title: Superadditivity of distillable entanglement From Quantum Teleportation

abstract:
  We show that the phenomenon of superadditivity of distillable entanglement
observed in multipartite quantum systems results from the consideration of
states created during the execution of the standard end-to-end quantum
teleportation protocol (and a few additional local operations and classical
communication (LOCC) steps) on a linear chain of singlets. Some of these
intermediate states are tensor products of bound-entangled (BE) states, and
hence, by construction possess distillable entanglement, which can be unlocked
by simply completing the rest of the LOCC operations required by the underlying
teleportation protocol. We use this systematic approach to construct both new
and known examples of superactivation of bound entanglement, and first examples
of activation of BE states using other BE states. A surprising outcome is the
construction of noiseless quantum relay channels with no distillable
entanglement between any two parties, except for that between the two end
nodes.


title: Comment on the sign of the Casimir force

abstract:
  I show that reflection positivity implies that the force between any mirror
pair of charge-conjugate probes of the quantum vacuum is attractive. This
generalizes a recent theorem of Kenneth and Klich to interacting quantum
fields, to arbitrary semiclassical bodies, and to quantized probes with
non-overlapping wavefunctions. I also prove that the torques on
charge-conjugate probes tend always to rotate them into a mirror-symmetric
position.


title:
Explicit form of correlation function three-settings tight Bell
  inequalities for three qubits

abstract:
  We present a method to derive explicit forms of tight correlation function
Bell inequalities for three systems and dichotomic observables, which involve
three settings for each observer. We also give sufficient conditions for
quantum predictions to satisfy the new inequalities.


title:
Coherent control in a decoherence-free subspace of a collective
  multi-level system

abstract:
  Decoherence-free subspaces (DFS) in systems of dipole-dipole interacting
multi-level atoms are investigated theoretically. It is shown that the
collective state space of two dipole-dipole interacting four-level atoms
contains a four-dimensional DFS. We describe a method that allows to populate
the antisymmetric states of the DFS by means of a laser field, without the need
of a field gradient between the two atoms. We identify these antisymmetric
states as long-lived entangled states. Further, we show that any single-qubit
operation between two states of the DFS can be induced by means of a microwave
field. Typical operation times of these qubit rotations can be significantly
shorter than for a nuclear spin system.


title: Generation of Long-Lived Isomeric States via Bremsstrahlung Irradiation

abstract:
  A method to generate long-lived isomeric states effectively for Mossbauer
applications is reported. We demonstrate that this method is better and easier
to provide highly sensitive Mossbauer effect of long-lived isomers (>1ms) such
as 103Rh. Excitation of (gamma,gamma) process by synchrotron radiation is
painful due mainly to their limited linewidth. Instead,(gamma,gamma') process
of bremsstrahlung excitation is applied to create these long-lived isomers.
Isomers of 45Sc, 107Ag, 109Ag, and 103Rh have been generated from this method.
Among them, 103Rh is the only one that we have obtained the gravitational
effect at room temperature.


title:
All tight multipartite Bell correlation inequalities for three
  dichotomic observables per observer

abstract:
  A derivation of the full set of Bell inequalities involving correlation
functions, for two parties, with binary observables, and three possible local
settings. The procedure can be extended straightforwardly to multiparty
correlations.


title:
Optimum unambiguous discrimination of two mixed states and application
  to a class of similar states

abstract:
  We study the measurement for the unambiguous discrimination of two mixed
quantum states that are described by density operators $\rho_1$ and $\rho_2$ of
rank d, the supports of which jointly span a 2d-dimensional Hilbert space.
Based on two conditions for the optimum measurement operators, and on a
canonical representation for the density operators of the states, two equations
are derived that allow the explicit construction of the optimum measurement,
provided that the expression for the fidelity of the states has a specific
simple form. For this case the problem is mathematically equivalent to
distinguishing pairs of pure states, even when the density operators are not
diagonal in the canonical representation. The equations are applied to the
optimum unambiguous discrimination of two mixed states that are similar states,
given by $\rho_2= U\rho_1 U^{\dag}$, and that belong to the class where the
unitary operator U can be decomposed into multiple rotations in the d mutually
orthogonal two-dimensional subspaces determined by the canonical
representation.


title: Quantum superchemistry in an output coupler of coherent matter waves

abstract:
  We investigate the quantum superchemistry or Bose-enhanced atom-molecule
conversions in a coherent output coupler of matter waves, as a simple
generalization of the two-color photo-association. The stimulated effects of
molecular output step and atomic revivals are exhibited by steering the rf
output couplings. The quantum noise-induced molecular damping occurs near a
total conversion in a levitation trap. This suggests a feasible two-trap scheme
to make a stable coherent molecular beam.


title: Quantum Zero-error Capacity

abstract:
  We define here a new kind of quantum channel capacity by extending the
concept of zero-error capacity for a noisy quantum channel. The necessary
requirement for which a quantum channel has zero-error capacity greater than
zero is given. Finally, we point out some directions on how to calculate the
zero-error capacity of such channels.


title:
Decoherence Free Subspace and entanglement by interaction with a common
  squeezed bath

abstract:
  In this work we find explicitly the decoherence free subspace (DFS) for a two
two-level system in a common squeezed vacuum bath. We also find an orthogonal
basis for the DFS composed of a symmetrical and an antisymmetrical (under
particle permutation) entangled state. For any initial symmetrical state, the
master equation has one stationary state which is the symmetrical entangled
decoherence free state. In this way, one can generate entanglement via common
squeezed bath of the two systems. If the initial state does not have a definite
parity, the stationary state depends strongly on the initial conditions of the
system and it has a statistical mixture of states which belong to the DFS. We
also study the effect of the coupling between the two-level systems on the DFS.


title: On the existence of negative energy states in QED in the temporal gauge

abstract:
  It is generally assumed that the vacuum state is the quantum state with the
lowest energy. However, it has been shown that this is not the case for a
Dirac-Maxwell field in the temporal gauge. In this paper we will present
another proof, different from that presented in previous work, which shows that
the vacuum state is not the minimum energy state for a Dirac-Maxwell field in
the temporal gauge.


title:
Linear dynamical entropy and free-independence for quantized maps on the
  torus

abstract:
  We study the relations between the averaged linear entropy production in
periodically measured quantum systems and ergodic properties of their classical
counterparts. Quantized linear automorphisms of the torus, both classically
chaotic and regular ones, are used as examples. Numerical calculations show
different entropy production regimes depending on the relation between the
Kolmogorov-Sinai entropy and the measurement entropy. The hypothesis of free
independence relations between the dynamics and measurement proposed to explain
the initial constant and maximal entropy production is tested numerically for
those models.


title: Quantum processing photonic states in optical lattices

abstract:
  The mapping of photonic states to collective excitations of atomic ensembles
is a powerful tool which finds a useful application in the realization of
quantum memories and quantum repeaters. In this work we show that cold atoms in
optical lattices can be used to perform an entangling unitary operation on the
transferred atomic excitations. After the release of the quantum atomic state,
our protocol results in a deterministic two qubit gate for photons. The
proposed scheme is feasible with current experimental techniques and robust
against the dominant sources of noise.


title: Operational Axioms for Quantum Mechanics

abstract:
  The mathematical formulation of Quantum Mechanics in terms of complex Hilbert
space is derived for finite dimensions, starting from a general definition of
"physical experiment" and from five simple Postulates concerning "experimental
accessibility and simplicity". For the infinite dimensional case, on the other
hand, a C*-algebra representation of physical transformations is derived,
starting from just four of the five Postulates via a Gelfand-Naimark-Segal
(GNS) construction. The present paper simplifies and sharpens the previous
derivation in version 1. The main ingredient of the axiomatization is the
postulated existence of "faithful states" that allows one to calibrate the
experimental apparatus. Such notion is at the basis of the operational
definitions of the scalar product and of the "transposed" of a physical
transformation. What is new in the present paper with respect to
quant-ph/0603011 is the operational deduction of an involution corresponding to
the "complex-conjugation" for effects, whose extension to transformations
allows to define the "adjoint" of a transformation when the extension is
composition-preserving.


title:
Unambiguous discrimination of mixed states: A description based on
  system-ancilla coupling

abstract:
  We propose a general description on the unambiguous discrimination of mixed
states according to the system-environment coupling, and present a procedure to
reduce this to a standard semidefinite programming problem. In the two states
case, we introduce the canonical vectors and partly simplify the problem to the
case of discrimination between pairs of canonical vectors. By considering the
positivity of the two by two matrices, we obtain a series of new upper bounds
of the total success probability which depends on both the prior probabilities
and specific state structures.


title: Physical accessible transformations on a finite number of quantum states

abstract:
  We consider to treat the usual probabilistic cloning, state separation,
unambiguous state discrimination, \emph{etc} in a uniform framework. All these
transformations can be regarded as special examples of generalized completely
positive trace non-increasing maps on a finite number of input states. From the
system-ancilla model we construct the corresponding unitary implementation of
pure $\to$ pure, pure $\to$ mixed, mixed $\to$ pure, and mixed $\to$ mixed
states transformations in the whole system and obtain the necessary and
sufficient conditions on the existence of the desired maps. We expect our work
will be helpful to explore what we can do on a finite set of input states.


title:
Ultraslow Propagation of Squeezed Vacuum Pulses with Electromagnetically
  Induced Transparency

abstract:
  We have succeeded in observing ultraslow propagation of squeezed vacuum
pulses with electromagnetically induced transparency. Squeezed vacuum pulses
(probe lights) were incident on a laser cooled 87Rb gas together with an
intense coherent light (control light). A homodyne method sensitive to the
vacuum state was employed for detecting the probe pulse passing through the
gas. A delay of 3.1us was observed for the probe pulse having a temporal width
of 10 us.


title:
Near field in quantum electrodynamics: Green functions, Lorentz
  condition, "nonlocality in the small", frustrated total reflection

abstract:
  Investigation of near field of QED requires the refuse from an averaging of
the Lorentz condition that smooths out some field peculiarities. Instead of it
Schwinger decomposition of the 4-potential with the Bogoliubov method of
interaction switching in time and in space regions is considered. At such
approach near field is describable by the part of covariant Green function of
QED, the fast-damping Schwinger function formed by longitudinal and scalar
components of Aμ none restricted by light cone. This description reveals
possibility of superluminal phenomena within the near field zone as a
"nonlocality in the small". Some specification of Bogoliubov method allows, as
examples, descriptions of near fields of point-like charge and at FTIR
phenomena. Precisely such possibilities of nonlocal interactions are revealed
in the common QED expressions for the Van-der-Waals and Casimir interactions
and in the F\"{o}rster law.
  Key words: Lorentz condition, near field, propagators, superluminal, FTIR.
  PACS: 03.30.+p, 12.20.-m, 13.40.-f, 68.37.Uv.


title:
Traceable 2D finite-element simulation of the whispering-gallery modes
  of axisymmetric electromagnetic resonators

abstract:
  This paper explains how a popular, commercially-available software package
for solving partial-differential-equations (PDEs), as based on the
finite-element method (FEM), can be configured to calculate, efficiently, the
frequencies and fields of the whispering-gallery (WG) modes of axisymmetric
dielectric resonators. The approach is traceable; it exploits the PDE-solver's
ability to accept the definition of solutions to Maxwell's equations in
so-called `weak form'. Associated expressions and methods for estimating a WG
mode's volume, filling factor(s) and, in the case of closed(open) resonators,
its wall(radiation) loss, are provided. As no transverse approximation is
imposed, the approach remains accurate even for quasi-transverse
magnetic/electric modes of low, finite azimuthal mode order. The approach's
generality and utility are demonstrated by modeling several non-trivial
structures: (i) two different optical microcavities [one toroidal made of
silica, the other an AlGaAs microdisk]; (ii) a 3rd-order sapphire:air Bragg
cavity; (iii) two different cryogenic sapphire WG-mode resonators; both (ii)
and (iii) operate in the microwave X-band. By fitting one of (iii) to a set of
measured resonance frequencies, the dielectric constants of sapphire at
liquid-helium temperature have been estimated.


title: Homodyne QPSK Detection for Quantum Key Distribution

abstract:
  We present a QKD system with fainted pulses using self-homodyne coherent
detection in optical fibers at 1543nm. BB84 protocol key is encoded in the
optical phase using a twoelectrode Mach-Zehnder modulator, producing a QPSK
modulation.


title:
Development of a high sensitivity torsional balance for the study of the
  Casimir force in the 1-10 micrometer range

abstract:
  We discuss a proposal to measure the Casimir force in the parallel plate
configuration in the $1-10\mu$m range via a high-sensitivity torsional balance.
This will allow to measure the thermal contribution to the Casimir force
therefore discriminating between the various approaches discussed so far. The
accurate control of the Casimir force in this range of distances is also
required to improve the limits to the existence of non-Newtonian forces in the
micrometer range predicted by unification models of fundamental interactions.


title: Integrating a QPSK Quantum Key Distribution Link

abstract:
  We present the integration of the optical and electronic subsystems of a
BB84-QKD fiber link. A highspeed FPGA MODEM generates the random QPSK sequences
for a fiber-optic delayed self-homodyne scheme using APD detectors.


title: The Casimir effect within scattering theory

abstract:
  We review the theory of the Casimir effect using scattering techniques. After
years of theoretical efforts, this formalism is now largely mastered so that
the accuracy of theory-experiment comparisons is determined by the level of
precision and pertinence of the description of experimental conditions. Due to
an imperfect knowledge of the optical properties of real mirrors used in the
experiment, the effect of imperfect reflection remains a source of uncertainty
in theory-experiment comparisons. For the same reason, the temperature
dependence of the Casimir force between dissipative mirrors remains a matter of
debate. We also emphasize that real mirrors do not obey exactly the assumption
of specular reflection, which is used in nearly all calculations of material
and temperature corrections. This difficulty may be solved by using a more
general scattering formalism accounting for non-specular reflection with
wavevectors and field polarizations mixed. This general formalism has already
been fruitfully used for evaluating the effect of roughness on the Casimir
force as well as the lateral Casimir force appearing between corrugated
surfaces. The commonly used `proximity force approximation' turns out to lead
to inaccuracies in the description of these two effects.


title: Force on a neutral atom near conducting microstructures

abstract:
  We derive the non-retarded energy shift of a neutral atom for two different
geometries. For an atom close to a cylindrical wire we find an integral
representation for the energy shift, give asymptotic expressions, and
interpolate numerically. For an atom close to a semi-infinite halfplane we
determine the exact Green's function of the Laplace equation and use it derive
the exact energy shift for an arbitrary position of the atom. These results can
be used to estimate the energy shift of an atom close to etched microstructures
that protrude from substrates.


title: Ab-initio path integral techniques for molecules

abstract:
  Path integral Monte Carlo with Green's function analysis allows the sampling
of quantum mechanical properties of molecules at finite temperature. While a
high-precision computation of the energy of the Born-Oppenheimer surface from
path integral Monte Carlo is quite costly, we can extract many properties
without explicitly calculating the electronic energies. We demonstrate how
physically relevant quantities, such as bond-length, vibrational spectra, and
polarizabilities of molecules may be sampled directly from the path integral
simulation using Matsubura (temperature) Green's functions (imaginary-time
correlation functions). These calculations on the hydrogen molecule are a
proof-of-concept, designed to motivate new work on fixed-node path-integral
calculations for molecules.


title: Pauli Diagonal Channels Constant on Axes

abstract:
  We define and study the properties of channels which are analogous to unital
qubit channels in several ways. A full treatment can be given only when the
dimension d is a prime power, in which case each of the (d+1) mutually unbiased
bases (MUB) defines an axis. Along each axis the channel looks like a
depolarizing channel, but the degree of depolarization depends on the axis.
When d is not a prime power, some of our results still hold, particularly in
the case of channels with one symmetry axis. We describe the convex structure
of this class of channels and the subclass of entanglement breaking channels.
We find new bound entangled states for d = 3.
  For these channels, we show that the multiplicativity conjecture for maximal
output p-norm holds for p=2. We also find channels with behavior not exhibited
by unital qubit channels, including two pairs of orthogonal bases with equal
output entropy in the absence of symmetry. This provides new numerical evidence
for the additivity of minimal output entropy.


title:
Optimal and covariant single-copy LOCC transformation between two
  two-qubit states

abstract:
  Given two two-qubit pure states characterized by their Schmidt numbers we
investigate an optimal strategy to convert the states between themselves with
respect to their local unitary invariance. We discuss the efficiency of this
transformation and its connection to LOCC convertibility properties between two
single-copy quantum states. As an illustration of the investigated
transformations we present a communication protocol where in spite of all
expectations a shared maximally entangled pair between two participants is the
worst quantum resource.


title: Environmental effects on electron spin relaxation in N@C60

abstract:
  We examine environmental effects of surrounding nuclear spins on the electron
spin relaxation of the N@C60 molecule (which consists of a nitrogen atom at the
centre of a fullerene cage). Using dilute solutions of N@C60 in regular and
deuterated toluene, we observe and model the effect of translational diffusion
of nuclear spins of the solvent molecules on the N@C60 electron spin relaxation
times. We also study spin relaxation in frozen solutions of N@C60 in CS2, to
which small quantities of a glassing agent, S2Cl2 are added. At low
temperatures, spin relaxation is caused by spectral diffusion of surrounding
nuclear 35Cl and 37Cl spins in the S2Cl2, but nevertheless, at 20 K, T2 times
as long as 0.23 ms are observed.


title:
Intercontinental quantum liaisons between entangled electrons in ion
  traps of thermoluminescent crystals

abstract:
  The experiments reported in this paper were carried out with space-separated
entangled thermoluminescent dosimetry (TLD) crystals in Baton Rouge, Louisiana
(USA) and Givarlais (France) at 8,182 km between entangled samples. Samples
consisted of doped lithium fluoride TLD's that were simultaneously irradiated
in pairs together at one location by Bremsstrahlung radiation generated by a
Varian CLINAC unit. One of the paired TLD crystals was then mailed to Baton
Rouge and its entangled counterpart remained in Givarlais. The crystal in Baton
Rouge (master) was then subjected to thermal stimulation which elicited a
measurable light emission response in the counterpart (slave) under a
photomultiplier in Givarlais. Highly correlated passive light emissions were
observed in the nonheated slave TLD while the master TLD was ramped up in
temperature and then allowed to cool to ambient temperature. Maximum
correlations in the slave TLD light emissions were observed at the turn around
temperature which is the point where the master TLD temperature is allowed to
decrease. The experimenter in Girvalais was thus able to determine with high
accuracy the point in time at which the master TLD heating oven was turned off
(turn around point) without any communication between the experimenters during
the heating-cooling phase of the experiment. The implications of these observed
results are of great significance for quantum communication technology.


title:
Coordinating quantum agents' perspectives: convex operational theories,
  quantum information, and quantum foundations

abstract:
  In this paper, I propose a project of enlisting quantum information science
as a source of task-oriented axioms for use in the investigation of operational
theories in a general framework capable of encompassing quantum mechanics,
classical theory, and more. Whatever else they may be, quantum states of
systems are compendia of probabilities for the outcomes of possible operations
we may perform on the systems: ``operational theories.'' I discuss appropriate
general frameworks for such theories, in which convexity plays a key role. Such
frameworks are appropriate for investigating what things look like from an
``inside view,'' i.e. for describing perspectival information that one
subsystem of the world can have about another. Understanding how such views can
combine, and whether an overall ``geometric'' picture (``outside view'')
coordinating them all can be had, even if this picture is very different in
nature from the structure of the perspectives within it, is the key to
understanding whether we may be able to achieve a unified, ``objective''
physical view in which quantum mechanics is the appropriate description for
certain perspectives, or whether quantum mechanics is truly telling us we must
go beyond this ``geometric'' conception of physics. The nature of information,
its flow and processing, as seen from various operational persepectives, is
likely to be key to understanding whether and how such coordination and
unification can be achieved.


title: Channel correction via quantum erasure

abstract:
  By exploiting a generalization of recent results on environment-assisted
channel correction, we show that, whenever a quantum system undergoes a channel
realized as an interaction with a probe, the more efficiently the information
about the input state can be erased from the probe, the higher is the
corresponding entanglement fidelity of the corrected channel, and vice-versa.
The present analysis applies also to channels for which perfect quantum erasure
is impossible, thus extending the original quantum eraser arrangement, and
naturally embodies a general information-disturbance tradeoff.


title:
Heralded single photon source at 1550 nm from pulsed parametric down
  conversion

abstract:
  Heralding of single photon at 1550 nm from pump pulsed non degenerate
spontaneous parametric downconversion is demonstrated. P(1) and P(2) of our
source are 0.1871 and 2.4 x 10 ^-3 respectively. Triggering of our source is
2.16 x 10^5 trigger.s^-1. This source may be used in QKD system.


title:
Non-Markovian Decay and Lasing Condition in an Optical Microcavity
  Coupled to a Structured Reservoir

abstract:
  The decay dynamics of the classical electromagnetic field in a leaky optical
resonator supporting a single mode coupled to a structured continuum of modes
(reservoir) is theoretically investigated, and the issue of threshold condition
for lasing in presence of an inverted medium is comprehensively addressed.
Specific analytical results are given for a single-mode microcavity resonantly
coupled to a coupled resonator optical waveguide (CROW), which supports a band
of continuous modes acting as decay channels. For weak coupling, the usual
exponential Weisskopf-Wigner (Markovian) decay of the field in the bare
resonator is found, and the threshold for lasing increases linearly with the
coupling strength. As the coupling between the microcavity and the structured
reservoir increases, the field decay in the passive cavity shows non
exponential features, and correspondingly the threshold for lasing ceases to
increase, reaching a maximum and then starting to decrease as the coupling
strength is further increased. A singular behavior for the "laser phase
transition", which is a clear signature of strong non-Markovian dynamics, is
found at critical values of the coupling between the microcavity and the
reservoir.


title:
New Inequalities for Tomograms in the Probability Representation of
  Quantum States

abstract:
  New inequalities for symplectic tomograms of quantum states and their
connection with entropic uncertainty relations are discussed within the
framework of the probability representation of quantum mechanics.


title: Probabilistic Teleportation of a Qudit

abstract:
  It is known that if the shared resource is a maximally entangled state then
it is possible to teleport an unknown state with unit fidelity and unit
probability. However, if the shared resource is a non-maximally entangled state
then one has to follow a probabilistic scheme where one can teleport a qubit
with unit fidelity and non-unit probability. In this work, we investigate the
feasibility of using partially entangled states as a resource for quantum
teleportation of a qudit. We also give an expression for the probability of
successful teleportation of an unknown qudit.


title:
Quantum mechanics on a circle: Husimi phase space distributions and
  semiclassical coherent state propagators

abstract:
  We discuss some basic tools for an analysis of one-dimensionalquantum systems
defined on a cyclic coordinate space. The basic features of the generalized
coherent states, the complexifier coherent states are reviewed. These states
are then used to define the corresponding (quasi)densities in phase space. The
properties of these generalized Husimi distributions are discussed, in
particular their zeros.Furthermore, the use of the complexifier coherent states
for a semiclassical analysis is demonstrated by deriving a semiclassical
coherent state propagator in phase space.


title:
Decay and storage of multiparticle entangled states of atoms in
  collective thermostat

abstract:
  We derive a master equation describing the collective decay of two-level
atoms inside a single mode cavity in the dispersive limit. By considering
atomic decay in the collective thermostat, we found a decoherence-free subspace
of the multiparticle entangled states of the W-like class. We present a scheme
for writing and storing these states in collective thermostat.


title: Thermal and dissipative effects in Casimir physics

abstract:
  We report on current efforts to detect the thermal and dissipative
contributions to the Casimir force. For the thermal component, two experiments
are in progress at Dartmouth and at the Institute Laue Langevin in Grenoble.
The first experiment will seek to detect the Casimir force at the largest
explorable distance using a cylinder-plane geometry which offers various
advantages with respect to both sphere-plane and parallel-plane geometries. In
the second experiment, the Casimir force in the parallel-plane configuration is
measured with a dedicated torsional balance, up to 10 micrometers. Parallelism
of large surfaces, critical in this configuration, is maintained through the
use of inclinometer technology already implemented at Grenoble for the study of
gravitationally bound states of ultracold neutrons, For the dissipative
component of the Casimir force, we discuss detection techniques based upon the
use of hyperfine spectroscopy of ultracold atoms and Rydberg atoms. Although
quite challenging, this triad of experimental efforts, if successful, will give
us a better knowledge of the interplay between quantum and thermal fluctuations
of the electromagnetic field and of the nature of dissipation induced by the
motion of objects in a quantum vacuum.


title: Basic Logic and Quantum Entanglement

abstract:
  As it is well known, quantum entanglement is one of the most important
features of quantum computing, as it leads to massive quantum parallelism,
hence to exponential computational speed-up. In a sense, quantum entanglement
is considered as an implicit property of quantum computation itself. But...can
it be made explicit? In other words, is it possible to find the connective
"entanglement" in a logical sequent calculus for the machine language? And
also, is it possible to "teach" the quantum computer to "mimic" the EPR
"paradox"? The answer is in the affirmative, if the logical sequent calculus is
that of the weakest possible logic, namely Basic logic. A weak logic has few
structural rules. But in logic, a weak structure leaves more room for
connectives (for example the connective "entanglement"). Furthermore, the
absence in Basic logic of the two structural rules of contraction and weakening
corresponds to the validity of the no-cloning and no-erase theorems,
respectively, in quantum computing.


title:
Continuous-Variables Quantum Key Distribution over Standard Telecom
  Fiber

abstract: This paper has been withdrawn


title:
A measurement-based measure of the size of macroscopic quantum
  superpositions

abstract:
  Recent experiments claiming formation of quantum superposition states in near
macroscopic sys- tems raise the question of how the sizes of general quantum
superposition states in an interacting system are to be quantified. We propose
here a measure of size for such superposition states that is based on what
measurements can be performed to probe and distinguish the different branches
of the state. The measure allows comparison of the effective size for
superposition states in very different physical systems. It can be applied to a
very general class of superposition states and reproduces known results for
near-ideal cases. Comparison with a prior measure based on analy- sis of
coherence between branches indicates that significantly smaller effective
superposition sizes result from our measurement-based measure. Application to a
system of interacting bosons in a double-well trapping potential shows that the
effective superposition size is strongly dependent on the relative magnitude of
the barrier height and interparticle interaction.


title: Discrimination between pure states and mixed states

abstract:
  In this paper, we discuss the problem of determining whether a quantum system
is in a pure state, or in a mixed state. We apply two strategies to settle this
problem: the unambiguous discrimination and the maximum confidence
discrimination. We also proved that the optimal versions of both strategies are
equivalent. The efficiency of the discrimination is also analyzed. This scheme
also provides a method to estimate purity of quantum states, and Schmidt
numbers of composed systems.


title:
Photon Counting and Super Homodyne Detection of Weak QPSK Signals for
  Quantum Key Distribution

abstract:
  We compare the principles and experimental results of two different QPSK
signal detection configurations, photon counting and super homodyning, for
applications in fiber-optic Quantum Key Distribution (QKD) systems operating at
telecom wavelength, using the BB84 protocol.


title: Stronger Violation of Local Theories with Equalities

abstract:
  Bell type inequalities are used to test local realism against quantum
theory.In this paper, we consider a two party system with two settings and two
possible outcomes on each side, and derive equalities in local theories which
are violated by quantum theory by a factor of 1.522 tolerating 0.586 fraction
of white noise admixture which is twice that of the previous results.


title: Structure of propagators for quantum nondemolition systems

abstract:
  In the scheme of a quantum nondemolition (QND) measurement, an observable is
measured without perturbing its evolution. In the context of studies of
decoherence in quantum computing, we examine the `open' quantum system of a
two-level atom, or equivalently, a spin-1/2 system, in interaction with quantum
reservoirs of either oscillators or spins, under the QND condition of the
Hamiltonian of the system commuting with the system-reservoir interaction. The
propagators for these QND Hamiltonians are shown to be connected to the
squeezing and rotation operators for the two baths, respectively. Squeezing and
rotation being both phase space area-preserving canonical transformations, this
brings out an interesting analogy between the energy-preserving QND
Hamiltonians and the homogeneous linear canonical transformations.


title: Variants of Bell inequalities

abstract:
  A family of Bell-type inequalities is present, which are constructed directly
from the "standard" Bell inequalities involving two dichotomic observables per
site. It is shown that the inequalities are violated by all the generalized
Greenberger-Horne-Zeilinger states of multiqubits. Remarkably, our new
inequalities can provide stronger non-locality tests in a sense that the local
reality inequalities are exponentially stronger than the corresponding
multipartite separability inequalities. This reveals that the exponential
violation of local realism by separable states is an interesting consequence of
quantum fluctuation of multipartite systems.


title: Functional integral treatment of some quantum nondemolition systems

abstract:
  In the scheme of a quantum nondemolition (QND) measurement, an observable is
measured without perturbing its evolution. In the context of studies of
decoherence in quantum computing, we examine the `open' quantum system of a
two-level atom, or equivalently, a spin-1/2 system, in interaction with quantum
reservoirs of either oscillators or spins, under the QND condition of the
Hamiltonian of the system commuting with the system-reservoir interaction. For
completeness, we also examine the well-known non-QND spin-Bose problem. For all
these many-body systems, we use the methods of functional integration to work
out the propagators. The propagators for the QND Hamiltonians are shown to be
analogous to the squeezing and rotation operators, respectively, for the two
kinds of baths considered. Squeezing and rotation being both phase space
area-preserving canonical transformations, this brings out an interesting
connection between the energy-preserving QND Hamiltonians and the homogeneous
linear canonical transformations.


title: Dual species matter qubit entangled with light

abstract:
  We propose and demonstrate an atomic qubit based on a cold
$^{85}$Rb-$^{87}$Rb isotopic mixture, entangled with a frequency-encoded
optical qubit. The interface of an atomic qubit with a single spatial light
mode, and the ability to independently address the two atomic qubit states,
should provide the basic element of an interferometrically robust quantum
network.


title: Continuous-Time Quantum Random Walks Require Discrete Space

abstract:
  Quantum random walks are shown to have non-intuitive dynamics which makes
them an attractive area of study for devising quantum algorithms for
long-standing open problems as well as those arising in the field of quantum
computing. In the case of continuous-time quantum random walks, such peculiar
dynamics can arise from simple evolution operators closely resembling the
quantum free-wave propagator. We investigate the divergence of quantum walk
dynamics from the free-wave evolution and show that in order for
continuous-time quantum walks to display their characteristic propagation, the
state space must be discrete. This behavior rules out many continuous quantum
systems as possible candidates for implementing continuous-time quantum random
walks.


title: Entanglement and optimal strings of qubits for memory channels

abstract:
  We investigate the problem of enhancement of mutual information by encoding
classical data into entangled input states of arbitrary length and show that
while there is a threshold memory or correlation parameter beyond which
entangled states outperform the separable states, resulting in a higher mutual
information, this memory threshold increases toward unity as the length of the
string increases. These observations imply that encoding classical data into
entangled states may not enhance the classical capacity of quantum channels.


title: Homological invariants of stabilizer states

abstract:
  We propose a new kind of invariant of multi-party stabilizer states with
respect to local Clifford equivalence. These homological invariants are
discrete entities defined in terms of the entanglement a state enjoys with
respect to arbitrary groupings of the parties, and they may be thought of as
reflecting entanglement in a qualitative way. We investigate basic properties
of the invariants and link them with known results on the extraction of GHZ
states.


title: Delocalized Entanglement of Atoms in optical Lattices

abstract:
  We show how to detect and quantify entanglement of atoms in optical lattices
in terms of correlations functions of the momentum distribution. These
distributions can be measured directly in the experiments. We introduce two
kinds of entanglement measures related to the position and the spin of the
atoms.


title:
Unambiguous State Discrimination of two density matrices in Quantum
  Information Theory

abstract:
  In this thesis we study the problem of unambiguously discriminating two mixed
quantum states. We first present reduction theorems for optimal unambiguous
discrimination of two generic density matrices. We show that this problem can
be reduced to that of two density matrices that have the same rank $r$ in a
2$r$-dimensional Hilbert space. These reduction theorems also allow us to
reduce USD problems to simpler ones for which the solution might be known. As
an application, we consider the unambiguous comparison of $n$ linearly
independent pure states with a simple symmetry. Moreover, lower bounds on the
optimal failure probability have been derived. For two mixed states they are
given in terms of the fidelity. Here we give tighter bounds as well as
necessary and sufficient conditions for two mixed states to reach these bounds.
We also construct the corresponding optimal measurement. With this result, we
provide analytical solutions for unambiguously discriminating a class of
generic mixed states. This goes beyond known results which are all reducible to
some pure state case. We however show that examples exist where the bounds
cannot be reached. Next, we derive properties on the rank and the spectrum of
an optimal USD measurement. This finally leads to a second class of exact
solutions. Indeed we present the optimal failure probability as well as the
optimal measurement for unambiguously discriminating any pair of geometrically
uniform mixed states in four dimensions. This class of problems includes for
example the discrimination of both the basis and the bit value mixed states in
the BB84 QKD protocol with coherent states.


title:
The One Dimensional Approachissimo Quantum Harmonic Oscillator: The
  Hilbert-Polya Hamiltonian for the Primes and the Zeros of the Riemann
  Function

abstract:
  I have made an ample study of one dimensional quantum oscillators, ranging
from logarithmic to exponential potentials. I have found that the eigenvalues
of the hamiltonian of the oscillator with the limiting (approachissimo)
harmonic potential (~ p(x)2) maps the zeros of the Riemann function height up
in the Riemann line. This is the potential created by the field of J(x) that is
the Riemann generator of the prime number counting function, p(x), that in turn
can be defined by an integral transformation of the Riemann zeta function. This
plays the role of the spring strength of the quantum limiting harmonic
oscillator. The number theory meaning of this result is that the roots height
up of the zeta function are the eigenvalues of a Hamiltonian whose potential is
the number of primes squared up to a given x. Therefore this may prove the
never published Hilbert-Polya conjecture. The conjecture is true but does not
imply the truth of the Riemann hypothesis. We can have complex conjugated zeros
off the Riemman line and map them with another hermitic operator and a general
expression is given for that. The zeros off the line affect the fluctuation of
the eigenvalues but not their mean values.


title: Entanglement of a microcanonical ensemble

abstract:
  We replace time-averaged entanglement by ensemble-averaged entanglement and
derive a simple expression for the latter. We show how to calculate the
ensemble average for a two-spin system and for the Jaynes-Cummings model. In
both cases the time-dependent entanglement is known as well so that one can
verify that the time average coincides with the ensemble average.


title:
Nonlocal field correlations and dynamical Casimir-Polder forces between
  one excited- and two ground-state atoms

abstract:
  The problem of nonlocality in the dynamical three-body Casimir-Polder
interaction between an initially excited and two ground-state atoms is
considered. It is shown that the nonlocal spatial correlations of the field
emitted by the excited atom during the initial part of its spontaneous decay
may become manifest in the three-body interaction. The observability of this
new phenomenon is discussed.


title: Minimum-energy pulses for quantum logic cannot be shared

abstract:
  We show that if an electromagnetic energy pulse with average photon number
<n> is used to carry out the same quantum logical operation on a set of N
atoms, either simultaneously or sequentially, the overall error probability in
the worst case scenario (i.e., maximized over all the possible initial atomic
states) scales as N^2/<n>. This means that in order to keep the error
probability bounded by N\epsilon, with \epsilon ~ 1/<n>, one needs to use
N/\epsilon photons, or equivalently N separate "minimum-energy'' pulses: in
this sense the pulses cannot, in general, be shared. The origin for this
phenomenon is found in atom-field entanglement. These results may have
important consequences for quantum logic and, in particular, for large-scale
quantum computation.


title: Atomic detection in microwave cavity experiments: a dynamical model

abstract:
  We construct a model for the detection of one atom maser in the context of
cavity Quantum Electrodynamics (QED) used to study coherence properties of
superpositions of electromagnetic modes. Analytic expressions for the atomic
ionization are obtained, considering the imperfections of the measurement
process due to the probabilistic nature of the interactions between the
ionization field and the atoms. Limited efficiency and false counting rates are
considered in a dynamical context, and consequent results on the information
about the state of the cavity modes are obtained.


title:
Fields of quantum reference frames based on different representations of
  rational numbers as states of qubit strings

abstract:
  In this paper fields of quantum reference frames based on gauge
transformations of rational string states are described in a way that,
hopefully, makes them more understandable than their description in an earlier
paper. The approach taken here is based on three main points: (1) There are a
large number of different quantum theory representations of natural numbers,
integers, and rational numbers as states of qubit strings. (2) For each
representation, Cauchy sequences of rational string states give a
representation of the real (and complex) numbers. A reference frame is
associated to each representation. (3) Each frame contains a representation of
all mathematical and physical theories that have the representations of the
real and complex numbers as a scalar base for the theories. These points and
other aspects of the resulting fields are then discussed and justified in some
detail. Also two different methods of relating the frame field to physics are
discussed.


title:
Percolation, renormalization, and quantum computing with
  non-deterministic gates

abstract:
  We apply a notion of static renormalization to the preparation of entangled
states for quantum computing, exploiting ideas from percolation theory. Such a
strategy yields a novel way to cope with the randomness of non-deterministic
quantum gates. This is most relevant in the context of optical architectures,
where probabilistic gates are common, and cold atoms in optical lattices, where
hole defects occur. We demonstrate how to efficiently construct cluster states
without the need for rerouting, thereby avoiding a massive amount of
conditional dynamics; we furthermore show that except for a single layer of
gates during the preparation, all subsequent operations can be shifted to the
final adapted single qubit measurements. Remarkably, cluster state preparation
is achieved using essentially the same scaling in resources as if deterministic
gates were available.


title: One qubit almost completely reveals the dynamics of two

abstract:
  From the time dependence of states of one of them, the dynamics of two
interacting qubits is determined to be one of two possibilities that differ
only by a change of signs of parameters in the Hamiltonian. The only exception
is a simple particular case where several parameters in the Hamiltonian are
zero and one of the remaining nonzero parameters has no effect on the time
dependence of states of the one qubit. The mean values that describe the
initial state of the other qubit and of the correlations between the two qubits
also are generally determined to within a change of signs by the time
dependence of states of the one qubit, but with many more exceptions. An
example demonstrates all the results. Feedback in the equations of motion that
allows time dependence in a subsystem to determine the dynamics of the larger
system can occur in both classical and quantum mechanics. The role of quantum
mechanics here is just to identify qubits as the simplest objects to consider
and specify the form that equations of motion for two interacting qubits can
take.


title: Two-Setting Bell Inequalities for Many Qubits

abstract:
  We present a family of Bell inequalities involving only two measurement
settings of each party for N>2 qubits. Our inequalities include all the
standard ones with fewer than N qubits and thus gives a natural generalization.
It is shown that all the Greenberger-Horne-Zeilinger states violate the
inequalities maximally, with an amount that grows exponentially as
2^{{(N-2)}/2}. The inequalities are also violated by some states that do
satisfy all the standard Bell inequalities. Remarkably, our results yield in an
efficient and simple way an implementation of nonlocality tests of many qubits
favorably within reach of the well-established technology of linear optics.


title: Infinite Correlation in Measured Quantum Processes

abstract:
  We show that quantum dynamical systems can exhibit infinite correlations in
their behavior when repeatedly measured. We model quantum processes using
quantum finite-state generators and take the stochastic language they generate
as a representation of their behavior. We analyze two spin-1 quantum systems
that differ only in how they are observed. The corresponding language generated
has short-range correlation in one case and infinite correlation in the other.


title: Geometrical structure of entangled states and secant variety

abstract:
  We show that the secant variety of the Segre variety gives useful information
about the geometrical structure of an arbitrary multipartite quantum system. In
particular, we investigate the relation between arbitrary bipartite and
three-partite entangled states and this secant variety. We also discuss the
geometry of an arbitrary general multipartite state.


title:
Deterministic Quantum Key Distribution Using Gaussian-Modulated Squeezed
  States

abstract:
  A continuous variable ping-pong scheme, which is utilized to generate
deterministically private key, is proposed. The proposed scheme is implemented
physically by using Gaussian-modulated squeezed states. The deterministic way,
i.e., no basis reconciliation between two parties, leads a two-times efficiency
comparing to the standard quantum key distribution schemes. Especially, the
separate control mode does not need in the proposed scheme so that it is
simpler and more available than previous ping-pong schemes. The attacker may be
detected easily through the fidelity of the transmitted signal, and may not be
successful in the beam splitter attack strategy.


title:
Voronoi Diagrams and a Numerical Estimation of a Quantum Channel
  Capacity

abstract:
  We give a new geometric interpretation of quantum pure states. Using
  Voronoi diagrams, we reinterpret the structure of the space of pure states as
a subspace of the quantum state space. In addition to the known coincidence of
some Voronoi diagrams for one-qubit pure states, we will show that even for
mixed one-qubit states, as far as sites are given as pure states, the Voronoi
diagram with respect to some distances -- the divergence, the Bures distance,
and the Euclidean distance -- are all the same.
  As to higher level pure quantum states, for the divergence, the
  Fubini-Study distance, and the Bures distance, the coincidence of the
diagrams still holds, while the coincidence of the diagrams with respect to the
divergence and the Euclidean distance no longer holds. That fact has a
significant meaning when we try to apply the method used for a numerical
estimation of a one-qubit quantum channel capacity to a higher level system.


title: How often is the coordinate of a harmonic oscillator positive?

abstract:
  The coordinate of a harmonic oscillator is measured at a time chosen at
random among three equiprobable instants: now, after one third of the period,
or after two thirds. The (total) probability that the outcome is positive
depends on the state of the oscillator. In the classical case the probability
varies between 1/3 and 2/3, but in the quantum case -- between 0.29 and 0.71.


title: Localization of atomic ensembles via superfluorescence

abstract:
  The sub-wavelength localization of an ensemble of atoms concentrated to a
small volume in space is investigated. The localization relies on the
interaction of the ensemble with a standing wave laser field. The light
scattered in the interaction of standing wave field and atom ensemble depends
on the position of the ensemble relative to the standing wave nodes. This
relation can be described by a fluorescence intensity profile, which depends on
the standing wave field parameters, the ensemble properties, and which is
modified due to collective effects in the ensemble of nearby particles. We
demonstrate that the intensity profile can be tailored to suit different
localization setups. Finally, we apply these results to two localization
schemes. First, we show how to localize an ensemble fixed at a certain position
in the standing wave field. Second, we discuss localization of an ensemble
passing through the standing wave field.


title:
(1+1)-Dirac particle with position-dependent mass in complexified
  Lorentz scalar interactions: effectively PT-symmetric

abstract:
  The effect of the built-in supersymmetric quantum mechanical language on the
spectrum of the (1+1)-Dirac equation, with position-dependent mass (PDM) and
complexified Lorentz scalar interactions, is re-emphasized. The signature of
the "quasi-parity" on the Dirac particles' spectra is also studied. A Dirac
particle with PDM and complexified scalar interactions of the form S(z)=S(x-ib)
(an inversely linear plus linear, leading to a PT-symmetric oscillator model),
and S(x)=S_{r}(x)+iS_{i}(x) (a PT-symmetric Scarf II model) are considered.
Moreover, a first-order intertwining differential operator and an
$\eta$-weak-pseudo-Hermiticity generator are presented and a complexified
PT-symmetric periodic-type model is used as an illustrative example.


title: Choice of a Metric for the Non-Hermitian Oscillator

abstract:
  The harmonic oscillator Hamiltonian, when augmented by a non-Hermitian
$\cal{PT}$-symmetric part, can be transformed into a Hermitian Hamiltonian.
This is achieved by introducing a metric which, in general, renders other
observables such as the usual momentum or position as non-Hermitian operators.
The metric depends on one real parameter, the full range of which is
investigated. The explicit functional dependence of the metric and each
associated Hamiltonian is given. A specific choice of this parameter determines
a specific combination of position and momentum as being an observable; this
can be in particular either standard position or momentum, but not both
simultaneously. Singularities of the metric are explored and their removability
is investigated. The physical significance of these findings is discussed


title: On single-photon quantum key distribution in the presence of loss

abstract:
  We investigate two-way and one-way single-photon quantum key distribution
(QKD) protocols in the presence of loss introduced by the quantum channel. Our
analysis is based on a simple precondition for secure QKD in each case. In
particular, the legitimate users need to prove that there exists no separable
state (in the case of two-way QKD), or that there exists no quantum state
having a symmetric extension (one-way QKD), that is compatible with the
available measurements results. We show that both criteria can be formulated as
a convex optimisation problem known as a semidefinite program, which can be
efficiently solved. Moreover, we prove that the solution to the dual
optimisation corresponds to the evaluation of an optimal witness operator that
belongs to the minimal verification set of them for the given two-way (or
one-way) QKD protocol. A positive expectation value of this optimal witness
operator states that no secret key can be distilled from the available
measurements results. We apply such analysis to several well-known
single-photon QKD protocols under losses.


title: Casimir-Polder force density between an atom and a conducting wall

abstract:
  In this paper we calculate the Casimir-Polder force density (force per unit
area acting on the elements of the surface) on a metallic plate placed in front
of a neutral atom. To obtain the force density we use the quantum operator
associated to the electromagnetic stress tensor. We explicitly show that the
integral of this force density over the plate reproduces the total force acting
on the plate. This result shows that, although the force is obtained as a sum
of surface element-atom contributions, the stress-tensor method includes also
nonadditive components of Casimir-Polder forces in the evaluation of the force
acting on a macroscopic object.


title: Security verification for continuous variable quantum key distribution

abstract: This paper has been withdrawn by the author


title:
Physical mechanism of superluminal traversal time: interference between
  multiple finite wave packets

abstract:
  The mechanism of superluminal traversal time through a potential well or
potential barrier is investigated from the viewpoint of interference between
multiple finite wave packets, due to the multiple reflections inside the well
or barrier. In the case of potential-well traveling that is classically
allowed, each of the successively transmitted constituents is delayed by a
subluminal time. When the thickness of the well is much smaller in comparision
with a characteristic length of the incident wave packet, the reshaped wave
packet in transmission maintains the profile of the incident wave packet. In
the case of potential-barrier tunneling that is classically forbidden, though
each of the successively transmitted constituents is delayed by a time that is
independent of the barrier thickness, the interference between multiple
transmitted constituents explains the barrier-thickness dependence of the
traversal time for thin barriers and its barrier-thickness independence for
thick barriers. This manifests the nature of Hartman effect.


title: Sample dependence of the Casimir forces

abstract:
  We have analyzed available optical data for Au in the mid-infrared range
which is important for a precise prediction of the Casimir force. Significant
variation of the data demonstrates genuine sample dependence of the dielectric
function. We demonstrate that the Casimir force is largely determined by the
material properties in the low frequency domain and argue that therefore the
precise values of the Drude parameters are crucial for an accurate evaluation
of the force. These parameters can be estimated by two different methods,
either by fitting real and imaginary parts of the dielectric function at low
frequencies, or via a Kramers-Kronig analysis based on the imaginary part of
the dielectric function in the extended frequency range. Both methods lead to
very similar results. We show that the variation of the Casimir force
calculated with the use of different optical data can be as large as 5% and at
any rate cannot be ignored. To have a reliable prediction of the force with a
precision of 1%, one has to measure the optical properties of metallic films
used for the force measurement.


title: The quantum FFT can be classically simulated

abstract:
  In this note we describe a simple and intriguing observation: the quantum
Fourier transform (QFT) over $Z_q$, which is considered the most ``quantum''
part of Shor's algorithm, can in fact be simulated efficiently by classical
computers.
  More precisely, we observe that the QFT can be performed by a circuit of
poly-logarithmic path-width, if the circuit is allowed to apply not only
unitary gates but also general linear gates. Recalling the results of Markov
and Shi [MaSh] and Jozsa [Jo] which provided classical simulations of such
circuits in time exponential in the tree-width, this implies the result stated
in the title.
  Classical simulations of the FFT are of course meaningless when applied to
classical input strings on which their result is already known; Our observation
might be interesting only in the context in which the QFT is used as a
subroutine and applied to more interesting superpositions. We discuss the
reasons why this idea seems to fail to provide an efficient classical
simulation of the entire factoring algorithm.
  In the course of proving our observation, we provide two alternative proofs
of the results of [MaSh,Jo] which we use. One proof is very similar in spirit
to that of [MaSh] but is more visual, and is based on a graph parameter which
we call the ``bubble width'', tightly related to the path- and tree-width. The
other proof is based on connections to the Jones polynomial; It is very short,
if one is willing to rely on several known results.


title:
On the role of entanglement and correlations in mixed-state quantum
  computation

abstract:
  In a quantum computation with pure states, the generation of large amounts of
entanglement is known to be necessary for a speedup with respect to classical
computations. However, examples of quantum computations with mixed states are
known, such as the deterministic computation with one quantum qubit (DQC1)
model [Knill and Laflamme, Phys. Rev. Lett. 81, 5672 (1998)], in which
entanglement is at most marginally present, and yet a computational speedup is
believed to occur. Correlations, and not entanglement, have been identified as
a necessary ingredient for mixed-state quantum computation speedups. Here we
show that correlations, as measured through the operator Schmidt rank, are
indeed present in large amounts in the DQC1 circuit. This provides evidence for
the preclusion of efficient classical simulation of DQC1 by means of a whole
class of classical simulation algorithms, thereby reinforcing the conjecture
that DQC1 leads to a genuine quantum computational speedup.


title: Lower Bounds of Concurrence for Tripartite Quantum Systems

abstract:
  We derive an analytical lower bound for the concurrence of tripartite quantum
mixed states. A functional relation is established relating concurrence and the
generalized partial transpositions.


title:
Coherent control of photon transmission : slowing light in coupled
  resonator waveguide doped with $\Lambda $ Atoms

abstract:
  In this paper, we propose and study a hybrid mechanism for coherent
transmission of photons in the coupled resonator optical waveguide (CROW) by
incorporating the electromagnetically induced transparency (EIT) effect into
the controllable band gap structure of the CROW. Here, the configuration setup
of system consists of a CROW with homogeneous couplings and the artificial
atoms with $\Lambda$-type three levels doped in each cavity. The roles of three
levels are completely considered based on a mean field approach where the
collection of three-level atoms collectively behave as two-mode spin waves. We
show that the dynamics of low excitations of atomic ensemble can be effectively
described by an coupling boson model. The exactly solutions show that the light
pulses can be stopped and stored coherently by adiabatically controlling the
classical field.


title:
Fault-tolerant Quantum Computation with Highly Verified Logical Cluster
  States

abstract:
  We investigate a scheme of fault-tolerant quantum computation based on the
cluster model. Logical qubits are encoded by a suitable code such as the
Steane's 7-qubit code. Cluster states of logical qubits are prepared by
post-selection through verification at high fidelity level, where the
unsuccessful ones are discarded without recovery operation. Then, gate
operations are implemented by transversal measurements on the prepared logical
cluster states. The noise threshold is improved significantly by making the
high fidelity preparation and transversal measurement. It is estimated to be
about 3% by a numerical simulation.


title: Geometric Phase of a qubit interacting with a squeezed-thermal bath

abstract:
  We study the geometric phase of an open two-level quantum system under the
influence of a squeezed, thermal environment for both non-dissipative as well
as dissipative system-environment interactions. In the non-dissipative case,
squeezing is found to have a similar influence as temperature, of suppressing
geometric phase, while in the dissipative case, squeezing tends to counteract
the suppressive influence of temperature in certain regimes. Thus, an
interesting feature that emerges from our work is the contrast in the interplay
between squeezing and thermal effects in non-dissipative and dissipative
interactions. This can be useful for the practical implementation of geometric
quantum information processing. By interpreting the open quantum effects as
noisy channels, we make the connection between geometric phase and quantum
noise processes familiar from quantum information theory.


title:
Nonexponential decoherence and momentum subdiffusion in a quantum L\'evy
  kicked rotator

abstract:
  We investigate decoherence in the quantum kicked rotator (modelling cold
atoms in a pulsed optical field) subjected to noise with power-law tail
waiting-time distributions of variable exponent (Levy noise). We demonstrate
the existence of a regime of nonexponential decoherence where the notion of a
decoherence rate is ill-defined. In this regime, dynamical localization is
never fully destroyed, indicating that the dynamics of the quantum system never
reaches the classical limit. We show that this leads to quantum subdiffusion of
the momentum, which should be observable in an experiment.


title: Ratcheting up energy by means of measurement

abstract:
  The destruction of quantum coherence can pump energy into a system. For our
examples this is paradoxical since the destroyed correlations are ordinarily
considered negligible. Mathematically the explanation is straightforward and
physically one can identify the degrees of freedom supplying this energy.
Nevertheless, the energy input can be calculated without specific reference to
those degrees of freedom.


title:
Non-exponential decay via tunneling in tight-binding lattices and the
  optical Zeno effect

abstract:
  An exactly-solvable model for the decay of a metastable state coupled to a
semi-infinite tight-binding lattice, showing large deviations from exponential
decay in the strong coupling regime, is presented. An optical realization of
the lattice model, based on discrete diffraction in a semi-infinite array of
tunneling-coupled optical waveguides, is proposed to test non-exponential decay
and for the observation of an optical analog of the quantum Zeno effect.


title:
Analysis of a quantum memory for photons based on controlled reversible
  inhomogeneous broadening

abstract:
  We present a detailed analysis of a quantum memory for photons based on
controlled and reversible inhomogeneous broadening (CRIB). The explicit
solution of the equations of motion is obtained in the weak excitation regime,
making it possible to gain insight into the dependence of the memory efficiency
on the optical depth, and on the width and shape of the atomic spectral
distributions. We also study a simplified memory protocol which does not
require any optical control fields.


title: Robust optimal quantum gates for Josephson charge qubits

abstract:
  Quantum optimal control theory allows to design accurate quantum gates. We
employ it to design high-fidelity two-bit gates for Josephson charge qubits in
the presence of both leakage and noise. Our protocol considerably increases the
fidelity of the gate and, more important, it is quite robust in the disruptive
presence of 1/f noise. The improvement in the gate performances discussed in
this work (errors of the order of 10^{-3}-10^{-4} in realistic cases) allows to
cross the fault tolerance threshold.


title:
Continuous Variable Quantum Cryptography using Two-Way Quantum
  Communication

abstract:
  Quantum cryptography has been recently extended to continuous variable
systems, e.g., the bosonic modes of the electromagnetic field. In particular,
several cryptographic protocols have been proposed and experimentally
implemented using bosonic modes with Gaussian statistics. Such protocols have
shown the possibility of reaching very high secret-key rates, even in the
presence of strong losses in the quantum communication channel. Despite this
robustness to loss, their security can be affected by more general attacks
where extra Gaussian noise is introduced by the eavesdropper. In this general
scenario we show a "hardware solution" for enhancing the security thresholds of
these protocols. This is possible by extending them to a two-way quantum
communication where subsequent uses of the quantum channel are suitably
combined. In the resulting two-way schemes, one of the honest parties assists
the secret encoding of the other with the chance of a non-trivial superadditive
enhancement of the security thresholds. Such results enable the extension of
quantum cryptography to more complex quantum communications.

categories: quant-ph cs.CR cs.IT math.IT physics.optics


title: Qubit portrait of qudit states and Bell inequalities

abstract:
  A linear map of qudit tomogram onto qubit tomogram (qubit portrait) is
proposed as a characteristics of the qudit state. Using the qubit portrait
method the Bell inequalities for two qubits and two qutrits are discussed in
framework of probability representation of quantum mechanics. Semigroup of
stochastic matrices is associated with tomographic probability distributions of
qubit and qutrit states. Bell-like inequalities are studied using the semigroup
of stochastic matrices. The qudit-qubit map of tomographic probability
distributions is discussed as ansatz to provide a necessary condition for
separability of quantum states.


title: Quantum Formulation of Fractional Orbital Angular Momentum

abstract:
  The quantum theory of rotation angles (S. M. Barnett and D. T. Pegg, Phys.
Rev. A, 41, 3427-3425 (1990)) is generalised to non-integer values of the
orbital angular momentum. This requires the introduction of an additional
parameter, the orientation of a phase discontinuity associated with fractional
values of the orbital angular momentum. We apply our formalism to the
propagation of light modes with fractional orbital angular momentum in the
paraxial and non-paraxial regime.


title:
Entanglement generation in atoms immersed in a thermal bath of external
  quantum scalar fields with a boundary

abstract:
  We examine the entanglement creation between two mutually independent
two-level atoms immersed in a thermal bath of quantum scalar fields in the
presence of a perfectly reflecting plane boundary. With the help of the master
equation that describes the evolution in time of the atom subsystem obtained,
in the weak-coupling limit, by tracing over environment (scalar fields) degrees
of freedom, we find that the presence of the boundary may play a significant
role in the entanglement creation in some circumstances and the new parameter,
the distance of the atoms from the boundary, besides the bath temperature and
the separation between the atoms, gives us more freedom in manipulating
entanglement generation. Remarkably, the final remaining entanglement in the
equilibrium state is independent of the presence of the boundary.


title: Selective entanglement breaking

abstract:
  We discuss the cases where local decoherence selectively degrades one type of
entanglement more than other types. A typical case is called state ordering
change, in which two input states with different amounts of entanglement
undergoes a local decoherence and the state with the larger entanglement
results in an output state with less entanglement than the other output state.
We are also interested in a special case where the state with the larger
entanglement evolves to a separable state while the other output state is still
entangled, which we call selective entanglement breaking. For three-level or
larger systems, it is easy to find examples of the state ordering change and
the selective entanglement breaking, but for two-level systems it is not
trivial whether such situations exist. We present a new strategy to construct
examples of two-qubit states exhibiting the selective entanglement breaking
regardless of entanglement measure. We also give a more striking example of the
selective entanglement breaking in which the less entangled input state has
only an infinitesimal amount of entanglement.


title: Quantum nonlocality of four-qubit entangled states

abstract:
  Quantum nonlocality of several four-qubit states is investigated by
constructing a new Bell inequality. These include the
Greenberger-Zeilinger-Horne (GHZ) state, W state, cluster state, and the state
$|\chi>$ that has been recently proposed in [PRL, {\bf 96}, 060502 (2006)]. The
Bell inequality is optimally violated by $|\chi>$ but not violated by the GHZ
state. The cluster state also violates the Bell inequality though not
optimally. The state $|\chi>$ can thus be discriminated from the cluster state
by using the inequality. Different aspects of four-partite entanglement are
also studied by considering the usefulness of a family of four-qubit mixed
states as resources for two-qubit teleportation. Our results generalize those
in [PRL, {\bf 72}, 797 (1994)].


title:
Thermodynamics of bipartite systems: Application to light-matter
  interactions

abstract:
  Heat and work for quantum systems governed by dissipative master equations
with a time-dependent driving field were introduced in the pioneering work of
Alicki [J. Phys. A 12, L103 (1979)]. Alicki's work was in the Schroedinger
picture; here we extend these definitions to the Heisenberg and interaction
pictures. We show that in order to avoid consistency problems, the full time
derivatives in the definitions for heat flux and power (work flux) should be
replaced by partial time derivatives. We also present an alternative approach
to the partitioning of the energy flux which differs from that of Alicki in
that the instantaneous interaction energy with the external field is not
included directly. We then proceed to generalize Alicki's definition of power
by replacing the original system and its external driving field with a larger,
bipartite system, governed by a time-independent Hamiltonian. Using the
definition of heat flux and the generalized definition of power, we derive the
first law of thermodynamics in differential form, both for the full bipartite
system and the partially traced subsystems. Although the second law (Clausius
formulation) is satisfied for the full bipartite system, we find that in
general there is no rigorous formulation of the second law for the partially
traced subsystem unless certain additional requirements are met. Once these
requirements are satisfied, however, both the Carnot and the Clausius
formulations of the second law are satisfied. We illustrate this thermodynamic
analysis on both the simple Jaynes-Cummings model (JCM) and an extended
dissipative Jaynes-Cummings model (ED-JCM), which is a model for a quantum
amplifier.


title: Controlled Creation of Spatial Superposition States for Single Atoms

abstract:
  We present a method for the controlled and robust generation of spatial
superposition states of single atoms in micro-traps. Using a counter-intuitive
positioning sequence for the individual potentials and appropriately chosen
trapping frequencies, we show that it is possible to selectively create two
different orthogonal superposition states, which can in turn be used for
quantum information purposes.


title: Thermodynamic analysis of quantum light amplification

abstract:
  Thermodynamics of a three-level maser was studied in the pioneering work of
Scovil and Schulz-DuBois [Phys. Rev. Lett. 2, 262 (1959)]. In this work we
consider the same three-level model, but treat both the matter and light
quantum mechanically. Specifically, we analyze an extended (three-level)
dissipative Jaynes-Cummings model (ED-JCM) within the framework of a quantum
heat engine, using novel formulas for heat flux and power in bipartite systems
introduced in our previous work [E. Boukobza and D. J. Tannor, PRA (in press)].
Amplification of the selected cavity mode occurs even in this simple model, as
seen by a positive steady state power. However, initial field coherence is
lost, as seen by the decaying off-diagonal field density matrix elements, and
by the Husimi-Kano Q function. We show that after an initial transient time the
field's entropy rises linearly during the operation of the engine, which we
attribute to the dissipative nature of the evolution and not to matter-field
entanglement. We show that the second law of thermodynamics is satisfied in two
formulations (Clausius, Carnot) and that the efficiency of the ED-JCM heat
engine agrees with that defined intuitively by Scovil and Schulz-DuBois.
Finally, we compare the steady state heat flux and power of the fully quantum
model with the semiclassical counterpart of the ED-JCM, and derive the engine
efficiency formula of Scovil and Schulz-DuBois analytically from fundamental
thermodynamic fluxes.


title: Schroedinger and Hamilton-Jacobi equations

abstract:
  Time-dependent Schroedinger equation represents the basis of any
quantum-theoretical approach. The question concerning its proper content in
comparison to the classical physics has not been, however, fully answered until
now. It will be shown that there is one-to-one physical correspondence between
basic solutions (represented always by one Hamiltonian eigenfunction only) and
classical ones, as the non-zero quantum potential has not any physical sense,
representing only the "numerical" difference between Hamilton principal
function and the phase of corresponding wave function in the case of
non-inertial motion. Possible interpretation of superposition solutions will be
then discussed in the light of this fact. And also different interpretation
alternatives of the quantum-mechanical model will be newly analyzed and new
attitude to them will be reasoned.


title: Quantum Dynamics of Bound Entangled States

abstract:
  By using the partial transpose and realignment method,we study the time
evolution of the bound entanglement under the bilinear-biquadratic Hamiltonian.
For the initial Horodecki's bound entangled state, it keeps bound entangled for
some time, while for the initial bound entangled states constructed from the
unextendable product basis, they become free once the time evolution begins.
The time evolution provides a new way to construct bound entangled states, and
also gives a method to free bound entanglement.


title:
Preparation and Determination of Spin-Polarized States in
  Multi-Zeeman-sublevel Atoms

abstract:
  We demonstrate a simple technique to prepare and determine the desired
internal quantum states in multi-Zeeman-sublevel atoms. By choosing appropriate
coupling and pumping laser beams, atoms can be easily prepared in a desired
Zeeman sublevel with high purity or in any chosen ground-state population
distributions. The population distributions or state purities of such prepared
atomic states can be determined by using a weak, circularly-polarized probe
beam due to differences in transition strengths among different Zeeman
sublevels. Preparing well-defined internal quantum states in
multi-Zeeman-sublevel atoms (or spin-polarized quantum-state engineering) will
be very important in demonstrating many interesting effects in quantum
information processing with multi-level atomic systems.


title: Linear optics and quantum maps

abstract:
  We present a theoretical analysis of the connection between classical
polarization optics and quantum mechanics of two-level systems. First, we
review the matrix formalism of classical polarization optics from a quantum
information perspective. In this manner the passage from the
Stokes-Jones-Mueller description of classical optical processes to the
representation of one- and two-qubit quantum operations, becomes
straightforward. Second, as a practical application of our
classical-\emph{vs}-quantum formalism, we show how two-qubit maximally
entangled mixed states (MEMS), can be generated by using polarization and
spatial modes of photons generated via spontaneous parametric down conversion.


title:
The Relation between Disentangled States and Algorithmic Information
  Theory

abstract:
  In this short communication it is proposed the general form of a n-qubit
disentangled state as a irreducible sentence, in the sense explained by the
algorithmic information theory, whose length increases in a non-polynomial way
when the number of qubits increases.


title:
Adiabatic frequency conversion of quantum optical information in atomic
  vapor

abstract:
  We experimentally demonstrate a quantum communication protocol that enables
frequency conversion and routing of quantum optical information in an adiabatic
and thus robust way. The protocol is based on electromagnetically-induced
transparency in systems with multiple excited levels: transfer and/or
distribution of optical states between different signal modes is implemented by
adiabatically changing the control fields. The proof-of-principle experiment is
performed using the hyperfine levels of the rubidium D1 line.


title: Bhattacharyya inequality for quantum state estimation

abstract:
  Using higher-order derivative with respect to the parameter, we will give
lower bounds for variance of unbiased estimators in quantum estimation
problems. This is a quantum version of the Bhattacharyya inequality in the
classical statistical estimation. Because of non-commutativity of operator
multiplication, we obtain three different types of lower bounds; Type S, Type R
and Type L. If the parameter is a real number, the Type S bound is useful. If
the parameter is complex, the Type R and L bounds are useful. As an
application, we will consider estimation of polynomials of the complex
amplitude of the quantum Gaussian state. For the case where the amplitude lies
in the real axis, a uniformly optimum estimator for the square of the amplitude
will be derived using the Type S bound. It will be shown that there is no
unbiased estimator uniformly optimum as a polynomial of annihilation and/or
creation operators for the cube of the amplitude. For the case where the
amplitude does not necessarily lie in the real axis, uniformly optimum
estimators for holomorphic, antiholomorphic and real-valued polynomials of the
amplitude will be derived. Those estimators for the holomorphic and real-valued
cases attains the Type R bound, and those for the antiholomorphic and
real-valued cases attains the Type L bound. This article clarifies what is the
best method to measure energy of laser.


title:
Exact Polynomial Eigensolutions of the Schrodinger Equation for the
  Pseudoharmonic Potential

abstract:
  The polynomial solution of the Schrodinger equation for the Pseudoharmonic
potential is found for any arbitrary angular momentum $l$. The exact
bound-state energy eigenvalues and the corresponding eigen functions are
analytically calculated. The energy states for several diatomic molecular
systems are calculated numerically for various principal and angular quantum
numbers. By a proper transformation, this problem is also solved very simply by
using the known eigensolutions of anharmonic oscillator potential.


title:
Comments on 'Teleportation of two quNit entanglement: Exploiting local
  resources'and 'Reply to Yang et al.'s comment'

abstract:
  This paper includes two comments. The first one is a comment on
'Teleportation of two quNit entanglement: Exploiting local resources'. Two
different efficient methods are presented to modify some details of N. Ba An's
protocol, so that teleportation of two quNit entanglement can be successfully
achieved in terms of the revised N. Ba An's protocol. The second one is a
comment on 'Reply to Yang et al.'s comment'.


title:
Test of the isotopic and velocity selectivity of a lithium atom
  interferometer by magnetic dephasing

abstract:
  A magnetic field gradient applied to an atom interferometer induces a
$M$-dependent phase shift which results in a series of decays and revivals of
the fringe visibility. Using our lithium atom interferometer based on Bragg
laser diffraction, we have measured the fringe visibility as a function of the
applied gradient. We have thus tested the isotopic selectivity of the
interferometer, the velocity selective character of Bragg diffraction for
different diffraction orders as well as the effect of optical pumping of the
incoming atoms. All these observations are qualitatively understood but a
quantitative analysis requires a complete model of the interferometer.


title:
Experimental Realization of Deutsch's Algorithm in a One-way Quantum
  Computer

abstract:
  We report the first experimental demonstration of an all-optical one-way
implementation of Deutsch's quantum algorithm on a four-qubit cluster state.
All the possible configurations of a balanced or constant function acting on a
two-qubit register are realized within the measurement-based model for quantum
computation. The experimental results are in excellent agreement with the
theoretical model, therefore demonstrating the successful performance of the
algorithm.


title: Philosophical Aspects of Quantum Information Theory

abstract:
  Quantum information theory represents a rich subject of discussion for those
interested in the philosphical and foundational issues surrounding quantum
mechanics for a simple reason: one can cast its central concerns in terms of a
long-familiar question: How does the quantum world differ from the classical
one? Moreover, deployment of the concepts of information and computation in
novel contexts hints at new (or better) means of understanding quantum
mechanics, and perhaps even invites re-assessment of traditional material
conceptions of the basic nature of the physical world. In this paper I review
some of these philosophical aspects of quantum information theory, begining
with an elementary survey of the theory, seeking to highlight some of the
principles and heuristics involved. We move on to a discussion of the nature
and definition of quantum information and deploy the findings in discussing the
puzzles surrounding teleportation. The final two sections discuss,
respectively, what one might learn from the development of quantum computation
(both about the nature of quantum systems and about the nature of computation)
and consider the impact of quantum information theory on the traditional
foundational questions of quantum mechanics (treating of the views of
Zeilinger, Bub and Fuchs, amongst others).


title:
Continuously decoupling single-qubit operations from a perturbing
  thermal bath of scalar bosons

abstract:
  We investigate the use of continuously-applied external fields to maximize
the fidelity of quantum logic operations performed on a decohering qubit.
Assuming a known error operator and an environment represented by a scalar
boson field at a finite temperature, we show how decoherence during logical
operations can be efficiently reduced by applying a superposition of two
external vector fields: one rotating orthogonally to the direction of the
other, which remains static. The required field directions, frequency of
rotation and amplitudes to decouple noise dynamically are determined by the
coupling constants and the desired logical operation. We illustrate these
findings numerically for a Hadamard quantum gate and an environment with ohmic
spectral density.


title:
Optimal Control of High-Fidelity Quantum Gates in the Presence of
  Decoherence

abstract:
  This paper has been withdrawn as it has been superseded by postings
arXiv:quant-ph/0702147 and arXiv:0712.2935


title: Precision measurement with an optical Josephson junction

abstract:
  We study a new type of Josephson device, the so-called "optical Josephson
junction" as proposed in Phys. Rev. Lett. {\bf 95}, 170402 (2005). Two
condensates are optically coupled through a waveguide by a pair of Bragg beams.
This optical Josephson junction is analogous to the usual Josephson junction of
two condensates weakly coupled via tunneling. We discuss the use of this
optical Josephson junction, for making precision measurements.


title: Dynamical chaos versus quantum interference

abstract:
  We discuss the dephasing induced by the internal classical chaotic motion in
the absence of any external environment. To this end a new extension of
fidelity for mixed states is introduced, which we name {\it allegiance}. Such
quantity directly accounts for quantum interference and is measurable in a
Ramsey interferometry experiment. We show that in the semiclassical limit the
decay of the allegiance is exactly expressed, due to the dephasing, in terms of
an appropriate classical correlation function. Our results are derived
analytically for the case of a nonlinear driven oscillator and then numerically
confirmed for the kicked rotor model.


title: Entanglement generation and protection by detuning modulation

abstract:
  We introduce a protocol for steady-state entanglement generation and
protection based on detuning modulation in the dissipative interaction between
a two-qubit system and a bosonic mode. The protocol is a global-addressing
scheme which only requires control over the system as a whole. We describe a
postselection procedure to project the register state onto a subspace of
maximally entangled states. We also outline how our proposal can be implemented
in a circuit-quantum electrodynamics setup.


title:
Relativistic invariant projectors on a complex spinor space and a rule
  of polarizations summation in a complex bispinor space

abstract:
  Relativistic invariant projectors of states in a complex bispinor space on a
complex spinor space are constructed. An expression for sections of bundle with
connection on group SU(4) in an explicit form has been obtained. Within the
framework of the proposed geometrical approach the rule of summation over
polarizations of states in a complex bispinor space has been derived. It has
been shown that states in a complex bispinor space always describe a pair of
Dirac's particles.


title:
Bopp operators and phase-space spin dynamics: Application to rotational
  quantum brownian motion

abstract:
  As already known for nonrelativistic spinless particles, Bopp operators give
an elegant and simple way to compute the dynamics of quasiprobability
distributions in the phase space formulation of Quantum Mechanics. In this
work, we present a generalization of Bopp operators for spins and apply our
results to the case of open spin systems. This approach allows to take the
classical limit in a transparent way, recovering the corresponding
Fokker-Planck equation.


title: A case concerning the improved transition probability

abstract:
  As is well known, the existed perturbation theory can be applied to
calculations of energy, state and transition probability in many quantum
systems. However, there are different paths and methods to improve its
calculation precision and efficiency in our view. According to an improved
scheme of perturbation theory proposed by [An Min Wang, quant-ph/0611217], we
reconsider the transition probability and perturbed energy for a Hydrogen atom
in a constant magnetic field. We find the results obtained by using Wang's
scheme are indeed more satisfying in the calculation precision and efficiency.
Therefore, Wang's scheme can be thought of as a powerful tool in the
perturbation calculation of quantum systems.


title:
Experimental entanglement verification and quantification via
  uncertainty relations

abstract:
  We report on experimental studies on entanglement quantification and
verification based on uncertainty relations for systems consisting of two
qubits. The new proposed measure is shown to be invariant under local unitary
transformations, by which entanglement quantification is implemented for
two-qubit pure states. The nonlocal uncertainty relations for two-qubit pure
states are also used for entanglement verification which serves as a basic
proposition and promise to be a good choice for verification of multipartite
entanglement.


title:
Determinable Solutions for One-dimensional Quantum Potentials:
  Scattering, Quasi-bound and Bound State Problems

abstract:
  We derive analytic expressions of the recursive solutions to the
Schr\"{o}dinger's equation by means of a cutoff potential technique for
one-dimensional piecewise constant potentials. These solutions provide a method
for accurately determining the transmission probabilities as well as the wave
function in both classically accessible region and inaccessible region for any
barrier potentials. It is also shown that the energy eigenvalues and the wave
functions of bound states can be obtained for potential-well structures by
exploiting this method. Calculational results of illustrative examples are
shown in order to verify this method for treating barrier and potential-well
problems.


title: Casimir interaction between gas media of excited atoms

abstract:
  The retarded dispersion interaction (Casimir interaction) between two dilute
dielectric media at high temperatures is considered. The excited atoms are
taken into account. It is shown that the perturbation technique can not be
applied to this problem due to the divergence of integrals. A non-perturbative
approach based on kinetic Green functions is implemented. We consider
interaction between two atoms (one of them is excited0 embedded in an absorbing
dielectric medium. We take into account possible absorption of photons in the
medium, which solves the problem of divergence. The force between two plane
dilute dielectric media is calculated at pair interaction approximation. We
show that the result of quantum electrodynamics differs from the Lifshitz
formula for dilute gas media at high temperatures (if the number of excited
atoms is significant). According to quantum electrodynamics, the interaction
may be either attractive or repulsive depending on the temperature and the
density numbers of the media.


title: Control of multiatom entanglement in a cavity

abstract:
  We propose a general formalism for analytical description of multiatomic
ensembles interacting with a single mode quantized cavity field under the
assumption that most atoms remain un-excited on average. By combining the
obtained formalism with the nilpotent technique for the description of
multipartite entanglement we are able to overview in a unified fashion
different probabilistic control scenarios of entanglement among atoms or
examine atomic ensembles. We then apply the proposed control schemes to the
creation of multiatom states useful for quantum information.


title: Gaussian Decoherence and Gaussian Echo from Spin Environments

abstract:
  We examine an exactly solvable model of decoherence -- a spin-system
interacting with a collection of environment spins. We show that in this simple
model (introduced some time ago to illustrate environment--induced
superselection) generic assumptions about the coupling strengths lead to a
universal (Gaussian) suppression of coherence between pointer states. We
explore the regime of validity of this result and discuss its relation to
spectral features of the environment. We also consider its relevance to the
experiments on the so-called Loschmidt echo (which measures, in effect, the
fidelity between the initial and time-reversed or "echo" signal). In
particular, we show that for partial reversals (e.g., when of only a part of
the total Hamiltonian changes sign) fidelity will exhibit a Gaussian dependence
on the time of reversal. In such cases echo may become independent of the
details of the reversal procedure or the specifics of the coupling to the
environment. This puzzling behavior was observed in several NMR experiments.
Natural candidates for such two environments (one of which is easily reversed,
while the other is ``irreversible'') are suggested for the experiment involving
ferrocene.


title: Power of unentangled measurements on two antiparallel spins

abstract:
  We consider a pair of antiparallel spins polarized in a random direction to
encode quantum information. We wish to extract as much information as possible
on the polarization direction attainable by an unentangled measurement, i.e.,
by a measurement, whose outcomes are associated with product states. We develop
analytically the upper bound 0.7935 bits to the Shannon mutual information
obtainable by an unentangled measurement, which is definitely less than the
value 0.8664 bits attained by an entangled measurement. This proves our main
result, that not every ensemble of product states can be optimally
distinguished by an unentangled measurement, if the measure of
distinguishability is defined in the sense of Shannon. We also present results
from numerical calculations and discuss briefly the case of parallel spins.


title: Intrinsic Quantum Computation

abstract:
  We introduce ways to measure information storage in quantum systems, using a
recently introduced computation-theoretic model that accounts for measurement
effects. The first, the quantum excess entropy, quantifies the shared
information between a quantum process's past and its future. The second, the
quantum transient information, determines the difficulty with which an observer
comes to know the internal state of a quantum process through measurements. We
contrast these with von Neumann entropy and quantum entropy rate and provide a
closed-form expression for the latter for the class of deterministic quantum
processes.


title: Language Diversity of Measured Quantum Processes

abstract:
  The behavior of a quantum system depends on how it is measured. How much of
what is observed comes from the structure of the quantum system itself and how
much from the observer's choice of measurement? We explore these questions by
analyzing the \emph{language diversity} of quantum finite-state generators. One
result is a new way to distinguish quantum devices from their classical
(stochastic) counterparts. While the diversity of languages generated by these
two computational classes is the same in the case of periodic processes,
quantum systems generally generate a wider range of languages than classical
systems.


title:
Squeezed light for bandwidth limited atom optics experiments at the
  Rubidium D1 line

abstract:
  We report on the generation of more than 5 dB of vacuum squeezed light at the
Rubidium D1 line (795 nm) using periodically poled KTiOPO$_{4}$ (PPKTP) in an
optical parametric oscillator. We demonstrate squeezing at low sideband
frequencies, making this source of non-classical light compatible with
bandwidth limited atom optics experiments. When PPKTP is operated as a
parametric amplifier, we show a noise reduction of 4 dB stably locked within
the 150 kHz-500 kHz frequency range. This matches the bandwidth of
Electromagnetically Induced Transparency (EIT) in Rubidium hot vapour cells
under the condition of large information delay.