged
in aperiodic sequences, in a manner that leads to a rich variety of possible
wave function evolutions. Quasiperiodic sequences, following the Fibonacci
prescription, are of particular interest, leading to a sub-ballistic
wavefunction spreading. In contrast, random sequences leads to diffusive
spreading, similar to the classical random walk behaviour. We also describe how
to experimentally implement these aperiodic sequences.


title: Convergence of coined quantum walks on d-dimensional Euclidean space

abstract:
  Coined quantum walks may be interpreted as the motion in position space of a
quantum particle with a spin degree of freedom; the dynamics are determined by
iterating a unitary transformation which is the product of a spin
transformation and a translation conditional on the spin state. Coined quantum
walks on the d-dimensional lattice can be treated as special cases of coined
quantum walks on d-dimensional Euclidean space. We study quantum walks on
d-dimensional Euclidean space and prove that the sequence of rescaled
probability distributions in position space associated to the unitary evolution
of the particle converges to a limit distribution.


title: Quantum computation in optical lattices via global laser addressing

abstract:
  A scheme for globally addressing a quantum computer is presented along with
its realisation in an optical lattice setup of one, two or three dimensions.
The required resources are mainly those necessary for performing quantum
simulations of spin systems with optical lattices, circumventing the necessity
for single qubit addressing. We present the control procedures, in terms of
laser manipulations, required to realise universal quantum computation. Error
avoidance with the help of the quantum Zeno effect is presented and a scheme
for globally addressed error correction is given. The latter does not require
measurements during the computation, facilitating its experimental
implementation. As an illustrative example, the pulse sequence for the
factorisation of the number fifteen is given.


title: A numerical study of the spectrum and eigenfunctions on a tubular arc

abstract:
  The Hamiltonian for a particle constrained to move on the surface of a curved
nanotube is derived using the methods of differential forms. A two-dimensional
Gram-Schmidt orthonormalization procedure is employed to calculate basis
functions for determining the eigenvalues and eigenstates of a tubular arc (a
nanotube in the shape of a hyperbolic cosine) with several hundred scattering
centers. The curvature of the tube is shown to induce bound states that are
dependent on the curvature parameters and bend location of the tube.


title:
Symmetric Triple Well with Non-Equivalent Vacua: Simple Quantum
  Mechanical Approach

abstract:
  The structure of the energy levels in a deep triple well is analyzed using
simple quantum mechanical considerations. The resultant spectra of the first
three energy levels are found to be composed of a ground state localized at the
central well and the two other states are distributed only among the left and
right well in anti-symmetric and symmetric way respectively. Due to the
tunneling effects the energy eigenvalue of the ground state is approximately
equal to the ground state energy for a harmonic oscillator localized at the
central well, while the two others are nearly degenerate with approximate
values equal to the ground state energy of a harmonic oscillator localized at
the left or right well. The resulting pattern of the spectra are confirmed
numerically. The failure of the instantonic approach recently applied for
predicting the correct spectra is commented


title: Resonances for coupled Bose-Einstein Condensates

abstract:
  We study some effects arising from periodic modulation of the asymmetry and
the barrier height of a two-well potential containing a Bose-Einstein
condensate. At certain modulation frequencies the system exhibits resonances,
which may lead to enhancement of the tunneling rate between the wells and which
can be used to control the particle distribution among the wells. Some of the
effects predicted for a two-well system can be carried over to the case of a
Bose-Einstein condensate in an optical lattice.


title: On the realization of Bell observables

abstract:
  We show how Bell observables on a bipartite quantum system can be obtained by
local observables via a controlled-unitary transformation. For continuous
variables this result holds for the Bell observable corresponding to the
non-conventional heterodyne measurement on two radiation modes, which is
connected through a 50-50 beam-splitter to two local observables given by
single-mode homodyne measurements. A simple scheme for a controlled-unitary
transformation of continuous variables is also presented, which needs only two
squeezers, a parametric downconverter and two beam splitters.


title: Fermionic concurrence in the extended Hubbard dimer

abstract:
  In this paper, we introduce and study the fermionic concurrence in a two-site
extended Hubbard model. Its behaviors both at the ground state and finite
temperatures as function of Coulomb interaction $U$ (on-site) and $V$
(nearest-neighbor) are obtained analytically and numerically. We also
investigate the change of the concurrence under a nonuniform field, including
local potential and magnetic field, and find that the concurrence can be
modulated by these fields.


title: Quantum mechanics as kinetics

abstract:
  Relations between Hamiltonian mechanics and quantum mechanics are studied. It
is stressed that classical mechanics possesses all the specific features of
quantum theory: operators, complex variables, probabilities (in case of ergodic
systems). The Planck constant and the Fock space appear after putting a
dynamical system in a thermal bath. For harmonic oscillator in a thermal bath,
the probability amplitudes can be identified with the complex valued phase
functions $f(q+ip)$ describing small deviations from the equilibrium state,
when the time of relaxation is large. A chain of such oscillators models both
the one-dimensional space (or string), and one-dimensional quantum field
theory.


title:
Method of convex rigid frames and applications in studies of
  multipartite quNit pure-states

abstract:
  In this Letter we suggest a method of convex rigid frames in the studies of
the multipartite quNit pure-states. We illustrate what are the convex rigid
frames and what is the method of convex rigid frames. As the applications we
use this method to solve some basic problems and give some new results (three
theorems): The problem of the partial separability of the multipartite quNit
pure-states and its geometric explanation; The problem of the classification of
the multipartite quNit pure-states, and give a perfect explanation of the local
unitary transformations; Thirdly, we discuss the invariants of classes and give
a possible physical explanation.


title:
Steady state entanglement of two atoms created by classical driving
  field

abstract:
  The stabilization of steady state entanglement caused by action of a
classical driving field in the system of two-level atoms with the dipole
interaction accompanied by spontaneous emission is discussed. An exact solution
shows that the maximum amount of concurrence that can be achieved in Lamb-Dicke
limit is 0.43, which corresponds to the entanglement $\mathcal{E}_{max}=0.285$
ebit. Dependence of entanglement on interatomic distance and classical driving
field is examined numerically.


title:
Deterministic secure direct communication using GHZ states and swapping
  quantum entanglement

abstract:
  We present a deterministic secure direct communication scheme via
entanglement swapping, where a set of ordered maximally entangled
three-particle states (GHZ states), initially shared by three spatially
separated parties, Alice, Bob and Charlie, functions as a quantum information
channel. After ensuring the safety of the quantum channel, Alice and Bob apply
a series local operations on their respective particles according to the
tripartite stipulation and the secret message they both want to send to
Charlie. By three Alice, Bob and Charlie's Bell measurement results, Charlie is
able to infer the secret messages directly. The secret messages are faithfully
transmitted from Alice and Bob to Charlie via initially shared pairs of GHZ
states without revealing any information to a potential eavesdropper. Since
there is not a transmission of the qubits carrying the secret message between
any two of them in the public channel, it is completely secure for direct
secret communication if perfect quantum channel is used.


title:
Quantum secure direct communication by Einstein-Podolsky-Rosen pairs and
  entanglement swapping

abstract:
  We present a quantum secure direct communication scheme achieved by swapping
quantum entanglement. In this scheme a set of ordered Einstein-Podolsky-Rosen
(EPR) pairs is used as a quantum information channel for sending secret
messages directly. After insuring the safety of the quantum channel, the sender
Alice encodes the secret messages directly by applying a series local
operations on her particles sequence according to their stipulation.
  Using three EPR pairs, three bits of secret classical information can be
faithfully transmitted from Alice to remote Bob without revealing any
information to a potential eavesdropper.
  By both Alice and Bob's GHZ state measurement results, Bob is able to read
out the encoded secret messages directly. The protocol is completely secure if
perfect quantum channel is used, because there is not a transmission of the
qubits carrying the secret message between Alice and Bob in the public channel.


title: Bremsstrahlung from Charged Bose-Einstein Condensates

abstract:
  Low energy bremsstrahlung formulae are derived for a particle beam of charged
bosons forming a Bose-Einsten condensate. The expression for energy radiated
consists of two terms in this case. One of them, the larger one in the limit of
large numbers of Bosons in the state, is proportional to the number of bosons
squared and has the same form as one obtains in a hydrodynamic model of quantum
wave mechanics. This term is sensitive to the size and shape of the wave packet
especially when the force field causing acceleration is localized in extent.
The second term, identical to the single particle scattering formula, is less
sensitive to the wave packet size and shape and is proportional to the number
of bosons in the condensate. The conclusion is that for a Bose-Einstein
condensate the radiated bremsstrahlung is a sensitive function of the wave
packet shape which is quite different than for a beam of incoherent particles
which do not show very much dependence on the wave packet. Only lowest order
radiation is calculated. It is also found that to lowest order there does not
exist any situation in which the radiation loss from a coherent state of bosons
vanishes completely, so that there is no analog of superconductivity in a
particle beam of this type at least within the net of assumptions made in this
paper. The amount of radiation can be greatly suppressed however, as it is a
sensitive function of the form of the wave function.


title: P-wave Feshbach resonances of ultra-cold $^6$Li

abstract:
  We report the observation of three p-wave Feshbach resonances of $^6$Li atoms
in the lowest hyperfine state $f=1/2$. The positions of the resonances are in
good agreement with theory. We study the lifetime of the cloud in the vicinity
of the Feshbach resonances and show that depending on the spin states, 2- or
3-body mechanisms are at play. In the case of dipolar losses, we observe a
non-trivial temperature dependence that is well explained by a simple model.


title:
Inequalities and separations among assisted capacities of quantum
  channels

abstract:
  We exhibit discrete memoryless quantum channels whose quantum capacity
assisted by two-way classical communication, $Q_2$, exceeds their unassisted
one-shot Holevo capacity $C_H$. These channels may be thought of as having a
data input and output, along with a control input that partly influences, and a
control output that partly reveals, which of a set of unitary evolutions the
data undergoes en route from input to output. The channel is designed so that
the data's evolution can be exactly inferred by a classically coordinated
processing of 1) the control output, and 2) a reference system entangled with
the control input, but not from either of these resources alone. Thus a two-way
classical side channel allows the otherwise noisy evolution of the data to be
corrected, greatly increasing the capacity. The same family of channels
provides examples where the classical capacity assisted by classical feedback,
$C_B$, and the quantum capacity assisted by classical feedback $Q_B$, both
exceed $C_H$. A related channel, whose data input undergoes dephasing before
interacting with the control input, has a classical capacity $C=C_H$ strictly
less than its $C_2$, the classical capacity assisted by independent classical
communication.


title: Sub shot noise phase quadrature measurement of intense light beams

abstract:
  We present a setup to perform sub shot noise measurements of the phase
quadrature for intense pulsed light without the use of a separate local
oscillator. A Mach--Zehnder interferometer with an unbalanced arm length is
used to detect the fluctuations of the phase quadrature at a single side band
frequency. Using this setup, the non--separability of a pair of quadrature
entangled beams is demonstrated experimentally.


title: Reconstruction of superoperators from incomplete measurements

abstract:
  We present strategies how to reconstruct (estimate) properties of a quantum
channel described by the map E based on incomplete measurements. In a
particular case of a qubit channel a complete reconstruction of the map E can
be performed via complete tomography of four output states E[rho_j ] that
originate from a set of four linearly independent test states j (j = 1, 2, 3,
4) at the input of the channel. We study the situation when less than four
linearly independent states are transmitted via the channel and measured at the
output. We present strategies how to reconstruct the channel when just one, two
or three states are transmitted via the channel. In particular, we show that if
just one state is transmitted via the channel then the best reconstruction can
be achieved when this state is a total mixture described by the density
operator rho = I/2. To improve the reconstruction procedure one has to send via
the channel more states. The best strategy is to complement the total mixture
with pure states that are mutually orthogonal in the sense of the Bloch-sphere
representation. We show that unitary transformations (channels) can be uniquely
reconstructed (determined) based on the information of how three properly
chosen input states are transformed under the action of the channel.


title:
The Fraunhofer Quantum Computing Portal - www.qc.fraunhofer.de - A
  web-based Simulator of Quantum Computing Processes

abstract:
  Fraunhofer FIRST develops a computing service and collaborative workspace
providing a convenient tool for simulation and investigation of quantum
algorithms. To broaden the twenty qubit limit of workstation-based simulations
to the next qubit decade we provide a dedicated high memorized Linux cluster
with fast Myrinet interconnection network together with a adapted parallel
simulator engine. This simulation service supplemented by a collaborative
workspace is usable everywhere via web interface and integrates both hardware
and software as collaboration and investigation platform for the quantum
community. The beta test version realizes all common one, two and three qubit
gates, arbitrary one and two bit gates, orthogonal measurements as well as
special gates like Oracle, Modulo function, Quantum Fourier Transformation and
arbitrary Spin-Hamiltonians up to 31 qubits. For a restricted gate set it
feasible to investigate circuits with up to sixty qubits. URL:
http://www.qc.fraunhofer.de


title:
Tomographic reconstruction of the single-photon Fock state by
  high-frequency homodyne detection

abstract:
  A single-photon Fock state has been generated by means of conditional
preparation from a two-photon state emitted in the process of spontaneous
parametric down-conversion. A recently developed high-frequency homodyne
tomography technique has been used to completely characterize the Fock state by
means of a pulse-to-pulse analysis of the detectors' difference photocurrent.
The density matrix elements of the generated state have been retrieved with a
final detection efficiency of about 57%. A comparison has been performed
between the phase-averaged tomographic reconstructions of the Wigner function
as obtained from the measured density-matrix elements and from a direct Abel
transform of the homodyne data. The ability of our system to work at the full
repetition rate of the pulsed laser (82 MHz) substantially simplifies the
detection scheme, allowing for more ``exotic'' quantum states to be generated
and analyzed.


title:
Spintronic single qubit gate based on a quantum ring with spin-orbit
  interaction

abstract:
  In a quantum ring connected with two external leads the spin properties of an
incoming electron are modified by the spin-orbit interaction resulting in a
transformation of the qubit state carried by the spin. The ring acts as a one
qubit spintronic quantum gate whose properties can be varied by tuning the
Rashba parameter of the spin-orbit interaction, by changing the relative
position of the junctions, as well as by the size of the ring. We show that a
large class of unitary transformations can be attained with already one ring --
or a few rings in series -- including the important cases of the Z, X, and
Hadamard gates. By choosing appropriate parameters the spin transformations can
be made unitary, which corresponds to lossless gates.


title: Variable stepsize Runge-Kutta methods for stochastic wave equations

abstract:
  We show that existing Runge-Kutta methods for ordinary differential equations
(odes) can be modified to solve stochastic differential equations (sdes) with
strong solutions provided that appropriate changes are made to the way
stepsizes are selected. The order of the resulting sde scheme is half the order
of the ode scheme. Specifically, we show that an explicit 9th order Runge-Kutta
method (with an embedded 8th order method) for odes yields an order 4.5 method
for sdes which can be implemented with variable stepsizes. This method is
tested by solving systems of sdes originating from stochastic wave equations
arising from master equations and the many-body Schroedinger equation.


title: Shaping quantum pulses of light via coherent atomic memory

abstract:
  We describe a technique for generating pulses of light with controllable
photon numbers, propagation direction, timing, and pulse shapes. The technique
is based on preparation of an atomic ensemble in a state with a desired number
of atomic spin excitations, which is later converted into a photon pulse.
Spatio-temporal control over the pulses is obtained by exploiting long-lived
coherent memory for photon states and electromagnetically induced transparency
(EIT) in an optically dense atomic medium. Using photon counting experiments we
observe generation and shaping of few-photon sub-Poissonian light pulses. We
discuss prospects for controlled generation of high-purity n-photon Fock states
using this technique.


title: A Relativistic Version of the Ghirardi-Rimini-Weber Model

abstract:
  Carrying out a research program outlined by John S. Bell in 1987, we arrive
at a relativistic version of the Ghirardi-Rimini-Weber (GRW) model of
spontaneous wavefunction collapse. The GRW model was proposed as a solution of
the measurement problem of quantum mechanics and involves a stochastic and
nonlinear modification of the Schr\"odinger equation. It deviates very little
from the Schr\"odinger equation for microscopic systems but efficiently
suppresses, for macroscopic systems, superpositions of macroscopically
different states. As suggested by Bell, we take the primitive ontology, or
local beables, of our model to be a discrete set of space-time points, at which
the collapses are centered. This set is random with distribution determined by
the initial wavefunction. Our model is nonlocal and violates Bell's inequality
though it does not make use of a preferred slicing of space-time or any other
sort of synchronization of spacelike separated points. Like the GRW model, it
reproduces the quantum probabilities in all cases presently testable, though it
entails deviations from the quantum formalism that are in principle testable.
Our model works in Minkowski space-time as well as in (well-behaved) curved
background space-times.


title: On interacting fermions and bosons with definite total momentum

abstract:
  Any {\it exact} eigenstate with a definite momentum of a many-body
Hamiltonian can be written as an integral over a {\it symmetry-broken} function
$\Phi$. For two particles, we solve the problem {\it exactly} for all energy
levels and any inter-particle interaction. Especially for the ground-state,
$\Phi$ is given by the simple Hartree-Fock/Hartree ansatz for fermions/bosons.
Implications for several and many particles as well as a numerical example are
provided.


title: Quantum Beat of Two Single Photons

abstract:
  The interference of two single photons impinging on a beam splitter is
measured in a time-resolved manner. Using long photons of different frequencies
emitted from an atom-cavity system, a quantum beat with a visibility close to
100% is observed in the correlation between the photodetections at the output
ports of the beam splitter. The time dependence of the beat amplitude reflects
the coherence properties of the photons. Most remarkably, simultaneous
photodetections are never observed, so that a temporal filter allows one to
obtain perfect two-photon coalescence even for non-perfect photons.


title:
Counterfactual errors and state reduction in relativistic quantum
  physics

abstract:
  We use the laws of relativistic physics to show that classically motivated
counterfactual statements are inadequate when discussing the principles of
quantum physics and that EPR style arguments against state reduction are
incorrect.


title: Why are probabilistic laws governing quantum mechanics and neurobiology?

abstract:
  We address the question: Why are dynamical laws governing in quantum
mechanics and in neuroscience of probabilistic nature instead of being
deterministic? We discuss some ideas showing that the probabilistic option
offers advantages over the deterministic one.


title: Quantum key distribution with asymmetric channel noise

abstract:
  We show that one may take advantages in both robusty and key rate of
asymmetric channel noise.


title:
Fault tolerant quantum key distribution protocol with collective random
  unitary noise

abstract:
  We propose an easy implementable prepare-and-measure protocol for robust
quantum key distribution with photon polarization. The protocol is fault
tolerant against collective random unitary channel noise. The protocol does not
need any collective quantum measurement or quantum memory. A security proof and
a specific linear optical realization using spontaneous parametric down
conversion are given.


title: Proposed Test of Quantum Nonlocality for Continuous Variables

abstract:
  We propose a test of nonlocality for continuous variables using a two-mode
squeezed state as the source of nonlocal correlations and a measurement scheme
based on conditional homodyne detection. Both the CHSH- and the CH-inequality
are constructed from the conditional homodyne data and found to be violated for
a squeezing parameter larger than $r\approx0.48$.


title:
Vacuum fluctuations and the conditional homodyne detection of squeezed
  light

abstract:
  Conditional homodyne detection of quadrature squeezing is compared with
standard nonconditional detection. Whereas the latter identifies
nonclassicality in a quantitative way, as a reduction of the noise power below
the shot noise level, conditional detection makes a qualitative distinction
between vacuum state squeezing and squeezed classical noise. Implications of
this comparison for the realistic interpretation of vacuum fluctuations
(stochastic electrodynamics) are discussed.


title: Multiparty Quantum Secret Sharing Based on Entanglement Swapping

abstract:
  A multiparty quantum secret sharing (QSS) protocol is proposed by using
swapping quantum entanglement of Bell states. The secret messages are imposed
on Bell states by local unitary operations. The secret messages are split into
several parts and each part is distributed to a party so that no action of a
subset of all the parties but their entire cooperation is able to read out the
secret messages. In addition, the dense coding is used in this protocol to
achieve a high efficiency. The security of the present multiparty QSS against
eavesdropping has been analyzed and confirmed even in a noisy quantum channel.


title: Probabilistically Cloning and Quantum Computation

abstract:
  We discuss the usefulness of quantum cloning and present examples of quantum
computation tasks for which cloning offers an advantage which cannot be matched
by any approach that does not resort to it. In these quantum computations, we
need to distribute quantum information contained in states about which we have
some partial information. To perform quantum computations, we use
state-dependent probabilistic quantum cloning procedure to distribute quantum
information in the middle of a quantum computation.


title:
Searching for degeneracies of real Hamiltonians using homotopy
  classification of loops in SO($n$)

abstract:
  Topological tests to detect degeneracies of Hamiltonians have been put
forward in the past. Here, we address the applicability of a recently proposed
test [Phys. Rev. Lett. {\bf 92}, 060406 (2004)] for degeneracies of real
Hamiltonian matrices. This test relies on the existence of nontrivial loops in
the space of eigenbases SO$(n)$. We develop necessary means to determine the
homotopy class of a given loop in this space. Furthermore, in cases where the
dimension of the relevant Hilbert space is large the application of the
original test may not be immediate. To remedy this deficiency, we put forward a
condition for when the test is applicable to a subspace of Hilbert space.
Finally, we demonstrate that applying the methodology of [Phys. Rev. Lett. {\bf
92}, 060406 (2004)] to the complex Hamiltonian case does not provide any new
information.


title: From Quantum State Targeting to Bell Inequalities

abstract:
  Quantum state targeting is a quantum game which results from combining
traditional quantum state estimation with additional classical information. We
consider a particular version of the game and show how it can be played with
maximally entangled states. The optimal solution of the game is used to derive
a Bell inequality for two entangled qutrits. We argue that the nice properties
of the inequality are direct consequences of the method of construction.


title: Quantum correlations and Nash equilibria of a bi-matrix game

abstract:
  Playing a symmetric bi-matrix game is usually physically implemented by
sharing pairs of 'objects' between two players. A new setting is proposed that
explicitly shows effects of quantum correlations between the pairs on the
structure of payoff relations and the 'solutions' of the game. The setting
allows a re-expression of the game such that the players play the classical
game when their moves are performed on pairs of objects having correlations
that satisfy the Bell's inequalities. If players receive pairs having quantum
correlations the resulting game cannot be considered another classical
symmetric bi-matrix game. Also the Nash equilibria of the game are found to be
decided by the nature of the correlations.


title:
Towards Quantum Superpositions of a Mirror: an Exact Open Systems
  Analysis

abstract:
  We analyze the recently proposed mirror superposition experiment of Marshall,
Simon, Penrose, and Bouwmeester, assuming that the mirror's dynamics contains a
non-unitary term of the Lindblad type proportional to -[q,[q,\rho]], with q the
position operator for the center of mass of the mirror, and \rho the
statistical operator. We derive an exact formula for the fringe visibility for
this system. We discuss the consequences of our result for tests of
environmental decoherence and of collapse models. In particular, we find that
with the conventional parameters for the CSL model of state vector collapse,
maintenance of coherence is expected to within an accuracy of at least 1 part
in 10^{8}. Increasing the apparatus coupling to environmental decoherence may
lead to observable modifications of the fringe visibility, with time dependence
given by our exact result.


title: Comparison of classical chaos with quantum chaos

abstract:
  We investigate chaotic behavior in a 2-D Hamiltonian system - oscillators
with anharmonic coupling. We compare the classical system with quantum system.
Via the quantum action, we construct Poincar\'e sections and compute Lyapunov
exponents for the quantum system. We find that the quantum system is globally
less chaotic than the classical system. We also observe with increasing energy
the distribution of Lyapunov exponts approaching a Gaussian with a strong
correlation between its mean value and energy.


title:
How quantum correlations enhance prediction of complementary
  measurements

abstract:
  If there are correlations between two qubits then the results of the
measurement on one of them can help to predict measurement results on the other
one. It is an interesting question what can be predicted about the results of
two complementary projective measurements on the first qubit. To quantify these
predictions the complementary \emph{knowledge excesses} are used. A non-trivial
constraint restricting them is derived. For any mixed state and for arbitrary
measurements the knowledge excesses are bounded by a factor depending only on
the maximal violation of Bell's inequalities. This result is experimentally
verified on two-photon Werner states prepared by means of spontaneous
parametric down-conversion.


title: Inequalities for quantum channels assisted by limited resources

abstract:
  The information capacities and ``distillability'' of a quantum channel are
studied in the presence of auxiliary resources. These include prior
entanglement shared between the sender and receiver and free classical bits of
forward and backward communication. Inequalities and trade-off curves are
derived. In particular an alternative proof is given that in the absence of
feedback and shared entanglement, forward classical communication does not
increase the quantum capacity of a channel.


title:
Conclusive and arbitrarily perfect quantum state transfer using parallel
  spin chain channels

abstract:
  We suggest a protocol for perfect quantum communication through spin chain
channels. By combining a dual-rail encoding with measurements only at the
receiving end, we can get conclusively perfect state transfer, whose
probability of success can be made arbitrarily close to unity. As an example of
such an amplitude delaying channel, we show how two parallel Heisenberg spin
chains can be used as quantum wires. Perfect state transfer with a probability
of failure lower than P in a Heisenberg chain of N spin-1/2 particles can be
achieved in a timescale of the order of N^1.7|ln(P)|. We demonstrate that our
scheme is more robust to decoherence and non-optimal timing than any scheme
using single spin chains.


title:
Generation of twin beams from an optical parametric oscillator pumped by
  a frequency-doubled diode laser

abstract:
  Quantum-correlated twin beams were generated from a triply resonant optical
parametric oscillator with an a-cut KTP crystal pumped by a frequency-doubled
diode laser. A total output of 5.1 mW was obtained in the
classical-to-nonclassical-light conversion system driven by a 50-mW diode laser
at 1080 nm. Quantum noise reduction of 4.3 dB (63%) in the intensity difference
between the twin beams was successfully observed at the detection frequency of
3 MHz.


title:
Generalized GHZ-class and W-class concurrence and entanglement vectors
  of the multipartite systems consisting of qubits

abstract:
  We propose two classes of the generalized concurrence vectors of the
multipartite systems consisting of qubits. Making use of them, we are able to,
respectively, describe and quantify GHZ-class and W-class entanglement both in
total and between arbitrary two partite in the multipartite system consisting
of qubits. In the case of pure state of three qubits that one partite is
separable, it is shown to exactly back to the usual Wootters' concurrence after
introduce a whole concurrence vector. In principle, our method is applicable to
any $N$-partite systems consisting of $N$ qubits.


title: Comment on "Secure direct communication with a quantum one-time pad"

abstract:
  In the paper [Phys. Rev. A \textbf{69}, 052319 (2004)], a quantum direct
communication protocol is proposed which is claimed to be unconditionally
secure even for the case of a noisy channel. We show that this is not the case
by giving an undetectable attack scheme.


title: Casimir interaction between cylinders

abstract:
  We compute the Casimir interaction energy between two perfectly conducting,
concentric cylinders, using the mode-by-mode summation technique. Then we
compare it with the approximate results obtained using the proximity theorem
and a semiclassical approximation based on classical periodic orbits. We show
that the proximity theorem with a particular choice for the effective area
coincides with the semiclassical approximation and reproduces the exact result
far beyond its expected range of validity. We also compute the force between
slightly eccentric cylinders and discuss the advantages of using a cylindrical
geometry to measure the Casimir force.


title: Quantum computing with magnetically interacting atoms

abstract:
  We propose a scalable quantum-computing architecture based on cold atoms
confined to sites of a tight optical lattice. The lattice is placed in a
non-uniform magnetic field and the resulting Zeeman sublevels define qubit
states. Microwave pulses tuned to space-dependent resonant frequencies are used
for individual addressing. The atoms interact via magnetic-dipole interactions
allowing implementation of a universal controlled-NOT gate. The resulting gate
operation times for alkalis are on the order of milliseconds, much faster then
the anticipated decoherence times. Single qubit operations take about 10
microseconds. Analysis of motional decoherence due to NOT operations is given.
We also comment on the improved feasibility of the proposed architecture with
complex open-shell atoms, such as Cr, Eu and metastable alkaline-earth atoms
with larger magnetic moments.


title: Robust Quantum Communication Using A Polarization-Entangled Photon Pair

abstract:
  Noise and imperfection of realistic devices are major obstacles for
implementing quantum cryptography. In particular birefringence in optical
fibers leads to decoherence of qubits encoded in polarization of photon. We
show how to overcome this problem by doing single qubit quantum communication
without a shared spatial reference frame and precise timing. Quantum
information will be encoded in pair of photons using ``tag'' operations which
corresponds to the time delay of one of the polarization modes. This method is
robust against the phase instability of the interferometers despite the use of
time-bins. Moreover synchronized clocks are not required in the ideal situation
no photon loss case as they are only necessary to label the different encoded
qubits.


title: On Quantum Cellular Automata

abstract:
  In recent work [quant-ph/0405174] by Schumacher and Werner was discussed an
abstract algebraic approach to a model of reversible quantum cellular automata
(CA) on a lattice. It was used special model of CA based on partitioning scheme
and so there is a question about quantum CA derived from more general, standard
model of classical CA. In present work is considered an approach to definition
of a scheme with "history", valid for quantization both irreversible and
reversible classical CA directly using local transition rules. It is used
language of vectors in Hilbert spaces instead of C*-algebras, but results may
be compared in some cases. Finally, the quantum lattice gases, quantum walk and
"bots" are also discussed briefly.


title: Comment on: "Measuring a Photonic Qubit without Destroying It"

abstract:
  Recently, Pryde et al reported the demonstration of a quantum non-demolition
scheme for single-photon polarization states with linear optics and projective
measurements [Phys. Rev. Lett. 92, 190402 (2004)]. Here, we argue that their
interpretation of the experiment is inconsistent with the fidelity measure they
use.


title: The time evolution of the Bohmian Pilot Wave

abstract:
  In this paper the Newton-Schrodinger-Bohm equation is solved for particles
with m>M_P. It is shown that the Bohmian pilot wave for particles with m>M_P
oscillates with frequency omega=tau^{-1}_P, where tau_P is the Planck time.
  Key words: Macroscopic particles; Pilot wave.


title:
Vacuum fluctuations and Brownian motion of a charged test particle near
  a reflecting boundary

abstract:
  We study the Brownian motion of a charged test particle coupled to
electromagnetic vacuum fluctuations near a perfectly reflecting plane boundary.
The presence of the boundary modifies the quantum fluctuations of the electric
field, which in turn modifies the motion of the test particle. We calculate the
resulting mean squared fluctuations in the velocity and position of the test
particle. In the case of directions transverse to the boundary, the results are
negative. This can be interpreted as reducing the quantum uncertainty which
would otherwise be present.


title:
Creating excitonic entanglement in quantum dots through the optical
  Stark effect

abstract:
  We show that two initially non-resonant quantum dots may be brought into
resonance by the application of a single detuned laser. This allows for control
of the inter-dot interactions and the generation of highly entangled excitonic
states on the picosecond timescale. Along with arbitrary single qubit
manipulations, this system would be sufficient for the demonstration of a
prototype excitonic quantum computer.


title: Stability of Quantum Dynamics

abstract:
  The stability of quantum systems to perturbations of the Hamiltonian is
studied. This stability is quantified by the fidelity. Dependence of fidelity
on the initial state as well as on the dynamical properties of the system is
considered. In particular, systems having a chaotic or regular classical limit
are analysed. The fidelity decay rate is given by an integral of the
correlation function of the perturbation and is thus smaller the faster
correlation function decays. If the perturbation can be written as a time
derivative of another operator, meaning that the time averaged perturbation
vanishes, fidelity freezes at a constant value and starts to decay only after a
long time inversely proportional to the perturbation strength. In composite
systems stability of entanglement to perturbations of the Hamiltonian is
analysed in terms of purity. For regular systems purity decay is shown to be
independent of Planck's constant for coherent initial states in the
semiclassical limit. The accelerated decoherence of macroscopic superpositions
is also explained. The theory of fidelity decay is applied to the stability of
quantum computation and an improved quantum Fourier transform algorithm is
designed.


title:
Energy level statistics for models of coupled single-mode Bose--Einstein
  condensates

abstract:
  We study the distribution of energy level spacings in two models describing
coupled single-mode Bose-Einstein condensates. Both models have a fixed number
of degrees of freedom, which is small compared to the number of interaction
parameters, and is independent of the dimensionality of the Hilbert space. We
find that the distribution follows a universal Poisson form independent of the
choice of coupling parameters, which is indicative of the integrability of both
models. These results complement those for integrable lattice models where the
number of degrees of freedom increases with increasing dimensionality of the
Hilbert space. Finally, we also show that for one model the inclusion of an
additional interaction which breaks the integrability leads to a non-Poisson
distribution.


title: Quantum Positioning System

abstract:
  A quantum positioning system (QPS) is proposed that can provide a user with
all four of his space-time coordinates. The user must carry a corner cube
reflector, a good clock, and have a two-way classical channel of communication
with the origin of the reference frame. Four pairs of entangled photons
(biphotons) are sent through four interferometers: three interferometers are
used to determine the user's spatial position, and an additional interferometer
is used to synchronize the user's clock to coordinate time in the reference
frame. The spatial positioning part of the QPS is similar to a classical
time-of-arrival (TOA) system, however, a classical TOA system (such as GPS)
must have synchronized clocks that keep coordinate time and therefore the
clocks must have long-term stability, whereas in the QPS only a photon
coincidence counter is needed and the clocks need only have short-term
stability. Several scenarios are considered for a QPS: one is a terrestrial
system and another is a space-based-system composed of low-Earth orbit (LEO)
satellites. Calculations indicate that for a space-based system, neglecting
atmospheric effects, a position accuracy below the 1 cm-level is possible for
much of the region near the Earth. The QPS may be used as a primary system to
define a global 4-dimensional reference frame.


title: Permanents in linear optical networks

abstract:
  We develop an abstract look at linear optical networks from the viewpoint of
combinatorics and permanents. In particular we show that calculation of matrix
elements of unitarily transformed photonic multi-mode states is intimately
linked to the computation of permanents. An implication of this remarkable fact
is that all calculations that are based on evaluating matrix elements are
generically computationally hard. Moreover, quantum mechanics provides simpler
derivations of certain matrix analysis results which we exemplify by showing
that the permanent of any unitary matrix takes its values across the unit disk
in the complex plane.


title: Continuous variable entanglement of phase locked light beams

abstract:
  We explore in detail the possibility of intracavity generation of
continuous-variable (CV) entangled states of light beams under mode
phase-locked conditions. We show that such quantum states can be generated in
self-phase locked nondegenerate optical parametric oscillator (NOPO) based on a
type-II phase-matched down-conversion combined with linear mixer of two
orthogonally polarized modes of the subharmonics in a cavity. A quantum theory
of this device, recently realized in the experiment, is developed for both
sub-threshold and above-threshold operational regimes. We show that the system
providing high level phase coherence between two generated modes, unlike to the
ordinary NOPO, also exhibits different types of quantum correlations between
photon numbers and phases of these modes. We quantify the CV entanglement as
two-mode squeezing and show that the maximal degree of the integral two-mode
squeezing(that is 50% relative to the level of vacuum fluctuations) is achieved
at the pump field intensity close to the generation threshold of self-phase
locked NOPO, provided that the constant of linear coupling between the two
polarizations is much less than the mode detunings. The peculiarities of CV
entanglement for the case of unitary, non-dissipative dynamics of the system
under consideration is also cleared up.


title: Quantum Game Theory in Finance

abstract:
  This is a short review of the background and recent development in quantum
game theory and its possible application in economics and finance. The
intersection of science and society is also discussed. The review is addressed
to non--specialists.


title: Quantum dialogue

abstract:
  We propose an entanglement-based protocol for two people to simultaneously
exchange their messages. We show that the protocol is asymptotically secure
against the disturbance attack, the intercept-and-resend attack and the
entangle-and-measure attack. Our protocol is experimentally feasible within
current technologies.


title: Quantum chaos viewed from quantum action

abstract:
  We consider a mixed chaotic Hamiltonian system and compare classical with
quantum chaos. As alternative to the methods of enegy level spacing statistics
and trace formulas, we construct a quantum action and a quantum analogue phase
space to analyse quantum chaos.


title: The complementarity of quantum observables: theory and experiment

abstract:
  Three notions of complementarity - operational, probabilistic, and value
complementarity - are reanalysed with respect to the question of joint
measurements and compared with reference to some examples of canonically
conjugate observables. It is shown that the joint measurability of noncommuting
observables is a consequence of the quantum formalism if unsharp observables
are taken into account; a fact not in conflict with the idea of
complementarity, which, in its strongest version, was originally formulated
only for sharp observables. As an illustration of the general theory, the
wave-particle duality of photons is analysed in terms of complementary path and
interference observables and their unsharp joint measurability.


title: Quantum Information Processing in Nanostructures

abstract:
  Semiconductor quantum dots integrated with ultrafast spectroscopy technology
are prime candidates for building scalable architectures for Quantum
Information Processing. In this review paper we survey the current state of
theoretical proposals concerning all-optical control of nanostructure qubits
and their interactions. These schemes offer potential for ultrafast optical
manipulation of quantum information in time scales within the coherence time.


title: A Newtonian Hidden Variable Theory

abstract:
  A new hidden variable theory is proposed, according to which particles
follows definite trajectories, as in Bohmian Mechanics or Nelson's stochastic
mechanics; in the new theory, however, the trajectories are classical, i.e.
Newtonian. This result is obtained by developing the following concepts: (i)
the essential elements of a hidden variable theory are a set of trajectories
and a measure defined on it; the Newtonian HCT will be defined by giving these
two elements. (ii) The universal wave function has a tree structure, whose
branches are generated by the measurement processes and are spatially
disjoined. (iii) The branches have a classical structure, i.e. classical paths
go along them; this property derives from the fact that the paths close to the
classical ones give the main contribution to the Feynman propagator. (iv)
Classical trajectories can give rise to quantum phenomena, like for instance
the interference phenomena of the two-slit experiment, by violating the so
called Independence Assumption, which is always implicitely made in the
conceptual analysis of these phenomena.


title:
A combinatorial approach for studying LOCC transformations of
  multipartite states

abstract:
  We develop graph theoretic methods for analysing maximally entangled pure
states distributed between a number of different parties. We introduce a
technique called {\it bicolored merging}, based on the monotonicity feature of
entanglement measures, for determining combinatorial conditions that must be
satisfied for any two distinct multiparticle states to be comparable under
local operations and classical communication (LOCC). We present several results
based on the possibility or impossibility of comparability of pure multipartite
states. We show that there are exponentially many such entangled multipartite
states among $n$ agents. Further, we discuss a new graph theoretic metric on a
class of multi-partite states, and its implications.


title: Normal-mode spectroscopy of a single bound atom-cavity system

abstract:
  The energy-level structure of a single atom strongly coupled to the mode of a
high-finesse optical cavity is investigated. The atom is stored in an
intracavity dipole trap and cavity cooling is used to compensate for inevitable
heating. Two well-resolved normal modes are observed both in the cavity
transmission and the trap lifetime. The experiment is in good agreement with a
Monte Carlo simulation, demonstrating our ability to localize the atom to
within $\lambda/10$ at a cavity antinode.


title: A possible hypercomputational quantum algorithm

abstract:
  The term `hypermachine' denotes any data processing device (theoretical or
that can be implemented) capable of carrying out tasks that cannot be performed
by a Turing machine. We present a possible quantum algorithm for a classically
non-computable decision problem, Hilbert's tenth problem; more specifically, we
present a possible hypercomputation model based on quantum computation. Our
algorithm is inspired by the one proposed by Tien D. Kieu, but we have selected
the infinite square well instead of the (one-dimensional) simple harmonic
oscillator as the underlying physical system. Our model exploits the quantum
adiabatic process and the characteristics of the representation of the
dynamical Lie algebra su(1,1) associated to the infinite square well.


title:
Ultralong coherence times in the purely electronic zero-phonon line
  emission of single molecules

abstract:
  We report the observation of ultralong coherence times in the purely
electronic zero-phonon line emission of single terrylenediimide molecules at
1.4 K. Vibronic excitation and spectrally resolved detection with a scanning
Fabry-Perot spectrum analyzer were used to measure a linewidth of 65 MHz. This
is within a factor of 1.6 of the transform limit. It therefore indicates that
single molecule emission may be suited for applications in linear optics
quantum computation. Additionally it is shown that high resolution spectra
taken with the spectrum analyzer allow for the investigation of fast spectral
dynamics in the emission of a single molecule.


title: Quantum states satisfying classical probability constraints

abstract:
  For linear combinations of quantum product averages in an arbitrary bipartite
state, we derive new quantum Bell-form and CHSH-form inequalities with the
right-hand sides expressed in terms of a bipartite state. This allows us to
specify in a general setting bipartite state properties sufficient for the
validity of a classical CHSH-form inequality and the perfect correlation form
of the original Bell inequality for any bounded quantum observables. We also
introduce a new general condition on a bipartite state and quantum observables
sufficient for the validity of the original Bell inequality, in its perfect
correlation or anticorrelation forms. Under this general sufficient condition,
a bipartite quantum state does not necessarily exhibit perfect correlations or
anticorrelations.


title:
Entanglement-assisted classical information capacity of the amplitude
  damping channel

abstract:
  In this paper, we calculate the entanglement-assisted classical information
capacity of amplitude damping channel and compare it with the particular mutual
information which is considered as the entanglement-assisted classical
information capacity of this channel in Ref. 6. It is shown that the difference
between them is very small. In addition, we point out that using partial
symmetry and concavity of mutual information derived from dense coding scheme
one can simplify the calculation of entanglement-assisted classical information
capacities for non-unitary-covariant quantum noisy channels.


title:
Entanglement convertibility for infinite dimensional pure bipartite
  states

abstract:
  It is shown that the order property of pure bipartite states under SLOCC
(stochastic local operations and classical communications) changes radically
when dimensionality shifts from finite to infinite. In contrast to finite
dimensional systems where there is no pure incomparable state, the existence of
infinitely many mutually SLOCC incomparable states is shown for infinite
dimensional systems even under the bounded energy and finite information
exchange condition. These results show that the effect of the infinite
dimensionality of Hilbert space, the ``infinite workspace'' property, remains
even in physically relevant infinite dimensional systems.


title: A logarithmic-depth quantum carry-lookahead adder

abstract:
  We present an efficient addition circuit, borrowing techniques from the
classical carry-lookahead arithmetic circuit. Our quantum carry-lookahead
(QCLA) adder accepts two n-bit numbers and adds them in O(log n) depth using
O(n) ancillary qubits. We present both in-place and out-of-place versions, as
well as versions that add modulo 2^n and modulo 2^n - 1.
  Previously, the linear-depth ripple-carry addition circuit has been the
method of choice. Our work reduces the cost of addition dramatically with only
a slight increase in the number of required qubits. The QCLA adder can be used
within current modular multiplication circuits to reduce substantially the
run-time of Shor's algorithm.


title:
Quantum Adaptive Protocol and its Application in Image Storage and
  Retrieval

abstract: This paper has been temporarily withdrawn by the authors.


title: Quantum computation with cold bosonic atoms in an optical lattice

abstract:
  We analyse an implementation of a quantum computer using bosonic atoms in an
optical lattice. We show that, even though the number of atoms per site and the
tunneling rate between neighbouring sites is unknown, one may perform a
universal set of gates by means of adiabatic passage.


abstract:
  A precise estimation of the computational complexity in Shor's factoring
algorithm under the condition that the large integer we want to factorize is
composed by the product of two prime numbers, is derived by the results related
to number theory. Compared with Shor's original estimation, our estimation
shows that one can obtain the solution under such a condition, by less
computational complexity.


title:
Spin squeezing and precision probing with light and samples of atoms in
  the gaussian approximation

abstract:
  We consider an ensemble of trapped atoms interacting with a continuous wave
laser field. For sufficiently polarized atoms and for a polarized light field,
we may approximate the non-classical components of the collective spin angular
momentum operator for the atoms and the Stokes vectors of the field by
effective position and momentum variables for which we assume a gaussian state.
Within this approximation, we present a theory for the squeezing of the atomic
spin by polarization rotation measurements on the probe light. We derive
analytical expressions for the squeezing with and without inclusion of the
noise effects introduced by atomic decay and by photon absorption. The theory
is readily adapted to the case of inhomogeneous light-atom coupling [A. Kuzmich
and T.A.B. Kennedy, Phys. Rev. Lett. Vol. 92, 030407 (2004)]. As a special
case, we show how to formulate the theory for an optically thick sample by
slicing the gas into pieces each having only small photon absorption
probability. Our analysis of a realistic probing and measurement scheme shows
that it is the maximally squeezed component of the atomic gas that determines
the accuracy of the measurement.


title: Quantum cryptography: a practical information security perspective

abstract:
  Quantum Key Exchange (QKE, also known as Quantum Key Distribution or QKD)
allows communicating parties to securely establish cryptographic keys. It is a
well-established fact that all QKE protocols require that the parties have
access to an authentic channel. Without this authenticated link, QKE is
vulnerable to man-in-the-middle attacks. Overlooking this fact results in
exaggerated claims and/or false expectations about the potential impact of QKE.
In this paper we present a systematic comparison of QKE with traditional key
establishment protocols in realistic secure communication systems.


title: Experimental realization of hyper-entangled two-photon states

abstract:
  We report on the the experimental realization of hyper-entangled two photon
states, entangled in polarization and momentum. These states are produced by a
high brilliance parametric source of entangled photon pairs with peculiar
characteristics of flexibility in terms of state generation. The quality of the
entanglement in the two degrees of freedom has been tested by multimode
Hong-Ou-Mandel interferometry.


title: A new perturbation method in quantum mechanics

abstract:
  We investigate the convergence properties of a perturbation method proposed
some time ago and reveal some of it most interesting features. Anharmonic
oscillators in the strong--coupling limit prove to be appropriate illustrative
examples and benchmark.


title: Efficient generation of graph states for quantum computation

abstract:
  We present an entanglement generation scheme which allows arbitrary graph
states to be efficiently created in a linear quantum register via an auxiliary
entangling bus. The dynamics of the entangling bus is described by an effective
non-interacting fermionic system undergoing mirror-inversion in which qubits,
encoded as local fermionic modes, become entangled purely by Fermi statistics.
We discuss a possible implementation using two species of neutral atoms stored
in an optical lattice and find that the scheme is realistic in its requirements
even in the presence of noise.


title:
A Subexponential Time Algorithm for the Dihedral Hidden Subgroup Problem
  with Polynomial Space

abstract:
  In a recent paper, Kuperberg described the first subexponential time
algorithm for solving the dihedral hidden subgroup problem. The space
requirement of his algorithm is super-polynomial. We describe a modified
algorithm whose running time is still subexponential and whose space
requirement is only polynomial.


title: Non-conditioned generation of Schroedinger cat states in a cavity

abstract:
  We investigate the dynamics of a two-level atom in a cavity filled with a
nonlinear medium. We show that the atom-field detuning $\delta$ and the
nonlinear parameter $\chi^{(3)}$ may be combined to yield a periodic dynamics
and allowing the generation of almost exact superpositions of coherent states
({\sl Schr\"odinger} cats). By analysing the atomic inversion and the field
purity, we verify that any initial atom-field state is recovered at each
revival time, and that a coherent field interacting with an excited atom
evolves to a superposition of coherent states at each collapse time. We show
that a mixed field state (statistical mixture of two coherent states) evolves
towards a pure field state ({\sl Schr\"odinger} cat) as well. We discuss the
validity of those results by using the field fidelity and the {\sl Wigner}
function.


title: The Theory of Two-Photon Interference in an EIT System

abstract:
  We examine the possibility of storing and retrieving a single photon using
electromagnetically induced transparency (EIT). We consider the theory of a
proof of principle two-photon interference experiment, in which an atomic vapor
cell is placed in one arm of a two-photon interferometer. Since the two-photon
state is entangled, we can examine the degree to which entanglement survives.
We show that while the experiment might be difficult, it should be possible to
perform. We also show that the two-photon interference pattern has oscillatory
behavior. pacs 42.50.Ct, 42.50.Dv, 42.50.Gy, 42.50.St


title: Qubits and Quantum Spaces

abstract:
  We consider the quantum computational process as viewed by an insider
observer: this is equivalent to an isomorphism between the quantum computer and
a quantum space, namely the fuzzy sphere. The result is the formulation of a
reversible quantum measurement scheme, with no hidden information.


title: Less eavesdropping losses induce more eavesdropping information gain

abstract:
  The eavesdropping scheme proposed by W\'{o}jcik [Phys. Rev. Lett. {\bf
90},157901(2003)] on the quantum communication protocol of Bostr\"{o}m and
Felbinger [Phys. Rev. Lett. {\bf 89}, 187902(2002)] is improved by constituting
a new set of attack operations. The improved scheme only induces half of the
eavesdropping losses in W\'{o}jcik's scheme, therefore, in a larger domain of
the quantum channel transmission efficiency $\eta$, i.e., [0,75%], the
eavesdropper Eve can attack all the transmitted bits. Comparing to W\'{o}jcik's
scheme, in the improved scheme the eavesdropping (legitimate) information gain
does not vary in the $\eta$ domain of [0, 50%], while in the $\eta$ domain of
(50%, 75%] the less eavesdropping losses induce more eavesdropping information
gains, for Eve can attack {\it all} the transmitted bits and accordingly
eavesdropping information gains do {\it not} decrease. Moreover, for the
Bostr\"{o}m-Felbinger protocol, the insecurity upper bound of $\eta$ presented
by W\'{o}jcik is pushed up in the this paper, that is, according to
W\'{o}jcik's eavesdropping scheme, the Bostr\"{o}m-Felbinger protocol is not
secure for transmission efficiencies lower than almost 60%, while according to
the improved scheme, it is not secure for transmission efficiencies lower than
almost 80%.


title:
Nonlocality tests of Bell Inequalities and of Hardy's "ladder theorem"
  without "supplementary assumptions"

abstract:
  We have experimentally tested the non local properties of the states
generated by a high brilliance source of entanglement which virtually allows
the direct measurement of the full set of photon pairs created by the basic QED
process implied by the parametric quantum scattering. Standard Bell
measurements and Bell's inequality violation test have been realized over the
entire cone of emission of the degenerate pairs. By the same source we have
verified the Hardy's ladder theory up to the 20th step and the contradiction
between the standard quantum theory and the local realism has been tested for
41% of entangled pairs. PACS: 03.65.Ud, 03.67.Mn, 42.65.Lm


title: Altruistic Contents of Quantum Prisoner's Dilemma

abstract:
  We examine the classical contents of quantum games. It is shown that a
quantum strategy can be interpreted as a classical strategies with effective
density-dependent game matrices composed of transposed matrix elements. In
particular, successful quantum strategies in dilemma games are interpreted in
terms of a symmetrized game matrix that corresponds to an altruistic game plan.

categories: quant-ph cond-mat.mes-hall math-ph math.MP q-bio.PE


title: Gauge fields, point interactions and few-body problems in one dimension

abstract:
  Point interactions for the second derivative operator in one dimension are
studied. Every operator from this family is described by the boundary
conditions which include a $ 2 \times 2 $ real matrix with the unit determinant
and a phase. The role of the phase parameter leading to unitary equivalent
operators is discussed in the present paper. In particular it is shown that the
phase parameter is not redundant (contrary to previous studies) if non
stationary problems are concerned. It is proven that the phase parameter can be
interpreted as the amplitude of a singular gauge field. Considering the
few-body problem we extend the range of parameters for which the exact solution
can be found using the Bethe Ansatz.


title: Quantum state transmission via a spin ladder as a robust data bus

abstract:
  We explore the physical mechanism to coherently transfer the quantum
information of spin by connecting two spins to an isotropic antiferromagnetic
spin ladder system as data bus. Due to a large spin gap existing in such a
perfect medium, the effective Hamiltonian of the two connected spins can be
archived as that of Heisenberg type, which possesses a ground state with
maximal entanglement. We show that the effective coupling strength is inversely
proportional to the distance of the two spins and thus the quantum information
can be transferred between the two spins separated by a longer distance, i.e.
the characteristic time of quantum state transferring linearly depends on the
distance.


title: When is the two-level approximation untenable in issues of Decoherence?

abstract:
  We examine the conditions in favor and necessity of a realistic multileveled
description of a decohering quantum system. Under these conditions approximate
techniques to simplify a multileveled system by its first two levels is
unreliable and a realistic multilevel description in the formulation of
decoherence is unavoidable. In this regard, our first crucial observation is
that, the validity of the two level approximation of a multileveled system is
not controlled purely by {\it sufficiently low temperatures}. We demonstrate
using three different environmental spectral models that the type of
system-environment coupling and the environmental spectrum have a dominant role
over the temperature. Particularly, zero temperature quantum fluctuations
induced by the Caldeira-Leggett type linear coordinate coupling can be
influential in a wide energy range in the systems allowed transitions. The
second crucial observation against the validity of the two level approximation
is that the decoherence times being among the system's short time scales are
found to be dominated not by the resonant but {\it non-resonant} processes. We
demonstrate this in three stages. Firstly, our zero temperature numerical
calculations reveal that, the calculated decoherence rates including
relaxation, dephasing and leakage phenomena show, a linear dependence on the
spectral area for all spectral models used, independent from the spectral shape
within a large environmental spectral range compared to the quantum system's
energies. Secondly, within the same range, the decoherence times only have a
marginal dependence on the translations of the entire frequency spectrum.
Finally, the same decoherence rates show strong dependence on the number of
coupled levels by the system-environment coupling.


title:
Quantum noise induced entanglement and chaos in the dissipative quantum
  model of brain

abstract:
  We discuss some features of the dissipative quantum model of brain in the
frame of the formalism of quantum dissipation. Such a formalism is based on the
doubling of the system degrees of freedom. We show that the doubled modes
account for the quantum noise in the fluctuating random force in the
system-environment coupling. Remarkably, such a noise manifests itself through
the coherent structure of the system ground state. The entanglement of the
system modes with the doubled modes is shown to be permanent in the infinite
volume limit. In such a limit the trajectories in the memory space are
classical chaotic trajectories.


title: A new inequality for the von Neumann entropy

abstract:
  Strong subadditivity of von Neumann entropy, proved in 1973 by Lieb and
Ruskai, is a cornerstone of quantum coding theory. All other known inequalities
for entropies of quantum systems may be derived from it. Here we prove a new
inequality for the von Neumann entropy which we prove is independent of strong
subadditivity: it is an inequality which is true for any four party quantum
state, provided that it satisfies three linear relations (constraints) on the
entropies of certain reduced states.


title: A note on the geometric phase in adiabatic approximation

abstract:
  The adiabatic theorem shows that the instantaneous eigenstate is a good
approximation of the exact solution for a quantum system in adiabatic
evolution. One may therefore expect that the geometric phase calculated by
using the eigenstate should be also a good approximation of exact geometric
phase. However, we find that the former phase may differ appreciably from the
latter if the evolution time is large enough.


title:
Quantum Computation and Quantum Information: Are They Related to Quantum
  Paradoxology?

abstract:
  We review both the Einstein, Podolsky, Rosen (EPR) paper about the
completeness of quantum theory, and Schrodinger's responses to the EPR paper.
We find that both the EPR paper and Schrodinger's responses, including the cat
paradox, are not consistent with the current understanding of quantum theory
and thermodynamics.
  Because both the EPR paper and Schrodinger's responses play a leading role in
discussions of the fascinating and promising fields of quantum computation and
quantum information, we hope our review will be helpful to researchers in these
fields.


title:
The laplacian of a graph as a density matrix: a basic combinatorial
  approach to separability of mixed states

abstract:
  We study entanglement properties of mixed density matrices obtained from
combinatorial Laplacians. This is done by introducing the notion of the density
matrix of a graph. We characterize the graphs with pure density matrices and
show that the density matrix of a graph can be always written as a uniform
mixture of pure density matrices of graphs. We consider the von Neumann entropy
of these matrices and we characterize the graphs for which the minimum and
maximum values are attained. We then discuss the problem of separability by
pointing out that separability of density matrices of graphs does not always
depend on the labelling of the vertices. We consider graphs with a tensor
product structure and simple cases for which combinatorial properties are
linked to the entanglement of the state. We calculate the concurrence of all
graph on four vertices representing entangled states. It turns out that for
some of these graphs the value of the concurrence is exactly fractional.


title:
Contextuality for preparations, transformations, and unsharp
  measurements

abstract:
  An operational definition of contextuality is introduced which generalizes
the standard notion in three ways: (1) it applies to arbitrary operational
theories rather than just quantum theory, (2) it applies to arbitrary
experimental procedures, rather than just sharp measurements, and (3) it
applies to a broad class of ontological models of quantum theory, rather than
just deterministic hidden variable models. We derive three no-go theorems for
ontological models, each based on an assumption of noncontextuality for a
different sort of experimental procedure; one for preparation procedures,
another for unsharp measurement procedures (that is, measurement procedures
associated with positive-operator valued measures), and a third for
transformation procedures. All three proofs apply to two-dimensional Hilbert
spaces, and are therefore stronger than traditional proofs of contextuality.


title: The conformally invariant Laplace-Beltrami operator and factor ordering

abstract:
  In quantum mechanics the kinetic energy term for a single particle is usually
written in the form of the Laplace-Beltrami operator. This operator is a factor
ordering of the classical kinetic energy. We investigate other relatively
simple factor orderings and show that the {\it only} other solution for a
conformally flat metric is the conformally invariant Laplace-Beltrami operator.
For non-conformally-flat metrics this type of factor ordering fails, by just
one term, to give the conformally invariant Laplace-Beltrami operator.


title: Entanglement in the stabilizer formalism

abstract:
  We define a multi-partite entanglement measure for stabilizer states, which
can be computed efficiently from a set of generators of the stabilizer group.
Our measure applies to qubits, qudits and continuous variables.


title: A New Relation between post and pre-optimal measurement states

abstract:
  When an optimal measurement is made on a qubit and what we call an Unbiased
Mixture of the resulting ensembles is taken, then the post measurement density
matrix is shown to be related to the pre-measurement density matrix through a
simple and linear relation. It is shown that such a relation holds only when
the measurements are made in Mutually Unbiased Bases- MUB. For Spin-1/2 it is
also shown explicitly that non-orthogonal measurements fail to give such a
linear relation no matter how the ensembles are mixed. The result has been
proved to be true for arbitrary quantum mechanical systems of finite
dimensional Hilbert spaces. The result is true irrespective of whether the
initial state is pure or mixed.