title:
Quantum controlled phase gate and cluster states generation via two
  superconducting quantum interference devices in a cavity

abstract:
  A scheme for implementing 2-qubit quantum controlled phase gate (QCPG) is
proposed with two superconducting quantum interference devices (SQUIDs) in a
cavity. The gate operations are realized within the two lower flux states of
the SQUIDs by using a quantized cavity field and classical microwave pulses.
Our scheme is achieved without any type of measurement, does not use the cavity
mode as the data bus and only requires a very short resonant interaction of the
SQUID-cavity system. As an application of the QCPG operation, we also propose a
scheme for generating the cluster states of many SQUIDs.


title: Thermodynamic and Quantum Thermodynamic Analyses of Brownian Movement

abstract:
  Thermodynamic and quantum thermodynamic analyses of Brownian movement of a
solvent and a colloid passing through neutral thermodynamic equilibrium states
only. It is shown that Brownian motors and E. coli do not represent Brownian
movement.


title: Hypercomplex Dirac Equation and Electrodynamics of Non-Conserved Charges

abstract:
  It is shown that the hypercomplex Dirac equation describes the system of
connected fields: 4-scalar, 4-pseudoscalar, 4-vector, 4-pseudo-vector and
antisymmetric 4-tensor second rank field. If mass is assumed to be zero this
system splits into two subsystems. Equations containing tensor, scalar and
pseudoscalar fields coincide with Maxwell equations complemented by scalar and
pseudoscalar fields. This system describes the electrodynamics of non-conserved
charges. The scalar and pseudoscalar fields are generated only by the
non-conserved charges - electric and hypothetical magnetic. The influence of
these fields on the charged particles is very unusual - it causes a change of
their rest mass. This allows us to give a new look at the Wigner paradox and
mechanism of mass renormalization.


title: Quantum Hidden Subgroup Algorithms: An Algorithmic Toolkit

abstract:
  One of the most promising and versatile approaches to creating new quantum
algorithms is based on the quantum hidden subgroup (QHS) paradigm, originally
suggested by Alexei Kitaev. This class of quantum algorithms encompasses the
Deutsch-Jozsa, Simon, Shor algorithms, and many more.
  In this paper, our strategy for finding new quantum algorithms is to
decompose Shor's quantum factoring algorithm into its basic primitives, then to
generalize these primitives, and finally to show how to reassemble them into
new QHS algorithms. Taking an "alphabetic building blocks approach," we use
these primitives to form an "algorithmic toolkit" for the creation of new
quantum algorithms, such as wandering Shor algorithms, continuous Shor
algorithms, the quantum circle algorithm, the dual Shor algorithm, a QHS
algorithm for Feynman integrals, free QHS algorithms, and more.
  Toward the end of this paper, we show how Grover's algorithm is most
surprisingly "almost" a QHS algorithm, and how this result suggests the
possibility of an even more complete "algorithmic tookit" beyond the QHS
algorithms.


title: Effective fault-tolerant quantum computation with slow measurements

abstract:
  How important is fast measurement for fault-tolerant quantum computation?
Using a combination of existing and new ideas, we argue that measurement times
as long as even 1,000 gate times or more have a very minimal effect on the
quantum accuracy threshold. This shows that slow measurement, which appears to
be unavoidable in many implementations of quantum computing, poses no essential
obstacle to scalability.


title: Nonlinear coherent dynamics of an atom in an optical lattice

abstract:
  We consider a simple model of lossless interaction between a two-level single
atom and a standing-wave single-mode laser field which creates a
one-dimensional optical lattice. Internal dynamics of the atom is governed by
the laser field which is treated to be classical with a large number of
photons. Center-of-mass classical atomic motion is governed by the optical
potential and the internal atomic degree of freedom. The resulting
Hamilton-Schr\"odinger equations of motion are a five-dimensional nonlinear
dynamical system with two integrals of motion. The main focus of the paper is
chaotic atomic motion that may be quantified strictly by positive values of the
maximal Lyapunov exponent. It is shown that atom, depending on the value of its
total energy, can either oscillate chaotically in a well of the optical
potential or fly ballistically with weak chaotic oscillations of its momentum
or wander in the optical lattice changing the direction of motion in a chaotic
way. In the regime of chaotic wandering atomic motion is shown to have fractal
properties. We find a useful tool to visualize complicated atomic motion --
Poincar\'e mapping of atomic trajectories in an effective three-dimensional
phase space onto planes of atomic internal variables and momentum. We find
common features with typical non-hyperbolic Hamiltonian systems -- chains of
resonant islands of different sizes embedded in a stochastic sea, stochastic
layers, bifurcations, and so on. The phenomenon of sticking of atomic
trajectories to boundaries of regular islands, that should have a great
influence to atomic transport in optical lattices, is found and demonstrated
numerically.


title: Asymptotic Entanglement of Two Independent Systems in a Common Bath

abstract:
  Two, non-interacting systems immersed in a common bath and evolving with a
Markovian, completely positive dynamics can become initially entangled via a
purely noisy mechanism. Remarkably, for certain, phenomenologically relevant
environments, the quantum correlations can persist even in the asymptotic
long-time regime.


title: Dynamics of Loschmidt echoes and fidelity decay

abstract:
  Fidelity serves as a benchmark for the relieability in quantum information
processes, and has recently atracted much interest as a measure of the
susceptibility of dynamics to perturbations. A rich variety of regimes for
fidelity decay have emerged. The purpose of the present review is to describe
these regimes, to give the theory that supports them, and to show some
important applications and experiments. While we mention several approaches we
use time correlation functions as a backbone for the discussion. Vanicek's
uniform approach to semiclassics and random matrix theory provides an important
alternative or complementary aspects. Other methods will be mentioned as we go
along. Recent experiments in micro-wave cavities and in elastodynamic systems
as well as suggestions for experiments in quantum optics shall be discussed.


title: One-mode quantum Gaussian channels

abstract:
  A classification of one-mode Gaussian channels is given up to canonical
unitary equivalence. A complementary to the quantum channel with additive
classical Gaussian noise is described providing an example of one-mode Gaussian
channel which is neither degradable nor anti-degradable.


title:
Reconstruction of photon statistics using low performance photon
  counters

abstract:
  The output of a photodetector consists of a current pulse whose charge has
the statistical distribution of the actual photon numbers convolved with a
Bernoulli distribution. Photodetectors are characterized by a nonunit quantum
efficiency, i.e. not all the photons lead to a charge, and by a finite
resolution, i.e. a different number of detected photons leads to a
discriminable values of the charge only up to a maximum value. We present a
detailed comparison, based on Monte Carlo simulated experiments and real data,
among the performances of detectors with different upper limits of counting
capability. In our scheme the inversion of Bernoulli convolution is performed
by maximum-likelihood methods assisted by measurements taken at different
quantum efficiencies. We show that detectors that are only able to discriminate
between zero, one and more than one detected photons are generally enough to
provide a reliable reconstruction of the photon statistics for single-peaked
distributions, while detectors with higher resolution limits do not lead to
further improvements. In addition, we demonstrate that, for semiclassical
states, even on/off detectors are enough to provide a good reconstruction.
Finally, we show that a reliable reconstruction of multi-peaked distributions
requires either higher quantum efficiency or better capability in
discriminating high number of detected photons.


title: Error-Resistant Distributed Quantum Computation in Trapped Ion Chain

abstract:
  We consider experimentally feasible chains of trapped ions with pseudo-spin
1/2, and find models that can potentially be used to implement error-resistant
quantum computation. Similar in spirit to classical neural networks, the
error-resistance of the system is achieved by encoding the qubits distributed
over the whole system. We therefore call our system a ''quantum neural
network'', and present a ''quantum neural network model of quantum
computation''. Qubits are encoded in a few quasi-degenerated low energy levels
of the whole system, separated by a large gap from the excited states, and
large energy barriers between themselves. We investigate protocols for
implementing a universal set of quantum logic gates in the system, by adiabatic
passage of a few low-lying energy levels of the whole system. Naturally
appearing and potentially dangerous distributed noise in the system leaves the
fidelity of the computation virtually unchanged, if it is not too strong. The
computation is also naturally resilient to local perturbations of the spins.


title:
Feasibility of a quantum memory for continuous variables based on
  trapped ions

abstract:
  We propose to use a large cloud of cold trapped ions as a medium for quantum
optics and quantum information experiments. Contrary to most recent
realizations of qubit manipulation based on a small number of trapped and
cooled ions, we study the case of traps containing a macroscopic number of
ions. We consider in particular the implementation of a quantum memory for
quantum information stored in continuous variables and study the impact of the
relevant physical parameters on the expected performances of the system.


title:
Photoionization of strontium for trapped-ion quantum information
  processing

abstract:
  We report a demonstration of simple and effective loading of strontium ions
into a linear radio frequency Paul trap using photoionization. The ionization
pathway is 5s2 1S0 -- 5s5p 1P1 -- 5p2 1D2, and the 5p2 1D2 final state is
auto-ionizing. Both transitions are driven using diode lasers: a
grating-stabilized 922 nm diode doubled in a single pass through potassium
niobate to 461 nm and a bare diode at 405 nm. Using this technique, we have
reduced the background pressure during the ion loading process by a factor of 2
compared to the conventional technique of electron bombardment. Initial ion
temperatures are low enough that the ions immediately form crystals. It is also
possible to observe the trapping region with a CCD camera during ion creation,
allowing specific ion number loading with high probability.


title:
Security proof of a three-state quantum key distribution protocol
  without rotational symmetry

abstract:
  Standard security proofs of quantum key distribution (QKD) protocols often
rely on symmetry arguments. In this paper, we prove the security of a
three-state protocol that does not possess rotational symmetry. The three-state
QKD protocol we consider involves three qubit states, where the first two
states, |0_z> and |1_z>, can contribute to key generation and the third state,
|+>=(|0_z>+|1_z>)/\sqrt{2}, is for channel estimation. This protocol has been
proposed and implemented experimentally in some frequency-based QKD systems
where the three states can be prepared easily. Thus, by founding on the
security of this three-state protocol, we prove that these QKD schemes are, in
fact, unconditionally secure against any attacks allowed by quantum mechanics.
The main task in our proof is to upper bound the phase error rate of the qubits
given the bit error rates observed. Unconditional security can then be proved
not only for the ideal case of a single-photon source and perfect detectors,
but also for the realistic case of a phase-randomized weak coherent light
source and imperfect threshold detectors. Our result on the phase error rate
upper bound is independent of the loss in the channel. Also, we compare the
three-state protocol with the BB84 protocol. For the single-photon source case,
our result proves that the BB84 protocol strictly tolerates a higher quantum
bit error rate than the three-state protocol; while for the coherent-source
case, the BB84 protocol achieves a higher key generation rate and secure
distance than the three-state protocol when a decoy-state method is used.


title: Against `Realism'

abstract:
  We examine the prevalent use of the phrase ``local realism'' in the context
of Bell's Theorem and associated experiments, with a focus on the question:
what exactly is the `realism' in `local realism' supposed to mean? Carefully
surveying several possible meanings, we argue that all of them are flawed in
one way or another as attempts to point out a second premise (in addition to
locality) on which the Bell inequalities rest, and (hence) which might be
rejected in the face of empirical data violating the inequalities. We thus
suggest that the phrase `local realism' should be banned from future
discussions of these issues, and urge physicists to revisit the foundational
questions behind Bell's Theorem.


title: The fidelity of general bosonic channels with pure state input

abstract:
  We first derive for the general form of the fidelity for various bosonic
channels. Thereby we give the fidelity of different quantum bosonic channel,
possibly with product input and entangled input respectively, as examples. The
properties of the fidelity are carefully examined.


title: Bell's inequalities in the tomographic representation

abstract:
  The tomographic approach to quantum mechanics is revisited as a direct tool
to investigate violation of Bell-like inequalities. Since quantum tomograms are
well defined probability distributions, the tomographic approach is emphasized
to be the most natural one to compare the predictions of classical and quantum
theory. Examples of inequalities for two qubits an two qutrits are considered
in the tomographic probability representation of spin states.


title: The efficiencies of generating cluster states with weak non-linearities

abstract:
  We propose a scalable approach to building cluster states of matter qubits
using coherent states of light. Recent work on the subject relies on the use of
single photonic qubits in the measurement process. These schemes can be made
robust to detector loss, spontaneous emission and cavity mismatching but as a
consequence the overhead costs grow rapidly, in particular when considering
single photon loss. In contrast, our approach uses continuous variables and
highly efficient homodyne measurements. We present a two-qubit scheme, with a
simple bucket measurement system yielding an entangling operation with success
probability 1/2. Then we extend this to a three-qubit interaction, increasing
this probability to 3/4. We discuss the important issues of the overhead cost
and the time scaling. This leads to a "no-measurement" approach to building
cluster states, making use of geometric phases in phase space.


title:
Class of PPT bound entangled states associated to almost any set of pure
  entangled states

abstract:
  We analyze a class of entangled states for bipartite $d \otimes d$ systems,
with $d$ non-prime. The entanglement of such states is revealed by the
construction of canonically associated entanglement witnesses. The structure of
the states is very simple and similar to the one of isotropic states: they are
a mixture of a separable and a pure entangled state whose supports are
orthogonal. Despite such simple structure, in an opportune interval of the
mixing parameter their entanglement is not revealed by partial transposition
nor by the realignment criterion, i.e. by any permutational criterion in the
bipartite setting. In the range in which the states are Positive under Partial
Transposition (PPT), they are not distillable; on the other hand, the states in
the considered class are provably distillable as soon as they are Nonpositive
under Partial Transposition (NPT). The states are associated to any set of more
than two pure states. The analysis is extended to the multipartite setting. By
an opportune selection of the set of multipartite pure states, it is possible
to construct mixed states which are PPT with respect to any choice of bipartite
cuts and nevertheless exhibit genuine multipartite entanglement. Finally, we
show that every $k$-positive but not completely positive map is associated to a
family of nondecomposable maps.


title: Measurements on Composite Qudits

abstract:
  We study measurements of the unitary generalization of Pauli operators.
First, an analytical (constructive) solution to the eigenproblem of these
operators is presented. Next, in the case of two subsystems, the Schmidt form
of the eigenvectors is derived to identify measurements which are easy to
implement. These results are utilized to show that quantum cryptography with
two bases, when operating on a two-component qudit, can be realized with
measurements on individual subsystems, assisted with classical communication.
We also discuss feasible devices which perform tomography of polarisation-path
qudits.


title: A Foundation Theory of Quantum Mechanics

abstract:
  The nRules are empirical regularities that were discovered in macroscopic
situations where the outcome is known. When they are projected theoretically
into the microscopic domain they predict a novel ontology including the
frequent collapse of an atomic wave function, thereby defining an nRule based
foundation theory. Future experiments can potentially discriminate between this
and other foundation theories of (non-relativistic) quantum mechanics.
Important features of the nRules are: (1) they introduce probability through
probability current rather than the Born rule, (2) they are valid independent
of size (micro or macroscopic), (3) they apply to individual trials, not just
to ensembles of trials. (4) they allow all observers to be continuously
included in the system without ambiguity, (5) they account for the collapse of
the wave function without introducing new or using old physical constants, and
(6) in dense environments they provide a high frequency of stochastic
localizations of quantum mechanical objects. Key words: measurement, stochastic
choice, state reduction.


title: Manipulation of matter waves using Bloch and Bloch-Zener oscillations

abstract:
  We present theoretical and numerical results on the dynamics of ultracold
atoms in an accelerated single- and double-periodic optical lattice. In the
single-periodic potential Bloch oscillations can be used to generate fast
directed transport with very little dispersion. The dynamics in the
double-periodic system is dominated by Bloch-Zener oscillations, i.e. the
interplay of Bloch oscillations and Zener tunneling between the subbands. Apart
from directed transport, the latter system permits various interesting
applications, such as widely tunable matter wave beam splitters and
Mach-Zehnder interferometry. As an application, a method for efficient probing
of small nonlinear mean-field interactions is suggested.


title:
Architecture of a Quantum Multicomputer Optimized for Shor's Factoring
  Algorithm

abstract:
  The quantum multicomputer consists of a large number of small nodes and a
qubus interconnect for creating entangled state between the nodes. The primary
metric chosen is the performance of such a system on Shor's algorithm for
factoring large numbers: specifically, the quantum modular exponentiation step
that is the computational bottleneck. This dissertation introduces a number of
optimizations for the modular exponentiation. My algorithms reduce the latency,
or circuit depth, to complete the modular exponentiation of an n-bit number
from O(n^3) to O(n log^2 n) or O(n^2 log n), depending on architecture.
Calculations show that these algorithms are one million times and thirteen
thousand times faster, when factoring a 6,000-bit number, depending on
architecture. Extending to the quantum multicomputer, five different qubus
interconnect topologies are considered, and two forms of carry-ripple adder are
found to be the fastest for a wide range of performance parameters. The links
in the quantum multicomputer are serial; parallel links would provide only very
modest improvements in system reliability and performance. Two levels of the
Steane [[23,1,7]] error correction code will adequately protect our data for
factoring a 1,024-bit number even when the qubit teleportation failure rate is
one percent.


title: Free electron in a laser field: The nonrelativistic solution

abstract:
  Schrodinger equation for a charged particle interacting with the plane wave
electromagnetic field is solved exactly. The exact analytic solution and the
perturbative solution up to second order are compared.


title:
Broadcasting of three qubit entanglement via local copying and
  entanglement swapping

abstract:
  In this work,We investigate the problem of secretly broadcasting of
three-qubit entangled state between two distant partners. The interesting
feature of this problem is that starting from two particle entangled state
shared between two distant partners we find that the action of local cloner on
the qubits and the measurement on the machine state vector generates
three-qubit entanglement between them. The broadcasting of entanglement is made
secret by sending the measurement result secretly using cryptographic scheme
based on orthogonal states. Further we show that this idea can be extended to
generate three particle entangled state between three distant partners.


title: Self-cooling of a micro-mirror by radiation pressure

abstract:
  We demonstrate passive feedback cooling of a mechanical resonator based on
radiation pressure forces and assisted by photothermal forces in a high-finesse
optical cavity. The resonator is a free-standing high-reflectance micro-mirror
(of mass m=400ng and mechanical quality factor Q=10^4) that is used as
back-mirror in a detuned Fabry-Perot cavity of optical finesse F=500. We
observe an increased damping in the dynamics of the mechanical oscillator by a
factor of 30 and a corresponding cooling of the oscillator modes below 10 K
starting from room temperature. This effect is an important ingredient for
recently proposed schemes to prepare quantum entanglement of macroscopic
mechanical oscillators.


title: Spatial Entanglement of a Free Bosonic Field

abstract:
  In this paper we discuss the entanglement properties of a thermal
non-relativistic free bosonic field. We demonstrate how to formally construct
spatial modes in order to use a continuous variable separability criterion and
show that the construction of the modes plays a significant role in the way the
entanglement manifests itself. For instance the presence of entanglement
depends on the shape of the modes and their separation. The temperature of the
gas is another crucial factor and for one choice of modes we show that
entanglement can be found at arbitrarily high temperatures as long as we can
zoom in on suitably small regions in space. Moreover, we show that the
entanglement here is useful as it can be extracted to a pair of localised
systems.


title: Quantum Capacities of Channels with small Environment

abstract:
  We investigate the quantum capacity of noisy quantum channels which can be
represented by coupling a system to an effectively small environment. A
capacity formula is derived for all cases where both system and environment are
two-dimensional--including all extremal qubit channels. Similarly, for channels
acting on higher dimensional systems we show that the capacity can be
determined if the channel arises from a sufficiently small coupling to a qubit
environment. Extensions to instances of channels with larger environment are
provided and it is shown that bounds on the capacity with unconstrained
environment can be obtained from decompositions into channels with small
environment.


title: Tests of Complete Positivity in Fiber Optics

abstract:
  We consider the propagation of polarized photons in optical fibers under the
action of randomly generated noise. In such situation, the change in time of
the photon polarization can be described by a quantum dynamical semigroup. We
show that the hierarchy among the decay constants of the polarization density
matrix elements as prescribed by complete positivity can be experimentally
probed using standard laboratory set-ups.


title: Quantum Many-Body Culling

abstract:
  We propose a method to produce a definite number of ground-state atoms by
adiabatic reduction of the depth of a potential well that confines a degenerate
Bose gas with repulsive interactions. Using a variety of methods, we map out
the maximum number of particles that can be supported by the well as a function
of the well depth and interaction strength, covering the limiting case of a
Tonks gas as well as the mean-field regime. We also estimate the time scales
for adiabaticity and discuss the recent observation of atomic number squeezing
(Chuu et al., Phys. Rev. Lett. {\bf 95}, 260403 (2005)).


title: Constructions of complex Hadamard matrices via tiling Abelian groups

abstract:
  Applications in quantum information theory and quantum tomography have raised
current interest in complex Hadamard matrices. In this note we investigate the
connection between tiling Abelian groups and constructions of complex Hadamard
matrices. First, we recover a recent very general construction of complex
Hadamard matrices due to Dita via a natural tiling construction. Then we find
some necessary conditions for any given complex Hadamard matrix to be
equivalent to a Dita-type matrix. Finally, using another tiling construction,
due to Szabo, we arrive at new parametric families of complex Hadamard matrices
of order 8, 12 and 16, and we use our necessary conditions to prove that these
families do not arise with Dita's construction. These new families complement
the recent catalogue of complex Hadamard matrices of small order.


title: Noncrossing normal ordering for functions of boson operators

abstract:
  Normally ordered forms of functions of boson operators are important in many
contexts in particular concerning Quantum Field Theory and Quantum Optics.
Beginning with the seminal work of Katriel [Lett. Nuovo Cimento,
10(13):565--567, 1974], in the last few years, normally ordered forms have been
shown to have a rich combinatorial structure, mainly in virtue of a link with
the theory of partitions. In this paper, we attempt to enrich this link. By
considering linear representations of noncrossing partitions, we define the
notion of noncrossing normal ordering. Given the growing interest in
noncrossing partitions, because of their many unexpected connections (like, for
example, with free probability), noncrossing normal ordering appears to be an
intriguing notion. We explicitly give the noncrossing normally ordered form of
the functions (a^{r}(a^{\dag})^{s})^{n}) and (a^{r}+(a^{\dag})^{s})^{n}, plus
various special cases. We are able to establish for the first time bijections
between noncrossing contractions of these functions, k-ary trees and sets of
lattice paths.


title:
Iterative Solutions for Low Lying Excited States of a Class of
  Schroedinger Equation

abstract:
  The convergent iterative procedure for solving the groundstate Schroedinger
equation is extended to derive the excitation energy and the wave function of
the low-lying excited states. The method is applied to the one-dimensional
quartic potential problem. The results show that the iterative solution
converges rapidly when the coupling $g$ is not too small.


title:
Onset of Entanglement and Noise Cross-Correlations in Two-Qubit System
  Interacting with Common Bosonic Bath

abstract:
  We summarize our recent results for the induced exchange interaction due to
thermal bosonic environment (bath) which also generates quantum noise. Our
focus here is on the onset of the interaction. We demonstrate that the induced
interaction can be used to manipulate and create entanglement over time scales
sufficiently large for controlling the two-qubit system for quantum computing
applications, though ultimately the noise effects will dominate.


title:
A single photon emitted by a single particle in free space vacuum modes
  and its resonant interaction with two- and three-level absorbers

abstract:
  We consider the time-delayed coincidence counting of two photons emitted in a
cascade by a single particle (atom, molecule, nucleus, etc). The
time-dependence of the probability amplitude of the second photon in the
cascade has a sharply rising leading edge due to the detection of the first
photon, as results from causality. If a macroscopic ensemble of resonant
two-level absorbers is placed in the path of the second photon between the
radiation source and the detector, the photon absorption does not follow Beer's
law due to the time-asymmetric shape of the photon. For very short delay times
almost no absorption takes place, even in an optically dense medium. We analyze
the propagation of such a second photon in a thick resonant three-level
absorber if a narrow electromagnetically induced transparency (EIT) window is
present at the center of the absorption line. It is shown that the EIT medium
can change the asymmetric time dependence of the photon probability amplitude
to a bell shape (EIT filtering). This bell-shaped photon interacts much more
efficiently with an other ensemble of two-level absorbers chosen, for example,
to store this photon and the information it carries.


title: Environment assisted entanglement enhancement

abstract:
  We consider dissipative atom-cavity systems and show that their collective
dynamics leads to the maximization of entanglement for intermediate values of
the cavity leakage parameter $\kappa$. We discuss possible ways the reservoir
influences entanglement. We first consider the entanglement of a single
two-level atom with a microwave cavity that is coupled to another cavity. We
show that the atom-cavity entanglement can be made to increase with cavity
leakage. We next show that the entanglement between two atoms passing
successively through a cavity can be maximised for intermediate values of
$\kappa$. We finally consider the micromaser where the increase of two-atom
entanglement for stronger cavity-environment coupling is demonstrated for
experimentally attainable values of the micromaser parameters.


title: Exact propagators for atom-laser interactions

abstract:
  A class of exact propagators describing the interaction of an $N$-level atom
with a set of on-resonance $\delta$-lasers is obtained by means of the Laplace
transform method. State-selective mirrors are described in the limit of strong
lasers. The ladder, V and $\Lambda$ configurations for a three-level atom are
discussed. For the two level case, the transient effects arising as result of
the interaction between both a semi-infinite beam and a wavepacket with the
on-resonance laser are examined.


title: Squeezed state purification with linear optics and feed forward

abstract:
  A scheme for optimal and deterministic linear optical purification of mixed
squeezed Gaussian states is proposed and experimentally demonstrated. The
scheme requires only linear optical elements and homodyne detectors, and allows
the balance between purification efficacy and squeezing degradation to be
controlled. One particular choice of parameters gave a ten-fold reduction of
the thermal noise with a corresponding squeezing degradation of only 11%. We
prove optimality of the protocol, and show that it can be used to enhance the
performance of quantum informational protocols such as dense coding and
entanglement generation.


title: Electromagnetic Casimir energy with extra dimensions

abstract:
  We calculate the energy-momentum tensor due to electromagnetic vacuum
fluctuations between two parallel hyperplanes in more than four dimensions,
considering both metallic and MIT boundary conditions. Using the axial gauge,
the problem can be mapped upon the corresponding problem with a massless,
scalar field satisfying respectively Dirichlet or Neumann boundary conditions.
The pressure between the plates is constant while the energy density is found
to diverge at the boundaries when there are extra dimensions. This can be
related to the fact that Maxwell theory is then no longer conformally
invariant. A similar behavior is known for the scalar field where a constant
energy density consistent with the pressure can be obtained by improving the
energy-momentum tensor with the Huggins term. This is not possible for the
Maxwell field. However, the change in the energy-momentum tensor with distance
between boundaries is finite in all cases.


title:
Unconditional security of the Bennett 1992 quantum key-distribution
  scheme with strong reference pulse

abstract:
  We prove the unconditional security of the original Bennett 1992 protocol
with strong reference pulse. We show that we may place a projection onto
suitably defined qubit spaces before the receiver, which makes the analysis as
simple as qubit-based protocols. Unlike the single-photon-based qubits, the
qubits identified in this scheme are almost surely detected by the receiver
even after a lossy channel. This leads to the key generation rate that is
proportional to the channel transmission rate for proper choices of
experimental parameters.


title: Generation of Entangled Photon Holes using Quantum Interference

abstract:
  In addition to photon pairs entangled in polarization or other variables,
quantum mechanics also allows optical beams that are entangled through the
absence of the photons themselves. These correlated absences, or ``entangled
photon holes'', can lead to counter-intuitive nonlocal effects analogous to
those of the more familiar entangled photon pairs. Here we report an
experimental observation of photon holes generated using quantum interference
effects to suppress the probability that two photons in a weak laser pulse will
separate at an optical beam splitter.


title:
Evaluable multipartite entanglement measures: are multipartite
  concurrences entanglement monotones?

abstract:
  We discuss the monotonicity under local operations and classical
communication (LOCC) of systematically constructed quantities aiming at
quantification of entanglement properties of multipartite quantum systems. The
so-called generalized multipartite concurrences can qualify as legitimate
entanglement measures if they are monotonous under LOCC. In the paper we give a
necessary and sufficient criterion for their monotonicity.


title: Master equation for a quantum particle in a gas

abstract:
  The equation for the quantum motion of a Brownian particle in a gaseous
environment is derived by means of S-matrix theory. This quantum version of the
linear Boltzmann equation accounts non-perturbatively for the quantum effects
of the scattering dynamics and describes decoherence and dissipation in a
unified framework. As a completely positive master equation it incorporates
both the known equation for an infinitely massive Brownian particle and the
classical linear Boltzmann equation as limiting cases.


title:
Performance of Deterministic Dynamical Decoupling Schemes: Concatenated
  and Periodic Pulse Sequences

abstract:
  Dynamical decoupling can be used to preserve arbitrary quantum states despite
undesired interactions with the environment, using control Hamiltonians
affecting the system only. We present a system-independent analysis of
dynamical decoupling based on leading order decoupling error estimates, valid
for bounded-strength environments. Using as a key tool a renormalization
transformation of the effective system-bath coupling Hamiltonian, we delineate
the reliability domain of dynamical decoupling used for quantum state
preservation, in a general setting for a single qubit. We specifically analyze
and compare two deterministic dynamical decoupling schemes -- periodic and
concatenated -- and distinguish between two limiting cases of fast versus slow
environments. We prove that concatenated decoupling outperforms periodic
decoupling over a wide range of parameters. These results are obtained for both
``ideal'' (zero-width) and realistic (finite-width) pulses This work extends
and generalizes our earlier work, Phys. Rev. Lett. 95, 180501 (2005).


title:
Continuous-variable teleportation in the characteristic-function
  description

abstract:
  We give a description of the continuous-variable teleportation protocol in
terms of the characteristic functions of the quantum states involved. The
Braunstein--Kimble protocol is written for an unbalanced homodyne measurement
and arbitrary input and resource states. We show that the output of the
protocol is a superposition between the input one-mode field and a classical
one induced by measurement and classical communication. We choose to describe
the input state distortion through teleportation by the average photon number
of the measurement-induced field. Only in the case of symmetric resource states
we find a relation between the optimal added noise and the minimal EPR
correlations used to define inseparability.


title: Local and Global Distinguishability in Quantum Interferometry

abstract:
  A statistical distinguishability based on relative entropy characterises the
fitness of quantum states for phase estimation. This criterion is employed in
the context of a Mach-Zehnder interferometer and used to interpolate between
two regimes, of local and global phase distinguishability. The scaling of
distinguishability in these regimes with photon number is explored for various
quantum states. It emerges that local distinguishability is dependent on a
discrepancy between quantum and classical rotational energy. Our analysis
demonstrates that the Heisenberg limit is the true upper limit for local phase
sensitivity. Only the `NOON' states share this bound, but other states exhibit
a better trade-off when comparing local and global phase regimes.


title:
Unconditionally Secure Quantum Bit Commitment Protocols Based on
  Correlation Immune Boolean Function

abstract:
  A class of quantum protocols of bit commitment is constructed based on the
nonorthogonal states coding and the correlation immunity of some Boolean
functions. The binding condition of these protocols is guaranteed mainly by the
law of causality and the concealing condition is guaranteed by the
indistinguishability between nonorthogonal quantum states and the correlation
immunity of Boolean functions. We also give out an oblivious transfer protocol
based on two-nonorthogonal states coding and build a bit commitment protocol on
top of it. The relationship between these protocols and the well known no-go
theorem is also discussed in details.


title: Efficient generation of entangled photons by cavity QED

abstract:
  A potential scheme is proposed to generate complete sets of entangled photons
in the context of cavity quantum electrodynamics (QED). The scheme includes
twice interactions of atoms with cavities, in which the first interaction is
made in two-mode optical cavities and the second one exists in a microwave
cavity. In the optical cavities the atoms are resonant with the cavity modes,
while the detuned interaction of the atoms with a single-mode of the microwave
cavity is driven by a classical field. We show that our scheme is carried out
with higher efficiency than previeous schemes, and is close to the reach of
current technique.


title: Time domain Einstein-Podolsky-Rosen correlation

abstract:
  We experimentally demonstrate creation and characterization of
Einstein-Podolsky-Rosen (EPR) correlation between optical beams in the time
domain. The correlated beams are created with two independent continuous-wave
optical parametric oscillators and a half beam splitter. We define temporal
modes using a square temporal filter with duration $T$ and make time-resolved
measurement on the generated state. We observe the correlations between the
relevant conjugate variables in time domain which correspond to the EPR
correlation. Our scheme is extendable to continuous variable quantum
teleportation of a non-Gaussian state defined in the time domain such as a
Schr\"odinger cat-like state.


title: Quantum walks with random phase shifts

abstract:
  We investigate quantum walks in multiple dimensions with different quantum
coins. We augment the model by assuming that at each step the amplitudes of the
coin state are multiplied by random phases. This model enables us to study in
detail the role of decoherence in quantum walks and to investigate the
quantum-to-classical transition. We also provide classical analogues of the
quantum random walks studied. Interestingly enough, it turns out that the
classical counterparts of some quantum random walks are classical random walks
with a memory and biased coin. In addition random phase shifts "simplify" the
dynamics (the cross interference terms of different paths vanish on average)
and enable us to give a compact formula for the dispersion of such walks.


title: Fundamentals for immediate implementation of a quantum secured Internet

abstract:
  This work shows how a secure Internet for users A and B can be implemented
through a fast key distribution system that uses physical noise to encrypt
information transmitted in deterministic form. Starting from a shared secret
random sequence between them, long sequences of fresh random bits can be shared
in a secure way and not involving a third party. The shared decrypted random
bits -encrypted by noise at the source- are subsequently utilized for
one-time-pad data encryption. The physical generated protection is not
susceptible to advances in computation or mathematics. In particular, it does
not depend on the difficulty of factoring numbers in primes. Also, there is no
use of Linear Feed Back Shift Registers. The attacker has free access to the
communication channels and may acquire arbitrary number of copies of the
transmitted signal without lowering the security level. No intrusion detection
method is needed.


title: Quantum simulations under translational symmetry

abstract:
  We investigate the power of quantum systems for the simulation of Hamiltonian
time evolutions on a cubic lattice under the constraint of translational
invariance. Given a set of translationally invariant local Hamiltonians and
short range interactions we determine time evolutions which can and those that
can not be simulated. Whereas for general spin systems no finite universal set
of generating interactions is shown to exist, universality turns out to be
generic for quadratic bosonic and fermionic nearest-neighbor interactions when
supplemented by all translationally invariant on-site Hamiltonians.


title:
Can Bohmian trajectories account for quantum recurrences having
  classical periodicities?

abstract:
  Quantum systems in specific regimes display recurrences at the period of the
periodic orbits of the corresponding classical system. We investigate the
excited hydrogen atom in a magnetic field -- a prototypical system of 'quantum
chaos' -- from the point of view of the de Broglie Bohm (BB) interpretation of
quantum mechanics. The trajectories predicted by BB theory are computed and
contrasted with the time evolution of the wavefunction, which shows pronounced
features at times matching the period of closed orbits of the classical
hydrogen in a magnetic field problem. Individual BB trajectories do not possess
these periodicities and cannot account for the quantum recurrences. These
recurrences can however be explained by BB theory by considering the ensemble
of trajectories compatible with an initial statistical distribution, although
none of the trajectories of the ensemble are periodic, rendering unclear the
dynamical origin of the classical periodicities.


title:
Specification of the Q Hypothesis: An Alternative Mathematical
  Foundation for Physics

abstract:
  Mathematical tools related to coherence theory and classical-quantum
equivalence, due to Wigner and Glauber, are essential to modern, practical and
empirical understanding of electromagnetics in areas like quantum optics and
nanoelectronics. This paper specifies how an extension of these same tools
(especially Glauber's "Q" mapping) can be applied to strong nuclear forces as
well, and provides a "bottom-up" approach to axiomatic unification of physics,
grounded in empirical reality (dice included). The Q hypothesis also has
implications for quantum measurement and quantum information technology. The
basic hypothesis is that density matrices across all of quantum field theory
can be "decoded" or mapped usefully into probability distributions for
"classical" fields, by using a generalization of Glauber's Q mapping, which
does the same for electromagnetics.


title:
A new type of solution of the Schr\"odinger equation on a self-similar
  fractal potential

abstract:
  Scattering a quantum particle by a self-similar fractal potential on a Cantor
set is investigated. We present a new type of solution of the functional
equation for the transfer matrix of this potential, which was derived earlier
from the Schr\"odinger equation.


title:
How to measure the wave-function absolute squared of a moving particle
  by using mirrors

abstract:
  We consider a slow particle with wave function $\psi_t(\vec{x})$, moving
freely in some direction. A mirror is briefly switched on around a time $T$ and
its position is scanned. It is shown that the measured reflection probability
then allows the determination of $|\psi_T(\vec{x})|^2$. Experimentally
available atomic mirrors should make this method applicable to the
center-of-mass wave function of atoms with velocities in the cm/s range.


title:
Prevent eavesdropping with bright reference pulses for practical quantum
  key distribution

abstract:
  We analyze the application of bright reference pulses to prevent the
photon-number-splitting attack in weak-pulse quantum key distribution. Under
the optimal eavesdropping strategy as far as we know, the optimal parameters of
bright reference and signal pulses can ensure a secure transmission distance up
to 146 km. To realize the quantum key distribution scenario with up-present
techniques, we present an experimentally feasible scheme to create a large
splitting ratio between bright reference and signal pulses, and to switch the
bright reference pulses away from signal pulses to avoid the after-pulse
disturbance.


title: Quantum Computation via Paraconsistent Computation

abstract:
  We present an original model of paraconsistent Turing machines (PTMs), a
generalization of the classical Turing machines model of computation using a
paraconsistent logic. Next, we briefl y describe the standard models of quantum
computation: quantum Turing machines and quantum circuits, and revise quantum
algorithms to solve the so-called Deutsch's problem and Deutsch-Jozsa problem.
Then, we show the potentialities of the PTMs model of computation simulating
the presented quantum algorithms via paraconsistent algorithms. This way, we
show that PTMs can resolve some problems in exponentially less time than any
classical deterministic Turing machine. Finally, We show that it is not
possible to simulate all characteristics (in particular entangled states) of
quantum computation by the particular model of PTMs here presented, therefore
we open the possibility of constructing a new model of PTMs by which it is
feasible to simulate such states.


title: Counterfactual computation revisited

abstract:
  Mitchison and Jozsa recently suggested that the "chained-Zeno" counterfactual
computation protocol recently proposed by Hosten et al. is counterfactual for
only one output of the computer. This claim was based on the existing abstract
algebraic definition of counterfactual computation, and indeed according to
this definition, their argument is correct. However, a more general definition
(physically adequate) for counterfactual computation is implicitly assumed by
Hosten et. al. Here we explain in detail why the protocol is counterfactual and
how the "history tracking" method of the existing description inadequately
represents the physics underlying the protocol. Consequently, we propose a
modified definition of counterfactual computation. Finally, we comment on one
of the most interesting aspects of the error-correcting protocol.


title:
Continous Measurement of the Energy Eigenstates of a Nanomechanical
  Resonator without a Non-Demolition Probe

abstract:
  We show that it is possible to perform a continuous measurement that
continually projects a nano-resonator into its energy eigenstates by employing
a linear coupling with a two-state system. This technique makes it possible to
perform a measurement that exposes the quantum nature of the resonator by
coupling it to a Cooper-pair Box and a superconducting transmission-line
resonator.


title:
Tripartite entanglement from interlinked $\chi^{(2)}$ parametric
  interactions

abstract:
  We examine the tripartite entanglement properties of an optical system using
interlinked $\chi^{(2)}$ interactions, recently studied experimentally in terms
of its phase-matching properties by Bondani et al [M. Bondani, A. Allevi, E.
Gevinti, A. Agliati, and A. Andreoni, arXiv:quant-ph/0604002.]. We show that
the system does produce output modes which are genuinely tripartite entangled
and that detection of this entanglement depends crucially on the correlation
functions which are measured, with a three-mode Einstein-Podolsky-Rosen
inequality being the most sensitive.


title:
Construction of a unique metric in quasi-Hermitian quantum mechanics:
  non-existence of the charge operator in a 2 x 2 matrix model

abstract:
  For a specific exactly solvable 2 by 2 matrix model with a PT-symmetric
Hamiltonian possessing a real spectrum, we construct all the eligible physical
metrics and show that none of them admits a factorization CP in terms of an
involutive charge operator C. Alternative ways of restricting the physical
metric to a unique form are briefly discussed.


title: Sequential Quantum Cloning

abstract:
  Not all unitary operations upon a set of qubits can be implemented by
sequential interactions between each qubit and an ancillary system. We analyze
the specific case of sequential quantum cloning 1->M and prove that the minimal
dimension D of the ancilla grows linearly with the number of clones M. In
particular, we obtain D = 2M for symmetric universal quantum cloning and D =
M+1 for symmetric phase-covariant cloning. Furthermore, we provide a recipe for
the required ancilla-qubit interactions in each step of the sequential
procedure for both cases.


title: Decoherence and Recoherence in a Vibrating RF SQUID

abstract:
  We study an RF SQUID, in which a section of the loop is a freely suspended
beam that is allowed to oscillate mechanically. The coupling between the RF
SQUID and the mechanical resonator originates from the dependence of the total
magnetic flux threading the loop on the displacement of the resonator. Motion
of the latter affects the visibility of Rabi oscillations between the two
lowest energy states of the RF SQUID. We address the feasibility of
experimental observation of decoherence and recoherence, namely decay and rise
of the visibility, in such a system.


title: Degradation of a quantum directional reference frame as a random walk

abstract:
  We investigate if the degradation of a quantum directional reference frame
through repeated use can be modeled as a classical direction undergoing a
random walk on a sphere. We demonstrate that the behaviour of the fidelity for
a degrading quantum directional reference frame, defined as the average
probability of correctly determining the orientation of a test system, can be
fit precisely using such a model. Physically, the mechanism for the random walk
is the uncontrollable back-action on the reference frame due to its use in a
measurement of the direction of another system. However, we find that the
magnitude of the step size of this random walk is not given by our classical
model and must be determined from the full quantum description.


title:
Teleportation with a Mixed State of Four Qubits and the Generalized
  Singlet Fraction

abstract:
  Recently, an explicit protocol ${\cal E}_0$ for faithfully teleporting
arbitrary two-qubit states using genuine four-qubit entangled states was
presented by us [Phys. Rev. Lett. {\bf 96}, 060502 (2006)]. Here, we show that
${\cal E}_0$ with an arbitrary four-qubit mixed state resource $\Xi$ is
equivalent to a generalized depolarizing bichannel with probabilities given by
the maximally entangled components of the resource. These are defined in terms
of our four-qubit entangled states. We define the generalized singlet fraction
${\cal G}[\Xi]$, and illustrate its physical significance with several
examples. We argue that in order to teleport arbitrary two-qubit states with
average fidelity better than is classically possible, we have to demand that
${\cal G}[\Xi] > 1/2$. In addition, we conjecture that when ${\cal G}[\Xi] <
1/4$ then no entanglement can be teleported. It is shown that to determine the
usefulness of $\Xi$ for ${\cal E}_0$, it is necessary to analyze ${\cal
G}[\Xi]$.


title:
Normalization of Collisional Decoherence: Squaring the Delta Function,
  and an Independent Cross-Check

abstract:
  We show that when the Hornberger--Sipe calculation of collisional decoherence
is carried out with the squared delta function a delta of energy instead of a
delta of the absolute value of momentum, following a method introduced by
Di\'osi, the corrected formula for the decoherence rate is simply obtained. The
results of Hornberger and Sipe and of Di\'osi are shown to be in agreement. As
an independent cross-check, we calculate the mean squared coordinate diffusion
of a hard sphere implied by the corrected decoherence master equation, and show
that it agrees precisely with the same quantity as calculated by a classical
Brownian motion analysis.


title:
Notes on Certain Newton Gravity Mechanisms of Wave Function Localisation
  and Decoherence

abstract:
  Both the additional non-linear term in the Schr\"odinger equation and the
additional non-Hamiltonian term in the von Neumann equation, proposed to ensure
localisation and decoherence of macro-objects, resp., contain the same
Newtonian interaction potential formally. We discuss certain aspects that are
common for both equations. In particular, we calculate the enhancement of the
proposed localisation and/or decoherence effects, which would take place if one
could lower the conventional length-cutoff and resolve the mass density on the
interatomic scale.


title: `Plausibilities of plausibilities': an approach through circumstances

abstract:
  Probability-like parameters appearing in some statistical models, and their
prior distributions, are reinterpreted through the notion of `circumstance', a
term which stands for any piece of knowledge that is useful in assigning a
probability and that satisfies some additional logical properties. The idea,
which can be traced to Laplace and Jaynes, is that the usual inferential
reasonings about the probability-like parameters of a statistical model can be
conceived as reasonings about equivalence classes of `circumstances' - viz.,
real or hypothetical pieces of knowledge, like e.g. physical hypotheses, that
are useful in assigning a probability and satisfy some additional logical
properties - that are uniquely indexed by the probability distributions they
lead to.


title: Reliable entanglement transfer between pure quantum states

abstract:
  The problem of the reliable transfer of entanglement from one pure bipartite
quantum state to another using local operations is analyzed. It is shown that
in the case of qubits the amount that can be transferred is restricted to the
difference between the entanglement of the two states. In the presence of a
catalytic state the range of the transferrable amount broadens to a certain
degree.


title: The Role of Resonances in the Stochastic Limit

abstract:
  In the stochastic limit the resonances play a fundamental role because they
determine the generalized susceptivities which are the building blocks of all
the physical information which survives in this limit. There are two sources of
possible divergences: one related to the singularities of the form factor;
another to the chaoticity of the spectrum. The situation will be illustrated
starting from the example of the discrete part of the hydrogen atom in
interaction with the electromagnetic field.


title:
On the Relationship between Resolution Enhancement and Multiphoton
  Absorption Rate in Quantum Lithography

abstract:
  The proposal of quantum lithography [Boto et al., Phys. Rev. Lett. 85, 2733
(2000)] is studied via a rigorous formalism. It is shown that, contrary to Boto
et al.'s heuristic claim, the multiphoton absorption rate of a ``NOON'' quantum
state is actually lower than that of a classical state with otherwise identical
parameters. The proof-of-concept experiment of quantum lithography [D'Angelo et
al., Phys. Rev. Lett. 87, 013602 (2001)] is also analyzed in terms of the
proposed formalism, and the experiment is shown to have a reduced multiphoton
absorption rate in order to emulate quantum lithography accurately. Finally,
quantum lithography by the use of a jointly Gaussian quantum state of light is
investigated, in order to illustrate the trade-off between resolution
enhancement and multiphoton absorption rate.


title:
Proposed realization of the Dicke-model quantum phase transition in an
  optical cavity QED system

abstract:
  The Dicke model consisting of an ensemble of two-state atoms interacting with
a single quantized mode of the electromagnetic field exhibits a
zero-temperature phase transition at a critical value of the dipole coupling
strength. We propose a scheme based on multilevel atoms and cavity-mediated
Raman transitions to realise an effective Dicke system operating in the phase
transition regime. Output light from the cavity carries signatures of the
critical behavior which is analyzed for the thermodynamic limit where the
number of atoms is very large.


title:
The expansion of orthogonal complete set and teleportation of an
  arbitrary two-qubit state

abstract:
  In accordance with the principle of superposition and operator rule, the
state of the whole system composed of the state of the particles to be
teleported and quantum channel can be expanded by Bell bases and transformation
operator. Theoretically, if determinant of transformation operators is not
zero, the teleportation can be realized only by performing an inverse
transformation. Actually, if the transformation operator is not a unitary
operation, then by using one auxiliary qubits, the teleportation can be
realized only by performing a collective unitary transformation. Moreover, the
further analysis of the relationship between collective unitary operation and
transformation operators is discussed.


title: Entanglement oscillations in open Heisenberg chains

abstract:
  We study pairwise entanglements in spin-half and spin-one Heisenberg chains
with an open boundary condition, respectively. We find out that the
ground-state and the first-excited-state entanglements are equal for the
three-site spin-one chain. When the number of sites L>3, the concurrences and
negativities display oscillatory behaviors, and the oscillations of the
ground-state and the first-excited-state entanglements are out of phase or in
phase.


title: General conditions for a quantum adiabatic evolution

abstract:
  The smallness of the variation rate of the hamiltonian matrix elements
compared to the (square of the) energy spectrum gap is usually believed to be
the key parameter for a quantum adiabatic evolution. However it is only
perturbatively valid for scaled timed hamiltonian and resonance processes as
well as off resonance possible constructive St\"{u}ckelberg interference
effects violate this usual condition for general hamiltionian. More general
adiabatic condition and exact bounds for adiabatic quantum evolution are
derived and studied in the framework of a two-level system. The usual criterion
is restored for real two level hamiltonian with small number of monotonicity
changes of the hamiltonian matrix elements and its derivative.


title: Bounding the set of quantum correlations

abstract:
  We introduce a hierarchy of conditions necessarily satisfied by any
distribution P(ab) representing the probabilities for two separate observers to
obtain outcomes a and b when making local measurements on a shared quantum
state. Each condition in this hierarchy is formulated as a semidefinite
program. Our approach can be used to obtain upper-bounds on the quantum
violation of an arbitrary Bell inequality. It yields, for instance, tight
bounds for the violations of the Collins et al. inequalities.


title:
Explicit Realization of Pseudo-Hermitian and Quasi-Hermitian Quantum
  Mechanics for Two-Level Systems

abstract:
  We give an explicit characterization of the most general quasi-Hermitian
operator H, the associated metric operators \eta_+, and \eta_+-pseudo-Hermitian
operators acting in two-dimensional complex Euclidean space C^2. These
operators represent the physical observables of a model whose Hamiltonian and
Hilbert space are respectively H and C^2 endowed with the inner product defined
by \eta_+. Our calculations allows for a direct demonstration of the fact that
the choice of an irreducible family of observables fixes the metric operator up
to a multiplicative factor.


title:
Scattering of two-level atoms by delta lasers: Exactly solvable models
  in atom optics

abstract:
  We study the scattering of two-level atoms at narrow laser fields, modeled by
a $\delta$-shape intensity profile. The unique properties of these potentials
allow us to give simple analytic solutions for one or two field zones. Several
applications are studied: a single $\delta$-laser may serve as a detector model
for atom detection and arrival-time measurements, either by means of
fluorescence or variations in occupation probabilities. We show that, in
principle, this ideal detector can measure the particle density, the quantum
mechanical flux, arrival time distributions or local kinetic energy densities.
Moreover, two spatially separated $\delta$-lasers are used to investigate
quantized-motion effects on Ramsey interferometry.


title:
Morse potential and its relationship with the Coulomb in a
  position-dependent mass background

abstract:
  We provide some explicit examples wherein the Schr\"odinger equation for the
Morse potential remains exactly solvable in a position-dependent mass
background. Furthermore, we show how in such a context, the map from the full
line $(- \infty, \infty)$ to the half line $(0, \infty)$ may convert an exactly
solvable Morse potential into an exactly solvable Coulomb one. This generalizes
a well-known property of constant-mass problems.


title: Towards optimization of quantum circuits

abstract:
  Any unitary operation in quantum information processing can be implemented
via a sequence of simpler steps - quantum gates. However, actual implementation
of a quantum gate is always imperfect and takes a finite time. Therefore,
seeking for a short sequence of gates - efficient quantum circuit for a given
operation, is an important task. We contribute to this issue by proposing
optimization of the well-known universal procedure proposed by Barenco et.al
[1]. We also created a computer program which realizes both Barenco's
decomposition and the proposed optimization. Furthermore, our optimization can
be applied to any quantum circuit containing generalized Toffoli gates,
including basic quantum gate circuits.


title: The "Unromantic Pictures" of Quantum Theory

abstract:
  I am concerned with two views of quantum mechanics that John S. Bell called
``unromantic'': spontaneous wave function collapse and Bohmian mechanics. I
discuss some of their merits and report about recent progress concerning
extensions to quantum field theory and relativity. In the last section, I
speculate about an extension of Bohmian mechanics to quantum gravity.


title: Phase-conjugate optical coherence tomography

abstract:
  Quantum optical coherence tomography (Q-OCT) offers a factor-of-two
improvement in axial resolution and the advantage of even-order dispersion
cancellation when it is compared to conventional OCT (C-OCT). These features
have been ascribed to the non-classical nature of the biphoton state employed
in the former, as opposed to the classical state used in the latter.
Phase-conjugate OCT (PC-OCT), introduced here, shows that non-classical light
is not necessary to reap Q-OCT's advantages. PC-OCT uses classical-state signal
and reference beams, which have a phase-sensitive cross-correlation, together
with phase conjugation to achieve the axial resolution and even-order
dispersion cancellation of Q-OCT with a signal-to-noise ratio that can be
comparable to that of C-OCT.


title: Random bipartite entanglement from W and W-like states

abstract:
  We describe a protocol for distilling maximally entangled bipartite states
between random pairs of parties from those sharing a tripartite W state, and
show that, rather surprisingly, the total distillation rate (the total number
of EPR pairs distilled per W, irrespective of who shares them) may be done at a
higher rate than distillation of bipartite entanglement between specified pairs
of parties. Specifically, the optimal distillation rate for specified
entanglement for the W has been previously shown to be the asymptotic
entanglement of assistance of 0.92 EPR pairs per W, while our protocol can
asymptotically distill 1 EPR pair per W between random pairs of parties, which
we conjecture to be optimal. We thus demonstrate a tradeoff between the overall
asymptotic rate of EPR distillation and the distribution of final EPR pairs
between parties. We further show that by increasing the number of parties in
the protocol that there exist states with fixed lower-bounded distillable
entanglement for random parties but arbitrarily small distillable entanglement
for specified parties.


title:
A modified approach to the measurement problem: Objective reduction in
  the retinal molecule prior to conformational change

abstract:
  A new analysis of the measurement problem reveals the possibility that
collapse of the wavefunction may now take place just before photoisomerization
of the rhodopsin molecule in the retinal rods. It is known that when a photon
is initially absorbed by the retinal molecule which, along with opsin comprises
the rhodopsin molecule, an electron in the highest pi orbital is immediately
excited to a pi* orbital. This means that a measurement or transfer of
information takes place at the quantum level before the retinal molecule
commences the conformational change from cis to trans. This could have profound
implications for resolving some of the foundational issues confronting quantum
mechanics.


title: Quantum heat engine with continuum working medium

abstract:
  We introduce a new quantum heat engine, in which the working medium is a
quantum system with a discrete level and a continuum. Net work done by this
engine is calculated and discussed. The results show that this quantum heat
engine behaves like the two-level quantum heat engine in both the
high-temperature and the low-temperature limits, but it operates differently in
temperatures between them. The efficiency of this quantum heat engine is also
presented and discussed.


title:
Experimental Long-Distance Decoy-State Quantum Key Distribution Based On
  Polarization Encoding

abstract:
  We demonstrate the decoy-state quantum key distribution (QKD) with one-way
quantum communication in polarization space over 102km. Further, we simplify
the experimental setup and use only one detector to implement the one-way
decoy-state QKD over 75km, with the advantage to overcome the security
loopholes due to the efficiency mismatch of detectors. Our experimental
implementation can really offer the unconditionally secure final keys. We use 3
different intensities of 0, 0.2 and 0.6 for the pulses of source in our
experiment. In order to eliminate the influences of polarization mode
dispersion in the long-distance single-mode optical fiber, an automatic
polarization compensation system is utilized to implement the active
compensation.


title:
Topological structures of adiabatic phase for multi-level quantum
  systems

abstract:
  The topological properties of adiabatic gauge fields for multi-level
(three-level in particular) quantum systems are studied in detail. Similar to
the result that the adiabatic gauge field for SU(2) systems (e.g. two-level
quantum system or angular momentum systems, etc) have a monopole structure, the
curvature two-forms of the adiabatic holonomies for SU(3) three-level and SU(3)
eight-level quantum systems are shown to have monopole-like (for all levels) or
instanton-like (for the degenerate levels) structures.


title:
Coherent acceleration of material wavepackets in modulated optical
  fields

abstract:
  We study the quantum dynamics of a material wavepacket bouncing off a
modulated atomic mirror in the presence of a gravitational field. We find the
occurrence of coherent accelerated dynamics for atoms beyond the familiar
regime of dynamical localization. The acceleration takes place for certain
initial phase space data and within specific windows of modulation strengths.
The realization of the proposed acceleration scheme is within the range of
present day experimental possibilities.


title: Two Derivations of the Master Equation of Quantum Brownian Motion

abstract:
  Central to many discussion of decoherence is a master equation for the
reduced density matrix of a massive particle experiencing scattering from its
surrounding environment, such as that of Joos and Zeh. Such master equations
enjoy a close relationship with spontaneous localization models, like the GRW
model. This aim of this paper is to present two derivations of the master
equation. The first derivation is a pedagogical model designed to illustrate
the origins of the master equation as simply as possible, focusing on physical
principles and without the complications of S-matrix theory. This derivation
may serve as a useful tutorial example for students attempting to learn this
subject area. The second is the opposite: a very general derivation using
non-relativistic many body field theory. It reduces to the equation of the type
given by Joos and Zeh in the one-particle sector, but correcting certain
numerical factors which have recently become significant in connection with
experimental tests of decoherence. This master equation also emphasizes the
role of local number density as the ``preferred basis'' for decoherence in this
model.


title: Quantum Lyapunov Exponents

abstract:
  We show that it is possible to associate univocally with each given solution
of the time-dependent Schroedinger equation a particular phase flow ("quantum
flow") of a non-autonomous dynamical system. This fact allows us to introduce a
definition of chaos in quantum dynamics (quantum chaos), which is based on the
classical theory of chaos in dynamical systems. In such a way we can introduce
quantities which may be appelled "Quantum Lyapunov Exponents". Our approach
applies to a non-relativistic quantum-mechanical system of n charged particles;
in the present work numerical calculations are performed only for the hydrogen
atom. In the computation of the trajectories we first neglect the spin
contribution to chaos, then we consider the spin effects in quantum chaos. We
show how the quantum Lyapunov exponents can be evaluated and give several
numerical results which describe some properties found in the present approach.
Although the system is very simple and the classical counterpart is regular,
the most non-stationary solutions of the corresponding Schroeodinger equation
are "chaotic" according to our definition.


title:
White noise approach to the low density limit of a quantum particle in a
  gas

abstract:
  The white noise approach to the investigation of the dynamics of a quantum
particle interacting with a dilute and in general non-equilibrium gaseous
environment in the low density limit is outlined. The low density limit is the
kinetic Markovian regime when only pair collisions (i.e., collisions of the
test particle with one particle of the gas at one time moment) contribute to
the dynamics. In the white noise approach one first proves that the appropriate
operators describing the gas converge in the sense of appropriate matrix
elements to certain operators of quantum white noise. Then these white noise
operators are used to derive quantum white noise and quantum stochastic
equations describing the approximate dynamics of the total system consisting of
the particle and the gas. The derivation is given ab initio, starting from the
exact microscopic quantum dynamics. The limiting dynamics is described by a
quantum stochastic equation driven by a quantum Poisson process. This equation
then applied to the derivation of quantum Langevin equation and linear
Boltzmann equation for the reduced density matrix of the test particle. The
first part of the paper describes the approach which was developed by L.
Accardi, I.V. Volovich and the author and uses the Fock-antiFock (or GNS)
representation for the CCR algebra of the gas. The second part presents the
approach to the derivation of the limiting equations directly in terms of the
correlation functions, without use of the Fock-antiFock representation. This
approach simplifies the derivation and allows to express the strength of the
quantum number process directly in terms of the one-particle $S$-matrix.


title:
N identical particles under quantum confinement: A many-body dimensional
  perturbation theory approach II, the lowest-order wave function II

abstract:
  In this paper, the second in a series of two, we complete the derivation of
the lowest-order wave function of a dimensional perturbation theory (DPT)
treatment for the N-body quantum-confined system. Taking advantage of the
symmetry of the zeroth-order configuration, we use group theoretic techniques
and the FG matrix method from quantum chemistry to obtain analytic results for
frequencies and normal modes. This method directly accounts for each two-body
interaction, rather than an average interaction so that even lowest-order
results include beyond-mean-field effects. It is thus appropriate for the study
of both weakly and strongly interacting systems and the transition between
them. While previous work has focused on energies, lowest-order wave functions
yield important information such as the nature of excitations and expectation
values of physical observables at low orders including density profiles. Higher
orders in DPT also require as input the zeroth-order wave functions. In the
earlier paper we presented a program for calculating the analytic normal-mode
coordinates of the large-D system and illustrated the procedure by deriving the
two simplest normal modes. In this paper we complete this analysis by deriving
the remaining, and more complex, normal coordinates of the system.


title: Coherent State Path Integrals in the Weyl Representation

abstract:
  We construct a representation of the coherent state path integral using the
Weyl symbol of the Hamiltonian operator. This representation is very different
from the usual path integral forms suggested by Klauder and Skagerstan in
\cite{Klau85}, which involve the normal or the antinormal ordering of the
Hamiltonian. These different representations, although equivalent quantum
mechanically, lead to different semiclassical limits. We show that the
semiclassical limit of the coherent state propagator in Weyl representation is
involves classical trajectories that are independent on the coherent states
width. This propagator is also free from the phase corrections found in
\cite{Bar01} for the two Klauder forms and provides an explicit connection
between the Wigner and the Husimi representations of the evolution operator.


title:
Dispersion spreading of polarization-entangled states of light and
  two-photon interference

abstract:
  We study the interference structure of the second-order intensity correlation
function for polarization-entangled two-photon light obtained from type-II
collinear frequency-degenerate spontaneous parametric down-conversion (SPDC).
The structure is visualised due to the spreading of the two-photon amplitude as
two-photon light propagates through optical fibre with group-velocity
dispersion (GVD). Because of the spreading, polarization-entangled Bell states
can be obtained without any birefringence compensation at the output of the
nonlinear crystal; instead, proper time selection of the intensity correlation
function is required. A birefringent material inserted at the output of the
nonlinear crystal (either reducing the initial o-e delay between the oppositely
polarized twin photons or increasing this delay) leads to a more complicated
interference structure of the correlation function.


title:
Continuous-Discrete Entanglement: An Example with Non-Relativistic
  Particles

abstract:
  This article discusses entanglement between two subsystems, one with discrete
degrees of freedom and the other with continuous degrees of freedom. The
overlap integral between continuous variable wave functions emerges as an
important parameter to characterize this kind entanglement. ``Beam-like''
entanglement and ``shape-like'' entanglement are contrasted. One example of
this kind of entanglement is between between the spin degrees of freedom and
the momentum degrees of freedom for a non-relativistic particle. This
intraparticle entanglement is Galilean invariant.


title:
Photon polarization entanglement induced by biexciton: experimental
  evidence for violation of Bell's inequality

abstract:
  We have investigated the polarization entanglement between photon pairs
generated from a biexciton in a CuCl single crystal via resonant hyper
parametric scattering. The pulses of a high repetition pump are seen to provide
improved statistical accuracy and the ability to test Bell's inequality. Our
results clearly violate the inequality and thus manifest the quantum
entanglement and nonlocality of the photon pairs. We also analyzed the quantum
state of our photon pairs using quantum state tomography.


title:
A semiclassical trace formula for the canonical partition function of
  one dimensional systems

abstract:
  We present a semiclassical trace formula for the canonical partition function
of arbitrary one-dimensional systems. The approximation is obtained via the
stationary exponent method applied to the phase-space integration of the
density operator in the coherent state representation. The formalism is valid
in the low temperature limit, presenting accurate results in this regime. As
illustrations we consider a quartic Hamiltonian that cannot be split into
kinetic and potential parts, and a system with two local minima. Applications
to spin systems are also presented.


title: Finite key analysis for symmetric attacks in quantum key distribution

abstract:
  We introduce a constructive method to calculate the achievable secret key
rate for a generic class of quantum key distribution protocols, when only a
finite number n of signals is given. Our approach is applicable to all
scenarios in which the quantum state shared by Alice and Bob is known. In
particular, we consider the six state protocol with symmetric eavesdropping
attacks, and show that for a small number of signals, i.e. below the order of
10^4, the finite key rate differs significantly from the asymptotic value for n
approaching infinity. However, for larger n, a good approximation of the
asymptotic value is found. We also study secret key rates for protocols using
higher-dimensional quantum systems.


title:
TEM10 homodyne detection as an optimal small displacement and tilt
  measurements scheme

abstract:
  We report an experimental demonstration of optimal measurements of small
displacement and tilt of a Gaussian beam - two conjugate variables - involving
a homodyne detection with a TEM10 local oscillator. We verify that the standard
split detection is only 64% efficient. We also show a displacement measurement
beyond the quantum noise limit, using a squeezed vacuum TEM10 mode within the
input beam.


title: Conditional Quantum Walk and Iterated Quantum Games

abstract:
  Iterated bipartite quantum games are implemented in terms of the
discrete-time quantum walk on the line. Our proposal allows for conditional
strategies, as two rational agents make a choice from a restricted set of
two-qubit unitary operations. Several frequently used classical strategies give
rise to families of corresponding quantum strategies. A quantum version of the
Prisoner's Dilemma in which both players use mixed strategies is presented as a
specific example. Since there are now quantum walk physical implementations at
a proof-of principle stage, this connection may represent a step towards the
experimental realization of quantum games.


title:
The basic principles to construct a generalized state-locking pulse
  field and simulate efficiently the reversible and unitary halting protocol of
  a universal quantum computer

abstract:
  It has been shown (Arxiv: quant-ph/0507236) that a universal quantum computer
could be powerful enough to solve efficiently the quantum search problem, and
the reversible and unitary halting protocol based on the state-locking pulse
field is the key component to construct the efficient quantum search processes,
while the state-locking pulse field is the key component to generate the
reversible and unitary halting protocol. In this paper the reversible and
unitary halting protocol and the generalized state-locking pulse field have
been extensively investigated theoretically. The basic principles to construct
the state-locking pulse field and design the reversible and unitary halting
protocol are studied in detail. A generalized state-locking pulse field is
generally dependent upon the time and space variables. It could be a sequence
of time- and space-dependent electromagnetic pulse fields and could also
contain the time- and space-dependent potential fields. Thus, the reversible
and unitary halting protocol built up out of the state-locking pulse field
generally consists of a sequence of time- and space-dependent unitary evolution
processes. It is shown how the quantum control process is constructed to
simulate efficiently the reversible and unitary halting protocol. An improved
subspace-reduction quantum program and circuit based on the reversible and
unitary halting protocol is proposed as the key component to construct further
an efficient quantum search process. A simple atomic physical system that is an
atomic ion or a neutral atom in the double-well potential field is proposed to
show how the state-locking pulse field is generated and how to implement the
reversible and unitary halting protocol.


title: Adiabatic Preparation of Topological Order

abstract:
  Topological order characterizes those phases of matter that defy a
description in terms of symmetry and cannot be distinguished in terms local
order parameters. This type of order plays a key role in the theory of the
fractional quantum Hall effect, as well as in topological quantum information
processing. Here we show that a system of n spins forming a lattice on a
Riemann surface can undergo a second order quantum phase transition between a
spin-polarized phase and a string-net condensed phase. This is an example of a
phase transition between magnetic and topological order. We furthermore show
how to prepare the topologically ordered phase through adiabatic evolution in a
time that is upper bounded by O(\sqrt{n}). This provides a physically plausible
method for constructing a topological quantum memory. We discuss applications
to topological and adiabatic quantum computing.


title: Sub-ballistic behaviour of the Quantum Kicked Rotor

abstract:
  We study the resonances of the quantum kicked rotor subjected to an
excitation that follows an aperiodic Fibonacci prescription. In such a case the
secondary resonances show a sub-ballistic behaviour like the quantum walk with
the same aperiodic prescription for the coin. The principal resonances maintain
the well-known ballistic behaviour. Then the parallelism previusly established
between the kicked rotor and the generalized quantum walk is retained only with
the secondary resonances.


title:
Coherent Population Trapping of Single Spins in Diamond Under Optical
  Excitation

abstract:
  Coherent population trapping is demonstrated in single nitrogen-vacancy
centers in diamond under optical excitation. For sufficient excitation power,
the fluorescence intensity drops almost to the background level when the laser
modulation frequency matches the 2.88 GHz splitting of the ground states. The
results are well described theoretically by a four-level model, allowing the
relative transition strengths to be determined for individual centers. The
results show that all-optical control of single spins is possible in diamond.


title: Sharp probability estimates for Shor's order-finding algorithm

abstract:
  Let N be a (large positive integer, let b > 1 be an integer relatively prime
to N, and let r be the order of b modulo N. Finally, let QC be a quantum
computer whose input register has the size specified in Shor's original
description of his order-finding algorithm. We prove that when Shor's algorithm
is implemented on QC, then the probability P of obtaining a (nontrivial)
divisor of r exceeds 0.7 whenever N exceeds 2^{11}-1 and r exceeds 39, and we
establish that 0.7736 is an asymptotic lower bound for P. When N is not a power
of an odd prime, Gerjuoy has shown that P exceeds 90 percent for N and r
sufficiently large. We give easily checked conditions on N and r for this 90
percent threshold to hold, and we establish an asymptotic lower bound for P of
(2/Pi) Si(4Pi), about .9499, in this situation. More generally, for any
nonnegative integer q, we show that when QC(q) is a quantum computer whose
input register has q more qubits than does QC, and Shor's algorithm is run on
QC(q), then an asymptotic lower bound on P is (2/Pi) Si(2^(q+2) Pi) (if N is
not a power of an odd prime). Our arguments are elementary and our lower bounds
on P are carefully justified.


title:
Experimental phase-covariant cloning of polarization states of single
  photons

abstract:
  The experimental realization of optimal symmetric phase-covariant 1->2
cloning of qubit states is presented. The qubits are represented by
polarization states of photons generated by spontaneous parametric
down-conversion. The experiment is based on the interference of two photons on
a custom-made beam splitter with different splitting ratios for vertical and
horizontal polarization components. From the measured data we have estimated
the implemented cloning transformation using the maximum-likelihood method. The
result shows that the realized transformation is very close to the ideal one
and the map fidelity reaches 94%.


title:
Comment on: "Electromagnetically induced left-handedness in optically
  excited four-level atomic media"

abstract:
  We discuss the recent proposal by Thommen and Mandel (Phys. Rev. Lett. {\bf
96}, 053601 (2006)) for electromagnetically induced negative refraction.
Although the main conclusion of the paper -- the possibility to achieve
negative refraction in an experimentally accessible atomic scheme -- remains
valid, we show that the weak-excitation approximation used is invalid in the
parameter regime studied and leads to quantitatively incorrect predictions. We
show that negative refraction is always accompanied by absorption rather than
by gain, and that the maximum value of the refraction-absorption ratio is of
order unity.


title: The sum-over-histories formulation of quantum computing

abstract:
  Since Deutsch (1985), quantum computers have been modeled exclusively in the
language of state vectors and the Schroedinger equation. We present a
complementary view of quantum circuits inspired by the path integral formalism
of quantum mechanics, and examine its application to some simple textbook
problems.


title: Exponentially Enhanced Quantum Metrology

abstract:
  We show that when a suitable entanglement generating unitary operator
depending on a parameter is applied on N qubits in parallel, and an appropriate
observable is measured, a precision of order 2 raised to the power (-N) in
estimating the parameter may be achieved. This exponentially improves the
precision achievable in classical and in quantum non-entangling parallel
strategies. We propose a quantum-optics model of laser light interacting with
an N-qubit system, say a polyatomic molecule, via a generalized Jaynes-Cummings
interaction which, in principle, could achieve the exponentially enhanced
precision.


title: Role of Bell Singlet State in the Suppression of Disentanglement

abstract:
  The stability of entanglement of two atoms in a cavity is analyzed in this
work. By studying the general Werner states we clarify the role of Bell-singlet
state in the problem of suppression of disentanglement due to spontaneous
emission. It is also shown explicitly that the final amount of entanglement
depends on the initial ingredients of the Bell-singlet state.


title: Isospectral Hamiltonians from Moyal products

abstract:
  Recently Scholtz and Geyer proposed a very efficient method to compute metric
operators for non-Hermitian Hamiltonians from Moyal products. We develop these
ideas further and suggest to use a more symmetrical definition for the Moyal
products, because they lead to simpler differential equations. In addition, we
demonstrate how to use this approach to determine the Hermitian counterpart for
a Pseudo-Hermitian Hamiltonian. We illustrate our suggestions with the
explicitly solvable example of the -x^4-potential and the ubiquitous harmonic
oscillator in a complex cubic potential.


title:
Resilient Quantum Computation in Correlated Environments: A Quantum
  Phase Transition Perspective

abstract:
  We analyze the problem of a quantum computer in a correlated environment
protected from decoherence by QEC using a perturbative renormalization group
approach. The scaling equation obtained reflects the competition between the
dimension of the computer and the scaling dimension of the correlations. For an
irrelevant flow, the error probability is reduced to a stochastic form for long
time and/or large number of qubits; thus, the traditional derivation of the
threshold theorem holds for these error models. In this way, the ``threshold
theorem'' of quantum computing is rephrased as a dimensional criterion.


title:
Ex-house 2D finite-element simulation of the whispering-gallery modes of
  arbitrarily shaped axisymmetric electromagnetic resonators

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


title:
Casimir attraction in multilayered plane parallel magnetodielectric
  systems

abstract:
  A powerful procedure is presented for calculating the Casimir attraction
between plane parallel multilayers made up of homogeneous regions with
arbitrary magnetic and dielectric properties by use of the Minkowski
energy-momentum tensor. The theory is applied to numerous geometries and shown
to reproduce a number of results obtained by other authors. Although the
various pieces of theory drawn upon are well known, the relative ease with
which the Casimir force density in even complex planar structures may be
calculated, appears not to be widely appreciated, and no single paper to the
author's knowledge renders explicitly the procedure demonstrated herein.
Results may be seen as an important building block in the settling of issues of
fundamental interest, such as the long-standing dispute over the thermal
behaviour of the Casimir force or the question of what is the correct stress
tensor to apply, a discussion re-quickened by the newly suggested alternative
theory due to Raabe and Welsch.


title:
Ordering ambiguity revisited via position dependent mass pseudo-momentum
  operators

abstract:
  Ordering ambiguity associated with the von Roos position dependent mass (PDM)
Hamiltonian is considered. An affine locally scaled first order differential
introduced, in Eq.(9), as a PDM-pseudo-momentum operator. Upon intertwining our
Hamiltonian, which is the sum of the square of this operator and the potential
function, with the von Roos d-dimensional PDM-Hamiltonian, we observed that the
so-called von Roos ambiguity parameters are strictly determined, but not
necessarily unique. Our new ambiguity parameters' setting is subjected to
Dutra's and Almeida's [11] reliability test and classified as good ordering.


title:
Experimental ancilla-assisted qubit transmission against correlated
  noise using quantum parity checking

abstract:
  We report the experimental demonstration of a transmission scheme of photonic
qubits over unstabilized optical fibers, which has the plug-and-play feature as
well as the ability to transmit any state of a qubit, regardless of whether it
is known, unknown, or entangled to other systems. A high fidelity to the
noiseless quantum channel was achieved by adding an ancilla photon after the
signal photon within the correlation time of the fiber noise and by performing
quantum parity checking. Simplicity, maintenance-free feature and robustness
against path-length mismatches among the nodes make our scheme suitable for
multi-user quantum communication networks.


title: Arithmetic on a Distributed-Memory Quantum Multicomputer

abstract:
  We evaluate the performance of quantum arithmetic algorithms run on a
distributed quantum computer (a quantum multicomputer). We vary the node
capacity and I/O capabilities, and the network topology. The tradeoff of
choosing between gates executed remotely, through ``teleported gates'' on
entangled pairs of qubits (telegate), versus exchanging the relevant qubits via
quantum teleportation, then executing the algorithm using local gates
(teledata), is examined. We show that the teledata approach performs better,
and that carry-ripple adders perform well when the teleportation block is
decomposed so that the key quantum operations can be parallelized. A node size
of only a few logical qubits performs adequately provided that the nodes have
two transceiver qubits. A linear network topology performs acceptably for a
broad range of system sizes and performance parameters. We therefore recommend
pursuing small, high-I/O bandwidth nodes and a simple network. Such a machine
will run Shor's algorithm for factoring large numbers efficiently.


title:
Generation of Three-Qubit Entangled W-State by Nonlinear Optical State
  Truncation

abstract:
  We propose an alternative scheme to generate W state via optical state
truncation using quantum scissors. In particular, these states may be generated
through three-mode optical state truncation in a Kerr nonlinear coupler. The
more general three-qubit state may be also produced if the system is driven by
external classical fields.


title: Quantum teleportation with pair-coherent states

abstract:
  Recently it has been argued that all presently performed continuous variable
quantum teleportation experiments could be explained using a local hidden
variable theory. In this paper we study a modification of the original protocol
which requires a fully quantum mechanical explanation even when coherent states
are teleported. Our calculations of the fidelity of teleportation using a
pair-coherent state under ideal conditions suggests that fidelity above the
required limit of 1/2 may be achievable in an experiment also.


title: Estimating entanglement measures in experiments

abstract:
  We present a method to estimate entanglement measures in experiments. We show
how a lower bound on a generic entanglement measure can be derived from the
measured expectation values of any finite collection of entanglement witnesses.
Hence witness measurements are given a quantitative meaning without the need of
further experimental data. We apply our results to a recent multi-photon
experiment [M. Bourennane et al., Phys. Rev. Lett. 92, 087902 (2004)], giving
bounds on the entanglement of formation and the geometric measure of
entanglement in this experiment.


title: Generalized qudit Choi maps

abstract:
  Following the linear programming prescription of Ref. \cite{PRA72}, the
$d\otimes d$ Bell diagonal entanglement witnesses are provided. By using
Jamiolkowski isomorphism, it is shown that the corresponding positive maps are
the generalized qudit Choi maps. Also by manipulating particular $d\otimes d$
Bell diagonal separable states and constructing corresponding bound entangled
states, it is shown that thus obtained $d\otimes d$ BDEW's (consequently qudit
Choi maps) are non-decomposable in certain range of their parameters.