title: The VIP Experiment

abstract:
  The Pauli Exclusion Principle (PEP) is a basic principle of Quantum
Mechanics, and its validity has never been seriously challenged. However, given
its importance, it is very important to check it as thoroughly as possible.
Here we describe the VIP (Violation of PEP) experiment, an improved version of
the Ramberg and Snow experiment (Ramberg and Snow, Phys. Lett. B238 (1990)
438); VIP shall be performed at the Gran Sasso underground laboratories, and
aims to test the Pauli Exclusion Principle for electrons with unprecedented
accuracy, down to $\frac{\beta^2}{2} \sim 10^{-30}$


title:
Robustness of two-way quantum communication protocols against Trojan
  horse attack

abstract:
  We discuss the robustness of two-way quantum communication protocols against
Trojan horse attack and introduce a novel attack, delay-photon Trojan horse
attack. Moreover, we present a practical way for two-way quantum communication
protocols to prevent the eavesdropper from stealing the information transmitted
with Trojan horse attacks. It means that two-way quantum communication
protocols is also secure in a practical application.


title: Coherent State Quantum Key Distribution Without Random Basis Switching

abstract:
  The random switching of measurement bases is commonly assumed to be a
necessary step of quantum key distribution protocols. In this paper we show
that switching is not required for coherent state continuous variable quantum
key distribution. We show this via the no-switching protocol which results in
higher information rates and a simpler experimental setup. We propose an
optimal eavesdropping attack against this protocol, for individual Gaussian
attacks, and we investigate and compare the no-switching protocol applied to
the original BB84 scheme.


title: Lossless Quantum Compression

abstract:
  We describe lossless quantum compression of unknown mixtures (of
non-orthogonal states) and give an expression of the optimal rate of
compression.


title:
Addendum to "Quantum secret sharing between multiparty and multiparty
  without entanglement"

abstract:
  Recently, Yan and Gao proposed a quantum secret sharing protocol between
multiparty ($m$ members in group 1) and multiparty ($n$ members in group 2)
using a sequence of single photons (Phys. Rev. A \textbf{72}, 012304 (2005)).
We find that it is secure if the quantum signal transmitted is only a single
photon but insecure with a multi-photon signal as some agents can get the
information about the others' message if they attack the communication with a
Trojan horse. However, security against this attack can be attained with a
simple modification.


title: Cold atom dynamics in a quantum optical lattice potential

abstract:
  We study a generalized cold atom Bose Hubbard model, where the periodic
optical potential is formed by a cavity field with quantum properties. On the
one hand the common coupling of all atoms to the same mode introduces cavity
mediated long range atom-atom interactions and on the other hand atomic
backaction on the field introduces atom-field entanglement. This modifies the
properties of the associated quantum phase transitions and allows for new
correlated atom-field states including superposition of different atomic
quantum phases. After deriving an approximative Hamiltonian including the new
long range interaction terms we exhibit central physical phenomena at generic
configurations of few atoms in few wells. We find strong modifications of
population fluctuations and next-nearest neighbor correlations near the phase
transition point.


title:
Field-free two-direction alignment alternation of linear molecules by
  elliptic laser pulses

abstract:
  We show that a linear molecule subjected to a short specific elliptically
polarized laser field yields postpulse revivals exhibiting alignment
alternatively located along the orthogonal axis and the major axis of the
ellipse. The effect is experimentally demonstrated by measuring the optical
Kerr effect along two different axes. The conditions ensuring an optimal
field-free alternation of high alignments along both directions are derived.


title: Structure of the Phase in Pure Two-Mode Gaussian States

abstract:
  The two-mode relative phase associated with Gaussian states plays an
important role in quantum information processes in optical, atomic and
electronic systems. In this work, the origin and structure of the two-mode
relative phase in pure Gaussian states is studied in terms of its dependences
on the quadratures of the modes. This is done by constructing local canonical
transformations to an associated two-mode squeezed state. The results are
illustrated by studying the time dependence of the phase under a nonlocal
unitary model evolution containing correlations between the modes. In a more
general context, this approach may allow the two-mode phase to be studied in
situations sensitive to different physical parameters within experimental
configurations relevant to quantum information processing tasks.


title: The quantum world is not built up from correlations

abstract:
  It is known that the global state of a composite quantum system can be
completely determined by specifying correlations between measurements performed
on subsystems only. Despite the fact that the quantum correlations thus suffice
to reconstruct the quantum state, we show, using a Bell inequality argument,
that they cannot be regarded as objective local properties of the composite
system in question. It is well known since the work of J.S. Bell, that one
cannot have locally preexistent values for all physical quantities, whether
they are deterministic or stochastic. The Bell inequality argument we present
here shows this is also impossible for correlations among subsystems of an
individual isolated composite system. Neither of them can be used to build up a
world consisting of some local realistic structure. As a corrolary to the
result we argue that entanglement cannot be considered ontologically robust.
The argument has an important advantage over others because it does not need
perfect correlations but only statistical correlations. It can therefore easily
be tested in currently feasible experiments using four particle entanglement.


title:
A flow-map model for analyzing pseudothresholds in fault-tolerant
  quantum computing

abstract:
  An arbitrarily reliable quantum computer can be efficiently constructed from
noisy components using a recursive simulation procedure, provided that those
components fail with probability less than the fault-tolerance threshold.
Recent estimates of the threshold are near some experimentally achieved gate
fidelities. However, the landscape of threshold estimates includes
pseudothresholds, threshold estimates based on a subset of components and a low
level of recursion. In this paper, we observe that pseudothresholds are a
generic phenomenon in fault-tolerant computation. We define pseudothresholds
and present classical and quantum fault-tolerant circuits exhibiting
pseudothresholds that differ by a factor of 4 from fault-tolerance thresholds
for typical relationships between component failure rates. We develop tools for
visualizing how reliability is influenced by recursive simulation in order to
determine the asymptotic threshold. Finally, we conjecture that refinements of
these methods may establish upper bounds on the fault-tolerance threshold for
particular codes and noise models.


title:
Geometric Phase Based Quantum Computation Applied to an NP-Complete
  Problem

abstract:
  We present a new approach to quantum computation involving the geometric
phase. In this approach, an entire computation is performed by adiabatically
evolving a suitably chosen quantum system in a closed circuit in parameter
space. The problem solved is the determination of the solubility of a 3-SAT
Boolean Satisfiability problem. The problem of non-adiabatic transitions to
higher levels is addressed in several ways. The avoided level crossings are
well defined and the interpolation can be slowed in this region, the
Hamiltonian can be scaled with problem dimension resulting in a constant gap
size and location, and the prescription here is sufficiently general as to
allow for other suitably chosen Hamiltonians. Finally, we show that with $n$
applications of this approach, the geometric phase based quantum computation
method may be used to find the solution to the 3-SAT problem in $n$ variables,
a member of the NP-complete complexity class.


title: Experimental purification of coherent states

abstract:
  We propose a scheme for optimal Gaussian purification of coherent states from
several imperfect copies. The proposal is experimentally demonstrated for the
case of two copies of a coherent state sent through independent noisy channels.
Our purification protocol relies on only linear optics and an ancilla vacuum
state, rendering this approach an interesting alternative to the more complex
protocols of entanglement distillation and quantum error correction.


title: Parametric down-conversion in photonic crystal waveguides

abstract:
  Photonic crystals create dramatic new possibilities for nonlinear optics.
Line defects are shown to support modes suitable for the production of pairs of
photons by the material's second order nonlinearity even if the phase-matching
conditions cannot be satisfied in the bulk. These structures offer the
flexibility to achieve specific dispersion characteristics and potentially very
high brightness. In this work, two phase matching schemes are identified and
analyzed regarding their dispersive properties.


title: Secret parameters in quantum bit commitment

abstract:
  The no-go theorem of unconditionally secure quantum bit commitment depends
crucially on the assumption that Alice knows in detail all the probability
distributions generated by Bob. We show that if a protocol is concealing, then
the cheating unitary transformation is independent of any parameters (including
probability distributions) secretly chosen by Bob, so that Alice can calculate
it without knowing Bob's secret choices. Otherwise the protocol cannot be
concealing. Our result shows that the original impossibility proof was based on
an incorrect assumption, despite the fact that its conclusion remains valid
within the adopted framework. Furthermore, our result eliminates a potential
loophole in the no-go theorem.


title: Controlled Quantum Secret Sharing

abstract:
  We present a new protocol in which a secret multiqubit quantum state
$\ket{\Psi}$ is shared by $n$ players and $m$ controllers, where $\ket{\Psi}$
is the encoding state of a quantum secret sharing scheme. The players may be
considered as field agents responsible for carrying out a task, using the
secret information encrypted in $\ket{\Psi}$, while the controllers are
superiors who decide if and when the task should be carried out and who to do
it. Our protocol only requires ancillary Bell states and Bell-basis
measurements.


title: Inertia of Intrinsic Spin

abstract:
  The state of a particle in space and time is characterized by its mass and
spin, which therefore determine the inertial properties of the particle. The
coupling of intrinsic spin with rotation is examined and the corresponding
inertial effects of intrinsic spin are studied. An experiment to measure
directly the spin-rotation coupling via neutron interferometry is analyzed in
detail.


title: Experimental Entangled Entanglement

abstract:
  All previous tests of local realism have studied correlations between
single-particle measurements. In the present experiment, we have performed a
Bell experiment on three particles in which one of the measurements corresponds
to a projection onto a maximally-entangled state. We show theoretically and
experimentally, that correlations between these entangled measurements and
single-particle measurements are too strong for any local-realistic theory and
are experimentally exploited to violate a CHSH-Bell inequality by more than 5
standard deviations. We refer to this possibility as "entangled entanglement".


title: Decoherence from spin environments

abstract:
  We examine two exactly solvable models of decoherence -- a central
spin-system, (i) with and (ii) without a self--Hamiltonian, interacting with a
collection of environment spins. In the absence of a self--Hamiltonian we show
that in this model (introduced some time ago to illustrate environment--induced
superselection) generic assumptions about the coupling strengths can lead to a
universal (Gaussian) suppression of coherence between pointer states. On the
other hand, we show that when the dynamics of the central spin is dominant a
different regime emerges, which is characterized by a non--Gaussian decay and a
dramatically different set of pointer states. We explore the regimes of
validity of the Gaussian--decay and discuss its relation to the spectral
features of the environment and to the Loschmidt echo (or fidelity).


title:
Dynamic response of interacting one-dimensional fermions in the harmonic
  atom trap: Phase response and the inhomogeneous mobility

abstract:
  The problem of the Kohn mode in bosonized theories of one-dimensional
interacting fermions in the harmonic trap is investigated and a suitable
modification of the interaction is proposed which preserves the Kohn mode. The
modified theory is used to calculate exactly the inhomogeneous linear mobility
at position z in response to a spatial force pulse at another position. It is
found the inhomogeneous particle mobility exhibits resonances not only at the
trap frequency but also at multiples of a new renormalized collective mode
frequency which depends on the strength of the interaction. In contrast, the
local response obtained by averaging over the pulse position remains that of
the non-interacting system.


title:
Atomic density of an harmonically trapped ideal gas near Bose-Einstein
  transition temperature

abstract:
  We have studied the atomic density of a cloud confined in an isotropic
harmonic trap at the vicinity of the Bose-Einstein transition temperature. We
show that, for a non-interacting gas and near this temperature, the
ground-state density has the same order of magnitude as the excited states
density at the centre of the trap. This holds in a range of temperatures where
the ground-state population is negligible compared to the total atom number. We
compare the exact calculations, available in a harmonic trap, to semi-classical
approximations. We show that these latter should include the ground-state
contribution to be accurate..


title: Relative states, quantum axes and quantum references

abstract:
  We address the problem of measuring the relative angle between two "quantum
axes" made out of N1 and N2 spins. Closed forms of our fidelity-like figure of
merit are obtained for an arbitrary number of parallel spins. The asymptotic
regimes of large N1 and/or N2 are discussed in detail. The extension of the
concept "quantum axis" to more general situations is addressed. We give optimal
strategies when the first quantum axis is made out of parallel spins whereas
the second is a general state made out of two spins.


title: Entanglement induced by a two-mode thermal field

abstract:
  The entanglement between two identical two-level atoms interacting with two
mode thermal field through a nondegenerate two-photon process has been
suggested. It has been shown that for some atomic initial state the
entanglement induced by nondegenerate two-photon interaction is larger than
that induced by one-photon and degenerate two-photon processes.


title: Lower bounds on the absorption probability of beam splitters

abstract:
  We derive a lower limit to the amount of absorptive loss present in passive
linear optical devices such as a beam splitter. We choose a particularly simple
beam splitter geometry, a single planar slab surrounded by vacuum, which
already reveals the important features of the theory. It is shown that, using
general causality requirements and statistical arguments, the lower bound
depends on the frequency of the incident light and the transverse resonance
frequency of a suitably chosen single-resonance model only. For symmetric beam
splitters and reasonable assumptions on the resonance frequency $\omega_T$, the
lower absorption bound is $p_{\min}\approx 10^{-6}(\omega/\omega_T)^4$.


title: Phase space contraction and quantum operations

abstract:
  We give a criterion to differentiate between dissipative and diffusive
quantum operations. It is based on the classical idea that dissipative
processes contract volumes in phase space. We define a quantity that can be
regarded as ``quantum phase space contraction rate'' and which is related to a
fundamental property of quantum channels: non-unitality. We relate it to other
properties of the channel and also show a simple example of dissipative noise
composed with a chaotic map. The emergence of attaractor-like structures is
displayed.


title: Factorizations and Physical Representations

abstract:
  A Hilbert space in M dimensions is shown explicitly to accommodate
representations that reflect the prime numbers decomposition of M.
Representations that exhibit the factorization of M into two relatively prime
numbers: the kq representation (J. Zak, Phys. Today, {\bf 23} (2), 51 (1970)),
and related representations termed $q_{1}q_{2}$ representations (together with
their conjugates) are analysed, as well as a representation that exhibits the
complete factorization of M. In this latter representation each quantum number
varies in a subspace that is associated with one of the prime numbers that make
up M.


title: Global Optical Control of a Quantum Spin Chain

abstract:
  Quantum processors which combine the long decoherence times of spin qubits
together with fast optical manipulation of excitons have recently been the
subject of several proposals. I show here that arbitrary single- and entangling
two-qubit gates can be performed in a chain of perpetually coupled spin qubits
solely by using laser pulses to excite higher lying states. It is also
demonstrated that universal quantum computing is possible even if these pulses
are applied {\it globally} to a chain; by employing a repeating pattern of four
distinct qubit units the need for individual qubit addressing is removed. Some
current experimental qubit systems would lend themselves to implementing this
idea.


title: Heat Capacity as A Witness of Entanglement

abstract:
  We demonstrate that the presence of entanglement in macroscopic bodies (e.g.
solids) in thermodynamical equilibrium could be revealed by measuring
heat-capacity. The idea is that if the system were in a separable state, then
for certain Hamiltonians heat capacity would not tend asymptotically to zero as
the temperature approaches absolute zero. Since this would contradict the third
law of thermodynamics, one concludes that the system must contain entanglement.
The separable bounds are obtained by minimization of the heat capacity over
separable states and using its universal low-temperature behavior. Our results
open up a possibility to use standard experimental techniques of solid state
physics -- namely, heat capacity measurements -- to detect entanglement in
macroscopic samples.


title: From Pure Schroedingerian to Statistical Dynamics

abstract:
  Many processes in nature seem to be entirely controlled by transition rates
and the corresponding statistical dynamics. Some of them are in essence
quantum, like the decay of excited states, the tunneling through barriers or
the decay of unstable nuclei. Thus, starting from first principles, those
systems should be analyzed on the basis of the Schroedinger equation. In the
present paper we consider a two level system coupled to an environment which is
basically described by an two-band energy scheme. For appropriately tuned
environment parameters, the excitation probability of the two level system
exhibits statistical dynamics, while the full system follows the coherent,
unitary pure state evolution generated by the Schroedinger equation.


title:
Metric Operator in Pseudo-Hermitian Quantum Mechanics and the Imaginary
  Cubic Potential

abstract:
  We present a systematic perturbative construction of the most general metric
operator (and positive-definite inner product) for quasi-Hermitian Hamiltonians
of the standard form, H= p^2/2 + v(x), in one dimension. We show that this
problem is equivalent to solving an infinite system of iteratively decoupled
hyperbolic partial differential equations in (1+1)-dimensions. For the case
that v(x) is purely imaginary, the latter have the form of a nonhomogeneous
wave equation which admits an exact solution. We apply our general method to
obtain the most general metric operator for the imaginary cubic potential,
v(x)=i \epsilon x^3. This reveals an infinite class of previously unknown CPT-
as well as non-CPT-inner products. We compute the physical observables of the
corresponding unitary quantum system and determine the underlying classical
system. Our results for the imaginary cubic potential show that, unlike the
quantum system, the corresponding classical system is not sensitive to the
choice of the metric operator. As another application of our method we give a
complete characterization of the pseudo-Hermitian canonical quantization of a
free particle moving in real line that is consistent with the usual choice for
the quantum Hamiltonian. Finally we discuss subtleties involved with higher
dimensions and systems having a fixed length scale.


title:
Realization of a Decoherence-free, Optimally Distinguishable Mesoscopic
  Quantum Superposition

abstract:
  We report the realization of an entangled quantum superposition of M=12
photons by a high gain, quantum-injected optical parametric amplification. The
system is found so highly resilient against decoherence to exhibit directly
accessible mesoscopic interference effects at normal temperature. By modern
tomographic methods the non-separability and the quantum superposition are
demonstrated for the overall mesoscopic output state of the dynamic ''closed
system''. The device realizes the condition conceived by Erwin Schroedinger
with his 1935 paradigmatic ''Cat'' apologue, a fundamental landmark in quantum
mechanics.


title:
Information transmission via entangled quantum states in Gaussian
  channels with memory

abstract:
  Gaussian quantum channels have recently attracted a growing interest, since
they may lead to a tractable approach to the generally hard problem of
evaluating quantum channel capacities. However, the analysis performed so far
has always been restricted to memoryless channels. Here, we consider the case
of a bosonic Gaussian channel with memory, and show that the classical capacity
can be significantly enhanced by employing entangled input symbols instead of
product symbols.


title:
Entanglement, Teleportation, And Single Photon Storage Using Two Level
  Atoms Inside An Optical Parametric Oscillator

abstract:
  I consider several interesting aspects of a new light source, a two-level
atom, or N two-level atoms inside an Optical Parametric Oscillator. We find
that in the weak driving limit, detection of a transmitted or fluorescent
photon generates a highly entangled state of the atom and the cavity. This
entanglement can be used with beamsplitters to create more complex quantum
states and implement teleportation protocols. Also, one can store a single
photon in the atoms, along the lines of recent slow and stopped light proposals
and experiments.


title: Mechanical similarity as a generalization of scale symmetry

abstract:
  In this paper we study the symmetry known as mechanical similarity (LMS) and
present for any monomial potential. We analyze it in the framework of the
Koopman-von Neumann formulation of classical mechanics and prove that in this
framework the LMS can be given a canonical implementation. We also show that
the LMS is a generalization of the scale symmetry which is present only for the
inverse square potential. Finally we study the main obstructions which one
encounters in implementing the LMS at the quantum mechanical level.


title:
A new quantum lower bound method, with an application to strong direct
  product theorem for quantum search

abstract:
  We present a new method for proving lower bounds on quantum query algorithms.
The new method is an extension of adversary method, by analyzing the eigenspace
structure of the problem.
  Using the new method, we prove a strong direct product theorem for quantum
search. This result was previously proven by Klauck, Spalek and de Wolf
(quant-ph/0402123) using polynomials method. No proof using adversary method
was known before.


title: Entanglement in Interactive Proof Systems with Binary Answers

abstract:
  If two classical provers share an entangled state, the resulting interactive
proof system is significantly weakened [quant-ph/0404076]. We show that for the
case where the verifier computes the XOR of two binary answers, the resulting
proof system is in fact no more powerful than a system based on a single
quantum prover: +MIP*[2] is contained in QIP(2). This also implies that
+MIP*[2] is contained in EXP which was previously shown using a different
method [Presentation of Cleve et al. at CCC'04]. This contrasts with an
interactive proof system where the two provers do not share entanglement. In
that case, +MIP[2] = NEXP for certain soundness and completeness parameters
[quant-ph/0404076].


title: Photon wave function

abstract:
  Photon wave function is a controversial concept. Controversies stem from the
fact that photon wave functions can not have all the properties of the
Schroedinger wave functions of nonrelativistic wave mechanics. Insistence on
those properties that, owing to peculiarities of photon dynamics, cannot be
rendered, led some physicists to the extreme opinion that the photon wave
function does not exist. I reject such a fundamentalist point of view in favor
of a more pragmatic approach. In my view, the photon wave function exists as
long as it can be precisely defined and made useful.


title: Note on the 8$_{18}$-Knot

abstract:
  We define some physical variables associated with the traversing sequences of
electrons along the orbit which is a 2D projection of 8$_{18}$-knot. The
configuration is regular but the resulting contributions, which are related to
the physical variable, of those combinations from all the possible states to
the fixed spatial sites show certain irregular behavior near the over- or
under-crossing points of this knot. The possible explanation for this kind of
direct geometric consequences is made to linked to the physical insight.


title:
Resonant States of Wave Propagation in Disordered System of Bosonic
  Particles

abstract:
  We demonstrate the effects of an induced disorder (or a free-orientation :
$\theta$ which is related to the relative direction of scattering of particles
w.r.t. to the normal of the propagating plane-wave front) upon the possible
resonance of the plane (sound) wave propagating in Bose gases by using the
quantum kinetic equations. We firstly present the diverse dispersion relations
obtained by the relevant Pauli-blocking parameter $B$ (which describes the Bose
particles when $B$ is positive) and the free-orientation $\theta$ and then,
based on the acoustic analog, address the possible resonant states.


title: Quantum Algorithms for Matching and Network Flows

abstract:
  We present quantum algorithms for the following graph problems: finding a
maximal bipartite matching in time O(n sqrt{m+n} log n), finding a maximal
non-bipartite matching in time O(n^2 (sqrt{m/n} + log n) log n), and finding a
maximal flow in an integer network in time O(min(n^{7/6} sqrt m * U^{1/3},
sqrt{n U} m) log n), where n is the number of vertices, m is the number of
edges, and U <= n^{1/4} is an upper bound on the capacity of an edge.


title:
Modified Bennett-Brassard 1984 Quantum Key Distribution With Two-way
  Classical Communications

abstract:
  The quantum key distribution protocol without public announcement of bases is
equipped with a two-way classical communication symmetric entanglement
purification protocol. This modified key distribution protocol is
unconditionally secure and has a higher tolerable error rate of 20%, which is
higher than previous scheme without public announcement of bases.


title:
Density Matrix in Quantum Mechanics and Distinctness of Ensembles Having
  the Same Compressed Density Matrix

abstract:
  We clarify different definitions of the density matrix by proposing the use
of different names, the full density matrix for a single-closed quantum system,
the compressed density matrix for the averaged single molecule state from an
ensemble of molecules, and the reduced density matrix for a part of an
entangled quantum system, respectively. We show that ensembles with the same
compressed density matrix can be physically distinguished by observing
fluctuations of various observables. This is in contrast to a general belief
that ensembles with the same compressed density matrix are identical. Explicit
expression for the fluctuation of an observable in a specified ensemble is
given. We have discussed the nature of nuclear magnetic resonance quantum
computing. We show that the conclusion that there is no quantum entanglement in
the current nuclear magnetic resonance quantum computing experiment is based on
the unjustified belief that ensembles having the same compressed density matrix
are identical physically. Related issues in quantum communication are also
discussed.


title: Entanglement properties in (1/2,1) mixed-spin Heisenberg systems

abstract:
  By using the concept of negativity, we investigate entanglement in (1/2,1)
mixed-spin Heisenberg systems. We obtain the analytical results of entanglement
in small isotropic Heisenberg clusters with only nearest-neighbor (NN)
interactions up to four spins and in the four-spin Heisenberg model with both
NN and next-nearest-neighbor (NNN) interactions. For more spins, we numerically
study effects of temperature, magnetic fields, and NNN interactions on
entanglement. We study in detail the threshold value of the temperature, after
which the negativity vanishes.


title:
Simultaneous amplitude and phase damping of a kind of Gaussian states
  and their separability

abstract:
  We give out the time evolution solution of simultaneous amplitude and phase
damping for any continuous variable state. For the simultaneous amplitude and
phase damping of a wide class of two- mode entangled Gaussian states, two
analytical conditions of the separability are given. One is the sufficient
condition of separability. The other is the condition of PPT separability where
the Peres-Horodecki criterion is applied. Between the two conditions there may
exist bound entanglement. The simplest example is the simultaneous amplitude
and phase damping of a two-mode squeezed vacuum state. The damped state is
non-Gaussian.


title:
Bell inequalities stronger than the CHSH inequality for 3-level
  isotropic states

abstract:
  We show that some two-party Bell inequalities with two-valued observables are
stronger than the CHSH inequality for 3 \otimes 3 isotropic states in the sense
that they are violated by some isotropic states in the 3 \otimes 3 system that
do not violate the CHSH inequality. These Bell inequalities are obtained by
applying triangular elimination to the list of known facet inequalities of the
cut polytope on nine points. This gives a partial solution to an open problem
posed by Collins and Gisin. The results of numerical optimization suggest that
they are candidates for being stronger than the I_3322 Bell inequality for 3
\otimes 3 isotropic states. On the other hand, we found no Bell inequalities
stronger than the CHSH inequality for 2 \otimes 2 isotropic states. In
addition, we illustrate an inclusion relation among some Bell inequalities
derived by triangular elimination.


title: Information processing in generalized probabilistic theories

abstract:
  I introduce a framework in which a variety of probabilistic theories can be
defined, including classical and quantum theories, and many others. From two
simple assumptions, a tensor product rule for combining separate systems can be
derived. Certain features, usually thought of as specifically quantum, turn out
to be generic in this framework, meaning that they are present in all except
classical theories. These include the non-unique decomposition of a mixed state
into pure states, a theorem involving disturbance of a system on measurement
(suggesting that the possibility of secure key distribution is generic), and a
no-cloning theorem. Two particular theories are then investigated in detail,
for the sake of comparison with the classical and quantum cases. One of these
includes states that can give rise to arbitrary non-signalling correlations,
including the super-quantum correlations that have become known in the
literature as Nonlocal Machines or Popescu-Rohrlich boxes. By investigating
these correlations in the context of a theory with well-defined dynamics, I
hope to make further progress with a question raised by Popescu and Rohrlich,
which is, why does quantum theory not allow these strongly nonlocal
correlations? The existence of such correlations forces much of the dynamics in
this theory to be, in a certain sense, classical, with consequences for
teleportation, cryptography and computation. I also investigate another theory
in which all states are local. Finally, I raise the question of what further
axiom(s) could be added to the framework in order uniquely to identify quantum
theory, and hypothesize that quantum theory is optimal for computation.


title: Comment on "Quantum Physics from A to Z"

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


title: Localization for free ultrametric quantum particle

abstract:
  Free evolution for quantum particle in general ultrametric space is
considered. We find that if mean zero wave packet is localized in some ball in
the ultrametric space then its evolution remains localized in the same ball.


title: Pseudo-Hermiticity, PT-symmetry, and the Metric Operator

abstract:
  The main achievements of Pseudo-Hermitian Quantum Mechanics and its
distinction with the indefinite-metric quantum theories are reviewed. The issue
of the non-uniqueness of the metric operator and its consequences for defining
the observables are discussed. A systematic perturbative expression for the
most general metric operator is offered and its application for a toy model is
outlined.


title:
Generation of 1.5-um band time-bin entanglement using spontaneous fiber
  four-wave mixing and planar lightwave circuit interferometers

abstract:
  This paper reports 1.5-um band time-bin entanglement generation. We employed
a spontaneous four-wave mixing process in a dispersion shifted fiber, with
which correlated photon pairs with very narrow bandwidths were generated
efficiently. To observe two-photon interference, we used planar lightwave
circuit based interferometers that were operated stably without feedback
control. As a result, we obtained coincidence fringes with 99 % visibilities
after subtracting accidental coincidences, and successfully distributed
entangled photons over 20-km standard single-mode fiber without any
deterioration in the quantum correlation.


title:
First-order intertwining operators and position-dependent mass
  Schrodinger equations in d dimensions

abstract:
  The problem of d-dimensional Schrodinger equations with a position-dependent
mass is analyzed in the framework of first-order intertwining operators. With
the pair (H, H_1) of intertwined Hamiltonians one can associate another pair of
second-order partial differential operators (R, R_1), related to the same
intertwining operator and such that H (resp. H_1) commutes with R (resp. R_1).
This property is interpreted in superalgebraic terms in the context of
supersymmetric quantum mechanics (SUSYQM). In the two-dimensional case, a
solution to the resulting system of partial differential equations is obtained
and used to build a physically-relevant model depicting a particle moving in a
semi-infinite layer. Such a model is solved by employing either the
commutativity of H with some second-order partial differential operator L and
the resulting separability of the Schrodinger equation or that of H and R
together with SUSYQM and shape-invariance techniques. The relation between both
approaches is also studied.


title: Quantum noise and stochastic reduction

abstract:
  In standard nonrelativistic quantum mechanics the expectation of the energy
is a conserved quantity. It is possible to extend the dynamical law associated
with the evolution of a quantum state consistently to include a nonlinear
stochastic component, while respecting the conservation law. According to the
dynamics thus obtained, referred to as the energy-based stochastic Schrodinger
equation, an arbitrary initial state collapses spontaneously to one of the
energy eigenstates, thus describing the phenomenon of quantum state reduction.
In this article, two such models are investigated: one that achieves state
reduction in infinite time, and the other in finite time. The properties of the
associated energy expectation process and the energy variance process are
worked out in detail. By use of a novel application of a nonlinear filtering
method, closed-form solutions--algebraic in character and involving no
integration--are obtained for both these models. In each case, the solution is
expressed in terms of a random variable representing the terminal energy of the
system, and an independent noise process. With these solutions at hand it is
possible to simulate explicitly the dynamics of the quantum states of
complicated physical systems.


title:
Repeat-Until-Success quantum computing using stationary and flying
  qubits

abstract:
  We introduce an architecture for robust and scalable quantum computation
using both stationary qubits (e.g. single photon sources made out of trapped
atoms, molecules, ions, quantum dots, or defect centers in solids) and flying
qubits (e.g. photons). Our scheme solves some of the most pressing problems in
existing non-hybrid proposals, which include the difficulty of scaling
conventional stationary qubit approaches, and the lack of practical means for
storing single photons in linear optics setups. We combine elements of two
previous proposals for distributed quantum computing, namely the efficient
photon-loss tolerant build up of cluster states by Barrett and Kok [Phys. Rev.
A 71, 060310(R) (2005)] with the idea of Repeat-Until-Success (RUS) quantum
computing by Lim et al. [Phys. Rev. Lett. 95, 030505 (2005)]. This idea can be
used to perform eventually deterministic two-qubit logic gates on spatially
separated stationary qubits via photon pair measurements. Under non-ideal
conditions, where photon loss is a possibility, the resulting gates can still
be used to build graph states for one-way quantum computing. In this paper, we
describe the RUS method, present possible experimental realizations, and
analyse the generation of graph states.


title: A Representation of Real and Complex Numbers in Quantum Theory

abstract:
  A quantum theoretic representation of real and complex numbers is described
here as equivalence classes of Cauchy sequences of quantum states of finite
strings of qubits. There are 4 types of qubits each with associated single
qubit annihilation creation (a-c) operators that give the state and location of
each qubit type on a 2 dimensional integer lattice. The string states, defined
as finite products of creation operators acting on the qubit vacuum state
correspond to complex rational numbers with real and imaginary components.
These states span a Fock space. Arithmetic relations and operations are defined
for the string states. Cauchy sequences of these states are defined, and the
arithmetic relations and operations lifted to apply to these sequences. Based
on these, equivalence classes of these sequences are seen to have the requisite
properties of real and complex numbers. The representations have some
interesting aspects. Quantum equivalence classes are larger than their
corresponding classical classes, but no new classes are created. There exist
Cauchy sequences such that each state in the sequence is an entangled
superposition of the real and imaginary components, yet the sequence is a real
number. Also, except for coefficients of superposition states, the construction
is done with no reference to the real and complex number base, R and C, of the
Fock space.


title:
Observation of polarization quantum noise of laser radiation in Rb vapor
  cell

abstract:
  We present experimental study of polarization quantum noise of laser
radiation passed through optically think vapor of Rb87. We observe a step-like
noise spectrum. We discuss various factor which may result in such noise
spectrum and prevent observation of squeezing of quantum fluctuations predicted
in Matsko et al. PRA 63, 043814 (2001).


title:
Implications of Qudit Superselection rules for the Theory of
  Decoherence-free Subsystems

abstract:
  The use of d-state systems, or qudits, in quantum information processing is
discussed. Three-state and higher dimensional quantum systems are known to have
very different properties from two-state systems, i.e., qubits. In particular
there exist qudit states which are not equivalent under local unitary
transformations unless a selection rule is violated. This observation is shown
to be an important factor in the theory of decoherence-free, or noiseless,
subsystems. Experimentally observable consequences and methods for
distinguishing these states are also provided, including the explicit
construction of new decoherence-free or noiseless subsystems from qutrits.
Implications for simulating quantum systems with quantum systems are also
discussed.


title: Cryptography In the Bounded Quantum-Storage Model

abstract:
  We initiate the study of two-party cryptographic primitives with
unconditional security, assuming that the adversary's quantum memory is of
bounded size. We show that oblivious transfer and bit commitment can be
implemented in this model using protocols where honest parties need no quantum
memory, whereas an adversarial player needs quantum memory of size at least n/2
in order to break the protocol, where n is the number of qubits transmitted.
This is in sharp contrast to the classical bounded-memory model, where we can
only tolerate adversaries with memory of size quadratic in honest players'
memory size. Our protocols are efficient, non-interactive and can be
implemented using today's technology. On the technical side, a new entropic
uncertainty relation involving min-entropy is established.


title:
A multi-mode model of a non-classical atom laser produced by outcoupling
  from a Bose-Einstein condensate with squeezed light

abstract:
  We examine the properties of an atom laser produced by outcoupling from a
Bose-Einstein condensate with squeezed light. We introduce a method which
allows us to model the full multimode dynamics of the squeezed optical field
and the outcoupled atoms. We show that for experimentally reasonable parameters
that the quantum statistics of the optical field are almost completely
transferred to the outcoupled atoms, and investigate the robustness to the
coupling strength and the two-photon detuning.


title:
On the evaluation of the ac-Stark shift and Kramers-Heisenberg
  dispersion formula

abstract:
  We describe a unified approach for the determination of ac-Stark correction
and Kramers-Heisenberg dispersion formula. In both cases the contribution from
infinite intermediate summation appearing in the expression for the
corresponding matrix elements are evaluated exactly in the dipole approximation
for the ground state of hydrogen atom using a variation of the Dalgarno-Lewis
method. The analytical expressions obtained can be efficiently used for the
numerical evaluation of matrix element for all values of incident photon energy
and comparison is made with results obtained by different methods.


title: A Topos Perspective on State-Vector Reduction

abstract:
  A preliminary investigation is made of possible applications in quantum
theory of the topos formed by the collection of all $M$-sets, where $M$ is a
monoid. Earlier results on topos aspects of quantum theory can be rederived in
this way. However, the formalism also suggests a new way of constructing a
`neo-realist' interpretation of quantum theory in which the truth values of
propositions are determined by the actions of the monoid of strings of finite
projection operators. By these means, a novel topos perspective is gained on
the concept of state-vector reduction.


title: Approximations by graphs and emergence of global structures

abstract:
  We study approximations of billiard systems by lattice graphs. It is
demonstrated that under natural assumptions the graph wavefunctions approximate
solutions of the Schroedinger equation with energy rescaled by the billiard
dimension. As an example, we analyze a Sinai billiard with attached leads. The
results illustrate emergence of global structures in large quantum graphs and
offer interesting comparisons with patterns observed in complex networks of a
different nature.


title:
Hilbert-Schmidt Geometry of n-Level Jakobczyk-Siennicki Two-Dimensional
  Quantum Systems

abstract:
  Jakobczyk and Siennicki studied two-dimensional sections of a set of
(generalized) Bloch vectors corresponding to n x n density matrices of
two-qubit systems (that is, the case n = 4). They found essentially five
different types of (nontrivial) separability regimes. We compute the
Euclidean/Hilbert-Schmidt (HS) separability probabilities assigned to these
regimes, and conduct parallel two-dimensional sectional analyses for the
higher-level cases n=6,8,9 and 10. Making use of the newly-introduced
capability for integration over implicitly defined regions of version 5.1 of
Mathematica -- also fruitfully used in our n=4 three-parameter
entropy-maximization-based study quant-ph/0507203 -- we obtain a wide-ranging
variety of exact HS-probabilities. For n>6, the probabilities are those of
having a partial positive transpose (PPT). For the n=6 case, we also obtain
biseparability probabilities; in the n=8,9 instances, bi-PPT probabilities; and
for n=8, tri-PPT probabilities. By far, the most frequently recorded
probability for n>4 is \pi/4 = 0.785398. We also conduct a number of related
analyses, pertaining to the (one-dimensional) boundaries (both exterior and
interior) of the separability and PPT domains, and attempt (with limited
success) some exact calculations pertaining to the 9-dimensional (real) and
15-dimensional (complex) convex sets of two-qubit density matrices -- for which
HS-separability probabilities have been conjectured, but not verified.


title: Decoherence by Correlated Noise and Quantum Error Correction

abstract:
  We study the decoherence of a quantum computer in an environment which is
inherently correlated in time and space. We first derive the nonunitary time
evolution of the computer and environment in the presence of a stabilizer error
correction code, providing a general way to quantify decoherence for a quantum
computer. The general theory is then applied to the spin-boson model. Our
results demonstrate that effects of long-range correlations can be
systematically reduced by small changes in the error correction codes.


title:
Precision characterisation of two-qubit Hamiltonians via entanglement
  mapping

abstract:
  We show that the general Heisenberg Hamiltonian with non-uniform couplings
can be characterised by mapping the entanglement it generates as a function of
time. Identification of the Hamiltonian in this way is possible as the
coefficients of each operator control the oscillation frequencies of the
entanglement function. The number of measurements required to achieve a given
precision in the Hamiltonian parameters is determined and an efficient
measurement strategy designed. We derive the relationship between the number of
measurements, the resulting precision and the ultimate discrete error
probability generated by a systematic mis-characterisation, when implementing
two-qubit gates for quantum computing.


title: On Spontaneous Wave Function Collapse and Quantum Field Theory

abstract:
  One way of obtaining a version of quantum mechanics without observers, and
thus of solving the paradoxes of quantum mechanics, is to modify the
Schroedinger evolution by implementing spontaneous collapses of the wave
function. An explicit model of this kind was proposed in 1986 by Ghirardi,
Rimini, and Weber (GRW), involving a nonlinear, stochastic evolution of the
wave function. We point out how, by focussing on the essential mathematical
structure of the GRW model and a clear ontology, it can be generalized to
(regularized) quantum field theories in a simple and natural way.


title: Detecting multimode entanglement by symplectic uncertainty relations

abstract:
  A hierarchy of multimode uncertainty relations on the second moments of n
pairs of canonical operators is derived in terms of quantities invariant under
linear canonical (i.e. symplectic) transformations. Conditions for the
separability of multimode continuous variable states are derived from the
uncertainty relations, generalizing the inequalities obtained in [Phys. Rev.
Lett. 96, 110402 (2006)] to states with some transposed symplectic eigenvalues
equal to 1. Finally, to illustrate the methodology proposed for the detection
of continuous variable entanglement, the separability of multimode noisy
GHZ-like states is analysed in detail with the presented techniques, deriving a
necessary and sufficient condition for the separability of such states under an
`even' bipartition of the modes.


title: High fidelity measurement and quantum feedback control in circuit QED

abstract:
  Circuit QED is a promising solid-state quantum computing architecture. It
also has excellent potential as a platform for quantum control -- especially
quantum feedback control -- experiments. However, the current scheme for
measurement in circuit QED is low efficiency and has low signal-to-noise ratio
for single shot measurements. The low quality of this measurement makes the
implementation of feedback difficult, and here we propose two schemes for
measurement in circuit QED architectures that can significantly improve
signal-to-noise, and potentially achieve quantum limited measurement. Such
measurements would enable the implementation of quantum feedback protocols and
we illustrate this with a simple entanglement stabilization scheme.


title: Bit Commitment from Weak Non-Locality

abstract:
  So-called non-local boxes, which have been introduced as an idealization-in
different respects-of the behavior of entangled quantum states, have been known
to allow for unconditional bit commitment between the two involved parties. We
show that, actually, any possible non-local correlation which produces random
bits on both sides can be used to implement bit commitment, and that this holds
even when the parties are allowed to delay their inputs to the box. Since a
particular example is the behavior of an EPR pair, this resource allows for
implementing unconditionally secure bit commitment as long as the parties
cannot entangle their Qbits with any other system.


title: Description of Quantum Entanglement with Nilpotent Polynomials

abstract:
  We propose a general method for introducing extensive characteristics of
quantum entanglement. The method relies on polynomials of nilpotent raising
operators that create entangled states acting on a reference vacuum state. By
introducing the notion of tanglemeter, the logarithm of the state vector
represented in a special canonical form and expressed via polynomials of
nilpotent variables, we show how this description provides a simple criterion
for entanglement as well as a universal method for constructing the invariants
characterizing entanglement. We compare the existing measures and classes of
entanglement with those emerging from our approach. We derive the equation of
motion for the tanglemeter and, in representative examples of up to four-qubit
systems, show how the known classes appear in a natural way within our
framework. We extend our approach to qutrits and higher-dimensional systems,
and make contact with the recently introduced idea of generalized entanglement.
Possible future developments and applications of the method are discussed.


title:
Local uncertainty relations serving as measures of entanglement in a
  bipartite two-level system

abstract:
  We comment on the recent suggestion to use a family of local uncertainty
relations as a standard way of quantifying entanglement in two-qubit systems.
Some statements made on the applicability of the proposed "measures" are overly
optimistic. We exemplify how these specific "measures" fall short, and present
a minor modification of the general theory which uses the same experimentally
gathered information, but in a slightly different, better way.


title:
Local Measures of Entanglement and Critical Exponents at Quantum Phase
  Transitions

abstract:
  We discuss on general grounds some local indicators of entanglement, that
have been proposed recently for the study and classification of quantum phase
transitions. In particular, we focus on the capability of entanglement in
detecting quantum critical points and related exponents. We show that the
singularities observed in all local measures of entanglement are a consequence
of the scaling hypothesis. In particular, as every non-trivial local observable
is expected to be singular at criticality, we single out the most relevant one
(in the renormalization group sense) as the best-suited for finite-size scaling
analysis. The proposed method is checked on a couple of one-dimensional spin
systems. The present analysis shows that the singular behaviour of local
measures of entanglement is fully encompassed in the usual statistical
mechanics framework.


title: Quantum Pattern Matching

abstract:
  We propose a quantum algorithm for closest pattern matching which allows us
to search for as many distinct patterns as we wish in a given string
(database), requiring a query function per symbol of the pattern alphabet. This
represents a significant practical advantage when compared to Grover's search
algorithm as well as to other quantum pattern matching methods, which rely on
building specific queries for particular patterns. Our method makes arbitrary
searches on long static databases much more realistic and implementable. Our
algorithm, inspired by Grover's, returns the position of the closest substring
to a given pattern of size $M$ with non-negligible probability in $O(\sqrt{N})$
queries, where $N$ is the size of the string. Furthermore, we give the full
recipe to implement our algorithm (together with its total circuit complexity),
thus offering an oracle-based quantum algorithm ready to be implemented.


title: Correlations in Entangled States

abstract:
  Entangled EPR spin pairs can be treated using the statistical ensemble
interpretation of quantum mechanics. As such the singlet state results from an
ensemble of spin pairs each with its own specific axis of quantization. This
axis acts like a quantum mechanical hidden variable. If the spins lose
coherence they disentangle into a mixed state that contains classical
correlations. In this paper an infinitesimal phase decoherence is introduced to
the singlet state in order to reveal more clearly some of the correlations. It
is shown that a singlet state has no classical correlations.


title:
Locally observable conditions for the successful implementation of
  entangling multi-qubit quantum gates

abstract:
  The information obtained from the operation of a quantum gate on only two
complementary sets of input states is sufficient to estimate the quantum
process fidelity of the gate. In the case of entangling gates, these conditions
can be used to predict the multi qubit entanglement capability from the
fidelities of two non-entangling local operations. It is then possible to
predict highly non-classical features of the gate such as violations of local
realism from the fidelities of two completely classical input-output relations,
without generating any actual entanglement.


title: On single-copy entanglement

abstract:
  The largest eigenvalue of the reduced density matrix for quantum chains is
shown to have a simple physical interpretation and power-law behaviour in
critical systems. This is verified numerically for XXZ spin chains.


title: Entanglement in resonant photon scattering

abstract:
  Single--photon which is initially uncorrelated with atom, will evolve to be
entangled with the atom on their continuous kinetic variables in the process of
resonant scattering. We find the relations between the entanglement and their
physical control parameters, which indicates that high entanglement can be
reached by broadening the scale of the atomic wave or squeezing the linewidth
of the incident single--photon pulse.


title:
A perturbative treatment for the exponential-cosine-screened Coulomb
  potential

abstract:
  An alternative approximation scheme has been used in solving the Schroedinger
equation for the exponential-cosine-screened Coulomb potential. The bound state
energies for various eigenstates and the corresponding wave functions are
obtained analytically up to the second perturbation term.


title:
Generation of Atomic Cluster States through the Cavity Input-Output
  Process

abstract:
  We propose a scheme to implement a two-qubit controlled-phase gate for single
atomic qubits, which works in principle with nearly ideal success probability
and fidelity. Our scheme is based on the cavity input-output process and the
single photon polarization measurement. We show that, even with the practical
imperfections such as atomic spontaneous emission, weak atom-cavity coupling,
violation of the Lamb-Dicke condition, cavity photon loss, and detection
inefficiency, the proposed gate is feasible for generation of a cluster state
in that it meets the scalability criterion and it operates in a conclusive
manner. We demonstrate a simple and efficient process to generate a cluster
state with our high probabilistic entangling gate.


title:
Partial scaling transform of multiqubit states as a criterion of
  separability

abstract:
  The partial scaling transform of the density matrix for multiqubit states is
introduced to detect entanglement of quantum states. The transform contains
partial transposition as a special case. The scaling transform corresponds to
partial time scaling of subsystem (or partial Planck's constant scaling) which
was used to formulate recently separability criterion for continous variables.A
measure of entanglement which is a generalization of negativity measure is
introduced being based on tomographic probability description of spin states.


title: Decay of Loschmidt Echo Enhanced by Quantum Criticality

abstract:
  We study the transition of a quantum system $S $ from a pure state to a mixed
one, which is induced by the quantum criticality of the surrounding system $E$
coupled to it. To characterize this transition quantitatively, we carefully
examine the behavior of the Loschmidt echo (LE) of $E$ modelled as an Ising
model in a transverse field, which behaves as a measuring apparatus in quantum
measurement. It is found that the quantum critical behavior of $E$ strongly
affects its capability of enhancing the decay of LE: near the critical value of
the transverse field entailing the happening of quantum phase transition, the
off-diagonal elements of the reduced density matrix describing $S$ vanish
sharply.


title: On the structure of the body of states with positive partial transpose

abstract:
  We show that the convex set of separable mixed states of the 2 x 2 system is
a body of constant height. This fact is used to prove that the probability to
find a random state to be separable equals 2 times the probability to find a
random boundary state to be separable, provided the random states are generated
uniformly with respect to the Hilbert-Schmidt (Euclidean) distance. An
analogous property holds for the set of positive-partial-transpose states for
an arbitrary bipartite system.


title:
Thermal entanglement in a two-qubit Heisenberg XXZ spin chain under an
  inhomogeneous magnetic field

abstract:
  The thermal entanglement in a two-qubit Heisenberg \emph{XXZ} spin chain is
investigated under an inhomogeneous magnetic field \emph{b}. We show that the
ground-state entanglement is independent of the interaction of
\emph{z}-component $J_{z}$. The thermal entanglement at the fixed temperature
can be enhanced when $J_{z}$ increases. We strictly show that for any
temperature \emph{T} and $J_{z}$ the entanglement is symmetric with respect to
zero inhomogeneous magnetic field, and the critical inhomogeneous magnetic
field $b_{c}$ is independent of $J_{z}$. The critical magnetic field $B_{c}$
increases with the increasing $|b|$ but the maximum entanglement value that the
system can arrive becomes smaller.


title:
A microscopic derivation of the quantum mechanical formal scattering
  cross section

abstract:
  We prove that the empirical distribution of crossings of a "detector''
surface by scattered particles converges in appropriate limits to the
scattering cross section computed by stationary scattering theory. Our result,
which is based on Bohmian mechanics and the flux-across-surfaces theorem, is
the first derivation of the cross section starting from first microscopic
principles.


title: A photonic quantum information interface

abstract:
  Quantum communication is the art of transferring quantum states, or quantum
bits of information (qubits), from one place to another. On the fundamental
side, this allows one to distribute entanglement and demonstrate quantum
nonlocality over significant distances. On the more applied side, quantum
cryptography offers, for the first time in human history, a provably secure way
to establish a confidential key between distant partners. Photons represent the
natural flying qubit carriers for quantum communication, and the presence of
telecom optical fibres makes the wavelengths of 1310 and 1550 nm particulary
suitable for distribution over long distances. However, to store and process
quantum information, qubits could be encoded into alkaline atoms that absorb
and emit at around 800 nm wavelength. Hence, future quantum information
networks made of telecom channels and alkaline memories will demand interfaces
able to achieve qubit transfers between these useful wavelengths while
preserving quantum coherence and entanglement. Here we report on a qubit
transfer between photons at 1310 and 710 nm via a nonlinear up-conversion
process with a success probability greater than 5%. In the event of a
successful qubit transfer, we observe strong two-photon interference between
the 710 nm photon and a third photon at 1550 nm, initially entangled with the
1310 nm photon, although they never directly interacted. The corresponding
fidelity is higher than 98%.


title: Quantum Localization

abstract:
  The auxiliary q-rules of quantum mechanics developed in other papers are
applied to the problem of the location of material objects, both macroscopic
and microscopic. All objects tend to expand in space due to the uncertainty in
their momentum. The q-rules are found to oppose this perpetual expansion with a
collapse mechanism that insures the dependable localization of objects in
ordinary human experience.


title:
Limits on entanglement in rotationally-invariant scattering of spin
  systems

abstract:
  This paper investigates the dynamical generation of entanglement in
scattering systems, in particular two spin systems that interact via
rotationally-invariant scattering. The spin degrees of freedom of the in-states
are assumed to be in unentangled, pure states, as defined by the entropy of
entanglement. Because of the restriction of rotationally-symmetric
interactions, perfectly-entangling S-matrices, i.e. those that lead to a
maximally entangled out-state, only exist for a certain class of separable
in-states. Using Clebsch-Gordan coefficients for the rotation group, the
scattering phases that determine the S-matrix are determined for the case of
spin systems with $\sigma = 1/2$, 1, and 3/2.


title:
Thermal entanglement in a two-spin-qutrit system under a nonuniform
  external magnetic field

abstract:
  The thermal entanglement in a two-spin-qutrit system with two spins coupled
by exchange interaction under a magnetic field in an arbitrary direction is
investigated. Negativity, the measurement of entanglement, is calculated. We
find that for any temperature the evolvement of negativity is symmetric with
respect to magnetic field. The behavior of negativity is presented for four
different cases. The results show that for different temperature, different
magnetic field give maximum entanglement. Both the parallel and antiparallel
magnetic field cases are investigated qualitatively (not quantitatively) in
detail, we find that the entanglement may be enhanced under an antiparallel
magnetic field.


title: Non-local properties of a symmetric two-qubit system

abstract:
  Non-local properties of symmetric two-qubit states are quantified in terms of
a complete set of entanglement invariants. We prove that negative values of
some of the invariants are signatures of quantum entanglement. This leads us to
identify sufficient conditions for non-separability in terms of entanglement
invariants. Non-local properties of two-qubit states extracted from (i) Dicke
state (ii) state generated by one-axis twisting Hamiltonian, and (iii)
one-dimensional Ising chain with nearest neighbour interaction are analyzed in
terms of the invariants characterizing them.


title: A dynamical model for quantum memory channels

abstract:
  A dynamical model for quantum channel is introduced which allows one to pass
continuously from the memoryless case to the case in which memory effects are
present. The quantum and classical communication rates of the model are defined
and explicit expression are provided in some limiting case. In this context we
introduce noise attenuation strategies where part of the signals are sacrificed
to modify the channel environment. The case of qubit channel with phase damping
noise is analyzed in details.


title: Geometrical Statistics--Classical and Quantum

abstract:
  This is a review of the ideas behind the Fisher--Rao metric on classical
probability distributions, and how they generalize to metrics on density
matrices. As is well known, the unique Fisher--Rao metric then becomes a large
family of monotone metrics. Finally I focus on the Bures--Uhlmann metric, and
discuss a recent result that connects the geometric operator mean to a geodesic
billiard on the set of density matrices.


title: Optimal phase measurements with pure Gaussian states

abstract:
  We analyze the Heisenberg limit on phase estimation for Gaussian states. In
the analysis, no reference to a phase operator is made. We prove that the
squeezed vacuum state is the most sensitive for a given average photon number.
We provide two adaptive local measurement schemes that attain the Heisenberg
limit asymptotically. One of them is described by a positive operator-valued
measure and its efficiency is exhaustively explored. We also study Gaussian
measurement schemes based on phase quadrature measurements. We show that
homodyne tomography of the appropriate quadrature attains the Heisenberg limit
for large samples. This proves that this limit can be attained with local
projective Von Neuman measurements.


title: Classical Vs Quantum Probability in Sequential Measurements

abstract:
  We demonstrate in this paper that the probabilities for sequential
measurements have features very different from those of single-time
measurements. First, they cannot be modelled by a classical stochastic process.
Second, they are contextual, namely they depend strongly on the specific
measurement scheme through which they are determined. We construct
Positive-Operator-Valued measures (POVM) that provide such probabilities. For
observables with continuous spectrum, the constructed POVMs depend strongly on
the resolution of the measurement device, a conclusion that persists even if we
consider a quantum mechanical measurement device or the presence of an
environment. We then examine the same issues in alternative interpretations of
quantum theory. We first show that multi-time probabilities cannot be naturally
defined in terms of a frequency operator. We next prove that local hidden
variable theories cannot reproduce the predictions of quantum theory for
sequential measurements, even when the degrees of freedom of the measuring
apparatus are taken into account. Bohmian mechanics, however, does not fall in
this category. We finally examine an alternative proposal that sequential
measurements can be modelled by a process that does not satisfy the Kolmogorov
axioms of probability. This removes contextuality without introducing
non-locality, but implies that the empirical probabilities cannot be always
defined (the event frequencies do not converge). We argue that the predictions
of this hypothesis are not ruled out by existing experimental results
(examining in particular the "which way" experiments); they are, however,
distinguishable in principle.


title: Time-of-arrival probabilities and quantum measurements

abstract:
  We study the construction of probability densities for time-of-arrival in
quantum mechanics. Our treatment is based upon the facts that (i) time appears
in quantum theory as an external parameter to the system, and (ii) propositions
about the time-of-arrival appear naturally when one considers histories. The
definition of time-of-arrival probabilities is straightforward in stochastic
processes. The difficulties that arise in quantum theory are due to the fact
that the time parameter of Schr\"odinger's equation does not naturally define a
probability density at the continuum limit, but also because the procedure one
follows is sensitive on the interpretation of the reduction procedure. We
consider the issue in Copenhagen quantum mechanics and in history-based schemes
like consistent histories. The benefit of the latter is that it allows a proper
passage to the continuous limit--there are however problems related to the
quantum Zeno effect and decoherence. We finally employ the histories-based
description to construct Positive-Operator-Valued-Measures (POVMs) for the
time-of-arrival, which are valid for a general Hamiltonian. These POVMs
typically depend on the resolution of the measurement device; for a free
particle, however, this dependence cancels in the physically relevant regime
and the POVM coincides with that of Kijowski.


title: Dynamics of momentum entanglement in lowest-order QED

abstract:
  We study the dynamics of momentum entanglement generated in the lowest-order
QED interaction between two massive spin-1/2 charged particles, which grows in
time as the two fermions exchange virtual photons. We observe that the degree
of generated entanglement between interacting particles with initial
well-defined momentum can be infinite. We explain this divergence in the
context of entanglement theory for continuous variables, and show how to
circumvent this apparent paradox. Finally, we discuss two different
possibilities of transforming momentum into spin entanglement, through
dynamical operations or through Lorentz boosts.


title: Renormalization scheme for a multi-qubit-network

abstract:
  We present a renormalization scheme which simplifies the dynamics of an
important class of interacting multi-qubit systems. We show that a wide class
of M+1 qubit systems can be reduced to an equivalent n+1 qubit system with n
equal to, or greater than, 2, for any M. Our renormalization scheme faithfully
reproduces the overall dynamics of the original system including the
entanglement properties. In addition to its direct application to atom-cavity
and nanostructure systems, the formalism offers insight into a variety of
situations ranging from decoherence due to a spin-bath with its own internal
entanglement, through to energy transfer processes in organic systems such as
biological photosynthetic units.


title:
Half the entanglement in critical systems is distillable from a single
  specimen

abstract:
  We establish that the leading critical scaling of the single-copy
entanglement is exactly one half of the entropy of entanglement of a block in
critical infinite spin chains in a general setting, using methods of conformal
field theory. Conformal symmetry imposes that the single-copy entanglement for
critical many-body systems scales as E_1(\rho_L)=(c/6) \log L- (c/6)
(\pi^2/\log L) + O(1/L), where L is the number of constituents in a block of an
infinite chain and c corresponds to the central charge. This proves that from a
single specimen of a critical chain, already half the entanglement can be
distilled compared to the rate that is asymptotically available. The result is
substantiated by a quantitative analysis for all translationally invariant
quantum spin chains corresponding to general isotropic quasi-free fermionic
models. An analytic example of the XY model shows that away from criticality
the above simple relation is only maintained near the quantum phase transition
point.

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


title: Mechanisms in Adaptive Feedback Control: Photoisomerization in a Liquid

abstract:
  The underlying mechanism for Adaptive Feedback Control in the experimental
photoisomerization of NK88 in methanol is exposed theoretically. With given
laboratory limitations on laser output, the complicated electric fields are
shown to achieve their targets in qualitatively simple ways. Further, control
over the cis population without laser limitations reveals an incoherent
pump-dump scenario as the optimal isomerization strategy. In neither case are
there substantial contributions from quantum multiple-path interference or from
nuclear wavepacket coherence. Environmentally induced decoherence is shown to
justify the use of a simplified theoretical model.


title:
Experimental Study of the Role of Atomic Interactions on Quantum
  Transport

abstract:
  We report an experimental study of quantum transport for atoms confined in a
periodic potential and compare between thermal and BEC initial conditions. We
observe ballistic transport for all values of well depth and initial
conditions, and the measured expansion velocity for thermal atoms is in
excellent agreement with a single-particle model. For weak wells, the expansion
of the BEC is also in excellent agreement with single-particle theory, using an
effective temperature. We observe a crossover to a new regime for the BEC case
as the well depth is increased, indicating the importance of interactions on
quantum transport.


title: Particle dynamics in a relativistic invariant stochastic medium

abstract:
  The dynamics of particles moving in a medium defined by its relativistically
invariant stochastic properties is investigated. For this aim, the force
exerted on the particles by the medium is defined by a stationary random
variable as a function of the proper time of the particles. The equations of
motion for a single one-dimensional particle are obtained and numerically
solved. A conservation law for the drift momentum of the particle during its
random motion is shown. Moreover, the conservation of the mean value of the
total linear momentum for two particles repelling each other according with the
Coulomb interaction is also following. Therefore, the results indicate the
realization of a kind of stochastic Noether theorem in the system under study.
Possible applications to the stochastic representation of Quantum Mechanics are
advanced.


abstract:
  Contrary to the usual assumption that the experimental preparation of pure
entangled states can be described by mixed states due to white noise, a more
realistic description for polarization-entangled states produced by parametric
down-conversion is that they are mixed states due to decoherence in a preferred
polarization basis. This distinction between white and colored noise is crucial
when we look for maximal violations of Bell's inequalities for two-qubit and
two-qutrit entangled states. We find that violations of Bell's inequalities
with realistic noise for polarization-entangled photons are extremely robust
for colored noise, whereas this is not the case for white noise. In addition,
we study the difference between white and colored noise for maximal violations
of Bell's inequalities for three and four-qubit entangled states.


title: Experimental test for violation of duality on a photon beam

abstract:
  We test for evidence violating the duality invariant ratio of photon beam
irradiance and wave intensity. Split beams from a 633 nm HeNe laser are
intersected at a diffraction grating complementary to the resultant
interference pattern. An output beam from the grating, depleted in irradiance
relative to wave intensity from the perspective of local realism, is
transiently intersected with a beam from an independent HeNe laser and measured
irradiance is amplified by ~4% in conflict with quantum mechanics.


title:
Quantum state sharing of an arbitrary two-qubit state with two-photon
  entanglements and Bell-state measurements

abstract:
  Two schemes for sharing an arbitrary two-qubit state based on entanglement
swapping are proposed with Bell-state measurements and local unitary
operations. One is based on the quantum channel with four
Einstein-Podolsky-Rosen (EPR) pairs shared in advance. The other is based on a
circular topological structure, i.e., each user shares an EPR pair with his
neighboring one. The advantage of the former is that the construction of the
quantum channel between the agents is controlled by the sender Alice, which
will improve the security of the scheme. The circular scheme reduces the
quantum resource largely when the number of the agents is large. Both of those
schemes have the property of high efficiency as almost all the instances can be
used to split the quantum information. They are more convenient in application
than the other schemes existing as they require only two-qubit entanglements
and two-qubit joint measurements for sharing an arbitrary two-qubit state.


title:
Numerical investigation of photon creation in a three-dimensional
  resonantly vibrating cavity: TE-modes

abstract:
  The creation of TE-mode photons in a three-dimensional perfectly conducting
cavity with one resonantly vibrating wall is studied numerically. We show that
the creation of TE-mode photons in a rectangular cavity is related to the
production of massive scalar particles on a time-dependent interval. The
equations of motion are solved numerically which allows to take into account
the intermode coupling. We compare the numerical results with analytical
predictions and discuss the effects of the intermode coupling in detail. The
numerical simulations reveal that photon creation in a three-dimensional
resonantly vibrating cavity can be maximized by arranging the size of the
cavity such that certain conditions are realized. In particular, the creation
of TE-mode photons in the lowest frequency mode $(1,1,1)$ is most efficient in
a non-cubic cavity where the size of the non-dynamical dimensions is roughly 11
times larger than the size of the dynamical dimension. We discuss this effect
and its relation to the intermode coupling in detail.


title: Generalized boson algebra and its entangled bipartite coherent states

abstract:
  Starting with a given generalized boson algebra U_<q>(h(1)) known as the
bosonized version of the quantum super-Hopf U_q[osp(1/2)] algebra, we employ
the Hopf duality arguments to provide the dually conjugate function algebra
Fun_<q>(H(1)). Both the Hopf algebras being finitely generated, we produce a
closed form expression of the universal T matrix that caps the duality and
generalizes the familiar exponential map relating a Lie algebra with its
corresponding group. Subsequently, using an inverse Mellin transform approach,
the coherent states of single-node systems subject to the U_<q>(h(1)) symmetry
are found to be complete with a positive-definite integration measure.
Nonclassical coalgebraic structure of the U_<q>(h(1)) algebra is found to
generate naturally entangled coherent states in bipartite composite systems.


title: Interference of mesoscopic particles: quantum--classical transition

abstract:
  We analyze the double slit interference of a mesoscopic particle. We
calculate the visibility of the interference pattern, introduce a
characteristic temperature that defines the onset to decoherence and scrutinize
the conditions that must be satisfied for an interference experiment to be
possible.


title:
A Simultaneous Quantum Secure Direct Communication Scheme between the
  Central Party and Other M Parties

abstract:
  We propose a simultaneous quantum secure direct communication scheme between
one party and other three parties via four-particle GHZ states and swapping
quantum entanglement. In the scheme, three spatially separated senders, Alice,
Bob and Charlie, transmit their secret messages to a remote receiver Diana by
performing a series local operations on their respective particles according to
the quadripartite stipulation. From Alice, Bob, Charlie and Diana's Bell
measurement results, Diana can infer the secret messages. If a perfect quantum
channel is used, the secret messages are faithfully transmitted from Alice, Bob
and Charlie to Diana via initially shared pairs of four-particle GHZ states
without revealing any information to a potential eavesdropper. As there is no
transmission of the qubits carrying the secret message in the public channel,
it is completely secure for the direct secret communication. This scheme can be
considered as a network of communication parties where each party wants to
communicate secretly with a central party or server.


title: Semiclassical analysis of a complex quartic Hamiltonian

abstract:
  It is necessary to calculate the C operator for the non-Hermitian
PT-symmetric Hamiltonian H=\half p^2+\half\mu^2x^2-\lambda x^4 in order to
demonstrate that H defines a consistent unitary theory of quantum mechanics.
However, the C operator cannot be obtained by using perturbative methods.
Including a small imaginary cubic term gives the Hamiltonian H=\half p^2+\half
\mu^2x^2+igx^3-\lambda x^4, whose C operator can be obtained perturbatively. In
the semiclassical limit all terms in the perturbation series can be calculated
in closed form and the perturbation series can be summed exactly. The result is
a closed-form expression for C having a nontrivial dependence on the dynamical
variables x and p and on the parameter \lambda.


title: Fractional Supersymmetry as a Superposition of Ordinary Supersymmetry

abstract:
  It is shown how to derive fractional supersymmetric quantum mechanics of
order k as a superposition of k-1 copies of ordinary supersymmetric quantum
mechanics.


title: Sharing Polarization within Quantum Subspaces

abstract:
  Given an ensemble of n spins, at least some of which are partially polarized,
we investigate the sharing of this polarization within a subspace of k spins.
We assume that the sharing results in a pseudopure state, characterized by a
single purity parameter which we call the bias. As a concrete example we
consider ensembles of spin-1/2 nuclei in liquid-state nuclear magnetic
resonance (NMR) systems. The shared bias levels are compared with some current
entanglement bounds to determine whether the reduced subspaces can give rise to
entangled states.


title: Quantum correlations in (1+1)-dimensional systems

abstract:
  Contents: 1.- Introduction 2.- Scaling of entanglement in (1+1)-dimensional
systems 3.- Entanglement and RG-flows 4.- Matrix Product States Appendix A.-
Entanglement and order relations B.- Hilbert space in a conformal theory


title: Stochastic mean-field dynamics for fermions in the weak coupling limit

abstract:
  Assuming that the effect of the residual interaction beyond mean-field is
weak and has a short memory time, two approximate treatments of correlation in
fermionic systems by means of Markovian quantum jump are presented. A
simplified scenario for the introduction of fluctuations beyond mean-field is
first presented. In this theory, part of the quantum correlations between the
residual interaction and the one-body density matrix are neglected and jumps
occur between many-body densities formed of pairs of states $D=| \Phi_a > <
\Phi_b |/< \Phi_b . |\Phi_a >$ where $| \Phi_a >$ and $| \Phi_b >$ are
antisymmetrized products of single-particle states. The underlying Stochastic
Mean-Field (SMF) theory is discussed and applied to the monopole vibration of a
spherical $^{40}$Ca nucleus under the influence of a statistical ensemble of
two-body contact interaction. This framework is however too simplistic to
account for both fluctuation and dissipation. In the second part of this work,
an alternative quantum jump method is obtained without making the approximation
on quantum correlations. Restricting to two particles-two holes residual
interaction, the evolution of the one-body density matrix of a correlated
system is transformed into a Lindblad equation. The associated dissipative
dynamics can be simulated by quantum jumps between densities written as $D = |
\Phi >< \Phi |$ where $| \Phi >$ is a normalized Slater determinant. The
associated stochastic Schroedinger equation for single-particle wave-functions
is given.


title: Feedback cooling of atomic motion in cavity QED

abstract:
  We consider the problem of controlling the motion of an atom trapped in an
optical cavity using continuous feedback. In order to realize such a scheme
experimentally, one must be able to perform state estimation of the atomic
motion in real time. While in theory this estimate may be provided by a
stochastic master equation describing the full dynamics of the observed system,
integrating this equation in real time is impractical. Here we derive an
approximate estimation equation for this purpose, and use it as a drive in a
feedback algorithm designed to cool the motion of the atom. We examine the
effectiveness of such a procedure using full simulations of the cavity QED
system, including the quantized motion of the atom in one dimension.


title: A Quantum Field Theory Twist to Photon Angular Momentum

abstract:
  A quantum field theory approach is put forward to generalize the concept of
classical spatial light beams carrying orbital angular momentum to the
single-photon level. This quantization framework is carried out both in the
paraxial and nonparaxial regimes. Upon extension to the optical phase space,
closed-form expressions are found for a photon Wigner representation describing
transformations on the orbital Poincare sphere of unitarily related families of
paraxial spatial modes.


title:
Impact of positivity and complete positivity on accessibility of
  Markovian dynamics

abstract:
  We consider a two-dimensional quantum control system evolving under an
entropy-increasing irreversible dynamics in the semigroup form. Considering a
phenomenological approach to the dynamics, we show that the accessibility
property of the system depends on whether its evolution is assumed to be
positive or completely positive. In particular, we characterize the family of
maps having different accessibility and show the impact of that property on
observable quantities by means of a simple physical model.


title:
Does consciousness really collapse the wave function? A possible
  objective biophysical resolution of the measurement problem

abstract:
  An analysis has been performed of the theories and postulates advanced by von
Neumann, London and Bauer, and Wigner, concerning the role that consciousness
might play in the collapse of the wave function, which has become known as the
measurement problem. This reveals that an error may have been made by them in
the area of biology and its interface with quantum mechanics, when they called
for the reduction of any superposition states in the brain through the mind or
consciousness. Many years later Wigner changed his mind to reflect a simpler
and more realistic objective position, expanded upon by Shimony, which appears
to offer a way to resolve this issue. The argument is therefore made that the
wave function of any superposed photon state or states is always objectively
changed within the complex architecture of the eye in a continuous linear
process initially for most of the superposed photons, followed by a
discontinuous nonlinear collapse process later for any remaining superposed
photons, thereby guaranteeing that only final, measured information is
presented to the brain, mind or consciousness. An experiment to be conducted in
the near future may enable us to simultaneously resolve the measurement problem
and also determine if the linear nature of quantum mechanics is violated by the
perceptual process.


title: Coulomb entangler and entanglement testing network for waveguide qubits

abstract:
  We present a small network for the testing of the entanglement of two
ballistic electron waveguide qubits. The network produces different output
conditional on the presence or absence of entanglement. The structure of the
network allows for the determination of successful entanglement operations
through the measurement of the output of a single qubit. We also present a
simple model of a dynamic coulomb-like interaction and use it to describe some
characteristics of a proposed scheme for the entanglement of qubits in
ballistic electron waveguides.


title: Real-Valued Charged Fields and Interpretation of Quantum Mechanics

abstract:
  Schroedinger (Nature, v.169, p.538 (1952)) demonstrated that, contrary to the
widespread belief, charged particles may be described by real fields. Therefore
the sets of solutions with real-valued charged fields are considered in the
present work for some versions of (non-second-quantized) quantum
electrodynamics (for Dirac spinors "real-valued" is understood as "satisfying
the Majorana condition"). In some of the versions any solution may be obtained
from a solution from those sets by a gauge transform. The solutions from those
sets have common features suggesting a natural interpretation along the lines
of the Bohm interpretation, but no quantum potentials arise, and it is the
electromagnetic field, not the wave function, that plays the role of the
guiding field.


title: Heisenberg's wave packet reconsidered

abstract:
  This note shows that Heisenberg's choice for a wave function in his original
paper on the uncertainty principle is simply a renormalized characteristic
function of a stable distribution with certain restrictions on the parameters.
Relaxing Heisenberg's restrictions leads to a more general formulation of the
uncertainty principle. This reformulation shows quantum uncertainty can exist
at a macroscopic level. These modifications also give rise to a new form of
Schrodinger's wave equation as the equation of a vibrating string. Although a
heat equation version can also be given, the latter shows the traditional
formulation of Schrodinger's equation involves a hidden Cauchy amplitude
assumption.


title: NMR Quantum Information Processing with Para-Hydrogen

abstract:
  This thesis addresses the problems of initialization and separability in
liquid state NMR based quantum information processors. We prepare pure quantum
states lying above the entanglement threshold. Our pure state quantum computer
derives its purity from the highly polarized nuclear spin states in the
para-hydrogen molecule. The thesis begins with a critique of conventional NMR
based quantum information processing outlining the major strengths and
weaknesses of the technology. We describe the enhanced magnetic ordering of the
nuclear spin states in para-hydrogen and an initialization experiment
exploiting this effect to achieve pure, entangled states. These states can
indeed be used as initial states in implementing quantum algorithms: we
describe mplementations of the Deutsch and the Grover quantum algorithms. The
"twirl" operation converts a completely arbitrary input state to a Werner
singlet. The NMR implementation of this operation is taken up. We also analyze
the possibility of sharing the purity of some highly polarized qubits in a
quantum computer onto quantum subspaces of arbitrary dimensions, and whether
these sharing operations increase or decrease the likelihood of entanglement.


title:
A quantum protocol to win the graph colouring game on all Hadamard
  graphs

abstract:
  This paper deals with graph colouring games, an example of pseudo-telepathy,
in which two provers can convince a verifier that a graph $G$ is $c$-colourable
where $c$ is less than the chromatic number of the graph. They win the game if
they convince the verifier. It is known that the players cannot win if they
share only classical information, but they can win in some cases by sharing
entanglement. The smallest known graph where the players win in the quantum
setting, but not in the classical setting, was found by Galliard, Tapp and Wolf
and has 32,768 vertices. It is a connected component of the Hadamard graph
$G_N$ with $N=c=16$. Their protocol applies only to Hadamard graphs where $N$
is a power of 2. We propose a protocol that applies to all Hadamard graphs.
Combined with a result of Frankl, this shows that the players can win on any
induced subgraph of $G_{12}$ having 1609 vertices, with $c=12$. Combined with a
result of Frankl and Rodl, our result shows that all sufficiently large
Hadamard graphs yield pseudo-telepathy games.


title: The Grammar of Teleportation

abstract:
  Whilst a straightforward consequence of the formalism of non-relativistic
quantum mechanics, the phenomenon of quantum teleportation has given rise to
considerable puzzlement. In this paper, the teleportation protocol is reviewed
and these puzzles dispelled. It is suggested that they arise from two primary
sources: 1) the familiar error of hypostatizing an abstract noun (in this case,
`information') and 2) failure to differentiate interpretation dependent from
interpretation independent features of quantum mechanics. A subsidiary source
of error, the simulation fallacy, is also identified. The resolution presented
of the puzzles of teleportation illustrates the benefits of paying due
attention to the logical status of `information' as an abstract noun.


title: Entanglement as a quantum order parameter

abstract:
  We show that the quantum order parameters (QOP) associated with the
transitions between a normal conductor and a superconductor in the BCS and
eta-pairing models and between a Mott-insulator and a superfluid in the
Bose-Hubbard model are directly related to the amount of entanglement existent
in the ground state of each system. This gives a physical meaningful
interpretation to these QOP, which shows the intrinsically quantum nature of
the phase transitions considered.


title:
Discrete Wigner distribution for two qubits: a characterization of
  entanglement properties

abstract:
  We study the properties of the discrete Wigner distribution for two qubits
introduced by Wotters. In particular, we analyze the entanglement properties
within the Wigner distribution picture by considering the negativity of the
Wigner function (WF) and the correlations of the marginal distribution. We show
that a state is entangled if at least one among the values assumed by the
corresponding discrete WF is smaller than a certain critical (negative) value.
Then, based on the Partial Transposition criterion, we establish the relation
between the separability of a density matrix and the non-negativity of the WF's
relevant both to such a density matrix and to the partially transposed thereof.
  Finally, we derive a simple inequality --involving the covariance-matrix
elements of a given WF-- which appears to provide a separability criterion
stronger than the one based on the Local Uncertainty Relations.


title:
A Quantum Logic Array Microarchitecture: Scalable Quantum Data Movement
  and Computation

abstract:
  Recent experimental advances have demonstrated technologies capable of
supporting scalable quantum computation. A critical next step is how to put
those technologies together into a scalable, fault-tolerant system that is also
feasible. We propose a Quantum Logic Array (QLA) microarchitecture that forms
the foundation of such a system. The QLA focuses on the communication resources
necessary to efficiently support fault-tolerant computations. We leverage the
extensive groundwork in quantum error correction theory and provide analysis
that shows that our system is both asymptotically and empirically fault
tolerant. Specifically, we use the QLA to implement a hierarchical, array-based
design and a logarithmic expense quantum-teleportation communication protocol.
Our goal is to overcome the primary scalability challenges of reliability,
communication, and quantum resource distribution that plague current proposals
for large-scale quantum computing.


title: Implications of the Luders Postulate for Quantum Algorithms

abstract:
  The Luders postulate is reviewed and implications for quantum algorithms are
discussed. A search algorithm for an unstructured database is described.


title:
Absolute frequency measurement of an SF6 two-photon line using a
  femtosecond optical comb and sum-frequency generation

abstract:
  We demonstrate a new simple technique to measure IR frequencies near 30 THz
using a femtosecond (fs) laser optical comb and sum-frequency generation. The
optical frequency is directly compared to the distance between two modes of the
fs laser, and the resulting beat note is used to control this distance which
depends only on the repetition rate fr of the fs laser. The absolute frequency
of a CO2 laser stabilized onto an SF6 two-photon line has been measured for the
first time. This line is an attractive alternative to the usual saturated
absorption OsO4 resonances used for the stabilization of CO2 lasers. First
results demonstrate a fractional Allan deviation of 3.10-14 at 1 s.


title:
Further results on the observability of quantum systems under general
  measurement

abstract:
  In this paper, we present a collection of results on the observability of
quantum mechanical systems, in the case the output is the result of a discrete
nonselective measurement. By defining an effective observable we extend
previous results, on the Lie algebraic characterization of observable systems,
to general measurements. Further results include the characterization of a
`best probe' (i.e. a minimally disturbing probe) in indirect measurement and a
study of the relation between disturbance and observability in this case. We
also discuss how the observability properties of a quantum system relate to the
problem of state reconstruction. Extensions of the formalism to the case of
selective measurements are also given.