title:
Purification of Single Qubits and reconstruction from Post-Measurement
  State

abstract:
  Purification of mixed states in Quantum Mechanics, by which we mean the
transformation into pure states, has been viewed as an {\it Operation} in the
sense of Kraus et al and explicit {\it Kraus Operators} \cite{kra1,kra2,kra3}
have been constructed for two seperate purification protocols. The first one,
initially due to Schrodinger \cite{sch} and subsequently elaborated by
Sudarshan et al \cite{sudar}, is based on the {\it preservation of
probabilities}. We have constructed a second protocol here based on {\it
optimization of fidelities}. Both purification protocols have been implemented
on a single qubit in an attempt to improve the fidelity of the purified post
measurement state of the qubit with the initial pure state. We have considered
both {\it complete} and {\it partial} measurements and have established bounds
and inequalities for various fidelities. We show that our purification protocol
leads to better state reconstruction, most explicitly so, when partial
measurements are made.


title: Comment on 'quantum dialogue'

abstract:
  In this comment, we want to emphasize that when the state of the two qubit is
$|0>_{B}|1>_{A}$ or $|1>_{A}|0>_{B}$, Bob's Bell basis measurement outcome can
not be $|\phi ^{\pm}>$. Also, when the product state is $|0> |0>$ or $|1> |1>$,
Bob's measurement outcome can not be $|\psi ^{\pm}>$.


title: Violation of Bell's inequality for continuous variables

abstract:
  We construct a wide class of bounded continuous variables observables that
lead to violations of Bell inequalities for the EPR state and give an intuitive
Wigner function explanation how to predetermine which operators won't ever
exceed the bounds given by local theories.


title: Relativistic quantum mechanics and the Bohmian interpretation

abstract:
  Conventional relativistic quantum mechanics, based on the Klein-Gordon
equation, does not possess a natural probabilistic interpretation in
configuration space. The Bohmian interpretation, in which probabilities play a
secondary role, provides a viable interpretation of relativistic quantum
mechanics. We formulate the Bohmian interpretation of many-particle wave
functions in a Lorentz-covariant way. In contrast with the nonrelativistic
case, the relativistic Bohmian interpretation may lead to measurable
predictions on particle positions even when the conventional interpretation
does not lead to such predictions.


title: MUBs, polytopes, and finite geometries

abstract:
  A complete set of N+1 mutually unbiased bases (MUBs) exists in Hilbert spaces
of dimension N = p^k, where p is a prime number. They mesh naturally with
finite affine planes of order N, that exist when N = p^k. The existence of MUBs
for other values of N is an open question, and the same is true for finite
affine planes. I explore the question whether the existence of complete sets of
MUBs is directly related to the existence of finite affine planes. Both
questions can be shown to be geometrical questions about a convex polytope, but
not in any obvious way the same question.


title: On SIC-POVMs and MUBs in Dimension 6

abstract:
  We provide a partial solution to the problem of constructing mutually
unbiased bases (MUBs) and symmetric informationally complete POVMs (SIC-POVMs)
in non-prime-power dimensions.
  An algebraic description of a SIC-POVM in dimension six is given. Furthermore
it is shown that several sets of three mutually unbiased bases in dimension six
are maximal, i.e., cannot be extended.


title: Synthesis of Quantum Logic Circuits

abstract:
  We discuss efficient quantum logic circuits which perform two tasks: (i)
implementing generic quantum computations and (ii) initializing quantum
registers. In contrast to conventional computing, the latter task is nontrivial
because the state-space of an n-qubit register is not finite and contains
exponential superpositions of classical bit strings. Our proposed circuits are
asymptotically optimal for respective tasks and improve published results by at
least a factor of two.
  The circuits for generic quantum computation constructed by our algorithms
are the most efficient known today in terms of the number of expensive gates
(quantum controlled-NOTs). They are based on an analogue of the Shannon
decomposition of Boolean functions and a new circuit block, quantum
multiplexor, that generalizes several known constructions. A theoretical lower
bound implies that our circuits cannot be improved by more than a factor of
two. We additionally show how to accommodate the severe architectural
limitation of using only nearest-neighbor gates that is representative of
current implementation technologies. This increases the number of gates by
almost an order of magnitude, but preserves the asymptotic optimality of gate
counts.


title: Time Ordering in Kicked Qubits

abstract:
  We examine time ordering effects in strongly, suddenly perturbed two-state
quantum systems (kicked qubits) by comparing results with time ordering to
results without time ordering. Simple analytic expressions are given for state
occupation amplitudes and probabilities for singly and multiply kicked qubits.
We investigate the limit of no time ordering, which can differ in different
representations.


title: The field inside a random distribution of parallel dipoles

abstract:
  We determine the probability distribution for the field inside a random
uniform distribution of electric or magnetic dipoles.
  For parallel dipoles, simulations and an analytical derivation show that
although the average contribution from any spherical shell around the probe
position vanishes, the Levy stable distribution of the field is symmetric
around a non-vanishing field amplitude.
  In addition we show how omission of contributions from a small volume around
the probe leads to a field distribution with a vanishing mean, which, in the
limit of vanishing excluded volume, converges to the shifted distribution.


title:
The average detection probability of the quantum dialogue protocol under
  the disturbance attack is 3/4

abstract:
  We prove explicitly that the detection probability of the disturbance attack
in the recently proposed quantum dialogue protocol is 3/4 in average. The
purpose is not only to reply a comment but also to provide a deeper
understanding of a kind of tampering in an unauthorized communication.


title: The Complexity of the Local Hamiltonian Problem

abstract:
  The k-local Hamiltonian problem is a natural complete problem for the
complexity class QMA, the quantum analog of NP. It is similar in spirit to
MAX-k-SAT, which is NP-complete for k<=2. It was known that the problem is
QMA-complete for any k <= 3. On the other hand 1-local Hamiltonian is in P, and
hence not believed to be QMA-complete. The complexity of the 2-local
Hamiltonian problem has long been outstanding. Here we settle the question and
show that it is QMA-complete. We provide two independent proofs; our first
proof uses only elementary linear algebra. Our second proof uses a powerful
technique for analyzing the sum of two Hamiltonians; this technique is based on
perturbation theory and we believe that it might prove useful elsewhere. Using
our techniques we also show that adiabatic computation with two-local
interactions on qubits is equivalent to standard quantum computation.


title:
Economically Improving Message-Unilaterally-Transmitted Quantum Secure
  Direct Communication to Realize Two-Way Communication

abstract:
  We present a subtle idea to economically improve
message-unilaterally-transmitted quantum secure direct communication (QSDC)
protocols to realize two-way secure direct communication.


title: Entanglement versus energy in quantum spin models

abstract:
  We study entanglement properties of all eigenstates of the Heisenberg XXX
model, and find that the entanglement and mixedness for a pair of
nearest-neighbor qubits are completely determined by the corresponding
eigenenergies. Specifically, the negativity of the eigenenergy implies pairwise
entanglement. From the relation between entanglement and eigenenergy, we obtain
finite-size behaviors of the entanglement. We also study entanglement and
mixedness versus energy in the quantum Heisenberg XY model.


title: Giant Lamb Shift in Photonic Crystals

abstract:
  We obtain a general result for the Lamb shift of excited states of
multi-level atoms in inhomogeneous electromagnetic structures and apply it to
study atomic hydrogen in inverse-opal photonic crystals. We find that the
photonic-crystal environment can lead to very large values of the Lamb shift,
as compared to the case of vacuum. We also predict that the position-dependent
Lamb shift should extend from a single level to a mini-band for an assemble of
atoms with random distribution in space, similar to the velocity-dependent
Doppler effect in atomic/molecular gases.


title:
Explicit Form of the Evolution Operator for the Four Atoms
  Tavis-Cummings Model

abstract:
  In this letter the explicit form of evolution operator for the four atoms
Tavis-Cummings model is given.


title:
General formalism of Hamiltonians for realizing a prescribed evolution
  of a qubit

abstract:
  We investigate the inverse problem concerning the evolution of a qubit
system, specifically we consider how one can establish the Hamiltonians that
account for the evolution of a qubit along a prescribed path in the projected
Hilbert space. For a given path, there are infinite Hamiltonians which can
realize the same evolution. A general form of the Hamiltonians is constructed
in which one may select the desired one for implementing a prescribed
evolution. This scheme can be generalized to higher dimensional systems.


title:
Geometric quantum gate for trapped ions based on optical dipole forces
  induced by Gaussian laser beams

abstract:
  We present an implementation of quantum logic gates via internal state
dependent displacements of ions in a linear Paul trap caused by optical dipole
forces. Based on a general quantum analysis of the system dynamics we consider
specific implementations with alkaline earth ions. For experimentally realistic
parameters gate infidelities as low as $10^{-4}$ can be obtained.


title: Conditional Density Matrix in the Context of Noncontextuality

abstract:
  Conditional density matrix represents a quantum state of subsystem in
different schemes of quantum communication. Here we discuss some properties of
conditional density matrix and its place in general scheme of quantum
mechanics.


title:
Towards measuring variations of Casimir energy by a superconducting
  cavity

abstract:
  We consider a Casimir cavity, one plate of which is a thin superconducting
film. We show that when the cavity is cooled below the critical temperature for
the onset of superconductivity, the sharp variation (in the far infrared) of
the reflection coefficient of the film engenders a variation in the value of
the Casimir energy. Even though the relative variation in the Casimir energy is
very small, its magnitude can be comparable to the condensation energy of the
superconducting film, and this gives rise to a number of testable effects,
including a significant increase in the value of the critical magnetic field,
required to destroy the superconductivity of the film. The theoretical ground
is therefore prepared for the first experiment ever aimed at measuring
variations of the Casimir energy itself.


title: On the existence of Einstein-Podolsky-Rosen Channels

abstract:
  A physical theory without interpretation is mathematics. Since there are no
paradoxes in science, only incorrect interpretations of phenomena or inadequate
theories, it is necessary to use a consistent interpretation of quantum
mechanics that makes physical sense and satisfies the experimental facts.
Quantum "teleportation" provides such an example because the current treatments
rely on unexplained connections between separated correlated particles. In this
comment, a mechanism is suggested that avoids the instantaneous wave function
collapse between non-interacting entangled particles at space like separations.
This mechanism requires a statistical ensemble interpretation of the wave
function.


title: Phase-Modulus Relations for a Reflected Particle

abstract:
  We formulate analytically the reflection of a one dimensional, expanding free
wave-packet (wp) from an infinite barrier. Three types of wp's are considered,
representing an electron, a molecule and a classical object. We derive a
threshold criterion for the values of the dynamic parameters so that reciprocal
(Kramers-Kronig) relations hold {\it in the time domain} between the
log-modulus of the wp and the (analytic part of its) phase acquired during the
reflection. For an electron, in a typical case, the relations are shown to be
satisfied. For a molecule the modulus-phase relations take a more complicated
form, including the so called Blaschke term. For a classical particle
characterized by a large mean momentum ($\hbar K >> \frac{\hbar trajectory
length} {(size of wave-packet)^2} >>> \frac{\hbar}{size of wave-packet}$) the
rate of acquisition of the relative phase between different wp components is
enormous (for a bullet it is typically $10^{14}$ GHertz) with also a very large
value for the phase maximum.


title: Quantum Zeno effect by indirect measurement: The effect of the detector

abstract:
  We study the quantum Zeno effect in the case of indirect measurement, where
the detector does not interact directly with the unstable system. Expanding on
the model of Koshino and Shimizu [Phys. Rev. Lett., 92, 030401, (2004)] we
consider a realistic Hamiltonian for the detector with a finite bandwidth. We
also take explicitly into account the position, the dimensions and the
uncertainty in the measurement of the detector. Our results show that the
quantum Zeno effect is not expected to occur, except for the unphysical case
where the detector and the unstable system overlap.


title: Quantum Computing using Linear Optics

abstract:
  Quantum computers are expected to be able to solve mathematical problems that
cannot be solved using conventional computers. Many of these problems are of
practical importance, especially in the areas of cryptography and secure
communications. APL is developing an optical approach to quantum computing in
which the bits, or "qubits", are represented by single photons. Our approach
allows the use of ordinary (linear) optical elements that are available for the
most part as off-the-shelf components. Recent experimental demonstrations of a
variety of logic gates for single photons, a prototype memory device, and other
devices will be described.


title: Quantum Chaos: Reduced Density Matrix Fluctuations in Coupled Systems

abstract:
  Following a recent work (briefly reviewed below) we consider temporal
fluctuations in the reduced density matrix elements for a coupled system
involving a pair of kicked rotors as also one made up of a pair of Harper
Hamiltonians. These dynamical fluctuations are found to constitute a reliable
indicator of the degree of chaos in the quantum dynamics, and are related to
stationary features like the eigenvalue and eigenvector distributions of the
system under consideration. A brief comparison is made with the evolution of
the reduced distribution function in the classical phase space.


title: Signed Phases and Fields Associated with Degeneracies

abstract:
  In the first part, expressions are given for the {\it sign} of the
topological angle that is acquired upon making a loop around a degeneracy
("conical intersection") point of two molecular energy surfaces. The
expressions involve the partial derivatives (with respect to the nuclear
coordinates) of the matrix elements of the coupling Hamiltonian. Examples are
given of a few studied cases, such as of excited states that have topological
angles with a sign opposite to those in the ground states. In the second part,
the two dimensional (or two parameter) situation that characterizes a conical
intersection (ci) between potential surfaces in a polyatomic molecule is
constructed as a limiting case of the three dimensional Dirac-monopole
situation. For an electron occupying a twofold state, we obtain both the
"magnetic-field" (or curl-field) and the tensorial (or Yang-Mills-) field
(which is the sum of a curl and of a vector- product term). These pseudo-
fields represent the reaction of the electron on the nuclear motion via the
nonadiabatic coupling terms (NACTs). We find that both fields are aligned with
the orthogonal, (so called) seam directions of the ci and are zero everywhere
outside the seam, but they differ as regards the flux that they produce. In a
two-state situation, the fields are representation dependent and the values of,
e.g., the fluxes depend on the state that the electron occupies. The angular
dependence of the NACTs and the fields calculated from a general linearly
coupled model agrees with recently computed results for $C_2 H$ [A.M. Mebel, M.
Baer and S.H. Lin, J.Chem. Phys. {\bf 115} 3673 (2001)]. An
effective-Hamiltonian formalism is proposed for experimentally observing and
distinguishing between the different fields.


title: Phase Evolution in a Multi-Component System

abstract:
  We derive a general expression for the expectation value of the phase
acquired by a time dependent wave function in a multi component system, as
excursions are made in its coordinate space. We then obtain the mean phase for
the (linear dynamic $E \otimes \epsilon$) Jahn-Teller situation in an
electronically degenerate system. We interpret the phase-change as an
observable measure of the {\it effective} nodal structure of the wave function.


title: Improved Simulation of Stabilizer Circuits

abstract:
  The Gottesman-Knill theorem says that a stabilizer circuit -- that is, a
quantum circuit consisting solely of CNOT, Hadamard, and phase gates -- can be
simulated efficiently on a classical computer. This paper improves that theorem
in several directions. First, by removing the need for Gaussian elimination, we
make the simulation algorithm much faster at the cost of a factor-2 increase in
the number of bits needed to represent a state. We have implemented the
improved algorithm in a freely-available program called CHP
(CNOT-Hadamard-Phase), which can handle thousands of qubits easily. Second, we
show that the problem of simulating stabilizer circuits is complete for the
classical complexity class ParityL, which means that stabilizer circuits are
probably not even universal for classical computation. Third, we give efficient
algorithms for computing the inner product between two stabilizer states,
putting any n-qubit stabilizer circuit into a "canonical form" that requires at
most O(n^2/log n) gates, and other useful tasks. Fourth, we extend our
simulation algorithm to circuits acting on mixed states, circuits containing a
limited number of non-stabilizer gates, and circuits acting on general
tensor-product initial states but containing only a limited number of
measurements.


title: Adiabatic and non-adiabatic merging of independent Bose condensates

abstract:
  Motivated by a recent experiment [Chikkatur, et al. Science, 296, 2193
(2002)] on the merging of atomic condensates, we investigate how two
independent condensates with random initial phases can develop a unique
relative phase when we move them together. In the adiabatic limit, the uniting
of independent condensates can be understood from the eigenstate evolution of
the governing Hamiltonian, which maps degenerate states (corresponding to
fragmented condensates) to a single state (corresponding to a united
condensate) . In the non-adiabatic limit corresponding to the practical
experimental configurations, we give an explanation on why we can still get a
large condensate fraction with a unique relative phase. Detailed numerical
simulations are then performed for the non-adiabatic merging of the
condensates, which confirm our explanation and qualitative estimation. The
results may have interesting implications for realizing a continuous atom laser
based on merging of condensates.


title: Fault-Tolerant Quantum Computation via Exchange interactions

abstract:
  Quantum computation can be performed by encoding logical qubits into the
states of two or more physical qubits, and controlling a single effective
exchange interaction and possibly a global magnetic field. This "encoded
universality" paradigm offers potential simplifications in quantum computer
design since it does away with the need to perform single-qubit rotations. Here
we show that encoded universality schemes can be combined with quantum error
correction. In particular, we show explicitly how to perform fault-tolerant
leakage correction, thus overcoming the main obstacle to fault-tolerant encoded
universality.


title: Nonlocality and the Correlation of Measurement Bases

abstract:
  Nonlocal nature apparently shown in entanglement is one of the most striking
features of quantum theory. We examine the locality assumption in Bell-type
proofs for entangled qubits, i.e. the outcome of a qubit at one end is
independent of the basis choice at the other end. It has recently been claimed
that in order to properly incorporate the phenomenon of self-observation, the
Heisenberg picture with time going backwards provides a consistent description.
We show that, if this claim holds true, the assumption in nonlocality proofs
that basis choices at two ends are independent of each other may no longer be
true, and may pose a threat to the validity of Bell-type proofs.


title: Symmetric Triple Well with Non-Equivalent Vacua: Instantonic Approach

abstract:
  We show that for the triple well potential with non-equivalent vacua,
instantons generate for the low lying energy states a singlet and a doublet of
states rather than a triplet of equal energy spacing. Our energy splitting
formulae are also confirmed numerically. This splitting property is due to the
presence of non-equivalent vacua. A comment on its generality to multi-well is
presented.


title: A Quantum Theoretical Formulation of Proper Time for Spin 1/2 Particles

abstract:
  We find a quantum mechanical formulation of proper time for spin 1/2
particles within the framework of the Dirac theory. It is shown that an
operator corresponds to the rate of the proper time and that the operator
contains terms which oscillate with a very high frequency. We deduce, as an
effect of these terms, the existence of an interference between the magnetic
field and the rate of proper time. There is a possibility that the conclusion
derived in this letter has some implications for astrophysics.


title: Ground state entanglement and geometric entropy in the Kitaev's model

abstract:
  We study the entanglement properties of the ground state in Kitaev's model.
This is a two-dimensional spin system with a torus topology and nontrivial
four-body interactions between its spins. For a generic partition $(A,B)$ of
the lattice we calculate analytically the von Neumann entropy of the reduced
density matrix $\rho_A$ in the ground state. We prove that the geometric
entropy associated with a region $A$ is linear in the length of its boundary.
Moreover, we argue that entanglement can probe the topology of the system and
reveal topological order. Finally, no partition has zero entanglement and we
find the partition that maximizes the entanglement in the given ground state.


title: Fisher, Kaehler, Weyl and the quantum potential

abstract:
  Connections between Fisher information, Kaehler geometry of a quantum
projective Hilbert space, and the Weyl-Ricci scalar curvature of a Riemannian
flat spacetime with quantum matter are sketched.


title: Double-EIT ground-state laser cooling without blue-sideband heating

abstract:
  We discuss a laser cooling scheme for trapped atoms or ions which is based on
double electromagnetically induced transparency (EIT) and makes use of a
four-level atom in tripod configuration. The additional fourth atomic state is
coupled by a strong coupling laser field to the usual three-level setup of
single-EIT cooling. This effectively allows to create two EIT structures in the
absorption spectrum of the system to be cooled, which may be controlled by the
coupling laser field parameters to cancel both the carrier- and the
blue-sideband excitations. In leading order of the Lamb-Dicke expansion, this
suppresses all heating processes. As a consequence, the double-EIT scheme can
be used to lower the cooling limit by almost two powers of the Lamb-Dicke
parameter as compared to single-EIT cooling.


title: Almost Certain Escape from Black Holes

abstract:
  This paper examines how black holes might compute in light of recent models
of the black-hole final state. These models suggest that quantum information
can escape from the black hole by a process akin to teleportation. They require
a specific final state and restrictions on the interaction between the
collapsing matter and the incoming Hawking radiation for quantum information to
escape. This paper shows that for an arbitrary final state and for generic
interactions between matter and Hawking radiation, the quantum information
about how the hole was formed and the results of any computation performed by
the matter inside the hole escapes with fidelity exponentially close to 1.


title: Recursive Calculation of Effective Potential and Variational Resummation

abstract:
  We set up a method for a recursive calculation of the effective potential
which is applied to a cubic potential with imaginary coupling. The result is
resummed using variational perturbation theory (VPT), yielding an exponentially
fast convergence.


title: Quantum Searching via Entanglement and Partial Diffusion

abstract:
  In this paper, we will define a quantum operator that performs the inversion
about the mean only on a subspace of the system (Partial Diffusion Operator).
This operator is used in a quantum search algorithm that runs in O(sqrt{N/M})
for searching an unstructured list of size N with M matches such that 1<= M<=N.
We will show that the performance of the algorithm is more reliable than known
{fixed operators quantum search algorithms} especially for multiple matches
where we can get a solution after a single iteration with probability over 90%
if the number of matches is approximately more than one-third of the search
space. We will show that the algorithm will be able to handle the case where
the number of matches M is unknown in advance such that 1<=M<=N in
O(sqrt{N/M}). A performance comparison with Grover's algorithm will be
provided.


title:
Nuclear Magnetic Resonance Implementation of a Quantum Clock
  Synchronization Algorithm

abstract:
  The quantum clock synchronization algorithm proposed by I. L. Chuang (Phys.
Rev. Lett, 85, 2006(2000)) has been implemented in a three qubit nuclear
magnetic resonance quantum system. The effective-pure state is prepared by the
spatial averaging approach. The time difference between two separated clocks
can be determined by reading out directly through the NMR spectra.


title:
Modularization of multi-qubit controlled phase gate and its NMR
  implementation

abstract:
  Quantum circuit network is a set of circuits that implements a certain
computation task. Being at the center of the quantum circuit network, the
multi-qubit controlled phase shift is one of the most important quantum gates.
In this paper, we apply the method of modular structuring in classical computer
architecture to quantum computer and give a recursive realization of the
multi-qubit phase gate. This realization of the controlled phase shift gate is
convenient in realizing certain quantum algorithms. We have experimentally
implemented this modularized multi-qubit controlled phase gate in a three qubit
nuclear magnetic resonance quantum system. The network is demonstrated
experimentally using line selective pulses in nuclear magnetic resonance
technique. The procedure has the advantage of being simple and easy to
implement.


title: Computational Methods for Simulating Quantum Computers

abstract:
  This review gives a survey of numerical algorithms and software to simulate
quantum computers.It covers the basic concepts of quantum computation and
quantum algorithms and includes a few examples that illustrate the use of
simulation software for ideal and physical models of quantum computers.


title: Numerical simulations of mixed states quantum computation

abstract:
  We describe quantum-octave package of functions useful for simulations of
quantum algorithms and protocols. Presented package allows to perform
simulations with mixed states. We present numerical implementation of important
quantum mechanical operations - partial trace and partial transpose. Those
operations are used as building blocks of algorithms for analysis of
entanglement and quantum error correction codes. Simulation of Shor's algorithm
is presented as an example of package capabilities.


title: On Cyclic Harmonic Oscillators

abstract:
  It is proven that the energy of a quantum mechanical harmonic oscillator with
a generically time-dependent but cyclic frequency, $\omega_{0}(t_{0})=
\omega_{0}(0)$, cannot decrease on the average if the system is originally in a
stationary state, after the system goes through a full cycle. The energy
exchange always takes place in the direction from the macroscopic system
(environment) to the quantum microscopic system.


title: A Pathwise Ergodic Theorem for Quantum Trajectories

abstract:
  If the time evolution of an open quantum system approaches equilibrium in the
time mean, then on any single trajectory of any of its unravelings the time
averaged state approaches the same equilibrium state with probability 1. In the
case of multiple equilibrium states the quantum trajectory converges in the
mean to a random choice from these states.


title: Quantum Algorithm for SAT Problem and Quantum Mutual Entropy

abstract:
  It is von Neumann who opened the window for today's Information epoch. He
defined quantum entropy including Shannon's information more than 20 years
ahead of Shannon, and he introduced a concept what computation means
mathematically. In this paper I will report two works that we have recently
done, one of which is on quantum algorithum in generalized sense solving the
SAT problem (one of NP complete problems) and another is on quantum mutual
entropy properly describing quantum communication processes.


title: On Quantum Capacity and its Bound

abstract:
  The quantum capacity of a pure quantum channel and that of
classical-quantum-classical channel are discussed in detail based on the fully
quantum mechanical mutual entropy. It is proved that the quantum capacity
generalizes the so-called Holevo bound.


title: New quantum algorithm for studying NP-complete problems

abstract:
  Ordinary approach to quantum algorithm is based on quantum Turing machine or
quantum circuits. It is known that this approach is not powerful enough to
solve NP-complete problems. In this paper we study a new approach to quantum
algorithm which is a combination of the ordinary quantum algorithm with a
chaotic dynamical system. We consider the satisfiability problem as an example
of NP-complete problems and argue that the problem, in principle, can be solved
in polynomial time by using our new quantum algorithm.


title: Reciprocity between Moduli and Phases in Time-Dependent Wave-Functions

abstract:
  For time (t) dependent wave functions we derive rigorous conjugate relations
between analytic decompositions (in the complex t-plane) of the phases and of
the log moduli. We then show that reciprocity, taking the form of
Kramers-Kronig integral relations (but in the time domain), holds between
observable phases and moduli in several physically important instances. These
include the nearly adiabatic (slowly varying) case, a class of cyclic
wave-functions, wave packets and non-cyclic states in an "expanding potential".
The results exhibit the interdependence of geometric-phases and related decay
probabilities. Several known quantum mechanical theories possess the
reciprocity property obtained in the paper.


title: Phase-Modulus Relations in Cyclic Wave Functions

abstract:
  We derive reciprocal integral relations between phases and amplitude moduli
for a class of wave functions that are cyclically varying in time. The
relations imply that changes of a certain kind (e.g. not arising from the
dynamic phase) obligate changes in the other. Numerical results indicate the
approximate validity of the relationships for arbitrarily (non-cyclically)
varying states in the adiabatic (slowly changing) limit.


title:
Communicating continuous quantum variables between different Lorentz
  frames

abstract:
  We show how to communicate Heisenberg-limited continuous (quantum) variables
between Alice and Bob in the case where they occupy two inertial reference
frames that differ by an unknown Lorentz boost. There are two effects that need
to be overcome: the Doppler shift and the absence of synchronized clocks.
Furthermore, we show how Alice and Bob can share Doppler-invariant
entanglement, and we demonstrate that the protocol is robust under photon loss.


title: On superselection rules for macroscopic objects

abstract:
  It is shown that for "ideal" macroscopic objects there are superselection
rules forbidding superpositions of macroscopically distinguishable states of
the objects. For real macroscopic bodies the notion of "weak" superselection
rules is introduced. Some other aspects of the measurement problem are
discussed.


title:
Atoms near magnetodielectric bodies: van-der-Waals energy and
  Casimir-Polder force

abstract:
  Based on macroscopic QED in linear, causal media, we present a consistent
theory for the Casimir-Polder force acting on an atom positioned near
dispersing and absorbing magnetodielectric bodies. The perturbative result for
the van-der-Waals energy is shown to exhibit interesting new features in the
presence of magnetodielectric bodies. To go beyond perturbation theory, we
start with the center-of-mass equation of motion and derive a dynamical
expression for the Casimir-Polder force acting on an atom prepared in an
arbitrary electronic state. For a non-driven atom in the weak coupling regime,
the force as a function of time is shown to be a superposition of force
components that are related to the electronic density matrix elements at chosen
time. These force components depend on the position-dependent polarizability of
the atom that correctly accounts for the body-induced level shifts and
broadenings.


title:
Canonical and kinetic forms of the electromagnetic momentum in an ad hoc
  quantization scheme for a dispersive dielectric

abstract:
  An ad hoc quantization scheme for the electromagnetic field in a weakly
dispersive, transparent dielectric leads to the definition of canonical and
kinetic forms for the momentum of the electromagnetic field in a dispersive
medium. The canonical momentum is uniquely defined as the operator that
generates spatial translations in a uniform medium, but the quantization scheme
suggests two possible choices for the kinetic momentum operator, corresponding
to the Abraham or the Minkowski momentum in classical electrodynamics. Another
implication of this procedure is that a wave packet containing a single dressed
photon travels at the group velocity through the medium. The physical
significance of the canonical momentum has already been established by
considerations of energy and momentum conservation in the atomic recoil due to
spontaneous emission, the Cerenkov effect, the Doppler effect, and phase
matching in nonlinear optical processes. In addition, the data of the Jones and
Leslie radiation pressure experiment is consistent with the assignment of one
?k unit of canonical momentum to each dressed photon. By contrast, experiments
in which the dielectric is rigidly accelerated by unbalanced electromagnetic
forces require the use of the Abraham momentum.


title: Locality and information transfer in quantum operations

abstract:
  We investigate the situation in which no information can be transferred from
a quantum system B to a quantum system A, even though both interact with a
common system C.


title: Quantum decoherence in a supersymmetric mechanical system

abstract:
  We make a novel observation about the decoherence phenomenon of the fermion
in the Witten's supersymmetric (SUSY) quantum mechanical model. It is shown
that, when the bosonic partner in the SUNY model is unobservable in a certain
energy scale, the quantum coherence meant by the superposition of fermion
states can not be preserved for a long time due to the quantum decoherence
induced by the overlooked boson. This refers to a supercharge superselection
rule similar to the charge superselection. We numerically calculate the
decoherence factor characterizing the extent of decoherence . The obtained
result shows the periodic decoherence with finite quantum jump. Some analytic
results under the harmonic approximation for the superpotential can be obtained
to confirm the numerical calculations.


title: Complexity in Quantum System and Its Application to Brain Function

abstract:
  The complexity and the chaos degree can be used to examine the chaotic
aspects of not only several nonlinear classical and quantum physical physics
but also life sciences. We will construct a model describing the function of
brain in the context of Quantum Information Dynamics.


title: Recognition and Teleportation

abstract:
  We study a possible function of brain, in particular, we try to describe
several aspects of the process of recognition. In order to understand the
fundamental parts of the recognition process, the quantum teleportation scheme
seems to be useful. We consider a channel expression of the teleportation
process that serves for a simplified description of the recognition process in
brain.


title: A Description of Quantum Chaos

abstract:
  A measure describing the chaos of a dynamics was introduced by two
complexities in information dynamics, and it is called the chaos degree. In
particular, the entropic chaos degree has been used to characterized several
dynamical maps such that logistis, Baker's, Tinckerbel's in classical or
quantum systems. In this paper, we give a new treatment of quantum chaos by
defining the entropic chaos degree for quantum transition dynamics, and we
prove that every non-chaotic quantum dynamics, e.g., dissipative dynamics, has
zero chaos degree. A quantum spin 1/2 system is studied by our chaos degree,
and it is shown that this degree well describes the chaotic behavior of the
spin system.


title: Mutual Entropy in Quantum Information and Information Genetics

abstract:
  After Shannon, entropy becomes a fundamental quantity to describe not only
uncertainity or chaos of a system but also information carried by the system.
Shannon's important discovery is to give a mathematical expression of the
mutual entropy (information), information transmitted from an input system to
an output system, by which communication processes could be analyzed on the
stage of mathematical science. In this paper, first we review the quantum
mutual entropy and discuss its uses in quantum information theory, and secondly
we show how the classical mutual entropy can be used to analyze genomes, in
particular, those of HIV.


title: Information Dynamics and its Application to Recognition Process

abstract:
  We touch the problem of finding simplified models for the recognition
process. We are interested in how the input signal arriving at the brain is
entangled (connected) to the memory already stored and the consciousness that
existed in the brain, and how a part of the signal will be finally stored as a
memory.


title:
The 'quantum dialogue' can be eavesdropped under the
  intercept-and-resend attack

abstract:
  In this comment we point out that the 'quantum dialogue' (Phys. Lett. A (in
press); quant-ph/0406130) can be eavesdropped under the intercept-and-resend
attack. We also give a revised control mode to detect this attack.


title:
Nonlinear interaction of light with Bose-Einstein condensate: new
  methods to generate subpoissonian light

abstract:
  We consider $\Lambda$-type model of the Bose-Einstein condensate of sodium
atoms interacting with the light. Coefficients of the Kerr-nonlinearity in the
condensate can achieve large and negative values providing the possibility for
effective control of group velocity and dispersion of the probe pulse. We find
a regime when the observation of the "slow" and "fast" light propagating
without absorption becomes achievable due to strong nonlinearity. An effective
two-level quantum model of the system is derived and studied based on the su(2)
polynomial deformation approach. We propose an efficient way for generation of
subpoissonian fields in the Bose-Einstein condensate at time-scales much
shorter than the characteristic decay time in the system. We show that the
quantum properties of the probe pulse can be controlled in BEC by the classical
coupling field.


title: Highly Nonclassical Quantum States and Environment Induced Decoherence

abstract:
  In this thesis concrete quantum systems are investigated in the framework of
the environment induced decoherence. The focus is on the dynamics of highly
nonclassical quantum states, the Wigner function of which are negative over
some regions of their domains. One of the chosen physical systems is a diatomic
molecule, where the potential energy of the nuclei is an anharmonic function of
their distance. A system of two-level atoms, which can be important from the
viewpoint of quantum information technology, is also investigated. A method is
described that is valid in both systems and can determine the characteristic
time of the decoherence in a dynamical way. The direction of the decoherence
and its relation to energy dissipation is also studied. Finally, a scheme is
proposed that can prepare decoherence-free states using the experimental
techniques presently available.


title: A Path Integral Approach for Disordered Quantum Walks in One Dimension

abstract:
  The present letter gives a rigorous way from quantum to classical random
walks by introducing an independent random fluctuation and then taking
expectations based on a path integral approach.


title: Distillation of local purity from quantum states

abstract:
  Recently Horodecki et al. [Phys. Rev. Lett. 90, 100402 (2003)] introduced an
important quantum information processing paradigm, in which two parties sharing
many copies of the same bipartite quantum state distill local pure states, by
means of local unitary operations assisted by a one-way (two-way) completely
dephasing channel. Local pure states are a valuable resource from a
thermodynamical point of view, since they allow thermal energy to be converted
into work by local quantum heat engines. We give a simple
information-theoretical characterization of the one-way distillable local
purity, which turns out to be closely related to a previously known operational
measure of classical correlations, the one-way distillable common randomness.


title: Ergodic quantum computing

abstract:
  We propose a (theoretical ;-) model for quantum computation where the result
can be read out from the time average of the Hamiltonian dynamics of a
2-dimensional crystal on a cylinder. The Hamiltonian is a spatially local
interaction among Wigner-Seitz cells containing 6 qubits. The quantum circuit
that is simulated is specified by the initialization of program qubits. As in
Margolus' Hamiltonian cellular automaton (implementing classical circuits), a
propagating wave in a clock register controls asynchronously the application of
the gates. However, in our approach all required initializations are basis
states. After a while the synchronizing wave is essentially spread around the
whole crystal. The circuit is designed such that the result is available with
probability about 1/4 despite of the completely undefined computation step.
This model reduces quantum computing to preparing basis states for some qubits,
waiting, and measuring in the computational basis. Even though it may be
unlikely to find our specific Hamiltonian in real solids, it is possible that
also more natural interactions allow ergodic quantum computing.


title: Noise models for superoperators in the chord representation

abstract:
  We study many-qubit generalizations of quantum noise channels that can be
written as an incoherent sum of translations in phase space. Physical
description in terms of the spectral properties of the superoperator and the
action in phase space are provided. A very natural description of decoherence
leading to a preferred basis is achieved with diffusion along a phase space
line. The numerical advantages of using the chord representation are
illustrated in the case of coarse-graining noise.


title: Extremal covariant POVM's

abstract:
  We consider the convex set of positive operator valued measures (POVM) which
are covariant under a finite dimensional unitary projective representation of a
group. We derive a general characterization for the extremal points, and
provide bounds for the ranks of the corresponding POVM densities, also relating
extremality to uniqueness and stability of optimized measurements. Examples of
applications are given.


title: Probabilistic aspects of Wigner function

abstract:
  The Wigner function of quantum systems is an effective instrument to
construct the approximate classical description of the systems for which the
classical approximation is possible. During the last time, the Wigner function
formalism is applied as well to seek indications of specific quantum properties
of quantum systems leading to impossibility of the classical approximation
construction. Most of all, as such an indication the existence of negative
values in Wigner function for specific states of the quantum system being
studied is used. The existence of such values itself prejudices the
probabilistic interpretation of the Wigner function, though for an arbitrary
observable depending jointly on the coordinate and the momentum of the quantum
system just the Wigner function gives an effective instrument to calculate the
average value and the other statistical characteristics.
  In this paper probabilistic interpretation of the Wigner function based on
coordination of theoretical-probabilistic definition of the probability
density, with restrictions to a physically small domain of phase space due to
the uncertainty principle, is proposed.


title: Quantum Process Tomography of the Quantum Fourier Transform

abstract:
  The results of quantum process tomography on a three-qubit nuclear magnetic
resonance quantum information processor are presented, and shown to be
consistent with a detailed model of the system-plus-apparatus used for the
experiments. The quantum operation studied was the quantum Fourier transform,
which is important in several quantum algorithms and poses a rigorous test for
the precision of our recently-developed strongly modulating control fields. The
results were analyzed in an attempt to decompose the implementation errors into
coherent (overall systematic), incoherent (microscopically deterministic), and
decoherent (microscopically random) components. This analysis yielded a
superoperator consisting of a unitary part that was strongly correlated with
the theoretically expected unitary superoperator of the quantum Fourier
transform, an overall attenuation consistent with decoherence, and a residual
portion that was not completely positive - although complete positivity is
required for any quantum operation. By comparison with the results of computer
simulations, the lack of complete positivity was shown to be largely a
consequence of the incoherent errors during the quantum process tomography
procedure. These simulations further showed that coherent, incoherent, and
decoherent errors can often be identified by their distinctive effects on the
spectrum of the overall superoperator. The gate fidelity of the experimentally
determined superoperator was 0.64, while the correlation coefficient between
experimentally determined superoperator and the simulated superoperator was
0.79; most of the discrepancies with the simulations could be explained by the
cummulative effect of small errors in the single qubit gates.


title: Comparability of multipartite entanglement

abstract:
  We prove, in a multipartite setting, that it's always feasible to exactly
transform a genuinely $m$-partite entangled state with sufficient many copies
to any other $m$-partite state via local quantum operation and classical
communication. This result affirms the comparability of multipartite pure
entangled states.


title: Frequency and temporal effects in linear optical quantum computing

abstract:
  Typically linear optical quantum computing (LOQC) models assume that all
input photons are completely indistinguishable. In practice there will
inevitably be non-idealities associated with the photons and the experimental
setup which will introduce a degree of distinguishability between photons. We
consider a non-deterministic optical controlled-NOT gate, a fundamental LOQC
gate, and examine the effect of temporal and spectral distinguishability on its
operation. We also consider the effect of utilizing non-ideal photon counters,
which have finite bandwidth and time response.


title: Generalized Supersymmetric Perturbation Theory

abstract:
  Using the basic ingredient of supersymmetry, we develop a simple alternative
approach to perturbation theory in one-dimensional non-relativistic quantum
mechanics. The formulae for the energy shifts and wave functions do not involve
tedious calculations which appear in the available perturbation theories. The
model applicable in the same form to both the ground state and excited bound
states, unlike the recently introduced supersymmetric perturbation technique
which, together with other approaches based on logarithmic perturbation theory,
are involved within the more general framework of the present formalism.


title: Exact treatment of $\ell \neq 0$ states

abstract:
  Using the basic ingredient of supersymmetry, a general procedure for the
treatment of quantum states having nonzero angular momenta is presented.


title:
A Process Algebraic Approach to Concurrent and Distributed Quantum
  Computation: Operational Semantics

abstract:
  Full formal descriptions of algorithms making use of quantum principles must
take into account both quantum and classical computing components and assemble
them so that they communicate and cooperate. Moreover, to model concurrent and
distributed quantum computations, as well as quantum communication protocols,
quantum to quantum communications which move qubits physically from one place
to another must also be taken into account. Inspired by classical process
algebras, which provide a framework for modeling cooperating computations, a
process algebraic notation is defined, named QPAlg for Quantum Process Algebra,
which provides a homogeneous style to formal descriptions of concurrent and
distributed computations comprising both quantum and classical parts. On the
quantum side, QPAlg provides quantum variables, operations on quantum variables
(unitary operators and measurement observables), as well as new forms of
communications involving the quantum world. The operational semantics makes
sure that these quantum objects, operations and communications operate
according to the postulates of quantum mechanics.


title: Polarization squeezing in vertical-cavity surface-emitting lasers

abstract:
  We further elaborate the theory of quantum fluctuations in vertical-cavity
surface-emitting lasers (VCSELs), developed in Ref. \cite{Hermier02}. In
particular, we introduce the quantum Stokes parameters to describe the quantum
self- and cross-correlations between two polarization components of the
electromagnetic field generated by this type of lasers. We calculate
analytically the fluctuation spectra of these parameters and discuss
experiments in which they can be measured. We demonstrate that in certain
situations VCSELs can exhibit polarization squeezing over some range of
spectral frequencies. This polarization squeezing has its origin in
sub-Poissonian pumping statistics of the active laser medium.


title:
Teleportation with insurance of an entangled atomic state via cavity
  decay

abstract:
  We propose a scheme to teleport an entangled state of two $\Lambda$-type
three-level atoms via photons. The teleportation protocol involves the local
redundant encoding protecting the initial entangled state and allowing for
repeating the detection until quantum information transfer is successful. We
also show how to manipulate a state of many $\Lambda$-type atoms trapped in a
cavity.


title: Classically-Controlled Quantum Computation

abstract:
  Quantum computations usually take place under the control of the classical
world. We introduce a Classically-controlled Quantum Turing Machine (CQTM)
which is a Turing Machine (TM) with a quantum tape for acting on quantum data,
and a classical transition function for a formalized classical control. In
CQTM, unitary transformations and measurements are allowed. We show that any
classical TM is simulated by a CQTM without loss of efficiency. The gap between
classical and quantum computations, already pointed out in the framework of
measurement-based quantum computation is confirmed. To appreciate the
similarity of programming classical TM and CQTM, examples are given.


title: The Continuous Variable Quantum Teleportation Controversy

abstract:
  I argue that the objections by Rudolph and Sanders \cite{RS} to performing
continuous variable quantum teleportation experiments using lasers, as well as
the various rebuttals to their paper, are based on a misunderstanding of the
Partition Ensemble Fallacy.


title: Transformation of quantum states using uniformly controlled rotations

abstract:
  We consider a unitary transformation which maps any given state of an
$n$-qubit quantum register into another one. This transformation has
applications in the initialization of a quantum computer, and also in some
quantum algorithms. Employing uniformly controlled rotations, we present a
quantum circuit of $2^{n+2}-4n-4$ CNOT gates and $2^{n+2}-5$ one-qubit
elementary rotations that effects the state transformation. The complexity of
the circuit is noticeably lower than the previously published results.
Moreover, we present an analytic expression for the rotation angles needed for
the transformation.


title: A protocol for secure and deterministic quantum key expansion

abstract:
  In all existing protocols of private communication with encryption and
decryption, the pre-shared key can be used for only one time. We give a
deterministic quantum key expansion protocol where the pre-shared key can be
recycled. Our protocol is exponentially secure. Our protocol costs less qubits
and almost zero classical communications with authentication steps being
included. Since our protocol can distribute the deterministic bits, it can also
be used for direct communication.


title: Effect of magnetic fields on bound localized electron states

abstract:
  An approximate analytic solution for the ground electron state are found to
the Schroedinger equation for a combination of a uniform magnetic field and
single attractive delta-potential. Effect of the magnetic field on this bound
localized electron state is discussed. We show that above effect leads to
appearing the probability current density in some region centered in the point
of the delta-potential as well as to enlarging (for a number of physical
models) the localization region of the electron in the plane perpendicular to
the magnetic field. We are expected that these properties can be of importance
for real quantum mechanical systems.


title:
Breakdown of an Electric-Field Driven System: a Mapping to a Quantum
  Walk

abstract:
  Quantum transport properties of electron systems driven by strong electric
fields are studied by mapping the Landau-Zener transition dynamics to a quantum
walk on a semi-infinite one-dimensional lattice with a reflecting boundary,
where the sites correspond to energy levels and the boundary the ground state.
Quantum interference induces a distribution localized around the ground state,
and when the electric field is strengthened, a delocalization transition occurs
describing breakdown of the original electron system.


title: Cavity QED and Quantum Computation in the Weak Coupling Regime

abstract:
  In this paper we consider a model of quantum computation based on n atoms of
laser-cooled and trapped linearly in a cavity and realize it as the n atoms
Tavis-Cummings Hamiltonian interacting with n external (laser) fields.
  We solve the Schr{\" o}dinger equation of the model in the case of n=2 and
construct the controlled NOT gate by making use of a resonance condition and
rotating wave approximation associated to it. Our method is not heuristic but
completely mathematical, and the significant feature is a consistent use of
Rabi oscillations.
  We also present an idea of the construction of three controlled NOT gates in
the case of n=3 which gives the controlled-controlled NOT gate.


title: Limits to phase resolution in matter wave interferometry

abstract:
  We study the quantum dynamics of a two-mode Bose-Einstein condensate in a
time-dependent symmetric double-well potential using analytical and numerical
methods. The effects of internal degrees of freedom on the visibility of
interference fringes during a stage of ballistic expansion are investigated
varying particle number, nonlinear interaction sign and strength as well as
tunneling coupling. Expressions for the phase resolution are derived and the
possible enhancement due to squeezing is discussed. In particular, the role of
the superfluid - Mott insulator cross-over and its analog for attractive
interactions is recognized.


title:
Exponential Type Complex and non-Hermitian Potentials in PT-Symmetric
  Quantum Mechanics and Hamiltonian Hierarchy Method

abstract:
  The supersymmetric solutions of PT-/non-PT-symmetric and non-Hermitian
deformed Morse and P\"{o}schl-Teller potentials are obtained by solving the
Schr\"{o}dinger equation. The Hamiltonian hierarchy method is used to get the
real energy eigenvalues and corresponding eigenfunctions. \newline
{PACS:05.20.-y; 05.30.-d; 05.70. Ce; 03.65.-w}\newline {\it Keywords}{\small :
Supersymmetric quantum mechanics, Hamiltonian Hierarchy Method, Morse
potential, P\"{o}schl-Teller potential}


title: Partial deterministic non-demolition Bell measurement

abstract:
  We propose the scheme implementing partial deterministic non-demolition Bell
measurement. When it is used in quantum teleportation the information about an
unknown input state is optimally distributed among three outgoing qubits. The
optimality means that the output fidelities saturate the asymmetric cloning
inequality. The flow of information among the qubits is controlled by the
preparation of a pair of ancillary qubits used in the Bell measurement. It is
also demonstrated that the measurement is optimal two-qubit operation in the
sense of the trade-off between the state disturbance and the information gain.


title: The Extent of Multi-particle Quantum Non-locality

abstract:
  It is well known that entangled quantum states can be nonlocal: the
correlations between local measurements carried out on these states cannot
always be reproduced by local hidden variable models. Svetlichny, followed by
others, showed that multipartite quantum states are even more nonlocal than
bipartite ones in the sense that nonlocal classical models with (super-luminal)
communication between some of the parties cannot reproduce the quantum
correlations. Here we study in detail the kinds of nonlocality present in
quantum states. More precisely we enquire what kinds of classical communication
patterns cannot reproduce quantum correlations. By studying the extremal points
of the space of all multiparty probability distributions, in which all parties
can make one of a pair of measurements each with two possible outcomes, we find
a necessary condition for classical nonlocal models to reproduce the statistics
of all quantum states. This condition extends and generalises work of
Svetlichny and others in which it was shown that a particular class of
classical nonlocal models, the ``separable'' models, cannot reproduce the
statistics of all multiparticle quantum states. Our condition shows that the
nonlocality present in some entangled multiparticle quantum states is much
stronger than previously thought. We also study the sufficiency of our
condition.


title:
The effect of coordinate and momentum uncertainties on collision of
  coherent electrons

abstract:
  We have examined the head-on collision of two electrons in approximation of
coherent states. We have shown that the character of collision depends mainly
on ratio of initial relative electron's momentum to momentum uncertainty of
electrons. When this ratio becomes greater then 1, the Coulomb interaction does
not practically influence the scattering.


title: Coupled ion - nanomechanical systems

abstract:
  We study ions in a nanotrap, where the electrodes are nanomechanical
resonantors. The ions play the role of a quantum optical system which acts as a
probe and control, and allows entanglement with or between nanomechanical
resonators.


title: Error Filtration and Entanglement Purification for Quantum Communication

abstract:
  The key realisation which lead to the emergence of the new field of quantum
information processing is that quantum mechanics, the theory that describes
microscopic particles, allows the processing of information in fundamentally
new ways. But just as in classical information processing, errors occur in
quantum information processing, and these have to be corrected. A fundamental
breakthrough was the realisation that quantum error correction is in fact
possible. However most work so far has not been concerned with technological
feasibility, but rather with proving that quantum error correction is possible
in principle. Here we describe a method for filtering out errors and
entanglement purification which is particularly suitable for quantum
communication. Our method is conceptually new, and, crucially, it is easy to
implement in a wide variety of physical systems with present day technology and
should therefore be of wide applicability.


title: Arbitrarily accurate composite pulses

abstract:
  Systematic errors in quantum operations can be the dominating source of
imperfection in achieving control over quantum systems. This problem, which has
been well studied in nuclear magnetic resonance, can be addressed by replacing
single operations with composite sequences of pulsed operations, which cause
errors to cancel by symmetry. Remarkably, this can be achieved without
knowledge of the amount of error epsilon. Independent of the initial state of
the system, current techniques allow the error to be reduced to O(epsilon^3).
Here, we extend the composite pulse technique to cancel errors to O(epsilon^n),
for arbitrary n.


title:
An extension of Chaitin's halting probability \Omega to a measurement
  operator in an infinite dimensional quantum system

abstract:
  This paper proposes an extension of Chaitin's halting probability \Omega to a
measurement operator in an infinite dimensional quantum system. Chaitin's
\Omega is defined as the probability that the universal self-delimiting Turing
machine U halts, and plays a central role in the development of algorithmic
information theory. In the theory, there are two equivalent ways to define the
program-size complexity H(s) of a given finite binary string s. In the standard
way, H(s) is defined as the length of the shortest input string for U to output
s. In the other way, the so-called universal probability m is introduced first,
and then H(s) is defined as -log_2 m(s) without reference to the concept of
program-size.
  Mathematically, the statistics of outcomes in a quantum measurement are
described by a positive operator-valued measure (POVM) in the most general
setting. Based on the theory of computability structures on a Banach space
developed by Pour-El and Richards, we extend the universal probability to an
analogue of POVM in an infinite dimensional quantum system, called a universal
semi-POVM. We also give another characterization of Chaitin's \Omega numbers by
universal probabilities. Then, based on this characterization, we propose to
define an extension of \Omega as a sum of the POVM elements of a universal
semi-POVM. The validity of this definition is discussed.
  In what follows, we introduce an operator version \hat{H}(s) of H(s) in a
Hilbert space of infinite dimension using a universal semi-POVM, and study its
properties.


title:
Experimental Demonstration of Unconditional Entanglement Swapping for
  Continuous Variables

abstract:
  The unconditional entanglement swapping for continuous variables is
experimentally demonstrated. Two initial entangled states are produced from two
nondegenerate optical parametric amplifiers operating at deamplification.
Through implementing the direct measurement of Bell-state between two optical
beams from each amplifier the remaining two optical beams, which have never
directly interacted with each other, are entangled. The quantum correlation
degrees of 1.23dB and 1.12dB below the shot noise limit for the amplitude and
phase quadratures resulting from the entanglement swapping are straightly
measured.


title: Contextual objectivity and the quantum formalism

abstract:
  The new orthodoxy of quantum mechanics (QM) based on the decoherence approach
requires many-worlds as an essential ingredient for logical consistency, and
one may wonder what status to give to all these "other worlds". Here we
advocate that it is possible to build a consistent approach to QM where no
other worlds are needed, and where the quantum formalism appears as a
consequence of requiring the enumerability of physical properties. Such a
quantization hypothesis is closely related to indistinguishability, and is
deeply inconsistent with classical physics.


title:
Inducing complete population oscillations in systems with externally
  induced dipole moments

abstract:
  A two level system is considered which has no static dipole moment, e.g.
molecule $H_2$ in its ground electronic state. If strong enough external field
is applied, it will dynamically distort such a system and supply it with time
(and field) dependent dipole moment. Although it is impossible to do so in the
undistorted system which has no coupling to the dipole component of the
external field, having induced in it a dipole moment, the rotational and
vibrational dynamics of such system can be manipulated using lasers. In this
work, a system is considered in which the external perturbation dynamically
induces the transition dipole moment between only two distinct levels. The aim
of the work is to show how the driving pulse can be analytically designed, that
will produce Rabi-like complete population oscillations between the two levels.


title: The Loss of fidelity due to quantum leakage for Josephson charge qubits

abstract:
  In this paper we calculate the loss of fidelity due to quantum leakage for
the Josephson charge qubit (JCQ) in virtue of the Mathieu functions. It is
shown that for an present typical parameters of JCQ E_{J}/E_{ch}~0.02, the loss
of the fidelity per elementary operation is about 10^(-4) which satisfy the
DiVincenzo's low decoherence criterion. By appropriately improving the design
of the Josephson junction, namely, decreasing E_{J}/E_{ch} to 0.01, the loss of
fidelity per elementary operation can decrease to 10^(-6) even smaller.


title:
Transit time of a freely-falling quantum particle in a background
  gravitational field

abstract:
  Using a model quantum clock, I evaluate an expression for the time of a
non-relativistic quantum particle to transit a piecewise geodesic path in a
background gravitational field with small spacetime curvature (gravity
gradient), in the case that the apparatus is in free fall. This calculation
complements and extends an earlier one (Davies 2004) in which the apparatus is
fixed to the surface of the Earth. The result confirms that, for particle
velocities not too low, the quantum and classical transit times coincide, in
conformity with the principle of equivalence. I also calculate the quantum
corrections to the transit time when the de Broglie wavelengths are long enough
to probe the spacetime curvature. The results are compared with the calculation
of Chaio and Speliotopoulos (2003), who propose an experiment to measure the
foregoing effects.


title: Maximal violation of Mermin's inequalities

abstract:
  In this paper, it is proved that the maximal violation of Mermin's
inequalities of $n$ qubits occurs only for GHZ's states and the states obtained
from them by local unitary transformations. The key point of our argument
involved here is by using the certain algebraic properties that Pauli's
matrices satisfy, which is based on the determination of local spin observables
of the associated Bell-Mermin operators.


title: Quantum State of Entangled Photon Pairs

abstract:
  I show that the photon pairs used in experimental tests of quantum
non-locality based on Bell's theorem are not in the entangled quantum state.
The correct quantum state of the ``entangled'' photon pairs is suggested. Two
experiments for testing this quantum state are proposed.


title: Experimental quantum key distribution over highly noisy channels

abstract:
  Error filtration is a method for encoding the quantum state of a single
particle into a higher dimensional Hilbert space in such a way that it becomes
less sensitive to phase noise. We experimentally demonstrate this method by
distributing a secret key over an optical fiber whose noise level otherwise
precludes secure quantum key distribution. By filtering out the phase noise, a
bit error rate of 15.3% +/- 0.1%, which is beyond the security limit, can be
reduced to 10.6% +/- 0.1%, thereby guaranteeing the cryptographic security.


title: Casimir-Polder potentials as entanglement probe

abstract:
  We have considered the interaction of a pair of spatially separated two-level
atoms with the electromagnetic field in its vacuum state and we have analyzed
the amount of entanglement induced between the two atoms by the non local field
fluctuations. This has allowed us to characterize the quantum nature of the non
local correlations of the electromagnetic field vacuum state as well as to link
the induced quantum entanglement with Casimir-Polder potentials.


title:
Note on multiple additivity of minimal Renyi entropy output of the
  Werner-Holevo channels

abstract:
  We give an elementary self-contained proof that the minimal entropy output of
arbitrary products of channels $\rho \mapsto \frac{1}{d-1}(1-\rho^T)$ is
additive.


title:
High efficiency transfer of quantum information and multi-particle
  entanglement generation in translation invariant quantum chains

abstract:
  We demonstrate that a translation invariant chain of interacting quantum
systems can be used for high efficiency transfer of quantum entanglement and
the generation of multi-particle entanglement over large distances and between
arbitrary sites without the requirement of precise spatial or temporal control.
The scheme is largely insensitive to disorder and random coupling strengths in
the chain. We discuss harmonic oscillator systems both in the case of arbitrary
Gaussian states and in situations when at most one excitation is in the system.
The latter case which we prove to be equivalent to an xy-spin chain may be used
to generate genuine multi particle entanglement. Such a 'quantum data bus' may
prove useful in future solid state architectures for quantum information
processing.


title:
Lower bound on the number of Toffoli gates in a classical reversible
  circuit through quantum information concepts

abstract:
  The question of finding a lower bound on the number of Toffoli gates in a
classical reversible circuit is addressed. A method based on quantum
information concepts is proposed. The method involves solely concepts from
quantum information - there is no need for an actual physical quantum computer.
The method is illustrated on the example of classical Shannon data compression.


title: Exact Stochastic Reformulation of Non-Relativistic Quantum Dynamics

abstract:
  We show that any non-relativistic quantum N-body dynamics problem with
pairwise interactions can be exactly reformulated in terms of N well behaved
1-body stochastic density equations. Specifically, the time evolving N-body
density matrix is written as an average of tensor products of 1-body densities
each of which obeys a stochastic evolution equation. Such decompositions can be
constructed for any mixture of fermions and bosons. The evolution equations for
the 1-body densities preserve norm, Hermiticity and positivity.


title: Distributed quantum dense coding

abstract:
  We introduce the notion of distributed quantum dense coding, i.e. the
generalization of quantum dense coding to more than one sender and more than
one receiver. We show that global operations (as compared to local operations)
of the senders do not increase the information transfer capacity, in the case
of a single receiver. For the case of two receivers, using local operations and
classical communication, a non-trivial upper bound for the capacity is derived.
We propose a general classification scheme of quantum states according to their
usefulness for dense coding. In the bipartite case (for any dimensions), bound
entanglement is not useful for this task.


title:
Common origin of no-cloning and no-deleting principles - Conservation of
  information

abstract:
  We discuss the role of the notion of information in the description of
physical reality. We consider theories for which dynamics is linear with
respect to stochastic mixing. We point out that the no-cloning and no-deleting
principles emerge in any such theory, if law of conservation of information is
valid, and two copies contain more information than one copy. We then describe
the quantum case from this point of view.


title: Numerical Methods for Stochastic Differential Equations

abstract:
  Stochastic differential equations (sdes) play an important role in physics
but existing numerical methods for solving such equations are of low accuracy
and poor stability. A general strategy for developing accurate and efficient
schemes for solving stochastic equations in outlined here. High order numerical
methods are developed for integration of stochastic differential equations with
strong solutions. We demonstrate the accuracy of the resulting integration
schemes by computing the errors in approximate solutions for sdes which have
known exact solutions.


title:
Uniform semiclassical approach to fidelity decay in the deep Lyapunov
  regime

abstract:
  We use the uniform semiclassical approximation in order to derive the
fidelity decay in the regime of large perturbations.
  Numerical computations are presented which agree with our theoretical
predictions.
  Moreover, our theory allows to explain previous findings, such as the
deviation from the Lyapunov decay rate in cases where the classical finite-time
instability is non-uniform in phase space.


title:
Correlation functions, Bell's inequalities and the fundamental
  conservation laws

abstract:
  I derive the correlation function for a general theory of two-valued spin
variables that satisfy the fundamental conservation law of angular momentum.
The unique theory-independent correlation function is identical to the quantum
mechanical correlation function. I prove that any theory of correlations of
such discrete variables satisfying the fundamental conservation law of angular
momentum violates the Bell's inequalities. Taken together with the Bell's
theorem, this result has far reaching implications. No theory satisfying
Einstein locality, reality in the EPR-Bell sense, and the validity of the
conservation law can be constructed. Therefore, all local hidden variable
theories are incompatible with fundamental symmetries and conservation laws.
Bell's inequalities can be obeyed only by violating a conservation law. The
implications for experiments on Bell's inequalities are obvious. The result
provides new insight regarding entanglement, and its measures.


title:
Exact and approximate many-body dynamics with stochastic one-body
  density matrix evolution

abstract:
  We show that the dynamics of interacting fermions can be exactly replaced by
a quantum jump theory in the many-body density matrix space. In this theory,
jumps occur between densities formed of pairs of Slater determinants, $D_{ab}=|
\Phi_a > < \Phi_b |$, where each state evolves according to the Stochastic
Schr\"odinger Equation (SSE) given in ref. \cite{Jul02}. A stochastic
Liouville-von Neumann equation is derived as well as the associated
Bogolyubov-Born-Green-Kirwood-Yvon (BBGKY) hierarchy. Due to the specific form
of the many-body density along the path, the presented theory is equivalent to
a stochastic theory in one-body density matrix space, in which each density
matrix evolves according to its own mean field augmented by a one-body noise.
Guided by the exact reformulation, a stochastic mean field dynamics valid in
the weak coupling approximation is proposed. This theory leads to an
approximate treatment of two-body effects similar to the extended
Time-Dependent Hartree-Fock (Extended TDHF) scheme. In this stochastic mean
field dynamics, statistical mixing can be directly considered and jumps occur
on a coarse-grained time scale. Accordingly, numerical effort is expected to be
significantly reduced for applications.


title: Path summation and quantum measurements

abstract:
  We propose a general theoretical approach to quantum measurements based on
the path (histories) summation technique. For a given dynamical variable A, the
Schr\"odinger state of a system in a Hilbert space of arbitrary dimensionality
is decomposed into a set of substates, each of which corresponds to a
particular detailed history of the system. The coherence between the substates
may then be destroyed by meter(s) to a degree determined by the nature and the
accuracy of the measurement(s) which may be of von Neumann, finite-time or
continuous type. Transformations between the histories obtained for
non-commuting variables and construction of simultaneous histories for
non-commuting observables are discussed. Important cases of a particle
described by Feynman paths in the coordinate space and a qubit in a two
dimensional Hilbert space are studied in some detail.


title: The Relativistic Quantum Motions

abstract:
  Using the relativistic quantum stationary Hamilton-Jacobi equation within the
framework of the equivalence postulate, and grounding oneself on both
relativistic and quantum Lagrangians, we construct a Lagrangian of a
relativistic quantum system in one dimension and derive a third order equation
of motion representing a first integral of the relativistic quantum Newton's
law. Then, we plot the relativistic quantum trajectories of a particle moving
under the constant and the linear potentials. We establish the existence of
nodes and link them to the de Broglie's wavelength.


title: Field quantization in inhomogeneous absorptive dielectrics

abstract:
  The quantization of the electromagnetic field in a three-dimensional
inhomogeneous dielectric medium with losses is carried out in the framework of
a damped-polariton model with an arbitrary spatial dependence of its
parameters. The equations of motion for the canonical variables are solved
explicitly by means of Laplace transformations for both positive and negative
time. The dielectric susceptibility and the quantum noise-current density are
identified in terms of the dynamical variables and parameters of the model. The
operators that diagonalize the Hamiltonian are found as linear combinations of
the canonical variables, with coefficients depending on the electric
susceptibility and the dielectric Green function. The complete time dependence
of the electromagnetic field and of the dielectric polarization is determined.
Our results provide a microscopic justification of the phenomenological
quantization scheme for the electromagnetic field in inhomogeneous dielectrics.


title:
Faithful qubit distribution assisted by one additional qubit against
  collective noise

abstract:
  We propose a distribution scheme of polarization states of a single photon
over collective-noise channel. By adding one extra photon with a fixed
polarization, we can protect the state against collective noise via a
parity-check measurement and post-selection. While the scheme succeeds only
probabilistically, it is simpler and more flexible than the schemes utilizing
decoherence-free subspace. An application to BB84 protocol through collective
noise channel, which is robust to the Trojan horse attack, is also given.


title: Random Matrix Theory and Entanglement in Quantum Spin Chains

abstract:
  We compute the entropy of entanglement in the ground states of a general
class of quantum spin-chain Hamiltonians - those that are related to quadratic
forms of Fermi operators - between the first N spins and the rest of the system
in the limit of infinite total chain length. We show that the entropy can be
expressed in terms of averages over the classical compact groups and establish
an explicit correspondence between the symmetries of a given Hamiltonian and
those characterizing the Haar measure of the associated group. These averages
are either Toeplitz determinants or determinants of combinations of Toeplitz
and Hankel matrices. Recent generalizations of the Fisher-Hartwig conjecture
are used to compute the leading order asymptotics of the entropy as N -->
infinity . This is shown to grow logarithmically with N. The constant of
proportionality is determined explicitly, as is the next (constant) term in the
asymptotic expansion. The logarithmic growth of the entropy was previously
predicted on the basis of numerical computations and conformal-field-theoretic
calculations. In these calculations the constant of proportionality was
determined in terms of the central charge of the Virasoro algebra. Our results
therefore lead to an explicit formula for this charge. We also show that the
entropy is related to solutions of ordinary differential equations of
Painlev\'e type. In some cases these solutions can be evaluated to all orders
using recurrence relations.


title: Optical Lattices, Ultracold Atoms and Quantum Information Processing

abstract:
  We review novel methods to investigate, control and manipulate neutral atoms
in optical lattices. These setups allow unprecedented quantum control over
large numbers of atoms and thus are very promising for applications in quantum
information processing. After introducing optical lattices we discuss the
superfluid (SF) and Mott insulating (MI) states of neutral atoms trapped in
such lattices and investigate the SF-MI transition as recently observed
experimentally. In the second part of the paper we give an overview of
proposals for quantum information processing and show different ways to
entangle the trapped atoms, in particular the usage of cold collisions and
Rydberg atoms. Finally, we also briefly discuss the implementation of quantum
simulators, entanglement enhanced atom interferometers, and ideas for robust
quantum memory in optical lattices.


title: Aspects of generic entanglement

abstract:
  We study entanglement and other correlation properties of random states in
high-dimensional bipartite systems. These correlations are quantified by
parameters that are subject to the "concentration of measure" phenomenon,
meaning that on a large-probability set these parameters are close to their
expectation. For the entropy of entanglement, this has the counterintuitive
consequence that there exist large subspaces in which all pure states are close
to maximally entangled. This, in turn, implies the existence of mixed states
with entanglement of formation near that of a maximally entangled state, but
with negligible quantum mutual information and, therefore, negligible
distillable entanglement, secret key, and common randomness. It also implies a
very strong locking effect for the entanglement of formation: its value can
jump from maximal to near zero by tracing over a number of qubits negligible
compared to the size of total system. Furthermore, such properties are generic.
Similar phenomena are observed for random multiparty states, leading us to
speculate on the possibility that the theory of entanglement is much simplified
when restricted to asymptotically generic states. Further consequences of our
results include a complete derandomization of the protocol for universal
superdense coding of quantum states.


title:
Comment on "Quantum key distribution with 1.25 Gbps clock
  synchronization" by J.C. Bienfang et al., quant-ph/0405097

abstract:
  We analyze the significance for quantum key distribution (QKD) of free-space
quantum communications results reported in a recent paper (J. C. Bienfang et
al., quant-ph/0405097, hereafter referred to as "Bienfang et al."), who
contrast the quantum communications rate of their partial QKD implementation
(which does not produce cryptographically useful shared, secret keys) over a
short transmission distance, with the secret bit rates of previous full QKD
implementations over much longer distances. We show that when a
cryptographically relevant comparison with previous results is made, the system
described by Bienfang et al. would offer no advantages for QKD, contrary to
assertions in their paper and in spite of its high clock rate. Further, we show
that the claim made by Bienfang et al. that "high transmission rates serve ...
to extend the distance over which a QKD system can operate" is incorrect. Our
analysis illustrates an important aspect of QKD that is too often overlooked in
experiments: the sifted bit rate can be a highly misleading indicator of the
performance of a QKD system.


title:
Symplectic quantization, inequivalent quantum theories, and Heisenberg's
  principle of uncertainty

abstract:
  We analyze the quantum dynamics of the non-relativistic two-dimensional
isotropic harmonic oscillator in Heisenberg's picture. Such a system is taken
as toy model to analyze some of the various quantum theories that can be built
from the application of Dirac's quantization rule to the various symplectic
structures recently reported for this classical system. It is pointed out that
that these quantum theories are inequivalent in the sense that the mean values
for the operators (observables) associated with the same physical classical
observable do not agree with each other. The inequivalence does not arise from
ambiguities in the ordering of operators but from the fact of having several
symplectic structures defined with respect to the same set of coordinates. It
is also shown that the uncertainty relations between the fundamental
observables depend on the particular quantum theory chosen. It is important to
emphasize that these (somehow paradoxical) results emerge from the combination
of two paradigms: Dirac's quantization rule and the usual Copenhagen
interpretation of quantum mechanics.


title: Non-positivity of Groenewold operators

abstract:
  A central feature in the Hilbert space formulation of classical mechanics is
the quantisation of classical Liouville densities, leading to what may be
termed term Groenewold operators. We investigate the spectra of the Groenewold
operators that correspond to Gaussian and to certain uniform Liouville
densities. We show that when the classical coordinate-momentum uncertainty
product falls below Heisenberg's limit, the Groenewold operators in the
Gaussian case develop negative eigenvalues and eigenvalues larger than 1.
However, in the uniform case, negative eigenvalues are shown to persist for
arbitrarily large values of the classical uncertainty product.


title: Parallel Treatment of Estimation of SU(2) and Phase Estimation

abstract:
  We discuss the accuracy of the estimation of the $n$ identical unknown
actions of $\SU(2)$ by using entanglement. This problem has a similar structure
with the phase estimation problem, which was discussed by Bu\v{z}ek, Derka, and
Massar\cite{BDM}. The estimation error asymptotically decreases to zero with an
order of $\frac{1}{n^2}$ at least.


title: Implementation of group-covariant POVMs by orthogonal measurements

abstract:
  We consider group-covariant positive operator valued measures (POVMs) on a
finite dimensional quantum system. Following Neumark's theorem a POVM can be
implemented by an orthogonal measurement on a larger system. Accordingly, our
goal is to find an implementation of a given group-covariant POVM by a quantum
circuit using its symmetry. Based on representation theory of the symmetry
group we develop a general approach for the implementation of group-covariant
POVMs which consist of rank-one operators. The construction relies on a method
to decompose matrices that intertwine two representations of a finite group. We
give several examples for which the resulting quantum circuits are efficient.
In particular, we obtain efficient quantum circuits for a class of POVMs
generated by Weyl-Heisenberg groups. These circuits allow to implement an
approximative simultaneous measurement of the position and crystal momentum of
a particle moving on a cyclic chain.


title:
Influence of the signal light on the transient optical properties of a
  four-level EIT medium

abstract:
  General formulae for the transient evolution of the susceptibility
(absorption) induced by the quantum interference effect in a four-level N-type
EIT medium is presented. The influence of the signal light on the transient
susceptibility for the probe beam is studied for two typical cases when the
strength of the coupling beam is much greater or less than that of the signal
field. An interesting level reciprocity relationship between these two cases is
found.


title: Quantum Phase Transitions and Bipartite Entanglement

abstract:
  We develop a general theory of the relation between quantum phase transitions
(QPTs) characterized by nonanalyticities in the energy and bipartite
entanglement. We derive a functional relation between the matrix elements of
two-particle reduced density matrices and the eigenvalues of general two-body
Hamiltonians of $d$-level systems. The ground state energy eigenvalue and its
derivatives, whose non-analyticity characterizes a QPT, are directly tied to
bipartite entanglement measures. We show that first-order QPTs are signalled by
density matrix elements themselves and second-order QPTs by the first
derivative of density matrix elements. Our general conclusions are illustrated
via several quantum spin models.


title:
Basic Logic and Quantum Computing: Logical Judgements by an Insider
  Observer

abstract:
  We consider the logical assertions of a hypothetical observer who is inside a
quantum computer and performs a reversible quantum measurement, obtaining a
symmetric couple of new axioms, valid only inside the quantum computer. The
result is that, in this logical framework, symmetry and paraconsistency hold.


title: Quantum state transfer in imperfect artificial spin networks

abstract:
  High-fidelity quantum computation and quantum state transfer are possible in
short spin chains. We exploit a system based on a dispersive qubit-boson
interaction to mimic XY coupling. In this model, the usually assumed
nearest-neighbors coupling is no more valid: all the qubits are mutually
coupled. We analyze the performances of our model for quantum state transfer
showing how pre-engineered coupling rates allow for nearly optimal state
transfer. We address a setup of superconducting qubits coupled to a microstrip
cavity in which our analysis may be applied.


title:
Antilocalization in Coherent Backscattering of Light in a
  Multi-Resonance Atomic System

abstract:
  Theoretical prediction of antilocalization of light in ultracold atomic gas
samples, in the weak localization regime, is reported. Calculations and
Monte-Carlo simulations show that, for selected spectral ranges in the vicinity
of atomic 85Rb hyperfine transitions, quantum coherence in optical transitions
through nondegenerate hyperfine levels in multiple light scattering generates
destructive interference in otherwise reciprocal scattering paths. This effect
leads to enhancement factors less than unity in a coherent backscattering
geometry, and suggests the possibility of enhanced diffusion of light in
ultracold atomic vapors.


title: Theory of control of spin/photon interface for quantum networks

abstract:
  A cavity coupling a charged nanodot and a fiber can act as a quantum
interface, through which a stationary spin qubit and a flying photon qubit can
be inter-converted via cavity-assisted Raman process. This Raman process can be
controlled to generate or annihilate an arbitrarily shaped single-photon
wavepacket by pulse-shaping the controlling laser field. This quantum interface
forms the basis for many essential functions of a quantum network, including
sending, receiving, transferring, swapping, and entangling qubits at
distributed quantum nodes as well as a deterministic source and an efficient
detector of a single photon wavepacket with arbitrarily specified shape and
average photon number. Numerical study of noise effects on the operations shows
high fidelity.


title: Optimal superdense coding of entangled states

abstract:
  We present a one-shot method for preparing pure entangled states between a
sender and a receiver at a minimal cost of entanglement and quantum
communication. In the case of preparing unentangled states, an earlier paper
showed that a 2n-qubit quantum state could be communicated to a receiver by
physically transmitting only n+o(n) qubits in addition to consuming n ebits of
entanglement and some shared randomness. When the states to be prepared are
entangled, we find that there is a reduction in the number of qubits that need
to be transmitted, interpolating between no communication at all for maximally
entangled states and the earlier two-for-one result of the unentangled case,
all without the use of any shared randomness. We also present two applications
of our result: a direct proof of the achievability of the optimal superdense
coding protocol for entangled states produced by a memoryless source, and a
demonstration that the quantum identification capacity of an ebit is two
qubits.


title: Quantum stream cipher based on optical communications

abstract:
  In 2000, an attractive new quantum cryptography was discovered by H.P.Yuen
based on quantum communication theory. It is applicable to direct encryption,
for example quantum stream cipher based on Yuen protocol(Y-00), with high
speeds and for long distance by sophisticated optical devices which can work
under the average photon number per signal light pulse:$<n> = 1000 \sim 10000$.
In addition, it may provide information-theoretic security against known/chosen
plaintext attack, which has no classical analogue. That is, one can provide
secure communication, even the system has $H(K) << H(X)$.
  In this paper, first, we give a brief review on the general logic of Yuen's
theory. Then, we show concrete security analysis of quantum stream cipher to
quantum individual measurement attacks. Especially by showing the analysis of
Lo-Ko known plaintext attack, the feature of Y-00 is clarified. In addition, we
give a simple experimental result on the advantage distillation by scheme
consisting of intensity modulation/direct detection optical communication.


title: Optical Quantum Computation with Perpetually Coupled Spins

abstract:
  The possibility of using strongly and continuously interacting spins for
quantum computation has recently been discussed. Here we present a simple
optical scheme that achieves this goal while avoiding the drawbacks of earlier
proposals. We employ a third state, accessed by a classical laser field, to
create an effective barrier to information transfer. The mechanism proves to be
highly efficient both for continuous and pulsed laser modes; moreover it is
very robust, tolerating high decay rates for the excited states. The approach
is applicable to a broad range of systems, in particular dense structures such
as solid state self-assembled (e.g., molecular) devices. Importantly, there are
existing structures upon which `first step' experiments could be immediately
performed.


title: Basics of Quantum Computation (Part 1)

abstract:
  The aim of this textbook is to bridge in regard of quantum computation what
proves to be a considerable threshold even to the usual science trained
readership between the level of science popularization, and on the other hand,
the presently available more encyclopedic textbooks. In this respect the
present textbook is aimed to be a short, simple, rigorous and direct
introduction, addressing itself only to quantum computation proper.


title: Geometrical Optics in Coincidence Imaging System

abstract:
  We discuss the geometrical optics of coincidence imaging for two kinds of
spatial correlations which are related to a classical thermal light source and
a two-photon quantum entangled state.


title:
On the Key-Uncertainty of Quantum Ciphers and the Computational Security
  of One-way Quantum Transmission

abstract:
  We consider the scenario where Alice wants to send a secret (classical)
$n$-bit message to Bob using a classical key, and where only one-way
transmission from Alice to Bob is possible. In this case, quantum communication
cannot help to obtain perfect secrecy with key length smaller then $n$. We
study the question of whether there might still be fundamental differences
between the case where quantum as opposed to classical communication is used.
In this direction, we show that there exist ciphers with perfect security
producing quantum ciphertext where, even if an adversary knows the plaintext
and applies an optimal measurement on the ciphertext, his Shannon uncertainty
about the key used is almost maximal. This is in contrast to the classical case
where the adversary always learns $n$ bits of information on the key in a known
plaintext attack. We also show that there is a limit to how different the
classical and quantum cases can be: the most probable key, given matching
plain- and ciphertexts, has the same probability in both the quantum and the
classical cases. We suggest an application of our results in the case where
only a short secret key is available and the message is much longer.


title: Security of Y-00 and similar quantum cryptographic protocols

abstract:
  It is claimed in Phys. Lett. A by T. Nishioka et. al. 327 (2004) 28-32, that
the security of Y-00 is equivalent to that of a classical stream cipher. In
this paper it is shown that the claim is false in either the use of Y-00 for
direct encryption or key generation, in all the parameter ranges it is supposed
to operate including those of the experiments reported thus far. The security
of Y-00 type protocols is clarified.