title:
Magneto-optical rotation and cross-phase modulation via coherently
  driven tripod atoms

abstract:
  We study the interaction of a weak probe field, having two orthogonally
polarized components, with an optically dense medium of four-level atoms in a
tripod configuration. In the presence of a coherent driving laser,
electromagnetically induced transparency is attained in the medium,
dramatically enhancing its linear as well as nonlinear dispersion while
simultaneously suppressing the probe field absorption. We present the
semiclassical and fully quantum analysis of the system. We propose an
experimentally feasible setup that can induce large Faraday rotation of the
probe field polarization and therefore be used for ultra-sensitive optical
magnetometry. We then study the Kerr nonlinear coupling between the two
components of the probe, demonstrating a novel regime of symmetric, extremely
efficient cross-phase modulation, capable of fully entangling two single-photon
pulses. This scheme may thus pave the way to photon-based quantum information
applications, such as deterministic all-optical quantum computation, dense
coding and teleportation.


title: Quantum cloning in spin networks

abstract:
  We introduce an approach to quantum cloning based on spin networks and we
demonstrate that phase covariant cloning can be realized using no external
control but only with a proper design of the Hamiltonian of the system. In the
1 -> 2 cloning we find that the XY model saturates the value for the fidelity
of the optimal cloner and gives values comparable to it in the genera N -> M
case. We finally discuss the effect of external noise. Our protocol is much
more robust to decoherence than a conventional procedure based on quantum
gates.


title: Stern-Gerlach Experiment and Bohm Limit

abstract:
  Stern-Gerlach experiment is a paradigm of measurement theory in quantum
mechanics. Notwithstanding several analysis given in literature, no clear
understanding of the apparent collapse has been given so far. Indeed, one can
imagine a Stern-Gerlach device where all environmental effects are removed and
ask the question on how the measurement goes on. In this letter we will prove
that a the Stern-Gerlach device behaves as a true measurement apparatus, as
expected by the Copenaghen interpretation, by the Ehrenfest theorem. In this
way we recover, by other means, a limit on the Stern-Gerlach device, given by
Bohm, based on scrambling of phases due to a large oscillation frequency.


title: Two qubits entanglement dynamics in a symmetry-broken environment

abstract:
  We study the temporal evolution of entanglement pertaining to two qubits
interacting with a thermal bath. In particular we consider the simplest
nontrivial spin bath models where symmetry breaking occurs and treat them by
mean field approximation. We analytically find decoherence free entangled
states as well as entangled states with an exponential decay of the quantum
correlation at finite temperature.


title:
Entanglement Preserving in Quantum Copying of Three-qubit Entangled
  State

abstract:
  We study the degree to which quantum entanglement survives when a three-qubit
entangled state is copied by using local and non-local processes, respectively,
and investigate iterating quantum copying for the three-qubit system. There may
exist inter-three-qubit entanglement and inter-two-qubit entanglement for the
three-qubit system. We show that both local and non-local copying processes
degrade quantum entanglement in the three-particle system due to a residual
correlation between the copied output and the copying machine. We also show
that the inter-two-qubit entanglement is preserved better than the
inter-three-qubit entanglement in the local cloning process. We find that
non-local cloning is much more efficient than the local copying for
broadcasting entanglement, and output state via non-local cloning exhibits the
fidelity better than local cloning.


title:
Electromagnetically induced transparency in multi-level cascade scheme
  of cold rubidium atoms

abstract:
  We report an experimental investigation of electromagnetically induced
transparency in a multi-level cascade system of cold atoms. The absorption
spectral profiles of the probe light in the multi-level cascade system were
observed in cold Rb-85 atoms confined in a magneto-optical trap, and the
dependence of the spectral profile on the intensity of the coupling laser was
investigated. The experimental measurements agree with the theoretical
calculations based on the density matrix equations of the rubidium cascade
system.


title: There, and Back Again: Quantum Theory and Global Optimisation

abstract:
  We consider a problem in quantum theory that can be formulated as an
optimisation problem and present a global optimisation algorithm for solving
it, the foundation of which relies in turn on a theorem from quantum theory. To
wit, we consider the maximal output purity $\nu_q$ of a quantum channel as
measured by Schatten $q$-norms, for integer $q$. This quantity is of
fundamental importance in the study of quantum channel capacities in quantum
information theory. To calculate $\nu_q$ one has to solve a non-convex
optimisation problem that typically exhibits local optima. We show that this
particular problem can be approximated to arbitrary precision by an eigenvalue
problem over a larger matrix space, thereby circumventing the problem of local
optima. The mathematical proof behind this algorithm relies on the Quantum de
Finetti theorem, which is a theorem used in the study of the foundations of
quantum theory.
  We expect that the approach presented here can be generalised and will turn
out to be applicable to a larger class of global optimisation problems. We also
present some preliminary numerical results, showing that, at least for small
problem sizes, the present approach is practically realisable.


title: Single-spin readout for buried dopant semiconductor qubits

abstract:
  In the design of quantum computer architectures that take advantage of the
long coherence times of dopant nuclear and electron spins in the solid-state,
single-spin detection for readout remains a crucial unsolved problem. Schemes
based on adiabatically induced spin-dependent electron tunnelling between
individual donor atoms, detected using a single electron transistor (SET) as an
ultra-sensitive electrometer, are thought to be problematic because of the low
ionisaton energy of the final D- state. In this paper we analyse the adiabatic
scheme in detail. We find that despite significant stabilization due to the
presence of the D+, the field strengths required for the transition lead to a
shortened dwell-time placing severe constraints on the SET measurement time. We
therefore investigate a new method based on resonant electron transfer, which
operates with much reduced field strengths. Various issues in the
implementation of this method are also discussed.


title:
Collapses and revivals of exciton emission in a semiconductor
  microcavity: detuning and phase-space filling effects

abstract:
  We investigate exciton emission of quantum well embedded in a semiconductor
microcavity. The analytical expressions of the light intensity for the cases of
excitonic number state and coherent state are presented by using secular
approximation. Our results show that the effective exciton-exciton interaction
leads to the appearance of collapse and revival of the light intensity. The
revival time is twice compared the coherent state case with that of the number
state. The dissipation of the exciton-polariton lowers the revival amplitude
but does not alter the revival time. The influences of the detuning and the
phase-space filling are studied. We find that the effect of the higher-order
exciton-photon interaction may be removed by adjusting the detuning.


title: Spontaneous Collapse of Unstable Quantum Superposition State

abstract:
  On the basis of a proposed model of wave function collapse, we investigate
spontaneous localization of a quantum state. The model is similar to the
Ghirardi-Rimini-Weber model, while we postulate the localization functions to
depend on the quantum state to suffer collapse. According to the model, dual
dynamics in quantum mechanics, deterministic and stochastic time evolution, are
algorithmically implemented in tandem. After discussing the physical
implications of the model qualitatively, we present numerical results for
one-dimensional systems by way of example.


title: Isotropic quantum spin channels and additivity questions

abstract:
  The minimum entropy output is computed for rotationally invariant quantum
channels acting on spin-1/2 and spin-1 systems. For the case of two parallel
such channels and initial entangled (singlet) state the entropy of the output
is higher then the doubled minimal entropy output of the single channel. This
gives a certain moral support to the additivity hypothesis. Another related
simple function of the channel (minimum entropy gain) is shown to be additive
in general.


title: Maximum Likelihood Based Quantum Set Separation

abstract:
  In this paper we introduce a method, which is used for set separation based
on quantum computation. In case of no a-priori knowledge about the source
signal distribution, it is a challenging task to find an optimal decision rule
which could be implemented in the separating algorithm. We lean on the Maximum
Likelihood approach and build a bridge between this method and quantum
counting. The proposed method is also able to distinguish between disjunct sets
and intersection sets.


title: Normal Order: Combinatorial Graphs

abstract:
  A conventional context for supersymmetric problems arises when we consider
systems containing both boson and fermion operators. In this note we consider
the normal ordering problem for a string of such operators. In the general
case, upon which we touch briefly, this problem leads to combinatorial numbers,
the so-called Rook numbers. Since we assume that the two species, bosons and
fermions, commute, we subsequently restrict ourselves to consideration of a
single species, single-mode boson monomials. This problem leads to elegant
generalisations of well-known combinatorial numbers, specifically Bell and
Stirling numbers. We explicitly give the generating functions for some classes
of these numbers. In this note we concentrate on the combinatorial graph
approach, showing how some important classical results of graph theory lead to
transparent representations of the combinatorial numbers associated with the
boson normal ordering problem.


title:
Quantum lithography, entanglement and Heisenberg-limited parameter
  estimation

abstract:
  We explore the intimate relationship between quantum lithography,
Heisenberg-limited parameter estimation and the rate of dynamical evolution of
quantum states. We show how both the enhanced accuracy in measurements and the
increased resolution in quantum lithography follow from the use of
entanglement. Mathematically, the hyperresolution of quantum lithography
appears naturally in the derivation of Heisenberg-limited parameter estimation.
We also review recent experiments offering a proof of principle of quantum
lithography, and we address the question of state preparation and the
fabrication of suitable photoresists.


title: Laser linewidth effects in quantum state discrimination by EIT

abstract:
  We discuss the use of electromagnetically modified absorption to achieve
selective excitation in atoms: that is, the laser excitation of one transition
while avoiding simultaneously exciting another transition whose frequency is
the same as or close to that of the first. The selectivity which can be
achieved in the presence of electromagnetically induced transparency (EIT) is
limited by the decoherence rate of the dark state. We present exact analytical
expressions for this effect, and also physical models and approximate
expressions which give useful insights into the phenomena. When the laser
frequencies are near-resonant with the single-photon atomic transitions, EIT is
essential for achieving discrimination. When the laser frequencies are far
detuned, the `bright' two-photon Raman resonance is important for achieving
selective excitation, while the `dark' resonance (EIT) need not be. The
application to laser cooling of a trapped atom is also discussed.


title: Storing Images in Entangled Quantum Systems

abstract:
  We introduce a new method of storing visual information in Quantum Mechanical
systems which has certain advantages over more restricted classical memory
devices. To do this we employ uniquely Quantum Mechanical properties such as
Entanglement in order to store information concerning the position and shape of
simple objects.


title:
Non-Markovian Quantum Trajectories Versus Master Equations: Finite
  Temperature Heat Bath

abstract:
  The interrelationship between the non-Markovian stochastic Schr\"odinger
equations and the corresponding non-Markovian master equations is investigated
in the finite temperature regimes. We show that the general finite temperature
non-Markovian trajectories can be used to derive the corresponding
non-Markovian master equations. A simple, yet important solvable example is the
well-known damped harmonic oscillator model in which a harmonic oscillator is
coupled to a finite temperature reservoir in the rotating wave approximation.
The exact convolutionless master equation for the damped harmonic oscillator is
obtained by averaging the quantum trajectories relying upon no assumption of
coupling strength or time scale. The master equation derived in this way
automatically preserves the positivity, Hermiticity and unity.


title:
Observation of coherent oscillation of a single nuclear spin and
  realization of a two-qubit conditional quantum gate

abstract:
  Rabi nutations of a single nuclear spin in a solid have been observed. The
experiments were carried out on a single electron and a single 13C nuclear spin
of a single nitrogen vacancy defect center in diamond. The system was used for
implementation of quantum logical NOT and a conditional two-qubit gate (CROT).
Density matrix tomography of the CROT gate shows that the gate fidelity
achieved in our experiments is up to 0.9, good enough to be used in quantum
algorithms.


title:
Comment on "Enhancing Acceleration Radiation from Ground-State Atoms via
  Cavity Quantum Electrodynamics"

abstract:
  This is a comment on [Phys. Rev. Lett. 91, 243004 (2003)] by Marlan O.
Scully, Vitaly V. Kocharovsky, Alexey Belyanin, Edward Fry and Federico Capasso
(quant-ph/0305178).


title:
Quantum dense coding in multiparticle entangled states via local
  measurements

abstract:
  In this paper, we study quantum dense coding between two arbitrarily fixed
particles in a (N+2)-particle maximally-entangled states through introducing an
auxiliary qubit and carrying out local measurements. It is shown that the
transmitted classical information amount through such an entangled quantum
channel usually is less than two classical bits. However, the information
amount may reach two classical bits of information, and the classical
information capacity is independent of the number of the entangled particles in
the initial entangled state under certain conditions. The results offer deeper
insights to quantum dense coding via quantum channels of multi-particle
entangled states.


title: Towards Linear Optical Quantum Computers

abstract:
  Scalable quantum computation with linear optics was considered to be
impossible due to the lack of efficient two-qubit logic gates, despite its ease
of implementation of one-qubit gates. Two-qubit gates necessarily need a
nonlinear interaction between the two photons, and the efficiency of this
nonlinear interaction is typically very tiny in bulk materials. However, we
recently have shown that this barrier can be circumvented with effective
nonlinearities produced by projective measurements, and with this work
linear-optical quantum computing becomes a new possibility of scalable quantum
computation. We review several issues concerning its principles and
requirements.


title:
Quantum linear mutual information and classical correlations in globally
  pure bipartite systems

abstract:
  We investigate the correlations of initially separable probability
distributions in a globally pure bipartite system with two degrees of freedom
for classical and quantum systems. A classical version of the quantum linear
mutual information is introduced and the two quantities are compared for a
system of oscillators coupled with both linear and non-linear interactions. The
classical correlations help to understand how much of the quantum loss of
purity are due to intrinsic quantum effects and how much is related to the
probabilistic character of the initial states, a characteristic shared by both
the classical and quantum pictures. Our examples show that, for initially
localized Gaussian states, the classical statistical mutual linear entropy
follows its quantum counterpart for short times. For non-Gaussian states the
behavior of the classical and quantum measures of information are still
qualitatively similar, although the fingerprints of the non-classical nature of
the initial state can be observed in their different amplitudes of oscillation.


title: Inconsistency of quantum--classical dynamics, and what it implies

abstract:
  A new proof of the impossibility of a universal quantum-classical dynamics is
given. It has at least two consequences. The standard paradigm ``quantum system
is measured by a classical apparatus" is untenable, while a quantum matter can
be consistently coupled only with a quantum gravity.


title: Feasibility of detecting single atoms using photonic bandgap cavities

abstract:
  We propose an atom-cavity chip that combines laser cooling and trapping of
neutral atoms with magnetic microtraps and waveguides to deliver a cold atom to
the mode of a fiber taper coupled photonic bandgap (PBG) cavity. The
feasibility of this device for detecting single atoms is analyzed using both a
semi-classical treatment and an unconditional master equation approach.
Single-atom detection seems achievable in an initial experiment involving the
non-deterministic delivery of weakly trapped atoms into the mode of the PBG
cavity.


title: Proper and Improper Separability

abstract:
  The distinction between proper and improper mixtures is a staple of the
discussion of foundational questions in quantum mechanics. Here we note an
analogous distinction in the context of the theory of entanglement. The
terminology of `proper' versus `improper' separability is proposed to mark the
distinction.


title: Limitations of Quantum Advice and One-Way Communication

abstract:
  Although a quantum state requires exponentially many classical bits to
describe, the laws of quantum mechanics impose severe restrictions on how that
state can be accessed. This paper shows in three settings that quantum messages
have only limited advantages over classical ones.
  First, we show that $\mathsf{BQP/qpoly}\subseteq\mathsf{PP/poly}$, where
$\mathsf{BQP/qpoly}$ is the class of problems solvable in quantum polynomial
time, given a polynomial-size "quantum advice state" that depends only on the
input length. This resolves a question of Buhrman, and means that we should not
hope for an unrelativized separation between quantum and classical advice.
Underlying our complexity result is a general new relation between
deterministic and quantum one-way communication complexities, which applies to
partial as well as total functions.
  Second, we construct an oracle relative to which $\mathsf{NP}\not \subset
\mathsf{BQP/qpoly}$. To do so, we use the polynomial method to give the first
correct proof of a direct product theorem for quantum search. This theorem has
other applications; for example, it can be used to fix a result of Klauck about
quantum time-space tradeoffs for sorting.
  Third, we introduce a new trace distance method for proving lower bounds on
quantum one-way communication complexity. Using this method, we obtain optimal
quantum lower bounds for two problems of Ambainis, for which no nontrivial
lower bounds were previously known even for classical randomized protocols.
  A preliminary version of this paper appeared in the 2004 Conference on
Computational Complexity (CCC).


title:
Experimental Demonstration of Five-photon Entanglement and
  Open-destination Teleportation

abstract:
  Universal quantum error-correction requires the ability of manipulating
entanglement of five or more particles. Although entanglement of three or four
particles has been experimentally demonstrated and used to obtain the extreme
contradiction between quantum mechanics and local realism, the realization of
five-particle entanglement remains an experimental challenge. Meanwhile, a
crucial experimental challenge in multi-party quantum communication and
computation is the so-called open-destination teleportation. During
open-destination teleportation, an unknown quantum state of a single particle
is first teleported onto a N-particle coherent superposition to perform
distributed quantum information processing. At a later stage this teleported
state can be readout at any of the N particles for further applications by
performing a projection measurement on the remaining N-1 particles. Here, we
report a proof-of-principle demonstration of five-photon entanglement and
open-destination teleportation. In the experiment, we use two entangled photon
pairs to generate a four-photon entangled state, which is then combined with a
single photon state to achieve the experimental goals. The methods developed in
our experiment would have various applications e.g. in quantum secret sharing
and measurement-based quantum computation.


title: Quantum Logic Using Linear Optics

abstract:
  In order for quantum communications systems to become widely used, it will
probably be necessary to develop quantum repeaters that can extend the range of
quantum key distribution systems and correct for errors in the transmission of
quantum information. Quantum logic gates based on linear optical techniques
appear to be a promising approach for the development of quantum repeaters, and
they may have applications in quantum computing as well. Here we describe the
basic principles of logic gates based on linear optics, along with the results
from several experimental demonstrations of devices of this kind. A prototype
source of single photons and a quantum memory device for photons are also
discussed. These devices can be combined with a four-qubit encoding to
implement a quantum repeater.


title: Overview of Quantum Error Prevention and Leakage Elimination

abstract:
  Quantum error prevention strategies will be required to produce a scalable
quantum computing device and are of central importance in this regard. Progress
in this area has been quite rapid in the past few years. In order to provide an
overview of the achievements in this area, we discuss the three major classes
of error prevention strategies, the abilities of these methods and the
shortcomings. We then discuss the combinations of these strategies which have
recently been proposed in the literature. Finally we present recent results in
reducing errors on encoded subspaces using decoupling controls. We show how to
generally remove mixing of an encoded subspace with external states (termed
leakage errors) using decoupling controls. Such controls are known as ``leakage
elimination operations'' or ``LEOs.''


title:
Correct mutual information, quantum bit error rate and secure
  transmission efficiency in Wojcik's eavesdropping scheme on ping-pong
  protocol

abstract:
  The wrong mutual information, quantum bit error rate and secure transmission
efficiency in Wojcik's eavesdropping scheme [PRL90(03)157901]on ping-pong
protocol have been pointed out and corrected.


title:
Phase Space Correspondence between Classical Optics and Quantum
  Mechanics

abstract:
  The paper scrutinizes both the similarities and the differences between the
classical optics and quantum mechanical theories in phase space, especially
between the Wigner distribution functions defined in the respective phase
spaces. Classical optics is able to provide an understanding of either the
corpuscular or wave aspects of quantum mechanics, reflected in phase space
through the classical limit of the quantum Wigner distribution function or the
Wigner distribution function in classical optics, respectively. However,
classical optics, as any classical theory, cannot mimic the wave-particle
duality that is at the heart of quantum mechanics. Moreover, it is never enough
underlined that, although the mathematical phase space formalisms in classical
optics and quantum mechanics are very similar, the main difference between
these theories, evidenced in the results of measurements, is as deep as it can
get even in phase space. On the other hand, the phase space treatment allows an
unexpected similar treatment of interference phenomena, although quantum and
classical superpositions of wavefunctions and fields, respectively, have a
completely different behavior. This similarity originates in the bilinear
character of the Wigner distribution function in both quantum mechanics and
classical optical wave theory. Actually, the phase space treatment of the
quantum and classical wave theory is identical from the mathematical point of
view, if the Planck constant is replaced by the wavelength of light. Even
Wigner distribution functions of particular quantum states, such as the
Schrodinger cat state, can be mimicked by classical optical means, but not the
true quantum character, which resides in the probability significance of the
wavefunction, in comparison to the physical realness of classical wave fields.


title:
High-Precision Numerical Determination of Eigenvalues for a Double-Well
  Potential Related to the Zinn-Justin Conjecture

abstract:
  A numerical method of high precision is used to calculate the energy
eigenvalues and eigenfunctions for a symmetric double-well potential. The
method is based on enclosing the system within two infinite walls with a large
but finite separation and developing a power series solution for the
Schr$\ddot{o}$dinger equation. The obtained numerical results are compared with
those obtained on the basis of the Zinn-Justin conjecture and found to be in an
excellent agreement.


title: Simulating noisy quantum protocols with quantum trajectories

abstract:
  The theory of quantum trajectories is applied to simulate the effects of
quantum noise sources induced by the environment on quantum information
protocols. We study two models that generalize single qubit noise channels like
amplitude damping and phase flip to the many-qubit situation. We calculate the
fidelity of quantum information transmission through a chaotic channel using
the teleportation scheme with different environments. In this example, we
analyze the role played by the kind of collective noise suffered by the quantum
processor during its operation. We also investigate the stability of a quantum
algorithm simulating the quantum dynamics of a paradigmatic model of chaos, the
baker's map. Our results demonstrate that, using the quantum trajectories
approach, we are able to simulate quantum protocols in the presence of noise
and with large system sizes of more than 20 qubits.


title: Young's two-slits experiment with people

abstract:
  The two-slits and one-slit experiments with people are performed and show the
existence of wave component of women behavior.


title: Fault-Tolerant Quantum Computation For Local Non-Markovian Noise

abstract:
  We derive a threshold result for fault-tolerant quantum computation for local
non-Markovian noise models. The role of error amplitude in our analysis is
played by the product of the elementary gate time t_0 and the spectral width of
the interaction Hamiltonian between system and bath. We discuss extensions of
our model and the applicability of our analysis.


title: Nonclassicality of Thermal Radiation

abstract:
  It is demonstrated that thermal radiation of small occupation number is
strongly nonclassical. This includes most forms of naturally occurring
radiation. Nonclassicality can be observed as a negative weak value of a
positive observable. It is related to negative values of the Margenau-Hill
quasi-probability distribution.


title: Analytically Solvable PT-Invariant Periodic Potentials

abstract:
  Associated Lam\'e potentials $V(x)=a(a+1)m\sn^2(x,m)+b(b+1)m{\cn^2
(x,m)}/{\dn^2(x,m)}$ are used to construct complex, PT-invariant, periodic
potentials using the anti-isospectral transformation $x \to ix+\beta$, where
$\beta$ is any nonzero real number. These PT-invariant potentials are defined
by $V^{PT}(x) \equiv -V(ix+\beta)$, and have a different real period from
$V(x)$. They are analytically solvable potentials with a finite number of band
gaps, when $a$ and $b$ are integers. Explicit expressions for the band edges of
some of these potentials are given. For the special case of the complex
potential $V^{PT}(x)=-2m\sn^2(ix+\beta,m)$, we also analytically obtain the
dispersion relation. Additional new, solvable, complex, PT-invariant, periodic
potentials are obtained by applying the techniques of supersymmetric quantum
mechanics.


title: Coins Make Quantum Walks Faster

abstract:
  We show how to search N items arranged on a $\sqrt{N}\times\sqrt{N}$ grid in
time $O(\sqrt N \log N)$, using a discrete time quantum walk. This result for
the first time exhibits a significant difference between discrete time and
continuous time walks without coin degrees of freedom, since it has been shown
recently that such a continuous time walk needs time $\Omega(N)$ to perform the
same task. Our result furthermore improves on a previous bound for quantum
local search by Aaronson and Ambainis. We generalize our result to 3 and more
dimensions where the walk yields the optimal performance of $O(\sqrt{N})$ and
give several extensions of quantum walk search algorithms for general graphs.
The coin-flip operation needs to be chosen judiciously: we show that another
``natural'' choice of coin gives a walk that takes $\Omega(N)$ steps. We also
show that in 2 dimensions it is sufficient to have a two-dimensional coin-space
to achieve the time $O(\sqrt{N} \log N)$.


title:
Controlling qubit transitions during non-adiabatic rapid passage through
  quantum interference

abstract:
  In adiabatic rapid passage, the Bloch vector of a qubit is inverted by slowly
inverting an external field to which it is coupled, and along which it is
initially aligned. In non-adiabatic twisted rapid passage, the external field
is allowed to twist around its initial direction with azimuthal angle \phi(t)
at the same time that it is non-adiabatically inverted. For polynomial twist,
\phi(t) \sim Bt^{n}. We show that for n \ge 3, multiple qubit resonances can
occur during a single inversion of the external field, producing strong
interference effects in the qubit transition probability. The character of the
interference is controllable through variation of the twist strength B.
Constructive and destructive interference are possible, greatly enhancing or
suppressing qubit transitions. Experimental confirmation of these controllable
interference effects has already occurred. Application of this interference
mechanism to the construction of fast fault-tolerant quantum CNOT and NOT gates
is discussed.


abstract:
  For two gaussian states with given correlation matrices, in order that
relative entropy between them is practically calculable, I in this paper
describe the ways of transforming the correlation matrix to matrix in the
exponential density operator. Gaussian relative entropy of entanglement is
proposed as the minimal relative entropy of the gaussian state with respect to
separable gaussian state set. I prove that gaussian relative entropy of
entanglement achieves when the separable gaussian state is at the border of
separable gaussian state set and inseparable gaussian state set. For two mode
gaussian states, the calculation of gaussian relative entropy of entanglement
is greatly simplified from searching for a matrix with 10 undetermined
parameters to 3 variables. The two mode gaussian states are classified as four
types, numerical evidence strongly suggests that gaussian relative entropy of
entanglement for each type is realized by the separable state within the same
type.For symmetric gaussian state it is strictly proved that it is achieved by
symmetric gaussian state.


title:
Experimental open air quantum key distribution with a single photon
  source

abstract:
  We present a full implementation of a quantum key distribution (QKD) system
with a single photon source, operating at night in open air. The single photon
source at the heart of the functional and reliable setup relies on the pulsed
excitation of a single nitrogen-vacancy color center in diamond nanocrystal. We
tested the effect of attenuation on the polarized encoded photons for inferring
longer distance performance of our system. For strong attenuation, the use of
pure single photon states gives measurable advantage over systems relying on
weak attenuated laser pulses. The results are in good agreement with
theoretical models developed to assess QKD security.


title:
Entangling single and $N$ atom qubits for fast quantum state detection
  and transmission

abstract:
  We discuss the use of Rydberg blockade techniques for entanglement of 1 atom
qubits with collective $N$ atom qubits. We show how the entanglement can be
used to achieve fast readout and transmission of the state of single atom
qubits without the use of optical cavities.


title:
Bell inequalities versus entanglement and mixedness for a class of two -
  qubit states

abstract:
  For a class of mixed two -qubit states we show that it is not possible to
discriminate between states violating or non - violating Bell - CHSH
inequalities, knowing only their entanglement and mixedness. For a large set of
possible values of these quantities, we construct pairs of states with the same
entanglement and mixedness such that one state is violating but the other is
non - violating Bell - CHSH inequality.


title:
Marginal distributions in $(\bf 2N)$-dimensional phase space and the
  quantum $(\bf N+1)$ marginal theorem

abstract:
  We study the problem of constructing a probability density in 2N-dimensional
phase space which reproduces a given collection of $n$ joint probability
distributions as marginals. Only distributions authorized by quantum mechanics,
i.e. depending on a (complete) commuting set of $N$ variables, are considered.
A diagrammatic or graph theoretic formulation of the problem is developed. We
then exactly determine the set of ``admissible'' data, i.e. those types of data
for which the problem always admits solutions. This is done in the case where
the joint distributions originate from quantum mechanics as well as in the case
where this constraint is not imposed. In particular, it is shown that a
necessary (but not sufficient) condition for the existence of solutions is
$n\leq N+1$. When the data are admissible and the quantum constraint is not
imposed, the general solution for the phase space density is determined
explicitly. For admissible data of a quantum origin, the general solution is
given in certain (but not all) cases. In the remaining cases, only a subset of
solutions is obtained.


title:
Slow decay of dynamical correlation functions for nonequilibrium quantum
  states

abstract:
  A property of dynamical correlation functions for nonequilibrium states is
discussed. We consider arbitrary dimensional quantum spin systems with local
interaction and translationally invariant states with nonvanishing current over
them. A correlation function between local charge and local Hamiltonian at
different spacetime points is shown to exhibit slow decay.

categories: quant-ph cond-mat.stat-mech math-ph math.MP


title: Is Fresnel Optics Quantum Mechanics in Phase Space?

abstract:
  We formulate and argue in favor of the following conjecture: There exists an
intimate connection between Wigner's quantum mechanical phase space
distribution function and classical Fresnel optics.


title:
Quantum holography and multiparticle entanglement using ground-state
  coherences

abstract:
  A method of storing and retrieving quantum states of radiation fields using
the ground-state coherences is discussed. We demonstrate the generation of
multiparticle entangled states starting from atoms prepared in a coherent
state. Use is made of the j=-1/2 to +1/2 atomic transition and its interaction
with a far-detuned elliptically polarized field.


title:
Classification of Quantum Symmetric Non-zero Sum 2x2 Games in the Eisert
  Scheme

abstract:
  A quantum game in the Eisert scheme is defined by the payoff matrix, plus
some quantum entanglement parameters. In the symmetric nonzero-sum 2x2 games,
the relevant features of the game are given by two parameters in the payoff
matrix, and only one extra entanglement parameter is introduced by quantizing
it in the Eisert scheme.
  The criteria adopted in this work to classify quantum games are the amount
and characteristics of Nash equilibria and Pareto-optimal positions. A new
classification based on them is developed for symmetric nonzero-sum classical
2x2 games, as well as classifications for quantum games with different
restricted subsets of the total strategy set. Finally, a classification is
presented taking the whole set of strategies into account, both unitary
strategies and nonunitary strategies studied as convex mixures of unitary
strategies.
  The classification reproduces features which have been previously found in
other works, like appearance of multiple equilibria, changes in the character
of equilibria, and entanglement regime transitions.


title:
Reply to "Non-relativistic proofs of the spin-statistics connection," by
  Shaji and Sudarshan

abstract:
  I earlier proved under stated assumptions of nonrelativistic quantum
mechanics, that identical spin-zero particles with no internal degrees of
freedom must be bosons. Shaji and Sudarshan have criticized that proof,
asserting that it is "based on single-valuedness under rotation of the wave
functions of systems of identical particles." No such assumption was used.
Here, to remove all doubt, I exclude the fermion possibility through a proof in
which even the existence of rotation is not used. Following that, I assume that
the Hilbert space is invariant under rotation to exclude other symmetries which
correspond neither to bosons nor to fermions. In that proof no issue of
single-valuedness can arise.


title: Single-beam noise characteristics of quantum correlated twin beams

abstract:
  We investigated the intensity noise spectra of the single beam of a
pump-enhanced continuous-wave (cw) optical parametric oscillator (OPO), which
was used to generate quantum correlated twin beams, as a function of the pump
power. With a triply (pump-, signal-, and idler-) resonant cavity, the
oscillation threshold of our OPO was about 8.5+/-1.3 mW and the measured slope
conversion efficiency was 0.72+/-0.02. A twin beams with power of 240 mW were
generated at pump power of 350 mW. The relaxation oscillation frequencies,
which depend on the pump power, were observed when the pump power of OPO was
from 12.5 mW to 28 mW. The experimental results confirm the predicted increase
in OPO relaxation frequency with pump power. Squeezing of the single beam
intensity was for the first time inferred experimentally by exploiting nature
of quantum noise dependent on loss.


title:
Large negative and positive delay of optical pulses in coherently
  prepared dense Rb vapor with buffer gas

abstract:
  We experimentally study the group time delay for a light pulse propagating
through hot Rb vapor in the presence of a strong coupling field in a $\Lambda$
configuration. We demonstrate that the ultra-slow pulse propagation is
transformed into superluminal propagation as the one-photon detuning of the
light increases due to the change in the transmission resonance lineshape.
Negative group velocity as low as -c/10^6=-80 m/s is recorded. We also find
that the advance time in the regime of the superluminal propagation grows
linearly with increasing laser field power.


title: Consciousness and the Wigner's friend problem

abstract:
  It is generally agreed that decoherence theory is, if not a complete answer,
at least a great step forward towards a solution of the quantum measurement
problem. It is shown here however that in the cases in which a sentient being
is explicitly assumed to take cognizance of the outcome the reasons we have for
judging this way are not totally consistent, so that the question has to be
considered anew. It is pointed out that the way the Broglie-Bohm model solves
the riddle suggests a possible clue, consisting in assuming that even very
simple systems may have some sort of a proto-consciousness, but that their
``internal states of consciousness'' are not predictive. It is, next, easily
shown that if we imagine the systems get larger, in virtue of decoherence their
internal states of consciousness progressively gain in predictive value. So
that, for macro-systems, they may be identified (in practice) with the
predictive states of consciousness on which we ground our observational
predictions. The possibilities of carrying over this idea to standard quantum
mechanics are then investigated. Conditions of conceptual consistency are
considered and found rather strict, and, finally, two solutions emerge,
differing conceptually very much from one another but in both of which the,
possibly non-predictive, generalized internal states of consciousness play a
crucial role.


title:
Effects of the Bloch-Siegert Oscillation on the Precision of Qubit
  Rotations: Direct Two-Level vs. Off-Resonant Raman Excitation

abstract:
  In a direct two-level qubit system, when the Rabi frequency is comparable to
the resonance frequency, the rotating wave approximation is not appropriate. In
this case, the Rabi oscillation is accompanied by another oscillation at twice
the frequency of the driving field, the so called Bloch-Siegert oscillation
(BSO), which depends on the initial phase of the driving field. This
oscillation may restrict the precise rotation of a qubit made of direct
two-level system. Here, we show that in case of an effectively two-level lambda
system, the BSO is inherently negligible, implying a greater precision for
rotation of a qubit made of such a lambda system when compared to a direct
two-level qubit in a strong driving field.


title:
Quantum and Classical Strong Direct Product Theorems and Optimal
  Time-Space Tradeoffs

abstract:
  A strong direct product theorem says that if we want to compute k independent
instances of a function, using less than k times the resources needed for one
instance, then our overall success probability will be exponentially small in
k. We establish such theorems for the classical as well as quantum query
complexity of the OR function. This implies slightly weaker direct product
results for all total functions. We prove a similar result for quantum
communication protocols computing k instances of the Disjointness function.
  Our direct product theorems imply a time-space tradeoff T^2*S=Omega(N^3) for
sorting N items on a quantum computer, which is optimal up to polylog factors.
They also give several tight time-space and communication-space tradeoffs for
the problems of Boolean matrix-vector multiplication and matrix multiplication.


title: Extremal entanglement and mixedness in continuous variable systems

abstract:
  We investigate the relationship between mixedness and entanglement for
Gaussian states of continuous variable systems. We introduce generalized
entropies based on Schatten $p$-norms to quantify the mixedness of a state, and
derive their explicit expressions in terms of symplectic spectra. We compare
the hierarchies of mixedness provided by such measures with the one provided by
the purity (defined as ${\rm tr} \varrho^2$ for the state $\varrho$) for
generic $n$-mode states. We then review the analysis proving the existence of
both maximally and minimally entangled states at given global and marginal
purities, with the entanglement quantified by the logarithmic negativity. Based
on these results, we extend such an analysis to generalized entropies,
introducing and fully characterizing maximally and minimally entangled states
for given global and local generalized entropies. We compare the different
roles played by the purity and by the generalized $p$-entropies in quantifying
the entanglement and the mixedness of continuous variable systems. We introduce
the concept of average logarithmic negativity, showing that it allows a
reliable quantitative estimate of continuous variable entanglement by direct
measurements of global and marginal generalized $p$-entropies.


title: Unambiguous comparison of the states of multiple quantum systems

abstract:
  We consider N quantum systems initially prepared in pure states and address
the problem of unambiguously comparing them. One may ask whether or not all $N$
systems are in the same state. Alternatively, one may ask whether or not the
states of all N systems are different. We investigate the possibility of
unambiguously obtaining this kind of information. It is found that some
unambiguous comparison tasks are possible only when certain linear independence
conditions are satisfied. We also obtain measurement strategies for certain
comparison tasks which are optimal under a broad range of circumstances, in
particular when the states are completely unknown. Such strategies, which we
call universal comparison strategies, are found to have intriguing connections
with the problem of quantifying the distinguishability of a set of quantum
states and also with unresolved conjectures in linear algebra. We finally
investigate a potential generalisation of unambiguous state comparison, which
we term unambiguous overlap filtering.


title:
All quantum observables in a hidden-variables model must commute
  simultaneously

abstract:
  Under a standard set of assumptions for a hidden-variables model for quantum
events, we show that all observables must commute simultaneously. And, despite
Bell's complaint that a key condition of von Neumann's was quite unrealistic,
we show these these conditions are entirely equivalent to those later
introduced by Bell, Kochen and Specker. As is it known that these conditions
are also equivalent to those under which the Bell-Clauser-Horne inequalities
are derived, we see that any experimental violations of the inequalities
demonstrate only that quantum observables do not commute.The same conclusion
applies to the collection of elegant inequality-free no-go proofs of
Greenberger-Horne-Zeilinger, Mermin and Peres. Otherwise expressed, the usual
hidden-variable models have assumptions that are collectively too strong to be
interesting, and hence need modification or deletion.


title: Quantum Entanglement in Time

abstract:
  The temporal Bell inequalities are derived from the assumptions of realism
and locality in time. It is shown that quantum mechanics violates these
inequalities and thus is in conflict with the two assumptions. This can be used
for performing certain tasks that are not possible classically. Our results
open up a possibility for introducing the notion of entanglement in time in
quantum physics.


title:
Computable Functions, the Church-Turing Thesis and the Quantum
  Measurement Problem

abstract:
  It is possible in principle to construct quantum mechanical observables and
unitary operators which, if implemented in physical systems as measurements and
dynamical evolution, would contradict the Church-Turing thesis, which lies at
the foundation of computer science. Elsewhere we have argued that the quantum
measurement problem implies a finite, computational model of the measurement
and evolution of quantum states. If correct, this approach helps to identify
the key feature that can reconcile quantum mechanics with the Church-Turing
thesis: finitude of the degree of fine-graining of Hilbert space. This suggests
that the Church-Turing thesis constrains the physical universe and thereby
highlights a surprising connection between purely logical and algorithmic
considerations on the one hand and physical reality on the other.


title:
The classical capacity achievable by a quantum channel assisted by
  limited entanglement

abstract:
  We give the trade-off curve showing the capacity of a quantum channel as a
function of the amount of entanglement used by the sender and receiver for
transmitting information. The endpoints of this curve are given by the
Holevo-Schumacher-Westmoreland capacity formula and the entanglement-assisted
capacity, which is the maximum over all input density matrices of the quantum
mutual information. The proof we give is based on the
Holevo-Schumacher-Westmoreland formula, and also gives a new and simpler proof
for the entanglement-assisted capacity formula.


title: A categorical semantics of quantum protocols

abstract:
  We study quantum information and computation from a novel point of view. Our
approach is based on recasting the standard axiomatic presentation of quantum
mechanics, due to von Neumann, at a more abstract level, of compact closed
categories with biproducts. We show how the essential structures found in key
quantum information protocols such as teleportation, logic-gate teleportation,
and entanglement-swapping can be captured at this abstract level. Moreover,
from the combination of the --apparently purely qualitative-- structures of
compact closure and biproducts there emerge `scalars` and a `Born rule'. This
abstract and structural point of view opens up new possibilities for describing
and reasoning about quantum systems. It also shows the degrees of axiomatic
freedom: we can show what requirements are placed on the (semi)ring of scalars
C(I,I), where C is the category and I is the tensor unit, in order to perform
various protocols such as teleportation. Our formalism captures both the
information-flow aspect of the protocols (see quant-ph/0402014), and the
branching due to quantum indeterminism. This contrasts with the standard
accounts, in which the classical information flows are `outside' the usual
quantum-mechanical formalism.


title: A Generic Security Proof for Quantum Key Distribution

abstract:
  Quantum key distribution allows two parties, traditionally known as Alice and
Bob, to establish a secure random cryptographic key if, firstly, they have
access to a quantum communication channel, and secondly, they can exchange
classical public messages which can be monitored but not altered by an
eavesdropper, Eve. Quantum key distribution provides perfect security because,
unlike its classical counterpart, it relies on the laws of physics rather than
on ensuring that successful eavesdropping would require excessive computational
effort. However, security proofs of quantum key distribution are not trivial
and are usually restricted in their applicability to specific protocols. In
contrast, we present a general and conceptually simple proof which can be
applied to a number of different protocols. It relies on the fact that a
cryptographic procedure called privacy amplification is equally secure when an
adversary's memory for data storage is quantum rather than classical.


title:
Preparation of pseudopure state in a cluster of dipolar-coupled spins
  with "unresolved" spectrum

abstract:
  A method of creating pseudopure spin states in large clusters of coupled
spins is described. It is based on filtering multiple-quantum coherence of the
highest order followed by a time-reversal period and partial saturation.
Experimental demonstration is presented for a cluster of six dipolar-coupled
proton spins of a benzene molecule in liquid crystalline matrix, and the
details of spin dynamics are studied numerically.


title: Faithful Teleportation with Partially Entangled States

abstract:
  We write explicitly a general protocol for faithful teleportation of a
d-state particle (qudit) via a partially entangled pair of (pure) $n$-state
particles. The classical communication cost (CCC) of the protocol is
$\log_{2}(nd)$ bits, and it is implemented by a {\em projective} measurement
performed by Alice, and a unitary operator performed by Bob (after receiving
from Alice the measurement result). We prove the optimality of our protocol by
a comparison with the concentrate and teleport strategy. We also show that if
$d>n/2$ or if there is no residual entanglement left after the faithful
teleportation, the CCC of {\em any} protocol is at least $\log_{2}(nd)$ bits.
Furthermore, we find a lower bound on the CCC in the process transforming one
bipartite state to another by means of local operation and classical
communication (LOCC).


title:
Consistent measurement of a quantum dynamical variable using classical
  apparatus

abstract:
  The claim that there is an inconsistency of quantum-classical dynamics [1] is
investigated. We point out that a consistent formulation of quantum and
classical dynamics which can be used to describe quantum measurement processes
is already available in the literature [2]. An example in which a quantum
system is interacting with a classical system is worked out using this
formulation.


title: Spherical Code Key Distribution Protocols for Qubits

abstract:
  Recently spherical codes were introduced as potentially more capable
ensembles for quantum key distribution. Here we develop specific key creation
protocols for the two qubit-based spherical codes, the trine and tetrahedron,
and analyze them in the context of a suitably-tailored intercept/resend attack,
both in standard form, and a ``gentler'' version whose back-action on the
quantum state is weaker. When compared to the standard unbiased basis
protocols, BB84 and six-state, two distinct advantages are found. First, they
offer improved tolerance of eavesdropping, the trine besting its counterpart
BB84 and the tetrahedron the six-state protocol. Second, the key error rate may
be computed from the sift rate of the protocol itself, removing the need to
sacrifice key bits for this purpose. This simplifies the protocol and improves
the overall key rate.


title: Feedback control of quantum state reduction

abstract:
  Feedback control of quantum mechanical systems must take into account the
probabilistic nature of quantum measurement. We formulate quantum feedback
control as a problem of stochastic nonlinear control by considering separately
a quantum filtering problem and a state feedback control problem for the
filter. We explore the use of stochastic Lyapunov techniques for the design of
feedback controllers for quantum spin systems and demonstrate the possibility
of stabilizing one outcome of a quantum measurement with unit probability.


title:
Deterministic Dicke state preparation with continuous measurement and
  control

abstract:
  We characterize the long-time projective behavior of the stochastic master
equation describing a continuous, collective spin measurement of an atomic
ensemble both analytically and numerically. By adding state based feedback, we
show that it is possible to prepare highly entangled Dicke states
deterministically.


title:
Efficient many-party controlled teleportation of multi-qubit quantum
  information via entanglement

abstract:
  We present a way to teleport multi-qubit quantum information from a sender to
a distant receiver via the control of many agents in a network. We show that
the original state of each qubit can be restored by the receiver as long as all
the agents collaborate. However, even if one agent does not cooperate, the
receiver can not fully recover the original state of each qubit. The method
operates essentially through entangling quantum information during
teleportation, in such a way that the required auxiliary qubit resources, local
operation, and classical communication are considerably reduced for the present
purpose.


title:
Comments on "Fault-Tolerant Quantum Computation for Local Non-Markovian
  Noise"

abstract:
  In the recent paper Terhal and Burkard derived a threshold result for
fault-tolerant quantum computation under the assumption of the non-Markovian
noise and claimed to rebut the objections rised by Alicki and Horodecki's. The
purpose of this note is to show that the main condition used by Terhal and
Burkard, although looking quite innocently, implies the assumption of the
extremally low probability error per single quantum gate - p < 10^{-8} -
10^{-12} - i.e. the square of the expected threshold value for the case of
Markovian noise.


title:
Quantum Phase Space in Relativistic Theory: the Case of Charge-Invariant
  Observables

abstract:
  Mathematical method of quantum phase space is very useful in physical
applications like quantum optics and non-relativistic quantum mechanics.
However, attempts to generalize it for the relativistic case lead to some
difficulties. One of problems is band structure of energy spectrum for a
relativistic particle. This corresponds to an internal degree of freedom,
so-called charge variable. In physical problems we often deal with such of
dynamical variables that do not depend on this degree of freedom. These are
position, momentum, and any combination of them. Restricting our consideration
to this kind of observables we propose the relativistic Weyl--Wigner--Moyal
formalism that contains some surprising differences from its non-relativistic
counterpart.


title:
Some Novel Thought Experiments Involving Foundations of Quantum
  Mechanics and Quantum Information

abstract:
  In this thesis, we have proposed some novel thought experiments involving
foundations of quantum mechanics and quantum information theory, using quantum
entanglement property. Concerning foundations of quantum mechanics, we have
suggested some typical systems including two correlated particles which can
distinguish between the two famous theories of quantum mechanics, i.e. the
standard and Bohmian quantum mechanics, at the individual level of pair of
particles. Meantime, the two theories present the same predictions at the
ensemble level of particles. Regarding quantum information theory, two
theoretical quantum communication schemes including quantum dense coding and
quantum teleportation schemes have been proposed by using entangled spatial
states of two EPR particles shared between two parties. It is shown that the
rate of classical information gain in our dense coding scheme is greater than
some previously proposed multi-qubit protocols by a logarithmic factor
dependent on the dimension of Hilbert space. The proposed teleportation scheme
can provide a complete wave function teleportation of an object having other
degrees of freedom in our three-dimensional space, for the first time. All
required unitary operators which are necessary in our state preparation and
Bell state measurement processes are designed using symmetric normalized
Hadamard matrix, some basic gates and one typical conditional gate, which are
introduced here for the first time.


title:
Scattering of dark-state polaritons in optical lattices and quantum
  phase gate for photons

abstract:
  We discuss the quasi 1-D scattering of two counter-propagating, dark-state
polaritons (DSP), each containing a single excitation. DSPs are formed from
photons in media with electromagnetically induced transparency and are
associated with ultra-slow group velocities. State-dependent elastic collisions
of atoms at the same lattice site lead to a nonlinear interaction. It is shown
that the scattering process in a deep optical lattice filled by cold atoms
generates a large and homogeneous conditional phase shift between two
individual polaritons. The latter has potential applications for a photonic
phase gate. The quasi 1-D scattering problem is solved analytically and the
influence of degrading processes such as dephasing due to collisions with
ground-state atoms is discussed.


title:
Optimal Control of Quantum Dissipative Dynamics: Analytic solution for
  cooling the three level $\Lambda$ system

abstract:
  We study the problem of optimal control of dissipative quantum dynamics.
Although under most circumstances dissipation leads to an increase in entropy
(or a decrease in purity) of the system, there is an important class of
problems for which dissipation with external control can decrease the entropy
(or increase the purity) of the system. An important example is laser cooling.
In such systems, there is an interplay of the Hamiltonian part of the dynamics,
which is controllable and the dissipative part of the dynamics, which is
uncontrollable. The strategy is to control the Hamiltonian portion of the
evolution in such a way that the dissipation causes the purity of the system to
increase rather than decrease. The goal of this paper is to find the strategy
that leads to maximal purity at the final time. Under the assumption that
Hamiltonian control is complete and arbitrarily fast, we provide a general
framework by which to calculate optimal cooling strategies. These assumptions
lead to a great simplification, in which the control problem can be
reformulated in terms of the spectrum of eigenvalues of $\rho$, rather than
$\rho$ itself. By combining this formulation with the Hamilton-Jacobi-Bellman
theorem we are able to obtain an equation for the globaly optimal cooling
strategy in terms of the spectrum of the density matrix. For the three-level
$\Lambda$ system, we provide a complete analytic solution for the optimal
cooling strategy. For this system it is found that the optimal strategy does
not exploit system coherences and is a 'greedy' strategy, in which the purity
is increased maximally at each instant.


title:
Dissipative quantum dynamics with the Surrogate Hamiltonian approach. A
  comparison between spin and harmonic baths

abstract:
  The dissipative quantum dynamics of an anharmonic oscillator coupled to a
bath is studied with the purpose of elucidating the differences between the
relaxation to a spin bath and to a harmonic bath. Converged results are
obtained for the spin bath by the Surrogate Hamiltonian approach. This method
is based on constructing a system-bath Hamiltonian, with a finite but large
number of spin bath modes, that mimics exactly a bath with an infinite number
of modes for a finite time interval. Convergence with respect to the number of
simultaneous excitations of bath modes can be checked. The results are compared
to calculations that include a finite number of harmonic modes carried out by
using the multi-configuration time-dependent Hartree method of Nest and Meyer,
[J. Chem. Phys. 119, 24 (2003)]. In the weak coupling regime, at zero
temperature and for small excitations of the primary system, both methods
converge to the Markovian limit. When initially the primary system is
significantly excited, the spin bath can saturate restricting the energy
acceptance. An interaction term between bath modes that spreads the excitation
eliminates the saturation. The loss of phase between two cat states has been
analyzed and the results for the spin and harmonic baths are almost identical.
For stronger couplings, the dynamics induced by the two types of baths deviate.
The accumulation and degree of entanglement between the bath modes have been
characterized. Only in the spin bath the dynamics generate entanglement between
the bath modes.


title:
Etude du couplage optomecanique dans une cavite de grande finesse.
  Observation du mouvement Brownien d'un miroir

abstract:
  The topic of this thesis is the theoretical analysis of the optomechanical
coupling effects in a high-finesse optical cavity, and the experimental
realization of such a device. Radiation pressure exerted by light limits the
sensitivity of high precision optical measurements. In particular, the
sensitivity of interferometric measurements of gravitational wave is limited by
the so called standard quantum limit. cavity with a movable mirror. The
internal field stored in such cavity can be orders of magnitude greater than
the input field, and it's radiation pressure force can change the physical
length of the cavity. In turn, any change in the mirror's position changes the
phase of the out put field. This optomechanical coupling leads to an
intensity-dependent phase shift for the light equivalent to an optical Kerr
effect. Such a device can then be used for squeezing generation or quantum
nondemolition measurements. In our experiment, we send a laser beam in to a
high-finesse optical cavity with a movable mirror coated on a high Q-factor
mechanical resonator. Quantum effects of radiation pressure become therefore,
at low temperature, experimentally observable. However, we've shown that the
phase of the reflected field is very sensitive to small mirror displacements,
which indicate other possible applications of this type of device like high
precision displacements measurements. We've been able to observe the Brownian
motion of the moving mirror. We've also used an auxiliary intensity modulated
laser beam to optically excite the acoustic modes. We've finally obtained a
sensitivity of 2x10^(-19) m/sqrt(Hz), in agreement with theoretical prediction.


title: Decoherence of matter waves by thermal emission of radiation

abstract:
  Emergent quantum technologies have led to increasing interest in decoherence
- the processes that limit the appearance of quantum effects and turn them into
classical phenomena. One important cause of decoherence is the interaction of a
quantum system with its environment, which 'entangles' the two and distributes
the quantum coherence over so many degrees of freedom as to render it
unobservable. Decoherence theory has been complemented by experiments using
matter waves coupled to external photons or molecules, and by investigations
using coherent photon states, trapped ions and electron interferometers. Large
molecules are particularly suitable for the investigation of the
quantum-classical transition because they can store much energy in numerous
internal degrees of freedom; the internal energy can be converted into thermal
radiation and thus induce decoherence. Here we report matter wave
interferometer experiments in which C70 molecules lose their quantum behaviour
by thermal emission of radiation. We find good quantitative agreement between
our experimental observations and microscopic decoherence theory. Decoherence
by emission of thermal radiation is a general mechanism that should be relevant
to all macroscopic bodies.


title: On parametric type interaction between light and atomic ensembles

abstract:
  One-photon and Raman type interactions between two-level atoms and
narrow-band light are considered. We give some exactly solvable models of these
processes when only one-photon Fock states are involved in the evolution.
Possible application of these models for generation and transformation of
entangled states of the W-class, some of which demonstrate hierarchy structure,
are discussed. Finally, we consider preparation of entangled chains of atomic
ensembles.


title: Generalized GHZ States and Distributed Quantum Computing

abstract:
  A key problem in quantum computing is finding a viable technological path
toward the creation of a scalable quantum computer. One possible approach
toward solving part of this problem is distributed computing, which provides an
effective way of utilizing a network of limited capacity quantum computers.
  In this paper, we present two primitive operations, cat-entangler and
cat-disentangler, which in turn can be used to implement non-local operations,
e.g. non-local CNOT and quantum teleportation. We also show how to establish an
entangled pair, and use entangled pairs to efficiently create a generalized GHZ
state. Furthermore, we present procedures which allow us to reuse channel
qubits in a sequence of non-local operations.
  These non-local operations work on the principle that a cat-like state,
created by cat-entangler, can be used to distribute a control qubit among
multiple computers. Using this principle, we show how to efficiently implement
non-local control operations in many situation, including a parallel
implementation of a certain kind of unitary transformation. Finally, as an
example, we present a distributed version of the quantum Fourier transform.


title: Why the quantum?

abstract:
  This paper is a commentary on the foundational significance of the
Clifton-Bub-Halvorson theorem characterizing quantum theory in terms of three
information-theoretic constraints (Foundations of Physics 33, 1561-1591 (2003);
quant-ph/0211089). I argue that: (1) a quantum theory is best understood as a
theory about the possibilities and impossibilities of information transfer, as
opposed to a theory about the mechanics of nonclassical waves or particles, (2)
given the information-theoretic constraints, any mechanical theory of quantum
phenomena that includes an account of the measuring instruments that reveal
these phenomena must be empirically equivalent to a quantum theory, and (3)
assuming the information-theoretic constraints are in fact satisfied in our
world, no mechanical theory of quantum phenomena that includes an account of
measurement interactions can be acceptable, and the appropriate aim of physics
at the fundamental level then becomes the representation and manipulation of
information.


title: A General Method for Complete Population Transfer in Degenerate Systems

abstract:
  A simple theoretical solution to the design of a control field that generates
complete population transfer from an initial state, via $N$ nondegenerate
intermediate states, to one arbitrary member of $M$ ($M\leq N$) degenerate
states is constructed. The full control field exploits an $(M+N-1)$-node null
adiabatic state, created by designing the relative phases and amplitudes of the
component fields that together make up the full field. The solution found is
universal in the sense that it does not depend on the exact number of the
unwanted degenerate states or their properties. The results obtained suggest
that a class of multi-level quantum systems with degenerate states can be
completely controllable, even under extremely strong constraints, e.g., never
populating a Hilbert subspace that is only a few dimensions smaller than the
whole Hilbert space.


title: Mutual information, bit error rate and security in W\'{o}jcik's scheme

abstract:
  In this paper the correct calculations of the mutual information of the whole
transmission, the quantum bit error rate (QBER) are presented. Mistakes of the
general conclusions relative to the mutual information, the quantum bit error
rate (QBER) and the security in W\'{o}jcik's paper [Phys. Rev. Lett. {\bf 90},
157901(2003)] have been pointed out.


title: Collective effects in the dynamics of driven atoms in a high-Q resonator

abstract:
  We study the quantum dynamics of N coherently driven two-level atoms coupled
to an optical resonator. In the strong coupling regime the cavity field
generated by atomic scattering interferes destructively with the pump on the
atoms. This suppresses atomic excitation and even for strong driving fields
prevents atomic saturation, while the stationary intracavity field amplitude is
almost independent of the atom number. The magnitude of the interference effect
depends on the detuning between laser and cavity field and on the relative
atomic positions and is strongest for a wavelength spaced lattice of atoms
placed at the antinodes of the cavity mode. In this case three dimensional
intensity minima are created in the vicinity of each atom. In this regime
spontaneous emission is suppressed and the dominant loss channel is cavity
decay. Even for a cavity linewidth larger than the atomic natural width, one
regains strong interference through the cooperative action of a sufficiently
large number of atoms. These results give a new key to understand recent
experiments on collective cavity cooling and may allow to implement fast
tailored atom-atom interactions as well as nonperturbative particle detection
with very small energy transfer.


title: Quantum mechanics from two physical postulates

abstract:
  For an arbitrary preparation, quantum mechanical descriptions refer to the
complementary contexts set by incompatible measurements. We argue that an
arbitrary preparation, therefore, should be described with respect to such a
context by its degrees of disturbance (represented by real numbers) and their
probability distribution (postulate 1). Measurement contexts thus provide
reference frames for the preparation space of a physical system; a preparation
being described by a point in this space with the aforementioned as its
coordinates relative to a given measurement apparatus. However, all measurement
contexts are equivalent with regard to the description of a given preparation;
there is no preferred measurement (postulate 2). In the framework provided by
the preparation space, we show that quantum mechanics emerges naturally from
the above postulates in a new formulation which is manifestly canonical;
provided the degrees of disturbance are identified with the quantum phases of
the preparation with respect to (the basis furnished by) the measurement
apparatus.


title: Zitterbewegung and semiclassical observables for the Dirac equation

abstract:
  In a semiclassical context we investigate the Zitterbewegung of relativistic
particles with spin 1/2 moving in external fields. It is shown that the
analogue of Zitterbewegung for general observables can be removed to arbitrary
order in \hbar by projecting to dynamically almost invariant subspaces of the
quantum mechanical Hilbert space which are associated with particles and
anti-particles. This not only allows to identify observables with a
semiclassical meaning, but also to recover combined classical dynamics for the
translational and spin degrees of freedom. Finally, we discuss properties of
eigenspinors of a Dirac-Hamiltonian when these are projected to the almost
invariant subspaces, including the phenomenon of quantum ergodicity.


title: Two-Photon Absorption

abstract:
  Two-photon absorption is theoretically analyzed within the semiclassical
formalism of radiation-matter interaction. We consider an ensemble of
inhomogeneously broadened three-level atoms subjected to the action of two
counterpropagating fields of the same frequency. By concentrating in the limit
of large detuning in one-photon transitions, we solve perturbatively the Bloch
equations in a non-usual way. In this way we derive an analytical expression
for the width of the two-photon resonance that makes evident sub-Doppler
two--photon spectroscopy. We also derive an analytical expression for the Stark
shift of the two-photon resonance.


title:
Measurement-Based Quantum Turing Machines and Questions of
  Universalities

abstract:
  Quantum measurement is universal for quantum computation. This universality
allows alternative schemes to the traditional three-step organisation of
quantum computation: initial state preparation, unitary transformation,
measurement. In order to formalize these other forms of computation, while
pointing out the role and the necessity of classical control in
measurement-based computation, and for establishing a new upper bound of the
minimal resources needed to quantum universality, a formal model is introduced
by means of Measurement-based Quantum Turing Machines.


title: Pseudo-random operators of the circular ensembles

abstract:
  We demonstrate quantum algorithms to implement pseudo-random operators that
closely reproduce statistical properties of random matrices from the three
universal classes: unitary, symmetric, and symplectic. Modified versions of the
algorithms are introduced for the less experimentally challenging quantum
cellular automata. For implementing pseudo-random symplectic operators we
provide gate sequences for the unitary part of the time-reversal operator.


title:
Estimation of a classical parameter with gaussian probes: magnetometry
  with collective atomic spins

abstract:
  We present a theory for the estimation of a classical magnetic field by an
atomic sample with a gaussian distribution of collective spin components. By
incorporating the magnetic field and the probing laser field as quantum
variables with gaussian distributions on equal footing with the atoms, we
obtain a very versatile description which is readily adapted to include probing
with squeezed light, dissipation and loss and additional measurement
capabilities on the atomic system.


title:
Time evolution, cyclic solutions and geometric phases for the
  generalized time-dependent harmonic oscillator

abstract:
  The generalized time-dependent harmonic oscillator is studied. Though several
approaches to the solution of this model have been available, yet a new
approach is presented here, which is very suitable for the study of cyclic
solutions and geometric phases. In this approach, finding the time evolution
operator for the Schr\"odinger equation is reduced to solving an ordinary
differential equation for a c-number vector which moves on a hyperboloid in a
three-dimensional space. Cyclic solutions do not exist for all time intervals.
A necessary and sufficient condition for the existence of cyclic solutions is
given. There may exist some particular time interval in which all solutions
with definite parity, or even all solutions, are cyclic. Criterions for the
appearance of such cases are given. The known relation that the nonadiabatic
geometric phase for a cyclic solution is proportional to the classical Hannay
angle is reestablished. However, this is valid only for special cyclic
solutions. For more general ones, the nonadiabatic geometric phase may contain
an extra term. Several cases with relatively simple Hamiltonians are solved and
discussed in detail. Cyclic solutions exist in most cases. The pattern of the
motion, say, finite or infinite, can not be simply determined by the nature of
the Hamiltonian (elliptic or hyperbolic, etc.). For a Hamiltonian with a
definite nature, the motion can changes from one pattern to another, that is,
some kind of phase transition may occur, if some parameter in the Hamiltonian
goes through some critical value.


title: Ghost imaging using homodyne detection

abstract:
  We present a theoretical study of ghost imaging based on correlated beams
arising from parametric down-conversion, and which uses balanced homodyne
detection to measure both the signal and idler fields. We analytically show
that the signal-idler correlations contain the full amplitude and phase
information about an object located in the signal path, both in the near-field
and the far-field case. To this end we discuss how to optimize the optical
setups in the two imaging paths, including the crucial point regarding how to
engineer the phase of the idler local oscillator as to observe the desired
orthogonal quadrature components of the image. We point out an inherent link
between the far-field bandwidth and the near-field resolution of the reproduced
image, determined by the bandwidth of the source of the correlated beams.
However, we show how to circumvent this limitation by using a spatial averaging
technique which dramatically improves the imaging bandwidth of the far-field
correlations as well as speeds up the convergence rate. The results are backed
up by numerical simulations taking into account the finite size and duration of
the pump pulse.


title: Proposal for optically realizing quantum game

abstract:
  We present a proposal for optically implementing the quantum game of the
two-player quantum prisoner's dilemma involving nonmaximally entangled states
by using beam splitters, phase shifters, cross-Kerr medium, photon detector and
the single-photon representation of quantum bits.


title:
Diffraction of ultra-cold fermions by quantized light fields: Standing
  versus traveling waves

abstract:
  We study the diffraction of quantum degenerate fermionic atoms off of
quantized light fields in an optical cavity. We compare the case of a linear
cavity with standing wave modes to that of a ring cavity with two
counter-propagating traveling wave modes. It is found that the dynamics of the
atoms strongly depends on the quantization procedure for the cavity field. For
standing waves, no correlations develop between the cavity field and the atoms.
Consequently, standing wave Fock states yield the same results as a classical
standing wave field while coherent states give rise to a collapse and revivals
in the scattering of the atoms. In contrast, for traveling waves the scattering
results in quantum entanglement of the radiation field and the atoms. This
leads to a collapse and revival of the scattering probability even for Fock
states. The Pauli Exclusion Principle manifests itself as an additional
dephasing of the scattering probability.


title: Laser Polarization Effects in Supercontinuum Generation

abstract:
  We present experimental evidence of a fourth order process in electric field
in supercontinuum generation. We also show laser induced polarization
preference in the supercontinuum generating media. These results have become
possible through the choice of isotropic and anisotropic samples interacting
with ultrashort laser pulses of changing ellipticity. Laser polarization
emerges as an important control parameter for the highly nonlinear phenomenon
of supercontinuum generation.


title:
Quantum Fluctuations around Bistable Solitons in the Cubic-Quintic
  nonlinear Schr\"{o}dinger equation

abstract:
  Small quantum fluctuations in solitons described by the cubic-quintic
nonlinear Schr\"{o}dinger equation (CQNLSE) are studied with the linear
approximation. The cases of both self-defocusing and self-focusing quintic term
are considered (in the latter case, solitons may be effectively stable, despite
the possibility of collapse). The numerically implemented back-propagation
method is used to calculate the optimal squeezing ratio for the quantum
fluctuations vs. the propagation distance. In the case of the self-defocusing
quintic nonlinearity, opposite signs in front of the cubic and quintic terms
make the fluctuations around bistable pairs of solitons (which have different
energies for the same width) totally different. The fluctuations around
nonstationary Gaussian pulses in the CQNLSE model are studied too.


title: Holonomic Quantum Computing Based on the Stark Effect

abstract:
  We propose a spin manipulation technique based entirely on electric fields
applied to acceptor states in $p$-type semiconductors with spin-orbit coupling.
While interesting in its own right, the technique can also be used to implement
fault-resilient holonomic quantum computing. We explicitly compute adiabatic
transformation matrix (holonomy) of the degenerate states and comment on the
feasibility of the scheme as an experimental technique.


title: Quantum process tomography of a controlled-NOT gate

abstract:
  We demonstrate complete characterization of a two-qubit entangling process -
a linear optics controlled-NOT gate operating with coincident detection - by
quantum process tomography. We use maximum-likelihood estimation to convert the
experimental data into a physical process matrix. The process matrix allows
accurate prediction of the operation of the gate for arbitrary input states,
and calculation of gate performance measures such as the average gate fidelity,
average purity and entangling capability of our gate, which are 0.90, 0.83 and
0.73, respectively.


title:
Solitons Formed by Dark-State Polaritons in Electromagnetic Induced
  Transparency

abstract:
  We show the possible stable soliton generation for the dark-state polaritons
(DSPs) in an electromagnetic induced transparency (EIT) medium composed of
$\Lambda$-type atoms. Whether the solitons are dark or bright can be controlled
by the coupling field intensity and the one photon detuning of the probe field.
Besides, the velocity and time width of the solitons also can be adjusted by
the coupling light.


title:
The Interference Term in the Wigner Distribution Function and the
  Aharonov-Bohm Effect

abstract:
  A phase space representation of the Aharonov-Bohm effect is presented. It
shows that the shift of interference fringes is associated to the interference
term of the Wigner distribution function of the total wavefunction, whereas the
interference pattern is defined by the common projections of the Wigner
distribution functions of the interfering beams


title:
Bell's inequality for conditional probabilities as a test for
  quantum-like behaviour of mind

abstract:
  We define quantum-like probabilistic behaviour as behaviour which is
impossible to describe by using the classical probability model. We discuss the
conjecture that cognitive behaviour is quantum-like. There is presented the
scheme for an experimental test for quantum-like cognitive behaviour based on a
generalization of the famous Bell's inequality. This generalization is an
analogue of Bell's inequality, but for conditional probabilities. The use of
conditional probabilities (instead of simultaneous probability distributions
for pairs of observables in the original Bell's inequality) gives the
possibility to separate two problems which are mixed in the original Bell's
framework: nonlocality and nonclassical (quantum-like) probabilistic behaviour.
Our inequality for conditional probabilities can be used for experiments with a
single system (so we need not to prepare pairs of correlated systems) to find
quantum-like behaviour. This possibility is extremely important in cognitive
sciences where it is practically impossible to prepare pairs of precisely
correlated cognitive systems.


title: Unconditional Security of Practical Quantum Key Distribution

abstract:
  We present an extension of the first proof for the unconditional security of
the BB84 quantum key distribution protocol which was given by Mayers. We remove
the constraint that a perfect BB84 quantum source is required and the proof
given here covers a range of practical quantum sources. Nothing is assumed
about the detector except that the efficiency with which signals are detected
is basis independent.


title: Fault-Tolerant Postselected Quantum Computation: Schemes

abstract:
  Postselected quantum computation is distinguished from regular quantum
computation by accepting the output only if measurement outcomes satisfy
predetermined conditions. The output must be accepted with nonzero probability.
Methods for implementing postselected quantum computation with noisy gates are
proposed. These methods are based on error-detecting codes. Conditionally on
detecting no errors, it is expected that the encoded computation can be made to
be arbitrarily accurate. Although the probability of success of the encoded
computation decreases dramatically with accuracy, it is possible to apply the
proposed methods to the problem of preparing arbitrary stabilizer states in
large error-correcting codes with local residual errors. Together with
teleported error-correction, this may improve the error tolerance of
non-postselected quantum computation.


title: Charge Qubit Storage and Its Engineered Decoherence via Microwave Cavity

abstract:
  We study the entanglement of the superconducting charge qubit with the
quantized electromagnetic field in a microwave cavity. It can be controlled
dynamically by a classical external field threading the SQUID within the charge
qubit. Utilizing the controllable quantum entanglement, we can demonstrate the
dynamic process of the quantum storage of information carried by charge qubit.
On the other hand, based on this engineered quantum entanglement, we can also
demonstrate a progressive decoherence of charge qubit with quantum jump due to
the coupling with the cavity field in quasi-classical state.


title:
Generation of multi-photon Fock states by bichromatic adiabatic passage:
  topological analysis

abstract:
  We propose a robust scheme to generate multi-photon Fock states in an
atom-maser-cavity system using adiabatic passage techniques and topological
properties of the dressed eigenenergy surfaces. The mechanism is an exchange of
photons from the maser field into the initially empty cavity by bichromatic
adiabatic passage. The number of exchanged photons depends on the design of the
adiabatic dynamics through and around the conical intersections of dressed
eigenenergy surfaces.


title:
Categorical Foundations of Quantum Logics and Their Truth Values
  Structures

abstract:
  We introduce a foundational sheaf theoretical scheme for the comprehension of
quantum event structures, in terms of localization systems consisting of
Boolean coordinatization coverings induced by measurement. The scheme is based
on the existence of a categorical adjunction between presheaves of Boolean
event algebras and Quantum event algebras. On the basis of this adjoint
correspondence we prove the existence of an object of truth values in the
category of quantum logics, characterized as subobject classifier. This
classifying object plays the equivalent role that the two-valued Boolean truth
values object plays in classical event structures. We construct the object of
quantum truth values explicitly and argue that it constitutes the appropriate
choice for the valuation of propositions describing the behavior of quantum
systems.


title:
Quantum Memory Based on $\Lambda$-Atoms Ensemble with Two-Photon
  Resonance EIT

abstract:
  We study the $\Lambda$-atoms ensemble based quantum memory for the storage of
the quantum information carried by a probe light field. Two atomic Rabi
transitions of the ensemble are coupled to the quantum probe field and
classical control field respectively with a same detuning. Making use of the
hidden symmetry analysis developed recently for the on-resonance EIT case (Sun,
Li, and Liu, Phys. Rev. Lett. 91, 147903 (2003)), we show that the dark states
and dark-state polaritons can still exist for the case of two-photon resonance
EIT. Starting from these dark states we construct a complete class of
eigen-states of the total system. A explicit form of the adiabatic condition is
also given in order to achieve the memory and retrieve of quantum information.


title:
Loss of quantum coherence in a system coupled to a zero-temperature
  environment

abstract:
  We discuss the influence of a zero-temperature environment on a coherent
quantum system. First, we calculate the reduced density operator of the system
in the framework of the well-known, exactly solvable model of an oscillator
coupled to a bath of harmonic oscillators. Then, we propose the sketch of an
Aharonov--Bohm-like interferometer showing, through interference measurements,
the decrease of the coherence length of the system due to the interaction with
the environment, even in the zero temperature limit.


title: Non-Abelian Geometric Quantum Memory with Atomic Ensemble

abstract:
  We study a quantum information storage scheme based on an atomic ensemble
with near (also exact) three-photon resonance electromagnetically induced
transparency (EIT). Each 4-level-atom is coupled to two classical control
fields and a quantum probe field. Quantum information is adiabatically stored
in the associated dark polariton manifold. An intrinsic non-trivial topological
structure is discovered in our quantum memory implemented through the symmetric
collective atomic excitations with a hidden SU(3) dynamical symmetry. By
adiabatically changing the Rabi frequencies of two classical control fields,
the quantum state can be retrieved up to a non-abelian holonomy and thus
decoded from the final state in a purely geometric way.


title: On the structure of optimal sets for tensor product channel

abstract:
  For a given quantum channel we consider two optimal sets of states, related
to the Holevo capacity and to the minimal output entropy of this channel. Some
properties of these sets as well as the necessary and sufficient condition for
their coincidence are obtained. The relations between additivity properties for
two quantum channels and the structure of the optimal sets for tensor product
of these channels are considered. It turns out that these additivity properties
are connected with the special hereditary property of the optimal sets for
tensor product channel. We explore the structural properties of these optimal
sets under two different assumptions.
  The first assumption means that for tensor product of two channels with
arbitrary constraints there exists optimal ensemble with the product state
average. It turns out that exactly this assumption guarantees hereditary
property of the both optimal sets for tensor product channel and provides some
observations concerning the additivity problems.
  The second assumption means additivity of the Holevo capacity for two
channels with arbitrary constraints. It turns out that exactly this assumption
guarantees strong hereditary property of the both optimal sets for tensor
product channel and provides the natural "projective" relations between these
sets and the optimal sets for single channels.


title: Quantum tomography for solid state qubits

abstract:
  We propose a method for the tomographic reconstruction of qubit states for a
general class of solid state systems in which the Hamiltonians are represented
by spin operators, e.g., with Heisenberg-, $XXZ$-, or XY- type exchange
interactions. We analyze the implementation of the projective operator
measurements, or spin measurements, on qubit states. All the qubit states for
the spin Hamiltonians can be reconstructed by using experimental data.


title:
Nonlinear extensions of Schr\"odinger-von Neumann quantum dynamics: a
  list of conditions for compatibility with thermodynamics

abstract:
  We propose a list of conditions that consistency with thermodynamics imposes
on linear and nonlinear generalizations of standard unitary quantum mechanics
that assume a set of true quantum states without the restriction $\rho^2=\rho$
even for strictly isolated systems and that are to be considered in
experimental tests of the existence of intrinsic (spontaneous) decoherence at
the microscopic level.


title:
Sequential Probability Ratio Test for the detection of a single electron
  spin in the OSCAR setup

abstract:
  The MRFM device is a powerful setup for manipulating single electron spin in
resonance in a magnetic field. However, the real time observation of a
resonating spin is still an issue because of the very low SNR of the output
signal. This paper investigates the usability and the efficiency of sequential
detection schemes (the Sequential Probability Ratio Test) to decrease the
required integration time, in comparison to standard fixed time detection
schemes.


title:
Preparation and Entanglement Purification of Qubits through Zeno-like
  Measurements

abstract:
  A novel method of purification, purification through Zeno-like measurements
[H. Nakazato, T. Takazawa, and K. Yuasa, Phys. Rev. Lett. 90, 060401 (2003)],
is discussed extensively and applied to a few simple qubit systems. It is
explicitly demonstrated how it works and how it is optimized. As possible
applications, schemes for initialization of multiple qubits and entanglement
purification are presented, and their efficiency is investigated in detail.
Simplicity and flexibility of the idea allow us to apply it to various kinds of
settings in quantum information and computation, and would provide us with
useful and practical methods of state preparation.


title: Purification of Quantum State through Zeno-like Measurements

abstract:
  We present a novel procedure to purify quantum states, i.e., purification
through Zeno-like measurements. By simply repeating one and the same
measurement on a quantum system, one can purify another system in interaction
with the former. The conditions for the (efficient) purification are specified
on a rather general setting, and the framework of the method possesses wide
applicability. It is explicitly demonstrated on a specific setup that the
purification becomes very efficient by tuning relevant parameters.


title: Entanglement Purification through Zeno-like Measurements

abstract:
  We present a novel method to purify quantum states, i.e. purification through
Zeno-like measurements, and show an application to entanglement purification.


title:
Exactly Solvable Many-Body Systems and Pseudo-Hermitian Point
  Interactions

abstract:
  We study Hamiltonian systems with point interactions and give a systematic
description of the corresponding boundary conditions and the spectrum
properties for self-adjoint, PT-symmetric systems and systems with real
spectra. The integrability of one dimensional many body systems with these
kinds of point (contact) interactions are investigated for both bosonic and
fermionic statistics.


title: Simultaneous arrival of information in absorbing wave guides

abstract:
  We demonstrate that the temporal peak generated by specific electromagnetic
pulses may arrive at different positions simultaneously in an absorbing wave
guide. The effect can be used for triggering several devices all at once at
unknown distances from the sender or generally to transmit information so that
it arrives at the same time to receivers at different, unknown locations. This
simultaneity cannot be realized by the standard transmission methods.


title: Pseudo Memory Effects, Majorization and Entropy in Quantum Random Walks

abstract:
  A quantum random walk on the integers exhibits pseudo memory effects, in that
its probability distribution after N steps is determined by reshuffling the
first N distributions that arise in a classical random walk with the same
initial distribution. In a classical walk, entropy increase can be regarded as
a consequence of the majorization ordering of successive distributions. The
Lorenz curves of successive distributions for a symmetric quantum walk reveal
no majorization ordering in general. Nevertheless, entropy can increase, and
computer experiments show that it does so on average. Varying the stages at
which the quantum coin system is traced out leads to new quantum walks,
including a symmetric walk for which majorization ordering is valid but the
spreading rate exceeds that of the usual symmetric quantum walk.


title: Supersymmetry and Qubit Field Theory

abstract:
  Supersymmetric extension of the Deutsch's qubit field theory is presented and
a new solution of quantum information paradox via anticommuting qubit-fields
condensate is shown.


title:
Generation of nonclassical photon states using a superconducting qubit
  in a microcavity

abstract:
  Based on the interaction between the radiation field and a superconductor, we
propose a way to engineer quantum states using a SQUID charge qubit inside a
microcavity. This device can act as a deterministic single photon source as
well as generate any Fock states and an arbitrary superposition of Fock states
for the cavity field. The controllable interaction between the cavity field and
the qubit can be realized by the tunable gate voltage and classical magnetic
field applied to the SQUID.