title:
Adiabatic Quantum Computation is Equivalent to Standard Quantum
  Computation

abstract:
  Adiabatic quantum computation has recently attracted attention in the physics
and computer science communities, but its computational power was unknown. We
describe an efficient adiabatic simulation of any given quantum algorithm,
which implies that the adiabatic computation model and the conventional quantum
computation model are polynomially equivalent. Our result can be extended to
the physically realistic setting of particles arranged on a two-dimensional
grid with nearest neighbor interactions. The equivalence between the models
provides a new vantage point from which to tackle the central issues in quantum
computation, namely designing new quantum algorithms and constructing fault
tolerant quantum computers. In particular, by translating the main open
questions in the area of quantum algorithms to the language of spectral gaps of
sparse matrices, the result makes these questions accessible to a wider
scientific audience, acquainted with mathematical physics, expander theory and
rapidly mixing Markov chains.


title: Vibrational coherent quantum computation

abstract:
  A long-lived coherent state and non-linear interaction have been
experimentally demonstrated for the vibrational mode of a trapped ion. We
propose an implementation of quantum computation using coherent states of the
vibrational modes of trapped ions. Differently from earlier experiments, we
consider a far-off resonance for the interaction between external fields and
the ion in a bidimensional trap. By appropriate choices of the detunings
between the external fields, the adiabatic elimination of the ionic excited
level from the Hamiltonian of the system allows for beam splitting between
orthogonal vibrational modes, production of coherent states and non-linear
interactions of various kinds. In particular, this model enables the generation
of the four coherent Bell states. Furthermore, all the necessary operations for
quantum computation such as preparation of qubits, one-qubit and controlled
two-qubit operations, are possible. The detection of the state of a vibrational
mode in a Bell state is made possible by the combination of resonant and
off-resonant interactions between the ion and some external fields. We show
that our read-out scheme provides highly efficient discrimination between all
the four Bell states. We extend this to a quantum register composed of many
individually trapped ions. In this case, operations on two remote qubits are
possible through a cavity mode. We emphasize that our remote-qubit operation
scheme does not require a high quality factor resonator: the cavity field acts
as a catalyst for the gate operation.


title: Bell's Inequality Violation (BIQV) with Non-Negative Wigner Function

abstract:
  A Bell inequality violation (BIQV) allowed by the two-mode squeezed state
(TMSS), whose Wigner function is nonnegative, is shown to hold only for
correlations among dynamical variables (DV) that cannot be interpreted via a
local hidden variable (LHV) theory. Explicit calculations and interpretation
are given for Bell's suggestion that the EPR (Einstein, Podolsky and Rosen)
state will not allow for BIQV in conjuction with its Wigner representative
state being nonnegative.
  It is argued that Bell's theorem disallowing the violation of Bell's
inequality within a local hidden-variable theory depends on the DV's having a
definite value --assigned by the LHV-- even when they cannot be simultaneously
measured. The analysis leads us to conclude that BIQV is to be associated with
endowing these definite values to the DV's and {\it not} with their locality
attributes.


title: No Signalling and Quantum Key Distribution

abstract:
  Standard quantum key distribution protocols are provably secure against
eavesdropping attacks, if quantum theory is correct. It is theoretically
interesting to know if we need to assume the validity of quantum theory to
prove the security of quantum key distribution, or whether its security can be
based on other physical principles. The question would also be of practical
interest if quantum mechanics were ever to fail in some regime, because a
scientifically and technologically advanced eavesdropper could perhaps use
post-quantum physics to extract information from quantum communications without
necessarily causing the quantum state disturbances on which existing security
proofs rely. Here we describe a key distribution scheme provably secure against
general attacks by a post-quantum eavesdropper who is limited only by the
impossibility of superluminal signalling. The security of the scheme stems from
violation of a Bell inequality.


title: High Temperature Macroscopic Entanglement

abstract:
  In this paper I intend to show that macroscopic entanglement is possible at
high temperatures. I analyze multipartite entanglement produced by the $\eta$
pairing mechanism which features strongly in the fermionic lattice models of
high $T_c$ superconductivity. This problem is shown to be equivalent to
calculating multipartite entanglement in totally symmetric states of qubits. I
demonstrate that we can conclusively calculate the relative entropy of
entanglement within any subset of qubits in an overall symmetric state. Three
main results then follow. First, I show that the condition for
superconductivity, namely the existence of the off diagonal long range order
(ODLRO), is not dependent on two-site entanglement, but on just classical
correlations as the sites become more and more distant. Secondly, the
entanglement that does survive in the thermodynamical limit is the entanglement
of the total lattice and, at half filling, it scales with the log of the number
of sites. It is this entanglement that will exist at temperatures below the
superconducting critical temperature, which can currently be as high as 160
Kelvin. Thirdly, I prove that a complete mixture of symmetric states does not
contain any entanglement in the macroscopic limit. On the other hand, the same
mixture of symmetric states possesses the same two qubit entanglement features
as the pure states involved, in the sense that the mixing does not destroy
entanglement for finite number of qubits, albeit it does decrease it. Maximal
mixing of symmetric states also does not destroy ODLRO and classical
correlations. I discuss various other inequalities between different
entanglements as well as generalizations to the subsystems of any
dimensionality (i.e. higher than spin half).


title: Some useful combinatorial formulae for bosonic operators

abstract:
  We give a general expression for the normally ordered form of a function
F(w(a,a*)) where w is a function of boson annihilation and creation operators
satisfying [a,a*]=1. The expectation value of this expression in a coherent
state becomes an exact generating function of Feynman-type graphs associated
with the zero-dimensional Quantum Field Theory defined by F(w). This enables
one to enumerate explicitly the graphs of given order in the realm of
combinatorially defined sequences. We give several examples of the use of this
technique, including the applications to Kerr-type and superfluidity-type
hamiltonians.


title: Multigrid solution of a path integral formulation for the hydrogen atom

abstract:
  An efficient multigrid Monte-Carlo algorithm for calculating the ground state
of the hydrogen atom using path integral is presented. The algorithm uses a
unigrid approach. The action integral near r=0 is modified so that the correct
values of observables are obtained. It is demonstrated that the critical slow
down (CSD) is eliminated. Finally, the algorithm is compared to the staging
algorithm.


title: Quantum Cryptography without Switching

abstract:
  We propose a new coherent state quantum key distribution protocol that
eliminates the need to randomly switch between measurement bases. This protocol
provides significantly higher secret key rates with increased bandwidths than
previous schemes that only make single quadrature measurements. It also offers
the further advantage of simplicity compared to all previous protocols which,
to date, have relied on switching.


title: Acoustic Casimir Pressure for Arbitrary Media

abstract:
  In this paper we derive a general expression for the acoustic Casimir
pressure between two parallel slabs made of arbitrary materials and whose
acoustic reflection coefficients are not equal. The formalism is based on the
calculation of the local density of modes using a Green's function approach.
  The results for the Casimir acoustic pressure are generalized to a
sphere/plate configuration using the proximity theorem


title:
Finding a maximally correlated state - Simultaneous Schmidt
  decomposition of bipartite pure states

abstract:
  We consider a bipartite mixed state of the form, $\rho =\sum_{\alpha, \beta
=1}^{l}a_{\alpha \beta} | \psi_{\alpha}> < \psi_ \beta}| $, where $|
\psi_{\alpha}>$ are normalized bipartite state vectors, and matrix $(a_{\alpha
\beta})$ is positive semidefinite. We provide a necessary and sufficient
condition for the state $\rho $ taking the form of maximally correlated states
by a local unitary transformation. More precisely, we give a criterion for
simultaneous Schmidt decomposability of $| \psi_{\alpha}>$ for $\alpha
=1,2,..., l$. Using this criterion, we can judge completely whether or not the
state $\rho $ is equivalent to the maximally correlated state, in which the
distillable entanglement is given by a simple formula. For generalized Bell
states, this criterion is written as a simple algebraic relation between
indices of the states. We also discuss the local distinguishability of the
generalized Bell states that are simultaneously Schmidt decomposable.


title: Entanglement and Tensor Product Decomposition for Two Fermions

abstract:
  The problem of the choice of tensor product decomposition in a system of two
fermions with the help of Bogoliubov transformations of creation and
annihilation operators is discussed. The set of physical states of the
composite system is restricted by the superselection rule forbidding the
superposition of fermions and bosons. It is shown that the Wootters concurrence
is not proper entanglement measure in this case. The explicit formula for the
entanglement of formation is found and its dependence on tensor product
decompositions of the Hilbert space is discussed. It is shown that the set of
separable states is narrower than in two-qubit case. Moreover, there exist
states which are separable with respect to all tensor product decompositions of
the Hilbert space.


title: Entanglement and Entropy in a Spin-Boson Quantum Phase Transition

abstract:
  We investigate the entanglement properties of an ensemble of atoms
interacting with a single bosonic field mode via the Dicke (superradiance)
Hamiltonian. The model exhibits a quantum phase transition and a
well-understood thermodynamic limit, allowing the identification of both
quantum and semi-classical many-body features in the behaviour of the
entanglement. We consider the entanglement between the atoms and the field, an
investigation initiated in [N. Lambert, C. Emary and T. Brandes, Phys. Rev.
Lett. {\bf 92}, %073602 (2004)]. In the thermodynamic limit, we give exact
results for all entanglement partitions and observe a logarithmic divergence of
the atom-field entanglement, and discontinuities in the average linear entropy.


title: Information-capacity description of spin-chain correlations

abstract:
  Information capacities achievable in the multi-parallel-use scenarios are
employed to characterize the quantum correlations in unmodulated spin chains.
By studying the qubit amplitude damping channel, we calculate the quantum
capacity $Q$, the entanglement assisted capacity $C_E$, and the classical
capacity $C_1$ of a spin chain with ferromagnetic Heisenberg interactions.


title:
Security Trade-offs in Ancilla-Free Quantum Bit Commitment in the
  Presence of Superselection Rules

abstract:
  Security trade-offs have been established for one-way bit commitment in
quant-ph/0106019. We study this trade-off in two superselection settings. We
show that for an `abelian' superselection rule (exemplified by particle
conservation) the standard trade-off between sealing and binding properties
still holds. For the non-abelian case (exemplified by angular momentum
conservation) the security trade-off can be more subtle, which we illustrate by
showing that if the bit-commitment is forced to be ancilla-free an
asymptotically secure quantum bit commitment is possible.


title: Erasure Thresholds for Efficient Linear Optics Quantum Computation

abstract:
  Using an error models motivated by the Knill, Laflamme, Milburn proposal for
efficient linear optics quantum computing [Nature 409,46--52, 2001], error rate
thresholds for erasure errors caused by imperfect photon detectors using a 7
qubit code are derived and verified through simulation. A novel method -- based
on a Markov chain description of the erasure correction procedure -- is
developed and used to calculate the recursion relation describing the error
rate at different encoding levels from which the threshold is derived, matching
threshold predictions by Knill, Laflamme and Milburn [quant-ph/0006120, 2000].
In particular, the erasure threshold for gate failure rate in the same order as
the measurement failure rate is found to be above 1.78%.


title: Interference of an array of independent Bose-Einstein condensates

abstract:
  We have observed high-contrast matter wave interference between 30
Bose-Einstein condensates with uncorrelated phases. Interference patterns were
observed after independent condensates were released from a one-dimensional
optical lattice and allowed to expand and overlap. This initially surprising
phenomenon is explained with a simple theoretical model which generalizes the
analysis of the interference of two independent condensates.


title:
Volumes and hyperareas of the spaces of separable and nonseparable
  qubit-qutrit systems: Initial numerical analyses

abstract:
  Paralleling our recent computationally-intensive work for the case N=4
(quant-ph/0308037), we undertake the task for N=6 of computing to high
numerical accuracy, the formulas of Sommers and Zyczkowski (quant-ph/0304041)
for the (N^2-1)-dimensional volume and (N^2-2)-dimensional hyperarea of the
(separable and nonseparable) N x N density matrices, based on the Bures
(minimal monotone) metric. At the same time, we estimate the unknown volumes
and hyperareas based on a number of other monotone metrics of interest.
Additionally, we estimate -- but with perhaps unavoidably diminished accuracy
-- all these volume and hyperarea quantities, when restricted to the ``small''
subset of 6 x 6 density matrices that are separable (classically correlated) in
nature. The ratios of separable to separable plus nonseparable volumes, then,
yield corresponding estimates of the ``probabilities of separability''. We are
particularly interested in exploring the possibility that a number of the
various 35-dimensional volumes and 34-dimensional hyperareas, possess exact
values -- which we had, in fact, conjectured to be the case for the qubit-qubit
systems (N=4), with the ``silver mean'', sqrt{2}-1, appearing to play a
fundamental role as regards the separable states.


title:
Simulating Hamiltonian dynamics using many-qudit Hamiltonians and local
  unitary control

abstract:
  When can a quantum system of finite dimension be used to simulate another
quantum system of finite dimension? What restricts the capacity of one system
to simulate another? In this paper we complete the program of studying what
simulations can be done with entangling many-qudit Hamiltonians and local
unitary control. By entangling we mean that every qudit is coupled to every
other qudit, at least indirectly. We demonstrate that the only class of
finite-dimensional entangling Hamiltonians that aren't universal for simulation
is the class of entangling Hamiltonians on qubits whose Pauli operator
expansion contains only terms coupling an odd number of systems, as identified
by Bremner et. al. [Phys. Rev. A, 69, 012313 (2004)]. We show that in all other
cases entangling many-qudit Hamiltonians are universal for simulation.


title: Complementarity of Entanglement and Interference

abstract:
  A complementarity relation is shown between the visibility of interference
and bipartite entanglement in a two qubit interferometric system when the
parameters of the quantum operation change for a given input state. The
entanglement measure is a decreasing function of the visibility of
interference. The implications for quantum computation are briefly discussed.


title: Two-mode entanglement in two-component Bose-Einstein condensates

abstract:
  We study the generation of two-mode entanglement in a two-component
Bose-Einstein condensate trapped in a double-well potential. By applying the
Holstein-Primakoff transformation, we show that the problem is exactly solvable
as long as the number of excitations due to atom-atom interactions remains low.
In particular, the condensate constitutes a symmetric Gaussian system, thereby
enabling its entanglement of formation to be measured directly by the
fluctuations in the quadratures of the two constituent components [Giedke {\it
et al.}, Phys. Rev. Lett. {\bf 91}, 107901 (2003)]. We discover that
significant two-mode squeezing occurs in the condensate if the interspecies
interaction is sufficiently strong, which leads to strong entanglement between
the two components.


title:
Direct measurement and reconstruction of nonclassical features of twin
  beams generated in spontaneous parametric down-conversion

abstract:
  Correlations in twin beams composed of many photon pairs are studied using an
intensified CCD camera. Joint signal-idler photon-number distribution and
quantum phase-space quasi-distributions determined from experimental data have
nonclassical features.


title: Nonlocality of cluster states of qubits

abstract:
  We investigate cluster states of qubits with respect to their non-local
properties. We demonstrate that a Greenberger-Horne-Zeilinger (GHZ) argument
holds for any cluster state: more precisely, it holds for any partial, thence
mixed, state of a small number of connected qubits (five, in the case of
one-dimensional lattices). In addition, we derive a new Bell inequality that is
maximally violated by the 4-qubit cluster state and is not violated by the
4-qubit GHZ state.


title: Spatial search and the Dirac equation

abstract:
  We consider the problem of searching a d-dimensional lattice of N sites for a
single marked location. We present a Hamiltonian that solves this problem in
time of order sqrt(N) for d>2 and of order sqrt(N) log(N) in the critical
dimension d=2. This improves upon the performance of our previous quantum walk
search algorithm (which has a critical dimension of d=4), and matches the
performance of a corresponding discrete-time quantum walk algorithm. The
improvement uses a lattice version of the Dirac Hamiltonian, and thus requires
the introduction of spin (or coin) degrees of freedom.


title:
Efficiencies for the single mode operation of a quantum optical
  nonlinear shift gate

abstract:
  We investigate the single mode operation of a quantum optical nonlinear \pi
phase shift gate implemented by a single two-level atom in one-dimensional free
space. Since the single mode property of the input photons at the atom is not
preserved in the interaction at the atom, we analyze the effeciency of single
mode operation that can still be achieved. We show how the input pulse shape
can be optimized to obtain high efficiencies for the nonlinear single mode
operation. With this analysis, we obtain an optimal single mode transmittance
per photon of 78% for the successful nonliner \pi phase shift operation.


title:
A semiclassical derivation of Einstein's rate equations for light-matter
  interaction

abstract:
  Einstein's rate equations are derived from the semiclassical Bloch equations
describing the interaction of a classical broadband light field with a
two--level system.


title: Reflection Symmetries for Multiqubit Density Operators

abstract:
  For multiqubit density operators in a suitable tensorial basis, we show that
a number of nonunitary operations used in the detection and synthesis of
entanglement are classifiable as reflection symmetries, i.e., orientation
changing rotations. While one-qubit reflections correspond to antiunitary
symmetries, as is known for example from the partial transposition criterion,
reflections on the joint density of two or more qubits are not accounted for by
the Wigner Theorem and are well-posed only for sufficiently mixed states. One
example of such nonlocal reflections is the unconditional NOT operation on a
multiparty density, i.e., an operation yelding another density and such that
the sum of the two is the identity operator. This nonphysical operation is
admissible only for sufficiently mixed states.


title:
Characterizing 3-qubit UPB states: violations of LHV models, preparation
  via nonlocal unitaries and PPT entangled nonlocal orbits

abstract:
  For the 3-qubit UPB state, i.e., the bound entangled state constructed from
an Unextendable Product Basis of Bennett et al. (Phys. Rev. Lett. 82:5385,
1999), we provide a set of violations of Local Hidden Variable (LHV) models
based on the particular type of reflection symmetry encoded in this state. The
explicit nonlocal unitary operation needed to prepare the state from its
reflected separable mixture of pure states is given, as well as a nonlocal
one-parameter orbit of states with Positive Partial Transpositions (PPT) which
swaps the entanglement between a state and its reflection twice during a
period.


title: Models for Equilibrium BEC Superradiance

abstract:
  Motivated by recent experiments with superradiant Bose-Einstein Condensate
(BEC) we consider simple microscopic models describing rigorously the
interference of the two cooperative phenomena, BEC and radiation, in
thermodynamic equilibrium. Our resuts in equilibrium confirm the presence of
the observed superradiant light scattering from BEC: (a) the equilibrium
superradiance exists only below a certain transition temperature; (b) there is
superradiance and matter-wave (BEC) enhancement due to the coherent coupling
between light and matter.


title: Why Should we Interpret Quantum Mechanics?

abstract:
  The development of quantum information theory has renewed interest in the
idea that the state vector does not represent the state of a quantum system,
but rather the knowledge or information that we may have on the system. I argue
that this epistemic view of states appears to solve foundational problems of
quantum mechanics only at the price of being essentially incomplete.


title: Quantum information and general relativity

abstract:
  The Einstein-Podolsky-Rosen paradox (1935) is reexamined in the light of
Shannon's information theory (1948). The EPR argument did not take into account
that the observers' information was localized, like any other physical object.
General relativity introduces new problems: there are horizons which act as
one-way membranes for the propagation of quantum information, in particular
black holes which act like sinks.


title: Quantum Random Walks do not need a Coin Toss

abstract:
  Classical randomized algorithms use a coin toss instruction to explore
different evolutionary branches of a problem. Quantum algorithms, on the other
hand, can explore multiple evolutionary branches by mere superposition of
states. Discrete quantum random walks, studied in the literature, have
nonetheless used both superposition and a quantum coin toss instruction. This
is not necessary, and a discrete quantum random walk without a quantum coin
toss instruction is defined and analyzed here. Our construction eliminates
quantum entanglement from the algorithm, and the results match those obtained
with a quantum coin toss instruction.


title: Inconsistencies of the Adiabatic Theorem and the Berry Phase

abstract:
  The adiabatic theorem states that if we prepare a quantum system in one of
the instantaneous eigenstates then the quantum number is an adiabatic invariant
and the state at a later time is equivalent to the instantaneous eigenstate at
that time apart from phase factors. Recently, Marzlin and Sanders have pointed
out that this could lead to apparent violation of unitarity. We resolve the
Marzlin-Sanders inconsistency within the quantum adiabatic theorem. Yet, our
resolution points to another inconsistency, namely, that the cyclic as well as
non-cyclic adiabatic Berry phases may vanish under strict adiabatic condition.
We resolve this inconsistency and develop an unitary operator decomposition
method to argue for the validity of the adiabatic approximation.


title: Entanglement of eta-pairing state with off-diagonal long-range order

abstract:
  Off-diagonal long-range order (ODLRO) is a quantum phenomenon not describable
in classical mechanical terms. It is believed to be one characteristic of
superconductivity. The quantum state constructed by eta-pairing which
demonstrates ODLRO is an eigenstate of the three-dimensional Hubbard model.
Entanglement is a key concept of the quantum information processing and has no
classical counterpart. We study the entanglement property of eta-pairing
quantum state. The concurrence is a well-known measure of quantum entanglement.
We show that the concurrence of entanglement between one-site and the rest
sites is exactly the correlation function of the ODLRO for the eta-pairing
state in the thermodynamic limit. So, when the eta-pairing state is entangled,
it demonstrates ODLRO and is thus in superconducting phase, if it is a
separable state, there is no ODLRO. In the thermodynamic limit, the
entanglement between M-site and other sites of the eta-pairing state does not
vanish. Other types of ODLRO of eta-pairing state are presented. We show that
the behavior of the ODLRO correlation functions is equivalent to that of the
entanglement of the eta-pairing state. The scaling of the entropy of the
entanglement for the eta-pairing state is studied.


title: On-demand single-photon state generation via nonlinear absorption

abstract:
  We propose a method for producing on-demand single-photon states based on
collision-induced exchanges of photons and unbalanced linear absorption between
two single-mode light fields. These two effects result in an effective
nonlinear absorption of photons in one of the modes, which can lead to single
photon states. A quantum nonlinear attenuator based on such a mechanism can
absorb photons in a normal input light pulse and terminate the absorption at a
single-photon state. Because the output light pulses containing single photons
preserve the properties of the input pulses, we expect this method to be a
means for building a highly controllable single photon source.


title:
Spatial weak-light solitons in an electro-magnetically induced nonlinear
  waveguide

abstract:
  We show that a weak probe light beam can form spatial solitons in an
electro-magnetically induced transparency (EIT) medium composed of four-level
atoms and a coupling light field. We find that the coupling light beam can
induce a highly controllable nonlinear waveguide and exert very strong effects
on the dynamical behavior of the solitons. Hence, in the EIT medium, it is not
only possible to produce spatial solitons at very low light intensities but
also simultaneously control these solitons by using the coupling-light-induced
nonlinear waveguide.


abstract:
  A simple entanglement measure for multipartite pure states is formulated
based on the partial entropy of a series of reduced density matrices. Use of
the proposed new measure to distinguish disentangled, partially entangled, and
maximally entangled multipartite pure states is illustrated.


title: Fault-tolerant quantum computation with cluster states

abstract:
  The one-way quantum computing model introduced by Raussendorf and Briegel
[Phys. Rev. Lett. 86 (22), 5188-5191 (2001)] shows that it is possible to
quantum compute using only a fixed entangled resource known as a cluster state,
and adaptive single-qubit measurements. This model is the basis for several
practical proposals for quantum computation, including a promising proposal for
optical quantum computation based on cluster states [M. A. Nielsen,
arXiv:quant-ph/0402005, accepted to appear in Phys. Rev. Lett.]. A significant
open question is whether such proposals are scalable in the presence of
physically realistic noise. In this paper we prove two threshold theorems which
show that scalable fault-tolerant quantum computation may be achieved in
implementations based on cluster states, provided the noise in the
implementations is below some constant threshold value. Our first threshold
theorem applies to a class of implementations in which entangling gates are
applied deterministically, but with a small amount of noise. We expect this
threshold to be applicable in a wide variety of physical systems. Our second
threshold theorem is specifically adapted to proposals such as the optical
cluster-state proposal, in which non-deterministic entangling gates are used. A
critical technical component of our proofs is two powerful theorems which
relate the properties of noisy unitary operations restricted to act on a
subspace of state space to extensions of those operations acting on the entire
state space.


title: On compatibility and improvement of different quantum state assignments

abstract:
  When Alice and Bob have different quantum knowledges or state assignments
(density operators) for one and the same specific individual system, then the
problems of compatibility and pooling arise. The so-called first
Brun-Finkelstein-Mermin (BFM) condition for compatibility is reobtained in
terms of possessed or sharp (i. e., probability one) properties. The second BFM
condition is shown to be generally invalid in an infinite-dimensional state
space. An argument leading to a procedure of improvement of one state
assifnment on account of the other and vice versa is presented.


title: Coherent manipulation of motional states of trapped ions

abstract:
  The beam splitter and phase shifter, which are the key elements in the
experiments of light interference, are realized in the motion of trapped ions.
Some applications, such as the creation of quantum motional states and the
realization of Mach-Zehnder interferometer, are illustrated. Several detection
methods of motional states used in the interferometer are also discussed.


title: Squeezing in the audio gravitational wave detection band

abstract:
  We demonstrate the generation of broad-band continuous-wave optical squeezing
down to 200Hz using a below threshold optical parametric oscillator (OPO). The
squeezed state phase was controlled using a noise locking technique. We show
that low frequency noise sources, such as seed noise, pump noise and detuning
fluctuations, present in optical parametric amplifiers have negligible effect
on squeezing produced by a below threshold OPO. This low frequency squeezing is
ideal for improving the sensitivity of audio frequency measuring devices such
as gravitational wave detectors.


title: Generation of photon-number entangled soliton pairs through interactions

abstract:
  Two new simple schemes for generating macroscopic (many-photon)
continuous-variable entangled states by means of continuous interactions
(rather than collisions) between solitons in optical fibers are proposed.
First, quantum fluctuations around two time-separated single-component temporal
solitons are considered. Almost perfect correlation between the photon-number
fluctuations can be achieved after propagating a certain distance, with a
suitable initial separation between the solitons. The photon-number correlation
can also be achieved in a pair of vectorial solitons with two polarization
components. In the latter case, the photon-number-entangled pulses can be
easily separated by a polarization beam splitter. These results offer novel
possibilities to produce entangled sources for quantum communication and
computation.


title: Photon statistics without counting photons

abstract:
  We show how to obtain the photon distribution of a single-mode field using
only avalanche photodetectors. The method is based on measuring the field at
different quantum efficiencies and then inferring the photon distribution by
maximum-likelihood estimation. The convergence of the method and its robustness
against fluctuations are illustrated by means of numerically simulated
experiments.


title: Lindblad and non--Lindblad type dynamics of a quantum Brownian particle

abstract:
  The dynamics of a typical open quantum system, namely a quantum Brownian
particle in a harmonic potential, is studied focussing on its non-Markovian
regime. Both an analytic approach and a stochastic wave function approach are
used to describe the exact time evolution of the system. The border between two
very different dynamical regimes, the Lindblad and non-Lindblad regimes, is
identified and the relevant physical variables governing the passage from one
regime to the other are singled out. The non-Markovian short time dynamics is
studied in detail by looking at the mean energy, the squeezing, the Mandel
parameter and the Wigner function of the system.


title: Comment on Photon Exchange Interactions

abstract:
  We previously suggested that photon exchange interactions could be used to
produce nonlinear effects at the two-photon level, and similar effects have
been experimentally observed by Resch et al. (quant-ph/0306198). Here we note
that photon exchange interactions are not useful for quantum information
processing because they require the presence of substantial photon loss. This
dependence on loss is somewhat analogous to the postselection required in the
linear optics approach to quantum computing suggested by Knill, Laflamme, and
Milburn [Nature 409, 46 (2001)].


title:
Entropy, entanglement, and area: analytical results for harmonic lattice
  systems

abstract:
  We revisit the question of the relation between entanglement, entropy, and
area for harmonic lattice Hamiltonians corresponding to discrete versions of
real free Klein-Gordon fields. For the ground state of the d-dimensional cubic
harmonic lattice we establish a strict relationship between the surface area of
a distinguished hypercube and the degree of entanglement between the hypercube
and the rest of the lattice analytically, without resorting to numerical means.
We outline extensions of these results to longer ranged interactions, finite
temperatures and for classical correlations in classical harmonic lattice
systems. These findings further suggest that the tools of quantum information
science may help in establishing results in quantum field theory that were
previously less accessible.


title: Spin Microscope Based on Optically Detected Magnetic Resonance

abstract:
  We propose a scanning magnetic microscope which has a photoluminescence
nanoprobe implanted in the tip of an AFM or STM, or NSOM, and exhibits
optically detected magnetic resonance (ODMR). The proposed spin microscope has
nanoscale lateral resolution and the single spin sensitivity for AFM and STM.


title:
Improving the sensitivity of FM spectroscopy using nano-mechanical
  cantilevers

abstract:
  It is suggested that nano-mechanical cantilevers can be employed as high-Q
filters to circumvent laser noise limitations on the sensitivity of frequency
modulation spectroscopy. In this approach a cantilever is actuated by the
radiation pressure of the amplitude modulated light that emerges from an
absorber. Numerical estimates indicate that laser intensity noise will not
prevent a cantilever from operating in the thermal noise limit, where the high
Q's of cantilevers are most advantageous.


title:
Computing the wavefunction from trajectories: particle and wave pictures
  in quantum mechanics and their relation

abstract:
  We discuss the particle method in quantum mechanics which provides an exact
scheme to calculate the time-dependent wavefunction from a single-valued
continuum of trajectories where two spacetime points are linked by at most a
single orbit. A natural language for the theory is offered by the hydrodynamic
analogy, in which wave mechanics corresponds to the Eulerian picture and the
particle theory to the Lagrangian picture. The Lagrangian model for the quantum
fluid may be developed from a variational principle. The Euler-Lagrange
equations imply a fourth-order nonlinear partial differential equation to
calculate the trajectories of the fluid particles as functions of their initial
coordinates using as input the initial wavefunction. The admissible solutions
are those consistent with quasi-potential flow. The effect of the superposition
principle is represented via a nonclassical force on each particle. The
wavefunction is computed via the standard map between the Lagrangian
coordinates and the Eulerian fields, which provides the analogue in this model
of Huygens principle in wave mechanics. The method is illustrated by
calculating the time-dependence of a free Gaussian wavefunction. The Eulerian
and Lagrangian pictures are complementary descriptions of a quantum process in
that they have associated Hamiltonian formulations that are connected by a
canonical transformation. The de Broglie-Bohm interpretation, which employs the
same set of trajectories, should not be conflated with the Lagrangian version
of the hydrodynamic interpretation. The theory implies that the mathematical
results of the de Broglie-Bohm model may be regarded as statements about
quantum mechanics itself rather than about its interpretation.


title: Fast quantum algorithm for numerical gradient estimation

abstract:
  Given a blackbox for f, a smooth real scalar function of d real variables,
one wants to estimate the gradient of f at a given point with n bits of
precision. On a classical computer this requires a minimum of d+1 blackbox
queries, whereas on a quantum computer it requires only one query regardless of
d. The number of bits of precision to which f must be evaluated matches the
classical requirement in the limit of large n.


title:
Qubit Decoherence and Non-Markovian Dynamics at Low Temperatures via an
  Effective Spin-Boson Model

abstract:
  Quantum Brownian oscillator model (QBM), in the Fock-space representation,
can be viewed as a multi-level spin-boson model. At sufficiently low
temperature, the oscillator degrees of freedom are dynamically reduced to the
lowest two levels and the system behaves effectively as a two-level (E2L)
spin-boson model (SBM) in this limit. We discuss the physical mechanism of
level reduction and analyze the behavior of E2L-SBM from the QBM solutions. The
availability of close solutions for the QBM enables us to study the
non-Markovian features of decoherence and leakage in a SBM in the
non-perturbative regime (e.g. without invoking the Born approximation) in
better details than before. Our result captures very well the characteristic
non-Markovian short time low temperature behavior common in many models.


title: Quantum squeezing of optical dissipative structures

abstract:
  We show that any optical dissipative structure supported by degenerate
optical parametric oscillators contains a special transverse mode that is free
from quantum fluctuations when measured in a balanced homodyne detection
experiment. The phenomenon is not critical as it is independent of the system
parameters and, in particular, of the existence of bifurcations. This result is
a consequence of the spatial symmetry breaking introduced by the dissipative
structure. Effects that could degrade the squeezing level are considered.


title: Bounds on localisable information via semidefinite programming

abstract:
  We investigate so-called localisable information of bipartite states and a
parallel notion of information deficit. Localisable information is defined as
the amount of information that can be concentrated by means of classical
communication and local operations where only maximally mixed states can be
added for free. The information deficit is defined as difference between total
information contents of the state and localisable information. We consider a
larger class of operations: the so called PPT operations, which in addition
preserve maximally mixed state (PPT-PMM operations). We formulate the related
optimization problem as sedmidefnite program with suitable constraints. We then
provide bound for fidelity of transition of a given state into product pure
state on Hilbert space of dimension d. This allows to obtain general upper
bound for localisable information (and also for information deficit). We
calculated the bounds exactly for Werner states and isotropic states in any
dimension. Surprisingly it turns out that related bounds for information
deficit are equal to relative entropy of entanglement (in the case of Werner
states - regularized one). We compare the upper bounds with lower bounds based
on simple protocol of localisation of information.


title: Analytic Representation of The Square-Root Operator

abstract:
  In this paper, we use the theory of fractional powers of linear operators to
construct a general (analytic) representation theory for the square-root energy
operator of relativistic quantum theory, which is valid for all values of the
spin. We focus on the spin 1/2 case, considering a few simple yet solvable and
physically interesting cases, in order to understand how to interpret the
operator. Our general representation is uniquely determined by the Green's
function for the corresponding Schrodinger equation. We find that, in general,
the operator has a representation as a nonlocal composite of (at least) three
singularities. In the standard interpretation, the particle component has two
negative parts and one (hard core) positive part, while the antiparticle
component has two positive parts and one (hard core) negative part. This effect
is confined within a Compton wavelength such that, at the point of singularity,
they cancel each other providing a finite result. Furthermore, the operator
looks like the identity outside a few Compton wavelengths (cutoff). To our
knowledge, this is the first example of a physically relevant operator with
these properties.


title: Analytic Representation of The Dirac Equation

abstract:
  In this paper we construct an analytical separation (diagonalization) of the
full (minimal coupling) Dirac equation into particle and antiparticle
components. The diagonalization is analytic in that it is achieved without
transforming the wave functions, as is done by the Foldy-Wouthuysen method, and
reveals the nonlocal time behavior of the particle-antiparticle relationship.
We interpret the zitterbewegung and the result that a velocity measurement (of
a Dirac particle) at any instant in time is, as reflections of the fact that
the Dirac equation makes a spatially extended particle appear as a point in the
present by forcing it to oscillate between the past and future at speed c. From
this we infer that, although the form of the Dirac equation serves to make
space and time appear on an equal footing mathematically, it is clear that they
are still not on an equal footing from a physical point of view. On the other
hand, the Foldy-Wouthuysen transformation, which connects the Dirac and square
root operator, is unitary. Reflection on these results suggests that a more
refined notion (than that of unitary equivalence) may be required for physical
systems.


title:
Rotating frames and gauge invariance in three-dimensional many-body
  quantum systems

abstract:
  We study the quantization of many-body systems in three dimensions in
rotating coordinate frames using a gauge invariant formulation of the dynamics.
We consider reference frames defined by linear gauge conditions, and discuss
their Gribov ambiguities and commutator algebra. We construct the momentum
operators, inner-product and Hamiltonian in those gauges, for systems with and
without translation invariance. The analogy with the quantization of
non-Abelian Yang-Mills theories in non-covariant gauges is emphasized. Our
results are applied to quasi-rigid systems in the Eckart frame.


title: Generalized Pseudopotentials for Higher Partial Wave Scattering

abstract:
  We derive a generalized zero-range pseudopotential applicable to all partial
wave solutions to the Schroedinger equation based on a delta-shell potential in
the limit that the shell radius approaches zero. This properly models all
higher order multipole moments not accounted for with a monopolar delta
function at the origin, as used in the familiar Fermi pseudopotential for
s-wave scattering. By making the strength of the potential energy dependent, we
derive self-consistent solutions for the entire energy spectrum of the
realistic potential. We apply this to study two particles in an isotropic
harmonic trap, interacting through a central potential, and derive analytic
expressions for the energy eigenstates and eigenvalues.


title: Conveyor belt clock synchronization

abstract:
  A protocol for synchronizing distant clocks is proposed that does not rely on
the arrival times of the signals which are exchanged, and an optical
implementation based on coherent-state pulses is described. This protocol is
not limited by any dispersion that may be present in the propagation medium
through which the light signals are exchanged. Possible improvements deriving
from the use of quantum-mechanical effects are also addressed.


title: Proposed experiment on the continuity of quantum entanglement

abstract:
  We propose experiments on quantum entanglement for investigating the Einstein
Podolsky Rosen (EPR) problem with the polarization directions of photons. These
experiments are performed to investigate whether the defined polarization
directions in an entangled state are teleported between entangled photons.
EPR-type sequential experiments are performed using a twin-photon beam and two
pairs of linear polarization analyzers under the cross-Nicol condition (i.e.,
orthogonal to each other). If the third filter whose polarization angle is 45
degrees is set between the first cross-Nicol filters, the beam intensity is
changed from 0 to 12.5 %, and at the second cross-Nicol filters, the beam
intensity is changed from 0 to 25 %. In this experiment, we predict that the
"continuity of quantum entanglement" under a pure Hamiltonian evolution is
detected.


title:
Weak Measurement of the Arrival Times of Single Photons and Pairs of
  Entangled Photons

abstract:
  In this paper we propose a setup for the weak measurement of photon arrival
time. It is found that the weak values of this arrival time can lie far away
from the expectation value, and in principle also in regions forbidden by
special relativity. We discuss in brief the implications of these results as
well as their reconciliation with the principle of causality. Furthermore, an
analysis of the weak arrival times of a pair of photons in a Bell state shows
that these weak arrival times are correlated.


title: Resource-efficient linear optical quantum computation

abstract:
  We introduce a scheme for linear optics quantum computation, that makes no
use of teleported gates, and requires stable interferometry over only the
coherence length of the photons. We achieve a much greater degree of efficiency
and a simpler implementation than previous proposals. We follow the "cluster
state" measurement based quantum computational approach, and show how cluster
states may be efficiently generated from pairs of maximally polarization
entangled photons using linear optical elements. We demonstrate the
universality and usefulness of generic parity measurements, as well as
introducing the use of redundant encoding of qubits to enable utilization of
destructive measurements - both features of use in a more general context.


title:
Comment on the paper "Semiclassical Dirac Theory of Tunnel Ionization"
  (by N.Milosevic, V.P.Krainov, and T.Brabec)

abstract:
  Some remarks are made on the articles by N.Milosevic, V.P.Krainov, T.Brabec
and our papers are about problem of semiclassical theory of tunnel ionization.


title: Quantum Logic and Non-Commutative Geometry

abstract:
  We propose a general scheme for the "logic" of elementary propositions of
physical systems, encompassing both classical and quantum cases, in the
framework given by Non Commutative Geometry. It involves Baire*-algebras, the
non-commutative version of measurable functions, arising as envelope of the
C*-algebras identifying the topology of the (non-commutative) phase space. We
outline some consequences of this proposal in different physical systems. This
approach in particular avoids some problematic features appearing in the
definition of the state of "initial conditions" in the standard W*-algebraic
approach to classical systems.


title: Comment on "Universal Thermal Radiation Drag on Neutral Objects"

abstract:
  In a recent letter [Phys. Rev. Lett. 91, 220801 (2003)], V. Mkrtchian and
co-workers calculated the radiation pressure force on a moving body, assuming
the electromagnetic field to be at temperature $T,$ and the velocity to be much
smaller than $c.$ They considered both dielectrics and conductors, and related
the effect to Casimir dissipative forces. Here we claim that their approach may
only apply in the Rayleigh-Ganz scattering regime, which corresponds to very
small particles and/or electromagnetically rarefied media.
  Moreover, we argue that their interpretation in terms of the Casimir effect
is misleading, since vacuum fluctuations do not contribute in the (implicitly
assumed) regime of uniform motion.


title: Probabilities from Entanglement, Born's Rule from Envariance

abstract:
  I show how probabilities arise in quantum physics by exploring implications
of {\it environment - assisted invariance} or {\it envariance}, a recently
discovered symmetry exhibited by entangled quantum systems. Envariance of
perfectly entangled ``Bell-like'' states can be used to rigorously justify
complete ignorance of the observer about the outcome of any measurement on
either of the members of the entangled pair. For more general states,
envariance leads to Born's rule, $p_k \propto |\psi_k|^2$ for the outcomes
associated with Schmidt states. Probabilities derived in this manner are an
objective reflection of the underlying state of the system -- they represent
experimentally verifiable symmetries, and not just a subjective ``state of
knowledge'' of the observer. Envariance - based approach is compared with and
found superior to pre-quantum definitions of probability including the {\it
standard definition} based on the `principle of indifference' due to Laplace,
and the {\it relative frequency approach} advocated by von Mises. Implications
of envariance for the interpretation of quantum theory go beyond the derivation
of Born's rule: Envariance is enough to establish dynamical independence of
preferred branches of the evolving state vector of the composite system, and,
thus, to arrive at the {\it environment - induced superselection (einselection)
of pointer states}, that was usually derived by an appeal to decoherence.
Envariant origin of Born's rule for probabilities sheds a new light on the
relation between ignorance (and hence, information) and the nature of quantum
states.


title: Global entanglement and quantum criticality in spin chains

abstract:
  Entanglement in quantum XY spin chains of arbitrary length is investigated
via a recently-developed global measure suitable for generic quantum many-body
systems. The entanglement surface is determined over the phase diagram, and
found to exhibit structure richer than expected. Near the critical line, the
entanglement is peaked (albeit analytically), consistent with the notion that
entanglement--the non-factorization of wave functions--reflects quantum
correlations. Singularity does, however, accompany the critical line, as
revealed by the divergence of the field-derivative of the entanglement along
the line. The form of this singularity is dictated by the universality class
controlling the quantum phase transition.


title:
Casimir-Polder interaction between an atom and a cavity wall under the
  influence of real conditions

abstract:
  The Casimir-Polder interaction between an atom and a metal wall is
investigated under the influence of real conditions including the dynamic
polarizability of the atom, finite conductivity of the wall metal and nonzero
temperature of the system. Both analytical and numerical results for the free
energy and force are obtained over a wide range of the atom-wall distances.
Numerical computations are performed for an Au wall and metastable
He${}^{\ast}$, Na and Cs atoms. For the He${}^{\ast}$ atom we demonstrate, as
an illustration, that at short separations of about the Au plasma wavelength at
room temperature the free energy deviates up to 35% and the force up to 57%
from the classical Casimir-Polder result. Accordingly, such large deviations
should be taken into account in precision experiments on atom-wall
interactions. The combined account of different corrections to the
Casimir-Polder interaction leads to the conclusion that at short separations
the corrections due to the dynamic polarizability of an atom play a more
important role than -- and suppress -- the corrections due to the nonideality
of the metal wall. By the comparison of the exact atomic polarizabilities with
those in the framework of the single oscillator model, it is shown that the
obtained asymptotic expressions enable calculation of the free energy and force
for the atom-wall interaction under real conditions with a precision of one
percent.


title: Generation and detection of bound entanglement

abstract:
  We propose a method for the experimental generation of two different families
of bound entangled states of three qubits. Our method is based on the explicit
construction of a quantum network that produces a purification of the desired
state. We also suggest a route for the experimental detection of bound
entanglement, by employing a witness operator plus a test of the positivity of
the partial transposes.


title: Detecting Genuine Multipartite Entanglement with Two Local Measurements

abstract:
  We present entanglement witness operators for detecting genuine multipartite
entanglement. These witnesses are robust against noise and require only two
local measurement settings when used in an experiment, independent from the
number of qubits. This allows detection of entanglement for an increasing
number of parties without a corresponding increase in effort. The witnesses
presented detect states close to GHZ, cluster and graph states. Connections to
Bell inequalities are also discussed.


title: On separable states for composite systems of distinguishable fermions

abstract:
  We study characterization of separable (classically correlated) states for
composite systems of distinguishable fermions that are represented as CAR
algebras.


title: Byzantine Agreement with Two Quantum Key Distribution Setups

abstract:
  It is pointed out that two separated quantum channels and three classical
authenticated channels are sufficient resources to achieve detectable
broadcast.


title: Rethinking Renormalization for Quantum Phase Transitions

abstract:
  This is a conceptual paper that re-examines the principles underlying the
application of renormalization theory to quantum phase transitions in the light
of quantum information theory. We start by describing the intuitive argument
known as the Kadanoff ``block-spin'' construction for spins fixed on a lattice
and then outline some subsequent ideas by Wilson and White. We then reconstruct
these concepts for quantum phase transitions from first principles. This new
perspective offers some very natural explanations for some features of
renormalization theory that had previously seemed rather mysterious, even
contrived. It also offers some suggestions as to how we might modify
renormalization methods to make them more successful. We then discuss some
possible order parameters and a class of functionals that are analogues of the
correlation length in such systems.


title: Qutrit state engineering with biphotons

abstract:
  The novel experimental realization of three-level optical quantum systems is
presented. We use the polarization state of biphotons to generate a specific
sequence of states that are used in the extended version of BB84 protocol. We
experimentally verify the orthogonality of the basis states and demonstrate the
ability to easily switch between them. The tomography procedure is employed to
reconstruct the density matrices of generated states.


title: Experimental Realization of an NMR Quantum Switch

abstract:
  In this paper, we report an experimental realization of quantum switch using
nuclear spins and magnetic resonant pulses. The nuclear spins of H and C in
carbon-13 labelled chloroform are used to carry the information, then nuclear
magnetic resonance pulses are applied to perform either bypass or cross
function to achieve the switching. Compared with a traditional space or time
domain switch, this switching architecture is much more scalable, therefore a
high throughput switching device can be built simply by increasing the number
of I/O ports. In addition, it can be used not only as a device to switch
classical information, but also a building block of quantum information
networks.


title:
Characteristics of the Limit Cycle of a Reciprocating Quantum Heat
  Engine

abstract:
  When a reciprocating heat engine is started it eventually settles to a stable
mode of operation. The approach of a first principle quantum heat engine toward
this stable limit cycle is studied. The engine is based on a working medium
consisting of an ensemble of quantum systems composed of two coupled spins. A
four stroke cycle of operation is studied, with two {\em isochore} branches
where heat is transferred from the hot/cold baths and two {\em adiabats} where
work is exchanged. The dynamics is generated by a completely positive map. It
has been shown that the performance of this model resembles an engine with
intrinsic friction. The quantum conditional entropy is employed to prove the
monotonic approach to a limit cycle. Other convex measures, such as the quantum
distance display the same monotonic approach. The equations of motion of the
engine are solved for the different branches and are combined to a global
propagator that relates the state of the engine in the beginning of the cycle
to the state after one period of operation of the cycle. The eigenvalues of the
propagator define the rate of relaxation toward the limit cycle. A longitudinal
and transverse mode of approach to the limit cycle is identified. The entropy
balance is used to explore the necessary conditions which lead to a stable
limit cycle. The phenomena of friction can be identified with a zero change in
the von Neumann entropy of the working medium.


title: Scattering in highly singular potentials

abstract:
  Recently, in Quantum Field theory, there has been an interest in scattering
in highly singular potentials. Here, solutions to the stationary Schroedinger
equation are presented when the potential is a multiple of an arbitrary
positive power of the Dirac delta distribution. The one dimensional, and the
spherically symmetric three dimensional cases are dealt with.


title: Macroscopic entanglement of many-magnon states

abstract:
  We study macroscopic entanglement of various pure states of a one-dimensional
N-spin system with N>>1. Here, a quantum state is said to be macroscopically
entangled if it is a superposition of macroscopically distinct states. To judge
whether such superposition is hidden in a general state, we use an essentially
unique index p: A pure state is macroscopically entangled if p=2, whereas it
may be entangled but not macroscopically if p<2. This index is directly related
to the stability of the state. We calculate the index p for various states in
which magnons are excited with various densities and wavenumbers. We find
macroscopically entangled states (p=2) as well as states with p=1. The former
states are unstable in the sense that they are unstable against some local
measurements. On the other hand, the latter states are stable in the senses
that they are stable against local measurements and that their decoherence
rates never exceed O(N) in any weak classical noises. For comparison, we also
calculate the von Neumann entropy S(N) of a subsystem composed of N/2 spins as
a measure of bipartite entanglement. We find that S(N) of some states with p=1
is of the same order of magnitude as the maximum value N/2. On the other hand,
S(N) of the macroscopically entangled states with p=2 is as small as O(log N)<<
N/2. Therefore, larger S(N) does not mean more instability. We also point out
that these results are analogous to those for interacting many bosons.
Furthermore, the origin of the huge entanglement, as measured either by p or
S(N), is discussed to be due to the spatial propagation of magnons.


title: Reversible quantum cellular automata

abstract:
  We define quantum cellular automata as infinite quantum lattice systems with
discrete time dynamics, such that the time step commutes with lattice
translations and has strictly finite propagation speed. In contrast to earlier
definitions this allows us to give an explicit characterization of all local
rules generating such automata. The same local rules also generate the global
time step for automata with periodic boundary conditions. Our main structure
theorem asserts that any quantum cellular automaton is structurally reversible,
i.e., that it can be obtained by applying two blockwise unitary operations in a
generalized Margolus partitioning scheme. This implies that, in contrast to the
classical case, the inverse of a nearest neighbor quantum cellular automaton is
again a nearest neighbor automaton.
  We present several construction methods for quantum cellular automata, based
on unitaries commuting with their translates, on the quantization of
(arbitrary) reversible classical cellular automata, on quantum circuits, and on
Clifford transformations with respect to a description of the single cells by
finite Weyl systems. Moreover, we indicate how quantum random walks can be
considered as special cases of cellular automata, namely by restricting a
quantum lattice gas automaton with local particle number conservation to the
single particle sector.


title: Entanglement Witness Derived from NMR Superdense Coding

abstract:
  We show that it is possible to transfer two-bit information via encoding a
single qubit in a conventional nuclear magnetic resonance (NMR) experiment with
two very weakly polarized nuclear spins. Nevertheless, the experiment can not
be regarded as a demonstration of superdense coding by means of NMR because it
is based on the large number of molecules being involved in the ensemble state
rather than the entanglement of the NMR states. Following the discussions, an
entanglement witness, particularly applicable for NMR, is introduced based on
separate and simultaneous measurement of the individual nuclear spin
magnetizations.


title: The Evolution Operator of The Three Atoms Tavis-Cummings Model

abstract:
  The explicit form of evolution operator of the three atoms Tavis-Cummings
Model is given.


title: Secure Direct Communication with a Quantum One-Time-Pad

abstract:
  Quantum secure direct communication is the direct communication of secret
messages without first producing a shared secret key. It maybe used in some
urgent circumstances. Here we propose a quantum secure direct communication
protocol using single photons. The protocol uses batches of single photons
prepared randomly in one of four different states. These single photons serve
as a one-time-pad which are used directly to encode the secret messages in one
communication process. We also show that it is unconditionally secure. The
protocol is feasible with present-day technique.


title: Topological Test Spaces

abstract:
  A test space is the set of outcome-sets associated with a collection of
experiments. This notion provides a simple mathematical framework for the study
of probabilistic theories -- notably, quantum mechanics -- in which one is
faced with incommensurable random quantities. In the case of quantum mechanics,
the relevant test space, the set of orthonormal bases of a Hilbert space,
carries significant topological structure. This paper inaugurates a general
study of topological test spaces. Among other things, we show that any
topological test space with a compact space of outcomes is of finite rank. We
also generalize results of Meyer and Clifton-Kent by showing that, under very
weak assumptions, any second-countable topological test space contains a dense
semi-classical test space.


title: Efficient Multi-Party Quantum Secret Sharing Schemes

abstract:
  In this work, we generalize the quantum secret sharing scheme of Hillary,
Bu\v{z}ek and Berthiaume[Phys. Rev. A59, 1829(1999)] into arbitrary
multi-parties. Explicit expressions for the shared secret bit is given. It is
shown that in the Hillery-Bu\v{z}ek-Berthiaume quantum secret sharing scheme
the secret information is shared in the parity of binary strings formed by the
measured outcomes of the participants. In addition, we have increased the
efficiency of the quantum secret sharing scheme by generalizing two techniques
from quantum key distribution. The favored-measuring-basis Quantum secret
sharing scheme is developed from the Lo-Chau-Ardehali technique[H. K. Lo, H. F.
Chau and M. Ardehali, quant-ph/0011056] where all the participants choose their
measuring-basis asymmetrically, and the measuring-basis-encrypted Quantum
secret sharing scheme is developed from the Hwang-Koh-Han technique [W. Y.
Hwang, I. G. Koh and Y. D. Han, Phys. Lett. A244, 489 (1998)] where all
participants choose their measuring-basis according to a control key. Both
schemes are asymptotically 100% in efficiency, hence nearly all the GHZ-states
in a quantum secret sharing process are used to generate shared secret
information.


title: Compact Orthoalgebras

abstract:
  We initiate a study of topological orthoalgebras (TOAs), concentrating on the
compact case. Examples of TOAs include topological orthomodular lattices, and
also the projection lattice of a Hilbert space. As the latter example
illustrates, a lattice-ordered TOA need not be a topological lattice. However,
we show that a compact Boolean TOA is a topological Boolean algebra. Using
this, we prove that any compact regular TOA is atomistic, and has a compact
center. We prove also that any compact TOA with isolated 0 is of finite height.
We then focus on stably ordered TOAs: those in which the upper-set generated by
an open set is open. These include both topological orthomodular lattices and
interval orthoalgebras -- in particular, projection lattices. We show that the
topology of a compact stably-ordered TOA with isolated 0 is determined by that
of of its space of atoms.


title: A perturbative approach for the dynamics of the quantum Zeno subspaces

abstract:
  In this paper we investigate the dynamics of the quantum Zeno subspaces which
are the eigenspaces of the interaction Hamiltonian, belonging to different
eigenvalues. Using the perturbation theory and the adiabatic approximation, we
get a general expression of the jump probability between different Zeno
subspaces. We applied this result in some examples. In these examples, as the
coupling constant of the interactions increases, the measurement keeps the
system remaining in its initial subspace and the quantum Zeno effect takes
place.


title:
Perfect random number generator is unnecessary for secure quantum key
  distribution

abstract:
  Game G: Clare passes a string s which is either from perfect random number
generator R0 or from good imperfect number generator R1, with equal
probability. Alice's information about whether it is from R0 or R1 is bounded
by small value h. Alice use s as the input random numbers for QKD protocol with
Bob. Suppose Eve may have very small information about the final key if s is
from R0 and Eve has large information if s is from R1, then after the protocol,
Alice announce the final key, Eve's information about whether s is from R0 or
R1 is unreasonablly large, i.e., breaks the known bound, h. Explicit formulas
are given in the article.


title: A new mathematical representation of Game Theory II

abstract:
  In another paper with the same name\cite{frame}, we proposed a new
representation of Game Theory, but most results are given by specific examples
and argument. In this paper, we try to prove the conclusions as far as we can,
including a proof of equivalence between the new representation and the
traditional Game Theory, and a proof of Classical Nash Theorem in the new
representation. And it also gives manipulation definition of quantum game and a
proof of the equivalence between this definition and the general abstract
representation. A Quantum Nash Proposition is proposed but without a general
proof. Then, some comparison between Nash Equilibrium (NE) and the
pseudo-dynamical equilibrium (PDE) is discussed. At last, we investigate the
possibility that whether such representation leads to truly Quantum Game, and
whether such a new representation is helpful to Classical Game, as an answer to
the questions in \cite{enk}. Some discussion on continuous-strategy games are
also included.


title: The uncertainty relation for joint measurement of position and momentum

abstract:
  We prove an uncertainty relation, which imposes a bound on any joint
measurement of position and momentum. It is of the form $(\Delta P)(\Delta
Q)\geq C\hbar$, where the `uncertainties' quantify the difference between the
marginals of the joint measurement and the corresponding ideal observable.
Applied to an approximate position measurement followed by a momentum
measurement, the uncertainties become the precision $\Delta Q$ of the position
measurement, and the perturbation $\Delta P$ of the conjugate variable
introduced by such a measurement.
  We also determine the best constant $C$, which is attained for a unique phase
space covariant measurement.


title: Distillation protocols: Output entanglement and local mutual information

abstract:
  A complementary behavior between local mutual information and average output
entanglement is derived for arbitrary bipartite ensembles. This leads to bounds
on the yield of entanglement in distillation protocols that involve
disinguishing. This bound is saturated in the hashing protocol for
distillation, for Bell-diagonal states.


title: Speeding up gate operations through dissipation

abstract:
  It is commonly believed that decoherence is the main obstacle to quantum
information processing. In contrast to this, we show how decoherence in the
form of dissipation can improve the performance of certain quantum gates. As an
example we consider the realisations of a controlled phase gate and a two-qubit
SWAP operation with the help of a single laser pulse in atom-cavity systems. In
the presence of spontaneous decay rates, the speed of the gates can be improved
by a factor 2 without sacrificing high fidelity and robustness against
parameter fluctuations. Even though this leads to finite gate failure rates,
the scheme is comparable with other quantum computing proposals.


title:
Bound entanglement maximally violating Bell inequalities: quantum
  entanglement is not equivalent to quantum security

abstract:
  It is shown that Smolin four-qubit bound entangled states [Phys. Rev. A, 63
032306 (2001)] can maximally violate two-setting Bell inequality similar to
standard CHSH inequality. Surprisingly this entanglement does not allow for
secure key distillation, so neither entanglement nor violation of Bell
inequalities implies quantum security. It is also pointed out how that kind of
bound entanglement can be useful in reducing communication complexity.


title: Short-time decoherence of Josephson charge qubits

abstract:
  In this paper we investigate the short-time decoherence of single Josephson
charge qubit (JCQ). The measure of decoherence is chosen as the maximum norm of
the deviation density operator. It is shown that when the temperature low
enough (for example T=30mK), within the elementary gate-operation time
tau{g}~12.7ps, the decoherence is smaller than 0.0001 at present setup of JCQ.
The Josephson charge qubit is suitable to take the blocks for quantum
computations according to the DiVincenzo low decoherence criterion.


title:
Classical versus quantum errors in quantum computation of dynamical
  systems

abstract:
  We analyze the stability of a quantum algorithm simulating the quantum
dynamics of a system with different regimes, ranging from global chaos to
integrability. We compare, in these different regimes, the behavior of the
fidelity of quantum motion when the system's parameters are perturbed or when
there are unitary errors in the quantum gates implementing the quantum
algorithm. While the first kind of errors has a classical limit, the second one
has no classical analogue. It is shown that, whereas in the first case
(``classical errors'') the decay of fidelity is very sensitive to the dynamical
regime, in the second case (``quantum errors'') it is almost independent of the
dynamical behavior of the simulated system. Therefore, the rich variety of
behaviors found in the study of the stability of quantum motion under
``classical'' perturbations has no correspondence in the fidelity of quantum
computation under its natural perturbations. In particular, in this latter case
it is not possible to recover the semiclassical regime in which the fidelity
decays with a rate given by the classical Lyapunov exponent.


title:
Using of small-scale quantum computers in cryptography with many-qubit
  entangled states

abstract:
  We propose a new cryptographic protocol. It is suggested to encode
information in ordinary binary form into many-qubit entangled states with the
help of a quantum computer. A state of qubits (realized, e.g., with photons) is
transmitted through a quantum channel to the addressee, who applies a quantum
computer tuned to realize the inverse unitary transformation decoding of the
message. Different ways of eavesdropping are considered, and an estimate of the
time needed for determining the secret unitary transformation is given. It is
shown that using even small quantum computers can serve as a basis for very
efficient cryptographic protocols. For a suggested cryptographic protocol, the
time scale on which communication can be considered secure is exponential in
the number of qubits in the entangled states and in the number of gates used to
construct the quantum network.

categories: quant-ph cond-mat.mes-hall cond-mat.supr-con physics.atom-ph physics.optics


title:
Generalized Quantum Turing Machine and its Application to the SAT Chaos
  Algorithm

abstract:
  Ohya and Volovich have proposed a new quantum computation model with chaotic
amplification to solve the SAT problem, which went beyond usual quantum
algorithm. In this paper, we generalize quantum Turing machine, and we show in
this general quantum Turing machine (GQTM) that we can treat the Ohya-Volovich
(OV) SAT algorithm.


title: How Can We Observe and Describe Chaos?

abstract:
  We propose a new approach to define chaos in dynamical systems from the point
of view of Information Dynamics. Observation of chaos in reality depends upon
how to observe it, for instance, how to take the scale in space and time.
Therefore it is natural to abandon taking several mathematical limiting
procedures. We take account of them, and chaos degree previously introduced is
redefined in this paper.


title:
A general scheme for the effective-mass Schrodinger equation and the
  generation of the associated potentials

abstract:
  A systematic procedure to study one-dimensional Schr\"odinger equation with a
position-dependent effective mass (PDEM) in the kinetic energy operator is
explored. The conventional free-particle problem reveals a new and interesting
situation in that, in the presence of a mass background, formation of bound
states is signalled. We also discuss coordinate-transformed, constant-mass
Schr\"odinger equation, its matching with the PDEM form and the consequent
decoupling of the ambiguity parameters. This provides a unified approach to
many exact results known in the literature, as well as to a lot of new ones.


title: Decoherence by a chaotic many-spin bath

abstract:
  We numerically investigate decoherence of a two-spin system (central system)
by a bath of many spins 1/2. By carefully adjusting parameters, the dynamical
regime of the bath has been varied from quantum chaos to regular, while all
other dynamical characteristics have been kept practically intact. We
explicitly demonstrate that for a many-body quantum bath, the onset of quantum
chaos leads to significantly faster and stronger decoherence compared to an
equivalent non-chaotic bath. Moreover, the non-diagonal elements of the
system's density matrix decay differently for chaotic and non-chaotic baths.
Therefore, knowledge of the basic parameters of the bath (strength of the
system-bath interaction, bath's spectral density of states) is not always
sufficient, and much finer details of the bath's dynamics can strongly affect
the decoherence process.


title:
Proposed experiment of which-way detection by longitudinal momentum
  transfer in Young's double slit experiment

abstract:
  The momentum of a photon may reveal the answer to the "which way" problem of
Young's double slit experiments. A photon passing through the boundary between
two media, in which a photon travels at different velocities, undergoes a
momentum change according to the law of conservation of momentum. The momentum
of the photon is transferred locally to the medium, and the boundary between
the media receives stress, which determines the photon trajectory. An
experiment is performed using a crystal plate that can transform the stress to
electric charge. We are able to detect the electric charge after the detection
of the photon on the screen, and control the sensitivity of photon detection.
By means of this proposed experiment it is determined whether or not an attempt
to detect the "which way" of photon travel destroys the interference patterns.


title: Q-circuit Tutorial

abstract:
  Q-circuit is a list of macros that greatly simplifies the construction of
quantum circuit diagrams (QCDs) in LaTeX with the help of the Xy-pic package.
This tutorial should help the reader acquire the skill to render arbitrary QCDs
in a matter of minutes. Q-circuit is available for free at
http://info.phys.unm.edu/Qcircuit/.


title: George Boole and the Bell inequalities

abstract:
  As shown by Pitowsky, the Bell inequalities are related to certain classes of
probabilistic inequalities dealt with by George Boole, back in the 1850s. Here
a short presentation of this relationship is given. Consequently, the Bell
inequalities can be obtained without any assumptions of physical nature, and
merely through mathematical argument.


title: Quantum Mechanics without amplitudes

abstract:
  Amplitudes are the major logical object in Quantum Theory. Despite this fact
they presents no physical reality and in consequence only observables can be
experimetally checked. We discuss the possibility of a theory of Quantum
Probabilities capable of give full account to quantum phenomena. Advanteges of
this formulation are the evidence of physical processes not described by the
orotodox formulation using amplitudes and the possibility of a full
algoritimization of Quantum Mechanics.


title:
Characterization of entanglement of more than two qubits with Bell
  inequalities and global entanglement

abstract:
  It is shown that the entanglement-structure of 3- and 4-qubit states can be
characterized by optimized operators of the Mermin-Klyshko type. It is possible
to discriminate between pure 2-qubit entanglements and higher entanglements. A
comparison with a global entanglement measure and the i-concurrence is made.


title: Could Energy Decoherence due to Quantum Gravity be observed?

abstract:
  It has recently been proposed that quantum gravity might lead to the
decoherence of superpositions in energy, corresponding to a discretization of
time at the Planck scale. At first sight the proposal seems amenable to
experimental verification with methods from quantum optics and atomic physics.
However, we argue that the predicted decoherence is unobservable in such
experiments if it acts globally on the whole experimental setup. This is
related to the unobservability of the global phase in interference. We also
show how local energy decoherence, which acts separately on system and phase
reference, could be detected with remarkable sensitivity and over a wide range
of length scales by long-distance Ramsey interferometry with metastable atomic
states. The sensitivity of the experiments can be further enhanced using
multi-atom entanglement.


title: An efficient quantum filter for multiphoton states

abstract:
  We propose a scheme for implementing a multipartite quantum filter that uses
entangled photons as a resource. It is shown that the success probability for
the 2-photon parity filter can be as high as 1/2, which is the highest that has
so far been predicted without the help of universal two-qubit quantum gates.
Furthermore, the required number of ancilla photons is the least of all current
parity filter proposals. Remarkably, the quantum filter operates with
probability 1/2 even in the N-photon case, irregardless of the number of
photons in the input state.


title: Classical and Quantum Ensembles via Multiresolution. I. BBGKY Hierarchy

abstract:
  A fast and efficient numerical-analytical approach is proposed for modeling
complex behaviour in the BBGKY hierarchy of kinetic equations. We construct the
multiscale representation for hierarchy of reduced distribution functions in
the variational approach and multiresolution decomposition in polynomial tensor
algebras of high-localized states. Numerical modeling shows the creation of
various internal structures from localized modes, which are related to
localized or chaotic type of behaviour and the corresponding patterns
(waveletons) formation. The localized pattern is a model for energy confinement
state (fusion) in plasma.

categories: quant-ph cond-mat.stat-mech math-ph math.MP nlin.PS physics.plasm-ph


title:
Classical and Quantum Ensembles via Multiresolution. II. Wigner
  Ensembles

abstract:
  We present the application of the variational-wavelet analysis to the
analysis of quantum ensembles in Wigner framework. (Naive) deformation
quantization, the multiresolution representations and the variational approach
are the key points. We construct the solutions of Wigner-like equations via the
multiscale expansions in the generalized coherent states or high-localized
nonlinear eigenmodes in the base of the compactly supported wavelets and the
wavelet packets. We demonstrate the appearance of (stable) localized patterns
(waveletons) and consider entanglement and decoherence as possible
applications.


title: Gaussian Dynamics is Classical Dynamics

abstract:
  A direct comparison of quantum and classical dynamical systems can be
accomplished through the use of distribution functions. This is useful for both
fundamental investigations such as the nature of the quantum-classical
transition as well as for applications such as quantum feedback control. By
affording a clear separation between kinematical and dynamical quantum effects,
the Wigner distribution is particularly valuable in this regard. Here we
discuss some consequences of the fact that when closed-system classical and
quantum dynamics are treated in Gaussian approximation, they are in fact
identical. Thus, it follows that several results in the so-called `semiquantum'
chaos actually arise from approximating the classical, and not the quantum
dynamics. (Similarly, opposing claims of quantum suppression of chaos are also
suspect.) As a simple byproduct of the analysis we are able to show how the
Lyapunov exponent appears in the language of phase space distributions in a way
that clearly underlines the difference between quantum and classical dynamical
situations. We also informally discuss some aspects of approximations that go
beyond the Gaussian approximation, such as the issue of when quantum nonlinear
dynamical corrections become important compared to nonlinear classical
corrections.


title: Casimir Force between a Small Dielectric Sphere and a Dielectric Wall

abstract:
  The possibility of repulsive Casimir forces between small metal spheres and a
dielectric half-space is discussed. We treat a model in which the spheres have
a dielectric function given by the Drude model, and the radius of the sphere is
small compared to the corresponding plasma wavelength. The half-space is also
described by the same model, but with a different plasma frequency. We find
that in the retarded limit, the force is quasi-oscillatory. This leads to the
prediction of stable equilibrium points at which the sphere could levitate in
the Earth's gravitational field. This seems to lead to the possibility of an
experimental test of the model. The effects of finite temperature on the force
are also studied, and found to be rather small at room temperature. However,
thermally activated transitions between equilibrium points could be significant
at room temperature.


title: Entanglement of electron spins in superconductors

abstract:
  We investigate entanglement of two electron spins forming Cooper pairs in an
s-wave superconductor. The two-electron space-spin density matrix is obtained
from the BCS ground state using a two-particle Green's function. It is
demonstrated that a two spin state is not given by a spin singlet state but by
a Werner state. It is found that the entanglement length, within which two
spins are entangled, is not the order of the coherence length but the order of
the Fermi wave length.


title: On Counterfactuals and Contextuality

abstract:
  Counterfactual reasoning and contextuality is defined and critically
evaluated with regard to its nonempirical content. To this end, a uniqueness
property of states, explosion views and link observables are introduced. If
only a single context associated with a particular maximum set of observables
can be operationalized, then a context translation principle resolves
measurements of different contexts.


title: Negativity of the Wigner function as an indicator of nonclassicality

abstract:
  A measure of nonclassicality of quantum states based on the volume of the
negative part of the Wigner function is proposed. We analyze this quantity for
Fock states, squeezed displaced Fock states and cat-like states defined as
coherent superposition of two Gaussian wave packets.


title: Quantal interferometry with dissipative internal motion

abstract:
  In presence of dissipation, quantal states may acquire complex-valued phase
effects. We suggest a notion of dissipative interferometry that accommodates
this complex-valued structure and that may serve as a tool for analyzing the
effect of certain kinds of external influences on quantal interference. The
concept of mixed-state phase and concomitant gauge invariance is extended to
dissipative internal motion. The resulting complex-valued mixed-state
interference effects lead to well-known results in the unitary limit and in the
case of dissipative motion of pure quantal states. Dissipative interferometry
is applied to fault-tolerant geometric quantum computation.


title:
The equivalence of two approaches to the design of entanglement
  distillation protocols

abstract:
  We show the equivalence of two approaches to the design of entanglement
distillation protocols. The first approach is based on local unitary operations
that yield permutations of tensor products of Bell states. The second approach
is based on stabilizer codes.


title: Geometric phase for an adiabatically evolving open quantum system

abstract:
  We derive an elegant solution for a two-level system evolving adiabatically
under the influence of a driving field with a time-dependent phase, which
includes open system effects such as dephasing and spontaneous emission. This
solution, which is obtained by working in the representation corresponding to
the eigenstates of the time-dependent Hermitian Hamiltonian, enables the
dynamic and geometric phases of the evolving density matrix to be separated and
relatively easily calculated.


title:
Scattering, trapped modes and guided waves in waveguides and diffraction
  gratings

abstract:
  We suggest the numerical approach to detect eigenfrequencies of trapped modes
in waveguides or guided waves in diffraction gratings. At the same time, the
approach works perfectly for computation of systems with finitely many
scattering channels. The most attractive example concerns the possibility of
control on electron transport in nano-dimensions system consisting of a
resonator and finitely many adjoined channels due to external variable electric
field.


title: A Single Atom Transistor in a 1D Optical Lattice

abstract:
  We propose a scheme utilising a quantum interference phenomenon to switch the
transport of atoms in a 1D optical lattice through a site containing an
impurity atom. The impurity represents a qubit which in one spin state is
transparent to the probe atoms, but in the other acts as a single atom mirror.
This allows a single-shot quantum non-demolition measurement of the qubit spin.


title: On extremal quantum states of composite systems with fixed marginals

abstract:
  We study the convex set of all bipartite quantum states with fixed marginal
states. The extremal states in this set have recently been characterized by
Parthasarathy [Ann. Henri Poincar\'e (to appear), quant-ph/0307182, [1]]. Here
we present an alternative necessary and sufficient condition for a state with
given marginals to be extremal. Our approach is based on a canonical duality
between bipartite states and a certain class of completely positive maps and
has the advantage that it is easier to check and to construct explicit examples
of extremal states. In dimension 2 x 2 we give a simple new proof for the fact
that all extremal states with maximally mixed marginals are precisely the
projectors onto maximally entangled wave functions. We also prove that in
higher dimension this does not hold and construct an explicit example of an
extremal state with maximally mixed marginals in dimension 3 x 3 that is not
maximally entangled. Generalizations of this result to higher dimensions are
also discussed.


title:
Field Theory reformulated without Self-energy Parts. The dressing
  Operator

abstract:
  The reformulation of field theory for avoiding self-energy parts in the
dynamical evolution has been applied successfully in the framework of the Lee
model, [M. de Haan. Ann. Phys., 311, 314-349 (2004)] enabling a kinetic
extension of the description. The basic ingredient is the recognition of these
self-energy parts. [M. de Haan and C. George. Trends in Statistical Physics 3
(2000), 115] The original reversible description is embedded in the new one and
appears now as a restricted class of initial conditions. [M. de Haan and C.
George. Prog. Theor. Phys.,109, 881-909 (2003)] This program is realized here
in the reduced formalism for a scalar field, interacting with a two-level atom,
beyond the usual rotating wave approximation. The kinetic evolution operator,
previously surmised, [M. de Haan. Physica, A171 (1991), 159] is here derived
from first principles, justifying the usual practice in optics where the common
use of the so-called pole approximation should no longer be viewed as an
approximation but as an alternative description in the appropriate formalism.
That model illustrates how some dressing of the atomic levels (and vertices),
through an appropriate operator, finds its place naturally into the new
formalism since the bare and dressed ground states do no longer coincide.
Moreover, finite velocity for field propagation is now possible in all cases,
without the presence of precursors for multiple detections.


title: Non-full rank bound entangled states satisfying the range criterion

abstract:
  A systematic method for generating bound entangled states in any bipartite
system, with ranks ranging from five to full rank, is presented. These states
are constructed by mixing separable states with UPB (Unextendible Product
Basis) - generated PPT bound entangled states. A subset of this class of PPT
bound entangled states, having less than full rank, is shown to satisfy the
range criterion [Phys. Lett. A, vol. 232 (1997) 333].


title:
Measurement of an integral of a classical field with a single quantum
  particle

abstract:
  A method for measuring an integral of a classical field via local interaction
of a single quantum particle in a superposition of 2^N states is presented. The
method is as efficient as a quantum method with N qubits passing through the
field one at a time and it is exponentially better than any known classical
method that uses N bits passing through the field one at a time. A related
method for searching a string with a quantum particle is proposed.


title: Improved ancilla preparation scheme increases fault-tolerant threshold

abstract:
  We demonstrate an improved concatenated encoded ancilla preparation
procedure. Simulations show that this procedure significantly increases the
error threshold beneath which arbitrarily long quantum computations are
possible.


title: Phase-space approach to Berry's phases

abstract:
  We propose a new formula for the adiabatic Berry phase which is based on
phase-space formulation of quantum mechanics. This approach sheds a new light
into the correspondence between classical and quantum adiabatic phases -- both
phases are related with the averaging procedure: Hannay's angle with averaging
over the classical torus and Berry's phase with averaging over the entire
classical phase space with respect to the corresponding Wigner function.
Generalizations to the non-abelian Wilczek--Zee case and mixed states are also
included.


title:
A relaxationless demonstration of the Quantum Zeno Paradox on an
  individual atom

abstract:
  The driven evolution of the spin of an individual atomic ion on the
ground-state hyperfine resonance is impeded by the observation of the ion in
one of the pertaining eigenstates. Detection of resonantly scattered light
identifies the ion in its upper ``bright'' state. The lower ``dark'' ion state
is free of relaxation and correlated with the detector by a null signal. Null
events represent the straightforward demonstration of the quantum Zeno paradox.
Also, high probability of survival was demonstrated when the ion, driven by a
fractionated $\pi $ pulse, was probed {\em and monitored} during the
intermissions of the drive, such that the ion's evolution is completely
documented.


title: Geometric Dequantization

abstract:
  Dequantization is a set of rules which turn quantum mechanics (QM) into
classical mechanics (CM). It is not the WKB limit of QM. In this paper we show
that, by extending time to a 3-dimensional "supertime", we can dequantize the
system in the sense of turning the Feynman path integral version of QM into the
functional counterpart of the Koopman-von Neumann operatorial approach to CM.
Somehow this procedure is the inverse of geometric quantization and we present
it in three different polarizations: the Schroedinger, the momentum and the
coherent states ones.


title: Coadjoint Orbits, Spin and Dequantization

abstract:
  In this Letter we propose two path integral approaches to describe the
classical mechanics of spinning particles. We show how these formulations can
be derived from the associated quantum ones via a sort of geometrical
dequantization procedure proposed in a previous paper.


title: Degaussification of twin-beam and nonlocality in the phase space

abstract:
  We show that inconclusive photon subtraction (IPS) on twin-beam produces
non-Gaussian states that violate Bell's inequality in the phase-space. The
violation is larger than for the twin-beam itself irrespective of the IPS
quantum efficiency. The explicit expression of IPS map is given both for the
density matrix and the Wigner function representations.


title: Sub-shot-noise heterodyne polarimetry

abstract:
  We report the experimental demonstration of a heterodyne polarization
rotation measurement with a noise floor 4.8 dB below the optical shot noise, by
use of the classically phase-locked quantum twin beams emitted above threshold
by an ultrastable type-II Na:KTP CW optical parametric oscillator. We believe
this is the largest noise reduction achieved to date on optical
phase-difference measurements.


title: Quantum measurements and finite geometry

abstract:
  A complete set of mutually unbiased bases for a Hilbert space of dimension N
is analogous in some respects to a certain finite geometric structure, namely,
an affine plane. Another kind of quantum measurement, known as a symmetric
informationally complete positive-operator-valued measure, is, remarkably, also
analogous to an affine plane, but with the roles of points and lines
interchanged. In this paper I present these analogies and ask whether they shed
any light on the existence or non-existence of such symmetric quantum
measurements for a general quantum system with a finite-dimensional state
space.


title:
Relativistic Aharonov--Bohm effect in the presence of two-dimensional
  Coulomb potential

abstract:
  We obtain exact solutions to the Dirac equation and the relevant binding
energies in the combined Aharonov--Bohm--Coulomb potential in 2+1 dimensions.
By means of solutions obtained the quantum Aharonov--Bohm effect is studied for
free and bound electron states. We show that the total scattering amplitude in
the combined Aharonov--Bohm--Coulomb potential is a sum of the Aharonov--Bohm
and the Coulomb scattering amplitudes. This modifies expression for the
standard Aharonov--Bohm cross section due to the interference these two
amplitudes with each other.


title:
Photon Statistics of a Non-Stationary Periodically Driven Single-Photon
  Source

abstract:
  We investigate the photon statistics of a single-photon source that operates
under non-stationary conditions. The photons are emitted by shining a periodic
sequence of laser pulses on single atoms falling randomly through a
high-finesse optical cavity. Strong antibunching is found in the intensity
correlation of the emitted light, demonstrating that a single atom emits
photons one-by-one. However, the number of atoms interacting with the cavity
follows a Poissonian statistics so that, on average, no sub-Poissonian photon
statistics is obtained, unless the measurement is conditioned on the presence
of single atoms.