title:
Information and The Brukner-Zeilinger Interpretation of Quantum
  Mechanics: A Critical Investigation

abstract:
  In Brukner and Zeilinger's interpretation of quantum mechanics, information
is introduced as the most fundamental notion and the finiteness of information
is considered as an essential feature of quantum systems. They also define a
new measure of information which is inherently different from the Shannon
information and try to show that the latter is not useful in defining the
information content in a quantum object.
  Here, we show that there are serious problems in their approach which make
their efforts unsatisfactory. The finiteness of information does not explain
how objective results appear in experiments and what an instantaneous change in
the so-called information vector (or catalog of knowledge) really means during
the measurement. On the other hand, Brukner and Zeilinger's definition of a new
measure of information may lose its significance, when the spin measurement of
an elementary system is treated realistically. Hence, the sum of the individual
measures of information may not be a conserved value in real experiments.


title: Entanglement and Bell Inequalities

abstract:
  The entangled quantum states play a key role in quantum information. The
association of the quantum state vector with each individual physical system in
an attributive way is a source of many false paradoxes and inconsistencies. The
paradoxes are avoided if the purely statistical interpretation (SI) of the
quantum state vector is adopted. According the SI the quantum theory (QT) does
not provide any deterministic prediction for any individual experimental result
obtained for a free physical system, for a trapped ion or for a quantum dot. In
this article it is shown that if the SI is used then, contrary to the general
belief, the QT does not predict for the ideal spin singlet state perfect
anti-correlation of the coincidence coumts for the distant detectors.
Subsequently the various proofs of the Bell's theorem are reanalyzed and in
particular the importance and the implications of the use of the unique
probability space in these proofs are elucidated. The use of the unique
probability space is shown to be equivalent to the use of the joint probability
distributions for the non commuting observables. The experimental violation of
the Bell's inequalities proves that the naive realistic particle like spatio-
temporal description of the various quantum mechanical experiments is
impossible. Of course it does not give any argument for the action at the
distance and it does not provide the proof of the completeness of the QM. The
fact that the quantum state vector is not an attribute of a single quantum
system and that the quantum observables are contextual has to be taken properly
into account in any implementation of the quantum computing device.


title: Assisted Quantum Secret Sharing

abstract:
  A restriction on quantum secret sharing (QSS) that comes from the no-cloning
theorem is that any pair of authorized sets in an access structure should
overlap. From the viewpoint of application, this places an unnatural constraint
on secret sharing. We present a generalization, called assisted QSS (AQSS),
where access structures without pairwise overlap of authorized sets is
permissible, provided some shares are withheld by the share dealer. We show
that no more than $\lambda-1$ withheld shares are required, where $\lambda$ is
the minimum number of {\em partially linked classes} among the authorized sets
for the QSS. This is useful in QSS schemes where the share dealer is honest by
definition and is equivalent to a secret reconstructor. Our result means that
such applications of QSS need not be thwarted by the no-cloning theorem.


title: Entangled Two-Photon Wave Packet in a Dispersive Medium

abstract:
  We report an experimental study of group-velocity dispersion effect on an
entangled two-photon wavepacket, generated via spontaneous parametric
down-conversion and propagating through a dispersive medium. Even in the case
of using CW laser beam for pump, the biphoton wavepacket and the second-order
correlation function spread significantly. The study and understanding of this
phenomenon is of great importance for quantum information applications, such as
quantum communication and distant clock synchronization.


title:
Coherence-preserving trap architecture for long-term control of giant
  Rydberg atoms

abstract:
  We present a way to trap a single Rydberg atom, make it long-lived and
preserve an internal coherence over time scales reaching into the minute range.
We propose to trap using carefully designed electric fields, to inhibit the
spontaneous emission in a non resonant conducting structure and to maintain the
internal coherence through a tailoring of the atomic energies using an external
microwave field. We thoroughly identify and account for many causes of
imperfection in order to verify at each step the realism of our proposal.


title: Affine maps of density matrices

abstract:
  For quantum systems described by finite matrices, linear and affine maps of
matrices are shown to provide equivalent descriptions of evolution of density
matrices for a subsystem caused by unitary Hamiltonian evolution in a larger
system; an affine map can be replaced by a linear map, and a linear map can be
replaced by an affine map. There may be significant advantage in using an
affine map. The linear map is generally not completely positive, but the linear
part of an equivalent affine map can be chosen to be completely positive and
related in the simplest possible way to the unitary Hamiltonian evolution in
the larger system.


title: Distant Clock Synchronization Using Entangled Photon Pairs

abstract:
  We report a proof-of-principle experiment on distant clock synchronization.
Besides the achievement of picosecond resolution at 3 kilometer distance, this
experiment demonstrated a novel concept for high accuracy non-local timing and
positioning based on the quantum feature of entangled states.


title: Distinguishing between optical coherent states with imperfect detection

abstract:
  Several proposed techniques for distinguishing between optical coherent
states are analyzed under a physically realistic model of photodetection.
Quantum error probabilities are derived for the Kennedy receiver, the Dolinar
receiver and the unitary rotation scheme proposed by Sasaki and Hirota for
sub-unity detector efficiency. Monte carlo simulations are performed to assess
the effects of detector dark counts, dead time, signal processing bandwidth and
phase noise in the communication channel. The feedback strategy employed by the
Dolinar receiver is found to achieve the Helstrom bound for sub-unity detection
efficiency and to provide robustness to these other detector imperfections
making it more attractive for laboratory implementation than previously
believed.


title: Entanglement Sharing and Decoherence in the Spin-Bath

abstract:
  The monogamous nature of entanglement has been illustrated by the derivation
of entanglement sharing inequalities - bounds on the amount of entanglement
that can be shared amongst the various parts of a multipartite system.
Motivated by recent studies of decoherence, we demonstrate an interesting
manifestation of this phenomena that arises in system-environment models where
there exists interactions between the modes or subsystems of the environment.
We investigate this phenomena in the spin-bath environment, constructing an
entanglement sharing inequality bounding the entanglement between a central
spin and the environment in terms of the pairwise entanglement between
individual bath spins. The relation of this result to decoherence will be
illustrated using simplified system-bath models of decoherence.


title: New Ways to Solve the Schroedinger Equation

abstract:
  We discuss a new approach to solve the low lying states of the Schroedinger
equation. For a fairly large class of problems, this new approach leads to
convergent iterative solutions, in contrast to perturbative series expansions.
These convergent solutions include the long standing difficult problem of a
quartic potential with either symmetric or asymmetric minima.


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

abstract:
  Here we explore the possibility to obtain a non-relativistic proof of the
spin-statistics theorem. First, we examine the structure of axioms and theorems
involved in a relativistic Schwinger-like proof of the spin-statistics
relation. Second, starting from this structure we identify the relativistic
assumptions. Last, we reformulate these assumptions in order to obtain a
Galilean proof. We conclude that the spin-statistics theorem cannot be deduced
into the framework of Galilean quantum field theories for three space
dimensions because two of the assumptions needed to prove the theorem are
incompatible. We analyze, however, the conditions under which a non-relativist
proof could still be deduced.


title: Optimum Small Optical Beam Displacement Measurement

abstract:
  We derive the quantum noise limit for the optical beam displacement of a
TEM00 mode. Using a multimodal analysis, we show that the conventional split
detection scheme for measuring beam displacement is non-optimal with 80%
efficiency. We propose a new displacement measurement scheme that is optimal
for small beam displacement. This scheme utilises a homodyne detection setup
that has a TEM10 mode local oscillator. We show that although the quantum noise
limit to displacement measurement can be surpassed using squeezed light in
appropriate spatial modes for both schemes, the TEM10 homodyning scheme
out-performs split detection for all values of squeezing.


title: Securing QKD Links in the Full Hilbert Space

abstract:
  Many quantum key distribution analyses examine the link security in a subset
of the full Hilbert space that is available to describe the system. In reality,
information about the photon state can be embedded in correlations between the
polarization space and other dimensions of the Hilbert space in such a way that
Eve can determine the polarization of a photon without affecting it. This paper
uses the concept of suitability to quantify the available information for Eve,
and then describe a systematic way to calculate and measure these
possibilities.


title:
Detection of sub-shot-noise spatial correlation in high-gain parametric
  down-conversion

abstract:
  Using a 1GW-1ps pump laser pulse in high gain parametric down-conversion
allows us to detect sub-shot-noise spatial quantum correlation with up to one
hundred photoelectrons per mode, by means of a high efficiency CCD. The
statistics is performed in single-shot over independent spatial replica of the
system. The paper highlights the evidence of quantum correlation between
symmetrical signal and idler spatial areas in the far field, in the high gain
regime. In accordance with the predictions of numerical calculations the
observed transition from the quantum to the classical regime is interpreted as
a consequence of the narrowing of the down-converted beams in the very high
gain regime.


title:
Perfect State Transfer, Effective Gates and Entanglement Generation in
  Engineered Bosonic and Fermionic Networks

abstract:
  We show how to achieve perfect quantum state transfer and construct effective
two-qubit gates between distant sites in engineered bosonic and fermionic
networks. The Hamiltonian for the system can be determined by choosing an
eigenvalue spectrum satisfying a certain condition, which is shown to be both
sufficient and necessary in mirror-symmetrical networks. The natures of the
effective two-qubit gates depend on the exchange symmetry for fermions and
bosons. For fermionic networks, the gates are entangling (and thus universal
for quantum computation). For bosonic networks, though the gates are not
entangling, they allow two-way simultaneous communications. Protocols of
entanglement generation in both bosonic and fermionic engineered networks are
discussed.


title: PT-Symmetric Quantum Mechanics: A Precise and Consistent Formulation

abstract:
  The physical condition that the expectation values of physical observables
are real quantities is used to give a precise formulation of PT-symmetric
quantum mechanics. A mathematically rigorous proof is given to establish the
physical equivalence of PT-symmetric and conventional quantum mechanics. The
results reported in this paper apply to arbitrary PT-symmetric Hamiltonians
with a real and discrete spectrum. They hold regardless of whether the boundary
conditions defining the spectrum of the Hamiltonian are given on the real line
or a complex contour.


title:
Criteria of quantum correlation in the measurement of continuous
  variables in optics

abstract:
  The purpose of this short tutorial paper is to review various criteria that
have been used to characterize the quantum character of correlations in optical
systems, such as "gemellity", QND correlation, intrication, EPR correlation and
Bell correlation, to discuss and compare them. This discussion, restricted to
the case of measurements of continuous optical variables, includes also an
extension of known criteria for "twin beams" to the case of imbalanced
correlations.


title: QC Paulinesia

abstract:
  My goal in producing this document was to create a collection of qubit
circuit identities that are used in Quantum Computing. Mathematicians and
Physicists may consider it as being analogous to a Table of Integrals or a
Mathematical Handbook such as Gradshteyn-Ryzhik or Abramowitz-Stegun. Computer
Programmers may think of it as a scrapbook of code snippets that are elegant,
instructive, well documented, and useful. Electronics experts may view it as a
compendium of circuits for performing a large assortment of tasks. The vast
majority of the circuit identities collected in this work were not discovered
for the first time by me, and I take no credit for discovering them. In
producing this document, I am acting as a collector, not as a discoverer.


title:
Conservation and entanglement of Hermite-Gaussian modes in parametric
  down-conversion

abstract:
  We show that the transfer of the angular spectrum of the pump beam to the
two-photon state in spontaneous parametric down-conversion enables the
generation of entangled Hermite-Gaussian modes. We derive an analytical
expression for the two-photon state in terms of these modes and show that there
are restrictions on both the parity and order of the down-converted
Hermite-Gaussian fields. Using these results, we show that the two-photon state
is indeed entangled in Hermite-Gaussian modes. We propose experimental methods
of creating maximally-entangled Bell states and non-maximally entangled pure
states of first order Hermite-Gaussian modes.


title: Quantum search for multiple items using parallel queries

abstract:
  In the quantum database search problem we are required to search for an item
in a database. In this paper, we consider a generalization of this problem,
where we are provided d identical copes of a database each with N items which
we can query in parallel. Then, given k items, we are required to determine the
locations where these items are stored. We show that any quantum algorithm for
this task must perform Omega(sqrt{Nk/d min{d,k}}) parallel queries. We also
design an algorithm whose performance comes within a factor O(log d) of this
lower bound.


title: Field squeeze operators in optical cavities with atomic ensembles

abstract:
  We propose a method of generating unitarily single and two-mode field
squeezing in an optical cavity with an atomic cloud. Through a suitable laser
system, we are able to engineer a squeeze field operator decoupled from the
atomic degrees of freedom, yielding a large squeeze parameter that is scaled up
by the number of atoms, and realizing degenerate and non-degenerate parametric
amplification. By means of the input-output theory we show that ideal squeezed
states and perfect squeezing could be approached at the output. The scheme is
robust to decoherence processes.


title:
Quantum teleportation of an arbitrary two qubit state and its relation
  to multipartite entanglement

abstract:
  We explicitly show a protocol in which an arbitrary two qubit a|00> + b|01> +
c|10> + d|11> is faithfully and deterministically teleported from Alice to Bob.
We construct the 16 orthogonal generalized Bell states which can be used to
teleport the two qubits. The local operations Bob must perform on his qubits in
order to recover the teleported state is also constructed. They are restricted
only to single qubit gates. This means that a CNOT gate is not necessary to
complete the protocol. A generalization where N qubits is teleported is also
shown. We define a generalized magic basis, which possesses interesting
properties. These properties help us to suggest a generalized concurrence from
which we construct a new measure of entanglement that has a clear physical
interpretation: A multipartite state has maximum entanglement if it is a
genuine quantum teleportation channel.


title: Quantum computing on long-lived donor states of Li in Si

abstract:
  We predict a gigantically long lifetime of the first excited state of an
interstitial lithium donor in silicon. The nature of this effect roots in the
anomalous level structure of the {\em 1s} Li manifold under external stress.
Namely, the coupling between the lowest two states of the opposite parity is
very weak and occurs via intervalley phonon transitions only. We propose to use
these states under the controlled ac and dc stress to process quantum
information. We find an unusual form of the elastic-dipole interaction between
%the electronic transitions in different donors. This interaction scales with
the inter-donor distance $R$ as $R^{-3}$ or $R^{-5}$ for the transitions
between the states of the same or opposite parity, respectively. The long-range
$R^{-3}$ interaction provides a high fidelity mechanism for 2-qubit operations.


title: Quantum Pseudo-Telepathy

abstract:
  Quantum information processing is at the crossroads of physics, mathematics
and computer science. It is concerned with that we can and cannot do with
quantum information that goes beyond the abilities of classical information
processing devices. Communication complexity is an area of classical computer
science that aims at quantifying the amount of communication necessary to solve
distributed computational problems. Quantum communication complexity uses
quantum mechanics to reduce the amount of communication that would be
classically required.
  Pseudo-telepathy is a surprising application of quantum information
processing to communication complexity. Thanks to entanglement, perhaps the
most nonclassical manifestation of quantum mechanics, two or more quantum
players can accomplish a distributed task with no need for communication
whatsoever, which would be an impossible feat for classical players.
  After a detailed overview of the principle and purpose of pseudo-telepathy,
we present a survey of recent and no-so-recent work on the subject. In
particular, we describe and analyse all the pseudo-telepathy games currently
known to the authors.


title: The language of Einstein spoken by optical instruments

abstract:
  Einstein had to learn the mathematics of Lorentz transformations in order to
complete his covariant formulation of Maxwell's equations. The mathematics of
Lorentz transformations, called the Lorentz group, continues playing its
important role in optical sciences. It is the basic mathematical language for
coherent and squeezed states. It is noted that the six-parameter Lorentz group
can be represented by two-by-two matrices. Since the beam transfer matrices in
ray optics is largely based on two-by-two matrices or $ABCD$ matrices, the
Lorentz group is bound to be the basic language for ray optics, including
polarization optics, interferometers, lens optics, multilayer optics, and the
Poincar\'e sphere. Because the group of Lorentz transformations and ray optics
are based on the same two-by-two matrix formalism, ray optics can perform
mathematical operations which correspond to transformations in special
relativity. It is shown, in particular, that one-lens optics provides a
mathematical basis for unifying the internal space-time symmetries of massive
and massless particles in the Lorentz-covariant world.


title: Criteria for Exact Qudit Universality

abstract:
  We describe criteria for implementation of quantum computation in qudits. A
qudit is a d-dimensional system whose Hilbert space is spanned by states |0>,
|1>,... |d-1>. An important earlier work of Mathukrishnan and Stroud [1]
describes how to exactly simulate an arbitrary unitary on multiple qudits using
a 2d-1 parameter family of single qudit and two qudit gates. Their technique is
based on the spectral decomposition of unitaries. Here we generalize this
argument to show that exact universality follows given a discrete set of single
qudit Hamiltonians and one two-qudit Hamiltonian. The technique is related to
the QR-matrix decomposition of numerical linear algebra. We consider a generic
physical system in which the single qudit Hamiltonians are a small collection
of H_{jk}^x=\hbar\Omega (|k><j|+|j><k|) and H_{jk}^y =\hbar\Omega
(i|k><j|-i|j><k|). A coupling graph results taking nodes 0,... d-1 and edges
j<->k iff H_{jk}^{x,y} are allowed Hamiltonians. One qudit exact universality
follows iff this graph is connected, and complete universality results if the
two-qudit Hamiltonian H=-\hbar\Omega |d-1,d-1><d-1,d-1| is also allowed. We
discuss implementation in the eight dimensional ground electronic states of
^{87}Rb and construct an optimal gate sequence using Raman laser pulses.


title: Teleportation Topology

abstract:
  We discuss the structure of teleportation. By associating matrices to the
preparation and measurement states, we show that for a unitary transformation M
there is a full teleportation procedure for obtaining M|S> from a given state
|S>. The key to this construction is a diagrammatic intepretation of matrix
multiplication that applies equally well to a topological composition of a
maximum and a minimum that underlies the structure of the teleportation. This
paper is a preliminary report on joint work with H. Carteret and S. Lomonaco.


title: Entanglement of the Hermite-Gaussian modes states of photons

abstract:
  We found that the Hermit-Gaussian(HG) modes of the down converted beams from
the spontaneous parametric down conversion are quasi-conserved and the
generated photon pairs are HG modes entangled for some special cases. This is
valuable for either the investigation of fundament properties of
multi-dimensional entanglement or quantum information applications.


title: Entanglement and Quantum Phase Transition Revisited

abstract:
  We show that, for an exactly solvable quantum spin model, a discontinuity in
the first derivative of the ground state concurrence appears in the absence of
quantum phase transition. It is opposed to the popular belief that the
non-analyticity property of entanglement (ground state concurrence) can be used
to determine quantum phase transitions. We further point out that the
analyticity property of the ground state concurrence in general can be more
intricate than that of the ground state energy. Thus there is no one-to-one
correspondence between quantum phase transitions and the non-analyticity
property of the concurrence. Moreover, we show that the von Neumann entropy, as
another measure of entanglement, can not reveal quantum phase transition in the
present model. Therefore, in order to link with quantum phase transitions, some
other measures of entanglement are needed.


title: Compatibility of subsystem states

abstract:
  We examine the possible states of subsystems of a system of bits or qubits.
In the classical case (bits), this means the possible marginal distributions of
a probability distribution on a finite number of binary variables; we give
necessary and sufficient conditions for a set of probability distributions on
all proper subsets of the variables to be the marginals of a single
distribution on the full set. In the quantum case (qubits), we consider mixed
states of subsets of a set of qubits; in the case of three qubits, we find
quantum Bell inequalities -- necessary conditions for a set of two-qubit states
to be the reduced states of a single mixed state of three qubits. We conjecture
that these conditions are also sufficient.


title:
Entanglement and Quantum Phase Transition in Low Dimensional Spin
  Systems

abstract:
  Entanglement of the ground states in $XXZ$ and dimerized Heisenberg spin
chains as well as in a two-leg spin ladder is analyzed by using the spin-spin
concurrence and the entanglement entropy between a selected sublattice of spins
and the rest of the system. In particular, we reveal that quantum phase
transition points/boundaries may be identified based on the analysis on the
local extreme of this entanglement entropy, which is illustrated to be superior
over the concurrence scenario and may enable us to explore quantum phase
transitions in many other systems including higher dimensional ones.


title:
Single-atom detection using whispering gallery modes of microdisk
  resonators

abstract:
  We investigate the possibility of using dielectric microdisk resonators for
the optical detection of single atoms trapped and cooled in magnetic microtraps
near the surface of a substrate. The bound and evanescent fields of optical
whispering gallery modes are calculated and the coupling to straight waveguides
is investigated using finite-difference time domain solutions of Maxwell's
equations. Results are compared with semi-analytical solutions based on coupled
mode theory. We discuss atom detection efficiencies and the feasibility of
non-destructive measurements in such a system depending on key parameters such
as disk size, disk-waveguide coupling, and scattering losses.


title:
Entanglement in a Two-Qubit Ising Model Under a Site-Dependent External
  Magnetic Field

abstract:
  We investigate the ground state and the thermal entanglement in the two-qubit
Ising model interacting with a site-dependent magnetic field. The degree of
entanglement is measured by calculating the concurrence. For zero temperature
and for certain direction of the applied magnetic field, the quantum phase
transition observed under a uniform external magnetic field disappears once a
very small non-uniformity is introduced. Furthermore, we have shown
analytically and confirmed numerically that once the direction of one of the
magnetic field is along the Ising axis then no entangled states can be
produced, independently of the degree of non-uniformity of the magnetic fields
on each site.


title: Off-diagonal geometric phase in composite systems

abstract:
  The effect due to the inter-subsystem coupling on the off-diagonal geometric
phase in a composite system is investigated. We analyze the case where the
system undergo an adiabatic evolution. Two coupled qubits driven by
time-dependent external magnetic fields are presented as an example, the
off-diagonal geometric phase as well as the adiabatic condition are examined
and discussed.


title: Rotational invariance as an additional constraint on local realism

abstract:
  Rotational invariance of physical laws is a generally accepted principle. We
show that it leads to an additional external constraint on local realistic
models of physical phenomena involving measurements of multiparticle spin 1/2
correlations. This new constraint rules out such models even in some situations
in which standard Bell inequalities allow for explicit construction of such
models. The whole analysis is performed without any additional assumptions on
the form of local realistic models.


title:
Atom-photon, atom-atom and photon-photon entanglement preparation via
  fractional adiabatic passage

abstract:
  We propose a relatively robust scheme to generate maximally entangled states
of (i) an atom and a cavity photon, (ii) two atoms in their ground states, and
(iii) two photons in two spatially separate high-Q cavities. It is based on the
interaction via fractional adiabatic passage of a three-level atom traveling
through a cavity mode and a laser beam. The presence of optical phases is
emphasized.


title:
Physical Bounds to the Entropy-Depolarization Relation in Random Light
  Scattering

abstract:
  We present a theoretical study of multi-mode scattering of light by optically
random media, using the Mueller-Stokes formalism which permits to encode all
the polarization properties of the scattering medium in a real $4 \times 4$
matrix. From this matrix two relevant parameters can be extracted: the
depolarizing power $D_M$ and the polarization entropy $E_M$ of the scattering
medium. By studying the relation between $E_M$ and $D_M$, we find that {\em
all} scattering media must satisfy some {\em universal} constraints. These
constraints apply to both classical and quantum scattering processes. The
results obtained here may be especially relevant for quantum communication
applications, where depolarization is synonymous with decoherence.


title: Anharmonic Oscillator Equations:Treatment Parallel to Mathieu Equation

abstract:
  The treatment of anharmonic oscillators (including double-wells) by instanton
methods is wellknown. The alternative differential equation method is not so
wellknown. Here we reformulate the latter completely parallel to the strong
coupling case of the cosine potential and Mathieu equation for which extensive
literature and monographs exist.


title: Model for Entangled States with Spin-Spin Interaction

abstract:
  A system consisting of two neutral spin 1/2 particles is analyzed for two
magnetic field perturbations: 1) an inhomogeneous magnetic field over all
space, and 2) external fields over a half space containing only one of the
particles. The field is chosen to point from one particle to the other, which
results in essentially a one-dimensional problem. A number of interesting
features are revealed for the first case: the singlet, which has zero potential
energy in the unperturbed case, remains unstable in the perturbing field. The
spin zero component of the triplet evolves into a bound state with a double
well potential, with the possibility of tunneling. Superposition states can be
constructed which oscillate between entangled and unentangled states. For the
second case, we show that changes in the magnetic field around one particle
affect measurements of the spin of the entangled particle not in the magnetic
field nonlocally. By using protective measurements, we show it is possible in
principle to establish a nonlocal interaction using the two particles, provided
the dipole-dipole potential energy does not vanish and is comparable to the
potential energy of the particle in the external field.


title:
Quantum noise and mixedness of a pumped dissipative non-linear
  oscillator

abstract:
  Evolutions of quantum noise, characterized by quadrature squeezing parameter
and Fano factor, and of mixedness, quantified by quantum von Neumann and linear
entropies, of a pumped dissipative non-linear oscillator are studied. The model
can describe a signal mode interacting with a thermal reservoir in a
parametrically pumped cavity with a Kerr non-linearity. It is discussed that
the initial pure states, including coherent states, Fock states, and finite
superpositions of coherent states evolve into the same steady mixed state as
verified by the quantum relative entropy and the Bures metric. It is shown
analytically and verified numerically that the steady state can be well
approximated by a nonclassical Gaussian state exhibiting quadrature squeezing
and sub-Poissonian statistics for the cold thermal reservoir. A rapid increase
is found in the mixedness, especially for the initial Fock states and
superpositions of coherent states, during a very short time interval, and then
for longer evolution times a decrease in the mixedness to the same, for all the
initial states, and relatively low value of the nonclassical Gaussian state.


title:
Manipulation and storage of optical field and atomic ensemble quantum
  states

abstract:
  We study how to efficiently manipulate and store quantum information between
optical fields and atomic ensembles. We show how various non-dissipative
transfer schemes can be used to transfer and store quantum states such as
squeezed vacuum states or entangled states into the long-lived ground state
spins of atomic ensembles.


title: Quantum chaos in elementary quantum mechanics

abstract:
  We introduce an analytical solution to the one of the most familiar problems
from the elementary quantum mechanics textbooks. The following discussion
provides simple illustrations to a number of general concepts of quantum
chaology, along with some recent developments in the field and a historical
perspective on the subject.


title: Ghost Imaging with Blackbody Radiation

abstract:
  We present a theoretical study of ghost imaging by using blackbody radiation
source. A Gaussian thin lens equation for the ghost imaging, which depends on
both paths, is derived. The dependences of the visibility and quality of the
image on the transverse size and temperature of the blackbody are studied. The
main differences between the ghost imaging by using the blackbody radiation and
by using the entangled photon pairs are image-forming equation, and the
visibility and quality of the image


title: Scalable fault-tolerant quantum computation in DFS blocks

abstract:
  We investigate how to concatenate different decoherence-free subspaces (DFSs)
to realize scalable universal fault-tolerant quantum computation. Based on
tunable $XXZ$ interactions, we present an architecture for scalable quantum
computers which can fault-tolerantly perform universal quantum computation by
manipulating only single type of parameter. By using the concept of
interaction-free subspaces we eliminate the need to tune the couplings between
logical qubits, which further reduces the technical difficulties for
implementing quantum computation.


title: Quantum Superposition Principle and Geometry

abstract:
  If one takes seriously the postulate of quantum mechanics in which physical
states are rays in the standard Hilbert space of the theory, one is naturally
lead to a geometric formulation of the theory. Within this formulation of
quantum mechanics, the resulting description is very elegant from the
geometrical viewpoint, since it allows to cast the main postulates of the
theory in terms of two geometric structures, namely a symplectic structure and
a Riemannian metric. However, the usual superposition principle of quantum
mechanics is not naturally incorporated, since the quantum state space is
non-linear. In this note we offer some steps to incorporate the superposition
principle within the geometric description. In this respect, we argue that it
is necessary to make the distinction between a 'projective superposition
principle' and a 'decomposition principle' that extend the standard
superposition principle. We illustrate our proposal with two very well known
examples, namely the spin 1/2 system and the two slit experiment, where the
distinction is clear from the physical perspective. We show that the two
principles have also a different mathematical origin within the geometrical
formulation of the theory.


title:
Analytical results for entanglement in the five-qubit anisotropic
  Heisenberg model

abstract:
  We solve the eigenvalue problem of the five-qubit anisotropic Heisenberg
model, without use of Bethe's Ansatz, and give analytical results for
entanglement and mixedness of two nearest-neighbor qubits. The entanglement
takes its maximum at Delta= (Delta>1) for the case of zero (finite) temperature
with Delta being the anisotropic parameter. In contrast, the mixedness takes
its minimum at Delta=1 (Delta>1) for the case of zero (finite) temperature.


title:
Antilinearity in Bipartite Quantum Systems and Imperfect Quantum
  Teleportation

abstract:
  Antilinearity is quite natural in bipartite quantum systems. There is a
one-to-one correspondence between vectors and certain antilinear maps, here
called EPR-maps. Some of their properties and uses, including the factorization
of quantum teleportation maps, is explained. There is an elementary link to
twisted Kronecker products and to the modular objects of Tomita and Takesaki.


title: Theory of decoherence in a matter wave Talbot-Lau interferometer

abstract:
  We present a theoretical framework to describe the effects of decoherence on
matter waves in Talbot-Lau interferometry. Using a Wigner description of the
stationary beam the loss of interference contrast can be calculated in closed
form. The formulation includes both the decohering coupling to the environment
and the coherent interaction with the grating walls. It facilitates the
quantitative distinction of genuine quantum interference from the expectations
of classical mechanics. We provide realistic microscopic descriptions of the
experimentally relevant interactions in terms of the bulk properties of the
particles and show that the treatment is equivalent to solving the
corresponding master equation in paraxial approximation.


title: Quantum Noise in Multipixel Image Processing

abstract:
  We consider the general problem of the quantum noise in a multipixel
measurement of an optical image. We first give a precise criterium in order to
characterize intrinsic single mode and multimode light. Then, using a
transverse mode decomposition, for each type of possible linear combination of
the pixels' outputs we give the exact expression of the detection mode, i.e.
the mode carrying the noise. We give also the only way to reduce the noise in
one or several simultaneous measurements.


title: Deciding separability with a fixed error

abstract:
  We give a short proof of the cross norm characterization of separability due
to O. Rudolph and show how its computation, for a fixed chosen error, can be
reduced to a linear programming problem whose dimension grows polynomially with
the inverse of the error.


title: A Quantum Teleportation Game

abstract:
  We investigate a game where a sender (Alice) teleports coherent states to two
receivers (Bob and Charlie) through a tripartite Gaussian state. The aim of the
receivers is to optimize their teleportation fidelities by means of local
operations and classical communications. We show that a non-cooperative
strategy, corresponding to the standard telecloning protocol, can be
outperformed by a cooperative strategy, which gives rise to a novel
(cooperative) telecloning protocol.


title:
Application of the variational $R$-matrix method to one-dimensional
  quantum tunneling

abstract:
  We have applied the variational $R$-matrix method to calculate the reflection
and tunneling probabilities of particles tunneling through one-dimensional
potential barriers for five different types of potential profiles -- truncated
linear step, truncated exponential step, truncated parabolic, bell-shaped, and
Eckart. Our variational results for the transmission and reflection
coefficients are compared with exact analytical results and results obtained
from other numerical methods. We find that our results are in good agreement
with them. We conclude that the variational $R$-matrix method is a simple,
non-iterative, and effective method to solve one-dimensional quantum tunneling
problems.


title:
Cavity QED quantum phase gates for a single longitudinal mode of the
  intracavity field

abstract:
  A single three-level atom driven by a longitudinal mode of a high-Q cavity is
used to implement two-qubit quantum phase gates for the intracavity field. The
two qubits are associated to the zero-and one-photon Fock states of each of the
two opposite circular polarization states of the field. The three-level atom
yields the conditional phase gate provided the two polarization states and the
atom interact in a $V$-type configuration and the two photon resonance
condition is fulfilled. Microwave and optical implementations are discussed
with gate fidelities being evaluated against several decoherence mechanisms
such as atomic velocity fluctuations or the presence of a weak magnetic field.
The use of coherent states for both polarization states is investigated to
assess the entanglement capability of the proposed quantum gates.


title: Bell Gems: the Bell basis generalized

abstract:
  A class of self-similar sets of entangled quantum states is introduced, for
which a recursive definition is provided. These sets, the "Bell gems," are
defined by the subsystem exchange symmetry characteristic of the Bell states.
Each Bell gem is shown to be an orthonormal basis of maximally entangled
elements. A non-trivial example Bell gem is presented. Quantum circuits for
producing the elements of this example from the computational basis states are
provided.


title:
Generalized Coherent States for q-oscillator connected with discrete
  q-Hermite polynomials

abstract:
  We are continuing here the study of generalized coherent states of
Barut-Girardello type for the oscillator-like systems connected with the given
set of orthogonal polynomials. In this work we construct the family of coherent
states associated with discrete $q$-Hermite polynomials of the II-type and
prove the over-completeness of this family of states by constructing the
measure for unity decomposition for this family of coherent states.


title: Generalization of the Deutsch algorithm using two qudits

abstract:
  Deutsch's algorithm for two qubits (one control qubit plus one auxiliary
qubit) is extended to two $d$-dimensional quantum systems or qudits for the
case in which $d$ is equal to $2^n$, $n=1,2,...$ . This allows one to classify
a certain oracle function by just one query, instead of the $2^{n-1}+1$ queries
required by classical means. The given algorithm for two qudits also solves
efficiently the Bernstein-Vazirani problem. Entanglement does not occur at any
step of the computation.


title:
Quasideterministic generation of maximally entangled states of two
  mesoscopic atomic ensembles by adiabatic quantum feedback

abstract:
  We introduce an efficient, quasideterministic scheme to generate maximally
entangled states of two atomic ensembles. The scheme is based on quantum
nondemolition measurements of total atomic populations and on adiabatic quantum
feedback conditioned by the measurements outputs. The high efficiency of the
scheme is tested and confirmed numerically for ideal photodetection as well as
in the presence of losses.


title: Long-range quantum entanglement in noisy cluster states

abstract:
  We describe a phase transition for long-range entanglement in a
three-dimensional cluster state affected by noise. The partially decohered
state is modeled by the thermal state of a suitable Hamiltonian. We find that
the temperature at which the entanglement length changes from infinite to
finite is nonzero. We give an upper and lower bound to this transition
temperature.


title: Deterministic Plug-and-Play for Quantum Communication

abstract:
  We present a scheme for secure deterministic quantum communication without
using entanglement, in a Plug-and-Play fashion. The protocol is completely
deterministic, both in the encoding procedure and in the control one, thus
doubling the communication rate with respect to other setups; moreover,
deterministic nature of transmission, apart from rendering unnecessary bases
revelation on the public channel, allows the realization of protocols like
`direct communication' and `quantum dialogue'. The encoding exploits the phase
degree of freedom of a photon, thus paving the way to an optical fiber
implementation, feasible with present day technology.


title: Wigner-Weyl isomorphism for quantum mechanics on Lie groups

abstract:
  The Wigner-Weyl isomorphism for quantum mechanics on a compact simple Lie
group $G$ is developed in detail. Several New features are shown to arise which
have no counterparts in the familiar Cartesian case. Notable among these is the
notion of a `semiquantised phase space', a structure on which the Weyl symbols
of operators turn out to be naturally defined and, figuratively speaking,
located midway between the classical phase space $T^*G$ and the Hilbert space
of square integrable functions on $G$. General expressions for the star product
for Weyl symbols are presented and explicitly worked out for the angle-angular
momentum case.


title: Teleportation of an atomic ensemble quantum state

abstract:
  We propose a protocol to achieve high fidelity quantum state teleportation of
a macroscopic atomic ensemble using a pair of quantum-correlated atomic
ensembles. We show how to prepare this pair of ensembles using quasiperfect
quantum state transfer processes between light and atoms. Our protocol relies
on optical joint measurements of the atomic ensemble states and magnetic
feedback reconstruction.


title:
Relativistic quantum communication with bosonic and fermionic
  interferometers

abstract:
  We derive the relativistic transformation laws for the annihilation operators
of the scalar field, the massive spin-1 vector field, the electromagnetic field
and the spinor field. The technique developed here involves straightforward
mathematical techniques based on fundamental quantum field theory, and is
applicable to the study of entanglement in arbitrary coordinate
transformations. In particular, it predicts particle creation for non-inertial
motion. Furthermore, we present a unified description of relativistic
transformations and multi-particle interferometry with bosons and fermions,
which encompasses linear optical quantum computing.


title:
On the Dynamics of Generalized Coherent States. I. Exact and Stable
  Evolution

abstract:
  The exact and stable evolutions of generalized coherent states (GCS) for
quantum systems are considered by making use of the time-dependent integrals of
motion method and of the Klauder approach to the relationship between quantum
and classical mechanics. It is shown that one can construct for any quantum
system overcomplete family of states (OFS), related to the unitary
representations of the Lie group G by means of integral of motion generators,
and the possibility of using this group as a dynamical symmetry group is
pointed out. The relation of the OFS with quantum measurement theory is also
established.


title:
On the Dynamics of Generalized Coherent States. II. Classical Equations
  of Motions

abstract:
  Using the Klauder approach the stable evolution of generalized coherent
states (GCS) for some groups (SU(2), SU(1,1) and SU(N)) is considered and it is
shown that one and the same classical solution z(t) can correctly characterize
the quantum evolution of many different (in general nonequivalent) systems. As
examples some concrete systems are treated in greater detail: it is obtained
that the nonstationary systems of the singular oscillator, of the particle
motion in a magnetic field, and of the oscillator with friction all have stable
SU(1,1) GCS whose quantum evolution is determined by one and the same classical
function z(t). The physical properties of the constructed SU(1,1) GCS are
discussed and it is shown particularly that in the case of discrete series
D_k^+ they are those states for which the quantum mean values coincide with the
statistical ones for an oscillator in a thermostat.


title: Quantum error correction of coherent errors by randomization

abstract:
  A general error correction method is presented which is capable of correcting
coherent errors originating from static residual inter-qubit couplings in a
quantum computer. It is based on a randomization of static imperfections in a
many-qubit system by the repeated application of Pauli operators which change
the computational basis. This Pauli-Random-Error-Correction (PAREC)-method
eliminates coherent errors produced by static imperfections and increases
significantly the maximum time over which realistic quantum computations can be
performed reliably. Furthermore, it does not require redundancy so that all
physical qubits involved can be used for logical purposes.


title: Kraus representation of damped harmonic oscillator and its application

abstract:
  By definition, the Kraus representation of a harmonic oscillator suffering
from the environment effect, modeled as the amplitude damping or the phase
damping, is directly given by a simple operator algebra solution. As examples
and applications, we first give a Kraus representation of a single qubit whose
computational basis states are defined as bosonic vacuum and single particle
number states. We further discuss the environment effect on qubits whose
computational basis states are defined as the bosonic odd and even coherent
states. The environment effects on entangled qubits defined by two different
kinds of computational basis are compared with the use of fidelity.


title: Effects of decoherence and imperfections for quantum algorithms

abstract:
  We study effects of static inter-qubit interactions and random errors in
quantum gates on the stability of various quantum algorithms including the
Grover quantum search algorithm and the quantum chaos maps. For the Grover
algorithm our numerical and analytical results show existence of regular and
chaotic phases depending on the static imperfection strength $\epsilon$. The
critical border $\epsilon_c$ between two phases drops polynomially with the
number of qubits $n_q$ as $\epsilon_c \sim n_q^{-3/2}$. In the regular phase
$(\epsilon < \epsilon_c)$ the algorithm remains robust against imperfections
showing the efficiency gain $\epsilon_c / \epsilon$ for $\epsilon >
2^{-n_q/2}$. In the chaotic phase $(\epsilon > \epsilon_c)$ the algorithm is
completely destroyed. The results for the Grover algorithm are compared with
the imperfection effects for quantum algorithms of quantum chaos maps where the
universal law for the fidelity decay is given by the Random Matrix Theory
(RMT). We also discuss a new gyroscopic quantum error correction method which
allows to reduce the effect of static imperfections. In spite of this decay
GYQEC allows to obtain a significant gain in the accuracy of quantum
computations.


title: Two-photon "ghost" imaging with thermal light

abstract:
  We report the first experimental demonstration of two-photon imaging with a
pseudo-thermal source. Similarly to the case of entangled states, a two-photon
Gaussian thin lens equation is observed, indicating EPR type correlation in
position. We introduce the concepts of two-photon coherent and two-photon
incoherent imaging. The differences between the entangled and the thermal cases
are explained in terms of these concepts.


title: Contextual Observables and Quantum Information

abstract:
  In this short paper we present the main features of a new quantum programming
language proposed recently by Peter Selinger which gives a good idea about the
difficulties of constructing a scalable quantum computer. We show how some of
these difficulties are related to the contextuality of quantum observables and
to the abstract and statistical character of quantun theory (QT). We discuss
also, in some detail, the statistical interpretation (SI) of QT and the
contextuality of observables indicating the importance of these concepts for
the whole domain of quantum information.


title: Towards the theory of control in observable quantum systems

abstract:
  An operational description of the controlled Markov dynamics of
quantum-mechanical system is introduced. The feedback control strategies with
regard to the dynamical reduction of quantum states in the course of quantum
real-time measurements are discribed in terms of quantum filtering of these
states. The concept of sufficient coordinates for the description of the a
posteriori quantum states from a given class is introduced, and it is proved
that they form a classical Markov process with values in either state operators
or state vector space. The general problem of optimal control of a
quantum-mechanical system is discussed and the corresponding Bellman equation
in the space of sufficient coordinates is derived. The results are illustrated
in the example of control of the semigroup dynamics of a quantum system that is
instantaneously observed at discrete times and evolves between measurement
times according to the Schroedinger equation.


title: Remark on the additivity conjecture for the quantum depolarizing channel

abstract:
  We consider bistochastic quantum channels generated by unitary
representations of the discret group. The proof of the additivity conjecture
for the quantum depolarizing channel $\Phi$ based on the decreasing property of
the relative entropy is given. We show that the additivity conjecture is true
for the channel $\Xi =\Psi \circ \Phi $, where $\Psi $ is the phase damping.


title: Extremal Entanglement For Triqubit Pure States

abstract:
  A complete analysis of entangled triqubit pure states is carried out based on
a new simple entanglement measure. An analysis of all possible extremally
entangled pure triqubit states with up to eight terms is shown to reduce, with
the help of SLOCC transformations, to three distinct types. The analysis
presented are most helpful for finding different entanglement types in
multipartite pure state systems.


title: Fast Quantum Modular Exponentiation

abstract:
  We present a detailed analysis of the impact on modular exponentiation of
architectural features and possible concurrent gate execution. Various
arithmetic algorithms are evaluated for execution time, potential concurrency,
and space tradeoffs. We find that, to exponentiate an n-bit number, for storage
space 100n (twenty times the minimum 5n), we can execute modular exponentiation
two hundred to seven hundred times faster than optimized versions of the basic
algorithms, depending on architecture, for n=128. Addition on a neighbor-only
architecture is limited to O(n) time when non-neighbor architectures can reach
O(log n), demonstrating that physical characteristics of a computing device
have an important impact on both real-world running time and asymptotic
behavior. Our results will help guide experimental implementations of quantum
algorithms and devices.


title: Experimental Teleportation of a Quantum Controlled-NOT Gate

abstract:
  Teleportation of quantum gates is a critical step for implementation of
quantum networking and teleportation-based models of quantum computation. We
report an experimental demonstration of teleportation of the prototypical
quantum controlled-NOT (CNOT) gate. Assisted with linear optical manipulations,
photon entanglement produced from parametric down conversion, and coincidence
measurements, we teleport the quantum CNOT gate from acting on local qubits to
acting on remote qubits. The quality of the quantum gate teleportation is
characterized through the method of quantum process tomography, with an average
fidelity of 0.84 demonstrated for the teleported gate.


title:
Fano diagonalization of a polariton model for an inhomogeneous
  absorptive dielectric

abstract:
  The Hamiltonian of a polariton model for an inhomogeneous linear absorptive
dielectric is diagonalized by means of Fano's diagonalization method. The
creation and annihilation operators for the independent normal modes are
explicitly found as linear combinations of the canonical operators. The
coefficients in these combinations depend on the tensorial Green function that
governs the propagation of electromagnetic waves through the dielectric. The
time-dependent electromagnetic fields in the Heisenberg picture are given in
terms of the diagonalizing operators. These results justify the
phenomenological quantization of the electromagnetic field in an absorptive
dielectric.


title:
The Holevo capacity of infinite dimensional channels and the additivity
  problem

abstract:
  The notion of the Holevo capacity for arbitrarily constrained infinite
dimensional quantum channels is introduced. It is shown that despite
nonexistence of an optimal ensemble in this case it is possible to define the
notion of the output optimal average state for such a channel. The
characterization of the output optimal average state and a "minimax" expression
for the Holevo capacity are obtained. This makes it possible to prove
equivalence of several additivity properties for infinite dimensional quantum
channels.
  The notion of the $\chi$-function for an infinite dimensional channel is
considered, its strong concavity and lower semicontinuity are shown.
  The problem of continuity of the Holevo capacity is also discussed. It is
shown that the Holevo capacity is continuous function of a channel in the
finite dimensional case while in general it is only lower semicontinuous. This
conclusion is confirmed by the example.
  The main result of this note is the statement that additivity of the Holevo
capacity for all finite dimensional channels implies additivity of the Holevo
capacity for all infinite dimensional channels with arbitrary constraints. The
subadditivity of the $\chi$-function for two infinite dimensional channels with
one of them noiseless or entanglement breaking is also proved.


title:
Dynamical Casimir effect in a periodically changing domain: A dynamical
  systems approach

abstract:
  We study the problem of the behavior of a quantum massless scalar field in
the space between two parallel infinite perfectly conducting plates, one of
them stationary, the other moving periodically. We reformulate the physical
problem into a problem about the asymptotic behavior of the iterates of a map
of the circle, and then apply results from theory of dynamical systems to study
the properties of the map. Many of the general mathematical properties of maps
of the circle translate into properties of the field in the cavity. For
example, we give a complete classification of the possible resonances in the
system, and show that small enough perturbations do not destroy the resonances.
We use some mathematical identities to give transparent physical interpretation
of the processes of creation and amplification of the quantum field due to the
motion of the boundary and to elucidate the similarities and the differences
between the classical and quantum fields in domains with moving boundaries.


title:
General implementation of all possible positive-operator-value
  measurements of single photon polarization states

abstract:
  Positive Operator Value Measures (POVMs) are the most general class of
quantum measurements. We propose a setup in which all possible POVMs of a
single photon polarization state (corresponding to all possible sets of
two-dimensional Kraus operators) can be implemented easily using linear optics
elements. This method makes it possible to experimentally realize any
projective orthogonal, projective non-orthogonal or non-projective sets of any
number of POVM operators. Furthermore our implementation only requires vacuum
ancillas, and is deterministic rather than probabilistic. Thus it realizes
every POVM with the correct set of output states. We give the settings required
to implement two different well-known non-orthogonal projective POVMs.


title: Dynamical Casimir effect with cylindrical waveguides

abstract:
  I consider the quantum electromagnetic field in a coaxial cylindrical
waveguide, such that the outer cylindrical surface has a time-dependent radius.
The field propagates parallel to the axis, inside the annular region between
the two cylindrical surfaces. When the mechanical frequency and the thickness
of the annular region are small enough, only Transverse Electromagnetic (TEM)
photons may be generated by the dynamical Casimir effect. The photon emission
rate is calculated in this regime, and compared with the case of parallel
plates in the limit of very short distances between the two cylindrical
surfaces. The proximity force approximation holds for the transition matrix
elements in this limit, but the emission rate scales quadratically with the
mechanical frequency, as opposed to the cubic dependence for parallel plates.


title: A quantum algorithm for examining oracles

abstract:
  In this paper, we consider a quantum algorithm for solving the following
problem: ``Suppose $f$ is a function given as a black box (that is also called
an oracle) and $f$ is invariant under some AND-mask. Examine a property of $f$
by querying the oracle.'' We compare the efficiency of our quantum algorithm
with that of classical algorithms by evaluating the expected number of queries
for each algorithm. We show that our quantum algorithm is more efficient than
any classical algorithm in some cases. However, our quantum algorithm does not
exhibit an exponential speedup in the size of an input, compared with the best
classical algorithm. Our algorithm extracts a global property of $f$ (that is,
invariance of $f$) while it neglects local properties of $f$ (that is, outputs
of $f$). We can regard our algorithm as an application of Simon's algorithm.


title:
The Bloch-vector space for N-level systems -- the spherical-coordinate
  point of view

abstract:
  Bloch-vector spaces for $N$-level systems are investigated from the
spherical-coordinate point of view in order to understand their geometrical
aspects. We show that the maximum radius in each direction, which is due to the
construction of the Bloch-vector space, is determined by the minimum eigenvalue
of the corresponding observable (orthogonal generator of SU(N)). From this
fact, we reveal the dual property of the structure of the Bloch-vector space;
if in some direction the space reachs the large sphere (pure state), then in
the opposite direction the space can only get to the small sphere, and vice
versa. Another application is a parameterization with simple ranges of density
operators. We also provide three classes of quantum-state representation based
on actual measurements beyond the Bloch vector and discuss their state-spaces.


title:
Measurement of Coupling PDC photon sources with single-mode and
  multimode optical fibers

abstract:
  We investigate the coupling efficiency of parametric downconversion light
(PDC) into single and multi-mode optical fibers as a function of the pump beam
diameter, crystal length and walk-off. We outline two different theoretical
models for the preparation and collection of either single-mode or multi-mode
PDC light (defined by, for instance, multi-mode fibers or apertures,
corresponding to bucket detection). Moreover, we define the mode-matching
collection efficiency, important for realizing a single-photon source based on
PDC output into a well-defined single spatial mode. We also define a multimode
collection efficiency that is useful for single-photon detector calibration
applications.


title: On the quantum vacuum contribution to the momentum of dielectric media

abstract:
  More recently, Feigel has considered the quantum vacuum contribution to the
momentum of electromagnetic media. However, in Feigel's treatment he did not
take into account the relativistic transformation of the optical constants
(electric permittivity and magnetic permeability) of moving media. Here it is
shown that the effect arising from such a transformation will also provide a
quantum vacuum contribution to the velocity of media, in addition to the one
derived by Feigel himself.


title: Angular momentum and the geometrical gauge of localized photon staes

abstract:
  Localized photon states have non-zero angular momentum that varies with the
non-unique choice of a transverse basis and is changed by gauge transformations
of the geometric vector potential $\mathbf{a}$. The position operator must
depend on the choice of gauge, but a complete gauge transformation of a
physically distinct state has no observable effects. The potential $\mathbf{a}$
has a Dirac string singularity that is related to an optical vortex of the
electric field.


title: Decoherence in collective quantum memories for photons

abstract:
  The influence of decoherence on the fidelity of quantum memories for photonic
qubits based on dark-state polaritons in atomic ensembles is discussed. It is
shown that despite the large entanglement of the collective storage states
corresponding to single photons or nonclassical states of light the sensitivity
to decoherence does not scale with the number of atoms. This is due to the
existence of equivalence classes of storage states corresponding to states with
the same number of dark-state polariton excitations but arbitrary excitations
in other polariton modes. Several decoherence processes are discussed in
detail: single-atom spin-flips and dephasing, atom loss and motion of atoms.


title: Quantum Battle of the Sexes Revisited

abstract:
  Several quantum versions of the battle of the sexes game are analyzed. Some
of them are shown to reproduce the classical game. In some, there are no Nash
quantum pure equilibria. In some others, the payoffs are always equal to each
other. In others still, all equilibria favor Alice or Bob depending on a phase
shift of the initial state of the system. Explicit detailed calculations are
for the first time exhibited.


title: Quantum mechanics is about quantum information

abstract:
  I argue that quantum mechanics is fundamentally a theory about the
representation and manipulation of information, not a theory about the
mechanics of nonclassical waves or particles. The notion of quantum information
is to be understood as a new physical primitive -- just as, following
Einstein's special theory of relativity, a field is no longer regarded as the
physical manifestation of vibrations in a mechanical medium, but recognized as
a new physical primitive in its own right.


title:
Experimental evidence of high-resolution ghost imaging and ghost
  diffraction with classical thermal light

abstract:
  High-resolution ghost image and ghost diffraction experiments are performed
by using a single source of thermal-like speckle light divided by a beam
splitter. Passing from the image to the diffraction result solely relies on
changing the optical setup in the reference arm, while leaving untouched the
object arm. The product of spatial resolutions of the ghost image and ghost
diffraction experiments is shown to overcome a limit which was formerly thought
to be achievable only with entangled photons.


title:
Tomographic Characterization of Three-Qubit Pure States with Only
  Two-Qubit Detectors

abstract:
  A tomographic process for three-qubit pure states using only pairwise
detections is presented.


title:
On the Claim of the Observation of 'Macro-quantum effects' of Magnetic
  Vector Potential in the Classical Domain

abstract:
  I present conclusive arguments to show that a recent claim of observation of
quantum-like effects of the magnetic vector potential in the classical
macrodomain is spurious. The `one dimensional interference patterns' referred
to in the paper by R. K. Varma et al (Phys. Lett. A 303 (2002) 114-120) are not
due to any quantum-like wave phenomena. The data reported in the paper are not
consistent with the interpretation of interference, or with the topology of the
Aharonov-Bohm effect. The assertion that they are evidence of A-B like effect
in the classical macrodomain is based on inadequate appreciation of basic
physical facts regarding classical motion of electrons in magnetic fields,
interference phenomena, and the A-B effect.


title: Kerr nonlinear coupler and entanglement

abstract:
  We discuss a model comprising two coupled nonlinear oscillators (Kerr-like
nonlinear coupler) with one of them pumped by an external coherent excitation.
Applying the method of nonlinear quantum scissors we show that the quantum
evolution of the coupler can be closed within a finite set of n-photon Fock
states. Moreover, we show that the system is able to generate Bell-like states
and, as a consequence, the coupler discussed behaves as a two-qubit system. We
also analyze the effects of dissipation on entanglement of formation
parametrized by concurrence.


title:
Photonic crystals and inhibition of spontaneous emission: an
  introduction

abstract:
  In the first part of this introductory review we outline the developments in
photonic band gap materials from the physics of photonic band gap formation to
the fabrication and potential applications of photonic crystals. We briefly
describe the analogies between electron and photon localization, present a
simple model of a band structure calculation and describe some of the
techniques used for fabricating photonic crystals. Also some applications in
the field of photonics and optical circuitry are briefly presented. In the
second part, we discuss the consequences for the interaction between an atom
and the light field when the former is embedded in photonic crystals of a
specific type, exhibiting a specific form of a gap in the density of states.
  We first briefly review the standard treatment (Weisskopf-Wigner theory) in
describing the dynamics of spontaneous emission in free space from first
principles, and then proceed by explaining the alterations needed to properly
treat the case of a two-level atom embedded in a photonic band gap material.


title:
Exploiting the quantum Zeno effect to beat photon loss in linear optical
  quantum information processors

abstract:
  We devise a new technique to enhance transmission of quantum information
through linear optical quantum information processors. The idea is based on
applying the Quantum Zeno effect to the process of photon absorption. By
frequently monitoring the presence of the photon through a QND (quantum
non-demolition) measurement the absorption is suppressed. Quantum information
is encoded in the polarization degrees of freedom and is therefore not affected
by the measurement. Some implementations of the QND measurement are proposed.


title:
Thermal noise limitations to force measurements with torsion pendulums:
  Applications to the measurement of the Casimir force and its thermal
  correction

abstract:
  A general analysis of thermal noise in torsion pendulums is presented. The
specific case where the torsion angle is kept fixed by electronic feedback is
analyzed. This analysis is applied to a recent experiment that employed a
torsion pendulum to measure the Casimir force. The ultimate limit to the
distance at which the Casimir force can be measured to high accuracy is
discussed, and in particular the prospects for measuring the thermal correction
are elaborated upon.


title: Above potential barrier diffusion

abstract:
  The stationary phase method is applied to diffusion by a potential barrier
for an incoming wave packet with energies greater then the barrier height. It
is observed that a direct application leads to paradoxical results. The correct
solution, confirmed by numerical calculations is the creation of multiple peaks
as a consequence of multiple reflections. Lessons concerning the use of the
stationary phase method are drawn.


title: Hilbert's 17th problem and the quantumness of states

abstract:
  A state of a quantum systems can be regarded as {\it classical} ({\it
quantum}) with respect to measurements of a set of canonical observables iff
there exists (does not exist) a well defined, positive phase space
distribution, the so called Galuber-Sudarshan $P$-representation. We derive a
family of classicality criteria that require that averages of positive
functions calculated using $P$-representation must be positive. For polynomial
functions, these criteria are related to 17-th Hilbert's problem, and have
physical meaning of generalized squeezing conditions; alternatively, they may
be interpreted as {\it non-classicality witnesses}. We show that every generic
non-classical state can be detected by a polynomial that is a sum of squares of
other polynomials (sos). We introduce a very natural hierarchy of states
regarding their degree of quantumness, which we relate to the minimal degree of
a sos polynomial that detects them is introduced. Polynomial non-classicality
witnesses can be directly measured.


title: Quantum entanglement of a large number of photons

abstract:
  A bipartite multiphoton entangled state is created through stimulated
parametric down-conversion of strong laser pulses in a nonlinear crystal. It is
shown how detectors that do not resolve photon number can be used to analyze
such multiphoton states. Entanglement of up to 12 photons is detected using
both the positivity of the partially transposed density matrix and a newly
derived criteria. Furthermore, evidence is provided for entanglement of up to
100 photons. The multi-particle quantum state is such that even in the case of
an overall photon collection and detection efficiency as low as a few percent,
entanglement remains and can be detected.


title: Stability of Phase-modulated Quantum Key Distribution System

abstract:
  Phase drift and random fluctuation of interference visibility in double
unbalanced M-Z QKD system are observed and distinguished. It has been found
that the interference visibilities are influenced deeply by the disturbance of
transmission fiber. Theory analysis shows that the fluctuation is derived from
the envioronmental disturbance on polarization characteristic of fiber,
especially including transmission fiber. Finally, stability conditions of
one-way anti-disturbed M-Z QKD system are given out, which provides a
theoretical guide in pragmatic anti-disturbed QKD.


title:
Generation of polarization entangled photon pairs and violation of
  Bell's inequality using spontaneous four-wave mixing in fiber loop

abstract:
  We report the generation of polarization entangled photon pairs in the
1550-nm wavelength band using spontaneous four-wave mixing in a
dispersion-shifted fiber loop. The use of the fiber-loop configuration made it
possible to generate polarization entangled states very stably. With accidental
coincidences subtracted, we obtained coincidence fringes with >90 %
visibilities, and observed a violation of Bell's inequality by seven standard
deviations. We also confirmed the preservation of the quantum correlation
between the photons even after they had been separated by 20 km of optical
fiber.


title: Realizing Quantum Controlled Phase Flip through Cavity-QED

abstract:
  We propose a scheme to realize quantum controlled phase flip (CPF) between
two rare earth ions embedded in respective microsphere cavity via interacting
with a single-photon pulse in sequence. The numerical simulations illuminate
that the CPF gate between ions is robust and scalable with extremely high
fidelity and low error rate. Our scheme is more applicable than other schemes
presented before based on current laboratory cavity-QED technology, and it is
possible to be used as an applied unit gate in future quantum computation and
quantum communication.


title: On The Possibilities For Superluminal Signalling

abstract:
  This paper shall define and discuss two types of quantum process -
Disentangling and Entangling. The first type will be shown to contradict
Unitarity and is therefore ruled out as a possible signalling process within
standard linear Quantum Mechanics. The paper will argue that the second type -
the entangling process - is both allowed by the principles of Quantum
Mechanics, and can transmit superluminal signals. A proposal will be made for
addressing the objection to superluminal signalling by the Special Theory of
Relativity.


title:
Quantum Computing and Hidden Variables I: Mapping Unitary to Stochastic
  Matrices

abstract:
  This paper initiates the study of hidden variables from the discrete,
abstract perspective of quantum computing. For us, a hidden-variable theory is
simply a way to convert a unitary matrix that maps one quantum state to
another, into a stochastic matrix that maps the initial probability
distribution to the final one in some fixed basis. We list seven axioms that we
might want such a theory to satisfy, and then investigate which of the axioms
can be satisfied simultaneously. Toward this end, we construct a new
hidden-variable theory that is both robust to small perturbations and
indifferent to the identity operation, by exploiting an unexpected connection
between unitary matrices and network flows. We also analyze previous
hidden-variable theories of Dieks and Schrodinger in terms of our axioms. In a
companion paper, we will show that actually sampling the history of a hidden
variable under reasonable axioms is at least as hard as solving the Graph
Isomorphism problem; and indeed is probably intractable even for quantum
computers.


title:
Quantum Hamilton - Jacobi soluton for spectra of several one dimensional
  potentials with special properties

abstract:
  In this thesis the quantum Hamilton - Jacobi (QHJ) formalism is used for (i)
potentials which exhibit different spectra for different ranges of the
potential parameters, (ii) exactly solvable (ES) periodic potentials (iii)
quasi - exactly solvable (QES) periodic potentials and (iv) the PT symmetric
potentials (ES and QES). The QHJ formalism provides a simple and elegant method
to obtain the bound state and the band edge eigenvalues and the eigenfunctions.
For this purpose, a simple conjecture on the singularities of the logarithmic
derivative of the wave function in the complex plane is made and used in a
straight forward fashion to obtain the desired results.


title:
Energetic Suppression of Decoherence in Exchange-Only Quantum
  Computation

abstract:
  Universal quantum computation requiring only the Heisenberg exchange
interaction and suppressing decoherence via an energy gap is presented. The
combination of an always-on exchange interaction between the three physical
qubits comprising the encoded qubit and a global magnetic field generates an
energy gap between the subspace of interest and all other states. This energy
gap suppresses decoherence. Always-on exchange couplings greatly simplify
hardware specifications and the implementation of inter-logical-qubit gates. A
controlled phase gate can be implemented using only three Heisenberg exchange
operations all of which can be performed simultaneously.


title: Nonclassicality in Weak Measurements

abstract:
  We examine weak measurements of arbitrary observables where the object is
prepared in a mixed state and on which measurements with imperfect detectors
are made. The weak value of an observable can be expressed as a conditional
expectation value over an infinite class of different generalized Kirkwood
quasi-probability distributions. "Strange" weak values for which the real part
exceeds the eigenvalue spectrum of the observable can only be found if the
Terletsky-Margenau-Hill distribution is negative, or, equivalently, if the real
part of the weak value of the density operator is negative. We find that a
classical model of a weak measurement exists whenever the
Terletsky-Margenau-Hill representation of the observable equals the classical
representation of the observable and the Terletsky-Margenau-Hill distribution
is nonnegative. Strange weak values alone are not sufficient to obtain a
contradiction with classical models.
  We propose feasible weak measurements of photon number of the radiation
field. Negative weak values of energy contradicts all classical stochastic
models, whereas negative weak values of photon number contradict all classical
stochastic models where the energy is bounded from below by the zero-point
energy. We examine coherent states in particular, and find negative weak values
with probabilities of 16% for kinetic energy (or squared field quadrature), 8%
for harmonic oscillator energy and 50% for photon number. These experiments are
robust against detector inefficiency and thermal noise.


title:
Comment on ``All quantum observables in a hidden-variable model must
  commute simultaneously"

abstract:
  Malley discussed {[Phys. Rev. A {\bf 69}, 022118 (2004)]} that all quantum
observables in a hidden-variable model for quantum events must commute
simultaneously. In this comment, we discuss that Malley's theorem is indeed
valid for the hidden-variable theoretical assumptions, which were introduced by
Kochen and Specker. However, we give an example that the local hidden-variable
(LHV) model for quantum events preserves noncommutativity of quantum
observables. It turns out that Malley's theorem is not related with the LHV
model for quantum events, in general.


title:
Efficient high-fidelity quantum computation using matter qubits and
  linear optics

abstract:
  We propose a practical, scalable, and efficient scheme for quantum
computation using spatially separated matter qubits and single photon
interference effects. The qubit systems can be NV-centers in diamond,
Pauli-blockade quantum dots with an excess electron or trapped ions with
optical transitions, which are each placed in a cavity and subsequently
entangled using a double-heralded single-photon detection scheme. The fidelity
of the resulting entanglement is extremely robust against the most important
errors such as detector loss, spontaneous emission, and mismatch of cavity
parameters. We demonstrate how this entangling operation can be used to
efficiently generate cluster states of many qubits, which, together with single
qubit operations and readout, can be used to implement universal quantum
computation. Existing experimental parameters indicate that high fidelity
clusters can be generated with a moderate constant overhead.


title: An Optical Approach to the Dynamical Casimir Effect

abstract:
  We recently proposed a new approach to analyze the parametric resonance in a
vibrating cavity based on the analysis of classical optical paths. This
approach is used to examine various models of cavities with moving walls. We
prove that our method is useful to extract easily basic physical outcome.


title: Tunnelling Through Two Barriers

abstract:
  Recent studies of the tunnelling through two opaque barriers claim that the
transit time is independent of the barrier widths and of the separation
distance between the barriers. We observe, in contrast, that if multiple
reflections are allowed for correctly (infinite peaks) the transit time between
the barriers appears exactly as expected.


title: Repeat-Until-Success Quantum Computing

abstract:
  We demonstrate the possibility to perform distributed quantum computing using
only single photon sources (atom-cavity-like systems), linear optics and photon
detectors. The qubits are encoded in stable ground states of the sources. To
implement a universal two-qubit gate, two photons should be generated
simultaneously and pass through a linear optics network, where a measurement is
performed on them. Gate operations can be repeated until a success is heralded
without destroying the qubits at any stage of the operation. In contrast to
other schemes, this does not require explicit qubit-qubit interactions, a
priori entangled ancillas nor the feeding of photons into photon sources.


title: Light slowdown in the vicinity of cross-over resonances

abstract:
  Pulse propagation is considered in an inhomogeneously broadened medium of
three-level atoms in a V-configuration, dressed by a counter-propagating pump
pulse. A significant signal slowdown is demonstrated in this of the three
frequency windows of a reduced absorption and a steep normal dispersion, which
is due to a cross-over resonance. Particular properties of the group index in
the vicinity of such a resonance are demonstrated in the case of closely spaced
upper levels.


title: Efficient state preparation for a register of quantum bits

abstract:
  We describe a quantum algorithm to prepare an arbitrary pure state of a
register of a quantum computer with fidelity arbitrarily close to 1. Our
algorithm is based on Grover's quantum search algorithm. For sequences of
states with suitably bounded amplitudes, the algorithm requires resources that
are polynomial in the number of qubits. Such sequences of states occur
naturally in the problem of encoding a classical probability distribution in a
quantum register.


title: Slow-light solitons

abstract:
  A new type of soliton with controllable speed is constructed generalizing the
theory of slow-light propagation to an integrable regime of nonlinear dynamics.
The scheme would allow the quantum-information transfer between optical
solitons and atomic media.


title: Witnessing Entanglement with Second-Order Interference

abstract:
  Second-order interference and Hanbury-Brown and Twiss type experiments can
provide an operational framework for the construction of witness operators that
can test classical and nonclassical properties of a Gaussian squeezed state
(GSS), and provide entanglement witness operators to study the separability
properties of correlated Gaussian squeezed sates.


title:
Construction of exact solutions of Bloch-Maxwell equation based on
  Darboux transformation

abstract:
  A new strategy, using Darboux transformations, of finding self-switching
solutions of $i\dot{\rho} = [H, f({\rho})]$ is introduced. Unlike the previous
ones, working for any f but for Hamiltonians whose spectrum contains at least
three equally spaced eigenvalues, the strategy does not impose any restriction
on the discrete part of the spectrum of H. The strategy is applied to the
Bloch-Maxwell system.


title:
Analytic results for Gaussian wave packets in four model systems: I.
  Visualization of the kinetic energy

abstract:
  Using Gaussian wave packet solutions, we examine how the kinetic energy is
distributed in time-dependent solutions of the Schrodinger equation
corresponding to the cases of a free particle, a particle undergoing uniform
acceleration, a particle in a harmonic oscillator potential, and a system
corresponding to an unstable equilibrium. We find, for specific choices of
initial parameters, that as much as 90% of the kinetic energy can be localized
(at least conceptually) in the `front half' of such Gaussian wave packets, and
we visualize these effects.


title:
Analytic results for Gaussian wave packets in four model systems: II.
  Autocorrelation functions

abstract:
  The autocorrelation function, A(t), measures the overlap (in Hilbert space)
of a time-dependent quantum mechanical wave function, psi(x,t), with its
initial value, psi(x,0). It finds extensive use in the theoretical analysis and
experimental measurement of such phenomena as quantum wave packet revivals. We
evaluate explicit expressions for the autocorrelation function for
time-dependent Gaussian solutions of the Schrodinger equation corresponding to
the cases of a free particle, a particle undergoing uniform acceleration, a
particle in a harmonic oscillator potential, and a system corresponding to an
unstable equilibrium (the so-called `inverted' oscillator.) We emphasize the
importance of momentum-space methods where such calculations are often more
straightforwardly realized, as well as stressing their role in providing
complementary information to results obtained using position-space
wavefunctions.


title: Alternative Perspective on Quantum Tunneling and Instantons

abstract:
  We present a new way to compute and interpret quantum tunneling in a 1-D
double-well potential. For large transition time we show that the quantum
action functional gives an analytical expression for tunneling amplitudes. This
has been confirmed by numerical simulations giving relative errors in the order
of 1e-5. In contrast to the classical potential, the quantum potential has a
triple-well if the classical wells are deep enough. Its minima are located at
the position of extrema of the ground state wave function. The striking feature
is that a single trajectory with a double instanton reproduces the tunneling
amplitude. This is in contrast to the standard instanton approach, where
infinitely many instantons and anti-instatons have to be taken into account.
The quantum action functional is valid in the deep quantum regime in contrast
to the semi-classical regime where the standard instanton approach holds. We
compare both approaches via numerical simulations. While the standard instanton
picture describes only the transition between potential minima of equal depth,
the quantum action may give rise to instantons also for asymmetric potential
minima. Such case is illustrated by an example.


title:
Recasting Mermin's multi-player game into the framework of
  pseudo-telepathy

abstract:
  Entanglement is perhaps the most non-classical manifestation of quantum
mechanics. Among its many interesting applications to information processing,
it can be harnessed to reduce the amount of communication required to process a
variety of distributed computational tasks. Can it be used to eliminate
communication altogether? Even though it cannot serve to signal information
between remote parties, there are distributed tasks that can be performed
without any need for communication, provided the parties share prior
entanglement: this is the realm of pseudo-telepathy.
  One of the earliest uses of multi-party entanglement was presented by Mermin
in 1990. Here we recast his idea in terms of pseudo-telepathy: we provide a new
computer-scientist-friendly analysis of this game. We prove an upper bound on
the best possible classical strategy for attempting to play this game, as well
as a novel, matching lower bound. This leads us to considerations on how well
imperfect quantum-mechanical apparatus must perform in order to exhibit a
behaviour that would be classically impossible to explain. Our results include
improved bounds that could help vanquish the infamous detection loophole.


title: Contractive Entanglement in Squeezed State Evolution

abstract:
  The position variance of a single-mode Yuen states can go below the standard
quantum limit. For two-mode squeezed states, it is shown that the
time-dependent evolution of the entanglement of formation can be contractive,
going below that of the squeezed state with minimum Einstein-Podolsky-Rosen
dispersions, and increasing thereafter. The rate of change of the
Einstein-Podolsky-Rosen dispersions as a function of the two-mode phases
control this process. Contractive entanglement is shown to be equivalent to a
rotating phase space accompanied by time-dependent single-mode squeezing.


title:
Atom-field transfer of coherence in a two-photon micromaser assisted by
  a classical field

abstract:
  We investigate the transfer of coherence from atoms to a cavity field
initially in a statistical mixture in a two-photon micromaser arrangement. The
field is progressively modified from a maximum entropy state (thermal state)
towards an almost pure state (entropy close to zero) due to its interaction
with atoms sent across the cavity. We trace over the atomic variables, i.e.,
the atomic states are not collapsed by a detector after they leave the cavity.
We find that by applying an external classical driving field it is possible to
substantially increase the field purity without the need of previously
preparing the atoms in a superposition of their energy eigenstates. We also
discuss some of the nonclassical features of the resulting field.


title: Revised Iterative Solution of Ground State of Double-Well Potential

abstract:
  A revised new iterative method based on Green function defined by quadratures
along a single trajectory is developed and applied to solve the ground state of
the double-well potential. The result is compared to the one based on the
original iterative method. The limitation of the asymptotic expansion is also
discussed.


title: On the Generalized Dirac Equation for Fermions with Two Mass States

abstract:
  The generalized Dirac equation of the second order, describing particles with
spin 1/2 and two mass states, is analyzed. The projection operators extracting
states with definite energy and spin projections are obtained. The first order
generalized Dirac equation in the 20-dimensional matrix form and the
relativistically invariant bilinear form are derived. We obtain the canonical
energy-momentum tensor and density of the electromagnetic current expressed
through the 20-component wave function. Minimal and non-minimal electromagnetic
interactions of fermions are considered, and the quantum-mechanical Hamiltonian
is found. It is shown that there are only causal propagations of waves in the
approach considered.


title: Solid-State Quantum Communication With Josephson Arrays

abstract:
  Josephson junction arrays can be used as quantum channels to transfer quantum
information between distant sites. In this work we discuss simple protocols to
realize state transfer with high fidelity. The channels do not require
complicate gating but use the natural dynamics of a properly designed array. We
investigate the influence of static disorder both in the Josephson energies and
in the coupling to the background gate charges, as well as the effect of
dynamical noise. We also analyze the readout process, and its backaction on the
state transfer.


title: Probabilities are single-case, or nothing

abstract:
  Physicists have, hitherto, mostly adopted a frequentist conception of
probability, according to which probability statements apply only to ensembles.
It is argued that we should, instead, adopt an epistemic, or Bayesian
conception, in which probabilities are conceived as logical constructs rather
than physical realities, and in which probability statements do apply directly
to individual events. The question is closely related to the disagreement
between the orthodox school of statistical thought and the Bayesian school. It
has important technical implications (it makes a difference, what statistical
methodology one adopts). It may also have important implications for the
interpretation of the quantum state.


title: Detection of vacuum entanglement in a linear ion trap

abstract:
  We propose and study a method for detecting ground-state entanglement in a
chain of trapped ions, which realizes a suggested scheme for detecting vacuum
entanglement in quantum field theory. We show that the entanglement between
single ions or groups of ions can be swapped to the internal levels of two ions
by sending laser pulses that couple the internal and motional degrees of
freedom. This allows to entangle two ions without actually doing gates. A proof
of principle of the effect can be realized with two trapped ions and is
feasible with current technology.


title:
Entanglement within the Quantum Trajectory Description of Open Quantum
  Systems

abstract:
  The degree of entanglement in an open quantum system varies according to how
information in the environment is read. A measure of this contextual
entanglement is introduced based on quantum trajectory unravelings of the open
system dynamics. It is used to characterize the entanglement in a driven
quantum system of dimension $2\times\infty$ where the entanglement is induced
by the environmental interaction. A detailed mechanism for the
environment-induced entanglement is given.


title:
Potential Harmonics Expansion Method for Trapped Interacting Bosons :
  Inclusion of Two-Body Correlation

abstract:
  We study a system of $A$ identical interacting bosons trapped by an external
field by solving ab initio the many-body Schroedinger equation. A complete
solution by using, for example, the traditional hyperspherical harmonics (HH)
basis develops serious problems due to the large degeneracy of HH basis,
symmetrization of the wave function, calculation of the matrix elements, etc.
for large $A$. Instead of the HH basis, here we use the "potential harmonics"
(PH) basis, which is a subset of HH basis. We assume that the contribution to
the orbital and grand orbital [in $3(A-1)$-dimensional space of the reduced
motion] quantum numbers comes only from the interacting pair. This implies
inclusion of two-body correlations only and disregard of all higher-body
correlations. Such an assumption is ideally suited for the Bose-Einstein
condensate (BEC), which is extremely dilute. Unlike the $(3A-4)$ hyperspherical
variables in HH basis, the PH basis involves only three {\it{active}}
variables. It drastically reduces the number of coupled equations and
calculation of the potential matrix becomes tremendously simplified, as it
involves integrals over only three variables for any $A$. One can easily
incorporate realistic atom-atom interactions in a straight forward manner. We
study the ground and excited state properties of the condensate for both
attractive and repulsive interactions for various particle number.


title:
Effects of the interplay between interaction and disorder in bipartite
  entanglement

abstract:
  We use a disordered anti-ferromagnetic spin-1/2 chain with anisotropic
exchange coupling to model an array of interacting qubits. All qubits have the
same level spacing, except two, which are called the defects of the chain. The
level spacings of the defects are equal and much larger than all the others. We
investigate how the entanglement between the two defects depends on the
anisotropy of the system. When the anisotropy coupling is much larger than the
energy difference between a defect and an ordinary qubit, the two defects
become strongly entangled. Small anisotropies, on the contrary, may decrease
the entanglement, which is, in this case, also much affected by the number of
excitations. The analysis is made for nearest neighbor and next-nearest
neighbor defects. The decrease in the entanglement for nearest neighbor defects
is not very significant, especially in large chains.


title: Distance measures to compare real and ideal quantum processes

abstract:
  With growing success in experimental implementations it is critical to
identify a "gold standard" for quantum information processing, a single measure
of distance that can be used to compare and contrast different experiments. We
enumerate a set of criteria such a distance measure must satisfy to be both
experimentally and theoretically meaningful. We then assess a wide range of
possible measures against these criteria, before making a recommendation as to
the best measures to use in characterizing quantum information processing.


title: High-Fidelity Z-Measurement Error Correction of Optical Qubits

abstract:
  We demonstrate a quantum error correction scheme that protects against
accidental measurement, using an encoding where the logical state of a single
qubit is encoded into two physical qubits using a non-deterministic photonic
CNOT gate. For the single qubit input states |0>, |1>, |0>+|1>, |0>-|1>,
|0>+i|1>, and |0>-i|1> our encoder produces the appropriate 2-qubit encoded
state with an average fidelity of 0.88(3) and the single qubit decoded states
have an average fidelity of 0.93(5) with the original state. We are able to
decode the 2-qubit state (up to a bit flip) by performing a measurement on one
of the qubits in the logical basis; we find that the 64 1-qubit decoded states
arising from 16 real and imaginary single qubit superposition inputs have an
average fidelity of 0.96(3).