title:
Electronic energy transfer: vibrational control and nonlinear wavepacket
  interferometry

abstract:
  The time-development of photoexcitations in molecular aggregates exhibits
specific dynamics of electronic states and vibrational wavefunction. We discuss
the dynamical formation of entanglement between electronic and vibrational
degrees of freedom in molecular aggregates with theory of electronic energy
transfer and the method of vibronic 2D wavepackets [Cina, Kilin, Humble, J.
Chem. Phys. 118, 46 (2003)]. The vibronic dynamics is also described by
applying Jaynes-Cummings model to the electronic energy transfer [Kilin,
Pereverzev, Prezhdo, J. Chem. Phys. 120, 11209 (2004);math-ph/0403023].
Following the ultrafast excitation of donor[chem-ph/9411004] the population of
acceptor rises by small portions per each vibrational period, oscillates force
and back between donor and acceptor with later damping and partial revivals of
this oscillation. The transfer rate gets larger as donor wavepacket approaches
the acceptor equilibrium configuration, which is possible at specific energy
differences of donor and acceptor and at maximal amount of the vibrational
motion along the line that links donor and acceptor equilibria positions. The
four-pulse phase-locked nonlinear wavepacket 2D interferograms reflect the
shape of the relevant 2D vibronic wavepackets and have maxima at longer delay
between excitation pulses for dimers with equal donor-acceptor energy
difference compare to dimers with activationless energy configuration [Cina,
Fleming, J. Phys. Chem. A. 108, 11196 (2004)].


title: Entanglement Detection by Local Orthogonal Observables

abstract:
  We propose a family of entanglement witnesses and corresponding positive maps
that are not completely positive based on local orthogonal observables. As
applications the entanglement witness of the $3\times 3$ bound entangled state
[P. Horodecki, Phys. Lett. A {\bf 232}, 333 (1997)] is explicitly constructed
and a family of $d$-dimensional bound entangled states is designed so that the
entanglement can be detected by permuting local orthogonal observables. Further
the proposed physically not implementable positive maps can be physically
realized by measuring a Hermitian correlation matrix of local orthogonal
observables.


title: The most robust entangled state of light

abstract:
  We study how photon absorption losses degrade the bipartite entanglement of
entangled states of light. We consider two questions: (i) what state contains
the smallest average number of photons given a fixed amount of entanglement?
and (ii) what state is the most robust against photon absorption? We explain
why the two-mode squeezed state is the answer to the first question but not
quite to the second question.


title:
Manipulative resonant nonlinear optics: nonlinear interference effects
  and quantum control of nonlinearity, dispersion, transparency and
  inversionless amplification in an extended strongly-absorbing
  inhomogeneously-broadened medium

abstract:
  Specific features of nonlinear interference processes at quantum transitions
in near- and fully-resonant optically-dense Doppler-broadened medium are
studied. The feasibility of overcoming of the fundamental limitation on a
velocity-interval of resonantly coupled molecules imposed by the Doppler effect
is shown based on quantum coherence. This increases the efficiency of
nonlinear-optical processes in atomic and molecular gases that possess the most
narrow and strongest resonances. The possibility of all-optical switching of
the medium to opaque or, alternatively, to absolutely transparent, or even to
strongly-amplifying states is explored, which is controlled by a small
variation of two driving radiations. The required intensities of the control
fields are shown to be typical for cw lasers. These effects are associated with
four-wave mixing accompanied by Stokes gain and by their interference in fully-
and near-resonant optically-dense far-from-degenerate double-Lambda medium.
Optimum conditions for inversionless amplification of short-wavelength
radiation above the oscillation threshold at the expense of the
longer-wavelength control fields, as well as for Raman gain of the generated
idle infrared radiation, are investigated. The outcomes are illustrated with
numerical simulations applied to sodium dimer vapor. Similar schemes can be
realized in doped solids and in fiber optics.


title:
Tunable Whispering Gallery Mode Resonators for Cavity Quantum
  Electrodynamics

abstract:
  We theoretically study the properties of highly prolate shaped dielectric
microresonators. Such resonators sustain whispering gallery modes that exhibit
two spatially well separated regions with enhanced field strength. The field
per photon on the resonator surface is significantly higher than e.g. for
equatorial whispering gallery modes in microsphere resonators with a comparable
mode volume. At the same time, the frequency spacing of these modes is much
more favorable, so that a tuning range of several free spectral ranges should
be attainable. We discuss the possible application of such resonators for
cavity quantum electrodynamics experiments with neutral atoms and reveal
distinct advantages with respect to existing concepts.


title:
Entangled-state cryptographic protocol that remains secure even if
  nonlocal hidden variables exist and can be measured with arbitrary precision

abstract:
  Standard quantum cryptographic protocols are not secure if one assumes that
nonlocal hidden variables exist and can be measured with arbitrary precision.
The security can be restored if one of the communicating parties randomly
switches between two standard protocols.


title: Non-Bilocal Measurement via Entangled State

abstract:
  Two observers, who share a pair of particles in an entangled mixed state, can
use it to perform some non-bilocal measurement over another bipartite system.
In particular, one can construct a specific game played by the observers
against a coordinator, in which they can score better than a pair of observers
who only share a classical communication channel.


title: Unconditional quantum cloning of coherent states with linear optics

abstract:
  A scheme for optimal Gaussian cloning of optical coherent states is proposed
and experimentally demonstrated. Its optical realization is based entirely on
simple linear optical elements and homodyne detection. The optimality of the
presented scheme is only limited by detection inefficiencies. Experimentally we
achieved a cloning fidelity of about 65%, which almost touches the optimal
value of 2/3.


title: General Description of Mixed State Geometric Phase

abstract:
  We consider arbitrary mixed state in unitary evolution and provide a
comprehensive description of corresponding geometric phase in which two
different points of view prevailing currently can be unified. Introducing an
ancillary system and considering the purification of given mixed state, we find
that different results of mixed state geometric phase correspond to different
choice of the representation of Hilbert space of the ancilla. Moreover we
demonstrate that in order to obtain Uhlmann's geometric phase it is not
necessary to resort to the unitary evolution of ancilla.


title: Spin Chains as Perfect Quantum State Mirrors

abstract:
  Quantum information transfer is an important part of quantum information
processing. Several proposals for quantum information transfer along linear
arrays of nearest-neighbor coupled qubits or spins were made recently. Perfect
transfer was shown to exist in two models with specifically designed strongly
inhomogeneous couplings. We show that perfect transfer occurs in an entire
class of chains, including systems whose nearest-neighbor couplings vary only
weakly along the chain. The key to these observations is the Jordan-Wigner
mapping of spins to noninteracting lattice fermions which display perfectly
periodic dynamics if the single-particle energy spectrum is appropriate. After
a half-period of that dynamics any state is transformed into its mirror image
with respect to the center of the chain. The absence of fermion interactions
preserves these features at arbitrary temperature and allows for the transfer
of nontrivially entangled states of several spins or qubits.


title:
Distributing entanglement and single photons through an intra-city,
  free-space quantum channel

abstract:
  We have distributed entangled photons directly through the atmosphere to a
receiver station 7.8 km away over the city of Vienna, Austria at night.
Detection of one photon from our entangled pairs constitutes a triggered single
photon source from the sender. With no direct time-stable connection, the two
stations found coincidence counts in the detection events by calculating the
cross-correlation of locally-recorded time stamps shared over a public internet
channel. For this experiment, our quantum channel was maintained for a total of
40 minutes during which time a coincidence lock found approximately 60000
coincident detection events. The polarization correlations in those events
yielded a Bell parameter, S=2.27/pm0.019, which violates the CHSH-Bell
inequality by 14 standard deviations. This result is promising for
entanglement-based free-space quantum communication in high-density urban
areas. It is also encouraging for optical quantum communication between ground
stations and satellites since the length of our free-space link exceeds the
atmospheric equivalent.


title: Subjective and Objective Probabilities in Quantum Mechanics

abstract:
  We discuss how the apparently objective probabilities predicted by quantum
mechanics can be treated in the framework of Bayesian probability theory, in
which all probabilities are subjective. Our results are in accord with earlier
work by Caves, Fuchs, and Schack, but our approach and emphasis are different.
We also discuss the problem of choosing a noninformative prior for a density
matrix.


title: Quantum Eraser Using A Modified Stern-Gerlach Setup

abstract:
  We propose a new setup to demonstrate quantum eraser, using spin-1/2
particles in a modified Stern-Gerlach setup, with a double slit. The
"which-way" information can be erased simply by applying a transverse magnetic
field with an additional magnet, resulting in the emergence of two
complementary staggered interference patterns. Use of the classic Stern-Gerlach
setup, and the unweaving of the washed out interference without any coincident
counting, is what makes this proposal novel.


title:
Contribution from stochastic electrodynamics to the understanding of
  quantum mechanics

abstract:
  During the last decades there has been a relatively extensive attempt to
develop the theory of stochastic electrodynamics (SED) with a view to
establishing it as the foundation for quantum mechanics. The theory had several
important successes, but failed when applied to the study of particles subject
to nonlinear forces. An analysis of the failure showed that its reasons are not
to be ascribed to the principles of SED, but to the methods used to construct
the theory, particularly the use of a Fokker-Planck approximation and
perturbation theory. A new, non perturbative approach has been developed,
called linear stochastic electrodynamics (LSED), of which a clean form is
presented here. After introducing the fundamentals of SED, we discuss in detail
the principles on which LSED is constructed. We pay attention to the
fundamental issue of the mechanism that leads to the quantum behaviour of field
and matter, and demonstrate that indeed LSED is a natural way to the quantum
formalism by demanding its solutions to comply with a limited number of
principles, each one with a clear physical meaning. As a further application of
the principles of LSED we derive also the Planck distribution. In a final
section we revisit some of the most tantalizing quandaries of quantum mechanics
from the point of view offered by the present theory, and show that it offers a
clear physical answer to them.


title: Quantitative measures of entanglement in pair coherent states

abstract:
  The pair coherent states for a two-mode radiation field are known to belong
to a family of states with non-Gaussian wave function. The nature of quantum
entanglement between the two modes and some features of non-classicality are
studied for such states. The existing criteria for inseparability are examined
in the context of pair coherent states.


title:
Manipulation of the Spontaneous Emission Dynamics of Quantum Dots in 2D
  Photonic Crystals

abstract:
  We demonstrate the ability to control the spontaneous emission dynamics of
self-assembled quantum dots via the local density of optical modes in
2D-photonic crystals. We show that an incomplete 2D photonic bandgap is
sufficient to significantly lengthen the spontaneous emission lifetime
($>2\times$) over a wide bandwidth ($\Delta\lambda\geq40$ nm). For dots that
are both \textit{spectrally} and \textit{spatially} coupled to strongly
localized ($V_{mode}\sim1.5(\lambda/n)^{3}$), high $Q\sim2700$ optical modes,
we have directly measured a strong Purcell enhanced shortening of the emission
lifetime $\geq5.6\times$, limited only by our temporal resolution. Analysis of
the spectral dependence of the recombination dynamics shows a maximum lifetime
shortening of $19\pm4$. From the directly measured enhancement and suppression
we show that the single mode coupling efficiency for quantum dots in such
structures is at least $\beta=92%$ and is estimated to be as large as
$\sim97%$.


title: Quantum Key Distribution with Blind Polarization Bases

abstract:
  We propose a new quantum key distribution scheme that uses the blind
polarization basis. In our scheme the sender and the receiver share key
information by exchanging qubits with arbitrary polarization angles without
basis reconciliation. As only random polarizations are transmitted, our
protocol is secure even when a key is embedded in a not-so-weak coherent-state
pulse. We show its security against the photon number splitting attack and the
impersonation attack.


title: Thermodynamical Versus Optical Complementarity

abstract:
  We establish correspondence between macroscopic thermodynamical quantities
and complementarity in wave interference. The well known visibility and
predictability in a double slit--like experiment are shown to be connected to
magnetic susceptibility and magnetization of a general interacting spin chain.
This gives us the ability to analyze the tradeoff between thermodynamical
quantities in the same information--theoretic way that is used in analyzing the
wave--particle duality. We thus obtain new physical insights into usually
complicated thermodynamical models, such as viewing a phase transition simply
as a change from an effective single slit diffraction to a double slit
interference.


title: Illusion of quantum speed-up

abstract:
  Quantum computers are believed to surpass the classical ones. Moreover, it is
claimed that this belief reaches the level of a mathematically proven fact
within the oracle model of computation. Here we impair the whole class of the
so-called rigorist proofs of quantum speed-up obtained within this model.


title: Entanglement Energetics in the Ground State

abstract:
  We show how many-body ground state entanglement information may be extracted
from sub-system energy measurements at zero temperature. A precise relation
between entanglement and energy fluctuations is demonstrated in the weak
coupling limit. Examples are given with the two-state system and the harmonic
oscillator, and energy probability distributions are calculated. Comparisons
made with recent qubit experiments show this type of measurement provides
another method to quantify entanglement with the environment.


title: Ground State Entanglement Energetics

abstract:
  We consider the ground state of simple quantum systems coupled to an
environment. In general the system is entangled with its environment. As a
consequence, even at zero temperature, the energy of the system is not sharp: a
projective measurement can find the system in an excited state. We show that
energy fluctuation measurements at zero temperature provide entanglement
information. For two-state systems which exhibit a persistent current in the
ground state, energy fluctuations and persistent current fluctuations are
closely related. The harmonic oscillator serves to illustrate energy
fluctuations in a system with an infinite number of states. In addition to the
energy distribution we discuss the energy-energy time-correlation function in
the zero-temperature limit.


title:
Greenberger-Horne-Zeilinger correlation and Bell-type inequality seen
  from moving frame

abstract:
  The relativistic version of the Greenberger-Horne-Zeilinger experiment with
massive particles is proposed. We point out that, in the moving frame, GHZ
correlations of spins in original directions transfer to different directions
due to the Wigner rotation. Its effect on the degree of violation of Bell-type
inequality is also discussed.


title: Entanglement Detection in the Stabilizer Formalism

abstract:
  We investigate how stabilizer theory can be used for constructing sufficient
conditions for entanglement. First, we show how entanglement witnesses can be
derived for a given state, provided some stabilizing operators of the state are
known. These witnesses require only a small effort for an experimental
implementation and are robust against noise. Second, we demonstrate that also
nonlinear criteria based on uncertainty relations can be derived from
stabilizing operators. These criteria can sometimes improve the witnesses by
adding nonlinear correction terms. All our criteria detect states close to
Greenberger-Horne-Zeilinger states, cluster and graph states. We show that
similar ideas can be used to derive entanglement conditions for states which do
not fit the stabilizer formalism, such as the three-qubit W state. We also
discuss connections between the witnesses and some Bell inequalities.


title:
Classical and quantum fingerprinting with shared randomness and
  one-sided error

abstract:
  Within the simultaneous message passing model of communication complexity,
under a public-coin assumption, we derive the minimum achievable worst-case
error probability of a classical fingerprinting protocol with one-sided error.
We then present entanglement-assisted quantum fingerprinting protocols
attaining worst-case error probabilities that breach this bound.


title: Stimulated wave of polarization in 1D Ising chain

abstract:
  It is demonstrated that in one-dimensional Ising chain with nearest-neighbor
interactions, irradiated by a weak resonant transverse field, a stimulated wave
of flipped spins can be triggered by a flip of a single spin. This analytically
solvable model illustrates mechanisms of quantum amplification and quantum
measurement.


title: On PPT States in KxMxN Composite Quantum Systems

abstract:
  We study the general representations of positive partial transpose (PPT)
states in ${\cal C}^K \otimes {\cal C}^M \otimes {\cal C}^N$. For the PPT
states with rank-$N$ a canonical form is obtained, from which a sufficient
separability condition is presented.


title: Hamiltonian structures for Pais-Uhlenbeck oscillator

abstract:
  The hamiltonian structures for quartic oscillator are considered. All
structures admitting quadratic hamiltonians are classified.


title: Auto-compensating differential phase shift quantum key distribution

abstract:
  We propose an auto-compensating differential phase shift scheme for quantum
key distribution with a high key-creation efficiency, which skillfully makes
use of automatic alignment of the photon polarization states in optical fiber
with modified Michelson interferometers composed of unequal arms with Faraday
mirrors at the ends. The Faraday-mirrors-based Michelson interferometers not
only function as pulse splitters, but also enable inherent compensation of
polarization mode dispersion in the optic-fiber paths at both Alice's and Bob's
sites. The sequential pulses encoded by differential phase shifts pass through
the quantum channel with the same polarization states, resulting in a stable
key distribution immune to the polarization mode dispersion in the quantum
channel. Such a system features perfect stability and higher key creation
efficiency over traditional schemes.


title: Classical and quantum radiation reaction for linear acceleration

abstract:
  We investigate the effect of radiation reaction on the motion of a wave
packet of a charged scalar particle linearly accelerated in quantum
electrodynamics. We give the details of the calculations for the case where the
particle is accelerated by a static potential that were outlined in Phys.Rev. D
70 (2004) 081701(R) and present similar results in the case of a time-dependent
but space-independent potential. In particular, we calculate the expectation
value of the position of the charged particle after the acceleration, to first
order in the fine structure constant in the $\hbar \to 0$ limit, and find that
the change in the expectation value of the position (the position shift) due to
radiation reaction agrees exactly with the result obtained using the
Lorentz-Dirac force in classical electrodynamics for both potentials. We also
point out that the one-loop correction to the potential may contribute to the
position change in this limit.


title: On hybrid states of two and three level atoms

abstract:
  We calculate atom-photon resonances in the Wigner-Weisskopf model, admitting
two photons and choosing a particular coupling function. We also present a
rough description of the set of resonances in a model for a three-level atom
coupled to the photon field. We give a general picture of matter-field
resonances these results fit into.


title: Three-region vacuum nonlocality

abstract:
  The correlations between three arbitrarily far-apart regions of the vacuum
state of the free Klein-Gordon field are investigated by means of its finite
duration coupling to three localized detectors. It is shown that these
correlations cannot be reproduced in terms of a hybrid local-nonlocal
hidden-variable model, i.e., the correlations between three arbitrarily
separated regions of the vacuum are fully nonlocal.


title:
Thermal Entanglement between Alternate Qubits of a Four-qubit Heisenberg
  XX Chain in a Magnetic Field

abstract:
  The concurrence of two alternate qubits in a four-qubit Heisenberg XX chain
is investigated when a uniform magnetic field B is included. It is found that
there is no thermal entanglement between alternate qubits if B is close to
zero. Magnetic field can induce entanglement in a certain range both for the
antiferromagnetic and ferromagnetic cases. Near zero temperature, the
entanglement undergoes two sudden changes with increasing value of the magnetic
field B. This is due to the changes in the ground state. This novel property
may be used as quantum entanglement switch. The anisotropy in the system can
also induce the entanglement between two alternate qubits.


title: The simplest Bell's theorem, with or without locality

abstract:
  We prove here a version of Bell's Theorem that is simpler than any previous
one. The contradiction of Bell's inequality with Quantum Mechanics in the new
version is not cured by non-locality so that this version allows one to single
out classical realism, and not locality, as the common source of all false
inequalities of Bell's type.


title: Quantum non-demolition measurement saturates fidelity trade-off

abstract:
  A general quantum measurement on an unknown quantum state enables us to
estimate what the state originally was. Simultaneously, the measurement has a
destructive effect on a measured quantum state which is reflected by the
decrease of the output fidelity. We show for any $d$-level system that quantum
non-demolition (QND) measurement controlled by a suitably prepared ancilla is a
measurement in which the decrease of the output fidelity is minimal. The ratio
between the estimation fidelity and the output fidelity can be continuously
controlled by the preparation of the ancilla. Different measurement strategies
on the ancilla are also discussed. Finally, we propose a feasible scheme of
such a measurement for atomic and optical 2-level systems based on basic
controlled-NOT gate.


title: Entanglement in spin-one Heisenberg chains

abstract:
  By using the concept of negativity, we study entanglement in spin-one
Heisenberg chains. Both the bilinear chain and the bilinear-biquadratic chain
are considered. Due to the SU(2) symmetry, the negativity can be determined by
two correlators, which greatly facilitate the study of entanglement properties.
Analytical results of negativity are obtained in the bilinear model up to four
spins and the two-spin bilinear-biquadratic model, and numerical results of
negativity are presented. We determine the threshold temperature before which
the thermal state is doomed to be entangled.


title:
Tensor polarizability and dispersive quantum measurement of multilevel
  atoms

abstract:
  Optimally extracting information from measurements performed on a physical
system requires an accurate model of the measurement interaction. Continuously
probing the collective spin of an Alkali atom cloud via its interaction with an
off-resonant optical probe is an important example of such a measurement where
realistic modeling at the quantum level is possible using standard techniques
from atomic physics. Typically, however, tutorial descriptions of this
technique have neglected the multilevel structure of realistic atoms for the
sake of simplification. In this paper we account for the full multilevel
structure of Alkali atoms and derive the irreducible form of the polarizability
Hamiltonian describing a typical dispersive quantum measurement. For a specific
set of parameters, we then show that semiclassical predictions of the theory
are consistent with our experimental observations of polarization scattering by
a polarized cloud of laser-cooled Cesium atoms. We also derive the
signal-to-noise ratio under a single measurement trial and use this to predict
the rate of spin-squeezing with multilevel Alkali atoms for arbitrary detuning
of the probe beam.


title:
Modified Kaluza-Klein Theory, Quantum Hidden Variables and 3-Dimensional
  Time

abstract:
  In this paper, the basic quantum field equations of free particle with
0-spin, 1-spin (for case of massless and mass $>$ 0) and 1/2 spin are derived
from Einstein equations under modified Kaluza-Klein metric, it shows that the
equations of quantum fields can be interpreted as pure geometry properties of
curved higher-dimensional time-space . One will find that if we interpret the
5th and 6th dimension as ``extra'' time dimension, the particle's wave-function
can be naturally interpreted as a single particle moving along geodesic path in
6-dimensional modified Kaluza-Klein time-space. As the result, the fundamental
physical effect of quantum theory such as double-slit interference of single
particle, statistical effect of wave-function, wave-packet collapse, spin,
Bose-Einstein condensation, Pauli exclusive principle can be interpreted as
``classical'' behavior in new time-space. In the last part of this paper, we
will coupling field equations of 0-spin, 1-spin and 1/2-spin particles with
gravity equations.


title:
Quasi-exact Solvability of Planar Dirac Electron in Coulomb and Magnetic
  Fields

abstract:
  The Dirac equation for an electron in two spatial dimensions in the Coulomb
and homogeneous magnetic fields is a physical example of quasi-exactly solvable
systems. This model, however, does not belong to the classes based on the
algebra $sl(2)$ which underlies most one-dimensional and effectively
one-dimensional quasi-exactly solvable systems. In this paper we demonstrate
that the quasi-exactly solvable differential equation possesses a hidden
$osp(2,2)$ superalgebra.


title:
Experimental Construction of Optical Multi-qubit Cluster States From
  Bell States

abstract:
  Cluster states serve as the central physical resource for the
measurement-based quantum computation. We here present a simple experimental
demonstration of the scalable cluster-state-construction scheme proposed by
Browne and Rudolph. In our experiment, three-photon cluster states are created
from two Bell states using linear optical devices. By observing a violation of
three-particle Mermin inequality of $|< \textit{A}>| = 3.10\pm0.03 $, we also
for the first time report a genuine three-photon entanglement. In addition, the
entanglement properties of the cluster states are examined under $\sigma_z$ and
$\sigma_x$ measurements on a qubit.


title: Ion trap transducers for quantum electromechanical oscillators

abstract:
  An enduring challenge for contemporary physics is to experimentally observe
and control quantum behavior in macroscopic systems. We show that a single
trapped atomic ion could be used to probe the quantum nature of a mesoscopic
mechanical oscillator precooled to 4K, and furthermore, to cool the oscillator
with high efficiency to its quantum ground state. The proposed experiment could
be performed using currently available technology.


title: Continuous quantum error correction by cooling

abstract:
  We describe an implementation of quantum error correction that operates
continuously in time and requires no active interventions such as measurements
or gates. The mechanism for carrying away the entropy introduced by errors is a
cooling procedure. We evaluate the effectiveness of the scheme by simulation,
and remark on its connections to some recently proposed error prevention
procedures.


title: On tests of local realism by CP-violation parameters of K^0 mesons

abstract:
  Recently various papers have proposed to test local realism (LR) by
considering electroweak CP-violation parameters values in neutral pseudoscalar
meson systems. Considering the large interest for a conclusive test of LR and
the experimental accessibility to these tests, in this paper we critically
consider these results showing how they, albeit very interesting, require
anyway additional assumptions and therefore cannot be considered conclusive
tests of LR.


title: Geometric phase around exceptional points

abstract:
  A wave function picks up, in addition to the dynamic phase, the geometric
(Berry) phase when traversing adiabatically a closed cycle in parameter space.
We develop a general multidimensional theory of the geometric phase for
(double) cycles around exceptional degeneracies in non-Hermitian Hamiltonians.
We show that the geometric phase is exactly $\pi$ for symmetric complex
Hamiltonians of arbitrary dimension and for nonsymmetric non-Hermitian
Hamiltonians of dimension 2. For nonsymmetric non-Hermitian Hamiltonians of
higher dimension, the geometric phase tends to $\pi$ for small cycles and
changes as the cycle size and shape are varied. We find explicitly the leading
asymptotic term of this dependence, and describe it in terms of interaction of
different energy levels.


title:
The Pioneer anomaly: the measure of a topological phase defect of light
  in cosmology

abstract:
  It is shown that a wave vector representing a light pulse in an adiabatically
evolving expanding space should develop, after a round trip (back and forth to
the emitter) a geometric phase for helicity states at a given fixed position
coordinate of this expanding space.In a section of the Hopf fibration of the
Poincare sphere that identifies a projection to the physically allowed states,
the evolution defines a parallel transported state that can be joined
continuously with the initial state by means of the associated
Berry-Pancharatnam connection. The connection allows to compute an anomaly in
the frequency for the vector modes in terms of the scale factor of the
space-time background being identical to the reported Pioneer Anomaly.


title:
Towards deterministic optical quantum computation with coherently driven
  atomic ensembles

abstract:
  Scalable and efficient quantum computation with photonic qubits requires (i)
deterministic sources of single-photons, (ii) giant nonlinearities capable of
entangling pairs of photons, and (iii) reliable single-photon detectors. In
addition, an optical quantum computer would need a robust reversible photon
storage devise. Here we discuss several related techniques, based on the
coherent manipulation of atomic ensembles in the regime of electromagnetically
induced transparency, that are capable of implementing all of the above
prerequisites for deterministic optical quantum computation with single
photons.


title:
Photon number squeezing of ultra-broadband laser pulses generated by
  microstructure fibers

abstract:
  To the best of our knowledge, we demonstrate for the first time the
generation of photon number squeezing by spectral filtering for ultra-broadband
light generated by microstructure fibers at 800 nm. A maximum squeezing of 4.6
dB is observed, corresponding to 10.3 dB after correcting for detection losses.
We numerically analyzed the quantum dynamics of ultrashort laser pulse
propagation through optical fibers by solving a nonlinear quantum Schrodinger
equation that included Raman scattering, especially for the quantum correlation
of photon number fluctuation among frequency modes in broadband pulses.


title: Optimal measurements for the dihedral hidden subgroup problem

abstract:
  We consider the dihedral hidden subgroup problem as the problem of
distinguishing hidden subgroup states. We show that the optimal measurement for
solving this problem is the so-called pretty good measurement. We then prove
that the success probability of this measurement exhibits a sharp threshold as
a function of the density nu=k/log N, where k is the number of copies of the
hidden subgroup state and 2N is the order of the dihedral group. In particular,
for nu<1 the optimal measurement (and hence any measurement) identifies the
hidden subgroup with a probability that is exponentially small in log N, while
for nu>1 the optimal measurement identifies the hidden subgroup with a
probability of order unity. Thus the dihedral group provides an example of a
group G for which Omega(log|G|) hidden subgroup states are necessary to solve
the hidden subgroup problem. We also consider the optimal measurement for
determining a single bit of the answer, and show that it exhibits the same
threshold. Finally, we consider implementing the optimal measurement by a
quantum circuit, and thereby establish further connections between the dihedral
hidden subgroup problem and average case subset sum problems. In particular, we
show that an efficient quantum algorithm for a restricted version of the
optimal measurement would imply an efficient quantum algorithm for the subset
sum problem, and conversely, that the ability to quantum sample from subset sum
solutions allows one to implement the optimal measurement.


title:
Capacity Theorems for Quantum Multiple Access Channels:
  Classical-Quantum and Quantum-Quantum Capacity Regions

abstract:
  We consider quantum channels with two senders and one receiver. For an
arbitrary such channel, we give multi-letter characterizations of two different
two-dimensional capacity regions. The first region is comprised of the rates at
which it is possible for one sender to send classical information, while the
other sends quantum information. The second region consists of the rates at
which each sender can send quantum information. For each region, we give an
example of a channel for which the corresponding region has a single-letter
description. One of our examples relies on a new result proved here, perhaps of
independent interest, stating that the coherent information over any degradable
channel is concave in the input density operator. We conclude with connections
to other work and a discussion on generalizations where each user
simultaneously sends classical and quantum information.


title:
Cavity QED and Quantum Computation in the Weak Coupling Regime II :
  Complete Construction of the Controlled-Controlled NOT Gate

abstract:
  In this paper we treat a cavity QED quantum computation. Namely, we consider
a model of quantum computation based on n atoms of laser-cooled and trapped
linearly in a cavity and realize it as the n atoms Tavis-Cummings Hamiltonian
interacting with n external (laser) fields.
  We solve the Schr{\" o}dinger equation of the model in the weak coupling
regime to construct the controlled NOT gate in the case of n=2, and to
construct the controlled-controlled NOT gate in the case of n=3 by making use
of several resonance conditions and rotating wave approximation associated to
them. We also present an idea to construct general quantum circuits.
  The approach is more sophisticated than that of the paper [K. Fujii,
Higashida, Kato and Wada, Cavity QED and Quantum Computation in the Weak
Coupling Regime, J. Opt. B : Quantum Semiclass. Opt. {\bf 6} (2004), 502].
  Our method is not heuristic but completely mathematical, and the significant
feature is based on a consistent use of Rabi oscillations.


title:
Optical selection rules and phase-dependent adiabatic state control in a
  superconducting quantum circuit

abstract:
  We analyze the optical selection rules of the microwave-assisted transitions
in a flux qubit superconducting quantum circuit (SQC). We show that the
parities of the states relevant to the superconducting phase in the SQC are
well-defined when the external magnetic flux $\Phi_{e}=\Phi_{0}/2$, then the
selection rules are same as the ones for the electric-dipole transitions in
usual atoms. When $\Phi_{e}\neq \Phi_{0}/2$, the symmetry of the potential of
the artificial "atom'' is broken, a so-called $\Delta$-type "cyclic"
three-level atom is formed, where one- and two-photon processes can coexist. We
study how the population of these three states can be selectively transferred
by adiabatically controlling the electromagnetic field pulses. Different from
$\Lambda$-type atoms, the adiabatic population transfer in our three-level
$\Delta$-atom can be controlled not only by the amplitudes but also by the
phases of the pulses.


title: Four-level and two-qubit systems, sub-algebras, and unitary integration

abstract:
  Four-level systems in quantum optics, and for representing two qubits in
quantum computing, are difficult to solve for general time-dependent
Hamiltonians. A systematic procedure is presented which combines analytical
handling of the algebraic operator aspects with simple solutions of classical,
first-order differential equations. In particular, by exploiting $su(2) \oplus
su(2)$ and $su(2) \oplus su(2) \oplus u(1)$ sub-algebras of the full SU(4)
dynamical group of the system, the non-trivial part of the final calculation is
reduced to a single Riccati (first order, quadratically nonlinear) equation,
itself simply solved. Examples are provided of two-qubit problems from the
recent literature, including implementation of two-qubit gates with Josephson
junctions.


title: Continuous quantum error correction

abstract:
  We describe new implementations of quantum error correction that are
continuous in time, and thus described by continuous dynamical maps. We
evaluate the performance of such schemes using numerical simulations, and
comment on the effectiveness and applicability of continuous error correction
for quantum computing.


title: Symmetries of the Poincare sphere and decoherence matrices

abstract:
  The Stokes parameters form a Minkowskian four-vector under various optical
transformations. As a consequence, the resulting two-by-two density matrix
constitutes a representation of the Lorentz group. The associated Poincare
sphere is a geometric representation of the Lorentz group. Since the Lorentz
group preserves the determinant of the density matrix, it cannot accommodate
the decoherence process through the decaying off-diagonal elements of the
density matrix, which yields to an incerese in the value of the determinant. It
is noted that the O(3,2) deSitter group contains two Lorentz subgroups. The
change in the determinant in one Lorentz group can be compensated by the other.
It is thus possible to describe the decoherence process as a symmetry
transformation in the O(3,2) space. It is shown also that these two coupled
Lorentz groups can serve as a concrete example of Feynman's rest of the
universe.


title:
Quantum Tomographic Cryptography with a Semiconductor Single Photon
  Source

abstract:
  In this paper we analyze the security of the so-called quantum tomographic
cryptography with the source producing entangled photons via an experimental
scheme proposed in Phys. Rev. Lett. 92, 37903 (2004). We determine the range of
the experimental parameters for which the protocol is secure against the most
general incoherent attacks.


title: Introduction to PT-Symmetric Quantum Theory

abstract:
  In most introductory courses on quantum mechanics one is taught that the
Hamiltonian operator must be Hermitian in order that the energy levels be real
and that the theory be unitary (probability conserving). To express the
Hermiticity of a Hamiltonian, one writes $H=H^\dagger$, where the symbol
$\dagger$ denotes the usual Dirac Hermitian conjugation; that is, transpose and
complex conjugate. In the past few years it has been recognized that the
requirement of Hermiticity, which is often stated as an axiom of quantum
mechanics, may be replaced by the less mathematical and more physical
requirement of space-time reflection symmetry (PT symmetry) without losing any
of the essential physical features of quantum mechanics. Theories defined by
non-Hermitian PT-symmetric Hamiltonians exhibit strange and unexpected
properties at the classical as well as at the quantum level. This paper
explains how the requirement of Hermiticity can be evaded and discusses the
properties of some non-Hermitian PT-symmetric quantum theories.


title: New Quasi-Exactly Solvable Sextic Polynomial Potentials

abstract:
  A Hamiltonian is said to be quasi-exactly solvable (QES) if some of the
energy levels and the corresponding eigenfunctions can be calculated exactly
and in closed form. An entirely new class of QES Hamiltonians having sextic
polynomial potentials is constructed. These new Hamiltonians are different from
the sextic QES Hamiltonians in the literature because their eigenfunctions obey
PT-symmetric rather than Hermitian boundary conditions. These new Hamiltonians
present a novel problem that is not encountered when the Hamiltonian is
Hermitian: It is necessary to distinguish between the parametric region of
unbroken PT symmetry, in which all of the eigenvalues are real, and the region
of broken PT symmetry, in which some of the eigenvalues are complex. The
precise location of the boundary between these two regions is determined
numerically using extrapolation techniques and analytically using WKB analysis.


title:
A New Approach to Solve the Low-lying States of the Schroedinger
  Equation

abstract:
  We review a new iterative procedure to solve the low-lying states of the
Schroedinger equation, done in collaboration with Richard Friedberg. For the
groundstate energy, the $n^{th}$ order iterative energy is bounded by a finite
limit, independent of $n$; thereby it avoids some of the inherent difficulties
faced by the usual perturbative series expansions. For a fairly large class of
problems, this new procedure can be proved to give convergent iterative
solutions. These convergent solutions include the long standing difficult
problem of a quartic potential with either symmetric or asymmetric minima.


title: A scheme for quantum communication using EPR pairs and local measurement

abstract:
  We present a scheme for quantum communication, where a set of EPR pairs,
initially shared by the sender Alice and the receiver Bob, functions as a
quantum channel. After insuring the safety of the quantum channel, Alice
applies local measurement on her particles of the EPR pairs and informs Bob the
encoding classical information publicly. According to Alice's classical
information and his measurement outcomes on the EPR pairs Bob can infer the
secret messages directly. In this scheme, to transmit one bit secret message,
the sender Alice only needs to send one bit classical information to the
receiver Bob. We also show that this scheme is completely secure if perfect
quantum channel is used.


title: The Symmetric Group Defies Strong Fourier Sampling: Part I

abstract:
  We resolve the question of whether Fourier sampling can efficiently solve the
hidden subgroup problem. Specifically, we show that the hidden subgroup problem
over the symmetric group cannot be efficiently solved by strong Fourier
sampling, even if one may perform an arbitrary POVM on the coset state. Our
results apply to the special case relevant to the Graph Isomorphism problem.


title: The quantum adversary method and classical formula size lower bounds

abstract:
  We introduce two new complexity measures for Boolean functions, or more
generally for functions of the form f:S->T. We call these measures sumPI and
maxPI. The quantity sumPI has been emerging through a line of research on
quantum query complexity lower bounds via the so-called quantum adversary
method [Amb02, Amb03, BSS03, Zha04, LM04], culminating in [SS04] with the
realization that these many different formulations are in fact equivalent.
Given that sumPI turns out to be such a robust invariant of a function, we
begin to investigate this quantity in its own right and see that it also has
applications to classical complexity theory.
  As a surprising application we show that sumPI^2(f) is a lower bound on the
formula size, and even, up to a constant multiplicative factor, the
probabilistic formula size of f. We show that several formula size lower bounds
in the literature, specifically Khrapchenko and its extensions [Khr71, Kou93],
including a key lemma of [Has98], are in fact special cases of our method.
  The second quantity we introduce, maxPI(f), is always at least as large as
sumPI(f), and is derived from sumPI in such a way that maxPI^2(f) remains a
lower bound on formula size. While sumPI(f) is always a lower bound on the
quantum query complexity of f, this is not the case in general for maxPI(f). A
strong advantage of sumPI(f) is that it has both primal and dual
characterizations, and thus it is relatively easy to give both upper and lower
bounds on the sumPI complexity of functions. To demonstrate this, we look at a
few concrete examples, for three functions: recursive majority of three, a
function defined by Ambainis, and the collision problem.


title:
Continuous measurements in a composite quantum system and possible
  exchange of information between its parts

abstract:
  We study an influence of the continuous measurement in a composite quantum
system C on the evolution of the states of its parts. It is shown that the
character of the evolution (decoherence or recoherence) depends on the type of
the measured quantity and on the initial state of the system. A number of
conditions under which the states of the subsystems of C decohere during the
measuring process are established. We propose a model of the composite system
and specify the observable the measurement of which may result in the
recoherence of the state of one of the subsystems of C. In the framework of
this model we find the optimal regime for the exchange of information between
the parts of C during the measurement. The main characteristics of such a
process are computed. We propose a scheme of detection of the recoherence under
the measurement in a concrete physical experiment.


title: Star clusters dynamics in a laboratory: electrons in an ultracold plasma

abstract:
  Electrons in a spherical ultracold quasineutral plasma at temperature in the
Kelvin range can be created by laser excitation of an ultra-cold laser cooled
atomic cloud. The dynamical behavior of the electrons is similar to the one
described by conventional models of stars clusters dynamics. The single mass
component, the spherical symmetry and no stars evolution are here accurate
assumptions. The analog of binary stars formations in the cluster case is
three-body recombination in Rydberg atoms in the plasma case with the same
Heggie's law: soft binaries get softer and hard binaries get harder. We
demonstrate that the evolution of such an ultracold plasma is dominated by
Fokker-Planck kinetics equations formally identical to the ones controlling the
evolution of a stars cluster. The Virial theorem leads to a link between the
plasma temperature and the ions and electrons numbers. The Fokker-Planck
equation is approximate using gaseous and fluid models. We found that the
electrons are in a Kramers-Michie-King's type quasi-equilibrium distribution as
stars in clusters. Knowing the electron distribution and using forced fast
electron extraction we are able to determine the plasma temperature knowing the
trapping potential depth.

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


title: A quantum lower bound for the query complexity of Simon's problem

abstract:
  Simon in his FOCS'94 paper was the first to show an exponential gap between
classical and quantum computation. The problem he dealt with is now part of a
well-studied class of problems, the hidden subgroup problems. We study Simon's
problem from the point of view of quantum query complexity and give here a
first nontrivial lower bound on the query complexity of a hidden subgroup
problem, namely Simon's problem. Our bound is optimal up to a constant factor.


title: Optimal quantum repeaters for qubits and qudits

abstract:
  A class of optimal quantum repeaters for qubits is suggested. The schemes are
minimal, i.e. involve a single additional probe qubit, and optimal, i.e.
provide the maximum information adding the minimum amount of noise. Information
gain and state disturbance are quantified by fidelities which, for our schemes,
saturate the ultimate bound imposed by quantum mechanics for randomly
distributed signals. Special classes of signals are also investigated, in order
to improve the information-disturbance trade-off. Extension to higher
dimensional signals (qudits) is straightforward.


title:
Feasibility of the interlock protocol against man-in-the-middle attacks
  on quantum cryptography

abstract:
  If an eavesdropper succeeds in compromising the quantum as well as the
classical channels and mimics the receiver "Bob" for the sender "Alice" and
vice versa, one defence strategy is the successive, temporally interlocked
partial transmission of the entire encrypted message.


title:
Teleportation of the States of Motion of Atoms by Interaction with
  Two-Slit Screens and Cavities

abstract:
  We present a scheme of teleportation of the state of motion of atoms making
use of screens with slits and cavities. The fascinating aspects of quantum
mechanics are highlighted making use of entanglement and the interaciton of
atoms with slits.


title:
Using continuous measurement to protect a universal set of quantum gates
  within a perturbed decoherence-free subspace

abstract:
  We consider a universal set of quantum gates encoded within a perturbed
decoherence-free subspace of four physical qubits. Using second-order
perturbation theory and a measuring device modeled by an infinite set of
harmonic oscillators, simply coupled to the system, we show that continuous
observation of the coupling agent induces inhibition of the decoherence due to
spurious perturbations. We thus advance the idea of protecting or even creating
a decoherence-free subspace for processing quantum information.


title:
Equivalence between two-mode spin squeezed states and pure entangled
  states with equal spin

abstract:
  We prove that a pure entangled state of two subsystems with equal spin is
equivalent to a two-mode spin-squeezed state under local operations except for
a set of bipartite states with measure zero, and we provide a counterexample to
the generalization of this result to two subsystems of unequal spin.


title: The Symmetric Group Defies Strong Fourier Sampling: Part II

abstract:
  Part I of this paper showed that the hidden subgroup problem over the
symmetric group--including the special case relevant to Graph
Isomorphism--cannot be efficiently solved by strong Fourier sampling, even if
one may perform an arbitrary POVM on the coset state. In this paper, we extend
these results to entangled measurements. Specifically, we show that the hidden
subgroup problem on the symmetric group cannot be solved by any POVM applied to
pairs of coset states. In particular, these hidden subgroups cannot be
determined by any polynomial number of one- or two-register experiments on
coset states.


title:
A comparative analysis of tunneling time concepts: Where do transmitted
  particles start from, on the average?

abstract:
  In this paper we compare the concept of the tunneling time introduced in
quant-ph/0405028 with those of the phase and dwell times. As is shown, unlike
the latter our definition of the transmission time coincides, in the limit of
weak scattering potentials, with that for a free particle. This is valid for
all values of the particle's momentum, including the case of however slow
particles. All three times are also considered for a resonant tunneling. In all
the cases the main feature to distinguish our concept from others is that the
average starting point of transmitted (reflected) particles does not coincide
with that of all particles. One has to stress here that there is no such an
experiment which would give coordinates of all the three points,
simultaneously. For measuring the position of the average starting point of
transmitted particles we propose an experimental scheme based on the Larmor
precession effect.


title: Quantum parallel dense coding of optical images

abstract:
  We propose quantum dense coding protocol for optical images. This protocol
extends the earlier proposed dense coding scheme for continuous variables
[S.L.Braunstein and H.J.Kimble, Phys.Rev.A 61, 042302 (2000)] to an essentially
multimode in space and time optical quantum communication channel. This new
scheme allows, in particular, for parallel dense coding of non-stationary
optical images. Similar to some other quantum dense coding protocols, our
scheme exploits the possibility of sending a classical message through only one
of the two entangled spatially-multimode beams, using the other one as a
reference system. We evaluate the Shannon mutual information for our protocol
and find that it is superior to the standard quantum limit. Finally, we show
how to optimize the performance of our scheme as a function of the
spatio-temporal parameters of the multimode entangled light and of the input
images.


title: Bell's inequality tests with meson-antimeson pairs

abstract:
  Recent proposals to test Bell's inequalities with entangled pairs of
pseudoscalar mesons are reviewed. This includes pairs of neutral kaons or
B-mesons and offers some hope to close both the locality and the detection
loopholes. Specific difficulties, however, appear thus invalidating most of
those proposals. The best option requires the use of kaon regeneration effects
and could lead to a successful test if moderate kaon detection efficiencies are
achieved.


title: On Collective Effects in Cavity Quantum Electrodynamics

abstract:
  We investigate the role of collective effects in the micromaser system as
used in various studies of the physics of cavity electrodynamics. We focus our
attention on the effect on large-time correlations due to multi-atom
interactions. The influence of detection efficiencies and collective effects on
the appearance of trapping states at low temperatures is also found to be of
particular importance.


title:
Implementation Schemes for the Factorized Quantum Lattice-Gas Algorithm
  for the One Dimensional Diffusion Equation using Persistent-Current Qubits

abstract:
  We present two experimental schemes that can be used to implement the
Factorized Quantum Lattice-Gas Algorithm for the 1D Diffusion Equation with
Persistent-Current Qubits. One scheme involves biasing the PC Qubit at multiple
flux bias points throughout the course of the algorithm. An implementation
analogous to that done in Nuclear Magnetic Resonance Quantum Computing is also
developed. Errors due to a few key approximations utilized are discussed and
differences between the PC Qubit and NMR systems are highlighted.


title:
Practical measurement of joint weak values and their connection to the
  annihilation operator

abstract:
  Weak measurements are a new tool for characterizing post-selected quantum
systems during their evolution. Weak measurement was originally formulated in
terms of von Neumann interactions which are practically available for only the
simplest single-particle observables. In the present work, we extend and
greatly simplify a recent, experimentally feasible, reformulation of weak
measurement for multiparticle observables [Resch and Steinberg (2004, Phys.
Rev. Lett., 92, 130402)]. We also show that the resulting ``joint weak values''
take on a particularly elegant form when expressed in terms of annihilation
operators.


title: Impossibility of perfect quantum sealing of classical information

abstract:
  Sealing information means making it publicly available, but with the
possibility of knowing if it has been read. Commenting on [1], we will show
that perfect quantum sealing is not possible for perfectly retrievable
information, due to the possibility of performing a perfect measurement without
disturbance, even on unknown states. The measurement is a collective one, and
this makes the protocol of quantum sealing very interesting as the only example
of the power of collective measurements in breaking security.


title: Quantum Goppa Codes over Hyperelliptic Curves

abstract:
  This Diplom thesis provides an explicit construction of a quantum Goppa code
for any hyperelliptic curve over a non-binary field. Hyperelliptic curves have
conjugate pairs of rational places. We use these pairs to construct
self-orthogonal classical Goppa codes with respect to a weighted inner product.
These codes are also self-orthogonal with respect to a symplectic inner product
and therefore define quantum stabilizer codes. A final transformation leads to
a quantum Goppa code with respect to the standard symplectic inner product.
Some examples illustrate the described construction.
  Furthermore we present a projection of a higher dimensional code onto the
base field and a special case when the projected code is again weighted
self-orthogonal and symmetric.


title: Teleportation of bipartite states using a single entangled pair

abstract:
  A class of quantum protocols to teleport bipartite (entangled) states of two
qubits is suggested. Our schemes require a single entangled pair shared by the
two parties and the transmission of three bits of classical information, as
well as a two-qubit gate with an additional qubit at the receiver' location.
Noisy quantum channels are considered and the effects on both the teleportation
fidelity and the entanglement of the replica are evaluated.


title: Buckyball Quantum Computer: Realization of a Quantum Gate

abstract:
  We have studied a system composed by two endohedral fullerene molecules. We
have found that this system can be used as good candidate for the realization
of Quantum Gates Each of these molecules encapsules an atom carrying a
spin,therefore they interact through the spin dipole interaction. We show that
a phase gate can be realized if we apply on each encased spin static and time
dependent magnetic field. We have evaluated the operational time of a
$\pi$-phase gate, which is of the order of ns. We made a comparison between the
theoretical estimation of the gate time and the experimental decoherence time
for each spin. The comparison shows that the spin relaxation time is much
larger than the $\pi$-gate operational time. Therefore, this indicates that,
during the decoherence time, it is possible to perform some thousands of
quantum computational operations. Moreover, through the study of concurrence,
we get very good results for the entanglement degree of the two-qubit system.
This finding opens a new avenue for the realization of Quantum Computers.


title: Quantum-noise--randomized data-encryption for WDM fiber-optic networks

abstract:
  We demonstrate high-rate randomized data-encryption through optical fibers
using the inherent quantum-measurement noise of coherent states of light.
Specifically, we demonstrate 650Mbps data encryption through a 10Gbps
data-bearing, in-line amplified 200km-long line. In our protocol, legitimate
users (who share a short secret-key) communicate using an M-ry signal set while
an attacker (who does not share the secret key) is forced to contend with the
fundamental and irreducible quantum-measurement noise of coherent states.
Implementations of our protocol using both polarization-encoded signal sets as
well as polarization-insensitive phase-keyed signal sets are experimentally and
theoretically evaluated. Different from the performance criteria for the
cryptographic objective of key generation (quantum key-generation), one
possible set of performance criteria for the cryptographic objective of data
encryption is established and carefully considered.


title: Entanglement Swapping: Entangling Atoms That Never Interacted

abstract:
  In this paper we discuss four different proposals of entangling atomic states
of particles which have never interacted. The experimental realization proposed
makes use of the interaction of Rydberg atoms with a micromaser cavity prepared
in either a coherent state or in a superposition of the zero and one field Fock
states. We consider atoms in either a three-level cascade or lambda
configuration


title: Multipartite Entanglement Measure

abstract:
  In this paper, we generalize the residual entanglement to the case of
multipartite states in arbitrary dimensions by making use of a new method.
Through the introduction of a special entanglement measure, the residual
entanglement of mixed states takes on a form that is more elegant than that in
Ref.[7] (Phys.Rev.A 61 (2000) 052306) . The result obtained in this paper is
different from the previous one given in Ref.[8] (Phys.Rev.A 63 (2000) 044301).
Several examples demonstrate that our present result is a good measurement of
the multipartite entanglement. Furthermore, the original residual entanglement
is a special case of our result.


title:
Localization of Relative-Position of Two Atoms Induced by Spontaneous
  Emission

abstract:
  We revisit the back-action of emitted photons on the motion of the relative
position of two cold atoms. We show that photon recoil resulting from the
spontaneous emission can induce the localization of the relative position of
the two atoms through the entanglement between the spatial motion of individual
atoms and their emitted photons. The result provides a more realistic model for
the analysis of the environment-induced localization of a macroscopic object.


title: Quantum perfect correlations

abstract:
  The notion of perfect correlations between arbitrary observables, or more
generally arbitrary POVMs, is introduced in the standard formulation of quantum
mechanics, and characterized by several well-established statistical
conditions. The transitivity of perfect correlations is proved to generally
hold, and applied to a simple articulation for the failure of Hardy's
nonlocality proof for maximally entangled states. The notion of perfect
correlations between observables and POVMs is used for defining the notion of a
precise measurement of a given observable in a given state. A longstanding
misconception on the correlation made by the measuring interaction is resolved
in the light of the new theory of quantum perfect correlations.


title: Entanglement and spin squeezing properties for three bosons in two modes

abstract:
  We discuss the canonical form for a pure state of three identical bosons in
two modes, and classify its entanglement correlation into two types, the
analogous GHZ and the W types as well known in a system of three
distinguishable qubits. We have performed a detailed study of two important
entanglement measures for such a system, the concurrence $\mathcal{C}$ and the
triple entanglement measure $\tau$. We have also calculated explicitly the spin
squeezing parameter $\xi$ and the result shows that the W state is the most
``anti-squeezing'' state, for which the spin squeezing parameter cannot be
regarded as an entanglement measure.


title: Teleportation in the Background of Schwarzschild Space-time

abstract:
  Quantum teleportation is investigated between Alice who is far from the
horizon and Bob locates near the event horizon of a Schwarzschild black hole.
The results show that the fidelity of the teleportation is reduced in this
curved space-time. However, high fidelity can still be reached outside a
massive black hole.


title: Quantum Detection with Unknown States

abstract:
  We address the problem of distinguishing among a finite collection of quantum
states, when the states are not entirely known. For completely specified
states, necessary and sufficient conditions on a quantum measurement minimizing
the probability of a detection error have been derived. In this work, we assume
that each of the states in our collection is a mixture of a known state and an
unknown state. We investigate two criteria for optimality. The first is
minimization of the worst-case probability of a detection error. For the second
we assume a probability distribution on the unknown states, and minimize of the
expected probability of a detection error.
  We find that under both criteria, the optimal detectors are equivalent to the
optimal detectors of an ``effective ensemble''. In the worst-case, the
effective ensemble is comprised of the known states with altered prior
probabilities, and in the average case it is made up of altered states with the
original prior probabilities.


title: Geometric phase in dephasing systems

abstract:
  Beyond the quantum Markov approximation, we calculate the geometric phase of
a two-level system driven by a quantized magnetic field subject to phase
dephasing. The phase reduces to the standard geometric phase in the weak
coupling limit and it involves the phase information of the environment in
general. In contrast with the geometric phase in dissipative systems, the
geometric phase acquired by the system can be observed on a long time scale. We
also show that with the system decohering to its pointer states, the geometric
phase factor tends to a sum over the phase factors pertaining to the pointer
states.


title:
High-fidelity teleportation beyond the no-cloning limit and entanglement
  swapping for continuous variables

abstract:
  We experimentally demonstrate continuous-variable quantum teleportation
beyond the no-cloning limit. We teleport a coherent state and achieve the
fidelity of 0.70$\pm$0.02 that surpasses the no-cloning limit of 2/3.
Surpassing the limit is necessary to transfer the nonclassicality of an input
quantum state. By using our high-fidelity teleporter, we demonstrate
entanglement swapping, namely teleportation of quantum entanglement, as an
example of transfer of nonclassicality.


title:
Exactly solvable non-Hermitian Jaynes-Cummings-type Hamiltonian
  admitting entirely real spectra from supersymmetry

abstract:
  It is shown that for a given Hermitian Hamiltonian possessing supersymmetry,
there is alwayas a non-hermitian Jaynes-Cummings-type Hamiltonian(JCTH)
admitting entirely real spectra. The parent supersymmetric Hamiltonian and the
corresponding non-hermitian JCTH are simultaneously diagonalizable. The exact
eigenstates of these non-hermitian Hamiltonians are constructed algebraically
for certain shape-invariant potentials, including a non-hermitian version of
the standard Jaynes-Cummings model for which the parent supersymmetric
Hamiltonian is the superoscillator. The positive-definite metric operator in
the Hilbert space is constructed explicitly along with the introduction of a
new inner product structure, so that the eigenstates form a complete set of
orthonormal vectors and the time-evolution is unitary.


title: Hamiltotian Formalism of Game Theory

abstract:
  A new representation of Game Theory is developed in this paper. State of
players is represented by a density matrix, and payoff function is a set of
hermitian operators, which when applied onto the density matrix give the payoff
of players. By this formulism, a new way to find the equilibria of games is
given by generalizing the thermodynamical evolutionary process leading to
equilibrium in Statistical Mechanics. And in this formulism, when quantum
objects instead of classical objects are used as the objects in the game, it's
naturally leads to the so-called Quantum Game Theory, but with a slight
difference in the definition of strategy state of players: the probability
distribution is replaced with a density matrix. Further more, both games of
correlated and independent players can be reached in this single framework,
while traditionally, they are treated separately by Non-cooperative Game Theory
and Coalitional Game Theory. Because of the density matrix is used as state of
players, besides classical correlated strategy, quantum entangled states can
also be used as strategies, which is an entanglement of strategies between
players, and it is different with the entanglement of objects' states as in the
so-called Quantum Game Theory. At last, in the form of density matrix, a class
of quantum games, where the payoff matrixes are commutative, can be reduced
into classical games. In this sense, it will put the classical game as a
special case of our quantum game.


title: Local cloning of two product states

abstract:
  Local quantum operations and classical communication (LOCC) put considerable
constraints on many quantum information processing tasks such as cloning and
discrimination. Surprisingly however, discrimination of any two pure states
survives such constraints in some sense. In this paper, we show that cloning is
not that lucky; namely, conclusive LOCC cloning of two product states is
strictly less efficient than global cloning.


title: Uncertainty, Monogamy, and Locking of Quantum Correlations

abstract:
  Squashed entanglement and entanglement of purification are quantum mechanical
correlation measures and defined as certain minimisations of entropic
quantities. We present the first non-trivial calculations of both quantities.
Our results lead to the conclusion that both measures can drop by an arbitrary
amount when only a single qubit of a local system is lost. This property is
known as "locking" and has previously been observed for other correlation
measures, such as the accessible information, entanglement cost and the
logarithmic negativity.
  In the case of squashed entanglement, the results are obtained with the help
of an inequality that can be understood as a quantum channel analogue of
well-known entropic uncertainty relations. This inequality may prove a useful
tool in quantum information theory.
  The regularised entanglement of purification is known to equal the
entanglement needed to prepare a many copies of quantum state by local
operations and a sublinear amount of communication. Here, monogamy of quantum
entanglement (i.e., the impossibility of a system being maximally entangled
with two others at the same time) leads to an exact calculation for all quantum
states that are supported either on the symmetric or on the antisymmetric
subspace of a dxd-dimensional system.


title:
Controlling the Spontaneous Emission Rate of Single Quantum Dots in a 2D
  Photonic Crystal

abstract:
  We observe large spontaneous emission rate modification of individual InAs
Quantum Dots (QDs) in 2D a photonic crystal with a modified, high-Q single
defect cavity. Compared to QDs in bulk semiconductor, QDs that are resonant
with the cavity show an emission rate increase by up to a factor of 8. In
contrast, off-resonant QDs indicate up to five-fold rate quenching as the local
density of optical states (LDOS) is diminished in the photonic crystal. In both
cases we demonstrate photon antibunching, showing that the structure represents
an on-demand single photon source with pulse duration from 210 ps to 8 ns. We
explain the suppression of QD emission rate using Finite Difference Time Domain
(FDTD) simulations and find good agreement with experiment.


title:
Detection of atomic entanglement and electromagnetically induced
  transparency in velocity-selective coherent population trapping

abstract:
  We investigate theoretically the optical properties of an atomic gas which
has been cooled by the laser cooling method velocity-selective coherent
population trapping. We demonstrate that the application of a weak laser pulse
gives rise to a backscattered pulse, which is a direct signal for the
entanglement in the atomic system, and which leads to single-particle
entanglement on the few-photon level. If the pulse is applied together with the
pump lasers, it also displays the phenomenon of electromagnetically induced
transparency. We suggest that the effect should be observable in a gas of
Rubidium atoms.


title:
On nonlinear evolution and supraluminal communication between finite
  quantum systems

abstract:
  We revise the 'no-signaling' condition for the supraluminal communication
between two spatially separated finite quantum systems of arbitrary dimensions,
thus generalizing a similar preceding approach for two-qubits: non-linear
evolution does not necessarily imply the possibility of supraluminal
communication between any sort of finite quantum systems.


title: Mapping deformed hyperbolic potentials into nondeformed ones

abstract:
  In this work we introduce a mapping between the so called deformed hyperbolic
potentials, which are presenting a continuous interest in the last few years,
and the corresponding nondeformed ones. As a consequence, we conclude that
these deformed potentials do not pertain to a new class of exactly solvable
potentials, but to the same one of the corresponding nondeformed ones.
Notwithstanding, we can reinterpret this type of deformation as a kind of
symmetry of the nondeformed potentials.


title:
Calculations of Electric Capacitance in Carbon and BN Nanotubes, and
  Zigzag Nanographite (BN, BCN) Ribbons

abstract:
  Electronic states in nanographite ribbons with zigzag edges are studied using
the extended Hubbard model with nearest neighbor Coulomb interactions. The
electronic states with the opposite electric charges separated along both edges
are analogous as nanocondensers. Therefore, electric capacitance, defined using
a relation of polarizability, is calculated to examine nano-functionalities. We
find that the behavior of the capacitance is widely different depending on
whether the system is in the magnetic or charge polarized phases. In the
magnetic phase, the capacitance is dominated by the presence of the edge states
while the ribbon width is small. As the ribbon becomes wider, the capacitance
remains with large magnitudes as the system develops into metallic zigzag
nanotubes. It is proportional to the inverse of the width, when the system
corresponds to the semiconducting nanotubes and the system is in the charge
polarized phase also. The latter behavior could be understood by the presence
of an energy gap for charge excitations. In the BN (BCN) nanotubes and ribbons,
the electronic structure is always like of semiconductors. The calculated
capacitance is inversely proportional to the distance between the positive and
negative electrodes.

categories: quant-ph cond-mat.mtrl-sci cond-mat.str-el physics.chem-ph


title: Sequential generation of entangled multi-qubit states

abstract:
  We consider the deterministic generation of entangled multi-qubit states by
the sequential coupling of an ancillary system to initially uncorrelated
qubits. We characterize all achievable states in terms of classes of matrix
product states and give a recipe for the generation on demand of any
multi-qubit state. The proposed methods are suitable for any sequential
generation-scheme, though we focus on streams of single photon time-bin qubits
emitted by an atom coupled to an optical cavity. We show, in particular, how to
generate familiar quantum information states such as W, GHZ, and cluster
states, within such a framework.


title: Decoherence in Quantum Systems

abstract:
  We discuss various definitions of decoherence and how it can be measured. We
compare and contrast decoherence in quantum systems with an infinite number of
eigenstates (such as the free particle and the oscillator) and spin systems. In
the former case, we point out the essential difference between assuming
"entanglement at all times" and entanglement with the reservoir occuring at
some initial time. We also discuss optimum calculational techniques in both
arenas.


title: Quantum Zeno and anti-Zeno Effects: An Exact Model

abstract:
  Recent studies suggest that both the quantum Zeno (increase of the natural
lifetime of an unstable quantum state by repeated measurements) and anti-Zeno
(decrease of the natural lifetime) effects can be made manifest in the same
system by simply changing the dissipative decay rate associated with the
environment. We present an {\underline{exact}} calculation confirming this
expectation.


title:
Simple Rate-1/3 Convolutional and Tail-Biting Quantum Error-Correcting
  Codes

abstract:
  Simple rate-1/3 single-error-correcting unrestricted and CSS-type quantum
convolutional codes are constructed from classical self-orthogonal
$\F_4$-linear and $\F_2$-linear convolutional codes, respectively. These
quantum convolutional codes have higher rate than comparable quantum block
codes or previous quantum convolutional codes, and are simple to decode. A
block single-error-correcting [9, 3, 3] tail-biting code is derived from the
unrestricted convolutional code, and similarly a [15, 5, 3] CSS-type block code
from the CSS-type convolutional code.


title:
A two-step MaxLik-MaxEnt strategy to infer photon distribution from
  on/off measurement at low quantum efficiency

abstract:
  A method based on Maximum-Entropy (ME) principle to infer photon distribution
from on/off measurements performed with few and low values of quantum
efficiency is addressed. The method consists of two steps: at first some
moments of the photon distribution are retrieved from on/off statistics using
Maximum-Likelihood estimation, then ME principle is applied to infer the
quantum state and, in turn, the photon distribution. Results from simulated
experiments on coherent and number states are presented.


title: Stabilizing an atom laser using spatially selective pumping and feedback

abstract:
  We perform a comprehensive study of stability of a pumped atom laser in the
presence of pumping, damping and outcoupling. We also introduce a realistic
feedback scheme to improve stability by extracting energy from the condensate
and determine its effectiveness. We find that while the feedback scheme is
highly efficient in reducing condensate fluctuations, it usually does not alter
the stability class of a particular set of pumping, damping and outcoupling
parameters.


title:
Process reconstruction: From unphysical to physical maps via maximum
  likelihood

abstract:
  We show that the method of maximum likelihood (MML) provides us with an
efficient scheme for reconstruction of quantum channels from incomplete
measurement data. By construction this scheme always results in estimations of
channels that are completely positive. Using this property we use the MML for a
derivation of physical approximations of un-physical operations. In particular,
we analyze the optimal approximation of the universal NOT gate as well as a
physical approximation of a quantum nonlinear polarization rotation.


title:
Einstein's coefficients and the wave-particle duality in the theory of
  thermal radiation

abstract:
  It is shown that the concept of elementary resonator in the theory of thermal
radiation implies the indivisible connection between particles (photons) and
electromagnetic waves. This wave-particle duality covers both the Wien and
Rayleigh-Jeans regions of spectrum.


title: Wigner Functions and Separability for Finite Systems

abstract:
  A discussion of discrete Wigner functions in phase space related to mutually
unbiased bases is presented. This approach requires mathematical assumptions
which limits it to systems with density matrices defined on complex Hilbert
spaces of dimension p^n where p is a prime number. With this limitation it is
possible to define a phase space and Wigner functions in close analogy to the
continuous case. That is, we use a phase space that is a direct sum of n
two-dimensional vector spaces each containing p^2 points. This is in contrast
to the more usual choice of a two-dimensional phase space containing p^(2n)
points. A useful aspect of this approach is that we can relate complete
separability of density matrices and their Wigner functions in a natural way.
We discuss this in detail for bipartite systems and present the generalization
to arbitrary numbers of subsystems when p is odd. Special attention is required
for two qubits (p=2) and our technique fails to establish the separability
property for more than two qubits.


title:
Unconditional fidelity thresholds in single copy distillation and some
  aspects of quantum error correction

abstract:
  Various aspects of distillation of noisy entanglement and some associated
effects in quantum error correction are considered. In particular we prove that
if only one--way classical communication (from Alice to Bob) is allowed and the
shared $d \otimes d$ state is not pure then there is a threshold for optimal
entanglement fraction $F$ of the state (being an overlap between the shared
state and symmetric maximally entangled state) which can be obtained in single
copy distillation process. This implies that to get (probabilistically)
arbitrary good conclusive teleportation via mixed state at least one classical
bit of backward communication (for Bob to Alice) has to be sent. We provide
several other threshold properties in this context including in particular the
existence of ultimate threshold of optimal $F$ for states of full rank. Finally
the threshold results are linked to those of error correction. Namely it is
pointed out that in quantum computer working on fixed number of quantum bits
almost any kind of noise can be (probabilistically) corrected only to some
threshold error bar though there are some (rare) exceptions.


title:
Demonstration of the spatial separation of the entangled quantum
  side-bands of an optical field

abstract:
  Quantum optics experiments on "bright" beams typically probe correlations
between side-band modes. However the extra degree of freedom represented by
this dual mode picture is generally ignored. We demonstrate the experimental
operation of a device which can be used to separate the quantum side-bands of
an optical field. We use this device to explicitly demonstrate the quantum
entanglement between the side-bands of a squeezed beam.


title:
Probabilistically implementing nonlocal operation using non-maximally
  entangled state

abstract:
  We develop the probabilistic implementation of a nonlocal gate
$\exp{[i\xi{\sigma_{n_A}}\sigma_{n_B}]}$ and $\xi\in[0,\frac\pi4]$, by using a
single non-maximally entangled state. We prove that, nonlocal gates can be
implemented with a fidelity greater than 79.3% and a consumption of less than
0.969 ebits and 2 classical bits, when $\xi\leq0.353$. This provides a higher
bound for the feasible operation compared to the former techniques
\cite{Cirac,Groisman,Bennett-1}. Besides, gates with $\xi\geq0.353$ can be
implemented with the probability 79.3% and a consumption of 0.969 ebits, which
is the same efficiency as the distillation-based protocol
\cite{Groisman,Bennett-1}, while our method saves extra classical resource.
Gates with $\xi\to0$ can be implemented with nearly unit probability and a
small entanglement. We also generalize some application to the multiple system,
where we find it is possible to implement certain nonlocal gates between many
non-entangled partners using a non-maximally multiple entangled state.


title: Fault testing quantum switching circuits

abstract:
  Test pattern generation is an electronic design automation tool that attempts
to find an input (or test) sequence that, when applied to a digital circuit,
enables one to distinguish between the correct circuit behavior and the faulty
behavior caused by particular faults. The effectiveness of this classical
method is measured by the fault coverage achieved for the fault model and the
number of generated vectors, which should be directly proportional to test
application time. This work address the quantum process validation problem by
considering the quantum mechanical adaptation of test pattern generation
methods used to test classical circuits. We found that quantum mechanics allows
one to execute multiple test vectors concurrently, making each gate realized in
the process act on a complete set of characteristic states in space/time
complexity that breaks classical testability lower bounds.


title: A Unified Picture of Decoherence Control

abstract:
  The different methods of reducing decoherence in quantum devices are
discussed from the unified point of view based on the energy conservation
principle and the concept of forbidden transitions. Minimal decoherence model,
"bang-bang" techniques, Zeno effect and decoherence-free subspaces and
subsystems are studied as particular examples.


title: Entanglement and majorization in (1+1)-dimensional quantum systems

abstract:
  Motivated by the idea of entanglement loss along Renormalization Group flows,
analytical majorization relations are proven for the ground state of
(1+1)-dimensional conformal field theories. For any of these theories,
majorization is proven to hold in the spectrum of the reduced density matrices
in a bipartite system when changing the size L of one of the subsystems.
Continuous majorization along uniparametric flows is also proven as long as
part of the conformal structure is preserved under the deformation and some
monotonicity conditions hold as well. As particular examples of our
derivations, we study the cases of the XX, Heisenberg and XY quantum spin
chains. Our results provide in a rigorous way explicit proves for all the
majorization conjectures raised by Latorre, Lutken, Rico, Vidal and Kitaev in
previous papers on quantum spin chains.


title: Quantum theory of microworld and the reality

abstract:
  The mathematical model of orthodox quantum mechanics has been critically
examined and some deficiencies have been summarized. The model based on the
extended Hilbert space and free of these shortages has been proposed;
parameters being until now denoted as "hidden" have been involved. Some earlier
arguments against a hidden-variable theory have been shown to be false, too. In
the known Einstein-Bohr controversy Einstein has been shown to be true. The
extended model seems to be strongly supported also by the polarization
experiments performed by us ten years ago.


title: Cluster States for Continuous-Variable Multipartite Entanglement

abstract:
  We introduce a new class of continuous-variable (CV) multipartite entangled
states, the CV cluster states, which might be generated from squeezing and
kerr-like interaction. The entanglement properties of these states are studied
in terms of classical communication and local operations. The quantum
teleportation network with cluster states is investigated. The graph states as
the general forms of cluster states are presented, which may be used to
generate CV Greenberger-Horne-Zeilinger states by simply local measurements and
classical communication. A chain for one-dimensional example of cluster states
can be readily experimentally produced only with squeezed light and
beamsplitters.


title: Entanglement production in Quantized Chaotic Systems

abstract:
  Quantum chaos is a subject whose major goal is to identify and to investigate
different quantum signatures of classical chaos. Here we study entanglement
production in coupled chaotic systems as a possible quantum indicator of
classical chaos. We use coupled kicked tops as a model for our extensive
numerical studies. We find that, in general, presence of chaos in the system
produces more entanglement. However, coupling strength between two subsystems
is also very important parameter for the entanglement production. Here we show
how chaos can lead to large entanglement which is universal and describable by
random matrix theory (RMT). We also explain entanglement production in coupled
strongly chaotic systems by deriving a formula based on RMT. This formula is
valid for arbitrary coupling strengths, as well as for sufficiently long time.
Here we investigate also the effect of chaos on the entanglement production for
the mixed initial state. We find that many properties of the mixed state
entanglement production are qualitatively similar to the pure state
entanglement production. We however still lack an analytical understanding of
the mixed state entanglement production in chaotic systems.


title: Instantaneous Measurement of field quadrature moments and entanglement

abstract:
  We present a method of measuring expectation values of quadrature moments of
a multimode field through two-level probe ``homodyning''. Our approach is based
on an integral transform formalism of measurable probe observables, where
analytically derived kernels unravel efficiently the required field information
at zero interaction time, minimizing decoherence effects. The proposed scheme
is suitable for fields that, while inaccessible to a direct measurement, enjoy
one and two-photon Jaynes-Cummings interactions with a two-level probe, like
spin, phonon, or cavity fields. Available data from previous experiments are
used to confirm our predictions.


title: Synthesizing arbitrary two-photon polarization mixed states

abstract:
  Two methods for creating arbitrary two-photon polarization pure states are
introduced. Based on these, four schemes for creating two-photon polarization
mixed states are proposed and analyzed. The first two schemes can synthesize
completely arbitrary two-qubit mixed states, i.e., control all 15 free
parameters: Scheme I requires several sets of crystals, while Scheme II
requires only a single set, but relies on decohering the pump beam.
Additionally, we describe two further schemes which are much easier to
implement. Although the total capability of these is still being studied, we
show that they can synthesize all two-qubit Werner states, maximally entangled
mixed states, Collins-Gisin states, and arbitrary Bell-diagonal states.


title: Identifying an Experimental Two-State Hamiltonian to Arbitrary Accuracy

abstract:
  Precision control of a quantum system requires accurate determination of the
effective system Hamiltonian. We develop a method for estimating the
Hamiltonian parameters for some unknown two-state system and providing
uncertainty bounds on these parameters. This method requires only one
measurement basis and the ability to initialise the system in some arbitrary
state which is not an eigenstate of the Hamiltonian in question. The scaling of
the uncertainty is studied for large numbers of measurements and found to be
proportional to one on the square-root of the number of measurements.


title: Entanglement in spin-1/2 dimerized Heisenberg systems

abstract:
  We study entanglement in dimerized Heisenberg systems. In particular, we give
exact results of ground-state pairwise entanglement for the four-qubit model by
identifying a Z_2 symmetry. Although the entanglements cannot identify the
critical point of the system, the mean entanglement of nearest-neighbor qubits
really does, namely, it reaches a maximum at the critical point.


title: Radiative Corrections to Multi-Level Mollow-Type Spectra

abstract:
  This paper is concerned with two rather basic phenomena: the incoherent
fluorescence spectrum of an atom driven by an intense laser field and the
coupling of the atom to the (empty) modes of the radiation field. The sum of
the many-photon processes gives rise to the inelastic part of the atomic
fluorescence, which, for a two-level system, has a well-known characteristic
three-peak structure known as the Mollow spectrum. From a theoretical point of
view, the Mollow spectrum finds a natural interpretation in terms of
transitions among laser-dressed states which are the energy eigenstates of a
second-quantized two-level system strongly coupled to a driving laser field. As
recently shown, the quasi-energies of the laser-dressed states receive
radiative corrections which are nontrivially different from the results which
one would expect from an investigation of the coupling of the bare states to
the vacuum modes. In this article, we briefly review the basic elements
required for the analysis of the dynamic radiative corrections, and we
generalize the treatment of the radiative corrections to the incoherent part of
the steady-state fluorescence to a three-level system consisting of 1S, 3P and
2S states.


title: How the Quantum Universe Became Classical

abstract:
  This is an informal introduction to the ideas of decoherence and emergent
classicality, including a simple account of the decoherent histories approach
to quantum theory. It is aimed at undergraduates with a basic appreciation of
quantum theory. The emphasis is on simple physical ideas and pictures.


title:
Quantum chaos algorithms and dissipative decoherence with quantum
  trajectories

abstract:
  Using the methods of quantum trajectories we investigate the effects of
dissipative decoherence in a quantum computer algorithm simulating dynamics in
various regimes of quantum chaos including dynamical localization, quantum
ergodic regime and quasi-integrable motion.
  As an example we use the quantum sawtooth algorithm which can be implemented
in a polynomial number of quantum gates.
  It is shown that the fidelity of quantum computation decays exponentially
with time and that the decay rate is proportional to the number of qubits,
number of quantum gates and per gate dissipation rate induced by external
decoherence.
  In the limit of strong dissipation the quantum algorithm generates a quantum
attractor which may have complex or simple structure.
  We also compare the effects of dissipative decoherence with the effects of
static imperfections.


title:
Tradeoff between extractable mechanical work, accessible entanglement,
  and ability to act as a reference system, under arbitrary superselection
  rules

abstract:
  Superselection rules (SSRs) limit the mechanical and quantum processing
resources represented by quantum states. However SSRs can be violated using
reference systems to break the underlying symmetry. We show that there is a
duality between the ability of a system to do mechanical work and to act as a
reference system. Further, for a bipartite system in a globally symmetric pure
state, we find a triality between the system's ability to do local mechanical
work, its ability to do ``logical work'' due to its accessible entanglement,
and its ability to act as a shared reference system.


title:
Van der Waals interaction between microparticle and uniaxial crystal
  with application to hydrogen atoms and multiwall carbon nanotubes

abstract:
  The Lifshitz theory of the van der Waals force is extended for the case of an
atom (molecule) interacting with a plane surface of an uniaxial crystal or with
a long solid cylinder or cylindrical shell made of isotropic material or
uniaxial crystal. For a microparticle near a semispace or flat plate made of an
uniaxial crystal the exact expressions for the free energy of the van der Waals
and Casimir-Polder interaction are presented. An approximate expression for the
free energy of microparticle- cylinder interaction is obtained which becomes
precise for microparticle-cylinder separations much smaller than cylinder
radius. The obtained expressions are used to investigate the van der Waals
interaction between hydrogen atoms (molecules) and graphite plates or multiwall
carbon nanotubes. To accomplish this the behavior of graphite dielectric
permittivities along the imaginary frequency axis is found using the optical
data for the complex refractive index of graphite for the ordinary and
extraordinary rays. It is shown that the position of hydrogen atoms inside
multiwall carbon nanotubes is energetically preferable compared with outside.


title: Quantum state expansion

abstract:
  It is experimentally demonstrated that an arbitrary quantum state of a single
spin 1/2: a|u> + b|d> can be converted into a superposition of the two
ferromagnetic states of a spin cluster: a|uu...uu> + b|dd...dd>. The physical
system is a cluster of seven dipolar-coupled nuclear spins of single-labeled
13C-benzene molecules in a liquid-crystalline matrix. In this complex system,
the pseudopure ground state and the required controlled unitary transformations
have been implemented. The experimental scheme can be considered as an explicit
model of quantum measurement.


title:
Unfolding a degeneracy point of two unbound states: Crossings and
  anticrossings of energies and widths

abstract:
  We show that when an isolated doublet of unbound states of a physical system
becomes degenerate for some values of the control parameters of the system, the
energy hypersurfaces representing the complex resonance energy eigenvalues as
functions of the control parameters have an algebraic branch point of rank one
in parameter space. Associated with this singularity in parameter space, the
scattering matrix, S_l(E), and the Green's function, G_l^(+)(k; r,r'), have one
double pole in the unphysical sheet of the complex energy plane. We
characterize the universal unfolding or deformation of a typical degeneracy
point of two unbound states in parameter space by means of a universal
2-parameter family of functions which is contact equivalent to the pole
position function of the isolated doublet of resonances at the exceptional
point and includes all small perturbations of the degeneracy condition up to
contact equivalence.


title: Nonlocal Gate Of Quantum Network Via Cavity Quantum Electrodynamics

abstract:
  We propose an experimentally feasible scheme to realize the nonlocal gate
between two different quantum network nodes. With an entanglement-qubit (ebit)
acts as a quantum channel, our scheme is resistive to actual environment noise
and can get high fidelity in current cavity quantum electrodynamics (C-QED)
system.