title: Dynamic Quantum Logic for Quantum Programs

abstract:
  We present a way to apply quantum logic to the study of quantum programs.
This is made possible by using an extension of the usual propositional language
in order to make transformations performed on the system appear explicitly.
This way, the evolution of the system becomes part of the logical study. We
show how both unitary operations and two-valued measurements can be included in
this formalism and can thus be handled logically.


title: An experimental demonstration of single photon nonlocality

abstract:
  In this letter we experimentally implement a single photon Bell test based on
the ideas of S. Tan et al. [Phys. Rev. Lett., vol. 66, 252 (1991)] and L. Hardy
[Phys. Rev. Lett.,vol. 73, 2279 (1994)]. A double heterodyne measurement is
used to measure correlations in the Fock space spanned by zero and one photons.
Local oscillators used in the correlation measurement are distributed to two
observers by co-propagating it in an orthogonal polarization mode. This method
eliminates the need for interferometrical stability in the setup, consequently
making it a robust and scalable method.


title:
Disappearance of the Measurement Paradox in a Metaplectic Extension of
  Quantum Dynamics

abstract:
  It is shown that Schrodinger dynamics can be embedded in a larger dynamical
theory which extends its symmetry group from the unitary group to the full
metaplectic group, i.e. the group of linear canonical transformations. Among
the newly admitted non-unitary processes are analogues of the classical
measurement process which makes it possible to treat the wave-function as an
objective property of the quantum mechanical system on the same footing as the
phase-space coordinates of a classical system. The notion of "observables" that
in general have values only when measured can then be dispensed with, and the
measurement paradox disappears.


title:
Quantum nonlocality of Heisenberg XX model with Site-dependent Coupling
  Strength

abstract:
  We show that the generalized Bell inequality is violated in the extended
Heisenberg model when the temperature is below a threshold value. The threshold
temperature values are obtained by constructing exact solutions of the model
using the temperature-dependent correlation functions. The effect due to the
presence of external magnetic field is also illustrated.


title: Entangled-Photon Imaging of a Pure Phase Object

abstract:
  We demonstrate experimentally and theoretically that a coherent image of a
pure phase object may be obtained by use of a spatially incoherent illumination
beam. This is accomplished by employing a two-beam source of entangled photons
generated by spontaneous parametric down-conversion. Though each of the beams
is, in and of itself, spatially incoherent, the pair of beams exhibits
higher-order inter-beam coherence. One of the beams probes the phase object
while the other is scanned. The image is recorded by measuring the photon
coincidence rate using a photon-counting detector in each beam. Using a
reflection configuration, we successfully imaged a phase object implemented by
a MEMS micro-mirror array. The experimental results are in accord with
theoretical predictions.


title:
Randomized and Quantum Algorithms Yield a Speed-Up for Initial-Value
  Problems

abstract:
  Quantum algorithms and complexity have recently been studied not only for
discrete, but also for some numerical problems. Most attention has been paid so
far to the integration problem, for which a speed-up is shown by quantum
computers with respect to deterministic and randomized algorithms on a
classical computer. In this paper we deal with the randomized and quantum
complexity of initial-value problems. For this nonlinear problem, we show that
both randomized and quantum algorithms yield a speed-up over deterministic
algorithms. Upper bounds on the complexity in the randomized and quantum
settings are shown by constructing algorithms with a suitable cost, where the
construction is based on integral information. Lower bounds result from the
respective bounds for the integration problem.


title: The density matrix in the de Broglie-Bohm approach

abstract:
  If the density matrix is treated as an objective description of individual
systems, it may become possible to attribute the same objective significance to
statistical mechanical properties, such as entropy or temperature, as to
properties such as mass or energy. It is shown that the de Broglie-Bohm
interpretation of quantum theory can be consistently applied to density
matrices as a description of individual systems. The resultant trajectories are
examined for the case of the delayed choice interferometer, for which Bell
appears to suggest that such an interpretation is not possible. Bell's argument
is shown to be based upon a different understanding of the density matrix to
that proposed here.


title: Holevo-ordering and the continuous-time limit for open Floquet dynamics

abstract:
  We consider an atomic beam reservoir as a source of quantum noise. The atoms
are modelled as two-state systems and interact one-at-a-time with the system.
The Floquet operators are described in terms of the Fermionic creation,
annihilation and number operators associated with the two-state atom. In the
limit where the time between interactions goes to zero and the interaction is
suitably scaled, we show that we may obtain a causal (that is, adapted) quantum
stochastic differential equation of Hudson-Parthasarathy type, driven by
creation, annihilation and conservation processes. The effect of the Floquet
operators in the continuous limit is exactly captured by the Holevo ordered
form for the stochastic evolution.


title: Husimi's $Q(\alpha)$ function and quantum interference in phase space

abstract:
  We discuss a phase space description of the photon number distribution of non
classical states which is based on Husimi's $Q(\alpha)$ function and does not
rely in the WKB approximation. We illustrate this approach using the examples
of displaced number states and two photon coherent states and show it to
provide an efficient method for computing and interpreting the photon number
distribution . This result is interesting in particular for the two photon
coherent states which, for high squeezing, have the probabilities of even and
odd photon numbers oscillating independently.


title:
Analytic solutions for quantum logic gates and modeling pulse errors in
  a quantum computer with a Heisenberg interaction

abstract:
  We analyze analytically and numerically quantum logic gates in a
one-dimensional spin chain with Heisenberg interaction. Analytic solutions for
basic one-qubit gates and swap gate are obtained for a quantum computer based
on logical qubits. We calculated the errors caused by imperfect pulses which
implement the quantum logic gates. It is numerically demonstrated that the
probability error is proportional to $\epsilon^4$, while the phase error is
proportional to $\epsilon$, where $\epsilon$ is the characteristic deviation
from the perfect pulse duration.


title:
Uncertainty and information in classical mechanics formulation. Common
  ground for thermodynamics and quantum mechanics

abstract:
  Mechanics can be founded on a principle relating the uncertainty delta-q in
the trajectory of an observable particle to its motion relative to the
observer. From this principle, p.delta-q=const., p being the q-conjugated
momentum, mechanical laws are derived and the meaning of the Lagrangian and
Hamiltonian functions are discussed. The connection between the presented
principle and Hamilton's Least Action Principle is examined.
  Wave mechanics and Schrodinger equation appear without additional assumptions
by choosing the representation for delta-q in the case the motion is not
trajectory describable. The Cramer-Rao inequality serves that purpose. For a
particle hidden from direct observation, the position uncertainty determined by
the enclosing boundaries leads to thermodynamics in a straightforward extension
of the presented formalism.
  The introduction of uncertainty in classical mechanics formulation enables
the translation of mechanical laws into the wide ranging conceptual framework
of information theory. The boundaries between classical mechanics,
thermodynamics and quantum mechanics are defined in terms of informational
changes associated with the system evolution. As a direct application of the
proposed formulation upper bounds for the rate of information transfer are
derived.


title:
Distinguishing a set of full product bases needs only projective
  measurements and classical communication

abstract:
  Nonlocality without entanglement is an interesting field. A manifestation of
quantum nonlocality without entanglement is the local indistinguishability of a
set of orthogonal product states. In this paper we analyze the character of
operators to distinguish a set of full product bases in a multi-partite system,
and show that distinguishing perfectly a set of full product bases needs only
local projective measurements and classical communication, and these
measurements cannot damage each product basis. Employing these conclusions one
can discuss local distinguishability of full product bases easily. Finally we
discuss the generalization of these results to the locally distinguishability
of a set of incomplete product bases.


title: A classical analog to topological non-local quantum interference effect

abstract:
  The two main features of the Aharonov-Bohm effect are the topological
dependence of accumulated phase on the winding number around the magnetic
fluxon, and non-locality -- local observations at any intermediate point along
the trajectories are not affected by the fluxon. The latter property is usually
regarded as exclusive to quantum mechanics. Here we show that both the
topological and non-local features of the Aharonov-Bohm effect can be
manifested in a classical model that incorporates random noise. The model also
suggests new types of multi-particle topological non-local effects which have
no quantum analog.


title: Swapping and entangling qubits of single photons and atoms

abstract:
  A scheme is proposed here to achieve swapping and entangling of photonic and
atomic qubits with high fidelity. The mechanism is based on the scattering of a
single photon from a $\Lambda$-type three-level atom. The evolution of the
coupled system is analyzed by projecting the quantum state onto a `bright' and
a `dark' state. Quantum interference of these two states, which is determined
by a frequency-dependent phase angle, can be exploited to perform various
two-qubit transformations. It is remarkable that the probability of success of
such transformations can approach unity in the strong coupling cavity QED
regime.


title: Quantum Heat Engines, the Second Law and Maxwell's Daemon

abstract:
  We introduce a class of quantum heat engines which consists of
two-energy-eigenstate systems, the simplest of quantum mechanical systems,
undergoing quantum adiabatic processes and energy exchanges with heat baths,
respectively, at different stages of a cycle. Armed with this class of heat
engines and some interpretation of heat transferred and work performed at the
quantum level, we are able to clarify some important aspects of the second law
of thermodynamics. In particular, it is not sufficient to have the heat source
hotter than the sink, but there must be a minimum temperature difference
between the hotter source and the cooler sink before any work can be extracted
through the engines. The size of this minimum temperature difference is
dictated by that of the energy gaps of the quantum engines involved. Our new
quantum heat engines also offer a practical way, as an alternative to Szilard's
engine, to physically realise Maxwell's daemon. Inspired and motivated by the
Rabi oscillations, we further introduce some modifications to the quantum heat
engines with single-mode cavities in order to, while respecting the second law,
extract more work from the heat baths than is otherwise possible in thermal
equilibria. Some of the results above are also generalisable to quantum heat
engines of an infinite number of energy levels including 1-D simple harmonic
oscillators and 1-D infinite square wells.


title: On the Work of Henry P. Stapp

abstract:
  For many years, Henry Stapp and I have been working separately and
independently on mind-centered interpretations of quantum theory. In this
review, I discuss his work and contrast it with my own. There is much that we
agree on, both in the broad problems we have addressed and in some of the
specific details of our analyses of neural physics, but ultimately we disagree
fundamently in our views on mind, matter, and quantum mechanics. In particular,
I discuss our contrasting opinions about the nature and randomness of quantum
events, about relativity theory, and about the many-minds idea. I also suggest
that Stapp's theories are inadequately developed.


title: Quantization of non-Hamiltonian and Dissipative Systems

abstract:
  A generalization of canonical quantization which maps a dynamical operator to
a dynamical superoperator is suggested. Weyl quantization of dynamical
operator, which cannot be represented as Poisson bracket with some function, is
considered. The usual Weyl quantization of observables is a specific case of
suggested quantization. This approach allows to define consistent quantization
procedure for non-Hamiltonian and dissipative systems. Examples of the harmonic
oscillator with friction (generalized Lorenz-Rossler-Leipnik-Newton equation),
the Fokker-Planck-type system and Lorenz-type system are considered.

categories: quant-ph cond-mat.stat-mech hep-th math-ph math.MP physics.chem-ph


title:
Teleportation of the U-NOT Gate and Quantum Cloning: a Computational
  Network and Experimental Realization

abstract:
  We present a computational circuit which realizes contextually the Tele-UNOT
gate and the universal optimal quantum cloning machine (UOQCM). We report the
experimental realization of the probabilistic UOQCM with polarization encoded
qubits. This is achieved by combining on a symmetric beam-splitter the input
qubit with an ancilla in a fully mixed state.


title: Quantum Statistical Field Theory and Combinatorics

abstract:
  This is a set of review notes on combinatorial aspects of Bosonic quantum
field theory. We collect together several related issues concerning moments of
distributions, moments of stochastic processes and Ito's formula, and Green's
functions and cumulant moments in quantum field theory.


title:
Clock Synchronization based on Second-Order Quantum Coherence of
  Entangled Photons

abstract:
  We present an algorithm for synchronizing two clocks based on second-order
quantum interference between entangled photons generated by parametric
down-conversion. The procedure is distinct from the standard Einstein two-way
clock synchronization method in that photon correlations are used to define
simultaneous events in the frame of reference of a Hong-Ou-Mandel (HOM)
interferometer. Once the HOM interferometer is balanced, by use of an
adjustable optical delay in one arm, arrival times of simultaneously generated
photons are recorded by each clock. Classical information on the arrival times
is sent from one clock to the other, and a correlation of arrival times is done
to determine the clock offset.


title:
Demonstration Of A Continuously Guided Atomic Interferometer Using A
  Single-Zone Optical Excitation

abstract:
  We demonstrate an atomic interferometer in which the atom passes through a
single-zone optical beam, consisting of a pair of bichromatic
counter-propagating fields. During the passage, the atomic wave packets in two
distinct internal states trace out split trajectories, guided by the optical
beams, with the amplitude and spread of each wave-packet varying continuously,
producing fringes that can reach a visibility close to unity. We show that the
rotation sensitivity of this continuous interferometer (CI) can be comparable
to that of the Borde-Chu Interferometer (BCI). The relative simplicity of the
CI makes it a potentially better candidate for practical applications.


title: Holonomic quantum computation in the presence of decoherence

abstract:
  We present a scheme to study non-abelian adiabatic holonomies for open
Markovian systems. As an application of our framework, we analyze the
robustness of holonomic quantum computation against decoherence. We pinpoint
the sources of error that must be corrected to achieve a geometric
implementation of quantum computation completely resilient to Markovian
decoherence.


title:
Serial composition of quantum coin-flipping, and bounds on cheat
  detection for bit-commitment

abstract:
  Quantum protocols for coin-flipping can be composed in series in such a way
that a cheating party gains no extra advantage from using entanglement between
different rounds. This composition principle applies to coin-flipping protocols
with cheat sensitivity as well, and is used to derive two results: There are no
quantum strong coin-flipping protocols with cheat sensitivity that is linear in
the bias (or bit-commitment protocols with linear cheat detection) because
these can be composed to produce strong coin-flipping with arbitrarily small
bias. On the other hand, it appears that quadratic cheat detection cannot be
composed in series to obtain even weak coin-flipping with arbitrarily small
bias.


title: Concurrence Vectors in Arbitrary Multipartite Quantum Systems

abstract:
  For a given pure state of multipartite system, the concurrence vector is
defined by employing the defining representation of generators of the
corresponding rotation groups. The norm of concurrence vector is considered as
a measure of entanglement. For multipartite pure state, the concurrence vector
is regarded as the direct sum of concurrence subvectors in the sense that each
subvector is associated with a pair of particles. It is proposed to use the
norm of each subvector as the contribution of the corresponding pair in
entanglement of the system.


title:
Beating quantum limits in interferometers with quantum locking of
  mirrors

abstract:
  The sensitivity in interferometric measurements such as gravitational-wave
detectors is ultimately limited by quantum noise of light. We discuss the use
of feedback mechanisms to reduce the quantum effects of radiation pressure.
Recent experiments have shown that it is possible to reduce the thermal motion
of a mirror by cold damping. The mirror motion is measured with an
optomechanical sensor based on a high-finesse cavity, and reduced by a feedback
loop. We show that this technique can be extended to lock the mirror at the
quantum level. In gravitational-waves interferometers with Fabry-Perot cavities
in each arms, it is even possible to use a single feedback mechanism to lock
one cavity mirror on the other. This quantum locking greatly improves the
sensitivity of the interferometric measurement. It is furthermore insensitive
to imperfections such as losses in the interferometer.


title: Improving the capacity of the ping-pong protocol

abstract:
  We present a quantum communication protocol which keeps all the properties of
the ping-pong protocol [Phys. Rev. Lett. 89, 187902 (2002)] but improves the
capacity doubly as the ping-pong protocol. Alice and Bob can use the variable
measurement basises in control mode to detect Eve's eavesdropping attack. In
message mode, Alice can use one unitary operations to encode two bits
information. Bob only needs to perform a Bell type measurement to decode
Alice's information. A classical message authentification method can protect
this protocol against the eavesdropping hiding in the quantum channel losses
and the denial-of-service (DoS) attack.


title:
Reciprocal Schr\"{o}dinger Equation: Durations of Delay and of Final
  States Formation in Processes of Scattering

abstract:
  The reciprocal Schr\"{o}dinger equation $\partial S(\omega ,{\bf r}%
)/i\partial \omega =\hat{\tau}(\omega ,{\bf r}) S(\omega ,{\bf r})$ for
$S$-matrix with temporal operator instead the Hamiltonian is established via
the Legendre transformation of classical action function. Corresponding
temporal functions are expressed via propagators of interacting fields. Their
real parts $\tau_{1}$are equivalent to the Wigner-Smith delay durations at
process of scattering and imaginary parts $\tau_{2}$ express the duration of
final states formation (dressing). As an apparent example, they can be clearly
interpreted in the oscillator model via polarization ($% \tau_{1}$) and
conductivity ($\tau_{2}$) of medium. The $\tau $-functions are interconnected
by the dispersion relations of Kramers-Kr\"{o}nig type. From them follows, in
particular, that $\tau_{2}$ is twice bigger than the uncertainty value and
thereby is measurable; it must be negative at some tunnel transitions and thus
can explain the observed superluminal transfer of excitations at near field
intervals (M.E.Perel'man. In: arXiv. physics/0309123). The covariant
generalizations of reciprocal equation clarifies the adiabatic hypothesis of
scattering theory as the requirement: $% \tau_{2}\to 0$ at infinity future and
elucidate the physical sense of some renormalization procedures.


title: Improving performance of probabilistic programmable quantum processors

abstract:
  We present a systematic analysis how one can improve performance of
probabilistic programmable quantum processors. We generalize a simple
Vidal-Masanes-Cirac processor that realizes U(1) rotations on a qubit with the
phase of the rotation encoded in a state of the program register. We show how
the probability of success of the probabilistic processor can be enhanced by
using the processor in loops. In addition we show that the same strategy can be
utilized for a probabilistic implementation of non-unitary transformations on
qubits. In addtion, we show that an arbitrary SU(2) transformations of qubits
can be encoded in program state of a universal programmable probabilistic
quantum processor. The probability of success of this processor can be enhanced
by a systematic correction of errors via conditional loops. Finally, we show
that all our results can be generalized also for qudits. In particular, we show
how to implement SU (N) rotations of qudits via programmable quantum processor
and how the performance of the processor can be enhanced when it is used in
loops.


title:
A Hybrid Quantum Search Engine: A Fast Quantum Algorithm for Multiple
  Matches

abstract:
  In this paper we will present a quantum algorithm which works very
efficiently in case of multiple matches within the search space and in the case
of few matches, the algorithm performs classically. This allows us to propose a
hybrid quantum search engine that integrates Grover's algorithm and the
proposed algorithm here to have general performance better that any pure
classical or quantum search algorithm.


title: Generalized measurements via programmable quantum processor

abstract:
  We show that it is possible to control the trade-off between information gain
and disturbance in generalized measurements of qudits by utilizing the
programmable quantum processor. This universal quantum machine allows us to
perform a generalized measurement on the initial state of the input qudit to
construct a Husimi function of this state. The trade-off between the gain and
the disturbance of the qudit is controlled by the initial state of ancillary
system that acts as a program register for the quantum-information distributor.
The trade-off fidelity does not depend on the initial state of the qudit.


title:
Optimized time-dependent perturbation theory for pulse-driven quantum
  dynamics in atomic or molecular systems

abstract:
  We present a time-dependent perturbative approach adapted to the treatment of
intense pulsed interactions. We show there is a freedom in choosing secular
terms and use it to optimize the accuracy of the approximation. We apply this
formulation to a unitary superconvergent technique and improve the accuracy by
several orders of magnitude with respect to the Magnus expansion.


title: An atom fiber for guiding cold neutral atoms

abstract:
  We present an omnidirectional matter wave guide on an atom chip. The
rotational symmetry of the guide is maintained by a combination of two current
carrying wires and a bias field pointing perpendicular to the chip surface. We
demonstrate guiding of thermal atoms around more than two complete turns along
a spiral shaped 25mm long curved path (curve radii down to 200$\mu$m) at
various atom--surface distances (35-450$\mu$m). An extension of the scheme for
the guiding of Bose-Einstein condensates is outlined.


title: Transient time-domain resonances and the time scale for tunneling

abstract:
  Transient {\it time-domain resonances} found recently in time-dependent
solutions to Schr\"{o}dinger's equation are used to investigate the issue of
the tunneling time in rectangular potential barriers. In general, a time
frequency analysis shows that these transients have frequencies above the
cutoff frequency associated with the barrier height, and hence correspond to
non-tunneling processes. We find, however, a regime characterized by the
barrier opacity, where the peak maximum $t_{max}$ of the {\it time-domain
resonance} corresponds to under-the-barrier tunneling. We argue that $t_{max}$
represents the relevant tunneling time scale through the classically forbidden
region.


title: Anomalous quantum states in finite macroscopic systems

abstract:
  We consider finite macroscopic systems, i.e., systems of large but finite
degrees of freedom, which we believe are poorly understood as compared with
small systems and infinite systems. We focus on pure states that do not have
the `cluster property.' Such a pure state is entangled macroscopically, and is
quite anomalous in view of many-body physics because it does not approach any
pure state in the infinite-size limit. However, we often encounter such
anomalous states when studying finite macroscopic systems, such as quantum
computers with many qubits and finite systems that will exhibit symmetry
breaking in the infinite-size limit. We study stabilities of such anomalous
states in general systems. In contrast to the previous works, we obtain general
and universal results, by making full use of the locality of the theory. Using
the general results, we discuss roles of anomalous states in quantum computers,
and the mechanism of emergence of a classical world from quantum theory.


title: Pure Stationary States of Open Quantum Systems

abstract:
  Using Liouville space and superoperator formalism we consider pure stationary
states of open and dissipative quantum systems. We discuss stationary states of
open quantum systems, which coincide with stationary states of closed quantum
systems. Open quantum systems with pure stationary states of linear oscillator
are suggested. We consider stationary states for the Lindblad equation. We
discuss bifurcations of pure stationary states for open quantum systems which
are quantum analogs of classical dynamical bifurcations.

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


title:
Selective pulse implementation of two-qubit gates for spin-3/2 based
  fullerene quantum information processing

abstract:
  We propose two potentially practical schemes to carry out two-qubit quantum
gates on endohedral fullerenes $N@C_{60}$ or $P@C_{60}$. The qubits are stored
in electronic spin degrees of freedom of the doped atom $N$ or $P$. By means of
the magnetic dipolar coupling between two neighboring fullerenes, the two-qubit
controlled-NOT gate and the two-qubit conditional phase gate are performed by
selective microwave pulses assisted by refocusing technique. We will discuss
the necessary additional steps for the universality of our proposal. We will
also show that our proposal is useful for both quantum gating and the readout
of quantum information from the spin-based qubit state.


title: Quantitative complementarity in two-path interferometry

abstract:
  The quantitative formulation of Bohr's complementarity proposed by
Greenberger and Yasin is applied to some physical situations for which
analytical expressions are available. This includes a variety of conventional
double-slit experiments, but also particle oscillations, as in the case of the
neutral-kaon system, and Mott scattering of identical nuclei. For all these
cases, a unified description can be achieved including a new parameter, $\nu$,
which quantifies the effective number of fringes one can observe in each
specific interferometric set-up.


title: Gisin's Theorem for Three Qubits

abstract:
  We present a Theorem that all generalized Greenberger-Horne-Zeilinger states
of a three-qubit system violate a Bell inequality in terms of probabilities.
All pure entangled states of a three-qubit system are shown to violate a Bell
inequality for probabilities; thus, one has Gisin's theorem for three qubits.


title:
Dynamics of quantum correlations and linear entropy in a
  multi-qubit-cavity system

abstract:
  We present a theoretical study of the relationship between entanglement and
entropy in multi-qubit quantum optical systems. Specifically we investigate
quantitative relations between the concurrence and linear entropy for a
two-qubit mixed system, implemented as two two-level atoms interacting with a
single-mode cavity field. The dynamical evolutions of the entanglement and
entropy, are controlled via time-dependent cavity-atom couplings. Our
theoretical findings lead us to propose an alternative measure of entanglement,
which could be used to develop a much needed correlation measure for more
general multi-partite quantum systems.


title:
Extending the class of quantum states that do not allow local realistic
  description

abstract:
  We present the necessary and sufficient condition for the violation of a new
series of multipartite Bell's inequalities with many measurement settings.


title: Effective damping in the Raman cooling of trapped ions

abstract:
  We present a method of treating the interaction of a single three-level ion
with two laser beams. The idea is to apply a unitary transformation such that
the exact transformed Hamiltonian has one of the three levels decoupled for all
values of the detunings. When one takes into account damping, the evolution of
the system is governed by a master equation usually obtained via adiabatic
approximation under the assumption of far-detuned lasers. To go around the
drawbacks of this technique, we use the same unitary transformation to get an
effective master equation.


title: A Note on Quantum Separability

abstract:
  This short note describes a method to tackle the (bipartite) quantum
separability problem. The method can be used for solving the separability
problem in an experimental setting as well as in the purely mathematical
setting. The idea is to invoke the following characterization of entangled
states: A state is entangled if and only if there exists an entanglement
witness that detects it. The method is basically a search for an entanglement
witness that detects the given state.


title: Thermal entanglement in the two-qubit Heisenberg XYZ model

abstract:
  We study the entanglement of a two-qubit one dimensional XYZ Heisenberg chain
in thermal equilibrium at temperature T. We obtain an analytical expression for
the entanglement of formation for this system in terms of the parameters of the
Hamiltonian and T. We show that depending on the relation among the coupling
constants it is possible to increase the amount of entanglement of the system
increasing its anisotropy. We also show numerically that for all sets of the
coupling constants entanglement is a monotonically decreasing function of the
temperature T, proving that we must have at least an external magnetic field in
the z-direction to obtain a behavior where entanglement increases with T.


title:
Light reflection upon a movable mirror as a paradigm for continuous
  variable teleportation network

abstract:
  We present an optomechanical system as a paradigm of three-mode teleportation
network. Quantum state transfer among optical and vibrational modes becomes
possible by exploiting correlations established by radiation pressure.


title: Experimental Hamiltonian identification for controlled two-level systems

abstract:
  We present a strategy to empirically determine the internal and control
Hamiltonians for an unknown two-level system (black box) subject to various
(piecewise constant) control fields when direct readout by measurement is
limited to a single, fixed observable.


title: Time as a Dynamical Variable

abstract:
  There are enough reasons for us to consider time as a dynamical variable or
operator; but according to Pauli's argument the existence of a self-adjoint
time operator is incompatible with the semi-boundedness of Hamiltonian
spectrum. In this article, we study the expressions of time operator and the
definitions of mean time from a general and new viewpoint, make a new comment
on Pauli's objections, and clarify some possible confusion in existing theories
of time in quantum mechanics. From which we try to provide a new gateway for
the conundrum of time in quantum mechanics and reconstruct a unified foundation
for some issues of time.


title:
Lower bounds for randomized and quantum query complexity using
  Kolmogorov arguments

abstract:
  We prove a very general lower bound technique for quantum and randomized
query complexity, that is easy to prove as well as to apply. To achieve this,
we introduce the use of Kolmogorov complexity to query complexity. Our
technique generalizes the weighted, unweighted methods of Ambainis, and the
spectral method of Barnum, Saks and Szegedy. As an immediate consequence of our
main theorem, adversary methods can only prove lower bounds for boolean
functions $f$ in $O(\min(\sqrt{n C_0(f)},\sqrt{n C_1(f)}))$, where $C_0, C_1$
is the certificate complexity, and $n$ is the size of the input. We also derive
a general form of the ad hoc weighted method used by Hoyer, Neerbek and Shi to
give a quantum lower bound on ordered search and sorting.


title:
Molecular Spectra from Rotationally Invariant Hamiltonians Based on the
  Quantum Algebra SUq(2) and Irreducible Tensor Operators under SUq(2)

abstract:
  The rotational invariance under the usual physical angular momentum of the
SUq(2) Hamiltonian for the description of rotational molecular spectra is
explicitly proved and a connection of this Hamiltonian to the formalism of
Amal'sky is provided. In addition, a new Hamiltonian for rotational spectra is
introduced, based on the construction of irreducible tensor operators (ITOs)
under SUq(2) and use of q-deformed tensor products and q-deformed
Clebsch-Gordan coefficients. The rotational invariance of this SUq(2) ITO
Hamiltonian under the usual physical angular momentum is explicitly proved and
a simple closed expression for its energy spectrum (the ``hyperbolic tangent
formula'') is introduced. Numerical tests against an experimental rotational
band of HF are provided.


title: Deformed Harmonic Oscillators for Metal Clusters and Balian-Bloch Theory

abstract:
  The predictions for the shell structure of metal clusters of the
three-dimensional q-deformed harmonic oscillator (3D q-HO), utilizing
techniques of quantum groups and having the symmetry Uq(3)$\supset$SOq(3), are
compared to the restrictions imposed by the periodic orbit theory of Balian and
Bloch, of electrons moving in a spherical cavity. It is shown that agreement
between the predictions of the two models is established through the
introduction of an additional term to the Hamiltonian of the 3D q-HO, which
does not influence the predictions for supershells. This term preserves the
Uq(3)$\supset$SOq(3) symmetry, while in addition it can be derived through a
variational procedure, analogous to the one leading from the usual harmonic
oscillator to the Morse oscillator by introducing the concept of the Variable
Frequency Oscillator (VFO).


title: On the meaning of entanglement in quantum measurement

abstract:
  Measurement interaction between a measured object and a measuring instrument,
if both are initially in a pure state, produces a (final) bipartite entangled
state vector. The quasi-classical part of the correlations in it is connected
with transmission of information in the measurement. But, prior to "reading"
the instrument, there is also purely quantum entanglement in the final state
vector. It is shown that in repeatable measurement quantitatively the
entanglement equals the amount of incompatibility between the measured
observable and the final state. It also equals the amount of incompatibility of
the observable and the initial state of the object.


title:
On Mutual Information in Multipartite Quantum States and Equality in
  Strong Subadditivity of Entropy

abstract:
  The challenge of equality in the strong subadditivity inequality of entropy
is approached via a general additivity of correlation information in terms of
nonoverlapping clusters of subsystems in multipartite states (density
operators). A family of tripartite states satisfying equality is derived.


title:
Entanglement swapping of noisy states: A kind of superadditivity in
  nonclassicality

abstract:
  We address the question as to whether an entangled state that satisfies local
realism will give a violation of the same, after entanglement swapping in a
suitable scenario. We consider such possibility as a kind of superadditivity in
nonclassicality. Importantly, it will indicate that checking for violation of
local realism, in the state obtained after entanglement swapping, can be a
method for detecting entanglement in the input state of the swapping procedure.
We investigate various entanglement swapping schemes, which involve mixed
initial states. The strength of violation of local realism by the state
obtained after entanglement swapping, is compared with the one for the input
states. We obtain a kind of superadditivity of violation of local realism for
Werner states, consequent upon entanglement swapping involving
Greenberger-Horne-Zeilinger state measurements. We also discuss whether
entanglement swapping of specific states may be used in quantum repeaters with
a substantially reduced need to perform the entanglement distillation step.


title: On noise-induced superselection rules

abstract:
  The dynamical properties of a quantum system can be profoundly influenced by
its environment. Usually, the environment provokes decoherence and its action
on the system can often be schematized by adding a noise term in the
Hamiltonian. However, other scenarios are possible: we show that by increasing
the strength of the noise, the Hilbert space of the system gradually splits
into invariant subspaces, among which transitions become increasingly
difficult. The phenomenon is equivalent to the formation of the quantum Zeno
subspaces. We explore the possibility that noise can {\em prevent}, rather than
provoke decoherence.


title: Von Neumann Quantum Processors

abstract:
  Most modern classical processors support so-called von Neumann architecture
with program and data registers. In present work is revisited similar approach
to models of quantum processors. Deterministic programmable quantum gate arrays
are considered as an example. They are also called von Neumann quantum
processors here and use conditional quantum dynamics. Such devices have some
problems with universality, but consideration of hybrid quantum processors,
i.e., models with both continuous and discrete quantum variables resolves the
problems. It is also discussed comparison of such a model of quantum processors
with more traditional approach.


title: Coherence in Microchip Traps

abstract:
  We report the coherent manipulation of internal states of neutral atoms in a
magnetic microchip trap. Coherence lifetimes exceeding 1 s are observed with
atoms at distances of $5-130 \mu$m from the microchip surface. The coherence
lifetime in the chip trap is independent of atom-surface distance within our
measurement accuracy, and agrees well with the results of similar measurements
in macroscopic magnetic traps. Due to the absence of surface-induced
decoherence, a miniaturized atomic clock with a relative stability in the
$10^{-13}$ range can be realized. For applications in quantum information
processing, we propose to use microwave near-fields in the proximity of chip
wires to create potentials that depend on the internal state of the atoms.


title:
Generation of highly non-classical n-photon polarization states by
  super-bunching at a photon bottleneck

abstract:
  It is shown that coherent superpositions of two oppositely polarized n-photon
states can be created by post-selecting the transmission of n independently
generated photons into a single mode transmission line. It is thus possible to
generate highly non-classical n-photon polarization states using only the
bunching effects associated with the bosonic nature of photons. The effects of
mode-matching errors are discussed and the possibility of creating n-photon
entanglement by redistributing the photons into n separate modes is considered.


title: Phase dynamics of a multimode Bose condensate controlled by decay

abstract:
  The relative phase between two uncoupled BE condensates tends to attain a
specific value when the phase is measured. This can be done by observing their
decay products in interference. We discuss exactly solvable models for this
process in cases where competing observation channels drive the phases to
different sets of values. We treat the case of two modes which both emit into
the input ports of two beam splitters, and of a linear or circular chain of
modes. In these latter cases, the transitivity of relative phase becomes an
issue.


title: Quantum Dynamical Entropy of Spin Systems

abstract:
  We investigate a quantum dynamical entropy of one-dimesional quantum spin
systems. We show that the dynamical entropy is bounded from above by a quantity
which is related with group velocity determined by the interaction and mean
entropy of the state.


title:
An Algorithmic Argument for Nonadaptive Query Complexity Lower Bounds on
  Advised Quantum Computation

abstract:
  This paper employs a powerful argument, called an algorithmic argument, to
prove lower bounds of the quantum query complexity of a multiple-block ordered
search problem in which, given a block number i, we are to find a location of a
target keyword in an ordered list of the i-th block. Apart from much studied
polynomial and adversary methods for quantum query complexity lower bounds, our
argument shows that the multiple-block ordered search needs a large number of
nonadaptive oracle queries on a black-box model of quantum computation that is
also supplemented with advice. Our argument is also applied to the notions of
computational complexity theory: quantum truth-table reducibility and quantum
truth-table autoreducibility.


title:
Controlled and secure direct communication using GHZ state and
  teleportation

abstract:
  A theoretical scheme for controlled and secure direct communication is
proposed. The communication is based on GHZ state and controlled quantum
teleportation. After insuring the security of the quantum channel (a set of
qubits in the GHZ state), Alice encodes the secret message directly on a
sequence of particle states and transmits them to Bob supervised by Charlie
using controlled quantum teleportation. Bob can read out the encoded messages
directly by the measurement on his qubits. In this scheme, the controlled
quantum teleportation transmits Alice's message without revealing any
information to a potential eavesdropper. Because there is not a transmission of
the qubit carrying the secret messages between Alice and Bob in the public
channel, it is completely secure for controlled and direct secret communication
if perfect quantum channel is used. The feature of this scheme is that the
communication between two sides depends on the agreement of the third side.


title:
Accuracy of the time-dependent Hartree-Fock approximation for
  uncorrelated initial states

abstract:
  This article concerns the time-dependent Hartree-Fock (TDHF) approximation of
single-particle dynamics in systems of interacting fermions. We find that the
TDHF approximation is accurate when there are sufficiently many particles and
the initial many-particle state is any Gibbs equilibrium state for
noninteracting fermions (with Slater determinants as a special example).
Assuming a bounded two-particle interaction, we obtain a bound on the error of
the TDHF approximation, valid for short times. We further show that the error
of the TDHF approximation vanishes at all times in the mean field limit.


title: Noise and disturbance in quantum mechanics

abstract:
  The operational meaning of some measures of noise and disturbance in
measurements is analyzed and their limitations are pointed out. The cases of
minimal noise and least disturbance are characterized.


title:
Conditional preparation of a non-classical state in the continuous
  variable regime: theoretical study

abstract:
  We study the characteristics of the quantum state of light produced by a
conditional preparation protocol totally performed in the continuous variable
regime. It relies on conditional measurements on quantum intensity correlated
bright twin beams emitted by a non-degenerate OPO above threshold. Analytical
expressions as well as computer simulations of the selected state properties
and preparation efficiency are developed and show that a sub-Poissonian state
can be produced by this technique. Projection onto a given trigger value is
studied and then extended to a finite band. The continuous variable regime
offers the unique possibility to improve dramatically the preparation
efficiency by choosing multiple selection bands and thus to generate a great
number of sub-Poissonian states in parallel.


title:
Noisy Control, the Adiabatic Geometric Phase, and Destruction of the
  Efficiency of Geometric Quantum Computation

abstract:
  We examine the adiabatic dynamics of a quantum system coupled to a noisy
classical control field. A stochastic phase shift is shown to arise in the
off-diagonal elements of the system's density matrix which can cause
decoherence. We derive the condition for onset of decoherence, and identify the
noise properties that drive decoherence. We show how this decoherence mechansim
causes: (1) a dephasing of the observable consequences of the adiabatic
geometric phase; and (2) the loss of computational efficiency of the Shor
algorithm when run on a sufficiently noisy geometric quantum computer.


title:
Casimir effect between dissimilar materials: a test for the proximity
  theorem

abstract:
  We calculate exactly the Casimir force between a spherical particle and a
plane, both with arbitrary dielectric properties, in the non-retarded limit.
Using a Spectral Representation formalism, we show that the Casimir force of a
sphere made of a material A and a plane made of a material B, differ from the
case when the sphere is made of B, and the plane is made of A. The differences
in energy and force show the importance of the geometry, and make evident the
necessity of realistic descriptions of the sphere-plane system beyond the
Proximity Theorem approximation.

categories: quant-ph cond-mat.mtrl-sci cond-mat.other physics.chem-ph


title:
Trade-off between multiple-copy transformation and entanglement
  catalysis

abstract:
  We demonstrate that multiple copies of a bipartite entangled pure state may
serve as a catalyst for certain entanglement transformations while a single
copy cannot. Such a state is termed a "multiple-copy catalyst" for the
transformations. A trade-off between the number of copies of source state and
that of the catalyst is also observed. These results can be generalized to
probabilistic entanglement transformations directly.


title: Quantum cryptography with and without entanglement

abstract:
  Quantum cryptography is reviewed, first using entanglement both for the
intuition and for the experimental realizations. Next, the implementation is
simplified in several steps until it becomes practical. At this point
entanglement has disappeared. This method can be seen as a lesson of Applied
Physics. Finally, security issues, e.g. photon number splitting attacks, and
counter-measures are discussed.


title:
The new wave equations of relativistic and non-relativistic quantum
  mechanics

abstract:
  In this work, we give the wave equations of relativistic and non-relativistic
quantum mechanics which are different from the Schr\"{o}dinger and Klein-Gordon
equation, and we also give the new relativistic wave equation of a charged
particle in an electromagnetic field


title: Correspondence and analyticity

abstract:
  The analyticity properties of the S matrix in the physical region are
determined by the correspondence principle, which asserts that the predictions
of classical physics are generated by taking the classical limit of the
predictions of quantum theory. The analyticity properties deducible in this way
from classical properties include the locations of the singularity surfaces,
the rules for analytic continuation around these surfaces, and the analytic
character (e.g., pole, logarithmic, etc.) of these singulatities. These
important properties of the S matrix are thus derived without using stringent
locality assumptions. The quantum properties are derived by an analytic reverse
engineering of the classical properties.


title:
Preparing high purity initial states for nuclear magnetic resonance
  quantum computing

abstract:
  Here we demonstrate how para-hydrogen can be used to prepare a two-spin
system in an almost pure state which is suitable for implementing nuclear
magnetic resonance (NMR) quantum computation. A 12ns laser pulse is used to
initiate a chemical reaction involving pure para-hydrogn (the nuclear spin
singlet of H2). The product, formed on the microsecond timescale, contains a
hydrogen derived two-spin system with an effective spin-state purity of 0.916.
To achieve a comparable result by direct cooling would require an unmanageable
(in the liquid state) effective spin temperature of 6.4mK or an impractical
magnetic field of 0.45MT at room temperature. The resulting spin state has an
entanglement of formation of 0.822 and cannot be described by local hidden
variable models.


title:
Quantum Superposition of a Mirror and Relative Decoherence (as
  Spontaneous Superposition Breaking)

abstract:
  Marshall et al. gedanken experiment of the quantum superpposition of a mirror
(oscilating part of a Michelson interferometer) interacting with single photon
is consequently interpreted by relative decoherence.Such relative decoherence
(based on the spontaneous superposition breaking (effective hiding)) on the
photon (quantum object) caused by mirror (measurement device) is sufficient to
model real measurement.


title: Quantum key distribution using gaussian-modulated coherent states

abstract:
  Quantum continuous variables are being explored as an alternative means to
implement quantum key distribution, which is usually based on single photon
counting. The former approach is potentially advantageous because it should
enable higher key distribution rates. Here we propose and experimentally
demonstrate a quantum key distribution protocol based on the transmission of
gaussian-modulated coherent states (consisting of laser pulses containing a few
hundred photons) and shot-noise-limited homodyne detection; squeezed or
entangled beams are not required. Complete secret key extraction is achieved
using a reverse reconciliation technique followed by privacy amplification. The
reverse reconciliation technique is in principle secure for any value of the
line transmission, against gaussian individual attacks based on entanglement
and quantum memories. Our table-top experiment yields a net key transmission
rate of about 1.7 megabits per second for a loss-free line, and 75 kilobits per
second for a line with losses of 3.1 dB. We anticipate that the scheme should
remain effective for lines with higher losses, particularly because the present
limitations are essentially technical, so that significant margin for
improvement is available on both the hardware and software.


title:
On energy-momentum spectrum of stationary states with nonvanishing
  current on 1-d lattice systems

abstract:
  On one-dimensional two-way infinite quantum lattice system, a property of
translationally invariant stationary states with nonvanishing current
expectation is investigated. We consider GNS representation with respect to
such a state, on which we have a group of space-time translation unitary
operators. We show that spectrum of the unitary operators, energy-momentum
spectrum with respect to the state, has a singularity at the origin.


title:
Security of Quantum Key Distribution with Coherent States and Homodyne
  Detection

abstract:
  We assess the security of a quantum key distribution protocol relying on the
transmission of Gaussian-modulated coherent states and homodyne detection. This
protocol is shown to be equivalent to a squeezed state protocol based on a CSS
code construction, and is thus provably secure against any eavesdropping
strategy. We also briefly show how this protocol can be generalized in order to
improve the net key rate.


title: Phase shift analysis in Nimtz experiments on tunneling and transmission

abstract:
  For the wave representing particle traveling through any layer system we
calculate appropriate phase shifts comparing two methods. One bases on the
standard scattering theory and is well known another uses unimodular but not
unitary M-monodromy matrix. Both methods are not equivalent due to different
boundary condition - in the one barrier case there exist analytical expressions
showing difference. Authors generalize results to many barrier (layer) system.
Instead of speaking about superluminarity we introduce into the quantum
mechanics so called by us "hurdling problem": can a quantum hurdler in one
dimension be faster then a sprinter (without obstacles) at the same distance.
Relations between wavefunction arguments and delay or advance are shown for
Nimtz systems.


title: Nonlocality of a free atomic wave packet

abstract:
  A simple model allows us to study the nonclassical behavior of slowly moving
atoms interacting with a quantized field. Atom and field become entangled and
their joint state can be identified as a mesoscopic "Schroedinger-cat". By
introducing appropriate observables for atom and field and by analyzing
correlations between them based on a Bell-type inequality we can show the
corresponding nonclassical behavior.


title:
Quantum logic networks for probabilistic and controlled teleportation of
  unknown quantum states

abstract:
  We present simplification schemes for probabilistic and controlled
teleportation of the unknown quantum states of both one-particle and
two-particle and construct efficient quantum logic networks for implementing
the new schemes by means of the primitive operations consisting of single-qubit
gates, two-qubit controlled-not gates, Von Neumann measurement and classically
controlled operations. In these schemes the teleportation are not always
successful but with certain probability.


title:
Quantum Search Algorithm with more Reliable Behaviour using Partial
  Diffusion

abstract:
  In this paper, we will use a quantum operator which performs the inversion
about the mean operation only on a subspace of the system ({\it Partial
Diffusion Operator}) to propose a quantum search algorithm runs in $O(\sqrt
N/M})$ for searching unstructured list of size $N$ with $M$ matches such that,
$1\le M \le N$. We will show that the performance of the algorithm is more
reliable than known quantum search algorithms especially for multiple matches
within the search space. A performance comparison with Grover's algorithm will
be provided.


title: Mixed State Geometric Phase from Thomas Rotations

abstract:
  It is shown that Uhlmann's parallel transport of purifications along a path
of mixed states represented by $2\times 2$ density matrices is just the path
ordered product of Thomas rotations. These rotations are invariant under
hyperbolic translations inside the Bloch sphere that can be regarded as the
Poincar\'e ball model of hyperbolic geometry. A general expression for the
mixed state geometric phase for an {\it arbitrary} geodesic triangle in terms
of the Bures fidelities is derived. The formula gives back the solid angle
result well-known from studies of the pure state geometric phase. It is also
shown that this mixed state anholonomy can be reinterpreted as the pure state
non-Abelian anholonomy of entangled states living in a suitable restriction of
the quaternionic Hopf bundle. In this picture Uhlmann's parallel transport is
just Pancharatnam transport of quaternionic spinors.


title: Separability in Asymmetric Phase-Covariant Cloning

abstract:
  Here, asymmetric phase-covariant quantum cloning machines are defined and
trade-off between qualities of their outputs and its impact on entanglement
properties of the outputs are studies. In addition, optimal families among
these cloners are introduced and also their entanglement properties are
investigated. An explicit proof of optimality is presented for the case of
qubits, which is based on the no-signaling condition. Our optimality proof can
also be used to derive an upper bound on trade-off relations for a more general
class of optimal cloners which clone states on a specific orbit of the Bloch
sphere. It is shown that the optimal cloners of the equatorial states, as in
the case of symmetric phase-covariant cloning, give rise to two separable
clones, and in this sense these states are unique. For these cloners it is
shown that total output is of GHZ-type.


title: Generalised coherent states of spinning relativistic particles

abstract:
  We construct coherent states of the massless and massive representations of
the Poincar\'e group. They are parameterised by points on the classical state
space of spinning particles. Their properties are explored, with special
emphasis on the geometrical structures on the state space.


title: What is a quantum really like?

abstract:
  The hypothesis of quantum self-interference is not directly observable, but
has at least three necessary implications. First, a quantum entity must have no
less than two open paths. Second, the size of the interval between any two
consecutive quanta must be irrelevant. Third, which-path information must not
be available to any observer. All of these predictions have been tested and
found to be false. A similar demonstration is provided for the hypothesis of
quantum erasure. In contrast, if quanta are treated as real particles, acting
as sources of real waves, then all types of interference can be explained with
a single causal mechanism, without logical or experimental inconsistencies.


title: Stopping Light All-Optically

abstract:
  We show that light pulses can be stopped and stored all-optically, with a
process that involves an adiabatic and reversible pulse bandwidth compression
occurring entirely in the optical domain. Such a process overcomes the
fundamental bandwidth-delay constraint in optics, and can generate arbitrarily
small group velocities for light pulses with a given bandwidth, without the use
of any coherent or resonant light-matter interactions. We exhibit this process
in optical resonator systems, where the pulse bandwidth compression is
accomplished only by small refractive index modulations performed at moderate
speeds. (Accepted for publication in Phys. Rev. Lett. Submitted on Sept. 10th
2003)


title: Physics of singular points in quantum mechanics

abstract:
  Defects or junctions in materials serve as a source of interactions for
particles, and in idealized limits they may be treated as singular points
yielding contact interactions. In quantum mechanics, these singularities
accommodate an unexpectedly rich structure and thereby provide a variety of
physical phenomena, especially if their properties are controlled properly.
Based on our recent studies, we present a brief review on the physical aspects
of such quantum singularities in one dimension. Among the intriguing phenomena
that the singularities admit, we mention strong vs weak duality, supersymmetry,
quantum anholonomy (Berry phase), and a copying process by anomalous caustics.
We also show that a partition wall as a singularity in a potential well can
give rise to a quantum force which exhibits an interesting temperature behavior
characteristic to the particle statistics.


title: Reply to comment on Y-00 quantum secure communication protocol

abstract:
  In 2000, an attractive new quantum cryptography was discovered by H.P.Yuen,
which can realize secure communication with high speeds and at long distance by
conventional optical devices. Recently, a criticism of the Yuen protocol, so
called Y-00, was made by Nishioka, and Imai group(Mitsubishi and University of
Tokyo), and they claimed Y-00 is essentially a classical stream cipher. This
paper shows that the claim is incorrect. In particular, it is shown that the
relation $l_i=r_i\oplus \tilde{k}_i$, which is their basis for attack, has no
essential role for any security analysis. In addition, we give a brief
introduction of the general logic for the security of Y-00 as direct encryption
and also for key generation.
  Several industries have started to make a test-bed of Y-00 for digital
optical fiber highway, following Kumar's leading work. We hope that this
discussion encourages experimental works which realize a secure communication
against quantum computer and quantum attacks based on physical principle.


title:
Integrability and PT-symmetry of $N$-Body Systems with Spin-coupling
  $\delta$-Interactions

abstract:
  We study the PT-symmetric boundary conditions for "spin"-related
$\delta$-interactions and the corresponding integrability for both bosonic and
fermionic many-body systems. The spectra and bound states are discussed in
detail for spin-1/2 particle systems.


title:
Canonical Form and Separability of PPT States in 2xMxN Composite Quantum
  Systems

abstract:
  We investigate the canonical forms of positive partial transposition (PPT)
density matrices in ${\cal C}^2 \otimes {\cal C}^M \otimes {\cal C}^N$
composite quantum systems with rank $N$. A general expression for these PPT
states are explicitly obtained. From this canonical form a sufficient
separability condition is presented.


title: Bunching of Photons When Two Beams Pass Through a a Beam Splitter

abstract:
  Dirac has written "Each photon then interferes only with itself. Interference
between two different photons never occurs." Indeed, a practical definition is
that "classical" optics consists of phenomena due to the interference of
photons only with themselves. However, photons obey Bose statistics which
implies a ``nonclassical'' tendency for them to ``bunch''. For a simple example
of nonclassical optical behavior, we consider two pulses of photons of a single
frequency that are simultaneously incident on two sides of a lossless, 50:50
beam splitter.


title: Quantum Systems and Alternative Unitary Descriptions

abstract:
  Motivated by the existence of bi-Hamiltonian classical systems and the
correspondence principle, in this paper we analyze the problem of finding
  Hermitian scalar products which turn a given flow on a Hilbert space into a
unitary one. We show how different invariant Hermitian scalar products give
rise to different descriptions of a quantum system in the Ehrenfest and
  Heisenberg picture.


title: Time of arrival with resonances: Beyond scattering states

abstract:
  Assorted questions: Time as a parameter in Quantum Mechanics. No-Go theorems
for a time operator. Localization, time and causality. Causality violation.
Localization again. Lesson 1: Evading the troubles: Im E finite.
  Lights and shadows of a time operator:"Table-top Spacetime" quantum
mechanics. Biphotons at Berkeley. Time operator build-up. Good news - bad news.
  Lesson 2: We need the resonances to tell one.


title: Bell's inequality and the coincidence-time loophole

abstract:
  This paper analyzes effects of time-dependence in the Bell inequality. A
generalized inequality is derived for the case when coincidence and
non-coincidence [and hence whether or not a pair contributes to the actual
data] is controlled by timing that depends on the detector settings. Needless
to say, this inequality is violated by quantum mechanics and could be violated
by experimental data provided that the loss of measurement pairs through
failure of coincidence is small enough, but the quantitative bound is more
restrictive in this case than in the previously analyzed "efficiency loophole."


title:
Uncertainties in the Measurement of the Momentum and Position of an
  Electron

abstract:
  It has been suggested that the uncertainty in the measurement of a particle's
momentum could be made arbitrarily small by observing the particle at two ends
of an arbitrarily long flight path. However, consideration of the nature of the
detection process shows that the usual limits of the uncertainty principle hold
independent of the length of the flight path.


title: Electron Bubbles in Liquid Helium

abstract:
  When an electron (or positronium atom) is injected into liquid helium with
nearly zero energy, a bubble quickly forms around it. This phenomenon (which
also occurs in liquid hydrogen, liquid neon and possibly in solid helium)
lowers the mobility of the electron to a value similar to that for a positive
ion. We estimate the radius of the bubble at zero pressure and temperature
based on the zero point energy of the electron. If the liquid is held in a
state of negative pressure, the bubble will expand beyond the radius at zero
pressure. We also estimate the negative pressure such that a bubble once formed
will grow without limit.


title: Hertzian Dipole Radiation via the Weizsacker-Williams Model

abstract:
  We use the Weizsacker-Williams method to deduce the radiated power, and its
angular distribution, emitted by an electron of charge that undergoes simple
harmonic motion.


title:
Entanglement and Quantum Phases in the Anisotropic Ferromagnetic
  Heisenberg Chain in the Presence of Domain Walls

abstract:
  We discuss entanglement in the spin-1/2 anisotropic ferromagnetic Heisenberg
chain in the presence of a boundary magnetic field generating domain walls. By
increasing the magnetic field, the model undergoes a first-order quantum phase
transition from a ferromagnetic to a kink-type phase, which is associated to a
jump in the content of entanglement available in the system. Above the critical
point, pairwise entanglement is shown to be non-vanishing and independent of
the boundary magnetic field for large chains. Based on this result, we provide
an analytical expression for the entanglement between arbitrary spins. Moreover
the effects of the quantum domains on the gapless region and for
antiferromagnetic anisotropy are numerically analysed. Finally multiparticle
entanglement properties are considered, from which we establish a
characterization of the critical anisotropy separating the gapless regime from
the kink-type phase.


title:
Comment on "Quantum entropy and special relativity" [by A. Peres, P. F.
  Scudo, and D. R. Terno, Phys. Rev. Lett. 88, 230402 (2002)]

abstract: Two subtleties of this paper are discussed.


title:
Calculation of Band Edge Eigenfunctions and Eigenvalues of Periodic
  Potentials through the Quantum Hamilton - Jacobi Formalism

abstract:
  We obtain the band edge eigenfunctions and the eigenvalues of solvable
periodic potentials using the quantum Hamilton - Jacobi formalism. The
potentials studied here are the Lam{\'e} and the associated Lam{\'e} which
belong to the class of elliptic potentials. The formalism requires an
assumption about the singularity structure of the quantum momentum function
$p$, which satisfies the Riccati type quantum Hamilton - Jacobi equation, $
p^{2} -i \hbar \frac{d}{dx}p = 2m(E- V(x))$ in the complex $x$ plane. Essential
use is made of suitable conformal transformations, which leads to the
eigenvalues and the eigenfunctions corresponding to the band edges in a simple
and straightforward manner. Our study reveals interesting features about the
singularity structure of $p$, responsible in yielding the band edge
eigenfunctions and eigenvalues.


title: Statistical Inverse Problem: Root Approach

abstract:
  Multiparametric statistical model providing stable reconstruction of
parameters by observations is considered. The only general method of this kind
is the root model based on the representation of the probability density as a
squared absolute value of a certain function, which is referred to as a
psi-function in analogy with quantum mechanics. The psi-function is represented
by an expansion in terms of an orthonormal set of functions. It is shown that
the introduction of the psi-function allows one to represent the Fisher
information matrix as well as statistical properties of the estimator of the
state vector (state estimator) in simple analytical forms. The chi-square test
is considered to test the hypotheses that the estimated vector converges to the
state vector of a general population. The method proposed may be applied to its
full extent to solve the statistical inverse problem of quantum mechanics (root
estimator of quantum states). In order to provide statistical completeness of
the analysis, it is necessary to perform measurements in mutually complementing
experiments (according to the Bohr terminology). The maximum likelihood
technique and likelihood equation are generalized in order to analyze quantum
mechanical experiments. It is shown that the requirement for the expansion to
be of a root kind can be considered as a quantization condition making it
possible to choose systems described by quantum mechanics from all statistical
models consistent, on average, with the laws of classical mechanics.


title: New Challenges and Directions in Casimir Force Experiments

abstract:
  This article is divided in three sections. In the first section we briefly
review some high precision experiments on the Casimir force, underlying an
important aspect of the analysis of the data. In the second section we discuss
our recent results in the measurement of the Casimir force using non-trivial
materials. In the third section we present some original ideas for experiments
on new phenomena related to the Casimir effects.


title: Partiality in physics

abstract:
  We revisit the standard axioms of domain theory with emphasis on their
relation to the concept of partiality, explain how this idea arises naturally
in probability theory and quantum mechanics, and then search for a mathematical
setting capable of providing a satisfactory unification of the two.


title: Probabilistic cloning and signalling

abstract:
  We give a proof of impossibility of probabilistic exact $1\to 2$ cloning of
any three different states of a qubit. The simplicity of the proof is due to
the use of a surprising result of remote state preparation [M.-Yong Ye,
Y.-Sheng Zhang and G.-Can Guo, quant-ph/0307027 (2003)]. The result is extented
to higher dimentional cases for special ensemble of states.


title:
Comment on "Quantum nonlocality for a three-particle nonmaximally
  entangled state without inequalities"

abstract:
  In a recent Brief Report, Zheng [S-B. Zheng, PRA 66, 014103 (2002)] has given
a proof of nonlocality without using inequalities for three spin-1/2 particles
in the nonmaximally entangled state |psi> = cos\theta |+++> + i sin\theta |-->.
Here we show that Zheng's proof is not correct. Indeed it is the case that, for
the experiment considered by Zheng, the only state that admits a nonlocality
proof without inequalities is the maximally entangled state.


title: Entanglement as a signature of quantum chaos

abstract:
  We explore the dynamics of entanglement in classically chaotic systems by
considering a multiqubit system that behaves collectively as a spin system
obeying the dynamics of the quantum kicked top. In the classical limit, the
kicked top exhibits both regular and chaotic dynamics depending on the strength
of the chaoticity parameter $\kappa$ in the Hamiltonian. We show that the
entanglement of the multiqubit system, considered for both bipartite and
pairwise entanglement, yields a signature of quantum chaos. Whereas bipartite
entanglement is enhanced in the chaotic region, pairwise entanglement is
suppressed. Furthermore, we define a time-averaged entangling power and show
that this entangling power changes markedly as $\kappa$ moves the system from
being predominantly regular to being predominantly chaotic, thus sharply
identifying the edge of chaos. When this entangling power is averaged over
initial states, it yields a signature of global chaos. The qualitative behavior
of this global entangling power is similar to that of the classical Lyapunov
exponent.


title: Measuring a photonic qubit without destroying it

abstract:
  Measuring the polarisation of a single photon typically results in its
destruction. We propose, demonstrate, and completely characterise a
\emph{quantum non-demolition} (QND) scheme for realising such a measurement
non-destructively. This scheme uses only linear optics and photo-detection of
ancillary modes to induce a strong non-linearity at the single photon level,
non-deterministically. We vary this QND measurement continuously into the weak
regime, and use it to perform a non-destructive test of complementarity in
quantum mechanics. Our scheme realises the most advanced general measurement of
a qubit: it is non-destructive, can be made in any basis, and with arbitrary
strength.


title: Quantum fluctuations in superresolving microscopy with squeezed light

abstract:
  We numerically investigate the role of quantum fluctuations in
superresolution of optical objects. First, we confirm that when quantum
fluctuations are not taken into account, one can easily improve the resolution
by one order of magnitude beyond the diffraction limit. Then we investigate the
standard quantum limit of superresolution which is achieved for illumination of
an object by a light wave in a coherent state. We demonstrate that this limit
can be beyond the diffraction limit. Finally, we show that further improvement
of superresolution beyond the standard quantum limit is possible using the
object illumination by a multimode squeezed light.


title: The Langevin equation from Markovian Quantum Central Limits

abstract:
  This paper has been withdrawn by the author. The central result is now
included in quant-ph/0309056 (as in the journal publication!). An erratum on
the Heisenberg perturbation series estimate is also included therein.


title:
The general structure and ergodic properties of quantum and classical
  mechanics: A unified C*-algebraic approach

abstract:
  Quantum mechanics is essentially a statistical theory. Classical mechanics,
however, is usually not viewed as being inherently statistical. Nevertheless,
the latter can also be formulated statistically. Furthermore, a statistical
formulation of both can be expressed in terms of unital C*-algebras, in which
case it becomes clear that they have the same general structure, with quantum
mechanics a noncommutative generalization of the classical case. In purely
mathematical terms it is seen that quantum mechanics is a noncommutative
generalization of probability theory. The most important insight in this
respect is that the projection postulate of quantum mechanics is a
noncommutative conditional probability. This is the subject of Chapter 1 of the
thesis.
  As ergodic theory (the long term behaviour of a dynamical system) is done in
classical probability theory, it is then also done for noncommutative
probability theory in Chapter 2. In particular, generalizations of Khintchine's
recurrence theorem and a variation thereof for ergodic systems are proved, as
well as various characterizations of noncommutative ergodicity.
  Lastly, in Chapter 3, recurrence and ergodicity is then investigated from a
physical perspective in quantum and classical mechanics, by means of a quantum
mechanical analogue of Liouville's Theorem in classical mechanics which was
suggested in Chapter 1.


title: Inequalities for experimental tests of the Kochen-Specker theorem

abstract:
  We derive inequalities for $n$ spin-1/2 systems under the assumption that the
hidden-variable theoretical joint probability distribution for any pair of
commuting observables is equal to the quantum mechanical one. Fine showed that
this assumption is connected to the no-hidden-variables theorem of Kochen and
Specker (KS theorem). These inequalities give a way to experimentally test the
KS theorem. The fidelity to the Bell states which is larger than 1/2 is
sufficient for the experimental confirmation of the KS theorem. Hence, the
Werner state is enough to test experimentally the KS theorem. Furthermore, it
is possible to test the KS theorem experimentally using uncorrelated states. An
$n$-partite uncorrelated state violates the $n$-partite inequality derived here
by an amount that grows exponentially with $n$.


title:
A Stochastic Liouville Equation Approach for the Effect of Noise in
  Quantum Computations

abstract:
  We propose a model based on a generalized effective Hamiltonian for studying
the effect of noise in quantum computations. The system-environment
interactions are taken into account by including stochastic fluctuating terms
in the system Hamiltonian. Treating these fluctuations as Gaussian Markov
processes with zero mean and delta function correlation times, we derive an
exact equation of motion describing the dissipative dynamics for a system of n
qubits. We then apply this model to study the effect of noise on the quantum
teleportation and a generic quantum controlled-NOT (CNOT) gate. For the quantum
CNOT gate, we study the effect of noise on a set of one- and two-qubit quantum
gates, and show that the results can be assembled together to investigate the
quality of a quantum CNOT gate operation. We compute the averaged gate fidelity
and gate purity for the quantum CNOT gate, and investigate phase, bit-flip, and
flip-flop errors during the CNOT gate operation. The effects of direct
inter-qubit coupling and fluctuations on the control fields are also studied.
We discuss the limitations and possible extensions of this model. In sum, we
demonstrate a simple model that enables us to investigate the effect of noise
in arbitrary quantum circuits under realistic device conditions.


title: Statistical Reconstruction of Qutrits

abstract:
  We discuss a procedure of measurement followed by the reproduction of the
quantum state of a three-level optical system - a frequency- and spatially
degenerate two-photon field. The method of statistical estimation of the
quantum state based on solving the likelihood equation and analyzing the
statistical properties of the obtained estimates is developed. Using the root
approach of estimating quantum states, the initial two-photon state vector is
reproduced from the measured fourth moments in the field . The developed
approach applied to quantum states reconstruction is based on the amplitudes of
mutually complementary processes. Classical algorithm of statistical estimation
based on the Fisher information matrix is generalized to the case of quantum
systems obeying Bohr's complementarity principle. It has been experimentally
proved that biphoton-qutrit states can be reconstructed with the fidelity of
0.995-0.999 and higher.


title: Decoherence of number states in phase-sensitive reservoirs

abstract:
  The non-unitary evolution of initial number states in general Gaussian
environments is solved analytically. Decoherence in the channels is quantified
by determining explicitly the purity of the state at any time. The influence of
the squeezing of the bath on decoherence is discussed. The behavior of coherent
superpositions of number states is addressed as well.


title:
Coherent states superpositions in cavity quantum electrodynamics with
  trapped ions

abstract:
  We investigate how superpositions of motional coherent states naturally arise
in the dynamics of a two-level trapped ion coupled to the quantized field
inside a cavity. We extend our considerations including a more realistic set up
where the cavity is not ideal and photons may leak through its mirrors. We
found that a detection of a photon outside the cavity would leave the ion in a
pure state. The statistics of the ionic state still keeps some interference
effects that might be observed in the weak coupling regime.


title: Effects on the eigenvalues of the quantum bouncer due to dissipation

abstract:
  Effects on the spectra of the quantum bouncer due to dissipation are given
when a linear or quadratic dissipation is taken into account. Classical
constant of motions and Hamiltonians are deduced for these systems and their
quantized eigenvalues are estimated through perturbation theory. we found some
differences when we compare the eigenvalues of these two quantities.


title: On Zurek's derivation of the Born rule

abstract:
  Recently, W. H. Zurek presented a novel derivation of the Born rule based on
a mechanism termed environment-assisted invariance, or "envariance" [W. H.
Zurek, Phys. Rev. Lett. 90(2), 120404 (2003)]. We review this approach and
identify fundamental assumptions that have implicitly entered into it,
emphasizing issues that any such derivation is likely to face.


title:
Decoherence, the measurement problem, and interpretations of quantum
  mechanics

abstract:
  Environment-induced decoherence and superselection have been a subject of
intensive research over the past two decades, yet their implications for the
foundational problems of quantum mechanics, most notably the quantum
measurement problem, have remained a matter of great controversy. This paper is
intended to clarify key features of the decoherence program, including its more
recent results, and to investigate their application and consequences in the
context of the main interpretive approaches of quantum mechanics.


title: A Rabi Oscillation in Four and Five Level Systems

abstract:
  In this paper we consider a general model of an atom with n energy levels
interacting with n-1 external (laser) fields which is a natural extension in
the two and three level systems. We exactly solve the Schr{\" o}dinger equation
to obtain a Rabi oscillation when n = 4 and 5, which will constitute a quantum
logic gate of Quantum Computation based on four and five level systems.


title: Optimal quantum detectors for unambiguous detection of mixed states

abstract:
  We consider the problem of designing an optimal quantum detector that
distinguishes unambiguously between a collection of mixed quantum states. Using
arguments of duality in vector space optimization, we derive necessary and
sufficient conditions for an optimal measurement that maximizes the probability
of correct detection. We show that the previous optimal measurements that were
derived for certain special cases satisfy these optimality conditions. We then
consider state sets with strong symmetry properties, and show that the optimal
measurement operators for distinguishing between these states share the same
symmetries, and can be computed very efficiently by solving a reduced size
semidefinite program.


title: Scattering theory and discrete-time quantum walks

abstract:
  We study quantum walks on general graphs from the point of view of scattering
theory. For a general finite graph we choose two vertices and attach one half
line to each. We are interested in walks that proceed from one half line,
through the graph, to the other. The particle propagates freely on the half
lines but is scattered at each vertex in the original graph. The probability of
starting on one line and reaching the other after n steps can be expressed in
terms of the transmission amplitude for the graph. An example is presented.


title:
Quantum Metrology: Detection of weak forces using Schrodinger Cat
  resources

abstract:
  We investigate the utility of non classical states of simple harmonic
oscillators (a superposition of coherent states) for sensitive force detection.
We find that like squeezed states a superposition of coherent states allows the
detection of displacement measurements at the Heisenberg limit. Entangling many
superpositions of coherent states offers a significant advantage over a single
mode superposition states with the same mean photon number.


title: Spin decoherence by spacetime curvature

abstract:
  A decoherence mechanism caused by spacetime curvature is discussed. The spin
state of a particle is shown to decohere if only the particle moves in a curved
spacetime. In particular, when a particle is near the event horizon of a black
hole, an extremely rapid spin decoherence occurs for an observer who is static
in a Killing time, however slow the particle's motion is.


title: $J$-matrix and Isolated States

abstract:
  We show that a quantum system with nonlocal interaction can have bound states
of unusual type -- Isolated States (IS). IS is a bound state that is not in
correspondence with the $S$-matrix pole. IS can have a positive as well as a
negative energy and can be treated as a generalization of the bound states
embedded in continuum on the case of discrete spectrum states. The formation of
IS in the spectrum of a quantum system is studied using a simple rank--2
separable potential with harmonic oscillator form factors. Some physical
applications are discussed, in particular, we propose a separable $NN$
potential supporting IS that describes the deuteron binding energy and the
s-wave triplet and singlet scattering phase shifts. We use this potential to
examine the so-called problem of the three-body bound state collapse discussed
in literature. We show that the variation of the two-body IS energy causes
drastic changes of the binding energy and of the spectrum of excited states of
the three-nucleon system.


title: Performance of quantum phase gates with cold trapped atoms

abstract:
  We examine the performance of a quantum phase gate implemented with cold
neutral atoms in microtraps, when anharmonic traps are employed and the effects
of finite temperature are also taken into account. Both the anharmonicity and
the temperature are found to pose limitations to the performance of the quantum
gate. We present a quantitative analysis of the problem and show that the phase
gate has a high quality performance for the experimental values that are
presently or in the near future achievable in the laboratory.


title: Toward a Quantum Process Algebra

abstract:
  Quantum computations operate in the quantum world. For their results to be
useful in any way, there is an intrinsic necessity of cooperation and
communication controlled by the classical world. As a consequence, full formal
descriptions of algorithms making use of quantum principles must take into
account both quantum and classical computing components and assemble them so
that they communicate and cooperate. This paper aims at defining a high level
language allowing the description of classical and quantum programming, and
their cooperation. Since process algebras provide a framework to model
cooperating computations and have well defined semantics, they have been chosen
as a basis for this language. Starting with a classical process algebra, this
paper explains how to transform it for including quantum computation. The
result is a quantum process algebra with its operational semantics, which can
be used to fully describe quantum algorithms in their classical context.


title: Disentanglement and Decoherence by Open System Dynamics

abstract:
  The destruction of quantum interference, decoherence, and the destruction of
entanglement both appear to occur under the same circumstances. To address the
connection between these two phenomena, we consider the evolution of arbitrary
initial states of a two-particle system under open system dynamics described by
a class of master equations which produce decoherence of each particle. We show
that all initial states become separable after a finite time, and we produce
the explicit form of the separated state. The result extends and amplifies an
earlier result of Di\'osi. We illustrate the general result by considering the
case in which the initial state is an EPR state (in which both the positions
and momenta of a particle pair are perfectly correlated). This example clearly
illustrates how the spreading out in phase space produced by the environment
leads to certain disentanglement conditions becoming satisfied.


title:
Scalable register initialization for quantum computing in an optical
  lattice

abstract:
  The Mott insulator state created by loading an atomic Bose-Einstein
condensate (BEC) into an optical lattice may be used as a means to prepare a
register of atomic qubits in a quantum computer. Such architecture requires a
lattice commensurately filled with atoms, which corresponds to the insulator
state only in the limit of zero inter-well tunneling. We show that a lattice
with spatial inhomogeneity created by a quadratic magnetic trapping potential
can be used to isolate a subspace in the center which is impervious to
hole-hoping. Components of the wavefunction with more than one atom in any well
can be projected out by selective measurement on a molecular photo-associative
transition. Maintaining the molecular coupling induces a quantum Zeno effect
that can sustain a commensurately filled register for the duration of a quantum
computation.


title:
Phase-dependent decoherence of optical transitions in Pr3+:LaF3 in the
  presence of a driving field

abstract:
  The decoherence times of orthogonally phased components of the optical
transition dipole moment in a two-level system have been observed to differ by
an order of magnitude. This phase anisotropy is observed in coherent transient
experiments where an optical driving field is present during extended periods
of decoherence. The decoherence time of the component of the dipole moment in
phase with the driving field is extended compared to T_2, obtained from
two-pulse photon echoes, in analogy with the spin locking technique of NMR.


title:
Introduction to the basics of entanglement theory in continuous-variable
  systems

abstract:
  We outline the basic questions that are being studied in the theory of
entanglement. Following a brief review of some of the main achievements of
entanglement theory for finite-dimensional quantum systems such as qubits, we
will consider entanglement in infinite-dimensional systems. Asking for a theory
of entanglement in such systems under experimentally feasible operations leads
to the development of the theory of entanglement of Gaussian states. Results of
this theory are presented and the tools that have been developed for it are
applied to a number of problems.


title: Measuring Entangled Qutrits and Their Use for Quantum Bit Commitment

abstract:
  We produce and holographically measure entangled qudits encoded in transverse
spatial modes of single photons. With the novel use of a quantum state
tomography method that only requires two-state superpositions, we achieve the
most complete characterisation of entangled qutrits to date. Ideally, entangled
qutrits provide better security than qubits in quantum bit-commitment: we model
the sensitivity of this to mixture and show experimentally and theoretically
that qutrits with even a small amount of decoherence cannot offer increased
security over qubits.


title: Cyclotomic quantum clock

abstract:
  In the wake of our recent work on cyclotomic effects in quantum phase locking
[M. Planat and H. C. Rosu, Phys. Lett. A 315, 1 (2003)], we briefly discuss
here a cyclotomic extension of the Salecker and Wigner quantum clock. We also
hint on a possible cyclotomic structure of time at the Planck scales


title: Estimation of Complexity for the Ohya-Masuda-Volovich SAT Algorithm

abstract:
  Ohya and Volovich have been proposed a new quantum computation model with
chaos amplification to solve the SAT problem, which went beyond usual quantum
algorithm. In this paper we study the complexity of the SAT algorithm by
counting the steps of computation algorithm rigorously, which was mentioned in
the paper [1,2,3,5,7]For this purpose, we refine the quantum gates treating the
SAT problem step by step.


title: Graviton Emission and Loss of Coherence

abstract:
  The decoherence effect due to emission of gravitons is examined. It shows the
same qualitative features of the QED effect which has already been
investigated, it is obviously much weaker, wholly universal and shows a
stronger energy dependence. The result can be extended to photons, they also
may undergo decoherence due to graviton emission. For this limited aim the
incomplete status of the quantum gravity, in comparison with QED, is not source
of severe difficulties because all the effects are attributed to the infrared
sector of the dynamics.


title: Designing robust gate implementations for quantum information processing

abstract:
  Quantum information processing systems are often operated through time
dependent controls; choosing these controls in a way that makes the resulting
operation insensitive to variations in unknown or uncontrollable system
parameters is an important prerequisite for obtaining high-fidelity gate
operations. In this article we present a numerical method for constructing such
robust control sequences for a quite general class of quantum information
processing systems. As an application of the method we have designed a robust
implementation of a phase-shift operation central to rare earth quantum
computing, an ensemble quantum computing system proposed by Ohlsson et. al.
[Opt. Comm. 201, 71 (2002)]. In this case the method has been used to obtain a
high degree of insensitivity with respect to differences between ensemble
members, but it is equally well suited for quantum computing with a single
physical system.


title:
Quantum averaging and resonances: two-level atom in a one-mode quantized
  field

abstract:
  We construct a non-perturbative approach based on quantum averaging combined
with resonant transformations to detect the resonances of a given Hamiltonian
and to treat them. This approach, that generalizes the rotating-wave
approximation, takes into account the resonances at low field and also at high
field (non-linear resonances). This allows to derive effective Hamiltonians
that contain the qualitative features of the spectrum, i.e. crossings and
avoided crossings, as a function of the coupling constant. At a second stage
the precision of the spectrum can be improved quantitatively by standard
perturbative methods like contact transformations. We illustrate this method to
determine the spectrum of a two-level atom interacting with a single mode of a
quantized field.


title:
Conserved Current Densities, Localization Probabilities, and a New
  Global Gauge Symmetry of Klein-Gordon Fields

abstract:
  For free Klein-Gordon fields, we construct a one-parameter family of
conserved current densities $J_a^\mu$, with $a\in(-1,1)$, and use the latter to
yield a manifestly covariant expression for the most general positive-definite
and Lorentz-invariant inner product on the space of solutions of the
Klein-Gordon equation. Employing a recently developed method of constructing
the Hilbert space and observables for Klein-Gordon fields, we then obtain the
probability current density ${\cal J}_a^\mu$ for the localization of a
Klein-Gordon field in space. We show that in the nonrelativistic limit both
$J_a^\mu$ and ${\cal J}_a^\mu$ tend to the probability current density for the
localization of a nonrelativistic free particle in space, but that unlike
$J_a^\mu$ the current density ${\cal J}_a^\mu$ is neither covariant nor
conserved. Because the total probability may be obtained by integrating either
of these two current densities over the whole space, the conservation of the
total probability may be viewed as a consequence of the local conservation of
$J_a^\mu$. The latter is a manifestation of a previously unnoticed global gauge
symmetry of the Klein-Gordon fields. The corresponding gauge group is U(1) if
the parameter $a$ is rational. It is the multiplicative group of positive real
numbers if $a$ is irrational. We also discuss an extension of our results to
Klein-Gordon fields minimally coupled to an electromagnetic field.

categories: quant-ph gr-qc hep-th math-ph math.MP nucl-ex


title: Exact Solutions of Holonomic Quantum Computation

abstract:
  Holonomic quantum computation is analyzed from geometrical viewpoint. We
develop an optimization scheme in which an arbitrary unitary gate is
implemented with a small circle in a complex projective space. Exact solutions
for the Hadamard, CNOT and 2-qubit discrete Fourier transform gates are
explicitly constructed.


title:
Pulse-driven near-resonant quantum adiabatic dynamics: lifting of
  quasi-degeneracy

abstract:
  We study the quantum dynamics of a two-level system driven by a pulse that
starts near-resonant for small amplitudes, yielding nonadiabatic evolution, and
induces an adiabatic evolution for larger amplitudes. This problem is analyzed
in terms of lifting of degeneracy for rising amplitudes. It is solved exactly
for the case of linear and exponential rising. Approximate solutions are given
in the case of power law rising. This allows us to determine approximative
formulas for the lineshape of resonant excitation by various forms of pulses
such as truncated trig-pulses. We also analyze and explain the various
superpositions of states that can be obtained by the Half Stark Chirped Rapid
Adiabatic Passage (Half-SCRAP) process.


title: Continuity of quantum conditional information

abstract:
  We prove continuity of quantum conditional information $S(\rho^{12}| \rho^2)$
with respect to the uniform convergence of states and obtain a bound which is
independent of the dimension of the second party. This can, e.g., be used to
prove the continuity of squashed entanglement.


title:
Recurrent fourth-order interference dips and peaks with a comb-like
  two-photon entangled state

abstract:
  We demonstrate full selective control over the constructive or destructive
character of fourth-order recurring interferences in a modified version of a
HOM interferometer using comb-like two-photon states. The comb
spectral/temporal structure is obtained by inserting an etalon cavity in the
signal path of an entangled photon pair obtained by pulsed spontaneous
parametric down-conversion. Both a simple qualitative discussion and a complete
theoretical derivation are used to explain and analyze the experimental data.