title:
Comment on ``Testing integrability with a single bit of quantum
  information''

abstract:
  In quant-ph/0303042, Poulin, Laflamme, Milburn and Paz consider the problem
of distinguishing quantum chaos from quantum integrability for dynamics in an
$N$-dimensional Hilbert space. They claim that this can be done by
deterministic quantum computing with a single bit using $O(\sqrt{N})$ physical
resources, compared to O(N) physical resources classically. I point out what
seems to be a fatal flaw with their proposal.


title: On the Power of Quantum Memory

abstract:
  We address the question whether quantum memory is more powerful than
classical memory. In particular, we consider a setting where information about
a random n-bit string X is stored in r classical or quantum bits, for r<n,
i.e., the stored information is bound to be only partial. Later, a randomly
chosen binary question F about X is asked, which has to be answered using only
the stored information. The maximal probability of correctly guessing the
answer F(X) is then compared for the cases where the storage device is
classical or quantum mechanical, respectively.
  We show that, despite the fact that the measurement of quantum bits can
depend arbitrarily on the question F to be answered, the quantum advantage is
negligible already for small values of the difference n-r.
  An implication for cryptography is that privacy amplification by application
of a compression function mapping n-bit strings to s-bit strings (for some
s<n-r), chosen publicly from a two-universal class of hash functions, remains
essentially equally secure when the adversary's memory is allowed to be r
quantum rather than only r classical bits.


title: Cavity QED and Quantum Computation in the Strong Coupling Regime

abstract:
  In this paper we propose a Hamiltonian generalizing the interaction of the
two--level atom and both the single radiation mode and external field $...$ a
kind of cavity QED. We solve the Schrodinger equation in the strong coupling
regime by making use of rotating wave approximation under new resonance
conditions containing the Bessel functions and etc, and obtain unitary
transformations of four types corresponding to Rabi oscillations which perform
quantum logic gates in Quantum Computation.


title:
Finite-level systems, Hermitian operators, isometries, and a novel
  parameterization of Stiefel and Grassmann manifolds

abstract:
  In this paper we obtain a description of the Hermitian operators acting on
the Hilbert space $\C^n$, description which gives a complete solution to the
over parameterization problem. More precisely we provide an explicit
parameterization of arbitrary $n$-dimensional operators, operators that may be
considered either as Hamiltonians, or density matrices for finite-level quantum
systems. It is shown that the spectral multiplicities are encoded in a flag
unitary matrix obtained as an ordered product of special unitary matrices, each
one generated by a complex $n-k$-dimensional unit vector, $k=0,1,...,n-2$. As a
byproduct, an alternative and simple parameterization of Stiefel and Grassmann
manifolds is obtained.


title:
Improved LeRoy-Bernstein near-dissociation expansion formula. Tutorial
  application to photoassociation spectroscopy of long-range states

abstract:
  NDE (Near-dissociation expansion) including LeRoy-Bernstein formulas are
improved by taking into account the multipole expansion coefficients and the
non asymptotic part of the potential curve. Applying these new simple
analytical formulas to photoassociation spectra of cold alkali atoms, we
improve the determination of the asymptotic coefficient, reaching a 1%
accuracy, for long-range relativistic potential curve of diatomic molecules.


title:
A Simple Solution of the Time-Independent Schrodinger Equation in One
  Dimension

abstract:
  We found a simple procedure for the solution of the time - independent
Schrodinger equation in one dimension without making any approximation. The
wave functions are always periodic. Two difficulties may be encountered: one is
to solve the equation E=U(x), where E and U(x) are the total and potential
energies, respectively, and the other is to calculate the integral of the
square root of U(x). If these calculations cannot be made analytically, it
should then be performed by numerical methods. To find the energy and the wave
function of the ground state, there is no need to calculate this integral, it
is sufficient to find the classical turning points, that is to solve the
equation E=U(x).


title:
Reply to: ``Comment on `Polynomial-Time Simulation of Pairing Models on
  a Quantum Computer'''

abstract:
  We reply to Dukelsky, et al. regarding the article: L. A. Wu, M. S. Byrd and
D. A. Lidar, Phys. Rev. Lett. 89, 057904 (2002).


title:
A Simple General Solution of the Radial Schrodinger Equation for
  Spherically Symmetric Potentials

abstract:
  By using a simple procedure the general solution of the time-independent
radial Schrodinger Equation for spherical symmetric potentials was made without
making any approximation. The wave functions are always periodic. It appears
two diffucilties: one of them is the solution of the equation E= U(r), where E
and U(r) are the total an effective potential energies, respectively, and the
other is the calculation of the integral of the square root of U(r). If
analytical calculations are not possible, one must apply numerical methods. To
find the energy wave function of the ground state, there is no need for the
calculation of this integral, it is sufficient to find the classical turning
points, that is to solve the equation E=U(r).


title: Robust Steering of n-level Quantum Systems

abstract:
  Robust open-loop steering of a finite-dimensional quantum system is a central
problem in a growing number of applications of information engineering. In the
present paper, we reformulate the problem in the classical control-theoretic
setting, and provide a precise definition of {\em robustness} of the control
strategy. We then discuss and compare some significant problems from NMR in the
light of the given definition. We obtain quantitative results that are
consistent with the qualitative ones available in the physics literature.


title:
Generation of Nonclassical Photon Pairs for Scalable Quantum
  Communication with Atomic Ensembles

abstract:
  Quantum information science attempts to exploit capabilities from the quantum
realm to accomplish tasks that are otherwise impossible in the classical domain
[1]. Although sufficient conditions have been formulated for the physical
resources required to achieve quantum computation and communication [2], there
is an evolving understanding of the power of quantum measurement combined with
conditional evolution for accomplishing diverse tasks in quantum information
science [3-5]. In this regard, a significant advance is the invention of a
protocol by Duan, Lukin, Cirac, and Zoller (DLCZ) [6] for the realization of
scalable long distance quantum communication and the distribution of
entanglement over quantum networks. Here, we report the first enabling step in
the realization of the protocol of DLCZ, namely the observation of quantum
correlations for photon pairs generated in the collective emission from an
atomic ensemble. The nonclassical character of the fields is evidenced by the
violation of a Cauchy-Schwarz inequality for the two fields (1,2). As compared
to prior investigations of nonclassical correlations for photon pairs produced
in atomic cascades [7] and in parametric down conversion [8], our experiment is
distinct in that the correlated (1,2) photons are separated by a programmable
time interval delta t, with delta t ~ 400 nsec in our initial experiments.


title: Bohmian arrival time without trajectories

abstract:
  The computation of detection probabilities and arrival time distributions
within Bohmian mechanics in general needs the explicit knowledge of a relevant
sample of trajectories. Here it is shown how for one-dimensional systems and
rigid inertial detectors these quantities can be computed without calculating
any trajectories. An expression in terms of the wave function and its spatial
derivative, both restricted to the boundary of the detector's spacetime volume,
is derived for the general case, where the probability current at the
detector's boundary may vary its sign.


title:
Collective Atomic Motion in an Optical Lattice formed inside a High
  Finesse Cavity

abstract:
  We report on collective non-linear dynamics in an optical lattice formed
inside a high finesse ring cavity in a so far unexplored regime, where the
light shift per photon times the number of trapped atoms exceeds the cavity
resonance linewidth. We observe bistability and self-induced squeezing
oscillations resulting from the retro-action of the atoms upon the optical
potential wells. We can well understand most of our observations within a
simplified model assuming adiabaticity of the atomic motion. Non-adiabatic
aspects of the atomic motion are reproduced by solving the complete system of
coupled non-linear equations of motion for hundred atoms.


title: Optical Cavity Implementations of the Quantum Walk

abstract:
  We discuss how the coined quantum walk on the line or on the circle can be
implemented using optical waves. We propose several optical cavity
configurations for these implementations.


title: Spin squeezing as a measure of entanglement in a two qubit system

abstract:
  We show that two definitions of spin squeezing extensively used in the
literature [M. Kitagawa and M. Ueda, Phys. Rev. A {\bf 47}, 5138 (1993) and
D.J. Wineland {\it et al.}, Phys. Rev. A {\bf 50}, 67 (1994)] give different
predictions of entanglement in the two-atom Dicke system. We analyze
differences between the definitions and show that the Kitagawa and Ueda's spin
squeezing parameter is a better measure of entanglement than the commonly used
spectroscopic spin squeezing parameter. We illustrate this relation by
examining different examples of a driven two-atom Dicke system in which spin
squeezing and entanglement arise dynamically. We give an explanation of the
source of the difference in the prediction of entanglement using the negativity
criterion for entanglement. For the examples discussed, we find that the
Kitagawa and Ueda's spin squeezing parameter is the sufficient and necessary
condition for entanglement.


title: Generating optical nonlinearity using trapped atoms

abstract:
  We describe a scheme for producing an optical nonlinearity using an
interaction with one or more ancilla two-level atomic systems. The
nonlinearity, which can be implemented using high efficiency fluorescence
shelving measurements, together with general linear transformations is
sufficient for simulating arbitrary Hamiltonian evolution on a Fock state
qudit. We give two examples of the application of this nonlinearity, one for
the creation of nonlinear phase shifts on optical fields as required in single
photon quantum computation schemes, and the other for the preparation of
optical Schrodinger cat states.


title: Secure Key Distribution by Swapping Quantum Entanglement

abstract:
  We report two key distribution schemes achieved by swapping quantum
entanglement. Using two Bell states, two bits of secret key can be shared
between two distant parties that play symmetric and equal roles. We also
address eavesdropping attacks against the schemes.


title: A Twist (with Pike) for the Simple Harmonic Oscillator

abstract:
  Extension of the formalism of Q.M. to resolve mathematical anomalies in the
structure of anti-unitary operators; implications for vacuum structure and
spin-statistics arising from an analysis applied to the S.H.O. Outline of the
derived properties of the S.M. Higgs boson.


title: Cluster states, algorithms and graphs

abstract:
  The present paper is concerned with the concept of the one-way quantum
computer, beyond binary-systems, and its relation to the concept of stabilizer
quantum codes. This relation is exploited to analyze a particular class of
quantum algorithms, called graph algorithms, which correspond in the binary
case to the Clifford group part of a network and which can efficiently be
implemented on a one-way quantum computer. These algorithms can ``completely be
solved" in the sense that the manipulation of quantum states in each step can
be computed explicitly. Graph algorithms are precisely those which implement
encoding schemes for graph codes. Starting from a given initial graph, which
represents the underlying resource of multipartite entanglement, each step of
the algorithm is related to a explicit transformation on the graph.


title: Elliptic Rydberg states as direction indicators

abstract:
  The orientation in space of a Cartesian coordinate system can be indicated by
the two vectorial constants of motion of a classical Keplerian orbit: the
angular momentum and the Laplace-Runge-Lenz vector. In quantum mechanics, the
states of a hydrogen atom that mimic classical elliptic orbits are the coherent
states of the SO(4) rotation group.It is known how to produce these states
experimentally. They have minimal dispersions of the two conserved vectors and
can be used as direction indicators. We compare the fidelity of this
transmission method with that of the idealized optimal method.


title:
A quantitative theory-versus-experiment comparison for the intense laser
  dissociation of H2+

abstract:
  A detailed theory-versus-experiment comparison is worked out for H$_2^+$
intense laser dissociation, based on angularly resolved photodissociation
spectra recently recorded in H.Figger's group. As opposite to other
experimental setups, it is an electric discharge (and not an optical
excitation) that prepares the molecular ion, with the advantage for the
theoretical approach, to neglect without lost of accuracy, the otherwise
important ionization-dissociation competition. Abel transformation relates the
dissociation probability starting from a single ro-vibrational state, to the
probability of observing a hydrogen atom at a given pixel of the detector
plate. Some statistics on initial ro-vibrational distributions, together with a
spatial averaging over laser focus area, lead to photofragments kinetic
spectra, with well separated peaks attributed to single vibrational levels. An
excellent theory-versus-experiment agreement is reached not only for the
kinetic spectra, but also for the angular distributions of fragments
originating from two different vibrational levels resulting into more or less
alignment. Some characteristic features can be interpreted in terms of basic
mechanisms such as bond softening or vibrational trapping.


title:
Necesary and sufficient range-dimension conditions for bipartite quantum
  correlations

abstract:
  Necessary and sufficient conditions for the existence of a composite-system
statistical operator, and, separately, for the possibility of its being
correlated or uncorrelated, are derived in terms of its range dimension and the
range dimensions of its reduced statistical operators.


title:
Generalization of the Darboux transformation and generalized harmonic
  oscillators

abstract:
  The Darbroux transformation is generalized for time-dependent Hamiltonian
systems which include a term linear in momentum and a time-dependent mass. The
formalism for the $N$-fold application of the transformation is also
established, and these formalisms are applied for a general quadratic system (a
generalized harmonic oscillator) and a quadratic system with an inverse-square
interaction up to N=2. Among the new features found, it is shown, for the
general quadratic system, that the shape of potential difference between the
original system and the transformed system could oscillate according to a
classical solution, which is related to the existence of coherent states in the
system.


title: Uniqueness of conserved currents in quantum mechanics

abstract:
  It is proved by a functional method that the conventional expression for the
Dirac current is the only conserved 4-vector implied by the Dirac equation that
is a function of just the quantum state. The demonstration is extended to
derive the unique conserved currents implied by the coupled Maxwell-Dirac
equations and the Klein-Gordon equation. The uniqueness of the usual Pauli and
Schrodinger currents follows by regarding these as the non-relativistic limits
of the Dirac and Klein-Gordon currents, respectively. The existence and
properties of further conserved vectors that are not functions of just the
state is examined.


title: On Feedback and the Classical Capacity of a Noisy Quantum Channel

abstract:
  In Shannon information theory the capacity of a memoryless communication
channel cannot be increased by the use of feedback from receiver to sender. In
this paper the use of classical feedback is shown to provide no increase in the
unassisted classical capacity of a memoryless quantum channel when feedback is
used across non-entangled input states, or when the channel is an
entanglement--breaking channel. This gives a generalization of the Shannon
theory for certain classes of feedback protocols when transmitting through
noisy quantum communication channels.


title: Quantum Search for Zeros of Polynomials

abstract:
  A quantum mechanical search procedure to determine the real zeros of a
polynomial is introduced. It is based on the construction of a spin observable
whose eigenvalues coincide with the zeros of the polynomial. Subsequent quantum
mechanical measurements of the observable output directly the numerical values
of the zeros. Performing the measurements is the only computational resource
involved.


title:
Enhancing Acceleration Radiation from Ground-State Atoms via Cavity
  Quantum Electrodynamics

abstract:
  When ground state atoms are accelerated through a high Q microwave cavity,
radiation is produced with an intensity which can exceed the intensity of Unruh
acceleration radiation in free space by many orders of magnitude. The cavity
field at steady state is described by a thermal density matrix under most
conditions. However, under some conditions gain is possible, and when the atoms
are injected in a regular fashion, the radiation can be produced in a squeezed
state.


title:
Polynomial Degree and Lower Bounds in Quantum Complexity: Collision and
  Element Distinctness with Small Range

abstract:
  We give a general method for proving quantum lower bounds for problems with
small range. Namely, we show that, for any symmetric problem defined on
functions $f:\{1, ..., N\}\to\{1, ..., M\}$, its polynomial degree is the same
for all $M\geq N$. Therefore, if we have a quantum lower bound for some
(possibly, quite large) range $M$ which is shown using polynomials method, we
immediately get the same lower bound for all ranges $M\geq N$. In particular,
we get $\Omega(N^{1/3})$ and $\Omega(N^{2/3})$ quantum lower bounds for
collision and element distinctness with small range.


title: Physical realizations of quantum operations

abstract:
  Quantum operations (QO) describe any state change allowed in quantum
mechanics, such as the evolution of an open system or the state change due to a
measurement. We address the problem of which unitary transformations and which
observables can be used to achieve a QO with generally different input and
output Hilbert spaces. We classify all unitary extensions of a QO, and give
explicit realizations in terms of free-evolution direct-sum dilations and
interacting tensor-product dilations. In terms of Hilbert space dimensionality
the free-evolution dilations minimize the physical resources needed to realize
the QO, and for this case we provide bounds for the dimension of the ancilla
space versus the rank of the QO. The interacting dilations, on the other hand,
correspond to the customary ancilla-system interaction realization, and for
these we derive a majorization relation which selects the allowed unitary
interactions between system and ancilla.


title:
Hermitian Schmidt Decomposition and Twin Observables of Bipartite Mixed
  States

abstract:
  It is shown that for each mixed state there exists a Schmidt (super state
vector) decomposition in terms of Hermitian operators. Its utilization for
finding all twins is illustrated in full detail in the case of the two
spin-one-half-particle states with maximally disordered subsystems (mixtures of
Bell states).


title: Continuous-time quantum walks on the symmetric group

abstract:
  In this paper we study continuous-time quantum walks on Cayley graphs of the
symmetric group, and prove various facts concerning such walks that demonstrate
significant differences from their classical analogues. In particular, we show
that for several natural choices for generating sets, these quantum walks do
not have uniform limiting distributions, and are effectively blind to large
areas of the graphs due to destructive interference.


title:
Reappraisal of the causal interpretation of quantum mechanics and of the
  quantum potential concept

abstract:
  The causal interpretation of quantum mechanics, as originally stated by
deBroglie and Bohm, had several attractive features. Among these is the
possibility that it could address some of the most fundamental questions on
quantum phenomena. However, subsequent theoretical conjectures, which have now
been included in the orthodox view of the deBroglie Bohm theory, are unphysical
and have done much to undermine the original theory's appeal. We, therefore,
return to the original theory as our starting point and address one of its
perplexing areas: the quantum potential. By avoiding the unphysical conjectures
we are led to an understanding of the quantum potential which is distinctly
different from that of the orthodox deBroglie Bohm view.


title: Dispersion Compensation for Atom Interferometry

abstract:
  A new technique for maintaining high contrast in an atom interferometer is
used to measure large de Broglie wave phase shifts. Dependence of an
interaction induced phase on the atoms' velocity is compensated by applying an
engineered \emph{counter phase}. The counter phase is equivalent to a rotation
and precisely determined by a frequency, and can be used to measure phase
shifts due to interactions of unknown strength. Phase shifts of 150 radians (5
times larger than previously possible) have now been measured in an atom beam
interferometer and we suggest that this technique can enable comparisons of
atomic polarizability with precision of one part in 10,000.


title: Photon Added Detection

abstract:
  The production of conditional quantum states and quantum operations based on
the result of measurement is now seen as a key tool in quantum information and
metrology. We propose a new type of photon number detector. It functions
non-deterministically, but when successful, it has high fidelity. The detector,
which makes use of an n-photon auxiliary Fock state and high efficiency
Homodyne detection, allows a tunable tradeoff between fidelity and probability.
By sacrificing probability of operation, an excellent approximation to a photon
number detector is achieved.


title: Multiorder coherent Raman scattering of a quantum probe field

abstract:
  We study the multiorder coherent Raman scattering of a quantum probe field in
a far-off-resonance medium with a prepared coherence. Under the conditions of
negligible dispersion and limited bandwidth, we derive a Bessel-function
solution for the sideband field operators. We analytically and numerically
calculate various quantum statistical characteristics of the sideband fields.
We show that the multiorder coherent Raman process can replicate the
statistical properties of a single-mode quantum probe field into a broad comb
of generated Raman sidebands. We also study the mixing and modulation of photon
statistical properties in the case of two-mode input. We show that the prepared
Raman coherence and the medium length can be used as control parameters to
switch a sideband field from one type of photon statistics to another type, or
from a non-squeezed state to a squeezed state and vice versa.


title:
Chains of Quasi-Classical Informations for Bipartite Correlations and
  the Role of Twin Observables

abstract:
  Having the quantum correlations in a general bipartite state in mind, the
information accessible by simultaneous measurement on both subsystems is shown
never to exceed the information accessible by measurement on one subsystem,
which, in turn is proved not to exceed the von Neumann mutual information. A
particular pair of (opposite- subsystem) observables are shown to be
responsible both for the amount of quasi-classical correlations and for that of
the purely quantum entanglement in the pure-state case: the former via
simultaneous subsystem measurements, and the latter through the entropy of
coherence or of incompatibility, which is defined for the general case. The
observables at issue are so-called twin observables. A general definition of
the latter is given in terms of their detailed properties.


title: Dynamics of Distillability

abstract:
  The time evolution of a maximally entangled bipartite systems is presented in
this paper. The distillability criterion is given in terms of Kraus operators.
Using the criterion, we discuss the distillability of $2\times 2$ and $n\times
n (n>2)$ systems in their evolution process. There are two distinguished
processes, dissipation and decoherence, which may destroy the distillability.
We discuss the effects of those processes on distillability in details.


title:
An asymmetric double-slit interferometer for small and large quantum
  particles

abstract:
  Quantum theory of interference phenomena does not take the diameter of the
particle into account, since particles were much smaller than the width of the
slits in early observations. In recent experiments with large molecules, the
diameter of the particle has approached the width of the slits. Therefore,
analytical description of these cases should include a finite particle size.
The generic quantum interference setup is an asymmetric double slit
interferometer. We evaluate the wave function of the particle transverse motion
using two forms of the solution of Schrodinger's equation in an asymmetric
interferometer: the Fresnel-Kirchhoff form and the form derived from the
transverse wave function in the momentum representation. The transverse
momentum distribution is independent of the distance from the slits, while the
space distribution strongly depends on this distance. Based on the transverse
momentum distribution we determined the space distribution of particles behind
the slits. We will present two cases: a) when the diameter of the particle may
be neglected with respect to the width of both slits, and b) when the diameter
of the particle is larger than the width of the smaller slit.


title: Symmetries of the Bell correlation inequalities

abstract:
  Bell correlation inequalities for two sites and 2+n or 3+3 two-way
measurements ("dichotomic observables") are considered. In the 2+n case, any
facet of the classical experience polytope is defined by a CHSH inequality
involving only two pairs of the observables. In the 3+3 case, contrary to
earlier results, the action of the symmetry group reduces the set of all Bell
inequalities to just 3 orbits, only one of them being "new" (not known from the
2+2 case). A detailed calculation for the singlet state of two qubits reveals
the configurations of a maximal violation for this class of inequalities.


title: Fiber-assisted detection with photon number resolution

abstract:
  We report the development of a photon-number resolving detector based on a
fiber-optical setup and a pair of standard avalanche photodiodes. The detector
is capable of resolving individual photon numbers, and operates on the
well-known principle by which a single mode input state is split into a large
number (eight) of output modes. We reconstruct the photon statistics of weak
coherent input light from experimental data, and show that there is a high
probability of inferring the input photon number from a measurement of the
number of detection events on a single run.


title: Photon engineering for quantum information processing

abstract:
  We study distinguishing information in the context of quantum interference
involving more than one parametric downconversion (PDC) source and in the
context of polarization-entangled photon pairs based on PDC. We arrive at
specific design criteria for two-photon sources so that when used as part of
complex optical systems, such as photon-based quantum information processing
schemes, distinguishing information between the photons is eliminated
guaranteeing high visibility interference. We propose practical techniques
which lead to suitably engineered two-photon states that can be realistically
implemented with available technology. Finally, we study an implementation of
the nonlinear-sign shift (NS) logic gate with PDC sources and show the effect
of distinguishing information on the performance of the gate.


title: Photon number resolution using a time-multiplexed single-photon detector

abstract:
  Photon number resolving detectors are needed for a variety of applications
including linear-optics quantum computing. Here we describe the use of
time-multiplexing techniques that allows ordinary single photon detectors, such
as silicon avalanche photodiodes, to be used as photon number-resolving
detectors. The ability of such a detector to correctly measure the number of
photons for an incident number state is analyzed. The predicted results for an
incident coherent state are found to be in good agreement with the results of a
proof-of-principle experimental demonstration.


title: Quantum states of hierarchic systems

abstract:
  The density matrix formalism which is widely used in the theory of
measurements, quantum computing, quantum description of chemical and biological
systems always imply the averaging over the states of the environment. In
practice this is impossible because the environment $U\setminusS$ of the system
$S$ is the complement of this system to the whole Universe and contains
infinitely many degrees of freedom. A novel method of construction density
matrix which implies the averaging only over the direct environment is
proposed. The Hilbert space of state vectors for the hierarchic quantum systems
is constructed.


title:
Entangled graphs II: Classical correlations in multi-qubit entangled
  systems

abstract:
  Bipartite correlations in multi-qubit systems cannot be shared freely. The
presence of entanglement or classical correlation on certain pairs of qubits
may imply correlations on other pairs. We present a method of characterization
of bi-partite correlations in multi-qubit systems using a concept of entangled
graphs that has been introduced in our earlier work [M.Plesch and V.Buzek,
Phys. Rev. A 67, 012322 (2003)]. In entangled graphs each qubit is represented
by a vertex while the entanglement and classical correlations are represented
by two types of edges. We prove by construction that any entangled graph with
classical correlations can be represented by a mixed state of N qubits.
However, not all entangled graphs with classical correlations can be
represented by a pure state.


title:
Intrinsic Decoherence Dynamics in Smooth Hamiltonian Systems:
  Quantum-classical Correspondence

abstract:
  A direct classical analog of the quantum dynamics of intrinsic decoherence in
Hamiltonian systems, characterized by the time dependence of the linear entropy
of the reduced density operator, is introduced. The similarities and
differences between the classical and quantum decoherence dynamics of an
initial quantum state are exposed using both analytical and computational
results. In particular, the classicality of early-time intrinsic decoherence
dynamics is explored analytically using a second-order perturbative treatment,
and an interesting connection between decoherence rates and the stability
nature of classical trajectories is revealed in a simple approximate classical
theory of intrinsic decoherence dynamics. The results offer new insights into
decoherence, dynamics of quantum entanglement, and quantum chaos.


title:
Contextual approach to quantum mechanics and the theory of the
  fundamental prespace

abstract:
  We constructed a Hilbert space representation of a contextual Kolmogorov
model. This representation is based on two fundamental observables -- in the
standard quantum model these are position and momentum observables. This
representation has all distinguishing features of the quantum model. Thus in
spite all ``No-Go'' theorems (e.g., von Neumann, Kochen and Specker,..., Bell)
we found the realist basis for quantum mechanics. Our representation is not
standard model with hidden variables. In particular, this is not a reduction of
quantum model to the classical one. Moreover, we see that such a reduction is
even in principle impossible. This impossibility is not a consequence of a
mathematical theorem but it follows from the physical structure of the model.
By our model quantum states are very rough images of domains in the space of
fundamental parameters - PRESPACE. Those domains represent complexes of
physical conditions. By our model both classical and quantum physics describe
REDUCTION of PRESPACE-INFORMATION. Quantum mechanics is not complete. In
particular, there are prespace contexts which can be represented only by a so
called hyperbolic quantum model. We predict violations of the Heisenberg's
uncertainty principle and existence of dispersion free states.


title: Quantum computation with optical coherent states

abstract:
  We show that quantum computation circuits using coherent states as the
logical qubits can be constructed from simple linear networks, conditional
photon measurements and "small" coherent superposition resource states.


title: Some prospects for ensemble solid-state NMR quantum computers

abstract:
  As an ensemble scheme of solid-state NMR quantum computers the extension of
Kane's many-qubits silicon scheme based on the array of 31 P donor atoms are
spaced lengthwise of the strip gates is considered. The possible planar
topology of such ensemble quantum computer is suggested. The estimation of the
output NMR signal was performed and it was shown that for the number N>=10^5 of
ensemble elements involving L~10^3 qubits each, the standard NMR methods are
usable.
  As main mechanisms of decoherence for low temperature (<0.1K), the adiabatic
processes of random modulation of qubit resonance frequency determined by
secular part of nuclear spin hyperfine interaction with electron magnetic
moment of basic atom and dipole-dipole interaction with nuclear moments of
neighboring impurity atoms was considered, It was made estimations of allowed
concentrations of magnetic impurities and of spin temperature whereby the
required decoherence suppression is obtained. Semiclassical decoherence model
of two qubit entangled states is also presented.
  As another variant of the solid-state ensemble quantum computer, the gateless
architecture of cellular-automaton with antiferromagnetically ordered electron
spins is also discussed here.


title: Fast and robust two-qubit gates for scalable ion trap quantum computing

abstract:
  We propose a new concept for a two-qubit gate operating on a pair of trapped
ions based on laser coherent control techniques. The gate is insensitive to the
temperature of the ions, works also outside the Lamb-Dicke regime, requires no
individual addressing by lasers, and can be orders of magnitude faster than the
trap period.


title: A Stern-Gerlach Experiment in Time

abstract:
  In non-relativistic quantum mechanics, path integrals are normally derived
from the Schroedinger equation. This assumes the two formalisms are equivalent.
Since time plays a very different role in the Schroedinger equation and in path
integrals, this may not be the case.
  We here derive path integrals directly by imposing two requirements: correct
behavior in the classical limit and the most complete practicable symmetry
between time and space.
  With these requirements, the path integral formalism predicts quantum
fluctuations over the time dimension analogous to the quantum fluctuations seen
over the three space dimensions. For constant potentials there is no effect.
But the coupling between rapidly varying electromagnetic fields and the quantum
fluctuations in time should be detectable.
  We consider a variation on the Stern-Gerlach experiment in which a particle
with a non-zero electric dipole moment is sent through a rapidly varying
electric field, oriented parallel to the particle's trajectory. The
Schroedinger equation predicts changes to the precession frequency of the wave
function about the trajectory but no physical splitting of the beam. With the
approach here, path integrals predict the changes to the precession frequency
and in addition that the beam will be split in velocity and time.


title: Quantum system identification

abstract:
  We formulate and study, in general terms, the problem of quantum system
identification, i.e., the determination (or estimation) of unknown quantum
channels through their action on suitably chosen input density operators. We
also present a quantitative analysis of the worst-case performance of these
schemes.


title:
Entanglement Cost of Antisymmetric States and Additivity of Capacity of
  Some Quantum Channel

abstract:
  We study the entanglement cost of the states in the contragredient space,
which consists of $(d-1)$ $d$-dimensional systems. The cost is always $\log_2
(d-1)$ ebits when the state is divided into bipartite $\C^d \otimes
(\C^d)^{d-2}$. Combined with the arguments in \cite{Matsumoto02}, additivity of
channel capacity of some quantum channels is also shown.


title: Quantum Mechanics on the cylinder

abstract:
  A new approach to deformation quantization on the cylinder considered as
phase space is presented. The method is based on the standard Moyal formalism
for R^2 adapted to (S^1 x R) by the Weil--Brezin--Zak transformation. The
results are compared with other solutions of this problem presented by
Kasperkovitz and Peev (Ann. Phys. vol. 230, 21 (1994)0 and by Plebanski and
collaborators (Acta Phys. Pol. vol. B 31}, 561 (2000)). The equivalence of
these three methods is proved.


title: Special functions, raising and lowering operators

abstract:
  The Schrodinger equations which are exactly solvable in terms of associated
special functions are directly related to some self-adjoint operators defined
in the theory of hypergeometric type equations. The fundamental formulae
occurring in a supersymmetric approach to these Hamiltonians are consequences
of some formulae concerning the general theory of associated special functions.
We use this connection in order to obtain a general theory of Schrodinger
equations exactly solvable in terms of associated special functions, and to
extend certain results known in the case of some particular potentials.


title:
Orthogonal polynomials defined by hypergeometric type equations with
  application to quantum mechanics

abstract:
  A hypergeometric type equation satisfying certain conditions defines either a
finite or an infinite system of orthogonal polynomials. We present in a unified
and explicit way all these systems of orthogonal polynomials, the associated
special functions and the corresponding raising/lowering operators. The
considered equations are directly related to some Schrodinger type equations
(Poschl-Teller, Scarf, Morse, etc), and the defined special functions are
related to the corresponding bound-state eigenfunctions.


title: Shapes of Quantum States

abstract:
  The shape space of k labelled points on a plane can be identified with the
space of pure quantum states of dimension k-2. Hence, the machinery of quantum
mechanics can be applied to the statistical analysis of planar configurations
of points. Various correspondences between point configurations and quantum
states, such as linear superposition as well as unitary and stochastic
evolution of shapes, are illustrated. In particular, a complete
characterisation of shape eigenstates for an arbitrary number of points is
given in terms of cyclotomic equations.


title:
The role of self-induced decoherence in the problem of the classical
  limit of quantum mechanics

abstract:
  Our account of the problem of the classical limit of quantum mechanics
involves two elements. The first one is self-induced decoherence, conceived as
a process that depends on the own dynamics of a closed quantum system governed
by a Hamiltonian with continuous spectrum; the study of decoherence is
addressed by means of a formalism used to give meaning to the van Hove states
with diagonal singularities. The second element is macroscopicity: when the
macroscopic limit is applied to the Wigner transformation of the diagonal state
resulting from decoherence, the description of the quantum system becomes
equivalent to the description of an ensemble of classical trajectories on phase
space weighted by their corresponding probabilities.


title: Entanglement Sharing in the Two-Atom Tavis-Cummings Model

abstract:
  Individual members of an ensemble of identical systems coupled to a common
probe can become entangled with one another, even when they do not interact
directly. We investigate how this type of multipartite entanglement is
generated in the context of a system consisting of two two-level atoms
resonantly coupled to a single mode of the electromagnetic field. The dynamical
evolution is studied in terms of the entanglements in the different bipartite
partitions of the system, as quantified by the I-tangle. We also propose a
generalization of the so-called residual tangle that quantifies the inherent
three-body correlations in our tripartite system. This enables us to completely
characterize the phenomenon of entanglement sharing in the case of the two-atom
Tavis-Cummings model, a system of both theoretical and experimental interest.


title: Spectrum of One-Dimensional Multiple Well Oscillators

abstract:
  We apply power series expansion to symmetric multi-well oscillators bounded
by two infinite walls. The spectrum and expectation values obtained are
compared with available exact and approximate values for the unbounded ones. It
is shown that the method is capable of producing to a high accuracy the
eigenvalues, eigenfunctions, and the expectation values $<x^{2k}>$ of the
corresponding unbounded ones as the separation between the two infinite walls
becomes large.


title: On the communication complexity of establishing a shared reference frame

abstract:
  We discuss the communication complexity of establishing a shared reference
frame, in particular examining the case of aligning spatial axes via the
exchange of spin-1/2 particles. Unlike previous work we allow for multiple
rounds of communication, and we give several simple examples demonstrating that
nontrivial tradeoffs between the number of rounds and the type of communication
required exist. We then give an explicit protocol for aligning spatial axes via
the exchange of spin-1/2 particles which makes no use of either exchanged
entangled states or of joint measurements. Rather it works by performing a
simple type of distributed quantum computation. To facilitate comparison with
previous work, we show that this protocol achieves a worst case fidelity for
the much studied problem of "direction finding" that is asymptotically
equivalent (up to polylog factors) to the optimal average case fidelity
achievable via a single forward communication of entangled states.


title: Scalability of Shor's algorithm with a limited set of rotation gates

abstract:
  Typical circuit implementations of Shor's algorithm involve controlled
rotation gates of magnitude $\pi/2^{2L}$ where $L$ is the binary length of the
integer N to be factored. Such gates cannot be implemented exactly using
existing fault-tolerant techniques. Approximating a given controlled
$\pi/2^{d}$ rotation gate to within $\delta=O(1/2^{d})$ currently requires both
a number of qubits and number of fault-tolerant gates that grows polynomially
with $d$. In this paper we show that this additional growth in space and time
complexity would severely limit the applicability of Shor's algorithm to large
integers. Consequently, we study in detail the effect of using only controlled
rotation gates with $d$ less than or equal to some $d_{\rm max}$. It is found
that integers up to length $L_{\rm max} = O(4^{d_{\rm max}})$ can be factored
without significant performance penalty implying that the cumbersome techniques
of fault-tolerant computation only need to be used to create controlled
rotation gates of magnitude $\pi/64$ if integers thousands of bits long are
desired factored. Explicit fault-tolerant constructions of such gates are also
discussed.


title: Decoherence without dissipation?

abstract:
  In a recent article, Ford, Lewis and O'Connell (PRA 64, 032101 (2001))
discuss a thought experiment in which a Brownian particle is subjected to a
double-slit measurement. Analyzing the decay of the emerging interference
pattern, they derive a decoherence rate that is much faster than previous
results and even persists in the limit of vanishing dissipation. This result is
based on the definition of a certain attenuation factor, which they analyze for
short times. In this note, we point out that this attenuation factor captures
the physics of decoherence only for times larger than a certain time t_mix,
which is the time it takes until the two emerging wave packets begin to
overlap. Therefore, the strategy of Ford et al of extracting the decoherence
time from the regime t < t_mix is in our opinion not meaningful. If one
analyzes the attenuation factor for t > t_mix, one recovers familiar behaviour
for the decoherence time; in particular, no decoherence is seen in the absence
of dissipation. The latter conclusion is confirmed with a simple calculation of
the off-diagonal elements of the reduced density matrix.


title:
Experimental verification of energy correlations in entangled photon
  pairs

abstract:
  Properties of entangled photon pairs generated in spontaneous parametric
down-conversion are investigated in interference experiments. Strong energy
correlations are demonstrated in a direct way. If a signal photon is detected
behind a narrow spectral filter, then interference appears in the Mach-Zehnder
interferometer placed in the route of the idler photon, even if the path
difference in the interferometer exceeds the coherence length of the light.
Narrow time correlations of the detection instants are demonstrated for the
same photon-pair source using the Hong-Ou-Mandel interferometer. Both these two
effects may be exhibited only by an entangled state.


title:
The Model of the Theory of the Quantum Brain Dynamics can be cast on the
  Heisenberg Spin Hamiltonian

abstract:
  In this note, we show that the model of the quantum brain dynamics can be
cast on a kind of the Heisenberg spin Hamiltonian. Therefore, we would like to
emphasize that the quantum dynamics of brain should be understood by the
physics of quantum spin systems.

categories: quant-ph cond-mat.dis-nn cond-mat.stat-mech physics.med-ph


title: Complete population transfer in degenerate n-state atoms

abstract:
  We find a set of conditions to achieve complete population transfer, via
coherent population trapping, from an initial state to a designated final state
at a designated time in a degenerate n-state atom, where the transitions are
caused by an external interaction.


title:
Synchronizing quantum clocks with classical one-way communication:
  Bounds on the generated entropy

abstract:
  We describe separable joint states on bipartite quantum systems that cannot
be prepared by any thermodynamically reversible classical one-way communication
protocol. We argue that the joint state of two synchronized microscopic clocks
is always of this type when it is considered from the point of view of an
``ignorant'' observer who is not synchronized with the other two parties.
  We show that the entropy generation of a classical one-way synchronization
protocol is at least \Delta S = \hbar^2/(4\Delta E \Delta t)^2 if \Delta t is
the time accuracy of the synchronism and \Delta E is the energy bandwidth of
the clocks. This dissipation can only be avoided if the common time of the
microscopic clocks is stored by an additional classical clock.
  Furthermore, we give a similar bound on the entropy cost for resetting
synchronized clocks by a classical one-way protocol. The proof relies on
observations of Zurek on the thermodynamic relevance of quantum discord. We
leave it as an open question whether classical multi-step protocols may perform
better.
  We discuss to what extent our results imply problems for classical concepts
of reversible computation when the energy of timing signals is close to the
Heisenberg limit.


title: Another state entanglement measure

abstract:
  We propose a measure of state entanglement for states of the tensor-product
of C*-algebras.


title: Quantum universal detectors

abstract:
  We address the problem of estimating the expectation value <O> of an
arbitrary operator O via a universal measuring apparatus that is independent of
O, and for which the expectation values for different operators are obtained by
changing only the data-processing. The ``universal detector'' performs a joint
measurement on the system and on a suitably prepared ancilla. We characterize
such universal detectors, and show how they can be obtained either via Bell
measurements or via local measurements and classical communication between
system and ancilla.


title: Rotational Cooling of Polar Molecules by Stark-tuned Cavity Resonance

abstract:
  A general scheme for rotational cooling of diatomic heteronuclear molecules
is proposed. It uses a superconducting microwave cavity to enhance the
spontaneous decay via Purcell effect. Rotational cooling can be induced by
sequentially tuning each rotational transition to cavity resonance, starting
from the highest transition level to the lowest using an electric field.
Electrostatic multipoles can be used to provide large confinement volume with
essentially homogeneous background electric field.


title: Beam Splitter Entangler for Light Fields

abstract:
  We propose an experimentally feasible scheme to generate various types of
entangled states of light fields by using beam splitters and single-photon
detectors. Two light fields are incident on two beam splitters and are split
into strong and weak output modes respectively. A conditional joint measurement
on both weak output modes may result in an entanglement between the two strong
output modes. The conditions for the maximal entanglement are discussed based
on the concurrence. Several specific examples are also examined.


title:
Electromagnetic-field quantization and spontaneous decay in left-handed
  media

abstract:
  We present a quantization scheme for the electromagnetic field interacting
with atomic systems in the presence of dispersing and absorbing
magnetodielectric media, including left-handed material having negative real
part of the refractive index. The theory is applied to the spontaneous decay of
a two-level atom at the center of a spherical free-space cavity surrounded by
magnetodielectric matter of overlapping band-gap zones. Results for both big
and small cavities are presented, and the problem of local-field corrections
within the real-cavity model is addressed.


title: On Koopman-von Neumann Waves II

abstract:
  In this paper we continue the study, started in [1], of the operatorial
formulation of classical mechanics given by Koopman and von Neumann (KvN) in
the Thirties. In particular we show that the introduction of the KvN Hilbert
space of complex and square integrable "wave functions" requires an enlargement
of the set of the observables of ordinary classical mechanics. The possible
role and the meaning of these extra observables is briefly indicated in this
work. We also analyze the similarities and differences between non selective
measurements and two-slit experiments in classical and quantum mechanics.


title: An An exactly solvable PT symmetric potential from the Natanzon class

abstract:
  The ${\cal PT}$ symmetric version of the generalised Ginocchio potential, a
member of the general exactly solvable Natanzon potential class is analysed and
its properties are compared with those of ${\cal PT}$ symmetric potentials from
the more restricted shape-invariant class. It is found that the ${\cal PT}$
symmetric generalised Ginocchio potential has a number of properties in common
with the latter potentials: it can be generated by an imaginary coordinate
shift $x\to x+{\rm i}\epsilon$; its states are characterised by the
quasi-parity quantum number; the spontaneous breakdown of ${\cal PT}$ symmetry
occurs at the same time for all the energy levels; and it has two
supersymmetric partners which cease to be ${\cal PT}$ symmetric when the ${\cal
PT}$ symmetry of the original potential is spontaneously broken.


title: An exactly solvable PT symmetric potential from the Natanzon class


title:
Accessibility of physical states and non-uniqueness of entanglement
  measure

abstract:
  Ordering physical states is the key to quantifying some physical property of
the states uniquely. Bipartite pure entangled states are totally ordered under
local operations and classical communication (LOCC) in the asymptotic limit and
uniquely quantified by the well-known entropy of entanglement. However, we show
that mixed entangled states are partially ordered under LOCC even in the
asymptotic limit. Therefore, non-uniqueness of entanglement measure is
understood on the basis of an operational notion of asymptotic convertibility.


title: Non-relativistic proofs of the spin-statistics connection

abstract:
  A recent paper by Peshkin [1] has drawn attention again to the problem of
understanding the spin statistics connection in non-relativistic quantum
mechanics. Allen and Mondragon [2] has pointed out correctly some of the flaws
in Peshkin's arguments which are based on the single valuedness under rotation
of the wave functions of systems of identical particles. We examine carefully
the claim that is made in the title of [2] that there can be ``no
spin-statistics connection in non-relativistic quantum mechanics''. We show
that we can derive the spin statistics connection for non-relativistic quantum
field theories which have equations of motion of the Hamiltonian type based on
SU(2) invariance of the Lagrangian. The formalism and machinary of
non-relativistic quantum field theory as opposed to usual quantum mechanics is
necessary for constructing our proof.


title:
Implementation of generalized quantum measurements: superadditive
  quantum coding, accessible information extraction, and classical capacity
  limit

abstract:
  Quantum information theory predicts that when the transmission resource is
doubled in quantum channels, the amount of information transmitted can be
increased more than twice by quantum channel coding technique, whereas the
increase is at most twice in classical information theory. This remarkable
feature, the superadditive quantum coding gain, can be implemented by
appropriate choices of code words and corresponding quantum decoding which
requires a collective quantum measurement. Recently, the first experimental
demonstration was reported [Phys. Rev. Lett. 90, 167906 (2003)]. The purpose of
this paper is to describe our experiment in detail. Particularly, a design
strategy of quantum collective decoding in physical quantum circuits is
emphasized. We also address the practical implication of the gain on
communication performance by introducing the quantum-classical hybrid coding
scheme. We show how the superadditive quantum coding gain, even in a small code
length, can boost the communication performance of conventional coding
technique.


title: Entangled two cavity modes preparation via a two-photon process

abstract:
  We propose a scheme for entangling two field modes in two high-Q optical
cavities. Making use of a virtual two-photon process, our scheme achieves
maximally entangled states without any real transitions of atomic internal
states, hence it is immune to the atomic decay.


title: Cavity field ensembles from nonselective measurements

abstract:
  We continue our investigations of cavity QED with time dependent parameters.
In this paper we discuss the situation where the state of the atoms leaving the
cavity is reduced but the outcome is not recorded. In this case our knowledge
is limited to an ensemble description of the results only. By applying the
Demkov-Kunike level-crossing model, we show that even in this case, the
filtering action of the interaction allows us to prepare a preassigned Fock
state with good accuracy. The possibilities and limitations of the method are
discussed and some relations to earlier work are presented.


title: A Theory of Errors in Quantum Measurement

abstract:
  It is common to model random errors in a classical measurement by the normal
(Gaussian) distribution, because of the central limit theorem. In the quantum
theory, the analogous hypothesis is that the matrix elements of the error in an
observable are distributed normally. We obtain the probability distribution
this implies for the outcome of a measurement, exactly for the case of 2x2
matrices and in the steepest descent approximation in general. Due to the
phenomenon of `level repulsion', the probability distributions obtained are
quite different from the Gaussian.


title:
Surface-impedance approach solves problems with the thermal Casimir
  force between real metals

abstract:
  The surface impedance approach to the description of the thermal Casimir
effect in the case of real metals is elaborated starting from the free energy
of oscillators. The Lifshitz formula expressed in terms of the dielectric
permittivity depending only on frequency is shown to be inapplicable in the
frequency region where a real current may arise leading to Joule heating of the
metal. The standard concept of a fluctuating electromagnetic field on such
frequencies meets difficulties when used as a model for the zero-point
oscillations or thermal photons in the thermal equilibrium inside metals.
Instead, the surface impedance permits not to consider the electromagnetic
oscillations inside the metal but taking the realistic material properties into
account by means of the effective boundary condition. An independent derivation
of the Lifshitz-type formulas for the Casimir free energy and force between two
metal plates is presented within the impedance approach. It is shown that they
are free of the contradictions with thermodynamics which are specific to the
usual Lifshitz formula for dielectrics in combination with the Drude model. We
demonstrate that in the impedance approach the zero-frequency contribution is
uniquely fixed by the form of impedance function and does not need any of the
ad hoc prescriptions intensively discussed in the recent literature. As an
example, the computations of the Casimir free energy between two gold plates
are performed at different separations and temperatures. It is argued that the
surface impedance approach lays a reliable framework for the future
measurements of the thermal Casimir force.


title:
Efficient measurements, purification, and bounds on the mutual
  information

abstract:
  When a measurement is made on a quantum system in which classical information
is encoded, the measurement reduces the observers average Shannon entropy for
the encoding ensemble. This reduction, being the {\em mutual information}, is
always non-negative. For efficient measurements the state is also purified;
that is, on average, the observers von Neumann entropy for the state of the
system is also reduced by a non-negative amount. Here we point out that by
re-writing a bound derived by Hall [Phys. Rev. A {\bf 55}, 100 (1997)], which
is dual to the Holevo bound, one finds that for efficient measurements, the
mutual information is bounded by the reduction in the von Neumann entropy. We
also show that this result, which provides a physical interpretation for Hall's
bound, may be derived directly from the Schumacher-Westmoreland-Wootters
theorem [Phys. Rev. Lett. {\bf 76}, 3452 (1996)]. We discuss these bounds, and
their relationship to another bound, valid for efficient measurements on pure
state ensembles, which involves the subentropy.


title: Completeness and Orthonormality in PT-symmetric Quantum Systems

abstract:
  Some PT-symmetric non-hermitean Hamiltonians have only real eigenvalues.
There is numerical evidence that the associated PT-invariant energy eigenstates
satisfy an unconventional completeness relation. An ad hoc scalar product among
the states is positive definite only if a recently introduced `charge operator'
is included in its definition. A simple derivation of the conjectured
completeness and orthonormality relations is given. It exploits the fact that
PT-symmetry provides an additional link between the eigenstates of the
Hamiltonian and those of its adjoint, which form a dual pair of bases. The
`charge operator' emerges naturally upon expressing the properties of the dual
bases in terms of one basis only.


title:
Test for entanglement using physically observable witness operators and
  positive maps

abstract:
  Motivated by the Peres-Horodecki criterion and the realignment criterion we
develop a more powerful method to identify entangled states for any bipartite
system through a universal construction of the witness operator. The method
also gives a new family of positive but non-completely positive maps of
arbitrary high dimensions which provide a much better test than the witness
operators themselves. Moreover, we find there are two types of positive maps
that can detect 2xN and 4xN bound entangled states. Since entanglement
witnesses are physical observables and may be measured locally our construction
could be of great significance for future experiments.


title: Multi-Party Pseudo-Telepathy

abstract:
  Quantum entanglement, perhaps the most non-classical manifestation of quantum
information theory, cannot be used to transmit information between remote
parties. Yet, it can be used to reduce the amount of communication required to
process a variety of distributed computational tasks. We speak of
pseudo-telepathy when quantum entanglement serves to eliminate the classical
need to communicate. In earlier examples of pseudo-telepathy, classical
protocols could succeed with high probability unless the inputs were very
large. Here we present a simple multi-party distributed problem for which the
inputs and outputs consist of a single bit per player, and we present a perfect
quantum protocol for it. We prove that no classical protocol can succeed with a
probability that differs from 1/2 by more than a fraction that is exponentially
small in the number of players. This could be used to circumvent the detection
loophole in experimental tests of nonlocality.


title:
Coherent Manipulation of Quantum Delta-kicked Dynamics:
  Faster-than-classical Anomalous Diffusion

abstract:
  Large transporting regular islands are found in the classical phase space of
a modified kicked rotor system in which the kicking potential is reversed after
every two kicks. The corresponding quantum system, for a variety of system
parameters and over long time scales, is shown to display energy absorption
that is significantly faster than that associated with the underlying classical
anomalous diffusion. The results are of interest to both areas of quantum chaos
and quantum control.


title:
No-deleting and no-cloning principles as consequences of conservation of
  quantum information

abstract:
  We show that the no-deleting and no-cloning principles are implications of
information conservation principle. This is unlike in classical physics, where
cloning and deleting are possible, independently of information conservation.
Connections with the second law of thermodynamics are also discusssed.


title:
Testing the Fair Sampling Assumption for EPR-Bell Experiments with
  Polarizer Beamsplitters

abstract:
  In spite of many attempts, no local realistic model seems to be able to
reproduce EPR-Bell type correlations, unless non ideal detection is allowed.
The low efficiency of detectors in all experiments with photons makes the use
of the fair sampling assumption unavoidable. However, since this very
assumption is false in all existing local realistic models based on inefficient
detection, we thus question its validity. We show that it is no more reasonable
to assume fair sampling than it is impossible to test, and we actually propose
an experimental test which would provides clear cut results in case of unfair
sampling.


title:
Squeezing based on nondegenerate frequency doubling internal to a
  realistic laser

abstract:
  We investigate theoretically the quantum fluctuations of the fundamental
field in the output of a nondegenerate second harmonic generation process
occuring inside a laser cavity. Due to the nondegenerate character of the
nonlinear medium, a field orthogonal to the laser field is for some operating
conditions indepedent of the fluctuations produced by the laser medium. We show
that this fact may lead to perfect squeezing for a certain polarization mode of
the fundamental field. The experimental feasibility of the system is also
discussed.


title: Quantum jumps and entropy production

abstract:
  The irreversible motion of an open quantum system can be represented through
an ensemble of state vectors following a stochastic dynamics with piecewise
deterministic paths. It is shown that this representation leads to a natural
definition of the rate of quantum entropy production. The entropy production
rate is expressed in terms of the von Neumann entropy and of the numbers of
quantum jumps corresponding to the various decay channels of the open system.
The proof of the positivity and of the convexity of the entropy production rate
is given. Monte Carlo simulations of the stochastic dynamics of a driven qubit
and of a $\Lambda$-configuration involving a dark state are performed in order
to illustrate the general theory.


title:
Local invariants for multi-partite entangled states allowing for a
  simple entanglement criterion

abstract:
  We present local invariants of multi-partite pure or mixed states, which can
be easily calculated and have a straight-forward physical meaning. As an
application, we derive a new entanglement criterion for arbitrary mixed states
of $n$ parties. The new criterion is weaker than the partial transposition
criterion but offers advantages for the study of multipartite systems. A
straightforward generalization of these invariants allows for the construction
of a complete set of observable polynomial invariants.


title:
Characterizing the combinatorics of distributed EPR pairs for
  multi-partite entanglement

abstract:
  We develop protocols for preparing a GHZ state and, in general,a pure
multi-partite maximally entangled state in a distributed network with apriori
quantum entanglement between agents using classical communication and local
operations. We investigate and characterize the minimal combinatorics of the
sharing of EPR pairs required amongst agents in a network for the creation of
multi-partite entanglement. We also characterize the minimal combinatorics of
agents in the creation of pure maximal multi-partite entanglement amongst the
set $N$ of $n$ agents in a network using apriori multi-partite entanglement
states amongst subsets of $N$. We propose protocols for establishing
multi-partite entanglement in the above cases.


title:
Comment on "Does the transverse electric zero mode contribute to the
  Casimir effect for a metal?"

abstract:
  Recently J.S. Hoye, I.Brevik, J.B. Aarseth, and K.A. Milton [Phys. Rev. E
v.67, 056116 (2003); quant-ph/0212125] proposed that if the Lifshitz formula is
combined with the Drude model, the transverse electric zero mode does not
contribute to the result for real metals and there arises a linear temperature
correction to the Casimir force at small temperatures. The authors claim that
in spite of the fact that the Casimir entropy in their approach is negative,
the Nernst heat theorem is satisfied. In the present Comment we show that the
authors' conclusion regarding the Nernst heat theorem is in error. We
demonstrate also the resolution of this thermodynamic puzzle based on the use
of the surface impedance instead of the Drude dielectric function. The results
of numerical computations obtained by the authors are compared with those from
use of the surface impedance approach which are thermodynamically consistent.


title:
Quantum Merlin-Arthur Proof Systems: Are Multiple Merlins More Helpful
  to Arthur?

abstract:
  This paper introduces quantum ``multiple-Merlin''-Arthur proof systems in
which Arthur receives multiple quantum proofs that are unentangled with each
other. Although classical multi-proof systems are obviously equivalent to
classical single-proof systems (i.e., usual Merlin-Arthur proof systems), it is
unclear whether or not quantum multi-proof systems collapse to quantum
single-proof systems (i.e., usual quantum Merlin-Arthur proof systems). This
paper presents a necessary and sufficient condition under which the number of
quantum proofs is reducible to two. It is also proved that, in the case of
perfect soundness, using multiple quantum proofs does not increase the power of
quantum Merlin-Arthur proof systems.


title: Quantum Schroedinger bridges

abstract:
  Elaborating on M. Pavon, J.Math. Phys. 40 (1999), 5565-5577, we develop a
simplified version of a variational principle within Nelson stochastic
mechanics that produces the von Neumann wave packet reduction after a position
measurement. This stochastic control problem parallels, with a different
kinematics, the problem of the Schroedinger bridge. This gives a profound
meaning to what was observed by Schroedinger in 1931 concerning Schroedinger
bridges: "Merkwuerdige Analogien zur Quantenmechanik, die mir sehr des
Hindenkens wert erscheinen".


title:
Bures/statistical distinguishability probabilities of triseparable and
  biseparable Eggeling-Werner States

abstract:
  In a number of previous studies, we have investigated the use of the volume
element of the Bures (minimal monotone) metric -- identically, one-fourth of
the statistical distinguishability (SD) metric -- as a natural measure over the
(n^2-1)-dimensional convex set of n x n density matrices. This has led us for
the cases n = 4 and 6 to estimates of the prior (Bures/SD) probabilities that
qubit-qubit and qubit-qutrit pairs are separable. Here, we extend this work
from such bipartite systems to the tripartite "laboratory'' quantum systems
possessing U x U x U symmetry recently constructed by Eggeling and Werner
(Phys. Rev. A 63 [2001], 042324). We derive the associated SD metric tensors
for the three-qubit and three-qutrit cases, and then obtain estimates of the
various related Bures/SD probabilities using Monte Carlo methods.


title: Spatial search by quantum walk

abstract:
  Grover's quantum search algorithm provides a way to speed up combinatorial
search, but is not directly applicable to searching a physical database.
Nevertheless, Aaronson and Ambainis showed that a database of N items laid out
in d spatial dimensions can be searched in time of order sqrt(N) for d>2, and
in time of order sqrt(N) poly(log N) for d=2. We consider an alternative search
algorithm based on a continuous time quantum walk on a graph. The case of the
complete graph gives the continuous time search algorithm of Farhi and Gutmann,
and other previously known results can be used to show that sqrt(N) speedup can
also be achieved on the hypercube. We show that full sqrt(N) speedup can be
achieved on a d-dimensional periodic lattice for d>4. In d=4, the quantum walk
search algorithm takes time of order sqrt(N) poly(log N), and in d<4, the
algorithm does not provide substantial speedup.


title: Non-Ergodic Nuclear Depolarization in Nano-Cavities

abstract:
  Recently, it has been observed that the effective dipolar interactions
between nuclear spins of spin-carrying molecules of a gas in a closed
nano-cavities are independent of the spacing between all spins. We derive exact
time-dependent polarization for all spins in spin-1/2 ensemble with spatially
independent effective dipolar interactions. If the initial polarization is on a
single (first) spin,$P_1(0)= 1$ then the exact spin dynamics of the model is
shown to exhibit a periodical short pulses of the polarization of the first
spin, the effect being typical of the systems having a large number, $N$, of
spins. If $N \gg 1$, then within the period $4\pi/g$ ($2\pi/g$) for odd (even)
$N$-spin clusters, with $g$ standing for spin coupling, the polarization of
spin 1 switches quickly from unity to the time independent value, 1/3, over the
time interval about $(g\sqrt{N})^{-1}$, thus, almost all the time, the spin 1
spends in the time independent condition $P_1(t)= 1/3$. The period and the
width of the pulses determine the volume and the form-factor of the ellipsoidal
cavity. The formalism is adopted to the case of time varying nano-fluctuations
of the volume of the cavitation nano-bubbles. If the volume $V(t)$ is varied by
the Gaussian-in-time random noise then the envelope of the polarization peaks
goes irreversibly to 1/3. The polarization dynamics of the single spin exhibits
the Gaussian (or exponential) time dependence when the correlation time of the
fluctuations of the nano-volume is larger (or smaller) than the $<(\delta g)^2
>^{-1/2} $, where the $<(\delta g)^2>$ is the variance of the $g(V(t))$
coupling. Finally, we report the exact calculations of the NMR line shape for
the $N$-spin gaseous aggregate.


title: Entanglement Dynamics in 1D Quantum Cellular Automata

abstract:
  Several proposed schemes for the physical realization of a quantum computer
consist of qubits arranged in a cellular array. In the quantum circuit model of
quantum computation, an often complex series of two-qubit gate operations is
required between arbitrarily distant pairs of lattice qubits. An alternative
model of quantum computation based on quantum cellular automata (QCA) requires
only homogeneous local interactions that can be implemented in parallel. This
would be a huge simplification in an actual experiment. We find some minimal
physical requirements for the construction of unitary QCA in a 1 dimensional
Ising spin chain and demonstrate optimal pulse sequences for information
transport and entanglement distribution. We also introduce the theory of
non-unitary QCA and show by example that non-unitary rules can generate
environment assisted entanglement.


title: Quality of a Which-Way Detector

abstract:
  We introduce a measure Q of the "quality" of a quantum which-way detector,
which characterizes its intrinsic ability to extract which-way information in
an asymmetric two-way interferometer. The "quality" Q allows one to separate
the contribution to the distinguishability of the ways arising from the quantum
properties of the detector from the contribution stemming from a-priori
which-way knowledge available to the experimenter, which can be quantified by a
predictability parameter P. We provide an inequality relating these two sources
of which-way information to the value of the fringe visibility displayed by the
interferometer. We show that this inequality is an expression of duality,
allowing one to trace the loss of coherence to the two reservoirs of which-way
information represented by Q and P. Finally, we illustrate the formalism with
the use of a quantum logic gate: the Symmetric Quanton-Detecton System (SQDS).
The SQDS can be regarded as two qubits trying to acquire which way information
about each other. The SQDS will provide an illustrating example of the
reciprocal effects induced by duality between system and which-way detector.


title: Information between quantum systems via POVMs

abstract:
  The concepts of `conditional entropy' and `information' retain their validity
for quantum systems, but their properties differ somewhat from those of their
classical counterparts; specifically, some equalities and inequalities of
classical information theory are in general violated.
  In this paper the concepts are generalized to include arbitrary indirect
measurements (POVMs). Though the generalization is straightforward, it is
important to ascertain that the basic relationships between the generalized
quantitites remain the same for the POVMs as for direct measurements.


title:
Quantization with maximally degenerate Poisson brackets: The harmonic
  oscillator!

abstract:
  Nambu's construction of multi-linear brackets for super-integrable systems
can be thought of as degenerate Poisson brackets with a maximal set of Casimirs
in their kernel. By introducing privileged coordinates in phase space these
degenerate Poisson brackets are brought to the form of Heisenberg's equations.
We propose a definition for constructing quantum operators for classical
functions which enables us to turn the maximally degenerate Poisson brackets
into operators. They pose a set of eigenvalue problems for a new state vector.
The requirement of the single valuedness of this eigenfunction leads to
quantization. The example of the harmonic oscillator is used to illustrate this
general procedure for quantizing a class of maximally super-integrable systems.


title:
The band-gap structure and the singular character of the bounded large
  array of potential barriers

abstract:
  The bounded one dimensional multibarrier potential shows signs of chaos,
phase transition and a transmission probability of unity for certain values of
its total length $L$ and the ratio $c$ of total interval to total width. Like
the infinite Kronig-Penney system, which is arranged along the whole spatial
region, the bounded multibarrier potential has a band-gap structure in its
energy spectrum. But unlike the Kronig-Penney system, in which the gaps
disappear for large energies, these gaps do not disappear for certain values of
$L$ and $c$. The energy is discontinuous even in parts of the spectrum with no
gaps at all. These results imply that the energy spectrum of the bounded
multibarrier system is singular.


title:
Damped harmonic oscillator: pure states of the bath and exact master
  equations

abstract:
  Time evolution of a harmonic oscillator linearly coupled to a heat bath is
compared for three classes of initial states for the bath modes - grand
canonical ensemble, number states and coherent states. It is shown that for a
wide class of number states the behavior of the oscillator is similar to the
case of the equilibrium bath. If the bath modes are initially in coherent
states, then the variances of the oscillator coordinate and momentum, as well
as its entanglement to the bath, asymptotically approach the same values as for
the oscillator at zero temperature and the average coordinate and momentum show
a Brownian-like behavior. We derive an exact master equation for the
characteristic function of the oscillator valid for arbitrary factorized
initial conditions. In the case of the equilibrium bath this equation reduces
to an equation of the Hu-Paz-Zhang type, while for the coherent states bath it
leads to an exact stochastic master equation with a multiplicative noise.


title: Mode-Locked Two-Photon States

abstract:
  The concept of mode locking in laser is applied to a two-photon state with
frequency entanglement. Cavity enhanced parametric down-conversion is found to
produce exactly such a state. The mode-locked two-photon state exhibits a
comb-like correlation function. An unbalanced Hong-Ou-Mandel type
interferometer is used to measure the correlation function. A revival of the
typical interference dip is observed. We will discuss schemes for engineering
of quantum states in time domain.


title: Anyon computers with smaller groups

abstract:
  Anyons obtained from a finite gauge theory have a computational power that
depends on the symmetry group. The relationship between group structure and
computational power is discussed in this paper. In particular, it is shown that
anyons based on finite groups that are solvable but not nilpotent are capable
of universal quantum computation. This extends previously published results to
groups that are smaller, and therefore more practical. Additionally, a new
universal gate-set is built out of an operation called a probabilistic
projection, and a quasi-universal leakage correction scheme is discussed.


title: Proposal for realization of a Toffoli gate via cavity-assisted collision

abstract:
  Cavity QED is a versatile tool to explore small scale quantum information
processing. Within this setting, we describe a particular protocol for
implementing a Toffoli gate with Rydberg atoms and a cavity field. Our scheme
uses both resonant and non resonant interactions, and in particular a cavity
assisted atomic collision. The experimental feasibility of the protocol is
carefully analyzed with the help of numerical simulations and takes into
account the decoherence process. Moreover, we show that our protocol is optimal
within the constraints imposed by the experimental setting.


title:
Exact solvability of potentials with spatially dependent effective
  masses

abstract:
  We discuss the relationship between exact solvability of the Schroedinger
equation, due to a spatially dependent mass, and the ordering ambiguity. Some
examples show that, even in this case, one can find exact solutions.
Furthermore, it is demonstrated that operators with linear dependence on the
momentum are nonambiguous.


title:
Single-photon interference experiment over 100 km for quantum
  cryptography system using a balanced gated-mode photon detector

abstract:
  We demonstrate single-photon interference over 100 km using a balanced
gated-mode photon detector and a plug & play system for quantum key
distribution. The visibility with 0.1 photon/pulse was more than 80% after 100
km transmission. This corresponds to the fidelity of a quantum cryptography
system of more than 90% and a QBER of less than 10%, satisfying the security
criteria.


title: Quantum key distribution between two groups using secret sharing

abstract:
  In this paper, we investigate properties of some multi-particle entangled
states and, from the properties applying the secret sharing present a new type
of quantum key distribution protocols as generalization of quantum key
distribution between two persons. In the protocols each group can retrieve the
secure key string, only if all members in each group should cooperate with one
another. We also show that the protocols are secure against an external
eavesdropper using the intercept/resend strategy.


title: New encoding schemes for quantum authentication

abstract:
  We study the potential of general quantum operations, Trace-Preserving
Completely-Positive Maps (TPCPs), as encoding and decoding mechanisms in
quantum authentication protocols. The study shows that these general operations
do not offer significant advantage over unitary encodings. We also propose a
practical authentication protocol based on the use of two successive unitary
encodings.


title: Classical and quantum spaces as rough images of the fundamental prespace

abstract:
  In spite of all {\bf no-go} theorems (e.g., von Neumann, Kochen and
Specker,..., Bell,...) we constructed a realist basis of quantum mechanics. In
our model both classical and quantum spaces b are rough images of the
fundamental {\bf prespace.} Quantum mechanics cannot be reduced to classical
one. Both classical and quantum representations induce reductions of prespace
information.


title: A density functional perspective for one-particle systems

abstract:
  Density functional theory is discussed in the context of one-particle
systems. We show that the ground state density $\rho_0(x)$ and energy $E_0$ are
simply related to a family of external potential energy functions with ground
state wave functions $\psi_n(x) \propto \rho_0(x)^n$ and energies $E_n=2nE_0$
for certain integer values of $n$.


title: The problem of gauge invariance in Dirac field theory

abstract:
  Quantum field theory is assumed to be gauge invariant. However it is well
known that when certain quantities are calculated using perturbation theory the
results are not gauge invariant. The non-gauge invariant terms have to be
removed in order to obtain a physically correct result. In this paper we will
examine this problem and determine why a theory that is supposed to be gauge
invariant produces non-gauge invariant results.


title: Decoherence and the transition from quantum to classical -- REVISITED

abstract:
  The environment surrounding a quantum system can, in effect, monitor some of
the systems observables. As a result, the eigenstates of these observables
continuously decohere and can behave like classical states.


title: Rotationally invariant proof of Bell's theorem without inequalities

abstract:
  The singlet state of two spin-3/2 particles allows a proof of Bell's theorem
without inequalities with two distinguishing features: any local observable can
be regarded as an Einstein-Podolsky-Rosen element of reality, and the
contradiction with local realism occurs not only for some specific local
observables but for any rotation whereof.


title: Supersinglets

abstract:
  Supersinglets $|{\cal S}_N^{(d)}>$ are states of total spin zero of $N$
particles of $d$ levels. Some applications of the $|{\cal S}_N^{(N)}>$ and
$|{\cal S}_N^{(2)}>$ states are described. The $|{\cal S}_N^{(N)}>$ states can
be used to solve three problems which have no classical solution: The "$N$
strangers," "secret sharing," and "liar detection" problems. The $|{\cal
S}_N^{(2)}>$ (with $N$ even) states can be used to encode qubits in
decoherence-free subspaces.


title:
Greenberger-Horne-Zeilinger-like proof of Bell's theorem involving
  observers who do not share a reference frame

abstract:
  Vaidman described how a team of three players, each of them isolated in a
remote booth, could use a three-qubit Greenberger-Horne-Zeilinger state to
always win a game which would be impossible to always win without quantum
resources. However, Vaidman's method requires all three players to share a
common reference frame; it does not work if the adversary is allowed to
disorientate one player. Here we show how to always win the game, even if the
players do not share any reference frame. The introduced method uses a 12-qubit
state which is invariant under any transformation $R_a \otimes R_b \otimes R_c$
(where $R_a = U_a \otimes U_a \otimes U_a \otimes U_a$, where $U_j$ is a
unitary operation on a single qubit) and requires only single-qubit
measurements. A number of further applications of this 12-qubit state are
described.


title:
Photon-number superselection and the entangled coherent-state
  representation

abstract:
  We introduce the entangled coherent state representation, which provides a
powerful technique for efficiently and elegantly describing and analyzing
quantum optics sources and detectors while respecting the photon number
superselection rule that is satisfied by all known quantum optics experiments.
We apply the entangled coherent state representation to elucidate and resolve
the longstanding puzzles of the coherence of a laser output field, interference
between two number states, and dichotomous interpretations of quantum
teleportation of coherent states.


title: Quantum Domain Theory - Definitions and Applications

abstract:
  Classically domain theory is a rigourous mathematical structure to describe
denotational semantics for programming languages and to study the computability
of partial functions. Recently, the application of domain theory has also been
extended to the quantum setting. In this note we review these results and we
present some new thoughts in this field.


title: Distillation of secret key and entanglement from quantum states

abstract:
  We study and solve the problem of distilling secret key from quantum states
representing correlation between two parties (Alice and Bob) and an
eavesdropper (Eve) via one-way public discussion: we prove a coding theorem to
achieve the "wire-tapper" bound, the difference of the mutual information
Alice-Bob and that of Alice-Eve, for so-called cqq-correlations, via one-way
public communication.
  This result yields information--theoretic formulas for the distillable secret
key, giving ``ultimate'' key rate bounds if Eve is assumed to possess a
purification of Alice and Bob's joint state.
  Specialising our protocol somewhat and making it coherent leads us to a
protocol of entanglement distillation via one-way LOCC (local operations and
classical communication) which is asymptotically optimal: in fact we prove the
so-called "hashing inequality" which says that the coherent information (i.e.,
the negative conditional von Neumann entropy) is an achievable EPR rate.
  This result is well--known to imply a whole set of distillation and capacity
formulas which we briefly review.


title:
Elements of information-theoretic derivation of the formalism of quantum
  theory

abstract:
  Information-theoretic derivations of the formalism of quantum theory have
recently attracted much attention. We analyze the axioms underlying a few such
derivations and propose a conceptual framework in which, by combining several
approaches, one can retrieve more of the conventional quantum formalism.


title: Uncertainty relations in curved spaces

abstract:
  Uncertainty relations for particle motion in curved spaces are discussed. The
relations are shown to be topologically invariant. New coordinate system on a
sphere appropriate to the problem is proposed. The case of a sphere is
considered in details. The investigation can be of interest for string and
brane theory, solid state physics (quantum wires) and quantum optics.


title:
Experimental requirements for Grover's algorithm in optical quantum
  computation

abstract:
  The field of linear optical quantum computation (LOQC) will soon need a
repertoire of experimental milestones. We make progress in this direction by
describing several experiments based on Grover's algorithm. These experiments
range from a relatively simple implementation using only a single non-scalable
CNOT gate to the most complex, requiring two concatenated scalable CNOT gates,
and thus form a useful set of early milestones for LOQC. We also give a
complete description of basic LOQC using polarization-encoded qubits, making
use of many simplifications to the original scheme of Knill, Laflamme, and
Milburn.


title:
Sampling Weak Values: A Non-Linear Bayesian Model for Non-Ideal Quantum
  Measurements

abstract:
  A model is proposed for the statistical analysis of arbitrary-strength
quantum measurements, based on a picture of "sampling weak values" from
different configurations of the system. The model is comprised of two elements:
a "local weak value" and a "likelihood factor". The first describes the
response of an idealized weak measurement situation where the back-reaction on
the system is perfectly controlled. The second assigns a weight factor to
possible configurations of the system. The distribution of the data in a
measurement of arbitrary strength may the be viewed as the net result of
interfering different samples weighted by the likelihood factor, each of which
implements a weak measurement of a different local weak value. It is shown that
the mean and variance of the data can be connected directly to the means and
variances of the sampled weak values. The model is then applied to a situation
similar to a phase transition, where the distribution of the data exhibits two
qualitatively different shapes as the strength parameter is slightly varied
away from a critical value: one below the critical point, where an unusual weak
value is resolved, the other above the critical point, where the spectrum of
the measured observable is resolved. In the picture of sampling, the transition
corresponds to a qualitative change in the sampling profile brought about by
the competition between the prior sampling distribution and the likelihood
factor.


title:
Accuracy of quantum-state estimation utilizing Akaike's information
  criterion

abstract:
  We report our theoretical and experimental investigations into errors in
quantum state estimation, putting a special emphasis on their asymptotic
behavior. Tomographic measurements and maximum likelihood estimation are used
for estimating several kinds of identically prepared quantum states (bi-photon
polarization states) produced via spontaneous parametric down-conversion.
Excess errors in the estimation procedures are eliminated by introducing a new
estimation strategy utilizing Akaike's information criterion. We make a
quantitative comparision between the errors of the experimentally estimated
states and their asymptotic lower bounds, which are derived from the
Cram\'{e}r-Rao inequality. Our results reveal influence of entanglement on the
errors in the estimation. An alternative measurement strategy employing
inseparable measurements is also discussed, and its performance is numerically
explored.


title: Quantum coherence in the presence of unobservable quantities

abstract:
  State representations summarize our knowledge about a system. When
unobservable quantities are introduced the state representation is typically no
longer unique. However, this non-uniqueness does not affect subsequent
inferences based on any observable data. We demonstrate that the inference-free
subspace may be extracted whenever the quantity's unobservability is guaranteed
by a global conservation law. This result can generalize even without such a
guarantee. In particular, we examine the coherent-state representation of a
laser where the absolute phase of the electromagnetic field is believed to be
unobservable. We show that experimental coherent states may be separated from
the inference-free subspaces induced by this unobservable phase. These physical
states may then be approximated by coherent states in a relative-phase Hilbert
space.


title: Quantum Computing with Atomic Josephson Junction Arrays

abstract:
  We present a quantum computing scheme with atomic Josephson junction arrays.
The system consists of a small number of atoms with three internal states and
trapped in a far-off resonant optical lattice. Raman lasers provide the
"Josephson" tunneling, and the collision interaction between atoms represent
the "capacitive" couplings between the modes. The qubit states are collective
states of the atoms with opposite persistent currents. This system is closely
analogous to the superconducting flux qubit. Single qubit quantum logic gates
are performed by modulating the Raman couplings, while two-qubit gates result
from a tunnel coupling between neighboring wells. Readout is achieved by tuning
the Raman coupling adiabatically between the Josephson regime to the Rabi
regime, followed by a detection of atoms in internal electronic states.
Decoherence mechanisms are studied in detail promising a high ratio between the
decoherence time and the gate operation time.


title: Entanglement Detection Based on Interference and Particle Counting

abstract:
  A sufficient condition for entanglement in two-mode continuous systems is
constructed based on interference visibility and the uncertainty of the total
particle number. The observables to be measured (particle numbers and particle
number variances) are relatively easily accessible experimentally. The method
may be used to detect entanglement in light fields or in Bose-Einstein
condensates. In contrast to the standard approach based on entanglement
witnesses, the condition is expressed in terms of an inequality which is
nonlinear in expectation values.


title:
Mean field master equation for self-interacting baths: comparison with
  exact spin--spin-bath dynamics

abstract:
  A mean field approximation is employed to derive a master equation suitable
for self-interacting baths and strong system-bath coupling. Solutions of the
master equation are compared with exact solutions for a central spin
interacting with a spin-bath.


title:
Numerical method for integro-differential generalized Langevin and
  master equations

abstract:
  We show that integro-differential generalized Langevin and non-Markovian
master equations can be transformed into larger sets of ordinary differential
equations. .On the basis of this transformation we develop a numerical method
for solving such integro-differential equations. Physically motivated example
calculations are performed to demonstrate the accuracy and convergence of the
method.


title:
Mean field master equation for self-interacting baths II: extension to
  time-dependent subsystem Hamiltonians

abstract:
  A mean field argument is used to derive a master equation for systems
simultaneously interacting with external fields and coupled environmental
degrees of freedom. We prove that this master equation preserves positivity of
the reduced density matrix. Solutions of the master equation are compared with
exact solutions for a system consisting of three spins which is manipulated
with a sequence of laser pulses while interacting with a spin-bath. Exact
solutions appear to converge to the master equation result as the number of
bath spins increases.


title: Quantum Dynamics of Magnetic and Electric Dipoles and Berry's Phase

abstract:
  We study the quantum dynamics of neutral particle that posseses a permanent
magnetic and electric dipole moments in the presence of an electromagnetic
field. The analysis of this dynamics demonstrates the appearance of a quantum
phase that combines the Aharonov-Casher effect and the He-Mckellar-Wilkens
effect. We demonstrate that this phase is a special case of the Berry's quantum
phase. A series of field configurations where this phase would be found are
presented. A generalized Casella-type effect is found in one these
configurations. A physical scenario for the quantum phase in an interferometric
experiment is proposed.


title: Noise-assisted preparation of entangled atoms

abstract:
  We discuss the generation of entangled states of two two-level atoms inside
an optical cavity. The cavity mode is supposed to be coupled to a white noise
with adjustable intensity. We describe how the entanglement between the atoms
inside the cavity arise in such a situation. The entanglement is maximized for
intermediate values of the noise intensity, while it is a monotonic function of
the spontaneous rate. This resembles the phenomenon of stochastic resonance and
sheds more light on the idea to exploit white noise in quantum information
processing.


title: Testing the bounds on quantum probabilities

abstract:
  Bounds on quantum probabilities and expectation values are derived for
experimental setups associated with Bell-type inequalities. In analogy to the
classical bounds, the quantum limits are experimentally testable and therefore
serve as criteria for the validity of quantum mechanics.