title: The electrodynamic 2-body problem and the origin of quantum mechanics

abstract:
  We numerically solve the functional differential equations (FDE's) of
2-particle electrodynamics, using the full electrodynamic force obtained from
the retarded Lienard-Wiechert potentials and the Lorentz force law. In
contrast, the usual formulation uses only the Coulomb force (scalar potential),
reducing the electrodynamic 2-body problem to a system of ordinary differential
equations (ODE's). The ODE formulation is mathematically suspect since FDE's
and ODE's are known to be incompatible; however, the Coulomb approximation to
the full electrodynamic force has been believed to be adequate for physics. We
can now test this long-standing belief by comparing the FDE solution with the
ODE solution, in the historically interesting case of the classical hydrogen
atom. The solutions differ.
  A key qualitative difference is that the full force involves a `delay'
torque. Our existing code is inadequate to calculate the detailed interaction
of the delay torque with radiative damping. However, a symbolic calculation
provides conditions under which the delay torque approximately balances (3rd
order) radiative damping. Thus, further investigations are required, and it was
prematurely concluded that radiative damping makes the classical hydrogen atom
unstable. Solutions of FDE's naturally exhibit an_infinite_ spectrum of
_discrete_ frequencies. The conclusion is that (a) the Coulomb force is_not_ a
valid approximation to the full electrodynamic force, so that (b) the n-body
interaction needs to be reformulated in various current contexts such as
molecular dynamics.


title:
Non-Markovian Quantum State Diffusion: Application to Quantum Jumps in
  $^{24}$Mg$^+$

abstract:
  Non-Markovian quantum state diffusion (NMQSD) is an exact method for
calculating the reduced density matrix of an arbitrary subsystem interacting
linearly with the radiation field. Applications of the theory have however been
few due to the intractable nature of the variational-differential NMQSD
evolution equation. Recently, we argued that the variational-differential
equation can be rewritten as an integrodifferential equation which can be
readily solved numerically. This manuscript provides an explicit derivation of
the modified equations. Applications to intermittent fluorescence in
$^{24}$Mg$^+$ are discussed in detail. Earlier speculations that quantum jumps
occur on all time scales are verified on a picosecond timescale. We show that a
plot of the probability density of the signal vs signal strength shows the two
characteristic peaks associated with the bright and dark manifolds, and that
the ratio of the areas under the peaks is 16 as observed experimentally. We
also show that the shape of this distribution is sensitive to bath memory, but
has a mathematical form common to both the Markovian and non-Markovian cases.


title:
Detection of multipartite entanglement in the vicinity of symmetric
  Dicke states

abstract:
  We present methods for detecting entanglement around symmetric Dicke states.
In particular, we consider N-qubit symmetric Dicke states with N/2 excitations.
In the first part of the paper we show that for large N these states have the
smallest overlap possible with states without genuine multi-partite
entanglement. Thus these states are particulary well suited for the
experimental examination of multi-partite entanglement. We present
fidelity-based entanglement witness operators for detecting multipartite
entanglement around these states. In the second part of the paper we consider
entanglement criteria, somewhat similar to the spin squeezing criterion, based
on the moments or variances of the collective spin operators. Surprisingly,
these criteria are based on an upper bound for variances for separable states.
We present both criteria detecting entanglement in general and criteria
detecting only genuine multi-partite entanglement. The collective operator
measured for our criteria is an important physical quantity: Its expectation
value essentially gives the intensity of the radiation when a coherent atomic
cloud emits light.


title: Exact Propagators for Soliton Potentials

abstract:
  Using the method of Darboux transformations (or equivalently supersymmetric
quantum mechanics) we obtain an explicit expression for the propagator for the
one-dimensional Schr\"odinger equation with a multi-soliton potential.


title: Squeezing at 946nm with periodically-poled KTiOPO_4

abstract:
  We report generation of squeezed vacuum in sideband modes of continuous-wave
light at 946 nm using a periodically poled KTiOPO_4 crystal in an optical
parametric oscillator. At the pump power of 250 mW, we observe the squeezing
level of -5.6+/-0.1 dB and the anti-squeezing level of +12.7+/-0.1 dB. The pump
power dependence of the observed squeezing/anti-squeezing levels agrees with
the theoretically calculated values when the phase fluctuation of locking is
taken into account.


title: Can entanglement efficiently be weakend by symmetrization?

abstract:
  Consider a quantum system with $m$ subsystems with $n$ qubits each, and
suppose the state of the system is living in the symmetric subspace. It is
known that, in the limit of $m\to\infty$, entanglement between any two
subsystems vanishes.
  In this paper we study asymptotic behavior of the entanglement as $m$ and $n$
grows. Our conjecture is that if $m$ is a polynomially bounded function in $n$,
then the entanglement decreases polynomially.
  The motivation of this study is a study of quantum Merlin-Arthur game. If
this conjecture is ture, we can prove that bipartite separable certificate does
not increase the computational power of the proof system. protocol.
  In the paper, we provide two evidences which support the conjecture. First,
if $m$ is an exponential function, then entanglement decreases exponentially
fast. Second, in case of a maximally entangled state, our conjecture is true.


title:
Method for Solving the Bloch Equation from the Connection with
  Time-Dependent Oscillator

abstract:
  We introduce a novel method to find exact density operators for a spin-1/2
particle in time-dependent magnetic fields by using the one-mode bosonic
representation of $su(2)$ and the connection with a time-dependent oscillator.
As illustrative examples, we apply the method to find the density operators for
constant and/or oscillating magnetic fields, which turn out to be
time-dependent in general.


title:
Quantum Non-Demolition Bell State Measurement and N-party GHZ State
  Preparation in Quantum Dot

abstract:
  By exploiting the fermionic qubit parity measurement, we present a scheme to
realize quantum non-demolition (QND) measurement of Bell-states and generate
n-party GHZ state in quantum dot. Compared with the original protocol, the
required electron transfer before and after parity measurement can be
nonadiabatic, which may speed up the operation speed and make the omitting of
spin-orbit interaction more reasonable. This may help us to construct CNOT gate
without highly precise control of coupling as the way of D. Gottesman and I. L.
Chuang.


title: Entanglement and quantum phase transitions

abstract:
  We examine several well known quantum spin models and categorize behavior of
pairwise entanglement at quantum phase transitions. A unified picture on the
connection between the entanglement and quantum phase transition is given.


title: On partially entanglement breaking channels

abstract:
  Using well known duality between quantum maps and states of composite systems
we introduce the notion of Schmidt number of a quantum channel. It enables one
to define classes of quantum channels which partially break quantum
entanglement. These classes generalize the well known class of entanglement
breaking channels.


title: Elementary proof of the bound on the speed of quantum evolution

abstract: An elementary proof is given of the bound on "ortogonalization time".


title: Controlling quantum state transfer in spin chain with the confined field

abstract:
  As a demonstration of the spectrum-parity matching condition (SPMC) for
quantum state transfer, we investigate the propagation of single-magnon state
in the Heisenberg chain in the confined external tangent magnetic field
analytically and numerically. It shows that the initial Gaussian wave packet
can be retrieved at the counterpart location near-perfectly over a longer
distance if the dispersion relation of the system meets the SPMC approximately.


title: A classical analogue of negative information

abstract:
  Recently, it was discovered that the `quantum partial information' needed to
merge one party's state with another party's state is given by the conditional
entropy, which can be negative [Horodecki, Oppenheim, and Winter, Nature 436,
673 (2005)]. Here we find a classical analogue of this, based on a long known
relationship between entanglement and shared private correlations: namely, we
consider a private distribution held between two parties, and correlated to a
reference system, and ask how much secret communication is needed for one party
to send her distribution to the other. We give optimal protocols for this task,
and find that private information can be negative - the sender's distribution
can be transferred and the potential to send future distributions in secret is
gained through the distillation of a secret key. An analogue of `quantum state
exchange' is also discussed and one finds cases where exchanging a distribution
costs less than for one party to send it. The results give new classical
protocols, and also clarify the various relationships between entanglement and
privacy.


title:
A theory of electromagnetic fluctuations for metallic surfaces and van
  der Waals interactions between metallic bodies

abstract:
  A new general expression is derived for the fluctuating electromagnetic field
outside a metal surface, in terms of its surface impedance. It provides a
generalization to real metals of Lifshitz theory of molecular interactions
between dielectric solids. The theory is used to compute the radiative heat
transfer between two parallel metal surfaces at different temperatures. It is
shown that a measurement of this quantity may provide an experimental
resolution of a long-standing controversy about the effect of thermal
corrections on the Casimir force between real metal plates.


title:
Maximizing nearest neighbour entanglement in finitely correlated
  qubit--chains

abstract:
  We consider translationally invariant states of an infinite one dimensional
chain of qubits or spin-1/2 particles. We maximize the entanglement shared by
nearest neighbours via a variational approach based on finitely correlated
states. We find an upper bound of nearest neighbour concurrence equal to
C=0.434095 which is 0.09% away from the bound C_W=0.434467 obtained by a
completely different procedure. The obtained state maximizing nearest neighbour
entanglement seems to approximate the maximally entangled mixed states (MEMS).
Further we investigate in detail several other properties of the so obtained
optimal state.


title: Generators and Roots of Quantum Logic Gates

abstract:
  This is an exposition of some basic mathematical aspects of quantum logic
gates. At first we established some general formulas for the case of arbitrary
quantum gate A with unique restriction A^2=I. The explicit form of the
generators and roots of matrix A have been found . Then we apply general
results to the particular cases of one-qubit and multi-qubit quantum gates.
Some interesting properties of square roots of basic Pauli and Hadamard gates
are demonstrated.


title: Causal Incompleteness: A New Perspective on Quantum Non-locality

abstract:
  The mathematical notion of incompleteness (eg of rational numbers,
Turing-computable functions, and arithmetic proof) does not play a key role in
conventional physics. Here, a reformulation of the kinematics of quantum theory
is attempted, based on an inherently granular and discontinuous state space, in
which the quantum wavefunction is associated with a finite set of finite bit
strings, and the unitary transformations of complex Hilbert space are
reformulated as finite permutation and related operators incorporating complex
and hyper-complex structure. Such a reformulation, consistent with Wheeler's
`It from Bit' programme, provides the basis for a novel interpretation of the
Bell theorem: that the experimental violation of the Bell inequalities reveals
the inevitable incompleteness of the causal structure of physical theory. The
kinematic reformulation of quantum theory so developed, provides a new
perspective on the age-old dichotomy of free will versus determinism.


abstract:
  We study $(1+1)$ dimensional Dirac equation with non Hermitian interactions,
but real energies. In particular, we analyze the pseudoscalar and scalar
interactions in detail, illustrating our observations with some examples. We
also show that the relevant hidden symmetry of the Dirac equation with such an
interaction is pseudo supersymmetry.


title:
Quantum dense coding via atomic ensembles without prior distributed
  entangled state

abstract:
  We investigate a scheme for implementing quantum dense coding via atomic
ensembles, where prior distribution of the quantum entangled state is not
needed. Our scheme also combines another two distinct advantages: atomic
ensembles qubits with long coherent time serve as memory qubits and phtonic
qubit as flying qubit. Thus our suggestion may offer a simple and easy way of
demonstrating quantum dense coding experimentally via atomic ensembles.


title: Mid-Infrared Single Photon Counting

abstract:
  We report a procedure to detect mid-infrared single photons at 4.65 um via a
two-stage scheme based on Sum Frequency Generation, using a Periodically Poled
Lithium Niobate (PPLN) nonlinear crystal and a Silicon Avalanche Photodiode. An
experimental investigation shows that, in addition to a high timing resolution,
this technique yields a detection sensitivity of 1.24 pW with 63mW of net pump
power.


title:
Comment on "Peres-Horodecki Separability Criterion for Continuous
  Variable Systems"

abstract: The content of this paper is now available as part of arXiv:0902.1502


title: Creating and probing long-range order in atomic clouds

abstract:
  Ultracold atoms interacting with the optical modes of a high-Q optical ring
cavity can synchronize their motion. The collective behavior makes the system
interesting for quantum computing applications. This paper is devoted to the
study of the collective coupling. We report on the first observation of a
collective dynamics and on the realization of a laser, the gain mechanism of
which is based on collective atomic recoil. We show that, if the atoms are
subject to a friction force, starting from an unordered distribution they
spontaneously form a moving density grating. Furthermore, we demonstrate that a
1D atomic density grating can be probed via Bragg scattering. By heterodyning
the Bragg-reflected light with a reference beam, we obtain detailed information
on phase shifts induced by the Bragg scattering process.


title: Near-Perfect Simultaneous Measurement of a Qubit Register

abstract:
  Simultaneous measurement of multiple qubits stored in hyperfine levels of
trapped 111Cd+ ions is realized with an intensified charge-coupled device (CCD)
imager. A general theory of fluorescence detection for hyperfine qubits is
presented and applied to experimental data. The use of an imager for photon
detection allows for multiple qubit state measurement with detection fidelities
of greater than 98%. Improvements in readout speed and fidelity are discussed
in the context of scalable quantum computation architectures.


title:
Multiple Reflections and Diffuse Scattering in Bragg Scattering at
  Optical Lattices

abstract:
  We study Bragg scattering at 1D atomic lattices. Cold atoms are confined by
optical dipole forces at the antinodes of a standing wave generated inside a
laser-driven cavity. The atoms arrange themselves into an array of lens-shaped
layers located at the antinodes of the standing wave. Light incident on this
array at a well-defined angle is partially Bragg-reflected. We measure
reflectivities as high as 30%. In contrast to a previous experiment devoted to
the thin grating limit [S. Slama, et al., Phys. Rev. Lett. 94, 193901 (2005)]
we now investigate the thick grating limit characterized by multiple
reflections of the light beam between the atomic layers. In principle multiple
reflections give rise to a photonic stop band, which manifests itself in the
Bragg diffraction spectra as asymmetries and minima due to destructive
interference between different reflection paths. We show that close to
resonance however disorder favors diffuse scattering, hinders coherent multiple
scattering and impedes the characteristic suppression of spontaneous emission
inside a photonic band gap.


title:
Comment on the quantum nature of angular momentum using a coupled-boson
  representation

abstract:
  A simple approach for understanding the quantum nature of angular momentum
and its reduction to the classical limit is presented based on Schwinger's
coupled-boson representation. This approach leads to a straightforward
explanation of why the square of the angular momentum in quantum mechanics is
given by j(j+1) instead of just j^2, where j is the angular momentum quantum
number.


title: All Inequalities for the Relative Entropy

abstract:
  The relative entropy of two n-party quantum states is an important quantity
exhibiting, for example, the extent to which the two states are different. The
relative entropy of the states formed by reducing two n-party to a smaller
number $m$ of parties is always less than or equal to the relative entropy of
the two original n-party states. This is the monotonicity of relative entropy.
  Using techniques from convex geometry, we prove that monotonicity under
restrictions is the only general inequality satisfied by relative entropies. In
doing so we make a connection to secret sharing schemes with general access
structures.
  A suprising outcome is that the structure of allowed relative entropy values
of subsets of multiparty states is much simpler than the structure of allowed
entropy values. And the structure of allowed relative entropy values (unlike
that of entropies) is the same for classical probability distributions and
quantum states.


title:
Possibly novel demonstration of nonlocality for two maximally entangled
  particles

abstract:
  We present, at the gedanken level, a possibly novel non-statistical
demonstration of nonlocality for two maximally entangled particles. The
argument requires only two alternative experimental contexts, only one and the
same single-particle observable, and leads to a contradiction with a
constrained form of local realism in 50% of experimental runs. However, the
argument is based on a 'plausible' but not definitive notion of locality.


title: Decoherence in a system of many two--level atoms

abstract:
  I show that the decoherence in a system of $N$ degenerate two--level atoms
interacting with a bosonic heat bath is for any number of atoms $N$ governed by
a generalized Hamming distance (called ``decoherence metric'') between the
superposed quantum states, with a time--dependent metric tensor that is
specific for the heat bath.The decoherence metric allows for the complete
characterization of the decoherence of all possible superpositions of
many-particle states, and can be applied to minimize the over-all decoherence
in a quantum memory. For qubits which are far apart, the decoherence is given
by a function describing single-qubit decoherence times the standard Hamming
distance. I apply the theory to cold atoms in an optical lattice interacting
with black body radiation.


title: Comparison of some methods of quantum state estimation

abstract:
  In the paper the Bayesian and the least squares methods of quantum state
tomography are compared for a single qubit. The quality of the estimates are
compared by computer simulation when the true state is either mixed or pure.
The fidelity and the Hilbert-Schmidt distance are used to quantify the error.
  It was found that in the regime of low measurement number the Bayesian method
outperforms the least squares estimation. Both methods are quite sensitive to
the degree of mixedness of the state to be estimated, that is, their
performance can be quite bad near pure states.


title: Fault-Tolerant Landau-Zener Quantum Gates

abstract:
  We present a method to perform fault-tolerant single-qubit gate operations
using Landau-Zener tunneling. In a single Landau-Zener pulse, the qubit
transition frequency is varied in time so that it passes through the frequency
of the radiation field. We show that a simple three-pulse sequence allows
eliminating errors in the gate up to the third order in errors in the qubit
energies or the radiation frequency.


title:
Experimental noise-resistant Bell-inequality violations for
  polarization-entangled photons

abstract:
  We experimentally demonstrate that violations of Bell's inequalities for
two-photon polarization-entangled states with colored noise are extremely
robust, whereas this is not the case for states with white noise. Controlling
the amount of noise by using the timing compensation scheme introduced by Kim
et al. [Phys. Rev. A 67, 010301(R) (2003)], we have observed violations even
for states with very high noise, in excellent agrement with the predictions of
Cabello et al. [Phys. Rev. A 72, 052112 (2005)].


title: Statistical Zero Knowledge and quantum one-way functions

abstract:
  One-way functions are a very important notion in the field of classical
cryptography. Most examples of such functions, including factoring, discrete
log or the RSA function, can be, however, inverted with the help of a quantum
computer. In this paper, we study one-way functions that are hard to invert
even by a quantum adversary and describe a set of problems which are good such
candidates. These problems include Graph Non-Isomorphism, approximate Closest
Lattice Vector and Group Non-Membership. More generally, we show that any hard
instance of Circuit Quantum Sampling gives rise to a quantum one-way function.
By the work of Aharonov and Ta-Shma, this implies that any language in
Statistical Zero Knowledge which is hard-on-average for quantum computers,
leads to a quantum one-way function. Moreover, extending the result of
Impagliazzo and Luby to the quantum setting, we prove that quantum
distributionally one-way functions are equivalent to quantum one-way functions.
Last, we explore the connections between quantum one-way functions and the
complexity class QMA and show that, similarly to the classical case, if any of
the above candidate problems is QMA-complete then the existence of quantum
one-way functions leads to the separation of QMA and AvgBQP.


title:
Optimal Visible Compression Rate For Mixed States Is Determined By
  Entanglement Of Purification

abstract:
  Entanglement of purification was introduced by Terhal et al. for
characterizing the bound of the generation of correlated states from maximally
entangled states with sublinear size of classical communication. On the other
hand, M. Horodecki obtained the optimal compression rate with a mixed states
ensemble in the visible setting. In this paper, we prove that the optimal
visible compression rate for mixed states is equal to the limit of the
regularized entanglement of purification of the state corresponding to the
given ensemble. This result gives a new interpretation to the entanglement of
purification.


title: Practical limitations in optical entanglement purification

abstract:
  Entanglement purification protocols play an important role in the
distribution of entangled systems, which is necessary for various quantum
information processing applications. We consider the effects of photo-detector
efficiency and bandwidth, channel loss and mode-mismatch on the operation of an
optical entanglement purification protocol. We derive necessary detector and
mode-matching requirements to facilitate practical operation of such a scheme,
without having to resort to destructive coincidence type demonstrations.


title:
Distortion operator and Entanglement Information Rate Distortion of
  Quantum Gaussian Source

abstract:
  Quantum random variable, distortion operator are introduced based on
canonical operators. As the lower bound of rate distortion, the entanglement
information rate distortion is achieved by Gaussian map for Gaussian source.
General Gaussian maps are further reduced to unitary transformations and
additive noises from the physical meaning of distortion. The entanglement
information rate distortion function then are calculated for one mode Gaussian
source. The rate distortion is accessible at zero distortion point. For pure
state, the rate distortion function is always zero. In contrast to the
distortion defined via fidelity, our definition of the distortion makes it
possible to calculate the entanglement information rate distortion function for
Gaussian source.


title: Photon Position Operator and Localization of Photons inside a Waveguide

abstract:
  In this article, we show that in the level of quantum mechanics, a photon
position operator with commuting components can be obtained in a more natural
way; in the level of quantum field theory, the photon position operator
corresponds to the center of the photon number. It is most interesting for us
to show that, a photon inside a waveguide can be localized in the same sense
that a massive particle can be localized in free space, and just as that it is
impossible to localize a massive particle with a greater precision than its
Compton wavelength, one can also arrival at a similar conclusion: it is
impossible to localize a photon inside a waveguide with a greater precision
than its equivalent Compton wavelength, which owing to evanescent-wave or
photon-tunneling phenomena.


title:
Quantum thermodynamics with missing reference frames: Decompositions of
  free energy into non-increasing components

abstract:
  If an absolute reference frame with respect to time, position, or orientation
is missing one can only implement quantum operations which are covariant with
respect to the corresponding unitary symmetry group G. Extending observations
of Vaccaro et al., I argue that the free energy of a quantum system with
G-invariant Hamiltonian then splits up into the Holevo information of the orbit
of the state under the action of G and the free energy of its orbit average.
These two kinds of free energy cannot be converted into each other. The first
component is subadditive and the second superadditive; in the limit of
infinitely many copies only the usual free energy matters.
  Refined splittings of free energy into more than two independent
(non-increasing) terms can be defined by averaging over probability measures on
G that differ from the Haar measure.
  Even in the presence of a reference frame, these results provide lower bounds
on the amount of free energy that is lost after applying a covariant channel.
If the channel properly decreases one of these quantities, it decreases the
free energy necessarily at least by the same amount, since it is unable to
convert the different forms of free energies into each other.


title: Quantum Advantage without Entanglement

abstract:
  We study the advantage of pure-state quantum computation without entanglement
over classical computation. For the Deutsch-Jozsa algorithm we present the
maximal subproblem that can be solved without entanglement, and show that the
algorithm still has an advantage over the classical ones. We further show that
this subproblem is of greater significance, by proving that it contains all the
Boolean functions whose quantum phase-oracle is non-entangling. For Simon's and
Grover's algorithms we provide simple proofs that no non-trivial subproblems
can be solved by these algorithms without entanglement.


title: Quantum computation with abelian anyons on the honeycomb lattice

abstract:
  We consider a two-dimensional spin system that exhibits abelian anyonic
excitations. Manipulations of these excitations enable the construction of a
quantum computational model. While the one-qubit gates are performed
dynamically the model offers the advantage of having a two-qubit gate that is
of topological nature. The transport and braiding of anyons on the lattice can
be performed adiabatically enjoying the robust characteristics of geometrical
evolutions. The same control procedures can be used when dealing with
non-abelian anyons. A possible implementation of the manipulations with optical
lattices is developed.


title: Quantum Computation: A Computer Science Perspective

abstract:
  The theory of quantum computation is presented in a self contained way from a
computer science perspective. The basics of classical computation and quantum
mechanics is reviewed. The circuit model of quantum computation is presented in
detail. Throughout there is an emphasis on the physical as well as the abstract
aspects of computation and the interplay between them.
  This report is presented as a Master's thesis at the department of Computer
Science and Engineering at G{\"o}teborg University, G{\"o}teborg, Sweden.
  The text is part of a larger work that is planned to include chapters on
quantum algorithms, the quantum Turing machine model and abstract approaches to
quantum computation.


title: Realistic Picture of 2D Harmonic Oscillator Coherent States

abstract:
  We show that a 2D harmonic oscillator coherent state is a soliton which has
the same evolution as a spinning top: the center of mass follows a classical
trajectory and the particle rotates around its center of mass in the same
direction as its spin with the harmonic oscillator frequency.


title: A complete analysis of the Stern-Gerlach experiment using Pauli spinors

abstract:
  The Stern-Gerlach experiment is the fundamental experiment in order to
exhibit the quantization of spin and understand the measurement problem in
quantum mechanics. However, although the Stern-Gerlach experiment plays an
essential role in the teaching of quantum mechanics, no complete analysis of
this experiment using Pauli spinors is presented in the pedagogical literature.
This paper presents such an analysis and develops implications for the theory
of quantum measurement.
  We first propose an analytic expression of both the wave function and the
probability density in the Stern-Gerlach experiment. Our explicit solution is
obtained via a complete integration of the Pauli equation over time and space.
The probability density evolution describes a slipping of the wave packet into
two separate packets due to the measurement device, but it cannot account for
impacts.
  We therefore calculate the de Broglie-Bohm trajectories, which not only
explain impacts naturally, but also accounts for the spin quantization
following the magnetic field gradient. It is then possible to propose a clear
explanation of measurement effects in the Stern-Gerlach experiment.


title: Momentum diffusion for coupled atom-cavity oscillators

abstract:
  It is shown that the momentum diffusion of free-space laser cooling has a
natural correspondence in optical cavities when the internal state of the atom
is treated as a harmonic oscillator. We derive a general expression for the
momentum diffusion which is valid for most configurations of interest: The atom
or the cavity or both can be probed by lasers, with or without the presence of
traps inducing local atomic frequency shifts. It is shown that, albeit the
(possibly strong) coupling between atom and cavity, it is sufficient for
deriving the momentum diffusion to consider that the atom couples to a mean
cavity field, which gives a first contribution, and that the cavity mode
couples to a mean atomic dipole, giving a second contribution. Both
contributions have an intuitive form and present a clear symmetry. The total
diffusion is the sum of these two contributions plus the diffusion originating
from the fluctuations of the forces due to the coupling to the vacuum modes
other than the cavity mode (the so called spontaneous emission term). Examples
are given that help to evaluate the heating rates induced by an optical cavity
for experiments operating at low atomic saturation. We also point out
intriguing situations where the atom is heated although it cannot scatter
light.


title:
Optimal cloning of single photon polarization by coherent feedback of
  beam splitter losses

abstract:
  Light fields can be amplified by measuring the field amplitude reflected at a
beam splitter of reflectivity R and adding a coherent amplitude proportional to
the measurement result to the transmitted field. By applying the quantum
optical realization of this amplification scheme to single photon inputs, it is
possible to clone the polarization states of photons. We show that optimal
cloning of single photon polarization is possible when the gain factor of the
amplification is equal to the inverse squareroot of 1-R.


title:
Regularized One Dimensional Coulomb Potential Induced by the Presence of
  Minimal Lengths

abstract:
  In this paper we address the problem of a particle moving in singular one
dimensional potentials in the framework of quantum mechanics with minimal
length. Using the momentum space representation we solve exactly the
Schrodinger equation for the Dirac delta potential and Coulomb potential. The
effect of the minimal length is revealed by a computation of effective
generalized Dirac delta potential and Coulomb potential.


title: Remote preparation of arbitrary time-encoded single-photon ebits

abstract:
  We propose and experimentally verify a novel method for the remote
preparation of entangled bits (ebits) made of a single-photon coherently
delocalized in two well-separated temporal modes. The proposed scheme
represents a remotely tunable source for tailoring arbitrary ebits, whether
maximally or non-maximally entangled, which is highly desirable for
applications in quantum information technology. The remotely prepared ebit is
studied by performing homodyne tomography with an ultra-fast balanced homodyne
detection scheme recently developed in our laboratory.


title: Fidelities for transformations of unknown quantum states

abstract:
  We present a general theoretical formalism to compute the fidelity of
transformations of unknown quantum states. We then focus on the case of
Gaussian transformations of continuous variable quantum systems, where, for the
case of a Gaussian distribution of displaced coherent states, the theory is
readily tractable by a covariance matrix formalism. We present analytical
results for recently implemented teleportation and memory storage protocols for
continuous variables.


title:
Time-resolved and state-selective detection of single freely falling
  atoms

abstract:
  We report on the detection of single, slowly moving Rubidium atoms using
laser-induced fluorescence. The atoms move at 3 m/s while they are detected
with a time resolution of 60 microseconds. The detection scheme employs a
near-resonant laser beam that drives a cycling atomic transition, and a highly
efficient mirror setup to focus a large fraction of the fluorescence photons to
a photomultiplier tube. It counts on average 20 photons per atom.


title: Entanglement and dephasing of quantum dissipative systems

abstract:
  The von Neumann entropy of various quantum dissipative models is calculated
in order to discuss the entanglement properties of these systems. First,
integrable quantum dissipative models are discussed, i.e., the quantum Brownian
motion and the quantum harmonic oscillator. In case of the free particle, the
related entanglement of formation shows no non-analyticity. In case of the
dissipative harmonic oscillator, there is a non-analyticity at the transition
of underdamped to overdamped oscillations. We argue that this might be a
general property of dissipative systems. We show that similar features arise in
the dissipative two level system and study different regimes using sub-Ohmic,
Ohmic and and super-Ohmic baths, within a scaling approach.


title: A modified quantum adiabatic evolution for the Deutsch-Jozsa problem

abstract:
  Deutsch-Jozsa algorithm has been implemented via a quantum adiabatic
evolution by S. Das et al. [Phys. Rev. A 65, 062310 (2002)]. This adiabatic
algorithm gives rise to a quadratic speed up over classical algorithms. We show
that a modified version of the adiabatic evolution in that paper can improve
the performance to constant time.


title: Lasing without inversion in a closed four-level system

abstract:
  We show lasing without population inversion in a closed four-level atomic
system which is no longer incoherently pumped on the transition at which lasing
occurs. The four-level system can overcome some limits in the typical
lambda-type and V-type three-level systems, furthermore it is possible to
achieve lasing even in the absence of the inversion condition for the thermal
radiation fields or of the inversion of spontaneous decay rate. We find that
when the detuning of probe field is equal to that of coherent coupling field,
there is a peak value along with the increase of the detuning. To explain it,
we bring forward that there are adiabatic and nonadiabatic stimulated Raman
processes when a three-level atomic system is interacted with two coherent
fields, so there are two mechanisms influencing the change of probe field. We
believe that it can explain more general lasing without inversion processes
because of quantum interference induced by coherent fields.


title:
Generating conditional atomic entanglement by measuring photon number in
  a single output channel

abstract:
  The polarization analysis of quantized probe light transmitted through an
atomic ensemble has been used to prepare entangled collective atomic states. In
a "balanced" detection configuration, where the difference signal from two
detection ports is analyzed, the continuous monitoring of a component of the
Stokes field vector provides a means for conditional projective measurements on
the atomic system. Here, we make use of classical driving fields, in the pulsed
regime, and of an "unbalanced" detection setup (single detector) where the
effective photon number of scattered photons is the detected observable.
Conditional atomic spin squeezed states and superpositions of such squeezed
states can be prepared in this manner.


title: On Series of Multiqubit Bell's Inequalities

abstract:
  We overview series of multiqubit Bell's inequalities which apply to
correlation functions. We present conditions that quantum states must satisfy
to violate such inequalities.


title: Schmidt balls around the identity

abstract:
  Robustness measures as introduced by Vidal and Tarrach [PRA, 59, 141-155]
quantify the extent to which entangled states remain entangled under mixing.
Analogously, we introduce here the Schmidt robustness and the random Schmidt
robustness. The latter notion is closely related to the construction of Schmidt
balls around the identity. We analyse the situation for pure states and provide
non-trivial upper and lower bounds. Upper bounds to the random Schmidt-2
robustness allow us to construct a particularly simple distillability
criterion. We present two conjectures, the first one is related to the radius
of inner balls around the identity in the convex set of Schmidt number
n-states. We also conjecture a class of optimal Schmidt witnesses for pure
states.


title:
Efficiency of coherent state quantum cryptography in the presence of
  loss: Influence of realistic error correction

abstract:
  We investigate the performance of continuous variable quantum key
distribution scheme in a practical setting. More specifically, we take
non-ideal error reconciliation procedure into account. The quantum channel
connecting the two honest parties is assumed to be lossy but noiseless. Secret
key rates are given for the case that the measurement outcomes are postselected
or a reverse reconciliation scheme is applied. The reverse reconciliation
scheme loses its initial advantage in the practical setting. If one combines
postselection with reverse reconciliation however, much of this advantage can
be recovered.


title:
Quantum secure direct communication with quantum encryption based on
  pure entangled states

abstract:
  We present a scheme for quantum secure direct communication with quantum
encryption. The two authorized users use repeatedly a sequence of the pure
entangled pairs (quantum key) shared for encrypting and decrypting the secret
message carried by the traveling photons directly. For checking eavesdropping,
the two parties perform the single-photon measurements on some decoy particles
before each round. This scheme has the advantage that the pure entangled
quantum signal source is feasible at present and any eavesdropper cannot steal
the message.


title: A Resource Framework for Quantum Shannon Theory

abstract:
  Quantum Shannon theory is loosely defined as a collection of coding theorems,
such as classical and quantum source compression, noisy channel coding
theorems, entanglement distillation, etc., which characterize asymptotic
properties of quantum and classical channels and states. In this paper we
advocate a unified approach to an important class of problems in quantum
Shannon theory, consisting of those that are bipartite, unidirectional and
memoryless.
  We formalize two principles that have long been tacitly understood. First, we
describe how the Church of the larger Hilbert space allows us to move flexibly
between states, channels, ensembles and their purifications. Second, we
introduce finite and asymptotic (quantum) information processing resources as
the basic objects of quantum Shannon theory and recast the protocols used in
direct coding theorems as inequalities between resources. We develop the rules
of a resource calculus which allows us to manipulate and combine resource
inequalities. This framework simplifies many coding theorem proofs and provides
structural insights into the logical dependencies among coding theorems.
  We review the above-mentioned basic coding results and show how a subset of
them can be unified into a family of related resource inequalities. Finally, we
use this family to find optimal trade-off curves for all protocols involving
one noisy quantum resource and two noiseless ones.


title: New Structural Quantum Circuit Simulating a Toffoli Gate

abstract:
  A Toffoli gate ($C^{n}$-NOT gate) is regarded as an important unitary gate in
quantum computation, and is simulated by a quantum circuit composed of
$C^{2}$-NOT gates. This paper presents a quantum circuit with a new
configuration of $C^{2}$-NOT gates simulating a $C^{2m+1}$-NOT operation under
the condition $m=2^{n}$ ($n=1,2,...$). The circuit is composed of units called
multi-qubits gates (MQGs), each of which performs $m$ $C^{2}$-NOT operations
simultaneously on $3m$ qubits. Simultaneous operations eliminate the need to
manipulate qubits individually, as required in conventional quantum circuits.
The proposed circuit thus represents a more realistic mode of operation for
practical computing systems. A nuclear magnetic resonance implementation of the
circuit is presented as a demonstration of the feasibility of MQG operations
for practical systems.


title:
Default-off inter-qubit interactions in NMR quantum computing in
  rotating solids

abstract:
  A dipolar recoupling technique is introduced as a new approach to quantum
gate operation in solid-state NMR under magic angle spinning. The default-off
property of inter-qubit interaction provides a simple way to controlled
operation without requiring elaborate qubit-decoupling pulses.


title: Eigenlevel statistics of the quantum adiabatic algorithm

abstract:
  We study the eigenlevel spectrum of quantum adiabatic algorithm for
3-satisfiability problem, focusing on single-solution instances. The properties
of the ground state and the associated gap, crucial for determining the running
time of the algorithm, are found to be far from the predictions of random
matrix theory. The distribution of gaps between the ground and the first
excited state shows an abundance of small gaps. Eigenstates from the central
part of the spectrum are, on the other hand, well described by random matrix
theory.


title: Quantum Interrogation with particles

abstract:
  Interaction-free measurement and quantum interrogation schemes can help in
the detection of particles without interacting with them in a classical sense.
We present a density matrix study of a quantum interrogation system designed
for particles that need not to be perfectly absorptive and compare the results
to those of the usual setup.


title: An Interaction-Free Quantum Shutter

abstract:
  We give a proposal for the physical implementation of a quantum shutter. This
shutter is a mechanism able to close the passage of light through a slit when
present. As it must be a quantum mechanical object, able to show superposition,
interesting quantum effects are observed. Finally, we review some applications
of the quantum shutter to build quantum memories and a CNOT gate.


title:
Locking of accessible information and implications for the security of
  quantum cryptography

abstract:
  The unconditional security of a quantum key distribution protocol is often
defined in terms of the accessible information, that is, the maximum mutual
information between the distributed key S and the outcome of an optimal
measurement on the adversary's (quantum) system. We show that, even if this
quantity is small, certain parts of the key S might still be completely
insecure when S is used in applications, such as for one-time pad encryption.
This flaw is due to a locking property of the accessible information: one
additional (physical) bit of information might increase the accessible
information by more than one bit.


title: Quantum superposition and entanglement of mesoscopic plasmons

abstract:
  Quantum superpositions and entanglement are at the heart of the quantum
information science. There have been only a few investigations of these
phenomena at the mesoscopic level, despite the fact that these systems are
promising for quantum state storage and processing. Here we present two novel
experiments with surface plasmons propagating on cm-long metallic stripe
waveguides. We demonstrate that two plasmons can be entangled at remote places.
In addition, we create a single plasmon in a temporal superposition state: it
exists in a superposition of two widely separated moments. These quantum
states, created using photons at telecom wavelength, are collectively held by a
mesoscopic number of electrons coding a single quantum bit of information; They
are shown to be very robust against decoherence.


title: Characterization of single photons using two-photon interference

abstract:
  Adiabatic passage techniques allow the generation of single photons which are
very long compared to the typical detector time resolution. Therefore the
detection time of a photon can be measured within the duration of the
single-photon wavepacket. As a consequence, two-photon interference can be
investigated in a time-resolved manner, i.e., the coincidence rate can be
measured as a function of the time between photodetections. The theoretical
analysis shows that this method not only gives information about the duration
of single photons, but also about their coherence time. Here we discuss how to
use this method for a spectral or temporal characterization of a single-photon
source.


title:
Experimental implementation of heat-bath algorithmic cooling using
  solid-state nuclear magnetic resonance

abstract:
  We report here the experimental realization of multi-step cooling of a
quantum system via heat-bath algorithmic cooling. The experiment was carried
out using nuclear magnetic resonance (NMR) of a solid-state ensemble
three-qubit system.


title:
Nonlocality without inequality for almost all two-qubit entangled state
  based on Cabello's nonlocality argument

abstract:
  Here we deal with a nonlocality argument proposed by Cabello which is more
general than Hardy's nonlocality argument but still maximally entangled states
do not respond. However, for most of the other entangled states maximum
probability of success of this argument is more than that of the Hardy's
argument.


title: Comments on "The Casimir effect upon a single plate"

abstract:
  We make remarks on Hoodbhoy's paper [J. Phys. A: Math. Gen. 38 (2005)
10253-10256] by pointing out the single plate considered could be elastic and
there will be deformations once the net (one-sided) force being upon the plate
which will change the position-dependent potential imposed by Hoodbhoy.


title: Validity of adiabaticity in Cavity QED

abstract:
  This paper deals with the concept of adiabaticity for fully quantum
mechanically cavity QED models. The physically interesting cases of Gaussian
and standing wave shapes of the cavity mode are considered. An analytical
approximate measure for adiabaticity is given and compared with numerical wave
packet simulations. Good agreement is obtained where the approximations are
expected to be valid. Usually for cavity QED systems, the large atom-field
detuning case is considered as the adiabatic limit. We, however, show that
adiabaticity is also valid, for the Gaussian mode shape, in the opposite limit.
Effective semiclassical time dependent models, which do not take into account
the shape of the wave packet, are derived. Corrections to such an effective
theory, which are purely quantum mechanical, are discussed. It is shown that
many of the results presented can be applied to time dependent two-level
systems.


title: Non-Integer Quantum Transitions

abstract:
  We show that in the quantum transition of a system induced by the interaction
with an intense laser of circular frequency $\omega$, the energy difference
between the initial and the final states of the system is not necessarily being
an integer multiple of the quantum energy $\hbar\omega$.


title: Optimal universal programmable detectors for unambiguous discrimination

abstract:
  We discuss the problem of designing unambiguous programmable discriminators
for any n unknown quantum states in an m-dimensional Hilbert space. The
discriminator is a fixed measurement that has two kinds of input registers: the
program registers and the data register. The quantum state in the data register
is what users want to identify, which is confirmed to be among the n states in
program registers. The task of the discriminator is to tell the users which
state stored in the program registers is equivalent to that in the data
register. First, we give a necessary and sufficient condition for judging an
unambiguous programmable discriminator. Then, if $m=n$, we present an optimal
unambiguous programmable discriminator for them, in the sense of maximizing the
worst-case probability of success. Finally, we propose a universal unambiguous
programmable discriminator for arbitrary n quantum states.


title:
Collapse of the Entangled State and the Entropy Increase in an Isolated
  System

abstract:
  We show that the collapse of the entangled quantum state makes the entropy
increase in an isolated system. The second law of thermodynamics is thus proven
in its most general form.


title:
Linking entanglement and quantum phase transitions via density
  functional theory

abstract:
  Density functional theory (DFT) is shown to provide a novel conceptual and
computational framework for entanglement in interacting many-body quantum
systems. DFT can, in particular, shed light on the intriguing relationship
between quantum phase transitions and entanglement. We use DFT concepts to
express entanglement measures in terms of the first or second derivative of the
ground state energy. We illustrate the versatility of the DFT approach via a
variety of analytically solvable models. As a further application we discuss
entanglement and quantum phase transitions in the case of mean field
approximations for realistic models of many-body systems.


title:
Narrow-band single-photon emission in the near infrared for quantum key
  distribution

abstract:
  We report on the observation of single colour centers in natural diamond
samples emitting in the near infrared region when optically excited.
Photoluminescence of these single emitters have several striking features, such
as a narrow-band fully polarized emission (FWHM 2 nm) around 780 nm, a short
excited-state lifetime of about 2 ns, and perfect photostability at room
temperature under our excitation conditions. We present a detailed study of
their photophysical properties. Development of a triggered single-photon source
relying on this single colour centre is discussed in the prospect of its
application to quantum key distribution.


title:
Application of an all-solid-state, frequency-doubled Nd:YAP laser to the
  generation of twin beams at 1080 nm

abstract:
  A laser-diode-pumped intracavity frequency-doubled Nd:YAP/KTP laser is
presented. Over 110 mw of TEM00 single-frequency output power at 540-nm
wavelength was obtained. The output green laser was employed to pump a
semimonolithic nondegenerate optical parametric oscillator to produce intensity
quantum correlated twin beams at 1080 nm, and the maximum quantum noise
squeezing of 74 %(5.9 dB) on the intensity difference fluctuation between the
twin beams is observed. The threshold was reduced and the stability was
increased significantly when compared with similar lamp-pumped systems.


title: Quantum Information Processing with Low-Dimensional Systems

abstract:
  A `register' in quantum information processing -- is composition of k quantum
systems, `qudits'. The dimensions of Hilbert spaces for one qudit and whole
quantum register are d and d^k respectively, but we should have possibility to
prepare arbitrary entangled state of these k systems. Preparation and arbitrary
transformations of states are possible with universal set of quantum gates and
for any d may be suggested such gates acting only on single systems and
neighbouring pairs. Here are revisited methods of construction of Hamiltonians
for such universal set of gates and as a concrete new example is considered
case with qutrits. Quantum tomography is also revisited briefly.


title:
Explicit Solutions for N-Dimensional Schrodinger Equations with
  Position-Dependent Mass

abstract:
  With the consideration of spherical symmetry for the potential and mass
function, one-dimensional solutions of non-relativistic Schrodinger equations
with spatially varying effective mass are successfully extended to arbitrary
dimensions within the frame of recently developed elegant non-perturbative
technique, where the BenDaniel-Duke effective Hamiltonian in one-dimension is
assumed like the unperturbed piece, leading to well-known solutions, whereas
the modification term due to possible use of other effective Hamiltonians in
one-dimension and, together with, the corrections coming from the treatments in
higher dimensions are considered as an additional term like the perturbation.
Application of the model and its generalization for the completeness are
discussed.


title:
Experimental implementation of high-fidelity unconventional geometric
  quantum gates using NMR interferometer

abstract:
  Following a key idea of unconventional geometric quantum computation
developed earlier [Phys. Rev. Lett. 91, 197902 (2003)], here we propose a more
general scheme in such an intriguing way: $\gamma_{d}=\alpha_{g}+\eta \gamma
_{g}$, where $\gamma_{d}$ and $\gamma_{g}$ are respectively the dynamic and
geometric phases accumulated in the quantum gate operation, with $\eta$ as a
constant and $\alpha_{g}$ being dependent only on the geometric feature of the
operation. More arrestingly, we demonstrate the first experiment to implement a
universal set of such kind of generalized unconventional geometric quantum
gates with high fidelity in an NMR system.


title:
Optimal estimation of a physical observable's expectation value for pure
  states

abstract:
  We study the optimal way to estimate the quantum expectation value of a
physical observable when a finite number of copies of a quantum pure state are
presented. The optimal estimation is determined by minimizing the squared error
averaged over all pure states distributed in a unitary invariant way. We find
that the optimal estimation is "biased", though the optimal measurement is
given by successive projective measurements of the observable. The optimal
estimate is not the sample average of observed data, but the arithmetic average
of observed and "default nonobserved" data, with the latter consisting of all
eigenvalues of the observable.


title:
Thermal breakdown of coherent backscattering: a case study of quantum
  duality

abstract:
  We investigate coherent backscattering of light by two harmonically trapped
atoms in the light of quantitative quantum duality. Including recoil and
Doppler shift close to an optical resonance, we calculate the interference
visibility as well as the amount of which-path information, both for zero and
finite temperature.


title:
Joint spectrum of photon pairs measured by coincidence Fourier
  spectroscopy

abstract:
  We propose and demonstrate a method for measuring the joint spectrum of
photon pairs via Fourier spectroscopy. The biphoton spectral intensity is
computed from a two-dimensional interferogram of coincidence counts. The method
has been implemented for a type-I downconversion source using a pair of
common-path Mach-Zender interferometers based on Soleil compensators. The
experimental results agree well with calculated frequency correlations that
take into account the effects of coupling into single-mode fibers. The Fourier
method is advantageous over scanning spectrometry when detectors exhibit high
dark count rates leading to dominant additive noise.


title: An algebraic study of unitary one dimensional quantum cellular automata

abstract:
  We provide algebraic criteria for the unitarity of linear quantum cellular
automata, i.e. one dimensional quantum cellular automata. We derive these both
by direct combinatorial arguments, and by adding constraints into the model
which do not change the quantum cellular automata's computational power. The
configurations we consider have finite but unbounded size.


title: Trellises for stabilizer codes: definition and uses

abstract:
  Trellises play an important theoretical and practical role for classical
codes. Their main utility is to devise complexity-efficient error estimation
algorithms. Here, we describe trellis representations for quantum stabilizer
codes. We show that they share the same properties as their classical analogs.
In particular, for any stabilizer code it is possible to find a minimal trellis
representation. Our construction is illustrated by two fundamental error
estimation algorithms.


title:
Manipulation of Two-Color Stationary Light Using Coherence Moving
  Gratings

abstract:
  We propose the dynamic control of two-color stationary light in a
double-lambda four-level system using electromagnetically induced transparency.
We demonstrate the complete localization of two-color quantum fields inside a
medium using coherence moving gratings resulting from slow-light based
atom-field interactions in backward nondegenerate four-wave mixing processes.
The quantum coherent control of the two-color stationary light opens a door to
deterministic quantum information science which needs a quantum nondemolition
measurement, where the two-color stationary light scheme would greatly enhance
nonlinearity with increased interaction time.


title: Exploring surface interactions with atom chips

abstract:
  We review the current status of the field of atom-surface interactions, with
an emphasis on the regimes specific to atom chips. Recent developments in
theory and experiment are highlighted. In particular, atom-surface interactions
define physical limits for miniaturization and coherent operation. This implies
constraints for applications in quantum information processing or matter wave
interferometry. We focus on atom-surface interaction potentials induced by
vacuum fluctuations (Van der Waals and Casimir-Polder forces), and on
transitions between atomic quantum states that are induced by thermally excited
magnetic near fields. Open questions and current challenges are sketched.


title:
Reduction of Guided Acoustic Wave Brillouin Scattering in Photonic
  Crystal Fibers

abstract:
  Guided Acoustic Wave Brillouin Scattering (GAWBS) generates phase and
polarization noise of light propagating in glass fibers. This excess noise
affects the performance of various experiments operating at the quantum noise
limit. We experimentally demonstrate the reduction of GAWBS noise in a photonic
crystal fiber in a broad frequency range using cavity sound dynamics. We
compare the noise spectrum to the one of a standard fiber and observe a 10-fold
noise reduction in the frequency range up to 200 MHz. Based on our measurement
results as well as on numerical simulations we establish a model for the
reduction of GAWBS noise in photonic crystal fibers.


title: Noncyclic geometric changes of quantum states

abstract:
  Non-Abelian quantum holonomies, i.e., unitary state changes solely induced by
geometric properties of a quantum system, have been much under focus in the
physics community as generalizations of the Abelian Berry phase. Apart from
being a general phenomenon displayed in various subfields of quantum physics,
the use of holonomies has lately been suggested as a robust technique to obtain
quantum gates; the building blocks of quantum computers. Non-Abelian holonomies
are usually associated with cyclic changes of quantum systems, but here we
consider a generalization to noncyclic evolutions. We argue that this open-path
holonomy can be used to construct quantum gates. We also show that a structure
of partially defined holonomies emerges from the open-path holonomy. This
structure has no counterpart in the Abelian setting. We illustrate the general
ideas using an example that may be accessible to tests in various physical
systems.


title:
Hamiltonians with position-dependent mass, deformations and
  supersymmetry

abstract:
  A new method for generating exactly solvable Schr\"odinger equations with a
position-dependent mass is proposed. It is based on a relation with some
deformed Schr\"odinger equations, which can be dealt with by using a
supersymmetric quantum mechanical approach combined with a deformed
shape-invariance condition. The solvability of the latter is shown to impose
the form of both the deformed superpotential and the position-dependent mass.
The conditions for the existence of bound states are determined. A lot of
examples are provided and the corresponding bound-state spectrum and
wavefunctions are reviewed.


title: Some remarks about mass and Zitterbewegung substructure of electron

abstract:
  It is shown that the electron Zitterbewegung, that is, the high-frequency
microscopic oscillatory motion of electron about its centre of mass, originates
a spatial distribution of charge. This allows the point-like electron behave
like a particle of definite size whose self-energy, that is, energy of its
electromagnetic field, owns a finite value. This has implications for the
electron mass, which, in principle, might be derived from Zitterbewegung
physics.


title: Entangled Photon Pairs from Semiconductor Quantum Dots

abstract:
  Tomographic analysis demonstrates that the polarization state of pairs of
photons emitted from a biexciton decay cascade becomes entangled when spectral
filtering is applied. The measured density matrix of the photon pair satisfies
the Peres criterion for entanglement by more than 3 standard deviations of the
experimental uncertainty and violates Bell's inequality. We show that the
spectral filtering erases the ``which path'' information contained in the
photons color and that the remanent information in the quantum dot degrees of
freedom is negligible.


title:
On the conection between the Liouville equation and the Schrodinger
  equation

abstract:
  We derive a classical Schrodinger type equation from the classical Liouville
equation in phase space. The derivation is based on a Wigner type Fourier
transform of the classical phase space probability distribution, which depends
on an arbitrary constant $\alpha$ with dimension of action. In order to achieve
this goal two requirements are necessary: 1) It is assumed that the classical
probability amplitude $\Psi(x,t)$ can be expanded in a complete set of
functions $\Phi_n(x)$ defined in the configuration space; 2) the classical
phase space distribution $W(x,p,t)$ obeys the Liouville equation and is a real
function of the position, the momentum and the time. We show that the constant
$\alpha$ appearing in the Fourier transform of the classical phase space
distribution, and also in the classical Schrodinger type equation, has its
origin in the spectral distribution of the vacuum zero-point radiation, and is
identified with the Planck's constant $\hbar$.


title: A Question of Self-consistent Semifactuality

abstract:
  This article is intended as a compendium and guide to the variety of Bell
Inequality derivations that have appeared in the literature in recent years,
classifying them into six broad categories, revealing the underlying, often
hidden, assumption common to each - semifactuality. Evaluation of the attendant
conditional brings to light a significant EPR loophole that has not appeared in
the literature. Semantics for the inequality in the ongoing philosophic debate
that led to its discovery is discussed.


title: Quantum Direct Communication with Authentication

abstract:
  We propose two Quantum Direct Communication (QDC) protocols with user
authentication. Users can identify each other by checking the correlation of
Greenberger-Horne-Zeilinger (GHZ) states. Alice can directly send a secret
message to Bob using the remaining GHZ states after authentication. Our second
QDC protocol can be used even though there is no quantum link between Alice and
Bob. The security of the transmitted message is guaranteed by properties of
entanglement of GHZ states.


title: Contextual Value-definiteness and the Kochen-Specker Paradox

abstract:
  Compatibility between the realist tenants of value-definiteness and causality
is called into question by several realism impossibility proofs in which their
formal elements are shown to conflict. We review how this comes about in the
Kochen-Specker and von Neumann proofs and point out a connection between their
key assumptions: a constraint on realist causality via additivity in the latter
proof, noncontextuality in the former. We conclude that value-definiteness and
contextuality are indeed not mutually exclusive.


title: Syntactic and Semantic Distribution in Quantum Measurement

abstract:
  The nondistributivity of compound quantum mechanical propositions leads to a
theorem that rules out the possibility of microscopic deterministic hidden
variables, the Logical No-Go Theorem. We observe that there appear in fact two
distinct nondistributivity relations in the derivation: one with a semantics
governed by an empirical conjunctive syntax, the other composed of conjunctive
primitives in the quantum mechanical probability calculus. We venture to
speculate how the two come to be confused in the derivation of the theorem.


title: GHz QKD at telecom wavelengths using up-conversion detectors

abstract:
  We have developed a hybrid single photon detection scheme for telecom
wavelengths based on nonlinear sum-frequency generation and silicon
single-photon avalanche diodes (SPADs). The SPAD devices employed have been
designed to have very narrow temporal response, i.e. low jitter, which we can
exploit for increasing the allowable bit rate for quantum key distribution. The
wavelength conversion is obtained using periodically poled Lithium niobate
waveguides (W/Gs). The inherently high efficiency of these W/Gs allows us to
use a continuous wave laser to seed the nonlinear conversion so as to have a
continuous detection scheme. We also present a 1.27GHz qubit repetition rate,
one-way phase encoding, quantum key distribution experiment operating at
telecom wavelengths that takes advantage of this detection scheme. The proof of
principle experiment shows a system capable of MHz raw count rates with a QBER
less than 2% and estimated secure key rates greater than 100 kbit/s over 25 km.


title:
Operator equations and Moyal products -- metrics in quasi-hermitian
  quantum mechanics

abstract:
  The Moyal product is used to cast the equation for the metric of a
non-hermitian Hamiltonian in the form of a differential equation. For
Hamiltonians of the form $p^2+V(ix)$ with $V$ polynomial this is an exact
equation. Solving this equation in perturbation theory recovers known results.
Explicit criteria for the hermiticity and positive definiteness of the metric
are formulated on the functional level.


title: Paramagnetic Faraday rotation with spin-polarized ytterbium atoms

abstract:
  We report observation of the paramagnetic Faraday rotation of spin-polarized
ytterbium (Yb) atoms. As the atomic samples, we used an atomic beam, released
atoms from a magneto-optical trap (MOT), and trapped atoms in a
far-off-resonant trap (FORT). Since Yb is diamagnetic and includes a spin-1/2
isotope, it is an ideal sample for the spin physics, such as quantum
non-demolition measurement of spin (spin QND), for example. From the results of
the rotation angle, we confirmed that the atoms were almost perfectly
polarized.


title: Quantum algorithm for optical template recognition with noise filtering

abstract:
  We propose a probabilistic quantum algorithm that decides whether a
monochrome picture matches a given template (or one out of a set of templates).
As a major advantage to classical pattern recognition, the algorithm just
requires a few incident photons and is thus suitable for very sensitive
pictures (similar to the Elitzur-Vaidman problem). Furthermore, for a
$2^{n}\times 2^{m}$ image, $\ord(n+m)$ qubits are sufficient. Using the quantum
Fourier transform, it is possible to improve the fault tolerance of the quantum
algorithm by filtering out small-scale noise in the picture. For example images
with $512\times512$ pixels, we have numerically simulated the unitary
operations in order to demonstrate the applicability of the algorithm and to
analyze its fault tolerance.


title: A universally programmable Quantum Cellular Automaton

abstract:
  We discuss the role of classical control in the context of reversible quantum
cellular automata. Employing the structure theorem for quantum cellular
automata, we give a general construction scheme to turn an arbitrary cellular
automaton with external classical control into an autonomous one, thereby
proving the computational equivalence of these two models. We use this
technique to construct a universally programmable cellular automaton on a
one-dimensional lattice with single cell dimension 12.


title: Dephasing of quantum bits by a quasi-static mesoscopic environment

abstract:
  We examine coherent processes in a two-state quantum system that is strongly
coupled to a mesoscopic spin bath and weakly coupled to other environmental
degrees of freedom. Our analysis is specifically aimed at understanding the
quantum dynamics of solid-state quantum bits such as electron spins in
semiconductor structures and superconducting islands. The role of mesoscopic
degrees of freedom with long correlation times (local degrees of freedom such
as nuclear spins and charge traps) in qubit-related dephasing is discussed in
terms of a quasi-static bath. A mathemat- ical framework simultaneously
describing coupling to the slow mesoscopic bath and a Markovian environment is
developed and the dephasing and decoherence properties of the total system are
investigated. The model is applied to several specific examples with direct
relevance to current ex- periments. Comparisons to experiments suggests that
such quasi-static degrees of freedom play an important role in current qubit
implementations. Several methods of mitigating the bath-induced error are
considered.


title:
Time evolution of the classical and quantum mechanical versions of
  diffusive anharmonic oscillator: an example of Lie algebraic techniques

abstract:
  We present the general solutions for the classical and quantum dynamics of
the anharmonic oscillator coupled to a purely diffusive environment. In both
cases, these solutions are obtained by the application of the
Baker-Campbell-Hausdorff (BCH) formulas to expand the evolution operator in an
ordered product of exponentials. Moreover, we obtain an expression for the
Wigner function in the quantum version of the problem. We observe that the role
played by diffusion is to reduce or to attenuate the the characteristic quantum
effects yielded by the nonlinearity, as the appearance of coherent
superpositions of quantum states (Schr\"{o}dinger cat states) and revivals.


title:
Highly versatile atomic micro traps generated by multifrequency magnetic
  field modulation

abstract:
  We propose the realization of custom-designed adiabatic potentials for cold
atoms based on multimode radio frequency radiation in combination with static
inhomogeneous magnetic fields. For example, the use of radio frequency combs
gives rise to periodic potentials acting as gratings for cold atoms. In strong
magnetic field gradients the lattice constant can be well below 1 micrometer.
By changing the frequencies of the comb in time the gratings can easily be
propagated in space, which may prove useful for Bragg scattering atomic matter
waves. Furthermore, almost arbitrarily shaped potential are possible such as
disordered potentials on a scale of several 100 nm or lattices with a spatially
varying lattice constant. The potentials can be made state selective and, in
the case of atomic mixtures, also species selective. This opens new
perspectives for generating tailored quantum systems based on ultra cold single
atoms or degenerate atomic and molecular quantum gases.


title: Schmidt information and entanglement in quantum systems

abstract:
  The purpose of this paper is to study entanglement of quantum states by means
of Schmidt decomposition. The notion of Schmidt information which characterizes
the non-randomness of correlations between two observers that conduct
measurements of EPR-states is proposed. In two important particular cases - a
finite number of Schmidt modes with equal probabilities and Gaussian
correlations- Schmidt information is equal to Shannon information. A universal
measure of a dependence of two variables is proposed. It is based on Schmidt
number and it generalizes the classical Pearson correlation coefficient. It is
demonstrated that the analytical model obtained can be applied to testing the
numerical algorithm of Schmidt modes extraction. A thermodynamic interpretation
of Schmidt information is given. It describes the level of entanglement and
correlations of micro-system with its environment


title: Quantum Heat Engines Using Superconducting Quantum Circuits

abstract:
  We propose a quantum analog of the internal combustion engine used in most
cars. Specifically, we study how to implement the Otto-type quantum heat engine
(QHE) with the assistance of a Maxwell's demon. Three steps are required:
thermalization, quantum measurement, and quantum feedback controlled by the
Maxwell demon. We derive the positive-work condition of this composite QHE. Our
QHE can be constructed using superconducting quantum circuits. We explicitly
demonstrate the essential role of the demon in this macroscopic QHE.


title:
An Algorithm for Constructing Polynomial Systems Whose Solution Space
  Characterizes Quantum Circuits

abstract:
  An algorithm and its first implementation in C# are presented for assembling
arbitrary quantum circuits on the base of Hadamard and Toffoli gates and for
constructing multivariate polynomial systems over the finite field Z_2 arising
when applying the Feynman's sum-over-paths approach to quantum circuits. The
matrix elements determined by a circuit can be computed by counting the number
of common roots in Z_2 for the polynomial system associated with the circuit.
To determine the number of solutions in Z_2 for the output polynomial system,
one can use the Groebner bases method and the relevant algorithms for computing
Groebner bases.


title: Relativistic Bohmian interpretation of quantum mechanics

abstract:
  I present a relativistic covariant version of the Bohmian interpretation of
quantum mechanics and discuss the corresponding measurable predictions. The
covariance is incoded in the fact that the nonlocal quantum potential
transforms as a scalar, which is a consequence of the fact that the nonlocal
wave function transforms as a scalar. The measurable predictions that can be
obtained with the deterministic Bohmian interpretation cannot be obtained with
the conventional interpretation simply because the conventional probabilistic
interpretation does not work in the case of relativistic quantum mechanics.


title: Entanglement and Adiabatic Quantum Computation

abstract:
  Adiabatic quantum computation provides an alternative approach to quantum
computation using a time-dependent Hamiltonian. The time evolution of
entanglement during the adiabatic quantum search algorithm is studied, and its
relevance as a resource is discussed.


title:
Cluster-state preparation and multipartite entanglement analyzer with
  fermions

abstract:
  Quantum cluster states and entangled state analyzers are essential to
measurement-based quantum computing. We propose to generate a quantum
cluster-state and to make multipartite entanglement analyzer by using
noninteracting free electrons or conduction electrons in quantum dots, based on
polarizing beam splitters, charge detectors and single-spin rotations. Our
schemes are deterministic without the need of qubit-qubit interaction.


title:
The classical capacity for the quantum Markov channel of continuous
  variables

abstract:
  Quantum communications using continuous variables are quite mature
experimental techniques and the relevant theories have been extensively
investigated with various methods. In this paper, we study the continuous
variable quantum channels from a different angle, i.e., by exploring master
equations. And we finally give explicitly the capacity of the channel we are
studying. By the end of this paper, we derive the criterion for the optimal
capacities of the Gaussian channel versus its fidelity.


title:
Entanglement evaluation of non-Gaussian states generated by photon
  subtraction from squeezed states

abstract:
  We consider the problem of evaluating the entanglement of non-Gaussian mixed
states generated by photon subtraction from entangled squeezed states. The
entanglement measures we use are the negativity and the logarithmic negativity.
These measures possess the unusual property of being computable with linear
algebra packages even for high-dimensional quantum systems. We numerically
evaluate these measures for the non-Gaussian mixed states which are generated
by photon subtraction with on/off photon detectors. The results are compared
with the behavior of certain operational measures, namely the teleportation
fidelity and the mutual information in the dense coding scheme. It is found
that all of these results are mutually consistent, in the sense that whenever
the enhancement is seen in terms of the operational measures, the negativity
and the logarithmic negativity are also enhanced.


title:
Photon number resolving detection at telecom wavelength with a charge
  integration photon detector

abstract:
  We demonstrate multiphoton discrimination at telecom wavelength with the
readout frequency of 40 Hz by charge integration photon detector (CIPD). The
CIPD consists of an InGaAs pin photodiode and a GaAs junction field effect
transistor as a pre-amplifier in a charge integration circuit, which is cooled
to 4.2 K to reduce thermal noise. The quantum efficiency of the CIPD (detector
itself) is 80% for 1530 nm light, and the readout noise is measured as 0.26
electrons at 40 Hz. We can construct Poisson distributions of photo-carriers
with distinguished peaks at each photo-carrier number corresponding to the
signal to noise ratio of about 3.


title: Review article: Linear optical quantum computing

abstract:
  Linear optics with photon counting is a prominent candidate for practical
quantum computing. The protocol by Knill, Laflamme, and Milburn [Nature 409, 46
(2001)] explicitly demonstrates that efficient scalable quantum computing with
single photons, linear optical elements, and projective measurements is
possible. Subsequently, several improvements on this protocol have started to
bridge the gap between theoretical scalability and practical implementation. We
review the original theory and its improvements, and we give a few examples of
experimental two-qubit gates. We discuss the use of realistic components, the
errors they induce in the computation, and how these errors can be corrected.


title: Decay of Quantum Accelerator Modes

abstract:
  Experimentally observable Quantum Accelerator Modes are used as a test case
for the study of some general aspects of quantum decay from classical stable
islands immersed in a chaotic sea. The modes are shown to correspond to
metastable states, analogous to the Wannier-Stark resonances. Different regimes
of tunneling, marked by different quantitative dependence of the lifetimes on
1/hbar, are identified, depending on the resolution of KAM substructures that
is achieved on the scale of hbar. The theory of Resonance Assisted Tunneling
introduced by Brodier, Schlagheck, and Ullmo [9], is revisited, and found to
well describe decay whenever applicable.


title:
Multimode theory of measurement-induced non-Gaussian operation on
  wideband squeezed light

abstract:
  We present a multimode theory of non-Gaussian operation induced by an
imperfect on/off-type photon detector on a splitted beam from a wideband
squeezed light. The events are defined for finite time duration $T$ in the time
domain. The non-Gaussian output state is measured by the homodyne detector with
finite bandwidh $B$. Under this time- and band-limitation to the quantm states,
we develop a formalism to evaluate the frequency mode matching between the
on/off trigger channel and the conditional signal beam in the homodyne channel.
Our formalism is applied to the CW and pulsed schemes. We explicitly calculate
the Wigner function of the conditional non-Gaussian output state in a realistic
situation. Good mode matching is achieved for $BT\alt1$, where the discreteness
of modes becomes prominant, and only a few modes become dominant both in the
on/off and the homodyne channels. If the trigger beam is projected nearly onto
the single photon state in the most dominant mode in this regime, the most
striking non-classical effect will be observed in the homodyne statistics. The
increase of $BT$ and the dark counts degrades the non-classical effect.


title: Time Minimal Trajectories for a Spin 1/2 Particle in a Magnetic Field

abstract:
  In this paper we consider the minimum time population transfer problem for
the $z$-component of the spin of a (spin 1/2) particle driven by a magnetic
field, controlled along the x axis, with bounded amplitude. On the Bloch sphere
(i.e. after a suitable Hopf projection), this problem can be attacked with
techniques of optimal syntheses on 2-D manifolds. Let $(-E,E)$ be the two
energy levels, and $|\Omega(t)|\leq M$ the bound on the field amplitude. For
each couple of values $E$ and $M$, we determine the time optimal synthesis
starting from the level $-E$ and we provide the explicit expression of the time
optimal trajectories steering the state one to the state two, in terms of a
parameter that can be computed solving numerically a suitable equation. For
$M/E<<1$, every time optimal trajectory is bang-bang and in particular the
corresponding control is periodic with frequency of the order of the resonance
frequency $\omega_R=2E$. On the other side, for $M/E>1$, the time optimal
trajectory steering the state one to the state two is bang-bang with exactly
one switching. Fixed $E$ we also prove that for $M\to\infty$ the time needed to
reach the state two tends to zero. In the case $M/E>1$ there are time optimal
trajectories containing a singular arc. Finally we compare these results with
some known results of Khaneja, Brockett and Glaser and with those obtained by
controlling the magnetic field both on the $x$ and $y$ directions (or with one
external field, but in the rotating wave approximation). As byproduct we prove
that the qualitative shape of the time optimal synthesis presents different
patterns, that cyclically alternate as $M/E\to0$, giving a partial proof of a
conjecture formulated in a previous paper.


title: Local asymptotic normality for qubit states

abstract:
  We consider n identically prepared qubits and study the asymptotic properties
of the joint state \rho^{\otimes n}. We show that for all individual states
\rho situated in a local neighborhood of size 1/\sqrt{n} of a fixed state
\rho^0, the joint state converges to a displaced thermal equilibrium state of a
quantum harmonic oscillator. The precise meaning of the convergence is that
there exist physical transformations T_{n} (trace preserving quantum channels)
which map the qubits states asymptotically close to their corresponding
oscillator state, uniformly over all states in the local neighborhood.
  A few consequences of the main result are derived. We show that the optimal
joint measurement in the Bayesian set-up is also optimal within the pointwise
approach. Moreover, this measurement converges to the heterodyne measurement
which is the optimal joint measurement of position and momentum for the quantum
oscillator. A problem of local state discrimination is solved using local
asymptotic normality.


title: Dispersion and uncertainty in multislit matter wave diffraction

abstract:
  We show that single and multislit experiments involving matter waves may be
constructed to assess correlations between the position and momentum of a
single free particle. These correlations give rise to position dependent phases
which develop dynamically and may play an important role in the interference
patterns. For large enough transverse coherence lenght such interference
patterns are noticeably different from those of a classical dispersion free
wave.


title: Angular minimum uncertainty states with large uncertainties

abstract:
  The uncertainty relation for angle and angular momentum has a lower bound
which depends on the form of the state. Surprisingly, this lower bound can be
very large. We derive the states which have the lowest possible uncertainty
product for a given uncertainty in the angle or in the angular momentum. We
show that, if the given angle uncertainty is close to its maximum value, the
lowest possible uncertainty product tends to infinity.


title: Roots and Fruits of Decoherence

abstract:
  The concept of decoherence is defined, and discussed in a historical context.
This is illustrated by some of its essential consequences which may be relevant
for the interpretation of quantum theory. Various aspects of the formalism are
also reviewed for this purpose.
  Contents: 1. Definition of concepts. 2. Roots in nuclear physics. 3. The
quantum-to-classical transition. 4. Quantum mechanics without observables. 5.
Rules versus tools. 6. Nonlocality. 7. Information loss (paradox?). 8. Dynamics
of entanglement. 9. Irreversibility. 10. Concluding remarks.

categories: quant-ph cond-mat.mes-hall gr-qc nucl-th


title: Dynamical Consequences of Strong Entanglement

abstract:
  This review analyzes and compares two consequences of strong decoherence: The
quantum Zeno effect and macroscopic motion described by master equations.


title: Time-shift attack in practical quantum cryptosystems

abstract:
  Recently, a new type of attack, which exploits the efficiency mismatch of two
single photon detectors (SPD) in a quantum key distribution (QKD) system, has
been proposed. In this paper, we propose another "time-shift" attack that
exploits the same imperfection. In our attack, Eve shifts the arrival time of
either the signal pulse or the synchronization pulse or both between Alice and
Bob. In particular, in a QKD system where Bob employs time-multiplexing
technique to detect both bit "0" and bit "1" with the same SPD, Eve, in
principle, could acquire full information on the final key without introducing
any error. Finally, we discuss some counter measures against our and earlier
attacks.


title: Relativity and Lorentz Invariance of Entanglement Distillability

abstract:
  We study entanglement distillability of bipartite mixed spin states under
Wigner rotations induced by Lorentz transformations. We define weak and strong
criteria for relativistic isoentangled and isodistillable states to
characterize relative and invariant behavior of entanglement and
distillability. We exemplify these criteria in the context of Werner states,
where fully analytical methods can be achieved and all relevant cases
presented.


title: Qubit Complexity of Continuous Problems

abstract:
  The number of qubits used by a quantum algorithm will be a crucial
computational resource for the foreseeable future. We show how to obtain the
classical query complexity for continuous problems. We then establish a simple
formula for a lower bound on the qubit complexity in terms of the classical
query complexity


title: Multipartite states under local unitary transformations

abstract:
  The equivalence problem under local unitary transformation for $n$--partite
pure states is reduced to the one for $(n-1)$--partite mixed states. In
particular, a tripartite system
$\mathcal{H}_A\otimes\mathcal{H}_B\otimes\mathcal{H}_C$, where $\mathcal{H}_j$
is a finite dimensional complex Hilbert space for $j=A,B,C$, is considered and
a set of invariants under local transformations is introduced, which is
complete for the set of states whose partial trace with respect to
$\mathcal{H}_A$ belongs to the class of generic mixed states.


title:
Decoherence-free manipulation of photonic memories for quantum
  computation

abstract:
  We present a protocol to construct an arbitrary quantum circuit. The quantum
bits (qubits) are encoded in polarisation states of single photons. They are
stored in spatially separated dense media deposed in an optical cavity.
Specific sequences of pulses address individually the storage media to encode
the qubits and to implement a universal set of gates. The proposed protocol is
decoherence-free in the sense that spontaneous emission and cavity damping are
avoided. We discuss a coupling scheme for experimental implementation in Neon
atoms.


title:
Random measurement bases, quantum state distinction and applications to
  the hidden subgroup problem

abstract:
  We show that measuring any two quantum states by a random POVM, under a
suitable definition of randomness, gives probability distributions having total
variation distance at least a universal constant times the Frobenius distance
between the two states, with high probability. This result gives us the first
sufficient condition and an information-theoretic solution for the following
quantum state distinction problem: given an a priori known ensemble of quantum
states, is there a single POVM that gives reasonably large total variation
distance between every pair of states from the ensemble? Our random POVM method
also gives us the first information-theoretic upper bound on the number of
copies required to solve the quantum state identification problem for general
ensembles, i.e., given some number of independent copies of a quantum state
from an a priori known ensemble, identify the state. The standard quantum
approach to solving the hidden subgroup problem (HSP) is a special case of the
state identification problem where the ensemble consists of so-called coset
states of candidate hidden subgroups. Combining Fourier sampling with our
random POVM result gives us single register algorithms using polynomially many
copies of the coset state that identify hidden subgroups having polynomially
bounded rank in every representation of the ambient group. These HSP algorithms
complement earlier results about the powerlessness of random Fourier sampling
when the ranks are exponentially large, which happens for example in the HSP
over the symmetric group. The drawback of random Fourier sampling based
algorithms is that they are not efficient because measuring in a random basis
is not. This leads us to the open question of efficiently implementable
pseudo-random measurement bases.


title: Arnol'd Tongues and Quantum Accelerator Modes

abstract:
  The stable periodic orbits of an area-preserving map on the 2-torus, which is
formally a variant of the Standard Map, have been shown to explain the quantum
accelerator modes that were discovered in experiments with laser-cooled atoms.
We show that their parametric dependence exhibits Arnol'd-like tongues and
perform a perturbative analysis of such structures. We thus explain the
arithmetical organisation of the accelerator modes and discuss experimental
implications thereof.


title:
Application of the Frobenius method to the Schrodinger equation for a
  spherically symmetric potential: anharmonic oscillator

abstract:
  The power series method has been adapted to compute the spectrum of the
Schrodinger equation for central potential of the form $V(r)={d_{-2}\over
r^2}+{d_{-1}\over r}+\sum_{i=0}^{\infty} d_{i}r^i$. The bound-state energies
are given as zeros of a calculable function, if the potential is confined in a
spherical box. For an unconfined potential the interval bounding the energy
eigenvalues can be determined in a similar way with an arbitrarily chosen
precision. The very accurate results for various spherically symmetric
anharmonic potentials are presented.


title:
Genuine three-partite entangled states with a local hidden variable
  model

abstract:
  We present a family of three-qubit quantum states with a basic local hidden
variable model. Any von Neumann measurement can be described by a local model
for these states. We show that some of these states are genuine three-partite
entangled and also distillable. The generalization for larger dimensions or
higher number of parties is also discussed. As a byproduct, we present
symmetric extensions of two-qubit Werner states.


title:
Fast polarimetry system for the application to spin quantum
  non-demolition measurement

abstract:
  We report the development of a fast pulse polarimeter for the application to
quantum non-demolition measurement of atomic spin (Spin QND). The developed
system was tunable to the atomic resonance of ytterbium atom and has narrow
laser linewidth suitable for spin QND. Using the developed polarimeter, we
successfully demonstrated the measurement of the vacuum noise, with 10^6 to
10^7 photon number per pulse.


title: Quantum "Pathfinder"

abstract:
  It is discussed an opportunity to introduce new class of quantum algorithms
based on possibility to express amplitude of transition between two states of
quantum system as sum of some function along all possible classical paths.
Continuous analogue of the property with integral on all possible paths is well
known due to Feynman approach and ensures the correspondence with classical
minimal action principle. It is less technical and rather popular introduction
to earlier work about application of Lagrangian methods in quantum computing
[quant-ph/0308017].


title: Quantum Filtering of Markov Signals with White Quantum Noise

abstract:
  Time-continuous non-anticipating quantum processes of nondemolition
measurements are introduced as the dynamical realizations of the causal
quasi-measurements, which are described in this paper by the adapted
operator-valued probability measures on the trajectory spaces of the
generalized temporal observations in quantum open systems. In particular, the
notion of physically realizable quantum filter is defined and the problem of
its optimization to obtain the best a posteriori quantum state is considered.
It is proved that the optimal filtering of a quantum Markovian Gaussian signal
with the Gaussian white quantum noise is described as a coherent Markovian
linear filter generalizing the classical Kalman filter. As an example, the
problem of optimal measurement of complex amplitude for a quantum Markovian
open oscillator, loaded to a quantum wave communication line, is considered and
solved.


title:
Secure Ping-Pong quantum key distribution protocol with 33.3% bit error
  rate

abstract: This paper has been withdrawn and replaced by quant-ph/0609207.


title:
Experimental Demonstration of Quantum Entanglement Between
  Frequency-Nondegenerate Optical Twin Beams

abstract:
  The quantum entanglement of amplitude and phase quadratures between two
intense optical beams with the total intensity of 22mW and the frequency
difference of 1nm, which are produced from an optical parametric oscillator
operating above threshold, is experimentally demonstrated with two sets of
unbalanced Mach-Zehnder interferometers. The measured quantum correlations of
intensity and phase are in reasonable agreement with the results calculated
based on a semi-classical analysis of the noise characteristics given by C.
Fabre et al.


title: Quantum State Engineering with Continuous-Variable Post-Selection

abstract:
  We present a scheme to conditionally engineer an optical quantum system via
continuous-variable measurements. This scheme yields high-fidelity squeezed
single photon and superposition of coherent states, from input single and two
photon Fock states respectively. The input Fock state is interacted with an
ancilla squeezed vacuum state using a beam-splitter. We transform the quantum
system by post-selecting on the continuous-observable measurement outcome of
the ancilla state. We experimentally demonstrate the principles of this scheme
using displaced coherent states and measure experimentally fidelities that are
only achievable using quantum resources.


title:
Explaining the unobserved: why quantum mechanics is not only about
  information

abstract:
  A remarkable theorem by Clifton, Bub and Halvorson (2003)(CBH) characterizes
quantum theory in terms of information--theoretic principles. According to Bub
(2004, 2005) the philosophical significance of the theorem is that quantum
theory should be regarded as a ``principle'' theory about (quantum) information
rather than a ``constructive'' theory about the dynamics of quantum systems.
Here we criticize Bub's principle approach arguing that if the mathematical
formalism of quantum mechanics remains intact then there is no escape route
from solving the measurement problem by constructive theories. We further
propose a (Wigner--type) thought experiment that we argue demonstrates that
quantum mechanics on the information--theoretic approach is incomplete.