title: All Moments of the Uniform Ensemble of Quantum Density Matrices

abstract:
  Given a uniform ensemble of quantum density matrices $\rho$, it is useful to
calculate the mean value over this ensemble of a product of entries of $\rho$.
We show how to calculate such moments in this paper. The answer involves well
known results from Group Representation Theory and Random Matrix Theory. This
quantum problem has a well known classical counterpart: given a uniform
ensemble of probability distributions $P=(P_1, P_2, ..., P_N)$ where the $P_j$
are non-negative reals that sum to one, calculate the mean value over this
probability simplex of products of $P$ components. The answer to the classical
problem follows from an integral formula due to Dirichlet.


title:
Does classical mechanics always adequately describe "classical physical
  reality"

abstract:
  The article is dedicated to discussion of irreversibility and foundation of
statistical mechanics "from the first principles". Taking into account
infinitesimal and, as it seems, neglectful for classical mechanics fluctuations
of the physical vacuum, makes a deterministic motion of unstable dynamic
systems is broken ("spontaneous determinism breaking", "spontaneous
stochastization"). Vacuum fluctuations play part of the trigger, starting the
powerful mechanism of exponent instability. The motion of the dynamic systems
becomes irreversible and stochastic. Classical mechanics turns out to be
applicable only for a small class of stable dynamic systems with zero
Kolmogorov-Sinay entropy $h=0$. For alternative "Stochastic mechanics" there
are corresponding equations of motion and Master Equation, describing
irreversible evolution of the initial distribution function to equilibrium
state.


title: To Physical Foundation of Quantum Mechanics

abstract:
  On the base of years of experience of working on the problem of the physical
foundation of quantum mechanics the author offers principles of solving it.
Under certain pressure of mathematical formalism there has raised a hypothesis
of complexity of space and time by Minkovsky, being significant mainly for
quantum objects. In this eight-dimensional space and time with six space and
two time dimensions all the problems and peculiarities of quantum mechanical
formalism disappear, the reasons of their appearance become clear, and there
comes a clear and physically transparent picture of the foundations of quantum
mechanics.


title:
A first experimental test of de Broglie-Bohm theory against standard
  quantum mechanics

abstract:
  De Broglie - Bohm (dBB) theory is a deterministic theory, built for
reproducing almost all Quantum Mechanics (QM) predictions, where position plays
the role of a hidden variable. It was recently shown that different coincidence
patterns are predicted by QM and dBB when a double slit experiment is realised
under specific conditions and, therefore, an experiment can test the two
theories. In this letter we present the first realisation of such a double slit
experiment by using correlated photons produced in type I Parametric Down
Conversion. Our results confirm QM contradicting dBB predictions.


title: Entanglement of a two-mode squeezed state in a Gaussian environment

abstract:
  Some non-classical properties such as squeezing, sub-Poissonian photon
statistics or oscillations in photon-number distributions may survive longer in
a phase-sensitive environment than in a phase-insensitive environment. We
examine if entanglement, which is an inter-mode non-classical feature, can also
survive longer in a phase-sensitive environment. Differently from the
single-mode case, we find that making the environment phase-sensitive does not
aid in prolonging entanglement.


title: Suitability versus fidelity for rating single-photon guns

abstract:
  The creation of specified quantum states is important for most, if not all,
applications in quantum computation and communication. The quality of the state
preparation is therefore an essential ingredient in any assessment of a
quantum-state gun. We show that the fidelity, under the standard definitions is
not sufficient to assess quantum sources, and we propose a new measure of
suitability that necessarily depends on the application for the source. We
consider the performance of single-photon guns in the context of quantum key
distribution (QKD) and linear optical quantum computation. Single-photon
sources for QKD need radically different properties than sources for quantum
computing. Furthermore, the suitability for single-photon guns is discussed
explicitly in terms of experimentally accessible criteria.


title: Distant entanglement with nearest neighbor interactions

abstract:
  Preparing many body entangled states efficiently using available interactions
is a challenging task. One solution may be to couple a system collectively with
a probe that leaves residual entanglement in the system. We investigate the
entanglement produced between two possibly distant qubits 1 and 2 that interact
locally with a third qubit 3 under unitary evolution generated by pairwise
Hamiltonians. For the case where the Hamiltonians commute, relevant to certain
quantum nondemolition measurements, the entanglement between qubits 1 and 2 is
calculated explicitly for several classes of initial states and compared with
the case of noncommuting interaction Hamiltonians. This analysis can be helpful
to identify preferable physical system interactions for entangled state
synthesis.


title:
Quantitative conditional quantum erasure in two-atom resonance
  fluorescence

abstract:
  We present a conditional quantum eraser which erases the a priori knowledge
or the predictability of the path a photon takes in a Young-type double-slit
experiment with two fluorescent four-level atoms. This erasure violates a
recently derived erasure relation which must be satisfied for a conventional,
unconditional quantum eraser that aims to find an optimal sorting of the system
into subensembles with particularly large fringe visibilities. The conditional
quantum eraser employs an interaction-free, partial which-way measurement which
not only sorts the system into optimal subsystems with large visibility but
also selects the appropriate subsystem with the maximum possible visibility. We
explain how the erasure relation can be violated under these circumstances.


title: Reversible Logic Circuit Synthesis

abstract:
  Reversible or information-lossless circuits have applications in digital
signal processing, communication, computer graphics and cryptography. They are
also a fundamental requirement in the emerging field of quantum computation. We
investigate the synthesis of reversible circuits that employ a minimum number
of gates and contain no redundant input-output line-pairs (temporary storage
channels). We prove constructively that every even permutation can be
implemented without temporary storage using NOT, CNOT and TOFFOLI gates. We
describe an algorithm for the synthesis of optimal circuits and study the
reversible functions on three wires, reporting distributions of circuit sizes.
We study circuit decompositions of reversible circuits where gates of the same
type are next to each other. Finally, in an application important to quantum
computing, we synthesize oracle circuits for Grover's search algorithm, and
show a significant improvement over a previously proposed synthesis algorithm.


title: Exchange Gate on the Qudit Space and Fock Space

abstract:
  We construct the exchange gate with small elementary gates on the space of
qudits, which consist of three controlled shift gates and three "reverse"
gates. This is a natural extension of the qubit case.
  We also consider a similar subject on the Fock space, but in this case we
meet with some different situation. However we can construct the exchange gate
by making use of generalized coherent operator based on the Lie algebra su(2)
which is a well--known method in Quantum Optics. We moreover make a brief
comment on "imperfect clone".


title: Quantum Computers, Discrete Space, and Entanglement

abstract:
  We consider algebras underlying Hilbert spaces used by quantum information
algorithms. We show how one can arrive at equations on such algebras which
define n-dimensional Hilbert space subspaces which in turn can simulate quantum
systems on a quantum system. In doing so we use MMP diagrams and linear
algorithms. MMP diagrams are tractable since an n-block of an MMP diagram has n
elements while an n-block of a standard lattice diagram has 2^n elements. An
immediate test for such an approach is a generation of minimal and arbitrary
Kochen-Specker vectors and we present a minimal state-independent
Kochen-Specker set of seven vectors from a Hilbert space with more than four
dimensions.


title: Commutativity and the third Reidemeister movement

abstract:
  In quantum information theory, for $a,b$ two positive operators living in
$B(\mathcal{H})$, where $\mathcal{H}$ is a separable Hilbert space, the quantum
fidelity is denoted by $a*b =(b^{1/2}ab^{1/2})^{1/2}$. One of the aim of this
let ter is to interpret the quantum fidelity as an algebraic law. We remark
that if $a,b,c$ are three positive operators whi ch commute pairwise, the law *
becomes self-distributive, i.e. the third Reidemeister movement in knot theory
is verif ied. We study the converse. Let three positive operators be given,
does the fact that the third Reidemeister movement between them is possible
implie that they commute pairwise ? Though in general we only conjecture it for
the moment, we prove it in some par ticular but important cases. Should this
movement be not possible, we interpret it as an obstruction to comm utativity.
We give also new examples of quandle algebras and left distributive systems and
study the generalisation of Ito maps.


title: Conscious Pulse I: The rules of engagement

abstract:
  This paper elaborates on four previously proposed rules of engagement between
conscious states and physiological states. A new rule is proposed that applies
to a continuous model of conscious brain states that cannot precisely resolve
eigenvalues. If two apparatus states are in superposition, and if their
eigenvalues are so close together that they cannot be consciously resolved on
this model, then it is shown that observation will not generally reduce the
superposition to just one of its member eigenstates. In general, the
observation of a quantum mechanical superposition results in another
superposition. Key words: Brain states, consciousness, conscious observer,
macroscopic superposition, measurement, state reduction, state collapse, von
Neumann.


title: Easy Monitored Entangled States

abstract:
  We discuss the generation and monitoring of durable atomic entangled state
via Raman-type process, which can be used in the quantum information
processing.


title: Quantum entanglement of unitary operators on bi-partite systems

abstract:
  We study the entanglement of unitary operators on $d_1\times d_2$ quantum
systems. This quantity is closely related to the entangling power of the
associated quantum evolutions. The entanglement of a class of unitary operators
is quantified by the concept of concurrence.


title: One-dimensional quantum walks with absorbing boundaries

abstract:
  In this paper we analyze the behavior of quantum random walks. In particular
we present several new results for the absorption probabilities in systems with
both one and two absorbing walls for the one-dimensional case. We compute these
probabilites both by employing generating functions and by use of an
eigenfunction approach. The generating function method is used to determine
some simple properties of the walks we consider, but appears to have
limitations. The eigenfunction approach works by relating the problem of
absorption to a unitary problem that has identical dynamics inside a certain
domain, and can be used to compute several additional interesting properties,
such as the time dependence of absorption. The eigenfunction method has the
distinct advantage that it can be extended to arbitrary dimensionality. We
outline the solution of the absorption probability problem of a
(d-1)-dimensional wall in a d-dimensional space.


title: Extended phase-matching conditions for improved entanglement generation

abstract:
  Extended phase-matching conditions for spontaneous parametric down-conversion
are examined. By augmenting the conventional phase-matching conditions, they
permit the creation of a class of frequency-entangled states that generalizes
the usual twin-beam biphoton state. An experimental characterization of these
states is possible through interferometric coincidence counting.


title: Between entropy and subentropy

abstract:
  The von Neumann entropy and the subentropy of a mixed quantum state are upper
and lower bounds, respectively, on the accessible information of any ensemble
consistent with the given mixed state. Here we define and investigate a set of
quantities intermediate between entropy and subentropy.


title: Simulation of quantum dynamics with quantum optical systems

abstract:
  We propose the use of quantum optical systems to perform universal simulation
of quantum dynamics. Two specific implementations that require present
technology are put forward for illustrative purposes. The first scheme consists
of neutral atoms stored in optical lattices, while the second scheme consists
of ions stored in an array of micro--traps. Each atom (ion) supports a
two--level system, on which local unitary operations can be performed through a
laser beam. A raw interaction between neighboring two--level systems is
achieved by conditionally displacing the corresponding atoms (ions). Then,
average Hamiltonian techniques are used to achieve evolutions in time according
to a large class of Hamiltonians.


title: Efficiency and formalism of quantum games

abstract:
  We pursue a general theory of quantum games. We show that quantum games are
more efficient than classical games, and provide a saturated upper bound for
this efficiency. We demonstrate that the set of finite classical games is a
strict subset of the set of finite quantum games. We also deduce the quantum
version of the Minimax Theorem and the Nash Equilibrium Theorem.


title: Affine-Detection Loophole in Quantum Data Processing

abstract:
  Here is considered a specific detection loophole, that is relevant not only
to testing of quantum nonlocality, but also to some other applications of
quantum computations and communications. It is described by a simple affine
relation between different quantum "data structures" like pure and mixed state,
separable and inseparable one. It is shown also, that due to such relations
imperfect device for a classical model may mimic measurements of quantum
correlations on ideal equipment.


title: Relativistic Time-of-Arrival and Traversal Time

abstract:
  We compute in a relativistic way the time-of-arrival and the traversal time
through a region of a free particle with spin 1/2. We do this by applying the
relativistic extension of the Event-Enhanced Quantum Theory which we have
presented in a previous paper. We find a very good coincidence of the results
of our formalism and the results obtained by using classical relativistic
mechanics.


title: Consistent Resolution of Some Relativistic Quantum Paradoxes

abstract:
  A relativistic version of the (consistent or decoherent) histories approach
to quantum theory is developed on the basis of earlier work by Hartle, and used
to discuss relativistic forms of the paradoxes of spherical wave packet
collapse, Bohm's formulation of Einstein-Podolsky-Rosen, and Hardy's paradox.
It is argued that wave function collapse is not needed for introducing
probabilities into relativistic quantum mechanics, and in any case should never
be thought of as a physical process. Alternative approaches to stochastic time
dependence can be used to construct a physical picture of the measurement
process that is less misleading than collapse models. In particular, one can
employ a coarse-grained but fully quantum mechanical description in which
particles move along trajectories, with behavior under Lorentz transformations
the same as in classical relativistic physics, and detectors are triggered by
particles reaching them along such trajectories. States entangled between
spacelike separate regions are also legitimate quantum descriptions, and can be
consistently handled by the formalism presented here. The paradoxes in question
arise because of using modes of reasoning which, while correct for classical
physics, are inconsistent with the mathematical structure of quantum theory,
and are resolved (or tamed) by using a proper quantum analysis. In particular,
there is no need to invoke, nor any evidence for, mysterious long-range
superluminal influences, and thus no incompatibility, at least from this
source, between relativity theory and quantum mechanics.


title:
Quantum trajectory approach to stochastically-induced quantum
  interference effects in coherently-driven two-level atoms

abstract:
  Stochastic perturbation of two-level atoms strongly driven by a coherent
light field is analyzed by the quantum trajectory method. A new method is
developed for calculating the resonance fluorescence spectra from numerical
simulations. It is shown that in the case of dominant incoherent perturbation,
the stochastic noise can unexpectedly create phase correlation between the
neighboring atomic dressed states. This phase correlation is responsible for
quantum interference between the related transitions resulting in anomalous
modifications of the resonance fluorescence spectra.


title:
Spin and Statistics in Nonrelativistic Quantum Mechanics: The Zero Spin
  Case

abstract:
  It is proved from stated assumptions of nonrelativistic quantum mechanics
based on the Schroedinger equation that identical spin-zero particles must obey
symmetric statistics.


title:
Relativistic entanglement of quantum states and nonlocality of
  Einstein-Podolsky-Rosen(EPR) paradox

abstract:
  Relativistic bipartite entangled quantum states is studied to show that
Nature doesn't favor nonlocality for massive particles in the
ultra-relativistic limit. We found that to an observer (Bob) in a moving frame
S', the entangled Bell state shared by Alice and Bob appears as the
superposition of the Bell bases in the frame S' due to the requirement of the
special relativity. It is shown that the entangled pair satisfies the Bell's
inequality when the boost speed approaches the speed of light, thus providing a
counter example for nonlocality of Einstein-Podolsky-Rosen(EPR) paradox.


title: Semiconductor-based Geometrical Quantum Gates

abstract:
  We propose an implementation scheme for holonomic, i.e., geometrical, quantum
information processing based on semiconductor nanostructures. Our quantum
hardware consists of coupled semiconductor macroatoms addressed/controlled by
ultrafast multicolor laser-pulse sequences. More specifically, logical qubits
are encoded in excitonic states with different spin polarizations and
manipulated by adiabatic time-control of the laser amplitudes . The two-qubit
gate is realized in a geometric fashion by exploiting dipole-dipole coupling
between excitons in neighboring quantum dots.


title: What's Wrong with these Observables?

abstract:
  An imprecise measurement of a dynamical variable (such as a spin component)
does not, in general, give the value of another dynamical variable (such as a
spin component along a slightly different direction). The result of the
measurement cannot be interpreted as the value of any observable that has a
classical analogue.


title:
Scheme for preparation of nonmaximal entanglement between two atomic
  ensembles

abstract:
  We propose an experimentally feasible scheme to generate nonmaximal
entanglement between two atomic ensembles. The degree of entanglement is
readily tunable. The scheme involves laser manipulation of atomic ensembles,
adjustable quarter- and half-wave plates, beam splitter, polarizing beam
splitters, and single-photon detectors, and well fits the status of the current
experimental technology. Finally we use the nonmaximally entangled state of
ensembles to demonstrate quantum nonlocality by detecting the
Clauser-Horne-Shimony-Holt inequality.


title: Implementations of Nonadiabatic Geometric Quantum Computation using NMR

abstract:
  Recently, geometric phases, which is fault tolerate to certain errors
intrinsically due to its geometric property, are getting considerable attention
in quantum computing theoretically. So far, only one experiment about adiabatic
geometric gate with NMR through Berry phase has been reported. However, there
are two drawbacks in it. First, the adiabatic condition of Berry phase makes
such gate very slowly. Second, the extra operation to eliminate the dynamic
phase. As we know, geometric phase can exist both adiabatic(Berry phase) and
nonadiabatic(Aharonov-Anandan phase). In this letter, we reports the first
experimental realization of nonadiabatic geometric gate with NMR through
conditional-AA phase. In our experiment the gates can be made faster and more
easily, and the two drawbacks mentioned above are removed.


title:
Resonant Energy Exchange between Atoms in Dispersing and Absorbing
  Surroundings

abstract:
  Within the framework of quantization of the macroscopic electromagnetic
field, a master equation describing both the resonant dipole-dipole interaction
(RDDI) and the resonant atom-field interaction (RAFI) in the presence of
dispersing and absorbing macroscopic bodies is derived, with the relevant
couplings being expressed in terms of the surroundings-assisted Green tensor.
It is shown that under certain conditions the RDDI can be regarded as being
governed by an effective Hamiltonian. The theory, which applies to both weak
and strong atom-field coupling, is used to study the resonant energy exchange
between two (two-level) atoms sharing initially a single excitation. In
particular, it is shown that in the regime of weak atom-field coupling there is
a time window, where the energy transfer follows a transfer-rate law of the
type obtained by ordinary second-order perturbation theory. Finally, the
spectrum of the light emitted during the energy transfer is studied and the
line splittings are discussed.


title:
The geometry of entanglement witnesses and local detection of
  entanglement

abstract:
  Let $H^{[ N]}=H^{[ d_{1}]}\otimes ... \otimes H^{[ d_{n}]}$ be a tensor
product of Hilbert spaces and let $\tau_{0}$ be the closest separable state in
the Hilbert-Schmidt norm to an entangled state $\rho_{0}$. Let
$\tilde{\tau}_{0}$ denote the closest separable state to $\rho_{0}$ along the
line segment from $I/N$ to $\rho_{0}$ where $I$ is the identity matrix.
Following [pitrubmat] a witness $W_{0}$ detecting the entanglement of
$\rho_{0}$ can be constructed in terms of $I, \tau_{0}$ and $\tilde{\tau}_{0}$.
If representations of $\tau_{0}$ and $\tilde{\tau}_{0}$ as convex combinations
of separable projections are known, then the entanglement of $\rho_{0}$ can be
detected by local measurements. G\"{u}hne \textit{et. al.} in [bruss1] obtain
the minimum number of measurement settings required for a class of two qubit
states. We use our geometric approach to generalize their result to the
corresponding two qudit case when $d$ is prime and obtain the minimum number of
measurement settings. In those particular bipartite cases,
$\tau_{0}=\tilde{\tau}_{0}$. We illustrate our general approach with a two
parameter family of three qubit bound entangled states for which $\tau_{0} \neq
\tilde{\tau}_{0}$ and we show our approach works for $n$ qubits.
  In [pitt] we elaborated on the role of a ``far face'' of the separable states
relative to a bound entangled state $\rho_{0}$ constructed from an orthogonal
unextendible product base. In this paper the geometric approach leads to an
entanglement witness expressible in terms of a constant times $I$ and a
separable density $\mu_{0}$ on the far face from $\rho_{0}$. Up to a
normalization this coincides with the witness obtained in [bruss1] for the
particular example analyzed there.


title: A new type of complementarity between quantum and classical information

abstract:
  Physical systems contain information which can be divided between classical
and quantum information. Classical information is locally accessible and allows
one to perform tasks such as physical work, while quantum information allows
one to perform tasks such as teleportation. It is shown that these two kinds of
information are complementarity in the sense that two parties can either gain
access to the quantum information, or to the classical information but not
both. This complementarity has a form very similar to the complementarities
usually encountered in quantum mechanics. For pure states, the entanglement
plays the role of Planck's constant. We also find another class of
complementarity relations which applies to operators, and is induced when two
parties can only perform local operations and communicate classical. In order
to formalize this notion we define the restricted commutator. Observables such
as the parity and phase of two qubits commute, but their restricted commutator
is non-zero. It is also found that complementarity is pure in the sense that
can be ''decoupled'' from the uncertainty principle.


title: Entanglement and Concurrence in the BCS State

abstract:
  An extension of the notion of concurrence introduced by Wooters is used to
quantify the entanglement of certain multipartite pure states, namely, the BCS
state of superconducting compounds. This leads to a definition of the
macrocanonical entanglement of pairing (MEP) for which we compute an analytical
formula in terms of two adimensional numbers, the cut-off and gap numbers,
which depend on measurable physical quantities. We find that strongly coupled
BCS elements like Pb and Nb have much larger MEP values than more conventional
BCS transition metal superconductors.


title:
Low energy (e,2e) ionization of Argon in the equal energy sharing
  geometry

abstract:
  Quantum Defect theory is a well established theoretical concept in modern
spectroscopy. We show that this approach is useful in electron impact
ionization problems where state of the art theoretical methods are presently
restricted mostly to simple atomic targets. For the well documented Argon
ionization case in equal energy sharing geometry the approach suggested leads
to significant improvements compared to previous calculations.


title: Quantum quincunx in cavity quantum electrodynamics

abstract:
  We introduce the quantum quincunx, which physically demonstrates the quantum
walk and is analogous to Galton's quincunx for demonstrating the random walk.
In contradistinction to the theoretical studies of quantum walks over
orthogonal lattice states, we introduce quantum walks over nonorthogonal
lattice states (specifically, coherent states on a circle) to demonstrate that
the key features of a quantum walk are observable albeit for strict parameter
ranges. A quantum quincunx may be realized with current cavity quantum
electrodynamics capabilities, and precise control over decoherence in such
experiments allows a remarkable decrease in the position noise, or spread, with
increasing decoherence.


title: Is There More to T?

abstract:
  We present some novel results indicating that time's description in
present-day physics is deficient. We use Hawking's information-erasure
hypothesis to counter his own claim that time's arrow depends only on initial
conditions. Next, we propose quantum mechanical experiments that yield
inconsistent histories, suggesting that not only events but also entire
histories might be governed by a more fundamental dynamics.


title: Quantum Zeno subspaces and dynamical superselection rules

abstract:
  The quantum Zeno evolution of a quantum system takes place in a proper
subspace of the total Hilbert space. The physical and mathematical features of
the "Zeno subspaces" depend on the measuring apparatus: when this is included
in the quantum description, the Zeno effect becomes a mere consequence of the
dynamics and, remarkably, can be cast in terms of an adiabatic theorem, with a
dynamical superselection rule. We look at several examples and focus on quantum
computation and decoherence-free subspaces.


title:
Rates of asymptotic entanglement transformations for bipartite mixed
  states: Maximally entangled states are not special

abstract:
  We investigate the asymptotic rates of entanglement transformations for
bipartite mixed states by local operations and classical communication (LOCC).
We analyse the relations between the rates for different transitions and obtain
simple lower and upper bound for these transitions. In a transition from one
mixed state to another and back, the amount of irreversibility can be different
for different target states. Thus in a natural way, we get the concept of
"amount" of irreversibility in asymptotic manipulations of entanglement. We
investigate the behaviour of these transformation rates for different target
states. We show that with respect to asymptotic transition rates under LOCC,
the maximally entangled states do not have a special status. In the process, we
obtain that the entanglement of formation is additive for all maximally
correlated states. This allows us to show irreversibility in asymptotic
entanglement manipulations for maximally correlated states in 2x2. We show that
the possible nonequality of distillable entanglement under LOCC and that under
operations preserving the positivity of partial transposition, is related to
the behaviour of the transitions (under LOCC) to separable target states.


title: An Architecture of Deterministic Quantum Central Processing Unit

abstract:
  We present an architecture of QCPU(Quantum Central Processing Unit), based on
the discrete quantum gate set, that can be programmed to approximate any
n-qubit computation in a deterministic fashion. It can be built efficiently to
implement computations with any required accuracy. QCPU makes it possible to
implement universal quantum computation with a fixed, general purpose hardware.
Thus the complexity of the quantum computation can be put into the software
rather than the hardware.


title: Interference and entanglement: an intrinsic approach

abstract:
  An addition rule of impure density operators, which provides a pure state
density operator, is formulated. Quantum interference including visibility
property is discussed in the context of the density operator formalism. A
measure of entanglement is then introduced as the norm of the matrix equal to
the difference between a bipartite density matrix and the tensor product of
partial traces. Entanglement for arbitrary quantum observables for multipartite
systems is discussed. Star-product kernels are used to map the formulation of
the addition rule of density operators onto the addition rule of symbols of the
operators. Entanglement and nonlocalization of the pure state projector and
allied operators are discussed. Tomographic and Weyl symbols (tomograms and
Wigner functions) are considered as examples. The squeezed-states and some
spin-states (two qubits) are studied to illustrate the formalism.


title: Quantum fluctations of coherent light in nonlinear media

abstract:
  The interaction of coherent light with a nonlinear medium is modeled here by
a general quantum anharmonic oscillator. The model is not exactly solvable in a
closed analytical form. But we need operator solutions of the equations of
motion corresponding to these models in order to study the quantum fluctuations
of coherent light in nonlinear media. In the present work we derive approximate
operator solutions. From these solutions we observe that there exists an
apparent discrepancy between the solutions obtained by different techniques. We
compare different solutions and conclude that all correct solutions are
equivalent and the apparent discrepancy is due to the use of different ordering
of the operators. We use these solutions to investigate the possibilities of
observing different optical phenomena in a nonlinear dielectric medium. To be
precise, we have studied quantum phase fluctuations of coherent light in a
third order inversion symmetric nonlinear medium. Fluctuations in phase space
quadrature for the same system are studied and the possibility of generating
squeezed state is reported. Fluctuations in photon number are studied and the
nonclassical phenomenon of antibunching is predicted. We have generalized the
results obtained for third order nonlinear medium and have studied the
interaction of an intense laser beam with a general (m-1)-th order nonlinear
medium. Aharanov Anandan nonadiabatic geometric phase is also discussed in the
context of (m-1)-th order nonlinear medium.


title: Creating quanta with "annihilation" operator

abstract:
  An asymmetric nature of the boson `destruction' operator $\hat{a}$ and its
`creation' partner $\hat{a}^{\dagger}$ is made apparent by applying them to a
quantum state $|\psi>$ different from the Fock state $|n>$. We show that it is
possible to {\em increase} (by many times or by any quantity) the mean number
of quanta in the new `photon-subtracted' state $\hat{a}|\psi >$. Moreover, for
certain `hyper-Poissonian' states $|\psi>$ the mean number of quanta in the
(normalized) state $\hat{a}|\psi>$ can be much greater than in the
`photon-added' state $\hat{a}^{\dagger}|\psi > $. The explanation of this
`paradox' is given and some examples elucidating the meaning of Mandel's
$q$-parameter and the exponential phase operators are considered.


title:
A Study of Quasi-Exactly Solvable Models within the Quantum
  Hamilton-Jacobi Method

abstract:
  A few quasi-exactly solvable models are studied within the quantum
Hamilton-Jacobi formalism. By assuming a simple singularity structure of the
quantum momentum function, we show that the exact quantization condition leads
to the condition for quasi-exact solvability.


title:
Implementation of universal quantum gates based on nonadiabatic
  geometric phases

abstract:
  We propose an experimentally feasible scheme to achieve quantum computation
based on nonadiabatic geometric phase shifts, in which a cyclic geometric phase
is used to realize a set of universal quantum gates. Physical implementation of
this set of gates is designed for Josephson junctions and for NMR systems.
Interestingly, we find that the nonadiabatic phase shift may be independent of
the operation time under appropriate controllable conditions. A remarkable
feature of the present nonadiabatic geometric gates is that there is no
intrinsic limitation on the operation time, unlike adiabatic geometric gates.
Besides fundamental interest, our results may simplify the implementation of
geometric quantum computation based on solid state systems, where the
decoherence time may be very short.


title: Imaginary Phases in Two-Level Model with Spontaneous Decay

abstract:
  We study a two-level model coupled to the electromagnetic vacuum and to an
external classic electric field with fixed frequency. The amplitude of the
external electric field is supposed to vary very slow in time. Garrison and
Wright [{\it Phys. Lett.} {\bf A128} (1988) 177] used the non-hermitian
Hamiltonian approach to study the adiabatic limit of this model and obtained
that the probability of this two-level system to be in its upper level has an
imaginary geometric phase. Using the master equation for describing the time
evolution of the two-level system we obtain that the imaginary phase due to
dissipative effects is time dependent, in opposition to Garrison and Wright
result. The present results show that the non-hermitian hamiltonian method
should not be used to discuss the nature of the imaginary phases in open
systems.


title: Universal Quantum Cloning in Cavity QED

abstract:
  We propose an implementation of an universal quantum cloning machine [UQCM,
Hillery and Buzek, Phys. Rev. A {\bf 56}, 3446 (1997)] in a Cavity Quantum
Electrodynamics (CQED) experiment. This UQCM acts on the electronic states of
atoms that interact with the electromagnetic field of a high $Q$ cavity. We
discuss here the specific case of the $1 \to 2$ cloning process using either a
one- or a two-cavity configuration.


title: Hopf algebra, thermodynamics and entanglement in quantum field theory

abstract:
  The quantum deformation of the Hopf algebra describes the skeleton of quantum
field theory, namely its characterizing feature consisting in the existence of
infinitely many unitarily inequivalent representations of the canonical
commutation relations. From this we derive the thermal properties of quantum
field theory, with the entropy playing the role of the generator of the
non-unitary time evolution. The entanglement of the quantum vacuum appears to
be robust against interaction with the environment: on cannot ``unknot the
knots" in the infinite volume limit.


title: Cyclical Quantum Memory for Photonic Qubits

abstract:
  We have performed a proof-of-principle experiment in which qubits encoded in
the polarization states of single-photons from a parametric down-conversion
source were coherently stored and read-out from a quantum memory device. The
memory device utilized a simple free-space storage loop, providing a cyclical
read-out that could be synchronized with the cycle time of a quantum computer.
The coherence of the photonic qubits was maintained during switching operations
by using a high-speed polarizing Sagnac interferometer switch.


title: A General Framework for Nonlinear Quantum Dynamics

abstract:
  We outline an approach that streamlines considerably the construction and
analysis of well-behaved nonlinear quantum dynamics, with completely positive
extensions to entangled systems. A few notes are added on the issue of quantum
measurements under a nonlinear dynamics.


title:
Non-Local Quantum Gates: a Cavity-Quantum-Electro-Dynamics
  implementation

abstract:
  The problems related to the management of large quantum registers could be
handled in the context of distributed quantum computation: unitary non-local
transformations among spatially separated local processors are realized
performing local unitary transformations and exchanging classical
communication. In this paper, we propose a scheme for the implementation of
universal non-local quantum gates such as a controlled-$\gate{NOT}$ ($\cnot$)
and a controlled-quantum phase gate ($\gate{CQPG}$). The system we have chosen
for their physical implementation is a Cavity-Quantum-Electro-Dynamics (CQED)
system formed by two spatially separated microwave cavities and two trapped
Rydberg atoms. We describe the procedures to follow for the realization of each
step necessary to perform a specific non-local operation.


title: Shouldn't there be an antithesis to quantization?

abstract:
  We raise the possibility of developing a theory of constructing quantum
dynamical observables independent from quantization and deriving classical
dynamical observables from pure quantum mechanical consideration. We do so by
giving a detailed quantum mechanical derivation of the classical time of
arrival at arbitrary arrival points for a particle in one dimension.


title: A discrete and finite approach to past proper time

abstract:
  The function $\gamma(x)=\frac{1}{\sqrt{1-x^2}}$ plays an important role in
mathematical physics, e.g. as factor for relativistic time dilation in case of
$x=\beta$ with $\beta=\frac{v}{c}$ or $\beta=\frac{pc}{E}$. Due to former
considerations it is reasonable to study the power series expansion of
$\gamma(x)$. Here its relationship to the binomial distribution is shown,
especially the fact, that the summands of the power series correspond to the
return probabilities to the starting point (local coordinates, configuration or
state) of a Bernoulli random walk. So $\gamma(x)$ and with that also proper
time is proportional to the sum of the return probabilities. In case of $x=1$
or $v=c$ the random walk is symmetric. Random walks with absorbing barriers are
introduced in the appendix. Here essentially the basic mathematical facts are
shown and references are given, most interpretation is left to the reader.


title:
Pure Stationary States of non-Hamiltonian and Dissipative Quantum
  Systems

abstract:
  Using Liouville space and superoperator formalism we consider pure stationary
states of open and dissipative quantum systems. We discuss stationary states of
open quantum systems, which coincide with stationary states of closed quantum
systems. Open quantum systems with pure stationary states of linear oscillator
are suggested. We consider stationary states for the Lindblad equation. We
discuss bifurcations of pure stationary states for open quantum systems which
are quantum analogs of classical dynamical bifurcations.


title:
Generation of Pulsed Polarization Entangled Two-Photon State via
  Temporal and Spectral Engineering

abstract:
  The quantum state of the photon pair generated from type-II spontaneous
parametric down-conversion pumped by a ultrafast laser pulse exhibits strong
decoherence in its polarization entanglement, an effect which can be attributed
to the clock effect of the pump pulse or, equivalently, to distinguishing
spectral information in the two-photon state. Here, we propose novel temporal
and spectral engineering techniques to eliminate these detrimental decoherence
effects. The temporal engineering of the two-photon wavefunction results in a
universal Bell-state synthesizer that is independent of the choice of pump
source, crystal parameters, wavelengths of the interacting photons, and the
bandwidth of the spectral filter. In the spectral engineering technique, the
distinguishing spectral features of the two-photon state are eliminated through
modifications to the two-photon source. In addition, spectral engineering also
provides a means for the generation of polarization-entangled states with novel
spectral characteristics: the frequency-correlated state and the
frequency-uncorrelated state.


title: Remarks on time-energy uncertainty relations

abstract:
  Using a recent construction of observables characterizing the time of
occurence of an effect in quantum theory, we present a rigorous derivation of
the standard time-energy uncertainty relation. In addition, we prove an
uncertainty relation for time measurements only.


title:
Optical phase-space reconstruction of mirror position at the attometer
  level

abstract:
  We describe an experiment in which the quadratures of the position of an
harmonically-bound mirror are observed at the attometer level. We have studied
the Brownian motion of the mirror, both in the free regime and in the
cold-damped regime when an external viscous force is applied by radiation
pressure. We have also studied the thermal-noise squeezing when the external
force is parametrically modulated. We have observed both the 50% theoretical
limit of squeezing at low gain and the parametric oscillation of the mirror for
a large gain.


title: Quantum Information and Spacetime Structure

abstract:
  In modern quantum information theory one deals with an idealized situation
when the spacetime dependence of quantum phenomena is neglected. However the
transmission and processing of (quantum) information is a physical process in
spacetime. Therefore such basic notions in quantum information theory as qubit,
channel, composite systems and entangled states should be formulated in space
and time. In this paper some basic notions of quantum information theory are
considered from the point of view of quantum field theory and general
relativity. It is pointed out an important fact that in quantum field theory
there is a statistical dependence between two regions in spacetime even if they
are spacelike separated. A classical probabilistic representation for a family
of correlation functions in quantum field theory is obtained. A noncommutative
generalization of von Neumann`s spectral theorem is discussed. We suggest a new
physical principle describing a relation between the mathematical formalism of
Hilbert space and quantum physical phenomena which goes beyond the
superselection rules. Entangled states and the change of state associated with
the measurement process in space and time are discussed including the black
hole and the cosmological spacetime.
  It is shown that any reasonable state in relativistic quantum field theory
becomes disentangled at large spacelike distances if one makes local
observations. As a result a violation of Bell`s inequalities can be observed
without inconsistency with principles of relativistic quantum theory only if
the distance between detectors is rather small. We suggest a further
experimental study of entangled states in spacetime by studying the dependence
of the correlation functions on the distance between detectors.


title: Suppression of decoherence via strong intra-environmental coupling

abstract:
  We examine the effects of intra-environmental coupling on decoherence by
constructing a low temperature spin--spin-bath model of an atomic impurity in a
Debye crystal. The impurity interacts with phonons of the crystal through
anti-ferromagnetic spin-spin interactions. The reduced density matrix of the
central spin representing the impurity is calculated by dynamically integrating
the full Schroedinger equation for the spin--spin-bath model for different
thermally weighted eigenstates of the spin-bath. Exact numerical results show
that increasing the intra-environmental coupling results in suppression of
decoherence. This effect could play an important role in the construction of
solid state quantum devices such as quantum computers.


title:
Asymptotic entanglement capacity of the Ising and anisotropic Heisenberg
  interactions

abstract:
  We compute the asymptotic entanglement capacity of the Ising interaction ZZ,
the anisotropic Heisenberg interaction XX + YY, and more generally, any
two-qubit Hamiltonian with canonical form K = a XX + b YY. We also describe an
entanglement assisted classical communication protocol using the Hamiltonian K
with rate equal to the asymptotic entanglement capacity.


title:
Lorentz covariant reduced-density-operator theory for relativistic
  quantum information processing

abstract:
  In this paper, we derived Lorentz covariant quantum Liouville equation for
the density operator which describes the relativistic quantum information
processing from Tomonaga-Schwinger equation and an exact formal solution for
the reduced-density-operator is obtained using the projector operator technique
and the functional calculus. When all the members of the family of the
hypersurfaces become flat hyperplanes, it is shown that our results agree with
those of non-relativistic case which is valid only in some specified reference
frame. To show that our new formulation can be applied to practical problems,
we derived the polarization of the vacuum in quantum electrodynamics up to the
second order. The formulation presented in this work is general and might be
applied to related fields such as quantum electrodynamics and relativistic
statistical mechanics.


title: Bell's Theorem and Chemical Potential

abstract:
  Chemical potential is a property which involves the effect of interaction
between the components of a system, and it results from the whole system. In
this paper, we argue that for two particles which have interacted via their
spins and are now spatially separated, the so-called Bell's locality condition
implies that the chemical potential of each particle is an individual property.
Here is a point where quantum statistical mechanics and the local hidden
variable theories are in conflict. Based on two distinct concepts of chemical
potential, the two theories predict two different patterns for the energy
levels of a system of two entangled particles. In this manner, we show how one
can distinguish the non-separable features of a two-particle system.


title:
Experimental investigation of a coherent quantum measurement of the
  degree of polarization of a single mode light beam

abstract:
  A novel method for the direct measurement of the degree of polarization is
described. It is one of the first practical implementations of a coherent
quantum measurement, the projection on the singlet state. Our first results
demonstrate the successful operation of the method. However, due to the
nonlinear crystals used presently, its application is limited to spectral
widths larger than ~8nm.


title: Quantum Motion as the Motion in the Integrable Weyl Space

abstract:
  A new version of hidden variables theory founded on the generalisation of
world's geometry is proposed. The quantum-mechanical motion as the motion in
some "inner space", which has a structure of the integrable Weyl space is
examined. Equations of motion for a quantum particle as well as equations for
the quantum-mechanical field, "guiding" the particle, are deduced. The wave
equation for quantum ensembles, the Born's interpretation of wave function and
the "guiding equation" as consequences of proposed model are obtained.


title: Security of Quantum Key Distribution with Entangled Qutrits

abstract:
  The study of quantum cryptography and quantum non-locality have
traditionnally been based on two-level quantum systems (qubits). In this paper
we consider a generalisation of Ekert's cryptographic protocol [Ekert] where
qubits are replaced by qutrits. The security of this protocol is related to
non-locality, in analogy with Ekert's protocol. In order to study its
robustness against the optimal individual attacks, we derive the information
gained by a potential eavesdropper applying a cloning-based attack.


title: Combinatorial Topology Of Multipartite Entangled States

abstract:
  With any state of a multipartite quantum system its separability polytope is
associated. This is an algebro-topological object (non-trivial only for mixed
states) which captures the localisation of entanglement of the state.
Particular examples of separability polytopes for 3-partite systems are
explicitly provided. It turns out that this characterisation of entanglement is
associated with simulation of arbitrary unitary operations by 1- and 2-qubit
gates. A topological description of how entanglement changes in course of such
simulation is provided.


title: Quantum Computing with Electron Spins in Quantum Dots

abstract:
  We present a set of concrete and realistic ideas for the implementation of a
small-scale quantum computer using electron spins in lateral GaAs/AlGaAs
quantum dots. Initialization is based on leads in the quantum Hall regime with
tunable spin-polarization. Read-out hinges on spin-to-charge conversion via
spin-selective tunneling to or from the leads, followed by measurement of the
number of electron charges on the dot via a charge detector. Single-qubit
manipulation relies on a microfabricated wire located close to the quantum dot,
and two-qubit interactions are controlled via the tunnel barrier connecting the
respective quantum dots. Based on these ideas, we have begun a series of
experiments in order to demonstrate unitary control and to measure the
coherence time of individual electron spins in quantum dots.


title: Quantum comparison machines with one-sided error

abstract:
  It is always possible to decide, with one-sided error, whether two quantum
states are the same under a specific unitary transformation. However we show
here that it is {\em impossible} to do so if the transformation is anti-linear
and non-singular. This result implies that unitary and anti-unitary operations
exist on an unequal footing in quantum information theory.


title:
Solutions of the nonrelativistic wave equation with position-dependent
  effective mass

abstract:
  Given a spatially dependent mass distribution we obtain potential functions
for exactly solvable nonrelativistic problems. The energy spectrum of the bound
states and their wavefunctions are written down explicitly. This is
accomplished by mapping the wave equation for these systems into well-known
exactly solvable Schrodinger equations with constant mass using point canonical
transformation. The Oscillator, Coulomb, and Morse class of potentials are
considered.


title:
Relation of the Bell inequalities with quantum logic, hidden variables
  and information theory

abstract:
  I review the relation of the Bell inequalities - characteristic of
(classical) probabilities defined on Boolean logics - with noncontextual and
local hidden variables theories of quantum mechanics and with quantum
information.


title: Error prevention scheme with two pairs of qubits

abstract:
  A scheme is presented for protecting one-qubit quantum information against
decoherence due to a general environment and local exchange interactions. The
scheme operates essentially by distributing information over two pairs of
qubits and through error prevention procedures. In the scheme, quantum
information is encoded through a decoherence-free subspace for collective phase
errors and exchange errors affecting the qubits in pairs; leakage out of the
encoding space due to amplitude damping is reduced by quantum Zeno effect. In
addition, how to construct decoherence-free states for n-qubit information
against phase and exchange errors is discussed.


title: Digital Switching in the Quantum Domain

abstract:
  In this paper, we present an architecture and implementation algorithm such
that digital data can be switched in the quantum domain. First we define the
connection digraph which can be used to describe the behavior of a switch at a
given time, then we show how a connection digraph can be implemented using
elementary quantum gates. The proposed mechanism supports unicasting as well as
multicasting, and is strict-sense non-blocking. It can be applied to perform
either circuit switching or packet switching. Compared with a traditional space
or time domain switch, the proposed switching mechanism is more scalable.
Assuming an n-by-n quantum switch, the space consumption grows linearly, i.e.
O(n), while the time complexity is O(1) for unicasting, and O(log n) for
multicasting. Based on these advantages, a high throughput switching device can
be built simply by increasing the number of I/O ports.


title:
Relation between classical communication capacity and entanglement
  capability for two-qubit unitary operations

abstract:
  Two-qubit operations may be characterized by their capacities for
communication, both with and without free entanglement, and their capacity for
creating entanglement. We establish a set of inequalities that give an ordering
to the capacities of two-qubit unitary operations. Specifically, we show that
the capacities for entanglement creation and bidirectional communication
without entanglement assistance are at least as great as half the bidirectional
communication capacity with entanglement assistance. In addition, we show that
the bidirectional communication that can be performed using an ensemble may be
increased via a two-qubit unitary operation by twice the operation's capacity
for entanglement.


title:
The various power decays of the survival probability at long times for
  free quantum particle

abstract:
  The long time behaviour of the survival probability of initial state and its
dependence on the initial states are considered, for the one dimensional free
quantum particle. We derive the asymptotic expansion of the time evolution
operator at long times, in terms of the integral operators. This enables us to
obtain the asymptotic formula for the survival probability of the initial state
$\psi (x)$, which is assumed to decrease sufficiently rapidly at large $|x|$.
We then show that the behaviour of the survival probability at long times is
determined by that of the initial state $\psi$ at zero momentum $k=0$. Indeed,
it is proved that the survival probability can exhibit the various power-decays
like $t^{-2m-1}$ for an arbitrary non-negative integers $m$ as $t \to \infty $,
corresponding to the initial states with the condition $\hat{\psi} (k) =
O(k^m)$ as $k\to 0$.


title:
Free initial wave packets and the long-time behavior of the survival and
  nonescape probabilities

abstract:
  The behavior of both the survival S(t) and nonescape P(t) probabilities at
long times for the one-dimensional free particle system is shown to be closely
connected to that of the initial wave packet at small momentum. We prove that
both S(t) and P(t) asymptotically exhibit the same power-law decrease at long
times, when the initial wave packet in momentum representation behaves as O(1)
or O(k) at small momentum. On the other hand, if the integer m becomes greater
than 1, S(t) and P(t) decrease in different power-laws at long times.


title: Numerical simulation of information recovery in quantum computers

abstract:
  Decoherence is the main problem to be solved before quantum computers can be
built. To control decoherence, it is possible to use error correction methods,
but these methods are themselves noisy quantum computation processes. In this
work we study the ability of Steane's and Shor's fault-tolerant recovering
methods, as well a modification of Steane's ancilla network, to correct errors
in qubits. We test a way to measure correctly ancilla's fidelity for these
methods, and state the possibility of carrying out an effective error
correction through a noisy quantum channel, even using noisy error correction
methods.


title: Data compression limit for an information source of interacting qubits

abstract:
  A system of interacting qubits can be viewed as a non-i.i.d quantum
information source. A possible model of such a source is provided by a quantum
spin system, in which spin-1/2 particles located at sites of a lattice interact
with each other. We establish the limit for the compression of information from
such a source and show that asymptotically it is given by the von Neumann
entropy rate. Our result can be viewed as a quantum analog of Shannon's
noiseless coding theorem for a class of non - i.i.d. quantum information
sources.


title:
Shutters, Boxes, But No Paradoxes: Time Symmetry Puzzles in Quantum
  Theory

abstract:
  The ``N-Box Experiment'' is a much-discussed thought experiment in quantum
mechanics. It is claimed by some authors that a single particle prepared in a
superposition of N+1 box locations and which is subject to a final
``post-selection'' measurement corresponding to a different superposition can
be said to have occupied ``with certainty'' N boxes during the intervening
time. However, others have argued that under closer inspection, this surprising
claim fails to hold. Aharonov and Vaidman have continued their advocacy of the
claim in question by proposing a variation on the N-box experiment, in which
the boxes are replaced by shutters and the pre- and post-selected particle is
entangled with a photon. These authors argue that the resulting ``N-shutter
experiment'' strengthens their original claim regarding the N-box experiment.
It is argued in this paper that the apparently surprising features of this
variation are no more robust than those of the N-box experiment and that it is
not accurate to say that the particle is ``with certainty'' in all N shutters
at any given time.


title: There is No Violation of Conservation Laws in Quantum Measurement

abstract:
  For some ideal quantum measurements, conservation laws would seem to be
violated systematically. It is argued that the intrinsically non-"ideal" nature
of quantum measurements rescues the conservation laws.


title:
A practical scheme for quantum computation with any two-qubit entangling
  gate

abstract:
  Which gates are universal for quantum computation? Although it is well known
that certain gates on two-level quantum systems (qubits), such as the
controlled-not (CNOT), are universal when assisted by arbitrary one-qubit
gates, it has only recently become clear precisely what class of two-qubit
gates is universal in this sense. Here we present an elementary proof that any
entangling two-qubit gate is universal for quantum computation, when assisted
by one-qubit gates. A proof of this important result for systems of arbitrary
finite dimension has been provided by J. L. and R. Brylinski
[arXiv:quant-ph/0108062, 2001]; however, their proof relies upon a long
argument using advanced mathematics. In contrast, our proof provides a simple
constructive procedure which is close to optimal and experimentally practical
[C. M. Dawson and A. Gilchrist, online implementation of the procedure
described herein (2002), http://www.physics.uq.edu.au/gqc/].


title: How photon detectors remove vacuum fluctuations

abstract:
  The main argument against the reality of the electromagnetic quantum vacuum
fluctuations is that they do not activate photon detectors. In order to met
this objection I propose a model of photocounting which, in the simple case of
a light signal with constant intensity, predicts a counting rate proportional
to the intensity, in agreement with the standard quantum result.


title:
Obedient quantum mechanics: New status of the theory in the inverse
  problem approach

abstract:
  New status in quantum mechanics is connected with recent achievements in the
inverse problem. With its help instead of about ten exactly solvable models
which serve as a basis of the contemporary education there are infinite (!)
number, even complete sets of such models. So, the whole quantum mechanics is
embraced by them. They correspond to all possible variations of spectral
parameters which determine all properties of quantum systems. There appears a
possibility to change at wish quantum objects by variation of these parameters
as control levers and examine quantum systems in different thinkable
situations. As a result, we acquire a vision of the intrinsic logic of behavior
of any thinkable system, including real ones. The regularities revealed by
computer visualization of these models were reformulated into unexpectedly
simple universal rules of arbitrary transformations and what is more, their
elementary constituents were discovered (new breakthrough). Of these elementary
"bricks" it is possible in principle to construct objects with any given
properties. This book of inverse problem quantum pictures is utmost
intelligible and recommended to any physicists, chemists, mathematicians,
biologists from students to professors who are interested in laws of the
microworld.


title: Single photons on demand from 3D photonic band-gap structures

abstract:
  We describe a practical implementation of a (semi-deterministic) photon gun
based on stimulated Raman adiabatic passage pumping and the strong enhancement
of the photonic density of states in a photonic band-gap material. We show that
this device allows {\em deterministic} and {\em unidirectional} production of
single photons with a high repetition rate of the order of 100kHz. We also
discuss specific 3D photonic microstructure architectures in which our model
can be realized and the feasibility of implementing such a device using
${Er}^{3+}$ ions that produce single photons at the telecommunication
wavelength of $1.55 \mu$m.


title: Quantum teleportation of light beams

abstract:
  We experimentally demonstrate quantum teleportation for continuous variables
using squeezed-state entanglement. The teleportation fidelity for a real
experimental system is calculated explicitly, including relevant imperfection
factors such as propagation losses, detection inefficiencies and phase
fluctuations. The inferred fidelity for input coherent states is F = 0.61 +-
0.02, which when corrected for the efficiency of detection by the output
observer, gives a fidelity of 0.62. By contrast, the projected result based on
the independently measured entanglement and efficiencies is 0.69. The
teleportation protocol is explained in detail, including a discussion of
discrepancy between experiment and theory, as well as of the limitations of the
current apparatus.


title: Quantum Mechanics with Difference Operators

abstract:
  A formulation of quantum mechanics with additive and multiplicative
(q-)difference operators instead of differential operators is studied from
first principles. Borel-quantisation on smooth configuration spaces is used as
guiding quantisation method. After a short discussion this method is translated
step-by-step to a framework based on difference operators. To restrict the
resulting plethora of possible quantisations additional assumptions motivated
by simplicity and plausibility are required. Multiplicative difference
operators and the corresponding q-Borel kinematics are given on the circle and
its N-point discretisation; the connection to q-deformations of the Witt
algebra is discussed. For a "natural" choice of the q-kinematics a
corresponding q-difference evolution equation is obtained. This study shows
general difficulties for a generalisation of a physical theory from a known one
to a "new" framework.


title: Quantum games with a multi-slit electron diffraction setup

abstract:
  A setup is proposed to play a quantum version of the famous bimatrix game of
Prisoners' Dilemma. Multi-slit electron diffraction with each player's pure
strategy consisting of opening one of the two slits at his/her disposal are
essential features of the setup. Instead of entanglement the association of
waves with travelling material objects is suggested as another resource to play
quantum games.


title:
Experimental realization of a fetching algorithm in a 7 qubit NMR
  quantum computer

abstract:
  Searching for marked items from an unsorted database is an important
scientific problem and a benchmark for computing devices as well. Using a
7-qubit liquid NMR quantum computer, we have demonstrated successfully an
hybrid quantum fetching algorithm that finds marked items using only a single
query. The essential idea is the operation of quantum computers in parallel. We
gave the detailed pulse sequence for coherent control of the 7 qubits. The
pulse sequence demonstrated here is not only useful for ensemble quantum
computation, but also can be regarded as a general purpose control-gate which
is useful for experimental design of quantum algorithms and general quantum
information processing task in other quantum computer schemes. A generalization
of the algorithm that is scalable to arbitrary qubit number is also provided.


title: Exploiting Randomness in Quantum Information Processing

abstract:
  We consider how randomness can be made to play a useful role in quantum
information processing - in particular, for decoherence control and the
implementation of quantum algorithms. For a two-level system in which the
decoherence channel is non-dissipative, we show that decoherence suppression is
possible if memory is present in the channel. Random switching between two
potentially harmful noise sources can then provide a source of stochastic
control. Such random switching can also be used in an advantageous way for the
implementation of quantum algorithms.


title: Uncertainty reconciles complementarity with joint measurability

abstract:
  The fundamental principles of complementarity and uncertainty are shown to be
related to the possibility of joint unsharp measurements of pairs of
noncommuting quantum observables. A new joint measurement scheme for
complementary observables is proposed. The measured observables are represented
as positive operator valued measures (POVMs), whose intrinsic fuzziness
parameters are found to satisfy an intriguing pay-off relation reflecting the
complementarity. At the same time, this relation represents an instance of a
Heisenberg uncertainty relation for measurement imprecisions. A
model-independent consideration show that this uncertainty relation is
logically connected with the joint measurability of the POVMs in question.


title: Quantum Fluctuations in Josephson Junction Comparators

abstract:
  We have developed a method for calculation of quantum fluctuation effects, in
particular of the uncertainty zone developing at the potential curvature sign
inversion, for a damped harmonic oscillator with arbitrary time dependence of
frequency and for arbitrary temperature, within the Caldeira-Leggett model. The
method has been applied to the calculation of the gray zone width Delta Ix of
Josephson-junction balanced comparators driven by a specially designed
low-impedance RSFQ circuit. The calculated temperature dependence of Delta Ix
in the range 1.5 to 4.2K is in a virtually perfect agreement with experimental
data for Nb-trilayer comparators with critical current densities of 1.0 and 5.5
kA/cm^2, without any fitting parameters.


title: Ghost spinors in quantum particles interference

abstract:
  In this article a question of the ghost spinors influence to the quantum
particles interference is investigated. The interaction between spinors and
ghost spinors are considered. Furthermore the conditions of zero-point
energy-momentum tensor in private cases are found. Also we consider a question
about the experimental test of Deutsch shadow pacticles existense.


title: Fundamentos Matematicos De Teorias Sobre Espacios Noconmutativos

abstract:
  In the talk we investigate the question of commutation of the whole-partial
derivatives, which should be considered when the function, which is subjected
to differentiation, has both explicit and implicit dependence. We apply the
results to the quantum theories.
  --
  En la platica investigamos la cuestion de conmutacion de derivadas parciales
cuando la funcion sujeta a diferenciacion tiene dependencia tanto explicita
como implicita. Aplicamos los resultados a teorias cuanticas.


title: Quantum 2-player gambling and correlated pay-off

abstract:
  I give an analysis of the simplest non-commutative quantum game, which is a
gambling game much like Heads or Tails. The quantum gamespace displays
strategies which are not interpretable through direct-product strategies of the
two players. Yet the pay-off allows for correlations if states corresponding to
a different number of rounds played are superposed.


title: Retrodiction as a tool for micromaser field measurements

abstract:
  We use retrodictive quantum theory to describe cavity field measurements by
successive atomic detections in the micromaser. We calculate the state of the
micromaser cavity field prior to detection of sequences of atoms in either the
excited or ground state, for atoms that are initially prepared in the excited
state. This provides the POM elements, which describe such sequences of
measurements.


title: Phase-dependent interaction in a 4-level atomic configuration

abstract:
  We study a four-level atomic scheme interacting with four lasers in a
closed-loop configuration with a $\diamondsuit$ (diamond) geometry. We
investigate the influence of the laser phases on the steady state. We show
that, depending on the phases and the decay characteristic, the system can
exhibit a variety of behaviors, including population inversion and complete
depletion of an atomic state. We explain the phenomena in terms of multi-photon
interference. We compare our results with the phase-dependent phenomena in the
double-$\Lambda$ scheme, as studied in [Korsunsky and Kosachiov, Phys. Rev A
{\bf 60}, 4996 (1999)]. This investigation may be useful for developing
non-linear optical devices, and for the spectroscopy and laser-cooling of
alkali-earth atoms.


title:
Confinement-Higgs transition in a disordered gauge theory and the
  accuracy threshold for quantum memory

abstract:
  We study the +/- J random-plaquette Z_2 gauge model (RPGM) in three spatial
dimensions, a three-dimensional analog of the two-dimensional +/- J random-bond
Ising model (RBIM). The model is a pure Z_2 gauge theory in which randomly
chosen plaquettes (occuring with concentration p) have couplings with the
``wrong sign'' so that magnetic flux is energetically favored on these
plaquettes. Excitations of the model are one-dimensional ``flux tubes'' that
terminate at ``magnetic monopoles.'' Electric confinement can be driven by
thermal fluctuations of the flux tubes, by the quenched background of magnetic
monopoles, or by a combination of the two. Like the RBIM, the RPGM has enhanced
symmetry along a ``Nishimori line'' in the p-T plane (where T is the
temperature). The critical concentration p_c of wrong-sign plaquettes at the
confinement-Higgs phase transition along the Nishimori line can be identified
with the accuracy threshold for robust storage of quantum information using
topological error-correcting codes: if qubit phase errors, qubit bit-flip
errors, and errors in the measurement of local check operators all occur at
rates below p_c, then encoded quantum information can be protected perfectly
from damage in the limit of a large code block. Numerically, we measure p_{c0},
the critical concentration along the T=0 axis (a lower bound on p_c), finding
p_{c0}=.0293 +/- .0002. We also measure the critical concentration of
antiferromagnetic bonds in the two-dimensional RBIM on the T=0 axis, finding
p_{c0}=.1031 +/-.0001. Our value of p_{c0} is incompatible with the value of
p_c=.1093 +/-.0002 found in earlier numerical studies of the RBIM, in
disagreement with the conjecture that the phase boundary of the RBIM is
vertical (parallel to the T axis) below the Nishimori line.

categories: quant-ph cond-mat.stat-mech hep-lat hep-th


title: Quantum bit commitment and unconditional security

abstract:
  It is generally believed that unconditionally secure quantum bit commitment
is impossible, due to widespread acceptance of an impossibility proof that
utilizes quantum entaglement cheating. In this paper, we delineate how the
impossibiliy proof formulation misses various types of quantum bit commitment
protocols based on two-way quantum communications. We point out some of the
gaps in the impossibility proof reasoning, and present corresponding
counterexamples. Four different types of bit commitment protocols are
constructed with several new protocol techniques. A specific Type 4 protocol is
described and proved unconditionally secure. Security analysis of a Type 1
protocol and a Type 2 protocol are also sketched. The security of Type 3
protocols is as yet open. A development of quantum statistical decision theory
and quantum games is needed to provie a complete security analysis of many such
protocols.


title:
Towards the Classical Communication Complexity of Entanglement
  Distillation Protocols with Incomplete Information

abstract:
  Quantum entanglement distillation protocols are LOCC protocols between Alice
and Bob that convert imperfect EPR pairs, or, in general, partially entangled
bipartite states into perfect or near-perfect EPR pairs. The classical
communication complexity of these protocols is the minimal amount of classical
communication needed for the conversion. In this paper, we focus on the
communication complexity of protocols that operate with incomplete information,
i.e., where the inputs are mixed states and/or prepared adversarially.
  We study 3 models of imperfect EPR pairs. In the measure-r model, r out of n
EPR pairs are measured by an adversary; in the depolarization model, Bob's
share of qubits underwent a depolarization channel; in the fidelity model, the
only information Alice and Bob possess is the fidelity of the shared state.
  For the measure-r model and the depolarization model, we prove tight and
almost-tight bounds on the outcome of LOCC protocols that don't use
communication. For the fidelity model, we prove a lower bound on the
communication complexity that matches the upper bound given by Ambainis, Smith,
and Yang [ASY02].


title: Pulse compression by parametric beating with a prepared Raman coherence

abstract:
  We present a general analysis for the interaction of a probe laser radiation
with a coherently prepared molecular Raman medium. We describe a general
formalism which includes dispersive effects, such as group velocity and group
velocity dispersion (GVD). When dispersion is negligible, the analysis is
especially simple and insightful. We show that molecular oscillations result in
a modulated instantaneous susceptibility of the medium. The effect of the
time-varying susceptibility on a probe laser pulse is two-fold: the output
frequency becomes modulated because of the time-varying phase velocity, and the
pulse shape becomes deformed because of the time-varying group velocity. We
identify two mechanisms for pulse compression: (1) Frequency chirping with
subsequent pulse compression by normal linear GVD (possibly in the same medium)
and (2) Compression due to the time-varying group velocity. We analyze various
aspects of pulse compression in the coherent Raman medium and derive
conservation relations for this process. When we consider a probe laser pulse
which is much shorter than the molecular oscillation period, we observe
frequency chirping, compression, or stretching of this pulse, depending on its
relative timing with respect to the molecular oscillations. Based on our
analysis, we propose a method for selective compression or frequency conversion
of single ultrashort pulses.


title:
Frequency modulation and pulse compression by coherent multimode
  molecular motion

abstract:
  We study beating of a probe field with a time-varying susceptibility in a
coherently prepared Raman medium. We consider the general case of an arbitrary
variation of susceptibility, which corresponds to a superposition of an
arbitrary number of excited Raman transitions. We derive a general analytical
solution and conservation relations for this process. We show that the
interference between Raman polarizations may substantially affect frequency
modulation and pulse compression for the probe field.


title:
The beta-neurexin/neuroligin-1 interneuronal intrasynaptic adhesion is
  essential for quantum brain dynamics

abstract:
  There are many blank areas in understanding the brain dynamics and especially
how it gives rise to consciousness. Quantum mechanics is believed to be capable
of explaining the enigma of conscious experience, however till now there is not
good enough model considering both the data from clinical neurology and having
some explanatory power. In this paper is presented a novel model in defence of
macroscopic quantum events within and between neural cells. The synaptic
beta-neurexin/neuroligin-1 adhesive protein complex is claimed to be not just
the core of the excitatory glutamatergic CNS synapse, instead it is a device
mediating entanglement between the cytoskeletons of the cortical neurons. Thus
the macroscopic coherent quantum state can extend throughout large brain
cortical areas and the subsequent collapse of the wavefunction could affect
simultaneously the subneuronal events in millions of neurons. The
neuroligin-1/beta-neurexin/synaptotagmin-1 complex also controls the process of
exocytosis and provides an interesting and simple mechanism for retrograde
signalling during learning-dependent changes in synaptic connectivity. A brief
outlook of the molecular machinery driving neuromediator release through
exocytosis is provided with particular emphasis on the possibility for
vibrationally-assisted tunneling.


title:
Continuous variable teleportation of a quantum state onto a macroscopic
  body

abstract:
  We study the possibility to teleport an unkown quantum state onto the
vibrational degree of freedom of a movable mirror. The quantum channel between
the two parties is established by exploiting radiation pressure effects.


title: Torus quantization for spinning particles

abstract:
  We derive semiclassical quantization conditions for systems with spin. To
this end one has to define the notion of integrability for the corresponding
classical system which is given by a combination of the translational motion
and classical spin precession. We determine the geometry of the invariant
manifolds of this product dynamics which support semiclassical solutions of the
wave equation. The semiclassical quantization conditions contain a new term,
which is of the same order as the Maslov correction. This term is identified as
a rotation angle for a classical spin vector. Applied to the relativistic
Kepler problem the procedure sheds some light on the amazing success of
Sommerfeld's theory of fine structure [Ann. Phys. (Leipzig) 51 (1916) 1-94].


title: Measurement of qutrits

abstract:
  We proposed the procedure of measuring the unknown state of the three-level
system - the qutrit, which was realized as the arbitrary polarization state of
the single-mode biphoton field. This procedure is accomplished for the set of
the pure states of qutrits; this set is defined by the properties of SU(2)
transformations, that are done by the polarization transformers.


title: Observation of sub-Fourier resonances in a quantum-chaotic system

abstract:
  We experimentally show that the response of a quantum-chaotic system can
display resonance lines sharper than the inverse of the excitation duration.
This allows us to discriminate two neighboring frequencies with a resolution
nearly 40 times better than the limit set by the Fourier inequality.
Furthermore, numerical studies indicate that there is no limit, but the loss of
signal, to this resolution, opening ways for the development of sub-Fourier
quantum-chaotic signal processing.


title:
Triplet Superconductors from the Viewpoint of Basic Elements for Quantum
  Computers

abstract:
  We discuss possibilities of utilizing superconductors with Cooper condensates
in triplet pairing states (where the spin of condensate pairs is S=1) for
practical realization of quantum computers. Superconductors with triplet
pairing condensates have features that are unique and cannot be found in the
usual (singlet pairing, S=0) superconductors. The symmetry of the order
parameter in some triplet superconductors (e.g., ruthenates) corresponds to
doubly-degenerate chiral states. These states can serve as qubit base states
for quantum computing.


title: Fidelity Decay as an Efficient Indicator of Quantum Chaos

abstract:
  Recent work has connected the type of fidelity decay in perturbed quantum
models to the presence of chaos in the associated classical models. We
demonstrate that a system's rate of fidelity decay under repeated perturbations
may be measured efficiently on a quantum information processor, and analyze the
conditions under which this indicator is a reliable probe of quantum chaos and
related statistical properties of the unperturbed system. The type and rate of
the decay are not dependent on the eigenvalue statistics of the unperturbed
system, but depend on the system's eigenvector statistics in the eigenbasis of
the perturbation operator. For random eigenvector statistics the decay is
exponential with a rate fixed precisely by the variance of the perturbation's
energy spectrum. Hence, even classically regular models can exhibit an
exponential fidelity decay under generic quantum perturbations. These results
clarify which perturbations can distinguish classically regular and chaotic
quantum systems.


title: Entanglement Breaking Channels

abstract:
  This paper has been removed and is superceded by a pair of papers on
stochastic maps, or channels, which break entanglement (EBT). quant-ph/0302031
General Entanglement Breaking Channels by Michael Horodecki, Peter W. Shor and
Mary Beth Ruskai includes the results originally in section 3, as well as a
number of new results about the extreme points. (The remarks in previously in
section 3 about "sign change condition" are incorrect and have been omitted
from quant-ph/0302031.) The new paper contains a to the conjecture that all
extreme points of EBT maps are CQ when d > 2. quant-ph/0302032 Qubit
Entanglement Breaking Channels by Mary Beth Ruskai contains the results
oroginally in section 2. The main result is that for qubits, EBT maps are
precisely the convex hull of those known as classical-quantum channels. The
complete positivity conditions in a canonical parameterization are reviewed and
used to formulate EBT constraints.


title: Ambiguities in the derivation of retrodictive probability

abstract:
  The derivation of the quantum retrodictive probability formula involves an
error, an ambiguity. The end result is correct because this error appears
twice, in such a way as to cancel itself. In addition, however, the usual
expression for the probability itself contains the same ambiguity; this may
lead to errors in its application. A generally applicable method is given to
avoid such ambiguities altogether.


title:
On Geometric Realization of Quantum Computations in Externally Driven
  4-Level System

abstract:
  The possibility of realization of quantum gates by means of the non-adiabatic
geometric phase is considered.
  It is shown that the non-adiabatic phase can be used for quantum gates
realization as well as the adiabatic one.


title:
Error Rate of the Kane Quantum Computer CNOT Gate in the Presence of
  Dephasing

abstract:
  We study the error rate of CNOT operations in the Kane solid state quantum
computer architecture. A spin Hamiltonian is used to describe the system.
Dephasing is included as exponential decay of the off diagonal elements of the
system's density matrix. Using available spin echo decay data, the CNOT error
rate is estimated at approsimately 10^{-3}.


title: Towards an Autonomous Formalism for Quantum Mechanics

abstract:
  A formalism is presented in which quantum particle dynamics can be developed
on its own rather than `quantization' of an underlying classical theory. It is
proposed that the unification of probability and dynamics should be considered
as the basic feature of quantum theory. Arguments are given to show that when
such a unification is attempted at the configuration space level, the wave
funtions of Schr$\ddot{o}$dinger theory appear as the natural candidates for
the desired unification. A *-algebra $\mathcal{A}_{Q}$ of (not necessarily
bounded) linear operators acting on an appropriate dense set of these wave
functions appears as the arena for quantum kinematics. A simple generalization
of an existing formalism in noncommutative geometry is employed to develop the
notion of generalized algebraic symplectic structure (GASS) which can
accomodate classical and quantum symplectic structures as special cases.
Quantum kinematics and dynamics is developed in in the framework of a
noncommutative Hamiltonian system employing an appropriate GASS based in $
\mathcal{A}_{Q}$. The Planck constant is introduced at only one place -- in the
quantum symplectic form; its appearance at conventional places is then
automatic. Unitary Wigner symmetries appear as canonical transformations in the
noncommutative Hamiltonian system. A straightforward treatment of quantum -
classical correspondence is given in terms of appropriate GASSes.


title:
Ballistic matter waves with angular momentum: Exact solutions and
  applications

abstract:
  An alternative description of quantum scattering processes rests on
inhomogeneous terms amended to the Schroedinger equation. We detail the
structure of sources that give rise to multipole scattering waves of definite
angular momentum, and introduce pointlike multipole sources as their limiting
case. Partial wave theory is recovered for freely propagating particles. We
obtain novel results for ballistic scattering in an external uniform force
field, where we provide analytical solutions for both the scattering waves and
the integrated particle flux. Our theory directly applies to p-wave
photodetachment in an electric field. Furthermore, illustrating the effects of
extended sources, we predict some properties of vortex-bearing atom laser beams
outcoupled from a rotating Bose-Einstein condensate under the influence of
gravity.


title: Casimir force in absorbing multilayers

abstract:
  The Casimir effect in a dispersive and absorbing multilayered system is
considered adopting the (net) vacuum-field pressure point of view to the
Casimir force. Using the properties of the macroscopic field operators
appropriate for absorbing systems and a convenient compact form of the Green
function for a multilayer, a straightforward and transparent derivation of the
Casimir force in a lossless layer of an otherwise absorbing multilayer is
presented. The resulting expression in terms of the reflection coefficients of
the surrounding stacks of layers is of the same form as that obtained by Zhou
and Spruch for a purely dispersive multilayer using the (surface) mode
summation method [Phys. Rev. A {\bf 52}, 297 (1995)]. Owing to the recursion
relations which the generalized Fresnel coefficients satisfy, this result can
be applied to more complex systems with planar symmetry. This is illustrated by
calculating the Casimir force on a dielectric (metallic) slab in a planar
cavity with realistic mirrors. Also, a relationship between the Casimir force
and energy in two different layers is established.


title:
Adiabatic limit interference effects for two energy level transition
  amplitudes and Nikitin - Umanskii formula studied by fundamental solution
  method

abstract:
  A method of fundamental solutions has been used to study adiabatic transition
amplitudes in two energy level systems for a class of Hamiltonians allowing
some simplifications of Stokes graphs corresponding to such transitions. It has
been shown that for simplest such cases the amplitudes take the Nikitin -
Umanskii form but for more complicated ones they are formed by a sum of terms
strictly related to a structure of Stokes graph corresponding to such cases.
This paper corrects our previous one [Phys. Rev. A, 63 052101 (2001)] and its
results are in a full agreement with the ones of Joye, Mileti and Pfister
[Phys. Rev. A, 44 4280 (1991)].


title: Deutsch-Jozsa algorithm for continuous variables

abstract:
  We present an idealized quantum continuous variable analog of the
Deutsch-Jozsa algorithm which can be implemented on a perfect continuous
variable quantum computer. Using the Fourier transformation and XOR gate
appropriate for continuous spectra we show that under ideal operation to
infinite precision that there is an infinite reduction in number of query calls
in this scheme.


title: The power of reduced quantum states

abstract:
  In a system of n quantum particles, we define a measure of the degree of
irreducible n-way correlation, by which we mean the correlation that cannot be
accounted for by looking at the states of (n-1) particles. In the case of
almost all pure states of three qubits we show that there is no such
correlation: almost every pure state of three qubits is completely determined
by its two-particle reduced density matrices.


title: Comment on Mermin's Recent Proof of the Theorem of Bell

abstract:
  Mermin states in a recent paper that his nontechnical version of Bell's
theorem stands and is not invalidated by time and setting dependent instrument
parameters as claimed in one of our previous papers. We identify a number of
misinterpretations (of our definitions) and mathematical inconsistencies in
Mermin's paper and show that Mermin's conclusions are therefore not valid: his
proof does not go forward if certain possible time dependencies are taken into
account.


title: Single Atom and Two Atom Ramsey Interferometry with Quantized Fields

abstract:
  Implications of field quantization on Ramsey interferometry are discussed and
general conditions for the occurrence of interference are obtained.
Interferences do not occur if the fields in two Ramsey zones have precise
number of photons. However in this case we show how two atom (like two photon)
interferometry can be used to discern a variety of interference effects as the
two independent Ramsey zones get entangled by the passage of first atom.
Generation of various entangled states like |0,2>+|2,0> are discussed and in
far off resonance case generation of entangled state of two coherent states is
discussed.


title:
Linear optics implementation of general two-photon projective
  measurement

abstract:
  We will present a method of implementation of general projective measurement
of two-photon polarization state with the use of linear optics elements only.
The scheme presented succeeds with a probability of at least 1/16. For some
specific measurements, (e.g. parity measurement) this probability reaches 1/4.


title:
Information Rates Achievable with Algebraic Codes on Quantum Discrete
  Memoryless Channels

abstract:
  The highest information rate at which quantum error-correction schemes work
reliably on a channel, which is called the quantum capacity, is proven to be
lower bounded by the limit of the quantity termed coherent information
maximized over the set of input density operators which are proportional to the
projections onto the code spaces of symplectic stabilizer codes. Quantum
channels to be considered are those subject to independent errors and modeled
as tensor products of copies of a completely positive linear map on a Hilbert
space of finite dimension, and the codes that are proven to have the desired
performance are symplectic stabilizer codes. On the depolarizing channel, this
work's bound is actually the highest possible rate at which symplectic
stabilizer codes work reliably.


title: Non-classical Gaussian states in noisy environments

abstract:
  In this article we review properties of Gaussian states and describe
operations on them. The interaction of the electromagnetic field with an
absorbing dielectric as a special type of environmental interaction will serve
as the basis for the understanding of decoherence and entanglement degradation
of Gaussian states of light propagating through fibers. The main part of the
article is devoted to the study of quantum teleportation in noisy environments.
Special emphasis is put onto the question of choosing the correct displacement
on the receiver's side.


title: On the efficiency of solid-state single photon sources

abstract:
  We discuss the efficiency of single photon sources based on a single
quasi-monochromatic emitter (such as a semiconductor quantum dot) inserted in a
pillar microcavity. We show that their efficiency, which is in principle
excellent thanks to the Purcell effect, can be drastically limited by extrinsic
cavity losses, such as those related to the scattering by the sidewalls
roughness. We present novel design rules for micropillars in view of this
application and show that for the well-mastered GaAs/AlAs system more than 70%
of the emission can be concentrated into the collimated emission beam
associated with the fundamental cavity mode.


title: Classical Correlations and Entanglement in Quantum Measurements

abstract:
  We analyze a quantum measurement where the apparatus is initially in a mixed
state. We show that the amount of information gained in a measurement is not
equal to the amount of entanglement between the system and the apparatus, but
is instead equal to the degree of classical correlations between the two. As a
consequence, we derive an uncertainty-like expression relating the information
gain in the measurement and the initial mixedness of the apparatus. Final
entanglement between the environment and the apparatus is also shown to be
relevant for the efficiency of the measurement.


title: Conditional preparation of maximal polarization entanglement

abstract:
  A simple experimental setup consisting of a spontaneous parametric
down-conversion source and passive linear optics is proposed for conditional
preparation of a maximally entangled polarization state of two photons.
Successful preparation is unambiguously heralded by coincident detection of
four auxiliary photons. The proposed scheme utilizes the down-conversion term
corresponding to the generation of three pairs of photons. We analyze imperfect
detection of the auxiliary photons and demonstrate that its deleterious effect
on the fidelity of the prepared state can be suppressed at the cost of
decreasing the efficiency of the scheme.


title: From Cbits to Qbits: Teaching computer scientists quantum mechanics

abstract:
  A strategy is suggested for teaching mathematically literate students, with
no background in physics, just enough quantum mechanics for them to understand
and develop algorithms in quantum computation and quantum information theory.
Although the article as a whole addresses teachers of physics, well versed in
quantum mechanics, the central pedagogical development is addressed directly to
computer scientists and mathematicians, with only occasional asides to their
teacher. Physicists uninterested in quantum pedagogy may be amused (or
irritated) by some of the views of standard quantum mechanics that arise
naturally from this unorthodox perspective.


title: Overhead and noise threshold of fault-tolerant quantum error correction

abstract:
  Fault tolerant quantum error correction (QEC) networks are studied by a
combination of numerical and approximate analytical treatments. The probability
of failure of the recovery operation is calculated for a variety of CSS codes,
including large block codes and concatenated codes. Recent insights into the
syndrome extraction process, which render the whole process more efficient and
more noise-tolerant, are incorporated. The average number of recoveries which
can be completed without failure is thus estimated as a function of various
parameters. The main parameters are the gate (gamma) and memory (epsilon)
failure rates, the physical scale-up of the computer size, and the time t_m
required for measurements and classical processing. The achievable computation
size is given as a surface in parameter space. This indicates the noise
threshold as well as other information. It is found that concatenated codes
based on the [[23,1,7]] Golay code give higher thresholds than those based on
the [[7,1,3]] Hamming code under most conditions. The threshold gate noise
gamma_0 is a function of epsilon/gamma and t_m; example values are
{epsilon/gamma, t_m, gamma_0} = {1, 1, 0.001}, {0.01, 1, 0.003}, {1, 100,
0.0001}, {0.01, 100, 0.002}, assuming zero cost for information transport. This
represents an order of magnitude increase in tolerated memory noise, compared
with previous calculations, which is made possible by recent insights into the
fault-tolerant QEC process.