title: Momentum entanglement and disentanglement between atom and photon

abstract:
  With the quantum interference between two transition pathways, we demonstrate
a novel scheme to coherently control the momentum entanglement between a single
atom and a single photon. The unavoidable disentanglement is also studied from
the first principle, which indicates that the stably entangled atom--photon
system with superhigh degree of entanglement may be realized with this scheme
under certain conditions.


title:
Husimi operator and Husimi function for describing electron's
  probability distribution in uniform magnetic field derived by virtue of the
  entangled state representation

abstract:
  For the first time we introduce the Husimi operator
Delta_h(gamma,varepsilon;kappa) for studying Husimi distribution in phase
space(gamma,varepsilon) for electron's states in uniform magnetic field, where
kappa is the Gaussian spatial width parameter. Using the Wigner operator in the
entangled state |lambda> representation [Hong-Yi Fan, Phys. Lett. A 301
(2002)153; A 126 (1987) 145) we find that Delta_h(gamma,varepsilon;kappa) is
just a pure squeezed coherent state density operator |gamma,varepsilon>_kappa
kappa<gamma,varepsilon|, which brings convenience for studying and calculating
the Husimi distribution. We in many ways demonstrate that the Husimi
distributions are Gaussian-broadened version of the Wigner distributions.
Throughout our calculation we have fully employed the technique of integration
within an ordered product of operators.


title: Reply to Yang et al.'s comment

abstract:
  This is to reply to a recent comment by Yang, Yuan and Zhang on
``Teleportation of two-quNit entanglement: Exploiting local resorces''.


title:
Non-Markovian generalization of the Lindblad theory of open quantum
  systems

abstract:
  A systematic approach to the non-Markovian quantum dynamics of open systems
is given by the projection operator techniques of nonequilibrium statistical
mechanics. Combining these methods with concepts from quantum information
theory and from the theory of positive maps, we derive a class of correlated
projection superoperators that take into account in an efficient way
statistical correlations between the open system and its environment. The
result is used to develop a generalization of the Lindblad theory to the regime
of highly non-Markovian quantum processes in structured environments.


title:
Exponential separations for one-way quantum communication complexity,
  with applications to cryptography

abstract:
  We give an exponential separation between one-way quantum and classical
communication protocols for a partial Boolean function (a variant of the
Boolean Hidden Matching Problem of Bar-Yossef et al.) Earlier such an
exponential separation was known only for a relational problem. The
communication problem corresponds to a \emph{strong extractor} that fails
against a small amount of \emph{quantum} information about its random source.
Our proof uses the Fourier coefficients inequality of Kahn, Kalai, and Linial.
  We also give a number of applications of this separation. In particular, we
show that there are privacy amplification schemes that are secure against
classical adversaries but not against quantum adversaries; and we give the
first example of a key-expansion scheme in the model of bounded-storage
cryptography that is secure against classical memory-bounded adversaries but
not against quantum ones.


title:
Two electron entanglement in quasi-one dimensional system: Role of
  resonances

abstract:
  We analyze the role of resonances in two-fermion entanglement production for
a quasi one-dimensional two channel scattering problem. We solve exactly for
the problem of a two-fermion antisymmetric product state scattering off a
double delta well potential. It is shown that the two-particle concurrence of
the post-selected state has an oscillatory behavior where the concurrence
vanishes at the values of momenta for virtual bound states in the double well.
These concurrence zeros are interpreted in terms of the uncertainty in the
knowledge of the state of the one particle subspace reduced one particle
density matrix. Our results suggest manipulation of fermion entanglement
production through the resonance structure of quantum dots.


title: How Stands Collapse I

abstract:
  In this volume in honor of GianCarlo Ghirardi, I discuss my involvement with
ideas of dynamical collapse of the state vector. 10 problems are introduced, 9
of which were seen following my initial work. 4 of these problems had a
resolution in GianCarlo Ghirardi, Alberto Rimini and Tullio Weber's Spontaneous
Localization (SL) model (which added 1 more problem). This stimulated a
(somewhat different) resolution of these 5 problems in the Continuous
Spontaneous Localization (CSL) model, in which I combined my initial work with
SL. In an upcoming volume in honor of Abner Shimony I shall discuss the status
of the 5 remaining post-CSL problems.


title: How Stands Collapse II

abstract:
  I review ten problems associated with the dynamical wave function collapse
program, which were described in the first of these two papers. Five of these,
the \textit{interaction, preferred basis, trigger, symmetry} and
\textit{superluminal} problems, were discussed as resolved there. In this
volume in honor of Abner Shimony, I discuss the five remaining problems,
\textit{tails, conservation law, experimental, relativity, legitimization}.
Particular emphasis is given to the tails problem, first raised by Abner. The
discussion of legitimization contains a new argument, that the energy density
of the fluctuating field which causes collapse should exert a gravitational
force. This force can be repulsive, since this energy density can be negative.
Speculative illustrations of cosmological implications are offered.


title:
Quantum Query Algorithms for Certain Problems and Generalization of
  Algorithm Designing Techniques

abstract:
  Quantum algorithms can be analyzed in a query model to compute Boolean
functions where input is given in a black box, but the aim is to compute
function value for arbitrary input using as few queries as possible. In this
paper we concentrate on quantum query algorithm designing tasks. The main aim
of research was to find new efficient algorithms and develop general algorithm
designing techniques. We present several exact quantum query algorithms for
certain problems that are better than classical counterparts. Next we introduce
algorithm transformation methods that allow significant enlarging of sets of
exactly computable functions. Finally, we propose algorithm constructing
methods applicable for algorithms with specific properties that allow
constructing algorithms for more complex functions preserving acceptable error
probability and number of queries.


title:
Local unitary versus local Clifford equivalence of stabilizer and graph
  states

abstract:
  The equivalence of stabilizer states under local transformations is of
fundamental interest in understanding properties and uses of entanglement. Two
stabilizer states are equivalent under the usual stochastic local operations
and classical communication criterion if and only if they are equivalent under
local unitary (LU) operations. More surprisingly, under certain conditions, two
LU equivalent stabilizer states are also equivalent under local Clifford (LC)
operations, as was shown by Van den Nest et al. [Phys. Rev. \textbf{A71},
062323]. Here, we broaden the class of stabilizer states for which LU
equivalence implies LC equivalence ($LU\Leftrightarrow LC$) to include all
stabilizer states represented by graphs with neither cycles of length 3 nor 4.
To compare our result with Van den Nest et al.'s, we show that any stabilizer
state of distance $\delta=2$ is beyond their criterion. We then further prove
that $LU\Leftrightarrow LC$ holds for a more general class of stabilizer states
of $\delta=2$. We also explicitly construct graphs representing $\delta>2$
stabilizer states which are beyond their criterion: we identify all 58 graphs
with up to 11 vertices and construct graphs with $2^m-1$ ($m\geq 4$) vertices
using quantum error correcting codes which have non-Clifford transversal gates.


title: Controllability of open quantum systems with Kraus-map dynamics

abstract:
  This paper presents a constructive proof of complete kinematic state
controllability of finite-dimensional open quantum systems whose dynamics are
represented by Kraus maps. For any pair of states (pure or mixed) on the
Hilbert space of the system, we explicitly show how to construct a Kraus map
that transforms one state into another. Moreover, we prove by construction the
existence of a Kraus map that transforms all initial states into a predefined
target state (such a process may be used, for example, in quantum information
dilution). Thus, in sharp contrast to unitary control, Kraus-map dynamics
allows for the design of controls which are robust to variations in the initial
state of the system. The general formalism is illustrated with examples of
state-to-state Kraus transformations in a two-level system. In particular, we
construct a family of non-unitary Kraus maps, which transform one pure state
into another. The problem of dynamic state controllability of open quantum
systems (i.e., controllability of state-to-state transformations, given a set
of available dynamical resources such as coherent controls, incoherent
interactions with the environment, and measurements) is also discussed.


title: Quantum mechanics in general quantum systems (I): Exact solution

abstract:
  Starting from our idea of combining the Feynman path integral spirit and the
Dyson series kernel, we find an explicit and general form of time evolution
operator that is a $c$-number function and a power series of perturbation
including all order approximations in the unperturbed Hamiltonian
representation. Based on it, we obtain an exact solution of the Schr\"{o}dinger
equation in general quantum systems independent of time. Comparison of our
exact solution with the existed perturbation theory makes some features and
significance of our exact solution clear. The conclusions expressly indicate
that our exact solution is obviously consistent with the usual time-independent
perturbation theory at any order approximation, it explicitly calculates out
the expanding coefficients of the unperturbed state in the non-perturbation
method, and it fully solves the recurrence equation of the expansion
coefficients of final state in the unperturbed Hamiltonian representation from
a view of time-dependent perturbation theory. At the same time, the exact
solution of the von Neumann equation is also given. Our results can be thought
of as theoretical developments of quantum dynamics, and are helpful for
understanding the dynamical behavior and related subjects of general quantum
systems in both theory and application. Our exact solution, together with its
sequence studies on perturbation theory [An Min Wang, quant-ph/0611217] and
open system dynamics [An Min Wang, quant-ph/0601051] can be used to establish
the foundation of theoretical formulism of quantum mechanics in general quantum
systems. Further applications of our exact solution to quantum theory can be
expected.


title: Quantum mechanics in general quantum systems (II): Perturbation theory

abstract:
  We propose an improved scheme of perturbation theory based on our exact
solution [An Min Wang, quant-ph/0611216] in general quantum systems independent
of time. Our elementary start-point is to introduce the perturbing parameter as
late as possible. Our main skills are Hamiltonian redivision so as to overcome
a flaw of the usual perturbation theory, and the perturbing Hamiltonian matrix
product decomposition in order to separate the contraction and anti-contraction
terms. Our calculational technology is the limit process for eliminating
apparent divergences. Our central idea is ``dynamical rearrangement and
summation" for the sake of the partial contributions from the high order even
all order approximations absorbed in our perturbed solution. Consequently, we
obtain the improved forms of the zeroth, first, second and third order
perturbed solutions absorbing the partial contributions from the high order
even all order approximations of perturbation. Then we deduce the improved
transition probability. In special, we propose the revised Fermi's golden rule.
Moreover, we apply our scheme to obtain the improved forms of perturbed energy
and perturbed state. In addition, we study an easy understanding example of
two-state system to illustrate our scheme and show its advantages. All of this
implies the physical reasons and evidences why our improved scheme of
perturbation theory are actually calculable, operationally efficient,
conclusively more accurate. Our improved scheme is the further development and
interesting application of our exact solution, and it has been successfully
used to study on open system dynamics [An Min Wang, quant-ph/0601051].

categories: quant-ph math-ph math.MP physics.atom-ph


title:
Quantum simulation of spin ordering with nuclear spins in a solid state
  lattice

abstract:
  An experiment demonstrating the quantum simulation of a spin-lattice
Hamiltonian is proposed. Dipolar interactions between nuclear spins in a solid
state lattice can be modulated by rapid radio-frequency pulses. In this way,
the effective Hamiltonian of the system can be brought to the form of an
antiferromagnetic Heisenberg model with long range interactions. Using a
semiconducting material with strong optical properties such as InP, cooling of
nuclear spins could be achieved by means of optical pumping. An additional
cooling stage is provided by adiabatic demagnetization in the rotating frame
(ADRF) down to a nuclear spin temperature at which we expect a phase transition
from a paramagnetic to antiferromagnetic phase. This phase transition could be
observed by probing the magnetic susceptibility of the spin-lattice. Our
calculations suggest that employing current optical pumping technology,
observation of this phase transition is within experimental reach.


title: On experimental procedures for entanglement verification

abstract:
  We give an overview of different types of entanglement that can be generated
in experiments, as well as of various protocols that can be used to verify or
quantify entanglement. We propose several criteria that, we argue, should be
applied to experimental entanglement verification procedures. Explicit examples
demonstrate that not following these criteria will tend to result in
overestimating the amount of entanglement generated in an experiment or in
infering entanglement when there is none. We distinguish protocols meant to
refute or eliminate hidden-variable models from those meant to verify
entanglement.


title:
Complete "Born's rule" from "environment-assisted invariance" in terms
  of pure-state twin unitaries

abstract:
  Zurek's derivation of the Born rule from envariance (environment-assisted
invariance) is tightened up, somewhat generalized, and extended to encompass
all possibilities. By this, besides Zurek's most important work, also the works
of 5 other commentators of the derivation are taken into account, and selected
excerpts commented upon. All this is done after a detailed theory of twin
unitaries, which are the other face of envariance.


title:
Self ordering and superradiant backscattering of a gaseous beam in a
  ring cavity with counter propagating pump

abstract:
  We study the threshold conditions of spatial self organization combined with
collective coherent optical backscattering of a thermal gaseous beam moving in
a high Q ring cavity with counter propagating pump. We restrict ourselves to
the limit of large detuning between the particles optical resonances and the
light field, where spontaneous emission is negligible and the particles can be
treated as polarizable point masses. Using a linear stability analysis in the
accelerated rest frame of the particles we derive an analytic bounds for the
selforganization as a function of particle number,average velocity, temperature
and resonator parameters. We check our results by a numerical iteration
procedure as well as by direct simulations of the N-particle dynamics. Due to
momentum conservation the backscattered intensity determines the average force
on the cloud, which gives the conditions for stopping and cooling a fast
molecular beam.


title: Lindblad rate equations

abstract:
  In this paper we derive an extra class of non-Markovian master equations
where the system state is written as a sum of auxiliary matrixes whose
evolution involve Lindblad contributions with local coupling between all of
them, resembling the structure of a classical rate equation. The system
dynamics may develops strong non-local effects such as the dependence of the
stationary properties with the system initialization. These equations are
derived from alternative microscopic interactions, such as complex environments
described in a generalized Born-Markov approximation and tripartite
system-environment interactions, where extra unobserved degrees of freedom
mediates the entanglement between the system and a Markovian reservoir.
Conditions that guarantees the completely positive condition of the solution
map are found. Quantum stochastic processes that recover the system dynamics in
average are formulated. We exemplify our results by analyzing the dynamical
action of non-trivial structured dephasing and depolarizing reservoirs over a
single qubit.


title: Linear entropy as an entanglement measure in two-fermion systems

abstract:
  We describe an efficient theoretical criterion, suitable for
indistinguishable particles to quantify the quantum correlations of any pure
two-fermion state, based on the Slater rank concept. It represents the natural
generalization of the linear entropy used to treat quantum entanglement in
systems of non-identical particles. Such a criterion is here applied to an
electron-electron scattering in a two-dimensional system in order to perform a
quantitative evaluation of the entanglement dynamics for various spin
configurations and to compare the linear entropy with alternative approaches.
Our numerical results show the dependence of the entanglement evolution upon
the initial state of the system and its spin components. The differences with
previous analyses accomplished by using the von Neumann entropy are discussed.
The evaluation of the entanglement dynamics in terms of the linear entropy
results to be much less demanding from the computational point of view, not
requiring the diagonalization of the density matrix.


title: Compatibility conditions from multipartite entanglement measures

abstract:
  We consider an arbitrary d_{1}\otimes d_{2}\otimes ... \otimes d_{N}
composite quantum system and find necessary conditions for general m-party
subsystem states to be the reduced states of a common N-party state. These
conditions will lead to various monogamy inequalities for bipartite quantum
entanglement and partial disorder in multipartite states. Our results are
tightly connected with the measures of multipartite entanglement.


title:
Simple scheme for implementing the Deutsch-Jozsa algorithm in thermal
  cavity

abstract:
  We present a simple scheme to implement the Deutsch-Jozsa algorithm based on
two-atom interaction in a thermal cavity. The photon-number-dependent parts in
the evolution operator are canceled with the strong resonant classical field
added. As a result, our scheme is immune to thermal field, and does not require
the cavity to remain in the vacuum state throughout the procedure. Besides,
large detuning between the atoms and the cavity is not necessary neither,
leading to potential speed up of quantum operation. Finally, we show by
numerical simulation that the proposed scheme is equal to demonstrate the
Deutsch-Jozsa algorithm with high fidelity.


title: Entanglement of random vectors

abstract:
  We analytically calculate the average value of i-th largest Schmidt
coefficient for random pure quantum states. Schmidt coefficients, i.e.,
eigenvalues of the reduced density matrix, are expressed in the limit of large
Hilbert space size and for arbitrary bipartite splitting as an implicit
function of index i.


title: Physical propositions and quantum languages

abstract:
  The word \textit{proposition} is used in physics with different meanings,
which must be distinguished to avoid interpretational problems. We construct
two languages $\mathcal{L}^{\ast}(x)$ and $\mathcal{L}(x)$ with classical
set-theoretical semantics which allow us to illustrate those meanings and to
show that the non-Boolean lattice of propositions of quantum logic (QL) can be
obtained by selecting a subset of \textit{p-testable} propositions within the
Boolean lattice of all propositions associated with sentences of
$\mathcal{L}(x)$. Yet, the aforesaid semantics is incompatible with the
standard interpretation of quantum mechanics (QM) because of known no-go
theorems. But if one accepts our criticism of these theorems and the ensuing SR
(semantic realism) interpretation of QM, the incompatibility disappears, and
the classical and quantum notions of truth can coexist, since they refer to
different metalinguistic concepts (\textit{truth} and \textit{verifiability
according to QM}, respectively). Moreover one can construct a quantum language
$\mathcal{L}_{TQ}(x)$ whose Lindenbaum-Tarski algebra is isomorphic to QL, the
sentences of which state (testable) properties of individual samples of
physical systems, while standard QL does not bear this interpretation.


title:
Spectral theory of quantum memory and entanglement via Raman scattering
  of light by an atomic ensemble

abstract:
  We discuss theoretically quantum interface between light and a spin polarized
ensemble of atoms with the spin >= 1 based on an off-resonant Raman scattering.
We present the spectral theory of the light-atoms interaction and show how
particular spectral modes of quantum light couple to spatial modes of the
extended atomic ensemble. We show how this interaction can be used for quantum
memory storage and retrieval and for deterministic entanglement protocols. The
proposed protocols are attractive due to their simplicity since they involve
just a single pass of light through atoms without the need for elaborate pulse
shaping or quantum feedback. As a practically relevant example we consider the
interaction of a light pulse with hyperfine components of D1 line of 87Rb. The
quality of the proposed protocols is verified via analytical and numerical
analysis.


title: Lower bounds on concurrence and separability conditions

abstract:
  We obtain analytical lower bounds on the concurrence of bipartite quantum
systems in arbitrary dimensions related to the violation of separability
conditions based on local uncertainty relations and on the Bloch representation
of density matrices. We also illustrate how these results complement and
improve those recently derived [K. Chen, S. Albeverio, and S.-M. Fei, Phys.
Rev. Lett. 95, 040504 (2005)] by considering the Peres-Horodecki and the
computable cross norm or realignment criteria.


title: Tensor Product Structures, Entanglement, and Particle Scattering

abstract:
  Particle systems admit a variety of tensor product structures (TPSs)
depending on the complete system of commuting observables chosen for the
analysis. Different notions of entanglement are associated with these different
TPSs. Global symmetry transformations and dynamical transformations factor into
products of local unitary operators with respect to certain TPSs and not with
respect to others. Symmetry-invariant and dynamical-invariant TPSs and
corresponding measures of entanglement are defined for particle scattering
systems.


title: Approximate locality for quantum systems on graphs

abstract:
  In this Letter we make progress on a longstanding open problem of Aaronson
and Ambainis [Theory of Computing 1, 47 (2005)]: we show that if A is the
adjacency matrix of a sufficiently sparse low-dimensional graph then the
unitary operator e^{itA} can be approximated by a unitary operator U(t) whose
sparsity pattern is exactly that of a low-dimensional graph which gets more
dense as |t| increases. Secondly, we show that if U is a sparse unitary
operator with a gap \Delta in its spectrum, then there exists an approximate
logarithm H of U which is also sparse. The sparsity pattern of H gets more
dense as 1/\Delta increases. These two results can be interpreted as a way to
convert between local continuous-time and local discrete-time processes. As an
example we show that the discrete-time coined quantum walk can be realised as
an approximately local continuous-time quantum walk. Finally, we use our
construction to provide a definition for a fractional quantum fourier
transform.


title:
Photonic crystal fibre source of photon pairs for quantum information
  processing

abstract:
  We demonstrate two key components for optical quantum information processing:
a bright source of heralded single photons; and a bright source of entangled
photon pairs. A pair of pump photons produces a correlated pair of photons at
widely spaced wavelengths (583 nm and 900 nm), via a $\chi^{(3)}$ four-wave
mixing process. We demonstrate a non-classical interference between heralded
photons from independent sources with a visibility of 95%, and an entangled
photon pair source, with a fidelity of 89% with a Bell state.


title: Conditional Density Operators and the Subjectivity of Quantum Operations

abstract:
  Assuming that quantum states, including pure states, represent subjective
degrees of belief rather than objective properties of systems, the question of
what other elements of the quantum formalism must also be taken as subjective
is addressed. In particular, we ask this of the dynamical aspects of the
formalism, such as Hamiltonians and unitary operators. Whilst some operations,
such as the update maps corresponding to a complete projective measurement,
must be subjective, the situation is not so clear in other cases. Here, it is
argued that all trace preserving completely positive maps, including unitary
operators, should be regarded as subjective, in the same sense as a classical
conditional probability distribution. The argument is based on a reworking of
the Choi-Jamiolkowski isomorphism in terms of "conditional" density operators
and trace preserving completely positive maps, which mimics the relationship
between conditional probabilities and stochastic maps in classical probability.


title: Toward a general theory of quantum games

abstract:
  We study properties of quantum strategies, which are complete specifications
of a given party's actions in any multiple-round interaction involving the
exchange of quantum information with one or more other parties. In particular,
we focus on a representation of quantum strategies that generalizes the
Choi-Jamio{\l}kowski representation of quantum operations. This new
representation associates with each strategy a positive semidefinite operator
acting only on the tensor product of its input and output spaces. Various facts
about such representations are established, and two applications are discussed:
the first is a new and conceptually simple proof of Kitaev's lower bound for
strong coin-flipping, and the second is a proof of the exact characterization
QRG = EXP of the class of problems having quantum refereed games.


title:
Complete physical simulation of the entangling-probe attack on the BB84
  protocol

abstract:
  We have used deterministic single-photon two qubit (SPTQ) quantum logic to
implement the most powerful individual-photon attack against the
Bennett-Brassard 1984 (BB84) quantum key distribution protocol. Our measurement
results, including physical source and gate errors, are in good agreement with
theoretical predictions for the Renyi information obtained by Eve as a function
of the errors she imparts to Alice and Bob's sifted key bits. The current
experiment is a physical simulation of a true attack, because Eve has access to
Bob's physical receiver module. This experiment illustrates the utility of an
efficient deterministic quantum logic for performing realistic physical
simulations of quantum information processing functions.


title:
First measurements of the index of refraction of gases for lithium
  atomic waves

abstract:
  We report here the first measurements of the index of refraction of gases for
lithium waves. Using an atom interferometer, we have measured the real and
imaginary part of the index of refraction $n$ for argon, krypton and xenon, as
a function of the gas density for several velocities of the lithium beam. The
linear dependence of $(n-1)$ with the gas density is well verified. The total
collision cross-section deduced from the imaginary part is in very good
agreement with traditional measurements of this quantity. Finally, as predicted
by theory, the real and imaginary parts of $(n-1)$ and their ratio $\rho$
exhibit glory oscillations.


title: Entanglement without nonlocality

abstract:
  We consider the characterization of entanglement from the perspective of a
Heisenberg formalism. We derive an original two-party generalized separability
criteria, and from this describe a novel physical understanding of
entanglement. We find that entanglement may be considered as fundamentally a
local effect, and therefore as a separable computational resource from
nonlocality. We show how entanglement differs from correlation physically, and
explore the implications of this new conception of entanglement for the notion
of classicality. We find that this understanding of entanglement extends
naturally to multipartite cases.


title: Analytical solutions for two-level systems with damping

abstract:
  A method is proposed to transform any analytic solution of the Bloch equation
into an analytic solution of the Landau-Lifshitz-Gilbert equation. This allows
for the analytical description of the dynamics of a two level system with
damping. This method shows that damping turns the linear Schr\"{o}dinger
equation of a two-level system into a nonlinear Schr\"{o}dinger equation. As
applications, it is shown that damping has a relatively mild influence on
self-induced transparency but destroys dynamical localization.


title: Path integrals and wavepacket evolution for damped mechanical systems

abstract:
  Damped mechanical systems with various forms of damping are quantized using
the path integral formalism. In particular, we obtain the path integral kernel
for the linearly damped harmonic oscillator and a particle in a uniform
gravitational field with linearly or quadratically damped motion. In each case,
we study the evolution of Gaussian wavepackets and discuss the characteristic
features that help us distinguish between different types of damping. For
quadratic damping, we show that the action and equation of motion of such a
system has a connection with the zero dimensional version of a currently
popular scalar field theory. Furthermore we demonstrate that the equation of
motion (for quadratic damping) can be identified as a geodesic equation in a
fictitious two-dimensional space.


title:
Ultracold atoms in radio-frequency-dressed potentials beyond the
  rotating wave approximation

abstract:
  We study dressed Bose-Einstein condensates in an atom chip radio-frequency
trap. We show that in this system sufficiently strong dressing can be achieved
to cause the widely used rotating wave approximation (RWA) to break down. We
present a full calculation of the atom - field coupling which shows that the
non-RWA contributions quantitatively alter the shape of the emerging dressed
adiabatic potentials. The non-RWA contributions furthermore lead to additional
allowed transitions between dressed levels. We use RF spectroscopy of
Bose-Einstein condensates trapped in the dressed state potentials to directly
observe the transition from the RWA to the beyond-RWA regime.


title:
Efficient classical simulation of the approximate quantum Fourier
  transform

abstract:
  We present a method for classically simulating quantum circuits based on the
tensor contraction model of Markov and Shi (quant-ph/0511069). Using this
method we are able to classically simulate the approximate quantum Fourier
transform in polynomial time. Moreover, our approach allows us to formulate a
condition for the composability of simulable quantum circuits. We use this
condition to show that any circuit composed of a constant number of approximate
quantum Fourier transform circuits and log-depth circuits with limited
interaction range can also be efficiently simulated.


title: Decoherence induced by interacting quantum spin baths

abstract:
  We study decoherence induced on a two-level system coupled to a
one-dimensional quantum spin chain. We consider the cases where the dynamics of
the chain is determined by the Ising, XY, or Heisenberg exchange Hamiltonian.
This model of quantum baths can be of fundamental importance for the
understanding of decoherence in open quantum systems, since it can be
experimentally engineered by using atoms in optical lattices. As an example,
here we show how to implement a pure dephasing model for a qubit system coupled
to an interacting spin bath. We provide results that go beyond the case of a
central spin coupled uniformly to all the spins of the bath, in particular
showing what happens when the bath enters different phases, or becomes
critical; we also study the dependence of the coherence loss on the number of
bath spins to which the system is coupled and we describe a
coupling-independent regime in which decoherence exhibits universal features,
irrespective of the system-environment coupling strength. Finally, we establish
a relation between decoherence and entanglement inside the bath. For the Ising
and the XY models we are able to give an exact expression for the decay of
coherences, while for the Heisenberg bath we resort to the numerical
time-dependent Density Matrix Renormalization Group.


title:
Generalized Lifshitz formula for a cylindrical plasma sheet in front of
  a plane beyond proximity force approximation

abstract:
  We calculate the first correction beyond proximity force approximation for a
cylindrical graphene sheet in interaction with a flat graphene sheet or a
dielectric half space.


title: Diluted maximum-likelihood algorithm for quantum tomography

abstract:
  We propose a refined iterative likelihood-maximization algorithm for
reconstructing a quantum state from a set of tomographic measurements. The
algorithm is characterized by a very high convergence rate and features a
simple adaptive procedure that ensures likelihood increase in every iteration
and convergence to the maximum-likelihood state.
  We apply the algorithm to homodyne tomography of optical states and quantum
tomography of entangled spin states of trapped ions and investigate its
convergence properties.


title: What About Quantum Theory? Bayes and the Born Interpretation

abstract:
  I show that probabilities in quantum mechanics are a measure of belief in the
presence of human ignorance, just like all other probabilities. The Born
interpretation of the square of modulus of the wave function arises from the
interaction of a quantum system with an observer, and probabilities will not
arise unless the observer is unable to access all the information available in
the system's wave function. Quantum mechanics generally does not permit all the
information to be obtained, even in principle, just as in relativity
information from outside the past light cone cannot be obtained. But
probabilities do not imply indeterminism. Instead, quantum mechanics is more
deterministic than classical mechanics.


title:
Universal Quantum Computation in a Neutral Atom Decoherence Free
  Subspace

abstract:
  In this paper, we propose a way to achieve protected universal computation in
a neutral atom quantum computer subject to collective dephasing. Our proposal
relies on the existence of a Decoherence Free Subspace (DFS), resulting from
symmetry properties of the errors. After briefly describing the physical system
and the error model considered, we show how to encode information into the DFS
and build a complete set of safe universal gates. Finally, we provide numerical
simulations for the fidelity of the different gates in the presence of
time-dependent phase errors and discuss their performance and practical
feasibility.


title: One-step preparation of cluster states in quantum dot molecules

abstract:
  Cluster states, a special type of highly entangled states, are a universal
resource for measurement-based quantum computation. Here, we propose an
efficient one-step generation scheme for cluster states in semiconductor
quantum dot molecules, where qubits are encoded on singlet and triplet state of
two coupled quantum dots. By applying a collective electrical field or
simultaneously adjusting interdot bias voltages of all double-dot molecule, we
get a switchable Ising-like interaction between any two adjacent quantum
molecule qubits. The initialization, the single qubit measurement, and the
experimental parameters are discussed, which shows the large cluster state
preparation and one-way quantum computation implementable in semiconductor
quantum dots with the present techniques.


title:
Entanglement, avoided crossings and quantum chaos in an Ising model with
  a tilted magnetic field

abstract:
  We study a one-dimensional Ising model with a magnetic field and show that
tilting the field induces a transition to quantum chaos. We explore the
stationary states of this Hamiltonian to show the intimate connection between
entanglement and avoided crossings. In general entanglement gets exchanged
between the states undergoing an avoided crossing with an overall enhancement
of multipartite entanglement at the closest point of approach, simultaneously
accompanied by diminishing two-body entanglement as measured by concurrence. We
find that both for stationary as well as nonstationary states, nonintegrability
leads to a destruction of two-body correlations and distributes entanglement
more globally.


title:
High fidelity transfer of an arbitrary quantum state between harmonic
  oscillators

abstract:
  It is shown that by switching a specific time-dependent interaction between a
harmonic oscillator and a transmission line (a waveguide, an optical fiber,
etc.) the quantum state of the oscillator can be transferred into that of
another oscillator coupled to the distant other end of the line, with a
fidelity that is independent of the initial state of both oscillators. For a
transfer time $T$, the fidelity approaches 1 exponentially in $\gamma T$ where
$\gamma$ is a characteristic damping rate. Hence, a good fidelity is achieved
even for a transfer time of a few damping times. Some implementations are
discussed.


title: What is "system": the information-theoretic arguments

abstract:
  The problem of "what is 'system'?" is in the very foundations of modern
quantum mechanics. Here, we point out the interest in this topic in the
information-theoretic context. E.g., we point out the possibility to manipulate
a pair of mutually non-interacting, non-entangled systems to employ
entanglement of the newly defined '(sub)systems' consisting the one and the
same composite system. Given the different divisions of a composite system into
"subsystems", the Hamiltonian of the system may perform in general
non-equivalent quantum computations. Redefinition of "subsystems" of a
composite system may be regarded as a method for avoiding decoherence in the
quantum hardware. In principle, all the notions refer to a composite system as
simple as the hydrogen atom.


title:
A Nano-Quantum Photonic Model for Justification of Dispersion in Single
  Crystal Film of NPP

abstract:
  In this paper, we present a nano-quantum photonic model for justification of
normal dispersion in a thin crystal film of NPP. In this method, we assume a
laser beam consists of a flow of energetic particles. By precise analyzing of
photon interaction with pi-electron system of benzene ring in NPP crystal, we
will attain refractive index (RI) in any wavelength and compare the results
with experimental data.


title: Local control of remote entanglement

abstract:
  We address the problem of the generation of entanglement. We focus on the
control of entanglement shared by two non-interacting parties $A$ and $C$ via
interaction with a third party $B$. We show that, for certain physical models,
it is possible to have an asymptotically complete control of the entanglement
shared by $A$ and $C$ by changing parameters of the Hamiltonian local at site
$B$. We present an example where different models (propositions) of physical
situation, that lead to different descriptions of the system $B$, result into
different amount entanglement produced. In the end we discuss limits of the
procedure.


title: Bounds on action of local quantum channels

abstract:
  We derive an upper bound on the action of a direct product of two quantum
maps (channels) acting on multi-partite quantum states. We assume that the
individual channels $\Lambda_j$ affect single-particle states so, that for an
arbitrary input $\rho_j$, the distance $D_j (\Lambda_j [ \rho_j ], \rho_j)$
between the input $\rho_j$ and the output $\Lambda_j [ \rho_j ]$ of the channel
is less than $\epsilon$. Given this assumption we show that for an arbitrary
{\em separable} two-partite state $\rho_{12}$ the distance between the input
$\rho_{12}$ and the output $\Lambda_1\otimes\Lambda_2[\rho_{12} ]$ fulfills the
bound $D_{12} (\Lambda_1 \otimes \Lambda_2 [ \rho_{12} ], \rho_{12}) \leq
\sqrt{2+ 2 \sqrt{(1-1/d_1)(1-1/d_2)}} \epsilon$ where $d_1$ and $d_2$ are
dimensions of first and second quantum system respectively. On the contrary,
entangled states are transformed in such a way, that the bound on the action of
the local channels is $D_{12} (\Lambda_1 \otimes \Lambda_2 [ \rho_{12} ],
\rho_{12}) \leq 2 \sqrt{2 - 1/d} \: \epsilon$, where $d$ is the dimension of
the smaller of the two quantum systems passing through the channels. Our
results show that the fundamental distinction between the set of separable and
the set of entangled states results into two different bounds which in turn can
be exploited for a discrimination between the two sets of states. We generalize
our results to multi-partite channels.


title:
Laser-noise-induced correlations and anti-correlations in
  Electromagnetically Induced Transparency

abstract:
  High degrees of intensity correlation between two independent lasers were
observed after propagation through a rubidium vapor cell in which they generate
Electromagnetically Induced Transparency (EIT). As the optical field
intensities are increased, the correlation changes sign (becoming
anti-correlation). The experiment was performed in a room temperature rubidium
cell, using two diode lasers tuned to the $^{85}$Rb $D_2$ line ($\lambda =
780$nm). The cross-correlation spectral function for the pump and probe fields
is numerically obtained by modeling the temporal dynamics of both field phases
as diffusing processes. We explored the dependence of the atomic response on
the atom-field Rabi frequencies, optical detuning and Doppler width. The
results show that resonant phase-noise to amplitude-noise conversion is at the
origin of the observed signal and the change in sign for the correlation
coefficient can be explained as a consequence of the competition between EIT
and Raman resonance processes.


title:
Resonance phenomena in Macroscopic Quantum Tunneling: the small
  viscosity limit

abstract:
  We present a new theoretical approach to describe the quantum behavior of a
macroscopic system interacting with an external irradiation field, close to the
resonant condition. Here we consider the extremely underdamped regime for a
system described by a double well potential. The theory includes both:
transitions from one well to the other and relaxation processes. We simulate
resonant phenomena in a rf-SQUID, whose parameters lie in the range typically
used in the experiments. The dependence of the transition probability W on the
external drive of the system $\phi_x$ shows three resonance peaks. One peak is
connected with the resonant tunneling and the two others with the resonant
pumping. The relative position of the two peaks correlated to the resonant
pumping depends on the pumping frequency $\nu$ and on the system parameters.
The preliminary measurements on our devices show a low dissipation level and
assure that they are good candidates in order to realize new experiments on the
resonant phenomena in the presence of an external microwave irradiation of
proper frequency.


title:
Manipulating Multi-qudit Entanglement Witnesses by Using Linear
  Programming

abstract:
  A new class of entanglement witnesses (EWs) called reduction type
entanglement witnesses is introduced, which can detect some multi-qudit
entangeled states including PPT ones with Hilbert space of dimension
$d_{_{1}}\otimes d_{_{2}}\otimes...\otimes d_{_{n}}$. The novelty of this work
comes from the fact that the feasible regions turn out to be convex polygons,
hence the manipulation of these EWs reduces to linear programming which can be
solved \emph{exactly} by using simplex method. The decomposability and
non-decomposability of these EWs are studied and it is shown that it has a
close connection with eigenvalues and optimality of EWs. Also using the Jamio\l
kowski isomorphism, the corresponding possible positive maps, including the
generalized reduction maps of Ref. \cite{Hall1}, are obtained.


title:
Relation between phase space coverage and entanglement for spin-1/2
  systems

abstract:
  For systems of two and three spins 1/2 it is known that the second moment of
the Husimi function can be related to entanglement properties of the
corresponding states. Here, we generalize this relation to an arbitrary number
of spins in a pure state. It is shown that the second moment of the Husimi
function can be expressed in terms of the lengths of the concurrence vectors
for all possible partitions of the N-spin system in two subsystems. This
relation implies that the phase space distribution of an entangled state is
less localized than that of a non-entangled state. As an example, the second
moment of the Husimi function is analyzed for an Ising chain subject to a
magnetic field perpendicular to the chain axis.


title:
Generando entrelazamiento en cadenas XY - (Generating entanglement in XY
  chains)

abstract:
  Se estudia en este trabajo la capacidad de generar entrelazamiento de una
cadena de espines con acoplamiento de Heisenberg XY y un campo magnetico
uniforme a partir de un estado inicial en el que los espines estan
completamente alineados. Se encuentra que la capacidad de generar estados
entrelazados no muestra un comportamiento monotono con el campo presentando, en
cambio, plateaus y resonancias. Tambien se muestra que, a pesar de que la
anisotropia es necesaria para que se generen estados entrelazados, una mayor
anisotropia no implica necesariamente mejores condiciones para generar
entrelazamiento que sirva para usarse en una computadora cuantica. Inclusive,
se observa que, se genera una cantidad finita de entrelazamiento en el limite
de pequena anisotropia.
  (The maximum entanglement reached by an initially fully aligned state
evolving in a XY Heisenberg spin chain placed in a uniform transverse magnetic
field is studied. It is shown that the capacity to create entangled states
(both of one qubit with the rest of the chain and pairwise between two adjacent
qubits) is not a monotonous function of the magnetic field: it presents
plateaus and resonances. It is also shown that, though anisotropy in the
interaction is necessary for entanglement generation, the best conditions for
generating entanglement that may be suitable for use in quantum computation is
not that of biggest anisotropy. Moreover finite amounts of entanglement are
generated in the small anisotropy limit.)


title:
Chameleon effect, the range of values hypothesis and reproducing the
  EPR-Bohm correlations

abstract:
  We present a detailed analysis of assumptions that J. Bell used to show that
local realism contradicts QM. We find that Bell's viewpoint on realism is
nonphysical, because it implicitly assume that observed physical variables
coincides with ontic variables (i.e., these variables before measurement). The
real physical process of measurement is a process of dynamical interaction
between a system and a measurement device. Therefore one should check the
adequacy of QM not to ``Bell's realism,'' but to adaptive realism (chameleon
realism). Dropping Bell's assumption we are able to construct a natural
representation of the EPR-Bohm correlations in the local (adaptive) realistic
approach.


title: Symmetric Informationally Complete Measurements of Arbitrary Rank

abstract:
  There has been much interest in so-called SIC-POVMs: rank 1 symmetric
informationally complete positive operator valued measures. In this paper we
discuss the larger class of POVMs which are symmetric and informationally
complete but not necessarily rank 1. This class of POVMs is of some independent
interest. In particular it includes a POVM which is closely related to the
discrete Wigner function. However, it is interesting mainly because of the
light it casts on the problem of constructing rank 1 symmetric informationally
complete POVMs. In this connection we derive an extremal condition alternative
to the one derived by Renes et al.


title: Concerning Dice and Divinity

abstract:
  Einstein initially objected to the probabilistic aspect of quantum mechanics
- the idea that God is playing at dice. Later he changed his ground, and
focussed instead on the point that the Copenhagen Interpretation leads to what
Einstein saw as the abandonment of physical realism. We argue here that
Einstein's initial intuition was perfectly sound, and that it is precisely the
fact that quantum mechanics is a fundamentally probabilistic theory which is at
the root of all the controversies regarding its interpretation. Probability is
an intrinsically logical concept. This means that the quantum state has an
essentially logical significance. It is extremely difficult to reconcile that
fact with Einstein's belief, that it is the task of physics to give us a vision
of the world apprehended sub specie aeternitatis. Quantum mechanics thus
presents us with a simple choice: either to follow Einstein in looking for a
theory which is not probabilistic at the fundamental level, or else to accept
that physics does not in fact put us in the position of God looking down on
things from above. There is a widespread fear that the latter alternative must
inevitably lead to a greatly impoverished, positivistic view of physical
theory. It appears to us, however, that the truth is just the opposite. The
Einsteinian vision is much less attractive than it seems at first sight. In
particular, it is closely connected with philosophical reductionism.


title:
Reversible and irreversible dynamics of a qubit interacting with a small
  environment

abstract:
  We analyze the dynamics of a system qubit interacting by means a sequence of
pairwise collisions with an environment consisting of just two qubits. We show
that the density operator of the qubits approaches a common time averaged
equilibrium state, characterized by large fluctuations, only for a random
sequence of collisions. For a regular sequence of collisions the qubitstates of
the system and of the reservoir undergo instantaneous periodic oscillations and
do not relax to a common state. Furthermore we show that pure bipartite
entanglement is developed only when at least two qubits are initially in the
same purestate while otherwise also genuine multipartite entanglement builds
up.


title: An environment-mediated quantum deleter

abstract:
  Environment-induced decoherence presents a great challenge to realizing a
quantum computer. We point out the somewhat surprising fact that decoherence
can be useful, indeed necessary, for practical quantum computation, in
particular, for the effective erasure of quantum memory in order to initialize
the state of the quantum computer. The essential point behind the deleter is
that the environment, by means of a dissipative interaction, furnishes a
contractive map towards a pure state. We present a specific example of an
amplitude damping channel provided by a two-level system's interaction with its
environment in the weak Born-Markov approximation. This is contrasted with a
purely dephasing, non-dissipative channel provided by a two-level system's
interaction with its environment by means of a quantum nondemolition
interaction. We point out that currently used state preparation techniques, for
example using optical pumping, essentially perform as quantum deleters.


title: Statistics dependence of the entanglement entropy

abstract:
  The entanglement entropy of a distinguished region of a quantum many-body
system reflects the entanglement present in its pure ground state. In this
work, we establish scaling laws for this entanglement for critical quasi-free
fermionic and bosonic lattice systems, without resorting to numerical means. We
consider the geometrical setting of D-dimensional half-spaces which allows us
to exploit a connection to the one-dimensional case. Intriguingly, we find a
difference in the scaling properties depending on whether the system is bosonic
- where an area-law is first proven to hold - or fermionic, extending previous
findings for cubic regions. For bosonic systems with nearest neighbor
interaction we prove the conjectured area-law by computing the logarithmic
negativity analytically. We identify a length scale associated with
entanglement, different from the correlation length. For fermions we determine
the logarithmic correction to the area-law, which depends on the topology of
the Fermi surface. We find that Lifshitz quantum phase transitions are
accompanied with a non-analyticity in the prefactor of the leading order term.


title:
Quantum Optical Random Walk: Quantization Rules and Quantum Simulation
  of Asymptotics

abstract:
  Rules for quantizing the walker+coin parts of a classical random walk are
provided by treating them as interacting quantum systems. A quantum optical
random walk (QORW), is introduced by means of a new rule that treats quantum or
classical noise affecting the coin's state, as sources of quantization. The
long term asymptotic statistics of QORW walker's position that shows enhanced
diffusion rates as compared to classical case, is exactly solved. A quantum
optical cavity implementation of the walk provides the framework for quantum
simulation of its asymptotic statistics. The simulation utilizes interacting
two-level atoms and/or laser randomly pulsating fields with fluctuating
parameters.


title:
Entanglement evolution for excitons of two separate quantum dots in a
  cavity driven by magnetic field

abstract:
  The time evolution of entanglement for excitons in two quantum dots embedded
in a single mode cavity is studied in a ``spin-boson'' regime. It is found that
although with the dissipation from the boson mode, the excitons in the two
quantum dots can be entangled by only modulating their energy bias $\epsilon$
under the influence of external driving magnetic field. Initially, the two
excitons are prepared in a pure separate state. When the time-dependent
magnetic field is switched on, a highly entangled state is produced and
maintained even in a very long time interval. The mechanism may be used to
control the quantum devices in practical applications.


title: Suppression of decoherence by bath ordering

abstract:
  The dynamics of two coupled spins-1/2 coupled to a spin-bath is studied as an
extended model of the Tessieri-Wilkie Hamiltonian \cite{TWmodel}. The pair of
spins served as an open subsystem were prepared in one of the Bell states and
the bath consisted of some spins-1/2 is in a thermal equilibrium state from the
very beginning. It is found that with the increasing the coupling strength of
the bath spins, the bath forms a resonant antiferromagnetic order. The
polarization correlation between the two spins of the subsystem and the
concurrence are recovered in some extent to the isolated subsystem. This
suppression of the subsystem decoherence may be used to control the quantum
devices in practical applications.


title:
Single photon state generation from a continuous-wave non-degenerate
  optical parametric oscillator

abstract:
  We present a theoretical treatment of conditional preparation of one-photon
states from a continuous-wave non-degenerate optical parametric oscillator. We
obtain an analytical expression for the output state Wigner function, and we
maximize the one-photon state fidelity by varying the temporal mode function of
the output state. We show that a higher production rate of high fidelity Fock
states is obtained if we condition the outcome on dark intervals around trigger
photo detection events.


title: Polynomial scheme for time evolution of open and closed quantum systems

abstract:
  Based on the generation function of Laguerre polynomials, We proposed a new
Laguerre polynomial expansion scheme in the calculation of evolution of time
dependent Schr\"odinger equation. Theoretical analysis and numerical test show
that the method is equally as good as Chebyshev polynomial expansion method in
efficiency and accuracy, with extra merits that no scaling to Hamiltonian is
needed and wider suitability.


title: Level crossing and quantum phase transition of the XY model

abstract:
  The ground state of a one-dimensional spin-1/2 chain with periodical boundary
condition in the Heisenberg XY model is investigated. We consider the spatial
correlation and concurrence between any nearest-neighbor pair of spins under
the conditions of different coupling strength, anisotropic parameter and
magnitude of a transverse field. Quantum phase transitions due to the
competition between coupling and alignment which cause the abrupt changes of
the correlation and concurrence are observed. The transition are direct results
of the level crossing.


title: Entanglement entropy and the simulation of Quantum Mechanics

abstract:
  The relation between entanglement entropy and the computational difficulty of
classically simulating Quantum Mechanics is briefly reviewed. Matrix product
states are proven to provide an efficient representation of one-dimensional
quantum systems. Further applications of the techniques based on matrix product
states, some of their spin-off and their recent generalizations to scale
invariant theories and higher dimensions systems are also discussed.


title:
Improving information/disturbance and estimation/distortion trade-offs
  with non universal protocols

abstract:
  We analyze in details a conditional measurement scheme based on linear
optical components, feed-forward loop and homodyne detection. The scheme may be
used to achieve two different tasks. On the one hand it allows the extraction
of information with minimum disturbance about a set of coherent states. On the
other hand, it represents a nondemolitive measurement scheme for the
annihilation operator, i.e. an indirect measurement of the Q-function. We
investigate the information/disturbance trade-off for state inference and
introduce the estimation/distortion trade-off to assess estimation of the
Q-function. For coherent states chosen from a Gaussian set we evaluate both
information/disturbance and estimation/distortion trade-offs and found that non
universal protocols may be optimized in order to achieve better performances
than universal ones. For Fock number states we prove that universal protocols
do not exist and evaluate the estimation/distortion trade-off for a thermal
distribution.


title:
A direct approach to fault-tolerance in measurement-based quantum
  computation via teleportation

abstract:
  We discuss a simple variant of the one-way quantum computing model [R.
Raussendorf and H.-J. Briegel, PRL 86, 5188, 2001], called the Pauli
measurement model, where measurements are restricted to be along the eigenbases
of the Pauli X and Y operators, while auxiliary qubits can be prepared both in
the $\ket{+_{\pi\over 4}}:={1/\sqrt{2}}(\ket{0}+e^{i{\pi\over 4}}\ket{1})$
state, and the usual $\ket{+}:={1/ \sqrt{2}}(\ket{0}+\ket{1})$ state. We prove
the universality of this quantum computation model, and establish a
standardization procedure which permits all entanglement and state preparation
to be performed at the beginning of computation. This leads us to develop a
direct approach to fault-tolerance by simple transformations of the
entanglement graph and preparation operations, while error correction is
performed naturally via syndrome-extracting teleportations.


title: Quantum Brownian motion for periodic coupling to an Ohmic bath

abstract:
  We show theoretically how the periodic coupling between an engineered
reservoir and a quantum Brownian particle leads to the formation of a dynamical
steady state which is characterized by an effective temperature above the
temperature of the environment. The average steady state energy of the system
has a higher value than expected from the environmental properties. The system
experiences repeatedly a non-Markovian behavior -- as a consequence the
corresponding effective decay for long evolution times is always on average
stronger than the Markovian one. We also highlight the consequences of the
scheme to the Zeno--anti-Zeno crossover which depends, in addition to the
periodicity $\tau$, also on the total evolution time of the system.


title: Quantum Thermodynamic Cycles and quantum heat engines

abstract:
  In order to describe quantum heat engines, here we systematically study
isothermal and isochoric processes for quantum thermodynamic cycles. Based on
these results the quantum versions of both the Carnot heat engine and the Otto
heat engine are defined without ambiguities. We also study the properties of
quantum Carnot and Otto heat engines in comparison with their classical
counterparts. Relations and mappings between these two quantum heat engines are
also investigated by considering their respective quantum thermodynamic
processes. In addition, we discuss the role of Maxwell's demon in quantum
thermodynamic cycles. We find that there is no violation of the second law,
even in the existence of such a demon, when the demon is included correctly as
part of the working substance of the heat engine.


title: Efficient Dynamic Nuclear Polarization at High Magnetic Fields

abstract:
  By applying a new technique for dynamic nuclear polarization involving
simultaneous excitation of electronic and nuclear transitions, we have enhanced
the nuclear polarization of the nitrogen nuclei in 15N@C60 by a factor of 1000
at a fixed temperature of 3 K and a magnetic field of 8.6 T, more than twice
the maximum enhancement reported to date. This methodology will allow the
initialization of the nuclear qubit in schemes exploiting N@C60 molecules as
components of a quantum information processing device.


title:
Efficiency in Quantum Key Distribution Protocols with Entangled Gaussian
  States

abstract:
  Quantum key distribution (QKD) refers to specific quantum strategies which
permit the secure distribution of a secret key between two parties that wish to
communicate secretly. Quantum cryptography has proven unconditionally secure in
ideal scenarios and has been successfully implemented using quantum states with
finite (discrete) as well as infinite (continuous) degrees of freedom. Here, we
analyze the efficiency of QKD protocols that use as a resource entangled
gaussian states and gaussian operations only. In this framework, it has already
been shown that QKD is possible (M. Navascu\'es et al. Phys. Rev. Lett. 94,
010502 (2005)) but the issue of its efficiency has not been considered. We
propose a figure of merit (the efficiency $E$) to quantify the number of
classical correlated bits that can be used to distill a key from a sample of
$N$ entangled states. We relate the efficiency of the protocol to the
entanglement and purity of the states shared between the parties.


title: Constructing Operator Valued Probability Measures in Phase Space

abstract:
  Probability measures (quasi probability mass), given in the form of integrals
of Wigner function over areas of the underlying phase space, give rise to
operator valued probability measures (OVM). General construction methods of
OVMs, are investigated in terms of geometric positive trace increasing maps
(PTI), for general 1D domains, as well as 2D shapes e.g. circles, disks.
Spectral properties of OVMs and operational implementations of their
constructing PITs are discussed.


title: Cartoon Computation: Quantum-like computing without quantum mechanics

abstract:
  We present a computational framework based on geometric structures. No
quantum mechanics is involved, and yet the algorithms perform tasks analogous
to quantum computation. Tensor products and entangled states are not needed --
they are replaced by sets of basic shapes. To test the formalism we solve in
geometric terms the Deutsch-Jozsa problem, historically the first example that
demonstrated the potential power of quantum computation. Each step of the
algorithm has a clear geometric interpetation and allows for a cartoon
representation.


title:
Complex magnetic monopoles and geometric phases around diabolic and
  exceptional points

abstract:
  We study the geometric phase (GP)in presence of diabolic (DP) and exceptional
(EP) points. While the GP associated with the DP is the flux of the Dirac
monopole, the GP related to the EP, being complex one, is described by the flux
of complex magnetic monopole. For open systems, in week-coupling limit, the
leading environment-induced contribution to the real part of complex GP is
given by a quadrupole term, and to its imaginary part by a dipolelike field. We
find that the GP has a finite gap at the DP and infinite one at the EP.


title: Decoherence Control in Open Quantum System via Classical Feedback

abstract:
  In this work we propose a novel strategy using techniques from systems theory
to completely eliminate decoherence and also provide conditions under which it
can be done so. A novel construction employing an auxiliary system, the bait,
which is instrumental to decoupling the system from the environment is
presented. Our approach to decoherence control in contrast to other approaches
in the literature involves the bilinear input affine model of quantum control
system which lends itself to various techniques from classical control theory,
but with non-trivial modifications to the quantum regime. The elegance of this
approach yields interesting results on open loop decouplability and Decoherence
Free Subspaces(DFS). Additionally, the feedback control of decoherence may be
related to disturbance decoupling for classical input affine systems, which
entails careful application of the methods by avoiding all the quantum
mechanical pitfalls. In the process of calculating a suitable feedback the
system has to be restructured due to its tensorial nature of interaction with
the environment, which is unique to quantum systems. The results are
qualitatively different and superior to the ones obtained via master equations.
Finally, a methodology to synthesize feedback parameters itself is given, that
technology permitting, could be implemented for practical 2-qubit systems to
perform decoherence free Quantum Computing.


title: Covariance matrices and the separability problem

abstract:
  We propose a unifying approach to the separability problem using covariance
matrices of locally measurable observables. From a practical point of view, our
approach leads to strong entanglement criteria that allow to detect the
entanglement of many bound entangled states in higher dimensions and which are
at the same time necessary and sufficient for two qubits. From a fundamental
perspective, our approach leads to insights into the relations between several
known entanglement criteria -- such as the computable cross norm and local
uncertainty criteria -- as well as their limitations.


title: Comment on "The Free Will Theorem"

abstract:
  In a recent paper [quant-ph/0604079], Conway and Kochen claim to have
established that theories of the GRW type, i.e., of spontaneous wave function
collapse, cannot be made relativistic. On the other hand, relativistic GRW-type
theories have already been presented, in my recent paper [quant-ph/0406094] and
by Dowker and Henson [J. Statist. Phys. 115: 1327 (2004), quant-ph/0209051].
Here, I elucidate why these are not excluded by the arguments of Conway and
Kochen.


title: Finding flows in the one-way measurement model

abstract:
  The one-way measurement model is a framework for universal quantum
computation, in which algorithms are partially described by a graph G of
entanglement relations on a collection of qubits. A sufficient condition for an
algorithm to perform a unitary embedding between two Hilbert spaces is for the
graph G, together with input/output vertices I, O \subset V(G), to have a flow
in the sense introduced by Danos and Kashefi [quant-ph/0506062]. For the
special case of |I| = |O|, using a graph-theoretic characterization, I show
that such flows are unique when they exist. This leads to an efficient
algorithm for finding flows, by a reduction to solved problems in graph theory.


title: Distribution of bipartite entanglement for random pure states

abstract:
  We calculate analytic expressions for the distribution of bipartite
entanglement for pure random quantum states. All moments of the purity
distribution are derived and an asymptotic expansion for the purity
distribution itself is deduced. An approximate expression for moments and
distribution of Meyer-Wallach entanglement for random pure states is then
obtained.


title:
Intrinsic unsharpness and approximate repeatability of quantum
  measurements

abstract:
  The intrinsic unsharpness of a quantum observable is studied by introducing
the notion of resolution width. This quantification of accuracy is shown to be
closely connected with the possibility of making approximately repeatable
measurements. As a case study, the intrinsic unsharpness and approximate
repeatability of position and momentum measurements are examined in detail.


title:
From the Mendeleev periodic table to particle physics and back to the
  periodic table

abstract:
  We briefly describe in this paper the passage from Mendeleev's chemistry
(1869) to atomic physics (in the 1900's), nuclear physics (in the 1932's) and
particle physics (from 1953 to 2006). We show how the consideration of
symmetries, largely used in physics since the end of the 1920's, gave rise to a
new format of the periodic table in the 1970's. More specifically, this paper
is concerned with the application of the group SO(4,2)xSU(2) to the periodic
table of chemical elements. It is shown how the Madelung rule of the atomic
shell model can be used for setting up a periodic table that can be further
rationalized via the group SO(4,2)xSU(2) and some of its subgroups. Qualitative
results are obtained from this nonstandard table.


title:
Comment on "Position-dependent effective mass Dirac equations with PT-
  symmetric and non - PT- symmetric potentials" [J. Phys. A: Math. Gen. 39
  (2006) 11877--11887]

abstract:
  Jia and Dutra (J. Phys. A: Math. Gen. 39 (2006) 11877) have considered the
one-dimensional non-Hermitian complexified potentials with real spectra in the
context of position-dependent mass in Dirac equation. In their second example,
a smooth step shape mass distribution is considered and a non-Hermitian non -
PT- symmetric Lorentz vector potential is obtained. They have mapped this
problem into an exactly solvable Rosen-Morse Schrodinger model and claimed that
the energy spectrum is real. The energy spectrum they have reported is pure
imaginary or at best forms an empty set. Their claim on the reality of the
energy spectrum is fragile, therefore.


title: The Converse Part of The Theorem for Quantum Hoeffding Bound

abstract:
  We prove the converse part of the theorem for quantum Hoeffding bound on the
asymptotics of quantum hypothesis testing, essentially based on an argument
developed by Nussbaum and Szkola in proving the converse part of the quantum
Chernoff bound. Our result complements Hayashi's proof of the direct
(achievability) part of the theorem, so that the quantum Hoeffding bound has
now been established.


title:
Teleportation of arbitrary $n$-qudit state with multipartite
  entanglement

abstract:
  We propose a protocol ${\cal D}_n$ for faithfully teleporting an arbitrary
$n$-qudit state with the tensor product state (TPS) of $n$ generalized Bell
states (GBSs) as the quantum channel. We also put forward explicit protocol
${\cal D}'_n$ and ${\cal D}''_n$ for faithfully teleporting an arbitrary
$n$-qudit state with two classes of $2n$-qudit GESs as the quantum channel,
where the GESs are a kind of genuine entangled states we construct and can not
be reducible to the TPS of $n$ GBSs.


title:
Strong electromagnetically induced transparency in atomic media with
  large residual Doppler broadening

abstract:
  Electromagnetically induced transparency and coherent population trapping
were observed in a hot (1000 K) calcium vapor embedded into an electrical gas
discharge. Unexpectedly large transparencies (of up to 70%) were observed under
very unfavorable conditions: probe wavelength shorter than the coupling
wavelength, and coupling Rabi frequency significantly smaller than the residual
Doppler linewidth of the two photon transition. We developed a theoretical
model that shows that the observed results are due to the combined effects of a
strong probe beam and a small open character of the atomic system. Coherent
population trapping also manifests itself as a change in the impedance of the
gas discharge, and the phenomenon can be probed with high sensitivity via the
optogalvanic effect.


title: Full separability criterion for tripartite quantum systems

abstract:
  In this paper, an intuitive approach is employed to generalize the full
separability criterion of tripartite quantum states of qubits to the
higher-dimensional systems (Phys. Rev. A \textbf{72}, 022333 (2005)). A
distinct characteristic of the present generalization is that less restrictive
conditions are needed to characterize the properties of full separability.
Furthermore, the formulation for pure states can be conveniently extended to
the case of mixed states by utilizing the kronecker product approximate
technique. As applications, we give the analytic approximation of the criterion
for weakly mixed tripartite quantum states and investigate the full
separability of some weakly mixed states.


title: Decohering histories and open quantum systems

abstract:
  I briefly review the ''decohering histories'' or ''consistent histories''
formulation of quantum theory, due to Griffiths, Omnes, and Gell-Mann and
Hartle (and the subject of my graduate work with George Sudarshan). I also sift
through the many meanings that have been attached to decohering histories, with
an emphasis on the most basic one: Decoherence of appropriate histories is
needed to establish that quantum mechanics has the correct classical limit.
Then I will describe efforts to find physical mechanisms that do this. Since
most work has focused on density matrix versions of decoherence, I'll consider
the relation between the two formulations, which historically has not been
straightforward. Finally, I'll suggest a line of research that would use recent
results by Sudarshan to illuminate this aspect of the classical limit of
quantum theory.


title: Quantum Lower Bounds by Entropy Numbers

abstract:
  We use entropy numbers in combination with the polynomial method to derive a
new general lower bound for the n-th minimal error in the quantum setting of
information-based complexity. As an application, we improve some lower bounds
on quantum approximation of embeddings between finite dimensional L_p spaces
and of Sobolev embeddings.


title: Cloning and Broadcasting in Generic Probabilistic Theories

abstract:
  We prove generic versions of the no-cloning and no-broadcasting theorems,
applicable to essentially {\em any} non-classical finite-dimensional
probabilistic model that satisfies a no-signaling criterion. This includes
quantum theory as well as models supporting ``super-quantum'' correlations that
violate the Bell inequalities to a larger extent than quantum theory. The proof
of our no-broadcasting theorem is significantly more natural and more
self-contained than others we have seen: we show that a set of states is
broadcastable if, and only if, it is contained in a simplex whose vertices are
cloneable, and therefore distinguishable by a single measurement. This
necessary and sufficient condition generalizes the quantum requirement that a
broadcastable set of states commute.


title:
Using Disentangled States and Algorithmic Information Theory to
  Construct a Not P Problem

abstract:
  In this work, are used Chaitin number Omega and the fact that the general
decomposition of an N-way disentangled state is an irreducible sentence whose
number of coefficients grows in a non-polynomial way with N, to construct a
problem that can never be solved in P.


title: Algebraic Quantum Mechanics and Pregeometry

abstract:
  We discuss the relation between the q-number approach to quantum mechanics
suggested by Dirac and the notion of "pregeometry" introduced by Wheeler. By
associating the q-numbers with the elements of an algebra and regarding the
primitive idempotents as "generalized points" we suggest an approach that may
make it possible to dispense with an a priori given space manifold. In this
approach the algebra itself would carry the symmetries of translation,
rotation, etc. Our suggestion is illustrated in a preliminary way by using a
particular generalized Clifford Algebra proposed originally by Weyl, which
approaches the ordinary Heisenberg algebra in a suitable limit. We thus obtain
a certain insight into how quantum mechanics may be regarded as a purely
algebraic theory, provided that we further introduce a new set of
"neighbourhood operators", which remove an important kind of arbitrariness that
has thus far been present in the attempt to treat quantum mechanics solely in
terms of a Heisenberg algebra.


title: Displacement Echoes: Classical Decay and Quantum Freeze

abstract:
  Motivated by neutron scattering experiments, we investigate the decay of the
fidelity with which a wave packet is reconstructed by a perfect time-reversal
operation performed after a phase space displacement. In the semiclassical
limit, we show that the decay rate is generically given by the Lyapunov
exponent of the classical dynamics. For small displacements, we additionally
show that, following a short-time Lyapunov decay, the decay freezes well above
the ergodic value because of quantum effects. Our analytical results are
corroborated by numerical simulations.


title:
Error-detection-based quantum fault tolerance against discrete Pauli
  noise

abstract:
  A quantum computer -- i.e., a computer capable of manipulating data in
quantum superposition -- would find applications including factoring, quantum
simulation and tests of basic quantum theory. Since quantum superpositions are
fragile, the major hurdle in building such a computer is overcoming noise.
  Developed over the last couple of years, new schemes for achieving fault
tolerance based on error detection, rather than error correction, appear to
tolerate as much as 3-6% noise per gate -- an order of magnitude better than
previous procedures. But proof techniques could not show that these promising
fault-tolerance schemes tolerated any noise at all.
  With an analysis based on decomposing complicated probability distributions
into mixtures of simpler ones, we rigorously prove the existence of constant
tolerable noise rates ("noise thresholds") for error-detection-based schemes.
Numerical calculations indicate that the actual noise threshold this method
yields is lower-bounded by 0.1% noise per gate.


title: Quantum complimentarity, erasers and photons

abstract:
  Optical experiments designed to explore quantum complementarity are
reanalyzed. It is argued that, for each, a classical explanation is not only
possible, but more coherent and less contrived. The final conclusion is that
these experiments actually constitute support for criticism of the photon
paradigm of electric charged particle interaction. They offer little or nothing
to say about quantum complementarity once the photon concept is not imposed by
mandate.


title: Four-Photon Interference with Asymmetric Beam Splitters

abstract:
  Two experiments of four-photon interference are performed with two pairs of
photons from parametric down-conversion with the help of asymmetric beam
splitters. The first experiment is a generalization of the Hong-Ou-Mandel
interference effect to two pairs of photons while the second one utilizes this
effect to demonstrate a four-photon de Broglie wavelength of $\lambda/4$ by
projection measurement.


title: Effect of Crystal Quality on HCP-BCC Phase Transition in Solid 4He

abstract:
  The kinetics of HCP-BCC structure phase transition is studied by precise
pressure measurement technique in 4He crystals of different quality. An
anomalous pressure behavior in bad quality crystals under constant volume
conditions is detected just after HCP-BCC structure phase transition. A sharp
pressure drop of 0.2 bar was observed at constant temperature. The subsequent
pressure kinetics is a non-monotonic temperature function. The effect observed
can be explained if we suppose that microscopic liquid droplets appear on the
HCP-BCC interphase region in bad quality crystals. After the interphase region
disappearance, these droplets are crystallized with pressure reduction. It is
shown that this effect is absent in high quality thermal-treated crystals.


title: Mixed-state fidelity and quantum criticality at finite temperature

abstract:
  We extend to finite temperature the fidelity approach to quantum phase
transitions (QPTs). This is done by resorting to the notion of mixed-state
fidelity that allows one to compare two density matrices corresponding to two
different thermal states. By exploiting the same concept we also propose a
finite-temperature generalization of the Loschmidt echo. Explicit analytical
expressions of these quantities are given for a class of quasi-free fermionic
Hamiltonians. A numerical analysis is performed as well showing that the
associated QPTs show their signatures in a finite range of temperatures.


title:
Comment on "Dirac Quantization of Pais-Uhlenbeck Fourth Order
  Oscillator"

abstract:
  The structure of Pais-Uhlenbeck oscillator in the equal-frequency limit has
been recently studied by Mannheim and Davidson [Phys.Rev. A71 (2005), 042110].
It appears that taking this limit, as presented in the above paper, is quite
subtle and the resulting structure of space of states - involved. In order to
clarify the situation we present here the proper way of taking the
equal-frequency limit, first under the assumption that the scalar product in
the space of states is positive defined. We discuss also the case of indefinite
metric space of states. We show that, irrespective of the way the limit is
defined, the limiting theory can be hardly viewed as satisfactory.


title: Information gain in quantum continual measurements

abstract:
  Inspired by works on information transmission through quantum channels, we
propose the use of a couple of mutual entropies to quantify the efficiency of
continual measurement schemes in extracting information on the measured quantum
system. Properties of these measures of information are studied and bounds on
them are derived.


title: Boundary effect of a partition in a quantum well

abstract:
  The paper wishes to demonstrate that, in quantum systems with boundaries,
different boundary conditions can lead to remarkably different physical
behaviour. Our seemingly innocent setting is a one dimensional potential well
that is divided into two halves by a thin separating wall. The two half wells
are populated by the same type and number of particles and are kept at the same
temperature. The only difference is in the boundary condition imposed at the
two sides of the separating wall, which is the Dirichlet condition from the
left and the Neumann condition from the right. The resulting different energy
spectra cause a difference in the quantum statistically emerging pressure on
the two sides. The net force acting on the separating wall proves to be nonzero
at any temperature and, after a weak decrease in the low temperature domain, to
increase and diverge with a square-root-of-temperature asymptotics for high
temperatures. These observations hold for both bosonic and fermionic type
particles, but with quantitative differences. We work out several analytic
approximations to explain these differences and the various aspects of the
found unexpectedly complex picture.


title:
On the impossibility of extracting classical randomness using a quantum
  computer

abstract:
  In this work we initiate the question of whether quantum devices can provide
us with an almost perfect source of classical randomness, and more generally,
suffice for classical cryptographic tasks, such as encryption. Indeed, it is
well known that classical computers are insufficient for either one of these
tasks when all they have access to is a realistic imperfect source of
randomness. On the other hand, quantum physics is inherently probabilistic
which suggests that perhaps quantum computers can provide a reasonable way to
overcome the above mentioned impossibility results. However, we show that this
is not the case in a realistic setting where observations (measurements) are
subject to noise.


title: Local Distinguishability of Any Three Quantum States

abstract:
  We prove that any three linearly independent pure quantum states can always
be locally distinguished with nonzero probability regardless of their
dimension, entanglement, or multipartite structure. Almost always, all three
states can be unambiguously identified. The only exceptional case, where one
state is locally knowable but the other two are not, is found among multi-qubit
states.


title:
A Tight High-Order Entropic Quantum Uncertainty Relation With
  Applications

abstract:
  We derive a new entropic quantum uncertainty relation involving min-entropy.
The relation is tight and can be applied in various quantum-cryptographic
settings.
  Protocols for quantum 1-out-of-2 Oblivious Transfer and quantum Bit
Commitment are presented and the uncertainty relation is used to prove the
security of these protocols in the bounded quantum-storage model according to
new strong security definitions.
  As another application, we consider the realistic setting of Quantum Key
Distribution (QKD) against quantum-memory-bounded eavesdroppers. The
uncertainty relation allows to prove the security of QKD protocols in this
setting while tolerating considerably higher error rates compared to the
standard model with unbounded adversaries. For instance, for the six-state
protocol with one-way communication, a bit-flip error rate of up to 17% can be
tolerated (compared to 13% in the standard model).
  Our uncertainty relation also yields a lower bound on the min-entropy key
uncertainty against known-plaintext attacks when quantum ciphers are composed.
Previously, the key uncertainty of these ciphers was only known with respect to
Shannon entropy.


title:
A derivation of Bell's inequalities in Wigner form from the fundamental
  assumption of statistical mechanics

abstract:
  Bell's inequalities in the form given by Wigner are derived from the
so-called fundamental assumption of statistical mechanics. I then demonstrate
the possible relationship between these inequalities and the second law,
particularly if assuming the Pauli exclusion principle dictates the expected
outcomes.


title:
Improving the security of secure direct communication based on secret
  transmitting order of particles

abstract:
  We analyzed the security of the secure direct communication protocol based on
secret transmitting order of particles recently proposed by Zhu, Xia, Fan, and
Zhang [Phys. Rev. A 73, 022338 (2006)], and found that this scheme is insecure
if an eavesdropper, say Eve, wants to steal the secret message with Trojan
horse attack strategies. The vital loophole in this scheme is that the two
authorized users check the security of their quantum channel only once. Eve can
insert another spy photon, an invisible photon or a delay one in each photon
which the sender Alice sends to the receiver Bob, and capture the spy photon
when it returns from Bob to Alice. After the authorized users check the
security, Eve can obtain the secret message according to the information about
the transmitting order published by Bob. Finally, we present a possible
improvement of this protocol.


title: Multiparty Quantum Remote Secret Conference

abstract:
  We present two schemes for multiparty quantum remote secret conference in
which each legitimate conferee can read out securely the secret message
announced by another one, but a vicious eavesdropper can get nothing about it.
The first one is based on the same key shared efficiently and securely by all
the parties with Greenberger-Horne-Zeilinger (GHZ) states, and each conferee
sends his secret message to the others with one-time pad crypto-system. The
other one is based on quantum encryption with a quantum key, a sequence of GHZ
states shared among all the conferees and used repeatedly after confirming
their security. Both these schemes are optimal as their intrinsic efficiency
for qubits approaches the maximal value.


title: Circular quantum secret sharing

abstract:
  A circular quantum secret sharing protocol is proposed, which is useful and
efficient when one of the parties of secret sharing is remote to the others who
are in adjacent, especially the parties are more than three. We describe the
process of this protocol and discuss its security when the quantum information
carrying is polarized single photons running circularly. It will be shown that
entanglement is not necessary for quantum secret sharing. Moreover, the
theoretic efficiency is improved to approach 100% as almost all the instances
can be used for generating the private key, and each photon can carry one bit
of information without quantum storage. It is straightforwardly to utilize this
topological structure to complete quantum secret sharing with multi-level
two-particle entanglement in high capacity securely.


title:
Simultaneous quantum state teleportation via the locked entanglement
  channel

abstract:
  We present a simultaneous quantum state teleportation scheme, in which
receivers can not recover their quantum state separately. When they want to
recover their respective quantum state, they must perform an unlocking operator
together.


title:
Maximal violation of the CGLMP inequality for infinite dimensional
  states

abstract:
  We present a much simplified version of the CGLMP inequality for the 2 x 2 x
d Bell scenario. Numerical maximization of the violation of this inequality
over all states and measurements suggests that the optimal state is far from
maximally entangled, while the best measurements are the same as conjectured
best measurements for the maximally entangled state. For very large values of d
the inequality seems to reach its minimal value given by the probability
constraints. This gives numerical evidence for a tight quantum Bell inequality
(or generalized Csirelson inequality) for the 2 x 2 x inf scenario.


title:
Efficient classical simulation of the semi-classical Quantum Fourier
  Transform

abstract:
  A number of elegant approaches have been developed for the identification of
quantum circuits which can be efficiently simulated on a classical computer.
Recently, these methods have been employed to demonstrate the classical
simulability of the quantum Fourier transform (QFT). In this note, we show that
one can demonstrate a number of simulability results for QFT circuits in a
straightforward manner using Griffiths and Niu's semi-classical QFT
construction [Phys. Rev. Lett. 76, 3228 (1996)]. We then discuss the
consequences of these results in the context of Shor's factorisation algorithm.


title: Deformation Quantization of Confined Systems

abstract:
  The Weyl-Wigner formulation of quantum confined systems poses several
interesting problems. The energy stargenvalue equation, as well as the
dynamical equation does not display the expected solutions. In this paper we
review some previous results in the subject and add some new contributions. We
reformulate the confined energy eigenvalue equation by adding to the
Hamiltonian a new (distributional) boundary potential. The new Hamiltonian is
proved to be globally defined and self-adjoint. Moreover, it yields the correct
Weyl-Wigner formulation of the confined system.


title: The universe as an eigenstate: spacetime paths and decoherence

abstract:
  This paper describes how the entire universe might be considered an
eigenstate determined by classical limiting conditions within it. This
description is in the context of an approach in which the path of each
relativistic particle in spacetime represents a fine-grained history for that
particle, and a path integral represents a coarse-grained history as a
superposition of paths meeting some criteria. Since spacetime paths are
parametrized by an invariant parameter, not time, histories based on such paths
do not evolve in time but are rather histories of all spacetime. Measurements
can then be represented by orthogonal states that correlate with specific
points in such coarse-grained histories, causing them to decohere, allowing a
consistent probability interpretation. This conception is applied here to the
analysis of the two slit experiment, scattering and, ultimately, the universe
as a whole. The decoherence of cosmological states of the universe then
provides the eigenstates from which our "real" universe can be selected by the
measurements carried out within it.


title: Nonclassicality and decoherence of photon-subtracted squeezed states

abstract:
  We discuss nonclassical properties of single-photon subtracted squeezed
vacuum states in terms of the sub-Poissonian statistics and the negativity of
the Wigner function. We derive a compact expression for the Wigner function
from which we find the region of phase space where Wigner function is negative.
We find an upper bound on the squeezing parameter for the state to exhibit
sub-Poissonian statistics. We then study the effect of decoherence on the
single-photon subtracted squeezed states. We present results for two different
models of decoherence, viz. amplitude decay model and the phase diffusion
model. In each case we give analytical results for the time evolution of the
state. We discuss the loss of nonclassicality as a result of decoherence. We
show through the study of their phase-space properties how these states decay
to vacuum due to the decay of photons. We show that phase damping leads to very
slow decoherence than the photon-number decay.


title: The sixth Hilbert's problem and the principles of quantum informatics

abstract:
  By the example of a Fourier transform, the possibilities of Hilbert space
geometry applications for statistical model construction are analyzed. In
accordance with Bohr's complementarity principle, mutually-complementary
coordinate and momentum representations are presented. It was demonstrated that
the characteristic function of coordinate distribution may be considered as a
convolution of the psi-function in momentum representation and vice versa. The
naturalness of coordinate and momentum operators introduction is demonstrated.
A probabilistic interpretation of Hilbert space geometry is given.
Cauchy-Bunyakowsky (Cauchy-Schwartz), Cramer-Rao and uncertainty inequalities
are considered in the same framework. The principal postulates of quantum
informatics as a natural science are presented. It is demonstrated that quantum
informatics serves as a theoretic basis for both probability theory and quantum
mechanics.


title: Cooling in a Bistable Optical Cavity

abstract:
  We propose a generic approach to nonresonant laser cooling of atoms/molecules
in a bistable optical cavity. The method exemplifies a photonic version of
Sisyphus cooling, in which the matter-dressed cavity extracts energy from the
particles and discharges it to the external field as a result of sudden
transitions between two stable states.


title:
Approximate resonance states in the semigroup decomposition of resonance
  evolution

abstract:
  The semigroup decomposition formalism makes use of the functional model for
$C_{.0}$ class contractive semigroups for the description of the time evolution
of resonances. For a given scattering problem the formalism allows for the
association of a definite Hilbert space state with a scattering resonance. This
state defines a decomposition of matrix elements of the evolution into a term
evolving according to a semigroup law and a background term. We discuss the
case of multiple resonances and give a bound on the size of the background
term. As an example we treat a simple problem of scattering from a square
barrier potential on the half-line.


title:
Multiphoton communication in lossy channels with photon-number entangled
  states

abstract:
  We address binary and quaternary communication channels based on correlated
multiphoton two-mode states of radiation in the presence of losses. The
protocol are based on photon number correlations and realized upon choosing a
shared set of thresholds to convert the outcome of a joint photon number
measurement into a symbol from a discrete alphabet. In particular, we focus on
channels build using feasible photon-number entangled states (PNES) as two-mode
coherently-correlated (TMC) or twin-beam (TWB) states and compare their
performances with that of channels built using feasible classically correlated
(separable) states. We found that PNES provide larger channel capacity in the
presence of loss, and that TWB-based channels may transmit a larger amount of
information than TMC-based ones at fixed energy and overall loss. Optimized bit
discrimination thresholds, as well as the corresponding maximized mutual
information, are explicitly evaluated as a function of the beam intensity and
the loss parameter. The propagation of TMC and TWB in lossy channels is
analyzed and the joint photon number distribution is evaluated, showing that
the beam statistics, either sub-Poissonian for TMC or super-Poissonian for TWB,
is not altered by losses. Although entanglement is not strictly needed to
establish the channels, which are based on photon-number correlations owned
also by separable mixed states, purity of the support state is relevant to
increase security. The joint requirement of correlation and purity individuates
PNES as a suitable choice to build effective channels. The effects of losses on
channel security are briefly discussed.