title: Single qubit quantum secret sharing with improved security

abstract:
  Analyzing carefully an experimentally feasible non-entangled single qubit
quantum secret sharing protocol and its modified version [Phys. Rev. Lett. 95,
230505 (2005); ibid. 98, 028902 (2007)], it is found that both versions are
insecure against coherent attacks though the original idea is so remarkable. To
overcome this fatal flaw, here we propose a protocol with a distinct security
checking strategy, which still involves single qubit operations only, making it
possible to achieve better security of quantum secret sharing with current
technology.


title:
Quantum origin of quantum jumps: Breaking of unitary symmetry induced by
  information transfer and the transition from quantum to classical

abstract:
  Measurements transfer information about a system to the apparatus, and then
further on -- to observers and (often inadvertently) to the environment. I show
that even imperfect copying essential in such situations restricts possible
unperturbed outcomes to an orthogonal subset of all possible states of the
system, thus breaking the unitary symmetry of its Hilbert space implied by the
quantum superposition principle. Preferred outcome states emerge as a result.
They provide framework for the ``wavepacket collapse'', designating terminal
points of quantum jumps, and defining the measured observable by specifying its
eigenstates. In quantum Darwinism, they are the progenitors of multiple copies
spread throughout the environment -- the fittest quantum states that not only
survive decoherence, but subvert it into carrying information about them --
into becoming a witness.


title:
Aspects of nonlocality from a quantum trajectory perspective: A WKB
  approach to Bohmian mechanics

abstract:
  Nonlocality is a property of paramount importance both conceptually and
computationally exhibited by quantum systems, which has no classical
counterpart. Conceptually, it is important because it implies that the evolving
system has information on what happens at any space point and time.
Computationally, because such a knowledge makes any calculation intractable as
the number of degrees of freedom involved increases beyond a few of them.
Bohmian mechanics, with its trajectory-based formalism in real configuration
space, can help to better understand nonlocality. A detailed analysis of how
nonlocal information is transmitted to quantum trajectories in simple systems
(free particle and harmonic oscillator) turns out to be very interesting when
compared to analogous systems in classical mechanics.


title:
Time Asymmetry in Quantum Physics - I. Theoretical Conclusion from
  Resonance and Decay-Phenomenology

abstract:
  It is explained how the unification of resonance and decay phenomena into a
consistent mathematical theory leads to quantum mechanical time-asymmetry. This
provides the theoretical basis for a subsequent paper II in which the
interpretation and experimental demonstration of this time-asymmetry is
discussed.


title:
Time Asymmetry in Quantum Physics - II. Experimental Demonstration Using
  a Single Ion

abstract:
  Quantum physics involves an ensemble of quantum systems, usually one thinks
of a large ensemble of identical quantum systems at one single time. In single
ion experiments one has a single quantum system at an ensemble of different
times. This provides the means of demonstrating the beginning of time of a
semigroup evolution for a decaying state.


title: Deformed quantum harmonic oscillator with diffusion and dissipation

abstract:
  A master equation for the deformed quantum harmonic oscillator interacting
with a dissipative environment, in particular with a thermal bath, is derived
in the microscopic model by using perturbation theory. The coefficients of the
master equation and of equations of motion for observables depend on the
deformation function. The steady state solution of the equation for the density
matrix in the number representation is obtained and the equilibrium energy of
the deformed harmonic oscillator is calculated in the approximation of small
deformation.


title:
Lindblad master equation for the damped harmonic oscillator with
  deformed dissipation

abstract:
  In the framework of the Lindblad theory for open quantum systems, a master
equation for the quantum harmonic oscillator interacting with a dissipative
environment, in particular with a thermal bath, is derived for the case when
the interaction is based on deformed algebra. The equations of motion for
observables strongly depend on the deformation function. The expectation values
of the number operator and squared number operator are calculated in the limit
of a small deformation parameter for the case of zero temperature of the
thermal bath. The steady state solution of the equation for the density matrix
in the number representation is obtained and its independence of the
deformation is shown.


title:
Deformation of quantum oscillator and of its interaction with
  environment

abstract:
  A master equation for the deformed quantum harmonic oscillator interacting
with a dissipative environment, in particular with a thermal bath, is derived
in the microscopic model by using perturbation theory, for the case when the
interaction is deformed. The coefficients of the master equation and of
equations of motion for observables depend on the deformation function. The
steady state solution of the equation for the density matrix in the number
representation is obtained and shown that it satisfies the detailed balance
condition. The equilibrium energy of the deformed harmonic oscillator,
calculated in the approximation of small deformation, does not depend on the
deformation of interaction operators.


title: A Continuously Observed Two-level System Interacting with a Vacuum Field

abstract:
  A discussion of the quantum Zeno effect and paradox is given. The quantum
Zeno paradox claims that a continuously observed system, prepared in a state
which is not an eigenstate of the Hamiltonian operator, never decays. To
recover the classical behavior of unstable systems we consider a two-level
system interacting with a Bose field, respectively prepared in the excited
state and in the Poincare invariant vacuum state. Using time-dependent
perturbation theory, we evaluate for a finite time interval the probability of
spontaneous decay of the two-level system. Using the standard argument to
obtain the quantum Zeno paradox, we consider N measurements where N goes to
infinity and we obtain that the non-decay probability law is a pure
exponential, therefore recovering the classical behavior.


title:
Determination of the domain of the admissible matrix elements in the
  four-dimensional PT-symmetric anharmonic model

abstract:
  Many manifestly non-Hermitian Hamiltonians (typically, PT-symmetric complex
anharmonic oscillators) possess a strictly real, "physical" bound-state
spectrum. This means that they are (quasi-)Hermitian with respect to a suitable
non-standard metric. The domain D of the existence of this metric is studied
here for a nontrivial though still non-numerical four-parametric "benchmark"
matrix model.


title: Periodically driven Quantum Ratchets: Symmetries and Resonances

abstract:
  We study the quantum version of a tilting and flashing Hamiltonian ratchets,
consisting of a periodic potential and a time-periodic driving field. The
system dynamics is governed by a Floquet evolution matrix bearing the symmetry
of the corresponding Hamiltonian. The dc-current appears due to the
desymmetrization of Floquet eigenstates, which become transporting when all the
relevant symmetries are violated. Those eigenstates which mostly contribute to
a directed transport reside in phase space regions corresponding to classical
resonances. Quantum dynamics leads to the dependence of the average velocity on
the initial phase of the ac-field. A resonant enhancement (or suppression) of
the dc-current, due to avoided crossings between different Floquet states takes
place upon tuning some control parameters. Our studies are predominantly aimed
at experimental realizations of ac-driven quantum ratchets with cold atoms.


title:
The frictional Schr\"odinger-Newton equation in models of wave function
  collapse

abstract:
  Replacing the Newtonian coupling G by -iG, the Schrodinger-Newton equation
becomes ``frictional''. Instead of the reversible Schrodinger-Newton equation,
we advocate its frictional version to generate the set of pointer states for
macroscopic quantum bodies.


title: Erasable and unerasable correlations

abstract:
  We address the problem of removing correlation from sets of states while
preserving as much local quantum information as possible. We prove that states
obtained from universal cloning can only be decorrelated at the expense of
complete erasure of local information (i.e. information about the copied
state). We solve analytically the problem of decorrelation for two qubits and
two qumodes (harmonic oscillators in Gaussian states), and provide sets of
decorrelable states and the minimum amount of noise to be added for
decorrelation.


title: On the quantization of nonlocal theory

abstract:
  A simple nonlocal theory is put into Hamiltonian form and quantized by using
the modern version of Ostrogradski approach.


title:
Ultraslow propagation of matched pulses by four-wave mixing in an atomic
  vapor

abstract:
  We have observed the ultraslow propagation of matched pulses in nondegenerate
four-wave mixing in a hot atomic vapor. Probe pulses as short as 70 ns can be
delayed by a tunable time of up to 40 ns with little broadening or distortion.
During the propagation, a probe pulse is amplified and generates a conjugate
pulse which is faster and separates from the probe pulse before getting locked
to it at a fixed delay. The precise timing of this process allows us to
determine the key coefficients of the susceptibility tensor. The presence of
gain in this system makes this system very interesting in the context of
all-optical information processing.


title: Analytical and Numerical Verification of the Nernst Theorem for Metals

abstract:
  In view of the current discussion on the subject, an effort is made to show
very accurately both analytically and numerically how the Drude dispersion
model gives consistent results for the Casimir free energy at low temperatures.
Specifically, for the free energy near T=0 we find the leading term to be
proportional to T^2 and the next-to-leading term proportional to T^{5/2}. These
terms give rise to zero Casimir entropy as T approaches zero, and is thus in
accordance with Nernst's theorem.


title: Quantum states transfer by the analogous Bell states

abstract:
  Transmitting quantum states by channels of analogous Bell states is studied
in this paper. We analyse the transmitting process, constructed the
probabilitic unitary operator, and gain the largest successful transfer quantum
state probability.


title: A single photon produces general W state of N qubits and its application

abstract:
  Based on the Wu's scheme[1], We prepare the general N-qubit W state. We find
that the concurrence of two qubits in general N-qubit W state is only related
to their coefficients and we successfully apply the general N-qubit W state to
quantum state transfer and quantum state prepare like that in two-qubit system.


title: Transport and Entanglement Generation in the Bose-Hubbard Model

abstract:
  We study entanglement generation via particle transport across a
one-dimensional system described by the Bose-Hubbard Hamiltonian. We analyze
how the competition between interactions and tunneling affects transport
properties and the creation of entanglement in the occupation number basis.
Alternatively, we propose to use spatially delocalized quantum bits, where a
quantum bit is defined by the presence of a particle either in a site or in the
adjacent one. Our results can serve as a guidance for future experiments to
characterize entanglement of ultracold gases in one-dimensional optical
lattices.


title: Quantum phases of interacting phonons in ion traps

abstract:
  The vibrations of a chain of trapped ions can be considered, under suitable
experimental conditions, as an ensemble of interacting phonons, whose quantum
dynamics is governed by a Bose--Hubbard Hamiltonian. In this work we study the
quantum phases which appear in this system, and show that thermodynamical
properties, such as critical parameters and critical exponents, can be measured
in experiments with a limited number of ions. Besides that, interacting phonons
in trapped ions offer us the possibility to access regimes which are difficult
to study with ultracold bosons in optical lattices, like models with attractive
or site--dependent phonon-phonon interactions.


title: Disproof of Bell's Theorem by Clifford Algebra Valued Local Variables

abstract:
  It is shown that Bell's theorem fails for the Clifford algebra valued local
realistic variables. This is made evident by exactly reproducing quantum
mechanical expectation value for the EPR-Bohm type spin correlations observable
by means of a local, deterministic, Clifford algebra valued variable, without
necessitating either remote contextuality or backward causation. Since Clifford
product of multivector variables is non-commutative in general, the spin
correlations derived within our locally causal model violate the CHSH
inequality just as strongly as their quantum mechanical counterparts.


title: Update for "The collapse of Bell determinism"

abstract:
  We update our paper: The collapse of Bell determinism (Physics Letters A, 359
(2006): 122-125; available online 16 June 2006). First, we point out that
Olivier Brunet, using lattice theoretic methods, has recently, and quite
independently, derived the core technical lemma of our paper; see: A priori
knowledge and the Kochen-Specker theorem (Physics Letters A, available online 5
January 2007). He has also kindly pointed out a misstep in the last line of one
of our results. We discuss the correction and comment on slightly revised
notions of how Bell determinism collapses. We also correct a few typos in the
original paper.


title: Quantum Block and Convolutional Codes from Self-orthogonal Product Codes

abstract:
  We present a construction of self-orthogonal codes using product codes. From
the resulting codes, one can construct both block quantum error-correcting
codes and quantum convolutional codes. We show that from the examples of
convolutional codes found, we can derive ordinary quantum error-correcting
codes using tail-biting with parameters [[42N,24N,3]]_2. While it is known that
the product construction cannot improve the rate in the classical case, we show
that this can happen for quantum codes: we show that a code [[15,7,3]]_2 is
obtained by the product of a code [[5,1,3]]_2 with a suitable code.


title: Constructions of Quantum Convolutional Codes

abstract:
  We address the problems of constructing quantum convolutional codes (QCCs)
and of encoding them. The first construction is a CSS-type construction which
allows us to find QCCs of rate 2/4. The second construction yields a quantum
convolutional code by applying a product code construction to an arbitrary
classical convolutional code and an arbitrary quantum block code. We show that
the resulting codes have highly structured and efficient encoders. Furthermore,
we show that the resulting quantum circuits have finite depth, independent of
the lengths of the input stream, and show that this depth is polynomial in the
degree and frame size of the code.


title:
Distinguishability of apparatus states in quantum measurement in the
  Stern-Gerlach experiment

abstract:
  In the context of the quantum mechanical modelling of a measurement process
using the Stern-Gerlach setup, we critically examine the relationship between
the notion of `distinguishability' of apparatus states defined in terms of the
inner product and spatial separation among the emerging wave packets. We show
that, in general, the mutual orthogonality of these wave packets does not
necessarily imply their unambiguous spatial separation even in the asymptotic
limit. A testable scheme is formulated to quantify such departures from
`idealness' for a range of relevant parameters.


title:
Suppression of Bragg scattering by collective interference of spatially
  ordered atoms with a high-Q cavity mode

abstract:
  When N driven atoms emit in phase into a high-Q cavity mode, the intracavity
field generated by collective scattering interferes destructively with the pump
driving the atoms. Hence atomic fluorescence is suppressed and cavity loss
becomes the dominant decay channel for the whole ensemble. Microscopically 3D
light-intensity minima are formed in the vicinity of the atoms that prevent
atomic excitation and form a regular lattice. The effect gets more pronounced
for large atom numbers, when the sum of the atomic decay rates exceeds the rate
of cavity losses and one would expect the opposite behaviour. These results
provide new insight into recent experiments on collective atomic dynamics in
cavities.


title:
Addressing individual atoms in optical lattices with standing-wave
  driving fields

abstract:
  A scheme for addressing individual atoms in one- or two-dimensional optical
lattices loaded with one atom per site is proposed. The scheme is based on
position-dependent atomic population transfer induced by several standing-wave
driving fields. This allows various operations important in quantum information
processing, such as manipulation and measurement of any single atom, two-qubit
operations between any pair of adjacent atoms, and patterned loading of the
lattice with one atom per every nth site for arbitrary n. The proposed scheme
is robust against considerable imperfections and actually within reach of
current technology.


title: Atom-wall dispersive forces from master equation formalism

abstract:
  Using the general expressions for level shifts obtained from the master
equation for a small system interacting with a large one considered as a
reservoir, we calculate the dispersive potentials between an atom and a wall in
the dipole approximation. We analyze in detail the particular case of a
two-level atom in the presence of a perfectly conducting wall. We study the van
der Waals as well as the resonant interactions. All distance regimes as well as
the high and low temperature regimes are considered. We show that the
Casimir-Polder interaction can not be considered as a direct result of the
vacuum fluctuations only. Concerning the interaction between the atom and the
wall at high temperature, which show that a saturation of the potential for all
distances occurs. This saturated potential coincides exactly with that obtained
in the London-van der Waals limit.


title:
The conservation of orbital angular momentum and the two-photon
  detection amplitude in spontaneous parametric down-conversion

abstract:
  We study the two-photon detection amplitude of the down-converted beams in
spontaneous parametric down-conversion when the physical variable of orbital
angular momentum is involved, taking into account both conservation and
non-conservation of angular momentum. Agreeing with experimental observations,
our theoretical calculation shows that spatial structure of the two-photon
detection amplitude of the down-converted beams carries important information
about conservation or non-conservation of orbital angular momentum in
spontaneous parametric down-conversion.


title:
Synchronized Independent Narrow-band Single Photons and Efficient
  Generation of Photonic Entanglement

abstract:
  We create independent, synchronized single-photon sources with built-in
quantum memory based on two remote cold atomic ensembles. The synchronized
single photons are used to demonstrate efficient generation of entanglement.
The resulting entangled photon pairs violate a Bell's Inequality by 5 standard
deviations. Our synchronized single photons with their long coherence time of
25 ns and the efficient creation of entanglement serve as an ideal building
block for scalable linear optical quantum information processing.


title: Quantum synapse for cold atoms

abstract:
  In this paper, the quantum synaptic effect is studied that arisen in the
system of two crossed wires excited by the static (DC) and radio-frequency (RF)
currents. The potential barrier between the two orthogonal atom streams is
controlled electronically and the atoms can be transferred from one wire to
another under certain critical values of the RF and DC currents. The results
are interesting in the study of quantum interferometry and quantum registering
of cold atoms.


title: Decoherence of an $n$-qubit quantum memory

abstract:
  We analyze decoherence of a quantum register in the absence of non-local
operations i.e. of $n$ non-interacting qubits coupled to an environment. The
problem is solved in terms of a sum rule which implies linear scaling in the
number of qubits. Each term involves a single qubit and its entanglement with
the remaining ones. Two conditions are essential: first decoherence must be
small and second the coupling of different qubits must be uncorrelated in the
interaction picture. We apply the result to a random matrix model, and
illustrate its reach considering a GHZ state coupled to a spin bath.


title:
Realization and characterization of a 2-photon 4-qubit linear cluster
  state

abstract:
  We report on the experimental realization of a 4-qubit linear cluster state
via two photons entangled both in polarization and linear momentum. This state
was investigated by performing tomographic measurements and by evaluating an
entanglement witness. By use of this state we carried out a novel nonlocality
proof, the so-called ``stronger two observer all versus nothing'' test of
quantum nonlocality.


title: On EPR paradox, Bell's inequalities and experiments which prove nothing

abstract:
  This article shows that the there is no paradox. Violation of Bell's
inequalities should not be identified with a proof of non locality in quantum
mechanics. A number of past experiments is reviewed, and it is concluded that
the experimental results should be re-evaluated. The results of the experiments
with atomic cascade are shown not to contradict the local realism. The article
points out flaws in the experiments with down-converted photons. The
experiments with neutron interferometer on measuring the "contextuality" and
Bell-like inequalities are analyzed, and it is shown that the experimental
results can be explained without such notions. Alternative experiment is
proposed to prove the validity of local realism.


title:
Effects of imperfect noise correlations on decoherence-free subsystems:
  SU(2) diffusion model

abstract:
  We present a model of an N-qubit channel where consecutive qubits experience
correlated random rotations. Our model is an extension to the standard
decoherence-free subsystems approach (DFS) which assumes that all the qubits
experience the same disturbance. The variation of rotations acting on
consecutive qubits is modeled as diffusion on the SU(2) group. The model may be
applied to spins traveling in a varying magnetic field, or to photons passing
through a fiber whose birefringence fluctuates over the time separation between
photons. We derive an explicit formula describing the action of the channel on
an arbitrary N-qubit state. For N=3 we investigate the effects of diffusion on
both classical and quantum capacity of the channel. We observe that
nonorthogonal states are necessary to achieve the optimal classical capacity.
Furthermore we find the threshold for the diffusion parameter above which
coherent information of the channel vanishes.


title: Cooling algorithms based on the 3-bit majority

abstract:
  Algorithmic cooling is a potentially important technique for making scalable
NMR quantum computation feasible in practice. Given the constraints imposed by
this approach to quantum computing, the most likely cooling algorithms to be
practicable are those based on simple reversible polarization compression (RPC)
operations acting locally on small numbers of bits. Several different
algorithms using 2- and 3-bit RPC operations have appeared in the literature,
and these are the algorithms I consider in this note. Specifically, I show that
the RPC operation used in all these algorithms is essentially a majority vote
of 3 bits, and prove the optimality of the best such algorithm. I go on to
derive some theoretical bounds on the performance of these algorithms under
some specific assumptions about errors.


title:
Efficient Quantum Algorithm for Hidden Quadratic and Cubic Polynomial
  Function Graphs

abstract:
  We introduce the Hidden Polynomial Function Graph Problem as a natural
generalization of an abelian Hidden Subgroup Problem (HSP) where the subgroups
and their cosets correspond to graphs of linear functions over the finite field
F_p. For the Hidden Polynomial Function Graph Problem the functions are not
restricted to be linear but can also be multivariate polynomial functions of
higher degree.
  For a fixed number of indeterminates and bounded total degree the Hidden
Polynomial Function Graph Problem is hard on a classical computer as its black
box query complexity is polynomial in p. In contrast, this problem can be
reduced to a quantum state identification problem so that the resulting quantum
query complexity does not depend on p. For univariate polynomials we construct
a von Neumann measurement for distinguishing the states. We relate the success
probability and the implementation of this measurement to certain classical
problems involving polynomial equations. We present an efficient algorithm for
hidden quadratic and cubic function graphs by establishing that the success
probability of the measurement is lower bounded by a constant and that it can
be implemented efficiently.


title: Topological-Like Features in Diagrammatical Quantum Circuits

abstract:
  In this paper, we revisit topological-like features in the extended
Temperley--Lieb diagrammatical representation for quantum circuits including
the teleportation, dense coding and entanglement swapping. We perform these
quantum circuits and derive characteristic equations for them with the help of
topological-like operations. Furthermore, we comment on known diagrammatical
approaches to quantum information phenomena from the perspectives of both
tensor categories and topological quantum field theories. Moreover, we remark
on the proposal for categorical quantum physics and information to be described
by dagger ribbon categories.


title: Generalized Quantum Telecloning

abstract:
  We present a generalized telecloning (GTC) protocol where the quantum channel
is non-optimally entangled and we study how the fidelity of the telecloned
states depends on the entanglement of the channel. We show that one can
increase the fidelity of the telecloned states, achieving the optimal value in
some situations, by properly choosing the measurement basis at Alice's, albeit
turning the protocol to a probabilistic one. We also show how one can convert
the GTC protocol to the teleportation protocol via proper unitary operations.


title: Low-Threshold Surface-Passivated Photonic Crystal Nanocavity Laser

abstract:
  The efficiency and operating range of a photonic crystal laser is improved by
passivating the InGaAs quantum well (QW) gain medium and GaAs membrane using an
(NH4)S treatment. The passivated laser shows a four-fold reduction in
nonradiative surface recombination rate, resulting in a four-fold reduction in
lasing threshold. A three-level carrier dynamics model explains the results and
shows that lasing threshold is as much determined by surface recombination
losses as by the cavity quality factor (Q). Surface passivation therefore
appears crucial in operating such lasers under practical conditions.


title:
Bose-Einstein condensate coupled to a nanomechanical resonator on an
  atom chip

abstract:
  We theoretically study the coupling of Bose-Einstein condensed atoms to the
mechanical oscillations of a nanoscale cantilever with a magnetic tip. This is
an experimentally viable hybrid quantum system which allows one to explore the
interface of quantum optics and condensed matter physics. We propose an
experiment where easily detectable atomic spin-flips are induced by the
cantilever motion. This can be used to probe thermal oscillations of the
cantilever with the atoms. At low cantilever temperatures, as realized in
recent experiments, the backaction of the atoms onto the cantilever is
significant and the system represents a mechanical analog of cavity quantum
electrodynamics. With high but realistic cantilever quality factors, the strong
coupling regime can be reached, either with single atoms or collectively with
Bose-Einstein condensates. We discuss an implementation on an atom chip.


title:
An analytical relation between entropy production and quantum Lyapunov
  exponents for Gaussian bipartite systems

abstract:
  We study and compare the information loss of a large class of Gaussian
bipartite systems. It includes the usual Caldeira-Leggett type model as well as
Anosov models (parametric oscillators, the inverted oscillator environment,
etc), which exhibit instability, one of the most important characteristics of
chaotic systems. We establish a rigorous connection between the quantum
Lyapunov exponents and coherence loss, and show that in the case of unstable
environments coherence loss is completely determined by the upper quantum
Lyapunov exponent, a behavior which is more universal than that of the
Caldeira-Leggett type model.


title:
Covariant path integral formalism of relativistic quantum mechanics
  along proper time

abstract:
  A space-time symmetric and explicitly Lorentz covariant path integral
formalism of relativistic quantum mechanics is proposed, which produces partial
locally correlations of quantum processes of massive particles with the
velocity of light at low energy limits. A superluminal correlation is also
possible if anti-particles that moving along reverse time direction are
excited. This provides a new point of view to interpret EPR experiments, also
leaks a light of hope for hidden variable theories.


title: Qubit Teleportation and Transfer across Antiferromagnetic Spin Chains

abstract:
  We explore the capability of spin-1/2 chains to act as quantum channels for
both teleportation and transfer of qubits. Exploiting the emergence of
long-distance entanglement in low-dimensional systems [Phys. Rev. Lett. 96,
247206 (2006)], here we show how to obtain high communication fidelities
between distant parties. An investigation of protocols of teleportation and
state transfer is presented, in the realistic situation where temperature is
included. Basing our setup on antiferromagnetic rotationally invariant systems,
both protocols are represented by pure depolarizing channels. We propose a
scheme where channel fidelity close to one can be achieved on very long chains
at moderately small temperature.


title: Generalized spectroscopy; coherence, superposition, and loss

abstract:
  We analyze single particle coherence and interference in the presence of
particle loss and derive an inequality that relates the preservation of
coherence, the creation of superposition with the vacuum, and the degree of
particle loss. We find that loss channels constructed using linear optics form
a special subclass. We suggests a generalized spectroscopy where, in analogy
with the absorption spectrum, we measure a "coherence loss spectrum" and a
"superposition creation spectrum". The theory is illustrated with examples.


title:
von Neumann-Landau equation for wave functions, wave-particle duality
  and collapses of wave functions

abstract:
  It is shown that von Neumann-Landau equation for wave functions can present a
mathematical formalism of motion of quantum mechanics. The wave functions of
von Neumann-Landau equation for a single particle are `bipartite', in which the
associated Schr\"{o}dinger's wave functions correspond to those `bipartite'
wave functions of product forms. This formalism 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. Furthermore, this extension of Schr\"{o}dinger's form 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.


title:
The effect of atmospheric turbulence on entangled orbital angular
  momentum states

abstract:
  We analyze the effect of atmospheric Kolmogorov turbulence on entangled
orbital angular momentum states generated by parametric down-conversion. We
calculate joint and signal photon detection probabilities and obtain
numerically their dependence on the mode-width-to-Fried-parameter ratio. We
demonstrate that entangled photons are less robust to the effects of Kolmogorov
turbulence compared to single photons. In contrast, signal photons are more
robust than single photons in the lowest-order mode. We also obtain numerically
a scaling relation between the value of the mode-width-to-Fried-parameter ratio
for which the joint detection probabilities is a maximum and the momentum
mismatch between signal and idler photons after propagation through the medium.


title:
A local hidden variables model for the measured EPR-type flavour
  entanglement in upsilon(4S)->B0B0bar decays

abstract:
  A local hidden variables model is exhibited which gives predictions in
agreement with the quantum ones for the recent experiment by Go et al.
quant-ph/0702267


title: Spin chains with electrons in Penning traps

abstract:
  We demonstrate that spin chains are experimentally feasible using electrons
confined in micro-Penning traps, supplemented with local magnetic field
gradients. The resulting Heisenberg-like system is characterized by coupling
strengths showing a dipolar decay. These spin chains can be used as a channel
for short distance quantum communication. Our scheme offers high accuracy in
reproducing an effective spin chain with relatively large transmission rate.


title:
Controlled-NOT gate design for Josephson phase qubits with tunable
  inductive coupling: Weyl chamber steering and area theorem

abstract:
  Superconducting qubits with tunable coupling are ideally suited for fast and
accurate implementation of quantum logic. Here we present a simple approach,
based on Weyl chamber steering, to CNOT gate design for inductively coupled
phase qubits with tunable coupling strength g. In the presence of simultaneous
rf pulses on the individual qubits that appropriately track the coupling
strength as it is varied, we show that an infinite family of switching
sequences preserving the time integral or "area" of g can be used to generate
CNOT logic. We demonstrate our approach by considering time-dependencies most
likely to be used in actual implementations: trapezoidal, sine, and soft
quartic (also known as Landau's hat).


title: Bounds on Information Propagation in Disordered Quantum Spin Chains

abstract:
  We investigate the propagation of information through the disordered XY
model. We find, with a probability that increases with the size of the system,
that all correlations, both classical and quantum, are suppressed outside of an
effective lightcone whose radius grows at most polylogarithmically with |t|.


title: A most compendious and facile quantum de Finetti theorem

abstract:
  In its most basic form, the finite quantum de Finetti theorem states that the
reduced k-partite density operator of an n-partite symmetric state can be
approximated by a convex combination of k-fold product states. Variations of
this result include Renner's "exponential" approximation by "almost-product"
states, a theorem which deals with certain triples of representations of the
unitary group, and D'Cruz et al.'s result for infinite-dimensional systems. We
show how these theorems follow from a single, general de Finetti theorem for
representations of symmetry groups, each instance corresponding to a particular
choice of symmetry group and representation of that group. This gives some
insight into the nature of the set of approximating states, and leads to some
new results, including an exponential theorem for infinite-dimensional systems.


title:
Linear Depth Stabilizer and Quantum Fourier Transformation Circuits with
  no Auxiliary Qubits in Finite Neighbor Quantum Architectures

abstract:
  In this paper, we investigate how quantum architectures affect the efficiency
of the execution of the quantum Fourier transform (QFT) and linear
transformations, which are essential parts of the stabilizer/Clifford group
circuits. In particular, we show that in most common and realistic physical
architectures including the linear nearest neighbor, two-dimensional lattice,
and bounded degree graph (containing a chain of length n), n-qubit QFT and
n-qubit stabilizer circuits can be parallelized to linear depth using no
auxiliary qubits. We construct lower bounds that show the efficiency of our
approach.


title:
Is orbital angular momentum always conserved in spontaneous parametric
  down-conversion?

abstract:
  In the non-linear optical process of type-II spontaneous parametric
down-conversion, we present on an experiment showing that the two-photon
detection amplitude of the down-converted beams does not generally reproduce
the transverse profile of the pump beam that carries non-zero orbital angular
momentum. We explain this observation by that orbital angular momentum is not
conserved in the type-II non-linear process due to the broken rotational
symmetry of the Hamiltonian.


title: On Subsystem Codes Beating the Hamming or Singleton Bound

abstract:
  Subsystem codes are a generalization of noiseless subsystems, decoherence
free subspaces, and quantum error-correcting codes. We prove a Singleton bound
for GF(q)-linear subsystem codes. It follows that no subsystem code over a
prime field can beat the Singleton bound. On the other hand, we show the
remarkable fact that there exist impure subsystem codes beating the Hamming
bound. A number of open problems concern the comparison in performance of
stabilizer and subsystem codes. One of the open problems suggested by Poulin's
work asks whether a subsystem code can use fewer syndrome measurements than an
optimal MDS stabilizer code while encoding the same number of qudits and having
the same distance. We prove that linear subsystem codes cannot offer such an
improvement under complete decoding.


title:
Comment on ``Analytical and numerical verification of the Nernst heat
  theorem for metals''

abstract:
  Recently, H{\o}ye, Brevik, Ellingsen and Aarseth (quant-ph/0703174) claimed
that the use of the Drude dielectric function leads to zero Casimir entropy at
zero temperature in accordance with Nernst's theorem. We demonstrate that their
proof is not applicable to metals with perfect crystal lattices having no
impurities. Thus there is no any contradiction with previous results in the
literature proving that the Drude dielectric function violates the Nernst
theorem for the Casimir entropy in the case of perfect crystal lattices. We
also indicate mistakes in the coefficients of their asymptotic expressions for
metals with impurities.

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


title: Classical Interaction Cannot Replace a Quantum Message

abstract:
  We demonstrate a two-player communication problem that can be solved in the
one-way quantum model by a 0-error protocol of cost O (log n) but requires
exponentially more communication in the classical interactive (bounded error)
model.


title:
Disentanglement and decoherence in two-spin and three-spin systems under
  dephasing

abstract:
  We compare disentanglement and decoherence rates within two-spin and
three-spin entangled systems subjected to all possible combinations of local
and collective pure dephasing noise combinations. In all cases, the bipartite
entanglement decay rate is found to be greater than or equal to the
dephasing-decoherence rates and often significantly greater. This sharpens
previous results for two-spin systems [T. Yu and J. H. Eberly Phys. Rev. B 68,
165322 (2003)] and extends them to the three-spin context.


title: Semi-classical theory of quiet lasers. Short version

abstract:
  This article is the shorten version of quant-phys/0610106 with a supplemented
theory and new results concerning a single-electron laser driven by a
constant-potentiel battery. "Quiet (or sub-Poissonian) oscillators generate a
number of dissipation events whose variance is less than the mean. It was shown
in 1984 by Golubev and Sokolov that lasers driven by regular pumps are quiet in
that sense. We consider in the present paper two oscillators that should
exhibit in principle the same property. First, a reflex klystron, a vacuum tube
operating in the microwave range of frequency. Second a laser involving a
single electron permanently interacting with the field. It is unnecessary to
quantize the optical field, that is, the theory is semi-classical, yet exact.
As an example, the battery-driven one-electron laser delivers a detected noise
of 7/8 of the shot-noise level, and is therefore sub-Poissonian. Our
calculations are related to resonance-fluorescence treatments but with a
different physical interpretation. Previous theories considering excited-state
atoms regularly-injected in low-loss resonators, on the other hand, do require
light quantization. The theory presented here is restricted to above-threshold
stationary single-mode oscillators. The paper is written in such a way that
readers should be able to follow it without having to refer to quantum-optics
texts."


title:
Comment on: Disproof of Bell's Theorem by Clifford Algebra Valued Local
  Variables

abstract: We ''save'' Bell's Theorem by showing a flaw in Christian's argument.


title:
Strongly Correlated Polaritons in a Two-Dimensional Array of Photonic
  Crystal Microcavities

abstract:
  We propose a practical scheme to observe the polaritonic quantum phase
transition (QPT) from the superfluid (SF) to Bose-glass (BG) to Mott-insulator
(MI) states. The system consists of a two-dimensional array of photonic crystal
microcavities doped with substitutional donor/acceptor impurities. Using
realistic parameters, we show that such strongly correlated polaritonic systems
can be constructed using the state-of-art semiconductor technology.


title:
Overcoming the su(2^n) sufficient condition for the coherent control of
  n-qubit systems

abstract:
  We study quantum systems with even numbers N of levels that are completely
state-controlled by unitary transformations generated by Lie algebras
isomorphic to sp(N) of dimension N(N+1)/2. These Lie algebras are smaller than
the respective su(N) with dimension N^2-1. We show that this reduction
constrains the Hamiltonian to have symmetric energy levels. An example of such
a system is an n-qubit system. Using a geometric representation for the quantum
wave function of a finite system, we present an explicit example that shows a
two-qubit system can be controlled by the elements of the Lie algebra sp(4)
(isomorphic to spin(5) and so(5)) with dimension ten rather than su(4) with
dimension fifteen. These results enable one to envision more efficient
algorithms for the design of fields for quantum-state engineering, and they
provide more insight into the fundamental structure of quantum control.


title:
Microscopic physics of quantum self-organisation of optical lattices in
  cavities

abstract:
  We study quantum particles at zero temperature in an optical lattice coupled
to a resonant cavity mode. The cavity field substantially modifies the particle
dynamics in the lattice, and for strong particle-field coupling leads to a
quantum phase with only every second site occupied. We study the growth of this
new order out of a homogeneous initial distribution for few particles as the
microscopic physics underlying a quantum phase transition. Simulations reveal
that the growth dynamics crucially depends on the initial quantum many-body
state of the particles and can be monitored via the cavity fluorescence.
Studying the relaxation time of the ordering reveals inhibited tunnelling,
which indicates that the effective mass of the particles is increased by the
interaction with the cavity field. However, the relaxation becomes very quick
for large coupling.


title: Quantum mechanics, Bayes' theorem and the conjunction fallacy

abstract:
  In the present article we consider the conjunction fallacy, a well known
cognitive heuristic experimentally tested in cognitive science, which occurs
for intuitive judgments in situations of bounded rationality. We show that the
quantum formalism can be used to describe in a very simple way this fallacy in
terms of interference effect. We evidence that the quantum formalism leads
quite naturally to violations of Bayes' rule when considering the probability
of the conjunction of two events. Thus we suggest that in cognitive science the
formalism of quantum mechanics can be used to describe a \textit{quantum
regime}, the bounded-rationality regime, where the cognitive heuristics are
valid.


title:
Quantum algorithm for the hidden subgroup problem on a class of
  semidirect product groups

abstract:
  We present efficient quantum algorithms for the hidden subgroup problem (HSP)
on the semidirect product of cyclic groups $\Z_{p^r}\rtimes_{\phi}\Z_{p^2}$,
where $p$ is any odd prime number and $r$ is any integer such that $r>4$. We
also address the HSP in the group $\Z_{N}\rtimes_{\phi}\Z_{p^2}$, where $N$ is
an integer with a special prime factorization. These quantum algorithms are
exponentially faster than any classical algorithm for the same purpose.


title: Cascade atom in high-Q cavity: The spectrum for non-Markovian decay

abstract:
  The spontaneous emission spectrum for a three level cascade configuration
atom in a single mode high-Q cavity coupled to a zero temperature reservoir of
continuum external modes is determined from the atom-cavity mode master
equation using the quantum regression theorem. Initially the atom is in its
upper state and the cavity mode empty of photons. Following Glauber, the
spectrum is defined via the response of a detector atom. Spectra are calculated
for the detector located inside the cavity (case A), outside the cavity end
mirror (Case B-end emission), or placed for emission out the side of the cavity
(Case C). The spectra for case A and case B are found to be essentially the
same. In all the cases the predicted lineshapes are free of instrumental
effects and only due to cavity decay. Spectra are presented for intermediate
and strong coupling regime situations (where both atomic transitions are
resonant with the cavity frequency), for cases of non-zero cavity detuning, and
for cases where the two atomic transition frequencies differ. The spectral
features for Cases B(A) and C are qualitatively similar, with six spectral
peaks for resonance cases and eight for detuned cases. These general features
of the spectra can be understood via the dressed atom model. However, Case B
and C spectra differ in detail, with the latter exhibiting a deep spectral hole
at the cavity frequency due to quantum interference effects.


title: Gaussian quantum marginal problem

abstract:
  The quantum marginal problem asks what local spectra are consistent with a
given spectrum of a joint state of a composite quantum system. This setting,
also referred to as the question of the compatibility of local spectra, has
several applications in quantum information theory. Here, we introduce the
analogue of this statement for Gaussian states for any number of modes, and
solve it in generality, for pure and mixed states, both concerning necessary
and sufficient conditions. Formally, our result can be viewed as an analogue of
the Sing-Thompson Theorem (respectively Horn's Lemma), characterizing the
relationship between main diagonal elements and singular values of a complex
matrix: We find necessary and sufficient conditions for vectors (d1, ..., dn)
and (c1, ..., cn) to be the symplectic eigenvalues and symplectic main diagonal
elements of a strictly positive real matrix, respectively. More physically
speaking, this result determines what local temperatures or entropies are
consistent with a pure or mixed Gaussian state of several modes. We find that
this result implies a solution to the problem of sharing of entanglement in
pure Gaussian states and allows for estimating the global entropy of
non-Gaussian states based on local measurements. Implications to the actual
preparation of multi-mode continuous-variable entangled states are discussed.
We compare the findings with the marginal problem for qubits, the solution of
which for pure states has a strikingly similar and in fact simple form.


title:
Correlation coefficients and the Robertson-Schroedinger uncertainty
  relations

abstract:
  Calling the quantity; 2delta(A)delta(B)/|<[A, B]>|, with non-zero
denominator, the uncertainty product ratio or UPR for the pair of observables,
(A, B), it is shown that any non-zero correlation coefficient between two
observables raises, above unity, the lower bound of the UPR for each member of
an infinite collection of pairs of incompatible observables. Conversely, any
UPR is subject to lower bounds above unity determined by each of an infinite
collection of correlation coefficients. This result generalizes the well known
Schroedinger strengthening of the Robertson uncertainty relations (with the
former expressed in terms of the correlation coefficient rather than the
anticommutator) where the UPR and the correlation coefficient both involve the
same pair of observables. Two, independent, derivations of the result are
presented to clarify its origins and some examples of its use are examined.


title: Regional versus Global Entanglement in Resonating-Valence-Bond states

abstract:
  We investigate the entanglement properties of resonating-valence-bond states
on two and higher dimensional lattices, which play a significant role in our
understanding of various many-body systems. We show that these states are
genuinely multipartite entangled, while there is only a negligible amount of
two-site entanglement. We comment on possible physical implications of our
findings.


title: Quantum Decoherence Modulated by Special Relativity

abstract:
  By investigating the evolution of a moving spin-1/2 Dirac electron coupled
with a background magnetic noise, we demonstrate that the effects of special
relativity will significantly modify the decoherence properties of the spin
state. The dephasing could be much suppressed, and for a sufficiently long time
the decoherence even seems to halt. This interesting phenomenon stems from the
dressed environment induced by special relativity.


title:
The lateral Casimir force beyond the proximity force approximation: a
  nontrivial interplay between geometry and quantum vacuum

abstract:
  The lateral Casimir force between two corrugated metallic plates makes
possible a study of the nontrivial interplay of geometry and Casimir effect
appearing beyond the regime of validity of the Proximity Force Approximation
(PFA). Quantitative evaluations can be obtained by using scattering theory in a
perturbative expansion valid when the corrugation amplitudes are smaller than
the three other length scales: the mean separation distance $L$ of the plates,
the corrugation period $\lambda_\C$ and the plasma wavelength $\lambda_\P$.
Within this perturbative expansion, evaluations are obtained for arbitrary
relative values of $L$, $\lambda_\C$ and $\lambda_\P$ while limiting cases,
some of them already known, are recovered when these values obey some specific
orderings. The consequence of these results for comparison with existing
experiments is discussed in the end of the paper.


title: Level Reduction and the Quantum Threshold Theorem

abstract:
  The quantum threshold theorem shows that a noisy quantum computer can
accurately and efficiently simulate any ideal quantum computation provided that
noise is weakly correlated and its strength is below a critical value known as
the quantum accuracy threshold. This thesis provides a simpler and more
transparent non-inductive proof of this theorem based on the concept of level
reduction. This concept is also used in proving the quantum threshold theorem
for coherent and leakage noise and for quantum computation by measurements. In
addition, the proof provides a methodology which allows us to establish
improved rigorous lower bounds on the value of the quantum accuracy threshold.


title: Quantum Search in an Ordered List via Adaptive Learning

abstract:
  We use a Bayesian approach to optimally solve problems in noisy binary
search. We deal with two variants:
  1. Each comparison can be erroneous with some probability $1 - p$. 2. At each
stage $k$ comparisons can be performed in parallel and a noisy answer is
returned
  We present a (classic) algorithm which optimally solves both variants
together, up to an additive term of O(\log \log(n)), and prove matching
information theoretic lower bounds. We use the algorithm to improve the results
of Farhi et al \cite{FGGS99} presenting a quantum (error free) search algorithm
in an ordered list of expected complexity less than (\log_2n) / 3.


title: Electrino bubbles and relational entanglement

abstract:
  We argue that the phenomena exhibited by bubbles forming around free
electrons in liquid helium and examined by Maris in his controversial 2000
paper point to the experimental relevance of relational entanglement. An
experiment to verify/disprove the relevant argument is suggested.


title:
Decoherence and relaxation of a qubit coupled to an Ohmic bath directly
  and via an intermediate harmonic oscillator

abstract:
  Using the numerical path integral method we investigate the decoherence and
relaxation of qubits coupled to an Ohmic bath directly and via an intermediate
harmonic oscillator (IHO). Here, we suppose the oscillation frequencies of the
bath modes are higher than the IHO's. When we choose suitable parameters the
qubits in the two models may have almost same decoherence and relaxation times.
However, the decoherence and relaxation times of the qubit in the
qubit-IHO-bath model can be modulated through changing the coupling
coefficients of the qubit-IHO and IHO-bath and the oscillation frequency of the
IHO.


title: Inverted Oscillator

abstract:
  The inverted harmonic oscillator problem is investigated quantum
mechanically. The exact wave function for the confined inverted oscillator is
obtained and it is shown that the associated energy eigenvalues are discrete
and it is given as a linear function of the quantum number $n$.


title: Complex Spectrum of a Spontaneously Unbroken PT Symmetric Hamiltonian

abstract:
  It is believed that unbroken PT symmetry is sufficient to guarantee that the
spectrum of a non-Hermitian Hamiltonian is real. We prove that this is not
true. We study a Hamiltonian with complex spectrum for which PT symmetry is not
spontaneously broken.


title: Parameters of Integral Circulant Graphs and Periodic Quantum Dynamics

abstract:
  The intention of the paper is to move a step towards a classification of
network topologies that exhibit periodic quantum dynamics. We show that the
evolution of a quantum system, whose hamiltonian is identical to the adjacency
matrix of a circulant graph, is periodic if and only if all eigenvalues of the
graph are integers (that is, the graph is integral). Motivated by this
observation, we focus on relevant properties of integral circulant graphs.
Specifically, we bound the number of vertices of integral circulant graphs in
terms of their degree, characterize bipartiteness and give exact bounds for
their diameter. Additionally, we prove that circulant graphs with odd order do
not allow perfect state transfer.


title: The Multiplicative Quantum Adversary

abstract:
  We present a new variant of the quantum adversary method. All adversary
methods give lower bounds on the quantum query complexity of a function by
bounding the change of a progress function caused by one query. All previous
variants upper-bound the_difference_ of the progress function, whereas our new
variant upper-bounds the_ratio_ and that is why we coin it the multiplicative
adversary. The new method generalizes to all functions the new quantum
lower-bound method by Ambainis [Amb05, ASW06] based on the analysis of
eigenspaces of the density matrix. We prove a strong direct product theorem for
all functions that have a multiplicative adversary lower bound.


title:
Improving the fidelity of single photon preparation from conditional
  down-conversion via asymmetric multiport detection

abstract:
  We discuss the conditional preparation of single photons via parametric
down-conversion. This technique is commonly used as a single photon source in
modern quantum optics experiments. A significant problem facing this technique
is the inability of present day photo-detectors to resolve photon number. This
results in mixing with higher photon number terms. To overcome this several
techniques have been proposed, including multi-port detection and time-division
multiplexing. These techniques help approximate number resolving detection even
when using non-number resolving detectors. In this paper we focus on 2-port
detection, the simplest such scheme. We show that by making the 2-port device
asymmetric the fidelity of prepared photons can be improved.


title:
Eliminating interactions between non-neighboring qubits in the
  preparation of cluster states in quantum molecules

abstract:
  We propose a scheme to eliminate the effect of non-nearest-neighbor qubits in
preparing cluster state with double-dot molecules. As the interaction
Hamiltonians between qubits are Ising-model and mutually commute, we can get
positive and negative effective interactions between qubits to cancel the
effect of non-nearest-neighbor qubits by properly changing the electron charge
states of each quantum dot molecule. The total time for the present multi-step
cluster state preparation scheme is only doubled for one-dimensional qubit
chain and tripled for two-dimensional qubit array comparing with the time of
previous protocol leaving out the non-nearest-neighbor interactions.


title: A Decomposition of Separable Werner States

abstract:
  We derive an integral convex combination of product states for a range of
separable Werner states. Our method consists of expanding the sought-after
local density operators in terms of Wigner operators. For dimension d=2, our
decomposition holds for the whole separable range of Werner states, while for
d>2 it is valid for a subset of separable Werner states. We illustrate the
general method with the explicit examples d=2 and d=3.


title:
Irreversible decay of nonlocal entanglement via a reservoir of a single
  degree of freedom

abstract:
  Recently, it has been realized that nonlocal disentanglement may take a
finite time as opposite to the asymptotic decay of local coherences. We find in
this paper that a sudden irreversible death of entanglement takes place in a
two atom optical Stern-Gerlach model. In particular, the one degree non
dissipative environment here considered suddenly destroys the initial
entanglement of any Bell's states $\ket{\phi^{\pm}}$ superposition.


title:
On the implementation of a deterministic secure coding protocol using
  polarization entangled photons

abstract:
  We demonstrate a prototype-implementation of deterministic information
encoding for quantum key distribution (QKD) following the ping-pong coding
protocol [K. Bostroem, T. Felbinger, Phys. Rev. Lett. 89 (2002) 187902-1]. Due
to the deterministic nature of this protocol the need for post-processing the
key is distinctly reduced compared to non-deterministic protocols. In the
course of our implementation we analyze the practicability of the protocol and
discuss some security aspects of information transfer in such a deterministic
scheme.


title: Ensemble averaged entanglement of two-particle states in Fock space

abstract:
  Recent results, extending the Schmidt decomposition theorem to wavefunctions
of identical particles, are reviewed. They are used to give a definition of
reduced density operators in the case of two identical particles. Next, a
method is discussed to calculate time averaged entanglement. It is applied to a
pair of identical electrons in an otherwise empty band of the Hubbard model,
and to a pair of bosons in the the Bose-Hubbard model with infinite range
hopping. The effect of degeneracy of the spectrum of the Hamiltonian on the
average entanglement is emphasised.


title: Disproof of Bell's Theorem: Reply to Critics

abstract:
  This is a collection of my responses to the criticisms of my argument against
the impossibility proof of John Bell, which aims to undermine any conceivable
local realistic completion of quantum mechanics. I plan to periodically update
this preprint instead of creating a new one for each response.


title: On Entropy Transmission for Quantum Channels

abstract:
  In this paper a notion of entropy transmission of quantum channels is
introduced as a natural extension of Ohya's entropy. Here by quantum channel is
meant unital completely positive mappings (ucp) of $B(H)$ into itself, where
$H$ is an infinite dimensional Hilbert space. Using a representation theorem of
ucp mapping we associate to every ucp map a uniquely determined state, and
prove that entropy of ucp map is less then Ohya's entropy of the associated
state.


title: How many ebits can be unlocked with one classical bit?

abstract:
  We find an upper bound on the rate at which entanglement can be unlocked by
classical bits. In particular, we show that for quantum information sources
that are specified by ensambles of pure bipartite states, one classical bit can
unlock at most one ebit.


title: Coherent atom-trimer conversion in a repulsive Bose-Einstein condensate

abstract:
  We show that the use of a generalized atom-molecule dark state permits the
enhanced coherent creation of triatomic molecules in a repulsive atomic
Bose-Einstein condensate, with further enhancement being possible in the case
of heteronuclear trimers via the constructive interference between two chemical
reaction channels.


title: Vacuum Energy and Repulsive Casimir Forces in Quantum Star Graphs

abstract:
  Casimir pistons are models in which finite Casimir forces can be calculated
without any suspect renormalizations. It has been suggested that such forces
are always attractive, but we present several counterexamples, notably a simple
type of quantum graph in which the sign of the force depends upon the number of
edges. We also show that Casimir forces in quantum graphs can be reliably
computed by summing over the classical orbits, and study the rate of
convergence of the periodic orbit expansion. In generic situations where no
analytic expression is available, the sign and approximate magnitude of Casimir
forces can often be obtained using only the shortest classical orbits.


title: Bipartite entanglement measure based on covariances

abstract:
  We propose an entanglement measure for two qudits based on the covariances of
a set of generators of the su(N) algebra. In particular, we represent this
measure in terms of the mutually unbiased projectors for N prime. For pure
states this measure quantify entanglement, we obtain an explicit expression
which relates it to the concurrence hierarchy, specifically the I-concurrence
and the 3-concurrence. For mixed states we propose a separability criterion.


title:
Instantaneous processing of "slow light": amplitude-duration control,
  storage, and splitting

abstract:
  Nonadiabatic change of the control field or of the low-frequency coherence
allows for an almost instantaneous change of the signal field propagating in a
thick resonant absorber where electromagnetically induced transparency is
realized. This finding is applied for the storage and retrieval of the signal,
for the creation of a signal copy and separation of this copy from the original
pulse without its destruction.


title: Classical statistical distributions can violate Bell-type inequalities

abstract:
  We investigate two-particle phase-space distributions in classical mechanics
characterized by a well-defined value of the total angular momentum. We
construct phase-space averages of observables related to the projection of the
particles' angular momenta along axes with different orientations. It is shown
that for certain observables, the correlation function violates Bell's
inequality. The key to the violation resides in choosing observables impeding
the realization of the counterfactual event that plays a prominent role in the
derivation of the inequalities. This situation can have statistical (detection
related) or dynamical (interaction related) underpinnings, but non-locality
does not play any role.


title:
Improved practical decoy state method in quantum key distribution with
  parametric down conversion source

abstract:
  In this paper, a new decoy-state scheme for quantum key distribution with
parametric down-conversion source is proposed. We use both three-intensity
decoy states and their triggered and nontriggered components to estimate the
fraction of single-photon counts and quantum bit-error rate of single-photon,
and then deduce a more accurate value of key generation rate. The final key
rate over transmission distance is simulated, which shows that we can obtain a
higher key rate than that of the existing methods, including our own earlier
work.


title:
Narrowband polarization-entangled photon pairs distributed over a WDM
  link for qubit networks

abstract:
  We present a bright, narrowband, portable, quasi-phase-matched two-crystal
source generating polarization-entangled photon pairs at 809 nm and 1555 nm at
a maximum rate of 1.2 10^6 s-1 THz-1 mW-1 after coupling to single-mode fiber.
The quantum channel at 1555 nm and the synchronization signal gating the single
photon detector are multiplexed in the same optical fiber of length 27 km by
means of wavelength division multiplexers (WDM) having 100 GHz (0.8 nm) spacing
between channels. This implementation makes quantum communication applications
compatible with current high-speed optical networks.


title: The quantum brachistochrone problem for non-Hermitian Hamiltonians

abstract:
  Recently Bender, Brody, Jones and Meister found that in the quantum
brachistochrone problem the passage time needed for the evolution of certain
initial states into specified final states can be made arbitrarily small, when
the time-evolution operator is taken to be non-Hermitian but PT-symmetric. Here
we demonstrate that such phenomena can also be obtained for non-Hermitian
Hamiltonians for which PT-symmetry is completely broken, i.e. dissipative
systems. We observe that the effect of a tunable passage time can be achieved
by projecting between orthogonal eigenstates by means of a time-evolution
operator associated to a non-Hermitian Hamiltonian. It is not essential that
this Hamiltonian is PT-symmetric.


title: Quantum Communication

abstract:
  Quantum communication, and indeed quantum information in general, has changed
the way we think about quantum physics. In 1984 and 1991, the first protocol
for quantum cryptography and the first application of quantum non-locality,
respectively, attracted a diverse field of researchers in theoretical and
experimental physics, mathematics and computer science. Since then we have seen
a fundamental shift in how we understand information when it is encoded in
quantum systems. We review the current state of research and future directions
in this new field of science with special emphasis on quantum key distribution
and quantum networks.


title: Quantum Circuit Placement

abstract:
  We study the problem of the practical realization of an abstract quantum
circuit when executed on quantum hardware. By practical, we mean adapting the
circuit to particulars of the physical environment which restricts/complicates
the establishment of certain direct interactions between qubits. This is a
quantum version of the classical circuit placement problem. We study the
theoretical aspects of the problem and also present empirical results that
match the best known solutions that have been developed by experimentalists.
Finally, we discuss the efficiency of the approach and scalability of its
implementation with regards to the future development of quantum hardware.


title: Space-like energy momentum in quantum electrodynamics

abstract:
  A common assumption in quantum field theory is that the energy-momentum
4-vector of any quantum state must be time-like. However it has been recently
shown [4] that this is not the case for a Dirac-Maxwell field in the coulomb
gauge. Here we will present a proof that is simpler then the proof of Ref. [4]
that there must exist quantum states which are space-like for a Dirac-Maxwell
field in the coulomb gauge.


title:
Correctable noise of Quantum Error Correcting Codes under adaptive
  concatenation

abstract:
  We examine the transformation of noise under a quantum error correcting code
(QECC) concatenated repeatedly with itself, by analyzing the effects of a
quantum channel after each level of concatenation using recovery operators that
are optimally adapted to use error syndrome information from the previous
levels of the code. We use the Shannon entropy of these channels to estimate
the thresholds of correctable noise for QECCs and find considerable
improvements under this adaptive concatenation. Similar methods could be used
to increase quantum fault tolerant thresholds.


title: Bell inequalities, classical cryptography and fractals

abstract:
  The relation between the boolean functions and Bell inequalities for qubits
is analyzed. The connection between the maximal quantum violation of a Bell
inequality and the nonlinearity of the corresponding boolean function is
discussed. A visualization scheme of boolean functions is proposed. An attempt
to classify Bell inequalities for qubits is made, a weaker result
(classification with respect to Jevons group) is obtained. The fractal
structure of the classification is shown. All constructs are illustrated by
Mathematica code.


title:
Reinterpreting Quantum Probabilities in a Realistic and Local Framework:
  The Modified BCHSH Inequalities

abstract:
  Most physicists uphold that the tests of the Bell inequalities (BI) performed
up to now confirm the predictions of standard quantum mechanics (SQM) and
refute local realism. But some scholars criticize this conviction, defending
local realism in various ways. We present here a new viewpoint based on an
improved version of the extended semantic realism (ESR) model that has been
recently worked out by one of the authors. The ESR model embodies the
mathematical formalism of SQM into a more general framework in which not only
local realism but also objectivity of physical properties holds, and the
probabilities of SQM are reinterpreted as conditional instead of absolute.
Hence the ESR model provides some predictions that are formally identical to
those of SQM but have a different physical interpretation, and further
predictions that differ also formally from those of SQM. In particular, we show
here that the BI introduced by Clauser, Horne, Shimony and Holt (BCHSH
inequalities) must be replaced by modified BCHSH inequalities. These depend on
detection probabilities which may be such that the new inequalities are never
violated by the conditional expectation values predicted by the model. The
condition that no violation occurs implies the existence of upper bounds on
detection probabilities, which makes the ESR model falsifiable. These results
admit an intuitive explanation in terms of unfair sampling but basically differ
from the seemingly similar results obtained by other approaches in which the
efficiency problem is discussed in order to vindicate some kind of local
realism.


title: Quantum thermodynamic Carnot and Otto-like cycles for a two-level system

abstract:
  From the thermodynamic equilibrium properties of a two-level system with
variable energy-level gap $\Delta$, and a careful distinction between the Gibbs
relation $dE = T dS + (E/\Delta) d\Delta$ and the energy balance equation $dE =
\delta Q^\leftarrow - \delta W^\to$, we infer some important aspects of the
second law of thermodynamics and, contrary to a recent suggestion based on the
analysis of an Otto-like thermodynamic cycle between two values of $\Delta$ of
a spin-1/2 system, we show that a quantum thermodynamic Carnot cycle, with the
celebrated optimal efficiency $1 - (T_{low}/T_{high})$, is possible in
principle with no need of an infinite number of infinitesimal processes,
provided we cycle smoothly over at least three (in general four) values of
$\Delta$, and we change $\Delta$ not only along the isoentropics, but also
along the isotherms, e.g., by use of the recently suggested maser-laser tandem
technique. We derive general bounds to the net-work to high-temperature-heat
ratio for a Carnot cycle and for the 'inscribed' Otto-like cycle, and represent
these cycles on useful thermodynamic diagrams.


title: Solving the Anharmonic Oscillator: Tuning the Boundary Condition

abstract:
  We outline a remarkably efficient method for generating solutions to quantum
anharmonic oscillators with an x^{2M} potential. We solve the Schroedinger
equation in terms of a free parameter which is then tuned to give the correct
boundary condition by generating a power series expansion of the wavefunction
in x and applying a modified Borel resummation technique to obtain the large x
behaviour. The process allows us to calculate energy eigenvalues to an
arbitrary level of accuracy. High degrees of precision are achieved even with
modest computing power. Our technique extends to all levels of excitation and
produces the correct solution to the double well oscillators even though they
are dominated by non-perturbative effects.


title:
Overlapping resonances in the control of intramolecular vibrational
  redistribution

abstract:
  Coherent control of bound state processes via the interfering overlapping
resonances scenario [Christopher et al., J. Chem. Phys. 123, 064313 (2006)] is
developed to control intramolecular vibrational redistribution (IVR). The
approach is applied to the flow of population between bonds in a model of
chaotic OCS vibrational dynamics, showing the ability to significantly alter
the extent and rate of IVR by varying quantum interference contributions.


title: Accuracy threshold for postselected quantum computation

abstract:
  We prove an accuracy threshold theorem for fault-tolerant quantum computation
based on error detection and postselection. Our proof provides a rigorous
foundation for the scheme suggested by Knill, in which preparation circuits for
ancilla states are protected by a concatenated error-detecting code and the
preparation is aborted if an error is detected. The proof applies to
independent stochastic noise but (in contrast to proofs of the quantum accuracy
threshold theorem based on concatenated error-correcting codes) not to
strongly-correlated adversarial noise. Our rigorously established lower bound
on the accuracy threshold, 1.04 \times 10^{-3}, is well below Knill's numerical
estimates.


title: Local Quantum Dot Tuning on Photonic Crystal Chips

abstract:
  Quantum networks based on InGaAs quantum dots embedded in photonic crystal
devices rely on QDs being in resonance with each other and with the cavities
they are embedded in. We developed a new technique based on temperature tuning
to spectrally align different quantum dots located on the same chip. The
technique allows for up to 1.8nm reversible on-chip quantum dot tuning.


title: Quantum mechanics with respect to different reference frames

abstract:
  Geometric (Schrodinger) quantization of nonrelativistic mechanics with
respect to different reference frames is considered. In classical
nonrelativistic mechanics, a reference frame is represented by a connection on
a configuration space fibered over a time axis. Under quantization, it yields a
connection on the quantum algebra of Schrodinger operators. The operators of
energy with respect to different reference frames are examined.


title: Algebraic approach to the Hulthen potential

abstract:
  In this paper the energy eigenvalues and the corresponding eigenfunctions are
calculated for Hulthen potential. Then we obtain the ladder operators and show
that these operators satisfy SU(2) commutation relation.


title: All bipartite entangled states display some hidden nonlocality

abstract:
  We show that a violation of the Clauser-Horne-Shimony-Holt (CHSH) inequality
can be demonstrated in a certain kind of Bell experiment for all bipartite
entangled states. Our protocol allows local filtering measurements and involves
shared ancilla states that do not themselves violate CHSH. Our result follows
from two main steps. We first provide a simple characterization of the states
that violate the CHSH-inequality after local filtering operations in terms of
witness-like operators. Second, we prove that for each entangled state
$\sigma$, there exists another state $\rho$ not violating CHSH, such that
$\rho\otimes\sigma$ violates CHSH. Hence, in this scenario, $\sigma$ cannot be
substituted by classical correlations without changing the statistics of the
experiment; we say that $\sigma$ is not simulable by classical correlations and
our result is that entanglement is equivalent to non-simulability.


title:
Corrections to the ns-levels of hydrogen atom in deformed space with
  minimal length

abstract:
  We investigated the hydrogen atom problem with deformed Heisenberg algebra
leading to the existence of minimal length. Using modified perturbation theory
developed in our previous work [M. M. Stetsko and V. M. Tkachuk, Phys. Rev. A
74, 012101 (2006)] we calculated the corrections to the arbitrary s-levels for
hydrogen atom. We received a simple relation for the estimation of minimal
length. We also compared the estimation of minimal length obtained here with
the results obtained in the preceding investigations.


title: Topologically decoherence-protected qubits with trapped ions

abstract:
  We show that trapped ions can be used to simulate a highly symmetrical
Hamiltonian with eingenstates naturally protected against local sources of
decoherence. This Hamiltonian involves long range coupling between particles
and provides a more efficient protection than nearest neighbor models discussed
in previous works. Our results open the perspective of experimentally realizing
in controlled atomic systems, complex entangled states with decoherence times
up to nine orders of magnitude longer than isolated quantum systems.


title: A local model violating Bell's inequalities

abstract:
  A model for two entangled systems in an EPR setting is shown to reproduce the
quantum-mechanical outcomes and expectation values. Each system is represented
by a small sphere containing a point-like particle embedded in a field. A
quantum state appears as an equivalence class of several possible
particle-field configurations. Contrarily to Bell-type hidden variables models,
the fields account for the non-commutative aspects of the measurements and deny
the simultaneous reality of incompatible physical quantities, thereby allowing
to escape EPR's "completeness or locality" dilemma.


title: Optimal Resources for Topological 2D Stabilizer Codes: Comparative Study

abstract:
  We study the resources needed to construct topological 2D stabilizer codes as
a way to estimate in part their efficiency and this leads us to perform a
comparative study of surface codes and color codes. This study clarifies the
similarities and differences between these two types of stabilizer codes. We
compute the error correcting rate $C:=n/d^2$ for surface codes $C_s$ and color
codes $C_c$ in several instances. On the torus, typical values are $C_s=2$ and
$C_c=3/2$, but we find that the optimal values are $C_s=1$ and $C_c=9/8$. For
planar codes, a typical value is $C_s=2$, while we find that the optimal values
are $C_s=1$ and $C_c=3/4$. In general, a color code encodes twice as much
logical qubits as a surface code does.


title: Interference of Fock states in a single measurement

abstract:
  We study analytically the structure of an arbitrary order correlation
function for a pair of Fock states and prove without any approximations that in
a single measurement of particle positions interference effects must occur as
experimentally observed with Bose-Einstein condensates. We also show that the
noise level present in the statistics is slightly lower than for a respective
measurement of phase states.


title: Robust Quantum Error Correction via Convex Optimization

abstract:
  We present a semidefinite program optimization approach to quantum error
correction that yields codes and recovery procedures that are robust against
significant variations in the noise channel. Our approach allows us to optimize
the encoding, recovery, or both, and is amenable to approximations that
significantly improve computational cost while retaining fidelity. We
illustrate our theory numerically for optimized 5-qubit codes, using the
standard [5,1,3] code as a benchmark. Our optimized encoding and recovery
yields fidelities that are uniformly higher by 1-2 orders of magnitude against
random unitary weight-2 errors compared to the [5,1,3] code with standard
recovery. We observe similar improvement for a 4-qubit decoherence-free
subspace code.


title: Supersymmetry and quantum games

abstract:
  In this paper we show the connection between the supersymmetry and quantum
games.


title: An exercise in "anhomomorphic logic"

abstract:
  A classical logic exhibits a threefold inner structure comprising an algebra
of propositions `A', a space of ``truth values'' `V', and a distinguished
family of mappings `phi' from propositions to truth values. Classically A is a
Boolean algebra, V=Z_2, and the admissible maps phi:A-->Z_2 are {\it
homomorphisms}. If one admits a larger set of maps, one obtains an
anhomomorphic logic that seems better suited to quantal reality (and the needs
of quantum gravity). I explain these ideas and illustrate them with three
simple examples.


title:
Strong Monogamy of Bipartite and Genuine Multipartite Entanglement: The
  Gaussian Case

abstract:
  We demonstrate the existence of general constraints on distributed quantum
correlations, which impose a trade-off on bipartite and multipartite
entanglement at once. For all N-mode Gaussian states under permutation
invariance, we establish exactly a monogamy inequality, stronger than the
traditional one, that by recursion defines a proper measure of genuine
N-partite entanglement. Strong monogamy holds as well for subsystems of
arbitrary size, and the emerging multipartite entanglement measure is found to
be scale invariant. We unveil its operational connection with the optimal
fidelity of continuous variable teleportation networks.

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


title:
Graphical description of the action of Clifford operators on stabilizer
  states

abstract:
  We introduce a graphical representation of stabilizer states and translate
the action of Clifford operators on stabilizer states into graph operations on
the corresponding stabilizer-state graphs. Our stabilizer graphs are
constructed of solid and hollow nodes, with (undirected) edges between nodes
and with loops and signs attached to individual nodes. We find that local
Clifford transformations are completely described in terms of local
complementation on nodes and along edges, loop complementation, and change of
node type or sign. Additionally, we show that a small set of equivalence rules
generates all graphs corresponding to a given stabilizer state; we do this by
constructing an efficient procedure for testing the equality of any two
stabilizer graphs.


title: Photon Statistics of a Two-Mode Squeezed Vacuum

abstract:
  We investigate the general case of the photon distribution of a two-mode
squeezed vacuum and show that the distribution of photons among the two modes
depends on four parameters: two squeezing parameters, the relative phase
between the two oscillators and their spatial orientation. The distribution of
the total number of photons depends only on the two squeezing parameters. We
derive analytical expressions and present pictures for both distributions.


title:
The Emergence of Classicality via Decoherence: Beyond the
  Caldeira-Legget Environment

abstract:
  Maximally predictive states, as defined in recent work by Zurek, Habib and
Paz, are studied for more elaborate environment models than a linear coupling.
An environment model which includes spatial correlations in the noise is
considered in the non-dissipative regime. The Caldeira-Leggett model is also
reconsidered in the context of an averaging procedure which produces a
completely positive form for the quantum master equation. In both cases, the
maximally predictive states for the harmonic oscillator are the coherent
states, which is the same result found by Zurek,Habib and Paz for the
Caldeira-Legget environment.


title: String Theory, Black Holes and Klein's Lemma

abstract:
  This is a writeup of a talk given at the Oskar Klein Centenery Symposium,
Stockholm, September 19-21, 1994. It is an essay on the black hole information
paradox and its connection with thermodynamics and the foundations of quantum
mechanics.


title: Strong Determinism vs. Computability

abstract:
  Are minds subject to laws of physics? Are the laws of physics computable? Are
conscious thought processes computable? Currently there is little agreement as
to what are the right answers to these questions. Penrose goes one step further
and asserts that: {\it a radical new theory is indeed needed, and I am
suggesting, moreover, that this theory, when it is found, will be of an
essentially non-computational character.} The aim of this paper is three fold:
1) to examine the incompatibility between the hypothesis of strong determinism
and computability, 2) to give new examples of uncomputable physical laws, and
3) to discuss the relevance of G\"odel's Incompleteness Theorem in refuting the
claim that an algorithmic theory---like strong AI---can provide an adequate
theory of mind. Finally, we question the adequacy of the theory of computation
to discuss physical laws and thought processes.


title: On the Stronger Statement of Levinson's Theorem for the Dirac Equation

abstract:
  Recently a stronger statement of Levinson's theorem for the Dirac equation
was presented, where the limits of the phase shifts at $E=\pm M$ are related to
the numbers of nodes of radial functions at the same energies, respectively.
However, in this letter we show that this statement has to be modified because
the limits of the phase shifts may be negative for the Dirac equation.


title: Kochen-Specker theorem for 8-dimensional space

abstract:
  A Kochen-Specker contradiction is produced with 36 vectors in a real
8-dimensional Hilbert space. These vectors can be combined into 30 distinct
projection operators (14 of rank 2, and 16 of rank 1). A state-specific variant
of this contradiction requires only 13 vectors, a remarkably low number for 8
dimensions.


title: Infinite matrices may violate the associative law

abstract:
  The momentum operator for a particle in a box is represented by an infinite
order Hermitian matrix $P$. Its square $P^2$ is well defined (and diagonal),
but its cube $P^3$ is ill defined, because $P P^2\neq P^2 P$. Truncating these
matrices to a finite order restores the associative law, but leads to other
curious results.


title:
A general formulation of discrete-time quantum mechanics, restrictions
  on the action and the relation of unitarity to the existence theorem for
  initial-value problems

abstract:
  A general formlulation for discrete-time quantum mechanics, based on
Feynman's method in ordinary quantum mechanics, is presented. It is shown that
the ambiguities present in ordinary quantum mechanics (due to noncommutativity
of the operators), are no longer present here. Then the criteria for the
unitarity of the evolution operator is examined. It is shown that the unitarity
of the evolution operator puts restrictions on the form of the action, and also
implies the existence of a solution for the classical initial-value problem.


title: Superconvergent Perturbation Method in Quantum Mechanics

abstract:
  An analogue of Kolmogorov's superconvergent perturbation theory in classical
mechanics is constructed for self adjoint operators. It is different from the
usual Rayleigh--Schr\"odinger perturbation theory and yields expansions for
eigenvalues and eigenvectors in terms of functions of the perturbation
parameter.


title: Quantum Averaging I: Poincar\'e--von Zeipel is Rayleigh--Schr\"odinger

abstract:
  An exact analogue of the method of averaging in classical mechanics is
constructed for self--adjoint operators. It is shown to be completely
equivalent to the usual Rayleigh--Schr\"odinger perturbation theory but gives
the sums over intermediate states in closed form expressions. The anharmonic
oscillator and the Henon--Heiles system are treated as examples to illustrate
the quantum averaging method.


title:
Two Mode Quantum Systems: Invariant Classification of Squeezing
  Transformations and Squeezed States

abstract:
  A general analysis of squeezing transformations for two mode systems is given
based on the four dimensional real symplectic group $Sp(4,\Re)\/$. Within the
framework of the unitary metaplectic representation of this group, a
distinction between compact photon number conserving and noncompact photon
number nonconserving squeezing transformations is made. We exploit the
$Sp(4,\Re)-SO(3,2)\/$ local isomorphism and the $U(2)\/$ invariant squeezing
criterion to divide the set of all squeezing transformations into a two
parameter family of distinct equivalence classes with representative elements
chosen for each class. Familiar two mode squeezing transformations in the
literature are recognized in our framework and seen to form a set of measure
zero. Examples of squeezed coherent and thermal states are worked out. The need
to extend the heterodyne detection scheme to encompass all of $U(2)\/$ is
emphasized, and known experimental situations where all $U(2)\/$ elements can
be reproduced are briefly described.


title: Quantization Failure in Unified Field Theories

abstract:
  Studies of geometrical theories suggest that fundmental problems of
quantization arise from the disparate usage of displacement operators. These
may be the source of a concealed inconsistency in the accepted formalism of
quantum physics. General relativity and related theories cannot be quantized by
the classical procedure. It is necessary to avoid the construction of
differential equations by operators applied algebraically. For such theories,
Von Neumann's theorem concerning hidden variables is avoided. A specified
alternative class of gravitational-quantum-electrodynamic theories is possible.


title:
Critique of proposed limit to space--time measurement, based on Wigner's
  clocks and mirrors

abstract:
  Based on a relation between inertial time intervals and the Riemannian
curvature, we show that space--time uncertainty derived by Ng and van Dam
implies absurd uncertainties of the Riemannian curvature.


title: RESOLUTION OF A ``RETRODICTION PARADOX''

abstract:
  It is shown that the ``retrodiction paradox'' recently introduced by Peres
arises not because of the fallacy of the time-symmetric approach as he claimed,
but due to an inappropriate usage of retrodiction.


title: NONLOCALITY OF A SINGLE PHOTON REVISITED AGAIN

abstract:
  Comment on L. Hardy, Phys. Rev. Lett. {\bf 73}, 2279 (1994). It is argued
that the experiment proposed by Hardy should not be considered as a single
photon experiment.


title: Irreversibility in quantum field theory

abstract:
  It is shown how the programme of decoherence can be applied in the context of
quantum field theory. To illustrate the role of gauge invariance, we first
discuss the charge superselection rule in quantum electrodynamics in some
detail. We then present an example where macroscopic electromagnetic fields are
``measured" through interaction with charges and thereby rendered classical.


title:
Reply to the comment of Y. Aharonov and L. Vaidman on ``Time asymmetry
  in quantum mechanics: a retrodiction paradox''

abstract:
  In the standard physical interpretation of quantum theory, prediction and
retrodiction are not symmetric. The opposite assertion by some authors results
from their use of non-standard interpretations of the theory.


title: CAUSALITY, MEMORY ERASING AND DELAYED CHOICE EXPERIMENTS

abstract:
  Comment on [R.L. Ingraham, Phys. Rev. A 50, 4502 (1994)]. Ingraham suggested
``a delayed-choice experiment with partial, controllable memory erasing''. It
is shown that he cannot be right since his predictions contradict relativistic
causality. A subtle quantum effect which was overlooked by Ingraham is
explained.


title: Nonlinear Schroedinger equation and two-level atoms

abstract:
  General features of nonlinear quantum mechanics are discussed in the context
of applications to two-level atoms.


title: Triple bracket generalization of quantum mechanics

abstract:
  An extension of the Liouville-von Neumann dynamics to a Nambu-type dynamics
is proposed. The resulting theory is the first version of nonlinear QM which is
free from internal inconsistencies.


title:
Selective continuous quantum measurements: Restricted path integrals and
  wave equations

abstract:
  We discuss both the restricted path integral (RPI) and the wave equation (WE)
techniques in the theory of continuous quantum measurements. We intend to make
Mensky's fresh review complete by transforming his "effective" WE with complex
Hamiltonian into Ito-differential equations.


title: Dymanics of Generalized Coherent States

abstract:
  We show that generalized coherent states follow Schr\"{o}dinger dynamics in
time-dependent potentials. The normalized wave-packets follow a classical
evolution without spreading; in turn, the Schr\"{o}dinger potential depends on
the state through the classical trajectory. This feedback mechanism with
continuous dynamical re-adjustement allows the packets to remain coherent
indefinetely.


title: On a Time Symmetric Formulation of Quantum Mechanics

abstract:
  We explore further the suggestion to describe a pre- and post-selected system
by a two-state, which is determined by two conditions. Starting with a formal
definition of a two-state Hilbert space and basic operations, we systematically
recast the basics of quantum mechanics - dynamics, observables, and measurement
theory - in terms of two-states as the elementary quantities. We find a simple
and suggestive formulation, that ``unifies'' two complementary observables:
probabilistic observables and non-probabilistic `weak' observables.
Probabilities are relevant for measurements in the `strong coupling regime'.
They are given by the absolute square of a two-amplitude (a projection of a
two-state). Non-probabilistic observables are observed in sufficiently `weak'
measurements, and are given by linear combinations of the two-amplitude. As a
sub-class they include the `weak values' of hermitian operators. We show that
in the intermediate regime, one may observe a mixing of probabilities and weak
values. A consequence of the suggested formalism and measurement theory, is
that the problem of non-locality and Lorentz non-covariance, of the usual
prescription with a `reduction', may be eliminated. We exemplify this point for
the EPR experiment and for a system under successive observations.


title: Topology, Locality, and Aharonov-Bohm Effect with Neutrons

abstract:
  Recent neutron interferometry experiments have been interpreted as
demonstrating a new topological phenomenon similar in principle to the usual
Aharonov-Bohm (AB) effect, but with the neutron's magnetic moment replacing the
electron's charge. We show that the new phenomenon, called Scalar AB (SAB)
effect, follows from an ordinary local interaction, contrary to the usual AB
effect, and we argue that the SAB effect is not a topological effect by any
useful definition. We find that SAB actually measures an apparently novel spin
autocorrelation whose operator equations of motion contain the local torque in
the magnetic field. We note that the same remarks apply to the Aharonov-Casher
effect.


title:
Sub-femtosecond determination of transmission delay times for a
  dielectric mirror (photonic bandgap) as a function of angle of incidence

abstract:
  Using a two-photon interference technique, we measure the delay for
single-photon wavepackets to be transmitted through a multilayer dielectric
mirror, which functions as a ``photonic bandgap'' medium. By varying the angle
of incidence, we are able to confirm the behavior predicted by the group delay
(stationary phase approximation), including a variation of the delay time from
superluminal to subluminal as the band edge is tuned towards to the wavelength
of our photons. The agreement with theory is better than 0.5 femtoseconds (less
than one quarter of an optical period) except at large angles of incidence. The
source of the remaining discrepancy is not yet fully understood.


title: The Nine Lives of Schroedinger's Cat

abstract:
  This MSc dissertation surveys nine interpretations of non-relativistic
quantum mechanics. Extensive references are given. The interpretations covered
are: the orthodox interpretation, Bohr's interpretation, the idea that the mind
causes collapse, hidden variables, the many-worlds interpretation, the
many-minds interpretation, Bohm's interpretation and two interpretations based
on decoherent histories.


title: How much time does a tunneling particle spend in the barrier region?

abstract:
  The question in the title may be answered by considering the outcome of a
``weak measurement'' in the sense of Aharonov et al. Various properties of the
resulting time are discussed, including its close relation to the Larmor times.
It is a universal description of a broad class of measurement interactions, and
its physical implications are unambiguous.


title: Quantum Nonlocality in Two-Photon Experiments at Berkeley

abstract:
  We review some of our experiments performed over the past few years on
two-photon interference. These include a test of Bell's inequalities, a study
of the complementarity principle, an application of EPR correlations for
dispersion-free time-measurements, and an experiment to demonstrate the
superluminal nature of the tunneling process. The nonlocal character of the
quantum world is brought out clearly by these experiments. As we explain,
however, quantum nonlocality is not inconsistent with Einstein causality.