title: Optimal control of time-dependent targets

abstract:
  In this work, we investigate how and to which extent a quantum system can be
driven along a prescribed path in Hilbert space by a suitably shaped laser
pulse. To calculate the optimal, i.e., the variationally best pulse, a properly
defined functional is maximized. This leads to a monotonically convergent
algorithm which is computationally not more expensive than the standard
optimal-control techniques to push a system, without specifying the path, from
a given initial to a given final state. The method is successfully applied to
drive the time-dependent density along a given trajectory in real space and to
control the time-dependent occupation numbers of a two-level system and of a
one-dimensional model for the hydrogen atom.


title: Simulatable security for quantum protocols

abstract:
  The notion of simulatable security (reactive simulatability, universal
composability) is a powerful tool for allowing the modular design of
cryptographic protocols (composition of protocols) and showing the security of
a given protocol embedded in a larger one. Recently, these methods have
received much attention in the quantum cryptographic community.
  We give a short introduction to simulatable security in general and proceed
by sketching the many different definitional choices together with their
advantages and disadvantages.
  Based on the reactive simulatability modelling of Backes, Pfitzmann and
Waidner we then develop a quantum security model. By following the BPW
modelling as closely as possible, we show that composable quantum security
definitions for quantum protocols can strongly profit from their classical
counterparts, since most of the definitional choices in the modelling are
independent of the underlying machine model.
  In particular, we give a proof for the simple composition theorem in our
framework.


title: Einstein's Boxes: Quantum Mechanical Solution

abstract:
  In this paper we present a solution of the Einstein's boxes paradox by modern
Quantum Mechanics in which a notion of density matrix is equivalent to a notion
of a quantum state of a system. We use a secondary quantization formalism in
the attempt to make a description particularly clear. The aim of this paper is
to provide pedagogical help to the students of quantum mechanics.


title:
Noise resistance of adiabatic quantum computation using random matrix
  theory

abstract:
  Besides the traditional circuit-based model of quantum computation, several
quantum algorithms based on a continuous-time Hamiltonian evolution have
recently been introduced, including for instance continuous-time quantum walk
algorithms as well as adiabatic quantum algorithms. Unfortunately, very little
is known today on the behavior of these Hamiltonian algorithms in the presence
of noise. Here, we perform a fully analytical study of the resistance to noise
of these algorithms using perturbation theory combined with a theoretical noise
model based on random matrices drawn from the Gaussian Orthogonal Ensemble,
whose elements vary in time and form a stationary random process.


title:
On the representation of contextual probabilistic dynamics in the
  complex Hilbert space: linear and nonlinear evolutions, Schroedinger dynamics

abstract:
  We constructed the representation of contextual probabilistic dynamics in the
complex Hilbert space. Thus dynamics of the wave function can be considered as
Hilbert space projections of realistic dynamics in a ``prespace''. The basic
condition for representing of the prespace-dynamics is the law of statistical
conservation of energy -- conservation of probabilities. Construction of the
dynamical representation is an important step in the development of contextual
statistical viewpoint to quantum processes. But the contextual statistical
model is essentially more general than the quantum one. Therefore in general
the Hilbert space projection of the ``prespace'' dynamics can be nonlinear and
even irreversible (but it is always unitary). There were found conditions of
linearity and reversibility of the Hilbert space dynamical projection. We also
found conditions for the conventional Schroedinger dynamics (including
time-dependent Hamiltonians). We remark that in general even the Schroedinger
dynamics is based just on the statistical conservation of energy; for
individual systems the law of conservation of energy can be violated (at least
in our theoretical model).


title: Comment on "Bell's Theorem without Inequalities and without Alignments"

abstract:
  In this Comment we show that Cabello's proof of Bell's theorem without
inequalities [Phys. Rev. Lett. 91, 230403 (2003)] does not exhibit two of the
three "remarkable properties" which the proof is claimed to possess. More
precisely it is our purpose to show that property (c) of Cabello's proof,
stating that ``local observables can be measured by means of tests on
individual qubits" is definitely false since it has been obtained by a wrong
use of the basic rules of the quantum formalism, while property (b), stating
that ``distant local setups do not need to be aligned since the require perfect
correlations are achieved for any local rotation of the setups", is also wrong
due to the failure of validity of property (c).


title:
Quantum X-waves in Kerr media and the progressive undistorted squeezed
  vacuum

abstract:
  A quantum theory of 3D X-shaped optical bullets in Kerr media is presented.
The existence of progressive undistorted squeezed vacuum is predicted.
Applications to quantum non-demolition experiments, entanglement and
interferometers for gravitational waves detection are envisaged.


title: Web-assisted tunneling in the kicked harmonic oscillator

abstract:
  We show that heating of harmonically trapped ions by periodic delta kicks is
dramatically enhanced at isolated values of the Lamb-Dicke parameter. At these
values, quasienergy eigenstates localized on island structures undergo avoided
crossings with extended web-states.


title: Two measurement settings can suffice to verify multipartite entanglement

abstract:
  We present entanglement witnesses for detecting genuine multi-qubit
entanglement. Our constructions are robust against noise and require only two
local measurement settings, independent of the number of qubits. Thus they
allow to verify entanglement of many qubits in experiments while requiring only
a small effort. In contrast, usual methods need an effort which increases
exponentially with the number of qubits. The witnesses detect states close to
GHZ states and cluster states.


title: Entanglement vs. the quantum-to-classical transition

abstract:
  We analyze the quantum-to-classical transition (QCT) for coupled bipartite
quantum systems for which the position of one of the two subsystems is
continuously monitored. We obtain the surprising result that the QCT can emerge
concomitantly with the presence of highly entangled states in the bipartite
system. Furthermore the changing degree of entanglement is associated with the
back-action of the measurement on the system and is itself an indicator of the
QCT. Our analysis elucidates the role of entanglement in von Neumann's paradigm
of quantum measurements comprised of a system and a monitored measurement
apparatus.


title:
Quantum Communication Through a Spin-Ring with Twisted Boundary
  Conditions

abstract:
  We investigate quantum communication between the sites of a spin-ring with
twisted boundary conditions. Such boundary conditions can be achieved by a flux
through the ring. We find that a non-zero twist can improve communication
through finite odd numbered rings and enable high fidelity multi-party quantum
communication through spin rings (working near perfectly for rings of 5 and 7
spins). We show that in certain cases, the twist results in the complete
blockage of quantum information flow to a certain site of the ring. This effect
can be exploited to interface and entangle a flux qubit and a spin qubit
without embedding the latter in a magnetic field.


title: Equivalence of Decoupling Schemes and Orthogonal Arrays

abstract:
  We consider the problem of switching off unwanted interactions in a given
multi-partite Hamiltonian. This is known to be an important primitive in
quantum information processing and several schemes have been presented in the
literature to achieve this task. A method to construct decoupling schemes for
quantum systems of pairwise interacting qubits was introduced by M.
Stollsteimer and G. Mahler and is based on orthogonal arrays. Another approach
based on triples of Hadamard matrices that are closed under pointwise
multiplication was proposed by D. Leung. In this paper, we show that both
methods lead to the same class of decoupling schemes. Moreover, we establish a
characterization of orthogonal arrays by showing that they are equivalent to
decoupling schemes which allow a refinement into equidistant time-slots.
Furthermore, we show that decoupling schemes for networks of higher-dimensional
quantum systems with t-local Hamiltonians can be constructed from classical
error-correcting codes.


title: Controlled Unitary Operation between Two Distant Atoms

abstract:
  We propose a scheme for implementing a controlled unitary gate between two
distant atoms directly communicating through a quantum transmission line. To
achieve our goal, only a series of several coherent pulses are applied to the
atoms. Our scheme thus requires no ancilla atomic qubit. The simplicity of our
scheme may significantly improve the scalability of quantum computers based on
trapped neutral atoms or ions.


title: Quasilinear Approach to Summation of the WKB Series

abstract:
  It is shown that the quasilinearization method (QLM) sums the WKB series. The
method approaches solution of the Riccati equation (obtained by casting the
Schr\"{o}dinger equation in a nonlinear form) by approximating the nonlinear
terms by a sequence of the linear ones, and is not based on the existence of a
smallness parameter. Each p-th QLM iterate is expressible in a closed integral
form. Its expansion in powers of $\hbar$ reproduces the structure of the WKB
series generating an infinite number of the WKB terms. Coefficients of the
first $2^p$ terms of the expansion are exact while coefficients of a similar
number of the next terms are approximate. The quantization condition in any QLM
iteration, including the first, leads to exact energies for many well known
physical potentials such as the Coulomb, harmonic oscillator,
P\"{o}schl-Teller, Hulthen, Hyleraas, Morse, Eckart etc.


title:
Entropy, time irreversibility and Schroedinger equation in a primarily
  discrete space-time

abstract:
  In this paper we show that the existence of a primarily discrete space-time
may be a fruitful assumption from which we may develop a new approach of
statistical thermodynamics in pre-relativistic conditions. The discreetness of
space-time structure is determined by a condition that mimics the Heisenberg
uncertainty relations and the motion in this space-time model is chosen as
simple as possible. From these two assumptions we define a path-entropy that
measures the number of closed paths associated with a given energy of the
system preparation. This entropy has a dynamical character and depends on the
time interval on which we count the paths. We show that it exists an
like-equilibrium condition for which the path-entropy corresponds exactly to
the usual thermodynamic entropy and, more generally, the usual statistical
thermodynamics is reobtained. This result derived without using the Gibbs
ensemble method shows that the standard thermodynamics is consistent with a
motion that is time-irreversible at a microscopic level. From this change of
paradigm it becomes easy to derive a $H-theorem$. A comparison with the
traditional Boltzmann approach is presented. We also show how our approach can
be implemented in order to describe reversible processes. By considering a
process defined simultaneously by initial and final conditions a well defined
stochastic process is introduced and we are able to derive a Schroedinger
equation, an example of time reversible equation.


title: Causality and Localization Operators

abstract:
  The evolution of the expectation values of one and two points scalar field
operators and of positive localization operators, generated by an istantaneous
point source is non local. Non locality is attributed either to zero point
vacuum fluctuations, or to non local operations or to the microcausality
principle being no satisfied.


title: Separability Criteria from Uncertainty Relations

abstract:
  We explain several separability criteria which rely on uncertainty relations.
For the derivation of these criteria uncertainty relations in terms of
variances or entropies can be used. We investigate the strength of the
separability conditions for the case of two qubits and show how they can
improve entanglement witnesses.


title:
Wave functions in the neighborhood of a toroidal surface; hard vs. soft
  constraint

abstract:
  The curvature potential arising from confining a particle initially in
three-dimensional space onto a curved surface is normally derived in the hard
constraint $q \to 0$ limit, with $q$ the degree of freedom normal to the
surface. In this work the hard constraint is relaxed, and eigenvalues and wave
functions are numerically determined for a particle confined to a thin layer in
the neighborhood of a toroidal surface. The hard constraint and finite layer
(or soft constraint) quantities are comparable, but both differ markedly from
those of the corresponding two dimensional system, indicating that the
curvature potential continues to influence the dynamics when the particle is
confined to a finite layer. This effect is potentially of consequence to the
modelling of curved nanostructures.


title: Practical Implementations of Twirl Operations

abstract:
  Twirl operations, which convert impure singlet states into Werner states,
play an important role in many schemes for entanglement purification. In this
paper we describe strategies for implementing twirl operations, with an
emphasis on methods suitable for ensemble quantum information processors such
as nuclear magnetic resonance (NMR) quantum computers. We implement our twirl
operation on a general two-spin mixed state using liquid state NMR techniques,
demonstrating that we can obtain the singlet Werner state with high fidelity.


title: The Cat oRules

abstract:
  The oRules of state reduction are applied to the case of the Schrodinger cat
experiment. It is shown that these rules can unambiguously describe the
conscious state of the cat, as well as an outside observer at any time during
the experiment. Two versions of the experiment are considered. In version I,
the conscious cat is made unconscious by a mechanism that is triggered by a
radioactive decay. In version II, the sleeping cat is made conscious by an
alarm clock that is triggered by a radioactive decay.


title:
Properties of the frequency operator do not imply the quantum
  probability postulate

abstract:
  We review the properties of the frequency operator for an infinite number of
systems and disprove claims in the literature that the quantum probability
postulate can be derived from these properties.


title:
Quantum and Classical Effects in the Two-Frequency Kicked Rotor with
  Variable Initial Phase

abstract:
  We present an investigation into effects exhibited by the two-frequency
kicked rotor. Experiments were performed and in addition quantum and classical
dynamics were simulated and compared with the experimental results. The
experiments involved pulsing the optical standing wave with two pulsing periods
of differing frequencies and variable initial phase offset. The ratio of
pulsing periods was sampled for rational and irrational values for different
experimental runs. In this paper we present these results and examine the
measured momentum distributions for the cause of any structures that are seen
in the energy as the initial phase offset is changed. Irrational ratios exhibit
no significant quantum effects, whereas rational ratios show dynamical
localisation (DL) for certain values of the initial phase. However, most of the
observed structure is found to be due to classical effects, in particular KAM
boundaries, and is therefore not of uniquely quantum origin.


title: Strong Analytic Controllability for Hydrogen Control Systems

abstract:
  The realization and representation of so(4,2) associated with the hydrogen
atom Hamiltonian are derived. By choosing operators from the realization of
so(4,2) as interacting Hamiltonians, a hydrogen atom control system is
constructed, and it is proved that this control system is strongly analytically
controllable based on a time-dependent strong analytic controllability theorem.


title: Analytic Controllability of Time-Dependent Quantum Control Systems

abstract:
  The question of controllability is investigated for a quantum control system
in which the Hamiltonian operator components carry explicit time dependence
which is not under the control of an external agent. We consider the general
situation in which the state moves in an infinite-dimensional Hilbert space, a
drift term is present, and the operators driving the state evolution may be
unbounded. However, considerations are restricted by the assumption that there
exists an analytic domain, dense in the state space, on which solutions of the
controlled Schrodinger equation may be expressed globally in exponential form.
The issue of controllability then naturally focuses on the ability to steer the
quantum state on a finite-dimensional submanifold of the unit sphere in Hilbert
space -- and thus on analytic controllability. A relatively straightforward
strategy allows the extension of Lie-algebraic conditions for strong analytic
controllability derived earlier for the simpler, time-independent system in
which the drift Hamiltonian and the interaction Hamiltonia have no intrinsic
time dependence. Enlarging the state space by one dimension corresponding to
the time variable, we construct an augmented control system that can be treated
as time-independent. Methods developed by Kunita can then be implemented to
establish controllability conditions for the one-dimension-reduced system
defined by the original time-dependent Schrodinger control problem. The
applicability of the resulting theorem is illustrated with selected examples.


title: Coherent inelastic backscattering of intense laser light by cold atoms

abstract:
  We present a nonperturbative treatment of coherent backscattering of intense
laser light from cold atoms, and predict a nonvanishing backscattering signal
even at very large intensities, due to the constructive (self-)interference of
inelastically scattered photons.


title:
Position and Momentum Entanglement of Dipole-Dipole Interacting Atoms in
  Optical Lattices: The Einstein-Podolsky-Rosen Paradox on a Lattice

abstract:
  We study a possible realization of the position- and momentum-correlated
atomic pairs that are confined to adjacent sites of two mutually shifted
optical lattices and are entangled via laser-induced dipole-dipole
interactions. The Einstein-Podolsky-Rosen (EPR) ``paradox'' [Phys. Rev. 47, 777
(1935)] with translational variables is then modified by lattice-diffraction
effects. This ``paradox'' can be verified to a high degree of accuracy in this
scheme.


title:
Quantum-Information Content of Fractional Occupation Probabilities in
  Nuclei

abstract:
  Three measures of the information content of a probability distribution are
briefly reviewed. They are applied to fractional occupation probabilities in
light nuclei, taking into account short-range correlations. The effect of
short-range correlations is to increase the information entropy (or disorder)
of nuclei, comparing with the independent particle model. It is also indicated
that the information entropy can serve as a sensitive index of order and
short-range correlations in nuclei. It is concluded that increasing $Z$, the
information entropy increases i.e. the disorder of the nucleus increases for
all measures of information considered in the present work.


title:
Exact solution of Schrodinger equation with deformed Ring-Shaped
  Potential

abstract:
  Exact solution of the Schrodinger equation with deformed ring shaped
potential is obtained in the parabolic and spherical coordinates. The
Nikiforov-Uvarov method is used in the solution. Eigenfunctions and
corresponding energy eigenvalues are calculated analytically. The agreement of
our results is good.


title: A product formula and combinatorial field theory

abstract:
  We treat the problem of normally ordering expressions involving the standard
boson operators a, a* where [a,a*]=1. We show that a simple product formula for
formal power series - essentially an extension of the Taylor expansion - leads
to a double exponential formula which enables a powerful graphical description
of the generating functions of the combinatorial sequences associated with such
functions - in essence, a combinatorial field theory. We apply these techniques
to some examples related to specific physical Hamiltonians.


title:
A comparative study of relative entropy of entanglement, concurrence and
  negativity

abstract:
  The problem of ordering of two-qubit states imposed by relative entropy of
entanglement (E) in comparison to concurrence (C) and negativity (N) is
studied. Analytical examples of states consistently and inconsistently ordered
by the entanglement measures are given. In particular, the states for which any
of the three measures imposes order opposite to that given by the other two
measures are described. Moreover, examples are given of pairs of the states,
for which (i) N'=N'' and C'=C'' but E' is different from E'', (ii) N'=N'' and
E'=E'' but C' differs from C'', (iii) E'=E'', N'<N'' and C'>C'', or (iv) states
having the same E, C, and N but still violating the
Bell-Clauser-Horne-Shimony-Holt inequality to different degrees.


title: Dynamical creation of entanglement by homodyne-mediated feedback

abstract:
  For two two-level atoms coupled to a single-mode cavity field that is driven
and heavily damped, the steady-state can be entangled by shining an
un-modulated driving laser on the system [S.Schneider, G. J. Milburn Phys. Rev
A 65, 042107, 2002]. We present a scheme to significantly increase the
steady-state entanglement by using homodyne-mediated feedback, in which the
driving laser is modulated by the homodyne photocurrent derived from the cavity
output. Such feedback can increase the nonlinear response to both the
decoherence process of the two-qubit system and the coherent evolution of
individual qubits. We present the properties of the entangled states using the
SO(3) Q function.


title: Orientations of two coupled molecules

abstract:
  Orientation states of two coupled polar molecules controlled by laser pulses
are studied theoretically. By varying the period of a series of periodically
applied laser pulse, transition from regular to chaotic behavior may occur.
Schmidt decomposition is used to measure the degree of entanglement. It is
found that the entanglement can be enhanced by increasing the strength of laser
pulse.


title: Optimizing linear optics quantum gates

abstract:
  In this paper, the problem of finding optimal success probabilities of static
linear optics quantum gates is linked to the theory of convex optimization. It
is shown that by exploiting this link, upper bounds for the success probability
of networks realizing single-mode gates can be derived, which hold in
generality for linear optical networks followed by postselection, i.e., for
networks of arbitrary size, any number of auxiliary modes, and arbitrary photon
numbers. As a corollary, the previously formulated conjecture is proven that
the optimal success probability of a postselected non-linear sign shift without
feed-forward is 1/4, a gate playing the central role in the scheme of
Knill-Laflamme-Milburn for quantum computation with linear optics. The concept
of Lagrange duality is shown to be applicable to provide rigorous proofs for
such bounds for elementary gates, although the original problem is a difficult
non-convex problem in infinitely many objective variables. The versatility of
this approach to identify other optimal linear optical schemes is demonstrated.


title: Entanglement in Random Subspaces

abstract:
  The selection of random subspaces plays a role in quantum information theory
analogous to the role of random strings in classical information theory. Recent
applications have included protocols achieving the quantum channel capacity and
methods for extending superdense coding from bits to qubits. In addition,
random subspaces have proved useful for studying the structure of bipartite and
multipartite entanglement.


title: Chain Teleportation

abstract:
  By means of the idea of measurements on the crossed space-time nonlocal
observables, we extend the mechanism for the two-way quantum teleportation to
the chain teleportation among N spatially separated spin-1/2 systems. Since in
the process only the local interactions are used, the microcausality is
automatically satisfied.


title:
Verification of polarization selection rules and implementation of
  selective coherent manipulations of hydrogenic transitions in n-GaAs

abstract:
  Electrons bound to shallow donors in GaAs have orbital energy levels
analogous to those of the hydrogen atom. The polarization selection rules for
optical transitions between the states analogous to the 1s and 2p states of
hydrogen in a magnetic field are verified using Terahertz (THz) radiation from
the UCSB Free Electron Laser. A polarization-selective coherent manipulation of
the quantum states is demonstrated and the relevance to quantum information
processing schemes is discussed.


title: General limit to non-destructive optical detection of atoms

abstract:
  We demonstrate that there is a fundamental limit to the sensitivity of
phase-based detection of atoms with light for a given maximum level of
allowable spontaneous emission. This is a generalisation of previous results
for two-level and three-level atoms. The limit is due to an upper bound on the
phase shift that can be imparted on a laser beam for a given excited state
population. Specifially, we show that no single-pass optical technique using
classical light, based on any number of lasers or coherences between any number
of levels, can exceed the limit imposed by the two-level atom. This puts
significant restrictions on potential non-destructive optical measurement
schemes.


title: Oscillator model for dissipative QED in an inhomogeneous dielectric

abstract:
  The Ullersma model for the damped harmonic oscillator is coupled to the
quantised electromagnetic field. All material parameters and interaction
strengths are allowed to depend on position. The ensuing Hamiltonian is
expressed in terms of canonical fields, and diagonalised by performing a
normal-mode expansion. The commutation relations of the diagonalising operators
are in agreement with the canonical commutation relations. For the proof we
replace all sums of normal modes by complex integrals with the help of the
residue theorem. The same technique helps us to explicitly calculate the
quantum evolution of all canonical and electromagnetic fields. We identify the
dielectric constant and the Green function of the wave equation for the
electric field. Both functions are meromorphic in the complex frequency plane.
The solution of the extended Ullersma model is in keeping with well-known
phenomenological rules for setting up quantum electrodynamics in an absorptive
and spatially inhomogeneous dielectric. To establish this fundamental
justification, we subject the reservoir of independent harmonic oscillators to
a continuum limit. The resonant frequencies of the reservoir are smeared out
over the real axis. Consequently, the poles of both the dielectric constant and
the Green function unite to form a branch cut. Performing an analytic
continuation beyond this branch cut, we find that the long-time behaviour of
the quantised electric field is completely determined by the sources of the
reservoir. Through a Riemann-Lebesgue argument we demonstrate that the field
itself tends to zero, whereas its quantum fluctuations stay alive. We argue
that the last feature may have important consequences for application of
entanglement and related processes in quantum devices.


title: Efficient and spectrally bright source of polarization-entangled photons

abstract:
  We demonstrate an efficient fiber-coupled source of nondegenerate
polarization entangled photons at 795 and 1609 nm using bidirectionally pumped
parametric down-conversion in bulk periodically poled lithium niobate. The
single-mode source has an inferred bandwidth of 50 GHz and a spectral
brightness of 300 pairs/s/GHz/mW of pump power that is suitable for narrowband
applications such as entanglement transfer from photonic to atomic qubits.


title: Chaos assisted adiabatic passage

abstract:
  We study the exact dynamics underlying stimulated Raman adiabatic passage
(STIRAP) for a particle in a multi-level anharmonic system (the infinite
square-well) driven by two sequential laser pulses, each with constant carrier
frequency. In phase space regions where the laser pulses create chaos, the
particle can be transferred coherently into energy states different from those
predicted by traditional STIRAP. It appears that a transition to chaos can
provide a new tool to control the outcome of STIRAP.


title:
Entangled and disentangled evolution for a single atom in a driven
  cavity

abstract:
  For an atom in an externally driven cavity, we show that special initial
states lead to near-disentangled atom-field evolution, and superpositions of
these can lead to near maximally-entangled states. Somewhat counterintutively,
we find that (moderate) spontaneous emission in this system actually leads to a
transient increase in entanglement beyond the steady-state value. We also show
that a particular field correlation function could be used, in an experimental
setting, to track the time evolution of this entanglement.


title: Standing waves in the Lorentz-covariant world

abstract:
  When Einstein formulated his special relativity, he developed his dynamics
for point particles. Of course, many valiant efforts have been made to extend
his relativity to rigid bodies, but this subject is forgotten in history. This
is largely because of the emergence of quantum mechanics with wave-particle
duality. Instead of Lorentz-boosting rigid bodies, we now boost waves and have
to deal with Lorentz transformations of waves. We now have some understanding
of plane waves or running waves in the covariant picture, but we do not yet
have a clear picture of standing waves. In this report, we show that there is
one set of standing waves which can be Lorentz-transformed while being
consistent with all physical principle of quantum mechanics and relativity. It
is possible to construct a representation of the Poincar\'e group using
harmonic oscillator wave functions satisfying space-time boundary conditions.
This set of wave functions is capable of explaining the quantum bound state for
both slow and fast hadrons. In particular it can explain the quark model for
hadrons at rest, and Feynman's parton model hadrons moving with a speed close
to that of light.


title:
Effects of staggered magnetic field on entanglement in the anisotropic
  XY model

abstract:
  We investigate effects of staggered magnetic field on thermal entanglement in
the anisotropic XY model. The analytic results of entanglement for the two-site
cases are obtained. For the general case of even sites, we show that when the
anisotropic parameter is zero, the entanglement in the XY model with a
staggered magnetic field is the same as that with a uniform magnetic field.


title: Initial correlations effects on decoherence at zero temperature

abstract:
  We consider a free charged particle interacting with an electromagnetic bath
at zero temperature. The dipole approximation is used to treat the bath
wavelengths larger than the width of the particle wave packet. The effect of
these wavelengths is described then by a linear Hamiltonian whose form is
analogous to phenomenological Hamiltonians previously adopted to describe the
free particle-bath interaction. We study how the time dependence of decoherence
evolution is related with initial particle-bath correlations. We show that
decoherence is related to the time dependent dressing of the particle. Moreover
because decoherence induced by the T=0 bath is very rapid, we make some
considerations on the conditions under which interference may be experimentally
observed.


title:
Geometric phase accumulation-based effects in the quantum dynamics of an
  anisotropically trapped ion

abstract:
  New physical effects in the dynamics of an ion confined in an anisotropic
two-dimensional Paul trap are reported. The link between the occurrence of such
manifestations and the accumulation of geometric phase stemming from the
intrinsic or controlled lack of symmetry in the trap is brought to light. The
possibility of observing in laboratory these anisotropy-based phenomena is
briefly discussed.


title: On supersymmetric quantum mechanics

abstract:
  This paper constitutes a review on N=2 fractional supersymmetric Quantum
Mechanics of order k. The presentation is based on the introduction of a
generalized Weyl-Heisenberg algebra W_k. It is shown how a general Hamiltonian
can be associated with the algebra W_k. This general Hamiltonian covers various
supersymmetrical versions of dynamical systems (Morse system, Poschl-Teller
system, fractional supersymmetric oscillator of order k, etc.). The case of
ordinary supersymmetric Quantum Mechanics corresponds to k=2. A connection
between fractional supersymmetric Quantum Mechanics and ordinary supersymmetric
Quantum Mechanics is briefly described. A realization of the algebra W_k, of
the N=2 supercharges and of the corresponding Hamiltonian is given in terms of
deformed-bosons and k-fermions as well as in terms of differential operators.


title:
Bounds on the entanglability of thermal states in liquid-state nuclear
  magnetic resonance

abstract:
  The role of mixed state entanglement in liquid-state nuclear magnetic
resonance (NMR) quantum computation is not yet well-understood. In particular,
despite the success of quantum information processing with NMR, recent work has
shown that quantum states used in most of those experiments were not entangled.
This is because these states, derived by unitary transforms from the thermal
equilibrium state, were too close to the maximally mixed state. We are thus
motivated to determine whether a given NMR state is entanglable - that is, does
there exist a unitary transform that entangles the state? The boundary between
entanglable and nonentanglable thermal states is a function of the spin system
size $N$ and its temperature $T$. We provide new bounds on the location of this
boundary using analytical and numerical methods; our tightest bound scales as
$N \sim T$, giving a lower bound requiring at least $N \sim 22,000$ proton
spins to realize an entanglable thermal state at typical laboratory NMR
magnetic fields. These bounds are tighter than known bounds on the
entanglability of effective pure states.


title: Non-linear photonic crystals as a source of entangled photons

abstract:
  Non-linear photonic crystals can be used to provide phase-matching for
frequency conversion in optically isotropic materials. The phase-matching
mechanism proposed here is a combination of form birefringence and phase
velocity dispersion in a periodic structure. Since the phase-matching relies on
the geometry of the photonic crystal, it becomes possible to use highly
non-linear materials. This is illustrated considering a one-dimensional
periodic Al$_{0.4}$Ga$_{0.6}$As / air structure for the generation of 1.5
$\mu$m light. We show that phase-matching conditions used in schemes to create
entangled photon pairs can be achieved in photonic crystals.


title: On the Complementarity Principle and the Uncertainty Principle

abstract:
  We present a scheme in which we investigate the two-slit experiment and we
show that the principle of complementarity is more fundamental then the
uncertainty principle.


title: Extended Quantum Color Coding

abstract:
  The quantum color coding scheme proposed by Korff and Kempe
(quant-ph/0405086) is easily extended so that the color coding quantum system
is allowed to be entangled with an extra auxiliary quantum system. It is shown
that in the extended scheme we need only $\sim 2\sqrt{N}$ quantum colors to
order $N$ objects in large $N$ limit, whereas $\sim N/e$ quantum colors are
required in the original non-extended version. The maximum success probability
has asymptotics expressed by the Tracy-Widom distribution of the largest
eigenvalue of a random GUE matrix.


title: Deterministic Local Conversion of Incomparable States by Collective LOCC

abstract:
  Incomparability of pure bipartite entangled states under deterministic LOCC
is a very strange phenomena. We find two possible ways of getting our desired
pure entangled state which is incomparable with the given input state, by
collective LOCC with certainty. The first one is by providing some pure
entanglement through the lower dimensional maximally-entangled states or using
further less amount of entanglement and the next one is by collective operation
on two pairs which are individually incomparable. It is quite surprising that
we are able to achieve maximally entangled states of any Schmidt rank from a
finite number of $2\times 2$ pure entangled states only by deterministic LOCC.
We provide general theory for the case of $3\times3$ system of incomparable
states by the above processes where incomparability seems to be the most
hardest one.


title:
Algebraic Representation of Many-Particle Coulomb Green Function and
  Application in Atomic Calculations

abstract:
  Basing on the relation between the Coulomb Green function and the Green
function of harmonic oscillator, the algebraic representation of the
many-particle Coulomb Green function in the form of annihilation and creation
operators is established. These results allow us to construct purely algebraic
method for atomic calculations and thus to reduce rather complicated
calculations of matrix elements to algebraic procedures of transforming the
products of annihilation and creation operators to a normal form. Effectiveness
of the constructed method is demonstrated through the example problem: the
ground state of hydrogenic molecule. The calculation algorithm of this
algebraic approach is simple and suitable for symbolic calculation programs,
such as Mathematica, that widely enlarge the application field of the Coulomb
Green function

categories: quant-ph physics.atom-ph physics.chem-ph physics.comp-ph


title:
The improvement wave equations of relativistic and non-relativistic
  quantum mechanics

abstract:
  In this work, we follow the idea of the De Broglie's matter waves and the
analogous method that Schr\"{o}dinger founded wave equation, but we apply the
more essential Hamilton principle instead of the minimum action principle of
Jacobi which was used in setting up Schr\"{o}dinger wave equation. Thus, we
obtain a novel non-relativistic wave equation which is different from the
Schr\"{o}dinger equation, and relativistic wave equation including free and
non-free particle. In addition, we get the spin 1/2 particle wave equation in
potential field.


title: A q-deformed Aufbau Prinzip

abstract:
  A building principle working for both atoms and monoatomic ions is proposed
in this Letter. This principle relies on the q-deformed chain SO(4) > G where G
= SO(3)_q.


title: Reply to Comment on "Quantum Measurement and Decoherence"

abstract:
  While agreeing with our exact expression for the time dependence of the
motion of a free particle in an initial superposition state, corresponding to
two identical Gaussians separated by a distance $d$, at temperature $T$, Gobert
et al., in the preceding Comment [Phys. Rev. A xxx], dispute our conclusions on
decoherence time scales. However, the parameters they used to generate their
figures are outside the regime of validity of our interpretation of the results
and, moreover, are not of physical interest in that they correspond to
$T\approx 0$. The point is that in their figures they have chosen the thermal
de Broglie wavelength $\lambda_{th}=\hbar \sqrt{mkT}$ to be equal to slit
spacing \emph{d}, whereas we have clearly stated [in the paragraph preceding
Eq. (21) of our paper] that decoherence occurs and that our expression for the
decoherence time applies only in the limit where \emph{d} is large compared not
only with the slit width $\sigma $ but also with the thermal de Broglie
wavelength, $d\gg \lambda_{th},\sigma $.


title:
Superconducting Qubits Coupled to Nanoelectromechanical Resonators: An
  Architecture for Solid-State Quantum Information Processing

abstract:
  We describe the design for a scalable, solid-state
quantum-information-processing architecture based on the integration of
GHz-frequency nanomechanical resonators with Josephson tunnel junctions, which
has the potential for demonstrating a variety of single- and multi-qubit
operations critical to quantum computation. The computational qubits are
eigenstates of large-area, current-biased Josephson junctions, manipulated and
measured using strobed external circuitry. Two or more of these phase qubits
are capacitively coupled to a high-quality-factor piezoelectric
nanoelectromechanical disk resonator, which forms the backbone of our
architecture, and which enables coherent coupling of the qubits. The integrated
system is analogous to one or more few-level atoms (the Josephson junction
qubits) in an electromagnetic cavity (the nanomechanical resonator). However,
unlike existing approaches using atoms in electromagnetic cavities, here we can
individually tune the level spacing of the ``atoms'' and control their
``electromagnetic'' interaction strength. We show theoretically that quantum
states prepared in a Josephson junction can be passed to the nanomechanical
resonator and stored there, and then can be passed back to the original
junction or transferred to another with high fidelity. The resonator can also
be used to produce maximally entangled Bell states between a pair of Josephson
junctions. Many such junction-resonator complexes can assembled in a
hub-and-spoke layout, resulting in a large-scale quantum circuit. Our proposed
architecture combines desirable features of both solid-state and cavity quantum
electrodynamics approaches, and could make quantum information processing
possible in a scalable, solid-state environment.


title:
Large Quantum imaging of nonlocal spatial correlations induced by
  orbital angular momentum

abstract:
  Through scanned coincidence counting, we probe the quantum image produced by
parametric down conversion with a pump beam carrying orbital angular momentum.
Nonlocal spatial correlations are manifested through splitting of the
coincidence spot into two.


title: Multiplicativity properties of entrywise positive maps

abstract:
  Multiplicativity of certain maximal p -> q norms of a tensor product of
linear maps on matrix algebras is proved in situations in which the condition
of complete positivity (CP) is either augmented by, or replaced by, the
requirement that the entries of a matrix representative of the map are
non-negative (EP). In particular, for integer t, multiplicativity holds for the
maximal 2 -> 2t norm of a product of two maps, whenever one of the pair is EP;
for the maximal 1 -> t norm for pairs of CP maps when one of them is also EP;
and for the maximal 1 -> 2t norm for the product of an EP and a 2-positive map.
Similar results are shown in the infinite-dimensional setting of convolution
operators on L^2(R), with the pointwise positivity of an integral kernel
replacing entrywise positivity of a matrix. These results apply in particular
to Gaussian bosonic channels.


title: Criteria of partial separability of multipartite qubit mixed-states

abstract:
  In this paper, we discuss the partial separability and its criteria problems
of multipartite qubit mixed-states. First we strictly define what is the
partial separability of a multipartite qubit system. Next we give a reduction
way from N-partite qubit density matrixes to bipartite qubit density matrixes,
and prove a necessary condition that a N-partite qubit mixed-state to be
partially separable is its reduction to satisfy the PPT condition.
  PACC numbers: 03.67.Mn; 03.65.Ud; 03.67.Hk


title: Squeezing and entanglement delay using slow light

abstract:
  We examine the interaction of a weak probe with $N$ atoms in a lambda-level
configuration under the conditions of electromagnetically induced transparency
(EIT). In contrast to previous works on EIT, we calculate the output state of
the resultant slowly propagating light field while taking into account the
effects of ground state dephasing and atomic noise for a more realistic model.
In particular, we propose two experiments using slow light with a nonclassical
probe field and show that two properties of the probe, entanglement and
squeezing, characterizing the quantum state of the probe field, can be
well-preserved throughout the passage.


title: Sets of Mutually Unbiased Bases as Arcs in Finite Projective Planes?

abstract:
  This note is a short elaboration of the conjecture of Saniga et al (J. Opt.
B: Quantum Semiclass. 6 (2004) L19-L20) by regarding a set of mutually unbiased
bases (MUBs) in a d-dimensional Hilbert space, d being a power of a prime, as
an analogue of an arc in a (Desarguesian) projective plane of order d. Complete
sets of MUBs thus correspond to (d+1)-arcs, i.e., ovals. The existence of two
principally distinct kinds of ovals for d even and greater than four, viz.
conics and non-conics, implies the existence of two qualitatively different
groups of the complete sets of MUBs for the Hilbert spaces of corresponding
dimensions.


title:
Effective boson-spin model for nuclei ensemble based universal quantum
  memory

abstract:
  We study the collective excitation of a macroscopic ensemble of polarized
nuclei fixed in a quantum dot. Under the approximately homogeneous condition
that we explicitly present in this paper, this many-particle system behaves as
a single mode boson interacting with the spin of a single conduction band
electron confined in this quantum dot. Within this effective spin-boson system,
the quantum information carried by the electronic spin can be coherently
transferred into the collective bosonic mode of excitation in the ensemble of
nuclei. In this sense, the collective bosonic excitation can serve as a stable
quantum memory to store the quantum information of spin state of electron.


title:
Quantum information processing by NMR using a 5-qubit system formed by
  dipolar coupled spins in an oriented molecule

abstract:
  Quantum Information processing by NMR with small number of qubits is well
established. Scaling to higher number of qubits is hindered by two major
requirements (i) mutual coupling among qubits and (ii) qubit addressability. It
has been demonstrated that mutual coupling can be increased by using residual
dipolar couplings among spins by orienting the spin system in a liquid
crystalline matrix. In such a case, the heteronuclear spins are weakly coupled
but the homonuclear spins become strongly coupled. In such circumstances, the
strongly coupled spins can no longer be treated as qubits. However, it has been
demonstrated elsewhere, that the $2^N$ energy levels of a strongly coupled N
spin-1/2 system can be treated as an N-qubit system. For this purpose the
various transitions have to be identified to well defined energy levels. This
paper consists of two parts. In the first part, the energy level diagram of a
heteronuclear 5-spin system is obtained by using a newly developed
heteronuclear z-cosy (HET-Z-COSY) experiment. In the second part,
implementation of logic gates, preparation of pseudopure states, creation of
entanglement and entanglement transfer is demonstrated, validating the use of
such systems for quantum information processing.


title:
Information Theoretical Approach to Control of Quantum Mechanical
  Systems

abstract:
  Fundamental limits on the controllability of quantum mechanical systems are
discussed in the light of quantum information theory. It is shown that the
amount of entropy-reduction that can be extracted from a quantum system by
feedback controller is upper bounded by a sum of the decrease of entropy
achievable in open-loop control and the mutual information between the quantum
system and the controller. This upper bound sets a fundamental limit on the
performance of any quantum controllers whose designs are based on the
possibilities to attain low entropy states. An application of this approach
pertaining to quantum error correction is also discussed.

categories: quant-ph cond-mat.stat-mech math-ph math.MP


title:
Wigner Distribution Analysis of a Schrodinger Cat Superposition of
  Displaced Equilibrium Coherent States

abstract:
  Motivated by recent experiments, we consider a Schr\"{o}dinger cat
superposition of two widely separated coherent states in thermal equilibrium.
The time development of our system is obtained using Wigner distribution
functions. In contrast to our discussion for a two-Gaussian wave packet [Phys.
Lett. A 286 (2001) 87], we find that, in the absence of dissipation, the
interference term does not decay rapidly in time, but in common with the other
two terms, it oscillates in time and persists for all times


title:
Reply to Marinatto's comment on "Bell's theorem without inequalities and
  without alignments"

abstract:
  Marinatto claims that in the proof of Bell's theorem without inequalities and
without alignments [A. Cabello, Phys. Rev. Lett. 91, 230403 (2003)], local
observables cannot be measured by means of tests on individual qubits.
Marinatto's claim is incorrect. To support this, the proof is explicitly
rewritten in terms of tests on individual qubits.


title: Bell's theorem without inequalities and without unspeakable information

abstract:
  A proof of Bell's theorem without inequalities is presented in which distant
local setups do not need to be aligned, since the required perfect correlations
are achieved for any local rotation of the local setups.


title: Proposed experiment for the quantum "Guess my number" protocol

abstract:
  An experimental realization of the entanglement-assisted "Guess my number"
protocol for the reduction of communication complexity, introduced by Steane
and van Dam, would require producing and detecting three-qubit GHZ states with
an efficiency eta > 0.70, which would require single photon detectors of
efficiency sigma > 0.89. We propose a modification of the protocol which can be
translated into a real experiment using present-day technology. In the proposed
experiment, the quantum reduction of the multi-party communication complexity
would require an efficiency eta > 0.05, achievable with detectors of sigma >
0.47, for four parties, and eta > 0.17 (sigma > 0.55) for three parties.


title: How much larger quantum correlations are than classical ones

abstract:
  Considering as distance between two two-party correlations the minimum number
of half local results one party must toggle in order to turn one correlation
into the other, we show that the volume of the set of physically obtainable
correlations in the Einstein-Podolsky-Rosen-Bell scenario is (3 pi/8)^2 = 1.388
larger than the volume of the set of correlations obtainable in local
deterministic or probabilistic hidden-variable theories, but is only 3 pi^2/32
= 0.925 of the volume allowed by arbitrary causal (i.e., no-signaling)
theories.


title:
Experimental Implementation of a Concatenated Quantum Error-Correcting
  Code

abstract:
  Concatenated coding provides a general strategy to achieve the desired level
of noise protection in quantum information storage and transmission. We report
the implementation of a concatenated quantum error-correcting code able to
correct against phase errors with a strong correlated component. The experiment
was performed using liquid-state nuclear magnetic resonance techniques on a
four spin subsystem of labeled crotonic acid. Our results show that
concatenation between active and passive quantum error-correcting codes offers
a practical tool to handle realistic noise contributed by both independent and
correlated errors.


title: Teleportation of Atomic States via Cavity Quantum Electrodynamics

abstract:
  In this article we discuss a scheme of teleportation of atomic states. The
experimental realization proposed makes use of cavity Quatum Electrodynamics
involving the interaction of Rydberg atoms with a micromaser cavity prepared in
a coherent state. We start presenting a scheme to prepare atomic Bell states
via the interaction of atoms with a cavity. In our scheme the cavity and some
atoms play the role of auxiliary systems used to achieve the teleportation.


title: Teleportation of Atomic States for Atoms in a Lambda Configuration

abstract:
  In this article we discuss a scheme of teleportation of atomic states making
use of three-level lambda atoms. The experimental realization proposed makes
use of cavity QED involving the interaction of Rydberg atoms with a micromaser
cavity prepared in a coherent state. We start presenting a scheme to prepare
atomic EPR states involving two-level atoms via the interaction of these atoms
with a cavity. In our scheme the cavity and some atoms play the role of
auxiliary systems used to achieve the teleportation.


title: Realization of GHZ States and the GHZ Test via Cavity QED

abstract:
  In this article we discuss the realization of atomic GHZ states involving
three-level atoms and we show explicitly how to use this state to perform the
GHZ test in which it is possible to decide between local realism theories and
quantum mechanics. The experimental realizations proposed makes use of the
interaction of Rydberg atoms with a cavity prepared in a coherent state.


title:
Classical sources of non-classical physics: the case of linear
  superposition

abstract:
  Classical linear wave superposition produces the appearance of interference.
This observation can be interpreted in two equivalent ways: one can assume that
interference is an illusion because input components remain unperturbed, or
that interference is real and input components undergo energy redistribution.
Both interpretations entail the same observable consequences at the macroscopic
level, but the first approach is considerably more popular. This preference was
established before the emergence of quantum mechanics. Unfortunately, it
requires a non-classical underlying mechanism and fails to explain well-known
microscopic observations. Classical physics appears to collapse at the quantum
level. On the other hand, quantum superposition can be described as a classical
process if the second alternative is adopted. The gap between classical
mechanics and quantum mechanics is an interpretive problem.


title: Nonlinear quantum optical computing via measurement

abstract:
  We show how the measurement induced model of quantum computation proposed by
Raussendorf and Briegel [Phys. Rev. Letts. 86, 5188 (2001)] can be adapted to a
nonlinear optical interaction. This optical implementation requires a Kerr
nonlinearity, a single photon source, a single photon detector and fast feed
forward. Although nondeterministic optical quantum information proposals such
as that suggested by KLM [Nature 409, 46 (2001)] do not require a Kerr
nonlinearity they do require complex reconfigurable optical networks. The
proposal in this paper has the benefit of a single static optical layout with
fixed device parameters, where the algorithm is defined by the final
measurement procedure.


title:
Bound and resonance states of the nonlinear Schroedinger equation in
  simple model systems

abstract:
  The stationary nonlinear Schroedinger equation, or Gross-Pitaevskii equation,
is studied for the cases of a single delta potential and a delta-shell
potential. These model systems allow analytical solutions, and thus provide
useful insight into the features of stationary bound, scattering and resonance
states of the nonlinear Schroedinger equation. For the single delta potential,
the influence of the potential strength and the nonlinearity is studied as well
as the transition from bound to scattering states. Furthermore, the properties
of resonance states for a repulsive delta-shell potential are discussed.


title: Canonical entanglement for two indistinguishable particle

abstract:
  We determine the degree of entanglement for two indistinguishable particles
based on the two-qubit tensor product structure, which is a framework for
emphasizing entanglement founded on observational quantities. Our theory
connects canonical entanglement and entanglement based on occupation number for
two fermions and for two bosons and shows that the degree of entanglement,
based on linear entropy, is closely related to the correlation measure for both
the bosonic and fermionic cases.


title: Quantum Anonymous Transmissions

abstract:
  We consider the problem of hiding sender and receiver of classical and
quantum bits (qubits), even if all physical transmissions can be monitored. We
present a quantum protocol for sending and receiving classical bits
anonymously, which is completely traceless: it successfully prevents later
reconstruction of the sender. We show that this is not possible classically. It
appears that entangled quantum states are uniquely suited for traceless
anonymous transmissions. We then extend this protocol to send and receive
qubits anonymously. In the process we introduce a new primitive called
anonymous entanglement, which may be useful in other contexts as well.


title: Magnetometry with entangled atomic samples

abstract:
  We present a theory for the estimation of a scalar or a vector magnetic field
by its influence on an ensemble of trapped spin polarized atoms. The atoms
interact off-resonantly with a continuous laser field, and the measurement of
the polarization rotation of the probe light, induced by the dispersive
atom-light coupling, leads to spin-squeezing of the atomic sample which enables
an estimate of the magnetic field which is more precise than that expected from
standard counting statistics. For polarized light and polarized atoms, a
description of the non-classical components of the collective spin angular
momentum for the atoms and the collective Stokes vectors of the light-field in
terms of effective gaussian position and momentum variables is practically
exact. The gaussian formalism describes the dynamics of the system very
effectively and accounts explicitly for the back-action on the atoms due to
measurement and for the estimate of the magnetic field. Multi-component
magnetic fields are estimated by the measurement of suitably chosen atomic
observables and precision and efficiency is gained by dividing the atomic gas
in two or more samples which are entangled by the dispersive atom-light
interaction.


title: Are atoms waves or particles?

abstract:
  It is shown that the Kapitza-Dirac effect with atoms, which has been
considered to be evidence of their wavelike character, can be interpreted as a
scattering of pointlike objects by the periodic laser field.


title: Basis States for Relativistic, Dynamically-Entangled Particles

abstract:
  In several recent papers on entanglement in relativistic quantum systems and
relativistic Bell's inequalities, relativistic Bell-type two-particle states
have been constructed in analogy to non-relativistic states. These
constructions do not have the form suggested by relativistic invariance of the
dynamics. Two relativistic formulations of Bell-type states are shown for
massive particles, one using the standard Wigner spin basis and one using the
helicity basis. The construction hinges on the use of Clebsch-Gordan
coefficients of the Poincar\'e group to reduce the direct product of two
unitary irreducible representations (UIRs) into a direct sum of UIRs.


title:
Note on Non-relativistic proof of the spin-statistics connection in the
  Galilean frame

abstract:
  We reply to the critique by Pucchini and Vucetich of our construction of a
non-relativistic proof of the spin-statistics connection using SU(2) invariance
and a Weiss-Schwinger action principle.


title: Quantum mechanical description of Stern-Gerlach experiments

abstract:
  The motion of neutral particles with magnetic moments in an inhomogeneous
magnetic field is described in a quantum mechanical framework. The validity of
the semi-classical approximations which are generally used to describe these
phenomena is discussed. Approximate expressions for the evolution operator are
derived and compared to the exact calculations. Focusing and spin-flip
phenomena are predicted. The reliability of Stern-Gerlach experiments to
measure spin projections is assessed in this framework.


title: Classical feedback for quantum channels

abstract:
  We investigate whether the use of a noiseless, classical feedback channel
will increase the capacity of a quantum discrete memoryless channel to transmit
classical information. This problem has been previously analyzed by Bowen and
Nagarajan for the case of protocols restricted to product input states. They
showed that feedback did not increase the information capacity. In this paper
we introduce a quantum analogue of classical causality and prove a capacity
theorem (in regularized form) for the transmission of classical information.


title: Single-particle entanglement

abstract:
  Using the approach to quantum entanglement based on the quantum fluctuations
of observables, we show the existence of perfect entangled states of a single
"spin-1" particle. We give physical examples related to the photons, condensed
matter physics, and particle physics.


title: On the use of the group SO(4,2) in atomic and molecular physics

abstract:
  In this paper the dynamical noninvariance group SO(4,2) for a hydrogen-like
atom is derived through two different approaches. The first one is by an
established traditional ascent process starting from the symmetry group SO(3).
This approach is presented in a mathematically oriented original way with a
special emphasis on maximally superintegrable systems, N-dimensional extension
and little groups. The second approach is by a new symmetry descent process
starting from the noninvariance dynamical group Sp(8,R) for a four-dimensional
harmonic oscillator. It is based on the little known concept of a Lie algebra
under constraints and corresponds in some sense to a symmetry breaking
mechanism. This paper ends with a brief discussion of the interest of SO(4,2)
for a new group-theoretical approach to the periodic table of chemical
elements. In this connection, a general ongoing programme based on the use of a
complete set of commuting operators is briefly described. It is believed that
the present paper could be useful not only to the atomic and molecular
community but also to people working in theoretical and mathematical physics.

categories: quant-ph math-ph math.MP physics.atom-ph


title:
Experimental study of the momentum correlation of a pseudo-thermal field
  in the photon counting regime

abstract:
  Thermal (or pseudo-thermal) radiation has been recently proposed for imaging
and interference types of experiments to simulate entangled states. We report
an experimental study on the momentum correlation properties of a
pseudo-thermal field in the photon counting regime. The characterization and
understanding of such a light source in the context of two photon physics,
especially its similarities and differences compared to entangled two-photon
states, is useful in gaining knowledge of entanglement and may help us to
assess the real potential of applications of chaotic light in this context.


title:
Time-resolved homodyne characterization of individual
  quadrature-entangled pulses

abstract:
  We describe a simple and efficient setup to generate and characterize
femtosecond quadrature-entangled pulses. Quantum correlations equivalent to
about 2.5 dB squeezing are efficiently and easily reached using the
non-degenerate parametric amplification of femtosecond pulses through a
single-pass in a thin (0.1 mm) potassium niobate crystal. The entangled pulses
are then individually sampled to characterize the non-separability and the
entropy of formation of the states. The complete experiment is analysed in the
time-domain, from the pulsed source of quadrature entanglement to the
time-resolved homodyne detection. This particularity allows for applications in
quantum communication protocols using continuous-variable entanglement.


title:
An experiential formula for the energy eigenvalues of a particle in a
  one-dimension finite-deep square well potential

abstract:
  We propose an experiential formula for the calculation of the energy
eigenvalues of a particle moving in a one-dimension finite-deep square well
potential after some physical considerations. This formula shows a simple
relation between the energy eigenvalues and the potential papameters, and can
be used to estimate the energy eigenvalues in a very simple way.


title: Deterministic event-based simulation of quantum interference

abstract:
  We propose and analyse simple deterministic algorithms that can be used to
construct machines that have primitive learning capabilities. We demonstrate
that locally connected networks of these machines can be used to perform blind
classification on an event-by-event basis, without storing the information of
the individual events. We also demonstrate that properly designed networks of
these machines exhibit behavior that is usually only attributed to quantum
systems. We present networks that simulate quantum interference on an
event-by-event basis. In particular we show that by using simple geometry and
the learning capabilities of the machines it becomes possible to simulate
single-photon interference in a Mach-Zehnder interferometer. The interference
pattern generated by the network of deterministic learning machines is in
perfect agreement with the quantum theoretical result for the single-photon
Mach-Zehnder interferometer. To illustrate that networks of these machines are
indeed capable of simulating quantum interference we simulate, event-by-event,
a setup involving two chained Mach-Zehnder interferometers. We show that also
in this case the simulation results agree with quantum theory.


title: Quantum phase-space analysis of the pendular cavity

abstract:
  We perform a quantum mechanical analysis of the pendular cavity, using the
positive-P representation, showing that the quantum state of the moving mirror,
a macroscopic object, has noticeable effects on the dynamics. This system has
previously been proposed as a candidate for the quantum-limited measurement of
small displacements of the mirror due to radiation pressure, for the production
of states with entanglement between the mirror and the field, and even for
superposition states of the mirror. However, when we treat the oscillating
mirror quantum mechanically, we find that it always oscillates, has no
stationary steady-state, and exhibits uncertainties in position and momentum
which are typically larger than the mean values. This means that previous
linearised fluctuation analyses which have been used to predict these highly
quantum states are of limited use. We find that the achievable accuracy in
measurement is far worse than the standard quantum limit due to thermal noise,
which, for typical experimental parameters, is overwhelming even at 2 mK.


title: Ghost imaging schemes: fast and broadband

abstract:
  In ghost imaging schemes information about an object is extracted by
measuring the correlation between a beam that passed the object and a reference
beam. We present a spatial averaging technique that substantially improves the
imaging bandwidth of such schemes, which implies that information about
high-frequency Fourier components can be observed in the reconstructed
diffraction pattern. In the many-photon regime the averaging can be done in
parallel and we show that this leads to a much faster convergence of the
correlations. We also consider the reconstruction of the object image, and
discuss the differences between a pixel-like detector and a bucket detector in
the object arm. Finally, it is shown how to non-locally make spatial filtering
of a reconstructed image. The results are presented using entangled beams
created by parametric down-conversion, but they are general and can be extended
also to the important case of using classically correlated thermal-like beams.


title: Stairway Quantum Computer

abstract:
  In the paper is considered stairway-like design of quantum computer, i.e.,
array of double quantum dots or wells. The model is quite general to include
wide variety of physical systems from coupled quantum dots in experiments with
solid state qubits, to very complex one, like DNA molecule. At the same time it
is concrete enough, to describe main physical principles for implementation of
universal set of quantum gates, initialization, measurement, decoherence, etc.


title: POVM optimization of classical correlations

abstract:
  We study the problem of optimization over positive valued-operator measure to
extract classical correlation in a bipartite quantum system. The proposed
method is applied to binary states only. Moreover, to illustrate this method,
an explicit example is studied in details.


title: Nonclassicality of a photon-subtracted Gaussian field

abstract:
  Recently, a non-Gaussian field, which may be a useful basis for entanglement
distillation and efficient quantum teleportation, has been experimentally
produced by subtracting a photon from a squeezed Gaussian field. We investigate
the nonclassicality of such the field using negativity in the Wigner function
and the existence of positive well-defined $P$ function. We obtain the
condition to see negativity in the Wigner function for the case including the
mixed Gaussian incoming field, the threshold photodetection and the inefficient
homodyne measurement. We show how similar the photon-subtracted state is to a
superposition of coherent states.


title: Surface Acoustic Wave Single-Electron Interferometry

abstract:
  We propose an experiment to observe interference of a single electron as it
is transported along two parallel quasi-one-dimensional channels trapped in a
single minimum of a travelling periodic electric field. The experimental device
is a modification of the surface acoustic wave (SAW) based quantum processor.
Interference is achieved by creating a superposition of spatial wavefunctions
between the two channels and inducing a relative phase shift via either a
transverse electric field or a magnetic field. The interference can be used to
estimate the decoherence time of an electron in this type of solid-state
device.


title: Decoherence of Anyon Qubit

abstract:
  One mechanism of decoherence of anyon qubit due to interaction with edge
states is considered. The calculations are made at low temperature in Markovian
and "short-time" approximation. Two approximations are compared.


title: Quantum Gloves

abstract:
  The slogan "information is physical" has been so successful that it led to
some excess. Classical and quantum information can be thought of independently
of any physical implementation. Pure information tasks can be realized using
such abstract c- and qu-bits, but physical tasks require appropriate physical
realizations of c- or qu-bits. As illustration we consider the problem of
communicating chirality. We discuss in detail the physical resources this
necessitates, and introduce the natural concept of "quantum gloves", i.e.
rotationally invariant quantum states that encode as much as possible the
concept of chirality and nothing more.


title: Optimal two-qubit quantum circuits using exchange interactions

abstract:
  The Heisenberg exchange interaction is a natural method to implement
non-local (i.e., multi-qubit) quantum gates in quantum information processing.
We consider quantum circuits comprising of $(SWAP)^\alpha $ gates, which are
realized through the exchange interaction, and single-qubit gates. A universal
two-qubit quantum circuit is constructed from only three $(SWAP)^\alpha$ gates
and six single-qubit gates. We further show that three $(SWAP)^\alpha $ gates
are not only sufficient, but necessary. Since six single-qubit gates are known
to be necessary, our universal two-qubit circuit is optimal in terms of the
number of {\em both} $(SWAP)^\alpha $ and single-qubit gates.


title: Modeling and Simulations of a Single-Spin Measurement using MRFM

abstract:
  We review the quantum theory of a single spin magnetic resonance force
microscopy (MRFM). We concentrate on the novel technique called oscillating
cantilever-driven adiabatic reversals (OSCAR), which has been used for a single
spin detection (Dan Rugar, Talk on the 2004 IEEE NTC Quantum Device Technology
Workshop). First we describe the quantum dynamics of the cantilever-spin system
using simple estimates in the spirit of the mean field approximation. Then we
present the results of our computer simulations of the Schrodinger equation for
the wave function of the cantilever-spin system and of the master equation for
the density matrix of the system. We demonstrate that the cantilever behaves
like a quasi-classical measurement device which detects the spin projection
along the effective magnetic field. We show that the OSCAR technique provides
continuous monitoring of the single spin, which could be used to detect the
mysterious quantum collapses of the wave function of the cantilever-spin
system.


title: Quantum Diagonalization Method in the Tavis-Cummings Model

abstract:
  To obtain the explicit form of evolution operator in the Tavis-Cummings model
we must calculate the term ${e}^{-itg(S_{+}\otimes a+S_{-}\otimes
a^{\dagger})}$ explicitly which is very hard. In this paper we try to make the
quantum matrix $A\equiv S_{+}\otimes a+S_{-}\otimes a^{\dagger}$ diagonal to
calculate ${e}^{-itgA}$ and, moreover, to know a deep structure of the model.
  For the case of one, two and three atoms we give such a diagonalization which
is first nontrivial examples as far as we know, and reproduce the calculations
of ${e}^{-itgA}$ given in quant-ph/0404034. We also give a hint to an
application to a noncommutative differential geometry.
  However, a quantum diagonalization is not unique and is affected by some
ambiguity arising from the noncommutativity of operators in quantum physics.
  Our method may open a new point of view in Mathematical Physics or Quantum
Physics.


title:
Analytical technique for simplification of the encoder-decoder circuit
  for a perfect five-qubit error correction

abstract:
  Simpler encoding and decoding networks are necessary for more reliable
quantum error correcting codes (QECCs). The simplification of the
encoder-decoder circuit for a perfect five-qubit QECC can be derived
analytically if the QECC is converted from its equivalent one-way entanglement
purification protocol (1-EPP). In this work, the analytical method to simplify
the encoder-decoder circuit is introduced and a circuit that is as simple as
the existent simplest circuits is presented as an example. The encoder-decoder
circuit presented here involves nine single- and two-qubit unitary operations,
only six of which are controlled-NOT (CNOT) gates.


title: Quantum Information Processing and Entanglement in Solid State Devices

abstract:
  Control over electron-spin states, such as coherent manipulation, filtering
and measurement promises access to new technologies in conventional as well as
in quantum computation and quantum communication. In this paper, we review
recent theoretical proposal of using electron spins in quantum confined
structures as qubits. We also present a theoretical proposal for testing Bell's
inequality in nano-electronics devices. We show that the entanglement of two
electron spins can be detected in the spin filter effect in the nanostructure
semiconductor / ferromagnetic semiconductor / semiconductor junction. In
particular, we show how to test Bell's inequality via the measurement of the
current-current correlation function in this setup.


title:
A quantum symmetric key cipher(Y-00) and key generation (Quantum stream
  cipher-Part II)

abstract:
  What obstructs the realization of useful quantum cryptography is single
photon scheme, or entanglement which is not applicable to the current
infrastructure of optical communication network. We are concerned with the
following question: Can we realize the information theoretically secure
symmetric key cipher under "the finite secret key" based on quantum-optical
communications? A role of quantum information theory is to give an answer for
such a question. As an answer for the question, a new quantum cryptography was
proposed by H.P.Yuen, which can realize a secure symmetric key cipher with high
speeds(Gbps) and for long distance(1000 Km). Although some researchers claim
that Yuen protocol(Y-00) is equivalent to the classical cryptography, they are
all mistaken. Indeed it has no classical analogue, and also provides a
generalization even in the conventional cryptography.
  At present, it is proved that a basic model of Y-00 has at least the security
such as $H(X|Y_E)=H(K|Y_E)=H(K)$, $H(K|Y_E,X)\sim 0$ under the average photon
number per signal light pulse:$<n> \sim 10000$. Towards our final goal, in this
paper, we clarify a role of classical randomness(secret key) and quantum
randomness in Y-00, and give a rigorous quantum mechanical interpretation of
the security, showing an analysis of quantum collective attack.


title:
Proposal of Quantum Simulation of Pairing Model on an NMR Quantum
  Computer

abstract:
  We give out a proposal of quantum simulation of pairing model on an NMR
quantum computer. In our proposal, we choose an appropriate initial state which
can be easily prepared in experiment. Making use of feature of NMR measure and
the technology of the second (discrete) Fourier transformation, our theoretical
scheme can obtain the spectrum of paring model in principle. We concretely
discuss the case in the concerned subspaces of pairing model and then, as an
example, give out a simple initial state to get the gap of two the lowest
energy levels in the given subspace. The quantum simulation to get more
differences of energy levels is able to be discussed similarly.


title:
Quantum Hamilton - Jacobi study of wave functions and energy spectrum of
  solvable and quasi - exactly solvable models

abstract:
  In this thesis, the quantum Hamilton Jacobi (QHJ) formalism is used to study
various exactly solvable (ES) and quasi -exactly solvable (QES) models. Using
this method, we obtain the bound state eigenvalues and the eigenfunctions for
the models studied. The central entity of this formalism in the logarithmic
derivative of the wave function, known as the quantum momentum function
(QMF).It is assumed that the point at infinity is an isolated singular
point.The kowledge of the singularity structure of the QMF is used to arrive at
the required solutions. We show that there are marked differences between the
singularity structures of the ES and QES models.


title: Group Approach to the Quantization of the P\"oschl-Teller dynamics

abstract:
  The quantum dynamics of a particle in the Modified P\"oschl-Teller potential
is derived from the group $SL(2,R)$ by applying a Group Approach to
Quantization (GAQ). The explicit form of the Hamiltonian as well as the ladder
operators is found in the enveloping algebra of this basic symmetry group. The
present algorithm provides a physical realization of the non-unitary,
finite-dimensional, irreducible representations of the $SL(2,R)$ group. The
non-unitarity manifests itself in that only half of the states are
normalizable, in contrast with the representations of SU(2) where all the
states are physical.

categories: quant-ph math-ph math.MP physics.chem-ph


title: Generation of entanglement in regular systems

abstract:
  We study dynamical generation of entanglement in bipartite quantum systems,
characterized by purity (or linear entropy), and caused by the coupling between
the two subsystems. Explicit semiclassical theory of purity decay is derived
for integrable classical dynamics of the uncoupled system, and for localized
(general Gaussian wave-packet) initial states. Purity decays as an algebraic
function of time times strength of perturbation, independently of the Planck's
constant.


title:
Electromagnetically induced absorption and transparency in
  magneto-optical resonances in elliptically polarized field

abstract:
  Using a 1 to 2 transition as an analytically tractable model, we discuss in
detail magneto-optical resonances of both EIA (electromagnetically induced
absorption) and EIT (electromagnetically induced transparency) types in the
Hanle configuration. The analysis is made for arbitrary rate of depolarizing
collisions in the excited state and arbitrary elliptical field polarization.
The obtained results clearly show that the main reason for the EIA sub-natural
resonance is the spontaneous transfer of anisotropy from the excited level to
the ground one. In the EIA case we predict the negative structures in the
absortpion resonance at large field detuning. The role of the finite atom-light
interaction time is briefly discussed. In addition we study non-trivial
peculiarities of the resonance lineshape related to the velocity spread in a
gas.


title:
Pseudo-Hermitian Description of PT-Symmetric Systems Defined on a
  Complex Contour

abstract:
  We describe a method that allows for a practical application of the theory of
pseudo-Hermitian operators to PT-symmetric systems defined on a complex
contour. We apply this method to study the Hamiltonians $H=p^2+x^2(ix)^\nu$
with $\nu\in(-2,\infty)$ that are defined along the corresponding anti-Stokes
lines. In particular, we reveal the intrinsic non-Hermiticity of $H$ for the
cases that $\nu$ is an even integer, so that $H=p^2\pm x^{2+\nu}$, and give a
proof of the discreteness of the spectrum of $H$ for all $\nu\in(-2,\infty)$.
Furthermore, we study the consequences of defining a square-well Hamiltonian on
a wedge-shaped complex contour. This yields a PT-symmetric system with a finite
number of real eigenvalues. We present a comprehensive analysis of this system
within the framework of pseudo-Hermitian quantum mechanics. We also outline a
direct pseudo-Hermitian treatment of PT-symmetric systems defined on a complex
contour which clarifies the underlying mathematical structure of the
formulation of PT-symmetric quantum mechanics based on the charge-conjugation
operator. Our results provide a conclusive evidence that pseudo-Hermitian
quantum mechanics provides a complete description of general PT-symmetric
systems regardless of whether they are defined along the real line or a complex
contour.


title:
Quantum interference effects in degenerate systems. Spontaneous and
  stimulated radiation

abstract:
  We study the effect of quantum interference on the structure and properties
of spontaneous and stimulated transitions in a degenerate V-type three-level
atom with an arbitrary total momentum of each state. Explicit expressions for
the factors in the terms of the relaxation operator and stimulated transition
operator with account of quantum interference effects are obtained. It has been
demonstrated that the condition for the dipole transition moments to be
parallel is insufficient and for conventional atoms the interference cross
terms are zeros for both operators. The conditions when quantum interference
influences the properties of the relaxation operators have been analysed. For
the stimulated transitions this condition is shown to be anisotropy of the
photon field interacting with the atoms. This case is minutely studied for the
D-line of alkali metals.


title: Scaling of success probabilities for linear optics gates

abstract:
  By using the abstract linear-optical network derived in [S. Scheel and N.
L\"utkenhaus, New J. Phys. \textbf{6}, 51 (2004)] we show that for the lowest
possible ancilla photon numbers the probability of success of realizing a
(single-shot) generalized nonlinear sign shift gate on an ($N+1$)-dimensional
signal state scales as $1/N^2$. We limit ourselves to single-shot gates without
conditional feed-forward. We derive our results by using determinants of
Vandermonde-type over a polynomial basis which is closely related to the
well-known Jacobi polynomials.


title: Bell's theorem: Critique of proofs with and without inequalities

abstract:
  Most of the standard proofs of the Bell theorem are based on the Kolmogorov
axioms of probability theory. We show that these proofs contain mathematical
steps that cannot be reconciled with the Kolmogorov axioms. Specifically we
demonstrate that these proofs ignore the conclusion of a theorem of Vorob'ev on
the consistency of joint distributions. As a consequence Bell's theorem stated
in its full generality remains unproven, in particular, for extended parameter
spaces that are still objective local and that include instrument parameters
that are correlated by both time and instrument settings. Although the Bell
theorem correctly rules out certain small classes of hidden variables, for
these extended parameter spaces the standard proofs come to a halt. The
Greenberger-Horne-Zeilinger (GHZ) approach is based on similar fallacious
arguments. For this case we are able to present an objective local computer
experiment that simulates the experimental test of GHZ performed by Pan,
Bouwmeester, Daniell, Weinfurter and Zeilinger and that directly contradicts
their claim that Einstein-local elements of reality can neither explain the
results of quantum mechanical theory nor their experimental results.


title: Remote Preparation and Distribution of Bipartite Entangled States

abstract:
  We prove a powerful theorem for tripartite remote entanglement distribution
protocols that establishes an upper bound on the amount of entanglement of
formation that can be created between two single-qubit nodes of a quantum
network. Our theorem also provides an operational interpretation of concurrence
as a type of entanglement capacity.


title: Quantum seals

abstract:
  A quantum seal is a way of encoding a message into quantum states, so that
anybody may read the message with little error, while authorized verifiers can
detect that the seal has been broken. We present a simple extension to the
Bechmann-Pasquinucci majority-voting scheme that is impervious to coherent
attacks, and further, encompasses sealing quantum messages by means of quantum
encryption. The scheme is relatively easy to implement, requiring neither
entanglement nor controlled operations during the state preparation, reading or
verification stages.


title: Optimal feedback control for the rapid preparation of a single qubit

abstract:
  We consider the use of feedback control during a measurement to increase the
rate at which a single qubit is purified, and more generally the rate at which
near-pure states may be prepared. We derive the optimal bang-bang algorithm for
rapid state preparation from an initially completely mixed state when the
measurement basis is unrestricted, and evaluate its performance numerically. We
also consider briefly the case in which the measurement basis is fixed with
respect to the state to be prepared, and describe the qualitative structure of
the optimal bang-bang algorithm.


title:
Giant nonlinearity and entanglement of single photons in photonic
  bandgap structures

abstract:
  Giantly enhanced cross-phase modulation with suppressed spectral broadening
is predicted between optically-induced dark-state polaritons whose propagation
is strongly affected by photonic bandgaps of spatially periodic media with
multilevel dopants. This mechanism is shown to be capable of fully entangling
two single-photon pulses with high fidelity.


title:
Experimental Observation of Classical Sub-Wavelength Interference with
  Thermal-Like Light

abstract:
  We show the experimental observation of the classical sub-wavelength
double-slit interference with a pseudo-thermal light source. The experimental
results are in agreement with the recent theoretical prediction shown in
quant-ph/0404078 (to be appeared in Phys. Rev. A).


title:
The Meissner effect and massive particles as witnesses of macroscopic
  entanglement

abstract:
  We show how the spatial macroscopic entanglement equivalent to the off
diagonal long range order (ODLRO) implies the Meissner effect and flux
quantisation for a superconductor. It is argued by analogy with superconductors
that the Higgs field must also be entangled in the same way. Internal (spin)
entanglement is shown to be irrelevant within this context, although it can of
course also be computed.


title:
Production of superpositions of coherent states in traveling optical
  fields with inefficient photon detection

abstract:
  We develop an all-optical scheme to generate superpositions of
macroscopically distinguishable coherent states in traveling optical fields. It
non-deterministically distills coherent state superpositions (CSSs) with large
amplitudes out of CSSs with small amplitudes using inefficient photon
detection. The small CSSs required to produce CSSs with larger amplitudes are
extremely well approximated by squeezed single photons. We discuss some
remarkable features of this scheme: it effectively purifies mixed initial
states emitted from inefficient single photon sources and boosts negativity of
Wigner functions of quantum states.


title:
Trigonometric identities, angular Schr\"{o}dinger equations and a new
  family of solvable models

abstract:
  Angular parts of certain solvable models are studied. We find that an
extension of this class may be based on suitable trigonometric identities. The
new exactly solvable Hamiltonians are shown to describe interesting two- and
three-particle systems of the generalized Calogero, Wolfes and
Winternitz-Smorodinsky types.


title: Correlated Photon Emission from a Single II-VI Quantum Dot

abstract:
  We report correlation and cross-correlation measurements of photons emitted
under continuous wave excitation by a single II-VI quantum dot (QD) grown by
molecular-beam epitaxy. A standard technique of microphotoluminescence combined
with an ultrafast photon correlation set-up allowed us to see an antibunching
effect on photons emitted by excitons recombining in a single CdTe/ZnTe QD, as
well as cross-correlation within the biexciton ($X_{2}$)-exciton ($X$)
radiative cascade from the same dot. Fast microchannel plate photomultipliers
and a time-correlated single photon module gave us an overall temporal
resolution of 140 ps better than the typical exciton lifetime in II-VI QDs of
about 250ps.


title:
Superluminal hidden communication as the underlying mechanism for
  quantum correlations: constraining models

abstract:
  Since Bell's theorem, it is known that quantum correlations cannot be
described by local variables (LV) alone: if one does not want to abandon
classical mechanisms for correlations, a superluminal form of communication
among the particles must be postulated. A natural question is whether such a
postulate would imply the possibility of superluminal signaling. Here we show
that the assumption of finite-speed superluminal communication indeed leads to
signaling when no LV are present, and more generally when only LV derivable
from quantum statistics are allowed. When the most general LV are allowed, we
prove in a specific case that the model can be made again consistent with
relativity, but the question remains open in general.


title: Qubit state guidance without feedback

abstract:
  We study a protocol for two-qubit state guidance that does not rely on
feedback mechanisms. In our scheme, entanglement can be concentrated by
arranging the interactions of the qubits with a continuous variable ancilla. By
properly post-selecting the outcomes of repeated measurements of the state of
the ancilla, the qubit state is driven to have a desired amount of purity and
entanglement. We stress the primary role played by the first iterations of the
protocol. Inefficiencies in the detection operations can be fully taken into
account.


title: Quantum entanglement can be simulated without communication

abstract:
  It has recently been shown that all causal correlations between two parties
which output each one bit, a and b, when receiving each one bit, x and y, can
be expressed as convex combinations of local correlations (i.e., correlations
that can be simulated with local random variables) and non-local correlations
of the form a+b=xy mod 2. We show that a single instance of the latter
elementary non-local correlation suffices to simulate exactly all possible
projective measurements that can be performed on the singlet state of two
qubits, with no communication needed at all. This elementary non-local
correlation thus defines some unit of non-locality, which we call a nl-bit.


title: Entanglement, Bell Inequalities and Decoherence in Particle Physics

abstract:
  We demonstrate the relevance of entanglement, Bell inequalities and
decoherence in particle physics. In particular, we study in detail the features
of the ``strange'' $K^0 \bar K^0$ system as an example of entangled
meson--antimeson systems. The analogies and differences to entangled spin--1/2
or photon systems are worked, the effects of a unitary time evolution of the
meson system is demonstrated explicitly. After an introduction we present
several types of Bell inequalities and show a remarkable connection to CP
violation. We investigate the stability of entangled quantum systems pursuing
the question how possible decoherence might arise due to the interaction of the
system with its ``environment''. The decoherence is strikingly connected to the
entanglement loss of common entanglement measures. Finally, some outlook of the
field is presented.


title: Nanomechanical Quantum Memory for Superconducting Qubits

abstract:
  Many protocols for quantum computation require a quantum memory element to
store qubits. We discuss the accuracy with which quantum states prepared in a
Josephson junction qubit can be stored in a nanoelectromechanical resonator and
then transfered back to the junction. We find that the fidelity of the memory
operation depends on both the junction-resonator coupling strength and the
location of the state on the Bloch sphere. Although we specifically focus on a
large-area, current-biased Josesphson junction phase qubit coupled to the
dilatational mode of a piezoelectric nanoelectromechanical disk resonator, many
our results will apply to other qubit-oscillator models.


title: Quantum Charged Non-Linear Nano-String and Quantum Vacuum

abstract:
  The classical and quantum dynamic of a nonlinear chareged vibrating string
and its interaction with quantum vacuum field is investigated. Some probability
amplitudes for transitions between vacuum field and quantum states of the
string are obtained. The effect of nonlinearity on some probability amplitudes
is investigated and finally the corect equation for string containing the
vacuum and radiation reaction field is obtained.


title:
Secure Coherent-state Quantum Key Distribution Protocols with Efficient
  Reconciliation

abstract:
  We study the equivalence between a realistic quantum key distribution
protocol using coherent states and homodyne detection and a formal entanglement
purification protocol. Maximally-entangled qubit pairs that one can extract in
the formal protocol correspond to secret key bits in the realistic protocol.
More specifically, we define a qubit encoding scheme that allows the formal
protocol to produce more than one entangled qubit pair per coherent state, or
equivalently for the realistic protocol, more than one secret key bit. The
entanglement parameters are estimated using quantum tomography. We analyze the
properties of the encoding scheme and investigate its application to the
important case of the attenuation channel.


title:
Comment on "Separability of quantum states and the violation of
  Bell-type inequalities"

abstract:
  The statement of E.R. Loubenets, Phys. Rev. A 69, 042102 (2004), that
separable states can violate classical probabilistic constraints is based on a
misleading definition of classicality, which is much narrower than Bell's
concept of local hidden variables. In a Bell type setting the notion of
classicality used by Loubenets corresponds to the assumption of perfect
correlations if the same observable is measured on both sides. While it is
obvious that most separable states do not satisfy this assumption, this does
not constitute "non-classical" behaviour in any usual sense of the word.


title: Generation of long-living entanglement between two separate atoms

abstract:
  A scheme for non-conditional generation of long-living maximally entangled
states between two spatially well separated atoms is proposed. In the scheme,
$\Lambda$-type atoms pass a resonator-like equipment of dispersing and
absorbing macroscopic bodies giving rise to body-assisted electromagnetic field
resonances of well-defined heights and widths. Strong atom-field coupling is
combined with weak atom-field coupling to realize entanglement transfer from
the dipole-allowed transitions to the dipole-forbidden transitions, thereby the
entanglement being preserved when the atoms depart from the bodies and from
each other. The theory is applied to the case of the atoms passing by a
microsphere.


title: Additivity in Isotropic Quantum Spin Channels

abstract:
  We prove additivity of the minimum output entropy and the Holevo capacity for
rotationally invariant quantum channels acting on spin-1/2 and spin-1 systems.
The physical significance of these channels and their relations to other known
channels is also discussed.


title: The invariants of the local Clifford group

abstract:
  We study the algebra of complex polynomials which remain invariant under the
action of the local Clifford group under conjugation. Within this algebra, we
consider the linear spaces of homogeneous polynomials degree by degree and
construct bases for these vector spaces for each degree, thereby obtaining a
generating set of polynomial invariants. Our approach is based on the
description of Clifford operators in terms of linear operations over GF(2).
Such a study of polynomial invariants of the local Clifford group is mainly of
importance in quantum coding theory, in particular in the classification of
binary quantum codes. Some applications in entanglement theory and quantum
computing are briefly discussed as well.


title:
Informal Resource Letter - Nonlinear quantum mechanics on arXiv up to
  August 2004

abstract:
  I compiled a list of articles on arXiv that deal with possible fundamental
quantum nonlinearities or examine the origins of its linearity. The list
extends til August 2004.