title: Exact Performance of Concatenated Quantum Codes

abstract:
  When a logical qubit is protected using a quantum error-correcting code, the
net effect of coding, decoherence (a physical channel acting on qubits in the
codeword) and recovery can be represented exactly by an effective channel
acting directly on the logical qubit. In this paper we describe a procedure for
deriving the map between physical and effective channels that results from a
given coding and recovery procedure. We show that the map for a concatenation
of codes is given by the composition of the maps for the constituent codes.
This perspective leads to an efficient means for calculating the exact
performance of quantum codes with arbitrary levels of concatenation. We present
explicit results for single-bit Pauli channels. For certain codes under the
symmetric depolarizing channel, we use the coding maps to compute exact
threshold error probabilities for achievability of perfect fidelity in the
infinite concatenation limit.


title: Can "quantumness" be an origin of dissipation?

abstract:
  In their constructions of system of quantum stochastic differential
equations, mathematicians and/or several physicists interpret that the function
of random force operator is to preserve the canonical commutation relation in
time, i.e., to secure the unitarity of time evolution generator even for
dissipative systems. If this is the case, it means physically that the origin
of dissipation is attributed to quantum non-commutativity ({\it quantumness}).
The mechanism that the mathematician's approaches rest on will be investigated
from the unified view point of Non-Equilibrium Thermo Field Dynamics (NETFD)
which is a canonical operator formalism of quantum systems in
far-from-equilibrium state including the system of quantum {\it stochastic}
equations.


title:
Mixed state geometric phases, entangled systems, and local unitary
  transformations

abstract:
  The geometric phase for a pure quantal state undergoing an arbitrary
evolution is a ``memory'' of the geometry of the path in the projective Hilbert
space of the system. We find that Uhlmann's geometric phase for a mixed quantal
state undergoing unitary evolution not only depends on the geometry of the path
of the system alone but also on a constrained bi-local unitary evolution of the
purified entangled state. We analyze this in general, illustrate it for the
qubit case, and propose an experiment to test this effect. We also show that
the mixed state geometric phase proposed recently in the context of
interferometry requires uni-local transformations and is therefore essentially
a property of the system alone.


title: Consciousness: The rules of engagement

abstract:
  We examine the role of a conscious observer in a typical quantum mechanical
measurement. Four rules are given that govern stochastic choice and state
reduction in several cases of continuous and intermittent observation. It is
found that consciousness always accompanies a state reduction leading to
observation, but its presence is not sufficient to 'cause' a reduction. The
distinction is clarified and codified by the rules that are given below. This
is the first of several papers that lead to an experimental test of the rules,
and of the "parallel principle" that is described elsewhere. Key words: Brain
states, boundary conditions, consciousness, conscious observer, environment,
decoherence, macroscopic superposition, measurement, state reduction, state
collapse, von Neumann.


title: Schrodinger's cat: The rules of engagement

abstract:
  In a previous paper we examined the role of a conscious observer in a typical
quantum mechanical measurement. Four rules were given that were found to govern
the stochastic choice and state reduction in several cases of continuous and
intermittent observation. It was shown that consciousness always accompanies a
state reduction leading to observation, but its presence is not sufficient to
'cause' a reduction. The distinction is clarified and codified by the rules
that are repeated below. In this paper, these rules are successfully applied to
two different versions of the Schrodinger cat experiment. Key Words: Brain
states, boundary conditions, cat paradox, consciousness, conscious observer,
environment, decoherence, macroscopic superposition, measurement, state
reduction, state collapse, von Neumann.


title:
Quantum searching a classical database (or how we learned to stop
  worrying and love the bomb)

abstract:
  We show how to perform a quantum search for a classical object, specifically
for a classical object which performs no coherent evolution on the quantum
computer being used for the search. We do so by using interaction free
measurement as a subroutine in a quantum search algorithm. In addition to
providing a simple example of how non-unitary processes which approximate
unitary ones can be useful in a quantum algorithm, our procedure requires only
one photon regardless of the size of the database, thereby establishing an
upper bound on the amount of energy required to search an arbitrarily large
database. Alternatively, our result can be interpreted as showing how to
perform an interaction free measurement with a single photon on an arbitrarily
large number of possible bomb positions simultaneously. We also provide a
simple example demonstrating that in terms of the number of database queries,
the procedure outlined here can outperform the best classical one.


title: Complete eigenstates of identical qubits arranged in regular polygons

abstract:
  We calculate the energy eigenvalues and eigenstates corresponding to coherent
single and multiple excitations of an array of N identical qubits or two-level
atoms (TLA's) arranged on the vertices of a regular polygon. We assume only
that the coupling occurs via an exchange interaction which depends on the
separation between the qubits. We include the interactions between all pairs of
qubits, and our results are valid for arbitrary distances relative to the
radiation wavelength. To illustrate the usefulness of these states, we plot the
distance dependence of the decay rates of the n=2 (biexciton) eigenstates of an
array of 4 qubits, and tabulate the biexciton eigenvalues and eigenstates, and
absorption frequencies, line widths, and relative intensities for polygons
consisting of N=2,...,9 qubits in the long-wavelength limit.


title: Constructing physically intuitive graph invariants

abstract:
  In this brief note I try to give a simple example of where physical intuition
about a collection of interacting qubits can lead to the construction of
"natural" versions of what are, generically, quite abstract mathematical
objects - in this case graph invariants.
  This note is written primarily for physicists who do not want to go through
the painful process of trying to understand Ed Witten's vastly more complicated
construction of physically intuitive knot invariants, but who'd like some idea
of how physical intuition can play a role in such things.


title:
Parameter scaling in the decoherent quantum-classical transition for
  chaotic systems

abstract:
  The quantum to classical transition has been shown to depend on a number of
parameters. Key among these are a scale length for the action, $\hbar$, a
measure of the coupling between a system and its environment, $D$, and, for
chaotic systems, the classical Lyapunov exponent, $\lambda$. We propose
computing a measure, reflecting the proximity of quantum and classical
evolutions, as a multivariate function of $(\hbar,\lambda,D)$ and searching for
transformations that collapse this hyper-surface into a function of a composite
parameter $\zeta = \hbar^{\alpha}\lambda^{\beta}D^{\gamma}$. We report results
for the quantum Cat Map, showing extremely accurate scaling behavior over a
wide range of parameters and suggest that, in general, the technique may be
effective in constructing universality classes in this transition.


title:
A simple demonstration of Bell's theorem involving two observers and no
  probabilities or inequalities

abstract:
  This paper describes a device, consisting of a central source and two widely
separated detectors with six switch settings each, that provides a simple
gedanken demonstration of Bell's theorem without relying on either statistical
effects or the occurrence of rare events. The mechanism underlying the
operation of the device is revealed for readers with a knowledge of quantum
mechanics.


title:
On first-arrival-time distributions for a Dirac electron in 1+1
  dimensions

abstract:
  For the special case of freely evolving Dirac electrons in $1 + 1$
dimensions, Feynman checkerboard paths have previously been used to derive
Wigner's arrival-time distribution which includes all arrivals. Here, an
attempt is made to use these paths to determine the corresponding distribution
of first-arrival times. Simple analytic expressions are obtained for the
relevant components of the first-arrival propagator. These are used to
investigate the relative importance of the first-arrival contribution to the
Wigner arrival-time distribution and of the contribution arising from
interference between first and later (i.e. second, third, ...) arrivals. It is
found that a distribution of (intrinsic) first-arrival times for a Dirac
electron cannot in general be consistently defined using checkerboard paths,
not even approximately in the nonrelativistic regime.


title:
Pulse-mode quantum projection synthesis: Effects of mode mismatch on
  optical state truncation and preparation

abstract:
  Quantum projection synthesis can be used for phase-probability-distribution
measurement, optical-state truncation and preparation. The method relies on
interfering optical lights, which is a major challenge in experiments performed
by pulsed light sources. In the pulsed regime, the time frequency overlap of
the interfering lights plays a crucial role on the efficiency of the method
when they have different mode structures. In this paper, the pulsed mode
projection synthesis is developed, the mode structure of interfering lights are
characterized and the effect of this overlap (or mode match) on the fidelity of
optical-state truncation and preparation is investigated. By introducing the
positive-operator-valued measure (POVM) for the detection events in the scheme,
the effect of mode mismatch between the photon-counting detectors and the
incident lights are also presented.


title:
Laser induced breakdown of the magnetic field reversal symmetry in the
  propagation of unpolarized light

abstract:
  We show how a medium, under the influece of a coherent control field which is
resonant or close to resonance to an appropriate atomic transition, can lead to
very strong asymmetries in the propagation of unpolarized light when the
direction of the magnetic field is reversed. We show how EIT can be used to
mimic effects occuring in natural systems and that EIT can produce very large
asymmetries as we use electric dipole allowed transitions. Using density matrix
calculations we present results for the breakdown of the magnetic field
reversal symmetry for two different atomic configurations.


title: How One Shutter Can Close N Slits

abstract:
  It is shown that a quantum shutter, pre- and post-selected in particular
quantum states, can close simultaneously arbitrary number of slits preventing
the passage of a single photon in an arbitrary state. A set of K pre- and
post-selected shutters can close the slits preventing the passage of K or less
photons. This result indicates that the surprising properties of pre- and
post-selected quantum systems are even more robust than previously expected.


title: Two-electron quantum dots as scalable qubits

abstract:
  We show that two electrons confined in a square semiconductor quantum dot
have two isolated low-lying energy eigenstates, which have the potential to
form the basis of scalable computing elements (qubits). Initialisation,
one-qubit and two-qubit universal gates, and readout are performed using
electrostatic gates and magnetic fields. Two-qubit transformations are
performed via the Coulomb interaction between electrons on adjacent dots.
Choice of initial states and subsequent asymmetric tuning of the tunnelling
energy parameters on adjacent dots control the effect of this interaction.


title: Are simultaneous Bell measurements possible?

abstract:
  All experimental tests of Bell-type inequalities and
Greenberger-Horne-Zeilinger setups rely on the separate and successive
measurement of the terms involved. We discuss possibilities of experimental
setups to measure all relevant terms simultaneously in a single experiment and
find this to be impossible. One reason is the lack of multi-partite states
which are unique in the sense that a measurement of some observable on one
particle fixes the value of the corresponding observables of the other
particles as well.


title: On the P- and T-non-invariant two-component equation for the neutrino

abstract:
  The relativistic two-component equation describing the free motion of
particles with zero mass and spin 1/2, which is P- and T-non-invariant but
C-invariant, is found. The representation of the Poincare group for zero mass
and discrete spin is constructed. The position operator for such a particle is
defined.


title:
On the CP-noninvariant equations for the particle with zero mass and
  spin s=1/2

abstract:
  One of us quant-ph/0206077 (Nucl. Phys. B, 1970, 21, 321) has shown that for
the particle with zero mass and spin s=1/2 there are three types of
two-component equations (or one four-component equation with three different
subsidiary conditions) which differ from one another by P, T and C properties.
One of these equations is the two-component Weyl equation. In this note we give
two other relativistic invariant equations.


title: On two-component equations for zero mass particles

abstract:
  The paper presents a detailed theoretical-group analysis of three types of
two-component equations of motion which describe the particle with zero mass
and spin 1/2. There are studied P-, T- and C-propertias of the equations
obtained.


title: Spontaneous emission of an atom in front of a mirror

abstract:
  Motivated by a recent experiment [J. Eschner {\it et al.}, Nature {\bf 413},
495 (2001)], we now present a theoretical study on the fluorescence of an atom
in front of a mirror. On the assumption that the presence of the distant mirror
and a lens imposes boundary conditions on the electric field in a plane close
to the atom, we derive the intensities of the emitted light as a function of an
effective atom-mirror distance. The results obtained are in good agreement with
the experimental findings.


title: Comment on "Quantum optimization for combinatorial searches"

abstract:
  This is a comment on a recent publication claiming to have found a ``quantum
optimization'' algorithm which outperforms known algorithms for minimizing some
``cost function''. Unfortunately, this algorithm is no better than choosing a
state at random and checking whether it has low cost.


title:
Distributed entanglement as a probe for the quantum structure of
  spacetime

abstract:
  Simultaneity is a well-defined notion in special relativity once a Minkowski
metric structure is fixed on the spacetime continuum (manifold) of events. In
quantum gravity, however, the metric is not expected to be a fixed, classical
structure, but a fluctuating quantum operator which may assume a coherent
superposition of two classically-distinguishable values. A natural question to
ask is what happens to the notion of simultaneity and synchronization when the
metric is in a quantum superposition. Here we show that the resource of
distributed entanglement of the same kind as used by Jozsa et al. [Phys. Rev.
Lett. 85, 2010 (2000)] gives rise to an experimental probe that is sensitive to
coherent quantum fluctuations in the spacetime metric.


title: Fresnel Representation of the Wigner Function: An Operational Approach

abstract:
  We present an operational definition of the Wigner function. Our method
relies on the Fresnel transform of measured Rabi oscillations and applies to
motional states of trapped atoms as well as to field states in cavities. We
illustrate this technique using data from recent experiments in ion traps [D.
M. Meekhof et al., Phys. Rev. Lett. 76, 1796 (1996)] and in cavity QED [B.
Varcoe et al., Nature 403, 743 (2000)]. The values of the Wigner functions of
the underlying states at the origin of phase space are W(0)=+1.75 for the
vibrational ground state and W(0)=-1.4 for the one-photon number state. We
generalize this method to wave packets in arbitrary potentials.


title: Bell inequalities and distillability in N-quantum-bit systems

abstract:
  The relation between Bell inequalities with two two-outcome measurements per
site and distillability is analyzed in systems of an arbitrary number of
quantum bits. We observe that the violation of any of these inequalities by a
quantum state implies that pure-state entanglement can be distilled from it.
The corresponding distillation protocol may require that some of the parties
join into several groups. We show that there exists a link between the amount
of the Bell inequality violation and the size of the groups they have to form
for distillation. Thus, a strong violation is always sufficient for full
N-partite distillability. This result also allows for a security proof of
multi-partite quantum key distribution (QKD) protocols.


title:
Pseudo-Hermitian version of the charged harmonic oscillator and its
  "forgotten" exact solutions

abstract:
  An unusual type of the exact solvability is reported. It is exemplified by
the Coulomb plus harmonic oscillator in D dimensions after a complexification
of its Hamiltonian which keeps the energies real. Infinitely many bound states
are found in closed form which generalizes the even-parity harmonic-oscillator
states at zero charge. Apparently, the model is halfway between exact and
quasi-exact.


title: How to correct small quantum errors

abstract:
  The theory of quantum error correction is a cornerstone of quantum
information processing. It shows that quantum data can be protected against
decoherence effects, which otherwise would render many of the new quantum
applications practically impossible. In this paper we give a self contained
introduction to this theory and to the closely related concept of quantum
channel capacities. We show, in particular, that it is possible (using
appropriate error correcting schemes) to send a non-vanishing amount of quantum
data undisturbed (in a certain asymptotic sense) through a noisy quantum
channel T, provided the errors produced by T are small enough.


title: Reply to: Comment on "Quantum Optimization for Combinatorial Searches"

abstract:
  This is my reply to Zalka and Brun's criticism of my recent paper on quantum
optimization heuristics. Essentially, this criticism is shown to be utterly
irrelevant.


title: An introduction to quantum coin-tossing

abstract:
  We review the quantum version of a well known problem of cryptography called
coin tossing (``flipping a coin via telephone''). It can be regarded as a game
where two remote players (who distrust each other) tries to generate a
uniformly distributed random bit which is common to both parties. The only
resource they can use to perform this task is a classical or quantum
communication channel. In this paper we provide a general overview over such
coin tossing protocols, concerning in particular their security.


title: Book Review: 'A New Kind of Science'

abstract:
  This is a critical review of the book 'A New Kind of Science' by Stephen
Wolfram. We do not attempt a chapter-by-chapter evaluation, but instead focus
on two areas: computational complexity and fundamental physics. In complexity,
we address some of the questions Wolfram raises using standard techniques in
theoretical computer science. In physics, we examine Wolfram's proposal for a
deterministic model underlying quantum mechanics, with 'long-range threads' to
connect entangled particles. We show that this proposal cannot be made
compatible with both special relativity and Bell inequality violation.


title:
Some Reflections on the Status of Conventional Quantum Theory when
  Applied to Quantum Gravity

abstract:
  All current approaches to quantum gravity employ essentially standard quantum
theory including, in particular, continuum quantities such as the real or
complex numbers. However, I wish to argue that this may be fundamentally wrong
in so far as the use of these continuum quantities in standard quantum theory
can be traced back to certain {\em a priori} assumptions about the nature of
space and time: assumptions that may be incompatible with the view of space and
time adopted by a quantum gravity theory. My conjecture is that in, some yet to
be determined sense, to each type of space-time there is associated a
corresponding type of quantum theory in which continuum quantities do not
necessarily appear, being replaced with structures that are appropriate to the
specific space-time.
  Topos theory then arises as a possible tool for `gluing' together these
different theories associated with the different space-times. As a concrete
example of the use of topos ideas, I summarise recent work applying presheaf
theory to the Kochen-Specher theorem and the assignment of values to physical
quantities in a quantum theory.


title: Quantum Technology: The Second Quantum Revolution

abstract:
  We are currently in the midst of a second quantum revolution. The first
quantum revolution gave us new rules that govern physical reality. The second
quantum revolution will take these rules and use them to develop new
technologies. In this review we discuss the principles upon which quantum
technology is based and the tools required to develop it. We discuss a number
of examples of research programs that could deliver quantum technologies in
coming decades including; quantum information technology, quantum
electromechanical systems, coherent quantum electronics, quantum optics and
coherent matter technology.


title:
Practical free-space quantum key distribution over 10 km in daylight and
  at night

abstract:
  We have demonstrated quantum key distribution (QKD) over a 10-km, 1-airmass
atmospheric range during daylight and at night. Secret random bit sequences of
the quality required for the cryptographic keys used to initialize secure
communications devices were transferred at practical rates with realistic
security. By identifying the physical parameters that determine the system's
secrecy efficiency, we infer that free-space QKD will be practical over much
longer ranges under these and other atmospheric and instrumental conditions.


title:
A Semidefinite Programming Approach to Optimal Unambiguous
  Discrimination of Quantum States

abstract:
  In this paper we consider the problem of unambiguous discrimination between a
set of linearly independent pure quantum states. We show that the design of the
optimal measurement that minimizes the probability of an inconclusive result
can be formulated as a semidefinite programming problem. Based on this
formulation, we develop a set of necessary and sufficient conditions for an
optimal quantum measurement. We show that the optimal measurement can be
computed very efficiently in polynomial time by exploiting the many well-known
algorithms for solving semidefinite programs, which are guaranteed to converge
to the global optimum.
  Using the general conditions for optimality, we derive necessary and
sufficient conditions so that the measurement that results in an equal
probability of an inconclusive result for each one of the quantum states is
optimal. We refer to this measurement as the equal-probability measurement
(EPM). We then show that for any state set, the prior probabilities of the
states can be chosen such that the EPM is optimal.
  Finally, we consider state sets with strong symmetry properties and equal
prior probabilities for which the EPM is optimal. We first consider
geometrically uniform state sets that are defined over a group of unitary
matrices and are generated by a single generating vector. We then consider
compound geometrically uniform state sets which are generated by a group of
unitary matrices using multiple generating vectors, where the generating
vectors satisfy a certain (weighted) norm constraint.


title:
An Inverse-Problem Approach to Designing Photonic Crystals for Cavity
  QED Experiments

abstract:
  Photonic band gap (PBG) materials are attractive for cavity QED experiments
because they provide extremely small mode volumes and are monolithic,
integratable structures. As such, PBG cavities are a promising alternative to
Fabry-Perot resonators. However, the cavity requirements imposed by QED
experiments, such as the need for high Q (low cavity damping) and small mode
volumes, present significant design challenges for photonic band gap materials.
Here, we pose the PBG design problem as a mathematical inversion and provide an
analytical solution for a two-dimensional crystal. We then address a planar (2D
crystal with finite thickness) structure using numerical techniques.


title: Entwined Pairs and Schroedinger 's Equation

abstract:
  We show that a point particle moving in space-time on entwined-pair paths
generates Schroedinger's equation in a static potential in the appropriate
continuum linit. This provides a new realist context for the Schroedinger
equation within the domain of classical stochastic processes. It also suggests
that self-quantizing systems may provide considerable insight into conventional
quantum mechanics.


title:
Quantum computing in optical microtraps based on the motional states of
  neutral atoms

abstract:
  We investigate quantum computation with neutral atoms in optical microtraps
where the qubit is implemented in the motional states of the atoms, i.e., in
the two lowest vibrational states of each trap. The quantum gate operation is
performed by adiabatically approaching two traps and allowing tunneling and
cold collisions to take place. We demonstrate the capability of this scheme to
realize a square-root of swap gate, and address the problem of double
occupation and excitation to other unwanted states. We expand the two-particle
wavefunction in an orthonormal basis and analyze quantum correlations
throughout the whole gate process. Fidelity of the gate operation is evaluated
as a function of the degree of adiabaticity in moving the traps. Simulations
are based on rubidium atoms in state-of-the-art optical microtraps with quantum
gate realizations in the few tens of milliseconds duration range.


title: Error exponents for entanglement concentration

abstract:
  Consider entanglement concentration schemes that convert n identical copies
of a pure state into a maximally entangled state of a desired size with success
probability being close to one in the asymptotic limit. We give the distillable
entanglement, the number of Bell pairs distilled per copy, as a function of an
error exponent, which represents the rate of decrease in failure probability as
n tends to infinity. The formula fills the gap between the least upper bound of
distillable entanglement in probabilistic concentration, which is the
well-known entropy of entanglement, and the maximum attained in deterministic
concentration. The method of types in information theory enables the detailed
analysis of the distillable entanglement in terms of the error rate. In
addition to the probabilistic argument, we consider another type of
entanglement concentration scheme, where the initial state is deterministically
transformed into a (possibly mixed) final state whose fidelity to a maximally
entangled state of a desired size converges to one in the asymptotic limit. We
show that the same formula as in the probabilistic argument is valid for the
argument on fidelity by replacing the success probability with the fidelity.
Furthermore, we also discuss entanglement yield when optimal success
probability or optimal fidelity converges to zero in the asymptotic limit
(strong converse), and give the explicit formulae for those cases.


title: Fractional Schrodinger equation

abstract:
  Properties of the fractional Schrodinger equation have been studied. We have
proven the hermiticity of fractional Hamilton operator and established the
parity conservation law for the fractional quantum mechanics. As physical
applications of the fractional Schrodinger equation we have found the energy
spectrum for a hydrogen-like atom - fractional ''Bohr atom'' and the energy
spectrum of fractional oscillator in the semiclassical approximation. A new
equation for the fractional probability current density has been developed and
discussed. We also discuss the relationships between the fractional and the
standard Schrodinger equations.


title: Measurement as soft final-state interaction with a stochastic system

abstract:
  A small quantum scattering system (the microsystem) is studied in interaction
with a large quantum system (the macrosystem) described by unknown stochastic
variables. The interaction between the two systems is diagonal for the
microsystem in a certain basis, and it leads to an imprint on the macrosystem.
Moreover, the interaction is assumed to involve only small transfers of energy
and momentum between the two systems (as compared to typical energies/momenta
within the microsystem). This makes it suitable to carry out the analysis in
scattering theory, where the transition amplitude for the whole system
factorizes. The interaction taking place within the macrosystem is assumed to
depend on the stochastic variables in such a way that, on the average, no
particular channel is favoured. The result is then, in the thermodynamic limit
of the macrosystem, that the whole system bifurcates and the microsystem ends
up in a state described by one of the basis vectors (in the mentioned basis).
The macrosystem ends up in an entangled state tied to this basis vector. For
the ensemble of macrosystems, the interaction with the microsystem leads, on
the average, to the usual decoherence and diagonal density matrix for the
microsystem. The macrosystem can be interpreted as representing a measurement
device for performing a measurement on the microsystem. The whole discussion is
carried out within quantum mechanics itself without any modification or
generalization.


title:
Efficiency of free energy calculations of spin lattices by spectral
  quantum algorithms

abstract:
  Quantum algorithms are well-suited to calculate estimates of the energy
spectra for spin lattice systems. These algorithms are based on the efficient
calculation of the discrete Fourier components of the density of states. The
efficiency of these algorithms in calculating the free energy per spin of
general spin lattices to bounded error is examined. We find that the number of
Fourier components required to bound the error in the free energy due to the
broadening of the density of states scales polynomially with the number of
spins in the lattice. However, the precision with which the Fourier components
must be calculated is found to be an exponential function of the system size.


title: A pseudo-simulation of Shor's quantum factoring algorithm

abstract:
  This paper presents a computer program, written in Maple, that allows a user
to simulate certain aspects of Shor's quantum factoring algorithm on a desktop
or laptop computer. The program does not simulate the unitary operations
carried out by a quantum computer but does faithfully mimic its output at the
crucial "readout" step of the order-finding process. The program reqires only
two inputs from the user - the number to be factored (which can be up to 10
digits long) and the number of qubits to be used in the factoring (for which a
helpful hint is given). The program then returns a detailed history of all its
attempts at factoring the number, beginning with its various unsuccessful
attempts and ending with the final successful attempt that leads to the correct
factors. The structure of the simulation is described, a typical output
produced by it is shown, and the factors limiting its performance are
discussed. The purpose of this simulation is to provide the user with some
"hands-on" experience of how quantum factoring works on integers somewhat
larger than can be handled by today's quantum computers.


title:
Solving the quantum search problem in polynomial time on an NMR quantum
  computer

abstract:
  The quantum search problem is an important problem due to the fact that a
general NP problem can be solved efficiently by an unsorted quantum search
algorithm. Here it has been shown that the quantum search problem could be
solved in polynomial time on an NMR quantum computer. The NMR ensemble quantum
computation is based on the quantum mechanical unitary dynamics that both a
closed quantum system and its ensemble obey the same quantum mechanical unitary
dynamics instead of on the pseudopure state or the effective pure state of the
classical NMR quantum computation. Based on the new principle the conventional
NMR multiple-quantum spectroscopy has been developed to solve experimentally
the search problem. The solution information of the search problem is first
loaded on the unitary evolution propagator which is constructed with the oracle
unitary operation and oracle-independent unitary operations and is used to
excite the multiple-quantum coherence in a spin ensemble. Then the multiple-
quantum spectroscopy is used to extract experimentally the solution
information. It has been discussed how to enhance the output NMR signal of the
quantum search NMR multiple-quantum pulse sequence and some approaches to
enhancing the NMR signal are also proposed. The present work could be helpful
for conventional high field NMR machines to solve efficiently the quantum
search problem.


title: A New Type of Limit Theorems for the One-Dimensional Quantum Random Walk

abstract:
  In this paper we consider the one-dimensional quantum random walk X^{varphi}
_n at time n starting from initial qubit state varphi determined by 2 times 2
unitary matrix U. We give a combinatorial expression for the characteristic
function of X^{varphi}_n. The expression clarifies the dependence of it on
components of unitary matrix U and initial qubit state varphi. As a consequence
of the above results, we present a new type of limit theorems for the quantum
random walk. In contrast with the de Moivre-Laplace limit theorem, our
symmetric case implies that X^{varphi}_n /n converges in distribution to a
limit Z^{varphi} as n to infty where Z^{varphi} has a density 1 / pi (1-x^2)
sqrt{1-2x^2} for x in (- 1/sqrt{2}, 1/sqrt{2}). Moreover we discuss some known
simulation results based on our limit theorems.


title:
On the three types of relativistic equations for particles with nonzero
  mass

abstract:
  In previous papers quant-ph/0206077, quant-ph/0206078, quant-ph/0206079 we
have shown that there exist three types of the relativistic equations for the
massless particles. Here we show that for the free particles and antiparticles
with the mass m>0 and the arbitrary spin $s \geq 1/2$ there also exist three
types of nonequivalent equations.


title:
P, T, C properties of the Poincare invariant equations for massive
  particles

abstract:
  We have shown quant-ph/0206104 (Lett. Nuovo Cimento, 1972, 4, 344) that for
free particles and antiparticles with mass m>0 and arbitrary spin s>0, in the
framework of the Poincare group P(1,3), there exist three types of
nonequivalent equations. In the present paper we study the P, T, C properties
of these equations.


title:
Schemes of implementation in NMR of quantum processors and Deutsch-Jozsa
  algorithm by using virtual spin representation

abstract:
  Schemes of experimental realization of the main two qubit processors for
quantum computers and Deutsch-Jozsa algorithm are derived in virtual spin
representation. The results are applicable for every four quantum states
allowing the required properties for quantum processor implementation if for
qubit encoding virtual spin representation is used. Four dimensional Hilbert
space of nuclear spin 3/2 is considered in details for this aim


title:
Distorted Waves with Exact Non-Local Exchange: a Canonical Function
  Approach

abstract:
  It is shown how the Canonical Function approach can be used to obtain
accurate solutions for the distorted wave problem taking account of direct
static and polarisation potentials and exact non-local exchange. Calculations
are made for electrons in the field of atomic hydrogen and the phaseshifts are
compared with those obtained using a modified form of the DWPO code of McDowell
and collaborators: for small wavenumbers our approach avoids numerical
instabilities otherwise present. Comparison is also made with phaseshifts
calculated using local equivalent-exchange potentials and it is found that
these are inaccurate at small wavenumbers. Extension of our method to the case
of atoms having other than s-type outer shells is dicussed.


title: Damped Bloch oscillations of cold atoms in optical lattices

abstract:
  The paper studies Bloch oscillations of cold neutral atoms in the optical
lattice. The effect of spontaneous emission on the dynamics of the system is
analyzed both analytically and numerically. The spontaneous emission is shown
to cause (i) the decay of Bloch oscillations with the decrement given by the
rate of spontaneous emission and (ii) the diffusive spreading of the atoms with
a diffusion coefficient depending on {\em both} the rate of spontaneous
emission and the Bloch frequency.


title: On the possible types of equations for zero-mass particles

abstract:
  There are a number of papers dedicated to the description of free particles
and antiparticles with zero mass and spin 1/2. A great many equations with
different C, P, T properties have been proposed and the impression could be
formed that there are many nonequivalent theories for zero-mass particles. The
purpose or this paper is to show that it is not the case and to describe all
nonequivalent equations.


title: Conditions for compatibility of quantum state assignments

abstract:
  Suppose N parties describe the state of a quantum system by N possibly
different density operators. These N state assignments represent the beliefs of
the parties about the system. We examine conditions for determining whether the
N state assignments are compatible. We distinguish two kinds of procedures for
assessing compatibility, the first based on the compatibility of the prior
beliefs on which the N state assignments are based and the second based on the
compatibility of predictive measurement probabilities they define. The first
procedure leads to a compatibility criterion proposed by Brun, Finkelstein, and
Mermin [BFM, Phys. Rev. A 65, 032315 (2002)]. The second procedure leads to a
hierarchy of measurement-based compatibility criteria which is fundamentally
different from the corresponding classical situation. Quantum mechanically none
of the measurement-based compatibility criteria is equivalent to the BFM
criterion.


title:
Classification of multipartite entangled states by multidimensional
  determinants

abstract:
  We find that multidimensional determinants "hyperdeterminants", related to
entanglement measures (the so-called concurrence or 3-tangle for the 2 or 3
qubits, respectively), are derived from a duality between entangled states and
separable states. By means of the hyperdeterminant and its singularities, the
single copy of multipartite pure entangled states is classified into an onion
structure of every closed subset, similar to that by the local rank in the
bipartite case. This reveals how inequivalent multipartite entangled classes
are partially ordered under local actions. In particular, the generic entangled
class of the maximal dimension, distinguished as the nonzero hyperdeterminant,
does not include the maximally entangled states in Bell's inequalities in
general (e.g., in the $n \geq 4$ qubits), contrary to the widely known
bipartite or 3-qubit cases. It suggests that not only are they never locally
interconvertible with the majority of multipartite entangled states, but they
would have no grounds for the canonical n-partite entangled states. Our
classification is also useful for the mixed states.


title: Point Interactions: PT-Hermiticity and Reality of the Spectrum

abstract:
  General point interactions for the second derivative operator in one
dimension are studied. In particular, ${\mathcal P \mathcal T}$-self-adjoint
point interactions with the support at the origin and at points $\pm l$ are
considered. The spectrum of such non-Hermitian operators is investigated and
conditions when the spectrum is pure real are presented. The results are
compared with those for standard self-adjoint point interactions.


title: Geometric coupling thresholds in a two-dimensional strip

abstract:
  We consider the Laplacian in a strip $\mathbb{R}\times (0,d)$ with the
boundary condition which is Dirichlet except at the segment of a length $2a$ of
one of the boundaries where it is switched to Neumann. This operator is known
to have a non-empty and simple discrete spectrum for any $a>0$. There is a
sequence $0<a_1<a_2<...$ of critical values at which new eigenvalues emerge
from the continuum when the Neumann window expands. We find the asymptotic
behavior of these eigenvalues around the thresholds showing that the gap is in
the leading order proportional to $(a-a_n)^2$ with an explicit coefficient
expressed in terms of the corresponding threshold-energy resonance
eigenfunction.


title: Comments on Bouda and Djama's "Quantum Newton's law"

abstract:
  Discussion of the differences between the trajectory representation of Floyd
and that of Bouda and Djama [Phys. Lett. A 285 (2001) 27, quant-ph/0103071]
renders insight: while Floyd's trajectories are related to group velocities,
Bouda and Djama's are not. Bouda and Djama's reasons for these differences are
also addressed.


title:
Quantum theory of resonantly enhanced four-wave mixing: mean-field and
  exact numerical solutions

abstract:
  We present a full quantum analysis of resonant forward four-wave mixing based
on electromagnetically induced transparency (EIT). In particular, we study the
regime of efficient nonlinear conversion with low-intensity fields that has
been predicted from a semiclassical analysis. We derive an effective nonlinear
interaction Hamiltonian in the adiabatic limit. In contrast to conventional
nonlinear optics this Hamiltonian does not have a power expansion in the fields
and the conversion length increases with the input power. We analyze the
stationary wave-mixing process in the forward scattering configuration using an
exact numerical analysis for up to $10^3$ input photons and compare the results
with a mean-field approach. Due to quantum effects, complete conversion from
the two pump fields into the signal and idler modes is achieved only
asymptotically for large coherent pump intensities or for pump fields in
few-photon Fock states. The signal and idler fields are perfectly quantum
correlated which has potential applications in quantum communication schemes.
We also discuss the implementation of a single-photon phase gate for continuous
quantum computation.


title: Five Lectures On Dissipative Master Equations

abstract:
  1 First Lecture: Basics
  1.1 Physical Derivation of the Master Equation
  1.2 Some Simple Implications
  1.3 Steady State
  1.4 Action to the Left
  2 Second Lecture: Eigenvalues and Eigenvectors of L
  2.1 A Simple Case First
  2.2 The General Case
  3 Third Lecture: Completeness of the Damping Bases
  3.1 Phase Space Functions
  3.2 Completeness of the Eigenvectors of L
  3.3 Positivity Conservation
  3.4 Lindblad Form of Liouville Operators
  4 Fourth Lecture: Quantum-Optical Applications
  4.1 Periodically Driven Damped Oscillator
  4.2 Conditional and Unconditional Evolution
  4.3 Physical Signicance of Statistical Operators
  5 Fifth Lecture: Statistics of Detected Atoms
  5.1 Correlation Functions
  5.2 Waiting Time Statistics
  5.3 Counting Statistics


title:
A simple quantum equation for Decoherence and dissipation (through
  interaction with the environment)

abstract:
  Within the density matrix formalism, it is shown that a simple way to get
decoherence is through the introduction of a "quantum" of time (chronon): which
implies replacing the differential Liouville--von Neumann equation with a
finite-difference version of it. In this way, one is given the possibility of
using a rather simple quantum equation to describe the decoherence effects due
to dissipation. Namely, the mere introduction (not of a "time-lattice", but
simply) of a "chronon" allows us to go on from differential to
finite-difference equations; and in particular to write down the
quantum-theoretical equations (Schroedinger equation, Liouville--von Neumann
equation,...) in three different ways: "retarded", "symmetrical", and
"advanced". One of such three formulations --the retarded one-- describes in an
elementary way a system which is exchanging (and losing) energy with the
environment; and in its density-matrix version, indeed, it can be easily shown
that all non-diagonal terms go to zero very rapidly. [A much larger
presentation of the theoretical ground on which this paper is based appeared in
the e-print quant-ph/9706059, and in the preprint IC/98/74, ICTP; Trieste,
1998].


title: Shedding (red and green) light on "time related hidden parameters"

abstract:
  I explain in elementary terms why the critique of Hess and Philipp in Section
3.2 of quant-ph/0103028 fails to invalidate the nontechnical version of Bell's
theorem I gave twenty years ago, involving two detectors with 3-pole switches
and red and green lights.


title: On Casimir Forces for Media with Arbitrary Dielectric Properties

abstract:
  We derive an expression for the Casimir force between slabs with arbitrary
dielectric properties characterized by their reflection coefficients. The
formalism presented here is applicable to media with a local or a non-local
dielectric response, an infinite or a finite width, inhomogeneous dissipative,
etc. Our results reduce to the Lifshitz formula for the force between
semi-infinite dielectric slabs by replacing the reflection coefficients by the
Fresnel amplitudes.


title:
Boltzmann/Gibbs distribution for a two level system interacting with a
  thermal reservoir does not follow from Schrodinger equation

abstract:
  In this work, we consider a 2-state quantum system interacting with a thermal
reservoir. By computing the long time limit of the probability for the system
to be in the ground state according to the Schrodinger/Von Neumann equation, we
reach a contradiction with the prediction of equilibrium statistical mechanics.
The most likely explanation is that the Schrodinger equation is incomplete as a
description of such systems, because the other assumptions made herein have a
wider range of experimental support.


title: Weak coin flipping with small bias

abstract:
  This note presents a quantum protocol that demonstrates that_weak_ coin
flipping with bias approximately 0.239, less than 1/4, is possible. A bias of
1/4 was the smallest known, and followed from the strong coin flipping protocol
of [Ambainis 2001]. Protocols with yet smaller bias, approximately 0.207, have
independently been discovered [Ambainis 2001, Spekkens and Rudolph 2002]. We
also present an alternative strong coin flipping protocol with bias 1/4 with
analysis simpler than that of [Ambainis 2001].


title: On communication over an entanglement-assisted quantum channel

abstract:
  Shared entanglement is a resource available to parties communicating over a
quantum channel, much akin to public coins in classical communication
protocols. Whereas shared randomness does not help in the transmission of
information, or significantly reduce the classical complexity of computing
functions (as compared to private-coin protocols), shared entanglement leads to
startling phenomena such as ``quantum teleportation'' and ``superdense
coding.''
  The problem of characterising the power of prior entanglement has puzzled
many researchers. In this paper, we revisit the problem of transmitting
classical bits over an entanglement-assisted quantum channel. We derive a new,
optimal bound on the number of quantum bits required for this task, for any
given probability of error. All known lower bounds in the setting of bounded
error entanglement-assisted communication are based on sophisticated
information theoretic arguments. In contrast, our result is derived from first
principles, using a simple linear algebraic technique.


title: On bit-commitment based quantum coin flipping

abstract:
  In this paper, we focus on a special framework for quantum coin flipping
protocols,_bit-commitment based protocols_, within which almost all known
protocols fit. We show a lower bound of 1/16 for the bias in any such protocol.
We also analyse a sequence of multi-round protocol that tries to overcome the
drawbacks of the previously proposed protocols, in order to lower the bias. We
show an intricate cheating strategy for this sequence, which leads to a bias of
1/4. This indicates that a bias of 1/4 might be optimal in such protocols, and
also demonstrates that a cleverer proof technique may be required to show this
optimality.


title: Adaptive quantum-limited estimates of phase

abstract:
  Quantum-limited estimation of an optical phase using adaptive (i.e. real-time
feedback) techniques is reviewed. One case is explored in detail, as it can be
understood using only elementary concepts such as photonic shot-noise and error
analysis. Very recent experimental results are discussed.


title: Quantum Trajectories for Realistic Photodetection I: General Formalism

abstract:
  Quantum trajectories describe the stochastic evolution of an open quantum
system conditioned on continuous monitoring of its output, such as by an ideal
photodetector. In practice an experimenter has access to an output filtered
through various electronic devices, rather than the microscopic states of the
detector. This introduces several imperfections into the measurement process,
of which only inefficiency has previously been incorporated into quantum
trajectory theory. However, all electronic devices have finite bandwidths, and
the consequent delay in conveying the output signal to the observer implies
that the evolution of the conditional state of the quantum system must be
non-Markovian. We present a general method of describing this evolution and
apply it to avalanche photodiodes (APDs) and to photoreceivers. We include the
effects of efficiency, dead time, bandwidth, electronic noise, and dark counts.
The essential idea is to treat the quantum system and classical detector
jointly, and to average over the latter to obtain the conditional quantum
state.
  The significance of our theory is that quantum trajectories for realistic
detection are necessary for sophisticated approaches to quantum feedback, and
our approach could be applied in many areas of physics.


title:
Quantum Trajectories for Realistic Photodetection II: Application and
  Analysis

abstract:
  In the preceding paper [Warszawski and Wiseman] we presented a general
formalism for determining the state of a quantum system conditional on the
output of a realistic detector, including effects such as a finite bandwidth
and electronic noise. We applied this theory to two sorts of photodetectors:
avalanche photodiodes and photoreceivers. In this paper we present simulations
of these realistic quantum trajectories for a cavity QED scenario in order to
ascertain how the conditioned state varies from that obtained with perfect
detection. Large differences are found, and this is manifest in the average of
the conditional purity. Simulation also allows us to comprehensively
investigate how the quality of the the photoreceiver depends upon its physical
parameters. In particular, we present evidence that in the limit of small
electronic noise, the photoreceiver quality can be characterized by an {\em
effective} bandwidth, which depends upon the level of electronic noise and the
filter bandwidth. We establish this result as an appropriate limit for a
simpler, analytically solvable, system. We expect this to be a general result
in other applications of our theory.


title:
On the average fidelity criterion for the non-classicality of the
  continuous variable quantum teleportation

abstract:
  We show that the existing argument on the non-classicality of the continuous
variable quantum teleportation (CVQT) experiment by the average fidelity
criterion is incomplete therefore so far it is still unclear whether the CVQT
experiment(Furusawa et al, Science,282, 706(1998)) is really non-classical.


title:
Anyons from non-solvable finite groups are sufficient for universal
  quantum computation

abstract:
  We present a constructive proof that anyonic magnetic charges with fluxes in
a non-solvable finite group can perform universal quantum computations. The
gates are built out of the elementary operations of braiding, fusion, and
vacuum pair creation, supplemented by a reservoir of ancillas of known flux.
Procedures for building the ancilla reservoir and for correcting leakage are
also described. Finally, a universal qudit gate-set, which is ideally suited
for anyons, is presented. The gate-set consists of classical computation
supplemented by measurements of the X operator.


title:
Shape invariance, raising and lowering operators in hypergeometric type
  equations

abstract:
  The Schr\" odinger equations which are exactly solvable in terms of
associated special functions are directly related to some self-adjoint
operators defined in the theory of hypergeometric type equations. The
fundamental formulae occurring in a supersymmetric approach to these
Hamiltonians are consequences of some formulae concerning the general theory of
associated special functions. We use this connection in order to obtain a
general theory of Schr\" odinger equations exactly solvable in terms of
associated special functions, and to extend certain results known in the case
of some particular potentials.


title:
Statistical properties of eigenvalues for an operating quantum computer
  with static imperfections

abstract:
  We investigate the transition to quantum chaos, induced by static
imperfections, for an operating quantum computer that simulates efficiently a
dynamical quantum system, the sawtooth map. For the different dynamical regimes
of the map, we discuss the quantum chaos border induced by static imperfections
by analyzing the statistical properties of the quantum computer eigenvalues.
For small imperfection strengths the level spacing statistics is close to the
case of quasi-integrable systems while above the border it is described by the
random matrix theory. We have found that the border drops exponentially with
the number of qubits, both in the ergodic and quasi-integrable dynamical
regimes of the map characterized by a complex phase space structure. On the
contrary, the regime with integrable map dynamics remains more stable against
static imperfections since in this case the border drops only algebraically
with the number of qubits.


title: Trapped-Ion Quantum Logic Utilizing Position-Dependent ac Stark Shifts

abstract:
  We present a scheme utilizing position-dependent ac Stark shifts for doing
quantum logic with trapped ions. By a proper choice of direction, position and
size, as well as power and frequency of a far-off-resonant Gaussian laser beam,
specific ac Stark shifts can be assigned to the individual ions, making them
distinguishable in frequency-space. In contrast to previous all-optical based
quantum gates with trapped ions, the present scheme enables individual
addressing of single ions and selective addressing of any pair of ions for
two-ion quantum gates, without using tightly focused laser beams. Furthermore,
the decoherence rate due to off-resonant excitations can be made negligible as
compared with other sources of decoherence.


title: Blackbody-radiation-assisted molecular laser cooling

abstract:
  The translational motion of molecular ions can be effectively cooled
sympathetically to temperatures below 100 mK in ion traps through Coulomb
interactions with laser-cooled atomic ions. The distribution of internal
rovibrational states, however, gets in thermal equilibrium with the typically
much higher temperature of the environment within tens of seconds. We consider
a concept for rotational cooling of such internally hot, but translationally
cold heteronuclear diatomic molecular ions. The scheme relies on a combination
of optical pumping from a few specific rotational levels into a ``dark state''
with redistribution of rotational populations mediated by blackbody radiation.


title: Screening of qubit from zero-temperature reservoir

abstract:
  We suggest an application of dynamical Zeno effect to isolate a qubit in the
quantum memory unit against decoherence caused by coupling with the reservoir
having zero temperature. The method is based on using an auxiliary casing
system that mediate the qubit-reservoir interaction and is simultaneously
frequently erased to ground state. This screening procedure can be implemented
in the cavity QED experiments to store the atomic and photonic qubit states.


title:
Natural majorization of the Quantum Fourier Transformation in
  phase-estimation algorithms

abstract:
  We prove that majorization relations hold step by step in the Quantum Fourier
Transformation (QFT) for phase-estimation algorithms considered in the
canonical decomposition. Our result relies on the fact that states which are
mixed by Hadamard operators at any stage of the computation only differ by a
phase. This property is a consequence of the structure of the initial state and
of the QFT, based on controlled-phase operators and a single action of a
Hadamard gate per qubit. As a consequence, Hadamard gates order the probability
distribution associated to the quantum state, whereas controlled-phase
operators carry all the entanglement but are immaterial to majorization. We
also prove that majorization in phase-estimation algorithms follows in a most
natural way from unitary evolution, unlike its counterpart in Grover's
algorithm.


title: Entanglement of photons

abstract:
  It is argued that the title of this paper represents a misconception.
Contrary to widespread beliefs it is electromagnetic field modes that are
``systems'' and can be entangled, not photons. The amount of entanglement in a
given state is shown to depend on redefinitions of the modes; we calculate the
minimum and maximum over all such redefinitions for several examples.


title: Single photon quantum cryptography

abstract:
  We report the full implementation of a quantum cryptography protocol using a
stream of single photon pulses generated by a stable and efficient source
operating at room temperature. The single photon pulses are emitted on demand
by a single nitrogen-vacancy (NV) color center in a diamond nanocrystal. The
quantum bit error rate is less that 4.6% and the secure bit rate is 9500
bits/s. The overall performances of our system reaches a domain where single
photons have a measurable advantage over an equivalent system based on
attenuated light pulses.


title: Photons uncertainty solves Einstein-Podolsky-Rosen paradox

abstract:
  Einstein, Podolsky and Rosen (EPR) pointed out that the quantum-mechanical
description of "physical reality" implied an unphysical, instantaneous action
between distant measurements. To avoid such an action at a distance, EPR
concluded that Quantum Mechanics had to be incomplete. However, its extensions
involving additional "hidden variables", allowing for the recovery of
determinism and locality, have been disproved experimentally (Bell's theorem).
Here, I present an opposite solution of the paradox based on the greater
indeterminism of the modern Quantum Field Theory (QFT) description of Particle
Physics, that prevents the preparation of any state having a definite number of
particles. The resulting uncertainty in photons radiation has interesting
consequences in Quantum Information Theory (e.g. cryptography and
teleportation). Moreover, since it allows for less elements of EPR physical
reality than the old non-relativistic Quantum Mechanics, QFT satisfies the EPR
condition of completeness without the need of hidden variables. The residual
physical reality does never violate locality, thus the unique objective proof
of "quantum nonlocality" is removed in an interpretation-independent way. On
the other hand, the supposed nonlocality of the EPR correlations turns out to
be a problem of the interpretation of the theory. If we do not rely on hidden
variables or new physics beyond QFT, the unique viable interpretation is a
minimal statistical one, that preserves locality and Lorentz symmetry.


title: Secure Multi-party Quantum Computing

abstract:
  Secure multi-party computing, also called "secure function evaluation", has
been extensively studied in classical cryptography. We consider the extension
of this task to computation with quantum inputs and circuits. Our protocols are
information-theoretically secure, i.e. no assumptions are made on the
computational power of the adversary. For the weaker task of verifiable quantum
secret sharing, we give a protocol which tolerates any t < n/4 cheating parties
(out of n). This is shown to be optimal. We use this new tool to show how to
perform any multi-party quantum computation as long as the number of dishonest
players is less than n/6.


title: Entangling Two Bose-Einstein Condensates by Stimulated Bragg Scattering

abstract:
  We propose an experiment for entangling two spatially separated Bose-Einstein
condensates by Bragg scattering of light. When Bragg scattering in two
condensates is stimulated by a common probe, the resulting quasiparticles in
the two condensates get entangled due to quantum communication between the
condensates via probe beam. The entanglement is shown to be significant and
occurs in both number and quadrature phase variables. We present two methods of
detecting the generated entanglement.


title: Entanglement Properties of Some Fractional Quantum Hall Liquids

abstract:
  We study the entanglement properties of some fractional quantum Hall liquids.
We calculate the entanglement of the Laughlin wave function and the wave
functions that are generated by the K-matrix using the modified entanglement
measure of indistinguishable fermions that is first proposed by Pa\v{s}kauskas
and You[1].


title: Symmetric qubits from cavity states

abstract:
  Two-mode cavities can be prepared in quantum states which represent symmetric
multi-qubit states. However, the qubits are impossible to address individually
and as such cannot be independently measured or otherwise manipulated. We
propose two related schemes to coherently transfer the qubits which the cavity
state represents onto individual atoms, so that the qubits can then be
processed individually. In particular, our scheme can be combined with the
quantum cloning scheme of Simon and coworkers [C. Simon et al, PRL 84, 2993
(2000)] to allow the optimal clones which their scheme produces to be spatially
separated and individually utilized.


title: Probabilistic generation of entanglement in optical cavities

abstract:
  We propose to produce entanglement by measuring the transmission of an
optical cavity. Conditioned on the detection of a reflected photon, pairs of
atoms in the cavity are prepared in maximally entangled states. The success
probability depends on the cavity parameters, but high quality entangled states
may be produced with a high probability even for cavities of moderate quality.


title: Quantum Zeno effect in a multilevel molecule

abstract:
  We study the dynamics of the populations of a model molecule endowed with two
sets of rotational levels of different parity, whose ground levels are energy
degenerate and coupled by a constant interaction. The relaxation rate from one
set of levels to the other one has an interesting dependence on the average
collision frequency of the molecules in the gas. This is interpreted as a
quantum Zeno effect due to the decoherence effects provoked by the molecular
collisions.


title: General Qubit Errors Cannot Be Corrected

abstract:
  Error correction, in the standard meaning of the term, implies the ability to
correct all small analog errors and some large errors. Examining assumptions at
the basis of the recently proposed quantum error-correcting codes, it is
pointed out that these codes can correct only a subset of errors, and are
unable to correct small phase errors which can have disastrous consequences for
a quantum computation. This shortcoming will restrict their usefulness in real
applications.


title:
Relating the Lorentzian and exponential: Fermi's approximation,the
  Fourier transform and causality

abstract:
  The Fourier transform is often used to connect the Lorentzian energy
distribution for resonance scattering to the exponential time dependence for
decaying states. However, to apply the Fourier transform, one has to bend the
rules of standard quantum mechanics; the Lorentzian energy distribution must be
extended to the full real axis $-\infty<E<\infty$ instead of being bounded from
below $0\leq E <\infty$ (``Fermi's approximation''). Then the Fourier transform
of the extended Lorentzian becomes the exponential, but only for times $t\geq
0$, a time asymmetry which is in conflict with the unitary group time evolution
of standard quantum mechanics. Extending the Fourier transform from
distributions to generalized vectors, we are led to Gamow kets, which possess a
Lorentzian energy distribution with $-\infty<E<\infty$ and have exponential
time evolution for $t\geq t_0 =0$ only. This leads to probability predictions
that do not violate causality.


title: Feynman's Decoherence

abstract:
  Gell-Mann's quarks are coherent particles confined within a hadron at rest,
but Feynman's partons are incoherent particles which constitute a hadron moving
with a velocity close to that of light. It is widely believed that the quark
model and the parton model are two different manifestations of the same
covariant entity. If this is the case, the question arises whether the Lorentz
boost destroys coherence. It is pointed out that this is not the case, and it
is possible to resolve this puzzle without inventing new physics. It is shown
that this decoherence is due to the measurement processes which are less than
complete.


title: Are classical probabilities instances of quantum amplitudes?

abstract:
  A conjecture about the quantum nature of classical probabilites is set forth
and discussed.


title:
Remarks on additivity of the Holevo channel capacity and of the
  entanglement of formation

abstract:
  The purpose of these notes is to discuss the relation between the additivity
questions regarding the quantities (Holevo) capacity of a quantum channel T and
entanglement of formation of a given bipartite state.
  In particular, using the Stinespring dilation theorem, we give a formula for
the channel capacity involving entanglement of formation. This can be used to
show that additivity of the latter for some states can be inferred from the
additivity of capacity for certain channels.
  We demonstrate this connection for a family of group--covariant channels,
allowing us to calculate the entanglement cost for many states, including some
where a strictly smaller upper bound on the distillable entanglement is known.
Group symmetry is used for more sophisticated analysis, giving formulas valid
for a class of channels. This is presented in a general framework, extending
recent findings of Vidal, Dur and Cirac (e-print quant-ph/0112131).
  We speculate on a general relation of superadditivity of the entanglement of
formation, which would imply both the general additivity of this function under
tensor products and of the Holevo capacity (with or without linear cost
constraints).


title: Reply to "Comments on Bouda and Djama's 'Quantum Newton's Law'"

abstract:
  In this reply, we hope to bring clarifications about the reservations
expressed by Floyd in his comments, give further explanations about the choice
of the approach and show that our fundamental result can be reproduced by other
ways. We also establish that Floyd's trajectories manifest some ambiguities
related to the mathematical choice of the couple of solutions of
Schr\"odinger's equation.


title:
A balanced gated-mode photon detector for qubit discrimination in 1550
  nm

abstract:
  A photon detector combining the two avalanche photon diodes (APD) has been
demonstrated for qubit discrimination in 1550 nm. Spikes accompanied with the
signals in gated-mode were canceled by balanced output from the two APDs. The
spike cancellation enabled one to reduce the threshold in the discriminators,
and thus the gate pulse voltage. The dark count probability and afterpulse
probability were reduced to 7x10^-7 and 10^-4, respectively, without affecting
the detection efficiency (11 %) at 178 K.


title: On Error Exponents in Quantum Hypothesis Testing

abstract:
  In the simple quantum hypothesis testing problem, upper bounds on the error
probabilities are shown based on a key operator inequality between a density
operator and its pinching. Concerning the error exponents, the upper bounds
lead to a noncommutative analogue of the Hoeffding bound, which is identical
with the classical counter part if the hypotheses, composed of two density
operators, are mutually commutative. The upper bounds also provide a simple
proof of the direct part of the quantum Stein's lemma.


title: Quantum theory of heating of a single trapped ion

abstract:
  The heating of trapped ions due to the interaction with a {\it quantized
environment} is studied {\it without performing the Born-Markov approximation}.
A generalized master equation local in time is derived and a novel theoretical
approach to solve it analytically is proposed. Our master equation is in the
Lindblad form with time dependent coefficients, thus allowing the simulation of
the dynamics by means of the Monte Carlo Wave Function (MCWF) method.


title: On the additional invariance of the Dirac and Maxwell equations

abstract:
  In this note we show that there exists a new set of operators {Q} (this set
is different from the operators which satisfy the Lie algebra of the Poincare
group P(1,3) with respect to which the Dirac and Maxwell equations are
invariant. We shall give the detailed proof of our assertions only for the
Dirac equation, since for the Maxwell equations all the assertions are proved
analogously.


title: On a motion equation for two particles in relativistic quantum mechanics

abstract:
  The purpose of the present note is to propose, in the framework of
relativistic quantum mechanics, a new Poincare-invariant equation for two
particles with masses m_1, m_2 and spin s_1=s_2=1/2. It is a first-order linear
differential equation for the eight-component wave function. With the help of
this equation the description of the motion of two-particle systems is reduced
to the description of one-particle systems in the (1+6)-dimensional Minkowski
space which can be in two spin states (s=0 or s=1).


title: Interaction of bimodal fields with few-level atoms in cavities and traps

abstract:
  The spectacular experimental results of the last few years in cavity quantum
electrodynamics and trapped ions research has led to very high level laboratory
performances. Such a stimulating situation essentially stems from two decisive
advancements. The first is the invention of reliable protocols for the
manipulation of single atoms. The second is the ability to produce desired
bosonic environments on demand. These progresses have led to the possibility of
controlling the form of the coupling between individual atoms and an arbitrary
number of bosonic modes. As a consequence, fundamental matter-radiation
interaction models like, for instance, the JC model and most of its numerous
nonlinear multiphoton generalizations, have been realized or simulated in
laboratory and their dynamical features have been tested more or less in
detail. This topical paper reviews the state of the art of the theoretical
investigations and of the experimental observations concerning the dynamical
features of the coupling between single few-level atoms and two bosonic modes.
In the course of the paper we show that such a configuration provides an
excellent platform for investigating various quantum intermode correlation
effects tested or testable in the cavity quantum electrodynamics and trapped
ion experimental realms. In particular we discuss a mode-mode correlation
effect appearing in the dynamics of a two-level atom quadratically coupled to
two bosonic modes. This effect, named parity effect, consists in a high
sensitivity to the evenness or oddness of the total number of bosonic
excitations.


title: Poincare-invariant equations with a rising mass spectrum

abstract:
  In this note we shall construct, in the framework of relativistic quantum
mechanics, the Poincare-invariant motion equations with realistic mass spectra.
These equations describe a system with mass spectra of the form $m^2=a^2+b^2
s(s+1)$, where a and b are arbitrary parameters. Such equations are obtained by
a reduction of the motion equation for two particles to a one-particle equation
which describes the particle in various mass and spin states. It we impose a
certain condition on the wave function of the derived equation, such an
equation describes the free motion of a fixed-mass particle with arbitrary (but
fixed) spin s.


title:
Once again on the `demythologizing' of the clock-in-the-box debate of
  Bohr and Einstein

abstract:
  The reply of de la Torre, Daleo and Garcia-Mata [Eur. J. Phys. 23 (2002)
L15-L16] to a criticism of their `demythologizing' analysis of the
clock-in-the-box debate between Einstein and Bohr is commented on.


title: Dynamical fidelity of a solid-state quantum computation

abstract:
  In this paper we analyze the dynamics in a spin-model of quantum computer.
Main attention is paid to the dynamical fidelity (associated with dynamical
errors) of an algorithm that allows to create an entangled state for remote
qubits. We show that in the regime of selective resonant excitations of qubits
there is no any danger of quantum chaos. Moreover, in this regime a modified
perturbation theory gives an adequate description of the dynamics of the
system. Our approach allows to explicitly describe all peculiarities of the
evolution of the system under time-dependent pulses corresponding to a quantum
protocol. Specifically, we analyze, both analytically and numerically, how the
fidelity decreases in dependence on the model parameters.


title: Hydrogenic Spin Quantum Computing in Silicon: A Digital Approach

abstract:
  We suggest an architecture for quantum computing with spin-pair encoded
qubits in silicon. Electron-nuclear spin-pairs are controlled by a dc magnetic
field and electrode-switched on and off hyperfine interaction. This digital
processing is insensitive to tuning errors and easy to model. Electron
shuttling between donors enables multi-qubit logic. These hydrogenic spin
qubits are transferable to nuclear spin-pairs, which have long coherence times,
and electron spin-pairs, which are ideally suited for measurement and
initialization. The architecture is scalable to highly parallel operation.


title:
Anomalous power law of quantum reversibility for classically regular
  dynamics

abstract:
  The Loschmidt Echo M(t) (defined as the squared overlap of wave packets
evolving with two slightly different Hamiltonians) is a measure of quantum
reversibility. We investigate its behavior for classically quasi-integrable
systems. A dominant regime emerges where M(t) ~ t^{-alpha} with alpha=3d/2
depending solely on the dimension d of the system. This power law decay is
faster than the result ~ t^{-d} for the decay of classical phase space
densities.


title: Can quantum vacuum fluctuations be considered real?

abstract:
  The main argument against the assumption that quantum fluctuations of the
electromagnetic field are real is that they do not activate photon detectors.
In order to met this objection I study several models of photon counter
compatible with the reality of the fluctuations. The models predict a nonlinear
dependence of the counting rate with the light intensity and the existence of a
nonthermal dark rate, results which might explain the difficulty for performing
loophole-free optical tests of Bell's inequality.


title: Photon counting with loop detector

abstract:
  We propose a design for a photon counting detector capable of resolving
multiphoton events. The basic element of the setup is a fiber loop, which traps
the input field with the help of a fast electrooptic switch. A single weakly
coupled avalanche photodiode is used to detect small portions of the signal
field extracted from the loop. We analyze the response of the loop detector to
an arbitrary input field, and discuss both the reconstruction of the photon
number distribution of an unknown field from the count statistics measured in
the setup, and the application of the detector in conditional state
preparation.


title: Quantum Teleportation of Superposition State for Squeezed States

abstract:
  This paper proposes a scheme for teleporting an arbitrary coherent
superposition state of two equal-amplitude and opposite-phase squeezed vacuum
states (SVS) via a symmetric 50/50 beam splitter and photodetectors. It is
shown that the quantum teleportation scheme has the successful probability 1/4.
Maximally entangled SVS's are used as quantum channel for realizing the
teleportation scheme. It is shown that if an initial quantum channel is in a
pure but not maximally entangled SVS, the quantum channel may be distilled to a
maximally entangled SVS through entanglement concentration.


title: Time Contortions in Modern Physics

abstract:
  As a basis for epistemological study of ``time,'' we analyze three suspect
phenomena introduced by modern physics: non-locality, asymmetric aging and
advanced interaction. It is shown that all three arise in connection with what
has to be taken as arbitrary ideosyncrasies in formulation. It is shown that
minor changes result in internally consistent variations of both Quantum
Mechanics and Special Relativity devoid of these phenomena. The
reinterpretation of some experiments though to confirm the existence of
non-locality and asymmetric aging is briefly considered and a possible test is
proposed.


title:
Functional Bell inequalities can serve as a stronger entanglement
  witness

abstract:
  We consider a Bell inequality for a continuous range of settings of the
apparatus at each site. This "functional" Bell inequality gives a better range
of violation for generalized GHZ states. Also a family of N-qubit bound
entangled states violate this inequality for N>5.


title:
Experimental preparation of Werner state via spontaneous parametric
  down-conversion

abstract:
  We present an experiment of preparing Werner state via spontaneous parametric
down-conversion and controlled decoherence of photons in this paper. In this
experiment two independent BBO (beta-barium borate) crystals are used to
produce down-conversion light beams, which are mixed to prepare Werner state.


title: Optimal Guessing Strategies in a Quantum Card Game

abstract:
  Three different quantum cards which are non-orthogonal quantum bits are sent
to two different players, Alice and Bob, randomly. Alice receives one of the
three cards, and Bob receives the remaining two cards. We find that Bob could
know better than Alice does on guessing Alice's card, no matter what Bob
chooses to measure his two cards collectively or separately. We also find that
Bob's best strategy for guessing Alice's card is to measure his two cards
collectively.


title: A dilemma in representing observables in quantum mechanics

abstract:
  There are self-adjoint operators which determine both spectral and
semispectral measures. These measures have very different commutativity and
covariance properties. This fact poses a serious question on the physical
meaning of such a self-adjoint operator and its associated operator measures.


title: How to mix a density matrix

abstract:
  A given density matrix may be represented in many ways as a mixture of pure
states. We show how any density matrix may be realized as a uniform ensemble.
It has been conjectured that one may realize all probability distributions that
are majorized by the vector of eigenvalues of the density matrix. We show that
if the states in the ensemble are assumed to be distinct then it is not true,
but a marginally weaker statement may still be true.


title:
Quantum Cryptography Based On Bell Inequalities for Three-Dimensional
  System

abstract:
  We present a crytographic protocol based upon entangled qutrit pairs. We
analyse the scheme under a symmetric incoherent attack and plot the region for
which the protocol is secure and compare this with the region of violations of
certain Bell inequalities.


title: The entangling power of passive optical elements

abstract:
  We investigate the entangling capability of passive optical elements, both
qualitatively and quantitatively. We present a general necessary and sufficient
condition for the possibility of creating distillable entanglement in an
arbitrary multi-mode Gaussian state with the help of passive optical elements,
thereby establishing a general connection between squeezing and the
entanglement that is attainable by non-squeezing operations. Special attention
is devoted to general two-mode Gaussian states, for which we provide the
optimal entangling procedure, present an explicit formula for the attainable
degree of entanglement measured in terms of the logarithmic negativity, and
discuss several practically important special cases.


title: Inequalities that test locality in quantum mechanics

abstract:
  Quantum theory violates Bell's inequality, but not to the maximum extent that
is logically possible. We derive inequalities (generalizations of Cirel'son's
inequality) that quantify the upper bound of the violation, both for the
standard formalism and the formalism of generalized observables (POVMs). These
inequalities are quantum analogues of Bell inequalities, and they can be used
to test the quantum version of locality. We discuss the nature of this kind of
locality. We also go into the relation of our results to an argument by Popescu
and Rohrlich (Found. Phys. 24, 379 (1994)) that there is no general connection
between the existence of Cirel'son's bound and locality.


title: Fidelity of Quantum Teleportation through Noisy Channels

abstract:
  We investigate quantum teleportation through noisy quantum channels by
solving analytically and numerically a master equation in the Lindblad form. We
calculate the fidelity as a function of decoherence rates and angles of a state
to be teleported. It is found that the average fidelity and the range of states
to be accurately teleported depend on types of noise acting on quantum
channels. If the quantum channels is subject to isotropic noise, the average
fidelity decays to 1/2, which is smaller than the best possible value 2/3
obtained only by the classical communication. On the other hand, if the noisy
quantum channel is modeled by a single Lindblad operator, the average fidelity
is always greater than 2/3.


title:
A Family of Quantum Stabilizer Codes Based on the Weyl Commutation
  Relations over a Finite Field

abstract:
  Using the Weyl commutation relations over a finite field we introduce a
family of error-correcting quantum stabilizer codes based on a class of
symmetric matrices over the finite field satisfying certain natural conditions.
When the field is GF(2) the existence of a rich class of such symmetric
matrices is demonstrated by a simple probabilistic argument depending on the
Chernoff bound for i.i.d symmetric Bernoulli trials. If, in addition, these
symmetric matrices are assumed to be circulant it is possible to obtain
concrete examples by a computer program. The quantum codes thus obtained admit
elegant encoding circuits.


title:
Einstein was right: Proof of absence of spooky state reduction in
  quantum mechanics

abstract:
  The present standard interpretation of quantum mechanics invokes nonlocality
and state reduction at space-like separated points during measurements on
entangled systems. While there is no understanding of the physical mechanism of
such nonlocal state reduction, the experimental verifications of quantum
correlations different from that predicted by local realistic theories have
polarized the physicists' opinion in favour of nonlocality. I show conclusively
that there is no such spooky state reduction, vindicating the strong views
against nonlocality held by Einstein and Popper. Experimental support to this
proof is also discussed. The Bell's inequalities arise due to ignoring the
phase information in the correlation function and not due to nonlocality. This
result goes against the current belief of quantum nonlocality held by the
majority of physicists; yet the proof is transparent and rigorous, and
therefore demands a change in the interpretation of quantum mechanics and
quantum measurements. The hypothesis of wave function collapse is inconsistent
with experimental observations on entangled correlated systems.


title: Quantum mechanics of a free particle on a pointed plane revisited

abstract:
  The detailed study of a quantum free particle on a pointed plane is
performed. It is shown that there is no problem with a mysterious ``quantum
anticentrifugal force" acting on a free particle on a plane discussed in a very
recent paper: M. A. Cirone et al, Phys. Rev. A 65, 022101 (2002), but we deal
with a purely topological efect related to distinguishing a point on a plane.
The new results are introduced concerning self-adjoint extensions of operators
describing the free particle on a pointed plane as well as the role played by
discrete symmetries in the analysis of such extensions.


title: Dynamics of entanglement between two trapped atoms

abstract:
  We investigate the dynamics of entanglement between two continuous variable
quantum systems. The model system consists of two atoms in a harmonic trap
which are interacting by a simplified s-wave scattering. We show, that the
dynamically created entanglement changes in a steplike manner. Moreover, we
introduce local operators which allow us to violate a Bell-CHSH inequality
adapted to the continuous variable case. The correlations show nonclassical
behavior and almost reach the maximal quantum mechanical value. This is
interesting since the states prepared by this interaction are very different
from any EPR-like state.


title: Can the game be quantum?

abstract:
  The game in which acts of participants don't have an adequate description in
terms of Boolean logic and classical theory of probabilities is considered. The
model of the game interaction is constructed on the basis of a non-distributive
orthocomplemented lattice. Mixed strategies of the participants are calculated
by the use of probability amplitudes according to the rules of quantum
mechanics. A scheme of quantization of the payoff function is proposed and an
algorithm for the search of Nash equilibrium is given. It is shown that
differently from the classical case in the quantum situation a discrete set of
equilibrium is possible.


title: Zeroth WKB Approximation in Quantum Mechanics

abstract:
  Solution of the Schr\"odinger's equation in the zero order WKB approximation
is analyzed. We observe and investigate several remarkable features of the
WKB$_0$ method. Solution in the whole region is built with the help of simple
connection formulas we derive from basic requirements of continuity and
finiteness for the wave function in quantum mechanics. We show that, for
conservative quantum systems, not only total energy, but also momentum is the
constant of motion. We derive the quantization conditions for two and more
turning point problems. Exact energy eigenvalues for solvable and some
``insoluble'' potentials are obtained. The eigenfunctions have the form of a
standing wave, $A_n\cos(k_nx+\delta_n)$, and are the asymptote of the exact
solution.


title:
Relativistic coherent states and charge structure of the coordinate and
  momentum operators

abstract:
  We consider relativistic coherent states for a spin-0 charged particle that
satisfy the next additional requirements: (i) the expected values of the
standard coordinate and momentum operators are uniquely related to the real and
imaginary parts of the coherent state parameter; (ii) these states contain only
one charge component. Three cases are considered: free particle, relativistic
rotator, and particle in a constant homogeneous magnetic field. For the
rotational motion of the two latter cases, such a description leads to the
appearance of the so-called nonlinear coherent states.


title: Bounds and enhancements for the Hartman effect

abstract:
  The time of passage of the transmitted wave packet in a tunneling collision
of a quantum particle with a square potential barrier becomes independent of
the barrier width in a range of barrier thickness. This is the Hartman effect,
which has been frequently associated with ``superluminality''. A fundamental
limitation on the effect is set by non-relativistic ``causality conditions''.
We demonstrate first that the causality conditions impose more restrictive
bounds on the negative time delays (time advancements) when no bound states are
present. These restrictive bounds are in agreement with a naive, and generally
false, causality argument based on the positivity of the ``extrapolated phase
time'', one of the quantities proposed to characterize the duration of the
barrier's traversal. Nevertheless, square wells may in fact lead to much larger
advancements than square barriers. We point out that close to thresholds of new
bound states the time advancement increases considerably, while, at the same
time, the transmission probability is large, which facilitates the possible
observation of the enhanced time advancement.


title: Exact isolated solutions for the two-photon Rabi Hamiltonian

abstract:
  The two-photon Rabi Hamiltonian is a simple model describing the interaction
of light with matter, with the interaction being mediated by the exchange of
two photons. Although this model is exactly soluble in the rotating-wave
approximation, we work with the full Hamiltonian, maintaining the
non-integrability of the model. We demonstrate that, despite this
non-integrability, there exist isolated, exact solutions for this model
analogous to the so-called Juddian solutions found for the single-photon Rabi
Hamiltonian. In so doing we use a Bogoliubov transformation of the field mode,
as described by the present authors in an earlier publication.


title: A general scheme for ensemble purification

abstract:
  We exhibit a general procedure to purify any given ensemble by identifying an
appropriate interaction between the physical system S of the ensemble and the
reference system K. We show that the interaction can be chosen in such a way to
lead to a spatial separation of the pair S-K. As a consequence, one can use it
to prepare at a distance different equivalent ensembles. The argument
associates a physically precise procedure to the purely formal and fictitious
process usually considered in the literature. We conclude with an illuminating
example taken from quantum computational theory.


title: Casimir effect for the sphere revisited

abstract:
  In a recent work Brevik \emph{et al.} have offered formal proofs of two
results which figure prominently in calculations of the Casimir pressure on a
sphere. It is shown by means of simple counterexamples that each of those
proofs is necessarily incorrect.


title:
An Information-Spectrum Approach to Classical and Quantum Hypothesis
  Testing for Simple Hypotheses

abstract:
  The information-spectrum analysis made by Han for classical hypothesis
testing for simple hypotheses is extended to a unifying framework including
both classical and quantum hypothesis testing as well as fixed-length source
coding, whereby general formulas for several quantities concerning the
asymptotic optimality of tests/codes are established in terms of classical and
quantum information spectrum. Generality of theorems and simplicity of proofs
are fully pursued, and as byproducts some improvements on the original
classical results are also obtained.


title: General formulas for capacity of classical-quantum channels

abstract:
  The capacity of a classical-quantum channel (or in other words the classical
capacity of a quantum channel) is considered in the most general setting, where
no structural assumptions such as the stationary memoryless property are made
on a channel. A capacity formula as well as a characterization of the strong
converse property is given just in parallel with the corresponding classical
results of Verd\'{u}-Han which are based on the so-called information-spectrum
method. The general results are applied to the stationary memoryless case with
or without cost constraint on inputs, whereby a deep relation between the
channel coding theory and the hypothesis testing for two quantum states is
elucidated. no structural assumptions such as the stationary memoryless
property are made on a channel. A capacity formula as well as a
characterization of the strong converse property is given just in parallel with
the corresponding classical results of Verdu-Han which are based on the
so-called information-spectrum method. The general results are applied to the
stationary memoryless case with or without cost constraint on inputs, whereby a
deep relation between the channel coding theory and the hypothesis testing for
two quantum states is elucidated.


title: General formulas for fixed-length quantum entanglement concentration

abstract:
  General formulas of entanglement concentration are derived by using an
information-spectrum approach for the i.i.d. sequences and the general
sequences of partially entangled pure states. That is, we derive general
relations between the performance of the entanglement concentration and the
eigenvalues of the partially traced state. The achievable rates with constant
constraints and those with exponential constraints can be calculated from these
formulas.


title:
New q-deformed coherent states with an explicitly known resolution of
  unity

abstract:
  We construct a new family of q-deformed coherent states $|z>_q$, where $0 < q
< 1$. These states are normalizable on the whole complex plane and continuous
in their label $z$. They allow the resolution of unity in the form of an
ordinary integral with a positive weight function obtained through the analytic
solution of the associated Stieltjes power-moment problem and expressed in
terms of one of the two Jacksons's $q$-exponentials. They also permit exact
evaluation of matrix elements of physically-relevant operators. We use this to
show that the photon number statistics for the states is sub-Poissonian and
that they exhibit quadrature squeezing as well as an enhanced signal-to-quantum
noise ratio over the conventional coherent state value. Finally, we establish
that they are the eigenstates of some deformed boson annihilation operator and
study some of their characteristics in deformed quantum optics.

categories: quant-ph math-ph math.MP math.QA physics.optics


title: Decoherence in resonantly driven bistable systems

abstract:
  We study dissipative tunneling in a double well potential that is driven
close to a resonance between the lowest tunnel doublet and a singlet. While the
coherent dynamics can be described well within a three-level approximation,
dissipative transitions to levels outside the singlet and the doublet may play
a crucial role. Moreover, such transitions can enhance the entropy production
significantly.


title:
An experiment that proves the absence of nonlocality in quantum
  mechanics

abstract:
  I have been arguing that quantum nonlocality, deeply entrenched in the
present formalism of quantum mechanics and widely believed as a reality by
physicists, is in fact absent. Spooky nonlocal state reduction is the most, and
perhaps the only irrational feature of present day physics. There are
experimental results that reject nonlocal state reduction at a distance. Also,
there are arguments that show that signal locality itself can be violated if
there is true nonlocal collapse of the wavefunction. The Bell's inequalities,
the violation of which polarized physicists in favour of nonlocality, arise not
due to nonlocality, but due to ignoring prior information on correlations
encoded in the phase of local probability amplitudes. Here I discuss an
experiment involving particles entangled in energy and time variables that
shows that there is no nonlocal state reduction during measurements on
entangled particles. Quantum mechanics is inconsistent if it includes the
concept of wavefunction collapse as a physical process for entangled
multi-particle systems.


title:
All Optical Implementation of Multi-Spin Entanglement in a Semiconductor
  Quantum Well

abstract:
  We use ultrafast optical pulses and coherent techniques to create spin
entangled states of non-interacting electrons bound to donors (at least three)
and at least two Mn2+ ions in a CdTe quantum well. Our method, relying on the
exchange interaction between localized excitons and paramagnetic impurities,
can in principle be applied to entangle a large number of spins.


title: Local vs. Joint Measurements for the Entanglement of Assistance

abstract:
  We consider a variant of the entanglement of assistance, as independently
introduced by D.P. DiVincenzo {\em et al.} ({\tt quant-ph/9803033}) and O.
Cohen (Phys. Rev. Lett. {\bf 80}, 2493 (1998)). Instead of considering
three-party states in which one of the parties, the assistant, performs a
measurement such that the remaining two parties are left with on average as
much entanglement as possible, we consider four-party states where two parties
play the role of assistants. We answer several questions that arise naturally
in this scenario, such as (i) how much more entanglement can be produced when
the assistants are allowed to perform joint measurements, (ii) for what type of
states are local measurements sufficient, (iii) is it necessary for the second
assistant to know the measurement outcome of the first, and (iv) are projective
measurements sufficient or are more general POVMs needed?