title: Spin Bath Decoherence of Quantum Entanglement

abstract:
  We study an analytically solvable model for decoherence of a two spin system
embedded in a large spin environment. As a measure of entanglement, we evaluate
the concurrence for the Bell states (Einstein-Podolsky-Rosen pairs). We find
that while for two separate spin baths all four Bell states lose their
coherence with the same time dependence, for a common spin bath, two of the
states decay faster than the others. We explain this difference by the relative
orientation of the individual spins in the pair. We also examine how the Bell
inequality is violated in the coherent regime. Both for one bath and two bath
cases, we find that while two of the Bell states always obey the inequality,
the other two initially violates the inequality at early times.


title: Effects of single-qubit quantum noise on entanglement purification

abstract:
  We study the stability under quantum noise effects of the quantum privacy
amplification protocol for the purification of entanglement in quantum
cryptography. We assume that the E91 protocol is used by two communicating
parties (Alice and Bob) and that the eavesdropper Eve uses the isotropic
Bu\v{z}ek-Hillery quantum copying machine to extract information. Entanglement
purification is then operated by Alice and Bob by means of the quantum privacy
amplification protocol and we present a systematic numerical study of the
impact of all possible single-qubit noise channels on this protocol. We find
that both the qualitative behavior of the fidelity of the purified state as a
function of the number of purification steps and the maximum level of noise
that can be tolerated by the protocol strongly depend on the specific noise
channel. These results provide valuable information for experimental
implementations of the quantum privacy amplification protocol.


title: All-Versus-Nothing Violation of Local Realism by Swapping Entanglement

abstract:
  The usual entanglement swapping protocol can entangle two particles that
never interact. Here we demonstrate an all-versus-nothing violation of local
realism for a partial entanglement swapping process. A
Clauser-Horne-Shimony-Holt-type inequality for two particles entangled by
entanglement swapping is proposed for a practical experiment and can be
violated quantum mechanically up to 4 beyond Cirel'son's bound 2sqr(2). Our
result uncovers the intriguing nonlocal character of entanglement swapping and
may lead to a conclusive (loophole-free) test of local realism versus quantum
mechanics. The experimental test of the nonlocality under current technology is
also discussed.


title: Simplified diagrammatic expansion for effective operator

abstract:
  For a quantum many-body problem, effective Hamiltonians that give exact
eigenvalues in reduced model space usually have different expressions, diagrams
and evaluation rules from effective transition operators that give exact
transition matrix elements between effective eigenvectors in reduced model
space. By modifying these diagrams slightly and considering the linked diagrams
for all the terms of the same order, we find that the evaluation rules can be
made the same for both effective Hamiltonian and effective transition operator
diagrams, and in many cases it is possible to combine many diagrams into one
modified diagram. We give the rules to evaluate these modified diagrams and
show their validity.


title:
Combinatorial approach to generalized Bell and Stirling numbers and
  boson normal ordering problem

abstract:
  We consider the numbers arising in the problem of normal ordering of
expressions in canonical boson creation and annihilation operators. We treat a
general form of a boson string which is shown to be associated with
generalizations of Stirling and Bell numbers. The recurrence relations and
closed-form expressions (Dobiski-type formulas) are obtained for these
quantities by both algebraic and combinatorial methods. By extensive use of
methods of combinatorial analysis we prove the equivalence of the
aforementioned problem to the enumeration of special families of graphs. This
link provides a combinatorial interpretation of the numbers arising in this
normal ordering problem.


title: Effective mass in cavity QED

abstract:
  We consider propagation of a two-level atom coupled to one electro-magnetic
mode of a high-Q cavity. The atomic center-of-mass motion is treated quantum
mechanically and we use a standing wave shape for the mode. The periodicity of
the Hamiltonian leads to a spectrum consisting of bands and gaps, which is
studied from a Floquet point of view. Based on the band theory we introduce a
set of effective mass parameters that approximately describe the effect of the
cavity on the atomic motion, with the emphasis on one associated with the group
velocity and on another one that coincides with the conventional effective
mass. Propagation of initially Gaussian wave packets is also studied using
numerical simulations and the mass parameters extracted thereof are compared
with those predicted by the Floquet theory. Scattering and transmission of the
wave packet against the cavity are further analyzed, and the constraints for
the effective mass approach to be valid are discussed in detail.


title:
Local field effects on the radiative lifetimes of Ce$^{3+}$ in different
  hosts

abstract:
  For emitters embedded in media of various refractive indices, different
theoretical models predicted substantially different dependencies of the
spontaneous emission lifetime on refractive index. It has been claimed that
various measurements on $4f\to 4f$ radiative transition of Eu$^{3+}$ in hosts
with variable refractive index appear to favor the real-cavity model [J.
Fluoresc. 13, 201 (2003) and references therein, Phys. Rev. Lett. 91, 203903
(2003)]. We notice that $5d\to 4f$ radiative transition of rare-earth ions,
dominated by allowed electric-dipole transitions with line strengths less
perturbed by the ligands, serves as a better test of different models. We
analyze the lifetimes of $5d\to 4f$ transition of Ce$^{3+}$ in hosts of
refractive indices varying from 1.4 to 2.2. The results favor the macroscopic
virtual-cavity model based on Lorentz local field [J. Fluoresc. 13, 201
(2003)].


title: Optimal discrimination of quantum operations

abstract:
  We address the problem of discriminating with minimal error probability two
given quantum operations. We show that the use of entangled input states
generally improves the discrimination. For Pauli channels we provide a complete
comparison of the optimal strategies where either entangled or unentangled
input states are used.


title: Plasmon signatures in high harmonic generation

abstract:
  High harmonic generation in polarizable multi-electron systems is
investigated in the framework of multi-configuration time-dependent
Hartree-Fock. The harmonic spectra exhibit two cut offs. The first cut off is
in agreement with the well established, single active electron cut off law. The
second cut off presents a signature of multi-electron dynamics. The strong
laser field excites non-linear plasmon oscillations. Electrons that are ionized
from one of the multi-plasmon states and recombine to the ground state gain
additional energy, thereby creating the second plateau.


title: On the black-box complexity of Sperner's Lemma

abstract:
  We present several results on the complexity of various forms of Sperner's
Lemma in the black-box model of computing. We give a deterministic algorithm
for Sperner problems over pseudo-manifolds of arbitrary dimension. The query
complexity of our algorithm is linear in the separation number of the skeleton
graph of the manifold and the size of its boundary. As a corollary we get an
$O(\sqrt{n})$ deterministic query algorithm for the black-box version of the
problem {\bf 2D-SPERNER}, a well studied member of Papadimitriou's complexity
class PPAD. This upper bound matches the $\Omega(\sqrt{n})$ deterministic lower
bound of Crescenzi and Silvestri. The tightness of this bound was not known
before. In another result we prove for the same problem an
$\Omega(\sqrt[4]{n})$ lower bound for its probabilistic, and an
$\Omega(\sqrt[8]{n})$ lower bound for its quantum query complexity, showing
that all these measures are polynomially related.


title: EIT and diffusion of atomic coherence

abstract:
  We study experimentally the effect of diffusion of Rb atoms on
Electromagnetically Induced Transparency (EIT) in a buffer gas vapor cell. In
particular, we find that diffusion of atomic coherence in-and-out of the laser
beam plays a crucial role in determining the EIT resonance lineshape and the
stored light lifetime.


title: Quantum Physics from A to Z

abstract:
  This is a collection of statements gathered on the occasion of the Quantum
Physics of Nature meeting in Vienna.


title: Lower Bounds on Matrix Rigidity via a Quantum Argument

abstract:
  The rigidity of a matrix measures how many of its entries need to be changed
in order to reduce its rank to some value. Good lower bounds on the rigidity of
an explicit matrix would imply good lower bounds for arithmetic circuits as
well as for communication complexity. Here we reprove the best known bounds on
the rigidity of Hadamard matrices, due to Kashin and Razborov, using tools from
quantum computing. Our proofs are somewhat simpler than earlier ones (at least
for those familiar with quantum) and give slightly better constants. More
importantly, they give a new approach to attack this longstanding open problem.


title: Atom diode: Variants, stability, limits, and adiabatic interpretation

abstract:
  We examine and explain the stability properties of the ``atom diode'', a
laser device that lets the ground state atom pass in one direction but not in
the opposite direction. The diodic behavior and the variants that result by
using different laser configurations may be understood with an adiabatic
approximation. The conditions to break down the approximation, which imply also
the diode failure, are analyzed.


title:
Dynamical probability, particle trajectories and completion of
  traditional quantum mechanics

abstract:
  Maintaining the position that the wave function $\psi$ provides a complete
description of state, the traditional formalism of quantum mechanics is
augmented by introducing continuous trajectories for particles which are sample
paths of a stochastic process determined (including the underlying probability
space) by $\psi$. In the resulting formalism, problems relating to measurements
and objective reality are solved as in Bohmian mechanics (without sharing its
weak points). The pitfalls of Nelson's stochastic mechanics are also avoided.


title: Transverse localization and slow propagation of light

abstract:
  The effect of finite control beam on the transverse spatial profile of the
slow light propagation in an electromagnetically induced transparency medium is
studied. We arrive at a general criterion in terms of eigenequation, and
demonstrate the existence of a set of localized, stationary transverse modes
for the negative detuning of the probe signal field. Each of these
diffraction-free transverse modes has its own characteristic group velocity,
smaller than the conventional theoretical result without considering the
transverse spatial effect.


title:
Ultrahigh Precision Absolute and Relative Rotation Sensing using Fast
  and Slow Light

abstract:
  We describe a resonator based optical gyroscope whose sensitivity for
measuring absolute rotation is enhanced via use of the anomalous dispersion
characteristic of superluminal light propagation. The enhancement is given by
the inverse of the group index, saturating to a bound determined by the group
velocity dispersion. We also show how the offsetting effect of the concomitant
broadening of the resonator linewidth may be circumvented by using an active
cavity. For realistic conditions, the enhancement factor is as high as 106. We
also show how normal dispersion used for slow light can enhance relative
rotation sensing in a specially designed Sagnac interferometer, with the
enhancement given by the slowing factor


title:
Entanglement entropy and quantum field theory: a non-technical
  introduction

abstract:
  In these proceedings we give a pedagogical and non-technical introduction to
the Quantum Field Theory approach to entanglement entropy. Particular attention
is devoted to the one space dimensional case, with a linear dispersion
relation, that, at a quantum critical point, can be effectively described by a
two-dimensional Conformal Field Theory.


title: A simple quantum gate with atom chips

abstract:
  We present a simple scheme for implementing an atomic phase gate using two
degrees of freedom for each atom and discuss its realization with cold rubidium
atoms on atom chips. We investigate the performance of this collisional phase
gate and show that gate operations with high fidelity can be realized in
magnetic traps that are currently available on atom chips.


title:
Nonlocality without inequalities for almost all entangled states of any
  quantum system

abstract:
  It is shown that it is possible to rule out all local and stochastic hidden
variable models accounting for the quantum mechanical predictions implied by
almost any entangled quantum state vector of any number of particles whose
Hilbert spaces have arbitrary dimensions, without resorting to Bell-type
inequalities. The present proof makes use of the mathematically precise notion
of Bell locality and it involves only simple set theoretic arguments.


title:
Dynamical zero-temperature phase transitions and cosmic
  inflation/deflation

abstract:
  For a rather general class of scenarios, sweeping through a zero-temperature
phase transition by means of a time-dependent external parameter entails
universal behavior: In the vicinity of the critical point, excitations behave
as quantum fields in an expanding or contracting universe. The resulting
effects such as the amplification or suppression of quantum fluctuations (due
to horizon crossing, freezing, and squeezing) including the induced spectrum
can be derived using the curved space-time analogy. The observed similarity
entices the question of whether cosmic inflation itself might perhaps have been
such a phase transition. PACS: 73.43.Nq, 04.62.+v, 98.80.Cq, 04.80.-y.


title:
Mutual information--based approach to adaptive homodyne detection of
  quantum optical states

abstract:
  I propose an approach to adaptive homodyne detection of digitally modulated
quantum optical pulses in which the phase of the local oscillator is chosen to
maximize the average information gain, i.e., the mutual information, at each
step of the measurement. I study the properties of this adaptive detection
scheme by considering the problem of classical information content of ensembles
of coherent states. Using simulations of quantum trajectories and
visualizations of corresponding measurement operators, I show that the proposed
measurement scheme adapts itself to the features of each ensemble. For all
considered ensembles of coherent states, it consistently outperforms heterodyne
detection and Wiseman's adaptive scheme for phase measurements [H.M. Wiseman,
Phys. Rev. Lett. 75, 4587 (1995)].


title: Realisation of a Frequency Standard Using an Atomic Fountain

abstract:
  We report the realisation and preliminary study of a frequency standard using
a fountain of laser cooled caesium atoms. Our apparatus uses a magneto-optical
trap as a source of cold atoms and optical pumping to prepare the atoms in the
correct state before they enter the microwave cavity.


title: Engineering Entanglement: The Fast-Approach Phase Gate

abstract:
  Optimal-control techniques and a fast-approach scheme are used to implement a
collisional control phase gate in a model of cold atoms in an optical lattice,
significantly reducing the gate time as compared to adiabatic evolution while
maintaining high fidelity. New objective functionals are given for which
optimal paths are obtained for evolution that yields a control-phase gate up to
single-atom Rabi shifts. Furthermore, the fast-approach procedure is used to
design a path to significantly increase the fidelity of non-adiabatic transport
in a recent experiment. Also, the entanglement power of phase gates is
quantified.


title: Algorithmic complexity and entanglement of quantum states

abstract:
  We define the algorithmic complexity of a quantum state relative to a given
precision parameter, and give upper bounds for various examples of states. We
also establish a connection between the entanglement of a quantum state and its
algorithmic complexity.


title:
Extracting an arbitrary relative phase from a multiqubit two-component
  entangled state

abstract:
  We show that an arbitrary relative phase can be extracted from a multiqubit
two-component (MTC) entangled state by local Hadamard transformations and
measurements along a single basis only. In addition, how to distinguish a MTC
entangled state with an arbitrary entanglement degree and relative phase from a
class of multiqubit mixed states is discussed.


title:
Probabilistic programmable quantum processors with multiple copies of
  program state

abstract:
  We examine the execution of general U(1) transformations on programmable
quantum processors. We show that, with only the minimal assumption of
availability of copies of the one-qubit program state, that the apparent
advantage of existing schemes proposed by G.Vidal it et al. [Phys. Rev. Lett.
88, 047905 (2002)] and M.Hillery et al. [Phys. Rev. A. 65, 022301 (2003)] to
execute a general U(1) transformation with greater probability using complex
program states appears not to hold.


title: Phase Control of Trapped Ion Quantum Gates

abstract:
  There are several known schemes for entangling trapped ion quantum bits for
large-scale quantum computation. Most are based on an interaction between the
ions and external optical fields, coupling internal qubit states of
trapped-ions to their Coulomb-coupled motion. In this paper, we examine the
sensitivity of these motional gate schemes to phase fluctuations introduced
through noisy external control fields, and suggest techniques to suppress the
resulting phase decoherence.


title:
Superluminal telecommunication: an observable contradiction between
  quantum entanglement and relativistic causality

abstract:
  I present a schema for a superluminal telecommunication system based on
polarization entangled photon pairs. Binary signals can be transmitted at
superluminal speed in this system, if entangled photon pairs can really be
produced. The existence of the polarization entangled photon pairs is in direct
contradiction to the relativistic causality in this telecommunication system.
This contradiction implies the impossibility of generating entangled photon
pairs.


title:
Mesoscopic continuous and discrete channels for quantum information
  transfer

abstract:
  We study the possibility of realizing perfect quantum state transfer in
mesoscopic devices. We discuss the case of the Fano-Anderson model extended to
two impurities. For a channel with an infinite number of degrees of freedom, we
obtain coherent behavior in the case of strong coupling or in weak coupling
off-resonance. For a finite number of degrees of freedom, coherent behavior is
associated to weak coupling and resonance conditions.


title: Multi-quantum eigenstates of a linear chain of coupled qubits

abstract:
  We present a technique to identify exact analytic expressions for the
multi-quantum eigenstates of a linear chain of coupled qubits. A choice of
Hilbert subspaces is described which allows an exact solution of the stationary
Schr\"{o}dinger equation without imposing periodic boundary conditions and
without neglecting end effects, fully including the dipole-dipole
nearest-neighbor interaction between the atoms. The treatment is valid for an
arbitrary coherent excitation in the atomic system, any number of atoms, any
size of the chain relative to the resonant wavelength and arbitrary initial
conditions of the atomic system. The procedure we develop is general enough to
be adopted for the study of excitation in an arbitrary array of atoms including
spin chains and one-dimensional Bose-Einstein condensates.


title: Probabilistic cloning with supplementary information

abstract:
  We consider probabilistic cloning of a state chosen from a mutually
nonorthogonal set of pure states, with the help of a party holding
supplementary information in the form of pure states. When the number of states
is 2, we show that the best efficiency of producing m copies is always achieved
by a two-step protocol in which the helping party first attempts to produce m-1
copies from the supplementary state, and if it fails, then the original state
is used to produce m copies. On the other hand, when the number of states
exceeds two, the best efficiency is not always achieved by such a protocol. We
give examples in which the best efficiency is not achieved even if we allow any
amount of one-way classical communication from the helping party.


title: Multimode entanglement and telecloning in a noisy environment

abstract:
  We address generation, propagation and application of multipartite continuous
variable entanglement in a noisy environment. In particular, we focus our
attention on the multimode entangled states achievable by second order
nonlinear crystals, {\em i.e.} coherent states of ${\rm SU}(m,1)$ group. The
full inseparability in the ideal case is shown, whereas thresholds for
separability are given for the tripartite case in the presence of noise. We
then consider coherent states of ${\rm SU}(m,1)$ as support for a telecloning
protocol, providing the first example of a completely asymmetric $1 \to m$
telecloning. We derive explicitly the optimal relation among the different
fidelities of the clones. The effect of noise in the various stages of the
protocol is taken into account, thus permitting its adaptive modifications to
the noisy environment. In the optimized scheme the clones' fidelity remains
maximal even in the presence of losses (in the absence of thermal noise), for
propagation times that diverge as the number of modes increases, indicating
that telecloning is a more effective way to distribute quantum information then
direct transmission followed by local cloning.


title:
Noncyclic Pancharatnam phase for mixed state SU(2) evolution in neutron
  polarimetry

abstract:
  We have measured the Pancharatnam relative phase for spin-1/2 states. In a
neutron polarimetry experiment the minima and maxima of intensity modulations,
giving the Pancharatnam phase, were determined. We have also considered general
SU(2) evolution for mixed states. The results are in good agreement with
theory.


title: Nanoscale atomic waveguides with suspended carbon nanotubes

abstract:
  We propose an experimentally viable setup for the realization of
one-dimensional ultracold atom gases in a nanoscale magnetic waveguide formed
by single doubly-clamped suspended carbon nanotubes. We show that all common
decoherence and atom loss mechanisms are small guaranteeing a stable operation
of the trap. Since the extremely large current densities in carbon nanotubes
are spatially homogeneous, our proposed architecture allows to overcome the
problem of fragmentation of the atom cloud. Adding a second nanowire allows to
create a double-well potential with a moderate tunneling barrier which is
desired for tunneling and interference experiments with the advantage of
tunneling distances being in the nanometer regime.


title:
Efficient generation of correlated photon pairs in a microstructure
  fiber

abstract:
  We report efficient generation of correlated photon pairs through degenerate
four-wave mixing in microstructure fibers. With 735.7 nm pump pulses producing
conjugate signal (688.5 nm) and idler (789.8 nm) photons in a 1.8 m
microstructure fiber, we detect photon pairs at a rate of 37.6 kHz with a
coincidence/accidental contrast of 10:1 with a full-width-at-half-maximum
bandwidth of 0.7 nm. This is the highest rate reported to date in a fiber-based
photon source. The nonclassicality of this source, as defined by the
Zou-Wang-Mandel inequality, is violated by 1100 times the uncertainty.


title: Highly asymmetric quantum cloning in arbitrary dimension

abstract:
  We investigate the universal asymmetric cloning of states in a Hilbert space
of arbitrary dimension. We derive the class of optimal and fully asymmetric
1->3 cloners, which produce three copies, each having a different fidelity. A
simple parametric expression for the maximum achievable cloning fidelity
triplets is then provided. As a side-product, we also prove the optimality of
the 1->2 asymmetric cloning machines that have been proposed in the literature.


title: Entanglement and the Power of One Qubit

abstract:
  The "Power of One Qubit" refers to a computational model that has access to
only one pure bit of quantum information, along with n qubits in the totally
mixed state. This model, though not as powerful as a pure-state quantum
computer, is capable of performing some computational tasks exponentially
faster than any known classical algorithm. One such task is to estimate with
fixed accuracy the normalized trace of a unitary operator that can be
implemented efficiently in a quantum circuit. We show that circuits of this
type generally lead to entangled states, and we investigate the amount of
entanglement possible in such circuits, as measured by the multiplicative
negativity. We show that the multiplicative negativity is bounded by a
constant, independent of n, for all bipartite divisions of the n+1 qubits, and
so becomes, when n is large, a vanishingly small fraction of the maximum
possible multiplicative negativity for roughly equal divisions. This suggests
that the global nature of entanglement is a more important resource for quantum
computation than the magnitude of the entanglement.


title: A generalized form of Hamilton's principle

abstract:
  Many quantization schemes rely on analogs of classical mechanics where the
connections with classical mechanics are indirect. In this work I propose a new
and direct connection between classical mechanics and quantum mechanics where
the quantum mechanical propagator is derived from a variational principle. I
identify this variational principle as a generalized form of Hamilton's
principle. This proposed variational principle is unusual because the physical
system is allowed to have imperfect information, i.e., there is incomplete
knowledge of the physical state. Two distribution functionals over possible
generalized momentum paths a[p(t)] and generalized coordinates paths b[q(t)]
are defined. A generalized action is defined that corresponds to a contraction
of a[p(t)], b[q(t)], and a matrix of the action evaluated at all possible p and
q paths. Hamilton's principle is the extremization of the generalized action
over all possible distributions. The normalization of the two distributions
allows their values to be negative and they are shown to be the real and
imaginary parts of the complex amplitude. The amplitude in the Feynman path
integral is shown to be an optimal vector that extremizes the generalized
action. This formulation is also shown to be directly applicable to statistical
mechanics and I show how irreversible behavior and the micro-canonical ensemble
follows immediately.


title: Microscopic origin of the second law of thermodynamics

abstract:
  We proved when random-variable fluctuations obey the central limit theorem
the equality of the uncertainty relation corresponds to the thermodynamic
equilibrium state. The inequality corresponds to the thermodynamic
non-equilibrium state. The uncertainty relation is a quantum-mechanics
expression of the second law of thermodynamics originated in wave-particle
duality. Formulas of mean square-deviations changes adjusted by random
fluctuations under the minimal uncertainty relation are obtained. Finally, an
assumption is made which is waiting for examination. We except phase
transitions in our discussion.


title:
Stationary entanglement and nonlocality of two qubits or qutrits
  collectively interacting with the thermal environment: The role of Bell
  singlet state

abstract:
  We investigate the stationary entanglement and stationary nonlocality of two
qubits collectively interacting with a common thermal environment. We assume
two qubits are initially in Werner state or Werner-like state, and find that
thermal environment can make two qubits become stationary nonlocality. The
analytical relations among average thermal photon number of the environment,
entanglement and nonlocality of two qubits are given in details. It is shown
that the fraction of Bell singlet state plays a key role in the phenomenon that
the common thermal reservoir can enhance the entanglement of two qubits.
Moreover, we find that the collective decay of two qubits in a thermal
reservoir at zero-temperature can generate a stationary maximally entangled
mixed state if only the fraction of Bell singlet state in the initial state is
not smaller than 2/3. It provides us a feasible way to prepare the maximally
entangled mixed state in various physical systems such as the trapped ions,
quantum dots or Josephson Junctions. For the case in which two qutrits
collectively coupled with the thermal reservoir at zero-temperature, we find
that the collective decay can induce the entanglement of two qutrits initially
in the maximally mixed state. The collective decay of two qutrits can also
induce distillable entanglement from the initial conjectured negative partial
transpose bound entangled states.


title:
Dualism in Entanglement and Testing Quantum to Classical Transition of
  Identicity

abstract:
  We show a hitherto unexplored consequence of the property of identicity in
quantum mechanics. If two identical objects, distinguished by a dynamical
variable A, are in certain entangled states of another dynamical variable B,
then, for such states, they are also entangled in variable A when distinguished
from each other by variable B. This dualism is independent of quantum
statistics. Departures from identicity of the objects due to arbitrarily small
differences in their innate attributes destroy this dualism. A system
independent scheme to test the dualism is formulated which is readily
realizable with photons. This scheme can be performed without requiring the
entangled objects to be brought together. Thus whether two macro-systems behave
as quantum identical objects can be probed without the complications of
scattering. Such a study would complement the program of testing the validity
of quantum superposition principle in the macro-domain which has stimulated
considerable experimentation.


title:
Uncertainty relation and quantitative condition of inversion symmetry of
  time in many-particle system

abstract:
  We proved that the uncertainty relation fits in with many-particle system and
the equality of the relation corresponds to the thermodynamic equilibrium
state, the inequality of the relation corresponds to the thermodynamic
non-equilibrium state for any quantum system. The microscopic origin of the
second law of thermodynamics is certainly resulted in the wave-particle duality
of matter. A quantitative condition of inversion symmetry of time is obtained.


title:
On exact identity between thermodynamic and informatic entropies in a
  unitary model of friction

abstract:
  An elementary collision model of a molecular reservoir is considered upon
which an external field is applied and the work is dissipated into heat. To
realize macroscopic irreversibility at the microscopic level, we introduce a
``graceful'' irreversible map which randomly mixes the identities of the
molecules. This map is expected to generate informatic entropy exactly equal to
the independently calculable irreversible thermodynamic entropy.


title: Tomograms in the Quantum-Classical transition

abstract:
  The quantum-classical limits for quantum tomograms are studied and compared
with the corresponding classical tomograms, using two different definitions for
the limit. One is the Planck limit where $\hbar \to 0$ in all $\hbar
$-dependent physical observables, and the other is the Ehrenfest limit where
$\hbar \to 0$ while keeping constant the mean value of the energy.The Ehrenfest
limit of eigenstate tomograms for a particle in a box and a harmonic
oscillatoris shown to agree with the corresponding classical tomograms of
phase-space distributions, after a time averaging. The Planck limit of
superposition state tomograms of the harmonic oscillator demostrating the
decreasing contribution of interferences terms as $\hbar \to 0$.


title: $\cal{PT}$ symmetric models with nonlinear pseudo supersymmetry

abstract:
  By applying the higher order Darboux algorithm to an exactly solvable non
Hermitian ${\cal{PT}}$ symmetric potential, we obtain a hierarchy of new
exactly solvable non Hermitian ${\cal{PT}}$ symmetric potentials with real
spectra. It is shown that the symmetry underlying the potentials so generated
and the original one is {\it nonlinear pseudo supersymmetry}. We also show that
this formalism can be used to generate a larger class of new solvable
potentials when applied to non Hermitian systems.


title: Commuting multiparty quantum observables and local compatibility

abstract:
  A formula for the commutator of tensor product matrices is used to shows
that, for qubits, compatibility of quantum multiparty observables almost never
implies local compatibility at each site and to predict when this happens/does
not happen in a concise manner. In particular, it is shown that two ``fully
nontrivial'' $n$-qubit observables are compatible locally and globally if and
only if they are equal up to sign. In addition, the formula gives insight into
the construction of new paradoxes of the type of the Kochen-Specker Theorem,
which can then be easily rephrased into proposals for new no hidden variable
experiments of the type of the ``Bell Theorem without inequalities''.


title: Semiclassical propagation of spin coherent states

abstract:
  The semiclassical propagation of spin coherent states is considered in
complex phase space. For two time-independent systems we find the appropriate
classical trajectories and show that their combined contributions are able to
describe quantum interference with great accuracy. Not only the modulus but
also the phase of the quantum propagator, both dynamical and geometric terms
combined, are accurately reproduced.


title: Three-spin interactions and entanglement in optical lattices

abstract:
  The entanglement properties of some novel quantum systems are studied that
are inspired by recent developments in cold-atom technology. A triangular
optical lattice of two atomic species can be employed to generate a variety of
spin-1/2 Hamiltonians including effective three-spin interactions. A variety of
one or two dimensional systems can thus be realized that possess
multi-degenerate ground states or non-vanishing chirality. The properties of
these ground states and their phase transitions are probed with appropriate
measures such as the entropic entanglement and the spin chirality.


title: Comment on "Quantum key distribution via quantum encryption"

abstract:
  In the paper [Zhang, Li and Guo, Phys. Rev. A 64, 024302 (2001)], a quantum
key distribution protocol based on quantum encryption was proposed, in which
the quantum key can be reused. However, it is shown that, if Eve employs a
special strategy to attack, this protocol becomes insecure because of the
reused quantum key. That is, Eve can elicit partial information about the key
bits without being detected. Finally, a possible improvement of the
Zhang-Li-Guo protocol is proposed.


title: Twin-photon techniques for photo-detector calibration

abstract:
  The aim of this review paper is to enlighten some recent progresses in
quantum optical metrology in the part of quantum efficiency measurements of
photo-detectors performed with bi-photon states. The intrinsic correlated
nature of entangled photons from Spontaneous Parametric Down Conversion
phenomenon has opened wide horizons to a new approach for the absolute
measurement of photo-detector quantum efficiency, outgoing the requirement for
conventional standards of optical radiation; in particular the simultaneous
feature of the creation of conjugated photons led to a well known technique of
coincidence measurement, deeply understood and implemented for standard uses.
On the other hand, based on manipulation of entanglement developed for Quantum
Information protocols implementations, a new method has been proposed for
quantum efficiency measurement, exploiting polarisation entanglement in
addition to energy-momentum and time ones, that is based on conditioned
polarisation state manipulation. In this review, after a general discussion on
absolute photo-detector calibration, we compare these different methods, in
order to give an accurate operational sketch of the absolute quantum efficiency
measurement state of the art.


title:
Prequantum classical statistical model with infinite dimensional
  phase-space

abstract:
  We show that quantum mechanics can be represented as an asymptotic projection
of statistical mechanics of classical fields. Thus our approach does not
contradict to a rather common opinion that quantum mechanics could not be
reduced to statistical mechanics of classical particles. Notions of a system
and causality can be reestablished on the prequantum level, but the price is
sufficiently high -- the infinite dimension of the phase space. In our approach
quantum observables, symmetric operators in the Hilbert space, are obtained as
derivatives of the second order of functionals of classical fields. Statistical
states are given by Gaussian ensembles of classical fields with zero mean value
(so these are vacuum fluctuations) and dispersion $\alpha$ which plays the role
of a small parameter of the model (so these are small vacuum fluctuations). Our
approach might be called {\it Prequantum Classical Statistical Field Theory} -
PCSFT. Our model is well established on the mathematical level. However, to
obtain concrete experimental predictions -- deviations of real experimental
averages from averages given by the von Neumann trace formula - we should find
the energy scale $\alpha$ of prequantum classical fields.


title:
On distinguishability, orthogonality, and violations of the second law:
  contradictory assumptions, contrasting pieces of knowledge

abstract:
  Two statements by von Neumann and a thought-experiment by Peres prompts a
discussion on the notions of one-shot distinguishability, orthogonality,
semi-permeable diaphragm, and their thermodynamic implications. In the first
part of the paper, these concepts are defined and discussed, and it is
explained that one-shot distinguishability and orthogonality are contradictory
assumptions, from which one cannot rigorously draw any conclusion, concerning
e.g. violations of the second law of thermodynamics. In the second part, we
analyse what happens when these contradictory assumptions comes, instead, from
_two_ different observers, having different pieces of knowledge about a given
physical situation, and using incompatible density matrices to describe it.


title:
Prequantum classical statistical model with infinite dimensional
  phase-space-2: complex representation of symplectic phase-space model

abstract:
  We show that QM can be represented as a natural projection of a classical
statistical model on the phase space $\Omega= H\times H,$ where $H$ is the real
Hilbert space. Statistical states are given by Gaussian measures on $\Omega$
having zero mean value and dispersion of the Planck magnitude -- fluctuations
of the ``vacuum field.'' Physical variables (e.g., energy) are given by maps
$f: \Omega \to {\bf R}$ (functions of classical fields). The crucial point is
that statistical states and variables are symplectically invariant. The
conventional quantum representation of our prequantum classical statistical
model is constructed on the basis of the Teylor expansion (up to the terms of
the second order at the vacuum field point $\omega=0)$ of variables $f: \Omega
\to {\bf R}$ with respect to the small parameter $\kappa= \sqrt{h}.$ A Gaussian
symplectically invariant measure (statistical state) is represented by its
covariation operator (von Neumann statistical operator). A symplectically
invariant smooth function (variable) is represented by its second derivative at
the vacuum field point $\omega=0.$ From the statistical viewpoint QM is a
statistical approximation of the prequantum classical statistical field theory
(PCSFT). Such an approximation is obtained through neglecting by statistical
fluctuations of the magnitude $o(h), h\to 0,$ in averages of physical
variables. Equations of Schr\"odinger, Heisenberg and von Neumann are images of
dynamics on $\Omega$ with a symplectically invariant Hamilton function.


title: Auxiliary nRules of Quantum Mechanics

abstract:
  Standard quantum mechanics makes use of four auxiliary rules that allow the
Schrodinger solutions to be related to laboratory experience, such as the Born
rule that connects square modulus to probability. These rules (here called the
sRules) lead to some unacceptable results. They do not allow the primary
observer to be part of the system. They do not allow individual observations
(as opposed to ensembles) to be part of the system. They make a fundamental
distinction between microscopic and macroscopic things, and they are ambiguous
in their description of secondary observers such as Schrodingers cat.
  The nRules are an alternative set of auxiliary rules that avoid the above
difficulties. In this paper we look at a wide range of representative
experiments showing that the nRules adequately relate the Schrodinger solutions
to empirical experience. This suggests that the sRules should be abandoned in
favor of the more satisfactory nRules, or a third auxiliary rule-set called the
oRules. Keywords: brain states, consciousness, decoherence, epistemology,
measurement, ontology, stochastic, state reduction, wave collapse.


title: New binding-concealing trade-offs for quantum string commitment

abstract:
  String commitment schemes are similar to the well studied bit commitment
schemes in cryptography with the difference that the committing party, say
Alice, is supposed to commit a long string instead of a single bit, to another
party say Bob. Similar to bit commitment schemes, such schemes are supposed to
be binding, i.e Alice cannot change her choice after committing, and concealing
i.e. Bob cannot find Alice's committed string before Alice reveals it.
  Ideal commitment schemes are known to be impossible. Even if some degrees of
cheating is allowed, Buhrman, Christandl, Hayden, Lo and Wehner
(quant-ph/0504078) have recently shown that there are some binding-concealing
trade-offs that any quantum string commitment scheme (QSC) must follow. They
showed trade-offs both in the scenario of single execution of the protocol and
in the asymptotic regime of sufficiently large number of parallel executions of
the protocol.
  We present here new trade-offs in the scenario of single execution of a QSC
protocol. Our trade-offs also immediately imply the trade-off shown by Buhrman
et al. in the asymptotic regime. We show our results by making a central use of
an important information theoretic tool called the substate theorem due to
Jain, Radhakrishnan and Sen. Our techniques are quite different from that of
Buhrman et al. and may be of independent interest.


title:
Simulation of interaction Hamiltonians by quantum feedback: a comment on
  the dynamics of information exchange between coupled systems

abstract:
  Since quantum feedback is based on classically accessible measurement
results, it can provide fundamental insights into the dynamics of quantum
systems by making available classical information on the evolution of system
properties and on the conditional forces acting on the system. In this paper,
the feedback-induced interaction dynamics between a pair of quantum systems is
analyzed. It is pointed out that any interaction Hamiltonian can be simulated
by local feedback if the levels of decoherence are sufficiently high. The
boundary between genuine entanglement generating quantum interactions and
non-entangling classical interactions is identified and the nature of the
information exchange between two quantum systems during an interaction is
discussed.


title: Non-local thin films in Casimir force calculations

abstract:
  he Casimir force is calculated between plates with thin metallic coating.
Thin films are described with spatially dispersive (nonlocal) dielectric
functions. For thin films the nonlocal effects are more relevant than for
half-spaces. However, it is shown that even for film thickness smaller than the
mean free path for electrons, the difference between local and nonlocal
calculations of the Casimir force is of the order of a few tenths of a percent.
Thus the local description of thin metallic films is adequate within the
current experimental precision and range of separations.


title:
Coherence Phenomena in the Phase-sensitive Optical Parametric
  Amplification inside a Cavity

abstract:
  We theoretically and experimentally demonstrate coherence phenomena in
optical parametric amplification inside a cavity. The mode splitting in
transmission spectra of phase-sensitive optical parametric amplifier is
observed. Especially, we show a very narrow dip and peak, which are the shape
of $\delta $ function, appear in the transmission profile. The origin of the
coherence phenomenon in this system is the interference between the harmonic
pump field and the subharmonic seed field in cooperation with dissipation of
the cavity.


title:
Common Axioms for Inferring Classical Ensemble Dynamics and Quantum
  Theory

abstract:
  The same set of physically motivated axioms can be used to construct both the
classical ensemble Hamilton-Jacobi equation and Schrodingers equation. Crucial
roles are played by the assumptions of universality and simplicity (Occam's
Razor) which restrict the number and type of of arbitrary constants that appear
in the equations of motion. In this approach, non-relativistic quantum theory
is seen as the unique single parameter extension of the classical ensemble
dynamics. The method is contrasted with other related constructions in the
literature and some consequences of relaxing the axioms are also discussed: for
example, the appearance of nonlinear higher-derivative corrections possibly
related to gravity and spacetime fluctuations. Finally, some open research
problems within this approach are highlighted.


title: Compiling gate networks on an Ising quantum computer

abstract:
  Here we describe a simple mechanical procedure for compiling a quantum gate
network into the natural gates (pulses and delays) for an Ising quantum
computer. The aim is not necessarily to generate the most efficient pulse
sequence, but rather to develop an efficient compilation algorithm that can be
easily implemented in large spin systems. The key observation is that it is not
always necessary to refocus all the undesired couplings in a spin system.
Instead the coupling evolution can simply be tracked and then corrected at some
later time. Although described within the language of NMR the algorithm is
applicable to any design of quantum computer based on Ising couplings.


title: Quantum damped oscillator I: dissipation and resonances

abstract:
  Quantization of a damped harmonic oscillator leads to so called Bateman's
dual system. The corresponding Bateman's Hamiltonian, being a self-adjoint
operator, displays the discrete family of complex eigenvalues. We show that
they correspond to the poles of energy eigenvectors and the corresponding
resolvent operator when continued to the complex energy plane. Therefore, the
corresponding generalized eigenvectors may be interpreted as resonant states
which are responsible for the irreversible quantum dynamics of a damped
harmonic oscillator.


title:
Squeezing and amplitude-squared squeezing effects on the dynamics of two
  nonidentical two-level Squeezing and amplitude-squared squeezing effects on
  the dynamics of two nonidentical two-level atoms atoms

abstract:
  Squeezing and amplitude-squared squeezing for two two-level nonidentical
atoms in lossless cavity has been investigated assuming the field to be
initially in the coherent state. The time-dependent squeezing parameters has
been calculated. The influence of the relative differences of two coupling
constants on the squeezing parameters has been analyzed.


title:
On evolution of the purity of two nonidentical two-level atoms
  interecting with one-mode coherent field

abstract:
  The resonant interaction of two nonidentical two-level atoms with one mode of
the electromagnetic field has been considered. The pure-state evolution of the
atomic states for field initially in the coherent state and atoms in the ground
state has been investigated. It has been shown that for intermediate values of
the relative differences of two coupling constants the atoms as well as the
field are returned most closely to a pure state at the revival time. The
possibility of the maximally entangled states at the beginning of the collapse
time has been discussed.


title:
Coherent backscattering in nonlinear atomic media: quantum Langevin
  approach

abstract:
  In this theoretical paper, we investigate coherence properties of the
near-resonant light scattered by two atoms exposed to a strong monochromatic
field. To properly incorporate saturation effects, we use a quantum Langevin
approach. In contrast to the standard optical Bloch equations, this method
naturally provides the inelastic spectrum of the radiated light induced by the
quantum electromagnetic vacuum fluctuations. However, to get the right spectral
properties of the scattered light, it is essential to correctly describe the
statistical properties of these vacuum fluctuations. Because of the presence of
the two atoms, these statistical properties are not Gaussian : (i) the spatial
two-points correlation function displays a speckle-like behavior and (ii) the
three-points correlation function does not vanish. We also explain how to
incorporate in a simple way propagation with a frequency-dependent scattering
mean-free path, meaning that the two atoms are embedded in an average
scattering dispersive medium. Finally we show that saturation-induced
nonlinearities strongly modify the atomic scattering properties and, as a
consequence, provide a source of decoherence in multiple scattering. This is
exemplified by considering the coherent backscattering configuration where
interference effects are blurred by this decoherence mechanism. This leads to a
decrease of the so-called coherent backscattering enhancement factor.


title:
Coherent photons and macroscopic cat states generated by a cyclic
  artificial atom with broken symmetry

abstract:
  We propose a novel method to generate non-classical states of a single-mode
microwave field, and to produce macroscopic cat states by virtue of a
three-level system with $\Delta$-shaped (or cyclic) transitions. This exotic
system can be implemented by a superconducting quantum circuit with a broken
symmetry in its effective potential. Using the cyclic population transfer,
controllable single-mode photon states can be created in the third transition
when two classical fields are applied to induce the other two transitions. This
is because, for large detuning, two classical fields are equivalent to an
effective external force, which derives the quantized single mode. Our approach
is valid not only for superconducting quantum circuits but also for any
three-level quantum system with $\Delta$-shaped transitions


title: An Algebra of Pure Quantum Programming

abstract:
  We develop a sound and complete equational theory for the functional quantum
programming language QML. The soundness and completeness of the theory are with
respect to the previously-developed denotational semantics of QML. The
completeness proof also gives rise to a normalisation algorithm following the
normalisation by evaluation approach. The current work focuses on the pure
fragment of QML omitting measurements.


title: Information Complexity of Quantum Gates

abstract:
  This paper considers the realizability of quantum gates from the perspective
of information complexity. Since the gate is a physical device that must be
controlled classically, it is subject to random error. We define the complexity
of gate operation in terms of the difference between the entropy of the
variables associated with initial and final states of the computation. We argue
that the gate operations are irreversible if there is a difference in the
accuracy associated with input and output variables. It is shown that under
some conditions the gate operation may be associated with unbounded entropy,
implying impossibility of implementation.


title: Reconstruction of the optical potential from scattering data

abstract:
  We propose a method for reconstruction of the optical potential from
scattering data. The algorithm is a two-step procedure. In the first step the
real part of the potential is determined analytically via solution of the
Marchenko equation. At this point we use a diagonal Pad\'{e} approximant of the
corresponding unitary $S$-matrix. In the second step the imaginary part of the
potential is determined via the phase equation of the variable phase approach.
We assume that the real and the imaginary parts of the optical potential are
proportional. We use the phase equation to calculate the proportionality
coefficient. A numerical algorithm is developed for a single and for coupled
partial waves. The developed procedure is applied to analysis of $^{1}S_{0}$
$NN$, $^{3}SD_{1}$ $NN$, $P31$ $\pi^{-} N$ and $S01$ $K^{+}N$ data.


title: Reply To "Comment on 'Quantum Convolutional Error-Correcting Codes' "

abstract:
  In their comment, de Almedia and Palazzo \cite{comment} discovered an error
in my earlier paper concerning the construction of quantum convolutional codes
(quant-ph/9712029). This error can be repaired by modifying the method of code
construction.


title:
Measuring the quality factor of a microwave cavity using superconduting
  qubit devices

abstract:
  We propose a method to create superpositions of two macroscopic quantum
states of a single-mode microwave cavity field interacting with a
superconducting charge qubit. The decoherence of such superpositions can be
determined by measuring either the Wigner function of the cavity field or the
charge qubit states. Then the quality factor Q of the cavity can be inferred
from the decoherence of the superposed states. The proposed method is
experimentally realizable within current technology even when the $Q$ value is
relatively low, and the interaction between the qubit and the cavity field is
weak.


title: Solvable relativistic quantum dots with vibrational spectra

abstract:
  For Klein-Gordon equation a consistent physical interpretation of wave
functions is reviewed as based on a proper modification of the scalar product
in Hilbert space. Bound states are then studied in a deep-square-well model
where spectrum is roughly equidistant and where a fine-tuning of the levels is
mediated by PT-symmetric interactions composed of imaginary delta functions
which mimic creation/annihilation processes.


title: Optimal Quantum Filtering and Quantum Feedback Control

abstract:
  Quantum mechanical systems exhibit an inherently probabilistic nature upon
measurement. Using a quantum noise model to describe the stochastic evolution
of the open quantum system and working in parallel with classical
indeterministic control theory, we present the theory of nonlinear optimal
quantum feedback control. The resulting quantum Bellman equation is then
applied to the explicitly solvable quantum linear-quadratic-Gaussian (LQG)
problem which emphasizes many similarities with the corresponding classical
control problem.


title: Enhanced algorithms for Local Search

abstract:
  Let G=(V,E) be a finite graph, and f:V->N be any function. The Local Search
problem consists in finding a local minimum of the function f on G, that is a
vertex v such that f(v) is not larger than the value of f on the neighbors of v
in G. In this note, we first prove a separation theorem slightly stronger than
the one of Gilbert, Hutchinson and Tarjan for graphs of constant genus. This
result allows us to enhance a previously known deterministic algorithm for
Local Search with query complexity O(\log n)\cdot d+O(\sqrt{g})\cdot\sqrt{n},
so that we obtain a deterministic query complexity of
d+O(\sqrt{g})\cdot\sqrt{n}, where n is the size of G, d is its maximum degree,
and $g$ is its genus. We also give a quantum version of our algorithm, whose
query complexity is of O(\sqrt{d})+O(\sqrt[4]{g})\cdot\sqrt[4]{n}\log\log n.
Our deterministic and quantum algorithms have query complexities respectively
smaller than the generic algorithms of Aldous and of Aaronson for large classes
of graphs, including graphs of bounded genus and planar graphs. Independently
from this work, Zhang has recently given a quantum algorithm which finds a
local minimum on the planar grid over \{1,...,\sqrt{n}\}^2 using
O(\sqrt[4]{n}(\log\log n)^2) queries. Our quantum algorithm can be viewed as a
strongly generalized, and slightly enhanced version of this algorithm.


title:
Decoherence-free subspaces and subsystems for a collectively
  depolarizing bosonic channel

abstract:
  We discuss the structure of decoherence-free subsystems for a bosonic channel
affected by collective depolarization. A single use of the channel is defined
as a transmission of a pair of bosonic modes. Collective depolarization
consists in a random linear U(2) transformation of the respective mode
operators, which is assumed to be identical for $N$ consecutive uses of the
channel. We derive a recursion formula that characterizes the dimensionality of
available decoherence-free subsystems in such a setting.


title: Condition and capability of quantum state separation

abstract:
  The linearity of quantum operations puts many fundamental constraints on the
information processing tasks we can achieve on a quantum system whose state is
not exactly known, just as we observe in quantum cloning and quantum
discrimination. In this paper we show that in probabilistic manner, linearity
is in fact the only one that restricts the physically realizable tasks. To be
specific, if a system is prepared in a state secretly chosen from a linearly
independent pure state set, then any quantum state separation can be physically
realized with a positive probability. Furthermore, we derive a lower bound on
the average failure probability of any quantum state separation.


title: Entanglement of bosonic modes in symmetric graphs

abstract:
  The ground and thermal states of a quadratic hamiltonian representing the
interaction of bosonic modes or particles are always Gaussian states. We
investigate the entanglement properties of these states for the case where the
interactions are represented by harmonic forces acting along the edges of
symmetric graphs, i.e. 1, 2, and 3 dimensional rectangular lattices, mean field
clusters and platonic solids. We determine the Entanglement of Formation (EoF)
as a function of the interaction strength, calculate the maximum EoF in each
case and compare these values with the bounds found in \cite{wolf} which are
valid for any quadratic hamiltonian.


title:
Operator Quantum Error Correcting Subsystems for Self-Correcting Quantum
  Memories

abstract:
  The most general method for encoding quantum information is not to encode the
information into a subspace of a Hilbert space, but to encode information into
a subsystem of a Hilbert space. Recently this notion has led to a more general
notion of quantum error correction known as operator quantum error correction.
In standard quantum error correcting codes, one requires the ability to apply a
procedure which exactly reverses on the error correcting subspace any
correctable error. In contrast, for operator error correcting subsystems, the
correction procedure need not undo the error which has occurred, but instead
one must perform correction only modulo the subsystem structure. This does not
lead to codes which differ from subspace codes, but does lead to recovery
routines which explicitly make use of the subsystem structure. Here we present
two examples of such operator error correcting subsystems. These examples are
motivated by simple spatially local Hamiltonians on square and cubic lattices.
In three dimensions we provide evidence, in the form a simple mean field
theory, that our Hamiltonian gives rise to a system which is self-correcting.
Such a system will be a natural high-temperature quantum memory, robust to
noise without external intervening quantum error correction procedures.


title: Many worlds and the emergence of probability in quantum mechanics

abstract:
  The interpretation of the squared norm as probability and the apparent
stochastic nature of observation in quantum mechanics are derived from the
strong law of large numbers and the algebraic properties of infinite sequences
of simultaneous quantum observables. It is argued that this result validates
the many-worlds view of quantum reality.


title: What is the Temperature Dependence of the Casimir Effect?

abstract:
  There has been recent criticism of our approach to the Casimir force between
real metallic surfaces at finite temperature, saying it is in conflict with the
third law of thermodynamics and in contradiction with experiment. We show that
these claims are unwarranted, and that our approach has strong theoretical
support, while the experimental situation is still unclear.


title:
Standard and Non-Standard Quantum Models : A Non-Commutative Version of
  the Classical System of SU(2) and SU(1,1) Arising from Quantum Optics

abstract:
  This is a challenging paper including some review and new results.
  Since the non-commutative version of the classical system based on the
compact group SU(2) has been constructed in (quant-ph/0502174) by making use of
Jaynes-Commings model and so-called Quantum Diagonalization Method in
(quant-ph/0502147), we construct a non-commutative version of the classical
system based on the non-compact group SU(1,1) by modifying the compact case.
  In this model the Hamiltonian is not hermite but pseudo hermite, which causes
a big difference between two models. For example, in the classical
representation theory of SU(1,1), unitary representations are infinite
dimensional from the starting point. Therefore, to develop a unitary theory of
non-commutative system of SU(1,1) we need an infinite number of non-commutative
systems, which means a kind of {\bf second non-commutativization}. This is a
very hard and interesting problem.
  We develop a corresponding theory though it is not always enough, and present
some challenging problems concerning how classical properties can be extended
to the non-commutative case.
  This paper is arranged for the convenience of readers as the first subsection
is based on the standard model (SU(2) system) and the next one is based on the
non-standard model (SU(1,1) system). This contrast may make the similarity and
difference between the standard and non-standard models clear.


title: Quantum Theory Looks at Time Travel

abstract:
  We introduce a quantum mechanical model of time travel which includes two
figurative beam splitters in order to induce feedback to earlier times. This
leads to a unique solution to the paradox where one could kill one's
grandfather in that once the future has unfolded, it cannot change the past,
and so the past becomes deterministic. On the other hand, looking forwards
towards the future is completely probabilistic. This resolves the classical
paradox in a philosophically satisfying manner.


title: Comment on "Teleportation with a uniformly accelerated partner"

abstract:
  We comment on the paper "Teleportation with a uniformly accelerated partner"
(quant-ph/0302179).


title: Nonclassical Moments and their Measurement

abstract:
  Practically applicable criteria for the nonclassicality of quantum states are
formulated in terms of different types of moments. For this purpose the moments
of the creation and annihilation operators, of two quadratures, and of a
quadrature and the photon number operator turn out to be useful. It is shown
that all the required moments can be determined by homodyne correlation
measurements. An example of a nonclassical effect that is easily characterized
by our methods is amplitude-squared squeezing.


title: Cooling trapped atoms in optical resonators

abstract:
  We derive an equation for the cooling dynamics of the quantum motion of an
atom trapped by an external potential inside an optical resonator. This
equation has broad validity and allows us to identify novel regimes where the
motion can be efficiently cooled to the potential ground state. Our result
shows that the motion is critically affected by quantum correlations induced by
the mechanical coupling with the resonator, which may lead to selective
suppression of certain transitions for the appropriate parameters regimes,
thereby increasing the cooling efficiency.


title: Trapped-ion qutrit spin molecule quantum computer

abstract:
  We present a qutrit quantum computer design using trapped ions in the
presence of a magnetic field gradient. The magnetic field gradient induces a
"spin-spin" type coupling, similar to the J-coupling observed in molecules,
between the qutrits which allows conditional quantum logic to take place. We
describe in some detail, how one can execute specific one and two qutrit
quantum gates, required for universal qutrit quantum computing.


title: Quantum Dot Cluster State Computing with Encoded Qubits

abstract:
  A class of architectures is advanced for cluster state quantum computation
using quantum dots. These architectures include using single and multiple dots
as logical qubits. Special attention is given to the supercoherent qubits
introduced by Bacon, Brown, and Whaley [Phys. Rev. Lett. {\bf 87}, 247902
(2001)] for which we discuss the effects of various errors, and present means
of error protection.


title: Variations on Quantum Oblivious Transfer

abstract:
  This paper has been withdrawn by the author, due to the insecurity against
attacks received in quant-ph/0605027v5.


title: On the missing axiom of Quantum Mechanics

abstract:
  The debate on the nature of quantum probabilities in relation to Quantum Non
Locality has elevated Quantum Mechanics to the level of an "Operational
Epistemic Theory". In such context the quantum superposition principle has an
extraneous non epistemic nature. This leads us to seek purely operational
foundations for Quantum Mechanics, from which to derive the current
mathematical axiomatization based on Hilbert spaces.
  In the present work I present a set of axioms of purely operational nature,
based on a general definition of "the experiment", the operational/epistemic
archetype of information retrieval from reality. As we will see, this starting
point logically entails a series of notions [state, conditional state, local
state, pure state, faithful state, instrument, propensity (i.e. "effect"),
dynamical and informational equivalence, dynamical and informational
compatibility, predictability, discriminability, programmability, locality,
a-causality, rank of the state, maximally chaotic state, maximally entangled
state, informationally complete propensity, etc. ], along with a set of rules
(addition, convex combination, partial orderings, ...), which, far from being
of quantum origin as often considered, instead constitute the universal
"syntactic manual" of the operational/epistemic approach. The missing
ingredient is, of course, the quantum superposition axiom for probability
amplitudes: for this I propose some substitute candidates of purely
operational/epistemic nature.


title: How to fool the GHZ and W witnesses

abstract:
  In spite of the fact that there are only two classes of three-qubit genuine
entanglement: W and GHZ, we show that there are three-qubit genuinely entangled
states which can not be detected neither by W nor by GHZ entanglement
witnesses.


title:
Performance of various quantum key distribution systems using 1.55 um
  up-conversion single-photon detectors

abstract:
  We compare the performance of various quantum key distribution (QKD) systems
using a novel single-photon detector, which combines frequency up-conversion in
a periodically poled lithium niobate (PPLN) waveguide and a silicon avalanche
photodiode (APD). The comparison is based on the secure communication rate as a
function of distance for three QKD protocols: the Bennett-Brassard 1984 (BB84),
the Bennett, Brassard, and Mermin 1992 (BBM92), and the coherent differential
phase shift keying (DPSK). We show that the up-conversion detector allows for
higher communication rates and longer communication distances than the commonly
used InGaAs/InP APD for all the three QKD protocols.


title: Suppressing decoherence of quantum algorithms by jump codes

abstract:
  The stabilizing properties of one-error correcting jump codes are explored
under realistic non-ideal conditions. For this purpose the quantum algorithm of
the tent-map is decomposed into a universal set of Hamiltonian quantum gates
which ensure perfect correction of spontaneous decay processes under ideal
circumstances even if they occur during a gate operation. An entanglement gate
is presented which is capable of entangling any two logical qubits of different
one-error correcting code spaces. With the help of this gate simultaneous
spontaneous decay processes affecting physical qubits of different code spaces
can be corrected and decoherence can be suppressed significantly.


title: Controlling quantum systems by embedded dynamical decoupling schemes

abstract:
  A dynamical decoupling method is presented which is based on embedding a
deterministic decoupling scheme into a stochastic one. This way it is possible
to combine the advantages of both methods and to increase the suppression of
undesired perturbations of quantum systems significantly even for long
interaction times. As a first application the stabilization of a quantum memory
is discussed which is perturbed by one-and two-qubit interactions.


title: Two-way quantum communication channels

abstract:
  We consider communication between two parties using a bipartite quantum
operation, which constitutes the most general quantum mechanical model of
two-party communication. We primarily focus on the simultaneous forward and
backward communication of classical messages. For the case in which the two
parties share unlimited prior entanglement, we give inner and outer bounds on
the achievable rate region that generalize classical results due to Shannon. In
particular, using a protocol of Bennett, Harrow, Leung, and Smolin, we give a
one-shot expression in terms of the Holevo information for the
entanglement-assisted one-way capacity of a two-way quantum channel. As
applications, we rederive two known additivity results for one-way channel
capacities: the entanglement-assisted capacity of a general one-way channel,
and the unassisted capacity of an entanglement-breaking one-way channel.


title:
Hidden variables in quantum mechanics: Generic models, set-theoretic
  forcing, and the emergence of probability

abstract:
  The hidden-variables premise is shown to be equivalent to the existence of
generic filters for algebras of commuting propositions and for certain more
general propositional systems. The significance of this equivalence is
interpreted in light of the theory of generic filters and boolean-valued models
in set theory (the method of forcing). The apparent stochastic nature of
quantum observation is derived for these hidden-variables models.


title:
Quantum-Information Theoretic Properties of Nuclei and Trapped Bose
  Gases

abstract:
  Fermionic (atomic nuclei) and bosonic (correlated atoms in a trap) systems
are studied from an information-theoretic point of view. Shannon and Onicescu
information measures are calculated for the above systems comparing correlated
and uncorrelated cases as functions of the strength of short range
correlations. One-body and two-body density and momentum distributions are
employed. Thus the effect of short-range correlations on the information
content is evaluated. The magnitude of distinguishability of the correlated and
uncorrelated densities is also discussed employing suitable measures of
distance of states i.e. the well known Kullback-Leibler relative entropy and
the recently proposed Jensen-Shannon divergence entropy. It is seen that the
same information-theoretic properties hold for quantum many-body systems
obeying different statistics (fermions and bosons).

categories: quant-ph cond-mat.other nucl-th physics.atom-ph


title:
An Algorithmic Test for Diagonalizability of Finite-Dimensional
  PT-Invariant Systems

abstract:
  A non-Hermitean operator does not necessarily have a complete set of
eigenstates, contrary to a Hermitean one. An algorithm is presented which
allows one to decide whether the eigenstates of a given PT-invariant operator
on a finite-dimensional space are complete or not. In other words, the
algorithm checks whether a given PT-symmetric matrix is diagonalizable. The
procedure neither requires to calculate any single eigenvalue nor any numerical
approximation.


title:
Segre variety, conifold, Hopf fibration, and separable multi-qubit
  states

abstract:
  We establish relations between Segre variety, conifold, Hopf fibration, and
separable sets of pure two-qubit states. Moreover, we investigate the geometry
and topology of separable sets of pure multi-qubit states based on a complex
multi-projective Segre variety and higher order Hopf fibration.


title: Decoherence of Josephson charge qubit

abstract:
  In this paper we investigate decoherence time of superconducting Josephson
charge qubit (JCQ). Two kinds of methods, iterative tensor multiplication (ITM)
method derived from the qusiadiabatic propagator path integral (QUAPI) and
Bloch equations method are used. Using the non-Markovian ITM method we correct
the decoherence time predicted by Bloch equations method. By comparing the
exact theoretical result with the experimental data we suggest that the Ohmic
noise plays the central role to the decoherence of the JCQ.


title:
Generalized quantum measurements. Part I: Information properties of soft
  quantum measurements

abstract:
  A special class of soft quantum measurements as a physical model of the fuzzy
measurements widely used in physics is introduced and its information
properties are studied in detail.


title:
Generalized quantum measurements. Part II: Partially-destructive quantum
  measurements in finite-dimensional Hilbert spaces

abstract:
  A concept of the generalized quantum measurement is introduced as the
transformation, which establishes a correspondence between the initial states
of the object system and final states of the object--measuring device (meter)
system with the help of a classical informational index, unambiguously linked
to the classically compatible set of states of the object--meter system. It is
shown that the generalized measurement covers all the key known quantum
measurement concepts--standard projective, entangling, fuzzy and the
generalized measurement with the partial or complete destruction of the initial
information contained in the object. A special class of partially-destructive
measurements that map the continual set of the states in finite-dimensional
quantum systems to that one of the infinite-dimensional quantum systems is
considered. Their informational essence and some information characteristics
are discussed in detail.


title:
No-relationship between impossibility of faster-than-light quantum
  communication and distinction of ensembles with the same density matrix

abstract:
  It has been claimed in the literature that impossibility of faster-than-light
quantum communication has an origin of indistinguishability of ensembles with
the same density matrix. We show that the two concepts are not related. We
argue that: 1) even with an ideal single-atom-precision measurement, it is
generally impossible to produce two ensembles with exactly the same density
matrix; or 2) to produce ensembles with the same density matrix, classical
communication is necessary. Hence the impossibility of faster-than-light
communication does not imply the indistinguishability of ensembles with the
same density matrix.


title: Evanescence in Coined Quantum Walks

abstract:
  In this paper we complete the analysis begun by two of the authors in a
previous work on the discrete quantum walk on the line [J. Phys. A 36:8775-8795
(2003) quant-ph/0303105 ]. We obtain uniformly convergent asymptotics for the
"exponential decay'' regions at the leading edges of the main peaks in the
Schr{\"o}dinger (or wave-mechanics) picture. This calculation required us to
generalise the method of stationary phase and we describe this extension in
some detail, including self-contained proofs of all the technical lemmas
required. We also rigorously establish the exact Feynman equivalence between
the path-integral and wave-mechanics representations for this system using some
techniques from the theory of special functions. Taken together with the
previous work, we can now prove every theorem by both routes.


title:
Entanglement, Purity, and Information Entropies in Continuous Variable
  Systems

abstract:
  Quantum entanglement of pure states of a bipartite system is defined as the
amount of local or marginal ({\em i.e.}referring to the subsystems) entropy.
For mixed states this identification vanishes, since the global loss of
information about the state makes it impossible to distinguish between quantum
and classical correlations. Here we show how the joint knowledge of the global
and marginal degrees of information of a quantum state, quantified by the
purities or in general by information entropies, provides an accurate
characterization of its entanglement. In particular, for Gaussian states of
continuous variable systems, we classify the entanglement of two--mode states
according to their degree of total and partial mixedness, comparing the
different roles played by the purity and the generalized $p-$entropies in
quantifying the mixedness and bounding the entanglement. We prove the existence
of strict upper and lower bounds on the entanglement and the existence of
extremally (maximally and minimally) entangled states at fixed global and
marginal degrees of information. This results allow for a powerful, operative
method to measure mixed-state entanglement without the full tomographic
reconstruction of the state. Finally, we briefly discuss the ongoing extension
of our analysis to the quantification of multipartite entanglement in highly
symmetric Gaussian states of arbitrary $1 \times N$-mode partitions.

categories: quant-ph cond-mat.stat-mech math-ph math.MP physics.optics


title:
Simultaneous classical-quantum capacities of quantum multiple access
  channels

abstract:
  The rates at which classical and quantum information can be simultaneously
transmitted from two spatially separated senders to a single receiver over an
arbitrary quantum channel are characterized. Two main results are proved in
detail. The first describes the region of rates at which one sender can send
classical information while the other sends quantum information. The second
describes those rates at which both senders can send quantum information. For
each of these situations, an example of a channel is given for which the
associated region admits a single-letter description. This is the author's
Ph.D. dissertation, submitted to the Department of Electrical Engineering at
Stanford University in March, 2005. It represents an expanded version of the
paper quant-ph/0501045, containing a number of tutorial chapters which may be
of independent interest for those learning about quantum Shannon theory.


title:
Branch dependence in the "consistent histories" approach to quantum
  mechanics

abstract:
  In the consistent histories formalism one specifies a family of histories as
an exhaustive set of pairwise exclusive descriptions of the dynamics of a
quantum system. We define branching families of histories, which strike a
middle ground between the two available mathematically precise definitions of
families of histories, viz., product families and Isham's history projector
operator formalism. The former are too narrow for applications, and the
latter's generality comes at a certain cost, barring an intuitive reading of
the ``histories''. Branching families retain the intuitiveness of product
families, they allow for the interpretation of a history's weight as a
probability, and they allow one to distinguish two kinds of coarse-graining,
leading to reconsidering the motivation for the consistency condition.


title: Yet another additivity conjecture

abstract:
  It is known that the additivity conjecture of Holevo capacity, output minimum
entoropy, and the entanglement of formation (EoF), are equivalent with each
other. Among them, the output minimum entropy is simplest, and hence many
researchers are focusing on this quantity.
  Here, we suggest yet another entanglement measure, whose strong
superadditivity and additivity are equivalent to the additivity of the
quantities mentioned above. This quantity is as simple as the output minimum
entropy, and in existing proofs of additivity conjecture of the output minimum
entropy for the specific examples, they are essentially proving the strong
superadditivity of this quantity.


title: Superballistic Diffusion of Entanglement in Disordered Spin Chains

abstract:
  We study the dynamics of a single excitation in an infinite XXZ spin chain,
which is launched from the origin. We study the time evolution of the spread of
entanglement in the spin chain and obtain an expression for the second order
spatial moment of concurrence, about the origin, for both ordered and
disordered chains. In this way, we show that a finite central disordered region
can lead to sustained superballistic growth in the second order spatial moment
of entanglement within the chain.


title:
Improvement of stabilizer based entanglement distillation protocols by
  encoding operators

abstract:
  This paper presents a method for enumerating all encoding operators in the
Clifford group for a given stabilizer. Furthermore, we classify encoding
operators into the equivalence classes such that EDPs (Entanglement
Distillation Protocol) constructed from encoding operators in the same
equivalence class have the same performance. By this classification, for a
given parameter, the number of candidates for good EDPs is significantly
reduced. As a result, we find the best EDP among EDPs constructed from [[4,2]]
stabilizer codes. This EDP has a better performance than previously known EDPs
over wide range of fidelity.


title: Universality of quantum critical dynamics in a planar OPO

abstract:
  We analyze the critical quantum fluctuations in a coherently driven planar
optical parametric oscillator. We show that the presence of transverse modes
combined with quantum fluctuations changes the behavior of the `quantum image'
critical point. This zero-temperature non-equilibrium quantum system has the
same universality class as a finite-temperature magnetic Lifshitz transition.


title: Two-photon interference with true thermal light

abstract:
  Two-photon interference and "ghost" imaging with entangled light have
attracted much attention since the last century because of the novel features
such as non-locality and sub-wavelength effect. Recently, it has been found
that pseudo-thermal light can mimic certain effects of entangled light. We
report here the first observation of two-photon interference with true thermal
light.


title: Bloch Oscillators: motion of wave-packets

abstract:
  In this paper we review some known results on the motion of Bloch Oscillators
in the crystal momentum representation. We emphasize that the acceleration
theorem, as usually stated by most of the authors, is incomplete, but in the
case of pure Bloch states. In fact, the exact version of the acceleration
theorem should contain a phase factor depending on both the time and crystal
momentum variables. As we show this phase factor plays an essential role in
order to understand the motion of Bloch Oscillators in the position variable.


title: Algebraic invariants of five qubits

abstract:
  The Hilbert series of the algebra of polynomial invariants of pure states of
five qubits is obtained, and the simplest invariants are computed.


title:
Detection and characterization of multipartite entanglement in optical
  lattices

abstract:
  We investigate the detection and characterization of entanglement based on
the quantum network introduced in [Phys. Rev. Lett. 93, 110501 (2004)] for
different experimental scenarios. We first give a detailed discussion of the
ideal scheme where no errors are present and full spatial resolution is
available. Then we analyze the implementation of the network in an optical
lattice. We find that even without any spatial resolution entanglement can be
detected and characterized in various kinds of states including cluster states
and macroscopic superposition states. We also study the effects of detection
errors and imperfect dynamics on the detection network. For our scheme to be
practical these errors have to be on the order of one over the number of
investigated lattice sites. Finally, we consider the case of limited spatial
resolution and conclude that significant improvement in entanglement detection
and characterization compared to having no spatial resolution is only possible
if single lattice sites can be resolved.


title: Dimensional Crossover in Bragg Scattering from an Optical Lattice

abstract:
  We study Bragg scattering at 1D optical lattices. Cold atoms are confined by
the optical dipole force at the antinodes of a standing wave generated inside a
laser-driven high-finesse cavity. The atoms arrange themselves into a chain of
pancake-shaped layers located at the antinodes of the standing wave. Laser
light incident on this chain is partially Bragg-reflected. We observe an
angular dependence of this Bragg reflection which is different to what is known
from crystalline solids. In solids the scattering layers can be taken to be
infinitely spread (3D limit). This is not generally true for an optical lattice
consistent of a 1D linear chain of point-like scattering sites. By an explicit
structure factor calculation we derive a generalized Bragg condition, which is
valid in the intermediate regime. This enables us to determine the aspect ratio
of the atomic lattice from the angular dependance of the Bragg scattered light.


title:
A family of generalized `pretty good' measurements and the minimal-error
  pure-state discrimination problems for which they are optimal

abstract:
  Paper withdrawn. Results are similar to V. P. Belavkin, "Optimum distinction
of non-orthogonal quantum signals," Radio Engineering and Electronic Physics,
vol. 20, June 1975, p. 39-47.


title: Determinism in the one-way model

abstract:
  We introduce a flow condition on open graph states (graph states with inputs
and outputs) which guarantees globally deterministic behavior of a class of
measurement patterns defined over them. Dependent Pauli corrections are derived
for all such patterns, which equalize all computation branches, and only depend
on the underlying entanglement graph and its choice of inputs and outputs.
  The class of patterns having flow is stable under composition and
tensorization, and has unitary embeddings as realizations. The restricted class
of patterns having both flow and reverse flow, supports an operation of
adjunction, and has all and only unitaries as realizations.


title: Rotational Properties and GHZ Contradictions in N-Qubit Systems

abstract:
  Rotational symmetries of N-qubit Greenberger-Horne-Zeilinger (GHZ) states
directly exhibit their nonlocality and render transparent the many possible
measurements that produce absolute contradictions with local realism. While N
measurements fix the assumed elements of reality, an exponentially growing
number of absolute contradictions occurs. Operators that represent the detector
settings provide a faithful representation of the symmetry group, and
demonstrate Kochen-Specker contextuality.


title:
Reference potential approach to the quantum-mechanical inverse problem:
  I. Calculation of phase shift and Jost function

abstract:
  Elegant and mathematically rigorous methods of the quantum inverse theory are
difficult to put into practice because there is always some lack of needful
input information. In this situation, one may try to construct a reference
potential, whose spectral characteristics would be in a reasonable agreement
with the available data of the system's properties. Since the reference
potential is fixed, it is always possible to calculate all its spectral
characteristics, including phase shift for scattering states and Jost function,
the main key to solve the inverse problem. Thereafter, one can calculate a
Bargmann potential whose Jost function differs from the initial one only by a
rational factor. This way it is possible, at least in principle, to construct a
more reliable potential for the system. The model system investigated in this
paper is diatomic xenon molecule in ground electronic state. Its reference
potential is built up of several smoothly joined Morse type components, which
enables to solve the related energy eigenvalue problem exactly. Moreover, the
phase shift can also be calculated in part analytically, and the Jost function
can be acertained very accurately in the whole range of positive energies. Full
energy dependence of the phase shift has been determined and its excellent
agreement with the Levinson theorem demonstrated. In addition, asymptotically
exact analytic formulas for the phase shift and the Jost function, independent
of each other, are obtained and their physical background elucidated.


title:
Quantum Measurements and Information-Theoretical Restrictions in
  Observable Algebras Formalism

abstract:
  Information-Theoretical restrictions on the information transfer are applied
to Quantum Measurements. For the measurement of quantum object S by information
system O this restrictions are calculated in Algebraic QM formalism as the
inference map to the (sub)algebra of O observables.


title: Bell's Jump Process in Discrete Time

abstract:
  The jump process introduced by J. S. Bell in 1986, for defining a quantum
field theory without observers, presupposes that space is discrete whereas time
is continuous. In this letter, our interest is to find an analogous process in
discrete time. We argue that a genuine analog does not exist, but provide
examples of processes in discrete time that could be used as a replacement.


title: Vacuum-stimulated cooling of single atoms in three dimensions

abstract:
  Taming quantum dynamical processes is the key to novel applications of
quantum physics, e.g. in quantum information science. The control of
light-matter interactions at the single-atom and single-photon level can be
achieved in cavity quantum electrodynamics, in particular in the regime of
strong coupling where atom and cavity form a single entity. In the optical
domain, this requires permanent trapping and cooling of an atom in a
micro-cavity. We have now realized three-dimensional cavity cooling and
trapping for an orthogonal arrangement of cooling laser, trap laser and cavity
vacuum. This leads to average single-atom trapping times exceeding 15 seconds,
unprecedented for a strongly coupled atom under permanent observation.


title: Conditional probabilities and density operators in quantum modeling

abstract:
  Based on a recent proof of free choices in linking equations to the
experiments they describe, I clarify relations among some purely mathematical
entities featured in quantum mechanics (probabilities, density operators,
partial traces, and operator-valued measures), thereby allowing applications of
these entities to the modeling of a wider variety of physical situations.
Conditional probabilities associated with projection-valued measures are
expressed by introducing conditional density operators, identical in some but
not all cases to the usual reduced density operators. By lifting density
operators to the extended Hilbert space featured in Neumark's theorem, I show
an obstacle to extending conditional density operators to arbitrary positive
operator-valued measures (POVMs); however, tensor products of POVMs are
compatible with conditional density operators. By way of application,
conditional density operators together with the free choice of probe particles
allow the so-called postulate of state reductions to be replaced by a theorem.
A second application demonstrates an equivalence between one form of quantum
key distribution and another, allowing a formulation of individual
eavesdropping attacks against transmitted-state BB84 to work also for
entangled-state BB84.


title:
Algebraic and information-theoretic conditions for operator quantum
  error-correction

abstract:
  Operator quantum error-correction is a technique for robustly storing quantum
information in the presence of noise. It generalizes the standard theory of
quantum error-correction, and provides a unified framework for topics such as
quantum error-correction, decoherence-free subspaces, and noiseless subsystems.
This paper develops (a) easily applied algebraic and information-theoretic
conditions which characterize when operator quantum error-correction is
feasible; (b) a representation theorem for a class of noise processes which can
be corrected using operator quantum error-correction; and (c) generalizations
of the coherent information and quantum data processing inequality to the
setting of operator quantum error-correction.


title: Distributed measurement-based quantum computation

abstract:
  We develop a formal model for distributed measurement-based quantum
computations, adopting an agent-based view, such that computations are
described locally where possible. Because the network quantum state is in
general entangled, we need to model it as a global structure, reminiscent of
global memory in classical agent systems. Local quantum computations are
described as measurement patterns. Since measurement-based quantum computation
is inherently distributed, this allows us to extend naturally several concepts
of the measurement calculus, a formal model for such computations. Our goal is
to define an assembly language, i.e. we assume that computations are
well-defined and we do not concern ourselves with verification techniques. The
operational semantics for systems of agents is given by a probabilistic
transition system, and we define operational equivalence in a way that it
corresponds to the notion of bisimilarity. With this in place, we prove that
teleportation is bisimilar to a direct quantum channel, and this also within
the context of larger networks.


title: Teleportation via multi-qubit channels

abstract:
  We investigate the problem of teleporting an unknown qubit state to a
recipient via a channel of $2\L$ qubits. In this procedure a protocol is
employed whereby $\L$ Bell state measurements are made and information based on
these measurements is sent via a classical channel to the recipient. Upon
receiving this information the recipient determines a local gate which is used
to recover the original state. We find that the $2^{2\L}$-dimensional Hilbert
space of states available for the channel admits a decomposition into four
subspaces. Every state within a given subspace is a perfect channel, and each
sequence of Bell measurements projects $2\L$ qubits of the system into one of
the four subspaces. As a result, only two bits of classical information need be
sent to the recipient for them to determine the gate. We note some connections
between these four subspaces and ground states of many-body Hamiltonian
systems, and discuss the implications of these results towards understanding
entanglement in multi-qubit systems.


title: Minimum error discrimination of Pauli channels

abstract:
  We solve the problem of discriminating with minimum error probability two
given Pauli channels. We show that, differently from the case of discrimination
between unitary transformations, the use of entanglement with an ancillary
system can strictly improve the discrimination, and any maximally entangled
state allows to achieve the optimal discrimination. We also provide a simple
necessary and sufficient condition in terms of the structure of the channels
for which the ultimate minimum error probability can be achieved without
entanglement assistance. When such a condition is satisfied, the optimal input
state is simply an eigenstate of one of the Pauli matrices.


title:
Entanglement monotones and maximally entangled states in multipartite
  qubit systems

abstract:
  We present a method to construct entanglement measures for pure states of
multipartite qubit systems. The key element of our approach is an antilinear
operator that we call {\em comb} in reference to the {\em hairy-ball theorem}.
For qubits (or spin 1/2) the combs are automatically invariant under
$SL(2,\CC)$. This implies that the {\em filters} obtained from the combs are
entanglement monotones by construction. We give alternative formulae for the
concurrence and the 3-tangle as expectation values of certain antilinear
operators. As an application we discuss inequivalent types of genuine four-,
five- and six-qubit entanglement.


title:
Generation of Matrices with Specified Eigenvalues and their
  Diagonalization

abstract:
  We present a prescription for forming matrices with specified eigenvalues and
known eigenvectors. With this method, we can form Hermitian, anti-Hermitian,
symmetric and general matrices with arbitrary eigenvalues. In addition we
propose an algorithm for diagonalizing such matrices. The functions required
for the realization of this are probability amplitudes connecting observables
with discrete eigenvalue spectra and can be obtained from spin theory. For the
example case of $5\times 5$ matrices, these functions are given.


title: Some fundamental aspects of a quantum potential

abstract:
  We show that given a suitable but essentially arbitrary function Q(x,t,h)
there are "generalized" quantum theories having Q as a quantum potential.