title: Generation of maximally entangled states of qudits using twin photons

abstract:
  We report an experiment to generate maximally entangled states of
D-dimensional quantum systems, qudits, by using transverse spatial correlations
of two parametric down-converted photons. Apertures with D-slits in the arms of
the twin fotons define the qudit space. By manipulating the pump beam correctly
the twin photons will pass only by symmetrically opposite slits, generating
entangled states between these differents paths. Experimental results for
qudits with D=4 and D=8 are shown. We demonstrate that the generated states are
entangled states.


title:
Pulse retrieval and soliton formation in a non-standard scheme for
  dynamic electromagnetically induced transparency

abstract:
  We examine in detail an alternative method of retrieving the information
written into an atomic ensemble of three-level atoms using electromagnetically
induced transparency. We find that the behavior of the retrieved pulse is
strongly influenced by the relative collective atom-light coupling strengths of
the two relevant transitions. When the collective atom-light coupling strength
for the retrieval beam is the stronger of the two transitions, regeneration of
the stored pulse is possible. Otherwise, we show the retrieval process can lead
to creation of soliton-like pulses.


title:
Coherent states of P{\"o}schl-Teller potential and their revival
  dynamics

abstract:
  A recently developed algebraic approach for constructing coherent states for
solvable potentials is used to obtain the displacement operator coherent state
of the P\"{o}schl-Teller potential. We establish the connection between this
and the annihilation operator coherent state and compare their properties. We
study the details of the revival structure arising from different time scales
underlying the quadratic energy spectrum of this system.


title: Scaling of non-Markovian Monte Carlo wave-function methods

abstract:
  We demonstrate a scaling method for non-Markovian Monte Carlo wave-function
simulations used to study open quantum systems weakly coupled to their
environments. We derive a scaling equation, from which the result for the
expectation values of arbitrary operators of interest can be calculated, all
the quantities in the equation being easily obtainable from the scaled Monte
Carlo simulations. In the optimal case, the scaling method can be used, within
the weak coupling approximation, to reduce the size of the generated Monte
Carlo ensemble by several orders of magnitude. Thus, the developed method
allows faster simulations and makes it possible to solve the dynamics of the
certain class of non-Markovian systems whose simulation would be otherwise too
tedious because of the requirement for large computational resources.


title: Conditions for optimal construction of two-qubit non-local gates

abstract:
  Optimal implementation of quantum gates is crucial for designing a quantum
computer. The necessary condition for optimal construction of a two-qubit
unitary operation is obtained. It can be proved that the B gate is the unique
gate that can construct a two-qubit universal circuit with only two
applications, i.e. this condition is also sufficient in the case of two
applications of the elementary two-qubit gate. It is also shown that one half
of perfect entanglers can not simulate an arbitrary two-qubit gate with only 3
applications.


title: An atom interferometer enabled by spontaneous decay

abstract:
  We investigate the question whether Michelson type interferometry is possible
if the role of the beam splitter is played by a spontaneous process. This
question arises from an inspection of trajectories of atoms bouncing
inelastically from an evanescent-wave (EW) mirror. Each final velocity can be
reached via two possible paths, with a {\it spontaneous} Raman transition
occurring either during the ingoing or the outgoing part of the trajectory. At
first sight, one might expect that the spontaneous character of the Raman
transfer would destroy the coherence and thus the interference. We investigated
this problem by numerically solving the Schr\"odinger equation and applying a
Monte-Carlo wave-function approach. We find interference fringes in velocity
space, even when random photon recoils are taken into account.


title: Proposed Differential Equation for Spin 1/2

abstract:
  Intimate though the connection is between orbital angular momentum and
intrinsic spin, there are differences between them the two quantities. One of
them is the fact that while orbital angular momentum has a wave-mechanics
description by courtesy of Legendre's equation, spin lacks one. In this paper,
we show that there exists a differential eigenvalue equation whose solutions
are the spin-1/2 eigenvectors.


title: The Interpretation of Quantum Mechanics

abstract:
  In this paper, we demonstrate how the interpretation of quantum mechanics due
to Land\'e resolves the Schr\"odinger cat paradox and disposes of the problem
of wave function collapse.


title: Submicrometer position control of single trapped neutral atoms

abstract:
  We optically detect the positions of single neutral cesium atoms stored in a
standing wave dipole trap with a sub-wavelength resolution of 143 nm rms. The
distance between two simultaneously trapped atoms is measured with an even
higher precision of 36 nm rms. We resolve the discreteness of the interatomic
distances due to the 532 nm spatial period of the standing wave potential and
infer the exact number of trapping potential wells separating the atoms.
Finally, combining an initial position detection with a controlled transport,
we place single atoms at a predetermined position along the trap axis to within
300 nm rms.


title:
Continuous measurement of canonical observables and limit Stochastic
  Schrodinger equations

abstract:
  We derive the stochastic Schrodinger equation for the limit of continuous
weak measurement where the observables monitored are canonical position and
momentum. To this end we extend an argument due to Smolianov and Truman from
the von Neumann model of indirect measurement of position to the Arthurs and
Kelly model for simultaneous measurement of position and momentum. We only
require unbiasedness of the detector states and an integrability condition
sufficient to ensure a central limit effect. Despite taking a weak interaction,
as opposed to weak measurement limit, the resulting stochastic wave equation is
of the same form as that derived in a recent paper by Scott and Milburn for the
specific case of joint Gaussian states.


title: Feynman Diagrams and the Quantum Stochastic Calculus

abstract:
  We present quantum stochastic calculus in terms of diagrams taking weights in
the algebra of observables of some quantum system. In particular, we note the
absence of non-time-consecutive Goldstien diagrams. We review recent results in
Markovian limits in these terms.


title: Quantum Walk on a Line with Two Entangled Particles

abstract:
  We introduce the concept of a quantum walk with two particles and study it
for the case of a discrete time walk on a line. A quantum walk with more than
one particle may contain entanglement, thus offering a resource unavailable in
the classical scenario and which can present interesting advantages. In this
work, we show how the entanglement and the relative phase between the states
describing the coin degree of freedom of each particle will influence the
evolution of the quantum walk. In particular, the probability to find at least
one particle in a certain position after $N$ steps of the walk, as well as the
average distance between the two particles, can be larger or smaller than the
case of two unentangled particles, depending on the initial conditions we
choose. This resource can then be tuned according to our needs, in particular
to enhance a given application (algorithmic or other) based on a quantum walk.
Experimental implementations are briefly discussed.


title:
Tight multipartite Bell's inequalities involving many measurement
  settings

abstract:
  We derive tight Bell's inequalities for N>2 observers involving more than two
alternative measurement settings. We give a necessary and sufficient condition
for a general quantum state to violate the new inequalities. The inequalities
are violated by some classes of states, for which all standard Bell's
inequalities with two measurement settings per observer are satisfied.


title:
Purity-bounded uncertainty relations in multidimensional space --
  generalized purity

abstract:
  Uncertainty relations for mixed quantum states (precisely, purity-bounded
position-momentum relations, developed by Bastiaans and then by Man'ko and
Dodonov) are studied in general multi-dimensional case. An expression for
family of mixed states at the lower bound of uncertainty relation is obtained.
It is shown, that in case of entropy-bounded uncertainty relations, lower-bound
state is thermal, and a transition from one-dimensional problem to
multi-dimensional one is trivial. Results of numerical calculation of the
relation lower bound for different types of generalized purity are presented.
Analytical expressions for general purity-bounded relations for highly mixed
states are obtained.


title:
Spin-dependent forces on trapped ions for phase-stable quantum gates and
  motional Schrodinger-cat states

abstract:
  Favored schemes for trapped-ion quantum logic gates use bichromatic laser
fields to couple internal qubit states with external motion through a
"spin-dependent force." We introduce a new degree of freedom in this coupling
that reduces its sensitivity to phase decoherence. We demonstrate bichromatic
spin-dependent forces on a single trapped $^{111}$Cd$^+$ ion, and show that
phase coherence of the resulting "Schr\"{o}dinger-cat" states of motion depends
critically upon the spectral arrangement of the optical fields. This applies
directly to the operation of entangling gates on multiple ions.


title:
A presentation of the quantum Fourier transform from a recursive
  viewpoint

abstract:
  In this note we construct a quantum Fourier transform circuit in a recursive
way, by directly copying the 'divide and conquer' construction of the fast
Fourier transform algorithm, rather than using the explicit formula that is
given in most introductory texts to quantum computation. We do not pretend this
presentation to be original, nor claim for any anteriority. The aim of this
paper is purely pedagogical.


title: The Distance Between Classical and Quantum Systems

abstract:
  In a recent paper, a "distance" function, \cal D, was defined which measures
the distance between pure classical and quantum systems. In this work, we
present a new definition of a "distance", D, which measures the distance
between either pure or impure classical and quantum states. We also compare the
new distance formula with the previous formula, when the latter is applicable.
To illustrate these distances, we have used 2 \times 2 matrix examples and
2-dimensional vectors for simplicity and clarity. Several specific examples are
calculated.


title: Robust and parsimonious realisations of unitaries in the one-way model

abstract:
  We present a new set of generators for unitary maps over \otimes^n(C^2) which
differs from the traditional rotation-based generating set in that it uses a
single-parameter family of 1-qubit unitaries J(a), together with a single
2-qubit unitary controlled-Z.
  Each generator is implementable in the one-way model using only two qubits,
and this leads to both parsimonious and robust implementations of general
unitaries. As an illustration, we give an implementation of the controlled-U
family which uses only 14 qubits, and has a 2-colourable underlying
entanglement graph (known to yield robust entangled states).


title:
Joint Probabilities of Photon polarization Correlations in $e^{+}e^{-}$
  Annihilation

abstract:
  Joint probability distributions of photon polarization correlations are
computed, as well as those corresponding to the cases when only one of the
photon's polarization is measured in $e^{+}e^{-}$ annihilation, in flight, in
QED. This provides a dynamical, rather than a kinematical, description of
photon polarization correlations as stemming from the ever precise and
realistic QED theory. Such computations may be relevant to recent and future
experiments involved in testing Bell-like inequalities as described.


title:
Asymptotic performance of optimal state estimation in quantum two level
  system

abstract:
  We derived an asymptotic bound the accuracy of the estimation when we use the
quantum correlation in the measuring apparatus. It is also proved that this
bound can be achieved in any model in the quantum two-level system. Moreover,
we show that this bound of such a model cannot be attained by any quantum
measurement with no quantum correlation in the measuring apparatus. That is, in
such a model, the quantum correlation can improve the accuracy of the
estimation in an asymptotic setting.


title:
Universal uncertainty principle and quantum state control under
  conservation laws

abstract:
  Heisenberg's uncertainty principle, exemplified by the gamma ray thought
experiment, suggests that any finite precision measurement disturbs any
observables noncommuting with the measured observable. Here, it is shown that
this statement contradicts the limit of the accuracy of measurements under
conservation laws originally found by Wigner in 1950s, and should be modified
to correctly derive the unavoidable noise caused by the conservation law
induced decoherence. The obtained accuracy limit leads to an interesting
conclusion that a widely accepted, but rather naive, physical encoding of
qubits for quantum computing suffers significantly from the decoherence induced
by the angular momentum conservation law.


title: Replication and Evolution of Quantum Species

abstract:
  We dwell upon the physicist's conception of `life' since Schroedinger and
Wigner through to the modern-day language of living systems in the light of
quantum information. We discuss some basic features of a living system such as
ordinary replication and evolution in terms of quantum bio-information. We also
discuss the principle of no-culling of living replicas. We show that in a
collection of identical species there can be no entanglement between one of the
mutated copies and the rest of the species in a closed universe. Even though
these discussions revolve around `artificial life' they may still be applicable
in real biological systems under suitable conditions.


title:
Exciton photon strong-coupling regime for a single quantum dot in a
  microcavity

abstract:
  We report on the observation of the strong coupling regime between a single
GaAs quantum dot and a microdisk optical mode. Photoluminescence is performed
at various temperatures to tune the quantum dot exciton with respect to the
optical mode. At resonance, we observe an anticrossing, signature of the strong
coupling regime with a well resolved doublet. The Vacuum Rabi splitting amounts
to 400 μeV and is twice as large as the individual linewidths.


title: Notes on super-operator norms induced by Schatten norms

abstract:
  This paper considers basic properties of super-operator norms induced by
Schatten p-norms. Such super-operator norms arise in various contexts in the
study of quantum information. It is proved that for completely positive
super-operators, the value of any such norm is achieved by a positive
semidefinite input, answering a question recently posed by King and Ruskai.
However, for any choice of p, there exists a super-operator that is the
difference of two completely positive, trace-preserving super-operators such
that the value of the super-operator norm is not even achieved on a Hermitian
input operator. Also considered are the properties of the above norms for
super-operators tensored with the identity super-operator.


title:
Universal and deterministic manipulation of the quantum state of
  harmonic oscillators: a route to unitary gates for Fock State qubits

abstract:
  We present a simple quantum circuit that allows for the universal and
deterministic manipulation of the quantum state of confined harmonic
oscillators. The scheme is based on the selective interactions of the referred
oscillator with an auxiliary three-level system and a classical external
driving source, and enables any unitary operations on Fock states, two-by-two.
One circuit is equivalent to a single qubit unitary logical gate on Fock states
qubits. Sequences of similar protocols allow for complete, deterministic and
state-independent manipulation of the harmonic oscillator quantum state.


title: Speed dependent polarization correlations in QED and entanglement

abstract:
  Exact computations of polarizations correlations probabilities are carried
out in QED, to the leading order, for initially polarized as well as
unpolarized particles. Quite generally they are found to be speed dependent and
are in clear violation of Bells inequality of Local Hidden Variables (LHV)
theories. This dynamical analysis shows how speed dependent entangled states
are generated. These computations, based on QED are expected to lead to new
experiments on polarization correlations monitoring speed in the light of Bells
theorem. The paper provides a full QED treatment of the dynamics of
entanglement.


title: Multipartite entanglement in quantum spin chains

abstract:
  We study the occurrence of multipartite entanglement in spin chains. We show
that certain genuine multipartite entangled states, namely W states, can be
obtained as ground states of simple XX type ferromagnetic spin chains in a
transverse magnetic field, for any number of sites. Moreover, multipartite
entanglement is proven to exist even at finite temperatures. A transition from
a product state to a multipartite entangled state occurs when decreasing the
magnetic field to a critical value. Adiabatic passage through this point can
thus lead to the generation of multipartite entanglement.


title: Vectorlike representation of one-dimensional scattering

abstract:
  We present a self-contained discussion of the use of the transfer-matrix
formalism to study one-dimensional scattering. We elaborate on the geometrical
interpretation of this transfer matrix as a conformal mapping on the unit disk.
By generalizing to the unit disk the idea of turns, introduced by Hamilton to
represent rotations on the sphere, we develop a method to represent transfer
matrices by hyperbolic turns, which can be composed by a simple
parallelogramlike rule.


title: Bell-Correlated Activable Bound Entanglement in Multiqubit Systems

abstract:
  We show that the Hilbert space of even number ($\geq4$) of qubits can always
be decomposed as a direct sum of four orthogonal subspaces such that the
normalized projectors onto the subspaces are activable bound entangled (ABE)
states. These states also show a surprising recursive relation in the sense
that the states belonging to $2N+2$ qubits are Bell correlated to the states of
$2N$ qubits; hence, we refer to these states as Bell-Correlated ABE (BCABE)
states. We also study the properties of noisy BCABE states and show that they
are very similar to that of two qubit Bell-diagonal states.


title:
Multimode entanglement of light and atomic ensembles via off-resonant
  coherent forward scattering

abstract:
  Quantum theoretical treatment of coherent forward scattering of light in a
polarized atomic ensemble with an arbitrary angular momentum is developed. We
consider coherent forward scattering of a weak radiation field interacting with
a realistic multi-level atomic transition. Based on the concept of an effective
Hamiltonian and on the Heisenberg formalism, we discuss the coupled dynamics of
the quantum fluctuations of the polarization Stokes components of propagating
light and of the collective spin fluctuations of the scattering atoms. We show
that in the process of coherent forward scattering this dynamics can be
described in terms of a polariton-type spin wave created in the atomic sample.
Our work presents a general example of entangling process in the system of
collective quantum states of light and atomic angular momenta, previously
considered only for the case of spin-1/2 atoms. We use the developed general
formalism to test the applicability of spin-1/2 approximation for modelling the
quantum non-demolishing measurement of atoms with a higher angular momentum.


title:
Exact quantization of nonsolvable potentials: the role of the quantum
  phase beyond the semiclassical approximation

abstract:
  Semiclassical quantization is exact only for the so called \emph{solvable}
potentials, such as the harmonic oscillator. In the \emph{nonsolvable} case the
semiclassical phase, given by a series in $\hbar$, yields more or less
approximate results and eventually diverges due to the asymptotic nature of the
expansion. A quantum phase is derived to bypass these shortcomings. It achieves
exact quantization of nonsolvable potentials and allows to obtain the quantum
wavefunction while locally approaching the best pre-divergent semiclassical
expansion. An iterative procedure allowing to implement practical calculations
with a modest computational cost is also given. The theory is illustrated on
two examples for which the limitations of the semiclassical approach were
recently highlighted: cold atomic collisions and anharmonic oscillators in the
nonperturbative regime.


title:
A novel experimental approach for the detection of the dynamic Casimir
  effect

abstract:
  The Casimir effect is a well-known macroscopic consequence of quantum vacuum
fluctuations, but whereas the static effect (Casimir force) has long been
observed experimentally, the dynamic Casimir effect is up to now undetected.
  From an experimental viewpoint a possible detection would imply the vibration
of a mirror at gigahertz frequencies. Mechanical motions at such frequencies
turn out to be technically unfeasible. Here we present a different experimental
scheme where mechanical motions are avoided, and the results of laboratory
tests showing that the scheme is practically feasible. We think that at present
this approach gives the only possibility of detecting this phenomenon.


title: Decoherence in a N-qubit solid-state quantum computer

abstract:
  We investigate the decoherence process for a quantum register composed of N
qubits coupled to an environment. We consider an environment composed of one
common phonon bath and several electronic baths. This environment is relevant
to the implementation of a charge based solid-state quantum computer. We
explicitly compute the time evolution of all off-diagonal terms of the
register's reduced density matrix. We find that in realistic configurations,
"superdecoherence" and "decoherence free subspaces" do not exist for an N-qubit
system. This means that all off-diagonal terms decay like exp(-q(t)N), where
q(t) is of the same order as the decay function of a single qubit.


title:
Electromagnetically induced transparency and dynamic Stark effect in
  coupled quantum resonators

abstract:
  In this work we reproduce the phenomenology of the electromagnetically
induced transparency and dynamic Stark effect in a dissipative system composed
by two coupled bosonic fields under linear and nonlinear amplification process.
Such a system can be used as a quantum switch in networks of oscillators.


title: Many-agent controlled teleportation of multi-qubit quantum information

abstract:
  We present a general idea to construct methods for multi-qubit quantum
teleportation between two remote parties with control of many agents in the
network. Our methods seem to be much simpler than the existing method proposed
recently (Phys. Rev. A {\bf 70}, 022329(2004)). We then demonstrate our idea by
using several different protocols of quantum key distribution, including Ekert
91 and the deterministic secure communication protocol raised by Deng and Long.


title: On the quantum description of Einstein's Brownian motion

abstract:
  A fully quantum treatment of Einstein's Brownian motion is given, showing in
particular the role played by the two original requirements of translational
invariance and connection between dynamics of the Brownian particle and atomic
nature of the medium. The former leads to a clearcut relationship with Holevo's
result on translation-covariant quantum-dynamical semigroups, the latter to a
formulation of the fluctuation-dissipation theorem in terms of the dynamic
structure factor, a two-point correlation function introduced in seminal work
by van Hove, directly related to density fluctuations in the medium and
therefore to its atomistic, discrete nature. A microphysical expression for the
generally temperature dependent friction coefficient is given in terms of the
dynamic structure factor and of the interaction potential describing the single
collisions. A comparison with the Caldeira Leggett model is drawn, especially
in view of the requirement of translational invariance, further characterizing
general structures of reduced dynamics arising in the presence of symmetry
under translations.


title: Dynamics of a Raman coupled model: entanglement and quantum computation

abstract:
  The evolution of a Raman coupled three-level lambda atom with two quantized
cavity modes is studied in the large detuning case; i.e. when the upper atomic
level can be adiabatically eliminated. Particularly the situation when the two
modes are prepared in initial coherent or squeezed states, with a large average
number of photons, is investigated. It is found that the atom, after specific
interaction times, disentangles from the two modes, leaving them, in certain
cases, in entangled Schrodinger cat states. These disentanglement times can be
controlled by adjusting the ratio between average numbers of photons in the two
modes. It is also shown how this effective model may be used for implementing
quantum information processing. Especially it is demonstrated how to generate
various entangled states, such as EPR- and GHZ-states, and quantum logic
operations, such as the control-not and the phase-gate.


title:
On entropic quantities related to the classical capacity of infinite
  dimensional quantum channels

abstract:
  In this paper we consider the $\chi$-function (the Holevo capacity of
constrained channel) and the convex closure of the output entropy for arbitrary
infinite dimensional channel.
  It is shown that the $\chi$-function of an arbitrary channel is a concave
lower semicontinuous function on the whole state space, having continuous
restriction to any set of continuity of the output entropy.
  The explicit representation for the convex closure of the output entropy is
obtained and its properties are explored. It is shown that the convex closure
of the output entropy coincides with the convex hull of the output entropy on
the convex set of states with finite output entropy. Similarly to the case of
the $\chi$-function, it is proved that the convex closure of the output entropy
has continuous restriction to any set of continuity of output entropy. Some
applications of these results to the theory of entanglement are discussed.
  The obtained properties of the convex closure of the output entropy make it
possible to generalize to the infinite dimensional case the convex duality
approach to the additivity problem.


title: Secrecy content of two-qubit states

abstract:
  We analyze the set of two-qubit states from which a secret key can be
extracted by single-copy measurements plus classical processing of the
outcomes. We introduce a key distillation protocol and give the corresponding
necessary and sufficient condition for positive key extraction. Our results
imply that the critical error rate derived by Chau, Phys. Rev. A {\bf 66},
060302 (2002), for a secure key distribution using the six-state scheme is
tight. Remarkably, an optimal eavesdropping attack against this protocol does
not require any coherent quantum operation.


title:
Optimal Experiment Design for Quantum State and Process Tomography and
  Hamiltonian Parameter Estimation

abstract:
  A number of problems in quantum state and system identification are
addressed. Specifically, it is shown that the maximum likelihood estimation
(MLE) approach, already known to apply to quantum state tomography, is also
applicable to quantum process tomography (estimating the Kraus operator sum
representation (OSR)), Hamiltonian parameter estimation, and the related
problems of state and process (OSR) distribution estimation. Except for
Hamiltonian parameter estimation, the other MLE problems are formally of the
same type of convex optimization problem and therefore can be solved very
efficiently to within any desired accuracy.
  Associated with each of these estimation problems, and the focus of the
paper, is an optimal experiment design (OED) problem invoked by the Cramer-Rao
Inequality: find the number of experiments to be performed in a particular
system configuration to maximize estimation accuracy; a configuration being any
number of combinations of sample times, hardware settings, prepared initial
states, etc. We show that in all of the estimation problems, including
Hamiltonian parameter estimation, the optimal experiment design can be obtained
by solving a convex optimization problem.
  Software to solve the MLE and OED convex optimization problems is available
upon request from the first author.


title:
The Construction of the Dual Family of Gazeau-Klauder Coherent States
  via Temporally Stable Nonlinear Coherent States

abstract:
  Using the {\it analytic representation} of the so-called Gazeau-Klauder
coherent states(CSs), we shall demonstrate that how a new class of generalized
CSs namely the {\it family of dual states} associated with theses states can be
constructed through viewing these states as {\it temporally stable nonlinear
CSs}. Also we find that the ladder operators, as well as the displacement type
operator corresponding to these two pairs of generalized CSs, may be easily
obtained using our formalism, without employing the {\it supersymmetric quantum
mechanics}(SUSYQM) techniques. Then, we have applied this method to some
physical systems with known spectrum, such as P\"{o}schl-Teller, infinite well,
Morse potential and Hydrogen-like spectrum as some quantum mechanical systems.
Finally, we propose the generalized form of Gazeau-Klauder CS and the
corresponding dual family.


title: Non-decomposable quantum dynamical semigroups and bound entangled states

abstract:
  We use open quantum system techniques to construct one-parameter semigroups
of positive maps and apply them to study the entanglement properties of a class
of 16-dimensional density matrices, representing states of a 4x4 bipartite
system.


title: The Unruh effect in an Ion Trap: An Analogy

abstract:
  We propose an experiment in which the phonon excitation of ion(s) in a trap,
with a trap frequency exponentially modulated at rate $\kappa$, exhibits a
thermal spectrum with an "Unruh" temperature given by T=hbar*kappa. We discuss
the similarities of this experiment to the usual Unruh effect for quantum
fields and uniformly accelerated detectors. We demonstrate a new Unruh effect
for detectors that respond to anti-normally ordered moments using the ion's
first blue sideband transition.


title:
Quantum Fast Fourier Transform Viewed as a Special Case of Recursive
  Application of Cosine-Sine Decomposition

abstract:
  A quantum compiler is a software program for decomposing ("compiling") an
arbitrary unitary matrix into a sequence of elementary operations (SEO).
Coppersmith showed that the $\nb$-bit Discrete Fourier Transform matrix
$U_{FT}$ can be decomposed in a very efficient way, as a sequence of
order($\nb^2$) elementary operations. Can a quantum compiler that doesn't know
a priori about Coppersmith's decomposition nevertheless decompose $U_{FT}$ as a
sequence of order($\nb^2$) elementary operations? In other words, can it
rediscover Coppersmith's decomposition by following a much more general
algorithm? Yes it can, if that more general algorithm is the recursive
application of the Cosine-Sine Decomposition (CSD).


title:
A class of 2^N x 2^N bound entangled states revealed by non-decomposable
  maps

abstract:
  We use some general results regarding positive maps to exhibit examples of
non-decomposable maps and 2^N x 2^N, N >= 2, bound entangled states, e.g. non
distillable bipartite states of N + N qubits.


title:
Efficient decoupling and recoupling in solid state NMR for quantum
  computation

abstract:
  A scheme for decoupling and selectively recoupling large networks of
dipolar-coupled spins is proposed. The scheme relies on a combination of
broadband, decoupling pulse sequences applied to all the nuclear spins with a
band-selective pulse sequence for single spin rotations or recoupling. The
evolution-time overhead required for selective coupling is independent of the
number of spins, subject to time-scale constraints, for which we discuss the
feasibility. This scheme may improve the scalability of solid-state-NMR quantum
computing architectures.


title: Electric circuit networks equivalent to chaotic quantum billiards

abstract:
  We formulate two types of electric RLC resonance network equivalent to
quantum billiards. In the network of inductors grounded by capacitors squared
resonant frequencies are eigenvalues of the quantum billiard. In the network of
capacitors grounded by inductors squared resonant frequencies are given by
inverse eigen values of the billiard. In both cases local voltages play role of
the wave function of the quantum billiard. However as different from quantum
billiards there is a heat power because of resistance of the inductors. In the
equivalent chaotic billiards we derive the distribution of the heat power which
well describes numerical statistics.


title:
Pulse Control of Decoherence in a Qubit Coupled with a Quantum
  Environment

abstract:
  We study the time evolution of a qubit linearly coupled with a quantum
environment under a sequence of short pi pulses. Our attention is focused on
the case where qubit-environment interactions induce the decoherence with
population decay. We assume that the environment consists of a set of bosonic
excitations. The time evolution of the reduced density matrix for the qubit is
calculated in the presence of periodic short pi pulses. We confirm that the
decoherence is suppressed if the pulse interval is shorter than the correlation
time for qubit-environment interactions.


title: Electrode Configurations for Fast Separation of Trapped Ions

abstract:
  We study the problem of designing electrode structures that allow pairs of
ions to be brought together and separated rapidly in an array of linear Paul
traps. We show that it is desirable for the electrode structure to produce a
d.c. octupole moment with an a.c. radial quadrupole. For the case where
electrical breakdown limits the voltages that can be applied, we show that the
octupole is more demanding than the quadrupole when the characteristic distance
scale of the structure is larger than 1 to 10 microns (for typical materials).
We present a variety of approaches and optimizations of structures consisting
of one to three layers of electrodes. The three-layer structures allow the
fastest operation at given distance r from the trap centres to the nearest
electrode surface, but when the total thickness w of the structure is
constrained, leading to w < r, then two-layer structures may be preferable.


title: Coherent control of trapped ions using off-resonant lasers

abstract:
  In this paper we develop a unified framework to study the coherent control of
trapped ions subject to state-dependent forces. Taking different limits in our
theory, we can reproduce two different designs of a two-qubit quantum gate
--the pushing gate [1] and the fast gates based on laser pulses from Ref.
[2]--, and propose a new design based on continuous laser beams. We demonstrate
how to simulate Ising Hamiltonians in a many ions setup, and how to create
highly entangled states and induce squeezing. Finally, in a detailed analysis
we identify the physical limits of this technique and study the dependence of
errors on the temperature. [1] J.I. Cirac, P. Zoller, Nature, 404, 579, 2000.
[2] J.J. Garcia-Ripoll, P. Zoller, J.I. Cirac, PRL 67, 062318, 2003


title:
Global control and fast solid-state donor electron spin quantum
  computing

abstract:
  We propose a scheme for quantum information processing based on donor
electron spins in semiconductors, with an architecture complementary to the
original Kane proposal. We show that a naive implementation of electron spin
qubits provides only modest improvement over the Kane scheme, however through
the introduction of global gate control we are able to take full advantage of
the fast electron evolution timescales. We estimate that the latent clock speed
is 100-1000 times that of the nuclear spin quantum computer with the ratio
$T_{2}/T_{ops}$ approaching the $10^{6}$ level.


title: Optimal copying of entangled two-qubit states

abstract:
  We investigate the problem of copying pure two-qubit states of a given degree
of entanglement in an optimal way. Completely positive covariant quantum
operations are constructed which maximize the fidelity of the output states
with respect to two separable copies. These optimal copying processes hint at
the intricate relationship between fundamental laws of quantum theory and
entanglement.


title:
Hertz potentials approach to the dynamical Casimir effect in cylindrical
  cavities of arbitrary section

abstract:
  We study the creation of photons in resonant cylindrical cavities with time
dependent length. The physical degrees of freedom of the electromagnetic field
are described using Hertz potentials. We describe the general formalism for
cavities with arbitrary section. Then we compute explicitly the number of TE
and TM motion-induced photons for cylindrical cavities with rectangular and
circular sections. We also discuss the creation of TEM photons in non-simply
connected cylindrical cavities.


title: Framework for quantum modeling of fiber-optical networks: PART I

abstract:
  We formulate quantum optics to include frequency dependence in the modeling
of optical networks. Entangled light pulses available for quantum cryptography
are entangled not only in polarization but also, whether one wants it or not,
in frequency. We model effects of the frequency spectrum of faint
polarization-entangled light pulses on detection statistics. For instance, we
show how polarization entanglement combines with frequency entanglement in the
variation of detection statistics with pulse energy. Attention is paid not only
to single-photon light states but also to multi-photon states. These are needed
(1) to analyze the dependence of statistics on energy and (2) to help in
calibrating fiber couplers, lasers and other devices, even when their desired
use is for the generation of single-photon light.


title: Framework for quantum modeling of fiber-optical networks: PART II


title: Unitarily localizable entanglement of Gaussian states

abstract:
  We consider generic $m\times n$-mode bipartitions of continuous variable
systems, and study the associated bisymmetric multimode Gaussian states. They
are defined as $(m+n)$-mode Gaussian states invariant under local mode
permutations on the $m$-mode and $n$-mode subsystems. We prove that such states
are equivalent, under local unitary transformations, to the tensor product of a
two-mode state and of $m+n-2$ uncorrelated single-mode states. The entanglement
between the $m$-mode and the $n$-mode blocks can then be completely
concentrated on a single pair of modes by means of local unitary operations
alone. This result allows to prove that the PPT (positivity of the partial
transpose) condition is necessary and sufficient for the separability of $(m +
n)$-mode bisymmetric Gaussian states. We determine exactly their negativity and
identify a subset of bisymmetric states whose multimode entanglement of
formation can be computed analytically. We consider explicit examples of pure
and mixed bisymmetric states and study their entanglement scaling with the
number of modes.


title:
Distinguishing Bipartitite Orthogonal States using LOCC: Best and Worst
  Cases

abstract:
  Two types of results are presented for distinguishing pure bipartite quantum
states using Local Operations and Classical Communications. We examine sets of
states that can be perfectly distinguished, in particular showing that any
three orthogonal maximally entangled states in C^3 tensor C^3 form such a set.
In cases where orthogonal states cannot be distinguished, we obtain upper
bounds for the probability of error using LOCC taken over all sets of k
orthogonal states in C^n tensor C^m. In the process of proving these bounds, we
identify some sets of orthogonal states for which perfect distinguishability is
not possible.


title:
Threshold Error Penalty for Fault Tolerant Computation with Nearest
  Neighbour Communication

abstract:
  The error threshold for fault tolerant quantum computation with concatenated
encoding of qubits is penalized by internal communication overhead. Many
quantum computation proposals rely on nearest-neighbour communication, which
requires excess gate operations. For a qubit stripe with a width of L+1
physical qubits implementing L levels of concatenation, we find that the error
threshold of 2.1x10^-5 without any communication burden is reduced to 1.2x10^-7
when gate errors are the dominant source of error. This ~175X penalty in error
threshold translates to an ~13X penalty in the amplitude and timing of gate
operation control pulses.


title: Optimal population transfers in a quantum system for large transfer time

abstract:
  Transferring the state of a quantum system to a given distribution of
populations is an important problem with applications to Quantum Chemistry and
Atomic Physics. In this work we consider exact population transfers that
minimize the L^2 norm of the control which is typically the amplitude of an
electromagnetic field. This problem is analytically and numerically
challenging. Except for few exactly solvable cases, there is no general
understanding of the nature of optimal controls and trajectories. We find that
by examining the limit of large transfer times, we can uncover such general
properties. In particular, for transfer times large with respect to the time
scale of the free dynamics of the quantum system, the optimal control is a sum
of components, each being a Bohr frequency sinusoid modulated by a slow
amplitude, i.e. a profile that changes considerably only on the scale of the
transfer time. Moreover, we show that the optimal trajectory follows a "mean''
evolution modulated by the fast free dynamics of the system. The calculation of
the "mean'' optimal trajectory and the slow control profiles is done via an
"averaged'' two-point boundary value problem which we derive and which is much
easier to solve than the one expressing the necessary conditions for optimality
of the original optimal transfer problem.


title: Analyses of reflection and transmission at moving potential step

abstract:
  The reflection and transmission of wave functions at a potential step is a
well-known issue in a textbook of quantum mechanics. We studied the reflection
and transmission characteristics analytically when the potential step is moving
at a constant velocity $v$ in the same direction as an incident wave function
by means of solving the time-dependent Schr\"{o}dinger equation. As for an
infinite potential step, it is known that group velocity is the same as the
moving velocity of the potential step. We found two interesting results when
the potential step has a finite height of $V_0$. The transmission occurs when
the kinetic energy of incident wave function is larger than the effective
potential hight of $(\sqrt{\frac{m}{2}}v + \sqrt{V_0})^2$. The other result is
that the reflectivity depends on $x$, which derives from the interference
between the incident and the reflected wave functions.


title:
Non-deterministic approximation of photon number discriminating
  detectors using non-discriminating detectors

abstract:
  We present a scheme for non-deterministically approximating photon number
resolving detectors using non-discriminating detectors. The model is simple in
construction and employs very few physical resources. Despite its
non-determinism, the proposal may nonetheless be suitable for use in some
quantum optics experiments in which non-determinism can be tolerated. We
analyze the detection scheme in the context of an optical implementation of the
controlled-NOT gate, an inherently non-deterministic device. This allows the
gate's success probability to be traded away for improved gate fidelity,
assuming high efficiency detectors. The scheme is compared to two other
proposals, both deterministic, for approximating discriminating detectors using
non-discriminating detectors: the cascade and time division multiplexing
schemes.


title: Local unitary versus local Clifford equivalence of stabilizer states

abstract:
  We study the relation between local unitary (LU) equivalence and local
Clifford (LC) equivalence of stabilizer states. We introduce a large subclass
of stabilizer states, such that every two LU equivalent states in this class
are necessarily LC equivalent. Together with earlier results, this shows that
LC, LU and SLOCC equivalence are the same notions for this class of stabilizer
states. Moreover, recognizing whether two given stabilizer states in the
present subclass are locally equivalent only requires a polynomial number of
operations in the number of qubits.


title:
Optimizing stochastic trajectories in exact quantum jump approaches of
  interacting systems

abstract:
  The quantum jump approach, where pairs of state vectors follow Stochastic
Schroedinger Equation (SSE) in order to treat the exact quantum dynamics of two
interacting systems, is first described. In this work the non-uniqueness of
such stochastic Schroedinger equations is investigated to propose strategies to
optimize the stochastic paths and reduce statistical fluctuations. In the
proposed method, called the 'adaptative noise method', a specific SSE is
obtained for which the noise depends explicitly on both the initial state and
on the properties of the interaction Hamiltonian. It is also shown that this
method can be further improved by introduction of a mean-field dynamics. The
different optimization procedures are illustrated quantitatively in the case of
interacting spins. A significant reduction of the statistical fluctuations is
obtained. Consequently a much smaller number of trajectories is needed to
accurately reproduce the exact dynamics as compared to the SSE without
optimization.


title:
Semiclassical Propagation of Wavepackets with Real and Complex
  Trajectories

abstract:
  We consider a semiclassical approximation for the time evolution of an
originally gaussian wave packet in terms of complex trajectories. We also
derive additional approximations replacing the complex trajectories by real
ones. These yield three different semiclassical formulae involving different
real trajectories. One of these formulae is Heller's thawed gaussian
approximation. The other approximations are non-gaussian and may involve
several trajectories determined by mixed initial-final conditions. These
different formulae are tested for the cases of scattering by a hard wall,
scattering by an attractive gaussian potential, and bound motion in a quartic
oscillator. The formula with complex trajectories gives good results in all
cases. The non-gaussian approximations with real trajectories work well in some
cases, whereas the thawed gaussian works only in very simple situations.


title:
Talbot-Lau interferometry with fullerenes: Sensitivity to inertial
  forces and vibrational dephasing

abstract:
  We discuss matter wave experiments in a near-field interferometer and focus
on dephasing phenomena due to inertial forces. Their presence may result in a
significant reduction of the observed interference contrast, even though they
do not lead to genuine decoherence. We provide quantitative estimates for the
most important effects and demonstrate experimentally the strong influence of
acoustic vibrations. Since the effects of inertial forces get increasingly
important for the interferometry with more massive particles they have to be
identified and compensated in future experiments.


title: Conditions for strictly purity-decreasing quantum Markovian dynamics

abstract:
  The purity, Tr(rho^2), measures how pure or mixed a quantum state rho is. It
is well known that quantum dynamical semigroups that preserve the identity
operator (which we refer to as unital) are strictly purity-decreasing
transformations. Here we provide an almost complete characterization of the
class of strictly purity-decreasing quantum dynamical semigroups. We show that
in the case of finite-dimensional Hilbert spaces a dynamical semigroup is
strictly purity-decreasing if and only if it is unital, while in the infinite
dimensional case, unitality is only sufficient.

categories: quant-ph math-ph math.MP physics.chem-ph


title: Adiabatic Quantum State Manipulation of Single Trapped Atoms

abstract:
  We use microwave induced adiabatic passages for selective spin flips within a
string of optically trapped individual neutral Cs atoms. We
position-dependently shift the atomic transition frequency with a magnetic
field gradient. To flip the spin of a selected atom, we optically measure its
position and sweep the microwave frequency across its respective resonance
frequency. We analyze the addressing resolution and the experimental robustness
of this scheme. Furthermore, we show that adiabatic spin flips can also be
induced with a fixed microwave frequency by deterministically transporting the
atoms across the position of resonance.


title: Excited Field of Particle in Quantum Theory

abstract:
  A model about excited field of a particle is discussed. We found this model
will give wave-particle duality clearly and its Lagrangian is consistent with
Quantum Theory. A new interpretation of quantum mechanics but not statistical
interpretation[1] is presented.


title: Encoding a logical qubit into physical qubits

abstract:
  We propose two protocols to encode a logical qubit into physical qubits
relying on common types of qubit-qubit interactions in as simple forms as
possible. We comment on its experimental implementation in several quantum
computing architectures, e.g. with trapped atomic ion qubits, atomic qubits
inside a high Q optical cavity, solid state Josephson junction qubits, and
Bose-Einstein condensed atoms.


title: Localizable Entanglement

abstract:
  We consider systems of interacting spins and study the entanglement that can
be localized, on average, between two separated spins by performing local
measurements on the remaining spins. This concept of Localizable Entanglement
(LE) leads naturally to notions like entanglement length and entanglement
fluctuations. For both spin-1/2 and spin-1 systems we prove that the LE of a
pure quantum state can be lower bounded by connected correlation functions. We
further propose a scheme, based on matrix-product states and the Monte Carlo
method, to efficiently calculate the LE for quantum states of a large number of
spins. The virtues of LE are illustrated for various spin models. In
particular, characteristic features of a quantum phase transition such as a
diverging entanglement length can be observed. We also give examples for pure
quantum states exhibiting a diverging entanglement length but finite
correlation length. We have numerical evidence that the ground state of the
antiferromagnetic spin-1 Heisenberg chain can serve as a perfect quantum
channel. Furthermore, we apply the numerical method to mixed states and study
the entanglement as a function of temperature.


title:
Solution of a Hamiltonian of quantum dots with Rashba spin-orbit
  coupling: quasi-exact solution

abstract:
  We present a method to solve the problem of Rashba spin-orbit coupling in
semiconductor quantum dots, within the context of quasi-exactly solvable
spectral problems. We show that the problem possesses a hidden osp(2,2)
superalgebra. We constructed a general matrix whose determinant provide exact
eigenvalues. Analogous mathematical structures between the Rashba and some of
the other spin-boson physical systems are notified.


title: Universality in the entanglement structure of ferromagnets

abstract:
  Systems of exchange-coupled spins are commonly used to model ferromagnets.
The quantum correlations in such magnets are studied using tools from quantum
information theory. Isotropic ferromagnets are shown to possess a universal
low-temperature density matrix which precludes entanglement between spins, and
the mechanism of entanglement cancellation is investigated, revealing a core of
states resistant to pairwise entanglement cancellation. Numerical studies of
one-, two-, and three-dimensional lattices as well as irregular geometries
showed no entanglement in ferromagnets at any temperature or magnetic field
strength.


title:
Vacuum induced Stark shifts for quantum logic using a collective system
  in a high quality dispersive cavity

abstract:
  A collective system of atoms in a high quality cavity can be described by a
nonlinear interaction which arises due to the Lamb shift of the energy levels
due to the cavity vacuum [Agarwal et al., Phys. Rev. A 56, 2249 (1997)]. We
show how this collective interaction can be used to perform quantum logic. In
particular we produce schemes to realize CNOT gates not only for two-qubit but
also for three-qubit systems. We also discuss realizations of Toffoli gates.
Our effective Hamiltonian is also realized in other systems such as trapped
ions or magnetic molecules.


title: Concurrence of mixed multi-partite quantum states

abstract:
  We propose generalizations of concurrence for multi-partite quantum systems
that can distinguish qualitatively distinct quantum correlations. All
introduced quantities can be evaluated efficiently for arbitrary mixed sates.


title: Entanglement Assisted Metrology

abstract:
  We propose a new approach to the measurement of a single spin state, based on
nuclear magnetic resonance (NMR) techniques and inspired by the coherent
control over many-body systems envisaged by Quantum Information Processing
(QIP). A single target spin is coupled via the natural magnetic dipolar
interaction to a large ensemble of spins. Applying external radio frequency
(rf) pulses, we can control the evolution of the system so that the spin
ensemble reaches one of two orthogonal states whose collective properties
differ depending on the state of the target spin and are easily measured. We
first describe this measurement process using QIP gates; then we show how
equivalent schemes can be defined in terms of the Hamiltonian of the spin
system and thus implemented under conditions of real control, using well
established NMR techniques. We demonstrate this method with a proof of
principle experiment in ensemble liquid state NMR and simulations for small
spin systems.


title:
Drastic effects of damping mechanisms on the third-order optical
  nonlinearity

abstract:
  We have investigated the optical response of superradiant atoms, which
undergoes three different damping mechanisms: radiative dissipation
($\gamma_r$), dephasing ($\gamma_d$), and nonradiative dissipation
($\gamma_n$). Whereas the roles of $\gamma_d$ and $\gamma_n$ are equivalent in
the linear susceptibility, the third-order nonlinear susceptibility drastically
depends on the ratio of $\gamma_d$ and $\gamma_n$: When $\gamma_d \ll
\gamma_n$, the third-order susceptibility is essentially that of a single atom.
Contrarily, in the opposite case of $\gamma_d \gg \gamma_n$, the third-order
susceptibility suffers the size-enhancement effect and becomes proportional to
the system size.


title: Concurrence of quasi pure quantum states

abstract:
  We derive an analytic approximation for the concurrence of weakly mixed
bipartite quantum states - typical objects in state of the art experiments.
This approximation is shown to be a lower bound of the concurrence of arbitrary
states.


title:
Field intensity distributions and polarization orientations in a
  vacuum-clad subwavelength-diameter optical fiber

abstract:
  We study the properties of the field in the fundamental mode HE$_{11}$ of a
vacuum-clad \textit{subwavelength-diameter} optical fiber using the exact
solutions of Maxwell's equations. We obtain simple analytical expressions for
the total intensity of the electric field. We discuss the origin of the
deviations of the exact fundamental mode HE$_{11}$ from the approximate mode
LP$_{01}$. We show that the thin thickness of the fiber and the high contrast
between the refractive indices of the silica core and the vacuum clad
substantially modify the intensity distributions and the polarization
properties of the field and its components, especially in the vicinity of the
fiber surface. One of the promising applications of the field around the
subwavelength-diameter fiber is trapping and guiding of atoms by the optical
force of the evanescent field.


title:
Coherently manipulating two-qubit quantum information using a pair of
  simultaneous laser pulses

abstract:
  Several sequential operations are usually needed for implementing controlled
quantum gates and generating entanglement between a pair of quantum bits. Based
on the conditional quantum dynamics for a two-ion system beyond the Lamb-Dicke
limit, here we propose a theoretical scheme for manipulating two-qubit quantum
information, i.e., implementing a universal two-qubit quantum gate and
generating a two-qubit entangled state, by using a pair of simultaneous laser
pulses. Neither the Lamb-Dicke approximation nor the auxiliary atomic level are
required. The experimental realizability of this simple approach is also
discussed.


title: Optimal phase estimation for qubit mixed states

abstract:
  We address the problem of optimal estimation of the relative phase for
two-dimensional quantum systems in mixed states. In particular, we derive the
optimal measurement procedures for an arbitrary number of qubits prepared in
the same mixed state.


title:
An efficient single-step scheme for manipulating quantum information of
  two trapped ions beyond the Lamb-Dicke limit

abstract:
  Based on the exact conditional quantum dynamics for a two-ion system, we
propose an efficient {\it single-step} scheme for coherently manipulating
quantum information of two trapped cold ions by using a pair of synchronous
laser pulses. Neither the auxiliary atomic level nor the Lamb-Dicke
approximation are needed.


title:
Realization of POVMs using measurement-assisted programmable quantum
  processors

abstract:
  We study possible realizations of generalized quantum measurements on
measurement-assisted programmable quantum processors.
  We focus our attention on the realization of von Neumann measurements and
informationally complete POVMs. It is known that two unitary transformations
implementable by the same programmable processor require mutually {\it
orthogonal} states.
  It turns out that the situation with von Neumann measurements is different.
  Specifically, in order to realize two such measurements one does not have to
use orthogonal program states. On the other hand, the number of the
implementable von Neumann measurements is still limited. As an example of a
programmable processor we use the so-called quantum information distributor.


title: Notes on coarse grainings and functions of observables

abstract:
  Using the Naimark dilation theory we investigate the question under what
conditions an observable which is a coarse graining of another observable is a
function of it. To this end, conditions for the separability and for the
Boolean structure of an observable are given.


title: PT-Symmetric Cubic Anharmonic Oscillator as a Physical Model

abstract:
  We perform a perturbative calculation of the physical observables, in
particular pseudo-Hermitian position and momentum operators, the equivalent
Hermitian Hamiltonian operator, and the classical Hamiltonian for the
PT-symmetric cubic anharmonic oscillator, $ H=p^1/(2m)+\mu^2x^2/2+i\epsilon x^3
$. Ignoring terms of order $ \epsilon^4 $ and higher, we show that this system
describes an ordinary quartic anharmonic oscillator with a position-dependent
mass and real and positive coupling constants. This observation elucidates the
classical origin of the reality and positivity of the energy spectrum. We also
discuss the quantum-classical correspondence for this PT-symmetric system,
compute the associated conserved probability density, and comment on the issue
of factor-ordering in the pseudo-Hermitian canonical quantization of the
underlying classical system.


title: Bath Assisted Cooling of Spins

abstract:
  A suitable sequence of sharp pulses applied to a spin coupled to a bosonic
bath can cool its state, i.e., increase its polarization or ground state
occupation probability. Starting from an unpolarized state of the spin in
equilibrium with the bath, one can reach very low temperatures or sizeable
polarizations within a time shorter than the decoherence time. Both the bath
and external fields are necessary for the effect which comes from the
backreaction of the spin on the bath. This method can be applied to cool at
once a disordered ensemble of spins. Since the bath is crucial for this
mechanism, the cooling limits are set by the strength of its interaction with
the spin(s).


title:
Engineering Schr\"{o}dinger cat states through cavity-assisted
  interaction of coherent optical pulses

abstract:
  We propose a scheme to engineer Schr\"{o}dinger-cat states of propagating
optical pulses. Multi-dimensional and multi-partite cat states can be generated
simply by reflecting coherent optical pulses successively from a single-atom
cavity. The influences of various sources of noise, including the atomic
spontaneous emission and the pulse shape distortion, are characterized through
detailed numerical simulation, which demonstrates practicality of this scheme
within the reach of current experimental technology.


title: Improved Bounds on Quantum Learning Algorithms

abstract:
  In this article we give several new results on the complexity of algorithms
that learn Boolean functions from quantum queries and quantum examples.
  Hunziker et al. conjectured that for any class C of Boolean functions, the
number of quantum black-box queries which are required to exactly identify an
unknown function from C is $O(\frac{\log |C|}{\sqrt{{\hat{\gamma}}^{C}}})$,
where $\hat{\gamma}^{C}$ is a combinatorial parameter of the class C. We
essentially resolve this conjecture in the affirmative by giving a quantum
algorithm that, for any class C, identifies any unknown function from C using
$O(\frac{\log |C| \log \log |C|}{\sqrt{{\hat{\gamma}}^{C}}})$ quantum black-box
queries.
  We consider a range of natural problems intermediate between the exact
learning problem (in which the learner must obtain all bits of information
about the black-box function) and the usual problem of computing a predicate
(in which the learner must obtain only one bit of information about the
black-box function). We give positive and negative results on when the quantum
and classical query complexities of these intermediate problems are
polynomially related to each other.
  Finally, we improve the known lower bounds on the number of quantum examples
(as opposed to quantum black-box queries) required for $(\epsilon,\delta)$-PAC
learning any concept class of Vapnik-Chervonenkis dimension d over the domain
$\{0,1\}^n$ from $\Omega(\frac{d}{n})$ to $\Omega(\frac{1}{\epsilon}\log
\frac{1}{\delta}+d+\frac{\sqrt{d}}{\epsilon})$. This new lower bound comes
closer to matching known upper bounds for classical PAC learning.


title: Hydrodynamic construction of the electromagnetic field

abstract:
  We present an alternative Eulerian hydrodynamic model for the electromagnetic
field in which the discrete vector indices in Maxwell\s equations are replaced
by continuous angular freedoms, and develop the corresponding Lagrangian
picture in which the fluid particles have rotational and translational
freedoms. This enables us to extend to the electromagnetic field the exact
method of state construction proposed previously for spin 0 systems, in which
the time-dependent wavefunction is computed from a single-valued continuum of
deterministic trajectories where two spacetime points are linked by at most a
single orbit. The deduction of Maxwell\s equations from continuum mechanics is
achieved by generalizing the spin 0 theory to a general Riemannian manifold
from which the electromagnetic construction is extracted as a special case. In
particular, the flat-space Maxwell equations are represented as a curved-space
Schr\"odinger equation for a massive system. The Lorentz covariance of the
Eulerian field theory is obtained from the non-covariant Lagrangian-coordinate
model as a kind of collective effect. The method makes manifest the
electromagnetic analogue of the quantum potential that is tacit in Maxwell\s
equations. This implies a novel definition of the \classical limit\ of
Maxwell\s equations that differs from geometrical optics. It is shown that
Maxwell\s equations may be obtained by canonical quantization of the classical
model. Using the classical trajectories a novel expression is derived for the
propagator of the electromagnetic field in the Eulerian picture. The trajectory
and propagator methods of solution are illustrated for the case of a light
wave.


title: Generalised Smolin states and their properties

abstract:
  Four qubit bound entangled Smolin states are generalised in a natural way to
even number of qubits. They are shown to maximally violate simple correlation
Bell inequalities and, as such, to reduce communication complexity, though they
do not admit quantum security. They are also shown to serve for remote quantum
information concentration like in the case of the original four qubit states.
Application of the information concentration to the process of unlocking of
classical correlations and quantum entanglement by quantum bit is pointed out.


title:
Local copying of orthogonal maximally entangled states and its
  relationship to local discrimination

abstract:
  In the quantum system, perfect copying is impossible without prior knowledge.
But, perfect copying is possible, if it is known that unknown states to be
copied is contained by the set of orthogonal states, which is called the copied
set. However, if our operation is limited to local operations and classical
communications, this problem is not trivial. Recently, F. Anselmi, A. Chefles
and M.B. Plenio constructed theory of local copying when the copied set
consists of maximally entangled states. They also classified the copied set
when it consists of two orthogonal states (New. J. Phys. 6, 164 (2004)). In
this paper, we completely classify the copied set of local copying of the
maximally entangled states in the prime dimensional system. That is, we prove
that, in the prime dimensional system, the set of locally copiable maximally
entangled states is equivalent to the set of Simultaneously Schmidt
decomposable canonical form Bell states. As a result, we conclude that local
copying of maximally entangled states is much more difficult than local
discrimination at least in prime dimensional systems.


title: Quantum gate characterization in an extended Hilbert space

abstract:
  We describe an approach for characterizing the process of quantum gates using
quantum process tomography, by first modeling them in an extended Hilbert
space, which includes non-qubit degrees of freedom. To prevent unphysical
processes from being predicted, present quantum process tomography procedures
incorporate mathematical constraints, which make no assumptions as to the
actual physical nature of the system being described. By contrast, the
procedure presented here ensures physicality by placing physical constraints on
the nature of quantum processes. This allows quantum process tomography to be
performed using a smaller experimental data set, and produces parameters with a
direct physical interpretation. The approach is demonstrated by example of
mode-matching in an all-optical controlled-NOT gate. The techniques described
are non-specific and could be applied to other optical circuits or quantum
computing architectures.


title: Quantum Zeno effect by general measurements

abstract:
  It was predicted that frequently repeated measurements on an unstable quantum
state may alter the decay rate of the state. This is called the quantum Zeno
effect (QZE) or the anti-Zeno effect (AZE), depending on whether the decay is
suppressed or enhanced. In conventional theories of the QZE and AZE, effects of
measurements are simply described by the projection postulate, assuming that
each measurement is an instantaneous and ideal one. However, real measurements
are not instantaneous and ideal. For the QZE and AZE by such general
measurements, interesting and surprising features have recently been revealed,
which we review in this article. The results are based on the quantum
measurement theory, which is also reviewed briefly. As a typical model, we
consider a continuous measurement of the decay of an excited atom by a
photodetector that detects a photon emitted from the atom upon decay. This
measurement is an indirect negative-result one, for which the curiosity of the
QZE and AZE is emphasized. It is shown that the form factor is renormalized as
a backaction of the measurement, through which the decay dynamics is modified.
In a special case of the flat response, where the detector responds to every
photon mode with an identical response time, results of the conventional
theories are reproduced qualitatively. However, drastic differences emerge in
general cases where the detector responds only to limited photon modes. For
example, against predictions of the conventional theories, the QZE or AZE may
take place even for states that exactly follow the exponential decay law. We
also discuss relation to the cavity quantum electrodynamics.


title: Temporal shaping of entangled photons

abstract:
  We experimentally demonstrate shaping of the two-photon wavefunction of
entangled photon-pairs, utilizing coherent pulse-shaping techniques. By
performing spectral-phase manipulations we tailor the two-photon wavefunction
exactly like a coherent ultrashort pulse. To observe the shaping we perform
sum-frequency generation (SFG) with an ultrahigh flux of entangled photons. At
the appropriate conditions, SFG performs as a coincidence detector with an
ultrashort response time (~100 fs), enabling a direct observation of the
two-photon wavefunction. This property also enables us to demonstrate
background-free, high-visibility two-photon interference oscillations.

categories: quant-ph physics.atom-ph physics.class-ph physics.optics


title:
Observation of sub-Poisson photon statistics in the cavity-QED
  microlaser

abstract:
  We have measured the second-order correlation function of the cavity-QED
microlaser output and observed a transition from photon bunching to
antibunching with increasing average number of intracavity atoms. The observed
correlation times and the transition from super- to sub-Poisson photon
statistics can be well described by gain-loss feedback or enhanced/reduced
restoring action against fluctuations in photon number in the context of a
quantum microlaser theory and a photon rate equation picture. However, the
theory predicts a degree of antibunching several times larger than that
observed, which may indicate the inadequacy of its treatment of atomic velocity
distributions.


title: An exact solution of the slow-light problem

abstract:
  We investigate propagation of a slow-light soliton in atomic vapors and
Bose-Einstein condensates described by the nonlinear Lambda-model. We show that
the group velocity of the soliton monotonically decreases with the intensity of
the controlling laser field, which decays exponentially after the laser is
switched off. The shock wave of the vanishing controlling field overtakes the
slow soliton and stops it, while the optical information is recorded in the
medium in the form of spatially localized polarization. We find an explicit
exact solution describing the whole process within the slowly varying amplitude
and phase approximation. Our results point to the possibility of addressing
spatially localized memory formations and moving these memory bits along the
medium in a controllable fashion.


title:
Manipulation of optical memory bits in atomic vapors and Bose-Einstein
  condensates

abstract:
  We provide an exact analytic description of decelerating, stopping and
re-accelerating optical solitons in atomic media. By virtue of this solution we
describe in detail how spatially localized optical memory bits can be written
down, read and moved along the atomic medium in a prescribed manner. Dynamical
control over the solitons is realized via a background laser field whose
intensity controls the velocity of the slow light in a similar way as in the
linear theory of electromagnetically induced transparency (EIT). We solve the
nonlinear model when the controlling field and the solitons interact in an
inseparable nonlinear superposition process. This allows us to access results
beyond the limits of the linear theory of EIT.


title: Cylindrical quantum well of finite depth in external magnetic field

abstract:
  Energy spectrum of an electron confined by finite hard-wall potential in a
cylinder quantum dot placed in weak (up to 100 kOe) homogeneous external
magnetic field were calculated using the method of variation of vector
potential. Electronic motion along the cylinder axis is limited by
one-dimensional infinite potential barrier and electronic motion on the plane
perpendicular to the axis is limited by two-dimensional finite potential
barrier.


title: Quantum Walks with Entangled Coins

abstract:
  We present a mathematical formalism for the description of unrestricted
quantum walks with entangled coins and one walker. The numerical behaviour of
such walks is examined when using a Bell state as the initial coin state, two
different coin operators, two different shift operators, and one walker. We
compare and contrast the performance of these quantum walks with that of a
classical random walk consisting of one walker and two maximally correlated
coins as well as quantum walks with coins sharing different degrees of
entanglement.
  We illustrate that the behaviour of our walk with entangled coins can be very
different in comparison to the usual quantum walk with a single coin. We also
demonstrate that simply by changing the shift operator, we can generate widely
different distributions. We also compare the behaviour of quantum walks with
maximally entangled coins with that of quantum walks with non-entangled coins.
Finally, we show that the use of different shift operators on 2 and 3 qubit
coins leads to different position probability distributions in 1 and 2
dimensional graphs.


title: An Elementary Proof of the Quantum Adiabatic Theorem

abstract: We provide an elementary proof of the quantum adiabatic theorem.


title: Warp-Drive Quantum Computation

abstract:
  Recently it has been shown that time-optimal quantum computation is attained
by using the Cartan decomposition of a unitary matrix. We extend this approach
by noting that the unitary group is compact. This allows us to reduce the
execution time of a quantum algorithm $U_{\rm alg}$ further by adding an extra
gate $W$ to it. This gate $W$ sends $U_{\rm alg}$ to another algorithm $WU_{\rm
alg}$ which is executable in a shorter time than $U_{\rm alg}$. We call this
technique warp-drive. Here we show both theoretically and experimentally that
the execution time of Grover's algorithm is reduced in two-qubit NMR quantum
computer. Warp-drive is potentially a powerful tool in accelerating algorithms
and reducing the errors in any realization. of a quantum computer


title: Quantum Control with Level Set Method

abstract:
  We examine the relevance of Level Set Methods (LSM)in coherent control
quantum systems where the objective is to retain or attain a particular
expectation value of a given measurable. The differences with the usual
applications of LSM, where continuous closed interfaces are involved, and the
quantum case, where we may have a discrete number of points to deal with, are
noted. The question of optimization in this new context is also clarified.
Simple examples with symmetric and asymmetric multidimensional potentials are
briefly considered.


title: Evolution of Level Sets in Hamilton Parameter Space

abstract:
  We develop a formalism to study the use of Level Set Method (LSM) in the
investigation of evolution of observables in terms of parameters of the
Hamiltonian, both of the system itself and the control part. A simple example
with an analytic solution available perturbatively is examined. We show that B
splines can quite accurately and smoothly interpolate surfaces corresponding to
constant expectation value of observables which form the level sets as
projections on a finite mesh of data. Lastly we make a brief priliminary
scrutiny of teh possibility of using temperature as a relevant parameter in
ensembles of quantum systems.


title:
A succinct presentation of the quantized Klein-Gordon field, and a
  similar quantum presentation of the classical Klein-Gordon random field

abstract:
  A succinct presentation of the algebraic structure of the quantized
Klein-Gordon field can be given in terms of a Lorentz invariant inner product.
A presentation of a classical Klein-Gordon \emph{random} field at non-zero
temperature can be given in the same noncommutative algebraic style, allowing a
detailed comparison of the quantized Klein-Gordon field with a classical
Klein-Gordon random field.


title:
Entanglement concentration for unknown atomic entangled states via
  entanglement swapping

abstract:
  An entanglement concentration scheme for unknown atomic entanglement states
is proposed via entanglement swapping in cavity QED. Because the interaction
used here is a large-detuned one between two driven atoms and a quantized
cavity mode, the effects of the cavity decay and thermal field have been
eliminated. These advantages can warrant the experimental feasibility of the
current scheme.


title: Quantum Optimal Control and Level Sets

abstract:
  We investigate how the concepts of optimal control of measurables of a system
with a time dependent Hamiltonian may be mixed with the level set technique to
keep the desired entity invariant. We derive sets of equations for this purpose
and also algorithms for numerical use. The notion of constancy of measurables
in this context is also examined to make the techniques more useful in
real-life situation where some variability of the measurable may be tolerable.


title: Energy Level Sets for the Morse Potential

abstract:
  In continuation of our previous work investigating the possibility of the use
of the Level Set Method in quantum control, we here present some numerical
results for a Morse potential. We find that a proper treatment of the Morse
potential eigenfunctions and eigenvalues for the case of a system with a small
number of bound energy levels, the anharmonic perturbative approximation is
actually invalid. We, therefore, use a Runge-Kutta integration method that
gives more plausible results. We also calculate the dipole moment for the
transitions between the two levels with our eigenfunctions and find that there
is a critical depth of the potential. Finally we find the level sets giving
equal expectation values of the energy, and comment on the unitary operators
needed to make transitions from any level set to another.


title: Optimal Control of Quantum Systems and a Generalized Level Set Method

abstract:
  We study the application of a generalized form of the level set method used
in classical physical contexts to quantum optimal control situations. The set
of OCT equations needed to keep the expectation value of an observable constant
is first discussed and the dimensionality of the actual parameter space
carefully considered. Then we see how concepts of level set methods emerge that
may help solve the inverse problem associated with designing the control
Hamiltonian with greater speed. The formal equations and the algorithm are
presented.


title: Phase delay time and superluminal propagation in barrier tunneling

abstract:
  In this work we study the behaviour of Wigner phase delay time for tunneling
in the reflection mode. Our system consists of a circular loop connected to a
single wire of semi-infinite length in the presence of Aharonov-Bohm flux. We
calculate the analytical expression for the saturated delay time. This
saturated delay time is independent of Aharonov- Bohm flux and the width of the
opaque barrier thereby generalizing the Hartman effect. This effect implies
superluminal group velocities as a consequence. We also briefly discuss the
concept called "space collapse or space destroyer".


title: Quantum Fidelity Decay of Quasi-Integrable Systems

abstract:
  We show, via numerical simulations, that the fidelity decay behavior of
quasi-integrable systems is strongly dependent on the location of the initial
coherent state with respect to the underlying classical phase space. In
parallel to classical fidelity, the quantum fidelity generally exhibits
Gaussian decay when the perturbation affects the frequency of periodic phase
space orbits and power-law decay when the perturbation changes the shape of the
orbits. For both behaviors the decay rate also depends on initial state
location. The spectrum of the initial states in the eigenbasis of the system
reflects the different fidelity decay behaviors. In addition, states with
initial Gaussian decay exhibit a stage of exponential decay for strong
perturbations. This elicits a surprising phenomenon: a strong perturbation can
induce a higher fidelity than a weak perturbation of the same type.


title: Quantum lithography by coherent control of classical light pulses

abstract:
  The smallest spot in optical lithography and microscopy is generally limited
by diffraction. Quantum lithography, which utilizes interference between groups
of N entangled photons, was recently proposed to beat the diffraction limit by
a factor N. Here we propose a simple method to obtain N photons interference
with classical pulses that excite a narrow multiphoton transition, thus
shifting the "quantum weight" from the electromagnetic field to the
lithographic material. We show how a practical complete lithographic scheme can
be developed and demonstrate the underlying principles experimentally by
two-photon interference in atomic Rubidium, to obtain focal spots that beat the
diffraction limit by a factor of 2.


title: A computable measure of nonclassicality for light

abstract:
  We propose the Entanglement Potential (EP) as a measure of nonclassicality
for quantum states of a single-mode electromagnetic field. It is the amount of
two-mode entanglement that can be generated from the field using linear optics,
auxiliary classical states and ideal photodetectors. The EP detects
nonclassicality, has a direct physical interpretation, and can be computed
efficiently. These three properties together make it stand out from previously
proposed nonclassicality measures. We derive closed expressions for the EP of
important classes of states and analyze as an example the degradation of
nonclassicality in lossy channels.


title: Complex Moduli of Physical Quanta

abstract:
  Classical mechanics can be formulated using a symplectic structure on
classical phase space, while quantum mechanics requires a
complex-differentiable structure on that same space. Complex-differentiable
structures on a given real manifold are often not unique. This letter is
devoted to analysing the dependence of the notion of a quantum on the
complex-differentiable structure chosen on classical phase space.


title: Measurement Induced Localization of Relative Degrees of Freedom

abstract:
  We present a comprehensive study, using both analytical and numerical
methods, of measurement-induced localization of relational degrees of freedom.
Looking first at the interference of two optical modes, we find that the
localization of the relative phase can be as good for mixed states - in
particular for two initially Poissonian or thermal states - as for the
well-known case of two Fock states. In a realistic setup the localization for
mixed states is robust and experimentally accessible, and we discuss
applications to superselection rules. For an ideal setup and initial Fock
states we show how a relational Schr\"{o}dinger cat state emerges, and
investigate circumstances under which such a state is destroyed. In our second
example we consider the localization of relative atomic phase between two Bose
Einstein condensates, looking particularly at the build up of spatial
interference patterns, an area which has attracted much attention since the
work of Javanainen and Yoo. We show that the relative phase localizes much
faster than was intimated in previous studies focusing on the emerging
interference pattern itself. Finally, we explore the localization of relative
spatial parameters discussed in recent work by Rau, Dunningham and Burnett. We
retain their models of indistinguishable scattering but make different
assumptions. In particular we consider the case of a real distant observer
monitoring light scattering off two particles, who records events only from a
narrow field of view. The localization is only partial regardless of the number
of observations. This paper contributes to the wider debate on relationism in
quantum mechanics, which treats fundamental concepts - reference frames and
conservation laws - from a fully quantum and operational perspective.


title:
The Hypervirial-Pad\'e Summation Method Applied to the Anharmonic
  Oscillator

abstract:
  The energy eigenvalues of the anharmonic oscillator characterized by the
cubic potential for various eigenstates are determined within the framework of
the hypervirial-Pad\'e summation method. For this purpose the E[3,3] and E[3,4]
Pad\'e approximants are formed to the energy perturbation series and given the
energy eigenvalues up to fourth order in terms of the anharmonicity parameter
$\lambda$.


title: Quantum mechanics as a result of time broadening of the classical object

abstract:
  It is shown that the Schrodinger equation is a byproduct of more
deterministic Boltzmann-like equation. All physical information is derived from
the solution of this equation, which is a function of space and momentum. The
additional terms in this equation, when compared with the classical Boltzmann
equation, asserts that the particle should be broadened over the time axis,
i.e., it is partly present at the past, the present, and the future. The
solution of this equation itself is nonlocal. This solution requires an
especial time coordinate that is different from the usual time parameter
entering into the formalism of quantum mechanics. This time coordinate is
hidden in the structure of wave function. It is substantiated that the mixed
state is a natural result of time broadening. Furthermore, the interaction
between an object that is broadened in time and an observer that has a very
narrow time-width leads to the collapse of wave function.


title: Nonlinear Bogolyubov-Valatin transformations and quaternions

abstract:
  In introducing second quantization for fermions, Jordan and Wigner
(1927/1928) observed that the algebra of a single pair of fermion creation and
annihilation operators in quantum mechanics is closely related to the algebra
of quaternions H. For the first time, here we exploit this fact to study
nonlinear Bogolyubov-Valatin transformations (canonical transformations for
fermions) for a single fermionic mode. By means of these transformations, a
class of fermionic Hamiltonians in an external field is related to the standard
Fermi oscillator.

categories: quant-ph cond-mat.str-el hep-ph math-ph math.MP nucl-th


title: On Pseudo-Hermitian Hamiltonians and Their Hermitian Counterparts

abstract:
  In the context of two particularly interesting non-Hermitian models in
quantum mechanics we explore the relationship between the original Hamiltonian
H and its Hermitian counterpart h, obtained from H by a similarity
transformation, as pointed out by Mostafazadeh. In the first model, due to
Swanson, h turns out to be just a scaled harmonic oscillator, which explains
the form of its spectrum. However, the transformation is not unique, which also
means that the observables of the original theory are not uniquely determined
by H alone. The second model we consider is the original PT-invariant
Hamiltonian, with potential V=igx^3. In this case the corresponding h, which we
are only able to construct in perturbation theory, corresponds to a complicated
velocity-dependent potential. We again explore the relationship between the
canonical variables x and p and the observables X and P.


title: Information and Entropy in Quantum Theory

abstract:
  We look at certain thought experiments based upon the 'delayed choice' and
'quantum eraser' interference experiments, which present a complementarity
between information gathered from a quantum measurement and interference
effects. It has been argued that these experiments show the Bohm interpretation
of quantum theory is untenable. We demonstrate that these experiments depend
critically upon the assumption that a quantum optics device can operate as a
measuring device, and show that, in the context of these experiments, it cannot
be consistently understood in this way. By contrast, we then show how the
notion of 'active information' in the Bohm interpretation provides a coherent
explanation of the phenomena shown in these experiments.
  We then examine the relationship between information and entropy. The thought
experiment connecting these two quantities is the Szilard Engine version of
Maxwell's Demon, and it has been suggested that quantum measurement plays a key
role in this. We provide the first complete description of the operation of the
Szilard Engine as a quantum system. This enables us to demonstrate that the
role of quantum measurement suggested is incorrect, and further, that the use
of information theory to resolve Szilard's paradox is both unnecessary and
insufficient. Finally we show that, if the concept of 'active information' is
extended to cover thermal density matrices, then many of the conceptual
problems raised by this paradox appear to be resolved.


title: Semiclassical Husimi functions for spin systems

abstract:
  We derive a semiclassical approximation to the Husimi functions of stationary
states of spin systems. We rederive the Bohr-Sommerfeld quantization for spin
by locating the poles of the corresponding local Green function. The residues
correspond to the Husimi functions, which are seen to agree very well with
exact calculations.


title: Quantum theory as a statistical theory under symmetry

abstract:
  Both statistics and quantum theory deal with prediction using probability. We
will show that there can be established a connection between these two areas.
This will at the same time suggest a new, less formalistic way of looking upon
basic quantum theory.
  A total parameter space $\Phi$, equipped with a group $G$ of transformations,
gives the mental image of some quantum system, in such a way that only certain
components, functions of the total parameter $\phi$ can be estimated. Choose an
experiment/ question $a$, and get from this a parameter space $\Lambda^{a}$,
perhaps after some model reduction compatible with the group structure.
  The essentially statistical construction of this paper leads under natural
assumptions to the basic axioms of quantum mechanics, and thus implies a new
statistical interpretation of this traditionally very formal theory. The
probabilities are introduced via Born's formula, and this formula is proved
from general, reasonable assumptions, essentially symmetry assumptions.
  The theory is illustrated by a simple macroscopic example, and by the example
of a spin 1/2 particle. As a last example we show a connection to inference
between related macroscropic experiments under symmetry.


title: Basic Concepts for a Quantum Mechanical Theory of Events

abstract:
  A physical theory is proposed that obeys both the principles of special
relativity and of quantum mechanics. As a key feature, the laws are formulated
in terms of quantum events rather than of particle states. Temporal and spatial
coordinates of a quantum event are treated on equal footing, namely as
self-adjoint operators on a Hilbert space. The theory is not based upon
Lagrangian or Hamiltonian mechanics, and breaks with the concept of a
continuously flowing time. The physical object under consideration is a
spinless particle exposed to an external potential. The theory also accounts
for particle-antiparticle pair creation and annihilation, and is therefore not
a single-particle theory in the usual sense. The Maxwell equations are derived
as a straightforward consequence of certain fundamental commutation relations.
In the non-relativistic limit and in the limit of vanishing time uncertainty,
the Schr\"odinger equation of a spinless particle exposed to an external
electromagnetic field is obtained.


title:
Enhancement of NMR signal via phase whitening and quantum Fourier
  Transform

abstract:
  NNR siganal will be enhanced by phase pre-whitening of presession of spin
followed by quantum Fourier transform. FFT cannot the business as the phase is
random.


title:
Engineering antiferromagnetic Heisenberg spin chains for maximizing of
  the groundstate entanglement

abstract:
  We study the correlation function and concurrence for the eigenstates with
zero spin of engineered Heisenberg models to explore the entanglement property.
It is shown that the total nearest neighbor (NN) correlation function of
zero-spin eigenstates reaches its local extremum when the coupling strength is
uniform, and correspondingly the groundstate entanglement of $d$-D cubic AF
Heisenberg model is locally maximized. Moreover, numerical calculations for a
$N$-site quantum spin ring with cosinusoidally modulated exchange coupling,
i.e. $J_i=J(1+\cos(2\pi i/N))$, indicate that the uniform coupling is not the
unique optimal distribution for maximizing the groundstate entanglement and
this modulation of interactions can induce the longer range entanglement.


title:
State-insensitive trapping and guiding of cesium atoms using a two-color
  evanescent field around a subwavelength-diameter fiber

abstract:
  We calculate the optical potentials, i.e. the light shifts, of the ground and
excited states of atomic cesium in a two-color evanescent field around a
subwavelength-diameter fiber. We show that the light shifts of the
$6S_{1/2}\leftrightarrow 6P_{3/2}$ transitions can be minimized by tuning one
trapping light to around 934.5 nm in wavelength (central red-detuned magic
wavelength) and the other light to around 685.5 nm in wavelength (central
blue-detuned magic wavelength). The simultaneous use of the red- and
blue-detuned magic wavelengths allows state-insensitive two-color trapping and
guiding of cesium atoms along the thin fiber. Our results can be used to
efficiently load a two-color dipole trap by cesium atoms from a magneto-optical
trap and to perform continuous observations.


title: Multipartite Asymmetric Quantum Cloning

abstract:
  We investigate the optimal distribution of quantum information over
multipartite systems in asymmetric settings. We introduce cloning
transformations that take $N$ identical replicas of a pure state in any
dimension as input, and yield a collection of clones with non-identical
fidelities. As an example, if the clones are partitioned into a set of $M_A$
clones with fidelity $F^A$ and another set of $M_B$ clones with fidelity $F^B$,
the trade-off between these fidelities is analyzed, and particular cases of
optimal $N \to M_A+M_B$ cloning machines are exhibited. We also present an
optimal $1 \to 1+1+1$ cloning machine, which is the first known example of a
tripartite fully asymmetric cloner. Finally, it is shown how these cloning
machines can be optically realized.


title:
Experimental Demonstration of Quantum State Multi-meter and One-qubit
  Fingerprinting in a Single Quantum Device

abstract:
  We experimentally demonstrate in NMR a quantum interferometric multi-meter
for extracting certain properties of unknown quantum states without resource to
quantum tomography. It can perform direct state determinations,
eigenvalue/eigenvector estimations, purity tests of a quantum system, as well
as the overlap of any two unknown quantum states. Using the same device, we
also demonstrate one-qubit quantum fingerprinting.


title: Macroscopic tunnel splittings in superconducting phase qubits

abstract:
  Prototype Josephson-junction based qubit coherence times are too short for
quantum computing. Recent experiments probing superconducting phase qubits have
revealed previously unseen fine splittings in the transition energy spectra.
These splittings have been attributed to new microscopic degrees of freedom
(microresonators), a previously unknown source of decoherence. We show that
macroscopic resonant tunneling in the extremely asymmetric double well
potential of the phase qubit can have observational consequences that are
strikingly similar to the observed data.


title: Level Set Method for Quantum Control of Dipole Moment

abstract:
  We investigate the level set method (LSM) in a specific quantum context;
namely the dipole transition moment for a system with a nontrivial Morse
potential. We draw equal moment sets in the two-dimensional space of two
important parameters of the potential, namely the depth of the potential and
its width. Another variable is introduced as a scale and we see "motions" of
the level sets normal to the contours, as in classical contexts such as fluid
dynamics or in epitaxial crystal growth. Presumably interpolating the level
sets normally by smooth functions such as splines may give a fairly accurate
method of combining the variables to keep the dipole moment invariant.


title:
Experimental investigation of quantum key distribution with position and
  momentum of photon pairs

abstract:
  We investigate the utility of Einstein-Podolsky-Rosen correlations of the
position and momentum of photon pairs from parametric down-conversion in the
implementation of a secure quantum key distribution protocol. We show that
security is guaranteed by the entanglement between downconverted pairs, and can
be checked by either direct comparison of Alice and Bob's measurement results
or evaluation of an inequality of the sort proposed by Mancini et al. (Phys.
Rev. Lett. 88, 120401 (2002)).