title: Maximal output purity and capacity for asymmetric unital qudit channels

abstract:
  We consider generalizations of depolarizing channels to maps in which the
identity channel is replaced by a convex combinations of unitary conjugations.
  We show that one can construct unital channels of this type for which the
input which achieves maximal output purity is unique. We give conditions under
which multiplicativity of the maximal p-norm and additivity of the minimal
output entropy. We also show that the Holevo capacity need not equal log d -
the minimal entropy as one might expect for a convex combination of unitary
conjugations. Conversely, we give examples for which this condition holds, but
the channel has no evident covariance properties.


title:
Diffusion and entanglement of a kicked particle in an infinite square
  well under frequent measurements

abstract:
  We investigate the dynamics of a kicked particle in an infinite square well
undergoing frequent measurements of energy. For a large class of periodic
kicking force, constant diffusion is found in such a non-KAM system. The
influence of phase shift of the kicking potential on the short-time dynamical
behavior is discussed. The general asymptotical measurement-assisted diffusion
rate is obtained. The entanglement between the particle and the measuring
apparatus is investigated. There exist two distinct dynamical behaviors of
entanglement. The bipartite entanglement grows with the kicking steps and it
gains larger value for the more chaotic system. However, the pairwise
entanglement between the system of interest and the partial spins of the
measuring apparatus decreases with the kicking steps. The relation between the
entanglement and quantum diffusion is also analyzed.
  PACS numbers: 05.45.Mt, 03.65.Ta


title: Mesoscopic Fractional Quantum in Soft Matter

abstract:
  Soft matter (e.g., biomaterials, polymers, sediments, oil, emulsions) has
become an important bridge between physics and diverse disciplines. Its
fundamental physical mechanism, however, is largely obscure. This study made
the first attempt to connect fractional Schrodinger equation and soft matter
physics under a consistent framework from empirical power scaling to
phenomenological kinetics and macromechanics to mesoscopic quantum mechanics.
The original contributions are the fractional quantum relationships, which show
Levy statistics and fractional Brownian motion are essentially related to
momentum and energy, respectively. The fractional quantum underlies fractal
mesostructures and many-body interactions of macromolecules in soft matter and
is experimentally testable.


title: Theory of Initialization-Free Decoherence-Free Subspaces and Subsystems

abstract:
  We introduce a generalized theory of decoherence-free subspaces and
subsystems (DFSs), which do not require accurate initialization. We derive a
new set of conditions for the existence of DFSs within this generalized
framework. By relaxing the initialization requirement we show that a DFS can
tolerate arbitrarily large preparation errors. This has potentially significant
implications for experiments involving DFSs, in particular for the experimental
implementation, over DFSs, of the large class of quantum algorithms which can
function with arbitrary input states.


title:
Comment on "Quantum secret sharing based on reusable
  Greenberger-Horne-Zeilinger states as secure carriers" [Phys. Rev. A 67,
  044302 (2003)]

abstract:
  In a recent paper [S. Bagherinezhad and V. Karimipour, Phys. Rev. A 67,
044302 (2003)], a quantum secret sharing protocol based on reusable GHZ states
was proposed. However, in this Comment, it is shown that this protocol is
insecure if Eve employs a special strategy to attack.


title:
Comment on "Quantum key distribution for d-level systems with
  generalized Bell states" [Phys. Rev. A 65, 052331 (2002)]

abstract:
  In the paper [Phys. Rev. A 65, 052331(2002)], an entanglement-based quantum
key distribution protocol for d-level systems was proposed. However, in this
Comment, it is shown that this protocol is insecure for a special attack
strategy.


title:
Robust quantum gates on neutral atoms with cavity-assisted
  photon-scattering

abstract:
  We propose a scheme to achieve quantum computation with neutral atoms whose
interactions are catalyzed by single photons. Conditional quantum gates,
including an $N$-atom Toffoli gate and nonlocal gates on remote atoms, are
obtained through cavity-assisted photon scattering in a manner that is robust
to random variation in the atom-photon coupling rate and which does not require
localization in the Lamb-Dicke regime. The dominant noise in our scheme is
automatically detected for each gate operation, leading to signalled errors
which do not preclude efficient quantum computation even if the error
probability is close to the unity.


title:
Quantum Computing with Spin Qubits Interacting Through Delocalized
  Excitons: Overcoming Hole Mixing

abstract:
  As a candidate scheme for controllably coupled qubits, we consider two
quantum dots, each doped with a single electron. The spin of the electron
defines our qubit basis and trion states can be created by using polarized
light; we show that the form of the excited trion depends on the state of the
qubit. By using the Luttinger-Kohn Hamiltonian we calculate the form of these
trion states in the presence of light-heavy hole mixing, and show that they can
interact through both the F\"orster transfer and static dipole-dipole
interactions. Finally, we demonstrate that by using chirped laser pulses, it is
possible to perform a two-qubit gate in this system by adiabatically following
the eigenstates as a function of laser detuning. These gates are robust in that
they operate with any realistic degree of hole mixing, and for either type of
trion-trion coupling.


title: Decoherence at absolute zero

abstract:
  We present an analytical study of the loss of quantum coherence at absolute
zero. Our model consists of a harmonic oscillator coupled to an environment of
harmonic oscillators at absolute zero. We find that for an Ohmic bath, the
offdiagonal elements of the density matrix in the position representation decay
as a power law in time at late times. This slow loss of coherence in the
quantum domain is qualitatively different from the exponential decay observed
in studies of high temperature environments.


title: Macroscopic entanglement in Quantum Computation

abstract:
  We investigate macroscopic entanglement of quantum states in quantum
computers, where we say a quantum state is entangled macroscopically if the
state has superposition of macroscopically distinct states. The index $p$ of
the macroscopic entanglement is calculated as a function of the step of the
computation, for Grover's quantum search algorithm and Shor's factoring
algorithm. It is found that whether macroscopically entangled states are used
or not depends on the numbers and properties of the solutions to the problem to
be solved. When the solutions are such that the problem becomes hard in the
sense that classical algorithms take more than polynomial steps to find a
solution, macroscopically entangled states are always used in Grover's
algorithm and almost always used in Shor's algorithm. Since they are
representative algorithms for unstructured and structured problems,
respectively, our results support strongly the conjecture that quantum
computers utilize macroscopically entangled states when they solve hard
problems much faster than any classical algorithms.


title: Adiabatic geometric phases in hydrogenlike atoms

abstract:
  We examine the effect of spin-orbit coupling on geometric phases in
hydrogenlike atoms exposed to a slowly varying magnetic field. The marginal
geometric phases associated with the orbital angular momentum and the intrinsic
spin fulfill a sum rule that explicitly relates them to the corresponding
geometric phase of the whole system. The marginal geometric phases in the
Zeeman and Paschen-Back limit are analyzed. We point out the existence of nodal
points in the marginal phases that may be detected by topological means.


title: On the Everett programme and the Born rule

abstract:
  Proponents of the Everett interpretation of Quantum Theory have made efforts
to show that to an observer in a branch, everything happens as if the
projection postulate were true without postulating it. In this paper, we will
indicate that it is only possible to deduce this rule if one introduces another
postulate that is logically equivalent to introducing the projection postulate
as an extra assumption. We do this by examining the consequences of changing
the projection postulate into an alternative one, while keeping the unitary
part of quantum theory, and indicate that this is a consistent (although
strange) physical theory.


title:
Two-Party Bell Inequalities Derived from Combinatorics via Triangular
  Elimination

abstract:
  We establish a relation between the two-party Bell inequalities for
two-valued measurements and a high-dimensional convex polytope called the cut
polytope in polyhedral combinatorics. Using this relation, we propose a method,
triangular elimination, to derive tight Bell inequalities from facets of the
cut polytope. This method gives two hundred million inequivalent tight Bell
inequalities from currently known results on the cut polytope. In addition,
this method gives general formulas which represent families of infinitely many
Bell inequalities. These results can be used to examine general properties of
Bell inequalities.


title:
Generalized decoding, effective channels, and simplified security proofs
  in quantum key distribution

abstract:
  Prepare and measure quantum key distribution protocols can be decomposed into
two basic steps: delivery of the signals over a quantum channel and
distillation of a secret key from the signal and measurement records by
classical processing and public communication. Here we formalize the
distillation process for a general protocol in a purely quantum-mechanical
framework and demonstrate that it can be viewed as creating an ``effective''
quantum channel between the legitimate users Alice and Bob. The process of
secret key generation can then be viewed as entanglement distribution using
this channel, which enables application of entanglement-based security proofs
to essentially any prepare and measure protocol. To ensure secrecy of the key,
Alice and Bob must be able to estimate the channel noise from errors in the
key, and we further show how symmetries of the distillation process simplify
this task. Applying this method, we prove the security of several key
distribution protocols based on equiangular spherical codes.


title: Quantum information can be negative

abstract:
  Given an unknown quantum state distributed over two systems, we determine how
much quantum communication is needed to transfer the full state to one system.
This communication measures the "partial information" one system needs
conditioned on it's prior information. It turns out to be given by an extremely
simple formula, the conditional entropy. In the classical case, partial
information must always be positive, but we find that in the quantum world this
physical quantity can be negative. If the partial information is positive, its
sender needs to communicate this number of quantum bits to the receiver; if it
is negative, the sender and receiver instead gain the corresponding potential
for future quantum communication. We introduce a primitive "quantum state
merging" which optimally transfers partial information. We show how it enables
a systematic understanding of quantum network theory, and discuss several
important applications including distributed compression, multiple access
channels and multipartite assisted entanglement distillation (localizable
entanglement). Negative channel capacities also receive a natural
interpretation.


title:
Smooth Controllability of Infinite Dimensional Quantum Mechanical
  Systems

abstract:
  Manipulation of infinite dimensional quantum systems is important to
controlling complex quantum dynamics with many practical physical and chemical
backgrounds. In this paper, a general investigation is casted to the
controllability problem of quantum systems evolving on infinite dimensional
manifolds. Recognizing that such problems are related with infinite dimensional
controllability algebras, we introduce an algebraic mathematical framework to
describe quantum control systems possessing such controllability algebras. Then
we present the concept of smooth controllability on infinite dimensional
manifolds, and draw the main result on approximate strong smooth
controllability. This is a nontrivial extension of the existing controllability
results based on the analysis over finite dimensional vector spaces to analysis
over infinite dimensional manifolds. It also opens up many interesting problems
for future studies.


title: Quantum limits to the measurement of spacetime geometry

abstract:
  This letter analyzes the limits that quantum mechanics imposes on the
accuracy to which spacetime geometry can be measured. By applying the physics
of computation to ensembles of clocks, as in GPS, we present a covariant
version of the quantum geometric limit, which states that the total number of
ticks of clocks and clicks of detectors that can be contained in a four volume
of spacetime of radius r and temporal extent t is less than or equal to rt/pi
x_P t_P, where x_P, t_P are the Planck length and time. The quantum geometric
bound limits the number of events or `ops' that can take place in a four-volume
of spacetime and is consistent with and complementary to the holographic bound
which limits the number of bits that can exist within a three-volume of
spacetime.


title: Braid Topologies for Quantum Computation

abstract:
  In topological quantum computation, quantum information is stored in states
which are intrinsically protected from decoherence, and quantum gates are
carried out by dragging particle-like excitations (quasiparticles) around one
another in two space dimensions. The resulting quasiparticle trajectories
define world-lines in three dimensional space-time, and the corresponding
quantum gates depend only on the topology of the braids formed by these
world-lines. We show how to find braids that yield a universal set of quantum
gates for qubits encoded using a specific kind of quasiparticle which is
particularly promising for experimental realization.


title: On the notion of quantum Lyapunov exponent

abstract:
  Classical chaos refers to the property of trajectories to diverge
exponentially as time tends to infinity. It is characterized by a positive
Lyapunov exponent.
  There are many different descriptions of quantum chaos. The one related to
the notion of generalized (quantum) Lyapunov exponent is based either on
qualitative physical considerations or on the so-called symplectic tomography
map.
  The purpose of this note is to show how the definition of quantum Lyapunov
exponent naturally arises in the framework of the Moyal phase space formulation
of quantum mechanics and is based on the notions of quantum trajectories and
the family of quantizers. The role of the Heisenberg uncertainty principle in
the statement of the criteria for quantum chaos is made explicit.


title:
Dynamical symmetry and analytical solutions of the non-autonomous
  quantum master equation of the dissipative two-level system: decoherence of
  quantum register

abstract:
  Based on the non-autonomous quantum master equation, we investigate the
dissipative and decoherence properties of the two-level atom system interacting
with the environment of thermal quantum radiation fields. For this system, by a
novel algebraic dynamic method, the dynamical symmetry of the system is found,
the quantum master equation is converted into a Schr\"{o}dinger-like equation
and the non-Hermitian rate (quantum Liouville) operator of the master equation
is expressed as a linear function of the dynamical u(2) generators.
Furthermore, the integrability of the non-autonomous master equation has been
proved for the first time. Based on the time-dependent analytical solutions,
the asymptotic behavior of the solution has been examined and the approach to
the equilibrium state has been proved. Finally, we have studied the decoherence
property of the multiple two-level atom system coupled to the thermal radiation
fields, which are related to the quantum register.


title: Hadamard type operations for qubits

abstract:
  We obtain the most general ensemble of qubits, for which it is possible to
design a universal Hadamard gate. These states when geometrically represented
on the Bloch sphere, give a new trajectory. We further consider some Hadamard
`type' of operations and find ensembles of states for which such
transformations hold. Unequal superposition of a qubit and its orthogonal
complement is also investigated.


title: The Singapore Protocol: Incoherent Eavesdropping Attacks

abstract:
  We thoroughly analyse the novel quantum key distribution protocol introduced
recently in quant-ph/0412075, which is based on minimal qubit tomography. We
examine the efficiency of the protocol for a whole range of noise parameters
and present a general analysis of incoherent eavesdropping attacks with
arbitrarily many steps in the iterative key generation process. The comparison
with the tomographic 6-state protocol shows that our protocol has a higher
efficiency (up to 20%) and ensures the security of the established key even for
noise parameters far beyond the 6-state protocol's noise threshold.


title: Measurements and Decoherence

abstract:
  A pedagogical and reasonably self-contained introduction to the measurement
problems in quantum mechanics and their partial solution by environment-induced
decoherence (plus some other important aspects of dcoherence) is given. The
point that decoherence does not solve the measurement problems completely is
clearly brought out.The relevance of interpretation of quantum mechanics in
this context is briefly discussed.


title:
The logarithmic negativity: A full entanglement monotone that is not
  convex

abstract:
  It is proven that the logarithmic negativity does not increase on average
under positive partial transpose preserving (PPT) operation including
subselection (a set of operations that incorporate local operations and
classical communication (LOCC) as a subset) and, in the process, a further
proof is provided that the negativity does not increase on average under the
same set of operations. Given that the logarithmic negativity is obtained from
the negativity applying a concave function and is itself not a convex quantity
this result is surprising as convexity is generally considered as describing
the local physical process of losing information. The role of convexity and in
particular its relation (or lack thereof) to physical processes is discussed in
this context.


title:
Creation of Stable Multipartite Entangled States in Spin Chains with
  Defects

abstract:
  We show how defects in a spin chain described by the XXZ model may be used to
generate entangled states, such as Bell and W states, and how to maintain them
with high fidelity. In the presence of several excitations, we also discuss how
the anisotropy of the system may be combined with defects to effectively assist
in the creation of the desired states.


title:
On demand single photon source using a nanoscale
  metal-insulator-semiconductor capacitor

abstract:
  We propose an on-demand single photon source for quantum cryptography using a
metal-insulator-semiconductor quantum dot capacitor structure. The main
component in the semiconductor is a p-doped quantum well, and the cylindrical
gate under consideration is only nanometers in diameter. As in conventional
metal-insulator-semiconductor capacitors, our system can also be biased into
the inversion regime. However, due to the small gate area, at the onset of
inversion there are only a few electrons residing in a quantum dot. In
addition, because of the strong size quantization and large Coulomb energy, the
number of electrons can be precisely controlled by the gate voltage. After
holding just one electron in the inversion layer, the capacitor is quickly
biased back to the flat band condition, and the subsequent radiative
recombination across the bandgap results in single photon emission. We present
numerical simulation results of a semiconductor heterojunction and discuss the
merits of this single photon source.


title: Ultimate decoherence border for matter-wave interferometry

abstract:
  Stochastic backgrounds of gravitational waves are intrinsic fluctuations of
spacetime which lead to an unavoidable decoherence mechanism. This mechanism
manifests itself as a degradation of the contrast of quantum interferences. It
defines an ultimate decoherence border for matter-wave interferometry using
larger and larger molecules. We give a quantitative characterization of this
border in terms of figures involving the gravitational environment as well as
the sensitivity of the interferometer to gravitational waves. The known level
of gravitational noise determines the maximal size of the molecular probe for
which interferences may remain observable. We discuss the relevance of this
result in the context of ongoing progresses towards more and more sensitive
matter-wave interferometry.


title: Weak measurements in quantum mechanics

abstract:
  The article recapitulates the concept of weak measurement in its broader
sense encapsulating the trade between asymptotically weak measurement precision
and asymptotically large measurement statistics. Essential applications in
time-continuous measurement and in postselected measurement are presented both
in classical and in quantum contexts. We discuss the anomalous quantum weak
value in postselected measurement. We concentrate on the general mathematical
and physical aspects of weak measurements and we do not expand on their
interpretation. Particular applications, even most familiar ones, are not
subject of the article which was written for Elsevier's Encyclopedia of
Mathematical Physics.


title:
Bloch theory of entangled photon generation in non-linear photonic
  crystals

abstract:
  We present a quantum mechanical description of parametric down-conversion and
phase-matching of Bloch-waves in non-linear photonic crystals. We discuss the
theory in one-dimensional Bragg structures giving a recipe for calculating the
down-converted emission strength and direction. We exemplify the discussion by
making explicit analytical predictions for the emission amplitude and direction
from a one-dimensional structure that consists of alternating layers of
Al0.4Ga0.6As and Air. We show that the emission is suitable for the extraction
of polarization-entangled photons.


title: Quantum measurement driven by spontaneous symmetry breaking

abstract:
  The measurement process in quantum mechanics is usually described by the von
Neumann projection postulate, which forms a basic constituent of the laws of
quantum mechanics. Since this postulate requires the outside observer of the
system, it is hard to apply quantum mechanics to the whole Universe. Therefore
we propose that the quantum measurement process is actually a physical process
associated with the ubiquitous mechanism of spontaneous symmetry breaking.
Based on this proposal, we construct a quantum measurement model in which the
von Neumann projection is described as the dynamical pro-coherence process.
Furthermore, the classically distinguishable pointer parameter emerges as the
c-number order parameter in the formalism of closed time-path quantum filed
theory. We also discuss the precision of the measurement and the possible
deduction of the Born probability postulate.


title:
Production of squeezed state of single mode cavity field by the coupling
  of squeezed vacuum field reservoir in nonautonomous case

abstract:
  The dissipative and decoherence properties as well as the asymptotic behavior
of the single mode electromagnetic field interacting with the time-dependent
squeezed vacuum field reservoir are investigated in detail by using the
algebraic dynamical method. With the help of the left and right representations
of the relevant $hw(4)$ algebra, the dynamical symmetry of the nonautonomous
master equation of the system is found to be $su(1,1)$. The unique equilibrium
steady solution is found to be the squeezed state and any initial state of the
system is proved to approach the unique squeezed state asymptotically. Thus the
squeezed vacuum field reservoir is found to play the role of a squeezing mold
of the cavity field.


title: Wave mechanics without gauge fixing

abstract:
  By using the multipolar gauge it is shown that the quantum mechanics of an
electrically charged particle moving in a prescribed classical electromagnetic
field (wave mechanics) may be expressed in a manner that is gauge invariant,
except that the only gauge functions that are allowable in a gauge
transformation are those that consist of the sum of a function that depends
only on the space coordinates and a term that is the product of a constant and
the time coordinate. The multipolar gauge specifies the specific set of
potentials that are best suited for use in the Schroedinger equation.


title:
Quantum interferences from cross talk in $J=1/2\leftrightarrow J=1/2$
  transitions

abstract:
  We consider the possibility of a control field opening up multiple pathways
and thereby leading to new interference and coherence effects. We illustrate
the idea by considering the $J=1/2\leftrightarrow J=1/2$ transition. As a
result of the additional pathways, we show the possibilities of nonzero
refractive index without absorption and gain without inversion. We explain
these results in terms of the coherence produced by the opening of an extra
pathway.


title: Toy Model for a Relational Formulation of Quantum Theory

abstract:
  In the absence of an external frame of reference physical degrees of freedom
must describe relations between systems. Using a simple model, we investigate
how such a relational quantum theory naturally arises by promoting reference
systems to the status of dynamical entities. Our goal is to demonstrate using
elementary quantum theory how any quantum mechanical experiment admits a purely
relational description at a fundamental level, from which the original
"non-relational" theory emerges in a semi-classical limit. According to this
thesis, the non-relational theory is therefore an approximation of the
fundamental relational theory. We propose four simple rules that can be used to
translate an "orthodox" quantum mechanical description into a relational
description, independent of an external spacial reference frame or clock. The
techniques used to construct these relational theories are motivated by a
Bayesian approach to quantum mechanics, and rely on the noiseless subsystem
method of quantum information science used to protect quantum states against
undesired noise. The relational theory naturally predicts a fundamental
decoherence mechanism, so an arrow of time emerges from a time-symmetric
theory. Moreover, there is no need for a "collapse of the wave packet" in our
model: the probability interpretation is only applied to diagonal density
operators. Finally, the physical states of the relational theory can be
described in terms of "spin networks" introduced by Penrose as a combinatorial
description of geometry, and widely studied in the loop formulation of quantum
gravity. Thus, our simple bottom-up approach (starting from the semi-classical
limit to derive the fully relational quantum theory) may offer interesting
insights on the low energy limit of quantum gravity.


title: Entanglement from thermal black body radiation

abstract:
  Two non--interacting quantum systems which couple to a common environment
with many degrees of freedom initially in thermal equilibrium can become
entangled due to the indirect interaction mediated through this heat bath. I
examine here the dynamics of reservoir induced entanglement for a heat bath
consisting of a thermal electro--magnetic radiation field, such as black body
radiation or the cosmic microwave background, and show how the effect can be
understood as result of an effective induced interaction.


title:
Measuring the purity of a qubit state: entanglement estimation with
  fully separable measurements

abstract:
  Given a finite number $N$ of copies of a qubit state we compute the maximum
fidelity that can be attained using joint-measurement protocols for estimating
its purity. We prove that in the asymptotic $N\to\infty$ limit,
separable-measurement protocols can be as efficient as the optimal
joint-measurement one if classical communication is used. This in turn shows
that the optimal estimation of the entanglement of a two-qubit state can also
be achieved asymptotically with fully separable measurements. The relationship
between our global Bayesian approach and the quantum Cramer-Rao bound is also
discussed.


title: Purification of quantum trajectories

abstract:
  We prove that the quantum trajectory of repeated perfect measurement on a
finite quantum system either asymptotically purifies, or hits upon a family of
`dark' subspaces, where the time evolution is unitary.


title: Chaos and Quantum Mechanics

abstract:
  The relationship between chaos and quantum mechanics has been somewhat uneasy
-- even stormy, in the minds of some people. However, much of the confusion may
stem from inappropriate comparisons using formal analyses. In contrast, our
starting point here is that a complete dynamical description requires a full
understanding of the evolution of measured systems, necessary to explain actual
experimental results. This is of course true, both classically and quantum
mechanically. Because the evolution of the physical state is now conditioned on
measurement results, the dynamics of such systems is intrinsically nonlinear
even at the level of distribution functions. Due to this feature, the
physically more complete treatment reveals the existence of dynamical regimes
-- such as chaos -- that have no direct counterpart in the linear (unobserved)
case. Moreover, this treatment allows for understanding how an effective
classical behavior can result from the dynamics of an observed quantum system,
both at the level of trajectories as well as distribution functions. Finally,
we have the striking prediction that time-series from measured quantum systems
can be chaotic far from the classical regime, with Lyapunov exponents differing
from their classical values. These predictions can be tested in next-generation
experiments.


title: Casimir effect with rough metallic mirrors

abstract:
  We calculate the second order roughness correction to the Casimir energy for
two parallel metallic mirrors. Our results may also be applied to the
plane-sphere geometry used in most experiments. The metallic mirrors are
described by the plasma model, with arbitrary values for the plasma wavelength,
the mirror separation and the roughness correlation length, with the roughness
amplitude remaining the smallest length scale for perturbation theory to hold.
  From the analysis of the intracavity field fluctuations, we obtain the
Casimir energy correction in terms of generalized reflection operators, which
account for diffraction and polarization coupling in the scattering by the
rough surfaces. We present simple analytical expressions for several limiting
cases, as well as numerical results that allow for a reliable calculation of
the roughness correction in real experiments. The correction is larger than the
result of the Proximity Force Approximation, which is obtained from our theory
as a limiting case (very smooth surfaces).


title:
Entanglement production and decoherence-free subspace of two single-mode
  cavities embedded in a common environment

abstract:
  A system consisting of two identical single-mode cavities coupled to a common
environment is investigated within the framework of algebraic dynamics. Based
on the left and right representations of the Heisenberg-Weyl algebra, the
algebraic structure of the master equation is explored and exact analytical
solutions of this system are obtained. It is shown that for such a system, the
environment can produce entanglement in contrast to its commonly believed role
of destroying entanglement. In addition, the collective zero-mode eigen
solutions of the system are found to be free of decoherence against the
dissipation of the environment. These decoherence-free states may be useful in
quantum information and quantum computation.


title: A Note on Shared Randomness and Shared Entanglement in Communication

abstract:
  We consider several models of 1-round classical and quantum communication,
some of these models have not been defined before. We "almost separate" the
models of simultaneous quantum message passing with shared entanglement and the
model of simultaneous quantum message passing with shared randomness. We define
a relation which can be efficiently exactly solved in the first model but
cannot be solved efficiently, either exactly or in 0-error setup in the second
model. In fact, our relation is exactly solvable even in a more restricted
model of simultaneous classical message passing with shared entanglement.
  As our second contribution we strengthen a result by Yao that a "very short"
protocol from the model of simultaneous classical message passing with shared
randomness can be simulated in the model of simultaneous quantum message
passing: for a boolean function f, QII(f) \in exp(O(RIIp(f))) log n.
  We show a similar result for protocols from a (stronger) model of 1-way
classical message passing with shared randomness: QII(f) \in exp(O(RIp(f))) log
n.
  We demonstrate a problem whose efficient solution in the model of
simultaneous quantum message passing follows from our result but not from
Yao's.


title: Quantum key distribution with trusted quantum relay

abstract:
  A trusted quantum relay is introduced to enable quantum key distribution
links to form the basic legs in a quantum key distribution network. The idea is
based on the well-known intercept/resend eavesdropping. The same scheme can be
used to make quantum key distribution between several parties. No entanglement
is required.


title: Quantum measurements and entropic bounds on information transmission

abstract:
  While a positive operator valued measure gives the probabilities in a quantum
measurement, an instrument gives both the probabilities and the a posteriori
states. By interpreting the instrument as a quantum channel and by using the
monotonicity theorem for relative entropies many bounds on the classical
information extracted in a quantum measurement are obtained in a unified
manner. In particular, it is shown that such bounds can all be stated as
inequalities between mutual entropies. This approach based on channels gives
rise to a unified picture of known and new bounds on the classical information
(Holevo's, Shumacher-Westmoreland-Wootters', Hall's, Scutaru's bounds, a new
upper bound and a new lower one). Some examples clarify the mutual
relationships among the various bounds.


title: Deterministic Entanglement of Assistance and Monogamy Constraints

abstract:
  Certain quantum information tasks require entanglement of assistance, namely
a reduction of a tripartite entangled state to a bipartite entangled state via
local measurements. We establish that 'concurrence of assistance' (CoA)
identifies capabilities and limitations to producing pure bipartite entangled
states from pure tripartite entangled states and prove that CoA is an
entanglement monotone for $(2\times2\times n)$-dimensional pure states.
Moreover, if the CoA for the pure tripartite state is at least as large as the
concurrence of the desired pure bipartite state, then the former may be
transformed to the latter via local operations and classical communication, and
we calculate the maximum probability for this transformation when this
condition is not met.


title: An entanglement-area law for general bosonic harmonic lattice systems

abstract:
  We demonstrate that the entropy of entanglement and the distillable
entanglement of regions with respect to the rest of a general harmonic lattice
system in the ground or a thermal state scale at most as the boundary area of
the region. This area law is rigorously proven to hold true in non-critical
harmonic lattice system of arbitrary spatial dimension, for general
finite-ranged harmonic interactions, regions of arbitrary shape and states of
nonzero temperature. For nearest-neighbor interactions - corresponding to the
Klein-Gordon case - upper and lower bounds to the degree of entanglement can be
stated explicitly for arbitrarily shaped regions, generalizing the findings of
[Phys. Rev. Lett. 94, 060503 (2005)]. These higher dimensional analogues of the
analysis of block entropies in the one-dimensional case show that under general
conditions, one can expect an area law for the entanglement in non-critical
harmonic many-body systems. The proofs make use of methods from entanglement
theory, as well as of results on matrix functions of block banded matrices.
Disordered systems are also considered. We moreover construct a class of
examples for which the two-point correlation length diverges, yet still an area
law can be proven to hold. We finally consider the scaling of classical
correlations in a classical harmonic system and relate it to a quantum lattice
system with a modified interaction. We briefly comment on a general
relationship between criticality and area laws for the entropy of entanglement.

categories: quant-ph cond-mat.stat-mech gr-qc hep-th math-ph math.MP


title: Dimension-Independent Positive-Partial-Transpose Probability Ratios

abstract:
  We conduct quasi-Monte Carlo numerical integrations in two very high (80 and
79)-dimensional domains -- the parameter spaces of rank-9 and rank-8
qutrit-qutrit (9 x 9) density matrices. We, then, estimate the ratio of the
probability -- in terms of the Hilbert-Schmidt metric -- that a generic rank-9
density matrix has a positive partial transpose (PPT) to the probability that a
generic rank-8 density matrix has a PPT (a precondition to
separability/nonentanglement). Close examination of the numerical results
generated -- despite certain large fluctuations -- indicates that the true
ratio may, in fact, be 2. Our earlier investigation (eprint quant-ph/0410238)
also yielded estimates close to 2 of the comparable ratios for qubit-qubit and
qubit-qutrit pairs (the only two cases where the PPT condition fully implies
separability). Therefore, it merits conjecturing (as Zyczkowski was the first
to do) that such Hilbert-Schmidt (rank-NM/rank-(NM-1)) PPT probability ratios
are 2 for all NM-dimensional quantum systems.


title: Signatures of quantum behavior in single-qubit weak measurements

abstract:
  With the recent surge of interest in quantum computation, it has become very
important to develop clear experimental tests for ``quantum behavior'' in a
system. This issue has been addressed in the past in the form of the
inequalities due to Bell and those due to Leggett and Garg. These inequalities
concern the results of ideal projective measurements, however, which are
experimentally difficult to perform in many proposed qubit designs, especially
in many solid state qubit systems. Here, we show that weak continuous
measurements, which are often practical to implement experimentally, can yield
particularly clear signatures of quantum coherence, both in the measured
correlation functions and in the measured power spectrum.


title: Clean Positive Operator Valued Measures

abstract:
  In quantum mechanics the statistics of the outcomes of a measuring apparatus
is described by a positive operator valued measure (POVM). A quantum channel
transforms POVM's into POVM's, generally irreversibly, thus loosing some of the
information retrieved from the measurement. This poses the problem of which
POVM's are "undisturbed", namely they are not irreversibly connected to another
POVM. We will call such POVM clean. In a sense, the clean POVM's would be
"perfect", since they would not have any additional "extrinsical" noise. Quite
unexpectedly, it turns out that such cleanness property is largely unrelated to
the convex structure of POVM's, and there are clean POVM's that are not
extremal and vice-versa. In this paper we solve the cleannes classification
problem for number n of outcomes n<=d (d dimension of the Hilbert space), and
we provide a a set of either necessary or sufficient conditions for n>d, along
with an iff condition for the case of informationally complete POVM's for
n=d^2.


title: Depolarisation cooling of an atomic cloud

abstract:
  We propose a cooling scheme based on depolarisation of a polarised cloud of
trapped atoms. Similar to adiabatic demagnetisation, we suggest to use the
coupling between the internal spin reservoir of the cloud and the external
kinetic reservoir via dipolar relaxation to reduce the temperature of the
cloud. By optical pumping one can cool the spin reservoir and force the cooling
process. In case of a trapped gas of dipolar chromium atoms, we show that this
cooling technique can be performed continuously and used to approach the
critical phase space density for BEC


title: Unmodulated spin chains as universal quantum wires

abstract:
  We study a quantum state transfer between two qubits interacting with the
ends of a quantum wire consisting of linearly arranged spins coupled by an
excitation conserving, time-independent Hamiltonian. We show that if we control
the coupling between the source and the destination qubits and the ends of the
wire, the evolution of the system can lead to an almost perfect transfer even
in the case in which all nearest-neighbour couplings between the internal spins
of the wire are equal.


title:
"All-versus-nothing" nonlocality test of quantum mechanics by two-photon
  hyperentanglement

abstract:
  We report the experimental realization and the characterization of
polarization and momentum hyperentangled two photon states, generated by a new
parametric source of correlated photon pairs. By adoption of these states an
"all versus nothing" test of quantum mechanics was performed. The two photon
hyperentangled states are expected to find at an increasing rate a widespread
application in state engineering and quantum information. PACS: 03.65.Ud,
03.67.Mn, 42.65. Lm


title: Tunneling out of a time-dependent well

abstract:
  Solutions to explicit time-dependent problems in quantum mechanics are rare.
In fact, all known solutions are coupled to specific properties of the
Hamiltonian and may be divided into two categories: One class consists of
time-dependent Hamiltonians which are not higher than quadratic in the position
operator, like i.e the driven harmonic oscillator with time-dependent
frequency. The second class is related to the existence of additional
invariants in the Hamiltonian, which can be used to map the solution of the
time-dependent problem to that of a related time-independent one.
  In this article we discuss and develop analytic methods for solving
time-dependent tunneling problems, which cannot be addressed by using quadratic
Hamiltonians. Specifically, we give an analytic solution to the problem of
tunneling from an attractive time-dependent potential which is embedded in a
long-range repulsive potential.
  Recent progress in atomic physics makes it possible to observe experimentally
time-dependent phenomena and record the probability distribution over a long
range of time. Of special interest is the observation of macroscopical
quantum-tunneling phenomena in Bose-Einstein condensates with time-dependent
trapping potentials. We apply our model to such a case in the last section.


title:
Addendum to "Sufficient conditions for three-particle entanglement and
  their tests in recent experiments"

abstract:
  A recent paper [M. Seevinck and J. Uffink, Phys. Rev. A 65, 012107 (2002)]
presented a bound for the three-qubit Mermin inequality such that the violation
of this bound indicates genuine three-qubit entanglement. We show that this
bound can be improved for a specific choice of observables. In particular, if
spin observables corresponding to orthogonal directions are measured at the
qubits (e.g., X and Y spin coordinates) then the bound is the same as the bound
for states with a local hidden variable model. As a consequence, it can
straightforwardly be shown that in the experiment described by J.-W. Pan et al.
[Nature 403, 515 (2000)] genuine three-qubit entanglement was detected.


title:
Dynamics of Entanglement for One-Dimensional Spin Systems in an External
  Time-Dependent Magnetic Field

abstract:
  We study the dynamics of entanglement for the XY-model, one-dimensional spin
systems coupled through nearest neighbor exchange interaction and subject to an
external time-dependent magnetic field. Using the two-site density matrix, we
calculate the time-dependent entanglement of formation between nearest neighbor
qubits. We investigate the effect of varying the temperature, the anisotropy
parameter and the external time-dependent magnetic field on the entanglement.
We have found that the entanglement can be localized between nearest neighbor
qubits for certain values of the external time-dependent magnetic field.
Moreover, as known for the magnetization of this model, the entanglement shows
nonergodic behavior, it does not approach its equilibrium value at the infinite
time limit.


title: Scattering length determination from trapped pairs of atoms

abstract:
  A method is described for estimating effective scattering lengths via
spectroscopy on a trapped pair of atoms. The method relies on the phenomena
that the energy levels of two atoms in a harmonic trap are shifted by their
collisional interaction. The amount of shift depends on the strength of the
interaction (i.e. scattering length). By combining the spectra of the trap
state energy levels and a suitable model for the effective scattering length,
an estimate for the latter may be inferred. Two practical methods for measuring
the trap spectra are proposed and illustrated in examples. The accuracy of the
scheme is analyzed and requirements on measurement precision are given.


title: Optical analog of Rabi oscillation suppression due to atomic motion

abstract:
  The Rabi oscillations of a two-level atom illuminated by a laser on resonance
with the atomic transition may be suppressed by the atomic motion through
averaging or filtering mechanisms. The optical analogs of these velocity
effects are described. The two atomic levels correspond in the optical analogy
to orthogonal polarizations of light and the Rabi oscillations to polarization
oscillations in a medium which is optically active, naturally or due to a
magnetic field. In the later case, the two orthogonal polarizations could be
selected by choosing the orientation of the magnetic field, and one of them be
filtered out. It is argued that the time-dependent optical polarization
oscillations or their suppression are observable with current technology.


title:
Equations of Motion with Multiple Proper Time: A New Interpretation of
  Basic Quantum Physics

abstract:
  Equations of motion for single particle under two proper time model and three
proper time model have been proposed and analyzed. The motions of particle are
derived from pure classical method but they exhibit the same properties of
quantum physics: the quantum wave equation, de Broglie equations, uncertainty
relation, statistical result of quantum wave-function. This shows us a possible
new way to interpret quantum physics. We will also prove that physics with
multiple proper time does not cause causality problem.


title: Remote state preparation without entangled channels

abstract: This paper has been withdrawn


title:
Algebraic treatments of the problems of the spin-1/2 particles in the
  one and two-dimensional geometry: a systematic study

abstract:
  We consider solutions of the 2x2 matrix Hamiltonians of the physical systems
within the context of the su(2) and su(1,1) Lie algebra. Our technique is
relatively simple when compared with the others and treats those Hamiltonians
which can be treated in a unified framework of the $% Sp(4,R)$ algebra. The
systematic study presented here reproduces a number of earlier results in a
natural way as well as leads to a novel findings. Possible generalizations of
the method are also suggested.


title: A scheme for entanglement extraction from a solid

abstract:
  Some thermodynamical properties of solids, such as heat capacity and magnetic
susceptibility, have recently been shown to be linked to the amount of
entanglement in a solid. However this entanglement may appear a mere
mathematical artifact of the typical symmetrization procedure of many-body wave
function in solid state physics.
  Here we show that this entanglement is physical demonstrating the principles
of its extraction from a typical solid state system by scattering two particles
off the system. Moreover we show how to simulate this process using present-day
optical lattices technology. This demonstrates not only that entanglement
exists in solids but also that it can be used for quantum information
processing or for test of Bell's inequalities.


title:
Simple security proof of quantum key distribution via uncertainty
  principle

abstract:
  We present an approach to the unconditional security of quantum key
distribution protocols based on the uncertainty principle. The approach applies
to every case that has been treated via the argument by Shor and Preskill, and
relieve them from the constraints of finding quantum error correcting codes. It
can also treat the cases with uncharacterized apparatuses. We derive a secure
key rate for the Bennett-Brassard-1984 protocol with an arbitrary source
characterized only by a single parameter representing the basis dependence.


title: Concurrence classes for general pure multipartite states

abstract:
  We propose concurrence classes for general pure multipartite states based on
an orthogonal complement of a positive operator valued measure on quantum
phase. In particular, we construct $W^{m}$ class, $GHZ^{m}$, and $GHZ^{m-1}$
class concurrences for general pure $m$-partite states. We give explicit
expressions for $W^{3}$ and $GHZ^{3}$ class concurrences for general pure
three-partite states and for $W^{4}$, $GHZ^{4}$, and $GHZ^{3}$ class
concurrences for general pure four-partite states.


title: On quantum non-signalling boxes

abstract:
  A classical non-signalling (or causal) box is an operation on classical
bipartite input with classical bipartite output such that no signal can be sent
from a party to the other through the use of the box. The quantum counterpart
of such boxes, i.e. completely positive trace-preserving maps on bipartite
states, though studied in literature, have been investigated less intensively
than classical boxes. We present here some results and remarks about such maps.
In particular, we analyze: the relations among properties as causality,
non-locality and entanglement; the connection between causal and entanglement
breaking maps; the characterization of causal maps in terms of the
classification of states with fixed reductions. We also provide new proofs of
the fact that every non-product unitary transformation is not causal, as well
as for the equivalence of the so-called semicausality and semilocalizability
properties.


title:
Precision lifetime measurements of a single trapped ion with ultrafast
  laser pulses

abstract:
  We report precision measurements of the excited state lifetime of the $5p$
$^2P_{1/2}$ and $5p$ $^2P_{3/2}$ levels of a single trapped Cd$^+$ ion. The ion
is excited with picosecond laser pulses from a mode-locked laser and the
distribution of arrival times of spontaneously emitted photons is recorded. The
resulting lifetimes are 3.148 $\pm$ 0.011 ns and 2.647 $\pm$ 0.010 ns for
$^2P_{1/2}$ and $^2P_{3/2}$ respectively. With a total uncertainty of under
0.4%, these are among the most precise measurements of any atomic state
lifetimes to date.


title:
Quantum methods for clock synchronization: Beating the standard quantum
  limit without entanglement

abstract:
  We introduce methods for clock synchronization that make use of the adiabatic
exchange of nondegenerate two-level quantum systems: ticking qubits. Schemes
involving the exchange of N independent qubits with frequency $\omega$ give a
synchronization accuracy that scales as $(\omega\sqrt{N})^{-1}$, i.e., as the
standard quantum limit. We introduce a protocol that makes use of N coherent
exchanges of a single qubit at frequency $\omega$, leading to an accuracy that
scales as $(\omega N)^{-1}\log N$. This protocol beats the standard quantum
limit without the use of entanglement, and we argue that this scaling is the
fundamental limit for clock synchronization allowed by quantum mechanics. We
analyse the performance of these protocols when used with a lossy channel.


title: Some properties of $\lambda$ - solitons

abstract:
  We investigate some dynamical parameters of $\lambda$ - solitons which arises
in the family of implicit difference schemes for the diffusion equation with
fractional difference operator $ \frac{{\partial ^2}}{{\partial x^2}}$ and
imaginary diffusion coefficient. We suppose that such schemes may correspond to
fractional diffusion equation with imaginary diffusion coefficient.

categories: quant-ph cond-mat.mes-hall nlin.PS physics.comp-ph


title:
Localization and Pattern Formation in Quantum Physics. I. Phenomena of
  Localization

abstract:
  In these two related parts we present a set of methods, analytical and
numerical, which can illuminate the behaviour of quantum system, especially in
the complex systems. The key points demonstrating advantages of this approach
are: (i) effects of localization of possible quantum states, more proper than
"gaussian-like states"; (ii) effects of non-perturbative multiscales which
cannot be calculated by means of perturbation approaches; (iii) effects of
formation of complex quantum patterns from localized modes or classification
and possible control of the full zoo of quantum states, including (meta) stable
localized patterns (waveletons). We'll consider calculations of Wigner
functions as the solution of Wigner-Moyal-von Neumann equation(s) corresponding
to polynomial Hamiltonians. Modeling demonstrates the appearance of (meta)
stable patterns generated by high-localized (coherent) structures or
entangled/chaotic behaviour. We can control the type of behaviour on the level
of reduced algebraical variational system. At the end we presented the
qualitative definition of the Quantum Objects in comparison with their
Classical Counterparts, which natural domain of definition is the category of
multiscale/multiresolution decompositions according to the action of
internal/hidden symmetry of the proper realization of scales of functional
spaces. It gives rational natural explanation of such pure quantum effects as
``self-interaction''(self-interference) and instantaneous quantum interaction.

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


title:
Localization and Pattern Formation in Quantum Physics. II. Waveletons in
  Quantum Ensembles

abstract:
  In this second part we present a set of methods, analytical and numerical,
which can describe behaviour in (non) equilibrium ensembles, both classical and
quantum, especially in the complex systems, where the standard approaches
cannot be applied. The key points demonstrating advantages of this approach
are: (i) effects of localization of possible quantum states; (ii) effects of
non-perturbative multiscales which cannot be calculated by means of
perturbation approaches; (iii) effects of formation of complex/collective
quantum patterns from localized modes and classification and possible control
of the full zoo of quantum states, including (meta) stable localized patterns
(waveletons). We demonstrate the appearance of nontrivial localized (meta)
stable states/patterns in a number of collective models covered by the
(quantum)/(master) hierarchy of Wigner-von Neumann-Moyal-Lindblad equations,
which are the result of ``wignerization'' procedure (Weyl-Wigner-Moyal
quantization) of classical BBGKY kinetic hierarchy, and present the explicit
constructions for exact analytical/numerical computations (fast convergent
variational-wavelet representation). Numerical modeling shows the creation of
different internal structures from localized modes, which are related to the
localized (meta) stable patterns (waveletons), entangled ensembles (with
subsequent decoherence) and/or chaotic-like type of behaviour.


title: Relaxation Optimized Transfer of Spin Order in Ising Spin Chains

abstract:
  In this manuscript, we present relaxation optimized methods for transfer of
bilinear spin correlations along Ising spin chains. These relaxation optimized
methods can be used as a building block for transfer of polarization between
distant spins on a spin chain. Compared to standard techniques, significant
reduction in relaxation losses is achieved by these optimized methods when
transverse relaxation rates are much larger than the longitudinal relaxation
rates and comparable to couplings between spins. We derive an upper bound on
the efficiency of transfer of spin order along a chain of spins in the presence
of relaxation and show that this bound can be approached by relaxation
optimized pulse sequences presented in the paper.


title: QED of lossy cavities: operator and quantum-state input-output relations

abstract:
  Within the framework of exact quantization of the electromagnetic field in
dispersing and absorbing media the input-output problem of a high-$Q$ cavity is
studied, with special emphasis on the absorption losses in the coupling mirror.
As expected, the cavity modes are found to obey quantum Langevin equations,
which could be also obtained from quantum noise theories, by appropriately
coupling the cavity modes to dissipative systems, including the effect of the
mirror-assisted absorption losses. On the contrary, the operator input-output
relations obtained in this way would be incomplete in general, as the exact
calculation shows. On the basis of the operator input-output relations the
problem of extracting the quantum state of an initially excited cavity mode is
studied and input-output relations for the $s$-parameterized phase-space
function are derived, with special emphasis on the relation between the Wigner
functions of the quantum states of the outgoing field and the cavity field.


title: Continuous variable private quantum channel

abstract:
  In this paper we introduce the concept of quantum private channel within the
continuous variables framework (CVPQC) and investigate its properties. In terms
of CVPQC we naturally define a "maximally" mixed state in phase space together
with its explicit construction and show that for increasing number of
encryption operations (which sets the length of a shared key between Alice and
Bob) the encrypted state is arbitrarily close to the maximally mixed state in
the sense of the Hilbert-Schmidt distance. We bring the exact solution for the
distance dependence and give also a rough estimate of the necessary number of
bits of the shared secret key (i.e. how much classical resources are needed for
an approximate encryption of a generally unknown continuous-variable state).
The definition of the CVPQC is analyzed from the Holevo bound point of view
which determines an upper bound of information about an incoming state an
eavesdropper is able to get from his optimal measurement.


title: Entropy exchange and entanglement in the Jaynes-Cummings model

abstract:
  The Jaynes-Cummings model is the simplest fully quantum model that describes
the interaction between light and matter. We extend a previous analysis by
Phoenix and Knight (S. J. D. Phoenix, P. L. Knight, Annals of Physics 186,
381). of the JCM by considering mixed states of both the light and matter. We
present examples of qualitatively different entropic correlations. In
particular, we explore the regime of entropy exchange between light and matter,
i.e. where the rate of change of the two are anti-correlated. This behavior
contrasts with the case of pure light-matter states in which the rate of change
of the two entropies are positively correlated and in fact identical. We give
an analytical derivation of the anti-correlation phenomenon and discuss the
regime of its validity. Finally, we show a strong correlation between the
region of the Bloch sphere characterized by entropy exchange and that
characterized by minimal entanglement as measured by the negative eigenvalues
of the partially transposed density matrix.


title: How to play two-players restricted quantum games with 10 cards

abstract:
  We show that it is perfectly possible to play 'restricted' two-players,
two-strategies quantum games proposed originally by Marinatto and Weber having
as the only equipment a pack of 10 cards. The 'quantum board' of such a model
of these quantum games is an extreme simplification of 'macroscopic quantum
machines' proposed by one of the authors in numerous papers that allow to
simulate by macroscopic means various experiments performed on two entangled
quantum objects


title: Are all maximally entangled states pure?

abstract:
  We study if all maximally entangled states are pure through several
entanglement monotones. In the bipartite case, we find that the same conditions
which lead to the uniqueness of the entropy of entanglement as a measure of
entanglement, exclude the existence of maximally mixed entangled states. In the
multipartite scenario, our conclusions allow us to generalize the idea of
monogamy of entanglement: we establish the \textit{polygamy of entanglement},
expressing that if a general state is maximally entangled with respect to some
kind of multipartite entanglement, then it is necessarily factorized of any
other system.


title:
Quantum computation via translation-invariant operations on a chain of
  qubits

abstract:
  A scheme of universal quantum computation on a chain of qubits is described
that does not require local control. All the required operations, an Ising-type
interaction and spatially uniform simultaneous one-qubit gates, are
translation-invariant.


title: Mapping the Schrodinger picture of open quantum dynamics

abstract:
  For systems described by finite matrices, an affine form is developed for the
maps that describe evolution of density matrices for a quantum system that
interacts with another. This is established directly from the Heisenberg
picture. It separates elements that depend only on the dynamics from those that
depend on the state of the two systems. While the equivalent linear map is
generally not completely positive, the homogeneous part of the affine maps is,
and is shown to be composed of multiplication operations that come simply from
the Hamiltonian for the larger system. The inhomogeneous part is shown to be
zero if and only if the map does not increase the trace of the square of any
density matrix. Properties are worked out in detail for two-qubit examples.


title: Qubit entanglement in multimagnon states

abstract:
  The qubit entanglement induced by quasiparticle excitations in the Heisenberg
spin chain and its relationship to the Bethe Ansatz structure of the eigenmodes
is studied. A phenomenon called entanglement quenching, which suppresses
eigenstate entanglement, is described and shown to be mediated by Goldstone
magnons. Scattering states are characterized by short-range entanglement, and
never exhibit entanglement at the longest range. In contrast, bound states have
complex long-range entanglement structures.


title: Efficient Parity Encoded Optical Quantum Computing

abstract:
  We present a linear optics quantum computation scheme with a greatly reduced
cost in resources compared to KLM. The scheme makes use of elements from
cluster state computation and achieves comparable resource usage to those
schemes while retaining the circuit based approach of KLM.


title: Quantum Malware

abstract:
  When quantum communication networks proliferate they will likely be subject
to a new type of attack: by hackers, virus makers, and other malicious
intruders. Here we introduce the concept of "quantum malware" to describe such
human-made intrusions. We offer a simple solution for storage of quantum
information in a manner which protects quantum networks from quantum malware.
This solution involves swapping the quantum information at random times between
the network and isolated, distributed ancillas. It applies to arbitrary attack
types, provided the protective operations are themselves not compromised.


title: Vacuum force on an atom in a magnetodielectric cavity

abstract:
  We demonstrate that, according to a recently suggested Lorentz-force approach
to the Casimir effect, the vacuum force on an atom embedded in a material
cavity differs substantially from the force on an atom of the cavity medium.
The force on an embedded atom is of the familiar (van der Waals and
Casimir-Polder) type, however, more strongly modified by the cavity medium than
usually considered. The force on an atom of the cavity medium is of the
medium-assisted force type with rather unusual properties, as demonstrated very
recently [M. S. Tomas, Phys. Rev. A 71, 060101(R) (2005)]. This implies similar
properties of the vacuum force between two atoms in a medium.


title: A constructive algorithm for the Cartan decomposition of SU(2^N)

abstract:
  We present an explicit numerical method to obtain the Cartan-Khaneja-Glaser
decomposition of a general element G of SU(2^N) in terms of its `Cartan' and
`non-Cartan' components. This effectively factors G in terms of group elements
that belong in SU(2^n) with n<N, a procedure that can be iterated down to n=2.
We show that every step reduces to solving the zeros of a matrix polynomial,
obtained by truncation of the Baker-Campbell-Hausdorff formula, numerically.
All computational tasks involved are straightforward and the overall truncation
errors are well under control.


title:
Characterization of quantum computable decision problems by state
  discrimination

abstract:
  One advantage of quantum algorithms over classical computation is the
possibility to spread out, process, analyse and extract information in
multipartite configurations in coherent superpositions of classical states.
This will be discussed in terms of quantum state identification problems based
on a proper partitioning of mutually orthogonal sets of states. The question
arises whether or not it is possible to encode equibalanced decision problems
into quantum systems, so that a single invocation of a filter used for state
discrimination suffices to obtain the result.


title:
Observation of triply coincident nonlinearities in periodically poled
  KTiOPO4

abstract:
  We report the simultaneous quasi-phase-matching of all three possible
nonlinearities for propagation along the X axis of periodocally poled (PP)
KTiOPO4 (KTP) for second-harmonic generation of 745 nm pulsed light from 1490nm
subpicosecond pulses in a PPKTP crystal with a 45.65 micrometer poling period.
This confirms the recent Sellmeier fits of KTP by K. Kato and E. Takaoka [Appl.
Opt. 41, 5040 (2002)]. Such coincident nonlinearities are of importance for
realizing compact sources of multipartite continuous-variable entanglement
[Pfister et al., Phys. Rev. A 70, 020302 (2004)] and we propose a new simpler
method for entangling four fields, based on this triple coincidence.


title:
Bright tripartite entanglement in triply concurrent parametric
  oscillation

abstract:
  We show that a novel optical parametric oscillator, based on concurrent
$\chi^{(2)}$ nonlinearities, can produce, above threshold, bright output beams
of macroscopic intensities which exhibit strong tripartite continuous-variable
entanglement. We also show that there are {\em two} ways that the system can
exhibit a new three-mode form of the Einstein-Podolsky-Rosen paradox, and
calculate the extra-cavity fluctuation spectra that may be measured to verify
our predictions.


title: Unconditionally Secure Quantum Bit Commitment

abstract:
  The ``impossibility proof'' on unconditionally secure quantum bit commitment
is examined. It is shown that the possibility of juxtaposing quantum and
classical randomness has not been properly taken into account. A specific
protocol that beats entanglement cheating with entanglement is proven to be
unconditionally secure.


title:
Comment on 'Quantum inversion of cold atoms in a microcavity: spatial
  dependence'

abstract:
  In a recent work, Abdel-Aty and Obada [2002 J. Phys. B 35 807-813] analyzed
the quantum inversion of cold atoms in a microcavity, the motion of the atoms
being described quantum mechanically. Two-level atoms were assumed to interact
with a single mode of the cavity, and the off-resonance case was considered
(namely the atomic transition frequency is detuned from the single mode cavity
frequency). We demonstrate in this paper that this case is incorrectly treated
by these authors and we question therefore their conclusions.


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

abstract:
  We refute in this Reply the criticisms made by M. Abdel-Aty [Phys. Rev. A 70,
047801 (2004)]. We show that none of them are founded and we demonstrate very
explicitly what is wrong in the arguments developed by this author.


title: Quantum phase-space description of light polarization

abstract:
  We present a method to characterize the polarization state of a light field
in the continuous-variable regime. Instead of using the abstract formalism of
SU(2) quasidistributions, we model polarization in the classical spirit by
superposing two harmonic oscillators of the same angular frequency along two
orthogonal axes. By describing each oscillator by a $s$-parametrized
quasidistribution, we derive in a consistent way the final function for the
polarization. We compare with previous approaches and discuss how this
formalism works in some relevant examples.


title:
Calculation of the Casimir Force between Similar and Dissimilar Metal
  Plates at Finite Temperature

abstract:
  The Casimir pressure is calculated between parallel metal plates, containing
the materials Au, Cu, or Al. Our motivation for making this calculation is the
need of comparing theoretical predictions, based on the Lifshitz formula, with
experiments that are becoming gradually more accurate. In particular, the
finite temperature correction is considered, in view of the recent discussion
in the literature on this point. A special attention is given to the case where
the difference between the Casimir pressures at two different temperatures,
T=300 K and T=350 K, is involved. This seems to be a case that will be
experimentally attainable in the near future, and it will be a critical test of
the temperature correction.


title:
Distillation Protocols that Involve Local Distinguishing: Composing
  Upper and Lower Bounds on Locally Accessible Information

abstract:
  We find a universal lower bound on locally accessible information for
arbitrary bipartite quantum ensembles, when one of the parties is
two-dimensional. In higher dimensions and in higher number of parties, the
lower bound is on accessible information by separable operations. We show that
for any given density matrix (of arbitrary number of parties and dimensions),
there exists an ensemble, the ''Scrooge ensemble'', which averages to the given
density matrix and whose locally accessible information saturates the lower
bound. Moreover, we use this lower bound along with a previously obtained upper
bound to obtain bounds on the yield of singlets in distillation protocols that
involve local distinguishing.


title:
A triangle of dualities: reversibly decomposable quantum channels,
  source-channel duality, and time reversal

abstract:
  Two quantum information processing protocols are said to be dual under
resource reversal if the resources consumed (generated) in one protocol are
generated (consumed) in the other. Previously known examples include the
duality between entanglement concentration and dilution, and the duality
between coherent versions of teleportation and super-dense coding. A quantum
feedback channel is an isometry from a system belonging to Alice to a system
shared between Alice and Bob. We show that such a resource may be reversibly
decomposed into a perfect quantum channel and pure entanglement, generalizing
both of the above examples. The dual protocols responsible for this
decomposition are the ``feedback father'' (FF) protocol and the ``fully quantum
reverse Shannon'' (FQRS) protocol. Moreover, the ``fully quantum Slepian-Wolf''
protocol (FQSW), a generalization of the recently discovered ``quantum state
merging'', is related to FF by source-channel duality, and to FQRS by time
reversal duality, thus forming a triangle of dualities. The source-channel
duality is identified as the origin of the previously poorly understood
``mother-father'' duality. Due to a symmetry breaking, the dualities extend
only partially to classical information theory.


title: Optimal photons for quantum information processing

abstract:
  Photonic quantum information processing schemes, such as linear optics
quantum computing, and other experiments relying on single-photon interference,
inherently require complete photon indistinguishability to enable the desired
photonic interactions to take place. Mode-mismatch is the dominant cause of
photon distinguishability in optical circuits. Here we study the effects of
photon wave-packet shape on tolerance against the effects of mode-mismatch in
linear optical circuits, and show that Gaussian distributed photons with large
bandwidth are optimal. The result is general and holds for arbitrary linear
optical circuits, including ones which allow for post-selection and classical
feed-forward.


title: Detuning effects on the mazer

abstract:
  A short review of the theoretical studies of the cold atom micromaser (mazer)
is presented. Existing models are then improved by considering more general
working conditions. Especially, the mazer physics is investigated in the
situation where a detuning between the cavity mode and the atomic transition
frequency is present. Interesting new effects are pointed out. Especially, it
is shown that the cavity may slow down or speed up the atoms according to the
sign of the detuning and that the induced emission process may be completely
blocked by use of a positive detuning. The transmission probability of
ultracold atoms through a micromaser is also studied and we generalize previous
results established in the resonant case. In particular, it is shown that the
velocity selection of cold atoms passing through the micromaser can be very
easily tuned and enhanced using a nonresonant field inside the cavity. This
manuscript is a summary of Martin's master thesis and articles [Phys. Rev. A
67, 053804 (2003)] and [Eur. Phys. J. D 29, 133 (2004)].


title: A time representation

abstract:
  The paper contains a proposal for an energy and time representation. We
construct modes that correspond to fuzzy distributions around discrete values
of energy or time. The modes form an orthogonal and complete set in the space
of square integrable functions. Energy and time are self adjoint in the space
spanned by the modes. The widths of the modes are analyzed as well as their
energy-time uncertainty relations. The lower uncertainty attainable for the
modes is shown. We also show times of arrival for massless particles.


title: Aligning Spatial Frames Through Quantum Channels

abstract:
  We review the optimal protocols for aligning spatial frames using quantum
systems. The communication problem addressed here concerns a type of
information that cannot be digitalized. Asher Peres referred to it as
"unspeakable information". We comment on his contribution to this subject and
give a brief account of his scientific interaction with the authors.


title: Classical mechanics without determinism

abstract:
  Classical statistical particle mechanics in the configuration space can be
represented by a nonlinear Schrodinger equation. Even without assuming the
existence of deterministic particle trajectories, the resulting quantum-like
statistical interpretation is sufficient to predict all measurable results of
classical mechanics. In the classical case, the wave function that satisfies a
linear equation is positive, which is the main source of the fundamental
difference between classical and quantum mechanics.


title: Cotangent bundle quantization: Entangling of metric and magnetic field

abstract:
  For manifolds $\cal M$ of noncompact type endowed with an affine connection
(for example, the Levi-Civita connection) and a closed 2-form (magnetic field)
we define a Hilbert algebra structure in the space $L^2(T^*\cal M)$ and
construct an irreducible representation of this algebra in $L^2(\cal M)$. This
algebra is automatically extended to polynomial in momenta functions and
distributions. Under some natural conditions this algebra is unique. The
non-commutative product over $T^*\cal M$ is given by an explicit integral
formula. This product is exact (not formal) and is expressed in invariant
geometrical terms. Our analysis reveals this product has a front, which is
described in terms of geodesic triangles in $\cal M$. The quantization of
$\delta$-functions induces a family of symplectic reflections in $T^*\cal M$
and generates a magneto-geodesic connection $\Gamma$ on $T^*\cal M$. This
symplectic connection entangles, on the phase space level, the original affine
structure on $\cal M$ and the magnetic field. In the classical approximation,
the $\hbar^2$-part of the quantum product contains the Ricci curvature of
$\Gamma$ and a magneto-geodesic coupling tensor.


title: Optimal squeezing and entanglement from noisy Gaussian operations

abstract:
  We investigate the creation of squeezing via operations subject to noise and
losses and ask for the optimal use of such devices when supplemented by
noiseless passive operations. Both single and repeated uses of the device are
optimized analytically and it is proven that in the latter case the squeezing
converges exponentially fast to its asymptotic optimum, which we determine
explicitly. For the case of multiple iterations we show that the optimum can be
achieved with fixed intermediate passive operations. Finally, we relate the
results to the generation of entanglement and derive the maximal two-mode
entanglement achievable within the considered scenario.


title:
Robustness of a quantum key distribution with two and three mutually
  unbiased bases

abstract:
  We study the robustness of various protocols for quantum key distribution. We
first consider the case of qutrits and study quantum protocols that employ two
and three mutually unbiased bases. We then derive the optimal eavesdropping
strategy for two mutually unbiased bases in dimension four and generalize the
result to a quantum key distribution protocol that uses two mutually unbiased
bases in arbitrary finite dimension.


title:
A symmetric treatment of damped harmonic oscillator in extended phase
  space

abstract:
  Extended phase space (EPS) formulation of quantum statistical mechanics
treats the ordinary phase space coordinates on the same footing and thereby
permits the definite the canonical momenta conjugate to these coordinates . The
extended lagrangian and extended hamiltonian are defined in EPS by the same
procedure as one does for ordinary lagrangian and hamiltonian. The combination
of ordinary phase space and their conjugate momenta exhibits the evolution of
particles and their mirror images together. The resultant evolution equation in
EPS for a damped harmonic oscillator, is such that the energy dissipated by the
actual oscillator is absorbed in the same rate by the image oscillator leaving
the whole system as a conservative system. We use the EPS formalism to obtain
the dual hamiltonian of a damped harmonic oscillator, first proposed by
Batemann, by a simple extended canonical transformations in the extended phase
space. The extended canonical transformations are capable of converting the
damped system of actual and image oscillators to an undamped one, and transform
the evolution equation into a simple form. The resultant equation is solved and
the eigenvalues and eigenfunctions for damped oscillator and its mirror image
are obtained. The results are in agreement with those obtained by Bateman. At
last, the uncertainty relation are examined for above system.


title: Gaussian State Description of Squeezed Light

abstract:
  We present a Gaussian state description of squeezed light generated in an
optical parametric oscillator. Using the Gaussian state description we describe
the dynamics of the system conditioned on homodyne detection on the output
field. Our theory shows that the output field is squeezed only if observed for
long enough times or by a detector with finite bandwidth. As an application of
the present approach we consider the use of finite bandwidth squeezed light
together with a sample of spin-polarized atoms to estimate a magnetic field.


title: Multi-particle entanglement

abstract:
  We review the theory of multi-particle entanglement. In this book chapter we
aim at briefly ``setting the coordinates'' and guiding through the extensive
literature in this field. Our coordinate system chosen for this chapter has the
axes labeled pure and mixed states on the one hand, entanglement in single
specimens and the asymptotic setting on the other hand. We very briefly mention
ways to detect multi-particle entanglement, and introduce the concepts of
stabilizer and graph states.


title: A critical analysis of the hydrino model

abstract:
  Recently, spectroscopic and calorimetric observations of hydrogen plasmas and
chemical reactions with them have been interpreted as evidence for the
existence of electronic states of the hydrogen atom with a binding energy of
more than 13.6 eV. The theoretical basis for such states, that have been dubbed
hydrinos, is investigated. We discuss both, the novel deterministic model of
the hydrogen atom, in which the existence of hydrinos was predicted, and
standard quantum mechanics. Severe inconsistencies in the deterministic model
are pointed out and the incompatibility of hydrino states with quantum
mechanics is reviewed.


title: Gaussian quantum channels

abstract:
  This article provides an elementary introduction to Gaussian channels and
their capacities. We review results on the classical, quantum, and entanglement
assisted capacities and discuss related entropic quantities as well as
additivity issues. Some of the known results are extended. In particular, it is
shown that the quantum conditional entropy is maximized by Gaussian states and
that some implications for additivity problems can be extended to the Gaussian
setting.


title: Generalised Asymmetric Quantum Cloning

abstract:
  We study machines that take N identical replicas of a pure qudit state as
input and output a set of M_A clones of a given fidelity and another set of
$M_B$ clones of another fidelity. The trade-off between these two fidelities is
investigated, and numerous examples of optimal N -> M_A+M_B cloning machines
are exhibited using a generic method. A generalisation to more than two sets of
clones is also discussed. Finally, an optical implementation of some such
machines is proposed. This paper is an extended version of
[xxx.arxiv.org/abs/quant-ph/0411179].


title: Quantum Error Correction in Spatially Correlated Quantum Noise

abstract:
  We consider quantum error correction of quantum-noise that is created by a
local interaction of qubits with a common bosonic bath. The possible exchange
of bath bosons between qubits gives rise to spatial and temporal correlations
in the noise. We find that these kind of noise correlations have a strong
negative impact on quantum error correction.


title:
Photothermal Fluctuations as a Fundamental Limit to Low-Frequency
  Squeezing in a Degenerate Optical Parametric Amplifier

abstract:
  We study the effect of photothermal fluctuations on squeezed states of light
through the photo-refractive effect and thermal expansion in a degenerate
optical parametric amplifier (OPA). We also discuss the effect of the
photothermal noise in various cases and how to minimize its undesirable
consequences. We find that the photothermal noise in the OPA introduces a
significant amount of noise on phase squeezed beams, making them less than
ideal for low frequency applications such as gravitational wave (GW)
interferometers, whereas amplitude squeezed beams are relatively immune to the
photothermal noise and may represent the best choice for application in GW
interferometers.


title:
A Uniform Approximation for the Coherent State Propagator using a
  Conjugate Application of the Bargmann Representation

abstract:
  We propose a conjugate application of the Bargmann representation of quantum
mechanics. Applying the Maslov method to the semiclassical connection formula
between the two representations, we derive a uniform semiclassical
approximation for the coherent state propagator which is finite at phase space
caustics.


title: Local Invariants for a Class of Mixed States

abstract:
  We investigate the equivalence of quantum states under local unitary
transformations. A complete set of invariants under local unitary
transformations is presented for a class of mixed states. It is shown that two
states in this class are locally equivalent if and only if all these invariants
have equal values for them.


title: Geometric Phase in Entangled Bipartite Systems

abstract:
  The geometric phase (GP) for bipartite systems in transverse external
magnetic fields is investigated in this paper. Two different situations have
been studied. We first consider two non-interacting particles. The results show
that because of entanglement, the geometric phase is very different from that
of the non-entangled case. When the initial state is a Werner state, the
geometric phase is, in general, zero and moreover the singularity of the
geometric phase may appear with a proper evolution time. We next study the
geometric phase when intra-couplings appear and choose Werner states as the
initial states to entail this discussion. The results show that unlike our
first case, the absolute value of the GP is not zero, and attains its maximum
when the rescaled coupling constant $J$ is less than 1. The effect of
inhomogeneity of the magnetic field is also discussed.


title: Analysis of Popper's Experiment and its Realization

abstract:
  An experiment proposed by Karl Popper to test the standard interpretation of
quantum mechanics was realized by Kim and Shih. We use a quantum mechanical
calculation to analyze Popper's proposal, and find a surprising result for the
location of the virtual slit. We also analyze Kim and Shih's experiment, and
demonstrate that although it ingeniously overcomes the problem of temporal
spreading of the wave-packet, it is inconclusive about Popper's test. We point
out that another experiment which (unknowingly) implements Popper's test in a
conclusive way, has actually been carried out. Its results are in contradiction
with Popper's prediction, and agree with our analysis.


title: Periodically Pulsed Strong Squeezing

abstract:
  We report on specific signatures of squeezing for time-modulated light
fields. We show that application of periodically-modulated driving fields
instead of continuous wave fields drastically improves the degree of quadrature
integral squeezing in an optical parametric oscillator. These results
particularly allow for applications in time-resolved quantum communication
protocols.


title: Quantum Information Processing Without Joint Measurement

abstract:
  We propose a linear optical scheme for the teleportation of unknown ionic
states, the entanglement concentration for nonmaximally entangled states for
ions via entanglement swapping and the remote preparation for ionic entangled
states. The unique advantage of the scheme is that the joint Bell-state
measurement needed in the previous schemes is not needed in the current scheme,
i.e. the joint Bell-state measurement has been converted into the product of
separate measurements on single ions and photons. In addition, the current
scheme can realize the quantum information processes for ions by using linear
optical elements, which simplify the implementation of quantum information
processing for ions.


title: On the temperature dependence of the interaction-induced entanglement

abstract:
  Both direct and indirect weak nonresonant interactions are shown to produce
entanglement between two initially disentangled systems prepared as a tensor
product of thermal states, provided the initial temperature is sufficiently
low. Entanglement is determined by the Peres-Horodeckii criterion, which
establishes that a composite state is entangled if its partial transpose is not
positive. If the initial temperature of the thermal states is higher than an
upper critical value $T_{uc}$ the minimal eigenvalue of the partially
transposed density matrix of the composite state remains positive in the course
of the evolution. If the initial temperature of the thermal states is lower
than a lower critical value $T_{lc}\leq T_{uc}$ the minimal eigenvalue of the
partially transposed density matrix of the composite state becomes negative
which means that entanglement develops. We calculate the lower bound $T_{lb}$
for $T_{lc}$ and show that the negativity of the composite state is negligibly
small in the interval $T_{lb}<T<T_{uc}$. Therefore the lower bound temperature
$T_{lb}$ can be considered as \textit{the} critical temperature for the
generation of entanglement.


title: Measures and dynamics of entangled states

abstract:
  We develop an original approach for the quantitative characterisation of the
entanglement properties of, possibly mixed, bi- and multipartite quantum states
of arbitrary finite dimension. Particular emphasis is given to the derivation
of reliable estimates which allow for an efficient evaluation of a specific
entanglement measure, concurrence, for further implementation in the monitoring
of the time evolution of multipartite entanglement under incoherent environment
coupling. The flexibility of the technical machinery established here is
illustrated by its implementation for different, realistic experimental
scenarios.


title: Fast SWAP gate by adiabatic passage

abstract:
  We present a process for the construction of a SWAP gate which does not
require a composition of elementary gates from a universal set. We propose to
employ direct techniques adapted to the preparation of this specific gate. The
mechanism, based on adiabatic passage, constitutes a decoherence-free method in
the sense that spontaneous emission and cavity damping are avoided.


title: Quantum Noise Locking

abstract:
  Quantum optical states which have no coherent amplitude, such as squeezed
vacuum states, can not rely on standard readout techniques to generate error
signals for control of the quadrature phase. Here we investigate the use of
asymmetry in the quadrature variances to obtain a phase-sensitive readout and
to lock the phase of a squeezed vacuum state, a technique which we call noise
locking (NL). We carry out a theoretical derivation of the NL error signal and
the associated stability of the squeezed and anti-squeezed lock points.
Experimental data for the NL technique both in the presence and absence of
coherent fields are shown, including a comparison with coherent locking
techniques. Finally, we use NL to enable a stable readout of the squeezed
vacuum state on a homodyne detector.


title:
Self-organization effects and light amplification of collective atomic
  recoil motion in a harmonic trap

abstract:
  Self-organization effects related to light amplification in the collective
atomic recoil laser system with the driven atoms confined in a harmonic trap
are investigated further. In the dispersive parametric region, our study
reveals that the spontaneously formed structures in the phase space contributes
an important role to the light amplification of the probe field under the
atomic motion being modified by the trap.


title: Characterization of multiqubit pure-state entanglement

abstract:
  A necessary and sufficient entanglement criterion based on variances of
Mermin-Klyshko's Bell operators is proved for multiqubit pure states. Contrary
to Bell's inequalities, entangled pure states strictly satisfy a quadratic
inequality but product ones can attain the equality under some local unitary
transformations, which can be obtained by solving a quadratic maximum problem.
This presents a characterization of multiqubit pure-state entanglement.


title: Perfect Quantum Error-Correcting Condition Revisited

abstract:
  A simple and unifying method to show the perfect error-correcting condition
is provided based on the quantum mutual information. The one-to-one
parameterization of quantum operations and the properties of the quantum
relative entropy are used effectively in this paper, where the equivalence
between the subspace transmission and the entanglement transmission is clearly
presented. We also revisit a variant of the no-cloning and no-deleting theorem
based on an information-theoretical tradeoff between two parties for the
reversibility of quantum operations, and demonstrate that the no-cloning and
no-deleting theorem leads to the perfect error-correcting condition on Kraus
operators.


title:
Singular behavior of an entangled state for a one-dimensional quantum
  spin system

abstract:
  We studied the entangled state for a one-dimensional $S=1/2$
antiferromagnetic quantum spin chain in a transverse field. We calculate the
ground state using the density matrix renormalization group and discuss how the
entangled state changes around a quantum phase transition (QPT) point. By
analyzing concurrence $C(\rho)$ for two-qubit density matrix $\rho$ after the
Lewenstein-Sanpera decomposition, $\rho=\Lambda \rho_s + (1-\Lambda) \rho_e $,
where $\rho_s$ is a separable density matrix and $\rho_e$ is a pure entangled
state obtained by a linear combination of Bell states, we find singular
behaviors both in $C(\rho_e)$ and $1-\Lambda$ at the QPT point.
$C(\rho_e)$ includes the effects of quantum fluctuations, which manifest the
competition between the antiferromagnetic spin fluctuation and the effect of
transverse field in the transverse Ising model. The quantum fluctuation shows
the singular maximum at the QPT point as expected from the general picture of
phase transition. In contrast, $1-\Lambda$ reveals the singular minimum at QPT
point.


title: Wave packet transmission of Bloch electron manipulated by magnetic field

abstract:
  We study the phenomenon of wave packet revivals of Bloch electrons and
explore how to control them by a magnetic field for quantum information
transfer. It is showed that the single electron system can be modulated into a
linear dispersion regime by the "quantized" flux and then an electronic wave
packet with the components localized in this regime can be transferred without
spreading. This feature can be utilized to perform the high-fidelity transfer
of quantum information encoded in the polarization of the spin. Beyond the
linear approximation, the re-localization and self-interference occur as the
novel phenomena of quantum coherence.


title: Light Qubit Storage and Retrieval using Macroscopic Atomic Ensembles

abstract:
  We present an experimentally feasible protocol for the complete
  storage and retrieval of arbitrary light states in an atomic quantum
  memory using the well-established Faraday interaction between light
  and matter. Our protocol relies on multiple passages of a single
  light pulse through the atomic ensemble without the impractical
  requirement of kilometer long delay lines between the passages.
  Furthermore, we introduce a time dependent interaction strength
  which enables storage and retrieval of states with arbitrary pulse
  shapes. The fidelity approaches unity exponentially without squeezed
  or entangled initial states, as illustrated by explicit calculations
  for a photonic qubit.


title:
New approach to (quasi)-exactly solvable Schrodinger equations with a
  position-dependent effective mass

abstract:
  By using the point canonical transformation approach in a manner distinct
from previous ones, we generate some new exactly solvable or quasi-exactly
solvable potentials for the one-dimensional Schr\"odinger equation with a
position-dependent effective mass. In the latter case, SUSYQM techniques
provide us with some additional new potentials.


title: Collapse challenge for interpretations of quantum mechanics

abstract:
  The collapse challenge for interpretations of quantum mechanics is to build
from first principles and your preferred interpretation a complete,
observer-free quantum model of the described experiment (involving a photon and
two screens), together with a formal analysis that completely explains the
experimental result. The challenge is explained in detail, and discussed in the
light of the Copenhagen interpretation and the decoherence setting.


title:
Rydberg wavepackets in terms of hidden-variables: de Broglie-Bohm
  trajectories

abstract:
  The dynamics of highly excited radial Rydberg wavepackets is analyzed in
terms of de Broglie-Bohm (BB) trajectories. Although the wavepacket evolves
along classical motion, the computed BB trajectories are markedly different
from the classical dynamics: in particular none of the trajectories initially
near the atomic core reach the outer turning point where the wavepacket
localizes periodically. The reasons for this behavior, that we suggest to be
generic for trajectory-based hidden variable theories, are discussed.


title:
Entanglement can enhance the distinguishability of entanglement-breaking
  channels

abstract:
  We show the rather counterintuitive result that entangled input states can
strictly enhance the distinguishability of two entanglement-breaking channels.


title: Relational time for systems of oscillators

abstract:
  Using an elementary example based on two simple harmonic oscillators, we show
how a relational time may be defined that leads to an approximate Schrodinger
dynamics for subsystems, with corrections leading to an intrinsic decoherence
in the energy eigenstates of the subsystem.