title: Minimal Informationally Complete Measurements for Pure States

abstract:
  We consider measurements, described by a positive-operator-valued measure
(POVM), whose outcome probabilities determine an arbitrary pure state of a
D-dimensional quantum system. We call such a measurement a pure-state
informationally complete (PSI-complete) POVM. We show that a measurement with
2D-1 outcomes cannot be PSI-complete, and then we construct a POVM with 2D
outcomes that suffices, thus showing that a minimal PSI-complete POVM has 2D
outcomes. We also consider PSI-complete POVMs that have only rank-one POVM
elements and construct an example with 3D-2 outcomes, which is a generalization
of the tetrahedral measurement for a qubit. The question of the minimal number
of elements in a rank-one PSI-complete POVM is left open.


title: Diffraction and trapping in circular lattices

abstract:
  When a single two-level atom interacts with a pair of Laguerre-Gaussian beams
with opposite helicity, this leads to an efficient exchange of angular momentum
between the light field and the atom. When the radial motion is trapped by an
additional potential, the wave function of a single localized atom can be split
into components that rotate in opposite direction. This suggests a novel scheme
for atom interferometry without mirror pulses. Also atoms in this configuration
can be bound into a circular lattice.


title: Conditional sign flip via teleportation

abstract:
  We present a model to realize a probabilistic conditional sign flip gate
using only linear optics. The gate operates in the space of number state qubits
and is obtained by a nonconventional use of the teleportation protocol. Both a
destructive and a nondestructive version of the gate are presented. In the
former case an Hadamard gate on the control qubit is combined with a projective
teleportation scheme mixing control and target. The success probability is 1/2.
In the latter case we need a quantum encoder realized via the interaction of
the control qubit with an ancillary state composed of two maximally entangled
photons. The success probability is 1/4.


title: Entangling two mode thermal fields through quantum erasing

abstract:
  We investigate a possible scheme for entangling two mode thermal fields
through the quantum erasing process, in which an atom is coupled with two mode
fields via the interaction governed by the two-mode two-photon Jaynes-Cummings
model. The influence of phase decoherence on the entanglement of two mode
fields is discussed. It is found that quantum erasing process can transfer part
of entanglement between the atom and fields to two mode fields initially in the
thermal states. The entanglement achieved by fields heavily depends on their
initial temperature and the detuning. The entanglement of stationary state is
also investigated.


title: Probabilistic implementation of universal Hadamard and Unitary gates

abstract:
  We show that the Hadamard and Unitary gates could be implemented by a unitary
evolution together with a measurement for any unknown state chosen from a set
$A = {{| {\Psi_i} > ,| {\bar {\Psi}_i} >}}({i = 1,2})$ if and only if $|
{\Psi_1} > ,| {\Psi_2} > ,| {\bar {\Psi}_1} > ,| {\bar {\Psi}_2} >$ are
linearly independent. We also derive the best transformation efficiencies.


title: Zero-Point cooling and low heating of trapped 111Cd+ ions

abstract:
  We report on ground state laser cooling of single 111Cd+ ions confined in
radio-frequency (Paul) traps. Heating rates of trapped ion motion are measured
for two different trapping geometries and electrode materials, where no effort
was made to shield the electrodes from the atomic Cd source. The low measured
heating rates suggest that trapped 111Cd+ ions may be well-suited for
experiments involving quantum control of atomic motion, including applications
in quantum information science.


title:
An algorithm for simulating the Ising model on a type-II quantum
  computer

abstract:
  Presented here is an algorithm for a type-II quantum computer which simulates
the Ising model in one and two dimensions. It is equivalent to the Metropolis
Monte-Carlo method and takes advantage of quantum superposition for random
number generation. This algorithm does not require the ensemble of states to be
measured at the end of each iteration, as is required for other type-II
algorithms. Only the binary result is measured at each node which means this
algorithm could be implemented using a range of different quantum computing
architectures. The Ising model provides an example of how cellular automata
rules can be formulated to be run on a type-II quantum computer.


title:
Spin Gauge Fields: from Berry Phase to Topological Spin Transport and
  Hall Effects

abstract:
  The paper examines the emergence of gauge fields during the evolution of a
particle with a spin that is described by a matrix Hamiltonian with n different
eigenvalues. It is shown that by introducing a spin gauge field a particle with
a spin can be described as a spin multiplet of scalar particles situated in a
non-Abelian pure gauge (forceless) field U(n). As the result, one can create a
theory of particle evolution that is gauge invariant with regards to the group
U^n(1). Due to this, in the adiabatic (Abelian) approximation the spin gauge
field is an analogue of n electromagnetic fields U(1) on the extended phase
space of the particle. These fields are force ones, and the forces of their
action enter the particle motion equations that are derived in the paper in the
general form. The motion equations describe the topological spin transport,
pumping and splitting. The Berry phase is represented in this theory
analogously to the Dirac phase of a particle in an electromagnetic field. Due
to the analogy with the electromagnetic field, the theory becomes natural in
the four-dimensional form. Besides the general theory the article considers a
number of important particular examples, both known and new.


title: Knowledge excess duality and violation of Bell inequalities

abstract:
  A constraint on two complementary knowledge excesses by maximal violation of
Bell inequalities for a single copy of any mixed state of two qubits $S,M$ is
analyzed. The complementary knowledge excesses ${\bf \Delta K}(\Pi_{M}\to
\Pi_{S})$ and ${\bf \Delta K}(\Pi'_{M}\to \Pi'_{S})$ quantify an enhancement of
ability to predict results of the complementary projective measurements
$\Pi_{S},\Pi'_{S}$ on the qubit $S$ from the projective measurements
$\Pi_{M},\Pi'_{M}$ performed on the qubit $M$. For any state $\rho_{SM}$ and
for arbitrary $\Pi_{S},\Pi'_{S}$ and $\Pi_{M},\Pi'_{M}$, the knowledge excesses
satisfy the following inequality ${\bf \Delta K}^{2}(\Pi_{M}\to \Pi_{S})+{\bf
\Delta K}^{2} (\Pi'_{M}\to \Pi'_{S})\leq (B_{max}/2)^2$, where $B_{max}$ is
maximum of violation of Bell inequalities under single-copy local operations
(local filtering and unitary transformations). Particularly, for the
Bell-diagonal states only an appropriate choice of the measurements
$\Pi_{S},\Pi'_{S}$ and $\Pi_{M},\Pi'_{M}$ are sufficient to saturate the
inequality.


title: Measurement-Based Quantum Turing Machines and their Universality

abstract:
  Quantum measurement is universal for quantum computation. This universality
allows alternative schemes to the traditional three-step organisation of
quantum computation: initial state preparation, unitary transformation,
measurement. In order to formalize these other forms of computation, while
pointing out the role and the necessity of classical control in
measurement-based computation, and for establishing a new upper bound of the
minimal resources needed to quantum universality, a formal model is introduced
by means of Measurement-based Quantum Turing Machines.


title: Adiabatic approximation in open quantum systems

abstract:
  We generalize the standard quantum adiabatic approximation to the case of
open quantum systems. We define the adiabatic limit of an open quantum system
as the regime in which its dynamical superoperator can be decomposed in terms
of independently evolving Jordan blocks. We then establish validity and
invalidity conditions for this approximation and discuss their applicability to
superoperators changing slowly in time. As an example, the adiabatic evolution
of a two-level open system is analysed.


title: Multiple-copy entanglement transformation and entanglement catalysis

abstract:
  We prove that any multiple-copy entanglement transformation [S.
Bandyopadhyay, V. Roychowdhury, and U. Sen, Phys. Rev. A \textbf{65}, 052315
(2002)] can be implemented by a suitable entanglement-assisted local
transformation [D. Jonathan and M. B. Plenio, Phys. Rev. Lett. \textbf{83},
3566 (1999)]. Furthermore, we show that the combination of multiple-copy
entanglement transformation and the entanglement-assisted one is still
equivalent to the pure entanglement-assisted one. The mathematical structure of
multiple-copy entanglement transformations then is carefully investigated. Many
interesting properties of multiple-copy entanglement transformations are
presented, which exactly coincide with those satisfied by the
entanglement-assisted ones. Most interestingly, we show that an arbitrarily
large number of copies of state should be considered in multiple-copy
entanglement transformations.


title: Quantum metastability in a class of moving potentials

abstract:
  In this paper we consider quantum metastability in a class of moving
potentials introduced by Berry and Klein. Potential in this class has its
height and width scaled in a specific way so that it can be transformed into a
stationary one. In deriving the non-decay probability of the system, we argue
that the appropriate technique to use is the less known method of scattering
states. This method is illustrated through two examples, namely, a moving
delta-potential and a moving barrier potential. For expanding potentials, one
finds that a small but finite non-decay probability persists at large times.
Generalization to scaling potentials of arbitrary shape is briefly indicated.


title: Stabilizing quantum metastable states in a time-periodic potential

abstract:
  Metastability of a particle trapped in a well with a time-periodically
oscillating barrier is studied in the Floquet formalism. It is shown that the
oscillating barrier causes the system to decay faster in general. However,
avoided crossings of metastable states can occur with the less stable states
crossing over to the more stable ones. If in the static well there exists a
bound state, then it is possible to stabilize a metastable state by
adiabatically increasing the oscillating frequency of the barrier so that the
unstable state eventually cross-over to the stable bound state. It is also
found that increasing the amplitude of the oscillating field may change a
direct crossing of states into an avoided one.


title:
Entangled states that cannot reproduce original classical games in their
  quantum version

abstract:
  A model of a quantum version of classical games should reproduce the original
classical games in order to be able to make a comparative analysis of quantum
and classical effects. We analyze a class of symmetric multipartite entangled
states and their effect on the reproducibility of the classical games. We
present the necessary and sufficient condition for the reproducibility of the
original classical games. Satisfying this condition means that complete
orthogonal bases can be constructed from a given multipartite entangled state
provided that each party is restricted to two local unitary operators. We prove
that most of the states belonging to the class of symmetric states with respect
to permutations, including the N-qubit W state, do not satisfy this condition.


title:
Optimal state encoding for quantum walks and quantum communication over
  spin systems

abstract:
  Recent work has shown that a simple chain of interacting spins can be used as
a medium for high-fidelity quantum communication. We describe a scheme for
quantum communication using a spin system that conserves z-spin, but otherwise
is arbitrary. The sender and receiver are assumed to directly control several
spins each, with the sender encoding the message state onto the larger
state-space of her control spins. We show how to find the encoding that
maximises the fidelity of communication, using a simple method based on the
singular-value decomposition. Also, we show that this solution can be used to
increase communication fidelity in a rather different circumstance: where no
encoding of initial states is used, but where the sender and receiver control
exactly two spins each and vary the interactions on those spins over time. The
methods presented are computationally efficient, and numerical examples are
given for systems having up to 300 spins.


title: Information Systems Self-description and Quantum Measurement Problem

abstract:
  Information-Theoretical restrictions on the systems self-descriptions are
applied to Quantum Measurements Theory. For the quantum object S measurement by
information system O such restrictions are described by the restricted states
formalism by Breuer. The analogous restrictions obtained in Algebraic QM from
the analysis of Segal algebra U of O observables; O restricted states set is
defined as U dual space. From Segal theorem for the associative subalgebra it's
shown that such states describe the random pointer outcomes observed by O in
the individual events.


title: Catalyst-assisted Probabilistic Entanglement Transformation

abstract:
  We are concerned with catalyst-assisted probabilistic entanglement
transformations. A necessary and sufficient condition is presented under which
there exist partial catalysts that can increase the maximal transforming
probability of a given entanglement transformation. We also design an algorithm
which leads to an efficient method for finding the most economical partial
catalysts with minimal dimension. The mathematical structure of
catalyst-assisted probabilistic transformation is carefully investigated.


title: Quantum bit detector

abstract:
  We propose and analyze an experimental scheme of quantum nondemolition
detection of monophotonic and vacuum states in a superconductive toroidal
cavity by means of Rydberg atoms.


title: Unknown Quantum States and Operations, a Bayesian View

abstract:
  The classical de Finetti theorem provides an operational definition of the
concept of an unknown probability in Bayesian probability theory, where
probabilities are taken to be degrees of belief instead of objective states of
nature. In this paper, we motivate and review two results that generalize de
Finetti's theorem to the quantum mechanical setting: Namely a de Finetti
theorem for quantum states and a de Finetti theorem for quantum operations. The
quantum-state theorem, in a closely analogous fashion to the original de
Finetti theorem, deals with exchangeable density-operator assignments and
provides an operational definition of the concept of an "unknown quantum state"
in quantum-state tomography. Similarly, the quantum-operation theorem gives an
operational definition of an "unknown quantum operation" in quantum-process
tomography. These results are especially important for a Bayesian
interpretation of quantum mechanics, where quantum states and (at least some)
quantum operations are taken to be states of belief rather than states of
nature.


title: Resonant cavity photon creation via the dynamical Casimir effect

abstract:
  Motivated by a recent proposal for an experimental verification of the
dynamical Casimir effect, the macroscopic electromagnetic field within a
perfect cavity containing a thin slab with a time-dependent dielectric
permittivity is quantized in terms of the dual potentials. For the resonance
case, the number of photons created out of the vacuum due to the dynamical
Casimir effect is calculated for both polarizations (TE and TM). PACS:
42.50.Lc, 03.70.+k, 42.50.Dv, 42.60.Da.


title: Two-dimensional Nanolithography Using Atom Interferometry

abstract:
  We propose a novel scheme for the lithography of arbitrary, two-dimensional
nanostructures via matter-wave interference. The required quantum control is
provided by a pi/2-pi-pi/2 atom interferometer with an integrated atom lens
system. The lens system is developed such that it allows simultaneous control
over atomic wave-packet spatial extent, trajectory, and phase signature. We
demonstrate arbitrary pattern formations with two-dimensional 87Rb wavepackets
through numerical simulations of the scheme in a practical parameter space.
Prospects for experimental realizations of the lithography scheme are also
discussed.


title: A new mathematical representation of Game Theory, I

abstract:
  In this paper, we introduce a framework of new mathematical representation of
Game Theory, including static classical game and static quantum game. The idea
is to find a set of base vectors in every single-player strategy space and to
define their inner product so as to form them as a Hilbert space, and then form
a Hilbert space of system state. Basic ideas, concepts and formulas in Game
Theory have been reexpressed in such a space of system state. This space
provides more possible strategies than traditional classical game and
traditional quantum game. So besides those two games, more games have been
defined in different strategy spaces. All the games have been unified in the
new representation and their relation has been discussed. General Nash
Equilibrium for all the games has been proposed but without a general proof of
the existence. Besides the theoretical description, ideas and technics from
Statistical Physics, such as Kinetics Equation and Thermal Equilibrium can be
easily incorporated into Game Theory through such a representation. This
incorporation gives an endogenous method for refinement of Equilibrium State
and some hits to simplify the calculation of Equilibrium State. The more
privileges of this new representation depends on further application on more
theoretical and real games. Here, almost all ideas and conclusions are shown by
examples and argument, while, we wish, lately, we can give mathematical proof
for most results.


title: Cooling many particles at once

abstract:
  We propose a mechanism for the collective cooling of a large number N of
trapped particles to very low temperatures by applying red-detuned laser fields
and coupling them to the quantized field inside an optical resonator. The
dynamics is described by what appears to be rate equations, but where some of
the major quantities are coherences and not populations. The cooperative
behavior of the system provides cooling rates of the same order of magnitude as
the cavity decay rate kappa. This constitutes a significant speed-up compared
to other cooling mechanisms since kappa can, in principle, be as large as
square root of N times the single-particle cavity or laser coupling constant.


title: Finite-Time Disentanglement via Spontaneous Emission

abstract:
  We show that under the influence of pure vacuum noise two entangled qubits
become completely disentangled in a finite time, and in a specific example we
find the time to be given by $\ln \Big(\frac{2 +\sqrt 2}{2}\Big)$ times the
usual spontaneous lifetime.


title: Mixed state non-Abelian holonomy for subsystems

abstract:
  Non-Abelian holonomy in dynamical systems may arise in adiabatic transport of
energetically degenerate sets of states. We examine such a holonomy structure
for mixtures of energetically degenerate quantal states. We demonstrate that
this structure has a natural interpretation in terms of the standard
Wilczek-Zee holonomy associated with a certain class of Hamiltonians that
couple the system to an ancilla. The mixed state holonomy is analysed for
holonomic quantum computation using ion traps.


title: Efficient quantum computation within a disordered Heisenberg spin-chain

abstract:
  We show that efficient quantum computation is possible using a disordered
Heisenberg spin-chain with `always-on' couplings. Such disorder occurs
naturally in nanofabricated systems. Considering a simple chain setup, we show
that an arbitrary two-qubit gate can be implemented using just three
relaxations of a controlled qubit, which amounts to switching the on-site
energy terms at most twenty-one times.


title:
Local Versus Global Thermal States: Correlations and the Existence of
  Local Temperatures

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


title: Interpretation of Uncertainty Relations for Three or More Observables

abstract:
  Conventional quantum uncertainty relations (URs) contain dispersions of two
observables. Generalized URs are known which contain three or more dispersions.
They are derived here starting with suitable generalized Cauchy inequalities.
It is shown what new information the generalized URs provide. Similar
interpretation is given to generalized Cauchy inequalities.


title: Path integral formulation of quantized fields

abstract:
  Physical path integral formulation of motion of particles in Riemannian
spaces is outlined and extended to deduce the corresponding field theoretical
formulation. For the special case of a zero rest mass particle in Minkowski
manifold, it is shown that the underlying space can be reduced to
three-dimensional while similar formulation for a massive particle requires a
four-dimensional structure. The solutions obtained by the present procedure are
compared with the results obtained from its counterpart for a massive particle
by setting the mass equal to zero. Although similar, the present formulation
has some additional implications pertaining to the energy spectrum and the
associated free field quantization, particularly it yields zero vacuum energy.
The results are naturally extended to higher dimensions.


title:
A General Criterion of Quantum Integrability Accommodating Central
  Charges and Anomalies

abstract:
  A simple quantum generalisation of the Liouville-Arnold criterion of
classical integrability is proposed: A system is quantum-integrable if it has
an abelian Lie group of Wigner symmetries of dimension equal to the number of
degrees of freedom. The criterion goes significantly beyond the familiar case
of involutive conserved operators to cover systems with anomalies in which
involutivity is modified by central charges. "Anomalous" quantum integrability
is shown to have all the expected consequences including exact
diagonalisability. The approach throws new light on the origin of Weyl group
invariance.


title: Cliffordization, Spin and Fermionic Star Products

abstract:
  Deformation quantization is a powerful tool for quantizing theories with
bosonic and fermionic degrees of freedom. The star products involved generate
the mathematical structures which have recently been used in attempts to
analyze the algebraic properties of quantum field theory. In the context of
quantum mechanics they provide a canonical quantization procedure for systems
with either bosonic of fermionic degrees of freedom. We illustrate this
procedure for a number a physical examples, including bosonic, fermionic and
supersymmetric oscillators. We show how non-relativistic and relativistic
particles with spin can be naturally described in this framework.


title: Comments on Shimony's Analysis

abstract:
  Shimony's method of analysis does not distinguish adequately between a
legitimate assumption of no faster-than-light action in one direction and the
to-be-proved assertion of faster-than-light transfer of information in the
other direction. The virtue is noted of replacing the logical framework based
on counterfactual concepts by one based on the concept of fixed past open
future.


title:
A genetic algorithm for finding pulse sequences for NMR quantum
  computing

abstract:
  We present a genetic algorithm for finding a set of pulse sequences, or
rotations, for a given quantum logic gate, as implemented by NMR. We
demonstrate the utility of the method by showing that shorter sequences than
have been previously published can be found for both a CNOT and for the central
part of Shor's algorithm (for N=15.) Artificial intelligence techniques like
the genetic algorithm here presented have an enormous potential for simplifying
the implementation of working quantum computers.


title: Entanglement dynamics in chaotic systems

abstract:
  We study quantum chaos for systems with more than one degree of freedom, for
which we present an analysis of the dynamics of entanglement. Our analysis
explains the main features of entanglement dynamics and identifies
entanglement-based signatures of quantum chaos. We discuss entanglement
dynamics for a feasible experiment involving an atom in a magneto-optical trap
and compare the results with entanglement dynamics for the well-studied quantum
kicked top.


title:
Generalized Measurement and Conclusive Teleportation with Nonmaximal
  Entanglement

abstract:
  We present linear optical schemes to perform generalized measurements for
conclusive teleportation when the sender and the receiver share nonmaximal
entanglement resulting from amplitude errors during propagation or generation.
Three different cases are considered for which the states to be teleported are
unknown superpositions of (a) single-photon and vacuum states, (b)
vertically-polarized and horizontally-polarized photon states, and (c) two
coherent states of opposite phases. The generalized measurement scheme for each
case is analyzed, which indicates that the success probability is much more
resistant to amplitude errors for case (c) than for case (a) or (b).


title: Near-Complete Teleportation of a Superposed Coherent State

abstract:
  The four Bell-type entangled coherent states, |\alpha>|-\alpha> \pm |-\alpha>
|\alpha> and |\alpha>|\alpha> \pm |-\alpha> |-\alpha>, can be discriminated
with a high probability using only linear optical means, as long as |\alpha| is
not too small. Based on this observation, we propose a simple scheme to almost
completely teleport a superposed coherent state. The nonunitary transformation,
that is required to complete the teleportation, can be achieved by embedding
the receiver's field state in a larger Hilbert space consisting of the field
and a single atom and performing a unitary transformation on this Hilbert
space.


title: Comment on "Geometric phases for mixed states during cyclic evolutions"

abstract:
  It is shown that a recently suggested concept of mixed state geometric phase
in cyclic evolutions [2004 {\it J. Phys. A} {\bf 37} 3699] is gauge dependent.


title: 3D Quantum Trajectories. Quantum orbits of the Hydrogen's electron

abstract:
  In this paper, we introduced the 3D-Quantum Stationary Hamilton Jacobi
Equation for a central potential, and established the 3D quantum law of motion
of an electron in the presence of such a potential. We established a system of
three differential equations from which, and as a numerical application, we
plotted the 3D quantum trajectories of the Hydrogen's electron for the ground
state and two excited states. we did a comparison between a supposed classical
trajectory of the hydrogen electron and the corresponding QTs. We found out
that for a particular set of the hidden variables, the QT is close in its shape
to the classical one.


title: BCS-like Modewise Entanglement of Fermion Gaussian States

abstract:
  We show that with respect to any bipartite division of modes, pure fermion
gaussian states display the same type of structure in its entanglement of modes
as that of the BCS wave function, namely, that of a tensor product of entangled
two-mode squeezed fermion states. We show that this structure applies to a
wider class of "isotropic" mixed fermion states, for which we derive necessary
and sufficient conditions for mode entanglement.


title: Large geometric phases and non-elementary monopoles

abstract:
  Degeneracies in the spectrum of an adiabatically transported quantum system
are important to determine the geometrical phase factor, and may be interpreted
as magnetic monopoles. We investigate the mechanism by which constraints acting
on the system, related to local symmetries, can create arbitrarily large
monopole charges. These charges are associated with different geometries of the
degeneracy. An explicit method to compute the charge as well as several
illustrative examples are given.


title: Polarization Tensors and the Photon Field

abstract:
  A direct calculation of the elements of the photon polarization vector for
arbitrary momentum in the helicity basis shows that it is not a vector but a
complex bivector. The bivector real and imaginary parts can be directly equated
with electromagnetic field amplitudes and the associated field equations are
the Maxwell equations in time-imaginary space. The bivector field exhibits a
phase freedom (Berry, or geometric phase) dependent on the rotation history of
the field or observer. Phase freedom is not intrinsically present in the
longitudinal excitations of the field and a general argument connects
quantization of angular momentum with the observation of phase changes
associated with frame rotation. Current and translation operators can be
defined for bivector fields that are free of defects associated with a
quantized vector potential.


title: Entanglement flow in multipartite systems

abstract:
  We investigate entanglement dynamics in multipartite systems, establishing a
quantitative concept of entanglement flow: both flow through individual
particles, and flow along general networks of interacting particles. In the
former case, the rate at which a particle can transmit entanglement is shown to
depend on that particle's entanglement with the rest of the system. In the
latter, we derive a set of entanglement rate equations, relating the rate of
entanglement generation between two subsets of particles to the entanglement
already present further back along the network. We use the rate equations to
derive a lower bound on entanglement generation in qubit chains, and compare
this to existing entanglement creation protocols.


abstract:
  Notions of a Gaussian state and a Gaussian linear map are generalized to the
case of anticommuting (Grassmann) variables. Conditions under which a Gaussian
map is trace preserving and (or) completely positive are formulated. For any
Gaussian map an explicit formula relating correlation matrices of input and
output states is presented. This formalism allows to develop the Lagrangian
representation for fermionic linear optics (FLO). It covers both unitary
operations and the single-mode projectors associated with FLO measurements.
Using the Lagrangian representation we reduce a classical simulation of FLO to
a computation of Gaussian integrals over Grassmann variables. Explicit formulas
describing evolution of a quantum state under FLO operations are put forward.


title: Is Quantum Search Practical?

abstract:
  Quantum algorithms and circuits can, in principle, outperform the best
non-quantum (classical) techniques for some hard computational problems.
However, this does not necessarily lead to useful applications. To gauge the
practical significance of a quantum algorithm, one must weigh it against the
best conventional techniques applied to useful instances of the same problem.
Grover's quantum search algorithm is one of the most widely studied.
  We identify requirements for Grover's algorithm to be useful in practice: (1)
a search application S where classical methods do not provide sufficient
scalability; (2) an instantiation of Grover's algorithm Q(S) for S that has a
smaller asymptotic worst-case runtime than any classical algorithm C(S) for S;
(3) Q(S) with smaller actual runtime for practical instances of S than that of
any C(S).
  We show that several commonly-suggested applications fail to satisfy these
requirements, and outline directions for future work on quantum search.


title:
Connections between relative entropy of entanglement and geometric
  measure of entanglement

abstract:
  As two of the most important entanglement measures--the entanglement of
formation and the entanglement of distillation--have so far been limited to
bipartite settings, the study of other entanglement measures for multipartite
systems appears necessary. Here, connections between two other entanglement
measures--the relative entropy of entanglement and the geometric measure of
entanglement--are investigated. It is found that for arbitrary pure states the
latter gives rise to a lower bound on the former. For certain pure states, some
bipartite and some multipartite, this lower bound is saturated, and thus their
relative entropy of entanglement can be found analytically in terms of their
known geometric measure of entanglement. For certain mixed states, upper bounds
on the relative entropy of entanglement are also established. Numerical
evidence strongly suggests that these upper bounds are tight, i.e., they are
actually the relative entropy of entanglement.


title: An artificial game with equilibrium state of entangled strategy

abstract:
  Using the representation introduced in \cite{frame}, an artificial game in
quantum strategy space is proposed and studied. Although it has well-known
classical correspondence, which has classical mixture strategy Nash Equilibrium
states, the equilibrium state of this quantum game is an entangled strategy
(operator) state of the two players. By discovering such behavior, it partially
shows the independent meaning of the new representation. The idea of
entanglement of strategies, instead of quantum states, is proposed, and in some
sense, such entangled strategy state can be regarded as a cooperative behavior
between game players.


title: Adiabatic Evolution in XXX Spin Chain is Fast

abstract: This paper has been withdrawn.


title: The Physics of Information

abstract:
  Information theory is a statistical theory concerned with the relative state
of detectors and physical systems. As a consequence, the classical framework of
Shannon needs to be extended to deal with quantum detectors, possibly moving at
relativistic speeds, conceivably within curved space-time. Considerable
progress toward such a theory has been achieved in the last ten years, while
much is still not understood. This review recapitulates some milestones along
this road, and speculates about future ones.


title: On an Alternative Parametrization for the Theory of Complex Spectra

abstract:
  The purpose of this letter is threefold : (i) to derive, in the framework of
a new parametrization, some compact formulas of energy averages for the
electrostatic interaction within an (nl)N configuration, (ii) to describe a new
generating function for obtaining the number of states with a given spin
angular momentum in an (nl)N configuration, and (iii) to report some apparently
new sum rules, actually a by-product of (i), for SU(2) > U(1) coupling
coefficients.


title: On the exit statistics theorem of many particle quantum scattering

abstract:
  We review the foundations of the scattering formalism for one particle
potential scattering and discuss the generalization to the simplest case of
many non interacting particles. We point out that the "straight path motion" of
the particles, which is achieved in the scattering regime, is at the heart of
the crossing statistics of surfaces, which should be thought of as detector
surfaces. We sketch a proof of the relevant version of the many particle flux
across surfaces theorem and discuss what needs to be proven for the foundations
of scattering theory in this context.


title: A Robust Semidefinite Programming Approach to the Separability Problem

abstract:
  We express the optimization of entanglement witnesses for arbitrary bipartite
states in terms of a class of convex optimization problems known as Robust
Semidefinite Programs (RSDP). We propose, using well known properties of RSDP,
several new sufficient tests for the separability of mixed states. Our results
are then generalized to multipartite density operators.


title:
Environmental effects in the quantum-classical transition for the
  delta-kicked harmonic oscillator

abstract:
  We discuss the roles of the macroscopic limit and of different
system-environment interactions in the quantum-classical transition for a
chaotic system. We consider the kicked harmonic oscillator subject to
reservoirs that correspond in the classical case to purely dissipative or
purely diffusive behavior, in a situation that can be implemented in ion trap
experiments. In the dissipative case, we derive an expression for the time at
which quantum and classical predictions become different (breaking time) and
show that a complete quantum-classical correspondence is not possible in the
chaotic regime. For the diffusive environment we estimate the minimum value of
the diffusion coefficient necessary to retrieve the classical limit and also
show numerical evidence that, for diffusion below this threshold, the breaking
time behaves, essentially, as in the case of the system without a reservoir.


title: Coherent Control of Resonance-Mediated Reactions: F + HD

abstract:
  Cross sections resulting from scattering that proceeds via an intermediate
resonance are shown to be exceptionally controllable using a coherent
superposition of only two intial states. Full quantum computations on
F+HD(v=0;j=0,1) --> H+DF, D+HF, which exhibits a resonance in one of the
reactive channels, support the formal arguments, showing that control is indeed
vast. In this case the ratio of reactive integral cross sections can be altered
by a factor of 62 (compared to a noncoherent factor of only 3.3), while the
ratio of reactive differential cross sections can be altered by a factor of
over 6000 (compared to a noncoherent factor of less than 7). These results
constitute the first prediction of extensive quantum control in a collisional
process.


title: Decoherence alias Loschmidt echo of the environment

abstract:
  Entanglement between a quantum system and its environment leads to loss of
coherence in the former. In general, the temporal fate of coherences is
complicated. Here, we establish the connection between decoherence of a central
system and fidelity decay in the environment for a variety of situations,
including both, energy conserving and dissipative couplings. We show how
properties of unitary time evolution of the environment can be inferred from
the non-unitary evolution of coherences in the central system. This opens up
promising ways for measuring Loschmidt echoes in a variety of situations.


title:
Effect of ancilla's structure on quantum error correction using the
  7-qubit Calderbank-Shor-Steane code

abstract:
  In this work we discuss the ability of different types of ancillas to control
the decoherence of a qubit interacting with an environment. The error is
introduced into the numerical simulation via a depolarizing isotropic channel.
After the correction we calculate the fidelity as a quality criterion for the
qubit recovered. We observe that a recovery method with a three-qubit ancilla
provides reasonable good results bearing in mind its economy. If we want to go
further, we have to use fault-tolerant ancillas with a high degree of
parallelism, even if this condition implies introducing new ancilla
verification qubits.


title:
Strong many-particle localization and quantum computing with perpetually
  coupled qubits

abstract:
  We demonstrate the onset of strong on-site localization in a one-dimensional
many-particle system. The localization is obtained by constructing, in an
explicit form, a bounded sequence of on-site energies that eliminates resonant
hopping between both nearest and remote sites. This sequence leads to
quasi-exponential decay of the single-particle transition amplitude. It also
leads to strong localization of stationary many-particle states in a
finite-length chain. For an {\it infinite} chain, we instead study the time
during which {\it all} many-particle states remain strongly localized. We show
that, for any number of particles, this time exceeds the reciprocal frequency
of nearest-neighbor hopping by a factor $\sim 10^5$ already for a moderate
bandwidth of on-site energies. The proposed energy sequence is robust with
respect to small errors. The formulation applies to fermions as well as
perpetually coupled qubits. The results show viability of quantum computing
with time-independent qubit coupling.


title: Ensemble Quantum Computation with atoms in periodic potentials

abstract:
  We show how to perform universal quantum computation with atoms confined in
optical lattices which works both in the presence of defects and without
individual addressing. The method is based on using the defects in the lattice,
wherever they are, both to ``mark'' different copies on which ensemble quantum
computation is carried out and to define pointer atoms which perform the
quantum gates. We also show how to overcome the problem of scalability on this
system.


title: Finite-Element Simulation of Casimir Forces in Arbitrary Geometries

abstract:
  A 3D finite-element numerical simulation was developed to investigate Casimir
forces in arbitrary geometries. The code was verified comparing it with results
obtained from analytical equations. Appling the simulation to previously not
assessed configurations, new Casimir properties were found such as repulsive
Casimir forces in groove like structures.


title:
Unconditionally Secure Key Distribution In Higher Dimensions By
  Depolarization

abstract:
  This paper presents a prepare-and-measure scheme using $N$-dimensional
quantum particles as information carriers where $N$ is a prime power. One of
the key ingredients used to resist eavesdropping in this scheme is to
depolarize all Pauli errors introduced to the quantum information carriers.
Using the Shor-Preskill-type argument, we prove that this scheme is
unconditionally secure against all attacks allowed by the laws of quantum
physics. For $N = 2^n > 2$, each information carrier can be replaced by $n$
entangled qubits. In this case, there is a family of eavesdropping attacks on
which no unentangled-qubit-based prepare-and-measure quantum key distribution
scheme known to date can generate a provably secure key. In contrast, under the
same family of attacks, our entangled-qubit-based scheme remains secure
whenever $2^n \geq 4$. This demonstrates the advantage of using entangled
particles as information carriers and of using depolarization of Pauli errors
to combat eavesdropping attacks more drastic than those that can be handled by
unentangled-qubit-based prepare-and-measure schemes.


title:
Periodicity and Quasi-Periodicity for Super-Integrable Hamiltonian
  Systems

abstract:
  Classical trajectories are calculated for two Hamiltonian systems with ring
shaped potentials. Both systems are super-integrable, but not maximally
super-integrable, having four globally defined single valued integrals of
motion each. All finite trajectories are quasi-periodical; they become truly
periodical if a commensurability condition is imposed on an angular momentum
component.


title:
Improved Bounds on the Randomized and Quantum Complexity of
  Initial-Value Problems

abstract:
  We deal with the problem, initiated in [8], of finding randomized and quantum
complexity of initial-value problems. We showed in [8] that a speed-up in both
settings over the worst-case deterministic complexity is possible. In the
present paper we prove, by defining new algorithms, that further improvement in
upper bounds on the randomized and quantum complexity can be achieved. In the
H\"older class of right-hand side functions with r continuous bounded partial
derivatives, with r-th derivative being a H\"older function with exponent \rho,
the \epsilon-complexity is shown to be O((1/\epsilon)^{1/(r+\rho+1/3)}) in the
randomized setting, and O((1/\epsilon)^{1/(r+\rho+1/2)}) on a quantum computer
(up to logarithmic factors). This is an improvement for the general problem
over the results from [8]. The gap still remaining between upper and lower
bounds on the complexity is further discussed for a special problem. We
consider scalar autonomous problems, with the aim of computing the solution at
the end point of the interval of integration. For this problem, we fill up the
gap by establishing (essentially) matching upper and lower complexity bounds.
We show that the complexity in this case is of order
(1/\epsilon)^{1/(r+\rho+1/2)} in the randomized setting, and
(1/\epsilon)^{1/(r+\rho+1)} in the quantum setting (again up to logarithmic
factors).


title: Parametric Oscillation with Squeezed Vacuum Reservoirs

abstract:
  Employing the quantum Hamiltonian describing the interaction of two-mode
light (signal-idler modes) generated by a nondegenerate parametric oscillator
(NDPO) with two uncorrelated squeezed vacuum reservoirs (USVR), we derive the
master equation. The corresponding Fokker-Planck equation for the Q-function is
then solved employing a propagator method developed in Ref. \cite{1}. Making
use of this Q-function, we calculate the quadrature fluctuations of the optical
system. From these results we infer that the signal-idler modes are in squeezed
states and the squeezing occurs in the first quadrature. When the NDPO operates
below threshold we show that, for a large squeezing parameter, a squeezing
amounting to a noise suppression approaching 100% below the vacuum level in the
first quadrature can be achieved.


title: Entanglement storage in atomic ensembles

abstract:
  We propose to entangle macroscopic atomic ensembles in cavity using
EPR-correlated beams. We show how the field entanglement can be almost
perfectly mapped onto the long-lived atomic spins associated with the ground
states of the ensembles, and how it can be retrieved in the fields exiting the
cavities after a variable storage time. Such a continuous variable quantum
memory is of interest for manipulating entanglement in quantum networks.


title:
Generating Polarization-Entangled Photon Pairs with Arbitrary Joint
  Spectrum

abstract:
  We present a scheme for generating polarization-entangled photons pairs with
arbitrary joint spectrum. Specifically, we describe a technique for spontaneous
parametric down-conversion in which both the center frequencies and the
bandwidths of the down-converted photons may be controlled by appropriate
manipulation of the pump pulse. The spectral control offered by this technique
permits one to choose the operating wavelengths for each photon of a pair based
on optimizations of other system parameters (loss in optical fiber, photon
counter performance, etc.). The combination of spectral control, polarization
control, and lack of group-velocity matching conditions makes this technique
particularly well-suited for a distributed quantum information processing
architecture in which integrated optical circuits are connected by spans of
optical fiber.


title: Decay rate and decoherence control in coupled dissipative cavities

abstract:
  We give a detailed account of the derivation of a master equation for two
coupled cavities in the presence of dissipation. The analytical solution is
presented and physical limits of interest are discussed. Firstly we show that
the decay rate of initial coherent states can be significantly modified if the
two cavities have different decay rates and are weakly coupled through a wire.
Moreover, we show that also decoherence rates can be substantially altered by
manipulation of physical parameters. Conditions for experimental realizations
are discussed.


title:
An efficient algorithm to recognize local Clifford equivalence of graph
  states

abstract:
  In [Phys. Rev. A 69, 022316 (2004)] we presented a description of the action
of local Clifford operations on graph states in terms of a graph transformation
rule, known in graph theory as \emph{local complementation}. It was shown that
two graph states are equivalent under the local Clifford group if and only if
there exists a sequence of local complementations which relates their
associated graphs. In this short note we report the existence of a polynomial
time algorithm, published in [Combinatorica 11 (4), 315 (1991)], which decides
whether two given graphs are related by a sequence of local complementations.
Hence an efficient algorithm to detect local Clifford equivalence of graph
states is obtained.


title: Some Comments on Entanglement and Local Thermofield Theory

abstract:
  We combine recent results of Clifton and Halvorson [1] with structural
results of the author [2--5] concerning the local observables in thermofield
theory. An number of interesting consequences are discussed.


title: Implementing quantum information processing with atoms, ions and photons

abstract:
  Lecture Notes of the Les Houches Summer School 2003 on Quantum entanglement
and information processing to be published as P. Zoller, J. I. Cirac, Luming
Duan and J. J. Garcia-Ripoll, in "Quantum entanglement and information
processing", Proceedings of the Les Houches Summer School, Session 79, edited
by D. Est{\`e}ve, J.M. Raimond and J. Dalibard (Elsevier, Amsterdam, 2004).
  This is an updated although shortened version of J. I. Cirac, L. M. Duan, and
P. Zoller, in "Quantum optical implementation of quantum information
processing", Proceedings of the International School of Physics "Enrico Fermi",
Course CXLVIII, p. 263, edited by F. Di Martini and C. Monroe (IOS Press,
Amsterdam, 2002).


title: A complete characterization of phase space measurements

abstract:
  We characterize all the phase space measurements for a non-relativistic
particle.


title:
Scattering by one-dimensional smooth potentials: between WKB and Born
  approximation

abstract:
  The paper discusses the applicability of WKB and Born (small perturbations)
approximations in the problem of the backscattering of quantum particles and
classical waves by one-dimensional smooth potentials with amplitudes small
compared to the energy of the incident particle (above-barrier scattering).
Both deterministic and random potentials are considered. The dependence of the
reflection coefficient and localization length on the amplitude and the
longitudinal scale of the scattering potential is investigated. It is shown
that perturbation and WKB theories are inconsistent in the above-barrier
backscattering problem. Not only the solutions but the regions of validity of
both methods as well depend strongly on the details of the potential profile,
and are individual for each potential. A simple criterion that allows
determining the boundary between the applicability domains of WKB and Born
approximations is found.


title: Wave functions and characteristic times for transmission and reflection

abstract:
  We present a renewed wave-packet analysis based on the following ideas: if a
quantum one-particle scattering process and the corresponding state are
described by an indivisible wave packet to move as a whole at all stages of
scattering, then they are elementary; otherwise, they are combined; each
combined process consists from several alternative elementary ones to proceed
simultaneously; the corresponding (normed) state can be uniquely presented as
the sum of elementary ones whose (constant) norms give unit, in sum; Born's
formula intended for calculating the {\it expectation} values of physical
observables, as well as the standard timing procedure are valid only for
elementary states and processes; only an elementary time-dependent state can be
considered as the quantum counterpart to some classical one-particle rajectory.
By our approach, tunneling a non-relativistic particle through a static
one-dimensional potential barrier is a combined process consisting from two
elementary ones, transmission and reflection. In the standard setting of the
problem, we find an unique pair of solutions to the Schr\"odinger equation,
which describe separately transmission and reflection. On this basis we
introduce (exact and asymptotic) characteristic times for transmission and
reflection.


title: Mirror Inversion of Quantum States in Linear Registers

abstract:
  Transfer of data in linear quantum registers can be significantly simplified
with pre-engineered but not dynamically controlled inter-qubit couplings. We
show how to implement a mirror inversion of the state of the register in each
excitation subspace with respect to the centre of the register. Our
construction is especially appealing as it requires no dynamical control over
individual inter-qubit interactions. If, however, individual control of the
interactions is available then the mirror inversion operation can be performed
on any substring of qubits in the register. In this case a sequence of mirror
inversions can generate any permutation of a quantum state of the involved
qubits.


title: Quantum optical implementation of quantum information processing

abstract:
  By popular request we post these old (from 2001) lecture notes of the Varenna
Summer School Proceedings. The original was published as J. I. Cirac, L. M.
Duan, and P. Zoller, in "Experimental Quantum Computation and Information"
Proceedings of the International School of Physics "Enrico Fermi", Course
CXLVIII, p. 263, edited by F. Di Martini and C. Monroe (IOS Press, Amsterdam,
2002).


title: Statistical properties of random density matrices

abstract:
  Statistical properties of ensembles of random density matrices are
investigated. We compute traces and von Neumann entropies averaged over
ensembles of random density matrices distributed according to the Bures
measure. The eigenvalues of the random density matrices are analyzed: we derive
the eigenvalue distribution for the Bures ensemble which is shown to be broader
then the quarter--circle distribution characteristic of the Hilbert--Schmidt
ensemble. For measures induced by partial tracing over the environment we
compute exactly the two-point eigenvalue correlation function.


title: Quantum Prisoner's Dilemma in the new representation

abstract:
  Using the representation introduced in our another paper\cite{frame}, the
well-known Quantum Prisoner's Dilemma proposed in \cite{jens}, is reexpressed
and calculated. By this example and the works in \cite{frame} on classical
games and Quantum Penny Flip game, which first proposed in \cite{meyer}, we
show that our new representation can be a general framework for games
originally in different forms.


title: Quantum thermal waves in quantum corrals

abstract:
  In this paper the possibility of the generation of the thermal waves in 2D
electron gas is investigated. In the frame of the quantum heat transport theory
the 2D quantum hyperbolic heat transfer equation is formulated and numerically
solved. The obtained solutions are the thermal waves in electron 2D gases. As
an exapmle the thermal waves in quantum corrals are described.
  Key words: 2D electron gas, quantum corrals, thermal waves.


title: Orienting coupled quantum rotors by ultrashort laser pulses

abstract:
  We point out that the non-adiabatic orientation of quantum rotors, produced
by ultrashort laser pulses, is remarkably enhanced by introducing dipolar
interaction between the rotors. This enhanced orientation of quantum rotors is
in contrast with the behavior of classical paired rotors, in which dipolar
interactions prevent the orientation of the rotors. We demonstrate also that a
specially designed sequence of pulses can most efficiently enhances the
orientation of quantum paired rotors.


title: Comment on "Quantum dense key distribution"

abstract:
  In this Comment we question the security of recently proposed by Degiovanni
et al. [Phys. Rev. A 69 (2004) 032310] scheme of quantum dense key
distribution.


title:
Complementarity and Information in "Delayed-choice for entanglement
  swapping"

abstract:
  Building on Peres's idea of "Delayed-choice for extanglement swapping" we
show that even the degree to which quantum systems were entangled can be
defined after they have been registered and may even not exist any more. This
does not arise as a paradox if the quantum state is viewed as just a
representative of information. Moreover such a view gives a natural
quantification of the complementarity between the measure of information about
the input state for teleportation and the amount of entanglement of the
resulting swapped entangled state.


title: Inter-valley interactions in Si quantum dots

abstract:
  In this paper, we studied the inter-valley interactions between the orbital
functions associated with multi-valley of silicon (Si) quantum dots. Numerical
calculations show that the inter-valley coupling between orbital functions
increases rapidly with an applied electric field. We also considered the
potential applications to the quantum bit operation utilizing controlled
inter-valley interactions. Quantum bits are the multi-valley symmetric and
anti-symmetric orbitals. Evolution of these orbitals would be controlled by an
external electric field which turns on and off the inter-valley interactions.
Estimates of the decoherence time are made for the longitudinal acoustic phonon
process. Elementary single and two qubit gates are also proposed.


title:
Creating Boolean Functions for the Five-EPR-Pair,
  Single-Error-Correcting Code

abstract:
  A quantum single-error-correcting scheme can be derived from a one-way
entanglement purification protocol in purifying one Bell state from a finite
block of five Bell states. The main issue to be concerned with in the theory of
such an error-correction is to create specific linear Boolean functions that
can transform the sixteen error syndromes occurring in the error-correcting
code onto their mappings so that one Bell state is corrected whenever the other
four in the finite block are measured. The Boolean function is performed under
the effect of its associated sequence of basic quantum unilateral and bilateral
operations. Previously, the Boolean function is created in use of the Monte
Carlo computer search method. We introduce here a systematic scenario for
creating the Boolean function and its associated sequence of operations so that
we can do the job in an analytical way without any trial and error effort.
Consequently, all possible Boolean functions can in principle be created by
using our method. Furthermore, for a deduced Boolean function, we can also in
the spirit of our method derive its best associated sequence of operations
which may contain the least number of total operations or the least number of
the bilateral XOR operations alone. Some results obtained in this work show the
capability of our method in creating the Boolean function and its sequence of
operations.


title: Are All Particles Identical?

abstract:
  We consider the possibility that all particles in the world are fundamentally
identical, i.e., belong to the same species. Different masses, charges, spins,
flavors, or colors then merely correspond to different quantum states of the
same particle, just as spin-up and spin-down do. The implications of this
viewpoint can be best appreciated within Bohmian mechanics, a precise
formulation of quantum mechanics with particle trajectories. The implementation
of this viewpoint in such a theory leads to trajectories different from those
of the usual formulation, and thus to a version of Bohmian mechanics that is
inequivalent to, though arguably empirically indistinguishable from, the usual
one. The mathematical core of this viewpoint is however rather independent of
the detailed dynamical scheme Bohmian mechanics provides, and it amounts to the
assertion that the configuration space for N particles, even N
``distinguishable particles,'' is the set of all N-point subsets of physical
3-space.


title:
Effect of inter-subsystem couplings on the evolution of composite
  systems

abstract:
  The effect of inter-subsystem coupling on the adiabaticity of composite
systems and that of its subsystems is investigated. Similar to the adiabatic
evolution defined for pure states, non-transitional evolution for mixed states
is introduced; conditions for the non-transitional evolution are derived and
discussed. An example that describes two coupled qubits is presented to detail
the general presentation. The effects due to non-adiabatic evolution on the
geometric phase are also presented and discussed.


title: Generation of macroscopic superposition states with small nonlinearity

abstract:
  We suggest a scheme to generate a macroscopic superposition state
(Schrodinger cat state) of a free-propagating optical field using a beam
splitter, homodyne measurement and a very small Kerr nonlinear effect. Our
scheme makes it possible to considerably reduce the required nonlinear effect
to generate an optical cat state using simple and efficient optical elements.


title: Gates for the Kane Quantum Computer in the Presence of Dephasing

abstract:
  In this paper we investigate the effect of dephasing on proposed quantum
gates for the solid-state Kane quantum computing architecture. Using a simple
model of the decoherence, we find that the typical error in a CNOT gate is $8.3
\times 10^{-5}$. We also compute the fidelities of Z, X, Swap, and Controlled Z
operations under a variety of dephasing rates. We show that these numerical
results are comparable with the error threshold required for fault tolerant
quantum computation.


title:
Scheme for the implementation of optimal cloning of two pairs of
  orthogonal states

abstract:
  We propose a feasible scheme to implement the 1-to-2 optimal cloning
transformation for two pairs of orthogonal states of two-dimensional quantum
systems in the context of cavity QED. The copied qubits are shown to be
inseparable by using Peres-Horodecki criterion.


title:
Localized states ("qubits"), entanglement and decoherence from Wigner
  zoo

abstract:
  We present the application of the variational-wavelet approach to the
construction and analysis of solutions of Wigner/von Neumann/Moyal and related
equations corresponding to the nonlinear (polynomial) dynamical problems.
(Naive) deformation quantization, the multiresolution representation (exact
multiscale decompostion) and the variational approach are the key points. We
construct the solutions via the high-localized nonlinear eigenmodes in the base
of the compactly supported wavelets and the wavelet packets. We demonstrate the
appearance of (stable) localized patterns (waveletons) and consider
entanglement and decoherence as possible applications.


title: Multiparticle entanglement purification for two-colorable graph states

abstract:
  We investigate multiparticle entanglement purification schemes which allow
one to purify all two colorable graph states, a class of states which includes
e.g. cluster states, GHZ states and codewords of various error correction
codes. The schemes include both recurrence protocols and hashing protocols. We
analyze these schemes under realistic conditions and observe for a generic
error model that the threshold value for imperfect local operations depends on
the structure of the corresponding interaction graph, but is otherwise
independent of the number of parties. The qualitative behavior can be
understood from an analytically solvable model which deals only with a
restricted class of errors. We compare direct multiparticle entanglement
purification protocols with schemes based on bipartite entanglement
purification and show that the direct multiparticle entanglement purification
is more efficient and the achievable fidelity of the purified states is larger.
We also show that the purification protocol allows one to produce private
entanglement, an important aspect when using the produced entangled states for
secure applications. Finally we discuss an experimental realization of a
multiparty purification protocol in optical lattices which is issued to improve
the fidelity of cluster states created in such systems.


title: Characterization of two-qubit perfect entanglers

abstract:
  Here we consider perfect entanglers from another perspective. It is shown
that there are some {\em special} perfect entanglers which can maximally
entangle a {\em full} product basis. We have explicitly constructed a
one-parameter family of such entanglers together with the proper product basis
that they maximally entangle. This special family of perfect entanglers
contains some well-known operators such as {\textsc{cnot}} and
{\textsc{dcnot}}, but {\em not} ${\small{\sqrt{\rm{\textsc{swap}}}}}$. In
addition, it is shown that all perfect entanglers with entangling power equal
to the maximal value, 2/9, are also special perfect entanglers. It is proved
that the one-parameter family is the only possible set of special perfect
entanglers. Also we provide an analytic way to implement any arbitrary
two-qubit gate, given a proper special perfect entangler supplemented with
single-qubit gates. Such these gates are shown to provide a minimum universal
gate construction in that just two of them are necessary and sufficient in
implementation of a generic two-qubit gate.


title: Key distillation from Gaussian states by Gaussian operations

abstract:
  We study the secrecy properties of Gaussian states under Gaussian operations.
Although such operations are useless for quantum distillation, we prove that it
is possible to distill a secret key secure against any attack from sufficiently
entangled Gaussian states with non-positive partial transposition. Moreover,
all such states allow for key distillation, when Eve is assumed to perform
finite-size coherent attacks before the reconciliation process.


title:
Kinematic approach to off-diagonal geometric phases of nondegenerate and
  degenerate mixed states

abstract:
  Off-diagonal geometric phases have been developed in order to provide
information of the geometry of paths that connect noninterfering quantal
states. We propose a kinematic approach to off-diagonal geometric phases for
pure and mixed states. We further extend the mixed state concept proposed in
[Phys. Rev. Lett. {\bf 90}, 050403 (2003)] to degenerate density operators. The
first and second order off-diagonal geometric phases are analyzed for unitarily
evolving pairs of pseudopure states.


title: A bipartite class of entanglement monotones for N-qubit pure states

abstract:
  We construct a class of algebraic invariants for N-qubit pure states based on
bipartite decompositions of the system.
  We show that they are entanglement monotones, and that they differ from the
well know linear entropies of the sub-systems. They therefore capture new
information on the non-local properties of multipartite systems.


title: Demonstrating quantum algorithm acceleration with NMR quantum computer

abstract:
  In general, a quantum circuit is constructed with elementary gates, such as
one-qubit gates and CNOT gates. It is possible, however, to speed up the
execution time of a given circuit by merging those elementary gates together
into larger modules, such that the desired unitary matrix expressing the
algorithm is directly implemented. We demonstrate this by taking the two-qubit
Grover's algorithm implemented in NMR quantum computation, whose pseudopure
state is generated by cyclic permutations of the state populations. This is the
first exact time-optimal solution, to our knowledge, obtained for a
self-contained quantum algorithm.


title:
Nonclassical-light generation in a photonic band-gap nonlinear planar
  waveguide

abstract:
  Optical parametric process occurring in a photonic band-gap planar waveguide
is studied from the point of view of nonclassical-light generation. Nonlinearly
interacting optical fields are described by the generalized superposition of
coherent signals and noise using the method of operator linear corrections to a
classical strong solution. Scattered backward-propagating fields are taken into
account. Squeezed light as well as light with sub-Poissonian statistics can be
obtained in two-mode fields under the specified conditions.


title: Geometric phase distributions for open quantum systems

abstract:
  In an open system, the geometric phase should be described by a distribution.
We show that a geometric phase distribution for open system dynamics is in
general ambiguous, but the imposition of reasonable physical constraints on the
environment and its coupling with the system yields a unique geometric phase
distribution that applies even for mixed states, non-unitary dynamics, and
non-cyclic evolutions.


title: Matrix Element Randomness, Entanglement, and Quantum Chaos

abstract:
  We demonstrate the connection between an operator's matrix element
distribution and entangling power via numerical simulations of random,
pseudo-random, and quantum chaotic operators. Creating operators with a random
distribution of matrix elements is more difficult than creating operators that
reproduce other statistical properties of random matrices. Thus, operators that
fulfill many random matrix statistical properties may not generate states of
high multi-partite entanglement. To quantify the randomness of various
statistical distributions and, by extension, entangling power, we use
properties of interpolating ensembles that transition between integrable and
random matrix ensembles.


title:
Dynamically localized systems: entanglement exponential sensitivity and
  efficient quantum simulations

abstract:
  We study the pairwise entanglement present in a quantum computer that
simulates a dynamically localized system. We show that the concurrence is
exponentially sensitive to changes in the Hamiltonian of the simulated system.
Moreover, concurrence is exponentially sensitive to the ``logic'' position of
the qubits chosen. These sensitivities could be experimentally checked
efficiently by means of quantum simulations with less than ten qubits. We also
show that the feasibility of efficient quantum simulations is deeply connected
to the dynamical regime of the simulated system.


title: Realistic model for radiation-matter interaction

abstract:
  This paper presents a realistic model that describes radiation-matter
interactions. This is achieved by a generalization of first quantization, where
the Maxwell equations are interpreted as the electromagnetic component of the
Schroedinger equation. This picture is complemented by the consideration of
electrons and photons as real particles in three-dimensional space, following
guiding conditions derived from the particle-wave-functions to which they are
associated. The guiding condition for the electron is taken from Bohmian
mechanics, while the photon velocity is defined as the ratio between the
Poynting vector and the electromagnetic energy density. The case of many
particles is considered, taking into account their statistical properties. The
formalism is applied to a two level system, providing a satisfactory
description for spontaneous emission, Lamb shift, scattering, absorption,
dispersion, resonance fluorescence and vacuum fields. This model adequately
describes quantum jumps by the entanglement between the photon and the atomic
system and it will prove to be very useful in the simulation of quantum devices
for quantum computers and quantum information systems. A possible relativistic
generalization is presented, together with its relationship to QED.


title: Ghost imaging, quantum and classical aspects

abstract:
  We analytically show that it is possible to perform coherent imaging by using
the classical correlation of two beams obtained by splitting incoherent thermal
radiation. A formal analogy is demonstrated between two such classically
correlated beams and two entangled beams produced by parametric
down-conversion. Because of this analogy, the classical beams can mimic
qualitatively all the imaging properties of the entangled beams, even in ways
which up to now were not believed possible. A key feature is that these
classical beams are spatially correlated both in the near-field and in the
far-field. Using realistic numerical simulations the performances of a
quasi-thermal and a parametric down-conversion source are shown to be closely
similar, both for what concerns the resolution and statistical properties. The
results of this paper provide a new scenario for the discussion of what role
the entanglement plays in correlated imaging.


title: Understanding Popper's Experiment

abstract:
  An experiment proposed by Karl Popper is considered by many to be a crucial
test of quantum mechanics. Although many loopholes in the original proposal
have been pointed out, they are not crucial to the test. We use only the
standard interpretation of quantum mechanics to point out what is fundamentally
wrong with the proposal, and demonstrate that Popper's basic premise was
faulty.


title: Error in an argument regarding "improper" mixtures

abstract:
  An argument, perhaps originating with Feyerabend a half century ago, and
repeated many times since, purporting to establish that an "ignorance
interpretation" of a bipartite pure entangled state leads to logical
inconsistency, is incorrect: the argument fails to account for the effects of
indistinguishability.


title: Consistency of the Adiabatic Theorem

abstract:
  We review the quantum adiabatic approximation for closed systems, and its
recently introduced generalization to open systems (M.S. Sarandy and D.A.
Lidar, e-print quant-ph/0404147). We also critically examine a recent argument
claiming that there is an inconsistency in the adiabatic theorem for closed
quantum systems [K.P. Marzlin and B.C. Sanders, Phys. Rev. Lett. 93, 160408
(2004)] and point out how an incorrect manipulation of the adiabatic theorem
may lead one to obtain such an inconsistent result.


title:
Thermal entanglement of spins in the Heisenberg model at low
  temperatures

abstract:
  We calculate the entanglement between two spins in the ferromagnetic
Heisenberg chain at low temperatures, and show that when only the ground state
and the one particle states are populated, the entanglement profile is a
gaussian with a characteristic length depending on the temperature and the
coupling between spins. The magnetic field only affects the amplitude of the
profile and not its characteristic length.


title:
Complete quantum control of the population transfer branching ratio
  between two degenerate target states

abstract:
  A five-level four-pulse phase-sensitive extended stimulated Raman adiabatic
passage scheme is proposed to realize complete control of the population
transfer branching ratio between two degenerate target states. The control is
achieved via a three-node null eigenstate that can be correlated with an
arbitrary superposition of the target states. Our results suggest that complete
suppression of the yield of one of two degenerate product states, and therefore
absolute selectivity in photochemistry, is achievable and predictable, even
without studying the properties of the unwanted product state beforehand.


title:
Ordered and periodic chaos of the bounded one dimensinal multibarrier
  potential

abstract:
  Numerical analysis indicates that there exists an unexpected new ordered
chaos for the bounded one-dimensional multibarrier potential. For certain
values of the number of barriers, repeated identical forms (periods) of the
wavepackets result upon passing through the multibarrier potential.


title:
Separable Multipartite Mixed States - Operational Asymptotically
  Necessary and Sufficient Conditions

abstract:
  We introduce an operational procedure to determine, with arbitrary
probability and accuracy, optimal entanglement witness for every multipartite
entangled state. This method provides an operational criterion for separability
which is asymptotically necessary and sufficient. Our results are also
generalized to detect all different types of multipartite entanglement.


title: Stabilizer Codes for Continuous-variable Quantum Error Correction

abstract:
  A generalization of the stabilizer code construction presented by Gottesman
is described, which allows for the construction of quantum error-correcting
codes for continuous-variable systems. This formalism describes all
continuous-variable codes presented to date, and can be used to construct new
codes based on discrete-variable codes or classical codes. We use it to
describe the nine-mode code given by Lloyd and Slotine, and a five-mode code
described by Braunstein. In addition, we construct a new continuous-variable
code based an code of Gottesman which encodes three logical modes of
information into eight physical modes and corrects one error. This
generalization is a step toward an independent understanding of
continuous-variable quantum information and a theory of fault-tolerant quantum
information processing.


title: Analysis of Superoscillatory Wave Functions

abstract:
  Surprisingly, differentiable functions are able to oscillate arbitrarily
faster than their highest Fourier component would suggest. The phenomenon is
called superoscillation. Recently, a practical method for calculating
superoscillatory functions was presented and it was shown that superoscillatory
quantum mechanical wave functions should exhibit a number of counter-intuitive
physical effects. Following up on this work, we here present more general
methods which allow the calculation of superoscillatory wave functions with
custom-designed physical properties. We give concrete examples and we prove
results about the limits to superoscillatory behavior. We also give a simple
and intuitive new explanation for the exponential computational cost of
superoscillations.


title:
Adaptive phase estimation is more accurate than non-adaptive phase
  estimation for continuous beams of light

abstract:
  We consider the task of estimating the randomly fluctuating phase of a
continuous-wave beam of light. Using the theory of quantum parameter
estimation, we show that this can be done more accurately when feedback is used
(adaptive phase estimation) than by any scheme not involving feedback
(non-adaptive phase estimation) in which the beam is measured as it arrives at
the detector. Such schemes not involving feedback include all those based on
heterodyne detection or instantaneous canonical phase measurements. We also
demonstrate that the superior accuracy adaptive phase estimation is present in
a regime conducive to observing it experimentally.


title: Entanglement and quantum phase transition in the extended Hubbard model

abstract:
  We study quantum entanglement in one-dimensional correlated fermionic system.
Our results show, for the first time, that entanglement can be used to identify
quantum phase transitions in fermionic systems.


title: Dark evolution in a time-varying Zeno subspace

abstract:
  We investigate the evolution of a quantum system under the influence of
sequential measurements. The measurement scheme distinguishes whether or not
the system is in a specified state $| {f_n}>$ at the $n^{\rm th}$ step, where
$| {f_n}>$ varies with $n$. Dark evolution corresponds to the situation when
all measurements give negative results. We show that dark evolution is unitary
in the continuous measurement limit. We derive the effective Hamiltonian, and
indicate how $| {f_n}>$ controls quantum state transport.


title: On the principles of quantum mechanics

abstract:
  We propose six principles as the fundamental principles of quantum mechanics:
principle of space and time, Galilean principle of relativity, Hamilton's
principle, wave principle, probability principle, and principle of
indestructibility and increatiblity of particles. We deductively develop the
formalism of quantum mechanics on the basis of them: we determine the form of
the Lagrangian that satisfies requirements of these principles, and obtain the
Schroedinger equation from the Lagrangian. We also derive the canonical
commutation relations. Then we adopt the following four guide lines. First, we
do not premise the relations between dynamical variables in classical
mechanics. Second, we define energy, momentum, and angular momentum as the
constants of motion that are derived from homogeneity and isotropy in space and
time on the basis of principle of space and time. Since energy and momentum are
quantitatively defined in classical mechanics, we define them in quantum
mechanics so that the corresponding conservation laws are satisfied in a
coupling system of a quantum particle and a classical particle. Third, we
define Planck's constant and the mass of a particle as proportionality
constants between energy and frequency due to one of Einstein-de Broglie
formulas and between momentum and velocity, respectively. We shall obtain the
canonical commutation relations and the Schroedinger equation for a particle in
an external field in the definitive form. We shall also prove that relations
between dynamical variables in quantum mechanics have the same forms for those
in classical mechanics.


title: Discrete Wigner functions and quantum computational speedup

abstract:
  In [Phys. Rev. A 70, 062101 (2004)] Gibbons et al. defined a class of
discrete Wigner functions W to represent quantum states in a finite Hilbert
space dimension d. I characterize a set C_d of states having non-negative W
simultaneously in all definitions of W in this class. For d<6 I show C_d is the
convex hull of stabilizer states. This supports the conjecture that negativity
of W is necessary for exponential speedup in pure-state quantum computation.


title: Conditional Aharonov-Bohm Phases with Double Quantum Dots

abstract:
  A quantum dot proposal for the implementation of topological quantum
computation is presented. The coupling of the electron charge to an external
magnetic field via the Aharonov-Bohm effect, combined with the control dynamics
of a double dot, results in a two-qubit control phase gate. The physical
mechanisms of the system are analysed in detail and the conditions for
performing quantum computation resilient to control errors are outlined and
found to be realisable with present technology.


title:
s-ordered phase-sum and phase-difference distribuitons of entangled
  coherent states

abstract:
  The $s$-ordered phase-sum and phase-difference distributions are considered
for Bell-like superpositions of two-mode coherent states. The distributions are
sensitive, respectively, to the sum and difference of the phases of the
entangled coherent states. They show loss of information about the entangled
state and may take on negative values for some orderings $s$.


title:
Experimental Investigation of Nonideal Two-qubit Quantum-state Filter by
  Quantum Process Tomography

abstract:
  We used quantum process tomography to investigate and identify the function
of a nonideal two-qubit quantum-state filters subject to various degree of
decoherence. We present a simple decoherence model that explains the
experimental results and point out that a beamsplitter followed by a
post-selection process is not, as commonly believed, a singlet-state filter. In
the ideal case it is a triplet-state filter.


title:
Distillation of Entanglement between Distant Systems by Repeated
  Measurements on Entanglement Mediator

abstract:
  A recently proposed purification method, in which the Zeno-like measurements
of a subsystem can bring about a distillation of another subsystem in
interaction with the former, is utilized to yield entangled states between
distant systems. It is shown that the measurements of a two-level system
locally interacting with other two spatially separated not coupled subsystems,
can distill entangled states from the latter irrespectively of the initial
states of the two subsystems.


title: High performance guided-wave asynchronous heralded single photon source

abstract:
  We report on a guided wave heralded photon source based on the creation of
non-degenerate photon pairs by spontaneous parametric down conversion in a
Periodically Poled Lithium Niobate waveguide. Using the signal photon at 1310
nm as a trigger, a gated detection process permits announcing the arrival of
single photons at 1550 nm at the output of a single mode optical fiber with a
high probability of 0.38. At the same time the multi-photon emission
probability is reduced by a factor of 10 compared to poissonian light sources.
Relying on guided wave technologies such as integrated optics and fiber optics
components, our source offers stability, compactness and efficiency and can
serve as a paradigm for guided wave devices applied to quantum communication
and computation using existing telecom networks.


title: Remarks on Aharanov-Bohm effect and geometric phase

abstract:
  Recently the physical mechanism for geometric phase in optics has been
elucidated in terms of the angular momentum holonomy proposed in 1992. Aharonov
and Kaufherr (PRL, 92, 070404, 2004) revisit the Aharonov-Bohm effect, and
propose non-local exchange of a conserved, gauge invariant quantity that
changes the modular momentum of the particle that is responsible for the AB
phase shift. We suggest that the net angular momentum shifts proposed for GP
may be analogous to the shift in the modulus momentum for the AB effect. At a
single photon or electron level such non-trivial, geometric effects seem to
hint at a new physics.


title:
On the maximal dimension of a completely entangled subspace for finite
  level quantum systems

abstract:
  Let $\mathcal{H}_i$ be a finite dimensional complex Hilbert space of
dimension $d_i$ associated with a finite level quantum system $A_i$ for $i = i,
1,2, ..., k$. A subspace $S \subset \mathcal{H} = \mathcal{H}_{A_{1}
  A_{2}... A_{k}} = \mathcal{H}_1 \otimes \mathcal{H}_2 \otimes ... \otimes
\mathcal{H}_k $ is said to be {\it completely entangled} if it has no nonzero
product vector of the form $u_1 \otimes u_2 \otimes ... \otimes u_k$ with $u_i$
in $\mathcal{H}_i$ for each $i$. Using the methods of elementary linear algebra
and the intersection theorem for projective varieties in basic algebraic
geometry we prove that $$\max_{S \in \mathcal{E}} \dim S = d_1 d_2... d_k -
(d_1 + ... + d_k) + k - 1$$ where $\mathcal{E} $ is the collection of all
completely entangled subspaces.
  When $\mathcal{H}_1 = \mathcal{H}_2 $ and $k = 2$ an explicit orthonormal
basis of a maximal completely entangled subspace of $\mathcal{H}_1 \otimes
\mathcal{H}_2$ is given.
  We also introduce a more delicate notion of a {\it perfectly entangled}
subspace for a multipartite quantum system, construct an example using the
theory of stabilizer quantum codes and pose a problem.


title: A Class of Special Matrices and Quantum Entanglement

abstract:
  We present a kind of construction for a class of special matrices with at
most two different eigenvalues, in terms of some interesting multiplicators
which are very useful in calculating eigenvalue polynomials of these matrices.
This class of matrices defines a special kind of quantum states --
$d$-computable states. The entanglement of formation for a large class of
quantum mixed states is explicitly presented.


title: Distilling angular momentum nonclassical states in trapped ions

abstract:
  In the spirit of Quantum Non-Demolition Measurements, we show that exploiting
suitable vibronic couplings and repeatedly measuring the atomic population of a
confined ion, it is possible to distill center of mass vibrational states with
well defined square of angular momentum or, alternatively, angular momentum
projection Schr\"odinger cat states.


title: Initial states and decoherence of histories

abstract:
  We study decoherence properties of arbitrarily long histories constructed
from a fixed projective partition of a finite dimensional Hilbert space. We
show that decoherence of such histories for all initial states that are
naturally induced by the projective partition implies decoherence for arbitrary
initial states. In addition we generalize the simple necessary decoherence
condition [Scherer et al., Phys. Lett. A (2004)] for such histories to the case
of arbitrary coarse-graining.


title: Quantum Computing and Zeroes of Zeta Functions

abstract:
  A possible connection between quantum computing and Zeta functions of finite
field equations is described. Inspired by the 'spectral approach' to the
Riemann conjecture, the assumption is that the zeroes of such Zeta functions
correspond to the eigenvalues of finite dimensional unitary operators of
natural quantum mechanical systems. The notion of universal, efficient quantum
computation is used to model the desired quantum systems.
  Using eigenvalue estimation, such quantum circuits would be able to
approximately count the number of solutions of finite field equations with an
accuracy that does not appear to be feasible with a classical computer. For
certain equations (Fermat hypersurfaces) it is show that one can indeed model
their Zeta functions with efficient quantum algorithms, which gives some
evidence in favor of the proposal of this article.


title:
Quantum reverse-engineering and reference frame alignment without
  non-local correlations

abstract:
  Estimation of unknown qubit elementary gates and alignment of reference
frames are formally the same problem. Using quantum states made out of $N$
qubits, we show that the theoretical precision limit for both problems, which
behaves as $1/N^{2}$, can be asymptotically attained with a covariant protocol
that exploits the quantum correlation of internal degrees of freedom instead of
the more fragile entanglement between distant parties. This cuts by half the
number of qubits needed to achieve the precision of the dense covariant coding
protocol.


title: Secure Deterministic Communication Without Entanglement

abstract:
  We propose a protocol for deterministic communication that does not make use
of entanglement. It exploits nonorthogonal states in a two-way quantum channel
attaining significant improvement of security and efficiency over already known
cryptographic protocols. The presented scheme, being deterministic, can be
devoted to direct communication as well as to key distribution, and its
experimental realization is feasible with present day technology.


title: Minimal qubit tomography

abstract:
  We present, and analyze thoroughly, a highly symmetric and efficient scheme
for the determination of a single-qubit state, such as the polarization
properties of photons emitted by a single-photon source. In our scheme there
are only four measured probabilities, just enough for the determination of the
three parameters that specify the qubit state, whereas the standard procedure
would measure six probabilities.


title:
Scaling of entanglement in finite arrays of exchange-coupled quantum
  dots

abstract:
  We present a finite-size scaling analysis of the entanglement in a
two-dimensional arrays of quantum dots modeled by the Hubbard Hamiltonian on a
triangular lattice. Using multistage block renormalization group approach, we
have found that there is an abrupt jump of the entanglement when a first-order
quantum phase transition occurs. At the critical point, the entanglement is
constant, independent of the block size.


title: Quantum Color-Coding Is Better

abstract:
  We describe a quantum scheme to ``color-code'' a set of objects in order to
record which one is which. In the classical case, N distinct colors are
required to color-code N objects. We show that in the quantum case, only N/e
distinct ``colors'' are required, where e = 2.71828 . . . If the number of
colors is less than optimal, the objects may still be correctly distinguished
with some success probability less than 1. We show that the success probability
of the quantum scheme is better than the corresponding classical one and is
information-theoretically optimal.


title:
Scaling of entanglement at quantum phase transition for two-dimensional
  array of quantum dots

abstract:
  With Hubbard model, the entanglement scaling behavior in a two-dimensional
itinerant system is investigated. It has been found that, on the two sides of
the critical point denoting an inherent quantum phase transition (QPT), the
entanglement follows different scalings with the size just as an order
parameter does. This fact reveals the subtle role played by the entanglement in
QPT as a fungible physical resource.


title:
Size effects in the electronic properties of finite arrays of exchange
  coupled quantum dots: A renormalization group approach

abstract:
  Transport properties of arrays of metallic quantum dots are governed by the
distance-dependent exchange coupling between the dots. It is shown that the
effective value of the exchange coupling, as measured by the charging energy
per dot, depends monotonically on the size of the array. The effect saturates
for hexagonal arrays of over 7^5 unit cells. The discussion uses a multi-stage
block renormalization group approach applied to the Hubbard Hamiltonian. A
first order phase transition occurs upon compression of the lattice and the
size dependence is qualitatively different for the two phases.


title: Combinatorial Approaches in Quantum Information Theory

abstract:
  We investigate the exploitation of various combinatorial properties of graphs
and set systems to study several issues in quantum information theory. We
characterize the combinatorics of distributed EPR pairs for preparing
multi-partite entanglement in a real communication network. This combinatorics
helps in the study of various problems in multi-party case by just reducing to
the two-party case. Particularly, we use this combinatorics to (1) study
various possible and impossible transformations of multi-partite states under
LOCC, thus presenting an entirely new approach, not based on entropic
criterion, to study such state transformations. (2) present a protocol and
proof of its unconditional security for quantum key distribution amongst
several trusted parties. (3) propose an idea to combine the features of quantum
key distribution and quantum secret sharing. We investigate all the above
issues in great detail and finally conclude briefly with some open research
directions based on our research.


title: The spectrum of a quantum potential

abstract:
  We apply a method recently devised by one of the authors to obtain an
approximate analytical formula for the spectrum of a quantum anharmonic
potential. Due to its general features the method can be applied with minimal
effort to general quantum potentials thus allowing very promising applications.


title:
New intensity and visibility aspects of a double loop neutron
  interferometer

abstract:
  Various phase shifters and absorbers can be put into the arms of a double
loop neutron interferometer. The mean intensity levels of the forward and
diffracted beams behind an empty four plate interferometer of this type have
been calculated. It is shown that the intensities in the forward and diffracted
direction can be made equal using certain absorbers. In this case the
interferometer can be regarded as a 50/50 beam splitter. Furthermore the
visibilities of single and double loop interferometers are compared to each
other by varying the transmission in the first loop using different absorbers.
It can be shown that the visibility becomes exactly 1 using a phase shifter in
the second loop. In this case the phase shifter in the second loop must be
strongly correlated to the transmission coefficient of the absorber in the
first loop. Using such a device homodyne-like measurements of very weak signals
should become possible.


title:
Kinematic approach to the mixed state geometric phase in nonunitary
  evolution

abstract:
  A kinematic approach to the geometric phase for mixed quantal states in
nonunitary evolution is proposed. This phase is manifestly gauge invariant and
can be experimentally tested in interferometry. It leads to well-known results
when the evolution is unitary.


title: How Quantum Entanglement Helps to Coordinate Non-Communicating Players

abstract:
  We consider a coalitional game with the same payoff for all players. To
maximize the payoff, the players need to use one collective strategy, if all
players are in certain states, and the other strategy otherwise. The current
state of each player changes according to external conditions and is not known
to the other players. In one example of such a game, quantum entanglement
between players results in the optimal payoff thrice the maximal payoff for
unentangled players.


title: Experimental Quantum Cloning with Prior Partial Information

abstract:
  When prior partial information about a state to be cloned is available, it
can be cloned with a fidelity higher than that of universal quantum cloning. We
experimentally verify this intriguing relationship between the cloning fidelity
and the prior information by reporting the first experimental optimal quantum
state-dependent cloning, using nuclear magnetic resonance techniques. Our
experiments may further have important implications into many quantum
information processing protocols.


title:
Efficient use of quantum resources for the transmission of a reference
  frame

abstract:
  We propose a covariant protocol for transmitting reference frames encoded on
$N$ spins, achieving sensitivity $N^{-2}$ without the need of a pre-established
reference frame and without using entanglement between sender and receiver. The
protocol exploits the use of equivalent representations, which were overlooked
in the previous literature.


title: Witnessed Entanglement

abstract:
  We present a new measure of entanglement for mixed states. It can be
approximately computable for every state and can be used to quantify all
different types of multipartite entanglement. We show that it satisfies the
usual properties of a good entanglement quantifier and derive relations between
it and other entanglement measures.


title: Quantum key distribution with 1.25 Gbps clock synchronization

abstract:
  We have demonstrated the exchange of sifted quantum cryptographic key over a
730 meter free-space link at rates of up to 1.0 Mbps, two orders of magnitude
faster than previously reported results. A classical channel at 1550 nm
operates in parallel with a quantum channel at 845 nm. Clock recovery
techniques on the classical channel at 1.25 Gbps enable quantum transmission at
up to the clock rate. System performance is currently limited by the timing
resolution of our silicon avalanche photodiode detectors. With improved
detector resolution, our technique will yield another order of magnitude
increase in performance, with existing technology.