title: Canonical Form and Separability of PPT States on Multiple Quantum Spaces

abstract:
  By using the "subtracting projectors" method in proving the separability of
PPT states on multiple quantum spaces, we derive a canonical form of PPT states
in ${\Cb}^{K_1} \otimes {\Cb}^{K_2} \otimes ... \otimes {\Cb}^{K_m} \otimes
{\Cb}^N$ composite quantum systems with rank $N$, from which a sufficient
separability condition for these states is presented.


title: Large-D Expansion from Variational Perturbation Theory

abstract:
  We derive recursively the perturbation series for the ground-state energy of
the D-dimensional anharmonic oscillator and resum it using variational
perturbation theory (VPT). From the exponentially fast converging approximants,
we extract the coefficients of the large-D expansion to higher orders. The
calculation effort is much smaller than in the standard field-theoretic
approach based on the Hubbard-Stratonovich transformation.


title:
Phase-dependent interference mechanisms in a three-level Lambda system
  driven by a quantized laser field

abstract:
  The dynamics of an atomic few-level system can depend on the phase of driving
fields coupled to the atom if certain conditions are satisfied. This is of
particular interest to control interference effects, which can alter the system
properties considerably. In this article, we discuss the mechanisms of such
phase control and interference effects in an atomic three-level system in
$\Lambda$ configuration, where the upper state spontaneously decays into the
two lower states. The lower states are coupled by a driving field, which we
treat as quantized. This allows for an interpretation on the single photon
level for both the vacuum and the driving field. By analyzing the system
behavior for a driving field initially in non-classical states with only few
Fock number states populated, we find that even though the driving field is
coupled to the lower states only, it induces a multiplet of upper states. Then
interference occurs independently in three-level subsystems in $V$
configuration, each formed by two adjacent upper states and a single dressed
lower state.


title: Work extraction from tripartite entanglement

abstract:
  The work extractable from correlated bipartite quantum systems can be used to
distinguish entanglement from classical correlation. A natural question is now
whether it can be generalised to multipartite systems. In this paper, we devise
a protocol to distinguish the GHZ, the W, and separable states in terms of the
thermodynamically extractable work under local operations and classical
communication, and compare the results with those obtained from Mermin's
inequalities.


title: Algebraic analysis of quantum search with pure and mixed states

abstract:
  An algebraic analysis of Grover's quantum search algorithm is presented for
the case in which the initial state is an arbitrary pure quantum state of n
qubits. This approach reveals the geometrical structure of the quantum search
process, which turns out to be confined to a four-dimensional subspace of the
Hilbert space. This work unifies and generalizes earlier results on the time
evolution of the amplitudes during the quantum search, the optimal number of
iterations and the success probability. Furthermore, it enables a direct
generalization to the case in which the initial state is a mixed state,
providing an exact formula for the success probability.


title: Quantum Key Distribution with vacuum--one-photon entangled states

abstract:
  We present a scheme to realize a quantum key distribution using vacuum-one
photon entangled states created both from Alice and Bob. The protocol consists
in an exchange of spatial modes between Alice and Bob and in a recombination
which allows one of them to reconstruct the bit encoded by the counterpart in
the phase of the entangled state. The security of the scheme is analyzed
against some simple kind of attack. The model is shown to reach higher
efficiency with respect to prior schemes using phase encoding methods.


title:
Entanglement entropy of fermions in any dimension and the Widom
  conjecture

abstract:
  We show that entanglement entropy of free fermions scales faster then area
law, as opposed to the scaling $L^{d-1}$ for the harmonic lattice, for example.
We also suggest and provide evidence in support of an explicit formula for the
entanglement entropy of free fermions in any dimension $d$, $S\sim
c(\partial\Gamma,\partial\Omega)\cdot L^{d-1}\log L$ as the size of a subsystem
$L\to\infty$, where $\partial\Gamma$ is the Fermi surface and $\partial\Omega$
is the boundary of the region in real space. The expression for the constant
$c(\partial\Gamma,\partial\Omega)$ is based on a conjecture due to H. Widom. We
prove that a similar expression holds for the particle number fluctuations and
use it to prove a two sided estimates on the entropy $S$.


title: Long-lived quantum memory with nuclear atomic spins

abstract:
  We propose to store non-classical states of light into the macroscopic
collective nuclear spin ($10^{18}$ atoms) of a $^3$He vapor, using
metastability exchange collisions. These collisions, commonly used to transfer
orientation from the metastable state $2^{3}S\_1$ to the ground state state of
$^3$He, can also transfer quantum correlations. This gives a possible
experimental scheme to map a squeezed vacuum field state onto a nuclear spin
state with very long storage times (hours).


title:
Entanglement and teleportation of macroscopic continuous variables by
  superconducting devices

abstract:
  A current-biased low-temperature superconducting Josephson junction (JJ) is
dynamically describable by the quantized motion of a fictitious particle in a
"washboard" potential. The long coherence time of tightly-bound states in the
washboard potential of a JJ has prompted the effort to couple JJs and operate
them as entangled qubits, capable of forming building blocks of a scalable
quantum computer. Here we consider a hitherto unexplored quantum aspect of
coupled JJs: the ability to produce Einstein-Podolsky-Rosen (EPR) entanglement
of their continuous variables, namely, their magnetic fluxes and induced
charges. Such entanglement, apart from its conceptual novelty, is the
prerequisite for a far-reaching goal: teleportation of the flux and charge
variables between JJs, implementing the transfer of an unknown quantum state
along a network of such devices.


title: Multipartite reduction criteria for separability

abstract:
  The reduction criterion is a well known necessary condition for separable
states, and states violating this condition are entangled and also
1-distillable. In this paper we introduce a new set of necessary conditions for
separability of multipartite states, obtained from a set of positive but not
completely positive maps. These conditions can be thought of as generalisations
of the reduction criterion to multipartite systems. We use tripartite Werner
states as an example to investigate the entanglement detecting powers of some
of these new conditions, and we also look at what these conditions mean in
terms of distillation. Finally, we show that these maps can be used to give a
partial solution to the subsystem problem, as described in Ref. [14].


title: Decoherent histories and generalized measurements

abstract:
  The theory of decoherent histories allows one to talk of the behavior of
quantum systems in the absence of measurement. This paper generalizes the idea
of decoherent histories to arbitrary open system operations and proposes
experimentally testable criteria for decoherence.


title: General model of quantum key distribution

abstract:
  A general mathematical framework for quantum key distribution based on the
concepts of quantum channel and Turing machine is suggested. The security for
its special case is proved. The assumption is that the adversary can perform
only individual (in essence, classical) attacks. For this case an advantage of
quantum key distribution over classical one is shown.


title: Simple Algorithm for Partial Quantum Search

abstract:
  Quite often in database search, we only need to extract portion of the
information about the satisfying item. Recently Radhakrishnan & Grover [RG]
considered this problem in the following form: the database of $N$ items was
divided into $K$ equally sized blocks. The algorithm has just to find the block
containing the item of interest. The queries are exactly the same as in the
standard database search problem. [RG] invented a quantum algorithm for this
problem of partial search that took about $0.33\sqrt{N/K}$ fewer iterations
than the quantum search algorithm. They also proved that the best any quantum
algorithm could do would be to save $0.78 \sqrt(N/K)$ iterations. The main
limitation of the algorithm was that it involved complicated analysis as a
result of which it has been inaccessible to most of the community. This paper
gives a simple analysis of the algorithm. This analysis is based on three
elementary observations about quantum search, does not require a single
equation and takes less than 2 pages.


title:
Multiparty quantum state sharing of an arbitrary two-particle state with
  Einstein-Podolsky-Rosen pairs

abstract:
  A scheme for multiparty quantum state sharing of an arbitrary two-particle
state is presented with Einstein-Podolsky-Rosen pairs. Any one of the $N$
agents has the access to regenerate the original state with two local unitary
operations if he collaborates with the other agents, say the controllers.
Moreover, each of the controllers is required to take only a product
measurement $\sigma_x \otimes \sigma_x$ on his two particles, which makes this
scheme more convenient for the agents in the applications on a network than
others. As all the quantum source can be used to carry the useful information,
the intrinsic efficiency of qubits approaches the maximal value. With a new
notation for the multipartite entanglement, the sender need only publish two
bits of classical information for each measurement, which reduces the
information exchanged largely.


title: Spontaneous emission and teleportation in cavity QED

abstract:
  In this work, we consider atomic spontaneous emission in a system consisting
of two identical two-level atoms interacting dispersively with the quantized
electromagnetic field in a high-Q cavity. We investigate the destructive effect
of the atomic decay on the generation of maximally entangled states, following
the proposal by Zheng S B and Guo G C (2000 Phys. Rev. Lett. 85 2392). In
particular, we analyze the fidelity of teleportation performed using such a
noisy channel and calculatethe maximum spontaneous decay rate we may have in
order to realize teleportation.


title: On independent permutation separability criteria

abstract:
  Recently P. Wocjan and M. Horodecki [quant-ph/0503129] gave a
characterization of combinatorially independent permutation separability
criteria. Combinatorial independence is a necessary condition for permutations
to yield truly independent criteria meaning that that no criterion is strictly
stronger that any other. In this paper we observe that some of these criteria
are still dependent and analyze why these dependencies occur. To remove them we
introduce an improved necessary condition and give a complete classification of
the remaining permutations. We conjecture that the remaining class of criteria
only contains truly independent permutation separability criteria. Our
conjecture is based on the proof that for two, three and four parties all these
criteria are truly independent and on numerical verification of their
independence for up to 8 parties. It was commonly believed that for three
parties there were 9 independent criteria, here we prove that there are exactly
6 independent criteria for three parties and 22 for four parties.


title: Quantum protocols for anonymous voting and surveying

abstract:
  We describe quantum protocols for voting and surveying. A key feature of our
schemes is the use of entangled states to ensure that the votes are anonymous
and to allow the votes to be tallied. The entanglement is distributed over
separated sites; the physical inaccessibility of any one site is sufficient to
guarantee the anonymity of the votes. The security of these protocols with
respect to various kinds of attack is discussed. We also discuss classical
schemes and show that our quantum voting protocol represents a N-fold reduction
in computational complexity, where N is the number of voters.


title:
Tailoring laser pulses with spectral and fluence constraints using
  optimal control theory

abstract:
  Within the framework of optimal control theory we develop a simple iterative
scheme to determine optimal laser pulses with spectral and fluence constraints.
The algorithm is applied to a one-dimensional asymmetric double well where the
control target is to transfer a particle from the ground state, located in the
left well, to the first excited state, located in the right well. Extremely
high occupations of the first excited state are obtained for a variety of
spectral and/or energetic constraints. Even for the extreme case where no
resonance frequency is allowed in the pulse the algorithm achieves an
occupation of almost 100%.


title: An introduction to entanglement measures

abstract:
  We review the theory of entanglement measures, concentrating mostly on the
finite dimensional two-party case. Topics covered include: single-copy and
asymptotic entanglement manipulation; the entanglement of formation; the
entanglement cost; the distillable entanglement; the relative entropic
measures; the squashed entanglement; log-negativity; the robustness monotones;
the greatest cross-norm; uniqueness and extremality theorems. Infinite
dimensional systems and multi-party settings will be discussed briefly.


title:
Effects of mismatched transmissions on two-mode squeezing and EPR
  correlations with a slow light medium

abstract:
  We theoretically discuss the preservation of squeezing and continuous
variable entanglement of two mode squeezed light when the two modes are
subjected to unequal transmission. One of the modes is transmitted through a
slow light medium while the other is sent through an optical fiber of unit
transmission. Balanced homodyne detection is used to check the presence of
squeezing. It is found that loss of squeezing occurs when the mismatch in the
transmission of the two modes is greater than 40% while near ideal squeezing is
preserved when the transmissions are equal. We also discuss the effect of this
loss on continuous variable entanglement using strong and weak EPR criteria and
possible applications for this experimental scheme.


title:
Few-body spin couplings and their implications for universal quantum
  computation

abstract:
  Electron spins in semiconductor quantum dots are promising candidates for the
experimental realization of solid-state qubits. We analyze the dynamics of a
system of three qubits arranged in a linear geometry and a system of four
qubits arranged in a square geometry. Calculations are performed for several
quantum dot confining potentials. In the three-qubit case, three-body effects
are identified that have an important quantitative influence upon quantum
computation. In the four-qubit case, the full Hamiltonian is found to include
both three-body and four-body interactions that significantly influence the
dynamics in physically relevant parameter regimes. We consider the implications
of these results for the encoded universality paradigm applied to the
four-electron qubit code; in particular, we consider what is required to
circumvent the four-body effects in an encoded system (four spins per encoded
qubit) by the appropriate tuning of experimental parameters.


title: Some Open Problems in Quantum Information Theory

abstract:
  This article is a snap-shot of a web site, which has been collecting open
problems in quantum information for several years, and documenting the progress
made on these problems. By posting it we make the complete collection available
in one printout. We also hope to draw more attention to this project, inviting
every researcher in the field to raise to the challenges, but also to suggest
new problems.


title:
Ambiguities on the quantization of a one-dimensional dissipative system
  with position depending dissipative coefficient

abstract:
  For a one-dimensional dissipative system with position depending coefficient,
two constant of motion are deduce. These constants of motion bring about two
Hamiltonians to describe the dynamics of same classical system. However, their
quantization describe the dynamics of two completely different quantum systems.


title: On the coexistence of position and momentum observables

abstract:
  We investigate the problem of coexistence of position and momentum
observables. We characterize those pairs of position and momentum observables
which have a joint observable.


title: An asymptotical separability criterion for bipartite density operators

abstract:
  For a given density matrix $\rho$ of a bipartite quantum system an
asymptotical separability criterion is suggested. Using the continuous ensemble
method, a sequence of separable density matrices is built which converges to
$\rho$ if and only if $\rho$ is separable. The convergence speed is evaluated
and for any given tolerance parameter $\kappa$ an iterative procedure is
suggested which decides in finite number of steps if there exists a separable
density matrix $\rho_\kappa$ which differs from the matrix $\rho$ by at most
$\kappa$.


title:
The relationship between two flavors of oblivious transfer at the
  quantum level

abstract:
  Though all-or-nothing oblivious transfer and one-out-of-two oblivious
transfer are equivalent in classical cryptography, we here show that due to the
nature of quantum cryptography, a protocol built upon secure quantum
all-or-nothing oblivious transfer cannot satisfy the rigorous definition of
quantum one-out-of-two oblivious transfer.


title:
A realistic device that simulates the non-local PR box without
  communication

abstract:
  A black box with two input bits and two output bits is called a non-local PR
box, if the XOR of the output bits equals the AND of the input bits. In a
recent article, Cerf et al. show that Alice and Bob, using such a PR box, can
effectively simulate entanglement without the need of communication. We show
that an adaptation of a model due to Dirk Aerts, yields a realistic simulation
of the non-local PR box without communication. Because the model is entirely
realistic, it cannot violate relativistic constraints. Like a non-local box, it
can be used to simulate the singlet state coincidence probabilities, but the
time to complete the observation of the outcome will exceed the time it takes a
photon to travel one arm in an EPRB setup. The model explicitly shows how to
produce an outcome that is locally perfectly random, but nevertheless
determines what happens in the other wing of the experiment, without
communication taking place between the two wings. In this sense, it can serve
as an accurate metaphor for the mechanism of entanglement. The model
considerably strengthens the claim that no communication is necessary to
simulate entanglement.


title: Classical Statistics Inherent in a Quantum Density Matrix

abstract:
  A density matrix formulation of classical bipartite correlations is
constructed. This leads to an understanding of the appearance of classical
statistical correlations intertwined with the quantum correlations as well as a
physical underpinning of these correlations. As a byproduct of this analysis, a
physical basis of the classical statistical correlations leading to additive
entropy in a bipartite system discussed recently by Tsallis et al emerges as
inherent classical spin fluctuations. It is found that in this example, the
quantum correlations shrink the region of additivity in phase space.


title: Quantum algorithms without initializing the auxiliary qubits

abstract:
  In this Letter, we construct the quantum algorithms for the Simon problem and
the period-finding problem, which do not require initializing the auxiliary
qubits involved in the process of functional evaluation but are as efficient as
the original algorithms. In these quantum algorithms, one can use any
arbitrarily mixed state as the auxiliary qubits, and furthermore can recover
the state of the auxiliary qubits to the original one after completing the
computations. Since the recovered state can be employed in any other
computations, we obtain that a single preparation of the auxiliary qubits in an
arbitrarily mixed state is sufficient to implement the iterative procedure in
the Simon algorithm or the period-finding algorithm.


title: Theory of Confined Quantum Time of Arrivals

abstract:
  We extend the concept of confined quantum time of arrival operators, first
developed for the free particle [E.A. Galapon, R. Caballar, R. Bahague {\it
Phys. Rev. Let.} {\bf 93} 180406 (2004)], to arbitrary potentials.


title:
Perfect transfer of many-particle quantum state via high-dimensional
  systems with spectrum-matched symmetry

abstract:
  The quantum state transmission (QST) through the medium of high-dimensional
many-particle system is studied with a symmetry analysis. We discover that, if
the spectrum matches the symmetry of a fermion or boson system in a certain
fashion, a perfect quantum state transfer can be implemented without any
operation on the medium. Based on this observation the well-established results
for the QST via quantum spin chains can be generalized to the high-dimensional
many-particle systems with pre-engineered nearest neighbor (NN) hopping
constants. By investigating a simple but realistic near half-filled
tight-binding fermion system with uniform NN hopping integral, we show that an
arbitrary many-particle state near the fermi surface can be perfectly
transferred to its translational counterpart.


title: Laser photon statistics in the feedback loop

abstract:
  A mere correspondence between the electron statistics and the photon one
vanishes in the feedback loop (FBL). It means that the direct photodetection,
supplying us with the electron statistics, does not provide us with a wished
information about the laser photon statistics. For getting this information we
should think up another measurement procedure, and we in the article suggest
applying the three-level laser as a auxiliary measuring device. This laser has
impressive property, namely, its photon statistics survive information about
the initial photon statistics of the laser which excites coherently the
three-level medium. Thus, if we choose the laser in the FBL as exciting the
three-level laser, then we have an possibility to evaluate its initial photon
statistics by means of direct detecting the three-level laser emission.
Finally, this approach allows us to conclude the feedback is not capable of
creating a regularity in the laser light beam. Contrary, the final photon
fluctuations turn out to be always even bigger. The mentioned above feature of
the three-level laser takes place only for the strong interaction between the
lasers (exciting and excited). It means the initial state of the exciting laser
is changed dramatically, so our measurement procedure can not be identified
with some non-demolition one.


title:
How the quality of a received EPR pair depends on the distances from an
  EPR source?

abstract:
  Let an EPR source which generates maximally entangled pairs be located so
that it has distances $L_1$ and $L_2$ to two users. After taking into account
various effects like loss of photons, deficiencies in the source and detectors,
an entangled pair traveling through the channel may loose its perfect
correlation due to errors in the channel. How the entanglement of the received
pair depends on the above distances and the local properties of the channels
used for this transmission? What is the best location of the source if we want
to achieve the highest fidelity? What is the threshold distance beyond which
the entanglement of the pair vanishes and becomes useless for using in
teleportation. We discuss these problems for the Pauli channel which simulates
the effect of optical fibers and possibly the atmosphere on the
polarization-entangled photons.


title: Spectrum of the Relativistic Particles in Various Potentials

abstract:
  We extend the notion of Dirac oscillator in two dimensions, to construct a
set of potentials. These potentials becomes exactly and quasi-exactly solvable
potentials of non-relativistic quantum mechanics when they are transformed into
a Schr\"{o}dinger-like equation. For the exactly solvable potentials,
eigenvalues are calculated and eigenfunctions are given by confluent
hypergeometric functions. It is shown that, our formulation also leads to the
study of those potentials in the framework of the supersymmetric quantum
mechanics.


title:
Entanglement in Quantum Spin Chains, Symmetry Classes of Random
  Matrices, and Conformal Field Theory

abstract:
  We compute the entropy of entanglement between the first $N$ spins and the
rest of the system in the ground states of a general class of quantum
spin-chains. We show that under certain conditions the entropy can be expressed
in terms of averages over ensembles of random matrices. These averages can be
evaluated, allowing us to prove that at critical points the entropy grows like
$\kappa\log_2 N + {\tilde \kappa}$ as $N\to\infty$, where $\kappa$ and ${\tilde
\kappa}$ are determined explicitly. In an important class of systems, $\kappa$
is equal to one-third of the central charge of an associated Virasoro algebra.
Our expression for $\kappa$ therefore provides an explicit formula for the
central charge.


title: On the quantum (in)stability in cavity QED

abstract:
  The stability and instability of quantum motion is studied in the context of
cavity quantum electrodynamics (QED). It is shown that the Jaynes-Cummings
dynamics can be unstable in the regime of chaotic walking of an atom in the
quantized field of a standing wave in the absence of any other interaction with
environment. This quantum instability manifests itself in strong variations of
quantum purity and entropy and in exponential sensitivity of fidelity of
quantum states to small variations in the atom-field detuning. It is quantified
in terms of the respective classical maximal Lyapunov exponent that can be
estimated in appropriate in-out experiments.


title:
Analytical Approach to Wave Packet Dynamics of Laser-Driven Particles
  beyond the Dipole Approximation

abstract:
  An analytical approach to quantum mechanical wave packet dynamics of
laser-driven particles is presented. The time-dependent Schroedinger equation
is solved for an electron exposed to a linearly polarized plane wave of
arbitrary shape. The calculation goes beyond the dipole approximation, such
that magnetic field effects like wave packet shearing are included. Analytical
expressions for the time-dependent widths of the wave packet and its
orientation are established. These allow for a simple understanding of the wave
packet dynamics.


title:
Quantum Message Exchange Based on Entanglement and Bell-State
  Measurements

abstract:
  We propose a scheme by which two parties can secretely and simultaneously
exchange messages. The scheme requires the two parties to share entanglement
and both to perform Bell-state measurements. Only two out of the four Bell
states are required to be distinguished in the Bell-state measurements, and
thus the scheme is experimentally feasible using only linear optical means.
Generalizations of the scheme to high-dimensional systems and to multipartite
entanglement are considered. We show also that the proposed scheme works even
if the two parties do not possess shared reference frames.


title: Deterministic Bell State Discrimination

abstract:
  We make use of local operations with two ancilla bits to deterministically
distinguish all the four Bell states, without affecting the quantum channel
containing these Bell states.


title: Dissipative dynamics of nondegenerate two-photon Jaynes-Cummings model

abstract:
  A nondegenerate two-photon Jaynes-Cummings model is investigated where the
leakage of photon through the cavity is taken into account. The effect of
cavity damping on the mean photon number, atomic populations, field statistics
and both field and atomic squeezing is considered on the basis of master
equation in dressed-state approximation for initial coherent fields and excited
atom.


title: Times of arrival: Bohm beats Kijowski

abstract:
  We prove that the Bohmian arrival time of the 1D Schroedinger evolution
violates the quadratic form structure on which Kijowski's axiomatic treatment
of arrival times is based. Within Kijowski's framework, for a free right moving
wave packet, the various notions of arrival time (at a fixed point x on the
real line) all yield the same average arrival time. We derive an inequality
relating the average Bohmian arrival time to the one of Kijowksi. We prove that
the average Bohmian arrival time is less than Kijowski's one if and only if the
wave packet leads to position probability backflow through x. Otherwise the two
average arrival times coincide.


title: Macroscopic thermal entanglement due to radiation pressure

abstract:
  Can entanglement and the quantum behavior in physical systems survive at
arbitrary high temperatures? In this Letter we show that this is the case for a
electromagnetic field mode in an optical cavity with a movable mirror in a
thermal state. We also identify two different dynamical regimes of generation
of entanglement separated by a critical coupling strength.


title:
Note on "Hyperfine coherence in the presence of spontaneous photon
  scattering" (quant-ph/0502063) and minimum energy requirements for quantum
  logic

abstract:
  The goal of this short note is to show that the formulas I derived originally
in [Phys. Rev. A 65, 022308 (2002)] regarding the errors introduced in quantum
logical operations by the quantum nature of the control fields apply even in
the situation discussed recently by Ozeri et al. in quant-ph/0502063, where the
decoherence-inducing spontaneous Raman scattering is considerably suppressed.


title:
A n-qubit controlled phase gate with superconducting quantum
  interference devices coupled to a resonator

abstract:
  We present a way to realize a $n$-qubit controlled phase gate with
superconducting quantum interference devices (SQUIDs) by coupling them to a
superconducting resonator. In this proposal, the two logical states of a qubit
are represented by the two lowest levels of a SQUID. An intermediate level of
each SQUID is utilized to facilitate coherent control and manipulation of
quantum states of the qubits. It is interesting to note that a $n$-qubit
controlled phase gate can be achieved with $n$ SQUIDs by successively applying
a $\pi /2$ Jaynes-Cummings pulse to each of the $n-1$ control SQUIDs before and
after a $\pi$ Jaynes-Cummings pulse on the target SQUID.


title: Operator quantum error correction

abstract:
  This paper is an expanded and more detailed version of our recent work in
which the Operator Quantum Error Correction formalism was introduced. This is a
new scheme for the error correction of quantum operations that incorporates the
known techniques - i.e. the standard error correction model, the method of
decoherence-free subspaces, and the noiseless subsystem method - as special
cases, and relies on a generalized mathematical framework for noiseless
subsystems that applies to arbitrary quantum operations. We also discuss a
number of examples and introduce the notion of ``unitarily noiseless
subsystems''.


title: Nonlocality without inequality for spin-s system

abstract:
  We analyze Hardy's non-locality argument for two spin-s systems and show that
earlier solution in this regard was restricted due to imposition of some
conditions which have no role in the argument of non-locality. We provide a
compact form of non-locality condition for two spin-s systems and extend it to
n number of spin-s particles. We also apply more general kind of non-locality
argument still without inequality, to higher spin system.


title: Two theorems of Jhon Bell and Communication Complexity

abstract:
  John Bell taught us that quantum mechanics can not be reproduced by
non-contextual and local Hidden variable theory. The impossibility of replacing
quantum mechanics by non-contextual Hidden Variable Theory can be turned to a
impossible coloring pseudo-telepathy game to be played by two distant players.
The game can not be won without communication in the classical world. But if
the players share entangled state (quantum correlation) the game can be won
deterministically using no communication. This again shows that though quantum
correlation can not be used for communication, two parties can not simulate
quantum correlation without classical communication. The motivation of the
article is to present the earlier works on Hidden Variable Theory and recently
developed pseudo-telepathy problem in a simpler way, which may be helpful for
the beginners in this area.


title: An Entangled Web of Crime: Bell's Theorem as a Short Story

abstract:
  Non-locality of the type first elucidated by Bell in 1964 is a difficult
concept to explain to non-specialists and undergraduates. Here we attempt this
by showing how such non-locality can be used to solve a problem in which
someone might find themselves as the result of a collection of normal, even if
somewhat unlikely, events. Our story is told in the style of a Sherlock Holmes
mystery, and is based on Mermin's formulation of the "paradoxical" illustration
of quantum non-locality discovered by Greenberger, Horne and Zeilinger.


title:
On the Interpretation of the 'Extra' Variables in the Hurwitz
  Transformation

abstract:
  The Hurwits transformation is generalized by introducing three new variables
called 'extra'. Interpretation of these extra variables allows us to establish
relation between isotropic harmonic oscillator and five-dimensional
hydrogen-like atom in 'electromagnetic' fields of various configurations. For
the Schrodinger equations, the scheme of separation of extra variables is
suggested.


title:
The Sturm-Liouville eigenvalue problem and NP-complete problems in the
  quantum setting with queries

abstract:
  We show how a number of NP-complete as well as NP-hard problems can be
reduced to the Sturm-Liouville eigenvalue problem in the quantum setting with
queries. We consider power queries which are derived from the propagator of a
system evolving with a Hamiltonian obtained from the discretization of the
Sturm-Liouville operator. We show that the number of power queries as well the
number of qubits needed to solve the problems studied in this paper is a low
degree polynomial. The implementation of power queries by a polynomial number
of elementary quantum gates is an open issue. If this problem is solved
positively for the power queries used for the Sturm-Liouville eigenvalue
problem then a quantum computer would be a very powerful computation device
allowing us to solve NP-complete problems in polynomial time.


title: Inverting quantum decoherence by classical feedback from the environment

abstract:
  We show that for qubits and qutrits it is always possible to perfectly
recover quantum coherence by performing a measurement only on the environment,
whereas for dimension d>3 there are situations where recovery is impossible,
even with complete access to the environment. For qubits, the minimal amount of
classical information to be extracted from the environment equals the entropy
exchange.


title:
Limit quantum efficiency for violation of Clauser-Horne Inequality for
  qutrits

abstract:
  In this paper we present the results of numerical calculations about the
minimal value of detection efficiency for violating the Clauser - Horne
inequality for qutrits. Our results show how the use of non-maximally entangled
states largely improves this limit respect to maximally entangled ones. A
stronger resistance to noise is also found.


title: Error Correcting the Control Unit in Global Control Schemes

abstract:
  Recent studies of globally controlled structures have culminated in a
theoretical demonstration that fault-tolerant quantum computation can be
carried out on a one--dimensional chain with control over two global fields
only. This required some patterns of classical states to localise operations,
which were stabilised with the Zeno effect. However, it is impossible to
achieve perfect stabilisation using this method, so error correction of these
states is desirable, and is the focus this paper.


title: Quantum vs. Classical Read-once Branching Programs

abstract:
  The paper presents the first nontrivial upper and lower bounds for
(non-oblivious) quantum read-once branching programs. It is shown that the
computational power of quantum and classical read-once branching programs is
incomparable in the following sense: (i) A simple, explicit boolean function on
2n input bits is presented that is computable by error-free quantum read-once
branching programs of size O(n^3), while each classical randomized read-once
branching program and each quantum OBDD for this function with bounded
two-sided error requires size 2^{\Omega(n)}. (ii) Quantum branching programs
reading each input variable exactly once are shown to require size
2^{\Omega(n)} for computing the set-disjointness function DISJ_n from
communication complexity theory with two-sided error bounded by a constant
smaller than 1/2-2\sqrt{3}/7. This function is trivially computable even by
deterministic OBDDs of linear size. The technically most involved part is the
proof of the lower bound in (ii). For this, a new model of quantum
multi-partition communication protocols is introduced and a suitable extension
of the information cost technique of Jain, Radhakrishnan, and Sen (2003) to
this model is presented.


title: Quantum measurement act as a "speech act"

abstract:
  I show that the quantum measurement problem can be understood if the
measurement is seen as a ``speech act'' in the sense of modern language theory.
The reduction of the state vector is in this perspective an intersubjectice --
or better a-subjective -- symbolic process. I then give some perspectives on
applications to the ``Mind-Body problem''.


title:
Quantum Behavior of Deterministic Systems with Information Loss. Path
  Integral Approach

abstract:
  't Hooft's derivation of quantum from classical physics is analyzed by means
of the classical path integral of Gozzi et al.. It is shown how the key element
of this procedure - the loss of information constraint - can be implemented by
means of Faddeev-Jackiw's treatment of constrained systems. It is argued that
the emergent quantum systems are identical with systems obtained in [Phys.Rev.
A 71 (2005) 052507] through Dirac-Bergmann's analysis. We illustrate our
approach with two simple examples - free particle and linear harmonic
oscillator. Potential Liouville anomalies are shown to be absent.


title:
A universal programmable quantum state discriminator that is optimal for
  unambiguously distinguishing between unknown states

abstract:
  We construct a device that can unambiguously discriminate between two unknown
quantum states. The unknown states are provided as inputs, or programs, for the
program registers and a third system, which is guaranteed to be prepared in one
of the states stored in the program registers, is fed into the data register of
the device. The device will then, with some probability of success, tell us
whether the unknown state in the data register matches the state stored in the
first or the second program register. We show that the optimal device, i. e.
the one that maximizes the probability of success, is universal. It does not
depend on the actual unknown states that we wish to discriminate.


title: Optimal Control of Coupled Josephson Qubits

abstract:
  Quantum optimal control theory is applied to two and three coupled Josephson
charge qubits. It is shown that by using shaped pulses a CNOT gate can be
obtained with a trace fidelity > 0.99999 for the two qubits, and even when
including higher charge states, the leakage is below 1%. Yet, the required time
is only a fifth of the pioneering experiment [T. Yamamoto et al., Nature 425
(2003), 941] for otherwise identical parameters. The controls have palindromic
smooth time courses representable by superpositions of a few harmonics. We
outline schemes to generate these shaped pulses such as simple network
synthesis. The approach is easy to generalise to larger systems as shown by a
fast realisation of TOFFOLI's gate in three linearly coupled charge qubits.
Thus it is to be anticipated that this method will find wide application in
coherent quantum control of systems with finite degrees of freedom whose
dynamics are Lie-algebraically closed.


title:
A study of LOCC-detection of a maximally entangled state using
  hypothesis testing

abstract:
  We study how well answer to the question ``Is the given quantum state equal
to a certain maximally entangled state?'' using LOCC, in the context of
hypothesis testing. Under several locality and invariance conditions, optimal
tests will be derived for several special cases by using basic theory of group
representations. Some optimal tests are realized by performing quantum
teleportation and checking whether the state is teleported. We will also give a
finite process for realizing some optimal tests. The performance of the tests
will be numerically compared.


title: Separability and Entanglement-Breaking in Infinite Dimensions

abstract:
  In this paper we give a general integral representation for separable states
in the tensor product of infinite dimensional Hilbert spaces and provide the
first example of separable states that are not countably decomposable. We also
prove the structure theorem for the quantum communication channels that are
entanglement-breaking, generalizing the finite-dimensional result of M.
Horodecki, Ruskai and Shor. In the finite dimensional case such channels can be
characterized as having the
  Kraus representation with operators of rank 1. The above example implies
existence of infinite-dimensional entanglement-breaking channels having no such
representation.


title: Holonomic Quantum Computation

abstract:
  In this brief review we describe the idea of holonomic quantum computation.
The idea of geometric phase and holonomy is introduced in a general way and we
provide few examples that should help the reader understand the issues
involved.


title: Long distance tunneling

abstract:
  Quantum tunneling between two potential wells in a magnetic field can be
strongly increased when the potential barrier varies in the direction
perpendicular to the line connecting the two wells and remains constant along
this line. A periodic structure of the wave function is formed in the direction
joining the wells. The resulting motion can be coherent like motion in a
conventional narrow band periodic structure. A particle penetrates the barrier
over a long distance which strongly contrasts to WKB-like tunneling. The whole
problem is stationary. The coherent process can be influenced by dissipation.


title: Applications of quantum message sealing

abstract:
  In 2003, Bechmann-Pasquinucci introduced the concept of quantum seals, a
quantum analogue to wax seals used to close letters and envelopes. Since then,
some improvements on the method have been found. We first review the current
quantum sealing techniques, then introduce and discuss potential applications
of quantum message sealing, and conclude with some discussion of the
limitations of quantum seals.


title: GHZ extraction yield for multipartite stabilizer states

abstract:
  Let $|\Psi>$ be an arbitrary stabilizer state distributed between three
remote parties, such that each party holds several qubits. Let $S$ be a
stabilizer group of $|\Psi>$. We show that $|\Psi>$ can be converted by local
unitaries into a collection of singlets, GHZ states, and local one-qubit
states. The numbers of singlets and GHZs are determined by dimensions of
certain subgroups of $S$. For an arbitrary number of parties $m$ we find a
formula for the maximal number of $m$-partite GHZ states that can be extracted
from $|\Psi>$ by local unitaries. A connection with earlier introduced measures
of multipartite correlations is made. An example of an undecomposable
four-party stabilizer state with more than one qubit per party is given. These
results are derived from a general theoretical framework that allows one to
study interconversion of multipartite stabilizer states by local Clifford group
operators. As a simple application, we study three-party entanglement in
two-dimensional lattice models that can be exactly solved by the stabilizer
formalism.


title: Phase randomization improves the security of quantum key distribution

abstract:
  Ideal quantum key distribution (QKD) protocols call for a source that emits
single photon signals, but the sources used in typical practical realizations
emit weak coherent states instead. A weak coherent state may contain more than
one photon, which poses a potential security risk. QKD with weak coherent state
signals has nevertheless been proven to be secure, but only under the
assumption that the phase of each signal is random (and completely unknown to
the adversary). Since this assumption need not be fully justified in practice,
it is important to know whether phase randomization is really a requirement for
security rather than a convenient technical assumption that makes the security
proof easier. Here, we exhibit an explicit attack in which the eavesdropper
exploits knowledge of the phase of the signals, and show that this attack
allows the eavesdropper to learn every key bit in a parameter regime where a
protocol using phase-randomized signals is provably secure. Thus we demonstrate
that phase randomization really does enhance the security of QKD using weak
coherent states. This result highlights the importance of a careful
characterization of the source for proofs of the security of quantum key
distribution.


title: Wigner function for discrete phase space: exorcising ghost images

abstract:
  We construct, using simple geometrical arguments, a Wigner function defined
on a discrete phase space of arbitrary integer Hilbert-space dimension that is
free of redundancies. ``Ghost images'' plaguing other Wigner functions for
discrete phase spaces are thus revealed as artifacts. It allows to devise a
corresponding phase-space propagator in an unambiguous manner. We apply our
definitions to eigenstates and propagator of the quantum baker map. Scars on
unstable periodic points of the corresponding classical map become visible with
unprecedented resolution.


title: Decoherence models and their effects on quantum maps and algorithms

abstract:
  In this work we study several models of decoherence and how different quantum
maps and algorithms react when perturbed by them. Following closely Ref. [1],
generalizations of the three paradigmatic one single qubit quantum channels
(these are the depolarizing channel, the phase damping channel and the
amplitude damping channel) for the case of an arbitrarily-sized
finite-dimensional Hilbert space are presented, as well as other types of noise
in phase space. More specifically, Grover's search algorithm's response to
decoherence is analyzed; together with those of a family of quantum versions of
chaotic and regular classical maps (the baker's map and the cat maps). A
relationship between how sensitive to decoherence a quantum map is and the
degree of complexity in the dynamics of its associated classical counterpart is
observed; resulting in a clear tendency to react the more decoherently the more
complex the associated classical dynamics is.


title:
Experimental reversion of the optimal quantum cloning and flipping
  processes

abstract:
  The quantum cloner machine maps an unknown arbitrary input qubit into two
optimal clones and one optimal flipped qubit. By combining linear and
non-linear optical methods we experimentally implement a scheme that, after the
cloning transformation, restores the original input qubit in one of the output
channels, by using local measurements, classical communication and feedforward.
This significant teleportation-like method demonstrates how the information is
preserved during the cloning process. The realization of the reversion process
is expected to find useful applications in the field of modern multi-partite
quantum cryptography.


title: A method for teleporting an unknown quantum state and its application

abstract:
  We suggest a method for teleporting an unknown quantum state. In this method
the sender Alice first uses a Controlled-Not operation on the particle in the
unknown quantum state and an ancillary particle which she wants to send to the
receiver Bob. Then she sends ancillary particle to Bob.
  When Alice is informed by Bob that the ancillary particle is received, she
performs a local measurement on the particle and sends Bob the outcome of the
local measurement via a classical channel. Depending on the outcome Bob can
restore the unknown quantum state, which Alice destroyed, on the ancillary
particle successfully. As an application of this method we propose a quantum
secure direct communication protocol.


title:
Book Review: "Quantum Theory as an Emergent Phenomenon", by Stephen L.
  Adler

abstract:
  This is a book review of the book: "Quantum Theory as an Emergent
Phenomenon", by Stephen L. Adler (Cambridge University Press - 2004)


title: Universal Quantum Computation with Shutter Logic

abstract:
  We show that universal quantum logic can be achieved using only linear optics
and a quantum shutter device. With these elements, we design a quantum memory
for any number of qubits and a CNOT gate which are the basis of a universal
quantum computer. An interaction-free model for a quantum shutter is given.


title: Local Cloning of Arbitrarily Entangled Multipartite States

abstract:
  We examine the perfect cloning of non-local, orthogonal states with only
local operations and classical communication. We provide a complete
characterisation of the states that can be cloned under these restrictions, and
their relation to distinguishability. We also consider the case of catalytic
cloning, which we show provides no enhancement to the set of clonable states.


title:
Recursive proof of the Bell-Kochen-Specker theorem in any dimension
  $n>3$

abstract:
  We present a method to obtain sets of vectors proving the Bell-Kochen-Specker
theorem in dimension $n$ from a similar set in dimension $d$ ($3\leq d<n\leq
2d$). As an application of the method we find the smallest proofs known in
dimension five (29 vectors), six (31) and seven (34), and different sets
matching the current record (36) in dimension eight.


title: Quantum accuracy threshold for concatenated distance-3 codes

abstract:
  We prove a new version of the quantum threshold theorem that applies to
concatenation of a quantum code that corrects only one error, and we use this
theorem to derive a rigorous lower bound on the quantum accuracy threshold
epsilon_0. Our proof also applies to concatenation of higher-distance codes,
and to noise models that allow faults to be correlated in space and in time.
The proof uses new criteria for assessing the accuracy of fault-tolerant
circuits, which are particularly conducive to the inductive analysis of
recursive simulations. Our lower bound on the threshold, epsilon_0 > 2.73
\times 10^{-5} for an adversarial independent stochastic noise model, is
derived from a computer-assisted combinatorial analysis; it is the best lower
bound that has been rigorously proven so far.


title:
Atom optics kicked rotor: experimental evidence for a pendulum
  description of the quantum resonance

abstract:
  We present measurements of the mean energy for an atom optics kicked rotor
ensemble close to quantum resonance. Oscillations in the mean energy in this
regime are are shown to be in agreement with a quasi--classical pendulum
approximation. The period of the oscillations is shown to scale with a single
variable, which depends on the number of kicks.


title:
Network for transfer of an arbitrary $n$-qubit atomic state via cavity
  QED

abstract:
  I show a scheme which allows a perfect transfer of an unknown single-qubit
atomic state from one atom to another by letting two atoms interact
simultaneously with a cavity QED. During the interaction between atom and
cavity, the cavity is only virtually excited and accordingly the scheme is
insensitive to the cavity field states and cavity decay. Based on this scheme,
a network for transfer of an arbitrary single-qubit atomic state between atoms
is engineered. Then the scheme is generalized to perfectly transfer an
arbitrary 2-qubit atomic state and accordingly a network for transfer of an
arbitrary 2-qubit atomic state is designed. At last, it is proven that the
schemes can be generalized to an arbitrary $n(n\ge 3)$-qubit atomic state
transfer case and a corresponding network is also proposed.


title:
Optimal Experimental Scheme for Bennett-Brassard 1984 Quantum Key
  Distribution Protocol with Weak Coherent Sources, Noisy and Lossy Channel

abstract:
  It is the first scheme which allows the detection apparatus to achieve both
the photon number of arriving signals and quantum bit error rate of the
multiphoton pulses precisely. We show that the upper bound of the fraction of
the tagged multiphoton pulses counts is $\mu $, which is independent of the
channel loss and the intensity of the decoy source. Such upper bound is
$inherent$ and cannot be reduced any longer as long as the weak coherent
scouces and high lossy channel are used. We show that our scheme can be
implemented even if the channel loss is very high. A stronger intensity of the
pulse source is allowable to improve the rate of quantum key distribution. Both
the signal pulses and decoy pulses can be used to generate the raw key after
verified the security of the communication. We analyze that our scheme is
optimal under today's technology.
  PACS: 03.67.Dd


title:
Concurrence classes for an arbitrary multi-qubit state based on positive
  operator valued measure

abstract:
  In this paper, we propose concurrence classes for an arbitrary multi-qubit
state based on orthogonal complement of a positive operator valued measure, or
POVM in short, on quantum phase. In particular, we construct concurrence for an
arbitrary two-qubit state and concurrence classes for the three- and four-qubit
states. And finally, we construct $W^{m}$ and $GHZ^{m}$ class concurrences for
multi-qubit states. The unique structure of our POVM enables us to distinguish
different concurrence classes for multi-qubit states.


title: Perspectives for a mixed two-qubit system with binomial quantum states

abstract:
  The problem of the relationship between entanglement and two-qubit systems in
which it is embedded is central to the quantum information theory. This paper
suggests that the concurrence hierarchy as an entanglement measure provides an
alternative view of how to think about this problem. We consider mixed states
of two qubits and obtain an exact solution of the time-dependent master
equation that describes the evolution of two two-level qubits (or atoms) within
a perfect cavity for the case of multiphoton transition. We consider the
situation for which the field may start from a binomial state. Employing this
solution, the significant features of the entanglement when a second qubit is
weakly coupled to the field and becomes entangled with the first qubit, is
investigated. We also describe the response of the atomic system as it varies
between the Rabi oscillations and the collapse-revival mode and investigate the
atomic inversion and the Q-function. We identify and numerically demonstrate
the region of parameters where significantly large entanglement can be
obtained. Most interestingly, it is shown that features of the entanglement is
influenced significantly when the multi-photon process is involved. Finally, we
obtain illustrative examples of some novel aspects of this system and show how
the off-resonant case can sensitize entanglement to the role of initial state
setting.


title: From quantum graphs to quantum random walks

abstract:
  We give a short overview over recent developments on quantum graphs and
outline the connection between general quantum graphs and so-called quantum
random walks.


title: Comment on "Detuning effects in the one-photon mazer"

abstract:
  In a recent work, Bastin and Martin (B-M) [Phys. Rev. A 67, 053804 (2003)]
have analyzed the quantum theory of the mazer in the off-resonant case.
However, our analysis of this case refutes their claim by showing that their
evaluation of the coupled equations for the off-resonant case is not
satisfactory. The correct expression can be obtained by applying an appropriate
formulae for the involved dressed-state parameters.


title: Distance-based degrees of polarization for a quantum field

abstract:
  It is well established that unpolarized light is invariant with respect to
any SU(2) polarization transformation. This requirement fully characterizes the
set of density matrices representing unpolarized states. We introduce the
degree of polarization of a quantum state as its distance to the set of
unpolarized states. We use two different candidates of distance, namely the
Hilbert-Schmidt and the Bures metric, showing that they induce fundamentally
different degrees of polarization. We apply these notions to relevant field
states and we demonstrate that they avoid some of the problems arising with the
classical definition.


title:
Quantum control of entanglement and information of two solid state
  qubits: remote control of dephasing

abstract:
  We investigate the scheme for controlling information characterized by
Von-Neumann entropy and the stationary state entanglement characterized by
concurrence of two solid state qubits in the collective dephasing channel. It
is shown that the local maximal value of the stationary state concurrence
always corresponds to the local minimal value of information. We also propose a
scheme for remotely controlling the entanglement of two solid state qubits
against the collective dephasing. This idea may open a door to remotely
suppress the detrimental effects of decoherence.


title:
Stationary state entanglement and total correlation of two qubits or
  qutrits

abstract:
  We investigate the mutual information and entanglement of stationary state of
two locally driven qubits under the influence of collective dephasing. It is
shown that both the mutual information and the entanglement of two qubits in
the stationary state exhibit damped oscillation with the scaled action time
$\gamma{T}$ of the local external driving field. It means that we can control
both the entanglement and total correlation of the stationary state of two
qubits by adjusting the action time of the driving field. We also consider the
influence of collective dephasing on entanglement of two qutrits and obtain the
sufficient condition that the stationary state is entangled.


title: On the truncation of the harmonic oscillator wavepacket

abstract:
  We present an interesting result regarding the implication of truncating the
wavepacket of the harmonic oscillator. We show that disregarding the
non-significant tails of a function which is the superposition of
eigenfunctions of the harmonic oscillator has a remarkable consequence: namely,
there exist infinitely many different superpositions giving rise to the same
function on the interval. Uniqueness, in the case of a wavepacket, is restored
by a postulate of quantum mechanics.


title: Intermediate quantum maps for quantum computation

abstract:
  We study quantum maps displaying spectral statistics intermediate between
Poisson and Wigner-Dyson. It is shown that they can be simulated on a quantum
computer with a small number of gates, and efficiently yield information about
fidelity decay or spectral statistics. We study their matrix elements and
entanglement production, and show that they converge with time to distributions
which differ from random matrix predictions. A randomized version of these maps
can be implemented even more economically, and yields pseudorandom operators
with original properties, enabling for example to produce fractal random
vectors. These algorithms are within reach of present-day quantum computers.


title: Quantum Secret Sharing Schemes and Reversibility of Quantum Operations

abstract:
  Quantum secret sharing schemes encrypting a quantum state into a multipartite
entangled state are treated. The lower bound on the dimension of each share
given by Gottesman [Phys. Rev. A \textbf{61}, 042311 (2000)] is revisited based
on a relation between the reversibility of quantum operations and the Holevo
information. We also propose a threshold ramp quantum secret sharing scheme and
evaluate its coding efficiency.


title: Spin dynamics in a dissipative environment: from quantal to classical

abstract:
  We address the problem of spin dynamics in the presence of a thermal bath, by
solving exactly the appropriate quantum master equations with
continued-fraction methods. The crossover region between the quantum and
classical domains is studied by increasing the spin value S, and the asymptote
for the classical absorption spectra is eventually recovered. Along with the
recognized relevance of the coupling strength, we show the critical role played
by the structure of the system-environment interaction in the emergence of
classical phenomenology.


title:
Investigation of the Casimir force between metal and semiconductor test
  bodies

abstract:
  The measurement of the Casimir force between a large gold coated sphere and
single crystal silicon plate is performed with an atomic force microscope. A
rigorous statistical comparison of data with theory is done, without use of the
concept of root-mean-square deviation, and excellent agreement is obtained. The
Casimir force between metal and semiconductor is demonstrated to be
significantly different than between two similar or dissimilar metals.


title: How to model quantum plasmas

abstract:
  Traditional plasma physics has mainly focused on regimes characterized by
high temperatures and low densities, for which quantum-mechanical effects have
virtually no impact. However, recent technological advances (particularly on
miniaturized semiconductor devices and nanoscale objects) have made it possible
to envisage practical applications of plasma physics where the quantum nature
of the particles plays a crucial role. Here, I shall review different
approaches to the modeling of quantum effects in electrostatic collisionless
plasmas. The full kinetic model is provided by the Wigner equation, which is
the quantum analog of the Vlasov equation. The Wigner formalism is particularly
attractive, as it recasts quantum mechanics in the familiar classical phase
space, although this comes at the cost of dealing with negative distribution
functions. Equivalently, the Wigner model can be expressed in terms of $N$
one-particle Schr{\"o}dinger equations, coupled by Poisson's equation: this is
the Hartree formalism, which is related to the `multi-stream' approach of
classical plasma physics. In order to reduce the complexity of the above
approaches, it is possible to develop a quantum fluid model by taking
velocity-space moments of the Wigner equation. Finally, certain regimes at
large excitation energies can be described by semiclassical kinetic models
(Vlasov-Poisson), provided that the initial ground-state equilibrium is treated
quantum-mechanically. The above models are validated and compared both in the
linear and nonlinear regimes.


title:
The dynamics of coupled atom and field assisted by continuous external
  pumping

abstract:
  The dynamics of a coupled system comprised of a two-level atom and cavity
field assisted by continuous external classical field (driving Jaynes-Cummings
model) is studied. When the initial field is prepared in a coherent state, the
dynamics strongly depends on the algebraic sum of both fields. If this sum is
zero (the compensative case) in the system only the vacuum Rabi oscillations
occur. The results with the dissipation and external field detuning from the
cavity field are also discussed.


title:
Dynamical phase transitions and temperature induced quantum correlations
  in an infinite spin chain

abstract:
  We study the dynamics of entanglement in the infinite asymmetric XY spin
chain, in an applied transverse field. The system is prepared in a thermal
equilibrium state (ground state at zero temperature) at the initial instant,
and it starts evolving after the transverse field is completely turned off. We
investigate the evolved state of the chain at a given fixed time, and show that
the nearest neighbor entanglement in the chain exhibits a critical behavior
(which we call a dynamical phase transition), controlled by the initial value
of the transverse field. The character of the dynamical phase transition is
qualitatively different for short and long evolution times. We also find a
nonmonotonic behavior of entanglement with respect to the temperature of the
initial equilibrium state. Interestingly, the region of the initial field for
which we obtain a nonmonotonicity of entanglement (with respect to temperature)
is directly related to the position and character of the dynamical phase
transition in the model.


title: A new algorithm for fixed point quantum search

abstract:
  The standard quantum search lacks a feature, enjoyed by many classical
algorithms, of having a fixed point, i.e. monotonic convergence towards the
solution. Recently a fixed point quantum search algorithm has been discovered,
referred to as the Phase-$\pi/3$ search algorithm, which gets around this
limitation. While searching a database for a target state, this algorithm
reduces the error probability from $\epsilon$ to $\epsilon^{2q+1}$ using $q$
oracle queries, which has since been proved to be asymptotically optimal. A
different algorithm is presented here, which has the same worst-case behavior
as the Phase-$\pi/3$ search algorithm but much better average-case behavior.
Furthermore the new algorithm gives $\epsilon^{2q+1}$ convergence for all
integral $q$, whereas the Phase-$\pi/3$ search algorithm requires $q$ to be
$(3^{n}-1)/2$ with $n$ a positive integer. In the new algorithm, the operations
are controlled by two ancilla qubits, and fixed point behavior is achieved by
irreversible measurement operations applied to these ancillas. It is an example
of how measurement can allow us to bypass some restrictions imposed by
unitarity on quantum computing.


title: On Quantum A/D and D/A Conversion

abstract:
  An algorithm is proposed which transfers the quantum information of a wave
function (analogue signal) into a register of qubits (digital signal) such that
$n$ qubits describe the amplitudes and phases of $2^n$ points of a sufficiently
smooth wave function. We assume that the continuous degree of freedom couples
to one or more qubits of a quantum register via a Jaynes Cummings Hamiltonian
and that we have universal quantum computation capabilities on the register as
well as the possibility to perform bang-bang control on the qubits. The
transfer of information is mainly based on the application of the quantum
phase-estimation algorithm in both directions. Here, the running time increases
exponentially with the number of qubits. We pose it as an open question which
interactions would allow polynomial running time. One example would be
interactions which enable exact squeezing operations.


title: A Discussion on the Teleportation Protocol for States of N Qubits

abstract:
  In this paper, we want to present a simple and comprehensive method to
implement teleportation of a system of N qubits and its discussion on the
corresponding quantum circuit. The paper can be read for nonspecialists in
quantum information.


title: Quantum metastability in time-periodic potentials

abstract:
  In this paper we investigate quantum metastability of a particle trapped in
between an infinite wall and a square barrier, with either a time-periodically
oscillating barrier (Model A) or bottom of the well (Model B). Based on the
Floquet theory, we derive in each case an equation which determines the
stability of the metastable system. We study the influence on the stability of
two Floquet states when their Floquet energies (real part) encounter a direct
or an avoided crossing at resonance. The effect of the amplitude of oscillation
on the nature of crossing of Floquet energies is also discussed. It is found
that by adiabatically changing the frequency and amplitude of the oscillation
field, one can manipulate the stability of states in the well. By means of a
discrete transform, the two models are shown to have exactly the same Floquet
energy spectrum at the same oscillating amplitude and frequency. The
equivalence of the models is also demonstrated by means of the principle of
gauge invariance.

categories: quant-ph cond-mat.mes-hall hep-th math-ph math.MP physics.chem-ph


title: Phase-space analysis of bosonic spontaneous emission

abstract:
  We present phase-space techniques for the modelling of spontaneous emission
in two-level bosonic atoms. The positive-P representation is shown to give a
full and complete description and can be further developed to give exact
treatments of the interaction of degenerate bosons with the electromagnetic
field in a given experimental situation. The Wigner representation, even when
truncated at second order, is shown to need a doubling of the phase-space to
allow for a positive-definite diffusion matrix in the appropriate Fokker-Planck
equation and still fails to agree with the full quantum results of the
positive-P representation. We show that quantum statistics and correlations
between the ground and excited states affect the dynamics of the emission
process, so that it is in general non-exponential.


title: The Schwinger Representation of a Group: Concept and Applications

abstract:
  The concept of the Schwinger Representation of a finite or compact simple Lie
group is set up as a multiplicity-free direct sum of all the unitary
irreducible representations of the group. This is abstracted from the
properties of the Schwinger oscillator construction for SU(2), and its
relevance in several quantum mechanical contexts is highlighted. The Schwinger
representations for $SU(2), SO(3)$ and SU(n) for all $n$ are constructed via
specific carrier spaces and group actions. In the SU(2) case connections to the
oscillator construction and to Majorana's theorem on pure states for any spin
are worked out. The role of the Schwinger Representation in setting up the
Wigner-Weyl isomorphism for quantum mechanics on a compact simple Lie group is
brought out.


title: Periodically-modulated entangled light

abstract:
  We propose periodically-modulated entangled states of light and show that
they can be generated in two experimentally feasible schemes of nondegenerate
optical parametric oscillator (NOPO): (i) driven by continuously modulated pump
field; (ii) under action of a periodic sequence of identical laser pulses. We
show that the time-modulation of the pump field amplitude essentially improves
the degree of continuous-variable entanglement in NOPO. We develop
semiclassical and quantum theories of these devices for both below- and
above-threshold regimes. Our analytical results are in well agrement with
numerical simulation and support a concept of time-modulated entangled states.


title:
Extreme sub-wavelength atom localization via coherent population
  trapping

abstract:
  We demonstrate an atom localization scheme based on monitoring of the atomic
coherences. We consider atomic transitions in a Lambda configuration where the
control field is a standing wave field. The probe field and the control field
produce coherence between the two ground states. We show that this coherence
has the same fringe pattern as produced by a Fabry-Perot interferometer and
thus measurement of the atomic coherence would localize the atom. Interestingly
enough the role of the cavity finesse is played by the ratio of the intensities
of the pump and probe. This is in fact the reason for obtaining extreme
subwavelenth localization. We suggest several methods to monitor the produced
localization.


title: Experimental Quantum Error Rejection for Quantum Communication

abstract:
  We report an experimental realization of bit-flip error rejection for
error-free transfer of quantum information through a noisy quantum channel. In
the experiment, an unknown state to be transmitted is encoded into a two-photon
entangled state, which is then sent through an engineered noisy quantum
channel. At the final stage, the unknown state is decoded by a quantum parity
measurement, successfully rejecting the erroneous transmission over the noisy
quantum channel.


title: Noncommuting Observables and Local Realism

abstract:
  A standard approach in the foundations of quantum mechanics studies local
realism and hidden variables models exclusively in terms of violations of
Bell-like inequalities. Thus quantum nonlocality is tied to the celebrated
no-go theorems, and these comprise a long list that includes the Kochen-Specker
and Bell theorems, as well as elegant refinements by Mermin, Peres, Hardy, GHZ,
and many others. Typically entanglement or carefully prepared multipartite
systems have been considered essential for violations of local realism and for
understanding quantum nonlocality. Here we show, to the contrary, that sharp
violations of local realism arise almost everywhere without entanglement. The
pivotal fact driving these violations is just the noncommutativity of quantum
observables. We demonstrate how violations of local realism occur for arbitrary
noncommuting projectors, and for arbitrary quantum pure states. Finally, we
point to elementary tests for local realism, using single particles and without
reference to entanglement, thus avoiding experimental loopholes and efficiency
issues that continue to bedevil the Bell inequality related tests.


title:
The Inner Limit of Quantum Brownian Evolution and its Relevance to
  Positivity

abstract:
  The conventional quantum Brownian propagator, which describes the evolution
of a system of interest bilinearly coupled to and initially uncorrelated with a
reservoir, does not preserve positivity of density operators, prompting workers
to modify the propagator by the ad hoc addition of time-independent terms to
the corresponding generator. We show that no such terms need be added to the
generator to preserve positivity provided one accounts for the rapid
entanglement of the system of interest and the reservoir on a time scale too
short for the conventional propagator to be valid.


title:
Performance of cavity-parametric amplifiers, employing Kerr
  nonlinearites, in the presence of two-photon loss

abstract:
  Two-photon loss mechanisms often accompany a Kerr nonlinearity. The kinetic
inductance exhibited by superconducting transmission lines provides an example
of a Kerr-like nonlinearity that is accompanied by a nonlinear resistance of
the two-photon absorptive type. Such nonlinear dissipation can degrade the
performance of amplifiers and mixers employing a Kerr-like nonlinearity as the
gain or mixing medium. As an aid for parametric amplifier design, we provide a
quantum analysis of a cavity parametric amplifier employing a Kerr nonlinearity
that is accompanied by a two-photon absorptive loss. Because of their
usefulness in diagnostics, we obtain expressions for the pump amplitude within
the cavity, the reflection coefficient for the pump amplitude reflected off of
the cavity, the parametric gain, and the intermodulation gain. Expressions by
which of the degree of squeezing can be computed are also presented.


title:
Optical switching and inversionless amplification controlled by
  state-dependent alignment of molecules

abstract:
  Switching anisotropic molecules from strongly-absorbing to
strongly-amplifying through a transparent state is shown to be possible by
application of dc or ac control electric fields without the requirement of the
population inversion. It is based on decoupling of the lower-level molecules
from the resonant light while the excited ones remain emitting due to their
state-dependent alignment. The amplification index may become dependent only on
a number of excited molecules and not on the population of the lower state. A
suitable class of molecules and applications in optoelectronics, fiberoptics
and nanophotonics are outlined.


title: Entanglement and Self-Pulsing Instability

abstract:
  We study the Wigner function of phase-locked nondegenerate optical parametric
oscillator and find the signatures of both phase-locking and self-pulsing
phenomena in phase space. We also analyze the problem of continuous-variable
entanglement in the self-pulsing instability regime.


title:
Inter-dimensional Degeneracies in van der Waals Clusters and Quantum
  Monte Carlo Computation of Rovibrational States

abstract:
  Quantum Monte Carlo estimates of the spectrum of rotationally invariant
states of noble gas clusters suggest inter-dimensional degeneracy in $N-1$ and
$N+1$ spacial dimensions. We derive this property by mapping the Schr\"odinger
eigenvalue problem onto an eigenvalue equation in which $D$ appears as a
continuous variable. We discuss implications for quantum Monte Carlo and
dimensional scaling methods.


title: Extending additivity from symmetric to asymmetric channels

abstract:
  We prove a lemma which allows one to extend results about the additivity of
the minimal output entropy from highly symmetric channels to a much larger
class. A similar result holds for the maximal output $p$-norm. Examples are
given showing its use in a variety of situations. In particular, we prove the
additivity and the multiplicativity for the shifted depolarising channel.


title: Deformation quantization of linear dissipative systems

abstract:
  A simple pseudo-Hamiltonian formulation is proposed for the linear
inhomogeneous systems of ODEs. In contrast to the usual Hamiltonian mechanics,
our approach is based on the use of non-stationary Poisson brackets, i.e.
corresponding Poisson tensor is allowed to explicitly depend on time. Starting
from this pseudo-Hamiltonian formulation we develop a consistent deformation
quantization procedure involving a non-stationary star-product $*_t$ and an
``extended'' operator of time derivative $D_t=\partial_t+...$, differentiating
the $\ast_t$-product. As in the usual case, the $\ast_t$-algebra of physical
observables is shown to admit an essentially unique (time dependent) trace
functional $\mathrm{Tr}_t$. Using these ingredients we construct a complete and
fully consistent quantum-mechanical description for any linear dynamical system
with or without dissipation. The general quantization method is exemplified by
the models of damped oscillator and radiating point charge.


title: Indistinguishable Photons from a Single Molecule

abstract:
  We report the results of coincidence counting experiments at the output of a
Michelson interferometer using the zero-phonon-line emission of a single
molecule at $1.4 K$. Under continuous wave excitation, we observe the absence
of coincidence counts as an indication of two-photon interference. This
corresponds to the observation of Hong-Ou-Mandel correlations and proves the
suitability of the zero-phonon-line emission of single molecules for
applications in linear optics quantum computation.


title:
High fidelity simulations of ion trajectories in miniature ion traps
  using the boundary-element method

abstract:
  In this paper we present numerical modeling results for endcap and linear ion
traps, used for experiments at the National Physical Laboratory in the UK and
Innsbruck University respectively. The secular frequencies for Strontium-88 and
Calcium-40 ions were calculated from ion trajectories, simulated using
boundary-element and finite-difference numerical methods. The results were
compared against experimental measurements. Both numerical methods showed high
accuracy with boundary-element method being more accurate. Such simulations can
be useful tools for designing new traps and trap arrays. They can also be used
for obtaining precise trapping parameters for desired ion control when no
analytical approach is possible as well as for investigating the ion heating
rates due to thermal electronic noise.


title: A matrix representation of graphs and its spectrum as a graph invariant

abstract:
  We use the line digraph construction to associate an orthogonal matrix with
each graph. From this orthogonal matrix, we derive two further matrices. The
spectrum of each of these three matrices is considered as a graph invariant.
For the first two cases, we compute the spectrum explicitly and show that it is
determined by the spectrum of the adjacency matrix of the original graph. We
then show by computation that the isomorphism classes of many known families of
strongly regular graphs (up to 64 vertices) are characterized by the spectrum
of this matrix. We conjecture that this is always the case for strongly regular
graphs and we show that the conjecture is not valid for general graphs. We
verify that the smallest regular graphs which are not distinguished with our
method are on 14 vertices.


title:
Reply to "Comment on 'A linear optics implementation of weak values in
  Hardy's paradox'"

abstract:
  The comment by Lundeen et al. contains two criticisms of our proposal. While
we agree that the state-preparation procedure could be replaced by a simpler
setup as proposed by the authors of the comment, we do not agree with the
authors on their second, and more important point regarding two-particle weak
measurements. We believe this to be the result of a misunderstanding of our
original paper.


title: Capacities of Quantum Channels for Massive Bosons and Fermions

abstract:
  We consider the capacity of classical information transfer for noiseless
quantum channels carrying a finite average number of massive bosons and
fermions. The maximum capacity is attained by transferring the Fock states
generated from the grand-canonical ensemble. Interestingly, the channel
capacity for a Bose gas indicates the onset of a Bose-Einstein condensation, by
changing its qualitative behavior at the criticality, while for a channel
carrying weakly attractive fermions, it exhibits the signatures of
Bardeen-Cooper-Schrieffer transition. We also show that for noninteracting
particles, fermions are better carriers of information than bosons.


title: Increasing identical particle entanglement by fuzzy measurements

abstract:
  We investigate the effects of fuzzy measurements on spin entanglement for
identical particles, both fermions and bosons. We first consider an ideal
measurement apparatus and define operators that detect the symmetry of the
spatial and spin part of the density matrix as a function of particle distance.
Then, moving on to realistic devices that can only detect the position of the
particle to within a certain spread, it was surprisingly found that the
entanglement between particles increases with the broadening of detection.


title: The Solovay-Kitaev algorithm

abstract:
  This pedagogical review presents the proof of the Solovay-Kitaev theorem in
the form of an efficient classical algorithm for compiling an arbitrary
single-qubit gate into a sequence of gates from a fixed and finite set. The
algorithm can be used, for example, to compile Shor's algorithm, which uses
rotations of $\pi / 2^k$, into an efficient fault-tolerant form using only
Hadamard, controlled-{\sc not}, and $\pi / 8$ gates. The algorithm runs in
$O(\log^{2.71}(1/\epsilon))$ time, and produces as output a sequence of
$O(\log^{3.97}(1/\epsilon))$ quantum gates which is guaranteed to approximate
the desired quantum gate to an accuracy within $\epsilon > 0$. We also explain
how the algorithm can be generalized to apply to multi-qubit gates and to gates
from $SU(d)$.


title:
Quantum Darwinism: Entanglement, branches, and the emergent classicality
  of redundantly stored quantum information

abstract:
  We lay a comprehensive foundation for the study of redundant information
storage in decoherence processes. Redundancy has been proposed as a
prerequisite for objectivity, the defining property of classical objects. We
consider two ensembles of states for a model universe consisting of one system
and many environments: the first consisting of arbitrary states, and the second
consisting of ``singly-branching'' states consistent with a simple decoherence
model. Typical states from the random ensemble do not store information about
the system redundantly, but information stored in branching states has a
redundancy proportional to the environment's size. We compute the specific
redundancy for a wide range of model universes, and fit the results to a simple
first-principles theory. Our results show that the presence of redundancy
divides information about the system into three parts: classical (redundant);
purely quantum; and the borderline, undifferentiated or ``nonredundant,''
information.


title:
Unusual scalar products in Hilbert space of Quantum Mechanics:
  non-Hermitian square-well model with two coupled channels

abstract:
  A pseudo-Hermitian coupled-channel square-well model is proposed, solved and
discussed. The domain of parameters is determined where all the bound-state
energies (twice degenerate with respect to the second observable which we call
"spin") remain real. The standard probabilistic interpretation of the wave
functions is achieved by the transition from the original elementary
non-physical indefinite pseudo-metric $\theta$ to another, more involved but
correct positive-definite physical metric $\Theta$. In our model the latter
step remains comparatively easily feasible. Another fortunate circumstance
emerges during the perturbative construction of the metric $\Theta$ and, hence,
of the correct scalar product and of the physical norm. One finds that at the
higher energy levels the influence of all the non-Hermiticities of our model
becomes very strongly and progressively suppressed.


title: Isoholonomic Problem and Holonomic Quantum Computation

abstract:
  Geometric phases accompanying adiabatic processes in quantum systems can be
utilized as unitary gates for quantum computation. Optimization of control of
the adiabatic process naturally leads to the isoholonomic problem. The
isoholonomic problem in a homogeneous fiber bundle is formulated and solved
completely.


title: Generalised Hong-Ou-Mandel Experiments with Bosons and Fermions

abstract:
  The Hong-Ou-Mandel (HOM) dip plays an important role in recent linear optics
experiments. It is crucial for quantum computing with photons and can be used
to characterise the quality of single photon sources and linear optics setups.
In this paper, we consider generalised HOM experiments with $N$ bosons or
fermions passing simultaneously through a symmetric Bell multiport beam
splitter. It is shown that for even numbers of bosons, the HOM dip occurs
naturally in the coincidence detection in the output ports. In contrast,
fermions always leave the setup separately exhibiting perfect coincidence
detection. Our results can be used to verify or employ the quantum statistics
of particles experimentally.


title:
Security of two quantum cryptography protocols using the same four qubit
  states

abstract:
  The first quantum cryptography protocol, proposed by Bennett and Brassard in
1984 (BB84), has been widely studied in the last years. This protocol uses four
states (more precisely, two complementary bases) for the encoding of the
classical bit. Recently, it has been noticed that by using the same four
states, but a different encoding of information, one can define a new protocol
which is more robust in practical implementations, specifically when attenuated
laser pulses are used instead of single-photon sources [V. Scarani et al.,
Phys. Rev. Lett. {\bf 92}, 057901 (2004); referred to as SARG04]. We present a
detailed study of SARG04 in two different regimes. In the first part, we
consider an implementation with a single-photon source: we derive bounds on the
error rate $Q$ for security against all possible attacks by the eavesdropper.
The lower and the upper bound obtained for SARG04 ($Q\lesssim 10.95%$ and
$Q\gtrsim 14.9%$ respectively) are close to those obtained for BB84 ($Q\lesssim
12.4%$ and $Q\gtrsim 14.6%$ respectively). In the second part, we consider the
realistic source consisting of an attenuated laser and improve on previous
analysis by allowing Alice to optimize the mean number of photons as a function
of the distance. SARG04 is found to perform better than BB84, both in secret
key rate and in maximal achievable distance, for a wide class of Eve's attacks.


title:
Entanglement on mixed stabiliser states--I: Normal Forms and Reduction
  Procedures

abstract:
  The stabiliser formalism allows the efficient description of a sizeable class
of pure as well as mixed quantum states of N-qubit systems. That same formalism
has important applications in the field of quantum error correcting codes,
where mixed stabiliser states correspond to projectors on subspaces associated
with stabiliser codes. Here, we derive efficient reduction procedures to obtain
various useful normal forms for stabiliser states. We explicitly prove that
these procedures will always converge to the correct result and that these
procedures are efficient in that they only require a polynomial number of
operations on the generators of the stabilisers.
  We obtain two single-party normal forms. The simplest, the row-reduced
echelon form, is useful to calculate partial traces of stabiliser states. The
second is the fully reduced form and allows for the efficient calculation of
the overlap between two stabiliser states, as well as of the Uhlmann fidelity
between them, and their Bures distance.
  We also find a reduction procedure of bipartite stabiliser states, where the
operations involved are restricted to be local ones. Using this two-party
normal form, we prove that every bipartite mixed stabiliser state is locally
equivalent to a direct product of a number of maximally entangled states and a
separable state. Using this normal form we can efficiently calculate every
reasonable bipartite entanglement measure of mixed stabiliser states.


title: Information transfer in leaky atom-cavity systems

abstract:
  We consider first a system of two enatangled cavities and a single two-level
atom passing through one of them. A ``monogamy'' inequality for this tripartite
system is quantitatively studied and verified in the presence of cavity
leakage. We next consider the simultaneous passage of two-level atoms through
both the cavities. Entanglement swapping is observed between the two-cavity and
the two-atom system. Cavity dissipation leads to the quantitative reduction of
information transfer though preserving the basic swapping property.


title: Entanglement of assistance and multipartite state distillation

abstract:
  We find that the asymptotic entanglement of assistance of a general bipartite
mixed state is equal to the smaller of its two local entropies. Our protocol
gives rise to the asymptotically optimal EPR pair distillation procedure for a
given tripartite pure state, and we show that it actually yields EPR and GHZ
states; in fact, under a restricted class of protocols, which we call "one-way
broadcasting", the GHZ-rate is shown to be optimal.
  This result implies a capacity theorem for quantum channels where the
environment helps transmission by broadcasting the outcome of an optimally
chosen measurement. We discuss generalisations to m parties, and show (for m=4)
that the maximal amount of entanglement that can be localised between two
parties is given by the smallest entropy of a group of parties of which the one
party is a member, but not the other. This gives an explicit expression for the
asymptotic localisable entanglement, and shows that any nontrivial ground state
of a spin system can be used as a perfect quantum repeater if many copies are
available in parallel.
  Finally, we provide evidence that any unital channel is asymptotically
equivalent to a mixture of unitaries, and any general channel to a mixture of
partial isometries.


title: Comment on "On infinite walls in deformation quantization"

abstract:
  We discuss a recent method proposed by Kryukov and Walton to address
boundary-value problems in the context of deformation quantization. We compare
their method with our own approach and establish a connection between the two
formalisms.


title:
All (qubit) decoherences: Complete characterization and physical
  implementation

abstract:
  We investigate decoherence channels that are modelled as a sequence of
collisions of a quantum system (e.g., a qubit) with particles (e.g., qubits) of
the environment. We show that collisions induce decoherence when a bi-partite
interaction between the system qubit and an environment (reservoir) qubit is
described by the controlled-U unitary transformation (gate). We characterize
decoherence channels and in the case of a qubit we specify the most general
decoherence channel and derive a corresponding master equation. Finally, we
analyze entanglement that is generated during the process of decoherence
between the system and its environment.


title: Towards quantum-based privacy and voting

abstract:
  The privacy of communicating participants is often of paramount importance,
but in some situations it is an essential condition. A typical example is a
fair (secret) voting. We analyze in detail communication privacy based on
quantum resources, and we propose new quantum protocols. Possible
generalizations that would lead to voting schemes are discussed.


title:
Effective potential analytic continuation approach for real time quantum
  correlation functions involving nonlinear operators

abstract:
  We apply the effective potential analytic continuation (EPAC) method to the
calculation of real time quantum correlation functions involving operators
nonlinear in the position operator $\hat{q}$. For a harmonic system the EPAC
method provides the exact correlation function at all temperature ranges, while
the other quantum dynamics methods, the centroid molecular dynamics and the
ring polymer molecular dynamics, become worse at lower temperature. For an
asymmetric anharmonic system, the EPAC correlation function is in very good
agreement with the exact one at $t=0$. When the time increases from zero, the
EPAC method gives good coincidence with the exact result at lower temperature.
Finally, we propose a simplified version of the EPAC method to reduce the
computational cost required for the calculation of the standard effective
potential.


title:
Spectral Approach to Chaos and Quantum-Classical Correspondence in
  Quantum Mapas

abstract:
  Correspondence in quantum chaotic systems is lost in short time scales.
Introducing some noise we study the spectrum of the resulting coarse grained
propagaor of density matrices. Some differen methods to compute the spectrum
are reviewed. Moreover, the relationship between the eigenvalues of the
coarse-grained superoperator and the classical Ruelle-Pollicott resonances is
remarked. As a concequence, classical decay rates in quantum time dependent
quantities appear.


title:
Spectral Conditions on the State of a Composite Quantum System Implying
  its Separability

abstract:
  For any unitarily invariant convex function F on the states of a composite
quantum system which isolates the trace there is a critical constant C such
that F(w)<= C for a state w implies that w is not entangled; and for any
possible D > C there are entangled states v with F(v)=D. Upper- and lower
bounds on C are given. The critical values of some F's for qubit/qubit and
qubit/qutrit bipartite systems are computed. Simple conditions on the spectrum
of a state guaranteeing separability are obtained. It is shown that the thermal
equilbrium states specified by any Hamiltonian of an arbitrary compositum are
separable if the temperature is high enough.


title:
Stochasticity, decoherence and an arrow of time from the discretization
  of time?

abstract:
  Certain intriguing consequences of the discreteness of time on the time
evolution of dynamical systems are discussed. In the discrete-time classical
mechanics proposed here, there is an {\it arrow of time} that follows from the
fact that the replacement of the time derivative by the backward difference
operator alone can preserve the non-negativity of the phase space density. It
is seen that, even for free particles, all the degrees of freedom are {\it
correlated} in principle. The forward evolution of functions of phase space
variables by a finite number of time steps, in this discrete-time mechanics,
depends on the entire continuous-time history in the interval $[0, \infty]$. In
this sense, discrete time evolution is {\it nonlocal} in time from a
continuous-time point of view. A corresponding quantum mechanical treatment is
possible {\it via} the density matrix approach. The interference between
non-degenerate quantum mechanical states decays exponentially. This {\it
decoherence} is present, in principle, for all systems; however, it is of
practical importance only in macroscopic systems, or in processes involving
large energy changes.


title: Nonlinear Quantum Dynamics

abstract:
  The vast majority of the literature dealing with quantum dynamics is
concerned with linear evolution of the wave function or the density matrix. A
complete dynamical description requires a full understanding of the evolution
of measured quantum systems, necessary to explain actual experimental results.
The dynamics of such systems is intrinsically nonlinear even at the level of
distribution functions, both classically as well as quantum mechanically. Aside
from being physically more complete, this treatment reveals the existence of
dynamical regimes, such as chaos, that have no counterpart in the linear case.
Here, we present a short introductory review of some of these aspects, with a
few illustrative results and examples.


title:
A two-level atom coupled to a controllable squeezed vacuum field
  reservoir

abstract:
  The dissipative and decoherence properties of the two-level atom interacting
with the squeezed vacuum field reservoir are investigated based on the
nonautonomous master equation of the atomic density matrix in the framework of
algebraic dynamics. The nonautonomous master equation is converted into a
Schr\"{o}dinger-like equations and its dynamical symmetry is found based on the
left and right representations of the relevant algebra. The time-dependent
solution and the steady solution are obtained analytically. The asymptotic
behavior of the solution is examined and the approach to the equilibrium state
is proved. Based on the analytic solution the response of the system to the
squeezed vacuum field reservoir is studied numerically.