title: Maser Oscillation in a Whispering-Gallery-Mode Microwave Resonator

abstract:
  We report the first observation of above-threshold maser oscillation in a
whispering-gallery(WG)-mode resonator, whose quasi-transverse-magnetic, 17th
azimuthal-order WG mode, at a frequency of approx. 12.038 GHz, with a loaded Q
of several hundred million, is supported on a cylinder of mono-crystalline
sapphire. An electron spin resonance (ESR) associated with Fe3+ ions, that are
substitutively included within the sapphire at a concentration of a few parts
per billion, coincides in frequency with that of the (considerably narrower) WG
mode. By applying a c.w. `pump' to the resonator at a frequency of approx.
31.34 GHz, with no applied d.c. magnetic field, the WG (`signal') mode is
energized through a three-level maser scheme. Preliminary measurements
demonstrate a frequency stability (Allan deviation) of a few times 1e-14 for
sampling intervals up to 100 s.


title: Localisation of a wave-function by superposition of different histories

abstract:
  Quantum state diffusion shows how stochastic interaction with the environment
may cause localisation of the wave-function, and thereby demonstrates that
quantum mechanics need not invoke a separate axiom of measurement to explain
the emergence of the classical world. It has not been clear whether quantum
state diffusion requires some new physics. We set up an explicit numerical
calculation of the evolution of the wave-function of a two-state system under
interaction using only the physics explicitly contained in quantum mechanics
without an axiom of measurement. The wave-function does indeed localise, as
proposed by quantum state diffusion, on eigenstates of the perturbation. The
mechanism appears to be the superposition of histories evolving under different
Hamiltonians.


title:
Generalization of Classical Statistical Mechanics to Quantum Mechanics
  and Stable Property of Condensed Matter

abstract:
  Classical statistical average values are generally generalized to average
values of quantum mechanics, it is discovered that quantum mechanics is direct
generalization of classical statistical mechanics, and we generally deduce both
a new general continuous eigenvalue equation and a general discrete eigenvalue
equation in quantum mechanics, and discover that a eigenvalue of quantum
mechanics is just an extreme value of an operator in possibility distribution,
the eigenvalue f is just classical observable quantity. A general classical
statistical uncertain relation is further given, the general classical
statistical uncertain relation is generally generalized to quantum uncertainty
principle, the two lost conditions in classical uncertain relation and quantum
uncertainty principle, respectively, are found. We generally expound the
relations among uncertainty principle, singularity and condensed matter
stability, discover that quantum uncertainty principle prevents from the
appearance of singularity of the electromagnetic potential between nucleus and
electrons, and give the failure conditions of quantum uncertainty principle.
Finally, we discover that the classical limit of quantum mechanics is classical
statistical mechanics, the classical statistical mechanics may further be
degenerated to classical mechanics, and we discover that only saying that the
classical limit of quantum mechanics is classical mechanics is mistake. As
application examples, we deduce both Shrodinger equation and state
superposition principle, deduce that there exist decoherent factor from a
general mathematical representation of state superposition principle, and the
consistent difficulty between statistical interpretation of quantum mechanics
and determinant property of classical mechanics is overcome.


title:
A treatment of the quantum partial entropies in the atom-field
  interaction with a class of Schrodinger cat states

abstract:
  This communication is an enquiry into the circumstances under which entropy
and subentropy methods can give an answer to the question of quantum
entanglement in the composite state. Using a general quantum dynamical system
we obtain the analytical solution when the atom initially starts from its
excited state and the field in different initial states. Different features of
the entanglement are investigated when the field is initially assumed to be in
a coherent state, an even coherent state (Schrodinger cate state) and a
statistical mixture of coherent states. Our results show that the setting of
the initial state and the Stark shift play important role in the evolution of
the sub-entropies and entanglement.


title: Entropy and Quantum Kolmogorov Complexity: A Quantum Brudno's Theorem

abstract:
  In classical information theory, entropy rate and Kolmogorov complexity per
symbol are related by a theorem of Brudno. In this paper, we prove a quantum
version of this theorem, connecting the von Neumann entropy rate and two
notions of quantum Kolmogorov complexity, both based on the shortest qubit
descriptions of qubit strings that, run by a universal quantum Turing machine,
reproduce them as outputs.

categories: quant-ph cs.IT math-ph math.DS math.IT math.MP


title: Quantum optical device accelerating dynamic programming

abstract:
  In this paper we discuss analogue computers based on quantum optical systems
accelerating dynamic programming for some computational problems. These
computers, at least in principle, can be realized by actually existing devices.
We estimate an acceleration in resolving of some NP-hard problems that can be
obtained in such a way versus deterministic computers


title: Between classical and quantum

abstract:
  The relationship between classical and quantum theory is of central
importance to the philosophy of physics, and any interpretation of quantum
mechanics has to clarify it. Our discussion of this relationship is partly
historical and conceptual, but mostly technical and mathematically rigorous,
including over 500 references. On the assumption that quantum mechanics is
universal and complete, we discuss three ways in which classical physics has so
far been believed to emerge from quantum physics, namely in the limit h -> 0 of
small Planck's constant (in a finite system), in the limit of a large system,
and through decoherence and consistent histores. The first limit is closely
related to modern quantization theory and microlocal analysis, whereas the
second involves methods of C*-algebras and the concepts of superselection
sectors and macroscopic observables. In these limits, the classical world does
not emerge as a sharply defined objective reality, but rather as an approximate
appearance relative to certain "classical" states and observables. Decoherence
subsequently clarifies the role of such states, in that they are "einselected",
i.e. robust against coupling to the environment. Furthermore, the nature of
classical observables is elucidated by the fact that they typically define
(approximately) consistent sets of histories. We make the point that
classicality results from the elimination of certain states and observables
from quantum theory. Thus the classical world is not created by observation (as
Heisenberg once claimed), but rather by the lack of it.


title: On the Energy Increase in Space-Collapse Models

abstract:
  A typical feature of spontaneous collapse models which aim at localizing
wavefunctions in space is the violation of the principle of energy
conservation. In the models proposed in the literature the stochastic field
which is responsible for the localization mechanism causes the momentum to
behave like a Brownian motion, whose larger and larger fluctuations show up as
a steady increase of the energy of the system. In spite of the fact that, in
all situations, such an increase is small and practically undetectable, it is
an undesirable feature that the energy of physical systems is not conserved but
increases constantly in time, diverging for $t \to \infty$. In this paper we
show that this property of collapse models can be modified: we propose a model
of spontaneous wavefunction collapse sharing all most important features of
usual models but such that the energy of isolated systems reaches an asymptotic
finite value instead of increasing with a steady rate.


title: Conditions for spin squeezing in a cold 87Rb ensemble

abstract:
  We study the conditions for generating spin squeezing via a quantum
non-demolition measurement in an ensemble of cold 87Rb atoms. By considering
the interaction of atoms in the 5S_{1/2}(F=1) ground state with probe light
tuned near the D2 transition, we show that, for large detunings, this system is
equivalent to a spin-1/2 system when suitable Zeeman substates and quantum
operators are used to define a pseudo-spin. The degree of squeezing is derived
for the rubidium system in the presence of scattering causing decoherence and
loss. We describe how the system can decohere and lose atoms, and predict as
much as 75% spin squeezing for atomic densities typical of optical dipole
traps.


title:
Characterization of complex quantum dynamics with a scalable NMR
  information processor

abstract:
  We present experimental results on the measurement of fidelity decay under
contrasting system dynamics using a nuclear magnetic resonance quantum
information processor. The measurements were performed by implementing a
scalable circuit in the model of deterministic quantum computation with only
one quantum bit. The results show measurable differences between regular and
complex behaviour and for complex dynamics are faithful to the expected
theoretical decay rate. Moreover, we illustrate how the experimental method can
be seen as an efficient way for either extracting coarse-grained information
about the dynamics of a large system, or measuring the decoherence rate from
engineered environments.


title: Holonomic quantum computation in decoherence-free subspaces

abstract:
  We show how to realize, by means of non-abelian quantum holonomies, a set of
universal quantum gates acting on decoherence-free subspaces and subsystems. In
this manner we bring together the quantum coherence stabilization virtues of
decoherence-free subspaces and the fault-tolerance of all-geometric holonomic
control. We discuss the implementation of this scheme in the context of quantum
information processing using trapped ions and quantum dots.


title: Dissipation and decoherence in a quantum oscillator

abstract:
  The time development of the reduced density matrix for a quantum oscillator
damped by coupling it to an ohmic environment is calculated via an identity of
the Debye-Waller form. Results obtained some years ago by Hakim and the author
in the free particle limit [1] are thus recovered. The evolution of a free
particle in a prepared initial state is examined, and a previously published
exchange [2,3] is illuminated with figures showing no decoherence without
dissipation.
  [1] V. Hakim and V. Ambegaokar, Phys. Rev. A 32, 423 (1985). [2] G.W. Ford
and R.F. O'Connell, Phys. Rev. A 70, 026102 (2004). [3] D. Gobert, J. von
Delft, and V. Ambegaokar, Phys. Rev. A 70, 0261001 (2004).


title: Nano Positioning of Single Atoms in a Micro Cavity

abstract:
  The coupling of individual atoms to a high-finesse optical cavity is
precisely controlled and adjusted using a standing-wave dipole-force trap, a
challenge for strong atom-cavity coupling. Ultracold Rubidium atoms are first
loaded into potential minima of the dipole trap in the center of the cavity.
Then we use the trap as a conveyor belt that we set into motion perpendicular
to the cavity axis. This allows us to repetitively move atoms out of and back
into the cavity mode with a repositioning precision of 135 nm. This makes
possible to either selectively address one atom of a string of atoms by the
cavity, or to simultaneously couple two precisely separated atoms to a higher
mode of the cavity.


title: Mutually Unbiased Bases and Orthogonal Decompositions of Lie Algebras

abstract:
  We establish a connection between the problem of constructing maximal
collections of mutually unbiased bases (MUBs) and an open problem in the theory
of Lie algebras. More precisely, we show that a collection of m MUBs in K^n
gives rise to a collection of m Cartan subalgebras of the special linear Lie
algebra sl_n(K) that are pairwise orthogonal with respect to the Killing form,
where K=R or K=C. In particular, a complete collection of MUBs in C^n gives
rise to a so-called orthogonal decomposition (OD) of sl_n(C). The converse
holds if the Cartan subalgebras in the OD are also *-closed, i.e., closed under
the adjoint operation. In this case, the Cartan subalgebras have unitary bases,
and the above correspondence becomes equivalent to a result relating
collections of MUBs to collections of maximal commuting classes of unitary
error bases, i.e., orthogonal unitary matrices.
  It is a longstanding conjecture that ODs of sl_n(C) can only exist if n is a
prime power. This corroborates further the general belief that a complete
collection of MUBs can only exist in prime power dimensions. The connection to
ODs of sl_n(C) potentially allows the application of known results on (partial)
ODs of sl_n(C) to MUBs.


title: Chaotic Quantum Decay in Driven Biased Optical Lattices

abstract:
  Quantum decay in an ac driven biased periodic potential modeling cold atoms
in optical lattices is studied for a symmetry broken driving. For the case of
fully chaotic classical dynamics the classical exponential decay is quantum
mechanically suppressed for a driving frequency \omega in resonance with the
Bloch frequency \omega_B, q\omega=r\omega_B with integers q and r.
Asymptotically an algebraic decay ~t^{-\gamma} is observed. For r=1 the
exponent \gamma agrees with $q$ as predicted by non-Hermitian random matrix
theory for q decay channels. The time dependence of the survival probability
can be well described by random matrix theory. The frequency dependence of the
survival probability shows pronounced resonance peaks with sub-Fourier
character.


title:
Quantum damped oscillator II: Bateman's Hamiltonian vs. 2D Parabolic
  Potential Barrier

abstract:
  We show that quantum Bateman's system which arises in the quantization of a
damped harmonic oscillator is equivalent to a quantum problem with 2D parabolic
potential barrier known also as 2D inverted isotropic oscillator. It turns out
that this system displays the family of complex eigenvalues corresponding to
the poles of analytical continuation of the resolvent operator to the complex
energy plane. It is shown that this representation is more suitable than the
hyperbolic one used recently by Blasone and Jizba.


title:
Distillation of multipartite entanglement by complementary stabilizer
  measurements

abstract:
  We propose a scheme of multipartite entanglement distillation driven by a
complementary pair of stabilizer measurements, to distill directly a wider
range of states beyond the stabilizer code states (such as the
Greenberger-Horne-Zeilinger states). We make our idea explicit by constructing
a recurrence protocol for the 3-qubit W state. Noisy W states resulting from
typical decoherence can be directly purified in a few steps, if their initial
fidelity is larger than a threshold. For general input mixed states, we observe
distillations to hierarchical fixed points, i.e., not only to the W state but
also to the 2-qubit Bell pair, depending on their initial entanglement.


title: Modified Grover's Search Algorithm in O(M+logN)Steps

abstract:
  The present letter proposes a modification in the well known Grover's search
algorithm, which searches a database of $N$ unsorted items in $O(\sqrt{N/M})$
steps, where $M$ represents the number of solutions to the search problem.
Concurrency control techniques and extra registers for marking and storing the
solutions are used in the modified algorithm. This requires additional space
but it is shown that the use of extra register and marking techniques can
reduce the time complexity to $O(M+\log N)$.


title:
Application of Pseudo-Hermitian Quantum Mechanics to a PT-Symmetric
  Hamiltonian with a Continuum of Scattering States

abstract:
  We extend the application of the techniques developed within the framework of
the pseudo-Hermitian quantum mechanics to study a unitary quantum system
described by an imaginary PT-symmetric potential v(x) having a continuous real
spectrum. For this potential that has recently been used, in the context of
optical potentials, for modelling the propagation of electromagnetic waves
travelling in a wave guide half and half filed with gain and absorbing media,
we give a perturbative construction of the physical Hilbert space, observables,
localized states, and the equivalent Hermitian Hamiltonian. Ignoring terms of
order three or higher in the non-Hermiticity parameter zeta, we show that the
equivalent Hermitian Hamiltonian has the form
$\frac{p^2}{2m}+\frac{\zeta^2}{2}\sum_{n=0}^\infty\{\alpha_n(x),p^{2n}\}$ with
$\alpha_n(x)$ vanishing outside an interval that is three times larger than the
support of $v(x)$, i.e., in 2/3 of the physical interaction region the
potential $v(x)$ vanishes identically. We provide a physical interpretation for
this unusual behavior and comment on the classical limit of the system.


title: Schmidt states and positivity of linear maps

abstract:
  Using pure entangled Schmidt states, we show that m-positivity of a map is
bounded by the ranks of its negative Kraus matrices. We also give an algebraic
condition for a map to be m-positive. We interpret these results in the context
of positive maps as entanglement witnesses, and find that only 1-positive maps
are needed for testing entanglement.


title: Continuous-time Quantum Walks on a Cycle Graph

abstract:
  We present analytical treatment of quantum walks on a cycle graph. The
investigation is based on a realistic physical model of the graph in which
decoherence is induced by continuous monitoring of each graph vertex with
nearby quantum point contact. We derive the analytical expression of the
probability distribution along the cycle. Upper bound estimate to mixing time
is shown.


title: Fast and simple one-way Quantum Key Distribution

abstract:
  We present and demonstrate a new protocol for practical quantum cryptography,
tailored for an implementation with weak coherent pulses to obtain a high key
generation rate. The key is obtained by a simple time-of-arrival measurement on
the data line; the presence of an eavesdropper is checked by an interferometer
on an additional monitoring line. The setup is experimentally simple; moreover,
it is tolerant to reduced interference visibility and to photon number
splitting attacks, thus featuring a high efficiency in terms of distilled
secret bit per qubit.


title:
Measure of phonon-number moments and motional quadratures through
  infinitesimal-time probing of trapped ions

abstract:
  A method for gaining information about the phonon-number moments and the
generalized nonlinear and linear quadratures in the motion of trapped ions (in
particular, position and momentum) is proposed, valid inside and outside the
Lamb-Dicke regime. It is based on the measurement of first time derivatives of
electronic populations, evaluated at the motion-probe interaction time t=0. In
contrast to other state-reconstruction proposals, based on measuring Rabi
oscillations or dispersive interactions, the present scheme can be performed
resonantly at infinitesimal short motion-probe interaction times, remaining
thus insensitive to decoherence processes.


title:
Generalization of entanglement to convex operational theories:
  Entanglement relative to a subspace of observables

abstract:
  We define what it means for a state in a convex cone of states on a space of
observables to be generalized-entangled relative to a subspace of the
observables, in a general ordered linear spaces framework for operational
theories. This extends the notion of ordinary entanglement in quantum
information theory to a much more general framework. Some important special
cases are described, in which the distinguished observables are subspaces of
the observables of a quantum system, leading to results like the identification
of generalized unentangled states with Lie-group-theoretic coherent states when
the special observables form an irreducibly represented Lie algebra. Some open
problems, including that of generalizing the semigroup of local operations with
classical communication to the convex cones setting, are discussed.


title: A New Ontological View of the Quantum Measurement Problem

abstract:
  A new ontological view of the quantum measurement processes is given, which
has bearings on many broader issues in the foundations of quantum mechanics as
well. In this scenario a quantum measurement is a non-equilibrium phase
transition in a ``resonant cavity'' formed by the entire physical universe
including all of its material and energy content. A quantum measurement
involves the energy and matter exchange among not only the system being
measured and the measuring apparatus but also the global environment of the
universe resonant cavity, which together constrain the nature of the phase
transition. Strict realism, including strict energy and angular momentum
conservation, is recovered in this view of the quantum measurement process
beyond the limit set by the uncertainty relations, which are themselves derived
from the exact commutation relations for quantum conjugate variables. Both the
amplitude and the phase of the quantum mechanical wavefunction acquire
substantial meanings in the new ontology, and the probabilistic element is
removed from the foundations of quantum mechanics, its apparent presence in the
quantum measurement being solely a result of the sensitive dependence on
initial/boundary conditions of the phase transitions of a many
degree-of-freedom system which is effectively the whole universe. Vacuum
fluctuations are viewed as the ``left over'' fluctuations after forming the
whole numbers of nonequilibrium resonant modes in the universe cavity. This new
view on the quantum processes helps to clarify many puzzles in the foundations
of quantum mechanics.


title:
Trapping and coherent manipulation of a Rydberg atom on a
  microfabricated device: a proposal

abstract:
  We propose to apply atom-chip techniques to the trapping of a single atom in
a circular Rydberg state. The small size of microfabricated structures will
allow for trap geometries with microwave cut-off frequencies high enough to
inhibit the spontaneous emission of the Rydberg atom, paving the way to
complete control of both external and internal degrees of freedom over very
long times. Trapping is achieved using carefully designed electric fields,
created by a simple pattern of electrodes. We show that it is possible to
excite, and then trap, one and only one Rydberg atom from a cloud of ground
state atoms confined on a magnetic atom chip, itself integrated with the
Rydberg trap. Distinct internal states of the atom are simultaneously trapped,
providing us with a two-level system extremely attractive for atom-surface and
atom-atom interaction studies. We describe a method for reducing by three
orders of magnitude dephasing due to Stark shifts, induced by the trapping
field, of the internal transition frequency. This allows for, in combination
with spin-echo techniques, maintenance of an internal coherence over times in
the second range. This method operates via a controlled light shift rendering
the two internal states' Stark shifts almost identical. We thoroughly identify
and account for sources of imperfection in order to verify at each step the
realism of our proposal.


title:
Photon-number fluctuation and correlation of bound soliton pairs in
  mode-locked fiber lasers

abstract:
  Quantum photon-number fluctuation and correlation of bound soliton pairs in
mode-locked fiber lasers are studied based on the complex Ginzburg-Landau
equation model. We find that, depending on their phase difference, the total
photon-number noise of the bound soliton pair can be larger or smaller than
that of a single soliton and the two solitons in the soliton pairs are with
positive or negative photon-number correlation, correspondingly. It is
predicted for the first time that out-of-phase soliton pairs can exhibit less
noises due to negative correlation.


title: Matter wave pulses characteristics

abstract:
  We study the properties of quantum single-particle wave pulses created by
sharp-edged or apodized shutters with single or periodic openings. In
particular, we examine the visibility of diffraction fringes depending on
evolution time and temperature; the purity of the state depending on the
opening-time window; the accuracy of a simplified description which uses
``source'' boundary conditions instead of solving an initial value problem; and
the effects of apodization on the energy width.


title: Canonical quantization of nonlinear many body systems

abstract:
  We study the quantization of a classical system of interacting particles
obeying a recently proposed kinetic interaction principle (KIP) [G. Kaniadakis,
Physica A {\bf 296}, 405 (2001)]. The KIP fixes the expression of the
Fokker-Planck equation describing the kinetic evolution of the system and
imposes the form of its entropy. In the framework of canonical quantization, we
introduce a class of nonlinear Schr\"odinger equations (NSEs) with complex
nonlinearities, describing, in the mean field approximation, a system of
collectively interacting particles whose underlying kinetics is governed by the
KIP. We derive the Ehrenfest relations and discuss the main constants of motion
arising in this model. By means of a nonlinear gauge transformation of third
kind it is shown that in the case of constant diffusion and linear drift the
class of NSEs obeying the KIP is gauge-equivalent to another class of NSEs
containing purely real nonlinearities depending only on the field
$\rho=|\psi|^2$.


title:
Modified Grover's search algorithm for the cases where the number of
  solutions is known

abstract:
  Grover's search algorithm searches a database of $N$ unsorted items in
$O(\sqrt{N/M})$ steps where $M$ represents the number of solutions to the
search problem. This paper proposes a scheme for searching a database of $N$
unsorted items in $O(logN)$ steps, provided the value of $M$ is known. It is
also shown that when $M$ is unknown but if we can estimate an upper bound of
possible values of $M$, then an improvement in the time complexity of
conventional Grover's algorithm is possible. In that case, the present scheme
reduces the time complexity to $O(MlogN)$.


title:
Measurement of the electric polarizability of lithium by atom
  interferometry

abstract:
  We have built an atom interferometer and, by applying an electric field on
one of the two interfering beams, we have measured the static electric
polarizability of lithium with a 0.66 % uncertainty. Our experiment is similar
to an experiment done on sodium in 1995 by D. Pritchard and co-workers, with
several improvements: the electric field can be calculated analytically and our
phase measurements are very accurate. This experiment illustrates the extreme
sensitivity of atom interferometry: when the atom enters the electric field,
its velocity increases and the fractional change, equal to 4 x 10^(-9) for our
largest field, is measured with a 10^(-3) accuracy.


title: Limits and restrictions of private quantum channel

abstract:
  We study private quantum channels on a single qubit, which encrypt given set
of plaintext states $P$. Specifically, we determine all achievable states
$\rho^{(0)}$ (average output of encryption) and for each particular set $P$ we
determine the entropy of the key necessary and sufficient to encrypt this set.
It turns out that single bit of key is sufficient when the set $P$ is two
dimensional. However, the necessary and sufficient entropy of the key in case
of three dimensional $P$ varies continuously between 1 and 2 bits depending on
the state $\rho^{(0)}$. Finally, we derive private quantum channels achieving
these bounds. We show that the impossibility of universal NOT operation on
qubit can be derived from the fact that one bit of key is not sufficient to
encrypt qubit.


title: Squeezed Fock state by inconclusive photon subtraction

abstract:
  We analyze in details the properties of the conditional state recently
obtained by J. Wenger, et al. [Phys. Rev. Lett. {\bf 92}, 153601 (2004)] by
means of inconclusive photon subtraction (IPS) on a squeezed vacuum state
$S(r)\ket{0}$. The IPS process can be characterized by two parameters: the IPS
transmissivity $\tau$ and the photodetection quantum efficiency $\eta$. We
found that the conditional state approaches the squeezed Fock state
$S(r)\ket{1}$ when $\tau,\eta \to 1$, i.e., in the limit of single-photon
subtraction. For non-unit IPS transmissivity and efficiency, the conditioned
state remains close to the target state, {\em i.e.} shows a high fidelity for a
wide range of experimental parameters. The nonclassicality of the conditional
state is also investigated and a nonclassicality threshold on the IPS
parameters is derived.


title: Hyperspheres and control of spin chains

abstract:
  Here is considered application of Spin(m) groups in theory of quantum control
of chain with spin-1/2 systems. It may be also compared with m-dimensional
analogues of Bloch sphere, but has nontrivial distinctions for chain with more
than one spin system.


title: Optical generation of matter qubit graph states

abstract:
  We present a scheme for rapidly entangling matter qubits in order to create
graph states for one-way quantum computing. The qubits can be simple 3-level
systems in separate cavities. Coupling involves only local fields and a static
(unswitched) linear optics network. Fusion of graph state sections occurs with,
in principle, zero probability of damaging the nascent graph state. We avoid
the finite thresholds of other schemes by operating on two entangled pairs, so
that each generates exactly one photon. We do not require the relatively slow
single qubit local flips to be applied during the growth phase: growth of the
graph state can then become a purely optical process. The scheme naturally
generates graph states with vertices of high degree and so is easily able to
construct minimal graph states, with consequent resource savings. The most
efficient approach will be to create new graph state edges even as qubits
elsewhere are measured, in a `just in time' approach. An error analysis
indicates that the scheme is relatively robust against imperfections in the
apparatus.


title:
Examples of bosonic de Finetti states over finite dimensional Hilbert
  spaces

abstract:
  According to the Quantum de Finetti Theorem, locally normal infinite particle
states with Bose-Einstein symmetry can be represented as mixtures of infinite
tensor powers of vector states. This note presents examples of
infinite-particle states with Bose-Einstein symmetry that arise as limits of
Gibbs ensembles on finite dimensional spaces, and displays their de Finetti
representations. We consider Gibbs ensembles for systems of bosons in a finite
dimensional setting and discover limits as the number of particles tends to
infinity, provided the temperature is scaled in proportion to particle number.


title:
Correspondence principle for the diffusive dynamics of a quartic
  oscillator: deterministic aspects and the role of temperature

abstract:
  The correspondence principle is investigated in the framework of
deterministic predictions for individual systems. Exact analytical results are
obtained for the quantum and the Liouvillian dynamics of a nonlinear oscillator
coupled to a phase-damping reservoir at a finite temperature. In this context,
the time of critical wave function spreading - the Ehrenfest time - emerges as
the characteristic time scale within which the concept of deterministic
behavior is admissible in physics. A scenario of "quasi-determinism" may be
then defined within which the motion is experimentally indistinguishable from
the truly deterministic motion of Newtonian mechanics. Beyond this time scale,
predictions for individual systems can be given only statistically and, in this
case, it is shown that diffusive decoherence is indeed a necessary ingredient
to establish the quantum-classical correspondence. Moreover, the
high-temperature regime is shown to be an additional condition for the
quantum-classical transition and, accordingly, a lower bound for the reservoir
temperature is derived for our model.


title: Entanglement in an expanding spacetime

abstract:
  We show that a dynamical spacetime generates entanglement between modes of a
quantum field. Conversely, the entanglement encodes information concerning the
underlying spacetime structure, which hints at the prospect of applications of
this observation to cosmology. Here we illustrate this point by way of an
analytically exactly soluble example, that of a scalar quantum field on a
two-dimensional asymptotically flat Robertson-Walker expanding spacetime. We
explicitly calculate the entanglement in the far future, for a quantum field
residing in the vacuum state in the distant past. In this toy universe, it is
possible to fully reconstruct the parameters of the cosmic history from the
entanglement entropy.


title: Single-photon quantum error rejection and correction with linear optics

abstract:
  We present single-photon schemes for quantum error rejection and correction
with linear optics. In stark contrast to other known proposals, our schemes do
not require multi-photon entangled states, are not probabilistic, and their
application is not restricted to single bit-flip errors.


title: Generalizations of Quantum Mechanics

abstract:
  We review realistic models that reproduce quantum theory in some limit and
yield potentially new physics outside that limit. In particular, we consider
deterministic hidden-variables theories (such as the pilot-wave model) and
their extension to 'quantum nonequilibrium', and we consider the continuous
spontaneous localization model of wave function collapse. Other models are
briefly discussed.


title: Efficient optical quantum information processing

abstract:
  Quantum information offers the promise of being able to perform certain
communication and computation tasks that cannot be done with conventional
information technology (IT). Optical Quantum Information Processing (QIP) holds
particular appeal, since it offers the prospect of communicating and computing
with the same type of qubit. Linear optical techniques have been shown to be
scalable, but the corresponding quantum computing circuits need many auxiliary
resources. Here we present an alternative approach to optical QIP, based on the
use of weak cross-Kerr nonlinearities and homodyne measurements. We show how
this approach provides the fundamental building blocks for highly efficient
non-absorbing single photon number resolving detectors, two qubit parity
detectors, Bell state measurements and finally near deterministic control-not
(CNOT) gates. These are essential QIP devices


title: Quantum Optics with Surface Plasmons

abstract:
  We describe a technique that enables strong, coherent coupling between
individual optical emitters and guided plasmon excitations in conducting
nano-structures at optical frequencies. We show that under realistic
conditions, optical emission can be almost entirely directed into the plasmon
modes. As an example, we describe an application of this technique involving
efficient generation of single photons on demand, in which the plasmon is
efficiently out-coupled to a dielectric waveguide.


title: Microscopic models of quantum jump super-operators

abstract:
  We discuss the quantum jump operation in an open system, and show that jump
super-operators related to a system under measurement can be derived from the
interaction of that system with a quantum measurement apparatus. We give two
examples for the interaction of a monochromatic electromagnetic field in a
cavity (the system) with 2-level atoms and with a harmonic oscillator
(representing two different kinds of detectors). We show that derived quantum
jump super-operators have `nonlinear' form which depends on assumptions made
about the interaction between the system and the detector. A continuous
transition to the standard Srinivas--Davies form of the quantum jump
super-operatoris shown.


title: Weak cloning of an unknown quantum state

abstract:
  The impossibility to clone an unknown quantum state is a powerful principle
to understand the nature of quantum mechanics, especially within the context of
quantum computing and quantum information. This principle has been generalized
to quantitative statements as to what extent imperfect cloning is possible. We
delineate an aspect of the border between the possible and the impossible
concerning quantum cloning, by putting forward an entanglement-assisted scheme
for simulating perfect cloning in the context of weak measurements. This
phenomenon we call weak cloning of an unknown quantum state.


title:
Casimir Effect as a Test for Thermal Corrections and Hypothetical
  Long-Range Interactions

abstract:
  We have performed a precise experimental determination of the Casimir
pressure between two gold-coated parallel plates by means of a micromachined
oscillator. In contrast to all previous experiments on the Casimir effect,
where a small relative error (varying from 1% to 15%) was achieved only at the
shortest separation, our smallest experimental error ($\sim 0.5$%) is achieved
over a wide separation range from 170 nm to 300 nm at 95% confidence. We have
formulated a rigorous metrological procedure for the comparison of experiment
and theory without resorting to the previously used root-mean-square deviation,
which has been criticized in the literature. This enables us to discriminate
among different competing theories of the thermal Casimir force, and to resolve
a thermodynamic puzzle arising from the application of Lifshitz theory to real
metals. Our results lead to a more rigorous approach for obtaining constraints
on hypothetical long-range interactions predicted by extra-dimensional physics
and other extensions of the Standard Model. In particular, the constraints on
non-Newtonian gravity are strengthened by up to a factor of 20 in a wide
interaction range at 95% confidence.


title:
Comment on quant-ph/0506105: The modified Grover algorithm cannot
  speedup unsorted database search

abstract:
  In a recent paper (quant-ph/0506105), A S Gupta, M. Gupta and A. Pathak
proposed a modified Grover algorithm that would exponentially accelerate the
unsorted database search problem if the number of marked items is known. If
this were true, it would represent a major fundamental breakthrough in computer
science, mathematics, quantum information and other related branches of
sciences.
  However the algorithm is not valid. We will explain it in this brief comment.


title: A Quantum-Classical Brackets from p-Mechanics

abstract:
  We provide an answer to the long standing problem of mixing quantum and
classical dynamics within a single formalism. The construction is based on
p-mechanical derivation (quant-ph/0212101, quant-ph/0304023) of quantum and
classical dynamics from the representation theory of the Heisenberg group. To
achieve a quantum-classical mixing we take the product of two copies of the
Heisenberg group which represent two different Planck's constants. In
comparison with earlier guesses our answer contains an extra term of analytical
nature, which was not obtained before in purely algebraic setup.
  Keywords: Moyal brackets, Poisson brackets, commutator, Heisenberg group,
orbit method, representation theory, Planck's constant, quantum-classical
mixing


title:
Decoherence of tripartite states - a trapped ion coupled to an optical
  cavity

abstract:
  We investigate the non-dissipative decoherence of three qubit system obtained
by manipulating the state of a trapped two-level ion coupled to an optical
cavity. Modelling the environment as a set of noninteracting harmonic
oscillators, analytical expressions for the state operator of tripartite
composite system, the probability of generating maximally entangled GHZ state,
and the population inversion have been obtained. The pointer observable is the
energy of the isolated quantum system. Coupling to environment results in
exponential decay of off diagonal matrix elements of the state operator with
time as well as a phase decoherence of the component states.
  Numerical calculations to examine the time evolution of GHZ state generation
probability and population inversion for different system environment coupling
strengths are performed. Using negativity as an entanglement measure and linear
entropy as a measure of mixedness, the entanglement dynamics of the tripartite
system in the presence of decoherence is analysed.


title:
Gaussian measures of entanglement versus negativities: the ordering of
  two-mode Gaussian states

abstract:
  In this work we focus on entanglement of two--mode Gaussian states of
continuous variable systems. We first review the formalism of Gaussian measures
of entanglement, adopting the framework developed in [M. M. Wolf {\em et al.},
Phys. Rev. A {\bf 69}, 052320 (2004)], where the Gaussian entanglement of
formation was defined. We compute Gaussian measures explicitely for two
important families of nonsymmetric two--mode Gaussian states, namely the states
of extremal (maximal and minimal) negativities at fixed global and local
purities, introduced in [G. Adesso {\em et al.}, Phys. Rev. Lett. {\bf 92},
087901 (2004)]. This allows us to compare the {\em orderings} induced on the
set of entangled two--mode Gaussian states by the negativities and by the
Gaussian entanglement measures. We find that in a certain range of global and
local purities (characterizing the covariance matrix of the corresponding
extremal states), states of minimum negativity can have more Gaussian
entanglement than states of maximum negativity. Thus Gaussian measures and
negativities are definitely inequivalent on nonsymmetric two--mode Gaussian
states (even when restricted to extremal states), while they are completely
equivalent on symmetric states, where moreover the Gaussian entanglement of
formation coincides with the true one. However, the inequivalence between these
two families of continuous-variable entanglement measures is somehow limited.
In fact we show rigorously that, at fixed negativities, the Gaussian
entanglement measures are bounded from below, and we provide strong evidence
that they are also bounded from above.

categories: quant-ph cond-mat.other math-ph math.MP physics.optics


title:
Numerical simulation of Bell inequality's violation using optical
  transverse modes in multimode waveguides

abstract:
  We numerically demonstrate that "mode-entangled states" based on the
transverse modes of classical optical fields in multimode waveguides violate
Bell's inequality. Numerically simulating the correlation measurement scheme of
Bell's inequality, we obtain the normalized correlation functions of the
intensity fluctuations for the two entangled classical fields. By using the
correlation functions, the maximum violations of Bell's inequality are
obtained. This implies that the two classical fields in the mode-entangled
states, although spatially separated, present a nonlocal correlation.


title: Towards Large-Scale Quantum Computation

abstract:
  This thesis deals with a series of quantum computer implementation issues
from the Kane 31P in 28Si architecture to Shor's integer factoring algorithm
and beyond. The discussion begins with simulations of the adiabatic Kane CNOT
and readout gates, followed by linear nearest neighbor implementations of
5-qubit quantum error correction with and without fast measurement. A linear
nearest neighbor circuit implementing Shor's algorithm is presented, then
modified to remove the need for exponentially small rotation gates. Finally, a
method of constructing optimal approximations of arbitrary single-qubit
fault-tolerant gates is described and applied to the specific case of the
remaining rotation gates required by Shor's algorithm.


title: The entanglement criterion of multiqubits

abstract:
  We present an entanglement criterion for multiqubits by using the quantum
correlation tensors which rely on the expectation values of the Pauli operators
for a multiqubit state. Our criterion explains not only the total entanglement
of the system but also the partial entanglement in subsystems. It shows that we
have to consider the subsystem entanglements in order to obtain the full
description for multiqubit entanglements. Furthermore, we offer an extension of
the entanglement to multiqudits.


title: Mutually unbiased phase states, phase uncertainties, and Gauss sums

abstract:
  Mutually unbiased bases (MUBs), which are such that the inner product between
two vectors in different orthogonal bases is a constant equal to 1/sqrt{d),
with d the dimension of the finite Hilbert space, are becoming more and more
studied for applications such as quantum tomography and cryptography, and in
relation to entangled states and to the Heisenberg-Weil group of quantum
optics. Complete sets of MUBs of cardinality d+1 have been derived for prime
power dimensions d=p^m using the tools of abstract algebra. Presumably, for non
prime dimensions the cardinality is much less. Here we reinterpret MUBs as
quantum phase states, i.e. as eigenvectors of Hermitean phase operators
generalizing those introduced by Pegg & Barnett in 1989. We relate MUB states
to additive characters of Galois fields (in odd characteristic p) and to Galois
rings (in characteristic 2). Quantum Fourier transforms of the components in
vectors of the bases define a more general class of MUBs with multiplicative
characters and additive ones altogether. We investigate the complementary
properties of the above phase operator with respect to the number operator. We
also study the phase probability distribution and variance for general pure
quantum electromagnetic states and find them to be related to the Gauss sums,
which are sums over all elements of the field (or of the ring) of the product
of multiplicative and additive characters. Finally, we relate the concepts of
mutual unbiasedness and maximal entanglement. This allows to use well studied
algebraic concepts as efficient tools in the study of entanglement and its
information aspects


title: A General Relativistic Generalization of Bell Inequality

abstract:
  In this work a general relativistic generalization of Bell inequality is
suggested. Namely,it is proved that practically in any general relativistic
metric there is a generalization of Bell inequality.It can be satisfied within
theories of local (subluminal) hidden variables, but it cannot be satisfied in
the general case within standard quantum mechanical formalism or within
theories of nonlocal (superluminal) hidden variables. It is shown too that
within theories of nonlocal hidden variables but not in the standard quantum
mechanical formalism a paradox appears in the situation when one of the
correlated subsystems arrives at a Schwarzschild black hole. Namely, there is
no way that black hole horizon obstructs superluminal influences between spin
of the subsystem without horizon and spin of the subsystem within horizon,or
simply speaking,there is none black hole horizon nor "no hair" theorem for
subsystems with correlated spins. It implies that standard quantum mechanical
formalism yields unique consistent and complete description of the quantum
mechanical phenomenons.


title:
Reference potential approach to the quantum-mechanical inverse problem:
  II. Solution of Krein equation

abstract:
  A reference potential approach to the one-dimensional quantum-mechanical
inverse problem is developed. All spectral characteristics of the system,
including its discrete energy spectrum, the full energy dependence of the phase
shift, and the Jost function, are expected to be known. The technically most
complicated task in ascertaining the potential, solution of a relevant integral
equation, has been decomposed into two relatively independent problems. First,
one uses Krein method to calculate an auxiliary potential with exactly the same
spectral density as the initial reference potential, but with no bound states.
Thereafter, using Gelfand-Levitan method, it is possible to introduce, one by
one, all bound states, along with calculating another auxiliary potential of
the same spectral density at each step. For the system under study (diatomic
xenon molecule), the kernel of the Krein integral equation can be accurately
ascertained with the help of solely analytic means. At small distances the
calculated auxiliary potential with no bound states practically coincides with
the initial reference potential, which is in full agreement with general
theoretical considerations. Several possibilities of solving the Krein equation
are proposed and the prospects of further research discussed.


title: Black-body radiation in extra dimensions

abstract:
  The general form of the Stefan-Boltzmann law for the energy density of
black-body radiation is generalized to a spacetime with extra dimensions using
standard kinetic and thermodynamic arguments. From statistical mechanics one
obtains an exact formula. In a field-theoretic derivation, the Maxwell field
must be quantized. The notion of electric and magnetic fields is different in
spacetimes with more than four dimensions. While the energy-momentum tensor for
the Maxwell field is traceless in four dimensions, it is not so when there are
extra dimensions. But it is shown that its thermal average is traceless and in
agreement with the thermodynamic results.


title: Quantum information-flow, concretely, and axiomatically

abstract:
  These lecture notes survey some joint work with Samson Abramsky. Somewhat
informally I will discuss the main results in a pedestrian not too technical
way. These include: (1) `The logic of entanglement', that is, the
identification and abstract axiomatization of the `quantum information-flow'
which enables protocols such as quantum teleportation. To this means we define
strongly compact closed categories which abstractly capture the behavioral
properties of quantum entanglement. (2) `Postulates for an abstract quantum
formalism' in which classical information-flow (e.g. token exchange) is part of
the formalism. As an example, we provide a purely formal description of quantum
teleportation and prove correctness in abstract generality. In this formalism
types reflect kinds, contra the essentially typeless von Neumann formalism.
Hence even concretely this formalism manifestly improves on the usual one. (3)
`A high-level approach to quantum informatics'.


title: The cryptographic power of misaligned reference frames

abstract:
  Suppose that Alice and Bob define their coordinate axes differently, and the
change of reference frame between them is given by a probability distribution
mu over SO(3). We show that this uncertainty of reference frame is of no use
for bit commitment when mu is uniformly distributed over a (sub)group of SO(3),
but other choices of mu can give rise to a partially or even asymptotically
secure bit commitment.


title:
De-linearizing Linearity: Projective Quantum Axiomatics from Strong
  Compact Closure

abstract:
  Elaborating on our joint work with Abramsky in quant-ph/0402130 we further
unravel the linear structure of Hilbert spaces into several constituents. Some
prove to be very crucial for particular features of quantum theory while others
obstruct the passage to a formalism which is not saturated with physically
insignificant global phases.
  First we show that the bulk of the required linear structure is purely
multiplicative, and arises from the strongly compact closed tensor which,
besides providing a variety of notions such as scalars, trace, unitarity,
self-adjointness and bipartite projectors, also provides Hilbert-Schmidt norm,
Hilbert-Schmidt inner-product, and in particular, the preparation-state
agreement axiom which enables the passage from a formalism of the vector space
kind to a rather projective one, as it was intended in the (in)famous Birkhoff
& von Neumann paper.
  Next we consider additive types which distribute over the tensor, from which
measurements can be build, and the correctness proofs of the protocols
discussed in quant-ph/0402130 carry over to the resulting weaker setting. A
full probabilistic calculus is obtained when the trace is moreover linear and
satisfies the \em diagonal axiom, which brings us to a second main result,
characterization of the necessary and sufficient additive structure of a both
qualitatively and quantitatively effective categorical quantum formalism
without redundant global phases. Along the way we show that if in a category a
(additive) monoidal tensor distributes over a strongly compact closed tensor,
then this category is always enriched in commutative monoids.


title: Equivalence of Quantum States under Local Unitary Transformations

abstract:
  In terms of the analysis of fixed point subgroup and tensor decomposability
of certain matrices, we study the equivalence of of quantum bipartite mixed
states under local unitary transformations. For non-degenerate case an
operational criterion for the equivalence of two such mixed bipartite states
under local unitary transformations is presented.


title: Concurrence of arbitrary dimensional bipartite quantum states

abstract:
  We derive an analytical lower bound for the concurrence of a bipartite
quantum state in arbitrary dimension. A functional relation is established
relating concurrence, the Peres-Horodecki criterion and the realignment
criterion. We demonstrate that our bound is exact for some mixed quantum
states. The significance of our method is illustrated by giving a quantitative
evaluation of entanglement for many bound entangled states, some of which fail
to be identified by the usual concurrence estimation method.


title:
A fast quantum search algorithm by controlled measurement and qubit
  comparison implying the complexity class inclusion NP in BQP

abstract:
  This paper has been withdrawn by the author due to an apparent
misunderstanding of quantum feedback control.


title: Irreversibility for all bound entangled states

abstract:
  We derive a new inequality for entanglement for a mixed four-partite state.
Employing this inequality, we present a one-shot lower bound for entanglement
cost and prove that entanglement cost is strictly larger than zero for any
entangled state. We demonstrate that irreversibility occurs in the process of
formation for all non-distillable entangled states. In this way we solve a long
standing problem, of how "real" is entanglement of bound entangled states.
Using the new inequality we also prove impossibility of local-cloning of a
known entangled state.


title: Generation of Bright Two-Color Continuous Variable Entanglement

abstract:
  We present the first measurement of squeezed-state entanglement between the
twin beams produced in an Optical Parametric Oscillator (OPO) operating above
threshold. Besides the usual squeezing in the intensity difference between the
twin beams, we have measured squeezing in the sum of phase quadratures. Our
scheme enables us to measure such phase anti-correlations between fields of
different frequencies. In the present measurements, wavelengths differ by ~1
nm. Entanglement is demonstrated according to the Duan et al. criterion [Phys.
Rev. Lett. 84, 2722 (2000)] $\Delta^2\hat{p}_- +\Delta^2\hat{q}_+=1.47(2)<2$.
This experiment opens the way for new potential applications such as the
transfer of quantum information between different parts of the electromagnetic
spectrum.


title:
Phase Space Tomography of Classical and Nonclassical Vibrational States
  of Atoms in an Optical Lattice

abstract:
  Atoms trapped in optical lattice have long been a system of interest in the
AMO community, and in recent years much study has been devoted to both short-
and long-range coherence in this system, as well as to its possible
applications to quantum information processing. Here we demonstrate for the
first time complete determination of the quantum phase space distributions for
an ensemble of $^{85}Rb$ atoms in such a lattice, including a negative Wigner
function for atoms in an inverted state.


title: Quantum mechanics and the time travel paradox

abstract:
  The closed causal chains arising from backward time travel do not lead to
paradoxes if they are self consistent. This raises the question as to how
physics ensures that only self-consistent loops are possible. We show that, for
one particular case at least, the condition of self consistency is ensured by
the interference of quantum mechanical amplitudes associated with the loop. If
this can be applied to all loops then we have a mechanism by which inconsistent
loops eliminate themselves.


title:
Dephasing representation of quantum fidelity for general pure and mixed
  states

abstract:
  General semiclassical expression for quantum fidelity (Loschmidt echo) of
arbitrary pure and mixed states is derived. It expresses fidelity as an
interference sum of dephasing trajectories weighed by the Wigner function of
the initial state, and does not require that the initial state be localized in
position or momentum. This general dephasing representation is special in that,
counterintuitively, all of fidelity decay is due to dephasing and none due to
the decay of classical overlaps. Surprising accuracy of the approximation is
justified by invoking the shadowing theorem: twice--both for physical
perturbations and for numerical errors. It is shown how the general expression
reduces to the special forms for position and momentum states and for wave
packets localized in position or momentum. The superiority of the general over
the specialized forms is explained and supported by numerical tests for wave
packets, non-local pure states, and for simple and random mixed states. The
tests are done in non-universal regimes in mixed phase space where detailed
features of fidelity are important. Although semiclassically motivated, present
approach is valid for abstract systems with a finite Hilbert basis provided
that the discrete Wigner transform is used. This makes the method applicable,
via a phase space approach, e. g., to problems of quantum computation.


title: Long-distance frequency dissemination with a resolution of 10-17

abstract:
  We use a new technique to disseminate microwave reference signals along
ordinary optical fiber. The fractional frequency resolution of a link of 86 km
in length is 10-17 for a one day integration time, a resolution higher than the
stability of the best microwave or optical clocks. We use the link to compare
the microwave reference and a CO2/OsO4 frequency standard that stabilizes a
femtosecond laser frequency comb. This demonstrates a resolution of 3.10-14 at
1 s. An upper value of the instability introduced by the femtosecond
laser-based synthesizer is estimated as 1.10-14 at 1 s.


title: Quantum storage and information transfer with superconducting qubits

abstract:
  We design theoretically a new device to realize the general quantum storage
based on dcSQUID charge qubits. The distinct advantages of our scheme are
analyzed in comparison with existing storage scenarios. More arrestingly, the
controllable XY-model spin interaction has been realized for the first time in
superconducting qubits, which may have more potential applications besides
those in quantum information processing. The experimental feasibility is also
elaborated.


title:
Reply to Comment on ''Quantum secret sharing based on a reusable
  Greenberger-Horne -Zeilinger states as secure carriers''

abstract:
  We show that the criticism of a recent comment \cite{ch2} on the insecurity
of a quantum secret sharing protocol proposed in \cite{v2} is based on a
misconception about the meaning of security and hence is invalid. The same
misconception also appears in another comment of the authors \cite{ch1} on the
security of an entangled-based quantum key distribution protocol
\cite{zhang,v1}.


title:
Frequency modulated laser beam with highly efficient intensity
  stabilisation

abstract:
  We analyse the limitation of the amplitude modulation rejection due to the
spatial modulation of the output beam of an acousto-optic modulator used in an
active laser beam stabilisation system when a frequency modulation of a few
megahertz is applied to this modulator. We show how to overcome this problem,
using a single mode optical fibre at the output of the modulator. A residual
amplitude modulation of 10-5 is achieved.


title: Slow light, superadiabaticity and shape-preserving pulses

abstract:
  We analyze the nonlinear dynamics of atomic dark states in Lambda
configuration that interact with light at exact resonance. We found a
generalization of shape-preserving pulses [R. Grobe, F. T. Hioe, and J. H.
Eberly, Phys. Rev. Lett. 73, 3183 (1994)] and show that the condition for
adiabaticity of the atomic dynamics is never violated, as long as spontaneous
emission is negligible.


title: Entanglement generation in harmonic chains: tagging by squeezing

abstract:
  We address the problem of spring-like coupling between bosons in an open
chain configuration where the counter-rotating terms are explicitly included.
We show that fruitful insight can be gained by decomposing the time-evolution
operator of this problem into a pattern of linear-optics elements. This allows
us to provide a clear picture of the effects of the counter-rotating terms in
the important problem of long-haul entanglement distribution. The analytic
control over the variance matrix of the state of the bosonic register allows us
to track the dynamics of the entanglement. This helps in designing a global
addressing scheme, complemented by a proper initialization of the register,
which quantitatively improves the entanglement between the extremal oscillators
in the chain, thus providing a strategy for feasible long distance entanglement
distribution.


title: Distinguishing unentangled states with an unentangled measurement

abstract:
  In a 1991 paper, Asher Peres and the author theoretically analyzed a set of
unentangled bipartite quantum states that could apparently be distinguished
better by a global measurement than by any sequence of local measurements on
the individual subsystems. The present paper returns to the same example, and
shows that the best result so far achieved can alternatively be attained by a
measurement that, while still global, is "unentangled" in the sense that the
operator associated with each measurement outcome is a tensor product.


title: Quantum Computing, Metrology, and Imaging

abstract:
  Information science is entering into a new era in which certain subtleties of
quantum mechanics enables large enhancements in computational efficiency and
communication security. Naturally, precise control of quantum systems required
for the implementation of quantum information processing protocols implies
potential breakthoughs in other sciences and technologies. We discuss recent
developments in quantum control in optical systems and their applications in
metrology and imaging.


title: Entanglement of Formation of Rotationally Symmetric States

abstract:
  Computing the entanglement of formation of a bipartite state is generally
difficult, but special symmetries of a state can simplify the problem. For
instance, this allows one to determine the entanglement of formation of Werner
states and isotropic states. We consider a slightly more general class of
states, rotationally symmetric states, also known as SU(2)-invariant states.
These states are invariant under global rotations of both subsystems, and one
can examine entanglement in cases where the subsystems have different
dimensions. We derive an analytic expression for the entanglement of formation
of rotationally symmetric states of a spin-$j$ particle and a spin-${1\over2}$
particle. We also give expressions for the I-concurrence, I-tangle, and
convex-roof-extended negativity.


title:
Range criterion and classification of true entanglement in
  $2\times{M}\times{N}$ system

abstract:
  We propose a range criterion which is a sufficient and necessary condition
satisfied by two pure states transformable with each other under reversible
stochastic local operations assisted with classical communication. We also
provide a systematic method for seeking all kinds of true entangled states in
the $2\times{M}\times{N}$ system, and can effectively distinguish them by means
of the range criterion. The efficiency of the criterion and the method is
exhibited by the classification of true entanglement in some types of the
tripartite systems.


title:
On the transfer matrix method and WKB approximation for Schr\"{o}dinger
  equation with position-dependent effective mass

abstract:
  We have obtained a set of coupled differential equations from the continuous
limit of the transfer matrix method. Decoupling such a set of equations yields
an extension to the Wentzel-Kramers-Brillouin (WKB) approximation for the
Schr\"{o}dinger equation with the position-dependent effective mass (PDEM). In
the classically allowed region, the decoupling is to ignore the reflection
resulting from the variations of both the potential and effective mass. By
considering an infinite-well example with the PDEM, it is shown that the
extended WKB approximation can provide not only an estimation to eigenenergies,
but also an analytic form to approximate wavefunctions.


title:
No-Flipping as a consequence of No-Signalling and Non-increase of
  Entanglement under LOCC

abstract:
  Non existence of Universal NOT gate for arbitrary quantum mechanical states
is a fundamental constraint on the allowed operations performed on physical
systems. The largest set of states that can be flipped by using a single NOT
gate is the set of states lying on a great circle of the Bloch-sphere. In this
paper, we show the impossibility of universal exact-flipping operation, first
by using the fact that no faster than light communication is possible and then
by using the principle of "non-increase of entanglement under LOCC".
Interestingly, exact flipping of the states of any great circle does not
violate these two principles, as expected.


title: Quantum Robot: Structure, Algorithms and Applications

abstract:
  A kind of brand-new robot, quantum robot, is proposed through fusing quantum
theory with robot technology. Quantum robot is essentially a complex quantum
system and it is generally composed of three fundamental parts: MQCU (multi
quantum computing units), quantum controller/actuator, and information
acquisition units. Corresponding to the system structure, several learning
control algorithms including quantum searching algorithm and quantum
reinforcement learning are presented for quantum robot. The theoretic results
show that quantum robot can reduce the complexity of O(N^2) in traditional
robot to O(N^(3/2)) using quantum searching algorithm, and the simulation
results demonstrate that quantum robot is also superior to traditional robot in
efficient learning by novel quantum reinforcement learning algorithm.
Considering the advantages of quantum robot, its some potential important
applications are also analyzed and prospected.


title: Quantum State Transfer Characterized by Mode Entanglement

abstract:
  We study the quantum state transfer (QST) of a class of tight-bonding Bloch
electron systems with mirror symmetry by considering the mode entanglement.
Some rigorous results are obtained to reveal the intrinsic relationship between
the fidelity of QST and the mirror mode concurrence (MMC), which is defined to
measure the mode entanglement with a certain spatial symmetry and is just the
overlap of a proper wave function with its mirror image. A complementarity is
discovered as the maximum fidelity is accompanied by a minimum of MMC. And at
the instant, which is just half of the characteristic time required to
accomplish a perfect QST, the MMC can reach its maximum value one. A large
class of perfect QST models with a certain spectrum structure are discovered to
support our analytical results.


title: Quantum mechanics helps in learning for more intelligent robot

abstract:
  A learning algorithm based on state superposition principle is presented. The
physical implementation analysis and simulated experiment results show that
quantum mechanics can give helps in learning for more intelligent robot.


title:
A Continuous Non-demolition Measurement of the Cs Clock Transition
  Pseudo-spin

abstract:
  We demonstrate a weak continuous measurement of the pseudo-spin associated
with the clock transition in a sample of Cs atoms. Our scheme uses an optical
probe tuned near the D1 transition to measure the sample birefringence, which
depends on the z-component of the collective pseudospin. At certain probe
frequencies the differential light shift of the clock states vanishes and the
measurement is non-perturbing. In dense samples the measurement can be used to
squeeze the collective clock pseudo-spin, and has potential to improve the
performance of atomic clocks and interferometers.


title: Conditionalizing and commutativity: a note on Malley

abstract: This paper has been withdrawn.


title:
Mutual Information of Bipartite States and Quantum Discord in Terms of
  Coherence Information

abstract:
  In relation of observable and quantum state, the entity $I_C$ from previous
work quantifies simultaneously coherence, incompatibility and quantumness. In
this article its application to quantum correlations in bipartite states is
studied. It is shown that Zurek's quantum discord can always be expressed as
excess coherence information (global minus local). Strong and weak zero-discord
cases are distinguished and investigated in terms of necessary and sufficient
and sufficient conditions respectively. A unique string of relevant subsystem
observables, each a function of the next, for "interrogating" the global state
about the state of the opposite subsystem is derived with detailed entropy and
information gain discussion. The apparent disappearance of discord in
measurement is investigated, and it is shown that it is actually shifted from
between subsystems 1 and 2 to between subsystems 1 and $(2+3)$, where 3 is the
measuring instrument. Finally, it is shown that the global coherence
information $I_C(A_2,\rho_{12})$ is shifted into the global coherence
information $I_C(A_2,\rho_{123}^f)$ in the final state $\rho_{123}^f$ of the
measurement interaction.


title:
Three-qubit Thermal Entanglement via Entanglement Swapping on Two-qubit
  Heisenberg XY chains

abstract:
  In this paper we consider the generation of a three-qubit GHZ-like thermal
state by applying the entanglement swapping scheme of Zukowski {\it et al.}
[Ann. N. Y. Acad. Sci. {\bf 755}, 91 (1995)] to three pairs of two-qubit
Heisenberg XY chains. The quality of the resulting three-qubit entanglement is
studied by analyzing the teleportation fidelity, when it is used as a resource
in the teleportation protocol of Karlsson {\it et al.}[Phys. Rev. A {\bf 58},
4394 (1998)]. We show that even though thermal noise in the original two-qubit
states is amplified by the entanglement swapping process, we are still able to
achieve nonclassical fidelities for the anisotropic Heisenberg XY chains at
finitely higher and higher temperatures by adjusting the strengths of an
external magnetic field. This has a positive implication on the solid-state
realization of a quantum computer.


title: Iterative entanglement distillation for finite resources

abstract:
  We discuss a specific entanglement distillation scheme under the constraint
of finite samples of entangled qubit pairs. It is shown that an iterative
process can be explicitly formulated. The average fidelity of this process can
be enhanced by introducing conditional storing of entangled qubit pairs in each
step of the iteration. We investigate the corresponding limitations on the size
and the initial fidelity of the sample.


title: Microcanonical distributions for quantum systems

abstract:
  The standard assumption for the equilibrium microcanonical state in quantum
mechanics, that the system must be in one of the energy eigenstates, is
weakened so as to allow superpositions of states. The weakened form of the
microcanonical postulate thus asserts that all quantum states giving rise to
the same energy expectation value must be realised with equal probability. The
consequences that follow from this assertion are investigated. In particular, a
closed-form expression for the density of states associated with any system
having a nondegenerate energy spectrum is obtained. The result is applied to a
variety of examples, for which the behaviour of the state density, as well as
the relation between energy and temperature, are determined. Numerical studies
indicate that the density of states converges to a distribution when the number
of energy levels approaches infinity.


title: Preparing multi-partite entanglement of photons and matter qubits

abstract:
  We show how to make event-ready multi-partite entanglement between qubits
which may be encoded on photons or matter systems. Entangled states of matter
systems, which can also act as single photon sources, can be generated using
the entangling operation presented in quant-ph/0408040. We show how to entangle
such sources with photon qubits, which may be encoded in the dual rail,
polarization or time-bin degrees of freedom. We subsequently demonstrate how
projective measurements of the matter qubits can be used to create entangled
states of the photons alone. The state of the matter qubits is inherited by the
generated photons. Since the entangling operation can be used to generate
cluster states of matter qubits for quantum computing, our procedure enables us
to create any (entangled) photonic quantum state that can be written as the
outcome of a quantum computer.


title: The certainty principle

abstract:
  The notion of the quantum angle is introduced. The quantum angle turns out to
be a metric on the set of physical states of a quantum system. Its kinematics
and dynamics is studied. The certainty principle for quantum systems is
formulated and proved. It turns out that the certainty principle is closely
connected with the Heisenberg uncertainty principle (it presents, in some
sense, an opposite point of view). But at the same time the certainty principle
allows to give rigorous formulations for wider class of problems (it allows to
rigorously interpret and ground the analogous inequalities for the pairs of
quantities like time - energy, angle - angular momentum etc.)


title:
Absolute Quantum Efficiency Measurements by Means of Conditioned
  Polarization Rotation

abstract:
  We propose a new scheme for measuring the quantum efficiency of photon
counting detectors by using correlated pho-tons. The measurement technique is
based on a 90 rotation of the polarization of one photon member of a correlated
pair produced by parametric down-conversion, conditioned on the detection of
the other correlated photon after polarization selection. We present
experimental results obtained with this scheme.


title: Pseudoresonance mechanism of all-optical frequency standard operation

abstract:
  We propose a novel approach to all-optical frequency standard design, based
on a counterintuitive combination of the coherent population trapping effect
and signal discrimination at the maximum of absorption for the probe radiation.
The short-term stability of such a standard can achieve the level of
10^-14/(\tau)^1/2. The physics beyond this approach is dark resonance splitting
caused by interaction of the nuclear magnetic moment with the external magnetic
field.


title: Local vs Nonlocal cloning in a noisy environment

abstract:
  We address the distribution of quantum information among many parties in the
presence of noise. In particular, we consider how to optimally send to m
receivers the information encoded into an unknown coherent state. On one hand,
a local strategy is considered, consisting in a local cloning process followed
by direct transmission. On the other hand, a telecloning protocol based on
nonlocal quantum correlations is analyzed. Both the strategies are optimized to
minimize the detrimental effects due to losses and thermal noise along the
propagation. The comparison between the local and the nonlocal protocol shows
that telecloning is more effective than local cloning for a wide range of noise
parameters. Our results indicate that nonlocal strategies can be more robust
against noise than local ones, thus being suitable candidates to play a major
role in quantum information networks.


title:
Entanglement and criticality in translational invariant harmonic lattice
  systems with finite-range interactions

abstract:
  We discuss the relation between entanglement and criticality in
translationally invariant harmonic lattice systems with non-randon,
finite-range interactions. We show that the criticality of the system as well
as validity or break-down of the entanglement area law are solely determined by
the analytic properties of the spectral function of the oscillator system,
which can easily be computed. In particular for finite-range couplings we find
a one-to-one correspondence between an area-law scaling of the bi-partite
entanglement and a finite correlation length. This relation is strict in the
one-dimensional case and there is strog evidence for the multi-dimensional
case. We also discuss generalizations to couplings with infinite range.
Finally, to illustrate our results, a specific 1D example with nearest and
next-nearest neighbor coupling is analyzed.


title:
Bounds on Multipartite Entangled Orthogonal State Discrimination Using
  Local Operations and Classical Communication

abstract:
  We show that entanglement guarantees difficulty in the discrimination of
orthogonal multipartite states locally. The number of pure states that can be
discriminated by local operations and classical communication is bounded by the
total dimension over the average entanglement. A similar, general condition is
also shown for pure and mixed states. These results offer a rare operational
interpretation for three abstractly defined distance like measures of
multipartite entanglement.


title: Multiple-Access Bosonic Communications

abstract:
  The maximum rates for reliably transmitting classical information over
Bosonic multiple-access channels (MACs) are derived when the transmitters are
restricted to coherent-state encodings. Inner and outer bounds for the ultimate
capacity region of the Bosonic MAC are also presented. It is shown that the
sum-rate upper bound is achievable with a coherent-state encoding and that the
entire region is asymptotically achievable in the limit of large mean input
photon numbers.


title:
Controlled Flow of Spin-Entangled Electrons via Adiabatic Quantum
  Pumping

abstract:
  We propose a method to dynamically generate and control the flow of
spin-entangled electrons, each belonging to a spin-singlet, by means of
adiabatic quantum pumping. The pumping cycle functions by periodic time
variation of localized two-body interactions. We develop a generalized approach
to adiabatic quantum pumping as traditional methods based on scattering matrix
in one dimension cannot be applied here. We specifically compute the flow of
spin-entangled electrons within a Hubbard-like model of quantum dots, and
discuss possible implementations and identify parameters that can be used to
control the singlet flow.


title: Quantum Mechanics in Multiply-Connected Spaces

abstract:
  We explain why, in a configuration space that is multiply connected, i.e.,
whose fundamental group is nontrivial, there are several quantum theories,
corresponding to different choices of topological factors. We do this in the
context of Bohmian mechanics, a quantum theory without observers from which the
quantum formalism can be derived. What we do can be regarded as generalizing
the Bohmian dynamics on $\mathbb{R}^{3N}$ to arbitrary Riemannian manifolds,
and classifying the possible dynamics that arise. This approach provides a new
understanding of the topological features of quantum theory, such as the
symmetrization postulate for identical particles. For our analysis we employ
wave functions on the universal covering space of the configuration space.


title: Quantumness of ensemble from no-broadcasting principle

abstract:
  Quantum information, though not precisely defined, is a fundamental concept
of quantum information theory which predicts many fascinating phenomena and
provides new physical resources. A basic problem is to recognize the features
of quantum systems responsible for those phenomena. One of such important
features is that non-commuting quantum states cannot be broadcast: two copies
cannot be obtained out of a single copy, not even reproduced marginally on
separate systems. We focus on the difference of information contents between
one copy and two copies which is a basic manifestation of the gap between
quantum and classical information. We show that if the chosen information
measure is the Holevo quantity, the difference between the information contents
of one copy and two copies is zero if and only if the states can be broadcast.
We propose a new approach in defining measures of quantumness of ensembles
based on the difference of information contents between the original ensemble
and the ensemble of duplicated states. We comment about the permanence property
of quantum states and the recently introduced superbroadcasting operation. We
also provide an Appendix where we discuss the status of quantum information in
quantum physics, basing on the so-called isomorphism principle.


title: Noise-free high-efficiency photon-number-resolving detectors

abstract:
  High-efficiency optical detectors that can determine the number of photons in
a pulse of monochromatic light have applications in a variety of physics
studies, including post-selection-based entanglement protocols for linear
optics quantum computing and experiments that simultaneously close the
detection and communication loopholes of Bell's inequalities. Here we report on
our demonstration of fiber-coupled, noise-free, photon-number-resolving
transition-edge sensors with 88% efficiency at 1550 nm. The efficiency of these
sensors could be made even higher at any wavelength in the visible and
near-infrared spectrum without resulting in a higher dark-count rate or
degraded photon-number resolution.


title:
Equations of Motion with Multiple Proper Time: A New Interpretation of
  Spin

abstract:
  The purpose of this paper is to show that: when a single particle moving
under 3-proper time (three-dimensional time), the trajectories of a classical
particle are equivalent to a quantum field with spin. Three-proper time models
are built for spinless particle, particles with integer spin and half-integer
spin respectively. The models recreate the same physical behavior as quantum
field theory of free particles -- by using pure classical methods with three
proper time. A new interpretation of spin is given. It provides us more evident
that it is possible to interpret quantum physics by using multiple dimensional
time. In the last part of this paper, Bose-Einstein statistics and Fermi-Dirac
statistics are derived under classical method.


title: Completely Quantized Collapse and Consequences

abstract:
  Promotion of quantum theory from a theory of measurement to a theory of
reality requires an unambiguous specification of the ensemble of realizable
states (and each state's probability of realization). Although not yet achieved
within the framework of standard quantum theory, it has been achieved within
the framework of the Continuous Spontaneous Localization (CSL) wave function
collapse model. In this paper, I consider a previously presented model, which
is predictively equivalent to CSL. In this Completely Quantized Collapse (CQC)
model, the classical random field which causes collapse in CSL is quantized.
The ensemble of realizable states is described by a single state vector, the
"ensemble vector," the sum of the direct product of an eigenstate of the
quantized field and the CSL state corresponding to that eigenstate. Using this
description, a long standing problem is resolved: it is shown how to define
energy of the random field and its energy of interaction with particles so that
total energy is conserved for the ensemble of realizable states. As a
byproduct, since the random field energy spectrum is unbounded, its canonical
conjugate, a self-adjoint time operator, is discussed. Finally, CSL is a
phenomenological description, whose connection to, or derivation from, more
conventional physics has not yet appeared. We suggest that, because CQC is
fully quantized, it is a natural framework for replacement of the classical
field of CSL by a quantized physical entity. Two illustrative examples are
given.


title: A degenerate three-level laser with a parametric amplifier

abstract:
  The aim of this paper is to study the squeezing and statistical properties of
the light produced by a degenerate three-level laser whose cavity contains a
degenerate parametric amplifier. In this quantum optical system the top and
bottom levels of the three-level atoms injected into the laser cavity are
coupled by the pump mode emerging from the parametric amplifier. For a linear
gain coefficient of 100 and for a cavity damping constant of 0.8, the maximum
intracavity squeezing is found at steady state and at threshold to be 93%.


title:
Velocity quantization approach of the one-dimensional dissipative
  harmonic oscillator

abstract:
  Given a constant of motion for the one-dimensional harmonic oscillator with
linear dissipation in the velocity, the problem to get the Hamiltonian for this
system is pointed out, and the quantization up to second order in the
perturbation approach is used to determine the modification on the eigenvalues
when dissipation is taken into consideration. This quantization is realized
using the constant of motion instead of the Hamiltonian.


title: Popescu-Rohrlich correlations as a unit of nonlocality

abstract:
  A set of nonlocal correlations that have come to be known as a PR box suggest
themselves as a natural unit of nonlocality, much as a singlet is a natural
unit of entanglement. We present two results relevant to this idea. One is that
a wide class of multipartite correlations can be simulated using local
operations on PR boxes only. We show this with an explicit scheme, which has
the interesting feature that the number of PR boxes required is related to the
computational resources necessary to represent a function defining the
multipartite box. The second result is that there are quantum multipartite
correlations, arising from measurements on a cluster state, that cannot be
simulated with n PR boxes, for any n.


title:
Infinitesimal local operations and differential conditions for
  entanglement monotones

abstract:
  Much of the theory of entanglement concerns the transformations that are
possible to a state under local operations with classical communication (LOCC);
however, this set of operations is complicated and difficult to describe
mathematically. An idea which has proven very useful is that of the {\it
entanglement monotone}: a function of the state which is invariant under local
unitary transformations and always decreases (or increases) on average after
any local operation. In this paper we look on LOCC as the set of operations
generated by {\it infinitesimal local operations}, operations which can be
performed locally and which leave the state little changed. We show that a
necessary and sufficient condition for a function of the state to be an
entanglement monotone under local operations that do not involve information
loss is that the function be a monotone under infinitesimal local operations.
We then derive necessary and sufficient differential conditions for a function
of the state to be an entanglement monotone. We first derive two conditions for
local operations without information loss, and then show that they can be
extended to more general operations by adding the requirement of {\it
convexity}. We then demonstrate that a number of known entanglement monotones
satisfy these differential criteria. Finally, as an application, we use the
differential conditions to construct a new polynomial entanglement monotone for
three-qubit pure states. It is our hope that this approach will avoid some of
the difficulties in the theory of multipartite and mixed-state entanglement.


title: Interconversion of Nonlocal Correlations

abstract:
  In this paper we study the correlations that arise when two separated parties
perform measurements on systems they hold locally. We restrict ourselves to
those correlations with which arbitrarily fast transmission of information is
impossible. These correlations are called nonsignaling. We allow the
measurements to be chosen from sets of an arbitrary size, but promise that each
measurement has only two possible outcomes. We find the structure of this
convex set of nonsignaling correlations by characterizing its extreme points.
Taking an information-theoretic view, we prove that all of these extreme
correlations are interconvertible. This suggests that the simplest extremal
nonlocal distribution (called a PR box) might be the basic unit of nonlocality.
We also show that this unit of nonlocality is sufficient to simulate all
quantum states when measured with two outcome measurements.


title: Unstable particles as open quantum systems

abstract:
  We present the probability preserving description of the decaying particle
within the framework of quantum mechanics of open systems taking into account
the superselection rule prohibiting the superposition of the particle and
vacuum. In our approach the evolution of the system is given by a family of
completely positive trace preserving maps forming one-parameter dynamical
semigroup. We give the Kraus representation for the general evolution of such
systems which allows one to write the evolution for systems with two or more
particles. Moreover, we show that the decay of the particle can be regarded as
a Markov process by finding explicitly the master equation in the Lindblad
form. We also show that there are remarkable restrictions on the possible
strength of decoherence.


title: The Arrow of Time in Rigged Hilbert Space Quantum Mechanics

abstract:
  Arno Bohm and Ilya Prigogine's Brussels-Austin Group have been working on the
quantum mechanical arrow of time and irreversibility in rigged Hilbert space
quantum mechanics. A crucial notion in Bohm's approach is the so-called
preparation/registration arrow. An analysis of this arrow and its role in
Bohm's theory of scattering is given. Similarly, the Brussels-Austin Group uses
an excitation/de-excitation arrow for ordering events, which is also analyzed.
The relationship between the two approaches is initially discussed focusing on
their semi-group operators and time arrows. Finally a possible realist
interpretation of the rigged Hilbert space formulation of quantum mechanics is
considered.


title:
Quantum limited velocity readout and quantum feedback cooling of a
  trapped ion via electromagnetically induced transparency

abstract:
  We discuss continuous observation of the momentum of a single atom by
employing the high velocity sensitivity of the index of refraction in a driven
$\Lambda$-system based on electromagnetically induced transparency (EIT). In
the ideal limit of unit collection efficiency this provides a quantum limited
measurement with minimal backaction on the atomic motion. A feedback loop,
which drives the atom with a force proportional to measured signal, provides a
cooling mechanism for the atomic motion. We derive the master equation which
describes the feedback cooling and show that in the Lamb-Dicke limit the steady
state energies are close to the ground state, limited only by the photon
collection efficiency. Outside of the Lamb-Dicke regime the predicted
temperatures are well below the Doppler limit.


title: Resonances, Unstable Systems and Irreversibility: Matter Meets Mind

abstract:
  The fundamental time-reversal invariance of dynamical systems can be broken
in various ways. One way is based on the presence of resonances and their
interactions giving rise to unstable dynamical systems, leading to well-defined
time arrows. Associated with these time arrows are semigroups bearing time
orientations. Usually, when time symmetry is broken, two time-oriented
semigroups result, one directed toward the future and one directed toward the
past. If time-reversed states and evolutions are excluded due to resonances,
then the status of these states and their associated backwards-in-time oriented
semigroups is open to question. One possible role for these latter states and
semigroups is as an abstract representation of mental systems as opposed to
material systems. The beginnings of this interpretation will be sketched.


title: Quantum feedback cooling of a single trapped ion in front of a mirror

abstract:
  We develop a theory of quantum feedback cooling of a single ion trapped in
front of a mirror. By monitoring the motional sidebands of the light emitted
into the mirror mode we infer the position of the ion, and act back with an
appropriate force to cool the ion. We derive a feedback master equation along
the lines of the quantum feedback theory developed by Wiseman and Milburn,
which provides us with cooling times and final temperatures as a function of
feedback gain and various system parameters.


title:
Scattering induced dynamical entanglement and the quantum-classical
  correspondence

abstract:
  The generation of entanglement produced by a local potential interaction in a
bipartite system is investigated. The degree of entanglement is contrasted with
the underlying classical dynamics for a Rydberg molecule (a charged particle
colliding on a kicked top). Entanglement is seen to depend on the structure of
classical phase-space rather than on the global dynamical regime. As a
consequence regular classical dynamics can in certain circumstances be
associated with higher entanglement generation than chaotic dynamics. In
addition quantum effects also come into play: for example partial revivals,
which are expected to persist in the semiclassical limit, affect the long time
behaviour of the reduced linear entropy. These results suggest that
entanglement may not be a pertinent universal signature of chaos.


title: General paradigm for distilling classical key from quantum states

abstract:
  We develop a formalism for distilling a classical key from a quantum state in
a systematic way, expanding on our previous work on secure key from bound
entanglement [K. Horodecki et. al., Phys. Rev. Lett. 94 (2005)]. More detailed
proofs, discussion and examples are provided of the main results. Namely, we
demonstrate that all quantum cryptographic protocols can be recast in a way
which looks like entanglement theory, with the only change being that instead
of distilling EPR pairs, the parties distill private states. The form of these
general private states are given, and we show that there are a number of useful
ways of expressing them. Some of the private states can be approximated by
certain states which are bound entangled. Thus distillable entanglement is not
a requirement for a private key. We find that such bound entangled states are
useful for a cryptographic primitive we call a controlled private quantum
channel. We also find a general class of states which have negative partial
transpose (are NPT), but which appear to be bound entangled. The relative
entropy distance is shown to be an upper bound on the rate of key. This allows
us to compute the exact value of distillable key for a certain class of private
states.


title:
Local Conversion of Greenberger-Horne-Zeilinger States to Approximate W
  States

abstract:
  Genuine 3-qubit entanglement comes in two different inconvertible types
represented by the Greenberger-Horne-Zeilinger (GHZ) state and the W state. We
describe a specific method based on local positive operator valued measures and
classical communication that can convert the ideal N-qubit GHZ state to a state
arbitrarily close to the ideal N-qubit W state. We then experimentally
implement this scheme in the 3-qubit case and characterize the input and the
final state using 3-photon quantum state tomography.


title: The noncommutative degenerate electron gas

abstract:
  The quantum dynamics of nonrelativistic single particle systems involving
noncommutative coordinates, usually referred to as noncommutative quantum
mechanics, has lately been the object of several investigations. In this note
we pursue these studies for the case of multi-particle systems. We use as a
prototype the degenerate electron gas whose dynamics is well known in the
commutative limit. Our central aim here is to understand qualitatively, rather
than quantitatively, the main modifications induced by the presence of
noncommutative coordinates. We shall first see that the noncommutativity
modifies the exchange correlation energy while preserving the electric
neutrality of the model. By employing time-independent perturbation theory
together with the Seiberg-Witten map we show, afterwards, that the ionization
potential is modified by the noncommutativity. It also turns out that the
noncommutative parameter acts as a reference temperature. Hence, the
noncommutativity lifts the degeneracy of the zero temperature electron gas.


title: Quantum Scattering in Quasi-1D Cylindrical Confinement

abstract:
  Finite size effects alter not only the energy levels of small systems, but
can also lead to new effective interactions within these systems. Here the
problem of low energy quantum scattering by a spherically symmetric short range
potential in the presence of a general cylindrical confinement is investigated.
A Green's function formalism is developed which accounts for the full 3D nature
of the scattering potential by incorporating all phase-shifts and their
couplings. This quasi-1D geometry gives rise to scattering resonances and
weakly localized states, whose binding energies and wavefunctions can be
systematically calculated. Possible applications include e.g. impurity
scattering in ballistic quasi-1D quantum wires in mesoscopic systems and in
atomic matter wave guides. In the particular case of parabolic confinement, the
present formalism can also be applied to pair collision processes such as
two-body interactions. Weakly bound pairs and quasi-molecules induced by the
confinement and having zero or higher orbital angular momentum can be
predicted, such as p- and d-wave pairings.