title: Electron impact ionization loading of a surface electrode ion trap

abstract:
  We demonstrate a method for loading surface electrode ion traps by electron
impact ionization. The method relies on the property of surface electrode
geometries that the trap depth can be increased at the cost of more
micromotion. By introducing a buffer gas, we can counteract the rf heating
assocated with the micromotion and benefit from the larger trap depth. After an
initial loading of the trap, standard compensation techniques can be used to
cancel the stray fields resulting from charged dielectric and allow for the
loading of the trap at ultra-high vacuum.


title: Generating set for general multipartite entangled states

abstract:
  We propose a entanglement generating set for a general multipartite state
based on the of concurrence. In particular, we show that concurrence for
general multipartite states can be constructed by different classes of local
operators which are defined by complement of positive operator valued measure
on quantum phases. The entanglement generating set consists of different
classes of entanglement that are detected by these classes of operators and
contributes to the degree of entanglement for a general multipartite state.


title: Quantum secure communication scheme with W state

abstract:
  Recently, Cao et al. proposed a new quantum secure direct communication
scheme using W state. In their scheme, the error rate introduced by an
eavesdropper who takes intercept-resend attack, is only 8.3%. Actually, their
scheme is just a quantum key distribution scheme because the communication
parties first create a shared key and then encrypt the secret message using
one-time pad. We then present a quantum secure communication scheme using
three-qubit W state. In our scheme, the error rate is raised to 25% and it is
not necessary for the present scheme to use alternative measurement or Bell
basis measurement. We also show our scheme is unconditionally secure.


title: Multipartite Quantum Secret Sharing using Not-So-Weak Qubits

abstract:
  We propose a new quantum secret sharing scheme using a single non-entangled
qubit. In the scheme, by transmitting a qubit to the next party sequentially, a
sender can securely transmit a secret message to $N$ receivers who could only
decode the message cooperatively after randomly shuffling the polarization of
the qubit. We explain this quantum secret sharing scheme into the one between a
sender and two receivers, and generalize the scheme between a sender and $N$
receivers. Since our scheme is capable of using a faint coherent pulse as a
qubit, it is experimentally feasible within current technology.


title: Sir Arthur Eddington and the Foundations of Modern Physics

abstract:
  This doctoral dissertation presents an in-depth analysis of the first six
chapters of Eddington's Fundamental Theory, sometimes referred to as his
'statistical' theory, in the context of discoveries and advancements made since
its original publication in 1946. In particular, the analysis is focused on
being in the context of the foundations of quantum mechanics and quantum field
theory. The results of the analysis illuminate a number of novel methods and
techniques that, though not always correct, may prove to be enlightening and
even useful for future research in quantum foundations.


title: Test of the quantumness of atom-atom correlations in a bosonic gas

abstract:
  It is shown how the quantumness of atom-atom correlations in a trapped
bosonic gas can be made observable. Application of continuous feedback control
of the center of mass of the atomic cloud is shown to generate oscillations of
the spatial extension of the cloud, whose amplitude can be directly used as a
characterization of atom-atom correlations. Feedback parameters can be chosen
such that the violation of a Schwarz inequality for atom-atom correlations can
be tested at noise levels much higher than the standard quantum limit.


title: Quantum Communication in Spin Systems With Long-Range Interactions

abstract:
  We calculate the fidelity of transmission of a single qubit between distant
sites on semi-infinite and finite chains of spins coupled via the magnetic
dipole interaction. We show that such systems often perform better than their
Heisenberg nearest-neighbour coupled counterparts, and that fidelities closely
approaching unity can be attained between the ends of finite chains without any
special engineering of the system, although state transfer becomes slow in long
chains. We discuss possible optimization methods, and find that, for any
length, the best compromise between the quality and the speed of the
communication is obtained in a nearly uniform chain of 4 spins.


title:
Two-mode optical state truncation and generation of maximally entangled
  states in pumped nonlinear couplers

abstract:
  Schemes for optical-state truncation of two cavity modes are analysed. The
systems, referred to as the nonlinear quantum scissors devices, comprise two
coupled nonlinear oscillators (Kerr nonlinear coupler) with one or two of them
pumped by external classical fields. It is shown that the quantum evolution of
the pumped couplers can be closed in a two-qubit Hilbert space spanned by
vacuum and single-photon states only. Thus, the pumped couplers can behave as a
two-qubit system. Analysis of time evolution of the quantum entanglement shows
that Bell states can be generated. A possible implementation of the couplers is
suggested in a pumped double-ring cavity with resonantly enhanced Kerr
nonlinearities in an electromagnetically-induced transparency scheme. The
fragility of the generated states and their entanglement due to the standard
dissipation and phase damping are discussed by numerically solving two types of
master equations.


title:
Analysis of a Quantum Nondemolition Measurement Scheme Based on Kerr
  Nonlinearity in Photonic Crystal Waveguides

abstract:
  We discuss the feasibility of a quantum nondemolition measurement (QND) of
photon number based on cross phase modulation due to the Kerr effect in
Photonic Crystal Waveguides (PCWs). In particular, we derive the equations for
two modes propagating in PCWs and their coupling by a third order nonlinearity.
The reduced group velocity and small cross-sectional area of the PCW lead to an
enhancement of the interaction relative to bulk materials. We show that in
principle, such experiments may be feasible with current photonic technologies,
although they are limited by material properties. Our analysis of the
propagation equations is sufficiently general to be applicable to the study of
soliton formation, all-optical switching and can be extended to processes
involving other orders of the nonlinearity.


title: Notes on optimality of direct characterisation of quantum dynamics

abstract:
  We argue that the claimed optimality of a new process tomography method
suggested in [quant-ph/0601033] and [quant-ph/0601034] is based on not
completely fair comparison that does not take into account the available
information in an equal way. We also argue that the method is not a new process
tomography scheme, but rather represents an interesting modification of ancilla
assisted process tomography method. In our opinion these modifications require
deeper understanding and further investigation.


title:
Electromagnetic manipulation for anti-Zeno effect in an engineered
  quantum tunneling process

abstract:
  We investigate the quantum Zeno and anti-Zeno effects for the irreversible
quantum tunneling from a quantum dot to a ring array of quantum dots. By
modeling the total system with the Anderson-Fano-Lee model, it is found that
the transition from the quantum Zeno effect to quantum anti-Zeno effect can
happen as the magnetic flux and the gate voltage were adjusted.


title: Light transport in cold atoms and thermal decoherence

abstract:
  By using the coherent backscattering interference effect, we investigate
experimentally and theoretically how coherent transport of light inside a cold
atomic vapour is affected by the residual motion of atomic scatterers. As the
temperature of the atomic cloud increases, the interference contrast
dramatically decreases emphazising the role of motion-induced decoherence for
resonant scatterers even in the sub-Doppler regime of temperature. We derive
analytical expressions for the corresponding coherence time.


title: Secure Quantum Bit Commitment Using Unstable Particles

abstract:
  Using unstable particles which decay by emitting neutrinos, we propose a
quantum bit commitment protocol that is humanly impossible to break. Neutrinos
carry away quantum information, but their interaction with matter is so weak
that it would take an astronomically-sized machine just to catch them, not to
mention performing controlled unitary operations on them. As a result quantum
information is lost, and cheating is not possible even if the participants had
access to the most powerful quantum computers that could ever be built.
Therefore, for all practical purposes, our new protocol is as good as
unconditionally secure.


title: The certainty principle II: Proof of the uncertainty principle

abstract:
  A more detailed derivation of the Heisenberg uncertainty principle from the
certainty principle is given.


title:
Implementing the one-dimensional quantum (Hadamard) walk using a
  Bose-Einstein Condensate

abstract:
  We propose a scheme to implement the simplest and best-studied version of
quantum random walk, the discrete Hadamard walk, in one dimension using
coherent macroscopic sample of ultracold atoms, Bose-Einstein condensate (BEC).
Implementation of quantum walk using BEC gives access to the familiar quantum
phenomena on a macroscopic scale. This paper uses rf pulse to implement
Hadamard operation (rotation) and stimulated Raman transition technique as
unitary shift operator. The scheme suggests implementation of Hadamard
operation and unitary shift operator while the BEC is trapped in long Rayleigh
range optical dipole trap. The Hadamard rotation and a unitary shift operator
on BEC prepared in one of the internal state followed by a bit flip operation,
implements one step of the Hadamard walk. To realize a sizable number of steps,
the process is iterated without resorting to intermediate measurement. With
current dipole trap technology it should be possible to implement enough steps
to experimentally highlight the discrete quantum random walk using a BEC
leading to further exploration of quantum random walks and its applications.


title: Fidelity and Coherence Measures from Interference

abstract:
  By utilizing single particle interferometry, the fidelity or coherence of a
pair of quantum states is identified with their capacity for interference. We
consider processes acting on the internal degree of freedom (e.g., spin or
polarization) of the interfering particle, preparing it in states \rho_{A} or
\rho_{B} in the respective path of the interferometer. The maximal visibility
depends on the choice of interferometer, as well as the locality or
non-locality of the preparations, but otherwise depends only on the states
\rho_{A} and \rho_{B} and not the individual preparation processes themselves.
This allows us to define interferometric measures which probe locality and
correlation properties of spatially or temporally separated processes, and can
be used to differentiate between processes that cannot be distinguished by
direct process tomography using only the internal state of the particle.


title:
N Identical Particles Under Quantum Confinement: A Many-Body Dimensional
  Perturbation Theory Approach II, The Lowest-Order Wave Function I

abstract:
  In this paper we continue our development of a dimensional perturbation
theory (DPT) treatment of N identical particles under quantum confinement. DPT
is a beyond-mean-field method which is applicable to both weakly and
strongly-interacting systems and can be used to connect both limits. In a
previous paper we developed the formalism for low-order energies and excitation
frequencies. This formalism has been applied to atoms, Bose-Einstein
condensates and quantum dots. One major advantage of the method is that N
appears as a parameter in the analytical expressions for the energy and so
results for N up to a few thousand are easy to obtain. Other properties
however, are also of interest, for example the density profile in the case of a
BEC,and larger N results are desirable as well. The latter case requires us to
go to higher orders in DPT. These calculations require as input zeroth-order
wave functions and this paper, along with a subsequent paper, addresses this
issue.


title: A naturally error suppressing quantum memory

abstract:
  We propose a method to construct quantum storage wherein the phase error due
to decoherence is naturally suppressed without constant error detection and
correction. As an example, we describe a quantum memory made of two physical
qubits encoded in the ground state of a two-qubit phase-error detecting code.
Such a system can be simulated by introducing a coupling between the two
physical qubits. This method is effective for physical systems in which the
$T_1$ decay process is negligible but coherence is limited by the $T_2$ decay
process. We take trapped ions as a possible example to apply the natural
suppression method and show that the $T_2$ decay time due to slow ambient
fluctuating fields at the physical qubits can be lengthened as much as $10^4$.


title: Optimal control, geometry, and quantum computing

abstract:
  We prove upper and lower bounds relating the quantum gate complexity of a
unitary operation, U, to the optimal control cost associated to the synthesis
of U. These bounds apply for any optimal control problem, and can be used to
show that the quantum gate complexity is essentially equivalent to the optimal
control cost for a wide range of problems, including time-optimal control and
finding minimal distances on certain Riemannian, subriemannian, and Finslerian
manifolds. These results generalize the results of Nielsen, Dowling, Gu, and
Doherty, Science 311, 1133-1135 (2006), which showed that the gate complexity
can be related to distances on a Riemannian manifold


title: Quantum Computation as Geometry

abstract:
  Quantum computers hold great promise, but it remains a challenge to find
efficient quantum circuits that solve interesting computational problems. We
show that finding optimal quantum circuits is essentially equivalent to finding
the shortest path between two points in a certain curved geometry. By recasting
the problem of finding quantum circuits as a geometric problem, we open up the
possibility of using the mathematical techniques of Riemannian geometry to
suggest new quantum algorithms, or to prove limitations on the power of quantum
computers.


title: Strong monotonicity in mixed-state entanglement manipulation

abstract:
  A strong entanglement monotone, which never increases under local operations
and classical communications (LOCC), restricts quantum entanglement
manipulation more strongly than the usual monotone since the usual one does not
increase on average under LOCC. We propose new strong monotones in mixed-state
entanglement manipulation under LOCC. These are related to the decomposability
and 1-positivity of an operator constructed from a quantum state, and reveal
geometrical characteristics of entangled states. These are lower bounded by the
negativity or generalized robustness of entanglement.


title: On the simulation of quantum circuits

abstract:
  We consider recent works on the simulation of quantum circuits using the
formalism of matrix product states and the formalism of contracting tensor
networks. We provide simplified direct proofs of many of these results,
extending an explicit class of efficiently simulable circuits (log depth
circuits with 2-qubit gates of limited range) to the following: let C be any
poly sized quantum circuit (generally of poly depth too) on n qubits comprising
1- and 2- qubit gates and 1-qubit measurements (with 2-qubit gates acting on
arbitrary pairs of qubit lines). For each qubit line j let D_j be the number of
2-qubit gates that touch or cross the line j i.e. the number of 2-qubit gates
that are applied to qubits i,k with i \leq j \leq k. Let D=max_j D_j. Then the
quantum process can be classically simulated in time n poly(2^D). Thus if
D=O(log n) then C may be efficiently classically simulated.


title: Continuous Wave Function Collapse in Quantum-Electrodynamics?

abstract:
  Time-continuous wavefunction collapse mechanisms n o t restricted to
markovian approximation have been found only a few years ago, and have left
many issues open. The results apply formally to the standard relativistic
quantum-electrodynamics. I present a generalized Schrodinger equation driven by
a certain complex stochastic field. The equation reproduces the e x a c t
dynamics of the interacting fermions in QED. The state of the fermions appears
to collapse continuously, due to their interaction with the photonic degrees of
freedom. Even the formal study is instructive for the foundations of quantum
mechanics and of field theory as well.


title: The equilibrium limit of the Casati-Prosen model

abstract:
  An alternative explanation of the decoherence in the Casati-Prosen model is
presented. It is based on the Self Induced Decoherence formalism extended to
non-integrable systems.


title: Quantum process tomography: the role of initial correlations

abstract:
  We address the problem of quantum process tomography with the preparators
producing states correlated with the environmental degrees of freedom that play
role in the system-environment interactions. We discuss the physical
situations, in which the dynamics is described by nonlinear, or noncompletely
positive transformations. In particular, we show that arbitrary mapping
$\varrho_{\rm in}\to\varrho_{\rm out}$ can be realized by using appropriate set
of preparators and applying the unitary operation SWAP. The experimental
``realization'' of perfect NOT operation is presented. We address the problem
of the verification of the compatibility of the preparator devices with the
estimating process. The evolution map describing the dynamics in arbitrary time
interval is known not to be completely positive, but still linear. The
tomography and general properties of these maps are discussed.


title: Spatial Orientation using Quantum Telepathy

abstract:
  We implemented the protocol of entanglement assisted orientation in the space
proposed by Brukner et al (quant-ph/0603167). We used min-max principle to
evaluate the optimal entangled state and the optimal direction of polarization
measurements which violate the classical bound.


title: Extremal covariant measurements

abstract:
  We characterize the extremal points of the convex set of quantum measurements
that are covariant under a finite-dimensional projective representation of a
compact group, with action of the group on the measurement probability space
which is generally non-transitive. In this case the POVM density is made of
multiple orbits of positive operators, and, in the case of extremal
measurements, we provide a bound for the number of orbits and for the rank of
POVM elements. Two relevant applications are considered, concerning state
discrimination with mutually unbiased bases and the maximization of the mutual
information.


title: An open quantum system approach to EPR correlations in K0-K0 system

abstract:
  We find the time evolution of the system of two non-interacting unstable
particles, distinguishable as well as identical ones, in arbitrary reference
frame having only the Kraus operators governing the evolution of its components
in the rest frame. We than calculate in the rigorous way
Einstein-Podolsky-Rosen quantum correlation functions for K0-K0 system in the
singlet state taking into account CP-violation and decoherence and show that
the results are exactly the same despite the fact we treat kaons as
distinguishable or identical particles which means that the statistics of the
particles plays no role, at least in considered cases.


title: Supersymmetric Biorthogonal Quantum Systems

abstract:
  We discuss supersymmetric biorthogonal systems, with emphasis given to the
periodic solutions that occur at spectral singularities of PT symmetric models.
For these periodic solutions, the dual functions are associated polynomials
that obey inhomogeneous equations. We construct in detail some explicit
examples for the supersymmetric pairs of potentials V_{+/-}(z) = -U(z)^2 +/-
z(d/(dz))U(z) where U(z) = \sum_{k>0}u_{k}z^{k}. In particular, we consider the
cases generated by U(z) = z and z/(1-z). We also briefly consider the effects
of magnetic vector potentials on the partition functions of these systems.


title: Hardy's criterion of nonlocality for mixed states

abstract:
  We generalize Hardy's proof of nonlocality to the case of bipartite mixed
statistical operators, and we exhibit a necessary condition which has to be
satisfied by any given mixed state $\sigma$ in order that a local and realistic
hidden variable model exists which accounts for the quantum mechanical
predictions implied by $\sigma$. Failure of this condition will imply both the
impossibility of any local explanation of certain joint probability
distributions in terms of hidden variables and the nonseparability of the
considered mixed statistical operator. Our result can be also used to determine
the maximum amount of noise, arising from imperfect experimental
implementations of the original Hardy's proof of nonlocality, in presence of
which it is still possible to put into evidence the nonlocal features of
certain mixed states.


title: On the Notion of Proposition in Classical and Quantum Mechanics

abstract:
  The term proposition usually denotes in quantum mechanics (QM) an element of
(standard) quantum logic (QL). Within the orthodox interpretation of QM the
propositions of QL cannot be associated with sentences of a language stating
properties of individual samples of a physical system, since properties are
nonobjective in QM. This makes the interpretation of propositions
problematical. The difficulty can be removed by adopting the objective
interpretation of QM proposed by one of the authors (semantic realism, or SR,
interpretation). In this case, a unified perspective can be adopted for QM and
classical mechanics (CM), and a simple first order predicate calculus L(x) with
Tarskian semantics can be constructed such that one can associate a physical
proposition (i.e., a set of physical states) with every sentence of L(x). The
set $P^{f}$ of all physical propositions is partially ordered and contains a
subset $P^{f}_{T}$ of testable physical propositions whose order structure
depends on the criteria of testability established by the physical theory. In
particular, $P^{f}_{T}$ turns out to be a Boolean lattice in CM, while it can
be identified with QL in QM. Hence the propositions of QL can be associated
with sentences of L(x), or also with the sentences of a suitable quantum
language $L_{TQ}(x)$, and the structure of QL characterizes the notion of
testability in QM. One can then show that the notion of quantum truth does not
conflict with the classical notion of truth within this perspective.
Furthermore, the interpretation of QL propounded here proves to be equivalent
to a previous pragmatic interpretation worked out by one of the authors, and
can be embodied within a more general perspective which considers states as
first order predicates of a broader language with a Kripkean semantics.


title: Strengths and Weaknesses of Quantum Fingerprinting

abstract:
  We study the power of quantum fingerprints in the simultaneous message
passing (SMP) setting of communication complexity. Yao recently showed how to
simulate, with exponential overhead, classical shared-randomness SMP protocols
by means of quantum SMP protocols without shared randomness
($Q^\parallel$-protocols). Our first result is to extend Yao's simulation to
the strongest possible model: every many-round quantum protocol with unlimited
shared entanglement can be simulated, with exponential overhead, by
$Q^\parallel$-protocols. We apply our technique to obtain an efficient
$Q^\parallel$-protocol for a function which cannot be efficiently solved
through more restricted simulations. Second, we tightly characterize the power
of the quantum fingerprinting technique by making a connection to arrangements
of homogeneous halfspaces with maximal margin. These arrangements have been
well studied in computational learning theory, and we use some strong results
obtained in this area to exhibit weaknesses of quantum fingerprinting. In
particular, this implies that for almost all functions, quantum fingerprinting
protocols are exponentially worse than classical deterministic SMP protocols.


title: Optical implementation of the encoding of two qubits to a single qutrit

abstract:
  We have devised an optical scheme for the recently proposed protocol for
encoding two qubits into one qutrit. In this protocol, Alice encodes an
arbitrary pure product state of two qubits into a state of one qutrit. Bob can
then restore error-free any of the two encoded qubit states but not both of
them simultaneously. We have successfully realized this scheme experimentally
using spatial-mode encoding. Each qubit (qutrit) was represented by a single
photon that could propagate through two (three) separate fibers. We
theoretically propose two generalizations of the original protocol. We have
found a probabilistic operation that enables to retrieve both qubits
simultaneously with the average fidelity above 90% and we have proposed
extension of the original encoding transformation to encode N qubits into one
(N+1)-dimensional system.


title:
Bounds for the adiabatic approximation with applications to quantum
  computation

abstract:
  We present straightforward proofs of estimates used in the adiabatic
approximation. The gap dependence is analyzed explicitly. We apply the result
to interpolating Hamiltonians of interest in quantum computing.


title:
The rigged Hilbert space approach to the Lippmann-Schwinger equation.
  Part I

abstract:
  We exemplify the way the rigged Hilbert space deals with the
Lippmann-Schwinger equation by way of the spherical shell potential. We
explicitly construct the Lippmann-Schwinger bras and kets along with their
energy representation, their time evolution and the rigged Hilbert spaces to
which they belong. It will be concluded that the natural setting for the
solutions of the Lippmann-Schwinger equation--and therefore for scattering
theory--is the rigged Hilbert space rather than just the Hilbert space.


title:
The rigged Hilbert space approach to the Lippmann-Schwinger equation.
  Part II: The analytic continuation of the Lippmann-Schwinger bras and kets

abstract:
  The analytic continuation of the Lippmann-Schwinger bras and kets is obtained
and characterized. It is shown that the natural mathematical setting for the
analytic continuation of the solutions of the Lippmann-Schwinger equation is
the rigged Hilbert space rather than just the Hilbert space. It is also argued
that this analytic continuation entails the imposition of a time asymmetric
boundary condition upon the group time evolution, resulting into a semigroup
time evolution. Physically, the semigroup time evolution is simply a (retarded
or advanced) propagator.


title: Yangian and Applications

abstract:
  In this paper, the Yangian relations are tremendously simplified for Yangians
associated to SU(2), SU(3), SO(5) and SO(6) based on RTT relations that much
benefit the realization of Yangian in physics. The physical meaning and some
applications of Yangian have been shown.


title: Quantum Speed Limit for Perfect State Transfer in One Dimension

abstract:
  The basic idea of spin chain engineering for perfect quantum state transfer
(QST) is to find a set of coupling constants in the Hamiltonian, such that a
particular state initially encoded on one site will evolve freely to the
opposite site without any dynamical controls. The minimal possible evolution
time represents a speed limit for QST. We prove that the optimal solution is
the one simulating the precession of a spin in a static magnetic field. We also
argue that, at least for solid-state systems where interactions are local, it
is more realistic to characterize the computation power by the couplings than
the initial energy.


title: Factorization of large numbers and the suggestion of an algorithm

abstract:
  In this paper, we intend to present a new algorithm to factorize large
numbers. According to the algorithm proposed here, we prove that there is a
common factor between p and q. With this procedure, the time of factorization
considerably decreases. The algorithm is based on a graphic representation and,
when the corresponding graph is drawn, coordinate pairs will originate two
straight lines that intercept one another. These coordinate pairs are formed by
prime numbers in the x-axis, and factors in the y-axis, including the factor in
common.


title: A search on the Klein-Gordon equation

abstract:
  The s-wave Klein-Gordon equation for the bound states is separated in two
parts to see clearly the relativistic contributions to the solution in the
non-relativistic limit. The reliability of the model is discussed with the
specifically chosen two examples.


title: Maximally entangled mixed states made easy

abstract:
  We show that it is possible to achieve maximally entangled mixed states of
two qubits from the singlet state via the action of local non-trace-preserving
quantum channels. Moreover, we present a simple, feasible linear optical
implementation of one of such channels.


title: Quantum description of spin tunneling in magnetic molecules: a new view

abstract:
  Starting from a phenomenological Hamiltonian originally written in terms of
angular momentum operators we derive a new quantum angle-based Hamiltonian that
allows for a discussion on the quantum spin tunneling. The study of the
applicability of the present approach, carried out in calculations with a
soluble quasi-spin model, shows that we are allowed to use our method in the
description of physical systems such as the Mn12-acetate molecule, as well as
the octanuclear iron cluster, Fe8, in a reliable way. With the present
description the interpretation of the spin tunneling is seen to be direct, the
spectra and energy barriers of those systems are obtained, and it is shown that
they agree with the experimental ones.


title:
Completely positive covariant two-qubit quantum processes and optimal
  quantum NOT operations for entangled qubit pairs

abstract:
  The structure of all completely positive quantum operations is investigated
which transform pure two-qubit input states of a given degree of entanglement
in a covariant way. Special cases thereof are quantum NOT operations which
transform entangled pure two-qubit input states of a given degree of
entanglement into orthogonal states in an optimal way. Based on our general
analysis all covariant optimal two-qubit quantum NOT operations are determined.
In particular, it is demonstrated that only in the case of maximally entangled
input states these quantum NOT operations can be performed perfectly.


title: Lasing and cooling in a hot cavity

abstract:
  We present a microscopic laser model for many atoms coupled to a single
cavity mode, including the light forces resulting from atom-field momentum
exchange. Within a semiclassical description, we solve the equations for atomic
motion and internal dynamics to obtain analytic expressions for the optical
potential and friction force seen by each atom. When optical gain is maximum at
frequencies where the light field extracts kinetic energy from the atomic
motion, the dynamics combines optical lasing and motional cooling. From the
corresponding momentum diffusion coefficient we predict sub-Doppler
temperatures in the stationary state. This generalizes the theory of cavity
enhanced laser cooling to active cavity systems. We identify the gain induced
reduction of the effective resonator linewidth as key origin for the faster
cooling and lower temperatures, which implys that a bad cavity with a gain
medium can replace a high-Q cavity. In addition, this shows the importance of
light forces for gas lasers in the low-temperature limit, where atoms can
arrange in a periodic pattern maximizing gain and counteracting spatial hole
burning. Ultimately, in the low temperature limit, such a setup should allow to
combine optical lasing and atom lasing in single device.


title: Electrostatic pair creation and recombination in quantum plasmas

abstract:
  The collective production of electron-positron pairs by electrostatic waves
in quantum plasmas is investigated. In particular, a semi-classical governing
set of equation for a self-consistent treatment of pair creation by the
Schwinger mechanism in a quantum plasma is derived.


title:
Self-adjoint differential operators assosiated with self-adjoint
  differential expressions

abstract:
  Considerable attention has been recently focused on quantum-mechanical
systems with boundaries and/or singular potentials for which the construction
of physical observables as self-adjoint (s.a.) operators is a nontrivial
problem. We present a comparative review of various methods of specifying
ordinary s.a. differential operators generated by formally s.a. differential
expressions based on the general theory of s.a. extensions of symmetric
operators. The exposition is untraditional and is based on the concept of
asymmetry forms generated by adjoint operators. The main attention is given to
a specification of s.a. extensions by s.a. boundary conditions. All the methods
are illustrated by examples of quantum-mechanical observables like momentum and
Hamiltonian. In addition to the conventional methods, we propose a possible
alternative way of specifying s.a. differential operators by explicit s.a.
boundary conditions that generally have an asymptotic form for singular
boundaries. A comparative advantage of the method is that it allows avoiding an
evaluation of deficient subspaces and deficiency indices. The effectiveness of
the method is illustrated by a number of examples of quantum-mechanical
observables.


title: Quantum Recurrences in Driven Power-law Potentials

abstract:
  The recurrence phenomena of an initially well localized wave packet are
studied in periodically driven power-law potentials. For our general study we
divide the potentials in two kinds, namely tightly binding and loosely binding
potentials. In the presence of an external periodically modulating force, these
potentials may exhibit classical and quantum chaos. The dynamics of a quantum
wave packet in the modulated potentials displays recurrences at various time
scales. We develop general analytical relations for these times and discuss
their parametric dependence.


title: Adaptive phase measurements for narrowband squeezed beams

abstract:
  We have previously [Phys. Rev. A 65, 043803 (2002)] analyzed adaptive
measurements for estimating the continuously varying phase of a coherent beam,
and a broadband squeezed beam. A real squeezed beam must have finite photon
flux N and hence can be significantly squeezed only over a limited frequency
range. In this paper we analyze adaptive phase measurements of this type for a
realistic model of a squeezed beam. We show that, provided it is possible to
suitably choose the parameters of the beam, a mean-square phase uncertainty
scaling as (N/kappa)^{-5/8} is possible, where kappa is the linewidth of the
beam resulting from the fluctuating phase. This is an improvement over the
(N/kappa)^{-1/2} scaling found previously for coherent beams. In the
experimentally realistic case where there is a limit on the maximum squeezing
possible, the variance will be reduced below that for coherent beams, though
the scaling is unchanged.


title: A Scheme of Cartan Decomposition for su(N)

abstract:
  A scheme to perform the Cartan decomposition for the Lie algebra su(N) of
arbitrary finite dimensions is introduced. The schme is based on two algebraic
structures, the conjugate partition and the quotient algebra, that are easily
generated by a Cartan subalgebra and generally exist in su(N). In particular,
the Lie algebras su(2^p) and every su(2^{p-1} < N < 2^p) share the isomorphic
structure of the quotient algebra. This structure enables an efficient
algorithm for the recursive and exhaustive construction of Cartan
decompositions. Further with the scheme, a unitary transformation in SU(N) can
be recursively decomposed into a product of certain designated operators, e.g.,
local and nonlocal gates. Such a recursive decomposition of a transformation
implies an evolution path on the manifold of the group.


title:
Entanglement and decoherence of N atoms and a mesoscopic field in a
  cavity

abstract:
  We investigate the behavior of N atoms resonantly coupled to a single
electromagnetic field mode sustained by a high quality cavity, containing a
mesoscopic coherent field. We show with a simple effective hamiltonian model
that the strong coupling between the cavity and the atoms produces an
atom-field entangled state, involving N+1 nearly-coherent components slowly
rotating at different paces in the phase plane. The periodic overlap of these
components results in a complex collapse and revival pattern for the Rabi
oscillation. We study the influence of decoherence due to the finite cavity
quality factor. We propose a simple analytical model, based on the Monte Carlo
approach to relaxation. We compare its predictions with exact calculations and
show that these interesting effects could realistically be observed on a two or
three atoms sample in a 15 photons field with circular Rydberg atoms and
superconducting cavities.


title:
Investigations of Optical Coherence Properties in an Erbium-doped
  Silicate Fiber for Quantum State Storage

abstract:
  We studied optical coherence properties of the 1.53 $\mu$m telecommunication
transition in an Er$^{3+}$-doped silicate optical fiber through spectral
holeburning and photon echoes. We find decoherence times of up to 3.8 $\mu$s at
a magnetic field of 2.2 Tesla and a temperature of 150 mK. A strong
magnetic-field dependent optical dephasing was observed and is believed to
arise from an interaction between the electronic Er$^{3+}$ spin and the
magnetic moment of tunneling modes in the glass. Furthermore, we observed
fine-structure in the Erbium holeburning spectrum originating from
superhyperfine interaction with $^{27}$Al host nuclei. Our results show that
Er$^{3+}$-doped silicate fibers are promising material candidates for quantum
state storage.


title: Microscopic origin of Casimir-Polder forces

abstract:
  We establish a general relation between dispersion forces. First, based on
QED in causal media, leading-order perturbation theory is used to express both
the single-atom Casimir-Polder and the two-atom van der Waals potentials in
terms of the atomic polarizabilities and the Green tensor for the body-assisted
electromagnetic field. Endowed with this geometry-independent framework, we
then employ the Born expansion of the Green tensor together with the
Clausius-Mosotti relation to prove that the macroscopic Casimir-Polder
potential of an atom in the presence of dielectric bodies is due to an infinite
sum of its microscopic many-atom van der Waals interactions with the atoms
comprising the bodies. This theorem holds for inhomogeneous, dispersing, and
absorbing bodies of arbitrary shapes and arbitrary atomic composition on an
arbitrary background of additional magnetodielectric bodies.


title:
Controlled Stark shifts in Er$^{3+}$-doped crystalline and amorphous
  waveguides for quantum state storage

abstract:
  We present measurements of the linear Stark effect on the $^{4}$I$_{15/2}
\to$ $^{4}$I$_{13/2}$ transition in an Er$^{3+}$-doped proton-exchanged
LiNbO$_{3}$ crystalline waveguide and an Er$^{3+}$-doped silicate fiber. The
measurements were made using spectral hole burning techniques at temperatures
below 4 K. We measured an effective Stark coefficient
$(\Delta\mu_{e}\chi)/(h)=25\pm1$kHz/Vcm$^{-1}$ in the crystalline waveguide and
$(\bar{\Delta\mu_{e}}\chi)/(h)=15\pm1$kHz/Vcm$^{-1}$ in the silicate fiber.
These results confirm the potential of Erbium doped waveguides for quantum
state storage based on controlled reversible inhomogeneous broadening.


title: Novel designs for Penning ion traps

abstract:
  We present a number of alternative designs for Penning ion traps suitable for
quantum information processing (QIP) applications with atomic ions. The first
trap design is a simple array of long straight wires which allows easy optical
access. A prototype of this trap has been built to trap Ca+ and a simple
electronic detection scheme has been employed to demonstrate the operation of
the trap. Another trap design consists of a conducting plate with a hole in it
situated above a continuous conducting plane. The final trap design is based on
an array of pad electrodes. Although this trap design lacks the open geometry
of the traps described above, the pad design may prove useful in a hybrid
scheme in which information processing and qubit storage take place in
different types of trap. The behaviour of the pad traps is simulated
numerically and techniques for moving ions rapidly between traps are discussed.
Future experiments with these various designs are discussed. All of the designs
lend themselves to the construction of multiple trap arrays, as required for
scalable ion trap QIP.


title:
Greenberger-Horne-Zeilinger argument of nonlocality without inequalities
  for mixed states

abstract:
  We generalize the Greenberger-Horne-Zeilinger nonlocality without
inequalities argument to cover the case of arbitrary mixed statistical
operators associated to three-qubits quantum systems. More precisely, we
determine the radius of a ball (in the trace distance topology) surrounding the
pure GHZ state and containing arbitrary mixed statistical operators which
cannot be described by any local and realistic hidden variable model and which
are, as a consequence, noncompletely separable. As a practical application, we
focus on certain one-parameter classes of mixed states which are commonly
considered in the experimental realization of the original GHZ argument and
which result from imperfect preparations of the pure GHZ state. In these cases
we determine for which values of the parameter controlling the noise a
nonlocality argument can still be exhibited, despite the mixedness of the
considered states. Moreover, the effect of the imperfect nature of measurement
processes is discussed.


title: Spin-Squeezing and Light Entanglement in Coherent Population Trapping

abstract:
  We show that high squeezing and entanglement can be generated at the output
of a cavity containing atoms interacting with two fields in a Coherent
Population Trapping situation, on account of a non-linear Faraday effect
experienced by the fields close to a dark-state resonance in a cavity.
Moreover, the cavity provides a feedback mechanism allowing to reduce the
quantum fluctuations of the ground state spin, resulting in strong steady state
spin-squeezing.


title: Asymmetrical two-atom entanglement in a coated microsphere

abstract:
  We study evolution of entanglement of two two-level atoms placed inside a
multilayered microsphere. We show that due to inhomogeneity of the field modes
this entanglement essentially depends on the atomic positions (asymmetrical
entanglement) and also on the detuning between the atomic transitions and field
frequencies. The robust and complete entanglement can be achieved even in the
resonant case when the atoms have different effective coupling constants, and
it can be extended in time if the detuning is large enough. We study
analytically the lossless case and estimate numerically the effect of
dissipative processes.


title: Computational complexity of the quantum separability problem

abstract:
  Ever since entanglement was identified as a computational and cryptographic
resource, researchers have sought efficient ways to tell whether a given
density matrix represents an unentangled, or separable, state. This paper gives
the first systematic and comprehensive treatment of this (bipartite) quantum
separability problem, focusing on its deterministic (as opposed to randomized)
computational complexity. First, I review the one-sided tests for separability,
paying particular attention to the semidefinite programming methods. Then, I
discuss various ways of formulating the quantum separability problem, from
exact to approximate formulations, the latter of which are the paper's main
focus. I then give a thorough treatment of the problem's relationship with the
complexity classes NP, NP-complete, and co-NP. I also discuss extensions of
Gurvits' NP-hardness result to strong NP-hardness of certain related problems.
A major open question is whether the NP-contained formulation (QSEP) of the
quantum separability problem is Karp-NP-complete; QSEP may be the first natural
example of a problem that is Turing-NP-complete but not Karp-NP-complete.
Finally, I survey all the proposed (deterministic) algorithms for the quantum
separability problem, including the bounded search for symmetric extensions
(via semidefinite programming), based on the recent quantum de Finetti theorem;
and the entanglement-witness search (via interior-point algorithms and global
optimization). These two algorithms have the lowest complexity, with the latter
being the best under advice of asymptotically optimal point-coverings of the
sphere.


title:
A Crucial Experiment To Test The Broglie-Bohm Trajectories For
  Indistinguishable Particles

abstract:
  The standard quantum theory has not taken into account the size of quantum
particles, the latter being implicitly treated as material points. The recent
interference experiments of Zeilinger [3] with large molecules like fullerenes
and the thought experiments of Bozic et al [7] with asymmetrical Young slits
make it possible today to take into account the particle size.
  We present here a complete study of this phenomenon where our simulations
show differences between the particles density after the slits and the modulus
square of the wave function. Then we propose a crucial experiment that allows
us to reconsider the wave-particle duality and to test the existence of the
Broglie-Bohm trajectories for indistinguishable particles.


title: On tight multiparty Bell inequalities for many settings

abstract:
  A derivation method is given which leads to a series of tight Bell
inequalities for experiments involving N parties, with binary observables, and
three possible local settings. The approach can be generalized to more
settings. Ramifications are presented.


title: Emergent Probabilities in Quantum Mechanics

abstract:
  The transition from the quantum to the classical is governed by randomizing
devices (RD), i.e., dynamical systems that are very sensitive to the
environment. We show that, in the presence of RDs, the usual arguments based on
the linearity of quantum mechanics that lead to the measurement problem do not
apply. RDs are the source of probabilities in quantum mechanics. Hence, the
reason for probabilities in quantum mechanics is the same as the reason for
probabilities in other parts of physics, namely our ignorance of the state of
the environment. This should not be confused with decoherence. The environment
here plays several, equally important roles: it is the dump for energy and
entropy of the RD, it puts the RD close to its transition point and it is the
reason for probabilities in quantum mechanics. We show that, even though the
state of the environment is unknown, the probabilities can be calculated and
are given by the Born rule. We then discuss what this view of quantum mechanics
means for the search of a quantum theory of gravity.


title: Gravitational quantum states of neutrons in a rough waveguide

abstract:
  A theory of gravitational quantum states of ultracold neutrons in waveguides
with absorbing/scattering walls is presented. The theory covers recent
experiments in which the ultracold neutrons were beamed between a mirror and a
rough scatterer/absorber. The analysis is based on a recently developed theory
of quantum transport along random rough walls which is modified in order to
include leaky (absorbing) interfaces and, more importantly, the low-amplitude
high-aperture roughness. The calculations are focused on a regime when the
direct transitions into the continuous spectrum above the absorption threshold
dominate the depletion of neutrons from the gravitational states and are more
efficient than the processes involving the intermediate states. The theoretical
results for the neutron count are sensitive to the correlation radius (lateral
size) of surface inhomogeneities and to the ratio of the particle energy to the
absorption threshold in a weak roughness limit. The main impediment for
observation of the higher gravitational states is the "overhang" of the
particle wave functions which can be overcome only by use scatterers with
strong roughness. In general, the strong roughness with high amplitude is
preferable if one wants just to detect the individual gravitational states,
while the strong roughness experiments with small amplitude and high aperture
are preferable for the quantitative analysis of the data. We also discuss the
ways to further improve the accuracy of calculations and to optimize the
experimental regime.


title: Performance of Equal Phase-Shift Search for One Iteration

abstract:
  Grover presented the phase-shift search by replacing the selective inversions
by selective phase shifts of $\pi /3$. In this paper, we investigate the
phase-shift search with general equal phase shifts. We show that for small
uncertainties, the failure probability of the Phase-$\pi /3$ search is smaller
than the general phase-shift search and for large uncertainties, the success
probability of the large phase-shift search is larger than the Phase-$\pi /3$
search. Therefore, the large phase-shift search is suitable for large-size of
databases.


title: A perturbative treatment for the bound states of the Hellmann potential

abstract:
  A new approximation formalism is applied to study the bound states of the
Hellmann potential, which represents the superposition of the attractive
Coulomb potential $-a/r$ and the Yukawa potential $b\exp (-\delta r)/r$ of
arbitrary strength $b$ and screening parameter $\delta $. Although the analytic
expressions for the energy eigenvalues $E_{n,l\text{}}$ yield quite accurate
results for a wide range of $n,\ell $ in the limit of very weak screening, the
results become gradually worse as the strength $b$ and the screening
coefficient $\delta $ increase. This is because that the expansion parameter is
not sufficiently small enough to guarantee the convergence of the expansion
series for the energy levels.


title:
The Projective Line Over the Finite Quotient Ring GF(2)[$x$]/$< x^{3} -
  x>$ and Quantum Entanglement II. The Mermin "Magic" Square/Pentagram

abstract:
  In 1993, Mermin (Rev. Mod. Phys. 65, 803--815) gave lucid and strikingly
simple proofs of the Bell-Kochen-Specker (BKS) theorem in Hilbert spaces of
dimensions four and eight by making use of what has since been referred to as
the Mermin(-Peres) "magic square" and the Mermin pentagram, respectively. The
former is a $3 \times 3$ array of nine observables commuting pairwise in each
row and column and arranged so that their product properties contradict those
of the assigned eigenvalues. The latter is a set of ten observables arranged in
five groups of four lying along five edges of the pentagram and characterized
by similar contradiction. An interesting one-to-one correspondence between the
operators of the Mermin-Peres square and the points of the projective line over
the product ring ${\rm GF}(2) \otimes \rm{GF}(2)$ is established. Under this
mapping, the concept "mutually commuting" translates into "mutually distant"
and the distinguishing character of the third column's observables has its
counterpart in the distinguished properties of the coordinates of the
corresponding points, whose entries are both either zero-divisors, or units.
The ten operators of the Mermin pentagram answer to a specific subset of points
of the line over GF(2)[$x$]/$<x^{3} - x>$. The situation here is, however, more
intricate as there are two different configurations that seem to serve equally
well our purpose. The first one comprises the three distinguished points of the
(sub)line over GF(2), their three "Jacobson" counterparts and the four points
whose both coordinates are zero-divisors; the other features the neighbourhood
of the point ($1, 0$) (or, equivalently, that of ($0, 1$)). Some other ring
lines that might be relevant for BKS proofs in higher dimensions are also
mentioned.


title: Many-fingered time Bohmian mechanics

abstract:
  The many-fingered time (MFT) formulation of many-particle quantum mechanics
and quantum field theory is a natural framework that overcomes the problem of
"instantaneous collapse" in entangled systems that exhibit nonlocalities. The
corresponding Bohmian interpretation can also be formulated in terms of MFT
beables, which alleviates the problem of instantaneous action at a distance by
using an ontology that differs from that in the standard Bohmian
interpretation. The appearance of usual single-time particle-positions and
fields is recovered by quantum measurements.


title: Entanglement distribution revealed by macroscopic observations

abstract:
  What can we learn about entanglement between individual particles in
macroscopic samples by observing only the collective properties of the
ensembles? Using only a few experimentally feasible collective properties, we
establish an entanglement measure between two samples of spin-1/2 particles (as
representatives of two-dimensional quantum systems). This is a tight lower
bound for the average entanglement between all pairs of spins in general and is
equal to the average entanglement for a certain class of systems. We compute
the entanglement measures for explicit examples and show how to generalize the
method to more than two samples and multi-partite entanglement.


title:
Communication cost of simulating quantum correlations in a class of
  integer spin singlet states

abstract:
  We give a classical protocol to exactly simulate quantum correlations implied
by a spin $s$ singlet state for the infinite sequence of \textit{integer} spin
$s={1,4,13,...}$ satisfying $(2s+1)=3^{n}$,where $n$ is a positive integer.The
required amount of communication is found to increase as $log_{3}d^{2}$ where
$d=2s+1$ is the dimension of the spin $s$ Hilbert space.


title:
A summary on two new algorithms for Grover's unsorted database search
  problem

abstract:
  In this summary we discuss two new algorithms for Grover's unsorted database
search problem that claimed to have reached exponential speedup over Grover's
original algorithm. One is in the quantum setting with "power queries" that
allow for exponential reduction in the number of queries over Grover's original
algorithm with "bit queries". The other is to use "dubit queries" on a duality
computer - a new computing model uses a quantum system's wave-particle duality,
which is able to achieve even greater computing power and better capability
than existent quantum computers we have been discussing. We discuss the
shortages and difficulties of both schemes as well.


title: Exchange Energy in Coupled Quantum Dots

abstract:
  In this work, the exchange energy J for a system of two laterally-coupled
quantum dots, each one with an electron, is calculated analytically and in a
detailed form, considering them as hydrogen-like atoms, under the
Heitler-London approach. The atomic orbitals, associated to each quantum dot,
are obtained from translation relations, as functions of the Fock-Darwin
states. Our results agree with the reported ones by Burkard, Loss and
DiVincenzo in their model of quantum gates based on quantum dots, as well as
with some recent experimental reports.


title: Quantum baker maps with controlled-NOT coupling

abstract:
  The characteristic stretching and squeezing of chaotic motion is linearized
within the finite number of phase space domains which subdivide a classical
baker map. Tensor products of such maps are also chaotic, but a more
interesting generalized baker map arises if the stacking orders for the factor
maps are allowed to interact. These maps are readily quantized, in such a way
that the stacking interaction is entirely attributed to primary qubits in each
map, if each subsystem has power-of-two Hilbert space dimension. We here study
the particular example of two baker maps that interact via a controlled-not
interaction. Numerical evidence indicates that the control subspace becomes an
ideal Markovian environment for the target map in the limit of large Hilbert
space dimension.


title: Generalized Quantum State Sharing

abstract:
  We present two quantum state sharing protocols where the channels are not
maximally entangled states. By properly choosing the measurement basis it is
possible to achieve unity fidelity transfer of the state if the parties
collaborate. We also show that contrary to the protocols where we have
maximally entangled channels these protocols are probabilistic. We then compare
the efficiency of both protocols and sketch the generalization of the protocols
to N parties.


title:
Generation of a squeezed vacuum resonant on Rubidium D_1 line with
  periodically-poled KTiOPO_4

abstract:
  We report generation of a continuous-wave squeezed vacuum resonant on the Rb
D_1 line (795 nm) using periodically poled KTiOPO_4 (PPKTP) crystals. With a
frequency doubler and an optical parametric oscillator based on PPKTP crystals,
we observed a squeezing level of -2.75 +- 0.14 dB and an anti-squeezing level
of +7.00 +- 0.13 dB. This system could be utilized for demonstrating storage
and retrieval of the squeezed vacuum, which is important for the ultra-precise
measurement of atomic spins as well as quantum information processing.


title:
Operational Classification and Quantification of Multipartite Entangled
  States

abstract:
  We formalize and extend an operational multipartite entanglement measure
introduced by T. R. Oliveira, G. Rigolin, and M. C. de Oliveira, Phys. Rev. A
73, 010305(R) (2006), through the generalization of global entanglement (GE)
[D. A. Meyer and N. R. Wallach, J. Math. Phys. 43, 4273 (2002)]. Contrarily to
GE the main feature of this measure lies in the fact that we study the mean
linear entropy of all possible partitions of a multipartite system. This allows
the construction of an operational multipartite entanglement measure which is
able to distinguish among different multipartite entangled states that GE
failed to discriminate. Furthermore, it is also maximum at the critical point
of the Ising chain in a transverse magnetic field, being thus able to detect a
quantum phase transition.


title:
Improving Raman velocimetry of laser-cooled cesium atoms by
  spin-polarization

abstract:
  We study the peformances of Raman velocimetry applied to laser-cooled,
spin-polarized, cesium atoms. Atoms are optically pumped into the F=4, m=0
ground-state Zeeman sublevel, which is insensitive to magnetic perturbations.
High resolution Raman stimulated spectroscopy is shown to produce
Fourier-limited lines, allowing, in realistic experimental conditions, atomic
velocity selection to one-fiftieth of a recoil velocity.

categories: quant-ph cond-mat.other physics.atom-ph physics.optics


title: Scalable multi-particle entanglement of trapped ions

abstract:
  Among the various kinds of entangled states, the 'W state' plays an important
role as its entanglement is maximally persistent and robust even under particle
loss. Such states are central as a resource in quantum information processing
and multiparty quantum communication. Here we report the scalable and
deterministic generation of four-, five-, six-, seven- and eight-particle
entangled states of the W type with trapped ions. We obtain the maximum
possible information on these states by performing full characterization via
state tomography, using individual control and detection of the ions. A
detailed analysis proves that the entanglement is genuine. The availability of
such multiparticle entangled states, together with full information in the form
of their density matrices, creates a test-bed for theoretical studies of
multiparticle entanglement. Independently, -Greenberger-Horne-Zeilinger-
entangled states with up to six ions have been created and analysed in Boulder.


title: A Stern-Gerlach experiment for slow light

abstract:
  Electromagnetically induced transparency allows for light transmission
through dense atomic media by means of quantum interference. Media exhibiting
electromagnetically induced transparency have very interesting properties, such
as extremely slow group velocities. Associated with the slow light propagation
are quasiparticles, the so-called dark polaritons, which are mixtures of a
photonic and an atomic contribution. We here demonstrate that these excitations
behave as particles with a nonzero magnetic moment, which is in clear contrast
to the properties of a free photon. It is found that light passing through a
rubidium gas cell under the conditions of electromagnetically induced
transparency is deflected by a small magnetic field gradient. The deflection
angle is proportional to the optical propagation time through the cell. The
observed beam deflection can be understood by assuming that dark state
polaritons have an effective magnetic moment. Our experiment can be described
in terms of a Stern-Gerlach experiment for the polaritons.


title: A Deterministic and Nondestructively-Verifiable Photon Number Source

abstract:
  We present a deterministic approach based on continuous measurement and
real-time quantum feedback control to prepare arbitrary photon number states of
a cavity mode. The procedure passively monitors the number state actually
achieved in each feedback stabilized measurement trajectory, thus providing a
nondestructively verifiable photon source. The feasibility of a possible cavity
QED implementation in the many-atom good-cavity coupling regime is analyzed.


title:
Plasmon assisted transmission of high dimensional orbital angular
  momentum entangled state

abstract:
  We present an experimental evidence that high dimensional orbital angular
momentum entanglement of a pair of photons can be survived after a
photon-plasmon-photon conversion. The information of spatial modes can be
coherently transmitted by surface plasmons. This experiment primarily studies
the high dimensional entangled systems based on surface plasmon with
subwavelength structures. It maybe useful in the investigation of spatial mode
properties of surface plasmon assisted transmission through subwavelength hole
arrays.


title: Strong coupling of single emitters to surface plasmons

abstract:
  We propose a method that enables strong, coherent coupling between individual
optical emitters and electromagnetic excitations in conducting nano-structures.
The excitations are optical plasmons that can be localized to sub-wavelength
dimensions. Under realistic conditions, the tight confinement causes optical
emission to be almost entirely directed into the propagating plasmon modes via
a mechanism analogous to cavity quantum electrodynamics. We first illustrate
this result for the case of a nanowire, before considering the optimized
geometry of a nanotip. We describe an application of this technique involving
efficient single-photon generation on demand, in which the plasmons are
efficiently out-coupled to a dielectric waveguide. Finally we analyze the
effects of increased scattering due to surface roughness on these
nano-structures.


title:
Scalable solid-state quantum computation in decoherence-free subspaces
  with trapped ions

abstract:
  We propose a decoherence-free subspaces (DFS) scheme to realize scalable
quantum computation with trapped ions. The spin-dependent Coulomb interaction
is exploited, and the universal set of unconventional geometric quantum gates
is achieved in encoded subspaces that are immune from decoherence by collective
dephasing. The scalability of the scheme for the ion array system is
demonstrated, either by an adiabatic way of switching on and off the
interactions, or by a fast gate scheme with comprehensive DFS encoding and
noise decoupling techniques.


title:
Transition behavior in the capacity of correlated-noisy channels in
  arbitrary dimensions

abstract:
  We construct a class of quantum channels in arbitrary dimensions for which
entanglement improves the performance of the channel. The channels have
correlated noise and when the level of correlation passes a critical value we
see a sharp transition in the optimal input states (states which minimize the
output entropy) from separable to maximally entangled states. We show that for
a subclass of channels with some extra conditions, including the examples which
we consider, the states which minimize the output entropy are the ones which
maximize the mutual information.


title: Quantum computing with superconductors I: Architectures

abstract:
  Josephson junctions have demonstrated enormous potential as qubits for
scalable quantum computing architectures. Here we discuss the current
approaches for making multi-qubit circuits and performing quantum information
processing with them.


title: Spin entanglement loss by local correlation transfer to the momentum

abstract:
  We show the decrease of spin-spin entanglement between two s=1/2 fermions or
two photons due to local transfer of correlations from the spin to the momentum
degree of freedom of one of the two particles. We explicitly show how this
phenomenon operates in the case where one of the two fermions (photons) passes
through a local homogeneous magnetic field (optically-active medium), losing
its spin correlations with the other particle.


title: One-way Quantum Computation - a tutorial introduction

abstract:
  In this book chapter, we provide a tutorial introduction to one-way quantum
computation and many of the techniques one can use to understand it. The
techniques which are described include the stabilizer formalism and the logical
Heisenberg picture. We highlight ways in which it is useful to understand
one-way computation beyond simple equivalence with the quantum circuit model.
We briefly review current proposals of implementations and experimental
progress and summarize some recent related theoretical developments.
  Although the chapter is primarily didactic in focus, we include a number of
new methods and observations. These include: a simpler and more compact
formulation of one-way quantum computation in the stabilizer formalism; A new
way of implementing unitaries diagonal in the computational basis; New results
on the family of operations which may be implemented in a single round of
measurements; A method for constructing compact one-way patterns by decomposing
unitaries in terms of diagonal unitaries and Clifford group transformations.


title:
Creation of entanglement in a scalable spin quantum computer with
  long-range dipole-dipole interaction between qubits

abstract:
  Creation of entanglement is considered theoretically and numerically in an
ensemble of spin chains with dipole-dipole interaction between the spins. The
unwanted effect of the long-range dipole interaction is compensated by the
optimal choice of the parameters of radio-frequency pulses implementing the
protocol. The errors caused by (i) the influence of the environment,(ii)
non-selective excitations, (iii) influence of different spin chains on each
other, (iv) displacements of qubits from their perfect locations, and (v)
fluctuations of the external magnetic field are estimated analytically and
calculated numerically. For the perfectly entangled state the z component, M,
of the magnetization of the whole system is equal to zero. The errors lead to a
finite value of M. If the number of qubits in the system is large, M can be
detected experimentally. Using the fact that M depends differently on the
parameters of the system for each kind of error, varying these parameters would
allow one to experimentally determine the most significant source of errors and
to optimize correspondingly the quantum computer design in order to decrease
the errors and M. Using our approach one can benchmark the quantum computer,
decrease the errors, and prepare the quantum computer for implementation of
more complex quantum algorithms.


title: Decoherence effects on the quantum spin channels

abstract:
  An open ended spin chain can serves as a quantum data bus for the coherent
transfer of quantum state information. In this paper, we investigate the
efficiency of such quantum spin channels which work in a decoherence
environment. Our results show that, the decoherence will significantly reduce
the fidelity of quantum communication through the spin channels. Generally
speaking, as the distance increases, the decoherence effects become more
serious, which will put some constraints on the spin chains for long distance
quantum state transfer.


title: Spin Decoherence in Superconducting Atom Chips

abstract:
  Using a consistent quantum-mechanical treatment for the electromagnetic
radiation, we theoretically investigate the magnetic spin-flip scatterings of a
neutral two-level atom trapped in the vicinity of a superconducting body. We
derive a simple scaling law for the corresponding spin-flip lifetime for such
an atom trapped near a superconducting thick slab. For temperatures below the
superconducting transition temperature T_c, the lifetime is found to be
enhanced by several orders of magnitude in comparison to the case of a normal
conducting slab. At zero temperature the spin-flip lifetime is given by the
unbounded free-space value.


title: Post-Quantum Key Exchange Protocols

abstract:
  If an eavesdropper Eve is equipped with quantum computers, she can easily
break the public key exchange protocols used today. In this paper we will
discuss the post-quantum Diffie-Hellman key exchange and private key exchange
protocols.


title: Deutsch and Jozsa's Algorithm Revisited

abstract:
  A classical analogue of Deutsch and Jozsa's algorithm is given and its
implications on quantum computing is discussed


title:
The Quantum Mechanics Problem of the Schroedinger Equation with the
  Trigonometric Rosen-Morse Potential

abstract:
  We present the quantum mechanics problem of the one-dimensional Schroedinger
equation with the trigonometric Rosen-Morse potential. This potential is of
possible interest to quark physics in so far as it captures the essentials of
the QCD quark-gluon dynamics and (i) interpolates between a Coulomb-like
potential (associated with one-gluon exchange) and the infinite wall potential
(associated with asymptotic freedom), (ii) reproduces in the intermediary
region the linear confinement potential (associated with multi-gluon
self-interactions) as established by lattice QCD calculations of hadron
properties. Moreover, its exact real solutions given here display a new class
of real orthogonal polynomials and thereby interesting mathematical entities in
their own.


title: Wave Mechanics of Two Hard Core Quantum Particles in 1-D Box

abstract:
  The wave mechanics of two impenetrable hard core particles in 1-D box is
analyzed. Each particle in the box behaves like an independent entity
represented by a {\it macro-orbital} (a kind of pair waveform). While the
expectation value of their interaction, $<V_{HC}(x)>$, vanishes for every state
of two particles, the expectation value of their relative separation, $<x>$,
satisfies $<x> \ge \lambda/2$ (or $q \ge \pi/d$, with $2d = L$ being the size
of the box). The particles in their ground state define a close-packed
arrangement of their wave packets (with $<x> = \lambda/2$, phase position
separation $\Delta\phi = 2\pi$ and momentum $|q_o| = \pi/d$) and experience a
mutual repulsive force ({\it zero point repulsion}) $f_o = h^2/2md^3$ which
also tries to expand the box. While the relative dynamics of two particles in
their excited states represents usual collisional motion, the same in their
ground state becomes collisionless. These results have great significance in
determining the correct microscopic understanding of widely different many body
systems.


title: A fundamental threat to quantum cryptography: gravitational attacks

abstract:
  An attack on the ``Bennett-Brassard 84''(BB84) quantum key-exchange protocol
in which Eve exploits the action of gravitation to infer information about the
quantum-mechanical state of the qubit exchanged between Alice and Bob, is
described. It is demonstrated that the known laws of physics do not allow to
describe the attack. Without making assumptions that are not based on broad
consensus, the laws of quantum gravity, unknown up to now, would be needed even
for an approximate treatment. Therefore, it is currently not possible to
predict with any confidence if information gained in this attack will allow to
break BB84. Contrary to previous belief, a proof of the perfect security of
BB84 cannot be based on the assumption that the known laws of physics are
strictly correct, yet.


title: The Yukawa Lagrangian Density is Inconsistent with the Hamiltonian

abstract:
  It is proved that no Hamiltonian exists for the real Klein-Gordon field used
in the Yukawa interaction. The experimental side supports this conclusion.


title: Quantum secure communication protocols based on entanglement swapping

abstract:
  We present a quantum secure direct communication protocol and a multiparty
quantum secret sharing protocol based on Einstein-Podolsky-Rosen pairs and
entanglement swapping. The present quantum secure direct communication protocol
makes use of the ideal of block transmission. We also point out that the sender
can encode his or her secret message without ensuring the security of the
quantum channel firstly. In the multiparty quantum secret sharing protocol, the
communication parties adopt checking mode or encoding mode with a certain
probability. It is not necessary for the protocol to perform local unitary
operation. In both the protocols, one party transmits only one photon for each
Einstein-Podolsky-Rosen pair to another party and the security for the
transmitting photons is ensured by selecting Z-basis or X-basis randomly to
measure the sampling photons.


title: State estimation from pair of conjugate qudits

abstract:
  We show that, for $N$ parallel input states, an anti-linear map with respect
to a specific basis is essentially a classical operator. We also consider the
information contained in phase-conjugate pairs $|\phi > |\phi^*>$, and prove
that there is more information about a quantum state encoded in phase-conjugate
pairs than in parallel pairs.


title:
Non-Hermitian quantum mechanics: the case of bound state scattering
  theory

abstract:
  Excited bound states are often understood within scattering based theories as
resulting from the collision of a particle on a target via a short-range
potential. We show that the resulting formalism is non-Hermitian and describe
the Hilbert spaces and metric operator relevant to a correct formulation of
such theories. The structure and tools employed are the same that have been
introduced in current works dealing with PT-symmetric and quasi-Hermitian
problems. The relevance of the non-Hermitian formulation to practical
computations is assessed by introducing a non-Hermiticity index. We give a
numerical example involving scattering by a short-range potential in a Coulomb
field for which it is seen that even for a small but non-negligible
non-Hermiticity index the non-Hermitian character of the problem must be taken
into account. The computation of physical quantities in the relevant Hilbert
spaces is also discussed.


title:
Comment on ``Inseparability Criteria for Continuous Bipartite Quantum
  States''

abstract:
  A Comment on the Letter by E. Shchukin and W. Vogel, Phys. Rev. Lett. 95,
230502 (2005).


title: Anomalous diffusion in quantum Brownian motion with colored noise

abstract:
  Anomalous diffusion is discussed in the context of quantum Brownian motion
with colored noise. It is shown that earlier results follow simply and directly
from the fluctuation-dissipation theorem. The limits on the long-time
dependence of anomalous diffusion are shown to be a consequence of the second
law of thermodynamics. The special case of an electron interacting with the
radiation field is discussed in detail. We apply our results to wave-packet
spreading.


title: On the Thermodynamic Limit of the Lipkin Model

abstract:
  The thermodynamic limit of the Lipkin model is investigated. While the limit
turns out to be rather elusive, the analysis gives strong indications that the
limit yields two analytically dissociated operators, one for the normal and one
for the deformed phase. While the Lipkin Hamiltonian is hermitian and has a
second order phase transition in finite dimensions (finite particle number),
both properties seem to be destroyed in the thermodynamic limit.


title: Quantum channel for light based on integrals of motion

abstract:
  A method based on integrals of motion for collective processes has been
introduced to achieve physical schemes in which one of the systems is
insensitive to interaction. Decoherence-free quantum channels that allow
sending any state of light, particulary the Fock states, through an absorbing
medium are considered as an example.


title: Inductive classification of multipartite entanglement under SLOCC

abstract:
  We propose an inductive procedure to classify N-partite entanglement under
stochastic local operations and classical communication (SLOCC) provided such a
classification is known for N-1 qubits. The method is based upon the analysis
of the coefficient matrix of the state in an arbitrary product basis. We
illustrate this approach in detail with the well-known bi- and tripartite
systems, obtaining as a by-product a systematic criterion to establish the
entanglement class of a given pure state without resourcing to any entanglement
measure. The general case is proved by induction, allowing us to find an upper
bound for the number of N-partite entanglement classes in terms of the number
of entanglement classes for N-1 qubits.


title: Probabilistic coding of quantum states

abstract:
  We discuss properties of probabilistic coding of two qubits to one qutrit and
generalize the scheme to higher dimensions. We show that the protocol
preservers entanglement between qubits to be encoded and environment and can be
also applied to mixed states. We present the protocol which enables encoding of
n qudits to one qudit of dimension smaller than the Hilbert space of the
original system and then probabilistically but error-free decode any subset of
k qudits. We give a formula for the probability of successful decoding.


title:
On the statistics of quantum expectations for systems in thermal
  equilibrium

abstract:
  The recent remarkable developments in quantum optics, mesoscopic and cold
atom physics have given reality to wave functions. It is then interesting to
explore the consequences of assuming ensembles over the wave functions simply
related to the canonical density matrix. In this note we analyze a previously
introduced distribution over wave functions which naturally arises considering
the Schroedinger equation as an infinite dimensional dynamical system. In
particular, we discuss the low temperature fluctuations of the quantum
expectations of coordinates and momenta for a particle in a double well
potential. Our results may be of interest in the study of chiral molecules.

categories: quant-ph cond-mat.stat-mech hep-th math-ph math.MP physics.chem-ph


title: On Dynamical Quantization

abstract:
  In this article we review some results obtained from a generalization of
quantum mechanics obtained from modification of the canonical commutation
relation $[q,p]={\rm i}\hbar$. We present some new results concerning
relativistic generalizations of previous works, and we calculate the energy
spectrum of some simple quantum systems, using the position and momentum
operators of this new formalism.


title:
Dispersion spreading of biphotons in optical fibres and two-photon
  interference

abstract:
  We present the first observation of two-photon polarization interference
structure in the second-order Glauber's correlation function of two-photon
light generated via type-II spontaneous parametric down-conversion. In order to
obtain this result, two-photon light is transmitted through an optical fibre
and the coincidence distribution is analyzed by means of the START-STOP method.
Beyond the experimental demonstration of an interesting effect in quantum
optics, these results also have considerable relevance for quantum
communications.


title: Benchmarking quantum control methods on a 12-qubit system

abstract:
  In this letter, we present an experimental benchmark of operational control
methods in quantum information processors extended up to 12 qubits. We
implement universal control of this large Hilbert space using two complementary
approaches and discuss their accuracy and scalability. Despite decoherence, we
were able to reach a 12-coherence state (or 12-qubits pseudo-pure cat state),
and decode it into an 11 qubit plus one qutrit labeled observable pseudo-pure
state using liquid state nuclear magnetic resonance quantum information
processors.


title:
Generation of long-living entanglement using cold trapped ions with pair
  cat states

abstract:
  With the reliance in the processing of quantum information on a cold trapped
ion, we analyze the entanglement entropy in the ion-field interaction with pair
cat states. We investigate a long-living entanglement allowing the
instantaneous position of the center-of-mass motion of the ion to be explicitly
time dependent. An analytic solution for the system operators is obtained. We
show that different nonclassical effects arise in the dynamics of the
population inversion, depending on the initial states of the vibrational
motion. We study in detail the entanglement degree and demonstrate how the
input pair cat state is required for initiating the long living entanglement.
This long living entanglement is damp out with an increase in the number
difference $q$. Owing to the properties of entanglement measures, the results
are checked using another entanglement measure (high order linear entropy).


title: Ground state cooling in a bad cavity

abstract:
  We study the mechanical effects of light on an atom trapped in a harmonic
potential when an atomic dipole transition is driven by a laser and it is
strongly coupled to a mode of an optical resonator. We investigate the cooling
dynamics in the bad cavity limit, focussing on the case in which the effective
transition linewidth is smaller than the trap frequency, hence when sideband
cooling could be implemented. We show that quantum correlations between the
mechanical actions of laser and cavity field can lead to an enhancement of the
cooling efficiency with respect to sideband cooling. Such interference effects
are found when the resonator losses prevail over spontaneous decay and over the
rates of the coherent processes characterizing the dynamics.


title: Quantum Algorithms for Simon's Problem Over General Groups

abstract:
  Daniel Simon's 1994 discovery of an efficient quantum algorithm for solving
the hidden subgroup problem (HSP) over Z_2^n provided one of the first
algebraic problems for which quantum computers are exponentially faster than
their classical counterparts. In this paper, we study the generalization of
Simon's problem to arbitrary groups. Fixing a finite group G, this is the
problem of recovering an involution m = (m_1, ..., m_n) in G^n from an oracle f
with the property that f(x) = f(xy) iff y equals m or the identity. In the
current parlance, this is the hidden subgroup problem (HSP) over groups of the
form G^n, where G is a nonabelian group of constant size, and where the hidden
subgroup is either trivial or has order two. Although groups of the form G^n
have a simple product structure, they share important representation-theoretic
properties with the symmetric groups S_n, where a solution to the HSP would
yield a quantum algorithm for Graph Isomorphism. In particular, solving their
HSP with the so-called ``standard method'' requires highly entangled
measurements on the tensor product of many coset states. Here we give quantum
algorithms with time complexity 2^O(sqrt(n log n)) that recover hidden
involutions m = (m_1, ..., m_n) in G^n where, as in Simon's problem, each m_i
is either the identity or the conjugate of a known element k, and there is a
character X of G for which X(k) = -X(1)$. Our approach combines the general
idea behind Kuperberg's sieve for dihedral groups with the ``missing harmonic''
approach of Moore and Russell. These are the first nontrivial hidden subgroup
algorithms for group families that require highly entangled multiregister
Fourier sampling.


title: On Protected Realizations of Quantum Information

abstract:
  There are two complementary approaches to realizing quantum information so
that it is protected from a given set of error operators. Both involve encoding
information by means of subsystems. One is initialization-based error
protection, which involves a quantum operation that is applied before error
events occur. The other is operator quantum error correction, which uses a
recovery operation applied after the errors. Together, the two approaches make
it clear how quantum information can be stored at all stages of a process
involving alternating error and quantum operations. In particular, there is
always a subsystem that faithfully represents the desired quantum information.
We give a definition of faithful realization of quantum information and show
that it always involves subsystems. This justifies the "subsystems principle"
for realizing quantum information. In the presence of errors, one can make use
of noiseless, (initialization) protectable, or error-correcting subsystems. We
give an explicit algorithm for finding optimal noiseless subsystems. Finding
optimal protectable or error-correcting subsystems is in general difficult.
Verifying that a subsystem is error-correcting involves only linear algebra. We
discuss the verification problem for protectable subsystems and reduce it to a
simpler version of the problem of finding error-detecting codes.


title: Factorizing the time evolution operator

abstract:
  There is a widespread belief in the quantum physical community, and in
textbooks used to teach Quantum Mechanics, that it is a difficult task to apply
the time evolution operator Exp{-itH/h} on an initial wave function. That is to
say, because the hamiltonian operator generally is the sum of two operators,
then it is a difficult task to apply the time evolution operator on an initial
wave function f(x,0), for it implies to apply terms operators like (a+b)^n. A
possible solution of this problem is to factorize the time evolution operator
and then apply successively the individual exponential operator on the initial
wave function. However, the exponential operator does not directly factorize,
i. e. Exp{a+b} is not equal to Exp{a}Exp{b}. In this work we present a useful
procedure for factorizing the time evolution operator when the argument of the
exponential is a sum of two operators, which obey specific commutation
relations. Then, we apply the exponential operator as an evolution operator for
the case of elementary unidimensional potentials, like the particle subject to
a constant force and the harmonic oscillator. Also, we argue about an apparent
paradox concerning the time evolution operator and non-spreading wave packets
addressed previously in the literature.


title:
Hypothesis testing for an entangled state produced by spontaneous
  parametric down conversion

abstract:
  Generation and characterization of entanglement are crucial tasks in quantum
information processing. A hypothesis testing scheme for entanglement has been
formulated. Three designs were proposed to test the entangled photon states
created by the spontaneous parametric down conversion. The time allocations
between the measurement vectors were designed to consider the anisotropic
deviation of the generated photon states from the maximally entangled states.
The designs were evaluated in terms of the p-value based on the observed data.
It has been experimentally demonstrated that the optimal time allocation
between the coincidence and anti-coincidence measurement vectors improves the
entanglement test. A further improvement is also experimentally demonstrated by
optimizing the time allocation between the anti-coincidence vectors. Analysis
on the data obtained in the experiment verified the advantage of the
entanglement test designed by the optimal time allocation.


title: Generation of NPT Entanglement from Nonclassical Photon Statistics

abstract:
  With a product state of the form $\rho_{in} = \rho_a\otimes|0>_b_b< 0|$ as
input, the output two-mode state $\rho_{{\rm out}}$, of the beam splitter is
shown to be NPT whenever the photon number distribution (PND) statistics
$\{p(n_a) \}$ associated with the possibly mixed state $\rho_a$ of the a_mode
is antibunched or otherwise nonclassical, i.e., if $\{p(n_a)\}$ fails to
respect any one of an infinite of classicality conditions.


title: Quantum Open System Theory: Bipartite Aspects

abstract:
  We demonstrate in straightforward calculations that even under ideally weak
noise the relaxation of bipartite open quantum systems contains elements not
previously encountered in quantum noise physics. While additivity of decay
rates is known to be generic for decoherence of a single system, we demonstrate
that it breaks down for bipartite coherence of even the simplest composite
systems.


title: Degradability of Bosonic Gaussian channels

abstract:
  The notion of weak-degradability of quantum channels is introduced by
generalizing the degradability definition given by Devetak and Shor. Exploiting
the unitary equivalence with beam-splitter/amplifier channels we then prove
that a large class of one-mode Bosonic Gaussian channels are either weakly
degradable or anti-degradable. In the latter case this implies that their
quantum capacity Q is null. In the former case instead, this allows us to
establish the additivity of the coherent information for those maps which admit
unitary representation with single-mode pure environment.


title:
Ultrafast Coherent Coupling of Atomic Hyperfine and Photon Frequency
  Qubits

abstract:
  We demonstrate ultrafast coherent coupling between an atomic qubit stored in
a single trapped cadmium ion and a photonic qubit represented by two resolved
frequencies of a photon. Such ultrafast coupling is crucial for entangling
networks of remotely-located trapped ions through photon interference, and is
also a key component for realizing ultrafast quantum gates between
Coulomb-coupled ions.


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

abstract:
  In Phys. Rev. A 70, 032104 (2004), M. Montesinos and G. F. Torres del
Castillo consider various symplectic structures on the classical phase space of
the two-dimensional isotropic harmonic oscillator. Using Dirac's quantization
condition, the authors investigate how these alternative symplectic forms
affect this system's quantization. They claim that these symplectic structures
result in mutually inequivalent quantum theories. In fact, we show here that
there exists a unitary map between the two representation spaces so that the
various quantizations are equivalent.


title:
Can one build a quantum hard drive? A no-go theorem for storing quantum
  information in equilibrium systems

abstract:
  We prove a no-go theorem for storing quantum information in equilibrium
systems. Namely, quantum information cannot be stored in a system with
time-independent Hamiltonian interacting with heat bath of temperature $T>0$
during time that grows with the number of used qubits. We prove it by showing,
that storing quantum information for macroscopic time would imply existence of
perpetuum mobile of the second kind. The general results are illustrated by the
Kitaev model of quantum memory. In contrast, classical information can be
stored in equilibrium states for arbitrary long times. We show how it is
possible via phase-transition type phenomena. Our result shows that there is a
fundamental difference between quantum and classical information in {\it
physical} terms.


title: Entangling power of baker's map: Role of symmetries

abstract:
  The quantum baker map possesses two symmetries: a canonical "spatial"
symmetry, and a time-reversal symmetry. We show that, even when these features
are taken into account, the asymptotic entangling power of the baker's map does
not always agree with the predictions of random matrix theory. We have verified
that the dimension of the Hilbert space is the crucial parameter which
determines whether the entangling properties of the baker are universal or not.
For power-of-two dimensions, i.e., qubit systems, an anomalous entangling power
is observed; otherwise the behavior of the baker is consistent with random
matrix theories. We also derive a general formula that relates the asymptotic
entangling power of an arbitrary unitary with properties of its reduced
eigenvectors.


title: Noisy Preprocessing and the Distillation of Private States

abstract:
  We provide a simple security proof for prepare & measure quantum key
distribution protocols employing noisy processing and one-way postprocessing of
the key. This is achieved by showing that the security of such a protocol is
equivalent to that of an associated key distribution protocol in which, instead
of the usual maximally-entangled states, a more general {\em private state} is
distilled. Besides a more general target state, the usual entanglement
distillation tools are employed (in particular, Calderbank-Shor-Steane
(CSS)-like codes), with the crucial difference that noisy processing allows
some phase errors to be left uncorrected without compromising the privacy of
the key.


title: Quantum Noise Randomized Ciphers

abstract:
  We review the notion of a classical random cipher and its advantages. We
sharpen the usual description of random ciphers to a particular mathematical
characterization suggested by the salient feature responsible for their
increased security. We describe a concrete system known as AlphaEta and show
that it is equivalent to a random cipher in which the required randomization is
effected by coherent-state quantum noise. We describe the currently known
security features of AlphaEta and similar systems, including lower bounds on
the unicity distances against ciphertext-only and known-plaintext attacks. We
show how AlphaEta used in conjunction with any standard stream cipher such as
AES (Advanced Encryption Standard) provides an additional, qualitatively
different layer of security from physical encryption against known-plaintext
attacks on the key. We refute some claims in the literature that AlphaEta is
equivalent to a non-random stream cipher.


title: Direct Use of Secret Key in Quantum Cryptography

abstract:
  For single-photon quantum key generation between two users, it is shown that
the use of a shared secret key extended via a pseudo-random number generator
may simultaneously enhance the security and efficiency of the cryptosystem.
This effect arises from the intrinsic performance difference between quantum
detectors with versus without knowledge of the key, a purely quantum effect and
a new principle for key generation. No intrusion level estimation is needed and
the method is directly applicable to realistic systems with imperfections.
Quantum direct encryption is also made possible by such use of a secret key.


title:
Entanglement between remote continuous variable quantum systems: effects
  of transmission loss

abstract:
  We study the effects of losses on the entanglement created between two
separate atomic gases by optical probing and homodyne detection of the
transmitted light. The system is well-described in the Gaussian state
formulation. Analytical results quantifying the degree of entanglement between
the two gases are derived and compared with the entanglement in a pair of light
pulses generated by an EPR source. For low (high) transmission losses the
highest degree of entanglement is obtained by probing with squeezed
(antisqueezed) light. In an asymmetric setup where light is only sent one way
through the atomic samples, we find that the logarithmic negativity of
entanglement attains a constant value $-\log_2(N)$ with $N=1/3$ irrespectively
of the loss along the transmission line.


title: Phase map decompositions for unitaries

abstract:
  We propose a universal decomposition of unitary maps over a tensorial power
of C^2, introducing the key concept of "phase maps", and investigate how this
decomposition can be used to implement unitary maps directly in the
measurement-based model for quantum computing. Specifically, we show how to
extract from such a decomposition a matching entangled graph state (with
inputs), and a set of measurements angles, when there is one. Next, we check
whether the obtained graph state verifies a "flow" condition, which guarantees
an execution order such that the dependent measurements and corrections of the
pattern yield deterministic results. Using a graph theoretic characterization
of flows, we can determine whether a flow can be constructed for a graph state
in polynomial time. This approach yields an algorithmic procedure which, when
it succeeds, may produce an efficient pattern for a given unitary.


title: Scaling behavior in the adiabatic Dicke Model

abstract:
  We analyze the quantum phase transition for a set of $N$-two level systems
interacting with a bosonic mode in the adiabatic regime. Through the
Born-Oppenheimer approximation, we obtain the finite-size scaling expansion for
many physical observables and, in particular, for the entanglement content of
the system.