title: Oxide-Semiconductor Materials for Quantum Computation

abstract:
  Scalable fault-tolerant quantum computer architectures require quantum gates
that operate within a small fraction of the qubit decoherence time and with
high accuracy over a bandwidth set by the decoherence rate. Electron spin
quantum bits in Si are promising because of long decoherence times (~0.5 ms),
but electrical gating schemes still seem problematic. Oxide-semiconductor
heterostructures have the potential for precise electrical control of gate
operations in the semiconductor, using optical rectification in the
ferroelectric oxide. Accurate (~80 dB), local (~10 nm), dynamic (>THz) and
programmable optical control over electric polarization in the ferroelectric
can be achieved using existing technology. Optical techniques may also be
useful in rapid initialization of the quantum computer, and for providing a
source of initialized qubits to use for quantum error correction. Advantages of
optical methods will be discussed within a framework proposed for a quantum
information processor using ferroelectrically coupled electron spins and Ge
quantum dots in Si.


title: Binary Decision Diagrams are a Subset of Bayesian Nets

abstract:
  The goal of this brief pedagogical article is to show that Binary Decision
Diagrams are a special kind of Bayesian Net. This observation is obvious to
workers in these two fields, but it might not be too obvious to others.


title:
Scheme for demonstrating Bell theorem in tripartite entanglement between
  atomic ensembles

abstract:
  We propose an experimentally feasible scheme to demonstrate quantum
nonlocality, using Greenberger-Horne-Zeilinger (GHZ) and $W$ entanglement
between atomic ensembles generated by a new developed method based on laser
manipulation and{} single-photon detection.


title:
Temporal build-up of electromagnetically induced transparency and
  absorption resonances in degenerate two-level transitions

abstract:
  The temporal evolution of electromagnetically induced transparency (EIT) and
absorption (EIA) coherence resonances in pump-probe spectroscopy of degenerate
two-level atomic transition is studied for light intensities below saturation.
Analytical expression for the transient absorption spectra are given for simple
model systems and a model for the calculation of the time dependent response of
realistic atomic transitions, where the Zeeman degeneracy is fully accounted
for, is presented. EIT and EIA resonances have a similar (opposite sign) time
dependent lineshape, however, the EIA evolution is slower and thus narrower
lines are observed for long interaction time. Qualitative agreement with the
theoretical predictions is obtained for the transient probe absorption on the
$^{85}Rb$ $D_{2}$ line in an atomic beam experiment.


title:
Lax-Phillips evolution as an evolution of Gell-Mann-Hartle-Griffiths
  histories and emergence of the Schr\"oedinger equation for a stable history

abstract:
  Using the Gell-Mann-Hartle-Griffiths formalism in the framework of the
Flesia-Piron form of the Lax-Phillips theory we show that the Schr\"oedinger
equation may be derived as a condition of stability of histories. This
mechanism is realized in a mathematical structure closely related to the Zeno
effect.


title: Effective Hamiltonians in quantum optics: a systematic approach

abstract:
  We discuss a general and systematic method for obtaining effective
Hamiltonians that describe different nonlinear optical processes. The method
exploits the existence of a nonlinear deformation of the usual su(2) algebra
that arises as the dynamical symmetry of the original model. When some physical
parameter, dictated by the process under consideration, becomes small, we
immediately get a diagonal effective Hamiltonian that correctly represents the
dynamics for arbitrary states and long times. We extend the technique to su(3)
and su(N), finding the corresponding effective Hamiltonians when some resonance
conditions are fulfilled.


title: Stationary entanglement between macroscopic mechanical oscillators

abstract:
  We show that the optomechanical coupling between an optical cavity mode and
the two movable cavity end mirrors is able to entangle two different
macroscopic oscillation modes of the mirrors. This continuous variable
entanglement is maintained by the light bouncing between the mirrors and is
robust against thermal noise. In fact, it could be experimentally demonstrated
using present technology.


title: Realization of Arbitrary Gates in Holonomic Quantum Computation

abstract:
  Among the many proposals for the realization of a quantum computer, holonomic
quantum computation (HQC) is distinguished from the rest in that it is
geometrical in nature and thus expected to be robust against decoherence. Here
we analyze the realization of various quantum gates by solving the inverse
problem: Given a unitary matrix, we develop a formalism by which we find loops
in the parameter space generating this matrix as a holonomy. We demonstrate for
the first time that such a one-qubit gate as the Hadamard gate and such
two-qubit gates as the CNOT gate, the SWAP gate and the discrete Fourier
transformation can be obtained with a single loop.


title: Spin network quantum simulator

abstract:
  We propose a general setting for a universal representation of the quantum
structure on which quantum information stands, whose dynamical evolution
(information manipulation) is based on angular momentum recoupling theory. Such
scheme complies with the notion of 'quantum simulator' in the sense of
Feynmann, and is shown to be related with the topological quantum field theory
approach to quantum computation.


title:
Decoherence of entangled kaons and its connection to entanglement
  measures

abstract:
  We study the time evolution of the entangled kaon system by considering the
Liouville - von Neumann equation with an additional term which allows for
decoherence. We choose as generators of decoherence the projectors to the
2-particle eigenstates of the Hamiltonian. Then we compare this model with the
data of the CPLEAR experiment and find in this way an upper bound on the
strength $\lambda$ of the decoherence. We also relate $\lambda$ to an effective
decoherence parameter $\zeta$ considered previously in literature. Finally we
discuss our model in the light of different measures of entanglement, i.e. the
von Neumann entropy $S$, the entanglement of formation $E$ and the concurrence
$C$, and we relate the decoherence parameter $\zeta$ to the loss of
entanglement: $1 - E$.


title: Multibarrier tunneling

abstract:
  We study the tunneling through an arbitrary number of finite rectangular
opaque barriers and generalize earlier results by showing that the total
tunneling phase time depends neither on the barrier thickness nor on the
inter-barrier separation. We also predict two novel peculiar features of the
system considered, namely the independence of the transit time (for non
resonant tunneling) and the resonant frequency on the number of barriers
crossed, which can be directly tested in photonic experiments. A thorough
analysis of the role played by inter-barrier multiple reflections and a
physical interpretation of the results obtained is reported, showing that
multibarrier tunneling is a highly non-local phenomenon.


title:
Optimal Entropic Uncertainty Relation for Successive Measurements in
  Quantum Information Theory

abstract:
  We derive an optimal bound on the sum of entropic uncertainties of two or
more observables when they are sequentially measured on the same ensemble of
systems. This optimal bound is shown to be greater than or equal to the bounds
derived in the literature on the entropic uncertainties of two observables
which are measured on distinct but identically prepared ensembles of systems.
In the case of a two-dimensional Hilbert Space, the optimal bound for
successive measurements of two spin components, is seen to be strictly greater
than the optimal bound for the case when they are measured on distinct
ensembles, except when the spin components are mutually parallel or
perpendicular.


title: Soliton back-action evading measurement using spectral filtering

abstract:
  We report on a back-action evading (BAE) measurement of the photon number of
fiber optical solitons operating in the quantum regime. We employ a novel
detection scheme based on spectral filtering of colliding optical solitons. The
measurements of the BAE criteria demonstrate significant quantum state
preparation and transfer of the input signal to the signal and probe outputs
exiting the apparatus, displaying the quantum-nondemolition (QND) behavior of
the experiment.


title: Quantum mechanics as electrodynamics of curvilinear waves

abstract:
  The suggested theory is the new quantum mechanics (QM) interpretation.The
research proves that QM represents the electrodynamics of the curvilinear
closed (non-linear) waves. It is entirely according to the modern
interpretation and explains the particularities and the results of the quantum
field theory.


title: Quantum mechanics and discontinuous motion of particles

abstract:
  We discuss a new realistic interpretation of quantum mechanics based on
discontinuous motion of particles. The historical and logical basis of
discontinuous motion of particles is given. It proves that if there exists only
one kind of physical reality--particles, then the realistic motion of particles
described by quantum mechanics should be discontinuous motion. We further
denote that protective measurement may provide a direct method to confirm the
existence of discontinuous motion of particles.


title: Atomic squeezing in a Lambda system

abstract:
  Using a quantum theory for an ensemble of three-level atoms (lambda) placed
in an optical cavity abd driven by electromagnetic fields, we show that the
long-lived spin associated with the ground state sublevels can be squeezed. Two
kinds of squeezing are obtained: self-spin squeezing, when the input fields are
coherent states and the atomic ensemble exhibit a large non-linearity;
squeezing transfer, when one of the incoming fields is squeezed.


title:
Comment on ``Quantum noise influencing human behaviour could fake
  effectiveness of drugs in clinical trials''

abstract: Here are discussed some problems concerning quant-ph/0208006.


title: Quantum Lost and Found

abstract:
  We consider the problem of correcting the errors incurred from sending
classical or quantum information through a noisy quantum environment by schemes
using classical information obtained from a measurement on the environment. We
give a conditions for quantum or classical information (prepared in a specified
input basis B) to be corrigible based on a measurement M. Based on these
criteria we give examples of noisy channels such that (1) no information can be
corrected by such a scheme (2) for some basis B there is a correcting
measurement M (3) for all bases B there is an M (4) there is a measurement M
which allows perfect correction for all bases B. The last case is equivalent to
the possibility of correcting quantum information, and turns out to be
equivalent to the channel allowing a representation as a convex combination of
isometric channels. Such channels are doubly stochastic but not conversely.


title: Mode-Wise Entanglement of Gaussian States

abstract:
  We address the decomposition of a multi-mode pure Gaussian state with respect
to a bi-partite division of the modes. For any such division the state can
always be expressed as a product state involving entangled two-mode squeezed
states and single mode local states at each side. The character of entanglement
of the state can therefore be understood modewise; that is, a given mode on one
side is entangled with only one corresponding mode of the other, and therefore
the total bi-partite entanglement is the sum of the modewise entanglement. This
decomposition is generally not applicable to all mixed Gaussian states.
However, the result can be extended to a special family of "isotropic" states,
characterized by a phase space covariance matrix with a completely degenerate
symplectic spectrum.


title: A measurement-based approach to quantum arrival times

abstract:
  For a quantum-mechanically spread-out particle we investigate a method for
determining its arrival time at a specific location. The procedure is based on
the emission of a first photon from a two-level system moving into a
laser-illuminated region. The resulting temporal distribution is explicitly
calculated for the one-dimensional case and compared with axiomatically
proposed expressions. As a main result we show that by means of a deconvolution
one obtains the well known quantum mechanical probability flux of the particle
at the location as a limiting distribution.


title:
Trapped-Ion Quantum Simulator: Experimental Application to Nonlinear
  Interferometers

abstract:
  We show how an experimentally realized set of operations on a single trapped
ion is sufficient to simulate a wide class of Hamiltonians of a spin-1/2
particle in an external potential. This system is also able to simulate other
physical dynamics. As a demonstration, we simulate the action of an $n$-th
order nonlinear optical beamsplitter. Two of these beamsplitters can be used to
construct an interferometer sensitive to phase shifts in one of the
interferometer beam paths. The sensitivity in determining these phase shifts
increases linearly with $n$, and the simulation demonstrates that the use of
nonlinear beamsplitters ($n$=2,3) enhances this sensitivity compared to the
standard quantum limit imposed by a linear beamsplitter ($n$=1).


title: Classical data compression with quantum side information

abstract:
  The problem of classical data compression when the decoder has quantum side
information at his disposal is considered. This is a quantum generalization of
the classical Slepian-Wolf theorem. The optimal compression rate is found to be
reduced from the Shannon entropy of the source by the Holevo information
between the source and side information.


title: Universal distortion-free entanglement concentration

abstract:
  Entanglement concentration from many copies of unknown pure states is
discussed, and we propose the protocol which not only achieves entropy rate,
but also produces the perfect maximally entangled state. Our protocol is
induced naturally from symmetry of $n$-tensored pure state, and is optimal for
all the protocols which concentrates entanglement from unknown pure states, in
the sense of failure probability. In the proof of optimality, the statistical
estimation theory plays a key role, for concentrated entanglement gives a
natural estimate of the entropy of entanglement.


title:
Generation of pulsed polarization entangled photon pair via a space
  cascaded two-crystal geometry

abstract:
  The generation of pulsed polarization entangled photon pair has been realized
using type-I phase matching in the spontaneous parametirc downconversion
process in a space cascaded two-crystal geometry. The optical axes of the
crystal are aligned in such a way that the horizontal polarization photon pair
is produced from up crystal, and the vertical polarization photon pair is
produced from down crystal. These two processes are simultaneous, but are
separable in space. We get the high entangled polarization photon pair by using
the single mode fiber to erase the spatial information between these two
processes. This photon pair exhibits more than 86% high-visibility quantum
interference for polarization variable without the narrowband filter and
temporal compensation.


title:
Comment on "On the uncertainty relations and squeezed states for the
  quantum mechanics on a circle"

abstract:
  It is shown by examples that the position uncertainty on a circle, proposed
recently by Kowalski and Rembieli\'nski [J. Phys. A 35 (2002) 1405] is not
consistent with the state localization. We argue that the relevant
uncertainties and uncertainty relations (UR's) on a circle are that based on
the Gram-Robertson matrix. Several of these generalized UR's are displayed and
related criterions for squeezed states are discussed.


title:
Nonrelativistic Quantum Mechanics with Spin in the Framework of a
  Classical Subquantum Kinetics

abstract:
  Recently it has been shown that the spinnless one particle quantum mechanics
can be obtained in the framework of entirely classical subquantum kinetics. In
the present paper we argue that, within the same scheme and without any extra
assumption, it is possible to obtain both the non relativistic quantum
mechanics with spin, in the presence of an arbitrary external electromagnetic
field, as well as the nonlinear quantum mechanics.
  Pacs: 03.65.Ta, 05.20.Dd
  KEY WORDS: monads, subquantum physics, quantum potential, foundations of
quantum mechanics


title: Energy density and localization of particles

abstract:
  We consider the use of the energy density for describing a localization of
relativistic particles. This method is consistent with the causality
requirements. The related positive operator valued measure is presented. The
probability distributions for one particle states are given explicitly.


title: Natural two-qubit gate for quantum computation using the XY interaction

abstract:
  The two-qubit interaction Hamiltonian of a given physical implementation
determines whether or not a two-qubit gate such as the CNOT gate can be
realized easily. It can be shown that, e.g., with the XY interaction more than
one two-qubit operation is required in order to realize CNOT. Here we propose a
two-qubit gate for the XY interaction which combines CNOT with the SWAP
operation. By using this gate quantum circuits can be implemented efficiently,
even if only nearest-neighbor coupling between the qubits is available.


title: Diffusive-ballistic crossover in 1D quantum walks

abstract:
  We show that particle transport in a uniform, quantum multi-baker map, is
generically ballistic in the long time limit, for any fixed value of Planck's
constant. However, for fixed times, the semi-classical limit leads to
diffusion. Random matrix theory provides explicit analytical predictions for
the mean square displacement of a particle in the system. These results exhibit
a crossover from diffusive to ballistic motion, with crossover time from
diffusive to ballistic motion on the order of the inverse of Planck's constant.
We can argue, that for a large class of 1D quantum random walks, similar to the
quantum multi-baker, a sufficient condition for diffusion in the semi-classical
limit is classically chaotic dynamics in each cell. Using an initial
equilibrium density matrix, we find that diffusive behavior is recovered in the
semi-classical limit for such systems, without further interactions with the
environment.


title: Phonon decoherence of quantum entanglement: Robust and fragile states

abstract:
  We study the robustness and fragility of entanglement of open quantum systems
in some exactly solvable models in which the decoherence is caused by a pure
dephasing process. In particular, for the toy models presented in this paper,
we identify two different time scales, one is responsible for local dephasing,
while the other is for entanglement decay. For a class of fragile entangled
states defined in this paper, we find that the entanglement of two qubits, as
measured by concurrence, decays faster asymptotically than the quantum
dephasing of an individual qubit.


title:
Realization of All-or-nothing-type Kochen-Specker Experiment with Single
  Photons

abstract:
  Using the spontaneous parametric down-conversion process in a type-I phase
matching BBO crystal as single photon source, we perform an all-or-nothing-type
Kochen-Specker experiment proposed by Simon \QTR{it}{et al}. [Phys. Rev. Lett.
\QTR{bf}{85}, 1783 (2000)] to verify whether noncontextual hidden variables or
quantum mechanics is right. The results strongly agree with quantum mechanics.


title: Search for Gravity in Quantum Evolution

abstract:
  The possibilities of curvature of space-time in the metric of quantum states
are investigated. The curvature of the metric corresponding to a wave function
of Hydrogen atom is determined. Also, Einstein tensor is described for a given
quantum state.


title: Deterministic Secure Direct Communication Using Entanglement

abstract:
  A novel secure communication protocol is presented, based on an entangled
pair of qubits and allowing asymptotically secure key distribution and
quasi-secure direct communication. Since the information is transferred in a
deterministic manner, no qubits have to be discarded. The transmission of
information is instantaneous, i.e. the information can be decoded during the
transmission. The security against arbitrary eavesdropping attacks is provided.
In case of eavesdropping attacks with full information gain, the detection rate
is 50% per control transmission. The experimental realization of the protocol
is feasible with relatively small effort, which also makes commercial
applications conceivable.


title:
Experimental Entanglement Concentration and Universal Bell-state
  Synthesizer

abstract:
  We report a novel Bell-state synthesizer in which an interferometric
entanglement concentration scheme is used. An initially mixed polarization
state from type-II spontaneous parametric down-conversion becomes entangled
after the interferometric entanglement concentrator. This Bell-state
synthesizer is universal in the sense that the output polarization state is not
affected by spectral filtering, crystal thickness, and, most importantly, the
choice of pump source. It is also robust against environmental disturbance and
a more general state, partially mixed$-$partially entangled state, can be
readily generated as well.


title: Integrability in 1D Quantum Chaos

abstract:
  Explicit, exact periodic orbit expansions for individual eigenvalues exist
for a subclass of quantum networks called regular quantum graphs. We prove that
all linear chain graphs have a regular regime.


title: Singular potentials and annihilation

abstract:
  We discuss the regularization of attractive singular potentials $-\alpha
_{s}/r^{s}$, $s\geq 2$ by infinitesimal imaginary addition to interaction
constant $\alpha_{s}=\alpha_{s}\pm i0$. Such a procedure enables unique
definition of scattering observables and is equal to an absorption (creation)
of particles in the origin. It is shown, that suggested regularization is an
analytical continuation of the scattering amplitudes of repulsive singular
potential in interaction constant $\alpha_{s}$. The nearthreshold properties of
regularized in a mentioned way singular potential are examined. We obtain
expressions for the scattering lengths, which turn to be complex even for
infinitesimal imaginary part of interaction constant. The problem of
perturbation of nearthreshold states of regular potential by a singular one is
treated, the expressions for level shifts and widths are obtained. We show,
that the physical sense of suggested regularization is that the scattering
observables are insensitive to any details of the short range modification of
singular potential, if there exists sufficiently strong inelastic short range
interaction. In this case the scattering observables are determined by
solutions of Schrodinger equation with regularized potential $-(\alpha_{s}\pm
i0)/r^{s}$. We point out that the developed formalism can be applied for the
description of systems with short range annihilation, in particular low energy
nucleon-antinucleon scattering.


title:
A physical distinction between a covariant and non covariant reduction
  process in relativistic quantum theories

abstract:
  Causality imposes strong restrictions on the type of operators that may be
observables in relativistic quantum theories. In fact, causal violations arise
when computing conditional probabilities for certain partial causally connected
measurements using the standard non covariant procedure. Here we introduce
another way of computing conditional probabilities, based on an intrinsic
covariant relational order of the events, which differs from the standard one
when these type of measurements are included. This alternative procedure is
compatible with a wider and very natural class of operators without breaking
causality. If some of these measurements could be implemented in practice as
predicted by our formalism, the non covariant, conventional approach should be
abandoned. Furthermore, the description we promote here would imply a new
physical effect where interference terms are suppressed as a consequence of the
covariant order in the measurement process.


title: Lewenstein-Sanpera Decomposition for $2\otimes 2$ Systems

abstract:
  As it is well known, every bipartite $2\otimes 2$ density matrix can be
obtained from Bell decomposable states via local quantum operations and
classical communications (LQCC). Using this fact, the Lewenstein-Sanpera
decomposition of an arbitrary bipartite $2\otimes 2$ density matrix has been
obtained through LQCC action upon Lewenstein-Sanpera decomposition of Bell
decomposable states of $2\otimes 2$ quantum systems, where the product states
introduced by Wootters in [W. K. Wootters, Phys. Rev. Lett. {\bf 80} 2245
(1998)] form the best separable approximation ensemble for Bell decomposable
states. It is shown that in these systems the average concurrence of the
Lewenstein-Sanpera decomposition is equal to the concurrence of these states.


title: Stationary Perturbation Theory with Spatially Well-separated Potentials

abstract:
  We present a new perturbation theory for quantum mechanical energy
eigenstates when the potential equals the sum of two localized, but not
necessarily weak potentials $V_{1}(\vec{r})$ and $V_{2}(\vec{r})$, with the
distance $L$ between the respective centers of the two taken to be quite large.
It is assumed that complete eigenfunctions of the local Hamiltonians (i.e., in
the presence of $V_{1}(\vec{r})$ or $V_{2}(\vec{r})$ only) are available as
inputs to our perturbation theory. If the two local Hamiltonians have
degenerate bound-state energy levels, a systematic extension of the molecular
orbital theory (or the tight-binding approximation) follows from our formalism.
Our approach can be viewed as a systematic adaptation of the multiple
scattering theory to the problem of bound states.


title: A quantum method to test the existence of consciousness

abstract:
  As we know, "Who can be said to be a conscious being?" is one of the hard
problems in present science, and no method has been found to strictly
differentiate the conscious being from the being without consciousness or usual
matter. In this short paper, we present a strict physical method based on
revised quantum dynamics to test the existence of consciousness, and the
principle is to use the distinguishability of nonorthogonal single states. We
demonstrate that although the dynamical collapse time can't be measured by a
physical measuring device, a conscious being can perceive it under the assumed
QSC condition, thus can distinguish the nonorthogonal single states in the
framework of revised quantum dynamics This in principle provides a quantum
method to differentiate man and machine, or to test the existence of
consciousness. We further discuss the rationality of the assumed QSC condition,
and denote that some experimental evidences have indicated that our human being
can satisfy the condition. This not only provides some confirmation of our
method, but also indicates that the method is a practical proposal, which can
be implemented in the near future experiments.


title:
Orthogonality And Distinguishability: Criterion For Local
  Distinguishability of Arbitrary Orthogonal States

abstract:
  We consider deeply the relation between the orthogonality and the
distinguishability of a set of arbitrary states (including multi-partite
states). It is shown that if a set of arbitrary states can be distinguished by
local operations and classical communication (LOCC), \QTR{it}{\}each of the
states can be written as a linear combination of product vectors such that all
product vectors of one of the states are orthogonal to the other states. With
this result we then prove a simple necessary condition for LOCC
distinguishability of a class of orthogonal states. These conclusions may be
useful in discussing the distinguishability of orthogonal quantum states
further, understanding the essence of nonlocality and discussing the
distillation of entanglement.


title: Robust Ising Gates for Practical Quantum Computation

abstract:
  I describe the use of techniques based on composite rotations to combat
systematic errors in controlled phase gates, which form the basis of two qubit
quantum logic gates. Although developed and described within the context of
Nuclear Magnetic Resonanace (NMR) quantum computing these sequences should be
applicable to any implementation of quantum computation based on Ising
couplings. In combination with existing single qubit gates this provides a
universal set of robust quantum logic gates.


title: A quantum key distribution protocol without classical communications

abstract:
  We provide a quantum key distribution protocol based on the correlations of
the Greenburger-Horne-Zeilinger(GHZ) state. No classical communication is
needed in the process of the establishment of the key. Our protocol is useful
when an unjammable classical communication channel is unavailable. We prove
that the protocol is secure.


title: A spontaneous collapse model on a lattice

abstract:
  We present a spontaneous collapse model of a field theory on a 1+1 null
lattice, in which the causal structure of the lattice plays a central role.
Issues such as ``locality,'' ``non-locality'' and superluminal signaling are
addressed in the context of the model which has the virtue of extreme
simplicity. The formalism of the model is related to that of the consistent
histories approach to quantum mechanics.


title: Combinatorics and Quantum Nonlocality

abstract:
  We use techniques for lower bounds on communication to derive necessary
conditions (in terms of detector efficiency or amount of super-luminal
communication) for being able to reproduce the quantum correlations occurring
in EPR-type experiments with classical local hidden-variable theories. As an
application, we consider n parties sharing a GHZ-type state and show that the
amount of super-luminal classical communication required to reproduce the
correlations is at least n(log n - 3) bits and the maximum detector efficiency
eta* for which the resulting correlations can still be reproduced by a local
hidden-variable theory is upper bounded by eta* <= 8/n and thus decreases with
n.


title: Monotonicity of quantum relative entropy revisited

abstract:
  Monotonicity under coarse-graining is a crucial property of the quantum
relative entropy. The aim of this paper is to investigate the condition of
equality in the monotonicity theorem and in its consequences such as the strong
sub-additivity of the von Neumann entropy, the Golden-Thompson trace inequality
and the monotonicity of the Holevo quantity.The relation to quantum Markovian
states is briefly indicated.


title: PT-symmetry and its spontaneous breakdown explained by anti-linearity

abstract:
  The impact of an anti-unitary symmetry on the spectrum of non-hermitean
operators is studied. Wigner's normal form of an anti-unitary operator is shown
to account for the spectral properties of non-hermitean, PT-symmetric
Hamiltonians. Both the occurrence of single real or complex conjugate pairs of
eigenvalues follows from this theory. The corresponding energy eigenstates span
either one- or two-dimensional irreducible representations of the symmetry PT.
In this framework, the concept of a spontaneously broken PT-symmetry is not
needed.


title:
Relative Frequency and Probability in the Everett Interpretation of
  Heisenberg-Picture Quantum Mechanics

abstract:
  The existence of probability in the sense of the frequency interpretation,
i.e. probability as "long term relative frequency," is shown to follow from the
dynamics and the interpretational rules of Everett quantum mechanics in the
Heisenberg picture. This proof is free of the difficulties encountered in
applying to the Everett interpretation previous results regarding relative
frequency and probability in quantum mechanics. The ontology of the Everett
interpretation in the Heisenberg picture is also discussed.


title: A modern review of the two-level approximation

abstract:
  The paradigm of the two-level atom is revisited and its perturbative analysis
is discussed in view of the principle of duality in perturbation theory. The
models we consider are a two-level atom and an ensemble of two-level atoms both
interacting with a single radiation mode. The aim is to see how the latter can
be actually used as an amplifier of quantum fluctuations to the classical level
through the thermodynamic limit of a very large ensemble of two-level atoms [M.
Frasca, Phys. Lett. A {\bf 283}, 271 (2001)] and how can remove Schr\"odinger
cat states. The thermodynamic limit can be very effective for producing both
classical states and decoherence on a quantum system that evolves without
dissipation. Decoherence without dissipation is indeed an effect of a single
two-level atom interacting with an ensemble of two-level atoms, a situation
that proves to be useful to understand recent experiments on nanoscale devices
showing unexpected disappearance of quantum coherence at very low temperatures.

categories: quant-ph cond-mat.mes-hall math-ph math.MP physics.atom-ph


title: On Questions of Non-Locality in our EPR model

abstract:
  The Einstein-locality of our EPR model has been questioned. We show that our
model obeys Einstein locality and that the questions are without basis.


title: Probabilistic quantum error correction

abstract:
  There are well known necessary and sufficient conditions for a quantum code
to correct a set of errors. We study weaker conditions under which a quantum
code may correct errors with probabilities that may be less than one. We work
with stabilizer codes and as an application study how the nine qubit code, the
seven qubit code, and the five qubit code perform when there are errors on more
than one qubit. As a second application, we discuss the concept of syndrome
quality and use it to suggest a way that quantum error correction can be
practically improved.


title:
Reply to Cen LiXiang, Li XinQi, and Yan YiJing: No adiabatic conditional
  geometrical phase shift

abstract:
  Much long before the appearing time of the Comment by Cen, Li, and Yan,, the
main issue addresed there by Cen et al had been resolved already. The
information offered by the Comment is selective and misleading.


title: Quantum Lower Bound for Recursive Fourier Sampling

abstract:
  One of the earliest quantum algorithms was discovered by Bernstein and
Vazirani, for a problem called Recursive Fourier Sampling. This paper shows
that the Bernstein-Vazirani algorithm is not far from optimal. The moral is
that the need to "uncompute" garbage can impose a fundamental limit on
efficient quantum computation. The proof introduces a new parameter of Boolean
functions called the "nonparity coefficient," which might be of independent
interest.


title: Quantum authentication with unitary coding sets

abstract:
  A general class of authentication schemes for arbitrary quantum messages is
proposed. The class is based on the use of sets of unitary quantum operations
in both transmission and reception, and on appending a quantum tag to the
quantum message used in transmission. The previous secret between partners
required for any authentication is a classical key. We obtain the minimal
requirements on the unitary operations that lead to a probability of failure of
the scheme less than one. This failure may be caused by someone performing a
unitary operation on the message in the channel between the communicating
partners, or by a potential forger impersonating the transmitter.


title: Antisymmetric multi-partite quantum states and their applications

abstract:
  Entanglement is a powerful resource for processing quantum information. In
this context pure, maximally entangled states have received considerable
attention. In the case of bipartite qubit-systems the four orthonormal
Bell-states are of this type. One of these Bell states, the singlet Bell-state,
has the additional property of being antisymmetric with respect to particle
exchange. In this contribution we discuss possible generalizations of this
antisymmetric Bell-state to cases with more than two particles and with
single-particle Hilbert spaces involving more than two dimensions. We review
basic properties of these totally antisymmetric states. Among possible
applications of this class of states we analyze a new quantum key sharing
protocol and methods for comparing quantum states.


title:
Generalized Zero Range Potentials and Multi-Channel Electron-Molecule
  Scattering

abstract:
  A multi-channel scattering problem is studied from a point of view of
integral equations system. The system appears while natural one-particle wave
function equation of the electron under action of a potential with
non-intersecting ranges is considered.
  Spherical functions basis expansion of the potentials introduces partial
amplitudes and corresponding radial functions. The approach is generalized to
multi-channel case by a matrix formulation in which a state vector component is
associated with a scattering channel.
  The zero-range potentials naturally enter the scheme when the class of
operators of multiplication is widen to distributions. %Analog of multipolar
expansion is treated. Spin variables, o Oscillations and rotations are
incorporated into the scheme.


title:
Frequency Locking Via Phase Mapping Of Remote Clocks Using Quantum
  Entanglement

abstract:
  Recently, we have shown 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
frequency-locking of remote oscillators, thereby facilitating the process of
synchronizing distant clocks.


title: Minimum energy needed to perform a quantum logical gate

abstract:
  A lower bound on the amount of energy needed to carry out an elementary
logical operation on a qubit system, with a given accuracy and in a given time,
has been recently postulated. This paper is an attempt to formalize this bound
and explore the conditions under which it may be expected to hold. This is a
work in progress and any contributions will be appreciated.


title: Hopf algebras of canonical commutation relations

abstract:
  Given a Heisenberg algebra A of canonical commutation relations modelled over
an infinite-dimensional nuclear space, a Hopf algebra of its quantum
deformations is also an algebra of canonical commutation relations whose Fock
representation recovers some non-Fock representation of A.


title:
Intensity-dependent dispersion under conditions of electromagnetically
  induced transparency in coherently prepared multi-state atoms

abstract:
  Interest in lossless nonlinearities has focussed on the the dispersive
properties of $\Lambda $ systems under conditions of electromagnetically
induced transparency (EIT). We generalize the lambda system by introducing
further degenerate states to realize a `Chain $ \Lambda $' atom where multiple
coupling of the probe field significantly enhances the intensity dependent
dispersion without compromising the EIT condition.


title:
An analysis of semiclassical radiation from single particle quantum
  currents shows surprising results

abstract:
  Classical electromagnetic radiation from quantum currents and densities are
calculated. For the free Schrodinger equation with no external force it's found
that the classical radiation is zero to all orders of the multipole expansion.
This is true of mixed or pure states for the charged particle. It is a
non-trivial and surprising result. A similar result is found for the
Klein-Gordon currents when the wave function consists of only positive energy
solutions. For the Dirac equation it is found that radiation is suppressed at
lower frequencies but is not zero at all frequencies. Implications of these
results for the interpretation of quantum mechanics are discussed.


title: Multi-output programmable quantum processor

abstract:
  By combining telecloning and programmable quantum gate array presented by
Nielsen and Chuang [Phys.Rev.Lett. 79 :321(1997)], we propose a programmable
quantum processor which can be programmed to implement restricted set of
operations with several identical data outputs. The outputs are
approximately-transformed versions of input data. The processor successes with
certain probability.


title:
Spin-based quantum information processing with semiconductor quantum
  dots and cavity QED

abstract:
  A quantum information processing scheme is proposed with semiconductor
quantum dots located in a high-Q single mode QED cavity. The spin degrees of
freedom of one excess conduction electron of the quantum dots are employed as
qubits. Excitonic states, which can be produced ultrafastly with optical
operation, are used as auxiliary states in the realization of quantum gates. We
show how properly tailored ultrafast laser pulses and Pauli-blocking effects,
can be used to achieve a universal encoded quantum computing.


title: Scaling of Decoherence Effects in Quantum Computers

abstract:
  The scaling of decoherence rates with the number of q-bits is studied for a
simple quantum computer model. Two state q-bits are localised around
well-separated positions via trapping potentials, but vibrational motion of
q-bits centre of mass motion occurs. Coherent one and two q-bit gating
processes are controlled by external classical fields and facilitated by a high
Q cavity mode. Decoherence due to q-bit and cavity mode coupling to a bath of
spontaneous emission modes, cavity decay modes and to the vibrational modes is
treated. A non-Markovian treatment of the short time behaviour of the fidelity
is presented, enabling time scales for decoherence to be determined, together
with their dependence on q-bit number for the case where the q-bit/cavity mode
system is in a pure state and the baths are in thermal states.


title: Tunneling in asymmetric double well: instanton calculus

abstract:
  The level splitting formula of an asymmetric double well potential is
calculated taking into account the multi-instanton contributions (dilute gas
approximation). Results can be related with known semiclassical ones obtained
with a truncated hamiltonian, and the symmetric case is easily recovered.


title: An improved phase error tolerance in Quantum search algorithm

abstract:
  As the matching condition in Grover search algorithm is transgressed due to
inevitable errors in phase inversions, it gives a reduction in maximum
probability of success. With a given degree of maximum success, we have derive
the generalized and imroved criterion for tolerated error and corresponding
size of quantum database under the inevitable gate imperfections. The vanished
inaccurancy to this condition has also been shown. Besides, a concise formula
for evaluating minimum number of iterations is also presented in this work.


title:
A New Strategy of Quantum-State Estimation for Achieving the Cramer-Rao
  Bound

abstract:
  We experimentally analyzed the statistical errors in quantum-state estimation
and examined whether their lower bound, which is derived from the Cramer-Rao
inequality, can be truly attained or not. In the experiments, polarization
states of bi-photons produced via spontaneous parametric down-conversion were
estimated employing tomographic measurements. Using a new estimation strategy
based on Akaike's information criterion, we demonstrated that the errors
actually approach the lower bound, while they fail to approach it using the
conventional estimation strategy.


title:
Nonlinear gauge transformation for a class of Schroedinger equations
  containing complex nonlinearities

abstract:
  We consider a wide class of nonlinear canonical quantum systems described by
a one-particle Schroedinger equation containing a complex nonlinearity. We
introduce a nonlinear unitary transformation which permits us to linearize the
continuity equation. In this way we are able to obtain a new quantum system
obeying to a nonlinear Schroedinger equation with a real nonlinearity. As an
application of this theory we consider a few already studied Schroedinger
equations as that containing the nonlinearity introduced by the
exclusion-inclusion principle, the Doebner-Goldin equation and others.
  PACS numbers: 03.65.-w, 11.15.-q


title: Quantum Feedback Channels

abstract:
  In Shannon information theory the capacity of a memoryless communication
channel cannot be increased by the use of feedback. In quantum information
theory the no-cloning theorem means that noiseless copying and feedback of
quantum information cannot be achieved. In this paper, quantum feedback is
defined as the unlimited use of a noiseless quantum channel from receiver to
sender. Given such quantum feedback, it is shown to provide no increase in the
entanglement--assisted capacities of a memoryless quantum channel, in direct
analogy to the classical case. It is also shown that in various cases of
non-assisted capacities, feedback may increase the capacity of memoryless
quantum channels.


title: Frequency down conversion through Bose condensation of light

abstract:
  We propose an experimental set up allowing to convert an input light of
wavelengths about $1-2 \mu m$ into an output light of a lower frequency. The
basic principle of operating relies on the nonlinear optical properties
exhibited by a microcavity filled with glass. The light inside this material
behaves like a 2D interacting Bose gas susceptible to thermalise and create a
quasi-condensate. Extension of this setup to a photonic bandgap material (fiber
grating) allows the light to behave like a 3D Bose gas leading, after
thermalisation, to the formation of a Bose condensate. Theoretical estimations
show that a conversion of $1 \mu m$ into $1.5 \mu m$ is achieved with an input
pulse of about $1 ns$ with a peak power of $10^3 W$, using a fiber grating
containing an integrated cavity of size about $500 \mu m \times 100 \mu m^2$.


title: Resonant Transfer of Excitons and Quantum Computation

abstract:
  Resonant energy transfer mechanisms have been observed in the sensitized
luminescence of solids, in quantum dots and in molecular nanostructures, and
they also play a central role in light harvesting processes in photosynthetic
organisms. We demonstrate that such mechanisms, together with the
exciton-exciton binding energy shift typical of these nanostructures, can be
used to perform universal quantum logic. In particular, we show how to generate
controlled exciton entanglement and identify two different regimes of quantum
behaviour.


title:
Decoherence in a spin--spin-bath model with environmental
  self-interaction

abstract:
  A low temperature model system consisting of a central spin coupled to a
spin-bath is studied to determine whether interaction among bath spins has an
effect on central spin dynamics. In the absence of intra-environmental
coupling, decoherence of the central spin is fast and irreversible. Strong
intra-environmental interaction results in an effective decoupling of the
central spin from the bath and suppression of decoherence. Weaker
intra-environmental coupling reduces but does not eliminate decoherence. We
believe that similar behaviour will be observed in any system with a
self-interacting environment.


title:
A General Approach of Quasi-Exactly Solvable Schroedinger Equations with
  Three Known Eigenstates

abstract:
  We propose a general method for constructing quasi-exactly solvable
potentials with three analytic eigenstates. These potentials can be real or
complex functions but the spectrum is real. A comparison with other methods is
also performed.


title: Broken Symmetries in the Entanglement of Formation

abstract:
  We compare some recent computations of the entanglement of formation in
quantum information theory and of the entropy of a subalgebra in quantum
ergodic theory. Both notions require optimization over decompositions of
quantum states. We show that both functionals are strongly related for some
highly symmetric density matrices. We discuss the presence of broken symmetries
in relation with the structure of the optimal decompositions.


title: A possible explanation of the superposition principle

abstract:
  I tentatively suggest that the superposition principle of quantum mechanics
is explicable in a mathematically natural way if it is possible to understand
probability amplitudes as complex-valued logarithms. This notion is inspired by
the fact that the quantum state may be interpreted as a measure of information.


title: Dynamical maps and measurements

abstract:
  We show how a set of POVMs, expressed as a set of $\mu$ linear maps, can be
performed with a unitary transformation followed by a von-Neumann measurement
with an ancillary system of no more than $\mu N^2$ dimensions. This result
shows that all generalized linear transformations and measurements on density
matrices can be performed by unitary transformations and von-Neumann
measurements by coupling a suitably large ancillary system.


title: Quantum ground-state computation with static gates

abstract:
  We develop a computation model for solving Boolean networks that implements
wires through quantum ground-state computation and implements gates through
identities following from angular momentum algebra and statistics. The gates
are static in the sense that they contribute Hamiltonian 0 and hold as
constants of the motion; only the wires are dynamic. Just as a spin 1/2 makes
an ideal 1-bit memory element, a spin 1 makes an ideal 3-bit gate. Such gates
cost no computation time: relaxing the wires alone solves the network. We
compare computation time with that of an easier Boolean network where all the
gate constraints are simply removed. This computation model is robust with
respect to decoherence and yields a generalized quantum speed-up for all NP
problems.


title:
One-qubit reduced states of a pure many-qubit state: polygon
  inequalities

abstract:
  We show that a necessary and sufficient condition for a set of $n$ one-qubit
mixed states to be the reduced states of a pure $n$-qubit state is that their
smaller eigenvalues should satisfy polygon inequalities: no one of them can
exceed the sum of the others.


title:
Saturation of the production of quantum entanglement between weakly
  coupled mapping systems in strongly chaotic region

abstract:
  The production of quantum entanglement between weakly coupled mapping
systems, whose classical counterparts are both strongly chaotic, is
investigated. In the weak coupling regime, it is shown that time correlation
functions of the unperturbed systems determine the entanglement production. In
particular, we elucidate that the increment of nonlinear parameter of coupled
kicked tops does not accelerate the entanglement production in the strongly
chaotic region. An approach to the dynamical inhibition of entanglement is
suggested.


title: Off-diagonal geometric phase for mixed states

abstract:
  We extend the off-diagonal geometric phase [Phys. Rev. Lett. {\bf 85}, 3067
(2000)] to mixed quantal states. The nodal structure of this phase in the qubit
(two-level) case is compared with that of the diagonal mixed state geometric
phase [Phys. Rev. Lett. {\bf 85}, 2845 (2000)]. Extension to higher dimensional
Hilbert spaces is delineated. A physical scenario for the off-diagonal mixed
state geometric phase in polarization-entangled two-photon interferometry is
proposed.


title:
Dynamical detailed balance and local KMS condition for non-equilibrium
  states

abstract:
  The principle of detailed balance is at the basis of equilibrium physics and
is equivalent to the Kubo-Martin-Schwinger (KMS) condition (under quite general
assumptions). In the present paper we prove that a large class of open quantum
systems satisfies a dynamical generalization of the detailed balance condition
({\it dynamical detailed balance}) expressing the fact that all the
micro-currents, associated to the Bohr frequencies are constant. The usual
(equilibrium) detailed balance condition is characterized by the property that
this constant is identically zero. From this we deduce a simple and
experimentally measurable relation expressing the microcurrent associated to a
transition between two levels $\epsilon_m\to\epsilon_n$ as a linear combination
of the occupation probabilities of the two levels, with coefficients given by
the generalized susceptivities (transport coefficients). Finally, using a
master equation characterization of the dynamical detailed balance condition,
we show that this condition is equivalent to a "local" generalization of the
usual KMS condition.


title:
Finite automata models of quantized systems: conceptual status and
  outlook

abstract:
  Since Edward Moore, finite automata theory has been inspired by physics, in
particular by quantum complementarity. We review automaton complementarity,
reversible automata and the connections to generalized urn models. Recent
developments in quantum information theory may have appropriate formalizations
in the automaton context.


title:
The role of entanglement in quantum measurement and information
  processing

abstract:
  The significance of the quantum feature of entanglement between physical
systems is investigated in the context of quantum measurements. It is shown
that, while there are measurement couplings that leave the object and probe
systems non-entangled, no information transfer from object to probe can take
place unless there is there is at least some intermittent period where the two
systems are entangled. PACS numbers: 03.65.Ca; 03.65.Ta; 03.65.Wj; 03.65.Ud.


title:
Conversion of a general quantum stabilizer code to an entanglement
  distillation protocol

abstract:
  We show how to convert a quantum stabilizer code to a one-way or two-way
entanglement distillation protocol. The proposed conversion method is a
generalization of those of Shor-Preskill and Nielsen-Chuang. The recurrence
protocol and the quantum privacy amplification protocol are equivalent to the
protocols converted from [[2,1]] stabilizer codes. We also give an example of a
two-way protocol converted from a stabilizer better than the recurrence
protocol and the quantum privacy amplification protocol. The distillable
entanglement by the class of one-way protocols converted from stabilizer codes
for a certain class of states is equal to or greater than the achievable rate
of stabilizer codes over the channel corresponding to the distilled state, and
they can distill asymptotically more entanglement from a very noisy Werner
state than the hashing protocol.


title: Hamiltonian and measuring time for analog quantum search

abstract:
  We derive in this study a Hamiltonian to solve with certainty the analog
quantum search problem analogue to the Grover algorithm. The general form of
the initial state is considered. Since the evaluation of the measuring time for
finding the marked state by probability of unity is crucially important in the
problem, especially when the Bohr frequency is high, we then give the exact
formula as a function of all given parameters for the measuring time.


title: Optimal Remote State Preparation

abstract:
  We prove that it is possible to remotely prepare an ensemble of non-commuting
mixed states using communication equal to the Holevo information for this
ensemble. This remote preparation scheme may be used to convert between
different ensembles of mixed states in an asymptotically lossless way,
analogous to concentration and dilution for entanglement.


title: Bounds for state-dependent quantum cloning

abstract:
  Due to the no-cloning theorem, the unknown quantum state can only be cloned
approximately or exactly with some probability. There are two types of cloners:
universal and state-dependent cloner. The optimal universal cloner has been
found and could be viewed as a special state-dependent quantum cloner which has
no information about the states. In this paper, we investigate the
state-dependent cloning when the state-set contains more than two states. We
get some bounds of the global fidelity for these processes. This method is not
dependent on the number of the states contained in the state-set. It is also
independent of the numbers of copying.


title: Bell's Inequality and quantum probability trees

abstract:
  The relations between Bell's inequality and quantum probability trees are
explained against the background offered by the concept of a quantum
probability tree built in others works. It is shown that f we use a concept of
probability tree it will not be necessary we set aside the principle of
separability and principle of locality.


title: Quantum logic between atoms inside a high Q optical cavity

abstract:
  We propose a protocol for conditional quantum logic between two 4-state atoms
inside a high Q optical cavity. The process detailed in this paper utilizes a
direct 4-photon 2-atom resonant process and has the added advantage of commonly
addressing the two atoms when they are inside the high Q optical cavity.


title:
Casimir interaction between two concentric cylinders: exact versus
  semiclassical results

abstract:
  The Casimir interaction between two perfectly conducting, infinite,
concentric cylinders is computed using a semiclassical approximation that takes
into account families of classical periodic orbits that reflect off both
cylinders. It is then compared with the exact result obtained by the
mode-by-mode summation technique. We analyze the validity of the semiclassical
approximation and show that it improves the results obtained through the
proximity theorem.


title:
Entanglement gauge and the non-Abelian geometric phase with two photonic
  qubits

abstract:
  We introduce the entanglement gauge describing the combined effects of local
operations and nonlocal unitary transformations on bipartite quantum systems.
The entanglement gauge exploits the invariance of nonlocal properties for
bipartite systems under local (gauge) transformations. This new formalism
yields observable effects arising from the gauge geometry of the bipartite
system. In particular, we propose a non-Abelian gauge theory realized via two
separated spatial modes of the quantized electromagnetic field manipulated by
linear optics. In this linear optical realization, a bi-partite state of two
separated spatial modes can acquire a non-Abelian geometric phase.


title: From Storage and Retrieval of Pulses to Adiabatons

abstract:
  We investigate whether it is possible to store and retrieve the intense probe
pulse from a $\Lambda$-type homogeneous medium of cold atoms. Through numerical
simulations we show that it is possible to store and retrieve the probe pulse
which are not necessarily weak. As the intensity of the probe pulse increases,
the retrieved pulse remains a replica of the original pulse, however there is
overall broadening and loss of the intensity. These effects can be understood
in terms of the dependence of absorption on the intensity of the probe. We
include the dynamics of the control field, which becomes especially important
as the intensity of the probe pulse increases. We use the theory of adiabatons
[Grobe {\it et al.} Phys. Rev. Lett. {\bf 73}, 3183 (1994)] to understand the
storage and retrieval of light pulses at moderate powers.


title: Coassociative grammar, periodic orbits and quantum random walk over Z

abstract:
  We prove that the combinatorics generated by the quantum random walk on Z can
be interpreted from the reading of periodic orbits of the classical chaotic map
$x \mapsto 2x mod 1$.


title: Covariant phase difference observables

abstract:
  Covariant phase difference observables are determined in two different ways,
by a direct computation and by a group theoretical method. A characterization
of phase difference observables which can be expressed as difference of two
phase observables is given. Classical limit of such phase difference
observables are determined and the Pegg-Barnett phase difference distribution
is obtained from the phase difference representation. The relation of Ban's
theory to the covariant phase theories is exhibited.


title: Time evolution of the Rabi Hamiltonian from the unexcited vacuum

abstract:
  The Rabi Hamiltonian describes a single mode of electromagnetic radiation
interacting with a two-level atom. Using the coupled cluster method, we
investigate the time evolution of this system from an initially empty field
mode and an unexcited atom. We give results for the atomic inversion and field
occupation, and find that the virtual processes cause the field to be squeezed.
No anti-bunching occurs.


title: Single Photon Source with Individualized Single Photon Certifications

abstract:
  As currently implemented, single-photon sources cannot be made to produce
single photons with high probability, while simultaneously suppressing the
probability of yielding two or more photons. Because of this, single photon
sources cannot really produce single photons on demand. We describe a
multiplexed system that allows the probabilities of producing one and more
photons to be adjusted independently, enabling a much better approximation of a
source of single photons on demand. The scheme uses a heralded photon source
based on parametric downconversion, but by effectively breaking the trigger
detector area into multiple regions, we are able to extract more information
about a heralded photon than is possible with a conventional arrangement. This
scheme allows photons to be produced along with a quantitative
``certification'' that they are single photons. Some of the single-photon
certifications can be significantly better than what is possible with
conventional downconversion sources (using a unified trigger detector region),
as well as being better than faint laser sources. With such a source of more
tightly certified single photons, it should be possible to improve the maximum
secure bit rate possible over a quantum cryptographic link. We present an
analysis of the relative merits of this method over the conventional
arrangement.


title: Bohmian Histories and Decoherent Histories

abstract:
  The predictions of the Bohmian and the decoherent (or consistent) histories
formulations of the quantum mechanics of a closed system are compared for
histories -- sequences of alternatives at a series of times. For certain kinds
of histories, Bohmian mechanics and decoherent histories may both be formulated
in the same mathematical framework within which they can be compared. In that
framework, Bohmian mechanics and decoherent histories represent a given history
by different operators. Their predictions for the probabilities of histories
therefore generally differ. However, in an idealized model of measurement, the
predictions of Bohmian mechanics and decoherent histories coincide for the
probabilities of records of measurement outcomes. The formulations are thus
difficult to distinguish experimentally. They may differ in their accounts of
the past history of the universe in quantum cosmology.


title: Entanglement Produced by Using Some Biparticle Bose Systems

abstract:
  In this paper, the generating of entanglement by using some biparticle Bose
systems acting on vacuum state are investigated. These systems include two-mode
squeezed system, thermal system of a free single particle (where the fictitious
tilde system is regarded another particle), and the system of two coupling
harmonic oscillators. The technique of integration within an ordered product
(IWOP) of operators is used.


title: Mixing in Continuous Quantum Walks on Graphs

abstract:
  Classical random walks on well-behaved graphs are rapidly mixing towards the
uniform distribution. Moore and Russell showed that a continuous quantum walk
on the hypercube is instantaneously uniform mixing. We show that the
continuous-time quantum walks on other well-behaved graphs do not exhibit this
uniform mixing. We prove that the only graphs amongst balanced complete
multipartite graphs that have the instantaneous uniform mixing property are the
complete graphs on two, three and four vertices, and the cycle graph on four
vertices. Our proof exploits the circulant structure of these graphs.
Furthermore, we conjecture that most complete cycles and Cayley graphs lack
this mixing property as well.


title: On the measure of nonclassicality of field states

abstract:
  The degree of nonclassicality of states of a field mode is analysed
considering both phase-space and distance-type measures of nonclassicality. By
working out some general examples, it is shown explicitly that the phase-space
measure is rather sensitive to superposition of states, with finite
superpositions possessing maximum nonclassical depth (the highest degree of
nonclassicality) irrespective to the nature of the component states. Mixed
states are also discussed and examples with nonclassical depth varying between
the minimum and the maximum allowed values are exhibited. For pure Gaussian
states, it is demonstrated that distance-type measures based on the
Hilbert-Schmidt metric are equivalent to the phase-space measure. Analyzing
some examples, it is shown that distance-type measures are efficient to
quantify the degree of nonclassicality of non-Gaussian pure states.


title: Momentum transfer using chirped standing wave fields: Bragg scattering

abstract:
  We consider momentum transfer using frequency-chirped standing wave fields.
Novel atom-beam splitter and mirror schemes based on Bragg scattering are
presented. It is shown that a predetermined number of photon momenta can be
transferred to the atoms in a single interaction zone.


title: Modal Interpretations and Relativity

abstract:
  A proof is given, at a greater level of generality than previous 'no-go'
theorems, of the impossibility of formulating a modal interpretation that
exhibits 'serious' Lorentz invariance at the fundamental level. Particular
attention is given to modal interpretations of the type proposed by Bub.


title: Connection Conditions and the Spectral Family under Singular Potentials

abstract:
  To describe a quantum system whose potential is divergent at one point, one
must provide proper connection conditions for the wave functions at the
singularity. Generalizing the scheme used for point interactions in one
dimension, we present a set of connection conditions which are well-defined
even if the wave functions and/or their derivatives are divergent at the
singularity. Our generalized scheme covers the entire U(2) family of
quantizations (self-adjoint Hamiltonians) admitted for the singular system. We
use this scheme to examine the spectra of the Coulomb potential $V(x) = - e^2 /
| x |$ and the harmonic oscillator with square inverse potential $V(x) = (m
\omega^2 / 2) x^2 + g/x^2$, and thereby provide a general perspective for these
models which have previously been treated with restrictive connection
conditions resulting in conflicting spectra. We further show that, for any
parity invariant singular potentials $V(-x) = V(x)$, the spectrum is determined
solely by the eigenvalues of the characteristic matrix $U \in U(2)$.

categories: quant-ph cond-mat.mes-hall math-ph math.MP


title:
Wronskian Approach and the One-dimensional Schrodinger Equation with
  Double-well Potential

abstract:
  A Wronskian determinant approach is suggested to study the energy and the
wave function for one-dimensional Schrodinger equation. An integral equation
and the corresponding Green's function are constructed. As an example, we
employed this approach to study the problem of double-well potential with
strong coupling. A series expansion of ground state energy up to the second
order approximation of iterative procedure is given.


title: Fiber-Optic Sources of Quantum Entanglement

abstract:
  We present a fiber-based source of polarization-entangled photon pairs that
is well suited for quantum communication applications in the 1.5$\mu$m band of
standard telecommunication fiber. Quantum-correlated signal and idler photon
pairs are produced when a nonlinear-fiber Sagnac interferometer is pumped in
the anomalous-dispersion region of the fiber. Recently, we have demonstrated
nonclassical properties of such photon pairs by using Geiger-mode InGaAs/InP
avalanche photodiodes. Polarization entanglement in the photon pairs can be
created by pumping the Sagnac interferometer with two orthogonally polarized
pulses. In this case the parametrically scattered signal-idler photons yield
biphoton interference with $>$90% visibility in coincidence detection, while no
interference is observed in direct detection of either the signal or the idler
photons.


title: Diameters of Homogeneous Spaces

abstract:
  Let G be a compact connected Lie group with trivial center. Using the action
of G on its Lie algebra, we define an operator norm | |_{G} which induces a
bi-invariant metric d_G(x,y)=|Ad(yx^{-1})|_{G} on G. We prove the existence of
a constant \beta \approx .12 (independent of G) such that for any closed
subgroup H \subsetneq G, the diameter of the quotient G/H (in the induced
metric) is \geq \beta.


title: Variability of fundamental constants

abstract:
  If the fine structure constant is not really constant, is this due to a
variation of $e$, $\hbar$, or $c$? It is argued that the only reasonable
conclusion is a variable speed of light.


title:
Robust generation of entanglement between two cavities mediated by short
  interactions with an atom

abstract:
  Current methods of preparing maximally entangled states in the laboratory
need an extremely accurate control of interaction times, requiring
sophisticated experimental techniques.
  Here, we show that such precise control is not necessary when one utilizes
short or weak interactions followed by measurements. We present a scheme for
the probabilistic generation of Bell states in a pair of cavities, after each
has interacted briefly with an atom. The advantage of this proposal, as
compared to present schemes, is its low sensitivity to the exact values of
experimental parameters such as atomic velocity and coupling strength, in fact,
for a large range of parameters the fidelity of the Bell states generated
remains close to unity.


title: Probabilities and Quantum Reality: Are There Correlata?

abstract:
  Any attempt to introduce probabilities into quantum mechanics faces
difficulties due to the mathematical structure of Hilbert space, as reflected
in Birkhoff and von Neumann's proposal for a quantum logic. The (consistent or
decoherent) histories solution is provided by its single framework rule, an
approach that includes conventional (Copenhagen) quantum theory as a special
case. Mermin's Ithaca interpretation addresses the same problem by defining
probabilities which make no reference to a sample space or event algebra
(``correlations without correlata''). But this leads to severe conceptual
difficulties, which almost inevitably couple quantum theory to unresolved
problems of human consciousness. Using histories allows a sharper quantum
description than is possible with a density matrix, suggesting that the latter
provides an ensemble rather than an irreducible single-system description as
claimed by Mermin. The histories approach satisfies the first five of Mermin's
desiderata for a good interpretation of quantum mechanics, including Einstein
locality, but the Ithaca interpretation seems to have difficulty with the first
(independence of observers) and the third (describing individual systems).


title: Storage of energy in confined quantum systems

abstract:
  Using the non-perturbative method of {\it dressed} states introduced in
previous publications [N.P.Andion, A.P.C. Malbouisson and A. Mattos Neto,
J.Phys.{\bf A34}, 3735, (2001); G. Flores-Hidalgo, A.P.C. Malbouisson, Y.W.
Milla, Phys. Rev. A, {\bf 65}, 063314 (2002)], we study the evolution of a
confined quantum mechanical system embedded in a {\it ohmic} environment. Our
approach furnishes a theoretical mechanism to control inhibition of the decay
of excited quantum systems in cavities, in both weak and strong coupling
regimes.

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


title: Relativistic Symmetry and Entangled Correlations

abstract:
  It is argued that the standard quantum mechanical description of the Bell
correlations between entangled subsystems is in conflict with relativistic
space-time symmetry. Proposals to abandon relativistic symmetry, in the sense
of explicitly returning to an absolute time and preferred frame, are rejected
on the grounds that the preferred frame is not empirically detectable, so the
asymmetry is an unsatisfactory feature in physical theory. A "symmetric view"
is proposed in which measurement events on space-like separated entangled
subsystems are connected by a symmetric two-way mutual influence. Because of
this reciprocity, there is complete symmetry of the description: Einsteinian
relativity of simultaneity and space-time symmetry are completely preserved.
The nature of the two-way influence is considered, as well as the possibility
of an empirical test.


title: A Renormalisation Approach to Effective Interactions in Hilbert Space

abstract:
  The low-lying bound states of a microscopic quantum many-body system of $n$
particles and the related physical observables can be worked out in a truncated
$n$--particle Hilbert space. We present here a non-perturbative analysis of
this problem which relies on a renormalisation concept and work out the link
with perturbative approaches.


title: A geometric theory of non-local two-qubit operations

abstract:
  We study non-local two-qubit operations from a geometric perspective. By
applying a Cartan decomposition to su(4), we find that the geometric structure
of non-local gates is a 3-Torus. We derive the invariants for local
transformations, and connect these local invariants to the coordinates of the
3-Torus. Since different points on the 3-Torus may correspond to the same local
equivalence class, we use the Weyl group theory to reduce the symmetry. We show
that the local equivalence classes of two-qubit gates are in one-to-one
correspondence with the points in a tetrahedron except on the base. We then
study the properties of perfect entanglers, that is, the two-qubit operations
that can generate maximally entangled states from some initially separable
states. We provide criteria to determine whether a given two-qubit gate is a
perfect entangler and establish a geometric description of perfect entanglers
by making use of the tetrahedral representation of non-local gates. We find
that exactly half the non-local gates are perfect entanglers. We also
investigate the non-local operations generated by a given Hamiltonian. We first
study the gates that can be directly generated by a Hamiltonian. Then we
explicitly construct a quantum circuit that contains at most three non-local
gates generated by a two-body interaction Hamiltonian, together with at most
four local gates generated by single qubit terms. We prove that such a quantum
circuit can simulate any arbitrary two-qubit gate exactly, and hence it
provides an efficient implementation of universal quantum computation and
simulation.


title: Advantage of a quantum player over a classical one in 2x2 quantum games

abstract:
  We study a general $2 \times 2$ symmetric, entangled, quantum game. When one
player has access only to classical strategies while the other can use the full
range of quantum strategies, there are ``miracle'' moves available to the
quantum player that can direct the result of the game towards the quantum
player's preferred result regardless of the classical player's strategy. The
advantage pertaining to the quantum player is dependent on the degree of
entanglement. Below a critical level, dependent on the payoffs in the game, the
miracle move is of no advantage.


title: Entanglement of two-mode Bose-Einstein condensates

abstract:
  We investigate the entaglement characteristics of two general bimodal
Bose-Einstein condensates - a pair of tunnel-coupled Bose-Einstein condensates
and the atom-molecule Bose-Einstein condensate. We argue that the entanglement
is only physically meaningful if the system is viewed as a bipartite system,
where the subsystems are the two modes. The indistinguishibility of the
particles in the condensate means that the atomic constituents are physically
inaccessible and thus the degree of entanglement between individual particles,
unlike the entanglement between the modes, is not experimentally relevant so
long as the particles remain in the condensed state. We calculate the
entanglement between the modes for the exact ground state of the two bimodal
condensates and consider the dynamics of the entanglement in the tunnel-coupled
case.


title: Do we really understand quantum mechanics?

abstract:
  This article presents a general discussion of several aspects of our present
understanding of quantum mechanics. The emphasis is put on the very special
correlations that this theory makes possible: they are forbidden by very
general arguments based on realism and local causality. In fact, these
correlations are completely impossible in any circumstance, except the very
special situations designed by physicists especially to observe these purely
quantum effects. Another general point that is emphasized is the necessity for
the theory to predict the emergence of a single result in a single realization
of an experiment. For this purpose, orthodox quantum mechanics introduces a
special postulate: the reduction of the state vector, which comes in addition
to the Schrodinger evolution postulate. Nevertheless, the presence in parallel
of two evolution processes of the same object (the state vector) maybe a
potential source for conflicts; various attitudes that are possible to avoid
this problem are discussed in this text. After a brief historical introduction,
recalling how the very special status of the state vector has emerged in
quantum mechanics, various conceptual difficulties are introduced and
discussed. The Einstein Podolsky Rosen (EPR) theorem is presented with the help
of a botanical parable, in a way that emphasizes how deeply the EPR reasoning
is rooted into what is often called "scientific method''. In another section
the GHZ argument, the Hardy impossibilities, as well as the BKS theorem are
introduced in simple terms.


title: Simple construction of quantum universal variable-length source coding

abstract:
  We simply construct a quantum universal variable-length source code in which,
independent of information source, both of the average error and the
probability that the coding rate is greater than the entropy rate $H(rho_p)$,
tend to 0. If $H(rho_p)$ is estimated, we can compress the coding rate to the
admissible rate $H(rho_p)$ with a probability close to 1. However, when we
perform a naive measurement for the estimation of $H(rho_p)$, the input state
is demolished. By smearing the measurement, we successfully treat the trade-off
between the estimation of $H(rho_p)$ and the non-demolition of the input state.
Our protocol can be used not only for the Schumacher's scheme but also for the
compression of entangled states.


title: Separability of pure n-qubit states: two characterizations

abstract:
  This paper characterizes two forms of separability of pure states of systems
of n qubits: (i) into a tensor product of n qubit states, and (ii), into a
tensor product of 2 subsystems states of p and q qubits respectively with
p+q=n. For both cases, necessary and sufficient conditions are proved in the
form of minimal sets of equalities among pair-wise products of amplitudes.
These conditions bear some relation with entanglement measures, and a number of
more refined questions about separability in n qubit systems can be studied on
the basis of these results.


title:
A remark on quantum key distribution with two way communication: the
  classical complexity in decoding the CSS code can be removed

abstract:
  So far all the proven unconditionally secure prepare and measure protocols
for the quantum key distribution(QKD) must solve the very complex problem of
decoding the classical CSS code. In the decoding stage, Bob has to compare his
string with an exponentially large number of all the strings in certain code
space to find out the closest one. Here we have spotted that, in an
entanglement purification protocol(EPP), the random basis in the state
preparation stage is only necessary to those check qubits, but uncessary to the
code qubits. In our modified two way communication EPP(2-EPP) protocol, Alice
and Bob may first take all the parity checks on $Z$ basis to reduce the bit
flip error to strictly zero with a high probability, e.g., $1-2^{-30}$, and
then use the CSS code to obtain the final key. We show that, this type of 2-EPP
protocol can be reduced to an equivalent prepare and measure protocol. In our
protocol, the huge complexity of decoding the classical CSS code is totally
removed.


title:
Ascertaining the Values of $\sigma_x$, $\sigma_y$, and $\sigma_z$ of a
  Polarization Qubit

abstract:
  In the 1987 spin retrodiction puzzle of Vaidman, Aharonov, and Albert one is
challenged to ascertain the values of $\sigma_x$, $\sigma_y$, and $\sigma_z$ of
a spin-1/2 particle by utilizing entanglement. We report the experimental
realization of a quantum-optical version in which the outcome of an
intermediate polarization projection is inferred by exploiting single-photon
two-qubit quantum gates. The experimental success probability is consistently
above the 90.2% threshold of the optimal one-qubit strategy, with an average
success probability of 95.6%.


title: One-atom maser: Non-separable atom pairs

abstract:
  When a one-atom maser is operated in the standard way -- excited, resonant
two-level atoms traverse the resonator at random times -- the emerging atoms
are entangled with the cavity field. As a consequence, the results of
measurements on different atoms tend to be correlated. We show that truly
non-classical correlations can be found between two successive atoms by
studying the properties of the reduced state of such an atom pair. In
particular, we calculate its degree of separability and find parameter ranges
in which it is markedly less than unity.


title:
Non-Hamiltonian Nature of Low Energy Nucleon Dynamics in an Effective
  Field Theory

abstract:
  Problems connected with non-Hamiltonian nature of low energy nucleon dynamics
in the effective field theory (EFT) of nuclear forces is investigated by using
the formalism of the generalized quantum dynamics (GQD) developed in [J. Phys.
A, 5657 (1999)]. This formalism is based on a generalized dynamical equation
derived as the most general equation of motion consistent with the current
concepts of quantum physics. By using the example of the EFT of nuclear forces,
we demonstrate that a theory, which, being formulated in terms of Hamiltonian
formalism, leads to ultraviolet divergences, may manifest itself as a perfectly
consistent theory free from infinities, if it is considered from the more
general point of view provided by the GQD. We show that non-Hamiltonian
character of nucleon dynamics gives rise to some new problems connected with
discontinuity of the evolution operator. This discontinuity results in the fact
that the Hilbert space of nucleon states cannot be realized in the standard
way. A space which allows one to realize, in a natural way, the Hilbert space
of nucleon states is investigated. The structure of this space reflects the
existence of the high energy degrees of freedom which affect on low energy
nucleon dynamics.


title:
Nonlinear Transformation for a Class of Gauged Schroedinger Equations
  with Complex Nonlinearities

abstract:
  In the present contribution we consider a class of Schroedinger equations
containing complex nonlinearities, describing systems with conserved norm
$|\psi|^2$ and minimally coupled to an abelian gauge field. We introduce a
nonlinear transformation which permits the linearization of the source term in
the evolution equations for the gauge field, and transforms the nonlinear
Schroedinger equations in another one with real nonlinearities. We show that
this transformation can be performed either on the gauge field $A_\mu$ or,
equivalently, on the matter field $\psi$. Since the transformation does not
change the quantities $|\psi|^2$ and $F_{\mu\nu}$, it can be considered a
generalization of the gauge transformation of third kind introduced some years
ago by other authors.
  Pacs numbers: 03.65.-w, 11.15.-q


title: Exponential algorithmic speedup by quantum walk

abstract:
  We construct an oracular (i.e., black box) problem that can be solved
exponentially faster on a quantum computer than on a classical computer. The
quantum algorithm is based on a continuous time quantum walk, and thus employs
a different technique from previous quantum algorithms based on quantum Fourier
transforms. We show how to implement the quantum walk efficiently in our
oracular setting. We then show how this quantum walk can be used to solve our
problem by rapidly traversing a graph. Finally, we prove that no classical
algorithm can solve this problem with high probability in subexponential time.


title: Measuring the elements of the optical density matrix

abstract:
  Most methods for experimentally reconstructing the quantum state of light
involve determining a quasiprobability distribution such as the Wigner
function. In this paper we present a scheme for measuring individual density
matrix elements in the photon number state representation. Remarkably, the
scheme is simple, involving two beam splitters and a reference field in a
coherent state.


title:
Efficient classical simulation of measurements in optical quantum
  information

abstract:
  We present conditions for the efficient simulation of a broad class of
optical quantum circuits on a classical machine: this class includes unitary
transformations, amplification, noise, and measurements. Various proposed
schemes for universal quantum computation using optics are assessed against
these conditions, and we consider the minimum resource requirements needed in
any optical scheme to generate optical nonlinear processes and perform
universal quantum computation.


title:
Dynamical suppression of radiative decay via atomic deflection by a
  standing light wave

abstract:
  We consider the radiative decay of atoms scattered by a resonant standing
light wave. Scattering is shown to suppress the Rabi oscillations and to slow
down the atomic radiative decay giving rise to a power law behavior of the
time-dependent level populations rather than the exponential one.


title:
Entanglement of resonantly coupled field modes in cavities with
  vibrating boundaries

abstract:
  We study time dependence of various measures of entanglement (covariance
entanglement coefficient, purity entanglement coefficient, normalized distance
coefficient, entropic coefficients) between resonantly coupled modes of the
electromagnetic field in ideal cavities with oscillating boundaries. Two types
of cavities are considered: a three-dimensional cavity possessing
eigenfrequencies $\omega_3=3\omega_1$, whose wall oscillates at the frequency
$\omega_w=2\omega_1$, and a one-dimensional (Fabry--Perot) cavity with an
equidistant spectrum $\omega_n= n\omega_1$, when the distance between perfect
mirrors oscillates at the frequencies $\omega_1$ and $2\omega_1$. The behaviour
of entanglement measures in these cases turns out to be completely different,
although all three coefficients demonstrate qualitatively similar time
dependences in each case (except for some specific situations, where the
covariance entanglement coefficient, based on traces of covariance submatrices,
seems to be essentially more sensitive to entanglement than other measures,
which are based on determinants of covariance submatrices). Different initial
states of the field are considered: vacuum, squeezed vacuum, thermal, Fock, and
even/odd coherent states.


title: Logical equivalence between generalized urn models and finite automata

abstract:
  To every generalized urn model there exists a finite (Mealy) automaton with
identical propositional calculus. The converse is true as well.


title: Entanglement between an electron and a nuclear spin 1/2

abstract:
  We report on the preparation and detection of entangled states between an
electron spin 1/2 and a nuclear spin 1/2 in a molecular single crystal. These
were created by applying pulses at ESR (9.5 GHz) and NMR (28 MHz) frequencies.
Entanglement was detected by using a special entanglement detector sequence
based on a unitary back transformation including phase rotation.


title: Maximum Fidelity Retransmission of Mirror Symmetric Qubit States

abstract:
  In this paper we address the problem of optimal reconstruction of a quantum
state from the result of a single measurement when the original quantum state
is known to be a member of some specified set. A suitable figure of merit for
this process is the fidelity, which is the probability that the state we
construct on the basis of the measurement result is found by a subsequent test
to match the original state. We consider the maximisation of the fidelity for a
set of three mirror symmetric qubit states. In contrast to previous examples,
we find that the strategy which minimises the probability of erroneously
identifying the state does not generally maximise the fidelity.


title: Nonclassical paths in the recurrence spectrum of diamagnetic atoms

abstract:
  Using time-independent scattering matrices, we study how the effects of
nonclassical paths on the recurrence spectra of diamagnetic atoms can be
extracted from purely quantal calculations. This study reveals an intimate
relationship between two types of nonclassical paths: exotic ghost orbits and
diffractive orbits. This relationship proves to be a previously unrecognized
reason for the success of semiclassical theories, like closed-orbit theory, and
permits a comprehensive reformulation of the semiclassical theory that
elucidates its convergence properties.


title: Teleportation improvement by inconclusive photon subtraction

abstract:
  Inconclusive photon subtraction (IPS) is a conditional measurement scheme to
force nonlinear evolution of a given state. In IPS the input state is mixed
with the vacuum in a beam splitter and then the reflected beam is revealed by
ON/OFF photodetection. When the detector clicks we have the (inconclusive)
photon subtracted state. We show that IPS on both channels of an entangled
twin-beam of radiation improves the fidelity of coherent state teleportation if
the energy of the incoming twin-beam is below a certain threshold, which
depends on the beam splitter transmissivity and the quantum efficiency of
photodetectors. We show that the energy threshold diverges when the
transmissivity and the efficiency approach unit and compare our results with
that of previous works on {\em conclusive} photon subtraction.


title:
Hamiltonian Systems on Complex Grassmann Manifold. Holonomy and
  Schrodinger Equation

abstract:
  Differential geometric structures such as the principal bundle for the
canonical vector bundle on a complex Grassmann manifold, the canonical
connection form on this bundle, the canonical symplectic form on a complex
Grassmann manifold and the corresponding dynamical systems are investigated.
The Grassmann manifold is considered as an orbit of the co-adjoint action and
the symplectic form is described as the restriction of the canonical Poisson
structure on a Lie coalgebra. The holonomy of the connection on the principal
bundle over Grassmannian and its relation with Berry phase is considered and
investigated for the integral curves of Hamiltonian dynamical systems.


title: Quantum computation with un-tunable couplings

abstract:
  Most quantum computer realizations require the ability to apply local fields
and tune the couplings between qubits, in order to realize single bit and two
bit gates which are necessary for universal quantum computation. We present a
scheme to remove the necessity of switching the couplings between qubits for
two bit gates, which are more costly in many cases. Our strategy is to compute
in and out of carefully designed interaction free subspaces analogous to
decoherence free subspaces, which allows us to effectively turn off and turn on
the interactions between the encoded qubits. We give two examples to show how
universal quantum computation is realized in our scheme with local
manipulations to physical qubits only, for both diagonal and off diagonal
interactions.


title: Entropy-energy balance in noisy quantum computers

abstract:
  We use entropy-energy arguments to assess the limitations on the running time
and on the system size, as measured in qubits, of noisy macroscopic
circuit-based quantum computers.


title:
Multidimensional quasi-exactly solvable potentials with two known
  eigenstates

abstract:
  A general approach for constructing multidimensional quasi-exactly solvable
(QES) potentials with explicitly known eigenfunctions for two energy levels is
proposed. Examples of new QES potentials are presented.


title: Fidelity and Purity Decay in Weakly Coupled Composite Systems

abstract:
  We study the stability of unitary quantum dynamics of composite systems (for
example: central system + environment) with respect to weak interaction between
the two parts. Unified theoretical formalism is applied to study different
physical situations: (i) coherence of a forward evolution as measured by purity
of the reduced density matrix, (ii) stability of time evolution with respect to
small coupling between subsystems, and (iii) Loschmidt echo measuring dynamical
irreversibility. Stability has been measured either by fidelity of pure states
of a composite system, or by the so-called reduced fidelity of reduced density
matrices within a subsystem. Rigorous inequality among fidelity,
reduced-fidelity and purity is proved and a linear response theory is developed
expressing these three quantities in terms of time correlation functions of the
generator of interaction. The qualitatively different cases of regular
(integrable) or mixing (chaotic in the classical limit) dynamics in each of the
subsystems are discussed in detail. Theoretical results are demonstrated and
confirmed in a numerical example of two coupled kicked tops.


title: Gamow Functionals on Operator Algebras

abstract:
  We obtain the precise form of two Gamow functionals, representing the
exponentially decaying part of a quantum resonance and its mirror image that
grows exponentially, as a linear, positive and continuous functional on an
algebra containing observables. These functionals do not admit normalization
and, with an appropiate choice of the algebra, are time reversal of each other.


title: The Gamow Functional

abstract:
  We present a formalism that represents pure states, mixtures and generalized
states as functionals on an algebra containing the observables of the system.
Along these states, there are other functionals that decay exponentially at all
times and therefore can be used to describe resonance phenomena.


title:
Shortening Grover's search algorithm for an expectation value quantum
  computer

abstract:
  Quantum algorithms are conventionally formulated for implementation on a
single system of qubits amenable to projective measurements. However, in
expectation value quantum computation, such as nuclear magnetic resonance
realizations, the computer consists of an ensemble of identical qubit-systems
amenable only to expectation value measurements. The prevalent strategy in such
expectation value implementations of quantum algorithms has been to retain the
conventional formulation's unitary operations but modify its initialization and
measurement steps appropriately. This naive approach is not optimal for
Grover's algorithm and a shortened version for expectation value quantum
computers is presented.


title:
The quantum bit commitment: a finite open system approach for a complete
  classification of protocols

abstract:
  Mayers, Lo and Chau argued that all quantum bit commitment protocols are
insecure, because there is no way to prevent an Einstein-Podolsky-Rosen (EPR)
cheating attack. However, Yuen presented some protocols which challenged the
previous impossibility argument. Up to now, it is still debated whether there
exist or not unconditionally secure protocols. In this paper the above
controversy is addressed. For such purpose, a complete classification of all
possible bit commitment protocols is given, including all possible cheating
attacks. Focusing on the simplest class of protocols (non-aborting and with
complete and perfect verification), it is shown how naturally a
game-theoretical situation arises. For these protocols, bounds for the cheating
probabilities are derived, involving the two quantum operations encoding the
bit values and their respective alternate Kraus decompositions. Such bounds are
different from those given in the impossibility proof. The whole classification
and analysis has been carried out using a "finite open system" approach. The
discrepancy with the impossibility proof is explained on the basis of the
implicit adoption of a "closed system approach"--equivalent to modeling the
commitment as performed by two fixed machines interacting unitarily in a
overall "closed system"--according to which it is possible to assume as "openly
known" both the initial state and the probability distributions for all secret
parameters, which can be then "purified". This approach is also motivated by
existence of unitary extensions for any open system. However, it is shown that
the closed system approach for the classification of commitment protocols
unavoidably leads to infinite dimensions, which then invalidate the continuity
argument at the basis of the impossibility proof.