title: Observation of Ultraslow Group Velocity of Light in a Solid

abstract:
  We report ultraslow group velocities of light in a solid. Light speeds as
slow as 45 m/s were observed, corresponding to a group delay of 66 ms in a 3-mm
thick crystal. Reduction of the group velocity is accomplished by using a sharp
spectral feature in absorption and dispersion, produced by a Raman excited spin
coherence in an optically dense Pr doped Y2SiO5 crystal.


title: Deduction of Planck's Formula from Multiphoton States

abstract:
  We obtain the black body radiation formula of Planck by considering
independent contributions of multiphoton states


title:
Environment-Induced Decoherence and the Transition From Quantum to
  Classical

abstract:
  We study dynamics of quantum open systems, paying special attention to those
aspects of their evolution which are relevant to the transition from quantum to
classical. We begin with a discussion of the conditional dynamics of simple
systems. The resulting models are straightforward but suffice to illustrate
basic physical ideas behind quantum measurements and decoherence. To discuss
decoherence and environment-induced superselection einselection in a more
general setting, we sketch perturbative as well as exact derivations of several
master equations valid for various systems. Using these equations we study
einselection employing the general strategy of the predictability sieve.
Assumptions that are usually made in the discussion of decoherence are
critically reexamined along with the ``standard lore'' to which they lead.
Restoration of quantum-classical correspondence in systems that are classically
chaotic is discussed. The dynamical second law -it is shown- can be traced to
the same phenomena that allow for the restoration of the correspondence
principle in decohering chaotic systems (where it is otherwise lost on a very
short time-scale). Quantum error correction is discussed as an example of an
anti-decoherence strategy. Implications of decoherence and einselection for the
interpretation of quantum theory are briefly pointed out.


title: Entanglement vs. Noncommutativity in Teleportation

abstract:
  We provide an alternative simple proof of the necessity of entanglement in
quantum teleportation by using the no-disentanglement theorem. We show that
this is true even when the state to be teleported is known to be among two
noncommuting qubits. We further show that to teleport any set of commuting
qubits, it is sufficient to have a classically correlated channel. Using this
result we provide a simple proof of the fact that any set of bipartite
entangled states can be exactly disentangled if the single particle density
matrices of any one party commute.


title: Geometry and Product States

abstract:
  As separable states are a convex combination of product states, the geometry
of the manifold of product states is studied. Prior results by Sanpera, Vidal
and Tarrach are extended. Furthermore, it is proven that states in the set
tangent to the manifold of product states, at the maximally mixed state are
separable; the set normal constains, among others, all maximally entangled
states. A canonical decomposition is given. A surprising result is that for the
case of two particles, the closest product state to the maximally entangled
state is the maximally mixed state. An algorithm is provided to find the
closest product state.


title:
Feynman Clocks, Casual Networks, and the Origin of Hierarchical 'Arrows
  of Time' in Complex Systems from the Big Bang to the Brain

abstract:
  A theory of 'time' as a form of 'information' is proposed. New tools such as
Feynman Clocks, Collective Excitation Networks, Sequential Excitation Networks,
Plateaus of Complexity, and Causal Networks are used to unify previously
separate 'arrows of time'.


title: q-deformed dynamics and Josephson junction

abstract:
  We define a generalized rate equation for an observable in quantum mechanics,
that involves a parameter q and whose limit $q\to 1$ gives the standard
Heisenberg equation. The generalized rate equation is used to study dynamics of
current biased Josephson junction. It is observed that this toy model
incorporates diffraction like effects in the critical current. Physical
interpretation for q is provided which is also shown to be q-deformation
parameter.


title: A Mach-Zehnder Interferometer for a Two-Photon Wave Packet

abstract:
  We propose an experiment that permits observation of the de Broglie
two-photon wave packet behavior for a pair of photons, using a Mach-Zehnder
interferometer. It is based on the use of pulsed lasers to generate pairs of
photons via spontaneous parametric down-conversion and the post-selection of
events. It differs from previous realizations by the use of a third
time-correlated photon to engineer the state of the photons. The same technique
can give us which-path information via an ``interaction-free'' experiment and
can be used in other experiments on the foundations of quantum mechanics
related to wave-particle duality and to nonlocality.


title:
Quantum Coherence in a Single Ion due to strong Excitation of a
  metastable Transition

abstract:
  We consider pump-probe spectroscopy of a single ion with a highly metastable
(probe) clock transition which is monitored by using the quantum jump
technique. For a weak clock laser we obtain the well known Autler-Townes
splitting. For stronger powers of the clock laser we demonstrate the transition
to a new regime. The two regimes are distinguished by the transition of two
complex eigenvalues to purely imaginary ones which can be very different in
magnitude. The transition is controlled by the power of the clock laser. For
pump on resonance we present simple analytical expressions for various
linewidths and line positions.


title: Number Partitioning on a Quantum Computer

abstract:
  We present an algorithm to compute the number of solutions of the
(constrained) number partitioning problem. A concrete implementation of the
algorithm on an Ising-type quantum computer is given.


title:
Absolute frequency measurement of the In$^{+}$ clock transition with a
  mode-locked laser

abstract:
  The absolute frequency of the In$^{+}$ $5s^{2 1}S_{0}$ - $5s5p^{3}P_{0}$
clock transition at 237 nm was measured with an accuracy of 1.8 parts in
$10^{13}$. Using a phase-coherent frequency chain, we compared the $^{1}S_{0}$
- $^{3}P_{0}$ transition with a methane-stabilized He-Ne laser at 3.39 $\mu$m
which was calibrated against an atomic cesium fountain clock. A frequency gap
of 37 THz at the fourth harmonic of the He-Ne standard was bridged by a
frequency comb generated by a mode-locked femtosecond laser. The frequency of
the In$^{+}$ clock transition was found to be $1 267 402 452 899.92 (0.23)$
kHz, the accuracy being limited by the uncertainty of the He-Ne laser
reference. This represents an improvement in accuracy of more than 2 orders of
magnitude on previous measurements of the line and now stands as the most
accurate measurement of an optical transition in a single ion.


title:
Quantum trajectories, real, surreal or an approximation to a deeper
  process?

abstract:
  The proposal that the one-parameter solutions of the real part of the
Schrodinger equation (quantum Hamilton-Jacobi equation) can be regarded as
`quantum particle trajectories' has received considerable attention recently.
Opinions as to their significance differ. Some argue that they do play a
fundamental role as actual particle trajectories, others regard them as mere
metaphysical appendages without any physical significance. Recent work has
claimed that in some cases the Bohm approach gives results that disagree with
those obtained from standard quantum mechanics and, in consequence, with
experiment. Furthermore it is claimed that these trajectories have such
unacceptable properties that they can only be considered as `surreal'. We
re-examine these questions and show that the specific objections raised by
Englert, Scully, Sussmann and Walther cannot be sustained. We also argue that
contrary to their negative view, these trajectories can provide a deeper
insight into quantum processes.


title:
Finding Solutions to NP Problems: Philosophical Difference Between
  Quantum and Evolutionary Search Algorithms

abstract:
  There is no known polynomial-time algorithm that can solve an NP problem.
Evolutionary search has been shown to be a viable method of finding acceptable
solutions within a reasonable time period. Recently quantum computers have
surfaced as another alternative method. But these two methods use radically
different philosophies for solving NP problems even though both search methods
are non-deterministic. This paper uses instances of {\bf SAT}, {\bf 3SAT} and
{\bf TSP} to describe how these two methods differ in their approach to solving
NP problems.


title: Information and noise in photon entanglement

abstract:
  By using finite resolution measurements it is possible to simultaneously
obtain noisy information on two non-commuting polarization components of a
single photon. This method can be applied to a pair of entangled photons with
polarization statistics that violate Bell's inequalities. The theoretically
predicted results show that the non-classical nature of entanglement arises
from negative joint probabilities for the non-commuting polarization
components. These negative probabilities allow a "disentanglement" of the
statistics, providing new insights into the non-classical properties of quantum
information.


title:
Macroscopic Local Realism Incompatible with Quantum Mechanics: Failure
  of Local Realism where Measurements give Macroscopic Uncertainties

abstract:
  We show that quantum mechanics predicts a contradiction with local hidden
variable theories for photon number measurements which have limited resolving
power, to the point of imposing an uncertainty in the photon number result
which is macroscopic in absolute terms. We show how this can be interpreted as
a failure of a new premise, macroscopic local realism.


title:
Contradiction of Quantum Mechanics with Local Hidden Variables for
  Continuous Variable Quadrature Phase Amplitude Measurements

abstract:
  We demonstrate a contradiction of quantum mechanics with local hidden
variable theories for continuous variable quadrature phase amplitude
(``position'' and ``momentum'') measurements, by way of a violation of a Bell
inequality. For any quantum state, this contradiction is lost for situations
where the quadrature phase amplitude results are always macroscopically
distinct. We show that for optical realisations of this experiment, where one
uses homodyne detection techniques to perform the quadrature phase amplitude
measurement, one has an amplification prior to detection, so that macroscopic
fields are incident on photodiode detectors. The high efficiencies of such
detectors may open a way for a loophole-free test of local hidden variable
theories.


title: Image and Coherence Transfer in the Stimulated Down-conversion Process

abstract:
  The intensity transverse profile of the light produced in the process of
stimulated down-conversion is derived. A quantum-mechanical treatment is used.
We show that the angular spectrum of the pump laser can be transferred to the
stimulated down-converted beam, so that images can also be transferred from the
pump to the down-converted beam. We also show that the transfer can occur from
the stimulating beam to the down-converted one. Finally, we study the process
of diffraction through an arbitrarily shaped screen. For the special case of a
double-slit, the interference pattern is explicitly obtained. The visibility
for the spontaneous emitted light is in accordance with the van Cittert -
Zernike theorem for incoherent light, while the visibility for the stimulated
emitted light is unity. The overall visibility is in accordance with previous
experimental results.


title: Superluminal Phenomena and the Quantum Preferred Frame

abstract:
  Motivated by a number of recent experiments, we discuss in this paper a
speculative but physically admissible form and solutions of effective
Maxwell-like equations describing propagation of electromagnetic field in a
medium which ``feels'' a quantum preferred frame.


title: Time, what is it? Dynamical Properties of Time

abstract:
  The phenomenon of local dynamical inhomogeneity of time is predicted, which
implies that the course of time along the trajectory of motion of a particle in
the inertial reference frames moving relative to each other depends on the
state of motion of the particle under the influence of a force field. As is
seen from the results obtained, the ability to influence the course of time
represents one of the most fundamental properties of any material system
intrinsically inherent in it by the very nature of things, which manifests
itself when the system interacts with force fields. The inferences of the paper
are not based on the use of any hypotheses and strictly follow from
relativistic equations of motion. The dependence of the course of time upon the
behaviour of physical system is, thus, a direct consequence of causality
principle, relativity principle and the pseudoeuclidity of space-time. The
results obtained confirm the Kozyrev hypothesis that time has physical
properties and open up radically new opportunities for the efficient control of
physical processes. It is demonstrated with point particle that the change in
the course of time results in the appearance of an additional force acting on
the particle. A general conclusion is drawn on the basis of the theory advanced
that relativistic equations of motion for any kind of matter contain
information about the physical properties of time, which are, thus, of
dynamical nature.


title: A New Formulation of Grover's Algorithm

abstract: Withdrawn; replaced by longer, more detailed paper quant-ph/0010065.


title: The importance of quantum decoherence in brain processes

abstract:
  It is shown how environmental decoherence plays an essential and constructive
role in a quantum mechanical theory of brain process that has significant
explanatory power.


title:
An extension of "Popper's experiment" can test interpretations of
  quantum mechanics

abstract:
  Karl Popper proposed a way to test whether a proposed relation of a
quantum-mechanical state to perceived reality in the Copenhagen interpretation
(CI) of quantum mechanics - namely that the state of a particle is merely an
expression of ``what is known'' about the system - is in agreement with all
experimental facts. A conceptual flaw in Popper's proposal is identified and an
improved version of his experiment (called ``Extension step 1'') - which fully
serves its original purpose - is suggested. The main purpose of this paper is
to suggest to perform this experiment. The results of this experiment predicted
under the alternative assumptions that the CI - together with the above
connection of the state function with reality - or the ``many-worlds''
interpretation (MWI) is correct are shown to be identical. Only after a further
modification (called ``Extension step 2'') - the use of an ion isolated from
the macroscopic environment as particle detector - the predictions using the
respective interpretations become qualitatively different.


title: Complete controllability of quantum systems

abstract:
  Sufficient conditions for complete controllability of $N$-level quantum
systems subject to a single control pulse that addresses multiple allowed
transitions concurrently are established. The results are applied in particular
to Morse and harmonic-oscillator systems, as well as some systems with
degenerate energy levels. Morse and harmonic oscillators serve as models for
molecular bonds, and the standard control approach of using a sequence of
frequency-selective pulses to address a single transition at a time is either
not applicable or only of limited utility for such systems.


title:
Limits of control for quantum systems: kinematical bounds on the
  optimization of observables and the question of dynamical realizability

abstract:
  In this paper we investigate the limits of control for mixed-state quantum
systems. The constraint of unitary evolution for non-dissipative quantum
systems imposes kinematical bounds on the optimization of arbitrary
observables. We summarize our previous results on kinematical bounds and show
that these bounds are dynamically realizable for completely controllable
systems. Moreover, we establish improved bounds for certain partially
controllable systems. Finally, the question of dynamical realizability of the
bounds for arbitary partially controllable systems is shown to depend on the
accessible sets of the associated control system on the unitary group U(N) and
the results of a few control computations are discussed briefly.


title: Quantum computing via measurements only

abstract:
  A quantum computer promises efficient processing of certain computational
tasks that are intractable with classical computer technology. While basic
principles of a quantum computer have been demonstrated in the laboratory,
scalability of these systems to a large number of qubits, essential for
practical applications such as the Shor algorithm, represents a formidable
challenge. Most of the current experiments are designed to implement sequences
of highly controlled interactions between selected particles (qubits), thereby
following models of a quantum computer as a (sequential) network of quantum
logic gates. Here we propose a different model of a scalable quantum computer.
In our model, the entire resource for the quantum computation is provided
initially in form of a specific entangled state (a so-called cluster state) of
a large number of qubits. Information is then written onto the cluster,
processed, and read out form the cluster by one-particle measurements only. The
entangled state of the cluster thus serves as a universal substrate for any
quantum computation. Cluster states can be created efficiently in any system
with a quantum Ising-type interaction (at very low temperatures) between
two-state particles in a lattice configuration.


title: Shor's Quantum Factoring Algorithm

abstract:
  This paper is a written version of a one hour lecture given on Peter Shor's
quantum factoring algorithm.


title:
Improving the Fidelity of Quantum Cloning by Fast Cycling away the
  Unwanted Transition

abstract:
  The fidelity of quantum cloning is very often limited by the accompanying
unwanted transitions. We show how the fidelity can be improved by using a
coherent field to cycle away the unwanted transitions. We demonstrate this
explicitly in the context of the model of Simon {\it et al.} [J. Mod. Opt. {\bf
47}, 233 (2000) ; Phys. Rev. Lett. {\bf 84}, 2993 (2000)]. We also investigate
the effects of the number of atoms on the quality of quantum cloning. We show
that the universality of the scheme can be maintained by choosing the cycling
field according to the input state of the qubit.


title:
Coherent quantum measurement for the direct determination of the degree
  of polarization and Polarization Mode Dispersion compensation

abstract:
  An example of a coherent measurement for the direct evaluation of the degree
of polarization of a single-mode optical beam is presented. It is applied to
the case of great practical importance where depolarization is caused by
polarization mode dispersion. It is demonstrated that coherent measurement has
the potential of significantly increasing the information gain, compared to
standard incoherent measurements.


title: Quantum nondemolition measurements in a Paul trap

abstract:
  In this work a family of quantum nondemolition variables for the case of a
particle caught in a Paul trap is obtained. Afterwards, in the context of the
so called restricted path integral formalism, a continuous measuring process
for this family of parameters is considered, and then the corresponding
propagators are calculated. In other words, the time evolution of a particle in
a Paul trap, when the corresponding quantum nondemolition parameter is being
continuously monitored, is deduced. The probabilities associated with the
possible measurement outputs are also obtained, and in this way new theoretical
results emerge, which could allow us to confront the predictions of this
restricted path integral formalism with the readouts of some future
experiments.


title: Entanglement purification of unknown quantum states

abstract:
  A concern has been expressed that ``the Jaynes principle can produce fake
entanglement'' [R. Horodecki et al., Phys. Rev. A {\bf 59}, 1799 (1999)]. In
this paper we discuss the general problem of distilling maximally entangled
states from $N$ copies of a bipartite quantum system about which only partial
information is known, for instance in the form of a given expectation value. We
point out that there is indeed a problem with applying the Jaynes principle of
maximum entropy to more than one copy of a system, but the nature of this
problem is classical and was discussed extensively by Jaynes. Under the
additional assumption that the state $\rho^{(N)}$ of the $N$ copies of the
quantum system is exchangeable, one can write down a simple general expression
for $\rho^{(N)}$. We show how to modify two standard entanglement purification
protocols, one-way hashing and recurrence, so that they can be applied to
exchangeable states. We thus give an explicit algorithm for distilling
entanglement from an unknown or partially known quantum state.


title: Dirac's hole theory versus quantum field theory

abstract:
  Dirac's hole theory and quantum field theory are usually considered
equivalent to each other. For models of a certain type, however, the
equivalence may not hold as we discuss in this Letter. This problem is closely
related to the validity of the Pauli principle in intermediate states of
perturbation theory.


title: Grover's quantum search algorithm

abstract:
  This paper is a written version of a one hour lecture given on Lov Grover's
database search algorithm.
  Table Of Contents
  1 ... Problem Definition
  2 ... The quantum mechanical perspective
  3 ... Properties of the inversion I_|psi>
  4 ... The method in Lov's "madness"
  5 ... Summary of Grover's algorithm
  6 ... An example of Grover's algorithm
  References


title: Entanglement of Formation and Conditional Information Transmission

abstract:
  We show that the separability of states in quantum mechanics has a close
counterpart in classical physics, and that conditional mutual information
(a.k.a. conditional information transmission) is a very useful quantity in the
study of both quantum and classical separabilities. We also show how to define
entanglement of formation in terms of conditional mutual information. This
paper lays the theoretical foundations for a sequel paper which will present a
computer program that can calculate a decomposition of any separable quantum or
classical state.


title: On a Physical Metatheory of Consciousness

abstract:
  We show that the modern quantum mechanics, and particularly the theory of
decoherence, allows for formulating a sort of a physical metatheory of
consciousness. Particularly, the analysis of the necessary conditions for the
occurrence of decoherence, along with the hypothesis that consciousness bears
(more-or-less) well definable physical origin, leads to a wider physical
picture naturally involving consciousness. This can be considered as a sort of
a psycho-physical parallelism, but on rather wide scales bearing some
cosmological relevance.


title: Quantum Factor Graphs

abstract:
  The natural Hilbert Space of quantum particles can implement
maximum-likelihood (ML) decoding of classical information. The 'Quantum Product
Algorithm' (QPA) is computed on a Factor Graph, where function nodes are
unitary matrix operations followed by appropriate quantum measurement. QPA is
like the Sum-Product Algorithm (SPA), but without summary, giving optimal
decode with exponentially finer detail than achievable using SPA. Graph cycles
have no effect on QPA performance. QPA must be repeated a number of times
before successful and the ML codeword is obtained only after repeated quantum
'experiments'. ML amplification improves decoding accuracy, and Distributed QPA
facilitates successful evolution.


title: Bunching and antibunching from single NV color centers in diamond

abstract:
  We investigate correlations between fluorescence photons emitted by single
N-V centers in diamond with respect to the optical excitation power. The
autocorrelation function shows clear photon antibunching at short times,
proving the uniqueness of the emitting center. We also report on a photon
bunching effect, which involves a trapping level. An analysis using
rate-equations for the populations of the N-V center levels shows the intensity
dependence of the rate equation coefficients.


title:
Self-Binding Transition in Bose Condensates with Laser-Induced
  ``Gravitation''

abstract:
  In our recent publication (D. O'Dell, et al, Phys. Rev. Lett. 84, 5687
(2000)) we proposed a scheme for electromagnetically generating a self-bound
Bose-Einstein condensate with 1/r attractive interactions: the analog of a Bose
star. Here we focus upon the conditions neccessary to observe the transition
from external trapping to self-binding. This transition becomes manifest in a
sharp reduction of the condensate radius and its dependence on the laser
intensity rather that the trap potential.


title: Quantum Teleportation with a Complete Bell State Measurement

abstract:
  We report a quantum teleportation experiment in which nonlinear interactions
are used for the Bell state measurements. The experimental results demonstrate
the working principle of irreversibly teleporting an unknown arbitrary quantum
state from one system to another distant system by disassembling into and then
later reconstructing from purely classical information and nonclassical EPR
correlations. The distinct feature of this experiment is that \emph{all} four
Bell states can be distinguished in the Bell state measurement. Teleportation
of a quantum state can thus occur with certainty in principle.


title: Bell's Theorem Without Hidden Variables

abstract:
  Experiments motivated by Bell's theorem have led some physicists to conclude
that quantum theory is nonlocal. However, the theoretical basis for such claims
is usually taken to be Bell's Theorem, which shows only that if certain
predictions of quantum theory are correct, and a strong hidden-variable
assumption is valid, then a certain locality condition must fail. This locality
condition expresses the idea that what an experimenter freely chooses to
measure in one spacetime region can have no effect of any kind in a second
region situated spacelike relative to the first. The experimental results
conform closely to the predictions of quantum theory in such cases, but the
most reasonable conclusion to draw is not that locality fails, but rather that
the hidden-variable assumption is false. For this assumption conflicts with the
quantum precept that unperformed experiments have no outcomes. The present
paper deduces the failure of this locality condition directly from the precepts
of quantum theory themselves, in a way that generates no inconsistency or any
conflict with the predictions of relativistic quantum field theory.


title: Zero knowledge convincing protocol on quantum bit is impossible

abstract:
  Consider two parties: Alice and Bob and suppose that Bob is given a qubit
system in a quantum state $\phi$, unknown to him. Alice knows $\phi$ and she is
supposed to convince Bob that she knows $\phi$ sending some test message. Is it
possible for her to convince Bob providing him "zero knowledge" i. e. no
information about $\phi$ he has? We prove that there is no "zero knowledge"
protocol of that kind. In fact it turns out that basing on Alice message, Bob
(or third party - Eve - who can intercept the message) can synthetize a copy of
the unknown qubit state $\phi$ with nonzero probability. This "no-go" result
puts general constrains on information processing where information {\it about}
quantum state is involved.


title: Unitary transformations for testing Bell inequalities

abstract:
  It is shown that optical experimental tests of Bell inequality violations can
be described by SU(1,1) transformations of the vacuum state, followed by photon
coincidence detections. The set of all possible tests are described by various
SU(1,1) subgroups of Sp(8,$\Bbb R$). In addition to establishing a common
formalism for physically distinct Bell inequality tests, the similarities and
differences of post--selected tests of Bell inequality violations are also made
clear. A consequence of this analysis is that Bell inequality tests are
performed on a very general version of SU(1,1) coherent states, and the
theoretical violation of the Bell inequality by coincidence detection is
calculated and discussed. This group theoretical approach to Bell states is
relevant to Bell state measurements, which are performed, for example, in
quantum teleportation.


title: Nash Equilibrium in the Quantum Battle of Sexes Game

abstract:
  We investigate Nash Equilibrium in the quantum Battle of Sexes Game. We find
the game has infinite Nash Equilibria and all of them leads to the asymmetry
result. We also show that there is no unique but infinite Nash Equilibrium in
it if we use the quantizing scheme proposed by Eisert et al and the two players
are allowed to adopt any unitary operator as his/her strategies.


title: Quantum Dynamics of Three Coupled Atomic Bose-Einstein Condensates

abstract:
  The simplest model of three coupled Bose-Einstein Condensates (BEC) is
investigated using a group theoretical method. The stationary solutions are
determined using the SU(3) group under the mean field approximation. This
semiclassical analysis using the system symmetries shows a transition in the
dynamics of the system from self trapping to delocalization at a critical value
for the coupling between the condensates. The global dynamics are investigated
by examination of the stable points and our analysis shows the structure of the
stable points depends on the ratio of the condensate coupling to the
particle-particle interaction, undergoes bifurcations as this ratio is varied.
This semiclassical model is compared to a full quantum treatment, which also
displays the dynamical transition. The quantum case has collapse and revival
sequences superposed on the semiclassical dynamics reflecting the underlying
discreteness of the spectrum. Non-zero circular current states are also
demonstrated as one of the higher dimensional effects displayed in this system.


title: A potential multipartide entanglement measure

abstract:
  We discuss entanglement of multiparticle quantum systems. We propose a
potential measure of a type of entanglement of pure states of n qubits, the
n-tangle. For a system of two qubits the n-tangle is equal to the square of the
concurrence, and for systems of three qubits it is equal to the ''residual
entanglement''. We show that the n -tangle, is also equal to the generalization
of concurrence squared for even n, and use this fact to prove that the n-tangle
is an entanglement monotone. However, the n-tangle is undefined for odd n>3.
Finally we propose a measure related to the n-tangle for mixed state systems of
n qubits, and find an analytical formula for this measure for even n.


title: Entanglement transformation at dielectric four-port devices

abstract:
  Quantum communication schemes widely use dielectric four-port devices as
basic elements for constructing optical quantum channels. Since for causality
reasons the permittivity is necessarily a complex function of frequency,
dielectrics are typical examples of noisy quantum channels in which quantum
coherence will not be preserved. Basing on quantization of the phenomenological
electrodynamics, we construct the transformation relating the output quantum
state to the input quantum state without placing frequency restrictions.
Knowledge of the full transformed quantum state enables us to compute the
entanglement contained in the output quantum state. We apply the formalism to
some typical examples in quantum communication.


title:
Nonadditivity of Bipartite Distillable Entanglement follows from
  Conjecture on Bound Entangled Werner States

abstract:
  Assuming the validity of a conjecture in quant-ph/9910026 and
quant-ph/9910022 we show that the distillable entanglement for two bipartite
states, each of which individually has zero distillable entanglement, can be
nonzero. We show that this also implies that the distillable entanglement is
not a convex function. Our example consists of the tensor product of a bound
entangled state based on an unextendible product basis with a Werner state
which lies in the class of conjectured undistillable states.


title: Scharnhorst effect at oblique incidence

abstract:
  We consider the Scharnhorst effect (anomalous photon propagation in the
Casimir vacuum) at oblique incidence, calculating both photon speed and
polarization states as functions of angle. The analysis is performed in the
framework of nonlinear electrodynamics and we show that many features of the
situation can be extracted solely on the basis of symmetry considerations.
Although birefringence is common in nonlinear electrodynamics it is not
universal; in particular we verify that the Casimir vacuum is not birefringent
at any incidence angle. On the other hand, group velocity is typically not
equal to phase velocity, though the distinction vanishes for special directions
or if one is only working to second order in the fine structure constant. We
obtain an ``effective metric'' that is subtly different from previous results.
The disagreement is due to the way that ``polarization sums'' are implemented
in the extant literature, and we demonstrate that a fully consistent
polarization sum must be implemented via a bootstrap procedure using the
effective metric one is attempting to define. Furthermore, in the case of
birefringence, we show that the polarization sum technique is intrinsically an
approximation.


title: Time Correlation in Tunneling of Photons

abstract:
  I propose to consider photon tunneling as a space-time correlation phenomenon
between the emission and absorption of a photon on the two sides of a barrier.
Standard technics based on an appropriate counting rate formula may then be
applied to derive the tunneling time distribution without any {\em ad hoc}
definition of this quantity. General formulae are worked out for a potential
model using Wigner-Weisskopf method. For a homogeneous square barrier in the
limit of zero tunneling probability a vanishing tunneling time is obtained.


title: Theory of quantum computation

abstract:
  Short review article on quantum computation accepted for Supplement III,
Encyclopaedia of Mathematics (publication expected Summer 2001). See also
http://www.wkap.nl/series.htm/ENM


title: Quantum information processing

abstract:
  Short review article on quantum information processing accepted for
Supplement III, Encyclopaedia of Mathematics (publication expected Summer
2001). See also http://www.wkap.nl/series.htm/ENM


title: The effects of time delays in adaptive phase measurements

abstract:
  It is not possible to make measurements of the phase of an optical mode using
linear optics without introducing an extra phase uncertainty. This extra phase
variance is quite large for heterodyne measurements, however it is possible to
reduce it to the theoretical limit of log(n)/(4n^2) using adaptive
measurements. These measurements are quite sensitive to experimental
inaccuracies, especially time delays and inefficient detectors. Here it is
shown that the minimum introduced phase variance when there is a time delay of
tau is tau/(8n). This result is verified numerically, showing that the phase
variance introduced approaches this limit for most of the adaptive schemes
using the best final phase estimate. The main exception is the adaptive mark II
scheme with simplified feedback, which is extremely sensitive to time delays.
The extra phase variance due to time delays is considered for the mark I case
with simplified feedback, verifying the tau/2 result obtained by Wiseman and
Killip both by a more rigorous analytic technique and numerically.


title: Quantum Information Theory

abstract:
  In this thesis I present a short review of ideas in quantum information
theory. The first chapter contains introductory material, sketching the central
ideas of probability and information theory. Quantum mechanics is presented at
the level of advanced undergraduate knowledge, together with some useful tools
for quantum mechanics of open systems. In the second chapter I outline how
classical information is represented in quantum systems and what this means for
agents trying to extract information from these systems. The final chapter
presents a new resource: quantum information. This resource has some
bewildering applications which have been discovered in the last ten years, and
continually presents us with unexpected insights into quantum theory and the
universe.
  The treatment is pedagogical and suitable for beginning graduates in the
field.


title: Semiclassical theory of cavity-assisted atom cooling

abstract:
  We present a systematic semiclassical model for the simulation of the
dynamics of a single two-level atom strongly coupled to a driven high-finesse
optical cavity. From the Fokker-Planck equation of the combined atom-field
Wigner function we derive stochastic differential equations for the atomic
motion and the cavity field. The corresponding noise sources exhibit strong
correlations between the atomic momentum fluctuations and the noise in the
phase quadrature of the cavity field. The model provides an effective tool to
investigate localisation effects as well as cooling and trapping times. In
addition, we can continuously study the transition from a few photon quantum
field to the classical limit of a large coherent field amplitude.


title: Homodyne Bell's inequalities for entangled mesoscopic superpositions

abstract:
  We present a scheme for demonstrating violation of Bell's inequalities using
a spin-1/2 system entangled with a pair of classically distinguishable wave
packets in a harmonic potential. In the optical domain, such wave packets can
be represented by coherent states of a single light mode. The proposed scheme
involves standard spin-1/2 projections and measurements of the position and the
momentum of the harmonic oscillator system, which for a light mode can be
realized by means of homodyne detection. We discuss effects of imperfections,
including non-unit efficiency of the homodyne detector, and point out a close
link between the visibility of interference and violation of Bell's
inequalities in the described scheme.


title:
Vacuum-Stimulated Raman Scattering based on Adiabatic Passage in a
  High-Finesse Optical Cavity

abstract:
  We report on the first observation of stimulated Raman scattering from a
Lambda-type three-level atom, where the stimulation is realized by the vacuum
field of a high-finesse optical cavity. The scheme produces one intracavity
photon by means of an adiabatic passage technique based on a counter-intuitive
interaction sequence between pump laser and cavity field. This photon leaves
the cavity through the less-reflecting mirror. The emission rate shows a
characteristic dependence on the cavity and pump detuning, and the observed
spectra have a sub-natural linewidth. The results are in excellent agreement
with numerical simulations.


title: Quantum Jumps on a Circle

abstract:
  It is demonstrated that in contrast to the well-known case with a quantum
particle moving freely in a real line, the wave packets corresponding to the
coherent states for a free quantum particle on a circle do not spread but
develop periodically in time. The discontinuous changes during the course of
time in the phase representing the position of a particle can be interpreted as
the quantum jumps on a circle.


title: Artificial Orbitals and a Solution to Grover's Problem

abstract:
  By allowing measurements of observables other than the state of the qubits in
a quantum computer, one can find eigenvectors very quickly. If a unitary
operation U is implemented as a time-independent Hamiltonian, for instance, one
can collapse the state of the computer to a nearby eigenvector of U with a
measurement of the energy. We examine some recent proposals for quantum
computation using time-independent Hamiltonians and show how to convert them
into ``artificial orbitals'' whose energy eigenstates match those of U. This
system can be used to find eigenvectors and eigenvalues with a single
measurement. We apply this technique to Grover's algorithm and the continuous
variant proposed by Farhi and Gutmann.


title: Vacuum Induced Coherences in Radiatively Coupled Multilevel Systems

abstract:
  We show that radiative coupling between two multilevel atoms having
near-degenerate states can produce new interference effects in spontaneous
emission. We explicitly demonstrate this possibility by considering two
identical V systems each having a pair of transition dipole matrix elements
which are orthogonal to each other. We discuss in detail the origin of the new
interference terms and their consequences. Such terms lead to the evolution of
certain coherences and excitations which would not occur otherwise. The special
choice of the orientation of the transition dipole matrix elements enables us
to illustrate the significance of vacuum induced coherence in multi-atom
multilevel systems. These coherences can be significant in energy transfer
studies.


title: Laser Cooling of Trapped Fermi Gases deeply below the Fermi Temperature

abstract:
  We study the collective Raman cooling of a polarized trapped Fermi gas in the
Festina Lente regime, when the heating effects associated with photon
reabsorptions are suppressed. We predict that by adjusting the spontaneous
Raman emission rates and using appropriately designed anharmonic traps,
temperatures of the order of 2.7% of the Fermi temperature can be achieved in
3D.


title: Geometric Phase of Three-level Systems in Interferometry

abstract:
  We present the first scheme for producing and measuring an Abelian geometric
phase shift in a three-level system where states are invariant under a
non-Abelian group. In contrast to existing experiments and proposals for
experiments, based on U(1)-invariant states, our scheme geodesically evolves
U(2)-invariant states in a four-dimensional SU(3)/U(2) space and is physically
realized via a three-channel optical interferometer.


title: Quantum mechanics with the permitted hidden parameters

abstract:
  Within the framework of the algebraic approach the problem of hidden
parameters in quantum mechanics is surveyed. It is shown that the algebraic
formulation of quantum mechanics permits introduction of a specific hidden
parameter, which has the form of nonlinear functional on the algebra of
observables. It is found out that the reasoning of von Neumann and of Bell
about incompatibility of quantum mechanics with hidden parameters is
inapplicable to the present case.


title:
Approximate quantum cloning and the impossibility of superluminal
  information transfer

abstract:
  We show that nonlocality of quantum mechanics cannot lead to superluminal
transmission of information, even if most general local operations are allowed,
as long as they are linear and trace preserving. In particular, any quantum
mechanical approximate cloning transformation does not allow signalling. On the
other hand, the no-signalling constraint on its own is not sufficient to
prevent a transformation from surpassing the known cloning bounds. We
illustrate these concepts on the basis of some examples.


title: Clifford algebras and universal sets of quantum gates

abstract:
  In this paper is shown an application of Clifford algebras to the
construction of computationally universal sets of quantum gates for $n$-qubit
systems. It is based on the well-known application of Lie algebras together
with the especially simple commutation law for Clifford algebras, which states
that all basic elements either commute or anticommute.


title: Decoherence in a superconducting ring

abstract:
  A superconducting ring has different sectors of states corresponding to
different values of the trapped magnetic flux; this multitude of states can be
used for quantum information storage. If a current supporting a nonzero flux is
set up in the ring, fluctuations of electromagnetic field will be able to
``detect'' that current and thus cause a loss of quantum coherence. We estimate
the decoherence exponent for a ring of a round type-II wire and find that it
contains a macroscopic suppression factor $(\delta / R_{1})^{2}$, where $R_{1}$
is the radius of the wire, and $\delta$ is the London penetration depth. We
present some encouraging numerical estimates based on this result.


title: Singular Potentials and Limit Cycles

abstract:
  We show that a central $1/r^n$ singular potential (with $n\geq 2$) is
renormalized by a one-parameter square-well counterterm; low-energy observables
are made independent of the square-well width by adjusting the square-well
strength. We find a closed form expression for the renormalization-group
evolution of the square-well counterterm.


title: Entanglement of Quantum Evolutions

abstract:
  The notion of entanglement can be naturally extended from quantum-states to
the level of general quantum evolutions. This is achieved by considering
multi-partite unitary transformations as elements of a multi-partite Hilbert
space and then extended to general quantum operations. We show some connection
between this entanglement and the entangling capabilities of the quantum
evolution.


title: Computational leakage: Grover's algorithm with imperfections

abstract:
  We study the effects of dissipation or leakage on the time evolution of
Grover's algorithm for a quantum computer. We introduce an effective two-level
model with dissipation and randomness (imperfections), which is based upon the
idea that ideal Grover's algorithm operates in a 2-dimensional Hilbert space.
The simulation results of this model and Grover's algorithm with imperfections
are compared, and it is found that they are in good agreement for appropriately
tuned parameters. It turns out that the main features of Grover's algorithm
with imperfections can be understood in terms of two basic mechanisms, namely,
a diffusion of probability density into the full Hilbert space and a stochastic
rotation within the original 2-dimensional Hilbert space.


title: Beyond Stabilizer Codes II: Clifford Codes

abstract:
  Knill introduced a generalization of stabilizer codes, in this note called
Clifford codes. It remained unclear whether or not Clifford codes can be
superior to stabilizer codes. We show that Clifford codes are stabilizer codes
provided that the abstract error group has an abelian index group. In
particular, if the errors are modelled by tensor products of Pauli matrices,
then the associated Clifford codes are necessarily stabilizer codes.


title:
Analysis of Generalized Grover's Quantum Search Algorithms Using
  Recursion Equations

abstract:
  The recursion equation analysis of Grover's quantum search algorithm
presented by Biham et al. [PRA 60, 2742 (1999)] is generalized. It is applied
to the large class of Grover's type algorithms in which the Hadamard transform
is replaced by any other unitary transformation and the phase inversion is
replaced by a rotation by an arbitrary angle. The time evolution of the
amplitudes of the marked and unmarked states, for any initial complex amplitude
distribution is expressed using first order linear difference equations. These
equations are solved exactly. The solution provides the number of iterations T
after which the probability of finding a marked state upon measurement is the
highest, as well as the value of this probability, P_max. Both T and P_max are
found to depend on the averages and variances of the initial amplitude
distributions of the marked and unmarked states, but not on higher moments.


title: On the partial wave amplitude of Coulomb scattering in three dimensions

abstract:
  The partial wave series for the Coulomb scattering amplitude in three
dimensions is evaluated in a very simple way to give the closed result.


title: Semiclassical description of Stern-Gerlach experiments

abstract:
  The motion of neutral particles with magnetic moments in an inhomogeneous
magnetic field is described in a semi-classical framework. The concept of
Coherent Internal States is used in the formulation of the semiclassical
approximation from the full quantum mechanical expression. The classical
trajectories are defined only for certain spin state, that satusfy the
conditions for being Coherent Internal State. The raalibility of Stern-Gerlach
experiments to measure spin projectons is assessed in this framework.


title: Bounds on relative entropy of entanglement for multi-party systems

abstract:
  We present upper and lower bounds to the relative entropy of entanglement of
multi-party systems in terms of the bi-partite entanglements of formation and
distillation and entropies of various subsystems. We point out implications of
our results to the local reversible convertibility of multi-party pure states
and discuss their physical basis in terms of deleting of information.


title: Theory of the Quantum Speed Up

abstract:
  Insofar as quantum computation is faster than classical, it appears to be
irreversible. In all quantum algorithms found so far the speed-up depends on
the extra-dynamical irreversible projection representing quantum measurement.
Quantum measurement performs a computation that dynamical computation cannot
accomplish as efficiently.


title: Beyond Stabilizer Codes I: Nice Error Bases

abstract:
  Nice error bases have been introduced by Knill as a generalization of the
Pauli basis. These bases are shown to be projective representations of finite
groups. We classify all nice error bases of small degree, and all nice error
bases with abelian index groups. We show that in general an index group of a
nice error basis is necessarily solvable.


title: Can We Obtain Quantum Theory From Reasonable Axioms?

abstract: See quant-ph/0101012


title: Classical solution of the wave equation

abstract:
  The classical limit of wave quantum mechanics is analyzed. It is shown that
the general requirements of continuity and finiteness to the solution
$\psi(x)=Ae^{i\phi(x)}+ Be^{-i\phi(x)}$, where $\phi(x)=\frac 1\hbar W(x)$ and
$W(x)$ is the reduced classical action of the physical system, result in the
asymptote of the exact solution and general quantization condition for $W(x)$,
which yields the exact eigenvalues of the system.


title: Entangled quantum states as direction indicators

abstract:
  We consider the use of N spin-1/2 particles for indicating a direction in
space. If N>2, their optimal state is entangled. For large N, the mean square
error decreases as N^{-2} (rather than N^{-1} for parallel spins).


title: Reply to "On Stapp's`Nonlocal Character of Quantum Theory' "

abstract:
  The question raised by Shimony and Stein is examined and used to explain in
more detail a key point of my proof that any theory that conforms to certain
general ideas of orthodox relativistic quantum field theory must permit
transfers of information over spacelike intervals. It is also explained why
this result is not a problem for relativistic quantum theory, but, on the
contrary, opens the door to a satisfactory realistic relativistic quantum
theory based on the ideas of Tomonaga, Schwinger, and von Neumann.


title: Complex Calogero model with real energies

abstract:
  We show that and how PT symmetry (interpreted as a "weakened Hermiticity")
can be extended to the exactly solvable two- and three-particle Calogero model.


title: Spin squeezing of mixed systems

abstract:
  The notion of spin squeezing has been discussed in this paper using the
density matrix formalism. Extending the definition of squeezing for pure states
given by Kitagawa and Ueda in an appropriate manner and employing the spherical
tensor representation, we show that mixed spin states which are non-oriented
and possess vector polarization indeed exhibit squeezing. We construct a mixed
state of a spin 1 system using two spin 1/2 states and study its squeezing
behaviour as a function of the individual polarizations of the two spinors.


title: Understanding quantum effects from classical principles

abstract:
  Dynamics of a particle is formulated from classical principles that are
amended by the uncertainty principle. Two best known quantum effects:
interference and tunneling are discussed from these principles. It is shown
that identical to quantum results are obtained by solving only classical
equations of motion. Within the context of interference Aharonov-Bohm effect is
solved as a local action of magnetic force on the particle. On the example of
tunneling it is demonstrated how uncertainty principle amends traditional
classical mechanics: it allows the momentum of the particle to change without
the force being the cause of it.


title: Decoherence: A dynamical approach to superselection rules?

abstract:
  It is well known that the dynamical mechanism of decoherence may cause
apparent superselection rules, like that of molecular chirality. These
`environment-induced' or `soft' superselection rules may be contrasted with
`hard' superselection rules, like that of electric charge, whose existence is
usually rigorously demonstrated by means of certain symmetry principles. We
address the question of whether this distinction between `hard' and `soft' is
well founded and argue that, despite first appearance, it might not be. For
this we give a detailed and somewhat pedagogical exposition of the basic
structural properties of the spaces of states and observables in order to
establish a fairly precise notion of superselection rules. We then discuss two
examples: the Bargmann superselection rule for overall mass in ordinary quantum
mechanics, and the superselection rule for charge in quantum electrodynamics.


title: Local hidden-variable models and negative-probability measures

abstract:
  Elaborating on a previous work by Han et al., we give a general,
basis-independent proof of the necessity of negative probability measures in
order for a class of local hidden-variable (LHV) models to violate the
Bell-CHSH inequality. Moreover, we obtain general solutions for LHV-induced
probability measures that reproduce any consistent set of probabilities.


title: Multi-Player and Multi-Choice Quantum Game

abstract:
  We investigate a multi-player and multi-choice quantum game. We start from
two-player and two-choice game and the result is better than its classical
version. Then we extend it to N-player and N-choice cases. In the quantum
domain, we provide a strategy with which players can always avoid the worst
outcome. Also, by changing the value of the parameter of the initial state, the
probabilities for players to obtain the best payoff will be much higher that in
its classical version.


title: The Quantum-Classical Transition in Nonlinear Dynamical Systems

abstract:
  Viewed as approximations to quantum mechanics, classical evolutions can
violate the positive-semidefiniteness of the density matrix. The nature of this
violation suggests a classification of dynamical systems based on
classical-quantum correspondence; we show that this can be used to identify
when environmental interaction (decoherence) will be unsuccessful in inducing
the quantum-classical transition. In particular, the late-time Wigner function
can become positive without any corresponding approach to classical dynamics.
In the light of these results, we emphasize key issues relevant for experiments
studying the quantum-classical transition.


title: The Dirac algebra and its physical interpretation

abstract:
  A version of the Dirac equation is derived from first principles using a
combination of quaternions and multivariate 4-vectors. The nilpotent form of
the operators used allows us to derive explicit expressions for the
wavefunctions of free fermions, vector bosons, scalar bosons; Bose-Einstein
condensates, and baryons;annihilation, creation and vacuum operators; the
quantum field integrals; and C, P and T transformations; and to suggest new
insights into the meaning of supersymmetry and renormalization.


title: Entanglement Measures under Symmetry

abstract:
  We show how to simplify the computation of the entanglement of formation and
the relative entropy of entanglement for states, which are invariant under a
group of local symmetries. For several examples of groups we characterize the
state spaces, which are invariant under these groups. For specific examples we
calculate the entanglement measures. In particular, we derive an explicit
formula for the entanglement of formation for UU-invariant states, and we find
a counterexample to the additivity conjecture for the relative entropy of
entanglement.


title: Separability properties of tripartite states with UxUxU-symmetry

abstract:
  We study separability properties in a 5-dimensional set of states of quantum
systems composed of three subsystems of equal but arbitrary finite Hilbert
space dimension d. These are the states, which can be written as linear
combinations of permutation operators, or, equivalently, commute with unitaries
of the form UxUxU. We compute explicitly the following subsets and their
extreme points: (1) triseparable states, which are convex combinations of
triple tensor products, (2) biseparable states, which are separable for a
twofold partition of the system, and (3) states with positive partial transpose
with respect to such a partition. Tripartite entanglement is investigated in
terms of the relative entropy of tripartite entanglement and of the trace norm.


title:
A Lorentz-invariant look at quantum clock synchronization protocols
  based on distributed entanglement

abstract:
  Recent work has raised the possibility that quantum information theory
techniques can be used to synchronize atomic clocks nonlocally. One of the
proposed algorithms for quantum clock synchronization (QCS) requires
distribution of entangled pure singlets to the synchronizing parties. Such
remote entanglement distribution normally creates a relative phase error in the
distributed singlet state which then needs to be purified asynchronously. We
present a fully relativistic analysis of the QCS protocol which shows that
asynchronous entanglement purification is not possible, and, therefore, that
the proposed QCS scheme remains incomplete. We discuss possible directions of
research in quantum information theory which may lead to a complete, working
QCS protocol.


title: Quantum clock synchronization and quantum error correction

abstract:
  I consider quantum protocols for clock synchronization, and investigate in
particular whether entanglement distillation or quantum error-correcting codes
can improve the robustness of these protocols. I also draw attention to some
unanswered questions about the relativistic theory of quantum measurement. This
paper is based on a talk given at the NASA-DoD Workshop on Quantum Information
and Clock Synchronization for Space Applications (QuICSSA), September 25-26,
2000.


title:
Experimental Test for Absence of R-Term in Schroedinger Equation in
  Curved Space

abstract:
  We point out that the presence of a term proportional to the scalar curvature
in the Schroedinger equation in curved space can easily be detected in atomic
spectra with Russel-Saunders coupling by a violation of the Lande interval rule
for adjacent levels E_J-E_{J-1} propto J.


title:
Cartan Decomposition of SU(2^n), Constructive Controllability of Spin
  systems and Universal Quantum Computing

abstract:
  In this paper we provide an explicit parameterization of arbitrary unitary
transformation acting on n qubits, in terms of one and two qubit quantum gates.
The construction is based on successive Cartan decompositions of the
semi-simple Lie group, SU(2^n). The decomposition highlights the geometric
aspects of building an arbitrary unitary transformation out of quantum gates
and makes explicit the choice of pulse sequences for the implementation of
arbitrary unitary transformation on $n coupled spins. Finally we make
observations on the optimality of the design procedure.


title: Invariant Polynomial Functions on k qudits

abstract:
  We study the polynomial functions on tensor states in $(C^n)^{\otimes k}$
which are invariant under $SU(n)^k$. We describe the space of invariant
polynomials in terms of symmetric group representations. For $k$ even, the
smallest degree for invariant polynomials is $n$ and in degree $n$ we find a
natural generalization of the determinant. For $n,d$ fixed, we describe the
asymptotic behavior of the dimension of the space of invariants as
$k\to\infty$. We study in detail the space of homogeneous degree 4 invariant
polynomial functions on $(C^2)^{\otimes k}$.


title:
Localization of Macroscopic Object Induced by the Factorization of
  Internal Adiabatic Motion

abstract:
  To account for the phenomenon of quantum decoherence of a macroscopic object,
such as the localization and disappearance of interference, we invoke the
adiabatic quantum entanglement between its collective states(such as that of
the center-of-mass (C.M)) and its inner states based on our recent
investigation. Under the adiabatic limit that motion of C.M dose not excite the
transition of inner states, it is shown that the wave function of the
macroscopic object can be written as an entangled state with correlation
between adiabatic inner states and quasi-classical motion configurations of the
C.M. Since the adiabatic inner states are factorized with respect to each parts
composing the macroscopic object, this adiabatic separation can induce the
quantum decoherence. This observation thus provides us with a possible solution
to the Schroedinger cat paradox


title: Quantum Gambling Using Two Nonorthogonal States

abstract:
  We give a (remote) quantum gambling scheme that makes use of the fact that
quantum nonorthogonal states cannot be distinguished with certainty. In the
proposed scheme, two participants Alice and Bob can be regarded as playing a
game of making guesses on identities of quantum states that are in one of two
given nonorthogonal states: if Bob makes a correct (an incorrect) guess on the
identity of a quantum state that Alice has sent, he wins (loses). It is shown
that the proposed scheme is secure against the nonentanglement attack. It can
also be shown heuristically that the scheme is secure in the case of the
entanglement attack.


title: Decomposition of pure states of a quantum register

abstract:
  Using the leading vector method, we show that any vector $h\in(C^2)^{\otimes
l}$ can be decomposed as a sum of at most (and at least in the generic case)
$2^l-l$ product vectors using local bitwise unitary transformations. The method
is based on representing the vectors by chains of appropriate simplicial
complex. This generalizes the Scmidt decomposition of pure states of a 2-bit
register to registers of arbitrary length $l$.


title: Entangling ions in arrays of microscopic traps

abstract:
  We consider a system of particles in an array of microscopic traps, coupled
to each other via electrostatic interaction, and pushed by an external
state-dependent force. We show how to implement a two-qubit quantum gate
between two such particles with a high fidelity.


title: Photon-photon correlations and entanglement in doped photonic crystals

abstract:
  We consider a photonic crystal (PC) doped with four-level atoms whose
intermediate transition is coupled near-resonantly with a photonic band-gap
edge. We show that two photons, each coupled to a different atomic transition
in such atoms, can manifest strong phase or amplitude correlations: One photon
can induce a large phase shift on the other photon or trigger its absorption
and thus operate as an ultrasensitive nonlinear photon-switch. These features
allow the creation of entangled two-photon states and have unique advantages
over previously considered media: (i) no control lasers are needed; (ii) the
system parameters can be chosen to cause full two-photon entanglement via
absorption; (iii) a number of PCs can be combined in a network.


title: Evaluating quantum teleportation of coherent states

abstract:
  We discuss the criteria for teleporting coherent states from simple
considerations about information exchange during the teleportation process.


title: Spatial entanglement of twin quantum images

abstract:
  We show that spatial entanglement of two twin images obtained by parametric
down-conversion is complete, i.e. concerns both amplitude and phase. This is
realised through a homodyne detection of these images which allows for
measurement of the field quadrature components. EPR correlations are shown to
exist between symmetrical pixels of the two images. The best possible
correlation is obtained by adjusting the phase of the local oscillator field
(LO) in the area of maximal amplification. The results for quadrature
components hold unchanged even in absence of any input image i.e. for pure
parametric fluorescence. In this case they are not related to intensity and
phase fluctuations.


title: Statistical Constraints on State Preparation for a Quantum Computer

abstract:
  Quantum computing algorithms require that the quantum register be initially
present in a superposition state. To achieve this, we consider the practical
problem of creating a coherent superposition state of several qubits. Owing to
considerations of quantum statistics, this requires that the entropy of the
system go down. This, in turn, has two practical implications: (i) the initial
state cannot be controlled; (ii) the temperature of the system must be reduced.
These factors, in addition to decoherence and sensitivity to errors, must be
considered in the implementation of quantum computers.


title:
Quantum-State Engineering of Multiple Trapped Ions for Center-of-Mass
  Mode

abstract:
  We propose a scheme to generate a superposition with arbitrary coefficients
on a line in phase space for the center-of-mass vibrational mode of N ions by
means of isolating all other spectator vibrational modes from the
center-of-mass mode. It can be viewed as the generation of previous methods for
preparing motional states of one ion. For large number of ions, we need only
one cyclic operatin to generate such a superposition of many coherent states.


title: Quantum cryptographic three party protocols

abstract:
  Due to the impossibility results of Mayers and Lo/Chau it is generally
thought that a quantum channel is cryptographically strictly weaker than
oblivious transfer. In this paper we prove that in a three party scenario a
quantum channel can be strictly stronger than oblivious transfer. With the
protocol introduced in this paper we can completely classify the cryptographic
strength of quantum multi party protocols.


title: Temporary Assumptions for Quantum Multiparty Secure Computations

abstract:
  This paper introduces quantum multiparty protocols which allow the use of
temporary assumptions. We prove that secure quantum multiparty computations are
possible if and only if classical multi party computations work. But these
strict assumptions are necessary only during the execution of the protocol and
can be loosened after termination of the protocol.
  We consider two settings:
  1. A collusion of players tries to learn the secret inputs of honest players
or tries to modify the result of the computation.
  2. A collusion of players cheats in the above way or tries to disrupt the
protocol, i.e., the collusion tries to abort the computation or leaks
information to honest players. We give bounds on the collusions tolerable after
a protocol has terminated and we state protocols reaching these bounds.


title: Realistic teleportation with linear optical elements

abstract:
  We calculate the highest possible information gain in a measurement of
entangled states when employing a beamsplitter. The result is used to evaluate
the fidelity, averaged over all unknown inputs, in a realistic teleportation
protocol that takes account of the imperfect detection of Bell states. Finally,
we introduce a probabilistic teleportation scheme, where measurements are made
in a partially entangled basis.


title: Quantum State Discrimination

abstract:
  There are fundamental limits to the accuracy with which one can determine the
state of a quantum system. I give an overview of the main approaches to quantum
state discrimination. Several strategies exist. In quantum hypothesis testing,
a quantum system is prepared in a member of a known, finite set of states, and
the aim is to guess which one with the minimum probability of error. Error free
discrimination is also sometimes possible, if we allow for the possibility of
obtaining inconclusive results. If no prior information about the state is
provided, then it is impractical to try to determine it exactly, and it must be
estimated instead. In addition to reviewing these various strategies, I
describe connections between state discrimination, the manipulation of quantum
entanglement, and quantum cloning. Recent experimental work is also discussed.


title: Classical Versus Quantum Ontology

abstract:
  This is an essay review of the book by D. Home: "Conceptual Foundations of
Quantum Physics: An Overview from Modern Perspectives" (New York: Plenum Press,
1997), xvii+386 pp., ISBN 0-306-45660-5.


title:
Quantum dynamical properties of a two-photon non-linear Jaynes-Cummings
  model

abstract:
  In this paper we study a two-photon time-dependent Jaynes-Cummings model
interacting with a Kerr-like medium. We assumed that the electromagnetic field
is in different states such as coherent, squeezed vacuum and pair coherent, and
that the atom is initially in the excited state. We studied the temporal
evolution of the population of the excited level, and the second order
coherence function. The results obtained show that this system has some
similarities with the two-mode Stark system. We analize two photon entanglement
for different initial conditions.


title: Quantum Walk on the Line

abstract:
  Motivated by the immense success of random walk and Markov chain methods in
the design of classical algorithms, we consider_quantum_ walks on graphs. We
analyse in detail the behaviour of unbiased quantum walk on the line, with the
example of a typical walk, the ``Hadamard walk''. We show that after t time
steps, the probability distribution on the line induced by the Hadamard walk is
almost uniformly distributed over the interval [-t/sqrt(2),t/sqrt(2)]. This
implies that the same walk defined on the circle mixes in_linear_ time. This is
in direct contrast with the quadratic mixing time for the corresponding
classical walk. We conclude by indicating how our techniques may be applied to
more general graphs.


title: Quantum computing by pairing trapped ultracold ions

abstract:
  The superpositional wave function oscillations for finite-time implementation
of quantum algorithms modifies the desired interference required for quantum
computing. We propose a scheme with trapped ultracold ion-pairs being qubits to
diminish the detrimental effect of the wave function oscillations, and apply
the scheme to the two-qubit Grover's search. It can be also found that the
qubits in our scheme are more robust against the decoherence caused by the
environment, and the model is scalable.


title: Quantum entanglement and classical separability in NMR computing

abstract:
  In the discussion about the quantumness of NMR computation a conclusion is
done that computational states are separable and therefore can not be
entangled. This conclusion is based on the assumption that the initial density
matrix of an individual molecule coincides with whole sample molecules
distribution over single molecule energy levels. This means that quantum
stochasticity is replaced by classical stochasticity. In the present paper it
is shown, that quantum NMR computation can create genuine entangled states if
initial system states are thermodynamical equilibrium ones. A separability
analysis problem can arise when one interprets the readout signal from whole
sample.


title:
Quantum-limited linewidth of a good-cavity laser: An analytical theory
  from near to far above threshold

abstract:
  The problem of the quantum-limited or intrinsic linewidth of a good-cavity
laser is revisited. Starting from the Scully-Lamb master equation, we present a
fully analytical treatment to determine the correlation function and the
spectrum of the cavity field at steady state. For this purpose, we develop an
analytical approximation method that implicitly incorporates the microscopic
fluctuations of both the phase and intensity of the field, and, in addition,
takes full account of the saturation of the nonlinear gain. Our main result is
a simple formula for the quantum-limited linewidth that is valid from near to
far above threshold and also includes the presence of thermal photons. Close to
the threshold, the linewidth is twice as large as predicted by the standard
phase-diffusion treatment neglecting intensity fluctuations, and even 50% above
threshold the increase is still considerable. In general, quantum fluctuations
of the intensity are present and continue to influence the linewidth as long as
the photon-number distribution is not strictly Poissonian. This inherent
relationship is displayed by a formula relating the linewidth and the Mandel
Q-parameter. More than 100% above treshold the linewidth is found to be smaller
than predicted by the standard treatment, since the simple phase-diffusion
model increasingly overestimates the rate of phase fluctuations by neglecting
gain saturation. In the limit of a very large mean photon number the expected
perfectly coherent classical field is obtained.


title: Polynomial Heisenberg algebras and higher order supersymmetry

abstract:
  It is shown that the higher order supersymmetric partners of the harmonic
oscillator Hamiltonian provide the simplest non-trivial realizations of the
polynomial Heisenberg algebras. A linearized version of the corresponding
annihilation and creation operator leads to a Fock representation which is the
same as for the harmonic oscillator Hamiltonian


title:
Polynomial procedure of avoiding multiqubit errors arising due to
  qubit-qubit interaction

abstract:
  Recently proposed implementations of quantum computer suffer from unavoidable
interaction between quantum bits depending upon data being written in them.
Novel procedure of avoiding multiqubit errors arising due to uncontrollable
qubit-qubit interaction by using addititonal intermediate qubits is proposed.
It is shown that the scheme requires only polynomial increase in number of
qubits and algorithmic steps.


title: Entanglement of coherent states and decoherence

abstract:
  A possibility to produce entangled superpositions of strong coherent states
is discussed. A recent proposal by Howell and Yazell [Phys. Rev. A 62, 012102
(2000)] of a device which entangles two strong coherent coherent states is
critically examined. A serious flaw in their design is found. New modified
scheme is proposed and it is shown that it really can generate non-classical
states that can violate Bell inequality. Moreover, a profound analysis of the
effect of losses and decoherence on the degree of entanglement is accomplished.
It reveals the high sensitivity of the device to any disturbances and the
fragility of generated states.


title:
The effect of multi-pair signal states in quantum cryptography with
  entangled photons

abstract:
  Real sources of entangled photon pairs (like parametric down conversion) are
not perfect. They produce quantum states that contain more than only one photon
pair with some probability. In this paper it is discussed what happens if such
states are used for the purpose of quantum key distribution. It is shown that
the presence of "multi-pair" signals (together with low detection efficiencies)
causes errors in transmission even if there is no eavesdropper. Moreover, it is
shown that even the eavesdropping, that draws information only from these
"multi-pair" signals, increases the error rate. Information, that can be
obtained by an eavesdropper from these signals, is calculated.


title:
Branch points in the complex plane and information loss in quantum
  systems at high level density

abstract:
  The mechanism of avoided level crossings in quantum systems is studied. It is
traced back to the existence of branch points in the complex plane which
influence the properties of resonance states as well as of discrete states. An
avoided level crossing of two states causes not only an exchange of the two
wave functions but, above all, correlations between them. The correlations play
an important role at high level density since they cause the loss of
information on the individual properties of the states.


title: N-fold Supersymmetry for a Periodic Potential

abstract:
  We report a new type of supersymmetry, "N-fold supersymmetry", in
one-dimensional quantum mechanics. Its supercharges are N-th order polynomials
of momentum: It reduces to ordinary supersymmetry for N=1, but for other values
of N the anticommutator of the supercharges is not the ordinary Hamiltonian,
but is a polynomial of the Hamiltonian. (For this reason, the original
Hamiltonian is referred to as the "Mother Hamiltonian".) This supersymmetry
shares some features with the ordinary variety, the most notable of which is
the non-renormalization theorem. An N-fold supersymmetry was earlier found for
a quartic potential whose supersymmetry is spontaneously broken. Here we report
that it also holds for a periodic potential, albeit with somewhat different
supercharges, whose supersymmetry is not broken.


title: Relativity of Entanglement

abstract:
  It has recently been suggested that various entanglement measures for
bipartite mixed states do not in general give the same ordering even in the
asymptotic cases [S. Virmani and M. B. Plenio, Phys. Lett. A {\bf 268}, 31
(2000)]. That is, for two certain mixed states, the order of the degree of
entanglement depends on the measures. Therefore, incomparable pairs of mixed
states which cannot be transformed to each other with unit efficiency by any
combinations of local quantum operations and classical communications exist. We
make an analogy of the relativity of the order of the degree of entanglement to
the relativity of temporal orders in the special theory of relativity.


title: Classical analysis of correlated multiple ionization in strong fields

abstract:
  We discuss the final stages of the simultaneous ionization of two or more
electrons due to a strong laser pulse. An analysis of the classical dynamics
suggests that the dominant pathway for non-sequential escape has the electrons
escaping in a symmetric arrangement. Classical trajectory models within and
near to this symmetry subspace support the theoretical considerations and give
final momentum distributions in close agreement with experiments.


title:
On the equivalence of the Langevin and auxiliary field quantization
  methods for absorbing dielectrics

abstract:
  Recently two methods have been developed for the quantization of the
electromagnetic field in general dispersing and absorbing linear dielectrics.
The first is based upon the introduction of a quantum Langevin current in
Maxwell's equations [T. Gruner and D.-G. Welsch, Phys. Rev. A 53, 1818 (1996);
Ho Trung Dung, L. Kn\"{o}ll, and D.-G. Welsch, Phys. Rev. A 57, 3931 (1998); S.
Scheel, L. Kn\"{o}ll, and D.-G. Welsch, Phys. Rev. A 58, 700 (1998)], whereas
the second makes use of a set of auxiliary fields, followed by a canonical
quantization procedure [A. Tip, Phys. Rev. A 57, 4818 (1998)]. We show that
both approaches are equivalent.


title: Basic concepts in quantum computation

abstract:
  Section headings: 1 Qubits, gates and networks 2 Quantum arithmetic and
function evaluations 3 Algorithms and their complexity 4 From interferometers
to computers 5 The first quantum algorithms 6 Quantum search 7 Optimal phase
estimation 8 Periodicity and quantum factoring 9 Cryptography 10 Conditional
quantum dynamics 11 Decoherence and recoherence 12 Concluding remarks


title: Indeterminate-length quantum coding

abstract:
  The quantum analogues of classical variable-length codes are
indeterminate-length quantum codes, in which codewords may exist in
superpositions of different lengths. This paper explores some of their
properties. The length observable for such codes is governed by a quantum
version of the Kraft-McMillan inequality. Indeterminate-length quantum codes
also provide an alternate approach to quantum data compression.


title:
Semiclassical dressed states of two-level quantum systems driven by
  nonresonant and/or strong laser fields

abstract:
  Analytical expressions for the semiclassical dressed states and corresponding
quasienergies are obtained for a two-level quantum system driven by a
nonresonant and/or strong laser field in a coherent state. These expressions
are of first order in a proper perturbative expansion, and already contain all
the relevant physical information on the dynamical and spectroscopic properties
displayed by these systems under such particular conditions. The influence of
the laser field parameters on transition frequencies, selection rules, and line
intensities can be easily understood in terms of the quasienergy-level diagram
and the allowed transitions between the different semiclassical dressed states.


title: The EPR Argument in a Relational Interpretation of Quantum Mechanics

abstract:
  It is shown that in the Rovelli relational interpretation of quantum
mechanics, in which the notion of absolute or observer independent state is
rejected, the conclusion of the ordinary EPR argument turns out to be
frame-dependent, provided the conditions of the original argument are suitably
adapted to the new interpretation. The consequences of this result for the
`peaceful coexistence' of quantum mechanics and special relativity are briefly
discussed.


title: Comment on `Decomposition of pure states of a quantum register'

abstract:
  I. Raptis and R. Zapatrin in the quant-ph/0010104 show possibility to express
general state of $l$-qubits quantum register as sum at most $2^l-l$ product
states. In the comment is suggested more simple construction with possibility
of generalization for decomposition of tensor product of Hilbert spaces with
arbitrary dimension $n$ (here simplicial complexes used in the article
mentioned above would not be applied directly). In this case it is
decomposition with $n^l-(n^2-n)l/2$ product states.


title: Optical Transition and Momentum Transfer in Atomic Wave Packets

abstract:
  It is shown that the population Rabi-floppings in a lossless two-level atom,
interacting with a monochromatic electromagnetic field, in general are
convergent in time. The well-known continuous floppings take place because the
restricted choosing of initial conditions, that is when the atom initially is
chosen on ground or excited level before the interaction, simultaneously having
a definite value of momentum there. The convergence of Rabi-floppings in atomic
wave-packet-states is a direct consequence of Doppler effect on optical
transition rates (Rabi-frequencies): it gradually leads to ''irregular''
chaotic-type distributions of momentum in ground and excited energy levels,
smearing the amplitudes of Rabi-floppings. Conjointly with Rabi-floppings, the
coherent accumulation of momentum on each internal energy level monotonically
diminishes too.


title:
Quantum Multi-object Search Algorithm with the Availability of Partial
  Information

abstract:
  Consider the unstructured search of an unknown number l of items in a large
unsorted database of size N. The multi-object quantum search algorithm consists
of two parts. The first part of the algorithm is to generalize Grover's
single-object search algorithm to the multi-object case and the second part is
to solve a counting problem to determine l.
  In this paper, we study the multi-object quantum search algorithm (in
continuous time), but in a more structured way by taking into account the
availability of partial information. The modeling of available partial
information is done simply by the combination of several prescribed, possibly
overlapping, information sets with varying weights to signify the reliability
of each set. The associated statistics is estimated and the algorithm
efficiency and complexity are analyzed.
  Our analysis shows that the search algorithm described here may not be more
efficient than the unstructured (generalized) multi-object Grover search if
there is ``misplaced confidence''. However, if the information sets have a
``basic confidence'' property in the sense that each information set contains
at least one search item, then a quadratic speedup holds on a much smaller data
space, which further expedite the quantum search for the first item.


title:
Characteristics of Quantum-Classical Correspondence for Two Interacting
  Spins

abstract:
  The conditions of quantum-classical correspondence for a system of two
interacting spins are investigated. Differences between quantum expectation
values and classical Liouville averages are examined for both regular and
chaotic dynamics well beyond the short-time regime of narrow states. We find
that quantum-classical differences initially grow exponentially with a
characteristic exponent consistently larger than the largest Lyapunov exponent.
We provide numerical evidence that the time of the break between the quantum
and classical predictions scales as log(${\cal J}/ \hbar$), where ${\cal J}$ is
a characteristic system action. However, this log break-time rule applies only
while the quantum-classical deviations are smaller than order hbar. We find
that the quantum observables remain well approximated by classical Liouville
averages over long times even for the chaotic motions of a few
degree-of-freedom system. To obtain this correspondence it is not necessary to
introduce the decoherence effects of a many degree-of-freedom environment.


title: Duality and Decoherence Free Subspaces

abstract:
  Quantum error avoiding codes are constructed by exploiting a geometric
interpretation of the algebra of measurements of an open quantum system. The
notion of a generalized Dirac operator is introduced and used to naturally
construct families of decoherence free subspaces for the encoding of quantum
information. The members of the family are connected to each other by the
discrete Morita equivalences of the algebra of observables, which render
possible several choices of noiseless code in which to perform quantum
computation. The construction is applied to various examples of discrete and
continuous quantum systems.


title: Quantum algorithms for solvable groups

abstract:
  In this paper we give a polynomial-time quantum algorithm for computing
orders of solvable groups. Several other problems, such as testing membership
in solvable groups, testing equality of subgroups in a given solvable group,
and testing normality of a subgroup in a given solvable group, reduce to
computing orders of solvable groups and therefore admit polynomial-time quantum
algorithms as well. Our algorithm works in the setting of black-box groups,
wherein none of these problems can be computed classically in polynomial time.
As an important byproduct, our algorithm is able to produce a pure quantum
state that is uniform over the elements in any chosen subgroup of a solvable
group, which yields a natural way to apply existing quantum algorithms to
factor groups of solvable groups.


title: Raman Sideband Cooling in presence of Multiple Decay Channels

abstract:
  We have investigated the efficiency of pulsed Raman sideband cooling in the
presence of multiple decay and excitation channels. By applying sum rules we
identify parameter regimes in which multiple scattering of photons can be
described by an effective wave vector. Using this method we determine the rate
of heating caused by optical pumping inside and outside the Lamb-Dicke regime.
On this basis we discuss also the efficiency of a recently proposed scheme for
ground-state cooling outside the Lamb-Dicke regime [G. Morigi, J.I. Cirac, M.
Lewenstein, and P. Zoller, Europhys. Lett. {\bf 39}, 13
  (1997)].


title: Multiparticle entanglement and its experimental detection

abstract:
  We discuss several aspects of multiparticle mixed state entanglement and its
experimental detection. First we consider entanglement between two particles
which is robust against disposals of other particles. To completely detect
these kinds of entanglement, full knowledge of the multiparticle density matrix
(or of all reduced density matrixes) is required. Then we review the relation
of the separability properties of l-partite splittings of a state $\rho$ to its
multipartite entanglement properties. We show that it suffices to determine the
diagonal matrix elements of $\rho$ in a certain basis in order to detect
multiparticle entanglement properties of $\rho$. We apply these observations to
analyze two recent experiments, where multiparticle entangled states of 3 (4)
particles were produced. Finally, we focus on bound entangled states
(non-separable, non-distillable states) and show that they can be activated by
joint actions of the parties. We also provide several examples which show the
activation of bound entanglement with bound entanglement.


title: Classical behaviour of the Dirac bispinor

abstract:
  It is usually supposed that the Dirac and radiation equations predict that
the phase of a fermion will rotate through half the angle through which the
fermion is rotated, which means, via the measured dynamical and geometrical
phase factors, that the fermion must have a half-integral spin. We demonstrate
that this is not the case and that the identical relativistic quantum mechanics
can also be derived with the phase of the fermion rotating through the same
angle as does the fermion itself. Under spatial rotation and Lorentz
transformation the bispinor transforms as a four-vector like the potential and
Dirac current. Previous attempts to provide this form of transformational
behaviour have foundered because a satisfactory current could not be
derived.(14)


title:
Possible peculiarities of synchrotron radiation in a strong magnetic
  field

abstract:
  Relativistic quantum effects on physical observables of scalar charged
particles are studied. Possible peculiarities of their behavior that can be
verified in an experiment can confirm several fundamental conceptions of
quantum mechanics. For observables independent of charge variable, we propose
relativistic Wigner function formalism that contains explicitly the measurement
device frame. This approach can provide the description of charged particles
gas (plasma). It differs from the traditional one but is consistent with the
Copenhagen interpretation of quantum mechanics. The effects that are connected
with this approach can be observed in astrophysical objects - neutron stars.


title:
Dipole Blockade and Quantum Information Processing in Mesoscopic Atomic
  Ensembles

abstract:
  We describe a technique for manipulating quantum information stored in
collective states of mesoscopic ensembles. Quantum processing is accomplished
by optical excitation into states with strong dipole-dipole interactions. The
resulting ``dipole blockade'' can be used to inhibit transitions into all but
singly excited collective states. This can be employed for a controlled
generation of collective atomic spin states as well as non-classical photonic
states and for scalable quantum logic gates. An example involving a cold
Rydberg gas is analyzed.


title:
The Wigner Kernel of a Particle obtained from the Wigner Kernel of a
  Spin by Group Theoretical Contraction

abstract:
  The Moyal formalism for a particle can be derived from the Moyal formalism
for a spin. This is done by contracting the group of rotations to the
oscillator group. A new derivation is given for the contraction of the spin
Wigner-kernel to the Wigner kernel of a particle.


title: Formal context for cryptographic models

abstract:
  To clarify what is involved in linking models to instruments, we adapt
quantum mechanics to define models that display explicitly the points at which
they can be linked to statistics of results of the use of instruments.
Extending an earlier proof that linking models to instruments takes guesswork,
we show: Any model of cryptographic instruments can be *enveloped*,
nonuniquely, by another model that expresses conditions of instruments that
must be met if the first model is to fit a set of measured outcomes. As a
result, model A of key distribution can be enveloped in various ways to reveal
alternative models that Eve can try to implement, in conflict with model A and
its promise of security. A different enveloping model can help Alice and Bob by
expressing necessities of synchronization that they manipulate to improve their
detection of eavesdropping. Finally we show that models based on pre-quantum
physics are also open to envelopment.


title:
Comment on ``Manipulating the frequency entangled states by an
  acoutic-optical modulator''

abstract:
  A recent theoretical paper [1] proposes a scheme for entanglement swapping
utilizing acousto-optic modulators without requiring a Bell-state measurement.
In this comment, we show that the proposal is flawed and no entanglement
swapping can occur without measurement.


title: Dynamical analysis of the buildup process near resonance

abstract:
  The time evolution of the buildup process inside a double-barrier system for
off-resonance incidence energies is studied by considering the analytic
solution of the time dependent Schr\"{o}dinger equation with cutoff plane wave
initial conditions. We show that the buildup process exhibits invariances under
arbitrary changes on the system parameters, which can be successfully described
by a simple and easy-to-use one-level formula. We find that the buildup of the
off-resonant probability density is characterized by an oscillatory pattern
modulated by the resonant case which governs the duration of the transient
regime. This is evidence that off-resonant and resonant tunneling are two
correlated processes, whose transient regime is characterized by the same
transient time constant of two lifetimes.


title: Lorentz Invariant Superluminal Tunneling

abstract:
  It is shown that superluminal optical signalling is possible without
violating Lorentz invariance and causality via tunneling through photonic band
gaps in inhomogeneous dielectrics of a special kind.


title: Quantum key distribution via quantum encryption

abstract:
  A quantum key distribution protocol based on quantum encryption is presented
in this Brief Report. In this protocol, the previously shared
Einstein-Podolsky-Rosen pairs act as the quantum key to encode and decode the
classical cryptography key. The quantum key is reusable and the eavesdropper
cannot elicit any information from the particle Alice sends to Bob. The concept
of quantum encryption is also discussed.


title: Entanglement and Extreme Spin Squeezing

abstract:
  For any mean value of a cartesian component of a spin vector we identify the
smallest possible uncertainty in any of the orthogonal components. The
corresponding states are optimal for spectroscopy and atomic clocks. We show
that the results for different spin J can be used to identify entanglement and
to quantity the depth of entanglement in systems with many particles. With the
procedure developed in this letter, collective spin measurements on an ensemble
of particles can be used as an experimental proof of multi-particle
entanglement


title: Quantum information theory

abstract:
  Quantum information theory is the study of the achievable limits of
information processing within quantum mechanics. Many different types of
information can be accommodated within quantum mechanics, including classical
information, coherent quantum information, and entanglement. Exploring the rich
variety of capabilities allowed by these types of information is the subject of
quantum information theory, and of this Dissertation. In particular, I
demonstrate several novel limits to the information processing ability of
quantum mechanics. Results of especial interest include: the demonstration of
limitations to the class of measurements which may be performed in quantum
mechanics; a capacity theorem giving achievable limits to the transmission of
classical information through a two-way noiseless quantum channel; resource
bounds on distributed quantum computation; a new proof of the quantum noiseless
channel coding theorem; an information-theoretic characterization of the
conditions under which quantum error-correction may be achieved; an analysis of
the thermodynamic limits to quantum error-correction, and new bounds on channel
capacity for noisy quantum channels.


title:
An expectation value expansion of Hermitian operators in a discrete
  Hilbert space

abstract:
  We discuss a real-valued expansion of any Hermitian operator defined in a
Hilbert space of finite dimension N, where N is a prime number, or an integer
power of a prime. The expansion has a direct interpretation in terms of the
operator expectation values for a set of complementary bases. The expansion can
be said to be the complement of the discrete Wigner function.
  We expect the expansion to be of use in quantum information applications
since qubits typically are represented by a discrete, and finite-dimensional
physical system of dimension N=2^p, where p is the number of qubits involved.
As a particular example we use the expansion to prove that an intermediate
measurement basis (a Breidbart basis) cannot be found if the Hilbert space
dimension is 3 or 4.


title:
Comment on "Foundations of quantum mechanics: Connection with stochastic
  processes"

abstract:
  Recently, Olavo has proposed several derivations of the Schrodinger equation
from different sets of hypothesis ("axiomatizations") [Phys. Rev. A 61, 052109
(2000)]. One of them is based on the infinitesimal inverse Weyl transform of a
classically evolved phase space density. We show however that the Schrodinger
equation can only be obtained in that manner for linear or quadratic potential
functions.


title: Einselection and Decoherence from an Information Theory Perspective

abstract:
  We introduce and investigate a simple model of conditional quantum dynamics.
It allows for a discussion of the information-theoretic aspects of quantum
measurements, decoherence, and environment-induced superselection
(einselection).