title: Hot entanglement in a simple dynamical model

abstract:
  How mixed can one component of a bi-partite system be initially and still
become entangled through interaction with a thermalized partner? We address
this question here. In particular, we consider the question of how mixed a
two-level system and a field mode may be such that free entanglement arises in
the course of the time evolution according to a Jaynes-Cummings type
interaction. We investigate the situation for which the two-level system is
initially in mixed state taken from a one-parameter set, whereas the field has
been prepared in an arbitrary thermal state. Depending on the particular choice
for the initial state and the initial temperature of the quantised field mode,
three cases can be distinguished: (i) free entanglement will be created
immediately, (ii) free entanglement will be generated, but only at a later time
different from zero, (iii) the partial transpose of the joint state remains
positive at all times. It will be demonstrated that increasing the initial
temperature of the field mode may cause the joint state to become distillable
during the time evolution, in contrast to a non-distillable state at lower
initial temperatures. We further assess the generated entanglement
quantitatively, by evaluating the logarithmic negativity numerically, and by
providing an analytical upper bound.


title:
Universally valid reformulation of the Heisenberg uncertainty principle
  on noise and disturbance in measurement

abstract:
  The Heisenberg uncertainty principle states that the product of the noise in
a position measurement and the momentum disturbance caused by that measurement
should be no less than the limit set by Planck's constant, hbar/2, as
demonstrated by Heisenberg's thought experiment using a gamma-ray microscope.
Here I show that this common assumption is false: a universally valid trade-off
relation between the noise and the disturbance has an additional correlation
term, which is redundant when the intervention brought by the measurement is
independent of the measured object, but which allows the noise-disturbance
product much below Planck's constant when the intervention is dependent. A
model of measuring interaction with dependent intervention shows that
Heisenberg's lower bound for the noise-disturbance product is violated even by
a nearly nondisturbing, precise position measuring instrument. An experimental
implementation is also proposed to realize the above model in the context of
optical quadrature measurement with currently available linear optical devices.


title: Propagation of the Angular Spectrum in the Up-conversion

abstract:
  We study theoretical and experimentally the propagation of the angular
spectrum of the light produced in the up-conversion process. The connection
between the angular spectrum of the fundamental and the second harmonic is
derived and measured. We show that even though they are connected, it is not
possible to directly transfer images from the fundamental to the second
harmonic.


title:
Slow Light Propagation in a Thin Optical Fiber via Electromagnetically
  Induced Transparency

abstract:
  We propose a novel configuration that utilizes electromagnetically induced
transparency (EIT) to tailor a fiber mode propagating inside a thin optical
fiber and coherently control its dispersion properties to drastically reduce
the group velocity of the fiber mode. The key to this proposal is: the
evanescent-like field of the thin fiber strongly couples with the surrounding
active medium, so that the EIT condition is met by the medium. We show how the
properties of the fiber mode is modified due to the EIT medium, both
numerically and analytically. We demonstrate that the group velocity of the new
modified fiber mode can be drastically reduced (approximately 44 m/sec) using
the coherently prepared orthohydrogen doped in a matrix of parahydrogen crystal
as the EIT medium.


title: Classical Three-Box "paradox"

abstract:
  A simple classical probabilistic system (a simple card game) classically
exemplifies Aharonov and Vaidman's "Three-Box 'paradox'" [J. Phys. A 24, 2315
(1991)], implying that the Three-Box example is neither quantal nor a paradox
and leaving one less difficulty to busy the interpreters of quantum mechanics.
An ambiguity in the usual expression of the retrodiction formula is shown to
have misled Albert, Aharonov, and D'Amato [Phys. Rev. Lett. 54, 5 (1985)] to a
result not, in fact, "curious"; the discussion illustrates how to avoid this
ambiguity.


title: Two-level laser light statistics

abstract:
  The statistics of the light emitted by two-level lasers is evaluated on the
basis of generalized rate equations. According to that approach, all
fluctuations are interpreted as being caused by the jumps that occur in active
and detecting atoms. The intra-cavity Fano factor and the photo-current
spectral density are obtained analytically for Poissonian and quiet pumps. The
algebra is simple and the formulas hold for small as well as large pumping
rates. Lasers exhibit excess noise at low pumping levels.


title: Comparison of two entangled states

abstract:
  This paper has been withdrawn by the authors, due Eqn. 8 is not a corollary
of ref. 3 and does not hold generally.


title: Quantum Interference in the Kirkwood-Rihaczek representation

abstract:
  We discuss the Kirkwood-Rihaczek phase space distribution and analyze a whole
new class of quasi-distributions connected with this function. All these
functions have the correct marginals. We construct a coherent state
representation of such functions, discuss which operator ordering corresponds
to the Kirkwood-Rihaczek distribution and their generalizations, and show how
such states are connected to squeezed states. Quantum interference in the
Kirkwood-Rihaczek representation is discussed.


title:
Relative Entropy and Single Qubit Holevo-Schumacher-Westmoreland Channel
  Capacity

abstract:
  The relative entropy description of Holevo-Schumacher-Westmoreland (HSW)
classical channel capacity is applied to single qubit channels. A simple
formula for the relative entropy of qubit density matrices in the Bloch sphere
representation is derived. This formula is combined with the
King-Ruskai-Szarek-Werner qubit channel ellipsoid picture to analyze several
unital and non-unital qubit channels in detail. An alternate proof that the
optimal HSW signalling states for single qubit unital channels are those states
with the minimum channel output entropy is presented. The derivation is based
on the symmetries of the qubit relative entropy formula and the
King-Ruskai-Szarek-Werner qubit channel ellipsoid picture. A proof is given
that the average output density matrix of any set of optimal HSW signalling
states for a (qubit or non-qubit) quantum channel is unique.


title:
Conditional q-Entropies and Quantum Separability: A Numerical
  Exploration

abstract:
  We revisit the relationship between quantum separability and the sign of the
relative q-entropies of composite quantum systems. The q-entropies depend on
the density matrix eigenvalues p_i through the quantity omega_q = sum_i p_i^q.
Renyi's and Tsallis' measures constitute particular instances of these
entropies. We perform a systematic numerical survey of the space of mixed
states of two-qubit systems in order to determine, as a function of the degree
of mixture, and for different values of the entropic parameter q, the volume in
state space occupied by those states characterized by positive values of the
relative entropy. Similar calculations are performed for qubit-qutrit systems
and for composite systems described by Hilbert spaces of larger dimensionality.
We pay particular attention to the limit case q --> infinity. Our numerical
results indicate that, as the dimensionalities of both subsystems increase,
composite quantum systems tend, as far as their relative q-entropies are
concerned, to behave in a classical way.


title:
Algebraic-geometric separability criterion and low rank mixed state
  entanglement

abstract:
  We first propose a new separability criterion based on algebraic-geometric
invariants of bipartite mixed states introduced in [1], then prove that for all
low ranks r <m+n-2, generic rank r mixed states in mxn systems have relatively
high Schmidt numbers (thus entangled) by this separability criterion. This also
means that the algebraic-geometric separability criterion proposed here can be
used to dectect all low rank entangled mixed states outside a measure zero set.


title: Efficient Quantum Algorithms for Estimating Gauss Sums

abstract:
  We present an efficient quantum algorithm for estimating Gauss sums over
finite fields and finite rings. This is a natural problem as the description of
a Gauss sum can be done without reference to a black box function. With a
reduction from the discrete logarithm problem to Gauss sum estimation we also
give evidence that this problem is hard for classical algorithms. The workings
of the quantum algorithm rely on the interaction between the additive
characters of the Fourier transform and the multiplicative characters of the
Gauss sum.


title: PT-invariant one-dimensional Coulomb problem

abstract:
  The one-dimensional Coulomb-like potential with a real coupling constant
beta, and a centrifugal-like core of strength G = alpha^2 - {1/4}, viz. V(x) =
{alpha^2 - (1/4)}/{(x-ic)^2} + beta/|x-ic|, is discussed in the framework of
PT-symmetry. The PT-invariant exactly solvable model so formed, is found to
admit a double set of real and discrete energies, numbered by a quasi-parity q
= +/- 1.


title: Doppler cooling to the recoil limit using sharp atomic transitions

abstract:
  In this paper, we develop an analytical approach to Doppler cooling of atoms
by one- or two-photon transitions when the natural width of the excited level
is so small that the process leads to a Doppler temperature comparable to the
recoil temperature. A ``quenching'' of the sharp line is introduced in order to
allow control of the time scale of the problem. In such limit, the usual
Fokker-Planck equation does not correctly describe the cooling process. We
propose a generalization of the Fokker-Planck equation and derive a new model
which is able to reproduce correctly the numerical results, up to the recoil
limit. Two cases of practical interest, one-photon Doppler cooling of strontium
and two-photon Doppler cooling of hydrogen are considered.


title:
Performing joint measurements and transformations on several qubits by
  operating on a single control qubit

abstract:
  An n-qubit quantum register can in principle be completely controlled by
operating on a single qubit that interacts with the register via an appropriate
fixed interaction. We consider a hypothetical system consisting of n spin-1/2
nuclei that interact with an electron spin via a magnetic interaction. We
describe algorithms that measure non-trivial joint observables on the register
by acting on the control spin only. For large n this is not an efficient model
for universal quantum computation but it can be modified to an efficient one if
one allows n possible positions of the control particle.
  This toy model of measurements illustrates in which way specific interactions
between the register and a probe particle support specific types of joint
measurements in the sense that some joint observables can be measured by simple
sequences of operations on the probe particle.


title: The Partition Ensemble Fallacy Fallacy

abstract:
  The Partition Ensemble Fallacy was recently applied to claim no quantum
coherence exists in coherent states produced by lasers. We show that this claim
relies on an untestable belief of a particular prior distribution of absolute
phase. One's choice for the prior distribution for an unobservable quantity is
a matter of `religion'. We call this principle the Partition Ensemble Fallacy
Fallacy. Further, we show an alternative approach to construct a
relative-quantity Hilbert subspace where unobservability of certain quantities
is guaranteed by global conservation laws. This approach is applied to coherent
states and constructs an approximate relative-phase Hilbert subspace.


title:
Separability criterion for pure states in multipartite and high
  dimensional systems

abstract:
  We propose a sufficient and necessary separability criterion for pure states
in multipartite and high dimensional systems. Its main advantage is operational
and computable. The obvious expressions of this criterion can be given out by
the coefficients of components of the pure state. In the end, we simply mention
a principle method how to define and obtain the measures of entanglement in
multipartite and high dimensional systems.


title: Quantum random walks in optical lattices

abstract:
  We propose an experimental realization of discrete quantum random walks using
neutral atoms trapped in optical lattices. The random walk is taking place in
position space and experimental implementation with present day technology
--even using existing set-ups-- seems feasible. We analyze the influence of
possible imperfections in the experiment and investigate the transition from a
quantum random walk to the classical random walk for increasing errors and
decoherence.


title:
Unambiguous State Discrimination of Coherent States with Linear Optics:
  Application to Quantum Cryptography

abstract:
  We discuss several methods for unambiguous state discrimination of N
symmetric coherent states using linear optics and photodetectors. One type of
measurements is shown to be optimal in the limit of small photon numbers for
any N. For the special case of N=4 this measurement can be fruitfully used by
the receiving end (Bob) in an implementation of the BB84 quantum key
distribution protocol using faint laser pulses. In particular, if Bob detects
only a single photon the procedure is equivalent to the standard measurement
that he would have to perform in a single-photon implementation of BB84, if he
detects two photons Bob will unambiguously know the bit sent to him in 50% of
the cases without having to exchange basis information, and if three photons
are detected, Bob will know unambiguously which quantum state was sent.


title: Game-theoretic discussion of quantum state estimation and cloning

abstract:
  We present a game-theoretic perspective on the problems of quantum state
estimation and quantum cloning. This enables us to show why the focus on
universal machines and the different measures of success, as employed in
previous works, are in fact legitimite.


title: Bell's theorem in the presence of classical communication

abstract:
  I explain what kinds of correlation or even direct classical communication
between detectors invalidate Bell's theorem, and what kinds do not.


title: Enhancement of Kerr nonlinearity via multi-photon coherence

abstract:
  We propose a new method of resonant enhancement of optical Kerr nonlinearity
using multi-level atomic coherence. The enhancement is accompanied by
suppression of the other linear and nonlinear susceptibility terms of the
medium. We show that the effect results in a modification of the nonlinear
Faraday rotation of light propagating in an Rb87 vapor cell by changing the
ellipticity of the light.


title: Phase measurements with weak reference pulses

abstract:
  Quantum state discrimination for two coherent states with opposite phases as
measured relative to a reference pulse is analyzed as functions of the
intensities of both the signal states and of the reference pulse. This problem
is relevant for Quantum Key Distribution with phase encoding. We consider both
the optimum measurements and simple measurements that require only
beamsplitters and photodetectors.


title: Pauli Tomography: complete characterization of a single qubit device

abstract:
  The marriage of Quantum Physics and Information Technology, originally
motivated by the need for miniaturization, has recently opened the way to the
realization of radically new information-processing devices, with the
possibility of guaranteed secure cryptographic communications, and tremendous
speedups of some complex computational tasks. Among the many problems posed by
the new information technology there is the need of characterizing the new
quantum devices, making a complete identification and characterization of their
functioning. As we will see, quantum mechanics provides us with a powerful tool
to achieve the task easily and efficiently: this tools is the so called quantum
entanglement, the basis of the quantum parallelism of the future computers. We
present here the first full experimental quantum characterization of a
single-qubit device. The new method, we may refer to as ''quantum
radiography'', uses a Pauli Quantum Tomography at the output of the device, and
needs only a single entangled state at the input, which works on the test
channel as all possible input states in quantum parallel. The method can be
easily extended to any n-qubits device.


title: Quantum Limit on Computational Time and Speed

abstract:
  We investigate if physical laws can impose limit on computational time and
speed of a quantum computer built from elementary particles. We show that the
product of the speed and the running time of a quantum computer is limited by
the type of fundamental interactions present inside the system. This will help
us to decide as to what type of interaction should be allowed in building
quantum computers in achieving the desired speed.


title: Relativistic Spontaneous Localization: A Proposal

abstract:
  A new proposal for a Lorentz-invariant spontaneous localization theory is
presented. It is based on the choice of a suitable set of macroscopic
quantities to be stochastically induced to have definite values. Such
macroscopic quantities have the meaning of a time-integrated amount of a
microscopically defined quantity called stuff related to the presence of
massive particles.


title:
The entanglement cost under operations preserving the positivity of
  partial transpose

abstract:
  We study the entanglement cost under quantum operations preserving the
positivity of the partial transpose (PPT-operations). We demonstrate that this
cost is directly related to the logarithmic negativity, thereby providing the
operational interpretation for this easily computable entanglement measure. As
examples we discuss general Werner states and arbitrary bi-partite Gaussian
states. Equipped with this result we then prove that for the anti-symmetric
Werner state PPT-cost and PPT-entanglement of distillation coincide giving the
first example of a truly mixed state for which entanglement manipulation is
asymptotically reversible.


title: Hiding Quantum Data

abstract:
  Recent work has shown how to use the laws of quantum mechanics to keep
classical and quantum bits secret in a number of different circumstances. Among
the examples are private quantum channels, quantum secret sharing and quantum
data hiding. In this paper we show that a method for keeping two classical bits
hidden in any such scenario can be used to construct a method for keeping one
quantum bit hidden, and vice--versa. In the realm of quantum data hiding, this
allows us to construct bipartite and multipartite hiding schemes for qubits
from the previously known constructions for hiding bits. Our method also gives
a simple proof that two bits of shared randomness are required to construct a
private quantum channel hiding one qubit.


title: On resumming periodic orbits in the spectra of integrable systems

abstract:
  Spectral determinants have proven to be valuable tools for resumming the
periodic orbits in the Gutzwiller trace formula of chaotic systems. We
investigate these tools in the context of integrable systems to which these
techniques have not been previously applied. Our specific model is a
stroboscopic map of an integrable Hamiltonian system with quadratic action
dependence, for which each stage of the semiclassical approximation can be
controlled. It is found that large errors occur in the semiclassical traces due
to edge corrections which may be neglected if the eigenvalues are obtained by
Fourier transformation over the long time dynamics. However, these errors cause
serious harm to the spectral approximations of an integrable system obtained
via the spectral determinants. The symmetry property of the spectral
determinant does not generally alleviate the error, since it sometimes sheds a
pair of eigenvalues from the unit circle. By taking into account the leading
order asymptotics of the edge corrections, the spectral determinant method
makes a significant recovery.


title: Generalizations of entanglement based on coherent states and convex sets

abstract:
  Unentangled pure states on a bipartite system are exactly the coherent states
with respect to the group of local transformations. What aspects of the study
of entanglement are applicable to generalized coherent states? Conversely, what
can be learned about entanglement from the well-studied theory of coherent
states? With these questions in mind, we characterize unentangled pure states
as extremal states when considered as linear functionals on the local Lie
algebra. As a result, a relativized notion of purity emerges, showing that
there is a close relationship between purity, coherence and (non-)entanglement.
To a large extent, these concepts can be defined and studied in the even more
general setting of convex cones of states. Based on the idea that entanglement
is relative, we suggest considering these notions in the context of partially
ordered families of Lie algebras or convex cones, such as those that arise
naturally for multipartite systems. The study of entanglement includes notions
of local operations and, for information-theoretic purposes, entanglement
measures and ways of scaling systems to enable asymptotic developments. We
propose ways in which these may be generalized to the Lie-algebraic setting,
and to a lesser extent to the convex-cones setting. One of our original
motivations for this program is to understand the role of entanglement-like
concepts in condensed matter. We discuss how our work provides tools for
analyzing the correlations involved in quantum phase transitions and other
aspects of condensed-matter systems.


title: Quantum estimation for non-differentiable models

abstract:
  State estimation is a classical problem in quantum information. In
optimization of estimation scheme, to find a lower bound to the error of the
estimator is a very important step. So far, all the proposed tractable lower
bounds use derivative of density matrix. However, sometimes, we are interested
in quantities with singularity, e.g. concurrence etc. In the paper, lower
bounds to a Mean Square Error (MSE) of an estimator are derived for a quantum
estimation problem without smoothness assumptions. Our main idea is to replace
the derivative by difference, as is done in classical estimation theory. We
applied the inequalities to several examples, and derived optimal estimator for
some of them.


title:
Evolution of the N ion Jaynes-Cummings model beyond the standard
  rotating wave approximation

abstract:
  A unitary transformation of the N-ion Jaynes-Cummings hamiltonian is
proposed. It is shown that any approximate expression of the evolution operator
associated with the transformed hamiltonian retains its validity independently
from the intensity of the external driving field. In particular, using the
rotating wave approximation, one obtains a solution for the N-ion
Jaynes-Cummings model which improves the standard rotating wave approximation
solution.


title: Minimal measurements of the gate fidelity of a qudit map

abstract:
  We obtain a simple formula for the average gate fidelity of a linear map
acting on qudits. It is given in terms of minimal sets of pure state
preparations alone, which may be interesting from the experimental point of
view. These preparations can be seen as the outcomes of certain minimal
positive operator valued measures. The connection of our results with these
generalized measurements is briefly discussed.


title: The Magnetic Casimir Effect

abstract:
  The Casimir effect results from alterations of the zero-point electromagnetic
energy introduced by boundary-conditions. For ferromagnetic layers separated by
vacuum (or a dielectric) such boundary-conditions are influenced by the
magneto-optical Kerr effect. We will show that this gives rise to a long-range
magnetic interaction and discuss the effect for two different configurations
(magnetization parallel and perpendicular to the layers). Analytical
expressions are derived for two models and compared to numerical calculations.
Numerical calculations of the effect for Fe are also presented and the
possibility of an experimental observation of the Casimir magnetic interaction
is discussed.


title:
Local permutations of products of Bell states and entanglement
  distillation

abstract:
  We present new algorithms for mixed-state multi-copy entanglement
distillation for pairs of qubits. Our algorithms perform significantly better
than the best known algorithms. Better algorithms can be derived that are tuned
for specific initial states. The new algorithms are based on a characterization
of the group of all locally realizable permutations of the 4^n possible tensor
products of n Bell states.


title: Scalable Ion Trap Quantum Computation with Pairwise Interactions Only

abstract:
  Universal ion trap computation on Decoherence Free Subspaces (DFS) using only
two qubit operations is presented. The DFS is constructed for the collective
dephasing model. Encoded single and two-qubit logical operations are
implemented via the Sorensen-Molmer interaction. Alternation of the effective
Hamiltonians for two particular phase configurations of control fields
approximates an anisotropic exchange interaction. This is universal over
suitable encodings of one logical qubit into three physical qubits which are
also DFS under collective decoherence.


title: Efficient generation of tunable photon pairs at 0.8 and 1.6 micrometer

abstract:
  We demonstrate efficient generation of collinearly propagating, highly
nondegenerate photon pairs in a periodically-poled lithium niobate cw
parametric downconverter with an inferred pair generation rate of 1.4*10^7/s/mW
of pump power. Detection of an 800-nm signal photon triggers a
thermoelectrically-cooled 20%-efficient InGaAs avalanche photodiode for the
detection of the 1600-nm conjugate idler photon. Using single-mode fibers as
spatial mode filters, we obtain a signal-conditioned idler-detection
probability of about 3.1%.


title: Optimal Realizations of Controlled Unitary Gates

abstract:
  The controlled-not gate and the single qubit gates are considered elementary
gates in quantum computing. It is natural to ask how many such elementary gates
are needed to implement more elaborate gates or circuits. Recall that a
controlled-U gate can be realized with two controlled-not gates and four single
qubit gates. We prove that this implementation is optimal if and only if the
matrix U satisfies the conditions tr U != 0, tr UX != 0, and det U != 1. We
also derive optimal implementations in the non-generic cases.


title: Non-Interactive Quantum Statistical and Perfect Zero-Knowledge

abstract:
  This paper introduces quantum analogues of non-interactive perfect and
statistical zero-knowledge proof systems. Similar to the classical cases, it is
shown that sharing randomness or entanglement is necessary for non-trivial
protocols of non-interactive quantum perfect and statistical zero-knowledge. It
is also shown that, with sharing EPR pairs a priori, the class of languages
having one-sided bounded error non-interactive quantum perfect zero-knowledge
proof systems has a natural complete problem. Non-triviality of such a proof
system is based on the fact proved in this paper that the Graph
Non-Automorphism problem, which is not known in BQP, can be reduced to our
complete problem. Our results may be the first non-trivial quantum
zero-knowledge proofs secure even against dishonest quantum verifiers, since
our protocols are non-interactive, and thus the zero-knowledge property does
not depend on whether the verifier in the protocol is honest or not. A
restricted version of our complete problem derives a natural complete problem
for BQP.


title:
Transient response of a quantum wave to an instantaneous potential step
  switching

abstract:
  The transient response of a stationary state of a quantum particle in a step
potential to an instantaneous change in the step height (a simplified model for
a sudden bias switch in an electronic semiconductor device) is solved exactly
by means of a semianalytical expression. The characteristic times for the
transient process up to the new stationary state are identified. A comparison
is made between the exact results and an approximate method.


title:
Generation of entangled coherent states via cross phase modulation in a
  double electromagnetically induced transparency regime

abstract:
  The generation of an entangled coherent state is one of the most important
ingredients of quantum information processing using coherent states. Recently,
numerous schemes to achieve this task have been proposed. In order to generate
travelling-wave entangled coherent states, cross phase modulation, optimized by
optical Kerr effect enhancement in a dense medium in an electromagnetically
induced transparency (EIT) regime, seems to be very promising. In this
scenario, we propose a fully quantized model of a double-EIT scheme recently
proposed [D. Petrosyan and G. Kurizki, {\sl Phys. Rev. A} {\bf 65}, 33833
(2002)]: the quantization step is performed adopting a fully Hamiltonian
approach. This allows us to write effective equations of motion for two
interacting quantum fields of light that show how the dynamics of one field
depends on the photon-number operator of the other. The preparation of a
Schr\"odinger cat state, which is a superposition of two distinct coherent
states, is briefly exposed. This is based on non-linear interaction via
double-EIT of two light fields (initially prepared in coherent states) and on a
detection step performed using a $50:50$ beam splitter and two photodetectors.
In order to show the entanglement of a generated entangled coherent state, we
suggest to measure the joint quadrature variance of the field. We show that the
entangled coherent states satisfy the sufficient condition for entanglement
based on quadrature variance measurement. We also show how robust our scheme is
against a low detection efficiency of homodyne detectors.


title: Lewenstein-Sanpera decomposition For Bell Decomposable

abstract:
  We propose a simple geometrical approach for finding the Lewenstein-Sanpera
decomposition of Bell decomposable states of 2 otimes 2 quantum systems. We
show that in these systems, the weight of the pure entangled part in the
decomposition is equal to the concurrence of the state. It is also shown that
the optimized separable part of L-S decomposition minimizes the von Neumann
relative entropy. We also obtain the decomposition for a class of mixed states
by using some LQCC actions. It is also shown that for these states the average
concurrence of L-S decomposition is equal to their concurrence.


title: Dynamical Aspects of Information Storage in Quantum-Mechanical Systems

abstract:
  We study information storage in noisy quantum registers and computers using
the methods of statistical dynamics. We develop the concept of a strictly
contractive quantum channel in order to construct mathematical models of
physically realizable, i.e., nonideal, quantum registers and computers.
Strictly contractive channels are simple enough, yet exhibit very interesting
features, which are meaningful from the physical point of view. In particular,
they allow us to incorporate the crucial assumption of finite precision of all
experimentally realizable operations. Strict contractivity also helps us gain
insight into the thermodynamics of noisy quantum evolutions (approach to
equilibrium). Our investigation into thermodynamics focuses on the
entropy-energy balance in quantum registers and computers under the influence
of strictly contractive noise. Using entropy-energy methods, we are able to
appraise the thermodynamical resources needed to maintain reliable operation of
the computer. We also obtain estimates of the largest tolerable error rate.
Finally, we explore the possibility of going beyond the standard circuit model
of error correction, namely constructing quantum memory devices on the basis of
interacting particle systems at low temperatures.


title:
Pseudohermitian Hamiltonians, time-reversal invariance and Kramers
  degeneracy

abstract:
  A necessary and sufficient condition in order that a (diagonalizable)
pseudohermitian operator admits an antilinear symmetry T such that T^{2}=-1 is
proven. This result can be used as a quick test on the T-invariance properties
of pseudohermitian Hamiltonians, and such test is indeed applied, as an
example, to the Mashhoon-Papini Hamiltonian.


title:
Constructing the Davies process of Resonance Fluorescence with Quantum
  Stochastic Calculus

abstract:
  Starting point is a given semigroup of completely positive maps on the 2
times 2 matrices. This semigroup describes the irreversible evolution of a
decaying 2-level atom. Using the integral-sum kernel approach to quantum
stochastic calculus we couple the 2-level atom to an environment, which in our
case will be interpreted as the electromagnetic field. The irreversible time
evolution of the 2-level atom then stems from the reversible time evolution of
atom and field together. Mathematically speaking, we have constructed a Markov
dilation of the semigroup. The next step is to drive the atom by a laser and to
count the photons emitted into the field by the decaying 2-level atom. For
every possible sequence of photon counts we construct a map that gives the time
evolution of the 2-level atom inferred by that sequence. The family of maps
that we obtain in this way forms a so-called Davies process. In his book Davies
describes the structure of these processes, which brings us into the field of
quantum trajectories. Within our model we calculate the jump operators and we
briefly describe the resulting counting process.


title: Conscious Pulse II: The rules of engagement

abstract:
  This is the final paper in a series that considers the rules of engagement
between conscious states and physiological states. In this paper, we imagine
that an endogenous quantum mechanical superposition is created by a classical
stimulus, and that this leads to a `physiological pulse' of states that are in
superposition with one another. This pulse is correlated with a `conscious
pulse' of the kind discussed in a previous paper (Conscious Pulse I). We then
add a rule (5) to the four rules previously given. This rule addresses the
effect of `pain' consciousness on both of these pulses, and in doing so, it
validates the "Parallel Principle" applied to pain. Key words: Brain states,
consciousness, conscious observer, macroscopic superposition, measurement,
state reduction, state collapse, von Neumann.


title:
Controllable Frequency Entanglement via Auto-Phase-Matched Spontaneous
  Parametric Down-Conversion

abstract:
  A new method for generating entangled photons with controllable frequency
correlation via spontaneous parametric down-conversion (SPDC) is presented. The
method entails initiating counter-propagating SPDC in a single-mode nonlinear
waveguide by pumping with a pulsed beam perpendicular to the waveguide. The
method offers several advantages over other schemes, including the ability to
generate frequency-correlated photon pairs regardless of the dispersion
characteristics of the system. Numerical evidence demonstrates the improvement
provided by this source in the special case of frequency-correlated two-photon
states.


title: One-Way Entangled-Photon Autocompensating Quantum Cryptography

abstract:
  A new quantum cryptography implementation is presented that combines one-way
operation with an autocompensating feature that has hitherto only been
available in implementations that require the signal to make a round trip
between the users. Using the concept of advanced waves, it is shown that this
new implementation is related to the round-trip implementations in the same way
that Ekert's two-particle scheme is related to the original one-particle scheme
of Bennett and Brassard. The practical advantages and disadvantages of the
proposed implementation are discussed in the context of existing schemes.


title: Information as a resource in distributed quantum systems

abstract:
  A new paradigm for distributed quantum systems where information is a
valuable resource is developed. After finding a unique measure for information,
we construct a scheme for it's manipulation in analogy with entanglement
theory. In this scheme instead of maximally entangled states, two parties
distill local states. We show that, surprisingly, the main tools of
entanglement theory are general enough to work in this opposite scheme. Up to
plausible assumptions, we show that the amount of information that must be lost
during the protocol of concentration of local information can be expressed as
the relative entropy distance from some special set of states.


title: Are there phase transitions in information space?

abstract:
  The interplay between two basic quantities -- quantum communication and
information -- is investigated. Quantum communication is an important resource
for quantum states shared by two parties and is directly related to
entanglement. Recently, the amount of local information that can be drawn from
a state has been shown to be closely related to the non-local properties of the
state. Here we consider both formation and extraction processes, and analyze
informational resources as a function of quantum communication. The resulting
diagrams in information space allow us to observe phase-like transitions when
correlations become classical.


title: Introduction to Quantum Error Correction

abstract:
  In this introduction we motivate and explain the ``decoding'' and
``subsystems'' view of quantum error correction. We explain how quantum noise
in QIP can be described and classified, and summarize the requirements that
need to be satisfied for fault tolerance. Considering the capabilities of
currently available quantum technology, the requirements appear daunting. But
the idea of ``subsystems'' shows that these requirements can be met in many
different, and often unexpected ways.


title: Introduction to Quantum Information Processing

abstract:
  As a result of the capabilities of quantum information, the science of
quantum information processing is now a prospering, interdisciplinary field
focused on better understanding the possibilities and limitations of the
underlying theory, on developing new applications of quantum information and on
physically realizing controllable quantum devices. The purpose of this primer
is to provide an elementary introduction to quantum information processing, and
then to briefly explain how we hope to exploit the advantages of quantum
information. These two sections can be read independently. For reference, we
have included a glossary of the main terms of quantum information.


title: Introduction to NMR Quantum Information Processing

abstract:
  After a general introduction to nuclear magnetic resonance (NMR), we give the
basics of implementing quantum algorithms. We describe how qubits are realized
and controlled with RF pulses, their internal interactions, and gradient
fields. A peculiarity of NMR is that the internal interactions (given by the
internal Hamiltonian) are always on. We discuss how they can be effectively
turned off with the help of a standard NMR method called ``refocusing''. Liquid
state NMR experiments are done at room temperature, leading to an extremely
mixed (that is, nearly random) initial state. Despite this high degree of
randomness, it is possible to investigate QIP because the relaxation time (the
time scale over which useful signal from a computation is lost) is sufficiently
long. We explain how this feature leads to the crucial ability of simulating a
pure (non-random) state by using ``pseudopure'' states. We discuss how the
``answer'' provided by a computation is obtained by measurement and how this
measurement differs from the ideal, projective measurement of QIP. We then give
implementations of some simple quantum algorithms with a typical experimental
result. We conclude with a discussion of what we have learned from NMR QIP so
far and what the prospects for future NMR QIP experiments are.


title:
Quantization of scalar fields in curved background, deformed Hopf
  algebra and entanglement

abstract:
  A suitable deformation of the Hopf algebra of the creation and annihilation
operators for a complex scalar field, initially quantized in Minkowski
space--time, induces the canonical quantization of the same field in a generic
gravitational background. The deformation parameter q turns out to be related
to the gravitational field. The entanglement of the quantum vacuum appears to
be robust against interaction with the environment.


title: Quantum theory of preparation and measurement

abstract:
  The conventional postulate for the probabilistic interpretation of quantum
mechanics is asymmetric in preparation and measurement, making retrodiction
reliant on inference by use of Bayes' theorem. Here, a more fundamental
symmetric postulate is presented, from which both predictive and retrodictive
probabilities emerge immediately, even where measurement devices more general
than those usually considered are involved. It is shown that the new postulate
is perfectly consistent with the conventional postulate.


title: Retrodiction with two-level atoms: atomic previvals

abstract:
  In the Jaynes-Cummings model a two-level atom interacts with a single-mode
electromagnetic field. Quantum mechanics predicts collapses and revivals in the
probability that a measurement will show the atom to be excited at various
times after the initial preparation of the atom and field. In retrodictive
quantum mechanics we seek the probability that the atom was prepared in a
particular state given the initial state of the field and the outcome of a
later measurement on the atom. Although this is not simply the time reverse of
the usual predictive problem, we demonstrate in this paper that retrodictive
collapses and revivals also exist. We highlight the differences between
predictive and retrodictive evolutions and describe an interesting situation
where the prepared state is essentially unretrodictable.


title:
Modelling Quantum Mechanics by the Quantumlike Description of the
  Electric Signal Propagation in Transmission Lines

abstract:
  It is shown that the transmission line technology can be suitably used for
simulating quantum mechanics. Using manageable and at the same time
non-expensive technology, several quantum mechanical problems can be simulated
for significant tutorial purposes. The electric signal envelope propagation
through the line is governed by a Schrodinger-like equation for a complex
function, representing the low-frequency component of the signal, In this
preliminary analysis, we consider two classical examples, i.e. the Frank-Condon
principle and the Ramsauer effect.


title: Are the laws of entanglement theory thermodynamical?

abstract:
  We argue that on its face, entanglement theory satisfies laws equivalent to
thermodynamics if the theory can be made reversible by adding certain bound
entangled states as a free resource during entanglement manipulation. Subject
to plausible assumptions, we prove that this is not the case in general, and
discuss the implications of this for the thermodynamics of entanglement.


title: Coherent States and Some Topics in Quantum Information Theory : Review

abstract:
  In the first half we make a short review of coherent states and generalized
coherent ones based on Lie algebras su(2) and su(1,1), and the Schwinger's
boson method to construct representations of the Lie algebras. In the second
half we make a review of recent developments on both swap of coherent states
and cloning of coherent states which are important subjects in Quantum
Information Theory.


title: Experimental investigation of continuous variable quantum teleportation

abstract:
  We report the experimental demonstration of quantum teleportation of the
quadrature amplitudes of a light field. Our experiment was stably locked for
long periods, and was analyzed in terms of fidelity, F; and with signal
transfer, T_{q}=T^{+}+T^{-}, and noise correlation, V_{q}=V_{in|out}^{+}
V_{in|out}^{-}. We observed an optimum fidelity of 0.64 +/- 0.02, T_{q}= 1.06
+/- 0.02 and V_{q} =0.96 +/- 0.10. We discuss the significance of both T_{q}>1
and V_{q}<1 and their relation to the teleportation no-cloning limit.


title: Causality implies formal state collapse

abstract:
  A physical theory of experiments carried out in a space-time region can
accommodate a detector localized in another space-like separated region, in
three, not necessarily exclusive, ways: 1) the detector formally collapses
physical states across space-like separations, 2) the detector enables
superluminal signals, and 3) the theory becomes logically inconsistent. If such
a theory admits autonomous evolving states, the space-like collapse must be
instantaneous. Time-like separation does not allow such conclusions. We also
prove some simple results on structural stability: within the set of all
possible theories, under a weak empirical topology, the set of all theories
with superluminal signals and the set of all theories with retrograde signals
are both open and dense.


title: A priori probability that two qubits are unentangled

abstract:
  In a previous study (quant-ph/9911058), several remarkably simple exact
results were found, in certain specialized m-dimensional scenarios (m<5), for
the a priori probability that a pair of qubits is unentangled/separable. The
measure used was the volume element of the Bures metric (identically one-fourth
the statistical distinguishability [SD] metric). Here, making use of a
newly-developed (Euler angle) parameterization of the 4 x 4 density matrices
(math-ph/0202002), we extend the analysis to the complete 15-dimensional convex
set (C) of arbitrarily paired qubits -- the total SD volume of which is known
to be \pi^8 / 1680 = \pi^8 / (2^4 3 5 7) = 5.64794. Using advanced quasi-Monte
Carlo procedures (scrambled Halton sequences) for numerical integration in this
high-dimensional space, we approximately (5.64851) reproduce that value, while
obtaining an estimate of .416302 for the SD volume of separable states. We
conjecture that this is but an approximation to \pi^6 /2310 = \pi^6 / (2 3 5 7
11) = .416186. The ratio of the two volumes, 8 / (11 \pi^2) = .0736881, would
then constitute the exact Bures/SD probability of separability. The SD area of
the 14-dimensional boundary of C is 142 \pi^7 / 12285 = 142 \pi^7 /(3^3 5 7 13)
= 34.911, while we obtain a numerical estimate of 1.75414 for the SD area of
the boundary of separable states.


title: Weak Values and Consistent Histories in Quantum Theory

abstract:
  A relation is obtained between weak values of quantum observables and the
consistency criterion for histories of quantum events. It is shown that
``strange'' weak values for projection operators (such as values less than
zero) always correspond to inconsistent families of histories. It is argued
that using the ABL rule to obtain probabilities for counterfactual measurements
corresponding to those strange weak values gives inconsistent results. This
problem is shown to be remedied by using the conditional weight, or
pseudo-probability, obtained from the multiple-time application of Luders'
Rule. It is argued that an assumption of reverse causality (a form of time
symmetry) implies that weak values obtain, in a restricted sense, at the time
of the weak measurement as well as at the time of post-selection. Finally, it
is argued that weak values are more appropriately characterised as
multiple-time amplitudes than expectation values, and as such can have little
to say about counterfactual questions.


title:
Computational Model for the One-Way Quantum Computer: Concepts and
  Summary

abstract:
  The one-way quantum computer (QCc) is a universal scheme of quantum
computation consisting only of one-qubit measurements on a particular entangled
multi-qubit state, the cluster state. The computational model underlying the
QCc is different from the quantum logic network model and it is based on
different constituents. It has no quantum register and does not consist of
quantum gates. The QCc is nevertheless quantum mechanical since it uses a
highly entangled cluster state as the central physical resource. The scheme
works by measuring quantum correlations of the universal cluster state.


title:
Concurrence as a Relative Entropy with Hilbert-Schmidt Distance in Bell
  Decomposable States

abstract:
  Hilbert-Schmidt distance reduces to Euclidean distance in Bell decomposable
states. Based on this, entanglement of these states are obtained according to
the protocol proposed in Ref. [V. Vedral et al, Phys. Rev. Lett. 78, 2275
(1995)] with Hilbert-Schmidt distance. It is shown that this measure is equal
to the concurrence and thus can be used to generate entanglement of formation.
We also introduce a new measure of distance and show that under the action of
restricted LQCC operations, the associated measure of entanglement transforms
in the same way as the concurrence transforms .


title: I-concurrence and tangle for isotropic states

abstract:
  We discuss properties of entanglement measures called I-concurrence and
tangle. For a bipartite pure state, I-concurrence and tangle are simply related
to the purity of the marginal density operators. The I-concurrence (tangle) of
a bipartite mixed state is the minimum average I-concurrence (tangle) of
ensemble decompositions of pure states of the joint density operator. Terhal
and Vollbrecht [Phys. Rev. Lett. 85, 2625 (2000)] have given an explicit
formula for the entanglement of formation of isotropic states in arbitrary
dimensions. We use their formalism to derive comparable expressions for the
I-concurrence and tangle of isotropic states.


title: Gate-Level Simulation of Quantum Circuits

abstract:
  While thousands of experimental physicists and chemists are currently trying
to build scalable quantum computers, it appears that simulation of quantum
computation will be at least as critical as circuit simulation in classical
VLSI design. However, since the work of Richard Feynman in the early 1980s
little progress was made in practical quantum simulation. Most researchers
focused on polynomial-time simulation of restricted types of quantum circuits
that fall short of the full power of quantum computation. Simulating quantum
computing devices and useful quantum algorithms on classical hardware now
requires excessive computational resources, making many important simulation
tasks infeasible. In this work we propose a new technique for gate-level
simulation of quantum circuits which greatly reduces the difficulty and cost of
such simulations. The proposed technique is implemented in a simulation tool
called the Quantum Information Decision Diagram (QuIDD) and evaluated by
simulating Grover's quantum search algorithm. The back-end of our package,
QuIDD Pro, is based on Binary Decision Diagrams, well-known for their ability
to efficiently represent many seemingly intractable combinatorial structures.
This reliance on a well-established area of research allows us to take
advantage of existing software for BDD manipulation and achieve unparalleled
empirical results for quantum simulation.


title: Entwined Paths, Difference Equations and the Dirac Equation

abstract:
  Entwined space-time paths are bound pairs of trajectories which are traversed
in opposite directions with respect to macroscopic time. In this paper we show
that ensembles of entwined paths on a discrete space-time lattice are simply
described by coupled difference equations which are discrete versions of the
Dirac equation. There is no analytic continuation, explicit or forced, involved
in this description. The entwined paths are `self-quantizing'. We also show
that simple classical stochastic processes that generate the difference
equations as ensemble averages are stable numerically and converge at a rate
governed by the details of the stochastic process. This result establishes the
Dirac equation in one dimension as a phenomenological equation describing an
underlying classical stochastic process in the same sense that the Diffusion
and Telegraph equations are phenomenological descriptions of stochastic
processes.


title:
Magnetic Properties of Scalar Particles --The Scalar Aharonov-Casher
  Effect and Supersymmetry

abstract:
  The original topological Aharonov-Casher (AC) effect is due to the
interaction of the anomalous magnetic dipole moment (MDM) with certain
configurations of electric field. Naively one would not expect an AC effect for
a scalar particle for which no anomalous MDM can be defined in the usual sense.
In this letter we study the AC effect in supersymmetric systems. In this
framework there is the possibility of deducing the AC effect of a scalar
particle from the corresponding effect for a spinor particle. In 3+1 dimensions
such a connection is not possible because the anomalous MDM is zero if
supersymmetry is an exact symmetry. However, in 2+1 dimensions it is possible
to have an anomalous MDM even with exact supersymmetry.
  Having demonstrated the relationship between the spinor and the scalar MDM,
we proceed to show that the scalar AC effect is uniquely defined. We then
compute the anomalous MDM at the one loop level, showing how the scalar form
arises in 2+1 dimensions from the coupling of the scalar to spinors. This model
shows how an AC effect for a scalar can be generated for non-supersymmetric
theories, and we construct such a model to illustrate the mechanism.


title:
Quantum noise influencing human behaviour could fake effectiveness of
  drugs in clinical trials

abstract:
  To test the effectiveness of a drug one can advice two randomly selected
groups of patients to take or not to take it, respectively. It is well-known
that the causal effect cannot be identified if not all patients comply. This
holds even when the non-compliers can be identified afterwards since latent
factors like patient's personality can influence both his decision and his
physical response. However, one can still give bounds on the effectiveness of
the drug depending on the rate of compliance. Remarkably, the proofs of these
bounds given in the literature rely on models that represent all relevant
latent factors (including noise) by hidden classical variables. In strong
analogy to the violation of Bell's inequality, some of these bounds fail if
patient's behavior is influenced by latent quantum processes (e.g. in his
nervous system). Quantum effects could fake an increase of the recovery rate by
about 13% although the drug would hurt as many patients as it would help if
everyone took it. The other bounds are true even in the quantum case.
  We do not present any realistic model showing this effect, we only point out
that the physics of decision making could be relevant for the causal
interpretation of every-day life statistical data.


title:
The fully entangled fraction as an inclusive measure of entanglement
  applications

abstract:
  Characterizing entanglement in all but the simplest case of a two qubit pure
state is a hard problem, even understanding the relevant experimental
quantities that are related to entanglement is difficult. It may not be
necessary, however, to quantify the entanglement of a state in order to
quantify the quantum information processing significance of a state. It is
known that the fully entangled fraction has a direct relationship to the
fidelity of teleportation maximized under the actions of local unitary
operations. In the case of two qubits we point out that the fully entangled
fraction can also be related to the fidelities, maximized under the actions of
local unitary operations, of other important quantum information tasks such as
dense coding, entanglement swapping and quantum cryptography in such a way as
to provide an inclusive measure of these entanglement applications. For two
qubit systems the fully entangled fraction has a simple known closed-form
expression and we establish lower and upper bounds of this quantity with the
concurrence. This approach is readily extendable to more complicated systems.


title: From Qubits to Continuous-Variable Quantum Computation

abstract:
  By encoding a qudit in a harmonic oscillator and investigating the d -->
infinity limit, we give an entirely new realization of continuous-variable
quantum computation. The generalized Pauli group is generated by number and
phase operators for harmonic oscillators.


title: Wavefunction Collapse and Random Walk

abstract:
  Wavefunction collapse models modify Schrodinger's equation so that it
describes the rapid evolution of a superposition of macroscopically
distinguishable states to one of them. This provides a phenomenological basis
for a physical resolution to the so-called "measurement problem." Such models
have experimentally testable differences from standard quantum theory. The most
well developed such model at present is the Continuous Spontaneous Localization
(CSL) model in which a fluctuating classical field interacts with particles to
cause collapse. One "side effect" of this interaction is that the field imparts
momentum to particles, causing a small blob of matter to undergo random walk.
Here we explore this in order to supply predictions which could be
experimentally tested. We examine the translational diffusion of a sphere and a
disc, and the rotational diffusion of a disc, according to CSL. For example, we
find that a disc of radius 2 cdot 10^{-5} cm and thickness 0.5 cdot 10^{-5} cm
diffuses through 2 pi rad in about 70sec (this assumes the "standard" CSL
parameter values). The comparable rms diffusion of standard quantum theory is
smaller than this by a factor 10^-3. At the reported pressure of < 5
cdot10^{-17} Torr, achieved at 4.2^{circ} K, the mean time between air molecule
collisions with the disc is approximately 45min (and the diffusion caused by
photon collisons is utterly negligible). This is ample time for observation of
the putative CSL diffusion over a wide range of parameters.
  This encourages consideration of how such an experiment may actually be
performed, and the paper closes with some thoughts on this subject


title: Quarticles and the Identity of Indiscernibles

abstract:
  The principle of the identity of indiscernibles (PII) states that if two
systems are qualitatively identical then they are logically identical. French
and Redhead (1988) and Butterfield (1993) have shown the sense in which bosons
and fermions violate the PII, but did not investigate the issue for particles
of other kinds of statistics: i.e., for the (p,q) particles -- or `quarticles'
-- of Hartle, Stolt and Taylor (1970). This paper shows that for any type of
indistinguishable quarticle the PII is violated but that for distinguishable
quarticles there are states in which it is violated by any pair of particles,
states in which it is violated only by some pairs of particles and states in
which it is violated by no pairs of particles. The updated version corrects a
minor statement of mathematical fact, and provides a short proof for a
conjecture made in the original.


title: An interpretation of the black-body radiation

abstract:
  The black-body radiation is reinterpreted in terms of the photon's many-body
wave functions in analogy with the condensed matter physics. This
interpretation has implications on the wave-particle duality, and on the
difference between the photon and the matter wave.


title: Quantization, group contraction and zero point energy

abstract:
  We study algebraic structures underlying 't Hooft's construction relating
classical systems with the quantum harmonic oscillator. The role of group
contraction is discussed. We propose the use of SU(1,1) for two reasons:
because of the isomorphism between its representation Hilbert space and that of
the harmonic oscillator and because zero point energy is implied by the
representation structure. Finally, we also comment on the relation between
dissipation and quantization.


title:
Intrinsic decoherence and classical-quantum correspondence in two
  coupled delta-kicked rotors

abstract:
  We show that classical-quantum correspondence of center of mass motion in two
coupled delta-kicked rotors can be obtained from intrinsic decoherence of the
system itself which occurs due to the entanglement of the center of mass motion
to the internal degree of freedom without coupling to external environment.


title: Quantum Communication and Decoherence

abstract:
  In this contribution we will give a brief overview on the methods used to
overcome decoherence in quantum communication protocols. We give an
introduction to quantum error correction, entanglement purification and quantum
cryptography. It is shown that entanglement purification can be used to create
``private entanglement'', which makes it a useful tool for cryptographic
protocols.


title: Optimal Teleportation Based on Bell Measurement

abstract:
  We study optimal teleportation based on the Bell measurements. An explicit
expression for the quantum channel associated with the optimal teleportation
with an arbitrary mixed state resource is presented. The optimal transmission
fidelity of the corresponding quantum channel is calculated and shown to be
related to the fully entangled fraction of the quantum resource, rather than
the singlet fraction as in the standard teleportation protocol.


title:
Trapping and cooling single atoms with far-off resonance intracavity
  doughnut modes

abstract:
  We investigate cooling and trapping of single atoms inside an optical cavity
using a quasi-resonant field and a far-off resonant mode of the Laguerre-Gauss
type. The far-off resonant doughnut mode provides an efficient trapping in the
case when it shifts the atomic internal ground and excited state in the same
way, which is particularly useful for quantum information applications of
cavity quantum electrodynamics (QED) systems. Long trapping times can be
achieved, as shown by full 3-D simulations of the quasi-classical motion inside
the resonator.


title: Radiation reaction in quantum field theory

abstract:
  We investigate radiation-reaction effects for a charged scalar particle
accelerated by an external potential realized as a space-dependent mass term in
quantum electrodynamics. In particular, we calculate the position shift of the
final-state wave packet of the charged particle due to radiation at lowest
order in the fine structure constant alpha and in the small h-bar
approximation. We show that it disagrees with the result obtained using the
Lorentz-Dirac formula for the radiation-reaction force, and that it agrees with
the classical theory if one assumes that the particle loses its energy to
radiation at each moment of time according to the Larmor formula in the static
frame of the potential. However, the discrepancy is much smaller than the
Compton wavelength of the particle. We also point out that the electromagnetic
correction to the potential has no classical limit. (Correction. Surface terms
were erroneously discarded to arrive at Eq. (59). By correcting this error we
find that the position shift according to the Lorentz-Dirac theory obtained
from Eq. (12) is reproduced by quantum field theory in the hbar -> 0 limit. We
also find that the small V(z) approximation is unnecessary for this agreement.
See Sec. VII.)


title: Realism and the wave-function

abstract:
  Realism -- the idea that the concepts in physical theories refer to 'things'
existing in the real world -- is introduced as a tool to analyze the status of
the wave-function. Although the physical entities are recognized by the
existence of invariant quantities, examples from classical and quantum physics
suggest that not all the theoretical terms refer to the entities: some terms
refer to properties of the entities, and some terms have only an epistemic
function. In particular, it is argued that the wave-function may be written in
terms of classical non-referring and epistemic terms. The implications for
realist interpretations of quantum mechanics and on the teaching of quantum
physics are examined.


title: Robustness of Entanglement for Bell Decomposable States

abstract:
  We propose a simple geometrical approach for finding the robustness of
entanglement for Bell decomposable states of 2 otimes 2 quantum systems. It is
shown that the robustness of entanglement is equal to the concurrence. We also
present an analytical expression for two separable states that wipe out all
entanglement of these states. Finally the random robustness of these states is
also obtained.


title:
Optimal sequence of quantum measurements in the sense of Stein's lemma
  in quantum hypothesis testing

abstract:
  We derive a necessary and sufficient condition for a sequence of quantum
measurements to achieve the optimal performance in quantum hypothesis testing.
Using an information-spectrum method, we discuss what quantum measurement we
should perform in order to attain the optimal exponent of the second error
probability under the condition that the first error probability goes to 0. As
an asymptotically optimal measurement, we propose a projection measurement
characterized by the irreducible representation theory of the special linear
group SL(H). Specially, in spin 1/2 system, it is realized by the simultaneous
measurement of the total momentum and a momentum of a specified direction. As a
byproduct, we obtain another proof of quantum Stein's lemma.


title: Wave Packets can Factorize Numbers

abstract:
  We draw attention to various aspects of number theory emerging in the time
evolution of elementary quantum systems with quadratic phases. Such model
systems can be realized in actual experiments. Our analysis paves the way to a
new, promising and effective method to factorize numbers.


title: Quantum Teleportation of Optical Quantum Gates

abstract:
  We show that a universal set of gates for quantum computation with optics can
be quantum teleported through the use of EPR entangled states, homodyne
detection, and linear optics and squeezing operations conditioned on
measurement outcomes. This scheme may be used for fault-tolerant quantum
computation in any optical scheme (qubit or continuous variable). The
teleportation of nondeterministic nonlinear gates employed in linear optics
quantum computation is discussed.


title: Complete positivity and dissipative factorized dynamics

abstract:
  We show that any Hermiticity and trace preserving continuous semigroup
gamma_t in d dimensions is completely positive if and only if the semigroup
gamma_t otimes gamma_t is positivity preserving.


title: Complementarity, measurement and information in interference experiments

abstract:
  Different criteria (Shannon's entropy, Bayes' average cost, Durr's normalized
rms spread) have been introduced to measure the "which-way" information present
in interference experiments where, due to non-orthogonality of the detector
states, the path determination is incomplete. For each of these criteria, we
determine the optimal measurement to be carried on the detectors, in order to
read out the maximum which-way information. We show that, while in two-beam
experiments, the optimal measurement is always provided by an observable
involving the detector only, in multibeam experiments, with equally populated
beams and two-state detectors, this is the case only for the Durr criterion, as
the other two require the introduction of an ancillary quantum system, as part
of the read-out apparatus.


title: Quantum Computation with Vibrationally Excited Molecules

abstract:
  A new physical implementation for quantum computation is proposed. The
vibrational modes of molecules are used to encode qubit systems. Global quantum
logic gates are realized using shaped femtosecond laser pulses which are
calculated applying optimal control theory. The scaling of the system is
favourable, sources for decoherence can be eliminated. A complete set of one
and two quantum gates is presented for a specific molecule. Detailed analysis
regarding experimental realization shows that the structural resolution of
today's pulse shapers is easiliy sufficient for pulse formation.


title: Path Integrals and Their Application to Dissipative Quantum Systems

abstract:
  Introduction
  Path Integrals
  - Introduction
  - Propagator
  - Free Particle
  - Path Integral Representation of Quantum Mechanics
  - Particle on a Ring
  - Particle in a Box
  - Driven Harmonic Oscillator
  - Semiclassical Approximation
  - Imaginary Time Path Integral
  Dissipative Systems
  - Introduction
  - Environment as Collection of Harmonic Oscillators
  - Effective Action
  Damped Harmonic Oscillator
  - Partition Function
  - Ground State Energy and Density of States
  - Position Autocorrelation Function


title: Measurement of the phase diffusion dynamics in the micromaser

abstract:
  We propose a realistic scheme for measuring the micromaser linewidth by
monitoring the phase diffusion dynamics of the cavity field. Our strategy
consists in exciting an initial coherent state with the same photon number
distribution as the micromaser steady-state field, singling out a purely
diffusive process in the system dynamics. After the injection of a
counter-field, measurements of the population statistics of a probe atom allow
us to derive the micromaser linewidth. Our proposal aims at solving a classic
and relevant decoherence problem in cavity quantum electrodynamics, allowing to
establish experimentally the distinctive features appearing in the micromaser
spectrum due to the discreteness of the electromagnetic field.


title: Unextendible product bases and the construction of inseparable states

abstract:
  Let H[N] denote the tensor product of n finite dimensional Hilbert spaces
H(r). A state |phi> of H[N] is separable if |phi> is the tensor product of
states in the respective product spaces. An orthogonal unextendible product
basis is a finite set B of separable orthonormal states |phi(k)> such that the
non-empty space B9perp), the set of vectors orthogonal to B, contains no
separable projection. Examples of orthogonal UPB sets were first constructed by
Bennett et al [1] and other examples appear, for example, in [2] and [3]. If F
denotes the set of convex combinations of the projections |phi(k)><phi(k)|,
then F is a face in the set S of separable densities. In this note we show how
to use F to construct families of positive partial transform states (PPT) which
are not separable. We also show how to make an analogous construction when the
condition of orthogonality is dropped. The analysis is motivated by the
geometry of the faces of the separable states and leads to a natural
construction of entanglement witnesses separating the inseparable PPT states
from S.


title: Optimized phase switching using a single atom nonlinearity

abstract:
  We show that a nonlinear phase shift of pi can be obtained by using a single
two level atom in a one sided cavity with negligible losses. This result
implies that the use of a one sided cavity can significantly improve the pi/18
phase shift previously observed by Turchette et al. [Phys. Rev. Lett. 75, 4710
(1995)].


title: Decay process accelerated by tunneling in its very early stage

abstract:
  We examine a fast decay process that arises in the transition period between
the Gaussian and exponential decay processes in quantum decay systems. It is
usually expected that the decay is decelerated by a confinement potential
barrier. However, we find a case where the decay in the transition period is
accelerated by tunneling through a confinement potential barrier. We show that
the acceleration gives rise to an appreciable effect on the time evolution of
the nonescape probability of the decay system.


title: Intermediate states in quantum cryptography and Bell inequalities

abstract:
  Intermediate states are known from intercept/resend eavesdropping in the BB84
quantum cryptographic protocol. But they also play fundamental roles in the
optimal eavesdropping strategy on BB84 and in the CHSH inequality. We
generalize the intermediate states to arbitrary dimension and consider
intercept/resend eavesdropping, optimal eavesdropping on the generalized BB84
protocol and present a generalized CHSH inequality for two entangled quNits
based on these states.


title: Decoherence induced continuous pointer states

abstract:
  We investigate the reduced dynamics in the Markovian approximation of an
infinite quantum spin system linearly coupled to a phonon field at positive
temperature. The achieved diagonalization leads to a selection of the
continuous family of pointer states corresponding to a configuration space of
the one-dimensional Ising model. Such a family provides a mathematical
description of an apparatus with continuous readings.


title:
Neural Unpredictability, the Interpretation of Quantum Theory, and the
  Mind-Body Problem

abstract:
  It has been suggested, on the one hand, that quantum states are just states
of knowledge; and, on the other, that quantum theory is merely a theory of
correlations. These suggestions are confronted with problems about the nature
of psycho-physical parallelism and about how we could define probabilities for
our individual future observations given our individual present and previous
observations. The complexity of the problems is underlined by arguments that
unpredictability in ordinary everyday neural functioning, ultimately stemming
from small-scale uncertainties in molecular motions, may overwhelm, by many
orders of magnitude, many conventionally recognized sources of observed
``quantum'' uncertainty. Some possible ways of avoiding the problems are
considered but found wanting. It is proposed that a complete understanding of
the relationship between subjective experience and its physical correlates
requires the introduction of mathematical definitions and indeed of new
physical laws.


title: State reconstruction of the kicked rotor

abstract:
  We propose two experimentally feasible methods based on atom interferometry
to measure the quantum state of the kicked rotor.


title: Kicked Rotor in Wigner Phase Space

abstract:
  We develop the Wigner phase space representation of a kicked particle for an
arbitrary but periodic kicking potential. We use this formalism to illustrate
quantum resonances and anti--resonances.


title:
Simplified and obvious expression of concurrence in Wootters' measure of
  entanglement of a pair of qubits

abstract:
  We obtain a simplified and obvious expression of "concurrence" in Wootters'
measure of entanglement of a pair of qubits having no more than two non-zero
eigenvalues in terms of concurrences of eigenstates and their simple
combinations. It not only simplifies the calculation of Wootters' measure of
entanglement, but also reveals some its general and important features. Our
conclusions are helpful to understand and use quantum entanglement further.


title: Quartic Anharmonicity in Different Spatial Dimensions

abstract:
  A path-integral method effective beyond the perturbation expansion approach
is suggested to consider the quartic anharmonicity in different spatial
dimensions. Due to an optimal representation of the partition function, the
leading term has already taken into account the correct strong-coupling
behaviour. In the simplest cases of zero and one dimension we have obtained
reasonable results in a simple way. Then, this technique is applied to the
superrenormalizable scalar theory phi^4_2 in two dimensions. This results in an
accurate estimation of the ground-state energy that provides exact weak- and
strong-coupling behaviour already in the leading-order approximation. The
next-to-leading terms give rise in insignificant corrections.


title: Master equations for effective Hamiltonians

abstract:
  We reelaborate on a general method for obtaining effective Hamiltonians that
describe different nonlinear optical processes. The method exploits the
existence of a nonlinear deformation of the su(2) algebra that arises as the
dynamical symmetry of the original model. When some physical parameter (usually
related to the dispersive limit) becomes small, we immediately get a diagonal
effective Hamiltonian that represents correctly the dynamics for arbitrary
states and long times. We apply the same technique to obtain how the noise
terms in the original model transform under this scheme, providing a systematic
way of including damping effects in processes described in terms of effective
Hamiltonians.


title: Energy Requirements for Quantum Data Compression and 1-1 Coding

abstract:
  By looking at quantum data compression in the second quantisation, we present
a new model for the efficient generation and use of variable length codes. In
this picture lossless data compression can be seen as the {\em minimum energy}
required to faithfully represent or transmit classical information contained
within a quantum state.
  In order to represent information we create quanta in some predefined modes
(i.e. frequencies) prepared in one of two possible internal states (the
information carrying degrees of freedom). Data compression is now seen as the
selective annihilation of these quanta, the energy of whom is effectively
dissipated into the environment. As any increase in the energy of the
environment is intricately linked to any information loss and is subject to
Landauer's erasure principle, we use this principle to distinguish lossless and
lossy schemes and to suggest bounds on the efficiency of our lossless
compression protocol.
  In line with the work of Bostr\"{o}m and Felbinger \cite{bostroem}, we also
show that when using variable length codes the classical notions of prefix or
uniquely decipherable codes are unnecessarily restrictive given the structure
of quantum mechanics and that a 1-1 mapping is sufficient. In the absence of
this restraint we translate existing classical results on 1-1 coding to the
quantum domain to derive a new upper bound on the compression of quantum
information. Finally we present a simple quantum circuit to implement our
scheme.


title:
Spin-dependent recombination - an electronic readout mechanism for solid
  state quantum computers

abstract:
  It is shown that coherent spin motion of electron-hole pairs localized in
band gap states of silicon can influence charge carrier recombination. Based on
this effect, a readout concept for silicon based solid-state spin--quantum
computers as proposed by Kane is suggested. The 31P quantum bit (qbit) is
connected via hyperfine coupling to the spin of the localized donor electron.
When a second localized and singly occupied electronic state with an energy
level deep within the band gap or close to the valence edge is in proximity, a
gate controlled exchange between the 31P nucleus and the two electronic states
can be activated that leaves the donor-deep level pair either unchanged in a
|T->-state or shifts it into a singlet state |S>. Since the donor deep level
transition is spin-dependent, the deep level becomes charged or not, depending
on the nuclear spin orientation of the donor nucleus. Thus, the state of the
qbit can be read with a sequence of light pulses and photo conductivity
measurements.


title: Remarks on the Cross Norm Criterion for Separability

abstract:
  Recently in Reference [ quant-ph/0202121] a computational criterion of
separability induced by greatest cross norm is proposed by Rudolph. There,
Rudolph conjectured that the new criterion is not weaker than positive partial
transpose criterion for separability. We show that there exist counterexample
to this claim, that is, proposed criterion is weaker than the positive partial
transpose criterion.


title:
The Low-Frequency Character of the Thermal Correction to the Casimir
  Force between Metallic Films

abstract:
  The frequency spectrum of the finite temperature correction to the Casimir
force can be determined by the use of the Lifshitz formalism for metallic
plates of finite conductivity. We show that the correction for the TE
electromagnetic modes is dominated by frequencies so low that the plates cannot
be modelled as ideal dielectrics. We also address the issues relating to the
behavior of electromagnetic fields at the surfaces an within metallic
conductors, and claculate the surface modes using appropriate low-frequency
metallic boundary conditions. Our result brings the tehrmal correction into
agreement with experimental results that were previously obtained.


title: Quantum Circuits with Unbounded Fan-out

abstract:
  We demonstrate that the unbounded fan-out gate is very powerful.
Constant-depth polynomial-size quantum circuits with bounded fan-in and
unbounded fan-out over a fixed basis (denoted by QNCf^0) can approximate with
polynomially small error the following gates: parity, mod[q], And, Or,
majority, threshold[t], exact[q], and Counting. Classically, we need
logarithmic depth even if we can use unbounded fan-in gates. If we allow
arbitrary one-qubit gates instead of a fixed basis, then these circuits can
also be made exact in log-star depth. Sorting, arithmetical operations, phase
estimation, and the quantum Fourier transform with arbitrary moduli can also be
approximated in constant depth.


title: Arbitrary qubit rotation through nonadiabatic evolution

abstract:
  We show how one can perform arbitrary rotation of any qubit, using delayed
laser pulses through nonadiabatic evolution, i.e., via transitions among the
adiabatic states. We use a double-Lambda scheme and use a set of control
parameters such as detuning, ratio of pulse amplitudes, time-separation of two
pulses for realizing different rotations of the qubit. We also investigate the
effect of different kinds of chirping, namely linear chirping and hyperbolic
tangent chirping. Our work using nonadiabatic evolution adds to the flexibility
in the implementation of logic gate operations and show how to achieve control
of quantum systems by using different types of pulses.


title:
Can the Wigner function be determined by properties for translation and
  parity transformation on lattice phase space?

abstract:
  We show that the Fano operator for one dimensional quantum system is uniquely
determined by assuming the reasonable behavior under translation and parity
transformation on phase space. Contrarily, for the system with lattice phase
space the same procedure does not work.


title: Hilbert Space Structure in Classical Mechanics: (I)

abstract:
  In this paper we study the Hilbert space structure underlying the Koopman-von
Neumann (KvN) operatorial formulation of classical mechanics. KvN limited
themselves to study the Hilbert space of zero-forms that are the square
integrable functions on phase space. They proved that in this Hilbert space the
evolution is unitary for every system. In this paper we extend the KvN Hilbert
space to higher forms which are basically functions of the phase space points
and the differentials on phase space. We prove that if we equip this space with
a positive definite scalar product the evolution can turn out to be non-unitary
for some systems. Vice versa if we insist in having a unitary evolution for
every system then the scalar product cannot be positive definite. Identifying
the one-forms with the Jacobi fields we provide a physical explanation of these
phenomena. We also prove that the unitary/non unitary character of the
evolution is invariant under canonical transformations.


title: Hilbert Space Structure in Classical Mechanics: (II)

abstract:
  In this paper we analyze two different functional formulations of classical
mechanics. In the first one the Jacobi fields are represented by bosonic
variables and belong to the vector (or its dual) representation of the
symplectic group. In the second formulation the Jacobi fields are given as
condensates of Grassmannian variables belonging to the spinor representation of
the metaplectic group. For both formulations we shall show that, differently
from what happens in the case presented in paper no. (I), it is possible to
endow the associated Hilbert space with a positive definite scalar product and
to describe the dynamics via a Hermitian Hamiltonian. The drawback of this
formulation is that higher forms do not appear automatically and that the
description of chaotic systems may need a further extension of the Hilbert
space.


title: Superluminal transmission is possible from now on

abstract:
  It is known that superluminal transmission of information and energy
contradicts Einstein's relativity. Here we announce an unusual TOE called
'nature theory' in which impossible things become possible. We present the
scheme of an apparatus for sending signals over arbitrarily large distances
with speeds arbitrarily exceeding the light speed in vacuum. Introducing the
notions of effective speed and reliability of superluminal devices, we
encourage experimenters to set and break world records in this new branch. At
the same time we outline a mechanism (termed 'particle encapsulation') owing to
which nature theory remains Lorentz invariant and so consistent with
experiments. From among other numerous applications of nature theory we discuss
briefly local antigravitation and new computing machines, called 'vacuum
computers', applying 'cat principle'. They are of great interest because should
enable humans to overcome the Goedel-Turing barrier.


title: Critique of "No-Signaling Condition and Quantum Dynamics"

abstract:
  We comment on the article of Ch. Simon, V. Buzek and N. Gisin: ``No-Signaling
Condition and Quantum Dynamics'', Phys. Rev. Lett. 87 (2001) 170405, which
argues that linearity of quantum mechanics follows from lack of superluminal
signals and some usual hypotheses about measurements. We argue that such
assumptions in the end are ineffective as an explanation of the linearity of
quantum mechanics.


title: Optimal Control of Spin Dynamics in the Presence of Relaxation

abstract:
  Experiments in coherent spectroscopy correspond to control of quantum
mechanical ensembles guiding them from initial to final target states. The
control inputs (pulse sequences) that accomplish these transformations should
be designed to minimize the effects of relaxation and to optimize the
sensitivity of the experiments. For example in nuclear magnetic resonance (NMR)
spectroscopy, a question of fundamental importance is what is the maximum
efficiency of coherence or polarization transfer between two spins in the
presence of relaxation. Furthermore, what is the optimal pulse sequence which
achieves this efficiency? In this letter, we initiate the study of a class of
control systems, which leads to analytical answers to the above questions.
Unexpected gains in sensitivity are reported for the most commonly used
experiments in NMR spectroscopy.


title: Cavity QED and quantum information processing with "hot" trapped atoms

abstract:
  We propose a method to implement cavity QED and quantum information
processing in high-Q cavities with a single trapped but non-localized atom. The
system is beyond the Lamb-Dick limit due to the atomic thermal motion. Our
method is based on adiabatic passages, which make the relevant dynamics
insensitive to the randomness of the atom position with an appropriate
interaction configuration. The validity of this method is demonstrated from
both approximate analytical calculations and exact numerical simulations. We
also discuss various applications of this method based on the current
experimental technology.


title: Echo spectroscopy and quantum stability of trapped atoms

abstract:
  We investigate the dephasing of ultra cold ^{85}Rb atoms trapped in an
optical dipole trap and prepared in a coherent superposition of their two
hyperfine ground states by interaction with a microwave pulse. We demonstrate
that the dephasing, measured as the Ramsey fringe contrast, can be reversed by
stimulating a coherence echo with a pi-pulse between the two pi/2 pulses, in
analogy to the photon echo. We also demonstrate that the failure of the echo
for certain trap parameters is due to dynamics in the trap, and thereby that
''echo spectroscopy'' can be used to study the quantum dynamics in the trap
even when more than 10^6 states are thermally populated, and to study the
crossover from quantum (where dynamical decoherence is supressed) to classical
dynamics.


title:
Photons Do Collapse In the Retina Not in the Brain Cortex: Evidence from
  Visual Illusions

abstract:
  While looking for evidence of quantum coherent states within the brain, many
quantum mind advocates proposed experiments based on the assumption that the
coherent state of a photon entering the visual system could somehow be
preserved through the neural processing, or in other words they suppose that
photons collapse not in the retina, but in the brain cortex. In this paper we
show that photons do collapse within the retina and subsequent processing of
information at the level of neural membranes proceeds. Moreover, we explicitly
stress on the fact that due to existent amplification of the signal produced by
each photon, a basic quantum mechanical theorem forbids the photon state to be
teleported from the retina to the brain cortex. The changes of the membrane
potential of the neurons in the primary visual cortex are shown to be relevant
to inputting visual sensory information that is already processed and is not
identical to the visual image entering in the retina. A striking evidence for
the existent processing of the incoming visual information by the retina is
provided by visual illusions resulting from the lateral inhibition mechanism.


title:
An Efficient Source of Single Photons: A Single Quantum Dot in a
  Micropost Microcavity

abstract:
  We have demonstrated efficient production of triggered single photons by
coupling a single semiconductor quantum dot to a three-dimensionally confined
optical mode in a micropost microcavity. The efficiency of emitting single
photons into a single-mode travelling wave is approximately 38%, which is
nearly two orders of magnitude higher than for a quantum dot in bulk
semiconductor material. At the same time, the probability of having more than
one photon in a given pulse is reduced by a factor of seven as compared to
light with Poissonian photon statistics.


title: Generalized (s-Parameterized) Weyl Transformation

abstract:
  A general canonical transformation of mechanical operators of position and
momentum is considered. It is shown that it automatically generates a parameter
s which leads to a generalized (or s-parameterized) Wigner function. This
allows one to derive a generalized (s-parameterized) Moyal brackets for any
dimensions. In the classical limit the s-parameterized Wigner averages of the
momentum and its square yield the respective classical values. Interestingly
enough,in the latter case the classical Hamilton-Jacobi equation emerges as a
consequence of such a transition only if there is a non-zero parameter s.


title: Robust dynamical decoupling with bounded controls

abstract:
  We propose a general procedure for implementing dynamical decoupling without
requiring arbitrarily strong, impulsive control actions. This is accomplished
by designing continuous decoupling propagators according to Eulerian paths in
the decoupling group for the system. Such Eulerian decoupling schemes offer two
important advantages over their impulsive counterparts: they are able to
enforce the same dynamical symmetrization but with more realistic control
resources and, at the same time, they are intrinsically tolerant against a
large class of systematic implementation errors.


title: Bell Inequalities with Auxiliary Communication

abstract:
  What is the communication cost of simulating the correlations produced by
quantum theory? We generalize Bell inequalities to the setting of local
realistic theories augmented by a fixed amount of classical communication.
Suppose two parties choose one of M two-outcome measurements and exchange 1 bit
of information. We present the complete set of inequalities for M = 2, and the
complete set of inequalities for the joint correlation observable for M = 3. We
find that correlations produced by quantum theory satisfy both of these sets of
inequalities. One bit of communication is therefore sufficient to simulate
quantum correlations in both of these scenarios.


title:
The generalized partial transposition criterion for separability of
  multipartite quantum states

abstract:
  We present a generalized partial transposition separability criterion for the
density matrix of a multipartite quantum system. This criterion comprises as
special cases the famous Peres-Horodecki criterion and the recent realignment
criterion in [O. Rudolph, quant-ph/0202121] and [K. Chen, L.A. Wu,
quant-ph/0205017]. It involves only straightforward matrix manipulations and is
easy to apply. A quantitative measure of entanglement based on this criterion
is also obtained.


title:
Criterion for distinguishability of arbitrary bipartite orthogonal
  states

abstract:
  In this paper we present a necessary and sufficient condition of
distinguishability of bipartite quantum states. It is shown that the operators
to reliably distinguish states need only rounds of projective measurements and
classical comunication. We also present a necessary condition of
distinguishability of bipartite quantum states which is simple and general.
With this condition one can get many cases of indistinguishability. The
conclusions may be useful in understanding the essence of nonlocality and
calculating the distillable entanglement and the bound of distillable
entanglement.


title:
Inferring superposition and entanglement from measurements in a single
  basis

abstract:
  We discuss what can be inferred from measurements on one- and two-qubit
systems using a single measurement basis at various times. We show that, given
reasonable physical assumptions, carrying out such measurements at
quarter-period intervals is enough to demonstrate coherent oscillations of one
or two qubits between the relevant measurement basis states. One can thus infer
from such measurements alone that an approximately equal superposition of two
measurement basis states has been created in a coherent oscillation experiment.
Similarly, one can infer that a near maximally entangled state of two qubits
has been created in an experiment involving a putative SWAP gate. These results
apply even if the relevant quantum systems are only approximate qubits. We
discuss applications to fundamental quantum physics experiments and quantum
information processing investigations.


title: On the Completeness of Quantum Mechanics

abstract:
  Quantum cryptography, quantum computer project, space-time quantization
program and recent computer experiments reported by Accardi and his
collaborators show the importance and actuality of the discussion of the
completeness of quantum mechanics (QM) started by Einstein more than 70 years
ago. Many years ago we pointed out that the violation of Bell's inequalities is
neither a proof of completeness of QM nor an indication of the violation of
Einsteinian causality. We also indicated how and in what sense a completeness
of QM might be tested with the help of statistical nonparametric purity tests.
In this paper we review and refine our arguments. We also point out that the
statistical predictions of QM for two-particle correlation experiments do not
give any deterministic prediction for a single pair. After beam is separated we
obtain two beams moving in opposite directions. If the coincidence is reported
it is only after the beams had interacted with corresponding measuring devices
and two particles had been detected. This fact has implications for quantum
cryptography. Namely a series of the measurements performed on the beam by Bob
and converted into a string of bits (secret key) will in general differ, due to
lack of strict anti-correlations, from a secret key found by Alice using the
same procedure.


title:
Exponential Lower Bound for 2-Query Locally Decodable Codes via a
  Quantum Argument

abstract:
  A locally decodable code encodes n-bit strings x in m-bit codewords C(x), in
such a way that one can recover any bit x_i from a corrupted codeword by
querying only a few bits of that word. We use a quantum argument to prove that
LDCs with 2 classical queries need exponential length: m=2^{Omega(n)}.
Previously this was known only for linear codes (Goldreich et al. 02). Our
proof shows that a 2-query LDC can be decoded with only 1 quantum query, and
then proves an exponential lower bound for such 1-query locally
quantum-decodable codes. We also show that q quantum queries allow more
succinct LDCs than the best known LDCs with q classical queries. Finally, we
give new classical lower bounds and quantum upper bounds for the setting of
private information retrieval. In particular, we exhibit a quantum 2-server PIR
scheme with O(n^{3/10}) qubits of communication, improving upon the O(n^{1/3})
bits of communication of the best known classical 2-server PIR.


title: Pattern recognition on a quantum computer

abstract:
  By means of a simple example it is demonstrated that the task of finding and
identifying certain patterns in an otherwise (macroscopically) unstructured
picture (data set) can be accomplished efficiently by a quantum computer.
Employing the powerful tool of the quantum Fourier transform the proposed
quantum algorithm exhibits an exponential speed-up in comparison with its
classical counterpart. The digital representation also results in a
significantly higher accuracy than the method of optical filtering. PACS:
03.67.Lx, 03.67.-a, 42.30.Sy, 89.70.+c.


title:
Recovering classical dynamics from coupled quantum systems through
  continuous measurement

abstract:
  We study the role of continuous measurement in the quantum to classical
transition for a system with coupled internal (spin) and external (motional)
degrees of freedom. Even when the measured motional degree of freedom can be
treated classically, entanglement between spin and motion causes strong
measurement backaction on the quantum spin subsystem so that classical
trajectories are not recovered in this mixed quantum-classical regime. The
measurement can extract localized quantum trajectories that behave classically
only when the internal action also becomes large relative to h-bar.


title: Structured Adiabatic Quantum Search

abstract:
  We examine the use of adiabatic quantum algorithms to solve structured, or
nested, search problems. We construct suitable time dependent Hamiltonians and
derive the computation times for a general class of nested searches involving n
qubits. As expected, we find that as additional structure is included, the
Hamiltonians become more local and the computation times decrease.