title:
Anomalies of Wave-Particle Duality due to Translational-Internal
  Entanglement

abstract:
  We predict that if internal and momentum states of an interfering object are
correlated (entangled), then by measuring its internal state we may infer both
path (corpuscular) and phase (wavelike) information with much higher precision
than for objects lacking such entanglement. We thereby partly circumvent the
standard complementarity constraints of which-path detection.


title: Radiation reaction and quantum damped harmonic oscillator

abstract:
  By taking a Klein-Gordon field as the environment of an harmonic oscillator
and using a new method for dealing with quantum dissipative systems (minimal
coupling method), the quantum dynamics and radiation reaction for a quantum
damped harmonic oscillator investigated. Applying perturbation method, some
transition probabilities indicating the way energy flows between oscillator,
reservoir and quantum vacuum, obtained


title:
Electromagnetic field quantization in a linear polarizable and
  magnetizable medium

abstract:
  By modeling a linear polarizable and magnetizable medium (magneto-dielectric)
with two quantum fields, namely E and M, electromagnetic field is quantized in
such a medium consistently and systematically. A Hamiltonian is proposed from
which, using the Heisenberg equations, Maxwell and constitutive equations of
the medium are obtained. For a homogeneous medium, the equation of motion of
the quantum vector potential, $\vec{A}$, is derived and solved analytically.
Two coupling functions which describe the electromagnetic properties of the
medium are introduced. Four examples are considered showing the features and
the applicability of the model to both absorptive and nonabsorptive
magneto-dielectrics.


title: Quantum information transport to multiple receivers

abstract:
  The importance of transporting quantum information and entanglement with high
fidelity cannot be overemphasized. We present a scheme based on adiabatic
passage that allows for transportation of a qubit, operator measurements and
entanglement, using a 1-D array of quantum sites with a single sender (Alice)
and multiple receivers (Bobs). Alice need not know which Bob is the receiver,
and if several Bobs try to receive the signal, they obtain a superposition
state which can be used to realize two-qubit operator measurements for the
generation of maximally entangled states.


title:
Hidden Variables, Non Contextuality and Einstein-Locality in Quantum
  Mechanics

abstract:
  We discuss the problem of hidden variables and the motivation for
introducting them in quantum mechanics. These include determinism, and the
problem of meassurement and incompleteness. We first discuss Von-Neumann's
imposisbility proof and then analyse it's weakness in terms of Bell's explicit
hidden variable model of spin one-half particles. We next discuss Gleason's
theorem and Kochen-Specker theorem and bring out the troublems with non
contextual hidden variable theories. An important role is played by Einstein
locality in the discussion of hidden variable theories as was first brought out
by Einstein, Podolsky and Rosen. We elaborate it's various implications such as
Bell's theorem in terms of Bell's inequalities as well as later work in which
Bell's theorem follows without using inequalities.


title: Quantification of complementarity in multi-qubit systems

abstract:
  Complementarity was originally introduced as a qualitative concept for the
discussion of properties of quantum mechanical objects that are classically
incompatible. More recently, complementarity has become a \emph{quantitative}
relation between classically incompatible properties, such as visibility of
interference fringes and "which-way" information, but also between purely
quantum mechanical properties, such as measures of entanglement. We discuss
different complementarity relations for systems of 2-, 3-, or \textit{n}
qubits. Using nuclear magnetic resonance techniques, we have experimentally
verified some of these complementarity relations in a two-qubit system.


title: Entanglement measurement with discrete multiple coin quantum walks

abstract:
  Within a special multi-coin quantum walk scheme we analyze the effect of the
entanglement of the initial coin state. For states with a special entanglement
structure it is shown that this entanglement can be meausured with the mean
value of the walk, which depends on the i-concurrence of the initial coin
state. Further on the entanglement evolution is investigated and it is shown
that the symmetry of the probability distribution is reflected by the symmetry
of the entanglement distribution.


title:
Mean-field dynamics of a Bose-Einstein condensate in a time-dependent
  triple-well trap: Nonlinear eigenstates, Landau-Zener models and STIRAP

abstract:
  We investigate the dynamics of a Bose--Einstein condensate (BEC) in a
triple-well trap in a three-level approximation. The inter-atomic interactions
are taken into account in a mean-field approximation (Gross-Pitaevskii
equation), leading to a nonlinear three-level model. New eigenstates emerge due
to the nonlinearity, depending on the system parameters. Adiabaticity breaks
down if such a nonlinear eigenstate disappears when the parameters are varied.
The dynamical implications of this loss of adiabaticity are analyzed for two
important special cases: A three level Landau-Zener model and the STIRAP
scheme. We discuss the emergence of looped levels for an equal-slope
Landau-Zener model. The Zener tunneling probability does not tend to zero in
the adiabatic limit and shows pronounced oscillations as a function of the
velocity of the parameter variation. Furthermore we generalize the STIRAP
scheme for adiabatic coherent population transfer between atomic states to the
nonlinear case. It is shown that STIRAP breaks down if the nonlinearity exceeds
the detuning.


title: Pseudopotential method for higher partial wave scattering

abstract:
  We present a zero-range pseudopotential applicable for all partial wave
interactions between neutral atoms. For p- and d-waves we derive effective
pseudopotentials, which are useful for problems involving anisotropic external
potentials. Finally, we consider two nontrivial applications of the p-wave
pseudopotential: we solve analytically the problem of two interacting
spin-polarized fermions confined in a harmonic trap, and analyze the scattering
of p-wave interacting particles in a quasi-two-dimensional system.


title:
What is the second law of thermodynamics and are there any limits to its
  validity?

abstract:
  In the scientific and engineering literature, the second law of
thermodynamics is expressed in terms of the behavior of entropy in reversible
and irreversible processes. According to the prevailing statistical mechanics
interpretation the entropy is viewed as a nonphysical statistical attribute, a
measure of either disorder in a system, or lack of information about the
system, or erasure of information collected about the system, and a plethora of
analytic expressions are proposed for the various measures. In this paper, we
present two expositions of thermodynamics (both 'revolutionary' in the sense of
Thomas Kuhn with respect to conventional statistical mechanics and traditional
expositions of thermodynamics) that apply to all systems (both macroscopic and
microscopic, including single particle or single spin systems), and to all
states (thermodynamic or stable equilibrium, nonequilibrium, and other states).


title: Diffusion-induced Ramsey narrowing

abstract:
  A novel form of Ramsey narrowing is identified and characterized. For
long-lived coherent atomic states coupled by laser fields, the diffusion of
atoms in-and-out of the laser beam induces a spectral narrowing of the atomic
resonance lineshape. Illustrative experiments and an intuitive analytical model
are presented for this diffusion-induced Ramsey narrowing, which occurs
commonly in optically-interrogated systems.


title: |0>|1>+|1>|0>

abstract:
  I give a simple argument that demonstrates that the state |0>|1>+|1>|0>, with
|0> denoting a state with 0 particles and |1> a 1-particle state, is entangled
in spite of recent claims to the contrary. I also discuss new viewpoints on the
old controversy about whether the above state can be said to display
single-particle or single-photon nonlocality.


title: Quantum algorithm for a generalized hidden shift problem

abstract:
  Consider the following generalized hidden shift problem: given a function f
on {0,...,M-1} x Z_N satisfying f(b,x)=f(b+1,x+s) for b=0,1,...,M-2, find the
unknown shift s in Z_N. For M=N, this problem is an instance of the abelian
hidden subgroup problem, which can be solved efficiently on a quantum computer,
whereas for M=2, it is equivalent to the dihedral hidden subgroup problem, for
which no efficient algorithm is known. For any fixed positive epsilon, we give
an efficient (i.e., poly(log N)) quantum algorithm for this problem provided M
> N^epsilon. The algorithm is based on the "pretty good measurement" and uses
H. Lenstra's (classical) algorithm for integer programming as a subroutine.


title: An Effective Two-component Entanglement in Double-well Condensation

abstract:
  We propose a spin-half approximation method for two-component condensation in
double wells to discuss the quantum entanglement of two components. This
approximation is presented to be valid under stationary tunneling effect for
odd particle number of each component. The evolution of the entanglement is
found to be affected by the particle number both quantitatively and
qualitatively. In detail, the maximal entanglement are shown to be hyperbolic
like with respect to tunneling rate and time. To successively obtain large and
long time sustained entanglement, the particle number should not be large.


title: Adaptive Phase Measurements in Linear Optical Quantum Computation

abstract:
  Photon counting induces an effective nonlinear optical phase shift on certain
states derived by linear optics from single photons. Although this no
nlinearity is nondeterministic, it is sufficient in principle to allow scalable
linear optics quantum computation (LOQC). The most obvious way to encode a
qubit optically is as a superposition of the vacuum and a single photon in one
mode -- so-called "single-rail" logic. Until now this approach was thought to
be prohibitively expensive (in resources) compared to "dual-rail" logic where a
qubit is stored by a photon across two modes. Here we attack this problem with
real-time feedback control, which can realize a quantum-limited phase
measurement on a single mode, as has been recently demonstrated experimentally.
We show that with this added measurement resource, the resource requirements
for single-rail LOQC are not substantially different from those of dual-rail
LOQC. In particular, with adaptive phase measurements an arbitrary qubit state
$\alpha \ket{0} + \beta\ket{1}$ can be prepared deterministically.


title: Quantum basin hopping with gradient-based local optimisation

abstract:
  The quantum basin hopping algorithm for continuous global optimisation
combines a local search with Grover's algorithm, and can locate the global
optimum using effort proportional to the square root of the number of basins.
This article establishes that Jordan's quantum gradient estimation method can
be incorporated into the quantum basin hopper, providing an extra acceleration
proportional to the domain dimension.


title: On decaying rate of a quantum state

abstract:
  Decaying rate of a quantum system investigated using the Fubini-Study
definition of distance between states.


title: A minimal coupling method for dissipative quantum systems

abstract:
  Quantum dynamics of a general dissipative system investigated by its coupling
to a Klein-Gordon type field as the environment by introducing a minimal
coupling method. As an example, the quantum dynamics of a damped three
dimensional harmonic oscillator investigated and some transition probabilities
indicating the way energy flows between the subsystems obtained. The quantum
dynamics of a dissipative two level system considered.


abstract:
  On the basis of quant-ph/0405028 we define the Larmor times for transmission
and reflection. These times are valid both for the stationary and
time-dependent scattering processes, without any restrictions on the shape of
Gaussian-like wave packets. We show that in the stationary case both the Larmor
times coincide with the corresponding dwell times obtained in quant-ph/0502073.
The Larmor-time concept gives the way to verify the approach presented in
quant-ph/0405028.


title: Quantum limit of measurement and the projection postulate

abstract:
  We study an electrostatic qubit monitored by a point-contact detector.
Projecting an entire qubit-detector wave function on the detector eigenstates
we determine the precision limit for the qubit measurements, allowed by quantum
mechanics. We found that this quantity is determined by qubit dynamics as well
as decoherence, generated by the measurement. Our results show how the quantum
precision limit can be improved by a proper design of a measurement procedure.


title: Asymmetries in symmetric quantum walks on two-dimensional networks

abstract:
  We study numerically the behavior of continuous-time quantum walks over
networks which are topologically equivalent to square lattices. On short time
scales, when placing the initial excitation at a corner of the network, we
observe a fast, directed transport through the network to the opposite corner.
This transport is not ballistic in nature, but rather produced by quantum
mechanical interference. In the long time limit, certain walks show an
asymmetric limiting probability distribution; this feature depends on the
starting site and, remarkably, on the precise size of the network. The limiting
probability distributions show patterns which are correlated with the initial
condition. This might have consequences for the application of continuous time
quantum walk algorithms.


title: Quasi-exactly solvable periodic potentials with three known eigenstates

abstract:
  Supersymmetric method of the constructing well-like quasi exactly solvable
(QES) potentials with three known eigenstates has been extended to the case of
periodic potentials. The explicit examples are presented. New QES potential
with two known eigenstates has been obtained.


title: Symmetry, model reduction, and quantum mechanics

abstract:
  Taking several statistical examples, in particular one involving a choice of
experiment, as points of departure, and making symmetry assumptions, the link
towards quantum theory developed in Helland (2005a,b) is surveyed and
clarified. The quantum Hilbert space is constructed from the parameters of the
various experiments using group representation theory. It is shown under
natural assumptions that a subset of the set of unit vectors of this space, the
generalized coherent state vectors, can be put in correspondence with questions
of the kind: What is the value of the (complete) parameter? - together with a
crisp answer to that question. Links are made to statistical models in general,
to model reduction of overparametrized models and to the design of experiments.
It turns out to be essential that the range of the statistical parameter is an
invariant set under the relevant symmetry group.


title: Fermi-Dirac statistics and the number theory

abstract:
  We relate the Fermi-Dirac statistics of an ideal Fermi gas in a harmonic trap
to partitions of given integers into distinct parts, studied in number theory.
Using methods of quantum statistical physics we derive analytic expressions for
cumulants of the probability distribution of the number of different
partitions.


title:
How to Test for Diagonalizability: The Discretized PT-Invariant
  Square-Well Potential

abstract:
  Given a non-hermitean matrix M, the structure of its minimal polynomial
encodes whether M is diagonalizable or not. This note will explain how to
determine the minimal polynomial of a matrix without going through its
characteristic polynomial. The approach is applied to a quantum mechanical
particle moving in a square well under the influence of a piece-wise constant
PT-symmetric potential. Upon discretizing the configuration space, the system
is decribed by a matrix of dimension three. It turns out not to be
diagonalizable for a critical strength of the interaction, also indicated by
the transition of two real into a pair of complex energy eigenvalues. The
systems develops a three-fold degenerate eigenvalue, and two of the three
eigenfunctions disappear at this exceptional point, giving a difference between
the algebraic and geometric multiplicity of the eigenvalue equal to two.


title:
Hilbert-Schmidt Separability Probabilities and Noninformativity of
  Priors

abstract:
  The Horodecki family employed the Jaynes maximum-entropy principle, fitting
the mean (b_{1}) of the Bell-CHSH observable (B). This model was extended by
Rajagopal by incorporating the dispersion (\sigma_{1}^2) of the observable, and
by Canosa and Rossignoli, by generalizing the observable (B_{\alpha}). We
further extend the Horodecki one-parameter model in both these manners,
obtaining a three-parameter (b_{1},\sigma_{1}^2,\alpha) two-qubit model, for
which we find a highly interesting/intricate continuum (-\infty < \alpha <
\infty) of Hilbert-Schmidt (HS) separability probabilities -- in which, the
golden ratio is featured. Our model can be contrasted with the three-parameter
(b_{q}, \sigma_{q}^2,q) one of Abe and Rajagopal, which employs a
q(Tsallis)-parameter rather than $\alpha$, and has simply q-invariant HS
separability probabilities of 1/2. Our results emerge in a study initially
focused on embedding certain information metrics over the two-level quantum
systems into a q-framework. We find evidence that Srednicki's recently-stated
biasedness criterion for noninformative priors yields rankings of priors fully
consistent with an information-theoretic test of Clarke, previously applied to
quantum systems by Slater.


title: Distributed quantum computation via optical fibres

abstract:
  We investigate the possibility of realising effective quantum gates between
two atoms in distant cavities coupled by an optical fibre. We show that highly
reliable swap and entangling gates are achievable. We exactly study the
stability of these gates in presence of imperfections in coupling strengths and
interaction times and prove them to be robust. Moreover, we analyse the effect
of spontaneous emission and losses and show that such gates are very promising
in view of the high level of coherent control currently achievable in optical
cavities.


title: Quantum phenomena may have macroscopic counterparts

abstract:
  From various points of view it is argued that one may find phenomena similar
to the quantum effects also in macroscopic cases. This forces one to give up as
a general requirement the assumption of realism as formulated by Gill and
others. For any potential set of experiments on a limited set of units, we find
it useful to introduce for these units the concept of a total parameter, a set
of parameters which is so large that a joint value is meaningless.


title: Combinatorics of boson normal ordering and some applications

abstract:
  We provide the solution to the normal ordering problem for powers and
exponentials of two classes of operators. The first one consists of boson
strings and more generally homogeneous polynomials, while the second one treats
operators linear in one of the creation or annihilation operators. Both
solutions generalize Bell and Stirling numbers arising in the number operator
case. We use the advanced combinatorial analysis to provide closed form
expressions, generating functions, recurrences, etc. The analysis is based on
the Dobi\'nski-type relations and the umbral calculus methods. As an
illustration of this framework we point out the applications to the
construction of generalized coherent states, operator calculus and ordering of
deformed bosons.


title: One-Dimensional Three-State Quantum Walk

abstract:
  We study a generalized Hadamard walk in one dimension with three inner
states. The particle governed by the three-state quantum walk moves, in
superposition, both to the left and to the right according to the inner state.
In addition to these two degrees of freedom, it is allowed to stay at the same
position. We calculate rigorously the wavefunction of the particle starting
from the origin for any initial qubit state, and show the spatial distribution
of probability of finding the particle. In contrast with the Hadamard walk with
two inner states on a line, the probability of finding the particle at the
origin does not converge to zero even after infinite time steps except special
initial states. This implies that the particle is trapped near the origin after
long time with high probability.


title: Topology of the three-qubit space of entanglement types

abstract:
  The three-qubit space of entanglement types is the orbit space of the local
unitary action on the space of three-qubit pure states, and hence describes the
types of entanglement that a system of three qubits can achieve. We show that
this orbit space is homeomorphic to a certain subspace of R^6, which we
describe completely. We give a topologically based classification of
three-qubit entanglement types, and we argue that the nontrivial topology of
the three-qubit space of entanglement types forbids the existence of standard
states with the convenient properties of two-qubit standard states.


title:
Model-Theoretic Investigations into Consequence Operation (Cn) in
  Quantum Logics. An Algebraic Approach

abstract:
  In this paper we present the fundamentals of the so-called algebraic approach
to propositional quantum logics. We define the set of formulas describing
quantum reality as a free algebra freely generated by the set of quantum
propositional variables. We define the general notion of logic as a structural
consequence operation. Next we introduce the concept of logical matrices
understood as a models of quantum logics. We give the definitions of two
quantum consequence operations defined in these models.


title: Microscopic Theory of Spontaneous Decay in a Dielectric

abstract:
  The local field correction to the spontanous dacay rate of an impurity source
atom imbedded in a disordered dielectric is calculated to second order in the
dielectric density. The result is found to differ from predictions associated
with both "virtual" and "real" cavity models of this decay process. However, if
the contributions from two dielectric atoms at the same position are included,
the virtual cavity result is reproduced.


title: Hamiltonian Based nRules, Time's Arrow

abstract:
  The auxiliary rules of quantum mechanics can be written without the Born rule
by using what are called the nRules. The nRules are understood in part by
making certain modifications in the Hamiltonian. In this paper, those
modifications are written directly into the nRules, reducing their number from
four to three. It is shown that the nRules in either form provide for a
definite direction in time, guaranteeing that a statistically irreversible
interaction is absolutely irreversible.


title:
Entanglement versus mixedness for coupled qubits under a phase damping
  channel

abstract:
  Quantification of entanglement against mixing is given for a system of
coupled qubits under a phase damping channel. A family of pure initial joint
states is defined, ranging from pure separable states to maximally entangled
state. An ordering of entanglement measures is given for well defined initial
state amount of entanglement.


title: A method to find quantum noiseless subsystems

abstract:
  We develop a structure theory for decoherence-free subspaces and noiseless
subsystems that applies to arbitrary (not necessarily unital) quantum
operations. The theory can be alternatively phrased in terms of the
superoperator perspective, or the algebraic noise commutant formalism. As an
application, we propose a method for finding all such subspaces and subsystems
for arbitrary quantum operations. We suggest that this work brings the
fundamental passive technique for error correction in quantum computing an
important step closer to practical realization.


title:
Dialogue Concerning Two Views on Quantum Coherence: Factist and
  Fictionist

abstract:
  A controversy that has arisen many times over in disparate contexts is
whether quantum coherences between eigenstates of certain quantities are fact
or fiction. We present a pedagogical introduction to the debate in the form of
a hypothetical dialogue between proponents from each of the two camps: a
factist and a fictionist. A resolution of the debate can be achieved, we argue,
by recognizing that quantum states do not only contain information about the
intrinsic properties of a system but about its extrinsic properties as well,
that is, about its relation to other systems external to it. Specifically, the
coherent quantum state of the factist is the appropriate description of the
relation of the system to one reference frame, while the incoherent quantum
state of the fictionist is the appropriate description of the relation of the
system to another, uncorrelated, reference frame. The two views, we conclude,
are alternative but equally valid paradigms of description.


title: Atemporal diagrams for quantum circuits

abstract:
  A system of diagrams is introduced that allows the representation of various
elements of a quantum circuit, including measurements, in a form which makes no
reference to time (hence ``atemporal''). It can be used to relate quantum
dynamical properties to those of entangled states (map-state duality), and
suggests useful analogies, such as the inverse of an entangled ket. Diagrams
clarify the role of channel kets, transition operators, dynamical operators
(matrices), and Kraus rank for noisy quantum channels. Positive (semidefinite)
operators are represented by diagrams with a symmetry that aids in
understanding their connection with completely positive maps. The diagrams are
used to analyze standard teleportation and dense coding, and for a careful
study of unambiguous (conclusive) teleportation. A simple diagrammatic argument
shows that a Kraus rank of 3 is impossible for a one-qubit channel modeled
using a one-qubit environment in a mixed state.


title: Interconvertibility of single-rail optical qubits

abstract:
  We show how to convert between partially coherent superpositions of a single
photon with the vacuum using linear optics and postselection based on homodyne
measurements. We introduce a generalized quantum efficiency for such states and
show that any conversion that decreases this quantity is possible. We also
prove that our scheme is optimal by showing that no linear optical scheme with
generalized conditional measurements, and with one single-rail qubit input can
improve the generalized efficiency.


title: Quantum control limited by quantum decoherence

abstract:
  We describe quantum controllability under the influences of the quantum
decoherence induced by the quantum control itself. It is shown that, when the
controller is considered as a quantum system, it will entangle with its
controlled system and then cause quantum decoherence in the controlled system.
In competition with this induced decoherence, the controllability will be
limited by some uncertainty relation in a well-armed quantum control process.
In association with the phase uncertainty and the standard quantum limit, a
general model is studied to demonstrate the possibility of realizing a
decoherence-free quantum control with a finite energy within a finite time. It
is also shown that if the operations of quantum control are to be determined by
the initial state of the controller, then due to the decoherence which results
from the quantum control itself, there exists a low bound for quantum
controllability.


title: Can We Believe in a Purely Unitary Quantum Dynamics?

abstract:
  It is pointed out that the question of a purely unitary quantum dynamics
amounts to the question if von Neumann entropy of a dynamically closed quantum
system is preserved in evolution.


title:
Large alphabet quantum key distribution with two-mode coherently
  correlated beams

abstract:
  The large-alphabet quantum cryptography protocol based on the two-mode
coherently correlated multi-photon beams is proposed. The alphabet extension
for the protocol is shown to result in the increase of the QKD effectiveness
and security.


title: When champions meet: Rethinking the Bohr--Einstein debate

abstract:
  Einstein's philosophy of physics (as clarified by Fine, Howard, and Held) was
predicated on his Trennungsprinzip, a combination of separability and locality,
without which he believed objectification, and thereby "physical thought" and
"physical laws", to be impossible. Bohr's philosophy (as elucidated by Hooker,
Scheibe, Folse, Howard, Held, and others), on the other hand, was grounded in a
seemingly different doctrine about the possibility of objective knowledge,
namely the necessity of classical concepts. In fact, it follows from Raggio's
Theorem in algebraic quantum theory that - within an appropriate class of
physical theories - suitable mathematical translations of the doctrines of Bohr
and Einstein are equivalent. Thus - upon our specific formalization - quantum
mechanics accommodates Einstein's Trennungsprinzip if and only if it is
interpreted a la Bohr through classical physics. Unfortunately, the
protagonists themselves failed to discuss their differences in this
constructive way, since their debate was dominated by Einstein's ingenious but
ultimately flawed attempts to establish the "incompleteness" of quantum
mechanics.
  This aspect of their debate may still be understood and appreciated, however,
as reflecting a much deeper and insurmountable disagreement between Bohr and
Einstein on the knowability of Nature. Using the theological controversy on the
knowability of God as a analogy, Einstein was a Spinozist, whereas Bohr could
be said to be on the side of Maimonides. Thus Einstein's off-the-cuff
characterization of Bohr as a 'Talmudic philosopher' was spot-on.


title:
Security bound of two-bases quantum key-distribution protocols using
  qudits

abstract:
  We investigate the security bounds of quantum cryptographic protocols using
$d$-level systems. In particular, we focus on schemes that use two mutually
unbiased bases, thus extending the BB84 quantum key distribution scheme to
higher dimensions. Under the assumption of general coherent attacks, we derive
an analytic expression for the ultimate upper security bound of such quantum
cryptography schemes. This bound is well below the predictions of optimal
cloning machines. The possibility of extraction of a secret key beyond
entanglement distillation is discussed. In the case of qutrits we argue that
any eavesdropping strategy is equivalent to a symmetric one. For higher
dimensions such an equivalence is generally no longer valid.


title: Path Integrals, and Classical and Quantum Constraints

abstract:
  Systems with constraints pose problems when they are quantized. Moreover, the
Dirac procedure of quantization prior to reduction is preferred. The projection
operator method of quantization, which can be most conveniently described by
coherent state path integrals, enables one to directly impose a regularized
form of the quantum constraints. This procedure also overcomes conventional
difficulties with normalization and second class constraints that invalidate
conventional Dirac quantization procedures.


title: Generalized concurrence and limits of separability for two qutrits

abstract:
  We present an extension of the Wootters concurrence for the case of two
qutrits in mixed states. The reduction of our extension to the case of two
levels shows complete agreement with Wootters concurrence for two qubits. As an
explicit example, we compute the concurrence for a family of symmetric states
and we obtain the bounds on the limit for separability. Our results are
compared with those of the negativity.


title: What is the Fastest Speed at which a Single Electron Can Be Detected?

abstract:
  Electrometers measure electric charge, but there must be a fundamental speed
limit to measuring one electric charge. Since there are no dimensional inputs
to this question, the answer must be expressible in terms of the fundamental
physical constants of Nature, e,h,m,c. In general the question should be posed
without reference to any specific technology, but for definiteness, we analyze
the field effect transistor, which is essentially an electrometer. In spite of
selecting a specific technology, we find that the speed limit is related to a
fundamental constant, the Rydberg frequency, or as appropriate, the
semiconductor Rydberg frequency including the electron effective mass, and the
relative dielectric constant. We do not know whether the Rydberg frequency
represents the upper speed limit, but on dimensional grounds we claim that the
final limit can only differ by some power of the fine-structure-constant.


title: The state |IV> (Babel of entanglement)

abstract:
  Maybe active discussions about entanglement in quantum information science
demonstrate some immaturity of this rather young area. So recent tries to look
for more accurate ways of classification devote rather encouragement than
criticism.


title: Initial Decoherence and Loss of Entanglement of Open Two-Qubit Systems

abstract:
  In this paper we investigate a open two-qubit model whose dynamics is not
exactly solvable. When the initial state is the maximum entangled state, as the
exactly solvable open two-qubit model [D. Tolkunov and V. Privman, Phys. Rev. A
71, 060308(R) (2005)], the decay of entanglement of formation of the model,
expressed by concurrence is also governed by the product of suppression factors
describing decoherence of the subsystems (qubits). However, if the initial
state is not the maximum entangled state, its concurrence will decrease faster
than the product of the suppression factors describing decoherence of the
qubits.


title: Violating Bell Inequalities Maximally for Two $d$-Dimensional Systems

abstract:
  We investigate the maximal violation of Bell inequalities for two
$d$-dimensional systems by using the method of Bell operator. The maximal
violation corresponds to the maximal eigenvalue of the Bell operator matrix.
The eigenvectors corresponding to these eigenvalues are described by asymmetric
entangled states. We estimate the maximum value of the eigenvalue for large
dimension. A family of elegant entangled states $|\Psi>_{\rm app}$ that violate
Bell inequality more strongly than the maximally entangled state but are
somewhat close to these eigenvectors is presented. These approximate states can
potentially be useful for quantum cryptography as well as many other important
fields of quantum information.


title: Thermal corrections in Casimir interaction between metal and dielectric

abstract:
  The Casimir interaction between two thick parallel plates, one made of metal
and the other of dielectric, is investigated at nonzero temperature. It is
shown that in some temperature intervals the Casimir pressure and the free
energy of a fluctuating field are the nonmonotonous functions of temperature
and the respective Casimir entropy can be negative. The physical interpretation
for these conclusions is given. At the same time we demonstrate that the
entropy vanishes when the temperature goes to zero, i.e., in the Casimir
interaction between metal and dielectric the Nernst heat theorem is satisfied.
The investigation is performed both analytically, by using the model of an
ideal metal and dilute dielectric or dielectric with a frequency-independent
dielectric permittivity, and numerically for real metal (Au) and dielectrics
with different behavior of the dielectric permittivity along the imaginary
frequency axis (Si and Al_2O_3).


title: Coherent evolution via reservoir driven holonomy

abstract:
  We show that in the limit of strongly interacting environment a system
initially prepared in a Decoherence Free Subspace (DFS) coherently evolves in
time, adiabatically following the changes of the DFS. If the reservoir cyclicly
evolves in time, the DFS states acquire an holonomy.


title:
Quantum mechanical description of measurement and the basic properties
  of state transformation

abstract:
  In this paper, without any priori assumption about the post-measurement state
of system, we will examine how this state is restricted by assuming each of
these following assumptions. First, by using this reasonable assumption that
two successive measurements should be describable as one measurement. Second,
by assuming the impossibility of faster than light signaling, "No-signaling
condition". However, only by using these assumptions it is not possible to
obtain the usual projection postulate. Instead, by means of a simple lemma, we
will show that the density operator of system after a measurement is a linear
function of the density operator determined by the usual post-measurement
postulate. Furthermore we will show this linear function has a Kraus
representation. Finally, we will discuss about the physical meaning of this
consequence.


title: Algebras of Measurements: the logical structure of Quantum Mechanics

abstract:
  In Quantum Physics, a measurement is represented by a projection on some
closed subspace of a Hilbert space. We study algebras of operators that
abstract from the algebra of projections on closed subspaces of a Hilbert
space. The properties of such operators are justified on epistemological
grounds. Commutation of measurements is a central topic of interest. Classical
logical systems may be viewed as measurement algebras in which all measurements
commute. Keywords: Quantum measurements, Measurement algebras, Quantum Logic.
PACS: 02.10.-v.


title:
Universal quantum computation on the power of quantum non-demolition
  measurements

abstract:
  In this paper we investigate the linear and nonlinear models of optical
quantum computation and discuss their scalability and efficiency. We show how
there are significantly different scaling properties in single photon
computation when weak cross-Kerr nonlinearities are allowed to supplement the
usual linear optical set. In particular we show how quantum non-demolition
measurements are an efficient resource for universal quantum computation.


title: Alternative Algebraic Structures from Bi-Hamiltonian Quantum Systems

abstract:
  We discuss the alternative algebraic structures on the manifold of quantum
states arising from alternative Hermitian structures associated with quantum
bi-Hamiltonian systems. We also consider the consequences at the level of the
Heisenberg picture in terms of deformations of the associative product on the
space of observables.


title: Grover's Quantum Search Algorithm and Diophantine Approximation

abstract:
  In a fundamental paper [Phys. Rev. Lett. 78, 325 (1997)] Grover showed how a
quantum computer can find a single marked object in a database of size N by
using only O(N^{1/2}) queries of the oracle that identifies the object. His
result was generalized to the case of finding one object in a subset of marked
elements. We consider the following computational problem: A subset of marked
elements is given whose number of elements is either M or K, M<K, our task is
to determine which is the case. We show how to solve this problem with a high
probability of success using only iterations of Grover's basic step (and no
other algorithm). Let m be the required number of iterations; we prove that
under certain restrictions on the sizes of M and K the estimation m <
(2N^{1/2})/(K^{1/2}-M^{1/2}) obtains. This bound sharpens previous results and
is known to be optimal up to a constant factor. Our method involves
simultaneous Diophantine approximations, so that Grover's algorithm is
conceptualized as an orbit of an ergodic automorphism of the torus. We comment
on situations where the algorithm may be slow, and note the similarity between
these cases and the problem of small divisors in classical mechanics.


title:
Upper bound on the secret key rate distillable from effective quantum
  correlations with imperfect detectors

abstract:
  We provide a simple method to obtain an upper bound on the secret key rate
that is particularly suited to analyze practical realizations of quantum key
distribution protocols with imperfect devices. We consider the so-called
trusted device scenario where Eve cannot modify the actual detection devices
employed by Alice and Bob. The upper bound obtained is based on the available
measurements results, but it includes the effect of the noise and losses
present in the detectors of the legitimate users.


title: Quantum search processes in the cyclic group state spaces

abstract:
  The hardness to solve an unstructured quantum search problem by a standard
quantum search algorithm mainly originates from the low efficiency to amplify
the amplitude of the marked state by the oracle unitary operation associated
with other known quantum operations. In order to bypass the square speedup
limitation of a standard quantum search algorithm it is necessary to develop
other type of quantum search algorithms. It is described in detail in the paper
for a quantum dynamical method to solve the quantum search problems in the
cyclic group state space. The binary dynamical representation for a quantum
state in the Hilbert space of the n-qubit quantum system is generalized to the
multi-base dynamical representation for a quantum state in the cyclic group
state space. Thus, any quantum state in the cyclic group state space may be
described completely in terms of a set of dynamical parameters that are closely
related to the symmetric property and structure of the cyclic group. The
quantum search problem therefore could be solved by determining the set of
dynamical parameters that describe completely the unknown marked state of the
search problem instead by directly measuring the marked state which is a
necessary step in the standard quantum search algorithm. An unstructured
quantum search problem in the Hilbert space is inevitably affected greatly by
the symmetric property and structure of a group. The main attempt of the paper
is to make use of the symmetric properties and structures of groups to help
solving the quantum search problems in the group state spaces. It is shown how
the quantum search process could be reduced from the cyclic group state space
to these cyclic group state subspaces with the help of the symmetric property
and structure of the cyclic group on a universal quantum computer.


title: Quantum pure-state identification

abstract:
  We address a problem of identifying a given pure state with one of two
reference pure states, when no classical knowledge on the reference states is
given, but a certain number of copies of them are available. We assume the
input state is guaranteed to be either one of the two reference states. This
problem, which we call quantum pure state identification, is a natural
generalization of the standard state discrimination problem. The two reference
states are assumed to be independently distributed in a unitary invariant way
in the whole state space. We give a complete solution for the averaged maximal
success probability of this problem for an arbitrary number of copies of the
reference states in general dimension. It is explicitly shown that the obtained
mean identification probability approaches the mean discrimination probability
as the number of the reference copies goes to infinity.


title: A frequency-doubled, pulsed laser system for rubidium manipulation

abstract:
  We have constructed a pulsed laser system for the manipulation of cold Rb
atoms. The system combines optical telecommunications components and frequency
doubling to generate light at 780 nm. Using a fast, fibre-coupled intensity
modulator, output from a continuous laser diode is sliced into pulses with a
length between 1.3 and 6.1 ns and a repetition frequency of 5 MHz. These pulses
are amplified using an erbium-doped fibre amplifier, and frequency-doubled in a
periodically poled lithium niobate crystal, yielding a peak power up to 12 W.
Using the resulting light at 780 nm, we demonstrate Rabi oscillations on the F
= 2 <-> F=3-transition of a single 87Rb atom.


title: Interpretation of Electron Tunneling from Uncertainty Principle

abstract:
  Beginners studying quantum mechanics are often baffled with electron
tunneling.Hence an easy approach for comprehension of the topic is presented
here on the basis of uncertainty principle.An estimate of the tunneling time is
also derived from the same method.


title: Demonstration of quantum telecloning of optical coherent states

abstract:
  Quantum cryptography promises in-principle secure communication between two
parties via a quantum channel, with the ability to discover eavesdropping when
it occurs. In 1999, a telecloning protocol was invented [M. Murao {\it et al}.,
Phys. Rev. A {\bf 59}, 156 (1999)] that provides a way for an eavesdropper to
remotely monitor a quantum cryptographic channel such that even if
eavesdropping is discovered, the identity and location of the eavesdropper is
guaranteed uncompromised. Here we demonstrate unconditional telecloning
experimentally for the first time. We symmetrically teleclone coherent states
of light, achieving a fidelity for each clone of $F = 0.58 \pm 0.01$.


title: A Femtosecond Nanometer Free Electron Source

abstract:
  We report a source of free electron pulses based on a field emission tip
irradiated by a low-power femtosecond laser. The electron pulses are shorter
than 70 fs and originate from a tip with an emission area diameter down to 2
nm. Depending on the operating regime we observe either photofield emission or
optical field emission with up to 200 electrons per pulse at a repetition rate
of 1 GHz. This pulsed electron emitter, triggered by a femtosecond oscillator,
could serve as an efficient source for time-resolved electron interferometry,
for time-resolved nanometric imaging and for synchrotrons.


title: Are Quantum States Exponentially Long Vectors?

abstract:
  I'm grateful to Oded Goldreich for inviting me to the 2005 Oberwolfach
Meeting on Complexity Theory. In this extended abstract, which is based on a
talk that I gave there, I demonstrate that gratitude by explaining why
Goldreich's views about quantum computing are wrong.


title:
Calculations of time-dependent observables in non-Hermitian quantum
  mechanics: The problem and a possible solution

abstract:
  The solutions of the time independent Schrodinger equation for non-Hermitian
(NH) Hamiltonians have been extensively studied and calculated in many
different fields of physics by using L^2 methods that originally have been
developed for the calculations of bound states. The existing non-Hermitian
formalism breaks down when dealing with wavepackets(WP). An open question is
how time dependent expectation values can be calculated when the Hamiltonian is
NH ? Using the F-product formalism, which was recently proposed, [J. Phys.
Chem., 107, 7181 (2003)] we calculate the time dependent expectation values of
different observable quantities for a simple well known study test case model
Hamiltonian. We carry out a comparison between these results with those
obtained from conventional(i.e., Hermitian) quantum mechanics (QM)
calculations. The remarkable agreement between these results emphasizes the
fact that in the NH-QM, unlike standard QM, there is no need to split the
entire space into two regions; i.e., the interaction region and its
surrounding. Our results open a door for a type of WP propagation calculations
within the NH-QM formalism that until now were impossible.


title: Rapid refractive index enhancements via laser-mediated collectivity

abstract:
  The collective interaction via the environmental vacuum is investigated for a
mixture of two deviating multi-atom ensembles in a moderately intense laser
field. Due to the numerous inter-atomic couplings, the laser-dressed system may
react sensitively and rapidly with respect to changes in the atomic and laser
parameters. We show for weak probe fields that in the absence of absorption
both the index of refraction and the group velocity may be modified strongly
and rapidly due to the collectivity.


title: Quantum correlations of an atomic ensemble via a classical bath

abstract:
  Somewhat surprisingly, quantum features can be extracted from a classical
bath. For this, we discuss a sample of three-level atoms in ladder
configuration interacting only via the surrounding bath, and show that the
fluorescence light emitted by this system exhibits non-classical properties.
Typical realizations for such an environment are thermal baths for microwave
transition frequencies, or incoherent broadband fields for optical transitions.
In a small sample of atoms, the emitted light can be switched from sub- to
super-poissonian and from anti-bunching to super-bunching controlled by the
mean number of atoms in the sample. Larger samples allow to generate
super-bunched light over a wide range of bath parameters and thus fluorescence
light intensities. We also identify parameter ranges where the fields emitted
on the two transitions are correlated or anti-correlated, such that the
Cauchy-Schwarz inequality is violated. As in a moderately strong baths this
violation occurs also for larger numbers of atoms, such samples exhibit
mesoscopic quantum effects.


title: PT-supersymmetric partner of a short-range square well

abstract:
  In a box of size $L$, a spatially antisymmetric square-well potential of a
purely imaginary strength ${\rm i}g$ and size $l < L$ is interpreted as an
initial element of the SUSY hierarchy of solvable Hamiltonians, the energies of
which are all real for $g < g_c(l)$. The first partner potential is constructed
in closed form and discussed.


title:
Room Temperature Lasing of InAs/GaAs Quantum Dots in the Whispering
  Gallery Modes of a Silica Microsphere

abstract:
  We have achieved low threshold lasing of self-assembled InAs/GaAs quantum
dots coupled to the evanescent wave of the high-$Q$ whispering gallery modes of
a silica microsphere. In spite of high temperature and $Q$-spoiling of
whispering gallery modes due to diffusion and refraction on the high index
semiconductor sample, room temperature lasing is obtained with less than 100
quantum dots. This result highlights the feasibility and interest of combining
self-assembled quantum dots and microspheres in view of cavity-quantum
electrodynamics experiments.


title: Quantum History cannot be Copied

abstract:
  We show that unitarity does not allow cloning of any two points in a ray.
This has implication for cloning of the geometric phase information in a
quantum state. In particular, the quantum history which is encoded in the
geometric phase during cyclic evolution of a quantum system cannot be copied.
We also prove that the generalized geometric phase information cannot be copied
by a unitary operation. We argue that our result also holds in the consistent
history formulation of quantum mechanics.


title: Continuous Variable Solution for Byzantine Agreement

abstract:
  We demonstrate that the Byzantine Agreement Problem (BAP) in its weaker
version with detectable broadcast, can be solved using continuous variables
Gaussian states with Gaussian operations. The protocol uses genuine tripartite
symmetric entanglement, but differs from protocols proposed for qutrits or
qubits. Contrary to the quantum key distribution (QKD) which is possible with
all Gaussian states, for the BAP entanglement is needed, but not all tripartite
entangled symmetric states can be used to solve the problem.


title:
An Exactly Solvable Model of an Avalanche-Type Measuring Device:
  Macroscopic Distinctiveness and Wavefunction Collapse

abstract:
  A quantum many-body model is presented with features similar to those of
certain particle detectors. The energy spectrum contains a single metastable
'ready'-state and macroscopically-distinct 'pointer' states. Measurements do
not pose paradoxes or require interventions outside the field theory formalism.
Transitions into classical-like states can be triggered by a single particle
with help of the thermal bath. Schroedinger cat states are associated with
superpositions of inequivalent vacua, thus relating wavefunction collapse to
the dynamics of symmetry breaking in phase transformations.


title:
Constraints for quantum logic arising from conservation laws and field
  fluctuations

abstract:
  We explore the connections between the constraints on the precision of
quantum logical operations that arise from a conservation law, and those
arising from quantum field fluctuations. We show that the conservation-law
based constraints apply in a number of situations of experimental interest,
such as Raman excitations, and atoms in free space interacting with the
multimode vacuum. We also show that for these systems, and for states with a
sufficiently large photon number, the conservation-law based constraint
represents an ultimate limit closely related to the fluctuations in the quantum
field phase.


title: Nonunitary Quantum Theory with a Field Cutoff

abstract:
  We consider a scalar quantum field theory, in which the interaction takes the
form of a field cutoff; the energy diverges to infinity whenever the value of
the field at some point falls outside a finite interval. In a simple
(1+1)-dimensional version of this theory, we may calculate the results of
certain scattering processes exactly. The main feature of the nontrivial
solutions is the appearance of shock fronts, whose time development is
irreversible. The resulting nonunitarity implies that these theories are, at a
minimum, radically different from conventional quantum field theories.


title: Genuine Multipartite Entanglement in Quantum Phase Transitions

abstract:
  We demonstrate that the Global Entanglement (GE) measure defined by Meyer and
Wallach, J. Math. Phys. 43, 4273 (2002), is maximal at the critical point for
the Ising chain in a transverse magnetic field. Our analysis is based on the
equivalence of GE to the averaged linear entropy, allowing the understanding of
multipartite entanglement (ME) features through a generalization of GE for
bipartite blocks of qubits. Moreover, in contrast to GE, the proposed ME
measure can distinguish three paradigmatic entangled states: $GHZ_{N}$,
$W_{N}$, and $EPR^{\otimes N/2}$. As such the generalized measure can detect
genuine ME and is maximal at the critical point.


title: Quantum Arnol'd diffusion in a rippled waveguide

abstract:
  We study the quantum Arnol'd diffusion for a particle moving in a quasi-1D
waveguide bounded by a periodically rippled surface, in the presence of the
time-periodic electric field. It was found that in a deep semiclassical region
the diffusion-like motion occurs for a particle in the region corresponding to
a stochastic layer surrounding the coupling resonance. The rate of the quantum
diffusion turns out to be less than the corresponding classical one, thus
indicating the influence of quantum coherent effects. Another result is that
even in the case when such a diffusion is possible, it terminates in time due
to the mechanism similar to that of the dynamical localization. The quantum
Arnol'd diffusion represents a new type of quantum dynamics, and may be
experimentally observed in measurements of a conductivity of low-dimensional
mesoscopic structures.


title:
Ballistic and localized transport for the atom optics kicked rotor in
  the limit of vanishing kicking period

abstract:
  We present mean energy measurements for the atom optics kicked rotor as the
kicking period tends to zero. A narrow resonance is observed marked by
quadratic energy growth, in parallel with a complete freezing of the energy
absorption away from the resonance peak. Both phenomena are explained by
classical means, taking proper account of the atoms' initial momentum
distribution.


title:
An approximate method for treating dispersion in one-way quantum
  channels

abstract:
  Coupling the output of a source quantum system into a target quantum system
is easily treated by cascaded systems theory if the intervening quantum channel
is dispersionless. However, dispersion may be important in some transfer
protocols, especially in solid-state systems. In this paper we show how to
generalize cascaded systems theory to treat such dispersion, provided it is not
too strong. We show that the technique also works for fermionic systems with a
low flux, and can be extended to treat fermionic systems with large flux. To
test our theory, we calculate the effect of dispersion on the fidelity of a
simple protocol of quantum state transfer. We find good agreement with an
approximate analytical theory that had been previously developed for this
example.


title:
Manifestations the hidden symmetry of Coulomb problem in the
  relativistic quantum mechanics - from Pauli to Dirac electron

abstract:
  The theorem known from Pauli equation about operators that anticommute with
Dirac's $K$-operator is generalized to the Dirac equation. By means of this
theorem the operator is constructed which governs the hidden symmetry in
relativistic Coulomb problem (Dirac equation). It is proved that this operator
coincides with the familiar Johnson-Lippmann one and is intimately connected to
the famous Laplace-Runge-Lenz (LRL) vector. Our derivation is very simple and
informative. It does not require a longtime and tedious calculations, as is
offten underlined in most papers.


title: Quantum Ordered Binary Decision Diagrams with Repeated Tests

abstract:
  Quantum branching programs (quantum binary decision diagrams, respectively)
are a convenient tool for examining quantum computations using only a
logarithmic amount of space. Recently several types of restricted quantum
branching programs have been considered, e. g. read--once quantum branching
programs. This paper considers quantum ordered binary decision diagrams
(QOBDDs) and answers the question: How does the computational power of QOBDDs
increase, if we allow repeated tests. Additionally it is described how to
synthesize QOBDDs according to Boolean operations.


title: Stronger two-observer all-versus-nothing violation of local realism

abstract:
  We introduce a two-observer all-versus-nothing proof of Bell's theorem which
reduces the number of required quantum predictions from 9 [A. Cabello, Phys.
Rev. Lett. 87, 010403 (2001); Z.-B. Chen et al., Phys. Rev. Lett. 90, 160408
(2003)] to 4, provides a greater amount of evidence against local realism,
reduces the detection efficiency requirements for a conclusive experimental
test of Bell's theorem, and leads to a Bell's inequality which resembles
Mermin's inequality for three observers [N. D. Mermin, Phys. Rev. Lett. 65,
1838 (1990)] but requires only two observers.


title:
Loophole-free Bell's experiment and two-photon all-versus-nothing
  violation of local realism

abstract:
  We introduce an all-versus-nothing proof of impossibility of
Einstein-Podolsky-Rosen's local elements of reality for two photons entangled
both in polarization and path degrees of freedom, which leads to a Bell's
inequality where the classical bound is 8 and the quantum prediction is 16. A
simple estimation of the detection efficiency required to close the detection
loophole using this proof gives eta > 0.69. This efficiency is lower than that
required for previous proposals.


title: Multiphoton Antiresonance and Quantum Activation in Driven Systems

abstract:
  We show that nonlinear response of a quantum oscillator displays antiresonant
dips and resonant peaks with varying frequency of the driving field. The effect
is a consequence of special symmetry and is related to resonant multiphoton
mixing of several pairs of oscillator states at a time. We also discuss escape
from a metastable state of forced vibrations. Two important examples show that
the probability of escape via diffusion over quasienergy is larger than via
dynamical tunneling provided the relaxation rate exceeds both of them.
Diffusion dominates even for zero temperature, so that escape occurs via
quantum rather than thermal activation. The effects can be studied using
Josephson junctions and Josephson-junction based systems.


title: Fundamental gravitational limitations to quantum computing

abstract:
  Lloyd has considered the ultimate limitations physics places on quantum
computers. He concludes in particular that for an ``ultimate laptop'' (a
computer of one liter of volume and one kilogram of mass) the maximum number of
operations per second is bounded by $10^{51}$. The limit is derived considering
ordinary quantum mechanics. Here we consider additional limits that are placed
by quantum gravity ideas, namely the use of a relational notion of time and
fundamental gravitational limits that exist on time measurements. We then
particularize for the case of an ultimate laptop and show that the maximum
number of operations is further constrained to $10^{47}$ per second.


title: Negativity and Concurrence for two qutrits

abstract:
  Two measures of entanglement, negativity and concurrence are studied for two
qutrits. An operator origin of negativity is presented and an analytic formula
connecting the two measures is derived.


title: Quantum walk on the line: entanglement and non-local initial conditions

abstract:
  The conditional shift in the evolution operator of a quantum walk generates
entanglement between the coin and position degrees of freedom. This
entanglement can be quantified by the von Neumann entropy of the reduced
density operator (entropy of entanglement). In the long time limit, it
converges to a well defined value which depends on the initial state. Exact
expressions for the asymptotic (long-time) entanglement are obtained for (i)
localized initial conditions and (ii) initial conditions in the position
subspace spanned by the +1 and -1 position eigenstates.


title: Continuous Dynamical Decoupling with Bounded Controls

abstract:
  We develop a theory of continuous decoupling with bounded controls from a
geometric perspective. Continuous decoupling with bounded controls can
accomplish the same decoupling effect as the bang-bang control while using
realistic control resources and it is robust against systematic implementation
errors. We show that the decoupling condition within this framework is
equivalent to average out error vectors whose trajectories are determined by
the control Hamiltonian. The decoupling pulses can be intuitively designed once
the structure function of the corresponding SU(n) is known and is represented
from the geometric perspective. Several examples are given to illustrate the
basic idea. From the physical implementation point of view we argue that the
efficiency of the decoupling is determined not by the order of the decoupling
group but by the minimal time required to finish a decoupling cycle.


title:
Quantum entanglement dynamics and decoherence wave in spin chains at
  finite temperatures

abstract:
  We analyze the quantum entanglement at the equilibrium in a class of exactly
solvable one-dimensional spin models at finite temperatures and identify a
region where the quantum fluctuations determine the behavior of the system. We
probe the response of the system in this region by studying the spin dynamics
after projective measurement of one local spin which leads to the appearance of
the ``decoherence wave''. We investigate time-dependent spin correlation
functions, the entanglement dynamics, and the fidelity of the quantum
information transfer after the measurement.


title:
Experimental Implementation of Discrete Time Quantum Random Walk on an
  NMR Quantum Information Processor

abstract:
  We present an experimental implementation of the coined discrete time quantum
walk on a square using a three qubit liquid state nuclear magnetic resonance
(NMR) quantum information processor (QIP). Contrary to its classical
counterpart, we observe complete interference after certain steps and a
periodicity in the evolution. Complete state tomography has been performed for
each of the eight steps making a full period. The results have extremely high
fidelity with the expected states and show clearly the effects of quantum
interference in the walk. We also show and discuss the importance of choosing a
molecule with a natural Hamiltonian well suited to NMR QIP by implementing the
same algorithm on a second molecule. Finally, we show experimentally that
decoherence after each step makes the statistics of the quantum walk tend to
that of the classical random walk.


title: Adiabatic Quantum Computing with Phase Modulated Laser Pulses

abstract:
  Implementation of quantum logical gates for multilevel system is demonstrated
through decoherence control under the quantum adiabatic method using simple
phase modulated laser pulses. We make use of selective population inversion and
Hamiltonian evolution with time to achieve such goals robustly instead of the
standard unitary transformation language.


abstract:
  We suggest an interpretation of quantum mechanics, inspired by the ideas of
Aharonov et al. of a time-symmetric description of quantum theory. We show that
a special final boundary condition for the Universe, may be consistently
defined as to determine single classical-like measurement outcomes, thus
solving the "measurement problem". No other deviation is made from standard
quantum mechanics, and the resulting theory is deterministic (in a two-time
sense) and local. Quantum mechanical probabilities are recovered in general,
but are eliminated from the description of any single measurement. We call this
the Two-time interpretation of quantum mechanics. We analyze ideal
measurements, showing how the quantum superposition is, in effect, dynamically
reduced to a single classical state via a "two-time decoherence" process. We
discuss some philosophical aspects of the suggested interpretation. We also
discuss weak measurements using the two-time formalism, and remark that in
these measurement situations, special final boundary conditions for the
Universe, might explain some unaccounted for phenomena.


title:
Quantum Minimal One Way Information: Relative Hardness and Quantum
  Advantage of Combinatorial Tasks

abstract:
  Two-party one-way quantum communication has been extensively studied in the
recent literature. We target the size of minimal information that is necessary
for a feasible party to finish a given combinatorial task, such as distinction
of instances, using one-way communication from another party. This type of
complexity measure has been studied under various names: advice complexity,
Kolmogorov complexity, distinguishing complexity, and instance complexity. We
present a general framework focusing on underlying combinatorial takes to study
these complexity measures using quantum information processing. We introduce
the key notions of relative hardness and quantum advantage, which provide the
foundations for task-based quantum minimal one-way information complexity
theory.


title: Open Quantum Dynamics: Complete Positivity and Entanglement

abstract:
  We review the standard treatment of open quantum systems in relation to
quantum entanglement, analyzing, in particular, the behaviour of bipartite
systems immersed in a same environment. We first focus upon the notion of
complete positivity, a physically motivated algebraic constraint on the quantum
dynamics, in relation to quantum entanglement, i.e. the existence of
statistical correlations which can not be accounted for by classical
probability. We then study the entanglement power of heat baths versus their
decohering properties, a topic of increasing importance in the framework of the
fast developing fields of quantum information, communication and computation.
The presentation is self contained and, through several examples, it offers a
detailed survey of the physics and of the most relevant and used techniques
relative to both quantum open system dynamics and quantum entanglement.


title:
Polynomial Solution of PT-/Non-PT-Symmetric and Non-Hermitian
  Generalized Woods-Saxon Potential via Nikiforov-Uvarov Method

abstract:
  Using the Nikiforov-Uvarov method, the bound state energy eigenvalues and
eigenfunctions of the PT-/non-PT-symmetric and non-Hermitian generalized
Woods-Saxon (WS) potential with the real and complex-valued energy levels are
obtained in terms of the Jacobi polynomials. According to the PT-symmetric
quantum mechanics, we exactly solved the time-independent Schrodinger equation
with same potential for the s-states and also for any l-state as well. It is
shown that the results are in good agreement with the ones obtained before.


title:
Generation and manipulation of nonclassical light using photonic
  crystals

abstract:
  Photonic crystal cavities can localize light into nanoscale volumes with high
quality factors. This permits a strong interaction between light and matter,
which is important for the construction of classical light sources with
improved properties (e.g., low threshold lasers) and of nonclassical light
sources (such as single and entangled photon sources) that are crucial pieces
of hardware of quantum information processing systems. This article will review
some of our recent experimental and theoretical results on the interaction
between single quantum dots and photonic crystal cavity fields, and on the
integration of multiple photonic crystal devices into functional circuits for
quantum information processing.


title:
Entanglement creation and distribution on a graph of exchange-coupled
  qutrits

abstract:
  We propose a protocol that allows both the creation and distribution of
entanglement, resulting in two distant parties (Alice and Bob) conclusively
sharing a bipartite Bell State. The system considered is a graph of three-level
objects ("qutrits") coupled by SU(3) exchange operators. The protocol begins
with a third party (Charlie) encoding two lattice sites in unentangled states,
and allowing unitary evolution under time. Alice and Bob perform a projective
measurement on their respective qutrits at a given time, and obtain a
maximally-entangled Bell state with a certain probablility. We also consider
two further protocols, one based on simple repetition and the other based on
successive measurements and conditional resetting, and show that the cumulative
probability of creating a Bell state between Alice and Bob tends to unity.


title: Entangling movable mirrors in a double cavity system

abstract:
  We propose a double-cavity set-up capable of generating a stationary
entangled state of two movable mirrors at cryogenic temperatures. The scheme is
based on the optimal transfer of squeezing of input optical fields to
mechanical vibrational modes of the mirrors, realized by the radiation pressure
of the intracavity light. We show that the presence of macroscopic entanglement
can be demonstrated by an appropriate read out of the output light of the two
cavities.


title: Diffraction of a Bose-Einstein Condensate in the Time Domain

abstract:
  We have observed the diffraction of a Bose-Einstein condensate of rubidium
atoms on a vibrating mirror potential. The matter wave packet bounces back at
normal incidence on a blue-detuned evanescent light field after a 3.6 mm free
fall. The mirror vibrates at a frequency of 500 kHz with an amplitude of 3.0
nm. The atomic carrier and sidebands are directly imaged during their ballistic
expansion. The locations and the relative weights of the diffracted atomic wave
packets are in very good agreement with the theoretical prediction of Carsten
Henkel et al. [1].


title:
Bell's inequality violation for entangled generalized Bernoulli states
  in two spatially separate cavities

abstract:
  We consider the entanglement of orthogonal generalized Bernoulli states in
two separate single-mode high-$Q$ cavities. The expectation values and the
correlations of the electric field in the cavities are obtained. We then
define, in each cavity, a dichotomic operator expressible in terms of the field
states which can be, in principle, experimentally measured by a probe atom that
``reads'' the field. Using the quantum correlations of couples of these
operators, we construct a Bell's inequality which is shown to be violated for a
wide range of the degree of entanglement and which can be tested in a simple
way. Thus the cavity fields directly show quantum non-local properties. A
scheme is also sketched to generate entangled orthogonal generalized Bernoulli
states in the two separate cavities.


title:
Born expansion of the Casimir-Polder interaction of a ground-state atom
  with dielectric bodies

abstract:
  Within leading-order perturbation theory, the Casimir-Polder potential of a
ground-state atom placed within an arbitrary arrangement of dispersing and
absorbing linear bodies can be expressed in terms of the polarizability of the
atom and the scattering Green tensor of the body-assisted electromagnetic
field. Based on a Born series of the Green tensor, a systematic expansion of
the Casimir-Polder potential in powers of the susceptibilities of the bodies is
presented. The Born expansion is used to show how and under which conditions
the Casimir-Polder force can be related to microscopic many-atom van der Waals
forces, for which general expressions are presented. As an application, the
Casimir-Polder potentials of an atom near a dielectric ring and an
inhomogeneous dielectric half space are studied and explicit expressions are
presented that are valid up to second order in the susceptibility.


title:
Atom interferometry measurement of the electric polarizability of
  lithium

abstract:
  Using an atom interferometer, we have measured the static electric
polarizability of $^7$Li $\alpha =(24.33 \pm 0.16)\times10^{-30} $ m$^3$ $=
164.19\pm 1.08 $ atomic units with a 0.66% uncertainty. Our experiment, which
is similar to an experiment done on sodium in 1995 by D. Pritchard and
co-workers, consists in applying an electric field on one of the two
interfering beams and measuring the resulting phase-shift. With respect to D.
Pritchard's experiment, we have made several improvements which are described
in detail in this paper: the capacitor design is such that the electric field
can be calculated analytically; the phase sensitivity of our interferometer is
substantially better, near 16 mrad/$\sqrt{Hz}$; finally our interferometer is
species selective it so that impurities present in our atomic beam (other
alkali atoms or lithium dimers) do not perturb our measurement. The extreme
sensitivity of atom interferometry is well illustrated by our experiment: our
measurement amounts to measuring a slight increase $\Delta v$ of the atom
velocity $v$ when it enters the electric field region and our present
sensitivity is sufficient to detect a variation $\Delta v/v \approx 6 \times
10^{-13}$.


title:
A General Setting for Geometric Phase of Mixed States Under an Arbitrary
  Nonunitary Evolution

abstract:
  The problem of geometric phase for an open quantum system is reinvestigated
in a unifying approach. Two of existing methods to define geometric phase, one
by Uhlmann's approach and the other by kinematic approach, which have been
considered to be distinct, are shown to be related in this framework. The
method is based upon purification of a density matrix by its uniform
decomposition and a generalization of the parallel transport condition obtained
from this decomposition. It is shown that the generalized parallel transport
condition can be satisfied when Uhlmann's condition holds. However, it does not
mean that all solutions of the generalized parallel transport condition are
compatible with those of Uhlmann's one. It is also shown how to recover the
earlier known definitions of geometric phase as well as how to generalize them
when degeneracy exists and varies in time.


title: Dirac's principle in multimode interference of independent sources

abstract:
  The extended Dirac's principle describes the interference between different
particles as an effect of the multiparticle system with itself. In this paper
we present a novel example, based on the detection of particles emitted in
multimode states by independent sources, which illustrates in a simple way the
necessity of extending the original Dirac's criterion.


title: Classical capacity of fermionic product channels

abstract:
  We study multi-qubit quantum channels that can be represented as a product of
one-mode fermionic attenuation channels. An explicit formula for the classical
capacity $C_1$ and for the minimum output entropy $S_{min}$ of these channels
is proposed. We compute $S_{min}$ analytically for any number of qubits under
assumption that the minimum is achieved on a Gaussian input. Apart from that, a
simple numerical method for evaluating $S_{min}$ is developed. The method is
applicable to any channels that are sufficiently noisy. For fermionic product
channels the proposed formula for $S_{min}$ agrees with the numerical results
with a precision about $10^{-9}$.


title: On the entanglement concentration of three-partite states

abstract:
  We investigate the concentration of multi-party entanglement by focusing on
simple family of three-partite pure states, superpositions of
Greenberger-Horne-Zeilinger states and singlets. Despite the simplicity of the
states, we show that they cannot be reversibly concentrated by the standard
entanglement concentration procedure, to which they seem ideally suited. Our
results cast doubt on the idea that for each N there might be a finite set of
N-party states into which any pure state can be reversibly transformed. We
further relate our results to the concept of locking of entanglement of
formation.


title:
Eavesdropping on the "ping-pong" type quantum communication protocols
  with invisible photon

abstract:
  The crucial issue of quantum communication protocol is its security. In this
paper, we show that all the deterministic and direct two-way quantum
communication protocols, sometimes called ping-pong (PP) protocols, are
insecure when an eavesdropper uses the invisible photon to eavesdrop on the
communication. With our invisible photon eavesdropping (IPE) scheme, the
eavesdropper can obtain full information of the communication with zero risk of
being detected. We show that this IPE scheme can be implemented experimentally
with current technology. Finally, a possible improvement of PP communication
protocols security is proposed. PACS:03.67.Hk


title: Simple Minimal Informationally Complete Measurements for Qudits

abstract:
  Simple minimal but informationally complete positive operator-valued measures
are constructed out of the expectation-value representation for qudits. Upon
suitable modification, the procedure transforms any set of d^2 linearly
independent hermitean operators into such an observable. Minor changes in the
construction lead to closed-form expressions for informationally complete
positive measures in d-dimensional Hilbert spaces.


title: Solvable model of quantum microcanonical states

abstract:
  This letter examines the consequences of a recently proposed modification of
the postulate of equal {\it a priori} probability in quantum statistical
mechanics. This modification, called the {\it quantum microcanonical postulate}
(QMP), asserts that for a system in microcanonical equilibrium all pure quantum
states having the same energy expectation value are realised with equal
probability. A simple model of a quantum system that obeys the QMP and that has
a nondegenerate spectrum with equally spaced energy eigenvalues is studied.
This model admits a closed-form expression for the density of states in terms
of the energy eigenvalues. It is shown that in the limit as the number of
energy levels approaches infinity, the expression for the density of states
converges to a $\delta$ function centred at the intermediate value $(E_{\rm
max}+E_{\rm min})/ 2$ of the energy. Determining this limit requires an
elaborate asymptotic study of an infinite sum whose terms alternate in sign.


title: Star-quantization of an infinite wall

abstract:
  In deformation quantization (a.k.a. the Wigner-Weyl-Moyal formulation of
quantum mechanics), we consider a single quantum particle moving freely in one
dimension, except for the presence of one infinite potential wall. Dias and
Prata pointed out that, surprisingly, its stationary-state Wigner function does
not obey the naive equation of motion, i.e. the naive stargenvalue (*-genvalue)
equation. We review our recent work on this problem, that treats the infinite
wall as the limit of a Liouville potential. Also included are some new results:
(i) we show explicitly that the Wigner-Weyl transform of the usual density
matrix is the physical solution, (ii) we prove that an effective-mass treatment
of the problem is equivalent to the Liouville one, and (iii) we point out that
self-adjointness of the operator Hamiltonian requires a boundary potential, but
one different from that proposed by Dias and Prata.


title: Jarlskog's Parametrization of Unitary Matrices and Qudit Theory

abstract:
  In the paper (math-ph/0504049) Jarlskog gave an interesting simple
parametrization to unitary matrices, which was essentially the canonical
coordinate of the second kind in the Lie group theory (math-ph/0505047).
  In this paper we apply the method to a quantum computation based on
multi-level system (qudit theory). Namely, by considering that the
parametrization gives a complete set of modules in qudit theory, we construct
the generalized Pauli matrices which play a central role in the theory and also
make a comment on the exchange gate of two-qudit systems.
  Moreover we give an explicit construction to the generalized Walsh-Hadamard
matrix in the case of n=3, 4 and 5. For the case of n=5 its calculation is
relatively complicated. In general, a calculation to construct it tends to
become more and more complicated as n becomes large.
  To perform a quantum computation the generalized Walsh-Hadamard matrix must
be constructed in a quick and clean manner. From our construction it may be
possible to say that a qudit theory with $n\geq 5$ is not realistic.
  This paper is an introduction towards Quantum Engineering.


title: Rapid State-Reduction of Quantum Systems Using Feedback Control

abstract:
  We consider using Hamiltonian feedback control to increase the speed at which
a continuous measurement purifies (reduces) the state of a quantum system, and
thus to increase the speed of the preparation of pure states. For a measurement
of an observable with N equispaced eigenvalues, we show that there exists a
feedback algorithm which will speed up the rate of state-reduction by at least
a factor of 2(N+1)/3.


title:
Quantum-Classical Transition of Photon-Carnot Engine Induced by Quantum
  Decoherence

abstract:
  We study the physical implementation of the Photon Carnot engine (PCE) based
on the cavity QED system [M. Scully et al, Science, \textbf{299}, 862 (2003)].
Here, we analyze two decoherence mechanisms for the more practical systems of
PCE, the dissipation of photon field and the pure dephasing of the input atoms.
As a result we find that (I) the PCE can work well to some extent even in the
existence of the cavity loss (photon dissipation); and (II) the short-time
atomic dephasing, which can destroy the PCE, is a fatal problem to be overcome.


title:
Exact Eigenvalues and Eigenfunctions of the Hulthen Potential in the
  PT-Symmetry for Any Angular Momentum

abstract:
  The Schrodinger equation with the PT-symmetric Hulthen potential is solved
exactly by taking into account effect of the centrifugal barrier for any
l-state. Eigenfunctions are obtained in terms of the Jacobi polynomials. The
Nikiforov-Uvarov method is used in the computations. Our numerical results are
in good agreement with the ones obtained before.
  Keywords: Energy Eigenvalues and Eigenfunctions; Hulthen potential;
PT-symmetry; Nikiforov-Uvarov Method.


title: Measurement of the separation between atoms beyond diffraction limit

abstract:
  Precision measurement of small separations between two atoms or molecules has
been of interest since the early days of science. Here, we discuss a scheme
which yields spatial information on a system of two identical atoms placed in a
standing wave laser field. The information is extracted from the collective
resonance fluorescence spectrum, relying entirely on far-field imaging
techniques. Both the interatomic separation and the positions of the two
particles can be measured with fractional-wavelength precision over a wide
range of distances from bout lambda/550 to lambda/2.


title: Twelve-spin "Schrodinger cat"

abstract:
  Pseudopure "cat" state, a superposition of quantum states with all spins up
and all spins down, is experimentally demonstrated for a system of twelve
dipolar-coupled nuclear spins of fully 13C-labeled benzene molecule oriented in
a liquid-crystalline matrix.


title: Quantum Bayesian methods and subsequent measurements

abstract:
  After a derivation of the quantum Bayes theorem, and a discussion of the
reconstruction of the unknown state of identical spin systems by repeated
measurements, the main part of this paper treats the problem of determining the
unknown phase difference of two coherent sources by photon measurements. While
the approach of this paper is based on computing correlations of actual
measurements (photon detections), it is possible to derive indirectly a
probability distribution for the phase difference. In this approach, the
quantum phase is not an observable, but a parameter of an unknown quantum
state. Photon measurements determine a probability distribution for the phase
difference. The approach used in this paper takes into account both photon
statistics and the finite efficiency of the detectors.


title:
Bright tripartite entanglement in triply concurrent parametric
  oscillation

abstract:
  This paper has been withdrawn since it was an inadvertant double submission.
An updated version of the original submission can be found at quant-ph/0505131


title: Orthodox Quantum Mechanics Free from Paradoxes

abstract:
  A formulation of quantum mechanics based on an operational definition of
state is presented. This formulation, which includes explicitly the macroscopic
systems, assumes the probabilistic interpretation and is nevertheless
objective. The classical paradoxes of quantum mechanics are analyzed and their
origin is found to be the fictitious properties that are usually attributed to
quantum-mechanical states. The hypothesis that any mixed state can always be
considered as an incoherent superposition of pure states is found to contradict
quantum mechanics. A solution of EPR paradox is proposed. It is shown that
entanglement of quantum states is compatible with realism and locality of
events, but implies non-local encoding of information.


title:
Quantum secure direct communication network with Einstein-Podolsky-Rosen
  pairs

abstract:
  We discuss the four requirements for a real point-to-point quantum secure
direct communication (QSDC) first, and then present two efficient QSDC network
schemes with an N ordered Einstein-Podolsky-Rosen pairs. Any one of the
authorized users can communicate another one on the network securely and
directly.


title: General properties of Nonsignaling Theories

abstract:
  This article identifies a series of properties common to all theories that do
not allow for superluminal signaling and predict the violation of Bell
inequalities. Intrinsic randomness, uncertainty due to the incompatibility of
two observables, monogamy of correlations, impossibility of perfect cloning,
privacy of correlations, bounds in the shareability of some states; all these
phenomena are solely a consequence of the no-signaling principle and
nonlocality. In particular, it is shown that for any distribution, the
properties of (i) nonlocal, (ii) no arbitrarily shareable and (iii) positive
secrecy content are equivalent.


title:
A new perturbative expansion of the time evolution operator associated
  with a quantum system

abstract:
  A novel expansion of the evolution operator associated with a -- in general,
time-dependent -- perturbed quantum Hamiltonian is presented. It is shown that
it has a wide range of possible realizations that can be fitted according to
computational convenience or to satisfy specific requirements. As a remarkable
example, the quantum Hamiltonian describing a laser-driven trapped ion is
studied in detail.


title: Disordered complex systems using cold gases and trapped ions

abstract:
  We report our research on disordered complex systems using cold gases and
trapped ions, and address the possibility of using complex systems for quantum
information processing. Two simple paradigmatic models of disordered complex
systems are revisited here. The first one corresponds to a short range
disordered Ising Hamiltonian (spin glasses), which can be implemented with a
Bose-Fermi (Bose-Bose) mixture in a disordered optical lattice. The second
model we address here is a long range disordered Hamiltonian, characteristic of
neural networks (Hopfield model), which can be implemented in a chain of
trapped ions with appropriately designed interactions.


title: Schmidt Analysis of Pure-State Entanglement

abstract:
  We examine the application of Schmidt-mode analysis to pure state
entanglement. Several examples permitting exact analytic calculation of Schmidt
eigenvalues and eigenfunctions are included, as well as evaluation of the
associated degree of entanglement.


title:
Some families of density matrices for which separability is easily
  tested

abstract:
  We reconsider density matrices of graphs as defined in [quant-ph/0406165].
The density matrix of a graph is the combinatorial laplacian of the graph
normalized to have unit trace. We describe a simple combinatorial condition
(the "degree condition") to test separability of density matrices of graphs.
The condition is directly related to the PPT-criterion. We prove that the
degree condition is necessary for separability and we conjecture that it is
also sufficient. We prove special cases of the conjecture involving nearest
point graphs and perfect matchings. We observe that the degree condition
appears to have value beyond density matrices of graphs. In fact, we point out
that circulant density matrices and other matrices constructed from groups
always satisfy the condition and indeed are separable with respect to any
split. The paper isolates a number of problems and delineates further
generalizations.


title: Robust entanglement

abstract:
  It is common belief among physicists that entangled states of quantum systems
loose their coherence rather quickly. The reason is that any interaction with
the environment which distinguishes between the entangled sub-systems collapses
the quantum state. Here we investigate entangled states of two trapped Ca$^+$
ions and observe robust entanglement lasting for more than 20 seconds.


title: Improved transfer of quantum information using a local memory

abstract:
  We demonstrate that the quantum communication between two parties can be
significantly improved if the receiver is allowed to store the received signals
in a quantum memory before decoding them. In the limit of an infinite memory,
the transfer is perfect. We prove that this scheme allows the transfer of
arbitrary multipartite states along Heisenberg chains of spin-1/2 particles
with random coupling strengths.


title: Lie Algebras and Suppression of Decoherence in Open Quantum Systems

abstract:
  Since there are many examples in which no decoherence-free subsystems exist
(among them all cases where the error generators act irreducibly on the system
Hilbert space), it is of interest to search for novel mechanisms which suppress
decoherence in these more general cases. Drawing on recent work
(quant-ph/0502153) we present three results which indicate decoherence
suppression without the need for noiseless subsystems. There is a certain
trade-off; our results do not necessarily apply to an arbitrary initial density
matrix, or for completely generic noise parameters. On the other hand, our
computational methods are novel and the result--suppression of decoherence in
the error-algebra approach without noiseless subsystems--is an interesting new
direction.


title:
Homodyne detection and parametric down-conversion: a classical approach
  applied to proposed "loophole-free" Bell tests

abstract:
  A classical model is presented for the features of parametric down-conversion
and homodyne detection relevant to recent proposed ``loophole-free'' Bell
tests. The Bell tests themselves are uncontroversial: there are no obvious
loopholes that might cause bias and hence, if the world does, after all, obey
local realism, no violation of a Bell inequality will be observed. Interest
centres around the question of whether or not the proposed criterion for
``non-classical'' light is valid. If cit is not, then the experiments will fail
in their initial concept, since both quantum theorists and local realists will
agree that we are seeing a purely classical effect. The Bell test, though, is
not the only criterion by which the quantum-mechanical and local realist models
can be judged. If the experiments are extended by including a range of
parameter values and by analysing, in addition to the proposed digitised
voltage differences, the raw voltages, the models can be compared in their
overall performance and plausibility.


title:
Quantum control, quantum information processing, and quantum-limited
  metrology with trapped ions

abstract:
  We briefly discuss recent experiments on quantum information processing using
trapped ions at NIST. A central theme of this work has been to increase our
capabilities in terms of quantum computing protocols, but we have also applied
the same concepts to improved metrology, particularly in the area of frequency
standards and atomic clocks. Such work may eventually shed light on more
fundamental issues, such as the quantum measurement problem.


title:
From EIT photon correlations to Raman anti-correlations in coherently
  prepared Rb vapor

abstract:
  We have experimentally observed switching between photon-photon correlations
(bunching) and anti-correlations (anti-bunching) between two orthogonally
polarized laser beams in an EIT configuration in Rb vapor. The bunching and
anti-bunching sswitching occurs at a specific magnetic field strength.


title: Macroscopic Einstein-Podolsky-Rosen pairs in superconducting circuits

abstract:
  We propose an efficient approach to prepare Einstein-Podolsky-Rosen (EPR)
pairs in currently existing Josephson nanocircuits with capacitive couplings.
In these fixed coupling circuits, two-qubit logic gates could be easily
implemented while, strictly speaking, single-qubit gates cannot be easily
realized. For a known two-qubit state, conditional single-qubit operation could
still be designed to evolve only the selected qubit and keep the other qubit
unchanged; the rotation of the selected qubit depends on the state of the other
one. These conditional single-qubit operations allow to deterministically
generate the well-known Einstein-Podolsky-Rosen pairs, represented by EPR-Bell
(or Bell) states. Quantum-state tomography is further proposed to
experimentally confirm the generation of these states. The decays of the
prepared EPR pairs are analyzed using numerical simulations. Possible
application of the generated EPR pairs to test Bell's Inequality is also
discussed.


title:
Security against the Invisible Photon Attack for the Quantum Key
  Distribution with Blind Polarization Bases

abstract:
  In this paper, we briefly show how the quantum key distribution with blind
polarization bases [Kye et al., Phys. Rev. Lett. 95, 040501 (2005)] can be made
secure against the invisible photon attack.


title:
There is no simulation of n-qubit operations by a single Hamiltonian
  with 2-spin interaction

abstract:
  Today's devices for quantum computing are still far from implementing useful
and powerful quantum algorithms. Decoherence and the wish to resist the effects
of errors in a system of quantum bits incurs a lot of overhead in the number of
gates and qubits. From a theoretical perspective, controlled quantum simulation
raises the hope to simulate the unitary quantum operationes generated by a
Hamiltonian with 3-body interaction with a suitably designed element that is
constructed of only 2-body interactions. That replacement would happen without
any additional gates, and its possibility would be due to the ambiguity of the
unit element of the Lie group connected with the algebra of traceless hermitian
matrices. We show that this hope is void, and give a general proof for this for
any order of interaction.


title: Comment on "Quantum Key Distribution with the Blind Polarization Bases"

abstract:
  We show an eavesdropping scheme, by which the eavesdropper can achieve the
full information of the key against the protocol [Kye et al., PRL 95 040501
(2005)] with a probability of unity and will not be discovered by the the
legitimate users, even in the case that they have the perfect single-photon
source and the loseless channel.


title: The Dynamics of 1D Quantum Spin Systems Can Be Approximated Efficiently

abstract:
  In this Letter we show that an arbitrarily good approximation to the
propagator e^{itH} for a 1D lattice of n quantum spins with hamiltonian H may
be obtained with polynomial computational resources in n and the error
\epsilon, and exponential resources in |t|. Our proof makes use of the finitely
correlated state/matrix product state formalism exploited by numerical
renormalisation group algorithms like the density matrix renormalisation group.
There are two immediate consequences of this result. The first is that the
Vidal's time-dependent density matrix renormalisation group will require only
polynomial resources to simulate 1D quantum spin systems for logarithmic |t|.
The second consequence is that continuous-time 1D quantum circuits with
logarithmic |t| can be simulated efficiently on a classical computer, despite
the fact that, after discretisation, such circuits are of polynomial depth.


title: The separability versus entanglement problem

abstract:
  We review the problem of discriminating entangled states from separable
states for bipartite systems. We formally define what entangled states are,
present some important criteria to detect entanglement, and show how they can
be classified according to their capability to perform some precisely defined
tasks.


title: Universality in Dynamical Formation of Entanglement for Quantum Chaos

abstract:
  Dynamical formation of entanglement is studied for quantum chaotic
bi-particle systems. We find that statistical properties of the Schmidt
eigenvalues for strong chaos are well described by the random matrix theory of
the Laguerre ensemble. This implies that entanglement formation for quantum
chaos has universal properties, and does not depend on specific aspects of the
systems.