title: Continuous variable encoding by ponderomotive interaction

abstract:
  Recently it has been proposed to construct quantum error-correcting codes
that embed a finite-dimensional Hilbert space in the infinite-dimensional
Hilbert space of a system described by continuous quantum variables [D.
Gottesman et al., Phys. Rev. A 64, 012310 (2001)]. The main difficulty of this
continuous variable encoding relies on the physical generation of the quantum
codewords. We show that ponderomotive interaction suffices to this end. As a
matter of fact, this kind of interaction between a system and a meter causes a
frequency change on the meter proportional to the position quadrature of the
system. Then, a phase measurement of the meter leaves the system in an
eigenstate of the stabilizer generators, provided that system and meter's
initial states are suitably prepared. Here we show how to implement this
interaction using trapped ions, and how the encoding can be performed on their
motional degrees of freedom. The robustness of the codewords with respect to
the various experimental imperfections is then analyzed.


title: Getting Something Out of Nothing

abstract:
  We study quantum key distribution with standard weak coherent states and
show, rather counter-intuitively, that the detection events originated from
vacua can contribute to secure key generation rate, over and above the best
prior art result. Our proof is based on a communication complexity/quantum
memory argument.


title: Practical Decoy State for Quantum Key Distribution

abstract:
  Decoy states have recently been proposed as a useful method for substantially
improving the performance of quantum key distribution. Here, we present a
general theory of the decoy state protocol based on only two decoy states and
one signal state. We perform optimization on the choice of intensities of the
two decoy states and the signal state. Our result shows that a decoy state
protocol with only two types of decoy states--the vacuum and a weak decoy
state--asymptotically approaches the theoretical limit of the most general type
of decoy state protocols (with an infinite number of decoy states). We also
present a one-decoy-state protocol. Moreover, we provide estimations on the
effects of statistical fluctuations and suggest that, even for long distance
(larger than 100km) QKD, our two-decoy-state protocol can be implemented with
only a few hours of experimental data. In conclusion, decoy state quantum key
distribution is highly practical.


title: Any is not all: EPR and the Einstein-Tolman-Podolsky paper

abstract:
  In Bohm's version of the EPR gedanken experiment, the spin of the second
particle along any vector is minus the spin of the other particle along the
same vector. It seems that either the choice of vector along which one projects
the spin of the first particle influences at superluminal speed the state of
the second particle, or naive realism holds true i.e., the projections of the
spin of any EPR particle along all the vectors are determined before any
measurement occurs). Naive realism is negated by Bell's theory that originated
and is still most often presented as related to non-locality, a relation whose
necessity has recently been proven to be false. I advocate here that the
solution of the apparent paradox lies in the fact that the spin of the second
particle is determined along any vector, but not along all vectors. Such an
any-all distinction was already present in quantum mechanics, for instance in
the fact that the spin can be measured along any vector but not at once along
all vectors, as a result of the Uncertainty Principle. The time symmetry of the
any-all distinction defended here is in fact reminiscent of (and I claim, due
to) the time symmetry of the Uncertainty Principle described by Einstein,
Tolman, and Podolsky in 1931, in a paper entitled ``Knowledge of Past and
Future in Quantum Mechanics" that is enough to negate naive realism and to hint
at the any-all distinction. A simple classical model is next built, which
captures aspects of the any-all distinction: the goal is of course not to have
a classical exact model, but to provide a caricature that might help some
people.


title: How come the Correlations?

abstract:
  In relativity there is space-time out there. In quantum mechanics there is
entanglement. Entanglement manifests itself by producing correlations between
classical events (e.g. the firing of some detectors) at any two space-time
locations. If the locations are time-like separated, i.e. one is in the future
of the other, then there is no specific difficulty to understand the
correlations. But if the two locations are space-like separated, the problem is
different. How can the two space-time locations out there know about what
happens in each other without any sort of communication? If space-time really
exists, the locations must do something like communicating. Or it was all set
up at the Beginning. But the correlations depend also on the free choice of the
experimentalists, one in each space-time location. This allowed John Bell to
derive his inequality and the experimentalists to violate it, thus refuting the
assumption that it was all set up at the beginning: the Correlations can't be
explained by common causes.


title: Postulates of quantum mechanics and phenomenology

abstract:
  We describe a system of axioms that, on one hand, is sufficient for
constructing the standard mathematical formalism of quantum mechanics and, on
the other hand, is necessary from the phenomenological standpoint. In the
proposed scheme, the Hilbert space and linear operators are only secondary
structures of the theory, while the primary structures are the elements of a
noncommutative algebra (observables) and the functionals on this algebra,
associated with the results of a single observation.


title: Quantum Measurement, a coherent description

abstract:
  It is widely known that `collapse of the wave function' on a quantum system A
may be brought about by an interaction with another quantum system B. We will
prove that this is not just a possible, but a necessary consequence of
information transfer from A to B. We generalize this in order to explain why
coherences are normally not observed in macroscopic quantum systems. Finally,
we provide a quantitative insight into the balance between information gain and
state disturbance. We define the quality of an information transfer. For all
information transfers of a certain quality, we find a minimum amount of state
collapse. Along the way, we obtain generalizations of the Joint Measurement
Theorem and of the Heisenberg Principle.


title: Information-technology approach to quantum feedback control

abstract:
  Quantum control theory is profitably reexamined from the perspective of
quantum information, two results on the role of quantum information technology
in quantum feedback control are presented and two quantum feedback control
schemes, teleportation-based distant quantum feedback control and quantum
feedback control with quantum cloning, are proposed. In the first feedback
scheme, the output from the quantum system to be controlled is fed back into
the distant actuator via teleportation to alter the dynamics of system. The
result theoretically shows that it can accomplish some tasks such as distant
feedback quantum control that Markovian or Bayesian quantum feedback can't
complete. In the second feedback strategy, the design of quantum feedback
control algorithms is separated into a state recognition step, which gives
"on-off" signal to the actuator through recognizing some copies from the
cloning machine, and a feedback (control) step using another copies of cloning
machine. A compromise between information acquisition and measurement
disturbance is established, and this strategy can perform some quantum control
tasks with coherent feedback.


title: Analysis of the SGA method for obtaining energy spectra

abstract:
  We analyze and clarify how the SGA (spectrum generating algebra) method has
been applied to different potentials. We emphasize that each energy level
$E_\nu$ obtained originally by Morse belongs to a {\em different} ${\mathfrak
{so}}(2,1)$ multiplet. The corresponding wavefunctions $\Psi_\nu$ are
eigenfuntions of the compact generators $J^\nu_0$ with the same eigenvalue
$k_0$, but with different eigenvalues $q_\nu$ of the Casimir operators $Q$. We
derive a general expression for all effective potentials which have
$\Psi_{\lambda_\nu,\nu+m}(r) \propto (J_+^\nu)^m ~\Psi_{\lambda_\nu,\nu}(r)$ as
eigenfunctions, without using super-symmetry formalism. The different actions
of SGA is further illustrated by two diagrams.


title: On the generalization of quantum state comparison

abstract:
  We investigate the unambiguous comparison of quantum states in a scenario
that is more general than the one that was originally suggested by Barnett et
al. First, we find the optimal solution for the comparison of two states taken
from a set of two pure states with arbitrary a priori probabilities. We show
that the optimal coherent measurement is always superior to the optimal
incoherent measurement. Second, we develop a strategy for the comparison of two
states from a set of N pure states, and find an optimal solution for some
parameter range when N=3. In both cases we use the reduction method for the
corresponding problem of mixed state discrimination, as introduced by Raynal et
al., which reduces the problem to the discrimination of two pure states only
for N=2. Finally, we provide a necessary and sufficient condition for
unambiguous comparison of mixed states to be possible.


title: Entanglement Distillation Protocols and Number Theory

abstract:
  We show that the analysis of entanglement distillation protocols for qudits
of arbitrary dimension $D$ benefits from applying basic concepts from number
theory, since the set $\zdn$ associated to Bell diagonal states is a module
rather than a vector space. We find that a partition of $\zdn$ into divisor
classes characterizes the invariant properties of mixed Bell diagonal states
under local permutations. We construct a very general class of recursion
protocols by means of unitary operations implementing these local permutations.
We study these distillation protocols depending on whether we use twirling
operations in the intermediate steps or not, and we study them both
analitically and numerically with Monte Carlo methods. In the absence of
twirling operations, we construct extensions of the quantum privacy algorithms
valid for secure communications with qudits of any dimension $D$. When $D$ is a
prime number, we show that distillation protocols are optimal both
qualitatively and quantitatively.

categories: quant-ph cond-mat.str-el hep-th physics.comp-ph


title:
Comment on ``Kepler problem in Dirac theory for a particle with
  position-dependent mass''

abstract:
  Based on easy-to-follow considerations it is not difficult to be vehemently
opposed not only the solutions found in that paper but also the conclusions
manifested there.


title:
Exploring super-radiant phase transitions via coherent control of a
  multi-qubit--cavity system

abstract:
  We propose the use of coherent control of a multi-qubit--cavity QED system in
order to explore novel phase transition phenomena in a general class of
multi-qubit--cavity systems. In addition to atomic systems, the associated
super-radiant phase transitions should be observable in a variety of
solid-state experimental systems, including the technologically important case
of interacting quantum dots coupled to an optical cavity mode.


title: Static and Dynamic Casimir Effect Instabilities

abstract:
  The static Casimir effect concerns quantum electrodynamic induced Lamb shifts
in the mode frequencies and thermal free energies of condensed matter systems.
Sometimes, the condensed matter constitutes the boundaries of a vacuum region.
The static frequency shift effects have been calculated in the one photon loop
perturbation theory approximation. The dynamic Casimir effect concerns two
photon radiation processes arising from time dependent frequency modulations
again computed in the one photon loop approximation. Under certain conditions
the one photon loop computation may become unstable and higher order terms must
be invoked to achieve stable solutions. This stability calculation is discussed
for a simple example dynamical Casimir effect system.


title: Weak measurements are universal

abstract:
  It is well known that any projective measurement can be decomposed into a
sequence of weak measurements, which cause only small changes to the state.
Similar constructions for generalized measurements, however, have relied on the
use of an ancilla system. We show that any generalized measurement can be
decomposed into a sequence of weak measurements without the use of an ancilla,
and give an explicit construction for these weak measurements. The measurement
procedure has the structure of a random walk along a curve in state space, with
the measurement ending when one of the end points is reached. This shows that
any measurement can be generated by weak measurements, and hence that weak
measurements are universal. This may have important applications to the theory
of entanglement.


title:
Comment on recent Physical Review Letter by Gaidarzhy, et al. "Evidence
  for quantized displacement in macroscopic nanomechanical oscillator."

abstract:
  In a recent letter, Gaidarzhy et al. claim to have observed evidence for
quantized displacements of a nanomechanical oscillator. We contend that the
evidence, analysis, claims, and conclusions presented are contrary to
expectations from fundamentals of quantum mechanics and elasticity theory, and
that the method used by the authors is unsuitable in principle to observe the
quantized energy states of a nanomechanical structure.


title:
Control of non-controllable quantum systems: A quantum control algorithm
  based on Grover iteration

abstract:
  A new notion of controllability, eigenstate controllability, is defined for
finite-dimensional bilinear quantum mechanical systems which are neither
strongly completely controllably nor completely controllable. And a quantum
control algorithm based on Grover iteration is designed to perform a quantum
control task of steering a system, which is eigenstate controllable but may not
be (strongly) completely controllable, from an arbitrary state to a target
state.


title: S-wave quantum entanglement in a harmonic trap

abstract:
  We analyze the quantum entanglement between two interacting atoms trapped in
a spherical harmonic potential. At ultra-cold temperature, ground state
entanglement is generated by the dominated s-wave interaction. Based on a
regularized pseudo-potential Hamiltonian, we examine the quantum entanglement
by performing the Schmidt decomposition of low-energy eigenfunctions. We
indicate how the atoms are paired and quantify the entanglement as a function
of a modified s-wave scattering length inside the trap.


title: Comment on "Practical Decoy State for Quantum Key Distribution"

abstract: Please goto the "Note Added" part of v6, quant-ph/0501143


title:
Characterization of tomographically faithful states in terms of their
  Wigner function

abstract:
  A bipartite quantum state is tomographically faithful when it can be used as
an input of a quantum operation acting on one of the two quantum systems, such
that the joint output state carries a complete information about the operation
itself. Tomographically faithful states are a necessary ingredient for
tomography of quantum operations and for complete quantum calibration of
measuring apparatuses. In this paper we provide a complete classification of
such states for continuous variables in terms of the Wigner function of the
state. For two-mode Gaussian states faithfulness simply resorts to correlation
between the modes.


title:
Understanding light quanta:Construction of the free electromagnetic
  field

abstract:
  The free electromagnetic field, solution of Maxwell's equations and carrier
of energy, momentum and spin, is construed as an emergent collective property
of an ensemble of photons, and with this, the consistency of an interpretation
that considers that the photons, and not the electromagnetic fields, are the
primary ontology is established.


title:
A QKD Protocol Extendable to Support Entanglement and Reduce
  Unauthorized Information Gain by Randomizing the Bases Lists with Key Values
  and Invalidate Explicit Privacy Amplification

abstract:
  This paper suggests an improvement to the BB84 scheme in Quantum key
distribution. The original scheme has its weakness in letting quantifiably more
information gain to an eavesdropper during public announcement of unencrypted
bases lists. The security of the secret key comes at the expense of the final
key length. We aim at exploiting the randomness of preparation (measurement)
basis and the bit values encoded (observed), so as to randomize the bases lists
before they are communicated over the public channel. A proof of security is
given for our scheme and proven that our protocol results in lesser information
gain by Eve in comparison with BB84 and its other extensions. Moreover, an
analysis is made on the feasibility of our proposal as such and to support
entanglement based QKD. The performance of our protocol is compared in terms of
the upper and lower bounds on the tolerable bit error rate. We also quantify
the information gain (by Eve) mathematically using the familiar approach of the
concept of Shannon entropy. The paper models the attack by Eve in terms of
interference in a multi-access quantum channel. Besides, this paper also hints
at the invalidation of a separate privacy amplification step in the
"prepare-and-measure" protocols in general.


title:
Multi-particle entanglement under asymptotic positive partial transpose
  preserving operations

abstract:
  We demonstrate that even under positive partial transpose preserving
operations in an asymptotic setting GHZ and W states are not reversibly
interconvertible. We investigate the structure of minimal reversible
entanglement generating set (MREGS) for tri-partite states under positive
partial transpose (ppt) preserving operations. We demonstrate that the set
consisting of W and EPR states alone cannot be an MREGS. In this context we
prove the surprising result that the relative entropy of entanglement can be
strictly sub-additive for certain pure tri-partite states which is crucial to
keep open the possibility that the set of GHZ-state and EPR states together
constitute an MREGS under ppt-preserving operations.


title:
Quantum Mechanics in Space--Time: the Feynman Path Amplitude Description
  of Physical Optics, de Broglie Matter Waves and Quark and Neutrino Flavour
  Oscillations

abstract:
  Feynman's laws of quantum dynamics are concisely stated, discussed in
comparison with other formulations of quantum mechanics and applied to selected
problems in the physical optics of photons and massive particles as well as
flavour oscillations. The classical wave theory of light is derived from these
laws for the case in which temporal variation of path amplitudes may be
neglected, whereas specific experiments, sensitive to the temporal properties
of path amplitudes, are suggested. The reflection coefficient of light from the
surface of a transparent medium is found to be markedly different to that
predicted by the classical Fresnel formula. Except for neutrino oscillations,
good agreement is otherwise found with previous calculations of spatially
dependent quantum interference effects.


title: A Three-Stage Quantum Cryptography Protocol

abstract:
  We present a three-stage quantum cryptographic protocol guaranteeing security
in which each party uses its own secret key. Unlike the BB84 protocol, where
the qubits are transmitted in only one direction and classical information
exchanged thereafter, the communication in the proposed protocol remains
quantum in each stage. A related system of key distribution is also described.


title:
Classical and Quantum Analysis of One Dimensional Velocity Selection for
  Ultracold Atoms

abstract:
  We discuss a velocity selection technique for obtaining cold atoms, in which
all atoms below a certain energy are spatially selected from the surrounding
atom cloud. Velocity selection can in some cases be more efficient than other
cooling techniques for the preparation of ultracold atom clouds in one
dimension. With quantum mechanical and classical simulations and theory we
present a scheme using a dipole force barrier to select the coldest atoms from
a magnetically trapped atom cloud. The dipole and magnetic potentials create a
local minimum which traps the coldest atoms. A unique advantage of this
technique is the sharp cut-off in the velocity distribution of the sample of
selected atoms. Such a non-thermal distribution should prove useful for a
variety of experiments, including proposed studies of atomic tunneling and
scattering from quantum potentials. We show that when the rms size of the atom
cloud is smaller than the local minimum in which the selected atoms are
trapped, the velocity selection technique can be more efficient in 1-D than
some common techniques such as evaporative cooling. For example, one simulation
shows nearly 6% of the atoms retained at a temperature 100 times lower than the
starting condition.


title:
Entanglement and conservation of orbital angular momentum in spontaneous
  parametric down-conversion

abstract:
  We show that the transfer of the plane wave spectrum of the pump beam to the
fourth-order transverse spatial correlation function of the two-photon field
generated by spontaneous parametric down-conversion leads to the conservation
and entanglement of orbital angular momentum of light. By means of a simple
experimental setup based on fourth-order (or two-photon) interferometry, we
show that our theoretical model provides a good description for down-converted
fields carrying orbital angular momentum.


title: Low Momentum Classical Mechanics with Effective Quantum Potentials

abstract:
  A recently introduced effective quantum potential theory is studied in a low
momentum region of phase space. This low momentum approximation is used to show
that the new effective quantum potential induces a space-dependent mass and a
smoothed potential both of them constructed from the classical potential. The
exact solution of the approximated theory in one spatial dimension is found.
The concept of effective transmission and reflection coefficients for effective
quantum potentials is proposed and discussed in comparison with an analogous
quantum statistical mixture problem. The results are applied to the case of a
square barrier.


title: Insecurity Of Imperfect Quantum Bit Seal

abstract:
  Quantum bit seal is a way to encode a classical bit quantum mechanically so
that everyone can obtain non-zero information on the value of the bit.
Moreover, such an attempt should have a high chance of being detected by an
authorized verifier. Surely, a reader looks for a way to get the maximum amount
of information on the sealed bit and at the same time to minimize her chance of
being caught. And a verifier picks a sealing scheme that maximizes his chance
of detecting any measurement of the sealed bit. Here, I report a strategy that
passes all measurement detection procedures at least half of the time for all
quantum bit sealing schemes. This strategy also minimizes a reader's chance of
being caught under a certain scheme. In this way, I extend the result of
Bechmann-Pasquinucci et al. by proving that quantum seal is insecure in the
case of imperfect sealed bit recovery.


title:
Use of non-adiabatic geometric phase for quantum computing by nuclear
  magnetic resonance

abstract:
  Geometric phases have stimulated researchers for its potential applications
in many areas of science. One of them is fault-tolerant quantum computation. A
preliminary requisite of quantum computation is the implementation of
controlled logic gates by controlled dynamics of qubits. In controlled
dynamics, one qubit undergoes coherent evolution and acquires appropriate
phase, depending on the state of other qubits. If the evolution is geometric,
then the phase acquired depend only on the geometry of the path executed, and
is robust against certain types of errors. This phenomenon leads to an
inherently fault-tolerant quantum computation.
  Here we suggest a technique of using non-adiabatic geometric phase for
quantum computation, using selective excitation. In a two-qubit system, we
selectively evolve a suitable subsystem where the control qubit is in state
|1>, through a closed circuit. By this evolution, the target qubit gains a
phase controlled by the state of the control qubit. Using these geometric phase
gates we demonstrate implementation of Deutsch-Jozsa algorithm and Grover's
search algorithm in a two-qubit system.


title:
On the observability of Bell's inequality violation in the optical
  Stern-Gerlach model

abstract:
  Using the optical Stern-Gerlach model, we have recently shown that the
non-local correlations between the internal variables of two atoms that
successively interact with the field of an ideal cavity in proximity of a nodal
region are affected by the atomic translational dynamics. As a consequence,
there can be some difficulties in observing violation of the Bell's inequality
for the atomic internal variables. These difficulties persist even if the atoms
travel an antinodal region, except when the spatial wave packets are exactly
centered in an antinodal point.


title: Unravelling Two-Photon High-Dimensional Entanglement

abstract:
  We propose an interferometric method to investigate the non-locality of
high-dimensional two-photon orbital angular momentum states generated by
spontaneous parametric down conversion. We incorporate two half-integer spiral
phase plates and a variable-reflectivity output beam splitter into a
Mach-Zehnder interferometer to build an orbital angular momentum analyzer. This
setup enables testing the non-locality of high-dimensional two-photon states by
repeated use of the Clauser-Horne inequality.


title: PT-symmetric supersymmetry in a solvable short-range model

abstract:
  The simplest purely imaginary and piecewise constant $\cal PT$-symmetric
potential located inside a larger box is studied. Unless its strength exceeds a
certain critical value, all the spectrum of its bound states remains real and
discrete. We interpret such a model as an initial element of the generalized
non-Hermitian Witten's hierarchy of solvable Hamiltonians and construct its
first supersymmetric (SUSY) partner in closed form.


title:
Pure superposition states of atoms generated by a bichromatic
  elliptically polarized filed

abstract:
  We find specific polarizations of components of a bichromatic field, which
allow one to prepare pure superposition states of atoms, using the coherent
population trapping effect. These $m$$-$$m$ states are prepared in the system
of Zeeman substates of the ground-state hyperfine levels with arbitrary angular
momenta $F_1$ and $F_2$. It is established that, in general case $m\ne 0$, the
use of waves with elliptical polarizations ($\epsilon_1$$\perp$$\epsilon_2$
field configuration for alkali metal atoms) is necessary for the pure state
preparation. We analytically show an unique advantage of the D1 line of alkali
metal atoms, which consists in the possibility to generate pure $m$$-$$m$
states even in the absence of spectral resolution of the excited-state
hyperfine levels, contrary to the D2 line.


title: Magnetic Susceptibility as a Macrosopic Entaglement Witness

abstract:
  We show that magnetic susceptibility can reveal spin entanglement between
individual constituents of a solid, while magnetisation describes their local
properties. We then show that these two thermodynamical quantities satisfy
complementary relation in the quantum-mechanical sense. It describes sharing of
(quantum) information in the solid between spin entanglement and local
properties of its individual constituents. Magnetic susceptibility is shown to
be a macroscopic spin entanglement witness that can be applied without complete
knowledge of the specific model (Hamiltonian) of the solid.


title:
Dependences of the van der Waals atom-wall interaction on atomic and
  material properties

abstract:
  The 1%-accurate calculations of the van der Waals interaction between an atom
and a cavity wall are performed in the separation region from 3 nm to 150 nm.
The cases of metastable He${}^{\ast}$ and Na atoms near the metal,
semiconductor or dielectric walls are considered. Different approximations to
the description of wall material and atomic dynamic polarizability are
carefully compared. The smooth transition to the Casimir-Polder interaction is
verified. It is shown that to obtain accurate results for the atom-wall van der
Waals interaction at shortest separations with an error less than 1% one should
use the complete optical tabulated data for the complex refraction index of the
wall material and the accurate dynamic polarizability of an atom. The obtained
results may be useful for the theoretical interpretation of recent experiments
on quantum reflection and Bose-Einstein condensation of ultracold atoms on or
near surfaces of different nature.


title:
A Group-Theoretical Approach to the Periodic Table of Chemical Elements:
  Old and New Developments

abstract:
  This paper is a companion article to the review paper by the present author
devoted to the classification of matter constituents (chemical elements and
particles) and published in the first part of the proceedings of The Second
Harry Wiener International Memorial Conference (see quant-ph/0310155). It is
mainly concerned with a group-theoretical approach to the Periodic Table of the
neutral elements based on the noncompact group SO(4,2)xSU(2).

categories: quant-ph math-ph math.MP physics.atom-ph


title: Is the CPT-norm always positive?

abstract:
  We give an explicit example of an exactly solvable PT-symmetric Hamiltonian
with the unbroken PT symmetry which has one eigenfunction with the zero
PT-norm. The set of its eigenfunctions is not complete in corresponding Hilbert
space and it is non-diagonalizable. In the case of a regular Sturm-Liouville
problem any diagonalizable PT-symmetric Hamiltonian with the unbroken PT
symmetry has a complete set of positive CPT-normalazable eigenfunctions. For
non-diagonalizable
  Hamiltonians a complete set of CPT-normalazable functions is possible but the
functions belonging to the root subspace corresponding to multiple zeros of the
characteristic determinant are not eigenfunctions of the Hamiltonian anymore.


title: Differential geometry of density states

abstract:
  We consider a geometrization, i.e., we identify geometrical structures, for
the space of density states of a quantum system. We also provide few comments
on a possible application of this geometrization for composite systems.


title:
Minimax determination of the energy spectrum of the Dirac equation in a
  Schwarzschild background

abstract:
  We calculate the bound-state energy spectrum of the Dirac Equation in a
Schwarzschild black hole background using a minimax variational method. Our
method extends that of Talman to the case of non-Hermitian interactions, such
as a black hole. The trial function is expressed in terms of a basis set that
takes into account both the Hermitian limit of the interaction in the
non-relativistic approximation, and the general behaviour of the solutions at
the origin, the horizon and infinity. Using this trial function an
approximation to the full complex energy bound-state spectrum is computed. We
study the behaviour of the method as the coupling constant of the interaction
is increased, which increases both the relativistic effects and the size of the
non-Hermitian part of the interaction. Finally we confirm that the method
follows the expected Hylleraas-Undheim behaviour.


title:
Fresnel tomography: a novel approach to the wave function reconstruction
  based on Fresnel representation of tomograms

abstract:
  New type of tomographic probability distribution, which contains complete
information on the density matrix (wave function) related to the Fresnel
transform of the complex wave function, is introduced. Relation to symplectic
tomographic probability distribution is elucidated. Multimode generalization of
the Fresnel tomography is presented. Examples of applications of the present
approach are given.


title: The Bell Theorem as a Special Case of a Theorem of Bass

abstract:
  The theorem of Bell states that certain results of quantum mechanics violate
inequalities that are valid for objective local random variables. We show that
the inequalities of Bell are special cases of theorems found ten years earlier
by Bass and stated in full generality by Vorob'ev. This fact implies precise
necessary and sufficient mathematical conditions for the validity of the Bell
inequalities. We show that these precise conditions differ significantly from
the definition of objective local variable spaces and as an application that
the Bell inequalities may be violated even for objective local random
variables.


title: Experimental Characterization of 1-D Velocity Selection

abstract:
  We demonstrate a 1-D velocity selection technique which relies on combining
magnetic and optical potentials. We have selected atom clouds with temperatures
as low as 2.9% of the initial temperature, with an efficiency of 1%. The
efficiency (percentage of atoms selected) of the technique can vary as slowly
as the square root of the final temperature. In addition to selecting the
coldest atoms from a cloud, this technique imparts a sharp cut-off in the
velocity distribution. The cold selected atoms are confined in a small well,
spatially separated from higher energy atoms. Such a non-thermal distribution
may be useful for atom optics experiments, such as studies of atom tunneling.


title:
Effect of phase relaxation on quantum superpositions in complex
  collisions

abstract:
  We study the effect of phase relaxation on coherent superpositions of
rotating clockwise and anticlockwise wave packets in the regime of strongly
overlapping resonances of the intermediate complex. Such highly excited
deformed complexes may be created in binary collisions of heavy ions, molecules
and atomic clusters. It is shown that phase relaxation leads to a reduction of
the interference fringes, thus mimicking the effect of decoherence. This
reduction is crucial for the determination of the phase--relaxation width from
the data on the excitation function oscillations in heavy--ion collisions and
bimolecular chemical reactions. The difference between the effects of phase
relaxation and decoherence is discussed.


title:
Optimal classical-communication-assisted local model of n-qubit
  Greenberger-Horne-Zeilinger correlations

abstract:
  We present a model, motivated by the criterion of reality put forward by
Einstein, Podolsky, and Rosen and supplemented by classical communication,
which correctly reproduces the quantum-mechanical predictions for measurements
of all products of Pauli operators on an n-qubit GHZ state (or ``cat state'').
The n-2 bits employed by our model are shown to be optimal for the allowed set
of measurements, demonstrating that the required communication overhead scales
linearly with n. We formulate a connection between the generation of the local
values utilized by our model and the stabilizer formalism, which leads us to
conjecture that a generalization of this method will shed light on the content
of the Gottesman-Knill theorem.


title:
Ultrahigh-sensitivity high-linearity photodetection system using a
  low-gain avalanche photodiode with an ultralow-noise readout circuit

abstract:
  A highly sensitive photodetection system with a detection limit of 1 photon/s
was developed. This system uses a commercially available 200-mm-diameter
silicon avalanche photodiode (APD) and an in-house-developed ultralow-noise
readout circuit, which are both cooled to 77 K. When the APD operates at a low
gain of about 10, it has a high-linearity response to the number of incident
photons and a low excess noise factor. The APD also has high quantum efficiency
and a dark current of less than 1 e/s at 77 K. This photodetection system will
shorten the measurement time and enable higher spatial and wavelength
resolution for near-field scanning optical microscopes.


title:
Effective squeezing enhancement via measurement-induced non-Gaussian
  operation and its application to dense coding scheme

abstract:
  We study the measurement-induced non-Gaussian operation on the single- and
two-mode \textit{Gaussian} squeezed vacuum states with beam splitters and
on-off type photon detectors, with which \textit{mixed non-Gaussian} states are
generally obtained in the conditional process. It is known that the
entanglement can be enhanced via this non-Gaussian operation on the two-mode
squeezed vacuum state. We show that, in the range of practical squeezing
parameters, the conditional outputs are still close to Gaussian states, but
their second order variances of quantum fluctuations and correlations are
effectively suppressed and enhanced, respectively. To investigate an
operational meaning of these states, especially entangled states, we also
evaluate the quantum dense coding scheme from the viewpoint of the mutual
information, and we show that non-Gaussian entangled state can be advantageous
compared with the original two-mode squeezed state.


title:
Multiphoton discrimination at telecom wavelength with charge integration
  photon detector

abstract:
  We present a charge integration photon detector (CIPD) that enables the
efficient measurement of photon number states at the telecom-fiber wavelengths
with a quantum efficiency of 80% and a resolution less than 0.5 electrons at 1
Hz sampling. The CIPD consists of an InGaAs PIN photodiode and a GaAs JFET in a
charge integration amplifier, which is cooled to 4.2 K to reduce thermal noise
and leakage current. The charge integration amplifier exhibits a low noise
level of 470 nV/Hz1/2. The dark count is as low as 500 electrons/hour.


title: Quantum Information Processing using coherent states in cavity QED

abstract:
  Using the highly detuned interaction between three-level $\Lambda$-type atoms
and coherent optical fields, we can realize the C-NOT gates from atoms to
atoms, optical fields to optical fields, atoms to optical fields and optical
fields to atoms. Based on the realization of the C-NOT gates we propose an
entanglement purification scheme to purify a mixed entangled states of coherent
optical fields. The simplicity of the current scheme makes it possible that it
will be implemented in experiment in the near future.


title: Minimum orbit dimension for local unitary action on n-qubit pure states

abstract:
  The group of local unitary transformations partitions the space of n-qubit
quantum states into orbits, each of which is a differentiable manifold of some
dimension. We prove that all orbits of the n-qubit quantum state space have
dimension greater than or equal to 3n/2 for n even and greater than or equal to
(3n + 1)/2 for n odd. This lower bound on orbit dimension is sharp, since
n-qubit states composed of products of singlets achieve these lowest orbit
dimensions.


title: Moduli of Quanta

abstract:
  The classical phase of the matrix model of 11-dimensional M-theory is
complex, infinite-dimensional Hilbert space. As a complex manifold, the latter
admits a continuum of nonequivalent, complex-differentiable structures that can
be placed in 1-to-1 correspondence with families of coherent states in the
Hilbert space of quantum states. The moduli space of nonbiholomorphic complex
structures on classical phase space turns out to be an infinite-dimensional
symmetric space. We argue that each choice of a complex differentiable
structure gives rise to a physically different notion of an elementary quantum.


title:
Extended Cahill-Glauber formalism for finite dimensional spaces: I.
  Fundamentals

abstract:
  The Cahill-Glauber approach for quantum mechanics on phase-space is extended
to the finite dimensional case through the use of discrete coherent states. All
properties and features of the continuous formalism are appropriately
generalized. The continuum results are promptly recovered as a limiting case.
The Jacobi Theta functions are shown to have a prominent role in the context.


title:
Multimode Squeezing Properties of a Confocal Opo: Beyond the Thin
  Crystal Approximation

abstract:
  Up to now, transverse quantum effects (usually labelled as "quantum imaging"
effects) which are generated by nonlinear devices inserted in resonant optical
cavities have been calculated using the "thin crystal approximation", i.e.
taking into account the effect of diffraction only inside the empty part of the
cavity, and neglecting its effect in the nonlinear propagation inside the
nonlinear crystal. We introduce in the present paper a theoretical method which
is not restricted by this approximation. It allows us in particular to treat
configurations closer to the actual experimental ones, where the crystal length
is comparable to the Rayleigh length of the cavity mode. We use this method in
the case of the confocal OPO, where the thin crystal approximation predicts
perfect squeezing on any area of the transverse plane, whatever its size and
shape. We find that there exists in this case a "coherence length" which gives
the minimum size of a detector on which perfect squeezing can be observed, and
which gives therefore a limit to the improvement of optical resolution that can
be obtained using such devices.


title:
Terahertz-induced depletion of the ground-state population of neutral
  donors in GaAs measured by resonant elastic light scattering from donor-bound
  excitons

abstract:
  Strong resonant elastic light scattering (RELS) from the donor-bound exciton
transition in GaAs (1.514eV) occurs at neutral donors in the ground (1S) state,
but not at neutral donors in excited hydrogenic states. When 1.6 THz radiation
is incident on an ensemble of neutral donors, we observe up to a 30% decrease
in the RELS, corresponding to a decrease in the population of neutral donors in
their ground states. This optical detection method is similar to quantum
nondemolition measurement techniques used for readout of ion trap quantum
computers and diamond nitrogen-vacancy centers. In this scheme, Auger
recombination of the bound exciton, which changes the state of the donor during
measurement, limits the measurement fidelity and maximum NIR excitation
intensity.


title: Security of Quantum Key Distribution with Realistic Devices

abstract:
  We simulate quantum key distribution (QKD) experimental setups and give out
some improvement for QKD procedures. A new data post-processing protocol is
introduced, mainly including error correction and privacy amplification. This
protocol combines the ideas of GLLP and the decoy states, which essentially
only requires to turn up and down the source power. We propose a practical way
to perform the decoy state method, which mainly follows the idea of Lo's decoy
state. A new data post-processing protocol is then developed for the QKD scheme
with the decoy state. We first study the optimal expected photon number mu of
the source for the improved QKD scheme. We get the new optimal mu=O(1)
comparing with former mu=O(eta), where eta is the overall transmission
efficiency. With this protocol, we can then improve the key generation rate
from quadratic of transmission efficiency O(eta2) to O(eta). Based on the
recent experimental setup, we obtain the maximum secure transmission distance
of over 140 km.


title: Current status of the DARPA Quantum Network

abstract:
  This paper reports the current status of the DARPA Quantum Network, which
became fully operational in BBN's laboratory in October 2003, and has been
continuously running in 6 nodes operating through telecommunications fiber
between Harvard University, Boston University, and BBN since June 2004. The
DARPA Quantum Network is the world's first quantum cryptography network, and
perhaps also the first QKD systems providing continuous operation across a
metropolitan area. Four more nodes are now being added to bring the total to 10
QKD nodes. This network supports a variety of QKD technologies, including
phase-modulated lasers through fiber, entanglement through fiber, and freespace
QKD. We provide a basic introduction and rational for this network, discuss the
February 2005 status of the various QKD hardware suites and software systems in
the network, and describe our operational experience with the DARPA Quantum
Network to date. We conclude with a discussion of our ongoing work.


title: Generalized Spherical Harmonics for l=2

abstract:
  We extend to the case l=2 the study of the new generalized spherical
harmonics introduced recently, and worked out only for the case l=1. We present
some of the properties of the new quantities and clarify how they are related
to standard forms.


title:
Experimental implementation of local adiabatic evolution algorithms by
  an NMR quantum information processor

abstract:
  Quantum adiabatic algorithm is a method of solving computational problems by
evolving the ground state of a slowly varying Hamiltonian. The technique uses
evolution of the ground state of a slowly varying Hamiltonian to reach the
required output state. In some cases, such as the adiabatic versions of
Grover's search algorithm and Deutsch-Jozsa algorithm, applying the global
adiabatic evolution yields a complexity similar to their classical algorithms.
However, using the local adiabatic evolution, the algorithms given by J. Roland
and N. J. Cerf for Grover's search [ Phys. Rev. A. {\bf 65} 042308(2002)] and
by Saurya Das, Randy Kobes and Gabor Kunstatter for the Deutsch-Jozsa algorithm
[Phys. Rev. A. {\bf 65}, 062301 (2002)], yield a complexity of order $\sqrt{N}$
(where N=2$^{\rm n}$ and n is the number of qubits). In this paper we report
the experimental implementation of these local adiabatic evolution algorithms
on a two qubit quantum information processor, by Nuclear Magnetic Resonance.


title: The Description of Polarization

abstract:
  In this paper, we extend to polarization the method we have recently employed
to treat spin. We are led to a generalization of its treatment. Thus, we are
able to connect its matrix treatment to first principles, and we obtain the
most generalized probability amplitudes and operators for its description.


title:
Remote state preparation: arbitrary remote control of photon
  polarization

abstract:
  We experimentally demonstrate the first remote state preparation of arbitrary
single-qubit states, encoded in the polarization of photons generated by
spontaneous parametric downconversion. Utilizing degenerate and nondegenerate
wavelength entangled sources, we remotely prepare arbitrary states at two
wavelengths. Further, we derive theoretical bounds on the states that may be
remotely prepared for given two-qubit resources.


title: Van der Waals interaction of excited media

abstract:
  Casimir interaction between two media of ground-state atoms is well described
with the help of Lifshitz formula depending upon permittivity of media. We will
show that this formula is in contradiction with experimental evidence for
excited atoms. We calculate Casimir force between two atoms if one of them or
both atoms are excited. We use methods of quantum electrodynamics specially
derived for the problem. It enables us to take into account excited-state
radiation widths of atoms. Then we calculate the force between excited atom and
medium of ground-state atoms. The results are in agreement with the ones,
obtained by other authors using perturbation theory or linear response theory.
Generalization of our results to the case of interaction between two media of
excited atoms results in a formula, which is in not only in quantitative, but
in qualitative contradiction with Lifshits formula. This contradiction
disappears if media of ground-state atoms are taken. Moreover, our result does
not include permittivity of the media. It includes the quantity which differs
from the permittivity only for excited atoms. The main features of our results
are as follows. The interaction is resonant; the force may be either attractive
or repulsive depending on resonant frequencies of the atoms of different media;
the value of Casimir force may be several orders of magnitude lager than that
predicted by Lifshitz formula. The features mentioned here are in agreement
with known experimental and theoretical evidences obtained by many authors for
interaction of a single excited atom with dielectric media.


title:
Some Mathematical Aspects of the Lifshitz Formula for the Thermal
  Casimir Force

abstract:
  The applicability of the Lifshitz formula is discussed to the case of two
thick parallel plates made of real metal. The usual description of the
zero-point vacuum oscillations on the background of the frequency-dependent
dielectric permittivity is shown to be in contradiction with thermodynamics.
Instead, the Lifshitz formula for the Casimir free energy should be
reformulated in terms of the reflection coefficients containing the surface
impedance instead of the dielectric permittivity. This approach is presently
confirmed experimentally by precision measurements of the van der Waals and
Casimir forces in micromechanical systems and it is in agreement with
thermodynamics.


title: Deflating Quantum Mysteries via the Relational Blockworld

abstract:
  We present a spacetime setting for non-relativistic quantum mechanics that
deflates "quantum mysteries" and relates non-relativistic quantum mechanics to
special relativity. This is achieved by assuming spacetime symmetries are
fundamental in a blockworld setting, i.e., by interpreting spacetime relations
as fundamental to relata. To justify this Relational Blockworld (RBW), we adopt
a result due to G. Kaiser whereby the relativity of simultaneity, stemming from
the Poincare algebra of special relativity, is responsible for the canonical
commutation relations of non-relativistic quantum mechanics. And, we
incorporate a result due to A. Bohr, B. Mottelson and O. Ulfbeck whereby the
density matrix for a given experimental situation is obtained from its
spacetime symmetry group. We provide an example to illustrate the explanatory
nature of RBW and conclude by explaining how RBW deflates "quantum mysteries."


title: A Subjective Approach to Quantum Probability

abstract:
  A likelihood order is defined over linear subspaces of a finite dimensional
Hilbert space. It is shown that such an order that satisfies some plausible
axioms can be represented by a quantum probability in two cases: pure state and
uniform measure.


title: Hamiltonian of Homonucleus Molecules for NMR Quantum Computing

abstract:
  We derive the Hamiltonian in the rotating frame for NMR quantum computing
with homonucleus molecules as its computational resource. The Hamiltonian thus
obtained is different from conventional Hamiltonians that appear in literature.
It is shown that control pulses designed for heteronucleus spins can be
translated to pulses for homonucleus spins by simply replacing hard pulses by
soft pulses with properly chosen pulse width. To demonstrate the validity of
our Hamiltonian, we conducted several experiments employing cytosine as a
homonucleus molecule. All the experimental results demonstrate that our
Hamiltonian accurately describes the dynamics of the spins, while the
conventional Hamiltonian fails. Finally we use our Hamiltonian for precise
control of field inhomogeneity compensation with a pair of $\pi$-pulses.


title: The Future of Computation

abstract:
  ``The purpose of life is to obtain knowledge, use it to live with as much
satisfaction as possible, and pass it on with improvements and modifications to
the next generation.'' This may sound philosophical, and the interpretation of
words may be subjective, yet it is fairly clear that this is what all living
organisms--from bacteria to human beings--do in their life time. Indeed, this
can be adopted as the information theoretic definition of life. Over billions
of years, biological evolution has experimented with a wide range of physical
systems for acquiring, processing and communicating information. We are now in
a position to make the principles behind these systems mathematically precise,
and then extend them as far as laws of physics permit. Therein lies the future
of computation, of ourselves, and of life.


title: Optical implementability of the two-dimensional Quantum Walk

abstract:
  We propose an optical cavity implementation of the two-dimensional coined
quantum walk on the line. The implementation makes use of only classical
resources, and is tunable in the sense that a large number of different unitary
transformations can be implemented by tuning some parameters of the device.


title: Simple test for quantum channel capacity

abstract:
  Basing on states and channels isomorphism we point out that semidefinite
programming can be used as a quick test for nonzero one-way quantum channel
capacity. This can be achieved by search of symmetric extensions of states
isomorphic to a given quantum channel. With this method we provide examples of
quantum channels that can lead to high entanglement transmission but still have
zero one-way capacity, in particular, regions of symmetric extendibility for
isotropic states in arbitrary dimensions are presented. Further we derive {\it
a new entanglement parameter} based on (normalised) relative entropy distance
to the set of states that have symmetric extensions and show explicitly the
symmetric extension of isotropic states being the nearest to singlets in the
set of symmetrically extendible states. The suitable regularisation of the
parameter provides a new upper bound on one-way distillable entanglement.


title: Long-range interactions and entanglement of slow single-photon pulses

abstract:
  We show that very large nonlocal nonlinear interactions between pairs of
colliding slow-light pulses can be realized in atomic vapors in the regime of
electromagnetically induced transparency. These nonlinearities are mediated by
strong, long-range dipole--dipole interactions between Rydberg states of the
multi-level atoms in a ladder configuration. In contrast to previously studied
schemes, this mechanism can yield a homogeneous conditional phase shift of pi
even for weakly focused single-photon pulses, thereby allowing a deterministic
realization of the photonic phase gate.


title: Limit points of the monotonic schemes

abstract:
  Many numerical simulations in quantum (bilinear) control use the
monotonically convergent algorithms of Krotov (introduced by Tannor), Zhu &
Rabitz or the general form of Maday & Turinici. This paper presents an analysis
of the limit set of controls provided by these algorithms and a proof of
convergence in a particular case.


title: Quantum Erasure: Quantum Interference Revisited

abstract:
  Recent experiments in quantum optics have shed light on the foundations of
quantum physics. Quantum erasers - modified quantum interference experiments -
show that quantum entanglement is responsible for the complementarity
principle.


title: On the limit cycle for the 1/r^2 potential in momentum space

abstract:
  The renormalization of the attractive 1/r^2 potential has recently been
studied using a variety of regulators. In particular, it was shown that
renormalization with a square well in position space allows multiple solutions
for the depth of the square well, including, but not requiring a
renormalization group limit cycle. Here, we consider the renormalization of the
1/r^2 potential in momentum space. We regulate the problem with a momentum
cutoff and absorb the cutoff dependence using a momentum-independent
counterterm potential. The strength of this counterterm is uniquely determined
and runs on a limit cycle. We also calculate the bound state spectrum and
scattering observables, emphasizing the manifestation of the limit cycle in
these observables.


title:
SUSY transformations between digonalizable and non-diagonalizable
  Hamiltonians

abstract:
  Recently (see quant-ph/0503040) an explicit example has been given of a
PT-symmetric non-diagonalizable Hamiltonian. In this paper we show that such
Hamiltonians appear as supersymmetric (SUSY) partners of Hermitian (hence
diagonalizable) Hamiltonians and they can be turned back to diagonalizable
forms by appropriate SUSY transformations.


title:
k-Component q-deformed charge coherent states and their nonclassical
  properties

abstract:
  k-Component q-deformed charge coherent states are constructed, their
(over)completeness proved and their generation explored. The q-deformed charge
coherent states and the even (odd) q-deformed charge coherent states are the
two special cases of them as k becomes 1 and 2, respectively. A D-algebra
realization of the SU$_q$(1,1) generators is given in terms of them. Their
nonclassical properties are studied and it is shown that for $k\geq3$, they
exhibit two-mode q-antibunching, but neither SU$_q$(1,1) squeezing, nor one- or
two-mode q-squeezing.


title: A quantum measure of coherence and incompatibility

abstract:
  The well-known two-slit interference is understood as a special relation
between observable (localization at the slits) and state (being on both slits).
Relation between an observable and a quantum state is investigated in the
general case. It is assumed that the amount of ceherence equals that of
incompatibility between observable and state. On ground of this, an argument is
peresented that leads to a natural quantum measure of coherence, called
"coherence or incompatibility information". Its properties are studied in
detail making use of 'the mixing property of relative entropy' derived in this
article. A precise relation between the measure of coherence of an observable
and that of its coarsening is obtained and discussed from the intutitive point
of view. Convexity of the measure is proved, and thus the fact that it is an
information entity is established. A few more detailed properties of coherence
information are derived with a view to investigate final-state entanglement in
general repeatable measurement, and, more importantly, general bipartite
entanglement in follow ups of this study.


title: Quantum walks on Cayley graphs

abstract:
  We address the problem of the construction of quantum walks on Cayley graphs.
Our main motivation is the relationship between quantum algorithms and quantum
walks. In particular, we discuss the choice of the dimension of the local
Hilbert space and consider various classes of graphs on which the structure of
quantum walks may differ. We completely characterise quantum walks on free
groups and present partial results on more general cases. Some examples are
given, including a family of quantum walks on the hypercube involving a
Clifford Algebra.


title: Entanglement in SO(3)-invariant bipartite quantum systems

abstract:
  The structure of the state spaces of bipartite (N tensor N) quantum systems
which are invariant under product representations of the group SO(3) of
three-dimensional proper rotations is analyzed. The subsystems represent
particles of arbitrary spin j which transform according to an irreducible
representation of the rotation group. A positive map theta is introduced which
describes the time reversal symmetry of the local states and which is unitarily
equivalent to the transposition of matrices. It is shown that the partial time
reversal transformation theta_2 = (I tensor theta) acting on the composite
system can be expressed in terms of the invariant 6-j symbols introduced by
Wigner into the quantum theory of angular momentum. This fact enables a
complete geometrical construction of the manifold of states with positive
partial transposition and of the sets of separable and entangled states of (4
tensor 4) systems. The separable states are shown to form a three-dimensional
prism and a three-dimensional manifold of bound entangled states is identified.
A positive maps is obtained which yields, together with the time reversal, a
necessary and sufficient condition for the separability of states of (4 tensor
4) systems. The relations to the reduction criterion and to the recently
proposed cross norm criterion for separability are discussed.


title:
The Partition Function for the Anharmonic Oscillator in the
  Strong-Coupling Regime

abstract:
  We consider a single anharmonic oscillator with frequency $\omega$ and
coupling constant $\lambda$ respectively, in the strong-coupling regime. We are
assuming that the system is in thermal equilibrium with a reservoir at
temperature $\beta^{-1}$. Using the strong-coupling perturbative expansion, we
obtain the partition function for the oscillator in the regime
$\lambda>>\omega$, up to the order $\frac{1}{\sqrt{\lambda}}$. To obtain this
result, we use of a combination of Klauder's independent-value generating
functional (Acta Phys. Austr. {\bf 41}, 237 (1975)), and the generalized
zeta-function method. The free energy and the mean energy, up to the order
$\frac{1}{\sqrt{\lambda}}$, are also presented. We are showing that the
thermodynamics quantities are nonanalytic in the coupling constant.


title:
Dissipative quantum chaos: transition from wave packet collapse to
  explosion

abstract:
  Using the quantum trajectories approach we study the quantum dynamics of a
dissipative chaotic system described by the Zaslavsky map. For strong
dissipation the quantum wave function in the phase space collapses onto a
compact packet which follows classical chaotic dynamics and whose area is
proportional to the Planck constant. At weak dissipation the exponential
instability of quantum dynamics on the Ehrenfest time scale dominates and leads
to wave packet explosion. The transition from collapse to explosion takes place
when the dissipation time scale exceeds the Ehrenfest time. For integrable
nonlinear dynamics the explosion practically disappears leaving place to
collapse.


title: Compatibility and Separability for Classical and Quantum Entanglement

abstract:
  We study the concepts of compatibility and separability and their
implications for quantum and classical systems. These concepts are illustrated
on a macroscopic model for the singlet state of a quantum system of two
entangled spin 1/2 with a parameter reflecting indeterminism in the measurement
procedure. By varying this parameter we describe situations from quantum,
intermediate to classical and study which tests are compatible or separated. We
prove that for classical deterministic systems the concepts of separability and
compatibility coincide, but for quantum systems and intermediate systems these
concepts are generally different.


title: Decomposition Theorem for State Property Systems

abstract:
  We prove a decomposition theorem for orthocomplemented state property
systems. More specifically we prove that an orthocomplemented state property
system is isomorphic to the direct union of the non classical components of
this state property system over the state space of the classical state property
system of this state property system.


title: One- and two-dimensional quantum walks in arrays of optical traps

abstract:
  We propose a novel implementation of discrete time quantum walks for a
neutral atom in an array of optical microtraps or an optical lattice. We
analyze a one-dimensional walk in position space, with the coin, the additional
qubit degree of freedom that controls the displacement of the quantum walker,
implemented as a spatially delocalized qubit, i.e., the coin is also encoded in
position space. We analyze the dependence of the quantum walk on temperature
and experimental imperfections as shaking in the trap positions. Finally,
combining a spatially delocalized qubit and a hyperfine qubit, we also give a
scheme to realize a quantum walk on a two-dimensional square lattice with the
possibility of implementing different coin operators.


title: Spectral Narrowing via Quantum Coherence

abstract:
  We have studied the transmission of an optically thick Rb vapor that is
illuminated by monochromatic and noise broaden laser fields in Lambda
configuration. The spectral width of the beat signal between the two fields
after transmission through the atomic medium is more than 1000 times narrower
than the spectral width of this signal before the medium.


title: Sudden switching in qubits

abstract:
  Analytic solutions are developed for two-state systems (e.g. qubits) strongly
perturbed by a series of rapidly changing pulses, called `kicks'. The evolution
matrix may be expressed as a time ordered product of evolution matrices for
single kicks. Single, double, and triple kicks are explicitly considered, and
the onset of observability of time ordering is examined. The effects of
different order of kicks on the dynamics of the system are studied and compared
with effects of time ordering in general. To determine the range of validity of
this approach, the effect of using pulses of finite widths for 2s-2p
transitions in atomic hydrogen is examined numerically.


title: Quantum anharmonic oscillators: a new approach

abstract:
  The determination of the eigenenergies of a quantum anharmonic oscillator
consists merely in finding the zeros of a function of the energy, namely the
Wronskian of two solutions of the Schroedinger equation which are regular
respectively at the origin and at infinity. We show in this paper how to
evaluate that Wronskian exactly, except for numerical rounding errors. The
procedure is illustrated by application to the gx2+x2N (N a positive integer)
oscillator.


title: Remote Preparation of Mixed States via Noisy Entanglement

abstract:
  We present a practical and general scheme of remote preparation for pure and
mixed state, in which an auxiliary qubit and controlled-NOT gate are used. We
discuss the remote state preparation (RSP) in two important types of decoherent
channel (depolarizing and dephaseing). In our experiment, we realize RSP in the
dephaseing channel by using spontaneous parametric down conversion (SPDC),
linear optical elements and single photon detector.


title: Evolution from Entanglement to Decoherence of Bipartite Mixed "X" States

abstract:
  We examine a class of bipartite mixed states which we call X states and
report several analytic results related to the occurrence of so-called
entanglement sudden death (ESD) under time evolution in the presence of common
types of environmental noise. Avoidance of sudden death by application of
purely local operations is shown to be feasible in some cases.


title: Teleporting a rotation on remote photons

abstract:
  Quamtum remote rotation allows implement local quantum operation on remote
systems with shared entanglement. Here we report an experimental demonstration
of remote rotation on single photons using linear optical element. And the
local dephase is also teleported during the process. The scheme can be
generalized to any controlled rotation commutes with $\sigma_{z}$.


title:
The generalized coherent states for oscillators, Connected with Meixner
  and Meixner-Pollaczek polynomials % %

abstract:
  The investigation of the generalized coherent states for oscillator-like
systems connected with given family of orthogonal polynomials is continued. In
this work we consider oscillators connected with Meixner and Meixner-Pollaczek
polynomials and define generalized coherent states for these oscillators. The
completeness condition for these states is proved by the solution of the
related classical moment problem. The results are compared with the other
authors ones. In particular, we show that the Hamiltonian of the relativistic
model of linear harmonic oscillator can be thought of as the linearization of
the quadratic Hamiltonian which naturally arised in our formalism.


title: Bipartite subspaces having no LOCC-distinguishable bases

abstract:
  It is proved that there exist subspaces of bipartite tensor product spaces
that have no orthonormal bases that can be perfectly distinguished by means of
LOCC protocols. A corollary of this fact is that there exist quantum channels
having sub-optimal classical capacity even when the receiver may communicate
classically with a third party that represents the channel's environment.


title: Could a Classical Probability Theory Describe Quantum Systems?

abstract:
  Quantum Mechanics (QM) is a quantum probability theory based on the density
matrix. The possibility of applying classical probability theory, which is
based on the probability distribution function(PDF), to describe quantum
systems is investigated in this work. In a sense this is also the question
about the possibility of a Hidden Variable Theory (HVT) of Quantum Mechanics.
Unlike Bell's inequality, which need to be checked experimentally, here HVT is
ruled out by theoretical consideration. The approach taken here is to construct
explicitly the most general HVT, which agrees with all results from experiments
on quantum systems (QS), and to check its validity and acceptability. Our list
of experimental facts of quantum objects, which all quantum theories are
required to respect, includes facts on repeat quantum measurement. We show that
it plays an essential role at showing that it is very unlikely that a classical
theory can successfully reproduce all QS facts, even for a single spin-1/2
object. We also examine and rule out Bell's HVT and Bohm's HVT based on the
same consideration.


title: Theory of Games on Quantum Objects

abstract:
  Effect of replacing the classical game object with a quantum object is
analyzed. We find this replacement requires a throughout reformation of the
framework of Game Theory. If we use density matrix to represent strategy state
of players, they are full-structured density matrices with off-diagonal
elements for the new games, while reduced diagonal density matrix will be
enough for the traditional games on classical objects. In such formalism, the
payoff function of every player becomes Hermitian Operator acting on the
density matrix. Therefore, the new game looks really like Quantum Mechanics
while the traditional game becomes Classical Mechanics.


title:
The Power of Strong Fourier Sampling: Quantum Algorithms for Affine
  Groups and Hidden Shifts

abstract:
  Many quantum algorithms, including Shor's celebrated factoring and discrete
log algorithms, proceed by reduction to a Hidden Subgroup problem, in which an
unknown subgroup H of a group G must be determined from a uniform superposition
on a left coset of H. These hidden subgroup problems are typically solved by
Fourier sampling. When G is nonabelian, two important variants of Fourier
sampling have been identified: the weak standard method, where only
representation names are measured, and the strong standard method, where full
measurement (i.e., the row and column of the representation, in a suitably
chosen basis) occurs. It has remained open whether the strong standard method
is indeed stronger. In this article, we settle this question in the
affirmative. We show that hidden subgroups H of the q-hedral groups, i.e.,
semidirect products Z_q \ltimes Z_p where q | (p-1), and in particular the
affine groups A_p, can be information-theoretically reconstructed using the
strong standard method. Moreover, if |H| = p/ \polylog(p), these subgroups can
be fully reconstructed with a polynomial amount of quantum and classical
computation. We show that, for some q, neither the ``forgetful'' abelian method
nor measuring in a random basis succeeds, even information-theoretically. Thus,
at least for some groups, it is crucial to use the full power of representation
theory: namely, to measure the high-dimensional representations in an adapted
basis that respects the group's subgroup structure. We apply our algorithm for
the hidden subgroup problem to new families of cryptographically motivated
Hidden Shift problems, generalizing work of van Dam, Hallgren and Ip on shifts
of multiplicative characters.


title:
On multiparticle W states, their implementations and applications in
  quantum information problems

abstract:
  Large number of multimode entangled states of light generated in down
conversion processes belongs to a collection which is natural generalization of
the W class. A brief overview of these states, schemes for their preparation,
experimental implementations and possible applications are presented.


title:
One-dimensional relativistic dissipative system with constant force and
  its quantization

abstract:
  For a relativistic particle under a constant force and a linear velocity
dissipation force, a constant of motion is found. Problems are shown for
getting the Hamiltoninan of this system. Thus, the quantization of this system
is carried out through the constant of motion and using the quantization of the
velocity variable. The dissipative relativistic quantum bouncer is outlined
within this quantization approach.


title: New measure of electron correlation

abstract:
  We propose to quantify the "correlation" inherent in a many-electron (or
many-fermion) wavefunction by comparing it to the unique uncorrelated state
that has the same single-particle density operator as it does.


title:
Quantum entanglement and quantum nonlocality for $N$-photon entangled
  states

abstract:
  Quantum entanglement and quantum nonlocality of $N$-photon entangled states
$|\psi_{N m}> =\mathcal{N}_{m}[\cos\gamma|N-m>_{1}|m>_{2}
+e^{i\theta_{m}}\sin\gamma|m>_{1}|N-m>_{2}]$ and their superpositions are
studied. We indicate that the relative phase $\theta_{m}$ affects quantum
nonlocality but not quantum entanglement for the state $|\psi_{N m}>$. We show
that quantum nonlocality can be controlled and manipulated by adjusting the
state parameters of $|\psi_{N m}>$, superposition coefficients, and the
azimuthal angles of the Bell operator. We also show that the violation of the
Bell inequality can reach its maximal value under certain conditions. It is
found that quantum superpositions based on $|\psi_{N m}>$ can increase the
amount of entanglement, and give more ways to reach the maximal violation of
the Bell inequality.


title: Picture Invariance in Quantum Optics

abstract:
  We clarify the controversy over the coherent-state (CS) versus the
number-state (NS) pictures in quantum optics. The NS picture is equivalent to
the CS picture, as long as the phases $\phi$ in the laser fields are randomly
distributed, as M{\o}lmer argues [\pra {\bf 55}, 3195 (1997)]. However, the
claim by Rudolph and Sanders [Phys. Rev. Lett. {\bf 87}, 077903 (2001)] has a
few gaps. First, they make an assumption that is not necessarily true in the
calculation of a density operator involved with a two-mode squeezed state. We
show that there exists entanglement in the density operator without defying the
assumption that phases are randomly distributed. Moreover, using a concept of
picture-invariance, we argue that it is not that criteria for quantum
teleportation are not satisfied. We discuss an analogy between the controversy
on the CS versus NS pictures to that on the heliocentric versus geocentric
pictures.


title:
Spectral implementation of some quantum algorithms by one- and
  two-dimensional nuclear magnetic resonance

abstract:
  Quantum information processing has been effectively demonstrated on a small
number of qubits by nuclear magnetic resonance. An important subroutine in any
computing is the readout of the output. ``Spectral implementation'' originally
suggested by Z.L. Madi, R. Bruschweiler and R.R. Ernst,
  [J. Chem. Phys. 109, 10603 (1999)], provides an elegant method of readout
with the use of an extra `observer' qubit. At the end of computation, detection
of the observer qubit provides the output via the multiplet structure of its
spectrum. In "spectral implementation" by two-dimensional experiment the
observer qubit retains the memory of input state during computation, thereby
providing correlated information on input and output, in the same spectrum.
"Spectral implementation" of Grover's search algorithm, approximate quantum
counting, a modified version of Berstein-Vazirani problem, and Hogg's algorithm
is demonstrated here in three and four-qubit systems.


title:
Observability of the arrival time distribution using spin-rotator as a
  quantum clock

abstract:
  An experimentally realizable scheme is formulated which can test any
postulated quantum mechanical approach for calculating the arrival time
distribution. This is specifically illustrated by using the modulus of the
probability current density for calculating the arrival time distribution of
spin-1/2 neutral particles at the exit point of a spin-rotator(SR) which
contains a constant magnetic field. Such a calculated time distribution is then
used for evaluating the distribution of spin orientations along different
directions for these particles emerging from the SR. Based on this, the result
of spin measurement along any arbitrary direction for such an ensemble is
predicted.


title: Schwarz inequality and concurrence

abstract:
  We establish a relation between the Schwarz inequality and the generalized
concurrence of an arbitrary, pure, bipartite or tripartite state. This relation
places concurrence in a geometrical and functional-analytical setting.


title:
Photon subtracted states and enhancement of nonlocality in the presence
  of noise

abstract:
  We address nonlocality of continuous variable systems in the presence of
dissipation and noise. Three nonlocality tests have been considered, based on
the measurement of displaced-parity, field-quadrature and pseudospin-operator,
respectively. Nonlocality of twin beam has been investigated, as well as that
of its non-Gaussian counterparts obtained by inconclusive subtraction of
photons. Our results indicate that: i) nonlocality of twin beam is degraded but
not destroyed by noise; ii) photon subtraction enhances nonlocality in the
presence of noise, especially in the low-energy regime.


title:
Precise comparison of theory and new experiment for the Casimir force
  leads to stronger constraints on thermal quantum effects and long-range
  interactions

abstract:
  We report an improved dynamic determination of the Casimir pressure between
two plane plates obtained using a micromachined torsional oscillator. The main
improvements in the current experiment are a significant suppression of the
surface roughness of the Au layers deposited on the interacting surfaces, and a
decrease in the experimental error in the measurement of the absolute
separation. A metrological analysis of all data permitted us to determine both
the random and systematic errors, and to find the total experimental error as a
function of separation at the 95% confidence level. In contrast to all previous
experiments on the Casimir effect, our smallest experimental error ($\sim
0.5$%) is achieved over a wide separation range. The theoretical Casimir
pressures in the experimental configuration were calculated by the use of four
theoretical approaches suggested in the literature. All corrections to the
Casimir force were calculated or estimated. All theoretical errors were
analyzed and combined to obtain the total theoretical error at the 95%
confidence level. Finally, the confidence interval for the differences between
theoretical and experimental pressures was obtained as a function of
separation. Our measurements are found to be consistent with two theoretical
approaches utilizing the plasma model and the surface impedance over the entire
measurement region. Two other approaches to the thermal Casimir force,
utilizing the Drude model or a special prescription for the determination of
the zero-frequency contribution to the Lifshitz formula, are excluded on the
basis of our measurements at the 99% and 95% confidence levels, respectively.
Finally, constraints on Yukawa-type hypothetical interactions are strengthened
by up to a factor of 20 in a wide interaction range.


title:
Entanglement manipulation for mixed states in a multilevel atom
  interacting with a cavity field

abstract:
  We derive an explicit formula for an entanglement measure of mixed quantum
states in a multi-level atom interacting with a cavity field within the
framework of the quantum mutual entropy. We describe its theoretical basis and
discuss its practical relevance (especially in comparison with already known
pure state results). The effect of the number of levels involved on the
entanglement is demonstrated via examples of three-, four- and five-level atom.
Numerical calculations under current experimental conditions are performed and
it is found that the number of levels present changes the general features of
entanglement dramatically.


title:
Engineering massive quantum memories by topologically time-modulated
  spin rings

abstract:
  We introduce a general scheme to realize perfect storage of quantum
information in systems of interacting qubits. This novel approach is based on
{\it global} external controls of the Hamiltonian, that yield time-periodic
inversions in the dynamical evolution, allowing a perfect periodic quantum
state recontruction. We illustrate the method in the particularly interesting
and simple case of spin systems affected by XY residual interactions with or
without static imperfections. The global control is achieved by step
time-inversions of an overall topological phase of the Aharonov-Bohm type. Such
a scheme holds both at finite size and in the thermodynamic limit, thus
enabling the massive storage of arbitrarily large numbers of local states, and
is stable against several realistic sources of noise and imperfections.


title: Local Instruction-Set Model for the Experiment of Pan et al

abstract:
  We present a modified local realistic model, based on instruction sets that
can be used to approximately reproduce the data of the Pan et al experiment.
The data of our model are closer to the results of the actual experiment by Pan
et al than the predictions of their quantum mechanical model. As a consequence
the experimental results can not be used to support their claim that quantum
nonlocality has been proven.


title:
On the role of Planck's oscillator in the construction of Heisenberg's
  mechanics

abstract:
  In the present note we elucidate how physical considerations based on
Planck's oscillator led to the construction of Heisenberg's mechanics.


title: Propagation of Light in an Ensemble of "3+1"-Level Atoms

abstract:
  We study the propagation of a quantum probe light in an ensemble of
"3+1"-level atoms when the atoms are coupled to two other classical control
fields. First we calculate the dispersion properties, such as susceptibility
and group velocity, of the probe light within such an atomic medium under the
case of three-photon resonance via the dynamical algebra method of collective
atomic excitations. Then we calculate the dispersion of the probe light not
only under the case that two classical control fields have the same detunings
to the relative atomic transitions but also under the case that they have the
different detunings. Our results show in both cases the phenomenon of
electromagnetically induced transparency can accur. Especially use the second
case, we can find two transparency windows for the probe light.


title:
Conditional generation of arbitrary single-mode quantum states of light
  by repeated photon subtractions

abstract:
  We propose a scheme for the conditional generation of arbitrary finite
superpositions of (squeezed) Fock states in a single mode of a traveling
optical field. The suggested setup requires only a source of squeezed states,
beam splitters, strong coherent beams, and photodetectors with single-photon
sensitivity. The method does not require photodetectors with a high efficiency
nor with a single-photon resolution.


title:
Necessity of Combining Mutually Incompatible Perspectives in the
  Construction of a Global View: Quantum Probability and Signal Analysis

abstract:
  The scientific fields of quantum mechanics and signal-analysis originated
within different settings, aimed at different goals and started from different
scientific paradigms. Yet the development of the two subjects has become
increasingly intertwined. We argue that these similarities are rooted in the
fact that both fields of scientific inquiry had to deal with finding a single
description for a phenomenon that yields complete information about itself only
when we consider mutually incompatible accounts of that phenomenon.


title: Micro-Anthropic Principle for Quantum theory

abstract:
  Probabilistic models (developped by workers such as Boltzmann, on foundations
due to pioneers such as Bayes) were commonly regarded merely as approximations
to a deterministic reality before the roles were reversed by the quantum
revolution (under the leadership of Heisenberg and Dirac) whereby it was the
deterministic description that was reduced to the status of an approximation,
while the role of the observer became particularly prominent. The concomitant
problem of lack of objectivity in the original Copenhagen interpretation has
not been satisfactorily resolved in newer approaches of the kind pioneered by
Everett. The deficiency of such interpretations is attributable to failure to
allow for the anthropic aspect of the problem, meaning {\it a priori}
uncertainty about the identity of the observer. The required reconciliation of
subjectivity with objectivity is achieved here by distinguishing the concept of
an observer from that of a perceptor, whose chances of identification with a
particular observer need to be prescribed by a suitable anthropic principle. It
is proposed that this should be done by an entropy ansatz according to which
the relevant micro-anthropic weighting is taken to be proportional to the
logarithm of the relevant number of Everett type branch-channels.


title:
On the Power of Random Bases in Fourier Sampling: Hidden Subgroup
  Problem in the Heisenberg Group

abstract:
  The hidden subgroup problem (HSP) provides a unified framework to study
problems of group-theoretical nature in quantum computing such as order finding
and the discrete logarithm problem. While it is known that Fourier sampling
provides an efficient solution in the abelian case, not much is known for
general non-abelian groups. Recently, some authors raised the question as to
whether post-processing the Fourier spectrum by measuring in a random
orthonormal basis helps for solving the HSP. Several negative results on the
shortcomings of this random strong method are known. In this paper however, we
show that the random strong method can be quite powerful under certain
conditions on the group G. We define a parameter r(G) for a group G and show
that O((\log |G| / r(G))^2) iterations of the random strong method give enough
classical information to identify a hidden subgroup in G. We illustrate the
power of the random strong method via a concrete example of the HSP over finite
Heisenberg groups. We show that r(G) = \Omega(1) for these groups; hence the
HSP can be solved using polynomially many random strong Fourier samplings
followed by a possibly exponential classical post-processing without further
queries. The quantum part of our algorithm consists of a polynomial computation
followed by measuring in a random orthonormal basis. This gives the first
example of a group where random representation bases do help in solving the HSP
and for which no explicit representation bases are known that solve the problem
with (\log G)^O(1) Fourier samplings. As an interesting by-product of our work,
we get an algorithm for solving the state identification problem for a set of
nearly orthogonal pure quantum states.


title: On Quantum Interference: No Decoherence From an Actual Movable Mirror

abstract:
  There exists a commonly accepted viewpoint that a movable mirror in an
interferometer should cause interference breakdown due to a quantum jump to one
of the two components of a photon mode. That effect goes back to Dirac. We
argue that the conventional reasoning is inadequate: First, it would be more
circumspect to interpret interference breakdown as being due to the
entanglement of the photon with the mirror, not referring to quantum jumps.
Second--and crucial--even in such an interpretation, the reasoning does not
take into account the uncertainty of the mirror momentum. The effect of the
entanglement and interference breakdown would take place if the uncertainty
were much less than the recoil momentum, which is of the order of the photon
momentum. However, an examination leads to the conclusion that for an actual
mirror the opposite situation occurs. Thus there should be no such effect.


title: Entanglement Evolution in the Presence of Decoherence

abstract:
  The entanglement of two qubits, each defined as an effective two-level, spin
1/2 system, is investigated for the case that the qubits interact via a
Heisenberg XY interaction and are subject to decoherence due to population
relaxation and thermal effects. For zero temperature, the time dependent
concurrence is studied analytically and numerically for some typical initial
states, including a separable (unentangled) initial state. An analytical
formula for non-zero steady state concurrence is found for any initial state,
and optimal parameter values for maximizing steady state concurrence are given.
The steady state concurrence is found analytically to remain non-zero for low,
finite temperatures. We also identify the contributions of global and local
coherence to the steady state entanglement.


title: Generation of a two-photon singlet beam

abstract:
  Controlling the pump beam transverse profile in multimode Hong-Ou-Mandel
interference, we generate a ''localized" two-photon singlet state, in which
both photons propagate in the same beam. This type of multi-photon singlet beam
may useful in quantum communication to avoid decoherence. We show that although
the photons are part of the same beam, they are never in the same plane wave
mode, which is characterized by spatial antibunching behavior in the plane
normal to the propagation direction.


abstract:
  The classical limit of non-integrable quantum systems is studied. We define
non-integrable quantum systems as those which have, as their classical limit, a
non-integrable classical system. In order to obtain this limit, the
self-induced decoherence approach and the corresponding classical limit are
generalized from integrable to non-integrable systems. In this approach, the
lost of information, usually conceived as the result of a coarse-graining or
the trace of an environment, is produced by a particular choice of the algebra
of observables and the systematic use of mean values, that project the unitary
evolution onto an effective non-unitary one. The decoherence times computed
with this approach coincide with those of the literature. By means of our
method, we can obtain the classical limit of the quantum state of a
non-integrable system, which turns out to be a set of unstable, potentially
chaotic classical trajectories contained in the Wigner transformation of the
quantum state.


title: Extension maps

abstract:
  We define extension maps as maps that extend a system (through adding
ancillary systems) without changing the state in the original system. We show,
using extension maps, why a completely positive operation on an initially
entangled system results in a non positive mapping of a subsystem. We also show
that any trace preserving map, either positive or negative, can be decomposed
in terms of an extension map and a completely positive map.


title: Limitations of practical multi-photon decoherence-free states

abstract:
  It is shown in this paper that decoherence-free subspace (DFS) of practical
multi-photon polarization can not avoid the exponential decoherence even in the
same extra-environment if the photons are frequency-anticorrelated. The reason
lies in that the condition of collective decoherence is not satisfied in this
case. As an example, the evolution of biphoton's decoherence-free state is
given. Possible solution for feasible multi-photon's DFS state is also given.


title:
Dissipation of quantum information near the event horizon of
  Schwarzschild black holes

abstract:
  We found that the Wigner rotation of a particle with spin 1/2, which is
unitary in Minkowski spacetime, becomes non-unitary as the particle is falling
onto the black hole. This implies that, from the quantum information processing
point of view, any quantum information encoded in spins will be dissipated near
the black hole. The gravitational field around the black hole thus acts like a
dissipative quantum channel, especially the bit flip channel.


title: Correlated two-photon imaging with true thermal light

abstract:
  We report the first experimental demonstration of two-photon correlated
imaging with true thermal light from a hollow cathode lamp. The coherence time
of the source is much shorter than that of previous experiments using random
scattered light from a laser. A two-pinhole mask was used as object, and the
corresponding thin lens equation was well satisfied. Since thermal light
sources are easier to obtain and measure than entangled light it is conceivable
that they may be used in special imaging applications.


title:
Entanglement in SU(2)-invariant quantum systems: The positive partial
  transpose criterion and others

abstract:
  We study entanglement in mixed bipartite quantum states which are invariant
under simultaneous SU(2) transformations in both subsystems. Previous results
on the behavior of such states under partial transposition are substantially
extended. The spectrum of the partial transpose of a given SU(2)-invariant
density matrix $\rho$ is entirely determined by the diagonal elements of $\rho$
in a basis of tensor-product states of both spins with respect to a common
quantization axis. We construct a set of operators which act as entanglement
witnesses on SU(2)-invariant states. A sufficient criterion for $\rho$ having a
negative partial transpose is derived in terms of a simple spin correlator. The
same condition is a necessary criterion for the partial transpose to have the
maximum number of negative eigenvalues. Moreover, we derive a series of sum
rules which uniquely determine the eigenvalues of the partial transpose in
terms of a system of linear equations. Finally we compare our findings with
other entanglement criteria including the reduction criterion, the majorization
criterion, and the recently proposed local uncertainty relations.


title: Notes on Polarization Measurements

abstract:
  We review a few useful concepts about polarization measurements in the
quantum domain. Using a perfectly general formalism, we show how to build the
quantum counterpart of some classical quantities like Stokes parameters and
Mueller matrices, which are well known in classical polarization-measurement
theory.


title:
Limits in the characteristic function description of non-Lindblad-type
  open quantum systems

abstract:
  In this paper I investigate the usability of the characteristic functions for
the description of the dynamics of open quantum systems focussing on
non-Lindblad-type master equations. I consider, as an example, a non-Markovian
generalized master equation containing a memory kernel which may lead to
nonphysical time evolutions characterized by negative values of the density
matrix diagonal elements [S.M. Barnett and S. Stenholm, Phys. Rev. A {\bf 64},
033808 (2001)]. The main result of the paper is to demonstrate that there exist
situations in which the symmetrically ordered characteristic function is
perfectly well defined while the corresponding density matrix loses positivity.
Therefore nonphysical situations may not show up in the characteristic
function. As a consequence, the characteristic function cannot be considered an
{\it alternative complete} description of the non-Lindblad dynamics.


title: Experimental One-Way Quantum Computing

abstract:
  Standard quantum computation is based on sequences of unitary quantum logic
gates which process qubits. The one-way quantum computer proposed by
Raussendorf and Briegel is entirely different. It has changed our understanding
of the requirements for quantum computation and more generally how we think
about quantum physics. This new model requires qubits to be initialized in a
highly-entangled cluster state. From this point, the quantum computation
proceeds by a sequence of single-qubit measurements with classical feedforward
of their outcomes. Because of the essential role of measurement a one-way
quantum computer is irreversible. In the one-way quantum computer the order and
choices of measurements determine the algorithm computed. We have
experimentally realized four-qubit cluster states encoded into the polarization
state of four photons. We fully characterize the quantum state by implementing
the first experimental four-qubit quantum state tomography. Using this cluster
state we demonstrate the feasibility of one-way quantum computing through a
universal set of one- and two-qubit operations. Finally, our implementation of
Grover's search algorithm demonstrates that one-way quantum computation is
ideally suited for such tasks.


title: Revealing virtual processes in the phase space

abstract:
  The short time dynamics of a quantum Brownian particle in a harmonic
potential is studied in the phase space. An exact non-Markovian analytic
approach to calculate the time evolution of the Wigner function is presented.
The dynamics of the Wigner function of an initially squeezed state is analyzed.
It is shown that virtual exchanges of energy between the particle and the
reservoir, characterizing the non-Lindblad short time dynamics where
system-reservoir correlations are not negligible, show up in the phase space.


title: Cavity state preparation using adiabatic transfer

abstract:
  We show how to prepare a variety of cavity field states for multiple
cavities. The state preparation technique used is related to the method of
stimulated adiabatic Raman passage or STIRAP. The cavity modes are coupled by
atoms, making it possible to transfer an arbitrary cavity field state from one
cavity to another, and also to prepare non-trivial cavity field states. In
particular, we show how to prepare entangled states of two or more cavities,
such as an EPR state and a W state, as well as various entangled superpositions
of coherent states in different cavities, including Schrodinger cat states. The
theoretical considerations are supported by numerical simulations.