title: General entanglement

abstract:
  The paper contains a brief review of an approach to quantum entanglement
based on analysis of dynamic symmetry of systems and quantum uncertainties,
accompanying the measurement of mean value of certain basic observables. The
latter are defined in terms of the orthogonal basis of Lie algebra,
corresponding to the dynamic symmetry group. We discuss the relativity of
entanglement with respect to the choice of basic observables and a way of
stabilization of robust entanglement in physical systems.


title:
Infinite qubit rings with maximal nearest neighbor entanglement: the
  Bethe ansatz solution

abstract:
  We search for translationally invariant states of qubits on a ring that
maximize the nearest neighbor entanglement. This problem was initially studied
by O'Connor and Wootters [Phys. Rev. A {\bf 63}, 052302 (2001)]. We first map
the problem to the search for the ground state of a spin 1/2 Heisenberg XXZ
model. Using the exact Bethe ansatz solution in the limit of an infinite ring,
we prove the correctness of the assumption of O'Connor and Wootters that the
state of maximal entanglement does not have any pair of neighboring spins
``down'' (or, alternatively spins ``up''). For sufficiently small fixed
magnetization, however, the assumption does not hold: we identify the region of
magnetizations for which the states that maximize the nearest neighbor
entanglement necessarily contain pairs of neighboring spins ``down''.


title: Pulsed squeezed light: simultaneous squeezing of multiple modes

abstract:
  We analyze the spectral properties of squeezed light produced by means of
pulsed, single-pass degenerate parametric down-conversion. The multimode output
of this process can be decomposed into characteristic modes undergoing
independent squeezing evolution akin to the Schmidt decomposition of the
biphoton spectrum. The main features of this decomposition can be understood
using a simple analytical model developed in the perturbative regime. In the
strong pumping regime, for which the perturbative approach is not valid, we
present a numerical analysis, specializing to the case of one-dimensional
propagation in a beta-barium borate waveguide. Characterization of the
squeezing modes provides us with an insight necessary for optimizing homodyne
detection of squeezing. For a weak parametric process, efficient squeezing is
found in a broad range of local oscillator modes, whereas the intense
generation regime places much more stringent conditions on the local
oscillator. We point out that without meeting these conditions, the detected
squeezing can actually diminish with the increasing pumping strength, and we
expose physical reasons behind this inefficiency.


title: Equivalence of two alternative approaches to Schrodinger equations

abstract:
  Recently developed simple approach for the exact/approximate solution of
Schrodinger equations with constant/position-dependent mass, in which the
potential is considered as in the perturbation theory, is shown to be
equivalent to the one leading to the construction of exactly solvable
potentials via the solution of second-order differential equations in terms of
known special functions. The formalism in the former solves difficulties
encountered in the latter in revealing the corrections explicitly to the
unperturbed piece of the solutions whereas the other obviate cumbersome
procedures used in the calculations of the former.


title: Efficient Simulation of Random Quantum States and Operators

abstract:
  We investigate the generation of quantum states and unitary operations that
are ``random'' in certain respects. We show how to use such states to estimate
the average fidelity, an important measure in the study of implementations of
quantum algorithms. We re-discover the result that the states of a maximal set
of mutually-unbiased bases serve this purpose. An efficient circuit is
presented that generates an arbitrary state out of such a set.
  Later on, we consider unitary operations that can be used to turn any quantum
channel into a depolarizing channel. It was known before that the Clifford
group serves this and a related purpose, and we show that these are actually
the same. We also show that a small subset of the Clifford group is already
sufficient to accomplish this. We conclude with an efficient construction of
the elements of that subset.
  This thesis is based on joint work with Richard Cleve, Joseph Emerson, and
Etera Livine.


title: Quantum communication cost of preparing multipartite entanglement

abstract:
  We study the preparation and distribution of high-fidelity multi-party
entangled states via noisy channels and operations. In the particular case of
GHZ and cluster states, we study different strategies using bipartite or
multipartite purification protocols. The most efficient strategy depends on the
target fidelity one wishes to achieve and on the quality of transmission
channel and local operations. We show the existence of a crossing point beyond
which the strategy making use of the purification of the state as a whole is
more efficient than a strategy in which pairs are purified before they are
connected to the final state. We also study the efficiency of intermediate
strategies, including sequences of purification and connection. We show that a
multipartite strategy is to be used if one wishes to achieve high fidelity,
whereas a bipartite strategy gives a better yield for low target fidelity.


title:
Steady state entanglement in open and noisy quantum systems at high
  temperature

abstract:
  We show that quantum mechanical entanglement can prevail even in noisy open
quantum systems at high temperature and far from thermodynamical equilibrium,
despite the deteriorating effect of decoherence. The system consists of a
number N of interacting quantum particles, and it can interact and exchange
particles with some environment. The effect of decoherence is counteracted by a
simple mechanism, where system particles are randomly reset to some standard
initial state, e.g. by replacing them with particles from the environment. We
present a master equation that describes this process, which we can solve
analytically for small N. If we vary the interaction strength and the reset
against decoherence rate, we find a threshold below which the equilibrium state
is classically correlated, and above which there is a parameter region with
genuine entanglement.


title: Quantum mechanics as a space-time theory

abstract:
  We show how quantum mechanics can be understood as a space-time theory
provided that its spatial continuum is modelled by a variable real number
(qrumber) continuum. Such a continuum can be constructed using only standard
Hilbert space entities. The geometry of atoms and subatomic objects differs
from that of classical objects. The systems that are non-local when measured in
the classical space-time continuum may be localized in the quantum continuum.
We compare this new description of space-time with the Bohmian picture of
quantum mechanics.


title:
Coherent generation of EPR-entangled light pulses mediated by a single
  trapped atom

abstract:
  We show that a single, trapped, laser-driven atom in a high-finesse optical
cavity allows for the quantum-coherent generation of entangled light pulses on
demand. Schemes for generating simultaneous and temporally separated pulse
pairs are proposed. The mechanical effect of the laser excitation on the
quantum motion of the cold trapped atom mediates the entangling interaction
between two cavity modes and between the two subsequent pulses, respectively.
The entanglement is of EPR-type, and its degree can be controlled through
external parameters. At the end of the generation process the atom is
decorrelated from the light field. Possible experimental implementations of the
proposals are discussed.


title: A toolbox for lattice spin models with polar molecules

abstract:
  There is growing interest to investigate states of matter with topological
order, which support excitations in the form of anyons, and which underly
topological quantum computing. Examples of such systems include lattice spin
models in two dimensions. Here we show that relevant Hamiltonians can be
systematically engineered with polar molecules stored in optical lattices,
where the spin is represented by a single electron outside a closed shell of a
heteronuclear molecule in its rotational ground state. Combining microwave
excitation with the dipole-dipole interactions and spin-rotation couplings
allows us to build a complete toolbox for effective two-spin interactions with
designable range and spatial anisotropy, and with coupling strengths
significantly larger than relevant decoherence rates. As an illustration we
discuss two models: a 2D square lattice with an energy gap providing for
protected quantum memory, and another on stacked triangular lattices leading to
topological quantum computing.


title: The time-energy certainty relation

abstract:
  All energy measurements of a quantum system are prone to inaccuracies. In
particular, if such measurements are carried over a finite period of time the
accuracy of the result is affected by the length of that period. Here I show an
upper bound on such inaccuracies and point out that the bound can be
arbitrarily small if many copies of the system are available.


title: Are quantum correlations symmetric ?

abstract:
  To our knowledge, all known bipartite entanglement measures are symmetric
under exchange of subsystems. We ask if an entanglement measure that is not
symmetric can exist. A related question is if there is a state that cannot be
swapped by means of LOCC. We show, that in general one cannot swap states by
LOCC. This allows to construct nonsymmetric measure of entanglement, and a
parameter that reports asymmetry of entanglement contents of quantum state. We
propose asymptotic measure of asymmetry of entanglement, and show that states
for which it is nonzero, contain necessarily bound entanglement.


title: Husimi distribution function and one-dimensional Ising model

abstract:
  Husimi distribution function for the one-dimensional Ising model is obtained.
One-point and joint distribution functions are calculated and their thermal
behaviour are discussed.


title:
Efficient quantum memory and entanglement between light and an atomic
  ensemble using magnetic fields

abstract:
  We present two protocols, one for the storage of light in an atomic ensemble
and the subsequent retrieval, and another one for the generation of
entanglement between light and atoms. They rely on two passes of a single pulse
through the ensemble, Larmor precessing in an external field. Both protocols
work deterministically and the relevant figures of merit - such as the fidelity
or the EPR variance - scale exponentially in the coupling strength. We solve
the corresponding Maxwell-Bloch equations describing the scattering process and
determine the resulting input-output relations which only involve one relevant
light mode that, in turn, can be easily accessed experimentally.


title:
All possible bipartite positive-operator-value measurements of
  two-photon polarization states

abstract:
  Here we propose an implementation of all possible Positive Operator Value
Measures (POVMs) of two-photon polarization states. POVMs are the most general
class of quantum measurements. Our setup requires linear optics, Bell State
measurements and an entangled three-photon ancilla state, which can be prepared
separately and in advance (or 'off-line'). As an example we give the detailed
settings for a simultaneous measurement of all four Bell States for an
arbitrary two-photon polarization state, which is impossible with linear optics
alone.


title:
Representations of U(2\infty) and the Value of the Fine Structure
  Constant

abstract:
  A relativistic quantum mechanics is formulated in which all of the
interactions are in the four-momentum operator and Lorentz transformations are
kinematic. Interactions are introduced through vertices, which are bilinear in
fermion and antifermion creation and annihilation operators, and linear in
boson creation and annihilation operators. The fermion-antifermion operators
generate a unitary Lie algebra, whose representations are fixed by a first
order Casimir operator (corresponding to baryon number or charge). Eigenvectors
and eigenvalues of the four-momentum operator are analyzed and exact solutions
in the strong coupling limit are sketched. A simple model shows how the fine
structure constant might be determined for the QED vertex.


title:
Speedup of quantum state transfer by three- qubit interactions:
  Implementation by nuclear magnetic resonance

abstract:
  Universal quantum information processing requires single-qubit rotations and
two-qubit interactions as minimal resources. A possible step beyond this
minimal scheme is the use of three-qubit interactions. We consider such
three-qubit interactions and show how they can reduce the time required for a
quantum state transfer in an XY spin chain. For the experimental
implementation, we use liquid-state nuclear magnetic resonance (NMR), where
three-qubit interactions can be implemented by sequences of radio-frequency
pulses.


title: Implementation of quantum algorithms with resonant interactions

abstract:
  We propose a scheme for implementing quantum algorithms with resonant
interactions. Our scheme only requires resonant interactions between two atoms
and a cavity mode, which is simple and feasible. Moreover, the implementation
would be an important step towards the fabrication of quantum computers in
cavity QED system.


title: Proof of Security of a High-Capacity Quantum Key Distribution Protocol

abstract:
  We prove the security of a high-capacity quantum key distribution protocol
over noisy channels. By using entanglement purification protocol, we construct
a modified version of the protocol in which we separate it into two consecutive
stages. We prove their securities respectively and hence the security of the
whole protocol.


title: Giant enhancement of quantum decoherence by frustrated environments

abstract:
  This Letter studies the decoherence in a system of two antiferromagnetically
coupled spins that interact with a spin bath environment. Systems are
considered that range from the rotationally invariant to highly anisotropic
spin models, have different topologies and values of parameters that are fixed
or are allowed to fluctuate randomly. We explore the conditions under which the
two-spin system clearly shows an evolution from the initial spin-up - spin-down
state towards the maximally entangled singlet state. We demonstrate that
frustration and, especially, glassiness of the spin environment strongly
enhances the decoherence of the two-spin system.


title: Efficient data processing and quantum phenomena: Single-particle systems

abstract:
  We study the relation between the acquisition and analysis of data and
quantum theory using a probabilistic and deterministic model for photon
polarizers. We introduce criteria for efficient processing of data and then use
these criteria to demonstrate that efficient processing of the data contained
in single events is equivalent to the observation that Malus' law holds. A
strictly deterministic process that also yields Malus' law is analyzed in
detail. We present a performance analysis of the probabilistic and
deterministic model of the photon polarizer. The latter is an adaptive
dynamical system that has primitive learning capabilities. This additional
feature has recently been shown to be sufficient to perform event-by-event
simulations of interference phenomena, without using concepts of wave
mechanics. We illustrate this by presenting results for a system of two chained
Mach-Zehnder interferometers, suggesting that systems that perform efficient
data processing and have learning capability are able to exhibit behavior that
is usually attributed to quantum systems only.


title:
Spatially Separated and Correlated Atom-molecule Lasers from a Bose
  Condensate

abstract:
  We propose a feasible scheme to create two spatially separated atomic and
molecular beams from an atomic Bose-Einstein condensate by combining the
Raman-type atom laser output and the two-color photo-association processes. We
examine the quantum dynamics and statistical properties of the system under
short-time limits, especially the quadrature-squeezed and mode-correlated
behaviors of two output beams for different initial state of the condensate.
The possibility to generate the entangled atom-molecule lasers by an optical
technique was also discussed.


title:
Spatiotemporally-localized-stationary typical wave function satisfying
  Klein-Gordon equation with emergent mass

abstract:
  Starting from relativistic mass-less Madelung fluid, we shall develop a class
of typical wave functions by imposing it to maximize Shannon entropy given its
finite average quantum potential. We show that there is a class of solutions in
which the wave function is spatiotemporally localized with finite spacetime
support, uniformly moving hence stationary. It turns out that the quantum
amplitude satisfies Klein-Gordon equation with emergent mass term proportional
to the square root of average quantum potential. We show that there is physical
time uncertainty which decreases as the mass increases. We also rederive the
classical energy-momentum relation provided the de Broglie-Einstein relation
holds. In this case, the time uncertainty is proportional to the inverse of
classical energy.


title: Nonclassicality of noisy quantum states

abstract:
  Nonclassicality conditions for an oscillator-like system interacting with a
hot thermal bath are considered. Nonclassical properties of quantum states can
be conserved up to a certain temperature threshold only. In this case,
affection of the thermal noise can be compensated via transformation of an
observable, which tests the nonclassicality (witness function). Possibilities
for experimental implementations based on unbalanced homodyning are discussed.
At the same time, we demonstrate that the scheme based on balanced homodyning
cannot be improved for noisy states with proposed technique and should be
applied directly.


title:
Quadrature entanglement and photon-number correlations accompanied by
  phase-locking

abstract:
  We investigate quantum properties of phase-locked light beams generated in a
nondegenerate optical parametric oscillator (NOPO) with an intracavity
waveplate. This investigation continuous our previous analysis presented in
Phys.Rev.A 69, 05814 (2004), and involves problems of continuous-variable
quadrature entanglement in the spectral domain, photon-number correlations as
well as the signatures of phase-locking in the Wigner function. We study the
role of phase-localizing processes on the quantum correlation effects. The
peculiarities of phase-locked NOPO in the self-pulsing instability operational
regime are also cleared up. The results are obtained in both the
P-representation as a quantum-mechanical calculation in the framework of
stochastic equations of motion, and also by using numerical simulation based on
the method of quantum state diffusion.


title: Efficient Preparation of Quantum States With Exponential Precision

abstract:
  It has been shown that, starting from the state |0>, in the general case, an
arbitrary quantum state |\psi> cannot be prepared with exponential precision in
polynomial time. However, we show that for the important special case when
|\psi> represents discrete values of some real, continuous function \psi(x),
efficient preparation is possible by applying the eigenvalue estimation
algorithm to a Hamiltonian which has \psi(x) as an eigenstate. We construct the
required Hamiltonian explicitly and present an iterative algorithm for removing
unwanted superpositions from the output state in order to reach |\psi> within
exponential accuracy. The method works under very general conditions and can be
used to provide the quantum simulation algorithm with very accurate and general
starting states.


title: Irregular Dynamics in a Solvable One-Dimensional Quantum Graph

abstract:
  We show that the quantum single particle motion on a one-dimensional line
with Fulop-Tsutsui point interactions exhibits characteristics usually
associated with nonintegrable systems both in bound state level statistics and
scattering amplitudes. We argue that this is a reflection of the underlying
stochastic dynamics which persists in classical domain.

categories: quant-ph cond-mat.dis-nn hep-ph math-ph math.MP nlin.CD physics.atom-ph


title: Shelving according to the NRules

abstract:
  When a weak photon decay competes with a strong photon decay in a 3-level
atom, the weak occasionally prevails over the strong photon. This is the
shelving phenomenon. It is assumed in this paper to occur objectively and
autonomously, independent of external observation or lack of observation.
  We here consider an auxiliary rule-set called the nRules that triggers a
non-unitary mechanism that applies to all macroscopic measurements, as well as
to many microscopic processes including shelving. These rules are shown below
to describe the shelving process in complete detail without resorting to the
notion of null measurement, or to an external measurement of any kind. They
describe the V and the Lambda shelving configurations, as well as the two
cascade configurations.


title: The Quantum Query Complexity of Elliptic PDE

abstract:
  The complexity of the following numerical problem is studied in the quantum
model of computation: Consider a general elliptic partial differential equation
of order 2m in a smooth, bounded domain Q\subset \R^d with smooth coefficients
and homogeneous boundary conditions. We seek to approximate the solution on a
smooth submanifold M\subseteq Q of dimension 0\le d_1 \le d. With the right
hand side belonging to C^r(Q), and the error being measured in the L_\infty(M)
norm, we prove that the n-th minimal quantum error is (up to logarithmic
factors) of order n^{-min((r+2m)/d_1,r/d+1)}. For comparison, in the classical
deterministic setting the n-th minimal error is known to be of order n^{-r/d},
for all d_1, while in the classical randomized setting it is (up to logarithmic
factors) n^{-min((r+2m)/d_1,r/d+1/2)}.


title: Resonance expansions in quantum mechanics

abstract:
  The goal of this contribution is to discuss various resonance expansions that
have been proposed in the literature.


title: Spatial dispersion in Casimir forces: A brief review

abstract:
  We present the basic principles of non-local optics in connection with the
calculation of the Casimir force between half-spaces and thin films.
  At currently accessible distances $L$, non-local corrections amount to about
half a percent, but they increase roughly as 1/L at smaller separations. Self
consistent models lead to corrections with the opposite sign as models with
abrupt surfaces.


title:
The Effect of Local Structure and Non-uniformity on Decoherence-Free
  States of Charge Qubits

abstract:
  We analyze robustness of decoherence-free (DF) subspace in charge qubits when
there are a local structure and non-uniformity that violate collective
decoherence measurement condition. We solve master equations of up to four
charge qubits and a detector as two serially coupled quantum point contacts
(QPC) with an island structure. We show that robustness of DF states is
strongly affected by local structure as well as by non-uniformities of qubits.


title: Threshold bounds for noisy bipartite states

abstract:
  For a nonseparable bipartite quantum state violating the
Clauser-Horne-Shimony-Holt (CHSH) inequality, we evaluate amounts of noise
breaking the quantum character of its statistical correlations under any
generalized quantum measurements of Alice and Bob. Expressed in terms of the
reduced states, these new threshold bounds can be easily calculated for any
concrete bipartite state. A noisy bipartite state, satisfying the extended CHSH
inequality and the perfect correlation form of the original Bell inequality for
any quantum observables, neither necessarily admits a local hidden variable
model nor exhibits the perfect correlation of outcomes whenever the same
quantum observable is measured on both "sides".


title:
Equivalence of Tripartite Quantum States under Local Unitary
  Transformations

abstract:
  The equivalence of tripartite pure states under local unitary transformations
is investigated. The nonlocal properties for a class of tripartite quantum
states in $\Cb^K \otimes \Cb^M \otimes \Cb^N$ composite systems are
investigated and a complete set of invariants under local unitary
transformations for these states is presented. It is shown that two of these
states are locally equivalent if and only if all these invariants have the same
values.


title: Quantum state merging and negative information

abstract:
  We consider a quantum state shared between many distant locations, and define
a quantum information processing primitive, state merging, that optimally
merges the state into one location. As announced in [Horodecki, Oppenheim,
Winter, Nature 436, 673 (2005)], the optimal entanglement cost of this task is
the conditional entropy if classical communication is free. Since this quantity
can be negative, and the state merging rate measures partial quantum
information, we find that quantum information can be negative. The classical
communication rate also has a minimum rate: a certain quantum mutual
information. State merging enabled one to solve a number of open problems:
distributed quantum data compression, quantum coding with side information at
the decoder and sender, multi-party entanglement of assistance, and the
capacity of the quantum multiple access channel. It also provides an
operational proof of strong subadditivity. Here, we give precise definitions
and prove these results rigorously.


title: Can quantum computing solve classically unsolvable problems?

abstract:
  T. D. Kieu has claimed that a quantum computing procedure can solve a
classically unsolvable problem. Recent work of W. D. Smith has shown that
Kieu's central mathematical claim cannot be sustained. Here, a more general
critique is given of Kieu's proposal and some suggestions are made regarding
the Church-Turing thesis.


title: Ground state overlap and quantum phase transitions

abstract:
  We present a characterization of quantum phase transitions in terms of the
the overlap function between two ground states obtained for two different
values of external parameters. On the examples of the Dicke and XY models, we
show that the regions of criticality of a system are marked by the extremal
points of the overlap and functions closely related to it. Further, we discuss
the connections between this approach and the Anderson orthogonality
catastrophe as well as with the dynamical study of the Loschmidt echo for
critical systems.


title: Temperature effects on mixed state geometric phase

abstract:
  Geometric phase of an open quantum system that is interacting with a thermal
environment (bath) is studied through some simple examples. The system is
considered to be a simple spin-half particle which is weakly coupled to the
bath. It is seen that even in this regime the geometric phase can vary with
temperature. In addition, we also consider the system under an adiabatically
time-varying magnetic field which is weakly coupled to the bath. An important
feature of this model is that it reveals existence of a temperature-scale in
which adiabaticity condition is preserved and beyond which the geometric phase
is varying quite rapidly with temperature. This temperature is exactly the one
in which the geometric phase vanishes. This analysis has some implications in
realistic implementations of geometric quantum computation.


title: Entropy: An inherent, nonstatistical property of any system in any state

abstract:
  Entropy is the distinguishing and most important concept of our efforts to
understand and regularize our observations of a very large class of natural
phenomena, and yet, it is one of the most contentious concepts of physics. In
this article, we review two expositions of thermodynamics, one without
reference to quantum theory, and the other quantum mechanical without
probabilities of statistical mechanics. In the first, we show that entropy is
an inherent property of any system in any state, and that its analytical
expression must conform to eight criteria. In the second, we recognize that
quantum thermodynamics: (i) admits quantum probabilities described either by
wave functions or by nonstatistical density operators; and (ii) requires a
nonlinear equation of motion that is delimited by but more general than the
Schroedinger equation, and that accounts for both reversible and irreversible
evolutions of the state of the system in time. Both the more general quantum
probabilities, and the equation of motion have been defined, and the three laws
of thermodynamics are shown to be theorems of this equation.


title: Optical response of a misaligned and suspended Fabry-Perot cavity

abstract:
  The response to a probe laser beam of a suspended, misaligned and detuned
optical cavity is examined. A five degree of freedom model of the fluctuations
of the longitudinal and transverse mirror coordinates is presented. Classical
and quantum mechanical effects of radiation pressure are studied with the help
of the optical stiffness coefficients and the signals provided by an FM
sideband technique and a quadrant detector, for generic values of the product
$\varpi \tau $ of the fluctuation frequency times the cavity round trip. A
simplified version is presented for the case of small misalignments. Mechanical
stability, mirror position entanglement and ponderomotive squeezing are
accommodated in this model. Numerical plots refer to cavities under test at the
so-called Pisa LF facility.


title:
Degree of entanglement as a physically ill-posed problem: The case of
  entanglement with vacuum

abstract:
  We analyze an example of a photon in superposition of different modes, and
ask what is the degree of their entanglement with vacuum. The problem turns out
to be ill-posed since we do not know which representation of the algebra of
canonical commutation relations (CCR) to choose for field quantization. Once we
make a choice, we can solve the question of entanglement unambiguously. So the
difficulty is not with mathematics, but with physics of the problem. In order
to make the discussion explicit we analyze from this perspective a popular
argument based on a photon leaving a beam splitter and interacting with two
two-level atoms. We first solve the problem algebraically in Heisenberg
picture, without any assumption about the form of representation of CCR. Then
we take the $\infty$-representation and show in two ways that in two-mode
states the modes are maximally entangled with vacuum, but single-mode states
are not entangled. Next we repeat the analysis in terms of the representation
of CCR taken from Berezin's book and show that two-mode states do not involve
the mode-vacuum entanglement. Finally, we switch to a family of reducible
representations of CCR recently investigated in the context of field
quantization, and show that the entanglement with vacuum is present even for
single-mode states. Still, the degree of entanglement is here difficult to
estimate, mainly because there are $N+2$ subsystems, with $N$ unspecified and
large.


title:
How Time Works in Quantum Systems: Overview of time ordering and time
  correlation in weakly perturbed atomic collisions and in strongly perturbed
  qubits

abstract:
  Time ordering may be defined by first defining the limit of no time ordering
(NTO) in terms of a time average of an external interaction, V(t). Previously,
time correlation was defined in terms of a similar limit called the independent
time approximation (ITA). Experimental evidence for time correlation has not
yet been distinguished from experimental evidence for time ordering.


title:
Applications of coherent classical communication and the Schur transform
  to quantum information theory

abstract:
  Quantum mechanics has led not only to new physical theories, but also a new
understanding of information and computation. Quantum information began by
yielding new methods for achieving classical tasks such as factoring and key
distribution but also suggests a completely new set of quantum problems, such
as sending quantum information over quantum channels or efficiently performing
particular basis changes on a quantum computer. This thesis contributes two
new, purely quantum, tools to quantum information theory--coherent classical
communication in the first half and an efficient quantum circuit for the Schur
transform in the second half.


title: Quasi multipartite entanglement measure based on quadratic functions

abstract:
  We develop a new entanglement measure by extending Jaeger's Minkowskian norm
entanglement measure. This measure can be applied to a much wider class of
multipartite mixed states, although still "quasi" in the sense that it is still
incapable of dividing precisely the sets of all separable and entangled states.
As a quadratic scalar function of the system density matrix, the quasi measure
can be easily expressed in terms of the so-called coherence vector of the
system density matrix, by which we show the basic properties of the quasi
measure including (1) zero-entanglement for all separable states, (2)
invariance under local unitary operations, and (3) non-increasing under local
POVM (positive operator-valued measure) measurements. These results open up
perspectives in further studies of dynamical problems in open systems,
especially the dynamic evolution of entanglement, and the entanglement
preservation against the environment-induced decoherence effects.


title:
Optical-state truncation and teleportation of qudits by conditional
  eight-port interferometry

abstract:
  Quantum scissors device of Pegg et al. (1998 Phys. Rev. Lett. 81, 1604)
enables truncation of the Fock-state expansion of an input optical field to
qubit and qutrit (three-dimensional) states only. Here, a generalized scissors
device is proposed using an eight-port optical interferometer. Upon
post-selection based on photon counting results, the interferometer implements
generation and teleportation of qudit (d-dimensional) states by truncation of
an input field at the (d-1)th term of its Fock-state expansion up to d=6.
Examples of selective truncations, which can be interpreted as a Fock-state
filtering and hole burning in the Fock space of an input optical field, are
discussed. Deterioration of the truncation due to imperfect photon counting is
discussed including inefficiency, dark counts and realistic photon-number
resolutions of photodetectors.


title: Security of Quantum Key Distribution

abstract:
  We propose various new techniques in quantum information theory, including a
de Finetti style representation theorem for finite symmetric quantum states. As
an application, we give a proof for the security of quantum key distribution
which applies to arbitrary protocols.


title:
Experimental Constraints of Using Slow-Light in Sodium Vapor for
  Light-Drag Enhanced Relative Rotation Sensing

abstract:
  We report on experimental observation of electromagnetically induced
transparency and slow-light (vg ~ c/607) in atomic sodium vapor, as a potential
medium for a recently proposed experiment on slow-light enhanced relative
rotation sensing [11]. We have performed an interferometric measurement of the
index variation associated with a two-photon resonance to estimate the
dispersion characteristics of the medium that is relevant to the slow-light
based rotation sensing scheme. We also show that the presence of
counter-propagating pump beams in an optical Sagnac loop produces a backward
optical phase conjugation beam that can generate spurious signals, which may
complicate the measurement of small rotations in the slow-light enhanced
gyroscope. We identify techniques for overcoming this constraint.


title:
Controllable Anomalous Dispersion and Group Index Nulling via
  Bi-Frequency Raman Gain in Rb Vapor for application to Ultraprecision
  Rotation Sensing

abstract:
  We have recently proposed [9], the use of fast-light media to obtain
ultrahigh precision rotation sensing capabilities. The scheme relies on
producing a critically anomalous dispersion, in a suitable dispersive medium,
which is introduced in the arms of a Sagnac interferometer. We present here an
experimental investigation of the anomalous dispersion properties of
bi-frequency Raman gain in rubidium vapor, with the goal of using this medium
for producing the critically anomalous dispersion condition. A heterodyne phase
measurement technique is used to measure accurately the index variation
associated with the dispersion. The slope of the negative linear dispersion (or
group index) is experimentally varied by more than two orders of magnitude
while changing the frequency separation between pump fields, responsible for
producing gain. Using this result, we have identified the experimental
parameters for achieving a null value of the group index, corresponding to the
critically anomalous dispersion condition necessary for enhanced rotational
sensitivity.


title: The Quantum Schur Transform: I. Efficient Qudit Circuits

abstract:
  We present an efficient family of quantum circuits for a fundamental
primitive in quantum information theory, the Schur transform. The Schur
transform on n d-dimensional quantum systems is a transform between a standard
computational basis to a labelling related to the representation theory of the
symmetric and unitary groups. If we desire to implement the Schur transform to
an accuracy of epsilon, then our circuit construction uses a number of gates
which is polynomial in n, d and log(1/epsilon). The important insights we use
to perform this construction are the selection of the appropriate subgroup
adapted basis and the Wigner-Eckart theorem. Our efficient circuit construction
renders numerous protocols in quantum information theory computationally
tractable and is an important new efficient quantum circuit family which goes
significantly beyond the standard paradigm of the quantum Fourier transform.


title: General quantization

abstract:
  Segal's hypothesis that physical theories drift toward simple groups follows
from a general quantum principle and suggests a general quantization process. I
general-quantize the scalar meson field in Minkowski space-time to illustrate
the process. The result is a finite quantum field theory over a finite quantum
space-time with higher symmetry than the singular theory. Multiple
quantification connects the levels of the theory.


title:
The asymptotic iteration method for the angular spheroidal eigenvalues
  with arbitrary complex size parameter c

abstract:
  The asymptotic iteration method is applied, to calculate the angular
spheroidal eigenvalues $\lambda^{m}_{\ell}(c)$ with arbitrary complex size
parameter $c$. It is shown that, the obtained numerical results of
$\lambda^{m}_{\ell}(c)$ are all in excellent agreement with the available
published data over the full range of parameter values $\ell$, $m$, and $c$.
Some representative values of $\lambda^{m}_{\ell}(c)$ for large real $c$ are
also given.


title:
The coordinate transformation and the exact solutions of the
  Schr\"{o}dinger equation with position-dependent effective mass

abstract:
  Using the coordinate transformation method, we solve the one-dimensional
Schr\"{o}dinger equation with position-dependent mass(PDM). The explicit
expressions for the potentials, energy eigenvalues and eigenfunctions of the
systems are given. The eigenfunctions can be expressed in terms of the Jacobi,
Hemite and generalized Laguerre polynomials. All potentials for these solvable
systems have an extra term $V_m$ which produced from the dependence of mass on
the coordinate, compared with that for the systems of constant mass. The
properties of $V_m$ for several mass functions are discussed.


title:
The Localization of $s$-Wave and Quantum Effective Potential of a
  Quasi-Free Particle with Position-Dependent Mass

abstract:
  The properties of the s-wave for a quasi-free particle with
position-dependent mass(PDM) have been discussed in details. Differed from the
system with constant mass in which the localization of the s-wave for the free
quantum particle around the origin only occurs in two dimensions, the
quasi-free particle with PDM can experience attractive forces in $D$ dimensions
except D=1 when its mass function satisfies some conditions. The effective mass
of a particle varying with its position can induce effective interaction which
may be attractive in some cases. The analytical expressions of the
eigenfunctions and the corresponding probability densities for the s-waves of
the two- and three-dimensional systems with a special PDM are given, and the
existences of localization around the origin for these systems are shown.


title: Irreversible Performance of a Quantum Harmonic Heat Engine

abstract:
  The unavoidable irreversible losses of power in a heat engine are found to be
of quantum origin. Following thermodynamic tradition a model quantum heat
engine operating by the Otto cycle is analyzed. The working medium of the model
is composed of an ensemble of harmonic oscillators. A link is established
between the quantum observables and thermodynamical variables based on the
concept of canonical invariance. These quantum variables are sufficient to
determine the state of the system and with it all thermodynamical variables.
Conditions for optimal work, power and entropy production show that maximum
power is a compromise between the quasistatic limit of adiabatic following on
the compression and expansion branches and a sudden limit of very short time
allocation to these branches. At high temperatures and quasistatic operating
conditions the efficiency at maximum power coincides with the endoreversible
result. The optimal compression ratio varies from the square root of the
temperature ratio in the quasistatic limit where their reversibility is
dominated by heat conductance to the temperature ratio to the power of 1/4 in
the sudden limit when the irreversibility is dominated by friction. When the
engine deviates from adiabatic conditions the performance is subject to
friction. The origin of this friction can be traced to the noncommutability of
the kinetic and potential energy of the working medium.


title: Transient spectrum of a single-Cooper-pair box with binomial states

abstract:
  We present an analytical expression for the response of a transient spectrum
to a single-Cooper-pair box biased by a classical voltage and irradiated by a
single-mode quantized field. The exact solution of the model is obtained, by
means of which we analyze the analytic form of the fluorescence spectrum using
the transitions among the dressed states of the system. An interesting relation
between the fluorescence spectrum and the dynamical evolution is found when the
initial field states are prepared in binomial states.


title: Bang-bang control of fullerene qubits using ultra-fast phase gates

abstract:
  Quantum mechanics permits an entity, such as an atom, to exist in a
superposition of multiple states simultaneously. Quantum information processing
(QIP) harnesses this profound phenomenon to manipulate information in radically
new ways. A fundamental challenge in all QIP technologies is the corruption of
superposition in a quantum bit (qubit) through interaction with its
environment. Quantum bang-bang control provides a solution by repeatedly
applying `kicks' to a qubit, thus disrupting an environmental interaction.
However, the speed and precision required for the kick operations has presented
an obstacle to experimental realization. Here we demonstrate a phase gate of
unprecedented speed on a nuclear spin qubit in a fullerene molecule (N@C60),
and use it to bang-bang decouple the qubit from a strong environmental
interaction. We can thus trap the qubit in closed cycles on the Bloch sphere,
or lock it in a given state for an arbitrary period. Our procedure uses
operations on a second qubit, an electron spin, in order to generate an
arbitrary phase on the nuclear qubit. We anticipate the approach will be vital
for QIP technologies, especially at the molecular scale where other strategies,
such as electrode switching, are unfeasible.


title: Notes on phase space quantization

abstract:
  We consider questions related to a quantization scheme in which a classical
variable f:\Omega\to R on a phase space \Omega is associated with a
semispectral measure E^f, such that the moment operators of E^f are required to
be of the form \Gamma(f^k), with \Gamma a suitable mapping from the set of
classical variables to the set of (not necessarily bounded) operators in some
Hilbert space. In particular, we investigate the situation where the map \Gamma
is implemented by the operator integral with respect to some fixed positive
operator measure. The phase space \Omega is first taken to be an abstract
measurable space, then a locally compact unimodular group, and finally R^2,
where we determine explicitly the relevant operators \Gamma(f^k) for certain
variables f, in the case where the quantization map \Gamma is implemented by a
translation covariant positive operator measure. In addition, we consider the
question under what conditions a positive operator measure is projection
valued.


title: Optimal Encoding of Classical Information in a Quantum Medium

abstract:
  We investigate optimal encoding and retrieval of digital data, when the
storage/communication medium is described by quantum mechanics. We assume an
m-ary alphabet with arbitrary prior distribution, and an n-dimensional quantum
system. Under these constraints, we seek an encoding-retrieval setup, comprised
of code-states and a quantum measurement, which maximizes the probability of
correct detection. In our development, we consider two cases. In the first, the
measurement is predefined and we seek the optimal code-states. In the second,
optimization is performed on both the code-states and the measurement.
  We show that one cannot outperform `pseudo-classical transmission', in which
we transmit n symbols with orthogonal code-states, and discard the remaining
symbols. However, such pseudo-classical transmission is not the only optimum.
We fully characterize the collection of optimal setups, and briefly discuss the
links between our findings and applications such as quantum key distribution
and quantum computing. We conclude with a number of results concerning the
design under an alternative optimality criterion, the worst-case posterior
probability, which serves as a measure of the retrieval reliability.


title: Opposites Attract - A Theorem About The Casimir Force

abstract:
  We consider the Casimir interaction between (non-magnetic) dielectric bodies
or conductors. Our main result is a proof that the Casimir force between two
bodies related by reflection is always attractive, independent of the exact
form of the bodies or dielectric properties. Apart from being a fundamental
property of fields, the theorem and its corollaries also rule out a class of
suggestions to obtain repulsive forces, such as the two hemisphere repulsion
suggestion and its relatives.

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


title: Two-Mode Theory of BEC Interferometry

abstract:
  A theory of BEC interferometry in an unsymmetrical double-well trap has been
developed for small boson numbers, based on the two-mode approximation. The
bosons are initially in the lowest mode of a single well trap, which is split
into a double well and then recombined. Possible fragmentations into separate
BEC states in each well during the splitting/recombination process are allowed
for. The BEC is treated as a giant spin system, the fragmented states are
eigenstates of S^2 and Sz. Self-consistent sets of equations for the amplitudes
of the fragmented states and for the two single boson mode functions are
obtained. The latter are coupled Gross-Pitaevskii equations. Interferometric
effects may be measured via boson numbers in the first excited mode


title: Generalized Bloch Spheres for m-Qubit States

abstract:
  m-Qubit states are imbedded in $\mathfrak{Cl}_{2^m}$ Clifford algebras. Their
probability spectra then depend on $O(2m)$ or $O(2m+1)$ invariants. Parameter
domains for $O(2m(+1))-$ vector and tensor configurations, generalizing the
notion of a Bloch sphere, are derived.


title: Bunching of Bell states

abstract:
  The bunching of two single photons on a beam-splitter is a fundamental
quantum effect, first observed by Hong, Ou and Mandel. It is a unique
interference effect that relies only on the photons' indistinguishability and
not on their relative phase. We generalize this effect by demonstrating the
bunching of two Bell states, created in two passes of a nonlinear crystal, each
composed of two photons. When the two Bell states are indistinguishable, phase
insensitive destructive interference prevents the outcome of four-fold
coincidence between the four spatial-polarization modes. For certain
combinations of the two Bell states, we demonstrate the opposite effect of
anti-bunching. We relate this result to the number of distinguishable modes in
parametric down-conversion.


title:
Controversy between Einstein and Bohr and two erroneous arguments used
  in supporting Copenhagen quantum mechanics

abstract:
  The support of Copenhagen quantum mechanics in the discussion concerning EPR
experiments has been based fundamentally on two mistakes. First, quantum
mechanics as well as hidden-variable theory give the same predictions; the
statement of Belinfante from 1973 about the significant difference must be
denoted as mistake. Secondly, the experimental violation of Bell's inequalities
has been erroneously interpreted as excluding the hidden-variable alternative,
while they have been based on assumption corresponding to classical physics.
The EPR experiments cannot bring, therefore, any decision in the controversy
between Einstein and Bohr. However, the view of Einstein is strongly supported
by experimental results concerning the light transmission through three
polarizers.


title: Feedback control of spin systems

abstract:
  The feedback stabilization problem for ensembles of coupled spin 1/2 systems
is discussed from a control theoretic perspective. The noninvasive nature of
the bulk measurement allows for a fully unitary and deterministic closed loop.
The Lyapunov-based feedback design presented does not require spins that are
selectively addressable. With this method, it is possible to obtain control
inputs also for difficult tasks, like suppressing undesired couplings in
identical spin systems.


title: Quantum cloning of identical mixed qubits

abstract:
  Quantum cloning of two identical mixed qubits $\rho \otimes \rho $ is
studied. We propose the quantum cloning transformations not only for the
triplet (symmetric) states but also for the singlet (antisymmetric) state. We
can copy these two identical mixed qubits to M (M>2) copies. This quantum
cloning machine is optimal in the sense that the shrinking factor between the
input and the output single qubit achieves the upper bound. The result shows
that we can copy two identical mixed qubits with the same quality as that of
two identical pure states.


title: A scalable optical detection scheme for matter wave interferometry

abstract:
  Imaging of surface adsorbed molecules is investigated as a novel detection
method for matter wave interferometry with fluorescent particles. Mechanically
magnified fluorescence imaging turns out to be an excellent tool for recording
quantum interference patterns. It has a good sensitivity and yields patterns of
high visibility. The spatial resolution of this technique is only determined by
the Talbot gratings and can exceed the optical resolution limit by an order of
magnitude. A unique advantage of this approach is its scalability: for certain
classes of nano-sized objects, the detection sensitivity will even increase
significantly with increasing size of the particle.


title: Simulating adiabatic evolution of gapped spin systems

abstract:
  We show that adiabatic evolution of a low-dimensional lattice of quantum
spins with a spectral gap can be simulated efficiently. In particular, we show
that as long as the spectral gap \Delta E between the ground state and the
first excited state is any constant independent of n, the total number of
spins, then the ground-state expectation values of local operators, such as
correlation functions, can be computed using polynomial space and time
resources. Our results also imply that the local ground-state properties of any
two spin models in the same quantum phase can be efficiently obtained from each
other. A consequence of these results is that adiabatic quantum algorithms can
be simulated efficiently if the spectral gap doesn't scale with n. The
simulation method we describe takes place in the Heisenberg picture and does
not make use of the finitely correlated state/matrix product state formalism.


title: Quantum states and space-time causality

abstract:
  Space-time symmetries and internal quantum symmetries can be placed on equal
footing in a hyperspin geometry. Four-dimensional classical space-time emerges
as a result of a decoherence that disentangles the quantum and the space-time
degrees of freedom. A map from the quantum space-time to classical space-time
that preserves the causality relations of space-time events is necessarily a
density matrix.


title:
Limitations of Quantum Simulation Examined by Simulating a Pairing
  Hamiltonian using Nuclear Magnetic Resonance

abstract:
  Quantum simulation uses a well-known quantum system to predict the behavior
of another quantum system. Certain limitations in this technique arise,
however, when applied to specific problems, as we demonstrate with a
theoretical and experimental study of an algorithm to find the low-lying
spectrum of a Hamiltonian. While the number of elementary quantum gates does
scale polynomially with the size of the system, it increases inversely to the
desired error bound $\epsilon$. Making such simulations robust to decoherence
using fault-tolerance constructs requires an additional factor of $1/ \epsilon$
gates. These constraints are illustrated by using a three qubit nuclear
magnetic resonance system to simulate a pairing Hamiltonian, following the
algorithm proposed by Wu, Byrd, and Lidar.


title:
Composite Dipolar Recoupling: Anisotropy Compensated Coherence Transfer
  in Solid-State NMR

abstract:
  The efficiency of dipole-dipole coupling driven coherence transfer
experiments in solid-state NMR spectroscopy of powder samples is limited by
dispersion of the orientation of the internuclear vectors relative to the
external magnetic field. Here we introduce general design principles and
resulting pulse sequences that approach full polarization transfer efficiency
for all crystallite orientations in a powder in magic-angle-spinning
experiments. The methods compensate for the defocusing of coherence due to
orientation dependent dipolar coupling interactions and inhomogeneous
radio-frequency fields. The compensation scheme is very simple to implement as
a scaffold (comb) of compensating pulses in which the pulse sequence to be
improved may be inserted. The degree of compensation can be adjusted and should
be balanced as a compromise between efficiency and length of the overall pulse
sequence. We show by numerical and experimental data that the presented
compensation protocol significantly improves the efficiency of known dipolar
recoupling solid-state NMR experiment.


title: Quantum control and manipulations with stationary three-color lights

abstract:
  A dynamic quantum control of three-color lights in an optically dense medium
is presented. We discuss how effectively to stop traveling three-color light
pulses in the medium by using three control laser fields at near resonant
transitions satisfying electromagnetically induced transparency. This opens a
door to the quantum coherent control of multiple traveling light pulses for
quantum memory and quantum switching, which are essential components in
multi-party quantum optical communications.


title:
An alternative implementation of the Lanczos algorithm for wave function
  propagation

abstract:
  We reformulate the Lanczos algorithm for quantum wave function propagation in
terms of variational principle. By including some basis states of previous time
steps into the variational subspace, the resultant accuracy increases by
several orders. Numerical errors of the alternative method accumulate much
slower than that of the original Lanczos method. There is almost no extra
numeric cost for the gaining of the accuracy, i.e., the accuracy increase needs
no extra operations of the Hamiltonian acting on state vectors, which are the
major numeric cost for wave function propagation. A wave packet moving in a
2-dimensional H\'enon-Heiles model serves as an illustration. This method is
suitable for small time step propagation of quantum wave functions in large
scale time dependent calculations where the operations of the Hamiltonian
acting on state vectors are expensive.


title:
Decoherence and localization in tunneling process under influence of one
  external degree of freedom

abstract:
  We investigate numerically the tunneling effect under influence of another
particle in a double well system. Such influence from only one degree of
freedom makes decoherence and quantum-classical transition, i.e., suppression
of the tunneling effect. The decoherence happens even for cases that the
influence is from a particle of very small mass, and it has virtually no effect
in the corresponding classical dynamics. There are cases similar to dynamical
localization that the suppressed tunneling rate is several times smaller than
the classical counterpart. This result is relevant for understanding
quantitatively the dynamical process of decoherence and quantum to classical
transition.


title: Macroscopic Quantum Criticality in a Circuit QED

abstract:
  Cavity quantum electrodynamic (QED) is studied for two strongly-coupled
charge qubits interacting with a single-mode quantized field, which is provided
by a on-chip transmission line resonator. We analyze the dressed state
structure of this superconducting circuit QED system and the selection rules of
electromagnetic-induced transitions between any two of these dressed states.
Its macroscopic quantum criticality, in the form of ground state level
crossing, is also analyzed, resulting from competition between the Ising-type
inter-qubit coupling and the controllable on-site potentials.


title: On multipartite invariant states I. Unitary symmetry

abstract:
  We propose a natural generalization of bipartite Werner and isotropic states
to multipartite systems consisting of an arbitrary even number of d-dimensional
subsystems (qudits). These generalized states are invariant under the action of
local unitary operations. We study basic properties of multipartite invariant
states: separability criteria and multi-PPT conditions.


title:
Preparation of nondegenerate coherent superpositions in a three-state
  ladder system assisted by Stark Shifts

abstract:
  We propose a technique to prepare coherent superpositions of two
nondegenerate quantum states in a three-state ladder system, driven by two
simultaneous fields near resonance with an intermediate state. The technique,
of potential application to enhancement of nonlinear processes, uses adiabatic
passage assisted by dynamic Stark shifts induced by a third laser field. The
method offers significant advantages over alternative techniques: (\i) it does
not require laser pulses of specific shape and duration and (\ii) it requires
less intense fields than schemes based on two-photon excitation with
non-resonant intermediate states. We discuss possible experimental
implementation for enhancement of frequency conversion in mercury atoms.


title:
Generating controllable atom-light entanglement with a Raman atom laser
  system

abstract:
  We introduce a scheme for creating continuous variable entanglement between
an atomic beam and an optical field, by using squeezed light to outcouple atoms
from a BEC via a Raman transition. We model the full multimode dynamics of the
atom laser beam and the squeezed optical field, and show that with appropriate
two-photon detuning and two-photon Rabi frequency, the transmitted light is
entangled in amplitude and phase with the outcoupled atom laser beam. The
degree of entanglement is controllable via changes in the two-photon Rabi
frequency of the outcoupling process.


title:
Efficient solvability of Hamiltonians and limits on the power of some
  quantum computational models

abstract:
  We consider quantum computational models defined via a Lie-algebraic theory.
In these models, specified initial states are acted on by Lie-algebraic quantum
gates and the expectation values of Lie algebra elements are measured at the
end. We show that these models can be efficiently simulated on a classical
computer in time polynomial in the dimension of the algebra, regardless of the
dimension of the Hilbert space where the algebra acts. Similar results hold for
the computation of the expectation value of operators implemented by a
gate-sequence. We introduce a Lie-algebraic notion of generalized mean-field
Hamiltonians and show that they are efficiently ("exactly") solvable by means
of a Jacobi-like diagonalization method. Our results generalize earlier ones on
fermionic linear optics computation and provide insight into the source of the
power of the conventional model of quantum computation.


title:
Dependences of the Casimir-Polder interaction between an atom and a
  cavity wall on atomic and material properties

abstract:
  The Casimir-Polder and van der Waals interactions between an atom and a flat
cavity wall are investigated under the influence of real conditions including
the dynamic polarizability of the atom, actual conductivity of the wall
material and nonzero temperature of the wall. The cases of different atoms near
metal and dielectric walls are considered. It is shown that to obtain accurate
results for the atom-wall interaction at short separations, one should use the
complete tabulated optical data for the complex refractive index of the wall
material and the accurate dynamic polarizability of an atom. At relatively
large separations in the case of a metal wall, one may use the plasma model
dielectric function to describe the dielectric properties of wall material. The
obtained results are important for the theoretical interpretation of
experiments on quantum reflection and Bose-Einstein condensation.


title:
Casimir-Polder interaction between an atom and a cylinder with
  application to nanosystems

abstract:
  Recently the Lifshitz theory of dispersion forces was extended for the case
of an atom (molecule) interacting with a plane surface of a uniaxial crystal or
with a long solid cylinder or cylindrical shell made of isotropic material or
uniaxial crystal. The obtained results are applicable to nanosystems. In
particular, we investigate the Casimir-Polder interaction between hydrogen
atoms (molecules) and multi-wall carbon nanotubes. It is demonstrated that the
hydrogen atoms located inside multiwall carbon nanotubes have a lower free
energy compared to those located outside. We also perform comparison studies of
the interaction of hydrogen atoms between themselves and with multi-wall carbon
nanotube. The obtained results are important for the problem of hydrogen
storage.


title: Direct Characterization of Quantum Dynamics

abstract:
  The characterization of quantum dynamics is a fundamental and central task in
quantum mechanics. This task is typically addressed by quantum process
tomography (QPT). Here we present an alternative "direct characterization of
quantum dynamics" (DCQD) algorithm. In contrast to all known QPT methods, this
algorithm relies on error-detection techniques and does not require any quantum
state tomography. We illustrate that, by construction, the DCQD algorithm can
be applied to the task of obtaining partial information about quantum dynamics.
Furthermore, we argue that the DCQD algorithm is experimentally implementable
in a variety of prominent quantum information processing systems, and show how
it can be realized in photonic systems with present day technology.


title: Direct Characterization of Quantum Dynamics: General Theory

abstract:
  The characterization of the dynamics of quantum systems is a task of both
fundamental and practical importance. A general class of methods which have
been developed in quantum information theory to accomplish this task is known
as quantum process tomography (QPT). In an earlier paper [M. Mohseni and D. A.
Lidar, Phys. Rev. Lett. 97, 170501 (2006)] we presented a new algorithm for
Direct Characterization of Quantum Dynamics (DCQD) of two-level quantum
systems. Here we provide a generalization by developing a theory for direct and
complete characterization of the dynamics of arbitrary quantum systems. In
contrast to other QPT schemes, DCQD relies on quantum error-detection
techniques and does not require any quantum state tomography. We demonstrate
that for the full characterization of the dynamics of n d-level quantum systems
(with d a power of a prime), the minimal number of required experimental
configurations is reduced quadratically from d^{4n} in separable QPT schemes to
d^{2n} in DCQD.


title: Constraints on the uncertainties of entangled symmetric qubits

abstract:
  We derive necessary and sufficient inseparability conditions imposed on the
variance matrix of symmetric qubits. These constraints are identified by
examining a structural parallelism between continuous variable states and two
qubit states. Pairwise entangled symmetric multiqubit states are shown here to
obey these constraints. We also bring out an elegant local invariant structure
exhibited by our constraints.


title: Designing bound states in a band as a model for a quantum network

abstract:
  We provide a model of a one dimensional quantum network, in the framework of
a lattice using Von Neumann and Wigner's idea of bound states in a continuum.
The localized states acting as qubits are created by a controlled deformation
of a periodic potential. These wave functions lie at the band edges and are
defects in a lattice. We propose that these defect states, with atoms trapped
in them, can be realized in an optical lattice and can act as a model for a
quantum network.


title: Engineering Quantum Jump Superoperators for Single Photon Detectors

abstract:
  We study the back-action of a single photon detector on the electromagnetic
field upon a photodetection by considering a microscopic model in which the
detector is constituted of a sensor and an amplification mechanism. Using the
quantum trajectories approach we determine the Quantum Jump Superoperator (QJS)
that describes the action of the detector on the field state immediately after
the photocount. The resulting QJS consists of two parts: the bright counts
term, representing the real photoabsorptions, and the dark counts term,
representing the amplification of intrinsic excitations inside the detector.
First we compare our results for the counting rates to experimental data,
showing a good agreement. Then we point out that by modifying the field
frequency one can engineer the form of QJS, obtaining the QJS's proposed
previously in an ad hoc manner.


title:
Generalized spin squeezing inequalities in $N$ qubit systems: theory and
  experiment

abstract:
  We present detailed derivations, various improvements and application to
concrete experimental data of spin squeezing inequalities formulated recently
by some of us [Phys. Rev. Lett. {\bf 95}, 120502 (2005)]. These inequalities
generalize the concept of the spin squeezing parameter, and provide necessary
and sufficient conditions for genuine 2-, or 3- qubit entanglement for
symmetric states, and sufficient entanglement condition for general $N$-qubit
states. We apply our method to theoretical study of Dicke states, and, in
particular, to $W$-states of $N$ qubits. Then, we analyze the recently
experimentally generated 7- and 8-ion $W$-states [Nature {\bf 438}, 643
(2005)]. We also present some novel details concerning this experiment.
Finally, we improve criteria for detection of genuine tripartite entanglement
based on entanglement witnesses.


title:
Contributions of Vacuum and Plasmon Modes to the Force on a Small Sphere
  near a Plate

abstract:
  The force on a small sphere with a plasma model dielectric function and in
the presence of a perfectly reflecting plane is considered. The contribution of
both the vacuum modes of the quantized electromagnetic field and of plasmon
modes in the sphere are discussed. In the case that the plasmon modes are in
their ground state, quasi-oscillatory terms from the vacuum and plasmon parts
cancel one another, leading a monotonic attractive force. If the plasmon modes
are not in the ground state, the net force is quasi-oscillatory. In both cases,
the sphere behaves in the same way as does an atom in either its ground state
or an excited state.


title: Infinite Divisibility in Euclidean Quantum Mechanics

abstract:
  In simple -- but selected -- quantum systems, the probability distribution
determined by the ground state wave function is infinitely divisible. Like all
simple quantum systems, the Euclidean temporal extension leads to a system that
involves a stochastic variable and which can be characterized by a probability
distribution on continuous paths. The restriction of the latter distribution to
sharp time expectations recovers the infinitely divisible behavior of the
ground state probability distribution, and the question is raised whether or
not the temporally extended probability distribution retains the property of
being infinitely divisible. A similar question extended to a quantum field
theory relates to whether or not such systems would have nontrivial scattering
behavior.


title: Loss of purity by wave packet scattering at low energies

abstract:
  We study the quantum entanglement produced by a head-on collision between two
gaussian wave packets in three-dimensional space. By deriving the two-particle
wave function modified by s-wave scattering amplitudes, we obtain an
approximate analytic expression of the purity of an individual particle. The
loss of purity provides an indicator of the degree of entanglement. In the case
the wave packets are narrow in momentum space, we show that the loss of purity
is solely controlled by the ratio of the scattering cross section to the
transverse area of the wave packets.


title:
Probing tiny motions of nanomechanical resonators: classical or quantum
  mechanical?

abstract:
  We propose a spectroscopic approach to probe tiny vibrations of a
nanomechanical resonator (NAMR), which may reveal classical or quantum behavior
depending on the decoherence-inducing environment. Our proposal is based on the
detection of the voltage-fluctuation spectrum in a superconducting transmission
line resonator (TLR), which is {\it indirectly} coupled to the NAMR via a
controllable Josephson qubit acting as a quantum transducer. The classical
(quantum mechanical) vibrations of the NAMR induce symmetric (asymmetric) Stark
shifts of the qubit levels, which can be measured by the voltage fluctuations
in the TLR. Thus, the motion of the NAMR, including if it is quantum mechanical
or not, could be probed by detecting the voltage-fluctuation spectrum of the
TLR.


title: Quantum Discrete Cosine Transform for Image Compression

abstract:
  Discrete Cosine Transform (DCT) is very important in image compression.
Classical 1-D DCT and 2-D DCT has time complexity O(NlogN) and O(N²logN)
respectively. This paper presents a quantum DCT iteration, and constructs a
quantum 1-D and 2-D DCT algorithm for image compression by using the iteration.
The presented 1-D and 2-D DCT has time complexity O(sqrt(N)) and O(N)
respectively. In addition, the method presented in this paper generalizes the
famous Grover's algorithm to solve complex unstructured search problem.


title:
Another convex combination of product states for the separable Werner
  state

abstract:
  In this paper, we write down the separable Werner state in a two-qubit system
explicitly as a convex combination of product states, which is different from
the convex combination obtained by Wootters' method. The Werner state in a
two-qubit system has a single real parameter and varies from inseparable state
to separable state according to the value of its parameter. We derive a hidden
variable model that is induced by our decomposed form for the separable Werner
state. From our explicit form of the convex combination of product states, we
understand the following: The critical point of the parameter for separability
of the Werner state comes from positivity of local density operators of the
qubits.


title: Entanglement and Teleportation of Gaussian States of the Radiation Field

abstract:
  We propose a reliable entanglement measure for a two-mode squeezed thermal
state of the quantum electromagnetic field in terms of its Bures distance to
the set of all separable states of the same kind. The requisite Uhlmann
fidelity of a pair of two-mode squeezed thermal states is evaluated as the
maximal transition probability between two four-mode purifications. By applying
the Peres-Simon criterion of separability we find the closest separable state.
This enables us to derive an insightful expression of the amount of
entanglement. Then we apply this measure of entanglement to the study of the
Braunstein-Kimble protocol of teleportation. We use as input state in
teleportation a mixed one-mode Gaussian state. The entangled state shared by
the sender (Alice) and the receiver (Bob) is taken to be a two-mode squeezed
thermal state. We find that the properties of the teleported state depend on
both the input state and the entanglement of the two-mode resource state. As a
measure of the quality of the teleportation process, we employ the Uhlmann
fidelity between the input and output mixed one-mode Gaussian states.


title:
A hemispherical, high-solid-angle optical micro-cavity for cavity-QED
  studies

abstract:
  We report a novel hemispherical micro-cavity that is comprised of a planar
integrated semiconductor distributed Bragg reflector (DBR) mirror, and an
external, concave micro-mirror having a radius of curvature $50\mathrm{\mu m}$.
The integrated DBR mirror containing quantum dots (QD), is designed to locate
the QDs at an antinode of the field in order to maximize the interaction
between the QD and the cavity. The concave micro-mirror, with high-reflectivity
over a large solid-angle, creates a diffraction-limited (sub-micron) mode-waist
at the planar mirror, leading to a large coupling constant between cavity mode
and QD. The half-monolithic design gives more spatial and spectral tuning
abilities, relatively to fully monolithic structures. This unique micro-cavity
design will potentially enable us to both reach the cavity quantum
electrodynamics (QED) strong coupling regime and realize the deterministic
generation of single photons on demand.


title:
Universal destabilization and slowing of spin transfer functions by a
  bath of spins

abstract:
  We investigate the effect of a spin bath on the spin transfer functions of a
permanently coupled spin system. When each spin is coupled to a seperate
environment, the effect on the transfer functions in the first excitation
sector is amazingly simple: the group velocity is slowed down by a factor of
two, and the fidelity is destabilized by a modulation of |cos Gt|, where G is
the mean square coupling to the environment.


title: Strengthened PT-symmetry with P $\neq$ P$^\dagger$

abstract:
  Two alternative scenarios are shown possible in Quantum Mechanics working
with non-Hermitian $PT-$symmetric form of observables. While, usually, people
assume that $P$ is a self-adjoint indefinite metric in Hilbert space (and that
their $P-$pseudo-Hermitian Hamiltonians $H$ possess the real spectra etc), we
propose to relax the constraint $P=P^\dagger$ as redundant. Non-Hermitian
triplet of coupled square wells is chosen for illustration purposes. Its
solutions are constructed and the observed degeneracy of their spectrum is
attributed to the characteristic nontrivial symmetry $S={P}^{-1} {P}^\dagger
\neq I$ of the model $H$. Due to the solvability of the model the determination
of the domain where the energies remain real is straightforward. A few remarks
on the correct (albeit ambiguous) physical interpretation of the model are
added.


title: Atomic reflection off conductor walls as a tool in cold atom traps

abstract:
  We explain why a system of cold $^{85}Rb$ atoms at temperatures of the order
$T\approx 7.78\times 10^{-5}$ K and below, but not too low to lie in the
quantum reflection regime, should be automatically repelled from the surface of
a conductor without the need of an evanescent field, as in a typical atom
mirror, to counteract the van der Waals attraction. The repulsive potential
arises naturally outside the conductor and is effective at distances from the
conductor surface of about 400nm, intermediate between the van der Waals and
the Casimir-Polder regions of variation. We propose that such a field-free
reflection capability should be useful as a component in cold atom traps. It
should be practically free of undesirable field fluctuations and would be
operative at distances for which surface roughness, dissipative effects and
other finite conductivity effects should be negligibly small.


title:
Braid Group, Temperley--Lieb Algebra, and Quantum Information and
  Computation

abstract:
  In this paper, we explore algebraic structures and low dimensional topology
underlying quantum information and computation. We revisit quantum
teleportation from the perspective of the braid group, the symmetric group and
the virtual braid group, and propose the braid teleportation, the teleportation
swapping and the virtual braid teleportation, respectively. Besides, we present
a physical interpretation for the braid teleportation and explain it as a sort
of crossed measurement. On the other hand, we propose the extended
Temperley--Lieb diagrammatical approach to various topics including quantum
teleportation, entanglement swapping, universal quantum computation, quantum
information flow, and etc. The extended Temperley--Lieb diagrammatical rules
are devised to present a diagrammatical representation for the extended
Temperley--Lieb category which is the collection of all the Temperley--Lieb
algebras with local unitary transformations. In this approach, various
descriptions of quantum teleportation are unified in a diagrammatical sense,
universal quantum computation is performed with the help of topological-like
features, and quantum information flow is recast in a correct formulation. In
other words, we propose the extended Temperley--Lieb category as a mathematical
framework to describe quantum information and computation involving maximally
entangled states and local unitary transformations.


title: Quantum mechanics in general quantum systems (III): open system dynamics

abstract:
  We investigate the exact solution, perturbation theory and master equation of
open system dynamics based on our serial studies on quantum mechanics in
general quantum systems [An Min Wang, quant-ph/0611216 and quant-ph/0611217].
In a system-environment separated representation, a general and explicit
solution of open system dynamics is obtained, and it is an exact solution since
it includes all order approximations of perturbation. In terms of the cut-off
approximation of perturbation and our improved scheme of perturbation theory,
the improved form of the perturbed solution of open systems absorbing the
partial contributions from the high order even all order approximations is
deduced. Moreover, only under the factorizing initial condition, the exact
master equation including all order approximations is proposed.
Correspondingly, the perturbed master equation and its improved form different
from the existed master equation are given. In special, the Redfield master
equation is derived out without using Born-Markov approximation. The solution
of open system dynamics in the Milburn model is also gained. As examples, Zurek
model of two-state open system and its extension with two transverse fields are
studied.

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


title: Ion Trap in a Semiconductor Chip

abstract:
  The electromagnetic manipulation of isolated atoms has led to many advances
in physics, from laser cooling and Bose-Einstein condensation of cold gases to
the precise quantum control of individual atomic ion. Work on miniaturizing
electromagnetic traps to the micrometer scale promises even higher levels of
control and reliability. Compared with 'chip traps' for confining neutral
atoms, ion traps with similar dimensions and power dissipation offer much
higher confinement forces and allow unparalleled control at the single-atom
level. Moreover, ion microtraps are of great interest in the development of
miniature mass spectrometer arrays, compact atomic clocks, and most notably,
large scale quantum information processors. Here we report the operation of a
micrometer-scale ion trap, fabricated on a monolithic chip using semiconductor
micro-electromechanical systems (MEMS) technology. We confine, laser cool, and
measure heating of a single 111Cd+ ion in an integrated radiofrequency trap
etched from a doped gallium arsenide (GaAs) heterostructure.


title: Level crossings in a cavity QED model

abstract:
  In this paper I study the dynamics of a two-level atom interacting with a
standing wave field. When the atom is subjected to a weak linear force, the
problem can be turned into a time dependent one, and the evolution is
understood from the band structure of the spectrum. The presence of level
crossings in the spectrum gives rise to Bloch oscillations of the atomic
motion. Here I investigate the effects of the atom-field detuning parameter. A
variety of different level crossings are obtained by changing the magnitude of
the detuning, and the behaviour of the atomic motion is strongly affected due
to this. I also consider the situation in which the detuning is oscillating in
time and its impact on the atomic motion. Wave packet simulations of the full
problem are treated numerically and the results are compared with analytical
solutions given by the standard Landau-Zener and the three-level Landau-Zener
models.


title: Single spin measurement using cellular automata techniques

abstract:
  We propose an approach for single spin measurement. Our method uses
techniques from the theory of quantum cellular automata to correlate a large
amount of ancillary spins to the one to be measured. It has the distinct
advantage of being efficient, and to a certain extent fault-tolerant. Under
ideal conditions, it requires the application of only order of cube root of N
steps (each requiring a constant number of rf pulses) to create a system of N
correlated spins. It is also fairly robust against pulse errors, imperfect
initial polarization of the ancilla spin system, and does not rely on
entanglement. We study the scalability of our scheme through numerical
simulation.


title: Quantum telecommunication based on atomic cascade transitions

abstract:
  A quantum repeater at telecommunications wavelengths with long-lived atomic
memory is proposed, and its critical elements are experimentally demonstrated
using a cold atomic ensemble. Via atomic cascade emission, an entangled pair of
1530 nm and 780 nm photons is generated. The former is ideal for long-distance
quantum communication, and the latter is naturally suited for mapping to a
long-lived atomic memory. Together with our previous demonstration of
photonic-to-atomic qubit conversion, both of the essential elements for the
proposed telecommunications quantum repeater have now been realized.


title: Quantum Brownian motion and the Third Law of thermodynamics

abstract:
  The quantum thermodynamic behavior of small systems is investigated in
presence of finite quantum dissipation. We consider the archetype cases of a
damped harmonic oscillator and a free quantum Brownian particle. A main finding
is that quantum dissipation helps to ensure the validity of the Third Law. For
the quantum oscillator, finite damping replaces the zero-coupling result of an
exponential suppression of the specific heat at low temperatures by a power-law
behavior. Rather intriguing is the behavior of the free quantum Brownian
particle. In this case, quantum dissipation is able to restore the Third Law:
Instead of being constant down to zero temperature, the specific heat now
vanishes proportional to temperature with an amplitude that is inversely
proportional to the ohmic dissipation strength. A distinct subtlety of finite
quantum dissipation is the result that the various thermodynamic functions of
the sub-system do not only depend on the dissipation strength but depend as
well on the prescription employed in their definition.


title:
A Note to Article "Quantization of Light Energy Directly from Classical
  Electromagnetic Theory in Vacuum"

abstract:
  Very recently we present a theory [Wei-long She, Chin. Phys. 14, 2514(2005);
Online: http://www.jop.org/journals/cp; http://arxiv.org/abs/quant-ph/0512097]
to show that the quantization of light energy in vacuum can be derived directly
from classical electromagnetic theory through the consideration of statistics
based on classical physics and reveal that the quantization of energy is an
intrinsic property of light as a classical electromagnetic wave and has no need
of being related to particles. In this theory a key concept of stability of
statistical distribution was involved. Here is a note to the theory, which
would be helpful for understanding the concept of stability of statistical
distribution.


title:
EPR Beams and Photon Number Detector: Toward Synthesizing Optical
  Nonlinearity

abstract:
  We present the two kinds of experimental results. One is a continuous
variable dense coding experiment, and the other is a photon number detector
with high linearity response, the so called charge integration photon detector
(CIPD). They can be combined together to be a potential tool for implementing
the cubic phase gate which is an important gate element to synthesize the
measurement induced nonlinearity for photonic quantum information processing.


title: The Sampling Theorem and Coherent State Systems in Quantum Mechanics

abstract:
  The well known Poisson Summation Formula is analysed from the perspective of
the coherent state systems associated with the Heisenberg--Weyl group. In
particular, it is shown that the Poisson summation formula may be viewed
abstractly as a relation between two sets of bases (Zak bases) arising as
simultaneous eigenvectors of two commuting unitary operators in which geometric
phase plays a key role. The Zak bases are shown to be interpretable as
generalised coherent state systems of the Heisenberg--Weyl group and this, in
turn, prompts analysis of the sampling theorem (an important and useful
consequence of the Poisson Summation Formula) and its extension from a coherent
state point of view leading to interesting results on properties of von Neumann
and finer lattices based on standard and generalised coherent state systems.


title: Hamilton's Turns for the Lorentz Group

abstract:
  Hamilton in the course of his studies on quaternions came up with an elegant
geometric picture for the group SU(2). In this picture the group elements are
represented by ``turns'', which are equivalence classes of directed great
circle arcs on the unit sphere $S^2$, in such a manner that the rule for
composition of group elements takes the form of the familiar parallelogram law
for the Euclidean translation group. It is only recently that this construction
has been generalized to the simplest noncompact group $SU(1,1) = Sp(2, R) =
SL(2,R)$, the double cover of SO(2,1). The present work develops a theory of
turns for $SL(2,C)$, the double and universal cover of SO(3,1) and $SO(3,C)$,
rendering a geometric representation in the spirit of Hamilton available for
all low dimensional semisimple Lie groups of interest in physics. The geometric
construction is illustrated through application to polar decomposition, and to
the composition of Lorentz boosts and the resulting Wigner or Thomas rotation.


title: Quantum interference and Bayes theorem

abstract:
  This note is a comment on the "quantum interferometry" section of Reference
[1]. It points out that the methods of that section can be applied to more
general states than the ones that are discussed in Ref. [1].


title: The quantum measurement problem as a witness to "It from bit"

abstract:
  A conceptual difficulty in the foundations of quantum mechanics is the
quantum measurement problem (QMP), essentially concerned with the apparent
non-unitarity of the measurement process and the classicality of macroscopic
systems. In an information theoretic approach proposed by us earlier (Quantum
Information Processing 2, 153, 2003), which we clarify and elaborate here, QMP
is understood to signal a fundamental finite resolution of quantum states, or,
equivalently, a discreteness of Hilbert space. This was motivated by the notion
that physical reality is a manifestation of information stored and discrete
computations performed at a deeper, sub-physical layer. This model entails that
states of sufficiently complex, entangled systems will be unresolvable, or,
{\em computationally unstable}. Wavefunction collapse is postulated as an error
preventive response to such computational instability. In effect, sufficiently
complex systems turn classical because of the finiteness of the computational
resources available to the physical universe. We show that this model forms a
reasonable complement to decoherence for resolving QMP, both in respect of the
problem of definite outcomes and of the preferred basis problem. The model
suggests that QMP, as a window on the sub-physical universe, serves as a
witness to Wheeler's koan ``it from bit''. Some implications for quantum
computation and quantum gravity are examined.


title:
Dynamics of Open Bosonic Quantum Systems in Coherent State
  Representation

abstract:
  We consider the problem of decoherence and relaxation of open bosonic quantum
systems from a perspective alternative to the standard master equation or
quantum trajectories approaches. Our method is based on the dynamics of
expectation values of observables evaluated in a coherent state representation.
We examine a model of a quantum nonlinear oscillator with a density-density
interaction with a collection of environmental oscillators at finite
temperature. We derive the exact solution for dynamics of observables and
demonstrate a consistent perturbation approach.


title: Spectrum of spontaneous emission into the mode of a cavity QED system

abstract:
  We study the probe spectrum of light generated by spontaneous emission into
the mode of a cavity QED system. The probe spectrum has a maximum on-resonance
when the number of inverted atoms for an input drive is maximal. For a larger
number of atoms N, the maximum splits and develops into a doublet, but its
frequencies are different from those of the so-called vacuum Rabi splitting.


title: Black holes are almost optimal quantum cloners

abstract:
  If black holes were able to clone quantum states, a number of paradoxes in
black hole physics would disappear. However, the linearity of quantum mechanics
forbids exact cloning of quantum states. Here we show that black holes indeed
clone incoming quantum states with a fidelity that depends on the black hole's
absorption coefficient, without violating the no-cloning theorem because the
clones are only approximate. Perfectly reflecting black holes are optimal
universal "quantum cloning machines" and operate on the principle of stimulated
emission, exactly as their quantum optical counterparts. In the limit of
perfect absorption, the fidelity of clones is equal to what can be obtained via
quantum state estimation methods. But for any absorption probability less than
one, the cloning fidelity is nearly optimal as long as $\omega/T\geq10$, a
common parameter for modest-sized black holes.


title: Noise thresholds for optical cluster-state quantum computation

abstract:
  In this paper we do a detailed numerical investigation of the fault-tolerant
threshold for optical cluster-state quantum computation. Our noise model allows
both photon loss and depolarizing noise, as a general proxy for all types of
local noise other than photon loss noise. We obtain a threshold region of
allowed pairs of values for the two types of noise. Roughly speaking, our
results show that scalable optical quantum computing is possible for photon
loss probabilities less than 0.003, and for depolarization probabilities less
than 0.0001. Our fault-tolerant protocol involves a number of innovations,
including a method for syndrome extraction known as telecorrection, whereby
repeated syndrome measurements are guaranteed to agree. This paper is an
extended version of [Dawson et al., Phys. Rev. Lett. 96, 020501].


title:
Optimizing single-photon-source heralding efficiency at 1550 nm using
  periodically poled lithium niobate

abstract:
  We explore the feasibility of using high conversion-efficiency
periodically-poled crystals to produce photon pairs for photon-counting
detector calibrations at 1550 nm. The goal is the development of an appropriate
parametric down-conversion (PDC) source at telecom wavelengths meeting the
requirements of high-efficiency pair production and collection in single
spectral and spatial modes (single-mode fibers). We propose a protocol to
optimize the photon collection, noise levels and the uncertainty evaluation.
This study ties together the results of our efforts to model the single-mode
heralding efficiency of a two-photon PDC source and to estimate the heralding
uncertainty of such a source.


title: Experimental implementation of time-coding quantum key distribution

abstract:
  We have implemented an experimental set-up in order to demonstrate the
feasibility of time-coding protocols for quantum key distribution. Alice
produces coherent 20 ns faint pulses of light at 853 nm. They are sent to Bob
with delay 0 ns (encoding bit 0) or 10 ns (encoding bit 1). Bob directs at
random the received pulses to two different arms. In the first one, a 300 ps
resolution Si photon-counter allows Bob to precisely measure the detection
times of each photon in order to establish the key. Comparing them with the
emission times of the pulses sent by Alice allows to evaluate the quantum bit
error rate (QBER). The minimum obtained QBER is 1.62 %. The possible loss of
coherence in the set-up can be exploited by Eve to eavesdrop the line.
Therefore, the second arm of Bob set-up is a Mach-Zender interferometer with a
10 ns propagation delay between the two path. Contrast measurement of the
output beams allows to measure the autocorrelation function of the received
pulses that characterizes their average coherence. In the case of an ideal
set-up, the value expected with the pulses sent by Alice is 0.576. The
experimental value of the pulses autocorrelation function is found to be 0.541.
Knowing the resulting loss of coherence and the measured QBER, one can evaluate
the mutual information between Alice and Eve and the mutual information between
Alice and Bob, in the case of intercept-resend attacks and in the case of
attacks with intrication. With our values, Bob has an advantage on Eve of 0.43
bit per pulse. The maximum possible QBER corresponding to equal informations
for Bob and Eve is 5.8 %. With the usual attenuation of fibres at 850 nm, it
shows that secure key distribution is possible up to a distance of 2.75 km,
which is sufficient for local links.


title: Potential Barrier Classification by Short-Time Measurement

abstract:
  We investigate the short-time dynamics of a delta-function potential barrier
on an initially confined wave-packet. There are mainly two conclusions: A) At
short times the probability density of the first particles that passed through
the barrier is unaffected by it. B) When the barrier is absorptive (i.e., its
potential is imaginary) it affects the transmitted wave function at shorter
times than a real potential barrier. Therefore, it is possible to distinguish
between an imaginary and a real potential barrier by measuring its effect at
short times only on the transmitting wavefunction.


title: Dissipative Stern-Gerlach recombination experiment

abstract:
  The possibility of obtaining the initial pure state in a usual Stern-Gerlach
experiment through the recombination of the two emerging beams is investigated.
We have extended the previous work of Englert, Schwinger and Scully \cite{ISG1}
including the fluctuations of the magnetic field generated by a properly chosen
magnet. As a result we obtained an attenuation factor to the possible revival
of coherence when the beams are perfectly recombined . When the source of the
magnetic field is a SQUID(superconducting quantum interference device) the
attenuation factor can be controlled by external circuits and the spin
decoherence directly measured. For the proposed SQUID with dimensions in the
scale of microns the attenuation factor has been shown unimportant when
compared with the interaction time of the spin with the magnet.


title: A relation between completely bounded norms and conjugate channels

abstract:
  We show a relation between a quantum channel $\Phi$ and its conjugate
$\Phi^C$, which implies that the $p\to p$ Schatten norm of the channel is the
same as the $1\to p$ completely bounded norm of the conjugate. This relation is
used to give an alternative proof of the multiplicativity of both norms.


title:
Phase sensitive adiabatic states, or does the material phase recognize
  the physical reality?

abstract:
  Phase sensitive adiabatic states for a quantum system interacting with an
electromagnetic field have been derived taking into account all material phase
factors of the initial bare states. The adiabatic states so obtained show a
traceable phase behavior, causally depending on the initial conditions and the
relevant physical processes. Experimental appearance of the material phase
effects has been discussed.


title:
Does only the amplitude of the state vector carry the physical
  information?

abstract:
  We present theoretical and experimental evidences, which show that the
material phase of the state vector is causally related with the dynamic of the
quantum system and becomes carrier of physical information.