title: On the degree conjecture for separability of multipartite quantum states

abstract:
  We settle the so-called degree conjecture for the separability of
multipartite quantum states, which are normalized graph Laplacians, first given
by Braunstein {\it et al.} [Phys. Rev. A \textbf{73}, 012320 (2006)]. The
conjecture states that a multipartite quantum state is separable if and only if
the degree matrix of the graph associated with the state is equal to the degree
matrix of the partial transpose of this graph. We call this statement to be the
strong form of the conjecture. In its weak version, the conjecture requires
only the necessity, that is, if the state is separable, the corresponding
degree matrices match. We prove the strong form of the conjecture for {\it
pure} multipartite quantum states, using the modified tensor product of graphs
defined in [J. Phys. A: Math. Theor. \textbf{40}, 10251 (2007)], as both
necessary and sufficient condition for separability. Based on this proof, we
give a polynomial-time algorithm for completely factorizing any pure
multipartite quantum state. By polynomial-time algorithm we mean that the
execution time of this algorithm increases as a polynomial in $m,$ where $m$ is
the number of parts of the quantum system. We give a counter-example to show
that the conjecture fails, in general, even in its weak form, for multipartite
mixed states. Finally, we prove this conjecture, in its weak form, for a class
of multipartite mixed states, giving only a necessary condition for
separability.


title: Hermite Expansions of Elements of Generalized Gelfand-Shilov space

abstract:
  We characterize the elements of generalized Gelfand Shilov spaces in terms of
the coefficients of their Fourier-Hermite expansion. The technique we use can
be applied both in quasianalytic and nonquasianalytic case. The
characterizations imply the kernel theorems for the dual spaces. The cases when
the test space is quasianalytic are important in quantum field theory with a
fundamental length, see for example papers of E.Bruning and S.Nagamachi,where
it was conjectured that the properties of the space of Fourier hyper functions,
which is isomorphic with S^1_1 are well adapted for the use in the theory.


title: Lifshitz Interaction between Dielectric Bodies of Arbitrary Geometry

abstract:
  A formulation is developed for the calculation of the
electromagnetic--fluctuation forces for dielectric objects of arbitrary
geometry at small separations, as a perturbative expansion in the dielectric
contrast. The resulting Lifshitz energy automatically takes on the form of a
series expansion of the different many-body contributions. The formulation has
the advantage that the divergent contributions can be readily determined and
subtracted off, and thus makes a convenient scheme for realistic numerical
calculations, which could be useful in designing nano-scale mechanical devices.


title: Communication Links for Distributed Quantum Computation

abstract:
  Distributed quantum computation requires quantum operations that act over a
distance on error-correction encoded states of logical qubits, such as the
transfer of qubits via teleportation. We evaluate the performance of several
quantum error correction codes, and find that teleportation failure rates of
one percent or more are tolerable when two levels of the [[23,1,7]] code are
used. We present an analysis of performing quantum error correction (QEC) on
QEC-encoded states that span two quantum computers, including the creation of
distributed logical zeroes. The transfer of the individual qubits of a logical
state may be multiplexed in time or space, moving serially across a single
link, or in parallel across multiple links. We show that the performance and
reliability penalty for using serial links is small for a broad range of
physical parameters, making serial links preferable for a large, distributed
quantum multicomputer when engineering difficulties are considered. Such a
multicomputer will be able to factor a 1,024-bit number using Shor's algorithm
with a high probability of success.


title: Fractal Fidelity as a signature of Quantum Chaos

abstract:
  We analyze the fidelity of a quantum simulation and we show that it displays
fractal fluctuations iff the simulated dynamics is chaotic. This analysis
allows us to investigate a given simulated dynamics without any prior
knowledge. In the case of integrable dynamics, the appearance of fidelity
fractal fluctuations is a signal of a highly corrupted simulation. We
conjecture that fidelity fractal fluctuations are a signature of the appearance
of quantum chaos. Our analysis can be realized already by a few qubit quantum
processor.


title: Withdrawing this manuscript

abstract:
  Recently Z. S. Zhang et al [Phys. Lett. A 356(2006)199] have proposed an
one-way quantum identity authentication scheme and claimed that it can verify
the user's identity and update securely the initial authentication key for
reuse.


title:
Three-party simultaneous quantum secure direct communication scheme with
  EPR pairs

abstract:
  We present a scheme for three-party simultaneous quantum secure direct
communication by using EPR pairs. In the scheme, three legitimate parties can
simultaneously exchange their secret messages. It is also proved to be secure
against the intercept-and-resend attack, the disturbance attack and the
entangled-and-measure attack.


title:
The Possibility of Reconciling Quantum Mechanics with Classical
  Probability Theory

abstract:
  We describe a scheme for constructing quantum mechanics in which a quantum
system is considered as a collection of open classical subsystems. This allows
using the formal classical logic and classical probability theory in quantum
mechanics. Our approach nevertheless allows completely reproducing the standard
mathematical formalism of quantum mechanics and identifying its applicability
limits. We especially attend to the quantum state reduction problem.


title: Network implementation of covariant two-qubit quantum operations

abstract:
  A six-qubit quantum network consisting of conditional unitary gates is
presented which is capable of implementing a large class of covariant two-qubit
quantum operations. Optimal covariant NOT operations for one and two-qubit
systems are special cases contained in this class. The design of this quantum
network exploits basic algebraic properties which also shed new light onto
these covariant quantum processes.


title:
Electromagnetically Induced Transparency in optically trapped rubidium
  atoms

abstract:
  We demonstrate electromagnetically induced transparency (EIT) in a sample of
rubidium atoms, trapped in an optical dipole trap. Mixing a small amount of
$\sigma^-$-polarized light to the weak $\sigma^+$-polarized probe pulses, we
are able to measure the absorptive and dispersive properties of the atomic
medium at the same time. Features as small as 4 kHz have been detected on an
absorption line with 20 MHz line width.


title: Multiphoton Quantum Optics and Quantum State Engineering

abstract:
  We present a review of theoretical and experimental aspects of multiphoton
quantum optics. Multiphoton processes occur and are important for many aspects
of matter-radiation interactions that include the efficient ionization of atoms
and molecules, and, more generally, atomic transition mechanisms;
system-environment couplings and dissipative quantum dynamics; laser physics,
optical parametric processes, and interferometry. A single review cannot
account for all aspects of such an enormously vast subject. Here we choose to
concentrate our attention on parametric processes in nonlinear media, with
special emphasis on the engineering of nonclassical states of photons and
atoms. We present a detailed analysis of the methods and techniques for the
production of genuinely quantum multiphoton processes in nonlinear media, and
the corresponding models of multiphoton effective interactions. We review
existing proposals for the classification, engineering, and manipulation of
nonclassical states, including Fock states, macroscopic superposition states,
and multiphoton generalized coherent states. We introduce and discuss the
structure of canonical multiphoton quantum optics and the associated one- and
two-mode canonical multiphoton squeezed states. This framework provides a
consistent multiphoton generalization of two-photon quantum optics and a
consistent Hamiltonian description of multiphoton processes associated to
higher-order nonlinearities. Finally, we discuss very recent advances that by
combining linear and nonlinear optical devices allow to realize multiphoton
entangled states of the electromnagnetic field, that are relevant for
applications to efficient quantum computation, quantum teleportation, and
related problems in quantum communication and information.


title:
Canonical and micro-canonical typical entanglement of continuous
  variable systems

abstract:
  We present a framework, compliant with the general canonical principle of
statistical mechanics, to define measures on the set of pure Gaussian states of
continuous variable systems. Within such a framework, we define two specific
measures, referred to as `micro-canonical' and `canonical', and apply them to
study systematically the statistical properties of the bipartite entanglement
of n-mode pure Gaussian states (as quantified by the entropy of a subsystem).
We rigorously prove the "concentration of measure" around a finite average,
occurring for the entanglement in the thermodynamical limit in both the
canonical and the micro-canonical approach. For finite n, we determine
analytically the average and standard deviation of the entanglement (as
quantified by the reduced purity) between one mode and all the other modes.
Furthermore, we numerically investigate more general situations, clearly
showing that the onset of the concentration of measure already occurs at
relatively small n.


title: Decoherence and the appearance of classicality in physical phenomena

abstract:
  It is accepted that among the ways through which a quantum phenomenon
decoheres and becomes a classical one is what is termed in the literature the
Zeno effect. This effect, named after the ancient Greek philosopher Zeno of
Elea (born about 485 B.C), were used in 1977 to analytically predict that an
initial quantum state may be preserved in time by merely repeating a large
number of times, in a finite total time, the experiment of checking its state.
Since then this effect has been experimentally validated and has become an
established physical fact. It has been argued by Simonius that the Zeno effect
must be related not only to quantum phenomena but also to many macroscopic and
classical effects. Thus, since it operates in both quantum and classical
regimes it must cause to a more generalized kind of decoherence than the
restricted one that ``classicalizes'' a quantum phenomenon. We show that this
generalized decoherence, {\it obtained as a result of dense measurement}, not
only gives rise to new phenomena that are demonstrated through new responses of
the densely interacted-upon system but also may physically {\it establish}
them. For that matter we have found and established the analogous {\it space
Zeno effect} which leads to the necessity of an ensemble of related observers
(systems) for the remarked physical validation of new phenomena. As will be
shown in Chapters 3-5 of this work the new phenomena (new responses of the
system) that result from the space Zeno effect may be of an unexpected nature.
We use quantum field theory in addition to the more conventional methods of
analysis and also corroborate our analytical findings by numerical simulations.


title: Engineering correlation and entanglement dynamics in spin systems

abstract:
  We show that the correlation and entanglement dynamics of spin systems can be
understood in terms of propagation of spin waves. This gives a simple, physical
explanation of the behaviour seen in a number of recent works, in which a
localised, low-energy excitation is created and allowed to evolve. But it also
extends to the scenario of translationally invariant systems in states far from
equilibrium, which require less local control to prepare. Spin-wave evolution
is completely determined by the system's dispersion relation, and the latter
typically depends on a small number of external, physical parameters.
Therefore, this new insight into correlation dynamics opens up the possibility
not only of predicting but also of controlling the propagation velocity and
dispersion rate, by manipulating these parameters. We demonstrate this
analytically in a simple, example system.


title:
Comment on ``A quantum-classical bracket that satisfies the Jacobi
  identity'' [J. Chem. Phys. 124, 201104 (2006)]

abstract:
  It shown that the quantum-classical dynamical bracket recently proposed in J.
Chem. Phys. 124, 201104 (2006) fails to satisfy the Jacobi identity.


title: Blind encoding into qudits

abstract:
  We consider the problem of encoding classical information into unknown qudit
states belonging to any basis, of a maximal set of mutually unbiased bases, by
one party and then decoding by another party who has perfect knowledge of the
basis. Working with qudits of prime dimensions, we point out a no-go theorem
that forbids shift operations on arbitrary unknown states. We then provide the
necessary conditions for reliable encoding/decoding.


title:
Experimental demonstration of stimulated polarization wave in a chain of
  nuclear spins

abstract:
  A stimulated wave of polarization, which implements a simple mechanism of
quantum amplification, is experimentally demonstrated in a chain of four
J-coupled nuclear spins, irradiated by a weak radio-frequency transverse field.
The "quantum domino" dynamics, a wave of flipped spins triggered by a flip of
the first spin, has been observed in fully $^{13}$C-labeled sodium butyrate.


title: Hybrid Quantum Computation in Quantum Optics

abstract:
  We propose a hybrid quantum computing scheme where qubit degrees of freedom
for computation are combined with quantum continuous variables for
communication. In particular, universal two-qubit gates can be implemented
deterministically through qubit-qubit communication, mediated by a
continuous-variable bus mode ("qubus"), without direct interaction between the
qubits and without any measurement of the qubus. The key ingredients are
controlled rotations of the qubus and unconditional qubus displacements. The
controlled rotations are realizable through typical atom-light interactions in
quantum optics. For such interactions, our scheme is universal and works in any
regime, including the limits of weak and strong nonlinearities.


title:
A Matrix Factorization of Extended Hamiltonian Leads to $N$-Particle
  Pauli Equation

abstract:
  In this paper the Levy-Leblond procedure for linearizing the Schr\"odinger
equation to obtain the Pauli equation for one particle is generalized to obtain
an $N$-particle equation with spin. This is achieved by using the more
universal matrix factorization, $G\tilde{G} = |G| I = (-K)^l I$. Here the
square matrix $G$ is linear in the total energy E and all momenta, $\tilde G$
is the matrix adjoint of $G$, $I$ is the identity matrix, $|G|$ is the
determinant of $G$, $l$ is a positive integer and $K=H-E$ is Lanczos' extended
Hamiltonian where $H$ is the classical Hamiltonian of the electro-mechanical
system. $K$ is identically zero for all such systems, so that matrix $G$ is
singular. As a consequence there always exists a vector function
$\underline\theta$ with the property $G\underline\theta=0$. This factorization
to obtain the matrix $G$ and vector function $\underline\theta$ is illustrated
first for a one-dimensional particle in a simple potential well. This same
technique, when applied to the classical nonrelativistic Hamiltonian for $N$
interacting particles in an electromagnetic field, is shown to yield for N=1
the Pauli wave equation with spin and its generalization to $N$ particles.
Finally this nonrelativistic generalization of the Pauli equation is used to
treat the simple Zeeman effect of a hydrogen-like atom as a two-particle
problem with spin.


title: Ultrafast electro-nuclear dynamics of H2 double ionization

abstract:
  The ultrafast electronic and nuclear dynamics of H2 laser-induced double
ionization is studied using a time-dependent wave packet approach that goes
beyond the fixed nuclei approximation. The double ionization pathways are
analyzed by following the evolution of the total wave function during and after
the pulse. The rescattering of the first ionized electron produces a coherent
superposition of excited molecular states which presents a pronounced transient
H+H- character. This attosecond excitation is followed by field-induced double
ionization and by the formation of short-lived autoionizing states which decay
via double ionization. These two double ionization mechanisms may be identified
by their signature imprinted in the kinetic-energy distribution of the ejected
protons.


title: Quantum time of flight distribution for cold trapped atoms

abstract:
  The time of flight distribution for a cloud of cold atoms falling freely
under gravity is considered. We generalise the probability current density
approach to calculate the quantum arrival time distribution for the mixed state
describing the Maxwell-Boltzmann distribution of velocities for the falling
atoms. We find an empirically testable difference between the time of flight
distribution calculated using the quantum probability current and that obtained
from a purely classical treatment which is usually employed in analysing time
of flight measurements. The classical time of flight distribution matches with
the quantum distribution in the large mass and high temperature limits.


title: The differential information-geometry of quantum phase transitions

abstract:
  The manifold of coupling constants parametrizing a quantum Hamiltonian is
equipped with a natural Riemannian metric with an operational
distinguishability content. We argue that the singularities of this metric are
in correspondence with the quantum phase transitions featured by the
corresponding system. This approach provides a universal conceptual framework
to study quantum critical phenomena which is differential-geometric and
information-theoretic at the same time.


title: Superdecoherence through gate control noise

abstract:
  The external control circuits of quantum gates inevitably introduce a small
but finite noise to the operation of quantum computers. The complex modes of
decoherence introduced by this noise are not covered by the common error
models. Using the controlled-phase gate as an example, the effect of gate
control noise on decoherence is investigated for different quantum computer
architectures. It is shown that the decoherence rate rises faster than linearly
with the length of a quantum register for most cases considered, adding to the
challenge of implementing proposed error correcting and fault tolerant
computation schemes. Sometimes an unwanted effective inter-qubit coupling
associated with the noise appears.


title:
Exact Bound State Solutions of the Schrodinger Equation for Noncentral
  Potential via the Nikiforov-Uvarov Method

abstract:
  Exact bound state solutions of the Schrodinger equation for generalized
noncentral potential are examined by means of the Nikiforov-Uvarov method. The
wavefunctions and the corresponding energy eigenvalues of the system are
obtained analytically. The results examined for the potential are compatible
with those obtained by the other methods.


title: A dual de Finetti theorem

abstract:
  The quantum de Finetti theorem says that, given a symmetric state, the state
obtained by tracing out some of its subsystems approximates a convex sum of
power states. The more subsystems are traced out, the better this approximation
becomes. Schur-Weyl duality suggests that there ought to be a dual result that
applies to a unitarily invariant state rather than a symmetric state. Instead
of tracing out a number of subsystems, one traces out part of every subsystem.
The theorem then asserts that the resulting state approximates the fully mixed
state, and the larger the dimension of the traced-out part of each subsystem,
the better this approximation becomes. This paper gives a number of
propositions together with their dual versions, to show how far the duality
holds.


title: A simple family of nonadditive quantum codes

abstract:
  Most known quantum codes are additive, meaning the codespace can be described
as the simultaneous eigenspace of an abelian subgroup of the Pauli group. While
in some scenarios such codes are strictly suboptimal, very little is understood
about how to construct nonadditive codes with good performance. Here we present
a family of nonadditive quantum codes for all odd blocklengths, n, that has a
particularly simple form. Our codes correct single qubit erasures while
encoding a higher dimensional space than is possible with an additive code or,
for n of 11 or greater, any previous codes.


title:
Mesoscopic entanglement of noninteracting qubits using collective
  spontaneous emission

abstract:
  We describe an experimentally straightforward method for preparing an
entangled W state of up to 100 qubits. Our repeat-until-success protocol relies
on detection of single photons from collective spontaneous emission in free
space. Our method allows entanglement preparation in a wide range of qubit
implementations that lack entangling qubit-qubit interactions. We give detailed
numerical examples for entanglement of neutral atoms in optical lattices and of
nitrogen-vacancy centres in diamond. The simplicity of our method should enable
preparation of mesoscopic entangled states in a number of physical systems in
the near future.


title: Entanglement witnesses and concurrence for multi-qubit states

abstract:
  We establish a relation between concurrence and entanglement witnesses. In
particular, we construct entanglement witnesses for three-qubit W and GHZ
states in terms of concurrence and different set of operators that generate it.
We also generalize our construction for multi-qubit states.


title:
Strategies for the preparation of large cluster states using
  non-deterministic gates

abstract:
  The cluster state model for quantum computation has paved the way for schemes
that allow scalable quantum computing, even when using non-deterministic
quantum gates. Here the initial step is to prepare a large entangled state
using non-deterministic gates. A key question in this context is the relative
efficiencies of different `strategies', i.e. in what order should the
non-deterministic gates be applied, in order to maximize the size of the
resulting cluster states? In this paper we consider this issue in the context
of `large' cluster states. Specifically, we assume an unlimited resource of
qubits and ask what the steady state rate at which `large' clusters are
prepared from this resource is, given an entangling gate with particular
characteristics. We measure this rate in terms of the number of entangling gate
operations that are applied. Our approach works for a variety of different
entangling gate types, with arbitrary failure probability. Our results indicate
that strategies whereby one preferentially bonds together identical qubits are
considerably more efficient than those in which one does not. Additionally,
compared to earlier analytic results, our numerical study offers substantially
improved resource scaling.


title: Entanglement conditions for multi-mode states

abstract:
  We provide a class of inequalities for detecting entanglements in multi-mode
systems. Necessary conditions for fully separable, bi-separable and sufficient
conditions for fully entangled states are explicitly presented.


title: Towards Dynamical Qubit Controlling with Time-Dependent External Fields

abstract:
  For a flux qubit described by a two-level system of equations we propose a
special time dependent external control field. We show that for a qubit placed
in this field there exists a critical value of tunnel frequency. When the
tunnel frequency is close to its critical value, the probability value of a
definite direction of the current circulating in a Josephson-junction circuit
may be kept above 1/2 during a desirable time interval. We also show that such
a behavior is not much affected by a sufficiently small dissipation.


title: Research on Hidden Variable Theories: a review of recent progresses

abstract:
  Quantum Mechanics (QM) is one of the pillars of modern physics: an impressive
amount of experiments have confirmed this theory and many technological
applications are based on it. Nevertheless, at one century since its
development, various aspects concerning its very foundations still remain to be
clarified. Among them, the transition from a microscopic probabilistic world
into a macroscopic deterministic one and quantum non-locality. A possible way
out from these problems would be if QM represents a statistical approximation
of an unknown deterministic theory.
  This review is addressed to present the most recent progresses on the studies
related to Hidden Variable Theories (HVT), both from an experimental and a
theoretical point of view, giving a larger emphasis to results with a direct
experimental application.
  More in details, the first part of the review is a historical introduction to
this problem. The Einstein-Podolsky-Rosen argument and the first discussions
about HVT are introduced, describing the fundamental Bell's proposal for a
general experimental test of every Local HVT and the first attempts to realise
it.
  The second part of the review is devoted to elucidate the recent progresses
toward a conclusive Bell inequalities experiment obtained with entangled
photons and other physical systems.
  Finally, the last sections are targeted to shortly discuss Non-Local HVT.


title:
A Note on Pseudo-Hermitian Systems with Point Interactions and Quantum
  Separability

abstract:
  We study the quantum entanglement and separability of Hermitian and
pseudo-Hermitian systems of identical bosonic or fermionic particles with point
interactions. The separability conditions are investigated in detail.


title:
Frequency filtered storage of parametric fluorescence with
  electromagnetically induced transparency

abstract:
  The broadband parametric fluorescence pulse (probe light) with center
frequency resonant on 87Rb D1 line was injected into a cold atomic ensemble
with coherent light (control light). Due to the low gain in the parametric down
conversion process, the probe light was in a highly bunched photon-pair state.
By switching off the control light, the probe light within the
electromagnetically induced transparency window was mapped on the atoms. When
the control light was switched on, the probe light was retrieved and frequency
filtered storage was confirmed from the superbunching effect and an increase of
the coherence time of the retrieved light.


title: Continuous variable entanglement sharing in non-inertial frames

abstract:
  We study the distribution of entanglement between modes of a free scalar
field from the perspective of observers in uniform acceleration. We consider a
two-mode squeezed state of the field from an inertial perspective, and
analytically study the degradation of entanglement due to the Unruh effect, in
the cases of either one or both observers undergoing uniform acceleration. We
find that for two observers undergoing finite acceleration, the entanglement
vanishes between the lowest frequency modes. The loss of entanglement is
precisely explained as a redistribution of the inertial entanglement into
multipartite quantum correlations among accessible and unaccessible modes from
a non-inertial perspective. We show that classical correlations are also lost
from the perspective of two accelerated observers but conserved if one of the
observers remains inertial.

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


title: Wigner-function description of EPR experiment

abstract:
  We provide a detailed description of the EPR paradox (in the Bohm version)
for a two qubit-state in the discrete Wigner function formalism. We compare the
probability distributions for two qubit relevant to simultaneously-measurable
observables (computed from the Wigner function) with the probability
distributions representing two perfectly-correlated classic particles in a
discrete phase-space. We write in both cases the updating formulae after a
measure, thus obtaining a mathematical definition of \textit{classic collapse}
and \textit{quantum collapse}. We study, with the EPR experiment, the joint
probability distributions of Alice's and Bob's qubit before and after the
measure, analyzing the non-local effects. In particular, we give a more precise
definition of locality, which we call m-locality: we show that quantum systems
may violate this kind of locality, thus preserving, in an EPR-like argument,
the completeness of Quantum Mechanics.


title:
Non-Abelian gauge field theory of the spin-orbit interaction and a
  perfect spin filter

abstract:
  We point out that the Rashba and Dresselhaus spin-orbit interactions in two
dimensions can be regarded as a Yang-Mills non-Abelian gauge field. The
physical field generated by the gauge field gives the electron wave function a
spin-dependent phase which is frequently called the Aharonov-Casher phase.
Applying on an AB ring this non-Abelian field together with the usual vector
potential, we can make the interference condition completely destructive for
one component of the spin while completely constructive for the other component
of the spin over the entire energy range. This enables us to construct a
perfect spin filter.


title:
Fidelity, dynamic structure factor, and susceptibility in critical
  phenomena

abstract:
  Motivated by the growing importance of fidelity in quantum critical
phenomena, we establish a general relation between fidelity and structure
factor of the driving term in a Hamiltonian through a newly introduced concept:
fidelity susceptibility. Our discovery, as shown by some examples, facilitates
the evaluation of fidelity in terms of susceptibility using well developed
techniques such as density matrix renormalization group for the ground state,
or Monte Carlo simulations for the states in thermal equilibrium.


title: Input-output relations for multiport ring cavities

abstract:
  Quantum input-output relations for a generic $n$-port ring cavity are
obtained by modeling the ring as a cascade of $n$ interlinked beam splitters.
Cavity response to a beam impinging on one port is studied as a function of the
beam-splitter reflectivities and the internal phase-shifts. Interferometric
sensitivity and stability are analyzed as a function of the number of ports.


title: An Implementation of a Positive Operator Valued Measure

abstract:
  An implementation of the positive operator valued measure (POVM) is given. By
using this POVM one can realize the probabilistic teleportation of an unknown
two-particle state.


title:
Manipulating Quantum States of Molecules Created via Photoassociation of
  Bose-Einstein Condensates

abstract:
  We show the quantum state transfer technique in two-color photoassociation
(PA) of a Bose-Einstein condensate, where a quantized field is used to couple
the free-bound transition from atom state to excited molecular state. Under the
weak excitation condition, we find that quantum states of the quantized field
can be transferred to the created molecular condensate. The feasibility of this
technique is confirmed by considering the atomic and molecular decays
discovered in the current PA experiments. The present results allow us to
manipulate quantum states of molecules in the photoassociation of a
Bose-Einstein condensate.


title: Quantum Entanglement and Electron Correlation in Molecular Systems

abstract:
  We study the relation between quantum entanglement and electron correlation
in quantum chemistry calculations. We prove that the Hartree-Fock (HF) wave
function does not violate Bell's inequality, thus is not entangled while the
configuration interaction (CI) wave function is entangled since it violates
Bell's inequality. Entanglement is related to electron correlation and might be
used as an alternative measure of the electron correlation in quantum chemistry
calculations. As an example we show the calculations of entanglement for the
H$_2$ molecule and how it is related to electron correlation of the system,
which is the difference between the exact and the HF energies.


title: Wave-particle duality in the damped harmonic oscillator

abstract:
  Quantization of the damped harmonic oscillator is taken as leitmotiv to
gently introduce elements of quantum probability theory for physicists. To this
end, we take (graduate) students in physics as entry level and explain the
physical intuition and motivation behind the, sometimes overwhelming, math
machinery of quantum probability theory.
  The main text starts with the quantization of the (undamped) harmonic
oscillator from the Heisenberg and Schroedinger point of view. We show how both
treatments are special instances of a quantum probabilistic quantization
procedure: the second quantization functor. We then apply the second
quantization functor to the damped harmonic oscillator and interpret the
quantum dynamics of the position and energy operator as stochastic processes.


title: Evidence Against Klein Paradox in Graphene

abstract:
  It is demonstrated that both transmission and reflection coefficients
associated to the Klein paradox at a step barrier are positive and less than
unity, so that the particle-antiparticle pair creation mechanism commonly
linked to this phenomenon is not necessary. Because graphene is a solid-state
testing ground for quantum electrodynamics phenomena involving massless Dirac
fermions we suggest that the transport characteristic through a p-n graphene
junction can decide between the results obtained in this paper and the common
Klein paradox theory, which imply negative transmission and higher-than-unity
reflection coefficients. Recent experimental evidence supports our findings.


title: Average Fidelity in n-Qubit systems

abstract:
  This letter generalizes the expression for the average fidelity of single
qubits, as found by Bowdrey et al., to the case of n qubits. We use a simple
algebraic approach with basis elements for the density-matrix expansion
expressed as Kronecker products of n Pauli spin matrices. An explicit
integration over initial states is avoided by invoking the invariance of the
state average under unitary transformations of the initial density matrix. The
results have applications to measurements of quantum information, for example
in ion-trap and NMR experiments.


title: A Dirac sea pilot-wave model for quantum field theory

abstract:
  We present a pilot-wave model for quantum field theory in which the Dirac sea
is taken seriously. The model ascribes particle trajectories to all the
fermions, including the fermions filling the Dirac sea. The model is
deterministic and applies to the regime in which fermion number is
superselected. This work is a further elaboration of work by Colin, in which a
Dirac sea pilot-wave model is presented for quantum electrodynamics. We extend
his work to non-electromagnetic interactions, we discuss a cut-off
regularization of the pilot-wave model and study how it reproduces the standard
quantum predictions. The Dirac sea pilot-wave model can be seen as a possible
continuum generalization of a lattice model by Bell. It can also be seen as a
development and generalization of the ideas by Bohm, Hiley and Kaloyerou, who
also suggested the use of the Dirac sea for the development of a pilot-wave
model for quantum electrodynamics.


title:
Effect of system level structure and spectral distribution of the
  environment on the decoherence rate

abstract:
  Minimizing the effect of decoherence on a quantum register must be a central
part of any strategy to realize scalable quantum information processing. Apart
from the strength of the coupling to the environment, the decoherence rate is
determined by the the system level structure and by the spectral composition of
the noise trace that the environment generates. Here, we discuss a relatively
simple model that allows us to study these different effects quantitatively in
detail. We evaluate the effect that the perturbation has on an NMR system while
it performs a Grover search algorithm.


title:
Numerical Bayesian quantum-state assignment for a three-level quantum
  system. II. Average-value data with a constant, a Gaussian-like, and a Slater
  prior

abstract:
  This paper offers examples of concrete numerical applications of Bayesian
quantum-state assignment methods to a three-level quantum system. The
statistical operator assigned on the evidence of various measurement data and
kinds of prior knowledge is computed partly analytically, partly through
numerical integration (in eight dimensions) on a computer. The measurement data
consist in the average of outcome values of N identical von Neumann projective
measurements performed on N identically prepared three-level systems. In
particular the large-N limit will be considered. Three kinds of prior knowledge
are used: one represented by a plausibility distribution constant in respect of
the convex structure of the set of statistical operators; another one
represented by a prior studied by Slater, which has been proposed as the
natural measure on the set of statistical operators; the last prior is
represented by a Gaussian-like distribution centred on a pure statistical
operator, and thus reflecting a situation in which one has useful prior
knowledge about the likely preparation of the system. The assigned statistical
operators obtained with the first two kinds of priors are compared with the one
obtained by Jaynes' maximum entropy method for the same measurement situation.
In the companion paper the case of measurement data consisting in absolute
frequencies is considered.


title: Quantum Walks, Quantum Gates and Quantum Computers

abstract:
  The physics of quantum walks on graphs is formulated in Hamiltonian language,
both for simple quantum walks and for composite walks, where extra discrete
degrees of freedom live at each node of the graph. It is shown how to map
between quantum walk Hamiltonians and Hamiltonians for qubit systems and
quantum circuits; this is done for both a single- and multi-excitation coding,
and for more general mappings. Specific examples of spin chains, as well as
static and dynamic systems of qubits, are mapped to quantum walks, and walks on
hyperlattices and hypercubes are mapped to various gate systems. We also show
how to map a quantum circuit performing the quantum Fourier transform, the key
element of Shor's algorithm, to a quantum walk system doing the same. The
results herein are an essential preliminary to a Hamiltonian formulation of
quantum walks in which coupling to a dynamic quantum environment is included.


title: Joint measurements on qubits and cloning of observables

abstract:
  Cloning of observables, unlike standard cloning of states, aims at copying
the information encoded in the statistics of a class of observables rather then
on quantum states themselves. In such a process the emphasis is on the quantum
operation (evolution plus measurement) necessary to retrieve the original
information. We analyze, for qubit systems, the cloning of a class generated by
two noncommuting observables, elucidating the relationship between such a
process and joint measurements. This helps in establishing an optimality
criterion for cloning of observables. We see that, even if the cloning machine
is designed to act on the whole class generated by two noncommuting
observables, the same optimal performances of a joint measurement can be
attained. Finally, the connection with state dependent cloning is enlightened.


title:
Error propagation in loss- and failure-tolerant quantum computation
  schemes

abstract:
  Qubit loss and gate failure are significant obstacles for the implementation
of scalable quantum computation. Recently there have been several proposals for
overcoming these problems, including schemes based on parity and cluster
states. While effective at dealing with loss and gate failure, these schemes
typically lead to a blow-out in effective depolarizing noise rates. In this
supplementary paper we present a detailed analysis of this problem and
techniques for minimizing it.


title:
Multi-particle and High-dimension Controlled Order Rearrangement
  Encryption Protocols

abstract:
  Based on the controlled order rearrange encryption (CORE) for quantum key
distribution using EPR pairs[Fu.G.Deng and G.L.Long Phys.Rev.A68 (2003)
042315], we propose the generalized controlled order rearrangement encryption
(GCORE) protocols of $N$ qubits and $N$ qutrits, concretely display them in the
cases using 3-qubit, 2-qutrit maximally entangled basis states. We further
indicate that our protocols will become safer with the increase of number of
particles and dimensions. Moreover, we carry out the security analysis using
quantum covariant cloning machine for the protocol using qutrits. Although the
applications of the generalized scheme need to be further studied, the GCORE
has many distinct features such as great capacity and high efficiency.


title:
Exponentiation of certain Matrices related to the Four Level System by
  use of the Magic Matrix

abstract:
  In this paper we show how to calculate explicitly the exponential of certain
matrices, which are evolution operators governing the interaction of the four
level system of atoms and the radiation, etc. We present a consistent method in
terms of the magic matrix by Makhlin.
  As a closely related subject, we derive a closed form expression of the
Baker-Campbell-Hausdorff formula for a class of matrices in SU(4), by use of
the method developed by the present authors in quant-ph/0610009.


title: Decoherence in time evolution of bound entanglement

abstract:
  We study a dynamic process of disentanglement by considering the time
evolution of bound entanglement for a quantum open system, two qutrits coupling
to a common environment. Here, the initial quantum correlations of the two
qutrits are characterized by the bound entanglement. In order to show the
universality of the role of environment on bound entanglement, both bosonic and
spin environments are considered. We found that the bound entanglement displays
collapses and revivals, and it can be stable against small temperature and time
change. The thermal fluctuation effects on bound entanglement are also
considered.


title:
Optimal quantum control of Bose Einstein condensates in magnetic
  microtraps

abstract:
  Transport of Bose-Einstein condensates in magnetic microtraps, controllable
by external parameters such as wire currents or radio-frequency fields, is
studied within the framework of optimal control theory (OCT). We derive from
the Gross-Pitaevskii equation the optimality system for the OCT fields that
allow to efficiently channel the condensate between given initial and desired
states. For a variety of magnetic confinement potentials we study transport and
wavefunction splitting of the condensate, and demonstrate that OCT allows to
drastically outperfrom more simple schemes for the time variation of the
microtrap control parameters.


title:
Interference-induced enhancement of intensity and energy of a multimode
  quantum optical field by a subwavelength array of coherent light sources

abstract:
  Recently, we have showed a mechanism that could provide a great transmission
enhancement of the light waves passed through subwavelength aperture arrays in
thin metal films not by the plasmon-polariton waves, but by the constructive
interference of diffracted waves (beams generated by the apertures) at the
detector placed in the far-field zone. We now present a quantum reformulation
of the model. The Hamiltonian describing the interference-induced enhancement
of the intensity and energy of a multimode quantum optical field is derived.
Such a field can be produced, for instance, by a subwavelength array of
coherent light sources.


title:
Entanglement and Quantum Phase Transitions via Adiabatic Quantum
  Computation

abstract:
  For a finite XY chain and a finite two-dimensional Ising lattice, it is shown
that the paramagnetic ground state is adiabatically transformed to the GHZ
state in the ferromagnetic phase by slowly turning on the magnetic field. The
fidelity between the GHZ state and an adiabatically evolved state shows a
feature of the quantum phase transition.


title: The Free-Will Postulate in Quantum Mechanics

abstract:
  The so-called "free will axiom" is an essential ingredient in many
discussions concerning hidden variables in quantum mechanics. In this paper we
argue that "free will" can be defined in different ways. The definition usually
employed is clearly invalid in strictly deterministic theories. A different,
more precise formulation is proposed here, defining a condition that may well
be a more suitable one to impose on theoretical constructions and models. Our
axiom, to be referred to as the `unconstrained initial state' condition, has
consequences similar to "free will", but does not clash with determinism, and
appears to lead to different conclusions concerning causality and locality in
quantum mechanics. Models proposed earlier by this author fall in this
category. Imposing our `unconstrained initial state' condition on a
deterministic theory underlying Quantum Mechanics, appears to lead to a
restricted free-will condition in the quantum system: an observer has the free
will to modify the setting of a measuring device, but has no control over the
phase of its wave function. The dismissal of the usual "free will" concept does
not have any consequences for our views and interpretations of human activities
in daily life, and the way our minds function, but it requires a more careful
discussion on what, in practice, free will actually amounts to.


title: Optical Shelving: Suppressed Fluorescence

abstract:
  The shelving phenomenon of quantum optics, originally observed by Dehmelt, is
analyzed in terms of the qRules that are given in another paper. The heuristic
value of these rules is apparent because they not only describe the dark period
during shelving, but they reveal the mechanism that enforces the suppression of
fluorescence during that time.


title: Revised Geometric Measure of Entanglement

abstract:
  We present an revised geometric measure of entanglement (RGME). The revised
version is an entanglement monotone. Some useful inequalities about RGME are
deduced. For exemplification, we give the formulas of RGME for the
two-parameter class of states in $2 \otimes n$ quantum system, the two
particles high dimensional maximally entangled mixed state, the isotropic state
including $n$-particle $d$-level case and two multipartite bound entangled
states. The result shows there is a relation $\widetilde{E}_{\sin^2} \leq
E_{re}$, which indicates that the RGME is an appropriate measure of
entanglement.


title:
Proposed experiment with Rydberg atoms to test the wave function
  interpretation

abstract:
  Experiment{Fabre_1983} shows that Rydberg atoms do not pass through 1
micronmeter width slits if their principal quantum number is rather large(n >
60). Thus, the particle density measured after the slits is null while the wave
function calculated after the slits is not. This experiment is in contradiction
with the Born interpretation (the square of the wave function is proportional
to the probability density for the particle to be found at each point in
space). The classical interpretation of this experiment, which removes the
contradiction, is to suppose that if the particles do not pass, the wave
function does not pass either (classical assumption).
  An alternative interpretation of this experiment is to suppose that the wave
function passes through the slits, but that the Born interpretation is not
valid any more in this case (alternative assumption).
  The aim of this paper is to present an experiment testing this alternative
assumption compared to the classical assumption.


title:
Quantumness without quantumness: entanglement as classical correlations
  in higher dimensions

abstract:
  I exploit the formal equivalence between the ground state of a $d$
dimensional quantum system and a d+1 dimensional classical Ising chain to
represent quantum entanglement in terms of classical correlations only. This
offers a general "local hidden variable model" for all quantum phenomena
existing in one dimension lower than the (hidden variable) classical model
itself. The local hidden variable model is not contradicted by the implications
of Bell's theorem. Formal theory is presented first and then exemplified by the
quantum Ising spin chain in a transverse magnetic field. Here I explicitely
show how to derive any two site entanglement in the transverse model from the
partition function of the classical Ising spin chain existing in two
dimensions. Some speculations are then presented regarding possible fundamental
implications of these results.


title:
Approximate quantum error correction, random codes, and quantum channel
  capacity

abstract:
  We work out a theory of approximate quantum error correction that allows us
to derive a general lower bound for the entanglement fidelity of a quantum
code. The lower bound is given in terms of Kraus operators of the quantum
noise. This result is then used to analyze the average error correcting
performance of codes that are randomly drawn from unitarily invariant code
ensembles. Our results confirm that random codes of sufficiently large block
size are highly suitable for quantum error correction. Moreover, employing a
lemma of Bennett, Shor, Smolin, and Thapliyal, we prove that random coding
attains information rates of the regularized coherent information.


title:
Generalised Clifford groups and simulation of associated quantum
  circuits

abstract:
  Quantum computations that involve only Clifford operations are classically
simulable despite the fact that they generate highly entangled states; this is
the content of the Gottesman-Knill theorem. Here we isolate the ingredients of
the theorem and provide generalisations of some of them with the aim of
identifying new classes of simulable quantum computations. In the usual
construction, Clifford operations arise as projective normalisers of the first
and second tensor powers of the Pauli group. We consider replacing the Pauli
group by an arbitrary finite subgroup G of U(d). In particular we seek G such
that G tensor G has an entangling normaliser. Via a generalisation of the
Gottesman-Knill theorem the resulting normalisers lead to classes of quantum
circuits that can be classically efficiently simulated. For the qubit case d=2
we exhaustively treat all finite subgroups of U(2) and find that the only ones
(up to unitary equivalence and trivial phase extensions) with entangling
normalisers are the groups G_n generated by X and the n^th root of Z.


title: Graph states and carrier-envelope phase squeezing

abstract:
  We outline a new class of continuous-variable graph states that can be useful
to describe entangle- ment, and also multimode squeezing, in an optical
frequency comb. We show that a particular case of such states coincides with
the squeezing of the carrier-envelope phase, or of the total intensity, of a
mode-locked laser. We then discuss the experimental issues related to measuring
the quantum noise of the carrier-envelope phase of a frequency comb and show
that this can be carried out by use of quantum heterodyne multiplexing, a
technique that may be useful to measure multipartite entanglement as well.


title: Observation of ultra-broadband, beamlike parametric downconversion

abstract:
  We report spontaneous parametric downconversion having an unusually wide
spectral bandwidth. A collinear type-1 phase-matching configuration is employed
with degeneracy near the zero group velocity dispersion frequency. With a
spectral width of 1080 nm and degenerate wavelength 1885 nm, the source also
emits a high flux of photon pairs constrained to a cone of only 2 degree
half-angle. A rigorous theoretical approach is developed that confirms the
experimental observations. The source properties are consistent with an
ultra-short photon-pair correlation time and, for a narrowband pump, extremely
high-dimensional spectral entanglement.


title: Might EPR particles communicate through a wormhole?

abstract:
  We consider the two-particle wave function of an Einstein-Podolsky-Rosen
system, given by a two dimensional relativistic scalar field model. The Bohm-de
Broglie interpretation is applied and the quantum potential is viewed as
modifying the Minkowski geometry. In this way an effective metric, which is
analogous to a black hole metric in some limited region, is obtained in one
case and a particular metric with singularities appears in the other case,
opening the possibility, following Holland, of interpreting the EPR
correlations as being originated by an effective wormhole geometry, through
which the physical signals can propagate.


title:
Applications of The Information Model of The Collapse Phenomena: The
  Mathematical Model of Everett's Worlds in The Case of The Measurement of A
  Spin 1/2 Projection

abstract:
  The information model of the collapse phenomena is further advanced. We
discover an important property of the model - the death point effect. The P
function approach is presented to construct the manifest form of the function
of risk. We clarify a close connection of the model with the Extended Everett
Concept. The model is also reformulated as an automaton. Examples are
considered.


title: Deutsch's Universal Quantum Turing Machine (Revisited)

abstract:
  Deutsch, Feynman, and Manin viewed quantum computing as a kind of universal
physical simulation procedure. Much of the writing about quantum Turing
machines has shown how these machines can simulate an arbitrary unitary
transformation on a finite number of qubits. This interesting problem has been
addressed most famously in a paper by Deutsch, and later by Bernstein and
Vazirani. Quantum Turing machines form a class closely related to deterministic
and probabilistic Turing machines and one might hope to find a universal
machine in this class. A universal machine is the basis of a notion of
programmability. The extent to which universality has in fact been established
by the pioneers in the field is examined and a key notion in theoretical
computer science (universality) is scrutinised. In a forthcoming paper, the
authors will also consider universality in the quantum gate model.


title: Modified Two-Slit Experiments and Complementarity

abstract:
  Some modified two-slit interference experiments claim to demonstrate a
violation of Bohr's complementarity principle. A typical such experiment is
theoretically analyzed using wave-packet dynamics. The flaw in the analysis of
such experiments is pointed out and it is demonstrated that they do not violate
complementarity. In addition, it is quite generally proved that if the state of
a particle is such that the modulus square of the wave-function yields an
interference pattern, then it necessarily loses which-path information.


title: Preparing and probing atomic number states with an atom interferometer

abstract:
  We describe the controlled loading and measurement of number-squeezed states
and Poisson states of atoms in individual sites of a double well optical
lattice. These states are input to an atom interferometer that is realized by
symmetrically splitting individual lattice sites into double wells, allowing
atoms in individual sites to evolve independently. The two paths then
interfere, creating a matter-wave double-slit diffraction pattern. The time
evolution of the double-slit diffraction pattern is used to measure the number
statistics of the input state. The flexibility of our double well lattice
provides a means to detect the presence of empty lattice sites, an important
and so far unmeasured factor in determining the purity of a Mott state.


title: Pairwise Concurrence Dynamics: A Four-Qubit Model

abstract:
  We examine entanglement dynamics via concurrence among four two-state systems
labeled $A, ~a, ~B, ~b$. The four systems are arranged on an addressable
"lattice" in such a way that $A$ and $a$ at one location labeled $Aa$ can
interact with each other via excitation exchange, and the same for $B$ and $b$
at location $Bb$. The $Aa$ location is prepared entangled with the $Bb$
location, but their mutual complete isolation prevents interaction in the
interval between actions of an external addressing agent. There are six
pairwise concurrences on the lattice, and we follow their evolution in the
interval between external actions. We show how entanglement evolves and may
exhibit the non-analytic effect termed entanglement sudden death (ESD), with
periodic recovery. These loss and gain processes may be interpreted as
entanglement transfer between the subsystems.


title: Fault-Tolerant Quantum Computation

abstract:
  I give a brief overview of fault-tolerant quantum computation, with an
emphasis on recent work and open questions.


title: A presentation of Quantum Logic based on an "and then" connective

abstract:
  When a physicist performs a quantic measurement, new information about the
system at hand is gathered. This paper studies the logical properties of how
this new information is combined with previous information. It presents Quantum
Logic as a propositional logic under two connectives: negation and the "and
then" operation that combines old and new information. The "and then"
connective is neither commutative nor associative. Many properties of this
logic are exhibited, and some small elegant subset is shown to imply all the
properties considered. No independence or completeness result is claimed.
Classical physical systems are exactly characterized by the commutativity, the
associativity, or the monotonicity of the "and then" connective. Entailment is
defined in this logic and can be proved to be a partial order. In orthomodular
lattices, the operation proposed by Finch (1969) satisfies all the properties
studied in this paper. All properties satisfied by Finch's operation in modular
lattices are valid in Hilbert Space Quantum Logic. It is not known whether all
properties of Hilbert Space Quantum Logic are satisfied by Finch's operation in
modular lattices. Non-commutative, non-associative algebraic structures
generalizing Boolean algebras are defined, ideals are characterized and a
homomorphism theorem is proved.


title: Two-Photon Emission from Semiconductors

abstract:
  We report the first experimental observations of two-photon emission from
semiconductors, to the best of our knowledge, and develop a corresponding
theory for the room-temperature process. Spontaneous two-photon emission is
demonstrated in optically-pumped bulk GaAs and in electrically-driven
GaInP/AlGaInP quantum wells. Singly-stimulated two-photon emission measurements
demonstrate the theoretically predicted two-photon optical gain in
semiconductors - a necessary ingredient for any realizations of future
two-photon semiconductor lasers. Photon-coincidence experiment validates the
simultaneity of the electrically-driven GaInP/AlGaInP two-photon emission,
limited only by detector's temporal resolution.


title: A classical one-way function to confound quantum adversaries

abstract:
  The promise of quantum computation and its consequences for
complexity-theoretic cryptography motivates an immediate search for
cryptosystems which can be implemented with current technology, but which
remain secure even in the presence of quantum computers. Inspired by recent
negative results pertaining to the nonabelian hidden subgroup problem, we
present here a classical algebraic function $f_V(M)$ of a matrix $M$ which we
believe is a one-way function secure against quantum attacks. Specifically,
inverting $f_V$ reduces naturally to solving a hidden subgroup problem over the
general linear group (which is at least as hard as the hidden subgroup problem
over the symmetric group). We also demonstrate a reduction from Graph
Isomorphism to the problem of inverting $f_V$; unlike Graph Isomorphism,
however, the function $f_V$ is random self-reducible and therefore uniformly
hard.
  These results suggest that, unlike Shor's algorithm for the discrete
logarithm--which is, so far, the only successful quantum attack on a classical
one-way function--quantum attacks based on the hidden subgroup problem are
unlikely to work. We also show that reconstructing any entry of $M$, or the
trace of $M$, with nonnegligible advantage is essentially as hard as inverting
$f_V$. Finally, $f_V$ can be efficiently computed and the number of output bits
is less than $1+\epsilon$ times the number of input bits for any $\epsilon >
0$.


abstract:
  Relativistic free-motion time-of-arrival theory for massive spin-1/2
particles is systematically developed. Contrary to the nonrelativistic
time-of-arrival operator studied thoroughly in previous literatures, the
relativistic time-of-arrival operator possesses self-adjoint extensions because
of the particle-antiparticle symmetry. The nonrelativistic limit of our theory
is in agreement with the nonrelativistic time-of-arrival theory. By comparing
the time-of-arrival operator with the Hamiltonian operator of a free Dirac
particle, one can show a duality between position space and momentum space.


title:
Decomposing generalized measurements into continuous stochastic
  processes

abstract:
  One of the broadest concepts of measurement in quantum theory is the
generalized measurement. Another paradigm of measurement--arising naturally in
quantum optics, among other fields--is that of continuous-time measurements,
which can be seen as the limit of a consecutive sequence of weak measurements.
They are naturally described in terms of stochastic processes, or
time-dependent random variables. We show that any generalized measurement can
be decomposed as a sequence of weak measurements with a mathematical limit as a
continuous stochastic process. We give an explicit construction for any
generalized measurement, and prove that the resulting continuous evolution, in
the long-time limit, collapses the state of the quantum system to one of the
final states generated by the generalized measurement, being decomposed, with
the correct probabilities. A prominent feature of the construction is the
presence of a feedback mechanism--the instantaneous choice weak measurement at
a given time depends on the outcomes of earlier measurements. For a generalized
measurement with $n$ outcomes, this information is captured by a real
$n$-vector on an $n$-simplex, which obeys a simple classical stochastic
evolution.


title: The Lueders Postulate and the Distinguishability of Observables

abstract:
  The Lueders postulate is reviewed and implications for the distinguishability
of observables are discussed. As an example the distinguishability of two
similar observables for spin-1/2 particles is described. Implementation issues
are briefly analyzed.


title: Entanglement versus observables

abstract:
  A general scheme to seek for the relations between entanglement and
bservables is proposed in principle. In two-qubit systems with enough general
Hamiltonian, we find the entanglement to be the functions of observables for
six kinds of chosen state sets and verify how these functions be invariant with
time evolution. Moreover, we demonstrate and illustrate the cases with
entanglement versus a set of commutable observables under eight kinds of given
initial states. Our conclusions show how entanglement become observable even
measurable by experiment, and they are helpful for understanding of the nature
of entanglement in physics.


title:
Evidence for coherent collective Rydberg excitation in the strong
  blockade regime

abstract:
  Blockade effects on the single quantum level are at the heart of quantum
devices like single-electron transistors. The blockade mechanisms are based on
strong interactions like the Coulomb interaction in case of single electrons.
Neutral atoms excited into a Rydberg state experience abnormally strong
interactions that lead to the corresponding blockade effect for Rydberg atoms.
In this paper we report on our measurements of a strong van der Waals blockade,
showing that only one out of several thousand atoms within a blockade volume
can be excited. In addition, our experimental results demonstrate the coherent
nature of the excitation of magnetically trapped ultracold atoms into a Rydberg
state, confirming the predicted dependence of the collective Rabi frequency on
the square root of the mesoscopic system size. This collective coherent
behaviour is generic for all mesoscopic systems which are able to carry only
one single excitation quantum.


title:
Visualization of Coherent Destruction of Tunneling in an Optical Double
  Well System

abstract:
  We report on a direct visualization of coherent destruction of tunneling
(CDT) of light waves in a double well system which provides an optical analog
of quantum CDT as originally proposed by Grossmann, Dittrich, Jung, and Hanggi
[Phys. Rev. Lett. {\bf 67}, 516 (1991)]. The driven double well, realized by
two periodically-curved waveguides in an Er:Yb-doped glass, is designed so that
spatial light propagation exactly mimics the coherent space-time dynamics of
matter waves in a driven double-well potential governed by the Schr\"{o}dinger
equation. The fluorescence of Er ions is exploited to image the spatial
evolution of light in the two wells, clearly demonstrating suppression of light
tunneling for special ratios between frequency and amplitude of the driving
field.


title:
Numerical evidence for the maximum number of mutually unbiased bases in
  dimension six

abstract:
  The question of determining the maximal number of mutually unbiased bases in
dimension six has received much attention since their introduction to quantum
information theory, but a definitive answer has still not been found. In this
paper we move away from the traditional analytic approach and use a numerical
approach to attempt to determine this number. We numerically minimise a
non-negative function of a set of N+1 orthonormal bases in dimension d which
only evaluates to zero if the bases are mutually unbiased. As a result we find
strong evidence that (as has been conjectured elsewhere) there are no more than
three mutually unbiased bases in dimension six.


title: Quantum reconstruction of an intense polarization squeezed optical state

abstract:
  We perform a reconstruction of the polarization sector of the density matrix
of an intense polarization squeezed beam starting from a complete set of Stokes
measurements. By using an appropriate quasidistribution, we map this onto the
Poincare space providing a full quantum mechanical characterization of the
measured polarization state.


title:
Entanglement and Subsystems, Entanglement beyond Subsystems, and All
  That

abstract:
  Entanglement plays a pervasive role nowadays throughout quantum information
science, and at the same time provides a bridging notion between quantum
information science and fields as diverse as condensed-matter theory, quantum
gravity, and quantum foundations. In recent years, a notion of ''Generalized
Entanglement'' (GE) has emerged, based on the idea that entanglement may be
directly defined through expectation values of preferred observables -- without
reference to a preferred subsystem decomposition. Preferred observables capture
the physically relevant point of view, as defined by dynamical, operational, or
fundamental constraints. While reducing to the standard entanglement notion
when preferred observables are restricted to arbitrary local observables acting
on individual subsystems, GE substantially expands subsystem-based entanglement
theories, in terms of both conceptual foundations and range of applicability.
Remarkably, the GE framework allows for non-trivial entanglement to exist
within a single, indecomposable quantum system, demands in general a
distinction between quantum separability and absence of entanglement, and
naturally extends to situations where existing approaches may not be directly
useful -- such as entanglement in arbitrary convex-cones settings and
entanglement for indistinguishable quantum particles. In this paper, we revisit
the main motivations leading to GE, and summarize the accomplishments and
prospects of the GE program to date, with an eye toward conceptual developments
and implications. In particular, we explain how the GE approach both shares
strong points of contact with abstract operational quantum theories and,
ultimately, calls for an observer-dependent redefinition of concepts like
locality, completeness, and reality in quantum theory.


title: Emergence of typical entanglement in two-party random processes

abstract:
  We investigate the entanglement within a system undergoing a random, local
process. We find that there is initially a phase of very fast generation and
spread of entanglement. At the end of this phase the entanglement is typically
maximal. In previous work we proved that the maximal entanglement is reached to
a fixed arbitrary accuracy within $O(N^3)$ steps, where $N$ is the total number
of qubits. Here we provide a detailed and more pedagogical proof. We
demonstrate that one can use the so-called stabilizer gates to simulate this
process efficiently on a classical computer. Furthermore, we discuss three ways
of identifying the transition from the phase of rapid spread of entanglement to
the stationary phase: (i) the time when saturation of the maximal entanglement
is achieved, (ii) the cut-off moment, when the entanglement probability
distribution is practically stationary, and (iii) the moment block entanglement
scales exhibits volume scaling. We furthermore investigate the mixed state and
multipartite setting. Numerically we find that classical and quantum
correlations appear to behave similarly and that there is a well-behaved
phase-space flow of entanglement properties towards an equilibrium, We describe
how the emergence of typical entanglement can be used to create a much simpler
tripartite entanglement description. The results form a bridge between certain
abstract results concerning typical (also known as generic) entanglement
relative to an unbiased distribution on pure states and the more physical
picture of distributions emerging from random local interactions.


title: Quantum t-designs: t-wise independence in the quantum world

abstract:
  A t-design for quantum states is a finite set of quantum states with the
property of simulating the Haar-measure on quantum states, w.r.t. any test that
uses at most t copies of a state. We give efficient constructions for
approximate quantum t-designs for arbitrary t. We then show that an approximate
4-design provides a derandomization of the state-distinction problem considered
by Sen (quant-ph/0512085), which is relevant to solving certain instances of
the hidden subgroup problem.


title: The Physical Basis of the Gibbs-von Neumann entropy

abstract:
  We develop the argument that the Gibbs-von Neumann entropy is the appropriate
statistical mechanical generalisation of the thermodynamic entropy, for
macroscopic and microscopic systems, whether in thermal equilibrium or not, as
a consequence of Hamiltonian dynamics. The mathematical treatment utilises well
known results [Gib02, Tol38, Weh78, Par89], but most importantly, incorporates
a variety of arguments on the phenomenological properties of thermal states
[Szi25, TQ63, HK65, GB91] and of statistical distributions[HG76, PW78, Len78].
This enables the identification of the canonical distribution as the unique
representation of thermal states without approximation or presupposing the
existence of an entropy function. The Gibbs-von Neumann entropy is then
derived, from arguments based solely on the addition of probabilities to
Hamiltonian dynamics.


title: Periodic orbit theory and spectral statistics for scaling quantum graphs

abstract:
  The explicit solution to the spectral problem of quantum graphs found
recently in \cite{Anima}, is used to produce the exact periodic orbit theory
description for the probability distributions of spectral statistics, including
the distribution for the nearest neighbor separations, $s_{n}=k_{n}-k_{n-1}$,
and the distribution of the spectral oscillations around the average, $\delta
k_{n}=k_{n}-\bar k_{n}$.


title: Quantum interference with photon pairs using two micro-structured fibres

abstract:
  We demonstrate a quantum interference experiment between two photons coming
from non-degenerate pairs created by four-wave mixing in two separated
micro-structured fibres. When the two heralded photons are made
indistinguishable a 95% visibility is demonstrated.


title:
Entanglement between two fermionic atoms inside a cylindrical harmonic
  trap

abstract:
  We investigate quantum entanglement between two (spin-1/2) fermions inside a
cylindrical harmonic trap, making use of the von Neumann entropy for the
reduced single particle density matrix as the pure state entanglement measure.
We explore the dependence of pair entanglement on the geometry and strength of
the trap and on the strength of the pairing interaction over the complete range
of the effective BCS to BEC crossover. Our result elucidates an interesting
connection between our model system of two fermions and that of two interacting
bosons.


title:
Dissipative tunneling through a parabolic potential in the Lindblad
  theory of open quantum systems

abstract:
  By using the Lindblad theory for open quantum systems, an analytical
expression of the tunneling probability through an inverted parabola is
obtained. This penetration probability depends on the environment coefficients.
It is shown that the tunneling probability increases with the dissipation and
the temperature of the thermal bath.


title: Topological stability of stored optical vortices

abstract:
  We report an experiment in which an optical vortex is stored in a vapor of Rb
atoms. Due to its 2\pi phase twist, this mode, also known as the Laguerre-Gauss
mode, is topologically stable and cannot unwind even under conditions of strong
diffusion. To supplement our finding, we stored a flat phase Gaussian beam with
a dark center. Contrary to the optical vortex, which stays stable for over 100
microseconds, the dark center in the retrieved flat-phased image was filled
with light at storage times as small as 10 microseconds. This experiment proves
that higher electromagnetic modes can be converted into atomic coherences, and
that modes with phase singularities are robust to decoherence effects such as
diffusion. This opens the possibility to more elaborate schemes for two
dimensional information storage in atomic vapors.


title: Trapping of light pulses in ensembles of stationary Lambda atoms

abstract:
  We present a detailed theoretical description of the generation of stationary
light pulses by standing wave electromagnetically induced transparency in media
comprised of stationary atoms. We show that, contrary to thermal gas media, the
achievable storage times are limited only by the ground state dephasing rate of
the atoms, making such media ideally suited for nonlinear optical interactions
between stored pulses. Furthermore, we find significant quantitative and
qualitative differences between the two types of media, which are important for
quantum information processing schemes involving stationary light pulses.


title: On quantum advantage in dense coding

abstract:
  The quantum advantage of dense coding is studied, considering general
encoding quantum operations. Particular attention is devoted to the case of
many senders, and it is shown that restrictions on the possible operations on
the senders' side may make some quantum state useless for dense-coding. It is
shown, e.g., that some states are useful for dense coding if the senders can
communicate classically (but not quantumly), yet they cannot be used for dense
coding, if classical communication is not allowed. These no-go results are
actually independent of the particular quantification of the quantum advantage,
being valid for any reasonable choice. It is further shown that the quantum
advantage of dense coding satisfies a monogamy relation with the so-called
entanglement of purification.


title: Linearity and Quantum Adiabatic Theorem

abstract:
  We show that in a quantum adiabatic evolution, even though the adiabatic
approximation is valid, the total phase of the final state indicated by the
adiabatic theorem may evidently differ from the actual total phase. This
invalidates the application of the linearity and the adiabatic approximation
simultaneously. Besides, based on this observation we point out the mistake in
the traditional proof for the adiabatic theorem. This mistake is the root of
the troubles that the adiabatic theorem has met. We also show that a similar
mistake remains in some recent modifications of the traditional adiabatic
theorem attempting to eliminate the troubles.


title: Angular momentum of the physical electron

abstract:
  The angular momentum of the physical electron, modelled as a Dirac fermion
coupled to the electromagnetic field, is found to be hbar/2, the same as that
of a bare Dirac fermion and independent of the size of the electric charge.


title: Spontaneous decay of an excited atom placed near a rectangular plate

abstract:
  Using the Born expansion of the Green tensor, we consider the spontaneous
decay rate of an excited atom placed in the vicinity of a rectangular plate. We
discuss the limitations of the commonly used simplifying assumption that the
plate extends to infinity in the lateral directions and examine the effects of
the atomic dipole moment orientation, atomic position, and plate boundary and
thickness on the atomic decay rate. In particular, it is shown that in the
boundary region, the spontaneous decay rate can be strongly modified.


title: Fidelity of quantum operations

abstract:
  We present a derivation and numerous applications of a compact explicit
formula for the average fidelity of a quantum operation on a finite dimensional
quantum system. The formula can be applied to averages over particularly
relevant subspaces; it is easily generalized to multi-component systems, and as
a special result, we show that when the same completely positive
trace-preserving map is applied to a large number of qubits with one-bit
fidelity F close to unity, the average fidelity of the operation on the full
K-bit register scales as $F^{3K/2}$.


title:
High accuracy results for the energy levels of the molecular ions H2+,
  D2+ and HD+, up to J=2

abstract:
  We present a nonrelativistic calculation of the rotation-vibration levels of
the molecular ions H2+, D2+ and HD+, relying on the diagonalization of the
exact three-body Hamiltonian. The J=2 levels are obtained with a very high
accuracy of 10^{-14} a.u. (for most levels) representing an improvement by five
orders of magnitude over previous calculations. The accuracy is also improved
for the J=1 levels of H2+ and D2+ with respect to earlier works. Moreover, we
have computed the sensitivities of the energy levels with respect to the mass
ratios, allowing these levels to be used for metrological purposes.


title:
Quantum state reconstruction with imperfect rotations on an
  inhomogeneously broadened ensemble of qubits

abstract:
  We present a method for performing quantum state reconstruction on qubits and
qubit registers in the presence of decoherence and inhomogeneous broadening.
The method assumes only rudimentary single qubit rotations as well as knowledge
of decoherence and loss mechanisms. We show that full state reconstruction is
possible even in the case where single qubit rotations may only be performed
imperfectly. Furthermore we show that for ensemble quantum computing proposals,
quantum state reconstruction is possible even if the ensemble experiences
inhomogeneous broadening and if only imperfect qubit manipulations are
available during state preparation and reconstruction.


title: Dissipative Mechanics Using Complex-Valued Hamiltonians

abstract:
  We show that a large class of dissipative systems can be brought to a
canonical form by introducing complex co-ordinates in phase space and a
complex-valued hamiltonian. A naive canonical quantization of these systems
lead to non-hermitean hamiltonian operators. The excited states are unstable
and decay to the ground state . We also compute the tunneling amplitude across
a potential barrier.

categories: quant-ph cond-mat.stat-mech hep-th math-ph math.DS math.MP math.SG


title: No nonlocal box is universal

abstract:
  We show that standard nonlocal boxes, also known as Popescu-Rohrlich
machines, are not sufficient to simulate any nonlocal correlations that do not
allow signalling. This was known in the multipartite scenario, but we extend
the result to the bipartite case. We then generalize this result further by
showing that no finite set containing any finite-output-alphabet nonlocal boxes
can be a universal set for nonlocality.


title: Subnormalized states and trace-nonincreasing maps

abstract:
  We investigate the set of completely positive, trace-nonincreasing linear
maps acting on the set M_N of mixed quantum states of size N. Extremal point of
this set of maps are characterized and its volume with respect to the
Hilbert-Schmidt (Euclidean) measure is computed explicitly for an arbitrary N.
The spectra of partially reduced rescaled dynamical matrices associated with
trace-nonincreasing completely positive maps belong to the N-cube inscribed in
the set of subnormalized states of size N. As a by-product we derive the
measure in M_N induced by partial trace of mixed quantum states distributed
uniformly with respect to HS-measure in $M_{N^2}$.


title: Quantum Finite Automata and Weighted Automata

abstract:
  Quantum finite automata derive their strength by exploiting interference in
complex valued probability amplitudes. Of particular interest is the 2-way
model of Ambainis and Watrous that has both quantum and classical states
(2QCFA) [A. Ambainis and J. Watrous, Two-way finite automata with quantum and
classical state, Theoretical Computer Science, 287(1), pp. 299-311, 2002],
since it combines the advantage of the power of interference in a
constant-sized quantum system with a 2-way head.
  This paper is a step towards finding the least powerful model which is purely
classical and can mimic the dynamics of quantum phase. We consider weighted
automata with the Cortes-Mohri definition of language recognition [C. Cortes
and M. Mohri, Context-Free Recognition with Weighted Automata, Grammars 3(2/3),
pp. 133-150, 2000] as a candidate model for simulating 2QCFA.
  Given any 2QCFA that (i) uses the accept-reject-continue observable, (ii)
recognizes a language with one-sided error and (iii) the entries of whose
unitary matrices are algebraic complex numbers, we show a method of
constructing a weighted automaton over $\mathbb{C}$ that simulates it
efficiently.


title: Quantum Multiplexing for Quantum Computer Networks

abstract:
  In communication networks many different channels must share a limited amount
of resources. In order to allow for multiple simultaneous communications,
multiple access techniques are routinely employed. With quantum communication,
it is possible to share a new kind of resource. All of the system channels can
be accommodated into a single channel in a larger Hilbert space. In the scheme,
a single line combines the information of all the users, and, at the receiver,
the original quantum channels are recovered. The given
multiplexer/demultiplexer circuit can perform this n qubits to qudit
transformation. Connections with superdense coding and classical multiple
access schemes are discussed.


title:
Non-EPR pairs quantum channel for teleporting an arbitrary two-qubit
  state

abstract:
  Recently, Yeo and Chua [Phys. have given an explicit protocol for faithfully
teleporting an arbitrary two-qubit state via a genuine four-qubit entangled
state, which is not reducible to a pair of Bell states. Here, we present a
transformation operator to give the criterion of for faithfully teleporting
arbitrary two-qubit states. The theoretical explanations of some quantum
channels are given by transformation operators. Furthermore, a new four-qubit
entangled state quantum channel is presented.


title: Bounding Run-Times of Local Adiabatic Algorithms

abstract:
  A common trick for designing faster quantum adiabatic algorithms is to apply
the adiabaticity condition locally at every instant. However it is often
difficult to determine the instantaneous gap between the lowest two
eigenvalues, which is an essential ingredient in the adiabaticity condition. In
this paper we present a simple linear algebraic technique for obtaining a lower
bound on the instantaneous gap even in such a situation. As an illustration, we
investigate the adiabatic unordered search of van Dam et al. (How powerful is
adiabatic quantum computation? Proc. IEEE FOCS, pp. 279-287, 2001) and Roland
and Cerf (Physical Review A 65, 042308, 2002) when the non-zero entries of the
diagonal final Hamiltonian are perturbed by a polynomial (in $\log N$, where
$N$ is the length of the unordered list) amount. We use our technique to derive
a bound on the running time of a local adiabatic schedule in terms of the
minimum gap between the lowest two eigenvalues.


title:
Inelastic collisions in an exactly solvable two-mode Bose-Einstein
  Condensate

abstract:
  Inelastic collisions occur in Bose-Einstein condensates, in some cases,
producing particle loss in the system. Nevertheless, these processes have not
been studied in the case when particles do not escape the trap. We show that
such inelastic processes are relevant in quantum properties of the system such
as the evolution of the relative population, the self trapping effect and the
probability distribution of particles. Moreover, including inelastic terms in
the model of the two-mode condensate allows for an exact analytical solution.
Using this solution, we show that collisions favor the generation of
entanglement between the modes of the condensate as long as the collision rate
does not exceed the natural frequency of the system.


title: Conditional Entanglement

abstract:
  Based on the ideas of {\it quantum extension} and {\it quantum conditioning},
we propose a generic approach to construct a new kind of entanglement measures
called {\it conditional entanglement}. The new measures, built from the known
entanglement measures, are convex, automatically {\it super-additive}, and even
smaller than the regularized versions of the generating measures. More
importantly, new measures can also be built directly from measures of
correlations, enabling us to introduce an {\it additive} measure and generalize
it to a multipartite entanglement measure.


title: Scattering quantum random-walk search with errors

abstract:
  We analyze the realization of a quantum-walk search algorithm in a passive,
linear optical network. The specific model enables us to consider the effect of
realistic sources of noise and losses on the search efficiency. Photon loss
uniform in all directions is shown to lead to the rescaling of search time.
Deviation from directional uniformity leads to the enhancement of the search
efficiency compared to uniform loss with the same average. In certain cases
even increasing loss in some of the directions can improve search efficiency.
We show that while we approach the classical limit of the general search
algorithm by introducing random phase fluctuations, its utility for searching
is lost. Using numerical methods, we found that for static phase errors the
averaged search efficiency displays a damped oscillatory behaviour that
asymptotically tends to a non-zero value.


title:
Casimir-Polder forces on excited atoms in the strong atom-field coupling
  regime

abstract:
  Based on macroscopic quantum electrodynamics in linear media, we develop a
general theory of the resonant Casimir-Polder force on an excited two-level
atom in the presence of arbitrary linear bodies, with special emphasis on the
strong-coupling regime where reabsorption of an emitted photon can give rise to
(vacuum) Rabi oscillations. We first derive a simple time-independent
expression for the force by using a dressed-state approximation. For initially
single-quantum excited atom-field systems we then study the dynamics of the
force by starting from the Lorentz force and evaluating its average as a
function of time. For strong atom-field coupling, we find that the force may
undergo damped Rabi oscillations. The damping is due to the decay of both the
atomic excitation and the field excitation, and both amplitude and mean value
of the oscillations depend on the chosen initial state.


title: Comment on Afshar's expriments

abstract:
  Results of the experiments carried out in [Shahriar S. Afshar, Proc. SPIE bf
5866 (2005) 229-244] and [Shahriar S. Afshar, AIP Cof. Proc. 810, (2006)
294-299] are reviewed and their interpretation by the authors is questioned.
Arguments are supported by numerical simulations.


title:
Effect of finite detection efficiency on the observation of the
  dipole-dipole interaction of a few Rydberg atoms

abstract:
  We have developed a simple analytical model describing multi-atom signals
that are measured in experiments on dipole-dipole interaction at resonant
collisions of a few Rydberg atoms. It has been shown that finite efficiency of
the selective field-ionization detector leads to the mixing up of the spectra
of resonant collisions registered for various numbers of Rydberg atoms. The
formulas which help to estimate an appropriate mean Rydberg atom number for a
given detection efficiency are presented. We have found that a measurement of
the relation between the amplitudes of collisional resonances observed in the
one- and two-atom signals provides a straightforward determination of the
absolute detection efficiency and mean Rydberg atom number. We also performed a
testing experiment on resonant collisions in a small excitation volume of a
sodium atomic beam. The resonances observed for 1 to 4 detected Rydberg atoms
have been analyzed and compared with theory.


title:
An Extension of the Wigner-Araki-Yanase Theorem to Multiplicative
  Conserved Quantities

abstract:
  An extension of the Wigner-Araki-Yanase theorem to multiplicative conserved
quantities is presented and approximate versions of the theorem are discussed.


title:
Exponential Type Complex and non-Hermitian Potentials within Quantum
  Hamilton-Jacobi Formalism

abstract:
  PT-/non-PT-symmetric and non-Hermitian deformed Morse and Poschl-Teller
potentials are studied first time by quantum Hamilton-Jacobi approach. Energy
eigenvalues and eigenfunctions are obtained by solving quantum Hamilton-Jacobi
equation.


title: MLC No-go Theorems: Reinterpretation and Extension

abstract:
  In this article, we are interested in the physical model of general quantum
protocols implementing secure two-party computations in the light of Mayers'
and Lo's & Chau's no-go theorems of bit commitment and oblivious transfer. In
contrast to the commonly adobted quantum pure two-party model in the literature
where classical communication is normally ignored, we propose an alternative
interpretation for the purification of classical communication in two-party
protocols by introducing a quantum third party for the classical channel. This
interpretation leads to a global three-party model, involving Alice's and Bob's
machines and the environment coupled to the macroscopic channel, using the
decoherence scheme in quantum measurements. This model could give a more
general view on the concealing/binding trade-off of quantum bit commitment
protocols.
  Inspired from this three-party interpretation, we extend the no-go theorems
for denying some classes of two-party protocols having access to some
particular quantum trusted third-parties, known as quantum two-party oracles.
The extension implies that a quantum protocol for implementing secure two-party
computations musts have access to a trusted third-party which erases
information and thus makes dissipation of heat to the environment.


title: Redfield reduced dynamics and entanglement

abstract:
  In phenomenological applications the time evolution of subsystems immersed in
an external environment are sometimes described by Markovian semigroups of
Redfield type that result non-positive: the appearence of negative
probabilities is avoided by restricting the admissible initial conditions to
those states that remain positive under the action of the dynamics. We show
that this often adopted procedure may lead to physical inconsistencies in
presence of entanglement.


title:
Phase detection at the quantum limit with multi-photon Mach-Zehnder
  interferometry

abstract:
  We study a Mach-Zehnder interferometer fed by a coherent state in one input
port and vacuum in the other. We explore a Bayesian phase estimation strategy
to demonstrate that it is possible to achieve the standard quantum limit
independently from the true value of the phase shift and specific assumptions
on the noise of the interferometer. We have been able to implement the protocol
using parallel operation of two photon-number-resolving detectors and
multiphoton coincidence logic electronics at the output ports of a
weakly-illuminated Mach-Zehnder interferometer. This protocol is unbiased and
saturates the Cramer-Rao phase uncertainty bound and, therefore, is an optimal
phase estimation strategy.


title: The Three-Dimensional Quantum Hamilton-Jacobi Equation and Microstates

abstract:
  In a stationary case and for any potential, we solve the three-dimensional
quantum Hamilton-Jacobi equation in terms of the solutions of the corresponding
Schrodinger equation. Then, in the case of separated variables, by requiring
that the conjugate momentum be invariant under any linear transformation of the
solutions of the Schrodinger equation used in the reduced action, we clearly
identify the integration constants successively in one, two and three
dimensions. In each of these cases, we analytically establish that the quantum
Hamilton-Jacobi equation describes microstates not detected by the Schrodinger
equation in the real wave function case.


title:
Matrix product states and exactly solvable spin 1/2 Heisenberg chains
  with nearest neighbor interactions

abstract:
  Using the matrix product formalism, we introduce a two parameter family of
exactly solvable $xyz$ spin 1/2 Heisenberg chains in magnetic field (with
nearest neighbor interactions) and calculate the ground state and correlation
functions in compact form. The ground state has a very interesting property:
all the pairs of spins are equally entangled with each other. Therefore it is
possible to engineer long-range entanglement in experimentally realizable spin
systems on the one hand and study more closely quantum phase transition in such
systems on the other.


title: Modeling an adiabatic quantum computer

abstract:
  We map adiabatic quantum evolution on the classical Hamiltonian dynamics of a
1D gas (Pechukas gas) and simulate the latter numerically. This approach turns
out to be both insightful and numerically efficient, as seen from our example
of a CNOT gate simulation. For a general class of Hamiltonians we show that the
escape probability from the initial state scales no faster than
|\dot{\lambda}|^{\gamma}, where |\dot{\lambda}| is the adiabaticity parameter.
The scaling exponent for the escape probability is \gamma = 1/2 for all levels,
except the edge (bottom and top) ones, where \gamma <~1/3. In principle, our
method can solve arbitrarily large adiabatic quantum Hamiltonians.


title: How to check the one-count operator experimentally

abstract:
  We propose an experimental scheme to probe the form of one-count operation
used in the theory of continuous photodetection in cavities. Two main steps
are: 1) an absorption of a single photon by an atom passing through a high-Q
cavity containing electromagnetic field in a thermal or coherent state, 2) a
subsequent measurement of the photon statistics in the new field state arising
after the photon absorption. Then comparing the probabilities of finding 0 and
1 photons in the initial and final states of the field, one can make
conclusions on the form of the one-count operation. This method can be readily
applied in the microwave cavity QED with present technology.


title: Does Observation Create Reality?

abstract:
  It has been suggested that the locality of information transfer in quantum
entanglement indicates that reality is subjective, meaning that there is an
innate inseparability between the physical system and the conscious mind of the
observer. This paper attempts to outline the relation between macroscopic and
microscopic worlds in the measurement process in regards to whether observation
creates reality. Indeed, the Maxwell's demon thought experiment suggests a
correlation between a microscopic (quantum) system and a macroscopic
(classical) apparatus, which leads to an energy transfer from the quantum
vacuum to the physical world, similar to particle creation from a vacuum. This
explanation shows that observation in quantum theory conserves, rather than
creates, energy.


title:
Superluminal propagation of an optical pulse in a Doppler broadened
  three-state, single channel active Raman gain medium

abstract:
  Using a single channel active Raman gain medium we show a $(220\pm 20)$ns
advance time for an optical pulse of $\tau_{FWHM}=15.4 \mu$s propagating
through a 10 cm medium, a lead time that is comparable to what was reported
previously. In addition, we have verified experimentally all the features
associated with this single channel Raman gain system. Our results show that
the reported gain-assisted superluminal propagation should not be attributed to
the interference between the two frequencies of the pump field.


title:
A precise CNOT gate in the presence of large fabrication induced
  variations of the exchange interaction strength

abstract:
  We demonstrate how using two-qubit composite rotations a high fidelity
controlled-NOT (CNOT) gate can be constructed, even when the strength of the
interaction between qubits is not accurately known. We focus on the exchange
interaction oscillation in silicon based solid-state architectures with a
Heisenberg Hamiltonian. This method easily applies to a general two-qubit
Hamiltonian. We show how the robust CNOT gate can achieve a very high fidelity
when a single application of the composite rotations is combined with a modest
level of Hamiltonian characterisation. Operating the robust CNOT gate in a
suitably characterised system means concatenation of the composite pulse is
unnecessary, hence reducing operation time, and ensuring the gate operates
below the threshold required for fault-tolerant quantum computation.