title:
Wigner Function Evolution of Quantum States in Presence of Self-Kerr
  Interaction

abstract:
  A Fokker-Planck equation for the Wigner function evolution in a noisy Kerr
medium ($\chi^{(3)}$ non-linearity) is presented. We numerically solved this
equation taking a coherent state as an initial condition. The dissipation
effects are discussed. We provide examples of quantum interference, sub-Planck
phase space structures, and Gaussian versus non-Gaussian dynamical evolution of
the state. The results also apply to the description of a nanomechanical
resonator with an intrinsic Duffing nonlinearity.


title: Coherence theory and coherence phenomena in a closed spin-1/2 system

abstract:
  A simplified Heisenberg spin model is studied in order to examine the idea of
decoherence in closed quantum systems. For this purpose, we present a
quantifiable definition to quantum coherence $\Xi$, and discuss in some detail
a general coherence theory and its elementary results. As expected, decoherence
is understood as a statistical process that is caused by the dynamics of the
system, similar to the growth of entropy. It appears that coherence is an
important measure that helps to understand quantum properties of a system,
e.g., the decoherence time can be derived from the coherence function $\Xi(t)$,
but not from the entropy dynamics. Moreover, the concept of decoherence time is
applicable in closed and finite systems. However, in most cases, the decay of
off-diagonal elements differs from the usual $\exp(-t/\tau_{\rm d})$ behaviour.
For concreteness, we report the form of decoherence time $\tau_{\rm d}$ in a
finite Heisenberg model with respect to the number of particles $N$, density
$n_{\rho}$, spatial dimension $D$ and $\epsilon$ in a $\eta/r^{\epsilon}$-type
of potential.


title:
Field autocorrelations in electromagnetically induced transparency:
  Effects of a squeezed probe field

abstract:
  The interaction of a quantized field with three-level atoms in $\Lambda$
configuration inside a two mode cavity is analyzed. We calculate the stationary
quadrature noise spectrum of the field outside the cavity in the case where the
input probe field is in a squeezed state and the atoms show electromagnetically
induced transparency (EIT). If the Rabi frequencies of both dipole transitions
of the atoms are different from zero, we show that the output probe field have
four maxima of squeezing absorption. We show that in some cases two of these
frequencies can be very close to the transition frequency of the atom, in a
region where the mean value of the field entering the cavity is hardly altered.
Furthermore, part of the absorbed squeezing of the probe field is transfered to
the pump field. For some conditions this transfer of squeezing can be complete.


title: A Bell pair in a generic random matrix environment

abstract:
  Two non-interacting qubits are coupled to an environment. Both coupling and
environment are represented by random matrix ensembles. The initial state of
the pair is a Bell state, though we also consider arbitrary pure states.
Decoherence of the pair is evaluated analytically in terms of purity; Monte
Carlo calculations confirm these results and also yield the concurrence of the
pair. Entanglement within the pair accelerates decoherence. Numerics display
the relation between concurrence and purity known for Werner states, allowing
us to give a formula for concurrence decay.


title: A microfabricated surface-electrode ion trap in silicon

abstract:
  The prospect of building a quantum information processor underlies many
recent advances ion trap fabrication techniques. Potentially, a quantum
computer could be constructed from a large array of interconnected ion traps.
We report on a micrometer-scale ion trap, fabricated from bulk silicon using
micro-electromechanical systems (MEMS) techniques. The trap geometry is
relatively simple in that the electrodes lie in a single plane beneath the
ions. In such a trap we confine laser-cooled 24Mg+ ions approximately 40
microns above the surface. The fabrication technique and planar electrode
geometry together make this approach amenable to scaling up to large trap
arrays. In addition we observe that little laser cooling light is scattered by
the electrodes.


title: The classical capacity for continuous variable teleportation channel

abstract:
  The process of quantum teleportation can be considered as a quantum channel.
The exact classical capacity of the continuous variable teleportation channel
is given. Also, the channel fidelity is derived. Consequently, the properties
of the continuous variable quantum teleportation are discussed and interesting
results are obtained.


title: Trace distance from the viewpoint of quantum operation techniques

abstract:
  In the present paper, the trace distance is exposed within the quantum
operations formalism. The definition of the trace distance in terms of a
maximum over all quantum operations is given. It is shown that for any pair of
different states, there are an uncountably infinite number of maximizing
quantum operations. Conversely, for any operation of the described type, there
are an uncountably infinite number of those pairs of states that the maximum is
reached by the operation. A behavior of the trace distance under considered
operations is studied. Relations and distinctions between the trace distance
and the sine distance are discussed.


title:
Stronger no-cloning, no-signalling and conservation of quantum
  information

abstract:
  It is known that the stronger no-cloning theorem and the no-deleting theorem
taken together provide the permanence property of quantum information. Also, it
is known that the violation of the no-deletion theorem would imply signalling.
Here, we show that the violation of the stronger no-cloning theorem could lead
to signalling. Furthermore, we prove the stronger no-cloning theorem from the
conservation of quantum information. These observations imply that the
permanence property of quantum information is connected to the no-signalling
and the conservation of quantum information.


title:
High-sensitivity optical monitoring of a micro-mechanical resonator with
  a quantum-limited optomechanical sensor

abstract:
  We experimentally demonstrate the high-sensitivity optical monitoring of a
micro-mechanical resonator and its cooling by active control. Coating a
low-loss mirror upon the resonator, we have built an optomechanical sensor
based on a very high-finesse cavity (30000). We have measured the thermal noise
of the resonator with a quantum-limited sensitivity at the 10^-19 m/rootHz
level, and cooled the resonator down to 5K by a cold-damping technique.
Applications of our setup range from quantum optics experiments to the
experimental demonstration of the quantum ground state of a macroscopic
mechanical resonator.


title: Always On Non-Nearest-Neighbor Coupling in Scalable Quantum Computing

abstract:
  We investigate the non-nearest-neighbor interaction effect in 1-D spin-1/2
chain model. In many previous schemes this long-range coupling is omitted
because of its relative weak strength compared with the nearest-neighbor
coupling. We show that the quantum gate deviation induced by the omitted
long-range interaction depends on not only its strength but also the scale of
the system. This implies that omitting the long-range interaction may challenge
the scalability of previous schemes. We further consider how to suppress this
unwanted effect. We propose a quantum computation scheme. In this scheme, by
using appropriate encoding method, we effectively negate the influence of the
next-nearest-neighbor interaction in order to improve the precision of quantum
gates. We also discuss the feasibility of this scheme in 1-D Josephson charge
qubit array system. This work may offer improvement in scalable quantum
computing.


title: Two qubits can be entangled in two distinct temperature regions

abstract:
  We have found that for a wide range of two-qubit Hamiltonians the
canonical-ensemble thermal state is entangled in two distinct temperature
regions. In most cases the ground state is entangled; however we have also
found an example where the ground state is separable and there are still two
regions. This demonstrates that the qualitative behavior of entanglement with
temperature can be much more complicated than might otherwise have been
expected; it is not simply determined by the entanglement of the ground state,
even for the simple case of two qubits. Furthermore, we prove a finite bound on
the number of possible entangled regions for two qubits, thus showing that
arbitrarily many transitions from entanglement to separability are not
possible. We also provide an elementary proof that the spectrum of the thermal
state at a lower temperature majorizes that at a higher temperature, for any
Hamiltonian, and use this result to show that only one entangled region is
possible for the special case of Hamiltonians without magnetic fields.


title:
On Weyl channels being covariant with respect to the maximum commutative
  group of unitaries

abstract:
  We investigate the Weyl channels being covariant with respect to the maximum
commutative group of unitary operators. This class includes the quantum
depolarizing channel and the "two-Pauli" channel as well. Then, we show that
our estimation of the output entropy for a tensor product of the phase damping
channel and the identity channel based upon the decreasing property of the
relative entropy allows to prove the additivity conjecture for the minimal
output entropy for the quantum depolarizing channel in any prime dimesnsion and
for the "two Pauli" channel in the qubit case.


title: Generation of maximally entangled states of two cavity modes

abstract:
  In this letter we present a scheme for generating maximally entangled states
of two cavity modes which enables us to generate complete set of Bell basis
states having rather simple initial state preparation. Furthermore, we study
the interaction of a two-level atom with two modes of electromagnetic field in
a high Q cavity. The two-level atom acts as a control qubit and the two mode
electromagnetic field serves as a target qubit. This simple system of quantum
electrodynamics provides us experimentally feasible universal quantum logic
gates.


title: Polarization and readout of coupled single spins in diamond

abstract:
  We study the coupling of a single nitrogen-vacancy center in diamond to a
nearby single nitrogen defect at room temperature. The magnetic dipolar
coupling leads to a splitting in the electron spin resonance frequency of the
nitrogen-vacancy center, allowing readout of the state of a single nitrogen
electron spin. At magnetic fields where the spin splitting of the two centers
is the same we observe a strong polarization of the nitrogen electron spin. The
amount of polarization can be controlled by the optical excitation power. We
combine the polarization and the readout in time-resolved pump-probe
measurements to determine the spin relaxation time of a single nitrogen
electron spin. Finally, we discuss indications for hyperfine-induced
polarization of the nitrogen nuclear spin.


title: Lecture Notes in Quantum Mechanics

abstract:
  These lecture notes cover undergraduate textbook topics (e.g. as in Sakurai),
and also additional advanced topics at the same level of presentation. In
particular: EPR and Bell; Basic postulates; The probability matrix; Measurement
theory; Entanglement; Quantum computation; Wigner-Weyl formalism; The adiabatic
picture; Berry phase; Linear response theory; Kubo formula; Modern approach to
scattering theory with mesoscopic orientation; Theory of the resolvent and the
Green function; Gauge and Galilei Symmetries; Motion in magnetic field; Quantum
Hall effect; Quantization of the electromagnetic field; Fock space formalism.


title: The BQP-hardness of approximating the Jones Polynomial

abstract:
  A celebrated important result due to Freedman, Larsen and Wang states that
providing additive approximations of the Jones polynomial at the k'th root of
unity, for constant k=5 and k>6, is BQP-hard. Together with the algorithmic
results of Freedman et al and Aharonov et al, this gives perhaps the most
natural BQP-complete problem known today and motivates further study of the
topic. In this paper we focus on the universality proof; we extend the
universality result of Freedman et al to k's that grow polynomially with the
number of strands and crossings in the link, thus extending the BQP-hardness of
Jones polynomial approximations to all values for which the AJL algorithm
applies, proving that for all those values, the problems are BQP-complete. As a
side benefit, we derive a fairly elementary proof of the Freedman et al density
result, without referring to advanced results from Lie algebra representation
theory, making this important result accessible to computer science audience.
We make use of two general lemmas we prove, the Bridge lemma and the Decoupling
lemma, which provide tools for establishing density of subgroups in SU(n).
Those tools seem to be of independent interest in more general contexts of
proving quantum universality. Our result also implies a completely classical
statement, that the_multiplicative_ approximations of the Jones polynomial, at
exactly the same values, are #P-hard, via a recent result due to Kuperberg.
Since the first publication of those results in their preliminary form
(arXiv:quant-ph/0605181v2), the methods we present here were used in several
other contexts. This paper is an improved and extended version of the original
results, and also includes discussions of the developments since then.


title: Maximally Non-Local and Monogamous Quantum Correlations

abstract:
  We introduce a version of the chained Bell inequality for an arbitrary number
of measurement outcomes, and use it to give a simple proof that the maximally
entangled state of two d dimensional quantum systems has no local component.
That is, if we write its quantum correlations as a mixture of local
correlations and general (not necessarily quantum) correlations, the
coefficient of the local correlations must be zero. This suggests an
experimental programme to obtain as good an upper bound as possible on the
fraction of local states, and provides a lower bound on the amount of classical
communication needed to simulate a maximally entangled state in dxd dimensions.
We also prove that the quantum correlations violating the inequality are
monogamous among non-signalling correlations, and hence can be used for quantum
key distribution secure against post-quantum (but non-signalling)
eavesdroppers.


title:
Trade-off between the tolerance of located and unlocated errors in
  nondegenerate quantum error-correcting codes

abstract:
  In a recent study [Rohde et al., quant-ph/0603130 (2006)] of several quantum
error correcting protocols designed for tolerance against qubit loss, it was
shown that these protocols have the undesirable effect of magnifying the
effects of depolarization noise. This raises the question of which general
properties of quantum error-correcting codes might explain such an apparent
trade-off between tolerance to located and unlocated error types. We extend the
counting argument behind the well-known quantum Hamming bound to derive a bound
on the weights of combinations of located and unlocated errors which are
correctable by nondegenerate quantum codes. Numerical results show that the
bound gives an excellent prediction to which combinations of unlocated and
located errors can be corrected with high probability by certain large
degenerate codes. The numerical results are explained partly by showing that
the generalized bound, like the original, is closely connected to the
information-theoretic quantity the quantum coherent information. However, we
also show that as a measure of the exact performance of quantum codes, our
generalized Hamming bound is provably far from tight.


title:
The entanglement of damped noon-state and its performance in phase
  measurement

abstract:
  The state evolution of the initial optical \textit{noon} state is
investigated. The residue entanglement of the state is calculated after it is
damped by amplitude and phase damping. The relative entropy of entanglement of
the damped state is exactly obtained. The performance of direct application of
the damped \textit{noon} state is compared with that of firstly distilling the
docoherence damped state then applying it in measurement.


title: Single qubit, two qubit gates and no signalling principle

abstract:
  In this work we investigate that whether one can construct single and two
qubit gates for arbitrary quantum states from the principle of no signalling.
We considered the problem for Pauli gates, Hadamard gate, C-Not gate.


title:
Revisiting impossible quantum operations using principle of
  no-signalling and non increase of entanglement under LOCC

abstract:
  In this letter, we show the impossibility of the general operation introduced
by Pati [3] using two different but consistent principles (i) no-signalling
(ii) non increase of entanglement under LOCC.


title: The Significance of Density in the Structure of Quantum Theories

abstract:
  It is proved that density plays a crucial role in the structure of quantum
field theory. The Dirac and the Klein-Gordon equations are examined. The
results prove that the Dirac equation is consistent with density related
requirements whereas the Klein-Gordon equation fails to do that. Experimental
data support these conclusions.


title: Lensless Fourier-Transform Ghost Imaging with Classical Incoherent Light

abstract:
  The Fourier-Transform ghost imaging of both amplitude-only and pure-phase
objects was experimentally observed with classical incoherent light at Fresnel
distance by a new lensless scheme. The experimental results are in good
agreement with the standard Fourier-transform of the corresponding objects.
This scheme provides a new route towards aberration-free diffraction-limited 3D
images with classically incoherent thermal light, which have no resolution and
depth-of-field limitations of lens-based tomographic systems.


title: Photon statistics dispersion in excitonic composites

abstract:
  Linear media are predicted to exist whose relative permiability is an
operator in the space of quantum states of light. Such media are characterized
by a photon statistics--dependent refractive index. This indicates a new type
of optical dispersion -- the photon statistics dispersion. Interaction of
quantum light with such media modifies the photon number distribution and, in
particular, the degree of coherence of light. An excitonic composite -- a
collection of noninteracting quantum dots -- is considered as a realization of
the medium with the photon statistics dispersion. Expressions are derived for
generalized plane waves in an excitonic composite and input--output relations
for a planar layer of the material. Transformation rules for different photon
initial states are analyzed. Utilization of the photon statistics dispersion in
potential quantum--optical devices is discussed.


title: Quantum Computing Using Crossed Atomic Beams

abstract:
  A quantum computer is a hypothetical device in which the laws of quantum
mechanics are used to introduce a degree of parallelism into computations and
which could therefore significantly improve on the computational speed of a
classical computer at certain tasks. Cluster state quantum computing (recently
proposed by Raussendorf and Briegel) is a new paradigm in quantum information
processing and is a departure from the conventional model of quantum
computation. The cluster state quantum computer begins by creating a highly
entangled multi-particle state (the cluster state) which it uses as a quantum
resource during the computation. Information is processed in the computer via
selected measurements on individual qubits that form the cluster state. We
describe in detail how a scalable quantum computer can be constructed using
microwave cavity QED and, in a departure from the traditional understanding of
a computer as a fixed array of computational elements, we show that cluster
state quantum computing is well suited to atomic beam experiments. We show that
all of the necessary elements have been individually realised, and that the
construction of a truly scalable atomic beam quantum computer may be an
experimental reality in the near future.


title: Atomic entanglement mediated by a squeezed cavity field

abstract:
  We consider the coherent state radiation field inside a micromaser cavity and
study the entanglement mediated by it on a pair of two level atoms passing
though the cavity one after the other. We then investigate the effects of
squeezing of the cavity field on the atomic entanglement. We compute the
entanglement of formation for the emerging mixed two-atom state and show that
squeezing of the cavity radiation field can increase the atomic entanglement.


title: Fault-Tolerant Thresholds for Encoded Ancillae with Homogeneous Errors

abstract:
  I describe a procedure for calculating thresholds for quantum computation as
a function of error model given the availability of ancillae prepared in
logical states with independent, identically distributed errors. The thresholds
are determined via a simple counting argument performed on a single qubit of an
infinitely large CSS code. I give concrete examples of thresholds thus
achievable for both Steane and Knill style fault-tolerant implementations and
investigate their relation to threshold estimates in the literature.


title: Time Double-Slit Interference in Tunneling Ionization

abstract:
  We show that interference phenomena plays a big role for the electron yield
in ionization of atoms by an ultra-short laser pulse. Our theoretical study of
single ionization of atoms driven by few-cycles pulses extends the
photoelectron spectrum observed in the double-slit experiment by Lindner et al,
Phys. Rev. Lett. \textbf{95}, 040401 (2005) to a complete three-dimensional
momentum picture. We show that different wave packets corresponding to the same
single electron released at different times interfere, forming interference
fringes in the two-dimensional momentum distributions. These structures
reproduced by means of \textit{ab initio} calculations are understood within a
semiclassical model.


title:
Time and frequency domain solutions in an optical analogue of Grover's
  search algorithm

abstract:
  We present new results on an optical implementation of Grover's quantum
search algorithm. This extends previous work in which the transverse spatial
mode of a light beam oscillates between a broad initial input shape and a
highly localized spike, which reveals the position of the tagged item. The
spike reaches its maximum intensity after $\sim\sqrt N$ round trips in a cavity
equipped with two phase plates, where $N$ is the ratio of the surface area of
the original beam and the area of the phase spot or tagged item. In our
redesigned experiment the search space is now two-dimensional. In the time
domain we demonstrate for the first time a multiple item search where the items
appear directly as bright spots on the images of a gated camera. In a
complementary experiment we investigate the searching cavity in the frequency
domain. The oscillatory nature of the search algorithm can be seen as a
splitting of cavity eigenmodes, each of which concentrates up to 50% of its
power in the bright spot corresponding to the solution.


title: Free Dirac evolution as a quantum random walk

abstract:
  Any positive-energy state of a free Dirac particle that is initially
highly-localized, evolves in time by spreading at speeds close to the speed of
light. This general phenomenon is explained by the fact that the Dirac
evolution can be approximated arbitrarily closely by a quantum random walk,
where the roles of coin and walker systems are naturally attributed to the spin
and position degrees of freedom of the particle. Initially entangled and
spatially localized spin-position states evolve with asymptotic two-horned
distributions of the position probability, familiar from earlier studies of
quantum walks. For the Dirac particle, the two horns travel apart at close to
the speed of light.


title:
Classification of GHZ-type, W-type and GHZ-W-type multiqubit
  entanglements

abstract:
  We propose the concept of SLOCC-equivalent basis (SEB) in the multiqubit
space. In particular, two special SEBs, the GHZ-type and the W-type basis are
introduced. They can make up a more general family of multiqubit states, the
GHZ-W-type states, which is a useful kind of entanglement for quantum
teleporatation and error correction. We completely characterize the property of
this type of states, and mainly classify the GHZ-type states and the W-type
states in a regular way, which is related to the enumerative combinatorics.
Many concrete examples are given to exhibit how our method is used for the
classification of these entangled states.


title: The Generalized Lyapunov Theorem and its Application to Quantum Channels

abstract:
  We give a simple and physically intuitive necessary and sufficient condition
for a map acting on a compact metric space to be mixing (i.e. infinitely many
applications of the map transfer any input into a fixed convergency point).
This is a generalization of the "Lyapunov direct method". First we prove this
theorem in topological spaces and for arbitrary continuous maps. Finally we
apply our theorem to maps which are relevant in Open Quantum Systems and
Quantum Information, namely Quantum Channels. In this context we also discuss
the relations between mixing and ergodicity (i.e. the property that there exist
only a single input state which is left invariant by a single application of
the map) showing that the two are equivalent when the invariant point of the
ergodic map is pure.


title: Universal Quantum Computation with Continuous-Variable Cluster States

abstract:
  We describe a generalization of the cluster-state model of quantum
computation to continuous-variable systems, along with a proposal for an
optical implementation using squeezed-light sources, linear optics, and
homodyne detection. For universal quantum computation, a nonlinear element is
required. This can be satisfied by adding to the toolbox any single-mode
non-Gaussian measurement, while the initial cluster state itself remains
Gaussian. Homodyne detection alone suffices to perform an arbitrary multi-mode
Gaussian transformation via the cluster state. We also propose an experiment to
demonstrate cluster-based error reduction when implementing Gaussian
operations.


title: Nanometre-scale nuclear-spin device for quantum information processing

abstract:
  We have developed semiconductor point contact devices in which nuclear spins
in a nanoscale region are coherently controlled by all-electrical methods.
Different from the standard nuclear-magnetic resonance technique, the
longitudinal magnetization of nuclear spins is directly detected by measuring
resistance, resulting in ultra-sensitive detection of the microscopic quantity
of nuclear spins. All possible coherent oscillations have been successfully
demonstrated between two levels from four nuclear spin states of I = 3/2
nuclei. Quantum information processing is discussed based on two fictitious
qubits of an I = 3/2 system and methods are described for performing arbitrary
logical gates both on one and two qubits. A scheme for quantum state tomography
based on Mz-detection is also proposed. As the starting point of quantum
manipulations, we have experimentally prepared the effective pure states for
the I = 3/2 nuclear spin system.


title: Continuous observation of a squeezed coherent state

abstract:
  The main aim of the paper is to present the analytical solution of the
Belavkin quantum filtering equation for damped harmonic oscillator being
initially in the squeezed coherent state for diffusion observation with complex
white noise. The comparison of the a priori and a posteriori mean value of the
optical quadrature operators and the photon number operator is given.


title:
Polar molecules near superconducting resonators: a coherent,
  all-electrical, molecule-mesoscopic interface

abstract:
  The challenge of building a scalable quantum processor requires consolidation
of the conflicting requirements of achieving coherent control and preservation
of quantum coherence in a large scale quantum system. Moreover, the system
should be compatible with miniaturization and integration of quantum circuits.
Mesoscopic solid state systems such as superconducting islands and quantum dots
feature robust control techniques using local electrical signals and
self-evident scaling based on advances in fabrication; however, in general the
quantum states of solid state devices tend to decohere rapidly. In contrast,
quantum optical systems based on trapped ions and neutral atoms exhibit
dramatically better coherence properties, while miniaturization of atomic and
molecular systems, and their integration with mesoscopic electrical circuits,
remains an important challenge. Below we describe methods for the integration
of a single particle system -- an isolated polar molecule -- with mesoscopic
solid state devices in a way that produces robust, coherent, quantum-level
control. The methods described include the trapping, cooling, detection,
coherent manipulation and quantum coupling of isolated polar molecules at
sub-micron dimensions near cryogenic stripline microwave resonators. We show
that electrostatically trapped polar molecules can exhibit strong confinement
and fast, purely electrical gate control. Furthermore, the effect of electrical
noise sources, a key issue in quantum information processing, can be suppressed
to very low levels via appropriate preparation and manipulation of the polar
molecules. Our setup provides a scalable cavity QED-type quantum computer
architecture, where entanglement of distant qubits stored in long-lived
rotational molecular states is achieved via exchange of microwave photons.


title: Adiabatic Condition for Nonlinear Systems

abstract:
  We revisit the adiabatic criterion in stimulated Raman adiabatic passage for
the three-level $\Lambda$-system, and compare the situation with and without
nonlinearity. In linear systems, the adiabatic condition is derived with the
help of the instantaneous eigenvalues and eigenstates of the Hamiltonian, a
procedure that breaks down in the presence of nonlinearity. Using an explicit
example relevant to photoassociation of atoms into diatomic molecules, we
demonstrate that the proper way to derive the adiabatic condition for the
nonlinear systems is through a linearization procedure.


title: All-electrical control of single ion spins in a semiconductor

abstract:
  We propose a method for all-electrical initialization, control and readout of
the spin of single ions substituted into a semiconductor. Mn ions in GaAs form
a natural example. In the ion's ground state the Mn core spin magnetic moment
locks antiparallel to the spin and orbital magnetic moment of a bound valence
hole from the GaAs host. Direct electrical manipulation of the ion spin is
possible because electric fields manipulate the orbital wave function of the
hole, and through the spin-orbit coupling the spin is reoriented as well.
Coupling two or more ion spins can be achieved using electrical gates to
control the size of the valence hole wave function near the semiconductor
surface. This proposal for coherent manipulation of individual ionic spins and
controlled coupling of ionic spins via electrical gates alone may find
applications in extremely high density information storage and in scalable
coherent or quantum information processing.


title:
The complete basis with an unextendible product basis and
  exact-entanglement bases (CBUPB) of three qubit

abstract:
  In this paper, an unextendible product basis and exact-entanglement bases of
three qubit is given, and the properties of entanglement for exact-entanglement
bases are also discussed. In addition, the bound entangled mixed state is
obtained from the exact-entanglement bases.


title: Improving the Convergence of an Iterative Algorithm Proposed By Waxman

abstract:
  In the iterative algorithm recently proposed by Waxman for solving eigenvalue
problems, we point out that the convergence rate may be improved. For many
non-singular symmetric potentials which vanish asymptotically, a simple
analytical relationship between the coupling constant of the potential and the
ground state eigenvalue is obtained which can be used to make the algorithm
more efficient.


title: Experimental Quantum Communication without a Shared Reference Frame

abstract:
  We present an experimental realization of a robust quantum communication
scheme [Phys. Rev. Lett. 93, 220501 (2004)] using pairs of photons entangled in
polarization and time. Our method overcomes errors due to collective rotation
of the polarization modes (e.g., birefringence in optical fiber or
misalignment), is insensitive to the phase's fluctuation of the interferometer,
and does not require any shared reference frame including time reference,
except the need to label different photons. The practical robustness of the
scheme is further shown by implementing a variation of the Bennett-Brassard
1984 quantum key distribution protocol over 1 km optical fiber.


title: Non-invasive Measurements of Cavity Parameters by Use of Squeezed Vacuum

abstract:
  We propose and experimentally demonstrate a method for non-invasive
measurements of cavity parameters by injection of squeezed vacuum into an
optical cavity. The principle behind this technique is the destruction of the
correlation between upper and lower quantum sidebands with respect to the
carrier frequency when the squeezed field is incident on the cavity. This
method is especially useful for ultrahigh $Q$ cavities, such as whispering
gallery mode (WGM) cavities, in which absorption and scattering by
light-induced nonlinear processes inhibit precise measurements of the cavity
parameters. We show that the linewidth of a test cavity is measured to be
$\gamma = 844\pm40$ kHz, which agrees with the classically measured linewidth
of the cavity within the uncertainty ($\gamma=856\pm34$ kHz).


title:
Entanglement of Quantum States, Thermodynamical Statistical
  Distributions and Physical Nature of Temperature

abstract:
  Thermodynamical equilibrium is considered as an effect of quantum entangling
of the vacuum state of a system. An explicit mathematical model of multi-
particle entangled pure quantum states is developed and analyzed. In the
framework, the process of measurement results in probability distributions that
exactly correspond to the heat equilibrium of a system in a thermostat.


title:
Matching method and exact solvability of discrete PT-symmetric square
  wells

abstract:
  Discrete PT-symmetric square wells are studied. Their wave functions are
found proportional to classical Tshebyshev polynomials of complex argument. The
compact secular equations for energies are derived giving the real spectra in
certain intervals of non-Hermiticity strengths Z. It is amusing to notice that
although the known square well re-emerges in the usual continuum limit, a twice
as rich, upside-down symmetric spectrum is exhibited by all its present
discretized predecessors.


title: Ultracold Few-Boson Systems in a Double-Well Trap

abstract:
  We investigate the transition of a quasi-one-dimensional few-boson system
from a weakly correlated to a fragmented and finally a fermionized ground
state. Our numerically exact analysis, based on a multi-configurational method,
explores the interplay between different shapes of external and inter-particle
forces. Specifically, we demonstrate that the addition of a central barrier to
an otherwise harmonic trap may supports the system's fragmentation, with a
symmetry-induced distinction between even and odd atom numbers. Moreover, the
impact of inhomogeneous interactions is studied, where the effective coupling
strength is spatially modulated. It is laid out how the ground state can be
displaced in a controlled way depending on the trap and the degree of
modulation. We present the one- and two-body densities and, beyond that,
highlight the role of correlations on the basis of the natural occupations.


title: Momentum transfer for momentum transfer-free which-path experiments

abstract:
  We analyze the origin of interference disappearance in which-path double
aperture experiments. We show that we can unambiguously define an observable
momentum transfer between the quantum particle and the path detector and we
prove in particular that the so called ``momentum transfer free'' experiments
can be in fact logically interpreted in term of momentum transfer.


title:
Decoherence-based exploration of d-dimensional one-way quantum
  computation

abstract:
  We study the effects of amplitude and phase damping decoherence in
d-dimensional one-way quantum computation (QC). Our investigation shows how
information transfer and entangling gate simulations are affected for d>=2. To
understand motivations for extending the one-way model to higher dimensions, we
describe how d-dimensional qudit cluster states deteriorate under environmental
noise. In order to protect quantum information from the environment we consider
the encoding of logical qubits into physical qudits and compare entangled pairs
of linear qubit-cluster states with single qudit clusters of equal length and
total dimension. Our study shows a significant reduction in the performance of
one-way QC for d>2 in the presence of Markovian type decoherence models.


title: Wigner-Araki-Yanase theorem on Distinguishability

abstract:
  The presence of an additive conserved quantity imposes a limitation on the
measurement process. According to the Wigner-Araki-Yanase theorem, the perfect
repeatability and the distinguishability on the apparatus cannot be attained
simultaneously. Instead of the repeatability, in this paper, the
distinguishability on both systems is examined. We derive a trade-off
inequality between the distinguishability of the final states on the system and
the one on the apparatus. The inequality shows that the perfect
distinguishability of both systems cannot be attained simultaneously.


title:
Quantum secure direct communication network with superdense coding and
  decoy photons

abstract:
  A quantum secure direct communication network scheme is proposed with quantum
superdense coding and decoy photons. The servers on a passive optical network
prepare and measure the quantum signal, i.e., a sequence of the $d$-dimensional
Bell states. After confirming the security of the photons received from the
receiver, the sender codes his secret message on them directly. For preventing
a dishonest server from eavesdropping, some decoy photons prepared by measuring
one photon in the Bell states are used to replace some original photons. One of
the users on the network can communicate any other one. This scheme has the
advantage of high capacity, and it is more convenient than others as only a
sequence of photons is transmitted in quantum line.


title:
Unified Theory of Annihilation-Creation Operators for Solvable
  (`Discrete') Quantum Mechanics

abstract:
  The annihilation-creation operators $a^{(\pm)}$ are defined as the
positive/negative frequency parts of the exact Heisenberg operator solution for
the `sinusoidal coordinate'. Thus $a^{(\pm)}$ are hermitian conjugate to each
other and the relative weights of various terms in them are solely determined
by the energy spectrum. This unified method applies to most of the solvable
quantum mechanics of single degree of freedom including those belonging to the
`discrete' quantum mechanics.

categories: quant-ph hep-th math-ph math.CA math.MP nlin.SI


title: Finite size mean-field models

abstract:
  We characterize the two-site marginals of exchangeable states of a system of
quantum spins in terms of a simple positivity condition. This result is used in
two applications. We first show that the distance between two-site marginals of
permutation invariant states on N spins and exchangeable states is of order
1/N. The second application relates the mean ground state energy of a
mean-field model of composite spins interacting through a product pair
interaction with the mean ground state energies of the components.


title: Semiclassical structure of chaotic resonance eigenfunctions

abstract:
  We study the resonance (or Gamow) eigenstates of open chaotic systems in the
semiclassical limit, distinguishing between left and right eigenstates of the
non-unitary quantum propagator, and also between short-lived and long-lived
states. The long-lived left (right) eigenstates are shown to concentrate as
$\hbar\to 0$ on the forward (backward) trapped set of the classical dynamics.
The limit of a sequence of eigenstates $\{\psi(\hbar)\}_{\hbar\to 0}$ is found
to exhibit a remarkably rich structure in phase space that depends on the
corresponding limiting decay rate. These results are illustrated for the open
baker map, for which the probability density in position space is observed to
have self-similarity properties.


title: Quantum loop programs

abstract:
  Loop is a powerful program construct in classical computation, but its power
is still not exploited fully in quantum computation. The exploitation of such
power definitely requires a deep understanding of the mechanism of quantum loop
programs. In this paper, we introduce a general scheme of quantum loops and
describe its computational process. The notions of termination and almost
termination are proposed for quantum loops, and the function computed by a
quantum loop is defined. To show their expressive power, quantum loops are
applied in describing quantum walks. Necessary and sufficient conditions for
termination and almost termination of a general quantum loop on any mixed input
state are presented. A quantum loop is said to be (almost) terminating if it
(almost) terminates on any input state. We show that a quantum loop is almost
terminating if and only if it is uniformly almost terminating. It is observed
that a small disturbance either on the unitary transformation in the loop body
or on the measurement in the loop guard can make any quantum loop (almost)
terminating. Moreover, a representation of the function computed by a quantum
loop is given in terms of finite summations of matrices. To illustrate the
notions and results obtained in this paper, two simplest classes of quantum
loop programs, one qubit quantum loops, and two qubit quantum loops defined by
controlled gates, are carefully examined.


title: Classical simulators of quantum computers and no-go theorems

abstract:
  It is discussed, why classical simulators of quantum computers escape from
some no-go claims like Kochen-Specker, Bell, or recent Conway-Kochen "Free
Will" theorems.


title: A Global Optimization Approach to Quantum Mechanics

abstract:
  This paper presents a global optimization approach to quantum mechanics,
which describes the most fundamental dynamics of the universe. It suggests that
the wave-like behavior of (sub)atomic particles could be the critical
characteristic of a global optimization method deployed by nature so that
(sub)atomic systems can find their ground states corresponding to the global
minimum of some energy function associated with the system. The classic
time-independent Schrodinger equation is shown to be derivable from the global
optimization method to support this argument.


title:
Exact solution in the Heisenberg picture and annihilation-creation
  operators

abstract:
  The annihilation-creation operators of the harmonic oscillator, the basic and
most important tools in quantum physics, are generalised to most solvable
quantum mechanical systems of single degree of freedom including the so-called
`discrete' quantum mechanics. They admit exact Heisenberg operator solution. We
present unified definition of the annihilation-creation operators (a^{(\pm)})
as the positive/negative frequency parts of the exact Heisenberg operator
solution.


title:
A Short Wavelength GigaHertz Clocked Fiber-Optic Quantum Key
  Distribution System

abstract:
  A quantum key distribution system has been developed, using standard
telecommunications optical fiber, which is capable of operating at clock rates
of greater than 1 GHz. The quantum key distribution system implements a
polarization encoded version of the B92 protocol. The system employs
vertical-cavity surface-emitting lasers with emission wavelengths of 850 nm as
weak coherent light sources, and silicon single photon avalanche diodes as the
single photon detectors. A distributed feedback laser of emission wavelength
1.3 micro-metres, and a linear gain germanium avalanche photodiode was used to
optically synchronize individual photons over the standard telecommunications
fiber. The quantum key distribution system exhibited a quantum bit error rate
of 1.4%, and an estimated net bit rate greater than 100,000 bits-per-second for
a 4.2 km transmission range. For a 10 km fiber range a quantum bit error rate
of 2.1%, and estimated net bit rate of greater than 7,000 bits-per-second was
achieved.


title: Entanglement and statistics in Hong-Ou-Mandel interferometry

abstract:
  Hong-Ou-Mandel interferometry allows one to detect the presence of
entanglement in two-photon input states. The same result holds for
two-particles input states which obey to Fermionic statistics. In the latter
case however anti-bouncing introduces qualitative differences in the
interferometer response. This effect is analyzed in a Gedankenexperiment where
the particles entering the interferometer are assumed to belong to a
one-parameter family of quons which continuously interpolate between the
Bosonic and Fermionic statistics.


title: Reexamination of Quantum Bit Commitment: the Possible and the Impossible

abstract:
  Bit commitment protocols whose security is based on the laws of quantum
mechanics alone are generally held to be impossible. In this paper we give a
strengthened and explicit proof of this result. We extend its scope to a much
larger variety of protocols, which may have an arbitrary number of rounds, in
which both classical and quantum information is exchanged, and which may
include aborts and resets. Moreover, we do not consider the receiver to be
bound to a fixed "honest" strategy, so that "anonymous state protocols", which
were recently suggested as a possible way to beat the known no-go results are
also covered. We show that any concealing protocol allows the sender to find a
cheating strategy, which is universal in the sense that it works against any
strategy of the receiver. Moreover, if the concealing property holds only
approximately, the cheat goes undetected with a high probability, which we
explicitly estimate. The proof uses an explicit formalization of general two
party protocols, which is applicable to more general situations, and a new
estimate about the continuity of the Stinespring dilation of a general quantum
channel. The result also provides a natural characterization of protocols that
fall outside the standard setting of unlimited available technology, and thus
may allow secure bit commitment. We present a new such protocol whose security,
perhaps surprisingly, relies on decoherence in the receiver's lab.


title:
A quantum secret sharing scheme among three parties ultilizing
  four-qubit Smolin bound entangled state

abstract:
  Four-qubit Smolin bound entangled state has a distinct feature: the state is
not distillable when every qubit is seperated from each other; but it makes two
separated qubit entangled if the other qubits group together. Here the feature
is applied to quantum secret sharing, a QSS protocol similar to Ekert 91
protocol of QKD is proposed. The security problem, disadvantage and advantageof
this protocol are disscused.


title: Quantum Error Correction of Time-Correlated Errors

abstract:
  The complexity of the error correction circuitry forces us to design quantum
error correction codes capable of correcting a single error per error
correction cycle. Yet, time-correlated error are common for physical
implementations of quantum systems; an error corrected during the previous
cycle may reoccur later due to physical processes specific for each physical
implementation of the qubits. In this paper we study quantum error correction
for a restricted class of time-correlated errors in a spin-boson model. The
algorithm we propose allows the correction of two errors per error correction
cycle, provided that one of them is time-correlated. The algorithm can be
applied to any stabilizer code when the two logical qubits $\mid 0_L>$ and
$\mid 1_L>$ are entangled states of $2^{n}$ basis states in
$\mathcal{H}_{2^n}$.


title: Equilibration of a dissipative quantum oscillator

abstract:
  An explicit demonstration is given of a harmonic oscillator in equilibrium
approaching the equilibrium of a corresponding interacting system by coupling
it to a thermal bath consisting of a continuum of harmonic oscillators.


title: Purification of large bicolorable graph states

abstract:
  We describe novel purification protocols for bicolorable graph states. The
protocols scale efficiently for large graph states. We introduce a method of
analysis that allows us to derive simple recursion relations characterizing
their behavior as well as analytical expressions for their thresholds and fixed
point behavior. We introduce two purification protocols with high threshold.
They can, for graph degree four, tolerate 1% (3%) gate error or 20% (30%) local
error.


title: Deterministic Secure Communications using Two-Mode Squeezed States

abstract:
  We propose a scheme for quantum cryptography that uses the squeezing phase of
a two-mode squeezed state to transmit information securely between two parties.
The basic principle behind this scheme is the fact that each mode of the
squeezed field by itself does not contain any information regarding the
squeezing phase. The squeezing phase can only be obtained through a joint
measurement of the two modes. This, combined with the fact that it is possible
to perform remote squeezing measurements, makes it possible to implement a
secure quantum communication scheme in which a deterministic signal can be
transmitted directly between two parties while the encryption is done
automatically by the quantum correlations present in the two-mode squeezed
state.


title:
Bichromatic Local Oscillator for Detection of Two-Mode Squeezed States
  of Light

abstract:
  We present a new technique for the detection of two-mode squeezed states of
light that allows for a simple characterization of these quantum states. The
usual detection scheme, based on heterodyne measurements, requires the use of a
local oscillator with a frequency equal to the mean of the frequencies of the
two modes of the squeezed field. As a result, unless the two modes are close in
frequency, a high-frequency shot-noise-limited detection system is needed. We
propose the use of a bichromatic field as the local oscillator in the
heterodyne measurements. By the proper selection of the frequencies of the
bichromatic field, it is possible to arbitrarily select the frequency around
which the squeezing information is located, thus making it possible to use a
low-bandwidth detection system and to move away from any excess noise present
in the system.


title: A single cold atom as efficient stationary source of EPR-entangled light

abstract:
  The Stokes and anti-Stokes components of the spectrum of resonance
fluorescence of a single trapped atom, which originate from the mechanical
coupling between the scattered photons and the quantized motion of the atomic
center of mass, exhibit quantum correlations which are of two-mode-squeezing
type. We study and demonstrate the build-up of such correlations in a specific
setup, which is experimentally accessible, and where the atom acts as efficient
and continuous source of EPR-entangled, two-mode squeezed light.


title: Propagation of spatially entangled qudits through free space

abstract:
  We show the propagation of entangled states of high-dimensional quantum
systems. The qudits states were generated using the transverse correlation of
the twin photons produced by spontaneous parametric down-conversion. Their
free-space distribution was performed at the laboratory scale and the
propagated states maintained a high-fidelity with their original form. The use
of entangled qudits allow an increase in the quantity of information that can
be transmitted and may also guarantee more privacy for communicating parties.
Therefore, studies about propagating entangled states of qudits are important
for the effort of building quantum communication networks.


title: Pseudo-Hermitian Interactions in Dirac Theory: Examples

abstract:
  We consider a couple of examples to study the pseudo-Hermitian interaction in
relativistic quantum mechanics. Rasbha interaction, commonly used to study the
spin Hall effect, is considered with imaginary coupling. The corresponding
Dirac Hamiltonian is shown to be parity pseudo-Hermitian. In the other example
we consider parity pseudo-Hermitian scalar interaction with arbitrary parameter
in Dirac theory. In both the cases we show that the energy spectrum is real and
all the other features of non-relativistic pseudo-Hermitian formulation are
present. Using the spectral method the positive definite metric operator
($\eta$) has been calculated explicitly for both the models to ensure positive
definite norms for the state vectors.


title:
Integration of fiber coupled high-Q silicon nitride microdisks with atom
  chips

abstract:
  Micron scale silicon nitride (SiN_x) microdisk optical resonators are
demonstrated with Q = 3.6 x 10^6 and an effective mode volume of 15 (\lambda /
n)^3 at near visible wavelengths. A hydrofluoric acid wet etch provides
sensitive tuning of the microdisk resonances, and robust mounting of a fiber
taper provides efficient fiber optic coupling to the microdisks while allowing
unfettered optical access for laser cooling and trapping of atoms. Measurements
indicate that cesium adsorption on the SiN_x surfaces significantly red-detunes
the microdisk resonances. A technique for parallel integration of multiple (10)
microdisks with a single fiber taper is also demonstrated.


title: Decoherence-Insensitive Quantum Communication by Optimal C^*-Encoding

abstract:
  The central issue in this article is to transmit a quantum state in such a
way that after some decoherence occurs, most of the information can be restored
by a suitable decoding operation. For this purpose, we incorporate redundancy
by mapping a given initial quantum state to a messenger state on a
larger-dimensional Hilbert space via a $C^*$-algebra embedding. Our noise model
for the transmission is a phase damping channel which admits a noiseless or
decoherence-free subspace or subsystem. More precisely, the transmission
channel is obtained from convex combinations of a set of lowest rank yes/no
measurements that leave a component of the messenger state unchanged. The
objective of our encoding is to distribute quantum information optimally across
the noise-susceptible component of the transmission when the noiseless
component is not large enough to contain all the quantum information to be
transmitted. We derive simple geometric conditions for optimal encoding and
construct examples.


title:
$\hbar$-independent Universality of the Quantum-Classical Canonical
  Transformations

abstract:
  A theory of non-unitary-invertible as well as unitary canonical
transformations is formulated in the context of Weyl's phase space
representations. That all quantum canonical transformations without an explicit
$\hbar$ dependence are also classical mechanical and vice versa is demonstrated
in the phase space. Contrary to some earlier results, it is also shown that the
quantum generators and their classical counterparts are identical and
$\hbar$-independent. The latter is a powerful result bringing the theory of
classical canonical transformations and the $\hbar$-independent quantum ones on
an equal footing.


title: Estimation of fluctuating magnetic fields by an atomic magnetometer

abstract:
  We present a theoretical analysis of the ability of atomic magnetometers to
estimate a fluctuating magnetic field. Our analysis makes use of a Gaussian
state description of the atoms and the probing field, and it presents the
estimator of the field and a measure of its uncertainty which coincides in the
appropriate limit with the achievements for a static field. We show by
simulations that the estimator for the current value of the field
systematically lags behind the actual value of the field, and we suggest a more
complete theory, where measurement results at any time are used to update and
improve both the estimate of the current value and the estimate of past values
of the B-field.


title: Erratum : Squeezing and entanglement delay using slow light

abstract:
  An inconsistency was found in the equations used to calculate the variance of
the quadrature fluctuations of a field propagating through a medium
demonstrating electromagnetically induced transparency (EIT). The decoherence
term used in our original paper introduces inconsistency under weak probe
approximation. In this erratum we give the Bloch equations with the correct
dephasing terms. The conclusions of the original paper remain the same. Both
entanglement and squeezing can be delayed and preserved using EIT without
adding noise when the decoherence rate is small.


title: Quantum Entanglement and Projective Ring Geometry

abstract:
  The paper explores the basic geometrical properties of the observables
characterizing two-qubit systems by employing a novel projective ring geometric
approach. After introducing the basic facts about quantum complementarity and
maximal quantum entanglement in such systems, we demonstrate that the
15$\times$15 multiplication table of the associated four-dimensional matrices
exhibits a so-far-unnoticed geometrical structure that can be regarded as three
pencils of lines in the projective plane of order two. In one of the pencils,
which we call the kernel, the observables on two lines share a base of Bell
states. In the complement of the kernel, the eight vertices/observables are
joined by twelve lines which form the edges of a cube. A substantial part of
the paper is devoted to showing that the nature of this geometry has much to do
with the structure of the projective lines defined over the rings that are the
direct product of $n$ copies of the Galois field GF(2), with $n$ = 2, 3 and 4.


title: Generation of cluster states with Josephson charge qubits

abstract:
  A scheme for the generation of the cluster states based on the Josephson
charge qubit is proposed. The two-qubit generating case is first introduced,
and then generalized to multi-qubit case. The scheme is simple and easily
manipulated, because any two charge qubits can be selectively and effectively
coupled by a common inductance. More manipulations can be realized before
decoherence sets in. All the devices in the scheme are well within the current
technology.


title: Equivalent qubit dynamics under classical and quantum noise

abstract:
  We study the dynamics of quantum systems under classical and quantum noise,
focusing on decoherence in qubit systems. Classical noise is described by a
random process leading to a stochastic temporal evolution of a closed quantum
system, whereas quantum noise originates from the coupling of the microscopic
quantum system to its macroscopic environment. We derive deterministic master
equations describing the average evolution of the quantum system under
classical continuous-time Markovian noise and two sets of master equations
under quantum noise. Strikingly, these three equations of motion are shown to
be equivalent in the case of classical random telegraph noise and proper
quantum environments. Hence fully quantum-mechanical models within the Born
approximation can be mapped to a quantum system under classical noise.
Furthermore, we apply the derived equations together with pulse optimization
techniques to achieve high-fidelity one-qubit operations under random telegraph
noise, and hence fight decoherence in these systems of great practical
interest.


title: Microscopic Theory of the Photon Recoil of an Atom in a Dielectric

abstract:
  An atom recoils when it undergoes spontaneous decay. In this paper we present
a microscopic calculation of the recoil of a source atom imbedded in a
dielectric medium. We find that the source atom recoils with the canonical
photon momentum $n\hbar k_{0,}$ where $n$ is the index of refraction and $\hbar
k_{0}$ is the photon momentum calculated at the source atom atomic frequency
$\omega_{0}$. We also show explicitly how the energy is conserved with the
photon inside the medium.


title: Quantum computation from a quantum logical perspective

abstract:
  It is well-known that Shor's factorization algorithm, Simon's period-finding
algorithm, and Deutsch's original XOR algorithm can all be formulated as
solutions to a hidden subgroup problem. Here the salient features of the
information-processing in the three algorithms are presented from a different
perspective, in terms of the way in which the algorithms exploit the
non-Boolean quantum logic represented by the projective geometry of Hilbert
space. From this quantum logical perspective, the XOR algorithm appears
directly as a special case of Simon's algorithm, and all three algorithms can
be seen as exploiting the non-Boolean logic represented by the subspace
structure of Hilbert space in a similar way. Essentially, a global property of
a function (such as a period, or a disjunctive property) is encoded as a
subspace in Hilbert space representing a quantum proposition, which can then be
efficiently distinguished from alternative propositions, corresponding to
alternative global properties, by a measurement (or sequence of measurements)
that identifies the target proposition as the proposition represented by the
subspace containing the final state produced by the algorithm.


title: Perfect Cluster States from Imperfect Global Entanglement

abstract:
  The cluster state, the highly entangled state that is the central resource
for one-way quantum computing, can be efficiently generated in a variety of
physical implementations via global nearest-neighbor interactions. In practice,
a systematic phase error is expected in the entangling process, resulting in
imperfect cluster states. We present a stochastic measurement technique to
generate large perfect cluster states and other graph states with high
probability from imperfect cluster states even when their initial entanglement
is weak.


title: Manipulation of Single Neutral Atoms in Optical Lattices

abstract:
  We analyze a scheme to manipulate quantum states of neutral atoms at
individual sites of optical lattices using focused laser beams. Spatial
distributions of focused laser intensities induce position-dependent energy
shifts of hyperfine states, which, combined with microwave radiation, allow
selective manipulation of quantum states of individual target atoms. We show
that various errors in the manipulation process are suppressed below $10^{-4} $
with properly chosen microwave pulse sequences and laser parameters. A similar
idea is also applied to measure quantum states of single atoms in optical
lattices.


title:
Efficient quantum key distribution secure against no-signalling
  eavesdroppers

abstract:
  By carrying out measurements on entangled states, two parties can generate a
secret key which is secure not only against an eavesdropper bound by the laws
of quantum mechanics, but also against a hypothetical "post-quantum"
eavesdroppers limited by the no-signalling principle only. We introduce a
family of quantum key distribution protocols of this type, which are more
efficient than previous ones, both in terms of key rate and noise resistance.
Interestingly, the best protocols involve large number of measurements. We show
that in the absence of noise, these protocols can yield one secret bit per
entanglement bit, implying that the key rates in the no-signalling post-quantum
scenario are comparable to the key rates in usual quantum key distribution.


title: Non-Gaussian, Mixed Continuous-Variable Entangled States

abstract:
  We study a class of mixed non-Gaussian entangled states that, whilst closely
related to Gaussian entangled states, none-the-less exhibit distinct properties
previously only associated with more exotic, pure non-Gaussian states.


title: Dynamical Casimir effect for different geometries

abstract:
  We consider the problem of motion-induced photon creation from quantum vacuum
inside closed, perfectly conducting cavities with time-dependent geometries.
These include one dimensional Fabry-Perrot resonators with Dirichlet or Neumann
boundary conditions, three dimensional cylindrical waveguides, and a spherical
shell. The number of Casimir TE, TM and TEM photons is computed. We also
present a classical mechanical analogue of the one dimensional dynamical
Casimir effect.


title:
Classicality, the ensemble interpretation, and decoherence: Resolving
  the Hyperion dispute

abstract:
  We analyze seemingly contradictory claims in the literature about the role
played by decoherence in ensuring classical behavior for the chaotically
tumbling satellite Hyperion. We show that the controversy is resolved once the
very different assumptions underlying these claims are recognized. In doing so,
we emphasize the distinct notions of the problem of classicality in the
ensemble interpretation of quantum mechanics and in decoherence-based
approaches that are aimed at addressing the measurement problem.


title: An observable entanglement measure for unknown mixed quantum states

abstract:
  We show how an unknown mixed quantum state's entanglement can be quantified
by a suitable, local parity measurement on its two-fold copy.


title: Distribution of G-concurrence of random pure states

abstract:
  Average entanglement of random pure states of an N x N composite system is
analyzed. We compute the average value of the determinant D of the reduced
state, which forms an entanglement monotone. Calculating higher moments of the
determinant we characterize the probability distribution P(D). Similar results
are obtained for the rescaled N-th root of the determinant, called
G-concurrence. We show that in the limit $N\to\infty$ this quantity becomes
concentrated at a single point G=1/e. The position of the concentration point
changes if one consider an arbitrary N x K bipartite system, in the joint limit
$N,K\to\infty$, K/N fixed.


title:
Photoassociation adiabatic passage of ultracold Rb atoms to form
  ultracold Rb_2 molecules

abstract:
  We theoretically explore photoassociation by Adiabatic Passage of two
colliding cold ^{85}Rb atoms in an atomic trap to form an ultracold Rb_2
molecule. We consider the incoherent thermal nature of the scattering process
in a trap and show that coherent manipulations of the atomic ensemble, such as
adiabatic passage, are feasible if performed within the coherence time window
dictated by the temperature, which is relatively long for cold atoms. We show
that a sequence of ~2*10^7 pulses of moderate intensities, each lasting ~750
ns, can photoassociate a large fraction of the atomic ensemble at temperature
of 100 microkelvin and density of 10^{11} atoms/cm^3. Use of multiple pulse
sequences makes it possible to populate the ground vibrational state. Employing
spontaneous decay from a selected excited state, one can accumulate the
molecules in a narrow distribution of vibrational states in the ground
electronic potential. Alternatively, by removing the created molecules from the
beam path between pulse sets, one can create a low-density ensemble of
molecules in their ground ro-vibrational state.


title: Popescu-Rohrlich Boxes in Quantum Measure Theory

abstract:
  Two results are proved at the quantal level in Sorkin's hierarchy of measure
theories. One is a strengthening of an existing bound on the correlations in
the EPR-Bohm setup under the assumption that the probabilities admit a strongly
positive joint quantal measure. It is also proved that any set of no-signalling
probabilities, for two distant experimenters with a choice of two alternative
experiments each and two possible outcomes per experiment, admits a joint
quantal measure, though one that is not necessarily strongly positive.


title: Quantum state stability against decoherence

abstract:
  We study the stability of the coherence of a state of a quantum system under
the effect of an interaction with another quantum system at short time. We find
an expression for evaluating the order of magnitude of the time scale for the
onset of instability as a function of the initial state of both involved
systems and of the sort of interaction between them. As an application we study
the spin-boson interaction in the dispersive interaction regime, driven by a
classical field. We find, for this model, that the behavior of the time scale
for the onset of instability, with respect to the boson bath temperature,
changes depending on the intensity of the classical field.


title: Comment on "Quantum Strategy Without Entanglement"

abstract:
  We make remarks on the paper of Du et al (quant-ph/0011078) by pointing out
that the quantum strategy proposed by the paper is trivial to the card game and
proposing a simple classical strategy to make the game in classical sense fair
too.


title: Engineering multiphoton states for linear optics computation

abstract:
  Transformations achievable by linear optical components allow to generate the
whole unitary group only when restricted to the one-photon subspace of a
multimode Fock space. In this paper, we address the more general problem of
encoding quantum information by multiphoton states, and elaborating it via
ancillary extensions, linear optical passive devices and photodetection. Our
scheme stems in a natural way from the mathematical structures underlying the
physics of linear optical passive devices. In particular, we analyze an
economical procedure for mapping a fiducial 2-photon 2-mode state into an
arbitrary 2-photon 2-mode state using ancillary resources and linear optical
passive N-ports assisted by post-selection. We found that adding a single
ancilla mode is enough to generate any desired target state. The effect of
imperfect photodetection in post-selection is considered and a simple trade-off
between success probability and fidelity is derived.


title: Indirect decoherence in optical lattices and cold gases

abstract:
  The interaction of two--level atoms with a common heat bath leads to an
effective interaction between the atoms, such that with time the internal
degrees of the atoms become correlated or even entangled. If part of the atoms
remain unobserved this creates additional indirect decoherence for the selected
atoms, on top of the direct decoherence due to the interaction with the heat
bath. I show that indirect decoherence can drastically increase and even
dominate the decoherence for sufficiently large times. I investigate indirect
decoherence through thermal black body radiation quantitatively for atoms
trapped at regular positions in an optical lattice as well as for atoms at
random positions in a cold gas, and show how indirect coherence can be
controlled or even suppressed through experimentally accessible parameters.


title:
Noise-Free Measurement of Harmonic Oscillators with Instantaneous
  Interactions

abstract:
  We present a method of measuring the quantum state of a harmonic oscillator
through instantaneous probe-system selective interactions of the
Jaynes-Cummings type. We prove that this scheme is robust to general
decoherence mechanisms, allowing the possibility of measuring fast-decaying
systems in the weak-coupling regime. This method could be applied to different
setups: motional states of trapped ions, microwave fields in cavity/circuit
QED, and even intra-cavity optical fields.


title: Permutation and Its Partial Transpose

abstract:
  Permutation and its partial transpose play important roles in quantum
information theory. The Werner state is recognized as a rational solution of
the Yang--Baxter equation, and the isotropic state with an adjustable parameter
is found to form a braid representation. The set of permutation's partial
transposes is an algebra called the "PPT" algebra which guides the construction
of multipartite symmetric states. The virtual knot theory having permutation as
a virtual crossing provides a topological language describing quantum
computation having permutation as a swap gate. In this paper, permutation's
partial transpose is identified with an idempotent of the Temperley--Lieb
algebra. The algebra generated by permutation and its partial transpose is
found to be the Brauer algebra. The linear combinations of identity,
permutation and its partial transpose can form various projectors describing
tangles; braid representations; virtual braid representations underlying common
solutions of the braid relation and Yang--Baxter equations; and virtual
Temperley--Lieb algebra which is articulated from the graphical viewpoint. They
lead to our drawing a picture called the "ABPK" diagram describing knot theory
in terms of its corresponding algebra, braid group and polynomial invariant.
The paper also identifies nontrivial unitary braid representations with
universal quantum gates, and derives a Hamiltonian to determine the evolution
of a universal quantum gate, and further computes the Markov trace in terms of
a universal quantum gate for a link invariant to detect linking numbers.


title: Scattering by PT-symmetric non-local potentials

abstract:
  A general formalism is worked out for the description of one-dimensional
scattering by non-local separable potentials and constraints on transmission
and reflection coefficients are derived in the cases of P, T, or PT invariance
of the Hamiltonian. The case of a solvable Yamaguchi potential is discussed in
detail.


title:
Deterministic secure quantum communication without maximally entangled
  states

abstract:
  Two deterministic secure quantum communication schemes are proposed, one
based on pure entangled states and the other on $d$-dimensional single-photon
states. In these two schemes, only single-photon measurements are required for
the two authorized users, which makes the schemes more convenient than others
in practical applications. Although each qubit can be read out after a
transmission of additional classical bit, it is unnecessary for the users to
transmit qubits double the distance between the sender and the receiver, which
will increase their bit rate and their security. The parties use decoy photons
to check eavesdropping efficiently. The obvious advantage in the first scheme
is that the pure entangled source is feasible with present techniques.


title:
Economical Quantum Secure Direct Communication Network with Single
  Photons

abstract:
  A scheme for quantum secure direct communication (QSDC) network is proposed
with a sequence of polarized single photons. The single photons are prepared
originally in the same state |0> by the servers on the network, which will
reduce the difficulty for the legitimate users to check eavesdropping largely.
The users code the information on the single photons with two unitary
operations which do not change their measuring bases. Some decoy photons, which
are produced by operating the sample photons with a Hadamard, are used for
preventing a potentially dishonest server from eavesdropping the quantum lines
freely. This scheme is an economical one as it is the easiest way for QSDC
network communication securely.


title:
Untouched aspects of the wave mechanics of a particle in one dimensional
  box

abstract:
  Wave mechanics of a particle in 1-D box (size $= d$) is critically analyzed
to reveal its untouched aspects. When the particle rests in its ground state,
its zero-point force ($F_o$) produces non-zero strain by modifying the box size
from $d$ to $d' = d + \Delta d$ in all practical situations where the force
($F_a$) restoring $d$ is not infinitely strong. Assuming that $F_a$ originates
from a potential $\propto x^2$ ($x$ being a small change in $d$), we find that:
(i) the particle and strained box assume a mutually bound state (under the
equilibrium between $F_o$ and $F_a$) with binding energy $\Delta{E} =
-\epsilon_o'\Delta{d}/d'$ (with $\epsilon_o' = h^2/8md'^2$ being the ground
state energy of the particle in the strained box), (ii) the box size oscillates
around $d'$ when the said equilibrium is disturbed, (iii) an exchange of energy
between the particle and the strained box occurs during such oscillations, and
(iv) the particle, having collisional motion in its excited states, assumes
collisionless motion in its ground state. These aspects have desired
experimental support and proven relevance for understanding the physics of
widely different systems such as quantum dots, quantum wires, trapped single
particle/ion, clusters of particles, superconductors, superfluids, {\it etc.}
It is emphasized that the physics of such a system in its low energy states can
be truly revealed if the theory incorporates $F_o$ and related aspects.


title:
Generation of polarization entangled photon pairs by a single crystal
  interferometric source pumped by femtosecond laser pulses

abstract:
  Photon pairs, highly entangled in polarization have been generated under
femtosecond laser pulse excitation by a type I crystal source, operating in a
single arm interferometric scheme. The relevant effects of temporal walk-off
existing in these conditions between the ordinary and extraordinary photons
were experimentally investigated. By introducing a suitable temporal
compensation between the two orthogonal polarization components highly
entangled pulsed states were obtained.


title: Dynamical Entanglement in Non-Relativistic, Elastic Scattering

abstract:
  This article considers dynamical entanglement in non-relativistic particle
scattering. Three questions are explored: what kinds of entanglement occur in
this system, how do global symmetries constrain entanglement, and how do the
boundary conditions of scattering affect dynamical entanglement? First, a
simple model of scattering spin systems is considered, then the full system is
discussed.


title:
PT-symmetric quartic anharmonic oscillator and position-dependent mass
  in a perturbative approach

abstract:
  To lowest order of perturbation theory we show that an equivalence can be
established between a $\cal PT$-symmetric generalized quartic anharmonic
oscillator model and a Hermitian position-dependent mass Hamiltonian $h$. An
important feature of $h$ is that it reveals a domain of couplings where the
quartic potential could be attractive, vanishing or repulsive. We also
determine the associated physical quantities.


title: Collective coherent population trapping in a thermal field

abstract:
  We analyzed the efficiency of coherent population trapping (CPT) in a
superposition of the ground states of three-level atoms under the influence of
the decoherence process induced by a broadband thermal field. We showed that in
a single atom there is no perfect CPT when the atomic transitions are affected
by the thermal field. The perfect CPT may occur when only one of the two atomic
transitions is affected by the thermal field. In the case when both atomic
transitions are affected by the thermal field, we demonstrated that regardless
of the intensity of the thermal field the destructive effect on the CPT can be
circumvented by the collective behavior of the atoms. An analytic expression
was obtained for the populations of the upper atomic levels which can be
considered as a measure of the level of thermal decoherence. The results show
that the collective interaction between the atoms can significantly enhance the
population trapping in that the population of the upper state decreases with
increased number of atoms. The physical origin of this feature was explained by
the semiclassical dressed atom model of the system. We introduced the concept
of multiatom collective coherent population trapping by demonstrating the
existence of collective (entangled) states whose storage capacity is larger
than that of the equivalent states of independent atoms.


title: How `hot' are mixed quantum states?

abstract:
  Given a mixed quantum state $\rho$ of a qudit, we consider any observable $M$
as a kind of `thermometer' in the following sense. Given a source which emits
pure states with these or those distributions, we select such distributions
that the appropriate average value of the observable $M$ is equal to the
average Tr$M\rho$ of $M$ in the stare $\rho$. Among those distributions we find
the most typical one, namely, having the highest differential entropy. We call
this distribution conditional Gibbs ensemble as it turns out to be a Gibbs
distribution characterized by a temperature-like parameter $\beta$. The
expressions establishing the liaisons between the density operator $\rho$ and
its temperature parameter $\beta$ are provided. Within this approach, the
uniform mixed state has the highest `temperature', which tends to zero as the
state in question approaches to a pure state.


title: Majorization in Quantum Adiabatic Algorithms

abstract:
  The majorization theory has been applied to analyze the mathematical
structure of quantum algorithms. An empirical conclusion by numerical
simulations obtained in the previous literature indicates that step-by-step
majorization seems to appear universally in quantum adiabatic algorithms. In
this paper, a rigorous analysis of the majorization arrow in a special class of
quantum adiabatic algorithms is carried out. In particular, we prove that for
any adiabatic algorithm of this class, step-by-step majorization of the ground
state holds exactly. For the actual state, we show that step-by-step
majorization holds approximately, and furthermore that the longer the running
time of the algorithm, the better the approximation.


title: Decoherence in quantum walks - a review

abstract:
  The development of quantum walks in the context of quantum computation, as
generalisations of random walk techniques, led rapidly to several new quantum
algorithms. These all follow unitary quantum evolution, apart from the final
measurement. Since logical qubits in a quantum computer must be protected from
decoherence by error correction, there is no need to consider decoherence at
the level of algorithms. Nonetheless, enlarging the range of quantum dynamics
to include non-unitary evolution provides a wider range of possibilities for
tuning the properties of quantum walks. For example, small amounts of
decoherence in a quantum walk on the line can produce more uniform spreading (a
top-hat distribution), without losing the quantum speed up. This paper reviews
the work on decoherence, and more generally on non-unitary evolution, in
quantum walks and suggests what future questions might prove interesting to
pursue in this area.


title: Can measuring entanglement be easy?

abstract:
  I raise some doubts concerning a protocol recently applied in an experiment
(Walborn et al, Nature) to measure entanglement. The protocol is much simpler
than other known entanglement-verification methods, but, I argue, needs
assumptions (namely that the state generated is known and pure) that are too
strong to be allowed and that are not justified in most experiments. An
extension of the protocol suggested in quant-ph/0605250 is much harder to
implement and still relies on assumptions not justified in
entanglement-verification protocols, as demonstrated by an explicit example.


title: Transfer of trapped atoms between two optical tweezer potentials

abstract:
  Trapped, laser-cooled rubidium atoms are transferred between two strongly
focused, horizontal, orthogonally intersecting laser beams. The transfer
efficiency is studied as a function of the vertical distance between the beam
axes. Optimum transfer is found when the distance equals the beam waist radius.
Numerical simulations reproduce well the experimental results.


title: Control and Representation of n-qubit Quantum Systems

abstract:
  Just as any state of a single qubit or 2-level system can be obtained from
any other state by a rotation operator parametrized by three real Euler angles,
we show how any state of an n-qubit or 2^n-level system can be obtained from
any other by a compact unitary transformation with 2^(n+1)-1 real angles, 2^n
of which are azimuthal-like and the rest polar-like. The results follow from a
modeling of the Hilbert space of n-qubits by a minimal left ideal of an
associative algebra. This representation is expected to be useful in the design
of new compact control techniques or more efficient algorithms in quantum
computing.


title: Resonance asymptotics in the generalized Winter model

abstract:
  We consider a modification of the Winter model describing a quantum particle
in presence of a spherical barrier given by a fixed generalized point
interaction. It is shown that the three classes of such interactions correspond
to three different types of asymptotic behaviour of resonances of the model at
high energies.


title: Multiparty Quantum Secret Report

abstract:
  A multiparty quantum secret report scheme is proposed with quantum
encryption. The boss Alice and her $M$ agents first share a sequence of
($M$+1)-particle Greenberger--Horne--Zeilinger (GHZ) states that only Alice
knows which state each ($M$+1)-particle quantum system is in. Each agent
exploits a controlled-not (CNot) gate to encrypt the travelling particle by
using the particle in the GHZ state as the control qubit. The boss Alice
decrypts the travelling particle with a CNot gate after performing a $\sigma_x$
operation on her particle in the GHZ state or not. After the GHZ states (the
quantum key) are used up, the parties check whether there is a vicious
eavesdropper, say Eve, monitoring the quantum line, by picking out some samples
from the GHZ states shared and measure them with two measuring bases. After
confirming the security of the quantum key, they use the GHZ states remained
repeatedly for next round of quantum communication. This scheme has the
advantage of high intrinsic efficiency for qubits and the total efficiency.


title: Quantum Dynamics as an analog of Conditional Probability

abstract:
  Quantum theory can be regarded as a non-commutative generalization of
classical probability. From this point of view, one expects quantum dynamics to
be analogous to classical conditional probabilities. In this paper, a variant
of the well-known isomorphism between completely positive maps and bipartite
density operators is derived, which makes this connection much more explicit.
The new isomorphism is given an operational interpretation in terms of
statistical correlations between ensemble preparation procedures and outcomes
of measurements. Finally, the isomorphism is applied to elucidate the
connection between no-cloning/no-broadcasting theorems and the monogamy of
entanglement, and a simplified proof of the no-broadcasting theorem is obtained
as a byproduct.


title: Lower bounds for the fidelity of entangled state preparation

abstract:
  Estimating the fidelity of state preparation in multi-qubit systems is
generally a time-consuming task. Nevertheless, this complexity can be reduced
if the desired state can be characterized by certain symmetries measurable with
the corresponding experimental setup. In this paper we give simple expressions
to estimate the fidelity of multi-qubit state preparation for
rotational-invariant, stabilizer, and generalized coherent states. We
specifically discuss the cat, W-type, and generalized coherent states, and
obtain efficiently measurable lower bounds for the fidelity. We use these
techniques to estimate the fidelity of a quantum simulation of an Ising-like
interacting model using two trapped ions. These results are directly applicable
to experiments using fidelity-based entanglement witnesses, such as quantum
simulations and quantum computation.


title:
Quantum to classical transition in a system with a mixed classical
  dynamics

abstract:
  We study how decoherence rules the quantum-classical transition of the Kicked
Harmonic Oscillator (KHO). When the amplitude of the kick is changed the system
presents a classical dynamics that range from regular to a strong chaotic
behavior. We show that for regular and mixed classical dynamics, and in the
presence of noise, the distance between the classical and the quantum phase
space distributions is proportional to a single parameter $\chi\equiv
K\hbar_{\rm eff}^2/4D^{3/2}$ which relates the effective Planck constant
$\hbar_{\rm eff}$, the kick amplitude $K$ and the diffusion constant $D$. This
is valid when $\chi < 1$, a case that is always attainable in the semiclassical
regime independently of the value of the strength of noise given by $D$. Our
results extend a recent study performed in the chaotic regime.


title: Sending Sensitive Messages in Quantum Packages

abstract:
  A communication protocol is introduced that allows the receiver of a message
to place an a posteriori bound on the amount of information that an
eavesdropper could have obtained during transmission of that message. This
quantum cryptographic protocol is distinct from quantum key distribution. The
quantum states and measurements required by this protocol are simple enough
that it can be implemented using existing technology.


title:
Non-Gaussian states for continuous variable quantum computation via
  Gaussian maps

abstract:
  We investigate non-Gaussian states of light as ancillary inputs for
generating nonlinear transformations required for quantum computing with
continuous variables. We consider a recent proposal for preparing a cubic phase
state, find the exact form of the prepared state and perform a detailed
comparison to the ideal cubic phase state. We thereby identify the main
challenges to preparing an ideal cubic phase state and describe the gates
implemented with the non-ideal prepared state. We also find the general form of
operations that can be implemented with ancilla Fock states, together with
Gaussian input states, linear optics and squeezing transformations, and
homodyne detection with feed forward, and discuss the feasibility of continuous
variable quantum computing using ancilla Fock states.


title:
Scattering theory with a natural regularization: Rediscovering the
  J-matrix method

abstract:
  In three dimensional scattering, the energy continuum wavefunction is
obtained by utilizing two independent solutions of the reference wave equation.
One of them is typically singular (usually, near the origin of configuration
space). Both are asymptotically regular and sinusoidal with a phase difference
(shift) that contains information about the scattering potential. Therefore,
both solutions are essential for scattering calculations. Various
regularization techniques were developed to handle the singular solution
leading to different well-established scattering methods. To simplify the
calculation the regularized solutions are usually constructed in a space that
diagonalizes the reference Hamiltonian. In this work, we start by proposing
solutions that are already regular. We write them as infinite series of square
integrable basis functions that are compatible with the domain of the reference
Hamiltonian. However, we relax the diagonal constraint on the representation by
requiring that the basis supports an infinite tridiagonal matrix representation
of the wave operator. The hope is that by relaxing this constraint on the
solution space a larger freedom is achieved in regularization such that a
natural choice emerges as a result. We find that one of the resulting two
independent wavefunctions is, in fact, the regular solution of the reference
problem. The other is uniquely regularized in the sense that it solves the
reference wave equation only outside a dense region covering the singularity in
configuration space. However, asymptotically it is identical to the irregular
solution. We show that this natural and special regularization is equivalent to
that already used in the J-matrix method of scattering.


title: Quantum Interpretations

abstract:
  Difficulties and discomfort with the interpretation of quantum mechanics are
due to differences in language between it and classical physics. Analogies to
The Special Theory of Relativity, which also required changes in the basic
worldview and language of non-relativistic classical mechanics, may help in
absorbing the changes called for by quantum physics. There is no need to invoke
extravagances such as the many worlds interpretation or specify a central role
for consciousness or neural microstructures. The simple, but basic, acceptance
that what is meant by the state of a physical system is different in quantum
physics from what it is in classical physics goes a long way in explaining its
seeming peculiarities.


title:
Dynamic generation of entangling wave packets in XY spin system with
  decaying long range couplings

abstract:
  We study the dynamic generation of spin entanglement between two distant
sites in an XY model with $1/r^{2}$-decay long range couplings. Due to the
linear dispersion relation $\epsilon (k)\sim |k|$ of magnons in such model, we
show that a well-located spin state can be dynamically split into two moving
entangled local wave packets without changing their shapes. Interestingly, when
such two wave packets meet at the diametrically opposite site after the fast
period $\tau =\pi $, the initial well-located state can be recurrent
completely. Numerical calculation is performed to confirm the analytical result
even the ring system of sizes $N$ up to several thousands are considered. The
truncation approximation for the coupling strengths is also studied. Numerical
simulation shows that the above conclusions still hold even the range of the
coupling strength is truncated at a relative shorter scale comparing to the
size of the spin system.


title: Group Covariant Protocols for Quantum String Commitment

abstract:
  We study the security of quantum string commitment (QSC) protocols with group
covariant encoding scheme. First we consider a class of QSC protocol, which is
general enough to incorporate all the QSC protocols given in the preceding
literatures. Then among those protocols, we consider group covariant protocols
and show that the exact upperbound on the binding condition can be calculated.
Next using this result, we prove that for every irreducible representation of a
finite group, there always exists a corresponding nontrivial QSC protocol which
reaches a level of security impossible to achieve classically.


title: Photon Spin, Zero-point Energy and Black-body radiation

abstract:
  A critical review of the obscure nature of the contribution of spin energy to
the energy of the electromagnetic field is presented. It is proposed that the
total energy of photon h\nu comprises of kinetic and spin parts each equal to
h\nu/2. Classical magnetic field is reinterpreted as angular momentum flux of
photon fluid. The black-body radiation law is revisited in the light of new
significance of the zero-point energy proposed here.


title: Quantum Mechanics: 44 Admissible Questions? -Not only Fapp-

abstract:
  The words: determinism, hidden variables, subjectivism, information,
objectivism, informational-theoretic axioms,observers have some connection with
physical reality? What we mean with "description" of physical reality? When we
say that we understand this reality? Certain parameters:position, velocity are
sufficient? We will focus only to conceptual considerations regarding the
relation between the "questions" and the relative "answers" in general and
specifically in quantum mechanics. It is usually believed that the answers are
more important of the questions, for this reason we can read many answers
everywhere and in different field of knowledge. We need to add and clarify some
things: (i) usually an answers require a question, (ii) but, as we know, their
relation is not so simple and immediate, (iii) For instance: a)an epistemic
questions give us ontic answers? b)the answer has a connection with the
question and vice versa? c)we could to infer a question starting from an
answer? d)there are answers without questions? These answers could be in some
framework considered as ontic answers? The relative scientific works are the
same time ontic? Speaking of quantum mechanics we see around many answers in
the meantime we do not see the correspondent questions, these answers seem
completely independent, and this seem a right road, the road of the independent
nature unlinked from human thoughts. We retain instead that questions can
affect the possible answers. Exist "something" before the question?


title:
Observation of Strong Coupling between One Atom and a Monolithic
  Microresonator

abstract:
  Over the past decade, strong interactions of light and matter at the
single-photon level have enabled a wide set of scientific advances in quantum
optics and quantum information science. This work has been performed
principally within the setting of cavity quantum electrodynamics with diverse
physical systems, including single atoms in Fabry-Perot resonators, quantum
dots coupled to micropillars and photonic bandgap cavities, and Cooper-pairs
interacting with superconducting resonators. Experiments with single, localized
atoms have been at the forefront of these advances with the use of optical
resonators in high-finesse Fabry-Perot configurations. As a result of the
extreme technical challenges involved in further improving the multilayer
dielectric mirror coatings of these resonators and in scaling to large numbers
of devices, there has been increased interest in the development of alternative
microcavity systems. Here we show strong coupling between individual Cesium
atoms and the fields of a high-quality toroidal microresonator. From
observations of transit events for single atoms falling through the resonator's
evanescent field, we determine the coherent coupling rate for interactions near
the surface of the resonator. We develop a theoretical model to quantify our
observations, demonstrating that strong coupling is achieved, with the rate of
coherent coupling exceeding the dissipative rates of the atom and the cavity.
Our work opens the way for investigations of optical processes with single
atoms and photons in lithographically fabricated microresonators. Applications
include the implementation of quantum networks, scalable quantum logic with
photons, and quantum information processing on atom chips.


title: Improving the security of quantum exam against cheating

abstract:
  The security of quantum exam [Phys. Lett. A 350 (2006) 174] is analyzed and
it is found that this protocol is secure for any eavesdropper except for the
"students" who take part in the exam. Specifically, any student can steal other
examinees' solutions and then cheat in the exam. Furthermore, a possible
improvement of this protocol is presented.