title: De-Quantising the Solution of Deutsch's Problem

abstract:
  Probably the simplest and most frequently used way to illustrate the power of
quantum computing is to solve the so-called {\it Deutsch's problem}. Consider a
Boolean function $f: \{0,1\} \to \{0,1\}$ and suppose that we have a
(classical) black box to compute it. The problem asks whether $f$ is constant
(that is, $f(0) = f(1)$) or balanced ($f(0) \not= f(1)$). Classically, to solve
the problem seems to require the computation of $f(0)$ and $ f(1)$, and then
the comparison of results. Is it possible to solve the problem with {\em only
one} query on $f$? In a famous paper published in 1985, Deutsch posed the
problem and obtained a ``quantum'' {\em partial affirmative answer}. In 1998 a
complete, probability-one solution was presented by Cleve, Ekert, Macchiavello,
and Mosca. Here we will show that the quantum solution can be {\it
de-quantised} to a deterministic simpler solution which is as efficient as the
quantum one. The use of ``superposition'', a key ingredient of quantum
algorithm, is--in this specific case--classically available.


title: Optimal cloning of coherent states by linear optics

abstract:
  We describe an optical scheme for optimal Gaussian n to m cloning of coherent
states. The scheme, which generalizes a recently demonstrated scheme for 1 to 2
cloning, involves only linear optical components and homodyne detection.


title: Fuzzy de Sitter space-times via coherent states quantization

abstract:
  A construction of the 2d and 4d fuzzy de Sitter hyperboloids is carried out
by using a (vector) coherent state quantization. We get a natural
discretization of the dS "time" axis based on the spectrum of Casimir operators
of the respective maximal compact subgroups SO(2) and SO(4) of the de Sitter
groups SO\_0(1,2) and SO\_0(1,4). The continuous limit at infinite spins is
examined.


title:
Two-photon and EIT-assisted Doppler cooling in a three-level cascade
  system

abstract:
  Laser cooling is theoretically investigated in a cascade three-level scheme,
where the excited state of a laser-driven transition is coupled by a second
laser to a top, more stable level, as for alkali-earth atoms. The second laser
action modifies the atomic scattering cross section and produces temperatures
lower than those reached by Doppler cooling on the lower transition. When
multiphoton processes due to the second laser are relevant, an electromagnetic
induced transparency modifies the absorption of the first laser, and the final
temperature is controlled by the second laser parameters. When the intermediate
state is only virtually excited, the dynamics is dominated by the two-photon
process and the final temperature is determined by the spontaneous decay rate
of the top state.


title:
Quantum information processing using strongly-dipolar coupled nuclear
  spins

abstract:
  Dipolar coupled homonuclear spins present challenging, yet useful systems for
quantum information processing. In such systems, eigenbasis of the system
Hamiltonian is the appropriate computational basis and coherent control can be
achieved by specially designed strongly modulating pulses. In this letter we
describe the first experimental implementation of the quantum algorithm for
numerical gradient estimation on the eigenbasis of a four spin system.


title: Geometric phase of an atom in a magnetic storage ring

abstract:
  A magnetically trapped atom experiences an adiabatic geometric (Berry's)
phase due to changing field direction. We investigate theoretically such an
Aharonov-Bohm-like geometric phase for atoms adiabatically moving inside a
storage ring as demonstrated in several recent experiments. Our result shows
that this phase shift is easily observable in a closed loop interference
experiment, and thus the shift has to be accounted for in the proposed inertial
sensing applications. The spread in phase shift due to the atom transverse
distribution is quantified through numerical simulations.


title:
Programmable unknown quantum-state discriminators with multiple copies
  of program and data: A Jordan basis approach

abstract:
  The discrimination of any pair of unknown quantum states is performed by
devices processing three parts of inputs: copies of the pair of unknown states
we want to discriminate are respectively stored in two program systems and
copies of data, which is guaranteed to be one of the unknown states, in a third
system. We study the efficiency of such programmable devices with the inputs
prepared with $n$ and $m$ copies of unknown qubits used as programs and data,
respectively. By finding a symmetry in the average inputs, we apply the Jordan
basis method to derive their optimal unambiguous discrimination and the
minimum-error discrimination schemes. The dependence of the optimal solutions
on the a prior probabilities of the mean input states is also demonstrated.


title: Polarization-controlled single photons

abstract:
  Vacuum-stimulated Raman transitions are driven between two magnetic substates
of a rubidium-87 atom strongly coupled to an optical cavity. A magnetic field
lifts the degeneracy of these states, and the atom is alternately exposed to
laser pulses of two different frequencies. This produces a stream of single
photons with alternating circular polarization in a predetermined
spatio-temporal mode. MHz repetition rates are possible as no recycling of the
atom between photon generations is required. Photon indistinguishability is
tested by time-resolved two-photon interference.


title: Quantum Formalism: Brief Epistemological Considerations

abstract:
  We argue about a conceptual approach to quantum formalism. Starting from
philosophical conjectures (Platonism, Idealism and Realism) as basic ontic
elements (namely: math world, data world, and state of matter), we will analyze
the quantum superposition principle. This analysis bring us to demonstrate that
the basic assumptions affect in different ways:(a) the general problem of the
information and computability about a system, (b) the nature of the math tool
utilized and (c) the correspondent physical reality.


title: Classical Coding and the Cauchy-Schwarz Inequality

abstract:
  In classical coding, a single quantum state is encoded into classical
information. Decoding this classical information in order to regain the
original quantum state is known to be impossible. However, one can attempt to
construct a state which comes as close as possible. We give bounds on the
smallest possible trace distance between the original and the decoded state
which can be reached. We give two approaches to the problem: one starting from
Keyl and Werner's no-cloning theorem, and one starting from an operator-valued
Cauchy-Schwarz inequality.


title:
Squeezing the Local Oscillator Does Not Improve Signal-to-Noise Ratio in
  Heterodyne Laser Radar

abstract:
  The signal-to-noise ratio for heterodyne laser radar with a coherent
target-return beam and a squeezed local-oscillator beam is lower than that
obtained using a coherent local oscillator, regardless of the method employed
to combine the beams at the detector.


title: Reply to the comment in quant-ph/0609028 on "controlled teleportation"

abstract:
  This is to reply to the comment of Kenigsberg and Mor on our previous work
``Efficient many-party controlled teleportation of multi-qubit quantum
information via entanglement''[Phys. Rev. A 70, 022329 (2004)].


title: A minimum-disturbing quantum state discriminator

abstract:
  We propose two experimental schemes for quantum state discrimination that
achieve the optimal tradeoff between the probability of correct identification
and the disturbance on the quantum state.


title: Inductive Entanglement Classification of Four Qubits under SLOCC

abstract:
  Using an inductive approach to classify multipartite entangled states under
stochastic local operations and classical communication introduced recently by
the authors [Phys. Rev. A 74, 052336 (2006)], we give the complete
classification of four-qubit entangled pure states. Apart from the expected
degenerate classes, we show that there exist eight inequivalent ways to
entangle four qubits. In this respect, permutation symmetry is taken into
account and states with a structure differing only by parameters inside a
continuous set are considered to belong to the same class.


title:
On the Possibility of Nonlinearities and Chaos Underlying Quantum
  Mechanics

abstract:
  Some of the so-called imponderables and counterintuitive puzzles associated
with the Copenhagen interpretation of quantum mechanics appear to have
alternate, parallel explanations in terms of nonlinear dynamics and chaos.
These include the mocking up of exponetial decay in closed systems, possible
nonlinear extensions of Bell's inequalities, spontaneous symmetry breaking and
the existence of intrinsically preferred internal oscillation modes
(quantization) in nonlinear systems, and perhaps even the production of
diffraction-like patterns by "order in chaos." The existence of such parallel
explanations leads to an empirical, quasi-experimental approach to the question
of whether or not there might be fundamental nonlinearities underying quantum
mechanics. This will be contrasted with recent more theoretical approaches, in
which nonlinear extensions have been proposed rather as corrections to a
fundamentally linear quantum mechanics. Sources of nonlinearity, such as
special relativity and the measurement process itself, will be investigated, as
will possible implications of nonlinearities for entanglement and decoherence.
It is conceivable that in their debates both Einstein and Bohr could have been
right -- for chaos provides the fundamental determinism favored by Einstein,
yet for practical measurements it requires the probabilistic interpretation of
the Bohr school.


title:
BQP-complete Problems Concerning Mixing Properties of Classical Random
  Walks on Sparse Graphs

abstract:
  We describe two BQP-complete problems concerning properties of sparse graphs
having a certain symmetry. The graphs are specified by efficiently computable
functions which output the adjacent vertices for each vertex. Let i and j be
two given vertices. The first problem consists in estimating the difference
between the number of paths of length m from j to j and those which from i to
j, where m is polylogarithmic in the number of vertices. The scale of the
estimation accuracy is specified by some a priori known upper bound on the
growth of these differences with increasing m. The problem remains BQP-hard for
regular graphs with degree 4.
  The second problem is related to continuous-time classical random walks. The
walk starts at some vertex j. The promise is that the difference of the
probabilities of being at j and at i, respectively, decays with O(exp(-\mu t))
for some \mu>0. The problem is to decide whether this difference is greater
than a exp(-\mu T) or smaller than b exp(-\mu T) after some time instant T,
where T is polylogarithmic and the difference a-b is inverse polylogarithmic in
the number of vertices. Since the probabilities differ only by an exponentially
small amount, an exponential number of trials would be necessary if one tried
to answer this question by running the walk itself.
  A modification of this problem, asking whether there exists a pair of nodes
for which the probability difference is at least a exp(-\mu T), is
QCMA-complete.


title: Quantum-limited force measurement with an optomechanical device

abstract:
  We study the detection of weak coherent forces by means of an optomechanical
device formed by a highly reflecting isolated mirror shined by an intense and
highly monochromatic laser field. Radiation pressure excites a vibrational mode
of the mirror, inducing sidebands of the incident field, which are then
measured by heterodyne detection. We determine the sensitivity of such a scheme
and show that the use of an entangled input state of the two sideband modes
improves the detection, even in the presence of damping and noise acting on the
mechanical mode.


title:
Experimental Realization of Maximum Confidence State Discrimination for
  the Extraction of Quantum Information

abstract:
  We present the first experimental demonstration of the maximum confidence
measurement strategy for quantum state discrimination. Applying this strategy
to an arbitrary set of states assigns to each input state a measurement outcome
which, when realized, gives the highest possible confidence that the state was
indeed present. The theoretically optimal measurement for discriminating
between three equiprobable symmetric qubit states is implemented in a
polarization-based free-space interferometer. The maximum confidence in the
measurement result is 2/3. This is the first explicit demonstration that an
improvement in the confidence over the optimal minimum error measurement is
possible for linearly dependent states.


title: Entanglement of electrons in interacting molecules

abstract:
  Quantum entanglement is a concept commonly used with reference to the
existence of certain correlations in quantum systems that have no classical
interpretation. It is a useful resource to enhance the mutual information of
memory channels or to accelerate some quantum processes as, for example, the
factorization in Shor's Algorithm. Moreover, entanglement is a physical
observable directly measured by the von Neumann entropy of the system. We have
used this concept in order to give a physical meaning to the electron
correlation energy in systems of interacting electrons. The electronic
correlation is not directly observable, since it is defined as the difference
between the exact ground state energy of the many--electrons Schroedinger
equation and the Hartree--Fock energy. We have calculated the correlation
energy and compared with the entanglement, as functions of the nucleus--nucleus
separation using, for the hydrogen molecule, the Configuration Interaction
method. Then, in the same spirit, we have analyzed a dimer of ethylene, which
represents the simplest organic conjugate system, changing the relative
orientation and distance of the molecules, in order to obtain the configuration
corresponding to maximum entanglement.


title: Quantum Mechanical Reflection Resonances

abstract:
  Resonances in the reflection probability amplitude r(E) can occur in energy
ranges in which the reflection probability R(E)=|r(E)|^2 is 1. They occur as
the phase phi(E) defined by r(E) = t*(E)/t(E) = 1e^{i 2phi(E)} undergoes a
rapid change of pi radians. During this transition the phase angle exhibits a
Lorentzian profile in that d(phi(E))/dE ~= 1/[(E-E_0)^2+(hbar*gamma/2)^2]. The
energy E_0 identifies the location of a quasi-bound state, gamma measures the
lifetime of this state, and t(E) is a matrix element of the transfer operator.
Methods for computing and measuring these resonances are proposed.


title: Optimal computation with non-unitary quantum walks

abstract:
  Quantum versions of random walks on the line and the cycle show a quadratic
improvement over classical random walks in their spreading rates and mixing
times respectively. Non-unitary quantum walks can provide a useful optimisation
of these properties, producing a more uniform distribution on the line, and
faster mixing times on the cycle. We investigate the interplay between quantum
and random dynamics by comparing the resources required, and examining
numerically how the level of quantum correlations varies during the walk. We
show numerically that the optimal non-unitary quantum walk proceeds such that
the quantum correlations are nearly all removed at the point of the final
measurement. This requires only O(log T) random bits for a quantum walk of T
steps


title: Experimental realization of Wheeler's delayed-choice GedankenExperiment

abstract:
  The quantum "mystery which cannot go away" (in Feynman's words) of
wave-particle duality is illustrated in a striking way by Wheeler's
delayed-choice GedankenExperiment. In this experiment, the configuration of a
two-path interferometer is chosen after a single-photon pulse has entered it :
either the interferometer is \textit{closed} (\textit{i.e.} the two paths are
recombined) and the interference is observed, or the interferometer remains
\textit{open} and the path followed by the photon is measured. We report an
almost ideal realization of that GedankenExperiment, where the light pulses are
true single photons, allowing unambiguous which-way measurements, and the
interferometer, which has two spatially separated paths, produces high
visibility interference. The choice between measuring either the 'open' or
'closed' configuration is made by a quantum random number generator, and is
space-like separated -- in the relativistic sense -- from the entering of the
photon into the interferometer. Measurements in the closed configuration show
interference with a visibility of 94%, while measurements in the open
configuration allow us to determine the followed path with an error probability
lower than 1%.


title: Formation of ions by high energy photons

abstract:
  We calculate the electron energy spectrum of ionization by a high energy
photon, accompanied by creation of electron-positron pair. The total cross
section of the process is also obtained. The asymptotics of the cross section
does not depend on the photon energies. At the photon energies exceeding a
certain value $\omega_0$ this appeares to to be the dominant mechanism of
formation of the ions. The dependence of $\omega_0$ on the value of nuclear
charge is obtained. Our results are consistent with experimental data.


title:
Global stability criterion for a quantum feedback control process on a
  single qubit and exponential stability in case of perfect detection
  efficiency

abstract:
  Quantum feedback control is a technology which can be used to drive a quantum
system into a predetermined eigenstate. In this article, sufficient conditions
for the experiment parameters of a quantum feedback control process of a
homodyne QND measurement are given to guarantee feedback control of a spin-1/2
quantum system in case of imperfect detection efficiency. For the case of pure
states and perfect detection efficiency, time scales of feedback control
processes are calculated.


title:
Emergent quantum phases in a heteronuclear molecular Bose--Einstein
  condensate model

abstract:
  We study a three-mode Hamiltonian modelling a heteronuclear molecular
Bose--Einstein condensate. Two modes are associated with two distinguishable
atomic constituents, which can combine to form a molecule represented by the
third mode. Beginning with a semi-classical analogue of the model, we conduct
an analysis to determine the phase space fixed points of the system.
Bifurcations of the fixed points naturally separate the coupling parameter
space into different regions. Two distinct scenarios are found, dependent on
whether the imbalance between the number operators for the atomic modes is zero
or non-zero. This result suggests the ground-state properties of the model
exhibit an unusual sensitivity on the atomic imbalance. We then test this
finding for the quantum mechanical model. Specifically we use Bethe ansatz
methods, ground-state expectation values, the character of the quantum
dynamics, and ground-state wavefunction overlaps to clarify the nature of the
ground-state phases. The character of the transition is smoothed due to quantum
fluctuations, but we may nonetheless identify the emergence of a quantum phase
boundary in the limit of zero atomic imbalance.


title:
Probabilistic cloning with supplementary information contained in the
  quantum states of two auxiliary systems

abstract:
  In probabilistic cloning with two auxiliary systems, we consider and compare
three different protocols for the success probabilities of cloning. We show
that, in certain circumstances, it may increase the success probability to add
an auxiliary system to the probabilistic cloning machine having one auxiliary
system, but we always can find another cloning machine with one auxiliary
system having the same success probability as that with two auxiliary systems.


title: Information-disturbance tradeoff for spin coherent state estimation

abstract:
  We show how to quantify the optimal tradeoff between the amount of
information retrieved by a quantum measurement in estimating an unknown spin
coherent state and the disturbance on the state itself, and how to derive the
corresponding minimum-disturbing measurement.


title: Hardy's proof of nonlocality in the presence of noise

abstract:
  We extend the validity of Hardy's nonlocality without inequalities proof to
cover the case of special one-parameter classes of non-pure statistical
operators. These mixed states are obtained by mixing the Hardy states with a
completely chaotic noise or with a colored noise and they represent a realistic
description of imperfect preparation processes of (pure) Hardy states in
nonlocality experiments. Within such a framework we are able to exhibit a
precise range of values of the parameter measuring the noise affecting the
non-optimal preparation of an arbitrary Hardy state, for which it is still
possible to put into evidence genuine nonlocal effects. Equivalently, our work
exhibits particular classes of bipartite mixed states whose constituents do not
admit any local and deterministic hidden variable model reproducing the quantum
mechanical predictions.


title:
A new effective mass Hamiltonian and associated Lame equation: bound
  states

abstract:
  A new quantum model with rational functions for the potential and effective
mass is proposed in a stretchable region outside which both are constant.
Starting from a generalized effective mass kinetic energy operator the matching
and boundary conditions for the envelope wave functions are derived. It is
shown that in a mapping to an auxiliary constant-mass Schrodinger picture one
obtains one-period ``associated Lame'' well bounded by two delta-wells or
delta-barriers depending on the values of one ordering parameter. The results
for bound states of this new solvable model are provided for a wide variation
of the parameters.


title: Asymmetric quantum telecloning of multiqubit states

abstract:
  We propose a scheme of 1$\to$2 optimal universal asymmetric quantum
telecloning of pure multiqubit states. In particular, we first investigate the
asymmetric telecloning of arbitrary 2-qubit states and then extend it to the
case of multiqubit system. Many figures of merit for the telecloning process
are checked, including the entanglement of the quantum channel and fidelities
of the clones. Our scheme can be used for the 1$\to$4 universal telecloning of
mixed multiqubit states.


title:
Coupled cavity QED for coherent control of photon transmission (I):
  Green function approach for hybrid systems with two-level doping

abstract:
  This is the first one of a series of our papers theoretically studying the
coherent control of photon transmission along the coupled resonator optical
waveguide (CROW) by doping artificial atoms for various hybrid structures. We
will provide the several approaches correspondingly based on Green function,
the mean field method and spin wave theory et al. In the present paper we adopt
the two-time Green function approach to study the coherent transmission photon
in a CROW with homogeneous couplings, each cavity of which is doped by a
two-level artificial atom. We calculate the two-time correlation function for
photon in the weak-coupling case. Its poles predict the exact dispersion
relation, which results in the group velocity coherently controlled by the
collective excitation of the doping atoms. We emphasize the role of the
population inversion of doping atoms induced by some polarization mechanism.


title: Reply to Comment on 'Spin Decoherence in Superconducting Atom Chips'

abstract:
  This is a reply to the Comment on 'Spin Decoherence in Superconducting Atom
Chips' [arXiv:quant-ph/0610095 (2006)].


title:
Value Assignments to Observables Depending on a History of Context
  Change

abstract:
  The functional composition principle is generalized by taking into account of
history of context change. Analysis of Peres' example shows hysteresis of value
assignments. It is shown that value assignments which depend on the history of
context change are possible in the case that the Hilbert space of state vectors
is finite dimensional.


title: Entanglement in quantum information theory

abstract:
  This thesis covers several aspects of entanglement in the context of quantum
information theory.


title: Simple setups for quantum games in optical networks

abstract:
  In this work, we propose two optical setups for two-players, non-zero and
zero sum, quantum games in optical networks using light polarization of
single-photon pulses, single-photon detectors and linear optical devices. The
optical setups proposed can be easily implemented permitting a fast
experimental realization of quantum games with present technology.


title:
Comments on Proposed Gravitational Modifications of Schrodinger Dynamics
  and their Experimental Implications

abstract:
  We discuss aspects of gravitational modifications of Schrodinger dynamics
proposed by Diosi and Penrose. We consider first the Diosi-Penrose criterion
for gravitationally induced state vector reduction, and compute the reduction
time expected for a superposition of a uniform density cubical solid in two
positions displaced by a small fraction of the cube side. We show that the
predicted effect is much smaller than would be observable in the proposed
Marshall et al. mirror experiment. We then consider the ``Schrodinger -Newton''
equation for an N-particle system. We show that in the independent particle
approximation, it differs from the usual Hartree approximation applied to the
Newtonian potential by self-interaction terms, which do not have a consistent
Born rule interpretation. This raises doubts about the use of the
Schrodinger-Newton equation to calculate gravitational effects on molecular
interference experiments. When the effects of Newtonian gravitation on
molecular diffraction are calculated using the standard many-body Schrodinger
equation, no washing out of the interference pattern is predicted.


title: Effective generation of cat and kitten states

abstract:
  We present an effective method of coherent state superposition (cat state)
generation using single trapped ion in a Paul trap. The method is
experimentally feasible for coherent states with amplitude $\alpha \le 2$ using
available technology. It works both in and beyond the Lamb-Dicke regime.


title: Quantum state conversion between continuous variable and qubits systems

abstract:
  We investigate how quantum state can be converted between continuous variable
and qubits systems. Non-linear Jaynes-Cumings interaction Hamiltonian is
introduced to accomplish the conversion. Detail analysis on the conversion of
thermal state exhibits that pretty good fidelity can be achieved.


title: Conversion of entanglement between continuous variable and qubit systems

abstract:
  We investigate how entanglement can be transferred between continuous
variable and qubit systems. We find that a two-mode squeezed vacuum state and a
continuous variable Werner state can be converted to the product states of
infinitive number of two-qubit states while keeping the entanglement. The
reverse process is also possible.


title: Contractions, Matrix Paramatrizations, and Quantum Information

abstract:
  In this note, we discuss dilation-theoretic matrix parametrizations of
contractions and positive matrices. These parametrizations are then applied to
some problems in quantum information theory. First we establish some properties
of positive maps, or entanglement witnesses. Two further applications,
concerning concrete dilations of completely positive maps, in particular
quantum operations, are given.


title:
PT/Non-PT Symmetric and Non-Hermitian Poschl-Teller-Like Solvable
  Potentials via Nikiforov-Uvarov Method

abstract:
  The solutions of trigonometric Scarf potential, PT/non-PT-symmetric and
non-Hermitian q-deformed hyperbolic Scarf and Manning-Rosen potentials are
obtained by solving the Schrodinger equation. The Nikiforov-Uvarov method is
used to obtain the real energy spectra and corresponding eigenfunctions.


title:
Reply to Comment on "On the Original Proof by Reductio ad Absurdum of
  the Hohenberg-Kohn Theorem for Many-Electron Coulomb Systems" [Szczepanik, W;
  Dulak, M.; Wesolowski, T. A., Int. J. Quantum Chem. 106 (2006)]

abstract:
  It is shown that the title Comment by W. Szczepanik, M. Dulak, and T. A.
Wesolowski [Int. J. Quantum Chem. 106 (2006)] is unsatisfactory.

categories: quant-ph cond-mat.soft cond-mat.str-el physics.chem-ph


title:
An application of renormalization in Hilbert space at phase transition
  points in strongly interacting systems

abstract:
  We introduce an algorithm aimed to reduce the dimensions of Hilbert space. It
is used here in order to study the behaviour of low energy states of strongly
interacting quantum many-body systems at first order transitions and avoided
crossings. The method is tested on different frustrated quantum spin ladders
with two legs. The role and importance of symmetries are investigated by using
different bases of states.


title: Estimating the Degree of Entanglement of Unknown Gaussian States

abstract:
  We give an introduction to Gaussian states and operations. A discussion of
the entanglement properties of bipartite Gaussian states in terms of its
covariance matrix follows. It is explained how entanglement can be witnessed
using feasible measurements, e.g. homodyne measurements. We find that the
outcome of such a measurements cannot only witness but also quantify
entanglement.


title: Single-photon-emitting diodes: a review

abstract:
  Compact and reliable sources of non-classical light could find many
applications in emerging technologies such as quantum cryptography, quantum
imaging and also in fundamental tests of quantum physics. Single self-assembled
quantum dots have been widely studied for this reason, but the vast majority of
reported work has been limited to optically excited sources. Here we discuss
the progress made so far, and prospects for, electrically driven
single-photon-emitting diodes (SPEDs).


title: Suppression of Quantum Scattering in Strongly Confined Systems

abstract:
  We demonstrate that scattering of particles strongly interacting in three
dimensions (3D) can be suppressed at low energies in a quasi-one-dimensional
(1D) confinement. The underlying mechanism is the interference of the s- and
p-wave scattering contributions with large s- and p-wave 3D scattering lengths
being a necessary prerequisite. This low-dimensional quantum scattering effect
might be useful in "interacting" quasi-1D ultracold atomic gases, guided atom
interferometry, and impurity scattering in strongly confined quantum wire-based
electronic devices.


title:
Demonstration of three-photon de Broglie wavelength by projection
  measurement

abstract:
  Two schemes of projection measurement are realized experimentally to
demonstrate the de Broglie wavelength of three photons without the need for a
maximally entangled three-photon state (the NOON state). The first scheme is
based on the proposal by Wang and Kobayashi (Phys. Rev. A {\bf 71}, 021802)
that utilizes a couple of asymmetric beam splitters while the second one
applies the general method of NOON state projection measurement to three-photon
case. Quantum interference of three photons is responsible for projecting out
the unwanted states, leaving only the NOON state contribution in these schemes
of projection measurement.


title: Graph States Under the Action of Local Clifford Group in Non-Binary Case

abstract:
  Graph states are well-entangled quantum states that are defined based on a
graph. Of course, if two graphs are isomorphic their associated states are the
same. Also, we know local operations do not change the entanglement of quantum
states. Therefore, graph states that are either isomorphic or equivalent under
the local Clifford group have the same properties. In this paper, we first
establish a bound on the number of graph states which are neither isomorphic
nor equivalent under the action of local Clifford group.
  Also, we study graph states in non-binary case. We translate the action of
local Clifford group, as well as measurement of Pauli operators, into
transformations on their associated graphs.
  Finally, we present an efficient algorithm to verify whether two graph
states, in non-binary case, are locally equivalent or not.


title: Macroscopic Entanglement and Phase Transitions

abstract:
  This paper summarises the results of our research on macroscopic entanglement
in spin systems and free Bosonic gases. We explain how entanglement can be
observed using entanglement witnesses which are themselves constructed within
the framework of thermodynamics and thus macroscopic observables. These
thermodynamical entanglement witnesses result in bounds on macroscopic
parameters of the system, such as the temperature, the energy or the
susceptibility, below which entanglement must be present. The derived bounds
indicate a relationship between the occurrence of entanglement and the
establishment of order, possibly resulting in phase transition phenomena. We
give a short overview over the concepts developed in condensed matter physics
to capture the characteristics of phase transitions in particular in terms of
order and correlation functions. Finally we want to ask and speculate whether
entanglement could be a generalised order concept by itself, relevant in
(quantum induced) phase transitions such as BEC, and that taking this view may
help us to understand the underlying process of high-T superconductivity.


title: Subsystem Pseudo-pure States

abstract:
  A critical step in experimental quantum information processing (QIP) is to
implement control of quantum systems protected against decoherence via
informational encodings, such as quantum error correcting codes, noiseless
subsystems and decoherence free subspaces. These encodings lead to the promise
of fault tolerant QIP, but they come at the expense of resource overheads.
  Part of the challenge in studying control over multiple logical qubits, is
that QIP test-beds have not had sufficient resources to analyze encodings
beyond the simplest ones. The most relevant resources are the number of
available qubits and the cost to initialize and control them. Here we
demonstrate an encoding of logical information that permits the control over
multiple logical qubits without full initialization, an issue that is
particularly challenging in liquid state NMR. The method of subsystem
pseudo-pure state will allow the study of decoherence control schemes on up to
6 logical qubits using liquid state NMR implementations.


title: Monogamy of Bell correlations and Tsirelson's bound

abstract:
  We consider three parties, A, B, and C, each performing one of two local
measurements on a shared quantum state of arbitrary dimension. We characterize
the trade-off between the nonlocality of the Bell correlations observed by AB
and of those observed by AC. This generalizes Tsirelson's bound on the quantum
value of the CHSH inequality, the latter being recovered when C is completely
uncorrelated with AB. We also discuss the trade-off between Bell violations and
local expectation values of observables that anticommute with the ones used in
the Bell test.


title: Evenly distributed unitaries: on the structure of unitary designs

abstract:
  We clarify the mathematical structure underlying unitary $t$-designs. These
are sets of unitary matrices, evenly distributed in the sense that the average
of any $t$-th order polynomial over the design equals the average over the
entire unitary group. We present a simple necessary and sufficient criterion
for deciding if a set of matrices constitutes a design. Lower bounds for the
number of elements of 2-designs are derived. We show how to turn mutually
unbiased bases into approximate 2-designs whose cardinality is optimal in
leading order. Designs of higher order are discussed and an example of a
unitary 5-design is presented. We comment on the relation between unitary and
spherical designs and outline methods for finding designs numerically or by
searching character tables of finite groups. Further, we sketch connections to
problems in linear optics and questions regarding typical entanglement.


title:
A chaotic dynamical reduction model for the quantum mechanical state
  vector

abstract:
  A new model is proposed for the purpose of modelling the ``wave function
collapse'' of a two-state quantum system. The collapse to a classical state is
driven by a nonlinear evolution equation with an extreme sensitivity to
absolute phase. It is hypothesized that the phase, or part of it, is displaying
chaotic behaviour. This chaotic behaviour can then be responsible for the
apparent indeterminacy we are experiencing for a single quantum system. Through
this randomness, the statistical ``ensemble'' behaviour, due to Born, to
describe a single quantum system, is no longer needed.


title: Experimental Implementation of Logical Bell State Encoding

abstract:
  Liquid phase NMR is a general purpose test-bed for developing methods of
coherent control relevant to quantum information processing. Here we extend
these studies to the coherent control of logical qubits and in particular to
the unitary gates necessary to create entanglement between logical qubits. We
report an experimental implementation of a conditional logical gate between two
logical qubits that are each in decoherence free subspaces that protect the
quantum information from fully correlated dephasing.


title:
Properties of a coupled two species atom-heteronuclear molecule
  condensate

abstract:
  We study the coherent association of a two-species atomic condensate into a
condensate of heteronuclear diatomic molecules, using both a semiclassical
treatment and a quantum mechanical approach. The differences and connections
between the two approaches are examined. We show that, in this coupled
nonlinear atom-molecule system, the population difference between the two
atomic species plays a significant role in the ground-state stability
properties as well as in coherent population oscillation dynamics.


title:
Characterization of a noisy quantum process by complementary classical
  operations

abstract:
  One of the challenges in quantum information is the demonstration of quantum
coherence in the operations of experimental devices. While full quantum process
tomography can do the job, it is both cumbersome and unintuitive. In this
presentation, we show that a surprisingly detailed and intuitively accessible
characterization of errors is possible by measuring the error statistics of
only two complementary classical operations of a quantum gate.


title: Quantum Booth's array multiplier

abstract:
  A new quantum architecture for multiplying signed integers is presented based
on Booth's algorithm, which is well known in classical computation. It is shown
how a quantum binary chain might be encoded by its flank changes, giving the
final product in 2's-complement representation.


title: Channel simulation with quantum side information

abstract:
  We study and solve the problem of classical channel simulation with quantum
side information at the receiver. This is a generalization of both the
classical reverse Shannon theorem, and the classical-quantum Slepian-Wolf
problem. The optimal noiseless communication rate is found to be reduced from
the mutual information between the channel input and output by the Holevo
information between the channel output and the quantum side information.
  Our main theorem has two important corollaries. The first is a quantum
generalization of the Wyner-Ziv problem: rate-distortion theory with quantum
side information. The second is an alternative proof of the trade-off between
classical communication and common randomness distilled from a quantum state.
  The fully quantum generalization of the problem considered is quantum state
redistribution. Here the sender and receiver share a mixed quantum state and
the sender wants to transfer part of her state to the receiver using
entanglement and quantum communication. We present outer and inner bounds on
the achievable rate pairs.


title: Security aspects of the Authentication used in Quantum Cryptography

abstract:
  Unconditionally secure message authentication is an important part of quantum
cryptography (QC). In this correspondence, we analyze security effects of using
a key obtained from QC for authentication purposes in later rounds of QC. In
particular, the eavesdropper gains partial knowledge on the key in QC that may
have an effect on the security of the authentication in the later round. Our
initial analysis indicates that this partial knowledge has little effect on the
authentication part of the system, in agreement with previous results on the
issue. However, when taking the full QC protocol into account, the picture is
different. By accessing the quantum channel used in QC, the attacker can change
the message to be authenticated. This, together with partial knowledge of the
key, does incur a security weakness of the authentication. The underlying
reason for this is that the authentication used, which is insensitive to such
message changes when the key is unknown, becomes sensitive when used with a
partially known key. We suggest a simple solution to this problem, and stress
usage of this or an equivalent extra security measure in QC.


title: A study on black-body radiation: classical and binary photons

abstract:
  The present study gives a detailed analysis of the black-body radiation based
on classical random variables. It is shown that the energy of a mode of a
chaotic radiation field (Gauss variable) can be uniquely decomposed into a sum
of a discrete variable (Planck variable having the Planck-Bose distribution)
and a continuous dark variable (with a truncated exponential distribution of
finite support). The Planck variable is decomposed, on one hand, into a sum of
binary variables representing the binary photons of energies 2^s*h*nu with
s=0,1,2,etc. In this way the black-body radiation can be viewed as a mixture of
thermodinamically independent fermion gases. The Planck variable can also be
decomposed into a sum of independent Poisson components representing the
classical photo-molecules of energies m*h*nu with m=1,2,3,etc. These classical
photons have only particle-like fluctuations, on the other hand, the binary
photons have wave-particle fluctuations of fermionic character.


title: Genuine tripartite entanglement and quantum phase transition

abstract:
  A new formulation called as entanglement measure for simplification, is
presented to characterize genuine tripartite entanglement of $(2\times 2\times
n)-$dimensional quantum pure states. The formulation shows that the genuine
tripartite entanglement can be described only on the basis of the local
$(2\times 2)-$dimensional reduced density matrix. In particular, the two
exactly solvable models of spin system studied by Yang (Phys. Rev. A
\textbf{71}, 030302(R) (2005)) is reconsidered by employing the entanglement
measure. The results show that a discontinuity in the first derivative of the
entanglement measure or in the entanglement measure itself of the ground state
just corresponds to the existence of quantum phase transition, which is
obviously prior to concurrence. Hence, the given entanglement measure may
become a new alternate candidate to help study the connection between quantum
entanglement and quantum phase transitions.


title:
Phase equivalent potentials, Complex coordinates and Supersymmetric
  Quantum Mechanics

abstract:
  Supersymmetric Quantum Mechanics may be used to construct reflectionless
potentials and phase-equivalent potentials. The exactly solvable case of the
$\lambda sech^2$ potential is used to show that for certain values of the
strength $\lambda$ the phase-equivalent singular potential arising from the
elimination of all the boundstates is identical to the original potential
evaluated at a point shifted in the complex cordinate space. This equivalence
has the consequence that certain general relations valid for reflectionless
potentials and phase-equivalent potentials lead to hitherto unknown identities
satisfied by the Associated Legendre functions. This exactly solvable probelm
is used to demonstrate some aspects of scattering theory.


title:
Error Exponent in Asymmetric Quantum Hypothesis Testing and Its
  Application to Classical-Quantum Channel coding

abstract:
  In the simple quantum hypothesis testing problem, upper bound with asymmetric
setting is shown by using a quite useful inequality by Audenaert et al,
quant-ph/0610027, which was originally invented for symmetric setting. Using
this upper bound, we obtain the Hoeffding bound, which are identical with the
classical counter part if the hypotheses, composed of two density operators,
are mutually commutative. Our upper bound improves the bound by Ogawa-Hayashi,
and also provides a simpler proof of the direct part of the quantum Stein's
lemma. Further, using this bound, we obtain a better exponential upper bound of
the average error probability of classical-quantum channel coding.


title: Quantum entanglement of two flux qubits induced by an auxiliary SQUID

abstract:
  We revisit a theoretical scheme to create quantum entanglement of two
three-levels superconducting quantum interference devices (SQUIDs) with the
help of an auxiliary SQUID. In this scenario, two three-levels systems are
coupled to a quantized cavity field and a classical external field and thus
form dark states. The quantum entanglement can be produced by a quantum
measurement on the auxiliary SQUID. Our investigation emphasizes the quantum
effect of the auxiliary SQUID. For the experimental feasibility and
accessibility of the scheme, we calculate the time evolution of the whole
system including the auxiliary SQUID. To ensure the efficiency of generating
quantum entanglement, relations between the measurement time and dominate
parameters of the system are analyzed according to detailed calculations.


title: Single observable concurrence measurement without simultaneous copies

abstract:
  We present a protocol that allows us to obtain the concurrence of any two
qubit pure state by performing a minimal and optimal tomography of one of the
subsystems through measuring a single observable of an ancillary four
dimensional qudit. An implementation for a system of trapped ions is also
proposed, which can be achieved with present day experimental techniques.


title: Direct measurement of concurrence for atomic two-qubit pure states

abstract:
  We propose a general scheme to measure the concurrence of an arbitrary
two-qubit pure state in atomic systems. The protocol is based on one- and
two-qubit operations acting on two available copies of the bipartite system,
and followed by a global qubit readout. We show that it is possible to encode
the concurrence in the probability of finding all atomic qubits in the ground
state. Two possible scenarios are considered: atoms crossing 3D microwave
cavities and trapped ion systems.


title:
A non-adiabatic approach to entanglement distribution over long
  distances

abstract:
  Entanglement distribution between trapped-atom quantum memories, viz. single
atoms in optical cavities, is addressed. In most scenarios, the rate of
entanglement distribution depends on the efficiency with which the state of
traveling single photons can be transferred to trapped atoms. This loading
efficiency is analytically studied for two-level, $V$-level, $\Lambda$-level,
and double-$\Lambda$-level atomic configurations by means of a system-reservoir
approach. An off-resonant non-adiabatic approach to loading $\Lambda$-level
trapped-atom memories is proposed, and the ensuing trade-offs between the
atom-light coupling rate and input photon bandwidth for achieving a high
loading probability are identified. The non-adiabatic approach allows a broad
class of optical sources to be used, and in some cases it provides a higher
system throughput than what can be achieved by adiabatic loading mechanisms.
The analysis is extended to the case of two double-$\Lambda$ trapped-atom
memories illuminated by a polarization-entangled biphoton.


title:
Generation of two-photon states with arbitrary degree of entanglement
  via nonlinear crystal superlattices

abstract:
  We demonstrate a general method of engineering the joint quantum state of
photon pairs produced in spontaneous parametric downconversion (PDC). The
method makes use of a superlattice structure of nonlinear and linear materials,
in conjunction with a broadband pump, to manipulate the group delays of the
signal and idler photons relative to the pump pulse, and realizes a joint
spectral amplitude with arbitrary degree of entanglement for the generated
pairs. This method of group delay engineering has the potential of synthesizing
a broad range of states including factorizable states crucial for quantum
networking and states optimized for Hong-Ou-Mandel interferometry. Experimental
results for the latter case are presented, illustrating the principles of this
approach.


title:
Generation of Pure-State Single-Photon Wavepackets by Conditional
  Preparation Based on Spontaneous Parametric Downconversion

abstract:
  We study the conditional preparation of single photons based on parametric
downconversion, where the detection of one photon from a given pair heralds the
existence of a single photon in the conjugate mode. We derive conditions on the
modal characteristics of the photon pairs, which ensure that the conditionally
prepared single photons are quantum-mechanically pure. We propose specific
experimental techniques that yield photon pairs ideally suited for
single-photon conditional preparation.


title: Expectations of two-level telegraph noise

abstract:
  We find expectation values of functions of time integrated two-level
telegraph noise. Expectation values of this noise are evaluated under simple
control pulses. Both the Gaussian limit and $1/f$ noise are considered. We
apply the results to a specific superconducting quantum computing example,
which illustrates the use of this technique for calculating error
probabilities.


title: Merlin-Arthur Games and Stoquastic Complexity

abstract:
  MA is a class of decision problems for which `yes'-instances have a proof
that can be efficiently checked by a classical randomized algorithm. We prove
that MA has a natural complete problem which we call the stoquastic k-SAT
problem. This is a matrix-valued analogue of the satisfiability problem in
which clauses are k-qubit projectors with non-negative matrix elements, while a
satisfying assignment is a vector that belongs to the space spanned by these
projectors. Stoquastic k-SAT is the first non-trivial example of a MA-complete
problem. We also study the minimum eigenvalue problem for local stoquastic
Hamiltonians that was introduced in quant-ph/0606140, stoquastic LH-MIN. A new
complexity class StoqMA is introduced so that stoquastic LH-MIN is
StoqMA-complete. Lastly, we consider the average LH-MIN problem for local
stoquastic Hamiltonians that depend on a random or `quenched disorder'
parameter, stoquastic AV-LH-MIN. We prove that stoquastic AV-LH-MIN is
contained in the complexity class \AM, the class of decision problems for which
yes-instances have a randomized interactive proof with two-way communication
between prover and verifier.


title: Wigner formula of rotation matrices and quantum walks

abstract:
  Quantization of a random-walk model is performed by giving a qudit (a
multi-component wave function) to a walker at site and by introducing a quantum
coin, which is a matrix representation of a unitary transformation. In quantum
walks, the qudit of walker is mixed according to the quantum coin at each time
step, when the walker hops to other sites. As special cases of the quantum
walks driven by high-dimensional quantum coins generally studied by Brun,
Carteret, and Ambainis, we study the models obtained by choosing rotation as
the unitary transformation, whose matrix representations determine quantum
coins. We show that Wigner's $(2j+1)$-dimensional unitary representations of
rotations with half-integers $j$'s are useful to analyze the probability laws
of quantum walks. For any value of half-integer $j$, convergence of all moments
of walker's pseudovelocity in the long-time limit is proved. It is generally
shown for the present models that, if $(2j+1)$ is even, the probability measure
of limit distribution is given by a superposition of $(2j+1)/2$ terms of scaled
Konno's density functions, and if $(2j+1)$ is odd, it is a superposition of $j$
terms of scaled Konno's density functions and a Dirac's delta function at the
origin. For the two-, three-, and four-component models, the probability
densities of limit distributions are explicitly calculated and their dependence
on the parameters of quantum coins and on the initial qudit of walker is
completely determined. Comparison with computer simulation results is also
shown.

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


title: Einstein's fluctuation formula. A historical overview

abstract:
  A historical overview is given on the basic results which appeared by the
year 1926 concerning Einstein's fluctuation formula of black-body radiation, in
the context of light-quanta and wave-particle duality. On the basis of the
original publications (from Planck's derivation of the black-body spectrum and
Einstein's introduction of the photons up to the results of Born, Heisenberg
and Jordan on the quantization of a continuum) a comparative study is presented
on the first line of thoughts that led to the concept of quanta. The nature of
the particle-like fluctuations and the wave-like fluctuations are analysed by
using several approaches. With the help of the classical probability theory, it
is shown that the infinite divisibility of the Bose distribution leads to the
new concept of classical poissonian photo-multiplets or to the binary
photo-multiplets of fermionic character. As an application, Einstein's
fluctuation formula is derived as a sum of fermion type fluctuations of the
binary photo-multiplets.


title: Thermodynamic Limit and Decoherence: Rigorous Results

abstract:
  Time evolution operator in quantum mechanics can be changed into a
statistical operator by a Wick rotation. This strict relation between
statistical mechanics and quantum evolution can reveal deep results when the
thermodynamic limit is considered. These results translate in a set of theorems
proving that these effects can be effectively at work producing an emerging
classical world without recurring to any external entity that in some cases
cannot be properly defined. In a many-body system has been recently shown that
Gaussian decay of the coherence is the rule with a duration of recurrence more
and more small as the number of particles increases. This effect has been
observed experimentally. More generally, a theorem about coherence of bulk
matter can be proved. All this takes us to the conclusion that a well definite
boundary for the quantum to classical world does exist and that can be drawn by
the thermodynamic limit, extending in this way the deep link between
statistical mechanics and quantum evolution to a high degree.


title:
Electron Fabry-Perot interferometer with two entangled magnetic
  impurities

abstract:
  We consider a one-dimensional (1D) wire along which single conduction
electrons can propagate in the presence of two spin-1/2 magnetic impurities.
The electron may be scattered by each impurity via a contact-exchange
interaction and thus a spin-flip generally occurs at each scattering event.
Adopting a quantum waveguide theory approach, we derive the stationary states
of the system at all orders in the electron-impurity exchange coupling
constant. This allows us to investigate electron transmission for arbitrary
initial states of the two impurity spins. We show that for suitable electron
wave vectors, the triplet and singlet maximally entangled spin states of the
impurities can respectively largely inhibit the electron transport or make the
wire completely transparent for any electron spin state. In the latter case, a
resonance condition can always be found, representing an anomalous behaviour
compared to typical decoherence induced by magnetic impurities. We provide an
explanation for these phenomena in terms of the Hamiltonian symmetries.
Finally, a scheme to generate maximally entangled spin states of the two
impurities via electron scattering is proposed.


title: Introduction to error correcting codes in quantum computers

abstract:
  The goal of this paper is to review the theoretical basis for achieving a
faithful quantum information transmission and processing in the presence of
noise. Initially encoding and decoding, implementing gates and quantum error
correction will be considered error free. Finally we will relax this non
realistic assumption, introducing the quantum fault-tolerant concept. The
existence of an error threshold permits to conclude that there is no physical
law preventing a quantum computer from being built. An error model based on the
depolarizing channel will be able to provide a simple estimation of the storage
or memory computation error threshold: < 5.2 10-5. The encoding is made by
means of the [[7,1,3]] Calderbank-Shor-Steane quantum code and the Shor's
fault-tolerant method to measure the stabilizer's generators is used.


title:
The Importance of the Algorithmic Information Theory to Construct a
  Possible Example Where NP#P

abstract:
  In this short communication, it is shown a simple problem using quantum
circuits for which the algorithmic information theory guarantee that the
minimal length of the algorithm able to solve it grows exponentially with the
number of qubits.


title:
When Non-Gaussian States are Gaussian: Generalization of
  Non-Separability Criterion for Continuous Variables

abstract:
  We present a class of non-Gaussian two-mode continuous variable states for
which the separability criterion for Gaussian states can be employed to detect
whether they are separable or not. These states reduce to the two-mode Gaussian
states as a special case.


title: Quantum randomness and value indefiniteness

abstract:
  As computability implies value definiteness, certain sequences of quantum
outcomes cannot be computable.


title: Casimir Force on Real Materials - the Slab and Cavity Geometry

abstract:
  We analyse the potential of the geometry of a slab in a planar cavity for the
purpose of Casimir force experiments. The force and its dependence on
temperature, material properties and finite slab thickness are investigated
both analytically and numerically for slab and walls made of aluminium and
teflon FEP respectively. We conclude that such a setup is ideal for
measurements of the temperature dependence of the Casimir force. By numerical
calculation it is shown that temperature effects are dramatically larger for
dielectrics, suggesting that a dielectric such as teflon FEP whose properties
vary little within a moderate temperature range, should be considered for
experimental purposes. We finally discuss the subtle but fundamental matter of
the various Green's two-point function approaches present in the literature and
show how they are different formulations describing the same phenomenon.


title: Possible Stimulated Emission of Entangled Rhodium Mossbauer Gammas

abstract:
  Observation of possible stimulated emission of Mossbauer gamma is reported by
liquid-nitrogen quenching of rhodium sample from room temperature to 77K in the
time-resolved Mossbauer spectroscopy. Recently, we have demonstrated the
anomalous emission of three entangled gammas of the E3 Mossbauer transition
generated by bremsstrahlung irradiation. In this work, we further report the
high-speed decay of excited state. We conjecture that cooling shrinkage,
gravitational redshift and crystal lattice collimate entangled gammas in a
linear cavity. This opens up a new approach towards gamma lasing, if the
stimulated emission occurs at this obtained low excitation density.


title:
What you always wanted to know about Bohmian mechanics but were afraid
  to ask

abstract:
  Bohmian mechanics is an alternative interpretation of quantum mechanics. We
outline the main characteristics of its non-relativistic formulation. Most
notably it does provide a simple solution to the infamous measurement problem
of quantum mechanics. Presumably the most common objection against Bohmian
mechanics is based on its non-locality and its apparent conflict with
relativity and quantum field theory. However, several models for a quantum
field theoretical generalization do exist. We give a non-technical account of
some of these models.


title: Approximate Randomization of Quantum States With Fewer Bits of Key

abstract:
  Randomization of quantum states is the quantum analogue of the classical
one-time pad. We present an improved, efficient construction of an
approximately randomizing map that uses O(d/epsilon^2) Pauli operators to map
any d-dimensional state to a state that is within trace distance epsilon of the
completely mixed state. Our bound is a log d factor smaller than that of
Hayden, Leung, Shor, and Winter (2004), and Ambainis and Smith (2004).
  Then, we show that a random sequence of essentially the same number of
unitary operators, chosen from an appropriate set, with high probability form
an approximately randomizing map for d-dimensional states. Finally, we discuss
the optimality of these schemes via connections to different notions of
pseudorandomness, and give a new lower bound for small epsilon.


title:
Comment on ``Experimental realization of Wheeler's delayed-choice
  GedankenExperiment''

abstract:
  A shortcoming in the authors' interpretation of this beautiful new experiment
is pointed out and briefly discussed.


title:
Determination of ground state properties in quantum spin systems by
  single qubit unitary operations and entanglement excitation energies

abstract:
  We introduce a method for analyzing ground state properties of quantum many
body systems, based on the characterization of separability and entanglement by
single subsystem unitary operations. We apply the method to the study of the
ground state structure of several interacting spin-1/2 models, described by
Hamiltonians with different degrees of symmetry. We show that the approach
based on single qubit unitary operations allows to introduce {\it
``entanglement excitation energies''}, a set of observables that can
characterize ground state properties, including the quantification of
single-site entanglement and the determination of quantum critical points. The
formalism allows to identify the existence and location of factorization
points, and a purely quantum {\it ``transition of entanglement''} that occurs
at the approach of factorization. This kind of quantum transition is
characterized by a diverging ratio of excitation energies associated to
single-qubit unitary operations.


title: Quantum cards and quantum rods

abstract:
  Quantum mechanical analysis of a rigid rod with one end fixed to a flat table
is presented. It is shown, that for a macroscopic rod the ground state is
orientationally delocalized only if the table is absolutely horizontal. In this
latter case the rod, assumed to be initally in the upright orientation, falls
down symmetrically and simultaneously in both directions, as claimed by Tegmark
and Wheeler. In addition, the time of fall is calculated using WKB
wavefunctions representing energy eigenstates near the barrier summit.


title: Approximate Private Quantum Channels

abstract:
  This thesis includes a survey of the results known for private and
approximate private quantum channels. We develop the best known upper bound for
$\epsilon$-randomizing maps, $n+2\log(1/\epsilon)+c$ bits required to
$\epsilon$-randomize an arbitrary $n$-qubit state by improving a scheme of
Ambainis and Smith \cite{AS04} based on small bias spaces \cite{NN90, AGHP92}.
We show by a probabilistic argument that in fact the great majority of random
schemes using slightly more than this many bits of key are also
$\epsilon$-randomizing. We provide the first known non-trivial lower bound for
$\epsilon$-randomizing maps, and develop several conditions on them which we
hope may be useful in proving stronger lower bounds in the future.


title:
Stationary entanglement between two movable mirrors in a classically
  driven Fabry-Perot cavity

abstract:
  We consider a Fabry-Perot cavity made by two moving mirrors and driven by an
intense classical laser field. We show that stationary entanglement between two
vibrational modes of the mirrors, with effective mass of the order of
micrograms, can be generated by means of radiation pressure. The resulting
entanglement is however quite fragile with respect to temperature.


title: Quantum Network Coding for General Graphs

abstract:
  Network coding is often explained by using a small network model called
Butterfly. In this network, there are two flow paths, s_1 to t_1 and s_2 to
t_2, which share a single bottleneck channel of capacity one. So, if we
consider conventional flow (of liquid, for instance), then the total amount of
flow must be at most one in total, say 1/2 for each path. However, if we
consider information flow, then we can send two bits (one for each path) at the
same time by exploiting two side links, which are of no use for the liquid-type
flow, and encoding/decoding operations at each node. This is known as network
coding and has been quite popular since its introduction by Ahlswede, Cai, Li
and Yeung in 2000. In QIP 2006, Hayashi et al showed that quantum network
coding is possible for Butterfly, namely we can send two qubits simultaneously
with keeping their fidelity strictly greater than 1/2.
  In this paper, we show that the result can be extended to a large class of
general graphs by using a completely different approach. The underlying
technique is a new cloning method called entanglement-free cloning which does
not produce any entanglement at all. This seems interesting on its own and to
show its possibility is an even more important purpose of this paper. Combining
this new cloning with approximation of general quantum states by a small number
of fixed ones, we can design a quantum network coding protocol which
``simulates'' its classical counterpart for the same graph.


title: Semiclassical quantization of an N-particle Bose-Hubbard model

abstract:
  A semiclassical Bohr-Sommerfeld approximation is derived for an N-particle,
two-mode Bose-Hubbard system modeling a Bose-Einstein condensate in a
double-well potential. This semiclassical description is based on the
`classical' dynamics of the mean-field Gross-Pitaevskii equation and is
expected to be valid for large N. We demonstrate the possibility to reconstruct
quantum properties of the N-particle system from the mean-field dynamics. The
resulting semiclassical eigenvalues and eigenstates are found to be in very
good agreement with the exact ones, even for small values of N, both for
subcritical and supercritical particle interaction strength where tunneling has
to be taken into account.


title: Transmission area and two-photon correlated imaging

abstract:
  The relationship between transmission area of an object imaged and the
visibility of its image is investigated in a lensless system. We show that the
changes of the visibility are quite different when the transmission area is
varied by different manners. An increase of the transmission by adding the slit
number leads to a decrease of the visibility. While, the change is adverse when
the slit width is widened for a given distance between two slits.


title: Quantum states characterization for the zero-error capacity

abstract:
  The zero-error capacity of quantum channels was defined as the least upper
bound of rates at which classical information can be transmitted through a
quantum channel with probability of error equal to zero. This paper
investigates some properties of input states and measurements used to attain
the quantum zero-error capacity. We start by reformulating the problem of
finding the zero-error capacity in the language of graph theory. This
alternative definition is used to prove that the zero-error capacity of any
quantum channel can be reached by using tensor products of pure states as
channel inputs, and projective measurements in the channel output. We conclude
by presenting an example that illustrates our results.


title:
On the analytic structure of Green's function for the Fano - Anderson
  model

abstract:
  We study analytic structure of the Green's function (GF) for the exactly
solvable Fano-Anderson model. We analyze the GF poles, branch points and
Riemann surface, and show how the Fermi's Golden Rule, valid in perturbative
regime for not to large time, appears in this context. The knowledge of
analytic structure of the GF in frequency representation opens opportunities
for obtaining formulas for the GF in time representation alternative to the
standard one using the spectral density.


title: Quantum key distribution with "dual detectors"

abstract:
  To improve the performance of a quantum key distribution (QKD) system, high
speed, low dark count single photon detectors (or low noise homodyne detectors)
are required. However, in practice, a fast detector is usually noisy. Here, we
propose a "dual detectors" method to improve the performance of a practical QKD
system with realistic detectors: the legitimate receiver randomly uses either a
fast (but noisy) detector or a quiet (but slow) detector to measure the
incoming quantum signals. The measurement results from the quiet detector can
be used to bound eavesdropper's information, while the measurement results from
the fast detector are used to generate secure key. We apply this idea to
various QKD protocols. Simulation results demonstrate significant improvements
in both BB84 protocol with ideal single photon source and Gaussian-modulated
coherent states (GMCS) protocol; while for decoy-state BB84 protocol with weak
coherent source, the improvement is moderate. We also discuss various practical
issues in implementing the "dual detectors" scheme.


title: Entanglement properties in the Inhomogeneous Tavis-Cummings model

abstract:
  In this work we study the properties of the atomic entanglement in the
eigenstates spectrum of the inhomogeneous Tavis-Cummings Model. The
inhomogeneity is present in the coupling among the atoms with quantum
electromagnetic field. We calculate analytical expressions for the concurrence
and we found that this exhibits a strong dependence on the inhomogeneity.


title: Local Hidden Variables Underpinning of Entanglement and Teleportation

abstract:
  Entangled states whose Wigner functions are non-negative may be viewed as
being accounted for by local hidden variables (LHV). Recently, there were
studies of Bell's inequality violation (BIQV) for such states in conjunction
with the well known theorem of Bell that precludes BIQV for theories that have
LHV underpinning. We extend these studies to teleportation which is also based
on entanglement. We investigate if, to what extent, and under what conditions
may teleportation be accounted for via LHV theory. Our study allows us to
expose the role of various quantum requirements. These are, e.g., the
uncertainty relation among non-commuting operators, and the no-cloning theorem
which forces the complete elimination of the teleported state at its initial
port.


title: Context, spacetime loops, and the interpretation of quantum mechanics

abstract:
  Three postulates are discussed: first that well-defined properties cannot be
assigned to an isolated system, secondly that quantum unitary evolution is
atemporal, and thirdly that some physical processes are never reversed. It is
argued that these give useful insight into quantum behaviour. The first
postulate emphasizes the fundamental role in physics of interactions and
correlations, as opposed to internal properties of systems. Statements about
physical interactions can only be framed in a context of further interactions.
This undermines the possibility of objectivity in physics. However, quantum
mechanics retains objectivity through the combination of the second and third
postulates. A rule is given for determining the circumstances in which physical
evolution is non-unitary. This rule appeals to the absence of spacetime loops
in the future evolution of a set of interacting systems. A single universe
undergoing non-unitary evolution is a viable interpretation.


title: Errors in trapped-ion quantum gates due to spontaneous photon scattering

abstract:
  We analyze the error in trapped-ion, hyperfine qubit, quantum gates due to
spontaneous scattering of photons from the gate laser beams. We investigate
single-qubit rotations that are based on stimulated Raman transitions and
two-qubit entangling phase-gates that are based on spin-dependent optical
dipole forces. This error is compared between different ion species currently
being investigated as possible quantum information carriers. For both gate
types we show that with realistic laser powers the scattering error can be
reduced to below current estimates of the fault-tolerance error threshold.


title: How to Distinguish Identical Particles. the General Case

abstract:
  The many-identical-particle quantum correlations are revisited utilizing the
machinery of basic group theory, especially that of the group of permutations.
It is done with the purpose to obtain precise definitions of effective distinct
particles, and of the limitations involved. Namely, certain restrictions allow
one to distinguish identical particles in the general case of N of them, and of
J clusters of effectively distinct particles, where N and J are arbitrary
integers (but 1<J<(N+1)). Mutually orthogonal, single-particle distinguishing
projectors (events or ptoperties), J of them, are the backbone of the
construction. The general results are exemplified by local quantum mechanics,
and by the case of nucleons. The former example suits laboratory experiments,
and a critical view of it is presented.


title: The computational complexity of PEPS

abstract:
  We determine the computational power of preparing Projected Entangled Pair
States (PEPS), as well as the complexity of classically simulating them, and
generally the complexity of contracting tensor networks. While creating PEPS
allows to solve PP problems, the latter two tasks are both proven to be
#P-complete. We further show how PEPS can be used to approximate ground states
of gapped Hamiltonians, and that creating them is easier than creating
arbitrary PEPS. The main tool for our proofs is a duality between PEPS and
postselection which allows to use existing results from quantum compexity.


title: Superfast Algorithms and the Halting Problem in Geometric Algebra

abstract:
  A new type of algorithms is presented that combine the advantages of quantum
and classical ones. Those combined advantages along with aspects of Geometric
Algebra that open possibilities unavailable to both of these computations are
exploited to obtain database search and number factoring algorithms that are
faster than the quantum ones, and even to create a ''pseudoalgorithm'' that can
perform noncomputational tasks.


title: The Quantum Interference Computer: error correction

abstract:
  An error correcting mechanism is proposed in the context of the Quantum
Interference Computer approach


title: The Systematic Measurement Errors and Uncertainty Relation

abstract:
  Inertial effects in non-inertial reference frames are compared with quantum
properties of tests objects. The real space-time and perfect inertial reference
frame can be compared accurate to the uncertainty relation. Complexities if
describing micro-object in non-inertial reference-frames are avoidable using
Ostrogradski's Canonical Formalism.


title: Negative weights make adversaries stronger

abstract:
  The quantum adversary method is one of the most successful techniques for
proving lower bounds on quantum query complexity. It gives optimal lower bounds
for many problems, has application to classical complexity in formula size
lower bounds, and is versatile with equivalent formulations in terms of weight
schemes, eigenvalues, and Kolmogorov complexity. All these formulations rely on
the principle that if an algorithm successfully computes a function then, in
particular, it is able to distinguish between inputs which map to different
values.
  We present a stronger version of the adversary method which goes beyond this
principle to make explicit use of the stronger condition that the algorithm
actually computes the function. This new method, which we call ADV+-, has all
the advantages of the old: it is a lower bound on bounded-error quantum query
complexity, its square is a lower bound on formula size, and it behaves well
with respect to function composition. Moreover ADV+- is always at least as
large as the adversary method ADV, and we show an example of a monotone
function for which ADV+-(f)=Omega(ADV(f)^1.098). We also give examples showing
that ADV+- does not face limitations of ADV like the certificate complexity
barrier and the property testing barrier.


title:
Defending the Pondicherry interpretation: A response to Shafiee,
  Jafar-Aghdami, and Golshani

abstract:
  Recently Shafiee, Jafar-Aghdami, and Golshani (Studies in History and
Philosophy of Modern Physics, 37, 316--329) took issue with certain aspects of
the Pondicherry interpretation of quantum mechanics, especially its definitions
and uses of "objective probability," its conception of space, the role it
assigns to the macroworld in a universe governed by quantum laws, and its claim
for the completeness of quantum mechanics. These issues are addressed.


title: Laser stabilization using spectral hole burning

abstract:
  We have frequency stabilized a Coherent CR699-21 dye laser to a transient
spectral hole on the 606 nm transition in Pr^{+3}:Y_2SiO_5. A frequency
stability of 1 kHz has been obtained on the 10 microsecond timescale together
with a long-term frequency drift below 1 kHz/s. RF magnetic fields are used to
repopulate the hyperfine levels allowing us to control the dynamics of the
spectral hole. A detailed theory of the atomic response to laser frequency
errors has been developed which allows us to design and optimize the laser
stabilization feedback loop, and specifically we give a stability criterion
that must be fulfilled in order to obtain very low drift rates. The laser
stability is sufficient for performing quantum gate experiments in
Pr^{+3}:Y_2SiO_5.


title: Robustness of quantum Markov chains

abstract:
  If the conditional information of a classical probability distribution of
three random variables is zero, then it obeys a Markov chain condition. If the
conditional information is close to zero, then it is known that the distance
(minimum relative entropy) of the distribution to the nearest Markov chain
distribution is precisely the conditional information. We prove here that this
simple situation does not obtain for quantum conditional information. We show
that for tri-partite quantum states the quantum conditional information is
always a lower bound for the minimum relative entropy distance to a quantum
Markov chain state, but the distance can be much greater; indeed the two
quantities can be of different asymptotic order and may even differ by a
dimensional factor.


title: Simulating a single qubit channel using a mixed state environment

abstract:
  We analyze the class of single qubit channels with the environment modeled by
a one-qubit mixed state. The set of affine transformations for this class of
channels is computed analytically, employing the canonical form for the
two-qubit unitary operator. We demonstrate that, 3/8 of the generalized
depolarizing channels can be simulated by the one-qubit mixed state environment
by explicitly obtaining the shape of the volume occupied by this class of
channels within the tetrahedron representing the generalized depolarizing
channels. Further, as a special case, we show that the two-Pauli Channel cannot
be simulated by a one-qubit mixed state environment.


title: Experimental quantum key distribution with active phase randomization

abstract:
  Phase randomization is an important assumption made in many security proofs
of practical quantum key distribution (QKD) systems. Here, we present the first
experimental demonstration of QKD with reliable active phase randomization. One
key contribution is a polarization-insensitive phase modulator, which we added
to a commercial phase-coding QKD system to randomize the global phase of each
bit. We also proposed a simple but useful method to verify experimentally that
the phase is indeed randomized. Our result shows very low QBER (<1%). We expect
this active phase randomization process to be a standard part in future QKD
set-ups due to its significance and feasibility.


title:
Some Possibility and Impossibility Results related to Discrete Fourier
  type transforms in Quantum Information

abstract:
  Very recently the most general ensemble of qubits are identified using the
notion of linearity; any of these qubits gets accepted by a Hadamard gate to
generate the equal superposition of the qubit and its orthogonal. Towards more
generalization, we investigate the possibility and impossibility results
related to Discrete Fourier Transform (DFT) type of operations for a more
general set up of qutrits.


title: On the relationship between entropy and information

abstract:
  In this paper, we analyze the relationship between entropy and information in
the context of the mixing process of two identical ideal gases. We will argue
that entropy has a special information-based feature that is enfolded in the
statistical entropy, but the second law does not include it directly.
Therefore, in some given processes in thermodynamics where there is no matter
and energy interaction between the system and environment, the state of the
system may go towards a situation of lower probability to increase observer's
information in environment. This is a kind of an information-based interaction
in which the total entropy is not constrained by the second law.


title:
Lissajous curves and semiclassical theory: The two-dimensional harmonic
  oscillator

abstract:
  The semiclassical treatment of the two-dimensional harmonic oscillator
provides an instructive example of the relation between classical motion and
the quantum mechanical energy spectrum. We extend previous work on the
anisotropic oscillator with incommensurate frequencies and the isotropic
oscillator to the case with commensurate frequencies for which the Lissajous
curves appear as classical periodic orbits. Because of the three different
scenarios depending on the ratio of its frequencies, the two-dimensional
harmonic oscillator offers a unique way to explicitly analyze the role of
symmetries in classical and quantum mechanics.


title: Projective Ring Line Encompassing Two-Qubits

abstract:
  The projective line over the (non-commutative) ring of two-by-two matrices
with coefficients in GF(2) is found to fully accommodate the algebra of 15
operators - generalized Pauli matrices - characterizing two-qubit systems. The
relevant sub-configuration consists of 15 points each of which is either
simultaneously distant or simultaneously neighbor to (any) two given distant
points of the line. The operators can be identified with the points in such a
one-to-one manner that their commutation relations are exactly reproduced by
the underlying geometry of the points, with the ring geometrical notions of
neighbor/distant answering, respectively, to the operational ones of
commuting/non-commuting. This remarkable configuration can be viewed in two
principally different ways accounting, respectively, for the basic 9+6 and 10+5
factorizations of the algebra of the observables. First, as a disjoint union of
the projective line over GF(2) x GF(2) (the "Mermin" part) and two lines over
GF(4) passing through the two selected points, the latter omitted. Second, as
the generalized quadrangle of order two, with its ovoids and/or spreads
standing for (maximum) sets of five mutually non-commuting operators and/or
groups of five maximally commuting subsets of three operators each. These
findings open up rather unexpected vistas for an algebraic geometrical
modelling of finite-dimensional quantum systems and give their numerous
applications a wholly new perspective.


title: Perturbation theory for sextic doubly anharmonic oscillator

abstract:
  A simple method for the calculation of higher orders of the logarithmic
perturbation theory for bound states of the spherical anharmonic oscillator is
developed. The structure of the perturbation series for energy eigenvalues of
the sextic doubly anharmonic oscillator is investigated. The recursion
technique for deriving renormalized perturbation expansions is offered.


title:
Exact Polynomial Solutions of the Mie-Type Potential in the
  N-Dimensional Schrodinger Equation

abstract:
  The polynomial solution of the N-dimensional space Schrodinger equation for a
special case of Mie potential is obtained for any arbitrary $% l-state. The
exact bound-state energy eigenvalues and the corresponding eigenfunctions are
calculated for diatomic molecular systems in the Mie-type potential. Keywords:
Mie potential, Schrodinger equation, Eigenvalue, Eigenfunction, Diatomic
molecules


title: Sum rules for Confining Potentials

abstract:
  Using the Green's function associated with the one-dimensional Schroedinger
equation it is possible to establish a hierarchy of sum rules involving the
eigenvalues of confining potentials which have only a boundstate spectrum. For
some potentials the sum rules could lead to divergences. It is shown that when
this happens it is possible to examine the separate sum rules satisfied by the
even and odd eigenstates of a symmetric confining potential and by subtraction
cancel the divergences exactly and produce a new sum rule which is free of
divergences. The procedure is illustrated by considering symmetric power law
potentials and the use of several examples. One of the examples considered
shows that the zeros of the Airy function and its derivative obey a sum rule
and this sum rule is verified. It is also shown how the procedure may be
generalised to establish sum rules for arbitrary symmetric confining
potentials.


title: A Straightforward Introduction to Continuous Quantum Measurement

abstract:
  We present a pedagogical treatment of the formalism of continuous quantum
measurement. Our aim is to show the reader how the equations describing such
measurements are derived and manipulated in a direct manner. We also give
elementary background material for those new to measurement theory, and
describe further various aspects of continuous measurements that should be
helpful to those wanting to use such measurements in applications.
Specifically, we use the simple and direct approach of generalized measurements
to derive the stochastic master equation describing the continuous measurements
of observables, give a tutorial on stochastic calculus, treat multiple
observers and inefficient detection, examine a general form of the measurement
master equation, and show how the master equation leads to information gain and
disturbance. To conclude, we give a detailed treatment of imaging the resonance
fluorescence from a single atom as a concrete example of how a continuous
position measurement arises in a physical system.


title: Exact bound states in volcano potentials

abstract:
  Quantum mechanics in a one--parameter family of volcano potentials is
investigated. After a discussion on their construction and classical mechanics,
we obtain exact, normalisable bound states for specific values of the energy.
The nature of the wave functions and probability densities, as well as some
curious features of the solutions are highlighted.


title: Physical theory of the twentieth century and contemporary philosophy

abstract:
  It has been shown that the criticism of Pauli as well as of Susskind and
Glogover may be avoided if the standard quantum-mechanical mathematical model
has been suitably extended. There is not more any reason for Einstein's
citicism, either, if in addition to some new results concerning Bell's
inequalities and Belifante's argument are taken into account. The ensemble
interpretation of quantum mechanics (or the hidden-variable theory) should be
preferred, which is also supported by the already published results of
experiments with three polarizers. Greater space in the text has been devoted
also to the discussion of epistemological problems and some philosophical
consequences.


title: Quantum erasure of decoherence

abstract:
  We consider the classical algebra of observables that are diagonal in a given
orthonormal basis, and define a complete decoherence process as a completely
positive map that asymptotically converts any quantum observable into a
diagonal one, while preserving the elements of the classical algebra. For
quantum systems in dimension two and three any decoherence process can be
undone by collecting classical information from the environment and using such
an information to restore the initial system state. As a relevant example, we
illustrate the quantum eraser of Scully et al. [Nature 351, 111 (1991)] as an
example of environment-assisted correction. Moreover, we present the
generalization of the eraser setup for d-dimensional systems, showing that any
von Neumann measurement on a system can be undone by a complementary
measurement on the environment.


title: Breakdown of the few-level approximation in collective systems

abstract:
  The validity of the few-level approximation in dipole-dipole interacting
collective systems is discussed. As example system, we study the archetype case
of two dipole-dipole interacting atoms, each modelled by two complete sets of
angular momentum multiplets. We establish the breakdown of the few-level
approximation by first proving the intuitive result that the dipole-dipole
induced energy shifts between collective two-atom states depend on the length
of the vector connecting the atoms, but not on its orientation, if complete and
degenerate multiplets are considered. A careful analysis of our findings
reveals that the simplification of the atomic level scheme by artificially
omitting Zeeman sublevels in a few-level approximation generally leads to
incorrect predictions. We find that this breakdown can be traced back to the
dipole-dipole coupling of transitions with orthogonal dipole moments. Our
interpretation enables us to identify special geometries in which partial
few-level approximations to two- or three-level systems are valid.


title:
On the Physical Interpretation of Partial Traces: Two Nonstandard
  Viewpoints

abstract:
  Mixed states are introduced in physics in order to express our ignorance
about the actual state of a physical system and are represented in standard
quantum mechanics (QM) by density operators. Such operators also appear if one
considers a (pure) entangled state of a compound system $\Omega$ and performs
partial traces on the projection operator representing it. Yet, they do not
represent mixed states (or proper mixtures) of the subsystems in this case, but
improper mixtures, since the coefficients in the convex sums expressing them
never bear the ignorance interpretation. Hence, one cannot attribute states to
the subsystems of a compound physical system in QM (subentity problem). We
discuss here two alternative proposals that can be worked out within the
Brussels and Lecce approaches. We firstly summarize the general framework
provided by the former, which suggests that improper mixtures could be
considered as new pure states. Then, we show that improper mixtures can be
considered as true (yet nonpure) states also according to the latter. The two
proposals seem to be compatible notwithstanding their different terminologies.


title: Stationary light pulses in ultra cold atomic gasses

abstract:
  We present a theoretical treatment of electromagnetically induced
transparency and light storage using standing wave coupling fields in a medium
comprised of stationary atoms, such as an ultra cold atomic gas or a solid
state medium. We show that it is possible to create stationary pulses of light
which have a qualitatively different behavior than in the case of a thermal gas
medium, offering greater potential for quantum information processing
applications.


title:
Analysis of critical parameters in the scheme of Bj\"ork, Jonsson, and
  S\'anchez-Soto

abstract:
  Bj\"ork, Jonsson, and S\'anchez-Soto describe an interesting
(gedanken-)experiment which demonstrates that single photons can indeed lead to
effects which have no local realistic description. %It is one of few cases
where the experimental falsification of the conjecture of local realism has
been done with use of the CH inequality, rather that the CHSH inequality. We
study the critical values of parameters of some possible features of a
non-perfect realisation of the experiment (especially photon loss, which could
be looked at as the detection efficiency), that need to be satisfied so that
the experiment can be considered as a valid test of quantum mechanics versus
local realism. Interestingly, the scheme turns out to be robust against photon
loss.


title: The disentangling power of unitaries

abstract:
  We define the disentangling power of a unitary operator in a similar way as
the entangling power defined by Zanardi, Zalka and Faoro [PRA, 62, 030301]. A
general formula is derived and it is shown that both quantities are directly
proportional. All results concerning the entangling power can simply be
translated into similar statements for the disentangling power. In particular,
the disentangling power is maximal for certain permutations derived from
orthogonal latin squares. These permutations can therefore be interpreted as
those that distort entanglement in a maximal way.


title: Classicality in Quantum Mechanics

abstract:
  In this article we propose a solution to the measurement problem in quantum
mechanics. We point out that the measurement problem can be traced to an a
priori notion of classicality in the formulation of quantum mechanics. If this
notion of classicality is dropped and instead classicality is defined in purely
quantum mechanical terms the measurement problem can be avoided. We give such a
definition of classicality. It identifies classicality as a property of large
quantum system. We show how the probabilistic nature of quantum mechanics is a
result of this notion of classicality. We also comment on what the implications
of this view are for the search of a quantum theory of gravity.


title: Entanglement dynamics in chains of qubits with noise and disorder

abstract:
  The entanglement dynamics of arrays of qubits is analysed in the presence of
some general sources of noise and disorder. In particular, we consider linear
chains of Josephson qubits in experimentally realistic conditions.
Electromagnetic and other (spin or boson) fluctuations due to the background
circuitry and surrounding substrate, finite temperature in the external
environment, and disorder in the initial preparation and the control parameters
are embedded into our model. We show that the amount of disorder that is
typically present in current experiments does not affect the entanglement
dynamics significantly, while the presence of noise can have a drastic
influence on the generation and propagation of entanglement. We examine under
which circumstances the system exhibits steady-state entanglement for both
short (N < 10) and long (N > 30) chains and show that, remarkably, there are
parameter regimes where the steady-state entanglement is strictly non-monotonic
as a function of the noise strength. We also present optimized schemes for
entanglement verification and quantification based on simple correlation
measurements that are experimentally more economic than state tomography.


title:
Stability and entanglement in optical-atomic amplification of trapped
  atoms: the role of atomic collisions

abstract:
  Atomic collisions are included in an interacting system of optical fields and
trapped atoms allowing field amplification. We study the effects of collisions
on the system stability. Also a study of the degree of entanglement between
atomic and optical fields is made. We found that, for an atomic field initially
in a vacuum state and optical field in a coherent state, the degree of
entanglement does not depend on the optical field intensity or phase. We show
that in conditions of exponential instability the system presents at long times
two distinct stationary degree of entanglement with collisions affecting only
one of them.

categories: quant-ph cond-mat.other cond-mat.stat-mech physics.atom-ph


title: Robustness against parametric noise of non ideal holonomic gates

abstract:
  Holonomic gates for quantum computation are commonly considered to be robust
against certain kinds of parametric noise, the very motivation of this
robustness being the geometric character of the transformation achieved in the
adiabatic limit. On the other hand, the effects of decoherence are expected to
become more and more relevant when the adiabatic limit is approached. Starting
from the system described by Florio et al. [Phys. Rev. A 73, 022327 (2006)],
here we discuss the behavior of non ideal holonomic gates at finite operational
time, i.e., far before the adiabatic limit is reached. We have considered
several models of parametric noise and studied the robustness of finite time
gates. The obtained results suggest that the finite time gates present some
effects of cancellation of the perturbations introduced by the noise which
mimic the geometrical cancellation effect of standard holonomic gates.
Nevertheless, a careful analysis of the results leads to the conclusion that
these effects are related to a dynamical instead of geometrical feature.


title: Optimal, reliable estimation of quantum states

abstract:
  Accurately inferring the state of a quantum device from the results of
measurements is a crucial task in building quantum information processing
hardware. The predominant state estimation procedure, maximum likelihood
estimation (MLE), generally reports an estimate with zero eigenvalues. These
cannot be justified. Furthermore, the MLE estimate is incompatible with error
bars, so conclusions drawn from it are suspect. I propose an alternative
procedure, Bayesian mean estimation (BME). BME never yields zero eigenvalues,
its eigenvalues provide a bound on their own uncertainties, and it is the most
accurate procedure possible. I show how to implement BME numerically, and how
to obtain natural error bars that are compatible with the estimate. Finally, I
briefly discuss the differences between Bayesian and frequentist estimation
techniques.