title:
Quantum entanglement inferred by the principle of maximum Tsallis
  entropy

abstract:
  The problem of quantum state inference and the concept of quantum
entanglement are studied using a non-additive measure in the form of Tsallis
entropy indexed by the positive parameter q. The maximum entropy principle
associated with this entropy along with its thermodynamic interpretation are
discussed in detail for the Einstein-Podolosky-Rosen pair of two spin-1/2
particles. Given the data on the Bell-Clauser-Horne-Shimony-Holt observable,
the analytic expression is given for the inferred quantum entangled state. It
is shown that for q greater than unity, indicating the sub-additive feature of
the Tsalls entropy, the entangled region is small and enlarges as one goes into
super-additive regime where q is less than unity. It is also shown that quantum
entanglement can be quantified by the generalized Kullback-Leibler entropy.


title: Determination of the Wigner function from photon statistics

abstract:
  We present an experimental realisation of the direct scheme for measuring the
Wigner function of a single quantized light mode. In this method, the Wigner
function is determined as the expectation value of the photon number parity
operator for the phase space displaced quantum state.


title: Coherent States for Transparent Potentials

abstract:
  Darboux transformation operators that produce multisoliton potentials are
analyzed as operators acting in a Hilbert space. Isometric correspondence
between Hilbert spaces of states of a free particle and a particle moving in a
soliton potential is established. It is shown that the Darboux transformation
operator is unbounded but closed and can not realize an isometric mapping
between Hilbert spaces. A quasispectral representation of such an operator in
terms of continuum bases is obtained. Different types of coherent states of a
multisoliton potential are introduced. Measures that realize the resolution of
the identity operator in terms of the projectors on the coherent states vectors
are calculated. It is shown that when these states are related with free
particle coherent states by a bounded symmetry operator the measure is defined
by ordinary functions and in the case of a semibounded symmetry operator the
measure is defined by a generalized function.


title: A simple proof of the unconditional security of quantum key distribution

abstract:
  Quantum key distribution is the most well-known application of quantum
cryptography. Previous proposed proofs of security of quantum key distribution
contain various technical subtleties. Here, a conceptually simpler proof of
security of quantum key distribution is presented. The new insight is the
invariance of the error rate of a teleportation channel: We show that the error
rate of a teleportation channel is independent of the signals being
transmitted. This is because the non-trivial error patterns are permuted under
teleportation. This new insight is combined with the recently proposed quantum
to classical reduction theorem. Our result shows that assuming that Alice and
Bob have fault-tolerant quantum computers, quantum key distribution can be made
unconditionally secure over arbitrarily long distances even against the most
general type of eavesdropping attacks and in the presence of all types of
noises.


title: Binding entanglement channels

abstract:
  We define the binding entanglement channel as the quantum channel through
which quantum information cannot be reliably transmitted, but which can be used
to share bound entanglement. We provide a characterization of such class of
channels. We also show that any bound entangled state can be used to
construction of the map corresponding the binding entanglement channel.


title: Optimal lower bounds for quantum automata and random access codes

abstract:
  Consider the finite regular language L_n = {w0 : w \in {0,1}^*, |w| \le n}.
It was shown by Ambainis, Nayak, Ta-Shma and Vazirani that while this language
is accepted by a deterministic finite automaton of size O(n), any one-way
quantum finite automaton (QFA) for it has size 2^{Omega(n/log n)}. This was
based on the fact that the evolution of a QFA is required to be reversible.
When arbitrary intermediate measurements are allowed, this intuition breaks
down. Nonetheless, we show a 2^{Omega(n)} lower bound for such QFA for L_n,
thus also improving the previous bound. The improved bound is obtained by
simple entropy arguments based on Holevo's theorem. This method also allows us
to obtain an asymptotically optimal (1-H(p))n bound for the dense quantum codes
(random access codes) introduced by Ambainis et al. We then turn to Holevo's
theorem, and show that in typical situations, it may be replaced by a tighter
and more transparent in-probability bound.


title:
Constructing Hamiltonian quantum theories from path integrals in a
  diffeomorphism invariant context

abstract:
  Osterwalder and Schrader introduced a procedure to obtain a (Lorentzian)
Hamiltonian quantum theory starting from a measure on the space of (Euclidean)
histories of a scalar quantum field. In this paper, we extend that construction
to more general theories which do not refer to any background, space-time
metric (and in which the space of histories does not admit a natural linear
structure). Examples include certain gauge theories, topological field theories
and relativistic gravitational theories. The treatment is self-contained in the
sense that an a priori knowledge of the Osterwalder-Schrader theorem is not
assumed.


title:
A Discrete Phase-Space Calculus for Quantum Spins based on a
  Reconstruction Method using Coherent States

abstract:
  To reconstruct a mixed or pure quantum state of a spin s is possible through
coherent states: its density matrix is fixed by the probabilities to measure
the value s along 4s(s+1) appropriately chosen directions in space. Thus, after
inverting the experimental data, the statistical operator is parametrized
entirely by expectation values. On this basis, a symbolic calculus for quantum
spins is developed, the `expectation-value representation.' It resembles the
Moyal representation for SU(2) but two important differences exist. On the one
hand, the symbols take values on a discrete set of points in phase space only.
On the other hand, no quasi-probabilities - that is, phase-space distributions
with negative values - are encountered in this approach.


title: Quantum information processing using quantum dot spins and cavity-QED

abstract:
  The electronic spin degrees of freedom in semiconductors typically have
decoherence times that are several orders of magnitude longer than other
relevant timescales. A solid-state quantum computer based on localized electron
spins as qubits is therefore of potential interest. Here, a scheme that
realizes controlled interactions between two distant quantum dot spins is
proposed. The effective long-range interaction is mediated by the vacuum field
of a high finesse microcavity. By using conduction-band-hole Raman transitions
induced by classical laser fields and the cavity-mode, parallel controlled-not
operations and arbitrary single qubit rotations can be realized. Optical
techniques can also be used to measure the spin-state of each quantum dot.


title:
Testing Bell's inequality with two-level atoms via population
  spectroscopy

abstract:
  We propose a feasible experimental scheme, employing methods of population
spectroscopy with two-level atoms, for a test of Bell's inequality for massive
particles. The correlation function measured in this scheme is the joint atomic
$Q$ function. An inequality imposed by local realism is violated by any
entangled state of a pair of atoms.


title: On energy transfer by detection of a tunneling atom

abstract:
  We are in the process of building an experiment to study the tunneling of
laser-cooled Rubidium atoms through an optical barrier. A particularly thorny
set of questions arises when one considers the possibility of observing a
tunneling particle while it is in the ``forbidden'' region. In earlier work, we
have discussed how one might probe a tunneling atom ``weakly,'' so as to
prevent collapse. Here we make some observations about the implications of a
more traditional quantum measurement. Considerations of energy conservation
suggest that attempts to observe tunneling atoms will enhance inelastic
scattering, but not in a way which can be directly observed. It is possible
that attempts to make such measurements may lead to experimentally realizable
``observationally assisted barrier penetration.''


title: Quantum soliton generation using an interferometer

abstract:
  For the first time a method for realizing macroscopic quantum optical
solitons is presented. Simultaneous photon-number and momentum squeezing is
predicted using soliton propagation in an interferometer. Extraction of soliton
pulses closer to true quantum solitons than their coherent counterparts from
mode-locked lasers is possible. Moreover, it is a general method of reducing
photon-number fluctuations below the shot-noise level for non-soliton pulses as
well. It is anticipated that similar reductions in particle fluctuations could
occur for other forms of interfering bosonic fields whenever self-interaction
nonlinearities exist, for example, interacting ultracold atoms.


title:
Efficient implementation of selective recoupling in heteronuclear spin
  systems using Hadamard matrices

abstract:
  We present an efficient scheme which couples any designated pair of spins in
heteronuclear spin systems. The scheme is based on the existence of Hadamard
matrices. For a system of $n$ spins with pairwise coupling, the scheme
concatenates $cn$ intervals of system evolution and uses at most $c n^2$ pulses
where $c \approx 1$. Our results demonstrate that, in many systems, selective
recoupling is possible with linear overhead, contrary to common speculation
that exponential effort is always required.


title:
Hall Normalization Constants for the Bures Volumes of the n-State
  Quantum Systems

abstract:
  We report the results of certain integrations of quantum-theoretic interest,
relying, in this regard, upon recently developed parameterizations of Boya et
al of the n x n density matrices, in terms of squared components of the unit
(n-1)-sphere and the n x n unitary matrices. Firstly, we express the normalized
volume elements of the Bures (minimal monotone) metric for n = 2 and 3,
obtaining thereby "Bures prior probability distributions" over the two- and
three-state systems. Then, as an essential first step in extending these
results to n > 3, we determine that the "Hall normalization constant" (C_{n})
for the marginal Bures prior probability distribution over the
(n-1)-dimensional simplex of the n eigenvalues of the n x n density matrices
is, for n = 4, equal to 71680/pi^2. Since we also find that C_{3} = 35/pi, it
follows that C_{4} is simply equal to 2^{11} C_{3}/pi. (C_{2} itself is known
to equal 2/pi.) The constant C_{5} is also found. It too is associated with a
remarkably simple decompositon, involving the product of the eight consecutive
prime numbers from 2 to 23.
  We also preliminarily investigate several cases, n > 5, with the use of
quasi-Monte Carlo integration. We hope that the various analyses reported will
prove useful in deriving a general formula (which evidence suggests will
involve the Bernoulli numbers) for the Hall normalization constant for
arbitrary n. This would have diverse applications, including quantum inference
and universal quantum coding.


title: Semiclassical dynamics of a spin-1/2 in an arbitrary magnetic field

abstract:
  The spin coherent state path integral describing the dynamics of a
spin-1/2-system in a magnetic field of arbitrary time-dependence is considered.
Defining the path integral as the limit of a Wiener regularized expression, the
semiclassical approximation leads to a continuous minimal action path with
jumps at the endpoints. The resulting semiclassical propagator is shown to
coincide with the exact quantum mechanical propagator. A non-linear
transformation of the angle variables allows for a determination of the
semiclassical path and the jumps without solving a boundary-value problem. The
semiclassical spin dynamics is thus readily amenable to numerical methods.


title:
Centrifugal terms in the WKB approximation and semiclassical
  quantization of hydrogen

abstract:
  A systematic semiclassical expansion of the hydrogen problem about the
classical Kepler problem is shown to yield remarkably accurate results. Ad hoc
changes of the centrifugal term, such as the standard Langer modification where
the factor l(l+1) is replaced by (l+1/2)^2, are avoided. The semiclassical
energy levels are shown to be exact to first order in $\hbar$ with all higher
order contributions vanishing. The wave functions and dipole matrix elements
are also discussed.


title:
Phase transitions and the internal noise structure of nonlinear
  Schr\"odi nger equation solitons

abstract:
  We predict phase-transitions in the quantum noise characteristics of systems
described by the quantum nonlinear Schr\"odinger equation, showing them to be
related to the solitonic field transition at half the fundamental soliton
amplitude. These phase-transitions are robust with respect to Raman noise and
scattering losses. We also describe the rich internal quantum noise structure
of the solitonic fields in the vicinity of the phase-transition. For optical
coherent quantum solitons, this leads to the prediction that eliminating the
peak side-band noise due to the electronic nonlinearity of silica fiber by
spectral filtering leads to the optimal photon-number noise reduction of a
fundamental soliton.


title: Quantum Computation as a Dynamical Process

abstract:
  In this paper, we discuss the dynamical issues of quantum computation. We
demonstrate that fast wave function oscillations can affect the performance of
Shor's quantum algorithm by destroying required quantum interference. We also
show that this destructive effect can be routinely avoided by using
resonant-pulse techniques. We discuss the dynamics of resonant pulse
implementations of quantum logic gates in Ising spin systems. We also discuss
the influence of non-resonant excitations. We calculate the range of parameters
where undesirable non-resonant effects can be minimized. Finally, we describe
the ``$2\pi k$-method'' which avoids the detrimental deflection of non-resonant
qubits.


title: Mathematical Inconsistencies in Dirac Field Theory

abstract:
  If a mathematical theory contains incompatible postulates then it is likely
that the theory will produce theorems or results that are contradictory. It
will be shown that this is the case with Dirac field theory. An example of such
a contradiction is the problem asociated with evaluating the Schwinger term. It
is generally known that different ways of evaluating this quantity yield
different results. It will be shown that the reason for this is that Dirac
field theory is mathematically inconsistent, i.e., it contains incompatible
assumptions or postulates. The generally accepted definition of the vacuum
state must be modified in order to create a consistent theory.


title:
Lower bounds of quantum black-box complexity and degree of approximation
  polynomials by influence of Boolean variables

abstract:
  We prove that, to compute a Boolean function $f$ on $N$ variables with error
probability $\epsilon$, any quantum black-box algorithm has to query at least
$\frac{1 - 2\sqrt{\epsilon}}{2} \rho_f N = \frac{1 - 2\sqrt{\epsilon}}{2}
\bar{S}_f$ times, where $\rho_f$ is the average influence of variables in $f$,
and $\bar{S}_f$ is the average sensitivity. It's interesting to contrast this
result with the known lower bound of $\Omega (\sqrt{S_f})$, where $S_f$ is the
sensitivity of $f$. This lower bound is tight for some functions. We also show
for any polynomial $\tilde{f}$ that approximates $f$ with error probability
$\epsilon$, $deg(\tilde{f}) \ge 1/4 (1 - \frac{3 \epsilon}{1 + \epsilon})^2
\rho_f N$. This bound can be better than previous known lower bound of
$\Omega(\sqrt{BS_f})$ for some functions. Our technique may be of intest
itself: we apply Fourier analysis to functions mapping $\{0, 1\}^N$ to unit
vectors in a Hilbert space. From this viewpoint, the state of the quantum
computer at step $t$ can be written as $\sum_{s\in \{0, 1\}^N, |s| \le t}
\hat{\phi}_s (-1)^ {s \cdot x}$, which is handy for lower bound analysis.


title: Self-Testing of Universal and Fault-Tolerant Sets of Quantum Gates

abstract:
  We consider the design of self-testers for quantum gates. A self-tester for
the gates F_1,...,F_m is a classical procedure that, given any gates
G_1,...,G_m, decides with high probability if each G_i is close to F_i. This
decision has to rely only on measuring in the computational basis the effect of
iterating the gates on the classical states. It turns out that instead of
individual gates, we can only design procedures for families of gates. To
achieve our goal we borrow some elegant ideas of the theory of program testing:
we characterize the gate families by specific properties, we develop a theory
of robustness for them, and show that they lead to self-testers. In particular
we prove that the universal and fault-tolerant set of gates consisting of a
Hadamard gate, a c-NOT gate, and a phase rotation gate of angle pi/4 is
self-testable.


title: Explicit product ensembles for separable quantum states

abstract:
  We present a general method for constructing pure-product-state
representations for density operators of $N$ quantum bits. If such a
representation has nonnegative expansion coefficients, it provides an explicit
separable ensemble for the density operator. We derive the condition for
separability of a mixture of the Greenberger-Horne-Zeilinger state with the
maximally mixed state.


title: Nonlinear von Neumann-type equations

abstract:
  We review some recent developments in the theory of nonlinear von Neumann
equations. We distinguish between the von Neumann equation (which can be
nonlinear) and the Liouville equation (which should be linear). Explicit
examples illustrate the technique of binary Darboux integration of nonlinear
density matrix equations and special attention is payed to the problem of how
to find physically nontrivial `self-scattering' solutions.


title:
Bogoliubov dispersion relation and the possibility of superfluidity for
  weakly-interacting photons in a 2D photon fluid

abstract:
  The Bogoliubov dispersion relation for the elementary excitations of the
weakly-interacting Bose gas is shown to hold for the case of the
weakly-interacting photon gas (the ``photon fluid'') in a nonlinear Fabry-Perot
cavity. This relation, which is also derived by means of a linearized
fluctuation analysis in classical nonlinear optics, implies the possibility of
a new superfluid state of light. The theory underlying an experiment in
progress to measure this dispersion relation is described, and another
experiment to test the prediction of the superfluidity of this state of light
is proposed.


title:
Implications of invariance of the Hamiltonian under canonical
  transformations in phase space

abstract:
  We observe that, within the effective generating function formalism for the
implementation of canonical transformations within wave mechanics, non-trivial
canonical transformations which leave invariant the form of the Hamilton
function of the classical analogue of a quantum system manifest themselves in
an integral equation for its stationary state eigenfunctions. We restrict
ourselves to that subclass of these dynamical symmetries for which the
corresponding effective generating functions are necessaarily free of quantum
corrections. We demonstrate that infinite families of such transformations
exist for a variety of familiar conservative systems of one degree of freedom.
We show how the geometry of the canonical transformations and the symmetry of
the effective generating function can be exploited to pin down the precise form
of the integral equations for stationary state eigenfunctions. We recover
several integral equations found in the literature on standard special
functions of mathematical physics. We end with a brief discussion (relevant to
string theory) of the generalization to scalar field theories in 1+1
dimensions.


title: Generic Bohmian Trajectories of an Isolated Particle

abstract:
  The generic Bohmian trajectories are calculated for an isolated particle in
an approximate energy eigenstate, for an arbitrary one-dimensional potential
well. It is shown, that the necessary and sufficient condition for there to be
a negligible probability of the trajectory deviating significantly from the
classical trajectory at any stage in the motion is, that the state be a
narrowly localised wave packet. The properties of the Bohmian trajectories are
discussed in relationship to the theory of retrodictively optimal simultaneous
measurements of position and momentum which was presented in several previous
papers. It is shown that the Bohmian velocity at x is the expectation value of
the velocity which would be observed at x, if one were to make a retrodictively
optimal simultaneous measurement of x and p, in the limit as the error in the
measurement of x tends to zero. This explains the tendency of the Bohmian
particle to behave in a highly non-classical manner. It also explains why the
trajectories in the interpretation recently proposed by Garcia de Polavieja
tend to be much more nearly classical in the limit of large quantum number. The
implications for other trajectory interpretations are considered.


title: The Meaning of Decoherence

abstract:
  The conceptual and dynamical aspects of decoherence are analyzed, while their
consequences are discussed for several fundamental applications. This
mechanism, which is based on a universal Schr\"odinger equation, is furthermore
compared with the phenomenological description of open systems in terms of
`quantum dynamical maps'.

categories: quant-ph cond-mat math-ph math.MP physics.atom-ph physics.chem-ph


title:
The exact solution for the superfluorescence of low density Frenkel
  excitons in double and triple lattice-layers

abstract:
  In low density regime, the fluorescence of Frenkel exitons in crystal slab
can be studied without the aid of rotating wave and Mackoffian approximation.
The equations for the case of double and triple lattice-layers are now solved
exactly to give the eigen decay rates, frequency shifts and the statistical
properties of the fields.


title: Post-Markov master equation for the dynamics of open quantum systems

abstract:
  A systematic first-order correction to the standard Markov master equation
for open quantum systems interacting with a bosonic bath is presented. It
extends the Markov Lindblad master equation to the more general case of
non-Markovian evolution. The meaning and applications of our `post'-Markov
master equation are illustrated with several examples, including a damped
two-level atom, the spin-boson model and the quantum Brownian motion model.
Limitations of the Markov approximation, the problem of positivity violation
and initial slips are also discussed.


title: Spontaneous decay in the presence of absorbing dielectric bodies

abstract:
  We present a formalism for studying the influence of dispersive and absorbing
dielectric bodies on a radiating atom in the framework of quantization of the
phenomenological Maxwell equations for given complex permittivities of the
bodies. In Markov approximation, the rate of spontaneous decay and the line
shift associated with it can then be related to the complex permittivities and
geometries of the bodies via the dyadic Green function of the classical
boundary value problem of electrodynamics -- a result which is in agreement
with second-order calculations for microscopic model systems. The theory is
applied to an atom near a planar interface as well as to an atom in a spherical
cavity. The latter, also known as the real-cavity model for spontaneous decay
of an excited atom embedded in a dielectric, is compared with the
virtual-cavity model. Connections with other approaches are mentioned and the
results are compared.


title:
Efficient Refocussing of One Spin and Two Spin Interactions for NMR
  Quantum Computation

abstract:
  The use of spin echoes to refocus one spin interactions (chemical shifts) and
two spin interactions (spin-spin couplings) plays a central role in both
conventional NMR experiments and NMR quantum computation. Here we describe
schemes for efficient refocussing of such interactions in both fully and
partially coupled spin systems.


title: Practical quantum cryptography for secure free-space communications

abstract:
  Quantum cryptography is an emerging technology in which two parties may
simultaneously generate shared, secret cryptographic key material using the
transmission of quantum states of light. The security of these transmissions is
based on the inviolability of the laws of quantum mechanics and
information-theoretically secure post-processing methods. An adversary can
neither successfully tap the quantum transmissions, nor evade detection, owing
to Heisenberg's uncertainty principle. In this paper we describe the theory of
quantum cryptography, and the most recent results from our experimental
free-space system with which we have demonstrated for the first time the
feasibility of quantum key generation over a point-to-point outdoor atmospheric
path in daylight. We achieved a transmission distance of 0.5 km, which was
limited only by the length of the test range. Our results provide strong
evidence that cryptographic key material could be generated on demand between a
ground station and a satellite (or between two satellites), allowing a
satellite to be securely re-keyed on orbit. We present a feasibility analysis
of surface-to-satellite quantum key generation.


title: Coherent States for the Deformed Algebras

abstract:
  We provide a unified approach for finding the coherent states of various
deformed algebras, including quadratic, Higgs and q-deformed algebras, which
are relevant for many physical problems. For the non-compact cases, coherent
states, which are the eigenstates of the respective annihilation operators, are
constructed by finding the canonical conjugates of these operators. We give a
general procedure to map these deformed algebras to appropriate Lie algebras.
Generalized coherent states, in the Perelomov sense, follow from this
construction.


title: Quantum equivalent of the Bertrand's theorem

abstract:
  A procedure for constructing bound state potentials is given. We show that,
under the natural conditions imposed on a radial eigenvalue problem, the only
special cases of the general central potential, which are exactly solvable and
have infinite number of energy eigenvalues, are the Coulomb and harmonic
oscillator potentials.


title: Derivation of Spin Vectors and Operators From First Principles

abstract:
  The interpretation of quantum mechanics due to Lande' is applied to the
connection between wave mechanics and matrix mechanics. The connection between
the differential eigenvalue equation and the matrix eigenvalue equation for an
operator is elucidated. In particular, we show that the elements of a matrix
vector state are probability amplitudes with a structure rather than being mere
constants. We obtain the most general expressions for the probability
amplitudes for the description of spin-1/2 measurements. As a result, we derive
spin-1/2 operators and vectors from first principles. The procedure used is
analogous to that by which orbital angular momentum wavefunctions and operators
are transformed to matrix mechanics vectors and matrices. The most generalized
forms of the spin operators and their eigenvectors for spin-1/2 are derived and
shown to reduce to the Pauli spin matrices and vectors in an appropriate limit.


title:
Quantum gates with neutral atoms: Controlling collisional interactions
  in time dependent traps

abstract:
  We theoretically study specific schemes for performing a fundamental
two-qubit quantum gate via controlled atomic collisions by switching
microscopic potentials. In particular we calculate the fidelity of a gate
operation for a configuration where a potential barrier between two atoms is
instantaneously removed and restored after a certain time. Possible
implementations could be based on microtraps created by magnetic and electric
fields, or potentials induced by laser light.


title: On the classical hydrodynamic limit of quantum field theories

abstract:
  We discuss the transition from a quantum to a classical domain for a model
where a separation into environment and system is explicitely not given.
Utilizing the coarse graining procedure for free quantum fields we also apply
the projection method and the Hamiltonian principle to study possible cases of
emergent classicality. General conditions for classical dynamics are given.
Eventually, they lead to the equations of motion for a perfect classical fluid.


title: Wave Function Interpretation and Quantum Mechanics Equations

abstract:
  The quantum mechanics description of a physical object stretched in space and
stable in time from the relativistic space-time properties point of view,
introduced in special theory of relativity, is considered and analysed. The
mathematical model of physical objects is proposed. This model gives a
possibility to unite a description of corpuscular and wave properties of real
physical objects, i.e. fields and particles. There are substantiated an
approach and a mathematical pattern which give a possibility to describe
physical object not only in causal, but also in absolute remote fields of the
Minkowski space. Applying the proposed approach to the microcosm description,
one can get the equations that in passage to the limit transfer to such quantum
mechanics equations as Schrodinger, Klein-Gordon-Fock and in particular case -
the wave equation. The event nature of the received equations is discussed. It
is shown that all mentioned equations reflect the space-time relativistic
properties during the description of the invariant and non-invariant physics
object characteristics.


title: A simple formula for ground state energy of a two-electron atom

abstract:
  A simple expression for a ground state energy for a two-electron atom is
derived. For this, assumption based upon the Niels Bohr ''old'' quantum
mechanics idea about electron correlation in a two-electron atom is exploited.
Results are compared with experimental data and theoretical results based on a
variation approach.


title:
Temporal behavior and quantum Zeno time of an excited state of the
  hydrogen atom

abstract:
  The quantum "Zeno" time of the 2P-1S transition of the hydrogen atom is
computed and found to be approximately 3.59 10^{-15}s (the lifetime is
approximately 1.595 10^{-9}s). The temporal behavior of this system is analyzed
in a purely quantum field theoretical framework and is compared to the
exponential decay law.


title:
A local hidden variable model of quantum correlation exploiting the
  detection loophole

abstract:
  A local hidden variable model exploiting the detection loophole to reproduce
exactly the quantum correlation of the singlet state is presented. The model is
shown to be compatible with both the CHSH and the CH Bell inequalities.
Moreover, it bears the same rotational symmetry as spins. The reason why the
model can reproduce the quantum correlation without violating the Bell theorem
is that in the model the efficiency of the detectors depends on the local
hidden variable. On average the detector efficiency is limited to 75%.


title:
Quantum-mechanical model for particles carrying electric charge and
  magnetic flux in two dimensions

abstract:
  We propose a simple quantum mechanical equation for $n$ particles in two
dimensions, each particle carrying electric charge and magnetic flux. Such
particles appear in (2+1)-dimensional Chern-Simons field theories as charged
vortex soliton solutions, where the ratio of charge to flux is a constant
independent of the specific solution. As an approximation, the charge-flux
interaction is described here by the Aharonov-Bohm potential, and the
charge-charge interaction by the Coulomb one. The equation for two particles,
one with charge and flux ($q, \Phi/Z$) and the other with ($-Zq, -\Phi$) where
$Z$ is a pure number is studied in detail. The bound state problem is solved
exactly for arbitrary $q$ and $\Phi$ when $Z>0$. The scattering problem is
exactly solved in parabolic coordinates in special cases when $q\Phi/2\pi\hbar
c$ takes integers or half integers. In both cases the cross sections obtained
are rather different from that for pure Coulomb scattering.


title: PT -symmetric harmonic oscillators

abstract:
  Within the framework of the recently proposed formalism using non-hermitean
Hamiltonians constrained merely by their PT invariance we describe a new
exactly solvable family of the harmonic-oscillator-like potentials with
non-equidistant spectrum. Our one-dimensional superposition of the harmonic x^2
with the centrifugal-like G/x^2 is regularized by a purely imaginary shift of
$x$.


title: Gauge invariance and the vacuum state

abstract:
  Quantum field theory is assumed to be gauge invariant. It is shown that for a
Dirac field the assumption of gauge invariance impacts on the way the vacuum
state is defined. It is shown that the conventional definition of the vacuum
state must be modified to take into account the requirements of gauge
invariance.


title: Entanglement measure and distance

abstract:
  Recently a new entanglemenet dilution scheme has been constructed by Lo and
Popescu. This paper points out that this result has a deep implication that the
entanglement measure for bipartite pure states is independent of the distance
between entangled two systems.


title: Free motion time-of-arrival operator and probability distribution

abstract:
  We reappraise and clarify the contradictory statements found in the
literature concerning the time-of-arrival operator introduced by Aharonov and
Bohm in Phys. Rev. {\bf 122}, 1649 (1961). We use Naimark's dilation theorem to
reproduce the generalized decomposition of unity (or POVM) from any
self-adjoint extension of the operator, emphasizing a natural one, which arises
from the analogy with the momentum operator on the half-line. General time
operators are set within a unifying perspective. It is shown that they are not
in general related to the time of arrival, even though they may have the same
form.


title: No Signalling and Probabilistic Quantum Cloning

abstract:
  We show that the condition of no faster-than-light signalling restricts the
number of quantum states that can be cloned in a given Hilbert space. This
condition leads to the constraints on a probabilistic quantum cloning machine
(PQCM) recently found by Duan and Guo.


title:
Coherent Control of Atom-Atom Interactions and Entanglement using
  Optical Fields

abstract:
  Two-photon optical transitions combined with long-range dipole-dipole
interactions can be used for the coherent manipulation of collective metastable
states composed of different atoms. We show that it is possible to induce
optical resonances accompanied by the generation of entangled superpositions of
the atomic states. Resonances of this kind can be used to implement quantum
logic gates using optically excited single atoms (impurities) in the condensed
phase.


title: Quantum finite multitape automata

abstract:
  Quantum finite automata were introduced by C.Moore, J.P. Crutchfield, and by
A.Kondacs and J.Watrous. This notion is not a generalization of the
deterministic finite automata. Moreover, it was proved that not all regular
languages can be recognized by quantum finite automata. A.Ambainis and
R.Freivalds proved that for some languages quantum finite automata may be
exponentially more concise rather than both deterministic and probabilistic
finite automata. In this paper we introduce the notion of quantum finite
multitape automata and prove that there is a language recognized by a quantum
finite automaton but not by a deterministic or probabilistic finite automata.
This is the first result on a problem which can be solved by a quantum computer
but not by a deterministic or probabilistic computer. Additionally we discover
unexpected probabilistic automata recognizing complicated languages.


title: Toward fault-tolerant quantum computation without concatenation

abstract:
  It has been known that quantum error correction via concatenated codes can be
done with exponentially small failure rate if the error rate for physical
qubits is below a certain accuracy threshold. Other, unconcatenated codes with
their own attractive features-improved accuracy threshold, local
operations-have also been studied. By iteratively distilling a certain
two-qubit entangled state it is shown how to perform an encoded Toffoli gate,
important for universal computation, on CSS codes that are either
unconcatenated or, for a range of very large block sizes, singly concatenated.


title:
Experimental Demonstration of Greenberger-Horne-Zeilinger Correlations
  Using Nuclear Magnetic Resonance

abstract:
  The Greenberger-Horne-Zeilinger (GHZ) effect provides an example of quantum
correlations that cannot be explained by classical local hidden variables. This
paper reports on the experimental realization of GHZ correlations using nuclear
magnetic resonance (NMR). The NMR experiment differs from the originally
proposed GHZ experiment in several ways: it is performed on mixed states rather
than pure states; and instead of being widely separated, the spins on which it
is performed are all located in the same molecule. As a result, the NMR version
of the GHZ experiment cannot entirely rule out classical local hidden
variables. It nonetheless provides an unambiguous demonstration of the
"paradoxical" GHZ correlations, and shows that any classical hidden variables
must communicate by non-standard and previously undetected forces. The NMR
demonstration of GHZ correlations shows the power of NMR quantum information
processing techniques for demonstrating fundamental effects in quantum
mechanics.


title:
Phase properties of hypergeometric states and negative hypergeometric
  states

abstract:
  We show that the three quantum states (P$\acute{o}$lya states, the
generalized non-classical states related to Hahn polynomials and negative
hypergeometric states) introduced recently as intermediates states which
interpolate between the binomial states and negative binomial states are
essentially identical. By using the Hermitial-phase-operator formalism, the
phase properties of the hypergeometric states and negative hypergeometric
states are studied in detail. We find that the number of peaks of phase
probability distribution is one for the hypergeometric states and $M$ for the
negative hypergeometric states.


title: A Realizable, Non-Null Schroedinger's Cat Experiment

abstract:
  Working from the Schroedinger's Cat paradigm, a series of experiments are
constructed. The Bedford-Wang experiment is examined, and the ambiguity in its
meaning is addressed. We eliminate this ambiguity by abandoning the idea of the
triggering event, replacing the two-state system with a mirror that undergoes
wave packet spreading. This creates an experimentally testable version of a
modified Schroedinger's Cat experiment for which a null result is not the
obvious outcome.


title: A one-dimensional lattice model for a quantum mechanical free particle

abstract:
  Two types of particles, A and B with their corresponding antiparticles, are
defined in a one dimensional cyclic lattice with an odd number of sites. In
each step of time evolution, each particle acts as a source for the
polarization field of the other type of particle with nonlocal action but with
an effect decreasing with the distance: A -->...\bar{B} B \bar{B} B \bar{B} ...
; B --> A \bar{A} A \bar{A} A ... . It is shown that the combined distribution
of these particles obeys the time evolution of a free particle as given by
quantum mechanics.


title: Quantum Mechanics as a Classical Field Theory

abstract:
  The formalism of quantum mechanics is presented in a way that its
interpretation as a classical field theory is emphasized. Two coupled real
fields are defined with given equations of motion. Densities and currents
associated to the fields are found with their corresponding conserved
quantities. The behavior of these quantities under a galilean transformation
suggest the association of the fields with a quantum mechanical free particle.
An external potential is introduced in the Lagrange formalism. The description
is equivalent to the conventional Schr\"odinger equation treatment of a
particle. We discuss the attempts to build an interpretation of quantum
mechanics based on this scheme. The fields become the primary onthology of the
theory and the particles appear as emergent properties of the fields. These
interpretations face serious problems for systems with many degrees of freedom.


title: Minimal Length Uncertainty Relation and Hydrogen Atom

abstract:
  We propose a new approach to calculate perturbatively the effects of a
particular deformed Heisenberg algebra on energy spectrum. We use this method
to calculate the harmonic oscillator spectrum and find that corrections are in
agreement with a previous calculation. Then, we apply this approach to obtain
the hydrogen atom spectrum and we find that splittings of degenerate energy
levels appear. Comparison with experimental data yields an interesting upper
bound for the deformation parameter of the Heisenberg algebra.


title: Sonoluminescence as a QED vacuum effect. II: Finite Volume Effects

abstract:
  In a companion paper [quant-ph/9904013] we have investigated several
variations of Schwinger's proposed mechanism for sonoluminescence. We
demonstrated that any realistic version of Schwinger's mechanism must depend on
extremely rapid (femtosecond) changes in refractive index, and discussed ways
in which this might be physically plausible. To keep that discussion tractable,
the technical computations in that paper were limited to the case of a
homogeneous dielectric medium. In this paper we investigate the additional
complications introduced by finite-volume effects. The basic physical scenario
remains the same, but we now deal with finite spherical bubbles, and so must
decompose the electromagnetic field into Spherical Harmonics and Bessel
functions. We demonstrate how to set up the formalism for calculating Bogolubov
coefficients in the sudden approximation, and show that we qualitatively retain
the results previously obtained using the homogeneous-dielectric (infinite
volume) approximation.


title: A protocol to preserve quantum coherence in cavity QED

abstract:
  We consider the problem of quantum decoherence in cavity QED devices and
investigate the possibility to preserve a Schroedinger cat as a coherent
superposition along the time.


title: Optimal Universal Disentangling Machine for Two Qubit Quantum States

abstract:
  We derive the optimal curve satisfied by the reduction factors, in the case
of universal disentangling machine which uses only local operations.
Impossibility of constructing a better disentangling machine, by using
non-local operations, is discussed.


title: Quantum mechanics on a real Hilbert space

abstract:
  The complex Hilbert space of standard quantum mechanics may be treated as a
real Hilbert space. The pure states of the complex theory become mixed states
in the real formulation. It is then possible to generalize standard quantum
mechanics, keeping the same set of physical states, but admitting more general
observables. The standard time reversal operator involves complex conjugation,
in this sense it goes beyond the complex theory and may serve as an example to
motivate the generalization. Another example is unconventional canonical
quantization such that the harmonic oscillator of angular frequency $\omega$
has any given finite or infinite set of discrete energy eigenvalues, limited
below by $\hbar\omega/2$.


title: Gravity-Induced Interference and Continuous Quantum Measurements

abstract:
  Gravity-induced quantum interference is a remarkable effect that has already
been confirmed experimentally, and it is a phenomenon in which quantum
mechanics and gravity play simultaneously an important role. Additionally, a
generalized version of this interference experiment could offer the possibility
to confront against measurement outputs one of the formalisms that claim to
give an explanation to the so called quantum measurement problem, namely the
restricted path integral formalism. In this work we will analyze a possible
extension of Colella, Overhauser, and Werner experiment and find that in the
context of the restricted path integral formalism we obtain new interference
terms that could be measured in an extended version of this experimental
construction. These new terms not only show, as in the first experiment, that
at the quantum level gravity is not a purely geometric effect, it still depends
on mass, but also show that interference does depend on some parameters that
appear in the restricted path integral formalism, thus offering the possibility
to have a testing framework for its theoretical predictions.


title:
Experimental realization of Popper's Experiment: Violation of the
  Uncertainty Principle?

abstract:
  An entangled pair of photons (1 and 2) are emitted to opposite directions. A
narrow slit is placed in the path of photon 1 to provide precise knowledge of
its position on the $y$ axis and this also determines the precise $y$ position
of its twin, photon 2, due to quantum entanglement. Is photon 2 going to
experience a greater uncertainty in momentum, i.e., a greater $\Delta p_{y}$,
due to the precise knowledge of its position $y$? The experimental data shows
$\Delta y\Delta p_{y}<\hbar $ for photon 2. Can this recent realization of the
historical thought experiment of Karl Popper signal a violation of the
uncertainty principle?


title:
Quantum statistical properties of the radiation field in a cavity with a
  movable mirror

abstract:
  A quantum system composed of a cavity radiation field interacting with a
movable mirror is considered and quantum statistical properties of the field
are studied. Such a system can serve in principle as an idealized meter for
detection of a weak classical force coupled to the mirror which is modelled by
a quantum harmonic oscillator. It is shown that the standard quantum limit on
the measurement of the mirror position arises naturally from the properties of
the system during its dynamical evolution. However, the force detection
sensitivity of the system falls short of the corresponding standard quantum
limit. We also study the effect of the nonlinear interaction between the moving
mirror and the radiation pressure on the quadrature fluctuations of the
initially coherent cavity field.


title:
Realization of logically labeled effective pure states for bulk quantum
  computation

abstract:
  We report the first use of "logical labeling" to perform a quantum
computation with a room-temperature bulk system. This method entails the
selection of a subsystem which behaves as if it were at zero temperature -
except for a decrease in signal strength - conditioned upon the state of the
remaining system. No averaging over differently prepared molecules is required.
In order to test this concept, we execute a quantum search algorithm in a
subspace of two nuclear spins, labeled by a third spin, using solution nuclear
magnetic resonance (NMR), and employing a novel choice of reference frame to
uncouple nuclei.


title: Maximal Beable Subalgebras of Quantum-Mechanical Observables

abstract:
  Given a state on an algebra of bounded quantum-mechanical observables (the
self-adjoint part of a C*-algebra), we investigate those subalgebras that are
maximal with respect to the property that the given state's restriction to the
subalgebra is a mixture of dispersion-free states---what we call maximal
"beable" subalgebras (borrowing a terminology due to J. S. Bell). We also
extend our investigation to the theory of algebras of unbounded observables (as
developed by R. Kadison), and show how our results articulate a solid
mathematical foundation for central tenets of the orthodox Copenhagen
interpretation of quantum theory (such as the joint indeterminacy of
canonically conjugate observables, and Bohr's defense of the completeness of
quantum theory against the argument of Einstein, Podolsky, and Rosen).


title:
Operation of universal gates in a DXD superconducting solid state
  quantum computer

abstract:
  We demonstrate that complete set of gates can be realized in a DXD
superconducting solid state quantum computer (quamputer), thereby proving its
universality.

categories: quant-ph cond-mat.mes-hall cond-mat.other cond-mat.supr-con cs.GL


title: Klein paradox and antiparticle

abstract:
  The Klein paradox of Klein-Gordon (KG) equation is discussed to show that KG
equation is self-consistent even at one-particle level and the wave function
for antiparticle is uniquely determined by the reasonable explanation of Klein
paradox. No concept of ``hole'' is needed.


title: Quantum Simulations on a Quantum Computer

abstract:
  We present a general scheme for performing a simulation of the dynamics of
one quantum system using another. This scheme is used to experimentally
simulate the dynamics of truncated quantum harmonic and anharmonic oscillators
using nuclear magnetic resonance. We believe this to be the first explicit
physical realization of such a simulation.


title: Non-separability without Non-separability in Nonlinear Quantum Mechanics

abstract:
  We show an example of benign non-separability in an apparently separable
system consisting of $n$ free non-correlated quantum particles, solitonic
solutions to the nonlinear phase modification of the Schr\"{o}dinger equation
proposed recently. The non-separability manifests itself in the wave function
of a single particle being influenced by the very presence of other particles.
In the simplest case of identical particles, it is the number of particles that
affects the wave function of each particle and, in particular, the width of its
Gaussian probability density. As a result, this width, a local property, is
directly linked to the mass of the entire Universe in a very Machian manner. In
the realistic limit of large $n$ if the width in question is to be microscopic,
the coupling constant must be very small resulting in an ``almost linear''
theory. This provides a model explanation of why the linearity of quantum
mechanics can be accepted with such a high degree of certainty even if the more
fundamental underlying theory could be nonlinear. We also demonstrate that when
such non-correlated solitons are coupled to harmonic oscilators they lead to a
faster-than-light nonlocal telegraph since changing the frequency of one
oscillator affects instantaneously the probability density of particles
associated with other oscillators. This effect can be alleviated by fine-tuning
the parameters of the solution. Exclusion rules of a novel kind that we term
supersuperselection rules also emerge from these solutions. They are similar to
the mass and the univalence superselection rules in linear quantum mechanics.
The effects in question and the exclusion rules do not appear if a weakly
separable extension to $n$-particles is employed.


title: Transmission time of wave packets through tunneling barriers

abstract:
  The transmission of wave packets through tunneling barriers is studied in
detail by the method of quantum molecular dynamics. The distribution function
of the times describing the arrival of a tunneling packet in front of and
behind a barrier and the momentum distribution function of the packet are
calculated. The behavior of the average coordinate of a packet, the average
momentum, and their variances is investigated. It is found that under the
barrier a part of the packet is reflected and a Gaussian barrier increases the
average momentum of the transmitted packet and its variance in momentum space.


title:
Quantum Superposition of Parametrically Amplified Multiphoton Pure
  States whitin a Decoherence-Free Schroedinger-Cat Structure

abstract:
  The new process of quantum-injection into an optical parametric amplifier
operating in entangled configuration is adopted to amplify into a large
dimensionality spin 1/2 Hilbert space the quantum entanglement and
superposition properties of the photon-couples generated by parametric
down-conversion. The structure of the Wigner function and of the field's
correlation functions shows a decoherence-free, multiphoton Schroedinger-cat
behaviour of the emitted field which is largely detectable against the
squeezed-vacuum noise. Furthermore, owing to its entanglement character, the
system is found to exhibit multi-particle quantum nonseparability and Bell-type
nonlocality properties. These relevant quantum features are analyzed for
several travelling-wave optical configurations implying different input
quantum-injection schemes


title: Quantum synthesis of arbitrary unitary operators

abstract:
  Nature provides us with a restricted set of microscopic interactions. The
question is whether we can synthesize out of these fundamental interactions an
arbitrary unitary operator. In this paper we present a constructive algorithm
for realization of any unitary operator which acts on a (truncated) Hilbert
space of a single bosonic mode. In particular, we consider a physical
implementation of unitary transformations acting on 1-dimensional vibrational
states of a trapped ion. As an example we present an algorithm which realizes
the discrete Fourier transform.


title: Fundamental Limit on ``Interaction Free'' Measurements

abstract:
  In ``interaction free'' measurements, one typically wants to detect the
presence of an object without touching it with even a single photon. One often
imagines a bomb whose trigger is an extremely sensitive measuring device whose
presence we would like to detect without triggering it. We point out that all
such measuring devices have a maximum sensitivity set by the uncertainty
principle, and thus can only determine whether a measurement is ``interaction
free'' to within a finite minimum resolution. We further discuss exactly what
can be achieved with the proposed ``interaction free'' measurement schemes.


title:
A Numerical Investigation of the Effects of Classical Phase Space
  Structure on a Quantum System

abstract:
  We present a detailed numerical study of a chaotic classical system and its
quantum counterpart. The system is a special case of a kicked rotor and for
certain parameter values possesses cantori dividing chaotic regions of the
classical phase space. We investigate the diffusion of particles through a
cantorus; classical diffusion is observed but quantum diffusion is only
significant when the classical phase space area escaping through the cantorus
per kicking period greatly exceeds Planck's constant. A quantum analysis
confirms that the cantori act as barriers. We numerically estimate the
classical phase space flux through the cantorus per kick and relate this
quantity to the behaviour of the quantum system. We introduce decoherence via
environmental interactions with the quantum system and observe the subsequent
increase in the transport of quantum particles through the boundary.


title:
Bose-Einstein Partition Statistics of Photons Emitted from a
  Superradiant Active Microcavity

abstract:
  We report the results of the first investigation on the superradiant temporal
and spatial quantum dynamics of two dipoles excited in a planar symmetrical
microcavity by a controlled femtosecond two-pulse excitation. A superradiant
enhancement of the time decay of the dipole excitation for a decreasing
inter-dipole transverse distance R has been found. Furthermore, the photon
partition statistics of the emitted field is found to exhibit a striking
quantum behaviour for R<lc, the transverse extension of the single allowed
microcavity mode.


title: Quantum Ontologies and Mind-Matter Synthesis

abstract:
  Aspects of a quantum mechanical theory of a world containing efficacious
mental aspects that are closely tied to brains, but that are not identical to
brains.


title: Attention, Intention, and Will in Quantum Physics

abstract:
  The need for a self-observing quantum system to pose questions leads to a
tripartite quantum process involving a Schroedinger process that is local
deterministic, a Heisenberg process that poses the question, and a Dirac
process that picks the answer. In the classical limit where Planck's constant
is set to zero these three processes reduce to one single deterministic
classical process: the fine structure wherein lies the effect of mind upon
matter is obliterated.


title: Nonlocality, Counterfactual, and Consistent Histories

abstract:
  The paper develops a version of modal logic that stays completely within the
framework provided by quantum principles, and then proves, within the framework
of quantum thinking, and in particular without invoking "hidden variables", a
Bell-type nonlocality result.


title: Fast and User-friendly Quantum Key Distribution

abstract:
  Some guidelines for the comparison of different quantum key distribution
experiments are proposed. An improved 'plug & play' interferometric system
allowing fast key exchange is then introduced. Self-alignment and compensation
of birefringence remain. Original electronics implementing the BB84 protocol
and allowing user-friendly operation is presented. Key creation with 0.1 photon
per pulse at a rate of 486 Hz with a 5.4% QBER - corresponding to a net rate of
210Hz - over a 23 Km installed cable was performed.


title: The test particle motion equations metrical form in a potential field

abstract:
  It is shown in the present work that the three-dimensional trajectories of an
electrical test particle in potential fields may be regarded as geodesic lines
lying on isotropic surfaces of some four-dimensional configurational space, the
connection of which has tortion, while the transference is nonmetric.


title: Optimal compression for mixed signal states

abstract:
  We consider the problem of the optimal compression rate in the case of the
source producing mixed signal states within the {\it visible} scheme (where
Alice, who is to compress the signal, can know the identities of the produced
states). We show that a simple strategy based on replacing the signal states
with their {\it extensions} gives {\it optimal} compression. As a result we
obtain a considerable simplification of the formula for optimal compression
rate within visible scheme.


title: Monte Carlo Hamiltonian

abstract:
  We construct an effective Hamiltonian via Monte Carlo from a given action.
This Hamiltonian describes physics in the low energy regime. We test it by
computing spectrum, wave functions and thermodynamical observables (average
energy and specific heat) for the free system and the harmonic oscillator. The
method is shown to work also for other local potentials.


title: Quantum correlations are not contained in the initial state

abstract:
  Two proofs are presented which show that quantum mechanics is incompatible
with the following assumption: all possible correlations between subsystems of
an individual isolated composite quantum system are contained in the initial
quantum state of the whole system, although just a subset of them is revealed
by the actual experiment.


title: Observations of Quantum Dynamics by Solution-State NMR Spectroscopy

abstract:
  NMR is emerging as a valuable testbed for the investigation of foundational
questions in quantum mechanics. The present paper outlines the preparation of a
class of mixed states, called pseudo-pure states, that emulate pure quantum
states in the highly mixed environment typically used to describe
solution-state NMR samples. It also describes the NMR observation of spinor
behavior in spin 1/2 nuclei, the simulation of wave function collapse using a
magnetic field gradient, the creation of entangled (or Bell) pseudo-pure
states, and a brief discussion of quantum computing logic gates, including the
Quantum Fourier Transform. These experiments show that liquid-state NMR can be
used to demonstrate quantum dynamics at a level suitable for laboratory
exercises.


title: Bell inequality for arbitrary many settings of the analyzers

abstract:
  A generalization of the CHSH-Bell inequality to arbitrary many settings is
presented. The singlet state of two spin $\half$ violates this inequality for
all numbers of setting. In the limit of arbitrarily large number of settings,
the violation tends to the finite ratio $4/\pi \approx 1.27$.


title:
An approximation to the Lamb shift and hyperfine splitting as nonlinear
  effective Coulomb-like interactions in the Dirac equation

abstract:
  The Dirac equation for the Coulomb problem is restated by incorporating a
nonlinear effective interaction into the Dirac Hamiltonian: one keeps the $1/r$
dependence for the Coulomb field, but the coupling constant is modified by a
factor depending on the n (principal quantum number) power of the mean value of
the Hamiltonian. In this simple context we study the Lamb shift and the
hyperfine splitting of the s-levels of hydrogenic atoms. We discuss to what
extent the corresponding calculations fit the energy splittings to the
appropriate order in the fine structure constant.


title: Quantum feedback with weak measurements

abstract:
  The problem of feedback control of quantum systems by means of weak
measurements is investigated in detail. When weak measurements are made on a
set of identical quantum systems, the single-system density matrix can be
determined to a high degree of accuracy while affecting each system only
slightly. If this information is fed back into the systems by coherent
operations, the single-system density matrix can be made to undergo an
arbitrary nonlinear dynamics, including for example a dynamics governed by a
nonlinear Schr\"odinger equation. We investigate the implications of such
nonlinear quantum dynamics for various problems in quantum control and quantum
information theory, including quantum computation. The nonlinear dynamics
induced by weak quantum feedback could be used to create a novel form of
quantum chaos in which the time evolution of the single-system wave function
depends sensitively on initial conditions.


title: Losing Your Marbles in Wavefunction Collapse Theories

abstract:
  Peter Lewis ([1997]) has recently argued that the wavefunction collapse
theory of GRW (Ghirardi, Rimini, and Weber [1986]) can only solve the problem
of wavefunction tails at the expense of predicting that arithmetic does not
apply to ordinary macroscopic objects. More specifically, Lewis argues that the
GRW theory must violate the enumeration principle: that `if marble 1 is in the
box and marble 2 is in the box and so on through marble $n$, then all $n$
marbles are in the box' ([1997], p. 321). Ghirardi and Bassi ([1999]) have
replied that it is meaningless to say that the enumeration principle is
violated because the wavefunction Lewis uses to exhibit the violation cannot
persist, according to the GRW theory, for more than a split second ([1999], p.
709). On the contrary, we argue that Lewis's argument survives Ghirardi and
Bassi's criticism unscathed. We then go on to show that, while the enumeration
principle can fail in the GRW theory, the theory itself guarantees that the
principle can never be empirically falsified, leaving the applicability of
arithmetical reasoning to both micro- and macroscopic objects intact.


title:
su(1,1) coherent states and a normal extension of su(1,1) annihilation
  operator: squeezed states and the simultaneous measurement of Q^{-1}P+PQ^{-1}
  and Q^{-2}

abstract:
  The the over-complete eigenvector system of the operator Q^{-1}P (Q:position,
P:momentum) which consists of the squeezed states |0; s, t > with various s and
t are investigated from the viewpoint of the annihilation and creation
relations related to the algebra su(1,1). We derive a positive operator-valued
measure(POVM) for the simultaneous measurement between the self-adjoint and
anti-self-adjoint parts of PQ^{-1}.


title:
Statistics of Raman-Active Excitations via Masurement of
  Stokes-Anti-Stokes Correlations

abstract:
  A general fundamental relation connecting the correlation of Stokes and
anti-Stokes modes to the quantum statistical behavior of vibration and pump
modes in Raman-active materials is derived. We show that under certain
conditions this relation can be used to determine the equilibrium number
variance of phonons.Time and temperature ranges for which such conditions can
be satisfied are studied and found to be available in todays' experimental
standards. Furthermore, we examine the results in the presence of multi-mode
pump as well as for the coupling of pump to the many vibration modes and
discuss their validity in these cases.


title: Quantum mechanics without statistical postulates

abstract:
  The Bohmian formulation of quantum mechanics is used in order to describe the
measurement process in an intuitive way without a reduction postulate in the
framework of a deterministic single system theory. Thereby the motion of the
hidden classical particle is chaotic during almost all nontrivial measurement
processes. For the correct reproduction of experimental results, it is further
essential that the distribution function $P(x)$ of the results of a position
measurement is identical with $|\Psi|^2$ of the wavefunction $\Psi$ of the
single system under consideration. It is shown that this feature is not an
additional assumption, but can be derived strictly from the chaotic motion of a
single system during a sequence of measurements, providing a completely
deterministic picture of the statistical features of quantum mechanics.


title: Spontaneous emission between an unusual pair of plates

abstract:
  We compute the modification in the spontaneous emission rate for a two-level
atom when it is located between two parallel plates of different nature: a
perfectly conducting plate $(\epsilon\to \infty)$ and an infinitely permeable
one $(\mu\to \infty)$. We also discuss the case of two infinitely permeable
plates. We compare our results with those found in the literature for the case
of two perfectly conducting plates.


title: Perturbative study of multiphoton processes in the tunneling regime

abstract:
  A perturbative study of the Schr\"{o}dinger equation in a strong
electromagnetic field with dipole approximation is accomplished in the
Kramers-Henneberger frame. A prove that just odd harmonics appear in the
spectrum for a linear polarized laser field is given, assuming that the atomic
radius is much lesser than the free-electron quiver motion amplitude. Within
this approximation a perturbation series is obtained in the Keldysh parameter
giving a description of multiphoton processes in the tunneling regime. The
theory is applied to the case of hydrogen-like atoms: The spectrum of higher
order harmonics and the above-threshold ionization rate are derived. The
ionization rate computed in this way determines the amplitudes of the
harmonics. The wave function of the atom proves to be rigid with respect to the
perturbation so that the effect of the laser field on the Coulomb potential in
the computation of the probability amplitudes can be neglected as a first
approximation: This approximation improves as the ratio between the amplitude
of the quiver motion of the electron and the atom radius becomes larger. The
semiclassical description currently adopted for harmonic generation is so
rederived by solving perturbatively the Schr\"{o}dinger equation.


title: Entanglement-assisted local manipulation of pure quantum states

abstract:
  We demonstrate that local transformations on a composite quantum system can
be enhanced in the presence of certain entangled states. These extra states act
much like catalysts in a chemical reaction: they allow otherwise impossible
local transformations to be realised, without being consumed in any way. In
particular, we show that this effect can considerably improve the efficiency of
entanglement concentration procedures for finite states.


title:
Unified Expressions of Physical Fundamental Interactions and Variational
  Principles Based On The Quantitative Causal Principle

abstract:
  This paper has been withdrawn by the authors because the paper is largely
revised and improved.


title: Thermalized Displaced Squeezed Thermal States

abstract:
  In the coordinate representation of thermofield dynamics, we investigate the
thermalized displaced squeezed thermal state which involves two temperatures
successively. We give the wavefunction and the matrix element of the density
operator at any time, and accordingly calculate some quantities related to the
position, momentum and particle number operator, special cases of which are
consistent with the results in the literature. The two temperatures have
diffenent correlations with the squeeze and coherence components. Moreover,
different from the properties of the position and momentum, the average value
and variance of the particle number operator as well as the second-order
correlation function are time-independent.


title: Unbroken supersymmetry in the Aharonov-Casher effect

abstract:
  We consider the problem of the bound states of a spin 1/2 chargless particle
in a given Aharonov-Casher configuration. To this end we recast the description
of the system in a supersymmetric form. Then the basic physical requirements
for unbroken supersymmetry are established. We comment on the possibility of
neutron confinement in this system.


title:
The ground state of a spin-1/2 neutral particle with anomalous magnetic
  moment in a Aharonov-Casher configuration

abstract:
  We determine the (bound) ground state of a spin 1/2 chargless particle with
anomalous magnetic moment in certain Aharonov-Casher configurations. We recast
the description of the system in a supersymmetric form. Then the basic physical
requirements for unbroken supersymmetry are established. We comment on the
possibility of neutron trapping in these systems.


title: History and Physics of the Klein Paradox

abstract:
  The early papers by Klein, Sauter and Hund which investigate scattering off a
high step potential in the context of the Dirac equation are discussed to
derive the 'paradox' first obtained by Klein. The explanation of this effect in
terms of electron-positron production is reassessed. It is shown that a
potential well or barrier in the Dirac equation can become supercritical and
emit positrons or electrons spontaneously if the potential is strong enough. If
the well or barrier is wide enough, a seemingly constant current is emitted.
This phenomenon is transient whereas the tunnelling first calculated by Klein
is time-independent. It is shown that tunnelling without exponential
suppression occurs when an electron is incident on a high barrier, even when
the barrier is not high enough to radiate. Klein tunnelling is therefore a
property of relativistic wave equations and is not necessarily connected to
particle emission. The Coulomb potential is investigated and it is shown that a
heavy nucleus of sufficiently large $Z$ will bind positrons. Correspondingly,
as $Z$ increases the Coulomb barrier should become increasingly transparent to
positrons. This is an example of Klein tunnelling. Phenomena akin to
supercritical positron emission may be studied experimentally in superfluid
$^3$He


title: Extension of Quantum Mechanics to Individual Systems

abstract:
  The Copenhagen Interpretation describes individual systems, using the same
Hilbert space formalism as does the statistical ensemble interpretation (SQM).
This leads to the well-known paradoxes surrounding the Measurement Problem. We
extend this common mathematical structure to encompass certain natural bundles
with connections over the Hilbert sphere S. This permits a consistent extension
of the statistical interpretation to interacting individual systems, thereby
resolving these paradoxes.
  Suppose V is a physical system in interaction with another system W. The
state vector of V+W has a set of polar decompositions with a vector q of
complex coefficients. These are parameterized by the right toroid T of
amplitudes q, and comprise a singular toroidal bundle over S, which comprises
the enlarged state space of V+W. We prove that each T has a unique natural
convex partition yielding the correct SQM probabilities. In the extended theory
V and W synchronously assume pure spectral states according to which member of
the partition contains q. The apparent indeterminism of SQM is thus
attributable to the effectively random distribution of initial phases.


title: How to build a non-spreading wave packet in quantum mechanics

abstract:
  Quantum mechanics asserts that a wave packet must inevitably spread as time
progresses since the dispersion relation for the quantum waves is assumed to be
quadratic in the momentum k. However, this assumption does not consider the
standard frequency Doppler shift formula of Galilean relativity. In this
article a non-dispersive wave packet is constructed by appropriately
considering the transformation rules between the laboratory and the
(particle's) rest inertial reference frames.


title: Classical Dynamics of the Quantum Harmonic Chain

abstract:
  The origin of classical predictability is investigated for the one
dimensional harmonic chain considered as a closed quantum mechanical system. By
comparing the properties of a family of coarse-grained descriptions of the
chain, we conclude that local coarse-grainings in this family are more useful
for prediction than nonlocal ones. A quantum mechanical system exhibits
classical behavior when the probability is high for histories having the
correlations in time implied by classical deterministic laws. But approximate
classical determinism holds only for certain coarse-grainings and then only if
the initial state of the system is suitably restricted. Coarse-grainings by the
values of the hydrodynamic variables (integrals over suitable volumes of
densities of approximately conserved quantities) define the histories usually
used in classical physics. But what distinguishes this coarse-graining from
others? This paper approaches this question by analyzing a family of
coarse-grainings for the linear harmonic chain. At one extreme in the family
the chain is divided into local groups of $N$ atoms. At the other extreme the
$N$ atoms are distributed nonlocally over the whole chain. Each coarse-graining
follows the average (center of mass) positions of the groups and ignores the
``internal'' coordinates within each group, these constituting a different
environment for each coarse-graining. We conclude that noise, decoherence, and
computational complexity favor locality over nonlocality for deterministic
predictability.


title: Finite precision measurement nullifies the Kochen-Specker theorem

abstract:
  Only finite precision measurements are experimentally reasonable, and they
cannot distinguish a dense subset from its closure. We show that the rational
vectors, which are dense in S^2, can be colored so that the contradiction with
hidden variable theories provided by Kochen-Specker constructions does not
obtain. Thus, in contrast to violation of the Bell inequalities, no
quantum-over-classical advantage for information processing can be derived from
the Kochen-Specker theorem alone.


title: Discrete formulation of teleportation of continuous variables

abstract:
  Teleportation of continuous variables can be described in two different ways,
one in terms of Wigner functions, the other in terms of discrete basis states.
The latter formulation provides the connection between the theory of
teleportation of continuous degrees of freedom of a light field and the
standard description of teleportation of discrete variables.


title:
Exactly solvable path integral for open cavities in terms of quasinormal
  modes

abstract:
  We evaluate the finite-temperature Euclidean phase-space path integral for
the generating functional of a scalar field inside a leaky cavity. Provided the
source is confined to the cavity, one can first of all integrate out the fields
on the outside to obtain an effective action for the cavity alone.
Subsequently, one uses an expansion of the cavity field in terms of its
quasinormal modes (QNMs)-the exact, exponentially damped eigenstates of the
classical evolution operator, which previously have been shown to be complete
for a large class of models. Dissipation causes the effective cavity action to
be nondiagonal in the QNM basis. The inversion of this action matrix inherent
in the Gaussian path integral to obtain the generating functional is therefore
nontrivial, but can be accomplished by invoking a novel QNM sum rule. The
results are consistent with those obtained previously using canonical
quantization.


title:
Experimental Realization of Discrete Fourier Transformation on NMR
  Quantum Computer

abstract:
  We report experimental implementation of discrete Fourier transformation(DFT)
on a nuclear magnetic resonance(NMR) quantum computer. Experimental results
agree with theoretical results. Using the pulse sequences we introduced, DFT
can be realized on any L-bit quantum number in principle.


title: Bayesian analysis of Bell inequalities

abstract:
  Statistical tests are needed to determine experimentally whether a
hypothetical theory based on local realism can be an acceptable alternative to
quantum mechanics. It is impossible to rule out local realism by a single test,
as often claimed erroneously. The ``strength'' of a particular Bell inequality
is measured by the number of trials that are needed to invalidate local realism
at a given confidence level. Various versions of Bell's inequality are compared
from this point of view. It is shown that Mermin's inequality for
Greenberger-Horne-Zeilinger states requires fewer tests than the
Clauser-Horne-Shimony-Holt inequality or than its chained variants applied to a
singlet state, and also than Hardy's proof of nonlocality.


title: Conditional quantum state engineering at beam splitter arrays

abstract:
  The generation of arbitrary single-mode quantum states from the vacuum by
alternate coherent displacement and photon adding as well as the measurement of
the overlap of a signal with an arbitrarily chosen quantum state are studied.
With regard to implementations, the transformation of the quantum state of a
traveling optical field at an array of beam splitters is considered, using
conditional measurement. Allowing for arbitrary quantum states of both the
input reference modes and the output reference modes on which the measurements
are performed, the setup is described within the concept of two-port
non-unitary transformation, and the overall non-unitary transformation operator
is derived. It is shown to be a product of operators, where each operator is
assigned to one of the beam splitters and can be expressed in terms of an
s-ordered operator product, with s being determined by the beam splitter
transmittance or reflectance. As an example we discuss the generation of and
overlap measurement with Schroedinger-cat-like states.


title: Grover's search algorithm: An optical approach

abstract:
  The essential operations of a quantum computer can be accomplished using
solely optical elements, with different polarization or spatial modes
representing the individual qubits. We present a simple all-optical
implementation of Grover's algorithm for efficient searching, in which a
database of four elements is searched with a single query. By `compiling' the
actual setup, we have reduced the required number of optical elements from 24
to only 12. We discuss the extension to larger databases, and the limitations
of these techniques.


title:
Realization of a 5-bit NMR Quantum Computer Using a New Molecular
  Architecture

abstract:
  We demonstrate a five-bit nuclear-magnetic-resonance quantum computer that
distinguishes among various functions on four bits, making use of quantum
parallelism. Its construction draws on the recognition of the sufficiency of
linear coupling along a chain of nuclear spins, the synthesis of a suitably
coupled molecule, and the use of a multi-channel spectrometer.


title: Gaussian, Mean Field and Variational Approximation: the Equivalence

abstract:
  We show the equivalence between the three approximation schemes for
self-interacting (1+1)-D scalar field theories. Based on rigorous results of
[1, 2], we are able to prove that the Gaussian approximation is very precise
for certain limits of coupling constants. The $\lambda \phi ^{4}+\sigma \phi
^{2}$ model will be used as a concrete application.


title: Exact Classical Effective Potential

abstract:
  A quantum spin system can be modelled by an equivalent classical system, with
an effective Hamiltonian obtained by integrating all non-zero frequency modes
out of the path integral. The effective Hamiltonian H_eff(S_i) derived from the
coherent-state integral is highly singular: the quasiprobability density
exp(-beta H_eff), a Wigner function, imposes quantisation through derivatives
of delta functions. This quasiprobability is the distribution of the
time-averaged lower symbol of the spin in the coherent-state integral. We
relate the quantum Monte Carlo minus-sign problem to the non-positivity of this
quasiprobability, both analytically and by Monte Carlo integration.


title:
Theoretical challenges in atom optics: Atomic and molecular diffraction
  by transmission gratings

abstract:
  A few years ago, diffraction of atoms by double slits and gratings was
achieved for the first time, and standard optical wave-theory provided an
excellent description of the experiments. More recently, diffraction of weakly
bound molecules and even clusters has been observed. Due to their size and to
possible breakup processes optical wave-theory is no longer adequate and a more
sophisticated approach is needed. Moreover, surface effects, which can modify
the diffraction pattern of atoms and molecules, give rise to further
complications. In this article a fully quantum mechanical approach to these
questions is discussed.


title:
Quantum mechanics in the K-Field formalism: the basic principles of
  geometrization

abstract:
  The basic principles of the quantum mechanics in the K-field formalism are
stated in the paper. The basic distinction of this theory arises from that the
quantum theory equations (including well-known Schrodinger, Klein-Gordon and
quadratic Dirac equations) are obtained from de Broglie postulate geometric
generalization. Rather, they are obtained as the free wave equations on a
manifold metrizing force interactions of particles. Such view on the quantum
theory basic equations allows one to use semiclassical models for the quantum
system simulation. The quantization principle modifies as well. Namely, quantum
system stationary conditions are such conditions, at which test particles
motion is Lyapunov stable.


title: Quantum Finite One-Counter Automata

abstract:
  In this paper the notion of quantum finite one-counter automata (QF1CA) is
introduced. Introduction of the notion is similar to that of the 2-way quantum
finite state automata by A.Kondacs and J.Watrous. The well-formedness
conditions for the automata are specified ensuring unitarity of evolution. A
special kind of QF1CA, called simple, that satisfies the well-formedness
conditions is introduced. That allows to specify rules for constructing such
automata more naturally and simpler than in general case. Possible models of
language recognition by QF1CA are considered. The recognition of some languages
by QF1CA is shown and compared with recognition by probabilistic counterparts.


title: Quantum Hexaspherical Observables for Electrons

abstract:
  A new quantum algebraic description of relativistic electrons, built on a
conformal dynamical symmetry (SO(4,2)), has recently been proposed to treat
localization in space-time. It is shown here that localization of an electron
may be represented by components of a SO(4,2) vector which are quantum
generalizations of the hexaspherical coordinates of classical projective
geometry. The shift of this vector under transformations to uniformly
accelerated frames is described by SO(4,2) rotations. Hexaspherical observables
also allow one to represent the quantum law of free fall under a form
explicitly compatible with the same dynamical symmetry.


title: Decoherent Histories and the Emergent Classicality of Local Densities

abstract:
  In the context of the decoherent histories approach to quantum theory, it is
shown that a class of macroscopic configurations consisting of histories of
local densities (number, momentum, energy) exhibit negligible interference.
This follows from the close connection of the local densities with the
corresponding exactly conserved (and so exactly decoherent) quantities, and
also from the observation that the eigenstates of local densities (averaged
over a sufficiently large volume) remain approximate eigenstates under time
evolution. The result is relevant to the derivation of hydrodynamic equations
using the decoherent histories approach.


title:
Two-particle entanglement as a property of three-particle entangled
  states

abstract:
  In a recent article [Phys. Rev. A 54, 1793 (1996)] Krenn and Zeilinger
investigated the conditional two-particle correlations for the subensemble of
data obtained by selecting the results of the spin measurements by two
observers 1 and 2 with respect to the result found in the corresponding
measurement by a third observer. In this paper we write out explicitly the
condition required in order for the selected results of observers 1 and 2 to
violate Bell's inequality for general measurement directions. It is shown that
there are infinitely many sets of directions giving the maximum level of
violation. Further, we extend the analysis by the authors to the class of
triorthogonal states |Psi> = c_1 |z_1>|z_2>|z_3> + c_2 |-z_1>|-z_2>|-z_3>. It
is found that a maximal violation of Bell's inequality occurs provided the
corresponding three-particle state yields a direct ("all or nothing")
nonlocality contradiction.


title:
Electron Spin Resonance Transistors for Quantum Computing in
  Silicon-Germanium Heterostructures

abstract:
  We apply the full power of modern electronic band structure engineering and
epitaxial heterostructures to design a transistor that can sense and control a
single donor electron spin. Spin resonance transistors may form the
technological basis for quantum information processing. One and two qubit
operations are performed by applying a gate bias. The bias electric field pulls
the electron wave function away from the dopant ion into layers of different
alloy composition. Owing to the variation of the g-factor (Si:g=1.995,
Ge:g=1.563), this displacement changes the spin Zeeman energy, allowing
single-qubit operations. By displacing the electron even further, the overlap
with neighboring qubits is affected, which allows two-qubit operations. Certain
Silicon-Germanium alloys allow a qubit spacing as large as 200 nm, which is
well within the capabilities of current lithographic techniques. We discuss
manufacturing limitations and issues regarding scaling up to a large size
computer.


title: Bounds on Integrals of the Wigner Function

abstract:
  The integral of the Wigner function over a subregion of the phase-space of a
quantum system may be less than zero or greater than one. It is shown that for
systems with one degree of freedom, the problem of determining the best
possible upper and lower bounds on such an integral, over all possible states,
reduces to the problem of finding the greatest and least eigenvalues of an
hermitian operator corresponding to the subregion. The problem is solved
exactly in the case of an arbitrary elliptical region. These bounds provide
checks on experimentally measured quasiprobability distributions.


title: Resonance flourescence in atomic coherent systems: spectral features

abstract:
  We study resonance flourescence in a four level ladder system and illustrate
some novel features due to quantum interference and atomic coherence effects.
We find that under three photon resonant conditions, in some region of the
parameter space of the rabi frequencies $\Omega_1,\Omega_2,\Omega_3$, emission
is dominantly by the level 4 at the line center even though there is an almost
equal distribution of populations in all the levels. As one increases
$\Omega_3$ with $\Omega_1 and \Omega_2$ held fixed, the four level system
'dynamically collapses' to a two level system. The steady state populations and
the the resonance flourescence from all the levels provide adequate evidence to
this effect.


title: Quantum State Detection Via Elimination

abstract:
  We present the view of quantum algorithms as a search-theoretic problem. We
show that the Fourier transform, used to solve the Abelian hidden subgroup
problem, is an example of an efficient elimination observable which eliminates
a constant fraction of the candidate secret states with high probability.
Finally, we show that elimination observables do not always exist by
considering the geometry of the hidden subgroup states of the dihedral group
D_N.


title: Ladder operators for isospectral oscillators

abstract:
  We present, for the isospectral family of oscillator Hamiltonians, a
systematic procedure for constructing raising and lowering operators satisfying
any prescribed `distorted' Heisenberg algebra (including the
$q$-generalization). This is done by means of an operator transformation
implemented by a shift operator. The latter is obtained by solving an
appropriate partial isometry condition in the Hilbert space. Formal
representations of the non-local operators concerned are given in terms of
pseudo-differential operators. Using the new annihilation operators, new
classes of coherent states are constructed for isospectral oscillator
Hamiltonians. The corresponding Fock-Bargmann representations are also
considered, with specific reference to the order of the entire function family
in each case.


title: Geometric phases for generalized squeezed coherent states

abstract:
  A simple technique is used to obtain a general formula for the Berry phase
(and the corresponding Hannay angle) for an arbitrary Hamiltonian with an
equally-spaced spectrum and appropriate ladder operators connecting the
eigenstates. The formalism is first applied to a general deformation of the
oscillator involving both squeezing and displacement. Earlier results are shown
to emerge as special cases. The analysis is then extended to multiphoton
squeezed coherent states and the corresponding anholonomies deduced.


title: Supersymmetry of a spin 1/2 particle on the real line

abstract:
  We study one dimensional supersymmetric (SUSY) quantum mechanics of a spin
1/2 particle moving in a rotating magnetic field and scalar potential. We also
discuss SUSY breaking and it is shown that SUSY breaking essentially depends on
the strength and period of the magnetic field. For a purely rotating magnetic
field the eigenvalue problem is solved exactly and two band energy spectrum is
found.


title: QED in the Presence of Arbitrary Kramers-Kronig Dielectric Media

abstract:
  The phenomenological Maxwell field is quantized for arbitrarily space- and
frequency-dependent complex permittivity. The formalism takes account of the
Kramers--Kronig relation and the dissipation-fluctuation theorem and yields the
fundamental equal-time commutation relations of QED. Applications to the
quantum-state transformation at absorbing and amplifying four-port devices and
to the spontaneous decay of an excited atom in the presence of absorbing
dielectric bodies are discussed.


title: Generalized Spin-1/2 Operators and Their Eigenvectors

abstract:
  Recently, we have shown how the interpretation of quantum mechanics due to
Lande' can be used to derive from first principles generalized formulas for the
operators and some eigenvectors for spin 1/2 Though we gave the operators for
all the components of the spin, we did not give the eigenvectors of the
operators for the x and y components of the spin. We now give these vectors. In
addition, we present a new and simple method of deriving the operators for the
x and y components of the spin as well as their vectors from those for the z
component. We give a general proof that the operator for the square of the spin
is the unit matrix multiplied by the value of the square of the spin.


title: Feynman Integral Approach to Absorption in Quantum Mechanics

abstract:
  We propose a formulation of an absorbing boundary for a quantum particle. The
formulation is based on a Feynman-type integral over trajectories that are
confined by the absorbing boundary. Trajectories that reach the absorbing wall
are instantaneously terminated and their probability is discounted from the
population of the surviving trajectories. This gives rise to a unidirectional
absorption current at the boundary. We calculate the survival probability as a
function of time. Several modes of absorption are derived from our formalism:
total absorption, absorption that depends on energy levels, and absorption of
non-interacting particles. Several applications are given: the slit experiment
with an absorbing screen and with absorbing lateral walls, and one dimensional
particle between two absorbing walls. The survival probability of a particle
between absorbing walls exhibits decay with beats.


title: Unsharp measurements and the conceptual problems of Quantum Theory

abstract:
  The paper emphasis the role of unsharpness in the body of Quantum Theory and
the relations to the conceptual problems of the Theory. Key words: quantum
measurement, unsharpness, effect, positive operator-valued measure


title: Quantum measurement breaks Lorentz symmetry

abstract:
  Traditionally causes come before effects, but according to modern physics
things aren't that simple. Special relativity shows that `before' and `after'
are relative, and quantum measurement is even more subtle. Since the
nonlocality of Bell's theorem, it has been known that quantum measurement has
an uneasy relation with special relativity, described by Shimony as `peaceful
coexistence'. Hardy's theorem says that quantum measurement requires a
preferred Lorentz frame. The original proofs of the theorem depended on there
being no backward causality, even at the quantum level. In quant-ph/9803044
this condition was removed. It was only required that systems with classical
inputs and outputs had no causal loops. Here the conditions are weakened
further: there should be no forbidden causal loops as defined in the text. The
theory depends on a transfer function analysis, which is introduced in detail
before application to specific systems.


title: Non-Contextual Hidden Variables and Physical Measurements

abstract:
  For a hidden variable theory to be indistinguishable from quantum theory for
finite precision measurements, it is enough that its predictions agree for some
measurement within the range of precision. Meyer has recently pointed out that
the Kochen-Specker theorem, which demonstrates the impossibility of a
deterministic hidden variable description of ideal spin measurements on a spin
1 particle, can thus be effectively nullified if only finite precision
measurements are considered. We generalise this result: it is possible to
ascribe consistent outcomes to a dense subset of the set of projection valued
measurements, or to a dense subset of the set of positive operator valued
measurements, on any finite dimensional system. Hence no Kochen-Specker like
contradiction can rule out hidden variable theories indistinguishable from
quantum theory by finite precision measurements in either class.


title: Information Flow in Entangled Quantum Systems

abstract:
  All information in quantum systems is, notwithstanding Bell's theorem,
localised. Measuring or otherwise interacting with a quantum system S has no
effect on distant systems from which S is dynamically isolated, even if they
are entangled with S. Using the Heisenberg picture to analyse quantum
information processing makes this locality explicit, and reveals that under
some circumstances (in particular, in Einstein-Podolski-Rosen experiments and
in quantum teleportation) quantum information is transmitted through
'classical' (i.e. decoherent) information channels.


title:
Good dynamics versus bad kinematics. Is entanglement needed for quantum
  computation?

abstract:
  We consider quantum computing with pseudo-pure states. This framework arises
in certain implementations of quantum computing using NMR. We analyze quantum
computational protocols which aim to solve exponential classical problems with
polynomial resources and ask whether or not entanglement of the pseudo-pure
states is needed to achieve this aim. We show that for a large class of such
protocols, including Shor's factorization, entanglement is necessary. We also
show that achieving entanglement is not sufficient: if the noise in the state
is sufficiently large, exponential resources are needed even if entanglement is
present.


title:
On the Curvature of Monotone Metrics and a Conjecture Concerning the
  Kubo-Mori Metric

abstract:
  It is the aim of this article to determine curvature quantities of an
arbitrary Riemannian monotone metric on the space of positive matrices resp.
nonsingular density matrices. Special interest is focused on the scalar
curvature due to its expected quantum statistical meaning. The scalar curvature
is explained in more detail for three examples, the Bures metric, the largest
monotone metric and the Kubo-Mori metric. In particular, we show an important
conjecture of Petz concerning the Kubo-Mori metric up to a formal proof of the
concavity of a certain function on R_+^3. This concavity seems to be
numerically evident. The conjecture of Petz asserts that the scalar curvature
of the Kubo-Mori metric increases if one goes to more mixed states.


title: Quantum Tomography Approach in Signal Analysis

abstract:
  Some properties of the fractional Fourier transform, which is used in
information processing, are presented in connection with the tomography
transform of optical signals. Relation of the Green function of the quantum
harmonic oscillator to the fractional Fourier transform is elucidated.