title: Nonlocality - The party may be over

abstract:
  We demonstrate that the phenomenon known as Spontaneous Parametric Down
Conversion is really an amplification, in a nonlinear crystal pumped by a
laser, of certain pairs of modes of the electromagnetic zeropoint field. The
demonstration is achieved by showing the existence of a related phenomenon,
Spontaneous Parametric Up Conversion. This phenomenon, once observed, will cast
doubt on the quantum-optical theory, which treats photons as the elementary
objects of the light field. It will also lend greater credibility to the
zeropoint-field description of optical entanglement phenomena. That description
is based on the unquantized light field and is consistently local, in contrast
with the nonlocal description of Quantum Optics.


title: Parrondo Games and Quantum Algorithms

abstract:
  We pursue the possible connections between classical games and quantum
computation. The Parrondo game is one in which a random combination of two
losing games produces a winning game. We introduce novel realizations of this
Parrondo effect in which the player can `win' via random reflections and
rotations of the state-vector, and connect these to known quantum algorithms.


title: Quantum repeated games

abstract:
  In a two-stage repeated classical game of prisoners' dilemma the knowledge
that both players will defect in the second stage makes the players to defect
in the first stage as well. We find a quantum version of this repeated game
where the players decide to cooperate in the first stage while knowing that
both will defect in the second.


title: Quantum information is incompressible without errors

abstract:
  A classical random variable can be faithfully compressed into a sequence of
bits with its expected length lies within one bit of Shannon entropy. We
generalize this variable-length and faithful scenario to the general quantum
source producing mixed states $\rho_i$ with probability $p_i$. In contrast to
the classical case, the optimal compression rate in the limit of large block
length differs from the one in the fixed-length and asymptotically faithful
scenario. The amount of this gap is interpreted as the genuinely quantum part
being incompressible in the former scenario.


title: Quantum Imaging

abstract:
  We provide a brief overview of the newly born field of quantum imaging, and
discuss some concepts that lie at the root of this field.


title: Quantum computer architecture for fast entropy extraction

abstract:
  If a quantum computer is stabilized by fault-tolerant quantum error
correction (QEC), then most of its resources (qubits and operations) are
dedicated to the extraction of error information. Analysis of this process
leads to a set of central requirements for candidate computing devices, in
addition to the basic ones of stable qubits and controllable gates and
measurements. The logical structure of the extraction process has a natural
geometry and hierarchy of communication needs; a computer whose physical
architecture is designed to reflect this will be able to tolerate the most
noise. The relevant networks are dominated by quantum information transport,
therefore to assess a computing device it is necessary to characterize its
ability to transport quantum information, in addition to assessing the
performance of conditional logic on nearest neighbours and the passive
stability of the memory. The transport distances involved in QEC networks are
estimated, and it is found that a device relying on swap operations for
information transport must have those operations an order of magnitude more
precise than the controlled gates of a device which can transport information
at low cost.


title: A first test of Wigner function local realistic model

abstract:
  We perform a first experimental test of a local realistic model, recently
proposed, based on the Wigner function as probability distribution for the
hidden variable. Our results disfavour the model and confirm standard quantum
mechanics predictions.


title: Subquantum Information and Computation

abstract:
  It is argued that immense physical resources - for nonlocal communication,
espionage, and exponentially-fast computation - are hidden from us by quantum
noise, and that this noise is not fundamental but merely a property of an
equilibrium state in which the universe happens to be at the present time. It
is suggested that 'non-quantum' or nonequilibrium matter might exist today in
the form of relic particles from the early universe. We describe how such
matter could be detected and put to practical use. Nonequilibrium matter could
be used to send instantaneous signals, to violate the uncertainty principle, to
distinguish non-orthogonal quantum states without disturbing them, to eavesdrop
on quantum key distribution, and to outpace quantum computation (solving
NP-complete problems in polynomial time).


title:
Scheme to Measure Quantum Stokes Parameters and their Fluctuations and
  Correlations

abstract:
  We propose a scheme to measure quantum Stokes parameters, their fluctuations
and correlations. The proposal involves measurements of intensities and
intensity- intensity correlations for suitably defined modes, which can be
produced by a combination of half wave and quarter wave plates.


title: Lorentz Invariance of Entanglement

abstract:
  We study the transformation of maximally entangled states under the action of
Lorentz transformations in a fully relativistic setting. By explicit
calculation of the Wigner rotation, we describe the relativistic analog of the
Bell states as viewed from two inertial frames moving with constant velocity
with respect to each other. Though the finite dimensional matrices describing
the Lorentz transformations are non-unitary, each single particle state of the
entangled pair undergoes an effective, momentum dependent, local unitary
rotation, thereby preserving the entanglement fidelity of the bipartite state.
The details of how these unitary transformations are manifested are explicitly
worked out for the Bell states comprised of massive spin 1/2 particles and
massless photon polarizations. The relevance of this work to non-inertial
frames is briefly discussed.


title: Elements of Micromaser Physics

abstract:
  The elements of micromaser physics are reviewed in a tutorial way. The
emphasis is on the basic theoretical concepts, not on technical details or
experimental subtleties. After a brief treatment of the atom-photon interaction
according to the Jaynes-Cummings model, the master equation that governs the
dynamics of the one-atom maser is derived. Then the more important properties
of the steady states of the one-atom maser are discussed, including the trapped
states. The approximations, upon which the standard theoretical model of the
one-atom maser is based, are exhibited. The methods by which one calculates
statistical properties of the emerging atoms are hinted at.


title: Motions of Classical Charged Tachyons

abstract:
  It is shown by numerical simulation that classical charged tachyons have
self-orbiting helical solutions in a narrow neighborhood of certain discrete
values for the velocity when the electromagnetic interaction is described by
Feynman-Wheeler electrodynamics. The force rapidly oscillates between
attractive and repulsive as a function of velocity in this neighborhood. Causal
electrodynamics is also considered, and in this case it is found that when the
force is attractive the tachyon loses energy to radiation. Only certain narrow
ranges of velocity give attractive forces, and a geometrical derivation of
these special velocities is given. Possible implications of these results for
hidden variable theories of quantum mechanics are conjectured.


title: Reflection of matter waves by a moving wall

abstract:
  Reflection of a normal incident matter wave by a perfectly reflecting wall
moving with a constant velocity is investigated. A surprising phenomenon is
found-that if the the wall moves faster than the phase velocity of the incident
wave, both the reflected and incident waves propagate in the same direction.
This counter-intuitive result is an example which shows that common sense is
not always credible when one deals with quantum problems.


title: Remote generalized measurements (POVMs) require non-maximal entanglement

abstract:
  We show that non-maximal entangled states can be used for implementing, with
unit probability, remote generalized measurements (POVMs). We show how any
n-qubit POVM can be applied remotely and derive its entanglement cost. The
later turns out to be equal to the entanglement capability for a class of
POVMs. This suggests a one-to-one relation between sub-sets of POVM operations
and entanglement.


title: Quantum cloning with an optical fiber amplifier

abstract:
  It has been shown theoretically that a light amplifier working on the
physical principle of stimulated emission should achieve optimal quantum
cloning of the polarization state of light. We demonstrate close-to-optimal
universal quantum cloning of polarization in a standard fiber amplifier for
telecom wavelengths. For cloning 1 --> 2 we find a fidelity of 0.82, the
optimal value being 5/6 = 0.83.


title: Plasmon-assisted Quantum Entanglement

abstract:
  The state of a two-particle system is called entangled when its quantum
mechanical wave function cannot be factorized in two single-particle wave
functions. Entanglement leads to the strongest counter-intuitive feature of
quantum mechanics, namely nonlocality. Experimental realization of quantum
entanglement is relatively easy for the case of photons; a pump photon can
spontaneously split into a pair of entangled photons inside a nonlinear
crystal. In this paper we combine quantum entanglement with nanostructured
metal optics in the form of optically thick metal films perforated with a
periodic array of subwavelength holes. These arrays act as photonic crystals
that may convert entangled photons into surface-plasmon waves, i.e.,
compressive charge density waves. We address the question whether the
entanglement survives such a conversion. We find that, in principle, optical
excitation of the surface plasmon modes of a metal is a coherent process at the
single-particle level. However, the quality of the plasmon-assisted
entanglement is limited by spatial dispersion of the hole arrays. This spatial
dispersion is due to the nonlocal dielectric response of a metal, which is
particularly large in the plasmonic regime; it introduces "which way" labels,
that may kill entanglement.


title: The Nature and Location of Quantum Information

abstract:
  Quantum information is defined by applying the concepts of ordinary (Shannon)
information theory to a quantum sample space consisting of a single framework
or consistent family. A classical analogy for a spin-half particle and other
arguments show that the infinite amount of information needed to specify a
precise vector in its Hilbert space is not a measure of the information carried
by a quantum entity with a $d$-dimensional Hilbert space; the latter is,
instead, bounded by log d bits (1 bit per qubit). The two bits of information
transmitted in dense coding are located not in one but in the correlation
between two qubits, consistent with this bound. A quantum channel can be
thought of as a "structure" or collection of frameworks, and the physical
location of the information in the individual frameworks can be used to
identify the location of the channel. Analysis of a quantum circuit used as a
model of teleportation shows that the location of the channel depends upon
which structure is employed; for ordinary teleportation it is not (contrary to
Deutsch and Hayden) present in the two bits resulting from the Bell-basis
measurement, but in correlations of these with a distant qubit. In neither
teleportation nor dense coding does information travel backwards in time, nor
is it transmitted by nonlocal (superluminal) influences. It is (tentatively)
proposed that all aspects of quantum information can in principle be understood
in terms of the (basically classical) behavior of information in a particular
framework, along with the framework dependence of this information.


title: Qubitless Quantum Logic

abstract:
  We discuss the implementation of quantum logic in a system of strongly
interacting particles. The implementation is qubitless since ``logical qubits''
don't correspond to any physical two-state subsystems. As an illustration, we
present the results of simulations of the quantum controlled-NOT gate and
Shor's algorithm for a chain of spin-1/2 particles with Heisenberg coupling.
Our proposal extends the current theory of quantum information processing to
include systems with permanent strong coupling between the two-state
subsystems.


title: Quantum Correlations in Systems of Indistinguishable Particles

abstract:
  We discuss quantum correlations in systems of indistinguishable particles in
relation to entanglement in composite quantum systems consisting of well
separated subsystems. Our studies are motivated by recent experiments and
theoretical investigations on quantum dots and neutral atoms in microtraps as
tools for quantum information processing. We present analogies between
distinguishable particles, bosons and fermions in low-dimensional Hilbert
spaces. We introduce the notion of Slater rank for pure states of pairs of
fermions and bosons in analogy to the Schmidt rank for pairs of distinguishable
particles. This concept is generalized to mixed states and provides a
correlation measure for indistinguishable particles. Then we generalize these
notions to pure fermionic and bosonic states in higher-dimensional Hilbert
spaces and also to the multi-particle case. We review the results on quantum
correlations in mixed fermionic states and discuss the concept of fermionic
Slater witnesses. Then the theory of quantum correlations in mixed bosonic
states and of bosonic Slater witnesses is formulated. In both cases we provide
methods of constructing optimal Slater witnesses that detect the degree of
quantum correlations in mixed fermionic and bosonic states.


title:
Bounds on the number of time steps for simulating arbitrary interaction
  graphs

abstract:
  In previous papers we have considered mutual simulation of n-partite
pair-interaction Hamiltonians. We have focussed on the running time overhead of
general simulations, while considering the required number of time steps only
for special cases (decoupling and time-reversal). These two complexity measures
differ significantly. Here we derive lower bounds on the number of time steps
for general simulations. In particular, the simulation of interaction graphs
with irrational spectrum requires at least n steps. We discuss as examples
graphs that correspond to graph codes and nearest neighbor interactions in 1-
and 2-dimensional lattices. In the latter case the lower bounds are almost
tight.


title: Strange attractor simulated on a quantum computer

abstract:
  We show that dissipative classical dynamics converging to a strange attractor
can be simulated on a quantum computer. Such quantum computations allow to
investigate efficiently the small scale structure of strange attractors,
yielding new information inaccessible to classical computers. This opens new
possibilities for quantum simulations of various dissipative processes in
nature.


title: Dimensional enhancement of kinetic energies

abstract:
  Simple thermodynamics considers kinetic energy to be an extensive variable
which is proportional to the number, N, of particles. We present a quantum
state of N non-interacting particles for which the kinetic energy increases
quadratically with N. This enhancement effect is tied to the quantum
centrifugal potential whose strength is quadratic in the number of dimensions
of configuration space.


title: Secure direct communication using entanglement

abstract:
  A novel communication protocol based on an entangled pair of qubits is
presented, allowing secure direct communication from one party to another
without the need for a shared secret key. Since the information is transferred
in a deterministic manner, no qubits have to be discarded and every qubit
carries message information. The security of the transfer against active and
passive eavesdropping attacks is provided. The detection rate of active attacks
is at least 25%. The protocol works with a quantum efficiency of 1 bit per
qubit transmitted.


title: Generation and degree of entanglement in a relativistic formulation

abstract:
  The generation of entangled states and their degree of entanglement is
studied ab initio in a relativistic formulation for the case of two interacting
spin-1/2 charged particles. In the realm of quantum electrodynamics we derive
the interaction that produces entanglement between the spin components of
covariant Dirac spinors describing the two particles. Following this consistent
approach the relativistic invariance of the generated entanglement is
discussed.


title:
Temporal Interferometry: A Mechanism for Controlling Qubit Transitions
  During Twisted Rapid Passage with Possible Application to Quantum Computing

abstract:
  In an adiabatic rapid passage experiment, the Bloch vector of a two-level
system (qubit) is inverted by slowly inverting an external field to which it is
coupled, and along which it is initially aligned. In twisted rapid passage, the
external field is allowed to twist around its initial direction with azimuthal
angle $\phi (t)$ at the same time that it is inverted. For polynomial twist:
$\phi (t) \sim Bt^{n}$. We show that for $n \geq 3$, multiple avoided crossings
can occur during the inversion of the external field, and that these crossings
give rise to strong interference effects in the qubit transition probability.
The transition probability is found to be a function of the twist strength $B$,
which can be used to control the time-separation of the avoided crossings, and
hence the character of the interference. Constructive and destructive
interference are possible. The interference effects are a consequence of the
temporal phase coherence of the wavefunction. The ability to vary this
coherence by varying the temporal separation of the avoided crossings renders
twisted rapid passage with adjustable twist strength into a temporal
interferometer through which qubit transitions can be greatly enhanced or
suppressed. Possible application of this interference mechanism to construction
of fast fault-tolerant quantum CNOT and NOT gates is discussed.


title:
Experimental investigation of the robustness of partially entangled
  photons over 11km

abstract:
  We experimentally investigate the robustness of maximal and non-maximal
Time-Bin entangled photons over distances up to 11 km. The entanglement is
determined by controllable parameters and in all cases is shown to be robust,
in that the photons maintain their degree of entanglement after transmission.


title: Dynamical Casimir Effect in a Designed Leaky Cavity

abstract:
  The phenomenon of particle creation within a resonantly vibrating lossy
cavity is investigated for the example of a massless scalar field at finite
temperature. Leakage is provided by insertion of a dispersive mirror into a
larger ideal cavity. Via the rotating wave approximation we demonstrate that
for the case of parametric resonance the exponential growth of the number of
created particles and the strong enhancement at finite temperatures are
preserved in the presence of reasonable losses. The relevance for experimental
tests of quantum radiation via the dynamical Casimir effect is addressed.


title: Quantum Theory of Feedback of Bosonic Gases

abstract:
  A quantum theory of feedback of bosonic many-atom systems is formulated. The
feedback-induced many-atom correlations are treated by use of a parameterized
correlation function, for which closed equations of motion are derived.
Therefrom the dynamics of any additive property of the system, i.e., properties
derived from the reduced single-atom density operator, can be obtained. An
example is given that indicates the correlation effects of feedback.


title: The Nature of Information in Quantum Mechanics

abstract:
  A suitable unified statistical formulation of quantum and classical mechanics
in a *-algebraic setting leads us to conclude that information itself is
noncommutative in quantum mechanics. Specifically we refer here to an
observer's information regarding a physical system. This is seen as the main
difference from classical mechanics, where an observer's information regarding
a physical system obeys classical probability theory. Quantum mechanics is then
viewed purely as a mathematical framework for the probabilistic description of
noncommutative information, with the projection postulate being a
noncommutative generalization of conditional probability. This view clarifies
many problems surrounding the interpretation of quantum mechanics, particularly
problems relating to the measuring process.


title:
Ergodic Properties of Local Spectral Density for a Conservative System
  of Coupled Quantum States

abstract:
  The shape and the inverse participation ratio (IPR) of local spectral density
(LSD) are studied for a generic isolated system of coupled quantum states, the
Hamiltonian of which is represented by a band random matrix with the disordered
leading diagonal. We find for the matrices with arbitrary small band that the
lack of ergodicity for LSD can be associated with an exponential increase in
IPR with the ratio $v/\Delta_c$ ($v$ - the root of mean square for off-diagonal
matrix elements, $\Delta_c$ - the energy spacing between directly coupled basis
states). Criterions specifying transition to localization and ergodicity for
LSD are considered.


title:
Decoherence of collective atomic spin states due to inhomogeneous
  coupling

abstract:
  We investigate the decoherence of a superposition of symmetric collective
internal states of an atomic ensemble due to inhomogeneous coupling to external
control fields. For asymptotically large system, we find the characteristic
decoherence rate scales as $\sqrt{N}$ with $N$ being the total number of atoms.
Our results shed new light on attempts for quantum information processing and
storage with atomic ensembles.


title: On the fidelity of mixed states of two qubits

abstract:
  We consider a single copy of a mixed state of two qubits and show how its
fidelity or maximal singlet fraction is related to the entanglement measures
concurrence and negativity. We characterize the extreme points of the convex
set of states with constant fidelity, and use this to prove tight lower and
upper bounds on the fidelity for a given amount of entanglement.


abstract:
  A workshop sponsored by NSF C-CR and organized by the Center for Discrete
Math and Theoretical Computer Science (DIMACS) was held at Elmsford, New York,
January 17-18, 2002 where we had several discussions that gave structure to
this report. The workshop immediately followed the Fifth Annual Quantum
Information Processing Workshop that was held January 14-17 at IBM Yorktown.
  This is the report from that workshop. The report recommends that the NSF
Division of Computer-Communications Research (C-CR) develop a new initiative in
"Theory of Quantum Computing and Communication" and describes several research
directions that this initiative could support.


title: Storage of light: A useful concept?

abstract:
  We show both analytically and numerically that photons from a probe pulse are
not stored in several recent experiments. Rather, they are absorbed to produce
a two-photon excitation. More importantly, when an identical coupling pulse is
re-injected into the medium, we show that the regenerated optical field has a
pulse width that is very different from the original probe field. It is
therefore, not a faithful copy of the original probe pulse.


title:
New Consequences of Induced Transparency in a Double-Lambda scheme:
  Destructive Interference In Four-wave Mixing

abstract:
  We investigate a four-state system interacting with long and short laser
pulses in a weak probe beam approximation. We show that when all lasers are
tuned to the exact unperturbed resonances, part of the four-wave mixing (FWM)
field is strongly absorbed. The part which is not absorbed has the exact
intensity required to destructively interfere with the excitation pathway
involved in producing the FWM state. We show that with this three-photon
destructive interference, the conversion efficiency can still be as high as
25%. Contrary to common belief,our calculation shows that this process, where
an ideal one-photon electromagnetically induced transparency is established, is
not most suitable for high efficiency conversion. With appropriate
phase-matching and propagation distance, and when the three-photon destructive
interference does not occur, we show that the photon flux conversion efficiency
is independent of probe intensity and can be close to 100%. In addition, we
show clearly that the conversion efficiency is not determined by the maximum
atomic coherence between two lower excited states, as commonly believed. It is
the combination of phase-matching and constructive interference involving the
two terms arising in producing the mixing wave that is the key element for the
optimized FWM generation. Indeed, in this scheme no appreciable excited state
is produced, so that the atomic coherence between states |0> and |2> is always
very small.


title:
A low-rate bound on the reliability of a quantum discrete memoryless
  channel

abstract:
  We extend a low-rate improvement of the random coding bound on the
reliability of a classical discrete memoryless channel to its quantum
counterpart. The key observation that we make is that the problem of bounding
below the error exponent for a quantum channel relying on the class of
stabilizer codes is equivalent to the problem of deriving error exponents for a
certain symmetric classical channel.


title: Storage States in Ultracold Collective Atoms

abstract:
  We present a complete theoretical description of atomic storage states in the
multimode framework by including spatial coherence in atomic collective
operators and atomic storage states. We show that atomic storage states are
Dicke states with the maximum cooperation number. In some limits, a set of
multimode atomic storage states has been established in correspondence with
multimode Fock states of the electromagnetic field. This gives better
understanding of both the quantum and coherent information of optical field can
be preserved and recovered in ultracold medium. In this treatment, we discuss
in detail both the adiabatic and dynamic transfer of quantum information
between the field and the ultracold medium.


title: Ultra-broadband Heteronuclear Hartmann-Hahn polarization transfer

abstract:
  It is showed on the basis of the multiple-quantum operator algebra space
formalism that ultra-broadband heteronuclear Hartmann-Hahn polarization
transfer could be achieved by the amplitude- and frequency-modulation
quasi-adiabatic excitation (90 degree) pulses, while it is usually difficult
for the adiabatic inversion pulses to achieve effectively broadband
Hartmann-Hahn transfer in a heteronuclear coupled two-spin system. The
adiabatic and quasi-adiabatic pulses have an important property that within
their activation bandwidth flip angle of the pulses is independent of the pulse
duration and the bandwidth increases as the pulse duration. This property is
importanr for construction of the heteronuclear Hartmann-Hahn transfer
sequences with the quasi-adiabatic 90 degree pulses. Theoretic analysis and
numerical simulation show that the heteronuclear Hartmann-Hahn transfer is
performed in the even-order multiple- quantum operator algebra subspace of the
two-spin system. The multiple-quantum operator algebra space formalism may give
a powerful guide to the construction of ultra- broadband heteronuclear
Hartmann-Hahn transfer sequences with the quasi-adiabatic 90 degree pulses.


title:
Engineering single atom and single photon sources from entangled atomic
  ensembles

abstract:
  We discuss the application of dipole blockade techniques for the preparation
of single atom and single photon sources. A deterministic protocol is given for
loading a single atom in an optical trap as well as ejecting a controlled
number of atoms in a desired direction. A single photon source with an
optically controlled beam-like emission pattern is described.


title: Near-optimal two-mode spin squeezing via feedback

abstract:
  We propose a feedback scheme for the production of two-mode spin squeezing.
We determine a general expression for the optimal feedback, which is also
applicable to the case of single-mode spin squeezing. The two-mode spin
squeezed states obtained via this feedback are optimal for j=1/2 and are very
close to optimal for j>1/2. In addition, the master equation suggests a
Hamiltonian that would produce two-mode spin squeezing without feedback, and is
analogous to the two-axis countertwisting Hamiltonian in the single mode case.


title:
Understanding destruction of nth-order quantum coherence in terms of
  multi-path interference

abstract:
  The classic example of the destruction of interference fringes in a
``which-way'' experiment, caused by an environmental interaction, may be viewed
as the destruction of first-order coherence as defined by Glauber many years
ago (Glauber). However, the influence of an environment can also destroy the
$n$th-order quantum coherence in a quantum system, where this high order
coherence is captured. We refer to this phenomenon as the $n$% th-order
decoherence. In this paper we show that, just as the first-order coherence can
be understood as the interference of the amplitudes for two distinct paths, the
higher order coherence may be understood as the interference of multiple
amplitudes corresponding to multiple paths. To see this, we introduce the
concept of $n$th-order ``multi-particle wave amplitude''. It turns out that the
$n$th-order correlation function can be expressed as the square norm of some
``multi-particle wave amplitude'' for the closed system or as the sum of such
square norms for the open system. We also examine, as a specific example, how
an environment can destroy the second order coherence by eliminating the
interference between various multiple paths.


title: Slower-than-Light Spin-1/2 Particles Endowed with Negative Mass Squared

abstract:
  Extending in a straightforward way the standard Dirac theory, we study a
quantum mechanical wave-equation describing free spinning particles --which we
propose to call "Pseudotachyons" (PT's)-- which behave like tachyons in the
momentum space, but like subluminal particles (v<c) in the ordinary space. This
is allowed since, as it happens in every quantum theory for spin-1/2 particles,
the momentum operator (that is conserved) and the velocity operator (that is
not) are independent operators, which refer to independent quantities. As a
consequence, at variance with ordinary Dirac particles, for PT's the average
velocity is not equal to the classical velocity, but actually to the velocity
"dual" of the classical velocity. The speed of PT's is therefore smaller than
the speed of light. Since a lot of experimental data seems to involve a
negative mass squared for neutrinos, we suggest that these particles might be
PT's, travelling, because of their very small mass, at subluminal speeds very
close to c. The present theory is shown to be separately invariant under the C,
P, T transformations; the covariance under Lorentz transformations is also
proved. Furthermore, we derive the kinematical constraints linking 4-impulse,
4-velocity and 4-polarization of free PT's


title: The reduction of the closest disentangled states

abstract:
  We study the closest disentangled state to a given entangled state in any
system (multi-party with any dimension). We obtain the set of equations the
closest disentangled state must satisfy, and show that its reduction is
strongly related to the extremal condition of the local filtering on each
party. Although the equations we obtain are not still tractable, we find some
sufficient conditions for which the closest disentangled state has the same
reduction as the given entangled state. Further, we suggest a prescription to
obtain a tight upper bound of the relative entropy of entanglement in two-qubit
systems.


title: Quantum Entropy Bound by Information in Black Hole Spacetime

abstract:
  We show that the increase of the generalized entropy by a quantum process
outside the horizon of a black hole is more than the Holevo bound of the
classical information lost into the black hole and which could be obtained by
further observations outside the horizon. In the optimal case, the prepared
information can be completely retrieved.


title: Formal Verification of Quantum Protocols

abstract:
  We propose to analyse quantum protocols by applying formal verification
techniques developed in classical computing for the analysis of communicating
concurrent systems. One area of successful application of these techniques is
that of classical security protocols, exemplified by Lowe's discovery and fix
of a flaw in the well-known Needham-Schroeder authentication protocol. Secure
quantum cryptographic protocols are also notoriously difficult to design.
Quantum cryptography is therefore an interesting target for formal
verification, and provides our first example; we expect the approach to be
transferable to more general quantum information processing scenarios. The
example we use is the quantum key distribution protocol proposed by Bennett and
Brassard, commonly referred to as BB84. We present a model of the protocol in
the process calculus CCS and the results of some initial analyses using the
Concurrency Workbench of the New Century (CWB-NC).


title: Entanglement for rank-2 mixed states

abstract:
  In a recent paper, Rungta et. al. [Phys. Rev. A, 64, 042315, 2001] introduced
a measure of mixed-state entanglement called the I-concurrence for arbitrary
pairs of qudits. We find an exact formula for an entanglement measure closely
related to the I-concurrence, the I-tangle, for all mixed states of two qudits
having no more than two nonzero eigenvalues. We use this formula to provide a
tight upper bound for the entanglement of formation for rank-2 mixed states of
a qubit and a qudit.


title:
Two Coupled Qubits are Classically Correlated with an Exact Bures
  Probability of 2^(1/2)/24

abstract: This paper has been withdrawn.


title: Transmit Classical and Quantum Information Secretly

abstract:
  This note presents a practical cryptography protocol for transmitting
classical and quantum information secretly and directly.


title: Spin and Electron Structure

abstract:
  The recent literature shows a renewed interest, with various independent
approaches, in the classical models for spin. Considering the possible interest
of those results, at least for the electron case, we purpose in this paper to
explore their physical and mathematical meaning, by the natural and powerful
language of Clifford algebras (which, incidentally, will allow us to unify
those different approaches). In such models, the ordinary electron is in
general associated to the mean motion of a point--like "constituent" Q, whose
trajectory is a cylindrical helix. We find, in particular, that the object Q
obeys a new, non-linear Dirac--like equation, such that --when averaging over
an internal cycle (which corresponds to a linearization)-- it transforms into
the ordinary Dirac equation (valid, of course, for the electron as a whole).


title: Complex--Barrier Tunnelling Times

abstract:
  In this paper we calculate the analytic expression of the phase time for the
scattering of an electron off a complex square barrier. As is well known the
(negative) imaginary part of the potential takes into account,
phenomenologically, the absorption. We investigate the so-called
Hartman-Fletcher effect, and find that it is suppressed by the presence of a
(not negligible) imaginary potential. In fact, when a sufficiently large
absorption is present, the asymptotical transmission speed is finite. Actually,
the tunnelling time does increase linearly with the barrier width. A recent
optical experiment seems to be in agreement with our theoretical previsions.


title:
Local transformation of mixed states of two qubits to Bell diagonal
  states

abstract:
  The optimal entanglement manipulation for a single copy of mixed states of
two qubits is to transform it to a Bell diagonal state. In this paper we derive
an explicit form of the local operation that can realize such a transformation.
The result obtained is universal for arbitrary entangled two-qubit states and
it discloses that the corresponding local filter is not unique for density
matrices with rank $n=2$ and can be exclusively determined for that with $n=3$
and 4. As illustrations, a four-parameters family of mixed states are explored,
the local filter as well as the transformation probability are given
explicitly, which verify the validity of the general result.


title: Modification of relative entropy of entanglement

abstract:
  We present the modified relative entropy of entanglement (MRE) that is proved
to be a upper bound of distillable entanglement (DE), also relative entropy of
entanglement (RE), and a lower bound of entanglement of formation (EF). For a
pure state, MRE is found by the requirement that MRE is equal to EF. For a
mixed state, MRE is calculated by defining a total relative density matrix. We
obtain an explicit and "weak" closed expressions of MRE that depends on the
pure state decompositions for two qubit systems and give out an algorithm to
calculate MRE in principle for more qubit systems. MRE significantly improves
the computability of RE, decreases the sensitivity on the pure state
decompositions in EF, reveals the particular difference of similar departure
states from Bell's state and restore the logarithmic dependence on probability
of component states consistent with information theory. As examples, we
calculate MRE of the mixture of Bell's states and departure states from Bell's
states, and compare them with EF as well as Wootters' EF. Moreover we study the
important properties of MRE including the behavior under local general
measurement (LGM) and classical communication (CC).


title: Quantum-optical communication rates through an amplifying random medium

abstract:
  We study the competing effects of stimulated and spontaneous emission on the
information capacity of an amplifying disordered waveguide. At the laser
threshold the capacity reaches a "universal" limit, independent of the degree
of disorder. Whether or not this limit is larger or smaller than the capacity
without amplification depends on the disorder, as well as on the input power.
Explicit expressions are obtained for heterodyne detection of coherent states,
and generalized for arbitrary detection scheme.


title: Optimal dimensionality for quantum cryptography

abstract:
  We perform a comparison of two protocols for generating a cryptographic key
composed from d-valued symbols: one exploiting a string of independent qubits
and another one utilizing d-level systems prepared in states belonging to d+1
mutually unbiased bases. We show that the protocol based on qubits is optimal
for quantum cryptography, since it provides higher security and higher key
generation rate.


title: Fast, efficient error reconciliation for quantum cryptography

abstract:
  We describe a new error reconciliation protocol {\it Winnow} based on the
exchange of parity and Hamming's ``syndrome'' for $N-$bit subunits of a large
data set. {\it Winnow} was developed in the context of quantum key distribution
and offers significant advantages and net higher efficiency compared to other
widely used protocols within the quantum cryptography community. A detailed
mathematical analysis of Winnow is presented in the context of practical
implementations of quantum key distribution; in particular, the information
overhead required for secure implementation is one of the most important
criteria in the evaluation of a particular error reconciliation protocol. The
increase in efficiency for Winnow is due largely to the reduction in
authenticated public communication required for its implementation.


title: Optimized quantum nondemolition measurement of a field quadrature

abstract:
  We suggest an interferometric scheme assisted by squeezing and linear
feedback to realize the whole class of field-quadrature quantum nondemolition
measurements, from Von Neumann projective measurement to fully non-destructive
non-informative one. In our setup, the signal under investigation is mixed with
a squeezed probe in an interferometer and, at the output, one of the two modes
is revealed through homodyne detection. The second beam is then
amplitude-modulated according to the outcome of the measurement, and finally
squeezed according to the transmittivity of the interferometer. Using strongly
squeezed or anti-squeezed probes respectively, one achieves either a projective
measurement, i.e. homodyne statistics arbitrarily close to the intrinsic
quadrature distribution of the signal, and conditional outputs approaching the
corresponding eigenstates, or fully non-destructive one, characterized by an
almost uniform homodyne statistics, and by an output state arbitrarily close to
the input signal. By varying the squeezing between these two extremes, or
simply by tuning the internal phase-shift of the interferometer, the whole set
of intermediate cases can also be obtained. In particular, an optimal quantum
nondemolition measurement of quadrature can be achieved, which minimizes the
information gain versus state disturbance trade-off.


title: New, efficient and robust, fiber-based quantum key distribution schemes

abstract:
  We present a new fiber based quantum key distribution (QKD) scheme which can
be regarded as a modification of an idea proposed by Inoue, Waks and Yamamoto
(IWY) [1]. The scheme described here uses a single phase modulator and two
differential delay elements in series at the transmitter that form an
interferometer when combined with a third differential delay element at the
receiver. The protocol is characterized by a high efficiency, reduced exposure
to an attack by an eavesdropper, and higher sensitivity to such an attack when
compared to other QKD schemes. For example, the efficiency with which
transmitted data contribute to the private key is 3/4 compared with 1/4 for
BB84 [2]. Moreover, an eavesdropper can aquire a maximum of 1/3 of the key
which leads to an error probability in the private key of 1/3. This can be
compared to 1/2 and 1/4 for these same parameters in both BB84 and IWY. The
combination of these considerations should lead to increased range and key
distribution rate over present fiber-based QKD schemes.


title: Entangled States and Local Measurements

abstract:
  We show that the maximum entanglement in a composite system corresponds to
the maximum uncertainty and maximum correlation of local measurements.


title: Optimal Control of One-Qubit Gates

abstract:
  We consider the problem of carrying an initial Bloch vector to a final Bloch
vector in a specified amount of time under the action of three control fields
(a vector control field). We show that this control problem is solvable and
therefore it is possible to optimize the control. We choose the physically
motivated criteria of minimum energy spent in the control, minimum magnitude of
the rate of change of the control and a combination of both. We find exact
analytical solutions.


title: Weak pseudo-Hermiticity and antilinear commutant

abstract:
  We inquire into some properties of diagonalizable pseudo-Hermitian operators,
showing that their definition can be relaxed and that the pseudo-Hermiticity
property is strictly connected with the existence of an antilinear symmetry.
This result is then illustrated by considering the particular case of the
complex Morse potential.


title: Entropy and Wigner Functions

abstract:
  The properties of an alternative definition of quantum entropy, based on
Wigner functions, are discussed. Such definition emerges naturally from the
Wigner representation of quantum mechanics, and can easily quantify the amount
of entanglement of a quantum state. It is shown that smoothing of the Wigner
function induces an increase in entropy. This fact is used to derive some
simple rules to construct positive definite probability distributions which are
also admissible Wigner functions


title: A Kochen-Specker Theorem for Unsharp Spin 1 Observables

abstract:
  Unsharp spin observables are shown to arise from the fact that a residual
uncertainty about the actual alignment of the measurement device remains. If
the uncertainty is below a certain level, and if the distribution misalignments
is covariant under rotations, a Kochen-Specker theorem for the unsharp spin
observables follows: There are finite sets of directions such that not all the
unsharp spin observables in these directions can consistently be assigned
approximate truth-values in a non-contextual way.


title:
Identification of dynamical Lie algebras for finite-level quantum
  control systems

abstract:
  The problem of identifying the dynamical Lie algebras of finite-level quantum
systems subject to external control is considered, with special emphasis on
systems that are not completely controllable. In particular, it is shown that
the dynamical Lie algebra for an N-level system with equally spaced energy
levels and uniform transition dipole moments, is a subalgebra for $so(N)$ if
$N=2\ell+1$, and a subalgebra of $sp(\ell)$ if $N=2\ell$. General conditions
for obtaining either $so(2\ell+1)$ or $sp(\ell)$ are established.


title: The capacity of hybrid quantum memory

abstract:
  The general stable quantum memory unit is a hybrid consisting of a classical
digit with a quantum digit (qudit) assigned to each classical state. The shape
of the memory is the vector of sizes of these qudits, which may differ. We
determine when N copies of a quantum memory A embed in N(1+o(1)) copies of
another quantum memory B. This relationship captures the notion that B is as at
least as useful as A for all purposes in the bulk limit. We show that the
embeddings exist if and only if for all p >= 1, the p-norm of the shape of A
does not exceed the p-norm of the shape of B. The log of the p-norm of the
shape of A can be interpreted as the maximum of S(\rho) + H(\rho)/p (quantum
entropy plus discounted classical entropy) taken over all mixed states \rho on
A. We also establish a noiseless coding theorem that justifies these entropies.
The noiseless coding theorem and the bulk embedding theorem together say that
either A blindly bulk-encodes into B with perfect fidelity, or A admits a state
that does not visibly bulk-encode into B with high fidelity.
  In conclusion, the utility of a hybrid quantum memory is determined by its
simultaneous capacity for classical and quantum entropy, which is not a finite
list of numbers, but rather a convex region in the classical-quantum entropy
plane.

categories: quant-ph cs.IT math-ph math.IT math.MP math.OA


title:
Stability of quantum states of finite macroscopic systems against
  classical noises, perturbations from environments, and local measurements

abstract:
  We study the stability of quantum states of macroscopic systems of finite
volume V, against weak classical noises (WCNs), weak perturbations from
environments (WPEs), and local measurements (LMs). We say that a pure state is
`fragile' if its decoherence rate is anomalously great, and `stable against
LMs' if the result of a LM is not affected by another LM at a distant point. By
making full use of the locality and huge degrees of freedom, we show the
following: (i) If square fluctuation of every additive operator is O(V) or less
for a pure state, then it is not fragile in any WCNs or WPEs. (ii) If square
fluctuations of some additive operators are O(V^2) for a pure state, then it is
fragile in some WCNs or WPEs. (iii) If a state (pure or mixed) has the `cluster
property,' then it is stable against LMs, and vice versa. These results have
many applications, among which we discuss the mechanism of symmetry breaking in
finite systems.


title: On the continuity of asymptotic measures of entanglement

abstract:
  We show that, in any open set of distillable states, all asymptotic
entanglement measures $E(\rho)$ are continuous as a function of (a single copy
of) $\rho$, even though they quantify the entanglement properties of
$\rho^{\otimes N}$ is the large $N$ limit.


title: Stochastic gauges in quantum dynamics for many-body simulations

abstract:
  Quantum dynamics simulations can be improved using novel quasiprobability
distributions based on non-orthogonal hermitian kernel operators. This
introduces arbitrary functions (gauges) into the stochastic equations, which
can be used to tailor them for improved calculations. A possible application to
full quantum dynamic simulations of BEC's is presented.


title: Second central extension in Galilean covariant field theory

abstract:
  The second central extension of the planar Galilei group has been alleged to
have its origin in the spin variable. This idea is explored here by considering
local Galilean covariant field theory for free fields of arbitrary spin. It is
shown that such systems generally display only a trivial realization of the
second central extension. While it is possible to realize any desired value of
the extension parameter by suitable redefinition of the boost operator, such an
approach has no necessary connection to the spin of the basic underlying field.


title: The Landau-Zener Model with Decoherence: The case S=1/2

abstract:
  We study the dynamics of a spin coupled to an oscillating magnetic field, in
the presence of decoherence and dissipation. In this context we solve the
master equation for the Landau-Zener problem, both in the unitary and in the
irreversible case. We show that a single spin can be "magnetized" in the
direction parallel to the oscillating bias. When decay from upper to lower
level is taken into account, hysteretic behavior is obtained.


title:
Measuring the mean value of vibrational observables in trapped ion
  systems

abstract:
  The theoretical foundations of a new general approach to the measurement
problem of vibrational observables in trapped ion systems is reported. The
method rests upon the introduction of a simple vibronic coupling structure
appropriately conceived to link the internal ionic state measurement outcomes
to the mean value of a motional variable of interest. Some applications are
provided and discussed in detail, bringing to light the feasibility and the
wide potentiality of the proposal.


title:
A Quantum Measurement Scenario which Requires Exponential Classical
  Communication for Simulation

abstract:
  In this paper we consider the following question: how many bits of classical
communication and shared random bits are necessary to simulate a quantum
protocol involving Alice and Bob where they share k entangled quantum bits and
do not communicate at all. We prove that 2^k classical bits are necessary, even
if the classical protocol is allowed an \epsilon chance of failure.


title: Optimal parameter estimation of depolarizing channel

abstract:
  We investigate strategies for estimating a depolarizing channel for a finite
dimensional system. Our analysis addresses the double optimization problem of
selecting the best input probe state and the measurement strategy that
minimizes the Bayes cost of a quadratic function. In the qubit case, we derive
the Bayes optimal strategy for any finite number of input probe particles when
bipartite entanglement can be formed in the probe particles.


title: A general framework of nondistortion quantum interrogation

abstract:
  We present a general framework to study nondistortion quantum interrogation
which preserves the internal state of the quantum object being detected. We
obtain the necessary and sufficient condition for successful performing
nondistortion interrogation for unknown quantum object when the interaction
between the probe system and the detected system takes place only once. When
the probe system and interrogation process have been limited we develop a
mathematical frame to determine whether it is possible to realize NQI processes
only relying on the choice of the original probe state. We also consider NQI
process in iterative cases. A sufficient criterion for NQI is obtained.


title:
Generation of polarization-entangled photon pairs in a cascade of two
  type-I crystals pumped by femtosecond pulses

abstract:
  We report the generation of polarization-entangled photons by
femtosecond-pulse-pumped spontaneous parametric down-conversion in a cascade of
two type-I crystals. Highly entangled pulsed states were obtained by
introducing a temporal delay between the two orthogonal polarization components
of the pump field. They exhibited high-visibility quantum interference and a
large concurrence value, without the need of post-selection using
narrow-bandwidth-spectral filters. The results are well explained by the theory
which incorporates the space-time dependence of interfering two-photon
amplitudes if dispersion and birefringence in the crystals are appropriately
taken into account. Such a pulsed entangled photon well localized in time
domain is useful for various quantum communication experiments, such as quantum
cryptography and quantum teleportation.


title:
Irrversibility of Entanglement Manipulations: Vagueness of the
  Entanglement of Cost and Entanglement of Distillation

abstract:
  We show that the entanglement of cost and entanglement of distillation can be
vague when we consider a more general form of entanglement manipulation in
which we collectively deal with not only states of our concern but also other
states. We introduce the most general entanglement manipulation in which the
formation and distillation can be simultaneously performed. We show that in a
certain case entanglement manipulations are reversible with respect to the most
general entanglement manipulation. This broadens our scope of vision on the
irreversibility of entanglement manipulations.


title: Testing statistical bounds on entanglement using quantum chaos

abstract:
  Previous results indicate that while chaos can lead to substantial entropy
production, thereby maximizing dynamical entanglement, this still falls short
of maximality. Random Matrix Theory (RMT) modeling of composite quantum
systems, investigated recently, entails an universal distribution of the
eigenvalues of the reduced density matrices. We demonstrate that these
distributions are realized in quantized chaotic systems by using a model of two
coupled and kicked tops. We derive an explicit statistical universal bound on
entanglement, that is also valid for the case of unequal dimensionality of the
Hilbert spaces involved, and show that this describes well the bounds observed
using composite quantized chaotic systems such as coupled tops.


title: Quantum Key Distribution over 67 km with a plug & play system

abstract:
  We present a fibre-optical quantum key distribution system. It works at
1550nm and is based on the plug & play setup. We tested the stability under
field conditions using aerial and terrestrial cables and performed a key
exchange over 67 km between Geneva and Lausanne.


title: N-particle N-level singlet states: Some properties and applications

abstract:
  Three apparently unrelated problems which have no solution using classical
tools are described: the "N-strangers," "secret sharing," and "liar detection"
problems. A solution for each of them is proposed. Common to all three
solutions is the use of quantum states of total spin zero of N spin-(N-1)/2
particles.


title: Experimental Implementation of the Quantum Random-Walk Algorithm

abstract:
  The quantum random walk is a possible approach to construct new quantum
algorithms. Several groups have investigated the quantum random walk and
experimental schemes were proposed. In this paper we present the experimental
implementation of the quantum random walk algorithm on a nuclear magnetic
resonance quantum computer. We observe that the quantum walk is in sharp
contrast to its classical counterpart. In particular, the properties of the
quantum walk strongly depends on the quantum entanglement.


title: Quantum models related to fouled Hamiltonians of the harmonic oscillator

abstract:
  We study a pair of canonoid (fouled) Hamiltonians of the harmonic oscillator
which provide, at the classical level, the same equation of motion as the
conventional Hamiltonian. These Hamiltonians, say $K_{1}$ and $K_{2}$, result
to be explicitly time-dependent and can be expressed as a formal rotation of
two cubic polynomial functions, $H_{1}$ and $H_{2}$, of the canonical variables
(q,p).
  We investigate the role of these fouled Hamiltonians at the quantum level.
Adopting a canonical quantization procedure, we construct some quantum models
and analyze the related eigenvalue equations. One of these models is described
by a Hamiltonian admitting infinite self-adjoint extensions, each of them has a
discrete spectrum on the real line. A self-adjoint extension is fixed by
choosing the spectral parameter $\epsilon$ of the associated eigenvalue
equation equal to zero. The spectral problem is discussed in the context of
three different representations. For $\epsilon =0$, the eigenvalue equation is
exactly solved in all these representations, in which square-integrable
solutions are explicity found. A set of constants of motion corresponding to
these quantum models is also obtained. Furthermore, the algebraic structure
underlying the quantum models is explored. This turns out to be a nonlinear
(quadratic) algebra, which could be applied for the determination of
approximate solutions to the eigenvalue equations.


title: Field quantization for open optical cavities

abstract:
  We study the quantum properties of the electromagnetic field in optical
cavities coupled to an arbitrary number of escape channels. We consider both
inhomogeneous dielectric resonators with a scalar dielectric constant
$\epsilon({\bf r})$ and cavities defined by mirrors of arbitrary shape. Using
the Feshbach projector technique we quantize the field in terms of a set of
resonator and bath modes. We rigorously show that the field Hamiltonian reduces
to the system--and--bath Hamiltonian of quantum optics. The field dynamics is
investigated using the input--output theory of Gardiner and Collet. In the case
of strong coupling to the external radiation field we find spectrally
overlapping resonator modes. The mode dynamics is coupled due to the damping
and noise inflicted by the external field. For wave chaotic resonators the mode
dynamics is determined by a non--Hermitean random matrix. Upon including an
amplifying medium, our dynamics of open-resonator modes may serve as a starting
point for a quantum theory of random lasing.


title:
Theory for the optimal control of time-averaged quantities in open
  quantum systems

abstract:
  We present variational theory for optimal control over a finite time interval
in quantum systems with relaxation. The corresponding Euler-Lagrange equations
determining the optimal control field are derived. In our theory the optimal
control field fulfills a high order differential equation, which we solve
analytically for some limiting cases. We determine quantitatively how
relaxation effects limit the control of the system. The theory is applied to
open two level quantum systems. An approximate analytical solution for the
level occupations in terms of the applied fields is presented. Different other
applications are discussed.


title:
Separability and correlations in composite states based on entropy
  methods

abstract:
  This work is an enquiry into the circumstances under which entropy methods
can give an answer to the questions of both quantum separability and classical
correlations of a composite state. Several entropy functionals are employed to
examine the entanglement and correlation properties guided by the corresponding
calculations of concurrence. It is shown that the entropy difference between
that of the composite and its marginal density matrices may be of arbitrary
sign except under special circumstances when conditional probability can be
defined appropriately. This ambiguity is a consequence of the fact that the
overlap matrix elements of the eigenstates of the composite density matrix with
those of its marginal density matrices also play important roles in the
definitions of probabilities and the associated entropies, along with their
respective eigenvalues. The general results are illustrated using pure and
mixed state density matrices of two-qubit systems. Two classes of density
matrices are found for which the conditional probability can defined: (1)
density matrices with commuting decompositions and (2) those which are
decohered in the representation where the density matrices of the marginals are
diagonal. The first class of states encompass those whose separability is
currently understood as due to particular symmetries of the states. The second
are a new class of states which are expected to be useful for understanding
separability. Examples of entropy functionals of these decohered states
including the crucial isospectral case are discussed.


title:
Studies of group velocity reduction and pulse regeneration with and
  without the adiabatic approximation

abstract:
  We present a detailed semiclassical study on the propagation of a pair of
optical fields in resonant media with and without adiabatic approximation. In
the case of near and on resonance excitation, we show detailed calculation,
both analytically and numerically, on the extremely slowly propagating probe
pulse and the subsequent regeneration of a pulse via a coupling laser. Further
discussions on the adiabatic approximation provide many subtle understandings
of the process including the effect on the band width of the regenerated
optical field. Indeed, all features of the optical pulse regeneration and most
of the intricate details of the process can be obtained with the present
treatment without invoke a full field theoretical method. For very far off
resonance excitation, we show that the analytical solution is nearly detuning
independent, a surprising result that is vigorously tested and compared to
numerical calculations with very good agreement.


title: Mathematical Foundations of Regular Quantum Graphs

abstract:
  We define a class of quantum systems called regular quantum graphs. Although
their dynamics is chaotic in the classical limit with positive topological
entropy, the spectrum of regular quantum graphs is explicitly computable
analytically and exactly, state by state, by means of periodic orbit
expansions. We prove analytically that the periodic orbit series exist and
converge to the correct spectral eigenvalues. We investigate the convergence
properties of the periodic orbit series and prove rigorously that both
conditionally convergent and absolutely convergent cases can be found. We
compare the periodic orbit expansion technique with Lagrange's inversion
formula. While both methods work and yield exact results, the periodic orbit
expansion technique has conceptual value since all the terms in the expansion
have direct physical meaning and higher order corrections are obtained
according to physically obvious rules. In addition our periodic orbit
expansions provide explicit analytical solutions for many classic text-book
examples of quantum mechanics that previously could only be solved using
graphical or numerical techniques.


title: Comments on Adiabatic Quantum Algorithms

abstract:
  Recently a method for adiabatic quantum computation has been proposed and
there has been considerable speculation about its efficiency for NP-complete
problems. Heuristic arguments in its favor are based on the unproven assumption
of an eigenvalue gap.
  We show that, even without the assumption of an eigenvalue gap, other
standard arguments can be used to show that a large class of Hamiltonians
proposed for adiabatic quantum computation have unique ground states.
  We also discuss some of the issues which arise in trying to analyze the
behavior of the eigenvalue gap. In particular, we propose several mechanisms
for modifying to final Hamiltonian to perform an adiabatic search with
efficiency comparable to that for 3-SAT. We also propose the use of randomly
defined final Hamiltonians as a mechanism for analyzing the generic spectral
behavior of the interpolating Hamiltonians associated with problems which lack
sufficent structure to be amenable to efficient classical algorithms.


title:
Causality in quantum teleportation: information extraction and noise
  effects in entanglement distribution

abstract:
  Quantum teleportation is possible because entanglement allows a definition of
precise correlations between the non-commuting properties of a local system and
corresponding non-commuting properties of a remote system. In this paper, the
exact causality achieved by maximal entanglement is analyzed and the results
are applied to the transfer of effects acting on the entanglement distribution
channels to the teleported output state. In particular, it is shown how
measurements performed on the entangled system distributed to the sender
provide information on the teleported state while transferring the
corresponding back-action to the teleported quantum state.


title: Two-photon processes in faint biphoton fields

abstract:
  The goal of this research is to determine and study a physical system that
will enable a fast and intrinsically two-photon detector, which would be of
interest for quantum information and metrology applications. We consider two
types of two-photon processes that can be observed using a very faint, but
quantum-correlated biphoton field. These are optical up-conversion and an
external photoelectric effect. We estimate the correlation enhancement factor
for the biphoton light compared to coherent light, report and discuss the
preliminary experimental results.


title: Phase estimation as a quantum nondemolition measurement

abstract:
  The phase estimation algorithm, which is at the heart of a variety of quantum
algorithms, including Shor's factoring algorithm, allows a quantum computer to
accurately determine an eigenvalue of an unitary operator. Quantum
nondemolition measurements are a quantum mechanical procedure, used to overcome
the standard quantum limit when measuring an observable. We show that the phase
estimation algorithm, in both the discrete and continuous variable setting, can
be viewed as a quantum nondemolition measurement.


title: FAQ about the "contextual objectivity" point of view

abstract:
  We discuss some Frequently Asked Questions about the ``contextual
objectivity" point of view on quantum mechanics introduced in two previous
preprints quant-ph/0012122 and quant-ph/0111154.


title: Electromagnetic Transition in Waveguide with Application to Lasers

abstract:
  The electromagnetic transition of two-level atomic systems in a waveguide is
calculated. Compared with the result in free space, the spontaneous emission
rate decrease because the phase space is smaller, and meanwhile, some resonance
appears in some cases. Moreover, the influence of non-uniform electromagnetic
field in a waveguide on absorption and stimulated emission is considered.
Applying the results to lasers, a method to enhance the laser power is
proposed.


title: Classical Rules in Quantum Games

abstract:
  We consider two aspects of quantum game theory: the extent to which the
quantum solution solves the original classical game, and to what extent the new
solution can be obtained in a classical model.


title: Practical quantum repeaters with linear optics and double-photon guns

abstract:
  We show how to create practical, efficient, quantum repeaters, employing
double-photon guns, for long-distance optical quantum communication. The guns
create polarization-entangled photon pairs on demand. One such source might be
a semiconducter quantum dot, which has the distinct advantage over parametric
down-conversion that the probability of creating a photon pair is close to one,
while the probability of creating multiple pairs vanishes. The swapping and
purifying components are implemented by polarizing beam splitters and
probabilistic optical CNOT gates.


title:
Mathematical Structure of Rabi Oscillations in the Strong Coupling
  Regime

abstract:
  In this paper we generalize the Jaynes--Cummings Hamiltonian by making use of
some operators based on Lie algebras su(1,1) and su(2), and study a
mathematical structure of Rabi floppings of these models in the strong coupling
regime. We show that Rabi frequencies are given by matrix elements of
generalized coherent operators (quant--ph/0202081) under the rotating--wave
approximation.
  In the first half we make a general review of coherent operators and
generalized coherent ones based on Lie algebras su(1,1) and su(2). In the
latter half we carry out a detailed examination of Frasca (quant--ph/0111134)
and generalize his method, and moreover present some related problems.
  We also apply our results to the construction of controlled unitary gates in
Quantum Computation. Lastly we make a brief comment on application to Holonomic
Quantum Computation.


title: Motion-light parametric amplifier and entanglement distributor

abstract:
  We propose a scheme for entangling the motional mode of a trapped atom with a
propagating light field via a cavity-mediated parametric interaction. We then
show that if this light field is subsequently coupled to a second distant atom
via a cavity-mediated linear-mixing interaction, it is possible to transfer the
entanglement from the light beam to the motional mode of the second atom to
create an EPR-type entangled state of the positions and momenta of two
distantly-separated atoms.


title: Entanglement and entropy engineering of atomic two-qubit states

abstract:
  We propose a scheme employing quantum-reservoir engineering to controllably
entangle the internal states of two atoms trapped in a high finesse optical
cavity. Using laser and cavity fields to drive two separate Raman transitions
between metastable atomic ground states, a system is realized corresponding to
a pair of two-state atoms coupled collectively to a squeezed reservoir.
Phase-sensitive reservoir correlations lead to entanglement between the atoms,
and, via local unitary transformations and adjustment of the degree and purity
of squeezing, one can prepare entangled mixed states with any allowed
combination of linear entropy and entanglement of formation.


title: Multipulse Control of Decoherence

abstract:
  We present a general formulation to suppress pure dephasing by multipulse
control. The formula is free from a specific form of interaction and is
expressed in terms of the correlation function of arbitrary system-reservoir
interaction.


title: Dynamical Casimir Effect in a Leaky Cavity at Finite Temperature

abstract:
  The phenomenon of particle creation within an almost resonantly vibrating
cavity with losses is investigated for the example of a massless scalar field
at finite temperature. A leaky cavity is designed via the insertion of a
dispersive mirror into a larger ideal cavity (the reservoir). In the case of
parametric resonance the rotating wave approximation allows for the
construction of an effective Hamiltonian. The number of produced particles is
then calculated using response theory as well as a non-perturbative approach.
In addition we study the associated master equation and briefly discuss the
effects of detuning. The exponential growth of the particle numbers and the
strong enhancement at finite temperatures found earlier for ideal cavities turn
out to be essentially preserved. The relevance of the results for experimental
tests of quantum radiation via the dynamical Casimir effect is addressed.
Furthermore the generalization to the electromagnetic field is outlined.


title: A single atom-based generation of Bell states of two cavities

abstract:
  A new conditional scheme for generating Bell states of two spatially
separated high-Q cavities is reported. Our method is based on the passage of
one atom only through the two cavities. A distinctive feature of our treatment
is that it incorporates from the very beginning the unavoidable presence of
fluctuations in the atom-cavity interaction times. The possibility of
successfully implementing our proposal against cavity losses and atomic
spontaneous decay is carefully discussed.


title: Thermal and ground-state entanglement in Heisenberg XX qubit rings

abstract:
  We study the entanglement of thermal and ground states in Heisernberg $XX$
qubit rings with a magnetic field. A general result is found that for
even-number rings pairwise entanglement between nearest-neighbor qubits is
independent on both the sign of exchange interaction constants and the sign of
magnetic fields. As an example we study the entanglement in the four-qubit
model and find that the ground state of this model without magnetic fields is
shown to be a four-body maximally entangled state measured by the $N$-tangle.


title: The large radius limit for coherent states on spheres

abstract:
  This paper concerns the coherent states on spheres studied by the authors in
[J. Math. Phys. 43 (2002), 1211-1236]. We show that in the odd-dimensional case
the coherent states on the sphere approach the classical Gaussian coherent
states on Euclidean space as the radius of the sphere tends to infinity.


title: Programmable Logic Devices in Experimental Quantum Optics

abstract:
  We discuss the unique capabilities of programmable logic devices (PLD's) for
experimental quantum optics and describe basic procedures of design and
implementation. Examples of advanced applications include optical metrology and
feedback control of quantum dynamical systems. As a tutorial illustration of
the PLD implementation process, a field programmable gate array (FPGA)
controller is used to stabilize the output of a Fabry-Perot cavity.


title: Purification and detection of entangled coherent states

abstract:
  In [J. C. Howell and J. A. Yeazell, Phys. Rev. A 62, 012102 (2000)], a
proposal is made to generate entangled macroscopically distinguishable states
of two spatially separated traveling optical modes. We model the decoherence
due to light scattering during the propagation along an optical transmission
line and propose a setup allowing an entanglement purification from a number of
preparations which are partially decohered due to transmission. A purification
is achieved even without any manual intervention. We consider a nondemolition
configuration to measure the purity of the state as contrast of interference
fringes in a double-slit setup. Regarding the entangled coherent states as a
state of a bipartite quantum system, a close relationship between purity and
entanglement of formation can be obtained. In this way, the contrast of
interference fringes provides a direct means to measure entanglement.


title:
Quantum computation with coupled-quantum-dots embedded in optical
  microcavities

abstract:
  Based on an idea that spatial separation of charge states can enhance quantum
coherence, we propose a scheme for quantum computation with quantum bit (qubit)
constructed from two coupled quantum dots. Quantum information is stored in
electron-hole pair state with the electron and hole locating in different dots,
which enables the qubit state being very long-lived. Universal quantum gates
involving any pair of qubits are realized by coupling the quantum dots through
cavity photon which is a hopeful candidate to transfer long-range information.
Operation analysis is carried out by estimating the gate time versus the
decoherence time.


title: Quantum Search with Two-atom Collisions in Cavity QED

abstract:
  We propose a scheme to implement two-qubit Grover's quantum search algorithm
using Cavity Quantum Electrodynamics. Circular Rydberg atoms are used as
quantum bits (qubits). They interact with the electromagnetic field of a
non-resonant cavity . The quantum gate dynamics is provided by a
cavity-assisted collision, robust against decoherence processes. We present the
detailed procedure and analyze the experimental feasibility.


title: Laser driven atoms in half-cavities

abstract:
  The behavior of a two level atom in a half-cavity, i.e. a cavity with one
mirror, is studied within the framework of a one dimensional model with respect
to spontaneous decay and resonance fluorescence. The system under consideration
corresponds to the setup of a recently performed experiment [J. Eschner
\textit{et. al.}, Nature \textbf{413}, 495 (2001)] where the influence of a
mirror on a fluorescing single atom was revealed. In the present work special
attention is paid to a regime of large atom-mirror distances where intrinsic
memory effects cannot be neglected anymore. This is done with the help of delay
differential equations which contain, for small atom-mirror distances, the
Markovian limit with effective level shifts and decay rates leading to the
phenomenon of enhancement or inhibition of spontaneous decay. Several features
are recovered beyond an effective Markovian treatment, appearing in
experimental accessible quantities like intensity or emission spectra of the
scattered light.


title: Quantum Gambling Using Three Nonorthogonal States

abstract:
  We provide a quantum gambling protocol using three (symmetric) nonorthogonal
states. The bias of the proposed protocol is less than that of previous ones,
making it more practical. We show that the proposed scheme is secure against
non-entanglement attacks. The security of the proposed scheme against
entanglement attacks is shown heuristically.


title: Semiclassical theory of weak values

abstract:
  Aharonov-Albert-Vaidman's weak values are investigated by a semiclassical
method. Examples of the semiclassical calculation that reproduces "anomalous"
weak values are shown. Furthermore, a complex extension of Ehrenfest's
quantum-classical correspondence between quantum expectation values of the
states with small quantum fluctuation, and classical dynamics, is shown.


title: Quantum three body Coulomb problem in two dimensions

abstract:
  We study the three-body Coulomb problem in two dimensions and show how to
calculate very accurately its quantum properties. The use of a convenient set
of coordinates makes it possible to write the Schr\"{o}dinger equation only
using annihilation and creation operators of four harmonic oscillators, coupled
by various terms of degree up to twelve. We analyse in details the discrete
symmetry properties of the eigenstates. The energy levels and eigenstates of
the two-dimensional helium atom are obtained numerically, by expanding the
Schr\"{o}dinger equation on a convenient basis set, that gives sparse banded
matrices, and thus opens up the way to accurate and efficient calculations. We
give some very accurate values of the energy levels of the first bound Rydberg
series. Using the complex coordinate method, we are also able to calculate
energies and widths of doubly excited states, i.e. resonances above the first
ionization threshold. For the two-dimensional $H^{-}$ ion, only one bound state
is found.


title:
A scheme for direct observation of entanglement for Gaussian continuous
  variables

abstract:
  We suggest an experimentally realizable scheme to test entanglement of a
mixed Gaussian continuous variable state. We find that the entanglement
condition is simplified for the family of Gaussian states which are relevant to
experimental realization. The entanglement condition is then shown to be
directly related to joint homodyne measurements. We show how robust the
proposed test of entanglement is against imperfect detection efficiency.


title: Space Dependence of Entangled States and Franson-type EPR Experiments

abstract:
  We analyze some aspects of recently performed Franson-type experiments with
entangled photon pairs aimed to test Bell's inequalities. We point out that
quantum theory leads to the coincidence rate between detectors which includes
in fact a dependence on the distance. We study this dependence and obtain that
for large distances the correlation function vanishes. Therefore with taking
into account the space parts of wave functions of photons for large distances
quantum mechanical predictions are consistent with Bell's inequalities. We
propose an experimental study of space dependence of correlation functions in
Bell-type experiments.


title: Quantum nonlocality and quantum dynamics

abstract:
  We argue that usual quantum statics and the dynamical equivalence of mixed
quantum states to {\it probabilistic mixtures}suffice to guarantee a linear
evolution law, which necessarily complies with the no-signaling condition.
Alternatively, there are nonlinear dynamical extensions that treat mixed states
as {\it elementary mixtures} and evolve {\it every}pure state linearly and
unitarily. But if all {\it entangled} pure states evolve linearly, then
elementary mixtures cannot evolve nonlinearly without challenging quantum
locality. Conversely, any such extension that is relativistically well behaved
demands a nonlinear evolution [decoherence] of pure entangled states. Wherefrom
follows that the linear evolution of entangled pure states provides an
unequivocal signature of linear quantum dynamics.


title:
On relations between probabilities under quantum and classical
  measurements

abstract:
  We show that the so-called quantum probabilistic rule, usually presented in
the physical literature as an argument of the essential distinction between the
probability relations under quantum and classical measurements, is not, as it
is usually considered, in contrast to the rule for the addition of
probabilities of incompatible events. The latter is, of course, valid under all
measurement situations upon classical and quantum objects. We discuss also
quantum measurement situation, which is similar to the classical one,
corresponding to classical Bayes' formula for conditional probabilities. We
show the compatibility of description of this quantum measurement situation in
the frame of purely classical and experimentally justified straight ward
frequency arguments [Khrennikov (1999, 2000, 2001)] and in the frame of the
quantum stochastic approach to the description of generalized quantum
measurements (Loubenets (2000); Barndorff-Nielsen and Loubenets (2001)]. In
view of derived results, we argue that even in the classical probability the
classical Bayes' formula describes a particular case of the considered
measurement situation, which is specific for context-independent measurements.
The similarity of the forms of the relation between the transformation of
probabilities, which we derive in the frame of quantum stochastic approach and
in the frame of straight ward frequency arguments, underlines once more that
the projective (von Neumann) measurements correspond only to a very special
kind of measurement situations in quantum theory.


title: Parity Violation and a Preferred Frame

abstract:
  Based on parity violation in the weak interaction and evidences from neutrino
oscillation, a natural choice is that neutrinos may be spacelike particles with
a tiny mass. To keep causality for spacelike particles, a kinematic time under
a non-standard form of the Lorentz transformation is introduced, which is
related to a preferred frame. A Dirac-type equation for spacelike neutrinos is
further investigated and its solutions are discussed. This equation can be
written in two spinor equations coupled together via nonzero mass while
respecting maximum parity violation. As a consequence, parity violation implies
that the principle of relativity is violated in the weak interaction.


title: Quantum Secure Communication with W States

abstract:
  A W state is pair-wisely entangled, belonging to the different class from
Greenberger, Horne, and Zeilinger (GHZ) state. We show that the W state enables
three variant protocols, that is, quantum key distribution between several
parts, quantum secret sharing, and both of them at the same time. It is
discussed how one's cheating can be detected in each protocol.


title: The distillable entanglement of multiple copies of Bell states

abstract:
  It is impossible to discriminate four Bell states through local operations
and classical communication (LOCC), if only one copy is provided. To complete
this task, two copies will suffice and be necessary. When $n$ copies are
provided, we show that the distillable entanglement is exactly $n-2$.


title: Adaptive homodyne measurement of optical phase

abstract:
  We present an experimental demonstration of the power of real-time feedback
in quantum metrology, confirming a theoretical prediction by Wiseman regarding
the superior performance of an adaptive homodyne technique for single-shot
measurement of optical phase. For phase measurements performed on weak coherent
states with no prior knowledge of the signal phase, we show that the variance
of adaptive homodyne estimation approaches closer to the fundamental quantum
uncertainty limit than any previously demonstrated technique. Our results
underscore the importance of real-time feedback for reaching quantum
performance limits in coherent telecommunication, precision measurement and
information processing.


title: Do macroscopic properties dictate microscopic probabilities?

abstract:
  Aharonov and Reznik have recently (in quant-ph/0110093) argued that the form
of the probabilistic predictions of quantum theory can be seen to follow from
properties of macroscopic systems. An error in their argument is identified.


title: Biologic

abstract:
  In this paper we explore the boundary between biology and the study of formal
systems (logic). In the end, we arrive at a summary formalism, a chapter in
"boundary mathematics" where there are not only containers <> but also
extainers ><, entities open to interaction and distinguishing the space that
they are not. The boundary algebra of containers and extainers is to biologic
what boolean algebra is to classical logic. We show how this formalism
encompasses significant parts of the logic of DNA replication, the Dirac
formalism for quantum mechanics, formalisms for protein folding and the basic
structure of the Temperley Lieb algebra at the foundations of topological
invariants of knots and links.


title: Fundamental Principle for Quantum Theory

abstract:
  We propose the principle, the law of statistical balance for basic physical
observables, which specifies quantum statistical theory among all other
statistical theories of measurements. It seems that this principle might play
in quantum theory the role that is similar to the role of Einstein's relativity
principle.


title:
Manifestation of classical wave delays in a fully quantized model of the
  scattering of a single photon

abstract:
  We consider a fully quantized model of spontaneous emission, scattering, and
absorption, and study propagation of a single photon from an emitting atom to a
detector atom both with and without an intervening scatterer. We find an exact
quantum analog to the classical complex analytic signal of an electromagnetic
wave scattered by a medium of charged oscillators. This quantum signal exhibits
classical phase delays. We define a time of detection which, in the appropriate
limits, exactly matches the predictions of a classically defined delay for
light propagating through a medium of charged oscillators. The fully quantized
model provides a simple, unambiguous, and causal interpretation of delays that
seemingly imply speeds greater than c in the region of anomalous dispersion.


title: Quantum Optimization Problems

abstract:
  Krentel [J. Comput. System. Sci., 36, pp.490--509] presented a framework for
an NP optimization problem that searches an optimal value among
exponentially-many outcomes of polynomial-time computations. This paper expands
his framework to a quantum optimization problem using polynomial-time quantum
computations and introduces the notion of an ``universal'' quantum optimization
problem similar to a classical ``complete'' optimization problem. We exhibit a
canonical quantum optimization problem that is universal for the class of
polynomial-time quantum optimization problems. We show in a certain relativized
world that all quantum optimization problems cannot be approximated closely by
quantum polynomial-time computations. We also study the complexity of quantum
optimization problems in connection to well-known complexity classes.


title: Continuous-variable quantum teleportation of entanglement

abstract:
  Entangled coherent states can be used to determine the entanglement fidelity
for a device that is designed to teleport coherent states. This entanglement
fidelity is universal, in that the calculation is independent of the use of
entangled coherent states and applies generally to the teleportation of
entanglement using coherent states. The average fidelity is shown to be a poor
indicator of the capability of teleporting entanglement; i.e., very high
average fidelity for the quantum teleportation apparatus can still result in
low entanglement fidelity for one mode of the two-mode entangled coherent
state.


title:
Comment on ''Matter-Wave Interferometer for Large Molecules''
  (quant-ph/0202158)

abstract:
  Diffraction experiments have moved to ever heavier objects in recent years,
now standing at the level of large molecules. Experiments in materials science
on the other hand have come down to ever smaller sizes largely due to the
success of the scanning tunneling microscope (STM), which allows studying
single molecules on surfaces. Since both fields coming from opposite side of
the size spectrum are now meeting, there is a need of consistent theories. The
simulations presented in the article, which we are referring to, are not
consistent with molecular dynamics simulations which are routinely used to
interpret STM results. Our comment points this out in detail.


title: Quantum search by measurement

abstract:
  We propose a quantum algorithm for solving combinatorial search problems that
uses only a sequence of measurements. The algorithm is similar in spirit to
quantum computation by adiabatic evolution, in that the goal is to remain in
the ground state of a time-varying Hamiltonian. Indeed, we show that the
running times of the two algorithms are closely related. We also show how to
achieve the quadratic speedup for Grover's unstructured search problem with
only two measurements. Finally, we discuss some similarities and differences
between the adiabatic and measurement algorithms.


title: Topics on the stochastical treatement of an open quantum system

abstract:
  The paper shortly presents the role of Stochastic Processes Theory in the
present day Quantum Theory, and the relation to Operational Quantum Physics.
The dynamics of an open quantum system is studied on a usual example from
Quantum Optics, suggesting the definition of a Neumark-type dilation for the
non-thermal states.


title: Quantum Mechanics as a Simple Generalization of Classical Mechanics

abstract:
  A motivation is given for expressing classical mechanics in terms of diagonal
projection matrices and diagonal density matrices. Then quantum mechanics is
seen to be a simple generalization in which one replaces the diagonal real
matrices with suitable Hermitian matrices.


title: Encoded Universality for Generalized Anisotropic Exchange Hamiltonians

abstract:
  We derive an encoded universality representation for a generalized
anisotropic exchange Hamiltonian that contains cross-product terms in addition
to the usual two-particle exchange terms. The recently developed algebraic
approach is used to show that the minimal universality-generating encodings of
one logical qubit are based on three physical qubits. We show how to generate
both single- and two-qubit operations on the logical qubits, using suitably
timed conjugating operations derived from analysis of the commutator algebra.
The timing of the operations is seen to be crucial in allowing simplification
of the gate sequences for the generalized Hamiltonian to forms similar to that
derived previously for the symmetric (XY) anisotropic exchange Hamiltonian. The
total number of operations needed for a controlled-Z gate up to local
transformations is five. A scalable architecture is proposed.


title:
Surpassing the standard quantum limit for optical imaging using
  non-classical multimode light

abstract:
  Using continuous wave superposition of spatial modes, we demonstrate
experimentally displacement measurement of a light beam below the standard
quantum limit. Multimode squeezed light is obtained by mixing a vacuum squeezed
beam and a coherent beam that are spatially orthogonal. Although the resultant
beam is not squeezed, it is shown to have strong internal spatial correlations.
We show that the position of such a light beam can be measured using a split
detector with an increased precision compared to a classical beam. This method
can be used to improve the sensitivity of small displacement measurements.


title: Factorization and superpotential of the PT symmetric Hamiltonian

abstract:
  We study the factorization of the PT symmetric Hamiltonian. The general
expression for the superpotential corresponding to the PT symmetric potential
is obtained and explicit examples are presented.


title: Squeezing of a coupled state of two spinors

abstract:
  The notion of spin squeezing involves reduction in the uncertainty of a
component of the spin vector below a certain limit. This aspect has been
studied earlier for pure and mixed states of definite spin. In this paper, this
study has been extended to coupled spin states which do not possess sharp spin
value. A general squeezing criterion has been obtained by requiring that a
direct product state for two spinors is not squeezed. The squeezing aspect of
entangled states is studied in relation to their spin- spin correlations.


title: What is entanglement?

abstract:
  I conjecture that only those states of light whose Wigner function is
positive are real states, and give arguments suggesting that this is not a
serious restriction. Hence it follows that the Wigner formalism in quantum
optics is capable of interpretation as a classical wave field with the addition
of a zeropoint contribution. Thus entanglement between pairs of photons with a
common origin occurs because the two light signals have amplitudes and phases,
both below and above the zeropoint intensity level, which are correlated with
each other.


title:
QED-Cavity model of microtubules implies dissipationless energy transfer
  and biological quantum teleportation

abstract:
  We refine a QED-cavity model of microtubules (MTs), proposed earlier by two
of the authors (N.E.M. and D.V.N.), and suggest mechanisms for the formation of
biomolecular mesoscopic coherent and/or entangled quantum states, which may
avoid decoherence for times comparable to biological characteristic times. This
refined model predicts dissipationless energy transfer along such "shielded"
macromolecules at near room temperatures as well as quantum teleportation of
states across MTs and perhaps neurons.


title: A New Protocol and Lower Bounds for Quantum Coin Flipping

abstract:
  We present a new protocol and two lower bounds for quantum coin flipping. In
our protocol, no dishonest party can achieve one outcome with probability more
than 0.75. Then, we show that our protocol is optimal for a certain type of
quantum protocols.
  For arbitrary quantum protocols, we show that if a protocol achieves a bias
of at most $\epsilon$, it must use at least $\Omega(\log \log
\frac{1}{\epsilon})$ rounds of communication. This implies that the parallel
repetition fails for quantum coin flipping. (The bias of a protocol cannot be
arbitrarily decreased by running several copies of it in parallel.)


title:
Dark States and Interferences in Cascade Transitions of Ultra-Cold Atoms
  in a Cavity

abstract:
  We examine the competition among one- and two-photon processes in an
ultra-cold, three-level atom undergoing cascade transitions as a result of its
interaction with a bimodal cavity. We show parameter domains where two-photon
transitions are dominant and also study the effect of two-photon emission on
the mazer action in the cavity. The two-photon emission leads to the loss of
detailed balance and therefore we obtain the photon statistics of the cavity
field by the numerical integration of the master equation. The photon
distribution in each cavity mode exhibits sub- and super- Poissonian behaviors
depending on the strength of atom-field coupling. The photon distribution
becomes identical to a Poisson distribution when the atom-field coupling
strengths of the modes are equal.


title: Locality in the Everett Interpretation of Quantum Field Theory

abstract:
  Recently it has been shown that transformations of Heisenberg-picture
operators are the causal mechanism which allows Bell-theorem-violating
correlations at a distance to coexist with locality in the Everett
interpretation of quantum mechanics. A calculation to first order in
perturbation theory of the generation of EPRB entanglement in nonrelativistic
fermionic field theory in the Heisenberg picture illustrates that the same
mechanism leads to correlations without nonlocality in quantum field theory as
well. An explicit transformation is given to a representation in which
initial-condition information is transferred from the state vector to the field
operators, making the locality of the theory manifest.


title: Quantum communication complexity of symmetric predicates

abstract:
  We completely (that is, up to a logarithmic factor) characterize the
bounded-error quantum communication complexity of every predicate $f(x,y)$
depending only on $|x\cap y|$ ($x,y\subseteq [n]$). Namely, for a predicate $D$
on $\{0,1,...,n\}$ let $\ell_0(D)\df \max\{\ell : 1\leq\ell\leq n/2\land
D(\ell)\not\equiv D(\ell-1)\}$ and $\ell_1(D)\df \max\{n-\ell : n/2\leq\ell <
n\land D(\ell)\not\equiv D(\ell+1)\}$. Then the bounded-error quantum
communication complexity of $f_D(x,y) = D(|x\cap y|)$ is equal (again, up to a
logarithmic factor) to $\sqrt{n\ell_0(D)}+\ell_1(D)$. In particular, the
complexity of the set disjointness predicate is $\Omega(\sqrt n)$. This result
holds both in the model with prior entanglement and without it.


title:
Phase Control of Nonadiabaticity-induced Quantum Chaos in An Optical
  Lattice

abstract:
  The qualitative nature (i.e. integrable vs. chaotic) of the translational
dynamics of a three-level atom in an optical lattice is shown to be
controllable by varying the relative laser phase of two standing wave lasers.
Control is explained in terms of the nonadiabatic transition between optical
potentials and the corresponding regular to chaotic transition in mixed
classical-quantum dynamics. The results are of interest to both areas of
coherent control and quantum chaos.