title: Primitive Quantum BCH Codes over Finite Fields

abstract:
  An attractive feature of BCH codes is that one can infer valuable information
from their design parameters (length, size of the finite field, and designed
distance), such as bounds on the minimum distance and dimension of the code. In
this paper, it is shown that one can also deduce from the design parameters
whether or not a primitive, narrow-sense BCH contains its Euclidean or
Hermitian dual code. This information is invaluable in the construction of
quantum BCH codes. A new proof is provided for the dimension of BCH codes with
small designed distance, and simple bounds on the minimum distance of such
codes and their duals are derived as a consequence. These results allow us to
derive the parameters of two families of primitive quantum BCH codes as a
function of their design parameters.


title:
Nonequilibrium dynamics of moving mirrors in quantum fields: Influence
  functional and Langevin equation

abstract:
  We employ the Schwinger-Keldysh formalism to study the nonequilibrium
dynamics of the mirror with perfect reflection moving in a quantum field.
Within the regime of linear response in terms of a first order expansion of the
mirror's displacement, the coarse-grained effective action is obtained by
integrating out quantum fields with the method of influence functional. The
semiclassical Langevin equation is derived, and is found to involve two levels
of backreaction effects on the dynamics of mirrors: radiation reaction induced
by the motion of the mirror and backreaction dissipation arising from
fluctuations in quantum fields via a fluctuation-dissipation relation. Although
the theorem of fluctuation and dissipation in linear response theory is of
model independence, the study from the first principles derivation shows that
the obtained theorem is also {\it independent} of the regulators introduced to
deal with short-distance divergences from quantum fields. Thus, when the method
of regularization is introduced to compute the dissipation and fluctuation
effects, this theorem must be fulfilled as the results are obtained by taking
the short-distance limit in the end of calculations. The backreaction effects
from vacuum fluctuations on moving mirrors are found to be hardly detected
while those effects from thermal fluctuations may be detectable.


title:
Effect of cavity photons on the generation of multi-particle
  entanglement

abstract:
  A study on the cause of the multi-particle entanglement is presented in this
work. We investigate how dot-like single quantum well excitons, which are
independently coupled through a single microcavity mode, evolve into maximally
entangled state as a series of conditional measurements are taken on the cavity
field state. We first show how cavity photon affects the entanglement purity of
the double-exciton Bell state and the triple-exciton W state. Generalization to
multi-excitons W states is then derived analytically. Our results pave the way
for studying the crucial cause of multi-particle collective effect.


title:
Symmetric multiparty-controlled teleportation of an arbitrary
  two-particle entanglement

abstract:
  We present a way for symmetric multiparty-controlled teleportation of an
arbitrary two-particle entangled state based on Bell-basis measurements by
using two Greenberger-Horne-Zeilinger states, i.e., a sender transmits an
arbitrary two-particle entangled state to a distant receiver, an arbitrary one
of the $n+1$ agents via the control of the others in a network. It will be
shown that the outcomes in the cases that $n$ is odd or it is even are
different in principle as the receiver has to perform a controlled-not
operation on his particles for reconstructing the original arbitrary entangled
state in addition to some local unitary operations in the former. Also we
discuss the applications of this controlled teleporation for quantum secret
sharing of classical and quantum information. As all the instances can be used
to carry useful information, its efficiency for qubits approaches the maximal
value.


title: Quantum secure conditional direct communication via EPR pairs

abstract:
  Two schemes for quantum secure conditional direct communication are proposed,
where a set of EPR pairs of maximally entangled particles in Bell states,
initially made by the supervisor Charlie, but shared by the sender Alice and
the receiver Bob, functions as quantum information channels for faithful
transmission. After insuring the security of the quantum channel and obtaining
the permission of Charlie (i.e. Charlie is trustworthy and cooperative, which
means the 'conditional' in the two schemes), Alice and Bob begin their private
communication under the control of Charlie. In the first scheme, Alice
transmits secret message to Bob in a deterministic manner with the help of
Charlie by means of Alice's local unitary transformations, both Alice and Bob's
local measurements, and both of Alice and Charlie's public classical
communication. In the second scheme, the secure communication between Alice and
Bob can be achieved via public classical communication of Charlie and Alice,
and the local measurements of both Alice and Bob. The common feature of these
protocols is that the communications between two communication parties Alice
and Bob depend on the agreement of the third side Charlie. Moreover,
transmitting one bit secret message, the sender Alice only needs to apply a
local operation on her one qubit and send one bit classical information. We
also show that the two schemes are completely secure if quantum channels are
perfect.


title:
On the Probabilistic Compatibility of Special Relativity and Quantum
  Mechanics

abstract:
  In this paper we introduce the three main notions of probability used by
physicists and discuss how these are to be used when invoking spacelike
separated observers in a relativistic format. We discuss a standard EPRB
experiment and concentrate upon problems of the interpretation of
probabilities. We promote a particularly conservative interpretation of this
experiment (which need not invoke an objective notion of collapse) where
probabilities are, tentatively, passively Lorentz invariant. We also argue that
the Heisenberg picture is preferable in relativistic situations due to a
conflict between the Schrodinger picture and passive Lorentz transformations of
probabilities. Throughout most of this paper we discuss the relative frequency
interpretation of probability as this is most commonly used. We also introduce
the logically necessary notion of `prior-frequency' in discussing whether the
choice by an observer can have any causal effect upon the measurement results
of another. We also critically examine the foundational use of relative
frequency in no-signalling theorems. We argue that SQT and SR are
probabilistically compatible, although we do not discuss whether they are
compatible on the level of individual events.


title: Entanglement Teleportation Through 1D Heisenberg Chain

abstract:
  Information transmission of two qubits through two independent 1D Heisenberg
chains as a quantum channel is analyzed. It is found that the entanglement of
two spin-$\frac 12$ quantum systems is decreased during teleportation via the
thermal mixed state in 1D Heisenberg chain. The entanglement teleportation will
be realized if the minimal entanglement of the thermal mixed state is provided
in such quantum channel. High average fidelity of teleportation with values
larger than 2/3 is obtained when the temperature {\it T} is very low. The
mutual information $\mathcal{I}$ of the quantum channel declines with the
increase of the temperature and the external magnetic field. The entanglement
quality of input signal states cannot enhance mutual information of the quantum
channel.


title: Quantum mechanics as "space-time statistical mechanics"?

abstract:
  In this paper we discuss and analyse the idea of trying to see
(non-relativistic) quantum mechanics as a ``space-time statistical mechanics'',
by using the classical statistical mechanical method on objective microscopic
space-time configurations. It is argued that this could perhaps be accomplished
by giving up the assumption that the objective ``state'' of a system is
independent of a future measurement performed on the system. This idea is then
applied in an example of quantum state estimation on a qubit system.


title: Popper's test of Quantum Mechanics

abstract:
  A test of quantum mechanics proposed by K. Popper and dealing with
two-particle entangled states emitted from a fixed source has been criticized
by several authors. Some of them claim that the test becomes inconclusive once
all the quantum aspects of the source are considered. Moreover, another
criticism states that the predictions attributed to quantum mechanics in
Popper's analysis are untenable. We reconsider these criticisms and show that,
to a large extend, the `falsifiability' potential of the test remains
unaffected.


title: A theory of quantum gravity based on quantum computation

abstract:
  This paper proposes a method of unifying quantum mechanics and gravity based
on quantum computation. In this theory, fundamental processes are described in
terms of pairwise interactions between quantum degrees of freedom. The geometry
of space-time is a construct, derived from the underlying quantum information
processing. The computation gives rise to a superposition of four-dimensional
spacetimes, each of which obeys the Einstein-Regge equations. The theory makes
explicit predictions for the back-reaction of the metric to computational
`matter,' black-hole evaporation, holography, and quantum cosmology.


title:
Multi-Instantons and Exact Results I: Conjectures, WKB Expansions, and
  Instanton Interactions

abstract:
  We consider specific quantum mechanical model problems for which perturbation
theory fails to explain physical properties like the eigenvalue spectrum even
qualitatively, even if the asymptotic perturbation series is augmented by
resummation prescriptions to "cure" the divergence in large orders of
perturbation theory. Generalizations of perturbation theory are necessary which
include instanton configurations, characterized by nonanalytic factors
exp(-a/g) where a is a constant and g is the coupling. In the case of
one-dimensional quantum mechanical potentials with two or more degenerate
minima, the energy levels may be represented as an infinite sum of terms each
of which involves a certain power of a nonanalytic factor and represents itself
an infinite divergent series. We attempt to provide a unified representation of
related derivations previously found scattered in the literature. For the
considered quantum mechanical problems, we discuss the derivation of the
instanton contributions from a semi-classical calculation of the corresponding
partition function in the path integral formalism. We also explain the relation
with the corresponding WKB expansion of the solutions of the Schroedinger
equation, or alternatively of the Fredholm determinant det(H-E) (and some
explicit calculations that verify this correspondence). We finally recall how
these conjectures naturally emerge from a leading-order summation of
multi-instanton contributions to the path integral representation of the
partition function. The same strategy could result in new conjectures for
problems where our present understanding is more limited.


title:
Multi-Instantons and Exact Results II: Specific Cases, Higher-Order
  Effects, and Numerical Calculations

abstract:
  In this second part of the treatment of instantons in quantum mechanics, the
focus is on specific calculations related to a number of quantum mechanical
potentials with degenerate minima. We calculate the leading multi-instanton
constributions to the partition function, using the formalism introduced in the
first part of the treatise [J. Zinn-Justin and U. D. Jentschura, e-print
quant-ph/0501136]. The following potentials are considered: (i) asymmetric
potentials with degenerate minima, (ii) the periodic cosine potential, (iii)
anharmonic oscillators with radial symmetry, and (iv) a specific potential
which bears an analogy with the Fokker-Planck equation. The latter potential
has the peculiar property that the perturbation series for the ground-state
energy vanishes to all orders and is thus formally convergent (the ground-state
energy, however, is nonzero and positive). For the potentials (ii), (iii), and
(iv), we calculate the perturbative B-function as well as the instanton
A-function to fourth order in g. We also consider the double-well potential in
detail, and present some higher-order analytic as well as numerical
calculations to verify explicitly the related conjectures up to the order of
three instantons. Strategies analogous to those outlined here could result in
new conjectures for problems where our present understanding is more limited.


title:
Self-induced decoherence approach: Strong limitations on its validity in
  a simple spin bath model and on its general physical relevance

abstract:
  The "self-induced decoherence" (SID) approach suggests that (1) the
expectation value of any observable becomes diagonal in the eigenstates of the
total Hamiltonian for systems endowed with a continuous energy spectrum, and
(2), that this process can be interpreted as decoherence. We evaluate the first
claim in the context of a simple spin bath model. We find that even for large
environments, corresponding to an approximately continuous energy spectrum,
diagonalization of the expectation value of random observables does in general
not occur. We explain this result and conjecture that SID is likely to fail
also in other systems composed of discrete subsystems. Regarding the second
claim, we emphasize that SID does not describe a physically meaningful
decoherence process for individual measurements, but only involves destructive
interference that occurs collectively within an ensemble of presupposed
"values" of measurements. This leads us to question the relevance of SID for
treating observed decoherence effects.


title:
New method to simulate quantum interference using deterministic
  processes and application to event-based simulation of quantum computation

abstract:
  We demonstrate that networks of locally connected processing units with a
primitive learning capability exhibit behavior that is usually only attributed
to quantum systems. We describe networks that simulate single-photon
beam-splitter and Mach-Zehnder interferometer experiments on a causal,
event-by-event basis and demonstrate that the simulation results are in
excellent agreement with quantum theory. We also show that this approach can be
generalized to simulate universal quantum computers.


title: Simulation of Quantum Computation: A deterministic event-based approach

abstract:
  We demonstrate that locally connected networks of machines that have
primitive learning capabilities can be used to perform a deterministic,
event-based simulation of quantum computation. We present simulation results
for basic quantum operations such as the Hadamard and the controlled-NOT gate,
and for seven-qubit quantum networks that implement Shor's numbering factoring
algorithm.


title:
Event-based simulation of single-photon beam splitters and Mach-Zehnder
  interferometers

abstract:
  We demonstrate that networks of locally connected processing units with a
primitive learning capability exhibit behavior that is usually only attributed
to quantum systems. We describe networks that simulate single-photon
beam-splitter and Mach-Zehnder interferometer experiments on a causal,
event-by-event basis and demonstrate that the simulation results are in
excellent agreement with quantum theory.


title: On Randomized and Quantum Query Complexities

abstract:
  We study randomized and quantum query (a.k.a. decision tree) complexity for
all total Boolean functions, with emphasis to derandomization and
dequantization (removing quantumness from algorithms). Firstly, we show that
$D(f) = O(Q_1(f)^3)$ for any total function $f$, where $D(f)$ is the minimal
number of queries made by a deterministic query algorithm and $Q_1(f)$ is the
number of queries made by any quantum query algorithm (decision tree analog in
quantum case) with one-sided constant error; both algorithms compute function
$f$. Secondly, we show that for all total Boolean functions $f$ holds
$R_0(f)=O(R_2(f)^2 \log N)$, where $R_0(f)$ and $R_2(f)$ are randomized
zero-sided (a.k.a Las Vegas) and two-sided (a.k.a. Monte Carlo) error query
complexities.


title: Comment on "Decoy State Quantum Key Distribution"

abstract:
  The main claim by H.K. Lo et al that they have for the first time made the
decoy-state method efficiently work in practice is inappropriate. We show that,
prior to our work, actually (and obviously) none of proposals raised by H.K. Lo
et al can really work in practice. Their main protocol requires infinite number
of different coherent states which is in principle impossible for any set-up.
Their idea of using very weak coherent state as decoy state doesn't work either
by our detailed analysis. The idea implicitly requires an unreasonablly large
number of pulses which needs at least 14 days to produce, if they want to do
QKD over a distance of 120-130km.


title: Continuous-Variable Spatial Entanglement for Bright Optical Beams

abstract:
  A light beam is said to be position squeezed if its position can be
determined to an accuracy beyond the standard quantum limit. We identify the
position and momentum observables for bright optical beams and show that
position and momentum entanglement can be generated by interfering two
position, or momentum, squeezed beams on a beam splitter. The position and
momentum measurements of these beams can be performed using a homodyne detector
with local oscillator of an appropriate transverse beam profile. We compare
this form of spatial entanglement with split detection-based spatial
entanglement.


title: An elementary formula for entanglement entropies of fermionic systems

abstract:
  An elementary formula for the von Neumann and Renyi entropies describing
quantum correlations in two-fermionic systems having four single particle
states is presented. An interesting geometric structure of fermionic
entanglement is revealed. A connection with the generalized Pauli principle is
established.


title: Thermal entanglement of bosonic modes

abstract:
  We study the change of entanglement under general linear transformation of
modes in a bosonic system and determine the conditions under which entanglement
can be generated under such transformation. As an example we consider the
thermal entanglement between the vibrational modes of two coupled oscillators
and determine the temperature above which quantum correlations are destroyed by
thermal fluctuations.


title:
Surface-Electrode Architecture for Ion-Trap Quantum Information
  Processing

abstract:
  We investigate a surface-mounted electrode geometry for miniature linear
radio frequency Paul ion traps. The electrodes reside in a single plane on a
substrate, and the pseudopotential minimum of the trap is located above the
substrate at a distance on order of the electrodes' lateral extent or
separation. This architecture provides the possibility to apply standard
microfabrication principles to the construction of multiplexed ion traps, which
may be of particular importance in light of recent proposals for large-scale
quantum computation based on individual trapped ions.


title: Heuristic explanation of quantum interference experiments

abstract:
  A particle is described as a non-spreading wave packet satisfying a linear
equation within the framework of special relativity. Young's and other
interference experiments are explained with a hypothesis that there is a
coupling interaction between the peaked and non-peaked pieces of the wave
packet. This explanation of the interference experiments provides a realistic
interpretation of quantum mechanics. The interpretation implies that there is
physical reality of particles and no wave function collapse. It also implies
that neither classical mechanics nor current quantum mechanics is a complete
theory for describing physical reality and the Bell inequalities are not the
proper touchstones for reality and locality. The problems of the boundary
between the macro-world and micro-world and the de-coherence in the transition
region (meso-world) between the two are discussed. The present interpretation
of quantum mechanics is consistent with the physical aspects of the Copenhagen
interpretation, such as, the superposition principle, Heisenberg's uncertainty
principle and Born's probability interpretation, but does not favor its
philosophical aspects, such as, non-reality, non-objectivity, non-causality and
the complementary principle.


title: Atomic spin decoherence near conducting and superconducting films

abstract:
  We derive scaling laws for the spin decoherence of neutral atoms trapped near
conducting and superconducting plane surfaces. A new result for thin films
sheds light on the measurement of Y.J. Lin, I. Teper, C. Chin, and V.
Vuleti\'{c} [Phys. Rev. Lett. \textbf{92}, 050404 (2004)]. Our calculation is
based on a quantum-theoretical treatment of electromagnetic radiation near
metallic bodies [P.K. Rekdal, S. Scheel, P.L. Knight, and E.A. Hinds, Phys.
Rev. A \textbf{70}, 013811 (2004)]. We show that there is a critical
atom-surface distance that maximizes the spin relaxation rate and we show how
this depends on the skin depth and thickness of the metal surface. In the light
of this impedance-matching effect we discuss the spin relaxation to be expected
above a thin superconducting niobium layer.


title: Linear-algebraic lambda-calculus

abstract:
  With a view towards models of quantum computation and/or the interpretation
of linear logic, we define a functional language where all functions are linear
operators by construction. A small step operational semantic (and hence an
interpreter/simulator) is provided for this language in the form of a term
rewrite system. The linear-algebraic lambda-calculus hereby constructed is
linear in a different (yet related) sense to that, say, of the linear
lambda-calculus. These various notions of linearity are discussed in the
context of quantum programming languages. KEYWORDS: quantum lambda-calculus,
linear lambda-calculus, $\lambda$-calculus, quantum logics.


title: Structuring quantum effects: superoperators as arrows

abstract:
  We show that the model of quantum computation based on density matrices and
superoperators can be decomposed in a pure classical (functional) part and an
effectful part modeling probabilities and measurement. The effectful part can
be modeled using a generalization of monads called arrows. We express the
resulting executable model of quantum computing in the programming language
Haskell using its special syntax for arrow computations. The embedding in
Haskell is however not perfect: a faithful model of quantum computing requires
type capabilities which are not directly expressible in Haskell.


title: A generalized skew information and uncertainty relation

abstract:
  A generalized skew information is defined and a generalized uncertainty
relation is established with the help of a trace inequality which was recently
proven by J.I.Fujii. In addition, we prove the trace inequality conjectured by
S.Luo and Z.Zhang. Finally we point out that Theorem 1 in {\it S.Luo and
Q.Zhang, IEEE Trans.IT, Vol.50, pp.1778-1782 (2004)} is incorrect in general,
by giving a simple counter-example.


title: A model of deterministic detector with dynamical decoherence

abstract:
  We discuss a deterministic model of detector coupled to a two-level system (a
qubit). The detector is a quasi-classical object whose dynamics is described by
the kicked rotator Hamiltonian. We show that in the regime of quantum chaos the
detector acts as a chaotic bath and induces decoherence of the qubit. We
discuss the dephasing and relaxation rates and demonstrate that several
features of Ohmic baths can be reproduced by our fully deterministic model.
Moreover, we show that, for strong enough qubit-detector coupling, the
dephasing rate is given by the rate of exponential instability of the
detector's dynamics, that is, by the Lyapunov exponent of classical motion.
Finally, we discuss the measurement in the regimes of strong and weak
qubit-detector coupling. For the case of strong coupling the detector performs
a measurement of the up/down state of the qubit. In the case of weak coupling,
due to chaos, the dynamical evolution of the detector is strongly sensitive to
the state of the qubit. However, in this case it is unclear how to extract a
signal from any measurement with a coarse-graining in the phase space on a size
much larger than the Planck cell.


title:
Decoherence-free creation of atom-atom entanglement in cavity via
  fractional adiabatic passage

abstract:
  We propose a robust and decoherence insensitive scheme to generate
controllable entangled states of two three-level atoms interacting with an
optical cavity and a laser beam. Losses due to atomic spontaneous transitions
and to cavity decay are efficiently suppressed by employing fractional
adiabatic passage and appropriately designed atom-field couplings. In this
scheme the two atoms traverse the cavity-mode and the laser beam in opposite
directions as opposed to other entanglement schemes in which the atoms are
required to have fixed locations inside a cavity. We also show that the
coherence of a traveling atom can be transferred to the other one without
populating the cavity-mode.


title: Boson Normal Ordering via Substitutions and Sheffer-type Polynomials

abstract:
  We solve the boson normal ordering problem for (q(a*)a + v(a*))^n with
arbitrary functions q and v and integer n, where a and a* are boson
annihilation and creation operators, satisfying [a,a*]=1. This leads to
exponential operators generalizing the shift operator and we show that their
action can be expressed in terms of substitutions. Our solution is naturally
related through the coherent state representation to the exponential generating
functions of Sheffer-type polynomials. This in turn opens a vast arena of
combinatorial methodology which is applied to boson normal ordering and
illustrated by a few examples.


title: The Casimir zero-point radiation pressure

abstract:
  We analyze some consequences of the Casimir-type zero-point radiation
pressure. These include macroscopic "vacuum" forces on a metallic layer
in-between a dielectric medium and an inert ($\epsilon (\omega) = 1$) one. Ways
to control the sign of these forces, based on dielectric properties of the
media, are thus suggested. Finally, the large positive Casimir pressure, due to
surface plasmons on thin metallic layers, is evaluated and discussed.


title: Quantum Weakest Preconditions

abstract:
  We develop a notion of predicate transformer and, in particular, the weakest
precondition, appropriate for quantum computation. We show that there is a
Stone-type duality between the usual state-transformer semantics and the
weakest precondition semantics. Rather than trying to reduce quantum
computation to probabilistic programming we develop a notion that is directly
taken from concepts used in quantum computation. The proof that weakest
preconditions exist for completely positive maps follows immediately from the
Kraus representation theorem. As an example we give the semantics of Selinger's
language in terms of our weakest preconditions. We also cover some specific
situations and exhibit an interesting link with stabilizers.


title:
Scheme to measure squeezing and phase properties of a harmonic
  oscillator

abstract:
  We propose a simple scheme to measure squeezing and phase properties of a
harmonic oscillator. We treat in particular the case of a the field, but the
scheme may be easily realized in ion traps. It is based on integral transforms
of measured atomic properties as atoms exit a cavity. We show that by measuring
atomic polarizations it is possible, after a given integration, to measure
several properties of the field.


title: Implausible Consequences of Superstrong Nonlocality

abstract:
  This Letter looks at the consequences of so-called 'superstrong nonlocal
correlations', which are hypothetical violations of Bell/CHSH inequalities that
are stronger than quantum mechanics allows, yet weak enough to prohibit
faster-than-light communication. It is shown that the existence of maximally
superstrong correlated bits implies that all distributed computations can be
performed with a trivial amount of communication, i.e. with one bit. If one
believes that Nature does not allow such a computational 'free lunch', then the
result in the Letter gives a reason why superstrong correlation are indeed not
possible.


title:
Continuous operation of a one-way quantum key distribution system over
  installed telecom fibre

abstract:
  We demonstrate a robust, compact and automated quantum key distribution
system, based upon a one-way Mach-Zender interferometer, which is actively
compensated for temporal drifts in the photon phase and polarization. The
system gives a superior performance to passive compensation schemes with an
average quantum bit error rate of 0.87% and a duty cycle of 99.6% for a
continuous quantum key distribution session of 19 hours over a 20.3km installed
telecom fibre. The results suggest that actively compensated QKD systems are
suitable for practical applications.


title: Histories Approach to Quantum Mechanics

abstract:
  These lecture notes cover the important developments in histories approach to
quantum mechanics with overall content and emphasis somewhat different from
other reviews and books on the subject.The idea of Houtappel, Van Dam and
Wigner of employing objects based on primitive concepts of physical theories is
discussed in some detail and the fact that histories are such objects is
emphasized. Application of histories formalism to the problem of understanding
the quasiclassical domain is treated in some detail. Other topics discussed
include generalized histories-based quantum mechanics and its application to
the quantum mechanics of space-time,generalization of the notion of time
sequences employing partial semigroups,quasitemporal structures, history
projection operator (HPO) formalism, the algebraic scheme of Isham and Linden,
an axiomatic scheme for quasitemporal histories-based theories and symmetries
and conservation laws in histories-based theories.


title: Bell's Inequality, Random Sequence, and Quantum Key Distribution

abstract:
  The Ekert 91 quantum key distribution (QKD) protocol appears to be secure
whatever devices legitimate users adopt for the protocol, as long as the
devices give a result that violates Bell's inequality. However, this is not the
case if they ignore non-detection events because Eve can make use of the
detection-loophole, as Larrson showed. We show that even when legitimate users
take into account non-detection events Eve can successfully eavesdrop if the
QKD system has been appropriately designed by the manufacturer. A loophole
utilized here is that of `free-choice' (or `real randomness'). Local QKD
devices with pseudo-random sequence generator installed in them can apparently
violate Bell's inequality.


title: Wigner function and Schroedinger equation in phase space representation

abstract:
  We discuss a family of quasi-distributions (s-ordered Wigner functions of
Agarwal and Wolf) and its connection to the so called phase space
representation of the Schroedinger equation. It turns out that although Wigner
functions satisfy the Schroedinger equation in phase space they have completely
different interpretation.


title:
Experimental observation of transient velocity-selective coherent
  population trapping in one dimension

abstract:
  We report the observation of transient velocity-selective coherent population
trapping (VSCPT) in a beam of metastable neon atoms. The atomic momentum
distribution resulting from the interaction with counterpropagating sigma_+ and
sigma_- radiation which couples a J_g=2 <-> J_e=1 transition is measured via
the transversal beam profile. This transition exhibits a stable VSCPT dark
state formed by the two |J=2,m=+/-1> states, and a metastable dark state
containing the |J=2,m=+/-2> and |J=2,m=0> states. The dynamics of the formation
and decay of stable and metastable dark states is studied experimentally and
numerically and the finite lifetime of the metastable dark state is
experimentally observed. We compare the measured distribution with a numerical
solution of the master equation.


title: Tunnelling of condensate magnetization in a double-well potential

abstract:
  We study quantum dynamical properties of a spin-1 atomic Bose-Einstein
condensate in a double-well potential. Adopting a mean field theory and single
spatial mode approximation, we characterize our model system as two coupled
spins. For certain initial states, we find full magnetization oscillations
between wells not accompanied by mass (or atom numbers) exchange. We identify
dynamic regimes of collective spin variables arising from nonlinear
self-interactions that are different from the usual Josephson oscillations. We
also discuss magnetization beats and incomplete oscillations of collective spin
variables other than the magnetization. Our study points to an alternative
approach to observe coherent tunnelling of a condensate through a (spatial)
potential barrier.


title:
Graph State Preparation and Cluster Computation with Global Addressing
  of Optical Lattices

abstract:
  We present a novel way to manipulate ultra-cold atoms where four atomic
levels are trapped by appropriately tuned optical lattices. When employed to
perform quantum computation via global control, this unique structure
dramatically reduces the number of steps involved in the control procedures,
either for the standard, network, model, or for one-way quantum computation.
The use of a far-blue detuned lattice and a magnetically insensitive
computational basis makes the scheme robust against decoherence. The present
scheme is a promising candidate for experimental implementation of quantum
computation and for graph state preparation in one, two or three spatial
dimensions.


title: Feynman's Path Integrals and Bohm's Particle Paths

abstract:
  Both Bohmian mechanics, a version of quantum mechanics with trajectories, and
Feynman's path integral formalism have something to do with particle paths in
space and time. The question thus arises how the two ideas relate to each
other. In short, the answer is, path integrals provide a re-formulation of
Schroedinger's equation, which is half of the defining equations of Bohmian
mechanics. I try to give a clear and concise description of the various aspects
of the situation.


title: Casimir-Polder interaction of atoms with magnetodielectric bodies

abstract:
  A general theory of the Casimir-Polder interaction of single atoms with
dispersing and absorbing magnetodielectric bodies is presented, which is based
on QED in linear, causal media. Both ground-state and excited atoms are
considered. Whereas the Casimir-Polder force acting on a ground-state atom can
conveniently be derived from a perturbative calculation of the atom-field
coupling energy, an atom in an excited state is subject to transient force
components that can only be fully understood by a dynamical treatment based on
the body-assisted vacuum Lorentz force. The results show that the
Casimir-Polder force can be influenced by the body-induced broadening and
shifting of atomic transitions - an effect that is not accounted for within
lowest-order perturbation theory. The theory is used to study the
Casimir-Polder force of a ground-state atom placed within a magnetodielectric
multilayer system, with special emphasis on thick and thin plates as well as a
planar cavity consisting of two thick plates. It is shown how the competing
attractive and repulsive force components related to the electric and magnetic
properties of the medium, respectively, can - for sufficiently strong magnetic
properties - lead to the formation of potential walls and wells.


title: Decoherence in Quantum Walks on the Hypercube

abstract:
  We study a natural notion of decoherence on quantum random walks over the
hypercube. We prove that in this model there is a decoherence threshold beneath
which the essential properties of the hypercubic quantum walk, such as linear
mixing times, are preserved. Beyond the threshold, we prove that the walks
behave like their classical counterparts.


title: Casimir force acting on bodies in media: Why textbooks are wrong

abstract:
  This is the abstract of an invited contribution to be presented at the 9th
International Conference on Squeezed States and Uncertainty Relations, ICSSUR
'05, Besancon, France, May 2-6, 2005.


title: Deterministic and Efficient Quantum Cryptography Based on Bell's Theorem

abstract:
  We propose a novel double-entanglement-based quantum cryptography protocol
that is both efficient and deterministic. The proposal uses photon pairs with
entanglement both in polarization and in time degrees of freedom; each
measurement in which both of the two communicating parties register a photon
can establish one and only one perfect correlation and thus deterministically
create a key bit. Eavesdropping can be detected by violation of local realism.
A variation of the protocol shows a higher security, similarly to the six-state
protocol, under individual attacks. Our scheme allows a robust implementation
under current technology.


title:
Hawking radiation from Trojan states in muonic Hydrogen in strong laser
  field

abstract:
  We show that the Unruh-Davies effect is measurable from Trojan wavepackets in
muonic Hydrogen as the acceleration on the first muonic Bohr orbit reaches
10^25 of the earth acceleration. It is the biggest acceleration achievable in
the laboratory environment which have been ever predicted for the cyclotronic
configuration. We calculate the ratio between the power of Larmor radiation and
the power of Hawking radiation. The Hawking radiation is measurable even for
quantum numbers of the muon due to suppression of spontaneous emission in
Trojan Hydrogen.


title: Quantifying decoherence in continuous variable systems

abstract:
  We present a detailed report on the decoherence of quantum states of
continuous variable systems under the action of a quantum optical master
equation resulting from the interaction with general Gaussian uncorrelated
environments. The rate of decoherence is quantified by relating it to the decay
rates of various, complementary measures of the quantum nature of a state, such
as the purity, some nonclassicality indicators in phase space and, for two-mode
states, entanglement measures and total correlations between the modes.
Different sets of physically relevant initial configurations are considered,
including one- and two-mode Gaussian states, number states, and coherent
superpositions. Our analysis shows that, generally, the use of initially
squeezed configurations does not help to preserve the coherence of Gaussian
states, whereas it can be effective in protecting coherent superpositions of
both number states and Gaussian wave packets.


title:
Unambiguous discrimination of special sets of multipartite states using
  local measurements and classical communication

abstract:
  We initially consider a quantum system consisting of two qubits, which can be
in one of two nonorthogonal states, \Psi_0 or \Psi_1. We distribute the qubits
to two parties, Alice and Bob. They each measure their qubit and then compare
their measurement results to determine which state they were sent. This
procedure is error-free, which implies that it must sometimes fail. In
addition, no quantum memory is required; it is not necessary to store one of
the qubits until the result of the measurement on the other is known. We
consider the cases in which, should failure occur, both parties receive a
failure signal or only one does. In the latter case, if the states share the
same Schmidt basis, the states can be discriminated with the same failure
probability as would be obtained if the two qubits were measured together. This
scheme is sufficiently simple that it can be generalized to multipartite qubit
and qudit states. Applications to quantum secret sharing are discussed.
Finally, we present an optical scheme to experimenatlly realize the protocol in
the case of two qubits.


title:
Security improvement of using modified coherent state for quantum
  cryptography

abstract:
  Weak coherent states as a photon source for quantum cryptography have limit
in secure data rate and transmission distance because of the presence of
multi-photon events and loss in transmission line. Two-photon events in a
coherent state can be taken out by a two-photon interference scheme. We
investigate the security issue of utilizing this modified coherent state in
quantum cryptography. A 4 dB improvement in secure data rate or a nearly
two-fold increase in transmission distance over the coherent state are found.
With a recently proposed and improved encoding strategy, further improvement is
possible.


title: Simulations of Quantum Turing Machines by Quantum Multi-Stack Machines

abstract:
  As was well known, in classical computation, Turing machines, circuits,
multi-stack machines, and multi-counter machines are equivalent, that is, they
can simulate each other in polynomial time. In quantum computation, Yao [11]
first proved that for any quantum Turing machines $M$, there exists quantum
Boolean circuit $(n,t)$-simulating $M$, where $n$ denotes the length of input
strings, and $t$ is the number of move steps before machine stopping. However,
the simulations of quantum Turing machines by quantum multi-stack machines and
quantum multi-counter machines have not been considered, and quantum
multi-stack machines have not been established, either. Though quantum counter
machines were dealt with by Kravtsev [6] and Yamasaki {\it et al.} [10], in
which the machines count with $0,\pm 1$ only, we sense that it is difficult to
simulate quantum Turing machines in terms of this fashion of quantum computing
devices, and we therefore prove that the quantum multi-counter machines allowed
to count with $0,\pm 1,\pm 2,...,\pm n$ for some $n>1$ can efficiently simulate
quantum Turing machines.
  Therefore, our mail goals are to establish quantum multi-stack machines and
quantum multi-counter machines with counts $0,\pm 1,\pm 2,...,\pm n$ and $n>1$,
and particularly to simulate quantum Turing machines by these quantum computing
devices.


title:
For Distinguishing Conjugate Hidden Subgroups, the Pretty Good
  Measurement is as Good as it Gets

abstract:
  Recently Bacon, Childs and van Dam showed that the ``pretty good
measurement'' (PGM) is optimal for the Hidden Subgroup Problem on the dihedral
group D_n in the case where the hidden subgroup is chosen uniformly from the n
involutions. We show that, for any group and any subgroup H, the PGM is the
optimal one-register experiment in the case where the hidden subgroup is a
uniformly random conjugate of H. We go on to show that when H forms a Gel'fand
pair with its parent group, the PGM is the optimal measurement for any number
of registers. In both cases we bound the probability that the optimal
measurement succeeds. This generalizes the case of the dihedral group, and
includes a number of other examples of interest.


title:
A nonlinear model dynamics for closed-system, constrained,
  maximal-entropy-generation relaxation by energy redistribution

abstract:
  We discuss a nonlinear model for the relaxation by energy redistribution
within an isolated, closed system composed of non-interacting identical
particles with energy levels e_i with i=1,2,...,N. The time-dependent
occupation probabilities p_i(t) are assumed to obey the nonlinear rate
equations tau dp_i/dt=-p_i ln p_i+ alpha(t)p_i-beta(t)e_ip_i where alpha(t) and
beta(t) are functionals of the p_i(t)'s that maintain invariant the mean energy
E=sum_i e_ip_i(t) and the normalization condition 1=sum_i p_i(t). The entropy
S(t)=-k sum_i p_i(t) ln p_i(t) is a non-decreasing function of time until the
initially nonzero occupation probabilities reach a Boltzmann-like canonical
distribution over the occupied energy eigenstates. Initially zero occupation
probabilities, instead, remain zero at all times. The solutions p_i(t) of the
rate equations are unique and well-defined for arbitrary initial conditions
p_i(0) and for all times. Existence and uniqueness both forward and backward in
time allows the reconstruction of the primordial lowest entropy state. The time
evolution is at all times along the local direction of steepest entropy ascent
or, equivalently, of maximal entropy generation. These rate equations have the
same mathematical structure and basic features of the nonlinear dynamical
equation proposed in a series of papers ended with G.P.Beretta, Found.Phys.,
17, 365 (1987) and recently rediscovered in S. Gheorghiu-Svirschevski,
Phys.Rev.A, 63, 022105 and 054102 (2001). Numerical results illustrate the
features of the dynamics and the differences with the rate equations recently
considered for the same problem in M.Lemanska and Z.Jaeger, Physica D, 170, 72
(2002).


title:
Comments on "Evidence for Quantized Displacement in Macroscopic
  Nanomechanical Oscillators"

abstract:
  We make comments on Gaidarzhy {\it et al.}'s [{\it Phys. Rev. Lett.} 94,
030402 (2005)] letter.


title: Measurement-Based Teleportation Along Quantum Spin Chains

abstract:
  We consider teleportation of an arbitrary spin-1/2 target quantum state along
the ground state of a quantum spin chain. We present a decomposition of the
Hilbert space of the many body quantum state into 4 vector spaces. Within each
of these subspaces, it is possible to take any superposition of states, and use
projective measurements to perform unit fidelity teleportation. Any such
superposition is necessarily a spin liquid state. We also show that all total
spin-0 quantum states belong in the same space, so that it is possible to
perform unit fidelity teleportation over any one-dimensional spin-0 many body
quantum state. We generalise to $n$-Bell states, and present some general
bounds on fidelity of teleportation given a general state of a quantum spin
chain.


title:
The non dissipative damping of the Rabi oscillations as a "which-path"
  information

abstract:
  Rabi oscillations may be viewed as an interference phenomenon due to a
coherent superposition of different quantum paths, like in the Young's two-slit
experiment. The inclusion of the atomic external variables causes a non
dissipative damping of the Rabi oscillations. More generally, the atomic
translational dynamics induces damping in the correlation functions which
describe non classical behaviors of the field and internal atomic variables,
leading to the separability of these two subsystems. We discuss on the
possibility of interpreting this intrinsic decoherence as a "which-way"
information effect and we apply to this case a quantitative analysis of the
complementarity relation as introduced by Englert [Phys. Rev. Lett.
\textbf{77}, 2154 (1996)].


title:
The Quantum-Classical Comparison of the Arrival Time Distribution
  through the Probability Current

abstract:
  We consider the arrival time distribution defined through the quantum
probability current for a Gaussian wave packet representing free particles in
quantum mechanics in order to explore the issue of the classical limit of
arrival time. We formulate the classical analogue of the arrival time
distribution for an ensemble of free particles represented by a phase space
distribution function evolving under the classical Liouville's equation. The
classical probability current so constructed matches with the quantum
probability current in the limit of minimum uncertainty. Further, it is
possible to show in general that smooth transitions from the quantum mechanical
probability current and the mean arrival time to their respective classical
values are obtained in the limit of large mass of the particles.


title: Topological spin transport of relativistic electron

abstract:
  We derive the semiclassical (with accuracy of $\hbar$) motion equation for
relativistic electron, which follow from the Dirac equation. We determine both
the evolution equation for electron polarization, which takes the non-Abelian
Berry phase into account, and Hamiltonian equations for trajectories of the
particle (wave packet) center in the phase space. The equations have covariant
form with respect to U(2) gauge transformations and contain topological spin
terms that are connected to the Berry gauge field arising during the
diagonalization of the Dirac equation. The trajectory equations obtained are
substantially different from the traditional ones (for instance, those that
follow from the Pauli Hamiltonian) and correspond to contemporary ideas of
topological spin transport of particles.


title: Loss Tolerant Optical Qubits

abstract:
  We present a linear optics quantum computation scheme that employs a new
encoding approach that incrementally adds qubits and is tolerant to photon loss
errors. The scheme employs a circuit model but uses techniques from cluster
state computation and achieves comparable resource usage. To illustrate our
techniques we describe a quantum memory which is fault tolerant to photon loss.


title:
Considering relativistic symmetry as the first principle of quantum
  mechanics

abstract:
  On the basis of the relativistic symmetry of Minkowski space, we derive a
Lorentz invariant equation for a spread electron. This equation slightly
differs from the Dirac equation and includes additional terms originating from
the spread of an electron. Further, we calculate the anomalous magnetic moment
based on these terms. These calculations do not include any divergence;
therefore, renormalization procedures are unnecessary. In addition, the
relativistic symmetry existing among coordinate systems will provide a new
prospect for the foundations of quantum mechanics like the measurement process.


title: Nonbinary Quantum Reed-Muller Codes

abstract:
  We construct nonbinary quantum codes from classical generalized Reed-Muller
codes and derive the conditions under which these quantum codes can be
punctured. We provide a partial answer to a question raised by Grassl, Beth and
Roetteler on the existence of q-ary quantum MDS codes of length n with q\le
n\le q^2-1.


title: Free spin quantum computation with semiconductor nanostructures

abstract:
  Taking the excess electron spin in a unit cell of semiconductor multiple
quantum-dot structure as a qubit, we can implement scalable quantum computation
without resorting to spin-spin interactions. The technique of single electron
tunnelings and the structure of quantum-dot cellular automata (QCA) are used to
create a charge entangled state of two electrons which is then converted into
spin entanglement states by using single spin rotations. Deterministic
two-qubit quantum gates can also be manipulated using only single spin
rotations with help of QCA. A single-short read-out of spin states can be
realized by coupling the unit cell to a quantum point contact.


title:
Engineering of multi-dimensional entangled states of photon pairs using
  hyper-entanglement

abstract:
  Multi-dimensional entangled states have been proven to be more powerful in
some quantum information process. In this paper, down-converted photons from
spontaneous parametric down conversion(SPDC) are used to engineer
multi-dimensional entangled states. A kind of multi-degree multi-dimensional
Greenberger-Horne-Zeilinger(GHZ) state can also be generated. The
hyper-entangled photons, which are entangled in energy-time, polarization and
orbital angular momentum (OAM), is proved to be useful to increase the
dimension of systems and investigate higher-dimensional entangled states.


title: Spacetime structures of continuous time quantum walks

abstract:
  The propagation by continuous time quantum walks (CTQWs) on one-dimensional
lattices shows structures in the transition probabilities between different
sites reminiscent of quantum carpets. For a system with periodic boundary
conditions, we calculate the transition probabilities for a CTQW by
diagonalizing the transfer matrix and by a Bloch function ansatz. Remarkably,
the results obtained for the Bloch function ansatz can be related to results
from (discrete) generalized coined quantum walks. Furthermore, we show that
here the first revival time turns out to be larger than for quantum carpets.


title:
Generalized Probability Amplitudes for Spin Projection Measurements on
  Spin 2 Systems

abstract:
  In previous work, we have shown how the description of spin may be
generalized and we have worked out this generalization for the cases spin 1/2
and spin 1. In this paper, we deal with the case of spin 2 and give the
generalized probability amplitudes.


title:
Translational dynamics effects on the non-local correlations between two
  atoms

abstract:
  A pair of atoms interacting successively with the field of the same cavity
and exchanging a single photon, leave the cavity in an entangled state of
Einstein-Podolsky-Rosen (EPR) type (see, for example, [S.J.D. Phoenix, and S.M.
Barnett, J. Mod. Opt. \textbf{40} (1993) 979]). By implementing the model with
the translational degrees of freedom, we show in this letter that the
entanglement with the translational atomic variables can lead, under
appropriate conditions, towards the separability of the internal variables of
the two atoms. This implies that the translational dynamics can lead, in some
cases, to difficulties in observing the Bell's inequality violation for massive
particles.


title: Weak realism, counterfactuals, and decay of geometry at small scales

abstract:
  Two typical entanglements will be shown to stand on opposite sides on the
issue of \emph{instrumental realism}, the issue of whether (as for EPR in the
original form or EPRB, Bohm's versions using spin) or not (as for GHZ)
observables have values that preexist measurement. Instrumental realism in the
EPR context helps us prove that in some special circumstances, one can get
simultaneous knowledge of two conjugate quantities, which in particular make
sense together. This shatters the axiomatic presentation of Quantum Mechanics.
This simultaneous knowledge of two conjugate quantities elaborates on 1935 work
by Schr{\"o}dinger, hence the name, \emph{Schr{\"o}dinger Unorthodoxy Theorem},
given to the second main result that is easily deduced from the result on
instrumental realism. Once the axiomatic edifice of Quantum Mechanics is
broken, we can let go the completeness of the wave function as suggested in
EPR, and hold fast on locality. The EPR paper of mid-1935 gets here contrasted,
from a new point of view, with a 1936 text where Einstein avoids using
counterfactuals. Counterfactuals get a precise definition and the corresponding
concept is used all along, but may be new under an old name. We provide a short
critical review of Bell's 1964 paper. Then, a small modification of arguments
for the Schr{\"o}dinger Unorthodoxy Theorem will let appear a simple
conservation law, combined with the Malus Law, as the origin of the correlation
in Bell's version of EPRB: this is our third main result. As the fourth main
result, a last use of the concept of counterfactual yields the decay of
geometry at small enough scale. This opens a new world of interpretation of
aspects of Quantum Mechanics, aspects that range from measurement and the need
of classical physics to views on what realism should mean in microphysics.


title: Dynamics of tripartite entanglement

abstract:
  Maximally entangled states are of utmost importance to quantum communication,
dense coding, and quantum teleportation. With a trapped ion placed inside a
high finesse optical cavity, interacting with field of an external laser and
quantized cavity field, a scheme to generate a maximally entangled three qubit
GHZ state, is proposed. The dynamics of tripartite entanglement is
investigated, using negativity as an entanglement measure and linear entropy as
a measure of mixedness of a state. It is found that (a) the number of modes
available to the subsystem determines the maximum entanglement of a subsystem,
b) at entanglement maxima and minima, linear entropy and negativity uniquely
determine the nature of state, but the two measures do not induce the same
ordering of states, and c) for a special choice of system parameters maximally
entangled tripartite two mode GHZ state is generated. The scheme presented for
GHZ state generation is a single step process and is reduction free.
  PACS: 03.67.-a, 42.50.-p, 03.67.Dd


title:
Quantum Information Theoretical Analysis of Various Constructions for
  Quantum Secret Sharing

abstract:
  Recently, an information theoretical model for Quantum Secret Sharing (QSS)
schemes was introduced. By using this model, we prove that pure state Quantum
Threshold Schemes (QTS) can be constructed from quantum MDS codes and vice
versa. In particular, we consider stabilizer codes and give a constructive
proof of their relation with QTS. Furthermore, we reformulate the Monotone Span
Program (MSP) construction according to the information theoretical model and
check the recoverability and secrecy requirement. Finally, we consider QSS
schemes which are based on quantum teleportation.


title: Gaussian Operations and Privacy

abstract:
  We consider the possibilities offered by Gaussian states and operations for
two honest parties, Alice and Bob, to obtain privacy against a third
eavesdropping party, Eve. We first extend the security analysis of the protocol
proposed in M. Navascues et al., Phys. Rev. Lett. 94, 010502 (2005). Then, we
prove that a generalized version of this protocol does not allow to distill a
secret key out of bound entangled Gaussian states.


title:
Deterministic Entanglement via Molecular Dissociation in Integrated Atom
  Optics

abstract:
  Deterministic entanglement of neutral cold atoms can be achieved by combining
several already available techniques like the creation/dissociation of neutral
diatomic molecules, manipulating atoms with micro fabricated structures (atom
chips) and detecting single atoms with almost 100% efficiency. Manipulating
this entanglement with integrated/linear atom optics will open a new
perspective for quantum information processing with neutral atoms.


title: The Effects of Symmetries on Quantum Fidelity Decay

abstract:
  We explore the effect of a system's symmetries on fidelity decay behavior.
Chaos-like exponential fidelity decay behavior occurs in non-chaotic systems
when the system possesses symmetries and the applied perturbation is not tied
to a classical parameter. Similar systems without symmetries exhibit
faster-than-exponential decay under the same type of perturbation. This
counter-intuitive result, that extra symmetries cause the system to behave in a
chaotic fashion, may have important ramifications for quantum error correction.


title: Exact Entanglement Cost of Multi-Qubit Bound Entangled States

abstract:
  We report the exact entanglement cost of a class of multiqubit bound
entangled states, computed in the context of a universal model for multipartite
state preparation. The exact amount of entanglement needed to prepare such
states are determined by first obtaining lower bounds using a cut-set approach,
and then providing explicit local protocols achieving the lower bound.


title: Adiabatic Quantum Computation in Open Systems

abstract:
  We analyze the performance of adiabatic quantum computation (AQC) under the
effect of decoherence. To this end, we introduce an inherently open-systems
approach, based on a recent generalization of the adiabatic approximation. In
contrast to closed systems, we show that a system may initially be in an
adiabatic regime, but then undergo a transition to a regime where adiabaticity
breaks down. As a consequence, the success of AQC depends sensitively on the
competition between various pertinent rates, giving rise to optimality
criteria.


title: Yang--Baxterizations, Universal Quantum Gates and Hamiltonians

abstract:
  The unitary braiding operators describing topological entanglements can be
viewed as universal quantum gates for quantum computation. With the help of the
Brylinskis's theorem, the unitary solutions of the quantum Yang--Baxter
equation can be also related to universal quantum gates. This paper derives the
unitary solutions of the quantum Yang--Baxter equation via Yang--Baxterization
from the solutions of the braided relation. We study Yang--Baxterizations of
the non-standard and standard representations of the six-vertex model and the
complete solutions of the non-vanishing eight-vertex model. We construct
Hamiltonians responsible for the time-evolution of the unitary braiding
operators which lead to the Schr{\"o}dinger equations.


title: Bell's Theorem: A Critique

abstract:
  By implicitly assuming that all possible Bell-measurements occur
simultaneously, all proofs of Bell's Theorem violate Heisenberg's Uncertainty
Principle. This assumption is made in the original form of Bell's inequality,
in Wigner's probability inequalities, and in the ``nonlocality without
inequalities'' arguments. The introduction of time into derivations of these
variants of Bell's theorem results in extra terms related to the time order of
the measurements used in constructing correlation coefficients. Since the same
locality assumptions are made in the Heisenberg-compliant derivations of this
paper, only time-independent classical local hidden variable theories are
forbidden by violations of the original Bell inequalities; time-dependent
quantum local hidden variable theories can satisfy this new bound and agree
with experiment. We further point out that factorizable wavefunctions have been
used to describe some EPR experiments and can be used to describe others. These
will generate local de Broglie-Bohm trajectories in the description of the
data. This second, independent, line of argument also shows that violation of
Bell's inequality is only evidence that Heisenberg's Uncertainty Principle
cannot be ignored.


title: Entanglement and decoherence in spin gases

abstract:
  We study the dynamics of entanglement in spin gases. A spin gas consists of a
(large) number of interacting particles whose random motion is described
classically while their internal degrees of freedom are described
quantum-mechanically. We determine the entanglement that occurs naturally in
such systems for specific types of quantum interactions. At the same time,
these systems provide microscopic models for non--Markovian decoherence: the
interaction of a group of particles with other particles belonging to a
background gas are treated exactly, and differences between collective and
non--collective decoherence processes are studied. We give quantitative results
for the Boltzmann gas and also for a lattice gas, which could be realized by
neutral atoms hopping in an optical lattice. These models can be simulated
efficiently for systems of mesoscopic sizes (N ~ 10^5).


title: Semiclassical Expansions, the Strong Quantum Limit, and Duality

abstract:
  We show how to complement Feynman's exponential of the action so that it
exhibits a Z_2 duality symmetry. The latter illustrates a relativity principle
for the notion of quantum versus classical.


title: On the Computational Power of Molecular Heat Engines

abstract:
  A heat engine is a machine which uses the temperature difference between a
hot and a cold reservoir to extract work. Here both reservoirs are quantum
systems and a heat engine is described by a unitary transformation which
decreases the average energy of the bipartite system. On the molecular scale,
the ability of implementing such a unitary heat engine is closely connected to
the ability of performing logical operations and classical computing. This is
shown by several examples:
  (1) The most elementary heat engine is a SWAP-gate acting on 1 hot and 1 cold
two-level systems with different energy gaps.
  (2) An optimal unitary heat engine on a pair of 3-level systems can directly
implement OR and NOT gates, as well as copy operations. The ability to
implement this heat engine on each pair of 3-level systems taken from the hot
and the cold ensemble therefore allows universal classical computation.
  (3) Optimal heat engines operating on one hot and one cold oscillator mode
with different frequencies are able to calculate polynomials and roots
approximately.
  (4) An optimal heat engine acting on 1 hot and n cold 2-level systems with
different level spacings can even solve the NP-complete problem KNAPSACK.
Whereas it is already known that the determination of ground states of
interacting many-particle systems is NP-hard, the optimal heat engine is a
thermodynamic problem which is NP-hard even for n non-interacting spin systems.
This result suggest that there may be complexity-theoretic limitations on the
efficiency of molecular heat engines.


title: Collisional Properties of Cold Spin-Polarized Metastable Neon Atoms

abstract:
  We measure the rates of elastic and inelastic two-body collisions of cold
spin-polarized neon atoms in the metastable 3P2 state for 20^Ne and 22^Ne in a
magnetic trap. From particle loss, we determine the loss parameter of inelastic
collisions beta=6.5(18)x10^{-12} cm^3s^{-1} for 20^Ne and
beta=1.2(3)x10^{-11}cm^3{s}^{-1} for 22^Ne. These losses are caused by ionizing
(i.e. Penning) collisions %to more than and occur less frequently than for
unpolarized atoms. This proves the suppression of Penning ionization due to
spin-polarization. From cross-dimensional relaxation measurements, we obtain
elastic scattering lengths of a=-180(40) a_0 for 20^Ne and a=+150(+80/-50) a_0
for 22^Ne, where a_0=0.0529 nm.


title: Why the Afshar Experiment Does Not Refute Complementarity

abstract:
  A modified version of Young's experiment by Shahriar Afshar demonstrates
that, prior to what appears to be a ``which-way'' measurement, an interference
pattern exists. Afshar has claimed that this result constitutes a violation of
the Principle of Complementarity. This paper discusses the implications of this
experiment and considers how Cramer's Transactional Interpretation easily
accomodates the result. It is also shown that the Afshar experiment is
analogous in key respects to a spin one-half particle prepared as ``spin up
along $\bf x$'', subjected to a nondestructive confirmation of that
preparation, and post-selected in a specific state of spin along $\bf z$.
  The terminology ``which-way'' or ``which-slit'' is critiqued; it is argued
that this usage by both Afshar and his critics is misleading and has
contributed to confusion surrounding the interpretation of the experiment.
Nevertheless, it is concluded that
  Bohr would have had no more problem accounting for the Afshar result than he
would in accounting for the aforementioned pre- and post-selection spin
experiment, in which the particle's preparation state is confirmed by a
nondestructive measurement prior to post-selection. In addition, some new
inferences about the interpretation of delayed choice experiments are drawn
from the analysis.


title: Two Slightly-Entangled NP-Complete Problems

abstract:
  We perform a mathematical analysis of the classical computational complexity
of two genuine quantum-mechanical problems, which are inspired in the
calculation of the expected magnetizations and the entanglement between
subsystems for a quantum spin system. These problems, which we respectively
call SES and SESSP, are specified in terms of pure slightly-entangled quantum
states of n qubits, and rigorous mathematical proofs that they belong to the
NP-Complete complexity class are presented. Both SES and SESSP are, therefore,
computationally equivalent to the relevant 3-SAT problem, for which an
efficient algorithm is yet to be discovered.


title:
Nano-Thermodynamics: On the minimal length scale for the existence of
  temperature

abstract:
  We consider a quantum system consisting of a regular chain of elementary
subsystems with nearest neighbor interactions and assume that the total system
is in a canonical state with temperature $T$. We analyze under what condition
the state factors into a product of canonical density matrices with respect to
groups of $n$ subsystems each, and when these groups have the same temperature
$T$. While in classical mechanics the validity of this procedure only depends
on the size of the groups $n$, in quantum mechanics the minimum group size
$n_{\text{min}}$ also depends on the temperature $T $! As examples, we apply
our analysis to a harmonic chain and an Ising spin chain. We discuss various
features that show up due to the characteristics of the models considered. For
the harmonic chain, which successfully describes thermal properties of
insulating solids, our approach gives a first quantitative estimate of the
minimal length scale on which temperature can exist: This length scale is found
to be constant for temperatures above the Debye temperature and proportional to
$T^{-3}$ below.


title: Real Mutually Unbiased Bases

abstract:
  We tabulate bounds on the optimal number of mutually unbiased bases in R^d.
For most dimensions d, it can be shown with relatively simple methods that
either there are no real orthonormal bases that are mutually unbiased or the
optimal number is at most either 2 or 3. We discuss the limitations of these
methods when applied to all dimensions, shedding some light on the difficulty
of obtaining tight bounds for the remaining dimensions that have the form
d=16n^2, where n can be any number. We additionally give a simpler, alternative
proof that there can be at most d/2+1 real mutually unbiased bases in dimension
d instead of invoking the known results on extremal Euclidean line sets by
Cameron and Seidel, Delsarte, and Calderbank et al.


title: Unitary local invariance

abstract:
  We address unitary local (UL) invariance of bipartite pure states. Given a
bipartite state $|\Psi>>=\sum_{ij} \psi_{ij}\: |i>_1\otimes |j>_2$ the complete
characterization of the class of local unitaries $U_1\otimes U_2$ for which
$U_1\otimes U_2 |\Psi>>=|\Psi>>$ is obtained.The two relevant parameters are
the rank of the matrix $\Psi$, $[\Psi]_{ij}=\psi_{ij}$, and the number of its
equal singular values, {\em i.e.} the degeneracy of the eigenvalues of the
partial traces of $|\Psi>>$.


title:
Quantum nonlocality obtained from local states by entanglement
  purification

abstract:
  We have applied an entanglement purification protocol to produce a single
entangled pair of photons capable of violating a CHSH Bell inequality from two
pairs that individually could not. The initial poorly-entangled photons were
created by a controllable decoherence that introduced complex errors. All of
the states were reconstructed using quantum state tomography which allowed for
a quantitative description of the improvement of the state after purification.


title:
Unified Treatment of Heterodyne Detection: the Shapiro-Wagner and Caves
  Frameworks

abstract:
  A comparative study is performed on two heterodyne systems of photon
detectors expressed in terms of a signal annihilation operator and an image
band creation operator called Shapiro-Wagner and Caves' frame, respectively.
This approach is based on the introduction of a convenient operator $\hat \psi$
which allows a unified formulation of both cases. For the Shapiro-Wagner
scheme, where $[\hat \psi, \hat \psi^{\dag}] =0$, quantum phase and amplitude
are exactly defined in the context of relative number state (RNS)
representation, while a procedure is devised to handle suitably and in a
consistent way Caves' framework, characterized by $[\hat \psi, \hat
\psi^{\dag}] \neq 0$, within the approximate simultaneous measurements of
noncommuting variables. In such a case RNS phase and amplitude make sense only
approximately.


title: Theoretical investigation of moir\'e patterns in quantum images

abstract:
  Moir\'e patterns are produced when two periodic structures with different
spatial frequencies are superposed. The transmission of the resulting structure
gives rise to spatial beatings which are called moir\'e fringes. In classical
optics, the interest in moir\'e fringes comes from the fact that the spatial
beating given by the frequency difference gives information about details(high
spatial frequency) of a given spatial structure. We show that moir\'e fringes
can also arise in the spatial distribution of the coincidence count rate of
twin photons from the parametric down-conversion, when spatial structures with
different frequencies are placed in the path of each one of the twin beams. In
other words,we demonstrate how moir\'e fringes can arise from quantum images.


title: Scalable design of tailored soft pulses for coherent control

abstract:
  We present a scalable scheme to design optimized soft pulses and pulse
sequences for coherent control of interacting quantum many-body systems. The
scheme is based on the cluster expansion and the time dependent perturbation
theory implemented numerically. This approach offers a dramatic advantage in
numerical efficiency, and it is also more convenient than the commonly used
Magnus expansion, especially when dealing with higher order terms. We
illustrate the scheme by designing 2nd-order pi-pulses and a 6th-order 8-pulse
refocusing sequence for a chain of qubits with nearest-neighbor couplings. We
also discuss the performance of soft-pulse refocusing sequences in suppressing
decoherence due to low-frequency environment.


title: Oblivious transfer and quantum non-locality

abstract:
  Oblivious transfer, a central functionality in modern cryptography, allows a
party to send two one-bit messages to another who can choose one of them to
read, remaining ignorant about the other, whereas the sender does not learn the
receiver's choice. Oblivious transfer the security of which is
information-theoretic for both parties is known impossible to achieve from
scratch. - The joint behavior of certain bi-partite quantum states is
non-local, i.e., cannot be explained by shared classical information. In order
to better understand such behavior, which is classically explainable only by
communication, but does not allow for it, Popescu and Rohrlich have described a
"non-locality machine": Two parties both input a bit, and both get a random
output bit the XOR of which is the AND of the input bits. - We show a close
connection, in a cryptographic sense, between OT and the "PR primitive." More
specifically, unconditional OT can be achieved from a single realization of PR,
and vice versa. Our reductions, which are single-copy, information-theoretic,
and perfect, also lead to a simple and optimal protocol allowing for inverting
the direction of OT.


title: Mutually Unbiased Bases are Complex Projective 2-Designs

abstract:
  Mutually unbiased bases (MUBs) are a primitive used in quantum information
processing to capture the principle of complementarity. While constructions of
maximal sets of d+1 such bases are known for systems of prime power dimension
d, it is unknown whether this bound can be achieved for any non-prime power
dimension. In this paper we demonstrate that maximal sets of MUBs come with a
rich combinatorial structure by showing that they actually are the same objects
as the complex projective 2-designs with angle set {0,1/d}. We also give a new
and simple proof that symmetric informationally complete POVMs are complex
projective 2-designs with angle set {1/(d+1)}.


title:
Comment on 'Secure Communication using mesoscopic coherent states',
  Barbosa et al, Phys Rev Lett 90, 227901 (2003)

abstract:
  In a recent letter, Barbosa et al [PRL 90, 227901(2003)] claim that secure
communication is possible with bright coherent pulses, by using quantum noise
to hide the data from an eavesdropper. We show here that the secrecy in the
scheme of Barbosa et al is unrelated to quantum noise, but rather derives from
the secret key that sender and receiver share beforehand.


title: Spatial quantum correlations in multiple scattered light

abstract:
  We predict a new spatial quantum correlation in light propagating through a
multiple scattering random medium. The correlation depends on the quantum state
of the light illuminating the medium, is infinite range, and dominates over
classical mesoscopic intensity correlations. The spatial quantum correlation is
revealed in the quantum fluctuations of the total transmission or reflection
through the sample and should be readily observable experimentally.


title: Spin equation and its solutions

abstract:
  The aim of the present article is to study in detail the so-called spin
equation (SE) and present both the methods of generating new solution and a new
set of exact solutions. We recall that the SE with a real external field can be
treated as a reduction of the Pauli equation to the (0+1)-dimensional case.
Two-level systems can be described by an SE with a particular form of the
external field. In this article, we also consider associated equations that are
equivalent or (in one way or another) related to the SE. We describe the
general solution of the SE and solve the inverse problem for this equation. We
construct the evolution operator for the SE and consider methods of generating
new sets of exact solutions. Finally, we find a new set of exact solutions of
the SE.


title: Optimal estimates and joint measurement uncertainty relations

abstract:
  Often, one would like to determine some observable A, but can only measure
some (hopefully related) observable M. This can arise, for example, in quantum
eavesdropping, or when the research lab budget isn't large enough for that 100%
efficient photodetector. It also arises whenever one tries to jointly determine
two complementary observables via some measurement M.
  This raises three natural questions: (i) What is the best possible estimate
of A from M ? ; (ii) How 'noisy' is such an estimate ? ; and (iii) Are there
any universally valid uncertainty relations for joint estimates ? Quite general
answers, and applications to heterodyne detection and EPR joint measurements,
are briefly reviewed.


title: Relative entropy of entanglement of a kind of two qubit entangled states

abstract:
  We in this paper strictly prove that some block diagonalizable two qubit
entangled state with six none zero elements reaches its quantum relative
entropy entanglement by the a separable state having the same matrix structure.
The entangled state comprises local filtering result state as a special case.


title: A proposal for a two-slit experiment with a mirror

abstract:
  Summary. A modified version of the two-slit experiment is proposed in which
the moveable detector/counter used to obtain the fringe distribution by
counting single photons at different positions on the screen plane is replaced
with a mirror positioned at an angle to the screen plane. A single photon
arriving at the mirror through one of the slits is reflected at one of two
angles along one of two divergent paths. It is then detected by one of two
detectors, one on each of the paths, for different positions of the mirror on
the screen plane. This modification results in the ability to generate which
way information based on the path taken by the reflected photon as well as the
distribution of photon counts on the screen plane, one for each slit. Since the
photon does not experience any change other than the reflection at the mirror
the question whether fringes are also present because of a priori interference
effects when no mirror is present can be answered by examining the two a
posteriori distributions. Three possible outcomes are considered and
explanations proposed for them. The experiment is shown to be feasible with
existing technology and can be performed in most physics laboratories.


title: When the macroscopic game is the quantum game?

abstract:
  An example of the macroscopic game of two partners consisting of two
classical games played simultaneously with special dependence of strategies is
considered. The average profit of each partner is equal to the average profit
obtained in the quantum game with two noncommuting operators for the spin one
half system with strategies defined by the wave function. Nash equilibria in
the macroscopic game coincide with Nash equilibria of the quantum game.


title: Light storing in a medium of atoms in the tripod configuration

abstract:
  Light propagation and storing in a medium of atoms in the tripod
configuration driven by two control pulses are investigated theoretically in
terms of two polaritons and numerically. It is shown that a magnetic field
switched on at the pulse storage stage changes the phase relations between the
atomic coherences due to the stored pulse, which leads to an essential
modification of the released pulse. Quantitative relations concerning the
released pulse and the coherences are given. A general situation when the two
control fields are not proportional at the pulse release stage is also
examined. It is shown that in both cases a single dark state polariton is not
sufficient to account for the pulse evolution, which is connected with the fact
that a part of the signal remains in the medium after the release stage.


title: Entangling Power of Permutations

abstract:
  The notion of entangling power of unitary matrices was introduced by Zanardi,
Zalka and Faoro [PRA, 62, 030301]. We study the entangling power of
permutations, given in terms of a combinatorial formula. We show that the
permutation matrices with zero entangling power are, up to local unitaries, the
identity and the swap. We construct the permutations with the minimum nonzero
entangling power for every dimension. With the use of orthogonal latin squares,
we construct the permutations with the maximum entangling power for every
dimension. Moreover, we show that the value obtained is maximum over all
unitaries of the same dimension, with possible exception for 36. Our result
enables us to construct generic examples of 4-qudits maximally entangled states
for all dimensions except for 2 and 6. We numerically classify, according to
their entangling power, the permutation matrices of dimension 4 and 9, and we
give some estimates for higher dimensions.


title: Quantum toboggans

abstract:
  Among all the PT-symmetric potentials defined on complex coordinate contours
C(s), the name "quantum toboggan" is reserved for those whose C(s) winds around
a singularity and lives on at least two different Riemann sheets. An enriched
menu of prospective phenomenological models is then obtainable via the mere
changes of variables. We pay thorough attention to the harmonic oscillator
example with a fractional screening and emphasize the role of the existence and
invariance of its quasi-exact states for different tobogganic C(s).


title: Demonstration of Quantum Error Correction using Linear Optics

abstract:
  We describe a laboratory demonstration of a quantum error correction
procedure that can correct intrinsic measurement errors in linear-optics
quantum gates. The procedure involves a two-qubit encoding and fast
feed-forward-controlled single-qubit operations. In our demonstration the
qubits were represented by the polarization states of two single-photons from a
parametric down-conversion source, and the real-time feed-forward control was
implemented using an electro-optic device triggered by the output of
single-photon detectors.


title: Entanglement of zero angular momentum mixtures and black hole entropy

abstract:
  We calculate the entanglement of formation and the entanglement of
distillation for arbitrary mixtures of the zero spin states on an
arbitrary-dimensional bipartite Hilbert space. Such states are relevant to
quantum black holes and to decoherence-free subspaces based communication. The
two measures of entanglement are equal and scale logarithmically with the
system size. We discuss its relation to the black hole entropy law. Moreover,
these states are locally distinguishable but not locally orthogonal, thus
violating a conjecture that the entanglement measures coincide only on locally
orthogonal states. We propose a slightly weaker form of this conjecture.
Finally, we generalize our entanglement analysis to any unitary group.


title: Statistical Mechanics of Amplifying Apparatus

abstract:
  We implement Feynman's suggestion that the only missing notion needed for the
puzzle of Quantum Measurement is the statistical mechanics of amplifying
apparatus. We define a thermodynamic limit of quantum amplifiers which is a
classically describable system in the sense of Bohr, and define macroscopic
pointer variables for the limit system. Then we derive the probabilities of
Quantum Measurement from the deterministic Schroedinger equation by the usual
techniques of Classical Statistical Mechanics.


title:
Quantum secret sharing between multi-party and multi-party without
  entanglement

abstract:
  We propose a quantum secret sharing protocol between multi-party ($m$ members
in group 1) and multi-party ($n$ members in group 2) using a sequence of single
photons. These single photons are used directly to encode classical information
in a quantum secret sharing process. In this protocol, all members in group 1
directly encode their respective keys on the states of single photons via
unitary operations, then the last one (the $m^{th}$ member of group 1) sends
$1/n$ of the resulting qubits to each of group 2. Thus the secret message
shared by all members of group 1 is shared by all members of group 2 in such a
way that no subset of each group is efficient to read the secret message, but
the entire set (not only group 1 but also group 2) is. We also show that it is
unconditionally secure. This protocol is feasible with present-day techniques.


title:
Non-classical properties of quantum wave packets propagating in a
  Kerr-like medium

abstract:
  We investigate non-classical effects such as fractional revivals, squeezing
and higher-order squeezing of photon-added coherent states propagating through
a Kerr-like medium.The Wigner functions corresponding to these states at the
instants of fractional revivals are obtained, and the extent of
non-classicality quantified.


title:
Quantum spatial correlations in high-gain parametric down-conversion
  measured by means of a CCD camera

abstract:
  We consider travelling-wave parametric down-conversion in the high-gain
regime and present the experimental demonstration of the quantum character of
the spatial fluctuations in the system. In addition to showing the presence of
sub-shot noise fluctuations in the intensity difference, we demonstrate that
the peak value of the normalized spatial correlations between signal and idler
lies well above the line marking the boundary between the classical and the
quantum domain. This effect is equivalent to the apparent violation of the
Cauchy-Schwartz inequality, predicted by some of us years ago, which represents
a spatial analogue of photon antibunching in time. Finally, we analyse
numerically the transition from the quantum to the classical regime when the
gain is increased and we emphasize the role of the inaccuracy in the
determination of the symmetry center of the signal/idler pattern in the
far-field plane.


title:
Note on Possibility of obtaining a non-relativistic proof of the
  spin-statistics theorem in the Galilean frame

abstract:
  We replay to the critique by Sudarshan and Shaji of our argument of
impossibility to obtain a non-relativistic proof of the spin-statistics theorem
in the Galilean frame.


title: Bohmian trajectories and the ether: Where does the analogy fail?

abstract:
  Once considered essential to the explanation of electromagnetic phenomena,
the ether was eventually discarded after the advent of special relativity. The
lack of empirical signature of realist interpretative schemes of quantum
mechanics, like Bohmian trajectories, has led some to conclude that, just like
the ether, they can be dispensed with, replaced by the corresponding emergence
of the concept of information. Although devices like Bohmian trajectories and
the ether do present important analogies, I argue that there is also a crucial
difference, related to distinct explanatory functions of quantum mechanics.


title:
Meltdown in quantum computers needs not occur: Nuclear experiments show
  a way out

abstract:
  We show that phase memory can be much longer than energy relaxation in
systems with exponentially large dimensions of Hilbert space; this finding is
documented by fifty years of nuclear experiments, though the information is
somewhat hidden. For quantum computers Hilbert spaces of dimension $2^{100}$ or
larger will be typical and therefore this effect may contribute significantly
to reduce the problems of scaling of quantum computers to a useful number of
qubits.


title:
Analysis of a quantum logic device based on dipole-dipole interactions
  of optically trapped Rydberg atoms

abstract:
  We present a detailed analysis and design of a neutral atom quantum logic
device based on atoms in optical traps interacting via dipole-dipole coupling
of Rydberg states. The dominant physical mechanisms leading to decoherence and
loss of fidelity are enumerated. Our results support the feasibility of
performing single and two-qubit gates at MHz rates with decoherence probability
and fidelity errors at the level of $10^{-3}$ for each operation. Current
limitations and possible approaches to further improvement of the device are
discussed.


title: Temporal Bell inequalities without noninvasive measurability

abstract:
  Some temporal Bell inequalities are deduced under the assumption of realism
and perfect correlation. No locality condition is needed. When the system is
macroscopic, the perfect correlation assumption substitutes the noninvasive
measurability hypothesis advanteousgely. The new inequalities are violated
quantically. This violation is clearly more severe than the similar violation
in the case of ordinary Bell inequalities. Some microscopic and macroscopic
situations in which these inequalities could be tested are considered.


title: The role of the rigged Hilbert space in Quantum Mechanics

abstract:
  There is compelling evidence that, when continuous spectrum is present, the
natural mathematical setting for Quantum Mechanics is the rigged Hilbert space
rather than just the Hilbert space. In particular, Dirac's bra-ket formalism is
fully implemented by the rigged Hilbert space rather than just by the Hilbert
space. In this paper, we provide a pedestrian introduction to the role the
rigged Hilbert space plays in Quantum Mechanics, by way of a simple, exactly
solvable example. The procedure will be constructive and based on a recent
publication. We also provide a thorough discussion on the physical significance
of the rigged Hilbert space.


title:
Classical and Quantum Complexity of the Sturm-Liouville Eigenvalue
  Problem

abstract:
  We study the approximation of the smallest eigenvalue of a Sturm-Liouville
problem in the classical and quantum settings. We consider a univariate
Sturm-Liouville eigenvalue problem with a nonnegative function $q$ from the
class $C^2([0,1])$ and study the minimal number $n(\e)$ of function evaluations
or queries that are necessary to compute an $\e$-approximation of the smallest
eigenvalue. We prove that $n(\e)=\Theta(\e^{-1/2})$ in the (deterministic)
worst case setting, and $n(\e)=\Theta(\e^{-2/5})$ in the randomized setting.
The quantum setting offers a polynomial speedup with {\it bit} queries and an
exponential speedup with {\it power} queries. Bit queries are similar to the
oracle calls used in Grover's algorithm appropriately extended to real valued
functions. Power queries are used for a number of problems including phase
estimation. They are obtained by considering the propagator of the discretized
system at a number of different time moments. They allow us to use powers of
the unitary matrix $\exp(\tfrac12 {\rm i}M)$, where $M$ is an $n\times n$
matrix obtained from the standard discretization of the Sturm-Liouville
differential operator. The quantum implementation of power queries by a number
of elementary quantum gates that is polylog in $n$ is an open issue.


title: Classical Capacity of Quantum Binary Adder Channels

abstract:
  We analyze the quantum binary adder channel, i.e. the quantum generalization
of the classical, and well-studied, binary adder channel: in this model qubits
rather than classical bits are transmitted. This of course is as special case
of the general theory of quantum multiple access channels, and we may apply the
established formulas for the capacity region to it. However, the binary adder
channel is of particular interest classically, which motivates our generalizing
it to the quantum domain. It turns out to be a very nice case study not only of
multi-user quantum information theory, but also on the role entanglement plays
there. It turns out that the analogous classical situation, the multi-user
channel supported by shared randomness, is not distinct from the channel
without shared randomness, as far as rates are concerned. However, we discuss
the effect the new resource has on error probabilities, in an appendix.
  We focus specially on the effect entanglement between the senders as well as
between senders and receiver has on the capacity region. Interestingly, in some
of these cases one can devise rather simple codes meeting the capacity bounds,
even in a zero-error model, which is in marked difference to code construction
in the classical case.


title: Decoherence Effects in Reactive Scattering

abstract:
  Decoherence effects on quantum and classical dynamics in reactive scattering
are examined using a Caldeira-Leggett type model. Through a study of dynamics
of the collinear H+H2 reaction and the transmission over simple one-dimensional
barrier potentials, we show that decoherence leads to improved agreement
between quantum and classical reaction and transmission probabilities,
primarily by increasing the energy dispersion in a well defined way. Increased
potential nonlinearity is seen to require larger decoherence in order to attain
comparable quantum-classical agreement.


title: Entanglement of Pure Two-Mode Gaussian States

abstract:
  The entanglement of general pure Gaussian two-mode states is examined in
terms of the coefficients of the quadrature components of the wavefunction. The
entanglement criterion and the entanglement of formation are directly evaluated
as a function of these coefficients, without the need for deriving local
unitary transformations. These reproduce the results of other methods for the
special case of symmetric pure states which employ a relation between squeezed
states and Einstein-Podolsky-Rosen correlations. The modification of the
quadrature coefficients and the corresponding entanglement due to application
of various optical elements is also derived.


title:
Analytic solution to N vs. M photon phase control in an open two-level
  system

abstract:
  Decoherence effects on the traditional N vs. M photon coherent control of a
two-level system are investigated, with 1 vs. 3 used as a specific example. The
problem reduces to that of a two-level system interacting with a single mode
field, but with an effective Rabi frequency that depends upon the fundamental
and third harmonic fields. The resultant analytic control solution is explored
for a variety of parameters, with emphasis on the dependence of control on the
relative phase of the lasers. The generalization to off-resonant cases is
noted.


title:
Calculation of accurate permanent dipole moments of the lowest $^{1,3}
  \Sigma^+$ states of heteronuclear alkali dimers using extended basis sets

abstract:
  The obtention of ultracold samples of dipolar molecules is a current
challenge which requires an accurate knowledge of their electronic properties
to guide the ongoing experiments. In this paper, we systematically investigate
the ground state and the lowest triplet state of mixed alkali dimers (involving
Li, Na, K, Rb, Cs) using a standard quantum chemistry approach based on
pseudopotentials for atomic core representation, gaussian basis sets, and
effective terms for core polarization effects. We emphasize on the convergence
of the results for permanent dipole moments regarding the size of the gaussian
basis set, and we discuss their predicted accuracy by comparing to other
theoretical calculations or available experimental values. We also revisit the
difficulty to compare computed potential curves among published papers, due to
the differences in the modelization of core-core interaction.


title: Experimental reconstruction of photon statistics without photon counting

abstract:
  Experimental reconstructions of photon number distributions of both
continuous-wave and pulsed light beams are reported. Our scheme is based on
on/off avalanche photodetection assisted by maximum-likelihood estimation and
does not involve photon counting. Reconstructions of the distribution for both
semiclassical and quantum states of light are reported for single-mode as well
as for multimode beams.


title:
Barbosa et al. Reply to ``Comment on 'Secure Communication using
  mesoscopic coherent states', Barbosa et al, Phys Rev Lett 90, 227901", Yuan
  and Shields, Phys. Rev. Lett. 94, 048901(2005)

abstract:
  Yuan and Shields claim that our data-encryption protocol is entirely
equivalent to a classical stream cipher utilizing no quantum phenomena. Their
claim is, indeed, false. Yuan and Shields also claim that schemes similar to
the one presented in Phys. Rev. Lett. 90, 227901 are not suitable for key
generation. This claim is also refuted. In any event, we welcome the
opportunity to clarify the situation for a wider audience.


title: Separability conditions from the Landau-Pollak uncertainty relation

abstract:
  We obtain a collection of necessary (sufficient) conditions for a bipartite
system of qubits to be separable (entangled), which are based on the
Landau-Pollak formulation of the uncertainty principle. These conditions are
tested, and compared with previously stated criteria, by applying them to
states whose separability limits are already known. Our results are also
extended to multipartite and higher-dimensional systems.


title: Hyperfine Coherence in the Presence of Spontaneous Photon Scattering

abstract:
  The coherence of a hyperfine-state superposition of a trapped $^{9}$Be$^+$
ion in the presence of off-resonant light is experimentally studied. It is
shown that Rayleigh elastic scattering of photons that does not change state
populations also does not affect coherence. Coherence times exceeding the
average scattering time of 19 photons are observed. This result implies that,
with sufficient control over its parameters, laser light can be used to
manipulate hyperfine-state superpositions with very little decoherence.