title: A simple necessary decoherence condition for a set of histories

abstract:
  Within the decoherent histories formulation of quantum mechanics, we
investigate necessary conditions for decoherence of arbitrarily long histories.
We prove that fine-grained histories of arbitrary length decohere for all
classical initial states if and only if the unitary evolution preserves
classicality of states (using a natural formal definition of classicality). We
give a counterexample showing that this equivalence does not hold for
coarse-grained histories.


title: Quantum Logic Operations Using Single Photons and the Zeno Effect

abstract:
  We show that the quantum Zeno effect can be used to implement several quantum
logic gates for photonic qubits, including a gate that is similar to the
square-root of SWAP operation. The operation of these devices depends on the
fact that photons can behave as if they were non-interacting fermions instead
of bosons in the presence of a strong Zeno effect. These results are discussed
within the context of several no-go theorems for non-interacting fermions or
bosons.


title: Quantum convolutional codes: fundamentals

abstract:
  We describe the theory of quantum convolutional error correcting codes. These
codes are aimed at protecting a flow of quantum information over long distance
communication. They are largely inspired by their classical analogs which are
used in similar circumstances in classical communication. In this article, we
provide an efficient polynomial formalism for describing their stabilizer
group, derive an on-line encoding circuit with linear gate complexity and study
error propagation together with the existence of on-line decoding. Finally, we
provide a maximum likelihood error estimation algorithm with linear classical
complexity for any memoryless channel.


title:
Algorithmic Cooling of Spins: A Practicable Method for Increasing
  Polarization

abstract:
  An efficient technique to generate ensembles of spins that are highly
polarized by external magnetic fields is the Holy Grail in Nuclear Magnetic
Resonance (NMR) spectroscopy. Since spin-half nuclei have steady-state
polarization biases that increase inversely with temperature, spins exhibiting
high polarization biases are considered cool, even when their environment is
warm. Existing spin-cooling techniques are highly limited in their efficiency
and usefulness. Algorithmic cooling is a promising new spin-cooling approach
that employs data compression methods in open systems. It reduces the entropy
of spins on long molecules to a point far beyond Shannon's bound on reversible
entropy manipulations (an information-theoretic version of the 2nd Law of
Thermodynamics), thus increasing their polarization. Here we present an
efficient and experimentally feasible algorithmic cooling technique that cools
spins to very low temperatures even on short molecules. This practicable
algorithmic cooling could lead to breakthroughs in high-sensitivity NMR
spectroscopy in the near future, and to the development of scalable NMR quantum
computers in the far future. Moreover, while the cooling algorithm itself is
classical, it uses quantum gates in its implementation, thus representing the
first short-term application of quantum computing devices.


title: Quantum-"classical" correspondence in a nonadiabatic-transition system

abstract:
  A nonadiabatic-transition system which exhibits ``quantum chaotic'' behavior
[Phys. Rev. E {\bf 63}, 066221 (2001)] is investigated from quasi-classical
aspects. Since such a system does not have a naive classical limit, we take the
mapping approach by Stock and Thoss [Phys. Rev. Lett. {\bf 78}, 578 (1997)] to
represent the quasi-classical dynamics of the system. We numerically show that
there is a sound correspondence between the quantum chaos and classical chaos
for the system.


title:
Fast simulation of a quantum phase transition in an ion-trap realisable
  unitary map

abstract:
  We demonstrate a method of exploring the quantum critical point of the Ising
universality class using unitary maps that have recently been demonstrated in
ion trap quantum gates. We reverse the idea with which Feynman conceived
quantum computing, and ask whether a realisable simulation corresponds to a
physical system. We proceed to show that a specific simulation (a unitary map)
is physically equivalent to a Hamiltonian that belongs to the same universality
class as the transverse Ising Hamiltonian. We present experimental signatures,
and numerical simulation for these in the six-qubit case.


title:
Violation of the Nernst heat theorem in the theory of thermal Casimir
  force between Drude metals

abstract:
  We give a rigorous analytical derivation of low-temperature behavior of the
Casimir entropy in the framework of the Lifshitz formula combined with the
Drude dielectric function. An earlier result that the Casimir entropy at zero
temperature is not equal to zero and depends on the parameters of the system is
confirmed, i.e. the third law of thermodynamics (the Nernst heat theorem) is
violated. We illustrate the resolution of this thermodynamical puzzle in the
context of the surface impedance approach by several calculations of the
thermal Casimir force and entropy for both real metals and dielectrics.
Different representations for the impedances, which are equivalent for real
photons, are discussed. Finally, we argue in favor of the Leontovich boundary
condition which leads to results for the thermal Casimir force that are
consistent with thermodynamics.


title: Finite Supersymmetry Transformations

abstract:
  We investigate simple examples of supersymmetry algebras with real and
Grassmann parameters. Special attention is payed to the finite
supertransformations and their probability interpretation. Furthermore we look
for combinations of bosons and fermions which are invariant under
supertransformations. These combinations correspond to states that are highly
entangled.


title:
Nanosecond Dynamics of Single-Molecule Fluorescence Resonance Energy
  Transfer

abstract:
  Motivated by recent experiments on photon statistics from individual dye
pairs planted on biomolecules and coupled by fluorescence resonance energy
transfer (FRET), we show here that the FRET dynamics can be modelled by
Gaussian random processes with colored noise. Using Monte-Carlo numerical
simulations, the photon intensity correlations from the FRET pairs are
calculated, and are turned out to be very close to those observed in
experiment. The proposed stochastic description of FRET is consistent with
existing theories for microscopic dynamics of the biomolecule that carries the
FRET coupled dye pairs.


title: Nonclassical Imaging for a quantum search of trapped ions

abstract:
  We discuss a simple search problem which can be pursued with different
methods, either on a classical or on a quantum basis. The system is represented
by a chain of trapped ions. The ion to be searched is a member of that chain,
consists, however, of an isotopic species different to the others. It is shown
that the classical imaging may lead as fast to the final result as the quantum
imaging. However, for the discussed case the quantum method gives more
flexibility and higher precision when the number of ions considered in the
chain is increasing. In addition, interferences are observable even when the
distances between the ions is smaller than half a wavelength of the incident
light.


title: Quantum freeze of fidelity decay for chaotic dynamics

abstract:
  We show that the mechanism of quantum freeze of fidelity decay for
perturbations with zero time-average, recently discovered for a specific case
of integrable dynamics [New J. Phys. 5 (2003) 109], can be generalized to
arbitrary quantum dynamics. We work out explicitly the case of chaotic
classical counterpart, for which we find semi-classical expressions for the
value and the range of the plateau of fidelity. After the plateau ends, we find
explicit expressions for the asymptotic decay, which can be exponential or
Gaussian depending on the ratio of the Heisenberg time to the decay time.
Arbitrary initial states can be considered, e.g. we discuss coherent states and
random states.


title: Non-symmetric entanglement of atomic ensembles

abstract:
  The entanglement of multi-atom quantum states is considered. In order to
cancel noise due to inhomogeneous light atom coupling, the concept of matched
multi-atom observables is proposed. As a means to eliminate an important form
of decoherence this idea should be of broad relevance for quantum information
processing with atomic ensembles. The general approach is illustrated on the
example of rotation angle measurement, and it is shown that the multi-atom
states that were thought to be only weakly entangled can exhibit near-maximum
entanglement.


title: Quantum Phase Transitions in Anti-ferromagnetic Planar Cubic Lattices

abstract:
  Motivated by its relation to an $\cal{NP}$-hard problem, we analyze the
ground state properties of anti-ferromagnetic Ising-spin networks embedded on
planar cubic lattices, under the action of homogeneous transverse and
longitudinal magnetic fields. This model exhibits a quantum phase transition at
critical values of the magnetic field, which can be identified by the
entanglement behavior, as well as by a Majorization analysis. The scaling of
the entanglement in the critical region is in agreement with the area law,
indicating that even simple systems can support large amounts of quantum
correlations. We study the scaling behavior of low-lying energy gaps for a
restricted set of geometries, and find that even in this simplified case, it is
impossible to predict the asymptotic behavior, with the data allowing equally
good fits to exponential and power law decays. We can therefore, draw no
conclusion as to the algorithmic complexity of a quantum adiabatic ground-state
search for the system.


title: Survival law in a potential model

abstract:
  The radial equation of a simple potential model has long been known to yield
an exponential decay law in lowest order (Breit-Wigner) approximation. We
demonstrate that if the calculation is extended to fourth order the decay law
exhibits the quantum Zeno effect. This model has further been studied
numerically to characterize the extra exponential time parameter which
compliments the lifetime. We also investigate the inverse Zeno effect.


title: Quantum dynamical semigroups and non-decomposable positive maps

abstract:
  We study dynamical semigroups of positive, but not completely positive maps
on finite-dimensional bipartite systems and analyze properties of their
generators in relation to non-decomposability and bound-entanglement. An
example of non-decomposable semigroup leading to a 4x4-dimensional
bound-entangled density matrix is explicitly obtained.


title: Single-Photon Generation from Stored Excitation in an Atomic Ensemble

abstract:
  Single photons are generated from an ensemble of cold Cs atoms via the
protocol of Duan et al. [Nature \textbf{414}, 413 (2001)]. Conditioned upon an
initial detection from field 1 at 852 nm, a photon in field 2 at 894 nm is
produced in a controlled fashion from excitation stored within the atomic
ensemble. The single-quantum character of the field 2 is demonstrated by the
violation of a Cauchy-Schwarz inequality, namely $w(1_{2},1_{2}|1_{1})=0.24\pm
0.05\ngeq 1$, where $w(1_{2},1_{2}|1_{1})$ describes detection of two events
$(1_{2},1_{2})$ conditioned upon an initial detection $1_{1}$, with $w\to 0$
for single photons.


title:
Violation of local realism in a high-dimensional two-photon setup with
  non-integer spiral phase plates

abstract:
  We propose a novel setup to investigate the quantum non-locality of orbital
angular momentum states living in a high-dimensional Hilbert space. We
incorporate non-integer spiral phase plates in spatial analyzers, enabling us
to use only two detectors. The resulting setup is somewhat reminiscent of that
used to measure polarization entanglement. However, the two-photon states that
are produced, are not confined to a 2X2-dimensional Hilbert space, and the
setup allows the probing of correlations in a high-dimensional space. For the
special case of half-integer spiral phase plates, we predict a violation of the
Clauser-Horne-Shimony-Holt version of the Bell inequality (S<=2), that is even
stronger than achievable for two qubits (S=2*(2^1/2)), namely S=16/5


title: Classical phase-space descriptions of continuous-variable teleportation

abstract:
  The nonnegative Wigner function of all quantum states involved in
teleportation of Gaussian states using the standard continuous-variable
teleportation protocol means that there is a local realistic phase-space
description of the process. This includes the coherent states teleported up to
now in experiments. We extend the phase-space description to teleportation of
non-Gaussian states using the standard protocol and conclude that teleportation
of non-Gaussian states with fidelity of 2/3 is a "gold standard" for this kind
of teleportation.


title: Supersymmetry in quantum mechanics: An extended view

abstract:
  The concept of supersymmetry in a quantum mechanical system is extended,
permitting the recognition of many more supersymmetric systems, including very
familiar ones such as the free particle. Its spectrum is shown to be
supersymmetric, with space-time symmetries used for the explicit construction.
No fermionic or Grassmann variables need to be invoked. Our construction
extends supersymmetry to continuous spectra. Most notably, while the free
particle in one dimension has generally been regarded as having a doubly
degenerate continuum throughout, the construction clarifies taht there is a
single zero energy state at the base of the spectrum.


title: Luttinger liquid of polarons in one-dimensional boson-fermion mixtures

abstract:
  We use bosonization approach to investigate quantum phases in mixtures of
bosonic and fermionic atoms confined in one dimensional optical lattices. The
phase diagrams can be well understood in terms of polarons, which correspond to
atoms that are "dressed" by screening clouds of the other atom species. For a
mixture of single species of fermionic and bosonic atoms we find a charge
density wave phase, a phase with fermion pairing, and a regime of phase
separation. For a mixture of two species of fermionic atoms and one species of
bosonic atoms we obtain spin and charge density wave phases, a Wigner crystal
phase, singlet and triplet paired states of fermions, and a phase separation
regime. Equivalence between the Luttinger liquid description of polarons and
the canonical polaron transformation is established and the techniques to
detect the resulting quantum phases are discussed.


title: Mechanism and Lineshapes of Sub-Fourier Resonances

abstract:
  Subtle internal interference effects allow quantum-chaotic systems to display
"sub-Fourier" resonances, i.e. to distinguish two neighboring driving
frequencies in a time shorter than the inverse of the difference of the two
frequencies. We report experiments on the atomic version of the kicked rotor
showing the unusual properties of the sub-Fourier resonances, and develop a
theoretical approach (based on the Floquet theorem) explaining these
properties, and correctly predicting the widths and lineshapes.

categories: quant-ph cond-mat.soft nlin.CD physics.atom-ph


title:
Theory confronts experiment in the Casimir force measurements:
  quantification of errors and precision

abstract:
  We compare theory and experiment in the Casimir force measurement between
gold surfaces performed with the atomic force microscope. Both random and
systematic experimental errors are found leading to a total absolute error
equal to 8.5 pN at 95% confidence. In terms of the relative errors,
experimental precision of 1.75% is obtained at the shortest separation of 62 nm
at 95% confidence level (at 60% confidence the experimental precision of 1% is
confirmed at the shortest separation). An independent determination of the
accuracy of the theoretical calculations of the Casimir force and its
application to the experimental configuration is carefully made. Special
attention is paid to the sample-dependent variations of the optical tabulated
data due to the presence of grains, contribution of surface plasmons, and
errors introduced by the use of the proximity force theorem. Nonmultiplicative
and diffraction-type contributions to the surface roughness corrections are
examined. The electric forces due to patch potentials resulting from the
polycrystalline nature of the gold films are estimated. The finite size and
thermal effects are found to be negligible. The theoretical accuracy of about
1.69% and 1.1% are found at a separation 62 nm and 200 nm, respectively. Within
the limits of experimental and theoretical errors very good agreement between
experiment and theory is confirmed characterized by the root mean square
deviation of about 3.5 pN within all measurement range. The conclusion is made
that the Casimir force is stable relative to variations of the sample-dependent
optical and electric properties, which opens new opportunities to use the
Casimir effect for diagnostic purposes.


title: Optimal Database Search: Waves and Catalysis

abstract:
  Grover's database search algorithm, although discovered in the context of
quantum computation, can be implemented using any system that allows
superposition of states. A physical realization of this algorithm is described
using coupled simple harmonic oscillators, which can be exactly solved in both
classical and quantum domains. Classical wave algorithms are far more stable
against decoherence compared to their quantum counterparts. In addition to
providing convenient demonstration models, they may have a role in practical
situations, such as catalysis.


title: Discrete phase space based on finite fields

abstract:
  The original Wigner function provides a way of representing in phase space
the quantum states of systems with continuous degrees of freedom. Wigner
functions have also been developed for discrete quantum systems, one popular
version being defined on a 2N x 2N discrete phase space for a system with N
orthogonal states. Here we investigate an alternative class of discrete Wigner
functions, in which the field of real numbers that labels the axes of
continuous phase space is replaced by a finite field having N elements. There
exists such a field if and only if N is a power of a prime; so our formulation
can be applied directly only to systems for which the state-space dimension
takes such a value. Though this condition may seem limiting, we note that any
quantum computer based on qubits meets the condition and can thus be
accommodated within our scheme. The geometry of our N x N phase space also
leads naturally to a method of constructing a complete set of N+1 mutually
unbiased bases for the state space.


title: Entanglement bases and general structures of orthogonal complete bases

abstract:
  In quantum mechanics and quantum information, to establish the orthogonal
bases is a useful means. The existence of unextendible product bases impels us
to study the `entanglement bases' problems. In this paper, the concepts of
entanglement bases and exact-entanglement bases are defined, and a theorem
about exact-entanglement bases is given. We discuss the general structures of
the orthogonal complete bases. Two examples of applications are given. At last,
we discuss the problem of transformation of the general structure forms.


title:
Readout scheme of the fullerene-based quantum computer by a single
  electron transistor

abstract:
  The readout of the quantum spin state is a challenge for any spin-based
quantum computing implementation. We propose a scheme, based on the achieved
technique of single electron transistor (SET), to implement the readout of
electronic spin state inside a doped $C_{60}$ fullerene by means of the
magnetic dipole-dipole coupling and spin filters. In the presence of an
external magnetic field, we show how to perform the spin state detection by
transforming the information contained in the spin state into the tunneling
current. The robustness of our scheme against sources of error is discussed.


title: Quantum decoherence in the rotation of small molecules

abstract:
  The dynamics of non-polar diatomic molecules interacting with a far-detuned
narrow-band laser field, that only may drive rotational transitions, is
studied. The rotation of the molecule is considered both classically and
quantum mechanically, providing links to features known from the heavy
symmetric top. In particular, quantum decoherence in the molecular rotation,
being induced by spontaneous Raman processes, is addressed. It is shown how
this decoherence modifies the rotational dynamics in phase space.


title:
"Superluminal" tunnelling, quantum measurements and the speed of
  information transfer

abstract:
  We exploit the analogy between tunnelling across a potential barrier and
Aharonov's weak measurements to resolve the long standing paradox between the
impossibility to exceed the speed of light and the seemingly 'superluminal'
behaviur of the tunnelling particle in the barrier. We demonstrate that
'superluminality' occurs when the value of the duration $\tau$ spent in the
barrier is uncertain, whereas when $\tau$ is known accurately, no
'superluminal' behaviur is observed. In all cases only subluminal durations
contribute to the transmission, which precludes faster-than-light information
transfer, as shown by a recent experiment.


title: Non-Linear Canonical Transformations in Classical and Quantum Mechanics

abstract:
  $p$-Mechanics is a consistent physical theory which describes both classical
and quantum mechanics simultaneously through the representation theory of the
Heisenberg group. In this paper we describe how non-linear canonical
transformations affect $p$-mechanical observables and states. Using this we
show how canonical transformations change a quantum mechanical system. We seek
an operator on the set of $p$-mechanical observables which corresponds to the
classical canonical transformation. In order to do this we derive a set of
integral equations which when solved will give us the coherent state expansion
of this operator. The motivation for these integral equations comes from the
work of Moshinsky and a variety of collaborators. We consider a number of
examples and discuss the use of these equations for non-bijective
transformations.


title:
Observation of Robust Quantum Resonance Peaks in an Atom Optics Kicked
  Rotor with Amplitude Noise

abstract:
  The effect of pulse train noise on the quantum resonance peaks of the Atom
Optics Kicked Rotor is investigated experimentally. Quantum resonance peaks in
the late time mean energy of the atoms are found to be surprisingly robust
against all levels of noise applied to the kicking amplitude, whilst even small
levels of noise on the kicking period lead to their destruction. The robustness
to amplitude noise of the resonance peak and of the fall--off in mean energy to
either side of this peak are explained in terms of the occurence of stable,
$\epsilon$--classical dynamics [S. Wimberger, I. Guarneri, and S. Fishman,
\textit{Nonlin.} \textbf{16}, 1381 (2003)] around each quantum resonance.


title: Quantum Circuits for Incompletely Specified Two-Qubit Operators

abstract:
  While the question ``how many CNOT gates are needed to simulate an arbitrary
two-qubit operator'' has been conclusively answered -- three are necessary and
sufficient -- previous work on this topic assumes that one wants to simulate a
given unitary operator up to global phase. However, in many practical cases
additional degrees of freedom are allowed. For example, if the computation is
to be followed by a given projective measurement, many dissimilar operators
achieve the same output distributions on all input states. Alternatively, if it
is known that the input state is |0>, the action of the given operator on all
orthogonal states is immaterial. In such cases, we say that the unitary
operator is incompletely specified; in this work, we take up the practical
challenge of satisfying a given specification with the smallest possible
circuit. In particular, we identify cases in which such operators can be
implemented using fewer quantum gates than are required for generic completely
specified operators.


title: A BPS Interpretation of Shape Invariance

abstract:
  We show that shape invariance appears when a quantum mechanical model is
invariant under a centrally extended superalgebra endowed with an additional
symmetry generator, which we dub the shift operator. The familiar mathematical
and physical results of shape invariance then arise from the BPS structure
associated with this shift operator. The shift operator also ensures that there
is a one-to-one correspondence between the energy levels of such a model and
the energies of the BPS-saturating states. These findings thus provide a more
comprehensive algebraic setting for understanding shape invariance.


title: Complete Resolution of the Quantum Zeno Paradox for Outside Observers

abstract:
  The quantum Zeno paradox is fully resolved for purely indirect and incomplete
measurements performed by the detectors outside the system. If the outside
detectors are prepared to observe propagating signals of a decay event of an
excited state in the core region, the survival probability of the state is not
changed at all by the outside measurements, as long as the wavefunction of the
signals does not have reflectional-wave contributions going back to the core by
the outside detectors. The proof is independendent of the decay law of the
survival probabilities. Just watching frequently from outside (observation)
cannot be regarded as a measurement which yields the quantum Zeno effect.


title:
Positive P-Representations of the Thermal Operator from Quantum Control
  Theory

abstract:
  A positive P-representation for the spin-j thermal density matrix is given in
closed form. The representation is constructed by regarding the wave function
as the internal state of a closed-loop control system. A continuous
interferometric measurement process is proved to einselect coherent states, and
feedback control is proved to be equivalent to a thermal reservoir. Ito
equations are derived, and the P-representation is obtained from a
Fokker-Planck equation. Langevin equations are derived, and the force noise is
shown to be the Hilbert transform of the measurement noise. The formalism is
applied to magnetic resonance force microscopy (MRFM) and gravity wave (GW)
interferometry. Some unsolved problems relating to drift and diffusion on
Hilbert spaces are noted.


title:
Experimental realization of a programmable quantum-state discriminator
  and a phase-covariant quantum multimeter

abstract:
  We present an optical implementation of two programmable quantum measurement
devices. The first one serves for unambiguous discrimination of two
nonorthogonal states of a qubit. The particular pair of states to be
discriminated is specified by the quantum state of a program qubit. The second
device can perform von Neumann measurements on a single qubit in any basis
located on the equator of the Bloch sphere. Again, the basis is selected by the
state of a program qubit. In both cases the data and program qubits are
represented by polarization states of photons. The experimental apparatus
exploits the fact that two Bell states can be distinguished solely by means of
linear optics. The outcome corresponding to the remaining two Bell states
represents an inconclusive result.


title: About distillability of depolarized states

abstract:
  Reduction criteria for distillability is applied to general depolarized
states and an explicit condition is found in terms of a characteristic
polynomial of the density matrix. 3 $\times$ 3 bipartite systems are analyzed
in some details.


title: Random Solitons Realization of Quantum Mechanics and Stochastic Qubits

abstract:
  Stochastic realization of the wave function in quantum mechanics, with the
inclusion of soliton representation of extended particles, is discussed. The
concept of Stochastic Qubits is used for quantum computing modeling.


title:
Validation of entanglement purification by continuous variable
  polarization

abstract:
  We investigate the possibility of characterizing two-party entanglement by
measuring correlations of Stokes operators in polarized bright light beams. We
adapt a general separability criterion to such operators. We then show that
entanglement purification can only be singled out for a particular protocol.


title:
Simplified derivation of the Hawking-Unruh temperature for an
  accelerated observer in vacuum

abstract:
  A detector undergoing uniform acceleration $a$ in a vacuum field responds
just as though it were immersed in thermal radiation of temperature $T=\hbar
a/2\pi k c$. A simple, intuitive derivation of this result is given for the
case of a scalar field in one spatial dimension. The approach is then extended
to treat the case where the field seen by the accelerated observer is a
spin-1/2 Dirac field.


title: Jump-like unravelings for non-Markovian open quantum systems

abstract:
  Non-Markovian evolution of an open quantum system can be `unraveled' into
pure state trajectories generated by a non-Markovian stochastic (diffusive)
Schr\"odinger equation, as introduced by Di\'osi, Gisin, and Strunz. Recently
we have shown that such equations can be derived using the modal (hidden
variable) interpretation of quantum mechanics. In this paper we generalize this
theory to treat jump-like unravelings. To illustrate the jump-like behavior we
consider a simple system: A classically driven (at Rabi frequency $\Omega$)
two-level atom coupled linearly to a three mode optical bath, with a central
frequency equal to the frequency of the atom, $\omega_0$, and the two side
bands have frequencies $\omega_0\pm\Omega$. In the large $\Omega$ limit we
observed that the jump-like behavior is similar to that observed in this system
with a Markovian (broad band) bath. This is expected as in the Markovian limit
the fluorescence spectrum for a strongly driven two level atom takes the form
of a Mollow triplet. However the length of time for which the Markovian-like
behaviour persists depends upon {\em which} jump-like unraveling is used.


title:
Degradation of continuous variable entanglement in a phase-sensitive
  environment

abstract:
  We address the propagation of twin-beam of radiation through Gaussian
phase-sensitive channels, i.e. noisy channels with squeezed fluctuations. We
find that squeezing the environment always reduces the survival time of
entanglement in comparison to the case of simple dissipation and thermal noise.
We also show that the survival time is further reduced if the squeezing phase
of the fluctuations is different from the twin-beam phase.


title: Erasure versus teleportation scheme of optical CNOT gate

abstract:
  We clarify the connections between the erasure scheme of probabilistic CNOT
gate implementation recently proposed by Pittman, Jacobs and Franson [Phys.
Rev. A 64, 062311 (2001)] and quantum teleportation.


title: The Semiclassical Regime of the Chaotic Quantum-Classical Transition

abstract:
  An analysis of the semiclassical regime of the quantum-classical transition
is given for open, bounded, one dimensional chaotic dynamical systems.
Environmental fluctuations -- characteristic of all realistic dynamical systems
-- suppress the development of fine structure in classical phase space and damp
nonlocal contributions to the semiclassical Wigner function which would
otherwise invalidate the approximation. This dual regularization of the
singular nature of the semiclassical limit is demonstrated by a numerical
investigation of the chaotic Duffing oscillator.


title: Quantising on a category

abstract:
  We review the problem of finding a general framework within which one can
construct quantum theories of non-standard models for space, or space-time. The
starting point is the observation that entities of this type can typically be
regarded as objects in a category whose arrows are structure-preserving maps.
This motivates investigating the general problem of quantising a system whose
`configuration space' (or history-theory analogue) is the set of objects
$\Ob\Q$ in a category $\Q$.
  We develop a scheme based on constructing an analogue of the group that is
used in the canonical quantisation of a system whose configuration space is a
manifold $Q\simeq G/H$, where $G$ and $H$ are Lie groups. In particular, we
choose as the analogue of $G$ the monoid of `arrow fields' on $\Q$. Physically,
this means that an arrow between two objects in the category is viewed as an
analogue of momentum. After finding the `category quantisation monoid', we show
how suitable representations can be constructed using a bundle (or, more
precisely, presheaf) of Hilbert spaces over $\Ob\Q$. For the example of a
category of finite sets, we construct an explicit representation structure of
this type.


title: Polarization-sensitive quantum-optical coherence tomography

abstract:
  We set forth a polarization-sensitive quantum-optical coherence tomography
(PS-QOCT) technique that provides axial optical sectioning with
polarization-sensitive capabilities. The technique provides a means for
determining information about the optical path length between isotropic
reflecting surfaces, the relative magnitude of the reflectance from each
interface, the birefringence of the interstitial material, and the orientation
of the optical axis of the sample. PS-QOCT is immune to sample dispersion and
therefore permits measurements to be made at depths greater than those
accessible via ordinary optical coherence tomography. We also provide a general
Jones matrix theory for analyzing PS-QOCT systems and outline an experimental
procedure for carrying out such measurements.


title: Bloch Equations and Completely Positive Maps

abstract:
  The phenomenological dissipation of the Bloch equations is reexamined in the
context of completely positive maps. Such maps occur if the dissipation arises
from a reduction of a unitary evolution of a system coupled to a reservoir. In
such a case the reduced dynamics for the system alone will always yield
completely positive maps of the density operator. We show that, for Markovian
Bloch maps, the requirement of complete positivity imposes some Bloch
inequalities on the phenomenological damping constants. For non-Markovian Bloch
maps some kind of Bloch inequalities involving eigenvalues of the damping basis
can be established as well. As an illustration of these general properties we
use the depolarizing channel with white and colored stochastic noise.


title: Realization of a General Three-Qubit Quantum Gate

abstract:
  We prove that a generic three-qubit quantum logic gate can be implemented
using at most 98 one-qubit rotations about the $y$- and $z$-axes and 40 CNOT
gates, beating an earlier bound of 64 CNOT gates.


title: This elusive objective existence

abstract:
  Zurek's existential interpretation of quantum mechanics suffers from three
classical prejudices, including the belief that space and time are
intrinsically and infinitely differentiated. They compel him to relativize the
concept of objective existence in two ways. The elimination of these prejudices
makes it possible to recognize the quantum formalism's ontological implications
- the relative and contingent reality of spatiotemporal distinctions and the
extrinsic and finite spatiotemporal differentiation of the physical world -
which in turn makes it possible to arrive at an unqualified objective
existence. Contrary to a widespread misconception, viewing the quantum
formalism as being fundamentally a probability algorithm does not imply that
quantum mechanics is concerned with states of knowledge rather than states of
Nature. On the contrary, it makes possible a complete and strongly objective
description of the physical world that requires no reference to observers. What
objectively exists, in a sense that requires no qualification, is the
trajectories of macroscopic objects, whose fuzziness is empirically irrelevant,
the properties and values of whose possession these trajectories provide
indelible records, and the fuzzy and temporally undifferentiated states of
affairs that obtain between measurements and are described by counterfactual
probability assignments.


title: Probabilities from envariance?

abstract:
  Zurek claims to have derived Born's rule noncircularly in the context of an
ontological no-collapse interpretation of quantum states, without any "deus ex
machina imposition of the symptoms of classicality." After a brief review of
Zurek's derivation it is argued that this claim is exaggerated if not wholly
unjustified. In order to demonstrate that Born's rule arises noncircularly from
deterministically evolving quantum states, it is not sufficient to assume that
quantum states are somehow associated with probabilities and then prove that
these probabilities are given by Born's rule. One has to show how irreducible
probabilities can arise in the context of an ontological no-collapse
interpretation of quantum states. It is argued that the reason why all attempts
to do this have so far failed is that quantum states are fundamentally
algorithms for computing correlations between possible measurement outcomes,
rather than evolving ontological states.


title:
Signatures of wave packet revival phenomena in the expectation values of
  observables

abstract:
  Wave packet revivals and fractional revivals are striking quantum
interference phenomena that can occur under suitable conditions in a system
with a nonlinear spectrum. In the framework of a specific model (the
propagation of an initially coherent wave packet in a Kerr-like medium), it is
shown that distinctive signatures of these revivals and fractional revivals are
displayed by the time evolution of the expectation values of physical
observables and their powers, i.e., by experimentally measurable quantities.
Moreover, different fractional revivals can be selectively identified by
examining appropriate higher moments.


title: Exciton entanglement in two coupled semiconductor microcrystallites

abstract:
  Entanglement of the excitonic states in the system of two coupled
semiconductor microcrystallites, whose sizes are much larger than the Bohr
radius of exciton in bulk semiconductor but smaller than the relevant optical
wavelength, is quantified in terms of the entropy of entanglement. It is
observed that the nonlinear interaction between excitons increases the maximum
values of the entropy of the entanglement more than that of the linear coupling
model. Therefore, a system of two coupled microcrystallites can be used as a
good source of entanglement with fixed exciton number. The relationship between
the entropy of the entanglement and the population imbalance of two
microcrystallites is numerically shown and the uppermost envelope function for
them is estimated by applying the Jaynes principle.


title: Triggered qutrits for Quantum Communication protocols

abstract:
  A general protocol in Quantum Information and Communication relies in the
ability of producing, transmitting and reconstructing, in general, qunits. In
this letter we show for the first time the experimental implementation of these
three basic steps on a pure state in a three dimensional space, by means of the
orbital angular momentum of the photons. The reconstruction of the qutrit is
performed with tomographic techniques and a Maximum-Likelihood estimation
method. In this way we also demonstrate that we can perform any transformation
in the three dimensional space.


title:
Estimating mixing properties of local Hamiltonian dynamics and
  continuous quantum random walks is PSPACE-hard

abstract:
  A major topic of (classical) ergodic theory is to examine qualitatively how
the phase space of dynamical systems is penetrated by the orbits of their
dynamics. We consider interacting qubit systems with dynamics according to
4-local Hamiltonians and continuous quantum random walks. For these systems one
could use the von Neumann entropy of the time-average to characterize the
mixing properties of the corresponding orbits, i.e., what portion of the state
space and how uniformly it is filled out by the orbits. We show that the
problem of estimating this entropy is PSPACE-hard.


title: Scaling Ion Trap Quantum Computation through Fast Quantum Gates

abstract:
  We propose a method to achieve scalable quantum computation based on fast
quantum gates on an array of trapped ions, without the requirement of ion
shuttling. Conditional quantum gates are obtained for any neighboring ions
through spin-dependent acceleration of the ions from periodic photon kicks. The
gates are shown to be robust to influence of all the other ions in the array
and insensitive to the ions' temperature.


title: Testing Quantum Dynamics using Signaling

abstract:
  We consider a physical system in which the description of states and
measurements follow the usual quantum mechanical rules. We also assume that the
dynamics is linear, but may not be fully quantum (i.e unitary). We show that in
such a physical system, certain complementary evolutions, namely cloning and
deleting operations that give a better fidelity than quantum mechanically
allowed ones, in one (inaccessible) region, lead to signaling to a far-apart
(accessible) region. To show such signaling, one requires certain two-party
quantum correlated states shared between the two regions. Subsequent
measurements are performed only in the accessible part to detect such
phenomenon.


title: Quantum Estimation by Local Observables

abstract:
  Quantum estimation theory provides optimal observations for various
estimation problems for unknown parameters in the state of the system under
investigation. However, the theory has been developed under the assumption that
every observable is available for experimenters. Here, we generalize the theory
to problems in which the experimenter can use only locally accessible
observables. For such problems, we establish a Cram{\'e}r-Rao type inequality
by obtaining an explicit form of the Fisher information as a reciprocal lower
bound for the mean square errors of estimations by locally accessible
observables. Furthermore, we explore various local quantum estimation problems
for composite systems, where non-trivial combinatorics is needed for obtaining
the Fisher information.


title:
Vacuum fluctuations and moving atoms/detectors: From Casimir-Polder to
  Unruh effect

abstract:
  In this note we report on some new results \cite{SHP} on corrections to the
Casimir-Polder \cite{caspol} retardation force due to atomic motion and present
a preliminary (unpublished) critique on one recently proposed cavity QED
detection scheme of Unruh effect \cite{Unr76}. These two well-known effects
arise from the interaction between a moving atom or detector with a quantum
field under some boundary conditions introduced by a conducting mirror/cavity
or dielectric wall. The Casimir-Polder force is a retardation force on the atom
due to the dressing of the atomic ground state by the vacuum electromagnetic
field in the presence of a conducting mirror or dielectric wall. We have
recently provided an improved calculation by treating the mutual influence of
the atom and the (constrained) field in a self-consistent way. For an atom
moving adiabatically, perpendicular to a mirror, our result finds a coherent
retardation correction up to twice the stationary value. Unruh effect refers
loosely to the fact that a uniformly accelerated detector feels hot. Two prior
schemes have been proposed for the detection of `Unruh radiation', based on
charged particles in linear accelerators and storage rings. Here we are
interested in a third scheme proposed recently by Scully {\it et al}
\cite{Scully03} involving the injection of accelerated atoms into a microwave
or optical cavity. We analyze two main factors instrumental to the purported
success in this scheme, the cavity factor and the sudden switch-on factor. We
conclude that the effects engendered from these factors are unrelated to the
Unruh effect.


title: Bell's Theorem : The Naive View of an Experimentalist

abstract:
  In the first part of this presentation (sections 2 to 6), I show that Bell's
Inequalities provide a quantitative criterion to test "reasonable"
Supplementary Parameters Theories versus Quantum Mechanics. Following Bell, I
first explain the motivations for considering supplementary parameters
theories: the argument is based on an analysis of the famous
Einstein-Podolsky-Rosen (EPR) Gedankenexperiment . Introducing a reasonable
Locality Condition, we will then derive Bell's theorem, which states: (i) that
Local Supplementary Parameters Theories are constrained by Bell's Inequalities;
(ii) that certain predictions of Quantum Mechanics violate Bell's Inequalities,
and therefore that Quantum Mechanics is incompatible with Local Supplementary
Parameters Theories. I then point out that a fundamental assumption for this
conflict is the Locality assumption, and I show that in a more sophisticated
version of the E.P.R. thought experiment ("timing experiment"), the Locality
Condition may be considered a consequence of Einstein's Causality, preventing
faster-than-light interactions. The purpose of this first part is to convince
the reader that the formalism leading to Bell's Inequalities is very general
and reasonable. What is surprising is that such a reasonable formalism
conflicts with Quantum Mechanics. In fact, situations exhibiting a conflict are
very rare, and Quantum Optics is the domain where the most significant tests of
this conflict have been carried out, as presented in sections 7 to 11.


title:
Moving Charges, Detectors, and Mirrors in a Quantum Field with
  Backreaction

abstract:
  This is a progress report on our current work on moving charges, detectors,
and moving mirrors in a quantum field treated in a fully relativistic way via
the Feynman-Vernon influence functional method, which preserves maximal quantum
coherence of the system with self-consistent back-reaction from the field.


title: Berry and Pancharatnam Topological Phases of Atomic and Optical Systems

abstract:
  Theoretical and experimental studies of Berry and Pancharatnam phases are
reviewed. Basic elements of differential geometry are presented for
understanding the topological nature of these phases. The basic theory analyzed
by Berry in relation to magnetic monopoles is presented. The theory is
generalized to nonadiabatic processes and to noncyclic Pancharatnam phases.
Different systems are discussed including polarization optics, n-level atomic
systems, neutron interferometry and molecular topological phases.


title:
Dynamics and manipulation of entanglement in coupled harmonic systems
  with many degrees of freedom

abstract:
  We study the entanglement dynamics of a system consisting of a large number
of coupled harmonic oscillators in various configurations and for different
types of nearest neighbour interactions. For a one-dimensional chain we provide
compact analytical solutions and approximations to the dynamical evolution of
the entanglement between spatially separated oscillators. Key properties such
as the speed of entanglement propagation, the maximum amount of transferred
entanglement and the efficiency for the entanglement transfer are computed. For
harmonic oscillators coupled by springs, corresponding to a phonon model, we
observe a non-monotonic transfer efficiency in the initially prepared amount of
entanglement, i.e., an intermediate amount of initial entanglement is
transferred with the highest efficiency. In contrast, within the framework of
the rotating wave approximation (as appropriate e.g. in quantum optical
settings) one finds a monotonic behaviour. We also study geometrical
configurations that are analogous to quantum optical devices (such as
beamsplitters and interferometers) and observe characteristic differences when
initially thermal or squeezed states are entering these devices. We show that
these devices may be switched on and off by changing the properties of an
individual oscillator. They may therefore be used as building blocks of large
fixed and pre-fabricated but programmable structures in which quantum
information is manipulated through propagation. We discuss briefly possible
experimental realisations of systems of interacting harmonic oscillators in
which these effects may be confirmed experimentally.


title: Optical quantum computation using cluster states

abstract:
  We propose an approach to optical quantum computation in which a
deterministic entangling quantum gate may be performed using, on average, a few
hundred coherently interacting optical elements (beamsplitters, phase shifters,
single photon sources, and photodetectors with feedforward). This scheme
combines ideas from the optical quantum computing proposal of Knill, Laflamme
and Milburn [Nature 409 (6816), 46 (2001)], and the abstract cluster-state
model of quantum computation proposed by Raussendorf and Briegel [Phys. Rev.
Lett. 86, 5188 (2001)].


abstract:
  Applicability of Rydberg atoms to quantum computers is examined from
experimental point of view. In many theoretical proposals appeared recently,
excitation of atoms into highly excited Rydberg states was considered as a way
to achieve quantum entanglement in cold atomic ensembles via dipole-dipole
interaction that could be strong for Rydberg atoms. Appropriate conditions to
realize a conditional quantum phase gate have been analyzed. We also present
the results of modeling experiments on microwave spectroscopy of single- and
multi-atom excitations at the one-photon 37S-37P and two-photon 37S-38S
transitions in an ensemble of a few sodium Rydberg atoms. The microwave spectra
were investigated for various final states of the ensemble initially prepared
in its ground state. The quantum NOT operation with single atoms was found to
be affected by the Doppler effect and fluctuations of the microwave field. The
spectrum of full excitation of several Rydberg atoms was much narrower than
that of a single atom. This effect might be useful for the high-resolution
spectroscopy. The results may be also applied to the studies on collective
laser excitation of ground-state atoms aiming to realize quantum gates.


title:
Characterization of maximally entangled two-qubit states via the
  Bell-Clauser-Horne-Shimony-Holt inequality

abstract:
  Maximally entangled states should maximally violate the Bell inequality. In
this paper, it is proved that all two-qubit states that maximally violate the
Bell-Clauser-Horne-Shimony-Holt inequality are exactly Bell states and the
states obtained from them by local unitary transformations. The proof is
obtained by using the certain algebraic properties that Pauli's matrices
satisfy. The argument is extended to the three-qubit system. Since all states
obtained by local unitary transformations of a maximally entangled state are
equally valid entangled states, we thus give the characterizations of maximally
entangled states in both the two-qubit and three-qubit systems in terms of the
Bell inequality.


title: Real Observers and the Holographic Principle

abstract:
  The holographic principle asserts that the observable number of degrees of
freedom inside a volume is proportional not to the volume, but to the surface
area bounding the volume. There is currently a need to explain the principle in
terms of a more fundamental microscopic theory. This paper suggests a potential
explanation. This paper suggests that in general, for an observer to observe
the r coordinate of an event, the process of making that observation must
generate at least as much entropy as the information that the observation
gains. Following on from that, this paper sets out a simple argument that leads
to the result that observers on the surface of a sphere can observe an amount
of information about the enclosed system that is no more than an amount that is
proportional to the surface area of the sphere.


title: Transverse confinement in stochastic cooling of trapped atoms

abstract:
  Stochastic cooling of trapped atoms is considered for a laser-beam
configuration with beam waists equal or smaller than the extent of the atomic
cloud. It is shown, that various effects appear due to this transverse
confinement, among them heating of transverse kinetic energy. Analytical
results of the cooling in dependence on size and location of the laser beam are
presented for the case of a non-degenerate vapour.


title:
Quantum computing and information extraction for a dynamical quantum
  system

abstract:
  We discuss the simulation of a complex dynamical system, the so-called
quantum sawtooth map model, on a quantum computer. We show that a quantum
computer can be used to efficiently extract relevant physical information for
this model. It is possible to simulate the dynamical localization of classical
chaos and extract the localization length of the system with quadratic speed up
with respect to any known classical computation. We can also compute with
algebraic speed up the diffusion coefficient and the diffusion exponent both in
the regimes of Brownian and anomalous diffusion. Finally, we show that it is
possible to extract the fidelity of the quantum motion, which measures the
stability of the system under perturbations, with exponential speed up.


title: Weak Quantum Theory an the Emergence of Time

abstract:
  We present a scenario, how time could emerge in the framework of Weak Quantum
Theory. In a process, similar to the emergence of time in quantum cosmology,
time arises after an epistemic split of the unus mundus as a quality of the
individual conscious mind. Synchronization with matter and other mental systems
is achieved by entanglement correlations. In the course of its
operationalization, time loses its original quality of A-time and the B-time of
physics as measured by clocks will appear.


title:
Thermodynamics of quantum informational systems - Hamiltonian
  description

abstract:
  It is often claimed, that from a quantum system of d levels, and entropy S
and heat bath of temperature T one can draw kT(ln d -S) amount of work.
However, the usual arguments based on Szilard engine are not fully rigorous.
Here we prove the formula within Hamiltonian description of drawing work from a
quantum system and a heat bath, at a cost of entropy of the system. We base on
the derivation of thermodynamical laws and quantities in [R. Alicki, J. Phys.
A, 12, L103 (1979)] within a weak coupling limit. Our result provides fully
physical scenario for extracting thermodynamical work from quantum correlations
[J. Oppenheim et al. Phys. Rev. Lett. 89, 180402 (2002)]. We also derive
Landauer principle as a consquence of second law within the considered model.


title:
Selfsynchronization and dissipation-induced threshold in collective
  atomic recoil lasing

abstract:
  Networks of globally coupled oscillators exhibit phase transitions from
incoherent to coherent states. Atoms interacting with the counterpropagating
modes of a unidirectionally pumped high-finesse ring cavity form such a
globally coupled network. The coupling mechanism is provided by collective
atomic recoil lasing (CARL), i.e. cooperative Bragg scattering of laser light
at an atomic density grating, which is self-induced by the laser light. Under
the rule of an additional friction force, the atomic ensemble is expected to
undergo a phase transition to a state of synchronized atomic motion. We present
the experimental investigation of this phase transition by studying the
threshold behavior of the CARL process.


title: The logic of entanglement

abstract:
  We expose the information flow capabilities of pure bipartite entanglement as
a theorem -- which embodies the exact statement on the `seemingly acausal flow
of information' in protocols such as teleportation. We use this theorem to
re-design and analyze known protocols (e.g. logic gate teleportation and
entanglement swapping) and show how to produce some new ones (e.g. parallel
composition of logic gates). We also show how our results extend to the
multipartite case and how they indicate that entanglement can be measured in
terms of `information flow capabilities'. Ultimately, we propose a scheme for
automated design of protocols involving measurements, local unitary
transformations and classical communication.


title: Facts, Values and Quanta

abstract:
  Quantum mechanics is a fundamentally probabilistic theory (at least so far as
the empirical predictions are concerned). It follows that, if one wants to
properly understand quantum mechanics, it is essential to clearly understand
the meaning of probability statements. The interpretation of probability has
excited nearly as much philosophical controversy as the interpretation of
quantum mechanics. 20th century physicists have mostly adopted a frequentist
conception. In this paper it is argued that we ought, instead, to adopt a
logical or Bayesian conception. The paper includes a comparison of the orthodox
and Bayesian theories of statistical inference. It concludes with a few remarks
concerning the implications for the concept of physical reality.


title: Entanglement and bifurcations in Jahn-Teller models

abstract:
  We compare and contrast the entanglement in the ground state of two
Jahn-Teller models. The $E\otimes\beta$ system models the coupling of a
two-level electronic system, or qubit, to a single oscillator mode, while the
$E\otimes\epsilon$ models the qubit coupled to two independent, degenerate
oscillator modes. In the absence of a transverse magnetic field applied to the
qubit, both systems exhibit a degenerate ground state. Whereas there always
exists a completely separable ground state in the $E\otimes\beta$ system, the
ground states of the $E\otimes\epsilon$ model always exhibit entanglement. For
the $E\otimes\beta$ case we aim to clarify results from previous work, alluding
to a link between the ground state entanglement characteristics and a
bifurcation of a fixed point in the classical analogue. In the
$E\otimes\epsilon$ case we make use of an ansatz for the ground state. We
compare this ansatz to exact numerical calculations and use it to investigate
how the entanglement is shared between the three system degrees of freedom.


title: A practical scheme for error control using feedback

abstract:
  We describe a scheme for quantum error correction that employs feedback and
weak measurement rather than the standard tools of projective measurement and
fast controlled unitary gates. The advantage of this scheme over previous
protocols (for example Ahn et. al, PRA, 65, 042301 (2001)), is that it requires
little side processing while remaining robust to measurement inefficiency, and
is therefore considerably more practical. We evaluate the performance of our
scheme by simulating the correction of bit-flips. We also consider
implementation in a solid-state quantum computation architecture and estimate
the maximal error rate which could be corrected with current technology.


title:
Post-processing with linear optics for improving the quality of
  single-photon sources

abstract:
  Triggered single-photon sources produce the vacuum state with non-negligible
probability, but produce a much smaller multiphoton component. It is therefore
reasonable to approximate the output of these photon sources as a mixture of
the vacuum and single-photon states. We show that it is impossible to increase
the probability for a single photon using linear optics and photodetection on
fewer than four modes. This impossibility is due to the incoherence of the
inputs; if the inputs were pure-state superpositions, it would be possible to
obtain a perfect single-photon output. In the more general case, a chain of
beam splitters can be used to increase the probability for a single photon, but
at the expense of adding an additional multiphoton component. This improvement
is robust against detector inefficiencies, but is degraded by dark counts or
multiphoton components in the input.


title:
A scheme of measurement of quantum-vacuum geometric phases in the
  noncoplanar fibre system

abstract:
  We study the quantum-vacuum geometric phases resulting from the vacuum
fluctuation of photon fields in Tomita-Chiao-Wu noncoplanar curved fibre
system, and suggest a scheme to test the potential existence of such vacuum
effect. Since the signs of the quantum-vacuum geometric phases of left- and
right- handed (LRH) circularly polarized light are just opposite, the total
geometric phases at vacuum level is inescapably absent in the fibre experiments
performed previously by other authors. By using the present approach where the
fibre made of gyroelectric media is employed, the quantum-vacuum geometric
phases of LRH light cannot be exactly cancelled, and may therefore be achieved
test experimentally.


title: Holographic generation of micro-trap arrays for single atoms

abstract:
  We have generated multiple micron-sized optical dipole traps for neutral
atoms using holographic techniques with a programmable liquid crystal spatial
light modulator. The setup allows the storing of a single atom per trap, and
the addressing and manipulation of individual trapping sites.


title:
Phase Space Formulation of Quantum Mechanics. Insight into the
  Measurement Problem

abstract:
  A phase space mathematical formulation of quantum mechanical processes
accompanied by and ontological interpretation is presented in an axiomatic
form. The problem of quantum measurement, including that of quantum state
filtering, is treated in detail. Unlike standard quantum theory both quantum
and classical measuring device can be accommodated by the present approach to
solve the quantum measurement problem


title: Comment on 'Eavesdropping on the ping-pong communication protocol'

abstract: This is a comment on PRL90(03)157901 by Antoni Wojcik (quant-ph/0211199)


title: Violation of Bell inequality and entanglement of decaying Werner states

abstract:
  Bell-inequality violation and entanglement, measured by Wootters' concurrence
and negativity, of two qubits initially in Werner or Werner-like states coupled
to thermal reservoirs are analyzed within the master equation approach. It is
shown how this simple decoherence process leads to generation of states
manifesting the relativity of two-qubit entanglement measures.


title: Dissipation in systems of linear and nonlinear quantum scissors

abstract:
  We analyze truncation of coherent states up to a single-photon Fock state by
applying linear quantum scissors, utilizing the projection synthesis in a
linear optical system, and nonlinear quantum scissors, implemented by
periodically driven cavity with a Kerr medium. Dissipation effects on optical
truncation are studied in the Langevin and master equation approaches. Formulas
for the fidelity of lossy quantum scissors are found.


title:
Decoherence of two maximally entangled qubits in a lossy nonlinear
  cavity

abstract:
  Decoherence effect on quantum entanglement of two optical qubits in a lossy
cavity interacting with a nonlinear medium (Kerr nonlinearity) is analyzed. The
qubits are assumed to be initially in the maximally entangled states (Bell or
Bell-like states) or the maximally entangled mixed states, on the example of
Werner and Werner-like states. Two kinds of measures of the entanglement are
considered: the concurrence to describe a decay of the entanglement of
formation of the qubits, and the negativity to determine a decay of the
entanglement cost under positive-partial-transpose-preserving operations. It is
observed that the Kerr nonlinearity, in the discussed decoherence model, does
not affect the entanglement of the qubits initially in the Bell or Werner
states, although the evolution of the qubits can depend on this nonlinearity
explicitly. However, it is shown that for the initial Bell-like state and the
corresponding Werner-like state, the loss of the entanglement can be
periodically reduced by inserting the Kerr nonlinearity in the lossy cavity.
Moreover, the relativity of the entanglement measures is demonstrated, to our
knowledge for the first time, as a result of a physical process.


title: The C Operator in PT-Symmetric Quantum Theories

abstract:
  The Hamiltonian H specifies the energy levels and the time evolution of a
quantum theory. It is an axiom of quantum mechanics that H be Hermitian because
Hermiticity guarantees that the energy spectrum is real and that the time
evolution is unitary (probability preserving). This paper investigates an
alternative way to construct quantum theories in which the conventional
requirement of Hermiticity (combined transpose and complex conjugate) is
replaced by the more physically transparent condition of space-time reflection
(PT) symmetry. It is shown that if the PT symmetry of a Hamiltonian H is not
broken, then the spectrum of H is real. Examples of PT-symmetric non-Hermitian
quantum-mechanical Hamiltonians are H=p^2+ix^3 and H=p^2-x^4. The crucial
question is whether PT-symmetric Hamiltonians specify physically acceptable
quantum theories in which the norms of states are positive and the time
evolution is unitary. The answer is that a Hamiltonian that has an unbroken PT
symmetry also possesses a physical symmetry represented by a linear operator
called C. Using C it is shown how to construct an inner product whose
associated norm is positive definite. The result is a new class of fully
consistent complex quantum theories. Observables are defined, probabilities are
positive, and the dynamics is governed by unitary time evolution. After a
review of PT-symmetric quantum mechanics, new results are presented here in
which the C operator is calculated perturbatively in quantum mechanical
theories having several degrees of freedom.


title: The general boson normal ordering problem

abstract:
  We solve the boson normal ordering problem for F[(a*)^r a^s], with r,s
positive integers, where a* and a are boson creation and annihilation operators
satisfying [a,a*]=1. That is, we provide exact and explicit expressions for the
normal form wherein all a's are to the right. The solution involves integer
sequences of numbers which are generalizations of the conventional Bell and
Stirling numbers whose values they assume for r=s=1. A comprehensive theory of
such generalized combinatorial numbers is given including closed-form
expressions (extended Dobinski-type formulas)and generating functions. These
last are special expectation values in boson coherent states.


title: External Time-Varying Fields and Electron Coherence

abstract:
  The effect of time-varying electromagnetic fields on electron coherence is
investigated. A sinusoidal electromagnetic field produces a time varying
Aharonov-Bohm phase. In a measurement of the interference pattern which
averages over this phase, the effect is a loss of contrast. This is effectively
a form of decoherence. We calculate the magnitude of this effect for various
electromagnetic field configurations. The result seems to be sufficiently large
to be observable.


title:
Spontaneous emission of homogeneous broadening molecules on a
  micro-droplet's surface and local-field correction

abstract:
  We consider the spontaneous emission of a broadening molecule on the surface
of a micro-sphere in this paper. The density of states for the micro-cavity is
derived from quasi-normal models(QNM's) expansion of the correlation functions
of electromagnetic fields. Through detailed analysis we show that only weak
coupling between a broadening atom(molecule) and the electromagnetic fields
exists in a dielectric sphere cavity whether the sphere is small or big. From
these results we find the explicit expression of the spontaneous emission decay
rate for a surfactant broadening molecule on the surface of a micro-droplet
with radius $a$, in which only $1/a$ and $1/a^{2}$ components exhibit. Then we
apply this expression to a real experiment and obtain a consistent result with
the experiment. We also show that the real-cavity model of local field
correction is accurate, and reveal that the local-field correction factor can
be measured precisely and easily by fluorescence experiments of surfactant
molecules. Moreover, the spontaneous decay of a surfactant molecular on
droplet's surface is sensitive to the atomic broadening, so that the
fluorescence experiment in a micro-sphere cavity can be used to estimate the
radiative broadening.


title: Relativistic scattering with a singular potential in the Dirac equation

abstract:
  An elementary treatment of the Dirac equation in the presence of a three
dimensional spherically symmetric delta potential is presented. We show how to
calculate the cross section using the relativistic wave expansion method for a
one delta potential and two concentric delta potentials. We compare our results
with the cross section calculated in the Born approximation.


title:
Generation of atom-photon entangled states in atomic Bose-Einstein
  condensate via electromagnetically induced transparency

abstract:
  In this paper, we present a method to generate continuous-variable-type
entangled states between photons and atoms in atomic Bose-Einstein condensate
(BEC). The proposed method involves an atomic BEC with three internal states, a
weak quantized probe laser and a strong classical coupling laser, which form a
three-level Lambda-shaped BEC system. We consider a situation where the BEC is
in electromagnetically induced transparency (EIT) with the coupling laser being
much stronger than the probe laser. In this case, the upper and intermediate
levels are unpopulated, so that their adiabatic elimination enables an
effective two-mode model involving only the atomic field at the lowest internal
level and the quantized probe laser field. Atom-photon quantum entanglement is
created through laser-atom and inter-atomic interactions, and two-photon
detuning. We show how to generate atom-photon entangled coherent states and
entangled states between photon (atom) coherent states and atom-(photon-)
macroscopic quantum superposition (MQS) states, and between photon-MQS and
atom-MQS states.


title:
Bell Measurement and Local Measurement in the Modified Lo-Chau Quantum
  Key Distribution Protocol

abstract:
  We clarify the argument on the how (nonlocal) degenerate Bell measurement can
be replaced by local measurements in the modified Lo-Chau quantum key
distribution protocol. Discussing security criterion for users, we describe how
eavesdropper's refined information on the final state is not helpful. We argue
that current discussions on the equivalence of the Bell and the local
measurements are not clear. We show how the problem of equivalence can be
resolved using the fact that eavesdropper's refined information is not helpful
for her.


title: Quantum Theory of Fiber Bragg Grating Solitons

abstract:
  Following the pioneering work of Prof. Hermann A. Haus, a general quantum
theory for bi-directional nonlinear optical pulse propagation problems is
developed and applied to study the quantum properties of fiber Bragg grating
solitons. Fiber Bragg grating solitons are found to be automatically amplitude
squeezed after passing through the grating and the squeezing ratio saturates
after a certain grating length. The optimal squeezing ratio occurs when the
pulse energy is slightly above the fundamental soliton energy. One can also
compress the soliton pulsewidth and enhance the squeezing simultaneously by
using an apodized grating, as long as the solitons evolve adiabatically.


title:
The Correlations of Greenberger-Horne-Zeilinger States Described by
  Hilbert-Schmidt decomposition

abstract:
  Using simple quantum analysis we describe the correlations of
Greenberger-Horne-Zeilinger (GHZ) states by the use of Hilbert-Schmidt (HS)
representation. Our conclusion is that while these states disprove
local-realism they do not prove any nonlocality property.


title: p-Mechanics and Field Theory

abstract:
  The orbit method of Kirillov is used to derive the p-mechanical brackets
[math-ph/0007030, quant-ph/0212101]. They generate the quantum (Moyal) and
classic (Poisson) brackets on respective orbits corresponding to
representations of the Heisenberg group. The extension of p-mechanics to field
theory is made through the De Donder--Weyl Hamiltonian formulation. The
principal step is the substitution of the Heisenberg group with Galilean.
  Keywords: Classic and quantum mechanics, Moyal brackets, Poisson brackets,
commutator, Heisenberg group, orbit method, deformation quantisation,
representation theory, De Donder--Weyl field theory, Galilean group, Clifford
algebra, conformal M\"obius transformation, Dirac operator.


title: Entanglement induced by nonadiabatic chaos

abstract:
  We investigate entanglement between electronic and nuclear degrees of freedom
for a model nonadiabatic system. We find that entanglement (measured by the von
Neumann entropy of the subsystem for the eigenstates) is large in a statistical
sense when the system shows ``nonadiabatic chaos'' behavior which was found in
our previous work [Phys. Rev. E {\bf 63}, 066221 (2001)]. We also discuss
non-statistical behavior of the eigenstates for the regular cases.


title: Proposed magneto-electrostatic ring trap for neutral atoms

abstract:
  We propose a novel trap for confining cold neutral atoms in a microscopic
ring using a magneto-electrostatic potential. The trapping potential is derived
from a combination of a repulsive magnetic field from a hard drive atom mirror
and the attractive potential produced by a charged disk patterned on the hard
drive surface. We calculate a trap frequency of [29.7, 42.6, 62.8] kHz and a
depth of [16.1, 21.8, 21.8] MHz for [133Cs, 87Rb, 40K], and discuss a simple
loading scheme and a method for fabrication. This device provides a
one-dimensional potential in a ring geometry that may be of interest to the
study of trapped quantum degenerate one-dimensional gases.


title:
Quantum advantage does not survive in the presence of a corrupt source:
  Optimal strategies in simultaneous move games

abstract:
  Effects of a corrupt source on the dynamics of simultaneous move strategic
games are analyzed both for classical and quantum settings. The corruption rate
dependent changes in the payoffs and strategies of the players are observed. It
is shown that there is a critical corruption rate at which the players lose
their quantum advantage, and that the classical strategies are more robust to
the corruption in the source. Moreover, it is understood that the information
on the corruption rate of the source may help the players choose their optimal
strategy for resolving the dilemma and increase their payoffs. The study is
carried out in two different corruption scenarios for Prisoner's Dilemma,
Samaritan's Dilemma, and Battle of Sexes.


title:
Two-Mode Squeezed States and Their Superposition in the Motion of Two
  Trapped Ions

abstract:
  We propose a method to create two-mode squeezed states and their
superposition in the center-of-mass mode and breathing mode of two-trapped
ions. Each ion is illuminated simultaneously by two standing waves. One of the
fields is tuned to excite resonantly and simultaneously both upper sidebands of
the two normal modes, while the other field tuned to the corresponding lower
sidebands.


title:
Experimental demonstration of entanglement assisted coding using a
  two-mode squeezed vacuum state

abstract:
  We have experimentally realized the scheme initially proposed as quantum
dense coding with continuous variables [Ban, J. Opt. B \textbf{1}, L9 (1999),
and Braunstein and Kimble, \pra\textbf{61}, 042302 (2000)]. In our experiment,
a pair of EPR (Einstein-Podolski-Rosen) beams is generated from two independent
squeezed vacua. After adding two-quadrature signal to one of the EPR beams, two
squeezed beams that contain the signal were recovered. Although our squeezing
level is not sufficient to demonstrate the channel capacity gain over the
Holevo limit of a single-mode channel without entanglement, our channel is
superior to conventional channels such as coherent and squeezing channels. In
addition, optical addition and subtraction processes demonstrated are
elementary operations of universal quantum information processing on continuous
variables.


title:
Nonlinear optical properties of electromagnetically-induced-transparency
  medium interacting with two quantized fields

abstract:
  We study linear and nonlinear optical properties of electromagnetically
induced transparency (EIT) medium interacting with two quantized laser fields
for adiabatic EIT case. We show that EIT medium exhibits normal dispersion.
Kerr and higher order nonlinear refractive-index coefficients are also
calculated in a completely analytical form. It is indicated that EIT medium
exhibits giant resonantly enhanced nonlinearities. We discuss the response of
the EIT medium to nonclassical light fields and find that the polarization
vanishes when the probe laser is initially in a nonclassical state of no
single-photon coherence.


title: Observation of Antinormally Ordered Hanbury-Brown--Twiss Correlations

abstract:
  We have measured antinormally ordered Hanbury-Brown--Twiss correlations for
coherent states of electromagnetic field by using stimulated parametric
down-conversion process. Photons were detected by stimulated emission, rather
than by absorption, so that the detection responded not only to actual photons
but also to zero-point fluctuations via spontaneous emission. The observed
correlations were distinct from normally ordered ones as they showed excess
positive correlations, i.e., photon bunching effects, which arose from the
thermal nature of zero-point fluctuations.


title: Antinormally Ordered Photodetection of Continuous-mode Field

abstract:
  When the electromagnetic field is detected by stimulated emission, rather
than by absorption, antinormally ordered photodetection can be realized. One of
the distinct features of this photodetection scheme is its sensitivity to
zero-point fluctuation due to the existence of the spontaneous emission. We
have recently succeeded in experimentally demonstrating the antinormally
ordered photodetection by exploiting nondegenerate stimulated parametric
down-conversion process. To properly account for the experiment, the detection
process needs to be treated with time-dependent and continuous-mode operators
because of the broadband nature of the parametric down-conversion process and
the wide spectrum of the pump that we used. Here, we theoretically analyze the
antinormally ordered intensity correlation of the continuous-mode fields by
pursuing the detection process in the Heisenberg picture. It is shown that the
excess positive correlation due to zero-point fluctuation reduces because of
the frequency-distinguishability of the two emitted photon pairs.


title: Quantum Measurements and Information Restrictions in Algebraic QM

abstract:
  It's argued that Information-Theoretical restrictions for the systems
selfdescription are important for Quantum Measurement Problem. They are
described by O information system restricted states R formalism by Breuer and
can be obtained also in Algebraic QM considering Segal Algebra of O
observables. From Segal theorem it's shown that R describes the random
measurement outcomes in the individual events.


title: A three state invariant

abstract:
  For triples of probability measures, pure quantum states and mixed quantum
states we obtain the exact constraints on the fidelities of pairs in the
sequence. We show that it is impossible to decide between a quantum model,
either pure or mixed, and a classical model on the basis of the fidelities
alone. Next, we introduce a quantum three state invariant called phase and show
that any sequence of pure quantum states is uniquely reconstructible given the
fidelities and phases.


title: Note on the Dirac Field in Real Domain

abstract:
  Sufficient conditions for linear electrodynamics with real Dirac field are
derived.


title: Holism, Physical Theories and Quantum Mechanics

abstract:
  Motivated by the question what it is that makes quantum mechanics a holistic
theory (if so), I try to define for general physical theories what we mean by
`holism'. For this purpose I propose an epistemological criterion to decide
whether or not a physical theory is holistic, namely: a physical theory is
holistic if and only if it is impossible in principle to infer the global
properties, as assigned in the theory, by local resources available to an
agent. I propose that these resources include at least all local operations and
classical communication. This approach is contrasted with the well-known
approaches to holism in terms of supervenience. The criterion for holism
proposed here involves a shift in emphasis from ontology to epistemology. I
apply this epistemological criterion to classical physics and Bohmian mechanics
as represented on a phase and configuration space respectively, and for quantum
mechanics (in the orthodox interpretation) using the formalism of general
quantum operations as completely positive trace non-increasing maps.
Furthermore, I provide an interesting example from which one can conclude that
quantum mechanics is holistic in the above mentioned sense, although, perhaps
surprisingly, no entanglement is needed.


title: A Bell-type test of energy-time entangled qutrits

abstract:
  We have performed a Bell-type test for energy-time entangled qutrits. A
method of inferring the Bell violation in terms of an associated interference
visibility is derived. Using this scheme we obtained a Bell value of $2.784 \pm
0.023$, representing a violation of $34 \sigma$ above the limit for local
variables. The scheme has been developed for use at telecom wavelengths and
using proven long distance quantum communication architecture to optimize the
utility of this high dimensional entanglement resource.


title: Quasi dynamical symmetry in an interacting boson model phase transition

abstract:
  The oft-observed persistence of symmetry properties in the face of strong
symmetry-breaking interactions is examined in the SO(5)-invariant interacting
boson model. This model exhibits a transition between two phases associated
with U(5) and O(6) symmetries, respectively, as the value of a control
parameter progresses from 0 to 1. The remarkable fact is that, for intermediate
values of the control parameter, the model states exhibit the characteristics
of its closest symmetry limit for all but a relatively narrow transition region
that becomes progressively narrower as the particle number of the model
increases. This phenomenon is explained in terms of quasi-dynamical symmetry.


title: Weak Measurements with Arbitrary Pointer States

abstract:
  The exact conditions on valid pointer states for weak measurements are
derived. It is demonstrated that weak measurements can be performed with any
pointer state with vanishing probability current density. This condition is
found both for weak measurements of noncommuting observables and for $c$-number
observables. In addition, the interaction between pointer and object must be
sufficiently weak. There is no restriction on the purity of the pointer state.
For example, a thermal pointer state is fully valid.


title: Time Reversal and n-qubit Canonical Decompositions

abstract:
  For n an even number of qubits and v a unitary evolution, a matrix
decomposition v=k1 a k2 of the unitary group is explicitly computable and
allows for study of the dynamics of the concurrence entanglement monotone. The
side factors k1 and k2 of this Concurrence Canonical Decomposition (CCD) are
concurrence symmetries, so the dynamics reduce to consideration of the a
factor. In this work, we provide an explicit numerical algorithm computing v=k1
a k2 for n odd. Further, in the odd case we lift the monotone to a two-argument
function, allowing for a theory of concurrence dynamics in odd qubits. The
generalization may also be studied using the CCD, leading again to maximal
concurrence capacity for most unitaries. The key technique is to consider the
spin-flip as a time reversal symmetry operator in Wigner's axiomatization; the
original CCD derivation may be restated entirely in terms of this time
reversal. En route, we observe a Kramers' nondegeneracy: the existence of a
nondegenerate eigenstate of any time reversal symmetric n-qubit Hamiltonian
demands (i) n even and (ii) maximal concurrence of said eigenstate. We provide
examples of how to apply this work to study the kinematics and dynamics of
entanglement in spin chain Hamiltonians.


title: The ping-pong protocol can be attacked without eavesdropping

abstract: Attack the ping-pong protocol without eavesdropping.


title: Resilience of multi-photon entanglement under losses

abstract:
  We analyze the resilience under photon loss of the bi-partite entanglement
present in multi-photon states produced by parametric down-conversion. The
quantification of the entanglement is made possible by a symmetry of the states
that persists even under polarization-independent losses. We examine the
approach of the states to the set of states with a positive partial transpose
as losses increase, and calculate the relative entropy of entanglement. We find
that some bi-partite distillable entanglement persists for arbitrarily high
losses.


title:
Further quantum-gate methods using selective displacement of trapped
  ions

abstract:
  We consider quantum gates for trapped ions using state-selective displacement
of the ions. We generalize earlier work in order to treat arbitrary separations
between the traps. This requires the impact of anharmonicity arising from the
Coulomb interaction to be estimated. We show that its effects are always small
enough to allow high fidelity. In particular, the method can be applied to two
ions in the same trap. We also show that gates between non-neighbour ions, and
hence a Toffoli (three-qubit controlled-NOT) gate, can be achieved. We discuss
how the gate can be applied to logical qubits encoded in the
decoherence-free-subspace {|01>,|10>}, where each pair of ions stores a single
qubit. We also suggest alternatives to the spin-echo method to suppress
unwanted terms in the evolution.


title:
Convex Hulls of Varieties and Entanglement Measures Based on the Roof
  Construction

abstract:
  In this paper we study the problem of calculating the convex hull of certain
affine algebraic varieties. As we explain, the motivation for considering this
problem is that certain pure-state measures of quantum entanglement, which we
call polynomial entanglement measures, can be represented as affine algebraic
varieties. We consider the evaluation of certain mixed-state extensions of
these polynomial entanglement measures, namely convex and concave roofs. We
show that the evaluation of a roof-based mixed-state extension is equivalent to
calculating a hyperplane which is multiply tangent to the variety in a number
of places equal to the number of terms in an optimal decomposition for the
measure. In this way we provide an implicit representation of optimal
decompositions for mixed-state entanglement measures based on the roof
construction.


title:
Noiseless subsystems and the structure of the commutant in quantum error
  correction

abstract:
  The effect of noise on a quantum system can be described by a set of
operators obtained from the interaction Hamiltonian. Recently it has been shown
that generalized quantum error correcting codes can be derived by studying the
algebra of this set of operators. This led to the discovery of noiseless
subsystems. They are described by a set of operators obtained from the
commutant of the noise generators. In this paper we derive a general method to
compute the structure of this commutant in the case of unital noise.


title: Analysis of Quantum Light Memory in Atomic Systems (Final)

abstract:
  We extend the theory of quantum light memory in atomic ensemble of Lambda
type atoms with considering lower levels coherence decay rate and one and
two-photon detunings from resonances in low intensity and adiabatic passage
limit. We obtain that with considering these parameters, that there will be a
considerable decay of probe pulse and stored information; also, we obtain that
the group velocity of probe (light) pulse and its amplitude does not tend to
zero by turning off the control field. We propose a method to keep the probe
pulse in small values in turn off time of control field and to reduce the loss
of the stored probe pulse. In addition, we obtain that in the Off-resonance
case there will be a considerable distortion of the output light pulse that
causes in loss of the stored information, then we present limitations for
detunings and therefore for bandwidths of practical lasers also limitations for
maximum storage time to have negligible distortion of stored information. We
finally present the numerical calculations and compare them with analytical
results.


title:
Generation of entangled squeezed states in atomic Bose-Einstein
  condensates

abstract:
  A method for producing entangled squeezed states (ESSs) for atomic
Bose-Einstein condensates (BECs) is proposed by using a BEC with three internal
states and two classical laser beams. We show that it is possible to generate
two-state and multi-state ESSs under certain circumstances.


title: Nonmaximally-entangled-state quantum photolithography

abstract:
  Many previous works on quantum photolithography are based on
maximally-entangled states (MES). In this paper, we generalize the MES quantum
photolithography to the case where two light beams share a $N$-photon
nonmaximally-entangled state. we investigate the correlations between quantum
entanglement and quantum photolithography. It is shown that for nonlocal
entanglement between the two light beams the amplitude of the deposition rate
can be changed through varying the degree of entanglement described by an
entanglement angle while the resolution remains unchanged, and found that for
local entanglement between the two light beams the effective Rayleigh
resolution of quantum photolithography can be resonantly enhanced.


title: Non stabilizer Clifford codes with qupits

abstract:
  Clifford codes can be understood as a generalization of stabilizer codes. To
show the existence of a true Clifford code which is better than any stabilizer
code is a well known open problem in the theory of Clifford codes. One of the
main difficulties in solving this problem is that we know only about 110
examples of codes which are Clifford but not stabilizer codes. In this paper,
we obtain infinite examples of Clifford codes which are not stabilizer codes.
We expect our examples to be useful in the study of Clifford codes.


title:
Phase Space Formulation of Filtering. Insight into the Wave-Particle
  Duality

abstract:
  A phase space formulation of the filtering process upon an incident quantum
state is developed. This formulation can explain the results of both quantum
interference and delayed-choice experiments without making use of the
controversial wave-particle duality. Quantum particles are seen as localized
and indivisible concentrations of energy and/or mass, their probability
amplitude in phase space being described by the Wigner distribution function.
The wave or particle nature appears in experiments in which the interference
term of the Wigner distribution function is present or absent, respectively,
the filtering devices that modify the quantum wavefunction throughout the
set-up, from its generation to its final detection, being responsible for the
modification of the Wigner distribution function.


title: Temporal Fluctuations of Continuous-Time Quantum Random Walks on Circles

abstract:
  This work deals with both instantaneous uniform mixing property and temporal
standard deviation for continuous-time quantum random walks on circles in order
to study their fluctuations comparing with discrete-time quantum random walks,
and continuous- and discrete-time classical random walks.


title: A Model for Quantum Jumps in Magnetic Resonance Force Microscopy

abstract:
  We propose a simple model which describes the statistical properties of
quantum jumps in a single-spin measurement using the oscillating
cantilever-driven adiabatic reversals technique in magnetic resonance force
microscopy. Our computer simulations based on this model predict the average
time interval between two consecutive quantum jumps and the correlation time to
be proportional to the characteristic time of the magnetic noise and inversely
proportional to the square of the magnetic noise amplitude.


title: Squeezed light at sideband frequencies below 100 kHz from a single OPA

abstract:
  Quantum noise of the electromagnetic field is one of the limiting noise
sources in interferometric gravitational wave detectors. Shifting the spectrum
of squeezed vacuum states downwards into the acoustic band of gravitational
wave detectors is therefore of challenging demand to quantum optics
experiments. We demonstrate a system that produces nonclassical continuous
variable states of light that are squeezed at sideband frequencies below 100
kHz. A single optical parametric amplifier (OPA) is used in an optical noise
cancellation scheme providing squeezed vacuum states with coherent bright phase
modulation sidebands at higher frequencies. The system has been stably locked
for half an hour limited by thermal stability of our laboratory.


title:
Collective excitations in circular atomic configurations, and
  single-photon traps

abstract:
  Correlated excitations in a plane circular configuration of identical atoms
with parallel dipole moments are investigated. The collective energy
eigenstates, their level shifts and decay rates are computed utilizing a
decomposition of the atomic state space into carrier spaces for the irreducible
representations of the symmetry group $\ZZ_N$ of the circle. It is shown that
the index $p$ of these representations can be used as a quantum number
analogously to the orbital angular momentum quantum number $l$ in hydrogen-like
systems. Just as the hydrogen s-states are the only electronic wave functions
which can occupy the central region of the Coulomb potential, the
quasi-particle corresponding to a collective excitation of the atoms in the
circle can occupy the central atom only for vanishing $\ZZ_N$ quantum number
$p$. If a central atom is present, the $p=0$ state splits into two and shows
level-crossing at certain radii; in the regions between these radii, damped
Rabi oscillations between two "extreme" $p=0$ configurations occur. The
physical mechanisms behind super- and subradiance at a given radius and the
divergence of the level shifts at small interatomic distances are discussed. It
is shown that, beyond a certain critical number of atoms in the circle, the
lifetime of the maximally subradiant state increases exponentially with the
number of atoms in the configuration, making the system a natural candidate for
a {\it single-photon trap}.

categories: quant-ph cond-mat.mes-hall cond-mat.other physics.atm-clus


title: Stabilization of Ultracold Molecules Using Optimal Control Theory

abstract:
  In recent experiments on ultracold matter, molecules have been produced from
ultracold atoms by photoassociation, Feshbach resonances, and three-body
recombination. The created molecules are translationally cold, but
vibrationally highly excited. This will eventually lead them to be lost from
the trap due to collisions. We propose shaped laser pulses to transfer these
highly excited molecules to their ground vibrational level. Optimal control
theory is employed to find the light field that will carry out this task with
minimum intensity. We present results for the sodium dimer. The final target
can be reached to within 99% if the initial guess field is physically
motivated. We find that the optimal fields contain the transition frequencies
required by a good Franck-Condon pumping scheme. The analysis is able to
identify the ranges of intensity and pulse duration which are able to achieve
this task before other competing process take place. Such a scheme could
produce stable ultracold molecular samples or even stable molecular
Bose-Einstein condensates.


title:
Quantum error correction for continuously detected errors with any
  number of error channels per qubit

abstract:
  It was shown by Ahn, Wiseman, and Milburn [PRA {\bf 67}, 052310 (2003)] that
feedback control could be used as a quantum error correction process for errors
induced by weak continuous measurement, given one perfectly measured error
channel per qubit. Here we point out that this method can be easily extended to
an arbitrary number of error channels per qubit. We show that the feedback
protocols generated by our method encode $n-2$ logical qubits in $n$ physical
qubits, thus requiring just one more physical qubit than in the previous case.


title: EPR-Bohm experiment, interference of probabilities, imprecision of time

abstract:
  We demonstrate that the EPR-Bohm probabilities can be easily obtained in the
classical (but contextual) probabilistic framework by using the formula of
interference of probabilities. From this point of view the EPR-Bell experiment
is just an experiment on interference of probabilities. We analyse the time
structure of contextuality in the EPR-Bohm experiment. The conclusion is that
quantum mechanics does not contradict to a local realistic model in which
probabilities are calculated as averages over conditionings/measurements for
pairs of instances of time $t_1< t_2.$ If we restrict our consideration only to
simultaneous measurements at the fixed instance of time $t$ we would get
contradiction with Bell's theorem. One of implications of this fact might be
the impossibility to define instances of {\it time with absolute precision} on
the level of the contextual microscopic realistic model.


title: Schrodinger-Newton wave mechanics. The model

abstract:
  In the paper the Schrodinger equation (SE) with gravity term is developed and
discussed. It is shown that the modified SE is valid for particles with mass
m<M_P, M_P is the Planck mass, and contains the part which, we argue describes
the pilot wave. For m \to M_P the modified SE has the solution with oscillatory
term, i.e. strings. Key words: Schrodinger-Newton equation; Planck time; pilot
wave.