title: Minimum cbits for remote preperation and measurement of a qubit

abstract:
  We show that a qubit chosen from equatorial or polar great circles on a Bloch
spehere can be remotely prepared with one cbit from Alice to Bob if they share
one ebit of entanglement. Also we show that any single particle measurement on
an arbitrary qubit can be remotely simulated with one ebit of shared
entanglement and communication of one cbit.


title: Exact solutions of n-level systems and gauge theories

abstract:
  We find a relationship between unitary transformations of the dynamics of
quantum systems with time-dependent Hamiltonians and gauge theories. In
particular, we show that the nonrelativistic dynamics of spin-$\frac12$
particles in a magnetic field $B^i (t)$ can be formulated in a natural way as
an SU(2) gauge theory, with the magnetic field $B^i(t)$ playing the role of the
gauge potential A^i. The present approach can also be applied to systems of n
levels with time-dependent potentials, U(n) being the gauge group. This
geometric interpretation provides a powerful method to find exact solutions of
the Schr\"odinger equation. The root of the present approach rests in the
Hermiticity property of the Hamiltonian operators involved. In addition, the
relationship with true gauge symmetries of n-level quantum systems is
discussed.


title: Quantum Probability from Decision Theory?

abstract:
  In a recent paper (quant-ph/9906015), Deutsch claims to derive the
"probabilistic predictions of quantum theory" from the "non-probabilistic
axioms of quantum theory" and the "non-probabilistic part of classical decision
theory." We show that his derivation fails because it includes hidden
probabilistic assumptions.


title: Aharonov-Bohm Effect and Coordinate Transformations

abstract:
  Resorting to a Gedankenexperiment which is very similar to the famous
Aharonov-Bohm proposal it will be shown that, in the case of a Minkowskian
spacetime, we may use a nonrelativistic quantum particle and a noninertial
coordinate system and obtain geometric information of regions that are, to this
particle, forbidden. This shows that the outcome of a nonrelativistic quantum
process is determined not only by the features of geometry at those points at
which the process takes place, but also by geometric parameters of regions in
which the quantum system can not enter. From this fact we could claim that
geometry at the quantum level plays a non-local role. Indeed, the measurement
outputs of some nonrelativistic quantum experiments are determined not only by
the geometry of the region in which the experiment takes place, but also by the
geometric properties of spacetime volumes which are, in some way, forbidden in
the experiment.


title:
Approximate real time visualization of a Rabi transition by means of
  continuous fuzzy measurement

abstract:
  Continuous weak or fuzzy measurement of the Rabi oscillation of a two level
atom subjected to a $\pi-$pulse of a resonant light field is simulated
numerically. We thereby address the question whether it is possible to measure
characteristic features of the motion of the state of a single quantum system
in real time. We compare two schemes of continuous measurement: continuous
measurement with constant fuzziness and with fuzziness changing in the course
of the measurement. Because the sensitivity of the Rabi atom to the influence
of the measurement depends on the state of the atom, it is possible to optimize
the continuous fuzzy measurement by varying its fuzziness.


title:
The meaning of 'counterfactual' statements and non-locality in quantum
  mechanics

abstract:
  Recent discussions by Mermin [1] and Stapp [2] in this journal on
non-locality and counterfactuality are shown to contain linguistic problems
that require verification. As such they can at most provide us with two
subjective choices for the meaning of 'counterfactual statements' in quantum
mechanics. We shall show that the word 'counterfactual' is in fact
inappropriate here and should be replaced by the word 'hypothetical'. Mermin's
choice imposes a strictly contextual meaning based upon an interpretation of
counterfactuality which he used to refute, without proof as we shall see,
Stapp's logical proof [3] of non-locality in quantum theory. In linguistic
theory both authors' choices of meaning: counterfactual versus hypothetical are
equally acceptable and therefore some of the issues they discussed lie outside
the domain of physics. The issues they discussed are further confused by the
fact that in his reply Stapp [2] seems to have adopted Mermin's counterfactual
interpretation against his own original [3] hypothetical interpretation. In the
rest of this paper we shall adopt the hypothetical sense of Stapp's original
statements but we modify his crucial statement LOC2 appropriately, then
following his argumentations, we shall show that there is no conflict between
relativity and quantum mechanics. We suggest that this should be the natural
(pragmatic) choice of meaning in defining the predictions of events in the
Hardy experiment.


title:
Testing the symmetrization postulate of quantum mechanics and the
  spin-statistics connection

abstract:
  Recent experimental tests of the symmetrization postulate of quantum
mechanics are discussed. It is shown that in a strict sense these experiments
cannot test the validity of the symmetrization postulate, but in most cases do
test the spin-statistics connection. An experiment is proposed that would allow
to search for possible violations of the symmetrization postulate.


title:
Barrier Perturbation Induced Superarrivals and Nonlocality in a
  Time-Evolving Wave Packet

abstract:
  We compute the time evolving probability of a Gaussian wave packet to be
reflected from a rectangular potential barrier which is perturbed by reducing
its height. A time interval is found during which this probability of
reflection is enhanced (superarrivals) compared to the unperturbed case. Such a
time evolving reflection probability implies that the effect of perturbation
propagates across the wave packet faster than its group velocity - a curious
form of nonlocality.


title:
Bell's inequality for a single spin 1/2 particle and quantum
  contextuality

abstract:
  We argue that for a \emph{single particle} Bell's inequality is a consequence
of noncontextuality and is \emph{incompatible} with statistical predictions of
quantum mechanics. Thus noncontextual models can be empirically falsified,
\emph{independent} of locality condition. For this an appropriate entanglement
between \emph{disjoint} Hilbert spaces pertaining to translational and spin
degrees of freedom of a single spin-1/2 particle is invoked


title: Alternative Forms of Generalized Vectors and Operators for Spin 1/2

abstract:
  The forms of the generalized quantities that we have recently introduced are
dependent on the phase of the probability amplitudes for spin-projection
measurements. In this paper, we show explicitly that changing the phase gives
different forms for both the spin vectors and spin operators. Therefore, there
are as many forms of these quantities as there are different choices of phase.


title: Threshold and linewidth of a mirrorless parametric oscillator

abstract:
  We analyze the above-threshold behavior of a mirrorless parametric oscillator
based on resonantly enhanced four wave mixing in a coherently driven dense
atomic vapor. It is shown that, in the ideal limit, an arbitrary small flux of
pump photons is sufficient to reach the oscillator threshold. We demonstrate
that due to the large group-velocity delays associated with coherent media, an
extremely narrow oscillator linewidth is possible, making a narrow-band source
of non-classical radiation feasible.


title:
Spin Description and Calculation in the Lande' Interpretation of Quantum
  Mechanics

abstract:
  We explain the connection between the generalized spin quantities we have
recently introduced and standard forms. We show how the calculation of various
quantities of interest using these new forms is done. Focusing attention on
expectation values, we find that in every case, the standard results can be
obtained as special cases arising from the new generalized results.


title: Thermal noise of a plano-convex mirror

abstract:
  We study theoretically the internal thermal noise of a mirror coated on a
plano-convex substrate. The comparison with a cylindrical mirror of the same
mass shows that the effect on a light beam can be reduced by a factor 10,
improving the sensitivity of high-precision optical experiments such as
gravitational-wave interferometers.


title:
Three Approaches to the Quantitative Definition of Information in an
  Individual Pure Quantum State

abstract:
  In analogy of classical Kolmogorov complexity we develop a theory of the
algorithmic information in bits contained in any one of continuously many pure
quantum states: quantum Kolmogorov complexity. Classical Kolmogorov complexity
coincides with the new quantum Kolmogorov complexity restricted to the
classical domain. Quantum Kolmogorov complexity is upper bounded and can be
effectively approximated from above. With high probability a quantum object is
incompressible. There are two alternative approaches possible: to define the
complexity as the length of the shortest qubit program that effectively
describes the object, and to use classical descriptions with computable real
parameters.


title: Sufficient conditions for a disentanglement

abstract:
  We consider a disentanglement process in which local properties of an
entangled state are preserved, while the entanglement between the subsystems is
erased. Sufficient conditions for a perfect disentanglement (into product
states and into separable states) are derived, and connections to the
conditions for perfect cloning and for perfect broadcasting are observed.


title: Superintegrability on the two dimensional hyperboloid II

abstract:
  This work is devoted to the investigation of the quantum mechanical systems
on the two dimensional hyperboloid which admit separation of variables in at
least two coordinate systems. Here we consider two potentials introduced in a
paper of C.P.Boyer, E.G.Kalnins and P.Winternitz, which haven't yet been
studied. We give an example of an interbasis expansion and work out the
structure of the quadratic algebra generated by the integrals of motion.


title: Event-ready entanglement preparation

abstract:
  We present an experiment which prepare entanglement between photons that
nowhere interacted and whose paths nowhere crossed. The experiment puts
together two photons from two (non-maximal) singlet-like photon pairs and make
them interfere at an asymmetrical beam splitter. As a result one finds
polarization correlations between the other companion photons from the pairs
whose paths nowhere crossed each other even when no polarization measurements
have been carried out on the former photons. The latter set of photons that
nowhere interacted are therefore event-ready prepared by their pair-companion
photons. The result reveals nonlocality as a property of selection which can
even be a preselection. It also reveals that one can predict spin-correlated
behaviour of photons in space and at beam splitters by controlling a no-spin
observable. [From the Book of Abstracts as appeared at the Workshop.]


title:
A method for reaching detection efficiencies necessary for optical
  loophole-free Bell experiments

abstract:
  A method for preparing a loophole-free four-photon Bell experiments which
requires a detection efficiency of 67% is proposed. It enables realistic
detection efficiencies of 75% at a visibility of 85%. Two type-II crystals each
down convert one correlated photon pair and we entangle one photon from one
pair with one photon from the other pair on a highly transparent beam splitter.
The entanglement selects two other conjugate photons into a Bell state. Wide
solid angles for the conjugate photons then enable us to collect close to 100%
of them. The cases when both photon pairs come from only one of the two
crystals are successfully taken into account. Hardy's equalities are discussed.


title: Resonance energy-exchange-free detection and "welcher Weg" experiment

abstract:
  It is shown that a monolithic total-internal-reflection resonator can be used
for energy-exchange-free detections of objects without recoils. Related
energy-exchange-free detection of "welcher Weg" is discussed and an experiment
with an atom interferometer is proposed. The obtained results are in agreement
with quantum theory.


title: Entanglement Teleportation via Werner States

abstract:
  Transfer of entanglement and information is studied for quantum teleportation
of an unknown entangled state through noisy quantum channels. We find that the
quantum entanglement of the unknown state can be lost during the teleportation
even when the channel is quantum correlated. We introduce a fundamental
parameter of correlation information which dissipates linearly during the
teleportation through the noisy channel. Analyzing the transfer of correlation
information, we show that the purity of the initial state is important in
determining the entanglement of the replica state.


title: Fragility of a class of highly entangled states of many quantum-bits

abstract:
  We consider a Quantum Computer with n quantum-bits (`qubits'), where each
qubit is coupled independently to an environment affecting the state in a
dephasing or depolarizing way. For mixed states we suggest a quantification for
the property of showing {\it quantum} uncertainty on the macroscopic level. We
illustrate in which sense a large parameter can be seen as an indicator for
large entanglement and give hypersurfaces enclosing the set of separable
states. Using methods of the classical theory of maximum likelihood estimation
we prove that this parameter is decreasing with 1/\sqrt{n} for all those states
which have been exposed to the environment.
  Furthermore we consider a Quantum Computer with perfect 1-qubit gates and
2-qubit gates with depolarizing error and show that any state which can be
obtained from a separable initial state lies inbetween a family of pairs of
certain hypersurfaces parallel to those enclosing the separable ones.


title: Ballistic Single-Electron Quputer

abstract:
  We propose a new solid state implementation of a quantum computer (quputer)
using ballistic single electrons as flying qubits in 1D nanowires. We use a
single electron pump (SEP) to prepare the initial state and a single electron
transistor (SET) to measure the final state. Single qubit gates are implemented
using quantum dots as phase shifters and electron waveguide couplers as beam
splitters. A Coulomb coupler acts as a 2-qubit gate, using a mutual phase
modulation effect. Since the electron phase coherence length in GaAs/AlGaAs
heterostructures is of the order of 30$\mu$m, several gates (tens) can be
implemented before the system decoheres.


title:
Magnetic trapping of neutral particles: Classical and Quantum-mechanical
  study of a Ioffe-Pritchard type trap

abstract:
  Recently, we developed a method for calculating the lifetime of a particle
inside a magnetic trap with respect to spin flips, as a first step in our
efforts to understand the quantum-mechanics of magnetic traps. The 1D toy model
that was used in this study was physically unrealistic because the magnetic
field was not curl-free. Here, we study, both classically and
quantum-mechanically, the problem of a neutral particle with spin S, mass m and
magnetic moment mu, moving in 3D in an inhomogeneous magnetic field
corresponding to traps of the Ioffe-Pritchard, `clover-leaf' and `baseball'
type. Defining by omega_p, omega_z and omega_r the precessional, the axial and
the lateral vibrational frequencies, respectively, of the particle in the
adiabatic potential, we find classically the region in the $(\omega_{r}%
(omega_r -- omega_z) plane where the particle is trapped.
  Quantum-mechanically, we study the problem of a spin-one particle in the same
field. Treating omega_r / omega_p and omega_z / omega_p as small parameters for
the perturbation from the adiabatic Hamiltonian, we derive a closed-form
expression for the transition rate 1/T_{esc} of the particle from its trapped
ground-state. We find that in the extreme cases, the expression for 1/T_{esc}
is dominated by the largest of the two frequencies omega_r and omega_z.


title:
Microscopic theory of the interaction of ultracold dense Bose and Fermi
  gases with electromagnetic field

abstract:
  We present the rigorous microscopic quantum theory of the interaction of
ultracold Bose and Fermi gases with the electromagnetic field of vacuum and
laser photons. The main attention has been paid to the consistent consideration
of dynamical dipole-dipole interactions. The theory developed is shown to be
consistent with the general principles of the canonical quantization of
electromagnetic field in a medium. Starting from the first principles of QED we
have derived the general system of Maxwell-Bloch equations for atomic creation
and annihilation operators and the propagation equation for the laser field
which can be used for the self-consistent analysis of various linear and
nonlinear phenomena in atom optics at high densities of the atomic system. All
known equations which are used for the description of the behaviour of an
ultracold atomic ensemble in a radiation field can be obtained from our general
system of equations in a low-density limit.


title: Quantum state reconstruction in the presence of dissipation

abstract:
  We propose a realistic scheme to determine the quantum state of a single mode
cavity field even after it has started to decay due to the coupling with an
environment. Although dissipation destroys quantum coherences, we show that at
zero temperature enough information about the initial state remains, in an
observable quantity, to allow the reconstruction of its Wigner function.


title: Distributed Entanglement

abstract:
  Consider three qubits A, B, and C which may be entangled with each other. We
show that there is a trade-off between A's entanglement with B and its
entanglement with C. This relation is expressed in terms of a measure of
entanglement called the "tangle," which is related to the entanglement of
formation. Specifically, we show that the tangle between A and B, plus the
tangle between A and C, cannot be greater than the tangle between A and the
pair BC. This inequality is as strong as it could be, in the sense that for any
values of the tangles satisfying the corresponding equality, one can find a
quantum state consistent with those values. Further exploration of this result
leads to a definition of the "three-way tangle" of the system, which is
invariant under permutations of the qubits.


title:
Continuous-variable teleportation improvement by photon subtraction via
  conditional measurement

abstract:
  We show that the recently proposed scheme of teleportation of continuous
variables [S.L. Braunstein and H.J. Kimble, Phys. Rev. Lett. 80, 869 (1998)]
can be improved by a conditional measurement in the preparation of the
entangled state shared by the sender and the recipient. The conditional
measurement subtracts photons from the original entangled two-mode squeezed
vacuum, by transmitting each mode through a low-reflectivity beam splitter and
performing a joint photon-number measurement on the reflected beams. In this
way the degree of entanglement of the shared state is increased and so is the
fidelity of the teleported state.


title: A position-momentum EPR state of distantly-separated trapped atoms

abstract:
  We propose a scheme for preparing an EPR state in position and momentum of a
pair of distantly-separated trapped atoms. The scheme utilizes the entangled
light fields output from a nondegenerate optical parametric amplifier. Quantum
state exchange between these fields and the motional states of the trapped
atoms is accomplished via interactions in cavity QED.


title: More about Dynamical Reduction and the Enumeration Principle

abstract:
  In view of the arguments put forward by Clifton and Monton [1999] in a recent
preprint, we reconsider the alleged conflict of dynamical reduction models with
the enumeration principle. We prove that our original analysis of such a
problem is correct, that the GRW model does not meet any difficulty and that
the reasoning of the above authors is inappropriate since it does not take into
account the correct interpretation of the dynamical reduction theories.


title: Decoherence within a simple Model for the Environment

abstract:
  This article examines the decoherence of a macroscopic body using a simple
model of the environment and following the evolution of the pure state for the
whole system. We found that decoherence occurs for very general initial
conditions and were able to confirm a number of widely accepted features of the
process.


title: Can all neurobiological processes be described by classical physics?

abstract:
  We discuss results recently given in an article by M. Tegmark
(quant-ph/9907009) where he argues that neurons can be described appropriately
by pure classical physics. This letter is dedicated to the question if this is
really the case when the role of dissipation and noise -- the two concurrent
phenomena present in these biological structures -- is taken into account. We
argue that dissipation and noise might well be of quantum origin and give also
a possible reason why neural dynamics is not classical.


title:
Determination of Atom-Surface van der Waals Potentials from
  Transmission-Grating Diffraction Intensities

abstract:
  Molecular beams of rare gas atoms and D_2 have been diffracted from
  100 nm period SiN_x transmission gratings. The relative intensities of the
diffraction peaks out to the 8th order depend on the diffracting particle and
are interpreted in terms of effective slit widths. These differences have been
analyzed by a new theory which accounts for the long-range van der Waals
-C_3/l^3 interaction of the particles with the walls of the grating bars. The
values of the C_3 constant for two different gratings are in good agreement and
the results exhibit the expected linear dependence on the dipole
polarizability.


title:
Bound states in the Kratzer plus polynomial potentials and their new
  exact tractability via nonlinear algebraic equations

abstract:
  Schroedinger equation with potentials of the Kratzer plus polynomial type
(say, quartic V(r) = A r^4 +B r^3 + C r^2+D r + F/r + G/r^2 etc) is considered.
A new method of exact construction of some of its bound states is then
proposed. it is based on the Taylor series terminated rigorously after N+1
terms at specific couplings and energies. This enables us to find the exact,
complete and compact unperturbed solution of the Magyari's N+2 coupled and
nonlinear algebraic conditions of the termination in the strong-coupling regime
with G \to \infty. Next, at G < \infty, we adapt the Rayleigh-Schroedinger
perturbation theory and define the bound states via an innovated, triple
perturbation series. In tests we show that all the correction terms appear in
integer arithmetics and remain, therefore, exact.


title: Mutual coherence of optical and matter waves

abstract:
  We propose a scheme to measure the cross-correlations and mutual coherence of
optical and matter fields. It relies on the combination of a matter-wave
detector operating by photoionization of the atoms and a traditional absorption
photodetector. We show that the double-detection signal is sensitive to
cross-correlation functions of light and matter waves.


title: A quantum circuit for OR

abstract:
  We give the first quantum circuit for computing $f(0)$ OR $f(1)$ more
reliably than is classically possible with a single evaluation of the function.
OR therefore joins XOR (i.e. parity, $f(0) \oplus f(1)$) to give the full set
of logical connectives (up to relabeling of inputs and outputs) for which there
is quantum speedup. The XOR algorithm is of fundamental importance in quantum
computation; our OR algorithm (found with the aid of genetic programming), may
represent a new quantum computational effect, also useful as a ``subroutine''.


title:
On the correspondence between classical and quantum measurements on a
  bosonic field

abstract:
  We study the correspondence between classical and quantum measurements on a
harmonic oscillator that describes a one-mode bosonic field. We connect the
quantum measurement of an observable of the field with the possibility of
amplifying the observable ideally through a quantum amplifier. The
``classical'' measurement corresponds to the joint measurement of the position
$q$ and momentum $p$ of the harmonic oscillator, with following evaluation of a
function $f$ of the outcome $\alpha=q+ip$. For the electromagnetic field the
joint measurement is achieved by a heterodyne detector. The quantum measurement
of an observable $\hat O$ is obtained by preamplifying the heterodyne detector
through an ideal amplifier of $\hat O$, and rescaling the outcome by the gain
$g$. We give a general criterion which states when this preamplified heterodyne
detection scheme approaches the ideal quantum measurement of $\hat O$ in the
limit of infinite gain. We show that this criterion is satisfied and the ideal
measurement is achieved for the case of the photon number operator and for the
quadrature. For both operators the method is robust to nonunit quantum
efficiency of the heterodyne detector. On the other hand, we show that the
preamplified heterodyne detection scheme does not work for arbitrary observable
of the field. As a counterexample, we prove that the simple quadratic function
of the field $\hat K=i(a^{\dag 2}-a^2)/2$ has no corresponding polynomial
function $f(\alpha,\bar \alpha)$---including the obvious choice
$f=\hbox{Im}(\alpha^2)$---that allows the measurement of $\hat K$ through the
preamplified heterodyne measurement scheme.


title: Transition probability and preferential gauge

abstract:
  This paper is concerned with whether or not the preferential gauge can ensure
the uniqueness and correctness of results obtained from the standard
time-dependent perturbation theory, in which the transition probability is
formulated in terms of matrix elements of Hamiltonian.


title:
Von Neumann equations with time-dependent Hamiltonians and
  supersymmetric quantum mechanics

abstract:
  Starting with a time-independent Hamiltonian $h$ and an appropriately chosen
solution of the von Neumann equation $i\dot\rho(t)=[ h,\rho(t)]$ we construct
its binary-Darboux partner $h_1(t)$ and an exact scattering solution of
$i\dot\rho_1(t)=[h_1(t),\rho_1(t)]$ where $h_1(t)$ is time-dependent and not
isospectral to $h$. The method is analogous to supersymmetric quantum mechanics
but is based on a different version of a Darboux transformation. We illustrate
the technique by the example where $h$ corresponds to a 1-D harmonic
oscillator. The resulting $h_1(t)$ represents a scattering of a soliton-like
pulse on a three-level system.


title: The Possibility of Factorizable Contextual Hidden Variable Theories

abstract:
  Considering an extended type of Bohm's version of EPR thought experiment, we
derive Bell's inequality for the case of factorizable contextual hidden
variable theories which are consistent with the predictions of quantum theory.
Usually factorizability is associated with non-contextuality. Here, we show
that factorizability is consistent with contextuality, even for the ordinary
Bohm's version of EPR thought experiment.


title:
Comment on ``A local hidden variable model of quantum correlations
  exploiting the detection loophole''

abstract:
  We study in this short comment the analogies and the differences that exist
between several local hidden variable models.


title: Periodic Hamiltonian and Berry's phase in harmonic oscillators

abstract:
  For a time-dependent $\tau$-periodic harmonic oscillator of two linearly
independent homogeneous solutions of classical equation of motion which are
bounded all over the time (stable), it is shown, there is a representation of
states cyclic up to multiplicative constants under $\tau$-evolution or
$2\tau$-evolution depending on the model. The set of the wave functions is
complete. Berry's phase which could depend on the choice of representation can
be defined under the $\tau$- or $2\tau$-evolution in this representation. If a
homogeneous solution diverges as the time goes to infinity, it is shown that,
Berry's phase can not be defined in any representation considered. Berry's
phase for the driven harmonic oscillator is also considered. For the cases
where Berry's phase can be defined, the phase is given in terms of solutions of
the classical equation of motion.


title:
Nuclear Magnetic Resonance Quantum Computing Using Liquid Crystal
  Solvents

abstract:
  Liquid crystals offer several advantages as solvents for molecules used for
nuclear magnetic resonance quantum computing (NMRQC). The dipolar coupling
between nuclear spins manifest in the NMR spectra of molecules oriented by a
liquid crystal permits a significant increase in clock frequency, while short
spin-lattice relaxation times permit fast recycling of algorithms, and save
time in calibration and signal-enhancement experiments. Furthermore, the use of
liquid crystal solvents offers scalability in the form of an expanded library
of spin-bearing molecules suitable for NMRQC. These ideas are demonstrated with
the successful execution of a 2-qubit Grover search using a molecule
($^{13}$C$^{1}$HCl$_3$) oriented in a liquid crystal and a clock speed eight
times greater than in an isotropic solvent. Perhaps more importantly, five
times as many logic operations can be executed within the coherence time using
the liquid crystal solvent.


title:
Interaction of Radiation and a Relativistic Electron in Motion in a
  Constant Magnetic Field

abstract:
  The work examines the effect of multiple photon emission on the quantum
mechanical state of an electron emitting synchrotron radiation and on the
intensity of that radiation. Calculations are done with the variant of
perturbation theory based on the use of extended coherent states. A general
formula is derived for the number of emitted photons, which allows for taking
into account their mutual interaction. A model problem is used to demonstrate
the absence of the infrared catastrophe in the modified perturbation theory.
Finally, the electron density matrix is calculated, and the analysis of this
matrix makes it possible to conclude that the degree of the elecron's spatial
localization increases with the passage of time if the electron is being
accelerated.


title: Non-simultaneity in two-photon coincidence spectroscopy

abstract:
  Photon coincidence spectroscopy relies on detecting multiphoton emissions
from the combined atom-cavity system in atomic beam cavity quantum
electrodynamics experiments. These multiphoton emissions from the cavity are
nearly simultaneous approximately on the cavity lifetime scale. We determine
the optimal time for the detection window of photon pairs in two-photon
coincidence spectroscopy. If the window time is too short, some photon pairs
will not be detected; if the window time is too long, too many nearly
coincident independent single photons will be falsely interpreted as being a
photon pair. The paper has been submitted to The European Physical Journal D.


title: How much information can be obtained by a quantum measurement?

abstract:
  How much information about an unknown quantum state can be obtained by a
measurement? We propose a model independent answer: the information obtained is
equal to the minimum entropy of the outputs of the measurement, where the
minimum is taken over all measurements which measure the same ``property'' of
the state. This minimization is necessary because the measurement outcomes can
be redundant, and this redundancy must be eliminated. We show that this minimum
entropy is less or equal than the von Neumann entropy of the unknown states.
That is a measurement can extract at most one meaningful bit from every qubit
carried by the unknown states.


title: Algorithms on Ensemble Quantum Computers

abstract:
  In ensemble (or bulk) quantum computation, measurements of qubits in an
individual computer cannot be performed. Instead, only expectation values can
be measured. As a result of this limitation on the model of computation,
various important algorithms cannot be processed directly on such computers,
and must be modified. We provide modifications of various existing protocols,
including algorithms for universal fault--tolerant computation, Shor's
factorization algorithm (which can be extended to any algorithm computing an NP
function), and some search algorithms to enable processing them on ensemble
quantum computers.


title: Fidelity for Multimode Thermal Squeezed States

abstract:
  In the theory of quantum transmission of information the concept of fidelity
plays a fundamental role. An important class of channels, which can be
experimentally realized in quantum optics, is that of Gaussian quantum
channels. In this work we present a general formula for fidelity in the case of
two arbitrary Gaussian states. From this formula one can get a previous result
(H. Scutaru, J. Phys. A: Mat. Gen {\bf 31}, 3659 (1998)), for the case of a
single mode; or, one can apply it to obtain a closed compact expression for
multimode thermal states.


title: Mathematical surprises and Dirac's formalism in quantum mechanics

abstract:
  By a series of simple examples, we illustrate how the lack of mathematical
concern can readily lead to surprising mathematical contradictions in wave
mechanics. The basic mathematical notions allowing for a precise formulation of
the theory are then summarized and it is shown how they lead to an elucidation
and deeper understanding of the aforementioned problems. After stressing the
equivalence between wave mechanics and the other formulations of quantum
mechanics, i.e. matrix mechanics and Dirac's abstract Hilbert space
formulation, we devote the second part of our paper to the latter approach: we
discuss the problems and shortcomings of this formalism as well as those of the
bra and ket notation introduced by Dirac in this context. In conclusion, we
indicate how all of these problems can be solved or at least avoided.

categories: quant-ph cond-mat hep-th math-ph math.MP nucl-th physics.ed-ph


title: Formalisme de Dirac et surprises mathematiques en mecanique quantique

abstract:
  Differents formalismes sont utilises en mecanique quantique pour la
description des etats et des observables : la mecanique ondulatoire, la
mecanique matricielle et le formalisme invariant. Nous discutons les problemes
et inconvenients du formalisme invariant ainsi que ceux de la notation des bras
et kets introduite par Dirac dans ce contexte. Nous indiquons comment tous les
problemes peuvent etre resolus ou du moins evites. Une serie d'exemples
illustre les points souleves et montre comment l'insouciance mathematique peut
aisement conduire a des contradictions mathematiques surprenantes.


title:
Nonclassical effects in a driven atoms/cavity system in the presence of
  arbitrary driving field and dephasing

abstract:
  We investigate the photon statistics of light transmitted from a driven
optical cavity containing one or two atoms interacting with a single mode of
the cavity field. We treat arbitrary driving fields with emphasis on departure
from previous weak field results. In addition effects of dephasing due to
atomic transit through the cavity mode are included using two different models.
We find that both models show the nonclassical correlations are quite sensitive
to dephasing. The effect of multiple atoms on the system dynamics is
investigated by placing two atoms in the cavity mode at different positions,
therefore having different coupling strengths.


title:
Spin correlated interferometry for polarized and unpolarized photons on
  a beam splitter

abstract:
  Spin interferometry of the 4th order for independent polarized as well as
unpolarized photons arriving simultaneously at a beam splitter and exhibiting
spin correlation while leaving it, is formulated and discussed in the quantum
approach. Beam splitter is recognized as a source of genuine singlet photon
states. Also, typical nonclassical beating between photons taking part in the
interference of the 4th order is given a polarization dependent explanation.


title: Continuous variable quantum cryptography

abstract:
  We propose a quantum cryptographic scheme in which small phase and amplitude
modulations of CW light beams carry the key information. The presence of EPR
type correlations provides the quantum protection.


title:
Time-convolutionless reduced-density-operator theory of a noisy quantum
  channel: a two-bit quantum gate for quantum information processing

abstract:
  An exact reduced-density-operator for the output quantum states in
time-convolutionless form was derived by solving the quantum Liouville equation
which governs the dynamics of a noisy quantum channel by using a projection
operator method and both advanced and retarded propagators in time. The
formalism developed in this work is general enough to model a noisy quantum
channel provided specific forms of the Hamiltonians for the system, reservoir,
and the mutual interaction between the system and the reservoir are given.
Then, we apply the formulation to model a two-bit quantum gate composed of
coupled spin systems in which the Heisenberg coupling is controlled by the
tunneling barrier between neighboring quantum dots. Gate Characteristics
including the entropy, fidelity, and purity are calculated numerically for both
mixed and entangled initial states.


title:
Coherent States of Non-Linear Lie algebras: Application in Quantum
  Optics

abstract:
  We present a general unified approach for finding the coherent states of
polynomially deformed algebras such as the quadratic and Higgs algebras, which
are relevant for various multiphoton processes in quantum optics. We give a
general procedure to map these deformed algebras to appropriate Lie algebras.
This is used, for the non compact cases, to obtain the annihilation operator
coherent states, by finding the canonical conjugates of these operators.
  Generalized coherent states, in the Perelomov sense also follow from this
construction. This allows us to explicitly construct coherent states associated
with various quantum optical systems.


title:
Experimental Verifications of the Casimir Attractive Force between Solid
  Bodies

abstract:
  A brief review of the recent experimental verifications of the Casimir force
between extended bodies is presented. With modern techniques, it now appears
feasible to test the force law with 1% precision; I will address the issues
relating to the interpretation of experiments at this level of accuracy


title: Coding Theorems of Quantum Information Theory

abstract:
  Coding theorems and (strong) converses for memoryless quantum communication
channels and quantum sources are proved: for the quantum source the coding
theorem is reviewed, and the strong converse proven. For classical information
transmission via quantum channels we give a new proof of the coding theorem,
and prove the strong converse, even under the extended model of nonstationary
channels. As a by-product we obtain a new proof of the famous Holevo bound.
Then multi-user systems are investigated, and the capacity region for the
quantum multiple access channel is determined. The last chapter contains a
preliminary discussion of some models of compression of correlated quantum
sources, and a proposal for a program to obtain operational meaning for quantum
conditional entropy. An appendix features the introduction of a notation and
calculus of entropy in quantum systems.


title: On the Abnormal Type Anomalous Solutions of Quasipotential Equations

abstract:
  It is shown that there exist solutions of the quasipotential equations
exhibiting the abnormal type behaviour of the Bethe-Salpeter equation.


title: Quantum Gates and Clifford Algebras

abstract:
  Clifford algebras are used for definition of spinors. Because of using
spin-1/2 systems as an adequate model of quantum bit, a relation of the
algebras with quantum information science has physical reasons. But there are
simple mathematical properties of the algebras those also justifies such
applications.
  First, any complex Clifford algebra with 2n generators, Cl(2n,C), has
representation as algebra of all 2^n x 2^n complex matrices and so includes
unitary matrix of any quantum n-gate. An arbitrary element of whole algebra
corresponds to general form of linear complex transformation. The last property
is also useful because linear operators are not necessary should be unitary if
they used for description of restriction of some unitary operator to subspace.
  The second advantage is simple algebraic structure of Cl(2n) that can be
expressed via tenzor product of standard "building units" and similar with
behavior of composite quantum systems. The compact notation with 2n generators
also can be used in software for modeling of simple quantum circuits by modern
conventional computers.


title: Bargmann Invariants and Geometric Phases - a Generalised Connection

abstract:
  We develop the widest possible generalisation of the well-known connection
between quantum mechanical Bargmann invariants and geometric phases. The key
notion is that of null phase curves in quantum mechanical ray and Hilbert
spaces. Examples of such curves are developed. Our generalisation is shown to
be essential to properly understand geometric phase results in the cases of
coherent states and of Gaussian states. Differential geometric aspects of null
phase curves are also briefly explored.


title: Quantum Data Processing

abstract:
  We prove a data processing inequality for quantum communication channels,
which states that processing a received quantum state may never increase the
mutual information between input and output states.


title: New Treatment of Systems of Compounded Angular Momentum

abstract:
  The approach to quantum mechanics which we have used to derive the matrix
treatment of spin from first principles is now employed to treat systems of
compounded angular momentum. A general treatment is first given, which is then
applied to the concrete cases of a spin-0 and a spin-1 system obtained by
adding the spins of two spin-1/2 systems. Thus the probability amplitudes for
measurements on the systems are derived, as well as the matrix vectors and
operators corresponding to the systems. The matrix operators and states
obtained are different from the standard forms and are much more generalized.
The new results are applied to the case of joint measurements on the subsystems
of such a system; this is a problem that has been made very topical by the high
level of interest in the foundations of quantum mechanics. As a consequence of
the insights arising from this treatment, we show that the Clebsch-Gordan
coefficients are amenable to generalization, and we give the generalized forms
for these cases.


title: Parametric image amplification in optical cavities

abstract:
  We show the possibility of noiseless amplification of an optical image in
cavities containing a parametric oscillator. We consider a confocal ring cavity
with plane mirrors and compare with the case of planar cavity. In the latter
case the system operates with severe spatial limitations, while in the confocal
case, there is the possibility of preserving the signal-to-noise ratio while
amplifying uniformely the entire image.


title: Spectrum of light scattered from a deformed Bose-Einstein condensate

abstract:
  The spectrum of light scattered from a Bose-Einstein condensate is studied in
the limit of particle-number conservation. To this end, a description in terms
of deformed bosons is invoked and this leads to a deviation from the usual
predict spectrum's shape as soon as the number of particles decreases.


title: Output Spectrum of Single-Atom Lasers

abstract:
  We consider a laser composed of a single atom in a microcavity, with a
coherent or incoherent pump. We consider both three- and four-level gain
schemes, and examine the output spectrum of such lasers. We find that the
linewidth generally scales as the inverse of the photon number. For large
atom-field coupling, a vacuum-Rabi doublet structure is obtained. In the
three-level case, this vacuum-Rabi splitting is apparent only for small
intracavity photon numbers, and vanishes for large pumps. In the four-level
scheme, the vacuum-Rabi structure appears at a nonzero pump level, and is
maintained for large pumps, even when the intracavity photon number is larger
than unity. This behavior is explained utilizing the quantum trajectory
approach.


title:
EPR statistical mixture of correlated states with fractional brownian
  process induced by third party interaction

abstract: This paper has been withdrawn by the author(s)


title: Reliability function of general classical-quantum channel

abstract:
  In information theory the reliability function and its bounds, describing the
exponential behavior of the error probability, are the most important
quantitative characteristics of the channel performance. From a general point
of view, these bounds provide certain measures of distinguishability of a given
set of states. In an earlier paper we introduced quantum analogs of the random
coding and the expurgation lower bounds for the case of pure signal states.
Here we discuss the general case, in particular, we prove the previously
conjectured expurgation bound and find the quantum cutoff rate in the case of
arbitrary mixed signal states.


title: On Bures fidelity of displaced squeezed thermal states

abstract:
  Fidelity plays a key role in quantum information and communication theory.
Fidelity can be interpreted as the probability that a decoded message possesses
the same information content as the message prior to coding and transmission.
In this paper, we give a formula of Bures fidelity for displaced squeezed
thermal states directly by the displacement and squeezing parameters and
birefly discuss how the results can apply to quantum information theory.


title: Nonclassical correlations in damped N-solitons

abstract:
  The quantum statistics of damped higher-order optical solitons are analyzed
numerically, using cumulant-expansion techniques in Gaussian approximation. A
detailed analysis of nonclassical properties in both the time and the frequency
domain is given, with special emphasis on the role of absorption. Highly
nonclassical broadband spectral correlation is predicted.


title: Tomographic test of Bell's inequality

abstract:
  We present a homodyne detection scheme to verify Bell's inequality on
correlated optical beams at the output of a nondegenerate parametric amplifier.
Our approach is based on tomographic measurement of the joint detection
probabilities, which allows high quantum efficiency at detectors. A
self-homodyne scheme is suggested to simplify the experimental set-up.


title:
Classical Limit of the Trajectory Representation of Quantum Mechanics,
  Loss of Information and Residual Indeterminacy

abstract:
  The trajectory representation in the classical limit (\hbar \to 0) manifests
a residual indeterminacy. We show that the trajectory representation in the
classical limit goes to neither classical mechanics (Planck's correspondence
principle) nor statistical mechanics. This residual indeterminacy is contrasted
to Heisenberg uncertainty. We discuss the relationship between indeterminacy
and 't Hooft's information loss and equivalence classes.


title:
Two-Level Atom in an Optical Parametric Oscillator: Spectra of
  Transmitted and Fluorescent Fields in the Weak Driving Field Limit

abstract:
  We consider the interaction of a two-level atom inside an optical parametric
oscillator. In the weak driving field limit, we essentially have an atom-cavity
system driven by the occasional pair of correlated photons, or weakly squeezed
light. We find that we may have holes, or dips, in the spectrum of the
fluorescent and transmitted light. This occurs even in the strong-coupling
limit when we find holes in the vacuum-Rabi doublet. Also, spectra with a
sub-natural linewidth may occur. These effects disappear for larger driving
fields, unlike the spectral narrowing obtained in resonance fluorescence in a
squeezed vacuum; here it is important that the squeezing parameter $N$ tends to
zero so that the system interacts with only one correlated pair of photons at a
time. We show that a previous explanation for spectral narrowing and spectral
holes for incoherent scattering is not applicable in the present case, and
propose a new explanation. We attribute these anomalous effects to quantum
interference in the two-photon scattering of the system.


title: A simple proof of the converse of Hardy's theorem

abstract:
  In this paper we provide a simple proof of the fact that for a system of two
spin-1/2 particles, and for a choice of observables, there is a unique state
which shows Hardy-type nonlocality. Moreover, an explicit expression for the
probability that an ensemble of particle pairs prepared in such a state
exhibits a Hardy-type nonlocality contradiction is given in terms of two
independent parameters related to the observables involved. Incidentally, a
wrong statement expressed in Mermin's proof of the converse [N.D. Mermin, Am.
J. Phys. 62, 880 (1994)] is pointed out.


title: Quantum fluctuations for drag free geodesic motion

abstract:
  The drag free technique is used to force a proof mass to follow a geodesic
motion. The mass is protected from perturbations by a cage, and the motion of
the latter is actively controlled to follow the motion of the proof mass. We
present a theoretical analysis of the effects of quantum fluctuations for this
technique. We show that a perfect drag free operation is in principle possible
at the quantum level, in spite of the back action exerted on the mass by the
position sensor.


title:
Protecting Quantum Information Encoded in Decoherence Free States
  Against Exchange Errors

abstract:
  The exchange interaction between identical qubits in a quantum information
processor gives rise to unitary two-qubit errors. It is shown here that
decoherence free subspaces (DFSs) for collective decoherence undergo Pauli
errors under exchange, which however do not take the decoherence free states
outside of the DFS. In order to protect DFSs against these errors it is
sufficient to employ a recently proposed concatenated DFS-quantum error
correcting code scheme [D.A. Lidar, D. Bacon and K.B. Whaley, Phys. Rev. Lett.
{\bf 82}, 4556 (1999)].


title:
Realizing probabilistic identification and cloning of quantum states via
  universal quantum logic gates

abstract:
  Probabilistic quantum cloning and identifying machines can be constructed via
unitary-reduction processes [Duan and Guo, Phys. Rev. Lett. 80, 4999 (1998)].
Given the cloning (identifying) probabilities, we derive an explicit
representation of the unitary evolution and corresponding Hamiltonian to
realize probabilistic cloning (identification). The logic networks are obtained
by decomposing the unitary representation into universal quantum logic
operations. The robustness of the networks is also discussed. Our method is
suitable for a $k$-partite system, such as quantum computer, and may be
generalized to general state-dependent cloning and identification.


title: Universality of optimal measurements

abstract:
  We present optimal and minimal measurements on identical copies of an unknown
state of a qubit when the quality of measuring strategies is quantified with
the gain of information (Kullback of probability distributions). We also show
that the maximal gain of information occurs, among isotropic priors, when the
state is known to be pure. Universality of optimal measurements follows from
our results: using the fidelity or the gain of information, two different
figures of merits, leads to exactly the same conclusions. We finally
investigate the optimal capacity of $N$ copies of an unknown state as a quantum
channel of information.


title: Polarization state of a biphoton: quantum ternary logic

abstract:
  Polarization state of biphoton light generated via collinear
frequency-degenerate spontaneous parametric down-conversion is considered. A
biphoton is described by a three-component polarization vector, its arbitrary
transformations relating to the SU(3) group. A subset of such transformations,
available with retardation plates, is realized experimentally. In particular,
two independent orthogonally polarized beams of type-I biphotons are
transformed into a beam of type-II biphotons. Polarized biphotons are suggested
as ternary analogs of two-state quantum systems (qubits).


title: Quantum trajectories for Brownian motion

abstract:
  We present the stochastic Schroedinger equation for the dynamics of a quantum
particle coupled to a high temperature environment and apply it the dynamics of
a driven, damped, nonlinear quantum oscillator. Apart from an initial slip on
the environmental memory time scale, in the mean, our result recovers the
solution of the known non-Lindblad quantum Brownian motion master equation. A
remarkable feature of our approach is its localization property: individual
quantum trajectories remain localized wave packets for all times, even for the
classically chaotic system considered here, the localization being stronger the
smaller $\hbar$.


title:
Expanding Hermitean Operators in a Basis of Projectors on Coherent Spin
  States

abstract:
  The expectation values of a hermitean operator A in (2s+1)(2s+1) specific
coherent states of a spin are known to determine the operator unambiguously. As
shown here, (almost) any other (2s+1)(2s+1) coherent states also provide a
basis for self-adjoint operators. This is proven by considering the determinant
of the Gram matrix associated with the coherent state projectors as a
Hamiltonian of a fictitious classical spin system.


title: Quantization via hopping amplitudes: Schroedinger equation and free QED

abstract:
  Schroedinger's equation with scalar and vector potentials is shown to
describe "nothing but" hopping of a quantum particle on a lattice; any spatial
variation of the hopping amplitudes acts like an external electric and/or
magnetic field. The main point of the argument is the superposition principle
for state vectors; Lagrangians, path integrals, or classical Hamiltonians are
not (!) required. Analogously, the Hamiltonian of the free electromagnetic
field is obtained as a twofold continuum limit of unitary hopping in Z(N) link
configuration space, if gauge invariance and C and P symmetries are imposed.


title: Non-Abelian Berry connections for quantum computation

abstract:
  In the holonomic approach to quantum computation information is encoded in a
degenerate eigenspace of a parametric family of Hamiltonians and manipulated by
the associated holonomic gates. These are realized in terms of the non-abelian
Berry connection and are obtained by driving the control parameters along
adiabatic loops. We show how it is possible, for a specific model, to
explicitly determine the loops generating any desired logical gate, thus
producing a universal set of unitary transformations. In a multi-partite system
unitary transformations can be implemented efficiently by sequences of local
holonomic gates. Moreover a conceptual scheme for obtaining the required
Hamiltonian family, based on frequently repeated pulses, is discussed, together
with a possible process whereby the initial state can be prepared and the final
one can be measured.


title: Entanglement and the linearity of quantum mechanics

abstract:
  Optimal universal entanglement processes are discussed which entangle two
quantum systems in an optimal way for all possible initial states. It is
demonstrated that the linear character of quantum theory which enforces the
peaceful coexistence of quantum mechanics and relativity imposes severe
restrictions on the structure of the resulting optimally entangled states.
Depending on the dimension of the one-particle Hilbert space such a universal
process generates either a pure Bell state or mixed entangled states. In the
limit of very large dimensions of the one-particle Hilbert space the
von-Neumann entropy of the optimally entangled state differs from the one of
the maximally mixed two-particle state by one bit only.


title: Casimir force between metallic mirrors

abstract:
  We study the influence of finite conductivity of metals on the Casimir
effect. We put the emphasis on explicit theoretical evaluations which can help
comparing experimental results with theory. The reduction of the Casimir force
is evaluated for plane metallic plates. The reduction of the Casimir energy in
the same configuration is also calculated. It can be used to infer the
reduction of the force in the plane-sphere geometry through the `proximity
theorem'. Frequency dependent dielectric response functions of the metals are
represented either by the simple plasma model or, more accurately, by using the
optical data known for the metals used in recent experiments, that is Al, Au
and Cu. In the two latter cases, the results obtained here differ significantly
from those published recently.


title:
Semiclassical interferences and catastrophes in the ionization of
  Rydberg atoms by half-cycle pulses

abstract:
  A multi-dimensional semiclassical description of excitation of a Rydberg
electron by half-cycle pulses is developed and applied to the study of energy-
and angle-resolved ionization spectra. Characteristic novel phenomena
observable in these spectra such as interference oscillations and semiclassical
glory and rainbow scattering are discussed and related to the underlying
classical dynamics of the Rydberg electron. Modifications to the predictions of
the impulse approximation are examined that arise due to finite pulse
durations.


title: Excitation of weakly bound Rydberg electrons by half-cycle pulses

abstract:
  The interaction of a weakly bound Rydberg electron with an electromagnetic
half-cycle pulse (HCP) is described with the help of a multidimensional
semiclassical treatment. This approach relates the quantum evolution of the
electron to its underlying classical dynamics. The method is nonperturbative
and is valid for arbitrary spatial and temporal shapes of the applied HCP. On
the basis of this approach angle- and energy-resolved spectra resulting from
the ionization of Rydberg atoms by HCPs are analyzed. The different types of
spectra obtainable in the sudden-impact approximation are characterized in
terms of the appearing semiclassical scattering phenomena. Typical
modifications of the spectra originating from finite pulse effects are
discussed.


title: Destruction of quantum coherence and wave packet dynamic

abstract:
  The main aim of this article is to discuss characteristic physical phenomena
which govern the destruction of quantum coherence of material wave packets.


title:
Counterfactual entanglement and nonlocal correlations in separable
  states

abstract:
  It is shown that the outcomes of measurements on systems in separable mixed
states can be partitioned, via subsequent measurements on a disentangled
extraneous system, into subensembles that display the statistics of entangled
states. This motivates the introduction of the concept of "counterfactual"
entanglement, which can be associated with all separable mixed states including
those that are factorable. This type of entanglement gives rise to a new kind
of postselection-induced Bell inequality violation. The significance of
counterfactual entanglement, and its physical implications, are assessed.


title: Quantifying nonorthogonality

abstract:
  An exploratory approach to the possibility of analyzing nonorthogonality as a
quantifiable property is presented. Three different measures for the
nonorthogonality of pure states are introduced, and one of these measures is
extended to single-particle density matrices using methods that are similar to
recently introduced techniques for quantifying entanglement. Several
interesting special cases are considered. It is pointed out that a measure of
nonorthogonality can meaningfully be associated with a single mixed quantum
state. It is then shown how nonorthogonality can be unlocked with classical
information; this analysis reveals interesting inequalities and points to a
number of connections between nonorthogonality and entanglement.


title: General Strategies for Discrimination of Quantum States

abstract:
  We derive general discrimination of quantum states chosen from a certain set,
given initial $M$ copies of each state, and obtain the matrix inequality, which
describe the bound between the maximum probability of correctly determining and
that of error. The former works are special cases of our results.


title:
Realization of Probabilistic Identification and Clone of Quantum-States
  II Multiparticles System

abstract:
  We realize the probabilistic cloning and identifying linear independent
quantum states of multi-particles system, given prior probability, with
universal quantum logic gates using the method of unitary representation. Our
result is universal for separate state and entanglement. We also provide the
realization in the condition given $M$ initial copies for each state.


title: Quantum Clone and States Estimation for n-state System

abstract:
  We derive a lower bound for the optimal fidelity for deterministic cloning a
set of n pure states. In connection with states estimation, we obtain a lower
bound about average maximum correct states estimation probability.


title: Proposal for teleportation of an atomic state via cavity decay

abstract:
  We show how the state of an atom trapped in a cavity can be teleported to a
second atom trapped in a distant cavity simply by detecting photon decays from
the cavities. This is a rare example of a decay mechanism playing a
constructive role in quantum information processing. The scheme is
comparatively easy to implement, requiring only the ability to trap a single
three level atom in a cavity.


title: Berry phase in the simple harmonic oscillator

abstract:
  Berry phase of simple harmonic oscillator is considered in a general
representation. It is shown that, Berry phase which depends on the choice of
representation can be defined under evolution of the half of period of the
classical motions, as well as under evolution of the period. The Berry phases
do {\em not} depend on the mass or angular frequency of the oscillator. The
driven harmonic oscillator is also considered, and the Berry phase is given in
terms of Fourier coefficients of the external force and parameters which
determine the representation.


title: Quantum Communication Protocol Employing Weak Measurements

abstract:
  We propose a communication protocol exploiting correlations between two
events with a definite time-ordering: a) the outcome of a {\em weak
measurement} on a spin, and b) the outcome of a subsequent ordinary measurement
on the spin. In our protocol, Alice, first generates a "code" by performing
weak measurements on a sample of N spins.
  The sample is sent to Bob, who later performs a post-selection by measuring
the spin along either of two certain directions. The results of the
post-selection define the "key', which he then broadcasts publicly. Using both
her previously generated code and this key, Alice is able to infer the {\em
direction} chosen by Bob in the post-selection. Alternatively, if Alice
broadcasts publicly her code, Bob is able to infer from the code and the key
the direction chosen by Alice for her weak measurement. Two possible
experimental realizations of the protocols are briefly mentioned.


title: Trapping of single atoms in cavity QED

abstract:
  By integrating the techniques of laser cooling and trapping with those of
cavity quantum electrodynamics (QED), single Cesium atoms have been trapped
within the mode of a small, high finesse optical cavity in a regime of strong
coupling. The observed lifetime for individual atoms trapped within the cavity
mode is $\tau \approx 28$ms, and is limited by fluctuations of light forces
arising from the far-detuned intracavity field. This initial realization of
trapped atoms in cavity QED should enable diverse protocols in quantum
information science.


title: Elements of Environmental Decoherence

abstract:
  In this contribution I give a brief introduction to the essential concepts
and mechanisms of decoherence by the environment. The emphasis will be not so
much on technical details but rather on conceptual issues and the impact on the
interpretation problem of quantum theory.


title: Bell theorem involving all possible local measurements

abstract:
  The Bell theorem for a pair of two-state systems in a singlet state is
formulated for the entire range of measurement settings.


title: Quantum Teleportation is a Universal Computational Primitive

abstract:
  We present a method to create a variety of interesting gates by teleporting
quantum bits through special entangled states. This allows, for instance, the
construction of a quantum computer based on just single qubit operations, Bell
measurements, and GHZ states. We also present straightforward constructions of
a wide variety of fault-tolerant quantum gates.


title: Uncertainty is complementary to Complementarity

abstract:
  For any ideal two-path interferometer it is shown that the wave-particle
duality of quantum mechanics implies Heisenberg's uncertainty relation and vice
versa. It is conjectured that complementarity and uncertainty are two aspects
of the same general principle.


title:
Quantum simulation of a three-body interaction Hamiltonian on an NMR
  quantum computer

abstract:
  Extensions of average Hamiltonian theory to quantum computation permit the
design of arbitrary Hamiltonians, allowing rotations throughout a large Hilbert
space. In this way, the kinematics and dynamics of any quantum system may be
simulated by a quantum computer. A basis mapping between the systems dictates
the average Hamiltonian in the quantum computer needed to implement the desired
Hamiltonian in the simulated system. The flexibility of the procedure is
illustrated with NMR on 13-C labelled Alanine by creating the non-physical
Hamiltonian ZZZ corresponding to a three body interaction.


title: Measurement-induced interference in an inhomogeneous gravitational field

abstract:
  A very interesting quantum mechanical effect is the emergence of
gravity-induced interference, which has already been detected. This effect also
shows us that gravity is at the quantum level not a purely geometric effect,
the mass of the employed particles appears explicitly in the interference
expression. In this work we will generalize some previous results. It will be
shown that the introduction of a second order approximation in the propagator
of a particle, immersed in the Earth's gravitational field, and whose
coordinates are being continuously monitored, allows us to include, in the
corresponding complex oscillator, a frequency which now depends on the geometry
of the source of the gravitational field, a fact that is absent in the case of
a homogeneous field. Using this propagator we will analyze the interference
pattern of two particle beams whose coordinates are being continuously
monitored. We will compare our results againt the case of a homogeneous field,
and also against the measurement ouputs of the Colella, Overhauser, and Werner
experiment, and find that the difference in the dependence upon the geometry of
the source of the gravitational field could render detectable differences in
their respective measurement outputs.


title:
Topology Classes of Flat U(1) Bundles and Diffeomorphic Covariant
  Representations of the Heisenberg Algebra

abstract:
  The general construction of self-adjoint configuration space representations
of the Heisenberg algebra over an arbitrary manifold is considered. All such
inequivalent representations are parametrised in terms of the topology classes
of flat U(1) bundles over the configuration space manifold. In the case of
Riemannian manifolds, these representations are also manifestly diffeomorphic
covariant. The general discussion, illustrated by some simple examples in non
relativistic quantum mechanics, is of particular relevance to systems whose
configuration space is parametrised by curvilinear coordinates or is not simply
connected, which thus include for instance the modular spaces of theories of
non abelian gauge fields and gravity.


title:
Performing Quantum Measurement in Suitably Entangled States Originates
  the Quantum Computation Speed Up

abstract:
  We provide a justification of the quantum speed-up based on the complementary
roles played by the reversible preparation of an entangled state before
measurement and by the final measurement action.


title: Quantum Parabolic Sombrero

abstract:
  We have discussed the energy levels and probability distribution density for
a quantum particle placed in the two-dimensional sombrero-shaped potential
$V(\rho,\rho_0)=\mu\omega^2|\rho^2-\rho_0^2|/2$.


title: Probabilistic exact cloning and probabilistic no-signalling

abstract:
  We show that non-local resources cannot be used for probabilistic signalling
even if one can produce exact clones with the help of a probabilistic quantum
cloning machine (PQCM). We show that PQCM cannot help to distinguish two
statistical mixtures at a remote location. Thus quantum theory rules out the
possibility of sending superluminal signals not only deterministically but also
probabilistically. We give a bound on the success probability of producing
multiple clones in an entangled system.


title: Non-adiabatic transitions in multi-level systems

abstract:
  In a quantum system with a smoothly and slowly varying Hamiltonian, which
approaches a constant operator at times $t\to \pm \infty$, the transition
probabilities between adiabatic states are exponentially small. They are
characterized by an exponent that depends on a phase integral along a path
around a set of branch points connecting the energy level surfaces in complex
time. Only certain sequences of branch points contribute. We propose that these
sequences are determined by a topological rule involving the Stokes lines
attached to the branch points. Our hypothesis is supported by theoretical
arguments and results of numerical experiments.


title: Dualist interpretation of quantum mechanics

abstract:
  An interpretation of quantum mechanics is proposed which augments the
stochastic pilot-wave model, introduced by Nelson [E. Nelson, Phys. Rev., 150,
1079 (1966)], with a dual guidance condition. Namely, in addition to the
stochastic guidance condition which describes how the wave controls the
particle (wave-to-particle guidance condition), we introduce another stochastic
guidance condition describing how the particle guides the wave
(particle-to-wave guidance condition). We therefore introduce an
action-reaction principle in the pilot-wave formulation of quantum mechanics.
The particle-to-wave guidance condition takes the form of spontaneous
transitions of the wavefunction that are Poisson distributed in time. The
wavefunction selected by the transition is influenced by its likelyhood with
respect to the particle position, as well as certain conservation constraints.
It is shown that the stochastic particle-to-wave guidance condition reproduces
certain aspects of the spontaneous localization model of Ghirardi, Rimini and
Weber [G. C. Ghirardi, A. Rimini and T. Weber, Phys. Rev. D, 34, 470 (1986)],
and of Milburn's intrinsic decoherence model [G. J. Milburn, Phys. Rev. A, 44,
5401 (1991)], while avoiding some of the difficulties associated with them. The
macroscopic limit of the dualist interpretation is analyzed for the case of a
free body. Possible improvements of this interpretation are discussed.


title:
Laser Field Induced Birefringence and Enhancement of Magneto-optical
  Rotation

abstract:
  An initially isotropic medium, when subjected to either a magnetic field or a
coherent field, can induce anisotropy in the medium and can cause the
polarization of a probe field to rotate. Therefore the rotation of probe
polarization, due to magnetic field alone, can be controlled efficiently with
the use of a coherent control field. We demonstrate this enhancement of the
magneto-optical rotation (MOR) of a linearly polarized light, by doing detailed
calculations on a system with relevant transitions $j=0\leftrightarrow
j=1\leftrightarrow j=0$.


title:
Quantum gates by coupled quantum dots and measurement procedure in Si
  MOSFET

abstract:
  We investigated the quantum gates of coupled quantum dots, theoretically,
when charging effects can be observed. We have shown that the charged states in
the qubits can be observed by the channel current of the MOSFET structure.


title: Carmichael Numbers on a Quantum Computer

abstract:
  We present a quantum probabilistic algorithm which tests with a polynomial
computational complexity whether a given composite number is of the Carmichael
type. We also suggest a quantum algorithm which could verify a conjecture by
Pomerance, Selfridge and Wagstaff concerning the asymptotic distribution of
Carmichael numbers smaller than a given integer.


title:
Nonclassical interaction-free detection of objects in a monolithic
  total-internal-reflection resonator

abstract:
  We show that with an efficiency exceeding 99% one can use a monolithic
total-internal-reflection resonator in order to ascertain the presence of an
object without transferring a quantum of energy to it. We also propose an
experiment on the probabilistic meaning of the electric field that contains
only a very few photons.


title:
Spin-correlated interferometry with beam splitters: preselection of
  spin-correlated photons

abstract:
  A nonclassical feature of the fourth-order interference at a beam splitter,
that genuine photon spin singlets are emitted in predetermined directions even
when incident photons are unpolarized, has been used in a proposal for an
experiment that imposes quantum spin correlation on truly independent photons.
In the experiment, two photons from two such singlets interfere at a beam
splitter, and as a result the other two photons - which nowhere interacted and
whose paths nowhere crossed - exhibit a 100% correlation in polarization, even
when no polarization has been measured in the first two photons. The propsed
experiment permits closure of the remaining loopholes in the Bell theorem proof
and reveals the quantum nonlocality as a property of selection, and pioneers an
experimental procedure for exact preparation of unequal superposition.


title:
Physical Interpretation of Laser-Induced Suppression of Quantum
  Tunneling

abstract:
  We revisit the problem of laser-induced suppression of quantum dynamical
tunneling in a model system studied by Kilin et al. [Phys. Rev. Lett. 76 (1996)
3297]. This quantum system consists of a ground state symmetric double-well
potential which is coupled by a strong laser field to an excited state
asymmetric double-well potential. By analyzing the assumptions used in their
analysis we show that the suppression of quantum dynamical tunneling can be
explained with the use of dark and bright states of the system. We also
generalize the system and the conditions for suppression of quantum tunneling
and show that, in certain cases, suppression can occur regardless the
characteristics of the excited potential surface.


title: Transparency Near a Photonic Band Edge

abstract:
  We study the absorption and dispersion properties of a ${\bf \Lambda}$-type
atom which decays spontaneously near the edge of a photonic band gap (PBG).
Using an isotropic PBG model, we show that the atom can become transparent to a
probe laser field, even when other dissipative channels are present. This
transparency originates from the square root singularity of the density of
modes of the PBG material at threshold.


title: Galilean Lee Model of the Delta Function Potential

abstract:
  The scattering cross section associated with a two dimensional delta function
has recently been the object of considerable study. It is shown here that this
problem can be put into a field theoretical framework by the construction of an
appropriate Galilean covariant theory. The Lee model with a standard Yukawa
interaction is shown to provide such a realization. The usual results for delta
function scattering are then obtained in the case that a stable particle exists
in the scattering channel provided that a certain limit is taken in the
relevant parameter space. In the more general case in which no such limit is
taken finite corrections to the cross section are obtained which (unlike the
pure delta function case) depend on the coupling constant of the model.


title:
Bipartite Mixed States of Infinite-Dimensional Systems are Generically
  Nonseparable

abstract:
  Given a bipartite quantum system represented by a tensor product of two
Hilbert spaces, we give an elementary argument showing that if either component
space is infinite-dimensional, then the set of nonseparable density operators
is trace-norm dense in the set of all density operators (and the separable
density operators nowhere dense). This result complements recent detailed
investigations of separability, which show that when both component Hilbert
spaces are finite-dimensional, there is a separable neighborhood (perhaps very
small for large dimensions) of the maximally mixed state.


title: Bohmian Trajectories Post-Decoherence

abstract:
  The proposal that the interaction between a macroscopic body and its
environment plays a crucial role in producing the correct classical limit in
the Bohm interpretation of quantum mechanics is investigated, in the context of
a model of quantum Brownian motion. It is well known that one of the effects of
the interaction is to produce an extremely rapid approximate diagonalisation of
the reduced density matrix in the position representation. This effect is, by
itself, insufficient to produce generically quasi-classical behaviour of the
Bohmian trajectory. However, it is shown that, if the system particle is
initially in an approximate energy eigenstate, then there is a tendency for the
Bohmian trajectory to become approximately classical on a rather longer
time-scale. The relationship between this phenomenon and the behaviour of the
Wigner function post-decoherence (as analysed by Halliwell and Zoupas) is
discussed. It is also suggested that the phenomenon may be related to the
storage of information about the trajectory in the environment, and that it may
therefore be a general feature of every situation in which such environmental
monitoring occurs.


title: Are violations to temporal Bell inequalities there when somebody looks?

abstract:
  The possibility of observing violations of temporal Bell inequalities,
originally proposed by Leggett as a mean of testing the quantum mechanical
delocalization of suitably chosen macroscopic bodies, is discussed by taking
into account the effect of the measurement process. A general criterion
quantifying this possibility is defined and shown not to be fulfilled by the
various experimental configurations proposed so far to test inequalities of
different forms.


title: Simulating Quantum Mechanics by Non-Contextual Hidden Variables

abstract:
  No physical measurement can be performed with infinite precision. This leaves
a loophole in the standard no-go arguments against non-contextual hidden
variables. All such arguments rely on choosing special sets of
quantum-mechanical observables with measurement outcomes that cannot be
simulated non-contextually. As a consequence, these arguments do not exclude
the hypothesis that the class of physical measurements in fact corresponds to a
dense subset of all theoretically possible measurements with outcomes and
quantum probabilities that \emph{can} be recovered from a non-contextual hidden
variable model. We show here by explicit construction that there are indeed
such non-contextual hidden variable models, both for projection valued and
positive operator valued measurements.


title: Physics Beyond the Limits of Uncertainty Relations

abstract:
  The paper is a brief informal introduction to C*-algebraic foundations of
causal contextual subquantum theories. In particular, it is explained how the
contextuality property (which is a necessary consistency condition of all
causal subquantum theories) naturally appears within the framework of certain
C*-algebraic extensions of the quantum observables algebras. Furthermore, a
question of locality is discussed. It is explained that the appropriate
non-Kolmogorovian probability theory allows us to unify both locality and
causality with the principles of quantum mechanics, overcoming the obstacles
given by Bell's inequalities.


title:
Pauli's Theorem and Quantum Canonical Pairs: The Consistency Of a
  Bounded, Self-Adjoint Time Operator Canonically Conjugate to a Hamiltonian
  with Non-empty Point Spectrum

abstract:
  In single Hilbert space, Pauli's well-known theorem implies that the
existence of a self-adjoint time operator canonically conjugate to a given
Hamiltonian signifies that the time operator and the Hamiltonian possess
completely continuous spectra spanning the entire real line. Thus the
conclusion that there exists no self-adjoint time operator conjugate to a
semibounded or discrete Hamiltonian despite some well-known illustrative
counterexamples. In this paper we evaluate Pauli's theorem against the single
Hilbert space formulation of quantum mechanics, and consequently show the
consistency of assuming a bounded, self-adjoint time operator canonically
conjugate to a Hamiltonian with an unbounded, or semibounded, or finite point
spectrum. We point out Pauli's implicit assumptions and show that they are not
consistent in a single Hilbert space. We demonstrate our analysis by giving two
explicit examples. Moreover, we clarify issues sorrounding the different
solutions to the canonical commutation relations, and, consequently, expand the
class of acceptable canonical pairs beyond the solutions required by Pauli's
theorem.


title:
Recovering coherence from decoherence: a method of quantum state
  reconstruction

abstract:
  We present a feasible scheme for reconstructing the quantum state of a field
prepared inside a lossy cavity. Quantum coherences are normally destroyed by
dissipation, but we show that at zero temperature we are able to retrieve
enough information about the initial state, making possible to recover its
Wigner function as well as other quasiprobabilities. We provide a numerical
simulation of a Schroedinger cat state reconstruction.


title: Ground State Quantum Computation

abstract:
  We formulate a novel ground state quantum computation approach that requires
no unitary evolution of qubits in time: the qubits are fixed in stationary
states of the Hamiltonian. This formulation supplies a completely
time-independent approach to realizing quantum computers. We give a concrete
suggestion for a ground state quantum computer involving linked quantum dots.


title: Universal teleportation with a twist

abstract:
  We give a transfer theorem for teleportation based on twisting the
entanglement measurement. This allows one to say what local unitary operation
must be performed to complete the teleportation in any situation, generalizing
the scheme to include overcomplete measurements, non-abelian groups of local
unitary operations (e.g., angular momentum teleportation), and the effect of
non-maximally entangled resources.


title: Simulating nonlinear spin models in an ion trap

abstract:
  We show how a conditional displacement of the vibrational mode of trapped
ions can be used to simulate nonlinear collective and interacting spin systems
including nonlinear tops and Ising models (a universal two qubit gate),
independent of the vibrational state of the ion. Thus cooling to the
vibrational ground state is unnecessary provided the heating rate is not too
large.


title: Squeezing and photon distribution in a vibrating cavity

abstract:
  We obtain explicit analytical expressions for the quadrature variances and
the photon distribution functions of the electromagnetic field modes excited
from vacuum or thermal states due to the non-stationary Casimir effect in an
ideal one-dimensional Fabry--Perot cavity with vibrating walls, provided the
frequency of vibrations is close to a multiple frequency of the fundamental
unperturbed electromagnetic mode.


title: Quantum perfect correlations and Hardy's nonlocality theorem

abstract:
  In this paper the failure of Hardy's nonlocality proof for the class of
maximally entangled states is considered. A detailed analysis shows that the
incompatibility of the Hardy equations for this class of states physically
originates from the fact that the existence of quantum perfect correlations for
the three pairs of two-valued observables (D_11,D_21), (D_11,D_22) and
(D_12,D_21) [in the sense of having with certainty equal (different) readings
for a joint measurement of any one of the pairs (D_11,D_21), (D_11,D_22), and
(D_12,D_21)], necessarily entails perfect correlation for the pair of
observables (D_12,D_22) [in the sense of having with certainty equal
(different) readings for a joint measurement of the pair (D_12,D_22)]. Indeed,
the set of these four perfect correlations is found to satisfy the CHSH
inequality, and then no violations of local realism will arise for the
maximally entangled state as far as the four observables D_ij, i,j = 1,2, are
concerned. The connection between this fact and the impossibility for the
quantum mechanical predictions to give the maximum possible theoretical
violation of the CHSH inequality is pointed out. Moreover, it is generally
proved that the fulfillment of all the Hardy nonlocality conditions necessarily
entails a violation of the resulting CHSH inequality. The largest violation of
this latter inequality is determined.


title:
Classical limit in terms of symbolic dynamics for the quantum baker's
  map

abstract:
  We derive a simple closed form for the matrix elements of the quantum baker's
map that shows that the map is an approximate shift in a symbolic
representation based on discrete phase space. We use this result to give a
formal proof that the quantum baker's map approaches a classical Bernoulli
shift in the limit of a small effective Plank's constant.


title: Reversible quantum teleportation in an optical lattice

abstract:
  We propose a protocol, based on entanglement procedures recently suggested by
[D. Jaksch et al., Phys. Rev. Lett. 82, 1975 (1999)], which allows the
teleportation of an unknown state of a neutral atom in an optical lattice to
another atom in another site of the lattice, without any irreversible
detection.


title: On the Way to Submicroscopic Description of Nature

abstract:
  The outline analyzes the principal difficulties, which emerge at the applying
of modern quantum theory based on the Copenhagen School concept to phenomena
developed in the range close to 10^{-28} cm (the point of intersection of the
three fundamental interactions). It is shown that at this scale, the
interaction of a moving particle with space plays an essential role and just
space assigns wave and quantum properties to the particle. The main physical
aspects of space structure are discussed herein.


title: Ultimate physical limits to computation

abstract:
  Computers are physical systems: what they can and cannot do is dictated by
the laws of physics. In particular, the speed with which a physical device can
process information is limited by its energy and the amount of information that
it can process is limited by the number of degrees of freedom it possesses.
This paper explores the physical limits of computation as determined by the
speed of light $c$, the quantum scale $\hbar$ and the gravitational constant
$G$. As an example, quantitative bounds are put to the computational power of
an `ultimate laptop' with a mass of one kilogram confined to a volume of one
liter.


title: Semiclassical density of degeneracies in quantum regular systems

abstract:
  The spectrum of eigenenergies of a quantum integrable system whose
hamiltonian depends on a single parameter shows degeneracies (crossings) when
the parameter varies. We derive a semiclassical expression for the density of
crossings in the plane energy-parameter, that is the number of crossings per
unit of energy and unit of parameter, in terms of classical periodic orbits. We
compare the results of the semiclassical formula with exact quantum
calculations for two specific quantum integrable billiards.


title:
A novel realization of the Calogero-Moser scattering states as coherent
  states

abstract:
  A novel realization is provided for the scattering states of the $N$-particle
Calogero-Moser Hamiltonian. They are explicitly shown to be the coherent states
of the singular oscillators of the Calogero-Sutherland model. Our algebraic
treatment is straightforwardly extendable to a large number of few and
many-body interacting systems in one and higher dimensions.


title:
On the Relation Between Quantum Mechanical and Classical Parallel
  Transport

abstract:
  We explain how the kind of ``parallel transport'' of a wavefunction used in
discussing the Berry or Geometrical phase induces the conventional parallel
transport of certain real vectors. These real vectors are associated with
operators whose commutators yield diagonal operators; or in Lie algebras those
operators whose commutators are in the (diagonal) Cartan subalgebra.


title: On bound entanglement assisted distillation

abstract:
  We investigate asymptotic distillation of entanglement in the presence of an
unlimited amount of bound entanglement for bi-partite systems. We show that the
distillability is still bounded by the relative entropy of entanglement. This
offers a strong support to the fact that bound entanglement does not improve
distillation of entanglement.


title: Higher-Power Coherent and Squeezed States

abstract:
  A closed form expression for the higher-power coherent states (eigenstates of
$a^{j}$) is given. The cases j=3,4 are discussed in detail, including the
time-evolution of the probability densities. These are compared to the case
j=2, the even- and odd-coherent states. We give the extensions to the
"effective" displacement-operator, higher-power squeezed states and to the
ladder-operator/minimum-uncertainty, higher-power squeezed states. The
properties of all these states are discussed.


title: Quantum Teleportation Using Quantum Non-Demolition Technique

abstract:
  We propose a new scheme and protocol for quantum teleportation of a
single-mode field state, based on entanglement produced by quantum
non-demolition interaction. We show that the recently attained results in QND
technique allow to perform the teleportation in quantum regime. We also show
that applying QND coupling to squeezed fields will significantly improve the
quality of teleportation for a given degree of squeezing.


title: Optimal Ensemble Length of Mixed Separable States

abstract:
  The optimal (pure state) ensemble length of a separable state, A, is the
minimum number of (pure) product states needed in convex combination to
construct A. We study the set of all separable states with optimal (pure state)
ensemble length equal to k or fewer. Lower bounds on k are found below which
these sets have measure 0 in the set of separable states. In the bipartite case
and the multiparticle case where one of the particles has significantly more
quantum numbers than the rest, the lower bound for non-pure state ensembles is
sharp. A consequence of our results is that for all two particle systems,
except possibly those with a qubit or those with a nine dimensional Hilbert
space, and for all systems with more than two particles the optimal pure state
ensemble length for a randomly picked separable state is with probability 1
greater than the state's rank. In bipartite systems with probability 1 it is
greater than 1/4 the rank raised to the 3/2 power and in a system with p qubits
with probability 1 it is greater than (2^2p)/(1+2p), which is almost the square
of the rank.


title: A Cat-State Benchmark on a Seven Bit Quantum Computer

abstract:
  We propose and experimentally realize an algorithmic benchmark that
demonstrates coherent control with a sequence of quantum operations that first
generates and then decodes the cat state (|000...>+|111...>)/sqrt(2) to the
standard initial state |000...>. This is the first high fidelity experimental
quantum algorithm on the currently largest physical quantum register, which has
seven quantum bits (qubits) provided by the nuclei of crotonic acid. The
experiment has the additional benefit of verifying a seven coherence in a
generic system of coupled spins. Our implementation combines numerous nuclear
magnetic resonance (NMR) techniques in one experiment and introduces practical
methods for translating quantum networks to control operations. The
experimental procedure can be used as a reliable and efficient method for
creating a standard pseudo-pure state, the first step for implementing
traditional quantum algorithms in liquid state NMR. The benchmark and the
techniques can be adapted for use on other proposed quantum devices.


title: Implementing unitary operators in quantum computation

abstract:
  We present a general method which expresses a unitary operator by the product
of operators allowed by the Hamiltonian of spin-1/2 systems. In this method,
the generator of an operator is found first, and then the generator is expanded
by the base operators of the product operator formalism. Finally, the base
operators disallowed by the Hamiltonian, including more than two-body
interaction operators, are replaced by allowed ones by the axes transformation
and coupling order reduction technique. This method directly provides pulse
sequences for the nuclear magnetic resonance quantum computer, and can be
generally applied to other systems.


title:
MR spectroscopy with parabolic magnetic field: spin-oscillator coupling
  effect

abstract:
  The spectrum of the spin particle in oscillatory potential subjected to
external parabolic magnetic field ${\bf B}=(B_0+Gx+\tilde G x^2){\bf \hat z}$
is obtained. The structure of energy levels of the considered system allows to
identify the frequency of the oscillator via the spectrum of spin sublevels
coming only from {\it one} oscillatory level. The effect is due to the gradient
terms in the form of the field.


title: Effect of the measurement on the decay rate of a quantum system

abstract:
  We investigated the electron tunneling out of a quantum dot in the presence
of a continuous monitoring by a detector. It is shown that the Schr\"odinger
equation for the whole system can be reduced to new Bloch-type rate equations
describing the time-development of the detector and the measured system at
once. Using these equations we find that the continuous measurement of the
unstable system does not affect its exponential decay, $\exp (-\Gamma t)$,
contrary to expectations based on the Quantum Zeno effect . However, the width
of the energy distribution of the tunneling electron is no more $\Gamma$, but
increases due to the decoherence, generated by the detector.


title:
Gain Components in Autler-Townes Doublet from Quantum Interferences in
  Decay Channels

abstract:
  We consider non-degenerate pump-probe spectroscopy of V-systems under
conditions such that interference among decay channels is important. We
demonstrate how this interference can result in new gain features instead of
the usual absorption features. We relate this gain to the existence of a new
vacuum induced quasi-trapped-state. We further show how this also results in
large refractive index with low absorption.


title: Inseparability criterion for continuous variable systems

abstract:
  An inseparability criterion based on the total variance of a pair of
Einstein-Podolsky-Rosen type operators is proposed for continuous variable
systems. The criterion provides a sufficient condition for entanglement of any
two-party continuous variable states. Furthermore, for all the Gaussian states,
this criterion turns out to be a necessary and sufficient condition for
inseparability.


title: The Schwarz-Hora effect: present-day situation

abstract:
  The electron-diffraction pattern at a nonfluorescent target was observed by
Schwarz under attempts to modulate an electron beam by laser light. The pattern
was of the same color as the laser light. The analysis of the literature shows
there are the unresolved up to now significant contradictions between the
theory and the Schwarz experiments. To resolve these contradictions, the
interpretation of the Schwarz-Hora effect is considered, which is a development
of the idea formulated by Schwarz and Hora. It is supposed that the interaction
of electrons with the laser field inside a thin dielectric film is accompanied
not only by the processes of absorption and stimulated emission of photons but
also by formation of some metastable electron states in which the captured
photons can be transferred with a following emission at the target.


title: Casimir Effect on a Finite Lattice

abstract:
  This article presents a Hamiltonian lattice formulation of static Casimir
systems at a level of generality appropriate for an introductory investigation.
Background structure - represented by a lattice potential V(x) - is introduced
along one spatial direction with translation invariance in all other spatial
directions. Following some general analysis two specific finite one dimensional
lattice QFT systems are analyzed in detail.


title: Electrons above a Helium Surface and the One-Dimensional Rydberg Atom

abstract:
  Isolated electrons resting above a helium surface are predicted to have a
bound spectrum corresponding to a one-dimensional hydrogen atom. But in fact,
the observed spectrum is closer to that of a quantum-defect atom. Such a model
is discussed and solved in analytic closed form.


title:
Bogoliubov dispersion relation for a "photon fluid": Is this a
  superfluid?

abstract:
  We discuss the possibility that photons, which are bosons, can form a 2D
superfluid due to Bose-Einstein condensation inside a nonlinear Fabry-Perot
cavity filled with atoms in their ground states. A "photon fluid" forms inside
the cavity as a result of multiple photon-photon collisions mediated by the
atoms during a cavity ring-down time. The effective mass and chemical potential
for a photon inside this fluid are nonvanishing. This implies the existence of
a Bogoliubov dispersion relation for the low-lying elementary excitations of
the photon fluid, and in particular, that sound waves exist for
long-wavelength, low-frequency disturbances of this fluid. Possible experiments
to test for the superfluidity of the photon fluid based on the Landau
critical-velocity criterion will be discussed.