title: Medium-modified Casimir forces

abstract:
  We argue that the results for the vacuum forces on a slab and on an atom
embedded in a magnetodielectric medium near a mirror, obtained using a recently
suggested Lorentz-force approach to the Casimir effect, are equivalent to the
corresponding results obtained in a traditional way. We also derive a general
expression for the atom-atom force in a medium and extend a few classical
results concerning this force in vacuum and dielectrics to magnetodielectric
systems. This, for example, reveals that the (repulsive) interaction between
atoms of different polarizability type is at small distances unaffected by a
(weakly polarizable) medium.


title:
Entanglement, fidelity, and quantum-classical correlations with an atom
  walking in a quantized cavity field

abstract:
  Stability and instability of quantum evolution are studied in the interaction
between a two-level atom with photon recoil and a quantized field mode in an
ideal cavity, the basic model of cavity quantum electrodynamics (QED). It is
shown that the Jaynes-Cummings dynamics can be unstable in the regime of
chaotic walking of the atomic center-of-mass in the quantized field of a
standing wave in the absence of any kind of interaction with environment. This
kind of quantum instability manifests itself in strong variations of reduced
quantum purity and entropy, correlating with the respective classical Lyapunov
exponent, and in exponential sensitivity of fidelity of quantum states to small
variations in the atom-field detuning. The connection between quantum
entanglement and fidelity and the center-of-mass motion is clarified
analytically and numerically for a few regimes of that motion. The results are
illustrated with two specific initial field states: the Fock and coherent ones.
Numerical experiments demonstrate various manifestations of the
quantum-classical correspondence, including dynamical chaos and fractals, which
can be, in principle, observed in real experiments with atoms and photons in
high finesse cavities.


title:
Quantum gate for Q switching in monolithic photonic bandgap cavities
  containing two-level atoms

abstract:
  Photonic bandgap cavities are prime solid-state systems to investigate
light-matter interactions in the strong coupling regime. However, as the cavity
is defined by the geometry of the periodic dielectric pattern, cavity control
in a monolithic structure can be problematic. Thus, either the state coherence
is limited by the read-out channel, or in a high Q cavity, it is nearly
decoupled from the external world, making measurement of the state extremely
challenging. We present here a method for ameliorating these difficulties by
using a coupled cavity arrangement, where one cavity acts as a switch for the
other cavity, tuned by control of the atomic transition.


title: Eigensolutions of the kicked Harper model

abstract:
  The time-evolution operator for the kicked Harper model is reduced to block
matrix form when the effective Planck's constant hbar = 2 pi M/N and M and N
are integers. Each block matrix is spanned by an orthonormal set of N "kq"
(quasi-position/quasi-momentum) functions. This implies that the system's
eigenfunctions or stationary states are necessarily discrete and periodic. The
reduction allows, for the first time, an examination of the 2-dimensional
structure of the system's quasi-energy spectrum and the study of, with
unprecedented accuracy, the system's stationary states.


title: A new approach to quantum backflow

abstract:
  We derive some rigorous results concerning the backflow operator introduced
by Bracken and Melloy. We show that it is linear bounded, self adjoint, and not
compact. Thus the question is underlined whether the backflow constant is an
eigenvalue of the backflow operator. From the position representation of the
backflow operator we obtain a more efficient method to determine the backflow
constant. Finally, detailed position probability flow properties of a numerical
approximation to the (perhaps improper) wave function of maximal backflow are
displayed.


title: Discrete PT-symmetric square-well oscillators

abstract:
  Exact solvability of the discretized N-point version of the PT-symmetric
square-well model is pointed out. Its wave functions are found proportional to
the classical Tshebyshev polynomials of a complex argument. At all N a compact
secular equation is derived giving the real spectrum of energies at any
non-Hermiticity strength Z below its finite and weakly N-dependent critical
value. In the limit of vanishing Z the model degenerates to a Hermitian Hueckel
Hamiltonian.


title: On the SU(3) parametrization of qutrits

abstract:
  Parametrization of qutrits on the complex projective plane CP^2 = SU(3)/U(2)
is given explicitly. A set of constraints that characterize mixed state density
matrices is found.


title: Covariance and Frames of Reference

abstract:
  A transition of focus from state space to frames of reference and their
transformations is argued as being the appropriate setup for ensuring the
covariance of physical laws. Such an approach can not only simplify and clarify
aspects of General Relativity, but can possibly help in the development of a
Grand Unified Theory as well.


title:
Implementation of continuous variable quantum cryptography in optical
  fibres using a go-&-return configuration

abstract:
  We demonstrate an implementation of quantum key distribution with continuous
variables based on a go-&-return configuration over distances up to 14km. This
configuration leads to self-compensation of polarisation and phase
fluctuations. We observe a high degree of stability of our set-up over many
hours.


title:
Coupling bosonic modes with a qubit: entanglement dynamics at zero and
  finite-temperatures

abstract:
  We consider a system of two iso-spectral bosonic modes coupled with a single
two-level systems i.e., a qubit. The dynamics is described by a mode-symmetric
two-modes Jaynes-Cummings. The entanglement, induced between the two bosonic
modes, is analyzed and quantified by negativity. We computed the time evolution
of negativity starting from an initial thermal state of the bosonic sector for
both zero and finite temperature. We also studied the entangling power of the
interaction as a function of mode-qubit detuning and its resilience against
temperature increase. Finally a two-qubit gates based on bosonic virtual
subsystem is discussed.


title: Enhanced dynamical entanglement transfer with multiple qubits

abstract:
  We present two strategies to enhance the dynamical entanglement transfer from
continuous variable (CV) to finite dimensional systems by employing multiple
qubits. First, we consider the entanglement transfer to a composite finite
dimensional system of many qubits simultaneously interacting with a bipartite
CV field. We show that, considering realistic conditions in the generation of
CV entanglement, a small number of qubits resonantly coupled to the CV system
is sufficient for an almost complete dynamical transfer of the entanglement.
Our analysis also sheds further light on the transition between microscopic and
macroscopic behaviours of composite finite dimensional systems coupled to
bosonic fields (like atomic clouds interacting with light). Furthermore, we
present a protocol based on sequential interactions of the CV system with some
ancillary qubit systems and on subsequent measurements, allowing to
probabilistically convert CV entanglement into `almost perfect' Bell pairs of
two qubits. Our proposals are suited for realizations in various experimental
settings, ranging from cavity-QED to cavity-integrated superconducting devices.


title:
Experimental Entanglement Distillation of Two-qubit Mixed States Under
  Local filtering Operations

abstract:
  We experimentally demonstrate optimal entanglement distillation from two
forms of two-qubit mixed states under local filtering operations according to
the constructive method intruduced by F. Verstraete et al. [Phys. Rev. A 64,
010101(R) (2001)]. In principle, our set-up can be easily applied to distilling
entanglement from arbitrary two-qubit partially mixed states. We also test the
violation of the Clauser-Horne-Shinmony-Holt (CHSH) inequality for the
distilled state from the first form of mixed state to show its "hidden
non-locality".


title:
Perfect Computational Equivalence between Quantum Turing Machines and
  Finitely Generated Uniform Quantum Circuit Families

abstract:
  In order to establish the computational equivalence between quantum Turing
machines (QTMs) and quantum circuit families (QCFs) using Yao's quantum circuit
simulation of QTMs, we previously introduced the class of uniform QCFs based on
an infinite set of elementary gates, which has been shown to be computationally
equivalent to the polynomial-time QTMs (with appropriate restriction of
amplitudes) up to bounded error simulation. This result implies that the
complexity class BQP introduced by Bernstein and Vazirani for QTMs equals its
counterpart for uniform QCFs. However, the complexity classes ZQP and EQP for
QTMs do not appear to equal their counterparts for uniform QCFs. In this paper,
we introduce a subclass of uniform QCFs, the finitely generated uniform QCFs,
based on finite number of elementary gates and show that the class of finitely
generated uniform QCFs is perfectly equivalent to the class of polynomial-time
QTMs; they can exactly simulate each other. This naturally implies that BQP as
well as ZQP and EQP equal the corresponding complexity classes of the finitely
generated uniform QCFs.


title: Fast Rydberg gates without dipole blockade via quantum control

abstract:
  We propose a scheme for controlling interactions between Rydberg-excited
neutral atoms in order to perform a fast high-fidelity quantum gate. Unlike
dipole-blockade mechanisms already found in the literature, we drive resonantly
the atoms with a state-dependent excitation to Rydberg levels, and we exploit
the resulting dipole-dipole interaction to induce a controlled atomic motion in
the trap, in a similar way as discussed in recent ion-trap quantum computing
proposals. This leads atoms to gain the required gate phase, which turns out to
be a combination of a dynamic and a geometrical contribution. The fidelity of
this scheme is studied including small anharmonicity and temperature effects,
with promising results for reasonably achievable experimental parameters.


title:
Exact entanglement probability distribution of bi-partite randomised
  stabilizer states

abstract:
  We study the entanglement properties of random pure stabilizer states in
spin-1/2 particles. For two contiguous groups of spins of arbitrary size we
obtain a compact and exact expression for the probability distribution for the
entanglement values between the two sets. This allows for exact derivations of
the average entanglement and the degree of concentration of measure around this
average. We also give simple bounds on these quantities. We find that for large
systems the average entanglement is near maximal and the measure is
concentrated around it.


title:
Random decoupling schemes for quantum dynamical control and error
  suppression

abstract:
  We present a general control-theoretic framework for constructing and
analyzing random decoupling schemes, applicable to quantum dynamical control of
arbitrary finite-dimensional composite systems. The basic idea is to design the
control propagator according to a random rather than deterministic path on a
group. We characterize the performance of random decoupling protocols, and
identify control scenarios where they can significantly weaken time scale
requirements as compared to cyclic counterparts. Implications for reliable
quantum computation are discussed.


title:
Dynamical control of qubit coherence: Random versus deterministic
  schemes

abstract:
  We revisit the problem of switching off unwanted phase evolution and
decoherence in a single two-state quantum system in the light of recent results
on random dynamical decoupling methods [L. Viola and E. Knill, Phys. Rev. Lett.
{\bf 94}, 060502 (2005)]. A systematic comparison with standard cyclic
decoupling is effected for a variety of dynamical regimes, including the case
of both semiclassical and fully quantum decoherence models. In particular,
exact analytical expressions are derived for randomized control of decoherence
from a bosonic environment. We investigate quantitatively control protocols
based on purely deterministic, purely random, as well as hybrid design, and
identify their relative merits and weaknesses at improving system performance.
We find that for time-independent systems, hybrid protocols tend to perform
better than pure random and may improve over standard asymmetric schemes,
whereas random protocols can be considerably more stable against fluctuations
in the system parameters. Beside shedding light on the physical requirements
underlying randomized control, our analysis further demonstrates the potential
for explicit control settings where the latter may significantly improve over
conventional schemes.


title:
Continuum variable entangled state generated by an asymmetric beam
  splitter

abstract:
  The generating entangled state by using a 50/50 beamsplitter has been
discussed in the literature before . In this paper we explore how to use an
asymmetric beam-splitter to produce a new kind of entangled state. We construct
such kind of states theoretically and then prove that they make up a complete
and orthonormal representation in two-mode Fock space. Its application in
finding new squeezing operator and new squeezed state is introduced.


title:
Two-player quantum pseudo-telepathy based on recent all-versus-nothing
  violations of local realism

abstract:
  We introduce two two-player quantum pseudo-telepathy games based on two
recently proposed all-versus-nothing (AVN) proofs of Bell's theorem [A.
Cabello, Phys. Rev. Lett. 95, 210401 (2005); Phys. Rev. A 72, 050101(R)
(2005)]. These games prove that Broadbent and Methot's claim that these AVN
proofs do not rule out local-hidden-variable theories in which it is possible
to exchange unlimited information inside the same light-cone (quant-ph/0511047)
is incorrect.


title:
Gauge Invariant Quantization of Dissipative Systems of Charged Particles
  in Extended Phase Space

abstract:
  Recently, it is shown that the extended phase space formulation of quantum
mechanics is a suitable technique for studying the quantum dissipative systems.
Here, as a further application of this formalism, we consider a dissipative
system of charged particles interacting with an external time dependent
electric field. Such a system has been investigated by Buch and Denman, and two
distinct solutions with completely different structure have been obtained for
Schr\"odinger's equation in two different gauges. However, by generalizing the
gauge transformations to the phase space and using the extended phase space
technique to study the same system, we demonstrate how both gauges lead to the
same conductivity, suggesting the recovery of gauge invariance for this
physical quantity within the extended phase space approach.


title: Quantum Potential and Symmetries in Extended Phase Space

abstract:
  The behavior of the quantum potential is studied for a particle in a linear
and a harmonic potential by means of an extended phase space technique. This is
done by obtaining an expression for the quantum potential in momentum space
representation followed by the generalization of this concept to extended phase
space. It is shown that there exists an extended canonical transformation that
removes the expression for the quantum potential in the dynamical equation. The
situation, mathematically, is similar to disappearance of the centrifugal
potential in going from the spherical to the Cartesian coordinates that changes
the physical potential to an effective one. The representation where the
quantum potential disappears and the modified Hamilton-Jacobi equation reduces
to the familiar classical form, is one in which the dynamical equation turns
out to be the Wigner equation.


title: Study of Dissipative System of Charged Particles by Wigner's Functions

abstract:
  Recently, it is shown that the extended phase space formulation of quantum
mechanics is a suitable technique for studying the quantum dissipative system.
Here, as an application of this formalism, we consider a dissipative system of
charged particles interacting with an external time dependent electric field.
Such a system has been investigating by Buch and Denman, and two solutions with
completely different structure have been obtained for Schr\"{o}dinger's
equation in two different gauges. We demonstrate how both gauges lead to the
same conductivity by generalizing the gauge transformations to the phase space
and using the extended phase space technique.


title: A Generalized Rule For Non-Commuting Operators in Extended Phase Space

abstract:
  A generalized quantum distribution function is introduced. The corresponding
ordering rule for non-commuting operators is given in terms of a single
parameter. The origin of this parameter is in the extended canonical
transformations that guarantees the equivalence of different distribution
functions obtained by assuming appropriate values for this parameter.


title:
Analysis of the generation of photon pairs in periodically poled lithium
  niobate

abstract:
  The process of spontaneous parametric down-conversion (SPDC) in nonlinear
crystals makes it fairly easy to generate entangled photon states. It has been
known for some time that the conversion efficiency can be improved by employing
quasi-phase-matching in periodically poled crystals. Using two single-photon
detectors, we have analyzed the photon pairs generated by SPDC in a
periodically poled lithium niobate crystal pumped by a femtosecond laser.
Several parameters could be varied in our setup, allowing us to obtain data in
close agreement with both thermal and Poissonian photon-pair distributions.


title: How to Measure the Nonlocal Phase of a Single Photon

abstract:
  The relative phase between spatially separated component waves of a single
photon can be measured by joint interference with a second photon emitted by a
known source. In the case of a single such phase (i.e. two component waves),
the probability for a successful measurement is one half. This method can be
implemented with current experimental techniques.


title: Quantum key distribution without alternative measurements and rotations

abstract:
  A quantum key distribution protocol based on entanglement swapping is
proposed. Through choosing particles by twos from the sequence and performing
Bell measurements, two communicators can detect eavesdropping and obtain the
secure key. Because the two particles measured together are selected out
randomly, we need neither alternative measurements nor rotations of the Bell
states to obtain security.


title:
Adiabatic Quantum Computing in systems with constant inter-qubit
  couplings

abstract:
  We propose an approach suitable for solving NP-complete problems via
adiabatic quantum computation with an architecture based on a lattice of
interacting spins (qubits) driven by locally adjustable effective magnetic
fields. Interactions between qubits are assumed constant and
instance-independent, programming is done only by changing local magnetic
fields. Implementations using qubits coupled by magnetic-, electric-dipole and
exchange interactions are discussed.


title:
Observation of dynamic localization in periodically-curved waveguide
  arrays

abstract:
  We report on a direct experimental observation of dynamic localization (DL)
of light in sinusoidally-curved Lithium-Niobate waveguide arrays which provides
the optical analog of DL for electrons in periodic potentials subjected to ac
electric fields as originally proposed by Dunlap and Kenkre [D.H. Dunlap and
V.M. Kenkre, Phys. Rev. B 34, 3625 (1986)]. The theoretical condition for DL in
a sinusoidal field is experimentally demonstrated.


title:
On the information-splitting essence of two types of quantum key
  distribution protocols

abstract:
  With the help of a simple quantum key distribution (QKD) scheme, we discuss
the relation between BB84-type protocols and two-step-type ones. It is shown
that they have the same essence, i.e., information-splitting. More
specifically, the similarity between them includes (1) the carrier state is
split into two parts which will be sent one by one; (2) the possible states of
each quantum part are indistinguishable; (3) anyone who obtains both parts can
recover the initial carrier state and then distinguish it from several possible
states. This result is useful for related scheme designing and security
analyzing.


title: Untangling Quantum Entanglement

abstract:
  The phenomenon of quantum entanglement is explained in a way which is fully
consistent with Einstein's Special Theory of Relativity. A subtle flaw is
identified in the logic supporting the view that Bell's Inequality precludes
all local hidden-variable theories, and it is shown how EPR-type experiments
can be constructed to produce statistical correlation results in a purely
classical manner which match exactly the predictions made by quantum theory.


title:
Path Integral Approach for Spaces of Non-constant Curvature in Three
  Dimensions

abstract:
  In this contribution I show that it is possible to construct
three-dimensional spaces of non-constant curvature, i.e. three-dimensional
Darboux-spaces. Two-dimensional Darboux spaces have been introduced by Kalnins
et al., with a path integral approach by the present author. In comparison to
two dimensions, in three dimensions it is necessary to add a curvature term in
the Lagrangian in order that the quantum motion can be properly defined. Once
this is done, it turns out that in the two three-dimensional Darboux spaces,
which are discussed in this paper, the quantum motion is similar to the
two-dimensional case. In $\threedDI$ we find seven coordinate systems which
separate the Schr\"odinger equation. For the second space, $\threedDII$, all
coordinate systems of flat three-dimensional Euclidean space which separate the
Schr\"odinger equation also separate the Schr\"odinger equation in
$\threedDII$. I solve the path integral on $\threedDI$ in the $(u,v,w)$-system,
and on $\threedDII$ in the $(u,v,w)$-system and in spherical coordinates.


title: On the explanation for quantum statistics

abstract:
  The concept of classical indistinguishability is analyzed and defended
against a number of well-known criticisms, with particular attention to the
Gibbs' paradox. Granted that it is as much at home in classical as in quantum
statistical mechanics, the question arises as to why indistinguishability, in
quantum mechanics but not in classical mechanics, forces a change in
statistics. The answer, illustrated with simple examples, is that the
equilibrium measure on classical phase space is continuous, whilst on Hilbert
space it is discrete. The relevance of names, or equivalently, properties
stable in time that can be used as names, is also discussed.


title: Hall of Mirrors Scattering from an Impurity in a Quantum Wire

abstract:
  This paper develops a scattering theory to examine how point impurities
affect transport through quantum wires. While some of our new results apply
specifically to hard-walled wires, others--for example, an effective optical
theorem for two-dimensional waveguides--are more general. We apply the method
of images to the hard-walled guide, explicitly showing how scattering from an
impurity affects the wire's conductance. We express the effective cross section
of a confined scatterer entirely in terms of the empty waveguide's Green's
function, suggesting a way in which to use semiclassical methods to understand
transport properties of smooth wires. In addition to predicting some new
phenomena, our approach provides a simple physical picture for previously
observed effects such as conductance dips and confinement-induced resonances.


title: Generation of cluster states

abstract:
  We propose two schemes for the generation of the cluster states. One is based
on cavity quantum electrodynamics (QED) techniques. The scheme only requires
resonant interactions between two atoms and a single-mode cavity. The
interaction time is very short, which is important in view of decoherence.
Furthermore, we also discuss the cavity decay and atomic spontaneous emission
case. The other is based on atomic ensembles. The scheme has inherent fault
tolerance function and is robust to realistic noise and imperfections. All the
facilities used in our schemes are well within the current technology.


title: The Aladin2 experiment: status and perspectives

abstract:
  Aladin2 is an experiment devoted to the first measurement of variations of
Casimir energy in a rigid cavity. The main scientific motivation relies on the
possibility of the first demonstration of a phase transition influenced by
vacuum fluctuations. The guiding principle of the measurement, based on the
behaviour of the critical field for an in-cavity superconducting film, will be
only briefly recalled. In this paper, after an introduction to the long term
motivations, the experimental apparatus and the results of the first
measurement of sensitivity will be presented in detail, particularly in
comparison with the expected signal. Last, the most important steps towards the
final measurement will be discussed.


title: Stability, Gain, and Robustness in Quantum Feedback Networks

abstract:
  This paper concerns the problem of stability for quantum feedback networks.
We demonstrate in the context of quantum optics how stability of quantum
feedback networks can be guaranteed using only simple gain inequalities for
network components and algebraic relationships determined by the network.
Quantum feedback networks are shown to be stable if the loop gain is less than
one-this is an extension of the famous small gain theorem of classical control
theory. We illustrate the simplicity and power of the small gain approach with
applications to important problems of robust stability and robust
stabilization.


title:
Perturbative Analysis of Possible Failures in the Traditional Adiabatic
  Conditions

abstract:
  Recently, Marzlin and Sanders (2004) demonstrated an inconsistency when the
adiabatic approximation was applied to specific, "inverse" time-evolving
systems. Following that, Tong et al. (2005) showed that the widely used
traditional adiabatic conditions are insufficient to guarantee the validity of
the adiabatic approximation for this class of systems. In this article we
explore the origin of these observations by a perturbative approach and find
that in first order approximation certain nonzero terms appear in the solution
which gives rise to the breakdown of the adiabatic approximation (despite the
fact that the traditional adiabatic conditions are satisfied). We argue that in
this case the Hamiltonian of Marzlin and Sanders' inverse time evolving system
cannot be written in terms of t/T, where T denotes the total evolution time. It
is further demonstrated that the new qualitative adiabatic condition of Ye et
al. (2005) performs well in some cases when the traditional conditions fail to
describe properly non-adiabatic evolution.


title:
Statistical Mechanics of Quantum-Classical Systems with Holonomic
  Constraints

abstract:
  The statistical mechanics of quantum-classical systems with holonomic
constraints is formulated rigorously by unifying the classical Dirac bracket
and the quantum-classical bracket in matrix form.
  The resulting Dirac quantum-classical theory, which conserves the holonomic
constraints exactly, is then used to formulate time evolution and statistical
mechanics. The correct momentum-jump approximation for constrained system
arises naturally from this formalism. Finally, in analogy with what was found
in the classical case, it is shown that the rigorous linear response function
of constrained quantum-classical systems contains non-trivial additional terms
which are absent in the response of unconstrained systems.


title: Simulations of "Decoherence" with Noise Pulses

abstract:
  A simulation of decoherence as random noise in the Hamiltonian is studied.
The full Hamiltonian for the rf Squid is used, with the parameters chosen such
that there is a double-potential well configuration where the two
quasi-degenerate lowest levels are well separated from the rest. The results
for these first two levels are in quantitative agreement with expectations from
the ``spin 1/2'' picture for the behavior of a two-state system.


title: Classical Phase Space Density for the Relativistic Hydrogen Atom

abstract:
  Quantum mechanics is considered to arise from an underlying classical
structure (``hidden variable theory'', ``sub-quantum mechanics''), where
quantum fluctuations follow from a physical noise mechanism. The stability of
the hydrogen ground state can then arise from a balance between Lorentz damping
and energy absorption from the noise. Since the damping is weak, the ground
state phase space density should predominantly be a function of the conserved
quantities, energy and angular momentum.
  A candidate for this phase space density is constructed for ground state of
the relativistic hydrogen problem of a spinless particle. The first excited
states and their spherical harmonics are also considered in this framework. The
analytic expression of the ground state energy can be reproduced, provided
averages of certain products are replaced by products of averages. This
analysis puts forward that quantum mechanics may arise from an underlying
classical level as a slow variable theory, where each new quantum operator
relates to a new, well separated time interval.


title: Semantics and simulation of communication in quantum programming

abstract:
  We present the quantum programming language cQPL which is an extended version
of QPL [P. Selinger, Math. Struct. in Comp. Sci. 14(4):527-586, 2004]. It is
capable of quantum communication and it can be used to formulate all possible
quantum algorithms. Additionally, it possesses a denotational semantics based
on a partial order of superoperators and uses fixed points on a generalised
Hilbert space to formalise (in addition to all standard features expected from
a quantum programming language) the exchange of classical and quantum data
between an arbitrary number of participants. Additionally, we present the
implementation of a cQPL compiler which generates code for a quantum simulator.


title: Spatial decoherence near metallic surfaces

abstract:
  We present a first-principles derivation of spatial atomic-sublevel
decoherence near dielectric and metallic surfaces. The theory is based on the
electromagnetic-field quantization in absorbing dielectric media. We derive an
expression for the time-variation of the off-diagonal matrix element of the
atomic density matrix for arbitrarily shaped substrates. For planar
multilayered substrates we find that for small lateral separations of the
atom's possible positions the spatial coherence decreases quadratically with
the separation and inversely to the squared atom-surface distance.


title: Two-colorable graph states with maximal Schmidt measure

abstract:
  The Schmidt measure was introduced by Eisert and Briegel for quantifying the
degree of entanglement of multipartite quantum systems [Phys. Rev. A 64, 022306
(2001)]. Although generally intractable, it turns out that there is a bound on
the Schmidt measure for two-colorable graph states [Phys. Rev. A 69, 062311
(2004)]. For these states, the Schmidt measure is in fact directly related to
the number of nonzero eigenvalues of the adjacency matrix of the associated
graph. We remark that almost all two-colorable graph states have maximal
Schmidt measure and we construct specific examples. These involve perfect
trees, line graphs of trees, cographs, graphs from anti-Hadamard matrices, and
unyciclic graphs. We consider some graph transformations, with the idea of
transforming a two-colorable graph state with maximal Schmidt measure into
another one with the same property. In particular, we consider a transformation
introduced by Francois Jaeger, line graphs, and switching. By making appeal to
a result of Ehrenfeucht et al. [Discrete Math. 278 (2004)], we point out that
local complementation and switching form a transitive group acting on the set
of all graph states of a given dimension.


title: Limitations of Quantum Coset States for Graph Isomorphism

abstract:
  It has been known for some time that graph isomorphism reduces to the hidden
subgroup problem (HSP). What is more, most exponential speedups in quantum
computation are obtained by solving instances of the HSP. A common feature of
the resulting algorithms is the use of quantum coset states, which encode the
hidden subgroup. An open question has been how hard it is to use these states
to solve graph isomorphism. It was recently shown by Moore, Russell, and
Schulman that only an exponentially small amount of information is available
from one, or a pair of coset states. A potential source of power to exploit are
entangled quantum measurements that act jointly on many states at once. We show
that entangled quantum measurements on at least \Omega(n log n) coset states
are necessary to get useful information for the case of graph isomorphism,
matching an information theoretic upper bound. This may be viewed as a negative
result because highly entangled measurements seem hard to implement in general.
Our main theorem is very general and also rules out using joint measurements on
few coset states for some other groups, such as GL(n, F_{p^m}) and G^n where G
is finite and satisfies a suitable property.


title:
Tight Results on Multiregister Fourier Sampling: Quantum Measurements
  for Graph Isomorphism Require Entanglement

abstract:
  We establish a general method for proving bounds on the information that can
be extracted via arbitrary entangled measurements on tensor products of hidden
subgroup coset states. When applied to the symmetric group, the method yields
an Omega(n log n) lower bound on the number of coset states over which we must
perform an entangled measurement in order to obtain non-negligible information
about a hidden involution. These results are tight to within a multiplicative
constant and apply, in particular, to the case relevant for the Graph
Isomorphism problem.
  Part of our proof was obtained after learning from Hallgren, Roetteler, and
Sen that they had obtained similar results.


title: Valence-Bond-Solid state entanglement in a 2-D Cayley tree

abstract:
  The Valence-Bond-Solid (VBS) states are in general ground states for certain
gapped models. We consider the entanglement of VBS states on a two-dimensional
Cayley tree. We show that the entropy of the reduced density operator does not
depend on the whole size of the Cayley tree. We also show that asymptotically,
the entropy is liearly proportional to the number of singlet states cut by the
reduced density operator of the VBS state.


title:
On Integrability and Pseudo-Hermitian Systems with Spin-Coupling Point
  Interactions

abstract:
  We study the pseudo-Hermitian systems with general spin-coupling point
interactions and give a systematic description of the corresponding boundary
conditions for PT-symmetric systems. The corresponding integrability for both
bosonic and fermionic many-body systems with PT-symmetric contact interactions
is investigated.


title: The Application of Asymmetric Entangled States in Quantum Game

abstract:
  In the present letter, we propose a more general entangling operator to the
quantization of Cournot economic model, in which players can access to a
continuous set of strategies. By analyzing the relation between the von Neumann
entropy of the entangled state and the total profit of two players precisely,
we find that the total profit at the Nash equilibrium always achieves its
maximal value as long as the entropy tends to infinity. Moreover, since the
asymmetry is introduced in the entangled state, the quantum model shows some
kind of "encouraging" and "suppressing" effect in profit functions of different
players.


title:
Paramagnetic Materials and Practical Algorithmic Cooling for NMR Quantum
  Computing

abstract:
  Algorithmic Cooling is a method that uses novel data compression techniques
and simplecquantum computing devices to improve NMR spectroscopy, and to offer
scalable NMR quantum computers. The algorithm recursively employs two steps. A
reversible entropy compression of the computation quantum-bits (qubits) of the
system and an irreversible heat transfer from the system to the environment
through a set of reset qubits that reach thermal relaxation rapidly.
  Is it possible to experimentally demonstrate algorithmic cooling using
existing technology? To allow experimental algorithmic cooling, the
thermalization time of the reset qubits must be much shorter than the
thermalization time of the computation qubits. However such
thermalization-times ratios have yet to be reported.
  We investigate here the effect of a paramagnetic salt on the
thermalization-times ratio of computation qubits (carbons) and a reset qubit
(hydrogen). We show that the thermalization-times ratio is improved by
approximately three-fold. Based on this result, an experimental demonstration
of algorithmic cooling by thermalization and magnetic ions is currently
performed by our group and collaborators.


title:
Continuous measurements on continuous variable quantum systems: The
  Gaussian description

abstract:
  The Gaussian state description of continuous variables is adapted to describe
the quantum interaction between macroscopic atomic samples and continuous-wave
light beams. The formalism is very efficient: a non-linear differential
equation for the covariance matrix of the atomic system explicitly accounts for
both the unitary evolution, the dissipation and noise due to the atom-light
interaction, and the back-action due to homodyne optical detection on the beam
after its interaction with the atoms. Applications to atomic spin squeezing and
estimation of unknown classical parameters are presented, and extensions beyond
the Gaussian states are discussed.


title: Entanglement of Formation of Bipartite Quantum States

abstract:
  We give an explicit tight lower bound for the entanglement of formation for
arbitrary bipartite mixed states by using the convex hull construction of a
certain function. This is achieved by revealing a novel connection among the
entanglement of formation, the well-known Peres-Horodecki and realignment
criteria. The bound gives a quite simple and efficiently computable way to
evaluate quantitatively the degree of entanglement for any bipartite quantum
state.


title: Experimental Heat-Bath Cooling of Spins

abstract:
  Algorithmic cooling is a novel technique to generate ensembles of highly
polarized spins, which could significantly improve the signal strength in
Nuclear Magnetic Resonance (NMR) spectroscopy. It combines reversible
(entropy-preserving) manipulations and irreversible controlled interactions
with the environment, using simple quantum computing techniques to increase
spin polarization far beyond the Shannon entropy-conservation bound. Notably,
thermalization is beneficially employed as an integral part of the cooling
scheme, contrary to its ordinary destructive implications. We report the first
cooling experiments bypassing Shannon's entropy-conservation bound, performed
on a standard liquid-state NMR spectrometer. We believe that this experimental
success could pave the way for the first near-future application of quantum
computing devices.


title: Dobinski-type relations: Some properties and physical applications

abstract:
  We introduce a generalization of the Dobinski relation through which we
define a family of Bell-type numbers and polynomials. For all these sequences
we find the weight function of the moment problem and give their generating
functions. We provide a physical motivation of this extension in the context of
the boson normal ordering problem and its relation to an extension of the Kerr
Hamiltonian.


title:
Superposition and Entanglement: Pillars of Quantum Information
  Processing

abstract:
  This article discusses the important primitives of Superposition and
Entanglement in Quantum Information Processing from physics point of view.
System of spin-1/2 particles has been considered which presents itself as a
logical and conceptual candidate to understand these concepts. The article is
intended as a review of these important concepts and hopes to bring forth a
conceptual framework in this regard.


title: Nonlocal properties of dynamical three-body Casimir-Polder forces

abstract:
  We consider the three-body Casimir-Polder interaction between three atoms
during their dynamical self-dressing. We show that the time-dependent
three-body Casimir-Polder interaction energy displays nonlocal features related
to quantum properties of the electromagnetic field and to the nonlocality of
spatial field correlations. We discuss the measurability of this intriguing
phenomenon and its relation with the usual concept of stationary three-body
forces.


title:
Matter-screened Casimir force and Casimir-Polder force in planar
  structures

abstract:
  Using a recently developed theory of the Casimir force (Raabe C and Welsch
D-G 2005 Phys. Rev. A 71 013814), we calculate the force that acts on a plate
in front of a planar wall and the force that acts on the plate in the case
where the plate is part of matter that fills the space in front of the wall. We
show that in the limit of a dielectric plate whose permittivity is close to
unity, the force obtained in the former case reduces to the ordinary, i.e.,
unscreened Casimir-Polder force acting on isolated atoms. In the latter case,
the theory yields the Casimir-Polder force that is screened by the surrounding
matter.


title:
Inelastic scattering of light by a cold trapped atom: Effects of the
  quantum center-of-mass motion

abstract:
  The light scattered by a cold trapped ion, which is in the stationary state
of laser cooling, presents features due to the mechanical effects of
atom-photon interaction. These features appear as additional peaks (sidebands)
in the spectrum of resonance fluorescence. Among these sidebands the literature
has discussed the Stokes and anti-Stokes components, namely the sidebands of
the elastic peak. In this manuscript we show that the motion also gives rise to
sidebands of the inelastic peaks. These are not always visible, but, as we
show, can be measured in parameter regimes which are experimentally accessible.


title: Quantum phase transitions without thermodynamic limits

abstract:
  A new microcanonical equilibrium state is introduced for quantum systems with
finite-dimensional state spaces. Equilibrium is characterised by a uniform
distribution on a level surface of the expectation value of the Hamiltonian.
The distinguishing feature of the proposed equilibrium state is that the
corresponding density of states is a continuous function of the energy, and
hence thermodynamic functions are well defined for finite quantum systems. The
density of states, however, is not in general an analytic function. It is
demonstrated that generic quantum systems therefore exhibit second-order
(continuous) phase transitions at finite temperatures.


title: Experimental Quantum Cryptography with Qutrits

abstract:
  We produce two identical keys using, for the first time, entangled trinary
quantum systems (qutrits) for quantum key distribution. The advantage of
qutrits over the normally used binary quantum systems is an increased coding
density and a higher security margin. The qutrits are encoded into the orbital
angular momentum of photons, namely Laguerre-Gaussian modes with azimuthal
index l +1, 0 and -1, respectively. The orbital angular momentum is controlled
with phase holograms. In an Ekert-type protocol the violation of a
three-dimensional Bell inequality verifies the security of the generated keys.
A key is obtained with a qutrit error rate of approximately 10 %.


title: About what can be honestly called a Quantum Brudno's Theorem

abstract: This paper has been withdrawn.


title: Codekets

abstract:
  To every binary linear [n,k]-code C we associate a quantum state ("codeket")
belonging to the n-th tensor power of the 2-dimensional complex Hilbert space
associated to the spin 1/2 particle. We completely characterize the expectation
values of the products of x-, y- or z- spins measured in the state we define,
for each of the particles in a chosen subset. This establishes an interesting
relationship with the dual code. We also address the case of nonlinear codes,
and derive both a bound satisfied by the expectations of spin products, as well
as a nice algebraic identity.


title: On the number of representations providing noiseless subsystems

abstract:
  This paper studies the combinatoric structure of the set of all
representations, up to equivalence, of a finite-dimensional semisimple Lie
algebra. This has intrinsic interest as a previously unsolved problem in
representation theory, and also has applications to the understanding of
quantum decoherence. We prove that for Hilbert spaces of sufficiently high
dimension, decoherence-free subspaces exist for almost all representations of
the error algebra. For decoherence-free subsystems, we plot the function
$f_d(n)$ which is the fraction of all $d$-dimensional quantum systems which
preserve $n$ bits of information through DF subsystems, and note that this
function fits an inverse beta distribution. The mathematical tools which arise
include techniques from classical number theory.


title: Calculating state-to-state transition probabilities within TDDFT

abstract:
  The determination of the elements of the S-matrix within the framework of
time-dependent density-functional theory (TDDFT) has remained a widely open
question. We explore two different methods to calculate state-to-state
transition probabilities. The first method closely follows the extraction of
the S-matrix from the time-dependent Hartree-Fock approximation. This method
suffers from cross-channel correlations resulting in oscillating transition
probabilities in the asymptotic channels. An alternative method is proposed
which corresponds to an implicit functional in the time-dependent density. It
gives rise to stable and accurate transition probabilities. An exactly solvable
two-electron system serves as benchmark for a quantitative test.


title: Scheme for direct measurement of a general two-qubit Hamiltonian

abstract:
  The construction of two-qubit gates appropriate for universal quantum
computation is of enormous importance to quantum information processing.
Building such gates is dependent on accurate knowledge of the interaction
dynamics between two qubit systems. This letter will present a systematic
method for reconstructing the full two-qubit interaction Hamiltonian through
experimental measures of concurrence. This not only gives a convenient method
for constructing two qubit quantum gates, but can also be used to
experimentally determine various Hamiltonian parameters in physical systems. We
show explicitly how this method can be employed to determine the first and
second order spin-orbit corrections to the exchange coupling in quantum dots.


title: Fisher Information: Quantum Uncertainty Relation

abstract:
  The paper deals with the reformulation of quantum uncertainty relation
involving position and momentum of a particle on the basis of the Kerridge
measure of inaccuracy and the Fisher information.


title:
Reverse estimation theory, Complementality between RLD and SLD, and
  monotone distances

abstract:
  Many problems in quantum information theory can be vied as interconversion
between resources. In this talk, we apply this view point to state estimation
theory, motivated by the following observations.
  First, a monotone metric takes value between SLD and RLD Fisher metric. This
is quite analogous to the fact that entanglement measures are sandwiched by
distillable entanglement and entanglement cost. Second, SLD add RLD are
mutually complement via purification of density matrices, but its operational
meaning was not clear.
  To find a link between these observations, we define reverse estimation
problem, or simulation of quantum state family by probability distribution
family, proving that RLD Fisher metric is a solution to local reverse
estimation problem of quantum state family with 1-dim parameter. This result
gives new proofs of some known facts and proves one new fact about monotone
distances.
  We also investigate information geometry of RLD, and reverse estimation
theory of a multi-dimensional parameter family.


title: Shannon Entropy: Axiomatic Characterization and Application

abstract:
  We have presented a new axiomatic derivation of Shannon Entropy for a
discrete probability distribution on the basis of the postulates of additivity
and concavity of the entropy function.We have then modified shannon entropy to
take account of observational uncertainty.The modified entropy reduces, in the
limiting case, to the form of Shannon differential entropy. As an application
we have derived the expression for classical entropy of statistical mechanics
from the quantized form of the entropy.


title: On Classical Teleportation and Classical Nonlocality

abstract:
  An interesting protocol for classical teleportation of an unknown classical
state was recently suggested by Cohen, and by Gour and Meyer. In that protocol,
Bob can sample from a probability distribution P that is given to Alice, even
if Alice has absolutely no knowledge about P. Pursuing a similar line of
thought, we suggest here a limited form of nonlocality - "classical
nonlocality". Our nonlocality is the (somewhat limited) classical analogue of
the Hughston-Jozsa-Wootters (HJW) quantum nonlocality. The HJW nonlocality
tells us how, for a given density matrix rho, Alice can generate any
rho-ensemble on the North Star. This is done using surprisingly few resources -
one shared entangled state (prepared in advance), one generalized quantum
measurement, and no communication. Similarly, our classical nonlocality
presents how, for a given probability distribution P, Alice can generate any
P-ensemble on the North Star, using only one correlated state (prepared in
advance), one (generalized) classical measurement, and no communication.
  It is important to clarify that while the classical teleportation and the
classical non-locality protocols are probably rather insignificant from a
classical information processing point of view, they significantly contribute
to our understanding of what exactly is quantum in their well established and
highly famous quantum analogues.


title: Tomographic reconstruction of quantum states in N spatial dimensions

abstract:
  Most quantum tomographic methods can only be used for one-dimensional
problems. We show how to infer the quantum state of a non-relativistic
N-dimensional harmonic oscillator system by simple inverse Radon transforms.
The procedure is equally applicable to finding the joint quantum state of
several distinguishable particles in different harmonic oscillator potentials.
A requirement of the procedure is that the angular frequencies of the N
harmonic potentials are incommensurable. We discuss what kind of information
can be found if the requirement of incommensurability is not fulfilled and also
under what conditions the state can be reconstructed from finite time
measurements. As a further example of quantum state reconstruction in N
dimensions we consider the two related cases of an N-dimensional free particle
with periodic boundary conditions and a particle in an N-dimensional box, where
we find a similar condition of incommensurability and finite recurrence time
for the one-dimensional system.


title:
Teleportation and spin squeezing utilizing multimode entanglement of
  light with atoms

abstract:
  We present a protocol for the teleportation of the quantum state of a pulse
of light onto the collective spin state of an atomic ensemble. The entangled
state of light and atoms employed as a resource in this protocol is created by
probing the collective atomic spin, Larmor precessing in an external magnetic
field, off resonantly with a coherent pulse of light. We take here for the
first time full account of the effects of Larmor precession and show that it
gives rise to a qualitatively new type of multimode entangled state of light
and atoms. The protocol is shown to be robust against the dominating sources of
noise and can be implemented with an atomic ensemble at room temperature
interacting with free space light. We also provide a scheme to perform the
readout of the Larmor precessing spin state enabling the verification of
successful teleportation as well as the creation of spin squeezing.


title: A Proof of the Security of Quantum Key Distribution

abstract:
  We prove the security of theoretical quantum key distribution against the
most general attacks which can be performed on the channel, by an eavesdropper
who has unlimited computation abilities, and the full power allowed by the
rules of classical and quantum physics. A key created that way can then be used
to transmit secure messages such that their security is also unaffected in the
future.


title:
Non-Markovian decay beyond the Fermi Golden Rule: Survival Collapse of
  the polarization in spin chains

abstract:
  The decay of a local spin excitation in an inhomogeneous spin chain is
evaluated exactly: I) It starts quadratically up to a spreading time t_{S}. II)
It follows an exponential behavior governed by a self-consistent Fermi Golden
Rule. III) At longer times, the exponential is overrun by an inverse power law
describing return processes governed by quantum diffusion. At this last
transition time t_{R} a survival collapse becomes possible, bringing the
polarization down by several orders of magnitude. We identify this strongly
destructive interference as an antiresonance in the time domain. These general
phenomena are suitable for observation through an NMR experiment.


title: Quantum Computer Condition: Stability, Classical Computation and Norms

abstract:
  The Quantum Computer Condition (QCC) provides a rigorous and completely
general framework for carrying out analyses of questions pertaining to
fault-tolerance in quantum computers. In this paper we apply the QCC to the
problem of fluctuations and systematic errors in the values of characteristic
parameters in realistic systems. We show that fault-tolerant quantum
computation is possible despite variations in these parameters. We also use the
QCC to explicitly show that reliable classical computation can be carried out
using as input the results of fault-tolerant, but imperfect, quantum
computation. Finally, we consider the advantages and disadvantages of the
superoperator and diamond norms in connection with application of the QCC to
various quantum information-theoretic problems.


title:
Universal Quantum Computation with the nu=5/2 Fractional Quantum Hall
  State

abstract:
  We consider topological quantum computation (TQC) with a particular class of
anyons that are believed to exist in the Fractional Quantum Hall Effect state
at Landau level filling fraction nu=5/2. Since the braid group representation
describing statistics of these anyons is not computationally universal, one
cannot directly apply the standard TQC technique. We propose to use very noisy
non-topological operations such as direct short-range interaction between
anyons to simulate a universal set of gates. Assuming that all TQC operations
are implemented perfectly, we prove that the threshold error rate for
non-topological operations is above 14%. The total number of non-topological
computational elements that one needs to simulate a quantum circuit with $L$
gates scales as $L(\log L)^3$.


title:
Klein-Gordon Equation with Casimir Potential for Attosecond Laser Pulse
  Interaction with Matter

abstract:
  In this paper the Klein-Gordon equation (K-GE) is solved for the interaction
of attosecond laser pulses with medium in which Casimir force operates. It is
shown that for nanoscale structures, NEMS and MEMS, the attosecond laser pulses
can be used as the tool for the investigation of the role played by Casimir
force on the nanoscale. Key words: Casimir force; NEMS, MEMS, Attosecond laser
pulses.


title: A Simple Discussion on Wave Functional in Free QED

abstract: This paper has been withdrawn by the author due to some mistakes


title: Photon Wave Mechanics

abstract:
  In contrast to wave functions in nonrelativistic quantum mechanics
interpreted as probability amplitudes, wave functions in relativistic quantum
mechanics have generalized meanings such as charge-density amplitudes,
energy-density amplitudes as well as particle-number density amplitudes, etc.
Applying electromagnetic field intensities we construct a photon wave function,
it corresponds to the (1,0)+(0,1) spinor representation of the electromagnetic
field, and can be interpreted as the energy-density amplitude of photons
outside a source. In terms of photon wave functions we develop photon wave
mechanics, which provides us with a new quantum-mechanical description for
photons outside a source.


title:
Pseudo-Hermitian versus Hermitian position-dependent-mass Hamiltonians
  in a perturbative framework

abstract:
  We formulate a systematic algorithm for constructing a whole class of
Hermitian position-dependent-mass Hamiltonians which, to lowest order of
perturbation theory, allow a description in terms of PT-symmetric Hamiltonians.
The method is applied to the Hermitian analogue of the PT-symmetric cubic
anharmonic oscillator. A new example is provided by a Hamiltonian
(approximately) equivalent to a PT-symmetric extension of the one-parameter
trigonometric Poschl-Teller potential.


title: Atom-Photon Entanglement

abstract:
  We report the observation of entanglement between a single trapped atom and a
single photon at a wavelength suitable for low-loss communication over large
distances, thereby achieving a crucial step towards long range quantum
networks. To verify the entanglement we introduce a single atom state analysis.
This technique is used for full state tomography of the atom-photon qubit-pair.
The detection efficiency and the entanglement fidelity are high enough to allow
in a next step the generation of entangled atoms at large distances, ready for
a final loophole-free Bell experiment.


title: Semiclassical interpretation of microscopic processes

abstract:
  There are three upper limits (2, 2.sqrt{2}, 2.sqrt{3}) of the Bell operator
corresponding to different physical concepts: classical, hidden-variable and
quantum-mechanical. Only the classical concept corresponding to the lowest
limit has been excluded by experimental data, while the other two should be
regarded as acceptable for the interpretation of EPR experiments and all
microscopic processes. A corresponding hidden-variable or semiclassical model
(based on the extended Hilbert space) will be proposed and shortly described.


title: Excitation and Entanglement Transfer Versus Spectral Gap

abstract:
  We consider quantum many body systems as quantum channels and study the
relation between the transfer quality and the size of the spectral gap between
the system's ground and excited states. In our setup two ancillas are weakly
coupled to the quantum many body system at different sites, and we study the
propagation of an excitation and quantum information from one ancilla to the
other. We observe two different scenarios: a slow, but perfect transfer if the
gap large and a fast, but un-complete transfer otherwise. We provide a
numerical and analytical approach as well as a simplified physical model
explaining our findings. Our results relate the potential of spin chains acting
as quantum channels to the concept of quantum phase transitions and offer a
different approach to the characterisation of these.


title: Berry phase, topology, and diabolicity in quantum nano-magnets

abstract:
  A topological theory of the diabolical points (degeneracies) of quantum
magnets is presented. Diabolical points are characterized by their diabolicity
index, for which topological sum rules are derived. The paradox of the the
missing diabolical points for Fe8 molecular magnets is clarified. A new method
is also developed to provide a simple interpretation, in terms of destructive
interferences due to the Berry phase, of the complete set of diabolical points
found in biaxial systems such as Fe8.


title: Direct measurement of non-linear properties of bipartite quantum states

abstract:
  Non-linear properties of quantum states, such as entropy or entanglement,
quantify important physical resources and are frequently used in quantum
information science. They are usually calculated from a full description of a
quantum state, even though they depend only on a small number parameters that
specify the state. Here we extract a non-local and a non-linear quantity,
namely the Renyi entropy, from local measurements on two pairs of polarization
entangled photons. We also introduce a "phase marking" technique which allows
to select uncorrupted outcomes even with non-deterministic sources of entangled
photons. We use our experimental data to demonstrate the violation of entropic
inequalities. They are examples of a non-linear entanglement witnesses and
their power exceeds all linear tests for quantum entanglement based on all
possible Bell-CHSH inequalities.


title: Spatial Degrees of Freedom in Everett Quantum Mechanics

abstract:
  Stapp claims that, when spatial degrees of freedom are taken into account,
Everett quantum mechanics is ambiguous due to a "core basis problem." To
examine an aspect of this claim I generalize the ideal measurement model to
include translational degrees of freedom for both the measured system and the
measuring apparatus. Analysis of this generalized model using the Everett
interpretation in the Heisenberg picture shows that it makes unambiguous
predictions for the possible results of measurements and their respective
probabilities. The presence of translational degrees of freedom for the
measuring apparatus affects the probabilities of measurement outcomes in the
same way that a mixed state for the measured system would. Examination of a
measurement scenario involving several observers illustrates the consistency of
the model with perceived spatial localization of the measuring apparatus.


title:
Projection Measurement of the Maximally Entangled N-Photon State for a
  Demonstration of N-Photon de Broglie Wavelength

abstract:
  We construct a projection measurement process for the maximally entangled
N-photon state (the NOON-state) with only linear optical elements and
photodetectors. This measurement process will give null result for any N-photon
state that is orthogonal to the NOON state. We examine the projection process
in more detail for N=4 by applying it to a four-photon state from type-II
parametric down-conversion. This demonstrates an orthogonal projection
measurement with a null result. This null result corresponds to a dip in a
generalized Hong-Ou-Mandel interferometer for four photons. We find that the
depth of the dip in this arrangement can be used to distinguish a genuine
entangled four-photon state from two separate pairs of photons. We next apply
the NOON state projection measurement to a four-photon superposition state from
two perpendicularly oriented type-I parametric down-conversion processes. A
successful NOON state projection is demonstrated with the appearance of the
four-photon de Broglie wavelength in the interference fringe pattern.


title: Achieving Holevo Bound in Quantum Measurement

abstract:
  We show that information gain in a qubit measurement is optimal under a Von
Neumann measurement. For an initially mixed apparatus kept in touch with a
qubit, the conditions for achieving the equality sign of Holevo bound on the
information accessible to apparatus are derived. These constraints can be
identified as the conditions for the optimization of information gain in a
qubit measurement. At the end, we will generalize the idea to qudit
measurements using a phase-shift gate.


title: Dirac sextic oscillator in the constant magnetic field

abstract:
  We introduce a Dirac equation which reproduces the usual radial sextic
oscillator potential in the non-relativistic limit. We determine its energy
spectrum in the presence of the magnetic field. It is shown that the equation
is solved in the context of quasi-exactly-solvable problems. The equation
possesses hidden $sl_{2}$-algebra and the destroyed symmetry of the equation
can be recovered for a specific values of the magnetic field which leads to
exact determination of the eigenvalues.


title: Geometric phase for SU(N) through fiber bundles and unitary integration

abstract:
  Geometric phases are important in quantum physics and now central to fault
tolerant quantum computation. For spin-1/2 and SU(2), the Bloch sphere $S^2$,
together with a U(1) phase, provides a complete SU(2) description. We
generalize to $N$-level systems and SU($N$) in terms of a $2(N-1)$-dimensional
base space and reduction to a ($N$-1)-level problem, paralleling closely the
two-dimensional case. This iteratively solves the time evolution of an
$N$-level system and gives ($N$-1) geometric phases explicitly. A complete
analytical construction of a S^4 Bloch-like sphere for two qubits is given for
the Spin(5) or SO(5) subgroup of SU(4).


title: Corrections to the Nonrelativistic Ground Energy of a Helium Atom

abstract:
  Considering the nuclear motion, the authors give out the nonrelativistic
ground energy of a helium atom by using a simple but effective variational wave
function with a flexible parameter $k$. Based on this result, the relativistic
and radiative corrections to the nonrelativistic Hamiltonian are discussed. The
high precision value of the helium ground energy is evaluated to be -2.90338
a.u., and the relative error is 0.00034%.


title:
Exactly solvable models with PT-symmetry and with an asymmetric coupling
  of channels

abstract:
  Bound states generated by K coupled PT-symmetric square wells are studied in
a series of models where the Hamiltonians are assumed $R-$pseudo-Hermitian and
$R^2-$symmetric. Specific rotation-like generalized parities $R$ are considered
such that $R^N=I$ at some integers N. We show that and how our assumptions make
the models exactly solvable and quasi-Hermitian. This means that they possess
the real spectra as well as the standard probabilistic interpretation.


title: Three level atom optics in dipole traps and waveguides

abstract:
  An analogy is explored between a setup of three atomic traps coupled via
tunneling and an internal atomic three-level system interacting with two laser
fields. Within this scenario we describe a STIRAP like process which allows to
move an atom between the ground states of two trapping potentials and analyze
its robustness. This analogy is extended to other robust and coherent transport
schemes and to systems of more than a single atom. Finally it is applied to
manipulate external degrees of freedom of atomic wave packets propagating in
waveguides.


title:
Quantized Detector Networks: a Quantum Informational Approach to the
  Description and Interpretation of Quantum Physics

abstract:
  We discuss the QDN (quantized detector network) approach to the formulation
and interpretation of quantum mechanics. This approach gives us a system-free
approach to quantum physics. By this, we mean having a proper emphasis on those
aspects of physics which are observable and an avoidance of metaphysical
concepts, which by definition are incapable of verification and should play no
role in science on that account. By focusing on only what experimentalists deal
with, i.e., quantum information, we avoid the ambiguities and confusion
generated by the undue objectification of what are complex quantum processes.


title:
Reduced Dirac Equation and Lamb Shift as an Off-mass-shell effect in
  Quantum Electrodynamics

abstract:
  Based on the precision experimental data of energy-level differences in
hydrogenlike atoms, especially the 1S-2S transition of hydrogen and deuterium,
the necessity of establishing a reduced Dirac equation (RDE) with reduced mass
as the substitution of original electron mass is stressed. The theoretical
basis of RDE lies on two symmetries, the invariance under the space-time
inversion and that under the pure space inversion. Based on RDE and within the
framework of quantum electrodynamics in covariant form, the Lamb shift can be
evaluated (at one-loop level) as the radiative correction on a bound electron
staying in an off-mass-shell state--a new approach eliminating the infrared
divergence. Hence the whole calculation, though with limited accuracy, is
simplified, getting rid of all divergences and free of ambiguity.


title: Towards a fullerene-based quantum computer

abstract:
  Molecular structures appear to be natural candidates for a quantum
technology: individual atoms can support quantum superpositions for long
periods, and such atoms can in principle be embedded in a permanent molecular
scaffolding to form an array. This would be true nanotechnology, with
dimensions of order of a nanometre. However, the challenges of realising such a
vision are immense. One must identify a suitable elementary unit and
demonstrate its merits for qubit storage and manipulation, including input /
output. These units must then be formed into large arrays corresponding to an
functional quantum architecture, including a mechanism for gate operations.
Here we report our efforts, both experimental and theoretical, to create such a
technology based on endohedral fullerenes or 'buckyballs'. We describe our
successes with respect to these criteria, along with the obstacles we are
currently facing and the questions that remain to be addressed.


title:
The Quantum Governor: Automatic quantum control and reduction of the
  influence of noise without measuring

abstract:
  The problem of automatically protecting a quantum system against noise in a
closed circuit is analyzed. A general scheme is developed built from two steps.
At first, a distillation step is induced in which undesired components are
removed to another degree of freedom of the system. Later a recovering step is
employed which the system gains back its initial density. An Optimal-Control
method is used to generate the distilling operator. The scheme is demonstrated
by a simulation of a two level byte influenced by white noise. Undesired
deviations from the target were shown to be reduced by at least two orders of
magnitude on average. The relations between the quantum version of the
classical Watt's Governor and the field of quantum information are also
discussed.


title:
A New Quantum Lower Bound Method, with Applications to Direct Product
  Theorems and Time-Space Tradeoffs

abstract:
  We give a new version of the adversary method for proving lower bounds on
quantum query algorithms. The new method is based on analyzing the eigenspace
structure of the problem at hand. We use it to prove a new and optimal strong
direct product theorem for 2-sided error quantum algorithms computing k
independent instances of a symmetric Boolean function: if the algorithm uses
significantly less than k times the number of queries needed for one instance
of the function, then its success probability is exponentially small in k. We
also use the polynomial method to prove a direct product theorem for 1-sided
error algorithms for k threshold functions with a stronger bound on the success
probability. Finally, we present a quantum algorithm for evaluating solutions
to systems of linear inequalities, and use our direct product theorems to show
that the time-space tradeoff of this algorithm is close to optimal.


title:
Electromagnetically Induced Transparency using six level atoms doped in
  the crystalline medium

abstract:
  Using the density matrix theory of interaction between light and matter, and
relevant parameters of the relaxation rates for a six-level model, we have
shown theoretically the possibility of realizing Electromagnetically Induced
Transparency (EIT) and Slow Light effects in Pr(+3 ion) doped YSO crystal. In
addition, we have presented a simplified method to analyse EIT effect in such a
six level atomic system. Finally, we have demonstrated results of numerical
calculation and have compared them with experimental measurements reported
recently.


title: Verifying continuous variable entanglement of intense light pulses

abstract:
  Three different methods have been discussed to verify continuous variable
entanglement of intense light beams. We demonstrate all three methods using the
same set--up to facilitate the comparison. The non--linearity used to generate
entanglement is the Kerr--effect in optical fibres. Due to the brightness of
the entangled pulses, standard homodyne detection is not an appropriate tool
for the verification. However, we show that by using large asymmetric
interferometers on each beam individually, two non-commuting variables can be
accessed and the presence of entanglement verified via joint measurements on
the two beams. Alternatively, we witness entanglement by combining the two
beams on a beam splitter that yields certain linear combinations of quadrature
amplitudes which suffice to prove the presence of entanglement.


title: Joint estimation of real squeezing and displacement

abstract:
  We study the problem of joint estimation of real squeezing and amplitude of
the radiation field, deriving the measurement that maximizes the probability
density of detecting the true value of the unknown parameters. More generally,
we provide a solution for the problem of estimating the unknown unitary action
of a nonunimodular group in the maximum likelihood approach. Remarkably, in
this case the optimal measurements do not coincide with the so called
square-root measurements. In the case of squeezing and displacement we analyze
in detail the sensitivity of estimation for coherent states and displaced
squeezed states, deriving the asymptotic relation between the uncertainties in
the joint estimation and the corresponding uncertainties in the optimal
separate measurements of squeezing and displacement. A two-mode setup is also
analyzed, showing how entanglement between optical modes can be used to
approximate perfect estimation.


title: Entanglement of a qubit coupled to a resonator in the adiabatic regime

abstract:
  We discuss the ground state entanglement of a bi-partite system, composed by
a qubit strongly interacting with an oscillator mode, as a function of the
coupling strenght, the transition frequency and the level asymmetry of the
qubit. This is done in the adiabatic regime in which the time evolution of the
qubit is much faster than the oscillator one. Within the adiabatic
approximation, we obtain a complete characterization of the ground state
properties of the system and of its entanglement content.


title:
Dispersive properties and giant Kerr non-linearities in Dipole Induced
  Transparency

abstract:
  We calculate the dispersive properties of the reflected field from a cavity
coupled to a single dipole. We show that when a field is resonant with the
dipole it experiences a 90 degree phase shift relative to reflection from a
bare cavity if the Purcell factor exceeds the bare cavity reflectivity. We then
show that optically Stark shifting the dipole with a second field can be used
to achieve giant Kerr non-linearites. It is shown that currently achievable
cavity lifetimes and cavity quality factors can allow a single emitter in the
cavity to impose a nonlinear $\pi$ phase shift at the single photon level.


title:
Realization of an n-qubit controlled-U gate with superconducting quantum
  interference devices or atoms in cavity QED

abstract:
  We propose an approach to realize an $n$-qubit controlled-$U$ gate with
superconducting quantum interference devices (SQUIDs) in cavity QED. In this
approach, the two lowest levels of a SQUID represent the two logical states of
a qubit while a higher-energy intermediate level serves the gate manipulation.
Our method operates essentially by creating a single photon through one of the
control SQUIDs and then performing an arbitrary unitrary transformation on the
target SQUID with the assistance of the cavity photon. In addition, we show
that the method can be applied to implement an $n $% -qubit controlled-$U$ gate
with atomic qubits in cavity QED.


title:
Cloning and Optimal Gaussian individual attacks for continuous-variable
  quantum key distribution using coherent states and reverse reconciliation

abstract:
  We investigate the security of continuous-variable (CV) quantum key
distribution (QKD) using coherent states and reverse reconciliation against
Gaussian individual attacks based on an optimal Gaussian $1 \to 2$ cloning
machine. We provide an implementation of the optimal Gaussian individual
attack. we also find a Bell-measurement attack which works without delayed
choice of measurements and has better performance than the cloning attack.


title: Complex chaos in the conditional dynamics of qubits

abstract:
  We analyze the consequences of iterative measurement-induced nonlinearity on
the dynamical behavior of qubits. We present a one-qubit scheme where the
equation governing the time evolution is a complex-valued nonlinear map with
one complex parameter. In contrast to the usual notion of quantum chaos,
exponential sensitivity to the initial state occurs here. We calculate
analytically the Lyapunov exponent based on the overlap of quantum states, and
find that it is positive. We present a few illustrative examples of the
emerging dynamics.


title: A perturbative treatment for the energy levels of neutral atoms

abstract:
  Energy levels of neutral atoms have been re-examined by applying an
alternative perturbative scheme in solving the Schrodinger equation for the
Yukawa potential model with a modified screening parameter. The predicted shell
binding energies are found to be quite accurate over the entire range of the
atomic number $Z$ up to 84 and compare very well with those obtained within the
framework of hyper-virial-Pade scheme and the method of shifted large-N
expansion. It is observed that the new perturbative method may also be applied
to the other areas of atomic physics.


abstract:
  In this Letter we discuss the possibility of producing Bragg solitons in an
electromagnetically induced transparency medium. We show that this coherent
medium can be engineered to be a Bragg grating with a large Kerr nonlinearity
through proper arrangements of light fields. Unlike in previous studies, the
parameters of the medium can be easily controlled through adjusting the
intensities and detunings of lasers. Thus this scheme may provide an
opportunity to study the dynamics of Bragg solitons. And doing experiments with
low power lights is possible.


title: A Generalized Deletion Machine

abstract:
  In this work we prescribe a more generalized quantum-deleting machine (input
state dependent). The fidelity of deletion is dependent on some machine
parameters such that on alteration of machine parameters we get back to
standard deleting machines. We also carried out a various comparative study of
various kinds of quantum deleting machines. We also plotted graphs, making a
comparative study of fidelity of deletion of the deletion machines, obtained as
particular cases on changing the machine parameters of our machine.


title:
Transient Phenomena in Quantum Bound States Subjected to a Sudden
  Perturbation

abstract:
  Transient phenomena in quantum mechanics have been of interest to one of the
authors (MM) since long ago and, in this paper, we focus on the problem of a
potential V_- which for negative times gives rise to bound states and is
suddenly changed at t=0 to a potential V_+ which includes V_- plus a perturbed
term. An example will be the deuteron (where the proton and neutron are assumed
to interact through an oscillator potential) submitted to a sudden
electrostatic field. The analysis for t>0 can be carried out with the help of
appropriate Feynmann propagators and we arrive at the result that the
separation between the nucleons has an amplitude that depends on the intensity
of the electrostatic field, but its period continues to be related with the
inverse of the frequency of the oscillator proposed for the interaction. A
general approximate procedure for arbitrary problems of this type is also
presented at the end.


title:
Effective cross-Kerr nonlinearity and robust phase gates with trapped
  ions

abstract:
  We derive an effective Hamiltonian that describes a cross-Kerr type
interaction in a system involving a two-level trapped ion coupled to the
quantized field inside a cavity. We assume a large detuning between the ion and
field (dispersive limit) and this results in an interaction Hamiltonian
involving the product of the (bosonic) ionic vibrational motion and field
number operators. We also demonstrate the feasibility of operation of a phase
gate based on our hamiltonian. The gate is insensitive to spontaneous emission,
an important feature for the practical implementation of quantum computing.


title: Time-reversal and super-resolving phase measurements

abstract:
  We demonstrate phase super-resolution in the absence of entangled states. The
key insight is to use the inherent time-reversal symmetry of quantum mechanics:
our theory shows that it is possible to \emph{measure}, as opposed to prepare,
entangled states. Our approach is robust, requiring only photons that exhibit
classical interference: we experimentally demonstrate high-visibility phase
super-resolution with three, four, and six photons using a standard laser and
photon counters. Our six-photon experiment demonstrates the best phase
super-resolution yet reported with high visibility and resolution.


title:
Classical predictability and coarse-grained evolution of the quantum
  baker's map

abstract:
  We investigate how classical predictability of the coarse-grained evolution
of the quantum baker's map depends on the character of the coarse-graining. Our
analysis extends earlier work by Brun and Hartle [Phys. Rev. D 60, 123503
(1999)] to the case of a chaotic map. To quantify predictability, we compare
the rate of entropy increase for a family of coarse-grainings in the decoherent
histories formalism. We find that the rate of entropy increase is dominated by
the number of scales characterising the coarse-graining.


title:
Bayesian updating of a probability distribution encoded on a quantum
  register

abstract:
  We investigate the problem of Bayesian updating of a probability distribution
encoded in the quantum state of n qubits. The updating procedure takes the form
of a quantum algorithm that prepares the quantum register in the state
representing the posterior distribution. Depending on how the prior
distribution is given, we describe two implementations, one probabilistic and
one deterministic, of such an algorithm in the standard model of a quantum
computer.


title:
The entangling and disentangling power of unitary transformations are
  unequal

abstract:
  We consider two capacity quantities associated with bipartite unitary gates:
the entangling and the disentangling power. For two-qubit unitaries these two
capacities are always the same. Here we prove that these capacities are
different in general. We do so by constructing an explicit example of a
qubit-qutrit unitary whose entangling power is maximal (2 ebits), but whose
disentangling power is strictly less. A corollary is that there can be no
unique ordering for unitary gates in terms of their ability to perform
non-local tasks. Finally we show that in large dimensions, almost all bipartite
unitaries have entangling and disentangling capacities close to the maximal
possible (and thus in high dimensions the difference in these capacities is
small for almost all unitaries).


title:
Provable entanglement and information cost for qubit-based quantum
  key-distribution protocols

abstract:
  Provable entanglement has been shown to be a necessary precondition for
unconditionally secure key generation in the context of quantum cryptographic
protocols. We estimate the maximal threshold disturbance up to which the two
legitimate users can prove the presence of quantum correlations in their data,
in the context of the four- and six-state quantum key-distribution protocols,
under the assumption of coherent attacks. Moreover, we investigate the
conditions under which an eavesdropper can saturate these bounds, by means of
incoherent and two-qubit coherent attacks. A direct connection between
entanglement distillation and classical advantage distillation is also
presented.


title: Time reversal and exchange symmetries of unitary gate capacities

abstract:
  Unitary gates are an interesting resource for quantum communication in part
because they are always invertible and are intrinsically bidirectional. This
paper explores these two symmetries: time-reversal and exchange of Alice and
Bob. We will present examples of unitary gates that exhibit dramatic
separations between forward and backward capacities (even when the back
communication is assisted by free entanglement) and between
entanglement-assisted and unassisted capacities, among many others. Along the
way, we will give a general time-reversal rule for relating the capacities of a
unitary gate and its inverse that will explain why previous attempts at finding
asymmetric capacities failed. Finally, we will see how the ability to erase
quantum information and destroy entanglement can be a valuable resource for
quantum communication.


title:
Transfer of single photon polarization states by two-channel continuous
  variable teleportation

abstract:
  Superpositions of two orthogonal single-photon polarization states are
commonly used as optical qubits. If such qubits are sent by continuous variable
quantum teleportation, the modifications of the qubit states due to imperfect
entanglement cause an increase in the average photon number of the output
state. This effect can be interpreted as an accidental quantum cloning of the
single photon input. We analyze the output statistics of the single photon
teleportation and derive the transfer and cloning fidelities from the equations
of the polarization qubit.


title:
Optimal error tracking via quantum coding and continuous syndrome
  measurement

abstract:
  We revisit a scenario of continuous quantum error detection proposed by Ahn,
Doherty and Landahl [Phys. Rev. A 65, 042301 (2002)] and construct optimal
filters for tracking accumulative errors. These filters turn out to be of a
canonical form from hybrid control theory; we numerically assess their
performance for the bit-flip and five-qubit codes. We show that a tight upper
bound on the stochastic decay of encoded fidelity can be computed from the
measurement records. Our results provide an informative case study in
decoherence suppression with finite-strength measurement.


title:
Proposal of an experimentally accessible measure of many-fermion
  entanglement

abstract:
  We propose a measure of interaction-induced ground state entanglement in
many-fermion systems that is experimentally accessible. It is formulated in
terms of cross-correlations of currents through resonant fermion levels weakly
coupled to the probed system. The proposed entanglement measure vanishes in the
absence of many-body interactions and it is related to measures of occupation
number entanglement. We evaluate it for two examples of interacting electronic
nanostructures.


title:
Efficient symmetric multiparty quantum state sharing of an arbitrary
  m-qubit state

abstract:
  We present a scheme for symmetric multiparty quantum state sharing of an
arbitrary $m$-qubit state with $m$ Greenberger-Horne-Zeilinger states following
some ideas from the controlled teleportation [Phys. Rev. A \textbf{72}, 02338
(2005)]. The sender Alice performs $m$ Bell-state measurements on her $2m$
particles and the controllers need only to take some single-photon product
measurements on their photons independently, not Bell-state measurements, which
makes this scheme more convenient than the latter. Also it does not require the
parties to perform a controlled-NOT gate on the photons for reconstructing the
unknown $m$-qubit state and it is an optimal one as its efficiency for qubits
approaches the maximal value.


title: Quantum signature scheme with single photons

abstract:
  Quantum digital signature combines quantum theory with classical digital
signature. The main goal of this field is to take advantage of quantum effects
to provide unconditionally secure signature. We present a quantum signature
scheme with message recovery without using entangle effect. The most important
property of the proposed scheme is that it is not necessary for the scheme to
use Greenberger-Horne-Zeilinger states. The present scheme utilizes single
photons to achieve the aim of signature and verification. The security of the
scheme relies on the quantum one-time pad and quantum key distribution. The
efficiency analysis shows that the proposed scheme is an efficient scheme.


title:
The foundations of statistical mechanics from entanglement: Individual
  states vs. averages

abstract:
  We consider an alternative approach to the foundations of statistical
mechanics, in which subjective randomness, ensemble-averaging or time-averaging
are not required. Instead, the universe (i.e. the system together with a
sufficiently large environment) is in a quantum pure state subject to a global
constraint, and thermalisation results from entanglement between system and
environment. We formulate and prove a "General Canonical Principle", which
states that the system will be thermalised for almost all pure states of the
universe, and provide rigorous quantitative bounds using Levy's Lemma.


title:
Light induced effective magnetic fields for ultra-cold atoms in planar
  geometries

abstract:
  We propose a scheme to create an effective magnetic field for ultra-cold
atoms in a planar geometry. The set-up allows the experimental study of
classical and quantum Hall effects in close analogy to solid-state systems
including the possibility of finite currents. The present scheme is an
extention of the proposal in Phys. Rev. Lett. 93, 033602 (2004) where the
effective magnetic field is now induced for three-level Lambda-type atoms by
two counterpropagating laser beams with shifted spatial profiles. Under
conditions of electromagentically induced transparency the atom-light
interaction has a space dependent dark state, and the adiabatic center of mass
motion of atoms in this state experiences effective vector and scalar
potentials. The associated magnetic field is oriented perpendicular to the
propagation direction of the laser beams. The field strength achievable is one
flux quantum over an area given by the transverse beam separation and the laser
wavelength. For a sufficiently dilute gas the field is strong enough to reach
the lowest Landau level regime.


title: Semiquantum versus semiclassical mechanics for simple nonlinear systems

abstract:
  Quantum mechanics has been formulated in phase space, with the Wigner
function as the representative of the quantum density operator, and classical
mechanics has been formulated in Hilbert space, with the Groenewold operator as
the representative of the classical Liouville density function. Semiclassical
approximations to the quantum evolution of the Wigner function have been
defined, enabling the quantum evolution to be approached from a classical
starting point. Now analogous semiquantum approximations to the classical
evolution of the Groenewold operator are defined, enabling the classical
evolution to be approached from a quantum starting point. Simple nonlinear
systems with one degree of freedom are considered, whose Hamiltonians are
polynomials in the Hamiltonian of the simple harmonic oscillator. The behaviour
of expectation values of simple observables and of eigenvalues of the
Groenewold operator, are calculated numerically and compared for the various
semiclassical and semiquantum approximations.


title: Entanglement-Assisted Capacity of Quantum Multiple-Access Channels

abstract:
  We find a regularized formula for the entanglement-assisted (EA) capacity
region for quantum multiple access channels (QMAC). We illustrate the capacity
region calculation with the example of the collective phase-flip channel which
admits a single-letter characterization. On the way, we provide a
first-principles proof of the EA coding theorem based on a packing argument. We
observe that the Holevo-Schumacher-Westmoreland theorem may be obtained from a
modification of our EA protocol. We remark on the existence of a family
hierarchy of protocols for multiparty scenarios with a single receiver, in
analogy to the two-party case. In this way, we relate several previous results
regarding QMACs.


title: Quantum-Classical Dynamics of Wave Fields

abstract:
  An approach to the quantum-classical mechanics of phase space dependent
operators, which has been proposed recently, is remodeled as a formalism for
wave fields. Such wave fields obey a system of coupled non-linear equations
that can be written by means of a suitable non-Hamiltonian bracket. As an
example, the theory is applied to the relaxation dynamics of the spin-boson
model. In the adiabatic limit, a good agreement with calculations performed by
the operator approach is obtained. Moreover, the theory proposed in this paper
can take nonadiabatic effects into account without resorting to surface-hopping
approximations. Hence, the results obtained follow qualitatively those of
previous surface-hopping calculations and increase by a factor of (at least)
two the time length over which nonadiabatic dynamics can be propagated with
small statistical errors. Moreover, it is worth to note that the dynamics of
quantum-classical wave fields here proposed is a straightforward
non-Hamiltonian generalization of the formalism for non-linear quantum
mechanics that Weinberg introduced recently.


title:
Analytical regularisation for confined quantum fields between parallel
  surfaces

abstract:
  We review a simple technique for evaluating the vacuum energy stemming from
non-trivial boundary conditions and review results for the Casimir energy of a
massive fermionic field confined in a d+1-dimensional slab-bag and the effect
of a uniform magnetic field on the vacuum energy of confined massive bosonic
and fermionic fields. New results concerning the Casimir energy and the
evaluation of the rate of creation of quanta in kappa-deformed theories are
presented.


title: The Concept of Mass, Quarks, and Phase Space

abstract:
  We point out the conceptual problems related to the application of the
standard notion of mass to quarks and recall the arguments that there should be
a close connection between the properties of elementary particles and the arena
used for the description of classical macroscopic processes.
  Motivated by the above and the wish to introduce more symmetry between the
coordinates of position and momentum we concentrate on the classical
nonrelativistic phase space with ${\bf {p}}^2+{\bf {x}}^2$ as an invariant. A
symmetry-based argument on how to generalize the way in which mass enters into
our description of Nature is presented and placed into the context of the
phase-space scheme discussed.
  It is conjectured that the proposed non-standard way of relating "mass" to
the variables of the classical phase space is actually used in Nature, and that
it manifests itself through the existence of quarks. Some properties of this
proposal, including unobservability of "free quarks" and the emergence of
mesons, are discussed.


title: Analysis of a single-atom dipole trap

abstract:
  We describe a simple experimental technique which allows to store a single
Rubidium 87 atom in an optical dipole trap. Due to light-induced two-body
collisions during the loading stage of the trap the maximum number of captured
atoms is locked to one. This collisional blockade effect is confirmed by the
observation of photon anti-bunching in the detected fluorescence light. The
spectral properties of single photons emitted by the atom were studied with a
narrow-band scanning cavity. We find that the atomic fluorescence spectrum is
dominated by the spectral width of the exciting laser light field. In addition
we observe a spectral broadening of the atomic fluorescence light due to the
Doppler effect. This allows us to determine the mean kinetic energy of the
trapped atom corresponding to a temperature of 105 micro Kelvin. This simple
single-atom trap is the key element for the generation of atom-photon
entanglement required for future applications in quantum communication and a
first loophole-free test of Bell's inequality.


title: Cross-Kerr-based information transfer processes

abstract:
  The realization of nonclassical states is an important task for many
applications of quantum information processing. Usually, properly tailored
interactions, different from goal to goal, are considered in order to
accomplish specific tasks within the general framework of quantum state
engineering. In this paper we remark on the flexibility of a cross-Kerr
nonlinear coupling in hybrid systems as an important ingredient in the
engineering of nonclassical states. The general scenario we consider is the
implementation of high cross-Kerr nonlinearity in cavity-quantum
electrodynamics. In this context, we discuss the possibility of performing
entanglement transfer and swapping between a qubit and a continuous-variable
state. The recently introduced concept of entanglement reciprocation is also
considered and shown to be possible with our scheme. We reinterpret some of our
results in terms of applications of a generalized Ising interaction to systems
of different nature.


title: Periodic and discrete Zak bases

abstract:
  Weyl's displacement operators for position and momentum commute if the
product of the elementary displacements equals Planck's constant. Then, their
common eigenstates constitute the Zak basis, each state specified by two phase
parameters. Upon enforcing a periodic dependence on the phases, one gets a
one-to-one mapping of the Hilbert space on the line onto the Hilbert space on
the torus. The Fourier coefficients of the periodic Zak bases make up the
discrete Zak bases. The two bases are mutually unbiased. We study these bases
in detail, including a brief discussion of their relation to Aharonov's modular
operators, and mention how they can be used to associate with the single degree
of freedom of the line a pair of genuine qubits.