title:
Spectral Polarization and Spectral Phase Control of Time and Energy
  Entangled Photons

abstract:
  We demonstrate a scheme to spectrally manipulate a collinear, continuous
stream of time and energy entangled photons to generate beamlike,
bandwidth-limited fuxes of polarization-entangled photons with
nearly-degenerate wavelengths. Utilizing an ultrashort-pulse shaper to control
the spectral phase and polarization of the photon pairs, we tailor the shape of
the Hong-Ou-Mandel interference pattern, demonstrating the rules that govern
the dependence of this interference pattern on the spectral phases of the
photons. We then use the pulse shaper to generate all four polarization Bell
states. The singlet state generated by this scheme forms a very robust
decoherence-free subspace, extremely suitable for long distance fiber-optics
based quantum communication.


title:
Reply to M. Ziman's "Notes on optimality of direct characterization of
  quantum dynamics"

abstract:
  Recently M. Ziman [quant-ph/0603151] criticized our approach for quantifying
the required physical resources in the theory of Direct Characterization of
Quantum Dynamics (DCQD) [quant-ph/0601033, quant-ph/0601034] in comparison to
other quantum process tomography (QPT) schemes. Here we argue that Ziman's
comments regarding optimality, quantumness, and the novelty of DCQD are
inaccurate. Specifically, we demonstrate that DCQD is optimal with respect to
both the required number of experimental configurations and the number of
possible outcomes over all known QPT schemes in the 2^{2n} dimensional Hilbert
space of n system and n ancilla qubits. Moreover, we show DCQD is more
efficient than all known QPT schemes in the sense of overall required number of
quantum operations. Furthermore, we argue that DCQD is a new method for
characterizing quantum dynamics and cannot be considered merely as a subclass
of previously known QPT schemes.


title: How to Calculate the Exponential of Matrices

abstract:
  How to calculate the exponential of matrices in an explicit manner is one of
fundamental problems in almost all subjects in Science.
  Especially in Mathematical Physics or Quantum Optics many problems are
reduced to this calculation by making use of some approximations whether they
are appropriate or not. However, it is in general not easy.
  In this paper we give a very useful formula which is both elementary and
getting on with computer.


title:
Derivation of Master Equations in the Presence of Initial Correlations
  with Reservoir: Projection Method Revisited

abstract:
  We discuss the derivation of master equations in the presence of initial
correlations with the reservoir. In van Hove's limit, the total system behaves
as if it started from a factorized initial condition. A proper choice of
Nakajima-Zwanzig's projection operator is crucial and the reservoir should be
endowed with the mixing property.


title:
Tomography of one and two qubit states and factorisation of the Wigner
  distribution in prime power dimensions

abstract:
  We study different techniques that allow us to gain complete knowledge about
an unknown quantum state, e.g. to perform full tomography of this state. We
focus on two apparently simple cases, full tomography of one and two qubit
systems. We analyze and compare those techniques according to two figures of
merit. Our first criterion is the minimisation of the redundancy of the data
acquired during the tomographic process. In the case of two-qubits tomography,
we also analyze this process from the point of view of factorisability, so to
say we analyze the possibility to realise the tomographic process through local
operations and classical communications between local observers. This brings us
naturally to study the possibility to factorize the (discrete) Wigner
distribution of a composite system into the product of local Wigner
distributions. The discrete Heisenberg-Weyl group is an essential ingredient of
our approach. Possible extensions of our results to higher dimensions are
discussed in the last section and in the conclusions.


title:
Lorentz-covariant deformed algebra with minimal length and application
  to the 1+1-dimensional Dirac oscillator

abstract:
  The $D$-dimensional $(\beta, \beta')$-two-parameter deformed algebra
introduced by Kempf is generalized to a Lorentz-covariant algebra describing a
($D+1$)-dimensional quantized space-time. In the D=3 and $\beta=0$ case, the
latter reproduces Snyder algebra. The deformed Poincar\'e transformations
leaving the algebra invariant are identified. It is shown that there exists a
nonzero minimal uncertainty in position (minimal length). The Dirac oscillator
in a 1+1-dimensional space-time described by such an algebra is studied in the
case where $\beta'=0$. Extending supersymmetric quantum mechanical and
shape-invariance methods to energy-dependent Hamiltonians provides exact
bound-state energies and wavefunctions. Physically acceptable states exist for
$\beta < 1/(m^2 c^2)$. A new interesting outcome is that, in contrast with the
conventional Dirac oscillator, the energy spectrum is bounded.


title:
Application of the Asymptotic Iteration Method to a Perturbed Coulomb
  Model

abstract:
  We show that the asymptotic iteration method converges and yields accurate
energies for a perturbed Coulomb model. We also discuss alternative
perturbation approaches to that model.


title:
Light-Shift Imbalance Induced Blockade of Collective Excitations Beyond
  the Lowest Order

abstract:
  Current proposals focusing on neutral atoms for quantum computing are mostly
based on using single atoms as quantum bits (qubits), while using cavity
induced coupling or dipole-dipole interaction for two-qubit operations. An
alternative approach is to use atomic ensembles as qubits. However, when an
atomic ensemble is excited, by a laser beam matched to a two-level transition
(or a Raman transition) for example, it leads to a cascade of many states as
more and more photons are absorbed^1. In order to make use of an ensemble as a
qubit, it is necessary to disrupt this cascade, and restrict the excitation to
the absorption (and emission) of a single photon only. Here, we show how this
can be achieved by using a new type of blockade mechanism, based on the
light-shift imbalance (LSI) in a Raman transition. We describe first a simple
example illustrating the concept of light shift imbalanced induced blockade
(LSIIB) using a multi-level structure in a single atom, and show verifications
of the analytic prediction using numerical simulations. We then extend this
model to show how a blockade can be realized by using LSI in the excitation of
an ensemble. Specifically, we show how the LSIIB process enables one to treat
the ensemble as a two level atom that undergoes fully deterministic Rabi
oscillations between two collective quantum states, while suppressing
excitations of higher order collective states.


title:
Quantum Communication and Computing With Atomic Ensembles Using
  Light-Shift Imbalance Induced Blockade

abstract:
  Recently, we have shown that for conditions under which the so-called
light-shift imbalance induced blockade (LSIIB) occurs, the collective
excitation of an ensemble of a multi-level atom can be treated as a closed two
level system. In this paper, we describe how such a system can be used as a
quantum bit (qubit) for quantum communication and quantum computing.
Specifically, we show how to realize a C-NOT gate using the collective qubit
and an easily accessible ring cavity, via an extension of the so-called
Pellizzari scheme. We also describe how multiple, small-scale quantum computers
realized using these qubits can be linked effectively for implementing a
quantum internet. We describe the details of the energy levels and transitions
in 87Rb atom that could be used for implementing these schemes.


title: Notes on the Conway-Kochen Twin Argument

abstract:
  This is a revision of my original posting, in which I raised objections to
part of the Conway-Kochen argument. I now agree with them that their recent
reply answers my original concerns. In the first part of these notes (identical
to the original), I give a reformulation of the part of the Conway-Kochen
result that closes the contextuality loophole in the original Kochen-Specker
(KS) theorem. In the second part (modified in this revision) I review my
concerns connected with the finite time needed to make a measurement, and
briefly indicate how Conway and Kochen have reponded to them.


title:
A simple spectral condition implying separability for states of
  bipartite quantum systems

abstract:
  For two qubits and for general bipartite quantum systems, we give a simple
spectral condition in terms of the ordered eigenvalues of the density matrix
which guarantees that the corresponding state is separable.


title: What's wrong with this rebuttal?

abstract:
  A recent rebuttal to criticism of Bell's analysis is shown to be defective by
fault of failure to consider all hypothetical conditions input into the
derivation of Bell Inequalitites.


title:
Improving the security of quantum direct communication with
  authentication

abstract:
  Two protocols of quantum direct communication with authentication [Phys. Rev.
A {\bf 73}, 042305 (2006)] are recently proposed by Lee, Lim and Yang. In this
paper we will show that in the two protocols the authenticator Trent should be
prevented from knowing the secret message of communication. The first protocol
can be eavesdropped by Trent using the the intercept-measure-resend attack,
while the second protocol can be eavesdropped by Trent using single-qubit
measurement. To fix these leaks, I revise the original versions of the
protocols by using the Pauli-Z operation $\sigma_z$ instead of the original
bit-flip operation $X$. As a consequence, the protocol securities are improved.


title: Genetic algorithm optimization of entanglement

abstract:
  We present an application of a genetic algorithmic computational method to
the optimization of the concurrence measure of entanglement for the cases of
one dimensional chains, as well as square and triangular lattices in a simple
tight-binding approach in which the hopping of electrons is much stronger than
the phonon dissipation


title: 4D singular oscillator and generalized MIC-Kepler system

abstract:
  It is shown that the generalized MIC-Kepler system and four-dimensional
singular oscillator are dual to each other and the duality transformation is
the generalized version of the Kustaanheimo-Stiefel transformation.


title: Generating superposition and entanglement of squeezed vacuum states

abstract:
  We propose a scheme for generating the superposition and the entanglement of
squeezed vacuum states of electromagnetical fields. The scheme involves
single-photon source, single photon detector, and cross Kerr nonlinearity. The
Kerr nonlinearity required for generating the superposition of squeezed vacuum
states is 1/2 of that required for generating the superposition of coherent
states. The proposal can also be extended to generate the entanglement states
between two coherent states and that between one coherent state and one
squeezed vacuum state.


title:
Capacity of a simultaneous quantum secure direct communication scheme
  between the central party and other M parties

abstract:
  We analyze the capacity of a simultaneous quantum secure direct communication
scheme between the central party and other $M$ parties via $M+1$-particle GHZ
states and swapping quantum entanglement. It is shown that the encoding scheme
should be secret if other $M$ parties wants to transmit $M+1$ bit classical
messages to the center party secretly. However when the encoding scheme is
announced publicly, we prove that the capacity of the scheme in transmitting
the secret messages is 2 bits, no matter how big $M$ is.


title: Decoherence and reduction

abstract:
  A reduction mechanism resulting directly from the basic principles of quantum
mechanics is proposed, inseparably from decoherence. A rather consistent theory
of this effect is given and the next problems it raises are indicated.


title: Direct optical excitation of a fullerene-incarcerated metal ion

abstract:
  The endohedral fullerene Er3N@C80 shows characteristic 1.5 micron
photoluminescence at cryogenic temperatures associated with radiative
relaxation from the crystal-field split Er3+ 4I13/2 manifold to the 4I15/2
manifold. Previous observations of this luminescence were carried out by
photoexcitation of the fullerene cage states leading to relaxation via the
ionic states. We present direct non-cage-mediated optical interaction with the
erbium ion. We have used this interaction to complete a
photoluminescence-excitation map of the Er3+ 4I13/2 manifold. This ability to
interact directly with the states of an incarcerated ion suggests the
possibility of coherently manipulating fullerene qubit states with light.


title:
Beating the spatial standard quantum limits via adiabatic soliton
  expansion

abstract:
  Spatial quantum enhancement effects are studied under a unified framework. It
is shown that the multiphoton absorption rate of photons with a
quantum-enhanced lithographic resolution is reduced, not enhanced, contrary to
popular belief. The use of adiabatic soliton expansion is proposed to beat the
standard quantum limit on the optical beam displacement accuracy, as well as to
engineer an arbitrary multiphoton interference pattern for quantum lithography.
The proposed scheme provides a conceptually simple method that works for an
arbitrary number of photons.


title: Decision theory and information propagation in quantum physics

abstract:
  In recent papers, Zurek has objected to the decision-theoretic approach of
Deutsch and Wallace to deriving the Born rule for quantum probabilities on the
grounds that it courts circularity. Deutsch and Wallace assume that the many
worlds theory is true and that decoherence gives rise to a preferred basis.
However, decoherence arguments use the reduced density matrix, which relies
upon the partial trace and hence upon the Born Rule for its validity. Using the
Heisenberg Picture and quantum Darwinism - the notion that classical
information is quantum information that can proliferate in the environment
pioneered by Olliver et al - I show that measurement interactions between two
systems only create correlations between a specific set of commuting
observables of system 1 and a specific set of commuting observables of system
2. This argument picks out a unique basis in which information flows in the
correlations between those sets of commuting observables. I then derive the
Born rule for both pure and mixed states and answer some other criticisms of
the decision theoretic approach to quantum probability.


title:
Experimental continuous-variable entanglement from a
  phase-difference-locked optical parametric oscillator

abstract:
  We observed continuous-variable entanglement between the bright beams emitted
above threshold by an ultrastable optical parametric oscillator, classically
phase-locked at a frequency difference of 161.8273240(5) MHz. The
amplitude-difference squeezing is -3 dB and the phase-sum one is -1.35 dB.
Besides proving entanglement in a new physical system, the phase-locked OPO,
such unprecedented frequency-difference stability paves the way for
transferring entanglement between different optical frequencies and densely
implementing continuous-variable quantum information in the frequency domain.


title:
Fields of Iterated Quantum Reference Frames based on Gauge
  Transformations of Rational String States

abstract:
  This work is based on a description of quantum reference frames that seems
more basic than others in the literature. Here a frame is based on a set of
real and of complex numbers and a space time as a 4-tuple of the real numbers.
There are many isomorphic frames as there are many isomorphic sets of real
numbers. Each frame is suitable for construction of all physical theories as
mathematical structures over the real and complex numbers. The organization of
the frames into a field of frames is based on the representations of real and
complex numbers as Cauchy operators defined on complex rational states of
finite qubit strings. The structure of the field is based on noting that the
construction of real and complex numbers as Cauchy operators in a frame can be
iterated to create new frames coming from a frame. Gauge transformations on the
rational string states greatly expand the number of quantum frames as, for each
gauge U, there is one frame coming from the original frame. Forward and
backward iteration of the construction yields a two way infinite frame field
with satisfying properties. There is no background space time and there are no
real or complex numbers for the field as a whole. Instead these are relative
concepts associated with each frame in the field. Extension to include qukit
strings for different k bases, is described as is the problem of reconciling
the frame field to the existence of just one frame with one background space
time for the observable physical universe.


title:
Universal decoherence induced by an environmental quantum phase
  transition

abstract:
  Decoherence induced by coupling a system with an environment may display
universal features. Here we demostrate that when the coupling to the system
drives a quantum phase transition in the environment, the temporal decay of
quantum coherences in the system is Gaussian with a width independent of the
system-environment coupling strength. The existence of this effect opens the
way for a new type of quantum simulation algorithm, where a single qubit is
used to detect a quantum phase transition. We discuss possible implementations
of such algorithm and we relate our results to available data on universal
decoherence in NMR echo experiments.


title: State Transfer and Spin Measurement

abstract:
  We present a Hamiltonian that can be used for amplifying the signal from a
quantum state, enabling the measurement of a macroscopic observable to
determine the state of a single spin. We prove a general mapping between this
Hamiltonian and an exchange Hamiltonian for arbitrary coupling strengths and
local magnetic fields. This facilitates the use of existing schemes for perfect
state transfer to give perfect amplification. We further prove a link between
the evolution of this fixed Hamiltonian and classical Cellular Automata,
thereby unifying previous approaches to this amplification task.
  Finally, we show how to use the new Hamiltonian for perfect state transfer in
the, to date, unique scenario where total spin is not conserved during the
evolution, and demonstrate that this yields a significantly different response
in the presence of decoherence.


title: Quantum random walk of two photons in separable and entangled state

abstract:
  We discuss quantum random walk of two photons using linear optical elements.
We analyze the quantum random walk using photons in a variety of quantum states
including entangled states. We find that for photons initially in separable
Fock states, the final state is entangled. For polarization entangled photons
produced by type II downconverter, we calculate the joint probability of
detecting two photons at a given site. We show the remarkable dependence of the
two photon detection probability on the quantum nature of the state. In order
to understand the quantum random walk, we present exact analytical results for
small number of steps like five. We present in details numerical results for a
number of cases and supplement the numerical results with asymptotic analytical
results.


title: Creating large noon states with imperfect phase control

abstract:
  Optical noon states (|N,0> + |0,N>) are an important resource for
Heisenberg-limited metrology and quantum lithography. The only known methods
for creating noon states with arbitrary $N$ via linear optics and projective
measurments seem to have a limited range of application due to imperfect phase
control. Here, we show that bootstrapping techniques can be used to create
high-fidelity noon states of arbitrary size.


title:
Hybrid Quantum Processors: molecular ensembles as quantum memory for
  solid state circuits

abstract:
  We investigate a hybrid quantum circuit where ensembles of cold polar
molecules serve as long-lived quantum memories and optical interfaces for solid
state quantum processors. The quantum memory realized by collective spin states
(ensemble qubit) is coupled to a high-Q stripline cavity via microwave Raman
processes. We show that for convenient trap-surface distances of a few $\mu$m,
strong coupling between the cavity and ensemble qubit can be achieved. We
discuss basic quantum information protocols, including a swap from the cavity
photon bus to the molecular quantum memory, and a deterministic two qubit gate.
Finally, we investigate coherence properties of molecular ensemble quantum
bits.


title: New Limits on Fault-Tolerant Quantum Computation

abstract:
  We show that quantum circuits cannot be made fault-tolerant against a
depolarizing noise level of approximately 45%, thereby improving on a previous
bound of 50% (due to Razborov). Our precise quantum circuit model enables
perfect gates from the Clifford group (CNOT, Hadamard, S, X, Y, Z) and
arbitrary additional one-qubit gates that are subject to that much depolarizing
noise. We prove that this set of gates cannot be universal for arbitrary (even
classical) computation, from which the upper bound on the noise threshold for
fault-tolerant quantum computation follows.


title:
Gauge Symmetry of the Third Kind and Quantum Mechanics as an Infrared
  Phenomenon

abstract:
  We introduce functional degrees of freedom by a new gauge principle related
to the phase of the wave functional. Thereby, quantum mechanical systems are
seen as dissipatively embedded part of a nonlinear classical structure
producing universal correlations. There are a fundamental length and an
entropy/area parameter, besides standard couplings. For states that are
sufficiently spread over configuration space, quantum field theory is
recovered.


title:
Ghost imaging with intense fields from chaotically-seeded parametric
  downconversion

abstract:
  We present the first experimental demonstration of ghost imaging realized
with intense beams generated by a parametric downconversion interaction seeded
with pseudo-thermal light. As expected, the real image of the object is
reconstructed satisfying the thin-lens equation. We show that the experimental
visibility of the reconstructed image is in accordance with the theoretically
expected one.


title:
Electronic properties of Francium diatomic compounds and prospects for
  cold molecule formation

abstract:
  In this work we investigate the possibility to create cold Fr$\_2$, RbFr, and
CsFr molecules through photoassociation of cold atoms. Potential curves,
permanent and transition dipole moments for the Francium dimer and for the RbFr
and RbCs molecules are determined for the first time. The Francium atom is
modelled as a one valence electron moving in the field of the Fr$^+$ core,
which is described by a new pseudopotential with averaged relativistic effects,
and including effective core polarization potential. The molecular calculations
are performed for both the ionic species Fr$\_2^+$, RbFr$^+$, CsFr$^+$ and the
corresponding neutral, through the CIPSI quantum chemistry package where we
used new extended gaussian basis sets for Rb, Cs, and Fr atoms. As no
experimental data is available, we discuss our results by comparison with the
Rb$\_2$, Cs$\_2$, and RbCs systems. The dipole moment of CsFr reveals an
electron transfer yielding a Cs$^+$Fr$^-$ arrangement, while in all other mixed
alkali pairs the electron is transferred towards the lighter species. Finally
the perturbative treatment of the spin-orbit coupling at large distances
predicts that in contrast with Rb$\_2$ and Cs$\_2$, no double-well excited
potential should be present in Fr$\_2$, probably preventing an efficient
formation of cold dimers via photoassociation of cold Francium atoms.


title: Strengthened Bell inequalities for orthogonal spin directions

abstract:
  We strengthen the bound on the correlations of two spin-1/2 particles
(qubits) in separable (non-entangled) states for locally orthogonal spin
directions by much tighter bounds than the well-known Bell inequality. This
provides a sharper criterion for the experimental distinction between entangled
and separable states, and even one which is a necessary and sufficient
condition for separability. However, these improved bounds do not apply to
local hidden-variable theories, and hence they provide a criterion to test the
correlations allowed by local hidden-variable theories against those allowed by
separable quantum states. Furthermore, these bounds are stronger than some
recent alternative experimentally accessible entanglement criteria. We also
address the issue of finding a finite subset of these inequalities that would
already form a necessary and sufficient condition for non-entanglement. For
mixed state we have not been able to resolve this, but for pure states a set of
six inequalities using only three sets of orthogonal observables is shown to be
already necessary and sufficient for separability.


title: Generalized Continuous Variables Quantum Key Distribution

abstract:
  This paper has been withdrawn by the author due to the incorrect argument for
the security.


title:
The Necessary and Sufficient Conditions of Separability for Multipartite
  Pure States

abstract:
  In this paper we present the necessary and sufficient conditions of
separability for multipartite pure states. These conditions are very simple,
and they don't require Schmidt decomposition or tracing out operations. We also
give a necessary condition for a local unitary equivalence class for a
bipartite system in terms of the determinant of the matrix of amplitudes and
explore a variance as a measure of entanglement for multipartite pure states.


title: A tomographic setting for quasi-distribution functions

abstract:
  The method of constructing the tomographic probability distributions
describing quantum states in parallel with density operators is presented.
Known examples of Husimi-Kano quasi-distribution and photon number tomography
are reconsidered in the new setting. New tomographic schemes based on coherent
states and nonlinear coherent states of deformed oscillators, including
q-oscillators, are suggested. The associated identity decompositions providing
Gram-Schmidt operators are explicitly given, and contact with the Agarwal-Wolf
$\Omega$-operator ordering theory is made.


title: Optimal dense coding with arbitrary pure entangled states

abstract:
  We examine dense coding with an arbitrary pure entangled state sharing
between the sender and the receiver. Upper bounds on the average success
probability in approximate dense coding and on the probability of conclusive
results in unambiguous dense coding are derived. We also construct the optimal
protocol which saturates the upper bound in each case.


title:
Bohmian Mechanics with Complex Action: A New Trajectory-Based
  Formulation of Quantum Mechanics

abstract:
  In recent years there has been a resurgence of interest in Bohmian mechanics
as a numerical tool because of its local dynamics, which suggest the
possibility of significant computational advantages for the simulation of large
quantum systems. However, closer inspection of the Bohmian formulation reveals
that the nonlocality of quantum mechanics has not disappeared -- it has simply
been swept under the rug into the quantum force. In this paper we present a new
formulation of Bohmian mechanics in which the quantum action, S, is taken to be
complex. This leads to a single equation for complex S, and ultimately complex
x and p but there is a reward for this complexification -- a significantly
higher degree of localization. The quantum force in the new approach vanishes
for Gaussian wavepacket dynamics, and its effect on barrier tunneling processes
is orders of magnitude lower than that of the classical force. We demonstrate
tunneling probabilities that are in virtually perfect agreement with the exact
quantum mechanics down to 10^{-7} calculated from strictly localized quantum
trajectories that do not communicate with their neighbors. The new formulation
may have significant implications for fundamental quantum mechanics, ranging
from the interpretation of non-locality to measures of quantum complexity.


title: Quantization of noncommutative completely integrable Hamiltonian systems

abstract:
  Integrals of motion of a Hamiltonian system need not be commutative. The
classical Mishchenko-Fomenko theorem enables one to quantize a noncommutative
completely integrable Hamiltonian system around its invariant submanifold as an
abelian completely integrable Hamiltonian system.


title: Characterization of the entanglement of two squeezed states

abstract:
  We study a continuous-variable entangled state composed of two states which
are squeezed in two opposite quadratures in phase space. Various entanglement
conditions are tested for the entangled squeezed state and we study decoherence
models for noise, producing a mixed entangled squeezed state. We briefly
describe a probabilistic protocol for entanglement swapping based on the use of
this class of entangled states and the main features of a general generation
scheme.


title: Quantum Mechanics associated with a Finite Group

abstract:
  I describe, in the simplified context of finite groups and their
representations, a mathematical model for a physical system that contains both
its quantum and classical aspects. The physically observable system is
associated with the space containing elements fxf for f an element in the
regular representation of a given finite group G. The Hermitian portion of fxf
is the Wigner distribution of f whose convolution with a test function leads to
a mathematical description of the quantum measurement process. Starting with
the Jacobi group that is formed from the semidirect product of the Heisenberg
group with its automorphism group SL(2,F{N}) for N an odd prime number I show
that the classical phase space is the first order term in a series of subspaces
of the Hermitian portion of fxf that are stable under SL(2,F{N}). I define a
derivative that is analogous to a pseudodifferential operator to enable a
treatment that parallels the continuum case. I give a new derivation of the
Schrodinger-Weil representation of the Jacobi group. Keywords: quantum
mechanics, finite group, metaplectic. PACS: 03.65.Fd; 02.10.De; 03.65.Ta.


title:
Initial problem for heat equation with multisoliton inhomogeneity and
  one-loop quantum corrections

abstract:
  The generalized zeta-function is built by a dressing method based on the
Darboux covariance of the heat equation and used to evaluate the correspondent
functional integral in quasiclassical approximation. Quantum corrections to a
kink-like solutions of Landau-Ginzburg model are calculated.


title: A condition for any realistic theory of quantum systems

abstract:
  In quantum physics, the density operator completely describes the state.
Instead, in classical physics the mean value of every physical quantity is
evaluated by means of a probability distribution. We study the possibility to
describe pure quantum states and events with classical probability
distributions and conditional probabilities and prove that the distributions
can not be quadratic functions of the quantum state. Some examples are
considered. Finally, we deal with the exponential complexity problem of quantum
physics and introduce the concept of classical dimension for a quantum system.


title: Teleportation of entangled states without Bell-state measurement

abstract:
  In a recent paper [Phys. Rev. A 70, 025803 (2004)] we presented a scheme to
teleport an entanglement of zero- and one-photon states from a bimodal cavity
to another one, with 100% success probability. Here, inspired on recent results
in the literature, we have modified our previous proposal to teleport the same
entangled state without using Bell-state measurements. For comparison, the time
spent, the fidelity, and the success probability for this teleportation are
considered.


title:
Quest for quantum superpositions of a mirror: high and moderately low
  temperatures

abstract:
  The Born-Markov master equation analysis of the vibrating mirror and photon
experiment proposed by Marshall, Simon, Penrose and Bouwmeester is completed by
including the important issues of temperature and friction. We find that at the
level of cooling available to date, visibility revivals are purely classical,
and no quantum effect can be detected by the setup, no matter how strong the
photon-mirror coupling is. Checking proposals of universal nonenvironmental
decoherence is ruled out by dominating thermal decoherence; a conjectured
coordinate-diffusion contribution to decoherence may become observable on
reaching moderately low temperatures.


title:
Semiclassical approach to Bose-Einstein condensates in a triple well
  potential

abstract:
  We present a new approach for the analysis of Bose-Einstein condensates in a
few mode approximation. This method has already been used to successfully
analyze the vibrational modes in various molecular systems and offers a new
perspective on the dynamics in many particle bosonic systems. We discuss a
system consisting of a Bose-Einstein condensate in a triple well potential.
Such systems correspond to classical Hamiltonian systems with three degrees of
freedom. The semiclassical approach allows a simple visualization of the
eigenstates of the quantum system referring to the underlying classical
dynamics. From this classification we can read off the dynamical properties of
the eigenstates such as particle exchange between the wells and entanglement
without further calculations. In addition, this approach offers new insights
into the validity of the mean-field description of the many particle system by
the Gross-Pitaevskii equation, since we make use of exactly this correspondence
in our semiclassical analysis. We choose a three mode system in order to
visualize it easily and, moreover, to have a sufficiently interesting
structure, although the method can also be extended to higher dimensional
systems.


title: Fast Light in Fully Coherent Gain Media

abstract:
  We analyze the propagation of fast-light pulses through a finite-length
resonant gain medium both analytically and numerically. We find that intrinsic
instabilities can be avoided in attaining a substantial peak advance with an
ultra-short rather than a long or adiabatic probe.


title: The Simple Criteria of SLOCC Equivalence Classes

abstract:
  We put forward an alternative approach to the SLOCC classification of
entanglement states of three-qubit and four-qubit systems. By directly solving
matrix equations, we obtain the relations satisfied by the amplitudes of
states. The relations are readily tested since in them only addition,
subtraction and multiplication occur.


title: Clifford Code Constructions of Operator Quantum Error Correcting Codes

abstract:
  Recently, operator quantum error-correcting codes have been proposed to unify
and generalize decoherence free subspaces, noiseless subsystems, and quantum
error-correcting codes. This note introduces a natural construction of such
codes in terms of Clifford codes, an elegant generalization of stabilizer codes
due to Knill. Character-theoretic methods are used to derive a simple method to
construct operator quantum error-correcting codes from any classical additive
code over a finite field.


title: Spin-Charge Separation and the Pauli Electron

abstract:
  The separation between the spin and the charge converts the quantum
mechanical Pauli Hamiltonian into the Hamiltonian of the non-Abelian
Georgi-Glashow model, notorious for its magnetic monopoles and confinement. The
independent spin and charge fluctuations both lead to the Faddeev model,
suggesting the existence of a deep duality structure and indicating that the
fundamental carriers of spin and charge are knotted solitons.


title:
On the experimental feasibility of continuous-variable optical
  entanglement distillation

abstract:
  Entanglement distillation aims at preparing highly entangled states out of a
supply of weakly entangled pairs, using local devices and classical
communication only. In this note we discuss the experimentally feasible schemes
for optical continuous-variable entanglement distillation that have been
presented in [D.E. Browne, J. Eisert, S. Scheel, and M.B. Plenio, Phys. Rev. A
67, 062320 (2003)] and [J. Eisert, D.E. Browne, S. Scheel, and M.B. Plenio,
Annals of Physics (NY) 311, 431 (2004)]. We emphasize their versatility in
particular with regards to the detection process and discuss the merits of the
two proposed detection schemes, namely photo-detection and homodyne detection,
in the light of experimental realizations of this idea becoming more and more
feasible.


title: Combinatorics and field theory

abstract:
  For any given sequence of integers there exists a quantum field theory whose
Feynman rules produce that sequence. An example is illustrated for the Stirling
numbers. The method employed here offers a new direction in combinatorics and
graph theory.


title:
Entanglement of orbital angular momentum states between an ensemble of
  cold atoms and a photon

abstract:
  Recently, atomic ensemble and single photons were successfully entangled by
using collective enhancement [D. N. Matsukevich, \textit{et al.}, Phys. Rev.
Lett. \textbf{95}, 040405(2005).], where atomic internal states and photonic
polarization states were correlated in nonlocal manner. Here we experimentally
clarified that in an ensemble of atoms and a photon system, there also exists
an entanglement concerned with spatial degrees of freedom. Generation of
higher-dimensional entanglement between remote atomic ensemble and an
application to condensed matter physics are also discussed.


title: Consistency of Local Density Matrices is QMA-complete

abstract:
  Suppose we have an n-qubit system, and we are given a collection of local
density matrices rho_1,...,rho_m, where each rho_i describes a subset C_i of
the qubits. We say that the rho_i are ``consistent'' if there exists some
global state sigma (on all n qubits) that matches each of the rho_i on the
subsets C_i. This generalizes the classical notion of the consistency of
marginal probability distributions.
  We show that deciding the consistency of local density matrices is
QMA-complete (where QMA is the quantum analogue of NP). This gives an
interesting example of a hard problem in QMA. Our proof is somewhat unusual: we
give a Turing reduction from Local Hamiltonian, using a convex optimization
algorithm by Bertsimas and Vempala, which is based on random sampling. Unlike
in the classical case, simple mapping reductions do not seem to work here.


title: Features of Moyal Trajectories

abstract:
  We study the Moyal evolution of the canonical position and momentum
variables. We compare it with the classical evolution and show that, contrary
to what is commonly found in the literature, the two dynamics do not coincide.
We prove that this divergence is quite general by studying Hamiltonians of the
form $p^2 /2m + V(q)$. Several alternative formulations of Moyal dynamics are
then suggested. We introduce the concept of starfunction and use it to
reformulate the Moyal equations in terms of a system of ordinary differential
equations on the noncommutative Moyal plane. We then use this formulation to
study the semiclassical expansion of Moyal trajectories, which is cast in terms
of a (order by order in $\hbar$) recursive hierarchy of i) first order partial
differential equations as well as ii) systems of first order ordinary
differential equations. The latter formulation is derived independently for
analytic Hamiltonians as well as for the more general case of smooth local
integrable ones. We present various examples illustrating these results.


title: A simple proof of monogamy of entanglement

abstract:
  Monogamy of entanglement means that an entangled state cannot be shared with
many parties. The more parties, the less entanglement between them. In this
paper, we give a simple proof of this property and provide an upper bound of
the number of parties.


title: The Maxwell wave function of the photon

abstract:
  James Clerk Maxwell unknowingly discovered a correct relativistic, quantum
theory for the light quantum, forty-three years before Einstein postulated the
photon's existence. In this theory, the usual Maxwell field is the quantum wave
function for a single photon. When the non-operator Maxwell field of a single
photon is second quantized, the standard Dirac theory of quantum optics is
obtained. Recently, quantum-state tomography has been applied to experimentally
determine photon wave functions.


title: Introduction to the study of entropy in Quantum Games

abstract:
  The present work is an introductory study about entropy its properties and
its role in quantum information theory. In a next work, we will use these
results to the analysis of a quantum game described by a density operator and
with its entropy equal to von Neumann's.


title: Orthopositronium: "On the possible relation of gravity to electricity"

abstract:
  The resolve of the 'orthopositronium-lifetime puzzle' needs study of the
"isotope anomaly" in gaseous neon and also of the contribution ~ 0.002 of
nonperturbative mode into orthopositronium annihilation. The Michigan results
(2003) are considered as the first supervision of relation between gravitation
and electricity. For the decision of alternative in interpretation of new and
former results it is necessary to execute the program of additional
measurements.


title: Notes on the hidden subgroup problem on some semi-direct product groups

abstract:
  We consider the hidden subgroup problem on the semi-direct product of cyclic
groups $\Z_{N}\rtimes\Z_{p}$ with some restriction on $N$ and $p$. By using the
homomorphic properties, we present a class of semi-direct product groups in
which the structures of subgroups can be easily classified. Furthermore, we
show that there exists an efficient quantum algorithm for the hidden subgroup
problem on the class.


title: The Message of the Quantum?

abstract:
  We criticize speculations to the effect that quantum mechanics is
fundamentally about information. We do this by pointing out how unfounded such
speculations in fact are. Our analysis focuses on the dubious claims of this
kind recently made by Anton Zeilinger.


title:
Quantum Measurements for Hidden Subgroup Problems with Optimal Sample
  Complexity

abstract:
  One of the central issues in the hidden subgroup problem is to bound the
sample complexity, i.e., the number of identical samples of coset states
sufficient and necessary to solve the problem. In this paper, we present
general bounds for the sample complexity of the identification and decision
versions of the hidden subgroup problem. As a consequence of the bounds, we
show that the sample complexity for both of the decision and identification
versions is $\Theta(\log|\HH|/\log p)$ for a candidate set $\HH$ of hidden
subgroups in the case that the candidate subgroups have the same prime order
$p$, which implies that the decision version is at least as hard as the
identification version in this case. In particular, it does so for the
important instances such as the dihedral and the symmetric hidden subgroup
problems. Moreover, the upper bound of the identification is attained by the
pretty good measurement. This shows that the pretty good measurement can
identify any hidden subgroup of an arbitrary group with at most $O(\log|\HH|)$
samples.


title:
Bound State Solutions of Klein-Gordon Equation with the Kratzer
  Potential

abstract:
  The relativistic problem of spinless particle subject to a Kratzer potential
is analyzed. Bound state solutions for the s-wave are found by separating the
Klein-Gordon equation in two parts, unlike the similar works in the literature,
which provides one to see explicitly the relativistic contributions, if any, to
the solution in the non-relativistic limit.


title:
Geometric approach to nonlinear coherent states using the Higgs model
  for harmonic oscillator

abstract:
  In this paper, we investigate the relation between the curvature of the
physical space and the deformation function of the deformed oscillator algebra
using non-linear coherent states approach. For this purpose, we study
two-dimensional harmonic oscillators on the flat surface and on a sphere by
applying the Higgs modell. With the use of their algebras, we show that the
two-dimensional oscillator algebra on a surface can be considered as a deformed
one-dimensional oscillator algebra where the effect of the curvature of the
surface is appeared as a deformation function. We also show that the curvature
of the physical space plays the role of deformation parameter. Then we
construct the associated coherent states on the flat surface and on a sphere
and compare their quantum statistical properties, including quadrature
squeezing and antibunching effect.


title: Strong extinction of a laser beam by a single molecule

abstract:
  We present an experiment where a single molecule strongly affects the
amplitude and phase of a laser field emerging from a subwavelength aperture. We
achieve a visibility of -6% in direct and +10% in cross-polarized detection
schemes. Our analysis shows that a close to full extinction should be possible
using near-field excitation.


title: The Born Rule in Quantum and Classical Mechanics

abstract:
  Considerable effort has been devoted to deriving the Born rule (e.g. that
$|\psi(x)|^2 dx$ is the probability of finding a system, described by $\psi$,
between $x$ and $x + dx$) in quantum mechanics. Here we show that the Born rule
is not solely quantum mechanical; rather, it arises naturally in the Hilbert
space formulation of {\it classical} mechanics as well. These results provide
new insights into the nature of the Born rule, and impact on its understanding
in the framework of quantum mechanics.


title: Quantum broadcast communication

abstract:
  Broadcast encryption allows the sender to securely distribute his/her secret
to a dynamically changing group of users over a broadcast channel. In this
paper, we just consider a simple broadcast communication task in quantum
scenario, which the central party broadcasts his secret to multi-receiver via
quantum channel. We present three quantum broadcast communication schemes. The
first scheme utilizes entanglement swapping and Greenberger-Horne-Zeilinger
state to realize a task that the central party broadcasts his secret to a group
of receivers who share a group key with him. In the second scheme, based on
dense coding, the central party broadcasts the secret to multi-receiver who
share each of their authentication key with him. The third scheme is a quantum
broadcast communication scheme with quantum encryption, which the central party
can broadcast the secret to any subset of the legal receivers.


title:
Robustness of optimal working points for non-adiabatic holonomic quantum
  computation

abstract:
  Geometric phases are an interesting resource for quantum computation, also in
view of their robustness against decoherence effects. We study here the effects
of the environment on a class of one-qubit holonomic gates that have been
recently shown to be characterized by "optimal" working times. We numerically
analyze the behavior of these optimal points and focus on their robustness
against noise.


title:
The argument for an objective wave function collapse: Why spontaneous
  localization collapse or no-collapse decoherence cannot solve the measurement
  problem in a subjective fashion

abstract:
  A more detailed analysis of the measurement problem continues to support the
position taken by Shimony and the author that collapse of the wave function
takes place in an objective manner in the rhodopsin molecule of the retina.
This casts further doubts on the theories involving a spontaneous localization
collapse process or a no-collapse decoherence process taking place in the
visual cortex in a subjective fashion. The possibility is then raised, as per
Anandan, as to whether the solution of the measurement problem in quantum
theory allows one to address the problem of quantizing gravitation.


title:
Entanglement of a Single Spin-1 Object: An Example of Ubiquitous
  Entanglement

abstract:
  Using a single spin-1 object as an example, we discuss a recent approach to
quantum entanglement. The key idea of the approach consists in presetting of
basic observables in the very definition of quantum system. Specification of
basic observables defines the dynamic symmetry of the system. Entangled states
of the system are then interpreted as states with maximal amount of uncertainty
of all basic observables. The approach gives purely physical picture of
entanglement. In particular, it separates principle physical properties of
entanglement from inessential. Within the model example under consideration, we
show relativity of entanglement with respect to dynamic symmetry and argue
existence of single-particle entanglement. A number of physical examples are
considered.


title:
The Structure of Bipartite Quantum States - Insights from Group Theory
  and Cryptography

abstract:
  This thesis presents a study of the structure of bipartite quantum states. In
the first part, the representation theory of the unitary and symmetric groups
is used to analyse the spectra of quantum states. In particular, it is shown
how to derive a one-to-one relation between the spectra of a bipartite quantum
state and its reduced states, and the Kronecker coefficients of the symmetric
group. In the second part, the focus lies on the entanglement of bipartite
quantum states. Drawing on an analogy between entanglement distillation and
secret-key agreement in classical cryptography, a new entanglement measure,
`squashed entanglement', is introduced.


title:
Classification of multipartite entanglement containing infinitely many
  kinds of states

abstract:
  We give a further investigation of the range criterion and Low-to-High Rank
Generating Mode (LHRGM) introduced in \cite{Chen}, which can be used for the
classification of $2\times{M}\times{N}$ states under reversible local filtering
operations. By using of these techniques, we entirely classify the family of
$2\times4\times4$ states, which actually contains infinitely many kinds of
states. The classifications of true entanglement of $2\times(M+3)\times(2M+3)$
and $2\times(M+4)\times(2M+4)$ systems are briefly listed respectively.


title: New Approach to Quantum Key Distribution Via Quantum Encryption

abstract:
  In this Paper, we investigate the security of Zhang, Li and Guo quantum key
distribution via quantum encryption protocol [$\text{Phys. Rev. A} \textbf{64},
24302 (2001)$] and show that it is not secure against some of Eve's attacks and
with the probability one half she gets all of keys without being detected by
the two parties. The main defect in this protocol is that there is an attack
strategy by which Eve can change the previously shared Bell state between Alice
and Bob to two Bell states among herself and Alice and Bob. Hence, we show that
with probability $1/d$ its generalization to $d$-dimension systems is not
secure and show that its extension to the case of more partners based on the
reusable GHZ states is not secure and with probability one half Eve gets all of
keys without being detected by the two parties. In what follows, we show how in
going to higher dimensions those protocols can be repaired.


title: Renormalization Group and Quantum Information

abstract:
  The renormalization group is a tool that allows one to obtain a reduced
description of systems with many degrees of freedom while preserving the
relevant features. In the case of quantum systems, in particular,
one-dimensional systems defined on a chain, an optimal formulation is given by
White's "density matrix renormalization group". This formulation can be shown
to rely on concepts of the developing theory of quantum information.
Furthermore, White's algorithm can be connected with a peculiar type of
quantization, namely, angular quantization. This type of quantization arose in
connection with quantum gravity problems, in particular, the Unruh effect in
the problem of black-hole entropy and Hawking radiation. This connection
highlights the importance of quantum system boundaries, regarding the
concentration of quantum states on them, and helps us to understand the optimal
nature of White's algorithm.


title:
Quantum key distribution based on a Sagnac loop interferometer and
  polarization-insensitive phase modulators

abstract:
  We present a design for a quantum key distribution(QKD) system in a Sagnac
loop configuration, employing a novel phase modulation scheme based on
frequency shift, and demonstrate stable BB84 QKD operation with high
interference visibility and low quantum bit error rate (QBER). The phase
modulation is achieved by sending two light pulses with a fixed time delay (or
a fixed optical path delay) through a frequency shift element and by modulating
the amount of frequency shift. The relative phase between two light pulses upon
leaving the frequency-shift element is determined by both the time delay (or
the optical path delay) and the frequency shift, and can therefore be
controlled by varying the amount of frequency shift. To demonstrate its
operation, we used an acousto-optic modulator (AOM) as the frequency-shift
element, and vary the driving frequency of the AOM to encode phase
information.The interference visibility for a 40km and a 10km fiber loop is 96%
and 99%, respectively, at single photon level. We ran BB84 protocol in a 40-km
Sagnac loop setup continuously for one hour and the measured QBER remained
within the 2%~5% range. A further advantage of our scheme is that both phase
and amplitude modulation can be achieved simultaneously by frequency and
amplitude modulation of the AOM's driving signal, allowing our QKD system the
capability of implementing other protocols, such as the decoy-state QKD and the
continuous- variable QKD. We also briefly discuss a new type of Eavesdropping
strategy ("phaseremapping" attack) in bidirectional QKD system.


title: Investigations in quantum games using EPR-type set-ups

abstract:
  Research in quantum games has flourished during recent years. However, it
seems that opinion remains divided about their true quantum character and
content. For example, one argument says that quantum games are nothing but
'disguised' classical games and that to quantize a game is equivalent to
replacing the original game by a different classical game. The present thesis
contributes towards the ongoing debate about quantum nature of quantum games by
developing two approaches addressing the related issues. Both approaches take
Einstein-Podolsky-Rosen (EPR)-type experiments as the underlying physical
set-ups to play two-player quantum games. In the first approach, the players'
strategies are unit vectors in their respective planes, with the knowledge of
coordinate axes being shared between them. Players perform measurements in an
EPR-type setting and their payoffs are defined as functions of the
correlations, i.e. without reference to classical or quantum mechanics.
Classical bimatrix games are reproduced if the input states are classical and
perfectly anti-correlated, as for a classical correlation game. However, for a
quantum correlation game, with an entangled singlet state as input,
qualitatively different solutions are obtained. The second approach uses the
result that when the predictions of a Local Hidden Variable (LHV) model are
made to violate the Bell inequalities the result is that some probability
measures assume negative values. With the requirement that classical games
result when the predictions of a LHV model do not violate the Bell
inequalities, our analysis looks at the impact which the emergence of negative
probabilities has on the solutions of two-player games which are physically
implemented using the EPR-type experiments.


title: Graphical representation of generalized quantum measurements

abstract:
  We present graphical representation for genaralized quantum measurements
(POVM). We represent POVM elements as Bloch vectors and find the conditions
these vectors should satisfy in order to describe realizable physical
measurements. We show how to find probability of measurement outcome in a
graphical way. The whole formalism is applied to unambigous discrimination of
nonorthogonal quantum states.


title:
Multigraded Poincare series for mixed states of two qubits and the
  boundary of the set of separable states

abstract:
  Let M be the set of mixed states and S the set of separable states of the
two-qubit system, and G = SU(2) x SU(2) the group of local unitary
transformations (ignoring the overall phase factor). We compute the multigraded
Poincare series for the algebra of G-invariant polynomial functions on the
affine space of all Hermitian operators of trace 1. We check that this series
is consistent with the list of invariants computed by Makhlin. By using the
recent result of Augusiak et al., we show that the boundary of S decomposes
naturally into two pieces. We prove that the part of this boundary which is
contained in the relative interior of M is a smooth manifold.


title: Probability in the Everett World: Comments on Wallace and Greaves

abstract:
  It is often objected that the Everett interpretation of QM cannot make sense
of quantum probabilities, in one or both of two ways: either it can't make
sense of probability at all, or it can't explain why probability should be
governed by the Born rule. David Deutsch has attempted to meet these
objections. He argues not only that rational decision under uncertainty makes
sense in the Everett interpretation, but also that under reasonable
assumptions, the credences of a rational agent in an Everett world should be
constrained by the Born rule. David Wallace has developed and defended
Deutsch's proposal, and greatly clarified its conceptual basis. In particular,
he has stressed its reliance on the distinguishing symmetry of the Everett
view, viz., that all possible outcomes of a quantum measurement are treated as
equally real. The argument thus tries to make a virtue of what has usually been
seen as the main obstacle to making sense of probability in the Everett world.
In this note I outline some objections to the Deutsch-Wallace argument, and to
related proposals by Hilary Greaves about the epistemology of Everettian QM.
(In the latter case, my arguments include an appeal to an Everettian analogue
of the Sleeping Beauty problem.) The common thread to these objections is that
the symmetry in question remains a very significant obstacle to making sense of
probability in the Everett interpretation.


title:
Atomic entanglement generation with reduced decoherence via four-wave
  mixing

abstract:
  In most proposals for the generation of entanglement in large ensembles of
atoms via projective measurements, the interaction with the vacuum is
responsible for both the generation of the signal that is detected and the spin
depolarization or decoherence. In consequence, one has to usually work in a
regime where the information aquisition via detection is sufficiently slow
(weak measurement regime) such as not to strongly disturb the system. We
propose here a four-wave mixing scheme where, owing to the pumping of the
atomic system into a dark state, the polarization of the ensemble is not
critically affected by spontaneous emission, thus allowing one to work in a
strong measurement regime.


title: Simulated Quantum Computation of Molecular Energies

abstract:
  The calculation time for the energy of atoms and molecules scales
exponentially with system size on a classical computer but polynomially using
quantum algorithms. We demonstrate that such algorithms can be applied to
problems of chemical interest using modest numbers of quantum bits.
Calculations of the water and lithium hydride molecular ground-state energies
have been carried out on a quantum computer simulator using a recursive
phase-estimation algorithm. The recursive algorithm reduces the number of
quantum bits required for the readout register from about 20 to 4. Mappings of
the molecular wave function to the quantum bits are described. An adiabatic
method for the preparation of a good approximate ground-state wave function is
described and demonstrated for a stretched hydrogen molecule. The number of
quantum bits required scales linearly with the number of basis functions, and
the number of gates required grows polynomially with the number of quantum
bits.


title: Expectation Value in Bell's Theorem

abstract:
  We will demonstrate in this paper that Bell's theorem (Bell's inequality)
does not really conflict with quantum mechanics, the controversy between them
originates from the different definitions for the expectation value using the
probability distribution in Bell's inequality and the expectation value in
quantum mechanics. We can not use quantum mechanical expectation value measured
in experiments to show the violation of Bell's inequality and then further deny
the local hidden-variables theory. Considering the difference of their
expectation values, a generalized Bell's inequality is presented, which is
coincided with the prediction of quantum mechanics.


title: The Unitary Transformation in Quantum Teleportation

abstract:
  In the well known treatment of quantum teleportation, the receiver should
convert the state of his EPR particle into the replica of the unknown quantum
state by one of four possible unitary transformations. However, the importance
of these unitary transformations must be emphasized. We will show in this paper
that the receiver can not transform the state of his particle into an exact
replica of the unknown state which the sender want to transfer if he have not a
proper implementation of these unitary transformations. In the procedure of
converting state, the inevitable coupling between EPR particle and environment
which is needed by the implementation of unitary transformations will reduce
the accuracy of the replica.


title: The Role of Detector in Which-Way Experiment

abstract: I will replace this manuscript by a new paper


title: Momentum Dynamics of One Dimensional Quantum Walks

abstract:
  We derive the momentum space dynamic equations and state functions for one
dimensional quantum walks by using linear systems and Lie group theory. The
momentum space provides an analytic capability similar to that contributed by
the z transform in discrete systems theory. The state functions at each time
step are expressed as a simple sum of three Chebyshev polynomials. The
functions provide an analytic expression for the development of the walks with
time.


title: Discrete Rotational Symmetry and Quantum Key Distribution Protocols

abstract:
  We study the role of discrete rotational symmetry in quantum key distribution
by generalizing the well-known Bennett-Brassard 1984 (BB84) and
Scarani-Acin-Ribordy-Gisin 2004 (SARG04) protocols. We observe that discrete
rotational symmetry results in the protocol's invariance to continuous
rotations, thus leading to a simplified relation between bit and phase error
rates and consequently a straightforward security proof.


title: Energy Levels of "Hydrogen Atom" in Discrete Time Dynamics

abstract:
  We analyze dynamical consequences of a conjecture that there exists a
fundamental (indivisible) quant of time. In particular we study the problem of
discrete energy levels of hydrogen atom. We are able to reconstruct potential
which in discrete time formalism leads to energy levels of unperturbed hydrogen
atom. We also consider linear energy levels of quantum harmonic oscillator and
show how they are produced in the discrete time formalism. More generally, we
show that in discrete time formalism finite motion in central potential leads
to discrete energy spectrum, the property which is common for quantum
mechanical theory. Thus deterministic (but discrete time!) dynamics is
compatible with discrete energy levels.


title:
Exact solutions for vibrational levels of the Morse potential via the
  asymptotic iteration method

abstract:
  Exact solutions for vibrational levels of diatomic molecules via the Morse
potential are obtained by means of the asymptotic iteration method. It is shown
that, the numerical results for the energy eigenvalues of $^{7}Li_{2}$ are all
in excellent agreement with the ones obtained before. Without any loss of
generality, other states and molecules could be treated in a similar way.


title: Partial Observers

abstract:
  We attempt to dissolve the measurement problem using an anthropic principle
which allows us to invoke rational observers. We argue that the key feature of
such observers is that they are rational (we need not care whether they are
`classical' or `macroscopic' for example) and thus, since quantum theory can be
expressed as a rational theory of probabilistic inference, the measurement
problem is not a problem.


title: Unitary invariants of qubit systems

abstract:
  We give an algorithm allowing to construct bases of local unitary invariants
of pure k-qubit states from the knowledge of polynomial covariants of the group
of invertible local filtering operations. The simplest invariants obtained in
this way are explicited and compared to various known entanglement measures.
Complete sets of generators are obtained for up to four qubits, and the
structure of the invariant algebras is discussed in detail.


title: Principles of Control for Decoherence-Free Subsystems

abstract:
  Decoherence-Free Subsystems (DFS) are a powerful means of protecting quantum
information against noise with known symmetry properties. Although Hamiltonians
theoretically exist that can implement a universal set of logic gates on DFS
encoded qubits without ever leaving the protected subsystem, the natural
Hamiltonians that are available in specific implementations do not necessarily
have this property. Here we describe some of the principles that can be used in
such cases to operate on encoded qubits without losing the protection offered
by the DFS. In particular, we show how dynamical decoupling can be used to
control decoherence during the unavoidable excursions outside of the DFS. By
means of cumulant expansions, we show how the fidelity of quantum gates
implemented by this method on a simple two-physical-qubit DFS depends on the
correlation time of the noise responsible for decoherence. We further show by
means of numerical simulations how our previously introduced "strongly
modulating pulses" for NMR quantum information processing can permit
high-fidelity operations on multiple DFS encoded qubits in practice, provided
that the rate at which the system can be modulated is fast compared to the
correlation time of the noise. The principles thereby illustrated are expected
to be broadly applicable to many implementations of quantum information
processors based on DFS encoded qubits.


title:
Repeat-Until-Success Generation of Symmetric States without External
  Control

abstract:
  In the present paper, we propose a "repeat-until-success" scheme induced by
single particle measurement to generate arbitrary symmetric states based on
spin network. This protocol requires no modulated controls during the whole
process and it provides a persistent approach towards the desired symmetric
state. As a special case, we demonstrate that W state can be created with unit
probability within this framework.


title:
Analytical solutions for the dynamics of two trapped interacting
  ultracold atoms

abstract:
  We discuss exact solutions of the Schroedinger equation for the system of two
ultracold atoms confined in an axially symmetric harmonic potential. We
investigate different geometries of the trapping potential, in particular we
study the properties of eigenenergies and eigenfunctions for quasi-one- and
quasi-two-dimensional traps. We show that the quasi-one- and the
quasi-two-dimensional regimes for two atoms can be already realized in the
traps with moderately large (or small) ratios of the trapping frequencies in
the axial and the transverse directions. Finally, we apply our theory to
Feshbach resonances for trapped atoms. Introducing in our description an
energy-dependent scattering length we calculate analytically the eigenenergies
for two trapped atoms in the presence of a Feshbach resonance.


title:
Another Short and Elementary Proof of Strong Subadditivity of Quantum
  Entropy

abstract:
  A short and elementary proof of the joint convexity of relative entropy is
presented, using nothing beyond linear algebra. The key ingredients are an
easily verified integral representation and the strategy used to prove the
  Cauchy-Schwarz inequality in elementary courses. Several consequences are
proved in a way which allow an elementary proof of strong subadditivity in a
few more lines. Some expository material on Schwarz inequalities for operators
and the Holevo bound for partial measurements is also included.


title: Quantum mutual information and the one-time pad

abstract:
  Alice and Bob share a correlated composite quantum system AB. If AB is used
as the key for a one-time pad cryptographic system, we show that the maximum
amount of information that Alice can send securely to Bob is the quantum mutual
information of AB.


title: The characteristic function of optical evolution

abstract:
  The master equation of quantum optical density operator is transformed to the
equation of characteristic function. The parametric amplification and amplitude
damping as well as the phase damping are considered. The solution for the most
general initial quantum state is obtained for parametric amplification and
amplitude damping. The purity of one mode Gaussian system and the entanglement
of two mode Gaussian system are studied.


title:
Detecting a set of entanglement measures in an unknown tripartite
  quantum state by local operations and classical communication

abstract:
  We propose a more general method for detecting a set of entanglement
measures, i.e. negativities, in an \emph{arbitrary} tripartite quantum state by
local operations and classical communication. To accomplish the detection task
using this method, three observers, Alice, Bob and Charlie, do not need to
perform the partial transposition maps by the structural physical
approximation; instead, they are only required to collectively measure some
functions via three local networks supplemented by a classical communication.
With these functions, they are able to determine the set of negativities
related to the tripartite quantum state.


title: Representation of Qudits on a Riemann Sphere

abstract:
  In quantum computation and information science, the geometrical
representations based on the Bloch sphere representation for transformations of
two state systems have been traditionally used. While this representation is
very useful for the two state qubit, it cannot be generalized easily to
multiple states like that of qudits and when it is generalized, it looses its
simple geometrical representation.
  This paper proposes the use of an alternative representation in quantum
information and computation for qubits as well as qudits based on the Majorana
representation on the Riemann Sphere, which preserves the simple sphere
representation.


title: Experimental Quantum Teleportation with a 3-Bell-state Analyzer

abstract:
  We present a Bell-state analyzer for time-bin qubits allowing the detection
of three out of four Bell-states with linear optics, two detectors and no
auxiliary photons. The theoretical success rate of this scheme is 50%. A
teleportation experiment was performed to demonstrate its functionality. We
also present a teleportation experiment with a Fidelity larger than the cloning
limit of F=5/6.


title: Purity of states in the theory of open quantum systems

abstract:
  The condition of purity of states for a damped harmonic oscillator is
considered in the framework of Lindblad theory for open quantum systems. For a
special choice of the environment coefficients, the correlated coherent states
are shown to be the only states which remain pure all the time during the
evolution of the considered system. These states are also the most stable under
evolution in the presence of the environment.


title:
Photon Distribution Function for Long-Distance Propagation of Partially
  Coherent Beams through the Turbulent Atmosphere

abstract:
  The photon density operator function is used to calculate light beam
propagation through turbulent atmosphere. A kinetic equation for the photon
distribution function is derived and solved using the method of
characteristics. Optical wave correlations are described in terms of photon
trajectories that depend on fluctuations of the refractive index. It is shown
that both linear and quadratic disturbances produce sizable effects for
long-distance propagation. The quadratic terms are shown to suppress the
correlation of waves with different wave vectors. We examine the intensity
fluctuations of partially coherent beams (beams whose initial spatial coherence
is partially destroyed). Our calculations show that it is possible to
significantly reduce the intensity fluctuations by using a partially coherent
beam. The physical mechanism responsible for this pronounced reduction is
similar to that of the Hanbury-Braun, Twiss effect.


title: Wave-Particle Fluctuations, Coherence, and Bose-Einstein Condensation

abstract:
  By extending Einstein's separation of wave and particle parts of the second
order thermal fluctuation to encompass "generalized fluctuations" in any Bose
field, P. E. Gordon has proposed alternative definitions for nth order
coherence and nth order coherent states. The main point of this paper is to
explore some of the physical insights to be gained by extending dualism to
higher orders. Recent experiments have examined aspects of the coherence of
Bose-Einstein condensates. It has been argued that the condensate state is
coherent to (at least) second or third order, but the coherence properties of
Bose-Einstein condensates remain somewhat controversial. Using probability
distributions developed by M. O. Scully and V. V. Kocharovsky et. al., we apply
Gordon's dualistic expression of the coherence conditions to investigate
coherence properties in Bose-Einstein condensation. Via numerical calculations,
we present a graphical survey of wave-like and particle-like fluctuations in
condensed and uncondensed fractions. Near the critical point, we find a very
marked peak in the ratio of nth order wave to nth order particle fluctuations
in the condensate. Not surprisingly, n-point correlations between the positions
of condensate atoms also peak near the critical temperature, and this
apparently mirrors, to higher orders, the well-known relation between the
integral of the 2-point correlation function over a certain volume and the rms
fluctuation in the number of particles in that volume.


title: Long-distance entanglement in spin systems

abstract:
  Most quantum system with short-ranged interactions show a fast decay of
entanglement with the distance. In this Letter, we focus on the peculiarity of
some systems to distribute entanglement between distant parties. Even in
realistic models, like the spin-1 Heisenberg chain, sizable entanglement is
present between arbitrarily distant particles. We show that long distance
entanglement appears for values of the microscopic parameters which do not
coincide with known quantum critical points, hence signaling a transition
detected only by genuine quantum correlations.


title: Minimal Assumption Derivation of a weak Clauser-Horne Inequality

abstract:
  According to Bell's theorem a large class of hidden-variable models obeying
Bell's notion of local causality conflict with the predictions of quantum
mechanics. Recently, a Bell-type theorem has been proven using a weaker notion
of local causality, yet assuming the existence of perfectly correlated event
types. Here we present a similar Bell-type theorem without this latter
assumption. The derived inequality differs from the Clauser-Horne inequality by
some small correction terms, which render it less constraining.


title: Inseparability criteria based on matrices of moments

abstract:
  Inseparability criteria for continuous and discrete bipartite quantum states
based on moments of annihilation and creation operators are studied by
developing the idea of Shchukin-Vogel criterion [Phys. Rev. Lett. {\bf 95},
230502 (2005)]. If a state is separable, then the corresponding matrix of
moments is separable too. Thus, we derive generalized criteria, based on the
separability properties of the matrix of moments, are thus derived. In
particular, a new criterion based on realignment of moments in the matrix is
proposed as an analogue of the standard realignment criterion for density
matrices. Other inseparability inequalities are obtained by applying positive
maps to the matrix of moments. Usefulness of the Shchukin-Vogel criterion to
describe bipartite-entanglement of more than two modes is demonstrated: We
obtain some previously known three-mode inseparability criteria originally
derived from the Cauchy-Schwarz inequality, and we introduce new ones.


title:
Towards Heisenberg Limit in Magnetometry with Parametric Down Converted
  Photons

abstract:
  Recent theoretical and experimental papers have shown how one can achieve
Heisenberg limited measurements by using entangled photons. Here we show how
the photons in non-collinear down conversion process can be used for improving
the sensitivity of magneto-optical rotation by a factor of four which takes us
towards the Heisenberg limit. Our results apply to sources with arbitrary
pumping. We also present several generalizations of earlier results for the
collinear geometry. The sensitivity depends on whether the two-photon or
four-photon coincidence detection is used.


title: Solving mathematical problems with quantum search algorithm

abstract:
  Quantum computation has attracted much attention since it was shown by Shor
and Grover the possibility to implement quantum algorithms able to realize,
respectively, factoring and searching in a faster way than any other known
classical algorithm. It is possible to use Grover algorithm, taking profit of
its ability to find a specific value in a unordered database, to find, for
example, the zero of a logical function; the minimal or maximal value in a
database or to recognize if an odd number is prime or not. Here we show quantum
algorithms to solve those cited mathematical problems. The solution requires
the use of a quantum bit string comparator being used as oracle. This quantum
circuit compares two quantum states and identifies if they are equal or,
otherwise, which of them is the largest. Moreover, we also show the quantum bit
string comparator allow us to implement conditional statements in quantum
computation, a fundamental structure for designing of algorithms.


title: Topological Quantum Computing and the Jones Polynomial

abstract:
  In this paper, we give a description of a recent quantum algorithm created by
Aharonov, Jones, and Landau for approximating the values of the Jones
polynomial at roots of unity of the form exp(2$\pi$i/k). This description is
given with two objectives in mind. The first is to describe the algorithm in
such a way as to make explicit the underlying and inherent control structure.
The second is to make this algorithm accessible to a larger audience.


title: Thermal corrections to the Casimir effect

abstract:
  The Casimir effect, reflecting quantum vacuum fluctuations in the
electromagnetic field in a region with material boundaries, has been studied
both theoretically and experimentally since 1948. The forces between dielectric
and metallic surfaces both plane and curved have been measured at the 10 to 1
percent level in a variety of room-temperature experiments, and remarkable
agreement with the zero-temperature theory has been achieved. In fitting the
data various corrections due to surface roughness, patch potentials, curvature,
and temperature have been incorporated. It is the latter that is the subject of
the present article. We point out that, in fact, no temperature dependence has
yet been detected, and that the experimental situation is still too fluid to
permit conclusions about thermal corrections to the Casimir effect.
Theoretically, there are subtle issues concerning thermodynamics and
electrodynamics which have resulted in disparate predictions concerning the
nature of these corrections. However, a general consensus has seemed to emerge
that suggests that the temperature correction to the Casimir effect is
relatively large, and should be observable in future experiments involving
surfaces separated at the few micrometer scale.


title:
Multiparty simultaneous quantum identity authentication based on
  entanglement swapping

abstract:
  We present a multiparty simultaneous quantum identity authentication protocol
based on entanglement swapping. In our protocol, the multi-user can be
authenticated by a trusted third party simultaneously.


title:
Orthogonal polynomial solutions to the non-central modified Kratzer
  potential

abstract:
  We investigate the analytical solution of a new exactly solvable non-central
potential of $V(r,\theta) = D({\frac{r - a}{r}})^2+{\frac{\beta}{r^2\sin^2
\theta}}+{\frac{\gamma \cos \theta}{r^2\sin^2 \theta}}$ type, which may be
called as the modified non-central Kratzer potential. The energy eigenvalues as
well as the corresponding eigenfunctions are calculated for various values of
$n$ and $m$ quantum numbers within the framework of the Nikiforov-Uvarov and
Asymtotic Iteration Methods for the $CO$ diatomic molecule as an application of
this potential. In this paper, we first present the effect of the non-central
term on the bound-state energy eigenvalues: this effect is determined
explicitly for different $n$ and $m$ quantum numbers with $\beta=\gamma$=0.0,
0.1, 1.0 and 5.0 values and the results are compared with the findings of the
modified Kratzer potential for different $n$ and $l$ quantum numbers. Then, we
show that the angle-dependent non-central part behaves like a centrifugal
barrier and it reduces the depth of the attractive potential pocket, which
effects the bound-state energy eigenvalues.


title: A Bell Inequality Analog in Quantum Measure Theory

abstract:
  One obtains Bell's inequalities if one posits a hypothetical joint
probability distribution, or {\it measure}, whose marginals yield the
probabilities produced by the spin measurements in question. The existence of a
joint measure is in turn equivalent to a certain causality condition known as
``screening off''. We show that if one assumes, more generally, a joint {\it
quantal measure}, or ``decoherence functional'', one obtains instead an
analogous inequality weaker by a factor of $\sqrt{2}$. The proof of this
``Tsirel'son inequality'' is geometrical and rests on the possibility of
associating a Hilbert space to any strongly positive quantal measure. These
results lead both to a {\it question}: ``Does a joint measure follow from some
quantal analog of `screening off'?'', and to the {\it observation} that
non-contextual hidden variables are viable in histories-based quantum
mechanics, even if they are excluded classically.


title:
The Information-Disturbance Tradeoff and the Continuity of Stinespring's
  Representation

abstract:
  Stinespring's dilation theorem is the basic structure theorem for quantum
channels: it states that any quantum channel arises from a unitary evolution on
a larger system. Here we prove a continuity theorem for Stinespring's dilation:
if two quantum channels are close in cb-norm, then it is always possible to
find unitary implementations which are close in operator norm, with
dimension-independent bounds. This result generalizes Uhlmann's theorem from
states to channels and allows to derive a formulation of the
information-disturbance tradeoff in terms of quantum channels, as well as a
continuity estimate for the no-broadcasting theorem. We briefly discuss further
implications for quantum cryptography, thermalization processes, and the black
hole information loss puzzle.


title: Quantum teleportation over the Swisscom telecommunication network

abstract:
  We present a quantum teleportation experiment in the quantum relay
configuration using the installed telecommunication network of Swisscom. In
this experiment, the Bell state measurement occurs well after the entanglement
has been distributed, at a point where the photon upon which data is teleported
is already far away, and the entangled qubits are photons created from a
different crystal and laser pulse than the teleported qubit. A raw fidelity of
0.93+/-0.04 has been achieved using a heralded single-photon source.


title:
Coupled wave-particle dynamics as a possible ontology behind Quantum
  Mechanics and long-range interactions

abstract:
  The preliminary ideas presented in this paper have been fully developed in
arXiv:0902.4606v1 .


title:
Information hiding and retrieval in Rydberg wave packets using
  half-cycle pulses

abstract:
  We demonstrate an information hiding and retrieval scheme with the relative
phases between states in a Rydberg wave packet acting as the bits of a data
register. We use a terahertz half-cycle pulse (HCP) to transfer phase-encoded
information from an optically accessible angular momentum manifold to another
manifold which is not directly accessed by our laser pulses, effectively hiding
the information from our optical interferometric measurement techniques. A
subsequent HCP acting on these wave packets reintroduces the information back
into the optically accessible data register manifold which can then be `read'
out.


title: On Decoherence in Quantum Clock Synchronization

abstract:
  We study two quantum versions of the Eddington clock-synchronization protocol
in the presence of decoherence. The first protocol uses maximally entangled
states to achieve the Heisenberg limit for clock synchronization. The second
protocol achieves the limit without using entanglement. We show the equivalence
of the two protocols under any single-qubit decoherence model that does not
itself provide synchronization information.


title:
Potential and limits to cluster state quantum computing using
  probabilistic gates

abstract:
  We establish bounds to the necessary resource consumption when building up
cluster states for one-way computing using probabilistic gates. Emphasis is put
on state preparation with linear optical gates, as the probabilistic character
is unavoidable here. We identify rigorous general bounds to the necessary
consumption of initially available maximally entangled pairs when building up
one-dimensional cluster states with individually acting linear optical quantum
gates, entangled pairs and vacuum modes. As the known linear optics gates have
a limited maximum success probability, as we show, this amounts to finding the
optimal classical strategy of fusing pieces of linear cluster states. A formal
notion of classical configurations and strategies is introduced for
probabilistic non-faulty gates. We study the asymptotic performance of
strategies that can be simply described, and prove ultimate bounds to the
performance of the globally optimal strategy. The arguments employ methods of
random walks and convex optimization. This optimal strategy is also the one
that requires the shortest storage time, and necessitates the fewest
invocations of probabilistic gates. For two-dimensional cluster states, we
find, for any elementary success probability, an essentially deterministic
preparation of a cluster state with quadratic, hence optimal, asymptotic
scaling in the use of entangled pairs. We also identify a percolation effect in
state preparation, in that from a threshold probability on, almost all
preparations will be either successful or fail. We outline the implications on
linear optical architectures and fault-tolerant computations.


title: Applications of Feedback Control in Quantum Systems

abstract:
  We give an introduction to feedback control in quantum systems, as well as an
overview of the variety of applications which have been explored to date. This
introductory review is aimed primarily at control theorists unfamiliar with
quantum mechanics, but should also be useful to quantum physicists interested
in applications of feedback control. We explain how feedback in quantum systems
differs from that in traditional classical systems, and how in certain cases
the results from modern optimal control theory can be applied directly to
quantum systems. In addition to noise reduction and stabilization, an important
application of feedback in quantum systems is adaptive measurement, and we
discuss the various applications of adaptive measurements. We finish by
describing specific examples of the application of feedback control to cooling
and state-preparation in nano-electro-mechanical systems and single trapped
atoms.


title: Quantum manipulation and simulation using Josephson junction arrays

abstract:
  We discuss the prospect of using quantum properties of large scale Josephson
junction arrays for quantum manipulation and simulation. We study the
collective vibrational quantum modes of a Josephson junction array and show
that they provide a natural and practical method for realizing a high quality
cavity for superconducting qubit based QED. We further demonstrate that by
using Josephson junction arrays we can simulate a family of problems concerning
spinless electron-phonon and electron-electron interactions. These protocols
require no or few controls over the Josephson junction array and are thus
relatively easy to realize given currently available technology.


title:
Nonlinear coupling of nano mechanical resonators to Josephson quantum
  circuits

abstract:
  We propose a technique to couple the position operator of a nano mechanical
resonator to a SQUID device by modulating its magnetic flux bias. By tuning the
magnetic field properly, either linear or quadratic couplings can be realized,
with a discretely adjustable coupling strength. This provides a way to realize
coherent nonlinear effects in a nano mechanical resonator by coupling it to a
Josephson quantum circuit. As an example, we show how squeezing of the nano
mechanical resonator state can be realized with this technique. We also propose
a simple method to measure the uncertainty in the position of the nano
mechanical resonator without quantum state tomography.


title:
Poincare series for local unitary invariants of mixed states of the
  qubit-qutrit system

abstract:
  We consider the mixed states of the bipartite quantum system with the first
party a qubit and the second a qutrit. The group of local unitary
transformations of the system, ignoring the overall phase factor, is the direct
product G of SU(2) and SU(3). We compute the simply graded Poincare series of
the algebra of G-invariant polynomial functions on the set of mixed states of
the system, and construct several low degree invariants.


title: Nature of quantum recurrences in coupled higher dimensional systems

abstract:
  We investigate recurrence phenomena in coupled two degrees of freedom
systems. It is shown that an initial well localized wave packet displays
recurrences even in the presence of coupling in these systems. We discuss the
interdependence of these time scales namely, classical period and quantum
revival time, and explain significance of initial conditions.


title: The Super-Strong Coupling Regime of Cavity Quantum Electrodynamics

abstract:
  We describe a qualitatively new regime of cavity quantum electrodynamics, the
super strong coupling regime. This regime is characterized by atom-field
coupling strengths of the order of the free spectral range of the cavity,
resulting in a significant change in the spatial mode functions of the light
field. It can be reached in practice for cold atoms trapped in an optical
dipole potential inside the resonator. We present a nonperturbative scheme that
allows us to calculate the frequencies and linewidths of the modified field
modes, thereby providing a good starting point for a quantization of the
theory.


title: Monogamy inequality for distributed Gaussian entanglement

abstract:
  We show that for all n-mode Gaussian states of continuous variable systems,
the entanglement shared among n parties exhibits the fundamental monogamy
property. The monogamy inequality is proven by introducing the Gaussian tangle,
an entanglement monotone under Gaussian local operations and classical
communication, which is defined in terms of the squared negativity in complete
analogy with the case of n-qubit systems. Our results elucidate the structure
of quantum correlations in many-body harmonic lattice systems.

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


title: Quantum Trajectory method for the Quantum Zeno and anti-Zeno effects

abstract:
  We perform stochastic simulations of the quantum Zeno and anti-Zeno effects
for two level system and for the decaying one. Instead of simple projection
postulate approach, a more realistic model of a detector interacting with the
environment is used. The influence of the environment is taken into account
using the quantum trajectory method. The simulation of the measurement for a
single system exhibits the probabilistic behavior showing the collapse of the
wave-packet. When a large ensemble is analysed using the quantum trajectory
method, the results are the same as those produced using the density matrix
method. The results of numerical calculations are compared with the analytical
expressions for the decay rate of the measured system and a good agreement is
found. Since the analytical expressions depend on the duration of the
measurement only, the agreement with the numerical calculations shows that
otherparameters of the model are not important.


title: Possible approach to improve sensitivity of a Michelson interferometer

abstract:
  We propose a possible approach to achieve an 1/N sensitivity of Michelson
interferometer by using a properly designed random phase modulation. Different
from other approaches, the sensitivity improvement does not depend on
increasing optical powers or utilizing the quantum properties of light.
Moreover the requirements for optical losses and the quantum efficiencies of
photodetection systems might be lower than the quantum approaches and the
sensitivity improvement is frequency independent in all detection band.


title:
Entanglement-assisted Enhanced Information Transmission Over a Quantum
  Channel with Correlated Noise; A General Expression

abstract:
  Entanglement and entanglement-assisted are useful resources to enhance the
mutual information of the Pauli channels, when the noise on consecutive uses of
the channel has some partial correlations. In this paper, We study quantum
communication channels with correlated noise and derive a general expression
for the mutual information of quantum channel, for the product, maximally
entangled state coding and entanglement-assisted systems with correlated noise
in the Pauli quantum channels. Hence, we suggest more efficient coding in the
entanglement-assisted systems for the transmission of classical information and
derive a general expression for the entanglement-assisted classical capacity.
Our results show that in the presence of memory, a higher amount of classical
information is transmitted by two or four consecutive uses of
entanglement-assisted systems.


title: Remarks on quantization of Pais-Uhlenbeck oscillators

abstract:
  This work is concerned with a quantization of the Pais-Uhlenbeck oscillators
from the point of view of their multi-Hamiltonian structures. It is shown that
the 2n-th order oscillator with a simple spectrum is equivalent to the usual
anisotropic n - dimensional oscillator.


title: Classical and quantum q-deformed physical systems

abstract:
  On the basis of the non-commutative q-calculus, we investigate a
q-deformation of the classical Poisson bracket in order to formulate a
generalized q-deformed dynamics in the classical regime. The obtained
q-deformed Poisson bracket appears invariant under the action of the
q-symplectic group of transformations. In this framework we introduce the
q-deformed Hamilton's equations and we derive the evolution equation for some
simple q-deformed mechanical systems governed by a scalar potential dependent
only on the coordinate variable. It appears that the q-deformed Hamiltonian,
which is the generator of the equation of motion, is generally not conserved in
time but, in correspondence, a new constant of motion is generated. Finally, by
following the standard canonical quantization rule, we compare the well known
q-deformed Heisenberg algebra with the algebra generated by the q-deformed
Poisson bracket.


abstract:
  When submitting ``Coin-Flipping-based Quantum Oblivious Transfer''
(quant-ph/0605027v4) to Indocrypt-2006, I received valuable reviews. Due to the
attacks in these reviews, my major protocols, for cheat-sensitive and
coin-flipping-based 2-1 oblivious transfers, are insecure. I would have
withdrawn the paper. But I think I'd rather post the attacks made by the
reviewer. Besides, as ``coin-flipping-based 2-1 OT'' is an important case-study
in ``Two-party Models and the No-go Theorems'' (quant-ph/0608165), the paper is
also touched by the attack. It will be reconsidered, and the case-study will be
eliminated from it. I would like to thank the ``anonymous'' reviewer for those
valuable attacks and comments.


title: Correspondence of Bell State and One-Particle State Transformations

abstract:
  There is a direct correspondence between two-particle, entangled quantum
states, for example, Bell states, and the relative values of the component
one-particle states. This leads to a new rationale for quantum computing which
makes use of sequential processing of one-particle states rather than the
parallel processing associated with multiparticle states. It is shown that
deterministic transformations can correspond to certain Bell state operations.
There are some implications for the continuing discussion of quantum realism
and entanglement. A principle of relative realism is advocated.


title:
Field fluctuations near a conducting plate and Casimir-Polder forces in
  the presence of boundary conditions

abstract:
  We consider vacuum fluctuations of the quantum electromagnetic field in the
presence of an infinite and perfectly conducting plate. We evaluate how the
change of vacuum fluctuations due to the plate modifies the Casimir-Polder
potential between two atoms placed near the plate. We use two different methods
to evaluate the Casimir-Polder potential in the presence of the plate. They
also give new insights on the role of boundary conditions in the Casimir-Polder
interatomic potential, as well as indications for possible generalizations to
more complicated boundary conditions.