title: Moment operators of the Cartesian margins of the phase space observables

abstract:
  The theory of operator integrals is used to determine the moment operators of
the Cartesian margins of the phase space observables generated by the mixtures
of the number states. The moments of the $x$-margin are polynomials of the
position operator and those of the $y$-margin are polynomials of the momentum
operator.


title:
Spectroscopic properties of a two-level atom interacting with a complex
  spherical nanoshell

abstract:
  Frequency shifts, radiative decay rates, the Ohmic loss contribution to the
nonradiative decay rates, fluorescence yields, and photobleaching of a
two-level atom radiating anywhere inside or outside a complex spherical
nanoshell, i.e. a stratified sphere consisting of alternating silica and gold
concentric spherical shells, are studied. The changes in the spectroscopic
properties of an atom interacting with complex nanoshells are significantly
enhanced, often more than two orders of magnitude, compared to the same atom
interacting with a homogeneous dielectric sphere. The detected fluorescence
intensity can be enhanced by 5 or more orders of magnitude. The changes
strongly depend on the nanoshell parameters and the atom position. When an atom
approaches a metal shell, decay rates are strongly enhanced yet fluorescence
exhibits a well-known quenching. Rather contra-intuitively, the Ohmic loss
contribution to the nonradiative decay rates for an atomic dipole within the
silica core of larger nanoshells may be decreasing when the silica core - inner
gold shell interface is approached. The quasistatic result that the radial
frequency shift in a close proximity of a spherical shell interface is
approximately twice as large as the tangential frequency shift appears to apply
also for complex nanoshells. Significantly modified spectroscopic properties
(see computer program (pending publication of this manuscript) freely available
at http://www.wave-scattering.com) can be observed in a broad band comprising
all (nonresonant) optical and near-infrared wavelengths.

categories: quant-ph cond-mat.mtrl-sci physics.chem-ph physics.optics


title: Universal Quantum Gate, Yang--Baxterization and Hamiltonian

abstract:
  It is fundamental to view unitary braiding operators describing topological
entanglements as universal quantum gates for quantum computation. This paper
derives a unitary solution of the Quantum Yang--Baxter equation via
Yang--Baxterization and constructs the Hamiltonian responsible for the
time-evolution of the unitary braiding operator.


title:
Scalar and vector modulation instabilities induced by vacuum
  fluctuations in fibers: numerical study

abstract:
  We study scalar and vector modulation instabilities induced by the vacuum
fluctuations in birefringent optical fibers. To this end, stochastic coupled
nonlinear Schrodinger equations are derived. The stochastic model is equivalent
to the quantum field operators equations and allow for dispersion,
nonlinearity, and arbitrary level of birefringence. Numerical integration of
the stochastic equations is compared to analytical formulas in the case of
scalar modulation instability and non depleted pump approximation. The effect
of classical noise and its competition with vacuum fluctuations for inducing
modulation instability is also addressed.


title: Robust Multi-Partite Multi-Level Quantum Protocols

abstract:
  We present a tripartite three-level state that allows a secret sharing
protocol among the three parties, or a quantum key distribution protocol
between any two parties. The state used in this scheme contains entanglement
even after one system is traced out. We show how to utilize this residual
entanglement for quantum key distribution purposes, and propose a realization
of the scheme using entanglement of orbital angular momentum states of photons.


title:
Entanglement and factorized ground states in two-dimensional quantum
  antiferromagnets

abstract:
  Making use of exact results and quantum Monte Carlo data for the entanglement
of formation, we show that the ground state of anisotropic two-dimensional
S=1/2 antiferromagnets in a uniform field takes the classical-like form of a
product state for a particular value and orientation of the field, at which the
purely quantum correlations due to entanglement disappear. Analytical
expressions for the energy and the form of such states are given, and a novel
type of exactly solvable two-dimensional quantum models is therefore singled
out. Moreover, we show that the field-induced quantum phase transition present
in the models is unambiguously characterized by a cusp minimum in the
pairwise-to-global entanglement ratio R, marking the quantum-critical
enhancement of \emph{multipartite} entanglement. A detailed discussion is
provided on the universality of the cusp in R as a signature of quantum
critical behavior entirely based on entanglement.


title:
Entanglement Observables and Witnesses for Interacting Quantum Spin
  Systems

abstract:
  We discuss the detection of entanglement in interacting quantum spin systems.
First, thermodynamic Hamiltonian-based witnesses are computed for a general
class of one-dimensional spin-1/2 models. Second, we introduce optimal
bipartite entanglement observables. We show that a bipartite entanglement
measure can generally be associated to a set of independent two-body spin
observables whose expectation values can be used to witness entanglement. The
number of necessary observables is ruled by the symmetries of the model.
Illustrative examples are presented.


title: Cooling many particles to very low temperatures

abstract:
  In a recent paper [Beige, Knight, and Vitiello, quant-ph/0404160], we showed
that a large number N of particles can be cooled very efficiently using a
bichromatic interaction. The particles should be excited by red-detuned laser
fields while coupling to the quantized field mode inside a resonant and leaky
optical cavity. When the coupling constants are for all particles the same, a
collective behavior can be generated and the cooling rate can be as large as
square root of N times the single-particle coupling constants. Here we study
the algebraic structure of the dynamics and the origin of the collective
cooling process in more detail.


title: Estimation of pure qubits on circles

abstract:
  Gisin and Popescu [PRL, 83, 432 (1999)] have shown that more information
about their direction can be obtained from a pair of anti-parallel spins
compared to a pair of parallel spins, where the first member of the pair (which
we call the pointer member) can point equally along any direction in the Bloch
sphere. They argued that this was due to the difference in dimensionality
spanned by these two alphabets of states. Here we consider similar alphabets,
but with the first spin restricted to a fixed small circle of the Bloch sphere.
In this case, the dimensionality spanned by the anti-parallel versus parallel
alphabet is now equal. However, the anti-parallel alphabet is found to still
contain more information in general. We generalize this to having N parallel
spins and M anti-parallel spins. When the pointer member is restricted to a
small circle these alphabets again span spaces of equal dimension, yet in
general, more directional information can be found for sets with smaller |N-M|
for any fixed total number of spins. We find that the optimal POVMs for
extracting directional information in these cases can always be expressed in
terms of the Fourier basis. Our results show that dimensionality alone cannot
explain the greater information content in anti-parallel combinations of spins
compared to parallel combinations. In addition, we describe an LOCC protocol
which extract optimal directional information when the pointer member is
restricted to a small circle and a pair of parallel spins are supplied.


title: Free Entanglement Measure

abstract:
  In this paper, based on the classfication of multiparticle states and the
original definition of semiseparability, we give out the redefinition of
semiseparability and inseparability of multiparticle states. By virtue of the
redefinition, entanglement measure of multiparticle states can be converted
into bipartite entanglement measure in arbitrary dimension in mathematical
method. A simple expression of entanglement measure is given out. As examples,
a general three-particle pure state and an N-particle mixed state are
considered.


title:
Direct Derivation of the Schwinger Quantum Correction to the
  Thomas-Fermi Atom

abstract:
  The Schwinger quantum correction to the classic Thomas-Fermi atom is directly
derived by solving for the latter without recourse to a modeling after the
harmonic oscillator potential and without solving for the particle density.


title: Separability Criterion of Tripartite Qubit Systems

abstract:
  In this paper, we present a method to construct full separability criteria
for tripartite systems of qubits. The spirit of our approach is that a
tripartite pure state can be regarded as a three-order tensor that provides an
intuitionistic mathematical formulation for the full separability of pure
states. We extend the definition to mixed states and given out the
corresponding full separability criterion. As applications, we discuss the
separability of several bound entangled states, which shows that our criterion
is feasible.


title: A Derivation of the $Z\to\infty$ Limit for Atoms

abstract:
  Upper and lower bounds are derived for the ground-state energy of neutral
atoms which for $Z\to\infty$ both involve the limits of exact Green's functions
with one-body potentials. The limits of both bounds are shown to coincide with
the Thomas-Fermi ground-state energy.


title:
Entanglement and the Einstein-Podolsky-Rosen paradox with coupled
  intracavity optical downconverters

abstract:
  We show that two evanescently coupled $\chi^{(2)}$ parametric downconverters
inside a Fabry-Perot cavity provide a tunable source of quadrature squeezed
light, Einstein-Podolsky-Rosen correlations and quantum entanglement. Analysing
the operation in the below threshold regime, we show how these properties can
be controlled by adjusting the coupling strengths and the cavity detunings. As
this can be implemented with integrated optics, it provides a possible route to
rugged and stable EPR sources.


title:
Spontaneous emission of a photon: wave packet structures and atom-photon
  entanglement

abstract:
  Spontaneous emission of a photon by an atom is described theoretically in
three dimensions with the initial wave function of a finite-mass atom taken in
the form of a finite-size wave packet. Recoil and wave-packet spreading are
taken into account. The total atom-photon wave function is found in the
momentum and coordinate representations as the solution of an initial-value
problem. The atom-photon entanglement arising in such a process is shown to be
closely related to the structure of atom and photon wave packets which can be
measured in the coincidence and single-particle schemes of measurements. Two
predicted effects, arising under the conditions of high entanglement, are
anomalous narrowing of the coincidence wave packets and, under different
conditions, anomalous broadening of the single-particle wave packets.
Fundamental symmetry relations between the photon and atom single-particle and
coincidence wave packet widths are established. The relationship with the
famous scenario of Einstein-Podolsky-Rosen is discussed.


title:
Experimental investigation of amplitude and phase quantum correlations
  in a type II OPO above threshold: from the non-degenerate to the degenerate
  operation

abstract:
  We describe a very stable type II optical parametric oscillator operated
above threshold which provides 9.7 $\pm$ 0.5 dB (89%) of quantum noise
reduction on the intensity difference of the signal and idler modes. We also
report the first experimental study by homodyne detection of the generated
bright two-mode state in the case of frequency degenerate operation obtained by
introducing a birefringent plate inside the optical cavity.


title: Entanglement and non-locality are different resources

abstract:
  Bell's theorem states that, to simulate the correlations created by
measurement on pure entangled quantum states, shared randomness is not enough:
some "non-local" resources are required. It has been demonstrated recently that
all projective measurements on the maximally entangled state of two qubits can
be simulated with a single use of a "non-local machine". We prove that a
strictly larger amount of this non-local resource is required for the
simulation of pure non-maximally entangled states of two qubits
$\ket{\psi(\alpha)}= \cos\alpha\ket{00}+\sin\alpha\ket{11}$ with
$0<\alpha\lesssim\frac{\pi}{7.8}$.


title:
Electronic and center of mass transitions driven by Laguerre-Gaussian
  beams

abstract:
  We derive the interaction Hamiltonian of a Laguerre-Gaussian beam with a
simple atomic system, under the assumption of a small spread of the center of
mass wave function in comparison with the waist of the Laguerre-Gaussian beam.
The center of mass motion of the atomic system is taken into account. Using the
properties of regular spherical harmonics the internal and center of mass
coordinates are separated without making any multipolar expansion. Then the
selection rules of the internal and of the center of mass motion transitions
follow immediately. The influence of the winding number of the
Laguerre-Gaussian beams on the selection rules and transition probability of
the center of mass motion is discussed.


title: Driving the slow-light soliton by controlling laser field

abstract:
  In the framework of the nonlinear $\Lambda$-model we investigate propagation
of a slow-light soliton in atomic vapors and Bose-Einstein condensates. The
velocity of the slow-light soliton is controlled by a time-dependent background
field created by a controlling laser. For a fairly arbitrary time dependence of
the field we find the dynamics of the slow-light soliton inside the medium. We
provide an analytical description for the nonlinear dependence of the velocity
of the signal on the controlling field. If the background field is turned off
at some moment of time, the signal stops. We find the location and shape of the
spatially localized memory bit imprinted into the medium. We show that the
process of writing optical information can be described in terms of scattering
data for the underlying scattering problem.


title: Renormalization of the singular attractive $1/r^4$ potential

abstract:
  We study the radial Schr\"odinger equation for a particle of mass $m$ in the
field of a singular attractive $g^2/{r^4}$ potential with particular emphasis
on the bound states problem. Using the regularization method of Beane
\textit{et al.}, we solve analytically the corresponding ``renormalization
group flow" equation. We find in agreement with previous studies that its
solution exhibits a limit cycle behavior and has infinitely many branches. We
show that a continuous choice for the solution corresponds to a given fixed
number of bound states and to low energy phase shifts that vary continuously
with energy. We study in detail the connection between this regularization
method and a conventional method modifying the short range part of the
potential with an infinitely repulsive hard core. We show that both methods
yield bound states results in close agreement even though the regularization
method of Beane \textit{et al.} does not include explicitly any new scale in
the problem. We further illustrate the use of the regularization method in the
computation of electron bound states in the field of neutral polarizable
molecules without dipole moment. We find the binding energy of s-wave
polarization bound electrons in the field of C$_{60}$ molecules to be 17 meV
for a scattering length corresponding to a hard core radius of the size of the
molecule radius ($\sim 3.37$ \AA). This result can be further compared with
recent two-parameter fits using the Lennard-Jones potential yielding binding
energies ranging from 3 to 25 meV.

categories: quant-ph math-ph math.MP physics.atm-clus


title: Qubit channels that achieve capacity with two states

abstract:
  This paper considers a class of qubit channels for which three states are
always sufficient to achieve the Holevo capacity. For these channels it is
known that there are cases where two orthogonal states are sufficient, two
non-orthogonal states are required, or three states are necessary. Here a
systematic theory is given which provides criteria to distinguish cases where
two states are sufficient, and determine whether these two states should be
orthogonal or non-orthogonal. In addition, we prove a theorem on the form of
the optimal ensemble when three states are required, and present efficient
methods of calculating the Holevo capacity.


title: Pathwise Solution of a Class of Stochastic Master Equations

abstract:
  In this paper we consider an alternative formulation of a class of stochastic
wave and master equations with scalar noise that are used in quantum optics for
modelling open systems and continuously monitored systems. The reformulation is
obtained by applying J.M.C. Clark's pathwise reformulation technique from the
theory of classical nonlinear filtering. The pathwise versions of the
stochastic wave and master equations are defined for all driving paths and
depend continuously on them. In the case of white noise equations, we derive
analogs of Clark's robust approximations. The results in this paper may be
useful for implementing filters for the continuous monitoring and measurement
feedback control of quantum systems, and for developing new types of numerical
methods for unravelling master equations. The main ideas are illustrated by an
example.


title: Thermal effects on quantum communication through spin chains

abstract:
  We study the effect of thermal fluctuations in a recently proposed protocol
for transmission of unknown quantum states through quantum spin chains. We
develop a low temperature expansion for general spin chains. We then apply this
formalism to study exactly thermal effects on short spin chains of four spins.
We show that optimal times for extraction of output states are almost
independent of the temperature which lowers only the fidelity of the channel.
Moreover we show that thermal effects are smaller in the anti-ferromagnetic
chains than the ferromagnetic ones.


title: Instruments and mutual entropies in quantum information

abstract:
  General quantum measurements are represented by instruments. In this paper
the mathematical formalization is given of the idea that an instrument is a
channel which accepts a quantum state as input and produces a probability and
an a posteriori state as output. Then, by using mutual entropies on von Neumann
algebras and the identification of instruments and channels, many old and new
informational inequalities are obtained in a unified manner. Such inequalities
involve various quantities which characterize the performances of the
instrument under study; in particular, these inequalities include and
generalize the famous Holevo's bound.


title:
Quantum search algorithm by adiabatic evolution under a priori
  probability

abstract:
  Grover's algorithm is one of the most important quantum algorithms, which
performs the task of searching an unsorted database without a priori
probability. Recently the adiabatic evolution has been used to design and
reproduce quantum algorithms, including Grover's algorithm. In this paper, we
show that quantum search algorithm by adiabatic evolution has two properties
that conventional quantum search algorithm doesn't have. Firstly, we show that
in the initial state of the algorithm only the amplitude of the basis state
corresponding to the solution affects the running time of the algorithm, while
other amplitudes do not. Using this property, if we know a priori probability
about the location of the solution before search, we can modify the adiabatic
evolution to make the algorithm faster. Secondly, we show that by a factor for
the initial and finial Hamiltonians we can reduce the running time of the
algorithm arbitrarily. Especially, we can reduce the running time of adiabatic
search algorithm to a constant time independent of the size of the database.
The second property can be extended to other adiabatic algorithms.


title: Entanglement in the XX spin chain with an energy current

abstract:
  We consider the ground state of the XX chain that is constrained to carry a
current of energy. The von Neumann entropy of a block of $L$ neighboring spins,
describing entanglement of the block with the rest of the chain, is computed.
Recent calculations have revealed that the entropy in the XX model diverges
logarithmically with the size of the subsystem. We show that the presence of
the energy current increases the prefactor of the logarithmic growth. This
result indicates that the emergence of the energy current gives rise to an
increase of entanglement.


title: Why isn't every physicist a Bohmian?

abstract:
  This note collects, classifies and evaluates common criticism against the de
Broglie Bohm theory, including Ockham's razor, asymmetry in the de Broglie Bohm
theory, the ``surreal trajectory'' problem, the underdetermination of the de
Broglie Bohm theory and the question of relativistic and quantum field
theoretical generalizations of the de Broglie Bohm theory. We argue that none
of these objections provide a rigorous disproof, they rather highlight that
even in science theories can not solely be evaluated based on their empirical
confirmation.


title: Quantum limits of super-resolution in reconstruction of optical objects

abstract:
  We investigate analytically and numerically the role of quantum fluctuations
in reconstruction of optical objects from diffraction-limited images. Taking as
example of an input object two closely spaced Gaussian peaks we demonstrate
that one can improve the resolution in the reconstructed object over the
classical Rayleigh limit. We show that the ultimate quantum limit of resolution
in such reconstruction procedure is determined not by diffraction but by the
signal-to-noise ratio in the input object. We formulate a quantitative measure
of super-resolution in terms of the optical point-spread function of the
system.


title: A differential method for bounding the ground state energy

abstract:
  For a wide class of Hamiltonians, a novel method to obtain lower and upper
bounds for the lowest energy is presented. Unlike perturbative or variational
techniques, this method does not involve the computation of any integral (a
normalisation factor or a matrix element). It just requires the determination
of the absolute minimum and maximum in the whole configuration space of the
local energy associated with a normalisable trial function (the calculation of
the norm is not needed). After a general introduction, the method is applied to
three non-integrable systems: the asymmetric annular billiard, the many-body
spinless Coulombian problem, the hydrogen atom in a constant and uniform
magnetic field. Being more sensitive than the variational methods to any local
perturbation of the trial function, this method can used to systematically
improve the energy bounds with a local skilled analysis; an algorithm relying
on this method can therefore be constructed and an explicit example for a
one-dimensional problem is given.


title: Robust Charge-based Qubit Encoding

abstract:
  We propose a simple encoding of charge-based quantum dot qubits which
protects against fluctuating electric fields by charge symmetry. We analyse the
reduction of coupling to noise due to nearby charge traps and present single
qubit gates. The relative advantage of the encoding increases with lower charge
trap density.


title:
Quantum computation and analysis of Wigner and Husimi functions: toward
  a quantum image treatment

abstract:
  We study the efficiency of quantum algorithms which aim at obtaining phase
space distribution functions of quantum systems. Wigner and Husimi functions
are considered. Different quantum algorithms are envisioned to build these
functions, and compared with the classical computation. Different procedures to
extract more efficiently information from the final wave function of these
algorithms are studied, including coarse-grained measurements, amplitude
amplification and measure of wavelet-transformed wave function. The algorithms
are analyzed and numerically tested on a complex quantum system showing
different behavior depending on parameters, namely the kicked rotator. The
results for the Wigner function show in particular that the use of the quantum
wavelet transform gives a polynomial gain over classical computation. For the
Husimi distribution, the gain is much larger than for the Wigner function, and
is bigger with the help of amplitude amplification and wavelet transforms. We
also apply the same set of techniques to the analysis of real images. The
results show that the use of the quantum wavelet transform allows to lower
dramatically the number of measurements needed, but at the cost of a large loss
of information.


title: Robustness of the adiabatic quantum search

abstract:
  The robustness of the local adiabatic quantum search to decoherence in the
instantaneous eigenbasis of the search Hamiltonian is examined. We demonstrate
that the asymptotic time-complexity of the ideal closed case is preserved, as
long as the Hamiltonian dynamics is present. In the special case of pure
decoherence where the environment monitors the search Hamiltonian, it is shown
that the local adiabatic quantum search performs as the classical search.


title:
Equivalence between Entanglement and the Optimal Fidelity of Continuous
  Variable Teleportation

abstract:
  We devise the optimal form of Gaussian resource states enabling continuous
variable teleportation with maximal fidelity. We show that a nonclassical
optimal fidelity of $N$-user teleportation networks is {\it necessary and
sufficient} for $N$-party entangled Gaussian resources, yielding an estimator
of multipartite entanglement. This {\it entanglement of teleportation} is
equivalent to entanglement of formation in the two-user protocol, and to
localizable entanglement in the multi-user one. The continuous-variable tangle,
quantifying entanglement sharing in three-mode Gaussian states, is
operationally linked to the optimal fidelity of a tripartite teleportation
network.

categories: quant-ph cond-mat.other math-ph math.MP physics.optics


title: Bidirectional coherent classical communication

abstract:
  A unitary interaction coupling two parties enables quantum communication in
both the forward and backward directions.
  Each communication capacity can be thought of as a tradeoff between the
achievable rates of specific types of forward and backward communication.
  Our first result shows that for any bipartite unitary gate, coherent
classical communication is no more difficult than classical communication --
they have the same achievable rate regions. Previously this result was known
only for the unidirectional capacities (i.e., the boundaries of the tradeoff).
  We then relate the tradeoff curve for two-way coherent communication to the
tradeoff for two-way quantum communication and the tradeoff for coherent
communiation in one direction and quantum communication in the other.


title: Moir\'e patterns in quantum images

abstract:
  We observed moir\'e fringes in spatial quantum correlations between twin
photons generated by parametric down-conversion. Spatially periodic structures
were nonlocally superposed giving rise to beat frequencies typical of moir\'e
patterns. This result brings interesting perspectives regarding metrological
applications of such a quantum optical setup.


title: A procedure for calculating the many-particle Bohm quantum potential

abstract:
  In a recent work, M.Kohout (M.Kohout, Int.J.Quant.Chem. 87, 12 2002) raised
the important question of how to make a correct use of Bohm's approach for
defining a quantum potential. In this work, by taking into account Kohout's
results, we propose a general self-consistent iterative procedure for solving
this problem.


title: Process reconstruction from incomplete and/or inconsistent data

abstract:
  We analyze how an action of a qubit channel (map) can be estimated from the
measured data that are incomplete or even inconsistent. That is, we consider
situations when measurement statistics is insufficient to determine consistent
probability distributions. As a consequence either the estimation
(reconstruction) of the channel completely fails or it results in an unphysical
channel (i.e., the corresponding map is not completely positive). We present a
regularization procedure that allows us to derive physically reasonable
estimates (approximations) of quantum channels. We illustrate our procedure on
specific examples and we show that the procedure can be also used for a
derivation of optimal approximations of operations that are forbidden by the
laws of quantum mechanics (e.g., the universal NOT gate).


title: The Schrodinger Equation, Reversibility and the Grover Algorithm

abstract:
  Several aspects of the time-dependent Schrodinger equation are discussed in
the context of Quantum Information Theory.

categories: quant-ph cond-mat.str-el hep-th physics.atom-ph


title: Calculation of Positron Channeling Radiation with a Harmonic Potential

abstract:
  We present a detailed calculation of channeling radiation of planar-channeled
positrons from crystal targets in the framework of our approach, which was
proposed recently. In contrast to previous calculations of channeling radiation
in crystals, our calculation takes into account the interference between
different transition amplitudes. The development stemmed from the idea that the
amplitude for a given process is the sum of the transition amplitudes for each
transition to lower state of transverse energy with the same energy differences
between bound-bound transitions. It seems that a consistent interpretation is
only possible if positrons move in a nearly harmonic planar potential with
equidistant energy levels.


title: Brownian Entanglement

abstract:
  We show that for two classical brownian particles there exists an analog of
continuous-variable quantum entanglement: The common probability distribution
of the two coordinates and the corresponding coarse-grained velocities cannot
be prepared via mixing of any factorized distributions referring to the two
particles in separate. This is possible for particles which interacted in the
past, but do not interact in the present. Three factors are crucial for the
effect: 1) separation of time-scales of coordinate and momentum which motivates
the definition of coarse-grained velocities; 2) the resulting uncertainty
relations between the coordinate of the brownian particle and the change of its
coarse-grained velocity; 3) the fact that the coarse-grained velocity, though
pertaining to a single brownian particle, is defined on a common context of two
particles. The brownian entanglement is a consequence of a coarse-grained
description and disappears for a finer resolution of the brownian motion. We
discuss possibilities of its experimental realizations in examples of
macroscopic brownian motion.


title: Classical information capacity of a class of quantum channels

abstract:
  We consider the additivity of the minimal output entropy and the classical
information capacity of a class of quantum channels. For this class of channels
the norm of the output is maximized for the output being a normalized
projection. We prove the additivity of the minimal output Renyi entropies with
entropic parameters contained in [0, 2], generalizing an argument by Alicki and
Fannes, and present a number of examples in detail. In order to relate these
results to the classical information capacity, we introduce a weak form of
covariance of a channel. We then identify several instances of weakly covariant
channels for which we can infer the additivity of the classical information
capacity. Both additivity results apply to the case of an arbitrary number of
different channels. Finally, we relate the obtained results to instances of
bi-partite quantum states for which the entanglement cost can be calculated.


title:
Quantum electrodynamics of accelerated atoms in free space and in
  confined cavities

abstract:
  We consider a gedanken experiment with a beam of atoms in their ground state
that are accelerated through a single-mode microwave cavity. We show that
taking into account of the ''counter-rotating'' terms in the interaction
Hamiltonian leads to the excitation of an atom with simultaneous emission of a
photon into a field mode. In the case of a slow switching on of the
interaction, the ratio of emission and absorption probabilities is
exponentially small and is described by the Unruh factor. In the opposite case
of sharp cavity boundaries the above ratio is much greater and radiation is
produced with an intensity which can exceed the intensity of Unruh acceleration
radiation in free space by many orders of magnitude. In both cases real photons
are produced, contrary to the opinion that a uniformly accelerated atom does
not radiate. The cavity field at steady state is described by a thermal density
matrix. However, under some conditions laser gain is possible. We present a
detailed discussion of how the acceleration of atoms affects the generated
cavity field in different situations, progressing from a simple physical
picture of Unruh radiation to more complicated situations.


title: The Measurement Calculus

abstract:
  We propose a calculus of local equations over one-way computing patterns,
which preserves interpretations, and allows the rewriting of any pattern to a
standard form where entanglement is done first, then measurements, then local
corrections. We infer from this that patterns with no dependencies, or using
only Pauli measurements, can only realise unitaries belonging to the Clifford
group.


title: Minimum entangled state dimension required for pseudo-telepathy

abstract:
  Pseudo-telepathy provides an intuitive way of looking at Bell's inequalities,
in which it is often obvious that feats achievable by use of quantum
entanglement would be classically impossible. A two-player pseudo-telepathy
game proceeds as follows: Alice and Bob are individually asked a question and
they must provide an answer. They are not allowed any form of communication
once the questions are asked, but they may have agreed on a common strategy
prior to the execution of the game. We say that they win the game if the
questions and answers fulfil a specific relation. A game exhibits
pseudo-telepathy if there is a quantum strategy that makes Alice and Bob win
the game for all possible questions, provided they share prior entanglement,
whereas it would be impossible to win this game systematically in a classical
setting. In this paper, we show that any two-player pseudo-telepathy game
requires the quantum players to share an entangled quantum system of dimension
at least 3x3. This is optimal for two-player games, but the most efficient
pseudo-telepathy game possible, in terms of total dimension, involves three
players who share a quantum system of dimension 2x2x2.


title: Separability Criteria For Arbitrary Quantum Systems

abstract:
  The purpose of this paper is to obtain a sufficient and necessary condition
as a criteria to test whether an arbitrary multipartite state is entangled or
not. Based on the tensor expression of a multipartite pure state, the paper
shows that a state is separable iff $| \mathbf{C}(\rho)| $ =0 for pure states
and iff $C(\rho)$ vanishes for mixed states.


title: Observation of Nonspreading Wave Packets in an Imaginary Potential

abstract:
  We propose and experimentally demonstrate a method to prepare a nonspreading
atomic wave packet. Our technique relies on a spatially modulated absorption
constantly chiseling away from an initially broad de Broglie wave. The
resulting contraction is balanced by dispersion due to Heisenberg's uncertainty
principle. This quantum evolution results in the formation of a nonspreading
wave packet of Gaussian form with a spatially quadratic phase. Experimentally,
we confirm these predictions by observing the evolution of the momentum
distribution. Moreover, by employing interferometric techniques, we measure the
predicted quadratic phase across the wave packet. Nonspreading wave packets of
this kind also exist in two space dimensions and we can control their amplitude
and phase using optical elements.


title: On the positivity of Bloch-Boltzmann equations

abstract:
  In a large variety of spectroscopical applications Bloch-Boltzmann equations
(BBE) play an essential role. They describe the evolution of the reduced
density operator of an active atom which is coupled to radiation (Bloch part)
and which interacts collisionally with the perturber gas (Boltzmann part). The
standard approach to the collisional part is well-known from the literature. It
preserves hermiticity and normalization, but the question whether it preserves
positivity seems to remain open. The completely positive BBE were recently
derived via the general master equation techniques. These two approaches are
applied for a model of n-level nondegenerate atom. We show that within this
model both approaches to the collisional part of BBE are equivalent -- give the
same physical predictions. The approach based upon master equation techniques
guarantees the preservation of hermiticity, normalization and positivity. The
proven equivalence ascertains that the standard approach also preserves
positivity. Moreover, some aspects of the standard derivation (which atomic
states do contribute to the evolution) are clarified by the established
equivalence.


title: Entanglement and its Role in Shor's Algorithm

abstract:
  Entanglement has been termed a critical resource for quantum information
processing and is thought to be the reason that certain quantum algorithms,
such as Shor's factoring algorithm, can achieve exponentially better
performance than their classical counterparts. The nature of this resource is
still not fully understood: here we use numerical simulation to investigate how
entanglement between register qubits varies as Shor's algorithm is run on a
quantum computer. The shifting patterns in the entanglement are found to relate
to the choice of basis for the quantum Fourier transform.


title: Decoherence of a Measure of Entanglement

abstract:
  We demonstrate by an explicit model calculation that the decay of
entanglement of two two-state systems (two qubits) is governed by the product
of the factors that measure the degree of decoherence of each of the qubits,
subject to independent sources of quantum noise. This demonstrates an important
physical property that separated open quantum systems can evolve quantum
mechanically on time scales larger than the times for which they remain
entangled.


title:
Decoherence Rates in Large Scale Quantum Computers and Macroscopic
  Systems

abstract:
  Markovian regime decoherence effects in quantum computers are studied in
terms of the fidelity for the situation where the number of qubits N becomes
large. A general expression giving the decoherence time scale in terms of
Markovian relaxation elements and expectation values of products of system
fluctuation operators is obtained, which could also be applied to study
decoherence in other macroscopic systems such as Bose condensates and
superconductors. A standard circuit model quantum computer involving
three-state lambda system ionic qubits is considered, with qubits localised
around well-separated positions via trapping potentials. The centre of mass
vibrations of the qubits act as a reservoir. Coherent one and two qubit gating
processes are controlled by time dependent localised classical electromagnetic
fields that address specific qubits, the two qubit gating processes being
facilitated by a cavity mode ancilla, which permits state interchange between
qubits. With a suitable choice of parameters, it is found that the decoherence
time can be made essentially independent of N.


title: Limits on Efficient Computation in the Physical World

abstract:
  More than a speculative technology, quantum computing seems to challenge our
most basic intuitions about how the physical world should behave. In this
thesis I show that, while some intuitions from classical computer science must
be jettisoned in the light of modern physics, many others emerge nearly
unscathed; and I use powerful tools from computational complexity theory to
help determine which are which.


title:
Non-unitary evolution of a pure state into a mixed state in the
  measurement problem from standard Quantum Mechanics and its impact on complex
  space-time, no-boundary proposal and information loss paradox of
  singularity-free Quantum Cosmology

abstract:
  In order to resolve the measurement problem of Quantum Mechanics, non-unitary
time evolution has been derived from the unitarity of standard quantum
formalism. New wave functions of free and non-free quantum systems follow from
Schroedinger equation after inserting an ansatz. Quantum systems show up as
probability waves before measurement. A pure entangled state of a composite
system evolves non-unitarily, only to disentangle itself into a definite state
after reduction at the measurement point. A classical space-time point is
created momentarily in this event. Unitarity is restored at that point. The
non-Hermitian observables defined in the domain of rigged Hilbert space
transform into Hermitian ones at the measurement point. The problem of
preferred basis is resolved by the requirement of specifying the position of
measurement point. Two theorems prove that time is a non-Hermitian operator,
thus placing space and time on an equal footing. Bound states are found to need
discrete space-time, which supports its use in loop quantum gravity.
Non-unitarity in the theory helps buttress the no-boundary proposal; and
uncertainty relation makes a leeway to singularity-free Quantum Cosmology.
Quantum Mechanics also accommodates complex and negative probabilities.


title: Non-relativistic classical mechanics for spinning particles

abstract:
  We study the classical dynamics of non-relativistic particles endowed with
spin. Non-vanishing Zitterbewegung terms appear in the equation of motion also
in the small momentum limit. We derive a generalized work-energy theorem which
suggests classical interpretations for tunnel effect and quantum potential.


title: Tunnelling times: An elementary introduction

abstract:
  In this paper we examine critically and in detail some existing definitions
for the tunnelling times, namely: the phase-time; the centroid-based times; the
Buttiker and Landauer times; the Larmor times; the complex (path-integral and
Bohm) times; the dwell time, and finally the generalized (Olkhovsky and Recami)
dwell time, by adding also some numerical evaluations. Then, we pass to examine
the equivalence between quantum tunnelling and "photon tunnelling" (evanescent
waves propagation), with particular attention to tunnelling with Superluminal
group-velocities ("Hartman effect"). At last, in an Appendix, we add a bird-eye
view of all the experimental sectors of physics in which Superluminal motions
seem to appear.


title:
Is Quantum Suicide Painless? On an Apparent Violation of The Principal
  Principle

abstract:
  The experimental setup of the self-referential quantum measurement, jovially
known as the "quantum suicide" or the "quantum Russian roulette" is analyzed
from the point of view of the Principal Principle of David Lewis. It is shown
that the apparent violation of this principle--relating objective probabilities
and subjective chance--in this type of thought experiment is just an illusion
due to the usage of some terms and concepts ill-defined in the quantum context.
We conclude that even in the case that Everett's (or some other "no-collapse")
theory is a correct description of reality, we can coherently believe in
equating subjective credence with objective chance in quantum-mechanical
experiments. This is in agreement with results of the research on personal
identity in the quantum context by Parfit and Tappenden.


title: Quantum Mechanics and Reality

abstract:
  We begin by discussing ``What exists?'', i.e. ontology, in Classical Physics
which provided a description of physical phenomena at the macroscopic level.
The microworld however necessitates a introduction of Quantum ideas for its
understanding. It is almost certain that the world is quantum mechanical at
both microscopic as well as at macroscopic level. The problem of ontology of a
Quantum world is a difficult one. It also depends on which interpretation is
used. We first discuss some interpretations in which Quantum Mechanics does not
provide a complete framework but has to be supplemented by extra ingredients
e.g. (i) Copenhagen group of interpretations associated with the names of Niels
Bohr, Heisenberg, von-Neumann, and (ii) de-Broglie-Bohm interpretations. We
then look at some interpretations in which Quantum mechanics is supposed to
provide the entire framework such as (i) Everett-deWitt many world, (ii)
quantum histories interpretations. We conclude with some remarks on the
rigidity of the formalism of quantum mechanics, which is sharp contrast to it's
ontological fluidity.


title: Quantum Non-demolition Measurements on Qubits

abstract:
  We discuss the characterization and properties of quantum non-demolition
(QND) measurements on qubit systems. We introduce figures of merit which can be
applied to systems of any Hilbert space dimension thus providing universal
criteria for characterizing QND measurements. We discuss the controlled-NOT
gate and an optical implementation as examples of QND devices for qubits. We
also discuss the QND measurement of weak values.


title: Coherent states for the q-deformed quantum mechanics on a circle

abstract:
  The q-deformed coherent states for a quantum particle on a circle are
introduced and their properties investigated.


title:
Full characterization of a three-photon GHZ state using quantum state
  tomography

abstract:
  We have performed the first experimental tomographic reconstruction of a
three-photon polarization state. Quantum state tomography is a powerful tool
for fully describing the density matrix of a quantum system. We measured 64
three-photon polarization correlations and used a "maximum-likelihood"
reconstruction method to reconstruct the GHZ state. The entanglement class has
been characterized using an entanglement witness operator and the maximum
predicted values for the Mermin inequality was extracted.


title: Separability for lattice systems at high temperature

abstract:
  Equilibrium states of infinite extended lattice systems at high temperature
are studied with respect to their entanglement. Two notions of separability are
offered. They coincide for finite systems but differ for infinitely extended
ones. It is shown that for lattice systems with localized interaction for high
enough temperature there exists no local entanglement. Even more quasifree
states at high temperature are also not distillably entangled for all local
regions of arbitrary size. For continuous systems entanglement survives for all
temperatures. In mean field theories it is possible, that local regions are not
entangled but the entanglement is hidden in the fluctuation algebra.


title:
Modulational instability of two-component Bose-Einstein condensates in
  an optical lattice

abstract:
  We study modulational instability of two-component Bose-Einstein condensates
in an optical lattice, which is modelled as a coupled discrete nonlinear Schr
\"{o}dinger equation. The excitation spectrum and the modulational instability
condition of the total system are presented analytically. In the
long-wavelength limit, our results agree with the homogeneous two-component
Bose-Einstein condensates case. The discreteness effects result in the
appearance of the modulational instability for the condensates in miscible
region. The numerical calculations confirm our analytical results and show that
the interspecies coupling can transfer the instability from one component to
another.


title: Intrinsic time-uncertainties and decoherence: comparison of 4 models

abstract:
  Four models of energy decoherence are discussed from the common perspective
of intrinsic time-uncertainty. The four authors -- Milburn, Adler, Penrose, and
myself -- have four different approaches. The present work concentrates on
their common divisors at the level of the proposed equations rather than at the
level of the interpretations. General relationships between time-uncertainty
and energy-decoherence are presented in both global and local sense. Global and
local master equations are derived. (The local concept is favored.)


title: State estimation on correlated copies

abstract:
  State estimation is usually analyzed in the situation when copies are in a
product state, either mixed or pure. We investigate here the concept of state
estimation on correlated copies. We analyze state estimation on correlated N
qubit states, which are permutationally invariant. Using a correlated state we
try to estimate as good as possible the direction of the Bloch vector of a
single particle reduced density matrix. We derive the optimal fidelity for all
permutation invariant states. We find the optimal state, which yields the
highest estimation fidelity among the states with the same reduced density
matrix. Interestingly this state is not a product state. We also point out that
states produced by optimal universal cloning machines are the worst form the
point of view of estimating the reduced density matrix.


title: Toward a more economical cluster state quantum computation

abstract:
  We assess the effects of an intrinsic model for imperfections in cluster
states by introducing {\it noisy cluster states} and characterizing their role
in the one-way model for quantum computation. The action of individual
dephasing channels on cluster qubits is also studied. We show that the effect
of non-idealities is limited by using small clusters, which requires compact
schemes for computation. In light of this, we address an experimentally
realizable four-qubit linear cluster which simulates a controlled-{\sf NOT}
({\sf CNOT}).


title: A quantum version of Sanov's theorem

abstract:
  We present a quantum extension of a version of Sanov's theorem focussing on a
hypothesis testing aspect of the theorem: There exists a sequence of typical
subspaces for a given set $\Psi$ of stationary quantum product states
asymptotically separating them from another fixed stationary product state.
Analogously to the classical case, the exponential separating rate is equal to
the infimum of the quantum relative entropy with respect to the quantum
reference state over the set $\Psi$. However, while in the classical case the
separating subsets can be chosen universal, in the sense that they depend only
on the chosen set of i.i.d. processes, in the quantum case the choice of the
separating subspaces depends additionally on the reference state.


title:
Entanglement under restricted operations: Analogy to mixed-state
  entanglement

abstract:
  We show that the classification of bi-partite pure entangled states when
local quantum operations are restricted yields a structure that is analogous in
many respects to that of mixed-state entanglement. Specifically, we develop
this analogy by restricting operations through local superselection rules, and
show that such exotic phenomena as bound entanglement and activation arise
using pure states in this setting. This analogy aids in resolving several
conceptual puzzles in the study of entanglement under restricted operations. In
particular, we demonstrate that several types of quantum optical states that
possess confusing entanglement properties are analogous to bound entangled
states. Also, the classification of pure-state entanglement under restricted
operations can be much simpler than for mixed-state entanglement. For instance,
in the case of local Abelian superselection rules all questions concerning
distillability can be resolved.


title: The Quantum Emergence of Chaos

abstract:
  The dynamical status of isolated quantum systems, partly due to the linearity
of the Schrodinger equation is unclear: Conventional measures fail to detect
chaos in such systems. However, when quantum systems are subjected to
observation -- as all experimental systems must be -- their dynamics is no
longer linear and, in the appropriate limit(s), the evolution of expectation
values, conditioned on the observations, closely approaches the behavior of
classical trajectories. Here we show, by analyzing a specific example, that
microscopic continuously observed quantum systems, even far from any classical
limit, can have a positive Lyapunov exponent, and thus be truly chaotic.


title: Overcoming the EPR paradox

abstract:
  Quantum experiments detect particles, but they reveal information about wave
properties. No matter how quanta are detected, they always express the local
net state of the corresponding wave-function. The mechanism behind this process
is still a mystery. However, quantum wave-functions evolve like classical
waves. If they determine all the observations, why do they entail "weird"
phenomena? In particular, why do they produce conceptual problems, such as the
EPR paradox? It turns out that all the major interpretations in quantum
mechanics are particle-based ontologies. Variables of fundamental interest,
such as momentum and position, are assumed to reflect the input states of
discrete entities, even when their evolution is predicted by wave equations.
Here I show that a whole class of microscopic phenomena, including
entanglement, can be interpreted without contradictions if the variables of
quantum mechanics are actually treated as wave properties. The main
recommendation is to assume that sharp spectral states are transient outcomes
of superposition, rather than permanent input components.


title:
Remarks on exact solvability of quantum systems with spatially varying
  effective mass

abstract:
  Within the frame of a novel treatment we make a complete mathematical
analysis of exactly solvable one-dimensional quantum systems with non-constant
mass, involving their ordering ambiguities. This work extends the results
recently reported in the literature and clarifies the relation between
physically acceptable effective mass Hamiltonians.


title: A New Algebraic Approach to Perturbation Theory

abstract:
  An algebraic non-perturbative approach is proposed for the analytical
treatment of Schr\"{o}dinger equations with a potential that can be expressed
in terms of an exactly solvable piece with an additional potential. Avoiding
disadvantages of standard approaches, new handy recursion formulae with the
same simple form both for ground and excited states have been obtained. As an
illustration the procedure, well adapted to the use of computer algebra, is
successfully applied to quartic anharmonic oscillators by means of very simple
algebraic manipulations. The trend of the exact values of the energies is
rather well reproduced for a large range of values of the coupling constant
(g=0.001-10000).


title: Casimir Force on a Micrometer Sphere in a Dip: Proposal of an Experiment

abstract:
  The attractive Casimir force acting on a micrometer-sphere suspended in a
spherical dip, close to the wall, is discussed. This setup is in principle
directly accessible to experiment. The sphere and the substrate are assumed to
be made of the same perfectly conducting material.


title:
Bi-partite and global entanglement in a many-particle system with
  collective spin coupling

abstract:
  Bipartite and global entanglement are analyzed for the ground state of a
system of $N$ spin 1/2 particles interacting via a collective spin-spin
coupling described by the Lipkin-Meshkov-Glick (LMG) Hamiltonian. Under certain
conditions which includes the special case of a super-symmetry, the ground
state can be constructed analytically. In the case of an anti-ferromagnetic
coupling and for an even number of particles this state undergoes a smooth
crossover as a function of the continuous anisotropy parameter $\gamma $ from a
separable ($\gamma =\infty $) to a maximally entangled many-particle state
($\gamma =0$). From the analytic expression for the ground state, bipartite and
global entanglement are calculated. In the thermodynamic limit a discontinuous
change of the scaling behavior of the bipartite entanglement is found at the
isotropy point $\gamma =0$. For $% \gamma =0$ the entanglement grows
logarithmically with the system size with no upper bound, for $\gamma \neq 0$
it saturates at a level only depending on $\gamma $. For finite systems with
total spin $J=N/2$ the scaling behavior changes at $\gamma =\gamma
_{\mathrm{crit}}=1/J$.


title: How to build a 300 bit, 1 Giga-operation quantum computer

abstract:
  Experimental methods for laser-control of trapped ions have reached
sufficient maturity that it is possible to set out in detail a design for a
large quantum computer based on such methods, without any major omissions or
uncertainties. The main features of such a design are given, with a view to
identifying areas for study. The machine is based on 13000 ions moved via 20
micron vacuum channels around a chip containing 160000 electrodes and
associated classical control circuits; 1000 laser beam pairs are used to
manipulate the hyperfine states of the ions and drive fluorescence for readout.
The computer could run a quantum algorithm requiring 10^9 logical operations on
300 logical qubits, with a physical gate rate of 1 MHz and a logical gate rate
of 8 kHz, using methods for quantum gates that have already been experimentally
implemented. Routes for faster operation are discussed.


title:
Creating Ground State Molecules with Optical Feshbach Resonances in
  Tight Traps

abstract:
  We propose to create ultracold ground state molecules in an atomic
Bose-Einstein condensate by adiabatic crossing of an optical Feshbach
resonance. We envision a scheme where the laser intensity and possibly also
frequency are linearly ramped over the resonance. Our calculations for
$^{87}$Rb show that for sufficiently tight traps it is possible to avoid
spontaneous emission while retaining adiabaticity, and conversion efficiencies
of up to 50% can be expected.


title: Multi-distributed Entanglement in Finitely Correlated Chains

abstract:
  The entanglement-sharing properties of an infinite spin-chain are studied
when the state of the chain is a pure, translation-invariant state with a
matrix-product structure. We study the entanglement properties of such states
by means of their finitely correlated structure. These states are recursively
constructed by means of an auxiliary density matrix \rho on a matrix algebra B
and a completely positive map E: A \otimes B -> B, where A is the spin 2\times
2 matrix algebra. General structural results for the infinite chain are
therefore obtained by explicit calculations in (finite) matrix algebras. In
particular, we study not only the entanglement shared by nearest-neighbours,
but also, differently from previous works, the entanglement shared between
connected regions of the spin-chain. This range of possible applications is
illustrated and the maximal concurrence C=1/\sqrt{2} for the entanglement of
connected regions can actually be reached.


title:
A Study on the Noise Threshold of Fault-tolerant Quantum Error
  Correction

abstract:
  Quantum circuits implementing fault-tolerant quantum error correction (QEC)
for the three qubit bit-flip code and five-qubit code are studied. To describe
the effect of noise, we apply a model based on a generalized effective
Hamiltonian where the system-environment interactions are taken into account by
including stochastic fluctuating terms in the system Hamiltonian. This noise
model enables us to investigate the effect of noise in quantum circuits under
realistic device conditions and avoid strong assumptions such as maximal
parallelism and weak storage errors. Noise thresholds of the QEC codes are
calculated. In addition, the effects of imprecision in projective measurements,
collective bath, fault-tolerant repetition protocols, and level of parallelism
in circuit constructions on the threshold values are also studied with emphasis
on determining the optimal design for the fault-tolerant QEC circuit. These
results provide insights into the fault-tolerant QEC process as well as useful
information for designing the optimal fault-tolerant QEC circuit for particular
physical implementation of quantum computer.


title: Generation of quantum logic operations from physical Hamiltonians

abstract:
  We provide a systematic analysis of the physical generation of single- and
two-qubit quantum operations from Hamiltonians available in various quantum
systems for scalable quantum information processing. We show that generation of
one-qubit operations can be transformed into a steering problem on the Bloch
sphere, whereas the two-qubit problem can be generally transformed into a
steering problem in a tetrahedron representing all the local equivalence
classes of two-qubit operations (the Weyl chamber). We use this approach to
investigate several physical examples for the generation of two-qubit
operations. The steering approach provides useful guidance for the realization
of various quantum computation schemes.


title:
Criteria for quantum coherent transfer of excitons between chromophores
  in a polar solvent

abstract:
  We show that the quantum decoherence of Forster resonant energy transfer
between two optically active molecules can be described by a spin-boson model.
This allows us to give quantitative criteria, in terms of experimentally
measurable system parameters, that are necessary for coherent Bloch
oscillations of excitons between the chromophores. Experimental tests of our
results should be possible with Flourescent Resonant Energy Transfer (FRET)
spectroscopy. Although we focus on the case of protein-pigment complexes our
results are also relevant to quantum dots and organic molecules in a dielectric
medium.

categories: quant-ph cond-mat.other q-bio.BM q-bio.OT


title: Quantum key distribution over 122 km of standard telecom fiber

abstract:
  We report the first demonstration of quantum key distribution over a standard
telecom fiber exceeding 100 km in length. Through careful optimisation of the
interferometer and single photon detector, we achieve a quantum bit error ratio
of 8.9% for a 122km link, allowing a secure shared key to be formed after error
correction and privacy amplification. Key formation rates of up to 1.9 kbit/sec
are achieved depending upon fiber length. We discuss the factors limiting the
maximum fiber length in quantum cryptography.


title: Algorithmic complexity of quantum states

abstract:
  In this paper we give a definition for the Kolmogorov complexity of a pure
quantum state. In classical information theory the algorithmic complexity of a
string is a measure of the information needed by a universal machine to
reproduce the string itself. We define the complexity of a quantum state by
means of the classical description complexity of an (abstract) experimental
procedure that allows us to prepare the state with a given fidelity. We argue
that our definition satisfies the intuitive idea of complexity as a measure of
``how difficult'' it is to prepare a state. We apply this definition to give an
upper bound on the algorithmic complexity of a number of states.


title:
Unconditionally secure quantum key distribution over 50km of standard
  telecom fibre

abstract:
  We demonstrate a weak pulse quantum key distribution system using the BB84
protocol which is secure against all individual attacks, including photon
number splitting. By carefully controlling the weak pulse intensity we
demonstrate the maximum secure bit rate as a function of the fibre length.
Unconditionally secure keys can be formed for standard telecom fibres exceeding
50 km in length.


title: Quasi exactly solvable quantum lattice solitons

abstract:
  We extend the exactly solvable Hamiltonian describing $f$ quantum oscillators
considered recently by J. Dorignac et al. by means of a new interaction which
we choose as quasi exactly solvable. The properties of the spectrum of this new
Hamiltonian are studied as function of the new coupling constant. This
Hamiltonian as well as the original one are also related to adequate Lie
structures.


title: Quantum mechanics needs no interpretation

abstract:
  Probabilistic description of results of measurements and its consequences for
understanding quantum mechanics are discussed. It is shown that the basic
mathematical structure of quantum mechanics like the probability amplitude,
Born rule, probability density current, commutation relations, momentum
operator, uncertainty relations, rules for including the scalar and vector
potentials and existence of antiparticles can be derived from the definition of
the mean values of the space coordinates and time. Equations of motion of
quantum mechanics, the Klein-Gordon equation, Schroedinger equation and Dirac
equation are obtained from requirement of the relativistic invariance of the
theory. Limit case of localized probability densities leads to the
Hamilton-Jacobi equation of classical mechanics. Many particle systems are also
discussed.


title: Quantum simulator for the $\f{O(3)}$ nonlinear sigma model

abstract:
  We propose a design for the construction of a laboratory system based on
present-day technology which reproduces and thereby simulates the quantum
dynamics of the O(3) nonlinear sigma model. Apart from its relevance in
condensed-matter theory, this strongly interacting quantum field theory serves
as an important toy model for quantum chromo-dynamics (QCD) since it reproduces
many crucial properties of QCD. PACS: 03.67.-a, 03.67.Lx, 11.10.Kk, 68.65.-k.


title: The Computational Power of the W and GHZ states

abstract:
  It is well understood that the use of quantum entanglement significantly
enhances the computational power of systems. Much of the attention has focused
on Bell states and their multipartite generalizations. However, in the
multipartite case it is known that there are several inequivalent classes of
states, such as those represented by the W-state and the GHZ-state. Our main
contribution is a demonstration of the special computational power of these
states in the context of paradigmatic problems from classical distributed
computing. Concretely, we show that the W-state is the only pure state that can
be used to exactly solve the problem of leader election in anonymous quantum
networks. Similarly we show that the GHZ-state is the only one that can be used
to solve the problem of distributed consensus when no classical post-processing
is considered. These results generalize to a family of W- and GHZ-like states.
At the heart of the proofs of these impossibility results lie symmetry
arguments.


title: Pre- and Post-selection paradoxes and contextuality in quantum mechanics

abstract:
  Many seemingly paradoxical effects are known in the predictions for outcomes
of intermediate measurements made on pre- and post-selected quantum systems.
Despite appearances, these effects do not demonstrate the impossibility of a
noncontextual hidden variable theory, since an explanation in terms of
measurement-disturbance is possible. Nonetheless, we show that for every
paradoxical effect wherein all the pre- and post- selected probabilities are 0
or 1 and the pre- and post-selected states are nonorthogonal, there is an
associated proof of contextuality. This proof is obtained by considering all
the measurements involved in the paradoxical effect -- the pre-selection, the
post-selection, and the alternative possible intermediate measurements -- as
alternative possible measurements at a single time.


title:
Logical Pre- and Post-Selection paradoxes, measurement-disturbance and
  contextuality

abstract:
  Many seemingly paradoxical effects are known in the predictions for outcomes
of measurements made on pre- and post-selected quantum systems. A class of such
effects, which we call ``logical pre- and post-selection paradoxes'', bear a
striking resemblance to proofs of the Bell-Kochen-Specker theorem, which
suggests that they demonstrate the contextuality of quantum mechanics. Despite
the apparent similarity, we show that such effects can occur in noncontextual
hidden variable theories, provided measurements are allowed to disturb the
values of the hidden variables.


title: On the Complementarity Principle

abstract:
  We present a scheme in which we investigate the two-slit experiment and the
the principle of complementarity.


title: Extracting Atoms on Demand with Lasers

abstract:
  We propose a scheme that allows to coherently extract cold atoms from a
reservoir in a deterministic way. The transfer is achieved by means of
radiation pulses coupling two atomic states which are object to different
trapping conditions. A particular realization is proposed, where one state has
zero magnetic moment and is confined by a dipole trap, whereas the other state
with non-vanishing magnetic moment is confined by a steep microtrap potential.
We show that in this setup a predetermined number of atoms can be transferred
from a reservoir, a Bose-Einstein condensate, into the collective quantum state
of the steep trap with high efficiency in the parameter regime of present
experiments.


title: The Pondicherry interpretation of quantum mechanics: An overview

abstract:
  An overview of the Pondicherry interpretation of quantum mechanics is
presented. This interpretation proceeds from the recognition that the
fundamental theoretical framework of physics is a probability algorithm, which
serves to describe an objective fuzziness (the literal meaning of Heisenberg's
term "Unschaerfe," usually mistranslated as "uncertainty") by assigning
objective probabilities to the possible outcomes of unperformed measurements.
Although it rejects attempts to construe quantum states as evolving ontological
states, it arrives at an objective description of the quantum world that owes
nothing to observers or the goings-on in physics laboratories. In fact, unless
such attempts are rejected, quantum theory's true ontological implications
cannot be seen. Among these are the radically relational nature of space, the
numerical identity of the corresponding relata, the incomplete spatiotemporal
differentiation of the physical world, and the consequent top-down structure of
reality, which defies attempts to model it from the bottom up, whether on the
basis of an intrinsically differentiated spacetime manifold or out of a
multitude of individual building blocks.


title: Quantum information storage and state transfer based on spin systems

abstract:
  The idea of quantum state storage is generalized to describe the coherent
transfer of quantum information through a coherent data bus. In this universal
framework, we comprehensively review our recent systematical investigations to
explore the possibility of implementing the physical processes of quantum
information storage and state transfer by using quantum spin systems, which may
be an isotropic antiferromagnetic spin ladder system or a ferromagnetic
Heisenberg spin chain. Our studies emphasize the physical mechanisms and the
fundamental problems behind the various protocols for the storage and transfer
of quantum information in solid state systems.


title:
Characterization of the non-classical nature of conditionally prepared
  single photons

abstract:
  A reliable single photon source is a prerequisite for linear optical quantum
computation and for secure quantum key distribution. A criterion yielding a
conclusive test of the single photon character of a given source, attainable
with realistic detectors, is therefore highly desirable. In the context of
heralded single photon sources, such a criterion should be sensitive to the
effects of higher photon number contributions, and to vacuum introduced through
optical losses, which tend to degrade source performance. In this paper we
present, theoretically and experimentally, a criterion meeting the above
requirements.


title: Schroedinger Cat States of a Nanomechanical Resonator

abstract:
  We present a scheme of generating large-amplitude Schr\"{o}dinger cat states
and entanglement in a coupled system of nanomechanical resonator and single
Cooper pair box (SCPB), without being limited by the magnitude of the coupling.
It is shown that the entanglement between the resonator and the SCPB can be
detected by a spectroscopic method.


title: Scalable Ion Trap Quantum Computing without Moving Ions

abstract:
  A hybrid quantum computing scheme is studied where the hybrid qubit is made
of an ion trap qubit serving as the information storage and a solid-state
charge qubit serving as the quantum processor, connected by a superconducting
cavity. In this paper, we extend our previous work [1] and study the
decoherence, coupling and scalability of the hybrid system. We present our
calculations of the decoherence of the coupled ion - charge system due to the
charge fluctuations in the solid-state system and the dissipation of the
superconducting cavity under laser radiation. A gate scheme that exploits rapid
state flips of the charge qubit to reduce decoherence by the charge noise is
designed. We also study a superconducting switch that is inserted between the
cavity and the charge qubit and provides tunable coupling between the qubits.
The scalability of the hybrid scheme is discussed together with several
potential experimental obstacles in realizing this scheme.


title: Quantum Computing, Postselection, and Probabilistic Polynomial-Time

abstract:
  I study the class of problems efficiently solvable by a quantum computer,
given the ability to "postselect" on the outcomes of measurements. I prove that
this class coincides with a classical complexity class called PP, or
Probabilistic Polynomial-Time. Using this result, I show that several simple
changes to the axioms of quantum mechanics would let us solve PP-complete
problems efficiently. The result also implies, as an easy corollary, a
celebrated theorem of Beigel, Reingold, and Spielman that PP is closed under
intersection, as well as a generalization of that theorem due to Fortnow and
Reingold. This illustrates that quantum computing can yield new and simpler
proofs of major results about classical computation.


title: Asymmetric universal entangling machine

abstract:
  We give a definition of asymmetric universal entangling machine which
entangles a system in an unknown state to a specially prepared ancilla. The
machine produces a fixed state-independent amount of entanglement in exchange
to a fixed degradation of the system state fidelity. We describe explicitly
such a machine for any quantum system having $d$ levels and prove its
optimality. We show that a $d^2$-dimensional ancilla is sufficient for reaching
optimality. The introduced machine is a generalization to a number of widely
investigated universal quantum devices such as the symmetric and asymmetric
quantum cloners, the symmetric quantum entangler, the quantum information
distributor and the universal-NOT gate.


title: Interpolating statistics realized as Deformed Harmonic Oscillators

abstract:
  The idea that a system obeying interpolating statistics can be described by a
deformed oscillator algebra has been an outstanding issue. This original
concept introduced long ago by Greenberg is the motivation for this
investigation. We establish that a q-deformed algebra can be used to describe
the statistics of particles (anyons) interpolating continuously between Bose
and Fermi statistics, i.e., fractional statistics. We show that the generalized
intermediate statistics splits into the Boson-like and Fermion-like regimes,
each described by a unique oscillator algebra. The B-anyon thermostatistics is
described by employing the q-calculus based on the Jackson derivative but the
F-anyons are described by ordinary derivatives of thermodynamics. Thermodynamic
functions of both B-anyons and F-anyons are determined and examined.

categories: quant-ph cond-mat.stat-mech math-ph math.MP


title:
Four wave mixing with self-phase matching due to collective atomic
  recoil

abstract:
  We describe a method for non-degenerate four-wave mixing in a cold sample of
  4-level atoms. An integral part of the four-wave mixing process is a
collective instability which spontaneously generates a periodic density
modulation in the cold atomic sample with a period equal to half of the
wavelength of the generated high-frequency optical field. Due to the generation
of this density modulation, phase-matching between the pump and scattered
fields is not a necessary initial condition for this wave-mixing process to
occur, rather the density modulation acts to "self phase-match" the fields
during the course of the wave-mixing process. We describe a one-dimensional
model of this process, and suggest a proof-of-principle experiment which would
involve pumping a sample of cold Cs atoms with three infra-red pump fields to
produce blue light.


title: Quantum correlations in the soliton collisions

abstract:
  We study quantum correlations and quantum noise in the soliton collision
described by a general two-soliton solution of the nonlinear Schr\"odinger
equation, by using the back-propagation method. Our results include the
standard case of a $sech$-shaped initial pulse analyzed earlier. We reveal that
double-hump initial pulses can get more squeezed, and the squeezing ratio
enhancement is due to the long collision period in which the pulses are more
stationary. These results offer promising possibilities of using higher-order
solitons to generate strongly squeezed states for the quantum information
process and quantum computation.


title: Why is Schrodinger's Equation Linear?

abstract:
  Information-theoretic arguments are used to obtain a link between the
accurate linearity of Schrodinger's equation and Lorentz invariance: A possible
violation of the latter at short distances would imply the appearance of
nonlinear corrections to quantum theory. Nonlinear corrections can also appear
in a Lorentz invariant theory in the form of higher derivative terms that are
determined by a length scale, possibly the Planck length. It is suggested that
the best place to look for evidence of such quantum nonlinear effects is in
neutrino physics and cosmology.


title:
Multi-particle entanglement manipulation under positive partial
  transpose preserving operations

abstract:
  We consider the transformation of multi-partite states in the single copy
setting under positive-partial-transpose-preserving operations (PPT-operations)
and obtain both qualitative and quantitative results. Firstly, for some pure
state transformations that are impossible under local operations and classical
communication (LOCC), we demonstrate that they become possible with a
surprisingly large success probability under PPT-operations. Furthermore, we
clarify the convertibility of arbitrary multipartite pure states under
PPT-operations, and show that a drastic simplification in the classification of
pure state entanglement occurs when the set of operations is switched from LOCC
to PPT-operations. Indeed, the infinitely many types of LOCC-incomparable
entanglement are reduced to only one type under the action of PPT-operations.
This is a clear manifestation of the increased power afforded by the use of
PPT-bound entanglement. In addition, we further enlarge the set of operations
to clarify the effect of another type of bound entanglement, multipartite
unlockable bound entanglement, and show that a further simplification occurs.
As compared to pure states a more complicated situation emerges in the mixed
state settings. While single copy distillation becomes possible under
PPT-operations for some mixed states it remains impossible for other mixed
states.


title: What is Probability?

abstract:
  Probabilities may be subjective or objective; we are concerned with both
kinds of probability, and the relationship between them. The fundamental theory
of objective probability is quantum mechanics: it is argued that neither Bohr's
Copenhagen interpretation, nor the pilot-wave theory, nor stochastic
state-reduction theories, give a satisfactory answer to the question of what
objective probabilities are in quantum mechanics, or why they should satisfy
the Born rule; nor do they give any reason why subjective probabilities should
track objective ones.
  But it is shown that if probability only arises with decoherence, then they
must be given by the Born rule. That further, on the Everett interpretation, we
have a clear statement of what probabilities are, in terms of purely
categorical physical properties; and finally, along lines recently laid out by
Deutsch and Wallace, that there is a clear basis in the axioms of decision
theory as to why subjective probabilities should track these objective ones.
  These results hinge critically on the absence of hidden-variables or any
other mechanism (such as state-reduction) from the physical interpretation of
the theory.
  The account of probability has traditionally been considered the principal
weakness of the Everett interpretation; on the contrary it emerges as one of
its principal strengths.


title: Complementarity and Scientific Rationality

abstract:
  Bohr's interpretation of quantum mechanics has been criticized as incoherent
and opportunistic, and based on doubtful philosophical premises. If so Bohr's
influence, in the pre-war period of 1927-1939, is the harder to explain, and
the acceptance of his approach to quantum mechanics over de Broglie's had no
reasonable foundation. But Bohr's interpretation changed little from the time
of its first appearance, and stood independent of any philosophical
presuppositions. The principle of complementarity is itself best read as a
conjecture of unusually wide scope, on the nature and future course of
explanations in the sciences (and not only the physical sciences). If it must
be judged a failure today, it is not because of any internal inconsistency.


title: Algorithmic approach to quantum physics

abstract:
  Algorithmic approach is based on the assumption that any quantum evolution of
many particle system can be simulated on a classical computer with the
polynomial time and memory cost. Algorithms play the central role here but not
the analysis, and a simulation gives a "film" which visualizes many particle
quantum dynamics and is demonstrated to a user of the model. Restrictions
following from the algorithm theory are considered on a level of fundamental
physical laws. Born rule for the calculation of quantum probability as well as
the decoherence is derived from the existence of a nonzero minimal value of
amplitude module - a grain of amplitude. The limitation on the classical
computational resources gives the unified description of quantum dynamics that
is not divided to the unitary dynamics and measurements and does not depend on
the existence of observer. It is proposed the description of states based on
the nesting of particles in each other that permits to account the effects of
all levels in the same model. Algorithmic approach admits the possibility of
refutation, because it forbids the creation of a scalable quantum computer that
is allowed in the conventional quantum formalism.


title: Kochen-Specker Algorithms for Qunits

abstract:
  Algorithms for finding arbitrary sets of Kochen-Specker (KS) qunits (n-level
systems) as well as all the remaining vectors in a space of an arbitrary
dimension are presented. The algorithms are based on linear MMP diagrams which
generate orthogonalities of KS qunits, on an algebraic definition of states on
the diagrams, and on nonlinear equations corresponding to MMP diagrams whose
solutions are either KS qunits or the remaining vectors of a chosen space
depending on whether the diagrams allow 0-1 states or not. The complexity of
the algorithms is polynomial. New results obtained with the help of the
algorithms are presented.


title: Berry Phase and Adiabatic Breakdown in Optical Modulator

abstract:
  We consider an all in-fiber optical modulator based on a ring resonator
configuration. The case of adiabatic to nonadiabatic transition is considered,
where the geometrical (Berry) phase acquired in a round trip along the ring
changes abruptly by pi. Degradation of the responsivity of the modulator due to
finite linewidth of the optical input is discussed. We show that the
responsivity of the proposed modulator can be significantly enhanced with
optimum design and compare with other configurations.


title: The internal logic of Bell's states

abstract:
  We investigate the internal logic of a quantum computer with two qubits, in
the two particular cases of non-entanglement (separable states) and maximal
entanglement (Bell's states). To this aim, we consider an internal (reversible)
measurement which preserves the probabilities by mirroring the states. We then
obtain logical judgements for both cases of separable and Bell's states.


title:
Coherence Control of Adiabatic Decoherence in a Three-level Atom with
  Lambda Configuration

abstract:
  In this paper, we study the suppression of adiabatic decoherence in a
three-level atom with $\Lambda$ configuration using bang-bang control
technique. We have given the decoupling bang-bang operation group, and
programmed a sequence of periodic radio frequency twinborn pulses to realize
the control process. Moreover, we have studied the process with non-ideal
situation and established the condition for efficient suppression of adiabatic
decoherence.


title: Economical quantum cloning in any dimension

abstract:
  The possibility of cloning a d-dimensional quantum system without an ancilla
is explored, extending on the economical phase-covariant cloning machine found
in [Phys. Rev. A {\bf 60}, 2764 (1999)] for qubits. We prove the impossibility
of constructing an economical version of the optimal universal cloning machine
in any dimension. We also show, using an ansatz on the generic form of cloning
machines, that the d-dimensional phase-covariant cloner, which optimally clones
all uniform superpositions, can be realized economically only in dimension d=2.
The used ansatz is supported by numerical evidence up to d=7. An economical
phase-covariant cloner can nevertheless be constructed for d>2, albeit with a
lower fidelity than that of the optimal cloner requiring an ancilla. Finally,
using again an ansatz on cloning machines, we show that an economical version
of the Fourier-covariant cloner, which optimally clones the computational basis
and its Fourier transform, is also possible only in dimension d=2.


title:
Generation of Multi-atom Dicke States with Quasi-unit Probability
  through the Detection of Cavity Decay

abstract:
  We propose a scheme to create universal Dicke states of n largely detuned
atoms through detecting the leaky photons from an optical cavity. The
generation of entangled states in our scheme has quasi-unit success
probability, so it has potential practicability based on current or near coming
laboratory cavity QED technology.


title: Multiparty Quantum Secret Sharing

abstract:
  Based on a quantum secure direct communication (QSDC) protocol [Phys. Rev.
A69(04)052319], we propose a $(n,n)$-threshold scheme of multiparty quantum
secret sharing of classical messages (QSSCM) using only single photons. We take
advantage of this multiparty QSSCM scheme to establish a scheme of multiparty
secret sharing of quantum information (SSQI), in which only all quantum
information receivers collaborate can the original qubit be reconstructed. A
general idea is also proposed for constructing multiparty SSQI schemes from any
QSSCM scheme.


title: Quantum weak values

abstract:
  We experimentally determine weak values for a single photon's polarization,
obtained via a weak measurement that employs a two-photon entangling operation,
and postselection. The weak values cannot be explained by a semiclassical wave
theory, due to the two-photon entanglement. We observe the variation in size of
the weak value with measurement strength, obtaining an average measurement of
the S_1 Stokes parameter more than an order of magnitude outside of the
operator's spectrum for the smallest measurement strengths.


title:
Decoherence induced by zero point fluctuations in quantum Brownian
  motion

abstract:
  We show a completely analytical approach to the decoherence induced by a zero
temperature environment on a Brownian test particle. We consider an Omhic
environment bilinearly coupled to an oscillator and compute the master
equation. From diffusive coefficients, we evaluate the decoherence time for the
usual quantum Brownian motion and also for an upside-down oscillator, as a toy
model of a quantum phase transition.


title: No-clicking Event in Quantum Key Distribution

abstract:
  We discuss the `no-clicking' event, which is harmful for legitimate users in
Bennett-Brassard 1984 quantum key distribution: We describe an attack where
no-clicking events are utilized in the same way as double-clicking events are
utilized in the quantum Trojan-pony attack. We discuss how to deal with the
no-clicking events. We discuss how to estimate the security of the protocol
against the proposed attack by using a formula involved with the fraction of
adversarial removals of events.


title:
Exploring a New Post-Selection Condition for Efficient Linear Optics
  Quantum Computation

abstract:
  Recently, it was shown that fundamental gates for theoretically efficient
quantum information processing can be realized by using single photon sources,
linear optics and quantum counters. One of these fundamental gates is the
NS-gate, that is, the one-mode non-linear sign shift. In this work, firstly, we
prove by a new rigorous proof that the upper bound of success probability of
NS-gates with only one helper photon and an undefined number of ancillary modes
is bounded by 0.25. Secondly, we explore the upper bound of success probability
of NS-gate with a new post-selection measurement. The idea behind this new
post-selection measurement is to condition the success of NS-gate
transformation to the observation of only one helper photon in whichever of the
output modes.


title:
Influence of pump-field scattering to nonclassical-light generation in a
  photonic band-gap nonlinear planar waveguide

abstract:
  Optical parametric process occurring in a nonlinear planar waveguide can
serve as a source of light with nonclassical properties. Properties of the
generated fields are substantially modified by scattering of the nonlinearly
interacting fields in a photonic band-gap structure inside the waveguide. A
quantum model of linear operator amplitude corrections to amplitude mean-values
provides conditions for an efficient squeezed-light generation as well as
generation of light with sub-Poissonian photon-number statistics. Destructive
influence of phase mismatch of the nonlinear interaction can fully be
compensated using a suitable photonic-band gap structure inside the waveguide.
Also an increase of signal-to-noise ratio of an incident optical field can be
reached in the waveguide.


title: Local computer model emulating the results of the Pan et al. experiment

abstract:
  It is a widespread current belief that objective local models can not explain
the quantum optics experiment of Pan et al. By presenting a model that operates
on independent computers, we show that this belief is unfounded. Three remote
computers (Alice, Bob and Claire), that never communicate with each other, send
measurement results to a fourth computer that is in charge of collecting the
data and computing correlations. The result obtained by our local simulation is
in better agreement with the ideal quantum result than the Pan et al.
experiment. We also show that the local model presented by Pan et al. that can
not explain the quantum results contains inappropriate reasoning with profound
consequences for the possible results of any local model that uses probability
theory.


title: Simplifying monotonicity conditions for entanglement measures

abstract:
  We show that for a convex function the following, rather modest conditions,
are equivalent to monotonicity under local operations and classical
communication. The conditions are: 1)invariance under local unitaries, 2)
invariance under adding local ancilla in arbitrary state 3) on mixtures of
states possessing local orthogonal flags the function is equal to its average.
The result holds for multipartite systems. It is intriguing that the obtained
conditions are equalities. The only inequality is hidden in the condition of
convexity.


title: Time Dependent Quadratic Hamiltonians,SU(1,1), SU(2), SU(2,1) and SU(3)

abstract:
  The properties of SU(1,1) SU(2),SU(2,1) and SU(3) have often been used in
quantum optics. In this paper we demonstrate the use of these symmetries. The
group properties of SU(1,1) SU(2), and SU(2,1) are used to find the transition
probabilities of various time dependent quadratic Hamiltonians. We consider
Hamiltonians representing the frequency converter,parametric amplifier and
raman scattering.These Hamiltonians are used to describe optical coupling in
nonlinear crystals.


title:
And what if Bohm was right? The fundamental problem of quantum
  cryptography and a way to circumvent it

abstract:
  This paper has been withdrawn. The main technical result will reappear in the
new version of quant-ph/0501003.


title: Particle Statistics in Quantum Information Processing

abstract:
  Particle statistics is a fundamental part of quantum physics, and yet its
role and use in the context of quantum information have been poorly explored so
far. After briefly introducing particle statistics and the Symmetrization
Postulate, I will argue that this fundamental aspect of Nature can be seen as a
resource for quantum information processing and I will present examples showing
how it is possible to do useful and efficient quantum information processing
using only the effects of particles statistics.


title: Quantum displacements

abstract:
  In this paper, first we explain what are the `quantum displacements'. We
establish a group of bases, which contains the coupled bases coupling a ququart
and a bipartite qubit systems. By these bases, we can realize the quantum
displacements. We discuss some possible forms of them. At last, we point out
that a so-call ''non-imprecisely-cloning theorem'' also holds.


title: Quantization of Games: Towards Quantum Artificial Intelligence

abstract:
  On grounds of the discussed material, we reason about possible future
development of quantum game theory and its impact on information processing and
the emerging information society. The idea of quantum artificial intelligence
is explained.


title:
Measurement of geometric phase for mixed states using single photon
  interferometry

abstract:
  Geometric phase may enable inherently fault-tolerant quantum computation.
However, due to potential decoherence effects, it is important to understand
how such phases arise for {\it mixed} input states. We report the first
experiment to measure mixed-state geometric phases in optics, using a
Mach-Zehnder interferometer, and polarization mixed states that are produced in
two different ways: decohering pure states with birefringent elements; and
producing a nonmaximally entangled state of two photons and tracing over one of
them, a form of remote state preparation.


title: Nanomagnetic Planar Magnetic Resonance Microscopy Lens

abstract:
  The achievement of three-dimensional atomic resolution magnetic resonance
microscopy remains one of the main challenges in the visualization of
biological molecules. The prospects for single spin microscopy have come
tantalizingly close due to the recent developments in sensitive
instrumentation. Despite the single spin detection capability in systems of
spatially well-isolated spins, the challenge that remains is the creation of
conditions in space where only a single spin is resonant and detected in the
presence of other spins in its natural dense spin environment. We present a
nanomagnetic planar design where a localized Angstrom-scale point in
three-dimensional space is created above the nanostructure with a non-zero
minimum of the magnetic field magnitude. The design thereby represents a
magnetic resonance microscopy lens where potentially only a single spin located
in the focus spot of the structure is resonant. Despite the presence of other
spins in the Angstrom-scale vicinity of the resonant spin, the high gradient
magnetic field of the lens renders those spins inactive in the detection
process.


title:
Experimental Free-Space Distribution of Entangled Photon Pairs over a
  Noisy Ground Atmosphere of 13km

abstract:
  We report free-space distribution of entangled photon pairs over a noisy
ground atmosphere of 13km. It is shown that the desired entanglement can still
survive after the two entangled photons have passed through the noisy ground
atmosphere. This is confirmed by observing a space-like separated violation of
Bell inequality of $2.45 \pm 0.09$. On this basis, we exploit the distributed
entangled photon source to demonstrate the BB84 quantum cryptography scheme.
The distribution distance of entangled photon pairs achieved in the experiment
is for the first time well beyond the effective thickness of the aerosphere,
hence presenting a significant step towards satellite-based global quantum
communication.