title: Reducing the communication complexity with quantum entanglement

abstract:
  We propose a probabilistic two-party communication complexity scenario with a
prior nonmaximally entangled state, which results in less communication than
that is required with only classical random correlations. A simple all-optical
implementation of this protocol is presented and demonstrates our conclusion.


title:
Highly efficient photon-pair source using a Periodically Poled Lithium
  Niobate Waveguide

abstract:
  We report on a new kind of correlated photon-pair source based on a waveguide
integrated on a Periodically Poled Lithium Niobate substrate. Using a pump
laser of a few micro-Watts at 657 nm, we generate degenerate photon-pairs at
1314 nm. Detecting about 1500 coincidences per second, we can infer a
conversion rate of 10-6 pairs per pump photon, which is four orders of
magnitude higher than that obtained with previous bulk sources. These results
are very promising for the realization of sources for quantum communication and
quantum metrology experiments requiring a high signal-to-noise ratio or working
with more than one photon-pair at a time.


title: An Attack to Quantum Cryptography from Space

abstract:
  The promise of secure cryptographic quantum key distribution schemes is based
on the use of quantum effects in the spin space. We point out that in fact in
many current quantum cryptography protocols the space part of the wave function
is neglected. However exactly the space part of the wave function describes the
behaviour of particles in ordinary real three-dimensional space. As a result
such schemes can be secure against eavesdropping attacks in the abstract spin
space but could be insecure in the real three-dimensional space. We discuss an
approach to the security of quantum key distribution in space by using Bell's
inequality and a special preparation of the space part of the wave function.


title: Multi-bit gates for quantum computing

abstract:
  We present a general technique to implement products of many qubit operators
communicating via a joint harmonic oscillator degree of freedom in a quantum
computer. By conditional displacements and rotations we can implement
Hamiltonians which are trigonometric functions of qubit operators. With such
operators we can effectively implement higher order gates such as Toffoli gates
and C^n-NOT gates, and we show that the entire Grover search algorithm can be
implemented in a direct way.


title: Theoretical efficient high capacity Quantum Key Distribution Scheme

abstract:
  A theoretical quantum key distribution scheme using EPR pairs is presented.
This scheme is efficient in that it uses all EPR pairs in distributing the key
except those chosen for checking eavesdroppers. The high capacity is achieved
because each EPR pair carries 2 bits of key code.


title: Non-dissipative decoherence bounds on quantum computation

abstract:
  We investigate the capabilities of a quantum computer based on cold trapped
ions in presence of non-dissipative decoherence. The latter is accounted by
using the evolution time as a random variable and then averaging on a properly
defined probability distribution. Severe bounds on computational performances
are found.


title: Ponderomotive entanglement purification

abstract:
  It is shown that ponderomotive force can be used to purify entangled states.
The protocol is based on the possibility to exploit such force for a local
quantum nondemolition measurement of the total excitation number of continuos
variable entangled pairs.


title: Manipulating decay rates by entanglement and the Zeno effect

abstract:
  We analyse a class of quantum dynamical processes which may lead to the
hindering of the decay of a non-stationary state through appropriate
entanglement with an additional two-level system. In this case the process can
be considered as a module whose iteration is related to dynamical
implementations of the so called quantum Zeno effect.


title:
Quantum optical weak measurements can visualize photon dynamics in real
  time

abstract:
  An experiment is proposed to visualize stroboscopically in real time the
dynamics of a photon oscillating between two cavities. The visualization is
implemented by a sequence of weak measurements (POVM), which are carried out by
probing one of the cavities with a Rydberg atom and detecting a resulting phase
shift by Ramsey interferometry. This way to measure the number of photons in a
cavity was experimentally realized by Brune et al.. We suggest a feedback
mechanism which minimizes the disturbance due to the measurement and enables a
detection of the original evolution of the radiation field. PACS numbers:
03.65.Ta, 32.80.-t, 03.67.-a


title: Observation of decoherence with a movable mirror

abstract:
  Recently it has been proposed to use parity as a measure of the mechanism
behind decoherence or the transformation from quantum to classical. Here, we
show that the proposed experiment is more feasible than previously thought, as
even an initial thermal state would exhibit the hypothesized symmetry breaking.


title: Coherent Mechanism of Robust Population Inversion

abstract:
  A coherent mechanism of robust population inversion in atomic and molecular
systems by a chirped field is presented. It is demonstrated that a field of
sufficiently high chirp rate imposes a certain relative phase between a ground
and excited state wavefunction of a two-level system. The value of the relative
phase angle is thus restricted to be negative and close to 0 or $-\pi$ for
positive and negative chirp, respectively. This explains the unidirectionality
of the population transfer from the ground to the excited state. In a molecular
system composed of a ground and excited potential energy surface the symmetry
between the action of a pulse with a large positive and negative chirp is
broken. The same framwork of the coherent mechanism can explain the symmetry
breaking and the population inversion due to a positive chirped field.


title: Telecloning and multiuser quantum channels for continuous variables

abstract:
  We propose entangled (M+1)-mode quantum states as a multiuser quantum channel
for continuous-variable communication. Arbitrary quantum states can be sent via
this channel simultaneously to M remote and separated locations with equal
minimum excess noise in each output mode. For a set of coherent-state inputs,
the channel realizes optimum symmetric 1-to-M cloning at a distance
(``telecloning''). It also provides the optimal cloning of coherent states
without the need of amplifying the state of interest. The generation of the
multiuser quantum channel requires no more than two
10\log_{10}[(\sqrt{M}-1)/(\sqrt{M}+1)] dB squeezed states and M beam splitters.


title: Does nonorthogonality necessarily imply nonmaximal entanglement ?

abstract:
  We show that bipartite entanglements involving non-orthogonal states are {\it
necessarily nonmaximally} entangled, however {\it small} the non-orthogonality
may be. How the deviation from maximal entanglement is related to
nonorthogonality is quantified by using two independent measures of
entanglement corresponding to the violation of Bell's inequality and the
entropy measure respectively. This result is true even if one of the subsystems
has orthogonal states. An application of this is that a maximal violation of
Bell's inequality in entangled neutral kaons is not possible in the presence of
CP violation.


title: General Forms of a N-fold Supersymmetric Family

abstract:
  We report general forms of one family of the N-fold supersymmetry in
one-dimensional quantum mechanics. The N-fold supersymmetry is characterized by
the supercharges which are N-th order in differential operators. The family
reported here is defined as a particular form of the supercharges and is
referred to as ``type A''. We show that a quartic and a periodic potentials,
which were previously found to be N-fold supersymmetric by the authors, are
realized as special cases of this type A family.


title:
On generalized probabilities: correlation polytopes for automaton logic
  and generalized urn models, extensions of quantum mechanics and parameter
  cheats

abstract:
  Three extensions and reinterpretations of nonclassical probabilities are
reviewed. (i) We propose to generalize the probability axiom of quantum
mechanics to self-adjoint positive operators of trace one. Furthermore, we
discuss the Cartesian and polar decomposition of arbitrary normal operators and
the possibility to operationalize the corresponding observables. Thereby we
review and emphasize the use of observables which maximally represent the
context. (ii) In the second part, we discuss Pitowsky polytopes for automaton
logic as well as for generalized urn models and evaluate methods to find the
resulting Boole-Bell type (in)equalities. (iii) Finally, so-called ``parameter
cheats'' are introduced, whereby parameters are transformed bijectively and
nonlinearly in such a way that classical systems mimic quantum correlations and
vice versa. It is even possible to introduce parameter cheats which violate the
Boole-Bell type inequalities stronger than quantum ones, thereby trespassing
the Tsirelson limit. The price to be paid is nonuniformity.


title:
Storage of quantum dynamics on quantum states: a quasi-perfect
  programmable quantum gate

abstract:
  We show how quantum dynamics (a unitary transformation) can be captured in
the state of a quantum system, in such a way that the system can be used to
perform, at a later time, the stored transformation almost perfectly on some
other quantum system. Thus programmable quantum gates for quantum information
processing are feasible if some small degree of imperfection is allowed. We
discuss the possibility of using this fact for securely computing a secret
function on a public quantum computer. Finally, our scheme for storage of
operations also allows for a new form of quantum remote control.


title: A Scaling Law for the Energy Levels of a Nonlinear Schrodinger Equation

abstract:
  It is shown that the energy levels of the one-dimensional nonlinear
Schrodinger, or Gross-Pitaevskii, equation with the homogeneous trap potential
$x^{2p}$, $p\geq 1$, obey an approximate scaling law and as a consequence the
energy increases approximately linearly with the quantum number. Moreover, for
a quadratic trap, $p=1$, the rate of increase of energy with the quantum number
is independent of the nonlinearity: this prediction is confirmed with numerical
calculations. It is also shown that the energy levels computed using a
variational approximation do not satisfy this scaling law.


title:
100 Years of Quanta: Complex-Dynamical Origin of Planck's Constant and
  Causally Complete Extension of Quantum Mechanics

abstract:
  On 14 December 1900 Max Planck first formulated the idea of energy quanta
related to a new universal constant now known as Planck's constant. Despite the
following progress of thus initiated "quantum mechanics", the physical origin
of both quantization and universality of Planck's constant remains mysterious,
as well as other "peculiar" properties of quantum dynamics. In this paper we
review a recent causal extension of quantum mechanics consistently explaining
all its "mysteries" by irreducibly complex, "dynamically multivalued" behaviour
of the underlying system of two interacting protofields (quant-ph/9902015,
quant-ph/9902016). The theory contains no imposed postulates or entities except
one, unavoidable assumption about the physical nature of the protofields. All
the observed entities and their properties, starting from physically real
space, time, and elementary particle structure, are consistently derived, in
exact correspondence with their emergence in real, irreducibly complex system
dynamics. The latter provides also natural (dynamic) unification of the
causally extended versions of quantum mechanics, relativity, and field theory,
including unification and causal understanding of particle interaction forces.
Realism and wholeness of the obtained world picture are in agreement with the
"absolute reality" quest of Max Planck and confirm his famous doubts about the
conventional scheme of quantum mechanics (cf. quant-ph/9911107,
quant-ph/0101129). We describe various applications of the obtained results
providing successful solution to many fundamental and practical problems
dangerously stagnating within conventional approaches.


title: Spectral Oscillations, Periodic Orbits, and Scaling

abstract:
  The eigenvalue density of a quantum-mechanical system exhibits oscillations,
determined by the closed orbits of the corresponding classical system; this
relationship is simple and strong for waves in billiards or on manifolds, but
becomes slightly muddy for a Schrodinger equation with a potential, where the
orbits depend on the energy. We discuss several variants of a way to restore
the simplicity by rescaling the coupling constant or the size of the orbit or
both. In each case the relation between the oscillation frequency and the
period of the orbit is inspected critically; in many cases it is observed that
a characteristic length of the orbit is a better indicator. When these matters
are properly understood, the periodic-orbit theory for generic quantum systems
recovers the clarity and simplicity that it always had for the wave equation in
a cavity. Finally, we comment on the alleged "paradox" that semiclassical
periodic-orbit theory is more effective in calculating low energy levels than
high ones.


title: Tomography of Quantum Operations

abstract:
  Quantum operations describe any state change allowed in quantum mechanics,
including the evolution of an open system or the state change due to a
measurement. In this letter we present a general method based on quantum
tomography for measuring experimentally the matrix elements of an arbitrary
quantum operation. As input the method needs only a single entangled state. The
feasibility of the technique for the electromagnetic field is shown, and the
experimental setup is illustrated based on homodyne tomography of a twin-beam.


title: Generalized uncertainty relations and coherent and squeezed states

abstract:
  Characteristic uncertainty relations and their related squeezed states are
briefly reviewed and compared in accordance with the generalizations of three
equivalent definitions of the canonical coherent states. The standard SU(1,1)
coherent states are shown to be the unique states that minimize the
Schroedinger uncertainty relation for every pair of the three generators and
the Robertson relation for the three generators. The characteristic uncertainty
inequalities are naturally extended to the case of several states. It is shown
that these inequalities can be written in the equivalent complementary form.


title:
Coupled cavities for enhancing the cross-phase modulation in
  electromagnetically induced transparency

abstract:
  We propose an optical double-cavity resonator whose response to a signal is
similar to that of an Electromagnetically Induced Transparency (EIT) medium. A
combination of such a device with a four-level EIT medium can serve for
achieving large cross-Kerr modulation of a probe field by a signal field. This
would offer the possibility of building a quantum logic gate based on photonic
qubits. We discuss the technical requirements that are necessary for realizing
a probe-photon phase shift of Pi caused by a single-photon signal. The main
difficulty is the requirement of an ultra-low reflectivity beamsplitter and to
operate a sufficiently dense cool EIT medium in a cavity.


title: Negativity and Concurrence of mixed 2X2 states

abstract:
  We consider two measures of entanglement of mixed bipartite states of
dimension 2X2: concurrence and negativity. We first prove the conjecture of
Eisert and Plenio that concurrence can never be smaller than negativity. We
then characterise all states for which concurrence equals negativity and also
those states for which the difference between concurrence and negativity is
maximal (keeping either the concurrence fixed, or the participation ratio
R=1/trace(rho^2)).


title: Relativistic quantum protocols: "Bit Commitment" and "Coin Tossing"

abstract:
  The relativistic quantum protocols realizing the bit commitment and distant
coin tossing schemes are proposed. The protocols are based on the fact that the
non-stationary orthogonal extended quantum states cannot be reliably
distinguished if they are not fully accessible for the measurement. As the
states propagate from the domain controlled by one of the user to the domain
accessible for the measurements performed by the other user, they become
reliably distinguishable for the second user. Important for the protocol are
both the quantum nature of the states and the existence of a finite maximum
speed of the signal propagation imposed by the special relativity.


title:
Ultralocal Fields and their Relevance for Reparametrization Invariant
  Quantum Field Theory

abstract:
  Reparametrization invariant theories have a vanishing Hamiltonian and enforce
their dynamics through a constraint. We specifically choose the Dirac procedure
of quantization before the introduction of constraints. Consequently, for field
theories, and prior to the introduction of any constraints, it is argued that
the original field operator representation should be ultralocal in order to
remain totally unbiased toward those field correlations that will be imposed by
the constraints. It is shown that relativistic free and interacting theories
can be completely recovered starting from ultralocal representations followed
by a careful enforcement of the appropriate constraints. In so doing all
unnecessary features of the original ultralocal representation disappear.
  The present discussion is germane to a recent theory of affine quantum
gravity in which ultralocal field representations have been invoked before the
imposition of constraints.


title: Quantum Vernam Cipher

abstract:
  We discuss aspects of secure quantum communication by proposing and analyzing
a quantum analog of the Vernam cipher (one-time-pad). The quantum Vernam cipher
uses entanglement as the key to encrypt quantum information sent through an
insecure quantum channel. First, in sharp contrast with the classical Vernam
cipher, the quantum key can be recycled securely. We show that key recycling is
intrinsic to the quantum cipher-text, rather than using entanglement as the
key. Second, the scheme detects and corrects for arbitrary transmission errors,
and it does so using only local operations and classical communication (LOCC)
between the sender and the receiver. The application to quantum message
authentication is discussed. Quantum secret sharing schemes with similar
properties are characterized. We also discuss two general issues, the relation
between secret communication and secret sharing, the classification of secure
communication protocols.


title:
Security of Quantum Key Distribution with Entangled Photons Against
  Individual Attacks

abstract:
  Security of the Ekert protocol is proven against individual attacks where an
eavesdropper is allowed to share any density matrix with the two communicating
parties. The density matrix spans all of the photon number states of both
receivers, as well as a probe state of arbitrary dimensionality belonging to
the eavesdropper. Using this general eavesdropping strategy, we show that the
Shannon information on the final key, after error correction and privacy
amplification, can be made exponentially small. This is done by finding a bound
on the eavesdropper's average collision probability. We find that the average
collision probability for the Ekert protocol is the same as that of the BB84
protocol for single photons, indicating that there is no analog in the Ekert
protocol to photon splitting attacks. We then compare the communication rate of
both protocols as a function of distance, and show that the Ekert protocol has
potential for much longer communication distances, up to 170km, in the presence
of realistic detector dark counts and channel loss. Finally, we propose a
slightly more complicated scheme based on entanglement swapping that can lead
to even longer distances of communication. The limiting factor in this new
scheme is the fiber loss, which imposes very slow communication rates at longer
distances.


title: Laser-Induced Entangled Giant Quasi-Molecules in Optical Lattices

abstract:
  We analyze atom-atom interactions in optical lattices due to a laser-induced
long-range interatomic force which prevails over the usual London-van der-Waals
forces. This force, which can be generated by an intense laser field at a
wavelength longer than that of the lattice-generating laser, is shown to bind
pairs of cold atoms trapped at different lattice sites, and cause their
translational quantum correlations (spatial entanglement).


title: Idealization Second Quantization of Composite Particles

abstract:
  A practical method is developed to deal with the second quantization of the
many-body system containing the composite particles. In our treatment, the
modes associated with composite particles are regarded approximately as
independent ones compared with those of unbound particles. The field operators
of the composite particles thus arise naturally in the second quantization
Hamiltonian. To be emphasized, the second quantization Hamiltonian has the
regular structures which correspond clearly to different physical processes.


title: Black Hole Evaporation Entails an Objective Passage of Time

abstract:
  Time's apparent passage has long been debated by philosophers, with no
decisive argument for or against its objective existence. In this paper we show
that introducing the issue of determinism gives the debate a new, empirical
twist. We prove that any theory that states that the basic laws of physics are
time-symmetric must be strictly deterministic. It is only determinism that
enables time reversal, whether theoretical or experimental, of
anyentropy-increasing process. A contradiction therefore arises between
Hawking's argument that physical law is time-symmetric and his controversial
claim that black-hole evaporation introduces a fundamental unpredictability
into the physical world. The latter claim forcibly entails an intrinsic
time-arrow independent of boundary conditions. A simulation of a simple system
under time reversal shows how an intrinsic time arrow re-emerges, destroying
the time reversal, when even the slightest failure of determinism occurs. This
proof is then extended to the classical behavior of black holes. We conclude
with pointing out the affinity between time's arrow and its apparent passage.


title: Semiclassical Casimir Energies at Finite Temperature

abstract:
  We study the dependence on the temperature T of Casimir effects for a range
of systems, and in particular for a pair of ideal parallel conducting plates,
separated by a vacuum. We study the Helmholtz free energy, combining
Matsubara's formalism, in which the temperature appears as a periodic Euclidean
fourth dimension of circumference 1/T, with the semiclassical periodic orbital
approximation of Gutzwiller. By inspecting the known results for the Casimir
energy at T=0 for a rectangular parallelepiped, one is led to guess at the
expression for the free energy of two ideal parallel conductors without
performing any calculation. The result is a new form for the free energy in
terms of the lengths of periodic classical paths on a two-dimensional cylinder
section. This expression for the free energy is equivalent to others that have
been obtained in the literature. Slightly extending the domain of applicability
of Gutzwiller's semiclassical periodic orbit approach, we evaluate the free
energy at T>0 in terms of periodic classical paths in a four-dimensional cavity
that is the tensor product of the original cavity and a circle. The validity of
this approach is at present restricted to particular systems. We also discuss
the origin of the classical form of the free energy at high temperatures.


title: Geometrical Representation of Angular Momentum Coherence and Squeezing

abstract:
  A simple, and elegant geometrical representation is developed to describe the
concept of coherence and squeezing for angular momentum operators. Angular
momentum squeezed states were obtained by applying Bogoliubov transformation on
the angular momentum coherent states in Schwinger representation [Phys. Rev.
{\bf{A 51}} (1995)]. We present the geometrical phase space description of
angular momentum coherent and squeezed states and relate with the harmonic
oscillator. The unique feature of our geometric representation is the
portraying of the expectation values of the angular momentum components
accompanied by their uncertainties. The bosonic representation of the angular
momentum coherent and squeezed states is compared with the conventional one
mentioning the advantages of this representation of angular momentum in context
of coherence and squeezing. Extension of our work on single mode squeezing to
double mode squeezing is presented and compared with the single mode one. We
also point out the possible applications of the geometrical representation in
analyzing the accuracy of interferometers and in studying the behavior,
dynamics of an ensemble of quantum-mechanical two-level systems and its
interaction with radiation.


title: Quantum factoring, discrete logarithms and the hidden subgroup problem

abstract:
  Amongst the most remarkable successes of quantum computation are Shor's
efficient quantum algorithms for the computational tasks of integer
factorisation and the evaluation of discrete logarithms. In this article we
review the essential ingredients of these algorithms and draw out the unifying
generalization of the so-called abelian hidden subgroup problem. This involves
an unexpectedly harmonious alignment of the formalism of quantum physics with
the elegant mathematical theory of group representations and fourier transforms
on finite groups. Finally we consider the non-abelian hidden subgroup problem
mentioning some open questions where future quantum algorithms may be expected
to have a substantial impact.


title:
Continued fraction solution to the Raman interaction of a trapped
  ultracold ion with two traveling wave lasers

abstract:
  Raman interaction of a trapped ultracold ion with two traveling wave lasers
has been used extensively in the ion trap experiments. We solve this
interaction in the absence of the rotating wave approximation by a continued
fraction, without considering the restriction of the Lamb-Dicke limit and the
weak excitation regime. Some interesting characteristics of the ion-trap
system, particularly for the ion outside the weak excitation regime are found.
Finally, a comparison of our results with the solution under the rotating wave
approximation is made.


title: Entangled coherent states: teleportation and decoherence

abstract:
  When a superposition $(|\alpha>-|-\alpha>)$ of two coherent states with
opposite phase falls upon a 50-50 beamsplitter, the resulting state is
entangled. Remarkably, the amount of entanglement is exactly 1 ebit,
irrespective of $\alpha$, as was recently discovered by O. Hirota and M.
Sasaki. Here we discuss decoherence properties of such states and give a simple
protocol that teleports one qubit encoded in Schr\"odinger cat states


title: Quantum scattering in one dimension

abstract:
  A self-contained discussion of nonrelativistic quantum scattering is
presented in the case of central potentials in one space dimension, which will
facilitate the understanding of the more complex scattering theory in two and
three dimensions. The present discussion illustrates in a simple way the
concept of partial-wave decomposition, phase shift, optical theorem and
effective-range expansion.


title:
Effects of Imperfect Gate Operations in Shor's Prime Factorization
  Algorithm

abstract:
  The effects of imperfect gate operations in implementation of Shor's prime
factorization algorithm are investigated. The gate imperfections may be
classified into three categories: the systematic error, the random error, and
the one with combined errors. It is found that Shor's algorithm is robust
against the systematic errors but is vulnerable to the random errors. Error
threshold is given to the algorithm for a given number $N$ to be factorized.


title:
Bibliographic guide to the foundations of quantum mechanics and quantum
  information

abstract:
  This is a collection of references (papers, books, preprints, book reviews,
Ph. D. thesis, patents, web sites, etc.), sorted alphabetically and (some of
them) classified by subject, on foundations of quantum mechanics and quantum
information. Specifically, it covers hidden variables (``no-go'' theorems,
experiments), interpretations of quantum mechanics, entanglement, quantum
effects (quantum Zeno effect, quantum erasure, ``interaction-free''
measurements, quantum ``non-demolition'' measurements), quantum information
(cryptography, cloning, dense coding, teleportation), and quantum computation.


title: Quantum Walks On Graphs

abstract:
  We set the ground for a theory of quantum walks on graphs- the generalization
of random walks on finite graphs to the quantum world. Such quantum walks do
not converge to any stationary distribution, as they are unitary and
reversible.
  However, by suitably relaxing the definition, we can obtain a measure of how
fast the quantum walk spreads or how confined the quantum walk stays in a small
neighborhood. We give definitions of mixing time, filling time, dispersion
time. We show that in all these measures, the quantum walk on the cycle is
almost quadratically faster then its classical correspondent. On the other
hand, we give a lower bound on the possible speed up by quantum walks for
general graphs, showing that quantum walks can be at most polynomially faster
than their classical counterparts.


title: Nonlocal Effects of Partial Measurements and Quantum Erasure

abstract:
  Partial measurement turns the initial superposition not into a definite
outcome but into a greater probability for it. The probability can approach
100%, yet the measurement can undergo complete quantum erasure. In the EPR
setting, we prove that i) every partial measurement nonlocally creates the same
partial change in the distant particle; and ii) every erasure inflicts the same
erasure on the distant particle's state. This enables an EPR experiment where
the nonlocal effect does not vanish after a single measurement but keeps
"traveling" back and forth between particles. We study an experiment in which
two distant particles are subjected to interferometry with a partial "which
path" measurement. Such a measurement causes a variable amount of correlation
between the particles. A new inequality is formulated for same-angle
polarizations, extending Bell's inequality for different angles. The resulting
nonlocality proof is highly visualizable, as it rests entirely on the
interference effect. Partial measurement also gives rise to a new form of
entanglement, where the particles manifest correlations of multiple
polarization directions. Another novelty in that the measurement to be erased
is fully observable, in contrast to prevailing erasure techniques where it can
never be observed. Some profound conceptual implications of our experiment are
briefly pointed out.


title: Optimal conclusive teleportation of a d-dimensional unknown state

abstract:
  We formulate a conclusive teleportation protocol for a system in
d-dimensional Hilbert space utilizing the positive operator valued measurement
at the sending station. The conclusive teleportation protocol ensures some
perfect teleportation events when the channel is only partially entangled, at
the expense of lowering the overall average fidelity. We find the change of the
fidelity as optimizing the conclusive teleportation events and discuss how much
information remains in the inconclusive parts of the teleportation.


title: The Chirality of Exceptional Points

abstract:
  Exceptional points are singularities of the spectrum and wave functions which
occur in connection with level repulsion. They are accessible in experiments
using dissipative systems. It is shown that the wave function at an exceptional
point is one specific superposition of two wave functions which are themselves
specified by the exceptional point. The phase relation of this superposition
brings about a chirality which should be detectable in an experiment.


title: Quantum Correlations in Two-Fermion Systems

abstract:
  We characterize and classify quantum correlations in two-fermion systems
having 2K single-particle states. For pure states we introduce the Slater
decomposition and rank (in analogy to Schmidt decomposition and rank), i.e. we
decompose the state into a combination of elementary Slater determinants formed
by mutually orthogonal single-particle states. Mixed states can be
characterized by their Slater number which is the minimal Slater rank required
to generate them. For K=2 we give a necessary and sufficient condition for a
state to have a Slater number of 1. We introduce a correlation measure for
mixed states which can be evaluated analytically for K=2. For higher K, we
provide a method of constructing and optimizing Slater number witnesses, i.e.
operators that detect Slater number for some states.


title:
Proposal of an experimental scheme for realising a translucent
  eavesdropping on a quantum cryptographic channel

abstract:
  Purpose of this paper is to suggest a scheme, which can be realised with
today's technology and could be used for entangling a probe to a photon qubit
based on polarisation. Using this probe a translucent or a coherent
eavesdropping can be performed.


title: Time evolution and squeezing of the field amplitude in cavity QED

abstract:
  We present the conditional time evolution of the electromagnetic field
produced by a cavity QED system in the strongly coupled regime. We obtain the
conditional evolution through a wave-particle correlation function that
measures the time evolution of the field after the detection of a photon. A
connection exists between this correlation function and the spectrum of
squeezing which permits the study of squeezed states in the time domain. We
calculate the spectrum of squeezing from the master equation for the reduced
density matrix using both the quantum regression theorem and quantum
trajectories. Our calculations not only show that spontaneous emission degrades
the squeezing signal, but they also point to the dynamical processes that cause
this degradation.


title:
Comment on "Proposal for the Measurement of Bell-Type Correlations from
  Continuous Variables"

abstract:
  In a recent paper [T. C. Ralph, W. J. Munro, R. E. S. Polkinghorne, Phys.
Rev. Lett. 85, 2035 (2000)], the authors propose a test for Bell's inequalities
based on quadrature measurements for a correlated parametric source. We present
here a local hidden variable model which reproduces all the expectation values
predicted in the described experimental scheme, and point out that the proposal
is based on an inconsistent use of Bell's inequalities.


title:
Dynamical Stability of an Ion in a Linear Trap as a Solid-State Problem
  of Electron Localization

abstract:
  When an ion confined in a linear ion trap interacts with a coherent laser
field, the internal degrees of freedom, related to the electron transitions,
couple to the vibrational degree of freedom of the ion. As a result of this
interaction, quantum dynamics of the vibrational degree of freedom becomes
complicated, and in some ranges of parameters even chaotic. We analyze the
vibrational ion dynamics using a formal analogy with the solid-state problem of
electron localization. In particular, we show how the resonant approximation
used in analysis of the ion dynamics, leads to a transition from a
two-dimensional (2D) to a one-dimensional problem (1D) of electron
localization. The localization length in the solid-state problem is estimated
in cases of weak and strong interaction between the cites of the 2D cell by
using the methods of resonance perturbation theory, common in analysis of 1D
time-dependent dynamical systems.


title: Coherent States For SU(3)

abstract:
  We define coherent states for SU(3) using six bosonic creation and
annihilation operators. These coherent states are explicitly characterized by
six complex numbers with constraints. For the completely symmetric
representations (n,0) and (0,m), only three of the bosonic operators are
required. For mixed representations (n,m), all six operators are required. The
coherent states provide a resolution of identity, satisfy the continuity
property, and possess a variety of group theoretic properties. We introduce an
explicit parameterization of the group SU(3) and the corresponding integration
measure. Finally, we discuss the path integral formalism for a problem in which
the Hamiltonian is a function of SU(3) operators at each site.


title: Probabilistic Quantum Memories

abstract:
  Typical address-oriented computer memories cannot recognize incomplete or
noisy information. Associative (content-addressable) memories solve this
problem but suffer from severe capacity shortages. I propose a model of a
quantum memory that solves both problems. The storage capacity is exponential
in the number of qbits and thus optimal. The retrieval mechanism for incomplete
or noisy inputs is probabilistic, with postselection of the measurement result.
The output is determined by a probability distribution on the memory which is
peaked around the stored patterns closest in Hamming distance to the input.


title: Induced measures in the space of mixed quantum states

abstract:
  We analyze several product measures in the space of mixed quantum states. In
particular we study measures induced by the operation of partial tracing. The
natural, rotationally invariant measure on the set of all pure states of a N x
K composite system, induces a unique measure in the space of N x N mixed states
(or in the space of K x K mixed states, if the reduction takes place with
respect to the first subsystem). For K=N the induced measure is equal to the
Hilbert-Schmidt measure, which is shown to coincide with the measure induced by
singular values of non-Hermitian random Gaussian matrices pertaining to the
Ginibre ensemble. We compute several averages with respect to this measure and
show that the mean entanglement of $N \times N$ pure states behaves as lnN-1/2.


title: Non-holonomic Quantum Devices

abstract:
  We analyze the possibility and efficiency of non-holonomic control over
quantum devices with exponentially large number of Hilbert space dimensions. We
show that completely controllable devices of this type can be assembled from
elementary units of arbitrary physical nature, and can be employed efficiently
for universal quantum computations and simulation of quantum field dynamics.


title: Decay of an excited atom near an absorbing microsphere

abstract:
  Spontaneous decay of an excited atom near a dispersing and absorbing
microsphere of given complex permittivity that satisfies the Kramers-Kronig
relations is studied, with special emphasis on a Drude-Lorentz permittivity.
Both the whispering gallery field resonances below the band gap (for a
dielectric sphere) and the surface-guided field resonances inside the gap (for
a dielectric or a metallic sphere) are considered. Since the decay rate mimics
the spectral density of the sphere-assisted ground-state fluctuation of the
radiation field, the strengths and widths of the field resonances essentially
determine the feasible enhancement of spontaneous decay. In particular, strong
enhancement can be observed for transition frequencies within the interval in
which the surface-guided field resonances strongly overlap. When material
absorption becomes significant, then the highly structured emission pattern
that can be observed when radiative losses dominate reduces to that of a
strongly absorbing mirror. Accordingly, nonradiative decay becomes dominant. In
particular, if the distance between the atom and the surface of the microsphere
is small enough, the decay becomes purely nonradiative.


title: Finding cliques by quantum adiabatic evolution

abstract:
  Quantum adiabatic evolution provides a general technique for the solution of
combinatorial search problems on quantum computers. We present the results of a
numerical study of a particular application of quantum adiabatic evolution, the
problem of finding the largest clique in a random graph. An n-vertex random
graph has each edge included with probability 1/2, and a clique is a completely
connected subgraph. There is no known classical algorithm that finds the
largest clique in a random graph with high probability and runs in a time
polynomial in n. For the small graphs we are able to investigate (n <= 18), the
quantum algorithm appears to require only a quadratic run time.


title:
Geometric quantization and the generalized Segal-Bargmann transform for
  Lie groups of compact type

abstract:
  Let K be a connected Lie group of compact type and let T*(K) be its cotangent
bundle. This paper considers geometric quantization of T*(K), first using the
vertical polarization and then using a natural Kahler polarization obtained by
identifying T*(K) with the complexified group K_C. The first main result is
that the Hilbert space obtained by using the Kahler polarization is naturally
identifiable with the generalized Segal-Bargmann space introduced by the author
from a different point of view, namely that of heat kernels. The second main
result is that the pairing map of geometric quantization coincides with the
generalized Segal-Bargmann transform introduced by the author. This means that
in this case the pairing map is a constant multiple of a unitary map. For both
results it is essential that the half-form correction be included when using
the Kahler polarization. Together with results of the author with B. Driver,
these results may be seen as an instance of "quantization commuting with
reduction."


title: Avoiding Quantum Chaos in Quantum Computation

abstract:
  We study a one-dimensional chain of nuclear $1/2-$spins in an external
time-dependent magnetic field. This model is considered as a possible candidate
for experimental realization of quantum computation. According to the general
theory of interacting particles, one of the most dangerous effects is quantum
chaos which can destroy the stability of quantum operations. According to the
standard viewpoint, the threshold for the onset of quantum chaos due to an
interaction between spins (qubits) strongly decreases with an increase of the
number of qubits. Contrary to this opinion, we show that the presence of a
magnetic field gradient helps to avoid quantum chaos which turns out to
disappear with an increase of the number of qubits. We give analytical
estimates which explain this effect, together with numerical data supporting


title: Generalized Neutrino Equations

abstract:
  I discuss generalized spin-1/2 equations for neutrinos. They have been
obtained by means of the Gersten's method for derivation of arbitrary-spin
relativistic equations. Possible physical consequences are discussed.


title: Liquid state NMR Quantum Computing

abstract:
  This in an introduction on quantum computing and on the use of NMR to build
quantum computers, geared towards an NMR audience.


title: Interferometric Tests of Teleportation

abstract:
  We investigate a direct test of teleportation efficacy based on a
Mach-Zehnder interferometer. The analysis is performed for continuous variable
teleportation of both discrete and continuous observables.


title: Entanglement Teleportation Through Cat-like States

abstract:
  We first consider teleportation of entangled states shared between Claire and
Alice to Bob1 and Bob2 when Alice and the two Bobs share a single copy of a
GHZ-class state and where {\it all} the four parties are at distant locations.
We then generalize this situation to the case of teleportation of entangled
states shared between Claire1, Claire2, ....., Claire(N-1) and Alice to Bob1,
Bob2, ....., BobN when Alice and the N Bobs share a single copy of a Cat-like
state and where again {\it all} the 2N parties are at distant locations.


title: Quantum error-correcting codes associated with graphs

abstract:
  We present a construction scheme for quantum error correcting codes. The
basic ingredients are a graph and a finite abelian group, from which the code
can explicitly be obtained. We prove necessary and sufficient conditions for
the graph such that the resulting code corrects a certain number of errors.
This allows a simple verification of the 1-error correcting property of
fivefold codes in any dimension. As new examples we construct a large class of
codes saturating the singleton bound, as well as a tenfold code detecting 3
errors.


title: Quantum simulated annealing

abstract:
  The question of whether or not quantum computers can efficiently solve
NP-complete problems is open, although indications are that BQP does not
contain NP. Still, many of these problems are natural candidates for solution
on quantum computers. We outline a thermodynamical formalism for the traveling
salesman problem which allows for its expected polytime solution on a quantum
computer with probability arbitrarily close to unity, given sufficient energy
resources and subject to a weak nondegeneracy constraint on the distances.
Applications to other problems are also discussed.


title: Continuously induced coherence without induced emission

abstract:
  The coherence properties of two downconversion processes continuously coupled
via idler beams are analyzed. We find that the amount of which-way information
about a signal photon carried by idler beams periodically attains its maximum
and minimum in the course of evolution. In correspondence with the famous
experiments by Mandel's group on the induced coherence without induced emission
the coherence of signal beams is governed by that information. The ideal
which-way measurement is constructed.


title:
Implementation of a quantum algorithm to solve Bernstein-Vazirani's
  parity problem without entanglement on an ensemble quantum computer

abstract:
  Bernstein and Varizani have given the first quantum algorithm to solve parity
problem in which a strong violation of the classical imformation theoritic
bound comes about. In this paper, we refine this algorithm with fewer resource
and implement a two qubits algorithm in a single query on an ensemble quantum
computer for the first time.


title: Entropy and information gain in quantum continual measurements

abstract:
  The theory of measurements continuous in time in quantum mechanics (quantum
continual measurements) has been formulated by using the notions of instrument,
positive operator valued measure, etc., by using quantum stochastic
differential equations and by using classical stochastic differential equations
(SDE's) for vectors in Hilbert spaces or for trace-class operators. In the same
times Ozawa made developments in the theory of instruments and introduced the
related notions of a posteriori states and of information gain [1].
  In this paper we introduce a simple class of SDE's relevant to the theory of
continual measurements and we recall how they are related to instruments and a
posteriori states and, so, to the general formulation of quantum mechanics.
Then we introduce and use the notion of information gain and the other results
of the paper [1] inside the theory of continual measurements.
  [1] M. Ozawa, On information gain by quantum measurements of continuous
observables, J. Math. Phys. 27 (1986) 759-763.


title: Quantum Entanglement and Quantum Computational Algorithms

abstract:
  The existence of entangled quantum states gives extra power to quantum
computers over their classical counterparts. Quantum entanglement shows up
qualitatively at the level of two qubits. We show that if no entanglement is
envolved then whatever one can do with qubits can also be done with classical
optical systems. We demonstrate that the one- and the two-bit Deutsch-Jozsa
algorithm does not require entanglement and can be mapped onto a classical
optical scheme. It is only for three and more input bits that the DJ algorithm
requires the implementation of entangling transformations and in these cases it
is impossible to implement this algorithm classically.


title:
Exactly Solvable Three-body SUSY Systems with Internal Degrees of
  Freedom

abstract:
  The approach of multi-dimensional SUSY Quantum Mechanics is used in an
explicit construction of exactly solvable 3-body (and quasi-exactly-solvable
$N$-body) matrix problems on a line. From intertwining relations with
time-dependent operators, we build exactly solvable non-stationary scalar and
$2\times 2$ matrix 3-body models which are time-dependent extensions of the
Calogero model. Finally, we investigate the invariant operators associated to
these systems.


title: Teleporting Noncommuting Qubits Require Maximal Entanglement

abstract:
  Very recently, it was shown by Ghosh, Kar, Roy and Sen (\emph{Entanglement
vs. Noncommutativity in Teleportation}, quant-ph/0010012) that if it is \emph{a
priori} known that the state to be teleported is from a commuting set of
qubits, a separable channel is sufficient. We show that 1 ebit of entanglement
is a necessary resource to teleport a qubit even when it is known to be one of
two noncommuting states.


title: Limits to error correction in quantum chaos

abstract:
  We study the correction of errors that have accumulated in an entangled state
of spins as a result of unknown local variations in the Zeeman energy (B) and
spin-spin interaction energy (J). A non-degenerate code with error rate kappa
can recover the original state with high fidelity within a time kappa^1/2 /
max(B,J) -- independent of the number of encoded qubits. Whether the
Hamiltonian is chaotic or not does not affect this time scale, but it does
affect the complexity of the error-correcting code.


title: Satisfying Reality

abstract:
  It is claimed elsewhere that the conscious states of humans must have evolved
together with their biological states, and that an ongoing interaction between
the two must have occurred to insure that they mirror one another in any
species. A quantum mechanical mechanism and an evolutionary model for the
assumed mind/body interaction are described in those papers. The present paper
outlines the related ontological and epistemological assumptions, showing how
the claimed connection between conscious states and physical states should be
understood. Keywords: consciousness, Copenhagen, epistemology, form, Godel,
matter, monism, objective reality, post modernism, psychology, subjective
reality, TOE, undecidability, universals


title:
No-cloning theorem and teleportation criteria for quantum continuous
  variables

abstract:
  We discuss the criteria presently used for evaluating the efficiency of
quantum teleportation schemes for continuous variables. Using an argument based
upon the difference between 1-to-2 quantum cloning (quantum duplication) and
1-to-infinity cloning (classical measurement), we show that a fidelity value
larger than 2/3 is required for successful quantum teleportation of coherent
states. This value has not been reached experimentally so far.


title: Contextual objectivity : a realistic interpretation of quantum mechanics

abstract:
  An attempt is made to formulate quantum mechanics (QM) in physical rather
than in mathematical terms. It is argued that the appropriate conceptual
framework for QM is "contextual objectivity", which includes an objective
definition of the quantum state. This point of view sheds new light on topics
such as the reduction postulate and the quantum measurement process.


title:
Superluminal Motion in The interacting System of Electrons, Positrons
  and Photons

abstract:
  The coupled system of Boltzman equations for the interacting system of
electrons, positrons and photons in high external electric, E, and arbitrary
magnetic, H, fields is solved. The consideration is made under the conditions
of arbitrary heating and the mutual drag of carriers and photons. The
non-stationary and non-uniform distribution function of photons for the all
considered cases is obtained. It is shown that the distribution function of
photons has the stationary limit for the drift velocities $(\vec{u}\vec{q}/
\hbar \omega_q) <1 $ and grows exponentially with time for the drift velocities
$(\vec{u}\vec{q}/ \hbar \omega_q) \geq 1$. As a result of the analyses of the
phenomena of coupling of the mutual drag system of carriers with the photons,
we obtained some fundamental results: {\bf a)} the finiteness of the mass of
photon (i.e. the rest mass of the photons is not zero); {\bf b)} reality of
tachyons as a quasi-particles with a real mass in amplifying system (or
regime); {\bf c)} the mechanism of the theory of relativity and that of the
Doppler effect which coincides with the renormalization of frequency or mass as
a result of the mutual drag of carriers and photons at external field (force)
are the same. These conclusions were obtained as a result of the fact that the
relativistic factor enters the expressions of the distribution function of
photons and other physical expressions in the first order in the form
$[1-(u^2/c^2) ]^{-1}$, but not in the form $[1-(u^2/c^2) ]^{-1/2}$ as in
Einstein's theory. Also it is shown that the velocity of light is an average
velocity of photons in the ground state.


title:
Simultaneous minimum-uncertainty measurement of discrete-valued
  complementary observables

abstract:
  We have made the first experimental demonstration of the simultaneous minimum
uncertainty product between two complementary observables for a two-state
system (a qubit). A partially entangled two-photon state was used to perform
such measurements. Each of the photons carries (partial) information of the
initial state thus leaving a room for measurements of two complementary
observables on every member in an ensemble.


title:
Luttinger model approach to interacting one-dimensional fermions in a
  harmonic trap

abstract:
  A model of interacting one--dimensional fermions confined to a harmonic trap
is proposed. The model is treated analytically to all orders of the coupling
constant by a method analogous to that used for the Luttinger model. As a first
application, the particle density is evaluated and the behavior of Friedel
oscillations under the influence of interactions is studied. It is found that
attractive interactions tend to suppress the Friedel oscillations while strong
repulsive interactions enhance the Friedel oscillations significantly. The
momentum distribution function and the relation of the model interaction to
realistic pair interactions are also discussed.


title: Entanglement Optimization for Pairs of Qubits

abstract:
  Local Operations enhancing the entanglement of bipartite quantum states are
of great interest in quantum information processing. Subject of this paper are
local selective operations acting on single copies of states. Such operations
can lead to larger entanglement with respect to a certain measure as studies
before by the Horodeckis and A. Kent et al. (PRA 60,PRL 81 and 83). We present
a complete characterisation of all local operations yielding optimal
entanglement for pairs of qubits, extending former results of A. Kent et al. We
introduce a new technique for the classification of states according to their
behaviour under entanglement optimizing operations, using the entanglement
properties of the support of density matrices.


title: Strong Converse for Identification via Quantum Channels

abstract:
  In this paper we present a simple proof of the strong converse for
identification via discrete memoryless quantum channels, based on a novel
covering lemma. The new method is a generalization to quantum communication
channels of Ahlswede's recently discovered appoach to classical channels. It
involves a development of explicit large deviation estimates to the case of
random variables taking values in selfadjoint operators on a Hilbert space.
This theory is presented separately in an appendix, and we illustrate it by
showing its application to quantum generalizations of classical hypergraph
covering problems.


title: Compression of quantum measurement operations

abstract:
  We generalize recent work of Massar and Popescu dealing with the amount of
classical data that is produced by a quantum measurement on a quantum state
ensemble. In the previous work it was shown how spurious randomness generally
contained in the outcomes can be eliminated without decreasing the amount of
knowledge, to achieve an amount of data equal to the von Neumann entropy of the
ensemble. Here we extend this result by giving a more refined description of
what constitute equivalent measurements (that is measurements which provide the
same knowledge about the quantum state) and also by considering incomplete
measurements. In particular we show that one can always associate to a POVM
with elements a_j, an equivalent POVM acting on many independent copies of the
system which produces an amount of data asymptotically equal to the entropy
defect of an ensemble canonically associated to the ensemble average state and
the initial measurement (a_j). In the case where the measurement is not
maximally refined this amount of data is strictly less than the von Neumann
entropy, as obtained in the previous work. We also show that this is the best
achievable, i.e. it is impossible to devise a measurement equivalent to the
initial measurement (a_j) that produces less data. We discuss the
interpretation of these results. In particular we show how they can be used to
provide a precise and model independent measure of the amount of knowledge that
is obtained about a quantum state by a quantum measurement. We also discuss in
detail the relation between our results and Holevo's bound, at the same time
providing a new proof of this fundamental inequality.


title: Test Particle in a Quantum Gas

abstract:
  A master equation with a Lindblad structure is derived, which describes the
interaction of a test particle with a macroscopic system and is expressed in
terms of the operator valued dynamic structure factor of the system. In the
case of a free Fermi or Bose gas the result is evaluated in the Brownian limit,
thus obtaining a single generator master equation for the description of
quantum Brownian motion in which the correction due to quantum statistics is
explicitly calculated. The friction coefficients for Boltzmann and Bose or
Fermi statistics are compared.


title: Quantum Zeno and anti-Zeno effects in the Friedrichs model

abstract:
  We analyze the short-time behavior of the survival probability in the frame
of the Friedrichs model for different formfactors. We have shown that this
probability is not necessary analytic at the time origin. The time when the
quantum Zeno effect could be observed is found to be much smaller than usually
estimated. We have also studied the anti-Zeno era and have estimated its
duration.


title:
Optimum unambiguous discrimination among linearly-independent
  non-orthogonal quantum states and its optical realization

abstract:
  The problem of unambiguously distinguishing among nonorthogonal but linearly
independent quantum states can be solved by mapping the set of nonorthogonal
quantum states onto a set of orthogonal ones, which can then be distinguished
without error. Such nonunitary transformations can be performed conditionally
on quantum systems; a unitary transformation is carried out on a larger system
of which the system of interest is a subsytem, a measurement is performed, and
if the proper result is obtained, the desired nonunitary transformation will
have been performed on the subsystem. We show how to construct generalized
interferometers (multiports), which when combined with measurements on some of
the output ports, implement nonunitary transformations of this type. The input
states are single-photon states in which the photon is divided among several
modes. A number of explicit examples of distinguishing among three
nonorthogonal states are discussed, and the networks that optimally distinguish
among these states are presented.


title: Beyond Unruh Effect: Nonequilibrium Quantum Dynamics of Moving Charges

abstract:
  We discuss some common misconceptions in Unruh effect and Unruh radiation for
the cases of linear and circular uniform acceleration of a charged particle or
detector moving in a quantum field. We point to the need to go beyond Unruh
effect and develop a new theoretical framework for treating the stochastic
dynamics of particles interacting with quantum fields under more general
nonequilibrium conditions. This framework has been established in recent years
using the influence functional formalism and applied to relativistically moving
charged particles. Only with nonequilibrium concepts and methodology applied to
particle-field interaction can one grasp the full complexity of the problems of
beam physics under more realistic conditions, from electrons and heavy ions to
coherent atoms.


title: Possibility for Teleportation of Nuclei

abstract:
  Experimental realizations of QT have so far been limited to teleportation of
light. The present communication gives a new experimental scheme for QT of
heavy matter. We show that the standard experimental technique used in nuclear
physics may be successfully applied to teleportation of spin states of atomic
nuclei.
  It was shown that there are no theoretical prohibitions upon a possibility of
a complete Bell measurement, so that implementation of all four quantum
communication channels is at least theoretically available. A general
expression for scattering amplitude of two 1/2-spin particles was given in the
Bell operator basis, and peculiarities of Bell states registration are briefly
discussed.


title: Is there emitted radiation in Unruh effect?

abstract:
  The thermal radiance felt by a uniformly accelerated
detector/oscillator/atom--the Unruh effect-- is often mistaken to be some
emitted radiation detectable by an observer/probe/sensor. Here we show by an
explicit calculation of the energy momentum tensor of a quantum scalar field
that, at least in 1+1 dimension, while a polarization cloud is found to exist
around the particle trajectory, there is no emitted radiation from a uniformly
accelerated oscillator in equilibrium conditions. Under nonequilibrium
conditions which can prevail for non-uniformly accelerated trajectories or
before the atom or oscillator reaches equilibrium, there is conceivably
radiation emitted, but that is not what Unruh effect entails.


title:
Worldline Influence Functional: Abraham-Lorentz-Dirac-Langevin Equation
  from QED

abstract:
  We present a stochastic theory of charges moving in an electromagnetic field
using nonequilibrium quantum field theory. We give a first principles'
derivation of the Abraham-Lorentz-Dirac-Langevin equation which depicts the
quantum expectation value for a particle's trajectory and its stochastic
fluctuations by combining the worldline path integral quantization with the
Feynman-Vernon influence functional or closed-time-path effective action
methods. At lowest order, the equations of motion are approximated by a
stochastic Lorentz-Dirac equation.


title:
Resonantly enhanced nonlinear optics in semiconductor quantum wells: An
  application to sensitive infrared detection

abstract:
  A novel class of coherent nonlinear optical phenomena, involving induced
transparency in quantum wells, is considered in the context of a particular
application to sensitive long-wavelength infrared detection. It is shown that
the strongest decoherence mechanisms can be suppressed or mitigated, resulting
in substantial enhancement of nonlinear optical effects in semiconductor
quantum wells.


title:
Stochastic Theory of Relativistic Particles Moving in a Quantum Field:
  I. Influence Functional and Langevin Equation

abstract:
  We treat a relativistically moving particle interacting with a quantum field
from an open system viewpoint of quantum field theory by the method of
influence functionals or closed-time-path coarse-grained effective actions. The
particle trajectory is not prescribed but is determined by the backreaction of
the quantum field in a self-consistent way. Coarse-graining the quantum field
imparts stochastic behavior in the particle trajectory. The formalism is set up
here as a precursor to a first principles derivation of the
Abraham-Lorentz-Dirac (ALD) equation from quantum field theory as the correct
equation of motion valid in the semiclassical limit. This approach also
discerns classical radiation reaction from quantum dissipation in the motion of
a charged particle; only the latter is related to vacuum fluctuations in the
quantum field by a fluctuation-dissipation relation, which we show to exist for
nonequilibrim processes under this type of nonlinear coupling. This formalism
leads naturally to a set of Langevin equations associated with a generalized
ALD equation. These multiparticle stochastic differential equations feature
local dissipation (for massless quantum fields), nonlocal particle-particle
interactions, multiplicative noise, and nonlocal particle-particle
correlations, interrelated in ways characteristic of nonlinear theories,
through generalized fluctuation-dissipation relations.


title: Storage of light in atomic vapor

abstract:
  We report an experiment in which a light pulse is decelerated and trapped in
a vapor of Rb atoms, stored for a controlled period of time, and then released
on demand. We accomplish this storage of light by dynamically reducing the
group velocity of the light pulse to zero, so that the coherent excitation of
the light is reversibly mapped into a collective Zeeman (spin) coherence of the
Rb vapor.


title: Unconditionally Secure Quantum Coin Tossing via Entanglement Swapping

abstract:
  An unconditionally secure quantum cion tossing protocol for two remote
participants via entangled swapping is presented. The security of this protocol
is guaranteed by the nonlocal property of quantum entanglement and the
classical complexity.


title: Wigner Functions with Boundaries

abstract:
  We consider the general Wigner function for a particle confined to a finite
interval and subject to Dirichlet boundary conditions. We derive the boundary
corrections to the "star-genvalue" equation and to the time evolution equation.
These corrections can be cast in the form of a boundary potential contributing
to the total Hamiltonian which together with a subsidiary boundary condition is
responsible for the discretization of the energy levels. We show that a
completely analogous formulation (in terms of boundary potentials) is also
possible in standard operator quantum mechanics and that the Wigner and the
operator formulations are also in one-to-one correspondence in the confined
case. In particular, we extend Baker's converse construction to bounded
systems. Finally, we elaborate on the applications of the formalism to the
subject of Wigner trajectories, namely in the context of collision processes
and quantum systems displaying chaotic behavior in the classical limit.


title:
Classification of transformations of probabilities for preparation
  procedures: trigonometric and hyperbolic behaviours

abstract:
  We provide frequency probabilistic analysis of perturbations of physical
systems by preparation procedures. We obtained the classification of possible
probabilistic transformations connecting input and output probabilities that
can appear in physical experiments. We found that so called quantum
probabilistic rule is just one of possible rules. Besides the well known
trigonometric transformation (for example, for the polarization of light),
there exist the hyperbolic transformation of probabilities. In fact,
`hyperbolic polarization' have laready been observed in experiments with
elementary particles. However, it was not interpreted in such a way. The
situation is more complex with the hyperbolic interference of alternatives.


title: The information capacity of the lambda-system--photon field channel

abstract:
  The potentially attainable information capacity of a radiatively stable
lambda-system, viewed as the input of a quantum information channel, is
studied. The output of the channel is formed by the states of the photon field
with optical excitation frequencies, which are created as a result of Raman
pumping. The analysis is based on the notion of coherent information.


title: A Generalization of Grover's Algorithm

abstract:
  We investigate the necessary and sufficient conditions in order that a
unitary operator can amplify a pre-assigned component relative to a particular
basis of a generic vector at the expense of the other components. This leads to
a general method which allows, given a vector and one of its components we want
to amplify, to choose the optimal unitary operator which realizes that goal.
Grover's quantum algorithm is shown to be a particular case of our general
method. However the general structure of the unitary we find is remarkably
similar to that of Grover's one: a sign flip of one component combined with a
reflection with respect to a vector. In Grover's case this vector is fixed; in
our case it depends on a parameter and this allows optimization.


title: Quantum Fluctuations of Radiation Pressure

abstract:
  Quantum fluctuations of electromagnetic radiation pressure are discussed. We
use an approach based on the quantum stress tensor to calculate the
fluctuations in velocity and position of a mirror subjected to electromagnetic
radiation. Our approach reveals that radiation pressure fluctuations are due to
a cross term between vacuum and state dependent terms in a stress tensor
operator product. Thus observation of these fluctuations would entail
experimental confirmation of this cross term. We first analyze the pressure
fluctuations on a single, perfectly reflecting mirror, and then study the case
of an interferometer. This involves a study of the effects of multiple bounces
in one arm, as well as the correlations of the pressure fluctuations between
arms of the interferometer. In all cases, our results are consistent with those
previously obtained by Caves using different mehods.


title: Remarks on Lewenstein-Sanpera Decomposition

abstract:
  We discuss in this letter Lewenstein-Sanpera (L-S) decomposition for a
specific Werner state. Compared with the optimal case, we propose a
quasi-optimal one which in the view of concurrence leads to the same
entanglement measure for the entangled mixed state discussed. We think that in
order to obtain entanglement of given state the optimal L-S decomposition is
not necessary.


title: Landau Levels in the Presence of Topological Defects

abstract:
  We study the Landau levels associated with electrons moving in a magnetic
field in the presence of a continuous distribution of disclinations, a magnetic
screw dislocation and a dispiration. We focus on the influence of these
topological defects on the spectrum of the electron(or hole) in a magnetic
field in the framework of the geometric theory of defects in solids of Katanaev
and Volovich. The presence of the defects breaks the degeneracy of the Landau
levels in different ways depending on the defect. Exact expressions for
energies and eigenfuctions are found for all cases.


title: High-Precision Value for the Quartic Anharmonic Oscillator Ground State

abstract:
  We will describe how a new, quite simple, but highly effective algorithm,
together with the asymptotically fast FFT-based high-precision number
multiplication of Mathematica 4 can calculate the ground state of the x^4
anharmonic oscillator to the new record of more than 1000 digits.


title:
Non-local Operations: Purification, storage, compression, tomography,
  and probabilistic implementation

abstract:
  We provide several applications of a previously introduced isomorphism
between physical operations acting on two systems and entangled states [1]. We
show: (i) how to implement (weakly) non-local two qubit unitary operations with
a small amount of entanglement; (ii) that a known, noisy, non-local unitary
operation as well as an unknown, noisy, local unitary operation can be
purified; (iii) how to perform the tomography of arbitrary, unknown, non-local
operations; (iv) that a set of local unitary operations as well as a set of
non-local unitary operations can be stored and compressed; (v) how to implement
probabilistically two-qubit gates for photons. We also show how to compress a
set of bipartite entangled states locally, as well as how to implement certain
non-local measurements using a small amount of entanglement. Finally, we
generalize some of our results to multiparty systems.


title: Quantum Database Search can do without Sorting

abstract:
  Sorting is a fundamental computational process, which facilitates subsequent
searching of a database. It can be thought of as factorisation of the search
process. The location of a desired item in a sorted database can be found by
classical queries that inspect one letter of the label at a time. For an
unsorted database, no such classical quick search algorithm is available. If
the database permits quantum queries, however, then mere digitisation is
sufficient for efficient search. Sorting becomes redundant with the quantum
superposition of states. A quantum algorithm is written down which locates the
desired item in an unsorted database a factor of two faster than the best
classical algorithm can in a sorted database. This algorithm has close
resemblance to the assembly process in DNA replication.


title: Coherent Control of Quantum Chaotic Diffusion

abstract:
  Extensive coherent control over quantum chaotic diffusion using the kicked
rotor model is demonstrated and its origin in deviations from random matrix
theory is identified. Further, the extent of control in the presence of
external decoherence is established. The results are relevant to both areas of
quantum chaos and coherent control.


title: General approach to potentials with two known levels

abstract:
  We present the general form of potentials with two given energy levels
$E_{1}$, $E_{2}$ and find corresponding wave functions. These entities are
expressed in terms of one function $\xi (x)$ and one parameter $\Delta
E=E_{2}$-$E_{1}$. We show how the quantum numbers of both levels depend on
properties of the function $\xi (x)$. Our approach does not need resorting to
the technique of supersymmetric (SUSY) quantum mechanics but automatically
generates both the potential and superpotential.


title:
Stochastic Theory of Relativistic Particles Moving in a Quantum Field:
  II. Scalar Abraham-Lorentz-Dirac-Langevin Equation, Radiation Reaction and
  Vacuum Fluctuations

abstract:
  We apply the open systems concept and the influence functional formalism
introduced in Paper I to establish a stochastic theory of relativistic moving
spinless particles in a quantum scalar field. The stochastic regime resting
between the quantum and semi-classical captures the statistical mechanical
attributes of the full theory. Applying the particle-centric world-line
quantization formulation to the quantum field theory of scalar QED we derive a
time-dependent (scalar) Abraham-Lorentz-Dirac (ALD) equation and show that it
is the correct semiclassical limit for nonlinear particle-field systems without
the need of making the dipole or non-relativistic approximations. Progressing
to the stochastic regime, we derive multiparticle ALD-Langevin equations for
nonlinearly coupled particle-field systems. With these equations we show how to
address time-dependent dissipation/noise/renormalization in the semiclassical
and stochastic limits of QED. We clarify the the relation of radiation
reaction, quantum dissipation and vacuum fluctuations and the role that initial
conditions may play in producing non-Lorentz invariant noise. We emphasize the
fundamental role of decoherence in reaching the semiclassical limit, which also
suggests the correct way to think about the issues of runaway solutions and
preacceleration from the presence of third derivative terms in the ALD
equation. We show that the semiclassical self-consistent solutions obtained in
this way are ``paradox'' and pathology free both technically and conceptually.
This self-consistent treatment serves as a new platform for investigations into
problems related to relativistic moving charges.


title: Hyperbolic quantum mechanics

abstract:
  We start to develop the quantization formalism in a hyperbolic Hilbert space.
Generalizing Born's probability interpretation, we found that unitary
transformations in such a Hilbert space represent a new class of
transformations of probabilities which describe a kind of {\it hyperbolic
interference}. The most interesting problem which was generated by our
investigation is to find experimental evidence of hyperbolic interference. The
hyperbolic quantum formalism can also be interesting as a new theory of
probabilistic waves that can be developed parallely to the standard quantum
theory. Comparative analysis of these two wave theories could be useful for
understanding of the role of various structures of the standard quantum
formalism. In particular, one of distinguishing feature of the hyperbolic
quantum formalism is the restricted validity of the superposition principle.


title: An Analysis of Completely-Positive Trace-Preserving Maps on 2x2 Matrices

abstract:
  We give a useful new characterization of the set of all completely positive,
trace-preserving (i.e., stochastic) maps from 2x2 matrices to 2x2 matrices.
These conditions allow one to easily check any trace-preserving map for
complete positivity. We also determine explicitly all extreme points of this
set, and give a useful parameterization after reduction to a certain canonical
form. This allows a detailed examination of an important class of non-unital
extreme points which can be characterized as having exactly two images on the
Bloch sphere.
  We also discuss a number of related issues about the images and the geometry
of the set of stochastic maps, and show that any stochastic map on 2x2 matrices
can be written as a convex combination of two "generalized" extreme points.

categories: quant-ph math-ph math.MP math.OA math.RA


title:
Quantum Computing of Classical Chaos: Smile of the Arnold-Schrodinger
  Cat

abstract:
  We show on the example of the Arnold cat map that classical chaotic systems
can be simulated with exponential efficiency on a quantum computer. Although
classical computer errors grow exponentially with time, the quantum algorithm
with moderate imperfections is able to simulate accurately the unstable chaotic
classical dynamics for long times. The algorithm can be easily implemented on
systems of a few qubits.


title: Quantum Communication Complexity (A Survey)

abstract:
  Can quantum communication be more efficient than its classical counterpart?
Holevo's theorem rules out the possibility of communicating more than n bits of
classical information by the transmission of n quantum bits --- unless the two
parties are entangled, in which case twice as many classical bits can be
communicated but no more. In apparent contradiction, there are distributed
computational tasks for which quantum communication cannot be simulated
efficiently by classical means. In extreme cases, the effect of transmitting
quantum bits cannot be achieved classically short of transmitting an
exponentially larger number of bits.
  In a similar vein, can entanglement be used to save on classical
communication? It is well known that entanglement on its own is useless for the
transmission of information. Yet, there are distributed tasks that cannot be
accomplished at all in a classical world when communication is not allowed, but
that become possible if the non-communicating parties share prior entanglement.
This leads to the question of how expensive it is, in terms of classical
communication, to provide an exact simulation of the spooky power of
entanglement.


title:
Quantum-Liouville and Langevin Equations for Gravitational Radiation
  Damping

abstract:
  From a forward--backward path integral, we derive a master equation for the
emission and absorption of gravitons by a massive quantum object in a heat bath
of gravitons. Such an equation could describe collapse phenomena of dense
stars. We also present a useful approximate Langevin equation for such a
system.


title:
Formulation of Quantum Theory Using Computable and Non-Computable Real
  Numbers

abstract:
  It is shown that in two-state quantum theory, a generic quantum state can be
described by a non-computable real number. In terms of this, the criterion for
measurement outcome is simply and deterministically defined. This demonstration
is based on a construction of the Riemann sphere whose points represent, not
complex numbers, but divergent sequences with bivalent elements. Complex
structure arises from self-similar properties of a set of operators which
generate these sequences. In general, a rotation of (the coordinates of) the
sphere maps a computable real to a non-computable real. This is interpreted
physically as a mapping of a physically-measurable state to a counterfactual
state. Implications for non-locality, null measurements, many worlds and so on,
are discussed. The possible role of the Euler equation as the counterpart of
the Schrodinger equation for real-number quantum state evolution is also
outlined.


title: Charged particles in a rotating magnetic field

abstract:
  We study the valence electron of an alkaline atom or a general charged
particle with arbitrary spin and with magnetic moment moving in a rotating
magnetic field. By using a time-dependent unitary transformation, the
Schr\"odinger equation with the time-dependent Hamiltonian can be reduced to a
Schr\"odinger-like equation with a time-independent effective Hamiltonian.
Eigenstates of the effective Hamiltonian correspond to cyclic solutions of the
original Schr\"odinger equation. The nonadiabatic geometric phase of a cyclic
solution can be expressed in terms of the expectation value of the component of
the total angular momentum along the rotating axis, regardless of whether the
solution is explicitly available. For the alkaline atomic electron and a strong
magnetic field, the eigenvalue problem of the effective Hamiltonian is
completely solved, and the geometric phase turns out to be a linear combination
of two solid angles. For a weak magnetic field, the same problem is solved
partly. For a general charged particle, the problem is solved approximately in
a slowly rotating magnetic field, and the geometric phases are also calculated.


title: Quantum Networks for Concentrating Entanglement

abstract:
  If two parties, Alice and Bob, share some number, n, of partially entangled
pairs of qubits, then it is possible for them to concentrate these pairs into
some smaller number of maximally entangled states. We present a simplified
version of the algorithm for such entanglement concentration, and we describe
efficient networks for implementing these operations.


title:
Geometric Phases, Symmetries of Dynamical Invariants, and Exact Solution
  of the Schr\"odinger Equation

abstract:
  We introduce the notion of the geometrically equivalent quantum systems
(GEQS) as quantum systems that lead to the same geometric phases for a given
complete set of initial state vectors. We give a characterization of the GEQS.
These systems have a common dynamical invariant, and their Hamiltonians and
evolution operators are related by symmetry transformations of the invariant.
If the invariant is $T$-periodic, the corresponding class of GEQS includes a
system with a $T$-periodic Hamiltonian. We apply our general results to study
the classes of GEQS that include a system with a cranked Hamiltonian
$H(t)=e^{-iKt}H_0e^{iKt}$. We show that the cranking operator $K$ also belongs
to this class. Hence, in spite of the fact that it is time-independent, it
leads to nontrivial cyclic evolutions and geometric phases. Our analysis allows
for an explicit construction of a complete set of nonstationary cyclic states
of any time-independent simple harmonic oscillator. The period of these cyclic
states is half the characteristic period of the oscillator.


title: Photon position operators and localized bases

abstract:
  We extend a procedure for construction of the photon position operators with
transverse eigenvectors and commuting components [Phys. Rev. A 59, 954 (1999)]
to body rotations described by three Euler angles. The axial angle can be made
a function of the two polar angles, and different choices of the functional
dependence are analogous to different gauges of a magnetic field. Symmetries
broken by a choice of gauge are re-established by transformations within the
gauge group. The approach allows several previous proposals to be related.
Because of the coupling of the photon momentum and spin, our position operator,
like that proposed by Pryce, is a matrix that does not commute with the spin
operator. Unlike the Pryce operator, however, our operator has commuting
components, but the commutators of these components with the total angular
momentum require an extra term to rotate the matrices for each vector component
around the momentum direction. Several proofs of the nonexistence of a photon
position operator with commuting components are based on overly restrictive
premises that do not apply here.


title: Quantum Theory From Five Reasonable Axioms

abstract:
  The usual formulation of quantum theory is based on rather obscure axioms
(employing complex Hilbert spaces, Hermitean operators, and the trace rule for
calculating probabilities). In this paper it is shown that quantum theory can
be derived from five very reasonable axioms. The first four of these are
obviously consistent with both quantum theory and classical probability theory.
Axiom 5 (which requires that there exists continuous reversible transformations
between pure states) rules out classical probability theory. If Axiom 5 (or
even just the word "continuous" from Axiom 5) is dropped then we obtain
classical probability theory instead. This work provides some insight into the
reasons quantum theory is the way it is. For example, it explains the need for
complex numbers and where the trace formula comes from. We also gain insight
into the relationship between quantum theory and classical probability theory.


title: Entanglement in the Quantum Heisenberg XY model

abstract:
  We study the entanglement in the quantum Heisenberg XY model in which the
so-called W entangled states can be generated for 3 or 4 qubits. By the concept
of concurrence, we study the entanglement in the time evolution of the XY
model. We investigate the thermal entanglement in the two-qubit isotropic XY
model with a magnetic field and in the anisotropic XY model, and find that the
thermal entanglement exists for both ferromagnetic and antiferromagnetic cases.
Some evidences of the quantum phase transition also appear in these simple
models.


title: Inhibition of Decoherence due to Decay in a Continuum

abstract:
  We propose a scheme for slowing down decay into a continuum. We make use of a
sequence of ultrashort $2\pi$-pulses applied on an auxiliary transition of the
system so that there is a destructive interference between the two transition
amplitudes - one before the application of the pulse and the other after the
application of the pulse. We give explicit results for a structured continuum.
Our scheme can also inhibit unwanted transitions.


title: Entropy and Temperature of a Quantum Carnot Engine

abstract:
  It is possible to extract work from a quantum-mechanical system whose
dynamics is governed by a time-dependent cyclic Hamiltonian. An energy bath is
required to operate such a quantum engine in place of the heat bath used to run
a conventional classical thermodynamic heat engine. The effect of the energy
bath is to maintain the expectation value of the system Hamiltonian during an
isoenergetic expansion. It is shown that the existence of such a bath leads to
equilibrium quantum states that maximise the von Neumann entropy. Quantum
analogues of certain thermodynamic relations are obtained that allow one to
define the temperature of the energy bath.


title: On Ordering of Operators in Canonical Quantization in Curved Space

abstract:
  Ambiguities arising in different approaches (canonical, quasiclassical, path
integration) to quantization are discussed by an example of the mechanics of a
point-like particle in the Riemannian space (the geodesic dynamics). A way to
select a single rule of quantization is proposed by requirement of a
consistency of the quantum mechanics' following from the canonical and
quasiclassical approaches. This rule selects also a definition of the path
integration. A geometric interpretation of the noncovariance of the canonical
Hamilton operator of a particle with respect to the diffeomorphisms of the
Riemannian configuration space is proposed.


title: Measuring the Quantum State of a Large Angular Momentum

abstract:
  We demonstrate a general method to measure the quantum state of an angular
momentum of arbitrary magnitude. The (2F+1) x (2F+1) density matrix is
completely determined from a set of Stern-Gerlach measurements with (4F+1)
different orientations of the quantization axis. We implement the protocol for
laser cooled Cesium atoms in the 6S_{1/2}(F=4) hyperfine ground state and apply
it to a variety of test states prepared by optical pumping and Larmor
precession. A comparison of input and measured states shows typical
reconstruction fidelities of about 0.95.


title: Entangled state based on nonorthogonal state

abstract:
  Properties of entangled states based on nonorthogonal states are clarified.
Especially, it is shown that they can have complete degree of entanglement.


title: A review of Johnjoe McFadden's book ``Quantum Evolution''

abstract:
  In ``Quantum Evolution: Life in the Multiverse'' (HarperCollins, 2000), ISBN
0-00-255948-X, 0-00-655128-9, Johnjoe McFadden makes far-reaching claims for
the importance of quantum physics in the solution of problems in biological
science. In this review, I discuss the relevance of unitary wavefunction
dynamics to biological systems, analyse the inverse quantum Zeno effect, and
argue that McFadden's use of quantum theory is deeply flawed.


title: Nonlinear optics with less than one photon

abstract:
  We demonstrate suppression and enhancement of spontaneous parametric down-
conversion via quantum interference with two weak fields from a local
oscillator (LO). Pairs of LO photons are observed to upconvert with high
efficiency for appropriate phase settings, exhibiting an effective nonlinearity
enhanced by at least 10 orders of magnitude. This constitutes a two-photon
switch, and promises to be useful for a variety of nonlinear optical effects at
the quantum level.


title:
Analytical operator solution of master equations describing
  phase-sensitive processes

abstract:
  We present a method of solving master equations which may describe, in their
most general form, phase sensitive processes such as decay and amplification.
We make use of the superoperator technique.


title:
Do Quantum Nonlocal Correlations Imply Information Transfer?- A Gedanken
  Experiment

abstract:
  We study a generalization of the original Einstein-Podolsky-Rosen thought
experiment. It is essentially a delayed choice experiment as applied to
entangled particles. The basic idea is: given two observers sharing
position-momentum entangled photons, one party chooses whether she measures
position or momentum of her photons after the particles leave the source. The
other party should infer her action by checking for the absence or presence of
characteristic interference patterns after subjecting his particles to certain
optical pre-processing. An occurance of apparent signaling is attributed to the
difficulty in treating single photons simultaneously at a quantum mechanical
and quantum electrodynamic level, as required by the experiment, and points to
the need for a careful study of an aspect of the foundations of quantum
mechanics and electrodynamics.


title:
Do Quantum Nonlocal Correlations Imply Information Transfer?- A Simply
  Quantum Optical Test

abstract:
  In order to understand whether nonlocality implies information transfer, a
quantum optical experimental test, well within the scope of current technology,
is proposed. It is essentially a delayed choice experiment as applied to
entangled particles. The basic idea is: given two observers sharing
position-momentum entangled photons, one party chooses whether she measures
position or momentum of her photons after the particles leave the source. The
other party should infer her action by checking for the absence or presence of
characteristic interference patterns after subjecting his particles to certain
optical pre-processing. An occurance of signal transmission is attributed to
the breakdown of complementarity in incomplete measurements. Since the result
implies that the transferred information is classical, we discuss some
propositions for safeguarding causality.


title:
Supersymmetric and Shape-Invariant Generalization for Nonresonant and
  Intensity-Dependent Jaynes-Cummings Systems

abstract:
  A class of shape-invariant bound-state problems which represent transition in
a two-level system introduced earlier are generalized to include arbitrary
energy splittings between the two levels as well as intensity-dependent
interactions. We show that the couple-channel Hamiltonians obtained correspond
to the generalizations of the nonresonant and intensity-dependent nonresonant
Jaynes-Cummings Hamiltonians, widely used in quantized theories of laser. In
this general context, we determine the eigenstates, eigenvalues, the time
evolution matrix and the population inversion matrix factor.


title: Topological Quantum Computation

abstract:
  The theory of quantum computation can be constructed from the abstract study
of anyonic systems. In mathematical terms, these are unitary topological
modular functors. They underlie the Jones polynomial and arise in
Witten-Chern-Simons theory. The braiding and fusion of anyonic excitations in
quantum Hall electron liquids and 2D-magnets are modeled by modular functors,
opening a new possibility for the realization of quantum computers. The chief
advantage of anyonic computation would be physical error correction: An error
rate scaling like $e^{-\a\l}$, where $\l$ is a length scale, and $\alpha$ is
some positive constant. In contrast, the $\q$presumptive" qubit-model of
quantum computation, which repairs errors combinatorically, requires a
fantastically low initial error rate (about $10^{-4}$) before computation can
be stabilized.


title:
Quantum Information Processing with Ferroelectrically Coupled Quantum
  Dots

abstract:
  I describe a proposal to construct a quantum information processor using
ferroelectrically coupled Ge/Si quantum dots. The spin of single electrons form
the fundamental qubits. Small (<10 nm diameter) Ge quantum dots are optically
excited to create spin polarized electrons in Si. The static polarization of an
epitaxial ferroelectric thin film confines electrons laterally in the
semiconductor; spin interactions between nearest neighbor electrons are
mediated by the nonlinear process of optical rectification. Single qubit
operations are achieved through "g-factor engineering" in the Ge/Si structures;
spin-spin interactions occur through Heisenberg exchange, controlled by
ferroelectric gates. A method for reading out the final state, while required
for quantum computing, is not described; electronic approaches involving single
electron transistors may prove fruitful in satisfying this requirement.


title: Maximum-likelihood estimation of quantum measurement

abstract:
  Maximum likelihood estimation is applied to the determination of an unknown
quantum measurement. The measuring apparatus performs measurements on many
different quantum states and the positive operator-valued measures governing
the measurement statistics are then inferred from the collected data via
Maximum-likelihood principle. In contrast to the procedures based on linear
inversion, our approach always provides physically sensible result. We
illustrate the method on the case of Stern-Gerlach apparatus.


title: Quantum Logic

abstract:
  We investigate some forms of quantum logic arising from the standard and the
unsharp approach.


title: NMR Simulation of an Eight-State Quantum System

abstract:
  The propagation of excitation along a one-dimensional chain of atoms is
simulated by means of NMR. The physical system used as an analog quantum
computer is a nucleus of 133-Cs (spin 7/2) in a liquid crystalline matrix. The
Hamiltonian of migration is simulated by using a special 7-frequency pulse, and
the dynamics is monitored by following the transfer of population from one of
the 8 spin energy levels to the other.


title: On the measure of entanglement

abstract:
  In this paper we present the novel qualities of entanglement of formation for
general (so also infinite dimensional) quantum systems.
  A major benefit of our presentation is a rigorous description of entanglement
of formation. In particular, we indicate how this description may be used to
examine optimal decompositions. Illustrative examples showing the method of
estimation of entanglement of formation are givem\n.


title:
Computer Model of Quantum Zeno Effect in Spontaneous Decay with Distant
  Detector

abstract:
  A numerical model of spontaneous decay continuously monitored by a distant
detector of emitted particles is constructed. It is shown that there is no
quantum Zeno effect in such quantum measurement if the interaction between
emitted particle and detector is short-range and the mass of emitted particle
is not zero.


title: Detecting Quantum Entanglement

abstract:
  We review the criteria for separability and quantum entanglement, both in a
bipartite as well as a multipartite setting. We discuss Bell inequalities,
entanglement witnesses, entropic inequalities, bound entanglement and several
features of multipartite entanglement. We indicate how these criteria bear on
the experimental detection of quantum entanglement.


title: Evolution of entanglement for spin-flip dynamics

abstract:
  A model of evolution of bipartite quantum state entanglement is studied. It
involves recently introduced quantum block spin-flip dynamics on a lattice. We
find that for initially separable states the considered evolution leads, in
general, to entangled states. We also present a complete characterization of
two-point correlation functions for that type of dynamics to confirm
enhancement of quantum correlations for the considered system.