title:
On the Theoretical and Experimental Evidences of Material Phase
  Causality

abstract:
  We present theoretical and experimental evidences showing that the material
phase of the state vector is causally related with the dynamic of quantum
system and may have observable physical consequences.


title: Criticality, the area law, and the computational power of PEPS

abstract:
  The projected entangled pair state (PEPS) representation of quantum states on
two-dimensional lattices induces an entanglement based hierarchy in state
space. We show that the lowest levels of this hierarchy exhibit an enormously
rich structure including states with critical and topological properties as
well as resonating valence bond states. We prove, in particular, that coherent
versions of thermal states of any local 2D classical spin model correspond to
such PEPS, which are in turn ground states of local 2D quantum Hamiltonians.
This correspondence maps thermal onto quantum fluctuations, and it allows us to
analytically construct critical quantum models exhibiting a strict area law
scaling of the entanglement entropy in the face of power law decaying
correlations. Moreover, it enables us to show that there exist PEPS within the
same class as the cluster state, which can serve as computational resources for
the solution of NP-hard problems.


title: Topological Factors Derived From Bohmian Mechanics

abstract:
  We derive for Bohmian mechanics topological factors for quantum systems with
a multiply-connected configuration space Q. These include nonabelian factors
corresponding to what we call holonomy-twisted representations of the
fundamental group of Q. We employ wave functions on the universal covering
space of Q. As a byproduct of our analysis, we obtain an explanation, within
the framework of Bohmian mechanics, of the fact that the wave function of a
system of identical particles is either symmetric or anti-symmetric.


title: Decay of the Loschmidt echo in a time-dependent environment

abstract:
  We study the decay rate of the Loschmidt echo or fidelity in a chaotic system
under a time-dependent perturbation $V(q,t)$ with typical strength
$\hbar/\tau_{V}$. The perturbation represents the action of an uncontrolled
environment interacting with the system, and is characterized by a correlation
length $\xi_0$ and a correlation time $\tau_0$. For small perturbation
strengths or rapid fluctuating perturbations, the Loschmidt echo decays
exponentially with a rate predicted by the Fermi Golden Rule, $1/\tilde{\tau}=
\tau_{c}/\tau_{V}^2$, where typically $\tau_{c} \sim \min[\tau_{0},\xi_0/v]$
with $v$ the particle velocity. Whenever the rate $1/\tilde{\tau}$ is larger
than the Lyapunov exponent of the system, a perturbation independent Lyapunov
decay regime arises. We also find that by speeding up the fluctuations (while
keeping the perturbation strength fixed) the fidelity decay becomes slower, and
hence, one can protect the system against decoherence.


title: Universal quantum computation with unlabeled qubits

abstract:
  We show that an n-th root of the Walsh-Hadamard transform (obtained from the
Hadamard gate and a cyclic permutation of the qubits), together with two
diagonal matrices, namely a local qubit-flip (for a fixed but arbitrary qubit)
and a non-local phase-flip (for a fixed but arbitrary coefficient), can do
universal quantum computation on n qubits. A quantum computation, making use of
n qubits and based on these operations, is then a word of variable length, but
whose letters are always taken from an alphabet of cardinality three.
Therefore, in contrast with other universal sets, no choice of qubit lines is
needed for the application of the operations described here. A quantum
algorithm based on this set can be interpreted as a discrete diffusion of a
quantum particle on a de Bruijn graph, corrected on-the-fly by auxiliary
modifications of the phases associated to the arcs.


title: Entanglement of indistinguishable particles in condensed matter physics

abstract:
  The concept of entanglement in systems where the particles are
indistinguishable has been the subject of much recent interest and controversy.
In this paper we study the notion of entanglement of particles introduced by
Wiseman and Vaccaro [Phys. Rev. Lett. 91, 097902 (2003)] in several specific
physical systems, including some that occur in condensed matter physics. The
entanglement of particles is relevant when the identical particles are
itinerant and so not distinguished by their position as in spin models. We show
that entanglement of particles can behave differently to other approaches that
have been used previously, such as entanglement of modes (occupation-number
entanglement) and the entanglement in the two-spin reduced density matrix. We
argue that the entanglement of particles is what could actually be measured in
most experimental scenarios and thus its physical significance is clear. This
suggests entanglement of particles may be useful in connecting theoretical and
experimental studies of entanglement in condensed matter systems.


title: A new discrete view to the quantum world

abstract:
  Here I present a new discrete model of quantum mechanics for relativistic
1-electron systems, in which particle movement is described by a directed
space-time graph with attached 4-spinors, but without any continuous wave
functions. These graphs only consist of few space-like edges, e.g. the ground
state of atoms is described by two nodes and one edge, and interactions only
take place at the nodes.
  The fundament is an extremal principle for a relativistic invariant
"Lagrangian sum", from which "field-equations" and "equations of motion" are
derived, so the states (including the graph nodes) are completely determined.
  As important validations of the model, the corresponding graphs for the
stationary Dirac-equation for the atom are drawn and the correct spectra are
computed (Sommerfeld-levels).
  Also a discrete Schroedinger approximation and an associated "Hamiltonian
sum" are derived and the correct equation of a classical moving particle under
Lorentz-force is presented.
  I hope, that this new approach will help, to overcome some problems of
current quantum mechanics by making the wave function superfluous.


title: Driven harmonic oscillator as a quantum simulator for open systems

abstract:
  We show theoretically how a driven harmonic oscillator can be used as a
quantum simulator for non-Markovian damped harmonic oscillator. In the general
framework, the results demonstrate the possibility to use a closed system as a
simulator for open quantum systems. The quantum simulator is based on sets of
controlled drives of the closed harmonic oscillator with appropriately tailored
electric field pulses. The non-Markovian dynamics of the damped harmonic
oscillator is obtained by using the information about the spectral density of
the open system when averaging over the drives of the closed oscillator. We
consider single trapped ions as a specific physical implementation of the
simulator, and we show how the simulator approach reveals new physical insight
into the open system dynamics, e.g. the characteristic quantum mechanical
non-Markovian oscillatory behavior of the energy of the damped oscillator,
usually obtained by the non-Lindblad-type master equation, can have a simple
semiclassical interpretation.


title: Noiseless subsystems and Bell inequalities in curved spacetime

abstract:
  We examine the use of noiseless subsystems for quantum information processing
between two parties who do not share a common reference frame. In particular we
focus on Bell inequalities in curved spaces and outline a theoretical procedure
to test a Bell inequality, demonstrating the wide applicability of noiseless
subsystems.


title:
Simulating high-temperature superconductivity model Hamiltonians with
  atoms in optical lattices

abstract:
  We investigate the feasibility of simulating different model Hamiltonians
used in high-temperature superconductivity. We briefly discuss the most common
models and then focus on the simulation of the so-called t-J-U Hamiltonian
using ultra-cold atoms in optical lattices. For this purpose, previous
simulation schemes to realize the spin interaction term J are extended. We
especially overcome the condition of a filling factor of exactly one, which
otherwise would restrict the phase of the simulated system to a Mott-insulator.
Using ultra-cold atoms in optical lattices allows simulation of the discussed
models for a very wide range of parameters. The time needed to simulate the
Hamiltonian is estimated and the accuracy of the simulation process is
numerically investigated for small systems.


title: Quantitative aspects of entanglement in the driven Jaynes-Cummings model

abstract:
  Adopting the framework of the Jaynes-Cummings model with an external quantum
field, we obtain exact analytical expressions of the normally ordered moments
for any kind of cavity and driving fields. Such analytical results are
expressed in the integral form, with their integrands having a commom term that
describes the product of the Glauber-Sudarshan quasiprobability distribution
functions for each field, and a kernel responsible for the entanglement.
Considering a specific initial state of the tripartite system, the normally
ordered moments are then applied to investigate not only the squeezing effect
and the nonlocal correlation measure based on the total variance of a pair of
Einstein-Podolsky-Rosen type operators for continuous variable systems, but
also the Shchukin-Vogel criterion. This kind of numerical investigation
constitutes the first quantitative characterization of the entanglement
properties for the driven Jaynes-Cummings model.


title:
The meaning of group delay in barrier tunneling: A re-examination of
  superluminal group velocities

abstract:
  We show that the group delay in tunneling is not a traversal time but a
lifetime of stored energy or stored probability escaping through both ends of
the barrier. Because it is a lifetime associated with both forward
(transmitted) and backward (reflected) fluxes, it cannot be used to define a
group velocity for forward transit in cases where a wavepacket is mostly
reflected. For photonic tunneling barriers the group delay is identical to the
dwell time which is also a property of an entire wave function with reflected
and transmitted components. Theoretical predictions and experimental reports of
superluminal group velocities in barrier tunneling are re-interpreted.


title: What is semiquantum mechanics?

abstract:
  Semiclassical approximations to quantum dynamics are almost as old as quantum
mechanics itself. In the approach pioneered by Wigner, the evolution of his
quasiprobability density function on phase space is expressed as an asymptotic
series in increasing powers of Planck's constant, with the classical
Liouvillean evolution as leading term. Successive semiclassical approximations
to quantum dynamics are defined by successive terms in the series. We consider
a complementary approach, which explores the quantum-clssical interface from
the other direction. Classical dynamics is formulated in Hilbert space, with
the Groenewold quasidensity operator as the image of the Liouville density on
phase space. The evolution of the Groenewold operator is then expressed as an
asymptotic series in increasing powers of Planck's constant. Successive
semiquantum approximations to classical dynamics are defined by successive
terms in this series, with the familiar quantum evolution as leading term.


title:
An integrable optical-fiber source of polarization entangled
  photon-pairs in the telecom band

abstract:
  We demonstrate an optical-fiber based source of polarization entangled
photon-pairs with improved quality and efficiency, which has been integrated
with off-the-shelf telecom components and is, therefore, well suited for
quantum communication applications in the 1550 nm telecom band. Polarization
entanglement is produced by simultaneously pumping a loop of standard
dispersion-shifted fiber with two orthogonally-polarized pump pulses, one
propagating in the clockwise and the other in the counter-clockwise direction.
We characterize this source by investigating two-photon interference between
the generated signal-idler photon-pairs under various conditions. The
experimental parameters are carefully optimized to maximize the generated
photon-pair correlation and to minimize contamination of the entangled
photon-pairs from extraneously scattered background photons that are produced
by the pump pulses for two reasons: i) spontaneous Raman scattering causes
uncorrelated photons to be emitted in the signal/idler bands and ii) broadening
of the pump-pulse spectrum due to self-phase modulation causes pump photons to
leak into the signal/idler bands. We obtain two-photon interference with
visibility $>90$% without subtracting counts caused by the background photons
(only dark counts of the detectors are subtracted), when the mean photon number
in the signal (idler) channel is about 0.02/pulse, while no interference is
observed in direct detection of either the signal or the idler photons.


title: Quantum Network Coding

abstract:
  Since quantum information is continuous, its handling is sometimes
surprisingly harder than the classical counterpart. A typical example is
cloning; making a copy of digital information is straightforward but it is not
possible exactly for quantum information. The question in this paper is whether
or not quantum network coding is possible. Its classical counterpart is another
good example to show that digital information flow can be done much more
efficiently than conventional (say, liquid) flow.
  Our answer to the question is similar to the case of cloning, namely, it is
shown that quantum network coding is possible if approximation is allowed, by
using a simple network model called Butterfly. In this network, there are two
flow paths, s_1 to t_1 and s_2 to t_2, which shares a single bottleneck channel
of capacity one. In the classical case, we can send two bits simultaneously,
one for each path, in spite of the bottleneck. Our results for quantum network
coding include: (i) We can send any quantum state |psi_1> from s_1 to t_1 and
|psi_2> from s_2 to t_2 simultaneously with a fidelity strictly greater than
1/2. (ii) If one of |psi_1> and |psi_2> is classical, then the fidelity can be
improved to 2/3. (iii) Similar improvement is also possible if |psi_1> and
|psi_2> are restricted to only a finite number of (previously known) states.
(iv) Several impossibility results including the general upper bound of the
fidelity are also given.


title: Spatial Entanglement of Bosons in Thermal Equilibrium

abstract:
  We investigate spatial entanglement in a system of spinless non-interacting
bosons in a one dimensional harmonic trap described by the grand canonical
ensemble. We find the lower bound for the amount of spatial entanglement
contained in the average state of two bosons in the trap and show that it
exists for arbitrary high temperatures.


title: Identification and Distance Measures of Measurement Apparatus

abstract:
  We propose simple schemes that can perfectly identify projective measurement
apparatus secretly chosen from a finite set. Entanglements are used in these
schemes both to make possible the perfect identification and to improve the
efficiency significantly. A brief discussion on the problem of how to
appropriately define distance measures of measurements is also provided based
on the results of identification.


title: Realization of four-state qudits using biphotons

abstract:
  The novel experimental realization of four-level optical quantum systems
(ququarts) is presented. We exploit the polarization properties of frequency
non-degenerate biphoton field to obtain such systems. A simple method that does
not rely on interferometer is used to generate and measure the sequence of
states that can be used in quantum key distribution (QKD) protocol.


title: A SU(2) recipe for mutually unbiased bases

abstract:
  A simple recipe for generating a complete set of mutually unbiased bases in
dimension (2j+1)**e, with 2j + 1 prime and e positive integer, is developed
from a single matrix acting on a space of constant angular momentum j and
defined in terms of the irreducible characters of the cyclic group C(2j+1). As
two pending results, this matrix is used in the derivation of a polar
decomposition of SU(2) and of a FFZ algebra.


title:
Tensor Universality, Quantum Information Flow, Coecke's Theorem, and
  Generalizations

abstract:
  We show that Coecke's compositionality theorem for quantum information flow
follows by the universal property of tensor products from the case in which all
relevant states are totally disentangled, for which the proof is almost
trivial. With the same technique we deduce a PROP structure behind general
multipartite quantum information processing and show that all such are
equivalent to a canonical teleportation-type form. Some philosophical issues
concerning quantum information are also touched upon.


title: Casimir effect for curved geometries: PFA validity limits

abstract:
  We compute Casimir interaction energies for the sphere-plate and
cylinder-plate configuration induced by scalar-field fluctuations with
Dirichlet boundary conditions. Based on a high-precision calculation using
worldline numerics, we quantitatively determine the validity bounds of the
proximity force approximation (PFA) on which the comparison between all
corresponding experiments and theory are based. We observe the quantitative
failure of the PFA on the 1% level for a curvature parameter a/R > 0.00755.
Even qualitatively, the PFA fails to predict reliably the correct sign of
genuine Casimir curvature effects. We conclude that data analysis of future
experiments aiming at a precision of 0.1% must no longer be based on the PFA.


title: Causally Symmetric Bohm Model

abstract:
  A version of Bohm's model incorporating retrocausality is presented, the aim
being to explain the nonlocality of Bell's theorem while maintaining Lorentz
invariance in the underlying ontology. The strengths and weaknesses of this
alternative model are compared with those of the standard Bohm model.


title:
Entropy of a Quantum Oscillator coupled to a Heat Bath and implications
  for Quantum Thermodynamics

abstract:
  The free energy of a quantum oscillator in an arbitrary heat bath at a
temperature T is given by a "remarkable formula" which involves only a single
integral. This leads to a corresponding simple result for the entropy. The low
temperature limit is examined in detail and we obtain explicit results both for
the case of an Ohmic heat bath and a radiation heat bath. More general heat
bath models are also examined. This enables us to determine the entropy at zero
temperature in order to check the third law of thermodynamics in the quantum
regime


title: Shor's algorithm with fewer (pure) qubits

abstract:
  In this note we consider optimised circuits for implementing Shor's quantum
factoring algorithm. First I give a circuit for which none of the about 2n
qubits need to be initialised (though we still have to make the usual 2n
measurements later on). Then I show how the modular additions in the algorithm
can be carried out with a superposition of an arithmetic sequence. This makes
parallelisation of Shor's algorithm easier. Finally I show how one can factor
with only about 1.5n qubits, and maybe even fewer.


title: Cloning and Joint Measurements of Incompatible Components of Spin

abstract:
  A joint measurement of two observables is a {\it simultaneous} measurement of
both quantities upon the {\it same} quantum system. When two quantum-mechanical
observables do not commute, then a joint measurement of these observables
cannot be accomplished by projective measurements alone. In this paper we shall
discuss the use of quantum cloning to perform a joint measurement of two
components of spin associated with a qubit system. We introduce a cloning
scheme which is optimal with respect to this task. This cloning scheme may be
thought to work by cloning two components of spin onto its outputs. We compare
the proposed cloning machine to existing cloners.


title:
Decoherence induced by squeezing control errors in optical and ion trap
  holonomic quantum computations

abstract:
  We study decoherence induced by stochastic squeezing control errors
considering the particular implementation of Hadamard gate on optical and ion
trap holonomic quantum computers. We find the fidelity for Hadamard gate and
compute the purity of the final state when the control noise is modeled by
Ornstein-Uhlenbeck stochastic process. We demonstrate that in contradiction to
the case of the systematic control errors the stochastic ones lead to
decoherence of the final state. In the small errors limit we derive a simple
formulae connecting the gate fidelity and the purity of the final state.


title:
Single-qubit rotations in 2D optical lattices with multi-qubit
  addressing

abstract:
  Optical lattices with one atom on each site and interacting via cold
controlled collisions provide an efficient way to entangle a large number of
qubits with high fidelity. It has already been demonstrated experimentally that
this approach is especially suited for the generation of cluster states [O.
Mandel et al., Nature 425, 937 (2003)] which reduce the resource requirement
for quantum computing to the ability to perform single-qubit rotations and
qubit read out. In this paper, we describe how to implement these rotations in
1D and 2D optical lattices without having to address the atoms individually
with a laser field.


title:
Multichannel coupling with supersymmetric quantum mechanics and
  exactly-solvable model for Feshbach resonance

abstract:
  A new type of supersymmetric transformations of the coupled-channel radial
Schroedinger equation is introduced, which do not conserve the vanishing
behavior of solutions at the origin. Contrary to usual transformations, these
``non-conservative'' transformations allow, in the presence of thresholds, the
construction of potentials with coupled scattering matrices from uncoupled
potentials. As an example, an exactly-solvable potential matrix is obtained
which provides a very simple model of Feshbach-resonance phenomenon.


title:
Reduced Deadtime and Higher Rate Photon-Counting Detection using a
  Multiplexed Detector Array

abstract:
  We present a scheme for a photon-counting detection system that can be
operated at incident photon rates higher than otherwise possible by suppressing
the effects of detector deadtime. The method uses an array of N detectors and a
1-by-N optical switch with a control circuit to direct input light to live
detectors. Our calculations and models highlight the advantages of the
technique. In particular, using this scheme, a group of N detectors provides an
improvement in operation rate that can exceed the improvement that would be
obtained by a single detector with deadtime reduced by 1/N, even if it were
feasible to produce a single detector with such a large improvement in
deadtime. We model the system for continuous and pulsed light sources, both of
which are important for quantum metrology and quantum key distribution
applications.


title: Optimal estimation of squeezing

abstract:
  We present the optimal estimation of an unknown squeezing transformation of
the radiation field. The optimal estimation is unbiased and is obtained by
properly considering the degeneracy of the squeezing operator. For coherent
input states, the r.m.s. of the estimation scales as $(2\sqrt {\bar n})^{-1}$
versus the average photon number $\bar n$, while it can be enhanced to $(2\bar
n)^{-1}$ by using displaced squeezed states.


title: Nonlinear Interferometry via Fock State Projection

abstract:
  We use a photon-number resolving detector to monitor the photon number
distribution of the output of an interferometer, as a function of phase delay.
As inputs we use coherent states with mean photon number up to seven. The
postselection of a specific Fock (photon-number) state effectively induces
high-order optical non-linearities. Following a scheme by Bentley and Boyd
[S.J. Bentley and R.W. Boyd, Optics Express 12, 5735 (2004)] we explore this
effect to demonstrate interference patterns a factor of five smaller than the
Rayleigh limit.


title: Two-bit Deutsch-Jozsa algorithm using an atomic ensemble

abstract:
  The first optical proposal for the realization of the two-bit version of the
Deutsch-Jozsa algorithm [D. Deutsch and R. Jozsa, Proc. R. Soc. London A {\bf
493}, 553 (1992)] is presented. The proposal uses Stark shifts in an ensemble
of atoms and degenerate sources of photons. The photons interact dispersively
with an atomic ensemble, leading to an effective Hamiltonian in atom-field
basis, which is useful for performing the required two-qubit operations.
Combining these with a set of one-qubit operations, the algorithm can be
implemented. A discussion of the experimental feasibility of the proposal is
given.


title: Randomized control of open quantum systems

abstract:
  The problem of open-loop dynamical control of generic open quantum systems is
addressed. In particular, I focus on the task of effectively switching off
environmental couplings responsible for unwanted decoherence and dissipation
effects. After revisiting the standard framework for dynamical decoupling via
deterministic controls, I describe a different approach whereby the controller
intentionally acquires a random component. An explicit error bound on
worst-case performance of stochastic decoupling is presented.


title:
Bell inequality with an arbitrary number of settings and its
  applications

abstract:
  Based on a geometrical argument introduced by Zukowski, a new multisetting
Bell inequality is derived, for the scenario in which many parties make
measurements on two-level systems. This generalizes and unifies some previous
results. Moreover, a necessary and sufficient condition for the violation of
this inequality is presented. It turns out that the class of non-separable
states which do not admit local realistic description is extended when compared
to the two-setting inequalities. However, supporting the conjecture of Peres,
quantum states with positive partial transposes with respect to all subsystems
do not violate the inequality. Additionally, we follow a general link between
Bell inequalities and communication complexity problems, and present a quantum
protocol linked with the inequality, which outperforms the best classical
protocol.


title:
Emission spectra and quantum efficiency of single-photon sources in the
  cavity-QED strong-coupling regime

abstract:
  We derive analytical formulas for the forward emission and side emission
spectra of cavity-modified single-photon sources, as well as the corresponding
normal-mode oscillations in the cavity quantum electrodynamics (QED)
strong-coupling regime. We investigate the effects of pure dephasing, treated
in the phase-diffusion model based on a Wiener-Levy process, on the emission
spectra and normal-mode oscillations. We also extend our previous calculation
of quantum efficiency to include the pure dephasing process. All results are
obtained in the Weisskopf-Wigner approximation for an impulse-excited emitter.
We find that the spectra are broadened, the depths of the normal-mode
oscillations are reduced and the quantum efficiency is decreased in the
presence of pure dephasing.


title: On optimum Hamiltonians for state transformations

abstract:
  For a prescribed pair of quantum states |psi_I> and |psi_F> we establish an
elementary derivation of the optimum Hamiltonian, under constraints on its
eigenvalues, that generates the unitary transformation |psi_I> --> |psi_F> in
the shortest duration. The derivation is geometric in character and does not
rely on variational calculus.


title: On multipartite invariant states II. Orthogonal symmetry

abstract:
  We construct a new class of multipartite states possessing orthogonal
symmetry. This new class defines a convex hull of multipartite states which are
invariant under the action of local unitary operations introduced in our
previous paper "On multipartite invariant states I. Unitary symmetry". We study
basic properties of multipartite symmetric states: separability criteria and
multi-PPT conditions.


title: Quantum secret sharing between m-party and n-party with six states

abstract:
  We propose a quantum secret sharing scheme between $m$-party and $n$-party
using three conjugate bases, i.e. six states. A sequence of single photons,
each of which is prepared in one of the six states, is used directly to encode
classical information in the quantum secret sharing process. In this scheme,
each of all $m$ members in group 1 choose randomly their own secret key
individually and independently, and then directly encode their respective
secret information on the states of single photons via unitary operations, then
the last one (the $m$th member of group 1) sends $1/n$ of the resulting qubits
to each of group 2. By measuring their respective qubits, all members in group
2 share the secret information shared by all members in group 1. The secret
message shared by group 1 and group 2 in such a way that neither subset of each
group nor the union of a subset of group 1 and a subset of group 2 can extract
the secret message, but each whole group (all the members of each group) can.
The scheme is asymptotically 100% in efficiency. It makes the Trojan horse
attack with a multi-photon signal, the fake-signal attack with EPR pairs, the
attack with single photons, and the attack with invisible photons to be
nullification. We show that it is secure and has an advantage over the one
based on two conjugate bases. We also give the upper bounds of the average
success probabilities for dishonest agent eavesdropping encryption using the
fake-signal attack with any two-particle entangled states. This protocol is
feasible with present-day technique.


title: Frequency Spectra and Probability Distributions for Quantum Fluctuations

abstract:
  This paper discusses two distinct, but related issues in quantum fluctuation
effects. The first is the frequency spectrum which can be assigned to one loop
quantum processes. The formal spectrum is a flat one, but the finite quantum
effects can be associated with a rapidly oscillating spectrum, as in the case
of the Casimir effect. The leads to the speculation that one might be able to
dramatically change the final answer by upsetting the delicate cancellation
which usually occurs. The second issue is the probability distribution for
quantum fluctuations. It is well known that quantities which are linear in a
free quantum field have a Gaussian distribution. Here it will be argued that
quadratic quantities, such as the quantum stress tensor, must have a skewed
distribution. Some possible implications of this result for inflationary
cosmology will be discussed. In particular, this might be a source for
non-Gaussianity.


title: Shot noise in an electron waveguide square root of NOT gate

abstract:
  We present a calculation of the shot noise in a ballistic electron waveguide
square root of NOT gate. A general expression for the shot noise in the leads
connected to these types of gates is shown. We then parameterize an S-matrix
which qualitatively describes the action of a square root of NOT gate
previously found through numerical methods for GaAs/Al_xGa_{1-x}As based
waveguides systems. Using this S-matrix, the shot noise in a single output lead
and across two output leads is calculated. We find that the measurement of the
shot noise across two output leads allows for the determination of the fidelity
of the gate itself.


title: Superbroadcasting of continuous variables mixed states

abstract:
  We consider the problem of broadcasting quantum information encoded in the
average value of the field from N to M>N copies of mixed states of radiation
modes. We derive the broadcasting map that preserves the complex amplitude,
while optimally reducing the noise in conjugate quadratures. We find that from
two input copies broadcasting is feasible, with the possibility of simultaneous
purification (superbroadcasting). We prove similar results for purification
(M<=N) and for phase-conjugate broadcasting.


title: Phase-Remapping Attack in Practical Quantum Key Distribution Systems

abstract:
  Quantum key distribution (QKD) can be used to generate secret keys between
two distant parties. Even though QKD has been proven unconditionally secure
against eavesdroppers with unlimited computation power, practical
implementations of QKD may contain loopholes that may lead to the generated
secret keys being compromised. In this paper, we propose a phase-remapping
attack targeting two practical bidirectional QKD systems (the "plug & play"
system and the Sagnac system). We showed that if the users of the systems are
unaware of our attack, the final key shared between them can be compromised in
some situations. Specifically, we showed that, in the case of the
Bennett-Brassard 1984 (BB84) protocol with ideal single-photon sources, when
the quantum bit error rate (QBER) is between 14.6% and 20%, our attack renders
the final key insecure, whereas the same range of QBER values has been proved
secure if the two users are unaware of our attack; also, we demonstrated three
situations with realistic devices where positive key rates are obtained without
the consideration of Trojan horse attacks but in fact no key can be distilled.
We remark that our attack is feasible with only current technology. Therefore,
it is very important to be aware of our attack in order to ensure absolute
security. In finding our attack, we minimize the QBER over individual
measurements described by a general POVM, which has some similarity with the
standard quantum state discrimination problem.


title: Simulation of Quantum Adiabatic Search in the Presence of Noise

abstract:
  Results are presented of a large-scale simulation of the quantum adiabatic
search (QuAdS) algorithm in the presence of noise. The algorithm is applied to
the NP-Complete problem Exact Cover 3 (EC3). The noise is assumed to
Zeeman-couple to the qubits and its effects on the algorithm's performance is
studied for various levels of noise power, and for 4 different types of noise
polarization. We examine the scaling relation between the number of bits N (EC3
problem size) and the algorithm's noise-averaged median run-time <T(N)>. Clear
evidence is found of the algorithm's sensitivity to noise. Two fits to the
simulation results were done: (1) power-law scaling <T(N)> = aN**b; and (2)
exponential scaling <T(N)> = a[exp(bN) - 1]. Both types of scaling relations
provided excellent fits. We demonstrate how noise leads to decoherence in
QuAdS, estimate the amount of decoherence in our simulations, and derive an
upper bound for the noise-averaged QuAdS success probability in the weak noise
limit appropriate for our simulations.


title: Remarkable Degenerate Quantum Stabilizer Codes Derived from Duadic Codes

abstract:
  Good quantum codes, such as quantum MDS codes, are typically nondegenerate,
meaning that errors of small weight require active error-correction, which
is--paradoxically--itself prone to errors. Decoherence free subspaces, on the
other hand, do not require active error correction, but perform poorly in terms
of minimum distance. In this paper, examples of degenerate quantum codes are
constructed that have better minimum distance than decoherence free subspaces
and allow some errors of small weight that do not require active error
correction. In particular, two new families of [[n,1,>= sqrt(n)]]_q degenerate
quantum codes are derived from classical duadic codes.


title:
Temporal Distinguishability of an N-Photon State and Its
  Characterization by Quantum Interference

abstract:
  We present a multi-mode model to describe an arbitrary N-photon state with a
wide spectral range and some arbitrary temporal distribution. In general, some
of the $N$ photons are spread out in time while other may overlap and become
indistinguishable. From this model, we find that the temporal
(in)distinguishability of photons is related to the exchange symmetry of the
multi-photon wave function. We find that simple multi-photon detection scheme
gives rise to a more general photon bunching effect with the famous two-photon
effect as a special case. We then send this N-photon state into a recently
discovered multi-photon interference scheme. We calculate the visibility of the
multi-photon interference scheme and find that it is related to the temporal
distinguishability of the $N$ photons. Maximum visibility of one is achieved
for the indistinguishable N-photon state whereas the visibility degrades when
some of the photons are separated and become distinguishable. Thus we can
identify an experimentally measurable quantity that may quantitatively define
the degree of indistinguishability of an N-photon state. This presents a
quantitative demonstration of the complementary principle of quantum
interference.


title:
Secure Direct Communication Based on Secret Transmitting Order of
  Particles

abstract:
  We propose the schemes of quantum secure direct communication (QSDC) based on
secret transmitting order of particles. In these protocols, the secret
transmitting order of particles ensures the security of communication, and no
secret messages are leaked even if the communication is interrupted for
security. This strategy of security for communication is also generalized to
quantum dialogue. It not only ensures the unconditional security but also
improves the efficiency of communication.


title:
Globally controlled quantum wires for perfect qubit transport, mirroring
  and computing

abstract:
  It is expected that quantum wires (q-wires), will be required to transport
quantum information within many quantum computer implementations. Here we
describe a new design for a q-wire with perfect transmission using a uniformly
coupled Ising spin chain subject to global (homogeneously-applied) pulses.
Besides allowing for perfect transport of single qubits, the design also yields
the perfect ``mirroring'' of multiply encoded qubits within the wire. We
further utilise this global-pulse generated perfect mirror operation as a
``clock cycle'' to perform universal quantum computation on these multiply
encoded qubits. We demonstrate the operation of single and two-qubit quantum
logic gates and show that only $N-1$ complete mirror cycles are required to
execute a quantum Fourier transform on $N$ qubits encoded within the q-wire.


title: Superconducting Circuits and Quantum Information

abstract:
  Superconducting circuits can behave like atoms making transitions between two
levels. Such circuits can test quantum mechanics at macroscopic scales and be
used to conduct atomic-physics experiments on a silicon chip.


title: Quantum Mechanical Realization of a Popescu-Rohrlich Box

abstract:
  We consider quantum ensembles which are determined by pre- and
post-selection. Unlike the case of only pre-selected ensembles, we show that in
this case the probabilities for measurement outcomes at intermediate times
satisfy causality only rarely; such ensembles can in general be used to signal
between causally disconnected regions. We show that under restrictive
conditions, there are certain non-trivial bi-partite ensembles which do satisfy
causality. These ensembles give rise to a violation of the CHSH inequality,
which exceeds the maximal quantum violation given by Tsirelson's bound, $B_{\rm
CHSH}\le 2\sqrt2$, and obtains the Popescu-Rohrlich bound for the maximal
violation, $B_{\rm CHSH}\le 4$. This may be regarded as an a posteriori
realization of super-correlations, which have recently been termed
Popescu-Rohrlich boxes.


title:
Calculation of the Hidden Symmetry Operator for a $\cP\cT$-Symmetric
  Square Well

abstract:
  It has been shown that a Hamiltonian with an unbroken $\cP\cT$ symmetry also
possesses a hidden symmetry that is represented by the linear operator $\cC$.
This symmetry operator $\cC$ guarantees that the Hamiltonian acts on a Hilbert
space with an inner product that is both positive definite and conserved in
time, thereby ensuring that the Hamiltonian can be used to define a unitary
theory of quantum mechanics. In this paper it is shown how to construct the
operator $\cC$ for the $\cP\cT$-symmetric square well using perturbative
techniques.


title:
Entanglement distribution for a practical quantum-dot-based quantum
  processor architecture

abstract:
  We propose a quantum dot architecture for enabling universal quantum
information processing. Quantum registers, consisting of arrays of vertically
stacked self-assembled semiconductor quantum dots, are connected by chains of
in-plane self-assembled dots. We propose an entanglement distributor, a device
for producing and distributing maximally entangled qubits on demand,
communicated through in-plane dot chains. This enables the transmission of
entanglement to spatially separated register stacks, providing a resource for
the realisation of a sizeable quantum processor built from coupled register
stacks of practical size. Our entanglement distributor could be integrated into
many of the present proposals for self-assembled quantum dot-based quantum
computation. Our device exploits the properties of simple, relatively short,
spin-chains and does not require microcavities. Utilizing the properties of
self-assembled quantum dots, after distribution the entanglement can be mapped
into relatively long lived spin qubits and purified, providing a flexible,
distributed, off-line resource.


title:
Three-party quantum secure direct communication based on
  Greenberger-Horne-Zeilinger states

abstract:
  We present a three-party simultaneous quantum secure direct communication
(QSDC) scheme by using Greenberger-Horne-Zeilinger (GHZ) states. This scheme
can be directly generalized to $N$-party QSDC by using $n$-particle GHZ states.
We show that the many-party simultaneous QSDC scheme is secure not only against
the intercept-and-resend attack but also against the disturbance attack.


title:
Probabilistic Dense Coding Using a Non-symmetric Multipartite Quantum
  Channel

abstract:
  We investigate probabilistic dense coding in non-symmetric Hilbert spaces of
the sender's and the receiver's particles. The sender and the receiver share
the multipartite non-maximally quantum channel. We also discuss the average
information.


title: Quantum Dialogue by Using the GHZ State

abstract:
  We present a quantum dialogue protocol by using the
Greenberger-Horne-Zeilinger (GHZ) state. In this paper, we point out that the
`quantum dialogue' communication scheme recently introduced by Nguyen can be
eavesdropped on under an intercept-and-resend attack. We also give a revised
control mode to detect this attack. Hence, within the present version two users
can exchange their secret messages securely and simultaneously, and the
efficiency of information transmission can be successfully increased.


title: Multi-Receiver Quantum Dense Coding with Non-Symmetric Quantum Channel

abstract:
  A two-receiver quantum dense coding scheme and an $N$-receiver quantum dense
coding scheme, in the case of non-symmetric Hilbert spaces of the particles of
the quantum channel, are investigated in this paper. A sender can send his
messages to many receivers simultaneously. The scheme can be applied to quantum
secret sharing and controlled quantum dense coding.


title:
Swapping and permutation operators as entanglement witnesses for quantum
  Heisenberg spin-$s$ systems

abstract:
  Using the SU($N$) representation of the group theory, we derive the general
form of the spin swapping operator for the quantum Heisenberg spin-$s$ systems.
We further prove that such a spin swapping operator is equal to the spin
singlet pairing operator under the partial transposition. For SU(2) invariant
states, it is shown that the expectation value of the spin swapping operator
and its generalizations, the permutations, can be used as an entanglement
witness, especially, for the formulation of observable conditions of
entanglement.


title:
Experimentally realizable quantum comparison of coherent states and its
  applications

abstract:
  When comparing quantum states to each other, it is possible to obtain an
unambiguous answer, indicating that the states are definitely different,
already after a single measurement. In this paper we investigate comparison of
coherent states, which is the simplest example of quantum state comparison for
continuous variables. The method we present has a high success probability, and
is experimentally feasible to realize as the only required components are beam
splitters and photon detectors. An easily realizable method for quantum state
comparison could be important for real applications. As examples of such
applications we present a "lock and key" scheme and a simple scheme for quantum
public key distribution.


title: Time-optimal Control of Spin Systems

abstract:
  The paper discusses various aspects of time-optimal control of quantum spin
systems, modelled as right-invariant systems on a compact Lie group G. The main
results are the reduction of such a system to an equivalent system on a
homogeneous space G/H, and the explicit determination of optimal trajectories
on G/H in the case where G/H is a Riemannian symmetric space. These results are
mainly obtained by using methods from Lie theory and geometric control.


title: Decoherence in closed and open systems

abstract:
  A generalized formal framework for decoherence, that can be used both in open
and closed quantum systems, is sketched. In this context, the relationship
between the decoherence of a closed system and the decoherence of its
subsystems is studied, and the corresponding decoherence times are defined: for
macroscopic systems, the decoherence time of the closed system is much greater
than the decoherence time of its subsystems. Finally, it is shown that the
application of the new formal framework to a well-known model leads to
physically adequate results.


title:
Photon pair generation using four-wave mixing in a microstructured
  fibre: theory versus experiment

abstract:
  We develop a theoretical analysis of four-wave mixing used to generate photon
pairs useful for quantum information processing. The analysis applies to a
single mode microstructured fibre pumped by an ultra-short coherent pulse in
the normal dispersion region. Given the values of the optical propagation
constant inside the fibre, we can estimate the created number of photon pairs
per pulse, their central wavelength and their respective bandwidth. We use the
experimental results from a picosecond source of correlated photon pairs using
a micro-structured fibre to validate the model. The fibre is pumped in the
normal dispersion regime at 708nm and phase matching is satisfied for widely
spaced parametric wavelengths of 586nm and 894nm. We measure the number of
photons per pulse using a loss-independent coincidence scheme and compare the
results with the theoretical expectation. We show a good agreement between the
theoretical expectations and the experimental results for various fibre lengths
and pump powers.


title: State Tomography: Hidden Differences

abstract:
  We address the problem of completely characterizing multi-particle states
including loss of information to unobserved degrees of freedom. In systems
where non-classical interference plays a role, such as linear-optics quantum
gates, such information can degrade interference in two ways, by decoherence
and by distinguishing the particles. Distinguishing information, often the
limiting factor for quantum optical devices, is not correctly described by
previous state-reconstruction techniques, which account only for decoherence.
We extend these techniques and find that a single modified density matrix can
completely describe partially-coherent, partially-distinguishable states. We
use this observation to experimentally characterize two-photon polarization
states in single-mode optical fiber.


title:
Determination of the border between "shallow" and "deep" tunneling
  regions for Herman-Kluk method by asymptotic approach

abstract:
  The evaluation of a tunneling tail by the Herman-Kluk method, which is a
quasiclassical way to compute quantum dynamics, is examined by asymptotic
analysis. In the shallower part of the tail, as well as in the classically
allowed region, it is shown that the leading terms of semiclassical evaluations
of quantum theory and the Herman-Kluk formula agree, which is known as an
asymptotic equivalence. In the deeper part, it is shown that the asymptotic
equivalence breaks down, due to the emergence of unusual "tunneling
trajectory", which is an artifact of the Herman-Kluk method.


title: Boundary effect of deterministic dense coding

abstract:
  We present a rigorous proof of an interesting boundary effect of
deterministic dense coding first observed by Mozes et al. [Phys. Rev. A 71,
012311 (2005)]. Namely, it is shown that $d^2-1$ cannot be the maximal alphabet
size of any isometric deterministic dense coding schemes utilizing $d$-level
partial entanglement.


title: Theoretical study of a cold atom beam splitter

abstract:
  A theoretical model is presented for the study of the dynamics of a cold
atomic cloud falling in the gravity ﬁeld in the presence of two crossing
dipole guides. The cloud is split between the two branches of this laser guide,
and we compare experimental measurements of the splitting efﬁciency with
semiclassical simulations. We then explore the possibilities of optimization of
this beam splitter. Our numerical study also gives access to detailed
information, such as the atom temperature after the splitting.


title: Quantum and classical probability as Bayes-optimal observation

abstract:
  We propose a simple abstract formalisation of the act of observation, in
which the system and the observer are assumed to be in a pure state and their
interaction deterministically changes the states such that the outcome can be
read from the state of the observer after the interaction. If the observer
consistently realizes the outcome which maximizes the likelihood ratio that the
outcome pertains to the system under study (and not to his own state), he will
be called Bayes-optimal. We calculate the probability if for each trial of the
experiment the observer is in a new state picked randomly from his set of
states, and the system under investigation is taken from an ensemble of
identical pure states. For classical statistical mixtures, the relative
frequency resulting from the maximum likelihood principle is an unbiased
estimator of the components of the mixture. For repeated Bayes-optimal
observation in case the state space is complex Hilbert space, the relative
frequency converges to the Born rule. Hence, the principle of Bayes-optimal
observation can be regarded as an underlying mechanism for the Born rule. We
show the outcome assignment of the Bayes-optimal observer is invariant under
unitary transformations and contextual, but the probability that results from
repeated application is non-contextual. The proposal gives a concise
interpretation for the meaning of the occurrence of a single outcome in a
quantum experiment as the unique outcome that, relative to the state of the
system, is least dependent on the state of the observe at the instant of
measurement.


title: Quantum-classical correspondence on compact phase space

abstract:
  We propose to study the $L^2$-norm distance between classical and quantum
phase space distributions, where for the latter we choose the Wigner function,
as a global phase space indicator of quantum-classical correspondence. For
example, this quantity should provide a key to understand the correspondence
between quantum and classical Loschmidt echoes. We concentrate on fully chaotic
systems with compact (finite) classical phase space. By means of numerical
simulations and heuristic arguments we find that the quantum-classical fidelity
stays at one up to Ehrenfest-type time scale, which is proportional to the
logarithm of effective Planck constant, and decays exponentially with a maximal
classical Lyapunov exponent, after that time.


title: Quantum regression theorem for non-Markovian Lindblad equations

abstract:
  We find the conditions under which a quantum regression theorem can be
assumed valid for non-Markovian master equations consisting in Lindblad
superoperators with memory kernels. Our considerations are based on a
generalized Born-Markov approximation, which allows us to obtain our results
from an underlying Hamiltonian description. We demonstrate that a non-Markovian
quantum regression theorem can only be granted in a stationary regime if the
dynamics satisfies a quantum detailed balance condition. As an example we study
the correlations of a two level system embedded in a complex structured
reservoir and driven by an external coherent field.


title:
Scalable designs for quantum computing with rare-earth-ion-doped
  crystals

abstract:
  Due to inhomogeneous broadening, the absorption lines of rare-earth-ion
dopands in crystals are many order of magnitudes wider than the homogeneous
linewidths. Several ways have been proposed to use ions with different
inhomogeneous shifts as qubit registers, and to perform gate operations between
such registers by means of the static dipole coupling between the ions.
  In this paper we show that in order to implement high-fidelity quantum gate
operations by means of the static dipole interaction, we require the
participating ions to be strongly coupled, and that the density of such
strongly coupled registers in general scales poorly with register size.
Although this is critical to previous proposals which rely on a high density of
functional registers, we describe architectures and preparation strategies that
will allow scalable quantum computers based on rare-earth-ion doped crystals.


title:
Secret sharing of quantum information via entanglement swapping in
  cavity QED

abstract:
  We proposed a scheme on secret sharing of quantum information based on
entanglement swapping in cavity QED. In our scheme, the effects of cavity decay
and thermal field are all eliminated.


title:
Entangled States Generated via Two Superconducting Quantum Interference
  Devices(SQUIDs) in cavity QED

abstract:
  We propose a scheme for generating entangled states for two superconducting
quantum interference devices in a thermal cavity with the assistance of a
microwave pulse.


title:
Controlled Quantum Dense Coding in a Four-particle Non-maximally
  Entangled State via Local Measurements

abstract:
  A controlled quantum dense coding scheme is investigated with a four-particle
non-maximal quantum channel. The amount of classical information is shown to be
capable of being controlled by the controllers through adjustments of the local
measurement angles and to depend on the coefficients of the quantum channel; in
addition, the four particles are distributed in two inverse ways in such an
quantum channel. A restricted condition for distributing the particles to
realize quantum dense coding in an arbitrary ($N+2$)-particle quantum channel
is proposed.


title: Controlled Secure Direct Communication by Using GHZ Entangled State

abstract:
  We present a controlled secure direct communication protocol by using
Greenberger-Horne-Zeilinger (GHZ) entangled state via swapping quantum
entanglement and local unitary operations. Since messages transferred only by
using local operations and a public channel after entangled states are
successfully distributed, this protocol can protect the communication against a
destroying-travel-qubit-type attack. This scheme can also be generalized to a
multi-party control system.


title:
Fidelity of holonomic quantum computations in the case of random errors
  in the values of control parameters

abstract:
  We investigate the influence of random errors in external control parameters
on the stability of holonomic quantum computation in the case of arbitrary
loops and adiabatic connections. A simple expression is obtained for the case
of small random uncorrelated errors. Due to universality of mathematical
description our results are valid for any physical system which can be
described in terms of holonomies. Theoretical results are confirmed by
numerical simulations.


title:
Multi-step quantum secure direct communication using multi-particle
  Green-Horne-Zeilinger state

abstract:
  A multi-step quantum secure direct communication protocol using blocks of
multi-particle maximally entangled state is proposed. In this protocol, the
particles in a Green-Horne-Zeilinger state are sent from Alice to Bob in
batches in several steps. It has the advantage of high efficiency and high
source capacity.


title: Quantum Manipulation of Trapped Ions in Two Dimensional Coulomb Crystals

abstract:
  We show that a large number of ions stored in a Penning trap, and forming a
2D Coulomb crystal, provides an almost ideal system for scalable quantum
computation and quantum simulation. In particular, the coupling of the internal
states to the motion of the ions transverse to the crystal plane, allows one to
implement two qubit quantum gates. We analyze in detail the decoherence induced
by anharmonic couplings with in--plane hot vibrational modes, and show that
very high gate fidelities can be achieved with current experimental set--ups.


title: The photoelectric effect in external fields

abstract:
  Atoms and negative ions interacting with laser photons yield a coherent
source of photoelectrons. Applying external fields to photoelectrons gives rise
to interesting and valuable interference phenomena. We analyze the spatial
distribution of the photocurrent using elementary quantum methods. The
photoelectric effect is shown to be an interesting example for the use of
coherent particle sources in quantum mechanics.


title:
Entanglement Is Not Necessary for Perfect Discrimination between Unitary
  Operations

abstract:
  We show that a unitary operation (quantum circuit) secretely chosen from a
finite set of unitary operations can be determined with certainty by
sequentially applying only a finite amount of runs of the unknown circuit. No
entanglement or joint quantum operations is required in our scheme. We further
show that our scheme is optimal in the sense that the number of the runs is
minimal when discriminating only two unitary operations.


title: Quantum Adiabatic Computation and the Travelling Salesman Problem

abstract:
  The NP-complete problem of the travelling salesman (TSP) is considered in the
framework of quantum adiabatic computation (QAC). We first derive a remarkable
lower bound for the computation time for adiabatic algorithms in general as a
function of the energy involved in the computation. Energy, and not just time
and space, must thus be considered in the evaluation of algorithm complexity,
in perfect accordance with the understanding that all computation is physical.
We then propose, with oracular Hamiltonians, new quantum adiabatic algorithms
of which not only the lower bound in time but also the energy requirement do
not increase exponentially in the size of the input. Such an improvement in
both time and energy complexity, as compared to all other existing algorithms
for TSP, is apparently due to quantum entanglement. We also appeal to the
general theory of Diophantine equations in a speculation on physical
implementation of those oracular Hamiltonians.


title:
Bound states of the s-wave Klein-Gordon equation with equal scalar and
  vector Standard Eckart Potential

abstract:
  A supersymmetric technique for the bound-state solutions of the s-wave
Klein--Gordon equation with equal scalar and vector standard Eckart type
potential is proposed. Its exact solutions are obtained. Possible
generalization of our approach is outlined.


title: Detuned Electromagnetically Induced Transparency in an $N$-type Atom

abstract:
  The electromagnetically induced transparency (EIT) in an $N$ configuration is
studied under both resonant and off-resonant conditions. In a certain
off-resonant condition the dark state of the four level system, which is almost
the same as the resonant dark state in $\Lambda$ configuration, is rebuilt. The
actual system with damping is examined using optical Bloch equation, both
numerically and analytically. Based on this detuned dark state, some new
applications with frequency shifts can be realized.


title:
Simultaneous optical trapping and detection of atoms by microdisk
  resonators

abstract:
  We propose a scheme for simultaneously trapping and detecting single atoms
near the surface of a substrate using whispering gallery modes of a microdisk
resonator. For efficient atom-mode coupling the atom should be placed within
approximately 150 nm from the disk. We show that a combination of red and blue
detuned modes can form an optical trap at such distances while the back-action
of the atom on the field modes can simultaneously be used for atom detection.
We investigate these trapping potentials including van-der-Waals and
Casimir-Polder forces and discuss corresponding atom detection efficiencies,
depending on a variety of system parameters. Finally, we analyze the
feasibility of non-destructive detection.


title:
Noise sequences of infinite matrices and their applications to the
  characterization of the canonical phase and box localization observables

abstract:
  Noise sequences of infinite matrices associated with covariant phase and box
localization observables are defined and determined. The canonical observables
are characterized within the relevant classes of observables as those with
asymptotically minimal or minimal noise, i.e., the noise tending to 0 or having
the value 0.


title:
Information/disturbance trade-off in continuous variable Gaussian
  systems

abstract:
  We address the information/disturbance trade-off for state-measurements on
continuous variable Gaussian systems and suggest minimal schemes for
implementations. In our schemes, the symbols from a given alphabet are encoded
in a set of Gaussian signals which are coupled to a probe excited in a known
state. After the interaction the probe is measured, in order to infer the
transmitted state, while the conditional state of the signal is left for the
subsequent user. The schemes are minimal, {\em i.e.} involve a single
additional probe, and allow for the nondemolitive transmission of a continuous
real alphabet over a quantum channel. The trade-off between information gain
and state disturbance is quantified by fidelities and, after optimization with
respect to the measurement, analyzed in terms of the energy carried by the
signal and the probe. We found that transmission fidelity only depends on the
energy of the signal and the probe, whereas estimation fidelity also depends on
the alphabet size and the measurement gain. Increasing the probe energy does
not necessarily lead to a better trade-off, the most relevant parameter being
the ratio between the alphabet size and the signal width, which in turn
determine the allocation of the signal energy.


title: Oblivious transfer and quantum channels

abstract:
  We show that oblivious transfer can be seen as the classical analogue to a
quantum channel in the same sense as non-local boxes are for maximally
entangled qubits.


title: Quantum Probability Theory

abstract:
  The mathematics of classical probability theory was subsumed into classical
measure theory by Kolmogorov in 1933. Quantum theory as nonclassical
probability theory was incorporated into the beginnings of noncommutative
measure theory by von Neumann in the early thirties, as well. To precisely this
end, von Neumann initiated the study of what are now called von Neumann
algebras and, with Murray, made a first classification of such algebras into
three types. The nonrelativistic quantum theory of systems with finitely many
degrees of freedom deals exclusively with type I algebras. However, for the
description of further quantum systems, the other types of von Neumann algebras
are indispensable. The paper reviews quantum probability theory in terms of
general von Neumann algebras, stressing the similarity of the conceptual
structure of classical and noncommutative probability theories and emphasizing
the correspondence between the classical and quantum concepts, though also
indicating the nonclassical nature of quantum probabilistic predictions. In
addition, differences between the probability theories in the type I, II and
III settings are explained. A brief description is given of quantum systems for
which probability theory based on type I algebras is known to be insufficient.
These illustrate the physical significance of the previously mentioned
differences.


title: Trapped ion quantum computation with transverse phonon modes

abstract:
  We propose a scheme to implement quantum gates on any pair of trapped ions
immersed in a large linear crystal, using interaction mediated by the
transverse phonon modes. Compared with the conventional approaches based on the
longitudinal phonon modes, this scheme is much less sensitive to ion heating
and thermal motion outside of the Lamb-Dicke limit thanks to the stronger
confinement in the transverse direction. The cost for such a gain is only a
moderate increase of the laser power to achieve the same gate speed. We also
show how to realize arbitrary-speed quantum gates with transverse phonon modes
based on simple shaping of the laser pulses.


title: One-mirror Fabry-Perot and one-slit Young interferometry

abstract:
  We describe a new and distinctive interferometry in which a probe particle
scatters off a superposition of locations of a single free target particle. In
one dimension, probe particles incident on superposed locations of a single
"mirror" can interfere as if in a Fabry-Perot interferometer; in two
dimensions, probe particles scattering off superposed locations of a single
"slit" can interfere as if in a two-slit Young interferometer. The condition
for interference is loss of orthogonality of the target states and reduces, in
simple examples, to transfer of orthogonality from target to probe states. We
analyze experimental parameters and conditions necessary for interference to be
observed.


title: Localization of non-relativistic particles

abstract:
  This paper is a contribution to the problem of particle localization in
non-relativistic Quantum Mechanics. Our main results will be (1) to formulate
the problem of localization in terms of invariant subspaces of the Hilbert
space, and (2) to show that the rigged Hilbert space incorporates particle
localization in a natural manner.


title: Feedback control for communication with non-orthogonal states

abstract:
  Communicating classical information with a quantum system involves the
receiver making a measurement on the system so as to distinguish as well as
possible the alphabet of states used by the sender. We consider the situation
in which this measurement takes an appreciable time. In this case the
measurement must be described by a continuous measurement process. We consider
a continuous implementation of the optimal measurement for distinguishing
between two non-orthogonal states, and show that feedback control can be used
during this measurement to increase the rate at which the information regarding
the initial preparation is obtained. We show that while the maximum obtainable
increase is modest, the effect is purely quantum mechanical in the sense that
the enhancement is only possible when the initial states are non-orthogonal. We
find further that the enhancement in the rate of information gain is achieved
at the expense of reducing the total information which the measurement can
extract in the long-time limit.


title: Trapping atoms on a transparent permanent-magnet atom chip

abstract:
  We describe experiments on trapping of atoms in microscopic magneto-optical
traps on an optically transparent permanent-magnet atom chip. The chip is made
of magnetically hard ferrite-garnet material deposited on a dielectric
substrate. The confining magnetic fields are produced by miniature magnetized
patterns recorded in the film by magneto-optical techniques. We trap Rb atoms
on these structures by applying three crossed pairs of counter-propagating
laser beams in the conventional magneto-optical trapping (MOT) geometry. We
demonstrate the flexibility of the concept in creation and in-situ modification
of the trapping geometries through several experiments.


title: Cloning of Gaussian states by linear optics

abstract:
  We analyze in details a scheme for cloning of Gaussian states based on linear
optical components and homodyne detection recently demonstrated by U. L.
Andersen et al. [PRL 94 240503 (2005)]. The input-output fidelity is evaluated
for a generic (pure or mixed) Gaussian state taking into account the effect of
non-unit quantum efficiency and unbalanced mode-mixing. In addition, since in
most quantum information protocols the covariance matrix of the set of input
states is not perfectly known, we evaluate the average cloning fidelity for
classes of Gaussian states with the degree of squeezing and the number of
thermal photons being only partially known.


title: Wigner function statistics in classically chaotic systems

abstract:
  We have studied statistical properties of the values of the Wigner function
W(x) of 1D quantum maps on compact 2D phase space of finite area V. For this
purpose we have defined a Wigner function probability distribution P(w) = (1/V)
int delta(w-W(x)) dx, which has, by definition, fixed first and second moment.
In particular, we concentrate on relaxation of time evolving quantum state in
terms of W(x), starting from a coherent state. We have shown that for a
classically chaotic quantum counterpart the distribution P(w) in the
semi-classical limit becomes a Gaussian distribution that is fully determined
by the first two moments. Numerical simulations have been performed for the
quantum sawtooth map and the quantized kicked top. In a quantum system with
Hilbert space dimension N (similar 1/hbar) the transition of P(w) to a Gaussian
distribution was observed at times t proportional to log N. In addition, it has
been shown that the statistics of Wigner functions of propagator eigenstates is
Gaussian as well in the classically fully chaotic regime. We have also studied
the structure of the nodal cells of the Wigner function, in particular the
distribution of intersection points between the zero manifold and arbitrary
straight lines.


title: Entanglement of an impurity and conduction spins in the Kondo model

abstract:
  Based on Yosida's ground state of the single-impurity Kondo Hamiltonian, we
study three kinds of entanglement between an impurity and conduction electron
spins. First, it is shown that the impurity spin is maximally entangled with
all the conduction electrons. Second, a two-spin density matrix of the impurity
spin and one conduction electron spin is given by a Werner state. We find that
the impurity spin is not entangled with one conduction electron spin even
within the Kondo screening length $\xi_K$, although there is the spin-spin
correlation between them. Third, we show the density matrix of two conduction
electron spins is nearly same to that of a free electron gas. The single
impurity does not change the entanglement structure of the conduction electrons
in contrast to the dramatic change in electrical resistance.


title: Quantum process tomography and Linblad estimation of a solid state qubit

abstract:
  We present an example of quantum process tomography (QPT) performed on a
single solid state qubit. The qubit used is two energy levels of the triplet
state in the Nitrogen-Vacancy defect in Diamond. Quantum process tomography is
applied to a qubit which has been allowed to decohere for three different time
periods. In each case the process is found in terms of the chi matrix
representation and the affine map representation. The discrepancy between
experimentally estimated process and the closest physically valid process is
noted. The results of QPT performed after three different decoherence times are
used to find the error generators, or Lindblad operators, for the system, using
the technique introduced by Boulant et al. [N. Boulant, T.F. Havel, M.A. Pravia
and D.G. Cory, Phys. Rev. A 67, 042322 (2003)].


title:
Simulation and Implementation of Decoy State Quantum Key Distribution
  over 60km Telecom Fiber

abstract:
  Decoy state quantum key distribution (QKD) has been proposed as a novel
approach to improve dramatically both the security and the performance of
practical QKD set-ups. Recently, many theoretical efforts have been made on
this topic and have theoretically predicted the high performance of decoy
method. However, the gap between theory and experiment remains open. In this
paper, we report the first experiments on decoy state QKD, thus bridging the
gap. Two protocols of decoy state QKD are implemented: one-decoy protocol over
15km of standard telecom fiber, and weak+vacuum protocol over 60km of standard
telecom fiber. We implemented the decoy state method on a modified commercial
QKD system. The modification we made is simply adding commercial acousto-optic
modulator (AOM) on the QKD system. The AOM is used to modulate the intensity of
each signal individually, thus implementing the decoy state method. As an
important part of implementation, numerical simulation of our set-up is also
performed. The simulation shows that standard security proofs give a zero key
generation rate at the distance we perform decoy state QKD (both 15km and
60km). Therefore decoy state QKD is necessary for long distance secure
communication. Our implementation shows explicitly the power and feasibility of
decoy method, and brings it to our real-life.


title: Quantum automata, braid group and link polynomials

abstract:
  The spin--network quantum simulator model, which essentially encodes the
(quantum deformed) SU(2) Racah--Wigner tensor algebra, is particularly suitable
to address problems arising in low dimensional topology and group theory. In
this combinatorial framework we implement families of finite--states and
discrete--time quantum automata capable of accepting the language generated by
the braid group, and whose transition amplitudes are colored Jones polynomials.
The automaton calculation of the polynomial of (the plat closure of) a link L
on 2N strands at any fixed root of unity is shown to be bounded from above by a
linear function of the number of crossings of the link, on the one hand, and
polynomially bounded in terms of the braid index 2N, on the other. The growth
rate of the time complexity function in terms of the integer k appearing in the
root of unity q can be estimated to be (polynomially) bounded by resorting to
the field theoretical background given by the Chern-Simons theory.


title:
Decomposing a pulsed optical parametric amplifier into independent
  squeezers

abstract:
  We discuss the concept of characteristic squeezing modes applied to a
travelling-wave optical parametric amplifier pumped by an ultrashort pulse. The
characteristic modes undergo decoupled single-mode squeezing transformations,
and therefore they form a useful basis to describe the evolution of the entire
multimode system. This provides an elegant and intuitive picture of quantum
statistical properties of parametric fluorescence. We analyse the efficiency of
detecting quadrature squeezing, and present results of numerical calculations
for a realistic nonlinear medium.


title: Transmittivity measurements by means of squeezed vacuum light

abstract:
  A method for measuring the transmittivity of optical samples by using
squeezed--vacuum radiation is illustrated. A squeezed vacuum field generated by
a below--threshold optical parametric oscillator is propagated through a
nondispersive medium and detected by a homodyne apparatus. The variance of the
detected quadrature is used for measuring the transmittivity. With this method
it is drastically reduced the number of photons passing through the sample
during the measurement interval. The results of some tests are reported.


title: Monogamy of nonlocal quantum correlations

abstract:
  We describe a new technique for obtaining Tsirelson bounds, or upper bounds
on the quantum value of a Bell inequality. Since quantum correlations do not
allow signaling, we obtain a Tsirelson bound by maximizing over all
no-signaling probability distributions. This maximization can be cast as a
linear program. In a setting where three parties, A, B, and C, share an
entangled quantum state of arbitrary dimension, we: (i) bound the trade-off
between AB's and AC's violation of the CHSH inequality, and (ii) demonstrate
that forcing B and C to be classically correlated prevents A and B from
violating certain Bell inequalities, relevant for interactive proof systems and
cryptography.


title:
A microfabricated surface-electrode ion trap for scalable quantum
  information processing

abstract:
  We demonstrate confinement of individual atomic ions in a radio-frequency
Paul trap with a novel geometry where the electrodes are located in a single
plane and the ions confined above this plane. This device is realized with a
relatively simple fabrication procedure and has important implications for
quantum state manipulation and quantum information processing using large
numbers of ions. We confine laser-cooled Mg-24 ions approximately 40 micrometer
above planar gold electrodes. We measure the ions' motional frequencies and
compare them to simulations. From measurements of the escape time of ions from
the trap, we also determine a heating rate of approximately five motional
quanta per millisecond for a trap frequency of 5.3 MHz.


title:
Interpretation of Stationary States in Prequantum Classical Statistical
  Field Theory

abstract:
  We develop a prequantum classical statistical model in that the role of
hidden variables is played by classical (vector) fields. We call this model
Prequantum Classical Statistical Field Theory (PCSFT). The correspondence
between classical and quantum quantities is asymptotic, so we call our approach
asymptotic dequantization. In this note we pay the main attention to
interpretation of so called pure quantum states (wave functions) in PCSFT,
especially stationary states. We show, see Theorem 2, that pure states of QM
can be considered as labels for Gaussian measures concentrated on one
dimensional complex subspaces of phase space that are invariant with respect to
the Schr\"odinger dynamics. ``A quantum system in a stationary state $\psi$''
in PCSFT is nothing else than a Gaussian ensemble of classical fields
(fluctuations of the vacuum field of a very small magnitude) which is not
changed in the process of Schr\"odinger's evolution. We interpret in this way
the problem of {\it stability of hydrogen atom.


title: Measuring the photon distribution by ON/OFF photodectors

abstract:
  Reconstruction of photon statistics of optical states provide fundamental
information on the nature of any optical field and find various relevant
applications. Nevertheless, no detector that can reliably discriminate the
number of incident photons is available. On the other hand the alternative of
reconstructing density matrix by quantum tomography leads to various technical
difficulties that are particular severe in the pulsed regime (where mode
matching between signal an local oscillator is very challenging). Even if
on/off detectors, as usual avalanche PhotoDiodes operating in Geiger mode, seem
useless as photocounters, recently it was shown how reconstruction of photon
statistics is possible by considering a variable quantum efficiency. Here we
present experimental reconstructions of photon number distributions of both
continuous-wave and pulsed light beams in a scheme based on on/off avalanche
photodetection assisted by maximum-likelihood estimation. Reconstructions of
the distribution for both semiclassical and quantum states of light (as single
photon, coherent, pseudothermal and multithermal states) are reported for
single-mode as well as for multimode beams. The stability and good accuracy
obtained in the reconstruction of these states clearly demonstrate the
interesting potentialities of this simple technique.


title:
Borel quantization and nonlinear quantum mechanics. A review of
  developments in the series "Symmetries in Science" I - XIII

abstract:
  The results, different aspects and applications of our method of quantisation
on configuration manifolds - called Borel Quantisation - were presented at
meetings of the series `Symmetries in Science' and can be found in the
published proceedings. The developments with numerous coauthors, on Borel
quantisation and the related family of nonlinear Schr\"odinger equations called
Doebner-Goldin equations, are reviewed and commented here.


title:
Entanglement generation via scattering of two particles with hard-core
  repulsion

abstract:
  We analyse the entanglement generation in a one dimensional scattering
process. The two colliding particles have a Gaussian wave function and interact
by hard--core repulsion.In our analysis results on the entanglement of two mode
Gaussian states are used. The produced entanglement depends in a non-obvious
way on the parameters ratio of masses and initial widths. The asymptotic
wavefunction of the two particles and its associated ellipse yield additional
geometric insight into these conditions. The difference to the quantitative
analysis of the amount of entanglement generated by beam splitters with
squeezed light is discussed.


title: Classical simulation of limited-width cluster-state quantum computation

abstract:
  We present a classical protocol, using the matrix product state
representation, to simulate cluster-state quantum computation at a cost
polynomial in the number of qubits in the cluster and exponential in d -- the
width of the cluster. We use this result to show that any log-depth quantum
computation in the gate array model, with gates linking only nearby qubits, can
be simulated efficiently on a classical computer.


title:
Comment on "Protective measurements and Bohm's trajectories" by
  Y.Aharonov, B.G.Englert, and M.O.Scully

abstract:
  In the above paper [1], it is claimed that Bohm's mechanics can be challenged
by a simple gedanken experiment involving a protecting measurement. Here we
show that this gedanken experiment can be justified without problem and without
contradicting the axiom attributing a physical meaning to the Bohmian
trajectorie


title:
Analysis of photon-atom entanglement generated by Faraday rotation in a
  cavity

abstract:
  Faraday rotation based on AC Stark shifts is a mechanism that can entangle
the polarization variables of photons and atoms. We analyze the structure of
such entanglement by using the Schmidt decomposition method. The
time-dependence of entanglement entropy and the effective Schmidt number are
derived for Gaussian amplitudes. In particular we show how the entanglement is
controlled by the initial fluctuations of atoms and photons.


title: Broadcasting of Inseparability

abstract:
  Suppose we are given an entangled pair and then one can ask how well we can
produce two entangled pairs starting from a given entangled pair using only
local operations. To give response of the above asked question, we study
broadcasting of entanglement using state dependent quantum cloning machine as a
local copier. We show that the length of the interval for
probability-amplitude-squared for broadcasting of entanglement using state
dependent cloner can be made larger than the length of the interval for
probability-amplitude-squared for broadcasting entanglement using state
independent cloner. Further we show that there exists local state dependent
cloner which gives better quality copy (in terms of average fidelity) of an
entangled pair than the local universal cloner.


title:
Why special relativity should not be a template for a fundamental
  reformulation of quantum mechanics

abstract:
  In a comparison of the principles of special relativity and of quantum
mechanics, the former theory is marked by its relative economy and apparent
explanatory simplicity. A number of theorists have thus been led to search for
a small number of postulates - essentially information theoretic in nature -
that would play the role in quantum mechanics that the relativity principle and
the light postulate jointly play in Einstein's 1905 special relativity theory.
The purpose of the present paper is to resist this idea, at least in so far as
it is supposed to reveal the fundamental form of the theory. It is argued that
the methodology of Einstein's 1905 theory represents a victory of pragmatism
over explanatory depth; and that its adoption only made sense in the context of
the chaotic state state of physics at the start of the 20th century - as
Einstein well knew.


title: Arbitrary state controlled-unitary gate by adiabatic passage

abstract:
  We propose a robust scheme involving atoms fixed in an optical cavity to
directly implement the universal controlled-unitary gate. The present technique
based on adiabatic passage uses novel dark states well suited for the
controlled-rotation operation. We show that these dark states allow the robust
implementation of a gate that is a generalisation of the controlled-unitary
gate to the case where the control qubit can be selected to be an arbitrary
state. This gate has potential applications to the rapid implementation of
quantum algorithms such as of the projective measurement algorithm. This
process is decoherence-free since excited atomic states and cavity modes are
not populated during the dynamics.


title: A deterministic cavity-QED source of polarization entangled photon pairs

abstract:
  We present two cavity quantum electrodynamics proposals that, sharing the
same basic elements, allow for the deterministic generation of entangled
photons pairs by means of a three-level atom successively coupled to two single
longitudinal mode high-Q optical resonators presenting polarization degeneracy.
In the faster proposal, the three-level atom yields a polarization entangled
photon pair via two truncated Rabi oscillations, whereas in the adiabatic
proposal a counterintuitive Stimulated Raman Adiabatic Passage process is
considered. Although slower than the former process, this second method is very
efficient and robust under fluctuations of the experimental parameters and,
particularly interesting, almost completely insensitive to atomic decay.


title: Joint reality and temporal Bell inequalities

abstract:
  Some new temporal Bell inequalities are deduced under joint realism
assumption, using some perfect correlation property. No locality condition is
needed. When the measured system is a macroscopic system, joint realism
assumption substitutes the non-invasive measurabilioty hypothesis
advantegeously, provided that the system satisfies the perfect correlation
property. The new inequalities are violated quantically. This violation can be
more severe than the similar violation in the case of precedent temporal Bell
inequalities. Some microscopic and mesoscopic situations in which these
inequalities could be tested are roughly considered.


title: Deterministic atom-light quantum interface

abstract:
  The notion of an atom-light quantum interface has been developed in the past
decade, to a large extent due to demands within the new field of quantum
information processing and communication. A promising type of such interface
using large atomic ensembles has emerged in the past several years. In this
article we review this area of research with a special emphasis on
deterministic high fidelity quantum information protocols. Two recent
experiments, entanglement of distant atomic objects and quantum memory for
light are described in detail.


title:
Improved fidelity of triggered entangled photons from single quantum
  dots

abstract:
  We demonstrate the on-demand emission of polarisation-entangled photon pairs
from the biexciton cascade of a single InAs quantum dot embedded in a GaAs/AlAs
planar microcavity. Improvements in the sample design blue shifts the wetting
layer to reduce the contribution of background light in the measurements.
Results presented show that >70% of the detected photon pairs are entangled.
The high fidelity of the (|HxxHx>+|VxxVx>)/2^0.5 state that we determine is
sufficient to satisfy numerous tests for entanglement. The improved quality of
entanglement represents a significant step towards the realisation of a
practical quantum dot source compatible with applications in quantum
information.


title: An Equivalent Hermitian Hamiltonian for the non-Hermitian -x^4 Potential

abstract:
  The potential -x^4, which is unbounded below on the real line, can give rise
to a well-posed bound state problem when x is taken on a contour in the
lower-half complex plane. It is then PT-symmetric rather than Hermitian.
Nonetheless it has been shown numerically to have a real spectrum, and a proof
of reality, involving the correspondence between ordinary differential
equations and integral systems, was subsequently constructed for the general
class of potentials -(ix)^N. For PT-symmetric but non-Hermitian Hamiltonians
the natural PT metric is not positive definite, but a dynamically-defined
positive-definite metric can be defined, depending on an operator Q. Further,
with the help of this operator an equivalent Hermitian Hamiltonian h can be
constructed. This programme has been carried out exactly for a few soluble
models, and the first few terms of a perturbative expansion have been found for
the potential m^2x^2+igx^3. However, until now, the -x^4 potential has proved
intractable. In the present paper we give explicit, closed-form expressions for
Q and h, which are made possible by a particular parametrization of the contour
in the complex plane on which the problem is defined. This constitutes an
explicit proof of the reality of the spectrum. The resulting equivalent
Hamiltonian has a potential with a positive quartic term together with a linear
term.


title: Group theoretical approach to entanglement

abstract:
  We present a novel, universal description of quantum entanglement using group
theory and generalized characteristic functions. It leads to new reformulations
of the separability problem, and the positivity of partial transpose (PPT)
criterion. The latter turns out to be intimately related to a certain reality
condition for group representations. Within our formalism, we also show a
connection between existence of entanglement and group non-commutativity.


title: Minimal resources for linear optical one-way computing

abstract:
  We address the question of how many maximally entangled photon pairs are
needed in order to build up cluster states for quantum computing using the
toolbox of linear optics. As the needed gates in dual-rail encoding are
necessarily probabilistic with known optimal success probability, this question
amounts to finding the optimal strategy for building up cluster states, from
the perspective of classical control. We develop a notion of classical
strategies, and present rigorous statements on the ultimate maximal and minimal
use of resources of the globally optimal strategy. We find that this strategy -
being also the most robust with respect to decoherence - gives rise to an
advantage of already more than an order of magnitude in the number of maximally
entangled pairs when building chains with an expected length of L=40, compared
to other legitimate strategies. For two-dimensional cluster states, we present
a first scheme achieving the optimal quadratic asymptotic scaling. This
analysis shows that the choice of appropriate classical control leads to a very
significant reduction in resource consumption.


title:
Quantum secret sharing between multiparty and multiparty with four
  states

abstract:
  An protocol of quantum secret sharing between multiparty and multiparty with
four states is presented. We show that this protocol can make the Trojan horse
attack with a multi-photon signal, the fake-signal attack with EPR pairs, the
attack with single photons, and the attack with invisible photons to be
nullification. In addition, we also give the upper bounds of the average
success probabilities for dishonest agent eavesdropping encryption using the
fake-signal attack with any two-particle entangled states.


title: Variation Principle for Calculation of Many-Particle Effects in Crystals

abstract:
  Variation principle has been developed to calculate many-particle effects in
crystals. Within the framework of quasi-particle concept the variation
principle has been used to find one-electron states with taking into account of
effects due to non-locality of electronic density functional in electromagnetic
fields. A secondary quantized density matrix was used to find the Green
function of a quasiparticle and changes of its effective mass due to correlated
motion of interacting electrons.


title: The Interface between Quantum Mechanics and General Relativity

abstract:
  The generation, as well as the detection, of gravitational radiation by means
of charged superfluids is considered. One example of such a "charged
superfluid" consists of a pair of Planck-mass-scale, ultracold "Millikan oil
drops," each with a single electron on its surface, in which the oil of the
drop is replaced by superfluid helium. When levitated in a magnetic trap, and
subjected to microwave-frequency electromagnetic radiation, a pair of such
"Millikan oil drops" separated by a microwave wavelength can become an
efficient quantum transducer between quadrupolar electromagnetic and
gravitational radiation. This leads to the possibility of a Hertz-like
experiment, in which the source of microwave-frequency gravitational radiation
consists of one pair of "Millikan oil drops" driven by microwaves, and the
receiver of such radiation consists of another pair of "Millikan oil drops" in
the far field driven by the gravitational radiation generated by the first
pair. The second pair then back-converts the gravitional radiation into
detectable microwaves. The enormous enhancement of the conversion efficiency
for these quantum transducers over that for electrons arises from the fact that
there exists macroscopic quantum phase coherence in these charged superfluid
systems.


title:
Authenticated Multiuser Quantum Direct Communication using Entanglement
  Swapping

abstract:
  We present Authenticated Multiuser Quantum Direct Communication(MQDC)
protocols using entanglement swapping. Quantum direct communication is believed
to be a safe way to send a secret message without quantum key distribution. The
authentication process in our protocol allows only proper users to participate
in communication. In the communication stage after the authentication, any two
authorized users among n users can communicate each other even though there is
no quantum communication channels between them. For this protocol, we need only
n quantum communication channels between the authenticator and n users. It is
similar to the present telephone system in which there are n communication
channels between telephone company and users and any two designated users can
communicate each other using telephone line through the telephone company. The
securities of our protocols are analysed to be the same as those of other
quantum key distribution protocols.


title:
Theory of diatomic molecules in an external electromagnetic field from
  first quantum mechanical principles

abstract:
  We study a general problem of the
translational/rotational/vibrational/electronic dynamics of a diatomic molecule
exposed to an interaction with an arbitrary external electromagnetic field. The
theory developed in this paper is relevant to a variety of specific
applications. Such as, alignment or orientation of molecules by lasers,
trapping of ultracold molecules in optical traps, molecular optics and
interferometry, rovibrational spectroscopy of molecules in the presence of
intense laser light, or generation of high order harmonics from molecules.
Starting from the first quantum mechanical principles, we derive an appropriate
molecular Hamiltonian suitable for description of the center of mass,
rotational, vibrational and electronic molecular motions driven by the field
within the electric dipole approximation. Consequently, the concept of the
Born-Oppenheimer separation between the electronic and the nuclear degrees of
freedom in the presence of an electromagnetic field is introduced. Special
cases of the dc/ac field limits are then discussed separately. Finally, we
consider a perturbative regime of a weak dc/ac field, and obtain simple
analytic formulas for the associated Born-Oppenheimer
translational/rotational/vibrational molecular Hamiltonian.


title: The Quantum Setting with Randomized Queries for Continuous Problems

abstract:
  The standard setting of quantum computation for continuous problems uses
deterministic queries and the only source of randomness for quantum algorithms
is through measurement. This setting is related to the worst case setting on a
classical computer in the sense that the number of qubits needed to solve a
continuous problem must be at least equal to the logarithm of the worst case
information complexity of this problem. Since the number of qubits must be
finite, we cannot solve continuous problems on a quantum computer with infinite
worst case information complexity. This can even happen for continuous problems
with small randomized complexity on a classical computer. A simple example is
integration of bounded continuous functions. To overcome this bad property that
limits the power of quantum computation for continuous problems, we study the
quantum setting in which randomized queries are allowed. This type of query is
used in Shor's algorithm. The quantum setting with randomized queries is
related to the randomized classical setting in the sense that the number of
qubits needed to solve a continuous problem must be at least equal to the
logarithm of the randomized information complexity of this problem.