title:
A Two-Step Quantum Direct Communication Protocol Using
  Einstein-Podolsky-Rosen Pair Block

abstract:
  A protocol for quantum secure direct communication using blocks of EPR pairs
is proposed. A set of ordered $N$ EPR pairs is used as a data block for sending
secret message directly. The ordered $N$ EPR set is divided into two particle
sequences, a checking sequence and a message-coding sequence. After
transmitting the checking sequence, the two parties of communication check
eavesdropping by measuring a fraction of particles randomly chosen, with random
choice of two sets of measuring bases. After insuring the security of the
quantum channel, the sender, Alice encodes the secret message directly on the
message-coding sequence and send them to Bob. By combining the checking and
message-coding sequences together, Bob is able to read out the encoded messages
directly. The scheme is secure because an eavesdropper cannot get both
sequences simultaneously. We also discuss issues in a noisy channel.


title: Proof-of-Concept Experiments for Quantum Physics in Space

abstract:
  Quantum physics experiments in space using entangled photons and satellites
are within reach of current technology. We propose a series of fundamental
quantum physics experiments that make advantageous use of the space
infrastructure with specific emphasis on the satellite-based distribution of
entangled photon pairs. The experiments are feasible already today and will
eventually lead to a Bell-experiment over thousands of kilometers, thus
demonstrating quantum correlations over distances which cannot be achieved by
purely earth-bound experiments.


title: Capacity of nonlinear bosonic systems

abstract:
  We analyze the role of nonlinear Hamiltonians in bosonic channels.
  We show that the information capacity as a function of the channel energy is
increased with respect to the corresponding linear case, although only when the
energy used for driving the nonlinearity is not considered as part of the
energetic cost and when dispersive effects are negligible.


title: An Explicit Universal Gate-set for Exchange-Only Quantum Computation

abstract:
  A single physical interaction might not be universal for quantum computation
in general. It has been shown, however, that in some cases it can generate
universal quantum computation over a subspace. For example, by encoding logical
qubits into arrays of multiple physical qubits, a single isotropic or
anisotropic exchange interaction can generate a universal logical gate-set.
Recently, encoded universality for the exchange interaction was explicitly
demonstrated on three-qubit arrays, the smallest nontrivial encoding. We now
present the exact specification of a discrete universal logical gate-set on
four-qubit arrays. We show how to implement the single qubit operations exactly
with at most 3 nearest neighbor exchange operations and how to generate the
encoded controlled-not with 29 parallel nearest neighbor exchange interactions
or 54 serial gates, obtained from extensive numerical optimization using
genetic algorithms and Nelder-Mead searches. Our gate-sequences are immediately
applicable to implementations of quantum circuits with the exchange
interaction.


title:
Squeezed States of the Generalized Minimum Uncertainty State for the
  Caldirola-Kanai Hamiltonian

abstract:
  We show that the ground state of the well-known pseudo-stationary states for
the Caldirola-Kanai Hamiltonian is a generalized minimum uncertainty state,
which has the minimum allowed uncertainty $\Delta q \Delta p = \hbar
\sigma_0/2$, where $\sigma_0 (\geq 1)$ is a constant depending on the damping
factor and natural frequency. The most general symmetric Gaussian states are
obtained as the one-parameter squeezed states of the pseudo-stationary ground
state. It is further shown that the coherent states of the pseudo-stationary
ground state constitute another class of the generalized minimum uncertainty
states.


title: Quantum Entanglement in Heisenberg Antiferromagnets

abstract:
  Entanglement sharing among pairs of spins in Heisenberg antiferromagnets is
investigated using the concurrence measure. For a nondegenerate S=0 ground
state, a simple formula relates the concurrence to the diagonal correlation
function. The concurrence length is seen to be extremely short. A few finite
clusters are studied numerically, to see the trend in higher dimensions. It is
argued that nearest-neighbour concurrence is zero for triangular and Kagome
lattices. The concurrences in the maximal-spin states are explicitly
calculated, where the concurrence averaged over all pairs is larger than the
S=0 states.


title: The Quantum Self-eraser

abstract:
  A scheme for an atomic beam quantum self-eraser is presented. The proposal is
based on time reversal invariance on a quantum optical Ramsey fringes
experiment, where a realization of complementarity for atomic coherence can be
achieved. It consists of two high finesse resonators that are pumped and probed
by the same atom. This property relates quantum erasing with time reversal
symmetry, allowing for a full quantum erasing of the which-way information
stored in the cavity fields. The outlined scheme also prepares and observes a
non-local state in the fields of the resonators: a coherent superposition
between correlated states of macroscopically separated quantum systems. The
proposed scheme emphasizes the role of entanglement swapping in delayed-choice
experiments. Finally, we show that the quantum self-eraser violates temporal
Bell inequalities and analyze the relation between this violation and the
erasability of which-way information.


title: Duality in asymmetric Quantum optical Ramsey interferometers

abstract:
  A formalism have been recently derived [J. Martinez-Linares and D. Harmin,
quantum-ph/0306057] allowing one to separate different sources of which-way
information contributing to the total distinguishability D of the ways in a
two-way interferometer. Here we apply the formalism to a Quantum Optical Ramsey
Interferometer where both sources, the a-priori predictability of the ways P
and the quantum "Quality" Q of the which-way detector, stems from the same
physical interaction. We show that the formalism is able to separate both
sources of which-way information. Moreover, it is shown that Q succeeds in
quantifying the amount of quantum which-way information stored in the which-way
detector even in cases where D does not.


title:
An Experimental Realization of Quantum-vacuum Geometric Phases by Using
  the Gyrotropic-medium Optical Fiber

abstract:
  The connection between the quantum-vacuum geometric phases (which originates
from the vacuum zero-point electromagnetic fluctuation) and the non-normal
product procedure is considered in the present Letter. In order to investigate
this physically interesting geometric phases at quantum-vacuum level, we
suggest an experimentally feasible scheme to test it by means of a
noncoplanarly curved fiber made of gyrotropic media. A remarkable feature of
the present experimental realization is that one can easily extract the
nonvanishing and nontrivial quantum-vacuum geometric phases of left- and/or
right- handed circularly polarized light from the vanishing and trivial total
quantum-vacuum geometric phases.


title: A Supplement: on the Quantum-vacuum Geometric Phases

abstract:
  Three related topics on the quantum-vacuum geometric phases in a
noncoplanarly curved optical fiber is presented: (i) a brief review: the
investigation of vacuum effect and its experimental realization; (ii) the
sequence of ideas of geometric phases in the fiber; (iii) three derivations of
effective Hamiltonian that describes the wave propagation of photon field in a
curved fiber.


title: Atom-photon entanglement generation and distribution

abstract:
  We extend an earlier model by Law {\it et al.} \cite{law} for a cavity QED
based single-photon-gun to atom-photon entanglement generation and
distribution. We illuminate the importance of a small critical atom number on
the fidelity of the proposed operation in the strong coupling limit. Our result
points to a promisingly high purity and efficiency using currently available
cavity QED parameters, and sheds new light on constructing quantum computing
and communication devices with trapped atoms and high Q optical cavities.


title:
Experimental Scheme to Test the Fair Sampling Assumption in EPR-Bell
  Experiments

abstract:
  We propose here an experimental test of the fair sampling assumption in
two-channel EPR-Bell experiments for which the detection loophole holds.


title: Optimal control theory for unitary transformations

abstract:
  The dynamics of a quantum system driven by an external field is well
described by a unitary transformation generated by a time dependent
Hamiltonian. The inverse problem of finding the field that generates a specific
unitary transformation is the subject of study. The unitary transformation
which can represent an algorithm in a quantum computation is imposed on a
subset of quantum states embedded in a larger Hilbert space. Optimal control
theory (OCT) is used to solve the inversion problem irrespective of the initial
input state. A unified formalism, based on the Krotov method is developed
leading to a new scheme. The schemes are compared for the inversion of a
two-qubit Fourier transform using as registers the vibrational levels of the
$X^1\Sigma^+_g$ electronic state of Na$_2$. Raman-like transitions through the
$A^1\Sigma^+_u$ electronic state induce the transitions. Light fields are found
that are able to implement the Fourier transform within a picosecond time
scale. Such fields can be obtained by pulse-shaping techniques of a femtosecond
pulse. Out of the schemes studied the square modulus scheme converges fastest.
A study of the implementation of the $Q$ qubit Fourier transform in the Na$_2$
molecule was carried out for up to 5 qubits. The classical computation effort
required to obtain the algorithm with a given fidelity is estimated to scale
exponentially with the number of levels. The observed moderate scaling of the
pulse intensity with the number of qubits in the transformation is
rationalized.


title: Interference as a statistical consequence of conjecture on time quant

abstract:
  We analyze statistical consequences of a conjecture that there exists a
fundamental (indivisible) quant of time. We study particle dynamics with
discrete time. We show that a quantum-like interference pattern could appear as
a statistical effect for deterministic particles, i.e. particles that have
trajectories and obey deterministic dynamical equations, if one introduces a
discrete time. As a demonstration of this concept we consider particle
scattering on a screen with a slit. We study how resulting interference picture
depends on the parameters of the model. The resulting interference picture has
a nontrivial minimum-maximum distribution which vanishes, as the time
discreteness parameter goes to zero. This picture is qualitatively the same as
one obtained in quantum experiments. The picture includes some interesting
nonclassical properties such as a 'black' region behind the center of the slit.


title: A characterization of continuous variable entanglement

abstract:
  We present an experimental analysis of quadrature entanglement produced from
a pair of amplitude squeezed beams. The correlation matrix of the state is
characterized within a set of reasonable assumptions, and the strength of the
entanglement is gauged using measures of the degree of inseparability and the
degree of EPR paradox. We introduce controlled decoherence in the form of
optical loss to the entangled state, and demonstrate qualitative differences in
the response of the degrees of inseparability and EPR paradox to this loss. The
entanglement is represented on a photon number diagram that provides an
intuitive and physically relevant description of the state. We calculate
efficacy contours for several quantum information protocols on this diagram,
and use them to predict the effectiveness of our entanglement in those
protocols.


title: Hydrogen atom in strong magnetic field revisited

abstract:
  We derive in a straightforward way the spectrum of a hydrogen atom in a
strong magnetic field.


title: Entanglement Measure for Composite Systems

abstract:
  A general description of entanglement is suggested as an action realized by
an arbitrary operator over given disentangled states. The related entanglement
measure is defined. Because of its generality, this definition can be employed
for any physical systems, pure or mixed, equilibrium or nonequilibrium, and
characterized by any type of operators, whether these are statistical
operators, field operators, spin operators, or anything else. Entanglement of
any number of parts from their total ensemble forming a multiparticle composite
system can be determined. Interplay between entanglement and ordering,
occurring under phase transitions, is analysed by invoking the concept of
operator order indices.


title: The wave nature of biomolecules and fluorofullerenes

abstract:
  We demonstrate quantum interference for tetraphenylporphyrin, the first
biomolecule exhibiting wave nature, and for the fluorofullerene C60F48 using a
near-field Talbot-Lau interferometer. For the porphyrins, which are
distinguished by their low symmetry and their abundant occurence in organic
systems, we find the theoretically expected maximal interference contrast and
its expected dependence on the de Broglie wavelength. For C60F48 the observed
fringe visibility is below the expected value, but the high contrast still
provides good evidence for the quantum character of the observed fringe
pattern. The fluorofullerenes therefore set the new mark in complexity and mass
(1632 amu) for de Broglie wave experiments, exceeding the previous mass record
by a factor of two.


title:
Noncontextuality, Finite Precision Measurement and the Kochen-Specker
  Theorem

abstract:
  Meyer recently queried whether non-contextual hidden variable models can,
despite the Kochen-Specker theorem, simulate the predictions of quantum
mechanics to within any fixed finite experimental precision. Clifton and Kent
have presented constructions of non-contextual hidden variable theories which,
they argued, indeed simulate quantum mechanics in this way. These arguments
have evoked some controversy. One aim of this paper is to respond to and rebut
criticisms of the MCK papers. We thus elaborate in a little more detail how the
CK models can reproduce the predictions of quantum mechanics to arbitrary
precision. We analyse in more detail the relationship between classicality,
finite precision measurement and contextuality, and defend the claims that the
CK models are both essentially classical and non-contextual. We also examine in
more detail the senses in which a theory can be said to be contextual or
non-contextual, and in which an experiment can be said to provide evidence on
the point. In particular, we criticise the suggestion that a decisive
experimental verification of contextuality is possible, arguing that the idea
rests on a conceptual confusion.


title: Treatment of sound on quantum computers

abstract:
  We study numerically how a sound signal stored in a quantum computer can be
recognized and restored with a minimal number of measurements in presence of
random quantum gate errors. A method developed uses elements of MP3 sound
compression and allows to recover human speech and sound of complex quantum
wavefunctions.


title: Entanglement preservation in quantum cloning

abstract:
  Recently Galv\~{a}o and Hardy have shown that quantum cloning can improve the
performance of some quantum computation tasks. However such performance
enhancement is possible only if quantum correlations survive the cloning
process. We investigate preservation of the quantum correlations in the process
of non--local cloning of entangled pairs of two--level systems. We consider
different kinds of quantum cloning machines and compare their effectiveness in
the cloning of non--maximally entangled pure states. A mean entanglement is
introduced in order to obtain a quantitative evaluation of an average
efficiency for the different cloning machines. We show that a reduction of the
quantum correlations is significant and it strongly depends upon the kind of
cloning machine used. Losses of the entanglement are largest in the case of the
universal quantum cloning machine. Generally, in all cases considered the
losses of the entanglement are so drastic that the method of enhancement for
the performance of the quantum computation using the quantum cloning seems to
be questionable.


title: John Bell Across Space and Time

abstract:
  This is a review of the book Quantum [Un]speakables: From Bell to Quantum
Information. Reinhold A. Bertlmann and Anton Zeilinger (editors).


title:
On the Distribution of the Wave Function for Systems in Thermal
  Equilibrium

abstract:
  For a quantum system, a density matrix rho that is not pure can arise, via
averaging, from a distribution mu of its wave function, a normalized vector
belonging to its Hilbert space H. While rho itself does not determine a unique
mu, additional facts, such as that the system has come to thermal equilibrium,
might. It is thus not unreasonable to ask, which mu, if any, corresponds to a
given thermodynamic ensemble? To answer this question we construct, for any
given density matrix rho, a natural measure on the unit sphere in H, denoted
GAP(rho). We do this using a suitable projection of the Gaussian measure on H
with covariance rho. We establish some nice properties of GAP(rho) and show
that this measure arises naturally when considering macroscopic systems. In
particular, we argue that it is the most appropriate choice for systems in
thermal equilibrium, described by the canonical ensemble density matrix
rho_beta = (1/Z) exp(- beta H). GAP(rho) may also be relevant to quantum chaos
and to the stochastic evolution of open quantum systems, where distributions on
H are often used.


title:
Quantum Aspects of Semantic Analysis and Symbolic Artificial
  Intelligence

abstract:
  Modern approaches to semanic analysis if reformulated as Hilbert-space
problems reveal formal structures known from quantum mechanics. Similar
situation is found in distributed representations of cognitive structures
developed for the purposes of neural networks. We take a closer look at
similarites and differences between the above two fields and quantum
information theory.


title: Uncertainty, trajectories, and duality

abstract:
  We show explicitly how uncertainty can arise in a trajectory representation.
Then we show that the formal utilization of the WKB like hierarchy structure of
dKdV in the description of (X,psi) duality does not encounter norm constraints.


title:
Decoherence Rate of Semiconductor Charge Qubit Coupled to Acoustic
  Phonon Reservoir

abstract:
  We analyze decoherence of an electron in a double-dot due to the interaction
with acoustic phonons. For large tunneling rates between the quantum dots, the
main contribution to decoherence comes from the phonon emission relaxation
processes, while for small tunneling rates, the virtual-phonon, dephasing
processes dominate. Our results show that in common semiconductors, such as Si
and GaAs, the latter mechanism determines the upper limit for the double-dot
charge qubit performance measure.


title: Nonclassical Properties of Coherent States

abstract:
  It is demonstrated that a weak measurement of the squared quadrature
observable may yield negative values for coherent states. This result cannot be
reproduced by a classical theory where quadratures are stochastic $c$-numbers.
The real part of the weak value is a conditional moment of the Margenau-Hill
distribution. The nonclassicality of coherent states can be associated with
negative values of the Margenau-Hill distribution. A more general type of weak
measurement is considered, where the pointer can be in an arbitrary state, pure
or mixed.


title: Adiabatic Time Evolution in Spin-Systems

abstract:
  Adiabatic processes in the quantum Ising model and the anisotropic Heisenberg
model are discussed. The adiabatic processes are assumed to consist in the slow
variation of the strength of the magnetic field that environs the spin-systems.
These processes are of current interest in the treatment of cold atoms in
optical lattices and in Adiabatic Quantum Computation. We determine the
probability that, during an adiabatic passage starting from the ground state,
states with higher energy are excited.


title: Entanglement and the Phase Transition in Single Mode Superradiance

abstract:
  We consider the entanglement properties of the quantum phase transition in
the single-mode superradiance model, involving the interaction of a boson mode
and an ensemble of atoms. For infinite system size, the atom-field entanglement
of formation diverges logarithmically with the correlation length exponent.
Using a continuous variable representation, we compare this to the divergence
of the entropy in conformal field theories, and derive an exact expression for
the scaled concurrence and the cusp-like non-analyticity of the momentum
squeezing.


title: Quantum statistics of overlapping modes in open resonators

abstract:
  We study the quantum dynamics of optical fields in weakly confining
resonators with overlapping modes. Employing a recently developed quantization
scheme involving a discrete set of resonator modes and continua of external
modes we derive Langevin equations and a master equation for the resonator
modes. Langevin dynamics and the master equation are proved to be equivalent in
the Markovian limit. Our open-resonator dynamics may be used as a starting
point for a quantum theory of random lasers.


title: Rule (4) and Continuous Observation

abstract:
  The effect of rule (4) on a series or parallel sequence of quantum mechanical
steps is to insure that a conscious observer does not skip a step. This rule
effectively places the observer in continuous contact with the system. Key
Words: brain states, continuous observation, conscious observer, measurement,
probability current, state reduction, wave collapse.


title:
Action scales for quantum decoherence and their relation to structures
  in phase space

abstract:
  A characteristic action $\Delta S$ is defined whose magnitude determines some
properties of the expectation value of a general quantum displacement operator.
These properties are related to the capability of a given environmental
`monitoring' system to induce decoherence in quantum systems coupled to it. We
show that the scale for effective decoherence is given by $\Delta
S\approx\hbar$. We relate this characteristic action with a complementary
quantity, $\Delta Z$, and analyse their connection with the main features of
the pattern of structures developed by the environmental state in different
phase space representations. The relevance of the $\Delta S$-action scale is
illustrated using both a model quantum system solved numerically and a set of
model quantum systems for which analytical expressions for the time-averaged
expectation value of the displacement operator are obtained explicitly.


title:
Proof of Partial Equivalence of Classical and Quantum Dynamics in
  Bosonic Systems

abstract:
  Quantum Field Theory (QFT) makes predictions by combining assumptions about
(1) quantum dynamics, typically a Schrodinger or Liouville equation; (2)
quantum measurement, usually via a collapse formalism. Here I define a
"classical density matrix" rho to describe ensembles of states of ordinary
second-order classical systems (ODE or PDE). I prove that the dynamics of the
field observables, phi and pi, defined as operators over rho, obey precisely
the usual Louiville equation for the same field operators in QFT, following
Weinberg's treatment. I discuss implications in detail - particularly the
implication that the difference between quantum computing and quantum mechanics
versus classical systems lies mainly on the measurement side, not the dynamic
side. But what if measurement itself were to be derived from dynamics and
boundary conditions? An heretical realistic approach to building finite field
theory is proposed, linked to the backwards time interpretation of quantum
mechanics.


title: Collinear optical weak measurements with photonic crystals

abstract:
  We present a theoretical and experimental study of a photonic crystal based
optical system in terms of weak values that map polarization states onto
longitudinal spatial position and show fast and slow light behavior.


title: Correlated Equilibria of Classical Strategic Games with Quantum Signals

abstract:
  Correlated equilibria are sometimes more efficient than the Nash equilibria
of a game without signals. We investigate whether the availability of quantum
signals in the context of a classical strategic game may allow the players to
achieve even better efficiency than in any correlated equilibrium with
classical signals, and find the answer to be positive.


title: Closed-loop Quantum Parameter Estimation: Spins in a Magnetic Field

abstract:
  We present an experimental demonstration of closed-loop quantum parameter
estimation in which real-time feedback is used to achieve robustness to
modeling uncertainty. By performing broadband estimation of a magnetic field
acting on hyperfine spins in a cold atom ensemble, we show that accuracy is not
compromised by fluctuations in total atom number even though the measured
signal in our canonical configuration depends only on the product of the field
and atom number. This methodology could be essential for efforts to utilize
conditional squeezing in spin-resonance measurements.


title: Comment on "Entanglement Measure for Composite Systems"

abstract:
  A recent paper claimed to give an entanglement measure for composite systems,
including Bose condensation and superconductivity. It is not an entanglement
measure. It does not distinguish entanglement and non-factorization merely due
to (anti)symmetrization for identical particles. In fact, the nature of
entanglement, beyond the effect of (anti)symmetrization, in important states of
many identical particles such as Bose condensation and superconductivity, had
already been studied earlier.


title: One-qubit fingerprinting schemes

abstract:
  Fingerprinting is a technique in communication complexity in which two
parties (Alice and Bob) with large data sets send short messages to a third
party (a referee), who attempts to compute some function of the larger data
sets. For the equality function, the referee attempts to determine whether
Alice's data and Bob's data are the same. In this paper, we consider the
extreme scenario of performing fingerprinting where Alice and Bob both send
either one bit (classically) or one qubit (in the quantum regime) messages to
the referee for the equality problem. Restrictive bounds are demonstrated for
the error probability of one-bit fingerprinting schemes, and show that it is
easy to construct one-qubit fingerprinting schemes which can outperform any
one-bit fingerprinting scheme. The author hopes that this analysis will provide
results useful for performing physical experiments, which may help to advance
implementations for more general quantum communication protocols.


title: Evolutional Entanglement in Nonequilibrium Processes

abstract:
  Entanglement in nonequilibrium systems is considered. A general definition
for entanglement measure is introduced, which can be applied for characterizing
the level of entanglement produced by arbitrary operators. Applying this
definition to reduced density matrices makes it possible to measure the
entanglement in nonequilibrium as well as in equilibrium statistical systems.
An example of a multimode Bose-Einstein condensate is discussed.


title: Coherent states for exactly solvable potentials

abstract:
  A general algebraic procedure for constructing coherent states of a wide
class of exactly solvable potentials e.g., Morse and P{\"o}schl-Teller, is
given. The method, {\it a priori}, is potential independent and connects with
earlier developed ones, including the oscillator based approaches for coherent
states and their generalizations. This approach can be straightforwardly
extended to construct more general coherent states for the quantum mechanical
potential problems, like the nonlinear coherent states for the oscillators. The
time evolution properties of some of these coherent states, show revival and
fractional revival, as manifested in the autocorrelation functions, as well as,
in the quantum carpet structures.


title:
Intertwining technique for a system of difference Schroedinger equations
  and new exactly solvable multichannel potentials

abstract:
  The intertwining operator technique is applied to difference Schroedinger
equations with operator-valued coefficients. It is shown that these equations
appear naturally when a discrete basis is used for solving a multichannel
Schroedinger equation. New families of exactly solvable multichannel
Hamiltonians are found.


title: Solvable model of a strongly-driven micromaser

abstract:
  We study the dynamics of a micromaser where the pumping atoms are strongly
driven by a resonant classical field during their transit through the cavity
mode. We derive a master equation for this strongly-driven micromaser,
involving the contributions of the unitary atom-field interactions and the
dissipative effects of a thermal bath. We find analytical solutions for the
temporal evolution and the steady-state of this system by means of phase-space
techniques, providing an unusual solvable model of an open quantum system,
including pumping and decoherence. We derive closed expressions for all
relevant expectation values, describing the statistics of the cavity field and
the detected atomic levels. The transient regime shows the build-up of mixtures
of mesoscopic fields evolving towards a superpoissonian steady-state field
that, nevertheless, yields atomic correlations that exhibit stronger
nonclassical features than the conventional micromaser.


title:
Decoherence-Free Quantum Information Processing with Four-Photon
  Entangled States

abstract:
  Decoherence-free states protect quantum information from collective noise,
the predominant cause of decoherence in current implementations of quantum
communication and computation. Here we demonstrate that spontaneous parametric
down-conversion can be used to generate four-photon states which enable the
encoding of one qubit in a decoherence-free subspace. The immunity against
noise is verified by quantum state tomography of the encoded qubit. We show
that particular states of the encoded qubit can be distinguished by local
measurements on the four photons only.


title: Algebraic solution of master equations

abstract:
  We present a simple analytical method to solve master equations for finite
temperatures and any initial conditions, which consists in the expansion of the
density operator into normal modes. These modes and the expansion coefficients
are obtained algebraically by using ladder superoperators. This algebraic
technique is successful in cases in which the Liouville superoperator is
quadratic in the creation and annihilation operators.


title: Witnessing multipartite entanglement

abstract:
  We present the experimental detection of genuine multipartite entanglement
using entanglement witness operators. To this aim we introduce a canonical way
of constructing and decomposing witness operators so that they can be directly
implemented with present technology. We apply this method to three- and
four-qubit entangled states of polarized photons, giving experimental evidence
that the considered states contain true multipartite entanglement.


title: Orbital angular momentum of the down converted photons

abstract:
  We calculate the relative amplitude of orbital angular momentum (OAM)
entangled photon pairs from the spontaneous parametric down conversion. The
results show that the amplitude depends on both the two Laguerre indices l, p.
We also discuss the influences of the mostly used holograms and mono-mode
fibers for mode analyzation. We conclude that only a few dimensions can be
explored from the infinite OAM modes of the down-converted photon pairs.


title: Precis: Quantum Feedback

abstract:
  The following is the body of page ix of the PhD thesis Quantum Trajectories
and Feedback by H.M. Wiseman (Physics Department, University of Queensland,
1994), which is downloadable as a postscript file at
http://www.sct.gu.edu.au/~sctwiseh/PhDThesis.ps.z . It is (as it described
itself) a very brief technical summary of the most important results therein.


title:
Ferreting out the Fluffy Bunnies: Entanglement constrained by
  Generalized superselection rules

abstract:
  Entanglement is a resource central to quantum information (QI). In
particular, entanglement shared between two distant parties allows them to do
certain tasks that would otherwise be impossible. In this context, we study the
effect on the available entanglement of physical restrictions on the local
operations that can be performed by the two parties. We enforce these physical
restrictions by generalized superselection rules (SSRs), which we define to be
associated with a given group of physical transformations. Specifically the
generalized SSR is that the local operations must be covariant with respect to
that group. Then we operationally define the entanglement constrained by a SSR,
and show that it may be far below that expected on the basis of a naive (or
``fluffy bunny'') calculation. We consider two examples. The first is a
particle number SSR. Using this we show that for a two-mode BEC (with Alice
owning mode $A$ and Bob mode $B$), the useful entanglement shared by Alice and
Bob is identically zero. The second, a SSR associated with the symmetric group,
is applicable to ensemble QI processing such as in liquid-NMR. We prove that
even for an ensemble comprising many pairs of qubits, with each pair described
by a pure Bell state, the entanglement per pair constrained by this SSR goes to
zero for a large ensemble.


title:
A redefinition of Concurrence and its generalisation to bosonic
  subsystems of N qubit systems

abstract:
  We refine the notion of concurrence in this paper by a redefinition of the
concept. The new definition is simpler, computationally straight forward, and
allows the concurrence to be directly read off from the state. It has all the
positive features of the definiton given by Wootters over and above which it
can discriminate between different systems to which the Wootters prescription
would assign the same value. Finally, the definiton leads to a natural
extension of the notion to multiqubit concurrence, which we illustrate with
examples from quantum error correction codes.


title: The Case Against Objective Measurement

abstract:
  We consider two straightforward rules that govern the stochastic choice in a
single quantum mechanical event. They are shown to lead to absurd results if an
objective state reduction is allowed to compete with an observer state
reduction. Since the latter collapse is empirically verifiable, the existence
of the former is thrown into question. Key Words: Born Interpretation, brain
states, continuous observation, conscious observer, objective measurement,
observer measurement, probability current, state reduction, wave collapse.


title: A position and a time for the photon

abstract:
  This paper gives a constructive answer to the question whether photon states
can contain or not, and to what extent, the readings of rulers and clocks. The
paper first shows explicitly that, along with the momentum representation,
there is room in the one photon Hilbert space for an alternative position
representation. This is made possible by the existence of a self-adjoint,
involutive, position operator conjugate to the momentum operator (due to M.
Hawton). Position and momenta are shown to satisfy the Heisenberg-Weyl
quantization rules in the helicity basis, which is analyzed anew from this
point of view. The paper then turns to the photon's time of arrival. By picking
an appropriate photon Hamiltonian - using Maxwell equations as the photon
Schroedinger equation - a conjugate time of arrival operator is built. Its
interpretation, including the probability densities for the instant of arrival
(at arbitrary points of 3-D space) of photon states with different helicities
coming from arbitrary places, is discussed.


title: Symmetric Autocompensating Quantum Key Distribution

abstract:
  We present quantum key distribution schemes which are autocompensating
(require no alignment) and symmetric (Alice and Bob receive photons from a
central source) for both polarization and time-bin qubits. The primary benefit
of the symmetric configuration is that both Alice and Bob may have passive
setups (neither Alice nor Bob is required to make active changes for each run
of the protocol). We show that both the polarization and the time-bin schemes
may be implemented with existing technology. The new schemes are related to
previously described schemes by the concept of advanced waves.


title:
Implementing a Quantum Algorithm with Exchange-Coupled Quantum Dots: a
  Feasibility study

abstract:
  We present Monte Carlo wavefunction simulations for quantum computations
employing an exchange-coupled array of quantum dots. Employing a combination of
experimentally and theoretically available parameters, we find that gate
fidelities greater than 98 % may be obtained with current experimental and
technological capabilities. Application to an encoded 3 qubit (nine physical
qubits) Deutsch-Josza computation indicates that the algorithmic fidelity is
more a question of the total time to implement the gates than of the physical
complexity of those gates.


title: Cavity QED with optically transported atoms

abstract:
  Ultracold $^{87}$Rb atoms are delivered into a high-finesse optical
micro-cavity using a translating optical lattice trap and detected via the
cavity field. The atoms are loaded into an optical lattice from a magneto-optic
trap (MOT) and transported 1.5 cm into the cavity. Our cavity satisfies the
strong-coupling requirements for a single intracavity atom, thus permitting
real-time observation of single atoms transported into the cavity. This
transport scheme enables us to vary the number of intracavity atoms from 1 to
$>$100 corresponding to a maximum atomic cooperativity parameter of 5400, the
highest value ever achieved in an atom--cavity system. When many atoms are
loaded into the cavity, optical bistability is directly measured in real-time
cavity transmission.


title: Berry phase in a composite system

abstract:
  The Berry phase in a composite system with only one subsystem being driven
has been studied in this Letter. We choose two spin-$\frac 1 2 $ systems with
spin-spin couplings as the composite system, one of the subsystems is driven by
a time-dependent magnetic field. We show how the Berry phases depend on the
coupling between the two subsystems, and what is the relation between these
Berry phases of the whole system and those of the subsystems.


title: Quantifying Entanglement Production of Quantum Operations

abstract:
  The problem of entanglement produced by an arbitrary operator is formulated
and a related measure of entanglement production is introduced. This measure of
entanglement production satisfies all properties natural for such a
characteristic. A particular case is the entanglement produced by a density
operator or a density matrix. The suggested measure is valid for operations
over pure states as well as over mixed states, for equilibrium as well as
nonequilibrium processes. Systems of arbitrary nature can be treated, described
either by field operators, spin operators, or any other kind of operators,
which is realized by constructing generalized density matrices. The interplay
between entanglement production and phase transitions in statistical systems is
analysed by the examples of Bose-Einstein condensation, superconducting
transition, and magnetic transitions. The relation between the measure of
entanglement production and order indices is analysed.


title: Optical telecom networks as weak measurements with post-selection

abstract:
  In this work we establish a link between two apparently unrelated subjects:
polarization effects in optical fibers and devices, and the quantum theory of
weak measurements. We show that the abstract concept of weak measurements
followed by post-selection, introduced a decade ago by quantum theorists,
naturally appears in the everyday physics of telecom networks.


title:
Quantum Flows as Markovian Limit of Emission, Absorption and Scattering
  Interactions

abstract:
  We consider a Markovian approximation, of weak coupling type, to an open
system perturbation involving emission, absorption and scattering by reservoir
quanta. The result is the general form for a quantum stochastic flow driven by
creation, annihilation and gauge processes. A weak matrix limit is established
for the convergence of the interaction-picture unitary to a unitary, adapted
quantum stochastic process and of the Heisenberg dynamics to the corresponding
quantum stochastic flow: the convergence strategy is similar to the quantum
functional central limits introduced by Accardi, Frigerio and Lu$^{[ 1]}$. The
principal terms in the Dyson series expansions are identified and re-summed
after the limit to obtain explicit quantum stochastic differential equations
with renormalized coefficients. An extension of the Pul\UNICODE{0xe9}
inequalities$^{[ 2]}$ allows uniform estimates for the Dyson series expansion
for both the unitary operator and the Heisenberg evolution to be obtained.


title: Quantum Markovian Approximations for Fermionic Reservoirs

abstract:
  We establish a quantum functional central limit for the dynamics of a system
coupled to a Fermionic bath with a general interaction linear in the creation,
annihilation and scattering of the bath reservoir. Following a quantum
Markovian limit, we realize the open dynamical evolution of the system as an
adapted quantum stochastic process driven by Fermionic Noise.


title: Tailoring photonic entanglement in high-dimensional Hilbert spaces

abstract:
  We present an experiment where two photonic systems of arbitrary dimensions
can be entangled. The method is based on spontaneous parametric down conversion
with trains of d pump pulses with a fixed phase relation, generated by a
mode-locked laser. This leads to a photon pair created in a coherent
superposition of $d$ discrete emission times, given by the successive laser
pulses. Entanglement is shown by performing a two-photon interference
experiment and by observing the visibility of the interference fringes
increasing as a function of the dimension d. Factors limiting the visibility,
such as the presence of multiple pairs in one train, are discussed.


title: The geometry of quantum learning

abstract:
  Concept learning provides a natural framework in which to place the problems
solved by the quantum algorithms of Bernstein-Vazirani and Grover. By combining
the tools used in these algorithms--quantum fast transforms and amplitude
amplification--with a novel (in this context) tool--a solution method for
geometrical optimization problems--we derive a general technique for quantum
concept learning. We name this technique "Amplified Impatient Learning" and
apply it to construct quantum algorithms solving two new problems: BATTLESHIP
and MAJORITY, more efficiently than is possible classically.


title: Improving Gate-Level Simulation of Quantum Circuits

abstract:
  Simulating quantum computation on a classical computer is a difficult
problem. The matrices representing quantum gates, and the vectors modeling
qubit states grow exponentially with an increase in the number of qubits.
However, by using a novel data structure called the Quantum Information
Decision Diagram (QuIDD) that exploits the structure of quantum operators, a
useful subset of operator matrices and state vectors can be represented in a
form that grows polynomially with the number of qubits. This subset contains,
but is not limited to, any equal superposition of n qubits, any computational
basis state, n-qubit Pauli matrices, and n-qubit Hadamard matrices. It does
not, however, contain the discrete Fourier transform (employed in Shor's
algorithm) and some oracles used in Grover's algorithm. We first introduce and
motivate decision diagrams and QuIDDs. We then analyze the runtime and memory
complexity of QuIDD operations. Finally, we empirically validate QuIDD-based
simulation by means of a general-purpose quantum computing simulator QuIDDPro
implemented in C++. We simulate various instances of Grover's algorithm with
QuIDDPro, and the results demonstrate that QuIDDs asymptotically outperform all
other known simulation techniques. Our simulations also show that well-known
worst-case instances of classical searching can be circumvented in many
specific cases by data compression techniques.


title:
The Connection Between Density Matrix Method, Supersymmetric Quantum
  Mechanics and Lewis-Riesenfeld Invariant Theory

abstract:
  This paper is concerned with the connection between density matrix method,
supersymmetric quantum mechanics and Lewis-Riesenfeld invariant theory. It is
shown that these three formulations share the common mathematical structure:
specifically, all of them have the invariant operators which satisfies the
Liouville-Von Neumann equation and the solutions to the time-dependent
Schr\"{o}dinger equation and/or Schr\"{o}dinger eigenvalue equation can be
constructed in terms of the eigenstates of the invariants.


title:
Characterization of pure quantum states of multiple qubits using the
  Groverian entanglement measure

abstract:
  The Groverian entanglement measure, G(psi), is applied to characterize a
variety of pure quantum states |psi> of multiple qubits. The Groverian measure
is calculated analytically for certain states of high symmetry, while for
arbitrary states it is evaluated using a numerical procedure. In particular, it
is calculated for the class of Greenberger-Horne-Zeilinger states, the W states
as well as for random pure states of n qubits. The entanglement generated by
Grover's algorithm is evaluated by calculating G(psi) for the intermediate
states that are obtained after t Grover iterations, for various initial states
and for different sets of the marked states.


title: One dimensional quantum walk with unitary noise

abstract:
  The effect of unitary noise on the discrete one-dimensional quantum walk is
studied using computer simulations. For the noiseless quantum walk, starting at
the origin (n=0) at time t=0, the position distribution Pt(n) at time t is very
different from the Gaussian distribution obtained for the classical random
walk. Furthermore, its standard deviation, sigma(t) scales as sigma(t) ~ t,
unlike the classical random walk for which sigma(t) ~ sqrt{t}. It is shown that
when the quantum walk is exposed to unitary noise, it exhibits a crossover from
quantum behavior for short times to classical-like behavior for long times. The
crossover time is found to be T ~ alpha^(-2) where alpha is the standard
deviation of the noise.


title:
On the Quantum Computational Complexity of the Ising Spin Glass
  Partition Function and of Knot Invariants

abstract:
  It is shown that the canonical problem of classical statistical
thermodynamics, the computation of the partition function, is in the case of
+/-J Ising spin glasses a particular instance of certain simple sums known as
quadratically signed weight enumerators (QWGTs). On the other hand it is known
that quantum computing is polynomially equivalent to classical probabilistic
computing with an oracle for estimating QWGTs. This suggests a connection
between the partition function estimation problem for spin glasses and quantum
computation. This connection extends to knots and graph theory via the
equivalence of the Kauffman polynomial and the partition function for the Potts
model.


title: Contextualist viewpoint to Greenberger-Horne-Zeilinger paradox

abstract:
  We present probabilistic analysis of the Greenberger-Horne-Zeilinger (GHZ)
scheme in the contextualist framework, namely under the assumption that
distributions of hidden variables depend on settings of measurement devices. On
one hand, we found classes of probability distributions of hidden variables for
that the GHZ scheme does not imply a contradiction between the local realism
and quantum formalism. On the other hand, we found classes of probability
distributions of hidden variables for that the GHZ scheme still induce such a
contradiction (despite variations of distributions). It is also demonstrated
that (well known in probability theory) singularity/absolute continuity
dichotomy for probability distributions is closely related to the GHZ paradox.
Our conjecture is that this GHZ-coupling between singularity/absolute
continuity dichotomy and incompatible/compatible measurements might be a
general feature of quantum theory.


title: Probabilistic foundations of quantum mechanics and quantum information

abstract:
  We discuss foundation of quantum mechanics (interpretations, superposition,
principle of complementarity, locality, hidden variables) and quantum
information theory.


title: Measuring the Density Matrix by Local Addressing

abstract:
  We introduce a procedure to measure the density matrix of a material system.
The density matrix is addressed locally in this scheme by applying a sequence
of delayed light pulses. The procedure is based on the stimulated Raman
adiabatic passage (STIRAP) technique. It is shown that a series of population
measurements on the target state of the population transfer process yields
unambiguous information about the populations and coherences of the addressed
states, which therefore can be determined.


title: Quantum lower bounds for the set equality problems

abstract:
  The set equality problem is to decide whether two sets $A$ and $B$ are equal
or disjoint, under the promise that one of these is the case. Some other
problems, like the Graph Isomorphism problem, is solvable by reduction to the
set quality problem. It was an open problem to find any $w(1)$ query lower
bound when sets $A$ and $B$ are given by quantum oracles with functions $a$ and
$b$.
  We will prove $\Omega(\frac{n^{1/3}}{\log^{1/3} n})$ lower bound for the set
equality problem when the set of the preimages are very small for every element
in $A$ and $B$.


title:
Generation of entangled 3D localized quantum wave-packets via optical
  parametric amplification

abstract:
  In the framework of the paraxial and of the slowly varying envelope
approximations, with reference to a normally dispersive medium or to vacuum,
the electromagnetic field is given as a continuous quantum superposition of
non-dispersive and non-diffracting wave-packets (namely X-waves). Entangled
states as pairs of elementary excitations traveling at (approximately) the same
velocity are found in optical parametric amplification.


title:
Quantum convolution and quantum correlation algorithms are physically
  impossible

abstract:
  The key step in classical convolution and correlation algorithms, the
componentwise multiplication of vectors after initial Fourier Transforms, is
shown to be physically impossible to do on quantum states. Then this is used to
show that computing the convolution or correlation of quantum state
coefficients violates quantum mechanics, making convolution and correlation of
quantum coefficients physically impossible.


title:
Generation of ultrabright tunable polarization entanglement without
  spatial, spectral, or temporal constraints

abstract:
  The need for spatial and spectral filtering in the generation of polarization
entanglement is eliminated by combining two coherently-driven type-II
spontaneous parametric downconverters. The resulting ultrabright source emits
photon pairs that are polarization entangled over the entire spatial cone and
spectrum of emission. We detect a flux of $\sim$12 000 polarization-entangled
pairs/s per mW of pump power at 90% quantum-interference visibility, and the
source can be temperature tuned for 5 nm around frequency degeneracy. The
output state is actively controlled by precisely adjusting the relative phase
of the two coherent pumps.


title: About economic qubit cloning

abstract:
  In this paper we establish a deep connection between the 3 qubit one-to-two
phase-covariant quantum cloning network of Fuchs et al. [C. Fuchs, N. Gisin,
R.B. Griffiths, C.S. Niu, and A. Peres, Phys. Rev. A 56 no 4, 1163 (1997)], and
its economic 2 qubit counterpart due to Niu and Griffiths [Phys.Rev. A 60 no 4,
2764 (1999)]. A general, necessary and sufficient criterion is derived in order
to characterize the reducibility of 3 qubit cloners to 2 qubit cloners. When
this criterion is fulfilled, economic cloning is possible. We show that the
optimal isotropic or universal 3 qubit cloning machine is not reducible to a 2
qubit cloner.


title:
Entanglement for a two-parameter class of states in $2\otimes n$ quantum
  system

abstract:
  We exhibit a two-parameter class of states $\rho_{(\alpha,\gamma)}$, in
$2\otimes n$ quantum system for $n\ge 3$, which can be obtained from an
arbitrary state by means of local quantum operations and classical
communication, and which are invariant under all bilateral %unitary operations
%of the form $U\otimes U$ on $2\otimes n$ quantum system. We calculate the
negativity of $\rho_{(\alpha,\gamma)}$, and a lower bound and a tight upper
bound on its entanglement of formation. It follows from this calculation that
the entanglement of formation of $\rho_{(\alpha,\gamma)}$ cannot exceed its
negativity.


title:
Adiabatic creation of coherent superposition states via multiple
  intermediate states

abstract:
  We consider an adiabatic population transfer process that resembles the well
established stimulated Raman adiabatic passage (STIRAP). In our system, the
states have nonzero angular momentums $J$, therefore, the coupling laser fields
induce transitions among the magnetic sublevels of the states. In particular,
we discuss the possibility of creating coherent superposition states in a
system with coupling pattern $J=0\Leftrightarrow J=1$ and $J=1\Leftrightarrow
J=2$. Initially, the system is in the J=0 state. We show that by two delayed,
overlapping laser pulses it is possible to create any final superposition state
of the magnetic sublevels $|2,-2>$, $|2,0>$, $|2,+2>$. Moreover, we find that
the relative phases of the applied pulses influence not only the phases of the
final superposition state but the probability amplitudes as well. We show that
if we fix the shape and the time-delay between the pulses, the final state
space can be entirely covered by varying the polarizations and relative phases
of the two pulses. Performing numerical simulations we find that our transfer
process is nearly adiabatic for the whole parameter set.


title:
Analytic calculation of nonadiabatic transition probabilities from
  monodromy of differential equations

abstract:
  The nonadiabatic transition probabilities in the two-level systems are
calculated analytically by using the monodromy matrix determining the global
feature of the underlying differential equation. We study the time-dependent
2x2 Hamiltonian with the tanh-type plus sech-type energy difference and with
constant off-diagonal elements as an example to show the efficiency of the
monodromy approach. The application of this method to multi-level systems is
also discussed.


title:
Two interacting hard disks within a circular cavity: towards a quantal
  equation of states

abstract:
  We investigate a circular cavity billiard within which a pair of identical
hard disks of smaller but finite size is confined. Each disk shows a free
motion except when bouncing elastically with its partner and with the boundary
wall. Despite its circular symmetry, this system is nonintegrable and almost
chaotic because of the (short-range) interaction between the disks. We quantize
the system by incorporating the excluded volume effect for the wavefunction.
Eigenvalues and eigenfunctions are obtained by tuning the relative size between
the disks and the billiard. We define the volume V of the cavity and the
pressure P, i.e., the derivative of each eigenvalue with respect to V.
Reflecting the fact that the energy spectra of eigenvalues versus the disk size
show a multitude of level repulsions, P-V characteristics shows the anomalous
fluctuations accompanied by many van der Waals-like peaks in each of individual
excited eigenstates taken as a quasi-equilibrium. For each eigenstate, we
calculate the expectation values of the square distance between two disks, and
point out their relationship with the pressure fluctuations.


title:
A Constant Complexity Algorithm for Solving the Boolean Satisfiability
  Problem on Quantum Computers

abstract:
  This paper has been withdrawn by the author(s). Please refer to
quant-ph/0311171.


title:
Continuous variable entanglement and quantum state teleportation between
  optical and macroscopic vibrational modes through radiation pressure

abstract:
  We study an isolated, perfectly reflecting, mirror illuminated by an intense
laser pulse. We show that the resulting radiation pressure efficiently
entangles a mirror vibrational mode with the two reflected optical sideband
modes of the incident carrier beam. The entanglement of the resulting
three-mode state is studied in detail and it is shown to be robust against the
mirror mode temperature. We then show how this continuous variable entanglement
can be profitably used to teleport an unknown quantum state of an optical mode
onto the vibrational mode of the mirror.


title: Coherent oscillations in a single electron spin at room temperature

abstract:
  Rabi nutations of a single electron spin in a single defect center have been
detected. The coherent evolution of the spin quantum state is followed via
optical detection of the spin state. Coherence times up to several microseconds
at room temperature have been measured. Optical readout of the spin states
leads to decoherence, equivalent to the so-called Zeno effect. Quantum beats
between electron spin transitions in a single spin Hahn echo experiment are
observed. A closer analysis reveals that beats also result from the hyperfine
coupling of the electron spin to a single 14N nuclear spin. The results are
analysed in terms of a density matrix approach of an electron spin interacting
with two oscillating fields.


title: The Complexity of Probabilistic versus Quantum Finite Automata

abstract:
  We present a language $L_n$ which is recognizable by a probabilistic finite
automaton (PFA) with probability $1 - \epsilon$ for all $\epsilon > 0$ with
$O(log^2n)$ states, with a deterministic finite automaton (DFA) with O(n)
states, but a quantum finite automaton (QFA) needs at least $2^{\Omega(n/ \log
n)}$ states.


title: Completely positive maps with memory

abstract:
  The prevailing description for dissipative quantum dynamics is given by the
Lindblad form of a Markovian master equation, used under the assumption that
memory effects are negligible. However, in certain physical situations, the
master equation is essentially of a non-Markovian nature. This paper examines
master equations that possess a memory kernel, leading to a replacement of
white noise by colored noise. The conditions under which this leads to a
completely positive, trace-preserving map are discussed for an exponential
memory kernel. A physical model that possesses such an exponential memory
kernel is presented. This model contains a classical, fluctuating environment
based on random telegraph signal stochastic variables.


title: Decoherence in strongly coupled quantum oscillators

abstract:
  In this paper we present a comprehensive analysis of the coherence phenomenon
of two coupled dissipative oscillators. The action of a classical driving field
on one of the oscillators is also analyzed. Master equations are derived for
both regimes of weakly and strongly interacting oscillators from which
interesting results arise concerning the coherence properties of the joint and
the reduced system states. The strong coupling regime is required to achieve a
large frequency shift of the oscillator normal modes, making it possible to
explore the whole profile of the spectral density of the reservoirs. We show
how the decoherence process may be controlled by shifting the normal mode
frequencies to regions of small spectral density of the reservoirs. Different
spectral densities of the reservoirs are considered and their effects on the
decoherence process are analyzed. For oscillators with different damping rates,
we show that the worse-quality system is improved and vice-versa, a result
which could be useful for quantum state protection. State recurrence and swap
dynamics are analyzed as well as their roles in delaying the decoherence
process.


title: Dynamics of quantum trajectories in chaotic systems

abstract:
  Quantum trajectories defined in the de Broglie--Bohm theory provide a causal
way to interpret physical phenomena. In this Letter, we use this formalism to
analyze the short time dynamics induced by unstable periodic orbits in a
classically chaotic system, a situation in which scars are known to play a very
important role. We find that the topologies of the quantum orbits are much more
complicated than that of the scarring and associated periodic orbits, since the
former have quantum interference built in. Thus scar wave functions are
necessary to analyze the corresponding dynamics. Moreover, these topologies
imply different return routes to the vicinity of the initial positions, and
this reflects in the existence of different contributions in each peak of the
survival probability function.


title: Low-light-level nonlinear optics with slow light

abstract:
  Electromagnetically induced transparency in an optically thick, cold medium
creates a unique system where pulse-propagation velocities may be orders of
magnitude less than $c$ and optical nonlinearities become exceedingly large. As
a result, nonlinear processes may be efficient at low-light levels. Using an
atomic system with three, independent channels, we demonstrate a quantum
interference switch where a laser pulse with an energy density of $\sim23$
photons per $\lambda^2/(2\pi)$ causes a 1/e absorption of a second pulse.


title: Wavelength Teleportation via Distant Quantum Entanglement

abstract:
  Recently, we have shown theoretically [1] as well as experimentally [2] how
the phase of an electromagnetic field can be determined by measuring the
population of either of the two states of a two-level atomic system excited by
this field, via the so-called Bloch-Siegert oscillation resulting from the
interference between the co- and counter-rotating excitations. Here, we show
how a degenerate entanglement, created without transmitting any timing signal,
can be used to teleport this phase information. This phase-teleportation
process may be applied to achieve wavelength teleportation, which in turn may
be used for frequency-locking of remote oscillators.


title:
Long-time and unitary properties of semiclassical initial value
  representations

abstract:
  We numerically compare the semiclassical ``frozen Gaussian'' Herman-Kluk
propagator [Chem. Phys. 91, 27 (1984)] and the ``thawed Gaussian'' propagator
put forward recently by Baranger et al. [J. Phys. A 34, 7227 (2001)] by
studying the quantum dynamics in some nonlinear one-dimensional potentials. The
reasons for the lack of long time accuracy and norm conservation in the latter
method are uncovered. We amend the thawed Gaussian propagator with a global
harmonic approximation for the stability of the trajectories and demonstrate
that this revised propagator is a true alternative to the Herman-Kluk
propagator with similar accuracy.


title:
Equivalence of Classical Statistics and Quantum Dynamics of Well-Posed
  Bosonic Field Theories

abstract:
  Quantum Field Theory (QFT) makes predictions by combining two sets of
assumptions: (1) quantum dynamics, such as a Schrodinger or Liouville equation;
(2) quantum measurement, such as stochastic collapse to an eigenfunction of a
measurement operator. A previous paper defined a classical density matrix R
encoding the statistical moments of an ensemble of states of classical
second-order Hamiltonian field theory. It proved Tr(RQ)=E(Q), etc., for the
usual field operators as defined by Weinberg, and it proved that those
observables of the classical system obey the usual Heisenberg dynamic equation.
However, R itself obeys dynamics different from the usual Liouville equation!
This paper derives those dynamics and the discrepancy between CFT and normal
form QFT in predicting any observables g(Q,P). There is some preliminary
evidence for the conjecture that the discrepancies disappear in equilibrium
states (bound states and scattering states) for finite bosonic field theories.
Even if not, they appear small enough to warrant reconsideration of CFT as a
theory of dynamics. Appendix proposes alternative closure of turbulence based
on modified Bogliubov transforms, an application where ordinary ones become
undefined.


title: Entropy as a function of Geometric Phase

abstract:
  We give a closed-form solution of von Neumann entropy as a function of
geometric phase modulated by visibility and average distinguishability in
Hilbert spaces of two and three dimensions. We show that the same type of
dependence also exists in higher dimensions. We also outline a method for
measuring both the entropy and the phase experimentally using a simple
Mach-Zehnder type interferometer which explains physically why the two concepts
are related.


title:
Berry phase in entangled systems: a proposed experiment with single
  neutrons

abstract:
  The influence of the geometric phase, in particular the Berry phase, on an
entangled spin-1/2 system is studied. We discuss in detail the case, where the
geometric phase is generated only by one part of the Hilbert space. We are able
to cancel the effects of the dynamical phase by using the ``spin-echo'' method.
We analyze how the Berry phase affects the Bell angles and the maximal
violation of a Bell inequality. Furthermore we suggest an experimental
realization of our setup within neutron interferometry.


title: Optimal State Discrimination Using Particle Statistics

abstract:
  We present an application of particle statistics to the problem of optimal
ambiguous discrimination of quantum states. The states to be discriminated are
encoded in the internal degrees of freedom of identical particles, and we use
the bunching and antibunching of the external degrees of freedom to
discriminate between various internal states. We show that we can achieve the
optimal single-shot discrimination probability using only the effects of
particle statistics. We discuss interesting applications of our method to
detecting entanglement and purifying mixed states. Our scheme can easily be
implemented with the current technology.


title:
Prior information: how to circumvent the standard joint-measurement
  uncertainty relation

abstract:
  The principle of complementarity is quantified in two ways: by a universal
uncertainty relation valid for arbitrary joint estimates of any two observables
from a given measurement setup, and by a general uncertainty relation valid for
the_optimal_ estimates of the same two observables when the state of the system
prior to measurement is known. A formula is given for the optimal estimate of
any given observable, based on arbitrary measurement data and prior information
about the state of the system, which generalises and provides a more robust
interpretation of previous formulas for ``local expectations'' and ``weak
values'' of quantum observables. As an example, the canonical joint measurement
of position X and momentum P corresponds to measuring the commuting operators
X_J=X+X', P_J=P-P', where the primed variables refer to an auxilary system in a
minimum-uncertainty state. It is well known that Delta X_J Delta P_J >= hbar.
Here it is shown that given the_same_ physical experimental setup, and
knowledge of the system density operator prior to measurement, one can make
improved joint estimates X_est and P_est of X and P. These improved estimates
are not only statistically closer to X and P: they satisfy Delta X_est Delta
P_est >= hbar/4, where equality can be achieved in certain cases. Thus one can
do up to four times better than the standard lower bound (where the latter
corresponds to the limit of_no_ prior information). Other applications include
the heterodyne detection of orthogonal quadratures of a single-mode optical
field, and joint measurements based on Einstein-Podolsky-Rosen correlations.


title: Higher Order Measures, Generalized Quantum Mechanics and Hopf Algebras

abstract:
  We study Sorkin's proposal of a generalization of quantum mechanics and find
that the theories proposed derive their probabilities from $k$-th order
polynomials in additive measures, in the same way that quantum mechanics uses a
probability bilinear in the quantum amplitude and its complex conjugate. Two
complementary approaches are presented, a $C^*$ and a Hopf-algebraic one,
illuminating both algebraic and geometric aspects of the problem.


title:
The transition to classical chaos in a coupled quantum system through
  continuous measurement

abstract:
  Continuous observation of a quantum system yields a measurement record that
faithfully reproduces the classically predicted trajectory provided that the
measurement is sufficiently strong to localize the state in phase space but
weak enough that quantum backaction noise is negligible. We investigate the
conditions under which classical dynamics emerges, via continuous position
measurement, for a particle moving in a harmonic well with its position coupled
to internal spin. As a consequence of this coupling we find that classical
dynamics emerges only when the position and spin actions are both large
compared to $\hbar$. These conditions are quantified by placing bounds on the
size of the covariance matrix which describes the delocalized quantum coherence
over extended regions of phase space. From this result it follows that a mixed
quantum-classical regime (where one subsystem can be treated classically and
the other not) does not exist for a continuously observed spin 1/2 particle.
When the conditions for classicallity are satisfied (in the large-spin limit),
the quantum trajectories reproduce both the classical periodic orbits as well
as the classically chaotic phase space regions. As a quantitative test of this
convergence we compute the largest Lyapunov exponent directly from the measured
quantum trajectories and show that it agrees with the classical value.


title:
Model study on the photoassociation of a pair of trapped atoms into an
  ultralong-range molecule

abstract:
  Using the method of quantum-defect theory, we calculate the ultralong-range
molecular vibrational states near the dissociation threshold of a diatomic
molecular potential which asymptotically varies as $-1/R^3$. The properties of
these states are of considerable interest as they can be formed by
photoassociation (PA) of two ground state atoms. The Franck-Condon overlap
integrals between the harmonically trapped atom-pair states and the
ultralong-range molecular vibrational states are estimated and compared with
their values for a pair of untrapped free atoms in the low-energy scattering
state. We find that the binding between a pair of ground-state atoms by a
harmonic trap has significant effect on the Franck-Condon integrals and thus
can be used to influence PA. Trap-induced binding between two ground-state
atoms may facilitate coherent PA dynamics between the two atoms and the
photoassociated diatomic molecule.


title: Quantum force due to distinct boundary conditions

abstract:
  We calculate the quantum statistical force acting on a partition wall that
divides a one dimensional box into two halves. The two half boxes contain the
same (fixed) number of noninteracting bosons, are kept at the same temperature,
and admit the same boundary conditions at the outer walls; the only difference
is the distinct boundary conditions imposed at the two sides of the partition
wall. The net force acting on the partition wall is nonzero at zero temperature
and remains almost constant for low temperatures. As the temperature increases,
the force starts to decrease considerably, but after reaching a minimum it
starts to increase, and tends to infinity with a square-root-of-temperature
asympotics. This example demonstrates clearly that distinct boundary conditions
cause remarkable physical effects for quantum systems.


title:
Binary optical communication in single-mode and entangled quantum noisy
  channels

abstract:
  We address binary optical communication in single-mode and entangled quantum
noisy channels. For single-mode we present a systematic comparison between
direct photodetection and homodyne detection in realistic conditions, i.e.
taking into account the noise that occurs both during the propagation and the
detection of the signals. We then consider entangled channels based on
twin-beam state of radiation, and show that with realistic heterodyne detection
the error probability at fixed channel energy is reduced in comparison to the
single-mode cases for a large range of values of quantum efficiency and noise
parameters.


title: Optimized teleportation in Gaussian noisy channels

abstract:
  We address continuous variable quantum teleportation in Gaussian quantum
noisy channels, either thermal or squeezed-thermal. We first study the
propagation of twin-beam and evaluate a threshold for its separability. We find
that the threshold for purely thermal channels is always larger than for
squeezed-thermal ones. On the other hand, we show that squeezing the channel
improves teleportation of squeezed states and, in particular, we find the class
of squeezed states that are better teleported in a given noisy channel.
Finally, we find regimes where optimized teleportation of squeezed states
improves amplitude-modulated communication in comparison with direct
transmission.


title: The Stochastic Sector of Interacting-Free Quantum Field Theory

abstract:
  The Quantum Stochastic Limit of a quantum mechanical particle coupled to a
quantum field without the neglect of the response details of the interaction
(i.e. not making the dipole approximation) is made following the treatment of
Accardi and Lu [6] and the corresponding Quantum Stochastic Structure is
derived. The stochastic sector for the noise is constructed and is shown to be
of a qualitatively new type. We also include a physical discussion on the limit
noise which obeys Interacting-Free statistics and include a new shorter proof
of the noise convergence and also a new construction of Interacting-Free Fock
Space.


title: Anticrossings in Foerster Coupled Quantum Dots

abstract:
  We consider two coupled generic quantum dots, each modelled by a simple
potential which allows the derivation of an analytical expression for the
inter-dot Foerster coupling, in the dipole-dipole approximation. We investigate
the energy level behaviour of this coupled two-dot system under the influence
of an external applied electric field and predict the presence of anticrossings
in the optical spectra due to the Foerster interaction.


title: Experiments in PT-symmetric quantum mechanics

abstract:
  Extended quantum mechanics using non-Hermitian, pseudo-Hermitian Hamiltonians
is briefly reviewed. Supersymmetric regularizations, solvable simulations and
large-N expansion techniques are recollected as suitable means for the study of
non-local quasi-particles. Their many-particle or even, perhaps, unstable and
decaying generalizations are also considered. Picking up the Klein-Gordon
equation and its Feshbach-Villars' Hamiltonian for definiteness, we argue that
the PT symmetry might help to clarify experimental aspects of relativistic
quantum mechanics.


title: Robust quantum parameter estimation: coherent magnetometry with feedback

abstract:
  We describe the formalism for optimally estimating and controlling both the
state of a spin ensemble and a scalar magnetic field with information obtained
from a continuous quantum limited measurement of the spin precession due to the
field. The full quantum parameter estimation model is reduced to a simplified
equivalent representation to which classical estimation and control theory is
applied. We consider both the tracking of static and fluctuating fields in the
transient and steady state regimes. By using feedback control, the field
estimation can be made robust to uncertainty about the total spin number.


title: Quantum Stochastic Generators

abstract:
  We discuss stochastic derivations, stochastic Hamiltonians and the flows that
they generate, algebraic fluctuaion-dissipation theorems, etc., in a language
common to both classical and quantum algebras. It is convenient to define
distinct notions of time-ordered exponentials to take account of the breakdown
of the Leibniz rule in the Ito calculus. We introduce a notion of quantum
Stratonovich calculus and show how it relates to Stratonovich-Dyson time
ordered exponentials. We then use it to demonstrate a natural way to add
stochastic derivations.


title:
Quantum White Noises and The Master Equation for Gaussian Reference
  States

abstract:
  We show that a basic quantum white noise process formally reproduces quantum
stochastic calculus when the appropriate normal / chronological orderings are
prescribed. By normal ordering techniques for integral equations and a
generalization of the Araki-Woods representation, we derive the master and
random Heisenberg equations for an arbitrary Gaussian state: this includes
thermal and squeezed states.


title: Canonical Decompositions of n-qubit Quantum Computations and Concurrence

abstract:
  The two-qubit canonical decomposition SU(4) = [SU(2) \otimes SU(2)] Delta
[SU(2) \otimes SU(2)] writes any two-qubit quantum computation as a composition
of a local unitary, a relative phasing of Bell states, and a second local
unitary. Using Lie theory, we generalize this to an n-qubit decomposition, the
concurrence canonical decomposition (C.C.D.) SU(2^n)=KAK. The group K fixes a
bilinear form related to the concurrence, and in particular any computation in
K preserves the tangle |<phi^*|(-i sigma^y_1)...(-i sigma^y_n)|phi>|^2 for n
even. Thus, the C.C.D. shows that any n-qubit quantum computation is a
composition of a computation preserving this n-tangle, a computation in A which
applies relative phases to a set of GHZ states, and a second computation which
preserves it.
  As an application, we study the extent to which a large, random unitary may
change concurrence. The result states that for a randomly chosen a in A within
SU(2^{2p}), the probability that a carries a state of tangle 0 to a state of
maximum tangle approaches 1 as the even number of qubits approaches infinity.
Any v=k_1 a k_2 for such an a \in A has the same property. Finally, although
|<phi^*|(-i sigma^y_1)...(-i sigma^y_n)|phi>|^2 vanishes identically when the
number of qubits is odd, we show that a more complicated C.C.D. still exists in
which K is a symplectic group.


title: Time-frequency bases for BB84 protocol

abstract:
  The bases traditionally used for quantum key distribution (QKD) are a 0 or
pi/2 polarization or alternatively a 0 or pi/2 phase measured by
interferometry. We introduce a new set of bases, i.e. pulses sent in either a
frequency or time basis if the pulses are assumed to be transform limited. In
addition it is discussed how this scheme can be easily generalized from a
binary to an N-dimensional system, i.e., to ``quNdits.'' Optimal pulse
distribution and the chances for eavesdropping are discussed.


title: Mesoscopic Cavity Quantum Electrodynamics with Quantum Dots

abstract:
  We describe an electrodynamic mechanism for coherent, quantum mechanical
coupling between spacially separated quantum dots on a microchip. The technique
is based on capacitive interactions between the electron charge and a
superconducting transmission line resonator, and is closely related to atomic
cavity quantum electrodynamics. We investigate several potential applications
of this technique which have varying degrees of complexity. In particular, we
demonstrate that this mechanism allows design and investigation of an on-chip
double-dot microscopic maser. Moreover, the interaction may be extended to
couple spatially separated electron spin states while only virtually populating
fast-decaying superpositions of charge states. This represents an effective,
controllable long-range interaction, which may facilitate implementation of
quantum information processing with electron spin qubits and potentially allow
coupling to other quantum systems such as atomic or superconducting qubits.


title: Universal Signature of Non-Quantum Systems

abstract:
  It is shown that 'non-quantum systems', with anomalous statistical
properties, would carry a distinctive experimental signature. Such systems can
exist in deterministic hidden-variables theories (such as the pilot-wave theory
of de Broglie and Bohm). The signature consists of non-additive expectations
for non-commuting observables, breaking the sinusoidal modulation of quantum
probabilities for two-state systems. This effect is independent of the quantum
state (pure or mixed), or of the details of the hidden-variables model.
Experiments are proposed, testing polarisation probabilities for single
photons.


title: An one-time-pad key communication protocol with entanglement

abstract:
  We present an one-time-pad key communication protocol that allows secure
direct communication with entanglement. Alice can send message to Bob in a
deterministic manner by using local measurements and public communication. The
theoretical efficiency of this protocol is double compared with BB84 protocol.
We show this protocol is unconditional secure under arbitrary quantum attack.
And we discuss that this protocol can be perfectly implemented with current
technologies.


title:
Experimental observation of superluminal group velocities in bulk
  two-dimensional photonic bandgap crystals

abstract:
  We have experimentally observed superluminal and infinite group velocities in
bulk hexagonal two-dimensional photonic bandgap crystals with bandgaps in the
microwave region. The group velocities depend on the polarization of the
incident radiation and the air-filling fraction of the crystal.


title: Secure key from bound entanglement

abstract:
  We characterize the set of shared quantum states which contain a
cryptographically private key. This allows us to recast the theory of privacy
as a paradigm closely related to that used in entanglement manipulation. It is
shown that one can distill an arbitrarily secure key from bound entangled
states. There are also states which have less distillable private key than the
entanglement cost of the state. In general the amount of distillable key is
bounded from above by the relative entropy of entanglement. Relationships
between distillability and distinguishability are found for a class of states
which have Bell states correlated to separable hiding states. We also describe
a technique for finding states exhibiting irreversibility in entanglement
distillation.


title:
"Local Realism", Bell's Theorem and Quantum "Locally Realistic"
  Inequalities

abstract:
  Based on the new general framework for the probabilistic description of
experiments, introduced in quant-ph/0305126, quant-ph/0312199, we analyze in
mathematical terms the link between the validity of Bell-type inequalities
under joint experiments upon a system of any type and the physical concept of
"local realism". We prove that the violation of Bell-type inequalities in the
quantum case has no connection with the violation of "local realism". In a
general setting, we formulate in mathematical terms a condition on "local
realism" under a joint experiment and consider examples of quantum "locally
realistic" joint experiments. We, in particular, show that quantum joint
experiments of the Alice/Bob type are "locally realistic". For an arbitrary
bipartite quantum state, we derive quantum analogs of the original Bell
inequality. In view of our results, we argue that the violation of Bell-type
inequalities in the quantum case cannot be a valid argument in the discussion
on locality or non-locality of quantum interactions.


title: Remarks on the Bose description of the Pauli spin operators

abstract:
  Using both the fermionic-like and the bosonic-like properties of the Pauli
spin operators we discuss the Bose description of the Pauli spin operators
firstly proposed by Shigefumi Naka, and derive another new bosonic
representation of the Pauli spin operators. The eigenvector of $\sigma_{-}$ in
the bosonic representation is a nonlinear coherent state with the eigenvalues
being the Grassmann numbers.


title: Classical and quantum: some mutual clarifications

abstract:
  This paper presents two unconventional links between quantum and classical
physics. The first link appears in the study of quantum cryptography. In the
presence of a spy, the quantum correlations shared by Alice and Bob are
imperfect. One can either process the quantum information, recover perfect
correlations and finally measure the quantum systems; or, one can perform the
measurements first and then process the classical information. These two
procedures tolerate exactly the same error rate for a wide class of attacks by
the spy. The m second link is drawn between the quantum notions of "no-cloning
theorem" and "weak-measurements with post-selection", and simple experiments
using classical polarized light and ordinary telecom devices.


title:
The non-Markovian quantum behavior of open systems: An exact Monte Carlo
  method employing stochastic product states

abstract:
  It is shown that the exact dynamics of a composite quantum system can be
represented through a pair of product states which evolve according to a
Markovian random jump process. This representation is used to design a general
Monte Carlo wave function method that enables the stochastic treatment of the
full non-Markovian behavior of open quantum systems. Numerical simulations are
carried out which demonstrate that the method is applicable to open systems
strongly coupled to a bosonic reservoir, as well as to the interaction with a
spin bath. Full details of the simulation algorithms are given, together with
an investigation of the dynamics of fluctuations. Several potential
generalizations of the method are outlined.


title: A Dirac Particle in a Complex Potential

abstract:
  It has been observed that a quantum mechanical theory need not to be
Hermitian to have a real spectrum. In this paper we obtain the eigenvalues of a
Dirac charged particle in a complex static and spherically symmetric potential.
Furthermore, we study the Complex Morse and complex Coulomb potentials.


title:
Eigenvalue Spectrum of a Dirac Particle in Static and Spherical Complex
  Potential

abstract:
  It has been observed that a quantum theory need not to be Hermitian to have a
real spectrum. We study the non-Hermitian relativistic quantum theories for
many complex potentials, and we obtain the real relativistic energy eigenvalues
and corresponding eigenfunctions of a Dirac charged particle in complex static
and spherically symmetric potentials. Complex Dirac-Eckart, complex
Dirac-Rosen-Morse II, complex Dirac-Scarf and complex Dirac-Poschl-Teller
potential are investigated.


title:
Molecular Dynamics Simulation of Sympathetic Crystallization of
  Molecular Ions

abstract:
  It is shown that the translational degrees of freedom of a large variety of
molecules, from light diatomic to heavy organic ones, can be cooled
sympathetically and brought to rest (crystallized) in a linear Paul trap. The
method relies on endowing the molecules with an appropriate positive charge,
storage in a linear radiofrequency trap, and sympathetic cooling. Two
well--known atomic coolant species, ${}^9{\hbox{Be}}^+$ and
${}^{137}{\hbox{Ba}}^+$, are sufficient for cooling the molecular mass range
from 2 to 20,000 amu. The large molecular charge required for simultaneous
trapping of heavy molecules and of the coolant ions can easily be produced
using electrospray ionization. Crystallized molecular ions offer vast
opportunities for novel studies.


title: The Hilbert-Space Structure of Non-Hermitian Theories with Real Spectra

abstract:
  We investigate the quantum-mechanical interpretation of models with
non-Hermitian Hamiltonians and real spectra. After describing a general
framework to reformulate such models in terms of Hermitian Hamiltonians defined
on the Hilbert space $L_2(-\infty,\infty)$, we discuss the significance of the
algebra of physical observables.


title:
Unusual quantum states: nonlocality, entropy, Maxwell's daemon, and
  fractals

abstract:
  This paper analyses the mathematical properties of some unusual quantum
states that are constructed by inserting an impenetrable barrier into a chamber
confining a single particle.


title: Constructions of Mutually Unbiased Bases

abstract:
  Two orthonormal bases B and B' of a d-dimensional complex inner-product space
are called mutually unbiased if and only if |<b|b'>|^2=1/d holds for all b in B
and b' in B'. The size of any set containing (pairwise) mutually unbiased bases
of C^d cannot exceed d+1. If d is a power of a prime, then extremal sets
containing d+1 mutually unbiased bases are known to exist. We give a simplified
proof of this fact based on the estimation of exponential sums. We discuss
conjectures and open problems concerning the maximal number of mutually
unbiased bases for arbitrary dimensions.


title: Quantum Software Reusability

abstract:
  The design of efficient quantum circuits is an important issue in quantum
computing. It is in general a formidable task to find a highly optimized
quantum circuit for a given unitary matrix. We propose a quantum circuit design
method that has the following unique feature: It allows to construct efficient
quantum circuits in a systematic way by reusing and combining a set of highly
optimized quantum circuits. Specifically, the method realizes a quantum circuit
for a given unitary matrix by implementing a linear combination of representing
matrices of a group, which have known fast quantum circuits. We motivate and
illustrate this method by deriving extremely efficient quantum circuits for the
discrete Hartley transform and for the fractional Fourier transforms. The sound
mathematical basis of this design method allows to give meaningful and natural
interpretations of the resulting circuits. We demonstrate this aspect by giving
a natural interpretation of known teleportation circuits.


title: Momentum-position realization of the Einstein-Podolsky-Rosen paradox

abstract:
  We report on a momentum-position realization of the EPR paradox using direct
detection in the near and far fields of the photons emitted by collinear
type-II phase-matched parametric downconversion. Using this approach we
achieved a measured two-photon momentum-position variance product of
$0.01\hbar^2$, which dramatically violates the bounds for the EPR and
separability criteria.


title:
How significant are the known collision and element distinctness quantum
  algorithms?

abstract:
  Quantum search is a technique for searching N possibilities in only
O(sqrt(N)) steps. It has been applied in the design of quantum algorithms for
several structured problems. Many of these algorithms require significant
amount of quantum hardware. In this paper we observe that if an algorithm
requires O(P) hardware, it should be considered significant if and only if it
produces a speedup of at least O(sqrt(P)) over a simple quantum search
algorithm. This is because a speedup of $O(sqrt(P)) $ can be trivially obtained
by dividing the search space into $O(P)$ separate parts and handing the problem
to independent processors that do a quantum search. We argue that the known
algorithms for collision and element distinctness fail to be non-trivial in
this sense.


title: NuRules and Objective/Observer Measurement

abstract:
  New rules are proposed to govern the collapse of a wave function during
measurement. These rules apply with or without an observer in the system. They
overcome an absurdity that was previous found when an objective state reduction
is combined with an observer-based state reduction. Key Words: brain states,
conscious observer, detector, measurement, florescent pulsing, probability
current, state reduction, three-level atom, von Neumann, wave collapse.


title: NuRule (4) and the 3-Level Atom

abstract:
  When a weak decay competes with a strong decay in a 3-level atom, some
mechanism is necessary to occasionally stop the strong decay so the weak decay
can be completed. Rule (4) provides that mechanism. Using this rule, a weak
photon is emitted at the correct time for both the V and L configurations, as
well as for the two cascade configurations. Key Words: observer, detector,
measurement, florescent pulsing, probability current, state reduction,
three-level atom.


title:
Rabi spectra - a simple tool for analyzing the limitations of RWA in
  modelling of the selective population transfer in many-level quantum systems

abstract:
  The controlled manipulation of quantum systems is becoming an important
technological tool. Rabi oscillations are one of the simplest, yet quite useful
mechanisms for achieving such manipulation. However, the validity of simple
Rabi theory is limited and its upgrades are usually both technically involved,
as well as limited to the simple two-level systems. The aim of this paper is to
demonstrate a simple method for analyzing the applicability of the RWA Rabi
solutions to the controlled population transfer in complex many-level quantum
systems.


title: Some Attacks On Quantum-based Cryptographic Protocols

abstract:
  Quantum-based cryptographic protocols are often said to enjoy security
guaranteed by the fundamental laws of physics. However, even carefully designed
quantum-based cryptographic schemes may be susceptible to subtle attacks that
are outside the original design. As an example, we give attacks against a
recently proposed ``secure communication using mesoscopic coherent states'',
which employs mesoscopic states, rather than single-photon states. Our attacks
can be used either as a known-plaintext attack or in the case where the
plaintext has not been randomized. One of our attacks requires beamsplitters
and the replacement of a lossy channel by a lossless one. It is successful
provided that the original loss in the channel is so big that Eve can obtain
2^k copies of what Bob receives, where k is the length of the seed key
pre-shared by Alice and Bob. Substantial improvements over such an exhaustive
key search attack can be made, whenever a key is reused. Furthermore, we remark
that, under the same assumption of a known or non-random plaintext, Grover's
exhaustive key search attack can be applied directly to "secure communication
using mesoscopic coherent states", whenever the channel loss is more than 50
percent. Therefore, as far as information-theoretic security is concerned,
optically amplified signals necessarily degrade the security of the proposed
scheme, when the plaintext is known or non-random. Our attacks apply even if
the mesoscopic scheme is used only for key generation with a subsequent use of
the key for one-time-pad encryption.


title: Why Quantum Information Processing

abstract:
  In this brief note, I will consider the following questions: (1) What is QIP?
(2) Why QIP is interesting? (3) What QIP can do? (4) What QIP cannot do? (5)
What are the major challenges in QIP?


title: Certain conclusions of Gordon decomposition

abstract:
  We use a new view to the our reality which is presented by Guts-Deutsch
multiverse. In this article, we consider some conclusions of Gordon
decomposition of Dirac current.


title:
Relation between geometric phases of entangled bi-partite systems and
  their subsystems

abstract:
  This paper focuses on the geometric phase of entangled states of bi-partite
systems under bi-local unitary evolution. We investigate the relation between
the geometric phase of the system and those of the subsystems. It is shown that
(1) the geometric phase of cyclic entangled states with non-degenerate
eigenvalues can always be decomposed into a sum of weighted non-modular pure
state phases pertaining to the separable components of the Schmidt
decomposition, though the same cannot be said in the non-cyclic case, and (2)
the geometric phase of the mixed state of one subsystem is generally different
from that of the entangled state even by keeping the other subsystem fixed, but
the two phases are the same when the evolution operator satisfies conditions
where each component in the Schmidt decomposition is parallel transported.


title: Perfect state transfer in quantum spin networks

abstract:
  We propose a class of qubit networks that admit perfect transfer of any
quantum state in a fixed period of time. Unlike many other schemes for quantum
computation and communication, these networks do not require qubit couplings to
be switched on and off. When restricted to N-qubit spin networks of identical
qubit couplings, we show that 2 log_3 N is the maximal perfect communication
distance for hypercube geometries. Moreover, if one allows fixed but different
couplings between the qubits then perfect state transfer can be achieved over
arbitrarily long distances in a linear chain.


title:
Quantum Optical Construction of Generalized Pauli and Walsh-Hadamard
  Matrices in Three Level Systems

abstract:
  A set of generators of generalized Pauli matrices play a crucial role in
quantum computation based on n level systems of an atom. In this paper we show
how to construct them by making use of Rabi oscillations. We also construct the
generalized Walsh-Hadamard matrix in the case of three level systems and
present some related problems.