title:
Frontier between separability and quantum entanglement in a many spin
  system

abstract:
  We discuss the critical point $x_c$ separating the quantum entangled and
separable states in two series of N spins S in the simple mixed state
characterized by the matrix operator $\rho=x|\tilde{\phi}><\tilde{\phi}| +
\frac{1-x}{D^N} I_{D^N}$ where $x \in [0,1]$, $D =2S+1$, ${\bf I}_{D^N}$ is the
$D^N \times D^N$ unity matrix and $|\tilde {\phi}>$ is a special entangled
state. The cases x=0 and x=1 correspond respectively to fully random spins and
to a fully entangled state. In the first of these series we consider special
states $|\tilde{\phi}>$ invariant under charge conjugation, that generalizes
the N=2 spin S=1/2 Einstein-Podolsky-Rosen state, and in the second one we
consider generalizations of the Weber density matrices. The evaluation of the
critical point $x_c$ was done through bounds coming from the partial
transposition method of Peres and the conditional nonextensive entropy
criterion. Our results suggest the conjecture that whenever the bounds coming
from both methods coincide the result of $x_c$ is the exact one. The results we
present are relevant for the discussion of quantum computing, teleportation and
cryptography.


title: Estimation of qubit states in a factorizing basis

abstract:
  The optimal estimation of a quantum mechanical 2-state system (qubit) - with
N identically prepared qubits available - is obtained by measuring all qubits
simultaneously in an entangled basis. We report the experimental estimation of
qubits using a succession of N measurements on individual qubits where the
measurement basis is changed during the estimation procedure conditioned on the
outcome of previous measurements (self-learning estimation). The performance of
this adaptive algorithm is compared with other algorithms using measurements in
a factorizing basis.


title: Perturbation Approach for a Solid-State Quantum Computation

abstract:
  The dynamics of the nuclear-spin quantum computer with large number (L=1000)
of qubits is considered using a perturbation approach, based on approximate
diagonalization of exponentially large sparse matrices. Small parameters are
introduced and used to compute the error in implementation of entanglement
between remote qubits, by applying a sequence of resonant radio-frequency
pulses. The results of the perturbation theory are tested using exact numerical
solutions for small number of qubits.


title: Magnetic Resonance Force Microscopy Measurement of Entangled Spin States

abstract:
  We simulate magnetic resonance force microscopy measurements of an entangled
spin state. One of the entangled spins drives the resonant cantilever
vibrations, while the other remote spin does not interact directly with the
quasiclassical cantilever. The Schr\"odinger cat state of the cantilever
reveals two possible outcomes of the measurement for both entangled spins.


title: Quantum measurements and Paul traps in gravitational backgrounds

abstract:
  In the present work we solve the motion equations of a particle in a Paul
trap embeded in the gravitational field of a spherically symmetric mass. One of
the ideas behind this work concerns the analysis of the effects that the
gravity--induced quantum noise, stemming from the bodies in the neighborhood of
the Paul trap, could have upon the enhancement of the quantum behavior of this
system. This will be done considering a series expansion for the gravitational
field of the source, and including in the Hamiltonian of the Paul trap only the
first two terms. Higher--order contributions will be introduced as part of the
environment of the system, and in consequence will not appear in the
Hamiltonian. In other words, we put forward an argument that allows us to
differentiate those gravitational degrees of freedom that will appear as an
uncontrollable influence on the Paul trap. Along the ideas of the so called
restricted path integral formalism, we take into account the continuous
monitoring of the position of our particle, and in consequence the
corresponding propagators and probabilities, associated with the different
measurements outputs, are obtained.
  Afterwards, the differential equation related to a quantum nondemolition
variable is posed and solved, i.e., a family of quantum nondemolition
parameters is obtained. Finally, a qualitative analysis of the effects on the
system, of the gravity--induced environment, will be done.


title: Coherent radiation from neutral molecules moving above a grating

abstract:
  We predict and study the quantum-electrodynamical effect of parametric
self-induced excitation of a molecule moving above the dielectric or conducting
medium with periodic grating. In this case the radiation reaction force
modulates the molecular transition frequency which results in a parametric
instability of dipole oscillations even from the level of quantum or thermal
fluctuations. The present mechanism of instability of electrically neutral
molecules is different from that of the well-known Smith-Purcell and transition
radiation in which a moving charge and its oscillating image create an
oscillating dipole.
  We show that parametrically excited molecular bunches can produce an easily
detectable coherent radiation flux of up to a microwatt.


title: Superluminal influences, hidden variables, and signaling

abstract:
  We consider alternative models to quantum mechanics, that have been proposed
in the recent years in order to explain the EPR correlations between two
particles. These models allow in principle local hidden variables produced at
the source, and some superluminal "hidden communication" (or "influences") to
reproduce the non-local correlations. Moving to the case of three particles, we
show that these alternative models lead to signaling when "hidden
communication" alone is considered as the origin of the correlations.


title: Quantum Holography

abstract:
  We propose to make use of quantum entanglement for extracting holographic
information about a remote 3-D object in a confined space which light enters,
but from which it cannot escape. Light scattered from the object is detected in
this confined space entirely without the benefit of spatial resolution. Quantum
holography offers this possibility by virtue of the fourth-order quantum
coherence inherent in entangled beams.


title:
Realization of generalized quantum searching using nuclear magnetic
  resonance

abstract:
  According to the theoretical results, the quantum searching algorithm can be
generalized by replacing the Walsh-Hadamard(W-H) transform by almost any
quantum mechanical operation. We have implemented the generalized algorithm
using nuclear magnetic resonance techniques with a solution of chloroform
molecules. Experimental results show the good agreement between theory and
experiment.


title:
Similarity between Grover's quantum search algorithm and classical
  two-body collisions

abstract:
  By studying the attribute of the inversion about average operation in quantum
searching algorithm, we find the similarity between the quantum searching and
the course of two rigid bodies'collision. Some related questions are discussed
from this similarity.


title: Characterizing Entanglement

abstract:
  Quantum entanglement is at the heart of many tasks in quantum information.
Apart from simple cases (low dimensions, few particles, pure states), however,
the mathematical structure of entanglement is not yet fully understood. This
tutorial is an introduction to our present knowledge about how to decide
whether a given state is separable or entangled, how to characterize
entanglement via witness operators, how to classify entangled states according
to their usefulness (i.e. distillability), and how to quantify entanglement
with appropriate measures.


title:
Shor-Preskill Type Security-Proofs for Concatenated Bennett-Brassard
  1984 Quantum Key Distribution Protocol

abstract:
  We discuss long code problems in the Bennett-Brassard 1984 (BB84) quantum key
distribution protocol and describe how they can be overcome by concatenation of
the protocol. Observing that concatenated modified Lo-Chau protocol finally
reduces to the concatenated BB84 protocol, we give the unconditional security
of the concatenated BB84 protocol.


title:
Entanglement Measures with Asymptotic Weak-Monotonicity as Lower (Upper)
  Bound for the Entanglement of Cost (Distillation)

abstract:
  We propose entanglement measures with asymptotic weak-monotonicity. We show
that a normalized form of entanglement measures with the asymptotic
weak-monotonicity are lower (upper) bound for the entanglement of cost
(distillation).


title: Reflections upon separability and distillability

abstract:
  We present an abstract formulation of the so-called Innsbruck-Hannover
programme that investigates quantum correlations and entanglement in terms of
convex sets. We present a unified description of optimal decompositions of
quantum states and the optimization of witness operators that detect whether a
given state belongs to a given convex set. We illustrate the abstract
formulation with several examples, and discuss relations between optimal
entanglement witnesses and n-copy non-distillable states with non-positive
partial transpose.


title: Manifolds of interconvertible pure states

abstract:
  Local orbits of a pure state of an N x N bi-partite quantum system are
analyzed. We compute their dimensions which depends on the degeneracy of the
vector of coefficients arising by the Schmidt decomposition. In particular, the
generic orbit has 2N^2 -N-1 dimensions, the set of separable states is 4(N-1)
dimensional, while the manifold of maximally entangled states has N^2-1
dimensions.


title:
Comment on Book Review of `Quantum Evolution' (Johnjoe McFadden) by
  Mathew J. Donald

abstract:
  This paper is a response to the critical review by Mathew J. Donald of the
popular science book 'Quantum Evolution' by Johnjoe McFadden and our earlier
paper (by Johnjoe McFadden and Jim Al-Khalili), in which we propose a quantum
mechanical explanation for certain types of mutation. We here address Donald's
criticisms and reassert our proposal.


title:
From Davydov solitons to decoherence-free subspaces: self-consistent
  propagation of coherent-product states

abstract:
  The self-consistent propagation of generalized $D_{1}$ [coherent-product]
states and of a class of gaussian density matrix generalizations is examined,
at both zero and finite-temperature, for arbitrary interactions between the
localized lattice (electronic or vibronic) excitations and the phonon modes. It
is shown that in all legitimate cases, the evolution of $D_{1}$ states reduces
to the disentangled evolution of the component $D_{2}$ states. The
self-consistency conditions for the latter amount to conditions for
decoherence-free propagation, which complement the $D_{2}$ Davydov soliton
equations in such a way as to lift the nonlinearity of the evolution for the
on-site degrees of freedom. Although it cannot support Davydov solitons, the
coherent-product ansatz does provide a wide class of exact density-matrix
solutions for the joint evolution of the lattice and phonon bath in compatible
systems. Included are solutions for initial states given as a product of a
[largely arbitrary] lattice state and a thermal equilibrium state of the
phonons. It is also shown that external pumping can produce self-consistent
Frohlich-like effects. A few sample cases of coherent, albeit not solitonic,
propagation are briefly discussed.


title:
Ellipsometric measurements by use of photon pairs generated by
  spontaneous parametric down-conversion

abstract:
  We present a novel interferometric technique for performing ellipsometric
measurements. This technique relies on the use of a non-classical optical
source, namely, polarization-entangled twin photons generated by spontaneous
parametric down-conversion from a nonlinear crystal, in conjunction with a
coincidence-detection scheme. Ellipsometric measurements acquired with this
scheme are absolute; i.e., they do not require source and detector calibration.


title: The EPR correlations and the chameleon effect

abstract:
  We describe an experiment in which two non communicating computers, starting
from a common input in the form of sequences of pseudo--random numbers in the
interval $[0,2\pi]$, and computing deterministic $\{\pm 1\}$--valued functions,
chosen at random and independently, produce sequences of numbers whose
correlations coincide with the EPR correlations and therefore violate Bell's
inequality.
  The experiment is the practical implementation of a mathematical model of a
classical, deterministic system whose initial state is chosen at random from
its state space, with a known initial probability distribution, and whose
dynamics exhibits the chameleon effect described below. Such a system satisfies
the constraints of pre--determination, locality, causality, local independent
choices, singlet law and reproduces the EPR correlations.


title:
Dense Antihydrogen: Its Production and Storage to Envision Antimatter
  Propulsion

abstract:
  We discuss the possibility that dense antihydrogen could provide a path
towards a mechanism for a deep space propulsion system. We concentrate at
first, as an example, on Bose-Einstein Condensate (BEC) antihydrogen. In a
Bose-Einstein Condensate, matter (or antimatter) is in a coherent state
analogous to photons in a laser beam, and individual atoms lose their
independent identity. This allows many atoms to be stored in a small volume. In
the context of recent advances in producing and controlling BECs, as well as in
making antihydrogen, this could potentially provide a revolutionary path
towards the efficient storage of large quantities of antimatter, perhaps
eventually as a cluster or solid.


title: Thermalizing Quantum Machines: Dissipation and Entanglement

abstract:
  We study the relaxation of a quantum system towards the thermal equilibrium
using tools developed within the context of quantum information theory. We
consider a model in which the system is a qubit, and reaches equilibrium after
several successive two-qubit interactions (thermalizing machines) with qubits
of a reservoir. We characterize completely the family of thermalizing machines.
The model shows a tight link between dissipation, fluctuations, and the maximal
entanglement that can be generated by the machines. The interplay of quantum
and classical information processes that give rise to practical irreversibility
is discussed.


title:
On the role of coherent attacks in a type of strategic problem related
  to quantum key distribution

abstract:
  We consider a strategic problem of the Evesdropping to quantum key
distribution. Evesdropper hopes to obtain the maxium information given the
disturbance to the qubits is often For this strategy, the optimized individual
attack have been extensively constructed under various conditions. However, it
seems a difficult task in the case of coherent attack, i.e., Eve may treat a
number of intercepted qubits collectively, including the collective unitary
transformations and the measurements. It was conjectured by Cirac and Gisin
that no coherent attack can be more powerful for this strategy for BB84
protocol. In this paper we give a general conclusion on the role of coherent
attacks for the strategy of maxmizing the information given the disturbance.
Suppose in a quantum key distribution(QKD) protocol, all the transmitted bits
from Alice are independent and only the individual disturbances to each qubits
are examined by Alice and Bob. For this type of protocols(so far almost all QKD
protocols belong to this type), in principle no coherent attack is more
powerful than the product of optimized individual attack to each individual
qubits. All coherent attacks to the above QKD protocols can be disregarded for
the strategy above.


title: The Spin

abstract:
  It is shown that the spin is naturally introduced into classical mechanics if
the latter is formulated as dynamics of the phase space density. It is shown
that the uncertainty principle, as the amendment in this dynamics, restricts
possible spins, and in particular equation for the particle with the spin
$\hbar /2$ is derived. Also equation for the charge with this spin is derived
when electromagnetic field is included. In one example it is shown that the
modulus of the spin changes with the gradient of the magnetic field.


title: A note on Borromean correlations in multipartite quantum systems

abstract:
  If a pure state of a multipartite quantum system is Borromean, that is, its
density matrix becomes product after tracing out any its component then the
initial state is product itself. This shows the essentially classical nature of
Borromean correlations which can not be achieved by entangled pure states.


title: Cloning a Qutrit

abstract:
  We investigate several classes of state-dependent quantum cloners for
three-level systems. These cloners optimally duplicate some of the four
maximally-conjugate bases with an equal fidelity, thereby extending the
phase-covariant qubit cloner to qutrits. Three distinct classes of qutrit
cloners can be distinguished, depending on two, three, or four
maximally-conjugate bases are cloned as well (the latter case simply
corresponds to the universal qutrit cloner). These results apply to symmetric
as well as asymmetric cloners, so that the balance between the fidelity of the
two clones can also be analyzed.


title: How macroscopic properties dictate microscopic probabilities

abstract:
  We argue that the quantum probability law follows, in the large N limit, from
the compatibility of quantum mechanics with classical-like properties of
macroscopic objects. For a finite sample, we find that likely and unlikely
measurement outcomes are associated with distinct interference effects in a
sample weakly coupled to an environment.


title: Bell Inequalities and Pseudo-functional densities

abstract:
  A local hidden variable model with pseudo-functional density function
restricted to a binary probability event space is demonstrated to be able to
reproduce the quantum correlation in an Einstein Podolsky Rosen Bohm and
Aharonov type of experiment.
  In the density function use is made of Hadamard's finite part which disables
the possibility to derive Bell's inequality from models with such a type of
density function.


title:
Do all pure entangled states violate Bell's inequalities for correlation
  functions?

abstract:
  Any pure entangled state of two particles violates a Bell inequality for
two-particle correlation functions (Gisin's theorem). We show that there exist
pure entangled N>2 qubit states that do not violate any Bell inequality for N
particle correlation functions for experiments involving two dichotomic
observables per local measuring station. We also find that
Mermin-Ardehali-Belinskii-Klyshko inequalities may not always be optimal for
refutation of local realistic description.


title: Dilemma and Quantum Battle of Sexes

abstract:
  We analysed quantum version of the game battle of sexes using a general
initial quantum state. For a particular choice of initial entangled quantum
state it is shown that the classical dilemma of the battle of sexes can be
resolved and a unique solution of the game can be obtained.


title: Schroedinger discovery of wave mechanics

abstract:
  Schroedinger's great discovery of wave mechanics in 1926 - his annus
mirabilis - is discussed in detail. Beside the six most important papers that
appeared during the first half of 1926, letters between Schroedinger and
leading physicists at this time are main sources of this account.


title: Chen and Diao's quantum search algorithm is not exponentially fast

abstract: This paper has been withdrawn due to the withdrawal of the related paper.


title: Optimal cloning for two pairs of orthogonal states

abstract:
  We study the optimal cloning transformation for two pairs of orthogonal
states of two-dimensional quantum systems, and derive the corresponding optimal
fidelities.


title: Error sensitivity of a Quantum Simulator I: a first example

abstract:
  As the first useful Quantum Computers will be quantum simulators, here the
minimum number of qubits necessary for the solution of the Schroedinger
equation in simple test problems is evaluated. From the present preliminary
results it appears that it is possible to realize a useful quantum simulator
with a register of only 10-15 qbits. The intrinsic sensitivity to some errors
appears to be moderate without the need of error correcting methods. At present
there is at least some indication that the amplitude errors are more dangerous
than phase and decoherence errors. So corrections limited to amplitude errors
may be very useful.


title: Decoherence on Grover's quantum algorithm: perturbative approach

abstract:
  In this paper, we study decoherence on Grover's quantum searching algorithm
using a perturbative method. We assume that each two-state system (qubit)
suffers \sigma_{z} error with probability p (0\leq p\leq 1) independently at
every step in the algorithm. Considering an n-qubit density operator to which
Grover's operation is applied M times, we expand it in powers of 2Mnp and
derive its matrix element order by order under the n\to \infty limit. (In this
large n limit, we assume p is small enough, so that 2Mnp(\geq 0) can take any
real positive value or 0.) This approach gives us an interpretation about
creation of new modes caused by \sigma_{z} error and an asymptotic form of an
arbitrary order correction. Calculating the matrix element up to the fifth
order term numerically, we investigate a region of 2Mnp (perturbative
parameter) where the algorithm finds the correct item with a threshold of
probability P_{th} or more. It satisfies 2Mnp<(8/5)(1-P_{th}) around 2Mnp\simeq
0 and P_{th}\simeq 1, and this linear relation is applied to a wide range of
P_{th} approximately. This observation is similar to a result obtained by E.
Bernstein and U. Vazirani concerning accuracy of quantum gates for general
algorithms. We cannot investigate a quantum to classical phase transition of
the algorithm, because it is outside the reliable domain of our perturbation
theory.


title: Complementarity of representations in quantum mechanics

abstract:
  We show that Bohr's principle of complementarity between position and
momentum descriptions can be formulated rigorously as a claim about the
existence of representations of the CCRs. In particular, in any representation
where the position operator has eigenstates, there is no momentum operator, and
vice versa. Equivalently, if there are nonzero projections corresponding to
sharp position values, all spectral projections of the momentum operator map
onto the zero element.


title: Quantum entanglement and geometry of determinantal varieties

abstract:
  Quantum entanglement was first recognized as a feature of quantum mechanics
in the famous paper of Einstein, Podolsky and Rosen [18]. Recently it has been
realized that quantum entanglement is a key ingredient in quantum computation,
quantum communication and quantum cryptography ([16],[17],[6]). In this paper,
we introduce algebraic sets, which are determinantal varieties in the complex
projective spaces or the products of complex projective spaces, for the mixed
states in bipartite or multipartite quantum systems as their invariants under
local unitary transformations. These invariants are naturally arised from the
physical consideration of measuring mixed states by separable pure states. In
this way algebraic geometry and complex differential geometry of these
algebraic sets turn to be powerful tools for the understanding of quantum
enatanglement. Our construction has applications in the following important
topics in quantum information theory: 1) separability criterion, it is proved
the algebraic sets have to be the sum of the linear subspaces if the mixed
states are separable; 2) lower bound of Schmidt numbers, that is, generic low
rank bipartite mixed states are entangled in many degrees of freedom; 3)
simulation of Hamiltonians, it is proved the simulation of semi-positive
Hamiltonians of the same rank implies the projective isomorphisms of the
corresponding algebraic sets; 4) construction of bound enatanglement, examples
of the entangled mixed states which are invariant under partial transpositions
(thus PPT bound entanglement) are constructed systematically from our new
separability criterion. On the other hand many examples of entangled mixed
states with rich algebraic-geometric structure in their associated
determinantal varieties are constructed and studied from this point of view.


title: Superoscillations and tunneling times

abstract:
  It is proposed that superoscillations play an important role in the
interferences which give rise to superluminal effects. To exemplify that, we
consider a toy model which allows for a wave packet to travel, in zero time and
negligible distortion a distance arbitrarily larger than the width of the wave
packet. The peak is shown to result from a superoscillatory superposition at
the tail. Similar reasoning applies to the dwell time.


title:
Improving quantum interferometry by using entanglement (to take a
  decision you'd better use entanglement)

abstract:
  We address the use of entanglement to improve the precision of generalized
quantum interferometry, i.e. of binary measurements aimed to determine whether
or not a perturbation has been applied by a given device. For the most relevant
operations in quantum optics, we evaluate the optimal detection strategy and
the ultimate bounds to the minimum detectable perturbation. Our results
indicate that entanglement-assisted strategies improve the discrimination in
comparison with conventional schemes. A concrete setup to approach performances
of the optimal strategies is also suggested.


title:
Numerical Simulation of Decoherence-control Processes in Quantum
  Computers

abstract:
  Using a numerical simulation of the evolution of a qubit interacting with the
environment we show that quantum error detection and correction can work
effectively even when the recovery procedure introduces errors.


title: Reconsidering Bohr's reply to EPR

abstract:
  Although Bohr's reply to the EPR argument is supposed to be a watershed
moment in the development of his philosophy of quantum theory, it is difficult
to find a clear statement of the reply's philosophical point. Moreover, some
have claimed that the point is simply that Bohr is a radical positivist. In
this paper, we show that such claims are unfounded. In particular, we give a
mathematically rigorous reconstruction of Bohr's reply to the \emph{original}
EPR argument that clarifies its logical structure, and which shows that it does
not rest on questionable philosophical assumptions. Rather, Bohr's reply is
dictated by his commitment to provide "classical" and "objective" descriptions
of experimental phenomena.


title: The Current State of Coherent States

abstract:
  The original canonical coherent states could be defined in several ways. As
applications for other sets of coherent states arose, the rules of definition
were correspondingly changed. Among such rule changes were a change of group
and relaxation of the analytic nature of the labels. Recent developments have
done away with the group connections altogether and thereby allowed sets of
coherent states to be defined that are temporally stable for a wide variety of
dynamical systems including the hydrogen atom. This article outlines some of
the current trends in the definitions and properties of present-day coherent
states.


title:
Exact, convergent periodic-orbit expansions of individual energy
  eigenvalues of regular quantum graphs

abstract:
  We present exact, explicit, convergent periodic-orbit expansions for
individual energy levels of regular quantum graphs. One simple application is
the energy levels of a particle in a piecewise constant potential. Since the
classical ray trajectories (including ray splitting) in such systems are
strongly chaotic, this result provides the first explicit quantization of a
classically chaotic system.


title: Quantum tomography as a tool for the characterization of optical devices

abstract:
  We describe a novel tool for the quantum characterization of optical devices.
The experimental setup involves a stable reference state that undergoes an
unknown quantum transformation and is then revealed by balanced homodyne
detection. Through tomographic analysis on the homodyne data we are able to
characterize the signal and to estimate parameters of the interaction, such as
the loss of an optical component, or the gain of an amplifier. We present
experimental results for coherent signals, with application to the estimation
of losses introduced by simple optical components, and show how these results
can be extended to the characterization of more general optical devices.


title: Continuous quantum error correction via quantum feedback control

abstract:
  We describe a protocol for continuously protecting unknown quantum states
from decoherence that incorporates design principles from both quantum error
correction and quantum feedback control. Our protocol uses continuous
measurements and Hamiltonian operations, which are weaker control tools than
are typically assumed for quantum error correction. We develop a cost function
appropriate for unknown quantum states and use it to optimize our
state-estimate feedback. Using Monte Carlo simulations, we study our protocol
for the three-qubit bit-flip code in detail and demonstrate that it can improve
the fidelity of quantum states beyond what is achievable using quantum error
correction when the time between quantum error correction cycles is limited.


title: General Scheme for Super Dense Coding between Multi-Parties

abstract:
  Dense coding or super-dense coding in the case of high-dimension quantum
states between two parties and multi-parties has been studied in this paper. We
construct explicitly the measurement basis and the forms of the single-body
unitary operations corresponding to the basis chosen, and the rules for
selecting the one-body unitary operations in a multi-party case.


title:
AC-driven quantum spins: resonant enhancement of transverse DC
  magnetization

abstract:
  We consider s=1/2 spins in the presence of a constant magnetic field in
z-direction and an AC magnetic field in the x-z plane. A nonzero DC
magnetization component in y direction is a result of broken symmetries. A
pairwise interaction between two spins is shown to resonantly increase the
induced magnetization by one order of magnitude. We discuss the mechanism of
this enhancement, which is due to additional avoided crossings in the level
structure of the system.


title: Natural Entanglement in Bose-Einstein Condensates

abstract:
  Every Bose-Einstein condensate is in a highly entangled state, as a
consequence of the fact that the particles in a condensate are distributed over
space in a coherent way. It is proved that any two regions within a condensate
of finite particle number are entangled. This entanglement does not depend on
the distance between the two regions. Criteria for the presence of entanglement
are derived in the context of interference experiments. For separable states
there is a trade-off between fluctuations in particle number and interference
visibility.


title:
Quantum Computation with Coherent States, Linear Interactions and
  Superposed Resources

abstract:
  We show that quantum computation circuits with coherent states as the logical
qubits can be constructed using very simple linear networks, conditional
measurements and coherent superposition resource states.


title: The s-wave implosion and explosion of free particles

abstract:
  A free quantum particle in two dimensions with vanishing angular momentum
(s-wave) in the form of a ring-shaped wave packet feels an attractive force
towards the center of the ring, leading first to an implosion followed by an
explosion.


title: Quantum correlations versus Multisimultaneity: an experimental test

abstract:
  Multisimultaneity is a causal model of relativistic quantum physics which
assigns a real time ordering to any set of events, much in the spirit of the
pilot-wave picture. Contrary to standard quantum mechanics, it predicts a
disappearance of the correlations in a Bell-type experiment when both analysers
are in relative motion such that, each one in its own inertial reference frame,
is first to select the output of the photons. We tested this prediction using
acousto-optic modulators as moving beam-splitters and interferometers separated
by 55 m. We didn't observe any disappearance of the correlations, thus refuting
Multisimultaneity.


title: Limits to phase information in the parametric oscillator

abstract:
  Light in which the quantum fluctuations have been squeezed is often proposed
as a means of obtaining an improved phase reference compared to that available
from coherent light. Such a phase reference contains information about the
phase of the squeezed light, so it is important to calculate the limits to the
amount of ``phase information'' available. I define a phase resolution and show
how this scales as we increase the number of photons available when using
squeezed light generated by a parametric oscillator. Simple schemes for
creating squeezed coherent light using a beam-splitter and an interferometer
are analyzed, and it is shown that the results agree with earlier claims
[Kinsler] that when using squeezed coherent light as a phase reference, the
gain is not one of improved accuracy, but of lower power.


title: Quantum Information Processing with Microfabricated Optical Elements

abstract:
  We discuss a new direction in the field of quantum information processing
with neutral atoms. It is based on the use of microfabricated optical elements.
With these elements versatile and integrated atom optical devices can be
created in a compact fashion. This approach opens the possibility to scale,
parallelize, and miniaturize atom optics for new investigations in fundamental
research and applications towards quantum computing with neutral atoms. The
exploitation of the unique features of the quantum mechanical behavior of
matter waves and the capabilities of powerful state-of-the-art micro- and
nanofabrication techniques lend this approach a special attraction.


title:
Optimal Evaluation of Generalized Euler Angles with Applications to
  Classical and Quantum Control

abstract:
  Given two linearly independent matrices in $so(3)$, $Z_1$ and $Z_2$, every
rotation matrix $X_f \in SO(3)$ can be written as the product of alternate
elements from the one dimensional subgroups corresponding to $Z_1$ and $Z_2$,
namely $X_f=e^{Z_1 t_1}e^{Z_2 t_2}e^{Z_1t_3} \cdot \cdot \cdot e^{Z_1t_s}$. The
parameters $t_i$, $i=1,...,s$ are called {\it generalized Euler angles}.
  In this paper, we evaluate the minimum number of factors required for the
factorization of $X_f \in SO(3)$, as a function of $X_f$, and provide an
algorithm to determine the generalized Euler angles explicitly. The results can
be applied to the bang bang control with minimum number of switches of some
classical control systems and of two level quantum systems.


title: Bang-Bang Operations from a Geometric Perspective

abstract:
  Strong, fast pulses, called ``bang-bang'' controls can be used to eliminate
the effects of system-environment interactions. This method for preventing
errors in quantum information processors is treated here in a geometric setting
which leads to an intuitive perspective. Using this geometric description, we
clarify the notion of group symmetrization as an averaging technique, and
provide a geometric picture for evaluating errors due to imperfect bang-bang
controls. This will provide additional support for the usefulness of such
controls as a means for providing more reliable quantum information processing.


title: Entanglement Enhanced Multiplayer Quantum Games

abstract:
  We investigate the 3-player quantum Prisoner's Dilemma with a certain
strategic space, a particular Nash equilibrium that can remove the original
dilemma is found. Based on this equilibrium, we show that the game is enhanced
by the entanglement of its initial state.


title:
On the Experimental Incompatibility Between Standard and Bohmian Quantum
  Mechanics

abstract:
  Recently, three experiments have been proposed in order to show that the
standard and Bohmian quantum mechanics can have different predictions at the
individual level of particles. However, these thought experiments have
encountered some objections. In this work, it is our purpose to show that our
basic conclusions about those experiments are still intact.


title: Is there a real time ordering behind the nonlocal correlations?

abstract:
  It is argued that recent experiments with moving beam-splitters demonstrate
that there is no real time ordering behind the nonlocal correlations: In Bell's
world there is no "before" and "after".


title:
A New Proof of the Direct Part of Stein's Lemma in Quantum Hypothesis
  Testing

abstract:
  The direct part of Stein's lemma in quantum hypothesis testing is revisited
based on a key operator inequality between a density operator and its pinching.
The operator inequality is used to show a simple proof of the direct part of
Stein's lemma without using Hiai-Petz's theorem, along with an operator
monotone function, and in addition it is also used to show a new proof of
Hiai-Petz's theorem.


title:
Power-Law Energy Splitting Generated By Tunneling Between Non-smooth
  Tori

abstract:
  We discuss the energy level splitting $\Delta\epsilon$ due to quantum
tunneling between congruent tori in phase space. In analytic cases, it is well
known that $\Delta\epsilon$ decays faster than power of $\hbar$ in the
semi-classical limit. This is not true in non-smooth cases, specifically, when
the tori are connected by line on which the Hamiltonian is not smooth. Under
the assumption that the non-smoothness depends only upon the x- or
p-coordinate, the leading term in the semi-classical expansion of
$\Delta\epsilon$ is derived, which shows that $\Delta \epsilon$ decays as
$\hbar^{k+1}$ when $\hbar\to 0$ with k being the order of non-smoothness.


title:
Information extraction and quantum state distortions in continuous
  variable quantum teleportation

abstract:
  We analyze the loss of fidelity in continuous variable teleportation due to
non-maximal entanglement. It is shown that the quantum state distortions
correspond to the measurement back-action of a field amplitude measurement.
Results for coherent states and for photon number states are presented.


title: Surface impedance and the Casimir force

abstract:
  The impedance boundary condition is used to calculate the Casimir force in
configurations of two parallel plates and a shpere (spherical lens) above a
plate at both zero and nonzero temperature. The impedance approach allows one
to find the Casimir force between the realistic test bodies regardless of the
electromagnetic fluctuations inside the media. Although this approach is an
approximate one, it has wider areas of application than the Lifshitz theory of
the Casimir force. The general formulas of the impedance approach to the theory
of the Casimir force are given and the formal substitution is found for
connecting it with the Lifshitz formula. The range of micrometer separations
between the test bodies which is interesting from the experimental point of
view is investigated in detail. It is shown that at zero temperature the
results obtained on the basis of the surface impedance method are in agreement
with those obtained in framework of the Lifshitz theory within a fraction of a
percent. The temperature correction to the Casimir force from the impedance
method coincides with that from the Lifshitz theory up to four significant
figures. The case of millimeter separations which corresponds to the normal
skin effect is also considered. At zero temperature the obtained results have
good agreement with the Lifshitz theory. At nonzero temperature the impedance
approach is not subject to the interpretation problems peculiar to the
zero-frequency term of the Lifshitz formula in dissipative media.


title: Polarization Squeezing of Continuous Variable Stokes Parameters

abstract:
  We report the first direct experimental characterization of continuous
variable quantum Stokes parameters. We generate a continuous wave light beam
with more than 3 dB of simultaneous squeezing in three of the four Stokes
parameters. The polarization squeezed beam is produced by mixing two quadrature
squeezed beams on a polarizing beam splitter. Depending on the squeezed
quadrature of these two beams the quantum uncertainty volume on the
Poincar\'{e} sphere became a `cigar' or `pancake'-like ellipsoid.


title: Compressing Mixed-State Sources by Sending Classical Information

abstract:
  We consider visible compression for discrete memoryless sources of mixed
quantum states when only classical information can be sent from Alice to Bob.
We assume that Bob knows the source statistics, and that Alice and Bob have
identical random number generators. We put in an information theoretic
framework some recent results on visible compression for sources of states with
commuting density operators, and remove the commutativity requirement. We
derive a general achievable compression rate, which is for the noncommutative
case still higher than the known lower bound. We also present several related
problems of classical information theory, and show how they can be used to
answer some questions of the mixed state compression problem.


title: A GHZ-type proof of Bell's Theorem for A Two-particle Singlet State

abstract:
  For the case of two spin-1/2 particles in the singlet state, we provide a
GHZ-type proof of Bell's theorem by using the idea of postselected
measurements. Furthermore, we show that in spite of the low efficiency of the
detectors one can derive an inequality in the case of real experiments which is
violated by quantum mechanics.


title: Single particle nonlocality: A proposed experimental test

abstract:
  We show that controlled interference of a particle's wavefunction can be used
to perform a quantum mechanical measurement in an incomplete basis. This
happens because the measurement projects the particle into a lower dimensional
subspace of the Hilbert space of the incoming wave. It allows a sender (Alice)
to signal the receiver (Bob) nonlocally, by Alice's choosing to measure in a
complete or incomplete basis (in general: bases of differing incompleteness).
If experimentally confirmed, it furnishes a new quantum communication act:
nonlocal transmission of a bit without concomitant causal communication.
However, the question of its compatibility with special relativity remains
unresolved.


title:
Creation and annihilation operators and coherent states for the
  PT-symmetric oscillator

abstract:
  We construct two commuting sets of creation and annihilation operators for
the PT-symmetric oscillator. We then build coherent states of the latter as
eigenstates of such annihilation operators by employing a modified version of
the normalization integral that is relevant to PT-symmetric systems. We show
that the coherent states are normalizable only in the range (0, 1) of the
underlying coupling parameter $\alpha$.


title: On the concepts of radial and angular kinetic energies

abstract:
  We consider a general central-field system in D dimensions and show that the
division of the kinetic energy into radial and angular parts proceeds
differently in the wavefunction picture and the Weyl-Wigner phase-space
picture. Thus, the radial and angular kinetic energies are different quantities
in the two pictures, containing different physical information, but the
relation between them is well defined. We discuss this relation and illustrate
its nature by examples referring to a free particle and to a ground-state
hydrogen atom.


title: Semiclassical properties and chaos degree for the quantum baker's map

abstract:
  We study the chaotic behaviour and the quantum-classical correspondence for
the baker's map. Correspondence between quantum and classical expectation
values is investigated and it is numerically shown that it is lost at the
logarithmic timescale. The quantum chaos degree is computed and it is
demonstrated that it describes the chaotic features of the model. The
correspondence between classical and quantum chaos degrees is considered.


title: Quantum Algorithm for Hilbert's Tenth Problem

abstract:
  We explore in the framework of Quantum Computation the notion of {\em
Computability}, which holds a central position in Mathematics and Theoretical
Computer Science. A quantum algorithm for Hilbert's tenth problem, which is
equivalent to the Turing halting problem and is known to be mathematically
noncomputable, is proposed where quantum continuous variables and quantum
adiabatic evolution are employed. If this algorithm could be physically
implemented, as much as it is valid in principle--that is, if certain
hamiltonian and its ground state can be physically constructed according to the
proposal--quantum computability would surpass classical computability as
delimited by the Church-Turing thesis. It is thus argued that computability,
and with it the limits of Mathematics, ought to be determined not solely by
Mathematics itself but also by Physical Principles.


title: Accardi contra Bell (cum mundi): The Impossible Coupling

abstract:
  An experimentally observed violation of Bell's inequality is supposed to show
the failure of local realism to deal with quantum reality. However, finite
statistics and the time sequential nature of real experiments still allow a
loophole for local realism, known as the memory loophole. We show that the
randomized design of the Aspect experiment closes this loophole. Our main tool
is van de Geer's (2000) supermartingale version of the classical Bernstein
(1924) inequality guaranteeing, at the root n scale, a
not-heavier-than-Gaussian tail of the distribution of a sum of bounded
supermartingale differences. The results are used to specify a protocol for a
public bet between the author and L. Accardi, who in recent papers (Accardi and
Regoli, 2000a,b, 2001; Accardi, Imafuku and Regoli, 2002) has claimed to have
produced a suite of computer programmes, to be run on a network of computers,
which will simulate a violation of Bell's inequalites. At a sample size of
thirty thousand, both error probabilities are guaranteed smaller than one in a
million, provided we adhere to the sequential randomized design. The results
also show that Hess and Philipp's (2001a,b) recent claims are mistaken that
Bell's theorem fails because of time phenomena supposedly neglected by Bell.


title: Conditional generation of N-photon entangled states of light

abstract:
  We propose a scheme for conditional generation of two-mode N-photon
path-entangled states of traveling light field. These states may find
applications in quantum optical lithography and they may be used to improve the
sensitivity of interferometric measurements. Our method requires only
single-photon sources, linear optics (beam splitters and phase shifters), and
photodetectors with single photon sensitivity.


title: Quantifying entanglement with probabilities

abstract:
  We propose a new approach to the problem of defining the degree of
entanglement between two particles in a pure state with Hilbert spaces of
arbitrary finite dimensions. The central idea is that entanglement gives rise
to correlations between the particles that do not occur in separable states. We
individuate the contributions of these correlations to the joint and the
conditional probabilities of local measurements outcomes. We use these
probabilities to define the measure of entanglement. Our measure turns out to
be proportional to the so-called 2-entropy and therefore satisfies the
properties required for any measure of entanglement. We conclude with an
outlook on the problem of extending our approach to the case of multipartite
systems and mixed states.


title:
Microoptical Realization of Arrays of Selectively Addressable Dipole
  Traps: A Scalable Configuration for Quantum Computation with Atomic Qubits

abstract:
  We experimentally demonstrate novel structures for the realisation of
registers of atomic qubits: We trap neutral atoms in one and two-dimensional
arrays of far-detuned dipole traps obtained by focusing a red-detuned laser
beam with a microfabricated array of microlenses. We are able to selectively
address individual trap sites due to their large lateral separation of 125 mu
m. We initialize and read out different internal states for the individual
sites. We also create two interleaved sets of trap arrays with adjustable
separation, as required for many proposed implementations of quantum gate
operations.


title: Computational capacity of the universe

abstract:
  Merely by existing, all physical systems register information. And by
evolving dynamically in time, they transform and process that information. The
laws of physics determine the amount of information that a physical system can
register (number of bits) and the number of elementary logic operations that a
system can perform (number of ops). The universe is a physical system. This
paper quantifies the amount of information that the universe can register and
the number of elementary operations that it can have performed over its
history. The universe can have performed no more than $10^{120}$ ops on
$10^{90}$ bits.


title:
Efficient quantum computation of high harmonics of the Liouville density
  distribution

abstract:
  We show explicitly that high harmonics of the classical Liouville density
distribution in the chaotic regime can be obtained efficiently on a quantum
computer [1,2]. As was stated in [1], this provides information unaccessible
for classical computer simulations, and replies to the questions raised in
[3,4].


title: Topological quantum memory

abstract:
  We analyze surface codes, the topological quantum error-correcting codes
introduced by Kitaev. In these codes, qubits are arranged in a two-dimensional
array on a surface of nontrivial topology, and encoded quantum operations are
associated with nontrivial homology cycles of the surface. We formulate
protocols for error recovery, and study the efficacy of these protocols. An
order-disorder phase transition occurs in this system at a nonzero critical
value of the error rate; if the error rate is below the critical value (the
accuracy threshold), encoded information can be protected arbitrarily well in
the limit of a large code block. This phase transition can be accurately
modeled by a three-dimensional Z_2 lattice gauge theory with quenched disorder.
We estimate the accuracy threshold, assuming that all quantum gates are local,
that qubits can be measured rapidly, and that polynomial-size classical
computations can be executed instantaneously. We also devise a robust recovery
procedure that does not require measurement or fast classical processing;
however for this procedure the quantum gates are local only if the qubits are
arranged in four or more spatial dimensions. We discuss procedures for
encoding, measurement, and performing fault-tolerant universal quantum
computation with surface codes, and argue that these codes provide a promising
framework for quantum computing architectures.

categories: quant-ph cond-mat.stat-mech hep-lat hep-th


title: A Note on Linear Optics Gates by Post-Selection

abstract:
  Recently it was realized that linear optics and photo-detectors with feedback
can be used for theoretically efficient quantum information processing. The
first of three steps toward efficient linear optics quantum computation (eLOQC)
was to design a simple non-deterministic gate, which upon post-selection based
on a measurement result implements a non-linear phase shift on one mode. Here a
computational strategy is given for finding non-deterministic gates for bosonic
qubits with helper photons. A more efficient conditional sign flip gate is
obtained.


title: Motion of vortex lines in nonlinear wave mechanics

abstract:
  We extend our previous analysis of the motion of vortex lines [I.
Bialynicki-Birula, Z. Bialynicka-Birula and C. Sliwa, Phys. Rev. A 61, 032110
(2000)] from linear to a nonlinear Schroedinger equation with harmonic forces.
We also argue that under certain conditions the influence of the contact
nonlinearity on the motion of vortex lines is negligible. The present analysis
adds new weight to our previous conjecture that the topological features of
vortex dynamics are to a large extent universal.


title:
Quantum-optical states in finite-dimensional Hilbert space. II. State
  generation

abstract:
  In the second part of our review (for the first part see quant-ph/0108080),
we discuss a physical model for generation of "truncated" coherent and squeezed
states in finite-dimensional Hibert spaces.


title:
Controllability of quantum mechanical systems by root space
  decomposition of su(N)

abstract:
  The controllability property of the unitary propagator of an N-level quantum
mechanical system subject to a single control field is described using the
structure theory of semisimple Lie algebras. Sufficient conditions are provided
for the vector fields in a generic configuration as well as in a few degenerate
cases.


title: The basis problem in many-worlds theories

abstract:
  It is emphasized that a many-worlds interpretation of quantum theory exists
only to the extent that the associated basis problem is solved. The core basis
problem is that the robust enduring states specified by environmental
decoherence effects are essentially Gaussian wave packets that form continua of
non-orthogonal states. Hence they are not a discrete set of orthogonal basis
states to which finite probabilities can be assigned by the usual rules. The
natural way to get an orthogonal basis without going outside the Schroedinger
dynamics is to use the eigenstates of the reduced density matrix, and this idea
is the basis of some recent attempts by many-worlds proponents to solve the
basis problem. But these eigenstates do not enjoy the locality and
quasi-classicality properties of the states defined by environmental
decoherence effects, and hence are not satisfactory preferred basis states. The
basis problem needs to be addressed and resolved before a many-worlds-type
interpretation can be said to exist.


title:
Generation of entangled states of two traveling modes for fixed number
  of photon

abstract:
  We present an scheme to generate entangled state between two traveling modes
for fixed number of photon, which is based on beam splitter transformation and
conditional zero photon counters.


title: Bifurcation Phenomenon in a Spin Relaxation

abstract:
  Spin relaxation in a strong-coupling regime (with respect to the spin system)
is investigated in detail based on the spin-boson model in a stochastic limit.
We find a bifurcation phenomenon in temperature dependence of relaxation
constants, which is never observed in the weak-coupling regime. We also discuss
inequalities among the relaxation constants in our model and show the
well-known relation 2\Gamma_T >= \Gamma_L, for example, for a wider parameter
region than before.


title: Entanglement transfer from continuous variables to qubits

abstract:
  We show that two qubits can be entangled by local interactions with an
entangled two-mode continuous variable state. This is illustrated by the
evolution of two two-level atoms interacting with a two-mode squeezed state.
Two modes of the squeezed field are injected respectively into two spatially
separate cavities and the atoms are then sent into the cavities to resonantly
interact with the cavity field. We find that the atoms may be entangled even by
a two-mode squeezed state which has been decohered while penetrating into the
cavity.


title: Landau quantum systems: an approach based on symmetry

abstract:
  We show that the Landau quantum systems (or integer quantum Hall effect
systems) in a plane, sphere or a hyperboloid, can be explained in a complete
meaningful way from group-theoretical considerations concerning the symmetry
group of the corresponding configuration space. The crucial point in our
development is the role played by locality and its appropriate mathematical
framework, the fiber bundles. In this way the Landau levels can be understood
as the local equivalence classes of the symmetry group. We develop a unified
treatment that supplies the correct geometric way to recover the planar case as
a limit of the spherical or the hyperbolic quantum systems when the curvature
goes to zero. This is an interesting case where a contraction procedure gives
rise to nontrivial cohomology starting from a trivial one. We show how to
reduce the quantum hyperbolic Landau problem to a Morse system using horocyclic
coordinates. An algebraic analysis of the eigenvalue equation allow us to build
ladder operators which can help in solving the spectrum under different
boundary conditions.


title: Jahn-Teller Anharmonic Coupling for an Octahedral System

abstract:
  The coupling between doubly degenerate electronic states and doubly
degenerate vibrations is analysed for an octahedral system on the basis of the
introduction of an anharmonic Morse potential for the vibronic part. The
vibrations are described by anharmonic coherent states and their linear
coupling with the electronic states is considered. The matrix elements of the
vibronic interaction are builded and the energy levels corresponding to the
interaction Hamiltonian are derived.


title: Multi-Parameter Entanglement in Femtosecond Parametric Down-Conversion

abstract:
  A theory of spontaneous parametric down-conversion, which gives rise to a
quantum state that is entangled in multiple parameters, such as
three-dimensional wavevector and polarization, allows us to understand the
unusual characteristics of fourth-order quantum interference in many
experiments, including ultrafast type-II parametric down-conversion, the
specific example illustrated in this paper. The comprehensive approach provided
here permits the engineering of quantum states suitable for quantum information
schemes and new quantum technologies.


title: Matter waves from quantum sources in a force field

abstract:
  Localized scattering phenomena may result in the formation of stationary
matter waves originating from a compact region in physical space.
Mathematically, such waves are advantageously expressed in terms of quantum
sources that are introduced into the Schr\"odinger equation. The source
formalism yields direct access to the scattering wave function, particle
distribution, and total current. As an example, we study emission from
three-dimensional Gaussian sources into a homogeneous force field. This model
describes the behaviour of an atom laser supplied by an ideal Bose-Einstein
condensate under the influence of gravity. We predict a strong dependence of
the beam profile on the condensate size and the presence of interference
phenomena recently observed in photodetachment experiments.


title:
Limits to clock synchronization induced by completely dephasing
  communication channels

abstract:
  Clock synchronization procedures are analyzed in the presence of imperfect
communications. In this context we show that there are physical limitations
which prevent one from synchronizing distant clocks when the intervening medium
is completely dephasing, as in the case of a rapidly varying dispersive medium.


title: A new dynamical group approach to the modified Poschl-Teller potential

abstract:
  The properties of the modified P\"{o}schl-Teller (MPT) potential are
outlined. The ladder operators are constructed directly from the wave functions
without introducing any auxiliary variable. It is shown that these operators
are associated to the $su(2)$ algebra. Analytical expressions for the functions
$\sinh(\alpha x)$ and $\frac{\cosh(\alpha x)}{\alpha} \frac{d}{dx}$ are
evaluated from these ladder operators. The expansions of the coordinate $x$ and
momentum $\hat p$ in terms of the $su(2)$ generators are presented. This
analysis allows to establish an exact quantum-mechanical connection between the
$su(2)$ vibron model and the traditional descriptions of molecular vibron.


title: The (2+1) Dirac Equations with $\delta$ Potential

abstract:
  In this Letter the bound states of (2+1) Dirac equation with the
cylindrically symmetric $\delta (r-r_{0})$-potential are discussed. It is
surprisingly found that the relation between the radial functions at two sides
of $r_{0}$ can be established by an SO(2) transformation. We obtain a
transcendental equation for calculating the energy of the bound state from the
matching condition in the configuration space. The condition for existence of
bound states is determined by the Sturm-Liouville theorem.


title: The vacuum interpretation of quantum mechanics and the vacuum universe

abstract:
  Quantum mechanics is interpreted by the adjacent vacuum that behaves as a
virtual particle to be absorbed and emitted by its matter. As described in the
vacuum universe model, the adjacent vacuum is derived from the pre-inflationary
universe in which the pre-adjacent vacuum is absorbed by the pre-matter. This
absorbed pre-adjacent vacuum is emitted to become the added space for the
inflation in the inflationary universe whose space-time is separated from the
pre-inflationary universe. This added space is the adjacent vacuum. The
absorption of the adjacent vacuum as the added space results in the adjacent
zero space (no space), Quantum mechanics is the interaction between matter and
the three different types of vacuum: the adjacent vacuum, the adjacent zero
space, and the empty space. The absorption of the adjacent vacuum results in
the empty space superimposed with the adjacent zero space, confining the matter
in the form of particle. When the absorbed vacuum is emitted, the adjacent
vacuum can be anywhere instantly in the empty space superimposed with the
adjacent zero space where any point can be the starting point (zero point) of
space-time. Consequently, the matter that expands into the adjacent vacuum has
the probability to be anywhere instantly in the form of wavefunction. In the
vacuum universe model, the universe not only gains its existence from the
vacuum but also fattens itself with the vacuum. During the inflation, the
adjacent vacuum also generates the periodic table of elementary particles to
account for all elementary particles and their masses in a good agreement with
the observed values.


title: Some Advantages of a Local Realist, 3D Wave Soliton Approach to EPR

abstract:
  Not only are the foundation theories mutually compatible, they are also
compatible with local realism once this concept is properly formulated (without
presuming atomism in addition to locality). Relativity Theory is reconstructed
in the context of a preferred frame, but so as to secure the Relativity
Principle in every experiment except the measurement of the preferred frame,
defined by a null result for the dipole component of the Microwave Background
Radiation (MBR). The 3D soliton approach to physical modelling is found to be
consistent with both Special Relativity and Quantum Mechanical Nonlocality,
exemplified by the EPR paradox.


title: Quantising the Foliation in History Quantum Field Theory

abstract:
  As a preliminary to discussing the quantisation of the foliation in a history
form of general relativity, we show how the discussion in \cite{SavQFT1} of a
history version of free, scalar quantum field theory can be augmented in such a
way as to include the quantisation of the unit-length, time-like vector that
determines a Lorentzian foliation of Minkowski spacetime. We employ a Hilbert
bundle construction that is motivated by: (i) discussing the role of the
external Lorentz group in the existing history quantum field theory
\cite{SavQFT1}; and (ii) considering a specific representation of the extended
history algebra obtained from the multi-symplectic representation of scalar
field theory.


title: The two-body interaction with a circle in time

abstract:
  We complete our previous(1, 2) demonstration that there is a family of new
solutions to the photon and Dirac equations using spatial and temporal circles
and four-vector behaviour of the Dirac bispinor. We analyse one solution for a
bound state, which is equivalent to the attractive two-body interaction between
a charged point particle and a second, which remains at rest. We show this
yields energy and angular momentum eigenvalues that are identical to those
found by the usual method of solving of the Dirac equation,(4) including fine
structure. We complete our previous derivation(3) of QED from a set of rules
for the two-body interaction and generalise these. We show that QED may be
decomposed into a two-body interaction at every point in spacetime.


title: Quantum Effects in Matter-Wave Diffraction

abstract:
  Advances in micro-technology of the last years have made it possible to carry
optics textbooks experiments over to atomic and molecular beams, such as
diffraction by a double slit or transmission grating. The usual wave-optical
approach gives a good qualitative description of these experiments. However,
small deviations therefrom and sophisticated quantum mechanics yield new
surprising insights on the size of particles and on their interaction with
surfaces.


title: Quantum homogenization

abstract:
  We design a universal quantum homogenizer, which is a quantum machine that
takes as an input a system qubit initially in the state $\rho$ and a set of N
reservoir qubits initially prepared in the same state $\xi$. In the homogenizer
the system qubit sequentially interacts with the reservoir qubits via the
partial swap transformation. The homogenizer realizes, in the limit sense, the
transformation such that at the output each qubit is in an arbitratily small
neighbourhood of the state $\xi$ irrespective of the initial states of the
system and the reservoir qubits. This means that the system qubit undergoes an
evolution that has a fixed point, which is the reservoir state $\xi$. We also
study approximate homogenization when the reservoir is composed of a finite set
of identically prepared qubits. The homogenizer allows us to understand various
aspects of the dynamics of open systems interacting with environments in
non-equilibrium states. In particular, the reversibility vs or irreversibility
of the dynamics of the open system is directly linked to specific (classical)
information about the order in which the reservoir qubits interacted with the
system qubit. This aspect of the homogenizer leads to a model of a quantum safe
with a classical combination.We analyze in detail how entanglement between the
reservoir and the system is created during the process of quantum
homogenization. We show that the information about the initial state of the
system qubit is stored in the entanglement between the homogenized qubits.


title: Rigged Hilbert Space Approach to the Schrodinger Equation

abstract:
  It is shown that the natural framework for the solutions of any Schrodinger
equation whose spectrum has a continuous part is the Rigged Hilbert Space
rather than just the Hilbert space. The difficulties of using only the Hilbert
space to handle unbounded Schrodinger Hamiltonians whose spectrum has a
continuous part are disclosed. Those difficulties are overcome by using an
appropriate Rigged Hilbert Space (RHS). The RHS is able to associate an
eigenket to each energy in the spectrum of the Hamiltonian, regardless of
whether the energy belongs to the discrete or to the continuous part of the
spectrum. The collection of eigenkets corresponding to both discrete and
continuous spectra forms a basis system that can be used to expand any physical
wave function. Thus the RHS treats discrete energies (discrete spectrum) and
scattering energies (continuous spectrum) on the same footing.


title: Substituting a qubit for an arbitrarily large number of classical bits

abstract:
  We show that a qubit can be used to substitute for an arbitrarily large
number of classical bits. We consider a physical system S interacting locally
with a classical field phi(x) as it travels directly from point A to point B.
The field has the property that its integrated value is an integer multiple of
some constant. The problem is to determine whether the integer is odd or even.
This task can be performed perfectly if S is a qubit. On the otherhand, if S is
a classical system then we show that it must carry an arbitrarily large amount
of classical information. We identify the physical reason for such a huge
quantum advantage, and show that it also implies a large difference between the
size of quantum and classical memories necessary for some computations. We also
present a simple proof that no finite amount of one-way classical communication
can perfectly simulate the effect of quantum entanglement.


title:
A family of exact eigenstates for a single trapped ion interacting with
  a laser field

abstract:
  We show that, under certain combinations of the parameters governing the
interaction of a harmonically trapped ion with a laser beam, it is possible to
find one or more exact eigenstates of the Hamiltonian, with no approximations
except the optical rotating-wave approximation. These are related via a unitary
equivalence to exact eigenstates of the full Jaynes-Cummings model (including
counter-rotating terms) supplemented by a static driving term.


title:
The feasible generation of entangled photon states by using linear
  optical elements

abstract:
  We present a feasible scheme to produce a polarization-entangled photon
states $\frac{1}{\sqrt{2}}(|H>|V>+|V>|H>)$ in a controllable way. This scheme
requires single-photon sources, linear optical elements and photon detectors.
It generates the entanglement of spatially separated photons. The interaction
takes place in the photon detectors. We also show that the same idea can be
used to produce the entangled $N$-photon state
$\frac{1}{\sqrt{2}}(|0,N>+|N,0>)$


title: Spin-Statistics Theorem and Geometric Quantisation

abstract:
  We study how the spin-statistics theorem relates to the geometric structures
on phase space that are introduced in quantisation procedures (namely a U(1)
bundle and connection). The relation can be proved in both the relativistic and
the non-relativistic domain (in fact for any symmetry group including internal
symmetries) without quantum field theory, by the requirement that the exchange
can be implemented smoothly by a class of symmetry transformations that project
in the phase space of the joint system. We discuss the interpretation of this
requirement, stressing the fact that any distinction of identical particles
comes solely from the choice of coordinates. We then examine our construction
in the geometric and the coherent-state-path-integral quantisation schemes. In
the appendix we apply our results to exotic systems exhibiting continuous
``spin'' and ``fractional statistics''. This gives novel and unusual forms of
the spin-statistics relation.


title: Photon number conservation and photon interference

abstract:
  The group theoretical aspect of the description of passive lossless optical
four-ports (beam splitters) is revisited. It is shown through an example, that
this approach can be useful in understanding interferometric schemes where a
low number of photons interfere. The formalism is extended to passive lossless
optical six-ports, their SU(3)-theory is outlined.


title: Criteria for reachability of quantum states

abstract:
  We address the question of which quantum states can be inter-converted under
the action of a time-dependent Hamiltonian. In particular, we consider the
problem applied to mixed states, and investigate the difference between pure
and mixed-state controllability introduced in previous work. We provide a
complete characterization of the eigenvalue spectrum for which the state is
controllable under the action of the symplectic group. We also address the
problem of which states can be prepared if the dynamical Lie group is not
sufficiently large to allow the system to be controllable.


title: Quantum Entanglement and the Two-Photon Stokes Parameters

abstract:
  A formalism for two-photon Stokes parameters is introduced to describe the
polarization entanglement of photon pairs. This leads to the definition of a
degree of two-photon polarization, which describes the extent to which the two
photons act as a pair and not as two independent photons. This pair-wise
polarization is complementary to the degree of polarization of the individual
photons. The approach provided here has a number of advantages over the density
matrix formalism: it allows the one- and two-photon features of the state to be
separated and offers a visualization of the mixedness of the state of
polarization.


title: Quantum open systems and turbulence

abstract:
  We show that the problem of non conservation of energy found in the
spontaneous localization model developed by Ghirardi, Rimini and Weber is very
similar to the inconsistency between the stochastic models for turbulence and
the Navier-Stokes equation. This sort of analogy may be useful in the
development of both areas.


title:
Characterization of Distillable and Activable States using Entanglement
  Witnesses

abstract:
  We introduce a formalism that connections entanglement witnesses and the
distillation and activation properties of a state. We apply this formalism to
two cases: First, we rederive the results presented in quant-ph/0104095 by
Eggeling et al., namely that on copy of any bipartite state with non--positive
partial transpose (NPPT) is either distillable, or activable. Second, we show
that there exist three--partite NPPT states, with the property that two copies
can neither be distilled, nor activated.


title:
Spin 1/2 as propagation on a lattice with symmetries modulo gauge
  transformations

abstract:
  Relativistic spin 1/2, as represented by Susskind's 1977 discretization of
the Dirac equation on a spatial lattice, is shown to follow from basic, not
typically relativistic but essentially quantum theoretic assumptions: that
position eigenstates propagate to nearest neighbours while respecting lattice
symmetries modulo gauge transformations.


title: Room temperature stable single-photon source

abstract:
  We report on the realization of a stable solid state room temperature source
for single photons. It is based on the fluorescence of a single
nitrogen-vacancy (NV) color center in a diamond nanocrystal. Antibunching has
been observed in the fluorescence light under both continuous and pulsed
excitation. Our source delivers 2*10^4 single-photon pulses per second at an
excitation repetition rate of 10 MHz. The number of two-photon pulses is
reduced by a factor of five compared to strongly attenuated coherent sources.


title:
Teleportation implementation of non-deterministic quantum logic
  operations by using linear optical elements

abstract:
  We present a feasible scheme to implement the non-deterministic quantum logic
operation of Knill, Laflamme and Milburn (Nature, 409, 46-52(2001)) by using a
teleportation protocol, which requires only single-photon sources, linear
optical elements and photon detectors.


title:
Comment on "Fun and frustration with quarkonium in a 1+1 dimension," by
  R. S. Bhalerao and B. Ram [Am. J. Phys. 69 (7), 817-818 (2001)]

abstract:
  The Dirac equation in a 1+1 dimension with the Lorentz scalar potential g|x|
is approached. It is claimed that the eigenfunctions are proportional to the
parabolic cylinder functions instead Hermite polynomials. Numerical evaluation
of the quantization condition does not result in frustration.


title: Local Deterministic Transformations of Three-Qubit Pure States

abstract:
  The properties of deterministic LOCC transformations of three qubit pure
states are studied. We show that the set of states in the GHZ class breaks into
an infinite number of disjoint classes under this type of transformation. These
classes are characterized by the value of a quantity that is invariant under
these transformations, and is defined in terms of the coefficients of a
particular canonical form in which only states in the GHZ class can be
expressed. This invariant also imposes a strong constraint on any POVM that is
part of a deterministic protocol. We also consider a transformation generated
by a local 2-outcome POVM and study under what conditions it is deterministic,
i.e., both outcomes belong to the same orbit. We prove that for real states it
is always possible to find such a POVM and we discuss analytical and numerical
evidence that suggests that this result also holds for complex states. We study
the transformation generated in the space of orbits when one or more parties
apply several deterministic POVMs in succession and use these results to give a
complete characterization of the real states that can be obtained from the GHZ
state with probability 1.


title: Quantum filter for non-local polarization properties of photonic qubits

abstract:
  We present an optical filter that transmits photon pairs only if they share
the same horizontal or vertical polarization, without decreasing the quantum
coherence between these two possibilities. Various applications for
entanglement manipulations and multi-photon qubits are discussed.


title: Quantum Chaotic Environments, The Butterfly Effect, And Decoherence

abstract:
  We investigate the sensitivity of quantum systems that are chaotic in a
classical limit, to small perturbations of their equations of motion. This
sensitivity, originally studied in the context of defining quantum chaos, is
relevant to decoherence in situations when the environment has a chaotic
classical counterpart.


title: Exact and Approximate Performance of Concatenated Quantum Codes

abstract:
  We derive the effective channel for a logical qubit protected by an arbitrary
quantum error-correcting code, and derive the map between channels induced by
concatenation. For certain codes in the presence of single-bit Pauli errors, we
calculate the exact threshold error probability for perfect fidelity in the
infinite concatenation limit. We then use the control theory technique of
balanced truncation to find low-order non-asymptotic approximations for the
effective channel dynamics.


title: Random low rank mixed states are highly entangled

abstract:
  We prove that for many ranks r<2m-2, random rank r mixed states in bipartite
mxm systems have relatively high Schmidt numbers, which is based on
algebraic-geometric separability criterion proved in [1]. This also means that
the algebraic-geometric separability criterion can be used to detect all low
rank entangled mixed states outside a measure zero set.


title: Approach to Quantum Kramers' Equation and Barrier Crossing Dynamics

abstract:
  We have presented a simple approach to quantum theory of Brownian motion and
barrier crossing dynamics. Based on an initial coherent state representation of
bath oscillators and an equilibrium canonical distribution of quantum
mechanical mean values of their co-ordinates and momenta we have derived a
$c$-number generalized quantum Langevin equation. The approach allows us to
implement the method of classical non-Markovian Brownian motion to realize an
exact generalized non-Markovian quantum Kramers' equation. The equation is
valid for arbitrary temperature and friction. We have solved this equation in
the spatial diffusion-limited regime to derive quantum Kramers' rate of barrier
crossing and analyze its variation as a function of temperature and friction.
While almost all the earlier theories rest on quasi-probability distribution
functions (like Wigner function) and path integral methods, the present work is
based on {\it true probability distribution functions} and is independent of
path integral techniques. The theory is a natural extension of the classical
theory to quantum domain and provides a unified description of thermal
activated processes and tunneling.


title: Quantum-like formalism for cognitive measurements

abstract:
  We develop a kind of quantum formalism (Hilbert space probabilistic calculus)
for measurements performed over cognitive (in particular, conscious) systems.
By using this formalism we could predict averages of cognitive observables.
Reflecting the basic idea of neurophisiological and psychological studies on a
hierarchic structure of cognitive processes, we use p-adic hierarchic trees as
a mathematical model of a mental space. We also briefly discuss the general
problem of the choice of adequate mental geometry.


title:
Inhomogeneous boson realizations of the polynomial angular momentum
  algebra of arbitrary power

abstract:
  The inhomogeneous single-, two- and three-boson realizations of the more
general polynomial angular momentum algebra SU_n(2) are obtained from the Fock
representations of SU_n(2) that corresponds to the indecomposable
representation on the space of universal enveloping algebra U(SU_n(2)) and to
the induced representations on the quotient spaces U(SU_n(2))/I_i, with I_i
being some left ideals of U(SU_n(2)).


title: Path Integral Approach to the Non-Relativistic Electron Charge Transfer

abstract:
  A path integral approach has been generalized for the non-relativistic
electron charge transfer processes. The charge transfer - the capture of an
electron by an ion passing another atom or more generally the problem of
rearrangement collisions is formulated in terms of influence functionals. It
has been shown that the electron charge transfer process can be treated either
as electron transition problem or as elastic scattering of ion and atom in the
some effective potential field. The first-order Born approximation for the
electron charge transfer cross section has been reproduced to prove the
adequacy of the path integral approach for this problem.


title: The quantum algorithm of Kieu does not solve the Hilbert's tenth problem

abstract:
  Recently T. Kieu (arXiv:quant-ph/0110136) claimed a quantum algorithm
computing some functions beyond the Church-Turing class. He notes that "it is
in fact widely believed that quantum computation cannot offer anything new
about computability" and claims the opposite. However, his quantum algorithm
does not work, which is the point of my short note. I still believe that
quantum computation leads to new complexity but retains the old computability.


title: Temporal Windowing of Trapped States

abstract:
  Trapped state definition for 3-level atoms in Lambda configuration, is a very
restrictive one, and for the case of unpolarized beams, this definition no
longer holds.We introduce a more general definition by using a reference frame
rotating with the frequency of the control field, obtaining a temporal
windowing for the trapped population.This amounts to a time quantization of the
coherent population transfer, making possible to study the phase coherence in
trapped light.


title:
Solution of the one-dimensional Dirac equation with a linear scalar
  potential

abstract:
  We solve the Dirac equation in one space dimension for the case of a linear,
Lorentz-scalar potential. This extends earlier work of Bhalerao and Ram [Am. J.
Phys. 69 (7), 817-818 (2001)] by eliminating unnecessary constraints. The
spectrum is shown to match smoothly to the nonrelativistic spectrum in a
weak-coupling limit.


title: Measurements of semi-local and non-maximally entangled states

abstract:
  Consistency with relativistic causality narrows down dramatically the class
of measurable observables. We argue that by weakening the preparation role of
ideal measurements, many of these observables become measurable. Particularly,
we show by applying entanglement assisted remote operations, that all Hermitian
observable of a $2\times2$-dimensional bipartite system are measurable.


title: Adaptive Quantum Measurements of a Continuously Varying Phase

abstract:
  We analyze the problem of quantum-limited estimation of a stochastically
varying phase of a continuous beam (rather than a pulse) of the electromagnetic
field. We consider both non-adaptive and adaptive measurements, and both dyne
detection (using a local oscillator) and interferometric detection. We take the
phase variation to be \dot\phi = \sqrt{\kappa}\xi(t), where \xi(t) is
\delta-correlated Gaussian noise. For a beam of power P, the important
dimensionless parameter is N=P/\hbar\omega\kappa, the number of photons per
coherence time. For the case of dyne detection, both continuous-wave (cw)
coherent beams and cw (broadband) squeezed beams are considered. For a coherent
beam a simple feedback scheme gives good results, with a phase variance \simeq
N^{-1/2}/2. This is \sqrt{2} times smaller than that achievable by nonadaptive
(heterodyne) detection. For a squeezed beam a more accurate feedback scheme
gives a variance scaling as N^{-2/3}, compared to N^{-1/2} for heterodyne
detection. For the case of interferometry only a coherent input into one port
is considered. The locally optimal feedback scheme is identified, and it is
shown to give a variance scaling as N^{-1/2}. It offers a significant
improvement over nonadaptive interferometry only for N of order unity.


title:
The generation of an entangled four-photon state from two pairs of
  entangled two-photon states by using linear optical elements

abstract:
  We present a scheme to produce an entangled four-photon state from two pairs
of entangled two-photon states. Such entangled four-photon states are
equivalent to the quantum state of two maximally entangled spin-1 particles.
The scheme can also be generated to prepare an entangled 2N-photon state from N
pairs of entangled two-photon states. Such multi-photon states play a crucial
role in fundamental tests of quantum mechanics versus local realism and in many
quantum information and quantum computation schemes.


title: Purification of entangled coherent states

abstract:
  We suggest an entanglement purification scheme for mixed entangled coherent
states using 50-50 beam splitters and photodetectors. This scheme is directly
applicable for mixed entangled coherent states of the Werner type, and can be
useful for general mixed states using additional nonlinear interactions. We
apply our scheme to entangled coherent states decohered in a vacuum environment
and find the decay time until which they can be purified.


title: Quantum and classical correlations in quantum Brownian motion

abstract:
  We investigate the entanglement properties of the joint state of a
distinguished quantum system and its environment in the quantum Brownian motion
model. This model is a frequent starting point for investigations of
environment-induced superselection. Using recent methods from quantum
information theory, we show that there exists a large class of initial states
for which no entanglement will be created at all times between the system of
salient interest and the environment. If the distinguished system has been
initially prepared in a pure Gaussian state, then entanglement is created
immediately, regardless of the temperature of the environment and the
non-vanishing coupling.


title: Simulation of many-body interactions by conditional geometric phases

abstract:
  It is shown how to exactly simulate many-body interactions and multi-qubit
gates by coupling finite dimensional systems, e.g., qubits with a continuous
variable. Cyclic evolution in the phase space of such a variable gives rise to
a geometric phase, depending on a product of commuting operators. The latter
allows to simulate many-body Hamiltonians and nonlinear Hamiltonians, and to
implement a big variety of multi-qubit quantum gates on both qubits and encoded
qubits. An application to the quantum amplitude amplification algorithm will be
discussed.


title:
Destabilization of dark states and optical spectroscopy in
  Zeeman-degenerate atomic systems

abstract:
  We present a general discussion of the techniques of destabilizing dark
states in laser-driven atoms with either a magnetic field or modulated laser
polarization. We show that the photon scattering rate is maximized at a
particular evolution rate of the dark state. We also find that the atomic
resonance curve is significantly broadened when the evolution rate is far from
this optimum value. These results are illustrated with detailed examples of
destabilizing dark states in some commonly-trapped ions and supported by
insights derived from numerical calculations and simple theoretical models.


title: Geometric Quantum Computation on Solid-State Qubits

abstract:
  An adiabatic cyclic evolution of control parameters of a quantum system ends
up with a holonomic operation on the system, determined entirely by the
geometry in the parameter space. The operation is given either by a simple
phase factor (a Berry phase) or a non-Abelian unitary operator depending on the
degeneracy of the eigenspace of the Hamiltonian. Geometric quantum computation
is a scheme to use such holonomic operations rather than the conventional
dynamic operations to manipulate quantum states for quantum information
processing. Here we propose a geometric quantum computation scheme which can be
realized with current technology on nanoscale Josephson-junction networks,
known as a promising candidate for solid-state quantum computer.


title:
Reply to ``The quantum algorithm of Kieu does not solve the Hilbert's
  tenth problem"

abstract:
  The arguments employed in quant-ph/0111009, to claim that the quantum
algorithm in quant-ph/0110136 does not work, are so general that were they true
then the adiabatic theorem itself would have been wrong. As a matter of fact,
those arguments are only valid for the sudden approximation, not the adiabatic
process.


title: Driving Hamiltonian in a Quantum Search Problem

abstract:
  We examine the driving Hamiltonian in the analog analogue of Grover's
algorithm by Farhi and Gutmann. For a quantum system with a given Hamiltonian
$E|w> < w|$, we explicitly show that while the driving Hamiltonian $E|s> < s|$
optimally produces the state $|w>$ from an initial state $|s>$, the driving
Hamiltonian $E^{\prime}|s> < s|(E^{\prime} \ne E)$ does not provide any speedup
compared even with a classical computation.


title:
Non-maximal rank separable states are a set of measure 0 within the set
  of non-maximal rank states

abstract:
  This paper is being withdrawn due to an error in the proof. Hao Chen has
shown the author in the two qubit case that there is an open set of three
dimensional subspaces that are spanned by separable states. This means the
author's proof is in error. The problem is that homogeneity cannot be used and
so the Jacobian needs to be computed at all points


title:
What is the Temperature Dependence of the Casimir Force between Real
  Metals?

abstract:
  The situation with the temperature corrections to the Casimir force between
real metals of finite conductivity is reported. It is shown that the plasma
dielectric function is well adapted to the Lifshitz formula and leads to
reasonable results for real conductors. The Drude dielectric function which
describes media with dissipation is found not to belong to the application
range of the Lifshitz formula at nonzero temperature. For Drude metals the
special modification of the zero-frequency term of this formula is suggested.
The contradictory results on the subject in recent literature are analysed and
explained.


title: Multi-parameter Entanglement in Quantum Interferometry

abstract:
  The role of multi-parameter entanglement in quantum interference from
collinear type-II spontaneous parametric down-conversion is explored using a
variety of aperture shapes and sizes, in regimes of both ultrafast and
continuous-wave pumping. We have developed and experimentally verified a theory
of down-conversion which considers a quantum state that can be concurrently
entangled in frequency, wavevector, and polarization. In particular, we
demonstrate deviations from the familiar triangular interference dip, such as
asymmetry and peaking. These findings improve our capacity to control the
quantum state produced by spontaneous parametric down-conversion, and should
prove useful to those pursuing the many proposed applications of down-converted
light.


title:
Time and Information, The Origins of 'Time' from Information Flow In
  Causal Networks and Complex Systems

abstract:
  The 'problem of time' can be 'solved' in principle by taking the viewpoint
that information created by quantum systems or Feynman Clocks (FCs) is
transferred by signals to detectors as quantum 'infostates' and then used to
construct 'time' with a T-computer. This constructed quantum 'time' results
from the quantum computational process of coupling observed signals to standard
clock signals into time labeled infostates in an observers' T-computer. The
T-computer model is used to define standard 'time coordinates' for 'events' in
space-time maps. The 'direction' and 'dimension' of 'arrows of time' follow
from the ordering and properties of the numbers used to label event 'times'.


title: Smart Nanostructures and Synthetic Quantum Systems

abstract:
  So far proposed quantum computers use fragile and environmentally sensitive
natural quantum systems. Here we explore the notion that synthetic quantum
systems suitable for quantum computation may be fabricated from smart
nanostructures using topological excitations of a neural-type network that can
mimic natural quantum systems. These developments are a technological
application of process physics which is a semantic information theory of
reality in which space and quantum phenomena are emergent.


title: Classical nonlocal models for quantum field states

abstract: This paper is withdrawn.


title: Non-Locality and Theories of Causation

abstract:
  The aim of the paper is to investigate the characterization of an unambiguous
notion of causation linking single space-llike separated events in EPR-Bell
frameworks. This issue is investigated in ordinary quantum mechanics, with some
hints to no collapse formulations of the theory such as Bohmian mechanics.


title: Using Electrons on Liquid Helium for Quantum Computing

abstract:
  We describe a quantum computer based on electrons supported by a helium film
and localized laterally by small electrodes just under the helium surface. Each
qubit is made of combinations of the ground and first excited state of an
electron trapped in the image potential well at the surface. Mechanisms for
preparing the initial state of the qubit, operations with the qubits, and a
proposed readout are described. This system is, in principle, capable of
100,000 operations in a decoherence time.


title: Multi-party Quantum Computation

abstract:
  We investigate definitions of and protocols for multi-party quantum computing
in the scenario where the secret data are quantum systems. We work in the
quantum information-theoretic model, where no assumptions are made on the
computational power of the adversary. For the slightly weaker task of
verifiable quantum secret sharing, we give a protocol which tolerates any t <
n/4 cheating parties (out of n). This is shown to be optimal. We use this new
tool to establish that any multi-party quantum computation can be securely
performed as long as the number of dishonest players is less than n/6.


title: Efficient Discrete Approximations of Quantum Gates

abstract:
  Quantum compiling addresses the problem of approximating an arbitrary quantum
gate with a string of gates drawn from a particular finite set. It has been
shown that this is possible for almost all choices of base sets and furthermore
that the number of gates required for precision epsilon is only polynomial in
log 1/epsilon. Here we prove that using certain sets of base gates quantum
compiling requires a string length that is linear in log 1/epsilon, a result
which matches the lower bound from counting volume up to constant factor.


title: Adiabatic Quantum Computation and Deutsch's Algorithm

abstract:
  We show that by a suitable choice of a time dependent Hamiltonian, Deutsch's
algorithm can be implemented by an adiabatic quantum computer. We extend our
analysis to the Deutsch-Jozsa problem and estimate the required running time
for both global and local adiabatic evolutions.


title:
Superpositions of the Orbital Angular Momentum for Applications in
  Quantum Experiments

abstract:
  Two different experimental techniques for preparation and analyzing
superpositions of the Gaussian and Laguerre-Gassian modes are presented. This
is done exploiting an interferometric method on the one hand and using computer
generated holograms on the other hand. It is shown that by shifting the
hologram with respect to an incoming Gaussian beam different superpositions of
the Gaussian and the Laguerre-Gaussian beam can be produced. An analytical
expression between the relative phase and the amplitudes of the modes and the
displacement of the hologram is given. The application of such orbital angular
momenta superpositions in quantum experiments such as quantum cryptography is
discussed.


title: Intertwined isospectral potentials in an arbitrary dimension

abstract:
  The method of intertwining with n-dimensional (nD) linear intertwining
operator L is used to construct nD isospectral, stationary potentials. It has
been proven that differential part of L is a series in Euclidean algebra
generators. Integrability conditions of the consistency equations are
investigated and the general form of a class of potentials respecting all these
conditions have been specified for each n=2,3,4,5. The most general forms of 2D
and 3D isospectral potentials are considered in detail and construction of
their hierarchies is exhibited. The followed approach provides coordinate
systems which make it possible to perform separation of variables and to apply
the known methods of supersymmetric quantum mechanics for 1D systems. It has
been shown that in choice of coordinates and L there are a number of
alternatives increasing with $n$ that enlarge the set of available potentials.
Some salient features of higher dimensional extension as well as some
applications of the results are presented.


title:
Almost any quantum spin system with short-range interactions can support
  toric codes

abstract:
  Inspired by Kitaev's argument that physical error correction is possible in a
system of interacting anyons, we demonstrate that such "self-correction" is
fairly common in spin systems with classical Hamiltonians that admit the
Peierls argument and where errors are modelled by quantum perturbations.