title:
Teleportation fidelities of squeezed states from thermodynamical state
  space measures

abstract:
  We introduce a "microcanonical" measure (complying with the "general
canonical principle") over the second moments of pure bosonic Gaussian states
under an energy constraint. We determine the average fidelity for the
teleportation of states distributed according to such a measure and compare it
to a threshold obtained from a feasible classical strategy. Furthermore, we
show that, under the proposed measure, the distribution of the entanglement
concentrates around a finite value at the thermodynamical limit and, in
general, the typical entanglement of Gaussian states with maximal energy E is
not close to the maximum allowed by E.


title:
Quantum-classical transition for an analog of double-slit experiment in
  complex collisions: Dynamical decoherence in quantum many-body systems

abstract:
  We study coherent superpositions of clockwise and anti-clockwise rotating
intermediate complexes with overlapping resonances formed in bimolecular
chemical reactions. Disintegration of such complexes represents an analog of
famous double-slit experiment. The time for disappearance of the interference
fringes is estimated from heuristic arguments related to fingerprints of
chaotic dynamics of a classical counterpart of the coherently rotating complex.
Validity of this estimate is confirmed numerically for the H+D$_2$ chemical
reaction. Thus we demonstrate the quantum--classical transition in temporal
behavior of highly excited quantum many-body systems in the absence of external
noise and coupling to an environment.


title: Correcting Quantum Errors with Entanglement

abstract:
  We show how entanglement shared between encoder and decoder can simplify the
theory of quantum error correction. The entanglement-assisted quantum codes we
describe do not require the dual-containing constraint necessary for standard
quantum error correcting codes, thus allowing us to ``quantize'' all of
classical linear coding theory. In particular, efficient modern classical codes
that attain the Shannon capacity can be made into entanglement-assisted quantum
codes attaining the hashing bound (closely related to the quantum capacity).
For systems without large amounts of shared entanglement, these codes can also
be used as catalytic codes, in which a small amount of initial entanglement
enables quantum communication.


title:
A Model for Non-Linear Quantum Evolution based on Time Displaced
  Entanglement

abstract:
  We discuss a model for non-linear quantum evolution based on the idea of time
displaced entanglement, produced by taking one member of an entangled pair on a
round trip at relativistic speeds, thus inducing a time-shift between the pair.
We show that decoherence of the entangled pair is predicted. For non-maximal
entanglement this then implies the ability to induce a non-unitary, non-linear
quantum evolution. Although exhibiting unusual characteristics, we show that
these evolutions cannot be dismissed on the basis of entropic or causal
arguments.


title: Demonstration of optically modulated dispersion forces

abstract:
  We report the first experiment on the optical modulation of dispersion forces
through a change of the carrier density in a Si membrane. For this purpose a
high-vacuum based atomic force microscope and excitation light pulses from an
Ar laser are used. The experimental results are compared with two theoretical
models. The modulation of the dispersion force will find applications in
optomechanical micromachines.


title: Comment on `Spin Decoherence in Superconducting Atom Chips'

abstract:
  We comment on a recent paper [Phys. Rev. Lett. 97, 070401 (2006):
quant-ph/0603229] concerning rubidium atoms trapped near a superconducting
niobium surface at ~4K. This seeks to calculate the rate of atomic spin flips
induced by thermal magnetic noise. We point out that the calculation is in
error by a large factor because it is based on the two-fluid model of
superconductivity. This model gives a poor description of electromagnetic
dissipation just below the critical temperature because it cannot incorporate
the case II coherences of a fuller quantum theory.


title: Quantum key distribution network with wavelength addressing

abstract:
  Most traditional applications of quantum cryptography are point-to-point
communications, in which only two users can exchange keys. In this letter, we
present a network scheme that enable quantum key distribution between
multi-user with wavelength addressing. Considering the current state of
wavelength division multiplexing technique, dozens or hundreds of users can be
connected to such a network and directly exchange keys with each other. With
the scheme, a 4-user demonstration network was built up and key exchanges were
performed.


title: No quantum advantage for nonlocal computation

abstract:
  We investigate the problem of "nonlocal" computation, in which separated
parties must compute a function with nonlocally encoded inputs and output, such
that each party individually learns nothing, yet together they compute the
correct function output. We show that the best that can be done classically is
a trivial linear approximation. Surprisingly, we also show that quantum
entanglement provides no advantage over the classical case. On the other hand,
generalized (i.e. super-quantum) nonlocal correlations allow perfect nonlocal
computation. This gives new insights into the nature of quantum nonlocality and
its relationship to generalised nonlocal correlations.


title: On strong superadditivity for a class of quantum channels

abstract:
  Given a quantum channel $\Phi $ in a Hilbert space $H$ put $\hat
H_{\Phi}(\rho)=\min \limits_{\rho_{av}=\rho}\Sigma_{j=1}^{k}\pi_{j}S(\Phi
(\rho_{j}))$, where $\rho_{av}=\Sigma_{j=1}^{k}\pi_{j}\rho_{j}$, the minimum is
taken over all probability distributions $\pi =\{\pi_{j}\}$ and states
$\rho_{j}$ in $H$, $S(\rho)=-Tr\rho\log\rho$ is the von Neumann entropy of a
state $\rho$. The strong superadditivity conjecture states that $\hat H_{\Phi
\otimes \Psi}(\rho)\ge \hat H_{\Phi}(Tr_{K}(\rho))+\hat H_{\Psi}(Tr_{H}(\rho))$
for two channels $\Phi $ and $\Psi $ in Hilbert spaces $H$ and $K$,
respectively. We have proved the strong superadditivity conjecture for the
quantum depolarizing channel in prime dimensions. The estimation of the
quantity $\hat H_{\Phi\otimes \Psi}(\rho)$ for the special class of Weyl
channels $\Phi $ of the form $\Phi=\Xi \circ \Phi_{dep}$, where $\Phi_{dep}$ is
the quantum depolarizing channel and $\Xi $ is the phase damping is given.


title: A class of quantum many-body states that can be efficiently simulated

abstract:
  We introduce the multi-scale entanglement renormalization ansatz (MERA), an
efficient representation of certain quantum many-body states on a D-dimensional
lattice. Equivalent to a quantum circuit with logarithmic depth and distinctive
causal structure, the MERA allows for an exact evaluation of local expectation
values. It is also the structure underlying entanglement renormalization, a
coarse-graining scheme for quantum systems on a lattice that is focused on
preserving entanglement.


title: A special simplex in the state space for entangled qudits

abstract:
  Focus is on two parties with Hilbert spaces of dimension d, i.e. "qudits". In
the state space of these two possibly entangled qudits an analogue to the well
known tetrahedron with the four qubit Bell states at the vertices is presented.
The simplex analogue to this magic tetrahedron includes mixed states. Each of
these states appears to each of the two parties as the maximally mixed state.
Some studies on these states are performed, and special elements of this set
are identified. A large number of them is included in the chosen simplex which
fits exactly into conditions needed for teleportation and other applications.
Its rich symmetry - related to that of a classical phase space - helps to study
entanglement, to construct witnesses and perform partial transpositions. This
simplex has been explored in details for d=3. In this paper the mathematical
background and extensions to arbitrary dimensions are analysed.


title: Two-photon interference with two independent pseudo-thermal sources

abstract:
  The nature of two-photon interference is a subject that has aroused renewed
interest in recent years and is still under debate. In this paper we report the
first observation of two-photon interference with independent pseudo-thermal
sources in which sub-wavelength interference is observed. The phenomenon may be
described in terms of the classical statistical distribution of the two sources
and their optical transfer functions.


title:
Time-dependent Fr\"ohlich transformation approach for two-atom
  entanglement generated by successive passage through a cavity

abstract:
  Time-dependent Fr\"ohlich transformations can be used to derive an effective
Hamiltonian for a class of quantum systems with time-dependent perturbations.
We use such a transformation for a system with time-dependent atom-photon
coupling induced by the classical motion of two atoms in an inhomogeneous
electromagnetic field. We calculate the entanglement between the two atoms
resulting from their motion through a cavity as a function of their initial
position difference and velocity.


title:
Enhancement of entanglement transfer in a spin chain by phase
  shift-control

abstract:
  We study the effect of a phase shift on the amount of transferrable two-spin
entanglement in a spin chain. We consider a ferromagnetic Heisenberg/XY spin
chain, both numerically and analytically, and two mechanisms to generate a
phase shift, the Aharonov-Casher effect and the Dzyaloshinskii-Moriya
interaction. In both cases, the maximum attainable entanglement is shown to be
significantly enhanced, suggesting its potential usefulness in quantum
information processing.


title:
The role of degeneracy in the analogy between continuous variable and
  spin 1/2 systems

abstract:
  We point out limitations to the analogy between the continuous variable and
spin 1/2 systems and show that the maximal violation of Bell inequality is
related to an infinite degeneracy. We quantify non-maximal violation of the
Bell-CHSH inequality and comment potential experimental implications of our
work.


title:
Evolving Quantum Oracles with Hybrid Quantum-inspired Evolutionary
  Algorithm

abstract:
  Quantum oracles play key roles in the studies of quantum computation and
quantum information. But implementing quantum oracles efficiently with
universal quantum gates is a hard work. Motivated by genetic programming, this
paper proposes a novel approach to evolve quantum oracles with a hybrid
quantum-inspired evolutionary algorithm. The approach codes quantum circuits
with numerical values and combines the cost and correctness of quantum circuits
into the fitness function. To speed up the calculation of matrix multiplication
in the evaluation of individuals, a fast algorithm of matrix multiplication
with Kronecker product is also presented. The experiments show the validity and
the effects of some parameters of the presented approach. And some
characteristics of the novel approach are discussed too.


title: Semi-classical theory of quiet lasers. I: Principles

abstract:
  When light originating from a laser diode driven by non-fluctuating
electrical currents is incident on a photo-detector, the photo-current does not
fluctuate much. Precisely, this means that the variance of the number of
photo-electrons counted over a large time interval is much smaller that the
average number of photo-electrons. At non-zero Fourier frequency $\Omega$ the
photo-current power spectrum is of the form $\Omega^2/(1+\Omega^2)$ and thus
vanishes as $\Omega\to 0$, a conclusion equivalent to the one given above. The
purpose of this paper is to show that results such as the one just cited may be
derived from a (semi-classical) theory in which neither the optical field nor
the electron wave-function are quantized. We first observe that almost any
medium may be described by a circuit and distinguish (possibly non-linear)
conservative elements such as pure capacitances, and conductances that
represent the atom-field coupling. The theory rests on the non-relativistic
approximation. Nyquist noise sources (in which the Planck term $\hbar\omega/2$
is being restored) are associated with positive or negative conductances, and
the law of average-energy conservation is enforced. We consider mainly
second-order correlations in stationary linearized regimes.


title:
Non-destructive interferometric characterization of an optical dipole
  trap

abstract:
  A method for non-destructive characterization of a dipole trapped atomic
sample is presented. It relies on a measurement of the phase-shift imposed by
cold atoms on an optical pulse that propagates through a free space
Mach-Zehnder interferometer. Using this technique we are able to determine,
with very good accuracy, relevant trap parameters such as the atomic sample
temperature, trap oscillation frequencies and loss rates. Another important
feature is that our method is faster than conventional absorption or
fluorescence techniques, allowing the combination of high-dynamical range
measurements and a reduced number of spontaneous emission events per atom.


title: Characterizing and measuring multipartite Entanglement

abstract:
  A method is proposed to characterize and quantify multipartite entanglement
in terms of the probability density function of bipartite entanglement over all
possible balanced bipartitions of an ensemble of qubits. The method is tested
on a class of random pure states.


title: Quantum Kolmogorov complexity and its applications

abstract:
  Kolmogorov complexity is a measure of the information contained in a binary
string. We investigate here the notion of quantum Kolmogorov complexity, a
measure of the information required to describe a quantum state. We show that
for any definition of quantum Kolmogorov complexity measuring the number of
classical bits required to describe a pure quantum state, there exists a pure
n-qubit state which requires exponentially many bits of description. This is
shown by relating the classical communication complexity to the quantum
Kolmogorov complexity. Furthermore we give some examples of how quantum
Kolmogorov complexity can be applied to prove results in different fields, such
as quantum computation and thermodynamics, and we generalize it to the case of
mixed quantum states.


title: Observation of Faraday rotation from a single confined spin

abstract:
  Ability to read-out the state of a single confined spin lies at the heart of
solid-state quantum information processing. While all-optical spin measurements
using Faraday rotation has been successfully implemented in ensembles of
semiconductor spins, read-out of a single semiconductor spin has only been
achieved using transport measurements based on spin-charge conversion. Here, we
demonstrate an all-optical dispersive measurement of the spin-state of a single
electron trapped in a semiconductor quantum dot. We obtain information on the
spin state through conditional Faraday rotation of a spectrally detuned optical
field, induced by the polarization- and spin-selective trion (charged quantum
dot) transitions. To assess the sensitivity of the technique, we use an
independent resonant laser for spin-state preparation. An all-optical
dispersive measurement on single spins has the important advantage of
channeling the measurement back-action onto a conjugate observable, thereby
allowing for repetitive or continuous quantum nondemolition (QND) read-out of
the spin-state. We infer from our results that there are of order unity
back-action induced spin-flip Raman scattering events within our measurement
timescale. Therefore, straightforward improvements such as the use of a
solid-immersion lens and higher efficiency detectors would allow for
back-action evading spin measurements, without the need for a cavity.


title: Topological Quantum Compiling

abstract:
  A method for compiling quantum algorithms into specific braiding patterns for
non-Abelian quasiparticles described by the so-called Fibonacci anyon model is
developed. The method is based on the observation that a universal set of
quantum gates acting on qubits encoded using triplets of these quasiparticles
can be built entirely out of three-stranded braids (three-braids). These
three-braids can then be efficiently compiled and improved to any required
accuracy using the Solovay-Kitaev algorithm.


title: Experimental demonstration of four-party quantum secret sharing

abstract:
  Secret sharing is a multiparty cryptographic task in which some secret
information is splitted into several pieces which are distributed among the
participants such that only an authorized set of participants can reconstruct
the original secret. Similar to quantum key distribution, in quantum secret
sharing, the secrecy of the shared information relies not on computational
assumptions, but on laws of quantum physics. Here, we present an experimental
demonstration of four-party quantum secret sharing via the resource of
four-photon entanglement.


title: On the role of memory errors in quantum repeaters

abstract:
  We investigate the influence of memory errors in the quantum repeater scheme
for long-range quantum communication. We show that the communication distance
is limited in standard operation mode due to memory errors resulting from
unavoidable waiting times for classical signals. We show how to overcome these
limitations by (i) improving local memory, and (ii) introducing two new
operational modes of the quantum repeater. In both operational modes, the
repeater is run blindly, i.e. without waiting for classical signals to arrive.
In the first scheme, entanglement purification protocols based on one-way
classical communication are used allowing to communicate over arbitrary
distances. However, the error thresholds for noise in local control operations
are very stringent. The second scheme makes use of entanglement purification
protocols with two-way classical communication and inherits the favorable error
thresholds of the repeater run in standard mode. One can increase the possible
communication distance by an order of magnitude with reasonable overhead in
physical resources. We outline the architecture of a quantum repeater that can
possibly ensure intercontinental quantum communication.


title: Rotation of atoms in a two dimensional lattice with a harmonic trap

abstract:
  Rotation of atoms in a lattice is studied using a Hubbard model. It is found
that the atoms are still contained in the trap even when the rotation frequency
is larger than the trapping frequency. This is very different from the behavior
in continuum. Bragg scattering and coupling between angular and radial motion
are believed to make this stability possible. In this regime, density depletion
at the center of the trap can be developed for spin polarized fermions.


title:
Alternate Scheme for Optical Cluster-State Generation without
  Number-Resolving Photon Detectors

abstract:
  We design a controlled-phase gate for linear optical quantum computing by
using photodetectors that cannot resolve photon number. An intrinsic
error-correction circuit corrects errors introduced by the detectors. Our
controlled-phase gate has a 1/4 success probability. Recent development in
cluster-state quantum computing has shown that a two-qubit gate with non-zero
success probability can build an arbitrarily large cluster state with only
polynomial overhead. Hence, it is possible to generate optical cluster states
without number-resolving detectors and with polynomial overhead.


title: Electronic Noise in Optical Homodyne Tomography

abstract:
  The effect of the detector electronic noise in an optical homodyne tomography
experiment is shown to be equivalent to an optical loss if the detector is
calibrated by measuring the quadrature noise of the vacuum state. An explicit
relation between the electronic noise level and the equivalent optical
efficiency is obtained and confirmed in an experiment with a narrowband
squeezed vacuum source operating at an atomic rubidium wavelength.


title: Is Fault-Tolerant Quantum Computation Really Possible?

abstract:
  The so-called "threshold" theorem says that, once the error rate per qubit
per gate is below a certain value, indefinitely long quantum computation
becomes feasible, even if all of the qubits involved are subject to relaxation
processes, and all the manipulations with qubits are not exact. The purpose of
this article, intended for physicists, is to outline the ideas of quantum error
correction and to take a look at the proposed technical instruction for
fault-tolerant quantum computation. It seems that the mathematics behind the
threshold theorem is somewhat detached from the physical reality, and that some
ideal elements are always present in the construction. This raises serious
doubts about the possibility of large scale quantum computations, even as a
matter of principle.

categories: quant-ph cond-mat.other cond-mat.str-el physics.atom-ph


title:
Simple and efficient quantum key distribution with parametric
  down-conversion

abstract:
  We propose an efficient quantum key distribution protocol based on the
photon-pair generation from parametric down-conversion (PDC). It uses the same
experimental setup as the conventional protocol, but a refined data analysis
enables detection of photon-number splitting attacks by utilizing information
from a built-in decoy state. Assuming the use of practical detectors, we
analyze the unconditional security of the new scheme and show that it improves
the secure key generation rate by several orders of magnitude at long
distances, using a high intensity PDC source.


title: Building Gaussian Cluster States by Linear Optics

abstract:
  The linear optical creation of Gaussian cluster states, a potential resource
for universal quantum computation, is investigated. We show that for any
Gaussian cluster state, the canonical generation scheme in terms of QND-type
interactions, can be entirely replaced by off-line squeezers and beam
splitters. Moreover, we find that, in terms of squeezing resources, the
canonical states are rather wasteful and we propose a systematic way to create
cheaper states. As an application, we consider Gaussian cluster computation in
multiple-rail encoding. This encoding may reduce errors due to finite
squeezing, even when the extra rails are achieved through off-line squeezing
and linear optics.


title:
Implementation of adiabatic Abelian geometric gates with superconducting
  phase qubits

abstract:
  We have developed an adiabatic Abelian geometric quantum computation strategy
based on the non-degenerate energy eigenstates in (but not limited to)
superconducting phase-qubit systems. The fidelity of the designed quantum gate
was evaluated in the presence of simulated thermal fluctuation in
superconducting phase qubits circuit and was found to be rather robust against
the random errors. In addition, it was elucidated that the Berry phase in the
designed adiabatic evolution may be detected directly via the quantum state
tomography developed for superconducting qubits.


title: A modal-Hamiltonian interpretation of quantum mechanics

abstract:
  The aim of this paper is to introduce a new member of the family of the modal
interpretations of quantum mechanics. In this modal-Hamiltonian interpretation,
the Hamiltonian of the quantum system plays a decisive role in the
property-ascription rule that selects the definite-valued observables whose
possible values become actual. We show that this interpretation is effective
for solving the measurement problem, both in its ideal and its non-ideal
versions, and we argue for the physical relevance of the property-ascription
rule by applying it to well-known physical situations. Moreover, we explain how
this interpretation supplies a description of the elemental categories of the
ontology referred to by the theory, where quantum systems turn out to be
bundles of possible properties.


title:
Classical Representations of Quantum Mechanics Related to Statistically
  Complete Observables

abstract:
  We present a reformulation of quantum mechanics in terms of probability
measures and functions on a general classical sample space and in particular in
terms of probability densities and functions on phase space. The basis of our
proceeding is the existence of so-called statistically complete observables and
the duality between the state spaces and the spaces of the observables, the
latter holding in the quantum as well as in the classical case. In the
phase-space context, we further discuss joint position-momentum observables,
Hilbert spaces of infinitely differentiable functions on phase space, and
dequantizations. Finally, the relation of quantum dynamics to the classical
Liouville dynamics is investigated.


title: Interaction of a two-level atom with squeezed light

abstract:
  We consider a degenerate parametric oscillator whose cavity contains a
two-level atom. Applying the Heisenberg and quantum Langevin equations, we
calculate in the bad-cavity limit the mean photon number, the quadrature
variance, and the power spectrum for the cavity mode in general and for the
signal light and fluorescent light in particular. We also obtain the normalized
second-order correlation function for the fluorescent light. We find that the
presence of the two-level atom leads to a decrease in the degree of squeezing
of the signal light. It so turns out that the fluorescent light is in a
squeezed state and the power spectrum consists of a single peak only.


title: Point Estimation of States of Finite Quantum Systems

abstract:
  The estimation of the density matrix of a $k$-level quantum system is studied
when the parametrization is given by the real and imaginary part of the entries
and they are estimated by independent measurements. It is established that the
properties of the estimation procedure depend very much on the invertibility of
the true state. In particular, in case of a pure state the estimation is less
efficient. Moreover, several estimation schemes are compared for the unknown
state of a qubit when one copy is measured at a time. It is shown that the
average mean quadratic error matrix is the smallest if the applied observables
are complementary. The results are illustrated by computer simulations.


title: Local noise can enhance entanglement teleportation

abstract:
  Recently we have considered two-qubit teleportation via mixed states of four
qubits and defined the generalized singlet fraction. For single-qubit
teleportation, Badziag {\em et al.} [Phys. Rev. A {\bf 62}, 012311 (2000)] and
Bandyopadhyay [Phys. Rev. A {\bf 65}, 022302 (2002)] have obtained a family of
entangled two-qubit mixed states whose teleportation fidelity can be enhanced
by subjecting one of the qubits to dissipative interaction with the environment
via an amplitude damping channel. Here, we show that a dissipative interaction
with the local environment via a pair of time-correlated amplitude damping
channels can enhance fidelity of entanglement teleportation for a class of
entangled four-qubit mixed states. Interestingly, we find that this enhancement
corresponds to an enhancement in the quantum discord for some states.


title: Quantum tunneling through vacuum-multiparticle induced potentials

abstract:
  The vacuum cavity mode induces a potential barrier and a well when an
ultra-slow excited atom enters the interaction region so that it can be
reflected or transmitted with a certain probability. We demonstrate here that a
slow-velocity excited particle tunnels freely through a vacuum electromagnetic
field mode filled with $N-1$ ground state atoms. The reason for this is the
trapping of the moving atom into its upper state due to multiparticle
influences and the corresponding decoupling from the interaction with the
environment such that the emitter does not {\it see} the induced potentials.


title: Noisy Qutrit Channels

abstract:
  We present an analysis of spontaneous emission in a 3-level atom as an
example of a qutrit state under the action of noisy quantum channels. We choose
a 3-level atom with V-configuration to be the qutrit state. Gell-Mann matrices
and a generalized Bloch vector (8-dimensional) are used to describe the qutrit
density operator. Using the time-evolution equations of atomic variables we
find the Kraus representation of spontaneous emission quantum channel (SE
channel). Furthermore, we consider a generalized Werner state of two qutrits
and investigate the separability condition. We give similar analysis of
spontaneous emission for qubit channels. The influence of spontaneous emission
on the separability of Werner states for qutrit and qubit states is compared.


title: Regular and Chaotic Quantum Dynamic in Atom-Diatom Reactive Collisions

abstract:
  A new micro-irreversible 3D theory of quantum multichannel scattering in the
three-body system is developed. The quantum approach is constructed on the
generating trajectory tubes which allow taking into account influence of
classical non-integrability on the dynamical quantum system. It was shown that
when the volume of classical chaos in phase space is larger than quantum sell
in the main object of quantum system the wavefunction generates chaos (quantum
chaos). The probability of quantum transitions is constructed for this case. On
the example of collinear collision of Li+(FH) -> (LiF)+H system is carried out
the numerical calculation and was shown that in the system is generated quantum
(wave) chaos.


title:
Net Fisher information measure versus ionization potential and dipole
  polarizability in atoms

abstract:
  The net Fisher information measure, defined as the product of position and
momentum Fisher information measures and derived from the non-relativistic
Hartree-Fock wave functions for atoms with Z=1-102, is found to correlate well
with the inverse of the experimental ionization potential. Strong direct
correlations of the net Fisher information are also reported for the static
dipole polarizability of atoms with Z=1-88. The complexity measure, defined as
the ratio of the net Onicescu information measure and net Fisher information,
exhibits clearly marked regions corresponding to the periodicity of the atomic
shell structure. The reported correlations highlight the need for using the net
information measures in addition to either the position or momentum space
analogues. With reference to the correlation of the experimental properties
considered here, the net Fisher information measure is found to be superior
than the net Shannon information entropy.


title: Grover's algorithm on a Feynman computer

abstract:
  We present an implementation of Grover's algorithm in the framework of
Feynman's cursor model of a quantum computer. The cursor degrees of freedom act
as a quantum clocking mechanism, and allow Grover's algorithm to be performed
using a single, time-independent Hamiltonian. We examine issues of locality and
resource usage in implementing such a Hamiltonian. In the familiar language of
Heisenberg spin-spin coupling, the clocking mechanism appears as an excitation
of a basically linear chain of spins, with occasional controlled jumps that
allow for motion on a planar graph: in this sense our model implements the idea
of "timing" a quantum algorithm using a continuous-time random walk. In this
context we examine some consequences of the entanglement between the states of
the input/output register and the states of the quantum clock.


title: Quantum Physics and Human Language

abstract:
  Human languages employ constructions that tacitly assume specific properties
of the limited range of phenomena they evolved to describe. These assumed
properties are true features of that limited context, but may not be general or
precise properties of all the physical situations allowed by fundamental
physics. In brief, human languages contain `excess baggage' that must be
qualified, discarded, or otherwise reformed to give a clear account in the
context of fundamental physics of even the everyday phenomena that the
languages evolved to describe. The surest route to clarity is to express the
constructions of human languages in the language of fundamental physical
theory, not the other way around. These ideas are illustrated by an analysis of
the verb `to happen' and the word `reality' in special relativity and the
modern quantum mechanics of closed systems.


title: Entanglement of Collaboration

abstract:
  The entanglement of collaboration (EoC) quantifies the maximum amount of
entanglement, that can be generated between two parties, A and B, given
collaboration with N-2 other parties, when the N parties share a multipartite
(possibly mixed) state and where the collaboration consists of local operations
and classical communication (LOCC) by all parties. The localizable entanglement
(LE) is defined similarly except that A and B do not participate in the effort
to generate bipartite entanglement. We compare between these two operational
definitions and find sufficient conditions for which the EoC is equal to the
LE. In particular, we find that the two are equal whenever they are measured by
the concurrence or by one of its generalizations called the G-concurrence. We
also find a simple expression for the LE in terms of the Jamiolkowski
isomorphism and prove that it is convex.


title: Dissipation in Quantum Mechanics, Scalar and Vector Field Theory

abstract:
  A new minimal coupling method is introduced. A general dissipative quantum
system is investigated consistently and systematically. Some coupling functions
describing the interaction between the system and the environment are
introduced. Based on coupling functions, some susceptibility functions are
attributed to the environment explecitly. Transition probabilities relating the
way energy flows from the system to the environment are calculated and the
energy conservation is explecitly examined. This new formalism is generalized
to the dissipative scalar and vector field theories along the ideas developed
for the quantum dissipative systems


title:
Practical decoy state method in quantum key distribution with heralded
  single photon source

abstract:
  We propose a practical decoy state method with heralded single photon source
for quantum key distribution (QKD). In the protocol, 3 intensities are used and
one can estimate the fraction of single-photon counts. The final key rate over
transmission distance is simulated under various parameter sets. Due to the
lower dark count than that of a coherent state, it is shown that a 3-intensity
decoy-state QKD with a heralded source can work for a longer distance than that
of a coherent state.


title: Majorana spin-flip transitions in a magnetic trap

abstract:
  Atoms confined in a magnetic trap can escape by making spin-flip Majorana
transitions due to a breakdown of the adiabatic approximation. Several papers
have studied this process for atoms with spin $F = 1/2$ or $F= 1$. The present
paper calculates the escape rate for atoms with spin $F > 1$. This problem has
new features because the perturbation $\Delta T$ which allows atoms to escape
satisfies a selection rule $\Delta F_z = 0, \pm 1, \pm 2$ and multi-step
processes contribute in leading order. When the adiabatic approximation is
satisfied the leading order terms can be summed to yield a simple expression
for the escape rate.


title: Optical Nanofibers for Manipulating and Probing Single-Atom Fluorescence

abstract:
  We demonstrate how optical nanofibers can be used to manipulate and probe
single-atom fluorescence. We show that fluorescence photons from a very small
number of atoms, average atom number of less than 0.1, around the nanofiber can
readily be observed through single-mode optical fiber under resonant laser
irradiation. We show also that optical nanofibers enable us to probe the van
der Waals interaction between atoms and surface with high precision by
observing the fluorescence excitation spectrum.


title:
Simultaneous creations of discrete-variable entangle state and
  single-photon-added coherent state

abstract:
  The single-photon-added coherent state (SPACS), as an intermediate
classical-to-purely-quantum state, was first realized recently by Zavatta
\emph{$et al.$} (Science 306, 660 (2004)). We show here that the success
probability of their SPACS generation can be enhanced by a simple method which
leads to simultaneous creations of a discrete-variable entangled state and a
SPACS or even a hybrid-variable entangled SPACS in two different channels. The
impacts of the input thermal noise are also analyzed.


title: Asymmetric two-output quantum processor in any dimension

abstract:
  We propose two different implementations of an asymmetric two-output
probabilistic quantum processor, which can implement a restricted set of
one-qubit operations. One of them is constructed by combining asymmetric
telecloning with a quantum gate array. We analyze the efficiency of this
processor by evaluating the fidelities between the desired operation and the
one generated by the processor and show that the two output states are the same
as the ones produced by the optimal universal asymmetric Pauli cloning machine.
The schemes require only local operations and classical communication, they
have the advantage of transmitting the two output states directly to two
spatially separated receivers but they have a success probability of 1/2. We
show further that we can perform the same one-qubit operation with unity
probability at the cost of using nonlocal operations. We finally generalize the
two schemes for D-level systems and find that the local ones are successful
with a probability of 1/D and the nonlocal generalized scheme is always
successful.


title:
Observable geometric phase induced by a cyclically evolving dissipative
  process

abstract:
  In a prevous paper (Phys. Rev. Lett. 96, 150403 (2006)) we have proposed a
new way to generate an observable geometric phase on a quantum system by means
of a completely incoherent phenomenon. The basic idea was to force the ground
state of the system to evolve ciclically by "adiabatically" manipulating the
environment with which it interacts. The specific scheme we have previously
analyzed, consisting of a multilevel atom interacting with a broad-band
squeezed vacuum bosonic bath whose squeezing parameters are smoothly changed in
time along a closed loop, is here solved in a more direct way. This new
solution emphasizes how the geometric phase on the ground state of the system
is indeed due to a purely incoherent dynamics


title: Microscopic derivation of the Jaynes-Cummings model with cavity losses

abstract:
  In this paper we provide a microscopic derivation of the master equation for
the Jaynes-Cummings model with cavity losses. We single out both the
differences with the phenomenological master equation used in the literature
and the approximations under which the phenomenological model correctly
describes the dynamics of the atom-cavity system. Some examples wherein the
phenomenological and the microscopic master equations give rise to different
predictions are discussed in detail.


title:
High-temperature Bose-Einstein condensation of polaritons: realization
  under the intracavity laser pumping of matter condition

abstract:
  A quantum model of Bose-Einstein condensation based on processes involving
polaritons excited in an intracavity absorbing cell with resonance atoms, which
is manifested in the spectral characteristics of the system, is considered. It
is shown that the spectral 'condensation' appears which is directly related to
the degeneracy of a weakly interacting gas of polaritons resulting in
quasi-condensation at room temperature. The possibility of obtaining polariton
condensation as a new phase state by using the confinement of polaritons in an
atomic optical harmonic trap is discussed.


title: Applications of the group SU(1,1) for quantum computation and tomography

abstract:
  This paper collects miscellaneous results about the group SU(1,1) that are
helpful in applications in quantum optics. Moreover, we derive two new results,
the first is about the approximability of SU(1,1) elements by a finite set of
elementary gates, and the second is about the regularization of group
identities for tomographic purposes.


title:
Scheme for conditional generation of photon-added coherent state and
  optical entangled $W$ state

abstract:
  We propose a simple scheme to generate an arbitrary photon-added coherent
state of a travelling optical field by using only a set of degenerate
parametric amplifiers and single-photon detectors. Particularly, when the
single-photon-added coherent state (SPACS) is observed by following, e.g., the
novel technique of Zavatta \emph{$et al.$} (Science 306, 660 (2004)), we also
obtain the generalized optical entangled $W$ state. Finally, a qualitative
analysis of possible losses in our scheme is given.


title:
Driven single-band tight-binding dynamics, mesoscopic quantum circuits
  and realization of generalized parafermionic polynomial algebrasDeformed
  parafermionic algebra from single-band tight-binding dynamics

abstract:
  The quantum dynamics of a driven single-band tight-binding model with
different boundary conditions is considered. The relation between the
Hamiltonian describing the single-band tight-binding dynamics and the
Hamiltonian of a discrete-charge mesoscopic quantum circuit is elucidated. It
is shown that the former Hamiltonian, with Dirichlet boundary conditions, can
be considered as a realization of the deformed parafermionic polynomial
algebras.


title:
Demonstration of efficient scheme for generation of "Event Ready"
  entangled photon pairs from single photon source

abstract:
  We present a feasible and efficient scheme, and its proof-of-principle
demonstration, of creating entangled photon pairs in an event-ready way using
only simple linear optical elements and single photons. The quality of
entangled photon pair produced in our experiment is confirmed by a strict
violation of Bell's inequality. This scheme and the associated experimental
techniques present an important step toward linear optics quantum computation.


title: A Sharp Fannes-type Inequality for the von Neumann Entropy

abstract:
  We derive an inequality relating the entropy difference between two quantum
states to their trace norm distance, sharpening a well-known inequality due to
M. Fannes. In our inequality, equality can be attained for every prescribed
value of the trace norm distance.


title: On Adler's "Conway-Kochen Twin Argument"

abstract:
  We respond to Stephen Adler's comments on our paper "The Free Will
Theorem",and add some further observations.


title: Teleportation, Braid Group and Temperley--Lieb Algebra

abstract:
  We explore algebraic and topological structures underlying the quantum
teleportation phenomena by applying the braid group and Temperley--Lieb
algebra. We realize the braid teleportation configuration, teleportation
swapping and virtual braid representation in the standard description of the
teleportation. We devise diagrammatic rules for quantum circuits involving
maximally entangled states and apply them to three sorts of descriptions of the
teleportation: the transfer operator, quantum measurements and characteristic
equations, and further propose the Temperley--Lieb algebra under local unitary
transformations to be a mathematical structure underlying the teleportation. We
compare our diagrammatical approach with two known recipes to the quantum
information flow: the teleportation topology and strongly compact closed
category, in order to explain our diagrammatic rules to be a natural
diagrammatic language for the teleportation.


title:
Quantum interference between two single photons emitted by independently
  trapped atoms

abstract:
  When two indistinguishable single photons are fed into the two input ports of
a beam splitter, the photons will coalesce and leave together from the same
output port. This is a quantum interference effect, which occurs because the
two possible paths where the photons leave in different output ports interfere
destructively. This effect was first observed in parametric downconversion by
Hong, Ou and Mandel, and then with single photons produced one after the other
by the same quantum emitter. With the recent development of quantum
information, a lot of attention has been devoted to this coalescence effect as
a resource for quantum data processing using linear optics techniques. To
ensure the scalability of schemes based on these ideas, it is crucial that
indistinguishable photons are emitted by a collection of synchronized, but
otherwise independent sources. In this paper, we demonstrate the quantum
interference of two single photons emitted by two independently trapped single
atoms, bridging the gap towards the simultaneous emission of many
indistinguishable single photons by different emitters. Our data analysis shows
that the coalescence observed is mostly limited by the wavefront matching of
the light emitted by the two atoms, and to a lesser extent by the motion of
each atom in its own trap.


title: Atom in a coherently controlled squeezed vacuum

abstract:
  A broadband squeezed vacuum photon field is characterized by a complex
squeezing function. We show that by controlling the wavelength dependence of
its phase it is possible to change the dynamics of the atomic polarization
interacting with the squeezed vacuum. Such a phase modulation effectively
produces a finite range temporal interaction kernel between the two quadratures
of the atomic polarization yielding the change in the decay rates as well as
the appearance of additional oscillation frequencies. We show that decay rates
slower than the spontaneous decay rate can be achieved even for a squeezed bath
in the classic regime. For linear and quadratic phase modulations the power
spectrum of the scattered light exhibits narrowing of the central peak due to
the modified decay rates. For strong phase modulations side lobes appear
symmetrically around the central peak reflecting additional oscillation
frequencies.


title:
Security of quantum key distribution protocols using two-way classical
  communication or weak coherent pulses

abstract:
  We apply the techniques introduced in [Kraus et. al., Phys. Rev. Lett. 95,
080501, 2005] to prove security of quantum key distribution (QKD) schemes using
two-way classical post-processing as well as QKD schemes based on weak coherent
pulses instead of single-photon pulses. As a result, we obtain improved bounds
on the secret-key rate of these schemes.


title:
Quantum computers based on electron spins controlled by ultra-fast,
  off-resonant, single optical pulses

abstract:
  We describe a fast quantum computer based on optically controlled electron
spins in charged quantum dots that are coupled to microcavities. This scheme
uses broad-band optical pulses to rotate electron spins and provide the clock
signal to the system. Non-local two-qubit gates are performed by phase shifts
induced by electron spins on laser pulses propagating along a shared waveguide.
Numerical simulations of this scheme demonstrate high-fidelity single-qubit and
two-qubit gates with operation times comparable to the inverse Zeeman
frequency.


title: Subsystem Codes

abstract:
We investigate various aspects of operator quantum error-correcting codes or,
as we prefer to call them, subsystem codes. We give various methods to derive
subsystem codes from classical codes. We give a proof for the existence of
subsystem codes using a counting argument similar to the quantum
Gilbert-Varshamov bound. We derive linear programming bounds and other upper
bounds. We answer the question whether or not there exist
[[n,n-2d+2,r>0,d]]q subsystem codes. Finally, we compare stabilizer
and subsystem codes with respect to the required number of syndrome qudits.


title:
Hybrid quantum repeater based on dispersive CQED interactions between
  matter qubits and bright coherent light

abstract:
  We describe a system for long-distance distribution of quantum entanglement,
in which coherent light with large average photon number interacts dispersively
with single, far-detuned atoms or semiconductor impurities in optical cavities.
Entanglement is heralded by homodyne detection using a second bright light
pulse for phase reference. The use of bright pulses leads to a high success
probability for the generation of entanglement, at the cost of a lower initial
fidelity. This fidelity may be boosted by entanglement purification techniques,
implemented with the same physical resources. The need for more purification
steps is well compensated for by the increased probability of success when
compared to heralded entanglement schemes using single photons or weak coherent
pulses with realistic detectors. The principle cause of the lower initial
fidelity is fiber loss; however, spontaneous decay and cavity losses during the
dispersive atom/cavity interactions can also impair performance. We show that
these effects may be minimized for emitter-cavity systems in the weak-coupling
regime as long as the resonant Purcell factor is larger than one, the cavity is
over-coupled, and the optical pulses are sufficiently long. We support this
claim with numerical, semiclassical calculations using parameters for three
realistic systems: optically bright donor-bound impurities such as 19-F:ZnSe
with a moderate-Q microcavity, the optically dim 31-P:Si system with a high-Q
microcavity, and trapped ions in large but very high-Q cavities.


title: Field-theoretical space-time position operator of the Dirac electron

abstract:
  A field-theoretical space-time position operator can be properly introduced
for the Dirac field, it plays the role of a generalized Noether charge
associated with a local symmetry, and its second-quantized form shows that
quantum fields possess zero-point time, which implies that a zero-point
fluctuation of energy must be accompanied with a zero-point fluctuation of
time, and then in agreement with the time-energy uncertainty principle. In
practice, some possible physical effects owing to zero-point time remain to be
found.


title: Weak measurement and rapid state reduction in bipartite quantum systems

abstract:
  In this paper we consider feedback control algorithms for the rapid
purification of a bipartite state consisting of two qubits, when the observer
has access to only one of the qubits. We show 1) that the algorithm that
maximizes the average purification rate is not the same as that that for a
single qubit, and 2) that it is always possible to construct an algorithm that
generates a deterministic rate of purification for {\em both} qubits. We also
reveal a key difference between projective and continuous measurements with
regard to state-purification.


title: Classical spin models and the quantum stabilizer formalism

abstract:
  We relate a large class of classical spin models, including the inhomogeneous
Ising, Potts, and clock models of q-state spins on arbitrary graphs, to
problems in quantum physics. More precisely, we show how to express partition
functions as inner products between certain quantum stabilizer states and
product states. This connection allows us to use powerful techniques developed
in quantum information theory, such as the stabilizer formalism and classical
simulation techniques, to gain general insights into these models in a unified
way. We recover and generalize several symmetries and high-low temperature
dualities, and we provide an efficient classical evaluation of partition
functions for all interaction graphs with a bounded tree-width.


title:
Quantum Nondemolition Measurement of Discrete Fock States of a
  Nanomechanical Resonator

abstract:
  We study theoretically a radio frequency superconducting interference device
integrated with both a nanomechanical resonator and an LC one. By applying
adiabatic and rotating wave approximations, we obtain an effective Hamiltonian
that governs the dynamics of the mechanical and LC resonators. Nonlinear terms
in this Hamiltonian can be exploited for performing a quantum nondemolition
measurement of Fock states of the nanomechanical resonator. We address the
feasibility of experimental implementation and show that the nonlinear coupling
can be made sufficiently strong to allow the detection of discrete mechanical
Fock states.


title:
Are superluminal "signals" an acceptable hypothesis? - Difficulties in
  building a communication protocol with them

abstract:
  The local hidden variable assumption was repeatedly proved unable to explain
results of experiments in which contextuality is involved. Then, the correlated
results of measurements of entangled particles, began to be attributed to a
communication between particles through so-called "signals". These "signals"
need to possess superluminal velocity or move backward in time. No object that
has a rest-mass, not even the photons whose rest mass is zero, behave this way.
Still, as the nature of the presumed "sinals" is not known, people don't reject
the idea, despite the conflict with the theory of relativity. For this reason,
the present article examines the "signals" from another point of view: wherever
runs a communication, there has also to exist a communication protocol. The
article tries to outline a communication protocol between the space-separated,
entangled particles, and comes to a contradiction, making the idea of such a
communication highly doubtable.


title: De-Gaussification by inconclusive photon subtraction

abstract:
  We address conditional de-Gaussification of continuous variable states by
inconclusive photon subtraction (IPS) and review in details its application to
bipartite twin-beam state of radiation. The IPS map in the Fock basis has been
derived, as well as its counterpart in the phase-space. Teleportation assisted
by IPS states is analyzed and the corresponding fidelity evaluated as a
function of the involved parameters. Nonlocality of IPS states is investigated
by means of different tests including displaced parity, homodyne detection,
pseudospin, and displaced on/off photodetection. Dissipation and thermal noise
are taken into account, as well as non unit quantum efficiency in the detection
stage. We show that the IPS process, for a suitable choice of the involved
parameters, improves teleportation fidelity and enhances nonlocal properties.


title: Mubs and Hadamards of Order Six

abstract:
  We report on a search for mutually unbiased bases (MUBs) in 6 dimensions. We
find only triplets of MUBs, and thus do not come close to the theoretical upper
bound 7. However, we point out that the natural habitat for sets of MUBs is the
set of all complex Hadamard matrices of the given order, and we introduce a
natural notion of distance between bases in Hilbert space. This allows us to
draw a detailed map of where in the landscape the MUB triplets are situated. We
use available tools, such as the theory of the discrete Fourier transform, to
organise our results. Finally we present some evidence for the conjecture that
there exists a four dimensional family of complex Hadamard matrices of order 6.
If this conjecture is true the landscape in which one may search for MUBs is
much larger than previously thought.


title: On measurement-based quantum computation with the toric code states

abstract:
  We study measurement-based quantum computation (MQC) using as quantum
resource the planar code state on a two-dimensional square lattice (planar
analogue of the toric code). It is shown that MQC with the planar code state
can be efficiently simulated on a classical computer if at each step of MQC the
sets of measured and unmeasured qubits correspond to connected subsets of the
lattice.


title: Resonant Propagation of Entangled Rhodium Mossbauer Gammas

abstract:
  We report the resonant propagation of the long-lived Mossbauer gamma in the
time-resolved Mossbauer spectroscopy. Recently, three entangled gammas emitted
from the E3 rhodium Mossbauer transition has been proposed to interpret the
extraordinary observations in the previous report. Further observation reported
here is the dynamic beat of these entangled gammas at room temperature and 77K.
Apparent beat anisotropy reveals their long-distance resonant propagation,
which leads to suppressed Doppler shift of entangled photon transport in the
Borrmann channel.


title:
Zeroth order quantum coherences and preparation of pseudopure state in
  homonuclear dipolar coupling spin systems

abstract:
  Dynamics of zeroth order quantum coherences and preparation of the pseudopure
states in homonuclear systems of dipolar coupling spins is closely examined. It
has been shown an extreme important role of the non-diagonal part of zeroth
order coherence in construction of the pseudopure state. Simulations of the
preparation process of pseudopure states with the real molecular structures (a
rectangular (-chloro- -nitrobenzene molecule), a chain (hydroxyapatite
molecule), a ring (benzene molecule), and a double ring (cyclopentane
molecule)) open the way to experimental testing of the obtained results.


title:
Entanglement-enhanced classical capacity of two-qubit quantum channels
  with memory: the exact solution

abstract:
  The maximal amount of information which is reliably transmitted over two uses
of general Pauli channels with memory is proven to be achieved by maximally
entangled states beyond some memory threshold. In particular, this proves a
conjecture on the depolarizing channel by Macchiavello and Palma [Phys. Rev. A
{\bf 65}, 050301(R) (2002)]. Below the memory threshold, for arbitrary Pauli
channels, the two-use classical capacity is only achieved by a particular type
of product states.


title:
Dynamical Realization of Macroscopic Superposition States of Cold Bosons
  in a Tilted Double Well

abstract:
  We present exact expressions for the quantum sloshing of Bose-Einstein
condensates in a tilted two-well potential. Tunneling is suppressed by a small
potential difference between wells, or tilt. However, tunneling resonances
occur for critical values of the tilt when the barrier is high. At resonance,
tunneling times on the order of 10-100 ms are possible. Furthermore, such
tilted resonances lead to a dynamical scheme for creating few-body NOON-like
macroscopic superposition states which are protected by the many body
wavefunction against potential fluctuations.


title: High Fidelity Single-Qubit Gates Using Non-adiabatic Rapid Passage

abstract:
  Numerical simulation results are presented which suggest that a class of
non-adiabatic rapid passage sweeps first realized experimentally in 1991 should
be capable of implementing a set of quantum gates that is universal for
one-qubit unitary operations and whose elements operate with error
probabilities per operation P < 10^{-4}. The sweeps are non-composite and
generate controllable quantum interference effects which allow the one-qubit
gates produced to operate non-adiabatically while maintaining high accuracy.
The simulations suggest that the one-qubit gates produced by these sweeps show
promise as possible elements of a fault-tolerant scheme for quantum computing.


title: Local Operations in qubit arrays via global but periodic Manipulation

abstract:
  We provide a scheme for quantum computation in lattice systems via global but
periodic manipulation, in which only effective periodic magnetic fields and
global nearest neighbor interaction are required. All operations in our scheme
are attainable in optical lattice or solid state systems. We also investigate
universal quantum operations and quantum simulation in 2 dimensional lattice.
We find global manipulations are superior in simulating some nontrivial many
body Hamiltonians.


title: Loschmidt echo in a system of interacting electrons

abstract:
  We study the Loschmidt echo for a system of electrons interacting through
mean-field Coulomb forces. The electron gas is modeled by a self-consistent set
of hydrodynamic equations. It is observed that the quantum fidelity drops
abruptly after a time that is proportional to the logarithm of the perturbation
amplitude. The fidelity drop is related to the breakdown of the symmetry
properties of the wave function.


title: Geometric phases for mixed states and decoherence

abstract:
  The gauge invariance of geometric phases for mixed states is analyzed by
using the hidden local gauge symmetry which arises from the arbitrariness of
the choice of the basis set defining the coordinates in the functional space.
This approach gives a reformulation of the past results of adiabatic,
non-adiabatic and mixed state geometric phases. The geometric phases are
identified uniquely as the holonomy associated with the hidden local gauge
symmetry which is an exact symmetry of the Schr\"{o}dinger equation. The
purification and its inverse in the description of de-coherent mixed states are
consistent with the hidden local gauge symmetry. A salient feature of the
present formulation is that the total phase and visibility in the mixed state,
which are directly observable in the interference experiment, are manifestly
gauge invariant.


title: Probabilistic exact deletion and probabilistic no-signalling

abstract:
  In this work we show that one cannot use non-local resources for
probabilistic signalling even if one can delete a quantum state with the help
of probabilistic quantum deletion machine. Here we find that probabilistic
quantum deletion machine is not going to help us in identifying two statistical
mixture at remote location. Also we derive the bound on deletion probability
from no-signalling condition.


title:
Giant Optical Non-linearity induced by a Single Two-Level System
  interacting with a Cavity in the Purcell Regime

abstract:
  A two-level system that is coupled to a high-finesse cavity in the Purcell
regime exhibits a giant optical non-linearity due to the saturation of the
two-level system at very low intensities, of the order of one photon per
lifetime. We perform a detailed analysis of this effect, taking into account
the most important practical imperfections. Our conclusion is that an
experimental demonstration of the giant non-linearity should be feasible using
semiconductor micropillar cavities containing a single quantum dot in resonance
with the cavity mode.


title: Are quantum `irreality' and `nonlocality' ineluctable?

abstract:
  The early history of the development of Quantum Mechanics is surveyed to
discern the arguments leading to the introduction of the notions of `irreal'
wave functions and `nonlocal' correlations. It is argued that the assumption
that Quantum Mechanics is `complete', i.e., not just a variant of Statistical
Mechanics, is the feature compelling the introduction of these otherwise
problematic properties. Additionally, a consequence of the error first found by
Jaynes in proofs of Bell's ``theorem'', is illustrated. Finally, speculation on
the practical consequences of recognising that ``entanglement'' is a feature of
all hyperbolic differential equations is proposed.


title:
The Impossibility of the Counterfactual Computation for all Possible
  Outcomes

abstract:
  Recent proposal for counterfactual computation [Hosten et al., Nature, 439,
949 (2006)] is analyzed. It is argued that the method does not provide
counterfactual computation for all possible outcomes. The explanation involves
a novel paradoxical feature of pre- and post-selected quantum particles: the
particle can reach a certain location without being on the path that leads to
this location.


title: Pseudo-telepathy: input cardinality and Bell-type inequalities

abstract:
  Pseudo-telepathy is the most recent form of rejection of locality. Many of
its properties have already been discovered: for instance, the minimal
entanglement, as well as the minimal cardinality of the output sets, have been
characterized. This paper contains two main results. First, we prove that no
bipartite pseudo-telepathy game exists, in which one of the partners receives
only two questions; as a corollary, we show that the minimal "input
cardinality", that is, the minimal number of questions required in a bipartite
pseudo-telepathy game, is 3x3. Second, we study the Bell-type inequality
derived from the pseudo-telepathy game known as the Magic Square game: we
demonstrate that it is a tight inequality for 3 inputs and 4 outputs on each
side and discuss its weak resistance to noise.


title: Three-qubit dynamics of entanglement in magnetic field

abstract:
  A closed system of the equations for the local Bloch vectors and spin
correlation functions is obtained by decomplexification of the Liouville-von
Neumann equation for three magnetic qubits with the exchange interaction, that
takes place in an arbitrary time-dependent external magnetic field. The
numerical comparative analysis of entanglement is carried out depending on
initial conditions and the magnetic field modulation. The present study may be
useful for analysis of interference experiments and in the field of quantum
computing.


title: Boltzmann-Shannon Entropy: Generalization and Application

abstract:
  The paper deals with the generalization of both Boltzmann entropy and
distribution in the light of most-probable interpretation of statistical
equilibrium. The statistical analysis of the generalized entropy and
distribution leads to some new interesting results of significant physical
importance.


title: Analysis of the Entanglement Cost and Calculation of the Holevo Capacity

abstract:
  ``Beam me over,'' Alice: A cricket's quantum journey
  This thesis addresses two known quantities in quantum information science:
(1) entanglement cost, and (2) Holevo capacity. These quantities will be
crucial values when teleportation becomes common in daily life, perhaps
centuries from now.
  Assume that Alice desires to send a singing Japanese cricket to her friend
Bob in America, and that Alice and Bob already share a quantum entanglement.
First, Alice sends Bob a mass of information bits resulting from the
interaction between the cricket she holds in her hand and half of the
entanglement. Subsequently, Bob receives the information bits and manipulates
the other half of the entanglement, transforming them back into the original
cricket. Examining this situation from an instrumental engineering viewpoint,
quantifying the amount of the quantum entanglement and the number of
information bits is crucial for this transmission. If both values are enough,
Alice could even send herself to Bob's place instead of the tiny cricket.
  The topics of this thesis therefore are: (1) the mathematical properties of
the entanglement cost, such as whether it is an additive measure similar to
normal length or weight; and (2) how to calculate the Holevo capacity, an
ultimately achievable limit of the information conveyance capacity of an
information channel, such as of a single photon passing through an optical
fiber or space. These two distinct quantities are magically tied together by
several ``additive or not'' hypotheses, which await mathematical proof.


title:
Total Quantum Zeno effect and Intelligent States for a two level system
  in a squeezed bath

abstract:
  In this work we show that by frequent measurements of adequately chosen
observables, a complete suppression of the decay in an exponentially decaying
two level system interacting with a squeezed bath is obtained. The observables
for which the effect is observed depend on the the squeezing parameters of the
bath. The initial states which display Total Zeno Effect are intelligent states
of two conjugate observables associated to the electromagnetic fluctuations of
the bath.


title:
Strong Violations of Bell-type Inequalities for Path-Entangled Number
  States

abstract:
  We show that nonlocal correlation experiments on the two spatially separated
modes of a maximally path-entangled number state may be performed and lead to a
violation of a Clauser-Horne Bell inequality for any finite photon number N. We
present also an analytical expression for the two-mode Wigner function of a
maximally path-entangled number state and investigate a
Clauser-Horne-Shimony-Holt Bell inequality for such states. We test other
Bell-type inequalities. Some are violated by a constant amount for any N.


title: Exact zero-point interaction energy between cylinders

abstract:
  We calculate the exact Casimir interaction energy between two perfectly
conducting, very long, eccentric cylindrical shells using a mode summation
technique. Several limiting cases of the exact formula for the Casimir energy
corresponding to this configuration are studied both analytically and
numerically. These include concentric cylinders, cylinder-plane, and eccentric
cylinders, for small and large separations between the surfaces. For small
separations we recover the proximity approximation, while for large separations
we find a weak logarithmic decay of the Casimir interaction energy, typical of
cylindrical geometries.


title:
Optical Nondestructive Controlled-NOT Gate without Using Entangled
  Photons

abstract:
  We present and experimentally demonstrate a novel optical nondestructive
controlled-NOT gate without using entangled ancilla. With much fewer
measurements compared with quantum process tomography, we get a good estimation
of the gate fidelity. The result shows a great improvement compared with
previous experiments. Moreover, we also show that quantum parallelism is
achieved in our gate and the performance of the gate can not be reproduced by
local operations and classical communications.


title:
Exact Solution of the Klein-Gordon Equation for the PT-Symmetric
  Generalized Woods-Saxon Potential by the Nikiforov-Uvarov Method

abstract:
  The one-dimensional Klein-Gordon (KG) equation has been solved for the
PT-symmetric generalized Woods-Saxon (WS) potential. The Nikiforov-Uvarov(NU}
method which is based on solving the second-order linear differential equations
by reduction to a generalized equation of hypergeometric type is used to obtain
exact energy eigenvalues and corresponding eigenfunctions. We have also
investigated the positive and negative exact bound states of the s-states for
different types of complex generalized WS potentials.


title:
Irreducible decomposition of Gaussian distributions and the spectrum of
  black-body radiation

abstract:
  It is shown that the energy of a mode of a classical chaotic field, following
the continuous exponential distribution as a classical random variable, can be
uniquely decomposed into a sum of its fractional part and of its integer part.
The integer part is a discrete random variable (we call it Planck variable)
whose distribution is just the Bose distribution yielding the Planck law of
black-body radiation. The fractional part is the dark part (we call is dark
variable) with a continuous distribution, which is, of course, not observed in
the experiments. It is proved that the Bose distribution is infinitely
divisible, and the irreducible decomposition of it is given. The Planck
variable can be decomposed into an infinite sum of independent binary random
variables representing the binary photons (more accurately photo-molecules or
photo-multiplets) of energies 2^s*h*nu with s=0,1,2... . These binary photons
follow the Fermi statistics. Consequently, the black-body radiation can be
viewed as a mixture of statistically and thermodynamically independent fermion
gases consisting of binary photons. The binary photons give a natural tool for
the dyadic expansion of arbitrary (but not coherent) ordinary photon
excitations. It is shown that the binary photons have wave-particle
fluctuations of fermions. These fluctuations combine to give the wave-particle
fluctuations of the original bosonic photons expressed by the Einstein
fluctuation formula.


title:
Go and return propagation of biphotons in fiber and polarization
  entanglement

abstract:
  Propagation of entangled photons in optical fiber is one of the fundamental
issues for realizing quantum communication protocols. When entanglement in
polarization is considered, arises the problem of compensating for the fiber
effect on photons polarization. In this paper we demonstrate an effective
solution where a Faraday mirror allows to cancel undesired effects of
polarization drift in fiber. This technique is applied to a protocol for
generating Bell states by a narrow temporal selection of the second-order
intensity correlation function.


title: Sudden death and long-living of entanglement in an ion-trap laser

abstract:
  he dynamical properties of quantum entanglement in a time-dependent
three-level-trapped ion interacting with a laser field are studied in terms of
the reduced-density linear entropy considering two specific initial states of
the field. Allowing the instantaneous position of the center-of-mass motion of
the ion to be explicitly time-dependent, it is shown that either sudden death
of entanglement or survivability of quantum entanglement can be obtained with a
specific choice of the initial state parameters. The difference in evolution
picture corresponding to the multi-quanta processes is discussed.


title: Notes on Geometric-Algebra Quantum-Like Algorithms

abstract:
  In these notes we present preliminary results on quantum-like algorithms
where tensor product is replaced by geometric product. Such algorithms possess
the essential properties typical of quantum computation (entanglement,
parallelism) but employ additional algebraic structures typical of geometric
algebra -- structures absent in standard quantum computation. As a test we
reformulate in Geometric Algebra terms the Deutsch-Jozsa problem.


title:
Generation of maximally entangled mixed states of two atoms via
  on-resonance asymmetric atom-cavity couplings

abstract:
  A scheme for generating the maximally entangled mixed state of two atoms
on-resonance asymmetrically coupled to a single mode optical cavity field is
presented. The part frontier of both maximally entangled mixed states and
maximal Bell violating mixed states can be approximately reached by the
evolving reduced density matrix of two atoms if the ratio of coupling strengths
of two atoms is appropriately controlled. It is also shown that exchange
symmetry of global maximal concurrence is broken if and only if coupling
strength ratio lies between $\frac{\sqrt{3}}{3}$ and $\sqrt{3}$ for the case of
one-particle excitation and asymmetric coupling, while this partial
symmetry-breaking can not be verified by detecting maximal Bell violation.


title: Complementarity in quantum systems

abstract:
  Reduction of a state of a quantum system to a subsystem gives partial quantum
information about the true state of the total system. Two subalgebras A1 and A2
of B(H) are called complementary if the traceless subspaces of A1 and A2 are
orthogonal (with respect to the Hilbert-Schmidt inner product). When both
subalgebras are maximal Abelian, then the concept reduces to complementary
observables or mutually unbiased bases. In the paper several characterizations
of complementary subalgebras are given in the general case and several examples
are presented. For a 4-level quantum system, the structure of complementary
subalgebras can be described very well, the Cartan decomposition of unitaries
plays a role. It turns out that a measurement corresponding to the Bell basis
is complementary to any local measurement of the two-qubit-system.


title:
Recovery of classical chaotic-like behaviour in a quantum three-body
  problem

abstract:
  Recovering trajectories of quantum systems whose classical counterparts
display chaotic behavior has been a subject that has received a lot of interest
over the last decade. However, most of these studies have focused on driven and
dissipative systems. The relevance and impact of chaoticlike phenomena to
quantum systems has been highlighted in recent studies which have shown that
quantum chaos is significant in some aspects of quantum computation and
information processing. In this paper we study a three-body system comprising
of identical particles arranged so that the system's classical trajectories
exhibit Hamiltonian chaos. Here we show that it is possible to recover very
nearly classical-like, conservative, chaotic trajectories from such a system
through an unravelling of the master equation. First, this is done through
continuous measurement of the position of each system. Second, and perhaps
somewhat surprisingly, we demonstrate that we still obtain a very good match
between the classical and quantum dynamics by weakly measuring the position of
only one of the oscillators.


title:
Indirect control with quantum accessor (I): coherent control of
  multi-level system via qubit chain

abstract:
  Indirect controllability of an arbitrary finite dimensional quantum system
(N-dimensional qudit) through a quantum accessor is investigated. Here, The
qudit is coupled to a quantum accessor which is modeled as a fully controllable
spin chain with nearest neighbor (anisotropic) XY-coupling. The complete
controllability of such indirect control system is investigated in detail. The
general approach is applied to the indirect controllability of two and three
dimensional quantum systems. For two and three dimensional systems, a simpler
indirect control scheme is also presented.


title:
(p,q)-Deformations and (p,q)-Vector Coherent States of the
  Jaynes-Cummings Model in the Rotating Wave Approximation

abstract:
  Classes of (p,q)-deformations of the Jaynes-Cummings model in the rotating
wave approximation are considered. Diagonalization of the Hamiltonian is
performed exactly, leading to useful spectral decompositions of a series of
relevant operators. The latter include ladder operators acting between adjacent
energy eigenstates within two separate infinite discrete towers, except for a
singleton state. These ladder operators allow for the construction of
(p,q)-deformed vector coherent states. Using (p,q)-arithmetics, explicit and
exact solutions to the associated moment problem are displayed, providing new
classes of coherent states for such models. Finally, in the limit of decoupled
spin sectors, our analysis translates into (p,q)-deformations of the
supersymmetric harmonic oscillator, such that the two supersymmetric sectors
get intertwined through the action of the ladder operators as well as in the
associated coherent states.


title: Conjugate Codes and Applications to Cryptography

abstract:
  A conjugate code pair is defined as a pair of linear codes such that one
contains the dual of the other. The conjugate code pair represents the
essential structure of the corresponding Calderbank-Shor-Steane (CSS) quantum
code. It is argued that conjugate code pairs are applicable to quantum
cryptography in order to motivate studies on conjugate code pairs.


title: Concatenated Conjugate Codes

abstract:
  A conjugate code pair is defined as a pair of linear codes either of which
contains the dual of the other. A conjugate code pair represents the essential
structure of the corresponding Calderbank-Shor-Steane (CSS) quantum code. It is
known that conjugate code pairs are applicable to (quantum) cryptography. We
give a construction method for efficiently decodable conjugate code pairs.


title:
Concatenated Quantum Codes Constructible in Polynomial Time: Efficient
  Decoding and Error Correction

abstract:
  A method for concatenating quantum error-correcting codes is presented. The
method is applicable to a wide class of quantum error-correcting codes known as
Calderbank-Shor-Steane (CSS) codes. As a result, codes that achieve a high rate
in the Shannon theoretic sense and that are decodable in polynomial time are
presented. The rate is the highest among those known to be achievable by CSS
codes. Moreover, the best known lower bound on the greatest minimum distance of
codes constructible in polynomial time is improved for a wide range.


title: Information-Disturbance Tradeoff in Quantum State Discrimination

abstract:
  When discriminating between two pure quantum states, there exists a
quantitative tradeoff between the information retrieved by the measurement and
the disturbance caused on the unknown state. We derive the optimal tradeoff and
provide the corresponding quantum measurement. Such an optimal measurement
smoothly interpolates between the two limiting cases of maximal information
extraction and no measurement at all.


title: Experimental observation of three-color optical quantum correlations

abstract:
  Quantum correlations between bright pump, signal, and idler beams produced by
an optical parametric oscillator, all with different frequencies, are
experimentally demonstrated. We show that the degree of entanglement between
signal and idler fields is improved by using information of pump fluctuations.
This is the first observation of three-color optical quantum correlations.


title: Time delay for one-dimensional quantum systems with steplike potentials

abstract:
  This paper concerns time-dependent scattering theory and in particular the
concept of time delay for a class of one-dimensional anisotropic quantum
systems. These systems are described by a Schr\"{o}dinger Hamiltonian $H =
-\Delta + V$ with a potential $V(x)$ converging to different limits $V_{\ell}$
and $V_{r}$ as $x \to -\infty$ and $x \to +\infty$ respectively. Due to the
anisotropy they exhibit a two-channel structure. We first establish the
existence and properties of the channel wave and scattering operators by using
the modern Mourre approach. We then use scattering theory to show the identity
of two apparently different representations of time delay. The first one is
defined in terms of sojourn times while the second one is given by the
Eisenbud-Wigner operator. The identity of these representations is well known
for systems where $V(x)$ vanishes as $|x| \to \infty$ ($V_\ell = V_r$). We show
that it remains true in the anisotropic case $V_\ell \not = V_r$, i.e. we prove
the existence of the time-dependent representation of time delay and its
equality with the time-independent Eisenbud-Wigner representation. Finally we
use this identity to give a time-dependent interpretation of the
Eisenbud-Wigner expression which is commonly used for time delay in the
literature.


title: Asymptotic entanglement manipulation of bipartite pure states

abstract:
  Entanglement of pure states of bipartite quantum systems has been shown to
have a unique measure in terms of the von Neumann entropy of the reduced states
of either of its subsystems. The measure is established under entanglement
manipulation of an asymptotically large number of copies of the states. In this
paper, different asymptotic measures of entanglement assigned to arbitrary
sequences of bipartite pure states are shown to coincide only when the sequence
is information stable, in terms of the quantum spectral information rates of
the sequence of subsystem states. Additional bounds on the optimal rates of
entanglement manipulation protocols in quantum information are also presented,
including bounds given by generalizations of the coherent information and the
relative entropy of entanglement.


title:
Quantum List Decoding of Classical Block Codes of Polynomially Small
  Rate from Quantumly Corrupted Codewords

abstract:
  Given a classical error-correcting block code, the task of quantum list
decoding is to produce from any quantumly corrupted codeword a short list
containing all messages whose codewords exhibit high "presence" in the
quantumly corrupted codeword. Efficient quantum list decoders have been used to
prove a quantum hardcore property of classical codes. However, the code rates
of all known families of efficiently quantum list-decodable codes are,
unfortunately, too small for other practical applications. To improve those
known code rates, we prove that a specific code family of polynomially small
code rate over a fixed code alphabet, obtained by concatenating generalized
Reed-Solomon codes as outer codes with Hadamard codes as inner codes, has an
efficient quantum list-decoding algorithm if its codewords have relatively high
codeword presence in a given quantumly corrupted codeword. As an immediate
application, we use the quantum list decodability of this code family to solve
a certain form of quantum search problems in polynomial time. When the codeword
presence becomes smaller, in contrast, we show that the quantum list
decodability of generalized Reed-Solomon codes with high confidence is closely
related to the efficient solvability of the following two problems: the noisy
polynomial interpolation problem and the bounded distance vector problem.
Moreover, assuming that NP is not included in BQP, we also prove that no
efficient quantum list decoder exists for the generalized Reed-Solomon codes.


title:
Numerical simulation of a macroscopic quantum-like experiment:
  oscillating wave packet

abstract:
  We simulate the transformation of a classical fluid into a quantum-like
(super)-fluid by the application of a generalized quantum potential through a
retro-active loop. This numerical experiment is exemplified in the case of a
non-spreading oscillating wave packet in a harmonic potential. We find
signatures of a quantum-like behavior which are stable against various
perturbations.


title: Architecture of the Secoqc Quantum Key Distribution network

abstract:
  The European projet Secoqc (Secure Communication based on Quantum
Cryptography) aims at developing a global network for unconditionally secure
key distribution. This paper specifies the requirements and presents the
principles guiding the design of this network, and relevant to its architecture
and protocols.


title:
Security of quantum key distribution using weak coherent states with
  nonrandom phases

abstract:
  We prove the security of the Bennett-Brassard (BB84) quantum key distribution
protocol in the case where the key information is encoded in the relative phase
of a coherent-state reference pulse and a weak coherent-state signal pulse, as
in some practical implementations of the protocol. In contrast to previous
work, our proof applies even if the eavesdropper knows the phase of the
reference pulse, provided that this phase is not modulated by the source, and
even if the reference pulse is bright. The proof also applies to the case where
the key is encoded in the photon polarization of a weak coherent-state pulse
with a known phase, but only if the phases of the four BB84 signal states are
judiciously chosen. The achievable key generation rate scales quadratically
with the transmission in the channel, just as for BB84 with phase-randomized
weak coherent-state signals (when decoy states are not used). For the case
where the phase of the reference pulse is strongly modulated by the source, we
exhibit an explicit attack that allows the eavesdropper to learn every key bit
in a parameter regime where a protocol using phase-randomized signals is
provably secure.


title: Quantum Dynamics without the Wave Function

abstract:
  When suitably generalized and interpreted, the path-integral offers an
alternative to the more familiar quantal formalism based on state-vectors,
selfadjoint operators, and external observers. Mathematically one generalizes
the path-integral-as-propagator to a {\it quantal measure} $\mu$ on the space
$\Omega$ of all ``conceivable worlds'', and this generalized measure expresses
the dynamics or law of motion of the theory, much as Wiener measure expresses
the dynamics of Brownian motion. Within such ``histories-based'' schemes new,
and more ``realistic'' possibilities open up for resolving the philosophical
problems of the state-vector formalism. In particular, one can dispense with
the need for external agents by locating the predictive content of $\mu$ in its
sets of measure zero: such sets are to be ``precluded''. But unrestricted
application of this rule engenders contradictions. One possible response would
remove the contradictions by circumscribing the application of the preclusion
concept. Another response, more in the tradition of ``quantum logic'', would
accommodate the contradictions by dualizing $\Omega$ to a space of
``co-events'' and effectively identifying reality with an element of this dual
space.


title: Optimal multicopy asymmetric Gaussian cloning of coherent states

abstract:
  We investigate the asymmetric Gaussian cloning of coherent states which
produces M copies from N input replicas, such that the fidelity of all copies
may be different. We show that the optimal asymmetric Gaussian cloning can be
performed with a single phase-insensitive amplifier and an array of beam
splitters. We obtain a simple analytical expression characterizing the set of
optimal asymmetric Gaussian cloning machines.


title:
Implementation of quantum gates based on geometric phases accumulated in
  the eigenstates of periodic invariant operators

abstract:
  We propose a new strategy to physically implement a universal set of quantum
gates based on geometric phases accumulated in the nondegenerate eigenstates of
a designated invariant operator in a periodic physical system. The system is
driven to evolve in such a way that the dynamical phase shifts of the invariant
operator eigenstates are the same (or {\it mod} $2\pi$) while the corresponding
geometric phases are nontrivial.
  We illustrate how this strategy to work in a simple but typical NMR-type
qubit system.


title: Sum rules and the domain after the last node of an eigenstate

abstract:
  It is shown that it is possible to establish sum rules that must be satisfied
at the nodes and extrema of the eigenstates of confining potentials which are
functions of a single variable. At any boundstate energy the Schroedinger
equation has two linearly independent solutions one of which is normalisable
while the other is not. In the domain after the last node of a boundstate
eigenfunction the unnormalisable linearly independent solution has a simple
form which enables the construction of functions analogous to Green's functions
that lead to certain sum rules. One set of sum rules give conditions that must
be satisfied at the nodes and extrema of the boundstate eigenfunctions of
confining potentials. Another sum rule establishes a relation between an
integral involving an eigenfunction in the domain after the last node and a sum
involving all the eigenvalues and eigenstates. Such sum rules may be useful in
the study of properties of confining potentials. The exactly solvable cases of
the particle in a box and the simple harmonic oscillator are used to illustrate
the procedure. The relations between one of the sum rules and two-particle
densities and a construction based on Supersymmetric Quantum Mechanics are
discussed.


title: Entanglement in the above-threshold optical parametric oscillator

abstract:
  We investigate entanglement in the above-threshold Optical Parametric
Oscillator, both theoretically and experimentally, and discuss its potential
applications to quantum information. The fluctuations measured in the
subtraction of signal and idler amplitude quadratures are $\Delta^2 \hat
p_-=0.50(1)$, or $-3.01(9)$ dB, and in the sum of phase quadratures are
$\Delta^2 \hatq_+=0.73(1)$, or $-1.37(6)$ dB. A detailed experimental study of
the noise behavior as a function of pump power is presented, and discrepancies
with theory are discussed.


title: The Conway-Kochen argument and relativistic GRW models

abstract:
  In a recent paper, Conway and Kochen proposed what is now known as the "Free
Will theorem" which, among other things, should prove the impossibility of
combining GRW models with special relativity, i.e., of formulating
relativistically invariant models of spontaneous wavefunction collapse. Since
their argument basically amounts to a non-locality proof for any theory aiming
at reproducing quantum correlations, and since it was clear since very a long
time that any relativistic collapse model must be non-local in some way, we
discuss why the theorem of Conway and Kochen does not affect the program of
formulating relativistic GRW models.


title: Quantum State Transfer in Spin-1 Chains

abstract:
  We study the transfer of quantum information through a Heisenberg spin-1
chain prepared in its ground state. We measure the efficiency of such a quantum
channel {\em via} the fidelity of retrieving an arbitrarily prepared state and
{\em via} the transfer of quantum entanglement. The Heisenberg spin-1 chain has
a very rich quantum phase diagram. We show that the phase boundaries are
reflected in sharp variations of the transfer efficiency. In the vicinity of
the border between the dimer and the ferromagnetic phase (in the conjectured
spin-nematic region), we find strong indications for a qualitative change of
the excitation spectrum. Moreover, we identify two regions of the phase diagram
which give rise to particularly high transfer efficiency; the channel might be
non-classical even for chains of arbitrary length, in contrast to spin-1/2
chains.


title: Dissipation Effects in Hybrid Systems

abstract:
  The dissipation effect in a hybrid system is studied in this Letter. The
hybrid system is a compound of a classical magnetic particle and a quantum
single spin. Two cases are considered. In the first case, we investigate the
effect of the dissipative quantum subsystem on the motion of its classical
partner. Whereas in the second case we show how the dynamics of the quantum
single spin are affected by the dissipation of the classical particle.
Extension to general dissipative hybrid systems is discussed.


title: Quantum transport on two-dimensional regular graphs

abstract:
  We study the quantum-mechanical transport on two-dimensional graphs by means
of continuous-time quantum walks and analyse the effect of different boundary
conditions (BCs). For periodic BCs in both directions, i.e., for tori, the
problem can be treated in a large measure analytically. Some of these results
carry over to graphs which obey open boundary conditions (OBCs), such as
cylinders or rectangles. Under OBCs the long time transition probabilities
(LPs) also display asymmetries for certain graphs, as a function of their
particular sizes. Interestingly, these effects do not show up in the marginal
distributions, obtained by summing the LPs along one direction.


title: Superradiant Rayleigh scattering in a ring cavity

abstract:
  Collective interaction of light with an atomic gas can give rise to
superradiant instabilities. We experimentally study the sudden build-up of a
reverse light field in a laser-driven high-finesse ring cavity filled with
ultracold thermal or condensed atoms. While superradiant Rayleigh scattering
from atomic clouds is normally only observed at very low temperatures (i.e.
well below $1 \mu$K), the presence of the ring cavity enhances cooperativity
and allows for superradiance with thermal clouds as hot as several $10 \mu$K. A
characterization of the superradiance at various temperatures and cooperativity
parameters allows us to link it to the collective atomic recoil laser.


title:
Arbitrary accuracy iterative phase estimation algorithm as a two qubit
  benchmark

abstract:
  We discuss the implementation of an iterative quantum phase estimation
algorithm, with a single ancillary qubit. We suggest using this algorithm as a
benchmark for multi-qubit implementations. Furthermore we describe in detail
the smallest possible realization, using only two qubits, and exemplify with a
superconducting circuit. We discuss the robustness of the algorithm in the
presence of gate errors, and show that 7 bits of precision is obtainable, even
with very limited gate accuracies.

categories: quant-ph cond-mat.mes-hall cond-mat.supr-con gr-qc math-ph math.MP


title:
Restrictions on Information Transfer in Quantum Measurements and State
  Collapse

abstract:
  Information-theoretical restrictions on the information transfer in quantum
measurements are studied. They are derived for the measurement of system S by
detector D, registrated and processed by information system O. The formalism of
inference maps in Hilbert space is used for it; it permit to calculate O
restricted state which contains available for O information on S parameters.
It's shown that the principal information losses, inevitable in this formalism
and induced by Heisenberg commutation relations, stipulate the stochasticity of
measurement outcomes, registrated by O in the individual events.


title: Three ways to look at mutually unbiased bases

abstract:
  This is a review of the problem of Mutually Unbiased Bases in finite
dimensional Hilbert spaces, real and complex. Also a geometric measure of
"mubness" is introduced, and applied to some recent calculations in six
dimensions (partly done by Bjorck and by Grassl). Although this does not yet
solve any problem, some appealing structures emerge.


title:
Entanglement reciprocation between atomic qubits and entangled coherent
  state

abstract:
  Introducing classical fields, we can transfer entanglement completely from
discrete qubits into entangled coherent state. The entanglement also can be
retrieved from the continuous-variable state of the cavities to the atomic
qubits. Via postselection measure, atomic entangled state and entangled
coherent state can be mutual transformed fully.


title:
Loschmidt Echo and Berry phase of the quantum system coupled to the XY
  spin chain: Proximity to quantum phase transition

abstract:
  We study the Loschmidt echo (LE) of a coupled system consisting of a central
spin and its surrounding environment described by a general XY spin-chain
model. The quantum dynamics of the LE is shown to be remarkably influenced by
the quantum criticality of the spin chain. In particular, the decaying behavior
of the LE is found to be controlled by the anisotropy parameter of the spin
chain. Furthermore, we show that due to the coupling to the spin chain, the
ground-state Berry phase for the central spin becomes nonanalytical and its
derivative with respect to the magnetic parameter $\lambda$ in spin chain
diverges along the critical line $\lambda=1$, which suggests an alternative
measurement of the quantum criticality of the spin chain.


title: Concurrence of superposition

abstract:
  The bounds on concurrence of the superposition state in terms of those of the
states being superposed are studied in this paper. The bounds on concurrence
are quite different from those on the entanglement measure based on von Neumann
entropy (Phys. Rev. Lett. 97, 100502 (2006)). In particular, a nonzero lower
bound can be provided if the states being superposed are properly constrained.