title:
Convexity and the Separability Problem of Quantum Mechanical Density
  Matrices

abstract:
  A finite dimensional quantum mechanical system is modeled by a density rho, a
trace one, positive semi-definite matrix on a suitable tensor product space
H[N] . For the system to demonstrate experimentally certain non-classical
behavior, rho cannot be in S, a closed convex set of densities whose extreme
points have a specificed tensor product form. Two mathematical problems in the
quantum computing literature arise from this context: (1) the determination
whether a given rho is in S and (2) a measure of the ``entanglement'' of such a
rho in terms of its distance from S. In this paper we describe these two
problems in detail for a linear algebra audience, discuss some recent results
from the quantum computing literature, and prove some new results.We emphasize
the roles of densities rho as both operators on the Hilbert space H[N] and also
as points in a real Hilbert space M. We are able to compute the nearest
separable densities tau0 to rho0 in particular classes of inseparable densities
and we use the Euclidean distance between the two in M to quantify the
entanglement of rho0. We also show the role of tau0 in the construction of
separating hyperplanes, so-called entanglement witnesses in the quantum
computing literature.


title:
Power of Anisotropic Exchange Interactions: Universality and Efficient
  Codes for Quantum Computing

abstract:
  We study the quantum computational power of a generic class of anisotropic
solid state Hamiltonians. A universal set of encoded logic operations are found
which do away with difficult-to-implement single-qubit gates in a number of
quantum computer proposals, e.g., quantum dots and donor atom spins with
anisotropic exchange coupling, quantum Hall systems, and electrons floating on
helium.We show how to make the corresponding Hamiltonians universal by encoding
one qubit into two physical qubits, and by controlling nearest neighbor
interactions.

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


title: Entanglement of Bell Mixtures of Two Qubits

abstract:
  This paper is an appendix to a previous paper: quant-ph/0101123 ``Relaxation
Method for Calculating Quantum Entanglement", by Robert Tucci. For certain
mixtures of Bell basis states, namely the Werner States, we use the theoretical
machinery of our previous paper to derive algebraic formulas for: the pure and
mixed minimization entanglements (i.e., E_{pure} and E_{mixed}), their optimal
decompositions and their entanglement operators. This complements and
corroborates some results that were obtained numerically but not algebraically
in our previous paper. Some of the algebraic formulas presented here are new.
Others were first derived using a different method by Bennett et al in
quant-ph/9604024.


title: No place for particles in relativistic quantum theories?

abstract:
  Several recent arguments purport to show that there can be no relativistic,
quantum-mechanical theory of localizable particles and, thus, that relativity
and quantum mechanics can be reconciled only in the context of quantum field
theory. We point out some loopholes in the existing arguments, and we provide
two no-go theorems to close these loopholes. However, even with these loopholes
closed, it does not yet follow that relativity plus quantum mechanics
exclusively requires a field ontology, since relativistic quantum field theory
itself might permit an ontology of localizable particles supervenient on the
fundamental fields. Thus, we provide another no-go theorem to rule out this
possibility. Finally, we allay potential worries about this conclusion by
arguing that relativistic quantum field theory can nevertheless explain the
possibility of "particle detections," as well as the pragmatic utility of
"particle talk."


title:
Stabilizing distinguishable qubits against spontaneous decay by
  detected-jump correcting quantum codes

abstract:
  A new class of error-correcting quantum codes is introduced capable of
stabilizing qubits against spontaneous decay arising from couplings to
statistically independent reservoirs. These quantum codes are based on the idea
of using an embedded quantum code and exploiting the classical information
available about which qubit has been affected by the environment. They are
immediately relevant for quantum computation and information processing using
arrays of trapped ions or nuclear spins. Interesting relations between these
quantum codes and basic notions of design theory are established.


title: Quantum theory and the role of mind in nature

abstract:
  Orthodox Copenhagen quantum theory renounces the quest to understand the
reality in which we are imbedded, and settles for practical rules describing
connections between our observations. Many physicist have regarded this
renunciation of our effort to describe nature herself as premature, and John
von Neumann reformulated quantum theory as a theory of an evolving objective
universe interacting with human consciousness. This interaction is associated
both in Copenhagen quantum theory and in von Neumann quantum theory with a
sudden change that brings the objective physical state of a system in line with
a subjectively felt psychical reality. The objective physical state is thereby
converted from a material substrate to an informational and dispositional
substrate that carries both the information incorporated into it by the
psychical realities, and certain dispositions for the occurrence of future
psychical realities. The present work examines and proposes solutions to two
problems that have appeared to block the development of this conception of
nature. The first problem is how to reconcile this theory with the principles
of relativistic quantum field theory; the second problem is to understand
whether, strictly within quantum theory, a person's mind can affect the
activities of his brain, and if so how. Solving the first problem involves
resolving a certain nonlocality question. The proposed solution to the second
problem is based on a postulated connection between effort, attention, and the
quantum Zeno effect. This solution explains on the basis of quantum physics a
large amount of heretofore unexplained data amassed by psychologists.


title:
Theory of hybrid systems. I. The operator formulation of classical
  mechanics and semiclassical limit

abstract:
  The algebra of polynomials in operators that represent generalized coordinate
and momentum and depend on the Planck constant is defined. The Planck constant
is treated as the parameter taking values between zero and some nonvanishing
$h_0$. For the second of these two extreme values, introduced operatorial
algebra becomes equivalent to the algebra of observables of quantum mechanical
system defined in the standard manner by operators in the Hilbert space. For
the vanishing Planck constant, the generalized algebra gives the operator
formulation of classical mechanics since it is equivalent to the algebra of
variables of classical mechanical system defined, as usually, by functions over
the phase space. In this way, the semiclassical limit of kinematical part of
quantum mechanics is established through the generalized operatorial framework.


title: Local Realistic Theory for PDC Experiments Based on the Wigner Formalism

abstract:
  In this article we present a local hidden variables model for all experiments
involving photon pairs produced in parametric down conversion, based on the
Wigner representation of the radiation field. A modification of the standard
quantum theory of detection is made in order to give a local realist
explanation of the counting rates in photodetectors. This model involves the
existence of a real zeropoint field, such that the vacumm level of radiation
lies below the threshold of the detectors.


title:
Theory of hybrid systems. II. The symmetrized product and redefined Lie
  bracket of quantum mechanics

abstract:
  The symmetrized product for quantum mechanical observables is defined. It is
seen as consisting of the ordinary multiplication and the application of the
superoperator that orders the operators of coordinate and momentum. This
superoperator is defined in a way that allows obstruction free quantization
when the observables are considered from the point of view of the algebra.
Then, the operatorial version of the Poisson bracket is defined. It is shown
that it has all properties of the Lie bracket and that it can substitute the
commutator in the von Neumann equation.


title:
Critical Examination of the Conceptual Foundations of Classical
  Mechanics in the Light of Quantum Physics

abstract:
  As it is well known, classical mechanics consists of several basic features
like determinism, reductionism, completeness of knowledge and mechanicism. In
this article the basic assumptions are discussed which underlie those features.
It is shown that these basic assumptions - though universally assumed up the
beginnings of the XX century - are far from being obvious. Finally it is shown
that - to a certain extent - there is nothing wrong in assuming these basic
postulates. Rather, the error lies in the epistemological absolutization of the
theory, which was considered as a mirroring of Nature.


title: Greenberger-Horne-Zeilinger paradox for continuous variables

abstract:
  We show how to construct states for which a Greenberger-Horne-Zeilinger type
paradox occurs if each party measures either the position or momentum of his
particle. The paradox can be ascribed to the anticommutation of certain
translation operators in phase space. We then rephrase the paradox in terms of
modular and binary variables. The origin of the paradox is then due to the fact
that the associativity of addition of modular variables is true only for
c-numbers but does not hold for operators.


title: Four Photon Entanglement from Down Conversion

abstract:
  Double-pair emission from type-II parametric down conversion results in a
highly entangled 4-photon state. Due to interference, which is similar to
bunching from thermal emission, this state is not simply a product of two
pairs. The observation of this state can be achieved by splitting the two
emission modes at beam splitters and subsequent detection of a photon in each
output. Here we describe the features of this state and give a Bell theorem for
a 4-photon test of local realistic hidden variable theories.


title:
Quantum-Classical Correspondence for the Equilibrium Distributions of
  Two Interacting Spins

abstract:
  We consider the quantum and classical Liouville dynamics of a non-integrable
model of two coupled spins. Initially localised quantum states spread
exponentially to the system dimension when the classical dynamics are chaotic.
The long-time behaviour of the quantum probability distributions and, in
particular, the parameter-dependent rates of relaxation to the equilibrium
state are surprisingly well approximated by the classical Liouville mechanics
even for small quantum numbers. As the accessible classical phase space becomes
predominantly chaotic, the classical and quantum probability equilibrium
configurations approach the microcanonical distribution, although the quantum
equilibrium distributions exhibit characteristic `minimum' fluctuations away
from the microcanonical state. The magnitudes of the quantum-classical
differences arising from the equilibrium quantum fluctuations are studied for
both pure and mixed (dynamically entangled) quantum states. In both cases the
standard deviation of these fluctuations decreases as $(\hbar/{\mathcal
J})^{1/2}$, where ${\mathcal J}$ is a measure of the system size. In
conclusion, under a variety of conditions the differences between quantum and
classical Liouville mechanics are shown to become vanishingly small in the
classical limit (${\mathcal J}/\hbar \to \infty$) of a non-dissipative model
endowed with only a few degrees of freedom.


title: Maximal Violation of Bell's Inequalities for Continuous Variable Systems

abstract:
  We generalize Bell's inequalities to biparty systems with continuous quantum
variables. This is achieved by introducing the Bell operator in perfect analogy
to the usual spin-1/2 systems. It is then demonstrated that two-mode squeezed
vacuum states display quantum nonlocality by using the generalized Bell
operator. In particular, the original Einstein-Podolsky-Rosen entangled states,
which are the limiting case of the two-mode squeezed vacuum states, can
maximally violate Bell's inequality due to Clauser, Horne, Shimony and Holt.
The experimental aspect of our scheme and nonlocality of arbitrary biparticle
entangled pure states of continuous variables are briefly considered.


title:
Magnetic translation and Berry's phase factor through adiabatically
  rotating a magnetic field

abstract:
  For a spin subjected to an adiabatically changing magnetic field, the solid
angle result as embodied by a rotation operator is the only path-dependent
factor in the quantum evolution operator. For a charged particle, the infinite
degeneracy calls for a rigorous investigation. We find that in this case, it is
the product of the rotation operator and a path-ordered magnetic translation
operator that enters into the evolution operator and determines the geometric
phase. This result agrees with the fact that the instantaneous hamiltonian is
invariant under magnetic translation as well as rotation. Experimental
verification of the result is proposed.


title: Quantum Cloning Machines of a d-level System

abstract:
  The optimal N to M ($M>N$) quantum cloning machines for the d-level system
are presented. The unitary cloning transformations achieve the bound of the
fidelity.


title: Should PT symmetric quantum mechanics be interpreted as nonlinear?

abstract:
  The Feshbach-type reduction of the Hilbert space to the physically most
relevant "model" subspace is suggested as a means of a formal unification of
the standard quantum mechanics with its recently proposed PT symmetric
modification. The resulting "effective" Hamiltonians H(eff) are always
Hermitian, and the two alternative forms of their energy-dependence are
interpreted as a certain dynamical nonlinearity, responsible for the repulsion
and/or attraction of the levels in the Hermitian and/or PT symmetric cases,
respectively. The spontaneous PT symmetry breaking is then reflected by the
loss of the Hermiticity of H(eff) while the pseudo-unitary evolution law
persists in the unreduced Hilbert space.


title: Random Behaviour in Quantum Chaos

abstract:
  We investigate toy dynamical models of energy - level repulsion in quantum
(quasi)energy eigenvalue sequences.


title:
Interference enhanced polarization entanglement and the concept of an
  Entangled-Photon Laser

abstract:
  Inspired by laser operation, we address the question of whether stimulated
emission into polarization entangled modes can be achieved. We describe a state
produced by stimulated emission of the singlet Bell state and propose a setup
for creating it. As a first important step towards an entangled-photon laser we
demonstrate experimentally interference-enhanced polarization entanglement.


title: The Foundations of Quantum Information and Feasible Experiments

abstract:
  This thesis consists of four parts. In the first part it is shown that
optimal universal cloning of photons can be realized with the help of
stimulated emission. Possible schemes based on three-level systems and on
parametric down-conversion are analyzed in detail.
  The second part shows that the dynamical rules of quantum physics can be
derived from its static properties and the condition that superluminal
communication is forbidden.
  In the third part a very simple form of the Kochen-Specker theorem is
presented, which leads to a proposal for a simple experimental test of
non-contextual hidden variables.
  The fourth part shows how hidden-variable theorems can be derived in a
completely operational way. In particular a Kochen-Specker theorem is derived
that applies to experiments with finite precision.


title:
Security of EPR-based Quantum Cryptography against Incoherent Symmetric
  Attacks

abstract:
  We investigate a new strategy for incoherent eavesdropping in Ekert's
entanglement based quantum key distribution protocol. We show that under
certain assumptions of symmetry the effectiveness of this strategy reduces to
that of the original single qubit protocol of Bennett and Brassard.


title: Slow group velocity and Cherenkov radiation

abstract:
  We theoretically study the effect of ultraslow group velocities on the
emission of Vavilov-Cherenkov radiation in a coherently driven medium. We show
that in this case the aperture of the group cone on which the intensity of the
radiation peaks is much smaller than that of the usual wave cone associated
with the Cherenkov coherence condition. We show that such a singular behaviour
may be observed in a coherently driven ultracold atomic gas.


title: A Simple and Convenient Measure of NMR Rotor Fidelity

abstract:
  We describe a simple scheme for calculating the fidelity of a composite pulse
when considered as a universal rotor.


title: Spin squeezing in nonlinear spin coherent states

abstract:
  We introduce the nonlinear spin coherent state via its ladder operator
formalism and propose a type of nonlinear spin coherent state by the nonlinear
time evolution of spin coherent states. By a new version of spectroscopic
squeezing criteria we study the spin squeezing in both the spin coherent state
and nonlinear spin coherent state. The results show that the spin coherent
state is not squeezed in the x, y, and z directions, and the nonlinear spin
coherent state may be squeezed in the x and y directions.


title: Quantum mechanics vs. local realism, is that the question?

abstract:
  The conjecture is made that quantum mechanics is compatible with local hidden
variables (or local realism). The conjecture seems to be ruled out by the
theoretical argument of Bell, but it is supported by the empirical fact that
nobody has been able to perform a loophole-free test of local realism in spite
of renewed effort during almost 40 years.


title: Subsystem Purity as an Enforcer of Entanglement

abstract:
  We show that entanglement can always arise in the interaction of an
arbitrarily large system in any mixed state with a single qubit in a pure
state. This small initial purity is enough to enforce entanglement even when
the total entropy is close to maximum. We demonstrate this feature using the
Jaynes-Cummings interaction of a two level atom in a pure state with a field in
a thermal state at an arbitrarily high temperature. We find the time and
temperature variation of a lower bound on the amount of entanglement produced
and study the classical correlations quantified by the mutual information.


title: What Schr\"odinger's Cat is Telling

abstract:
  The recent experiment on superconducting quantum interference device provides
new opportunity to clarify the fundamental interpretation of quantum mechanics.
After analyzing this and relevant experiments, we claim the point of view that
the quantum state in abstract sense contains no information and the wave
function is the probability amplitude of fictitious measurement corresponding
to the real one which is prepared to be done by the observer. Talking about the
quantum state or wave function too materialized in ordinary language would
often cause misunderstanding. So basically, the Schr\"odinger's cat paradox is
over.


title: Quantum statistics via perturbation effects of preparation procedures

abstract:
  We study the following problem: Is it possible to explain the quantum
interference of probabilities in the purely corpuscular model for elementary
particles? We demonstrate that (by taking into account perturbation effects of
measurement and preparation procedures) we can obtain $\cos\theta$-perturbation
(interference term) in probabilistic rule connecting preparation procedures for
purely corpuscular objects. On one hand, our investigation demonstrated that
there is nothing special in so called `quantum probabilities': the right choice
of statistical ensembles gives the possibility to escape all `pathologies'. On
the other hand, we found that the standard trigonometric interference of
alternatives (observed, in particular, in quantum mechanics) is not the unique
possibility to extend (disturb) the conventional probabilistic rule for
addition of alternatives. There exist two other probabilistic rules that
connect three preparation procedures: hyperbolic and hyper-trigonometric
interferences.


title:
High-precision evaluation of the Vibrational spectra of long-range
  molecules

abstract:
  Vibrational spectra of long-range molecules are determined accurately and to
arbitrary accuracy with the Canonical Function Method. The energy levels of the
$0^-_g$ and $1_u$ electronic states of the $^{23}{\rm Na}_2$ molecule are
determined from the Ground state up to the continuum limit. The method is
validated by comparison with previous results obtained by Stwalley et al. using
the same potential and Trost et al. whose work is based on the Lennard-Jones
potential adapted to long-range molecules.


title:
Multiparticle entanglement with quantum logic networks: Application to
  cold trapped ions

abstract:
  We show how to construct a multi-qubit control gate on a quantum register of
an arbitrary size N. This gate performs a single-qubit operation on a specific
qubit conditioned by the state of other N-1 qubits. We provide an algorithm how
to build up an array of networks consisting of single-qubit rotations and
multi-qubit control-NOT gates for the synthesis of an arbitrary entangled
quantum state of N qubits. We illustrate the algorithm on a system of cold
trapped ions. This example illuminates the efficiency of the direct
implementation of the multi-qubit CNOT gate compared to its decomposition into
a network of two-qubit CNOT gates.


title: Spectral decomposition of Bell's operators for qubits

abstract:
  The spectral decomposition is given for the N-qubit Bell operators with two
observables per qubit. It is found that the eigenstates (when non-degenerate)
are N-qubit GHZ states even for those operators that do not allow the maximal
violation of the corresponding inequality. We present two applications of this
analysis. In particular, we discuss the existence of pure entangled states that
do not violate any Mermin-Klyshko inequality for $N\geq 3$.


title:
Simultaneous intraportation of many quantum states within the quantum
  computing network

abstract:
  A scheme is proposed for simultaneous intraportation of many unknown quantum
states within a quantum computing network. It is shown that our scheme, much
different from the teleportation in the strict sense, can be very similar to
the original teleportation proposal[Phys.Rev.Lett.{\bf 70} (1993)1895)] and the
efficiency of the scheme for quantum state transmission is very high. The
possible applications of our scheme are also discussed.


title: Observables in Relativistic Quantum Mechanics

abstract:
  We propose a quantum clock synchronization protocol in which Bob makes a
remote measurement on Alice's quantum clock via a third qubit acting as its
proxy. It is shown that the resulting correlations are dependent on the choice
of the hypersurface along which Bob's measurement of the proxy is deemed to
collapse the entangled state vector. A proper characterization of observables
in relativistic quantum mechanics is therefore constrained by relativistic
covariance as well as causality.


title: The Quantum Newton's Law

abstract:
  Using the quantum Hamilton-Jacobi equation within the framework of the
equivalence postulate, we construct a Lagrangian of a quantum system in one
dimension and derive a third order equation of motion representing a first
integral of the quantum Newton's law. We then integrate this equation in the
free particle case and compare our results to those of Floydian trajectories.
Finally, we propose a quantum version of Jacobi's theorem.


title: Exact uncertainty relations: technical details

abstract:
  The Heisenberg inequality \Delta X \Delta P \geq \hbar/2 can be replaced by
an exact equality, for suitably chosen measures of position and momentum
uncertainty, which is valid for all wavefunctions. The significance of this
"exact" uncertainty relation is discussed, and results are generalised to
angular momentum and phase, photon number and phase, time and frequency, and to
states described by density operators. Connections to optimal estimation of an
observable from the measurement of a second observable, Wigner functions,
energy bounds, and entanglement are also given.


title:
Quantum recognition of eigenvalues, structure of devices and
  thermodynamic properties

abstract:
  Quantum algorithms speeding up classical counterparts are proposed for the
problems:
  1. Recognition of eigenvalues with fixed precision. Given a quantum circuit
generating unitary mapping $U$ and a complex number the problem is to determine
is it an eigenvalue of $U$ or not.
  2. Given a molecular structure find thermodynamic functions like partitioning
function, entropy, etc. for a gas consisting of such molecules.
  3. Recognition of molecular structures. Find a molecular structure given its
spectrum.
  4. Recognition of electronic devices. Given an electronic device that can be
used only as a black box how to recognize its internal construction?
  We consider mainly structures generating sparse spectrums. These algorithms
require the time from about square root to logarithm of the time of classical
analogs and for the first three problems give exponential memory saving. Say,
the time required for distinguishing two devices with the same given spectrum
is about seventh root of the time of direct classical method, for the
recognition of eigenvalue - about sixth root. Thus microscopic quantum devices
can recognize molecular structures and physical properties of environment
faster than big classical computers.


title: Single-particle nonlocality and entanglement with the vacuum

abstract:
  We propose a single-particle experiment that is equivalent to the
conventional two-particle experiment used to demonstrate a violation of Bell's
inequalities. Hence, we argue that quantum mechanical nonlocality can be
demonstrated by single-particle states. The validity of such a claim has been
discussed in the literature, but without reaching a clear consensus. We show
that the disagreement can be traced to what part of the total state of the
experiment one assigns to the (macroscopic) measurement apparatus. However,
with a conventional and legitimate interpretation of the measurement process
one is led to the conclusion that even a single particle can show nonlocal
properties.


title:
On the Nonlocality of the Quantum Channel in the Standard Teleportation
  Protocol

abstract:
  By exhibiting a violation of a novel form of the Bell-CHSH inequality,
Zukowski has recently established that the quantum correlations exploited in
the standard perfect teleportation protocol cannot be recovered by any local
hidden variables model. Allowing the quantum channel state in the protocol to
be given by any density operator of two spin-1/2 particles, we show that a
violation of a generalized form of Zukowski's teleportation inequality can only
occur if the channel state, considered by itself, violates a Bell-CHSH
inequality. On the other hand, although it is sufficient for a teleportation
process to have a nonclassical fidelity-defined as a fidelity exceeding
2/3-that the channel state employed violate a Bell-CHSH inequality, we show
that such a violation does not imply a violation of Zukowski's teleportation
inequality or any of its generalizations. The implication does hold, however,
if the fidelity of the teleportation exceeds $2/3(1+1/2\sqrt{2})\approx .90$,
suggesting the existence of a regime of nonclassical values of the fidelity,
less than .90, for which the standard teleportation protocol can be modelled by
local hidden variables.


title: Bound entanglement for continuous variables is a rare phenomenon

abstract:
  We discuss the notion of bound entanglement (BE) for continuous variables
(CV). We show that the set of non--distillable states (NDS) for CV is nowhere
dense in the set of all states, i.e., the states of infinite--dimensional
bipartite systems are generically distillable. This automatically implies that
the sets of separable states, entangled states with positive partial transpose,
and bound entangled states are also nowhere dense in the set of all states. All
these properties significantly distinguish quantum CV systems from the spin
like ones. The aspects of the definition of BE for CV is also analysed,
especially in context of Schmidt numbers theory. In particular the main result
is generalised by means of arbitrary Schmidt number and single copy regime.


title:
K-quantum nonlinear coherent states: formulation, realization and
  nonclassical effects

abstract:
  We introduce a generalized class of states called K-quantum nonlinear
coherent states. Each K-state has K j-components corresponding to one and the
same eigenvalue. Each Kj-component can be composed of K K=1-states in a
correlated manner. The introduced states are shown to be realized in the
long-term behavior of the vibrational motion of an ion properly trapped and
laser-driven. Nonclassical properties of the states are studied in detail.


title: The Representation of Numbers in Quantum Mechanics

abstract:
  Earlier work on modular arithmetic of k-ary representations of length L of
the natural numbers in quantum mechanics is extended here to k-ary
representations of all natural numbers, and to integers and rational numbers.
Since the length L is indeterminate, representations of states and operators
using creation and annihilation operators for bosons and fermions are defined.
Emphasis is on definitions and properties of operators corresponding to the
basic operations whose properties are given by the axioms for each type of
number. The importance of the requirement of efficient implementability for
physical models of the axioms is emphasized. Based on this, successor
operations for each value of j corresponding to addition of k^{j-1} if j>0 and
k^{j} if j<0 are defined. It follows from the efficient implementability of
these successors, which is the case for all computers, that implementation of
the addition and multiplication operators, which are defined in terms of
polynomially many iterations of the successors, should be efficient. This is
not the case for definitions based on the successor for j=1 only. This is the
only successor defined in the usual axioms of arithmetic.


title:
Locality in the Everett Interpretation of Heisenberg-Picture Quantum
  Mechanics

abstract:
  Bell's theorem depends crucially on counterfactual reasoning, and is
mistakenly interpreted as ruling out a local explanation for the correlations
which can be observed between the results of measurements performed on
spatially-separated quantum systems. But in fact the Everett interpretation of
quantum mechanics, in the Heisenberg picture, provides an alternative local
explanation for such correlations. Measurement-type interactions lead, not to
many worlds but, rather, to many local copies of experimental systems and the
observers who measure their properties. Transformations of the
Heisenberg-picture operators corresponding to the properties of these systems
and observers, induced by measurement interactions, "label" each copy and
provide the mechanism which, e.g., ensures that each copy of one of the
observers in an EPRB or GHZM experiment will only interact with the "correct"
copy of the other observer(s). The conceptual problem of nonlocality is thus
replaced with a conceptual problem of proliferating labels, as correlated
systems and observers undergo measurement-type interactions with
newly-encountered objects and instruments; it is suggested that this problem
may be resolved by considering quantum field theory rather than the quantum
mechanics of particles.


title: Optimal quantum estimation of the coupling between two bosonic modes

abstract:
  We address the problem of the optimal quantum estimation of the coupling
parameter of a bilinear interaction, such as the transmittivity of a beam
splitter or the internal phase-shift of an interferometer. The optimal
measurement scheme exhibits Heisenberg scaling of the measurement precision
versus the total energy.


title: The Meaning of the Interaction-Free Measurements

abstract:
  Interaction-free measurements introduced by Elitzur and Vaidman [Found. Phys.
23, 987 (1993)] allow finding infinitely fragile objects without destroying
them. Many experiments have been successfully performed showing that indeed,
the original scheme and its modifications lead to reduction of the disturbance
of the observed systems. However, there is a controversy about the validity of
the term ``interaction-free'' for these experiments. Broad variety of such
experiments are reviewed and the meaning of the interaction-free measurements
is clarified.


title: Greenberger-Horne-Zeilinger nonlocality for continuous variable systems

abstract:
  As a development of our previous work, this paper is concerned with the
Greenberger-Horne-Zeilinger (GHZ) nonlocality for continuous variable cases.
The discussion is based on the introduction of a pseudospin operator, which has
the same algebra as the Pauli operator, for each of the $N$ modes of a light
field. Then the Bell-CHSH (Clauser, Horne, Shimony and Holt) inequality is
presented for the $N$ modes, each of which has a continuous degree of freedom.
Following Mermin's argument, it is demonstrated that for $N$-mode
parity-entangled GHZ states (in an infinite-dimensional Hilbert space) of the
light field, the contradictions between quantum mechanics and local realism
grow exponentially with $N$, similarly to the usual $N$-spin cases.


title: Decoherence effects of motion-induced radiation

abstract:
  The radiation pressure coupling with vacuum fluctuations gives rise to energy
damping and decoherence of an oscillating particle. Both effects result from
the emission of pairs of photons, a quantum effect related to the fluctuations
of the Casimir force. We discuss different alternative methods for the
computation of the decoherence time scale. We take the example of a spherical
perfectly-reflecting particle, and consider the zero and high temperature
limits. We also present short general reviews on decoherence and dynamical
Casimir effect.


title: Nonlocal variables with product states eigenstates

abstract:
  An alternative proof for existence of ``quantum nonlocality without
entanglement'', i.e. existence of variables with product-state eigenstates
which cannot be measured locally, is presented. A simple``nonlocal'' variable
for the case of one-way communication is given and the limit for its
approximate measurability is found.


title: Darwinism in quantum systems?

abstract:
  We investigate the role of quantum mechanical effects in the central
stability concept of evolutionary game theory i.e. an Evolutionarily Stable
Strategy (ESS). Using two and three-player symmetric quantum games we show how
the presence of quantum phenomenon of entanglement can be crucial to decide the
course of evolutionary dynamics in a population of interacting individuals.


title: Maximization of capacity and p-norms for some product channels

abstract:
  It is conjectured that the Holevo capacity of a product channel \Omega
\otimes \Phi is achieved when product states are used as input. Amosov, Holevo
and Werner have also conjectured that the maximal p-norm of a product channel
is achieved with product input states. In this paper we establish both of these
conjectures in the case that \Omega is arbitrary and \Phi is a CQ or QC channel
(as defined by Holevo). We also establish the Amosov, Holevo and Werner
conjecture when \Omega is arbitrary and either \Phi is a qubit channel and p=2,
or \Phi is a unital qubit channel and p is integer. Our proofs involve a new
conjecture for the norm of an output state of the half-noisy channel I \otimes
\Phi, when \Phi is a qubit channel. We show that this conjecture in some cases
also implies additivity of the Holevo capacity.


title: A new Bell Inequality for Werner states

abstract:
  Choosing four entangled stets to form an orthogonal and complete basis for a
two-particle system, we argue that a local hidden variable model should give
the probability of each entangled state if the two-particle system is described
by a mixed state. Under this condition, a new Bell inequality is derived for
the Werner states, and a nonlocality proof can be given for nonsequential
measurements PACS number: 03.65.Bz


title:
Quantum non-demolition measurement of nonlocal variables and its
  application in quantum authentication

abstract:
  Quantun non-demolition (QND) variables are generlized to the nonlocal ones by
proposing QND measurement networks of Bell states and multi-partite GHZ states,
which means that we can generate and measure them without any destruction. One
of its prospective applications in the quantum authentication system of the
Quantum Security Automatic Teller Machine (QSATM) which is much more reliable
than the classical ones is also presented.


title:
Geometrisation of electromagnetic field and topological interpretation
  of quantum formalism

abstract:
  A new approach is proposed for an electromagnetic field geometrisation. We
show that interacting Maxwell and Dirac fields can be considered as a single
connected space-time 4-manifold. The Dirac spinors appear wihtin such approach
as basic fanctions for the manifold fundamental group representation and
electric and magnertic fields appear as components of a curvature tensor of the
manifold covering space.


title: Fidelity of Single Qubit Maps

abstract:
  We give a simple way of characterising the average fidelity between a unitary
and a general operation on a single qubit which only involves calculating the
fidelities for a few pure input states.


title: Bound entangled states with not positive partial transposition exist

abstract:
  The paper has been withdrawn by the author due to crucial error in formula
(14) which does not satisfy conditions for S(x,y) domain if only d>2.


title: Worlds in the Everett Interpretation

abstract:
  This is a discussion of how we can understand the world-view given to us by
the Everett interpretation of quantum mechanics, and in particular the role
played by the concept of `world'. The view presented is that we are entitled to
use `many-worlds' terminology even if the theory does not specify the worlds in
the formalism; this is defended by means of an extensive analogy with the
concept of an `instant' or moment of time in relativity, with the lack of a
preferred foliation of spacetime being compared with the lack of a preferred
basis in quantum theory. Implications for identity of worlds over time, and for
relativistic quantum mechanics, are discussed.


title: Is It Possible To Clone Using An Arbitrary Blank State?

abstract:
  We show that in a cloning process, whether deterministic inexact or
probabilistic exact, one can take an arbitrary blank state while still using a
fixed cloning machine.


title: A single-mode quantum transport in serial-structure geometric scatterers

abstract:
  We study transport in quantum systems consisting of a finite array of N
identical single-channel scatterers. A general expression of the S matrix in
terms of the individual-element data obtained recently for potential scattering
is rederived in this wider context. It shows in particular how the band
spectrum of the infinite periodic system arises in the limit $N\to\infty$. We
illustrate the result on two kinds of examples. The first are serial graphs
obtained by chaining loops or T-junctions. A detailed discussion is presented
for a finite-periodic "comb"; we show how the resonance poles can be computed
within the Krein formula approach. Another example concerns geometric
scatterers where the individual element consists of a surface with a pair of
leads; we show that apart of the resonances coming from the decoupled-surface
eigenvalues such scatterers exhibit the high-energy behavior typical for the
delta' interaction for the physically interesting couplings.


title: A quantum evaporation effect

abstract:
  A small momentum transfer to a particle interacting with a steep potential
barrier gives rise to a quantum evaporation effect which increases the
transmission appreciably. This effect results from the unexpectedly large
population of quantum states with energies above the height of the barrier. Its
characteristic properties are studied and an example of physical system in
which it may be observed is given.


title: The asymptotic relative entropy of entanglement

abstract:
  We present an analytical formula for the asymptotic relative entropy of
entanglement for Werner states of arbitrary dimension. We then demonstrate its
validity using methods from convex optimization. To our knowledge, this is the
first case in which the value of a subadditive entanglement measure has been
obtained in the asymptotic limit.


title: Nondistortion Quantum Interrogation using EPR entangled Photons

abstract:
  We propose a novel scheme for nondistortion quantum interrogation (NQI),
defined as an interaction-free measurement which preserves the internal state
of the object being detected. In our scheme, two EPR entangled photons are used
as the probe and polarization sensitive measurements are performed at the four
ports of the Mach-Zehnder interferometer. In comparison with the previous
single photon scheme, it is shown that the two photon approach has a higher
probability of initial state preserving interrogation of an atom prepared in a
quantum superposition. In the case that the presence of the atom is not
successfully detected, the experiment can be repeated since the initial state
of the atom is unperturbed.


title: Quantum Data Hiding

abstract:
  We expand on our work on Quantum Data Hiding -- hiding classical data among
parties who are restricted to performing only local quantum operations and
classical communication (LOCC). We review our scheme that hides one bit between
two parties using Bell states, and we derive upper and lower bounds on the
secrecy of the hiding scheme. We provide an explicit bound showing that
multiple bits can be hidden bitwise with our scheme. We give a preparation of
the hiding states as an efficient quantum computation that uses at most one
ebit of entanglement. A candidate data hiding scheme that does not use
entanglement is presented. We show how our scheme for quantum data hiding can
be used in a conditionally secure quantum bit commitment scheme.


title:
Entangled qutrits violate local realism stronger than qubits - an
  analytical proof

abstract:
  In Kaszlikowski [Phys. Rev. Lett. {\bf 85}, 4418 (2000)], it has been shown
numerically that the violation of local realism for two maximally entangled
$N$-dimensional ($3 \leq N$) quantum objects is stronger than for two maximally
entangled qubits and grows with $N$. In this paper we present the analytical
proof of this fact for N=3.


title: Experiment can decide between standard and Bohmian quantum mechanics

abstract:
  In this investigation, we have considered two thought experiments to make a
comparison between predictions of the standard and the Bohmian quantum
mechanics. Concerning this, a two-particle system has been studied at two
various situations of the entangled and the unentangled states. In the first
experiment, the two theories can predict different results at the individual
level, while their statistical results are the same. In the other experiment,
not only they are in disagreement at the individual level, but their
equivalence at the statistical level also breaks down, if one uses selective
detection. Furthermore, we discuss about some objections that can be raised
against the results of the two suggested experiments.


title:
Bohmian prediction about a two double-slit experiment and its
  disagreement with standard quantum mechanics

abstract:
  The significance of proposals that can predict different results for standard
and Bohmian quantum mechanics have been the subject of many discussions over
the years. Here, we suggest a particular experiment (a two double-slit
experiment) and a special detection process, that we call selective detection,
to distinguish between the two theories. Using our suggested experiment, it is
shown that the two theories predict different observable results at the
individual level for a geometrically symmetric arrangement. However, their
predictions are the same at the ensemble level. On the other hand, we have
shown that at the statistical level, if we use our selective detection, then
either the predictions of the two theories differ or where standard quantum
mechanics is silent or vague, Bohmian quantum mechanics makes explicit
predictions.


title: Conditions for Nondistortion Interrogation of Quantum System

abstract:
  Under some physical considerations, we present a universal formulation to
study the possibility of localizing a quantum object in a given region without
disturbing its unknown internal state. When the interaction between the object
and probe wave function takes place only once, we prove the necessary and
sufficient condition that the object's presence can be detected in an initial
state preserving way. Meanwhile, a conditioned optimal interrogation
probability is obtained.


title:
Michelson interferometer operating at effects of first order with
  respect to v/c

abstract:
  In the first version of this paper (arXiv: 1003.2899v1, 15.03.2010) there is
described first, traditional method of measuring the non-zero shift of
interference fringe in the Michelson interferometer, operating on the effects
of second order with respect to v/c, and are revealed hidden causes of the
failure to measure the shift of interference fringe in the period from 1881
till 1960. It is shown that at the latitude of Obninsk within a 24-hour
observation period the horizontal projection of aether wind velocity varies
from 140 km/s to 480 km/s. The second version of this paper (arXiv:
1003.2899v2, 15.04.2010) is supplemented with a second method of finding the
velocity of the aether wind -- through measuring the largest seasonal decrease
in the ratio of the summer shift of the interference fringe to the winter one
(equaled ~12%). It gave the same interval of values of the projections of the
aether wind velocity as the first method. More than hundred years there
persists a belief that Michelson-type interferometer can not be adjusted such
as to detect effects of the first order with respect to v/c. Below I show that
it is possible, and more successfully than on the interferometer of the second
order. In contrast to the traditional approach, in the interferometer of the
first order the light after splitting on a semi-transparent plate propagates in
both arms to the reflecting mirrors in one optical medium, and returns after
reflection from the mirrors through another optical medium. The shift of the
interference fringe was found to be proportional to the difference of
dielectric permittivities of these media. The horizontal projection of the
Earth's velocity relative to luminiferous aether measured by the first order
interferometer agrees with that found by the first two methods.


title: Reply to Comment

abstract:
  In the comment(quant-ph/0103003) Eggeling,Vollbrecht and Wolf suspect our
method in quant-ph/0102133 is not practical. Here we explain our result and
method and show that our example can tell one how to judge a separable state,
and so our method is practical, at least for many mixed states.


title: Implications of Teleportation for Nonlocality

abstract:
  Adopting an approach similar to that of Zukowski [Phys. Rev. A 62, 032101
(2000)], we investigate connections between teleportation and nonlocality. We
derive a Bell-type inequality pertaining to the teleportation scenario and show
that it is violated in the case of teleportation using a perfect singlet. We
also investigate teleportation using `Werner states' of the form x P + (1-x)
I/4, where P is the projector corresponding to a singlet state and I is the
identity. We find that our inequality is violated, implying nonlocality, if x >
1/sqrt(2). In addition, we extend Werner's local hidden variable model to
simulation of teleportation with the x = 1/2 Werner state. Thus teleportation
using this state does not involve nonlocality even though the fidelity achieved
is 3/4 which is greater than the `classical limit' of 2/3. Finally, we comment
on a result of Gisin's and offer some philosophical remarks on teleportation
and nonlocality generally.


title:
Quantum nondemolition measurement of a light field component by a
  feedback compensated beam splitter

abstract:
  In conventional quantum nondemolition measurements, the interaction between
signal and probe preserves the measured variable. Alternatively, it is possible
to restore the original value of the variable by feedback. In this paper, we
describe a quantum nondemolition measurement of a quadrature component of the
light field using a feedback compensated beam splitter. The noise induced by
the vacuum port of the beam splitter is compensated by a linear feedback
resulting in an effective amplification of the observed variable. This
amplification is then be reversed by optical parametric amplification to
restore the original value of the field component.


title:
Functional approach to quantum decoherence and the classical final limit
  II: the pointer basis and the quantum measurements

abstract:
  A quantum system with discrete and continuos evolution spectrum is studied. A
final pointer basis is found, that can be defined in a presised mathematical
way. This result is use to explain the quantum measurement in the system.


title:
The physics of forgetting: Landauer's erasure principle and information
  theory

abstract:
  This article discusses the concept of information and its intimate
relationship with physics. After an introduction of all the necessary quantum
mechanical and information theoretical concepts we analyze Landauer's principle
that states that the erasure of information is inevitably accompanied by the
generation of heat. We employ this principle to rederive a number of results in
classical and quantum information theory whose rigorous mathematical
derivations are difficult. This demonstrates the usefulness of Landauer's
principle and provides an introduction to the physical theory of information.


title: Equivalent classes of closed three-level systems

abstract:
  In recent years a significant amount of research in quantum optics has been
devoted to the analysis of atomic three-level systems and for many physical
quantities the same effects have been predicted for different configurations.
These configurations can be split into essentially two classes. One for which
the system contains a metastable state and another where the system has two
close-lying levels and coherence effects become important. We demonstrate when
and why for a wide range of parameters these two classes are in fact equivalent
for many important physical quantities. A unified picture underlying a large
body of work on these categories of atomic three-level systems is presented and
applied to some examples.


title:
On the nature of spin, inertia and gravity of a moving canonical
  particle

abstract:
  It is suggested that a moving canonical particle interacts with a vacuum
regarded as a "soft" cellular space. The interaction results into the emergence
of elementary excitations of space - inertons - surrounding the particle. It is
assumed that such a motion leads not only to the spatial oscillation of the
particle along a path but to the oscillation of the particle centre-of-mass as
well. This phenomenon culminating in the anisotropic pulsation of the particle
is associated with the notion of spin. The particle-space interaction is
treated as the origin of the matter waves, which are identified with the
particle inertia and inertons surrounding the moving particle are considered as
carriers of its inert properties. Inertons are also identified with real
carriers of the gravitational interaction and the range of the particle
gravitational potential is evaluated by the inerton cloud amplitude
$\Lambda=\lambda c/v$, where $\lambda$ is the de Broglie wavelength, $c$ and
$v$ are the velocity of light and the particle respectively. The nature of the
phase transition that occurs in a quantum system when one should pass from the
description based on the Schroedinger formalism to that of resting on the Dirac
one is explained in detail.


title:
Optimal Conclusive Discrimination of Two Non-orthogonal Pure Product
  Multipartite States Locally

abstract:
  We consider one copy of a quantum system prepared in one of two
non-orthogonal pure product states of multipartite distributed among separated
parties. We show that there exist protocols which obtain optimal probability in
the sense of conclusive discrimination by means of local operations and
classical communications(LOCC) as good as by global operations. Also, we show a
protocol which minimezes the average number of local operations. Our result
implies that two product pure multipartite states might not have the non-local
property though more than two can have.


title:
Preparation of Schr\"odinger cat states with cold ions beyond the
  Lamb-Dicke limit

abstract:
  A scheme for preparing Schr\"odinger cat (SC) states is proposed beyond the
Lamb-Dicke limit in a Raman-$\Lambda$-type configuration. It is shown that SC
states can be obtained more efficiently with our scheme than with the former
ones.


title: Maximizing the entanglement of two mixed qubits

abstract:
  Two-qubit states occupy a large and relatively unexplored Hilbert space. Such
states can be succinctly characterized by their degree of entanglement and
purity. In this letter we investigate entangled mixed states and present a
class of states that have the maximum amount of entanglement for a given linear
entropy.


title:
Time-like flows of energy-momentum and particle trajectories for the
  Klein-Gordon equation

abstract:
  The Klein-Gordon equation is interpreted in the de Broglie-Bohm manner as a
single-particle relativistic quantum mechanical equation that defines unique
time-like particle trajectories. The particle trajectories are determined by
the conserved flow of the intrinsic energy density which can be derived from
the specification of the Klein-Gordon energy-momentum tensor in an
Einstein-Riemann space. The approach is illustrated by application to the
simple single-particle phenomena associated with square potentials.


title: Solvable analogue of the imaginary cubic oscillator

abstract:
  We prove that the purely imaginary square well generates an infinite number
of bound states with real energies. In the strong-coupling limit, our exact PT
symmetric solutions coincide, utterly unexpectedly, with their textbook, well
known Hermitian predecessors.


title:
Semiconductor quantum computer design with 100 nm separation of
  nuclear-spin qubits

abstract:
  We combine elements of the 1998 quantum computing proposals by Privman,
Vagner and Kventsel, and by Kane, with the new idea of nuclear-spin qubit
interactions mediated indirectly via the bound outer electrons of impurity
atoms whose nuclear spins 1/2 are the qubits. These electrons, in turn,
interact via the two-dimensional electron gas in the quantum Hall effect
regime. The resulting quantum computing scheme retains all the gate-control and
measurement aspects of the proposal by Kane, but allows qubit spacing at
distances of order 100 nm, attainable with the present-day
semiconductor-heterostructure device technologies.


title:
Quantum control of population inversion in the presence of spontaneous
  emission

abstract:
  The detrimental effect of spontaneous emission on the performance of control
schemes designed to achieve population inversion between the ground state and a
highly excited atomic state are studied using computer simulations.


title: Robust quantum memory via quantum control

abstract:
  A general scheme for building a quantum memory by transferring quantum
information to an essentially decoherence-free memory transition using quantum
control is presented and illustrated by computer simulations.


title:
Classical programmability is enough for quantum circuits universality in
  approximate sense

abstract:
  It was shown by M.A. Nielsen and I.L. Chuang 1997, that it is impossible to
build strictly universal programmable quantum gate array, that could perform
any unitary operation precisely and it was suggested to use probabilistic gate
arrays instead. In present work is shown, that if to use more physical and weak
condition of universality (suggested already in earliest work by D.Deutsch
1985) and to talk about simulation with arbitrary, but finite precision, then
it is possible to build universal programmable gate array. But now the same
no-go theorem by Nielsen and Chuang will have new interesting consequence ---
controlling programs for the gate arrays can be considered as pure classical.
More detailed design of such deterministic quantum gate arrays universal ``in
approximate sense'' is considered in the paper.


title: Low energy atomic collision with dipole interactions

abstract:
  We apply quantum defect theory to study low energy ground state atomic
collisions including aligned dipole interactions such as those induced by an
electric field. Our results show that coupled even ($l$) relative orbital
angular momentum partial wave channels exhibit shape resonance structures while
odd ($l$) channels do not. We analyze and interpret these resonances within the
framework of multichannel quantum defect theory (MQDT).


title: On the Measurement of Qubits

abstract:
  We describe in detail the theory underpinning the measurement of density
matrices of a pair of quantum two-level systems (``qubits''). Our particular
emphasis is on qubits realized by the two polarization degrees of freedom of a
pair of entangled photons generated in a down-conversion experiment; however
the discussion applies in general, regardless of the actual physical
realization. Two techniques are discussed, namely a tomographic reconstruction
(in which the density matrix is linearly related to a set of measured
quantities) and a maximum likelihood technique which requires numerical
optimization (but has the advantage of producing density matrices which are
always non-negative definite). In addition a detailed error analysis is
presented, allowing errors in quantities derived from the density matrix, such
as the entropy or entanglement of formation, to be estimated. Examples based on
down-conversion experiments are used to illustrate our results.


title: Quantum authentication of classical messages

abstract:
  Although key distribution is arguably the most studied context on which to
apply quantum cryptographic techniques, message authentication, i.e.,
certifying the identity of the message originator and the integrity of the
message sent, can also benefit from the use of quantum resources. Classically,
message authentication can be performed by techniques based on hash functions.
However, the security of the resulting protocols depends on the selection of
appropriate hash functions, and on the use of long authentication keys. In this
paper we propose a quantum authentication procedure that, making use of just
one qubit as the authentication key, allows the authentication of binary
classical messages in a secure manner.


title: Quantum message authentication codes

abstract:
  This paper has been withdrawn because of an error in the proof of Lemma 1.
Without Lemma 1, the result of the main theorem (with the same proof, but
omitting the use of Lemma 1 to extend things to general attacks) only holds for
attacks in which the attacker applies a tensor product of Pauli matrices (for
qubits) or generalized Pauli matrices. The resulting theorem is still relevant
for quantum message authentication, but the very strong result claimed in
earlier versions of this paper does not hold. These matters will be addressed
further in joint work with Claude Crepeau, Daniel Gottesman, Adam Smith, and
Alain Tapp. Thanks to them and to Debbie Leung for spotting the error.


title: A Dirac-type equation for spacelike neutrinos

abstract:
  Based on experimental evidences supporting the hypothesis that neutrinos
might be spacelike particles, a new Dirac-type equation is proposed and a
spin-1/2 spacelike quantum theory is developed. The new Dirac-type equation
provides a solution for the puzzle of negative mass-square of neutrinos. This
equation can be written in two spinor equations coupled together via nonzero
mass while respecting maximum parity violation, and it reduces to one Weyl
equation in the massless limit. Some peculiar features of spacelike neutrino
are discussed in this theoretical framework.


title: Polarization instabilities in a two-photon laser

abstract:
  We describe the operating characteristics of a new type of quantum oscillator
that is based on a two-photon stimulated emission process. This two-photon
laser consists of spin-polarized and laser-driven $^{39}$K atoms placed in a
high-finesse transverse-mode-degenerate optical resonator, and produces a beam
with a power of $\sim $0.2 $\mu $W at a wavelength of 770 nm. We observe
complex dynamical instabilities of the state of polarization of the two-photon
laser, which are made possible by the atomic Zeeman degeneracy. We conjecture
that the laser could emit polarization-entangled twin beams if this degeneracy
is lifted.


title:
On the Incompatibility of Standard Quantum Mechanics and the de
  Broglie-Bohm Theory

abstract:
  It is shown that the de Broglie-Bohm quantum theory of multi-particle systems
is incompatible with the standard quantum theory of such systems unless the
former is ergodic. A realistic experiment is suggested to distinguish between
the two theories.


title: Universal quantum processors with arbitrary radix n

abstract:
  Here is discussed the Hamiltonian approach to construction of deterministic
universal (in approximate sense) programmable quantum circuits with qubits or
any other quantum systems with dimension of Hilbert space is $n \ge 2$.


title: Compressibility of Mixed-State Signals

abstract:
  We present a formula that determines the optimal number of qubits per message
that allows asymptotically faithful compression of the quantum information
carried by an ensemble of mixed states. The set of mixed states determines a
decomposition of the Hilbert space into the redundant part and the irreducible
part. After removing the redundancy, the optimal compression rate is shown to
be given by the von Neumann entropy of the reduced ensemble.


title:
Entropy production and wave packet dynamics in the Fock space of closed
  chaotic many-body systems

abstract:
  Highly excited many-particle states in quantum systems such as nuclei, atoms,
quantum dots, spin systems, quantum computers etc., can be considered as
``chaotic'' superpositions of mean-field basis states (Slater determinants,
products of spin or qubit states). This is due to a very high level density of
many-body states that are easily mixed by a residual interaction between
particles (quasi-particles). For such systems, we have derived simple
analytical expressions for the time dependence of energy width of wave packets,
as well as for the entropy, number of principal basis components and inverse
participation ratio, and tested them in numerical experiments. It is shown that
the energy width $\Delta (t)$ increases linearly and very quickly saturates.
The entropy of a system increases quadratically, $S(t) \sim t^2$ at small
times, and after, can grow linearly, $S(t) \sim t$, before the saturation.
Correspondingly, the number of principal components determined by the entropy,
$N_{pc} \sim exp{(S(t))}$, or by the inverse participation ratio, increases
exponentially fast before the saturation. These results are explained in terms
of a cascade model which describes the flow of excitation in the Fock space of
basis components. Finally, a striking phenomenon of damped oscillations in the
Fock space at the transition to an equilibrium is discussed.

categories: quant-ph cond-mat.stat-mech nlin.CD nucl-th


title: Spatial evolution of short pulses under coherent population trapping

abstract:
  Spatial and temporal evolution is studied of two powerful short laser pulses
having different wavelengths and interacting with a dense three-level
Lambda-type optical medium under coherent population trapping. A general case
of unequal oscillator strengths of the transitions is considered. Durations of
the probe pulse and the coupling pulse $T_{1,2}$ ($T_2>T_1$) are assumed to be
shorter than any of the relevant atomic relaxation times. We propose analytical
and numerical solutions of a self-consistent set of coupled Schr\"{o}dinger
equations and reduced wave equations in the adiabatic limit with the account of
the first non-adiabatic correction. The adiabaticity criterion is also
discussed with the account of the pulse propagation. The dynamics of
propagation is found to be strongly dependent on the ratio of the transition
oscillator strengths. It is shown that envelopes of the pulses slightly change
throughout the medium length at the initial stage of propagation. This distance
can be large compared to the one-photon resonant absorption length. Eventually,
the probe pulse is completely reemitted into the coupling pulse during
propagation. The effect of localization of the atomic coherence has been
observed similar to the one predicted by Fleischhauer and Lukin (PRL, {\bf 84},
5094 (2000).


title: A classification of incomparable states

abstract:
  Let (\{| \psi> ,| \phi>}) be an incomparable pair of states ((| \psi
\nleftrightarrow | \phi>)), \emph, i.e., (| \psi>) and (| \phi>) cannot be
transformed to each other with probability one by local transformations and
classical communication (LOCC). We show that incomparable states can be
multiple-copy transformable, \emph, i.e., there can exist a \emph{k}, such that
(| \psi> ^{\otimes k+1}\to | \phi> ^{\otimes k+1}), i.e., (k+1) copies of (|
\psi>) can be transformed to (k+1) copies of (| \phi>) with probability one by
LOCC but (| \psi> ^{\otimes n}\nleftrightarrow | \phi> ^{\otimes n} \forall
n\leq k). We call such states \emph{k}-copy LOCC incomparable. We provide a
necessary condition for a given pair of states to be \emph{k}-copy LOCC
incomparable for some \emph{k}. We also show that there exist states that are
neither \emph{k}-copy LOCC incomparable for any \emph{k} nor catalyzable even
with multiple copies. We call such states strongly incomparable. We give a
sufficient condition for strong incomparability.
  We demonstrate that the optimal probability of a conclusive transformation
involving many copies, (p_{max}(| \psi> ^{\otimes m}\to | \phi> ^{\otimes m}))
can decrease exponentially with the number of source states (m), even if the
source state has \emph{more} entropy of entanglement.


title: A general mathematical structure for the time-reversal operator

abstract:
  The aim of this work is the mathematical analysis of the physical
time-reversal operator and its definition as a geometrical structure\QTR{bf}{,
}in such a way that it could be generalized to the purely mathematical realm.
Rigorously, only having such a ``time-reversal structure'' it can be decided
whether a dynamical system is time-symmetric or not.\QTR{it}{\}The
``time-reversal structures'' of several important physical and mathematical
examples are presented, showing that there are some mathematical categories
whose objects are the (classical or abstract) ``time-reversal systems'' and
whose morphisms generalize the Wigner transformation.


title: The survival of quantum coherence in deformed states superposition

abstract:
  We study the dissipative dynamics of deformed coherent states superposition.
We find that such kind of superposition can be more robust against decoherence
than the usual Schrodinger cat states.


title:
Comment on ``Experimental Separation of Geometric and Dynamical Phases
  Using Neutron Interferometry"

abstract:
  We analyse a recently reported neutron interference experiment to measure a
geometric phase and attempt to bring out the inadequacy of the ``phase modulo
2\pi" approach to the geometric phase. A modified neutron interferometer
experiment to observe 2n\pi phase changes of topological origin resulting from
closed circuits around singularities in the parameter space of the experiment
is proposed.


title: Orthomodular Lattices and a Quantum Algebra

abstract:
  We show that one can formulate an algebra with lattice ordering so as to
contain one quantum and five classical operations as opposed to the standard
formulation of the Hilbert space subspace algebra. The standard orthomodular
lattice is embeddable into the algebra. To obtain this result we devised
algorithms and computer programs for obtaining expressions of all quantum and
classical operations within an orthomodular lattice in terms of each other,
many of which are presented in the paper. For quantum disjunction and
conjunction we prove their associativity in an orthomodular lattice for any
triple in which one of the elements commutes with the other two and their
distributivity for any triple in which a particular one of the elements
commutes with the other two. We also prove that the distributivity of symmetric
identity holds in Hilbert space, although it remains an open problem whether it
holds in all orthomodular lattices, as it does not fail in any of over 50
million Greechie diagrams we tested.


title: The Covariant Stark Effect

abstract:
  This paper examines the Stark effect, as a first order perturbation of
manifestly covariant hydrogen-like bound states. These bound states are
solutions to a relativistic Schr\"odinger equation with invariant evolution
parameter, and represent mass eigenstates whose eigenvalues correspond to the
well-known energy spectrum of the non-relativistic theory. In analogy to the
nonrelativistic case, the off-diagonal perturbation leads to a lifting of the
degeneracy in the mass spectrum. In the covariant case, not only do the
spectral lines split, but they acquire an imaginary part which is lnear in the
applied electric field, thus revealing induced bound state decay in first order
perturbation theory. This imaginary part results from the coupling of the
external field to the non-compact boost generator. In order to recover the
conventional first order Stark splitting, we must include a scalar potential
term. This term may be understood as a fifth gauge potential, which compensates
for dependence of gauge transformations on the invariant evolution parameter.


title: Separability Properties of Three-mode Gaussian States

abstract:
  We derive a necessary and sufficient condition for the separability of
tripartite three mode Gaussian states, that is easy to check for any such
state. We give a classification of the separability properties of those systems
and show how to determine for any state to which class it belongs. We show that
there exist genuinely tripartite bound entangled states and point out how to
construct and prepare such states.


title: Squeezed light from spin squeezed atoms

abstract:
  We propose to produce pulses of strongly squeezed light by Raman scattering
of a strong laser pulse on a spin squeezed atomic sample. We prove that the
emission is restricted to a single field mode which perfectly inherits the
quantum correlations of the atomic system.


title: Just How Final are Today's Quantum Structures?

abstract:
  I present a selection of conceptual and mathematical problems in the
foundations of modern physics as they derive from the title question.
Contribution to a panel session, "Springer Forum: Quantum Structures --
Physical, Mathematical and Epistemological Problems", held at the Biannual
Symposium of the International Quantum Structures Association, Liptovsky Jan,
September 1998. To appear in journal: Soft Computing (2001).


title: Light-shift-induced quantum gates for ions in thermal motion

abstract:
  An effective interaction between trapped ions in thermal motion can be
generated by illuminating them simultaneously with a single laser resonant with
the ionic carrier frequency. The ac Stark-shift induces simultaneous `virtual'
two-phonon transitions via several motional modes. Within a certain laser
intensity range these transitions can interfere constructively, resulting in a
relatively fast, heating-resistant two-qubit logic gate.


title: Scaling properties of cavity-enhanced atom cooling

abstract:
  We extend an earlier semiclassical model to describe the dissipative motion
of N atoms coupled to M modes inside a coherently driven high-finesse cavity.
The description includes momentum diffusion via spontaneous emission and cavity
decay. Simple analytical formulas for the steady-state temperature and the
cooling time for a single atom are derived and show surprisingly good agreement
with direct stochastic simulations of the semiclassical equations for N atoms
with properly scaled parameters. A thorough comparison with standard free-space
Doppler cooling is performed and yields a lower temperature and a cooling time
enhancement by a factor of M times the square of the ratio of the atom-field
coupling constant to the cavity decay rate. Finally it is shown that laser
cooling with negligible spontaneous emission should indeed be possible,
especially for relatively light particles in a strongly coupled field
configuration.


title:
Inseparability criteria for demonstration of the Einstein-Podolsky-Rosen
  gedanken experiment

abstract:
  It is shown that a criterion used to demonstrate realization of the 1935
  Einstein-Podolsky-Rosen (EPR) gedanken experiment is sufficient to
demonstrate quantum entanglement.
  A further set of measurable criteria sufficient to demonstrate EPR gedanken
experiment is proposed, these being the set of criteria sufficient to
demonstrate entanglement, by way of a measured violation of a necessary
condition of separability. In this way, provided the spatial separation of
systems is sufficient to ensure EPR's locality hypothesis, it is shown how a
measured demonstration of entanglement will, at least, be equivalent to a
demonstration of the EPR gedanken experiment. Using hidden variables it is
explained how such demonstrations are a direct manifestation of the
inconsistency of local realism with quantum mechanics.


title: Tachyons, Quanta, and Chaos

abstract:
  It is shown using numerical simulation that classical charged tachyons have
several features normally thought to be unique to quantum mechanics. Spin-like
self-orbiting helical motions are shown to exist at discrete values for the
velocity of the tachyon in Feynman-Wheeler electrodynamics and in normal causal
electrodynamics more complex closed orbits also appear to exist. Tunneling
behavior of the classical tachyon is observed at classical turning points
depending on the angle of incidence. The equations of motion appear to be
chaotic and effectively indeterministic when the tachyon crosses its own past
light cone. It is argued that self-interacting tachyons moving in a tight helix
would behave causally, and that they could be a basis for a hidden variable
description of quantum mechanics. A procedure is proposed which could determine
the fine structure constant.


title: Time of occurrence observable in quantum mechanics

abstract:
  We propose a general construction of an observable measuring the time of
occurence of an effect in quantum theory. Time delay in potential scattering is
computed as a straightforward application.


title: Performance of Photon-Pair Quantum Key Distribution Systems

abstract:
  We analyze the quantitative improvement in performance provided by a novel
quantum key distribution (QKD) system that employs a correlated photon source
(CPS) and a photon-number resolving detector (PNR). Our calculations suggest
that given current technology, the CPR implementation offers an improvement of
several orders of magnitude in secure bit rate over previously described
implementations.


title: Ipso-information-transfer

abstract:
  A tripartite system with entangled non-orthogonal states is used to transfer
retrievable or usable information without requiring an external channel,
ipso-information-transfer (IIT). The non-Schmidt decomposable entanglement
couples two independent interactions through which the information is
transferred between a pair of non-interacting components of the system. We
outline a dynamical model of IIT using localised particles. Implications for
quantum nonlocality are raised.


title:
Requirement of optical coherence for continuous-variable quantum
  teleportation

abstract:
  We show that the sender (Alice) and the receiver (Bob) each require coherent
devices in order to achieve unconditional continuous variable quantum
teleportation (CVQT), and this requirement cannot be achieved with conventional
laser sources, even in principle. The appearance of successful CVQT in recent
experiments is due to interpreting the measurement record fallaciously in terms
of one preferred ensemble (or decomposition) of the correct density matrix
describing the state. Our analysis is unrelated to technical problems such as
laser phase drift or finite squeezing bandwidth.


title: Resonance absolute quantum reflection at selected energies

abstract:
  The possibility of the resonance reflection (100 % at maximum) is revealed.
The corresponding exactly solvable models with the controllable numbers of
resonances, their positions and widths are presented.


title: Transmission of a Cartesian Frame by a Quantum System

abstract:
  A single quantum system, such as a hydrogen atom, can transmit a Cartesian
coordinate frame (three axes). For this it has to be prepared in a
superposition of states belonging to different irreducible representations of
the rotation group. The algorithm for decoding such a state is presented, and
the fidelity of transmission is evaluated.


title: The zero energy quantum informtaiton processing

abstract:
  In contradistinction with some plausible statements of the information
theory, we point out the possibility of the zero energy quantum information
processing. Particularly, we investigate the rate of the entanglement formation
in the operation of the quantum "oracles" employing the "quantum parallelism",
and we obtain that the relative maximum of the rate of the operation
distinguishes the zero average energy of interaction in the composite system
"input register + output register". This result is reducible to neither of the
previously obtained bounds, and therefore represents a new bound for the
nonorthogonal state transformations in the quantum information theory.


title: Jump time and passage time: the duration of a quantum transition

abstract:
  Under unitary evolution, systems move gradually from state to state. An
unstable atom has amplitude in its original state after many lifetimes
($\tau_L$). But in the laboratory, transitions seem to go instantaneously, as
suggested by the term "quantum jump."
  The problem studied here is whether the "jump" can be assigned a duration, in
theory and in experiment. Two characteristic times are defined, jump time
($\tau_J$) and passage time ($\tau_P$). Both use Zeno time, $\tau_Z$, defined
in terms of $H$ and its initial state as $\tau_Z \equiv \hbar/\sqrt{<\psi|
(H-E_\psi)^2 |\psi>}$, with $E_\psi \equiv <\psi|H|\psi>$.
  $\tau_J$ is defined in terms of the time needed to slow (\`a la the quantum
Zeno effect) the decay: $\tau_J \equiv \tau_Z^2/\tau_L$. It appears in several
contexts. It is related to tunneling time in barrier penetration. Its inverse
is the bandwidth of the Hamiltonian, in a time-energy uncertainty principle.
$\tau_J$ is also an indicator of the duration of the quadratic decay regime in
both experiment and in numerical calculations (cf. Fig.~2 of PRA 57,1509
(1998).)
  The passage time, $\tau_P$, arises from unitary evolution sans
interpretation. It is based on a bound of Fleming (Nuov. Cim. 16 A, 232
(1973)): for any $H$ and $\psi$ a system cannot evolve to a state orthogonal to
$\psi$ for $t< \tau_P \equiv \pi \tau_Z/2$. By including apparatus in $H$,
$\tau_P$ limits the observation of decay according to the quantum measurement
ideas proposed in "Time's Arrows and Quantum Measurement," Cambridge U. Press,
1997, thereby allowing an experimental test of these ideas.


title:
Giant non-linearities accompanying electromagnetically induced
  transparency

abstract:
  We develop a fully quantum treatment of electromagnetically induced
transparency (EIT) in a vapor of three-level $\Lambda$-type atoms. Both the
probe and coupling lasers with arbitrary intensities are quantized, and treated
on the same footing. In addition to reproducing known results on ultraslow
pulse propagation at the lowest order in the ratio of their Rabi frequencies,
our treatment uncovers that the atomic medium with EIT exhibits giant Kerr as
well as higher order non-linearities. Enhancement of many orders of magnitude
is predicted for higher-order refractive-index coefficients.


title:
Self-adjoint extensions of operators and the teaching of quantum
  mechanics

abstract:
  For the example of the infinitely deep well potential, we point out some
paradoxes which are solved by a careful analysis of what is a truly
self-adjoint operator. We then describe the self-adjoint extensions and their
spectra for the momentum and the Hamiltonian operators in different physical
situations. Some consequences are worked out, which could lead to experimental
checks.


title: Quantum Theory Cannot Forbid Superluminal Signaling

abstract:
  This note analyzed the angle-distribution of the probabilities of two-photon
states come out of a single-photon's stimulated emission amplification, show
that one can exploit EPR photon pairs combined with stimulated emission to
realize superluminal signaling.


title: Monotones and invariants for multi-particle quantum states

abstract:
  We introduce new entanglement monotones which generalize, to the case of many
parties, those which give rise to the majorization-based partial ordering of
bipartite states' entanglement. We give some examples of restrictions they
impose on deterministic and probabilistic conversion between multipartite
states via local actions and classical communication. These include
restrictions which do not follow from any bipartite considerations. We derive
supermultiplicativity relations between each state's monotones and the
monotones for collective processing when the parties share several states. We
also investigate polynomial invariants under local unitary transformations, and
show that a large class of these are invariant under collective unitary
processing and also multiplicative, putting restrictions, for example, on the
exact conversion of multiple copies of one state to multiple copies of another.


title: Additivity for a class of unital qubit channels

abstract:
  Additivity of the Holevo capacity is proved for product channels, under the
condition that one of the channels is in a certain class of unital qubit
channels, with the other completely arbitrary. This qubit class includes the
depolarizing channel. As a byproduct this proves that the Holevo bound is the
ultimate information capacity of such qubit channels (assuming no prior
entanglement between sender and receiver). Additivity of minimal entropy and
multiplicativity of p-norms are also proved under the same assumptions.


title: Entanglement Induced Phase Transitions

abstract:
  Starting from the canonical ensemble over the space of pure quantum states,
we obtain an integral representation for the partition function. This is used
to calculate the magnetisation of a system of N spin-1/2 particles. The results
suggest the existence of a new type of first order phase transition that occurs
at zero temperature in the absence of spin-spin interactions. The transition
arises as a consequence of quantum entanglement. The effects of internal
interactions are analysed and the behaviour of the magnetic susceptibility for
a small number of interacting spins is determined.


title: More Is Not Necessarily Easier

abstract:
  In this brief comment, we consider the exact, deterministic, and
nonasymptotic transformation of multiple copies of pure states under LOCC. It
was conjectured in quant-ph/0103131 that, if $k$ copies of $|\psi\>$ can be
transformed to $k$ copies of $|\phi\>$, the same holds for all $r \geq k$. We
present counterexamples to the above conjecture.


title: Teleportation with the entangled states of a beam splitter

abstract:
  We present a teleportation protocol based upon the entanglement produced from
Fock states incident onto a beam splitter of arbitrary transmissivity. The
teleportation fidelity is analysed, its trends being explained from
consideration of a beam splitter's input/output characteristics.


title: Enhanced Estimation of a Noisy Quantum Channel Using Entanglement

abstract:
  We discuss the estimation of channel parameters for a noisy quantum channel -
the so-called Pauli channel - using finite resources. It turns out that prior
entanglement considerably enhances the fidelity of the estimation when we
compare it to an estimation scheme based on separable quantum states.


title: Quantum measurements, Information, and Dual States Representations

abstract:
  The quantum measurement problem considered for the model of measuring system
(MS) consist of measured state S (particle), detector D and information
processing device (observer) $O$ interacting with S,D.
  For 'external' observer $O'$ MS evolution obeys to Schrodinger equation (SE)
and $O$ (self)description of MS reconstructed from it in Breuer ansatz. MS
irreversible evolution (state collapse) for $O$ can be obtained if the true
quantum states manifold has the dual structure $L_T=\cal {H} \bigotimes L_V$
where $\cal H$ is Hilbert space and $\cal L_V$ is the set with elements
$V^O=|O_j> < O_j|$ describing random 'pointer' outcomes $O_j$ observed by $O$
in the individual events. Possible experimental tests of this dual states
structure described. The results interpretation in Quantum Information
framework and Relational Quantum Mechanics discussed.


title: A new proof for the existence of mutually unbiased bases

abstract:
  We develop a strong connection between maximally commuting bases of
orthogonal unitary matrices and mutually unbiased bases. A necessary condition
of the existence of mutually unbiased bases for any finite dimension is
obtained. Then a constructive proof of the existence of mutually unbiased bases
for dimensions which are power of a prime is presented. It is also proved that
in any dimension d the number of mutually unbiased bases is at most d+1. An
explicit representation of mutually unbiased observables in terms of Pauli
matrices are provided for d=2^m.


title: Collective Rabi Oscillations and Cold Collisions

abstract:
  We examine cold atomic collisions within a resonant optical cavity. The
quantized cavity mode can be used to manipulate the collisions between the cold
atoms, such that periodic exchange of excitations between the atoms and the
electromagnetic field strongly alters the collision dynamics. A colliding pair
of atoms can thereby oscillate between its ground and excited states during the
collision time. Using a semiclassical model, it can be predicted that such
Rabi-like oscillations are revealed in the atomic trap-loss probabilities,
which show maxima and minima as a function of the detuning between the
frequencies of the mode and the atomic transition.


title:
The Local Larmor Clock, Partial Densities of States, and Mesoscopic
  Physics

abstract:
  The local Larmor clock is used to derive a hierarchy of local densities of
states. At the bottom of this hierarchy are the partial density of states for
which represent the contribution to the local density of states if both the
incident and outgoing scattering channel are prescribed. On the next higher
level is the injectivity which represents the contribution to the local density
of states if only the incident channel is prescribed regardless of the final
scattering channel. The injectivity is related by reciprocity to the emissivity
of a point into a quantum channel. The sum of all partial density of states or
the sum of all injectivities or the sum of all emissivities is equal to the
local density of states. The use of the partial density of states is
illustrated for a number of different electron transport problems in mesoscopic
physics: The transmission from a tunneling tip into a mesoscopic conductor, the
discussion of inelastic or phase breaking scattering with a voltage probe, and
the ac-conductance of mesoscopic conductors. The transition from a capacitive
response (positive time-delay) to an inductive response (negative time-delay)
for a quantum point contact is used to illustrate the difficulty in associating
time-scales with a linear response analysis. A brief discussion of the
off-diagonal elements of a partial density of states matrix is presented. The
off-diagonal elements permit to investigate carrier fluctuations away from the
average carrier density. The work concludes with a discussion of the relation
between the partial density of states matrix and the Wigner-Smith delay time
matrix.


title:
Toward the Quantum Design of Multichannel Systems (The Inverse Problem
  Approach)

abstract:
  The multichannel generalization of the theory of spectral, scattering and
decay control is presented. New universal algorithms of construction of complex
quantum systems with given properties are suggested. Particularly,
transformations of interaction matrices leading to the concentration of waves
in a chosen partial channel and spatial localization are shown. The limiting
instructive cases illustrating different phenomena which occur with the
combination of 'incompatible' properties are considered. For example, the
scattering solutions with different resonance widths at the same energy for the
same interaction are revealed. Analogously, a 'paradoxical' coexistence of both
strong reflection and absolute transparency is explained. The case of the
violation of 'natural' asymptotic behavior of partial wave function is
demonstrated : it has a greater damping decrement for the channel with a lower
threshold. Peculiarities of the multichannel periodic structures, bound states
embedded into continuum, resonance tunneling and degeneracy of states are
described.


title: What is wrong with SLASH?

abstract:
  In an experiment featuring nonlinear optics, delayed choice and EPR-type
correlations, the possibility of faster-than-light communication appears not
totally implausible. Attempts are put forward and discussed to refute this
claim.


title: Entanglement generation and degradation by passive optical devices

abstract:
  The influence of losses in the interferometric generation and the
transmission of continuous-variable entangled light is studied, with special
emphasis on Gaussian states. Based on the theory of quantum-state
transformation at absorbing dielectric devices, the amount of entanglement is
quantified by means of the relative-entropy measure. Upper bounds of
entanglement and the distance to the set of separable Gaussian states are
calculated. Compared with the distance measure, the bounds can substantially
overestimate the entanglement. In particular, they do not show the drastic
decrease of entanglement with increasing mean photon number, as does the
distance measure.


title:
Interferometric Bell-state preparation using femtosecond-pulse-pumped
  Spontaneous Parametric Down-Conversion

abstract:
  We present theoretical and experimental study of preparing maximally
entangled two-photon polarization states, or Bell states, using femtosecond
pulse pumped spontaneous parametric down-conversion (SPDC). First, we show how
the inherent distinguishability in femtosecond pulse pumped type-II SPDC can be
removed by using an interferometric technique without spectral and amplitude
post-selection. We then analyze the recently introduced Bell state preparation
scheme using type-I SPDC. Theoretically, both methods offer the same results,
however, type-I SPDC provides experimentally superior methods of preparing Bell
states in femtosecond pulse pumped SPDC. Such a pulsed source of highly
entangled photon pairs is useful in quantum communications, quantum
cryptography, quantum teleportation, etc.


title: Localization and diffusion in Ising-type quantum networks

abstract:
  We investigate the effect of phase randomness in Ising-type quantum networks.
These networks model a large class of physical systems. They describe micro-
and nanostructures or arrays of optical elements such as beam splitters
(interferometers) or parameteric amplifiers. Most of these stuctures are
promising candidates for quantum information processing networks. We
demonstrate that such systems exhibit two very distinct types of behaviour. For
certain network configurations (parameters), they show quantum localization
similar to Anderson localization whereas classical stochastic behaviour is
observed in other cases. We relate these findings to the standard theory of
quantum localization.


title:
Detecting the inseparability and distillability of continuous variable
  states in Fock space

abstract:
  The partial transposition(PT) operation is an effecient tool in detecting the
inseparability of a mixed state. We give an explicit formula for the PT
operation for the continuous variable states in Fock space. We then give the
necessary and sufficient condition for the positivity of Gaussian operators.
Based on this, a number of creterions on the inseparability and distillability
for the multimode Gaussian states are naturally drawn. We finally give an
explicit formula for the state in a subspace of a global Gaussian state. This
formula, together with the known results for Gaussian states, gives the
criterions for the inseparability and distillability in a subspace of the
global Gaussian state.


title:
Optimally shaped terahertz pulses for phase retrieval in a Rydberg atom
  data register

abstract:
  We employ Optimal Control Theory to discover an efficient information
retrieval algorithm that can be performed on a Rydberg atom data register using
a shaped terahertz pulse. The register is a Rydberg wave packet with one
consituent orbital phase-reversed from the others (the ``marked bit''). The
terahertz pulse that performs the decoding algorithm does so by by driving
electron probability density into the marked orbital. Its shape is calculated
by modifying the target of an optimal control problem so that it represents the
direct product of all correct solutions to the algorithm.


title: Teleportation cost and hybrid compression of quantum signals

abstract:
  The amount of entanglement necessary to teleport quantum states drawn from
general ensemble $\{p_i,\rho_i\}$ is derived. The case of perfect transmission
of individual states and that of asymptotically faithful transmission are
discussed. Using the latter result, we also derive the optimum compression rate
when the ensemble is compressed into qubits and bits.


title: Raman cooling and heating of two trapped Ba+ ions

abstract:
  We study cooling of the collective vibrational motion of two 138Ba+ ions
confined in an electrodynamic trap and irradiated with laser light close to the
resonances S_1/2-P_1/2 (493 nm) and P_1/2-D_3/2 (650 nm). The motional state of
the ions is monitored by a spatially resolving photo multiplier. Depending on
detuning and intensity of the cooling lasers, macroscopically different
motional states corresponding to different ion temperatures are observed. We
also derive the ions' temperature from detailed analytical calculations of
laser cooling taking into account the Zeeman structure of the energy levels
involved. The observed motional states perfectly match the calculated
temperatures. Significant heating is observed in the vicinity of the dark
resonances of the Zeeman-split S_1/2-D_3/2 Raman transitions. Here two-photon
processes dominate the interaction between lasers and ions. Parameter regimes
of laser light are identified that imply most efficient laser cooling.


title: Gravitational bouncing of a quantum ball: Room for Airy's function Bi

abstract:
  I apply (i) a classical version of the Ermakov-Lewis procedure and (ii) the
strictly isospectral supersymmetric approach to the Schroedinger free fall of
the bouncing ball type. In both cases, the Airy function Bi, which in general
is eliminated as being unphysical, plays a well-defined role. Relevant plots
are displayed


title:
Comment on ``Requirement of optical coherence for continuous--variable
  quantum teleportation'' by Terry Rudolph and Barry C. Sanders

abstract:
  The argument of Rudolph and Sanders, while technically correct, raises
conceptual problems. In particular, if carried to its logical conclusion, it
would disallow the use in our theories of any time $t$ with implied resolution
beyond that of direct human experience.


title:
Photon coincidence spectroscopy for two-atom cavity quantum
  electrodynamics

abstract:
  We show that photon coincidence spectroscopy can provide an unambiguous
signature of two atoms simultaneously interacting with a quantised cavity field
mode. We also show that the single-atom Jaynes-Cummings model can be probed
effectively via photon coincidence spectroscopy, even with deleterious
contributions to the signal from two-atom events. In addition, we have
explicitly solved the eigenvectors and eigenvalues of two two-level atoms
coupled to a quantised cavity mode for differing coupling strengths.


title: Quantum Market Games

abstract:
  We propose a quantum-like description of markets and economics. The approach
has roots in the recently developed quantum game theory.


title: Mixedness in Bell-violation vs. Entanglement of Formation

abstract:
  Recently Munro, Nemoto and White (The Bell Inequality: A measure of
Entanglement?, quant-ph/0102119) tried to indicate that the reason behind a
state rho having higher amount of entanglement (as quantified by the
entanglement of formation) than a state eta, but producing the same amount of
Bell-violation, is due to the fact that the amount of mixedness (as quantified
by the linearised entropy) in rho is higher than that in eta. We counter their
argument with examples. We extend these considerations to the von Neumann
entropy. Our results suggest that the reason as to why equal amount of
Bell-violation requires different amounts of entanglement cannot, at least, be
explained by mixedness alone.