title:
Implementation of a Deutsch-like quantum algorithm utilizing
  entanglement at the two-qubit level, on an NMR quantum information processor

abstract:
  We describe the experimental implementation of a recently proposed quantum
algorithm involving quantum entanglement at the level of two qubits using NMR.
The algorithm solves a generalisation of the Deutsch problem and distinguishes
between even and odd functions using fewer function calls than is possible
classically. The manipulation of entangled states of the two qubits is
essential here, unlike the Deutsch-Jozsa algorithm and the Grover's search
algorithm for two bits.


title: Relational Quantum Measurements, Information and State Collapse

abstract:
  The quantum measurement problem considered for measuring system (MS) consist
of measured state S (particle), detector D and information processing device
(observer) O. It's shown that O states selfreference structure results in
principal nonobservability of MS interference terms which discriminate pure and
mixed S states. Such observables restriction permit to construct for MS states
subjective representation (SR) which describes probabilistic evolution for
measurement events observed by $O$ and his subjective information about S
values. SR is dual and nonequivalent to MS Hilbert space $H$ for external
observer $O'$. Due to it SR evolution is compatible with Schrodinger linear MS
evolution observed by $O'$. It's argued that SR evolution corresponds to S
state collapse for individual events observed by $O$.


title: Entanglement molecules

abstract:
  We investigate the entanglement properties of multiparticle systems,
concentrating on the case where the entanglement is robust against disposal of
particles. Two qubits -belonging to a multipartite system- are entangled in
this sense iff their reduced density matrix is entangled. We introduce a family
of multiqubit states, for which one can choose for any pair of qubits
independently whether they should be entangled or not as well as the relative
strength of the entanglement, thus providing the possibility to construct all
kinds of ''Entanglement molecules''. For some particular configurations, we
also give the maximal amount of entanglement achievable.


title: Entanglement, Information and Multiparticle Quantum Operations

abstract:
  Collective operations on a network of spatially-separated quantum systems can
be carried out using local quantum (LQ) operations, classical communication
(CC) and shared entanglement (SE). Such operations can also be used to
communicate classical information and establish entanglement between distant
parties. We show how these facts lead to measures of the inseparability of
quantum operations, and argue that a maximally-inseparable operation on 2
qubits is the SWAP operation. The generalisation of our argument to N qubit
operations leads to the conclusion that permutation operations are
maximally-inseparable. For even N, we find the minimum SE and CC resources
which are sufficient to perform an arbitrary collective operation. These
minimum resources are 2(N-1) ebits and 4(N-1) bits, and these limits can be
attained using a simple teleportation-based protocol. We also obtain lower
bounds on the minimum resources for the odd case. For all $N{\geq}4$, we show
that the SE/CC resources required to perform an arbitrary operation are
strictly greater than those that any operation can establish/communicate.


title:
Group-theoretical Structure of the Entangled States of N Identical
  Particles

abstract:
  We provide a group-theoretical classification of the entangled states of N
identical particles. The connection between quantum entanglement and the
exchange symmetry of the states of N identical particles is made explicit using
the duality between the permutation group and the simple unitary group. Each
particle has n-levels and spans the n-dimensional Hilbert space. We shall call
the general state of the particle as a qunit. The direct product of the N qunit
space is given a decomposition in terms of states with definite permutation
symmetry. The nature of fundamental entanglement of a state can be related to
the classes of partitions of the integer N. The maximally entangled states are
generated from linear combinations of the less entangled states of the direct
product space. We also discuss the nature of maximal entanglement and its
measures.


title: Error-free optical quantum communication

abstract:
  An optical scheme for the reliable transfer of quantum information through a
noisy quantum channel is proposed. The scheme is inspired by quantum
error-correction protocols, but it avoids the currently infeasible requirement
for a controlled-NOT operation between single photons. The quantum
communication scheme presented here rejects single bit-flip errors instead of
correcting them and combines quantum-measurement properties of three-particle
entangled (GHZ) states with properties of quantum teleportation.


title: Unconditionally Secure Quantum Bit Commitment Is Possible

abstract:
  Bit commitment involves the submission of evidence from one party to another
so that the evidence can be used to confirm a later revealed bit value by the
first party, while the second party cannot determine the bit value from the
evidence alone. It is widely believed that secure quantum bit commitment is
impossible due to quantum entanglement cheating, which is codified in a general
impossibility theorem. In this paper, the scope of this general impossibility
proof is extended and analyzed, and gaps are found. Three specific protocols
are described for which the entanglement cheating as given in the impossibility
proof fails to work. One of these protocols, QBC2, is proved to be
unconditionally secure.


title:
Low-energy relativistic effects and nonlocality in time-dependent
  tunneling

abstract:
  We consider exact time-dependent analytic solutions to the Schr\"odinger
equation for tunneling in one dimension with cut off wave initial conditions at
$t=0$. We obtain that as soon as $t \neq 0$ the transmitted probability density
at any arbitrary distance rises instantaneously with time in a linear manner.
Using a simple model we find that the above nonlocal effect of the
time-dependent solution is suppressed by consideration of low-energy
relativistic effects. Hence at a distance $x_0$ from the potential the
probability density rises after a time $t_0=x_0/c$ restoring Einstein
causality. This implies that the tunneling time of a particle can never be
zero.


title: Many-particle entanglement with Bose--Einstein condensates

abstract:
  We propose a method to produce entangled states of several particles starting
from a Bose-Einstein condensate. In the proposal, a single fast $\pi/2$ pulse
is applied to the atoms and due to the collisional interaction, the subsequent
free time evolution creates an entangled state involving all atoms in the
condensate. The created entangled state is a spin-squeezed state which could be
used to improve the sensitivity of atomic clocks.


title: Proposal for measurment of harmonic oscillator Berry phase in ion traps

abstract:
  We propose a scheme for measuring the Berry phase in the vibrational degree
of freedom of a trapped ion. Starting from the ion in a vibrational coherent
state we show how to reverse the sign of the coherent state amplitude by using
a purely geometric phase. This can then be detected through the internal
degrees of freedom of the ion. Our method can be applied to preparation of
Schr\"odinger cat states.


title:
Proposal for preparing delocalised mesoscopic states of material
  oscillators

abstract:
  A scheme is proposed for preparing delocalised mesoscopic states of the
motion of two or more atoms trapped at distantly-separated locations.
Generation of entanglement is achieved using interactions in cavity quantum
electrodynamics which facilitate motional quantum state transmission, via
light, between separate nodes of a quantum network. Possible applications of
the scheme are discussed.


title: Time Optimal Control in Spin Systems

abstract:
  In this paper, we study the design of pulse sequences for NMR spectroscopy as
a problem of time optimal control of the unitary propagator. Radio frequency
pulses are used in coherent spectroscopy to implement a unitary transfer of
state. Pulse sequences that accomplish a desired transfer should be as short as
possible in order to minimize the effects of relaxation and to optimize the
sensitivity of the experiments. Here, we give an analytical characterization of
such time optimal pulse sequences applicable to coherence transfer experiments
in multiple-spin systems. We have adopted a general mathematical formulation,
and present many of our results in this setting, mindful of the fact that new
structures in optimal pulse design are constantly arising. Moreover, the
general proofs are no more difficult than the specific problems of current
interest. From a general control theory perspective, the problems we want to
study have the following character. Suppose we are given a controllable right
invariant system on a compact Lie group, what is the minimum time required to
steer the system from some initial point to a specified final point? In NMR
spectroscopy and quantum computing, this translates to, what is the minimum
time required to produce a unitary propagator? We also give an analytical
characterization of maximum achievable transfer in a given time for the two
spin system.


title: Spin measurement retrodiction revisited

abstract:
  The retrodiction of spin measurements along a set of different axes is
revisited in detail. The problem is presented in two different pictures, a
geometric and a general algebraic one. Explicit measurement operators that
allow the retrodiction are given for the case of three and four axes. For the
Vaidman-Aharanov-Albert case of three orthogonal axes the quantum network is
constructed for two different initial Bell states.


title: Objective probabilities, quantum counterfactuals, and the ABL rule

abstract:
  The ABL rule is derived as a tool of standard quantum mechanics. The
ontological significance of the existence of objective probabilities is
discussed. Objections by Kastner and others to counterfactual uses of the ABL
rule are refuted. Metaphysical presumptions leading to such views as Kastner is
defending in her Comment are examined and shown to be unwarranted.


title: Universality of Decoherence

abstract:
  We consider environment induced decoherence of quantum superpositions to
mixtures in the limit in which that process is much faster than any competing
one generated by the Hamiltonian $H_{\rm sys}$ of the isolated system. While
the golden rule then does not apply we can discard $H_{\rm sys}$. By allowing
for simultaneous couplings to different reservoirs, we reveal decoherence as a
universal short-time phenomenon independent of the character of the system as
well as the bath and of the basis the superimposed states are taken from. We
discuss consequences for the classical behavior of the macroworld and quantum
measurement: For the decoherence of superpositions of macroscopically distinct
states the system Hamiltonian is always negligible.


title:
Relation of the oscillator and Coulomb systems on spheres and
  pseudospheres

abstract:
  It is shown, that oscillators on the sphere and the pseudosphere are related,
by the so-called Bohlin transformation, with the Coulomb systems on the
pseudosphere. The even states of an oscillator yield the conventional Coulomb
system on the pseudosphere, while the odd states yield the Coulomb system on
the pseudosphere in the presence of magnetic flux tube generating spin 1/2. A
similar relation is established for the oscillator on the (pseudo)sphere
specified by the presence of constant uniform magnetic field $B_0$ and the
Coulomb-like system on pseudosphere specified by the presence of the magnetic
field $\frac{B}{2r_0}(|\frac{x_3}{{\bf x}}|-\epsilon)$. The correspondence
between the oscillator and the Coulomb systems the higher dimensions is also
discussed.

categories: quant-ph gr-qc math-ph math.MP physics.atom-ph


title: New hydrogen-like potentials

abstract:
  Using the modified factorization method introduced by Mielnik, we construct a
new class of radial potentials whose spectrum for l=0 coincides exactly with
that of the hydrogen atom. A limiting case of our family coincides with the
potentials previously derived by Abraham and Moses


title: Thresholds for Linear Optics Quantum Computation

abstract:
  We previously established that in principle, it is possible to quantum
compute using passive linear optics with photo-detectors (quant-ph/0006088).
Here we describe techniques based on error detection and correction that
greatly improve the resource and device reliability requirements needed for
scalability. The resource requirements are analyzed for ideal linear optics
quantum computation (LOQC). The coding methods can be integrated both with loss
detection and phase error-correction to deal with the primary relaxation
processes in non-ideal optics, including detector inefficiencies. The main
conclusion of our work is that the resource requirements for implementing
quantum communication or computation with LOQC are reasonable. Furthermore,
this work clearly demonstrates how special knowledge of the error behavior can
be exploited for greatly improving the fault tolerance and overheads of a
physical quantum computer.


title: QED in dispersing and absorbing media

abstract:
  After giving an outline of the quantization scheme based on the microscopic
Hopfield model of a dielectric bulk material, we show how the classical
phenomenological Maxwell equations of the electromagnetic field in the presence
of dielectric matter of given space- and frequency-dependent complex
permittivity can be transferred to quantum theory. Including in the theory the
interaction of the medium-assisted field with atomic systems, we present both
the minimal-coupling Hamiltonian and the multipolar-coupling Hamiltonian in the
Coulomb gauge. To illustrate the concept, we discuss the input--output
relations of radiation and the transformation of radiation-field quantum states
at absorbing four-port devices, and the spontaneous decay of an excited atom
near the surface of an absorbing body and in a spherical micro-cavity with
intrinsic material losses. Finally, we give an extension of the quantization
scheme to other media such as amplifying media, magnetic media, and nonlinear
media.


title:
Construction of a Scalable, Uniform and Universal Quantum Network and
  Its Applications

abstract:
  We present a possible candidate of construction of a scalable, uniform and
universal quantum network, which is built from quantum gates to elements of
quantum circuit, again to quantum subnetworks and finally to an entire quantum
network. Our scheme can overcome some difficulties of the existing schemes and
makes improvements to different extent in the scale of quantum network, ability
of computation, implementation of engineering, efficiency of quantum network,
universality, compatibility, design principle, programmability, fault
tolerance, error control, industrialization and commercialization {\it et. al}
aspects. As the applications of this construction scheme, we obtain the entire
quantum networks for Shor's algorithm, Grover's algorithm and solving
Schr\"odinger equation in general. This implies that the scalable, uniform and
universal quantum networks are able to generally describe the known main
results and can be further applied to more interesting examples in quantum
computations.


title:
Quantum Computing by Two-Dimensional NMR using Spin- and
  Transition-Selective Pulses

abstract:
  Quantum computing using two-dimensional NMR has recently been described using
scalar coupling evolution technique [J. Chem. Phys.,109,10603 (1998)]. In the
present paper, we describe two-dimensional NMR quantum computing with the help
of selective pulses. A number of logic gates are implemented using two and
three qubits with one extra observer spin. Some many-in-one gates (or
Portmanteau gates) are implemented. Toffoli gate (or AND/NAND gate) and OR/NOR
gates are implemented on three qubits. Deutsch-Jozsa quantum algorithm for one
and two qubits, using one extra work qubit, has also been implemented using
selective pulses after creating a coherent superposition state, in the
two-dimensional methodology.


title: A method of enciphering quantum states

abstract:
  In this paper, we propose a method of enciphering quantum states of two-state
systems (qubits) for sending them in secrecy without entangled qubits shared by
two legitimate users (Alice and Bob). This method has the following two
properties. First, even if an eavesdropper (Eve) steals qubits, she can extract
information from them with certain probability at most. Second, Alice and Bob
can confirm that the qubits are transmitted between them correctly by measuring
a signature. If Eve measures m qubits one by one from n enciphered qubits and
sends alternative ones (the Intercept/Resend attack), a probability that Alice
and Bob do not notice Eve's action is equal to (3/4)^m or less. Passwords for
decryption and the signature are given by classical binary strings and they are
disclosed through a public channel. Enciphering classical information by this
method is equivalent to the one-time pad method with distributing a classical
key (random binary string) by the BB84 protocol. If Eve takes away qubits,
Alice and Bob lose the original quantum information. If we apply our method to
a state in iteration, Eve's success probability decreases exponentially. We
cannot examine security against the case that Eve makes an attack with using
entanglement. This remains to be solved in the future.


title: Multipartite generalisation of the Schmidt decomposition

abstract:
  We find a canonical form for pure states of a general multipartite system, in
which the constraints on the coordinates (with respect to a factorisable
orthonormal basis) are simply that certain ones vanish and certain others are
real. For identical particles they are invariant under permutations of the
particles. As an application, we find the dimension of the generic local
equivalence class.


title: A PT-Invariant Potential With Complex QES Eigenvalues

abstract:
  We show that the quasi-exactly solvable eigenvalues of the Schr\"odinger
equation for the PT-invariant potential $V(x) = -(\zeta \cosh 2x -iM)^2$ are
complex conjugate pairs in case the parameter M is an even integer while they
are real in case M is an odd integer. We also show that whereas the PT symmetry
is spontaneously broken in the former case, it is unbroken in the latter case.


title: Barnett-Pegg formalism of angle operators, revivals, and flux lines

abstract:
  We use the Barnett-Pegg formalism of angle operators to study a rotating
particle with and without a flux line. Requiring a finite dimensional version
of the Wigner function to be well defined we find a natural time quantization
that leads to classical maps from which the arithmetical basis of quantum
revivals is seen. The flux line, that fundamentally alters the quantum
statistics, forces this time quantum to be increased by a factor of a winding
number and determines the homotopy class of the path. The value of the flux is
restricted to the rational numbers, a feature that persists in the infinite
dimensional limit.


title:
Variational Characterisations of Separability and Entanglement of
  Formation

abstract:
  In this paper we develop a mathematical framework for the characterisation of
separability and entanglement of formation (EoF) of general bipartite states.
These characterisations are of the variational kind, meaning that separability
and EoF are given in terms of a function which is to be minimized over the
manifold of unitary matrices. A major benefit of such a characterisation is
that it directly leads to a numerical procedure for calculating EoF. We present
an efficient minimisation algorithm and an apply it to the bound entangled 3X3
Horodecki states; we show that their EoF is very low and that their distance to
the set of separable states is also very low. Within the same variational
framework we rephrase the results by Wootters (W. Wootters, Phys. Rev. Lett.
80, 2245 (1998)) on EoF for 2X2 states and present progress in generalising
these results to higher dimensional systems.


title: Two Qubits in the Dirac Representation

abstract:
  A general two qubit system expressed in terms of the complete set of unit and
fifteen traceless, Hermitian Dirac matrices, is shown to exhibit novel features
of this system. The well-known physical interpretations associated with the
relativistic Dirac equation involving the symmetry operations of time-reversal
T, charge conjugation C, parity P, and their products are reinterpreted here by
examining their action on the basic Bell states. The transformation properties
of the Bell basis states under these symmetry operations also reveal that C is
the only operator that does not mix the Bell states whereas all others do. In a
similar fashion, expressing the various logic gates introduced in the subject
of quantum computers in terms of the Dirac matrices shows for example, that the
NOT gate is related to the product of time-reversal and parity operators.


title: Inequalities for electron-field correlation functions

abstract:
  I show that there exists a class of inequalities between correlation
functions of different orders of a chaotic electron field. These inequalities
lead to the antibunching effect and are a consequence of the fact that
electrons are fermions -- indistinguishable particles with antisymmetric
states. The derivation of the inequalities is based on the known form of the
correlation functions for the chaotic state and on the properties of matrices
and determinants.


title:
Negative phase time for Scattering at Quantum Wells: A Microwave Analogy
  Experiment

abstract:
  If a quantum mechanical particle is scattered by a potential well, the wave
function of the particle can propagate with negative phase time. Due to the
analogy of the Schr\"odinger and the Helmholtz equation this phenomenon is
expected to be observable for electromagnetic wave propagation. Experimental
data of electromagnetic wells realized by wave guides filled with different
dielectrics confirm this conjecture now.


title: Entanglement Swapping Chains for General Pure States

abstract:
  We consider entanglement swapping schemes with general (rather than
maximally) entangled bipartite states of arbitary dimension shared pairwise
between three or more parties in a chain. The intermediate parties perform
generalised Bell measurements with the result that the two end parties end up
sharing a entangled state which can be converted into maximally entangled
states. We obtain an expression for the average amount of maximal entanglement
concentrated in such a scheme and show that in a certain reasonably broad class
of cases this scheme is provably optimal and that, in these cases, the amount
of entanglement concentrated between the two ends is equal to that which could
be concentrated from the weakest link in the chain.


title: Correlation Functions and Spin

abstract:
  The k-electron correlation function of a free chaotic electron beam is
derived with the spin degree of freedom taken into account. It is shown that it
can be expressed with the help of correlation functions for a polarized
electron beam of all orders up to k and the degree of spin polarization. The
form of the correlation function suggests that if the electron beam is not
highly polarized, observing multi-particle correlations should be difficult.
The result can be applied also to chaotic photon beams, the degree of spin
polarization being replaced by the degree of polarization.


title: A Kochen-Specker inequality

abstract:
  By probabilistic means, the concept of contextuality is extended so that it
can be used in non-ideal situations. An inequality is presented, which at least
in principle enables a test to discard non-contextual hidden-variable models at
low error rates, in the spirit of the Kochen-Specker theorem. Assuming that the
errors are independent, an explicit error bound of 1.42% is derived, below
which a Kochen-Specker contradiction occurs.


title:
Self-Consistent Effective Action for Quantum Particle with
  Space-Dependent Mass

abstract:
  We calculate the quantum corrections to the classical action of a particle
with coordinate-dependent mass. The result is made self-consistent by a
variational approach, thus making it applicable to strong-couplings and
singular potentials. By including thermal fluctuations, the we obtain an
effective action whose classical Euler-Lagrange equation describes the motion
of a particle including quantum and thermal effects.


title: Finding Matches between Two Databases on a Quantum Computer

abstract:
  Given two unsorted lists each of length N that have a single common entry, a
quantum computer can find that matching element with a work factor of
$O(N^{3/4}\log N)$ (measured in quantum memory accesses and accesses to each
list). The amount of quantum memory required is $O(N^{1/2})$. The quantum
algorithm that accomplishes this consists of an inner Grover search combined
with a partial sort all sitting inside of an outer Grover search.


title: Scaling Considerations in Ground State Quantum Computation

abstract:
  We study design challenges associated with realizing a ground state quantum
computer. In such a computer, the energy gap between the ground state and first
excited state must be sufficiently large to prevent disruptive excitations.
Here, an estimate is provided of this gap as a function of computer size. We
then address the problem of detecting the output of a ground state quantum
computer. It is shown that the exponential detection difficulties that appear
to be present at first can be overcome in a straightforward manner by small
design changes.


title: Quantum mechanical Carnot engine

abstract:
  A cyclic thermodynamic heat engine runs most efficiently if it is reversible.
Carnot constructed such a reversible heat engine by combining adiabatic and
isothermal processes for a system containing an ideal gas. Here, we present an
example of a cyclic engine based on a single quantum-mechanical particle
confined to a potential well. The efficiency of this engine is shown to equal
the Carnot efficiency because quantum dynamics is reversible. The quantum heat
engine has a cycle consisting of adiabatic and isothermal quantum processes
that are close analogues of the corresponding classical processes.


title: Continuous optical loading of a Bose-Einstein Condensate

abstract:
  The continuous pumping of atoms into a Bose-Einstein condensate via
spontaneous emission from a thermal reservoir is analyzed. We consider the case
of atoms with a three-level $\Lambda$ scheme, in which one of the atomic
transitions has a very much shorter life-time than the other one. We found that
in such scenario the photon reabsorption in dense clouds can be considered
negligible. If in addition inelastic processes can be neglected, we find that
optical pumping can be used to continuously load and refill Bose-Einstein
condensates, i.e. provides a possible way to achieve a continuous atom laser.


title: Phase-Control of Photoabsorption in Optically Dense Media

abstract:
  We present a self-consistent theory, as well as an illustrative application
to a realistic system, of phase control of photoabsorption in an optically
dense medium. We demonstrate that, when propagation effects are taken into
consideration, the impact on phase control is significant. Independently of the
value of the initial phase difference between the two fields, over a short
scaled distance of propagation, the medium tends to settle the relative phase
so that it cancels the atomic excitation. In addition, we find some rather
unusual behavior for an optically thin layer.


title: Locality and Bell's inequality

abstract:
  We prove that the locality condition is irrelevant to Bell in equality. We
check that the real origin of the Bell's inequality is the assumption of
applicability of classical (Kolmogorovian) probability theory to quantum
mechanics. We describe the chameleon effect which allows to construct an
experiment realizing a local, realistic, classical, deterministic and
macroscopic violation of the Bell inequalities.


title: Mermin's n-particle Bell inequality and operators' noncommutativity

abstract:
  The relationship between the noncommutativity of operators and the violation
of the Bell inequality is exhibited in the light of the n-particle Bell-type
inequality discovered by Mermin [PRL 65, 1838 (1990)]. It is shown, in
particular, that the maximal amount of violation of Mermin's inequality
predicted by quantum mechanics decreases exponentially by a factor of 2^{-m/2}
whenever any m among the n single-particle commutators happen to vanish.


title: Stochastic limit approximation for rapidly decaying systems

abstract:
  The stochastic limit approximation method for ``rapid'' decay is presented,
where the damping rate \gamma is comparable to the system frequency \Omega,
i.e., \gamma \sim \Omega, whereas the usual stochastic limit approximation is
applied only to the weak damping situation \gamma << \Omega. The key formulas
for rapid decay are very similar to those for weak damping, but the dynamics is
quite different. From a microscopic Hamiltonian, the spin-boson model, a Bloch
equation containing two independent time scales is derived. This is a useful
method to extract the minimal dissipative dynamics at high temperature kT >>
\hbar\Omega and the master equations obtained are of the Lindblad form even for
the Caldeira-Leggett model. The validity of the method is confirmed by
comparing the master equation derived through this method with the exact one.


title:
The speed of quantum information and the preferred frame: analysis of
  experimental data

abstract:
  The results of EPR experiments performed in Geneva are analyzed in the frame
of the cosmic microwave background radiation, generally considered as a good
candidate for playing the role of preferred frame. We set a lower bound for the
speed of quantum information in this frame at 1.5 x 10^4 c.


title:
Experimental test of non-local quantum correlation in relativistic
  configurations

abstract:
  We report on a new kind of experimental investigations of the tension between
quantum nonlocally and relativity. Entangled photons are sent via an optical
fiber network to two villages near Geneva, separated by more than 10 km where
they are analyzed by interferometers. The photon pair source is set as
precisely as possible in the center so that the two photons arrive at the
detectors within a time interval of less than 5 ps (corresponding to a path
length difference of less than 1 mm). One detector is set in motion so that
both detectors, each in its own inertial reference frame, are first to do the
measurement! The data always reproduces the quantum correlations, making it
thus more difficult to consider the projection postulate as a compact
description of real collapses of the wave-function.


title: Space-Time Structure as Hidden Variable

abstract:
  EPR correlations exist and can be observed independently of any a priori
given frame of reference. We can even construct a frame of reference that is
based on these correlations. This observation-based frame of reference is
equivalent to the customary a priori given frame of reference of the laboratory
when describing real EPR experiments. J.S. Bell has argued that local hidden
parameter theories that reproduce the predictions of Quantum Mechanics cannot
exist, but the counterfactual reasoning leading to Bell's conclusion is
physically meaningless if the frame of reference that is based on
EPR-correlations is accepted as the backdrop for EPR-type experiments. The
refutal of Bell's proof opens up for the construction of a viable hidden
parameter theory. A model of a spin h/2 particle in terms of a non-flat metric
of space-time is shown to be able to reproduce the predictions of quantum
mechanics in the Bohm-Aharonov version of the EPR experiment, without
introducing non-locality.


title: Entropic uncertainty relation for power-law wave packets

abstract:
  For the power-law quantum wave packet in configuration space, the variance of
the position observable may be divergent. Accordingly, the information-entropic
formulation of the uncertainty principle becomes more appropriate than the
Heisenberg-type formulation, since it involves only the finite quantities. It
is found that the total amount of entropic uncertainty converges to its lower
bound in the limit of a large value of the exponent.


title: Creating macroscopic atomic EPR states from Bose condensates

abstract:
  We present a scheme for creating quantum entangled atomic states through the
coherent spin-exchange collision of a spinor Bose-Einstein condensate. The
state generated possesses macroscopic Einstein-Podolsky-Roson correlation and
the fluctuation in one of its quasi-spin components vanishes. We show that an
elongated condensate with large aspect ratio is most suitable for creating such
a state.


title:
Decoherence-Free Subspaces for Multiple-Qubit Errors: (II) Universal,
  Fault-Tolerant Quantum Computation

abstract:
  Decoherence-free subspaces (DFSs) shield quantum information from errors
induced by the interaction with an uncontrollable environment. Here we study a
model of correlated errors forming an Abelian subgroup (stabilizer) of the
Pauli group (the group of tensor products of Pauli matrices). Unlike previous
studies of DFSs, this type of errors does not involve any spatial symmetry
assumptions on the system-environment interaction. We solve the problem of
universal, fault-tolerant quantum computation on the associated class of DFSs.


title: Quantum coherence and interaction-free measurements

abstract:
  We investigate the extent to which ``interaction-free'' measurements perturb
the state of quantum systems. We show that the absence of energy exchange
during the measurement is not a sufficient criterion to preserve that state, as
the quantum system is subject to measurement dependent decoherence. While it is
possible in general to design interaction-free measurement schemes that do
preserve that state, the requirement of quantum coherence preservation rapidly
leads to a very low efficiency. Our results, which have a simple interpretation
in terms of ``which-way'' arguments, open up the way to novel quantum
non-demolition techniques.


title: Stochastic mechanics and the Feynman integral

abstract:
  The Feynman integral is given a stochastic interpretation in the framework of
Nelson's stochastic mechanics employing a time-symmetric variant of Nelson's
kinematics recently developed by the author.


title: Quantum Algorithms for Element Distinctness

abstract:
  We present several applications of quantum amplitude amplification to finding
claws and collisions in ordered or unordered functions. Our algorithms
generalize those of Brassard, Hoyer, and Tapp, and imply an O(N^{3/4} log N)
quantum upper bound for the element distinctness problem in the comparison
complexity model (contrasting with Theta(N log N) classical complexity). We
also prove a lower bound of Omega(N^{1/2}) comparisons for this problem and
derive bounds for a number of related problems.


title:
Experimental Realization of an Order-Finding Algorithm with an NMR
  Quantum Computer

abstract:
  We report the realization of a nuclear magnetic resonance (NMR) quantum
computer which combines the quantum Fourier transform (QFT) with exponentiated
permutations, demonstrating a quantum algorithm for order-finding. This
algorithm has the same structure as Shor's algorithm and its speed-up over
classical algorithms scales exponentially. The implementation uses a
particularly well-suited five quantum bit molecule and was made possible by a
new state initialization procedure and several quantum control techniques.


title: Simultaneous decomposition of two states

abstract:
  Simultaneous decompositions of a pair of states into pure ones are examined.
There are privileged decompositions which are distinguished from all the other
ones.


title: Non-locality and quantum theory: new experimental evidence

abstract:
  Starting from the late 60's many experiments have been performed to verify
the violation Bell's inequality by Einstein-Podolsky-Rosen (EPR) type
correlations. The idea of these experiments being that: (i) Bell's inequality
is a consequence of locality, hence its experimental violation is an indication
of non locality; (ii) this violation is a typical quantum phenomenon because
any classical system making local choices (either deterministic or random) will
produce correlations satisfying this inequality. Both statements (i) and (ii)
have been criticized by quantum probability on theoretical grounds (not
discussed in the present paper) and the experiment discussed below has been
devised to support these theoretical arguments. We emphasize that the goal of
our experiment is not to reproduce classically the EPR correlations but to
prove that there exist perfectly local classical dynamical systems violating
Bell's inequality.


title: The decoherence criterion

abstract:
  The decoherence mechanism signals the limits beyond which the system dynamics
approaches the classical behavior. We show that in some cases decoherence may
also signal the limits beyond which the system dynamics has to be described by
quantum field theory, rather than by quantum mechanics.


title: The Quantum Complexity of Set Membership

abstract:
  We study the quantum complexity of the static set membership problem: given a
subset S (|S| \leq n) of a universe of size m (m \gg n), store it as a table of
bits so that queries of the form `Is x \in S?' can be answered. The goal is to
use a small table and yet answer queries using few bitprobes. This problem was
considered recently by Buhrman, Miltersen, Radhakrishnan and Venkatesh, where
lower and upper bounds were shown for this problem in the classical
deterministic and randomized models. In this paper, we formulate this problem
in the "quantum bitprobe model" and show tradeoff results between space and
time.In this model, the storage scheme is classical but the query scheme is
quantum.We show, roughly speaking, that similar lower bounds hold in the
quantum model as in the classical model, which imply that the classical upper
bounds are more or less tight even in the quantum case. Our lower bounds are
proved using linear algebraic techniques.


title: Quantum Recurrences: Probe to study Quantum Chaos

abstract:
  We study the phase space of periodically modulated gravitational cavity by
means of quantum recurrence phenomena. We report that the quantum recurrences
serve as a tool to connect phase space of the driven system with spectrum in
quantum domain. With the help of quantum recurrences we investigate the
quasi-energy spectrum of the system for a certain fixed modulation strength. In
addition, we study transition of spectrum from discrete to continuum as a
function of modulation strength.


title: What can biology bestow to quantum mechanics?

abstract:
  The biological hierarchy and the differences between living and non-living
matter are considered from the standpoint of quantum mechanics.


title: Security of Continuous Variable Quantum Cryptography

abstract:
  We discuss a quantum key distribution scheme in which small phase and
amplitude modulations of CW light beams carry the key information. The presence
of EPR type correlations provides the quantum protection. We identify universal
constraints on the level of shared information between the intended receiver
(Bob) and any eavesdropper (Eve) and use this to make a general evaluation of
security. We identify teleportation as an optimum eavesdropping technique.


title: Interference and the lossless lossy beam splitter

abstract:
  By directing the input light into a particular mode it is possible to obtain
as output all of the input light for a beam splitter that is 50% absorbing.
This effect is also responsible for nonlinear quantum interference when two
photons are incident on the beam splitter.


title: Quantum Holographic Teleportation of Light Fields

abstract:
  We describe a continious variable teleportation scheme that allows to
teleport the quantum state of distributed in space-time multimode
electromagnetic field. Our teleportation protocol uses the spatially-multimode
entangled Einstein-Podolsky-Rosen lihgt beams. We evaluate the "resolving
power" of teleportation in space-time and specify the degrees of freedom of
electromagnetic field whose quantum state can be effectively teleported. We
call this scheme "quantum holographic teleportation" because it can be
considered as an extension of conventional holography to quantum domain.


title:
Real inertons against hypothetical gravitons. Experimental proof of the
  existence of inertons

abstract:
  Previously, one of the authors has suggested [Phys. Essays, vol. 6, 554
(1993); vol. 10, 407 (1997)] a mechanism of the particle motion within the
framework of a vacuum regarded as an original cellular medium, i.e. quantum
aether. The existence of special elementary excitations of the aether medium --
inertons -- around the particle has been the main peculiarity of that
mechanism. The present paper treats the impact of inertons on the collective
behaviour of atoms in a solid. It is shown that inertons should contribute to
the effective potential of interaction of atoms in the crystal lattice. The
possibility of separating this inerton contribution from the value of the atom
vibration amplitude is analysed. The experiment which assumes the presence of
the hypothetical inerton field is performed. The expected changes in the
structure of the test specimens caused by this field are in fact convincingly
fixed in micrographs.

categories: quant-ph astro-ph cond-mat.mtrl-sci gr-qc hep-ex nucl-ex


title: Three dimensional time and energy operators and an uncertainty relation

abstract:
  Three dimensional time and energy operators are introduced and an uncertainty
relation between them is proved.


title:
Comment on "Thermal Effects on the Casimir Force in the 0.1-5 micrometer
  Range"

abstract:
  In a recent paper (M. Bostrom and Bo E. Sernelius, Phys. Rev. Lett. 84, 4757
(2000)) the combined effect of finite conductivity and finite temperature on
the Casimir force is analyzed, and significant deviations from other
theoretical results and a recent experiment are obtained.
  In this Comment, I show that the extrapolation to zero frequency is incorrect
because the authors have neglected that the wavenumber and frequency of the
electromagentic mode must simultaneously appraoch zero


title: A reply to "Quantum Clock Synchronization"

abstract:
  Recently a protocol for Quantum Clock Synchronization (QCS) of remote clocks
using quantum entanglement was proposed by Jozsa et al. This method has the
goal of eliminating the random noise present in classical synchronization
techniques. However, as stated QCS depends on the two members of each entangled
pair undergoing the same unitary evolution even while being transported to
different locations. This is essentially equivalent to a perfect Eddington Slow
Clock Transfer protocol and thus, not an improvement over classical techniques.
We will discuss this and suggest ways in which QCS may still be used.


title:
Analytic Expression for Exact Ground State Energy Based on an Operator
  Method for a Class of Anharmonic Potentials

abstract:
  A general procedure based on shift operators is formulated to deal with
anharmonic potentials. It is possible to extract the ground state energy
analytically using our method provided certain consistency relations are
satisfied. Analytic expressions for the exact ground state energy have also
been derived specifically for a large class of the one-dimensional oscillator
with cubic-quartic anharmonic terms. Our analytical results can be used to
check the accuracy of existing numerical methods, for instance the method of
state-dependent diagonalization. Our results also agree with the asymptotic
behavior in the divergent pertubative expansion of quartic harmonic oscillator.


title:
Photon antibunching in the fluorescence of individual color centers in
  diamond

abstract:
  We have observed photon antibunching in the fluorescence light emitted from a
single N-V center in diamond at room temperature. The possibility of generating
triggerable single photons with such a solide state system is discussed


title:
Squeezing of electromagnetic field in a cavity by electrons in Trojan
  states

abstract:
  The notion of the Trojan state of a Rydberg electron, introduced by
I.Bialynicki-Birula, M.Kali\'nski, and J.H.Eberly (Phys. Rev. Lett. 73, 1777
(1994)) is extended to the case of the electromagnetic field quantized in
acavity. The shape of the electronic wave packet describing the Trojan state is
practically the same as in the previously studied externally driven system. The
fluctuations of the quantized electromagnetic field around its classical value
exhibit strong squeezing. The emergence of Trojan states in the cylindrically
symmetrical system is attributed to spontaneous symmetry braking.


title: Quantum-Information Processing with Semiconductor Macroatoms

abstract:
  An all optical implementation of quantum information processing with
semiconductor macroatoms is proposed. Our quantum hardware consists of an array
of semiconductor quantum dots and the computational degrees of freedom are
energy-selected interband optical transitions. The proposed quantum-computing
strategy exploits exciton-exciton interactions driven by ultrafast sequences of
multi-color laser pulses. Contrary to existing proposals based on charge
excitations, the present all-optical implementation does not require the
application of time-dependent electric fields, thus allowing for a
sub-picosecond, i.e. decoherence-free, operation time-scale in realistic
state-of-the-art semiconductor nanostructures.


title: Quantum deletion is possible via a partial randomization procedure

abstract:
  An alternative kind of deleting/erasing operation is introduced which differs
from the commonly used {\it controlled-not} (C-not) conditional logical
operation $-$to flip to a standard, `zero' value the (classical or quantum)
state of the last copy in a chain, in a deletion process. It is completely
reversible, in the classical case, possessing a most natural cloning operation
counterpart. We call this deleting procedure R-deletion since, in a way, it can
be viewed as a `randomization' of the standard C-not operator. It has the
remarkable property of by-passing in a simple manner the `impossibility of
deletion of a quantum state' principle, put forward by Pati and Braunstein
recently \cite{pbn1}.


title: Quantum versus Classical Learnability

abstract:
  We consider quantum versions of two well-studied classical learning models:
Angluin's model of exact learning from membership queries and Valiant's
Probably Approximately Correct (PAC) model of learning from random examples. We
give positive and negative results for quantum versus classical learnability.
For each of the two learning models described above, we show that any concept
class is information-theoretically learnable from polynomially many quantum
examples if and only if it is information-theoretically learnable from
polynomially many classical examples. In contrast to this information-theoretic
equivalence betwen quantum and classical learnability, though, we observe that
a separation does exist between efficient quantum and classical learnability.
For both the model of exact learning from membership queries and the PAC model,
we show that under a widely held computational hardness assumption for
classical computation (the intractability of factoring), there is a concept
class which is polynomial-time learnable in the quantum version but not in the
classical version of the model.


title: Single photon generation by pulsed excitation of a single dipole

abstract:
  The fluorescence of a single dipole excited by an intense light pulse can
lead to the generation of another light pulse containing a single photon. The
influence of the duration and energy of the excitation pulse on the number of
photons in the fluorescence pulse is studied. The case of a two-level dipole
with strongly damped coherences is considered. The presence of a metastable
state leading to shelving is also investigated.


title: Multi-Player Quantum Games

abstract:
  Recently the concept of quantum information has been introduced into game
theory. Here we present the first study of quantum games with more than two
players. We discover that such games can possess a new form of equilibrium
strategy, one which has no analogue either in traditional games or even in
two-player quantum games. In these `pure' coherent equilibria, entanglement
shared among multiple players enables new kinds of cooperative behavior: indeed
it can act as a contract, in the sense that it prevents players from
successfully betraying one-another.


title: Loading a continuous-wave atom laser by optical pumping techniques

abstract:
  Demonstrating that despite loss processes, Bose-Einstein condensates can be
formed in steady state is a prerequisite for obtaining a coherent beam of atoms
in a continuous-wave atom laser. In this paper we propose a method for loading
atoms into the thermal component of a Bose condensed cloud confined in a
magnetic trap. This method is aimed at allowing steady state dynamics to be
achieved. The proposed scheme involves loading atoms into the conservative
magnetic potential using the spontaneous emission of photons. We show that the
probability for the reabsorption of these photons may be small .


title: Experimental realization of a highly structured search algorithm

abstract:
  The highly structured search algorithm proposed by Hogg[Phys.Rev.Lett.
80,2473(1998)] is implemented experimentally for the 1-SAT problem in a single
search step by using nuclear magnetic resonance technique with two-qubit
sample. It is the first demonstration of the Hogg's algorithm, and can be
readily extended to solving 1-SAT problem for more qubits in one step if the
appropriate samples possessing more qubits are experimentally feasible.


title: Why the Disjunction in Quantum Logic is Not Classical

abstract:
  The quantum logical `or' is analyzed from a physical perspective. We show
that it is the existence of EPR-like correlation states for the quantum
mechanical entity under consideration that make it nonequivalent to the
classical situation. Specifically, the presence of potentiality in these
correlation states gives rise to the quantum deviation from the classical
logical `or'. We show how this arises not only in the microworld, but also in
macroscopic situations where EPR-like correlation states are present. We
investigate how application of this analysis to concepts could alleviate some
well known problems in cognitive science.


title: Optimal distillation of a GHZ state

abstract:
  We present the optimal local protocol to distill a
Greenberger-Horne-Zeilinger (GHZ) state from a single copy of any pure state of
three qubits.


title:
Rapid solution of problems by nuclear-magnetic-resonance quantum
  computation

abstract:
  We offer an improved method for using a nuclear-magnetic-resonance quantum
computer (NMRQC) to solve the Deutsch-Jozsa problem. Two known obstacles to the
application of the NMRQC are exponential diminishment of density-matrix
elements with the number of bits, threatening weak signal levels, and the high
cost of preparing a suitable starting state. A third obstacle is a heretofore
unnoticed restriction on measurement operators available for use by an NMRQC.
Variations on the function classes of the Deutsch-Jozsa problem are introduced,
both to extend the range of problems advantageous for quantum computation and
to escape all three obstacles to use of an NMRQC. By adapting it to one such
function class, the Deutsch-Jozsa problem is made solvable without exponential
loss of signal. The method involves an extra work bit and a polynomially more
involved Oracle; it uses the thermal-equilibrium density matrix systematically
for an arbitrary number of spins, thereby avoiding both the preparation of a
pseudopure state and temporal averaging.


title: The Violation of Bell Inequalities in the Macroworld

abstract:
  We show that Bell inequalities can be violated in the macroscopic world. The
macroworld violation is illustrated using an example involving connected
vessels of water. We show that whether the violation of inequalities occurs in
the microworld or in the macroworld, it is the identification of nonidentical
events that plays a crucial role. Specifically, we prove that if nonidentical
events are consistently differentiated, Bell-type Pitowsky inequalities are no
longer violated, even for Bohm's example of two entangled spin 1/2 quantum
particles. We show how Bell inequalities can be violated in cognition,
specifically in the relationship between abstract concepts and specific
instances of these concepts. This supports the hypothesis that genuine quantum
structure exists in the mind. We introduce a model where the amount of
nonlocality and the degree of quantum uncertainty are parameterized, and
demonstrate that increasing nonlocality increases the degree of violation,
while increasing quantum uncertainty decreases the degree of violation.


title:
A Rosetta Stone for Quantum Mechanics with an Introduction to Quantum
  Computation

abstract:
  The purpose of these lecture notes is to provide readers, who have some
mathematical background but little or no exposure to quantum mechanics and
quantum computation, with enough material to begin reading the research
literature in quantum computation and quantum information theory. This paper is
a written version of the first of eight one hour lectures given in the American
Mathematical Society (AMS) Short Course on Quantum Computation held in
conjunction with the Annual Meeting of the AMS in Washington, DC, USA in
January 2000, and will appear in the AMS PSAPM volume entitled "Quantum
Computation."
  Part 1 of the paper is an introduction the to the concept of the qubit.
  Part 2 gives an introduction to quantum mechanics covering such topics as
Dirac notation, quantum measurement, Heisenberg uncertainty, Schrodinger's
equation, density operators, partial trace, multipartite quantum systems, the
Heisenberg versus the Schrodinger picture, quantum entanglement, EPR paradox,
quantum entropy.
  Part 3 gives a brief introduction to quantum computation, covering such
topics as elementary quantum computing devices, wiring diagrams, the no-cloning
theorem, quantum teleportation, Shor's algorithm, Grover's algorithm.
  Many examples are given. A table of contents as well as an index are provided
for readers who wish to "pick and choose." Since this paper is intended for a
diverse audience, it is written in an informal style at varying levels of
difficulty and sophistication from the very elementary to the more advanced.


title:
Calibration of Local Chiralities and Time-Arrows Based on Quantum
  Nonlocality

abstract:
  Unlike classical physics, quantum mechnanics is sensitive to mistaken choice
of chiralities or time-arrows of local reference systems. Quantum correlations
between distant electron spins, for instance, would reveal a mistaken local
chirality. Local polarization measurements and classical communication enable
the distant partners to compare their local chiralities. Local time-arrows can
be calibrated in a similar way.


title: The Liar-paradox in a Quantum Mechanical Perspective

abstract:
  In this paper we concentrate on the nature of the liar paradox as a cognitive
entity; a consistently testable configuration of properties. We elaborate
further on a quantum mechanical model [Aerts, Broekaert, Smets 1999] that has
been proposed to analyze the dynamics involved, and we focus on the
interpretation and concomitant philosophical picture. Some conclusions we draw
from our model favor an effective realistic interpretation of cognitive
reality.


title: Squeezing and entanglement of atomic beams

abstract:
  We propose and analyze a scheme for generating entangled atomic beams out of
a Bose-Einstein condensate using spin-exchanging collisions. In particular, we
show how to create both atomic squeezed states and entangled states of pairs of
atoms.


title: Reconstruction of SU(1,1) States

abstract:
  We show how group symmetries can be used to reconstruct quantum states. In
our scheme for SU(1,1) states, the input field passes through a non-degenerate
parametric amplifier and one measures the probability of finding the output
state with a certain number (usually zero) of photons in each mode. The density
matrix in the Fock basis is retrieved from the measured data by least squares
method after singular value decomposition of the design matrix. Several
illustrative examples involving the reconstruction of a pair coherent state, a
Perelomov coherent state, and a coherent superposition of pair coherent states
are considered.


title:
Conditional quantum state engineering in repeated 2-photon down
  conversion

abstract:
  The U(1,1) and U(2) transformations realized by three-mode interaction in the
respective parametric approximations are studied in conditional measurement,
and the corresponding non-unitary transformation operators are derived. As an
application, the preparation of single-mode quantum states using an optical
feedback loop is discussed, with special emphasis of Fock state preparation.
For that example, the influence of non-perfect detection and feedback is also
considered.


title:
Performances of Block Codes Attaining the Expurgated and Cutoff Rate
  Lower Bounds on the Error Exponent of Binary Classical-Quantum Channels

abstract:
  A conceptually simple method for derivation of lower bounds on the error
exponent of specific families of block codes used on classical-quantum channels
with arbitrary signal states over a finite Hilbert space is presented. It is
shown that families of binary block codes with appropriately rescaled binomial
multiplicity enumerators used on binary classical-quantum channels and decoded
by the suboptimal decision rule introduced by Holevo attain the expurgated and
cutoff rate lower bounds on the error exponent.


title:
Decoherence and the rate of entropy production in chaotic quantum
  systems

abstract:
  We show that for an open quantum system which is classically chaotic (a
quartic double well with harmonic driving coupled to a sea of harmonic
oscillators) the rate of entropy production has, as a function of time, two
relevant regimes: For short times it is proportional to the diffusion
coefficient (fixed by the system--environment coupling strength). For longer
times (but before equilibration) there is a regime where the entropy production
rate is fixed by the Lyapunov exponent. The nature of the transition time
between both regimes is investigated.


title: Remarks on 2-q-bit states

abstract:
  We distinguish six classes of families of locally equivalent states in a
straightforward scheme for classifying all 2-q-bit states; four of the classes
consist of two subclasses each. The simple criteria that we stated recently for
checking a given state's positivity and separability are justified, and we
discuss some important properties of Lewenstein-Sanpera decompositions. An
upper bound is conjectured for the sum of the degree of separability of a
2-q-bit state and its concurrence.


title: Quantum Message Disruption: A Two-State Model

abstract:
  A game in which one player makes unitary transformations of a simple system,
and another seeks to confound the resulting state by a randomly chosen action
is analyzed carefully. It is shown that the second player can reduce any system
to a completely random one by rotation through an angle of 120 degrees, about
an axis chosen at random. If, on the other hand, the second player is forced to
behave ``classically'' by reducing the wave function, then the first play
retains an advantage, which the second player may eliminate by repeated
measurement using randomly selected bases.


title: Landau levels on a torus

abstract:
  Landau levels have represented a very rich field of research, which has
gained widespread attention after their application to quantum Hall effect. In
a particular gauge, the holomorphic gauge, they give a physical implementation
of Bargmann's Hilbert space of entire functions. They have also been recognized
as a natural bridge between Feynman's path integral and Geometric Quantization.
We discuss here some mathematical subtleties involved in the formulation of the
problem when one tries to study quantum mechanics on a finite strip of sides
L_1, L_2 with a uniform magnetic field and periodic boundary conditions. There
is an apparent paradox here: infinitesimal translations should be associated to
canonical operators [p_x,p_y] \propto i\hslash B, and, at the same time, live
in a Landau level of finite dimension B L_1L_2/(hc/e), which is impossible from
Wintner's theorem. The paper shows the way out of this conundrum.


title: Coherent control of spin squeezing

abstract:
  We report an interaction that controls spin squeezing in a collection of spin
1/2 particles. We describe how spin squeezing can be generated and maintained
in time. Our scheme can be applied to control the spin squeezing in a Bose
condensate with two internal spin states.


title:
Entangling operations and their implementation using a small amount of
  entanglement

abstract:
  We study when a physical operation can produce entanglement between two
systems initially disentangled. The formalism we develop allows to show that
one can perform certain non-local operations with unit probability by
performing local measurement on states that are weakly entangled.


title: Maximum efficiency of a linear-optical Bell-state analyzer

abstract:
  In a photonic realization of qubits the implementation of quantum logic is
rather difficult due the extremely weak interaction on the few photon level. On
the other hand, in these systems interference is available to process the
quantum states. We formalize the use of interference by the definition of a
simple class of operations which include linear optical elements, auxiliary
states and conditional operations.
  We investigate an important subclass of these tools, namely linear optical
elements and auxiliary modes in the vacuum state. For this tools, we are able
to extend a previous quantitative result, a no-go theorem for perfect Bell
state analyzer on two qubits in polarization entanglement, by a quantitative
statement. We show, that within this subclass it is not possible to
discriminate unambiguously four equiprobable Bell states with a probability
higher than 50 %.


title: Exact solutions for a family of discretely spiked harmonic oscillators

abstract:
  Factorization method is developed for a family of discretely spiked harmonic
oscillators. Two sets of intertwining and ladder operators are presented to
algebraically generate eigenstates with energies isomorphic to those of the
ordinary harmonic oscillator. Normalization conditions are examined to reject
unphysical cases. Generic theory is specialized to one dimensional linear
oscillator and N dimensional radial oscillators in odd dimensions N=1,3,5..,
where orthogonal basis or sets of staggered orthogonal bases can be identified.
Even dimensions N=2,4,6.. are rejected as ill defined since they do not lead to
properly defined bases. The theory is augmented by a short Haskell program
Spike, which directly implements the intertwining and ladder operators,
generates the eigenfunctions, tests integrability of the solutions, verifies
orthogonality conditions and tests consistency of theoretical claims.


title:
Reeh-Schlieder Defeats Newton-Wigner: On alternative localization
  schemes in relativistic quantum field theory

abstract:
  Many of the "counterintuitive" features of relativistic quantum field theory
have their formal root in the Reeh-Schlieder theorem, which in particular
entails that local operations applied to the vacuum state can produce any state
of the entire field. It is of great interest, then, that I.E. Segal and, more
recently, G. Fleming (in a paper entitled "Reeh-Schlieder Meets Newton-Wigner")
have proposed an alternative "Newton-Wigner" localization scheme that avoids
the Reeh-Schlieder theorem. In this paper, I reconstruct the Newton-Wigner
localization scheme and clarify the limited extent to which it avoids the
counterintuitive consequences of the Reeh-Schlieder theorem. I also argue that
neither Segal nor Fleming has provided a coherent account of the physical
meaning of Newton-Wigner localization.


title:
All inseparable two-mode Gaussian continuous variable states are
  distillable

abstract:
  We show that all entangled Gaussian states of two infinite dimensional
systems can be distilled to maximally entangled states in finite dimensions.
The distillation protocol involves local squeezing operations, local homodyne
measurements with ancilla systems prepared in coherent states as well as local
joint measurements of the total number operator of two copies of the state.


title: Joint measurements via quantum cloning

abstract:
  We explore the possibility of achieving optimal joint measurements of
noncommuting observables on a single quantum system by performing conventional
measurements of commuting self adjoint operators on optimal clones of the
original quantum system. We consider the case of both finite dimensional and
infinite dimensional Hilbert spaces. In the former we study the joint
measurement of three orthogonal components of a spin 1/2, in the latter we
consider the case of the joint measurements of any pair of noncommuting
quadratures of one mode of the electromagnetic field. We show that universally
covariant cloning is not ideal for joint measurements, and a suitable non
universally covariant cloning is needed.


title: Experimental Demonstration of Optimal Unambiguous State Discrimination

abstract:
  We present the first full demonstration of unambiguous state discrimination
between non-orthogonal quantum states. Using a novel free space interferometer
we have realised the optimum quantum measurement scheme for two non-orthogonal
states of light, known as the Ivanovic-Dieks-Peres (IDP) measurement. We have
for the first time gained access to all three possible outcomes of this
measurement. All aspects of this generalised measurement scheme, including its
superiority over a standard von Neumann measurement, have been demonstrated
within 1.5% of the IDP predictions.


title: Quantum key distribution in the Holevo limit

abstract:
  A theorem by Shannon and the Holevo theorem impose that the efficiency of any
protocol for quantum key distribution, $\cal E$, defined as the number of
secret (i.e., allowing eavesdropping detection) bits per transmitted bit plus
qubit, is ${\cal E} \le 1$. The problem addressed here is whether the limit
${\cal E} =1$ can be achieved. It is showed that it can be done by splitting
the secret bits between several qubits and forcing Eve to have only a
sequential access to the qubits, as proposed by Goldenberg and Vaidman. A
protocol with ${\cal E} =1$ based on polarized photons and in which Bob's state
discrimination can be implemented with linear optical elements is presented.


title: Multiparty multilevel Greenberger-Horne-Zeilinger states

abstract:
  The proof of Bell's theorem without inequalities by Greenberger, Horne, and
Zeilinger (GHZ) is extended to multiparticle multilevel systems. The proposed
procedure generalizes previous partial results and provides an operational
characterization of the so-called GHZ states for multiparticle multilevel
systems.


title: Sources of quantum waves

abstract:
  Due to the space and time dependence of the wave function in the time
dependent Schroedinger equation, different boundary conditions are possible.
The equation is usually solved as an ``initial value problem'', by fixing the
value of the wave function in all space at a given instant. We compare this
standard approach to "source boundary conditions'' that fix the wave at all
times in a given region, in particular at a point in one dimension. In contrast
to the well-known physical interpretation of the initial-value-problem
approach, the interpretation of the source approach has remained unclear, since
it introduces negative energy components, even for ``free motion'', and a
time-dependent norm. This work provides physical meaning to the source method
by finding the link with equivalent initial value problems.


title:
Bell State Preparation using Pulsed Non-Degenerate Two-Photon
  Entanglement

abstract:
  We report a novel Bell state preparation experiment. High-purity Bell states
are prepared by using femtosecond pulse pumped \emph{nondegenerate} collinear
spontaneous parametric down-conversion. The use of femtosecond pump pulse {\em
does not} result in reduction of quantum interference visibility in our scheme
in which post-selection of amplitudes and other traditional mechanisms, such
as, using thin nonlinear crystals or narrow-band spectral filters are not used.
Another distinct feature of this scheme is that the pump, the signal, and the
idler wavelengths are all distinguishable, which is very useful for quantum
communications.


title: No contradictions between Bohmian and quantum mechanics

abstract:
  Two recent claims by A. Neumaier (quant-ph/0001011) and P. Ghose
(quant-ph/0001024) that Bohmian mechanics is incompatible with quantum
mechanics for correlations involving time are shown to be unfounded.


title: Triviality of GHZ operators of higher spin

abstract:
  We prove that local observables of the set of GHZ operators for particles of
spin higher than 1/2 reduce to direct sums of the spin 1/2 operators
$\sigma_x$, $\sigma_y$ and, therefore, no new contradictions with local realism
arise by considering them.


title: Sophisticated quantum search without entanglement

abstract:
  Although entanglement is widely considered to be necessary for quantum
algorithms to improve on classical ones, Lloyd has observed recently that
Grover's quantum search algorithm can be implemented without entanglement, by
replacing multiple particles with a single particle having exponentially many
states. We explain that this maneuver removes entanglement from any quantum
algorithm. But all physical resources must be accounted for to quantify
algorithm complexity, and this scheme typically incurs exponential costs in
some other resource(s). In particular, we demonstrate that a recent
experimental realization requires exponentially increasing precision. There is,
however, a quantum algorithm which searches a `sophisticated' database (not
unlike a Web search engine) with a single query, but which we show does not
require entanglement even for multiparticle implementations.


title:
A Numerical Study of the Performance of a Quantum Adiabatic Evolution
  Algorithm for Satisfiability

abstract:
  Quantum computation by adiabatic evolution, as described in quant-ph/0001106,
will solve satisfiability problems if the running time is long enough. In
certain special cases (that are classically easy) we know that the quantum
algorithm requires a running time that grows as a polynomial in the number of
bits. In this paper we present numerical results on randomly generated
instances of an NP-complete problem and of a problem that can be solved
classically in polynomial time. We simulate a quantum computer (of up to 16
qubits) by integrating the Schrodinger equation on a conventional computer. For
both problems considered, for the set of instances studied, the required
running time appears to grow slowly as a function of the number of bits.


title: Quantum codes on high-genus surfaces

abstract:
  An economy of scale is found when storing many qubits in one highly entangled
block of a topological quantum code. The code is defined by construction of a
topologically convoluted 2-d surface and does not work by compressing
redundancy in the encoded information.


title: Relativistic J-matrix method

abstract:
  The relativistic version of the J-matrix method for a scattering problem on
the potential vanishing faster than the Coulomb one is formulated. As in the
non-relativistic case it leads to a finite algebraic eigenvalue problem. The
derived expression for the tangent of phase shift is simply related to the
non-relativistic case formula and gives the latter as a limit case. It is due
to the fact that the used basis set satisfies the ``kinetic balance
condition''.


title: Solid State Quantum Computing Using Spectral Holes

abstract:
  A quantum computer that stores information on two-state systems called
quantum bits or qubits must be able to address and manipulate individual
qubits, to effect coherent interactions between pairs of qubits, and to read
out the value of qubits.1,2 Current methods for addressing qubits are divided
up into spatial methods, as when a laser beam is focused on an individual
qubit3,4,5 or spectral methods, as when a nuclear spin in a molecule is
addressed using NMR.6,7 The density of qubits addressable spatially is limited
by the wavelength of light, and the number of qubits addressable spectrally is
limited by spin linewidths. Here, we propose a method for addressing qubits
using a method that combines spatial and spectral selectivity. The result is a
design for quantum computation that provides the potential for a density of
quantum information storage and processing many orders of magnitude greater
than that afforded by ion traps or NMR. Specifically, this method uses an
ensemble of spectrally resolved atoms in a spectral holeburning solid. The
quantum oupling is provided by strong atom-cavity interaction. Using a thin
disc of diamond containing nitrogen-vacancy color centers as an example, we
present an explicit model for realizing up to 300 coupled qubits in a single
spot. We show how about 100 operations can take place in parallel, yielding
close to 40,000 operations before decoherence.


title: Quantum mechanics without interpretation

abstract:
  It is shown that the independence of the continuum hypothesis points to the
unique definite status of the set of intermediate cardinality: the intermediate
set exists only as a subset of continuum. This latent status is a consequence
of duality of the members of the set. Due to the structural inhomogeneity of
the intermediate set, its complete description falls into several "sections"
(theories) with their special main laws, dimensions, and directions, i.e., the
complete description of the one-dimensional intermediate set is
multidimensional. Quantum mechanics is one of these theories.


title: Schroedinger-Cat Entangled State Reconstruction in the Penning Trap

abstract:
  We present a tomographic method for the reconstruction of the full entangled
quantum state for the cyclotron and spin degrees of freedom of an electron in a
Penning trap. Numerical simulations of the reconstruction of several
significant quantum states show that the method turns out to be quite accurate.


title: On the quantum mechanical nature in liquid NMR quantum computing

abstract:
  The quantum nature of bulk ensemble NMR quantum computing --the center of
recent heated debate, is addressed. Concepts of the mixed state and
entanglement are examined, and the data in a 2 qubit liquid NMR quantum
computation are analyzed. It is pointed out that the key problem in the current
debate is the understanding of entanglement in a mixed state system. The
following points are concluded in this Letter: 1)Density matrix describes the
"state" of an average particle in an ensemble. It can not describe the state of
an individual particle in an ensemble in detail; 2) Entanglement is a property
of the wave function of a quantum particle(such as an molecule in a liquid NMR
sample). Separability of the density matrix can not be used to measure the
entanglement of mixed ensemble; 3)The evolution of states in bulk-ensemble NMR
quantum computation is quantum mechanical; 4) The coefficient before the
effective pure state density matrix, $\epsilon$, is an measure of the
simultaneity of the molecules in an ensemble. It reflects the intensity of the
NMR signal and has no significance in quantifying the entanglement in the bulk
ensemble NMR system. We conclude that the liquid NMR quantum computation is
genuine, not just classical simulations.


title:
Gravitationally induced transition to classical behavior above 10^10
  proton masses

abstract:
  The localization length for the center of mass motion of a matter lump,
induced by gravitation, is obtained, without using any phenomenological
constants. Its dependence from mass and volume is consistent both with unitary
evolution of microscopic particles and with the classical behavior of
macroscopic bodies required to account for wave function collapse in quantum
measurements, the transition between the two regimes being rather sharp. The
gravitational interaction of nonrelativistic matter is modelled by a Yukawa
Hamiltonian with vanishing pion mass, no gravitational background is needed and
the only hypothesis consists in assuming unentanglement between matter and the
Yukawa field.


title:
Effect of classically forbidden momenta in one dimensional quantum
  scattering

abstract:
  The transmitted wave that results from a collision of a wave packet which is
initially to the left of a potential barrier depends in general on the
amplitudes of negative momenta of the initial state. The exact form of this
dependence is shown and the importance of this classically forbidden effect is
illustrated with numerical examples. Special care is taken to account properly
for bound states.


title: Three-party entanglement from positronium

abstract:
  The decay of ortho-positronium into three photons produces a physical
realization of a pure state with three-party entanglement. Its quantum
correlations are analyzed using recent results on quantum information theory,
looking for the final state which has the maximal amount of GHZ-like
correlations. This state allows for a statistical dismissal of local realism
stronger than the one obtained using any entangled state of two spin one-half
particles.


title:
The Schmidt Measure as a Tool for Quantifying Multi-Particle
  Entanglement

abstract:
  We present a measure of quantum entanglement which is capable of quantifying
the degree of entanglement of a multi-partite quantum system. This measure,
which is based on a generalization of the Schmidt rank of a pure state, is
defined on the full state space and is shown to be an entanglement monotone,
that is, it cannot increase under local quantum operations with classical
communication and under mixing. For a large class of mixed states this measure
of entanglement can be calculated exactly, and it provides a detailed
classification of mixed states.


title: Entangling Atoms and Ions in Dissipative Environments

abstract:
  Quantum information processing rests on our ability to manipulate quantum
superpositions through coherent unitary transformations, and to establish
entanglement between constituent quantum components of the processor. The
quantum information processor (a linear ion trap, or a cavity confining the
radiation field for example) exists in a dissipative environment. We discuss
ways in which entanglement can be established within such dissipative
environments. We can even make use of a strong interaction of the system with
its environment to produce entanglement in a controlled way.


title: Coherent transport of matter waves

abstract:
  A transport theory for atomic matter waves in low-dimensional waveguides is
outlined. The thermal fluctuation spectrum of magnetic near fields leaking out
of metallic microstructures is estimated. The corresponding scattering rate for
paramagnetic atoms turns out to be quite large in micrometer-sized waveguides
(approx. 100/s). Analytical estimates for the heating and decoherence of a cold
atom cloud are given. We finally discuss numerical and analytical results for
the scattering from static potential imperfections and the ensuing spatial
diffusion process.


title:
Noise-free scattering of the quantized electromagnetic field from a
  dispersive linear dielectric

abstract:
  We study the scattering of the quantized electromagnetic field from a linear,
dispersive dielectric using the scattering formalism for quantum fields. The
medium is modeled as a collection of harmonic oscillators with a number of
distinct resonance frequencies. This model corresponds to the Sellmeir
expansion, which is widely used to describe experimental data for real
dispersive media. The integral equation for the interpolating field in terms of
the in field is solved and the solution used to find the out field. The
relation between the in and out creation and annihilation operators is found
which allows one to calculate the S-matrix for this system. In this model, we
find that there are absorption bands, but the input-output relations are
completely unitary. No additional quantum noise terms are required.


title: EPR effect in gravitational field: nature of non-locality

abstract:
  The realization of the Einstein-Podolsky-Rosen effect by the correlation of
spin projections of two particles created in the decay of a single scalar
particle is considered for particles propagating in gravitational field. The
absence of a global definition of spatial directions makes it unclear whether
the correlation may exist in this case and, if yes, what directions in distant
regions must be correlated. It is shown that in a gravitational field an
approximate correlation may exist and the correlated directions are connected
with each other by the parallel transport along the world lines of the
particles. The reason for this is that the actual origin of the quantum
non-locality is founded in local processes.


title: Entangled webs: Tight bound for symmetric sharing of entanglement

abstract:
  Quantum entanglement cannot be unlimitedly shared among arbitrary number of
qubits. Larger the number of entangled pairs in an N-qubit system, smaller the
degree of bi-partite entanglement is. We analyze a system of N qubits in which
an arbitrary pair of particles is entangled. We show that the maximum degree of
entanglement (measured in the concurrence) between any pair of qubits is 2/N.
This tight bound can be achieved when the qubits are prepared in a pure
symmetric (with respect to permutations) state with just one qubit in the basis
state |0> and the others in the basis state |1>.


title:
Local-field effect in atom optics of two-component Bose-Einstein
  condensates

abstract:
  Starting from the first principles of nonrelativistic QED we have developed
the quantum theory of the interaction of a two-component ultracold atomic
ensemble with the electromagnetic field of vacuum and laser photons. The main
attention has been paid to the consistent consideration of dynamical
dipole-dipole interactions in the radiation field. Taking into account
local-field effects we have derived the system of Maxwell-Bloch equations.
Optical properties of the two-component Bose gas are investigated. It is shown
that the refractive index of the gas is given by the Maxwell-Garnett formula.
All equations which are used up to now for the description of the behavior of
an ultracold atomic ensemble in a radiation field can be obtained from our
general system of equations in the low-density limit. Raman-Nath diffraction of
the two-component atomic beam is investigated on the basis of our general
system of equations.


title: Quantum Brain?

abstract:
  In order to create a novel model of memory and brain function, we focus our
approach on the sub-molecular (electron), molecular (tubulin) and
macromolecular (microtubule) components of the neural cytoskeleton. Due to
their size and geometry, these systems may be approached using the principles
of quantum physics. We identify quantum-physics derived mechanisms conceivably
underlying the integrated yet differentiated aspects of memory encoding/recall
as well as the molecular basis of the engram. We treat the tubulin molecule as
the fundamental computation unit (qubit) in a quantum-computational network
that consists of microtubules (MTs), networks of MTs and ultimately entire
neurons and neural networks.
  We derive experimentally testable predictions of our quantum brain hypothesis
and perform experiments on these.

categories: quant-ph cond-mat.dis-nn hep-ph physics.bio-ph


title: Continuously Measured Systems,Path Integrals and Information

abstract:
  A continuously measured quantum system may be described by restricted path
integrals (RPI) or equivalently by non-Hermitian Hamiltonians. The measured
system is then considered as an open system, the influence of the environment
being taken into account by restricting the path integral or by inclusion of an
imaginary part in the Hamiltonian. This way of description of measurements
naturally follows from the Feynman form of quantum mechanics without any
additional postulates and may be interpreted as an information approach to
continuous quantum measurements. This reveals deep features of quantum physics
concerning relations between quantum world and its classical appearance.


title: The quantum bit commitment theorem

abstract:
  Unconditionally secure two-party bit commitment based solely on the
principles of quantum mechanics (without exploiting special relativistic
signalling constraints, or principles of general relativity or thermodynamics)
has been shown to be impossible, but the claim is repeatedly challenged. The
quantum bit commitment theorem is reviewed here and the central conceptual
point, that an `Einstein-Podolsky-Rosen' attack or cheating strategy can always
be applied, is clarified. The question of whether following such a cheating
strategy can ever be disadvantageous to the cheater is considered and answered
in the negative. There is, indeed, no loophole in the theorem.


title:
Position Measurement for a Relativistic Particle:
  Restricted-Path-Integral Analysis

abstract:
  Measurements of the position of a relativistic particle is considered in the
framework of the Restricted-Path-Integral (RPI) approach. The amplitude
describing such a measurement is shown to be exponentially small outside the
light cone of the space-time point corresponding to the measurement output, in
a qualitative agreement with the Hellwig and Kraus' postulate of relativistic
state reduction. Theory of the measurement including the probability
distribution for different measurement outputs is suggested. It is shown that
correct theory does not exist (for arbitrary initial states) if the error of
the measurement is less than the Compton length. The physical reason is that
the picture of measurement is destroyed in this case by pair creation.


title: Measurability of Electromagnetic Field: Model and Path Integral Methods

abstract:
  The problem of the measurability of the electromagnetic field is investigated
1) in the framework of the abstract restricted-path-integral method, and 2) by
explicitly accounting the action of the field onto the meter and its back
reaction. The meaning of the previously obtained results as well as of the
classical results of Bohr and Rosenfeld are made clear. The
restricted-path-integral method with integration over field configurations is
shown to give an estimation on the measurability of the field by any device not
disturbing the measured field (in the process of measurement) more than by the
measurement error. Such method of measurement is necessary for the control of
the field in electronic devices.


title: Quantum mechanical histories and the Berry phase

abstract:
  We elaborate on the distinction between geometric and dynamical phase in
quantum theory and show that the former is intrinsically linked to the quantum
mechanical probabilistic structure. In particular, we examine the appearance of
the Berry phase in the consistent histories scheme and establish that it is the
basic building block of the decoherence functional.
  These results are consequences of the novel temporal structure of
histories-based theories.


title: The Kapitza - Dirac effect

abstract:
  The Kapitza - Dirac effect is the diffraction of a well - collimated particle
beam by a standing wave of light. Why is this interesting? Comparing this
situation to the introductory physics textbook example of diffraction of a
laser beam by a grating, the particle beam plays the role of the incoming wave
and the standing light wave the role of the material grating, highlighting
particle - wave duality. Apart from representing such a beautiful example of
particle - wave duality, the diffracted particle beams are coherent. This
allows the construction of matter interferometers and explains why the Kapitza
- Dirac effect is one of the workhorses in the field of atom optics. Atom
optics concerns the manipulation of atomic waves in ways analogous to the
manipulation of light waves with optical elements. The excitement and activity
in this new field of physics stems for a part from the realisation that the
shorter de Broglie wavelengths of matter waves allow ultimate sensitivities for
diffractive and interferometric experiments that in principle would far exceed
their optical analogues. Not only is the Kapitza - Dirac effect an important
enabling tool for this field of physics, but diffraction peaks have never been
observed for electrons, for which is was originally proposed in 1933. Why has
this not been observed? What is the relation between the interaction of laser
light with electrons and the interaction of laser light with atoms, or in other
words what is the relation between the ponderomotive potential and the
lightshift potential? Would it be possible to build interferometers using the
Kapitza - Dirac effect for other particles? These questions will be addressed
in this paper.


title: Note on reversibility of quantum jumps

abstract:
  It has been recently proved that a quantum jump may be reversed by a unitary
process provided the initial state is restricted by some conditions. The
application of such processes for preventing decoherence, for example in
quantum computers, was suggested. We shall show that in the situation when the
quantum jump is reversible it supplies no information about the initial state
additional to the information known beforehand. Therefore the reversibility of
this type does not contradict the general statement of quantum measurement
theory: a measurement cannot be reversed. As a consequence of this, the
coherence of a state (say, in a quantum computer) cannot be restored after it
is destroyed by dissipative processes having a character of measurement.


title:
Finite resolution of time in continuous measurements: phenomenology and
  the model

abstract:
  Definition of a quantum corridor describing monitoring a quantum observable
in the framework of the phenomenological restricted-path-integral (RPI)
approach is generalized for the case of a finite resolution of time. The
resulting evolution of the continuously measured system cannot be presented by
a differential equation. Monitoring of the position of a quantum particle is
also considered with the help of a model which takes into account a finite
resolution of time. The results based on the model are shown to coincide with
those of the phenomenological approach.


title:
Large-Angle, Single-Order, One and Two-Dimensional Atomic Interferometry
  for Creation of Nanostructures

abstract:
  We have shown via explicit analysis as well as numerical simulation the
design of two large angle interferometers employing two-photon pulses. The
first one uses the technique of adiabatic following in a dark state to produce
a large splitting angle atomic interferometer that is capable of producing one
dimensional gratings with spacings as small as 2 nm. Unlike other large angle
interferometers, this technique is not sensitive to errors in optical pulse
area and decoherence from excited state decay. This may lead to a nearly two
orders of magnitude improvement in the sensitivity of devices such as atomic
gyroscopes, which are already as good as the best laser gyroscopes. The second
interferometer uses the technique of Raman pulses to produce a two-dimensional
interferometer, with independent choice of grating spacings in each direction,
each being as small as 2 nm. This scheme may enable one to produce uniform
arrays of quantum dots with dimensions of only a few nm on each side. In
addition, it may be possible to generalize this process to produce arbitrary
patterns with the same type of resolution.


title: Local Distinguishability of Multipartite Orthogonal Quantum States

abstract:
  We consider one copy of a quantum system prepared in one of two orthogonal
pure states, entangled or otherwise, and distributed between any number of
parties. We demonstrate that it is possible to identify which of these two
states the system is in by means of local operations and classical
communication alone. The protocol we outline is both completely reliable and
completely general - it will correctly distinguish any two orthogonal states
100% of the time.


title: Unpolarized light in quantum optics

abstract:
  We present a new derivation of the unpolarized quantum states of light, whose
general form was first derived by Prakash and Chandra [Phys. Rev. A 4, 796
(1971)]. Our derivation makes use of some basic group theory, is
straightforward, and offers some new insights.


title: Evolutionarily Stable Strategies in Quantum Games

abstract:
  Evolutionarily Stable Strategy (ESS) in classical game theory is a refinement
of Nash equilibrium concept. We investigate the consequences when a small group
of mutants using quantum strategies try to invade a classical ESS in a
population engaged in symmetric bimatrix game of Prisoner's Dilemma. Secondly
we show that in an asymmetric quantum game between two players an ESS pair can
be made to appear or disappear by resorting to entangled or unentangled initial
states used to play the game even when the strategy pair remains a Nash
equilibrium in both forms of the game.


title: "Quantum mysteries for anyone," or classical verities for everyone?

abstract:
  Unarticulated, implicit hypotheses in Bell's analysis of Einstein, Podolsky
and Rosen (EPR) correlations are identified and examined. These relate to the
mathematical-analytical properties of random variables, the character of the
relevant sample spaces and physical interpretations. We shown that continuous
hidden variables are not precluded by Bell inequalities. Finally, we propose a
local realistic model of optical EPRB experiments and consider its
implications.


title: A local hidden variable theory for the GHZ experiment

abstract:
  A recent analysis by de Barros and Suppes of experimentally realizable GHZ
correlations supports the conclusion that these correlations cannot be
explained by introducing local hidden variables. We show, nevertheless, that
their analysis does not exclude local hidden variable models in which the
inefficiency in the experiment is an effect not only of random errors in the
detector equipment, but is also the manifestation of a pre-set, hidden property
of the particles ("prism models"). Indeed, we present an explicit prism model
for the GHZ scenario; that is, a local hidden variable model entirely
compatible with recent GHZ experiments.


title:
Scattering of relativistic particles with Aharonov-Bohm-Coulomb
  interaction in two dimensions

abstract:
  The Aharonov-Bohm-Coulomb potentials in two dimensions may describe the
interaction between two particles carrying electric charge and magnetic flux,
say, Chern--Simons solitons, or so called anyons. The scattering problem for
such two-body systems is extended to the relativistic case, and the scattering
amplitude is obtained as a partial wave series. The electric charge and
magnetic flux is ($-q$, $-\phi/Z$) for one particle and ($Zq$, $\phi$) for the
other. When $(Zq^2/\hbar c)^2\ll 1$, and $q\phi/2\pi\hbar c$ takes on integer
or half integer values, the partial wave series is summed up approximately to
give a closed form. The results exhibit some nonperturbative features and
cannot be obtained from perturbative quantum electrodynamics at the tree level.


title:
Possible consequences of introduction of the discrete carrier of
  material fields

abstract:
  In this work some probable consequences of introduction of discrete medium -
carrier for fields of a substance are considered. The given medium is thinked
as a set of harmonic oscillators filling all space. Discrete medium is
introduced on an image and similarity of a crystalline lattice of a condensed
matter. The particles - excitation are concentrated in space of a "reciprocal
lattice " of discrete medium. This leads to the violation of the energy and
momentum conservation laws at the wave number of particles higher then the wave
number of the reciprocal lattice. Some possible confirmation of the given
consideration may be the behavior of cosmic rays at high energy. The other
possible consequences of the proposed approach in physics of elementary
particles, cosmology, and quantum mechanics are also discussed.


title:
Property Attribution and the Projection Postulate in Relativistic
  Quantum Theory

abstract:
  State-vectors resulting from collapse along the forward light cone from a
measurement interaction can be used for the attribution of both local and
non-local properties.


title:
Quantum Teleportation and Bell's Inequality Using Single-Particle
  Entanglement

abstract:
  A single-particle entangled state can be generated by illuminating a beam
splitter with a single photon. Quantum teleportation utilizing such a
single-particle entangled state can be successfully achieved with a simple
setup consisting only of linear optical devices such as beam splitters and
phase shifters. Application of the locality assumption to a single-particle
entangled state leads to Bell's inequality, a violation of which signifies the
nonlocal nature of a single particle.


title:
Photon echo signature of vibrational cat states created by femtosecond
  excitation of molecules

abstract:
  A pair of coherent femtosecond pulse excitations applied to a molecule with
strong electron-phonon coupling creates a coherent superposition of a low
momentum and a high momentum wavepacket in the vibrational states of both the
excited state and the ground state of the coherent transition. As the excited
state is accelerated further, interference between the high momentum ground
state contribution and the low momentum excited state contribution causes a
photon echo. This photon echo is a direct consequence of quantum interference
between separate vibrational trajectories and can therefore provide
experimental evidence of the non-classical properties of molecular vibrations.