title:
Engineering phonon-photon interactions with a driven trapped ion in a
  cavity

abstract:
  We show how to generate quadratic and bi-quadratic phonon-photon interactions
through a driven three-level ion inside a cavity. With such a system it is
possible to squeeze the cavity-field state, the ion motional state or even the
entangled phonon-photon state. We present a detailed analysis of the
cavity-field squeezing process, distinguishing three different regimes of this
amplification mechanism: the subcritical, critical, and supercritical regimes,
which depend, apart from the coupling parameters, on the excitation of the
vibrational state. As an application of the engineered Hamiltonians, we show
how to implement a Fock-state filter for the vibrational mode. New aspects of
the technique of adiabatic elimination emerge in this analysis.


title: Fidelity of an optical memory based on stimulated photon echoes

abstract:
  We investigated the preservation of information encoded into the relative
phase and amplitudes of optical pulses during storage and retrieval in an
optical memory based on stimulated photon echo. By interfering photon echoes
produced in a Ti-indiffused single-mode Er-doped LiNbO$_{3}$ waveguiding
structure at telecom wavelength, we found that decoherence in the atomic medium
translates only as losses (and not as degradation) of information, as long as
the data pulse series is short compared to the atomic decoherence time. The
experimentally measured value of the visibility for interfering echoes is close
to 100 %. In addition to the expected three-pulse photon-echo interferences we
also observed interference due to a four-pulse photon echo. Our findings are of
particular interest for future long-distance quantum communication protocols,
which rely on the reversible transfer of quantum states between light and atoms
with high fidelity.


title:
Loschmidt echo in the Bose-Hubbard model: turning back time in an
  optical lattice

abstract:
  I show how to perform a Loschmidt echo (time reversal) in the Bose-Hubbard
model implemented with cold bosonic atoms in an optical lattice. The echo is
obtained by applying a linear phase imprint on the lattice and a change in
magnetic field to tune the boson-boson scattering length through a Feshbach
resonance. I discuss how the echo can measure the fidelity of the quantum
simulation, the intensity of an external potential (e.g. gravity), or the
critical point of the superfluid-insulator quantum phase transition.


title: Keeping a Quantum Bit Alive by Optimized $\pi$-Pulse Sequences

abstract:
  A general strategy to maintain the coherence of a quantum bit is proposed.
The analytical result is derived rigorously including all memory and
back-action effects. It is based on an optimized $\pi$-pulse sequence for
dynamic decoupling extending the Carr-Purcell-Meiboom-Gill (CPMG) cycle. The
optimized sequence is very efficient, in particular for strong couplings to the
environment.

categories: quant-ph cond-mat.mes-hall cond-mat.stat-mech cond-mat.str-el


title: Quantum speedup of classical mixing processes

abstract:
  Most approximation algorithms for #P-complete problems (e.g., evaluating the
permanent of a matrix or the volume of a polytope) work by reduction to the
problem of approximate sampling from a distribution $\pi$ over a large set
$\S$. This problem is solved using the {\em Markov chain Monte Carlo} method: a
sparse, reversible Markov chain $P$ on $\S$ with stationary distribution $\pi$
is run to near equilibrium. The running time of this random walk algorithm, the
so-called {\em mixing time} of $P$, is $O(\delta^{-1} \log 1/\pi_*)$ as shown
by Aldous, where $\delta$ is the spectral gap of $P$ and $\pi_*$ is the minimum
value of $\pi$. A natural question is whether a speedup of this classical
method to $O(\sqrt{\delta^{-1}} \log 1/\pi_*)$, the diameter of the graph
underlying $P$, is possible using {\em quantum walks}.
  We provide evidence for this possibility using quantum walks that {\em
decohere} under repeated randomized measurements. We show: (a) decoherent
quantum walks always mix, just like their classical counterparts, (b) the
mixing time is a robust quantity, essentially invariant under any smooth form
of decoherence, and (c) the mixing time of the decoherent quantum walk on a
periodic lattice $\Z_n^d$ is $O(n d \log d)$, which is indeed
$O(\sqrt{\delta^{-1}} \log 1/\pi_*)$ and is asymptotically no worse than the
diameter of $\Z_n^d$ (the obvious lower bound) up to at most a logarithmic
factor.


title: Group Theoretical Formulation of Quantum Partial Search Algorithm

abstract:
  Searching and sorting used as a subroutine in many important algorithms.
Quantum algorithm can find a target item in a database faster than any
classical algorithm. One can trade accuracy for speed and find a part of the
database (a block) containing the target item even faster, this is partial
search. An example is the following: exact address of the target item is given
by a sequence of many bits, but we need to know only some of them. More
generally partial search considers the following problem: a database is
separated into several blocks. We want to find a block with the target item,
not the target item itself. In this paper we reformulate quantum partial search
algorithm in terms of group theory.


title:
Light propagation through closed-loop atomic media beyond the
  multiphoton resonance condition

abstract:
  The light propagation of a probe field pulse in a four-level double-lambda
type system driven by laser fields that form a closed interaction loop is
studied. Due to the finite frequency width of the probe pulse, a
time-independent analysis relying on the multiphoton resonance assumption is
insufficient. Thus we apply a Floquet decomposition of the equations of motion
to solve the time-dependent problem beyond the multiphoton resonance condition.
We find that the various Floquet components can be interpreted in terms of
different scattering processes, and that the medium response oscillating in
phase with the probe field in general is not phase-dependent. The phase
dependence arises from a scattering of the coupling fields into the probe field
mode at a frequency which in general differs from the probe field frequency. We
thus conclude that in particular for short pulses with a large frequency width,
inducing a closed loop interaction contour may not be advantageous, since
otherwise the phase-dependent medium response may lead to a distortion of the
pulse shape. Finally, using our time-dependent analysis, we demonstrate that
both the closed-loop and the non-closed loop configuration allow for sub- and
superluminal light propagation with small absorption or even gain. Further, we
identify one of the coupling field Rabi frequencies as a control parameter that
allows to conveniently switch between sub- and superluminal light propagation.


title:
One-party Quantum Error Correcting Codes for Unbalanced Errors:
  Principles and Application to Quantum Dense Coding and Quantum Secure Direct
  Communications

abstract:
  In this article, we present the unbalanced quantum error correcting
codes(one-party-QECC), a novel idea for correcting unbalanced quantum errors.
In some quantum communication tasks using entangled pairs, the error
distributions between two parts of the pairs are unbalanced, and. one party
holds the whole entangled pairs at the final stage, and he or she is able to
perform joint measurements on the pairs. In this situation the proposed
one-party-QECC can improve error correction by allowing a higher tolerated
error rate. We have established the general correspondence between linear
classical codes and the one-party-QECC, and we have given the general
definition for this type quantum error correcting codes.It has been shown that
the one-party-QECC can correct errors as long as the error threshold is not
larger than 0.5. The one-party-QECC works even for fidelity less than 0.5 as
long as it is larger than 0.25. We give several concrete examples of the
one-party-QECC. We provide the applications of one-party-QECC in quantum dense
coding so that it can function in noisy channels. As a result, a large number
of quantum secure direct communication protocols based on dense coding is also
able to be protected by this new type of one-party-QECC.


title: Continuous transfer and laser guiding between two cold atom traps

abstract:
  We have demonstrated and modeled a simple and efficient method to transfer
atoms from a first Magneto-Optical Trap (MOT) to a second one. Two independent
setups, with cesium and rubidium atoms respectively, have shown that a high
power and slightly diverging laser beam optimizes the transfer between the two
traps when its frequency is red-detuned from the atomic transition. This
pushing laser extracts a continuous beam of slow and cold atoms out of the
first MOT and also provides a guiding to the second one through the dipolar
force. In order to optimize the transfer efficiency, the dependence of the
atomic flux on the pushing laser parameters (power, detuning, divergence and
waist) is investigated. The atomic flux is found to be proportional to the
first MOT loading rate. Experimentally, the transfer efficiency reaches 70%,
corresponding to a transfer rate up to 2.7x10^8 atoms/s with a final velocity
of 5.5 m/s. We present a simple analysis of the atomic motion inside the
pushing-guiding laser, in good agreement with the experimental data.


title: Contexts in quantum, classical and partition logic

abstract:
  Contexts are maximal collections of co-measurable observables "bundled
together" to form a "quasi-classical mini-universe." Different notions of
contexts are discussed for classical, quantum and generalized urn-automaton
systems.


title: Creating and probing macroscoping entanglement with light

abstract:
  We describe a scheme showing signatures of macroscopic optomechanical
entanglement generated by radiation pressure in a cavity system with a massive
movable mirror. The system we consider reveals genuine multipartite
entanglement. We highlight the way the entanglement involving the inaccessible
massive object is unravelled, in our scheme, by means of field-field quantum
correlations.


title: How to Introduce Time Operator

abstract:
  Time operator can be introduced by three different approaches: by pertaining
it to dynamical variables; by quantizing the classical expression of time;
taken as the restriction of energy shift generator to the Hilbert space of a
physical system.


title: Long-range entanglement in the Dirac vacuum

abstract:
  Recently, there have been a number of works investigating the entanglement
properties of distinct noncomplementary parts of discrete and continuous
Bosonic systems in ground and thermal states. The Fermionic case, however, has
yet to be expressly addressed. In this paper we investigate the entanglement
between a pair of far-apart regions of the 3+1 dimensional massless Dirac
vacuum via a previously introduced distillation protocol [B. Reznik et al.,
Phys. Rev. A 71, 042104 (2005)]. We show that entanglement persists over
arbitrary distances, and that as a function of L/R, where L is the distance
between the regions and R is their typical scale, it decays no faster than
exp(-(L/R)^2). We discuss the similarities and differences with analogous
results obtained for the massless Klein-Gordon vacuum.


title: Wigner functions, contact interactions, and matching

abstract:
  Quantum mechanics in phase space (or deformation quantization) appears to
fail as an autonomous quantum method when infinite potential walls are present.
The stationary physical Wigner functions do not satisfy the normal eigen
equations, the *-eigen equations, unless an ad hoc boundary potential is added
[Dias-Prata]. Alternatively, they satisfy a different, higher-order,
``*-eigen-* equation'', locally, i.e. away from the walls [Kryukov-Walton].
Here we show that this substitute equation can be written in a very simple
form, even in the presence of an additional, arbitrary, but regular potential.
The more general applicability of the -eigen- equation is then demonstrated.
First, using an idea from [Fairlie-Manogue], we extend it to a dynamical
equation describing time evolution. We then show that also for general contact
interactions, the -eigen- equation is satisfied locally. Specifically, we treat
the most general possible (Robin) boundary conditions at an infinite wall,
general one-dimensional point interactions, and a finite potential jump.
Finally, we examine a smooth potential, that has simple but different
expressions for x positive and negative. We find that the -eigen- equation is
again satisfied locally. It seems, therefore, that the -eigen- equation is
generally relevant to the matching of Wigner functions; it can be solved
piece-wise and its solutions then matched.


title: Long-distance teleportation of atomic qubit via optical interferometry

abstract:
  The problem of long-distance teleportation of single-atom qubits via a common
photonic channel is examined within the framework of a Mach-Zender optical
interferometer. As expected, when a coherent state is used as input, a
high-finesse optical cavity is required to overcome sensitivity to spontaneous
emission. However, we find that a number-squeezed light field in a twin-Fock
state can in principle create useful entanglement without cavity-enhancement.
Both approaches require single photon counting detectors, and best results are
obtained by combining cavity-feedback with twin-fock inputs. Such an approach
may allow a fidelity of $.99$ using a two-photon input and currently available
mirror and detector technology. In addition, the present approach can be
conveniently extended to generate multi-site entanglement and entanglement
swapping, both of which are necessities in quantum networks.


title:
Quantum interference of electromagnetic fields from remote quantum
  memories

abstract:
  We observe quantum, Hong-Ou-Mandel, interference of fields produced by two
remote atomic memories. High-visibility interference is obtained by utilizing
the finite atomic memory time in four-photon delayed coincidence measurements.
Interference of fields from remote atomic memories is a crucial element in
protocols for scalable generation of multi-node remote qubit entanglement.


title: A Quantum Approach to Classical Statistical Mechanics

abstract:
  We present a new approach to study the thermodynamic properties of
$d$-dimensional classical systems by reducing the problem to the computation of
ground state properties of a $d$-dimensional quantum model. This
classical-to-quantum mapping allows us to deal with standard optimization
methods, such as simulated and quantum annealing, on an equal basis.
Consequently, we extend the quantum annealing method to simulate classical
systems at finite temperatures. Using the adiabatic theorem of quantum
mechanics, we derive the rates to assure convergence to the optimal
thermodynamic state. For simulated and quantum annealing, we obtain the
asymptotic rates of $T(t) \approx (p N) /(k_B \log t)$ and $\gamma(t) \approx
(Nt)^{-\bar{c}/N}$, for the temperature and magnetic field, respectively. Other
annealing strategies, as well as their potential speed-up, are also discussed.

categories: quant-ph cond-mat.other cond-mat.stat-mech q-bio.OT


title: Resonant Scattering Can Enhance the Degree of Entanglement

abstract:
  Generation of entanglement between two qubits by scattering an entanglement
mediator is discussed. The mediator bounces between the two qubits and exhibits
a resonant scattering. It is clarified how the degree of the entanglement is
enhanced by the constructive interference of such bouncing processes. Maximally
entangled states are available via adjusting the incident momentum of the
mediator or the distance between the two qubits, but their fine tunings are not
necessarily required to gain highly entangled states and a robust generation of
entanglement is possible.


title: N-body-extended Channel Estimation for Low-Noise Parameters

abstract:
  The notion of low-noise channels was recently proposed and analyzed in detail
in order to describe noise-processes driven by environment [M. Hotta, T.
Karasawa and M. Ozawa, Phys. Rev. A72, 052334 (2005)]. An estimation theory of
low-noise parameters of channels has also been developed. In this report, we
address the low-noise parameter estimation problem for the $N$-body extension
of low-noise channels. We perturbatively calculate the Fisher information of
the output states in order to evaluate the lower-bound of the mean-square error
of the parameter estimation. We show that the maximum of the Fisher information
over all input states can be attained by a factorized input state in the
leading order of the low-noise parameter. Thus, to achieve optimal estimation,
it is not necessary for there to be entanglement of the $N$ subsystems, as long
as the true low-noise parameter is sufficiently small.


title: Eigenvalue problems for the complex PT-symmetric potential V(x)= igx

abstract:
  The spectrum of complex PT-symmetric potential, $V(x)=igx$, is known to be
null. We enclose this potential in a hard-box: $V(|x| \ge 1) =\infty $ and in a
soft-box: $V(|x|\ge 1)=0$. In the former case, we find real discrete spectrum
and the exceptional points of the potential. The asymptotic eigenvalues behave
as $E_n \sim n^2.$ The solvable purely imaginary PT-symmetric potentials
vanishing asymptotically known so far do not have real discrete spectrum. Our
solvable soft-box potential possesses two real negative discrete eigenvalues if
$|g|<(1.22330447)$. The soft-box potential turns out to be a scattering
potential not possessing reflectionless states.


title: Hidden Subhypergroup Problem

abstract:
  The Hidden Subgroup Problem is used in many quantum algorithms such as
Simon's algorithm and Shor's factoring and discrete log algorithms. A
polynomial time solution is known in case of abelian groups, and normal
subgroups of arbitrary finite groups. The general case is still open. An
efficient solution of the problem for symmetric group $S_n$ would give rise to
an efficient quantum algorithm for Graph Isomorphism Problem. We formulate a
hidden sub-hypergroup problem for finite hypergroups and solve it for finite
commutative hypergroups. The given algorithm is efficient if the corresponding
QFT could be calculated efficiently.


title: Persistence of Quantum Information

abstract:
  There is an increasing interest in the role of macroscopic environments to
our understanding of the basics of quantum theory. The knowledge of the
implications of the quantum theory to other theories, especially to the
statistical mechanics and the domain of validity has captivated scientists from
the beginning of quantum description. In such a context, the presence of an
environment is commonly thought as entanglement, decohering and mixing
properties of quantum system. Generically, an environment is assumed to be a
noisy reservoir or a heat bath. Whereas in common interpretation of statistical
mechanics the heat bath is unspecified, in quantum systems a heat bath can also
provide an indirect interaction between otherwise totally decoupled subsystems
and consequently a means to entangle them \cite{cdkl,dvclp,bfp}. In simple
example for the entanglement between two qubits due to the interaction with a
common heat bath has been explicitly shown in \cite{b}. Whereas in that paper
the bath is described by a collection of harmonic oscillators, it seems to be
more reasonable to specify the bath by stochastic forces represented by
stochastic fields. From a more general point of view we expect the bath should
be better described in a stochastic manner and not by deterministic forces. In
the present paper we consider a two level system (qubits) which are able to
perform flip processes by a coupling to classical stochastic fields. Thus we
bridge the gap between quantum and classical probability theory. This problem
is related to many other questions of quantum optics and quantum electronics
where quantum statistical aspects arising from the intrinsic quantum character
of the system while the possible time-dependence of system parameters may be
interpreted as the influence of classical thermal fluctuations.


title:
Dynamical decoupling induced renormalization of the non-Markovian
  dynamics

abstract:
  In this work we develop a numerical framework to investigate the
renormalization of the non-Markovian dynamics of an open quantum system to
which dynamical decoupling is applied. We utilize a non-Markovian master
equation which is derived from the non-Markovian quantum trajectories
formalism. It contains incoherent Markovian dynamics and coherent Schr\"odinger
dynamics as its limiting cases and is capable of capture the transition between
them. We have performed comprehensive simulations for the cases in which the
system is either driven by the Ornstein-Uhlenbeck noise or or is described by
the spin-boson model. The renormalized dynamics under bang-bang control and
continuous dynamical decoupling are simulated. Our results indicate that the
renormalization of the non-Markovian dynamics depends crucially on the spectral
density of the environment and the envelop of the decoupling pulses. The
framework developed in this work hence provides an unified approach to
investigate the efficiency of realistic decoupling pulses. This work also opens
a way to further optimize the decoupling via pulse shaping.


title: Pure entangled states probed by multipartite Bell experiments

abstract:
  The spectral decomposition of all (n,2,2) Bell operators ($2^{2^n}$ in
number, $n \ge 2$), as introduced by Werner and Wolf, is done. Its implications
on the characterization of Bell operators as probes of entanglement are
considered in detail.


title: Improving the Fidelity of Optical Zeno Gates via Distillation

abstract:
  We have modelled the Zeno effect Control-Sign gate of Franson et al (PRA 70,
062302, 2004) and shown that high two-photon to one-photon absorption ratios,
$\kappa$, are needed for high fidelity free standing operation. Hence we
instead employ this gate for cluster state fusion, where the requirement for
$\kappa$ is less restrictive. With the help of partially offline one-photon and
two-photon distillations, we can achieve a fusion gate with unity fidelity but
non-unit probability of success. We conclude that for $\kappa > 2200$, the Zeno
fusion gate will out perform the equivalent linear optics gate.


title: Generating squeezing in an atom laser through self-interaction

abstract:
  We describe a scheme for creating quadrature- and intensity-squeezed atom
lasers that do not require squeezed light as an input. The beam becomes
squeezed due to nonlinear interactions between the atoms in the beam in an
analogue to optical Kerr squeezing. We develop an analytic model of the process
which we compare to a detailed stochastic simulation of the system using phase
space methods. Finally we show that significant squeezing can be obtained in an
experimentally realistic system and suggest ways of increasing the tunability
of the squeezing.


title:
General Classes of Impossible Operations through the Existence of
  Incomparable States

abstract:
  In this work we show that the most general class of anti-unitary operators
are nonphysical in nature through the existence of incomparable pure bipartite
entangled states. It is also shown that a large class of
inner-product-preserving operations defined only on the three qubits having
spin-directions along x, y and z are impossible. If we perform such an
operation locally on a particular pure bipartite state then it will exactly
transform to another pure bipartite state that is incomparable with the
original one. As subcases of the above results we find the nonphysical nature
of universal exact flipping operation and existence of universal Hadamard gate.
Beyond the information conservation in terms of entanglement, this work shows
how an impossible local operation evolve with the joint system in a nonphysical
way.


title: E_7 and the tripartite entanglement of seven qubits

abstract:
  In quantum information theory, it is well known that the tripartite
entanglement of three qubits is described by the group [SL(2,C)]^3 and that the
entanglement measure is given by Cayley's hyperdeterminant. This has provided
an analogy with certain N=2 supersymmetric black holes in string theory, whose
entropy is also given by the hyperdeterminant. In this paper, we extend the
analogy to N=8. We propose that a particular tripartite entanglement of seven
qubits, encoded in the Fano plane, is described by the exceptional group E_7(C)
and that the entanglement measure is given by Cartan's quartic E_7 invariant.


title: Process tomography of ion trap quantum gates

abstract:
  A crucial building block for quantum information processing with trapped ions
is a controlled-NOT quantum gate. In this paper, two different sequences of
laser pulses implementing such a gate operation are analyzed using quantum
process tomography. Fidelities of up to 92.6(6)% are achieved for single gate
operations and up to 83.4(8)% for two concatenated gate operations. By process
tomography we assess the performance of the gates for different experimental
realizations and demonstrate the advantage of amplitude--shaped laser pulses
over simple square pulses. We also investigate whether the performance of
concatenated gates can be inferred from the analysis of the single gates.


title: Ergodic Classical-Quantum Channels: Structure and Coding Theorems

abstract:
  We consider ergodic causal classical-quantum channels (cq-channels) which
additionally have a decaying input memory. In the first part we develop some
structural properties of ergodic cq-channels and provide equivalent conditions
for ergodicity. In the second part we prove the coding theorem with weak
converse for causal ergodic cq-channels with decaying input memory. Our proof
is based on the possibility to introduce joint input-output state for the
cq-channels and an application of the Shannon-McMillan theorem for ergodic
quantum states. In the last part of the paper it is shown how this result
implies coding theorem for the classical capacity of a class of causal ergodic
quantum channels.


title: A Graphic Representation of States for Quantum Copying Machines

abstract:
  The aim of this paper is to introduce a new graphic representation of quantum
states by means of a specific application: the analysis of two models of
quantum copying machines. The graphic representation by diagrams of states
offers a clear and detailed visualization of quantum information's flow during
the unitary evolution of not too complex systems. The diagrams of states are
exponentially more complex in respect to the standard representation and this
clearly illustrates the discrepancy of computational power between quantum and
classical systems. After a brief introductive exposure of the general theory,
we present a constructive procedure to illustrate the new representation by
means of concrete examples. Elementary diagrams of states for single-qubit and
two-qubit systems and a simple scheme to represent entangled states are
presented. Quantum copying machines as imperfect cloners of quantum states are
introduced and the quantum copying machines of Griffiths and Niu and of Buzek
and Hillery are analyzed, determining quantum circuits of easier
interpretation. The method has indeed shown itself to be extremely successful
for the representation of the involved quantum operations and it has allowed to
point out the characteristic aspects of the quantum computations examined.


title:
Bound-state solutions of the Klein-Gordon equation for the generalized
  PT-symmetric Hulthen potential

abstract:
  The one-dimensional Klein-Gordon equation is solved for the PT-symmetric
generalized Hulthen potential in the scalar coupling scheme. The relativistic
bound-state energy spectrum and the corresponding wave functions are obtained
by using the Nikiforov-Uvarov method which is based on solving the second-order
linear differential equations by reduction to a generalized equation of
hypergeometric type.


title: Implementation of quantum operations on single photon qudits

abstract:
  We show that a general linear transformation from one single photon qudit to
another, the dimension of which can be either equal or unequal to that of the
first one, can be implemented by linear optics. As an application of the scheme
we elaborate a method to deterministically realize any finite-element Positive
Operator Value Measure (POVM) on single photon signals, which is also
generalizable to any quantum system in principle.


title: On Dalgarno and Lewis Perturbation Theory for Scattering States

abstract:
  We apply the method of Dalgarno and Lewis to scattering states and discuss
the choice of the unperturbed model in order to have a convergent perturbation
series for the phase shift.


title: Exploring scalar quantum walks on Cayley graphs

abstract:
  A quantum walk, \emph{i.e.}, the quantum evolution of a particle on a graph,
is termed \emph{scalar} if the internal space of the moving particle (often
called the coin) has a dimension one. Here, we study the existence of scalar
quantum walks on Cayley graphs, which are built from the generators of a group.
After deriving a necessary condition on these generators for the existence of a
scalar quantum walk, we present a general method to express the evolution
operator of the walk, assuming homogeneity of the evolution. We use this
necessary condition and the subsequent constructive method to investigate the
existence of scalar quantum walks on Cayley graphs of various groups presented
with two or three generators. In this restricted framework, we classify all
groups -- in terms of relations between their generators -- that admit scalar
quantum walks, and we also derive the form of the most general evolution
operator. Finally, we point out some interesting special cases, and extend our
study to a few examples of Cayley graphs built with more than three generators.


title: Use of dynamical coupling for improved quantum state transfer

abstract:
  We propose a method to improve quantum state transfer in transmission lines.
The idea is to localize the information on the last qubit of a transmission
line, by dynamically varying the coupling constants between the first and the
last pair of qubits. The fidelity of state transfer is higher then in a chain
with fixed coupling constants. The effect is stable against small fluctuations
in the system parameters.


title:
A fast and robust approach to long-distance quantum communication with
  atomic ensembles

abstract:
  Quantum repeaters create long-distance entanglement between quantum systems
while overcoming difficulties such as the attenuation of single photons in a
fiber. Recently, an implementation of a repeater protocol based on single
qubits in atomic ensembles and linear optics has been proposed [Nature 414, 413
(2001)]. Motivated by rapid experimental progress towards implementing that
protocol, here we develop a more efficient scheme compatible with active
purification of arbitrary errors. Using similar resources as the earlier
protocol, our approach intrinsically purifies leakage out of the logical
subspace and all errors within the logical subspace, leading to greatly
improved performance in the presence of experimental inefficiencies. Our
analysis indicates that our scheme could generate approximately one pair per 3
minutes over 1280 km distance with fidelity (F>78%) sufficient to violate
Bell's inequality.


title:
Security of quantum bit string commitment depends on the information
  measure

abstract:
  Unconditionally secure non-relativistic bit commitment is known to be
impossible in both the classical and the quantum world. However, when
committing to a string of n bits at once, how far can we stretch the quantum
limits? In this letter, we introduce a framework of quantum schemes where Alice
commits a string of n bits to Bob, in such a way that she can only cheat on a
bits and Bob can learn at most b bits of information before the reveal phase.
Our results are two-fold: we show by an explicit construction that in the
traditional approach, where the reveal and guess probabilities form the
security criteria, no good schemes can exist: a+b is at least n. If, however,
we use a more liberal criterion of security, the accessible information, we
construct schemes where a=4 log n+O(1) and b=4, which is impossible
classically. Our findings significantly extend known no-go results for quantum
bit commitment.


title: Perfect Teleportation and Superdense Coding With W-States

abstract:
  True tripartite entanglement of the state of a system of three qubits can be
classified on the basis of stochastic local operations and classical
communications (SLOCC). Such states can be classified in two categories: GHZ
states and W-states. It is known that GHZ states can be used for teleportation
and superdense coding, but the prototype W-state cannot be. However, we show
that there is a class of W-states that can be used for perfect teleportation
and superdense coding.


title: New perturbation theory in QED

abstract:
  The perturbation theory in QED used the new form of ground state as a state
of interacted particles is proposed. The mean electromagnetic field of charged
elementary particle is calculated. The possibility of elimination the problem
with ultraviolet as well as infrared divergences is shown. The electromagnetic
energy of the particle turns out to be regular and small.


title:
Quantum Coding Theorems for Arbitrary Sources, Channels and Entanglement
  Resources

abstract:
  The information spectrum approach gives general formulae for optimal rates of
various information theoretic protocols, under minimal assumptions on the
nature of the sources, channels and entanglement resources involved. This paper
culminates in the derivation of the dense coding capacity for a noiseless
quantum channel, assisted by arbitrary shared entanglement, using this
approach. We also review the currently known coding theorems, and their
converses, for protocols such as data compression for arbitrary quantum sources
and transmission of classical information through arbitrary quantum channels.
In addition, we derive the optimal rate of data compression for a mixed source


title: Semiclassics of the Chaotic Quantum-Classical Transition

abstract:
  We elucidate the basic physical mechanisms responsible for the
quantum-classical transition in one-dimensional, bounded chaotic systems
subject to unconditioned environmental interactions. We show that such a
transition occurs due to the dual role of noise in regularizing the
semiclassical Wigner function and averaging over fine structures in classical
phase space. The results are interpreted in the novel context of applying
recent advances in the theory of measurement and open systems to the
semiclassical quantum regime. We use these methods to show how a local
semiclassical picture is stabilized and can then be approximated by a classical
distribution at later times. The general results are demonstrated explicitly
via high-resolution numerical simulations of the quantum master equation for a
chaotic Duffing oscillator.


title:
A General Information Theoretical Proof for the Second Law of
  Thermodynamics

abstract:
  We show that the conservation and the non-additivity of the information,
together with the additivity of the entropy make the entropy increase in an
isolated system. The collapse of the entangled quantum state offers an example
of the information non-additivity. Nevertheless, the later is also true in
other fields, in which the interaction information is important. Examples are
classical statistical mechanics, social statistics and financial processes. The
second law of thermodynamics is thus proven in its most general form. It is
exactly true, not only in quantum and classical physics but also in other
processes, in which the information is conservative and non-additive.


title: Optimal control of entanglement via quantum feedback

abstract:
  It has recently been shown that finding the optimal measurement on the
environment for stationary Linear Quadratic Gaussian control problems is a
semi-definite program. We apply this technique to the control of the
EPR-correlations between two bosonic modes interacting via a parametric
Hamiltonian at steady state. The optimal measurement turns out to be nonlocal
homodyne measurement -- the outputs of the two modes must be combined before
measurement. We also find the optimal local measurement and control technique.
This gives the same degree of entanglement but a higher degree of purity than
the local technique previously considered [S. Mancini, Phys. Rev. A {\bf 73},
010304(R) (2006)].


title:
Quantum Entanglement under Non-Markovian Dynamics of Two Qubits
  Interacting with a common Electromagnetic Field

abstract:
  We study the non-equilibrium dynamics of a pair of qubits made of two-level
atoms separated in space with distance r and interacting with one common
electromagnetic field but not directly with each other. Our calculation makes a
weak coupling assumption, but no Born or Markov approximation. We derived a
non-Markovian master equation for the evolution of the reduced density matrix
of the two-qubit system after integrating out the electromagnetic field modes.
It contains a Markovian part with a Lindblad type operator and a nonMarkovian
contribution, the physics of which is the main focus of this study. We use the
concurrence function as a measure of quantum entanglement between the two
qubits. Two classes of states are studied in detail: Class A is a one parameter
family of states which are the superposition of the highest energy and Class B
states are linear combinations of the symmetric and the antisymmetric Bell
states. We obtain similar behavior for the Bell states as in earlier results
derived by using the Born-Markov approximation. However, in the Class A states
the behavior is qualitatively different: under the non-Markovian evolution we
do not see sudden death of quantum entanglement and subsequent revivals, except
when the qubits are sufficiently far apart. We provide explanations for such
differences of behavior both between these two classes of states and between
the predictions from the Markov and non-Markovian dynamics. We also study the
decoherence of this two-qubit system and find that the decoherence rate in the
case of one qubit initially in an excited state does not change significantly
with the qubits separation whereas it does for the case when one qubit is
initially in the ground state.


title: Quantumness of Josephson junctions reexamined

abstract:
  There exists an increasing evidence supporting the picture of the Josephson
junction (JJ) as a "macroscopic quantum system". On the other hand the
interpretation of experimental data strongly depends on the assumed theoretical
model. We analyse the possible states of a Cooper pair box ("charge qubit") for
the two types of models : two-mode Bose-Hubbard model with its large $N$
aproximations and the many-body description within the mean-field approximation
(Gross-Pitaevski equation). While the first class of models supports the
picture of JJ being a quantum subsystem of a single degree of freedom, the
second approach yields an essentially classical structure of accessible quantum
states which, in particular, implies the absence of entanglement for two
coupled JJ's. The arguments in favor of the mean-field theory are presented and
different experimental tests including a new proposal are briefly discussed.


title: On the Magic Matrix by Makhlin and the B-C-H Formula in SO(4)

abstract:
  A closed expression to the Baker-Campbell-Hausdorff (B-C-H) formula in SO(4)
is given by making use of the magic matrix by Makhlin. As far as we know this
is the {\bf first nontrivial example} on (semi-) simple Lie groups summing up
all terms in the B-C-H expansion.


title:
Essential state of the electromagnetic field and the double-slit
  experiment

abstract:
  A new class of generalized solutions related to the essential spectrum of
linear Maxwell's equations is presented. The essential modes are given in terms
of normalized singular Weyl's sequences, whose square represents Dirac's delta
functions in spatial and angular frequency domains. The action integral
associated with essential modes is well-defined. We claim that these modes
represent the collapsed state of the electromagnetic field and, with some
additional assumptions on the conservation of action, are suitable for
describing the double-slit experiment in accordance with the orthodox point of
view.


title: Variational Principle for Mixed Classical-Quantum Systems

abstract:
  An extended variational principle providing the equations of motion for a
system consisting of interacting classical, quasiclassical and quantum
components is presented, and applied to the model of bilinear coupling. The
relevant dynamical variables are expressed in the form of a quantum state
vector which includes the action of the classical subsystem in its phase
factor. It is shown that the statistical ensemble of Brownian state vectors for
a quantum particle in a classical thermal environment can be described by a
density matrix evolving according to a nonlinear quantum Fokker-Planck
equation. Exact solutions of this equation are obtained for a two-level system
in the limit of high temperatures, considering both stationary and
nonstationary initial states. A treatment of the common time shared by the
quantum system and its classical environment, as a collective variable rather
than as a parameter, is presented in the Appendix.


title:
CPT-like states in an ensemble of interacting fermions. On the
  possibility of new mechanism of superconductivity

abstract:
  Using the standard Hamiltonian of the BCS theory, we show that in an ensemble
of interacting fermions there exists a coherent state $|NC>$, which nullifies
the Hamiltonian of the interparticle interaction. This state has an analogy
with the well-known in quantum optics coherent population trapping effect
(CPT). A possible application of such CPT-like states in the superconductivity
theory is discussed.

categories: quant-ph cond-mat.str-el cond-mat.supr-con physics.atom-ph physics.optics


title: Quantum Property Testing for Solvable Groups

abstract:
  Property testing has been extensively studied and its target is to determine
whether a given object satisfies a certain property or it is far from the
property. In this paper, we construct an efficient quantum algorithm which
tests if a given quantum oracle performs the group multiplication of a solvable
group. Our work is strongly based on the efficient classical testing algorithm
for Abelian groups proposed by Friedl, Ivanyos and Santha. Since every Abelian
group is a solvable group, our result is in a sense a generalization of their
result.


title:
Optimal estimation of an observable's expectation value for pure states
  for general measure of deviation

abstract:
  We investigate the optimal estimation of quantum expectation value of a
physical observable, which minimizes a mean error with respect to general
measure of deviation, when a finite number of copies of a pure state are
prepared. If pure sates are uniformly distributed, the minimum value of mean
error for any measure of deviation is achieved by projective measurement on
each copy.


title:
Unconditionally secure one-way quantum key distribution using decoy
  pulses

abstract:
  We report here a complete experimental realization of one-way decoy-pulse
quantum key distribution, demonstrating an unconditionally secure key rate of
5.51 kbps for a 25.3 km fibre length. This is two orders of magnitudes higher
than the value that can be obtained with a non-decoy system. We introduce also
a simple test for detecting the photon number splitting attack and highlight
that it is essential for the security of the technique to fully characterize
the source and detectors used.


title:
Analysis of path integrals at low temperature : Box formula, occupation
  time and ergodic approximation

abstract:
  We study the low temperature behaviour of path integrals for a simple
one-dimensional model. Starting from the Feynman-Kac formula, we derive a new
functional representation of the density matrix at finite temperature, in terms
of the occupation times of Brownian motions constrained to stay within boxes
with finite sizes. From that representation, we infer a kind of ergodic
approximation, which only involves double ordinary integrals. As shown by its
applications to different confining potentials, the ergodic approximation turns
out to be quite efficient, especially in the low-temperature regime where other
usual approximations fail.


title: A pairwise additive strategy for quantifying multipartite entanglement

abstract:
  Based on the idea of measuring the factorizability of a given density matrix,
we propose a pairwise analysis strategy for quantifying and understanding
multipartite entanglement. The methodology proves very effective as it
immediately guarantees, in addition to the usual entanglement properties,
additivity and strong super additivity. We give a specific set of quantities
that fulfill the protocol and which, according to our numerical calculations,
make the entanglement measure an LOCC non-increasing function. The strategy
allows a redefinition of the structural concept of global entanglement.


title: Optimal quantum chain communication by end gates

abstract:
  The scalability of solid state quantum computation relies on the ability of
connecting the qubits to the macroscopic world. Quantum chains can be used as
quantum wires to keep regions of external control at a distance. However even
in the absence of external noise their transfer fidelity is too low to assure
reliable connections. We propose a method of optimizing the fidelity by minimal
usage of the available resources, consisting of applying a suitable sequence of
two-qubit gates at the end of the chain. Our scheme allows also the preparation
of states in the first excitation sector as well as cooling.


title: Realization of a superconducting atom chip

abstract:
  We have trapped rubidium atoms in the magnetic field produced by a
superconducting atom chip operated at liquid Helium temperatures. Up to
$8.2\cdot 10^5$ atoms are held in a Ioffe-Pritchard trap at a distance of 440
$\mu$m from the chip surface, with a temperature of 40 $\mu$K. The trap
lifetime reaches 115 s at low atomic densities. These results open the way to
the exploration of atom--surface interactions and coherent atomic transport in
a superconducting environment, whose properties are radically different from
normal metals at room temperature.


title: Parametrizations of Positive Matrices With Applications

abstract:
  This paper reviews some characterizations of positive matrices and discusses
which lead to useful parametrizations. It is argued that one of them, which we
dub the Schur-Constantinescu parametrization is particularly useful. Two new
applications of it are given. One shows all block-Toeplitz states are PPT. The
other application is to relaxation rates.


title:
Dilation Theoretic Parametrizations of Positive Matrices with
  Applications to Quantum Information

abstract:
  This paper, dedicated to the memory of late Professor Tiberiu Constantinescu,
discusses two parametrizations of positive matrices. The first, called the
Schur-Constantinescu parametrization, is used to construct several examples of
separable states (e.g., Hankel states). The second, called the Jacobi
parametrization, is used to present an alternative to the Bloch sphere
representation of qubits.


title:
Demonstration of a Tunable-Bandwidth White Light Interferometer using
  Anomalous Dispersion in Atomic Vapor

abstract:
  Recently, the design of a white-light-cavity has been proposed using negative
dispersion in an intra-cavity medium to make the cavity resonate over a large
range of frequencies and still maintain a high cavity build-up. This paper
presents the demonstration of this effect in a free-space cavity. The negative
dispersion of the intra-cavity medium is caused by bi-frequency Raman gain in
an atomic vapor cell. A significantly broad cavity response over a bandwidth
greater than 20 MHz has been observed. The experimental results agree well with
the theoretical model, taking into account effects of residual absorption. A
key application of this device would be in enhancing the sensitivity-bandwidth
product of the next generation gravitational wave detectors that make use of
the so-called signal-recycling mirror.


title:
Demonstration of Tunable Displacement-Measurement-Sensitivity using
  Variable Group Index in a Ring Resonator

abstract:
  We show that an intra-cavity medium with normal dispersion reduces the
sensitivity of the cavity resonance frequency to a change in its length by a
factor inversely proportional to the group index. Since the group index in an
atomic medium can be very large, this effect can help in constructing highly
frequency-stable cavities for various potential applications without taking
additional measures for mechanical stability. The results also establish
indirectly the opposite effect of enhanced sensitivity that can be realized for
a negative dispersion corresponding to a group index close to a null value.
This enhancement in turn can be employed to increase significantly the
sensitivity of a ring laser gyroscope.


title: Topological Computation without Braiding

abstract:
  We show that universal quantum computation can be performed within the ground
state of a topologically ordered quantum system, which is a naturally protected
quantum memory. In particular, we show how this can be achieved using brane-net
condensates in 3-colexes. The universal set of gates is implemented without
selective addressing of physical qubits and, being fully topologically
protected, it does not rely on quasiparticle excitations or their braiding.


title: KLM quantum computation as a measurement based computation

abstract:
  We show that the Knill Laflamme Milburn method of quantum computation with
linear optics gates can be interpreted as a one-way, measurement based quantum
computation of the type introduced by Briegel and Rausendorf. We also show that
the permanent state of n n-dimensional systems is a universal state for quantum
computation.


title: Vacuum induced torque between corrugated metallic plates

abstract:
  We study the torque arising between two corrugated metallic plates due to the
interaction with electromagnetic vacuum. This Casimir torque can be measured
with torsion pendulum techniques for separation distances as large as 1$\mu$m.
It allows one to probe the nontrivial geometry dependence of the Casimir energy
in a configuration which can be evaluated theoretically with accuracy. In the
optimal experimental configuration, the commonly used proximity force
approximation turns out to overestimate the torque by a factor 2 or larger.


title: The Quantum Chernoff Bound

abstract:
  We consider the problem of discriminating two different quantum states in the
setting of asymptotically many copies, and determine the optimal strategy that
minimizes the total probability of error. This leads to the identification of
the quantum Chernoff bound, thereby solving a long standing open problem. The
bound reduces to the classical Chernoff bound when the quantum states under
consideration commute. The quantum Chernoff bound is the natural symmetric
distance measure between quantum states because of its clear operational
meaning and because of the fact that it does not seem to share the undesirable
features of other distance measures like the fidelity, the trace norm and the
relative entropy.


title: Quantum Error Correction Beyond Completely Positive Maps

abstract:
  By introducing an operator sum representation for arbitrary linear maps, we
develop a generalized theory of quantum error correction (QEC) that applies to
any linear map, in particular maps that are not completely positive (CP). This
theory of "linear quantum error correction" is applicable in cases where the
standard and restrictive assumption of a factorized initial system-bath state
does not apply.


title: Cavity QED determination of atomic number statistics in optical lattices

abstract:
  We study the reflection of two counter-propagating modes of the light field
in a ring resonator by ultracold atoms either in the Mott insulator state or in
the superfluid state of an optical lattice. We obtain exact numerical results
for a simple two-well model and carry out statistical calculations appropriate
for the full lattice case. We find that the dynamics of the reflected light
strongly depends on both the lattice spacing and the state of the matter-wave
field. Depending on the lattice spacing, the light field is sensitive to
various density-density correlation functions of the atoms. The light field and
the atoms become strongly entangled if the latter are in a superfluid state, in
which case the photon statistics typically exhibit complicated multimodal
structures.


title: Reference frames, superselection rules, and quantum information

abstract:
  Recently, there has been much interest in a new kind of ``unspeakable''
quantum information that stands to regular quantum information in the same way
that a direction in space or a moment in time stands to a classical bit string:
the former can only be encoded using particular degrees of freedom while the
latter are indifferent to the physical nature of the information carriers. The
problem of correlating distant reference frames, of which aligning Cartesian
axes and synchronizing clocks are important instances, is an example of a task
that requires the exchange of unspeakable information and for which it is
interesting to determine the fundamental quantum limit of efficiency. There
have also been many investigations into the information theory that is
appropriate for parties that lack reference frames or that lack correlation
between their reference frames, restrictions that result in global and local
superselection rules. In the presence of these, quantum unspeakable information
becomes a new kind of resource that can be manipulated, depleted, quantified,
etcetera. Methods have also been developed to contend with these restrictions
using relational encodings, particularly in the context of computation,
cryptography, communication, and the manipulation of entanglement. This article
reviews the role of reference frames and superselection rules in the theory of
quantum information processing.


title: Compatibility of subsystem states and convex geometry

abstract:
  The subsystem compatibility problem, which concerns the question of whether a
set of subsystem states are compatible with a state of the entire system, has
received much study. Here we attack the problem from a new angle, utilising the
ideas of convexity that have been successfully employed against the
separability problem. Analogously to an entanglement witness, we introduce the
idea of a compatibility witness, and prove a number of properties about these
objects. We show that the subsystem compatibility problem can be solved
numerically and efficiently using semidefinite programming, and that the
numerical results from this solution can be used to extract exact analytic
results, an idea which we use to disprove a conjecture about the subsystem
problem made by Butterley et al. [Found. Phys. 36 83 (2006)]. Finally, we
consider how the ideas can be used to tackle some important variants of the
compatibility problem; in particular, the case of identical particles (known as
N-representability in the case of fermions) is considered.


title: Maximally polarized states for quantum light fields

abstract:
  The degree of polarization of a quantum state can be defined as its
Hilbert-Schmidt distance to the set of unpolarized states. We demonstrate that
the states optimizing this degree for a fixed average number of photons
$\bar{N}$ present a fairly symmetric, parabolic photon statistics, with a
variance scaling as $\bar{N}^2$. Although no standard optical process yields
such a statistics, we show that, to an excellent approximation, a highly
squeezed vacuum can be considered as maximally polarized.


title: Quantum mechanics and Leggett's principles of macroscopic realism

abstract:
  On the basis of our recent model of a one-dimensional (1D) completed
scattering we argue that Leggett's principles of macroscopic realism must and
can be extended onto the level of single electrons and atoms. These principles
need three quite feasible innovations in quantum mechanics (QM): (1) at the
conceptual level, QM must treat a pure time-dependent one-particle state to
involve two or more macroscopically distinct alternatives for a particle as a
pure {\it combined} one - the intermediate link between a pure {\it elementary}
state (indecomposable into macroscopically distinct parts) and statistical
mixture; (2) at the mathematical level, QM must provide the presentation of the
pure time-dependent combined state as a coherent superposition of
macroscopically distinct elementary states (MDESs); (3) at the experimental
level, QM must provide for such states two types of measurements - those for
observing the interference pattern resulting from the joint action of MDESs,
and non-demolishing 'which-way' measurements for scanning the individual
properties of MDESs.


title: Quantum nature of a strongly-coupled single quantum dot-cavity system

abstract:
  Cavity quantum electrodynamics (QED) studies the interaction between a
quantum emitter and a single radiation-field mode. When an atom is in strong
coupling with a cavity mode1,2, it is possible to realize key quantum
information processing (QIP) tasks, such as controlled coherent coupling and
entanglement of distinguishable quantum systems. Realizing these tasks in the
solid state is clearly desirable, and coupling semiconductor self-assembled
quantum dots (QDs) to monolithic optical cavities is a promising route to this
end. However, validating the efficacy of QDs in QIP applications requires
confirmation of the quantum nature of the QD-cavity system in the strong
coupling regime. Here we find a confirmation by observing quantum correlations
in photoluminescence (PL) from a photonic crystal (PC) nanocavity3-5
interacting with one, and only one, QD located precisely at the cavity electric
field maximum. When off-resonance, photon emission from the cavity mode and QD
excitons is anti-correlated at the level of single quanta, proving that the
mode is driven solely by the QD despite an energy mis-match between cavity and
excitons. When tuned into resonance, the exciton and photon enter the
strong-coupling regime of cavity-QED and the QD lifetime reduces by a factor of
120. The photon stream from the cavity becomes anti-bunched, proving that the
coupled exciton/photon system is in the quantum anharmonic regime. Our
observations unequivocally show that QIP tasks requiring the quantum nonlinear
regime are achievable in the solid state.


title:
Implementation of controlled SWAP gates for quantum fingerprinting and
  photonic quantum computation

abstract:
  We propose a scheme to implement quantum controlled SWAP gates by directing
single-photon pulses to a two-sided cavity with a single trapped atom. The
resultant gates can be used to realize quantum fingerprinting and universal
photonic quantum computation. The performance of the scheme is characterized
under realistic experimental noise with the requirements well within the reach
of the current technology.


title: Multiplexed Memory-Insensitive Quantum Repeaters

abstract:
  Long-distance quantum communication via distant pairs of entangled quantum
bits (qubits) is the first step towards more secure message transmission and
distributed quantum computing. To date, the most promising proposals require
quantum repeaters to mitigate the exponential decrease in communication rate
due to optical fiber losses. However, these are exquisitely sensitive to the
lifetimes of their memory elements. We propose a multiplexing of quantum nodes
that should enable the construction of quantum networks that are largely
insensitive to the coherence times of the quantum memory elements.


title: Quantum gambling using mesoscopic ring qubits

abstract:
  Quantum Game Theory provides us with new tools for practising games and some
other risk related enterprices like, for example, gambling. The two party
gambling protocol presented by Goldenberg {\it et al} is one of the simplest
yet still hard to implement applications of Quantum Game Theory. We propose
potential physical realisation of the quantum gambling protocol with use of
three mesoscopic ring qubits. We point out problems in implementation of such
game.


title: Using error correction to determine the noise model

abstract:
  Quantum error correcting codes have been shown to have the ability of making
quantum information resilient against noise. Here we show that we can use
quantum error correcting codes as diagnostics to characterise noise. The
experiment is based on a three-bit quantum error correcting code carried out on
a three-qubit nuclear magnetic resonance (NMR) quantum information processor.
Utilizing both engineered and natural noise, the degree of correlations present
in the noise affecting a two-qubit subsystem was determined. We measured a
correlation factor of c=0.5+/-0.2 using the error correction protocol, and
c=0.3+/-0.2 using a standard NMR technique based on coherence pathway
selection. Although the error correction method demands precise control, the
results demonstrate that the required precision is achievable in the
liquid-state NMR setting.


title: Medium effects on the van der Waals force

abstract:
  We consider the van der Waals interaction between two ground-state atoms
embedded in adjacent semi-infinite magnetodielectric media, with emphasis on
medium effects on it. We demonstrate that, in this case, at small atom-atom
distances the van der Waals interaction is screened by the surrounding media in
the same way as in an effective (single) medium. At larger atomic distances,
however, its dependence on the material parameters of the system and the
positions of the atoms is more complex. We also calculate the Casimir-Polder
potential of an atom A arising from a uniform distribution of atoms B in the
medium across the interface. Comparison of this potential with the
corresponding result deduced from the Casimir force on a thin composite slab in
front of a composite semi-infinite medium, both obeying the Clausius-Mossotti
relation, suggests a hint on how to improve a well-known formula for the van
der Waals potential with respect to the local-field effects.


title: Measurement-based quantum computation and undecidable logic

abstract:
  We establish a connection between measurement-based quantum computation and
the field of mathematical logic. We show that the computational power of an
important class of quantum states called graph states, representing resources
for measurement-based quantum computation, is reflected in the expressive power
of (classical) formal logic languages defined on the underlying mathematical
graphs. In particular, we show that for all graph state resources which can
yield a computational speed-up with respect to classical computation, the
underlying graphs--describing the quantum correlations of the states--are
associated with undecidable logic theories. Here undecidability is to be
interpreted in a sense similar to Goedel's incompleteness results, meaning that
there exist propositions, expressible in the above classical formal logic,
which cannot be proven or disproven.


title: Passage-time distributions from a spin-boson detector model

abstract:
  The passage-time distribution for a spread-out quantum particle to traverse a
specific region is calculated using a detailed quantum model for the detector
involved. That model, developed and investigated in earlier works, is based on
the detected particle's enhancement of the coupling between a collection of
spins (in a metastable state) and their environment. We treat the continuum
limit of the model, under the assumption of the Markov property, and calculate
the particle state immediately after the first detection. An explicit example
with 15 boson modes shows excellent agreement between the discrete model and
the continuum limit. Analytical expressions for the passage-time distribution
as well as numerical examples are presented. The precision of the measurement
scheme is estimated and its optimization discussed. For slow particles, the
precision goes like $E^{-3/4}$, which improves previous $E^{-1}$ estimates,
obtained with a quantum clock model.


title: Metric perturbation theory of quantum dynamics

abstract:
  A theory of quantum dynamics based on a discrete structure underlying the
space time manifold is developed for single particles. It is shown that at the
micro domain the interaction of particles with the underlying discrete
structure results in the quantum space time manifold. Regarding the resulting
quantum space-time as perturbation from the Lorentz metric it is shown it is
possible to discuss the dynamics of particles in the quantum domain.


title: KLM quantum computation with bosonic atoms

abstract:
  A Knill-Laflamme-Milburn (KLM) type quantum computation with bosonic neutral
atoms or bosonic ions is suggested. Crucially, as opposite to other quantum
computation schemes involving atoms (ions), no controlled interactions between
atoms (ions) involving their internal levels are required. Versus photonic KLM
computation this scheme has the advantage that single atom (ion) sources are
more natural than single photon sources, and single atom (ion) detectors are
far more efficient than single photon ones.


title: Rapid state purification protocols for a Cooper pair box

abstract:
  We propose techniques for implementing two different rapid state purification
schemes, within the constraints present in a superconducting charge qubit
system. Both schemes use a continuous measurement of charge (z) measurements,
and seek to minimize the time required to purify the conditional state. Our
methods are designed to make the purification process relatively insensitive to
rotations about the x-axis, due to the Josephson tunnelling Hamiltonian. The
first proposed method, based on the scheme of Jacobs [Phys. Rev. A 67,
030301(R) (2003)] uses the measurement results to control bias (z) pulses so as
to rotate the Bloch vector onto the x-axis of the Bloch sphere. The second
proposed method, based on the scheme of Wiseman and Ralph [New J. Phys. 8, 90
(2006)] uses a simple feedback protocol which tightly rotates the Bloch vector
about an axis almost parallel with the measurement axis. We compare the
performance of these and other techniques by a number of different measures.


title:
Superposition of coherent states prepared in one mode of a dissipative
  bimodal cavity

abstract:
  We solve the problem of the temporal evolution of one of two-modes embedded
in a same dissipative environment and investigate the role of the losses after
the preparation of a coherent state in only one of the two modes. Based on
current cavity QED technology, we present a calculation of the fidelity of a
superposition of coherent states engineered in a bimodal high-Q cavity. Our
calculation demonstrates that the engineered superposition retains coherence
for large times when the mean photon number of the prepared mode is on the
order of unity.


title: Stochastic Models of Quantum Mechanics - A Perspective

abstract:
  A subjective survey of stochastic models of quantum mechanics is given along
with a discussion of some key radiative processes, the clues they offer, and
the difficulties they pose for this program. An electromagnetic basis for
deriving quantum mechanics is advocated, and various possibilities are
considered. It is argued that only non-local or non-causal theories are likely
to be a successful basis for such a derivation.


title: Unsolvability of the Halting Problem in Quantum Dynamics

abstract:
  It is shown that the halting problem cannot be solved consistently in both
the Schrodinger and Heisenberg pictures of quantum dynamics. The existence of
the halting machine, which is assumed from quantum theory, leads into a
contradiction when we consider the case when the observer's reference frame is
the system that is to be evolved in both pictures. We then show that in order
to include the evolution of observer's reference frame in a physically sensible
way, the Heisenberg picture with time going backwards yields a correct
description.


title:
Key distillation from quantum channels using two-way communication
  protocols

abstract:
  We provide a general formalism to characterize the cryptographic properties
of quantum channels in the realistic scenario where the two honest parties
employ prepare and measure protocols and the known two-way communication
reconciliation techniques. We obtain a necessary and sufficient condition to
distill a secret key using this type of schemes for Pauli qubit channels and
generalized Pauli channels in higher dimension. Our results can be applied to
standard protocols such as BB84 or six-state, giving a critical error rate of
20% and 27.6%, respectively. We explore several possibilities to enlarge these
bounds, without any improvement. These results suggest that there may exist
weakly entangling channels useless for key distribution using prepare and
measure schemes.


title: Coding Theorem for a Class of Quantum Channels with Long-Term Memory

abstract:
  In this paper we consider the transmission of classical information through a
class of quantum channels with long-term memory, which are given by convex
combinations of product channels. Hence, the memory of such channels is given
by a Markov chain which is aperiodic but not irreducible. We prove the coding
theorem and weak converse for this class of channels. The main techniques that
we employ, are a quantum version of Feinstein's Fundamental Lemma and a
generalization of Helstrom's Theorem.


title: Teleportation of two-particle entangled state via cluster state

abstract:
  We present two schemes for teleporting an unknown two-particle entangled
state from a sender (Alice) to a receiver (Bob) via a four-particle entangled
cluster state. As it is shown, the unknown two-particle entangled state can be
teleported perfectly, and the successful possibilities and fidelities of the
two schemes both reach unit.


title:
A new link between Boson and Fermion algebras: an alternative to
  supersymmetry

abstract:
  In this paper we describe a new family of algebras which in the case of n = 2
reduces to the Fermion algebra and in the limiting case of n tends to infinity
reduces to the Boson algebra. These generalized algebras describe particles
which obey a generalized exclusion principle, limiting state occupation to
(n-1) and obeying a generalized particle statistics.


title:
What are Quantum Fluctuations? Round Table of the Third Conference on
  Quantum Theory: Reconsideration of Foundations

abstract:
  This is a transcript of the round table that took place during the conference
Quantum Theory: Reconsideration of Foundations - 3, June 2005, Vaxjo, Sweden.
There are presented opinions of leading experts in quantum foundations on such
fundamental problems as the origin of quantum fluctuations and completeness of
quantum mechanics.


title:
Features of Magneto-Optical Resonances in an Elliptically Polarized
  Traveling Light Wave

abstract:
  The parameters of nonlinear absorption magneto-optical resonances in the
Hanle configuration have been studied as functions of the ellipticity of a
traveling light wave. It has been found that these parameters (amplitude,
width, and amplitude-to-width ratio) depend strongly on the polarization of the
light wave. In particular, the resonance amplitude can increase by more than an
order of magnitude when the polarization changes from linear to optimal
elliptic. It has been shown that this effect is associated with the Doppler
frequency shift for atoms in a gas. The theoretical results have been
corroborated in experiments in Rb vapor.


title:
Effect of Atomic Coherence on Absorption in Four-level Systems: an
  Analytical study

abstract:
  Absorption profile of a four-level ladder atomic system interacting with
three driving fields is studied perturbatively and analytical results are
presented. Numerical results where the driving field strengths are treated upto
all orders are presented. The absorption features is studied in two regimes, i)
the weak middle transition coupling, i.e.
  $\Omega_2 << \Omega_{1,3}$ and ii) the strong middle transition coupling
  $\Omega_2 >>\Omega_{1,3}$. In case i), it is shown that the ground state
absorption and the saturation characteristics of the population of level 2
reveal deviation due to the presence of upper level couplings. In particular,
the saturation curve for the population of level 2 shows a dip for $\Omega_1 =
\Omega_3$. While the populations of levels 3 and 4 show a maxima when this
resonance condition is satisfied. Thus the resonance condition provides a
criterion for maximally populating the upper levels. A second order
perturbation calculation reveals the nature of this minima (maxima). In the
second case, I report two important features: a) Filtering of the Aulter-Townes
doublet in the three-peak absorption profile of the ground state, which is
achieved by detuning only the upper most coupling field, and b) control of
line-width by controlling the strength of the upper coupling fields. This
filtering technique coupled with the control of linewidth could prove to be
very useful for high resolution studies.


title: Entanglement localization by a single defect in a spin chain

abstract:
  We discuss the effect of a single diagonal defect on both the static and
dynamical properties of entanglement in a spin chain. We show that entanglement
localizes at the defect and discuss its localization length, arguing that this
can be used as a mean to store entanglement. We also show that the impurity
site can behave as an entanglement mirror and characterize the bouncing process
in terms of reflection and transmission coefficients.


title: Adiabatic Elimination in a Lambda System

abstract:
  This paper deals with different ways to extract the effective two-dimensional
lower level dynamics of a lambda system excited by off-resonant laser beams. We
present a commonly used procedure for elimination of the upper level, and we
show that it may lead to ambiguous results. To overcome this problem and better
understand the applicability conditions of this scheme, we review two rigorous
methods which allow us both to derive an unambiguous effective two-level
Hamiltonian of the system and to quantify the accuracy of the approximation
achieved: the first one relies on the exact solution of the Schrodinger
equation, while the second one resorts to the Green's function formalism and
the Feshbach projection operator technique.


title:
The Hatsopoulos-Gyftopoulos resolution of the Schroedinger-Park paradox
  about the concept of "state" in quantum statistical mechanics

abstract:
  A seldom recognized fundamental difficulty undermines the concept of
individual ``state'' in the present formulations of quantum statistical
mechanics (and in its quantum information theory interpretation as well). The
difficulty is an unavoidable consequence of an almost forgotten corollary
proved by E. Schroedinger in 1936 and perused by J.L. Park, Am. J. Phys., Vol.
36, 211 (1968). To resolve it, we must either reject as unsound the concept of
state, or else undertake a serious reformulation of quantum theory and the role
of statistics. We restate the difficulty and discuss a possible resolution
proposed in 1976 by G.N. Hatsopoulos and E.P. Gyftopoulos, Found. Phys., Vol.
6, 15, 127, 439, 561 (1976).


title: Optimal data processing for quantum measurements

abstract:
  We consider the general measurement scenario in which the ensemble average of
an operator is determined via suitable data-processing of the outcomes of a
quantum measurement described by a POVM. We determine the optimal processing
that minimizes the statistical error of the estimation.


title: Robust CNOT gates from almost any interaction

abstract:
  There are many cases where the interaction between two qubits is not
precisely known, but single qubit operations are available. In this paper we
show how, regardless of an incomplete knowledge of the strength or form of the
interaction between two qubits, it is often possible to construct a CNOT gate
which has arbitrarily high fidelity. In particular, we show that oscillations
in the strength of the exchange interaction in solid state Si and Ge structures
are correctable.


title: Parameter estimation of quantum channels

abstract:
  The efficiency of parameter estimation of quantum channels is studied in this
paper. We introduce the concept of programmable parameters to the theory of
estimation. It is found that programmable parameters obey the standard quantum
limit strictly; hence no speedup is possible in its estimation. We also
construct a class of non-unitary quantum channels whose parameter can be
estimated in a way that the standard quantum limit is broken. The study of
estimation of general quantum channels also enables an investigation of the
effect of noises on quantum estimation.


title:
Which Quantum Evolutions Can Be Reversed by Local Unitary Operations?
  Algebraic Classification and Gradient-Flow-Based Numerical Checks

abstract:
  Generalising in the sense of Hahn's spin echo, we completely characterise
those unitary propagators of effective multi-qubit interactions that can be
inverted solely by {\em local} unitary operations on $n$ qubits
(spins-$\tfrac{1}{2}$). The subset of $U\in \mathbf{SU}(2^n)$ satisfying
$U^{-1}=K_1 U K_2$ with pairs of local unitaries $K_1,
K_2\in\mathbf{SU}(2)^{\otimes n}$ comprises two classes: in type-I, $K_1$ and
$K_2$ are inverse to one another, while in type-II they are not. {Type-I}
consists of one-parameter groups that can jointly be inverted for all times
$t\in\R{}$ because their Hamiltonian generators satisfy $K H K^{-1} = \Ad K (H)
= -H$. As all the Hamiltonians generating locally invertible unitaries of
type-I are spanned by the eigenspace associated to the eigenvalue -1 of the
{\em local} conjugation map $\Ad K$, this eigenspace can be given in closed
algebraic form. The relation to the root space decomposition of
$\mathfrak{sl}(N,\C{})$ is pointed out. Special cases of type-I invertible
Hamiltonians are of $p$-quantum order and are analysed by the transformation
properties of spherical tensors of order $p$. Effective multi-qubit interaction
Hamiltonians are characterised via the graphs of their coupling topology.
  {Type-II} consists of pointwise locally invertible propagators, part of which
can be classified according to the symmetries of their matrix representations.
Moreover, we show gradient flows for numerically solving the decision problem
whether a propagator is type-I or type-II invertible or not by driving the
least-squares distance $\norm{K_1 e^{-itH} K_2 - e^{+itH}}^2_2$ to zero.


title:
Direct Production of Tripartite Pump-Signal-Idler Entanglement in the
  Above-Threshold Optical Parametric Oscillator

abstract:
  We calculate the quantum correlations existing among the three output fields
(pump, signal, and idler) of a triply resonant non-degenerate Optical
Parametric Oscillator operating above threshold. By applying the standard
criteria [P. van Loock and A. Furusawa, Phys. Rev. A 67, 052315 (2003)], we
show that strong tripartite continuous-variable entanglement is present in this
well-known and simple system. Furthermore, since the entanglement is generated
directly from a nonlinear process, the three entangled fields can have very
different frequencies, opening the way for multicolored quantum information
networks.


title: Subsystem fault tolerance with the Bacon-Shor code

abstract:
  We discuss how the presence of gauge sub-systems in the Bacon-Shor code [D.
Bacon, Phys. Rev. A 73, 012340 (2006)] leads to remarkably simple and efficient
methods for fault-tolerant error correction (FTEC). Most notably, FTEC does not
require entangled ancillary states and it can be implemented with
nearest-neighbor two-qubit measurements. By using these methods, we prove a
lower bound on the quantum accuracy threshold, 1.94 \times 10^{-4} for
adversarial stochastic noise, that improves previous lower bounds by nearly an
order of magnitude.


title: Measuring the quantum statistics of an atom laser beam

abstract:
  We propose and analyse a scheme for measuring the quadrature statistics of an
atom laser beam using extant optical homodyning and Raman atom laser
techniques. Reversal of the normal Raman atom laser outcoupling scheme is used
to map the quantum statistics of an incoupled beam to an optical probe beam. A
multimode model of the spatial propagation dynamics shows that the Raman
incoupler gives a clear signal of de Broglie wave quadrature squeezing for both
pulsed and continuous inputs. Finally, we show that experimental realisations
of the scheme may be tested with existing methods via measurements of Glauber's
intensity correlation function.


title:
Photon correlation vs interference of single-atom fluorescence in a
  half-cavity

abstract:
  Photon correlations are investigated for a single laser-excited ion trapped
in front of a mirror. Varying the relative distance between the ion and the
mirror, photon correlation statistics can be tuned smoothly from an
antibunching minimum to a bunching-like maximum. Our analysis concerns the
non-Markovian regime of the ion-mirror interaction and reveals the field
establishment in a half-cavity interferometer.


title: A priori Knowledge and the Kochen-Specker Theorem

abstract:
  We introduce and formalize a notion of "a priori knowledge" about a quantum
system, and show some properties about this form of knowledge. Finally, we show
that the Kochen-Specker theorem follows directly from this study. This version
is a draft version, the bibliography in particular is extremely scarce.
Comments welcome.


title: Optical excitation spectrum of an atom in a surface-induced potential

abstract:
  We study the optical excitation spectrum of an atom in the vicinity of a
dielectric surface. We calculate the rates of the total scattering and the
scattering into the evanescent modes. With a proper assessment of the
limitations, we demonstrate the portability of the flat-surface results to an
experimental situation with a nanofiber. The effect of the surface-induced
potential on the excitation spectrum for free-to-bound transitions is shown to
be weak. On the contrary, the effect for bound-to-bound transitions is
significant leading to a large excitation linewidth, a substantial negative
shift of the peak position, and a strong long tail on the negative side and a
small short tail on the positive side of the field--atom frequency detuning.


title:
Spontaneous radiative decay of translational levels of an atom near a
  dielectric surface

abstract:
  We study spontaneous radiative decay of translational levels of an atom in
the vicinity of a semi-infinite dielectric. We systematically derive the
microscopic dynamical equations for the spontaneous decay process. We calculate
analytically and numerically the radiative linewidths and the spontaneous
transition rates for the translational levels. The roles of the interference
between the emitted and reflected fields and of the transmission into the
evanescent modes are clearly identified. Our numerical calculations for the
silica--cesium interaction show that the radiative linewidths of the bound
excited levels with large enough but not too large vibrational quantum numbers
are moderately enhanced by the emission into the evanescent modes and those for
the deep bound levels are substantially reduced by the surface-induced red
shift of the transition frequency.


title: Bell Inequalities Classifying Bi-separable Three-qubit States

abstract:
  We present a set of Bell inequalities that gives rise to a finer
classification of the entanglement for tripartite systems. These inequalities
distinguish three possible bi-separable entanglements for three-qubit states.
The three Bell operators we employed constitute an external sphere of the
separable cube.


title:
Simultaneous amplification and non-symmetric amplitude damping of
  two-mode Gaussian state

abstract:
  The evolution of two-mode Gaussian state under symmetric amplification,
non-symmetric damping and thermal noise is studied. The time dependent solution
of the state characteristic function is obtained. The separability criterions
are given for the final state of weak amplification as well as strong
amplification.


title: Diffraction limited optics for single atom manipulation

abstract:
  We present an optical system designed to capture and observe a single neutral
atom in an optical dipole trap, created by focussing a laser beam using a large
numerical aperture N.A.=0.5 aspheric lens. We experimentally evaluate the
performance of the optical system and show that it is diffraction limited over
a broad spectral range (~ 200 nm) with a large transverse field (+/- 25
microns). The optical tweezer created at the focal point of the lens is able to
trap single atoms of 87Rb and to detect them individually with a large
collection efficiency. We measure the oscillation frequency of the atom in the
dipole trap, and use this value as an independent determination of the waist of
the optical tweezer. Finally, we produce with the same lens two dipole traps
separated by 2.2 microns and show that the imaging system can resolve the two
atoms.


title: Non-deterministic Two-Way Quantum Key Distribution using Coherent States

abstract:
  We propose a non-deterministic two-way quantum key distribution in which the
quantum correlation is established by transmitting the randomly polarized
photon. We analyze the security of the proposed quantum key distribution
against photon number splitting, impersonation, and Trojan horse attack and
quantify the security bound against mean photon number of the coherent state
pulse. Finally, we remark the characteristic features of the protocol.


title:
Cavity enhanced light scattering in optical lattices to probe atomic
  quantum statistics

abstract:
  Different quantum states of atoms in optical lattices can be nondestructively
monitored by off-resonant collective light scattering into a cavity. Angle
resolved measurements of photon number and variance give information about
atom-number fluctuations and pair correlations without single-site access.
Observation at angles of diffraction minima provides information on quantum
fluctuations insensitive to classical noise. For transverse probing, no photon
is scattered into a cavity from a Mott insulator phase, while the photon number
is proportional to the atom number for a superfluid.


title:
A Model for an Irreversible Bias Current in the Superconducting Qubit
  Measurement Process

abstract:
  The superconducting charge-phase `Quantronium' qubit is considered in order
to develop a model for the measurement process used in the experiment of Vion
et. al. [Science 296 886 (2002)]. For this model we propose a method for
including the bias current in the read-out process in a fundamentally
irreversible way, which to first order, is approximated by the Josephson
junction tilted-washboard potential phenomenology. The decohering bias current
is introduced in the form of a Lindblad operator and the Wigner function for
the current biased read-out Josephson junction is derived and analyzed. During
the read-out current pulse used in the Quantronium experiment we find that the
coherence of the qubit initially prepared in a symmetric superposition state is
lost at a time of 0.2 nanoseconds after the bias current pulse has been
applied. A timescale which is much shorter than the experimental readout time.
Additionally we look at the effect of Johnson-Nyquist noise with zero mean from
the current source during the qubit manipulation and show that the decoherence
due to the irreversible bias current description is an order of magnitude
smaller than that found through adding noise to the reversible tilted washboard
potential model. Our irreversible bias current model is also applicable to the
persistent current based qubits where the state is measured according to its
flux via a small inductance direct current superconducting quantum interference
device (DC-SQUID).


title: Quarkonium bound-state problem in momentum space revisited

abstract:
  A semi-spectral Chebyshev method for solving numerically singular integral
equations is presented and applied in the quarkonium bound-state problem in
momentum space. The integrals containing both, logarithmic and Cauchy singular
kernels, can be evaluated without subtractions by dedicated automatic
quadratures.
  By introducing a Chebyshev mesh and using the Nystrom algorithm the singular
integral equation is converted into an algebraic eigenvalue problem that can be
solved by standard methods.
  The proposed scheme is very simple to use, is easy in programming and highly
accurate.


title: From Objective Amplitudes to Bayesian Probabilities

abstract:
  We review the Consistent Amplitude approach to Quantum Theory and argue that
quantum probabilities are explicitly Bayesian. In this approach amplitudes are
tools for inference. They codify objective information about how complicated
experimental setups are put together from simpler ones. Thus, probabilities may
be partially subjective but the amplitudes are not.


title: Displacement deformed quantum fields

abstract:
  A displacement operator d_\zeta is introduced, verifying commutation
relations [d_\zeta, a_f^\dagger]=[d_\zeta, a_f]=\zeta(f)d_\zeta with field
creation and annihilation operators that verify [a_f,a_g]=0,
[a_f,a_g^\dagger]=(g,f), as usual. f and g are test functions, \zeta is a
Poincare invariant real-valued function on the test function space, and (g,f)
is a Poincare invariant Hermitian inner product. The *-algebra generated by all
these operators, and a state defined on it, nontrivially extends the *-algebra
of creation and annihilation operators and its Fock space representation. If
the usual requirement for linearity is weakened, as suggested in
quant-ph/0512190, we obtain a deformation of the free quantum field.


title:
Effective Quantum Dynamics of Interacting Systems with Inhomogeneous
  Coupling

abstract:
  We study the quantum dynamics of a single mode/particle interacting
inhomogeneously with a large number of particles and introduce an effective
approach to find the accessible Hilbert space where the dynamics takes place.
Two relevant examples are given: the inhomogeneous Tavis-Cummings model (e.g.,
N atomic qubits coupled to a single cavity mode, or to a motional mode in
trapped ions) and the inhomogeneous coupling of an electron spin to N nuclear
spins in a quantum dot.


title: Generalized Heisenberg relation and Quantum Harmonic Oscillators

abstract:
  We study the consequences of the generalized Heisenberg uncertainty relation
which admits a minimal uncertainty in length such as the case in a theory of
quantum gravity. In particular, the theory of quantum harmonic oscillators
arising from such a generalized uncertainty relation is examined. We
demonstrate that all the standard properties of the quantum harmonic
oscillators prevail when we employ a generalized momentum. We also show that
quantum electrodynamics and coherent photon states can be described in the
familiar standard manner despite the generalized uncertainty principle.


title: Fuzzy spheres from inequivalent coherent states quantizations

abstract:
  We present a new procedure which allows a coherent state (CS) quantization of
any set with a measure. It is manifest through the replacement of classical
observables by CS quantum observables, which acts on a Hilbert space of
prescribed dimension $N$. The algebra of CS quantum observables has the finite
dimension $N^2$. The application to the 2-sphere provides a family of
inequivalent CS quantizations, based on the spin spherical harmonics (the CS
quantization from usual spherical harmonics appears to give a trivial issue for
the cartesian coordinates). We compare these CS quantizations to the usual
(Madore) construction of the fuzzy sphere. The difference allows us to consider
our procedures as the constructions of new type of fuzzy spheres. The very
general character of our method suggests applications to construct fuzzy
versions of a variety of sets.


title: Teleporting a qubit a la Everett

abstract:
  If both Alice and Bob have access to a two-qubit "background state" then, by
simulating Everett's many worlds interpretation of measurement, Alice can
teleport a qubit to Bob, each using fixed unitaries. The Everett picture
unifies unitaries, measurements, and classical communication into just
unitaries, provided there are background states shared by all parties. We
review some of the literature on this theme and point out an additional fact:
If Bob has access to only one of the background qubits, teleportation is still
possible by Alice sending Bob one classical bit of information gained from a
measurement of the other qubit.


title: Fault-tolerant quantum computation with high threshold in two dimensions

abstract:
  We present a scheme of fault-tolerant quantum computation for a local
architecture in two spatial dimensions. The error threshold is 0.75% for each
source in an error model with preparation, gate, storage and measurement
errors.


title:
The readout of the fullerene-based quantum computing by a scanning
  tunneling microscope

abstract:
  We consider to detect the electron spin of a doped atom, i.e., a nitrogen or
a phosphorus, caged in a fullerene by currently available technique of the
scanning tunneling microscope (STM), which actually corresponds to the readout
of a qubit in the fullerene-based quantum computing. Under the conditions of
polarized STM current and Coulomb blockade, we investigate the tunneling matrix
elements involving the exchange coupling between the tunneling polarized
electrons and the encapsulated polarized electron, and calculate the variation
of the tunneling current with respect to different orientations of the
encapsulated electron spin. The experimental feasibility of our scheme is
discussed under the consideration of some imperfect factors.


title:
Nash equilibria in quantum games with generalized two-parameter
  strategies

abstract:
  In the Eisert protocol for 2 X 2 quantum games [Phys. Rev. Lett. 83, 3077], a
number of authors have investigated the features arising from making the
strategic space a two-parameter subset of single qubit unitary operators. We
argue that the new Nash equilibria and the classical-quantum transitions that
occur are simply an artifact of the particular strategy space chosen. By
choosing a different, but equally plausible, two-parameter strategic space we
show that different Nash equilibria with different classical-quantum
transitions can arise. We generalize the two-parameter strategies and also
consider these strategies in a multiplayer setting.


title: A lower bound on entanglement-assisted quantum communication complexity

abstract:
  We prove a general lower bound on the bounded-error entanglement-assisted
quantum communication complexity of Boolean functions. The bound is based on
the concept that any classical or quantum protocol to evaluate a function on
distributed inputs can be turned into a quantum communication protocol. As an
application of this bound, we give a very simple proof of the statement that
almost all Boolean functions on n+n bits have linear communication complexity,
even in the presence of unlimited entanglement.


title: Generalized Bell inequality for mixed states with variable constraints

abstract:
  In this paper, we present a generalized Bell inequality for mixed states. The
distinct characteristic is that the inequality has variable bound depending on
the decomposition of the density matrix. The inequality has been shown to be
more refined than the previous Bell inequality. It is possible that a separable
mixed state can violate the Bell inequality.


title: Controlled Single-Photon Emission from a Single Trapped Two-Level Atom

abstract:
  By illuminating an individual rubidium atom stored in a tight optical tweezer
with short resonant light pulses, we create an efficient triggered source of
single photons with a well-defined polarization. The measured intensity
correlation of the emitted light pulses exhibits almost perfect antibunching.
Such a source of high rate, fully controlled single photon pulses has many
potential applications for quantum information processing.


title: Quantum Error Correcting Subsystem Codes From Two Classical Linear Codes

abstract:
  The essential insight of quantum error correction was that quantum
information can be protected by suitably encoding this quantum information
across multiple independently erred quantum systems. Recently it was realized
that, since the most general method for encoding quantum information is to
encode it into a subsystem, there exists a novel form of quantum error
correction beyond the traditional quantum error correcting subspace codes.
These new quantum error correcting subsystem codes differ from subspace codes
in that their quantum correcting routines can be considerably simpler than
related subspace codes. Here we present a class of quantum error correcting
subsystem codes constructed from two classical linear codes. These codes are
the subsystem versions of the quantum error correcting subspace codes which are
generalizations of Shor's original quantum error correcting subspace codes. For
every Shor-type code, the codes we present give a considerable savings in the
number of stabilizer measurements needed in their error recovery routines.


title:
Possible considerations to teleport fermionic particles via studying on
  teleportation of two-particle state with a four fermionic-particle pure
  entangled state

abstract:
  In this paper we have firstly recapped some evolutionary debates on
conceptual quantum information matters, followed by an experiment done by
Lamei-Rashti and his collaborator, by which the bell inequality on p-p
scattering is violated. We then, by using the goal of his experiment, thought
to arrange POVM formalism for a possible teleportation of two particle states,
via nuclear magnetic spin of four entangled hydrogen like atoms.