title: An information-theoretic security proof for QKD protocols

abstract:
  We present a new technique for proving the security of quantum key
distribution (QKD) protocols. It is based on direct information-theoretic
arguments and thus also applies if no equivalent entanglement purification
scheme can be found. Using this technique, we investigate a general class of
QKD protocols with one-way classical post-processing. We show that, in order to
analyze the full security of these protocols, it suffices to consider
collective attacks. Indeed, we give new lower and upper bounds on the
secret-key rate which only involve entropies of two-qubit density operators and
which are thus easy to compute. As an illustration of our results, we analyze
the BB84, the six-state, and the B92 protocol with one-way error correction and
privacy amplification. Surprisingly, the performance of these protocols is
increased if one of the parties adds noise to the measurement data before the
error correction. In particular, this additional noise makes the protocols more
robust against noise in the quantum channel.


title: Quantum erasure within the Optical Stern-Gerlach Model

abstract:
  In the optical Stern-Gerlach effect the two branches in which the incoming
atomic packet splits up can display interference pattern outside the cavity
when a field measurement is made which erases the which-way information on the
quantum paths the system can follow. On the contrary, the mere possibility to
acquire this information causes a decoherence effect which cancels out the
interference pattern. A phase space analysis is also carried out to investigate
on the negativity of the Wigner function and on the connection between its
covariance matrix and the distinguishability of the quantum paths.


title: Experimental quantum communication complexity

abstract:
  We prove that the fidelity of two exemplary communication complexity
protocols, allowing for an N-1 bit communication, can be exponentially improved
by N-1 (unentangled) qubit communication. Taking into account, for a fair
comparison, all inefficiencies of state-of-the-art set-up, the experimental
implementation outperforms the best classical protocol, making it the candidate
for multi-party quantum communication applications.


title:
Decoherence-Free Emergence of Macroscopic Local Realism for entangled
  photons in a cavity

abstract:
  We investigate the influence of environmental noise on polarization entangled
light generated by parametric emission in a cavity. By adopting a recently
developed separability criterion, we show that: i) self-stimulation may
suppress the detrimental influence of noise on entanglement; ii) when
self-stimulation becomes effective, a classical model of parametric emission
incorporating noise provides the same results of quantum theory for the
expectation values involved in the separability criterion. Moreover we show
that, in the macroscopic limit, it is impossible to observe violations of local
realism with measurements of $n$-particle correlations, whatever n but finite.
These results provide an interesting example of the emergence of macroscopic
local realism in the presence of strong entanglement even in the absence of
decoherence.


title: A large family of quantum weak coin-flipping protocols

abstract:
  Each classical public-coin protocol for coin flipping is naturally associated
with a quantum protocol for weak coin flipping. The quantum protocol is
obtained by replacing classical randomness with quantum entanglement and by
adding a cheat detection test in the last round that verifies the integrity of
this entanglement. The set of such protocols defines a family which contains
the protocol with bias 0.192 previously found by the author, as well as
protocols with bias as low as 1/6 described herein. The family is analyzed by
identifying a set of optimal protocols for every number of messages. In the
end, tight lower bounds for the bias are obtained which prove that 1/6 is
optimal for all protocols within the family.


title: Quasirelativistic quasilocal finite wave-function collapse model

abstract:
  A Markovian wave function collapse model is presented where the
collapse-inducing operator, constructed from quantum fields, is a manifestly
covariant generalization of the mass density operator utilized in the
nonrelativistic Continuous Spontaneous Localization (CSL) wave function
collapse model. However, the model is not Lorentz invariant because two such
operators do not commute at spacelike separation, i.e., the time-ordering
operation in one Lorentz frame, the "preferred" frame, is not the time-ordering
operation in another frame. However, the characteristic spacelike distance over
which the commutator decays is the particle's Compton wavelength so, since the
commutator rapidly gets quite small, the model is "almost" relativistic. This
"QRCSL" model is completely finite: unlike previous, relativistic, models, it
has no (infinite) energy production from the vacuum state.
  QRCSL calculations are given of the collapse rate for a single free particle
in a superposition of spatially separated packets, and of the energy production
rate for any number of free particles: these reduce to the CSL rates if the
particle's Compton wavelength is small compared to the model's distance
parameter. One motivation for QRCSL is the realization that previous
relativistic models entail excitation of nuclear states which exceeds that of
experiment, whereas QRCSL does not: an example is given involving quadrupole
excitation of the $^{74}$Ge nucleus.


title: A geometric approach to quantum circuit lower bounds

abstract:
  What is the minimal size quantum circuit required to exactly implement a
specified n-qubit unitary operation, U, without the use of ancilla qubits? We
show that a lower bound on the minimal size is provided by the length of the
minimal geodesic between U and the identity, I, where length is defined by a
suitable Finsler metric on SU(2^n). The geodesic curves of such a metric have
the striking property that once an initial position and velocity are set, the
remainder of the geodesic is completely determined by a second order
differential equation known as the geodesic equation. This is in contrast with
the usual case in circuit design, either classical or quantum, where being
given part of an optimal circuit does not obviously assist in the design of the
rest of the circuit. Geodesic analysis thus offers a potentially powerful
approach to the problem of proving quantum circuit lower bounds. In this paper
we construct several Finsler metrics whose minimal length geodesics provide
lower bounds on quantum circuit size, and give a procedure to compute the
corresponding geodesic equation. We also construct a large class of solutions
to the geodesic equation, which we call Pauli geodesics, since they arise from
isometries generated by the Pauli group. For any unitary U diagonal in the
computational basis, we show that: (a) provided the minimal length geodesic is
unique, it must be a Pauli geodesic; (b) finding the length of the minimal
Pauli geodesic passing from I to U is equivalent to solving an exponential size
instance of the closest vector in a lattice problem (CVP); and (c) all but a
doubly exponentially small fraction of such unitaries have minimal Pauli
geodesics of exponential length.


title: Nondispersive two-electron Trojan wave packets

abstract:
  We demonstrate the existence of stable non-dispersing two-electron
Trojan-like wave packets in the helium atom in combined magnetic and circularly
polarized microwave fields. These packets follow circular orbits and we show
that they can also exist in quantum dots. Classically the two electrons follow
trajectories which resemble orbits discovered by Langmuir and which were used
in attempts at a Bohr-like quantization of the helium atom. Eigenvalues of a
generalized Hessian matrix are computed to investigate the classical stability
of these states. Diffusion Monte Carlo simulations demonstrate the quantum
stability of these two-electron wave packets in the helium atom and quantum dot
helium with an impurity center.


title: NP-complete Problems and Physical Reality

abstract:
  Can NP-complete problems be solved efficiently in the physical universe? I
survey proposals including soap bubbles, protein folding, quantum computing,
quantum advice, quantum adiabatic algorithms, quantum-mechanical
nonlinearities, hidden variables, relativistic time dilation, analog computing,
Malament-Hogarth spacetimes, quantum gravity, closed timelike curves, and
"anthropic computing." The section on soap bubbles even includes some
"experimental" results. While I do not believe that any of the proposals will
let us solve NP-complete problems efficiently, I argue that by studying them,
we can learn something not only about computation but also about physics.


title: Dwell times for transmission and reflection

abstract:
  As was shown in quant-ph/0405028, the state of a tunneling particle can be
uniquely presented as a coherent superposition of two states to describe
alternative sub-processes, transmission and reflection. In this paper, on the
basis of the stationary wave functions for these sub-processes, we give new
definitions of the dwell times for transmission and reflection. In the case of
rectangular potential barriers the dwell times are obtained explicitly. In
contrast with the well-known B\"{u}ttiker's dwell-time, our dwell time for
transmission increases exponentially, for the under-barrier tunneling, with
increasing the barrier's width. By our approach the well-known Hartman effect
is rather an artifact resulted from an improper interpretation of the
wave-packet tunneling and experimental data, but not a real physical effect
accompanying the tunneling phenomenon.


title: Riccati nonhermiticity with application to the Morse potential

abstract:
  A supersymmetric one-dimensional matrix procedure similar to relationships of
the same type between Dirac and Schrodinger equations in particle physics is
described at the general level. By this means we are able to introduce a
nonhermitic Hamiltonian having the imaginary part proportional to the solution
of a Riccati equation of the Witten type. The procedure is applied to the
exactly solvable Morse potential introducing in this way the corresponding
nonhermitic Morse problem. A possible application is to molecular diffraction
in evanescent waves over nanostructured surfaces


title:
Quantum Zeno and anti-Zeno effects in an Unstable System with Two Bound
  State

abstract:
  We analyze the experimental observations reported by Fischer et. al. [in
Phys. Rev. Lett. 87, 040402 (2001)] by considering a system of coupled unstable
bound quantum states A and B. The state B is coupled to a set of continuum
states C. We investigate the time evolution of A when it decays into C via B,
and find that frequent measurements on A leads to both the quantum Zeno effect
and the anti-Zeno effects depending on the frequency of measurements. We show
that it is the presence of B which allows for the anti-Zeno effect.


title:
A minimal coupling method for investigating one dimensional dissipative
  quantum systems

abstract:
  Quantum mechanics of a general one dimensional dissipative system
investigated by it's coupling to a Klein-Gordon field as the environment using
a minimal coupling method. Heisenberg equation for such a dissipative system
containing a dissipative term proportional to velocity obtained. As an example,
quantum dynamics of a damped harmonic oscillator as the prototype of some
important one dimensional dissipative models investigated consistently. Some
transition probabilities indicating the way energy flows between the subsystems
obtained.


title:
Scaling of running time of quantum adiabatic algorithm for propositional
  satisfiability

abstract:
  We numerically study quantum adiabatic algorithm for the propositional
satisfiability. A new class of previously unknown hard instances is identified
among random problems. We numerically find that the running time for such
instances grows exponentially with their size. Worst case complexity of quantum
adiabatic algorithm therefore seems to be exponential.


title: Screened Casimir forces

abstract:
  We demonstrate that a very recently obtained formula for the force on a slab
in a material planar cavity based on the calculation of the vacuum Lorentz
force [C. Raabe and D.-G. Welsch, Phys. Rev. A 71, 013814 (2005)] describes a
(medium) screened Casimir force and, in addition to it, a medium-assisted
force. The latter force also describes the force on the cavity medium. For
dilute media, it implies the atom-mirror interaction of the Casimir-Polder type
at large and of the Coulomb type at small atom-mirror distances of which the
sign is insensitive to the polarizability type (electric or magnetic) of the
atom.


title: Quantum kinetic energy densities: An operational approach

abstract:
  We propose and investigate a procedure to measure, at least in principle, a
positive quantum version of the local kinetic energy density. This procedure is
based, under certain idealized limits, on the detection rate of photons emitted
by moving atoms which are excited by a localized laser beam. The same type of
experiment, but in different limits, can also provide other non
positive-definite versions of the kinetic energy density. A connection with
quantum arrival time distributions is discussed.


title: Image transfer through a chaotic channel by intensity correlations

abstract:
  The three-wave mixing processes in a second-order nonlinear medium can be
used for imaging protocols, in which an object field is injected into the
nonlinear medium together with a reference field and an image field is
generated. When the reference field is chaotic, the image field is also chaotic
and does not carry any information about the object. We show that a clear image
of the object be extracted from the chaotic image field by measuring the
spatial intensity correlations between this field and one Fourier component of
the reference. We experimentally verify this imaging protocol in the case of
frequency downconversion.


title: Quantum information processing with noisy cluster states

abstract:
  We provide an analysis of basic quantum information processing protocols
under the effect of intrinsic non-idealities in cluster states. These
non-idealities are based on the introduction of randomness in the entangling
steps that create the cluster state and are motivated by the unavoidable
imperfections faced in creating entanglement using condensed-matter systems.
Aided by the use of an alternative and very efficient method to construct
cluster state configurations, which relies on the concatenation of fundamental
cluster structures, we address quantum state transfer and various fundamental
gate simulations through noisy cluster states. We find that a winning strategy
to limit the effects of noise, is the management of small clusters processed
via just a few measurements. Our study also reinforces recent ideas related to
the optical implementation of a one-way quantum computer.


title:
Class of bipartite quantum states satisfying the original Bell
  inequality

abstract:
  In a general setting, we introduce a new bipartite state property sufficient
for the validity of the perfect correlation form of the original Bell
inequality for any three bounded quantum observables. A bipartite quantum state
with this property does not necessarily exhibit perfect correlations. The class
of bipartite states specified by this property includes both separable and
nonseparable states. We prove analytically that, for any dimension d>2, every
Werner state, separable or nonseparable, belongs to this class.


title:
Entangling capacity and distinguishability of two-qubit unitary
  operators

abstract:
  We prove that the entangling capacity of a two-qubit unitary operator without
local ancillas, both with and without the restriction to initial product
states, as quantified by the maximum attainable concurrence, is directly
related to the distinguishability of a closely related pair of two-qubit
unitary operators. These operators are the original operator transformed into
its canonical form and the adjoint of this canonical form. The
distinguishability of these operators is quantified by the minimum overlap of
the output states over all possible input probe states. The entangling capacity
of the original unitary operator is therefore directly related to the degree of
non-Hermiticity of its canonical form, as quantified in an operationally
satisfactory manner in terms of the extent to which it can be distinguished, by
measurement, from its adjoint. Furthermore, the maximum entropy of
entanglement, again without local ancillas, that a given two-qubit unitary
operator can generate, is found to be closely related to the classical
capacities of certain quantum channels.


title:
Reply to "Comment on 'Separability of quantum states and the violation
  of Bell-type inequalities'"

abstract:
  It is stated by C. Simon, quant-ph/0410032, that the definition of
"classicality" used in quant-ph/0310116 is "much narrower than Bell's concept
of local hidden variables" and that, in the separable quantum case, the
validity of the perfect correlation form of the original Bell inequality is
necessarily linked with "the assumption of perfect correlations if the same
(quantum) observable is measured on both sides". Here, I prove that these and
other statements in quant-ph/0410032 are misleading.


title:
All-Versus-Nothing Violation of Local Realism by Two-Photon,
  Four-Dimensional Entanglement

abstract:
  We develop and exploit a source of two-photon four-dimensional entanglement
to report the first two-particle all-versus-nothing test of local realism with
a linear optics setup, but without resorting to a non-contextuality ssumption.
Our experimental results are in well agreement with quantum mechanics while in
extreme contradiction with local realism. Potential applications of our
experiment are briefly discussed.


title: Constructions and performance of classes of quantum LDPC codes

abstract:
  Two methods for constructing quantum LDPC codes are presented. We explain how
to overcome the difficulty of finding a set of low weight generators for the
stabilizer group of the code. Both approaches are based on some graph
representation of the generators of the stabilizer group and on simple local
rules that ensure commutativity. A message passing algorithm for generic
quantum LDPC codes is also introduced. Finally, we provide two specific
examples of quantum LDPC codes of rate 1/2 obtained by our methods, together
with a numerical simulation of their performance over the depolarizing channel.


title: Decoherence time in self-induced decoherence

abstract:
  A general method for obtaining the decoherence time in self-induced
decoherence is presented. In particular, it is shown that such a time can be
computed from the poles of the resolvent or of the initial conditions in the
complex extension of the Hamiltonian's spectrum. Several decoherence times are
estimated: $10^{-13}-$ $10^{-15}s$ for microscopic systems, and
$10^{-37}-10^{-39}s$ for macroscopic bodies. For the particular case of a
thermal bath, our results agree with those obtained by the einselection
(environment-induced decoherence) approach.


title:
Mathematical framework for simulation of quantum fields in complex
  interferometers using the two-photon formalism

abstract:
  We present a mathematical framework for simulation of optical fields in
complex gravitational-wave interferometers. The simulation framework uses the
two-photon formalism for optical fields and includes radiation pressure
effects, an important addition required for simulating signal and noise fields
in next-generation interferometers with high circulating power. We present a
comparison of results from the simulation with analytical calculation and show
that accurate agreement is achieved.


title: Partial positive scaling transform: a separability criterion

abstract:
  The problem of constructing a necessary and sufficient condition for
establishing the separability of continuous variable systems is revisited.
Simon [R. Simon, Phys. Rev. Lett. 84, 2726 (2000)] pointed out that such a
criterion may be constructed by drawing a parallel between the Peres' partial
transpose criterion for finite dimensional systems and partial time reversal
transformation for continuous variable systems. We generalize the partial time
reversal transformation to a partial scaling transformation and re-examine the
problem using a tomographic description of the continuous variable quantum
system. The limits of applicability of the entanglement criteria obtained from
partial scaling and partial time reversal are explored.


title: Nonadiabatic Geometric Quantum Computation Using A Single-loop Scenario

abstract:
  A single-loop scenario is proposed to realize nonadiabatic geometric quantum
computation. Conventionally, a so-called multi-loop approach is used to remove
the dynamical phase accumulated in the operation process for geometric quantum
gates. More intriguingly, we here illustrate in detail how to use a special
single-loop method to remove the dynamical phase and thus to construct a set of
universal quantum gates based on the nonadiabatic geometric phase shift. The
present scheme is applicable to NMR systems and may be feasible in other
physical systems.


title: Quantum bit string sealing

abstract:
  Though it was proven that secure quantum sealing of a single classical bit is
impossible in principle, here we propose an unconditionally secure quantum
sealing protocol which seals a classical bit string. Any reader can obtain each
bit of the sealed string with an arbitrarily small error rate, while reading
the string is detectable. The protocol is simple and easy to be implemented.
The possibility of using this protocol to seal a single bit in practical is
also discussed.


title:
Mean king's problem with mutually unbiased bases and orthogonal Latin
  squares

abstract:
  The mean king's problem with maximal mutually unbiased bases (MUB's) in
general dimension d is investigated. It is shown that a solution of the problem
exists if and only if the maximal number (d+1) of orthogonal Latin squares
exists. This implies that there is no solution in d=6 or d=10 dimensions even
if the maximal number of MUB's exists in these dimensions.


title: Information transmission through lossy bosonic memory channels

abstract:
  We study the information transmission through a quantum channel, defined over
a continuous alphabet and losing its energy en route, in presence of correlated
noise among different channel uses. We then show that entangled inputs improve
the rate of transmission of such a channel.


title: Helstrom Theorem by No-Signaling Condition

abstract:
  We prove a special case of Helstrom theorem by using no-signaling condition
in the special theory of relativity that faster-than-light communication is
impossible.


title:
Comment on "A simple experiment to test Bell's inequality", J.-M.
  Vigoureux

abstract:
  In the above paper, it is claimed that with a particular use of the Bell
inequality a simple single photon experiment could be performed to show the
impossibility of any deterministic hidden variable theory in quantum optics. A
careful analysis of the concept of probability for hidden variables and a
detailed discussion of the hidden variable model of de Broglie-Bohm show that
the reasoning and main conclusion of this paper are not correct.


title:
Coupled oscillators, entangled oscillators, and Lorentz-covariant
  harmonic oscillators

abstract:
  Other than scattering problems where perturbation theory is applicable, there
are basically two ways to solve problems in physics. One is to reduce the
problem to harmonic oscillators, and the other is to formulate the problem in
terms of two-by-two matrices. If two oscillators are coupled, the problem
combines both two-by-two matrices and harmonic oscillators. This method then
becomes a powerful research tool to cover many different branches of physics.
Indeed, the concept and methodology in one branch of physics can be translated
into another through the common mathematical formalism. Coupled oscillators
provide clear illustrative examples for some of the current issues in physics,
including entanglement and Feynman's rest of the universe. In addition, it is
noted that the present form of quantum mechanics is largely a physics of
harmonic oscillators. Special relativity is the physics of the Lorentz group
which can be represented by the group of two-by-two matrices commonly called
$SL(2,c)$. Thus the coupled harmonic oscillators can play the role of combining
quantum mechanics with special relativity. It is therefore possible to relate
the current issues of physics to the Lorentz-covariant formulation of quantum
mechanics.


title:
Simple examples of position-momentum correlated Gaussian free-particle
  wavepackets in one-dimension with the general form of the time-dependent
  spread in position

abstract:
  We provide simple examples of closed-form Gaussian wavepacket solutions of
the free-particle Schrodinger equation in one dimension which exhibit the most
general form of the time-dependent spread in position, namely (Delta x_t)^2 =
(Delta x_0)^2 + At + (Delta p_0)^2t^2/m^2, where A = <(xhat - <xhat>_0)(phat -
<phat>_0) + (phat - <phat>_0)(xhat - <xhat>_0)>_0 contains information on the
position-momentum correlation structure of the initial wave packet. We exhibit
straightforward examples corresponding to squeezed states, as well as
quasi-classical cases, for which A < 0 so that the position spread can (at
least initially) decrease in time because of such correlations. We discuss how
the initial correlations in these examples can be dynamically generated (at
least conceptually) in various bound state systems. Finally, we focus on
providing different ways of visualizing the x-p correlations present in these
cases, including the time-dependent distribution of kinetic energy and the use
of the Wigner quasi-probability distribution. We discuss similar results, both
for the time-dependent Delta x_t and special correlated solutions, for the case
of a particle subject to a uniform force.


title: Higher-order supersymmetric quantum mechanics

abstract:
  We review the higher-order supersymmetric quantum mechanics (H-SUSY QM),
which involves differential intertwining operators of order greater than one.
The iterations of first-order SUSY transformations are used to derive in a
simple way the higher-order case. The second order technique is addressed
directly, and through this approach unexpected possibilities for designing
spectra are uncovered. The formalism is applied to the harmonic oscillator: the
corresponding H-SUSY partner Hamiltonians are ruled by polynomial Heisenberg
algebras which allow a straight construction of the coherent states.


title: Forms of Quantum Nonseparability and Related Philosophical Consequences

abstract:
  Standard quantum mechanics unquestionably violates the separability principle
that classical physics (be it point-like analytic, statistical, or
field-theoretic) accustomed us to consider as valid. In this paper, quantum
nonseparability is viewed as a consequence of the Hilbert-space quantum
mechanical formalism, avoiding thus any direct recourse to the ramifications of
Kochen-Specker's argument or Bell's inequality. Depending on the mode of
assignment of states to physical systems (unit state vectors versus
non-idempotent density operators) we distinguish between strong/relational and
weak/deconstructional forms of quantum nonseparability. The origin of the
latter is traced down and discussed at length, whereas its relation to the all
important concept of potentiality in forming a coherent picture of the puzzling
entangled interconnections among spatially separated systems is also
considered. Finally, certain philosophical consequences of quantum
nonseparability concerning the nature of quantum objects, the question of
realism in quantum mechanics, and possible limitations in revealing the actual
character of physical reality in its entirety are explored.


title: Supersymmetric Models for Fermions on a Lattice

abstract:
  We investigate the large-N behaviour of simple examples of supersymmetric
interactions for fermions on a lattice. Witten's supersymmetric quantum
mechanics and the BCS model appear just as two different aspects of one and the
same model. For the BCS model, supersymmetry is only respected in a coherent
superposition of Bogoliubov states. In this coherent superposition mesoscopic
observables show better stability properties than in a Bogoliubov state.


title:
Thresholds for Linear Optics Quantum Computing with Photon Loss at the
  Detectors

abstract:
  We calculate the error threshold for the linear optics quantum computing
proposal by Knill, Laflamme and Milburn [Nature 409, pp. 46--52 (2001)] under
an error model where photon detectors have efficiency <100% but all other
components -- such as single photon sources, beam splitters and phase shifters
-- are perfect and introduce no errors. We make use of the fact that the error
model induced by the lossy hardware is that of an erasure channel, i.e., the
error locations are always known. Using a method based on a Markov chain
description of the error correction procedure, our calculations show that, with
the 7 qubit CSS quantum code, the gate error threshold for fault tolerant
quantum computation is bounded below by a value between 1.78% and 11.5%
depending on the construction of the entangling gates.


title: Quantum NOT Operation and Integrability in Two-Level Systems

abstract:
  We demonstrate the surprising integrability of the classical Hamiltonian
associated to a spin 1/2 system under periodic external fields. The one-qubit
rotations generated by the dynamical evolution is, on the one hand, close to
that of the rotating wave approximation (RWA), on the other hand to two
different ``average'' systems, according to whether a certain parameter is
small or large. Of particular independent interest is the fact that both the
RWA and the averaging theorem are seen to hold well beyond their expected
region of validity. Finally we determine conditions for the realization of the
quantum NOT operation by means of classical stroboscopic maps.


title:
Usefulness of classical communication for local cloning of entangled
  states

abstract:
  We solve the problem of the optimal cloning of pure entangled two-qubit
states with a fixed degree of entanglement using local operations and classical
communication. We show, that amazingly, classical communication between the
parties can improve the fidelity of local cloning if and only if the initial
entanglement is higher than a certain critical value. It is completely useless
for weakly entangled states. We also show that bound entangled states with
positive partial transpose are not useful as a resource to improve the best
local cloning fidelity.


title:
Optimal Control-Based Efficient Synthesis of Building Blocks of Quantum
  Algorithms Seen in Perspective from Network Complexity towards Time
  Complexity

abstract:
  In this paper, we demonstrate that optimal control algorithms can be used to
speed up the implementation of modules of quantum algorithms or quantum
simulations in networks of coupled qubits. The gain is most prominent in
realistic cases, where the qubits are not all mutually coupled. Thus the
shortest times obtained depend on the coupling topology as well as on the
characteristicratio of the time scales for local controls {\em vs} non-local
({\em i.e.} coupling) evolutions in the specific experimental setting. Relating
these minimal times to the number of qubits gives the tightest known upper
bounds to the actual time complexity of the quantum modules. As will be shown,
time complexity is a more realistic measure of the experimental cost than the
usual gate complexity.
  In the limit of fast local controls (as {\em e.g.} in NMR), time-optimised
realisations are shown for the quantum Fourier transform (QFT) and the multiply
controlled {\sc not}-gate ({\sc c$^{n-1}$not}) in various coupling topologies
of $n$ qubits. The speed-ups are substantial: in a chain of six qubits the
quantum Fourier transform so far obtained by optimal control is more than eight
times faster than the standard decomposition into controlled phase, Hadamard
and {\sc swap} gates, while the {\sc c$^{n-1}$not}-gate for completely coupled
network of six qubits is nearly seven times faster.


title:
Kepler problem in Dirac theory for a particle with position-dependent
  mass

abstract:
  Exact solution of Dirac equation for a particle whose potential energy and
mass are inversely proportional to the distance from the force centre has been
found. The bound states exist provided the length scale $a$ which appears in
the expression for the mass is smaller than the classical electron radius
$e^2/mc^2$. Furthermore, bound states also exist for negative values of $a$
even in the absence of the Coulomb interaction. Quasirelativistic expansion of
the energy has been carried out, and a modified expression for the fine
structure of energy levels has been obtained. The problem of kinetic energy
operator in the Schr\"odinger equation is discussed for the case of
position-dependent mass. In particular, we have found that for highly excited
states the mutual ordering of the inverse mass and momentum operator in the
non-relativistic theory is not important.


title: Quantum Channels with Memory

abstract:
  We present a general model for quantum channels with memory, and show that it
is sufficiently general to encompass all causal automata: any quantum process
in which outputs up to some time t do not depend on inputs at times t' > t can
be decomposed into a concatenated memory channel. We then examine and present
different physical setups in which channels with memory may be operated for the
transfer of (private) classical and quantum information. These include setups
in which either the receiver or a malicious third party have control of the
initializing memory. We introduce classical and quantum channel capacities for
these settings, and give several examples to show that they may or may not
coincide. Entropic upper bounds on the various channel capacities are given.
For forgetful quantum channels, in which the effect of the initializing memory
dies out as time increases, coding theorems are presented to show that these
bounds may be saturated. Forgetful quantum channels are shown to be open and
dense in the set of quantum memory channels.


title: Single Qubit Quantum Secret Sharing

abstract:
  We present a simple and practical protocol for the solution of a secure
multiparty communication task, the secret sharing, and its experimental
realization. In this protocol, a secret message is split among several parties
in a way that its reconstruction require the collaboration of the participating
parties. In the proposed scheme the parties solve the problem by a sequential
communication of a single qubit. Moreover we show that our scheme is equivalent
to the use of a multiparty entangled GHZ state but easier to realize and better
scalable in practical applications.


title: Motion of vortices implies chaos in Bohmian mechanics

abstract:
  Bohmian mechanics is a causal interpretation of quantum mechanics in which
particles describe trajectories guided by the wave function. The dynamics in
the vicinity of nodes of the wave function, usually called vortices, is regular
if they are at rest. However, vortices generically move during time evolution
of the system. We show that this movement is the origin of chaotic behavior of
quantum trajectories. As an example, our general result is illustrated
numerically in the two-dimensional isotropic harmonic oscillator.


title: Complete time-dependent treatment of a three-level system

abstract:
  Both unitary evolution and the effects of dissipation and decoherence for a
general three-level system are of widespread interest in quantum optics,
molecular physics, and elsewhere. A previous paper presented a technique for
solving the time-dependent operator equations involved but under certain
restrictive conditions. We now extend our results to a general three-level
system with arbitrary time-dependent Hamiltonians and Lindblad operators.
Analytical handling of the SU(3) algebra of the eight operators involved leaves
behind a set of coupled first-order differential equations for classical
functions. Solution of this set gives a complete solution of the quantum
problem, without having to invoke rotating-wave or other approximations.
Numerical illustrations are given.


title: Entanglement Generation of Nearly-Random Operators

abstract:
  We study the entanglement generation of operators whose statistical
properties approach those of random matrices but are restricted in some way.
These include interpolating ensemble matrices, where the interval of the
independent random parameters are restricted, pseudo-random operators, where
there are far fewer random parameters than required for random matrices, and
quantum chaotic evolution. Restricting randomness in different ways allows us
to probe connections between entanglement and randomness. We comment on which
properties affect entanglement generation and discuss ways of efficiently
producing random states on a quantum computer.


title: Quantum Indeterminism and First Passage Random Walks in Hilbert Space

abstract:
  We propose a new model for a measurement of a characteristic of a microscopic
quantum state by a large system that selects stochastically the different
eigenstates with appropriate quantum weights. Unlike previous works which
formulate a modified Schr\"odinger equation or an explicit modified
Hamiltonian, or more complicated mechanisms for reduction and decoherence to
introduce transition to classical stochasticity, we propose the novel use of
couplings to the environment, and random walks in the product Hilbert space of
the combined system, with first passage stopping rules, which seem intuitively
simple, as quantum weights and related stochasticity is a commonality that must
be preserved under the widest range of applications, independent of the
measured quantity and the specific properties of the measuring device.


title:
Fault-tolerant quantum repeaters with minimal physical resources, and
  implementations based on single photon emitters

abstract:
  We analyze a novel method that uses fixed, minimal physical resources to
achieve generation and nested purification of quantum entanglement for quantum
communication over arbitrarily long distances, and discuss its implementation
using realistic photon emitters and photonic channels. In this method, we use
single photon emitters with two internal degrees of freedom formed by an
electron spin and a nuclear spin to build intermediate nodes in a quantum
channel. State-selective fluorescence is used for probabilistic entanglement
generation between electron spins in adjacent nodes. We analyze in detail
several approaches which are applicable to realistic, homogeneously broadened
single photon emitters. Furthermore, the coupled electron and nuclear spins can
be used to efficiently implement entanglement swapping and purification. We
show that these techniques can be combined to generate high-fidelity
entanglement over arbitrarily long distances. We present a specific protocol
that functions in polynomial time and tolerates percent-level errors in
entanglement fidelity and local operations. The scheme has the lowest
requirements on physical resources of any current scheme for fully
fault-tolerant quantum repeaters.


title:
Electron interference in mesoscopic devices in the presence of
  nonclassical electromagnetic fields

abstract:
  The interaction of mesoscopic interference devices with nonclassical
electromagnetic fields is studied. The external quantum fields induce a phase
factor on the electric charges. This phase factor, which is a generalization of
the standard Aharonov-Bohm phase factor, is in the case of nonclassical
electromagnetic fields a quantum mechanical operator. Its expectation value
depends on the density matrix describing the nonclassical photons and
determines the interference. Several examples are discussed, which show that
the quantum noise of the nonclassical photons destroys slightly the electron
interference fringes. An interesting application arises in the context of
distant electron interference devices, irradiated with entangled photons. In
this case the interfering electrons in the two devices become entangled. The
same ideas are applied in the context of SQUID rings irradiated with
nonclassical electromagnetic fields. It is shown that the statistics of the
Cooper pairs tunneling through the Josephson junction depend on the statistics
of the photons.


title: Entanglement of topological phase factors

abstract:
  The topological phase factor induced on interfering electrons by external
quantum electromagnetic fields has been studied. Two and three electron
interference experiments inside distant cavities are considered and the
influence of correlated photons on the phase factors is investigated. It is
shown that the classical or quantum correlations of the irradiating photons are
transferred to the topological phases. The effect is quantified in terms of
Weyl functions for the density operators of the photons and illustrated with
particular examples. The scheme employs the generalized phase factor as a
mechanism for information transfer from the photons to the electric charges. In
this sense, the scheme may be useful in the context of flying qubits
(corresponding to the photons) and stationary qubits (electrons), and the
conversion from one type to the other.


title: Microscopic theory of the Casimir effect

abstract:
  Based on the photon-exciton Hamiltonian a microscopic theory of the Casimir
problem for dielectrics is developed. Using well-known many-body techniques we
derive a perturbation expansion for the energy which is free from divergences.
In the continuum limit we turn off the interaction at a distance smaller than a
cut-off distance $a$ to keep the energy finite. We will show that the
macroscopic theory of the Casimir effect with hard boundary conditions is not
well defined because it ignores the finite distance between the atoms, hence is
including infinite self-energy contributions. Nevertheless for disconnected
bodies the latter do not contribute to the force between the bodies. The
Lorentz-Lorenz relation for the dielectric constant that enters the force is
deduced in our microscopic theory without further assumptions.
  The photon Green's function can be calculated from a Dyson type integral
equation. The geometry of the problem only enters in this equation through the
region of integration which is equal to the region occupied by the dielectric.
The integral equation can be solved exactly for various plain and spherical
geometries without using boundary conditions. This clearly shows that the
Casimir force for dielectrics is due to the forces between the atoms.
  Convergence of the perturbation expansion and the metallic limit are
discussed. We conclude that for any dielectric function the transverse electric
(TE) mode does not contribute to the zero-frequency term of the Casimir force.


title: Continuous-wave phase-sensitive parametric image amplification

abstract:
  We study experimentally parametric amplification in the continuous regime
using a transverse-degenerate type-II Optical Parametric Oscillator operated
below threshold. We demonstrate that this device is able to amplify either in
the phase insensitive or phase sensitive way first a single mode beam, then a
multimode image. Furthermore the total intensities of the amplified image
projected on the signal and idler polarizations are shown to be correlated at
the quantum level.


title: Optimum unambiguous discrimination of two mixed quantum states

abstract:
  We investigate generalized measurements, based on positive-operator-valued
measures, and von Neumann measurements for the unambiguous discrimination of
two mixed quantum states that occur with given prior probabilities. In
particular, we derive the conditions under which the failure probability of the
measurement can reach its absolute lower bound, proportional to the fidelity of
the states. The optimum measurement strategy yielding the fidelity bound of the
failure probability is explicitly determined for a number of cases. One example
involves two density operators of rank d that jointly span a 2d-dimensional
Hilbert space and are related in a special way. We also present an application
of the results to the problem of unambiguous quantum state comparison,
generalizing the optimum strategy for arbitrary prior probabilities of the
states.


title: A role of topology and quantum information in physics

abstract:
  The role of topology in QIS with physical connection to the noncommutativity,
discretization,supersymmetry, entanglement, nonseparability and CP violation in
physics is discussed.


title: High Fidelity Single Qubit Operations using Pulsed EPR

abstract:
  Systematic errors in spin rotation operations using simple RF pulses place
severe limitations on the usefulness of the pulsed magnetic resonance methods
in quantum computing applications. In particular, the fidelity of quantum logic
operations performed on electron spin qubits falls well below the threshold for
the application of quantum algorithms. Using three independent techniques, we
demonstrate the use of composite pulses to improve this fidelity by several
orders of magnitude. The observed high-fidelity operations are limited by pulse
phase errors, but nevertheless fall within the limits required for the
application of quantum error correction.


title: Efficient quantum computation with probabilistic quantum gates

abstract:
  With a combination of the quantum repeater and the cluster state approaches,
we show that efficient quantum computation can be constructed even if all the
entangling quantum gates only succeed with an arbitrarily small probability
$p$. The required computational overhead scales efficiently both with $1/p$ and
$n$, where $n$ is the number of qubits in the computation. This approach
provides an efficient way to combat noise in a class of quantum computation
implementation schemes, where the dominant noise leads to probabilistic
signaled errors with an error probability $1-p$ far beyond any threshold
requirement.


title:
Characterizing entanglement by momentum-jump in the frustrated
  Heisenberg ring at quantum phase transition

abstract:
  We study the pairwise concurrences, a measure of entanglement, of the ground
states for the frustrated Heisenberg ring to explore the relation between
entanglement and quantum phase transition associated with the momentum jump.
The groundstate concurrences between any two sites are obtained analytically
and numerically. It shows that the summation of all possible pairwise
concurrences is an appropriate candidate to depict the phase transition. We
also investigate the role that the momentum takes in the jump of concurrence at
the critical points. We find that an abrupt momentum change rusults in the
maximal concurrence difference of two degenerate ground states.


title: Natural multiparticle entanglement in a Fermi gas

abstract:
  We investigate multipartite entanglement in a non-interacting fermion gas, as
a function of fermion separation, starting from the many particle fermion
density matrix. We prove that all multiparticle entanglement can be built only
out of two-fermion entanglement. Although from the Pauli exclusion principle we
would always expect entanglement to decrease with fermion distance, we
surprisingly find the opposite effect for certain fermion configurations. The
von Neumann entropy is found to be proportional to the volume for a large
number of particles even when they are arbitrarily close to each other. We will
illustrate our results using different configurations of two, three, and four
fermions at zero temperature although all our results can be applied to any
temperature and any number of particles.


title:
Observation of the skin-depth effect on the Casimir force between
  metallic surfaces

abstract:
  We have performed comparative measurements of the Casimir force between a
metallic plate and a transparent sphere coated with metallic films of different
thicknesses. We have observed that, if the thickness of the coating is less
than the skin-depth of the electromagnetic modes that mostly contribute to the
interaction, the force is significantly smaller than that measured with a thick
bulk-like film. Our results provide the first direct evidence of the skin-depth
effect on the Casimir force between metallic surfaces.


title: Problems of Quantum Measurement

abstract:
  We derive the probabilities of measurement results from Schroedinger's
equation plus a definition of macroscopic as a particular kind of thermodynamic
limit. Bohr's insight that a measurement apparatus must be classical in nature
and classically describable is made precise in a mathematical sense analogous
to the procedures of classical statistical mechanics and the study of
Hamiltonian heat baths.


title: Sufficient conditions for the anti-Zeno effect

abstract:
  The ideal anti-Zeno effect means that a perpetual observation leads to an
immediate disappearance of the unstable system. We present a straightforward
way to derive sufficient conditions under which such a situation occurs
expressed in terms of the decaying states and spectral properties of the
Hamiltonian. They show, in particular, that the gap between Zeno and anti-Zeno
effects is in fact very narrow.


title:
Convex rigid cover method in studies of quantum pure-states of many
  continuous variables

abstract:
  In this paper we prove that every pure-state $\Psi ^{(N)}$ of N (N$\geqslant
3)$ continuous variables corresponds to a pair of convex rigid covers (CRCs)
structures in the continuous-dimensional Hilbert-Schmidt space. Next we
strictly define what are the partial separability and ordinary separability,
and discuss how to use CRCs to describe various separability. We discuss the
problem of the classification of $\Psi ^{(N)}$ and give a kinematical
explanation of the local unitary operations acting upon $\Psi ^{(N)}$. Thirdly,
we discuss the invariants of classes and give a possible physical explanation.


title: A self-consistent theory of liquid He-4

abstract:
  A new method is proposed for the calculation of full density matrix and
thermodynamic functions of a many-boson system. Explicit expressions are
obtained in the pair correlations approximation for an arbitrary temperature.
The theory is self-consistent in the sence that the calculated properties at
low temperatures coincide with that of Bogoliubov theory and in the
high-temperature limit lead to the results for classical non-ideal gas in the
random phase approximation. The phase transition is also revealed as a
concequence of Bose-Einstein condensation deformed by interatomic interactions.
All the final formulae are written solely via the liquid structure factor taken
as a source information instead of the interatomic potential and, therefore,
interconnect only observable quantities. This gives also a possibility to study
such a strongly non-ideal system as liquid He-4.


title: Geometry of an adiabatic passage at a level crossing

abstract:
  We discuss adiabatic quantum phenomena at a level crossing. Given a path in
the parameter space which passes through a degeneracy point, we find a
criterion which determines whether the adiabaticity condition can be satisfied.
For paths that can be traversed adiabatically we also derive a differential
equation which specifies the time dependence of the system parameters, for
which transitions between distinct energy levels can be neglected. We also
generalize the well-known geometric connections to the case of adiabatic paths
containing arbitrarily many level-crossing points and degenerate levels.


title: Landau Levels Analog to Electric Dipole

abstract:
  In this letter we study the quantum dyamics of a neutral particle in the
presence of an external magnetic field. We demonstrate in a specific
field-dipole configuration that we have a quantization similar to the Landau
Levels. We investigate this quantization motivated by the recent analysis of
Landau-Aharonov-Casher(LAC) quantization of Ericsson and Sj\"oqvist[Phys Rev. A
{\bf 65} 013607 (2001)]. The energy eigenfuction and eigenvalues are obtained.


title:
Cavity-mediated long-range interaction for fast multiqubit quantum logic
  operations

abstract:
  Interactions among qubits are essential for performing two-qubit quantum
logic operations. However, nature gives us only nearest neighbor interactions
in simple and controllable settings. Here we propose a strategy to induce
interactions among two atomic entities that are not necessarily neighbors of
each other through their common coupling with a cavity field. This facilitates
fast multiqubit quantum logic operations through a set of two-qubit operations.
The ideas presented here are applicable to various quantum computing proposals
for atom based qubits such as, trapped ions, atoms trapped in optical cavities
and optical lattices.


title: Experimental Quantum Secret Sharing and Third-Man Quantum Cryptography

abstract:
  Quantum secret sharing (QSS) is a protocol to split a message into several
parts so that no subset of parts is sufficient to read the message, but the
entire set is. In the scheme, three parties Alice, Bob and Charlie first share
a three-photon entangled state, Charlie can then force Alice and Bob to
cooperate to be able to establish the secret key with him by performing proper
polarization measurements on his photon and announcing which polarization basis
he has chosen. In a similar manner, in third-man quantum cryptography (TQC) the
third-man, Charlie, can control whether Alice and Bob can communicate in a
secure way while he has no access whatsoever on the content of the
communication between Alice and Bob. Although QSS and TQC are essential for
advanced quantum communication, the low intensity multi-photon entanglement
source has made their realization an extreme experimental challenge. Here,
exploiting a high intensity four-photon entanglement source we report an
experimental realization of QSS and TQC . In the experiment, a key of low
quantum bit error rate (QBER) 0.35% is obtained using a simple error reduction
scheme.


title: Newtonian Adiabatics Unified

abstract:
  Newtonian adiabatics is the consistent truncation of the adiabatic
approximation to second order in small velocities. To be complete it must unify
two hitherto disjoint intellectual streams in the study of adiabatic motion.
The newer stream focuses on Berry's induced vector potential, or geometric
magnetism, and Provost and Vallee's induced scalar potential, reflecting
geometry in Hilbert space. The older stream focuses on Inglis' induced inertia,
influencing the geometry of adiabatic-parameter space. Starting with the
Hamiltonian of the newer stream, unification is simple: A naive or primitive
inertia, whose inverse appears in two terms of that Hamiltonian, is replaced by
the convention-independent sum of primitive and induced inertia tensors.


title: Theorem of Levinson Via The Spectral Density

abstract:
  We deduce Levinson\'{}s theorem in non-relativistic quantum mechanics in one
dimension as a sum rule for the spectral density constructed from asymptotic
data. We assume a self-adjoint hamiltonian which guarantees completeness; the
potential needs not to be isotropic and a zero-energy resonance is
automatically taken into account. Peculiarities of this one-dimension case are
explained because of the ``critical'' character of the free case $u(x) = 0$, in
the sense that any atractive potential forms at least a bound state. We believe
this method is more general and direct than the usual one in which one proves
the theorem first for single wave modes and performs analytical continuation.


title: Minimal coupling method and the dissipative scalar field theory

abstract:
  Quantum field theory of a damped vibrating string as the simplest dissipative
scalar field investigated by its coupling with an infinit number of
Klein-Gordon fields as the environment by introducing a minimal coupling
method. Heisenberg equation containing a dissipative term proportional to
velocity obtained for a special choice of coupling function and quantum
dynamics for such a dissipative system investigated. Some kinematical relations
calculated by tracing out the environment degrees of freedom. The rate of
energy flowing between the system and it's environment obtained.


title:
Enhancing the capacity and performance of collective atomic quantum
  memory

abstract:
  Present schemes involving the quantum non-demolition interaction between
atomic samples and off-resonant light pulses allow us to store quantum
information corresponding to a single harmonic oscillator (mode) in one
multiatomic system. We discuss the possibility to involve several coherences of
each atom so that the atomic sample can store information contained in several
quantum modes. This is achieved by the coupling of different magnetic sublevels
of the relevant hyperfine level by additional Raman pulses. This technique
allows us to design not only the quantum non-demolition coupling, but also beam
splitterlike and two-mode squeezerlike interactions between light and
collective atomic spin.


title:
Quantum entanglement, indistinguishability, and the absent-minded
  driver's problem

abstract:
  The absent-minded driver's problem illustrates that probabilistic strategies
can give higher pay-offs than deterministic ones. We show that there are
strategies using quantum entangled states that give even higher pay-offs, both
for the original problem and for the generalized version with an arbitrary
number of intersections and any possible set of pay-offs.


title: More On Grover's Algorithm

abstract:
  The goals of this paper are to show the following. First, Grover's algorithm
can be viewed as a digital approximation to the analog quantum algorithm
proposed in "An Analog Analogue of a Digital Quantum Computation", by E. Farhi
and S. Gutmann, Phys.Rev. A 57, 2403 - 2406 (1998), quant-ph/9612026. We will
call the above analog algorithm the Grover-Farhi-Gutmann or GFG algorithm.
Second, the propagator of the GFG algorithm can be written as a sum-over-paths
formula and given a sum-over-path interpretation, i.e., a Feynman path
sum/integral. We will use nonstandard analysis to do this. Third, in the
semi-classical limit $\hbar\to 0$, both the Grover and the GFG algorithms
(viewed in the setting of the approximation in this paper) must run
instantaneously. Finally, we will end the paper with an open question. In
"Semiclassical Shor's Algorithm", by P. Giorda, et al, Phys. Rev.A 70, 032303
(2004), quant-ph/0303037, the authors proposed building semi-classical quantum
computers to run Shor's algorithm because the success probability of Shor's
algorithm does not change much in the semi-classical limit. We ask the open
questions: In the semi-classical limit, does Shor's algorithm have to run
instantaneously?


title: New Tales of the Mean King

abstract:
  The Mean King's problem asks to determine the outcome of a measurement that
is randomly selected from a set of complementary observables. We review this
problem and offer a combinatorial solution. More generally, we show that
whenever an affine resolvable design exists, then a state reconstruction
problem similar to the Mean King's problem can be defined and solved. As an
application of this general framework we consider a problem involving three
qubits in which the outcome of nine different measurements can be determined
without using ancillary qubits. The solution is based on a measurement derived
from Hadamard designs.


title:
Quantum Mechanics without Complex Numbers: A Simple Model for the
  Electron Wavefunction Including Spin

abstract:
  A simple real-space model for the electron wavefunction is suggested, based
on a transverse wave with helicity, rotating at mc^2/h. The mapping of the real
two-dimensional vector phasor to the complex plane permits this to satisfy the
standard time-dependent Schroedinger equation. This model is extended to
provide an intuitive physical picture of electron spin. Implications of this
model are discussed.


title:
Higher Security Thresholds for Quantum Key Distribution by Improved
  Analysis of Dark Counts

abstract:
  We discuss the potential of quantum key distribution (QKD) for long distance
communication by proposing a new analysis of the errors caused by dark counts.
We give sufficient conditions for a considerable improvement of the key
generation rates and the security thresholds of well-known QKD protocols such
as Bennett-Brassard 1984, Phoenix-Barnett-Chefles 2000, and the six-state
protocol. This analysis is applicable to other QKD protocols like Bennett 1992.
We examine two scenarios: a sender using a perfect single-photon source and a
sender using a Poissonian source.


title:
Many-body-QED perturbation theory: Connection to the Bethe-Salpeter
  equation

abstract:
  The connection between many-body theory (MBPT)--in perturbative and
non-perturbative form--and quantum-electrodynamics (QED) is reviewed for
systems of two fermions in an external field. The treatment is mainly based
upon the recently developed covariant-evolution-operator method for QED
calculations [Lindgren et al. Phys. Rep. 389, 161 (2004)], which has a
structure quite akin to that of many-body perturbation theory. At the same time
this procedure is closely connected to the S-matrix and the Green's-function
formalisms and can therefore serve as a bridge between various approaches. It
is demonstrated that the MBPT-QED scheme, when carried to all orders, leads to
a Schroedinger-like equation, equivalent to the Bethe-Salpeter (BS) equation. A
Bloch equation in commutator form that can be used for an "extended" or
quasi-degenerate model space is derived. It has the same relation to the BS
equation as has the standard Bloch equation to the ordinary Schroedinger
equation and can be used to generate a perturbation expansion compatible with
the BS equation also for a quasi-degenerate model space.


title: Quantum Computing on Lattices using Global Two-Qubit Gate

abstract:
  We study the computation power of lattices composed of two dimensional
systems (qubits) on which translationally invariant global two-qubit gates can
be performed. We show that if a specific set of 6 global two qubit gates can be
performed, and if the initial state of the lattice can be suitably chosen, then
a quantum computer can be efficiently simulated


title:
Reversible universal quantum computation within translation invariant
  systems

abstract:
  We show how to perform reversible universal quantum computation on a
translationally invariant pure state, using only global operations based on
next-neighbor interactions. We do not need not to break the translational
symmetry of the state at any time during the computation. Since the proposed
scheme fulfills the locality condition of a quantum cellular automata, we
present a reversible quantum cellular automaton capable of universal quantum
computation.


title: Quantum Floyd-Warshall Alorithm

abstract:
  Classical Floyd-Warshall algorithm is used to solve all-pairs shortest path
problem on a directed graph. The classical algorithm runs in \mathcal{O}
(V^{3}) time where V represents the number of nodes. Here we have modified the
algorithm and proposed a quantum algorithm analogous to Floyd-Warshall
algorithm which exploits the superposition principle and runs in \mathcal{O}
(Vlog_{2}V) time.


title: Medium-assisted vacuum force

abstract:
  We discuss some implications of a very recently obtained result for the force
on a slab in a planar cavity based on the calculation of the vacuum Lorentz
force [C. Raabe and D.-G. Welsch, Phys. Rev. A 71, 013814 (2005)]. We
demonstrate that, according to this formula, the total force on the slab
consists of a medium-screened Casimir force and, in addition to it, a
medium-assisted force. The sign of of the medium-assisted force is determined
solely by the properties of the cavity mirrors. In the Lifshitz configuration,
this force is proportional to 1/d at small distances and is very small compared
with the corresponding van der Waals force. At large distances, however, it is
proportional to 1/d^4 and is comparable with the Casimir force, especially for
denser media. The exponents in these power laws decrease by 1 in the case of a
thin slab. The formula for the medium-assisted force also describes the force
on a layer of the cavity medium, which has similar properties. For dilute
media, it implies an atom-mirror interaction of the Coulomb type at small and
of the Casimir-Polder type at large atom-mirror distances. For a perfectly
reflecting mirror, the latter force is effectively only three-times smaller
than the Casimir-Polder force.


title: Experimental Synchronization of Independent Entangled Photon Sources

abstract:
  Realistic linear quantum information processing necessitates the ability to
synchronously generate entangled photon pairs either at the same or at distant
locations. Here, we report the experimental realization of synchronized
generation of independent entangled photon pairs. The quality of
synchronization is confirmed by observing a violation of Bell's inequality with
3.2 standard deviations in an entanglement swapping experiment. The techniques
developed in our experiment will be of great importance for future linear
optical realization of quantum repeaters and quantum computation.


title:
Wronskian formula for confluent second-order supersymmetric quantum
  mechanics

abstract:
  The confluent second-order supersymmetric quantum mechanics, for which the
factorization energies tend to a single value, is studied. We show that the
Wronskian formula remains valid if generalized eigenfunctions are taken as seed
solutions. The confluent algorithm is used to generate SUSY partners of the
Coulomb potential.


title: From perfect to fractal transmission in spin chains

abstract:
  Perfect state transfer is possible in modulated spin chains, imperfections
however are likely to corrupt the state transfer. We study the robustness of
this quantum communication protocol in the presence of disorder both in the
exchange couplings between the spins and in the local magnetic field. The
degradation of the fidelity can be suitably expressed, as a function of the
level of imperfection and the length of the chain, in a scaling form. In
addition the time signal of fidelity becomes fractal. We further characterize
the state transfer by analyzing the spectral properties of the Hamiltonian of
the spin chain.


title: Quantum relative positioning in Hilbert space

abstract:
  A new class of state transformations that are quantum mechanically prohibited
is introduced. These can be seen as the generalization of the universal-NOT
transformation which, for all pure inputs state of a given Hilbert space
produces pure outputs whose projection on the original state is fixed to a
value smaller than one. The case of not pure output states is also addressed.
We give an application of these transformations in the context of separability
criteria.


title:
Thermodynamic and quantum thermodynamic answers to Einstein's concerns
  about Brownian movement

abstract:
  On the occasion of the 100th anniversary of the beginning of the
revolutionary contributions to physics by Einstein, I am happy to respond to a
problem posed by him in 1905. He said: In this paper it will be shown that
according to the molecular-kinetic theory of heat, bodies of
microscopically-visible size suspended in a liquid will perform movements of
such magnitude that they can be easily observed in a microscope, on account of
the molecular motions of heat....that is, Brownian molecular motion. In this
article I provide incontrovertible evidence against molecular-kinetic
conception of heat, and a regularization of the Brownian movement that differs
from all the statistical procedures and/or analyses that exist in the archival
literature to date. The regularization is based on either of two distinct but
intimately interrelated revolutionary conceptions of thermodynamics, one is
purely thermodynamic and the other is quantum mechanical.


title: Time Minimal Trajectories for two-level Quantum Systems with Drift

abstract:
  On a two-level quantum system driven by an external field, we consider the
population transfer problem from the first to the second level, minimizing the
time of transfer, with bounded field amplitude. On the Bloch sphere (i.e. after
a suitable Hopf projection), this problem can be attacked with techniques of
optimal syntheses on 2-D manifolds.
  Let $(-E,E)$ be the two energy levels, and $|\Omega(t)|\leq M$ the bound on
the field amplitude. For each values of $E$ and $M$, we provide the explicit
expression of the time optimal trajectory steering the state one to the state
two in terms of a parameter that should be computed numerically.
  For $M<<E$, every time optimal trajectory is periodic (and in particular
bang-bang) with frequency of the order of the resonance frequency
$\omega_R=2E$.
  On the other side, for $M>E$ the time optimal trajectory steering the state
one to the state two is bang-bang with exactly one switching. Fixed $E$ we also
prove that for $M\to\infty$ the time needed to reach the state two tends to
zero.
  Finally we compare these results with some known results of Khaneja, Brockett
and Glaser and with those obtained in the Rotating Wave Approximation.


title: On the semiclassical evolution of quantum operators

abstract:
  The Heisenberg evolution of a given unitary operator corresponds classically
to a fixed canonical transformation that is viewed through a moving coordinate
system. The operators that form the bases of the Weyl representation and its
Fourier transform, the chord representation, are, respectively, unitary
reflection and translation operators. Thus, the general semiclassical study of
unitary operators allows us to propagate arbitrary operators, including density
operators, i.e. the Wigner function. The various propagation kernels are
different representations of the superoperators which act on the space of
operators of a closed quantum system. We here present the mixed semiclassical
propagator, that takes translation chords to reflection centres, or vice versa.
In contrast to the centre-centre propagator that directly evolves Wigner
functions, it is guaranteed to be caustic free, having a simple WKB-like
universal form for a finite time, whatever the number of degrees of freedom.
Special attention is given to the near-classical region of small chords, since
this dominates the averages of observables evaluated through the Wigner
function.


title: Quantum Channels and Representation Theory

abstract:
  In the study of d-dimensional quantum channels $(d \geq 2)$, an assumption
which is not very restrictive, and which has a natural physical interpretation,
is that the corresponding Kraus operators form a representation of a Lie
algebra. Physically, this is a symmetry algebra for the interaction
Hamiltonian. This paper begins a systematic study of channels defined by
representations; the famous Werner-Holevo channel is one element of this
infinite class. We show that the channel derived from the defining
representation of SU(n) is a depolarizing channel for all $n$, but for most
other representations this is not the case. Since the Bloch sphere is not
appropriate here, we develop technology which is a generalization of Bloch's
technique. Our method works by representing the density matrix as a polynomial
in symmetrized products of Lie algebra generators, with coefficients that are
symmetric tensors. Using these tensor methods we prove eleven theorems, derive
many explicit formulas and show other interesting properties of quantum
channels in various dimensions, with various Lie symmetry algebras. We also
derive numerical estimates on the size of a generalized ``Bloch sphere'' for
certain channels. There remain many open questions which are indicated at
various points through the paper.


title: Quantum Computation with Diatomic Bits in Optical Lattices

abstract:
  We propose a scheme for scalable and universal quantum computation using
diatomic bits with conditional dipole-dipole interaction, trapped within an
optical lattice. The qubit states are encoded by the scattering state and the
bound heteronuclear molecular state of two ultracold atoms per site. The
conditional dipole-dipole interaction appears between neighboring bits when
they both occupy the molecular state. The realization of a universal set of
quantum logic gates, which is composed of single-bit operations and a two-bit
controlled-NOT gate, is presented. The readout method is also discussed.


title: Hamilton-Jacobi-Bellman equations for Quantum Filtering and Control

abstract:
  We exploit the separation of the filtering and control aspects of quantum
feedback control to consider the optimal control as a classical stochastic
problem on the space of quantum states. We derive the corresponding
Hamilton-Jacobi-Bellman equations using the elementary arguments of classical
control theory and show that this is equivalent, in the Stratonovich calculus,
to a stochastic Hamilton-Pontryagin setup. We show that, for cost functionals
that are linear in the state, the theory yields the traditional Bellman
equations treated so far in quantum feedback.


title: Quantum interference with molecules: The role of internal states

abstract:
  Recent experiments have shown that fullerene and fluorofullerene molecules
can produce interference patterns. These molecules have both rotational and
vibrational degrees of freedom. This leads one to ask whether these internal
motions can play a role in degrading the interference pattern. We study this by
means of a simple model. Our molecule consists of two masses a fixed distance
apart. It scatters from a potential with two or several peaks, thereby
mimicking two or several slit interference. We find that in some parameter
regimes the entanglement between the internal states and the translational
degrees of freedom produced by the potential can decrease the visibility of the
interference pattern. In particular, different internal states correspond to
different outgoing wave vectors, so that if several internal states are
excited, the total interference pattern will be the sum of a number of
patterns, each with a different periodicity. The overall pattern is
consequently smeared out. In the case of two different peaks, the scattering
from the different peaks will excite different internal states so that the path
the molecule takes become entangled with its internal state. This will also
lead to degradation of the interference pattern. How these mechanisms might
lead to the emergence of classical behavior is discussed.


title:
Entanglement of three-qubit pure states in terms of teleportation
  capability

abstract:
  We define an entanglement measure, called the partial tangle, which
represents the residual two-qubit entanglement of a three-qubit pure state. By
its explicit calculations for three-qubit pure states, we show that the partial
tangle is closely related to the faithfulness of a teleportation scheme over a
three-qubit pure state.


title:
Subwavelength atom localization via amplitude and phase control of the
  absorption spectrum

abstract:
  We propose a scheme for subwavelength localization of an atom conditioned
upon the absorption of a weak probe field at a particular frequency.
Manipulating atom-field interaction on a certain transition by applying drive
fields on nearby coupled transitions leads to interesting effects in the
absorption spectrum of the weak probe field. We exploit this fact and employ a
four-level system with three driving fields and a weak probe field, where one
of the drive fields is a standing-wave field of a cavity. We show that the
position of an atom along this standing wave is determined when probe field
absorption is measured. We find that absorption of the weak probe field at a
certain frequency leads to subwavelength localization of the atom in either of
the two half-wavelength regions of the cavity field by appropriate choice of
the system parameters. We term this result as sub-half-wavelength localization
to contrast it with the usual atom localization result of four peaks spread
over one wavelength of the standing wave. We observe two localization peaks in
either of the two half-wavelength regions along the cavity axis.


title:
Phase control of electromagnetically induced transparency and its
  applications to tunable group velocity and atom localization

abstract:
  We show that, by simple modifications of the usual three-level $\Lambda$-type
scheme used for obtaining electromagnetically induced transparency (EIT), phase
dependence in the response of the atomic medium to a weak probe field can be
introduced. This gives rise to phase dependent susceptibility. By properly
controlling phase and amplitudes of the drive fields we obtain variety of
interesting effects. On one hand we obtain phase control of the group velocity
of a probe field passing through medium to the extent that continuous tuning of
the group velocity from subluminal to superluminal and back is possible. While
on the other hand, by choosing one of the drive fields to be a standing wave
field inside a cavity, we obtain sub-wavelength localization of moving atoms
passing through the cavity field.


title: Multipartite entanglement in spin chains

abstract:
  We investigate the presence of multipartite entanglement in macroscopic spin
chains. We discuss the Heisenberg and the XY model and derive bounds on the
internal energy for systems without multipartite entanglement. Based on this we
show that in thermal equilibrium the above mentioned spin systems contain
genuine multipartite entanglement, even at finite modest temperatures.


title: Quantum search with resonances

abstract:
  We present a continuous time quantum search algorithm analogous to Grover's.
In particular, the optimal search time for this algorithm is proportional to
$\sqrt{N}$, where $N$ is the database size. This search algorithm can be
implemented using any Hamiltonian with a discrete energy spectrum through
excitation of resonances between an initial and the searched state. This
algorithm is robust and, as in the case of Grover's, it allows for an error
$O(1/\sqrt{N})$ in the determination of the searched state. A discrete time
version of this continuous time search algorithm is built, and the connection
between the search algorithms with discrete and continuous times is
established.


title: Ghost Interference and Quantum Erasure

abstract:
  The two-photon ghost interference experiment, generalized to the case of
massive particles, is theoretically analyzed. It is argued that the experiment
is intimately connected to a double-slit interference experiment where, the
which-path information exists. The reason for not observing first order
interference behind the double-slit, is clarified.It is shown that the
underlying mechanism for the appearance of ghost interference is, the more
familiar, quantum erasure.


title:
A Perturbative Treatment Of A Generalized PT-Symmetric Quartic
  Anharmonic Oscillator

abstract:
  We examine a generalized script{PT}-Symmetric quartic anharmonic oscillator
model to determine the various physical variables perturbatively in powers of a
small quantity {\epsilon}. We make use of the Bender-Dunne operator basis
elements and exploit the properties of the totally symmetric operator T_{m,n}.


title: Exact Paraxial Quantization

abstract:
  A non-perturbative quantization of a paraxial electromagnetic field is
achieved via a generalized dispersion relation imposed on the longitudinal and
the transverse components of the photon wave vector. The theoretical formalism
yields a seamless transition between the paraxial- and the Maxwell-equation
solutions. This obviates the need to introduce either "ad hoc" or
perturbatively-defined field operators. In the limit of narrow beam-like
fields, the theory is in agreement with approximated quantization schemes
provided by other authors.


title:
Optimal Unambiguous State Discrimination of two density matrices and its
  link with the Fidelity

abstract:
  Recently the problem of Unambiguous State Discrimination (USD) of mixed
quantum states has attracted much attention. So far, bounds on the optimum
success probability have been derived [1]. For two mixed states they are given
in terms of the fidelity. Here we give tighter bounds as well as necessary and
sufficient conditions for two mixed states to reach these bounds. Moreover we
construct the corresponding optimal measurement strategies. With this result,
we provide analytical solutions for unambiguous discrimination of a class of
generic mixed states. This goes beyond known results which are all reducible to
some pure state case. Additionally, we show that examples exist where the
bounds cannot be reached.


title: Quantum Computational Representation of the Bosonic Oscillator

abstract:
  By the early eighties, Fredkin, Feynman, Minsky and others were exploring the
notion that the laws of physics could be simulated with computers. Feynman's
particular contribution was to bring quantum mechanics into the discussion and
his ideas played a key role in the development of quantum computation. It was
shown in 1995 by Barenco et al that all quantum computation processes could be
written in terms of local operations and CNOT gates. We show how one of the
most important of all physical systems, the quantized bosonic oscillator, can
be rewritten in precisely those terms and therefore described as a quantum
computational process, exactly in line with Feynman's ideas. We discuss single
particle excitations and coherent states.


title:
Quantum phase uncertainty in mutually unbiased measurements and Gauss
  sums

abstract:
  Mutually unbiased bases (MUBs), which are such that the inner product between
two vectors in different orthogonal bases is constant equal to the inverse
$1/\sqrt{d}$, with $d$ the dimension of the finite Hilbert space, are becoming
more and more studied for applications such as quantum tomography and
cryptography, and in relation to entangled states and to the Heisenberg-Weil
group of quantum optics. Complete sets of MUBs of cardinality $d+1$ have been
derived for prime power dimensions $d=p^m$ using the tools of abstract algebra
(Wootters in 1989, Klappenecker in 2003). Presumably, for non prime dimensions
the cardinality is much less. The bases can be reinterpreted as quantum phase
states, i.e. as eigenvectors of Hermitean phase operators generalizing those
introduced by Pegg & Barnett in 1989. The MUB states are related to additive
characters of Galois fields (in odd characteristic p) and of Galois rings (in
characteristic 2). Quantum Fourier transforms of the components in vectors of
the bases define a more general class of MUBs with multiplicative characters
and additive ones altogether. We investigate the complementary properties of
the above phase operator with respect to the number operator. We also study the
phase probability distribution and variance for physical states and find them
related to the Gauss sums, which are sums over all elements of the field (or of
the ring) of the product of multiplicative and additive characters. Finally we
relate the concepts of mutual unbiasedness and maximal entanglement. This
allows to use well studied algebraic concepts as efficient tools in our quest
of minimal uncertainty in quantum information primitives.


title:
Geometric Phase in Eigenspace Evolution of Invariant and Adiabatic
  Action Operators

abstract:
  The theory of geometric phase is generalized to a cyclic evolution of the
eigenspace of an invariant operator with $N$-fold degeneracy.
  The corresponding geometric phase is interpreted as a holonomy inherited from
the universal connection of a Stiefel U(N)-bundle over a Grassmann manifold.
Most significantly, for an arbitrary initial state, this geometric phase
captures the inherent geometric feature of the state evolution. Moreover, the
geometric phase in the evolution of the eigenspace of an adiabatic action
operator is also addressed, which is elaborated by a pullback U(N)-bundle.
Several intriguing physical examples are illustrated.


title:
A Fabry-Perot like two-photon interferometer for high-dimensional
  time-bin entanglement

abstract:
  We generate high-dimensional time-bin entanglement using a mode-locked laser
and analyze it with a 2-photon Fabry-Perot interferometer. The dimension of the
entangled state is limited only by the phase coherence between subsequent
pulses and is practically infinite. In our experiment a pico-second mode-locked
laser at 532 nm pumps a non-linear potassium niobate crystal to produce photon
pairs by spontaneous parametric down-conversion at 810 and 1550 nm.


title: PPT from spectra

abstract:
  In this contribution we solve the following problem. Let H_{nm} be a Hilbert
space of dimension nm, and let A be a positive semidefinite self-adjoint linear
operator on H_{nm}. Under which conditions on the spectrum has A a positive
partial transpose (is PPT) with respect to any partition H_n \otimes H_m of the
space H_{nm} as a tensor product of an n-dimensional and an m-dimensional
Hilbert space? We show that the necessary and sufficient conditions can be
expressed as a set of linear matrix inequalities on the eigenvalues of A.


title:
Fault-Tolerant Dissipative Preparation of Atomic Quantum Registers with
  Fermions

abstract:
  We propose a fault tolerant loading scheme to produce an array of fermions in
an optical lattice of the high fidelity required for applications in quantum
information processing and the modelling of strongly correlated systems. A cold
reservoir of Fermions plays a dual role as a source of atoms to be loaded into
the lattice via a Raman process and as a heat bath for sympathetic cooling of
lattice atoms. Atoms are initially transferred into an excited motional state
in each lattice site, and then decay to the motional ground state, creating
particle-hole pairs in the reservoir. Atoms transferred into the ground
motional level are no longer coupled back to the reservoir, and doubly occupied
sites in the motional ground state are prevented by Pauli blocking. This scheme
has strong conceptual connections with optical pumping, and can be extended to
load high-fidelity patterns of atoms.


title: New supersymmetric partners for the associated Lame potentials

abstract:
  We obtain exact solutions of the one-dimensional Schrodinger equation for
some families of associated Lame potentials with arbitrary energy through a
suitable ansatz, which may be appropriately extended for other such a families.
The formalism of supersymmetric quantum mechanics is used to generate new
exactly solvable potentials.


title: Time Optimal Control of Coupled Qubits Under Non-Stationary Interactions

abstract:
  In this article, we give a complete characterization of all the unitary
transformations that can be synthesized in a given time for a system of coupled
spin-1/2 in presence of general time varying coupling tensor. Our treatment is
quite general and our results help to characterize the reachable set at all
times for a class of bilinear control systems with time varying drift and
unbounded control amplitude. These results are of fundamental interest in
geometric control theory and have applications to control of coupled spins in
solid state NMR spectroscopy.


title:
From Quantum Optics to Non-Commutative Geometry : A Non-Commutative
  Version of the Hopf Bundle, Veronese Mapping and Spin Representation

abstract:
  In this paper we construct a non-commutative version of the Hopf bundle by
making use of Jaynes-Commings model and so-called Quantum Diagonalization
Method. The bundle has a kind of Dirac strings. However, they appear in only
states containing the ground one (${\cal F}\times \{\ket{0}\} \cup
\{\ket{0}\}\times {\cal F} \subset {\cal F}\times {\cal F}$) and don't appear
in remaining excited states. This means that classical singularities are not
universal in the process of non-commutativization.
  Based on this construction we moreover give a non-commutative version of both
the Veronese mapping which is the mapping from $\fukuso P^{1}$ to $\fukuso
P^{n}$ with mapping degree $n$ and the spin representation of the group SU(2).
  We also present some challenging problems concerning how classical
(beautiful) properties can be extended to the non-commutative case.


title: Quantum Correlations with a Classical Apparatus

abstract:
  A deterministic, relativistically local and thus classical Bell-type
apparatus is reported that violates the Bell-CHSH inequality by introducing a
simple local memory element in the detector and by requiring the detector
combinations to switch with unequal probabilities. This indicates that the
common notion of the fundamental impossibility of a classical-type theory
underlying quantum mechanics may need to be re-evaluated.


title: General Monogamy Inequality for Bipartite Qubit Entanglement

abstract:
  We consider multipartite states of qubits and prove that their bipartite
quantum entanglement, as quantified by the concurrence, satisfies a monogamy
inequality conjectured by Coffman, Kundu, and Wootters. We relate this monogamy
inequality to the concept of frustration of correlations in quantum spin
systems.


title:
Precise Creation, Characterization, and Manipulation of Single Optical
  Qubits

abstract:
  We present the theoretical basis for and experimental verification of
arbitrary single-qubit state generation, using the polarization of photons
generated via spontaneous parametric downconversion. Our precision measurement
and state reconstruction system has the capability to distinguish over 3
million states, all of which can be reproducibly generated using our state
creation apparatus. In order to complete the triumvirate of single qubit
control, there must be a way to not only manipulate single qubits after
creation and before measurement, but a way to characterize the manipulations
\emph{themselves}. We present a general representation of arbitrary processes,
and experimental techniques for generating a variety of single qubit
manipulations, including unitary, decohering, and (partially) polarizing
operations.


title: Collective Decoherence of Nuclear Spin Clusters

abstract:
  The problem of dipole-dipole decoherence of nuclear spins is considered for
strongly entangled spin cluster. Our results show that its dynamics can be
described as the decoherence due to interaction with a composite bath
consisting of fully correlated and uncorrelated parts. The correlated term
causes the slower decay of coherence at larger times. The decoherence rate
scales up as a square root of the number of spins giving the linear scaling of
the resulting error. Our theory is consistent with recent experiment reported
in decoherence of correlated spin clusters.


title: Upper bounds of a class of imperfect quantum sealing protocols

abstract:
  The model of the quantum protocols sealing a classical bit is studied. It is
shown that there exist upper bounds on its security. For any protocol where the
bit can be read correctly with the probability $\alpha $, and reading the bit
can be detected with the probability $\beta $, the upper bounds are $% \beta
\leqslant 1/2$ and $\alpha +\beta \leqslant 9/8$.


title:
Velocity selection of ultra-cold atoms with Fabry-Perot laser devices:
  improvements and limits

abstract:
  We discuss a method to select the velocities of ultra-cold atoms with a
modified Fabry-Perot type of device made of two effective barriers and a well
created, respectively, by blue and red detuned lasers. The laser parameters may
be used to select the peak and width of the transmitted velocity window. In
particular, lowering the central well provides a peak arbitrarily close to zero
velocity having a minimum but finite width. The low-energy atomic scattering
off this laser device is parameterized and approximate formulae are found to
describe and explain its behaviour.


title: Control of Local Relaxation Behavior in Closed Bipartite Quantum Systems

abstract:
  We investigate the decoherence of a spin 1/2 subsystem weakly coupled to an
environment of many spins 1/2 with and without mutual coupling. The total
system is closed, its state is pure and evolves under Schroedinger dynamics.
Nevertheless, the considered spin typically reaches a quasi-stationary
equilibrium state. Here we show that this state depends strongly on the
coupling to the environment on the one hand and on the coupling within the
environmental spins on the other. In particular we focus on spin star and spin
ring-star geometries to investigate the effect of intra-environmental coupling
on the central spin. By changing the spectrum of the environment its effect as
a bath on the central spin is changed also and may even be adjustable to some
degree. We find that the relaxation behavior is related to the distribution of
the energy eigenstates of the total system. For each of these relaxation modes
there is a dual mode for which the resulting subsystem approaches an inverted
state occupation probability (negative temperature).


title:
Rotational Correction on the Morse Potential Through the Pekeris
  Approximation and Nikiforov-Uvarov Method

abstract:
  The Nikiforov-Uvarov method is employed to calculate the the Schrodinger
equation with a rotation Morse potential. The bound state energy eigenvalues
and the corresponding eigenfunction are obtained. All of these calculation
present an effective and clear method under a Pekeris approximation to solve a
rotation Morse model. Meanwhile the results got here are in a good agreement
with ones before.


title:
Ground-state van der Waals forces in planar multilayer
  magnetodielectrics

abstract:
  Within the frame of lowest-order perturbation theory, the van der Waals
potential of a ground-state atom placed within an arbitrary dispersing and
absorbing magnetodielectric multilayer system is given. Examples of an atom
situated in front of a magnetodielectric plate or between two such plates are
studied in detail. Special emphasis is placed on the competing attractive and
repulsive force components associated with the electric and magnetic matter
properties, respectively, and conditions for the formation of repulsive
potential walls are given. Both numerical and analytical results are presented.


title:
Quantum memory for non-stationary light fields based on controlled
  reversible inhomogeneous broadening

abstract:
  We propose a new method for efficient storage and recall of non-stationary
light fields, e.g. single photon time-bin qubits, in optically dense atomic
ensembles. Our approach to quantum memory is based on controlled, reversible,
inhomogeneous broadening. We briefly discuss experimental realizations of our
proposal.


title: Quantum Implementation of Parrondo's Paradox

abstract:
  We propose a quantum implementation of a capital-dependent Parrondo's paradox
that uses $O(\log_2(n))$ qubits, where $n$ is the number of Parrondo games. We
present its implementation in the quantum computer language (QCL) and show
simulation results.


title: Perfect quantum state transfer with randomly coupled quantum chains

abstract:
  We suggest a scheme that allows arbitrarily perfect state transfer even in
the presence of random fluctuations in the couplings of a quantum chain. The
scheme performs well for both spatially correlated and uncorrelated
fluctuations if they are relatively weak (say 5%). Furthermore, we show that
given a quite arbitrary pair of quantum chains, one can check whether it is
capable of perfect transfer by only local operations at the ends of the chains,
and the system in the middle being a "black box". We argue that unless some
specific symmetries are present in the system, it will be capable of perfect
transfer when used with dual-rail encoding. Therefore our scheme puts minimal
demand not only on the control of the chains when using them, but also on the
design when building them.


title: Rabi spectroscopy for a two-level system

abstract:
  We have analyzed the interaction of a dissipative two level quantum system
with high and low frequency excitation. The system is continuously and
simultaneously irradiated by these two waves. If the frequency of the first
signal is close to the level separation and the second one is tuned to the Rabi
frequency, it is shown that the response of the system exhibits an undamped low
frequency oscillation. The method can be useful for low frequency Rabi
spectroscopy in various physical systems which are described by a two level
Hamiltonian, such as nuclei spins in NMR, double well quantum dots,
superconducting flux and charge qubits, etc. As an example, the application of
the method to the readout of a flux qubit is considered.


title:
Testing Gravity-Driven Collapse of the Wavefunction via Cosmogenic
  Neutrinos

abstract:
  It is pointed out that the Diosi-Penrose ansatz for gravity-induced quantum
state reduction can be tested by observing oscillations in the flavor ratios of
neutrinos originated at cosmological distances. Since such a test would be
almost free of environmental decoherence, testing the ansatz by means of a next
generation neutrino detector such as IceCube would be much cleaner than by
experiments proposed so far involving superpositions of macroscopic systems.
The proposed microscopic test would also examine the universality of
superposition principle at unprecedented cosmological scales.


title:
Enhancing practical security of quantum key distribution with a few
  decoy states

abstract:
  Quantum key distribution establishes a secret string of bits between two
distant parties. Of concern in weak laser pulse schemes is the especially
strong photon number splitting attack by an eavesdropper, but the decoy state
method can detect this attack with current technology, yielding a high rate of
secret bits. In this Letter, we develop rigorous security statements in the
case of finite statistics with only a few decoy states, and we present the
results of simulations of an experimental setup of a decoy state protocol that
can be simply realized with current technology.