title: Two atoms in an anisotropic harmonic trap

abstract:
  We consider the system of two interacting atoms confined in axially symmetric
harmonic trap. Within the pseudopotential approximation, we solve the
Schroedinger equation exactly, discussing the limits of quasi-one and
quasi-two-dimensional geometries. Finally, we discuss the application of an
energy-dependent pseudopotential, which allows to extend the validity of our
results to the case of tight traps and large scattering lengths.


title: Observation of the Vacuum-Rabi Spectrum for One Trapped Atom

abstract:
  The transmission spectrum for one atom strongly coupled to the field of a
high-finesse optical resonator is observed to exhibit a clearly resolved
vacuum-Rabi splitting characteristic of the normal modes in the eigenvalue
spectrum of the atom-cavity system. A new Raman scheme for cooling atomic
motion along the cavity axis enables a complete spectrum to be recorded for an
individual atom trapped within the cavity mode, in contrast to all previous
measurements in cavity QED that have required averaging over many atoms.


title:
Finite set of invariants to characterize local Clifford equivalence of
  stabilizer states

abstract:
  The classification of stabilizer states under local Clifford (LC) equivalence
is of particular importance in quantum error-correction and measurement-based
quantum computation. Two stabilizer states are called LC equivalent if there
exists a local Clifford operation which maps the first state to the second. We
present a finite set of invariants which completely characterizes the LC
equivalence class of any stabilizer state. Our invariants have simple
descriptions within the binary framework in which stabilizer states are usually
described.


title: Equation of state for Entanglement in a Fermi gas

abstract:
  Entanglement distance is the maximal separation between two entangled
electrons in a degenerate electron gas. Beyond that distance, all entanglement
disappears. We relate entanglement distance to degeneracy pressure both for
extreme relativistic and non-relativistic systems, and estimate the
entanglement distance in a white dwarf. Treating entanglement as a
thermodynamical quantity, we relate the entropy of formation and concurrence to
relative electron distance, pressure, and temperature, to form a new equation
of state for entanglement.


title:
Dealing with entanglement of continuous variables: Schmidt decomposition
  with discrete sets of orthogonal functions

abstract:
  We propose a method for obtaining the Schmidt decomposition of bipartite
systems with continuous variables. It approximates the modes to the prescribed
accuracy by well known orthogonal functions. We give some criteria for the
control of errors. We illustrate the method comparing its results with the
already published analysis for entanglement of biphotons. The agreement is
excellent.


title: External-Feedback Laser Cooling of Gases

abstract:
  We analyze the laser cooling of polarizable particles by continuous
dispersive position detection and active feedback. The magnitude of the
dissipative force is proportional to the particles' photon scattering rate into
the detector, while its velocity dependence is determined by the programmable
frequency dependence of the loop gain. The method combines final temperatures
near the recoil limit with large velocity capture range, and is applicable to
multilevel atoms or molecules.


title: Efficient universal programmable quantum measurements

abstract:
  A universal programmable detector is a device that can be tuned to perform
any desired measurement on a given quantum system, by changing the state of an
ancilla. With a finite dimension d for the ancilla only approximate universal
programmability is possible, with "size" d=f(1/e) increasing function of the
"accuracy" 1/e. In this letter we show that, much better than the exponential
size known in the literature, one can achieve polynomial size. An explicit
example with linear size is also presented. Finally, we show that for covariant
measurements exact programmability is feasible.


title: On the efficiency of nonlocal gates generation

abstract:
  We propose and study a method for using non-maximally entangled states to
implement probabilistically non-local gates. Unlike distillation-based
protocols, this method does not generate a maximally entangled state at
intermediate stages of the process. As a consequences, the method becomes more
efficient at a certain range of parameters. Gates of the form
$\exp[i\xi\sigma_{n_A}\sigma_{n_B}]$ with $\xi\ll1$, can be implemented with
nearly unit probability and with vanishingly small entanglement, while for the
distillation-based method the gate is produced with a vanishing success
probability. We also derive an upper bound to the optimal success probability
and show that in the small entanglement limit, the bound is tight.


title:
Understanding light quanta: First quantization of the free
  electromagnetic field

abstract:
  The quantization of the electromagnetic field in vacuum is presented without
reference to lagrangean quantum field theory. The equal time commutators of the
fields are calculated from basic principles. A physical discussion of the
commutators suggest that the electromagnetic fields are macroscopic emergent
properties of more fundamental physical system: the photons.


title: Alice falls into a black hole: Entanglement in non-inertial frames

abstract:
  Two observers determine the entanglement between two free bosonic modes by
each detecting one of the modes and observing the correlations between their
measurements. We show that a state which is maximally entangled in an inertial
frame becomes less entangled if the observers are relatively accelerated. This
phenomenon, which is a consequence of the Unruh effect, shows that entanglement
is an observer-dependent quantity in non-inertial frames. In the high
acceleration limit, our results can be applied to a non-accelerated observer
falling into a black hole while the accelerated one barely escapes. If the
observer escapes with infinite acceleration, the state's distillable
entanglement vanishes.


title:
Linear Canonical Transformations and the Hamilton-Jacobi Theory in
  Quantum Mechanics

abstract:
  We investigate two methods of constructing a solution of the Schr\"{o}dinger
equation from the canonical transformation in classical mechanics. One method
shows that we can formulate the solution of the Schr\"{o}dinger equation from
linear canonical transformations, the other focuses on the generating function
which satisfies the Hamilton-Jacobi equation in classical mechanics. We also
show that these two methods lead to the same solution of the Schr\"{o}dinger
equation.


title:
Exact supersymmetry in the relativistic hydrogen atom in general
  dimensions -- supercharge and the generalized Johnson-Lippmann operator

abstract:
  A Dirac particle in general dimensions moving in a 1/r potential is shown to
have an exact N = 2 supersymmetry, for which the two supercharge operators are
obtained in terms of (a D-dimensional generalization of) the Johnson-Lippmann
operator, an extension of the Runge-Lenz-Pauli vector that relativistically
incorporates spin degrees of freedom. So the extra symmetry (S(2))in the
quantum Kepler problem, which determines the degeneracy of the levels, is so
robust as to accommodate the relativistic case in arbitrary dimensions.


title: Efficient and perfect state transfer in quantum chains

abstract:
  We present a communication protocol for chains of permanently coupled qubits
which achieves perfect quantum state transfer and which is efficient with
respect to the number chains employed in the scheme. The system consists of $M$
uncoupled identical quantum chains. Local control (gates, measurements) is only
allowed at the sending/receiving end of the chains. Under a quite general
hypothesis on the interaction Hamiltonian of the qubits a theorem is proved
which shows that the receiver is able to asymptotically recover the messages by
repetitive monitoring of his qubits.


title: Bosonic Memory Channels

abstract:
  We discuss a Bosonic channel model with memory effects. It relies on a
multi-mode squeezed (entangled) environment's state. The case of lossy Bosonic
channels is analyzed in detail. We show that in the absence of input energy
constraints the memory channels are equivalent to their memoryless
counterparts. In the case of input energy constraint we provide lower and upper
bounds for the memory channel capacity.


title: Phase-sensitive detection of Bragg scattering at 1D optical lattices

abstract:
  We report on the observation of Bragg scattering at 1D atomic lattices. Cold
atoms are confined by optical dipole forces at the antinodes of a standing wave
generated by the two counter-propagating modes of a laser-driven high-finesse
ring cavity. By heterodyning the Bragg-scattered light with a reference beam,
we obtain detailed information on phase shifts imparted by the Bragg scattering
process. Being deep in the Lamb-Dicke regime, the scattered light is not
broadened by the motion of individual atoms. In contrast, we have detected
signatures of global translatory motion of the atomic grating.


title: Quasi exact solution of the Rabi Hamiltonian

abstract:
  A method is suggested to obtain the quasi exact solution of the Rabi
Hamiltonian. It is conceptually simple and can be easily extended to other
systems. The analytical expressions are obtained for eigenstates and
eigenvalues in terms of orthogonal polynomials.


title: Understanding light quanta: The Photon

abstract:
  An antisymmetric tensor, the photon tensor, is defined for the description of
the photon as a massless relativistic particle. The photon can be visualized as
an essentially two dimensional rotating object. The quantum mechanical
description of a single photon is presented and it is shown that it is wrong to
associate the quantum states of a photon with the macroscopic electromagnetic
fields. This work is part of a series devoted to the attempt to understand the
quantum of electromagnetic radiation, based on the assumption that the photons
are the primary ontology and that the electromagnetic fields are macroscopic
emergent properties of an ensemble of photons.


title: Quasi-exact-solution of the Generalized Exe Jahn-Teller Hamiltonian

abstract:
  We consider the solution of a generalized Exe Jahn-Teller Hamiltonian in the
context of quasi-exactly solvable spectral problems. This Hamiltonian is
expressed in terms of the generators of the osp(2,2) Lie algebra. Analytical
expressions are obtained for eigenstates and eigenvalues. The solutions lead to
a number of earlier results discussed in the literature. However, our approach
renders a new understanding of ``exact isolated'' solutions.


title: Spectra of Interacting Electrons in a Quantum Dot: Quasi-Exact Solution

abstract:
  We present a procedure to solve the Schroedinger equation of two interacting
electrons in a quantum dot in the presence of an external magnetic field within
the context of quasi-exactly-solvable spectral problems. We show that the
symmetries of the Hamiltonian can be recovered for specific values of the
magnetic field, which leads to an exact determination of the eigenvalues and
eigenfunctions. We show that the problem possesses a hidden sl_2-algebraic
structure.


title:
Lithium atom interferometer using laser diffraction : description and
  experiments

abstract:
  We have built and operated an atom interferometer of the Mach-Zehnder type.
The atomic wave is a supersonic beam of lithium seeded in argon and the mirrors
and beam-splitters for the atomic wave are based on elastic Bragg diffraction
on laser standing waves at 671 nm. We give here a detailed description of our
experimental setup and of the procedures used to align its components. We then
present experimental signals, exhibiting atomic interference effects with a
very high visibility, up to 84.5 %. We describe a series of experiments testing
the sensitivity of the fringe visibility to the main alignment defects and to
the magnetic field gradient.


title:
Berry phase in open quantum systems: a quantum Langevin equation
  approach

abstract:
  The evolution of a two level system with a slowly varying Hamiltonian,
modeled as s spin 1/2 in a slowly varying magnetic field, and interacting with
a quantum environment, modeled as a bath of harmonic oscillators is analyzed
using a quantum Langevin approach. This allows to easily obtain the dissipation
time and the correction to the Berry phase in the case of an adiabatic cyclic
evolution.


title: A new quantum ripple-carry addition circuit

abstract:
  We present a new linear-depth ripple-carry quantum addition circuit. Previous
addition circuits required linearly many ancillary qubits; our new adder uses
only a single ancillary qubit. Also, our circuit has lower depth and fewer
gates than previous ripple-carry adders.


title: Improving teleportation of continuous variables by local operations

abstract:
  We study a continuous-variable (CV) teleportation protocol based on a shared
entangled state produced by the quantum-nondemolition (QND) interaction of two
vacuum states. The scheme utilizes the QND interaction or an unbalanced beam
splitter in the Bell measurement. It is shown that in the non-unity gain regime
the signal transfer coefficient can be enhanced while the conditional variance
product remains preserved by applying appropriate local squeezing operation on
sender's part of the shared entangled state. In the unity gain regime it is
demonstrated that the fidelity of teleportation can be increased with the help
of the local squeezing operations on parts of the shared entangled state that
convert effectively our scheme to the standard CV teleportation scheme.
Further, it is proved analytically that such a choice of the local symplectic
operations minimizes the noise by which the mean number of photons in the input
state is increased during the teleportation. Finally, our analysis reveals that
the local symplectic operation on sender's side can be integrated into the Bell
measurement if the interaction constant of the interaction in the Bell
measurement can be adjusted properly.


title:
Amplification of Nonlocal Effects in Nonlinear Quantum Mechanics by
  Extreme Localization

abstract:
  Due to its connection to the diffeomorphism group, nonlinear quantum
mechanics may play an important role in quantum geometry. The Doebner-Goldin
nonlinearity (arising from representations of the diffeomorphism group)
amplifies nonlocal signaling effects under extreme localization, suggesting
that even if greatly suppressed at low energies, such effects may be
significant at the Planck scale. This offers new perspectives on Planck-scale
physics.


title: Universal Quantum Filter

abstract:
  Universal quantum filter (UQF) is introduced and proved to exist. Optical
realization of UQF is proposed in experiment.


title: A Cavity QED Implementation of Deutsch-Jozsa Algorithm

abstract:
  The Deutsch-Jozsa algorithm is a generalization of the Deutsch algorithm
which was the first algorithm written. We present schemes to implement the
Deutsch algorithm and the Deutsch-Jozsa algorithm via cavity QED.


title:
A proposal for the implementation of quantum gates with photonic-crystal
  coupled cavity waveguides

abstract:
  Quantum computers require technologies that offer both sufficient control
over coherent quantum phenomena and minimal spurious interactions with the
environment. We show, that photons confined to photonic crystals, and in
particular to highly efficient waveguides formed from linear chains of defects
doped with atoms can generate strong non-linear interactions which allow to
implement both single and two qubit quantum gates. The simplicity of the gate
switching mechanism, the experimental feasibility of fabricating two
dimensional photonic crystal structures and integrability of this device with
optoelectronics offers new interesting possibilities for optical quantum
information processing networks.


title: Remote state preparation using non-maximally entangled states

abstract:
  We present a scheme in which any pure qubit
$|\phi=\cos{\theta}|0+\sin{\theta}e^{i\varp hi}|1$ could be remotely prepared
by using minimum classical bits and the previously shared non-maximally
entangled states, on condition that the receiver holds the knowledge of
$\theta$. Several methods are available to check the trade-off between the
necessary entanglement resource and the achievable fidelity.


title: Feedback in quantum communication

abstract:
  In quantum communication feedback may be defined in a number of distinct
ways. An analysis of the effect feedback has on the rate information may be
communicated is given, and a number of results and conjectures are stated.


title:
On the effectiveness of the protocol creating the maximum entanglement
  of two charge-phase qubits by cavity field

abstract:
  We revisit the protocols to create maximally entangled states between two
Josephson junction (JJ) charge phase qubits coupled to a microwave field in a
cavity as a quantum data bus. We devote to analyze a novel mechanism of quantum
decoherence due to the adiabatic entanglement between qubits and the data bus,
the off-resonance microwave field. We show that even through the variable of
the data bus can be adiabatically eliminated, the entanglement between the
qubits and data bus remains and can decoher the superposition of two-particle
state. Fortunately we can construct a decoherence-free subspace of
two-dimension to against this adiabatic decoherence.To carry out the analytic
study for this decoherence problem, we develop Fr\H{o}hlich transformation to
re-derive the effective Hamiltonian of these system, which is equivalent to
that obtained from the adiabatic elimination approach .


title:
Bell's theorem and the experiments: Increasing empirical support to
  local realism

abstract:
  It is argued that local realism is a fundamental principle, which might be
rejected only if experiments clearly show that it is untenable. A critical
review is presented of the derivations of Bell's inequalities and the performed
experiments, with the conclusion that no valid, loophole-free, test exists of
local realism vs. quantum mechanics. It is pointed out that, without any
essential modification, quantum mechanics might be compatible with local
realism. This suggests that the principle may be respected by nature.


title: Nonlocality of two- and three-mode continuous variable systems

abstract:
  We address nonlocality of a class of fully inseparable three-mode Gaussian
states generated either by bilinear three-mode Hamiltonians or by a sequence of
bilinear two-mode Hamiltonians. Two different tests revealing strong
nonlocality are considered, in which the dichotomic Bell operator is
represented by displaced parity and by pseudospin operator respectively.
Three-mode states are also considered as a conditional source of two-mode non
Gaussian states, whose nonlocal properties are analyzed. We found that the non
Gaussian character of the conditional states allows violation of Bell's
inequalities (by parity and pseudospin tests) stronger than with a conventional
twin-beam state. However, the non Gaussian character is not sufficient to
reveal nonlocality thorough a dichotomized quadrature measurement strategy.


title: Additivity of minimal entropy output for a class of covariant channels

abstract:
  Additivity of minimal entropy output is proven for the class of quantum
channels $\Lambda_t (A):=t A^{T}+(1-t)\tau (A)$ in the parameter range
$-2/(d^2-2)\le t \le 1/(d+1)$.


title:
Solvable PT-symmetric model with a tunable interspersion of non-merging
  levels

abstract:
  We study the spectrum in such a PT-symmetric square well of a diameter L
where the "strength of the non-Hermiticity" is controlled by the two
parameters, viz., by an imaginary coupling ig and by the distance d of its
onset from the origin. We solve this problem and confirm that the spectrum is
discrete and real in a non-empty interval of g. Surprisingly, a specific
distinction between the bound states is found in their asymptotic
stability/instability with respect to an unlimited growth of g. In our model,
all of the low-lying levels remain asymptotically unstable at the small d and
finite L while only the stable levels survive for d near L or in the purely
imaginary well with infinite L. In between these two extremes, an unusual and
tunable, variable pattern of the interspersed "robust" and "fragile" subspectra
of the real levels is obtained.


title:
A novel variational approach for Quantum Field Theory: example of study
  of the ground state and phase transition in Nonlinear Sigma Model

abstract:
  We discuss a novel form of the variational approach in Quantum Field Theory
in which the trial quantum configuration is represented directly in terms of
relevant expectation values rather than, e.g., increasingly complicated
structure from Fock space. The quantum algebra imposes constraints on such
expectation values so that the variational problem is formulated here as an
optimization under constraints. As an example of application of such approach
we consider the study of ground state and critical properties in a variant of
nonlinear sigma model.


title: Simulation of the Burgers equation by NMR quantum information processing

abstract:
  We report on the implementation of Burgers equation as a type-II quantum
computation on an NMR quantum information processor. Since the flow field
evolving under the Burgers equation develops sharp features over time, this is
a better test of liquid state NMR implementations of type-II quantum computers
than the previous examples using the diffusion equation. In particular, we show
that Fourier approximations used in the encoding step are not the dominant
error. Small systematic errors in the collision operator accumulate and swamp
all other errors. We propose, and demonstrate, that the accumulation of this
error can be avoided to a large extent by replacing the single collision
operator with a set of operators with random errors and similar fidelities.
Experiments have been implemented on 16 two-qubit sites for eight successive
time steps for the Burgers equation.


title: Quantum Computing with Very Noisy Devices

abstract:
  In theory, quantum computers can efficiently simulate quantum physics, factor
large numbers and estimate integrals, thus solving otherwise intractable
computational problems. In practice, quantum computers must operate with noisy
devices called ``gates'' that tend to destroy the fragile quantum states needed
for computation. The goal of fault-tolerant quantum computing is to compute
accurately even when gates have a high probability of error each time they are
used. Here we give evidence that accurate quantum computing is possible with
error probabilities above 3% per gate, which is significantly higher than what
was previously thought possible. However, the resources required for computing
at such high error probabilities are excessive. Fortunately, they decrease
rapidly with decreasing error probabilities. If we had quantum resources
comparable to the considerable resources available in today's digital
computers, we could implement non-trivial quantum computations at error
probabilities as high as 1% per gate.


title:
Two cavity modes in a dissipative environment: cross decay rates and
  robust states

abstract:
  We investigate the role of the cross decay rates in a system composed by two
electromagnetic modes interacting with the same reservoir. Two feasible
experiments sensitive to the magnitudes and phases of these rates are
described. We show that if the cross decay rates are appreciable there are
states less exposed to decoherence and dissipation, and in limit situations a
decoherence free subspace appears.


title:
Jaynes-Cummings Model and a Non-Commutative "Geometry" : A Few Problems
  Noted

abstract:
  In this paper we point out that the Jaynes-Cummings model without taking a
renonance conditon gives a non-commutative version of the simple spin model
(including the parameters $x$, $y$ and $z$) treated by M. V. Berry. This model
is different from usual non-commutative ones because the x-y coordinates are
quantized, while the z coordinate is not.
  One of new and interesting points in our non-commutative model is that the
strings corresponding to Dirac ones in the Berry model exist only in states
containing the ground state (${\cal F}\times \{\ket{0}\} \cup \{\ket{0}\}\times
{\cal F}$), while for other excited states (${\cal F}\times {\cal F} \setminus
{\cal F}\times \{\ket{0}\} \cup \{\ket{0}\}\times {\cal F}$) they don't exist.
  It is probable that a non-commutative model makes singular objects (singular
points or singular lines or etc) in the corresponding classical model mild or
removes them partly.


title:
Dephasing representation: Employing the shadowing theorem to calculate
  quantum correlation functions

abstract:
  Due to the Heisenberg uncertainty principle, various classical systems
differing only on the scale smaller than Planck's cell correspond to the same
quantum system. This fact is used to find a unique semiclassical representation
without the Van Vleck determinant, applicable to a large class of correlation
functions expressible as quantum fidelity. As in the Feynman path integral
formulation of quantum mechanics, all contributing trajectories have the same
amplitude: that is why it is denoted the ``dephasing representation.'' By
relating the present approach to the problem of existence of true trajectories
near numerically-computed chaotic trajectories, the approximation is made
rigorous for any system in which the shadowing theorem holds. Numerical
implementation only requires computing actions along the unperturbed
trajectories and not finding the shadowing trajectories. While semiclassical
linear-response theory was used before in quasi-integrable and chaotic systems,
here its validity is justified in the most generic, mixed systems. Dephasing
representation appears to be a rare practical method to calculate quantum
correlation functions in nonuniversal regimes in many-dimensional systems where
exact quantum calculations are impossible.


title: Demonstrating quantum nonlocality without entanglement

abstract:
  We experimentally demonstrate the superior discrimination of separated,
unentangled two-qubit correlated states using nonlocal measurements, when
compared with measurements based on local operations and classical
communications. When predicted theoretically, this phenomenon was dubbed
"quantum nonlocality without entanglement." We characterize the performance of
the nonlocal, or joint, measurement with a payoff function, for which we
measure 0.72(2), compared with the maximum locally achievable value of 2/3 and
the overall optimal value of 0.75.


title:
Coherent state LOQC gates using simplified diagonal superposition
  resource states

abstract:
  In this paper we explore the possibility of fundamental tests for coherent
state optical quantum computing gates [T. C. Ralph, et. al, Phys. Rev. A
\textbf{68}, 042319 (2003)] using sophisticated but not unrealistic quantum
states. The major resource required in these gates are state diagonal to the
basis states. We use the recent observation that a squeezed single photon state
($\hat{S}(r) \ket{1}$) approximates well an odd superposition of coherent
states ($\ket{\alpha} - \ket{-\alpha}$) to address the diagonal resource
problem. The approximation only holds for relatively small $\alpha$ and hence
these gates cannot be used in a scaleable scheme. We explore the effects on
fidelities and probabilities in teleportation and a rotated Hadamard gate.


title:
Dephasing representation: Unified semiclassical framework for fidelity
  decay

abstract:
  This paper presents a unified semiclassical framework for five regimes of
quantum fidelity decay and conjectures a new universal regime. The theory is
based solely on the statistics of actions in the dephasing representation.
Counterintuitively, in this representation, all of the decay is due to
interference and none due to the decay of classical overlaps. Both rigorous and
numerical support of the theory is provided.


title:
Solution to the King's problem with observables being not mutually
  complementary

abstract:
  We investigate the King's problem of the measurement of operators $\vec{n}_k
\nobreak \cdot \nobreak \vec{\sigma} (k=1,2,3)$ instead of the three Cartesian
components $\sigma_x$, $\sigma_y$ and $\sigma_z$ of the spin operator
$\vec{\sigma}$. Here, $\vec{n}_k$ are three-dimensional real unit vectors. We
show the condition over three vectors $\vec{n}_k$ to ascertain the result for
measurement of any one of these operators.


title: Reexamination of optimal quantum state estimation of pure states

abstract:
  A direct derivation is given for the optimal mean fidelity of quantum state
estimation of a d-dimensional unknown pure state with its N copies given as
input, which was first obtained by M. Hayashi in terms of an infinite set of
covariant positive operator valued measures (POVM's) and by Bruss and
Macchiavello establishing a connection to optimal quantum cloning. An explicit
condition for POVM measurement operators for optimal estimators is obtained, by
which we construct optimal estimators with finite POVM using exact quadratures
on a hypersphere. These finite optimal estimators are not generally universal,
where universality means the fidelity is independent of input states. However,
any optimal estimator with finite POVM for M(>N) copies is universal if it is
used for N copies as input.


title: Decoherence and multipartite entanglement

abstract:
  We study the dynamics of multipartite entanglement under the influence of
decoherence. A suitable generalization of concurrence reveals distinct scaling
of the entanglement decay rate of GHZ and W states, for various environments.


title: Applications of Canonical Transformations

abstract:
  Canonical transformations are defined and discussed along with the
exponential, the coherent and the ultracoherent vectors. It is shown that the
single-mode and the $n$-mode squeezing operators are elements of the group of
canonical transformations. An application of canonical transformations is made,
in the context of open quantum systems, by studying the effect of squeezing of
the bath on the decoherence properties of the system. Two cases are analyzed.
In the first case the bath consists of a massless bosonic field with the bath
reference states being the squeezed vacuum states and squeezed thermal states
while in the second case a system consisting of a harmonic oscillator
interacting with a bath of harmonic oscillators is analyzed with the bath being
initially in a squeezed thermal state.


title:
Transfer of Nonclassical Properties from A Microscopic Superposition to
  Macroscopic Thermal States in The High Temperature Limit

abstract:
  We present several examples where prominent quantum properties are
transferred from a microscopic superposition to thermal states at high
temperatures. Our work is motivated by an analogy of Schrodinger's cat paradox,
where the state corresponding to the virtual cat is a mixed thermal state with
a large average photon number. Remarkably, quantum entanglement can be produced
between thermal states with nearly the maximum Bell-inequality violation even
when the temperatures of both modes approach infinity.


title: Cloning transformations in spin networks without external control

abstract:
  In this paper we present an approach to quantum cloning with unmodulated spin
networks. The cloner is realized by a proper design of the network and a choice
of the coupling between the qubits. We show that in the case of phase covariant
cloner the XY coupling gives the best results. In the 1->2 cloning we find that
the value for the fidelity of the optimal cloner is achieved, and values
comparable to the optimal ones in the general N->M case can be attained. If a
suitable set of network symmetries are satisfied, the output fidelity of the
clones does not depend on the specific choice of the graph. We show that spin
network cloning is robust against the presence of static imperfections.
Moreover, in the presence of noise, it outperforms the conventional approach.
In this case the fidelity exceeds the corresponding value obtained by quantum
gates even for a very small amount of noise. Furthermore we show how to use
this method to clone qutrits and qudits. By means of the Heisenberg coupling it
is also possible to implement the universal cloner although in this case the
fidelity is 10% off that of the optimal cloner.


title: Towards feedback control of entanglement

abstract:
  We provide a model to investigate feedback control of entanglement. It
consists of two distant (two-level) atoms which interact through a radiation
field and becomes entangled. We then show the possibility to stabilize such
entanglement against atomic decay by means of a feedback action.


title:
Conditioning two-party quantum teleportation within a three-party
  quantum channel

abstract:
  We consider an arbitrary continuous-variable three-party Gaussian quantum
state which is used to perform quantum teleportation of a pure Gaussian state
between two of the parties (Alice and Bob). In turn, the third party (Charlie)
can condition the process by means of local operations and classical
communication. We find the best measurement that Charlie can implement on his
own mode that preserves the Gaussian character of the three-mode state and
optimizes the teleportation fidelity between Alice and Bob.


title:
Raman-noise induced quantum limits for chi^3 nondegenerate
  phase-sensitive amplification and quadrature squeezing

abstract:
  We present a quantum theory of nondegenerate phase-sensitive parametric
amplification in a chi^3 nonlinear medium. The non-zero response time of the
Kerr chi^3 nonlinearity determines the quantum-limited noise figure of chi^3
parametric amplification, as well as the limit on quadrature squeezing. This
non-zero response time of the nonlinearity requires coupling of the parametric
process to a molecular-vibration phonon bath, causing the addition of excess
noise through spontaneous Raman scattering. We present analytical expressions
for the quantum-limited noise figure of frequency non-degenerate and frequency
degenerate chi^3 parametric amplifiers operated as phase-sensitive amplifiers.
We also present results for frequency non-degenerate quadrature squeezing. We
show that our non-degenerate squeezing theory agrees with the degenerate
squeezing theory of Boivin and Shapiro as degeneracy is approached. We have
also included the effect of linear loss on the phase-sensitive process.


title:
Lower and upper bounds on the secret key rate for QKD protocols using
  one--way classical communication

abstract:
  We investigate a general class of quantum key distribution (QKD) protocols
using one-way classical communication. We show that full security can be proven
by considering only collective attacks. We derive computable lower and upper
bounds on the secret key rate of those QKD protocol involving only entropies of
two--qubit density operators. As an illustration of our results, we determine
new bounds for the BB84, the six-state, and the B92 protocol. We show that in
all these cases the first classical processing that the legitimate partners
should apply consists in adding noise. This is precisely why any entanglement
based proof would generally fail here.


title: Two-photon coincident-frequency-entanglement via extended phase matching

abstract:
  We demonstrate a new class of frequency-entangled states generated via
spontaneous parametric down-conversion under extended phase matching
conditions. Biphoton entanglement with coincident signal and idler frequencies
is observed over a broad bandwidth in periodically poled KTiOPO$_4$. We
demonstrate high visibility in Hong-Ou-Mandel interferometric measurements
under pulsed pumping without spectral filtering, which indicates excellent
frequency indistinguishability between the down-converted photons. The
coincident-frequency entanglement source is useful for quantum information
processing and quantum measurement applications.


title: Two-photon interference with thermal light

abstract:
  The study of entangled states has greatly improved the basic understanding
about two-photon interferometry. Two-photon interference is not the
interference of two photons but the result of superposition among
indistinguishable two-photon amplitudes. The concept of two-photon amplitude,
however, has generally been restricted to the case of entangled photons. In
this letter we report an experimental study that may extend this concept to the
general case of independent photons. The experiment also shows interesting
practical applications regarding the possibility of obtaining high resolution
interference patterns with thermal sources.


title: Ultra-high-Q toroidal microresonators for cavity quantum electrodynamics

abstract:
  We investigate the suitability of toroidal microcavities for strong-coupling
cavity quantum electrodynamics (QED). Numerical modeling of the optical modes
demonstrate a significant reduction of modal volume with respect to the
whispering gallery modes of dielectric spheres, while retaining the high
quality factors representative of spherical cavities. The extra degree of
freedom of toroid microcavities can be used to achieve improved cavity QED
characteristics. Numerical results for atom-cavity coupling strength, critical
atom number N_0 and critical photon number n_0 for cesium are calculated and
shown to exceed values currently possible using Fabry-Perot cavities. Modeling
predicts coupling rates g/(2*pi) exceeding 700 MHz and critical atom numbers
approaching 10^{-7} in optimized structures. Furthermore, preliminary
experimental measurements of toroidal cavities at a wavelength of 852 nm
indicate that quality factors in excess of 100 million can be obtained in a 50
micron principal diameter cavity, which would result in strong coupling values
of (g/(2*pi),n_0,N_0)=(86 MHz,4.6*10^{-4},1.0*10^{-3}).


title:
In-Situ absolute phase detection of a microwave field via incoherent
  fluorescence

abstract:
  Measuring the amplitude and the absolute phase of a monochromatic microwave
field at a specific point of space and time has many potential applications,
including precise qubit rotations and wavelength quantum teleportation. Here we
show how such a measurement can indeed be made using resonant atomic probes,
via detection of incoherent fluorescence induced by a laser beam. This
measurement is possible due to self-interference effects between the positive
and negative frequency components of the field. In effect, the small cluster of
atoms here act as a highly localized pick-up coil, and the fluorescence channel
acts as a transmission line.


title: Physical implementation of entangling quantum measurements

abstract:
  We clarify the microscopic structure of the entangling quantum measurement
superoperators and examine their possible physical realization in a simple
three-qubit model, which implements the entangling quantum measurement with an
arbitrary degree of entanglement.


title: Reply to Comment on "Quantum dense key distribution"

abstract:
  In this Reply we propose a modified security proof of the Quantum Dense Key
Distribution protocol detecting also the eavesdropping attack proposed by
Wojcik in his Comment.


title: Collapse models: analysis of the free particle dynamics

abstract:
  We study a model of spontaneous wavefunction collapse for a free quantum
particle. We analyze in detail the time evolution of the single-Gaussian
solution and the double-Gaussian solution, showing how the reduction mechanism
induces the localization of the wavefunction in space; we also study the
asymptotic behavior of the general solution. With an appropriate choice for the
parameter $\lambda$ which sets the strength of the collapse mechanism, we prove
that: i) the effects of the reducing terms on the dynamics of microscopic
systems are negligible, the physical predictions of the model being very close
to those of standard quantum mechanics; ii) at the macroscopic scale, the model
reproduces classical mechanics: the wavefunction of the center of mass of a
macro-object behaves, with high accuracy, like a point moving in space
according to Newton's laws.


title:
Matter diffraction at oblique incidence: Higher resolution and the
  Helium Trimer Efimov state

abstract:
  We study the diffraction of atoms and weakly-bound three-atomic molecules
from a transmission grating at non-normal incidence. Due to the thickness of
the grating bars the slits are partially shadowed. Therefore, the projected
slit width decreases more strongly with the angle of incidence than the
projected period, increasing, in principle, the experimental resolution. The
shadowing, however, requires a revision of the theory of atom diffraction. We
derive an expression in the style of the Kirchhoff integral of optics and show
that the diffraction pattern exhibits a characteristic asymmetry which must be
accounted for when comparing with experimental data. We then analyze the
diffraction of weakly bound trimers and show that their finite size manifests
itself in a further reduction of the slit width by (3/4)<r> where <r> is the
average bond length. The improved resolution at non-normal incidence may in
particular allow to discern, by means of their bond lengths, between the small
ground state of the helium trimer (<r>=1 nm, Barletta and Kievsky, Phys. Rev. A
64, 042514 (2001)) and its predicted Efimov-type excited state (<r>=8 nm,
ibid.), and in this way to experimentally prove the existence of this
long-sought Efimov state.


title: Realization of the Optimal Universal Quantum Entangler

abstract:
  We present the first experimental demonstration of the ''optimal'' and
''universal'' quantum entangling process involving qubits encoded in the
polarization of single photons. The structure of the ''quantum entangling
machine'' consists of the quantum injected optical parametric amplifier by
which the contextual realization of the 1->2 universal quantum cloning and of
the universal NOT (U-NOT) gate has also been achieved.


title:
Realization of an Optimally Distinguishable Multi-photon Quantum
  Superposition

abstract:
  We report the successful generation of an entangled multiparticle quantum
superposition of pure photon states. They result from a multiple (universal}
cloning of a single photon qubit by a high gain, quantum-injected parametric
amplifier. The information preserving property of the process suggests for
these states the name of ''multi-particle qubits''. They are ideal objects for
investigating the emergence of the classical world in quantum systems with
increasing complexity, the decoherence processes and may allow the practical
implementation of the universal 2-qubit logic gates.


title: Deformed Bosons: Combinatorics of Normal Ordering

abstract:
  We solve the normal ordering problem for (A* A)^n where A* (resp. A) are one
mode deformed bosonic creation (resp. annihilation) operators satisfying
[A,A*]=[N+1]-[N]. The solution generalizes results known for canonical and
q-bosons. It involves combinatorial polynomials in the number operator N for
which the generating functions and explicit expressions are found. Simple
deformations provide examples of the method.


title: Renormalization group transformations on quantum states

abstract:
  We construct a general renormalization group transformation on quantum
states, independent of any Hamiltonian dynamics of the system. We illustrate
this procedure for translational invariant matrix product states in one
dimension and show that product, GHZ, W and domain wall states are special
cases of an emerging classification of the fixed points of this
coarse--graining transformation.


title: Results in Optimal Discrimination

abstract:
  We study the problem of discriminating between non-orthogonal quantum states
with least probability of error. We demonstrate that this problem can be
simplified if we solve for the error itself rather than solving directly for
the optimal measurement. This method enables us to derive solutions directly
and thus make definite statements about the uniqueness of an optimal strategy.
This approach immediately leads us to a state-discrimination analogue of Davies
Theorem.
  In the course of this, a complete solution for distinguishing equally likely
pure qubit states is presented.


title: A de Finetti representation for finite symmetric quantum states

abstract:
  Consider a symmetric quantum state on an n-fold product space, that is, the
state is invariant under permutations of the n subsystems. We show that,
conditioned on the outcomes of an informationally complete measurement applied
to a number of subsystems, the state in the remaining subsystems is close to
having product form. This immediately generalizes the so-called de Finetti
representation to the case of finite symmetric quantum states.


title: Nonlinear Quantum Mechanics at the Planck Scale

abstract:
  I argue that the linearity of quantum mechanics is an emergent feature at the
Planck scale, along with the manifold structure of space-time. In this regime
the usual causality violation objections to nonlinearity do not apply, and
nonlinear effects can be of comparable magnitude to the linear ones and still
be highly suppressed at low energies. This can offer alternative approaches to
quantum gravity and to the evolution of the early universe.


title: On the Temperature Dependence of the Casimir Effect

abstract:
  The temperature dependence of the Casimir force between a real metallic plate
and a metallic sphere is analyzed on the basis of optical data concerning the
dispersion relation of metals such as gold and copper. Realistic permittivities
imply, together with basic thermodynamic considerations, that the transverse
electric zero mode does not contribute. This results in observable differences
with the conventional prediction, which does not take this physical requirement
into account. The results are shown to be consistent with the third law of
thermodynamics, as well as being consistent with current experiments. However,
the predicted temperature dependence should be detectable in future
experiments. The inadequacies of approaches based on {\it ad hoc} assumptions,
such as the plasma dispersion relation and the use of surface impedance without
transverse momentum dependence, are discussed.


title: Single-qubit optical quantum fingerprinting

abstract:
  We analyze and demonstrate the feasibility and superiority of linear optical
single-qubit fingerprinting over its classical counterpart. For one-qubit
fingerprinting of two-bit messages, we prepare `tetrahedral' qubit states
experimentally and show that they meet the requirements for quantum
fingerprinting to exceed the classical capability. We prove that shared
entanglement permits 100% reliable quantum fingerprinting, which will
outperform classical fingerprinting even with arbitrary amounts of shared
randomness.


title: Comparison of Information Structures and Completely Positive Maps

abstract:
  A theorem of Blackwell about comparison between information structures in
classical statistics is given an analogue in the quantum probabilistic setup.
The theorem provides an operational interpretation for trace-preserving
completely positive maps, which are the natural quantum analogue of classical
stochastic maps. The proof of the theorem relies on the separation theorem for
convex sets and on quantum teleportation.


title:
Realization of GHZ States and the GHZ Test via Cavity QED for a Cavity
  Prepared in a Superposition of Zero and One Fock States

abstract:
  In this article we discuss the realization of atomic GHZ states involving
three-level atoms in a cascade and in a lambda configuration and we show
explicitly how to use this state to perform the GHZ test in which it is
possible to decide between local realism theories and quantum mechanics. The
experimental realizations proposed makes use the interaction of Rydberg atoms
with a cavity prepared in a state which is a superposition of zero and one Fock
states.


title:
Stabilization of three-dimensional matter-waves solitons in an optical
  lattice

abstract:
  We propose an experimentally relevant scheme to create stable solitons in a
three-dimensional Bose-Einstein condensate confined by a one-dimensional
optical lattice, using temporal modulation of the scattering length (through ac
magnetic field tuned close to the Feshbach resonance). Another physical
interpretation is a possibility to create stable 3D "light bullets" in an
optical medium with a longitudinal alternating self-focusing/defocusing
structure, and periodic modulation of the refractive index in a transverse
direction. We develop a variational approximation to identify a stability
region in the parametric space, and verify the existence of stable breathing
solitons in direct simulations. Both methods reveal that stable solitons may be
supported if the average value of the nonlinear coefficient (whose sign
corresponds to attraction between atoms) and the lattice's strength exceed
well-defined minimum values. Stable localized patterns may feature a multi-cell
structure.


title:
Classical Physics revisited: Derivation and explanation of the quantum
  mechanical superposition principle and Born's rule

abstract: The paper is withdrawn due to an error in Section 2.


title: Optimal finite measurements and Gauss quadratures

abstract:
  We exhibit measurements for optimal state estimation which have a finite
number of outcomes. This is achieved by a connection between finite optimal
measurements and Gauss quadratures. The example we consider to illustrate this
connection is that of state estimation on $N$ qubits, all in a same pure state.
Extensions to state estimation of mixed states are also discussed.


title: Qubit-Qutrit Separability-Probability Ratios

abstract:
  Paralleling our recent computationally-intensive (quasi-Monte Carlo) work for
the case N=4 (quant-ph/0308037), we undertake the task for N=6 of computing to
high numerical accuracy, the formulas of Sommers and Zyczkowski
(quant-ph/0304041) for the (N^2-1)-dimensional volume and (N^2-2)-dimensional
hyperarea of the (separable and nonseparable) N x N density matrices, based on
the Bures (minimal monotone) metric -- and also their analogous formulas
(quant-ph/0302197) for the (non-monotone) Hilbert-Schmidt metric. With the same
seven billion well-distributed (``low-discrepancy'') sample points, we estimate
the unknown volumes and hyperareas based on five additional (monotone) metrics
of interest, including the Kubo-Mori and Wigner-Yanase. Further, we estimate
all of these seven volume and seven hyperarea (unknown) quantities when
restricted to the separable density matrices. The ratios of separable volumes
(hyperareas) to separable plus nonseparable volumes (hyperareas) yield
estimates of the separability probabilities of generically rank-six (rank-five)
density matrices. The (rank-six) separability probabilities obtained based on
the 35-dimensional volumes appear to be -- independently of the metric (each of
the seven inducing Haar measure) employed -- twice as large as those (rank-five
ones) based on the 34-dimensional hyperareas. Accepting such a relationship, we
fit exact formulas to the estimates of the Bures and Hilbert-Schmidt separable
volumes and hyperareas.(An additional estimate -- 33.9982 -- of the ratio of
the rank-6 Hilbert-Schmidt separability probability to the rank-4 one is quite
clearly close to integral too.) The doubling relationship also appears to hold
for the N=4 case for the Hilbert-Schmidt metric, but not the others. We fit
exact formulas for the Hilbert-Schmidt separable volumes and hyperareas.


title:
Teleportation of Atomic States via Cavity QED for a Cavity Prepared in a
  Superposition of Zero and One Fock States

abstract:
  In this article we discuss two schemes of teleportation of atomic states. In
the first scheme we consider atoms in a three-level cascade configuration and
in the second scheme we consider atoms in a three-level lambda configuration.
The experimental realization proposed makes use of cavity Quatum
Electrodynamics involving the interaction of Rydberg atoms with a micromaser
cavity prepared in a state $|\psi >_{C}=(|0> +|1>)/\sqrt{2}$


title: Teleportation of Cavity Field States via Cavity QED

abstract:
  In this article we discuss two schemes of teleportation of cavity field
states. In the first scheme we consider cavities prepared in a coherent state
and in the second scheme we consider cavities prepared in a superposition of
zero and one Fock states.


title: Propagation of atomic matter waves inside an atom wave guide

abstract:
  The phenomenological description of propagation of atomic matter waves inside
a curved atom wave guide is presented based on the effective action principle.
The evolutions in both temporal and spatial domains for the atomic matter wave
in the presence of guiding potential fields (classical and quantized) are
considered. The coherent control of three-level atomic matter waves in a wave
guide by an external controlling light is briefly discussed. The concepts of
{\it atomic matter-wave bandgap structure} in a spatially periodic guiding
field ({\it e.g.}, the interior potential of carbon peapod which can trap both
atoms and light) and {\it optical lattice bandgap medium} are suggested.


title: The supersymmetric Pegg-Barnett oscillator

abstract:
  The su$(n)$ Lie algebraic structure of the Pegg-Barnett oscillator that
possesses a finite-dimensional number-state space is demonstrated. The
supersymmetric generalization of Pegg-Barnett oscillator is suggested. It is
shown that such a supersymmetric Pegg-Barnett oscillator may have some
potential applications, {\it e.g.}, the mass spectrum of the charged leptons.


title: Slow transport by continuous time quantum walks

abstract:
  Continuous time quantum walks (CTQW) do not necessarily perform better than
their classical counterparts, the continuous time random walks (CTRW). For one
special graph, where a recent analysis showed that in a particular direction of
propagation the penetration of the graph is faster by CTQWs than by CTRWs, we
demonstrate that in another direction of propagation the opposite is true; In
this case a CTQW initially localized at one site displays a slow transport. We
furthermore show that when the CTQW's initial condition is a totally symmetric
superposition of states of equivalent sites, the transport gets to be much more
rapid.


title: Experimental Realization of a Photonic Bell-State Analyzer

abstract:
  Efficient teleportation is a crucial step for quantum computation and quantum
networking. In the case of qubits, four different entangled Bell states have to
be distinguished. We have realized a probabilistic, but in principle
deterministic, Bell-state analyzer for two photonic quantum bits by the use of
a nondestructive controlled-NOT gate based on entirely linear optical elements.
This gate was capable of distinguishing between all of the Bell states with
higher than 75% fidelity without any noise substraction due to utilizing
quantum interference effects.


title:
On the relative quantum entanglement with respect to tensor product
  structure

abstract:
  Mathematical foundation of the novel concept of quantum tensor product by
Zanardi et al is rigorously established. The concept of relative quantum
entanglement is naturally introduced and its meaning is made clear both
mathematically and physically. For a finite or an infinite dimensional vector
space $W$ the so called tensor product partition (TPP) is introduced on
$End(W)$, the set of endmorphisms of $W$, and a natural correspondence is
constructed between the set of TPP's of $End(W)$ and the set of tensor product
structures (TPS's) of $W$. As a byproduct, it is shown that an arbitrarily
given wave function belonging to an n-dimensional Hilbert space, $n$ being not
a prime number, can be interpreted as a separable state with respect to some
man-made TPS, and thus a quantum entangled state of a many-body system with
respect to the "God-given" TPS can be regarded as a quantum state without
entanglement in some sense. The concept of standard set of observables is also
introduced to probe the underlying structure of the object TPP and to establish
its connection with practical physical measurement.


title: Entanglement Across a Transition to Quantum Chaos

abstract:
  We study the relation between entanglement and quantum chaos in one- and
two-dimensional spin-1/2 lattice models, which exhibit mixing of the
noninteracting eigenfunctions and transition from integrability to quantum
chaos. Contrary to what occurs in a quantum phase transition, the onset of
quantum chaos is not a property of the ground state but take place for any
typical many-spin quantum state. We study bipartite and pairwise entanglement
measures, namely the reduced Von Neumann entropy and the concurrence, and
discuss quantum entanglement sharing. Our results suggest that the behavior of
the entanglement is related to the mixing of the eigenfunctions rather than to
the transition to chaos.


title:
Distinguishability, contrast and complementarity in multimode
  two-particle interferences

abstract:
  Multimode two-particle systems show interference effects in one-particle
detections when both particles have common modes. We explore the possibility of
extending the usual concepts of distinguishability and visibility to these
types of systems. Distinguishability will refer now to the balance between
common and different modes of a two-particle system, instead of the standard
definition concerning available alternatives for a one-particle system. On the
other hand, the usual concept of visibility is not suitable for our problem and
must be replaced with that of contrast, measuring the ratio of detection
probabilities with and without interference effects. Finally, we show that for
the type of states considered in the paper there is a complementarity relation
between distinguishability and contrast for two-bosons systems. In contrast,
there is no two-fermion counterpart.


title: Electromagnetic field quantization in a linear dielectric medium

abstract:
  By modeling a dielectric medium with two independent reservoirs, i.e.,
electric and magnetic reservoirs, the electromagnetic field is quantized in a
linear dielectric medium consistently. A Hamiltonian is proposed from which
using the Heisenberg equations, not only the Maxwell equations but also the
structural equations can be obtained. Using the Laplace transformation, the
wave equation for the electromagnetic vector potential is solved in the case of
a homogeneous dielectric medium. Some examples are considered showing the
applicability of the model to both absorptive and nonabsorptive dielectrics.


title:
The controlled indirect coupling between spatially-separated qubits in
  antiferromagnet-based NMR quantum registers

abstract:
  It is considered the indirect inter-qubit coupling in 1D chain of atoms with
nuclear spins 1/2, which plays role of qubits in the quantum register. This
chain of the atoms is placed by regular way in easy-axis 3D antiferromagnetic
thin plate substrate, which is cleaned from the other nuclear spin containing
isotopes. It is shown that the range of indirect inter-spin coupling may run to
a great number of lattice constants both near critical point of quantum phase
transition in antiferromagnet of spin-flop type (control parameter is external
magnetic field) and/or near homogeneous antiferromagnetic resonance (control
parameter is microwave frequency).


title: Decoy State Quantum Key Distribution

abstract:
  There has been much interest in quantum key distribution. Experimentally,
quantum key distribution over 150 km of commercial Telecom fibers has been
successfully performed. The crucial issue in quantum key distribution is its
security. Unfortunately, all recent experiments are, in principle, insecure due
to real-life imperfections. Here, we propose a method that can for the first
time make most of those experiments secure by using essentially the same
hardware. Our method is to use decoy states to detect eavesdropping attacks. As
a consequence, we have the best of both worlds--enjoying unconditional security
guaranteed by the fundamental laws of physics and yet dramatically surpassing
even some of the best experimental performances reported in the literature.


title: A physical basis for the phase in Feynman path integration

abstract:
  In the path integral formulation of quantum mechanics, the phase factor
Exp[iS(x[t])] is associated with every path x[t]. Summing this factor over all
paths yields Feynman's propagator as a sum-over-paths. In the original
formulation, the complex phase was a mathematical device invoked to extract
wave behaviour in a particle framework. In this paper we show that the phase
itself can have a physical origin in time reversal, and that the propagator can
be drawn by a single deterministic path.


title: Topological properties of Berry's phase

abstract:
  By using a second quantized formulation of level crossing, which does not
assume adiabatic approximation, a convenient formula for geometric terms
including off-diagonal terms is derived. The analysis of geometric phases is
reduced to a simple diagonalization of the Hamiltonian in the present
formulation. If one diagonalizes the geometric terms in the infinitesimal
neighborhood of level crossing, the geometric phases become trivial for any
finite time interval $T$. The topological interpretation of Berry's phase such
as the topological proof of phase-change rule thus fails in the practical
Born-Oppenheimer approximation, where a large but finite ratio of two time
scales is involved.


title: Decoherence of a two-state atom driven by coherent light

abstract:
  Recent studies of the decoherence induced by the quantum nature of the laser
field driving a two-state atom [J. Gea-Banacloche, Phys. Rev. A 65, 022308
(2002); S. J. van Enk and H. J. Kimble, Quantum Inf. and Comp. 2, 1 (2002)]
have been questioned by Itano [W. M. Itano, Phys. Rev. A 68, 046301 (2003)] and
the proposal made that all decoherence is due to spontaneous emission. We
analyze the problem within the formalism of cascaded open quantum systems. Our
conclusions agree with the Itano proposal. We show that the decoherence,
nevertheless, may be divided into two parts--that due to forwards scattering
and to scattering out of the laser mode. Previous authors attribute the former
to the quantum nature of the laser field.


title: Quantum error correction fails for Hamiltonian models

abstract:
  It is argued that the existing schemes of fault-tolerant quantum computation
designed for discrete-time models and based on quantum error correction fail
for continuous-time Hamiltonian models even with Markovian noise.


title:
Molecular effects in the ionization of N$_2$, O$_2$ and F$_2$ by intense
  laser fields

abstract:
  In this paper we study the response in time of N$_2$, O$_2$ and F$_2$ to
laser pulses having a wavelength of 390nm. We find single ionization
suppression in O$_2$ and its absence in F$_2$, in accordance with experimental
results at $\lambda = 800$nm. Within our framework of time-dependent density
functional theory we are able to explain deviations from the predictions of
Intense-Field Many-Body $S$-Matrix Theory (IMST). We confirm the connection of
ionization suppression with destructive interference of outgoing electron waves
from the ionized electron orbital. However, the prediction of ionization
suppression, justified within the IMST approach through the symmetry of the
highest occupied molecular orbital (HOMO), is not reliable since it turns out
that, e.g. in the case of F$_2$, the electronic response to the laser pulse is
rather complicated and does not lead to dominant depletion of the HOMO.
Therefore, the symmetry of the HOMO is not sufficient to predict ionization
suppression. However, at least for F$_2$, the symmetry of the dominantly
ionized orbital is consistent with the non-suppression of ionization.


title:
State preparation based on Grover's algorithm in the presence of global
  information about the state

abstract:
  In a previous paper [quant-ph/0408045] we described a quantum algorithm to
prepare an arbitrary state of a quantum register with arbitrary fidelity. Here
we present an alternative algorithm which uses a small number of quantum
oracles encoding the most significant bits of the absolute value of the complex
amplitudes, and a small number of oracles encoding the most significant bits of
the phases. The algorithm given here is considerably simpler than the one
described in [quant-ph/0408045], on the assumption that a sufficient amount of
knowledge about the distribution of the absolute values of the complex
amplitudes is available.


title: Efficient tests for experimental quantum gates

abstract:
  Realistic quantum gates operate at non-vanishing noise levels. Therefore, it
is necessary to evaluate the performance of each device according to some
experimentally observable criteria of device performance. In this presentation,
the characteristic properties of quantum operations are discussed and efficient
measurement strategies are proposed.


title: Transporting, splitting and merging of atomic ensembles in a chip trap

abstract:
  We present a toolbox for cold atom manipulation with time-dependent magnetic
fields generated by an atom chip. Wire layouts, detailed experimental
procedures and results are presented for the following experiments: Use of a
magnetic conveyor belt for positioning of cold atoms and Bose-Einstein
condensates with a resolution of two nanometers; splitting of thermal clouds
and BECs in adjustable magnetic double well potentials; controlled splitting of
a cold reservoir. The devices that enable these manipulations can be combined
with each other. We demonstrate this by combining reservoir splitter and
conveyor belt to obtain a cold atom dispenser. We discuss the importance of
these devices for quantum information processing, atom interferometry and
Josephson junction physics on the chip. For all devices, absorption-image video
sequences are provided to demonstrate their time-dependent behaviour.


title: Towards Efficiently Solving Quantum Traveling Salesman Problem

abstract:
  We present a framework for efficiently solving Approximate Traveling Salesman
Problem (Approximate TSP) for Quantum Computing Models. Existing
representations of TSP introduce extra states which do not correspond to any
permutation. We present an efficient and intuitive encoding for TSP in quantum
computing paradigm. Using this representation and assuming a Gaussian
distribution on tour-lengths, we give an algorithm to solve Approximate TSP
(Euclidean) within BQP resource bounds. Generalizing this strategy for any
distribution, we present an oracle based Quantum Algorithm to solve Approximate
TSP. We present a realization of the oracle in the quantum counterpart of PP.


title:
Upper and lower bounds on the mean square radius and criteria for
  occurrence of quantum halo states

abstract:
  In the context of non-relativistic quantum mechanics, we obtain several upper
and lower limits on the mean square radius applicable to systems composed by
two-body bound by a central potential. A lower limit on the mean square radius
is used to obtain a simple criteria for the occurrence of S-wave quantum halo
sates.


title:
Storage and perpendicular retrieving of two-dimensional pulses in
  electromagnetically induced transparency media

abstract:
  Propagation of two dimensional pulses in electromagnetically induced
tranparency media in the case of perpendicular storing and retrieving pulses
has been analyzed. It has been shown that propagation control of the pulses in
optically thick media can be used for producing interchange between pulse
time-shape and intensity profile distribution. A simple obvious analytical
solution for the retrieved new field has been obtained.


title: Information Entropy and Correlation of the Hooke's Atom

abstract:
  We provide an algebraic procedure to find the eigenstates of two-charged
particles in an oscillator potential, known as {\it{Hooke's}} atom. For the
planar Hooke's atom, the exact eigenstates and single particle densities for
arbitrary azimuthal quantum number, are obtained analytically. Information
entropies associated with the wave functions for the relative motion are then
studied systematically, since the same incorporates the effect of the Coulomb
interaction. The {\it{quantum pottery}} of the information entropy density
reveals a number of intricate structures, which differ significantly for the
attractive and repulsive cases. We indicate the procedure to obtain the
approximate eigen states. Making use of the relationship of this dynamical
system with the quasi-exactly solvable systems, one can also develop a suitable
perturbation theory, involving the Coulomb coupling $Z$, for the approximate
wave functions.


title:
Harmonic Oscillators as Bridges between Theories: Einstein, Dirac, and
  Feynman

abstract:
  Other than scattering problems where perturbation theory is applicable, there
are basically two ways to solve problems in physics. One is to reduce the
problem to harmonic oscillators, and the other is to formulate the problem in
terms of two-by-two matrices. If two oscillators are coupled, the problem
combines both two-by-two matrices and harmonic oscillators. This method then
becomes a powerful research tool to cover many different branches of physics.
Indeed, the concept and methodology in one branch of physics can be translated
into another through the common mathematical formalism. Coupled oscillators
provide clear illustrative examples for some of the current issues in physics,
including entanglement, decoherence, and Feynman's rest of the universe. In
addition, it is noted that the present form of quantum mechanics is largely a
physics of harmonic oscillators. Special relativity is the physics of the
Lorentz group which can be represented by the group of by two-by-two matrices
commonly called $SL(2,c)$. Thus the coupled harmonic oscillators can therefore
play the role of combining quantum mechanics with special relativity. Both Paul
A. M. Dirac and Richard P. Feynman were fond of harmonic oscillators, while
they used different approaches to physical problems. Both were also keenly
interested in making quantum mechanics compatible with special relativity. It
is shown that the coupled harmonic oscillators can bridge these two different
approaches to physics.


title: Work extraction in the spin-boson model

abstract:
  We show that work can be extracted from a two-level system (spin) coupled to
a bosonic thermal bath. This is possible due to different initial temperatures
of the spin and the bath, both positive (no spin population inversion) and is
realized by means of a suitable sequence of sharp pulses applied to the spin.
The extracted work can be of the order of the response energy of the bath,
therefore much larger than the energy of the spin. Moreover, the efficiency of
extraction can be very close to its maximum, given by the Carnot bound, at the
same time the overall amount of the extracted work is maximal. Therefore, we
get a finite power at efficiency close to the Carnot bound.
  The effect comes from the backreaction of the spin on the bath, and it
survives for a strongly disordered (inhomogeneously broadened) ensemble of
spins. It is connected with generation of coherences during the work-extraction
process, and we derived it in an exactly solvable model. All the necessary
general thermodynamical relations are derived from the first principles of
quantum mechanics and connections are made with processes of lasing without
inversion and with quantum heat engines.


title:
Consequence for Wavefunction Collapse Model of the Sudbury Neutrino
  Observatory Experiment

abstract:
  It is shown that data on the dissociation rate of deuterium obtained in an
experiment at the Sudbury Neutrino Observatory provides evidence that the
Continuous Spontaneous Localization wavefunction collapse model should have
mass--proportional coupling to be viable.


title: Perfect Transfer of Arbitrary States in Quantum Spin Networks

abstract:
  We propose a class of qubit networks that admit perfect state transfer of any
two-dimensional quantum state in a fixed period of time. We further show that
such networks can distribute arbitrary entangled states between two distant
parties, and can, by using such systems in parallel, transmit the higher
dimensional systems states across the network. Unlike many other schemes for
quantum computation and communication, these networks do not require qubit
couplings to be switched on and off. When restricted to $N$-qubit spin networks
of identical qubit couplings, we show that $2\log_3 N$ is the maximal perfect
communication distance for hypercube geometries. Moreover, if one allows fixed
but different couplings between the qubits then perfect state transfer can be
achieved over arbitrarily long distances in a linear chain. This paper expands
and extends the work done in PRL 92, 187902.


title: Theory of multiwave mixing and decoherence control in qubit array system

abstract:
  We develop a theory to analyze the decoherence effect in a charged qubit
array system with photon echo signals in the multiwave mixing configuration. We
present how the decoherence suppression effect by the {\it bang-bang} control
with the $\pi$ pulses can be demonstrated in laboratory by using a bulk
ensemble of exciton qubits and optical pulses whose pulse area is even smaller
than $\pi$. Analysis is made on the time-integated multiwave mixing signals
diffracted into certain phase matching directions from a bulk ensemble.
Depending on the pulse interval conditions, the cross over from the decoherence
acceleration regime to the decoherence suppression regime, which is a peculiar
feature of the coherent interaction between a qubit and the reservoir bosons,
may be observed in the time-integated multiwave mixing signals in the realistic
case including inhomogeneous broadening effect. Our analysis will successfully
be applied to precise estimation of the reservoir parameters from experimental
data of the direction resolved signal intensities obtained in the multiwave
mixing technique.


title: Towards practical and fast Quantum Cryptography

abstract:
  We present a new protocol for practical quantum cryptography, tailored for an
implementation with weak coherent pulses. The key is obtained by a very simple
time-of-arrival measurement on the data line; an interferometer is built on an
additional monitoring line, allowing to monitor the presence of a spy (who
would break coherence by her intervention). Against zero-error attacks (the
analog of photon-number-splitting attacks), this protocol performs as well as
standard protocols with strong reference pulses: the key rate decreases only as
the transmission $t$ of the quantum channel. We present also two attacks that
introduce errors on the monitoring line: the intercept-resend, and a coherent
attack on two subsequent pulses. Finally, we sketch several possible variations
of this protocol.


title:
Nonlinear interactions with an ultrahigh flux of broadband entangled
  photons

abstract:
  We experimentally demonstrate sum-frequency generation (SFG) with entangled
photon-pairs, generating as many as 40,000 SFG photons per second, visible even
to the naked eye. The nonclassical nature of the interaction is exhibited by a
linear intensity-dependence of the nonlinear process. The key element in our
scheme is the generation of an ultrahigh flux of entangled photons while
maintaining their nonclassical properties. This is made possible by generating
the down-converted photons as broadband as possible, orders of magnitude wider
than the pump. This approach is readily applicable for other nonlinear
interactions, and may be applicable for various quantum-measurement tasks.


title: Quantum probability applied to the damped harmonic oscillator

abstract:
  In this introductory course we sketch the framework of quantum probability in
order to discuss open quantum systems, in particular the damped harmonic
oscillator.


title: Efficiency of Ground State Quantum Computer

abstract:
  The energy gap is calculated for the ground state quantum computer circuit,
which was recently proposed by Mizel et.al. When implementing a quantum
algorithm by Hamiltonians containing only pairwise interaction, the inverse of
energy gap $1/\Delta$ is proportional to $N^{4k}$, where $N$ is the number of
bits involved in the problem, and $N^k$ is the number of control operations
performed in a standard quantum paradigm. Besides suppressing decoherence due
to the energy gap, in polynomial time ground state quantum computer can finish
the quantum algorithms that are supposed to be implemented by standard quantum
computer in polynomial time.


title: The Strong Levinson Theorem for the Dirac Equation

abstract:
  We consider the Dirac equation in one space dimension in the presence of a
symmetric potential well. We connect the scattering phase shifts at E=+m and
E=-m to the number of states that have left the positive energy continuum or
joined the negative energy continuum respectively as the potential is turned on
from zero.


title:
Qubiter Algorithm Modification, Expressing Unstructured Unitary Matrices
  with Fewer CNOTs

abstract:
  A quantum compiler is a software program for decomposing ("compiling") an
arbitrary unitary matrix into a sequence of elementary operations (SEO). The
author of this paper is also the author of a quantum compiler called Qubiter.
Qubiter uses a matrix decomposition called the Cosine-Sine Decomposition (CSD)
that is well known in the field of Computational Linear Algebra. One way of
measuring the efficiency of a quantum compiler is to measure the number of
CNOTs it uses to express an unstructured unitary matrix (a unitary matrix with
no special symmetries). We will henceforth refer to this number as $\epsilon$.
In this paper, we show how to improve $\epsilon$ for Qubiter so that it matches
the current world record for $\epsilon$, which is held by another quantum
compiling algorithm based on CSD.


title: Bounds on integrals of the Wigner function: the hyperbolic case

abstract:
  Wigner functions play a central role in the phase space formulation of
quantum mechanics. Although closely related to classical Liouville densities,
Wigner functions are not positive definite and may take negative values on
subregions of phase space. We investigate the accumulation of these negative
values by studying bounds on the integral of an arbitrary Wigner function over
noncompact subregions of the phase plane with hyperbolic boundaries. We show
using symmetry techniques that this problem reduces to computing the bounds on
the spectrum associated with an exactly-solvable eigenvalue problem and that
the bounds differ from those on classical Liouville distributions. In
particular, we show that the total ``quasiprobability'' on such a region can be
greater than 1 or less than zero.


title:
A high-yield single photon source using gated spontaneous parametric
  down conversion

abstract:
  The construction of a single photon source using gated parametric
fluorescence is reported with the measurement results of the photon number
distribution. A beamlike twin-photon method is used in order to achieve high
collection efficiency. The estimated probability P(1) to find a single photon
in a collimated output pulse is 26.5 % at a repetition rate of 10 kHz when the
effective quantum efficiency of 27.4 % in the detection setup is compensated.


title:
Improved W\'{o}jcik's Eavesdropping Attack on Ping-Pong Protocol Without
  Eavesdropping-Induced Channel Loss

abstract:
  The eavesdropping scheme proposed by W\'{o}jcik [Phys. Rev. Lett. {\bf
90},157901(2003)] on the ping-pong protocol [Phys. Rev. Lett. {\bf 89},
187902(2002)] is improved by constituting a new set of attack operations. The
improved scheme has a zero eavesdropping-induced channel loss and produces
perfect anticorrelation. Therefore, the eavesdropper Eve can safely attack all
the transmitted bits and the eavesdropping information gain can always exceed
the legitimate user's information gain in the whole domain of the quantum
channel transmission efficiency $\eta$, i.e., [0,100%]. This means that the
ping-pong protocol can be completely eavesdropped in its original version. But
the improvement of the ping-pong protocol security produced by W\'{o}jcik is
also suitable for our eavesdropping attack.


title: The Casimir Effect Upon A Single Plate

abstract:
  In the presence of an external field, the imposition of specific boundary
conditions can lead to interesting new manifestations of the Casimir effect. In
particular, it is shown here that even a single conducting plate may experience
a non-zero force due to vacuum fluctuations. The origins of this force lie in
the change induced by the external potential in the density of available
quantum states.


title: Quantifying ground state entanglement of the BCS model

abstract:
  The concept of local concurrence is used to quantify the entanglement between
a single qubit and the remainder of a multi-qubit system. For the ground state
of the BCS model in the thermodynamic limit the set of local concurrences
completely describe the entanglement. As a measure for the entanglement of the
full system we investigate the Average Local Concurrence (ALC). We find that
the ALC satisfies a simple relation with the order parameter. We then show that
for finite systems with fixed particle number, a relation between the ALC and
the condensation energy exposes a threshold coupling. Below the threshold,
entanglement measures besides the ALC are significant.


title:
Vacuum field correlations and three-body Casimir-Polder potential with
  one excited atom

abstract:
  The three-body Casimir-Polder potential between one excited and two
ground-state atoms is evaluated. A physical model based on the dressed field
correlations of vacuum fluctuations is used, generalizing a model previously
introduced for three ground-state atoms. Although the three-body potential with
one excited atom is already known in the literature, our model gives new
insights on the nature of non-additive Casimir-Polder forces with one or more
excited atoms.


title:
From Fischer projections to quantum mechanics of tetrahedral molecules:
  new perspectives in chirality

abstract:
  The algebraic structure of central molecular chirality can be achieved
starting from the geometrical representation of bonds of tetrahedral molecules,
as complex numbers in polar form, and the empirical Fischer projections used in
organic chemistry. A general orthogonal O(4) algebra is derived from which we
obtain a chirality index related to the classification of a molecule as
achiral, diastereoisomer or enantiomer. Consequently, the chiral features of
tetrahedral chains can be predicted by means of a molecular Aufbau. Moreover, a
consistent Schroedinger equation is developed, whose solutions are the bonds of
tetrahedral molecules in complex number representation. Starting from this
result, the O(4) algebra can be considered as a quantum chiral algebra. It is
shown that the operators of such an algebra preserve the parity of the whole
system.


title: $k$-decomposability of positive maps

abstract:
  The problem of classification of decomposable (in the sense of Stormer)
positive maps between matrix algebras is presented. We propose the new notion
of "finite" version of decomposability ($k$-decomposabilty). The
characterisation of $k$-decomposability on the Hilbert space level is done. In
the case of low dimensional algebras the notion of local decomposability and
its applications for the description of decomposable maps are discussed.


title: Improved magic states distillation for quantum universality

abstract:
  Given stabilizer operations and the ability to repeatedly prepare a
single-qubit mixed state rho, can we do universal quantum computation? As
motivation for this question, "magic state" distillation procedures can reduce
the general fault-tolerance problem to that of performing fault-tolerant
stabilizer circuits.
  We improve the procedures of Bravyi and Kitaev in the Hadamard "magic"
direction of the Bloch sphere to achieve a sharp threshold between those rho
allowing universal quantum computation, and those for which any calculation can
be efficiently classically simulated. As a corollary, the ability to repeatedly
prepare any pure state which is not a stabilizer state (e.g., any single-qubit
pure state which is not a Pauli eigenstate), together with stabilizer
operations, gives quantum universality. It remains open whether there is also a
tight separation in the so-called T direction.


title: The Hidden Subgroup Problem - Review and Open Problems

abstract:
  An overview of quantum computing and in particular the Hidden Subgroup
Problem are presented from a mathematical viewpoint. Detailed proofs are
supplied for many important results from the literature, and notation is
unified, making it easier to absorb the background necessary to begin research
on the Hidden Subgroup Problem. Proofs are provided which give very concrete
algorithms and bounds for the finite abelian case with little outside
references, and future directions are provided for the nonabelian case. This
summary is current as of October 2004.


title: Read out of a Nuclear Spin Qubit

abstract:
  We propose a detector to read out the state of a single nuclear spin, with
potential application in future scalable NMR quantum computers. It is based on
a ``spin valve'' between bulk nuclear spin systems that is highly sensitive to
the state of the measured spin. We suggest a concrete realization of that
detector in a Silicon lattice. Transport of spin through the proposed spin
valve is analogous to that of charge through an electronic nanostructure, but
exhibits distinctive new features.


title: What was in the apparatus before the click

abstract:
  When a quantum system is described by a wave function consisting in a couple
of wave-packets, each wave packet traveling on a separate path, a commonly
asked question is why at a given time only one of the wave packets is able to
trigger a click in a detector. In the second half of the last century many
scientists considered the possibility that not all these wave-packets are
identical. Namely, that there exist full waves and empty waves. The two types
of waves were supposed to be identical in all the experiments, e.g. able to
produce interference when meeting, however, the full wave was supposed to be
able to trigger a detector, while the empty wave was supposed to leave the
detector silent. The present text describes an experiment which, for explaining
the results, seems to beg the hypothesis of full and empty waves.


title:
Quantum entanglement as an interpretation of bosonic character in
  composite two-particle systems

abstract:
  We consider a composite particle formed by two fermions or two bosons. We
discover that composite behavior is deeply related to the quantum entanglement
between the constituent particles. By analyzing the properties of creation and
annihilation operators, we show that bosonic character emerges if the
constituent particles become strongly entangled. Such a connection is
demonstrated explicitly in a class of two-particle wave functions.


title:
Intercept-resend attacks in the Bennett-Brassard 1984 quantum key
  distribution protocol with weak coherent pulses

abstract:
  Unconditional security proofs of the Bennett-Brassard protocol of quantum key
distribution have been obtained recently. These proofs cover also practical
implementations that utilize weak coherent pulses in the four signal
polarizations. Proven secure rates leave open the possibility that new proofs
or new public discussion protocols obtain larger rates over increased distance.
In this paper we investigate limits to error rate and signal losses that can be
tolerated by future protocols and proofs.


title: Error threshold estimation by means of the [[7,1,3]] CSS quantum code

abstract:
  The states needed in a quantum computation are extremely affected by
decoherence. Several methods have been proposed to control error spreading.
They use two main tools: fault-tolerant constructions and concatenated quantum
error correcting codes. In this work, we estimate the threshold conditions
necessary to make a long enough quantum computation. The [[7,1,3]] CSS quantum
code, together with the Shor method to measure the error syndrome is used. No
concatenation is included. The decoherence is introduced by means of the
depolarizing channel error model, obtaining several thresholds from the
numerical simulation. Regarding the maintenance of a qubit stabilized in the
memory, the error probability must be smaller than 2.9 10-5. In order to
implement a one or two qubit encoded gate in an effective fault-tolerant way,
it is possible to choose an adequate non-encoded noisy gate if the memory error
probability is smaller than 1.3 10-5. In addition, fulfilling this last
condition permits us to assume a more efficient behaviour compared to the
equivalent non-encoded process.


title:
Positive maps, states, entanglement and all that; some old and new
  problems

abstract:
  We outline a new approach to the characterization as well as to the
classification of positive maps. This approach is based on the facial
structures of the set of states and of the cone of positive maps. In
particular, the equivalence between Schroedinger's and Heisenberg's pictures is
reviewed in this more general setting. Furthermore, we discuss in detail the
structure of positive maps for two and three dimensional systems. In
particular, the explicit form of decomposition of a positive map and the
uniqueness of this decomposition for extremal positive maps for 2 dimensional
case are described. The difference of the structure of positive maps between 2
dimensional and 3 dimensional cases is clarified. The resulting
characterization of positive maps is applied to the study of quantum
correlations and entanglement


title: Berry's phase at quantum vacuum level

abstract:
  The quantum vacuum contribution to Berry's geometric phase of photon fields
inside a noncoplanarly curved (coiled) fiber is considered by means of the
second-quantization formulation. It is shown that the quantum vacuum Berry's
phases of left- and right-handed circularly polarized light have the equal
magnitudes but opposite signs, and are therefore eliminated entirely by each
other. In order to realize such a novel vacuum effect, a scheme to separate the
quantum vacuum Berry's phase of one polarized light from another by using the
chiral medium fiber is suggested. We think the present study might be the first
treatment for the time evolution of vacuum zero-point energy.


title:
Intrinsic decoherence effects on tripartite GHZ state generation using a
  trapped ion coupled to an optical cavity

abstract:
  We analyse the effects of intrinsic decoherence on the probability of
generating a tripartite GHZ state using a cool trapped ion coupled to a single
mode of the cavity field and interacting with a resonant laser field. Milburn
equation is solved for this tripartite system to obtain the time evolution of
density matrix as a function of cavity-ion coupling, laser-ion coupling, and
the size of unitary time step relative to the time scale determined by system
parameters. Starting with the system prepared initially in a separable state,
density matrix is used to calculate the probability of tripartite GHZ state
generation using coupling strengths reported in a recent experiment.


title:
Efficient multiple-quantum transition processes in an n-qubit spin
  system

abstract:
  The whole Hilbert state space of an n-qubit spin system can be divided into
(n+1) state subspaces according to the angular momentum theory of quantum
mechanics. Here it is shown that any unknown state in such a state subspace,
whose dimensional size is proportional to either a polynomial or exponential
function of the qubit number n, can be transferred efficiently into a larger
subspace with a dimensional size generally proportional to an exponential
function of the qubit number by the multiple-quantum unitary transformation
with a subspace-selective multiple-quantum unitary operator. The efficient
quantum circuits for the subspace-selective multiple-quantum unitary operators
are really constructed.


title:
A decoy-state protocol for quantum cryptography with 4 intensities of
  coherent states

abstract:
  In order to beat any type of photon-number-splitting attack, we propose a
protocol for quantum key distributoin (QKD) using 4 different intensities of
pulses. They are vacuum and coherent states with mean photon number $\mu,\mu'$
and $\mu_s$. $\mu_s$ is around 0.55 and this class of pulses are used as the
main signal states. The other two classes of coherent states ($\mu,\mu'$) are
also used signal states but their counting rates should be studied jointly with
the vacuum. We have shown that, given the typical set-up in practice, the key
rate from the main signal pulses is quite close to the theoretically allowed
maximal rate in the case given the small overall transmittance of $10^{-4}$.


title: Experimental creation of entanglement using separable state

abstract:
  We experimentally demonstrate the entanglement can be created on two distant
particles using separate state. We show that two data particles can share some
entanglement while one ancilla particle always remains separable from them
during the experimental evolution of the system. Our experiment can be viewed
as a benchmark to illustrate the idea that no prior entanglement is necessary
to create entanglement.


title:
Quantum Phase Transition of Ground-state Entanglement in a Heisenberg
  Spin Chain Simulated in an NMR Quantum Computer

abstract:
  Using an NMR quantum computer, we experimentally simulate the quantum phase
transition of a Heisenberg spin chain. The Hamiltonian is generated by a
multiple pulse sequence, the nuclear spin system is prepared in its
(pseudo-pure) ground state and the effective Hamiltonian varied in such a way
that the Heisenberg chain is taken from a product state to an entangled state
and finally to a different product state.


title: The Status of the Wave Function in Dynamical Collapse Models

abstract:
  The idea that in dynamical wave function collapse models the wave function is
superfluous is investigated. Evidence is presented for the conjecture that, in
a model of a field theory on a 1+1 lightcone lattice, knowing the field
configuration on the lattice back to some time in the past, allows the wave
function or quantum state at the present moment to be calculated, to arbitrary
accuracy so long as enough of the past field configuration is known.


title: Quantum Communication Cannot Simulate a Public Coin

abstract:
  We study the simultaneous message passing model of communication complexity.
Building on the quantum fingerprinting protocol of Buhrman et al., Yao recently
showed that a large class of efficient classical public-coin protocols can be
turned into efficient quantum protocols without public coin. This raises the
question whether this can be done always, i.e. whether quantum communication
can always replace a public coin in the SMP model. We answer this question in
the negative, exhibiting a communication problem where classical communication
with public coin is exponentially more efficient than quantum communication.
Together with a separation in the other direction due to Bar-Yossef et al.,
this shows that the quantum SMP model is incomparable with the classical
public-coin SMP model.
  In addition we give a characterization of the power of quantum fingerprinting
by means of a connection to geometrical tools from machine learning, a
quadratic improvement of Yao's simulation, and a nearly tight analysis of the
Hamming distance problem from Yao's paper.


title:
Negative group delay for Dirac particles traveling through a potential
  well

abstract:
  The properties of group delay for Dirac particles traveling through a
potential well are investigated. A necessary condition is put forward for the
group delay to be negative. It is shown that this negative group delay is
closely related to its anomalous dependence on the width of the potential well.
In order to demonstrate the validity of stationary-phase approach, numerical
simulations are made for Gaussian-shaped temporal wave packets. A restriction
to the potential-well's width is obtained that is necessary for the wave packet
to remain distortionless in the travelling. Numerical comparison shows that the
relativistic group delay is larger than its corresponding non-relativistic one.


title:
Stochastic Collapse and Decoherence of a Non-Dissipative Forced Harmonic
  Oscillator

abstract:
  Careful monitoring of harmonically bound (or as a limiting case, free) masses
is the basis of current and future gravitational wave detectors, and of
nanomechanical devices designed to access the quantum regime. We analyze the
effects of stochastic localization models for state vector reduction, and of
related models for environmental decoherence, on such systems, focusing our
analysis on the non-dissipative forced harmonic oscillator, and its free mass
limit. We derive an explicit formula for the time evolution of the expectation
of a general operator in the presence of stochastic reduction or
environmentally induced decoherence, for both the non-dissipative harmonic
oscillator and the free mass. In the case of the oscillator, we also give a
formula for the time evolution of the matrix element of the stochastic
expectation density matrix between general coherent states. We show that the
stochastic expectation of the variance of a Hermitian operator in any
unraveling of the stochastic process is bounded by the variance computed from
the stochastic expectation of the density matrix, and we develop a formal
perturbation theory for calculating expectation values of operators within any
unraveling. Applying our results to current gravitational wave interferometer
detectors and nanomechanical systems, we conclude that the deviations from
quantum mechanics predicted by the continuous spontaneous localization (CSL)
model of state vector reduction are at least five orders of magnitude below the
relevant standard quantum limits for these experiments. The proposed LISA
gravitational wave detector will be two orders of magnitude away from the
capability of observing an effect.