title:
Berry's phase in Cavity QED: proposal for observing an effect of field
  quantization

abstract:
  Geometric phases are well known in classical electromagnetism and quantum
mechanics since the early works of Pantcharatnam and Berry. Their origin relies
on the geometric nature of state spaces and has been studied in many different
systems such as spins, polarized light and atomic physics. Recent works have
explored their application in interferometry and quantum computation. Earlier
works suggest how to observe these phases in single quantum systems
adiabatically driven by external classical devices or sources, where, by
classical, we mean any system whose state does not change considerably during
the interaction time: an intense magnetic field interacting with a spin 1/2, or
a birefringent medium interacting with polarized light. Here we propose a
feasible experiment to investigate quantum effects in these phases, arising
when this classical source drives not a single quantum system, but two
interacting ones. In particular, we show how to observe a signature of field
quantization through a vacuum effect in Berry's phase. To do so, we describe
the interaction of an atom and a quantized cavity mode altogether driven by an
external quasi-classical field.


title: Creating high dimensional time-bin entanglement using mode-locked lasers

abstract:
  We present a new scheme to generate high dimensional entanglement between two
photonic systems. The idea is based on parametric down conversion with a
sequence of pump pulses generated by a mode-locked laser. We prove
experimentally the feasibility of this scheme by performing a Franson-type Bell
test using a 2-way interferometer with path-length difference equal to the
distance between 2 pump pulses. With this experiment, we can demonstrate
entanglement for a two-photon state of at least dimension D=11. Finally, we
propose a feasible experiment to show a Fabry-Perot like effect for a high
dimensional two-photon state.


title:
Dark state suppression and narrow fluorescent feature in a laser-driven
  lambda atom

abstract:
  We discuss quantum interference effects in a three-level atom in
lambda-configuration, where both transitions from the upper state to the lower
states are driven by a single monochromatic laser field. Although the system
has two lower states, quantum interference is possible because there are
interfering pathways to each of the two lower states. The additional
interference terms allow for interesting effects such as the suppression of a
dark state which is present without the interference. Finally we examine a
narrow spectral feature in the resonance fluorescence of the atom with quantum
interference.


title: Separability, Preferred Bases, and the Nonconservation of Quantities

abstract: This paper has been withdrawn.


title: What could be more practical than a good interpretation?

abstract:
  Quantum mechanics is more than the derivation of straightforward theorems
about vector spaces, Hilbert spaces and functional analysis. In order to be
applicable to experiment and technology, those theorems need interpretation and
meaning. Interpretation is to the formalism what a scaffolding in architecture
and building construction is to the completed building.


title:
Comment on "Exclusion of time in the theorem of Bell" by K. Hess and W.
  Philipp

abstract:
  A recent Letter by Hess and Philipp claims that Bell's theorem neglects the
possibility of time-like dependence in local hidden variables, hence is not
conclusive. Moreover the authors claim that they have constructed, in an
earlier paper, a local realistic model of the EPR correlations. However, they
themselves have neglected the experimenter's freedom to choose settings, while
on the other hand, Bell's theorem can be formulated to cope with time-like
dependence. This in itself proves that their toy model cannot satisfy local
realism, but we also indicate where their proof of its local realistic nature
fails.


title:
Laser-driven atom moving in a multimode cavity: strong enhancement of
  cavity-cooling efficiency

abstract:
  Cavity-mediated cooling of the center--of--mass motion of a transversally,
coherently pumped atom along the axis of a high--Q cavity is studied. The
internal dynamics of the atomic dipole strongly coupled to the cavity field is
treated by a non-perturbative quantum mechanical model, while the effect of the
cavity on the external motion is described classically in terms of the
analytically obtained linear friction and diffusion coefficients. Efficient
cavity-induced damping is found which leads to steady-state temperatures
well-below the Doppler limit. We reveal a mathematical symmetry between the
results here and for a similar system where, instead of the atom, the cavity
field is pumped. The cooling process is strongly enhanced in a degenerate
multimode cavity. Both the temperature and the number of scattered photons
during the characteristic cooling time exhibits a significant reduction with
increasing number of modes involved in the dynamics. The residual number of
spontaneous emissions in a cooling time for large mode degeneracy can reach and
even drop below the limit of a single photon.


title: A quantum phase gate implementation for trapped ions in thermal motion

abstract:
  We propose a novel scheme to implement a quantum controlled phase gate for
trapped ions in thermal motion with one standing wave laser pulse. Instead of
applying the rotating wave approximation this scheme makes use of the
counter-rotating terms of operators. We also demonstrate that the same scheme
can be used to generate maximally entangled states of $N$ trapped ions by a
single laser pulse.


title: The capacity of the quantum depolarizing channel

abstract:
  The information carrying capacity of the d-dimensional depolarizing channel
is computed. It is shown that this capacity can be achieved by encoding
messages as products of pure states belonging to an orthonormal basis of the
state space, and using measurements which are products of projections onto this
same orthonormal basis. In other words, neither entangled signal states nor
entangled measurements give any advantage for information capacity. The result
follows from an additivity theorem for the capacity of the product of the
depolarizing channel with an arbitrary channel. We establish the
Amosov-Holevo-Werner p-norm conjecture for this product channel for all p >= 1,
and deduce from this the additivity of the minimal entropy and of the Holevo
quantity.


title:
Electromagnetically induced transparency and reduced speeds for single
  photons in a fully quantized model

abstract:
  We introduce a simple model for electromagnetically induced transparency in
which all fields are treated quantum mechanically. We study a system of three
separated atoms at fixed positions in a one-dimensional multimode optical
cavity. The first atom serves as the source for a single spontaneously emitted
photon; the photon scatters from a three-level Lambda-configuration atom which
interacts with a single-mode field coupling two of the atomic levels; the third
atom serves as a detector of the total transmitted field. We find an analytical
solution for the quantum dynamics. From the quantum amplitude describing the
excitation of the detector atom we extract information that provides exact
single-photon analogs to wave delays predicted by semi-classical theories. We
also find complementary information in the expectation value of the electric
field intensity operator.


title: Power series Schrodinger eigenfunctions for a particle on the torus

abstract:
  The eigenvalues and a series representation of the eigenfunctions of the
Schrodinger equation for a particle on the surface of a torus are derived.


title:
Nonperturbative renormalization-group approach for quantum dissipative
  systems

abstract:
  We analyze the dissipative quantum tunneling in the Caldeira-Leggett model by
the nonperturbative renormalization-group method. We classify the dissipation
effects by introducing the notion of effective cutoffs. We calculate the
localization susceptibility to evaluate the critical dissipation for the
quantum-classical transition. Our results are consistent with previous
semiclassical arguments, but give considerably larger critical dissipation.


title: PT-symmetric square well and the associated SUSY hierarchies

abstract:
  The PT-symmetric square well problem is considered in a SUSY framework. When
the coupling strength $Z$ lies below the critical value $Z_0^{\rm (crit)}$
where PT symmetry becomes spontaneously broken, we find a hierarchy of SUSY
partner potentials, depicting an unbroken SUSY situation and reducing to the
family of $\sec^2$-like potentials in the $Z \to 0$ limit. For $Z$ above
$Z_0^{\rm (crit)}$, there is a rich diversity of SUSY hierarchies, including
some with PT-symmetry breaking and some with partial PT-symmetry restoration.


title: Quantum orders in an exact soluble model

abstract:
  We find all the exact eigenstates and eigenvalues of a spin-1/2 model on
square lattice: $H=16g \sum_i S^y_i S^x_{i+x} S^y_{i+x+y} S^x_{i+y}$. We show
that the ground states for $g<0$ and $g>0$ have different quantum orders
described by Z2A and Z2B projective symmetry groups. The phase transition at
$g=0$ represents a new kind of phase transitions that changes quantum orders
but not symmetry. Both the Z2A and Z2B states are described by $Z_2$ lattice
gauge theories at low energies. They have robust topologically degenerate
ground states and gapless edge excitations.


title:
Three-dimensional theory for interaction between atomic ensembles and
  free-space light

abstract:
  Atomic ensembles have shown to be a promising candidate for implementations
of quantum information processing by many recently-discovered schemes. All
these schemes are based on the interaction between optical beams and atomic
ensembles. For description of these interactions, one assumed either a
cavity-QED model or a one-dimensional light propagation model, which is still
inadequate for a full prediction and understanding of most of the current
experimental efforts which are actually taken in the three-dimensional free
space. Here, we propose a perturbative theory to describe the three-dimensional
effects in interaction between atomic ensembles and free-space light with a
level configuration important for several applications. The calculations reveal
some significant effects which are not known before from the other approaches,
such as the inherent mode-mismatching noise and the optimal mode-matching
conditions. The three-dimensional theory confirms the collective enhancement of
the signal-to-noise ratio which is believed to be one of the main advantage of
the ensemble-based quantum information processing schemes, however, it also
shows that this enhancement need to be understood in a more subtle way with an
appropriate mode matching method.


title:
Exact Quantum-Statistical Dynamics of Time-Dependent Generalized
  Oscillators

abstract:
  Using linear invariant operators in a constructive way we find the most
general thermal density operator and Wigner function for time-dependent
generalized oscillators. The general Wigner function has five free parameters
and describes the thermal Wigner function about a classical trajectory in phase
space. The contour of the Wigner function depicts an elliptical orbit with a
constant area moving about the classical trajectory, whose eccentricity
determines the squeezing of the initial vacuum.


title: Complete positivity and entangled degrees of freedom

abstract:
  We study how some recently proposed noncontextuality tests based on quantum
interferometry are affected if the test particles propagate as open systems in
presence of a gaussian stochastic background. We show that physical consistency
requires the resulting markovian dissipative time-evolution to be completely
positive.


title: Simulation of Markovian quantum dynamics on quantum logic networks

abstract:
  We study how generators of Markovian dynamics of a qubit can be simulated
using a programmable quantum processor.


title: Remote State Preparation without Oblivious Conditions

abstract:
  In quantum teleportation, neither Alice nor Bob acquires any classical
knowledge on teleported states. The teleportation protocol is said to be
oblivious to both parties. In remote state preparation (RSP) it is assumed that
Alice is given complete classical knowledge on the state that is to be prepared
by Bob. Recently, Leung and Shor showed that the same amount of classical
information as that in teleportation needs to be transmitted in any exact and
deterministic RSP protocol that is oblivious to Bob. We study similar RSP
protocols, but not necessarily oblivious to Bob. First it is shown that Bob's
quantum operation can be safely assumed to be a unitary transformation. We then
derive an equation that is a necessary and sufficient condition for such a
protocol to exist. By studying this equation, we show that one qubit RSP
requires 2 cbits of classical communication, which is the same amount as in
teleportation, even if the protocol is not assumed oblivious to Bob. For higher
dimensions, it is still open whether the amount of classical communication can
be reduced by abandoning oblivious conditions.


title: On classical models of spin

abstract:
  The reason for recalling this old paper is the ongoing discussion on the
attempts of circumventing certain assumptions leading to the Bell theorem
(Hess-Philipp, Accardi). If I correctly understand the intentions of these
Authors, the idea is to make use of the following logical loophole inherent in
the proof of the Bell theorem: Probabilities of counterfactual events A and A'
do not have to coincide with actually measured probabilities if measurements of
A and A' disturb each other, or for any other fundamental reason cannot be
performed simulaneously. It is generally believed that in the context of
classical probability theory (i.e. realistic hidden variables) probabilities of
counterfactual events can be identified with those of actually measured events.
In the paper I give an explicit counterexample to this belief. The "first
variation" on the Aerts model shows that counterfactual and actual problems
formulated for the same classical system may be unrelated. In the model the
first probability does not violate any classical inequality whereas the second
does. Pecularity of the Bell inequality is that on the basis of an in principle
unobservable probability one derives probabilities of jointly measurable random
variables, the fact additionally obscuring the logical meaning of the
construction. The existence of the loophole does not change the fact that I was
not able to construct a local model violating the inequality with all the other
loopholes eliminated.


title:
Non-canonical quantum optics (II): Poincar\'e covariant formalism and
  thermodynamic limit

abstract:
  The paper contains further development of the idea of field quantization
introduced in M. Czachor, J. Phys. A: Math. Gen. {\bf 33} (2000) 8081-8103. The
formalism is extended to the relativistic domain. The link to the standard
theory is obtained via a thermodynamic limit. Unitary representations of the
Poincar\'e group at the level of fields and states are explicitly given.
Non-canonical multi-photon and coherent states are introduced. In the
thermodynamic limit the statistics of photons in a coherent state is
Poissonian. The $S$ matrix of radiation fields produced by a classical current
is given by a non-canonical coherent-state displacement operator, a fact
automatically eliminating the infrared catastrophe. Field operators are shown
to be operators and not operator-valued distributions, and can be multiplied at
the same point in configuration space. An exactly solvable example is used to
compare predictions of the standard theory with those of non-canonical quantum
optics, and explicitly shows the mechanism of automatic ultraviolet
regularization occuring in the non-canonical theory. Similar conclusions are
obtained in perturbation theory, where one finds the standard Feynman diagrams,
but the Feynman rules are modified. A comparison with the Dicke-Hepp-Lieb model
allows to identify the physical structure behind the non-canonical algebra as
corresponding to an ensemble of indefinite-frequency oscillators with constant
density $N/V$.


title:
Quantum Surveying: How Entangled Pairs Act as Measuring Rods on
  Manifolds of Generalized Coherent States

abstract:
  Generalized coherent states arise from reference states by the action of
locally compact transformation groups and thereby form manifolds on which there
is an invariant measure. It is shown that this implies the existence of
canonically associated Bell states that serve as measuring rods by relating the
metric geometry of the manifold to the observed EPR correlations. It is further
shown that these correlations can be accounted for by a hidden variable theory
which is non-local but invariant under the stability group of the reference
state.


title: Bell's Inequalities and the Accardi-Gustafson Inequality

abstract:
  Many issues combine for consideration when speaking of Bell's Inequalities:
nonlocality, realism, hidden variables, incompatible measures, wave function
collapse, other. Each of these issues then may be viewed from several
viewpoints: historical, theoretical, physical, experimental, statistical,
communicational, cryptographical, and mathematical. From the mathematical
viewpoint, much of the Bell theory is ``just geometry''.


title:
Teleportation via thermally entangled state of a two-qubit Heisenberg XX
  chain

abstract:
  We find that quantum teleportation, using the thermally entangled state of
two-qubit Heisenberg XX chain as a resource, with fidelity better than any
classical communication protocol is possible. However, a thermal state with a
greater amount of thermal entanglement does not necessarily yield better
fidelity. It depends on the amount of mixing between the separable state and
maximally entangled state in the spectra of the two-qubit Heisenberg XX model.


title:
Narrowband frequency tunable light source of continuous quadrature
  entanglement

abstract:
  We report the observation of non-classical quantum correlations of continuous
light variables from a novel type of source. It is a frequency non-degenerate
optical parametric oscillator below threshold, where signal and idler fields
are separated by 740MHz corresponding to two free spectrum ranges of the
parametric oscillator cavity. The degree of entanglement observed, - 3.8 dB, is
the highest to-date for a narrowband tunable source suitable for atomic quantum
memory and other applications in atomic physics. Finally we use the latter to
visualize the Einstein-Podolsky-Rosen paradox.


title: Quantum nonlocality, Bell inequalities and the memory loophole

abstract:
  In the analysis of experiments designed to reveal violation of Bell-type
inequalities, it is usually assumed that any hidden variables associated with
the nth particle pair would be independent of measurement choices and outcomes
for the first $(n-1)$ pairs. Models which violate this assumption exploit what
we call the {\it memory loophole}. We focus on the strongest type of violation,
which uses the {\it 2-sided} memory loophole, in which the hidden variables for
pair $n$ can depend on the previous measurement choices and outcomes in both
wings of the experiment. We show that the 2-sided memory loophole allows a
systematic violation of the CHSH inequality when the data are analysed in the
standard way, but cannot produce a violation if a CHSH expression depending
linearly on the data is used. In the first case, the maximal CHSH violation
becomes small as the number of particle pairs tested becomes large. Hence,
although in principle the memory loophole implies a slight flaw in existing
analyses of Bell experiments, the data still strongly confirm quantum mechanics
against local hidden variables.
  We consider also a related loophole, the {\it simultaneous measurement
loophole}, which applies if all measurements on each side are carried out
simultaneously. We show that this can increase the probability of violating the
linearised CHSH inequality as well as other Bell-type inequalities.


title: A matrix realignment method for recognizing entanglement

abstract:
  Motivated by the Kronecker product approximation technique, we have developed
a very simple method to assess the inseparability of bipartite quantum systems,
which is based on a realigned matrix constructed from the density matrix. For
any separable state, the sum of the singular values of the matrix should be
less than or equal to 1. This condition provides a very simple, computable
necessary criterion for separability, and shows powerful ability to identify
most bound entangled states discussed in the literature. As a byproduct of the
criterion, we give an estimate for the degree of entanglement of the quantum
state.


title: Wigner's Spins, Feynman's Partons, and Their Common Ground

abstract:
  The connection between spin and symmetry was established by Wigner in his
1939 paper on the Poincar\'e group. For a massive particle at rest, the little
group is O(3) from which the concept of spin emerges. The little group for a
massless particle is isomorphic to the two-dimensional Euclidean group with one
rotational and two translational degrees of freedom. The rotational degree
corresponds to the helicity, and the translational degrees to the gauge degree
of freedom. The question then is whether these two different symmetries can be
united. Another hard-pressing problem is Feynman's parton picture which is
valid only for hadrons moving with speed close to that of light. While the
hadron at rest is believed to be a bound state of quarks, the question arises
whether the parton picture is a Lorentz-boosted bound state of quarks. We study
these problems within Einstein's framework in which the energy-momentum
relations for slow particles and fast particles are two different
manifestations one covariant entity.


title: Creation of Entanglement by Interaction with a Common Heat Bath

abstract:
  I show that entanglement between two qubits can be generated if the two
qubits interact with a common heat bath in thermal equilibrium, but do not
interact directly with each other. In most situations the entanglement is
created for a very short time after the interaction with the heat bath is
switched on, but depending on system, coupling, and heat bath, the entanglement
may persist for arbitrarily long times. This mechanism sheds new light on the
creation of entanglement. A particular example of two quantum dots in a closed
cavity is discussed, where the heat bath is given by the blackbody radiation.


title: Study of Optimization Problems by Quantum Annealing

abstract:
  We introduce quantum fluctuations into the simulated annealing process of
optimization problems, aiming at faster convergence to the optimal state. The
idea is tested by the two models, the transverse Ising model and the traveling
salesman problem (TSP). Adding the transverse field to the Ising model is a
simple way to introduce quantum fluctuations. The strength of the transverse
field is controlled as a function of time similarly to the temperature in the
conventional method. TSP can be described by the Ising spin, so that we also
apply the same technique to TSP. We directory solve the time-dependent
Schr\"odinger equation for small-size systems and perform the quantum Monte
Carlo simulation for large-size systems. Comparison with the results of the
corresponding classical (thermal) method reveals that the quantum method leads
to the ground state with much larger probability in almost all cases if we use
the same annealing schedule of the control parameters. We also find that the
relaxation time is quite short for quantum systems by numerical simulations. We
consider this is one of the reasons why the annealing in quantum systems have a
better performance of finding the optimal state in comparison with classical
systems.


title: Entanglement entropy of multipartite pure states

abstract:
  Consider a system consisting of $n$ $d$-dimensional quantum particles and
arbitrary pure state $\Psi$ of the whole system. Suppose we simultaneously
perform complete von Neumann measurements on each particle. One can ask: what
is the minimal possible value $S[\Psi]$ of the entropy of outcomes joint
probability distribution? We show that $S[\Psi]$ coincides with entanglement
entropy for bipartite states. We compute $S[\Psi]$ for two sample multipartite
states: the hexacode state ($n=6, d=2$) and determinant states ($n=d$). The
generalization of determinant states to the case $d<n$ is considered.


title:
Comment on Hess-Philipp anti-Bell and Gill-Weihs-Zeilinger-Zukowski
  anti-Hess-Philipp arguments

abstract:
  We present comparative analysis of Gill-Weihs-Zeilinger-Zukowski arguments
directed against Hess-Philipp anti-Bell arguments. In general we support
Hess-Philipp viewpoint to the sequence of measurements in the EPR-Bohm
experiments as stochastic time-like process. On the other hand, we support
Gill-Weihs-Zeilinger-Zukowski arguments against the use of time-like
correlations as the factor blocking the derivation of Bell-type inequalities.
We presented our own time-analysis of measurements in the EPR-Bohm experiments
based on the frequency approach to probability. Our analysis gives strong
arguments in favour of local realism. Moreover, our frequency analysis supports
the original EPR-idea that quantum mechnaics is not complete.


title: Statistics of dressed modes in a thermal state

abstract:
  By a Wigner-function calculation, we evaluate the trace of a certain Gaussian
operator arising in the theory of a boson system subject to both finite
temperature and (weak) interaction. Thereby we rederive (and generalize) a
recent result by Kocharovsky, Kocharovsky, and Scully [Phys. Rev. A, vol. 61,
art. 053606 (2000)] in a way that is technically much simpler. One step uses a
special case of the response of Wigner functions to linear transformations, and
we demonstrate the general case by simple means. As an application we extract
the counting statistics for each mode of the Bose gas.


title:
Quantum Measurement Problem, Decoherence, and Quantum Systems
  Selfdescription

abstract:
  Quantum Measurements regarded in Systems Selfdescription framework for
measuring system (MS) consist of measured state S environment E and observer
$O$ processing input S signal.
  $O$ regarded as quantum object which interaction with S,E obeys to
Schrodinger equation (SE) and from it and Breuer selfdescription formalism S
information for $O$ reconstructed. In particular S state collapse obtained if
$O$ selfdescription state has the dual structure $L_T=\cal H \bigotimes L_V$
where $\cal H$ is Hilbert space of MS states $\Psi_{MS}$. $\cal L_V$ is the set
with elements $V^O=|O_j> < O_j|$ describing random 'pointer' outcomes $O_j$
observed by $O$ in the individual events. The 'preferred' basis $|O_j>$ defined
by $O$ state decoherence via $O$ - E interactions. Zurek's Existential
Interpretation discussed in selfmeasurement framework.


title: Entanglement Properties of the Harmonic Chain

abstract:
  We study the entanglement properties of a closed chain of harmonic
oscillators that are coupled via a translationally invariant Hamiltonian, where
the coupling acts only on the position operators. We consider the ground state
and thermal states of this system, which are Gaussian states. The entanglement
properties of these states can be completely characterized analytically when
one uses the logarithmic negativity as a measure of entanglement.


title: Optimal scheme for estimating a pure qubit state via local measurements

abstract:
  We present the optimal scheme for estimating a pure qubit state by means of
local measurements on N identical copies. We give explicit examples for low N.
For large N, we show that the fidelity saturates the collective measurement
bound up to order 1/N. When the signal state lays on a meridian of the Bloch
sphere, we show that this can be achieved without classical communication.


title:
Relational Description of the Measurement Process in Quantum Field
  Theory

abstract:
  We have recently introduced a realistic, covariant, interpretation for the
reduction process in relativistic quantum mechanics. The basic problem for a
covariant description is the dependence of the states on the frame within which
collapse takes place. A suitable use of the causal structure of the devices
involved in the measurement process allowed us to introduce a covariant notion
for the collapse of quantum states. However, a fully consistent description in
the relativistic domain requires the extension of the interpretation to quantum
fields. The extension is far from straightforward. Besides the obvious
difficulty of dealing with the infinite degrees of freedom of the field theory,
one has to analyze the restrictions imposed by causality concerning the allowed
operations in a measurement process. In this paper we address these issues. We
shall show that, in the case of partial causally connected measurements, our
description allows us to include a wider class of causal operations than the
one resulting from the standard way for computing conditional probabilities.
This alternative description could be experimentally tested. A verification of
this proposal would give a stronger support to the realistic interpretations of
the states in quantum mechanics.


title:
The informational nature of quantum mechanics: A novel look at the
  interference experiment

abstract:
  It is argued that the nature of probability is essentially informational
rather than physical and that quantum mechanical predictions should be viewed
as logical inferences made on the basis of the information content of a given
experimental situation. By implementing such a viewpoint, it is possible to
maintain a sharp distinction between the physical and statistical aspects of
quantum mechanics. The idea is applied to the double-beam interference
experiment, reproducing the results of the standard formulation of quantum
mechanics in a manner that renders the notion of wave-particle duality
superfluous.


title:
Nonclassical statistics of intracavity coupled $\chi^{(2)}$ waveguides:
  the quantum optical dimer

abstract:
  A model is proposed where two $\chi^{(2)}$ nonlinear waveguides are contained
in a cavity suited for second-harmonic generation. The evanescent wave coupling
between the waveguides is considered as weak, and the interplay between this
coupling and the nonlinear interaction within the waveguides gives rise to
quantum violations of the classical limit. These violations are particularly
strong when two instabilities are competing, where twin-beam behavior is found
as almost complete noise suppression in the difference of the fundamental
intensities. Moreover, close to bistable transitions perfect twin-beam
correlations are seen in the sum of the fundamental intensities, and also the
self-pulsing instability as well as the transition from symmetric to asymmetric
states display nonclassical twin-beam correlations of both fundamental and
second-harmonic intensities. The results are based on the full quantum Langevin
equations derived from the Hamiltonian and including cavity damping effects.
The intensity correlations of the output fields are calculated
semi-analytically using a linearized version of the Langevin equations derived
through the positive-P representation. Confirmation of the analytical results
are obtained by numerical simulations of the nonlinear Langevin equations
derived using the truncated Wigner representation.


title:
Generation of maximum spin entanglement induced by cavity field in
  quantum-dot systems

abstract:
  Equivalent-neighbor interactions of the conduction-band electron spins of
quantum dots in the model of Imamoglu et al. [Phys. Rev. Lett. 83, 4204 (1999)]
are analyzed. Analytical solution and its Schmidt decomposition are found and
applied to evaluate how much the initially excited dots can be entangled to the
remaining dots if all of them are initially disentangled. It is demonstrated
that the perfect maximally entangled states (MES) can only be generated in the
systems of up to 6 dots with a single dot initially excited. It is also shown
that highly entangled states, approximating the MES with a good accuracy, can
still be generated in systems of odd number of dots with almost half of them
being excited. A sudden decrease of entanglement is observed by increasing the
total number of dots in a system with a fixed number of excitations.


title: Quantum information in base n defined by state partitions

abstract:
  We define a "nit" as a radix n measure of quantum information which is based
on state partitions associated with the outcomes of n-ary observables and
which, for n>2, is fundamentally irreducible to a binary coding. Properties of
this measure for entangled many-particle states are discussed. k particles
specify k nits in such a way that k mutually commuting measurements of
observables with n possible outcomes are sufficient to determine the
information.


title:
A Loophole in Bell's Theorem? Parameter Dependence in the Hess-Philipp
  Model

abstract:
  The hidden-variables model constructed by Karl Hess and Walter Philipp is
claimed by its authors to exploit a "loophole" in Bell's theorem; the claim is
made by Hess and Philipp that the parameters employed in their model extend
beyond those considered by Bell. Furthermore, they claim that their model
satisfies Einstein locality and is free of any "suspicion of spooky action at a
distance." Both of these claims are false; the Hess-Philipp model achieves
agreement with the quantum-mechanical predictions, not by circumventing Bell's
theorem, but via Parameter Dependence.


title: Compatibility of quantum states

abstract:
  We introduce a measure of the compatibility between quantum states--the
likelihood that two density matrices describe the same object. Our measure is
motivated by two elementary requirements, which lead to a natural definition.
We list some properties of this measure, and discuss its relation to the
problem of combining two observers' states of knowledge.


title:
Solving Shift Problems and Hidden Coset Problem Using the Fourier
  Transform

abstract:
  We give a quantum algorithm for solving a shifted multiplicative character
problem over Z/nZ and finite fields. We show that the algorithm can be
interpreted as a matrix factorization or as solving a deconvolution problem and
give sufficient conditions for a shift problem to be solved efficiently by our
algorithm. We also show that combining the shift problem with the hidden
subgroup problem results in a hidden coset problem. This naturally captures the
redundancy in the shift due to the periodic structure of multiplicative
characters over Z/nZ.


title:
A simple formula for the average gate fidelity of a quantum dynamical
  operation

abstract:
  This note presents a simple formula for the average fidelity between a
unitary quantum gate and a general quantum operation on a qudit, generalizing
the formula for qubits found by Bowdrey et al [Phys. Lett. A 294, 258 (2002)].
This formula may be useful for experimental determination of average gate
fidelity. We also give a simplified proof of a formula due to Horodecki et al
[Phys. Rev. A 60, 1888 (1999)], connecting average gate fidelity to
entanglement fidelity.


title:
Macroscopic quantum superpositions in highly-excited
  strongly-interacting many-body systems

abstract:
  We demonstrate a break-down in the macroscopic (classical-like) dynamics of
wave-packets in complex microscopic and mesoscopic collisions. This break-down
manifests itself in coherent superpositions of the rotating clockwise and
anticlockwise wave-packets in the regime of strongly overlapping many-body
resonances of the highly-excited intermediate complex. These superpositions
involve $\sim 10^4$ many-body configurations so that their internal interactive
complexity dramatically exceeds all of those previously discussed and
experimentally realized. The interference fringes persist over a time-interval
much longer than the energy relaxation-redistribution time due to the
anomalously slow phase randomization (dephasing). Experimental verification of
the effect is proposed.


title: Initial Decoherence of Open Quantum Systems

abstract:
  We present a new short-time approximation scheme for evaluation of
decoherence. At low temperatures, the approximation is argued to apply at
intermediate times as well. It then provides a tractable approach complementary
to Markovian-type approximations, and is appropriate for evaluation of
deviations from pure states in quantum computing models.


title: Easy Control over Fermionic Computations

abstract:
  Quantum fermionic computations on occupation numbers proposed in
quant-ph/0003137 are studied. It is shown that a control over external field
and tunneling would suffice to fulfill all quantum computations without
valuable slowdown in the framework of such model when an interaction of
diagonal type is fixed and permanent. Substantiation is given through a
reduction of some subset of this model to the conventional language of quantum
computing and application of the construction from quant-ph/0202030.


title: Quantum Mechanics as Quantum Information (and only a little more)

abstract:
  In this paper, I try once again to cause some good-natured trouble. The issue
remains, when will we ever stop burdening the taxpayer with conferences devoted
to the quantum foundations? The suspicion is expressed that no end will be in
sight until a means is found to reduce quantum theory to two or three
statements of crisp physical (rather than abstract, axiomatic) significance. In
this regard, no tool appears better calibrated for a direct assault than
quantum information theory. Far from a strained application of the latest fad
to a time-honored problem, this method holds promise precisely because a large
part--but not all--of the structure of quantum theory has always concerned
information. It is just that the physics community needs reminding.
  This paper, though taking quant-ph/0106166 as its core, corrects one mistake
and offers several observations beyond the previous version. In particular, I
identify one element of quantum mechanics that I would not label a subjective
term in the theory--it is the integer parameter D traditionally ascribed to a
quantum system via its Hilbert-space dimension.


title: Spin Processor

abstract:
  It is experimentally demonstrated that with multi-frequency excitation of
weak amplitude the dynamics of a cluster of dipolar-coupled spins can be
manipulated to perform parallel logic operations with long bit arrays.


title: Wigner functions for curved spaces I: On hyperboloids

abstract:
  We propose a Wigner quasiprobability distribution function for Hamiltonian
systems in spaces of constant curvature --in this paper on hyperboloids--,
which returns the correct marginals and has the covariance of the Shapiro
functions under SO(D,1) transformations. To the free systems obeying the
Laplace-Beltrami equation on the hyperboloid, we add a conic-oscillator
potential in the hyperbolic coordinate. As an example, we analyze the
1-dimensional case on a hyperbola branch, where this conic-oscillator is the
Poschl-Teller potential. We present the analytical solutions and plot the
computed results. The standard theory of quantum oscillators is regained in the
contraction limit to the space of zero curvature.


title: Dressed States Approach to Quantum Systems

abstract:
  Using the non-perturbative method of {\it dressed} states previously
introduced in JPhysA, we study effects of the environment on a quantum
mechanical system, in the case the environment is modeled by an ensemble of non
interacting harmonic oscillators. This method allows to separate the whole
system into the {\it dressed} mechanical system and the {\it dressed}
environment, in terms of which an exact, non-perturbative approach is possible.
When applied to the Brownian motion, we give explicit non-perturbative formulas
for the classical path of the particle in the weak and strong coupling regimes.
When applied to study atomic behaviours in cavities, the method accounts very
precisely for experimentally observed inhibition of atomic decay in small
cavities PhysLA, physics0111042.


title:
Experimental demonstration of a squeezing enhanced power recycled
  Michelson interferometer for gravitational wave detection

abstract:
  Interferometric gravitational wave detectors are expected to be limited by
shot noise at some frequencies. We experimentally demonstrate that a power
recycled Michelson with squeezed light injected into the dark port can overcome
this limit. An improvement in the signal-to-noise ratio of 2.3dB is measured
and locked stably for long periods of time. The configuration, control and
signal readout of our experiment are compatible with current gravitational wave
detector designs. We consider the application of our system to long baseline
interferometer designs such as LIGO.


title: Non-deterministic Gates for Photonic Single Rail Quantum Logic

abstract:
  We discuss techniques for producing, manipulating and measureing qubits
encoded optically as vacuum and single photon states. We show that a universal
set of non-deterministic gates can be constructed using linear optics and
photon counting. We investigate the efficacy of a test gate given realistic
detector efficiencies.


title: Analysis of Absorbing Times of Quantum Walks

abstract:
  Quantum walks are expected to provide useful algorithmic tools for quantum
computation. This paper introduces absorbing probability and time of quantum
walks and gives both numerical simulation results and theoretical analyses on
Hadamard walks on the line and symmetric walks on the hypercube from the
viewpoint of absorbing probability and time.


title: Multiple algebraisations of an elliptic Calogero-Sutherland model

abstract:
  Recently, Gomez-Ullate et al. (1) have studied a particular N-particle
quantum problem with an elliptic function potential supplemented by an external
field. They have shown that the Hamiltonian operator preserves a finite
dimensional space of functions and as such is quasi exactly solvable (QES). In
this paper we show that other types of invariant function spaces exist, which
are in close relation to the algebraic properties of the elliptic functions.
Accordingly, series of new algebraic eigenfunctions can be constructed.


title:
Derivation of the Schroedinger Equation and the Klein-Gordon Equation
  from First Principles

abstract:
  The Schroedinger- and Klein-Gordon equations are directly derived from
classical Lagrangians. The only inputs are given by the discreteness of energy
(E=hbar.w) and momentum (p=hbar.k), respectively, as well as the assumed
existence of a space-pervading field of "zero-point energy" (E_0=hbar.w/2 per
spatial dimension) associated to each particle of energy E. The latter leads to
an additional kinetic energy term in the classical Lagrangian, which alone
suffices to pass from classical to quantum mechanics. Moreover, Heisenberg's
uncertainty relations are also derived within this framework, i.e., without
referring to quantum mechanical or other complex-numbered functions.


title: Speedup in Quantum Adiabatic Evolution Algorithm

abstract:
  Quantum adiabatic evolution algorithm suggested by Farhi et al. was effective
in solving instances of NP-complete problems. The algorithm is governed by the
adiabatic theorem. Therefore, in order to reduce the running time, it is
essential to examine the minimum energy gap between the ground level and the
next one through the evolution. In this letter, we show a way of speedup in
quantum adiabatic evolution algorithm, using the extended Hamiltonian. We
present the exact relation between the energy gap and the elements of the
extended Hamiltonian, which provides the new point of view to reduce the
running time.


title:
Threshold temperature for pairwise and many-particle thermal
  entanglement in the isotropic Heisenberg model

abstract:
  We study the threshold temperature for pairwise thermal entanglement in the
spin-1/2 isotropic Heisenberg model up to 11 spins and find that the threshold
temperature for odd and even number of qubits approaches the thermal dynamical
limit from below and above, respectively. The threshold temperature in the
thermodynamical limit is estimated. We investigate the many-particle
entanglement in both ground states and thermal states of the system, and find
that the thermal state in the four-qubit model is four-particle entangled
before a threshold temperature.


title: Implementation of quantum maps by programmable quantum processors

abstract:
  A quantum processor is a device with a data register and a program register.
The input to the program register determines the operation, which is a
completely positive linear map, that will be performed on the state in the data
register. We develop a mathematical description for these devices, and apply it
to several different examples of processors. The problem of finding a processor
that will be able to implement a given set of mappings is also examined, and it
is shown that while it is possible to design a finite processor to realize the
phase-damping channel, it is not possible to do so for the amplitude-damping
channel.


title:
Structure and Parametrization of Generic Stochastic Maps of Density
  Matrices

abstract:
  The most general evolution of the density matrix of a quantum system with a
finite-dimensional state space is by stochastic maps which take a density
matrix linearly into the set of density matrices. These dynamical stochastic
maps form a linear convex set that may be viewed as supermatrices. The property
of hermiticity of density matrices renders an associated supermatrix hermitian
and hence diagonalizable. The positivity of the density matrix does not make
the associated supermatrix positive though. If the map itself is positive, it
is called completely positive and they have a simple parameterization. This is
extended to all positive (not completely positive) maps. A general dynamical
map that does not preserve the norm of the density matrices it acts on can be
thought of as the contraction of a norm-preserving map of an extended system.
The reconstruction of such extended dynamics is also given.


title: Alternative Structures and Bihamiltonian Systems

abstract:
  In the study of bi-Hamiltonian systems (both classical and quantum) one
starts with a given dynamics and looks for all alternative Hamiltonian
descriptions it admits.In this paper we start with two compatible Hermitian
structures (the quantum analog of two compatible classical Poisson brackets)
and look for all the dynamical systems which turn out to be bi-Hamiltonian with
respect to them.


title: A Realistic Deterministic Quantum Theory Using Borelian-Normal Numbers

abstract:
  The elements of a deterministic quantum theory are developed, which
reformulates and extends standard quantum theory. The proposed theory is
`realistic' in the sense that in it, a general M-level quantum state is
represented by a single real number r. Surprising as it may seem, this real
number is shown to contain the same probabilistic information as the standard
Hilbert-space state, plus additional information from which measurement outcome
is determined. A crucial concept in achieving this is that of Borelian
(number-theoretic) normality. The essential role of complex numbers in standard
quantum theory is subsumed by the action of a set of self-similar permutation
operators on the digits and places of the base-M expansion of a base-M
Borelian-normal r; these permutation operators are shown to have complex
structure and leave invariant the normality of the underlying real number. The
set of real numbers generated by these permutations defines not only the
Hilbert space of standard quantum theory, but also, in addition, the sample
space from which quantum measurement outcomes can be objectively determined.
Dynamical real-number state reduction is precisely described by deterministic
number-theoretic operators that reduce the degree of normality of r; from the
degree of normality one can infer the standard quantum-theoretic trace rule for
measurement probability. All the foundational difficulties of standard quantum
theory are described in terms of the proposed theory. It is shown that the
real-number states of the proposed theory are precisely its beables.


title:
A note on "A Matrix Realignment Method for Recognizing Entanglement,"
  quant-ph/0205017 v1

abstract:
  In quant-ph/0205017 v1 Chen, Wu and Yang formulated a necessary separability
criterion based on a realignment method for matrices. This note is to point out
that this criterion is identical to the necessary cross norm criterion
previously put forward by this author.


title: Optimal Entanglement Generation from Quantum Operations

abstract:
  We consider how much entanglement can be produced by a non-local two-qubit
unitary operation, $U_{AB}$ - the entangling capacity of $U_{AB}$. For a single
application of $U_{AB}$, with no ancillas, we find the entangling capacity and
show that it generally helps to act with $U_{AB}$ on an entangled state.
Allowing ancillas, we present numerical results from which we can conclude,
quite generally, that allowing initial entanglement typically increases the
optimal capacity in this case as well. Next, we show that allowing collective
processing does not increase the entangling capacity if initial entanglement is
allowed.


title:
Resonant dipole-dipole interaction in the presence of dispersing and
  absorbing surroundings

abstract:
  Within the framework of quantization of the macroscopic electromagnetic
field, equations of motion and an effective Hamiltonian for treating both the
resonant dipole-dipole interaction between two-level atoms and the resonant
atom-field interaction are derived, which can suitably be used for studying the
influence of arbitrary dispersing and absorbing material surroundings on these
interactions. The theory is applied to the study of the transient behavior of
two atoms that initially share a single excitation, with special emphasis on
the role of the two competing processes of virtual and real photon exchange in
the energy transfer between the atoms. In particular, it is shown that for weak
atom-field interaction there is a time window, where the energy transfer
follows a rate regime of the type obtained by ordinary second-order
perturbation theory. Finally, the resonant dipole-dipole interaction is shown
to give rise to a doublet spectrum of the emitted light for weak atom-field
interaction and a triplet spectrum for strong atom-field interaction.


title: On the capacities of bipartite Hamiltonians and unitary gates

abstract:
  We consider interactions as bidirectional channels. We investigate the
capacities for interaction Hamiltonians and nonlocal unitary gates to generate
entanglement and transmit classical information. We give analytic expressions
for the entanglement generating capacity and entanglement-assisted one-way
classical communication capacity of interactions, and show that these
quantities are additive, so that the asymptotic capacities equal the
corresponding 1-shot capacities. We give general bounds on other capacities,
discuss some examples, and conclude with some open questions.


title: Reflections on the Spatiotemporal Aspects of the Quantum World

abstract:
  The proper resolution of the so-called measurement problem requires a
"top-down" conception of the quantum world that is opposed to the usual
"bottom-up" conception, which builds on an intrinsically and maximally
differentiated manifold. The key to that problem is that the fuzziness of a
variable can manifest itself only to the extent that less fuzzy variables
exist. Inasmuch as there is nothing less fuzzy than the metric, this argues
against a quantum-gravity phenomenology and suggests that a quantum theory of
gravity is something of a contradiction in terms - a theory that would make it
possible to investigate the physics on scales that do not exist, or to study
the physical consequences of a fuzziness that has no physical consequences,
other than providing a natural cutoff for the quantum field theories of
particle physics.


title: Quantum Computing and Dynamical Quantum Models

abstract:
  A dynamical quantum model assigns an eigenstate to a specified observable
even when no measurement is made, and gives a stochastic evolution rule for
that eigenstate. Such a model yields a distribution over classical histories of
a quantum state. We study what can be computed by sampling from that
distribution, i.e., by examining an observer's entire history. We show that,
relative to an oracle, one can solve problems in polynomial time that are
intractable even for quantum computers; and can search an N-element list in
order N^{1/3} steps (though not fewer).


title:
Practical Scheme To Share A Secret Key Through An Up To 27.6% Bit Error
  Rate Quantum Channel

abstract:
  A secret key shared through quantum key distribution between two cooperative
players is secure against any eavesdropping attack allowed by the laws of
physics. Yet, such a key can be established only when the quantum channel error
rate due to eavesdropping or imperfect apparatus is low. Here, I report a
practical quantum key distribution scheme making use of an adaptive privacy
amplification procedure with two-way classical communication. Then, I prove
that the scheme generates a secret key whenever the bit error rate of the
quantum channel is less than $0.5-0.1\sqrt{5} \approx 27.6%$, thereby making it
the most error resistant scheme known to date.


title: First quantized electron and photon model of QED and radiative processes

abstract:
  In this study we combine the classical models of the massive and massless
spinning particles, derive the current-current interaction Lagrangian of the
particles from the gauge transformations of the classical spinors, and discuss
radiative processes in electrodynamics by using the solutions of the Dirac
equation and the quantum wave equations of the photon. The longitudinal
polarized photon states give a new idea about the vacuum concept in
electrodynamics.


title: Quantum Arnol'd Diffusion in a Simple Nonlinear System

abstract:
  We study the fingerprint of the Arnol'd diffusion in a quantum system of two
coupled nonlinear oscillators with a two-frequency external force. In the
classical description, this peculiar diffusion is due to the onset of a weak
chaos in a narrow stochastic layer near the separatrix of the coupling
resonance. We have found that global dependence of the quantum diffusion
coefficient on model parameters mimics, to some extent, the classical data.
However, the quantum diffusion happens to be slower that the classical one.
Another result is the dynamical localization that leads to a saturation of the
diffusion after some characteristic time. We show that this effect has the same
nature as for the studied earlier dynamical localization in the presence of
global chaos. The quantum Arnol'd diffusion represents a new type of quantum
dynamics and can be observed, for example, in 2D semiconductor structures
(quantum billiards) perturbed by time-periodic external fields.

categories: quant-ph astro-ph cond-mat.stat-mech nlin.CD physics.atom-ph


title: Generic Incomparability of Infinite-Dimensional Entangled States

abstract:
  In support of a recent conjecture by Nielsen (1999), we prove that the
phenomena of 'incomparable entanglement'--whereby, neither member of a pair of
pure entangled states can be transformed into the other via local operations
and classical communication (LOCC)--is a generic feature when the states at
issue live in an infinite-dimensional Hilbert space.


title: Inequalities for Quantum Entropy: A Review with Conditions for Equality

abstract:
  This paper presents self-contained proofs of the strong subadditivity
inequality for quantum entropy and some related inequalities for the quantum
relative entropy, most notably its convexity and its monotonicity under
stochastic maps. Moreover, the approach presented here, which is based on
Klein's inequality and one of Lieb's less well-known concave trace functions,
allows one to obtain conditions for equality. Using the fact that the Holevo
bound on the accessible information in a quantum ensemble can be obtained as a
consequence of the monotonicity of relative entropy, we show that equality can
be attained for that bound only when the states in the ensemble commute. The
paper concludes with an Appendix giving a short description of Epstein's
elegant proof of the relevant concavity theorem of Lieb.


title: Symmetry of distribution for the one-dimensional Hadamard walk

abstract:
  In this paper we study a one-dimensional quantum random walk with the
Hadamard transformation which is often called the Hadamard walk. We construct
the Hadamard walk using a transition matrix on probability amplitude and give
some results on symmetry of probability distributions for the Hadamard walk.


title: Realization of a space reversal operator

abstract:
  In this paper we propose the realization of a bosonic-fermionic interaction
in the context of trapped ions whose effect upon the ion center of mass degrees
of freedom is properly speaking a spatial inversion. The physical system and
its features are accurately described and some applications are briefly
discussed.


title:
Comment on "Feynman Effective Classical Potential in the Schrodinger
  Formulation"

abstract:
  We comment on the paper "Feynman Effective Classical Potential in the
Schrodinger Formulation"[Phys. Rev. Lett. 81, 3303 (1998)]. We show that the
results in this paper about the time evolution of a wave packet in a double
well potential can be properly explained by resorting to a variational
principle for the effective action. A way to improve on these results is also
discussed.


title: Multipartite entanglement

abstract:
  First, we show how the quantum circuits for generating and measuring
multi-party entanglement of qubits can be translated to continuous quantum
variables. We derive sufficient inseparability criteria for $N$-party
continuous-variable states and discuss their applicability. Then, we consider a
family of multipartite entangled states (multi-party multi-mode states with one
mode per party) described by continuous quantum variables and analyze their
properties. These states can be efficiently generated using squeezed light and
linear optics.


title: Quantum Entanglement of Identical Particles

abstract:
  We consider entanglement in a system of fixed number of identical particles.
Since any operation should be symmetrized over all the identical particles and
there is the precondition that the spatial wave functions overlap, the meaning
of identical-particle entanglement is fundamentally different from that of
distinguishable particles. The identical-particle counterpart of the Schmidt
basis is shown to be the single-particle basis in which the one-particle
reduced density matrix is diagonal. But it does not play a special role in the
issue of entanglement, which depends on the single-particle basis chosen. The
nonfactorization due to (anti)symmetrization is naturally excluded by using the
(anti)symmetrized basis or, equivalently, the particle number representation.
The natural degrees of freedom in quantifying the identical-particle
entanglement in a chosen single-particle basis are occupation numbers of
different single particle basis states. The entanglement between effectively
distinguishable spins is shown to be a special case of the occupation-number
entanglement.


title: Dissipation-assisted quantum gates with cold trapped ions

abstract:
  It is shown that a two-qubit phase gate and SWAP operation between ground
states of cold trapped ions can be realised in one step by simultaneously
applying two laser fields. Cooling during gate operations is possible without
perturbing the computation and the scheme does not require a second ion species
for sympathetic cooling. On the contrary, the cooling lasers even stabilise the
desired time evolution of the system. This affords gate operation times of
nearly the same order of magnitude as the inverse coupling constant of the ions
to a common vibrational mode.


title: Quantification of entanglement by means of convergent iterations

abstract:
  A convergent iterative procedure is proposed for the calculation of the
relative entropy of entanglement of a given bipartite quantum state. When this
state turns out to be non-separable the algorithm provides the corresponding
optimal entanglement witness measurement.


title: Manipulating Light Pulses via Dynamically Controlled Photonic Bandgap

abstract:
  When a resonance associated with electromagnetically induced transparency
(EIT) in an atomic ensemble is modulated by an off-resonant standing light
wave, a band of frequencies can appear for which light propagation is
forbidden. We show that dynamic control of such a bandgap can be used to
coherently convert a propagating light pulse into a stationary excitation with
non-vanishing photonic component. This can be accomplished with high efficiency
and negligble noise even at a level of few-photon quantum fields thereby
facilitating possible applications in quantum nonlinear optics and quantum
information.


title: Entanglement at distance: qubits versus continuous variables

abstract:
  We consider the problem of obtaining maximally entangled photon states at
distance in the presence of loss. We compare the efficiency of two different
schemes in establishing $N$ shared ebits: i) $N$ single ebit states with the
qubit encoded on polarization; ii) a single continuous variable entangled state
(emode) assisted by optimal local operation and classical communication (LOCC)
protocol in order to obtain a $2^N$-dimensional maximally entangled state, with
qubits encoded on the photon number.


title: ALEPH-QP: Universal hybrid quantum processors

abstract:
  A quantum processor (the programmable gate array) is a quantum network with a
fixed structure. A space of states is represented as tensor product of data and
program registers. Different unitary operations with the data register
correspond to "loaded" programs without any changing or "tuning" of network
itself. Due to such property and undesirability of entanglement between program
and data registers, universality of quantum processors is subject of rather
strong restrictions. By different authors was developed universal "stochastic"
quantum gate arrays. It was proved also, that "deterministic" quantum
processors with finite-dimensional space of states may be universal only in
approximate sense. In present paper is shown, that using hybrid system with
continuous and discrete quantum variables, it is possible to suggest a design
of strictly universal quantum processors. It is shown also that "deterministic"
limit of specific programmable "stochastic" U(1) gates (probability of success
becomes unit for infinite program register), discussed by other authors, may be
essentially same kind of hybrid quantum systems used here.


title: Electromagnetic interpretation of leptons

abstract:
  In the previous papers (quant-ph/0204037), (quant-ph/0204134) on the basis of
Dirac's equation we have considered the electromagnetic interpretation of the
quantum theory of electron. Here we continue the electron structure study.
Since the Dirac equation is also the equation of the other leptons, in the
present paper from the electromagnetic point of view we analyse the structure
of neutrino. We will show that not only the electron but also all the other
leptons have the electromagnetic structure.


title: How the no-cloning theorem got its name

abstract:
  I was the referee who approved the publication of Nick Herbert's FLASH paper,
knowing perfectly well that it was wrong. I explain why my decision was the
correct one, and I briefly review the progress to which it led.


title:
Quantum computation with two-level trapped cold ions beyond Lamb-Dicke
  limit

abstract:
  We propose a simple scheme for implementing quantum logic gates with a string
of two-level trapped cold ions outside the Lamb-Dicke limit. Two internal
states of each ion are used as one computational qubit (CQ) and the collective
vibration of ions acts as the information bus, i.e., bus qubit (BQ). Using the
quantum dynamics for the laser-ion interaction as described by a generalized
Jaynes-Cummings model, we show that quantum entanglement between any one CQ and
the BQ can be coherently manipulated by applying classical laser beams. As a
result, universal quantum gates, i.e. the one-qubit rotation and two-qubit
controlled gates, can be implemented exactly. The required experimental
parameters for the implementation, including the Lamb-Dicke (LD) parameter and
the durations of the applied laser pulses, are derived. Neither the LD
approximation for the laser-ion interaction nor the auxiliary atomic level is
needed in the present scheme.


title: Role of entanglement and correlations in dense coding

abstract:
  Usually it is assumed that quantum dense coding is due to quantum
entanglement between two parties. We show that this phenomenon has its origin
in {\em correlations} between two parties rather than simply in entanglement.
In order to justify our argument we evaluate a capacity of the noiseless
channel for two cases: (1) when Bob performs measurement just on the particle
received from Alice and (2) in the case when he utilizes the whole potential of
the dense coding, that is, he performs the measurement on the received particle
and the particle he had prior to the communication. We also present a simple
classical scenario which might serve as a prototype of the dense coding. We
generalize our results also for qudits.


title:
Gravitational Noise with Polarization Variables, the Nature of
  Entanglement States and Metrics Fluctuational Interpretation of Qauntum
  Mechanics

abstract:
  Discussed in the study are gravitational noise and the nature of entanglement
states. Their role in forming of entangled states. Nonlocal nature of entangled
states can be brought about by Polarization Variables. Polarization Variables
consists of sum a background of random classical gravitational fields and
waves, random electromagnets fields and so on known or unknown, distributed
average isotropically in the space. This background is capable of correlating
phases the oscillations of microobjects. From this follow, that entanglement
polarizations states is the functions of Polarization Variables in Vacuum.


title: Quantum communication complexity protocol with two entangled qutrits

abstract:
  We formulate a two-party communication complexity problem and present its
quantum solution that exploits the entanglement between two qutrits. We prove
that for a broad class of protocols the entangled state can enhance the
efficiency of solving the problem in the quantum protocol over any classical
one if and only if the state violates Bell's inequality for two qutrits.


title:
Event-independence, collective-independence,EPR-Bohm experiment and
  incompleteness of quantum mechanics

abstract:
  We analyse notion of independence in the EPR-Bohm framework by using
comparative analysis of independence in conventional and frequency probability
theories. Such an analysis is important to demonstrate that Bell's inequality
was obtained by using totally unjustified assumptions (e.g. the
Bell-Clauser-Horne factorability condition). Our frequency analysis also
demonstrated that Gill-Weihs-Zeilinger-Zukowski's arguments based on "the
experimenter's freedom to choose settings" to support the standard Bell
approach are neither justified by the structure of the EPR-Bohm experiment.
Finally, our analysis supports the original Einstein's viewpoint that quantum
mechanics is simply not complete.


title:
Preparation of spin squeezed atomic states by optical phase shift
  measurement

abstract:
  In this paper we present a state vector analysis of the generation of atomic
spin squeezing by measurement of an optical phase shift. The frequency
resolution is improved when a spin squeezed sample is used for spectroscopy in
place of an uncorrelated sample. When light is transmitted through an atomic
sample some photons will be scattered out of the incident beam, and this has a
destructive effect on the squeezing. We present quantitative studies for three
limiting cases: the case of a sample of atoms of size smaller than the optical
wavelength, the case of a large dilute sample and the case of a large dense
sample.


title: Quantum Random Walks Hit Exponentially Faster

abstract:
  We show that the hitting time of the discrete time quantum random walk on the
n-bit hypercube from one corner to its opposite is polynomial in n. This gives
the first exponential quantum-classical gap in the hitting time of discrete
quantum random walks. We provide the framework for quantum hitting time and
give two alternative definitions to set the ground for its study on general
graphs. We then give an application to random routing.


title: The Metric of Quantum States Revisited

abstract:
  A generalised definition of the metric of quantum states is proposed by using
the techniques of differential geometry. The metric of quantum state space
derived earlier by Anandan, is reproduced and verified here by this generalised
definition. The metric of quantum states in the configuration space and its
possible geometrical framework is explored. Also, invariance of the metric of
quantum states under local gauge transformations, coordinate transformations,
and the relativistic transformations is discussed.


title: Three Methods for Computing the Feynman Propagator

abstract:
  We present three methods for calculating the Feynman propagator for the
non-relativistic harmonic oscillator. The first method was employed by
Schwinger a half a century ago, but has rarely been used in non-relativistic
problems since. Also discussed is an algebraic method and a path integral
method so that the reader can compare the advantages and disadvantages of each
method.


title: Markov shift on non-commutative probability

abstract:
  We consider a class of quantum dissipative semigroup on a von-Neumann algebra
which admits a normal invariant state. We investigate asymptotic behavior of
the dissipative dynamics and their relation to that of the canonical Markov
shift. In case the normal invariant state is also faithful, we also extend the
notion of `quantum detailed balance' introduced by Frigerio-Gorini and prove
that forward weak Markov process and backward weak Markov process are
equivalent by an anti-unitary operator.


title: Interference of Quantum Market Strategies

abstract:
  Recent development in quantum computation and quantum information theory
allows to extend the scope of game theory for the quantum world. The paper is
devoted to the analysis of interference of quantum strategies in quantum market
games.


title:
Entanglement teleportation via thermally entangled states of two-qubit
  Heisenberg XX chain

abstract:
  Recently, entanglement teleportation has been investigated in Phys. Rev.
Lett. 84, 4236 (2000). In this paper we study entanglement teleportation via
two separate thermally entangled states of two-qubit Heisenberg XX chain. We
established the condition under which the parameters of the model have to
satisfy in order to teleport entanglement. The necessary minimum amount of
thermal entanglement for some fixed strength of exchange coupling is a function
of the magnetic field and temperature.


title: Detection of entanglement with few local measurements

abstract:
  We introduce a general method for the experimental detection of entanglement
by performing only few local measurements, assuming some prior knowledge of the
density matrix. The idea is based on the minimal decomposition of witness
operators into a pseudo-mixture of local operators. We discuss an
experimentally relevant case of two qubits, and show an example how bound
entanglement can be detected with few local measurements.


title: Producing the entangled photon pairs without type II down conversion

abstract:
  We propose a simple scheme to produce the polarization entangled photon pairs
without the type II phase match. The same scheme can also be used to produce
the macroscopic entangled photon states in both photon number space and the
polarization space(A. Lamas-Linares, J.C. Howell and D. Bouwmeester, Nature,
412887(2001)). Advantages and applications of our scheme in quantum key
distribution are discussed.


title: Retrodiction of Generalised Measurement Outcomes

abstract:
  If a generalised measurement is performed on a quantum system and we do not
know the outcome, are we able to retrodict it with a second measurement? We
obtain a necessary and sufficient condition for perfect retrodiction of the
outcome of a known generalised measurement, given the final state, for an
arbitrary initial state. From this, we deduce that, when the input and output
Hilbert spaces have equal (finite) dimension, it is impossible to perfectly
retrodict the outcome of any fine-grained measurement (where each POVM element
corresponds to a single Kraus operator) for all initial states unless the
measurement is unitarily equivalent to a projective measurement. It also
enables us to show that every POVM can be realised in such a way that perfect
outcome retrodiction is possible for an arbitrary initial state when the number
of outcomes does not exceed the output Hilbert space dimension. We then
consider the situation where the initial state is not arbitrary, though it may
be entangled, and describe the conditions under which unambiguous outcome
retrodiction is possible for a fine-grained generalised measurement. We find
that this is possible for some state if the Kraus operators are linearly
independent. This condition is also necessary when the Kraus operators are
non-singular. From this, we deduce that every trace-preserving quantum
operation is associated with a generalised measurement whose outcome is
unambiguously retrodictable for some initial state, and also that a set of
unitary operators can be unambiguously discriminated iff they are linearly
independent. We then examine the issue of unambiguous outcome retrodiction
without entanglement. This has important connections with the theory of locally
linearly dependent and locally linearly independent operators.


title: Brain as quantum-like computer

abstract:
  We present a contextualist statistical realistic model for quantum-like
representations in physics, cognitive science and psychology. We apply this
model to describe cognitive experiments to check quantum-like structures of
mental processes. The crucial role is played by interference of probabilities
for mental observables. Recently one of such experiments based on recognition
of images was performed. This experiment confirmed our prediction on
quantum-like behaviour of mind. In our approach ``quantumness of mind'' has no
direct relation to the fact that the brain (as any physical body) is composed
of quantum particles. We invented a new terminology ``quantum-like (QL) mind.''
Cognitive QL-behaviour is characterized by nonzero coefficient of interference
$\lambda.$ This coefficient can be found on the basis of statistical data.
There is predicted not only $\cos \theta$-interference of probabilities, but
also hyperbolic $\cosh \theta$-interference. This interference was never
observed for physical systems, but we could not exclude this possibility for
cognitive systems. We propose a model of brain functioning as QL-computer
(there is discussed difference between quantum and QL computers).


title: Quantum Principles and Mathematical Computability

abstract:
  Taking the view that computation is after all physical, we argue that
physics, particularly quantum physics, could help extend the notion of
computability. Here, we list the important and unique features of quantum
mechanics and then outline a quantum mechanical "algorithm" for one of the
insoluble problems of mathematics, the Hilbert's tenth and equivalently the
Turing halting problem. The key element of this algorithm is the {\em
computability} and {\em measurability} of both the values of physical
observables and of the quantum-mechanical probability distributions for these
values.


title: Transport of Quantum States and Separation of Ions in a Dual RF Ion Trap

abstract:
  We have investigated ion dynamics associated with a dual linear ion trap
where ions can be stored in and moved between two distinct locations. Such a
trap is a building block for a system to engineer arbitrary quantum states of
ion ensembles. Specifically, this trap is the unit cell in a strategy for
scalable quantum computing using a series of interconnected ion traps. We have
transferred an ion between trap locations 1.2 mm apart in 50 $\mu$s with near
unit efficiency ($> 10^{6}$ consecutive transfers) and negligible motional
heating, while maintaining internal-state coherence. In addition, we have
separated two ions held in a common trap into two distinct traps.


title: Circuit for Shor's algorithm using 2n+3 qubits

abstract:
  We try to minimize the number of qubits needed to factor an integer of n bits
using Shor's algorithm on a quantum computer. We introduce a circuit which uses
2n+3 qubits and O(n^3 lg(n)) elementary quantum gates in a depth of O(n^3) to
implement the factorization algorithm. The circuit is computable in polynomial
time on a classical computer and is completely general as it does not rely on
any property of the number to be factored.
  Keywords: Factorization, quantum circuits, modular arithmetics


title: A Bell-type Theorem Without Hidden Variables

abstract:
  It is shown that no theory that satisfies certain premises can exclude
faster-than-light influences. The premises include neither the existence of
hidden variables, nor counterfactual definiteness, nor any premise that
effectively entails the general existence of outcomes of unperformed local
measurements. All the premises are compatible with Copenhagen philosophy, and
the principles and predictions of relativistic quantum field theory. The
present proof is contrasted with an earlier one with the same objective.


title: Recovery of continuous wave squeezing at low frequencies

abstract:
  We propose and demonstrate a system that produces squeezed vacuum using a
pair of optical parametric amplifiers. This scheme allows the production of
phase sidebands on the squeezed vacuum which facilitate phase locking in
downstream applications. We observe strong, stably locked, continuous wave
vacuum squeezing at frequencies as low as 220 kHz. We propose an alternative
resonator configuration to overcome low frequency squeezing degradation caused
by the optical parametric amplifiers.


title: Tailoring teleportation to the quantum alphabet

abstract:
  We introduce a refinement of the standard continuous variable teleportation
measurement and displacement strategies. This refinement makes use of prior
knowledge about the target state and the partial information carried by the
classical channel when entanglement is non-maximal. This gives an improvement
in the output quality of the protocol. The strategies we introduce could be
used in current continuous variable teleportation experiments.


title: On the effective size of certain "Schr\"{o}dinger cat'' like states

abstract:
  Several experiments and experimental proposals for the production of
macroscopic superpositions naturally lead to states of the general form
$|\phi_1>^{\otimes N}+|\phi_2>^{\otimes N}$, where the number of subsystems $N$
is very large, but the states of the individual subsystems have large overlap,
$|{\l}\phi_1|\phi_2 \r|^2=1-\epsilon^2$. We propose two different methods for
assigning an effective particle number to such states, using ideal
Greenberger--Horne--Zeilinger (GHZ)-- states of the form $|0\r^{\otimes
n}+|1\r^{\otimes n}$ as a standard of comparison. The two methods are based on
decoherence and on a distillation protocol respectively. Both lead to an
effective size $n$ of the order of $N \epsilon^2$.


title: Characterization of non-local gates

abstract:
  A non-local unitary transformation of two qubits occurs when some Hamiltonian
interaction couples them. Here we characterize the amount, as measured by time,
of interaction required to perform two--qubit gates, when also arbitrarily
fast, local unitary transformations can be applied on each qubit. The minimal
required time of interaction, or interaction cost, defines an operational
notion of the degree of non--locality of gates. We characterize a partial order
structure based on this notion. We also investigate the interaction cost of
several communication tasks, and determine which gates are able to accomplish
them. This classifies two--qubit gates into four categories, differing in their
capability to transmit classical, as well as quantum, bits of information.


title: Dense coding and safety of quantum communications

abstract:
  The sending station being the classical device can be eavesdropped by
classical means. Dense coding and quantum nature of wave function give the
additional resource to raise the safety of the quantum channel as a whole.


title:
Multipartite Greenberger-Horne-Zeilinger paradoxes for continuous
  variables

abstract:
  We show how to construct Greenberger-Horn-Zeilinger type paradoxes for
continuous variable systems. We give two examples corresponding to 3 party and
5 party paradoxes. The paradoxes are revealed by carrying out position and
momentum measurements. The structure of the quantum states which lead to these
paradoxes is discussed.


title: Single Photons on Pseudo-Demand from Stored Parametric Down-Conversion

abstract:
  We describe the results of a parametric down-conversion experiment in which
the detection of one photon of a pair causes the other photon to be switched
into a storage loop. The stored photon can then be switched out of the loop at
a later time chosen by the user, providing a single photon for potential use in
a variety of quantum information processing applications. Although the stored
single photon is only available at periodic time intervals, those times can be
chosen to match the cycle time of a quantum computer by using pulsed
down-conversion. The potential use of the storage loop as a photonic quantum
memory device is also discussed.


title: Quantum electromagnetic field in a three dimensional oscillating cavity

abstract:
  We compute the photon creation inside a perfectly conducting, three
dimensional oscillating cavity, taking the polarization of the electromagnetic
field into account. As the boundary conditions for this field are both of
Dirichlet and (generalized) Neumann type, we analyze as a preliminary step the
dynamical Casimir effect for a scalar field satisfying generalized Neumann
boundary conditions. We show that particle production is enhanced with respect
to the case of Dirichlet boundary conditions. Then we consider the transverse
electric and transverse magnetic polarizations of the electromagnetic field.
For resonant frequencies, the total number of photons grows exponentially in
time for both polarizations, the rate being greater for transverse magnetic
modes.


title: Distinguishability of maximally entangled states

abstract:
  In $2 \otimes 2$, more than 2 orthogonal Bell states with single copy can
never be discriminated with certainty if only local operations and classical
communication (LOCC) are allowed. More than $d$ orthogonal maximally entangled
states in $d \otimed d$, which are in cannonical form, used by Bennett et. al.
[Phys. Rev. Lett. 70 (1993) 1895] can never be discriminated with certainty
when a single copy of the states is provided. Interestingly we show that all
orthogonal maximally entangled states, which are in cannonical form, can be
discriminated with certainty if and only if two copies of each of the states
are provided. The highly nontrivial problem of local discrimination of $d$ or
less no. of pairwise orthogonal maximally entangled states in $d \otimes d$ (in
single copy case), which are in cannonical form, is also discussed.


title:
Generation and control of resonance states in crossed magnetic and
  electric fields

abstract:
  A two-dimensional electron system interacting with an impurity and placed in
crossed magnetic and electric fields is under investigation. Since it is
assumed that an impurity center interacts as an attractive $\delta$-like
potential a renormalization procedure for the retarded Green's function has to
be carried out. For the vanishing electric field we obtain a close analytical
expression for the Green's function and we find one bound state localized
between Landau levels. It is also shown by numerical investigations that
switching on the electric field new long-living resonance states localized in
the vicinity of Landau levels can be generated.


title: Purification of two-qubit mixed states

abstract:
  We find the necessary and sufficient condition under which two two-qubit
mixed states can be purified into a pure maximally entangled state by local
operations and classical communication. The optimal protocol for such
transformation is obtained. This result leads to a necessary and sufficient
condition for the exact purification of $n$ copies of a two-qubit state.


title: Decoherence scenarios from micro- to macroscopic superpositions

abstract:
  Environment induced decoherence entails the absence of quantum interference
phenomena from the macroworld. The loss of coherence between superposed wave
packets depends on their separation. The precise temporal course depends on the
relative size of the time scales for decoherence and other processes taking
place in the open system and its environment. We use the exactly solvable model
of an harmonic oscillator coupled to a bath of oscillators to illustrate
various decoherence scenarios: These range from exponential golden-rule decay
for microscopic superpositions, system-specific decay for larger separations in
a crossover regime, and finally universal interaction-dominated decoherence for
ever more macroscopic superpositions.


title: A conditional-phase switch at the single-photon level

abstract:
  We present an experimental realization of a two-photon conditional-phase
switch, related to the ``$c$-$\phi $'' gate of quantum computation. This gate
relies on quantum interference between photon pairs, generating entanglement
between two optical modes through the process of spontaneous parametric
down-conversion (SPDC). The interference effect serves to enhance the effective
nonlinearity by many orders of magnitude, so it is significant at the quantum
(single-photon) level. By adjusting the relative optical phase between the
classical pump for SPDC and the pair of input modes, one can impress a large
phase shift on one beam which depends on the presence or absence of a single
photon in a control mode.


title: Zero-Range Potentials in Multi-Channel Diatomic Molecule Scattering

abstract:
  The method of zero-range potentials is generalized to account for the
molecular electron excitation process. It is made by a matrix formulation in
which a state vector components are associated with a scattering channel. The
multi-center target is considered and the model is applied to the example of $e
+ H_2$ low energy scattering. The results of evaluation of cross-sections are
compared with ones of the MCF and SMC methods.


title: Multiparty d-dimensional quantum information splitting

abstract:
  Generalization of quantum information splitting protocol from qubits to
qudits (quantum d-dimensional systems) is presented.


title:
A test of Local Realism with entangled kaon pairs and without
  inequalities

abstract:
  We propose the use of entangled pairs of neutral kaons, considered as a
promising tool to close the well known loopholes affecting generic Bell's
inequality tests, in a specific Hardy-type experiment. Hardy's contradiction
without inequalities between Local Realism and Quantum Mechanics can be
translated into a feasible experiment by requiring ideal detection efficiencies
for only one of the observables to be alternatively measured. Neutral kaons are
near to fulfil this requirement and therefore to close the efficiency loophole.


title: Embedding dissipation and decoherence in unitary evolution schemes

abstract:
  Dissipation and decoherence, and the evolution from pure to mixed states in
quantum physics are handled through master equations for the density matrix. By
embedding elements of this matrix in a higher-dimensional Liouville-Bloch
equation, the methods of unitary integration are adapted to solve for the
density matrix as a function of time, including the non-unitary effects of
dissipation and decoherence. The input requires only solutions of classical,
initial value time-dependent equations. Results are illustrated for a damped
driven two-level system.


title: Preparing encoded states in an oscillator

abstract:
  Recently a scheme has been proposed for constructing quantum error-correcting
codes that embed a finite-dimensional code space in the infinite-dimensional
Hilbert space of a system described by continuous quantum variables. One of the
difficult steps in this scheme is the preparation of the encoded states. We
show how these states can be generated by coupling a continuous quantum
variable to a single qubit. An ion trap quantum computer provides a natural
setting for a continuous system coupled to a qubit. We discuss how encoded
states may be generated in an ion trap.


title:
Both Toffoli and Controlled-NOT need little help to do universal quantum
  computation

abstract:
  What additional gates are needed for a set of classical universal gates to do
universal quantum computation? We answer this question by proving that any
single-qubit real gate suffices, except those that preserve the computational
basis.
  The result of Gottesman and Knill[quant-ph/9807006] implies that any quantum
circuit involving only the Controlled-NOT and Hadamard gates can be efficiently
simulated by a classical circuit. In contrast, we prove that Controlled-NOT
plus any single-qubit real gate that does not preserve the computational basis
and is not Hadamard (or its alike) are universal for quantum computing.
  Previously only a ``generic'' gate, namely a rotation by an angle
incommensurate with pi, is known to be sufficient in both problems, if only one
single-qubit gate is added.


title:
Elementary operations for quantum logic with a single trapped two-level
  cold ion beyond Lamb-Dicke limit

abstract:
  A simple alternative scheme for implementing quantum gates with a single
trapped cold two-level ion beyond the Lamb-Dicke (LD) limit is proposed. Basing
on the quantum dynamics for the laser-ion interaction described by a
generalized Jaynes-Cummings model, one can introduce two kinds of elementary
quantum operations i.e., the simple rotation on the bare atomic state,
generated by applying a resonant pulse, and the joint operation on the internal
and external degrees of the ion, performed by using an off-resonant pulse.
Several typical quantum gates, including Hadamard gate, controlled-Z and
controlled-NOT gates $etc.$, can thus be implemented exactly by using these
elementary operations. The experimental parameters including the LD parameter
and the durations of the applied laser pulses, for these implementation are
derived analytically and numerically. Neither the LD approximation for the
laser-ion interaction nor the auxiliary atomic level is needed in the present
scheme.


title:
Entanglement required in achieving entanglement-assisted channel
  capacities

abstract:
  Entanglement shared between the two ends of a quantum communication channel
has been shown to be a useful resource in increasing both the quantum and
classical capacities for these channels. The entanglement-assisted capacities
were derived assuming an unlimited amount of shared entanglement per channel
use. In this paper, bounds are derived on the minimum amount of entanglement
required per use of a channel, in order to asymptotically achieve the capacity.
This is achieved by introducing a class of entanglement-assisted quantum codes.
Codes for classes of qubit channels are shown to achieve the quantum
entanglement-assisted channel capacity when an amount of shared entanglement
per channel given by, E = 1 - Q_E, is provided. It is also shown that for very
noisy channels, as the capacities become small, the amount of required
entanglement converges for the classical and quantum capacities.


title:
Entanglement generation by adiabatic navigation in the space of
  symmetric multi-particle states

abstract:
  We propose a technique for robust and efficient navigation in the Hilbert
space of entangled symmetric states of a multiparticle system with externally
controllable linear and nonlinear collective interactions. A linearly changing
external field applied along the quantization axis creates a network of well
separated level crossings in the energy diagram of the collective states. One
or more transverse pulsed fields applied at the times of specific level
crossings induce adiabatic passage between these states. By choosing the timing
of the pulsed field appropriately, one can transfer an initial product state of
all N spins into (i) any symmetric state with n spin excitations and (ii) the
N-particle analog of the Greenberger-Horne-Zeilinger state. This technique,
unlike techniques using pulses of specific area, does not require precise
knowledge of the number of particles and is robust against variations in the
interaction parameters. We discuss potential applications in two-component Bose
condensates and ion-trap systems.


title:
Entanglement between two macroscopic fields by coherent atom-mediated
  exchange of photons

abstract:
  Using two different criteria for continuous variable systems we demonstrated
that pump and probe beams became quantum correlated in a situation of
Electromagnetically Induced Transparency in a sample of Rb atoms. Our result
combines two important features for practical implementations in the field of
quantum information processing. Namely, we proved the existence of entanglement
between two macroscopic light beams, and this entanglement is intrinsically
associated to a strong coherence in an atomic medium.


title:
Determination of the phase of an electromagnetic field via incoherent
  detection of fluorescence

abstract:
  We show that the phase of a field can be determined by incoherent detection
of the population of one state of a two-level system if the Rabi frequency is
comparable to the Bohr frequency so that the rotating wave approximation is
inappropriate. This implies that a process employing the measurement of
population is not a square-law detector in this limit. We discuss how the
sensitivity of the degree of excitation to the phase of the field may pose
severe constraints on precise rotations of quantum bits involving low-frequency
transitions. We present a scheme for observing this effect in an atomic beam,
despite the spread in the interaction time.


title: Strategies for Estimating Quantum Lossy Channels

abstract:
  Due to the anisotropy of quantum lossy channels one must choose optimal bases
of input states for best estimating them. In this paper, we obtain that the
equal probability Schr\"{o}dinger cat states are optimal for estimating a
single lossy channel and they are also the optimal bases of input states for
estimating composite lossy channels. On the other hand, by using the symmetric
logarithmic derivative (SLD) Fisher information of output states exported from
the lossy channels we obtain that if we take the equal probability
Schr\"{o}dinger cat states as the bases of input states the maximally entangled
inputs are not optimal, however if the bases of the input states are not the
equal probability Schr\"{o}dinger cat states the maximally entangled input
states may be optimal for the estimating composite lossy channel.


title: Three Dimensional Confinement : WKB Revisited

abstract:
  An alternate formalism is developed to determine the energy eigenvalues of
quantum mechanical systems, confined within a rigid impenetrable spherical box
of radius $r_0$, in the framework of Wentzel-Kramers-Brillouin (WKB)
approximation. Instead of considering the Langer correction for the centrifugal
term, the approach adopted here is that of Hainz and Grabert : The centrifugal
term is expanded perturbatively (in powers of $\hbar$), decomposing it into 2
terms -- the classical centrifugal potential and a quantum correction. Hainz
and Grabert found that this method reproduced the exact energies of the
hydrogen atom, to the first order in $\hbar$, with all higher order corrections
vanishing. In the present study, this formalism is extended to the case of
radial potentials under hard wall confinement, to check whether the same
argument holds good for such confined systems as well. As explicit examples, 3
widely known potentials are studied, which are of considerable importance in
the theoretical treatment of various atomic phenomena involving atomic
transitions, viz., the 3-dimensional harmonic oscillator, the hydrogen atom,
and the Hulthen potential.


title: Stabilization of decaying states and quantum vortices

abstract:
  A two-dimensional model of an electron moving under the influence of an
attractive zero-range potential as well as external magnetic and electric
fields is analyzed. We prove by numerical investigations that there are formed
such resonances which manifest a peculiar dependance on the electric field
intensity, i.e., although the electric field increases, the lifetime of these
resonance states grows up. It is explained that this phenomenon, called further
\textit{the stabilization}, is a close consequence of
\textit{quantum-mechanical vortices} induced by the magnetic field and
controlled by the electric field strength. In order to get more information
about these vortices the phase of wavefunctions as well as the probability
current for these stable resonances are investigated.


title: Isospectral partners for a complex PT-invariant potential

abstract:
  We construct isospectral partner potentials of a complex PT-invariant
potential, viz., V(x) = V_1 sech ^2 x - i V_2 sech x tanh x using Darboux's
method. Oneset of isospectral potentials are obatined which can be termed
'Satellite potentials', in the sense that they are pf the same form as the
original potential. In a particular case, the supersymmetric partner potential
has the same spectrum, including the zero energy ground state, a fact which
cannot occur in conventional supersymmetric quantum mechanics with real
potential. An explicit example of a non-trivial set of isospectral potential is
also obtained.