title: Sine distance for quantum states

abstract:
  We thoroughly analyse the distance between quantum states that has been
applied to state-dependent cloning and partly studied in the previous work of
the author [Phys. Rev. A 66, 042304 (2002)]. Elementary proofs of its
significant properties are given.


title: Practical Evaluation of Security for Quantum Key Distribution

abstract:
  Many papers proved the security of quantum key distribution (QKD) system, in
the asymptotic framework. The degree of the security has not been discussed in
the finite coding-length framework, sufficiently. However, to guarantee any
implemented QKD system requires, it is needed to evaluate a protocol with a
finite coding-length. For this purpose, we derive a tight upper bound of the
eavesdropper's information. This bound is better than existing bounds. We also
obtain the exponential rate of the eavesdropper's information. Further, we
approximate our bound by using the normal distribution.


title:
The quantum measurement problem and physical reality: a computation
  theoretic perspective

abstract:
  Is the universe computable? If yes, is it computationally a polynomial place?
In standard quantum mechanics, which permits infinite parallelism and the
infinitely precise specification of states, a negative answer to both questions
is not ruled out. On the other hand, empirical evidence suggests that
NP-complete problems are intractable in the physical world. Likewise,
computational problems known to be algorithmically uncomputable do not seem to
be computable by any physical means. We suggest that this close correspondence
between the efficiency and power of abstract algorithms on the one hand, and
physical computers on the other, finds a natural explanation if the universe is
assumed to be algorithmic; that is, that physical reality is the product of
discrete sub-physical information processing equivalent to the actions of a
probabilistic Turing machine. This assumption can be reconciled with the
observed exponentiality of quantum systems at microscopic scales, and the
consequent possibility of implementing Shor's quantum polynomial time algorithm
at that scale, provided the degree of superposition is intrinsically, finitely
upper-bounded. If this bound is associated with the quantum-classical divide
(the Heisenberg cut), a natural resolution to the quantum measurement problem
arises. From this viewpoint, macroscopic classicality is an evidence that the
universe is in BPP, and both questions raised above receive affirmative
answers. A recently proposed computational model of quantum measurement, which
relates the Heisenberg cut to the discreteness of Hilbert space, is briefly
discussed. A connection to quantum gravity is noted. Our results are compatible
with the philosophy that mathematical truths are independent of the laws of
physics.


title:
Is there a prescribed parameter's space for the adiabatic geometric
  phase?

abstract:
  The Aharonov-Anandan and Berry phases are determined for the cyclic motions
of a non-relativistic charged spinless particle evolving in the superposition
of the fields produced by a Penning trap and a rotating magnetic field.
Discussion about the selection of the parameter's space and the relationship
between the Berry phase and the symmetry of the binding potential is given.


title: An open--quantum--system formulation of particle decay

abstract:
  We consider an open quantum system which contains unstable states. The time
evolution of the system can be described by an effective non-hermitian
Hamiltonian H_{eff}, in accord with the Wigner--Weisskopf approximation, and an
additional term of the Lindblad form, the socalled dissipator. We show that,
after enlarging the original Hilbert space by states which represent the decay
products of the unstable states, the non-hermitian part of H_{eff} --the
``particle decay''-- can be incorporated into the dissipator of the enlarged
space via a specific Lindblad operator. Thus the new formulation of the time
evolution on the enlarged space has a hermitian Hamiltonian and is probability
conserving. The equivalence of the new formulation with the original one
demonstrates that the time evolution which is governed by a non-hermitian
Hamiltonian and a dissipator of the Lindblad form is nevertheless completely
positive, just as systems with hermitian Hamiltonians.


title:
A curvature dependent bound for entanglement change in classically
  chaotic systems

abstract:
  Using the Calogero-Moser model and the Nakamura equations for a multi-partite
quantum system, we prove an inequality between the mean bi-partite entanglement
rate of change under the variation of a critical parameter and the
level-curvature. This provides an upper bound for the rate of production or
distraction of entanglement induced dynamically. We then investigate the
dependence of the upper bound on the degree of chaos of the system, which in
turn, through the inequality, gives a measure of the stability of the entangled
state. Our analytical results are supported with extensive numerical
calculations.


title: Highly nonclassical photon statistics in parametric down conversion

abstract:
  We use photon counters to obtain the joint photon counting statistics from
twin-beam non-degenerate parametric down conversion, and we demonstrate
directly, and with no auxiliary assumptions, that these twin beams are
nonclassical.


title:
Test of nonlocality for a continuos-variable state based on arbitrary
  number of measurement outcomes

abstract:
  We propose a scheme to test Bell's inequalities for an arbitrary number of
measurement outcomes on entangled continuous variable states. The Bell
correlation functions are expressible in terms of phase-space quasiprobability
functions with complex ordering parameter, which can experimentally be
determined both directly via local CV-qubit interaction or indirectly via
tomographic reconstructions. We demonstrate that continuous-variable systems
can give stronger violations of these Bell's inequalities than of the ones
developed for two-outcome observables. Thus, while keeping the feasibility of
the phase-space approach, our scheme increases its efficiency.


title: Efficiency of quantum and classical transport on graphs

abstract:
  We propose a measure to quantify the efficiency of classical and quantum
mechanical transport processes on graphs. The measure only depends on the
density of states (DOS), which contains all the necessary information about the
graph. For some given (continuous) DOS, the measure shows a power law behavior,
where the exponent for the quantum transport is twice the exponent of its
classical counterpart. For small-world networks, however, the measure shows
rather a stretched exponential law but still the quantum transport outperforms
the classical one. Some finite tree-graphs have a few highly degenerate
eigenvalues, such that, on the other hand, on them the classical transport may
be more efficient than the quantum one.


title:
Theory for quantum state of photon pairs generated from spontaneous
  parametric down conversion nonlinear process

abstract:
  We present a theory for the quantum state of photon pairs generated from
spontaneous parametric down conversion nonlinear process in which the influence
of the final sizes of nonlinear optical crystals on eigen optical modes is
explicitly taken into consideration. We find that these photon pairs are not in
entangled quantum states. Polarization correlations between the signal beam and
the idler beam are explained. We also show that the two photons generated from
SPDC are not spatially separated, therefore the polarization correlation
between the signal and idler beams is not an evidence for quantum non-locality.


title: Finite Quantum Dynamics

abstract:
  We general-quantize the dynamics of the quantum harmonic oscillator to obtain
a covariant finite quantum dynamics in a finite quantum time. The usual central
(``superselected'') time results from a self-organization. Unitarity
necessarily fails, imperceptibly for middle times and grossly near the
beginning and end of time. Time and energy interconvert during space-time
decondensation or melt-down, at a rate governed by a constant like the Planck
power.


title: Fidelity of Quantum Interferometers

abstract:
  For a generic interferometer, the conditional probability density
distribution, $p(\phi|m)$, for the phase $\phi$ given measurement outcome $m$,
will generally have multiple peaks. Therefore, the phase sensitivity of an
interferometer cannot be adequately characterized by the standard deviation,
such as $\Delta\phi\sim 1/\sqrt{N}$ (the standard limit), or $\Delta\phi\sim
1/N$ (the Heisenberg limit). We propose an alternative measure of phase
sensitivity--the fidelity of an interferometer--defined as the Shannon mutual
information between the phase shift $\phi$\ and the measurement outcomes $m$.
As an example application of interferometer fidelity, we consider a generic
optical Mach-Zehnder interferometer, used as a sensor of a classical field. We
find the surprising result that an entangled {\it N00N} state input leads to a
lower fidelity than a Fock state input, for the same photon number.


title: Universal measurement of quantum correlations of radiation

abstract:
  A measurement technique is proposed which, in principle, allows one to
observe the general space-time correlation properties of a quantized radiation
field. Our method, called balanced homodyne correlation measurement, unifies
the advantages of balanced homodyne detection with those of homodyne
correlation measurements.


title: Universal and phase covariant superbroadcasting for mixed qubit states

abstract:
  We describe a general framework to study covariant symmetric broadcasting
maps for mixed qubit states. We explicitly derive the optimal N to M
superbroadcasting maps, achieving optimal purification of the single-site
output copy, in both the universal and the phase covariant cases. We also study
the bipartite entanglement properties of the superbroadcast states.


title: Optimization and Realization of Quantum Devices

abstract:
  This manuscript must be intended as an informal review of the research works
carried out during three years of PhD. "Informal" in the sense that technical
proofs are often omitted (they can be found in the papers) as one could do for
a presentation in a public talk. Clearly, some background of Quantum Mechanics
is needed, even if I tried to minimize the prerequisites.


title:
Entanglement dynamics of electron-electron scattering in low-dimensional
  semiconductor systems

abstract:
  We perform the quantitative evaluation of the entanglement dynamics in
scattering events between two insistinguishable electrons interacting via
Coulomb potential in 1D and 2D semiconductor nanostructures. We apply a
criterion based on the von Neumann entropy and the Schmidt decomposition of the
global state vector suitable for systems of identical particles. From the
timedependent numerical solution of the two-particle wavefunction of the
scattering carriers we compute their entanglement evolution for different spin
configurations: two electrons with the same spin, with different spin, singlet,
and triplet spin state. The procedure allows to evaluate the mechanisms that
govern entanglement creation and their connection with the characteristic
physical parameters and initial conditions of the system. The cases in which
the evolution of entanglement is similar to the one obtained for
distinguishable particles are discussed.


title: Local aspects of disentanglement induced by spontaneous emission

abstract:
  We consider spontaneous emission of two two-level atoms interacting with
vacuum fluctuations. We study the process of disentanglement in this system and
show the possibility of changing disentanglement time by local operations.


title:
Non-catastrophic Encoders and Encoder Inverses for Quantum Convolutional
  Codes

abstract:
  We present an algorithm to construct quantum circuits for encoding and
inverse encoding of quantum convolutional codes. We show that any quantum
convolutional code contains a subcode of finite index which has a
non-catastrophic encoding circuit. Our work generalizes the conditions for
non-catastrophic encoders derived in a paper by Ollivier and Tillich
(quant-ph/0401134) which are applicable only for a restricted class of quantum
convolutional codes. We also show that the encoders and their inverses
constructed by our method naturally can be applied online, i.e., qubits can be
sent and received with constant delay.


title: One-and-a-half quantum de Finetti theorems

abstract:
  We prove a new kind of quantum de Finetti theorem for representations of the
unitary group U(d). Consider a pure state that lies in the irreducible
representation U_{mu+nu} for Young diagrams mu and nu. U_{mu+nu} is contained
in the tensor product of U_mu and U_nu; let xi be the state obtained by tracing
out U_nu. We show that xi is close to a convex combination of states Uv, where
U is in U(d) and v is the highest weight vector in U_mu. When U_{mu+nu} is the
symmetric representation, this yields the conventional quantum de Finetti
theorem for symmetric states, and our method of proof gives near-optimal bounds
for the approximation of xi by a convex combination of product states. For the
class of symmetric Werner states, we give a second de Finetti-style theorem
(our 'half' theorem); the de Finetti-approximation in this case takes a
particularly simple form, involving only product states with a fixed spectrum.
Our proof uses purely group theoretic methods, and makes a link with the
shifted Schur functions. It also provides some useful examples, and gives some
insight into the structure of the set of convex combinations of product states.


title: Slow light in paraffin-coated Rb vapor cells

abstract:
  We present preliminary results from an experimental study of slow light in
anti-relaxation-coated Rb vapor cells, and describe the construction and
testing of such cells. The slow ground state decoherence rate allowed by coated
cell walls leads to a dual-structured electromagnetically induced transparency
(EIT) spectrum with a very narrow (<100 Hz) transparency peak on top of a broad
pedestal. Such dual-structure EIT permits optical probe pulses to propagate
with greatly reduced group velocity on two time scales. We discuss ongoing
efforts to optimize the pulse delay in such coated cell systems.


title: A quantum study of multi-bit phase coding for optical storage

abstract:
  We propose a scheme which encodes information in both the longitudinal and
spatial transverse phases of a continuous-wave optical beam. A split
detector-based interferometric scheme is then introduced to optimally detect
both encoded phase signals. In contrast to present-day optical storage devices,
our phase coding scheme has an information storage capacity which scales with
the power of the read-out optical beam. We analyse the maximum number of
encoding possibilities at the shot noise limit. In addition, we show that using
squeezed light, the shot noise limit can be overcome and the number of encoding
possibilities increased. We discuss a possible application of our phase coding
scheme for increasing the capacities of optical storage devices.


abstract:
  In this paper we investigate the occurrence of the Zeno and anti-Zeno effects
for quantum Brownian motion. We single out the parameters of both the system
and the reservoir governing the crossover between Zeno and anti-Zeno dynamics.
We demonstrate that, for high reservoir temperatures, the short time behaviour
of environment induced decoherence is the ultimate responsible for the
occurrence of either the Zeno or the anti-Zeno effect. Finally we suggest a way
to manipulate the decay rate of the system and to observe a controlled
continuous passage from decay suppression to decay acceleration using
engineered reservoirs in the trapped ion context .


title: Circuit analysis of quantum measurement

abstract:
  We develop a circuit theory that enables us to analyze quantum measurements
on a two-level system and on a continuous-variable system on an equal footing.
As a measurement scheme applicable to both systems, we discuss a swapping state
measurement which exchanges quantum states between the system and the measuring
apparatus before the apparatus meter is read out. This swapping state
measurement has an advantage in gravitational-wave detection over contractive
state measurement in that the postmeasurement state of the system can be set to
a prescribed one, regardless of the outcome of the measurement.


title:
On the Absence of Spurious Eigenstates in an Iterative Algorithm
  Proposed By Waxman

abstract:
  We discuss a remarkable property of an iterative algorithm for eigenvalue
problems recently advanced by Waxman that constitutes a clear advantage over
other iterative procedures. In quantum mechanics, as well as in other fields,
it is often necessary to deal with operators exhibiting both a continuum and a
discrete spectrum. For this kind of operators, the problem of identifying
spurious eigenpairs which appear in iterative algorithms like the Lanczos
algorithm does not occur in the algorithm proposed by Waxman.


title:
Matrix Tensor Product Approach to the Equivalence of Multipartite States
  under Local Unitary Transformations

abstract:
  The equivalence of multipartite quantum mixed states under local unitary
transformations is studied. A criterion for the equivalence of non-degenerate
mixed multipartite quantum states under local unitary transformations is
presented.


title: R Function Related to Entanglement of Formation

abstract:
  By investigating the convex property of the function R, appeared in computing
the entanglement of formation for isotropic states in Phys. Rev. Lett. 85, 2625
(2000), and a tight lower bound of entanglement of formation for arbitrary
bipartite mixed states in Phys. Rev. Lett. 95, 210501 (2005), we show
analytically that the very nice results in these papers are valid not only for
dimensions 2 and 3 but any dimensions.


title: Polarization ququarts

abstract:
  We discuss the concept of polarization states of four-dimensional quantum
systems based on frequency non-degenerate biphoton field. Several quantum
tomography protocols were developed and implemented for measurement of an
arbitrary state of ququart. A simple method that does not rely on
interferometric technique is used to generate and measure the sequence of
states that can be used for quantum communication purposes.


title:
Multi-level, multi-party singlets as ground states and their role in
  entanglement distribution

abstract:
  We show that a singlet of many multi-level quantum systems arises naturally
as the ground state of a physically-motivated Hamiltonian. The Hamiltonian
simply exchanges the states of nearest-neighbours in some network of qudits
(d-level systems); the results are independent of the strength of the couplings
or the network's topology. We show that local measurements on some of these
qudits project the unmeasured qudits onto a smaller singlet, regardless of the
choice of measurement basis at each measurement. It follows that the
entanglement is highly persistent, and that through local measurements, a large
amount of entanglement may be established between spatially-separated parties
for subsequent use in distributed quantum computation.


title: Information Transfer Implies State Collapse

abstract:
  We attempt to clarify certain puzzles concerning state collapse and
decoherence. In open quantum systems decoherence is shown to be a necessary
consequence of the transfer of information to the outside; we prove an upper
bound for the amount of coherence which can survive such a transfer. We claim
that in large closed systems decoherence has never been observed, but we will
show that it is usually harmless to assume its occurrence. An independent
postulate of state collapse over and above Schroedinger's equation and the
probability interpretation of quantum states, is shown to be redundant.


title: Detecting Broken PT-Symmetry

abstract:
  A fundamental problem in the theory of PT-invariant quantum systems is to
determine whether a given system `respects' this symmetry or not. If not, the
system usually develops non-real eigenvalues. It is shown in this contribution
how to algorithmically detect the existence of complex eigenvalues for a given
PT-symmetric matrix. The procedure uses classical results from stability theory
which qualitatively locate the zeros of real polynomials in the complex plane.
The interest and value of the present approach lies in the fact that it avoids
diagonalization of the Hamiltonian at hand.


title:
Generation of strong short coherent terahertz pulses in Ladder-Lambda
  and Double-Lambda systems

abstract:
  We show that coherently driven atomic or molecular media potentially yield
strong controllable short pulses of THz radiation. The method is based on
excitation of maximal quantum coherence in a gas medium by optical pulses and
coherent scattering of infra-red radiation to produce pulses of THz radiation.
The pulses have the energies range from several nJ to $\mu$J and time durations
from several fs to ns at room temperature.


title: A characterization of global entanglement

abstract:
  We define a set of $2^{n-1}-1$ entanglement monotones for $n$ qubits and give
a single measure of entanglement in terms of these. This measure is zero except
on globally entangled (fully inseparable) states. This measure is compared to
the Meyer-Wallach measure for two, three, and four qubits. We determine the
four-qubit state, symmetric under exchange of qubit labels, which maximizes
this measure. It is also shown how the elementary monotones may be computed as
a function of observable quantities. We compute the magnitude of our measure
for the ground state of the four-qubit superconducting experimental system
investigated in [M. Grajcar et al., Phys. Rev. Lett._96_, 047006 (2006)], and
thus confirm the presence of global entanglement in the ground state.


title: Self-Reduction Rate of a Microtubule

abstract:
  We formulate and study a quantum field theory of a microtubule, a basic
element of living cells. Following the quantum theory of consciousness by
Hameroff and Penrose, we let the system to reduce to one of the classical
states without measurement if certain conditions are
satisfied(self-reductions), and calculate the self-reduction time $\tau_N$ (the
mean interval between two successive self-reductions) of a cluster consisting
of more than $N$ neighboring tubulins (basic units composing a microtubule).
$\tau_N$ is interpreted there as an instance of the stream of consciousness. We
analyze the dependence of $\tau_N$ upon $N$ and the initial conditions, etc.
For relatively large electron hopping amplitude, $\tau_N$ obeys a power law
$\tau_N \sim N^b$, which can be explained by the percolation theory. For
sufficiently small values of the electron hopping amplitude, $\tau_N$ obeys an
exponential law, $\tau_N \sim \exp(c' N)$. By using this law, we estimate the
condition for $\tau_N $ to take realistic values $\tau_N$
\raisebox{-0.5ex}{$\stackrel{>}{\sim}$} $10^{-1}$ sec as $N$ \raisebox{-0.5ex}
{$\stackrel{>}{\sim}$} 1000.


title:
On the identification of the ground state based on occupation
  probabilities: An investigation of Smith's apparent counterexamples

abstract:
  We study a set of truncated matrices, given by Smith~\cite{Smith2005}, in
connection to an identification criterion for the ground state in our proposed
quantum adiabatic algorithm for Hilbert's tenth problem. We identify the origin
of the trouble for this truncated example and show that for a suitable choice
of some parameter it can always be removed. We also argue that it is only an
artefact of the truncation of the underlying Hilbert spaces, through showing
its sensitivity to different boundary conditions available for such a
truncation. It is maintained that the criterion, in general, should be
applicable provided certain conditions are satisfied. We also point out that,
apart from this one, other criteria serving the same identification purpose may
also be available.


title: A mathematical proof for a ground-state identification criterion

abstract:
  We give a mathematical proof for an identification criterion by a probability
measure for the ground state among an infinite number of available states, or a
finitely truncated number with appropriate boundary conditions, in a quantum
adiabatic algorithm for Hilbert's tenth problem.


title:
A possible general approach regarding the conformability of angular
  observables with mathematical rules of quantum mechanics

abstract:
  The conformability of angular observales (angular momentum and azimuthal
angle) with the mathematical rules of quantum mechanics is a question which
still rouses debates. It is valued negatively within the existing approaches
which are restricted by two amendable presumptions. If the respective
presumptions are removed one can obtain a general approach in which the
mentioned question is valued positively.


title:
One-step generation of high-quality squeezed and EPR states in cavity
  QED

abstract:
  We show how to generate bilinear (quadratic) Hamiltonians in cavity quantum
electrodynamics (QED) through the interaction of a single driven three-level
atom with two (one) cavity modes. With this scheme it is possible to generate
one-mode mesoscopic squeezed superpositions, two-mode entanglements, and
two-mode squeezed vacuum states (such the original EPR state), without the need
for Ramsey zones and external parametric amplification. The degree of squeezing
achieved is up to 99% with currently feasible experimental parameters and the
errors due to dissipative mechanisms become practically negligible.


title: Damped quantum harmonic oscillator

abstract:
  In the framework of the Lindblad theory for open quantum systems the damping
of the harmonic oscillator is studied. A generalization of the fundamental
constraints on quantum mechanical diffusion coefficients which appear in the
master equation for the damped quantum oscillator is presented; the
Schr\"odinger and Heisenberg representations of the Lindblad equation are given
explicitly. On the basis of these representations it is shown that various
master equations for the damped quantum oscillator used in the literature are
particular cases of the Lindblad equation and that the majority of these
equations are not satisfying the constraints on quantum mechanical diffusion
coefficients. Analytical expressions for the first two moments of coordinate
and momentum are also obtained by using the characteristic function of the
Lindblad master equation. The master equation is transformed into Fokker-Planck
equations for quasiprobability distributions. A comparative study is made for
the Glauber $P$ representation, the antinormal ordering $Q$ representation and
the Wigner $W$ representation. It is proven that the variances for the damped
harmonic oscillator found with these representations are the same. By solving
the Fokker-Planck equations in the steady state, it is shown that the
quasiprobability distributions are two-dimensional Gaussians with widths
determined by the diffusion coefficients. The density matrix is represented via
a generating function, which is obtained by solving a time-dependent linear
partial differential equation derived from the master equation. Illustrative
examples for specific initial conditions of the density matrix are provided.


title:
Tomographic test of Bell's inequality for a time-delocalized single
  photon

abstract:
  Time-domain balanced homodyne detection is performed on two well-separated
temporal modes sharing a single photon. The reconstructed density matrix of the
two-mode system is used to prove and quantify its entangled nature, while the
Wigner function is employed for an innovative tomographic test of Bell's
inequality based on the theoretical proposal by Banaszek and Wodkiewicz [Phys.
Rev. Lett. 82, 2009 (1999)]. Provided some auxiliary assumptions are made, a
clear violation of Banaszek-Bell's inequality is found.


title:
Quantum Mechanics of Klein-Gordon Fields I: Hilbert Space, Localized
  States, and Chiral Symmetry

abstract:
  We derive an explicit manifestly covariant expression for the most general
positive-definite and Lorentz-invariant inner product on the space of solutions
of the Klein-Gordon equation. This expression involves a one-parameter family
of conserved current densities J_a^\mu, with a\in(-1,1), that are analogous to
the chiral current density for spin half fields. The conservation of J_a^\mu is
related to a global gauge symmetry of the Klein-Gordon fields whose gauge group
is U(1) for rational a and the multiplicative group of positive real numbers
for irrational a. We show that the associated gauge symmetry is responsible for
the conservation of the total probability of the localization of the field in
space. This provides a simple resolution of the paradoxical situation resulting
from the fact that the probability current density for free scalar fields is
neither covariant nor conserved. Furthermore, we discuss the implications of
our approach for free real scalar fields offering a direct proof of the
uniqueness of the relativistically invariant positive-definite inner product on
the space of real Klein-Gordon fields. We also explore an extension of our
results to scalar fields minimally coupled to an electromagnetic field.


title: Spontaneous Pair Creation Revisited

abstract:
  Recently the so called Spontaneous Pair Creation of electron positron pairs
in a strong external field has been rigorously established. We give here the
heuristic core of the proof, since the results differ from those given in
earlier works.


title: Measurement master equation

abstract:
  We derive a master equation describing the evolution of a quantum system
subjected to a sequence of observations. These measurements occur randomly at a
given rate and can be of a very general form. As an example, we analyse the
effects of these measurements on the evolution of a two-level atom driven by an
electromagnetic field. For the associated quantum trajectories we find Rabi
oscillations, Zeno-effect type behaviour and random telegraph evolution spawned
by mini quantum jumps as we change the rates and strengths of measurement.


title: Geometric phases and criticality in spin systems

abstract:
  A general formalism of the relation between geometric phases produced by
circularly evolving interacting spin systems and their criticality behavior is
presented. This opens up the way for the use of geometric phases as a tool to
study regions of criticality without having to undergo a quantum phase
transition. As a concrete example a spin-1/2 chain with XY interactions is
presented and the corresponding geometric phases are analyzed. The
generalization of these results to the case of an arbitrary spin system
provides an explanation for the existence of such a relation.


title: Simulation of quantum dynamics via classical collective behavior

abstract:
  The simple algorithm for the simulation and visualization of non relativistic
quantum dynamics is proposed that is based on a collective behavior of
classical particles. Any quantum particle is represented as the swarm of its
classical samples which interact by simple rules including emission and
absorption of samples of tied photons. The quantum dynamics results from the
collective behavior of such a swarm where the eigenstates are treated as the
equilibrium states relatively to emission-absorption of photons. The
entanglement is treated as a correlation between samples of the different
swarms that is stored in the space-time part of the model inaccessible for a
user. The amplitude is always grained. The Coulomb field between quantum
particles is simulated, analogously to free flow of quantum package, by the
point wise interaction between its samples and scalar photon samples which
propagate by diffusion. It gives square root speedup in comparison to
each-with-each method. This method obviously includes decoherence and admits
the natural generalization on the QED of many particles with the linear
computational cost.


title: Quantum Predicative Programming

abstract:
  The subject of this work is quantum predicative programming -- the study of
developing of programs intended for execution on a quantum computer. We look at
programming in the context of formal methods of program development, or
programming methodology. Our work is based on probabilistic predicative
programming, a recent generalisation of the well-established predicative
programming. It supports the style of program development in which each
programming step is proven correct as it is made. We inherit the advantages of
the theory, such as its generality, simple treatment of recursive programs,
time and space complexity, and communication. Our theory of quantum programming
provides tools to write both classical and quantum specifications, develop
quantum programs that implement these specifications, and reason about their
comparative time and space complexity all in the same framework.


title: An Introduction to Error-Correcting Codes: From Classical to Quantum

abstract:
  This report surveys quantum error-correcting codes. As Preskill claimed, 21st
century would be the golden age of quantum error correction. Quantum channels
behave differently from classical channels, so researchers face difficulties in
developing robust quantum codes. Fortunately, the classical error control
methods have been well developed. If we can learn many lessons from classical
coding theory, we can expedite the development of quantum codes. Scientists
have discovered that quantum error correction shares many concepts with
classical counterpart. Both quantum and classical coding schemes add redundancy
to information to protect against noises. They also have similar conditions for
error detectability and correctability.


title:
Single photon continuous variable quantum key distribution based on
  energy-time uncertainty relation

abstract:
  In previous quantum key distribution (QKD) protocols, information is encoded
on either the discrete-variable of single-photon signal or continuous-variables
of multi-photon signal. Here, we propose a new QKD protocol by encoding
information on continuous-variables of a single photon. In this protocol, Alice
randomly encodes her information on either the central frequency of a
narrow-band single photon pulse or the time-delay of a broadband single photon
pulse, while Bob randomly chooses to do either frequency measurement or time
measurement. The security of this protocol rests on the energy-time uncertainty
relation, which prevents Eve from simultaneously determining both frequency and
time information with arbitrarily high resolution. In practice, this scheme may
be more robust against various channel noises, such as polarization and phase
fluctuations.


title: Secure quantum string seal exists

abstract:
  It was claimed that all quantum string seals are insecure [H. F. Chau,
quant-ph/0602099]. However, here it will be shown that for imperfect quantum
string seals, the information obtained by the measurement proposed in that
reference is trivial. Therefore imperfect quantum string seals can be
unconditionally secure.


title:
Efficient high-capacity quantum secret sharing with two-photon
  entanglement

abstract:
  An efficient high-capacity quantum secret sharing scheme is proposed
following some ideas in quantum dense coding with two-photon entanglement. The
message sender, Alice prepares and measures the two-photon entangled states,
and the two agents, Bob and Charlie code their information on their photons
with four local unitary operations, which makes this scheme more convenient for
the agents than others. This scheme has a high intrinsic efficiency for qubits
and a high capacity.


title:
Quantum Mechanics of Klein-Gordon Fields II: Relativistic Coherent
  States

abstract:
  We use the formulation of the quantum mechanics of first quantized
Klein-Gordon fields given in the first of this series of papers to study
relativistic coherent states. In particular, we offer an explicit construction
of coherent states for both charged and neutral (real) free Klein-Gordon fields
as well as for charged fields interacting with a constant magnetic field. Our
construction is free from the problems associated with charge-superselection
rule that complicated the previous studies. We compute various physical
quantities associated with our coherent states and present a detailed
investigation of their classical (nonquantum) and nonrelativistic limits.


title:
Quantum Fluctuations of Coulomb Potential as a Source of Flicker Noise.
  The Influence of External Electric Field

abstract:
  Fluctuations of the electromagnetic field produced by quantized matter in
external electric field are investigated. A general expression for the power
spectrum of fluctuations is derived within the long-range expansion. It is
found that in the whole measured frequency band, the power spectrum of
fluctuations exhibits an inverse frequency dependence. A general argument is
given showing that for all practically relevant values of the electric field,
the power spectrum of induced fluctuations is proportional to the field
strength squared. As an illustration, the power spectrum is calculated
explicitly using the kinetic model with the relaxation-type collision term.
Finally, it is shown that the magnitude of fluctuations produced by a sample
generally has a Gaussian distribution around its mean value, and its dependence
on the sample geometry is determined. In particular, it is demonstrated that
for geometrically similar samples, the power spectrum is inversely proportional
to the sample volume. Application of the obtained results to the problem of
flicker noise is discussed.


title:
Towards a Landau-Zener formula for an interacting Bose-Einstein
  condensate

abstract:
  We consider the Landau-Zener problem for a Bose-Einstein condensate in a
linearly varying two-level system, for the full many-particle system as well
and in the mean-field approximation. The many-particle problem can be solved
approximately within an independent crossings approximation, which yields an
explicit Landau-Zener formula.


title: Programmable quantum state discriminators with simple programs

abstract:
  We describe a class of programmable devices that can discriminate between two
quantum states. We consider two cases. In the first, both states are unknown.
One copy of each of the unknown states is provided as input, or program, for
the two program registers, and the data state, which is guaranteed to be
prepared in one of the program states, is fed into the data register of the
device. This device will then tell us, in an optimal way, which of the
templates stored in the program registers the data state matches. In the second
case, we know one of the states while the other is unknown. One copy of the
unknown state is fed into the single program register, and the data state which
is guaranteed to be prepared in either the program state or the known state, is
fed into the data register. The device will then tell us, again optimally,
whether the data state matches the template or is the known state. We determine
two types of optimal devices. The first performs discrimination with minimum
error, the second performs optimal unambiguous discrimination. In all cases we
first treat the simpler problem of only one copy of the data state and then
generalize the treatment to n copies. In comparison to other works we find that
providing n > 1 copies of the data state yields higher success probabilities
than providing n > 1 copies of the program states.


title:
Bilinear and quadratic Hamiltonians in two-mode cavity quantum
  electrodynamics

abstract:
  In this work we show how to engineer bilinear and quadratic Hamiltonians in
cavity quantum electrodynamics (QED) through the interaction of a single driven
two-level atom with cavity modes. The validity of the engineered Hamiltonians
is numerically analyzed even considering the effects of both dissipative
mechanisms, the cavity field and the atom. The present scheme can be used, in
both optical and microwave regimes, for quantum state preparation, the
implementation of quantum logical operations, and fundamental tests of quantum
theory.


title:
Multiparty controlled quantum secret direct communication using
  Greenberger-Horne-Zeilinger state

abstract:
  Base on the idea of dense coding of three-photon entangled state and qubit
transmission in blocks, we present a multiparty controlled quantum secret
direct communication scheme using Greenberger-Horne-Zeilinger state. In the
present scheme, the sender transmits her three bits of secret message to the
receiver directly and the secret message can only be recovered by the receiver
under the permission of all the controllers. All three-photon entangled states
are used to transmit the secret messages except those chosen for eavesdropping
check and the present scheme has a high source capacity because
Greenberger-Horne-Zeilinger state forms a large Hilbert space.


title: Selective recoupling and stochastic dynamical decoupling

abstract:
  An embedded selective recoupling method is proposed which is based on the
idea of embedding the recently proposed deterministic selective recoupling
scheme of Yamaguchi et al. [quant-ph/0411099] into a stochastic dynamical
decoupling method, such as the recently proposed
Pauli-random-error-correction-(PAREC) scheme [Eur. Phys. J. D 32, 153,
quant-ph/0407262]. The recoupling scheme enables the implementation of
elementary quantum gates in a quantum information processor by partial
suppression of the unwanted interactions. The random dynamical decoupling
method cancels a significant part of the residual interactions. Thus the time
scale of reliable quantum computation is increased significantly. Numerical
simulations are presented for a conditional two-qubit swap gate and for a
complex iterative quantum algorithm.


title:
Enhanced Convergence and Robust Performance of Randomized Dynamical
  Decoupling

abstract:
  We demonstrate the advantages of randomization in coherent quantum dynamical
control. For systems which are either time-varying or require decoupling cycles
involving a large number of operations, we find that simple randomized
protocols offer superior convergence and stability as compared to deterministic
counterparts. In addition, we show how randomization always allows to
outperform purely deterministic schemes at long times, including combinatorial
and concatenated methods. General criteria for optimally interpolating between
deterministic and stochastic design are proposed and illustrated in explicit
decoupling scenarios relevant to quantum information storage.


title:
Effects of Collisional Decoherence on Multipartite Entanglement - How
  would entanglement not be relatively common?

abstract:
  We consider the collision model of Ziman {\em et al.} and study the
robustness of $N$-qubit Greenberger-Horne-Zeilinger (GHZ), W, and linear
cluster states. Our results show that $N$-qubit entanglement of GHZ states
would be extremely fragile under collisional decoherence, and that of W states
could be more robust than of linear cluster states. We indicate that the
collision model of Ziman {\em et al.} could provide a physical mechanism to
some known results in this area of investigations. More importantly, we show
that it could give a clue as to how $N$-partite distillable entanglement would
be relatively rare in our macroscopic classical world.


title: Low-lying bifurcations in cavity quantum electrodynamics

abstract:
  The interplay of quantum fluctuations with nonlinear dynamics is a central
topic in the study of open quantum systems, connected to fundamental issues
(such as decoherence and the quantum-classical transition) and practical
applications (such as coherent information processing and the development of
mesoscopic sensors/amplifiers). With this context in mind, we here present a
computational study of some elementary bifurcations that occur in a driven and
damped cavity quantum electrodynamics (cavity QED) model at low intracavity
photon number. In particular, we utilize the single-atom cavity QED Master
Equation and associated Stochastic Schrodinger Equations to characterize the
equilibrium distribution and dynamical behavior of the quantized intracavity
optical field in parameter regimes near points in the semiclassical
(mean-field, Maxwell-Bloch) bifurcation set. Our numerical results show that
the semiclassical limit sets are qualitatively preserved in the quantum
stationary states, although quantum fluctuations apparently induce phase
diffusion within periodic orbits and stochastic transitions between attractors.
We restrict our attention to an experimentally realistic parameter regime.


title: Complementarity of Process and Substance

abstract:
  Process Philosophy endeavours to replace the classical ontology of substances
by a process ontology centered on notions of changes and transitions. We argue,
that the substantial and processual approach are mutually complementary. Here,
complementarity is to be understood in the sense of a "Generalized Quantum
Theory", which is not restricted to physical phenomena. From this point of
view, restricting oneself to either substance or process ontology would be as
ill-advised as exclusively relying on position or momentum observables in
physics. A new view on Zeno's paradox lends itself. The meaning of an "internal
energy observable", complementary to inner time, and its relationship to
"akategorial states" of the human mind will also be discussed.


title:
One-dimensional completed scattering and quantum nonlocality of
  entangled states

abstract:
  Entanglement is usually associated with compound systems. We first show that
a one-dimensional (1D) completed scattering of a particle on a static potential
barrier represents an entanglement of two alternative one-particle
sub-processes, transmission and reflection, macroscopically distinct at the
final stage of scattering. The wave function for the whole ensemble of
scattering particles can be uniquely presented as the sum of two isometrically
evolved wave packets to describe the (to-be-)transmitted and (to-be-)reflected
subensembles of particles at all stages of scattering. A noninvasive
Larmor-clock timing procedure adapted to either subensemble shows that namely
the dwell time gives the time spent, on the average, by a particle in the
barrier region, and it denies the Hartman effect. As regards the group time, it
cannot be measured and hence it cannot be accepted as a measure of the
tunneling time. We argue that nonlocality of entangled states appears in
quantum mechanics due to inconsistency of its superposition principle with the
corpuscular properties of a particle. For example, this principle associates a
1D completed scattering with a single (one-way) process, while a particle, as
an indivisible object, cannot take part in transmission and reflection,
simultaneously.


title: Quantum Mechanics without Nonlocality

abstract:
  We argue that quantum nonlocality of entangled states is not an actual
phenomenon. It appears in quantum mechanics as a consequence of the
inconsistency of its superposition principle with the corpuscular properties of
a quantum particle. In the existing form, this principle does not distinguish
between macroscopically distinct states of a particle and their superpositions:
it implies introducing observables for a particle, even if it is in an
entangled state. However, a particle cannot take part simultaneously in two or
more alternative macroscopically distinct sub-processes. Thus, calculating the
expectation values of the one-particle's observables, for entangled states, is
physically meaningless: Born's formula is not applicable to such states. The
same concerns the entangled states of compound quantum systems. In the {\it
existing} quantum mechanics, introducing Bell's inequalities is fully legal.
However, these inequalities imply averaging over an entangled state, and,
hence, they have no basis for their clear physical interpretation. Experiments
to confirm the violation of Bell's inequalities do not prove the existence of
nonlocality in microcosm. They confirm only that correlations introduced in the
existing theory of entangled states have no physical sense, for they contradict
special relativity.


title:
Universal quantum circuit for n-qubit quantum gate: A programmable
  quantum gate

abstract:
  Quantum computation has attracted much attention, among other things, due to
its potentialities to solve classical NP problems in polynomial time. For this
reason, there has been a growing interest to build a quantum computer. One of
the basic steps is to implement the quantum circuit able to realize a given
unitary operation. This task has been solved using decomposition of unitary
matrices in simpler ones till reach quantum circuits having only single-qubits
and CNOTs gates. Usually the goal is to find the minimal quantum circuit able
to solve a given problem. In this paper we go in a different direction. We
propose a general quantum circuit able to implement any specific quantum
circuit by just setting correctly the parameters. In other words, we propose a
programmable quantum circuit. This opens the possibility to construct a real
quantum computer where several different quantum operations can be realized in
the same hardware. The configuration is proposed and its optical implementation
is discussed.


title: Randomized Dynamical Decoupling Techniques for Coherent Quantum Control

abstract:
  The need for strategies able to accurately manipulate quantum dynamics is
ubiquitous in quantum control and quantum information processing. We
investigate two scenarios where randomized dynamical decoupling techniques
become more advantageous with respect to standard deterministic methods in
switching off unwanted dynamical evolution in a closed quantum system: when
dealing with decoupling cycles which involve a large number of control actions
and/or when seeking long-time quantum information storage. Highly effective
hybrid decoupling schemes, which combine deterministic and stochastic features
are discussed, as well as the benefits of sequentially implementing a
concatenated method, applied at short times, followed by a hybrid protocol,
employed at longer times. A quantum register consisting of a chain of spin-1/2
particles interacting via the Heisenberg interaction is used as a model for the
analysis throughout.


title: Quantum states representing perfectly secure bits are always distillable

abstract:
  It is proven that recently introduced states with perfectly secure bits of
cryptographic key (private states representing secure bit) [K. Horodecki et
al., Phys. Rev. Lett. 94, 160502 (2005)] as well as its multipartite and higher
dimension generalizations always represent distillable entanglement. The
corresponding lower bounds on distillable entanglement are provided. We also
present a simple alternative proof that for any bipartite quantum state
entanglement cost is an upper bound on distillable cryptographic key in
bipartite scenario.


title: Nonlinear Pseudo-Supersymmetry in the Framework of N-fold Supersymmetry

abstract:
  We recall the importance of recognizing the different mathematical nature of
various concepts relating to PT-symmetric quantum theories. After clarifying
the relation between supersymmetry and pseudo-supersymmetry, we prove
generically that nonlinear pseudo-supersymmetry, recently proposed by Sinha and
Roy, is just a special case of N-fold supersymmetry. In particular, we show
that all the models constructed by these authors have type A 2-fold
supersymmetry. Furthermore, we prove that an arbitrary one-body quantum
Hamiltonian which admits two (local) solutions in closed form belongs to type A
2-fold supersymmetry, irrespective of whether or not it is Hermitian,
PT-symmetric, pseudo-Hermitian, and so on.


title: Duality, Phase Structures and Dilemmas in Symmetric Quantum Games

abstract:
  Symmetric quantum games for 2-player, 2-qubit strategies are analyzed in
detail by using a scheme in which all pure states in the 2-qubit Hilbert space
are utilized for strategies. We consider two different types of symmetric games
exemplified by the familiar games, the Battle of the Sexes (BoS) and the
Prisoners' Dilemma (PD). These two types of symmetric games are shown to be
related by a duality map, which ensures that they share common phase structures
with respect to the equilibria of the strategies. We find eight distinct phase
structures possible for the symmetric games, which are determined by the
classical payoff matrices from which the quantum games are defined. We also
discuss the possibility of resolving the dilemmas in the classical BoS, PD and
the Stag Hunt (SH) game based on the phase structures obtained in the quantum
games. It is observed that quantization cannot resolve the dilemma fully for
the BoS, while it generically can for the PD and SH if appropriate correlations
for the strategies of the players are provided.


title:
Un-equivalency Theorem of Deformed Heisenberg-Weyl's Algebra in
  Noncommutative Space

abstract:
  An extensively tacit understandings of equivalency between the deformed
Heisenberg-Weyl algebra in noncommutative space and the undeformed
Heisenberg-Weyl algebra in commutative space is elucidated. Equivalency
conditions between two algebras are clarified. It is explored that the deformed
algebra related to the undeformed one by a non-orthogonal similarity
transformation. Furthermore, non-existence of a unitary similarity
transformation which transforms the deformed algebra to the undeformed one is
demonstrated. The un-equivalency theorem between the deformed and the
undeformed algebras is fully proved. Elucidation of this un-equivalency theorem
has basic meaning both in theory and practice.


title: On the detectability of quantum radiation in Bose-Einstein condensates

abstract:
  Based on doubly detuned Raman transitions between (meta) stable atomic or
molecular states and recently developed atom counting techniques, a detection
scheme for sound waves in dilute Bose-Einstein condensates is proposed whose
accuracy might reach down to the level of a few or even single phonons. This
scheme could open up a new range of applications including the experimental
observation of quantum radiation phenomena such as the Hawking effect in sonic
black-hole analogues or the acoustic analogue of cosmological particle
creation. PACS: 03.75.Kk, 04.70.Dy, 42.65.Dr.


title:
Geometric phase in open systems: beyond the Markov approximation and
  weak coupling limit

abstract:
  Beyond the quantum Markov approximation and the weak coupling limit, we
present a general theory to calculate the geometric phase for open systems with
and without conserved energy. As an example, the geometric phase for a
two-level system coupling both dephasingly and dissipatively to its environment
is calculated. Comparison with the results from quantum trajectory analysis is
presented and discussed.


title:
Contribution to understanding the mathematical structure of quantum
  mechanics

abstract:
  Probabilistic description of results of measurements and its consequences for
understanding quantum mechanics are discussed. It is shown that the basic
mathematical structure of quantum mechanics like the probability amplitudes,
Born rule, commutation and uncertainty relations, probability density current,
momentum operator, rules for including the scalar and vector potentials and
antiparticles can be obtained from the probabilistic description of results of
measurement of the space coordinates and time. Equations of motion of quantum
mechanics, the Klein-Gordon equation, Schrodinger equation and Dirac equation
are obtained from the requirement of the relativistic invariance of the
space-time Fisher information. The limit case of the delta-like probability
densities leads to the Hamilton-Jacobi equation of classical mechanics. Many
particle systems and the postulates of quantum mechanics are also discussed.


title: Simultaneous position and state measurement of Rydberg atoms

abstract:
  We present a technique for state-selective position detection of cold Rydberg
atoms. Ground state Rb atoms in a magneto-optical trap are excited to a Rydberg
state and are subsequently ionized with a tailored electric field pulse. This
pulse selectively ionizes only atoms in e.g. the 54d state and not in the 53d
state. The released electrons are detected after a slow flight towards a micro
channel plate. From the time of flight of the electrons the position of the
atoms is deduced. The state selectivity is about 20:1 when comparing 54d with
53d and the one-dimensional position resolution ranges from 6 to 40 $\mu$m over
a range of 300 $\mu$m. This state selectivity and position resolution are
sufficient to allow for the observation of coherent quantum excitation
transport.


title:
On the Assumption of Initial Factorization in the Master Equation for
  Weakly Coupled Systems I: General Framework

abstract:
  We analyze the dynamics of a quantum mechanical system in interaction with a
reservoir when the initial state is not factorized. In the weak-coupling (van
Hove) limit, the dynamics can be properly described in terms of a master
equation, but a consistent application of Nakajima-Zwanzig's projection method
requires that the reference (not necessarily equilibrium) state of the
reservoir be endowed with the mixing property.


title:
On the Assumption of Initial Factorization in the Master Equation for
  Weakly Coupled Systems II: Solvable Models

abstract:
  We analyze some solvable models of a quantum mechanical system in interaction
with a reservoir when the initial state is not factorized. We apply
Nakajima-Zwanzig's projection method by choosing a reference state of the
reservoir endowed with the mixing property. In van Hove's limit, the dynamics
is described in terms of a master equation. We observe that Markovianity
becomes a valid approximation for timescales that depend both on the form
factors of the interaction and on the observables of the reservoir that can be
measured.


title:
A simpler proof of zero-knowledge against quantum attacks using Grover's
  amplitude amplification

abstract:
  Watrous had presented the first proof of zero-knowledge property of a proof
system against a quantum verifier. The key of the proof is the construction of
a quantum simulator. In the construction, the 'failure state' is rotated to the
'success' state by a tricky operation which is initially developped for the
amplification of QMA proof systems.
  This manuscript presents a new and simpler construction of a simulator. In
the construction, we simply amplify the success probability of a classical
simulator using Grover's amplification.


title: Moyal products -- a new perspective on quasi-hermitian quantum mechanics

abstract:
  The rationale for introducing non-hermitian Hamiltonians and other
observables is reviewed and open issues identified. We present a new approach
based on Moyal products to compute the metric for quasi-hermitian systems. This
approach is not only an efficient method of computation, but also suggests a
new perspective on quasi-hermitian quantum mechanics which invites further
exploration. In particular, we present some first results which link the Berry
connection and curvature to non-perturbative properties and the metric.


title: Effects of non-local initial conditions in the Quantum Walk on the line

abstract:
  We report an enhancement of the decay rate of the survival probability when
non-local initial conditions in position space are considered in the Quantum
Walk on the line. It is shown how this interference effect can be understood
analytically by using previously derived results. Within a restricted position
subspace, the enhanced decay is correlated with a maximum asymptotic
entanglement level while the normal decay rate corresponds to initial relative
phases associated to a minimum entanglement level.


title: A probabilistic operator symbol framework for quantum information

abstract:
  Hilbert space operators may be mapped onto a space of ordinary functions
(operator symbols) equipped with an associative (but noncommutative)
star-product. A unified framework for such maps is reviewed. Because of its
clear probabilistic interpretation, a particular class of operator symbols
(tomograms) is proposed as a framework for quantum information problems. Qudit
states are identified with maps of the unitary group into the simplex. The
image of the unitary group on the simplex provides a geometrical
characterization of the nature of the quantum states. Generalized measurements,
typical quantum channels, entropies and entropy inequalities are discussed in
this setting.


title:
Comment on "Experimental motivation and empirical consistency in minimal
  no-collapse quantum mechanics"

abstract:
  Schlosshauer has advanced a theory of minimal no-collapse quantum mechanics
for a decoherence-based subjective resolution of the measurement problem. The
basic premise being that superposition states are maintained beyond the retinal
apparatus, becoming correlated with neuronal arrays located in the occipital
lobe of the brain. Decoherence for these neurons in a superposition of firing
and resting, leads to an irreversible dynamical decoupling of the two branches,
resulting in the emergence of a single subjective perception. Based upon prior
retinal research, it is shown that his theory is untenable for several reasons.


title: Decoherence of Dynamically Manipulated Qubits

abstract:
  We summarize our results on decoherence for short- to intermediate-time
dynamics of an externally controlled two-state quantum system - a qubit -
interacting with thermal bosonic environment. The developed approximation
schemes are illustrated for an adiabatic model with time-dependent gate
control, and for a model with rotating-wave gate function.


title: Stark effect on a geometry defined by a cake' slice

abstract:
  By using a variational calculation, we study the effect of an external
applied electric field on the ground state of electrons confined in a quantum
box with a geometry defined by a slice of a cake. This geometry is a first
approximation for a tip of a cantilever of an Atomic Force Microscope (AFM). By
modeling the tip with the slice, we calculate the electronic ground state
energy as function of the slice's diameter, its angular aperture, its thickness
and the intensity of the external electric field applied along the slice. For
the applied field pointing to the wider part of the slice, a confining
electronic effect in the opposite side is clearly observed. This effect is
sharper as the angular slice's aperture is smaller and there is more radial
space to manifest itself.


title: Stark effect in a wedge-shaped quantum box

abstract:
  The effect of an external applied electric field on the electronic ground
state energy of a quantum box with a geometry defined by a wedge is studied by
carrying out a variational calculation. This geometry could be used as an
approximation for a tip of a cantilever of an atomic force microscope. We study
theoretically the Stark effect as function of the parameters of the wedge: its
diameter, angular aperture and thickness; as well as function of the intensity
of the external electric field applied along the axis of the wedge in both
directions; pushing the carrier towards the wider or the narrower parts. A
confining electronic effect, which is sharper as the wedge dimensions are
smaller, is clearly observed for the first case. Besides, the sign of the Stark
shift changes when the angular aperture is changed from small angles to angles
theta>pi. For the opposite field, the electronic confinement for large
diameters is very small and it is also observed that the Stark shift is almost
independent with respect to the angular aperture.


title:
Supersymmetric solutions of PT-/non-PT-symmetric and non-Hermitian
  Screened Coulomb potential via Hamiltonian hierarchy inspired variational
  method

abstract:
  The supersymmetric solutions of PT-symmetric and Hermitian/non-Hermitian
forms of quantum systems are obtained by solving the Schrodinger equation for
the Exponential-Cosine Screened Coulomb potential. The Hamiltonian hierarchy
inspired variational method is used to obtain the approximate energy
eigenvalues and corresponding wave functions.


title:
Direct measurement of the quantum state of the electromagnetic field in
  a superconducting transmission line

abstract:
  We propose an experimental procedure to directly measure the state of an
electromagnetic field inside a resonator, corresponding to a superconducting
transmission line, coupled to a Cooper-pair box (CPB). The measurement protocol
is based on the use of a dispersive interaction between the field and the CPB,
and the coupling to an external classical field that is tuned to resonance with
either the field or the CPB. We present a numerical simulation that
demonstrates the feasibility of this protocol, which is within reach of present
technology.


title: Sudden Death of Entanglement: Classical Noise Effects

abstract:
  When a composite quantum state interacts with its surroundings, both quantum
coherence of individual particles and quantum entanglement will decay. We have
shown that under vacuum noise, i.e., during spontaneous emission, two-qubit
entanglement may terminate abruptly in a finite time [T. Yu and J. H. Eberly,
\prl {93}, 140404 (2004)], a phenomenon termed entanglement sudden death (ESD).
An open issue is the behavior of mixed-state entanglement under the influence
of classical noise. In this paper we investigate entanglement sudden death as
it arises from the influence of classical phase noise on two qubits that are
initially entangled but have no further mutual interaction.


title: Coherent Control of Stationary Light Pulses

abstract:
  We present a detailed analysis of the recently demonstrated technique to
generate quasi-stationary pulses of light [M. Bajcsy {\it et al.}, Nature
(London) \textbf{426}, 638 (2003)] based on electromagnetically induced
transparency. We show that the use of counter-propagating control fields to
retrieve a light pulse, previously stored in a collective atomic Raman
excitation, leads to quasi-stationary light field that undergoes a slow
diffusive spread. The underlying physics of this process is identified as pulse
matching of probe and control fields. We then show that spatially modulated
control-field amplitudes allow us to coherently manipulate and compress the
spatial shape of the stationary light pulse. These techniques can provide
valuable tools for quantum nonlinear optics and quantum information processing.


title:
Generation of a superposition of odd photon number states for quantum
  information networks

abstract:
  We report on the experimental observation of quantum-network-compatible light
described by a non-positive Wigner function. The state is generated by photon
subtraction from a squeezed vacuum state produced by a continuous wave optical
parametric amplifier. Ideally, the state is a coherent superposition of odd
photon number states, closely resembling a superposition of weak coherent
states (a Schroedinger cat), with the leading contribution from a single photon
state in the low parametric gain limit. Light is generated in a nearly perfect
spatial mode with a Fourier-limited frequency bandwidth which matches well
atomic quantum memory requirements. The source is also characterized by high
spectral brightness with about 10,000 and more events per second routinely
generated. The generated state of light is the ultimate input state for testing
quantum memories, quantum repeaters and linear optics quantum computers.


title: Determining the optical axes of entangled Laguerre Gauss modes

abstract:
  A method for determining the positions of hologram dislocations relative to
the optical axes of entangled Laguerre Gaussian modes is proposed. In our
method, the coincidence count rate distribution was obtained by scanning the
position of one of the holograms in two dimensions. Then, the relative position
of the hologram dislocation was determined quantitatively from the positions of
the minimum and maximum coincidence count rates. The validity of the method was
experimentally verified, and in addition, an experiment demonstrating the
violation of the Clauser-Horne-Shimony-Holt inequality was performed using the
well-identified optical axes of the entangled modes.


title: Quantum teleportation of optical images with frequency conversion

abstract:
  We describe a new version of continuous variables quantum holographic
teleportation of optical images. Unlike the previously proposed scheme, it is
based on the continuous variables quantum entanglement between the light fields
of different frequencies and allows for the wavelength conversion between the
original and the teleported images. The frequency tunable holographic
teleportation protocol can be used as a part of light-matter interface in
parallel quantum information processing and parallel quantum memory


title:
Density Matrix Tomography of Entangled Electron and Nuclear Spin States
  in 15N:C60

abstract:
  We discuss details of the preparation and detection of entangled
electron-nuclear spin states in 15N:C60 together with a quantitative evaluation
of the complete density matrix. All four Bell states of a two qubit subsystem
were analyzed. In addition we find a quantum critical temperature of Tq = 7.73
K for this system at an electron spin resonance frequency of 95 GHz.


title: Non-Gaussian states from continuous-wave Gaussian light sources

abstract:
  We present a general analysis of the state obtained by subjecting the output
from a continuous-wave (cw) Gaussian field to non-Gaussian measurements. The
generic multimode state of cw Gaussian fields is characterized by an infinite
dimensional covariance matrix involving the noise correlations of the source.
Our theory extracts the information relevant for detection within specific
temporal output modes from these correlation functions . The formalism is
applied to schemes for production of non-classical light states from a squeezed
beam of light.


title: Quantum information entropies of the eigenstates of the Morse potential

abstract:
  The position and momentum space information entropies for the Morse potential
are numerically obtained for different strengths of the potential. It is found
to satisfy the bound obtained by Beckner, Bialynicki-Birula, and Mycielski.
Interesting features of the entropy densities are graphically demonstrated.


title:
Quantum eigenstates and (pseudo-)classical accelerator modes of the
  Delta-Kicked Accelerator

abstract:
  The quantum dynamics of a periodically driven system, the delta-kicked
accelerator, is investigated in the semiclassical and pseudo-classical regimes,
where quantum accelerator modes are observed. We construct the evolution
operator of this classically chaotic system explicitly. If certain quantum
resonance conditions are fullfilled, we show one can reduce the evolution
operator to a finite matrix, whose eigenvectors are the quasi-eigenstates.
These are represented by their Husimi functions. In so doing, we are able to
directly compare the pure quantum states with the classical states. In the
semiclassical regime, the quantum states are found to be related to the
classical KAM tori and the classical accelerator modes. In contrast, the
quasi-eigenstates do not lie on the $\epsilon$-classical trajectories in the
pseudo-classical regime. This shows a clear and important distinction between
semiclassicality and the new type of pseudo-classicality found by Fishman,
Guarneri and Rebuzzini.


title: Asymptotic adaptive bipartite entanglement distillation protocol

abstract:
  We present a new asymptotic bipartite entanglement distillation protocol that
outperforms all existing asymptotic schemes. This protocol is based on the
breeding protocol with the incorporation of two-way classical communication.
Like breeding, the protocol starts with an infinite number of copies of a
Bell-diagonal mixed state. Breeding can be carried out as successive stages of
partial information extraction, yielding the same result: one bit of
information is gained at the cost (measurement) of one pure Bell state pair
(ebit). The basic principle of our protocol is at every stage to replace
measurements on ebits by measurements on a finite number of copies, whenever
there are two equiprobable outcomes. In that case, the entropy of the global
state is reduced by more than one bit. Therefore, every such replacement
results in an improvement of the protocol. We explain how our protocol is
organized as to have as many replacements as possible. The yield is then
calculated for Werner states.


title: Sudden Death of Entanglement of Two Jaynes-Cummings Atoms

abstract:
  We investigate entanglement dynamics of two isolated atoms, each in its own
Jaynes-Cummings cavity. We show analytically that initial entanglement has an
interesting subsequent time evolution, including the so-called sudden death
effect.


title:
Non-Hermitian Quantum Mechanics of Non-diagonalizable Hamiltonians:
  puzzles with self-orthogonal states

abstract:
  We consider QM with non-Hermitian quasi-diagonalizable Hamiltonians, i.e. the
Hamiltonians having a number of Jordan cells in particular biorthogonal bases.
The "self-orthogonality" phenomenon is clarified in terms of a correct spectral
decomposition and it is shown that "self-orthogonal" states never jeopardize
resolution of identity and thereby quantum averages of observables. The example
of a complex potential leading to one Jordan cell in the Hamiltonian is
constructed and its origin from level coalescence is elucidated. Some puzzles
with zero-binorm bound states in continuous spectrum are unraveled with the
help of a correct resolution of identity.

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


title: Collapse and Relativity

abstract:
  Ever since we have been in the possession of quantum theories without
observers, such as Bohmian mechanics or the Ghirardi-Rimini-Weber (GRW) theory
of spontaneous wave function collapse, a major challenge in the foundations of
quantum mechanics is to devise a relativistic quantum theory without observers.
One of the difficulties is to reconcile nonlocality with relativity. I report
about recent progress in this direction based on the GRW model: A relativistic
version of the model has been devised for the case of N noninteracting (but
entangled) particles. A key ingredient was to focus not on the evolution of the
wave function but rather on the evolution of the matter in space-time as
determined by the wave function.


title: Quantum dynamics of magnetically controlled network for Bloch electrons

abstract:
  We study quantum dynamics of wave packet motion of Bloch electrons in quantum
networks with the tight-binding approach for different types of
nearest-neighbor interactions. For various geometrical configurations, these
networks can function as some optical devices, such as beam splitters and
interferometers. When the Bloch electrons with the Gaussian wave packets input
these devices, various quantum coherence phenomena can be observed, e.g., the
perfect quantum state transfer without reflection in a Y-shaped beam, the
multi- mode entanglers of electron wave by star shaped network and Bloch
electron interferometer with the lattice Aharonov-Bohm effects. Behind these
conceptual quantum devices are the physical mechanism that, for hopping
parameters with some specific values, a connected quantum networks can be
reduced into a virtual network, which is a direct sum of some irreducible
subnetworks. Thus, the perfect quantum state transfer in each subnetwork in
this virtual network can be regarded as a coherent beam splitting process.
Analytical and numerical investigations show the controllability of wave packet
motion in these quantum networks by the magnetic flux through some loops of
these networks, or by adjusting the couplings on nodes. We find the essential
differences in these quantum coherence effects when the different wave packets
enter these quantum networks initially. With these quantum coherent features,
they are expected to be used as quantum information processors for the fermion
system based on the possible engineered solid state systems, such as the array
of quantum dots that can be implemented experimentally.


title:
Nonlinear Schr\"odinger equations from prequantum classical statistical
  field theory

abstract:
  We derive some important features of the standard quantum mechanics from a
certain classical-like model -- prequantum classical statistical field theory,
PCSFT. In this approach correspondence between classical and quantum quantities
is established through asymptotic expansions. PCSFT induces not only linear
Schr\"odinger's equation, but also its {\it nonlinear generalizations.} This
coupling with ``nonlinear wave mechanics'' is used to evaluate the small
parameter of PCSFT.


title:
Stochastic mechanics, trace dynamics, and differential space - a
  synthesis

abstract:
  It is shown how Adler's trace dynamics can be applied to stochastic mechanics
and other complex classical dynamical systems. Emergent non-commutivity due to
the fractal nature of sample trajectories is closely related to the fact that
the forward and backward time derivatives are different for these diffusions. A
new variational approach to stochastic mechanics based on trace dynamics is
introduced. It is shown that Yasue's method and Guerra and Morato's method can
both be generalized to allow for any diffusion constant in a stochastic model
of Schrodinger's equation, and that they can all also describe dissipative
diffusion. Then it is shown that the trace dynamical theory seems to only
describe dissipative diffusion unless an extra quantum mechanical potential
term is added to the Hamiltonian.
  The differential space theory of Wiener and Siegel is reconsidered as a
useful tool in this framework, and is generalized to stochastic processes
instead of deterministic ones for the hidden trajectories of observables. It is
proposed that the natural measure space for Wiener-Siegel theory is Haar
measure for random unitary matrices. A new interpretation of the polychotomic
algorithm is given.


title: A Realist Interpretation of the Quantum Measurement Problem

abstract:
  A new, realist interpretation of the quantum measurement processes is given.
In this scenario a quantum measurement is a non-equilibrium phase transition in
a ``resonant cavity'' formed by the entire physical universe including all its
material and energy content. Both the amplitude and the phase of the quantum
mechanical wavefunction acquire substantial meaning in this picture, and the
probabilistic element is removed from the foundations of quantum mechanics, its
apparent presence in the quantum measurement process is viewed as a result of
the sensitive dependence on initial/boundary conditions of the non-equilibrium
phase transitions in a many degree-of-freedom system. The implications of
adopting this realist ontology to the clarification and resolution of lingering
issues in the foundations of quantum mechanics, such as wave-particle duality,
Heisenberg's uncertainty relation, Schrodinger's Cat paradox, first and higher
order coherence of photons and atoms, virtual particles, the existence of
commutation relations and quantized behavior, etc., are also presented.


title: Continuous-Time Quantum Walks on Trees in Quantum Probability Theory

abstract:
  A quantum central limit theorem for a continuous-time quantum walk on a
homogeneous tree is derived from quantum probability theory. As a consequence,
a new type of limit theorems for another continuous-time walk introduced by the
walk is presented. The limit density is similar to that given by a
continuous-time quantum walk on the one-dimensional lattice.


title: Reply to Andrew Hodges

abstract:
  We separate the criticisms of Hodges \cite{Hodges2005} and others into those
against the algorithm itself and those against its physical implementation. We
then point out that {\em all} those against the algorithm are either misleading
or misunderstanding, and that the algorithm is self consistent. The only
central argument against physical implementations of the algorithm, on the
other hand, is based on an assumption that its Hamiltonians cannot be
effectively constructed due to a lack of infinite precision. However, so far
there is no known physical principle dictating why that cannot be done. To show
that the criticism may not be a forgone conclusion, we point out the virtually
unknown fact that, on the contrary, simple instances of Diophantine equations
with apparently {\em infinitely precisely} integer coefficients have {\em
already} been realised in experiments for certain quantum phase transitions. We
also speculate on how central limit theorem of statistics might be of some help
in the effective implementation of the required Hamiltonians.


title:
Three-Way Entanglement and Three-Qubit Phase Gate Based on a Coherent
  Six-Level Atomic System

abstract:
  We analyze the nonlinear optical response of a six-level atomic system under
a configuration of electromagnetically induced transparency. The giant
fifth-order nonlinearity generated in such a system with a relatively large
cross-phase modulation effect can produce efficient three-way entanglement and
may be used for realizing a three-qubit quantum phase gate. We demonstrate that
such phase gate can be transferred to a Toffoli gate, facilitating practical
applications in quantum information and computation.


title:
Quasiprobability distribution functions for periodic phase-spaces: I.
  Theoretical Aspects

abstract:
  An approach featuring $s$-parametrized quasiprobability distribution
functions is developed for situations where a circular topology is observed.
For such an approach, a suitable set of angle-angular momentum coherent states
must be constructed in appropriate fashion.


title: Control of decoherence in open quantum systems using feedback

abstract:
  Quantum feedback is assuming increasingly important role in quantum control
and quantum information processing. In this work we analyze the application of
such feedback techniques in eliminating decoherence in open quantum systems. In
order to apply such system theoretic methods we first analyze the invariance
properties of quadratic forms which corresponds to expected value of a
measurement and present conditions for decouplability of measurement outputs of
such time-varying open quantum systems from environmental effects.


title: Exact propagators for complex SUSY partners of real potentials

abstract:
  A method for calculating exact propagators for those complex potentials with
a real spectrum which are SUSY partners of real potentials is presented. It is
illustrated by examples of propagators for some complex SUSY partners of the
harmonic oscillator and zero potentials.


title:
Analogue of Cavity QED for Coupling between Spin and Nanomechanical
  Resonator

abstract:
  We describe a cavity QED analogue for the coupling system of a spin and a
nanomechanical resonator with a magnetic tip. For the quantized nanomechanical
resonator, a spin-boson model for this coupling system can refer to a
Jaynes-Cummings(JC) or an anti-JC model. These observations predict some
quantum optical phenomena, such as squeezing and "collapse-revival" in the
single oscillation mode of the nanomechanical resonator when it is initially
prepared in the quasi-classical state. By modulating the phase of RF magnetic
field one can switch the system between the JC and anti-JC model, which
provides a potential protocol for the detection of the single spin. A damping
mechanism is also analyzed.


title: Linear optics quantum Toffoli and Fredkin gates

abstract:
  We design linear optics multiqubit quantum logic gates. We assume the
traditional encoding of a qubit onto state of a single photon in two modes
(e.g. spatial or polarization). We suggest schemes allowing direct
probabilistic realization of the fundamental Toffoli and Fredkin gates without
resorting to a sequence of single- and two-qubit gates. This yields more
compact schemes and potentially reduces the number of ancilla photons. The
proposed setups involve passive linear optics, sources of auxiliary single
photons or maximally entangled pairs of photons, and single-photon detectors.
In particular, we propose an interferometric implementation of the Toffoli gate
in the coincidence basis, which does not require any ancilla photons and is
experimentally feasible with current technology.


title: Non locality and causal evolution in QFT

abstract:
  Non locality appearing in QFT during the free evolution of localized field
states and in the Feynman propagator function is analyzed. It is shown to be
connected to the initial non local properties present at the level of quantum
states and then it does not imply a violation of Einstein's causality. Then it
is investigated a simple QFT system with interaction, consisting of a classical
source coupled linearly to a quantum scalar field, that is exactly solved. The
expression for the time evolution of the state describing the system is given.
The expectation value of any arbitrary ``good'' local observable, expressed as
a function of the field operator and its space and time derivatives, is
obtained explicitly at all order in the field-matter coupling constant. These
expectation values have a source dependent part that is shown to be always
causally retarded, while the non local contributions are source independent and
related to the non local properties of zero point vacuum fluctuations.


title:
Quantum operational measurement of amplitude and phase parameters for
  SU(3)-symmetry optical field

abstract:
  We consider a new approach to describe a quantum optical Bose-system with
internal Gell-Mann symmetry by the SU(3)-symmetry polarization map in Hilbert
space. The operational measurement in density (or coherency) matrix elements
for the three mode optical field is discussed for the first time. We have
introduced a set of operators that describes the quantum measurement procedure
and the behavior of fluctuations for the amplitude and phase characteristics of
three level system. The original twelve-port interferometer for parallel
measurements of the Gell-Mann parameters is proposed. The quantum properties of
W-qutrit states under the measurement procedure are examined.


title:
Quantum Separability and Entanglement Detection via Entanglement-Witness
  Search and Global Optimization

abstract:
  We focus on determining the separability of an unknown bipartite quantum
state $\rho$ by invoking a sufficiently large subset of all possible
entanglement witnesses given the expected value of each element of a set of
mutually orthogonal observables. We review the concept of an entanglement
witness from the geometrical point of view and use this geometry to show that
the set of separable states is not a polytope and to characterize the class of
entanglement witnesses (observables) that detect entangled states on opposite
sides of the set of separable states. All this serves to motivate a classical
algorithm which, given the expected values of a subset of an orthogonal basis
of observables of an otherwise unknown quantum state, searches for an
entanglement witness in the span of the subset of observables. The idea of such
an algorithm, which is an efficient reduction of the quantum separability
problem to a global optimization problem, was introduced in PRA 70 060303(R),
where it was shown to be an improvement on the naive approach for the quantum
separability problem (exhaustive search for a decomposition of the given state
into a convex combination of separable states). The last section of the paper
discusses in more generality such algorithms, which, in our case, assume a
subroutine that computes the global maximum of a real function of several
variables. Despite this, we anticipate that such algorithms will perform
sufficiently well on small instances that they will render a feasible test for
separability in some cases of interest (e.g. in 3-by-3 dimensional systems).


title:
Anomalously Slow Cross Symmetry Phase Relaxation, Thermalized
  Non-Equilibrated Matter and Quantum Computing Beyond the Quantum Chaos Border

abstract:
  Thermalization in highly excited quantum many-body system does not
necessarily mean a complete memory loss of the way the system was formed. This
effect may pave a way for a quantum computing, with a large number of qubits
$n\simeq 100$--1000, far beyond the quantum chaos border. One of the
manifestations of such a thermalized non-equilibrated matter is revealed by a
strong asymmetry around 90$^\circ $ c.m. of evaporating proton yield in the
Bi($\gamma$,p) photonuclear reaction. The effect is described in terms of
anomalously slow cross symmetry phase relaxation in highly excited quantum
many-body systems with exponentially large Hilbert space dimensions. In the
above reaction this phase relaxation is about eight orders of magnitude slower
than energy relaxation (thermalization).

categories: quant-ph cond-mat.mes-hall nucl-ex nucl-th


title:
Production of heralded pure single photons from imperfect sources using
  cross phase modulation

abstract:
  Realistic single-photon sources do not generate single photons with
certainty. Instead they produce statistical mixtures of photons in Fock states
$\ket{1}$ and vacuum (noise). We describe how to eliminate the noise in the
output of the sources by means of another noisy source or a coherent state and
cross phase modulation (XPM). We present a scheme which announces the
production of pure single photons and thus eliminates the vacuum contribution.
This is done by verifying a XPM related phase shift with a Mach-Zehnder
interferometer.


title: Pre- and post-selection, weak values, and contextuality

abstract:
  By analyzing the concept of contextuality (Bell-Kochen-Specker) in terms of
pre-and-post-selection (PPS), it is possible to assign definite values to
observables in a new and surprising way. Physical reasons are presented for
restrictions on these assignments. When measurements are performed which do not
disturb the pre- and post-selection (i.e. weak measurements), then novel
experimental aspects of contextuality can be demonstrated including a proof
that every PPS-paradox with definite predictions implies contextuality. Certain
results of these measurements (eccentric weak values with e.g. negative values
outside the spectrum), however, cannot be explained by a "classical-like"
hidden variable theory.


title: Stationary quantum Markov process for the Wigner function

abstract:
  As a stochastic model for quantum mechanics we present a stationary quantum
Markov process for the time evolution of the Wigner function on a lattice phase
space Z_N x Z_N with N odd. By introducing a phase factor extension to the
phase space, each particle can be treated independently. This is an improvement
on earlier methods that require the whole distribution function to determine
the evolution of a constituent particle. The process has branching and
vanishing points, though a finite time interval can be maintained between the
branchings. The procedure to perform a simulation using the process is
presented.


title: Relations Between Quantum Maps and Quantum States

abstract:
  The relation between completely positive maps and compound states is
investigated in terms of the notion of quantum conditional probability.


title: A new pilot-wave model for quantum field theory

abstract:
  We present a way to construct a pilot-wave model for quantum field theory.
The idea is to introduce beables corresponding only to the bosonic degrees of
freedom and not to the fermionic degrees of freedom of the quantum state. We
illustrate this idea for quantum electrodynamics. The beables will be field
beables corresponding to the electromagnetic field and they will be introduced
in a similar way to that of Bohm's model for the free electromagnetic field.
Our approach is analogous to the situation in non-relativistic quantum theory,
where Bell treated spin not as a beable but only as a property of the
wavefunction.


title:
Ground state approximation for strongly interacting systems in arbitrary
  dimension

abstract:
  We introduce a variational method for the approximation of ground states of
strongly interacting spin systems in arbitrary geometries and spatial
dimensions. The approach is based on weighted graph states and superpositions
thereof. These states allow for the efficient computation of all local
observables (e.g. energy) and include states with diverging correlation length
and unbounded multi-particle entanglement. As a demonstration we apply our
approach to the Ising model on 1D, 2D and 3D square-lattices. We also present
generalizations to higher spins and continuous-variable systems, which allows
for the investigation of lattice field theories.


title: Quasi-exact minus-quartic oscillators in strong-core regime

abstract:
  PT-symmetric potentials $V({x}) = -{x}^4 +\j B {x}^3 + C {x}^2+\j D {x} +\j
F/{x} +G/{x}^2$ are quasi-exactly solvable, i.e., a specific choice of a small
$G=G^{(QES)}= integer/4$ is known to lead to wave functions $\psi^{(QES)}(x)$
in closed form at certain charges $F=F^{(QES)}$ and energies $E=E^{(QES)}$. The
existence of an alternative, simpler and non-numerical version of such a
construction is announced here in the new dynamical regime of very large
$G^{(QES)} \to \infty$.


title: Quantum exam

abstract:
  Absolutely and asymptotically secure protocols for organizing an exam in a
quantum way are proposed basing judiciously on multipartite entanglement. The
protocols are shown to stand against common types of eavesdropping attack.


title: Entanglement properties of quantum spin chains

abstract:
  We investigate the entanglement properties of a finite size 1+1 dimensional
Ising spin chain, and show how these properties scale and can be utilized to
reconstruct the ground state wave function. Even at the critical point, few
terms in a Schmidt decomposition contribute to the exact ground state, and to
physical properties such as the entropy. Nevertheless the entanglement here is
prominent due to the lower-lying states in the Schmidt decomposition.


title: First verification of generic fidelity recovery in a dynamical system

abstract:
  We study the time evolution of fidelity in a dynamical many body system,
namely a kicked Ising model, modified to allow for a time reversal invariance
breaking. We find good agreement with the random matrix predictions in the
realm of strong perturbations. In particular for the time-reversal symmetry
breaking case the predicted revival at Heisenberg time is clearly seen.


title: Robust observer for uncertain linear quantum systems

abstract:
  In the theory of quantum dynamical filtering, one of the biggest issues is
that the underlying system dynamics represented by a quantum stochastic
differential equation must be known exactly in order that the corresponding
filter provides an optimal performance; however, this assumption is generally
unrealistic. Therefore, in this paper, we consider a class of linear quantum
systems subjected to time-varying norm-bounded parametric uncertainties and
then propose a robust observer such that the variance of the estimation error
is guaranteed to be within a certain bound. Although in the linear case much of
classical control theory can be applied to quantum systems, the quantum robust
observer obtained in this paper does not have a classical analogue due to the
system's specific structure with respect to the uncertainties. Moreover, by
considering a typical quantum control problem, we show that the proposed robust
observer is fairly robust against a parametric uncertainty of the system even
when the other estimators--the optimal Kalman filter and risk-sensitive
observer--fail in the estimation.


title:
Quantum Revivals in Periodically Driven Systems close to nonlinear
  resonance

abstract:
  We calculate the quantum revival time for a wave-packet initially well
localized in a one-dimensional potential in the presence of an external
periodic modulating field. The dependence of the revival time on various
parameters of the driven system is shown analytically. As an example of
application of our approach, we compare the analytically obtained values of the
revival time for various modulation strengths with the numerically computed
ones in the case of a driven gravitational cavity. We show that they are in
very good agreement.


title: Does the wavefunction describe individual systems?

abstract:
  We analyze the issue of the interpretation of the wavefunction, namely
whether it should be interpreted as describing individual systems or ensembles
of identically prepared systems. We propose an experiment which can decide the
issue, based on the simultaneous measurement of the same observable with
different detectors, and we discuss the theoretical implications of the
possible experimental outcomes.


title: Quantum entanglement measure based on wedge product

abstract:
  We construct an entanglement measure that coincides with the generalized
concurrence for a general pure bipartite state based on wedge product.
Moreover, we construct an entanglement measure for pure multi-qubit states,
which are entanglement monotone. Furthermore, we generalize our result on a
general pure multipartite state.


title:
Comment on " Low-frequency character of the Casimir force between
  metallic films"

abstract:
  In Phys. Rev. {\bf E 70}, 047102 (2004), J.R. Torgerson and S.K. Lamoreaux
investigated for the first time the real-frequency spectrum of finite
temperature correction to the Casimir force, for metallic plates of finite
conductivity. The very interesting result of this study is that the correction
from the TE mode is dominated by low frequencies, for which the dielectric
description of the metal is invalid. However, their analysis of the problem,
based on more appropriate low-frequency metallic boundary conditions, uses an
incorrect form of boundary conditions for TE modes. We repeat their analysis,
using the correct boundary conditions. Our computations confirm their most
important result: contrary to the result of the dielectric model, the thermal
TE mode correction leads to an increase in the TE mode force of attraction
between the plates. The magnitude of the correction has a value about twenty
times larger than that quoted by them.


title:
Cavity-catalyzed deterministic generation of maximal entanglement
  between nonidentical atoms

abstract:
  By exactly solving the underlying Sch\"{o}dinger equation we show that one
and the same resonant cavity can be used as a catalyst to maximally entangle
atoms of two nonidentical groups. The generation scheme is realistic and
advantageous in the sense that it is deterministic, efficient, scalable and
immune from decoherence effects.