title:
Classical world arising out of quantum physics under the restriction of
  coarse-grained measurements

abstract:
  Conceptually different from the decoherence program, we present a novel
theoretical approach to macroscopic realism and classical physics within
quantum theory. It focuses on the limits of observability of quantum effects of
macroscopic objects, i.e., on the required precision of our measurement
apparatuses such that quantum phenomena can still be observed. First, we
demonstrate that for unrestricted measurement accuracy no classical description
is possible for arbitrarily large systems. Then we show for a certain time
evolution that under coarse-grained measurements not only macrorealism but even
the classical Newtonian laws emerge out of the Schroedinger equation and the
projection postulate.


title:
Complete and Deterministic discrimination of polarization Bell state
  assisted by momentum entanglement

abstract:
  A complete and deterministic Bell state measurement was realized by a simple
linear optics experimental scheme which adopts 2-photon polarization-momentum
hyperentanglement. The scheme, which is based on the discrimination among the
single photon Bell states of the hyperentangled state, requires the adoption of
standard single photon detectors. The four polarization Bell states have been
measured with average fidelity $F=0.889\pm0.010$ by using the linear momentum
degree of freedom as the ancilla. The feasibility of the scheme has been
characterized as a function of the purity of momentum entanglement.


title:
Secure and efficient decoy-state quantum key distribution with inexact
  pulse intensities

abstract:
  We present a general theorem for the efficient verification of the lower
bound of single-photon transmittance. We show how to do decoy-state quantum key
distribution efficiently with large random errors in the intensity control. In
our protocol, the linear terms of fluctuation disappear and only the quadratic
terms take effect. We then show the unconditional security of decoy-state
method with whatever error pattern in intensities of decoy pulses and signal
pulses provided that the intensity of each decoy pulse is less than $\mu$ and
the intensity of each signal pulse is larger than $\mu'$.


title:
Magnetic relaxation of superconducting quantum dot: two-dimensional
  false vacuum decay

abstract:
  Quantum tunneling of vortices has been found to be an important novel
phenomena for description of low temperature creep in high temperature
superconductors (HTSCs). We speculate that quantum tunneling may be also
exhibited in mesoscopic superconductors due to vortices trapped by the
Bean-Livingston barrier. The London approximation and method of images is used
to estimate the shape of the potential well in superconducting HTSC quantum
dot. To calculate the escape rate we use the instanton technique. We model the
vortex by a quantum particle tunneling from a two-dimensional ground state
under magnetic field applied in the transverse direction. The resulting decay
rates obtained by the instanton approach and conventional WKB are compared
revealing complete coincidence with each other.


title:
Foundations for Cooperating with Control Noise in the Manipulation of
  Quantum Dynamics

abstract:
  This paper develops the theoretical foundations for the ability of a control
field to cooperate with noise in the manipulation of quantum dynamics. The
noise enters as run-to-run variations in the control amplitudes, phases and
frequencies with the observation being an ensemble average over many runs as is
commonly done in the laboratory. Weak field perturbation theory is developed to
show that noise in the amplitude and frequency components of the control field
can enhance the process of population transfer in a multilevel ladder system.
The analytical results in this paper support the point that under suitable
conditions an optimal field can cooperate with noise to improve the control
outcome.


title: Observation-assisted optimal control of quantum dynamics

abstract:
  This paper explores the utility of instantaneous and continuous observations
in the optimal control of quantum dynamics. Simulations of the processes are
performed on several multilevel quantum systems with the goal of population
transfer. Optimal control fields are shown to be capable of cooperating or
fighting with observations to achieve a good yield, and the nature of the
observations may be optimized to more effectively control the quantum dynamics.
Quantum observations also can break dynamical symmetries to increase the
controllability of a quantum system. The quantum Zeno and anti-Zeno effects
induced by observations are the key operating principles in these processes.
The results indicate that quantum observations can be effective tools in the
control of quantum dynamics.


title: Developing the Deutsch-Hayden approach to quantum mechanics

abstract:
  The formalism of Deutsch and Hayden is a useful tool for describing quantum
mechanics explicitly as local and unitary, and therefore quantum information
theory as concerning a "flow" of information between systems. In this paper we
show that these physical descriptions of flow are unique, and develop the
approach further to include the measurement interaction and mixed states. We
then give an analysis of entanglement swapping in this approach, showing that
it does not in fact contain non-local effects or some form of superluminal
signalling.


title: Photon wave function and position eigenvectors

abstract:
  One and two photon wave functions are obtained by projection onto a basis of
simultaneous eigenvectors of the position and number operators.


title:
An Important Properties of Entanglement: Pairwise Entanglement can Only
  be Transferred by Entangled Pair

abstract:
  Basing on the calculation of all the pairwise entanglement in the $n$ ($n
\leq 6$)-qubit Heisenberg XX open chain with system impurity, we find an
important result: pairwise entanglement can only be transferred through
entangled pair. The non-nearest pairwise entanglement will has the possibility
to exist as long as there has even number qubit in their middle. This point
means that we can get longer distance entanglement in solid system.


title: Quantum Walks in an array of Quantum Dots

abstract:
  Quantum random walks are shown to have non-intuitive dynamics, which makes
them an attractive area of study for devising quantum algorithms for well-known
classical problems as well as those arising in the field of quantum computing.
In this work we propose a novel scheme for the physical implementation of a
discrete-time quantum random walk using laser excitations of the electronic
states of an array of quantum dots. These dots represent the discrete nodes of
the walk, while transitions between the energy levels inside each dot
correspond to the required coin operation and stimulated Raman adiabatic
passage (STIRAP) processes are employed to induce the steps of the walk. The
quantum dot design is tailored in such a way as to enable selective coupling of
the energy levels. Our simulation results show a close agreement with the ideal
quantum walk distribution as well as modest robustness towards noise
disturbance.


title:
Exact solutions of Schrodinger equation for PT-/non-PT-symmetric and
  non-Hermitian Exponential Type Potentials with the position-dependent
  effective mass

abstract:
  Exact solutions of Schrodinger equation for PT-/non-PT-symmetric and
non-Hermitian Morse and Poschl-Teller potentials are obtained with the
position-dependent effective mass by applying a point canonical transformation
method. Three kinds of mass distributions are used in order to construct
exactly solvable target potentials and obtain energy spectrum and corresponding
wave functions.


title:
Zero-Error Attacks and Detection Statistics in the Coherent One-Way
  Protocol for Quantum Cryptography

abstract:
  This is a study of the security of the Coherent One-Way (COW) protocol for
quantum cryptography, proposed recently as a simple and fast experimental
scheme. In the zero-error regime, the eavesdropper Eve can only take advantage
of the losses in the transmission. We consider new attacks, based on
unambiguous state discrimination, which perform better than the basic
beam-splitting attack, but which can be detected by a careful analysis of the
detection statistics. These results stress the importance of testing several
statistical parameters in order to achieve higher rates of secret bits.


title: Partial transposition on bi-partite system

abstract:
  Many of the properties of the partial transposition are not clear so far.
Here the number of the negative eigenvalues of K(T)(the partial transposition
of K) is considered carefully when K is a two-partite state. There are strong
evidences to show that the number of negative eigenvalues of K(T) is N(N-1)/2
at most when K is a state in Hilbert space N*N. For the special case, 2*2
system(two qubits), we use this result to give a partial proof of the
conjecture sqrt(K(T))(T)>=0. We find that this conjecture is strongly connected
with the entanglement of the state corresponding to the negative eigenvalue of
K(T) or the negative entropy of K.


title:
Wave-particle duality and `bipartite' wave functions for a single
  particle

abstract:
  It is shown that `bipartite' wave functions can present a mathematical
formalism of quantum theory for a single particle, in which the associated
Schr\"{o}dinger's wave functions correspond to those `bipartite' wave functions
of product forms. This extension of Schr\"{o}dinger's form establishes a
mathematical expression of wave-particle duality and that von Neumann's entropy
is a quantitative measure of complementarity between wave-like and
particle-like behaviors. In particular, this formalism suggests that collapses
of Schr\"{o}dinger's wave functions can be regarded as the simultaneous
transition of the particle from many levels to one. Our results shed
considerable light on the basis of quantum mechanics, including quantum
measurement.


title:
Entanglement and Disentanglement, Probabilistic Interpretation of
  Statevectors, and Transformation between Intrinsic Frames of Reference

abstract:
  We study a quantum theory based on two assumptions:
  In the intrinsic frame of reference of an isolated, macroscopic system,
  (i) the system has no global motion and is not entangled with any other
system, (ii) time evolution of statevectors of systems outside the system
satisfy Schr\"{o}dinger equation.
  A process of collision-type interaction between a microscopic system and a
macroscopic system is studied in an auxiliary frame of reference.
  In transforming the statevector of the two systems obtained in the auxiliary
frame of reference to the intrinsic frame of reference of the macroscopic
system, the above first assumption requires a discontinuous change of the
statevector. A probabilistic interpretation is given to the statevector for the
discontinuous change. For the microscopic system, the density matrix given in
the theory here is equal to the reduced density matrix given in the usual
quantum mechanics.


title:
Sequential attacks against differential-phase-shift quantum key
  distribution with weak coherent states

abstract:
  We investigate limitations imposed by sequential attacks on the performance
of differential-phase-shift quantum key distribution protocols that use pulsed
coherent light. In particular, we analyze two sequential attacks based on
unambiguous state discrimination and minimum error discrimination,
respectively, of the signal states emitted by the source. Sequential attacks
represent a special type of intercept-resend attacks and, therefore, they do
not allow the distribution of a secret key.


title: Optical signatures of quantum phase transitions in a light-matter system

abstract:
  Information about quantum phase transitions in conventional condensed matter
systems, must be sought by probing the matter system itself. By contrast, we
show that mixed matter-light systems offer a distinct advantage in that the
photon field carries clear signatures of the associated quantum critical
phenomena. Having derived an accurate, size-consistent Hamiltonian for the
photonic field in the well-known Dicke model, we predict striking behavior of
the optical squeezing and photon statistics near the phase transition. The
corresponding dynamics resemble those of a degenerate parametric amplifier. Our
findings boost the motivation for exploring exotic quantum phase transition
phenomena in atom-cavity, nanostructure-cavity, and
nanostructure-photonic-band-gap systems.


title:
Non-Hermitian Hamiltonians with real eigenvalues coupled to electric
  fields: from the time-independent to the time dependent quantum mechanical
  formulation

abstract:
  We provide a reviewlike introduction into the quantum mechanical formalism
related to non-Hermitian Hamiltonian systems with real eigenvalues. Starting
with the time-independent framework we explain how to determine an appropriate
domain of a non-Hermitian Hamiltonian and pay particular attention to the role
played by PT-symmetry and pseudo-Hermiticity. We discuss the time-evolution of
such systems having in particular the question in mind of how to couple
consistently an electric field to pseudo-Hermitian Hamiltonians. We illustrate
the general formalism with three explicit examples: i) the generalized Swanson
Hamiltonians, which constitute non-Hermitian extensions of anharmonic
oscillators, ii) the spiked harmonic oscillator, which exhibits explicit
supersymmetry and iii) the -x^4-potential, which serves as a toy model for the
quantum field theoretical phi^4-theory.


title: Teaching the Environment to Control Quantum Systems

abstract:
  A non-equilibrium, generally time-dependent, environment whose form is
deduced by optimal learning control is shown to provide a means for incoherent
manipulation of quantum systems. Incoherent control by the environment (ICE)
can serve to steer a system from an initial state to a target state, either
mixed or in some cases pure, by exploiting dissipative dynamics. Implementing
ICE with either incoherent radiation or a gas as the control is explicitly
considered, and the environmental control is characterized by its distribution
function. Simulated learning control experiments are performed with simple
illustrations to find the shape of the optimal non-equilibrium distribution
function that best affects the posed dynamical objectives.


title: Ellipses of Constant Entropy in the XY Spin Chain

abstract:
  Entanglement in the ground state of the XY model on the infinite chain can be
measured by the von Neumann entropy of a block of neighboring spins. We study a
double scaling limit: the size of the block is much larger then 1 but much
smaller then the length of the whole chain. The entropy of the block has an
asymptotic limit. We study this limiting entropy as a function of the
anisotropy and of the magnetic field. We identify its minima at product states
and its divergencies at the quantum phase transitions. We find that the curves
of constant entropy are ellipses and hyperbolas and that they all meet at one
point (essential critical point). Depending on the approach to the essential
critical point the entropy can take any value between 0 and infinity. In the
vicinity of this point small changes in the parameters cause large change of
the entropy.

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


title: Joint reality and Bell inequalities for consecutive measurements

abstract:
  Some new Bell inequalities for consecutive measurements 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 hypothesis advantageously,
provided that the system satisfies the perfect correlation property. The new
inequalities are violated quantically. This violation can be expected to be
more severe than in the case of precedent temporal Bell inequalities. Some
microscopic and mesoscopic situations, in which the new inequalities could be
tested, are roughly considered.


title:
Entanglement, EPR correlations and mesoscopic quantum superposition by
  the high-gain quantum injected parametric amplification

abstract:
  We investigate the multiparticle quantum superposition and the persistence of
multipartite entanglement of the quantum superposition generated by the quantum
injected high-gain optical parametric amplification of a single photon. The
physical configuration based on the optimal universal quantum cloning has been
adopted to investigate how the entanglement and the quantum coherence of the
system persists for large values of the nonlinear parametric gain g.


title: Anyonic Braiding in Optical Lattices

abstract:
  Topological quantum states of matter, both Abelian and non-Abelian, are
characterized by excitations whose wavefunctions undergo non-trivial
statistical transformations as one excitation is moved (braided) around
another. Topological quantum computation proposes to use the topological
protection and the braiding statistics of a non-Abelian topological state to
perform quantum computation. The enormous technological prospect of topological
quantum computation provides new motivation for experimentally observing a
topological state. Here we explicitly work out a realistic experimental scheme
to create and braid the Abelian topological excitations in the Kitaev model
built on a tunable robust system, a cold atom optical lattice. We also
demonstrate how to detect the key feature of these excitations, their braiding
statistics. Observation of this statistics would directly establish the
existence of anyons, quantum particles which are neither fermions nor bosons.
In addition to establishing topological matter, the experimental scheme we
develop here can also be adapted to a non-Abelian topological state, supported
by the same Kitaev model but in a different parameter regime, to eventually
build topologically protected quantum gates.


title: Entanglement and local information access for graph states

abstract:
  We exactly evaluate a number of multipartite entanglement measures for a
class of graph states, including d-dimensional cluster states (d = 1,2,3), the
Greenberger-Horne-Zeilinger states, and some related mixed states. The
entanglement measures that we consider are continuous, `distance from separable
states' measures, including the relative entropy, the so-called geometric
measure, and robustness of entanglement. We also show that for our class of
graph states these entanglement values give an operational interpretation as
the maximal number of graph states distinguishable by local operations and
classical communication (LOCC), as well as supplying a tight bound on the fixed
letter classical capacity under LOCC decoding.


title:
Opacity of electromagnetically induced transparency for quantum
  fluctuations

abstract:
  We analyze the propagation of a pair of quantized fields inside a medium of
three-level atoms in $\Lambda$ configuration. We calculate the stationary
quadrature noise spectrum of the field after propagating through the medium, in
the case where the probe field is in a squeezed state and the atoms show
electromagnetically induced transparency (EIT). We find an oscillatory transfer
of the initial quantum properties between the probe and pump fields which is
most strongly pronounced when both fields have comparable Rabi frequencies.
This implies that the quantum state measured after propagation can be
completely different from the initial state, even though the mean values of the
field are unaltered.


title:
Influence of qubit displacements on quantum logic operations in a
  silicon-based quantum computer with constant interaction

abstract:
  The errors caused by qubit displacements from their prescribed locations in
an ensemble of spin chains are estimated analytically and calculated
numerically for a quantum computer based on phosphorus donors in silicon. We
show that it is possible to polarize (initialize) the nuclear spins even with
displaced qubits by using Controlled NOT gates between the electron and nuclear
spins of the same phosphorus atom. However, a Controlled NOT gate between the
displaced electron spins is implemented with large error because of the
exponential dependence of exchange interaction constant on the distance between
the qubits. If quantum computation is implemented on an ensemble of many spin
chains, the errors can be small if the number of chains with displaced qubits
is small.


title: Implementation of optimal phase-covariant cloning machines

abstract:
  The optimal phase covariant cloning machine (PQCM) broadcasts the information
associated to an input qubit into a multi-qubit systems, exploiting a partial
a-priori knowledge of the input state. This additional a priori information
leads to a higher fidelity than for the universal cloning. The present article
first analyzes different experimental schemes to implement the 1->3 PQCM. The
method is then generalized to any 1->M machine for odd value of M by a
theoretical approach based on the general angular momentum formalism. Finally
different experimental schemes based either on linear or non-linear methods and
valid for single photon polarization encoded qubits are discussed.


title: Quantum games via search algorithms

abstract:
  We build new quantum games, similar to the spin flip game, where as a novelty
the players perform measurements on a quantum system associated to a continuous
time search algorithm. The measurements collapse the wave function into one of
the two possible states. These games are characterized by a continuous space of
strategies and the selection of a particular strategy is determined by the
moments when the players measure.


title: The Pauli equation in scale relativity

abstract:
  In standard quantum mechanics, it is not possible to directly extend the
Schrodinger equation to spinors, so the Pauli equation must be derived from the
Dirac equation by taking its non-relativistic limit. Hence, it predicts the
existence of an intrinsic magnetic moment for the electron and gives its
correct value. In the scale relativity framework, the Schrodinger, Klein-Gordon
and Dirac equations have been derived from first principles as geodesics
equations of a non-differentiable and continuous spacetime. Since such a
generalized geometry implies the occurence of new discrete symmetry breakings,
this has led us to write Dirac bi-spinors in the form of bi-quaternions
(complex quaternions). In the present work, we show that, in scale relativity
also, the correct Pauli equation can only be obtained from a non-relativistic
limit of the relativistic geodesics equation (which, after integration, becomes
the Dirac equation) and not from the non-relativistic formalism (that involves
symmetry breakings in a fractal 3-space). The same degeneracy procedure, when
it is applied to the bi-quaternionic 4-velocity used to derive the Dirac
equation, naturally yields a Pauli-type quaternionic 3-velocity. It therefore
corroborates the relevance of the scale relativity approach for the building
from first principles of the quantum postulates and of the quantum tools. This
also reinforces the relativistic and fundamentally quantum nature of spin,
which we attribute in scale relativity to the non-differentiability of the
quantum spacetime geometry (and not only of the quantum space). We conclude by
performing numerical simulations of spinor geodesics, that allow one to gain a
physical geometric picture of the nature of spin.


title: Foundation q-rules

abstract:
  The q-rules are three auxiliary rules that guide the application of
Schrodinger's equation. They are a set of instructions that describe how
stochastic choices cause the wave to collapse and "start over" with new
boundary conditions.


title: Could quantum mechanics be an approximation to another theory?

abstract:
  We consider the hypothesis that quantum mechanics is an approximation to
another, cosmological theory, accurate only for the description of subsystems
of the universe. Quantum theory is then to be derived from the cosmological
theory by averaging over variables which are not internal to the subsystem,
which may be considered non-local hidden variables. We find conditions for
arriving at quantum mechanics through such a procedure. The key lesson is that
the effect of the coupling to the external degrees of freedom introduces noise
into the evolution of the system degrees of freedom, while preserving a notion
of averaged conserved energy and time reversal invariance.
  These conditions imply that the effective description of the subsystem is
Nelson's stochastic formulation of quantum theory. We show that Nelson's
formulation is not, by itself, a classical stochastic theory as the conserved
averaged energy is not a linear function of the probability density. We also
investigate an argument of Wallstrom posed against the equivalence of Nelson's
stochastic mechanics and quantum mechanics and show that, at least for a simple
case, it is in error.


title:
Weak Fourier-Schur sampling, the hidden subgroup problem, and the
  quantum collision problem

abstract:
  Schur duality decomposes many copies of a quantum state into subspaces
labeled by partitions, a decomposition with applications throughout quantum
information theory. Here we consider applying Schur duality to the problem of
distinguishing coset states in the standard approach to the hidden subgroup
problem. We observe that simply measuring the partition (a procedure we call
weak Schur sampling) provides very little information about the hidden
subgroup. Furthermore, we show that under quite general assumptions, even a
combination of weak Fourier sampling and weak Schur sampling fails to identify
the hidden subgroup. We also prove tight bounds on how many coset states are
required to solve the hidden subgroup problem by weak Schur sampling, and we
relate this question to a quantum version of the collision problem.


title: Quantum logic via the exchange blockade in ultracold collisions

abstract:
  A nuclear spin can act as a quantum switch that turns on or off ultracold
collisions between atoms even when there is neither interaction between nuclear
spins nor between the nuclear and electron spins. This "exchange blockade" is a
new mechanism for implementing quantum logic gates that arises from the
symmetry of composite identical particles, rather than direct coupling between
qubits. We study the implementation of the entangling $\sqrt{\text{SWAP}}$ gate
based on this mechanism for a model system of two atoms with ground electron
configuration $^1S_0$, spin 1/2 nuclei, trapped in optical tweezers. We
evaluate a proof-of-principle protocol based on adiabatic evolution of a one
dimensional double Gaussian well, calculating fidelities of operation as a
function of interaction strength, gate time, and temperature.


title:
Stability of quantum motion in regular systems: a uniform semiclassical
  approach

abstract:
  We study the stability of quantum motion of classically regular systems in
presence of small perturbations. Onthe base of a uniform semiclassical theory
we derive the fidelity decay which displays a quite complexbehaviour, from
Gaussian to power law decay $t^{-\alpha}$ with $1 \le \alpha \le 2$.
Semiclassical estimates are given for the time scales separating the different
decaying regions and numerical results are presented which confirm our
theoretical predictions.


title:
Discrete time quantum walk model for single and entangled particles to
  retain entanglement in coin space

abstract:
  In most widely discussed discrete time quantum walk model, after every
unitary shift operator, the particle evolves into the superposition of position
space and settles down in one of its basis states, loosing entanglement in the
coin space in the new position. The Hadamard operation is applied to let the
particle to evolve into the superposition in the coin space and the walk is
iterated. We present a model with a additional degree of freedom for the
unitary shift operator $U^{\prime}$. The unitary operator with additional
degree of freedom will evolve the quantum particle into superposition of
position space retaining the entanglement in coin space. This eliminates the
need for quantum coin toss (Hadamard operation) after every unitary
displacement operation as used in most widely studied version of the discrete
time quantum walk model. This construction is easily extended to a multiple
particle quantum walk and in this article we extend it for a pair of particles
in pure state entangled in coin degree of freedom by simultaneously subjecting
it to a pair of unitary displacement operators which were constructed for
single particle. We point out that unlike for single particle quantum walk,
upon measurement of its position after $N$ steps, the entangled particles are
found together with 1/2 probability and at different positions with 1/2
probability. This can act as an advantage in applications of the quantum walk.
A special case is also treated using a complex physical system such as, inter
species two-particle entangled Bose-Einstein condensate, as an example.


title:
Photoionization Broadening of the 1S-2S Transition in a Beam of Atomic
  Hydrogen

abstract:
  We consider the excitation dynamics of the two-photon \sts transition in a
beam of atomic hydrogen by 243 nm laser radiation. Specifically, we study the
impact of ionization damping on the transition line shape, caused by the
possibility of ionization of the 2S level by the same laser field. Using a
Monte-Carlo simulation, we calculate the line shape of the \sts transition for
the experimental geometry used in the two latest absolute frequency
measurements (M. Niering {\it et al.}, PRL 84, 5496 (2000) and M. Fischer {\it
et al.}, PRL 92, 230802 (2004)). The calculated line shift and line width are
in excellent agreement with the experimentally observed values. From this
comparison we can verify the values of the dynamic Stark shift coefficient for
the \sts transition for the first time on a level of 15%. We show that the
ionization modifies the velocity distribution of the metastable atoms, the line
shape of the \sts transition, and has an influence on the derivation of its
absolute frequency.


title:
Shape invariance approach to exact solutions of the Klein-Gordon
  equation

abstract:
  Using the shape invariance property we obtain exact solutions of the
(1+1)dimensional Klein-Gordon equation for certain types of scalar and vector
potentials. We also discuss the possibility of obtaining real energy spectrum
with non-Hermitian interaction within this framework.


title:
Generation of continuous-wave broadband Einstein-Podolsky-Rosen beams
  using periodically-poled lithium niobate waveguides

abstract:
  Continuous-wave light beams with broadband Einstein-Podolsky-Rosen
correlation (Einstein-Podolsky-Rosen beams) are created with two independent
squeezed vacua generated by two periodically-poled lithium niobate waveguides
and a half beam splitter.


title: Quantum Pattern Retrieval by Qubit Networks with Hebb Interactions

abstract:
  Qubit networks with long-range interactions inspired by the Hebb rule can be
used as quantum associative memories. Starting from a uniform superposition,
the unitary evolution generated by these interactions drives the network
through a quantum phase transition at a critical computation time, after which
ferromagnetic order guarantees that a measurement retrieves the stored memory.
The maximum memory capacity p of these qubit networks is reached at a memory
density p/n=1.


title: Entangling spins by measuring charge: a parity-gate toolbox

abstract:
  The parity gate emerged recently as a promising resource for performing
universal quantum computation with fermions using only linear interactions.
Here we analyse the parity gate (P-gate) from a theoretical point of view in
the context of quantum networks. We present several schemes for entanglement
generation with P-gates and show that native networks simplify considerably the
resources required for producing multi-qubit entanglement, like n-GHZ states.
Other applications include a Bell-state analyser and teleportation. We also
show that cluster state fusion can be performed deterministically with parity
measurements. We then extend this analysis to hybrid quantum networks
containing spin and mode qubits. Starting from an easy-to-prepare resource
(spin-mode entanglement of single electrons) we show how to produce a spin
n-GHZ state with linear elements (beam-splitters and local spin-flips) and
charge-parity detectors; this state can be used as a resource in a spin quantum
computer or as a precursor for constructing cluster states. Finally, we
construct a novel spin CZ-gate by using the mode degrees of freedom as
ancillae.


title: Ultracold Bosons in a Tilted Multi-level Double-Well Potential

abstract:
  The N -body problem in a double well requires new features for quantum
information processing, macroscopic quantum superposition, and other
fundamental studies of quantum many body physics in ultracold atoms. One needs
(a) tilt, and (b) to go beyond the single-particle ground state in each well,
i.e., to two or more energy levels. For (a), we show that a small potential
difference between the wells, or tilt, causes the decoherence of Schrodinger
cat states. However, these states reappear when the tilt can be compensated by
atom-atom interactions; these tilted cat states constitute partial cats that
are protected from decoherence by the many body wavefunction. For (b), we
provide explicit criteria for when two energy levels are needed to describe the
state space. For typical experimental parameters, two levels are indeed
required for creation of cat states.


title:
Spontaneous emission into a planar optical waveguide mode by an atom
  outside the waveguide

abstract:
  An electronically excited atom or molecule located outside but near a planar
optical waveguide can decay by spontaneous emission of a photon into a guided
mode of the waveguide. We outline a QED theory for calculating the probability
for this process and describe general physical insights from that theory. A
couple of representative examples are discussed in detail.


title: Cooperative behavior of qutrits with dipole-dipole interactions

abstract:
  We have identified a class of many body problems with analytic solution
beyond the mean-field approximation. This is the case where each body can be
considered as an element of an assembly of interacting particles that are
translationally frozen multi-level quantum systems and that do not change
significantly their initial quantum states during the evolution. In contrast,
the entangled collective state of the assembly experiences an appreciable
change. We apply this approach to interacting three-level systems.


title: False qubits II. Entanglement of Josephson junctions

abstract:
  The recent experimental evidence for entangled states of two Josephson
junction qubits is briefly discussed. It is argued that the interpretation of
the experimental data strongly depends on the assumed theoretical model.
Namely, the qubit states are supposed to be the lowest lying eigenstates of a
certain effective Hamiltonian and hence automatically orthogonal, while the
simple analysis within a more fundamental many-particle model shows that those
states should strongly overlap. This makes the standard interpretation of the
measurement procedure questionable.


title:
Efficient one-step generation of large cluster states with solid-state
  circuits

abstract:
  Highly entangled states called cluster states are a universal resource for
measurement-based quantum computing (QC). Here we propose an efficient method
for producing large cluster states using superconducting quantum circuits. We
show that a large cluster state can be efficiently generated in just one step
by turning on the inter-qubit coupling for a short time. Because the
inter-qubit coupling is only switched on during the time interval for
generating the cluster state, our approach is also convenient for preparing the
initial state for each qubit and for implementing one-way QC via single-qubit
measurements. Moreover, the cluster state is robust against parameter
variations.


title: Quantum mechanics for military officers

abstract:
  We present a trivial probabilistic illustration for representation of quantum
mechanics as an algorithm for approximative calculation of averages.


title: N-representability is QMA-complete

abstract:
  We study the computational complexity of the N-representability problem in
quantum chemistry. We show that this problem is QMA-complete, which is the
quantum generalization of NP-complete. Our proof uses a simple mapping from
spin systems to fermionic systems, as well as a convex optimization technique
that reduces the problem of finding ground states to N-representability.


title: SU(2) Relativity and the EPR Paradox

abstract:
  In the normal presentation of the EPR problem a comparison is made between
the (weak) Copenhagen interpretation of quantum mechanics which seems to
suggest that at times action at a distance may take place, and the hidden
parameter interpretation which must satisfy Bell's inequality, in contradiction
to the predictions of quantum mechanics. In this paper, we consider a
relativistic approach to the paradox. However, the frame of reference under
consideration is not the usual Lorenz frame but rather the spin frame of
reference which is invariant with respect to the SU(2) group.


title: Can you do quantum mechanics without Einstein?

abstract:
  The present form of quantum mechanics is based on the Copenhagen school of
interpretation. Einstein did not belong to the Copenhagen school, because he
did not believe in probabilistic interpretation of fundamental physical laws.
This is the reason why we are still debating whether there is a more
deterministic theory. One cause of this separation between Einstein and the
Copenhagen school could have been that the Copenhagen physicists thoroughly
ignored Einstein's main concern: the principle of relativity. Paul A. M. Dirac
was the first one to realize this problem. Indeed, from 1927 to 1963, Paul A.
M. Dirac published at least four papers to study the problem of making the
uncertainty relation consistent with Einstein's Lorentz covariance. It is
interesting to combine those papers by Dirac to make the uncertainty relation
consistent with relativity. It is shown that the mathematics of two coupled
oscillators enables us to carry out this job. We are then led to the question
of whether the concept of localized probability distribution is consistent with
Lorentz covariance.


title: Distributions of continuous-time quantum walks

abstract:
  We study the distributions of the continuous-time quantum walk on a
one-dimensional lattice. In particular we will consider walks on unbounded
lattices, walks with one and two boundaries and Dirichlet boundary conditions,
and walks with periodic boundary conditions. We will prove that all
continuous-time quantum walks can be written as a series of Bessel functions of
the first kind and show how to approximate these series.


title: Experimental Quantum Teleportation of a Two-Qubit Composite System

abstract:
  Quantum teleportation, a way to transfer the state of a quantum system from
one location to another, is central to quantum communication and plays an
important role in a number of quantum computation protocols. Previous
experimental demonstrations have been implemented with photonic or ionic
qubits. Very recently long-distance teleportation and open-destination
teleportation have also been realized. Until now, previous experiments have
only been able to teleport single qubits. However, since teleportation of
single qubits is insufficient for a large-scale realization of quantum
communication and computation2-5, teleportation of a composite system
containing two or more qubits has been seen as a long-standing goal in quantum
information science. Here, we present the experimental realization of quantum
teleportation of a two-qubit composite system. In the experiment, we develop
and exploit a six-photon interferometer to teleport an arbitrary polarization
state of two photons. The observed teleportation fidelities for different
initial states are all well beyond the state estimation limit of 0.40 for a
two-qubit system. Not only does our six-photon interferometer provide an
important step towards teleportation of a complex system, it will also enable
future experimental investigations on a number of fundamental quantum
communication and computation protocols such as multi-stage realization of
quantum-relay, fault-tolerant quantum computation, universal quantum
error-correction and one-way quantum computation.


title: Experimental entanglement of six photons in graph states

abstract:
  Graph states are special kinds of multipartite entangled states that
correspond to mathematical graphs where the vertices take the role of quantum
spin systems and the edges represent interactions. They not only provide an
efficient model to study multiparticle entanglement, but also find wide
applications in quantum error correction, multi-party quantum communication and
most prominently, serve as the central resource in one-way quantum computation.
Here we report the creation of two special instances of graph states, the
six-photon Greenberger-Horne-Zeilinger states -- the largest photonic
Schr\"{o}dinger cat, and the six-photon cluster states-- a state-of-the-art
one-way quantum computer. Flexibly, slight modifications of our method allow
creation of many other graph states. Thus we have demonstrated the ability of
entangling six photons and engineering multiqubit graph states, and created a
test-bed for investigations of one-way quantum computation and studies of
multiparticle entanglement as well as foundational issues such as nonlocality
and decoherence.


title:
Observing quantum non-locality in the entanglement between modes of
  massive particles

abstract:
  We consider the question of whether it is possible to use the entanglement
between spatially separated modes of massive particles to observe nonlocal
quantum correlations. Mode entanglement can be obtained using a single
particle, indicating that it requires careful consideration before concluding
whether experimental observation, e.g. violation of Bell inequalities, is
possible or not. In the simplest setups analogous to optics experiments, that
observation is prohibited by fundamental conservation laws. However, we show
that using auxiliary particles, mode entanglement can be converted into forms
that allow the observation of quantum non-locality. The probability of
successful conversion depends on the nature and number of auxiliary particles
used. In particular, we find that an auxiliary Bose-Einstein condensate allows
the conversion arbitrarily many times with a small error that depends only on
the initial state of the condensate.


title: Classical billiards and double-slit quantum interference

abstract:
  We carry out a numerical simulation about the occurrence of interference
fringes in experiments where an initial Gaussian wave packet evolves inside a
billiard domain with two slits on the boundary. Our simulation extends a
previous work by Casati and Prosen and it is aimed to test their surprising
conclusion that the fringes disappear in the experiments with fully chaotic
billiards. According to the results found, we are led to reassess this
remarkable effect of classical dynamics on quantum interference. Actually, we
highlight another factor which acts on interference: a symmetry condition (SC)
concerning the experimental set-up. This condition seems even to play a role
more important than classical chaos. Indeed, when the SC is verified, classical
chaos has no effect, whereas when the SC is violated classical chaos turns out
to be an additional factor that causes dephasing at the slits. We explain the
respective roles of these two factors, by specifying the physical mechanism
through which they influence the interference patterns. This mechanism depends
both on the position and direction of the initial wave packet and on certain
its recurrences which occur especially in the regular billiards.


title:
Spontaneous emergence of angular momentum Josephson oscillations in
  coupled annular Bose-Einstein condensates

abstract:
  We investigate the nonlinear dynamics of two coupled annular Bose-Einstein
condensates (BECs). For certain values of the coupling strength the nonrotating
ground state is unstable with respect to fluctuations in the higher angular
momentum modes. The two branched Bogoliubov spectrum exhibits distinct regions
of instability enabling one to selectively occupy certain angular momentum
modes. For sufficiently long evolution times angular momentum Josephson
oscillations spontaneously appear, breaking the initial chiral symmetry of the
BECs.


title: Fast Quantum State Control of a Single Trapped Neutral Atom

abstract:
  We demonstrate the initialisation, read-out and high-speed manipulation of a
qubit stored in a single 87 Rb atom trapped in a submicron-size optical
tweezer. Single qubit rotations are performed on a sub-100 ns time scale using
two-photon Raman transitions. Using the ``spin-echo'' technique, we measure an
irreversible dephasing time of 34 ms. The read-out of the single atom qubit is
at the quantum projection noise limit when averaging up to 1000 individual
events.


title:
Non-local generation of entanglement of photons which do not meet each
  other

abstract:
  We report for the first time in an ancilla-free process a non-local
entanglement between two single photons which do not meet. For our experiment
we derive a simple and efficient method to entangle two single photons using
post-selection technology. The photons are guided into an interferometer setup
without the need for ancilla photons for projection into the Bell-states. After
passing the output ports, the photons are analyzed using a bell state analyzer
on each side. The experimental data clearly shows a non-local interaction
between these photons, surpassing the limit set by the CHSH-inequality with an
S-value of 2.54 and 24 standard deviations.


title: Optimal discrimination of mixed states: the quantum Chernoff bound

abstract:
  This paper has been withdrawn by the authors, due to a flaw in the proof of
Theorem 1. This preprint is superseded by quant-ph/0610027, where a correct
proof can be found. Thanks to Rainer Siegmund-Schultze for spotting the error.


title:
Simple protocol for secure decoy-state quantum key distribution with a
  loosely controlled source

abstract:
  The method of decoy-state quantum key distribution (QKD) requests different
intensities of light pulses. Existing theory has assumed exact control of
intensities. Here we propose a simple protocol which is secure and efficient
even there are errors in intensity control. In our protocol, decoy pulses and
signal pulses are generated from the same father pulses with a two-value
attenuation. Given the upper bound of fluctuation of the father pulses, our
protocol is secure provided that the two-value attenuation is done exactly. We
propose to use unbalanced beam-splitters for a stable attenuation. Given that
the intensity error is bounded by $\pm5%$, with the same key rate, our method
can achieve a secure distance only 1 km shorter than that of an ideal protocol
with exactly controlled source.


title: On the Impossibility of a Quantum Sieve Algorithm for Graph Isomorphism

abstract:
  It is known that any quantum algorithm for Graph Isomorphism that works
within the framework of the hidden subgroup problem (HSP) must perform highly
entangled measurements across Omega(n log n) coset states. One of the only
known models for how such a measurement could be carried out efficiently is
Kuperberg's algorithm for the HSP in the dihedral group, in which quantum
states are adaptively combined and measured according to the decomposition of
tensor products into irreducible representations. This ``quantum sieve'' starts
with coset states, and works its way down towards representations whose
probabilities differ depending on, for example, whether the hidden subgroup is
trivial or nontrivial.
  In this paper we give strong evidence that no such approach can succeed for
Graph Isomorphism. Specifically, we consider the natural reduction of Graph
Isomorphism to the HSP over the the wreath product S_n \wr Z_2. We show, modulo
a group-theoretic conjecture regarding the asymptotic characters of the
symmetric group, that no matter what rule we use to adaptively combine quantum
states, there is a constant b > 0 such that no algorithm in this family can
solve Graph Isomorphism in e^{b sqrt{n}} time. In particular, such algorithms
are essentially no better than the best known classical algorithms, whose
running time is e^{O(sqrt{n \log n})}.


title: Gravitationally Induced Decoherence of Optical Entanglement

abstract:
  We propose an experiment in which an entangled pair of optical pulses are
propagated through non-uniform gravitational fields. A field operator
calculation of this situation predicts decoherence of the optical entanglement
under experimentally realistic conditions.


title: Pairwise Entanglement and Local Polarization of Heisenberg Model

abstract:
  The pairwise entanglement and local polarization of the ground state are
discussed by studying the Heisenberg XX model in finite qubit case. The results
show that: the ground state is composed by the micro state with the minimal
total spin 0 (for even qubit) or 1/2 (for odd qubit), local polarization (LP)
has intimate relation with the probability of the micro state in the ground
state, the stronger the LP the smaller the probability, the same LP
corresponding to the same probability; the pairwise entanglement of the ground
state is the biggest in all the eigenvectors. We find when the qubit is small,
the degenerate of state will decrease the pairwise entanglement, there has
great different between the odd and the even qubit chain; when the qubit number
is big, the effect of qubit number to the pairwise entanglement will disappear,
the limited value will be round about 0.3424.


title:
Resonant purification of mixed states for closed and open quantum
  systems

abstract:
  Pure states are fundamental for the implementation of quantum technologies,
and several methods for the purification of the state of a quantum system S
have been developed in the past years. In this letter we present a new
approach, based on the interaction of S with an auxiliary system P, having a
wide range of applicability. Considering two-level systems S and P and assuming
a particular interaction between them, we prove that complete purifications can
be obtained under suitable conditions on the parameters characterizing P. Using
analytical and numerical tools, we show that the purification process exhibits
a resonant behavior in both the cases of system isolated from the external
environment or not.


title: The Geometry of Stochastic Reduction of an Entangled System

abstract:
  We show that the method of stochastic reduction of linear superpositions can
be applied to the process of disentanglement for the spin-0 state of two
spin-1/2 particles. We describe the geometry of this process in the framework
of the complex projective space


title:
Will spin-relaxation times in molecular magnets permit quantum
  information processing?

abstract:
  Using X-band pulsed electron spin resonance, we report the intrinsic
spin-lattice ($T_1$) and phase coherence ($T_2$) relaxation times in molecular
nanomagnets for the first time. In Cr$_7M$ heterometallic wheels, with $M$ = Ni
and Mn, phase coherence relaxation is dominated by the coupling of the electron
spin to protons within the molecule. In deuterated samples $T_2$ reaches 3
$\mu$s at low temperatures, which is several orders of magnitude longer than
the duration of spin manipulations, satisfying a prerequisite for the
deployment of molecular nanomagnets in quantum information applications.


title:
Quantum master equations from classical Lagrangians with two stochastic
  forces

abstract:
  We show how a large family of master equations, describing quantum Brownian
motion of a harmonic oscillator with translationally invariant damping, can be
derived within a phenomenological approach, based on the assumption that an
environment can be simulated by two classical stochastic forces. This family is
determined by three time-dependent correlation functions (besides the frequency
and damping coefficients), and it includes as special cases the known master
equations, whose dissipative part is bilinear with respect to the operators of
coordinate and momentum.


title: Experiment and theory in the Casimir effect

abstract:
  Casimir effect is the attractive force which acts between two plane parallel,
closely spaced, uncharged, metallic plates in vacuum. This phenomenon was
predicted theoretically in 1948 and reliably investigated experimentally only
in recent years. In fact, the Casimir force is similar to the familiar van der
Waals force in the case of relatively large separations when the relativistic
effects come into play. We review the most important experiments on measuring
the Casimir force by means of torsion pendulum, atomic force microscope and
micromechanical torsional oscillator. Special attention is paid to the puzzle
of the thermal Casimir force, i.e., to the apparent violation of the third law
of thermodynamics when the Lifshitz theory of dispersion forces is applied to
real metals. Thereafter we discuss the role of the Casimir force in nanosystems
including the stiction phenomenon, actuators, and interaction of hydrogen atoms
with carbon nanotubes. The applications of the Casimir effect for constraining
predictions of extra-dimensional unification schemes and other physics beyond
the standard model are also considered.


title: Fourier Analysis of Ghost Imaging

abstract:
  Fourier analysis of ghost imaging (FAGI) is proposed in this paper to analyze
the properties of ghost imaging with thermal light sources. This new theory is
compatible with the general correlation theory of intensity fluctuation and
could explain some amazed phenomena. Furthermore we design a series of
experiments to verify the new theory and investigate the inherent properties of
ghost imaging.


title: Compositeness effects, Pauli's principle and entanglement

abstract:
  We analyse some compositeness effects and their relation with entanglement.
We show that the purity of a composite system increases, in the sense of the
expectation values of the deviation operators, with large values of the
entanglement between the components of the system. We also study the validity
of Pauli's principle in composite systems. It is valid within the limits of
application of the approach presented here. We also present an example of two
identical fermions, one of them entangled with a distinguishable particle,
where the exclusion principle cannot be applied. This result can be important
in the description of open systems.


title:
A new perturbation technique for eigenenergies of the screened coulomb
  potential

abstract:
  The explicit semiclassical treatment of the logarithmic perturbation theory
for the bound-state problem of the radial Shrodinger equation with the screened
Coulomb potential is developed. Based upon h-expansions and new quantization
conditions a novel procedure for deriving perturbation expansions is offered.
Avoiding disadvantages of the standard approach, new handy recursion formulae
with the same simple form both for ground and excited states have been
obtained.


title: Novel schemes for measurement-based quantum computation

abstract:
  We establish a framework which allows one to construct novel schemes for
measurement-based quantum computation. The technique further develops tools
from many-body physics - based on finitely correlated or projected entangled
pair states - to go beyond the cluster-state based one-way computer. We
identify resource states that are radically different from the cluster state,
in that they exhibit non-vanishing correlation functions, can partly be
prepared using gates with non-maximal entangling power, or have very different
local entanglement properties. In the computational models, the randomness is
compensated in a different manner. It is shown that there exist resource states
which are locally arbitrarily close to a pure state. Finally, we comment on the
possibility of tailoring computational models to specific physical systems as,
e.g. cold atoms in optical lattices.


title:
Influence of a tight isotropic harmonic trap on photoassociation in
  ultracold homonuclear alkali gases

abstract:
  The influence of a tight isotropic harmonic trap on photoassociation of two
ultracold alkali atoms forming a homonuclear diatomic is investigated using
realistic atomic interaction potentials. Confinement of the initial atom pair
due to the trap leads to a uniform strong enhancement of the photoassociation
rate to most, but also to a strongly suppressed rate for some final states.
Thus tighter traps do not necessarily enhance the photoassociation rate. A
further massive enhancement of the rate is found for strong interatomic
interaction potentials. The details of this interaction play a minor role,
except for large repulsive interactions for which a sharp window occurs in the
photoassociation spectrum as is known from the trap-free case. A comparison
with simplified models describing the atomic interaction like the
pseudopotential approximation shows that they often provide reasonable
estimates for the trap-induced enhancement of the photoassociation rate even if
the predicted rates can be completely erroneous.


title: Fault-tolerant quantum repeater with atomic ensembles and linear optics

abstract:
  We present a detailed analysis of a new robust quantum repeater architecture
building on the original DLCZ protocol [L.M. Duan \textit{et al.}, Nature
(London) \textbf{414}, 413 (2001)]. The new architecture is based on two-photon
Hong-Ou-Mandel-type interference which relaxes the long-distance
interferometric stability requirements by about 7 orders of magnitude, from
sub-wavelength for the single photon interference required by DLCZ to the
coherence length of the photons, thereby removing the weakest point in the DLCZ
schema. Our proposal provides an exciting possibility for robust and realistic
long-distance quantum communication.


title:
Negative Polarization through Photon to Electron Spin Polarization
  Transfer in GaAs Quantum Wells

abstract:
  We demonstrate negative polarization created by light-hole exciton excitation
in g-factor engineered GaAs quantum wells measured by time-resolved Kerr
rotation and polarization-resolved photoluminescence. This negative
polarization is a result of polarization transfer from a photon to an electron
spin mediated by a light hole. This demonstration is an important step towards
achieving quantum media conversion from a photonic qubit to an electron spin
qubit required for building a quantum repeater.


title:
Controllable generation of highly nonclassical states from nearly pure
  squeezed vacua

abstract:
  We present controllable generation of various kinds of highly nonclassical
states of light, including the single photon state and superposition states of
mesoscopically distinct components. The high nonclassicality of the generated
states is measured by the negativity of the Wigner function, which is largest
ever observed to our knowledge. Our scheme is based on photon subtraction from
a nearly pure squeezed vacuum, generated from an optical parametric oscillator
with a periodically-poled KTiOPO$_4$ crystal as a nonlinear medium. This is an
important step to realize basic elements of universal quantum gates, and to
serve as a highly nonclassical input probe for spectroscopy and the study of
quantum memory.


title: Robust creation of entanglement between remote memory qubits

abstract:
  In this Letter we propose a robust quantum repeater architecture building on
the original DLCZ protocol [L.M. Duan \textit{et al.}, Nature \textbf{414}, 413
(2001)]. The architecture is based on two-photon Hong-Ou-Mandel-type
interference which relaxes the long distance stability requirements by about 7
orders of magnitude, from sub wavelength for the single photon interference
required by DLCZ to the coherence length of the photons. Our proposal provides
an exciting possibility for robust and realistic long distance quantum
communication.


title: Optical gratings induced by field-free alignment of molecules

abstract:
  We analyze the alignment of molecules generated by a pair of crossed
ultra-short pump pulses of different polarizations by a technique based on the
induced time-dependent gratings. Parallel polarizations yield an intensity
grating, while perpendicular polarizations induce a polarization grating. We
show that both configurations can be interpreted at moderate intensity as an
alignment induced by a single polarized pump pulse. The advantage of the
perpendicular polarizations is to give a signal of alignment that is free from
the plasma contribution. Experiments on femtosecond transient gratings with
aligned molecules were performed in CO2 at room temperature in a static cell
and at 30 K in a molecular expansion jet.


title:
A photon-like wavepacket with quantised properties based on classical
  Maxwell's equations

abstract:
  A photon-like wavepacket based on novel solutions of Maxwell's equations is
proposed. It is believed to be the first 'classical' model that contains so
many of the accepted quantum features. In this new work, novel solutions to
Maxwell's classical equations in dispersive guides are considered where local
helical twists with an arbitrary angular frequency W modulate a classical mode
(angular frequency w, group velocity vg). The modal field patterns are
unchanged, apart from the twist, provided that the helical velocity vh equals
vg. Pairs of resonating retarded and advanced waves with modal and helical
frequencies (w,W) and (w,-W)respectively, trap one temporal period of the
underlying classical mode forming a photon-like packet provided W = (M+1/2)w:
'Schrodinger' frequencies. This theory supports experimental evidence that the
photon velocity does not change with M in dispersive systems. Promotion and
demotion increase or decrease the helical frequencies in units of w. An energy
of interaction between retarded and advanced waves in the wave-packet is also
proportional to these helical frequencies W = (M+1/2)w similar to Planck's law.
Group velocity and polarisation are unaffected by the value of M. Advanced
waves enable phase and polarisation to be predicted along all future paths and
may help to explain the outcomes of experiments on delayed-choice interference
and entanglement, without causality being violated.


title: Information-theoretic significance of the Wigner distribution

abstract:
  A coarse grained Wigner distribution p_{W}(x,u) obeying positivity derives
out of information-theoretic considerations. Let p(x,u) be the unknown joint
PDF (probability density function) on position- and momentum fluctuations x,u
for a pure state particle. Suppose that the phase part Psi(x,z) of its Fourier
transform F.T.[p(x,u)]=|Z(x,z)|exp[iPsi(x,z)] is constructed as a hologram.
(Such a hologram is often used in heterodyne interferometry.) Consider a
particle randomly illuminating this phase hologram. Let its two position
coordinates be measured. Require that the measurements contain an extreme
amount of Fisher information about true position, through variation of the
phase function Psi(x,z). The extremum solution gives an output PDF p(x,u) that
is the convolution of the Wigner p_{W}(x,u) with an instrument function
defining uncertainty in either position x or momentum u. The convolution arises
naturally out of the approach, and is one-dimensional, in comparison with the
two-dimensional convolutions usually proposed for coarse graining purposes. The
output obeys positivity, as required of a PDF, if the one-dimensional
instrument function is sufficiently wide. The result holds for a large class of
systems: those whose amplitudes a(x) are the same at their boundaries
(Examples: states a(x) with positive parity; with periodic boundary conditions;
free particle trapped in a box).


title: Decay Processes in the Presence of Thin Superconducting Films

abstract:
  In a recent paper [Phys. Rev. Lett. 97, 070401 (2006)] the transition rate of
magnetic spin-flip of a neutral two-level atom trapped in the vicinity of a
thick superconducting body was studied. In the present paper we will extend
these considerations to a situation with an atom at various distances from a
dielectric film. Rates for the corresponding electric dipole-flip transition
will also be considered. The rates for these atomic flip transitions can be
reduced or enhanced, and in some situations they can even be completely
suppressed. For a superconducting film or a thin film of a perfect conducting
material various analytical expressions are derived that reveals the dependence
of the physical parameters at hand.


title: Does the Third Law of Thermodynamics hold in the Quantum Regime?

abstract:
  The first in a long series of papers by John T. Lewis,
  G. W. Ford and the present author, considered the problem of the most general
coupling of a quantum particle to a linear passive heat bath, in the course of
which they derived an exact formula for the free energy of an oscillator
coupled to a heat bath in thermal equilibrium at temperature T. This formula,
and its later extension to three dimensions to incorporate a magnetic field,
has proved to be invaluable in analyzing problems in quantum thermodynamics.
Here, we address the question raised in our title viz. Nernst's third law of
thermodynamics.


title: Optimal quantum circuits for general phase estimation

abstract:
  We address the problem of estimating the phase phi given N copies of the
phase rotation gate u(phi). We consider, for the first time, the optimization
of the general case where the circuit consists of an arbitrary input state,
followed by any arrangement of the N phase rotations interspersed with
arbitrary quantum operations, and ending with a POVM. Using the polynomial
method, we show that, in all cases where the measure of quality of the estimate
phi' for phi depends only on the difference phi'-phi, the optimal scheme has a
very simple fixed form. This implies that an optimal general phase estimation
procedure can be found by just optimizing the amplitudes of the initial state.


title: Quantum-classical transition in Scale Relativity

abstract:
  The theory of scale relativity provides a new insight into the origin of
fundamental laws in physics. Its application to microphysics allows us to
recover quantum mechanics as mechanics on a non-differentiable (fractal)
spacetime. The Schrodinger and Klein-Gordon equations are demonstrated as
geodesic equations in this framework. A development of the intrinsic properties
of this theory, using the mathematical tool of Hamilton's bi-quaternions, leads
us to a derivation of the Dirac equation within the scale-relativity paradigm.
The complex form of the wavefunction in the Schrodinger and Klein-Gordon
equations follows from the non-differentiability of the geometry, since it
involves a breaking of the invariance under the reflection symmetry on the
(proper) time differential element (ds <-> - ds). This mechanism is generalized
for obtaining the bi-quaternionic nature of the Dirac spinor by adding a
further symmetry breaking due to non-differentiability, namely the differential
coordinate reflection symmetry (dx^mu <-> - dx^mu) and by requiring invariance
under parity and time inversion. The Pauli equation is recovered as a
non-relativistic-motion approximation of the Dirac equation.


title: A Quantum Broadcasting Problem in Classical Low Power Signal Processing

abstract:
  We pose a problem called ``broadcasting Holevo-information'': given an
unknown state taken from an ensemble, the task is to generate a bipartite state
transfering as much Holevo-information to each copy as possible.
  We argue that upper bounds on the average information over both copies imply
lower bounds on the quantum capacity required to send the ensemble without
information loss. This is because a channel with zero quantum capacity has a
unitary extension transfering at least as much information to its environment
as it transfers to the output.
  For an ensemble being the time orbit of a pure state under a Hamiltonian
evolution, we derive such a bound on the required quantum capacity in terms of
properties of the input and output energy distribution. Moreover, we discuss
relations between the broadcasting problem and entropy power inequalities.
  The broadcasting problem arises when a signal should be transmitted by a
time-invariant device such that the outgoing signal has the same timing
information as the incoming signal had. Based on previous results we argue that
this establishes a link between quantum information theory and the theory of
low power computing because the loss of timing information implies loss of free
energy.


title: Quantum mechanics: Myths and facts

abstract:
  A common understanding of quantum mechanics (QM) among students and practical
users is often plagued by a number of "myths", that is, widely accepted claims
on which there is not really a general consensus among experts in foundations
of QM. These myths include wave-particle duality, time-energy uncertainty
relation, fundamental randomness, the absence of measurement-independent
reality, locality of QM, nonlocality of QM, the existence of well-defined
relativistic QM, the claims that quantum field theory (QFT) solves the problems
of relativistic QM or that QFT is a theory of particles, as well as myths on
black-hole entropy. The fact is that the existence of various theoretical and
interpretational ambiguities underlying these myths does not yet allow us to
accept them as proven facts. I review the main arguments and counterarguments
lying behind these myths and conclude that QM is still a
not-yet-completely-understood theory open to further fundamental research.


title: Quantum Key Distribution with Screening and Analyzing

abstract:
  We propose a quantum key distribution scheme by using screening angles and
analyzing detectors which enable to notice the presence of Eve who eavesdrops
the quantum channel, as the revised protocol of the recent quantum key
distribution [Phys. Rev. Lett. 95, 040501 (2005)]. We discuss the security of
the proposed quantum key distribution against various attacks including
impersonation attack and Trojan Horse attack.


title: Single-bit Feedback and Quantum Dynamical Decoupling

abstract:
  Synthesizing an effective identity evolution in a target system subjected to
unwanted unitary or non-unitary dynamics is a fundamental task for both quantum
control and quantum information processing applications. Here, we investigate
how single-bit, discrete-time feedback capabilities may be exploited to enact
or to enhance quantum procedures for effectively suppressing unwanted dynamics
in a finite-dimensional open quantum system. An explicit characterization of
the joint unitary propagators correctable by a single-bit feedback strategy for
arbitrary evolution time is obtained. For a two-dimensional target system, we
show how by appropriately combining quantum feedback with dynamical decoupling
methods, concatenated feedback-decoupling schemes may be built, which can
operate under relaxed control assumptions and can outperform purely closed-loop
and open-loop protocols.


title: No nonlocal box with uniform outputs is universal

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


title:
\epsilon-convertibility of entangled states and extension of Schmidt
  rank in infinite-dimensional systems

abstract:
  By introducing the concept of $\epsilon$-convertibility, we extend Nielsen's
and Vidal's theorems to the entanglement transformation of infinite-dimensional
systems. Using an infinite-dimensional version of Vidal's theorem we derive a
new stochastic-LOCC (SLOCC) monotone which can be considered as an extension of
the Schmidt rank. We show that states with polynomially-damped Schmidt
coefficients belong to a higher rank of entanglement class in terms of SLOCC
convertibility. For the case of Hilbert spaces of countable, but infinite
dimensionality, we show that there are actually an uncountable number of
classes of pure non-interconvertible bipartite entangled states.


title: Quantum search with variable times

abstract:
  Since Grover's seminal work, quantum search has been studied in great detail.
In the usual search problem, we have a collection of n items and we would like
to find a marked item. We consider a new variant of this problem in which
evaluating the i-th item may take a different number of time steps for
different i.
  Let t_i be the number of time steps required to evaluate the i-th item. If
the numbers t_i are known in advance, we give an algorithm that solves the
problem in O(\sqrt{t_1^2+t_2^2+...+t_n^2}) steps. This is optimal, as we also
show a matching lower bound. The case, when t_i are not known in advance, can
be solved with a polylogarithmic overhead. We also give an application of our
new search algorithm to computing read-once functions.


title: Design of Plasmon Cavities for Solid-State Cavity QED Applications

abstract:
  Research on photonic cavities with low mode volume and high quality factor
garners much attention because of applications ranging from optoelectronics to
cavity quantum electrodynamics (QED). We propose a cavity based on surface
plasmon modes confined by metallic distributed Bragg reflectors. We analyze the
structure with Finite Difference Time Domain simulations and obtain modes with
quality factor 1000 (including losses from metals), reduced mode volume
relative to photonic crystal cavities, Purcell enhancements of hundreds, and
even the capability of enabling cavity QED strong coupling.


title:
Optimal Gaussian $N$-to-$M$ cloning with linear optics and Gaussian
  cloning of known-phase coherent states

abstract:
  We show how to implement the optimal Gaussian $N$-to-$M$ cloning with linear
optics and homodyne detection. We also show that the Gaussian $N$-to-$M$
cloning of known-phase coherent states can be performed with the fidelity
$\sqrt \frac{2 M N}{2M N+M -N}$ by linear optics and homodyne detection, and
with $\frac{2}{\sqrt{1+\frac{1}{N}}+\sqrt {1-\frac{1}{M}}}$ by utilizing
quadrature squeezing. From the classical limit of the cloning (1-to-$\infty$
cloning), a necessary condition of continuous variable quantum key distribution
using known-phase coherent states is provided.


title:
Effects of ground state hyperfine shifts in quantum computing with
  optically hole burnt materials

abstract:
  We present an investigation of the effects of constant but random shifts of
the ground hyperfine qubit states in the setting of quantum computing with ion
doped crystals. Complex hyperbolic secant pulses can be used to transfer ions
reliably to electronically excited states, and a perturbative approach is used
to analyse the effect of ground state hyperfine shifts. This analysis shows
that the errors due to the hyperfine shift are dynamically supressed during
gate operation, a fact we attribute to the AC Stark shift. Furthermore we
present an implementation of a controlled phase gate which is resilient to the
effects of the hyperfine shift. Decoherence and decay effects are included in
simulations in order to show that a demonstration of quantum gates is feasible
over the relevant range of system parameters.


title:
Are Bohmian trajectories real? On the dynamical mismatch between de
  Broglie-Bohm and classical dynamics in semiclassical systems

abstract:
  The de Broglie-Bohm interpretation of quantum mechanics aims to give a
realist description of quantum phenomena in terms of the motion of point-like
particles following well-defined trajectories. This work is concerned by the de
Broglie-Bohm account of the properties of semiclassical systems. Semiclassical
systems are quantum systems that display the manifestation of classical
trajectories: the wavefunction and the observable properties of such systems
depend on the trajectories of the classical counterpart of the quantum system.
For example the quantum properties have a regular or disordered aspect
depending on whether the underlying classical system has regular or chaotic
dynamics. In contrast, Bohmian trajectories in semiclassical systems have
little in common with the trajectories of the classical counterpart, creating a
dynamical mismatch relative to the quantum-classical correspondence visible in
these systems. Our aim is to describe this mismatch (explicit illustrations are
given), explain its origin, and examine some of the consequences on the status
of Bohmian trajectories in semiclassical systems. We argue in particular that
semiclassical systems put stronger constraints on the empirical acceptability
and plausibility of Bohmian trajectories because the usual arguments given to
dismiss the mismatch between the classical and the de Broglie-Bohm motions are
weakened by the occurrence of classical trajectories in the quantum
wavefunction of such systems.


title: Photon Echo Quantum Memory for Arbitrary Non-Stationary Light Fields

abstract:
  We develop the theory of an optical quantum memory protocol based on the
three pulse photon echo (PE) in an optically dense medium with controlled
reversible inhomogeneous broadening (CRIB). The wave-function of the retrieved
photon echo field is derived explicitly as a function of an arbitrary input
Data light field. The storage and retrieval of time-bin qubit states based on
the described quantum memory is discussed, and it is shown that the memory
allows to measure the path length difference in an imbalanced interferometer
using short light pulses.


title: NMR experiment factors numbers with Gauss sums

abstract: We factor the number 157573 using an NMR implementation of Gauss sums.


title: Raw-data attacks in quantum cryptography with partial tomography

abstract:
  We consider a variant of the BB84 protocol for quantum cryptography, the
prototype of tomographically incomplete protocols, where the key is generated
by one-way communication rather than the usual two-way communication. Our
analysis, backed by numerical evidence, establishes thresholds for
eavesdropping attacks on the raw data and on the generated key at quantum bit
error rates of 10% and 6.15%, respectively. Both thresholds are lower than the
threshold for unconditional security in the standard BB84 protocol.


title: An Algorithm to Speed Up Quantum Search

abstract: The paper has been withdrawn because the research work is still in progress.


title:
Zero energy resonance and the logarithmically slow decay of unstable
  multilevel systems

abstract:
  The long time behavior of the reduced time evolution operator for unstable
multilevel systems is studied based on the N-level Friedrichs model in the
presence of a zero energy resonance.The latter means the divergence of the
resolvent at zero energy. Resorting to the technique developed by Jensen and
Kato [Duke Math. J. 46, 583 (1979)], the zero energy resonance of this model is
characterized by the zero energy eigenstate that does not belong to the Hilbert
space. It is then shown that for some kinds of the rational form factors the
logarithmically slow decay of the reduced time evolution operator can be
realized.


title:
Coexistence of unlimited bipartite and genuine multipartite
  entanglement: Promiscuous quantum correlations arising from discrete to
  continuous variable systems

abstract:
  Quantum mechanics imposes 'monogamy' constraints on the sharing of
entanglement. We show that, despite these limitations, entanglement can be
fully 'promiscuous', i.e. simultaneously present in unlimited two-body and
many-body forms in states living in an infinite-dimensional Hilbert space.
Monogamy just bounds the divergence rate of the various entanglement
contributions. This is demonstrated in simple families of N-mode (N >= 4)
Gaussian states of light fields or atomic ensembles, which therefore enable
infinitely more freedom in the distribution of information, as opposed to
systems of individual qubits. Such a finding is of importance for the
quantification, understanding and potential exploitation of shared quantum
correlations in continuous variable systems. We discuss how promiscuity
gradually arises when considering simple families of discrete variable states,
with increasing Hilbert space dimension towards the continuous variable limit.
Such models are somehow analogous to Gaussian states with asymptotically
diverging, but finite squeezing. In this respect, we find that non-Gaussian
states (which in general are more entangled than Gaussian states), exhibit also
the interesting feature that their entanglement is more shareable: in the
non-Gaussian multipartite arena, unlimited promiscuity can be already achieved
among three entangled parties, while this is impossible for Gaussian, even
infinitely squeezed states.

categories: quant-ph cond-mat.other math-ph math.MP physics.optics


title: Generalized Limits for Single-Parameter Quantum Estimation

abstract:
  We develop generalized bounds for quantum single-parameter estimation
problems for which the coupling to the parameter is described by intrinsic
multi-system interactions. For a Hamiltonian with $k$-system
parameter-sensitive terms, the quantum limit scales as $1/N^k$ where $N$ is the
number of systems. These quantum limits remain valid when the Hamiltonian is
augmented by any parameter independent interaction among the systems and when
adaptive measurements via parameter-independent coupling to ancillas are
allowed.


title: Efficient quantum key distribution with practical sources and detectors

abstract:
  We consider the security of a system of quantum key distribution (QKD) using
only practical devices. Currently, attenuated laser pulses are widely used and
considered to be the most practical light source. For the receiver of photons,
threshold (or on/off) photon detectors are almost the only choice. Combining
the decoy-state idea and the security argument based on the uncertainty
principle, we show that a QKD system composed of such practical devices can
achieve the unconditional security without any significant penalty in the key
rate and the distance limitation.


title: Rings with effects

abstract:
  A ring with effects (e-ring) is a generalization of the ring of bounded
linear operators on a Hilbert space and the subsystem of effect operators
(positive Hermitian operators dominated by the identity operator). The
POV-measures representing (perhaps fuzzy) quantum mechanical observables take
on their valued in the system of Hilbert-space effect operators. We study and
give several examples of e-rings, including von Neumann algebras and rings of
bounded measurable functions.


title: Optimal Broadcasting of Mixed States

abstract:
  The N to M (M>N) universal quantum broadcasting of mixed states are proposed
for qubits system. The broadcasting of mixed states is universal and optimal in
the sense that the shrinking factor is independent of input state and achieves
the upper bound. The quantum broadcasting of mixed qubits is a generalization
of the universal quantum cloning machine for identical pure input states. A new
pure state decompositions of the identical mixed qubits are obtained.


title:
Quantum cloning in coupled states of optical field and atomic ensemble
  by quasi-condensation of polaritons

abstract:
  We consider a new approach for storing quantum information by macroscopic
atomic excitations of two level atomic system. We offer the original scheme of
quantum cloning of optical field into the cavity polaritons containing the
phase insensitive parametrical amplifier and atomic cell placed in the cavity.
The high temperature quasi-condensation (and/or condensation) phenomenon for
polaritons arising in the cavity under the certain conditions is proposed for
the first time.


title:
Quantum Theory at the Crossroads: Reconsidering the 1927 Solvay
  Conference

abstract:
  We reconsider the crucial 1927 Solvay conference in the context of current
research in the foundations of quantum theory. Contrary to folklore, the
interpretation question was not settled at this conference and no consensus was
reached; instead, a range of sharply conflicting views were presented and
extensively discussed. Today, there is no longer an established or dominant
interpretation of quantum theory, so it is important to re-evaluate the
historical sources and keep the interpretation debate open. In this spirit, we
provide a complete English translation of the original proceedings (lectures
and discussions), and give background essays on the three main interpretations
presented: de Broglie's pilot-wave theory, Born and Heisenberg's quantum
mechanics, and Schroedinger's wave mechanics. We provide an extensive analysis
of the lectures and discussions that took place, in the light of current
debates about the meaning of quantum theory. The proceedings contain much
unexpected material, including extensive discussions of de Broglie's pilot-wave
theory (which de Broglie presented for a many-body system), and a "quantum
mechanics" apparently lacking in wave function collapse or fundamental time
evolution. We hope that the book will contribute to the ongoing revival of
research in quantum foundations, as well as stimulate a reconsideration of the
historical development of quantum physics. A more detailed description of the
book may be found in the Preface. (Copyright by Cambridge University Press
(ISBN: 9780521814218).)


title: Heisenberg's Uncertainty Principle

abstract:
  Heisenberg's uncertainty principle is usually taken to express a limitation
of operational possibilities imposed by quantum mechanics. Here we demonstrate
that the full content of this principle also includes its positive role as a
condition ensuring that mutually exclusive experimental options can be
reconciled if an appropriate trade-off is accepted. The uncertainty principle
is shown to appear in three manifestations, in the form of uncertainty
relations: for the widths of the position and momentum distributions in any
quantum state; for the inaccuracies of any joint measurement of these
quantities; and for the inaccuracy of a measurement of one of the quantities
and the ensuing disturbance in the distribution of the other quantity. Whilst
conceptually distinct, these three kinds of uncertainty relations are shown to
be closely related formally. Finally, we survey models and experimental
implementations of joint measurements of position and momentum and comment
briefly on the status of experimental tests of the uncertainty principle.


title: Fully multi-qubit entangled states

abstract:
  We investigate the properties of different levels of entanglement in graph
states which correspond to connected graphs. Combining the operational
definition of graph states and the postulates of entanglement measures, we
prove that in connected graph states of N>3 qubits there is no genuine
three-qubit entanglement. For certain classes of graph states, all genuine
k-qubit entanglement, 2\leq k\leq N-1, among every k qubits vanishes. These
results about connected graph states naturally lead to the definition of fully
multi-qubit entangled states. We also find that the connected graph states of
four qubits is one but not the only one class of fully four-qubit entangled
states.


title: Quantum theory of electronic double-slit diffraction

abstract:
  The phenomena of electron, neutron, atomic and molecular diffraction have
been studied by many experiments, and these experiments are explained by some
theoretical works. In this paper, we study electronic double-slit diffraction
with quantum mechanical approach. We can obtain the results: (1) When the slit
width $a$ is in the range of $3\lambda\sim 50\lambda$ we can obtain the obvious
diffraction patterns. (2) when the ratio of $\frac{d+a}{a}=n (n=1, 2,
3,\cdot\cdot\cdot)$, order $2n, 3n, 4n,\cdot\cdot\cdot$ are missing in
diffraction pattern. (3)When the ratio of $\frac{d+a}{a}\neq n (n=1, 2,
3,\cdot\cdot\cdot)$, there isn't missing order in diffraction pattern. (4) We
also find a new quantum mechanics effect that the slit thickness $c$ has a
large affect to the electronic diffraction patterns. We think all the
predictions in our work can be tested by the electronic double-slit diffraction
experiment.


title: Quantum limits in image processing

abstract:
  We determine the bound to the maximum achievable sensitivity in the
estimation of a scalar parameter from the information contained in an optical
image in the presence of quantum noise. This limit, based on the Cramer-Rao
bound, is valid for any image processing protocol. It is calculated both in the
case of a shot noise limited image and of a non-classical illumination. We also
give practical experimental implementations allowing us to reach this absolute
limit.


title: Quantum storage and cloning of light states in EIT-like medium

abstract:
  In the paper we consider a new approach for storage and cloning of quantum
information by three level atomic (molecular) systems in the presence of the
electromagnetically induced transparency (EIT) effect. For that, the various
schemes of transformation into the bright and dark polaritons for quantum
states of optical field in the medium are proposed. Physical conditions of
realization of quantum nondemolition (QND) storage of quantum optical state are
formulated for the first time. We have shown that the best storage and cloning
of can be achieved with the atomic ensemble in the Bose-Einstein condensation
state. We discuss stimulated Raman two-color photoassociation for experimental
realization of the schemes under consideration.


title: Quasiclassical Coarse Graining and Thermodynamic Entropy

abstract:
  Our everyday descriptions of the universe are highly coarse-grained,
following only a tiny fraction of the variables necessary for a perfectly
fine-grained description. Coarse graining in classical physics is made natural
by our limited powers of observation and computation. But in the modern quantum
mechanics of closed systems, some measure of coarse graining is inescapable
because there are no non-trivial, probabilistic, fine-grained descriptions.
This essay explores the consequences of that fact. Quantum theory allows for
various coarse-grained descriptions some of which are mutually incompatible.
For most purposes, however, we are interested in the small subset of
``quasiclassical descriptions'' defined by ranges of values of averages over
small volumes of densities of conserved quantities such as energy and momentum
and approximately conserved quantities such as baryon number. The
near-conservation of these quasiclassical quantities results in approximate
decoherence, predictability, and local equilibrium, leading to closed sets of
equations of motion. In any description, information is sacrificed through the
coarse graining that yields decoherence and gives rise to probabilities for
histories. In quasiclassical descriptions, further information is sacrificed in
exhibiting the emergent regularities summarized by classical equations of
motion. An appropriate entropy measures the loss of information. For a
``quasiclassical realm'' this is connected with the usual thermodynamic entropy
as obtained from statistical mechanics. It was low for the initial state of our
universe and has been increasing since.

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


title:
Conditional control of the quantum states of remote atomic memories for
  quantum networking

abstract:
  Quantum networks hold the promise for revolutionary advances in information
processing with quantum resources distributed over remote locations via
quantum-repeater architectures. Quantum networks are composed of nodes for
storing and processing quantum states, and of channels for transmitting states
between them. The scalability of such networks relies critically on the ability
to perform conditional operations on states stored in separated quantum
memories. Here we report the first implementation of such conditional control
of two atomic memories, located in distinct apparatuses, which results in a
28-fold increase of the probability of simultaneously obtaining a pair of
single photons, relative to the case without conditional control. As a first
application, we demonstrate a high degree of indistinguishability for remotely
generated single photons by the observation of destructive interference of
their wavepackets. Our results demonstrate experimentally a basic principle for
enabling scalable quantum networks, with applications as well to linear optics
quantum computation.


title: Mayet-Godowski Hilbert Lattice Equations

abstract:
  Several new results in the field of Hilbert lattice equations based on states
defined on the lattice as well as novel techniques used to arrive at these
results are presented. An open problem of Mayet concerning Hilbert lattice
equations based on Hilbert-space-valued states is answered.


title:
Photon Antibunching from a Single Quantum Dot-Microcavity System in the
  Strong Coupling Regime

abstract:
  We observe antibunching in the photons emitted from a strongly-coupled single
quantum dot and pillar microcavity in resonance. When the quantum dot was
spectrally detuned from the cavity mode, the cavity emission remained
antibunched, and also anticorrelated from the quantum dot emission. Resonant
pumping of the selected quantum dot via an excited state enabled these
observations by eliminating the background emitters that are usually coupled to
the cavity. This device demonstrates an on-demand single photon source
operating in the strong coupling regime, with a Purcell factor of 61 and
quantum effciency of 97%


title:
Demonstration of the difference Casimir force for samples with different
  charge carrier densities

abstract:
  A measurement of the Casimir force between a gold coated sphere and two Si
plates of different carrier densities is performed using a high vacuum based
atomic force microscope. The results are compared with the Lifshitz theory and
good agreement is found. Our experiment demonstrates that by changing the
carrier density of the semiconductor plate by several orders of magnitude it is
possible to modify the Casimir interaction. This result may find applications
in nanotechnology.


title:
Passive decoy state quantum key distribution: Closing the gap to perfect
  sources

abstract:
  We propose a quantum key distribution scheme which closely matches the
performance of a perfect single photon source. It nearly attains the physical
upper bound in terms of key generation rate and maximally achievable distance.
Our scheme relies on a practical setup based on a parametric downconversion
source and present-day, non-ideal photon-number detection. Arbitrary
experimental imperfections which lead to bit errors are included. We select
decoy states by classical post-processing. This allows to improve the effective
signal statistics and achievable distance.


title:
Asymptotic correctability of Bell-diagonal qudit states and lower bounds
  on tolerable error probabilities in quantum cryptography

abstract:
  The concept of asymptotic correctability of Bell-diagonal quantum states is
generalised to elementary quantum systems of higher dimensions. Based on these
results basic properties of quantum state purification protocols are
investigated which are capable of purifying tensor products of Bell-diagonal
states and which are based on $B$-steps of the Gottesman-Lo-type with the
subsequent application of a Calderbank-Shor-Steane quantum code. Consequences
for maximum tolerable error rates of quantum cryptographic protocols are
discussed.


title: Optomechanical entanglement between a movable mirror and a cavity field

abstract:
  We show how stationary entanglement between an optical cavity field mode and
a macroscopic vibrating mirror can be generated by means of radiation pressure.
We also show how the generated optomechanical entanglement can be quantified
and we suggest an experimental readout-scheme to fully characterize the
entangled state. Surprisingly, such optomechanical entanglement is shown to
persist for environment temperatures above 20K using state-of-the-art
experimental parameters.


title: Energy momentum flows for the massive vector field

abstract:
  We present a causal trajectory interpretation for the massive vector field,
based on the flows of rest energy and a conserved density defined using the
time-like eigenvectors and eigenvalues of the stress-energy-momentum tensor.
This work extends our previous work which used a similar procedure for the
scalar field. The massive, spin-one, complex vector field is discussed in
detail and solutions are classified using the Pauli-Lubanski spin vector. The
flows of energy-momentum are illustrated in a simple example of standing waves
in a plane.


title: Switching off the reservoir through nonstationary quantum systems

abstract:
  In this paper we demonstrate that the inevitable action of the environment
can be substantially weakened when considering appropriate nonstationary
quantum systems. Beyond protecting quantum states against decoherence, an
oscillating frequency can be engineered to make the system-reservoir coupling
almost negligible. Therefore, differently from previously-reported protecting
schemes, our technique does not require a previous knowledge of the state to be
protected. We show, in the domain of cavity quantum electrodynamics, how to
engineer such a nonstationary cavity mode through its dispersive interaction
with a driven two-level atom.