title: Semiclassical Approximations in Phase Space with Coherent States

abstract:
  We present a complete derivation of the semiclassical limit of the coherent
state propagator in one dimension, starting from path integrals in phase space.
We show that the arbitrariness in the path integral representation, which
follows from the overcompleteness of the coherent states, results in many
different semiclassical limits. We explicitly derive two possible semiclassical
formulae for the propagator, we suggest a third one, and we discuss their
relationships. We also derive an initial value representation for the
semiclassical propagator, based on an initial gaussian wavepacket. It turns out
to be related to, but different from, Heller's thawed gaussian approximation.
It is very different from the Herman--Kluk formula, which is not a correct
semiclassical limit. We point out errors in two derivations of the latter.
Finally we show how the semiclassical coherent state propagators lead to
WKB-type quantization rules and to approximations for the Husimi distributions
of stationary states.


title: Cooperative effects on Optical Forces- Dicke's Bullet

abstract:
  We investigate the cooperative effects on optical forces in a system of N two
level atoms occupying a volume of dimensions to within $\lambda ^3$, where
lambda is radiation wavelength and is driven by a coherent radiation field with
a spatial profile like Laguerre-Gaussian beam or ideal Bessel beam.We show a
dramatic enhancement on optical forces as well as the angular momentum imparted
to the atom by a factor of $N ^2$.


title: Quantum control using sequences of simple control pulses

abstract:
  Structured decompositions of a desired unitary operator are employed to
derive control schemes that achieve certain control objectives for finite-level
quantum systems using only sequences of simple control pulses such as square
waves with finite rise and decay times or Gaussian wavepackets. The technique
is applied to find control schemes that achieve population transfers for
pure-state systems, complete inversions of the ensemble populations for
mixed-state systems, create arbitrary superposition states and optimize the
ensemble average of observables.


title: Clock synchronization with dispersion cancellation

abstract:
  The dispersion cancellation feature of pulses which are entangled in
frequency is employed to synchronize clocks of distant parties. The proposed
protocol is insensitive to the pulse distortion caused by transit through a
dispersive medium. Since there is cancellation to all orders, also the effects
of slowly fluctuating dispersive media are compensated. The experimental setup
can be realized with currently available technology, at least for a proof of
principle.


title: Unconditional security in quantum bit commitment

abstract:
  The commitment of bits between two mutually distrustful parties is a powerful
cryptographic primitive with which many cryptographic objectives can be
achieved. It is widely believed that unconditionally secure quantum bit
commitment is impossible due to quantum entanglement cheating, which is
codified in a general impossibility theorem. Gaps in the proof of this
impossibility theorem are found. An unconditionally secure bit commitment
protocol utilizing anonymous quantum states and the no-clone theorem is
presented below with a full security proof.


title: The existance of unconditionally secure quantum bit commitment

abstract:
  This paper has been withdrawn by the author(s).The scheme presented is
insecure.


title: Is entanglement observer-dependent?

abstract:
  The properties of quantum entanglement are examined and the role of the
observer is pointed out.


title:
Kinetic theory of QED plasmas in a strong electromagnetic field. II. The
  mean-field description

abstract:
  Starting from a general relativistic kinetic equation, a self-consistent
mean-field equation for fermions is derived within a covariant density matrix
approach of QED plasmas in strong external fields. A Schr\"odinger picture
formulation on space-like hyperplanes is applied. The evolution of the
distribution function is described by the one-particle gauge-invariant 4x4
Wigner matrix, which is decomposed in spinor space. A coupled system of
equations for the corresponding Wigner components is obtained. The polarization
current is expressed in terms of the Wigner function. Charge conservation is
obeyed. In the quasi-classical limit for the Wigner components a relativistic
Vlasov equation is obtained, which is presented in an invariant, i.e.
hyperplane independent, form.


title:
Entanglement between photons and atoms coupled out from a
  Bose-Einstein-Condensate

abstract:
  We study the limitations to the relative number squeezing between photons and
atoms coupled out from a homogeneous Bose-Einstein-Condensate. We consider the
coupling between the translational atomic states by two photon Bragg processes,
with one of the photon modes involved in the Bragg process in a coherent state,
and the other initially unpopulated. We start with an interacting Bose-
condensate at zero temperature and compute the time evolution for the system.
We study the squeezing, i.e. the variance of the occupation number difference
between the second photon and the atomic c.m. mode. We discuss how collisions
between the atoms and photon rescattering affect the degree of squeezing which
may be reached in such experiments.


title: Decoherence : An irreversible Process

abstract:
  A wide-ranging theory of decoherence is derived from the quantum theory of
irreversible processes, with specific results having for their main limitation
the assumption of an exact pointer basis.


title:
Photon counting for quantum key distribution with Peltier cooled
  InGaAs/InP APD's

abstract:
  The performance of three types of InGaAs/InP avalanche photodiodes is
investigated for photon counting at 1550 nm in the temperature range of
thermoelectric cooling. The best one yields a dark count probability of $%
2.8\cdot 10^{-5}$ per gate (2.4 ns) at a detection efficiency of 10% and a
temperature of -60C. The afterpulse probability and the timing jitter are also
studied. The results obtained are compared with those of other papers and
applied to the simulation of a quantum key distribution system. An error rate
of 10% would be obtained after 54 kilometers.


title:
Second Quantization of Cini Model for High Order Quantum Decoherence in
  Quantum Measurement

abstract:
  By making the second quantization for the Cini Model of quantum measurement
without wave function collapse [M. Cini, Nuovo Cimento, B73 27(1983)], the
second order quantum decoherence (SOQD) is studied with a two mode boson system
interacting with an idealized apparatus composed by two quantum oscillators. In
the classical limit that the apparatus is prepared in a Fock state with a very
large quantum number, or in a coherent state with average quantum numbers large
enough, the SOQD phenomenon appears similar to the first order case of quantum
decoherence.


title: Dynamics of optically driven exciton and quantum decoherence

abstract:
  By using the normal ordering method, we study the state evolution of an
optically driven excitons in a quantum well immersed in a leaky cavity, which
was introduced by Yu-xi Liu et.al. [Phys. Rev. A {\bf 63}, 033816 (2001)]. The
influence of the external laser field on the quantum decoherence of a
mesoscopically superposed states of the excitons is investigated. Our result
shows that, thought the characteristic time of decoherence does not depend on
the external field, the phase of the decoherence factor can be well controlled
by adjusting the external parameters.


title: Clauser-Horne inequality for qutrits

abstract:
  In this brief report we show the new Bell-Clauser-Horne inequality for two
entangled three dimensional quantum systems (so called qutrits). This
inequality is violated by a maximally entangled state of two qutrits observed
via symmetric three input and three output port beamsplitter only if the amount
of noise in the system equals ${11-6\sqrt3 \over 2}\approx 0.308$. This is in
perfect agreement with the previous numerical calculations presented in
Kaszlikowski {\it et. al.} Phys. Rev. Lett. {\bf 85}, 4418 (2000).


title: Parametric Dynamics of Level Spacings in Quantum Chaos

abstract:
  We identify parametric (radial) Bessel-Ornstein-Uhlenbeck stochastic
processes as primitive dynamical models of energy level repulsion in irregular
quantum systems. Familiar GOE, GUE, GSE and non-Hermitian
  Ginibre universality classes of spacing distributions arise as special cases
in that formalism.


title: Entanglement in squeezed two-level atom

abstract:
  In the previous paper, we adopted the method using quantum mutual entropy to
measure the degree of entanglement in the time development of the
Jaynes-Cummings model. In this paper, we formulate the entanglement in the time
development of the Jaynes-Cummings model with squeezed states, and then show
that the entanglement can be controlled by means of squeezing.


title: Conclusive tests of local realism and pseudoscalar mesons

abstract:
  Many beautiful experiments have been addressed to test standard quantum
mechanics against local realistic models. Even if a strong evidence favouring
standard quantum mechanics is emerged, a conclusive experiment is still
lacking, because of low detection efficiencies. Recently, experiments based on
pseudoscalar mesons have been proposed as a way for obtaining a conclusive
experiment. In this paper, we investigate if this result can effectively be
obtained. Our conclusions, based on a careful analysis of the proposed set ups,
are that this will not be possible due to intrinsic limitations of these kind
of experiments.


title: Aligning Reference Frames Using Quantum States

abstract:
  We analyze the problem of sending, in a single transmission, the information
required to specify an orthogonal trihedron or reference frame through a
quantum channel made out of N elementary spins. We analytically obtain the
optimal strategy, i.e., the best encoding state and the best measurement. For
large N, we show that the average error goes to zero linearly in 1/N. Finally,
we discus the construction of finite optimal measurements.


title:
Relaxation of a two-level system strongly coupled to a reservoir:
  Anomalously slow decoherence

abstract:
  Relaxation of a two-level system (TLS) into a resonant infinite-temperature
reservoir with a Lorentzian spectrum is studied. The reservoir is described by
a complex Gaussian-Markovian field coupled to the nondiagonal elements of the
TLS Hamiltonian. The theory can be relevant for electromagnetic interactions in
microwave high-$Q$ cavities and muon spin depolarization. Analytical results
are obtained for the strong-coupling regime, $\Omega_0\gg\nu$, where $\Omega_0$
is the rms coupling amplitude (Rabi frequency) and $\nu$ is the width of the
reservoir spectrum. In this regime, the population difference and half of the
initial coherence decay with two characteristic rates: the most part of the
decay occurs at $t\sim\Omega_0^{-1}$, the relaxation being reversible for
$t\ll(\Omega_0^2\nu)^{-1/3}$, whereas for $t\gg(\Omega_0^2\nu)^{-1/3}$ the
relaxation becomes irreversible and is practically over. The other half of the
coherence decays with the rate on the order of $\nu$, which may be slower by
orders of magnitude than the time scale of the population relaxation. The above
features are explained by the fact that at $t\ll\nu^{-1}$ the reservoir
temporal fluctuations are effectively one-dimensional (adiabatic). Moreover, we
identify the pointer basis, in which the reduction of the state vector occurs.
The pointer states are correlated with the reservoir, being dependent on the
reservoir phase.


title: Renyi-Wehrl entropies as measures of localization in phase space

abstract:
  We generalize the concept of the Wehrl entropy of quantum states which gives
a basis-independent measure of their localization in phase space. We discuss
the minimal values and the typical values of these R{enyi-Wehrl entropies for
pure states for spin systems. According to Lieb's conjecture the minimal values
are provided by the spin coherent states. Though Lieb's conjecture remains
unproven, we give new proofs of partial results that may be generalized for
other systems. We also investigate random pure states and calculate the mean
Renyi-Wehrl entropies averaged over the natural measure in the space of pure
quantum states.


title: Counting, Fanout, and the Complexity of Quantum ACC

abstract:
  We propose definitions of $\QAC^0$, the quantum analog of the classical class
$\AC^0$ of constant-depth circuits with AND and OR gates of arbitrary fan-in,
and $\QACC[q]$, the analog of the class $\ACC[q]$ where $\Mod_q$ gates are also
allowed. We prove that parity or fanout allows us to construct quantum $\MOD_q$
gates in constant depth for any $q$, so $\QACC[2] = \QACC$. More generally, we
show that for any $q,p > 1$, $\MOD_q$ is equivalent to $\MOD_p$ (up to constant
depth). This implies that $\QAC^0$ with unbounded fanout gates, denoted
$\QACwf^0$, is the same as $\QACC[q]$ and $\QACC$ for all $q$. Since $\ACC[p]
\ne \ACC[q]$ whenever $p$ and $q$ are distinct primes, $\QACC[q]$ is strictly
more powerful than its classical counterpart, as is $\QAC^0$ when fanout is
allowed. This adds to the growing list of quantum complexity classes which are
provably more powerful than their classical counterparts.
  We also develop techniques for proving upper bounds for $\QACC^0$ in terms of
related language classes. We define classes of languages $\EQACC$, $\NQACC$ and
$\BQACC_{\rats}$. We define a notion of $\log$-planar $\QACC$ operators and
show the appropriately restricted versions of $\EQACC$ and $\NQACC$ are
contained in $\P/\poly$. We also define a notion of $\log$-gate restricted
$\QACC$ operators and show the appropriately restricted versions of $\EQACC$
and $\NQACC$ are contained in $\TC^0$.


title: Generalized Bell States and Quantum Teleportation

abstract:
  We make a brief comment on measurement of quantum operators with degenerate
eigenstates and apply to quantum teleportation. We also try extending the
quantum teleportation by Bennett et al [5] to more general situation by making
use of generalized Bell states.


title:
Degrees of concealment and bindingness in quantum bit commitment
  protocols

abstract:
  Although it is impossible for a bit commitment protocol to be both
arbitrarily concealing and arbitrarily binding, it is possible for it to be
both partially concealing and partially binding. This means that Bob cannot,
prior to the beginning of the unveiling phase, find out everything about the
bit committed, and Alice cannot, through actions taken after the end of the
commitment phase, unveil whatever bit she desires. We determine upper bounds on
the degrees of concealment and bindingness that can be achieved simultaneously
in any bit commitment protocol, although it is unknown whether these can be
saturated. We do, however, determine the maxima of these quantities in a
restricted class of bit commitment protocols, namely those wherein all the
systems that play a role in the commitment phase are supplied by Alice. We show
that these maxima can be achieved using a protocol that requires Alice to
prepare a pair of systems in an entangled state, submit one of the pair to Bob
at the commitment phase, and the other at the unveiling phase. Finally, we
determine the form of the trade-off that exists between the degree of
concealment and the degree of bindingness given various assumptions about the
purity and dimensionality of the states used in the protocol.


title: Jaynes-Cummings Model Dynamics in Two Trapped Ions

abstract:
  We showed that in Lamb-Dicke regime and under rotating wave approximation,
the dynamical behavior of two trapped ions interacting with a laser beam
resonant to the first red side-band of center-of-mass mode can be described by
Jaynes-Cummings Model. An exact analytic solution for this kind of
Jaynes-Cummings model is presented. The results showed that quantum collapses
and revivals for the occupation of two atoms, and squeezing for vibratic motion
of center-of-mass mode existed in both two different types of initial
conditions. The maximum momentum squeezing for center-of-mass mode in these two
types of conditions are found to be 42.4% and 43.8% respectively. The
coherence, in the first type of initial conditions can keeps long times, and in
the second type of initial conditions, a concrete form of coherent state is
obtained, when the initial average number is very small.


title:
Complexified PSUSY and SSUSY interpretations of some PT-symmetric
  Hamiltonians possessing two series of real energy eigenvalues

abstract:
  We analyze a set of three PT-symmetric complex potentials, namely harmonic
oscillator, generalized Poschl-Teller and Scarf II, all of which reveal a
double series of energy levels along with the corresponding superpotential.
Inspired by the fact that two superpotentials reside naturally in order-two
parasupersymmetry (PSUSY) and second-derivative supersymmetry (SSUSY) schemes,
we complexify their frameworks to successfully account for the three
potentials.


title: Continuous-variable quantum teleportation through lossy channels

abstract:
  The ultimate limits of continuous-variable single-mode quantum teleportation
due to absorption are studied, with special emphasis on (quasi-)monochromatic
optical fields propagating through fibers. It is shown that even if an
infinitely squeezed two-mode squeezed vacuum were used, the amount of
information that would be transferred quantum mechanically over a finite
distance is limited and effectively approaches to zero on a length scale that
is much shorter than the (classical) absorption length. Only for short
distances the state-dependent teleportation fidelity can be close to unity. To
realize the largest possibly fidelity, an asymmetrical equipment must be used,
where the source of the two-mode squeezed vacuum is nearer to Alice than to Bob
and in consequence the coherent displacement performed by Bob cannot be chosen
independently of the transmission lengths.


title: Typical entanglement in multi-qubit systems

abstract:
  Quantum entanglement and its paradoxical properties hold the key to an
information processing revolution. Much attention has focused recently on the
challenging problem of characterizing entanglement. Entanglement for a two
qubit system is reasonably well understood, however, the nature and properties
of multiple qubit systems are largely unexplored. Motivated by the importance
of such systems in quantum computing, we show that typical pure states of N
qubits are highly entangled but have decreasing amounts of pairwise
entanglement (measured using the Wootter's concurrence formula) as N increases.
Above six qubits very few states have any pairwise entanglement, and generally,
for a typical pure state of N qubits there is a sharp cut-off where its
subsystems of size m become PPT (positive partial transpose i.e., separable or
only bound entangled) around N >~ 2m + 3, based on numerical analysis up to
N=13.


title: Bell inequalities for arbitrarily high dimensional systems

abstract:
  We develop a novel approach to Bell inequalities based on a constraint that
the correlations exhibited by local realistic theories must satisfy. This is
used to construct a family of Bell inequalities for bipartite quantum systems
of arbitrarily high dimensionality which are strongly resistant to noise. In
particular our work gives an analytic description of numerical results of D.
Kaszlikowski, P. Gnacinski, M. Zukowski, W. Miklaszewski, A. Zeilinger, Phys.
Rev. Lett. {\bf 85}, 4418 (2000) and T. Durt, D. Kaszlikowski, M. Zukowski,
quant-ph/0101084, and generalises them to arbitrarily high dimensionality.


title: Preparation of decoherence-free, subradiant states in a cavity

abstract:
  The cause of decoherence in a quantum system can be traced back to the
interaction with the environment. As it has been pointed out first by Dicke, in
a system of N two-level atoms where each of the atoms is individually dipole
coupled to the environment, there are collective, subradiant states, that have
no dipole coupling to photon modes, and therefore they are expected to decay
slower. This property also implies that these type of states, which form an N-1
dimensional subspace of the atomic subsytem, also decohere slower. We propose a
scheme which will create such states. First the two-level atoms are placed in a
strongly detuned cavity and one of the atoms, called the control atom is
excited. The time evolution of the coupled atom-cavity system leads to an
appropriately entangled state of the atoms. By applying subsequent laser pulses
at a well defined time instant, it is possible to drive the atomic state into
the subradiant, i. e., decoherence free subspace. Up to a certain average
number of the photons, the result is independent of the state of the cavity.
The analysis of the conditions shows that this scheme is feasible with present
day techniques achieved in atom cavity interaction experiments.


title: Quantum Zeno phenomena: pulsed versus continuous measurement

abstract:
  The time evolution of an unstable quantum mechanical system coupled with an
external measuring agent is investigated. According to the features of the
interaction Hamiltonian, a quantum Zeno effect (hindered decay) or an inverse
quantum Zeno effect (accelerated decay) can take place, depending on the
response time of the apparatus. The transition between the two regimes is
analyzed for both pulsed and continuous measurements.


title: Decoherence and fluctuations in quantum interference experiments

abstract:
  We analyze the notion of quantum coherence in an interference experiment. We
let the phase shifts fluctuate according to a given statistical distribution
and introduce a decoherence parameter, defined in terms of a generalized
visibility of the interference pattern. One might naively expect that a
particle ensemble suffers a greater loss of quantum coherence by interacting
with an increasingly randomized distribution of shifts. As we shall see, this
is not always true.


title: Exact solutions of effective mass Schroedinger equations

abstract:
  We outline a general method of obtaining exact solutions of Schroedinger
equations with a position dependent effective mass. Exact solutions of several
potentials including shape invariant potentials have also been obtained.


title: Classical and quantum dynamics of a particle constrained on a circle

abstract:
  The Dirac method is used to analyze the classical and quantum dynamics of a
particle constrained on a circle. The method of Lagrange multipliers is
scrutinized, in particular in relation to the quantization procedure. Ordering
problems are tackled and solved by requiring the hermiticity of some operators.
The presence of an additional term in the quantum Hamiltonian is discussed.


title: A necessary and sufficient condition for optimal decompositions

abstract:
  An important measure of bipartite entanglement is the entanglement of
formation, which is defined as the minimum average pure state entanglement of
all decompositions realizing a given state. A decomposition which achieves this
minimum is called an optimal decomposition. However, as for the entanglement of
formation, there is not much known about the structure of such optimal
decompositions, except for some special cases, like states of two qubits or
isotropic states. Here we present a necessary and sufficient condition for a
set of pure states of a finite dimensional bipartite system to form an optimal
decomposition. This condition is well suited to treat the question, whether the
entanglement of formation is additive or not.


title: Kerr cat states from the four-photon Jaynes-Cummings model

abstract:
  We investigate the dynamics of a four-photon Jaynes-Cummings model for large
photon number. It is shown that at certain times the cavity field is in a pure
state which is a superposition of two Kerr states, analogous to the
Schr\"{o}dinger cat state (superposition of two coherent states) which occurs
in the one and two photon cases.


title:
A cesium gas strongly confined in one dimension : sideband cooling and
  collisional properties

abstract:
  We study one-dimensional sideband cooling of Cesium atoms strongly confined
in a far-detuned optical lattice. The Lamb-Dicke regime is achieved in the
lattice direction whereas the transverse confinement is much weaker. The
employed sideband cooling method, first studied by Vuletic et al.\cite{Vule98},
uses Raman transitions between Zeeman levels and produces a spin-polarized
sample. We present a detailed study of this cooling method and investigate the
role of elastic collisions in the system. We accumulate $83(5)%$ of the atoms
in the vibrational ground state of the strongly confined motion, and elastic
collisions cool the transverse motion to a temperature of $2.8 \mu $K=$0.7
\hbar\omega_{\rm osc}/k_{\rm B}$, where $\omega_{\rm osc}$ is the oscillation
frequency in the strongly confined direction. The sample then approaches the
regime of a quasi-2D cold gas. We analyze the limits of this cooling method and
propose a dynamical change of the trapping potential as a mean of cooling the
atomic sample to still lower temperatures. Measurements of the rate of
thermalization between the weakly and strongly confined degrees of freedom are
compatible with the zero energy scattering resonance observed previously in
weak 3D traps. For the explored temperature range the measurements agree with
recent calculations of quasi-2D collisions\cite{Petr01}. Transparent analytical
models reproduce the expected behavior for $k_{\rm B}T \gg \hbar \omega_{\rm
osc}$ and also for $k_{\rm B}T \ll \hbar \omega_{\rm osc}$ where the 2D
features are prominent.


title: The Secrecy Capacity of Practical Quantum Cryptography

abstract:
  Quantum cryptography has attracted much recent attention due to its potential
for providing secret communications that cannot be decrypted by any amount of
computational effort. This is the first analysis of the secrecy of a practical
implementation of the BB84 protocol that simultaneously takes into account and
presents the {\it full} set of complete analytical expressions for effects due
to the presence of pulses containing multiple photons in the attenuated output
of the laser, the finite length of individual blocks of key material, losses
due to error correction, privacy amplification, continuous authentication,
errors in polarization detection, the efficiency of the detectors, and
attenuation processes in the transmission medium. The analysis addresses
eavesdropping attacks on individual photons rather than collective attacks in
general. Of particular importance is the first derivation of the necessary and
sufficient amount of privacy amplification compression to ensure secrecy
against the loss of key material which occurs when an eavesdropper makes
optimized individual attacks on pulses containing multiple photons. It is shown
that only a fraction of the information in the multiple photon pulses is
actually lost to the eavesdropper.


title: Constraints on Eavesdropping on the BB84 Protocol

abstract:
  An undetected eavesdropping attack must produce count rate statistics that
are indistinguishable from those that would arise in the absence of such an
attack. In principle this constraint should force a reduction in the amount of
information available to the eavesdropper. In this paper we illustrate, by
considering a particular class of eavesdropping attacks, how the general
analysis of this problem may proceed.


title: The Geometry of Single-Qubit Maps

abstract:
  The physically allowed quantum evolutions on a single qubit can be described
in terms of their geometry. From a simple parameterisation of unital
single-qubit channels, the canonical form of all such channels can be given.
The related geometry can be used to understand how to approximate positive maps
by completely-positive maps, such as in the case of optimal eavesdropping
strategies. These quantum channels can be generated by the appropriate network
or through dynamical means. The Str{\o}mer-Woronowisc result can also be
understood in terms of this geometry.


title:
Conditional Dynamics of Open Quantum Systems: Conditional Dynamics of
  Open Quantum Systems: the Case of Multiple Observers

abstract:
  Many observers can simultaneously measure different parts of an environment
of a quantum system in order to find out its state. To study this problem we
generalize the formalism of conditional master equations to the multiple
observer case. To settle some issues of principle which arise in this context
(as the state of the system and of the environment are ultimately correlated),
we consider an example of a system qubit interacting through controlled nots
(CNOTs) with environmental qubits. The state of the system is the easiest to
find out for observers who measure in a basis of the environment which is most
correlated with the pointer basis of the system. In this case the observers
agree the most. Furthermore, the more predictable the pointers are, the easier
it is to find the state of the system, and the better is the agreement between
different observers.


title: N-fold Supersymmetry in Quantum Mechanics - General Formalism -

abstract:
  We report general properties of N-fold supersymmetry in one-dimensional
quantum mechanics. N-fold supersymmetry is characterized by supercharges which
are N-th polynomials of momentum. Relations between the anti-commutator of the
supercharges and the Hamiltonian, the spectra, the Witten index, the
non-renormalization theorems and the quasi-solvability are examined. We also
present further investigation about a particular class of N-fold supersymmetric
models which we dubbed type A. Algebraic equations which determine a part of
spectra of type A models are presented, and the non-renormalization theorem are
generalized. Finally, we present a possible generalization of N-fold
supersymmetry in multi-dimensional quantum mechanics.


title: On twin observables in entangled mixed states

abstract:
  It is pointed out that every mixed state statistical operator is, up to a
normalization constant, a super state vector in the Hilbert space of linear
Hilbert-Schmidt operators acting in the state space of the quantum system.
Hence, the well understood Schmidt canonical expansion of ordinary state
vectors can be carried over to mixed states. In particular, it can be utilized
for evaluating all the twins, i. e., the opposite-subsystem observables the
measurement of one of which is, on account of entanglement, ipso facto also a
measurement of the other. This is illustrated in full detail in the case of the
Horodecki two spin-one-half-particle states with maximally disordered
subsystems.


title: Regularization of a three-body problem with zero-range potentials

abstract:
  We propose a coordinate-space regularization of the three-body problem with
zero-range potentials. We include the effective range and the shape parameter
in the boundary condition of the zero-range potential. The proposed extended
zero-range model is tested against atomic helium trimers and is shown to
provide an adequate quantitative description of these systems.


title: Remarks on Noncontextual Hidden Variables and Physical Measurements

abstract:
  Kent [quant-ph/9906006] has constructed a hidden variable theory by taking
the finite precision of physical measurements into account. But its claim to
noncontextuality has been queried, and it shown here that there is a
particularly simple way of seeing why it cannot be noncontextual.


title:
The role of auxiliary states in state discrimination with linear optical
  evices

abstract:
  The role of auxiliary photons in the problem of identifying a state secretly
chosen from a given set of L-photon states is analyzed. It is shown that
auxiliary photons do not increase the ability to discriminate such states by
means of a global measurement using only optical linear elements, conditional
transformation and auxiliary photons.


title: Properties of Entanglement Monotones for Three-Qubit Pure States

abstract:
  Various parameterizations for the orbits under local unitary transformations
of three-qubit pure states are analyzed. The interconvertibility, symmetry
properties, parameter ranges, calculability and behavior under measurement are
looked at. It is shown that the entanglement monotones of any multipartite pure
state uniquely determine the orbit of that state under local unitary
transformations. It follows that there must be an entanglement monotone for
three-qubit pure states which depends on the Kempe invariant defined in [Phys.
Rev. A 60, 910 (1999)]. A form for such an entanglement monotone is proposed. A
theorem is proved that significantly reduces the number of entanglement
monotones that must be looked at to find the maximal probability of
transforming one multipartite state to another.


title: Secrecy, Computational Loads and Rates in Practical Quantum Cryptography

abstract:
  A number of questions associated with practical implementations of quantum
cryptography systems having to do with unconditional secrecy, computational
loads and effective secrecy rates in the presence of perfect and imperfect
sources are discussed. The different types of unconditional secrecy, and their
relationship to general communications security, are discussed in the context
of quantum cryptography. In order to actually carry out a quantum cryptography
protocol it is necessary that sufficient computational resources be available
to perform the various processing steps, such as sifting, error correction,
privacy amplification and authentication. We display the full computer machine
instruction requirements needed to support a practical quantum cryptography
implementation. We carry out a numerical comparison of system performance
characteristics for implementations that make use of either weak coherent
sources of light or perfect single photon sources, for eavesdroppers making
individual attacks on the quantum channel characterized by different levels of
technological capability. We find that, while in some circumstances it is best
to employ perfect single photon sources, in other situations it is preferable
to utilize weak coherent sources. In either case the secrecy level of the final
shared cipher is identical, with the relevant distinguishing figure-of-merit
being the effective throughput rate.


title:
Modelling the Recoherence of Mesoscopic Superpositions in Dissipative
  Environments

abstract:
  A model is presented to describe the recently proposed experiment (J.
Raimond,
  M. Brune and S. Haroche Phys. Rev. Lett {\bf 79}, 1964 (1997)) where a
mesoscopic superposition of radiation states is prepared in a high-Q cavity
which is coupled to a similar resonator. The dynamical coherence loss of such
state in the absence of dissipation is reversible and can in principle be
observed. We show how this picture is modified due to the presence of the
environmental couplings. Analytical expressions for the experimental
conditional probabilities and the linear entropy are given. We conclude that
the phenomenon can still be observed provided the ratio between the damping
constant and the inter-cavities coupling does not exceed about a few percent.
This observation is favored for superpositions of states with large overlap.


title: New Developments in the Casimir Effect

abstract:
  We provide a review of both new experimental and theoretical developments in
the Casimir effect. The Casimir effect results from the alteration by the
boundaries of the zero-point electromagnetic energy. Unique to the Casimir
force is its strong dependence on shape, switching from attractive to repulsive
as function of the size, geometry and topology of the boundary. Thus the
Casimir force is a direct manifestation of the boundary dependence of quantum
vacuum.
  We discuss in depth the general structure of the infinities in the field
theory which are removed by a combination of zeta-functional regularization and
heat kernel expansion. Different representations for the regularized vacuum
energy are given. The Casimir energies and forces in a number of configurations
of interest to applications are calculated. We stress the development of the
Casimir force for real media including effects of nonzero temperature, finite
conductivity of the boundary metal and surface roughness. Also the combined
effect of these important factors is investigated in detail on the basis of
condensed matter physics and quantum field theory at nonzero temperature.
  The experiments on measuring the Casimir force are also reviewed, starting
first with the older measurements and finishing with a detailed presentation of
modern precision experiments. The latter are accurately compared with the
theoretical results for real media.
  At the end of the review we provide the most recent constraints on the
corrections to Newtonian gravitational law and other hypothetical long-range
interactions at submillimeter range obtained from the Casimir force
measurements.


title:
Role of causality in ensuring unconditional security of relativistic
  quantum cryptography

abstract:
  The problem of unconditional security of quantum cryptography (i.e. the
security which is guaranteed by the fundamental laws of nature rather than by
technical limitations) is one of the central points in quantum information
theory. We propose a relativistic quantum cryptosystem and prove its
unconditional security against any eavesdropping attempts. Relativistic
causality arguments allow to demonstrate the security of the system in a simple
way. Since the proposed protocol does not employ collective measurements and
quantum codes, the cryptosystem can be experimentally realized with the present
state-of-art in fiber optics technologies. The proposed cryptosystem employs
only the individual measurements and classical codes and, in addition, the key
distribution problem allows to postpone the choice of the state encoding scheme
until after the states are already received instead of choosing it before
sending the states into the communication channel (i.e. to employ a sort of
``antedate'' coding).


title: Bound states in point-interaction star-graphs

abstract:
  We discuss the discrete spectrum of the Hamiltonian describing a
two-dimensional quantum particle interacting with an infinite family of point
interactions. We suppose that the latter are arranged into a star-shaped graph
with N arms and a fixed spacing between the interaction sites. We prove that
the essential spectrum of this system is the same as that of the infinite
straight "polymer", but in addition there are isolated eigenvalues unless N=2
and the graph is a straight line. We also show that the system has many
strongly bound states if at least one of the angles between the star arms is
small enough. Examples of eigenfunctions and eigenvalues are computed
numerically.


title: Quantum thermodynamic cooling cycle

abstract:
  The quantum-mechanical and thermodynamic properties of a 3-level molecular
cooling cycle are derived. An inadequacy of earlier models is rectified in
accounting for the spontaneous emission and absorption associated with the
coupling to the coherent driving field via an environmental reservoir. This
additional coupling need not be dissipative, and can provide a thermal driving
force - the quantum analog of classical absorption chillers. The dependence of
the maximum attainable cooling rate on temperature, at ultra-low temperatures,
is determined and shown to respect the recently-established fundamental bound
based on the second and third laws of thermodynamics.


title: Quantum Key Distribution using Multilevel Encoding: Security Analysis

abstract:
  We present security proofs for a protocol for Quantum Key Distribution (QKD)
based on encoding in finite high-dimensional Hilbert spaces. This protocol is
an extension of Bennett's and Brassard's basic protocol from two bases, two
state encoding to a multi bases, multi state encoding. We analyze the mutual
information between the legitimate parties and the eavesdropper, and the error
rate, as function of the dimension of the Hilbert space, while considering
optimal incoherent and coherent eavesdropping attacks. We obtain the upper
limit for the legitimate party error rate to ensure unconditional security when
the eavesdropper uses incoherent and coherent eavesdropping strategies. We have
also consider realistic noise caused by detector's noise.


title: Dissipative quantum theory: Implications for quantum entanglement

abstract:
  Three inter-related topics are discussed here. One, the Lindblad dynamics of
quantum dissipative systems; two, quantum entanglement in composite systems and
its quantification based on the Tsallis entropy; and three, robustness of
entanglement under dissipation. After a brief review of the Lindblad theory of
quantum dissipative systems and the idea of quantum entanglement in composite
quantum systems illustrated by describing the three particle systems, the
behavior of entanglement under the influence of dissipative processes is
discussed. These issues are of importance in the discussion of quantum
nanometric systems of current research.


title: Schroedinger-picture correlation functions for nonlinear evolutions

abstract:
  The well known interpretational difficulties with nonlinear Schr\"odinger and
von Neumann equations can be reduced to the problem of computing multiple-time
correlation functions in the absence of Heisenberg picture. Having no
Heisenberg picture one often resorts to Zeno-type reasoning which explicitly
involves the projection postulate as a means of computing conditional and joint
probabilities. Although the method works well in linear quantum mechanics, it
completely fails for nonlinear evolutions. We propose an alternative way of
performing the same task in linear quantum mechanics and show that the method
smoothly extends to the nonlinear domain. The trick is to use appropriate
time-dependent Hamiltonians which involve "switching-off functions". We apply
the technique to the EPR problem in nonlinear quantum mechanics and show that
paradoxes of Gisin and Polchinski disappear.


title:
Entanglement-assisted capacity of a quantum channel and the reverse
  Shannon theorem

abstract:
  The entanglement-assisted classical capacity of a noisy quantum channel is
the amount of information per channel use that can be sent over the channel in
the limit of many uses of the channel, assuming that the sender and receiver
have access to the resource of shared quantum entanglement, which may be used
up by the communication protocol. We show that this capacity is given by an
expression parallel to that for the capacity of a purely classical channel:
i.e., the maximum, over channel inputs $\rho$, of the entropy of the channel
input plus the entropy of the channel output minus their joint entropy, the
latter being defined as the entropy of an entangled purification of $\rho$
after half of it has passed through the channel. We calculate
entanglement-assisted capacities for two interesting quantum channels, the
qubit amplitude damping channel and the bosonic channel with
amplification/attenuation and Gaussian noise. We discuss how many independent
parameters are required to completely characterize the asymptotic behavior of a
general quantum channel, alone or in the presence of ancillary resources such
as prior entanglement. In the classical analog of entanglement assisted
communication--communication over a discrete memoryless channel (DMC) between
parties who share prior random information--we show that one parameter is
sufficient, i.e., that in the presence of prior shared random information, all
DMC's of equal capacity can simulate one another with unit asymptotic
efficiency.


title: Atomic position localization via dual measurement

abstract:
  We study localization of atomic position when a three-level atom interacts
with a quantized standing-wave field in the Ramsey interferometer setup. Both
the field quadrature amplitude and the atomic internal state are measured to
obtain the atomic position information. It is found that this dual measurement
scheme produces an interference pattern superimposed on a diffraction-like
pattern in the atomic position distribution, where the former pattern
originates from the state-selective measurement and the latter from the field
measurement. The present scheme results in a better resolution in the position
localization than the field-alone measurement schemes. We also discuss the
measurement-correlated mechanical action of the standing-wave field on the atom
in the light of Popper's test.


title: Squeezing enhancement by damping in a driven atom-cavity system

abstract:
  In a driven atom-cavity coupled system in which the two-level atom is driven
by a classical field, the cavity mode which should be in a coherent state in
the absence of its reservoir, can be squeezed by coupling to its reservoir. The
squeezing effect is enhanced as the damping rate of the cavity is increased to
some extent.


title: Analysis of 1 and 2 Particle Quantum Systems using Geometric Algebra

abstract:
  When two or more subsystems of a quantum system interact with each other they
can become entangled. In this case the individual subsystems can no longer be
described as pure quantum states. For systems with only 2 subsystems this
entanglement can be described using the Schmidt decomposition. This selects a
preferred orthonormal basis for expressing the wavefunction and gives a measure
of the degree of entanglement present in the system. The extension of this to
the more general case of n subsystems is not yet known. We present a review of
this process using the standard representation and apply this method to the
geometric algebra representation. This latter form has the advantage of
suggesting a generalisation to n subsystems.


title: Stability of mixed Nash equilibria in symmetric quantum games

abstract:
  In bi-matrix games the Bishop-Cannings theorem of the classical evolutionary
game theory does not permit pure evolutionarily stable strategies (ESSs) when a
mixed ESS exists. We find the necessary form of two-qubit initial quantum
states when a switch-over to a quantum version of the game also changes the
evolutionary stability of a mixed symmetric Nash equilibrium.


title: Experimental long-lived entanglement of two macroscopic objects

abstract:
  Entanglement is considered to be one of the most profound features of quantum
mechanics. An entangled state of a system consisting of two subsystems cannot
be described as a product of the quantum states of the two subsystems. In this
sense the entangled system is considered inseparable and nonlocal. It is
generally believed that entanglement manifests itself mostly in systems
consisting of a small number of microscopic particles. Here we demonstrate
experimentally the entanglement of two objects, each consisting of about 10^12
atoms. Entanglement is generated via interaction of the two objects - more
precisely, two gas samples of cesium atoms - with a pulse of light, which
performs a non-local Bell measurement on collective spins of the samples. The
entangled spin state can be maintained for 0.5 millisecond. Besides being of
fundamental interest, the robust, long-lived entanglement of material objects
demonstrated here is expected to be useful in quantum information processing,
including teleportation of quantum states of matter and quantum memory.


title: Entanglement sharing among qudits

abstract:
  Consider a system consisting of n d-dimensional quantum particles (qudits),
and suppose that we want to optimize the entanglement between each pair. One
can ask the following basic question regarding the sharing of entanglement:
what is the largest possible value Emax(n,d) of the minimum entanglement
between any two particles in the system? (Here we take the entanglement of
formation as our measure of entanglement.) For n=3 and d=2, that is, for a
system of three qubits, the answer is known: Emax(3,2) = 0.550. In this paper
we consider first a system of d qudits and show that Emax(d,d) is greater than
or equal to 1. We then consider a system of three particles, with three
different values of d. Our results for the three-particle case suggest that as
the dimension d increases, the particles can share a greater fraction of their
entanglement capacity.


title:
Multistability and Critical Fluctuation in a Split Bose-Einstein
  Condenstate

abstract:
  By using a two-mode description, we show that there exist the multistability,
phase transition and associated critical fluctuations in the macroscopic
tunneling process between the halves of a double-well trap containing a
Bose-Einstein condenstate. The phase transition that two of the triple stable
states and a unstable state merge into one stable state or a reverse process
takes place whenever the ratio of the mean field energy per particle to the
tunneling energy goes across a critical value of order one. The critical
fluctuation phenomenon corresponds to squeezed states for the phase difference
between the two wells accompanying with large fluctuations of atom numbers.


title:
Exciting Relative Number Squeezed Particles from Condensates Using
  Stimulated Light Scattering

abstract:
  We investigate the relative particle number squeezing produced in the excited
states of a weakly interacting condensate at zero temperature by stimulated
light scattering using a pair of lasers. We shall show that a modest number of
relative number squeezed particles can be achieved when atoms with momentum
$k$, produced in pairs through collisions in the condensate, are scattered out
by their interaction with the lasers. This squeezing is optimal when the
momentum $k$ is larger than the inverse healing length, $k>k_0$. This modest
number of relative number squeezed particles has the potential to be amplified
in four-wave-mixing experiments.


title:
Four-path interference and uncertainty principle in photodetachment
  microscopy

abstract:
  We study the quantal motion of electrons emitted by a pointlike monochromatic
isotropic source into parallel uniform electric and magnetic fields. The
two-path interference pattern in the emerging electron wave due to the electric
force is modified by the magnetic lens effect which periodically focuses the
beam into narrow filaments along the symmetry axis. There, four classical paths
interfere. With increasing electron energy, the current distribution changes
from a quantum regime governed by the uncertainty principle, to an intricate
spatial pattern that yields to a semiclassical analysis.


title: What do we know about wave function nodes?

abstract:
  Although quantum Monte Carlo is, in principal, an exact method for solving
the Schroedinger equation, it is well-known that systems of Fermions still pose
a challenge. Thus far all solutions to the "sign problem" remain inefficient
(or wrong). The fixed-node approach, however, is efficient, and in many
situations remains the best approach. If only we could find the exact nodes, or
at least a systematic way to improve the nodes, we would, in effect, bypass the
sign problem. Unfortunately, very little is known about wave function nodes,
and a systematic study has never been attempted, despite the obvious
consequences for improving quantum simulations that such knowledge might
generate. In this paper we study the nodal surfaces of simple atomic systems.


title: Interaction and Entanglement in the Multiparticle Spacetime Algebra

abstract:
  The multiparticle spacetime algebra (MSTA) is an extension of Dirac theory to
a multiparticle setting, which was first studied by Doran, Gull and Lasenby.
The geometric interpretation of this algebra, which it inherits from its
one-particle factors, possesses a number of physically compelling features,
including simple derivations of the Pauli exclusion principle and other
nonlocal effects in quantum physics. Of particular importance here is the fact
that all the operations needed in the quantum (statistical) mechanics of spin
1/2 particles can be carried out in the ``even subalgebra'' of the MSTA. This
enables us to ``lift'' existing results in quantum information theory regarding
entanglement, decoherence and the quantum/classical transition to space-time.
The full power of the MSTA and its geometric interpretation can then be used to
obtain new insights into these foundational issues in quantum theory. A system
of spin 1/2 particles located at fixed positions in space, and interacting with
an external magnetic field and/or with one another via their intrinsic magnetic
dipoles provides a simple paradigm for the study of these issues. This paradigm
can further be easily realized and studied in the laboratory by nuclear
magnetic resonance spectroscopy.


title:
Universal quantum computation and simulation using any entangling
  Hamiltonian and local unitaries

abstract:
  What interactions are sufficient to simulate arbitrary quantum dynamics in a
composite quantum system? We provide an efficient algorithm to simulate any
desired two-body Hamiltonian evolution using any fixed two-body entangling
n-qubit Hamiltonian and local unitaries. It follows that universal quantum
computation can be performed using any entangling interaction and local unitary
operations.


title: Spin Squeezing in the Ising Model

abstract:
  We analyze the collective spin noise in interacting spin systems. General
expressions are derived for the short time behaviour of spin systems with
general spin-spin interactions, and we suggest optimum experimental conditions
for the detection of spin squeezing. For Ising models with site dependent
nearest neighbour interactions general expressions are presented for the spin
squeezing parameter for all times. The reduction of collective spin noise can
be used to verify the entangling powers of quantum computer architectures based
on interacting spins.


title: Quantum memory for photons: I. Dark state polaritons

abstract:
  An ideal and reversible transfer technique for the quantum state between
light and metastable collective states of matter is presented and analyzed in
detail. The method is based on the control of photon propagation in coherently
driven 3-level atomic media, in which the group velocity is adiabatically
reduced to zero. Form-stable coupled excitations of light and matter
(``dark-state polaritons'') associated with the propagation of quantum fields
in Electromagnetically Induced Transparency are identified, their basic
properties discussed and their application for quantum memories for light
analyzed.


title: Quantum Computing and Nuclear Magnetic Resonance

abstract:
  Quantum information processing is the use of inherently quantum mechanical
phenomena to perform information processing tasks that cannot be achieved using
conventional classical information technologies. One famous example is quantum
computing, which would permit calculations to be performed that are beyond the
reach of any conceivable conventional computer. Initially it appeared that
actually building a quantum computer would be extremely difficult, but in the
last few years there has been an explosion of interest in the use of techniques
adapted from conventional liquid state nuclear magnetic resonance (NMR)
experiments to build small quantum computers. After a brief introduction to
quantum computing I will review the current state of the art, describe some of
the topics of current interest, and assess the long term contribution of NMR
studies to the eventual implementation of practical quantum computers capable
of solving real computational problems.


title: K-quantum Nonlinear Jaynes-Cummings Model in Two Trapped Ions

abstract:
  A k-quantum nonlinear Jaynes-Cummings model for two trapped ions interacting
with laser beams resonant to k-th red side-band of center-of-mass mode, far
from Lamb-Dicke regime, has been obtained. The exact analytic solution showed
the existence of quantum collapses and revivals of the occupation of two atoms.


title: Non-commutative space-time and the uncertainty principle

abstract:
  The full algebra of relativistic quantum mechanics (Lorentz plus Heisenberg)
is unstable. Stabilization by deformation leads to a new deformation parameter
$\epsilon \ell ^{2}$, $\ell $ being a length and $\epsilon$ a $\pm$ sign. The
implications of the deformed algebras for the uncertainty principle and the
density of states are worked out and compared with the results of past analysis
following from gravity and string theory.


title: Optimal Tight Frames and Quantum Measurement

abstract:
  Tight frames and rank-one quantum measurements are shown to be intimately
related. In fact, the family of normalized tight frames for the space in which
a quantum mechanical system lies is precisely the family of rank-one
generalized quantum measurements (POVMs) on that space. Using this
relationship, frame-theoretical analogues of various quantum-mechanical
concepts and results are developed.
  The analogue of a least-squares quantum measurement is a tight frame that is
closest in a least-squares sense to a given set of vectors. The least-squares
tight frame is found for both the case in which the scaling of the frame is
specified (constrained least-squares frame (CLSF)) and the case in which the
scaling is free (unconstrained least-squares frame (ULSF)). The well-known
canonical frame is shown to be proportional to the ULSF and to coincide with
the CLSF with a certain scaling.
  Finally, the canonical frame vectors corresponding to a geometrically uniform
vector set are shown to be geometrically uniform and to have the same
symmetries as the original vector set.


title: Grover Algorithm with zero theoretical failure rate

abstract:
  In standard Grover's algorithm for quantum searching, the probability of
finding the marked item is not exactly 1. In this Letter we present a modified
version of Grover's algorithm that searches a marked state with full successful
rate. The modification is done by replacing the phase inversion by two phase
rotation through angle $\phi$. The rotation angle is given analytically to be
$\phi=2 \arcsin(\sin{\pi\over (4J+6)}\over \sin\beta)$, where
$\sin\beta={1\over \sqrt{N}}$, $N$ the number of items in the database, and $J$
an integer equal to or greater than the integer part of $({\pi\over
2}-\beta)/(2\beta)$. Upon measurement at $(J+1)$-th iteration, the marked state
is obtained with certainty.


title: "Quantal" behavior in classical probability

abstract:
  A number of phenomena generally believed characteristic of quantum mechanics
and seen as interpretively problematic--the incompatibility and
value-indeterminacy of variables, the non-existence of dispersion-free states,
the failure of the standard marginal-probability formula, the failure of the
distributive law of disjunction and interference--are exemplified in an
emphatically non-quantal system: a deck of playing cards. Thus the appearance,
in quantum mechanics, of incompatibility and these associated phenomena
requires neither explanation nor interpretation.


title: `Quantum probabilities' as context depending probabilities

abstract:
  We study transformations of conventional (`classical') probabilities induced
by context transitions. It is demonstrated that the transition from one complex
of conditions to another induces a perturbation of the classical rule for the
addition of probabilistic alternatives. We classify such perturbations. It is
shown that there are two classes of perturbations: (a) trigonometric
interference; (b) hyperbolic interference. In particular, the well known
`quantum interference of probabilistic alternatives' can be obtained in
classical (but contextual) probabilistic framework. Therefore we need not apply
to wave arguments (or consider superposition principle) to get Quantum
Statistics. In particular, interference could be a feature of experiments with
purely corpuscular objects.


title: Relativistic Quantum Theory of Cyclotron Resonance in a Medium

abstract:
  In this paper the relativistic quantum theory of cyclotron resonance in an
arbitrary medium is presented. The quantum equation of motion for charged
particle in the field of plane electromagnetic wave and uniform magnetic field
in a medium is solved in the eikonal approximation. The probabilities of
induced multiphoton transitions between Landau levels in strong laser field is
calculated.


title: On entanglement-assisted classical capacity

abstract:
  This paper is essentially a lecture from the author's course on quantum
information theory, which is devoted to the result of C. H. Bennett, P. W.
Shor, J. A. Smolin and A. V. Thapliyal (quant-ph/0106052) concerning
entanglement-assisted classical capacity of a quantum channel. A modified proof
of this result is given and relation between entanglement-assisted and
unassisted classical capacities is discussed.


title: Magnetically induced vacuum decay

abstract:
  We study the fermionic vacuum energy of vacua with and without being applied
an external magnetic field. The energetic difference of two vacua leads to the
vacuum decaying and the vacuum-energy releasing. In the context of quantum
field theories, we discuss why and how the vacuum energy can be released by
spontaneous photon emissions and/or paramagnetically screening the external
magnetic field. In addition, we quantitatively compute the vacuum energy
released, the paramagnetic screening effect and the rate and spectrum of
spontaneous photon emissions. The possibilities of experimentally detecting
such an effect of vacuum-energy releasing and this effect accounting for the
anormalous X-ray pulsar are discussed.


title:
Simulating Arbitrary Pair-Interactions by a Given Hamiltonian:
  Graph-Theoretical Bounds on the Time Complexity

abstract:
  We use an n-spin system with permutation symmetric zz-interaction for
simulating arbitrary pair-interaction Hamiltonians. The calculation of the
required time overhead is mathematically equivalent to a separability problem
of n-qubit density matrices. We derive lower and upper bounds in terms of
chromatic index and the spectrum of the interaction graph. The complexity
measure defined by such a computational model is related to gate complexity and
a continuous complexity measure introduced in a former paper. We use
majorization of graph spectra for classifying Hamiltonians with respect to
their computational power.


title: A double-slit quantum eraser

abstract:
  We report a quantum eraser experiment which actually uses a Young double-slit
to create interference. The experiment can be considered an optical analogy of
an experiment proposed by Scully, Englert and Walther. One photon of an
entangled pair is incident on a Young double-slit of appropriate dimensions to
create an interference pattern in a distant detection region. Quarter-wave
plates, oriented so that their fast axes are orthogonal, are placed in front of
each slit to serve as which-path markers. The quarter-wave plates mark the
polarization of the interfering photon and thus destroy the interference
pattern. To recover interference, we measure the polarization of the other
entangled photon. In addition, we perform the experiment under delayed erasure
circumstances.


title: Lorentz transformations of open systems

abstract:
  We consider open dynamical systems, subject to external interventions by
agents that are not completely described by the theory (classical or quantal).
These interventions are localized in regions that are relatively spacelike.
Under these circumstances, no relativistic transformation law exists that
relates the descriptions of the physical system by observers in relative
motion. Still, physical laws are the same in all Lorentz frames.


title:
Classical capacity of a noiseless quantum channel assisted by noisy
  entanglement

abstract:
  We derive the general formula for the capacity of a noiseless quantum channel
assisted by an arbitrary amount of noisy entanglement. In this capacity
formula, the ratio of the quantum mutual information and the von Neumann
entropy of the sender's share of the noisy entanglement plays the role of
mutual information in the completely classical case. A consequence of our
results is that bound entangled states cannot increase the capacity of a
noiseless quantum channel.


title:
Nonstationary Casimir Effect and analytical solutions for quantum fields
  in cavities with moving boundaries

abstract:
  This is a review of publications on classical and quantum electrodynamics in
cavities with moving boundaries (in the quantum case this subject is labeled
frequently as "nonstationary Casimir effect" or "dynamical Casimir effect"),
from 1921 to October of 2000, with an emphasis on analytical results related to
cavities with resonantly oscillating boundaries.


title:
Entanglement and quantal coherence: a study of two limiting cases of
  rapid system--bath interactions

abstract:
  We consider the dynamics of a system coupled to a thermal bath, going beyond
the standard two-level system through the addition of an energy excitation
degree of freedom. Further extensions are to systems containing many fermions,
with the master equations modified to take Fermi-Dirac statistics into account,
and to potentials with a time-dependent bias that induce resonant avoided
crossing transitions. The limit $Q \to \infty$, where the interaction rate with
the bath is much greater than all free oscillation rates for the system, is
interrogated. Two behaviors are possible: freezing (quantum Zeno effect) or
synchronization (motional narrowing). We clarify the conditions that give rise
to each possibility, making an explicit connection with quantum measurement
theory. We compare the evolution of quantal coherence for the two cases as a
function of $Q$, noting that full coherence is restored as $Q \to \infty$.
Using an extended master equation, the effect of system-bath interactions on
entanglement in bipartite system states is computed. In particular, we show
that the sychronization case sees bipartite system entanglement fully preserved
in the large $Q$ limit.


title: `Circular type' quantum key distribution

abstract:
  `Circular type' interferometric system for quantum key distribution is
proposed. The system has naturally self-alignment and compensation of
birefringence and also has enough efficiency against polarisation dependence.
Moreover it is easily applicable to multi-party. Key creation with 0.1 photon
per pulse at a rate of 1.2KHz with a 5.4% QBER over a 200m fiber was realized.


title: The Pegg-Barnett Formalism and Covariant Phase Observables

abstract:
  We compare the Pegg-Barnett (PB) formalism with the covariant phase
observable approach to the problem of quantum phase and show that PB-formalism
gives essentially the same results as the canonical (covariant) phase
observable. We also show that PB-formalism can be extended to cover all
covariant phase observables including the covariant phase observable arising
from the angle margin of the Husimi Q-function.


title:
Complexity of decoupling and time-reversal for n spins with
  pair-interactions: Arrow of time in quantum control

abstract:
  Well-known Nuclear Magnetic Resonance experiments show that the time
evolution according to (truncated) dipole-dipole interactions between n spins
can be inverted by simple pulse sequences. Independent of n, the reversed
evolution is only two times slower than the original one. Here we consider more
general spin-spin couplings with long range. We prove that some are
considerably more complex to invert since the number of required time steps and
the slow-down of the reversed evolutions are necessarily of the order n.
Furthermore, the spins have to be addressed separately. We show for which
values of the coupling parameters the phase transition between simple and
complex time-reversal schemes occurs.


title: Removal of a single photon by adaptive absorption

abstract:
  We present a method to remove, using only linear optics, exactly one photon
from a field-mode. This is achieved by putting the system in contact with an
absorbing environment which is under continuous monitoring. A feedback
mechanism then decouples the system from the environment as soon as the first
photon is absorbed. We propose a possible scheme to implement this process and
provide the theoretical tools to describe it.


title: On the Existence of the Quantum Action

abstract:
  We have previously proposed a conjecture stating that quantum mechanical
transition amplitudes can be parametrized in terms of a quantum action. Here we
give a proof of the conjecture and establish the existance of a local quantum
action in the case of imaginary time in the Feynman-Kac limit (when temperature
goes to zero). Moreover we discuss some symmetry properties of the quantum
action.


title: Probabilistic implementation of universal quantum processors

abstract:
  We present a probabilistic quantum processor for qudits. The processor itself
is represented by a fixed array of gates. The input of the processor consists
of two registers. In the program register the set of instructions (program) is
encoded. This program is applied to the data register. The processor can
perform any operation on a single qudit of the dimension N with a certain
probability. If the operation is unitary, the probability is in general 1/N^2,
but for more restricted sets of operators the probability can be higher. In
fact, this probability can be independent of the dimension of the qudit Hilbert
space of the qudit under some conditions.


title: Towards a tomographic picture of a Bose-Einstein condensate

abstract:
  We investigate by means of numerical simulations the possibilities of
tomographic techniques applied to a Bose-Einstein condensate in order to
reconstruct its ground state. Essentially, two scenarios are considered for
which the density matrix elements can be retrieved from atom counting
probabilities. The methods presented here allow to distinguish among various
possible quantum states.


title:
Decoherence in a classically chaotic quantum system: entropy production
  and quantum--classical correspondence

abstract:
  We study the decoherence process for an open quantum system which is
classically chaotic (a quartic double well with harmonic driving coupled to a
sea of harmonic oscillators). We analyze the time dependence of the rate of
entropy production showing that it has two relevant regimes: For short times it
is proportional to the diffusion coefficient (fixed by the system--environment
coupling strength). For longer times (but before equilibration) it is fixed by
dynamical properties of the system (and is related to the Lyapunov exponent).
The nature of the transition time between both regimes is investigated and the
issue of quantum to classical correspondence is addressed. Finally, the impact
of the interaction with the environment on coherent tunneling is analyzed.


title:
Discrete Wigner functions and the phase space representation of quantum
  computers

abstract:
  We show how to represent the state and the evolution of a quantum computer
(or any system with an $N$--dimensional Hilbert space) in phase space. For this
purpose we use a discrete version of the Wigner function which, for arbitrary
$N$, is defined in a phase space grid of $2N\times 2N$ points. We compute such
Wigner function for states which are relevant for quantum computation. Finally,
we discuss properties of quantum algorithms in phase space and present the
phase space representation of Grover's quantum search algorithm.


title:
Non-oscillating solutions to uncoupled Ermakov systems and the
  semiclassical limit

abstract:
  The amplitude-phase formulation of the Schr\"{o}dinger equation is
investigated within the context of uncoupled Ermakov systems, whereby the
amplitude function is given by the auxiliary nonlinear equation. The classical
limit of the amplitude and phase functions is analyzed by setting up a
semiclassical Ermakov system. In this limit, it is shown that classical
quantities, such as the classical probability amplitude and the reduced action,
are obtained only when the semiclassical amplitude and the accumulated phase
are non-oscillating functions respectively of the space and energy variables.
Conversely, among the infinitely many arbitrary exact quantum amplitude and
phase functions corresponding to a given wavefunction, only the non-oscillating
ones yield classical quantities in the limit $\hbar \to 0$.


title: Algorithmic Cooling and Scalable NMR Quantum Computers

abstract:
  We present here algorithmic cooling (via polarization-heat-bath)- a powerful
method for obtaining a large number of highly polarized spins in liquid
nuclear-spin systems at finite temperature. Given that spin-half states
represent (quantum) bits, algorithmic cooling cleans dirty bits beyond the
Shannon's bound on data compression, by employing a set of rapidly
thermal-relaxing bits. Such auxiliary bits could be implemented using spins
that rapidly get into thermal equilibrium with the environment, e.g., electron
spins.
  Cooling spins to a very low temperature without cooling the environment could
lead to a breakthrough in nuclear magnetic resonance experiments, and our
``spin-refrigerating'' method suggests that this is possible.
  The scaling of NMR ensemble computers is probably the main obstacle to
building useful quantum computing devices, and our spin-refrigerating method
suggests that this problem can be resolved.


title:
Interaction representation method for Markov master equations in quantum
  optics

abstract:
  Conditions sufficient for a quantum dynamical semigroup (QDS) to be unital
are proved for a class of problems in quantum optics with Hamiltonians which
are self-adjoint polynomials of any finite order in creation and annihilation
operators. The order of the Hamiltonian may be higher than the order of
completely positive part of the formal generator of a QDS.
  The unital property of a minimal quantum dynamical semigroup implies the
uniqueness of the solution of the corresponding Markov master equation in the
class of quantum dynamical semigroups and, in the corresponding representation,
it ensures preservation of the trace or unit operator. We recall that only in
the unital case the formal generator of MME determines uniquely the
corresponding QDS.


title: Towards a Simulation of Quantum Computers by Classical Systems

abstract:
  We present a two-dimensional classical stochastic differential equation for a
displacement field of a point particle in two dimensions and show that its
components define real and imaginary parts of a complex field satisfying the
Schroedinger equation of a harmonic oscillator. In this way we derive the
discrete oscillator spectrum from classical dynamics. The model is then
generalized to an arbitrary potential. This opens up the possibility of
efficiently simulating quantum computers with the help of classical systems.


title: Langevin Equation for Particle in Thermal Photon Bath

abstract:
  The forward--backward path integral describing a charged particle moving in a
thermal bath of photons is expressed in terms of the solution of a
Langevin-type of equation. Approximate methods for solving this equation are
discussed.


title: Quantum and classical statistics of the electromagnetic zero-point field

abstract:
  A classical electromagnetic zero-point field (ZPF) analogue of the vacuum of
quantum field theory has formed the basis for theoretical investigations in the
discipline known as random or stochastic electrodynamics (SED) wherein quantum
measurements are imitated by the introduction of a stochastic classical
background EM field. Random EM fluctuations are assumed to provide
perturbations which can mimic some quantum phenomena while retaining a purely
classical basis, e.g. the Casimir force, the Van-der-Waals force, the Lamb
shift, spontaneous emission, the RMS radius of the harmonic oscillator, and the
radius of the Bohr atom. This classical ZPF is represented as a homogeneous,
isotropic ensemble of plane waves with fixed amplitudes and random phases.
Averaging over the random phases is assumed to be equivalent to taking the
ground-state expectation values of the corresponding quantum operator. We
demonstrate that this is not precisely correct by examining the statistics of
the classical ZPF in contrast to that of the EM quantum vacuum. We derive the
distribution for the individual mode amplitudes in the ground-state as
predicted by quantum field theory (QFT) and then carry out the same calculation
for the classical ZPF analogue, showing that the distributions are only in
approximate agreement, diverging as the density of k states decreases. We
introduce an alternative classical ZPF with a different stochastic character,
and demonstrate that it can exactly reproduce the statistics of the EM vacuum
of QED. Incorporated into SED, this new field is shown to give the correct (QM)
distribution for the amplitude of the ground-state of a harmonic oscillator,
suggesting the possibility of developing further successful correspondences
between SED and QED.


title: Signal-Locality in Hidden-Variables Theories

abstract:
  We prove that all deterministic hidden-variables theories, that reproduce
quantum theory for a 'quantum equilibrium' distribution of hidden variables,
predict the existence of instantaneous signals at the statistical level for
hypothetical 'nonequilibrium ensembles'. This signal-locality theorem
generalises yet another property of the pilot-wave theory of de Broglie and
Bohm. The theorem supports the hypothesis that in the remote past the universe
relaxed to a state of statistical equilibrium (at the hidden-variable level) in
which nonlocality happens to be masked by quantum noise.


title:
Sub-Riemannian Geometry and Time Optimal Control of Three Spin Systems:
  Quantum Gates and Coherence Transfer

abstract:
  Many coherence transfer experiments in Nuclear Magnetic Resonance
Spectroscopy, involving network of coupled spins, use temporary spin-decoupling
to produce desired effective Hamiltonians. In this paper, we show that
significant time can be saved in producing an effective Hamiltonian, if
spin-decoupling is avoided. We provide time optimal pulse sequences for
producing an important class of effective Hamiltonians in three spin networks.
These effective Hamiltonians are useful for coherence transfer experiments and
implementation of quantum logic gates in NMR quantum computing. It is
demonstrated that computing these time optimal pulse sequences can be reduced
to geometric problems that involve computing sub-Riemannian geodesics on
Homogeneous spaces.


title: On Using Quantum Protocols to Detect Traffic Analysis

abstract:
  We consider the problem of detecting whether an attacker measures the amount
of traffic sent over a communication channel-possibly without extracting
information about the transmitted data. A basic approach for designing a
quantum protocol for detecting a perpetual traffic analysis of this kind is
described.


title: Strong twin events in mixed-state entanglement

abstract:
  Continuing the study of mixed-state entanglement in terms of
opposite-subsystem observables the measurement of one of which amounts to the
same as that of the other (so-called twins), begun in a recent article,
so-called strong twin events, which imply biorthogonal mixing of states, are
defined and studied. It is shown that for each mixed state there exists a
Schmidt canonical (super state vector) expansion in terms of Hermitian
operators, and that it can be the continuation of the mentioned biorthogonal
mixing due to strong twins. The case of weak twins and nonhermitian Schmidt
canonical expansion is also investigated. A necessary and sufficient condition
for the existence of nontrivial twins for separable states is derived.


title: Soliton Squeezing in a Mach-Zehnder Fiber Interferometer

abstract:
  A new scheme for generating amplitude squeezed light by means of soliton
self-phase modulation is experimentally demonstrated. By injecting 180-fs
pulses into an equivalent Mach-Zehnder fiber interferometer, a maximum noise
reduction of $4.4 \pm 0.3$ dB is obtained ($6.3 \pm 0.6$ dB when corrected for
losses). The dependence of noise reduction on the interferometer splitting
ratio and fiber length is studied in detail.


title: Consciousness and Endogenous State Reduction: Two Experiments

abstract:
  It is normally claimed that physical systems create and influence
consciousness, but that consciousness cannot influence physical systems.
However, I believe that this idea is flawed, and I suggest the following
experiments as a way of demonstrating the influence of consciousness on
physical systems. The first uses Positron Emission Tomography (PET) with
humans, and the second uses autoradiography with rats. Background arguments are
given, where it is claimed that 'pain' consciousness is correlated in a certain
way with the binding of opiates to receptors in the brain. Key Words: state
reduction, state collapse, macroscopic superpositions, PET scan,
autoradiography.


title: Ultrahigh reflection from a medium with ultraslow group velocity

abstract:
  We show that an incident wavepacket at the boundary to a medium with
extremely slow group velocity, experiences enhanced reflection and a
substantial spatial and temporal distortion of the transmitted wave packet. In
the limit of vanishing group velocity, light cannot be transferred into the
medium due to its perfect reflectivity.


title: Dirac's equation in 1+1 D from a random walk

abstract:
  This paper is an investigation of the class of real classical Markov
processes without a birth process that will generate the Dirac equation in 1+1
dimensions. The Markov process is assumed to evolve in an extra (ordinal) time
dimension. The derivation requires that occupation by the Dirac particle of a
space-time lattice site is encoded in a 4 state classical probability vector.
Disregarding the state occupancy, the resulting Markov process is a homogeneous
and almost isotropic binary random walk in Dirac space and Dirac time
(including Dirac time reversals). It then emerges that the Dirac wavefunction
can be identified with a polarization induced by the walk on the Dirac
space-time lattice. The model predicts that QM observation must happen in
ordinal time, and that wavefunction collapse is due not to a dynamical
discontinuity, but to selection of a particular ordinal time. Consequently, the
model is more relativistically equitable in its treatment of space and time in
that the observer is attributed no special ability to specify the Dirac time of
observation.


title: Space Structure and Quantum Mechanics

abstract:
  A new concept of the constitution of Nature is proposed. The constructed
submicroscopic quantum mechanics is deterministic and is characterised by
elementary excitations of the space net that is treated as the tessellation of
balls, or superparticles. Said excitations called "inertons" accompany any
canonical particle when it moves. It is shown theoretically that the
introduction of inertons obviates all conceptual difficulties of orthodox
quantum mechanics. The theory has been verified experimentally. It is argued
that just inertons play the role of real carriers in the gravitational
interaction.


title: A Knob for Changing Light Propagation from Subluminal to Superluminal

abstract:
  We show how the application of a coupling field connecting the two lower
metastable states of a lambda-system can produce a variety of new results on
the propagation of a weak electromagnetic pulse. In principle the light
propagation can be changed from subluminal to superluminal. The negative group
index results from the regions of anomalous dispersion and gain in
susceptibility.


title: Irreversibility in the Halting Problem of Quantum Computer

abstract:
  The Halting problem of a quantum computer is considered. It is shown that if
halting of a quantum computer takes place the associated dynamics is described
by an irreversible operator.


title:
Wigner-function description of quantum teleportation in arbitrary
  dimensions and continuous limit

abstract:
  We present a unified approach to quantum teleportation in arbitrary
dimensions based on the Wigner-function formalism. This approach provides us
with a clear picture of all manipulations performed in the teleportation
protocol. In addition within the framework of the Wigner-function formalism all
the imperfections of the manipulations can be easily taken into account.


title: Nonlocal interactions and quantum dynamics

abstract:
  The problem is considered of describing the dynamics of quantum systems
generated by a nonlocal in time interaction. It is shown that the use of the
Feynman approach to quantum theory in combination with the canonical approach
allows one to extend quantum dynamics to describe the time evolution in the
case of such interactions. In this way, using only the current concepts of
quantum theory, a generalized equation of motion for state vectors is derived.
In the case, where the fundamental interaction generating the dynamics in a
system is local in time, this equation is equivalent to the Schr{\"o}dinger
equation. Explicit examples are given for an exactly solvable model. The
proposed formalism is shown to provide a new insight into the problem of the
description of nonlocal interactions in quantum field theory. It is shown that
such a property of the equation of motion as nonlocality in time may be
important for describing hadron-hadron interactions at low and intermediate
energies.


title: The Dirac algebra and grand unification

abstract:
  A representation of the Dirac algebra, derived from first principles, can be
related to the combinations of unit charges which determine particle
structures. The algebraic structure derives from a broken symmetry between
4-vectors and quaternions which can be applied to the broken symmetry between
the three nongravitational interactions. The significance of this relation for
Grand Unification is derived by explicit calculation of the running values of
the fine structure constants, with suggestions for the calculation of particle
masses.


title:
A Method for Justification of the View of Observables in Quantum
  Mechanics and Probability Distributions in Phase Space

abstract:
  There are considered some corollaries of certain hypotheses on the
observation process of microphenomena. We show that an enlargement of the phase
space and of its motion group and an account for the diffusion motions of
microsystems in the enlarged space, the motions which act by small random
translations along the enlarged group, lead to observable quantum effects. This
approach enables one to recover probability distributions in the phase space
for wave functions. The parameters of the model considered here are estimated
on the base of Lamb's shift in the spectrum of the hydrogen's atom.


title:
Spontaneous Parametric Down-Conversion Experiment to Measure Both Photon
  Trajectories and Double-Slit Interference

abstract:
  Recent work using spontaneous parametric down-conversion (SPDC) has made
possible investigations of the Einstein-Podolsky-Rosen paradox in its original
(position-momentum) form. We propose an experiment that uses SPDC photon pairs
to measure through which slit a photon passes while simultaneously observing
double-slit interference.


title: Chaos in a double driven dissipative nonlinear oscillator

abstract:
  We propose an anharmonic oscillator driven by two periodic forces of
different frequencies as a new time-dependent model for investigating quantum
dissipative chaos. Our analysis is done in the frame of statistical ensemble of
quantum trajectories in quantum state diffusion approach. Quantum dynamical
manifestation of chaotic behavior, including the emergence of chaos, properties
of strange attractors, and quantum entanglement are studied by numerical
simulation of ensemble averaged Wigner function and von Neumann entropy.


title: The Lie algebra Structure and Nonlinear Controllability of Spin Systems

abstract:
  In this paper, we study the controllability properties and the Lie algebra
structure of networks of particles with spin immersed in an electro-magnetic
field. We relate the Lie algebra structure to the properties of a graph whose
nodes represent the particles and an edge connects two nodes if and only if the
interaction between the two corresponding particles is active. For networks
with different gyromagnetic ratios, we provide a necessary and sufficient
condition of controllability in terms of the properties of the above mentioned
graph and describe the Lie algebra structure in every case. For these systems
all the controllability notions, including the possibility of driving the
evolution operator and/or the state, are equivalent. For general networks (with
possibly equal gyromagnetic ratios), we give a sufficient condition of
controllability. A general form of interaction among the particles is assumed
which includes both Ising and Heisenberg models as special cases.
  Assuming Heisenberg interaction we provide an analysis of low dimensional
cases (number of particles less then or equal to three) which include necessary
and sufficient controllability conditions as well as a study of their Lie
algebra structure. This also, provides an example of quantum mechanical systems
where controllability of the state is verified while controllability of the
evolution operator is not.


title: QSES's and the Quantum Jump

abstract:
  The stochastic methods in Hilbert space have been used both from a
fundamental and a practical point of view. The result we report here concerns
only the idea of applying these methods to model the evolution of quantum
systems and does not enter into the question of their fundamental or practical
status. It can be easily stated as follows: Once a quantum stochastic evolution
scheme is assumed, the incompatibility between the Markov property and the
notion of quantum jump is rapidly established.


title: Quantum Correlations in Two-Boson Wavefunctions

abstract:
  We present the Schmidt decomposition for arbitrary wavefunctions of two
indistinguishable bosons, extending the recent studies of entanglement or
quantum correlations for two fermion systems [J. Schliemann et al., Phys. Rev.
B {\bf 63}, 085311 (2001) and quant-ph/0012094]. We point out that the von
Neumann entropy of the reduced single particle density matrix remains to be a
good entanglement measure for two identical particles.


title:
Recoherence in the entanglement dynamics and classical orbits in the
  N-atom Jaynes-Cummings model

abstract:
  The rise in linear entropy of a subsystem in the N-atom Jaynes-Cummings model
is shown to be strongly influenced by the shape of the classical orbits of the
underlying classical phase space: we find a one-to-one correspondence between
maxima (minima) of the linear entropy and maxima (minima) of the expectation
value of atomic excitation J_z. Since the expectation value of this operator
can be viewed as related to the orbit radius in the classical phase space
projection associated to the atomic degree of freedom, the proximity of the
quantum wave packet to this atomic phase space borderline produces a maximum
rate of entanglement. The consequence of this fact for initial conditions
centered at periodic orbits in regular regions is a clear periodic recoherence.
For chaotic situations the same phenomenon (proximity of the atomic phase space
borderline) is in general responsible for oscillations in the entanglement
properties.


title: The essence of entanglement

abstract:
  Entanglement, according to Erwin Schroedinger the essence of quantum
mechanics, is at the heart of the Einstein-Podolsky-Rosen paradox and of the so
called quantum-nonlocality - the fact that a local realistic explanation of
quantum mechanics is not possible as quantitatively expressed by violation of
Bell's inequalities. Even as entanglement gains increasing importance in most
quantum information processing protocols, its conceptual foundation is still
widely debated. Among the open questions are: What is the conceptual meaning of
quantum entanglement? What are the most general constraints imposed by local
realism? Which general quantum states violate these constraints? Developing
Schroedinger's ideas in an information-theoretic context we suggest that a
natural understanding of quantum entanglement results when one accepts (1) that
the amount of information per elementary system is finite and (2) that the
information in a composite system resides more in the correlations than in
properties of individuals. The quantitative formulation of these ideas leads to
a rather natural criterion of quantum entanglement. Independently, extending
Bell's original ideas, we obtain a single general Bell inequality that
summarizes all possible constraints imposed by local realism on the
correlations for a multi-particle system. Violation of the general Bell
inequality results in an independent general criterion for quantum
entanglement. Most importantly, the two criteria agree in essence, though the
two approaches are conceptually very different. This concurrence strongly
supports the information-theoretic interpretation of quantum entanglement and
of quantum physics in general.


title:
Visibility, efficiency, and Bell violations in real
  Einstein-Podolsky-Rosen experiments

abstract:
  The violation of Bell's inequalities in Einstein-Podolsky-Rosen experiments
has been demonstrated for photons and ions. In all experiments of this kind the
relation between visibility, efficiency, and Bell violation is generally
unknown. In this paper we show that simulations based on a local hidden
variables models for entangled photons provide this information. It is
established that these properties are closely related by the way, in which
photons are detected after a polarizer beam splitter. On this basis we suggest
controlled experiments which, for the first time, subject the superposition
principle to experimental tests.


title: A feasible quantum communication complexity protocol

abstract:
  I show that a simple multi-party communication task can be performed more
efficiently with quantum communication than with classical communication, even
with low detection efficiency $\eta$. The task is a communication complexity
problem in which distant parties need to compute a function of the distributed
inputs, while minimizing the amount of communication between them. A realistic
quantum optical setup is suggested that can demonstrate a five-party quantum
protocol with higher-than-classical performance, provided $\eta>0.33$ .


title:
Novel Cascaded Ultra Bright Pulsed Source of Polarization Entangled
  Photons

abstract:
  A new ultra bright pulsed source of polarization entangled photons has been
realized using type-II phase matching in spontaneous parametric down conversion
process in two cascaded crystals. The optical axes of the crystals are aligned
in such a way that the extraordinarily (ordinarily) polarized cone from one
crystal overlaps with the ordinarily (extraordinarily) polarized cone from the
second crystal. This spatial overlapping removes the association between the
polarization and the output angle of the photons that exist in a single type-II
down conversion process. Hence, entanglement of photons originating from any
point on the output cones is possible if a suitable optical delay line is used.
This delay line is particularly simple and easy to implement.


title: Genetic information and quantum gas

abstract:
  Possible explanation of the 64/20 redundancy of the triplet genetic code
based on the assumption of quantum nature of genetic information is proposed.


title: Comments on the nonlinear Schrodinger equation

abstract:
  A proof is given that if the nonlinear Schrodinger wave function is
constrained to have support over only a finite volume in configuration space,
then the total energy is bounded from below for either sign of the logarithmic
term in the Hamiltonian. It is concluded that the usual assumption about the
sign of the logarithmic term made by Bialynicki-Birula and Mycielski is not the
only possibility, and that a sensible theory can be made with the opposite sign
as well.


title: Covariance and Fisher information in quantum mechanics

abstract:
  Variance and Fisher information are ingredients of the Cramer-Rao inequality.
We regard Fisher information as a Riemannian metric on a quantum statistical
manifold and choose monotonicity under coarse graining as the fundamental
property of variance and Fisher information. In this approach we show that
there is a kind of dual one-to-one correspondence between the candidates of the
two concepts. We emphasis that Fisher informations are obtained from relative
entropies as contrast functions on the state space and argue that the scalar
curvature might be interpreted as an uncertainty density on a statistical
manifold.


title:
Optimal eavesdropping in cryptography with three-dimensional quantum
  states

abstract:
  We study optimal eavesdropping in quantum cryptography with three-dimensional
systems, and show that this scheme is more secure than protocols using
two-dimensional states. We generalize the according eavesdropping
transformation to arbitrary dimensions, and discuss the connection with optimal
quantum cloning.


title: Control of one and two homonuclear spins

abstract:
  We consider the problem of steering control for the systems of one spin 1/2
particle and two interacting homonuclear spin 1/2 particles in an
electro-magnetic field. The describing models are bilinear systems whose state
varies on the Lie group of special unitary matrices of dimensions two and four,
respectively. By performing decompositions of Lie groups, taking into account
the describing equations at hand, we derive control laws to steer the state of
the system to any desired final configuration. Explicit formulas are given for
the parameters involved in the control algorithms. Moreover, the proposed
algorithms allow for arbitrary bounds on the magnitude of the controls and for
some flexibility in the specification of the final time which must be greater
than a given value but otherwise arbitrary.


title: Notions of controllability for quantum mechanical systems

abstract:
  In this paper, we define four different notions of controllability of
physical interest for multilevel quantum mechanical systems. These notions
involve the possibility of driving the evolution operator as well as the state
of the system. We establish the connections among these different notions as
well as methods to verify controllability.
  The paper also contains results on the relation between the controllability
in arbitrary small time of a system varying on a compact transformation Lie
group and the corresponding system on the associated homogeneous space. As an
application, we prove that, for the system of two interacting spin 1/2
particles, not every state transfer can be obtained in arbitrary small time.