title: Total correlations as fully additive entanglement monotones

abstract:
  We generalize the strategy presented in Refs. [1, 2], and propose general
conditions for a measure of total correlations to be an entanglement monotone
using its pure (and mixed) convex-roof extension. In so doing, we derive
crucial theorems and propose a concrete candidate for a total correlations
measure which is a fully additive entanglement monotone.


title: Resonances of Multichannel Systems

abstract:
  We describe structure of the T-matrices, scattering matrices, and Green
functions on unphysical energy sheets in multichannel scattering problems with
binary channels and in the three-body problem. Based on the explicit
representations obtained for the values of T- and S-matrices on the unphysical
sheets, we prove that the resonances belonging to an unphysical sheet are just
those energies where the correspondingly truncated scattering matrix, taken in
the physical sheet, has eigenvalue zero. We show, in addition, that
eigenvectors of the truncated scattering matrix associated with its zero
eigenvalue are formed of the breakup amplitudes for the respective resonant
states.


title:
Groverian Entanglement Measure of Pure Quantum States with Arbitrary
  Partitions

abstract:
  The Groverian entanglement measure of pure quantum states of $n$ qubits is
generalized to the case in which the qubits are divided into any $m \le n$
parties and the entanglement between these parties is evaluated. To demonstrate
this measure we apply it to general states of three qubits and to symmetric
states with any number of qubits such as the Greenberg-Horne-Zeiliner state and
the W state.


title: Dihedral symmetry of periodic chain: quantization and coherent states

abstract:
  Our previous work on quantum kinematics and coherent states over finite
configuration spaces is extended: the configuration space is, as before, the
cyclic group Z_n of arbitrary order n=2,3,..., but a larger group - the
non-Abelian dihedral group D_n - is taken as its symmetry group. The
corresponding group related coherent states are constructed and their
overcompleteness proved. Our approach based on geometric symmetry can be used
as a kinematic framework for matrix methods in quantum chemistry of ring
molecules.


title: Stationary states in a potential well

abstract:
  We review here in a historical context the topic of stationary states in the
quantum world, including the generalization to the primary ideas. We also
discuss the stationary states in one dimensional parabolic wells and the three
dimensional Coulomb and parabolic cases, as well as the stationary bound states
in the continuum


title: Su(2) intelligent states as coupled su(2) coherent states

abstract:
  We show how su(2) intelligent states can be obtained by coupling su(2)
coherent states. The construction is simple and efficient, and easily leads to
a discussion of some general properties of su(2) intelligent states.


title: Security Notions for Quantum Public-Key Cryptography

abstract:
  It is well known that Shor's quantum algorithm for integer factorization can
break down the RSA public-key cryptosystem, which is widely used in many
cryptographic applications. Thus, public-key cryptosystems in the quantum
computational setting are longed for cryptology. In order to define the
security notions of public-key cryptosystems, we have to model the power of the
sender, receiver, adversary and channel. While we may consider a setting where
quantum computers are available only to adversaries, we generally discuss what
are the right security notions for (quantum) public-key cryptosystems in the
quantum computational setting. Moreover, we consider the security of quantum
public-key cryptosystems known so far.


title:
Constructing CSS Codes with LDPC Codes for the BB84 Quantum Key
  Distribution Protocol

abstract:
  In this paper, we propose how to simply construct a pair of linear codes for
the BB84 quantum key distribution protocol. This protocol allows unconditional
security in the presence of an eavesdropper, and the pair of linear codes is
used for error correction and privacy amplification. Since their high decoding
performance implies low eavesdropper's mutual information, good design of the
two codes is required. The proposed method admits using arbitrary low-density
parity-check (LDPC) codes. Therefore, it has low complexity and high
performance for hardware implementation. Simulation results show that the pair
of codes performs well against practical and various noise levels.


title:
Violation of Energy Conservation in Boson and Fermion Fields on
  Subwavelength Nano-Scale

abstract:
  The Hamiltonians describing the energy nonconservation in boson and fermion
multimode fields under quantum interference have been derived. We show that
violation of the energy conservation is accompanied by the nonconservation of
momentum, number of particles and field charge. The phenomena could be observed
in Young's double-slit subwavelength (nanometer-scale) setup.

categories: quant-ph hep-th math-ph math.MP physics.optics


title: Polynomial Solution of Non-Central Potentials

abstract:
  We show that the exact energy eigenvalues and eigenfunctions of the
Schrodinger equation for charged particles moving in certain class of
non-central potentials can be easily calculated analytically in a simple and
elegant manner by using Nikiforov and Uvarov (NU) method. We discuss the
generalized Coulomb and harmonic oscillator systems. We study the Hartmann
Coulomb and the ring-shaped and compound Coulomb plus Aharanov-Bohm potentials
as special cases. The results are in exact agreement with other methods.


title: Privacy Amplification, Private States, and the Uncertainty Principle

abstract:
  We show that three principle means of treating privacy amplification in
quantum key distribution, private state distillation, classical privacy
amplification, and via the uncertainty principle, are equivalent and
interchangeable. By adapting the security proof based on the uncertainty
principle, we construct a new protocol for private state distillation which we
prove is identical to standard classical privacy amplification. Underlying this
approach is a new characterization of private states, related to their standard
formulation by the uncertainty principle, which gives a more physical
understanding of security in quantum key distribution.


title: Paradox in Wave-Particle Duality

abstract:
  We report on the simultaneous determination of complementary wave and
particle aspects of light in a double-slit type "welcher-weg" experiment beyond
the limitations set by Bohr's Principle of Complementarity. Applying classical
logic, we verify the presence of sharp interference in the single photon
regime, while reliably maintaining the information about the particular pinhole
through which each individual photon had passed. This experiment poses
interesting questions on the validity of Complementarity in cases where
measurements techniques that avoid Heisenberg's uncertainty principle and
quantum entanglement are employed. We further argue that the application of
classical concepts of waves and particles as embodied in Complementarity leads
to a logical inconsistency in the interpretation of this experiment.


title: Unbounded violation of tripartite Bell inequalities

abstract:
  We prove that there are tripartite quantum states (constructed from random
unitaries) that can lead to arbitrarily large violations of Bell inequalities
for dichotomic observables. As a consequence these states can withstand an
arbitrary amount of white noise before they admit a description within a local
hidden variable model. This is in sharp contrast with the bipartite case, where
all violations are bounded by Grothendieck's constant. We will discuss the
possibility of determining the Hilbert space dimension from the obtained
violation and comment on implications for communication complexity theory.
Moreover, we show that the violation obtained from generalized GHZ states is
always bounded so that, in contrast to many other contexts, GHZ states do in
this case not lead to extremal quantum correlations. The results are based on
tools from the theories of operator spaces and tensor norms which we exploit to
prove the existence of bounded but not completely bounded trilinear forms from
commutative C*-algebras.


title: Local encoding of classical information onto quantum states

abstract:
  In this article we investigate the possibility of encoding classical
information onto multipartite quantum states in the distant laboratory
framework. We show that for all states generated by Clifford operation there
always exist such an encoding, this includes all stabilizer states such as
cluster states and all graph states. We also show encoding for classes of
symmetric states (which cannot be generated by Clifford operations). We
generalise our approach using group theoretic methods introducing the unifying
notion of Pseudo Clifford operations. All states generated by Pseudo Clifford
operations are locally encodable (unifying all our examples), and we give a
general method for generating sets of many such locally encodable states.


title:
Quantum dynamics and breakdown of classical realism in nonlinear
  oscillators

abstract:
  The dynamics of a quantum nonlinear oscillator is studied in terms of its
quasi-flow, a dynamical mapping of the classical phase plane that represents
the time-evolution of the quantum observables. Explicit expressions are derived
for the deformation of the classical flow by the quantum nonlinearity in the
semiclassical limit. The breakdown of the classical trajectories under the
quantum nonlinear dynamics is quantified by the mismatch of the quasi-flow
carried by different observables. It is shown that the failure of classical
realism can give rise to a dynamical violation of Bell's inequalities.


title: Quantum Thought Experiments Can Define Nature

abstract:
  One would not think that thought experiments could matter to nature, for they
are a humble human device. Yet quantum mechanics very naturally frames thought
experiments (as distinct from precisely defining what exists). They exemplify
the informing powers of radiation. Though based on wave functions that have
time symmetry, these tableaux inevitably tell of irreversible behavior by
nature. The paper sketches how John von Neumann's measurement theory fits into
this and retells N. David Mermin's baseball story.


title:
Light scattering from ultracold atoms in optical lattices as an optical
  probe of quantum statistics

abstract:
  We study off-resonant collective light scattering from ultracold atoms
trapped in an optical lattice. Scattering from different atomic quantum states
creates different quantum states of the scattered light, which can be
distinguished by measurements of the spatial intensity distribution, quadrature
variances, photon statistics, or spectral measurements. In particular,
angle-resolved intensity measurements reflect global statistics of atoms (total
number of radiating atoms) as well as local statistical quantities (single-site
statistics even without an optical access to a single site) and pair
correlations between different sites. As a striking example we consider
scattering from transversally illuminated atoms into an optical cavity mode.
For the Mott insulator state, similar to classical diffraction, the number of
photons scattered into a cavity is zero due to destructive interference, while
for the superfluid state it is nonzero and proportional to the number of atoms.
Moreover, we demonstrate that light scattering into a standing-wave cavity has
a nontrivial angle dependence, including the appearance of narrow features at
angles, where classical diffraction predicts zero. The measurement procedure
corresponds to the quantum non-demolition (QND) measurement of various atomic
variables by observing light.


title:
Multiatom cooperative emission following single-photon absorption:
  Dicke-state dynamics

abstract:
  We investigate conditions under which multiatom absorption of a single photon
leads to cooperative decay. Our analysis reveals the symmetry properties of the
multiatom Dicke states underlying the cooperative decay dynamics and their
spatio-temporal manifestations, particularly, the forward-directed spontaneous
emission investigated by Scully et al.


title:
Scaling and better approximating quantum Fourier transform by higher
  radices

abstract:
  Quantum Fourier Transform (QFT) plays a principal role in the development of
efficient quantum algorithms. Since the number of quantum bits that can
currently built is limited, while many quantum technologies are inherently
three- (or more) valued, we consider extending the reach of the realistic
quantum systems by building a QFT over ternary quantum digits. Compared to
traditional binary QFT, the q-valued transform improves approximation
properties and increases the state space by a factor of (q/2)n. Further, we use
non-binary QFT derivation to generalize and improve the approximation bounds
for QFT.


title: Quantum search of partially ordered sets

abstract:
  We investigate the generalisation of quantum search of unstructured and
totally ordered sets to search of partially ordered sets (posets). Two models
for poset search are considered. In both models, we show that quantum
algorithms can achieve at most a quadratic improvement in query complexity over
classical algorithms, up to logarithmic factors; we also give quantum
algorithms that almost achieve this optimal reduction in complexity. In one
model, we give an improved quantum algorithm for searching forest-like posets;
in the other, we give an optimal O(sqrt(m))-query quantum algorithm for
searching posets derived from m*m arrays sorted by rows and columns. This leads
to a quantum algorithm that finds the intersection of two sorted lists of n
integers in O(sqrt(n)) time, which is optimal.


title: Particle Localization by Decoherence and Classical Lensing

abstract:
  A phenomenological model of the time evolution of a particle wavepacket is
presented that is subject to scattering event with small momentum transfer. It
is suited for three dimensions and allows for an additional potential. For a
random value of a phase parameter, it is equivalent to the decoherence by
scattering off plane wave field states. Imposing some condition on the same
phase parameter, the model delivers definite outcomes in a local pointer base.
Common sense notions about the quantum-classical transition are readily
obtained: Decoherent free particle solutions are localized constant-width
wavepackets, quantum barrier scattering results in random reflection and
transmission. The scattering process assumed in the phenomenological model is
realized by scattering of a particle pair interacting by a force-distance law
with small momentum transfer. Particles are localized not by decoherence but by
a classical lensing effect. Schmidt paths in the Schmidt representation of the
multiparticle state are products of single-particle wavepackets. Therefore the
model has a natural 'Many Worlds' interpretation, identifying each Schmidt path
with a classical branch. Different runs of the phenomenological model map the
state of the same particular particle within different branches.


title:
Influence of pure-dephasing by phonons on exciton-photon interfaces:
  Quantum microscopic theory

abstract:
  We have developed a full quantum microscopic theory to analyze the time
evolution of transversal and longitudinal components of an exciton-single
photon system coupled to bulk acoustic phonons. These components are subjected
to two decay processes. One is radiative relaxation and the other is
pure-dephasing due to exciton-phonon interaction. The former results in a decay
with an exponent linear to time, while the latter causes a faster initial decay
than the radiative decay. We analyzed the dependence of the components on the
duration of the input one-photon pulse, temperature, and radiative relaxation
rates. Such a quantitative analysis is important for the developments of
atom-photon interfaces which enable coherent transfer of quantum information
between photons and atomic systems. We found that, for a GaAs spherical quantum
dot in which the exciton interacts with bulk phonons, the maximal probability
of the excited state can be increased up to 75 %. This probability can be
considered as the efficiency for quantum information transfer from photon to
exciton.


title:
New Bell inequalities for the singlet state: Going beyond the
  Grothendieck bound

abstract:
  Contemporary versions of Bell's argument against local hidden variable (LHV)
theories are based on the Clauser Horne Shimony and Holt (CHSH) inequality, and
various attempts to generalize it. The amount of violation of these
inequalities cannot exceed the bound set by the Grothendieck constants.
However, if we go back to the original derivation by Bell, and use the perfect
anti-correlation embodied in the singlet spin state, we can go beyond these
bounds. In this paper we derive two-particle Bell inequalities for traceless
two-outcome observables, whose violation in the singlet spin state go beyond
the Grothendieck constants both for the two and three dimensional cases.
Moreover, creating a higher dimensional analog of perfect correlations, and
applying a recent result of Alon and his associates (Invent. Math. 163 499
(2006)) we prove that there are two-particle Bell inequalities for traceless
two-outcome observables whose violation increases to infinity as the dimension
and number of measurements grow. Technically these result are possible because
perfect correlations (or anti-correlations) allow us to transport the indices
of the inequality from the edges of a bipartite graph to those of the complete
graph. Finally, it is shown how to apply these results to mixed Werner states,
provided that the noise does not exceed 20%.


title: Spectral density matrix of a single photon measured

abstract:
  We propose and demonstrate a method for measuring the spectral density matrix
of a single photon pulse. The method is based on registering Hong-Ou-Mandel
interference between photon to be measured and a pair of attenuated and
suitably delayed laser pulses described by a known spectral amplitude. The
density matrix is retrieved from a two-dimensional interferogram of coincidence
counts. The method has been implemented for a type-I downconversion source,
pumped by ultrashort laser pulses. The experimental results agree well with a
theoretical model which takes into account the temporal as well as spatial
effects in the source.


title:
Universal fault tolerant quantum computation on bilinear nearest
  neighbor arrays

abstract:
  Assuming an array that consists of two parallel lines of qubits and that
permits only nearest neighbor interactions, we construct physical and logical
circuitry to enable universal fault tolerant quantum computation under the
[[7,1,3]] quantum code. A rigorous lower bound to the fault tolerant threshold
for this array is determined in a number of physical settings. Adversarial
memory errors, two-qubit gate errors, and readout errors are included in our
analysis. In the setting where the physical memory failure rate is equal to
one-tenth of the physical gate error rate, the physical readout error rate is
equal to the physical gate error rate, and the duration of physical readout is
ten times the duration of a physical gate, we obtain a lower bound to the
asymptotic threshold of 1.96x10^-6.


title: Quantum Cryptography: from Theory to Practice

abstract:
  Quantum cryptography can, in principle, provide unconditional security
guaranteed by the law of physics only. Here, we survey the theory and practice
of the subject and highlight some recent developments.


title: Purple bacteria and quantum Fourier transform

abstract:
  The LH-II of purple bacteria Rhodospirillum (Rs.) molischianum and
Rhodopseudomonas (Rps.) acidophila adopts a highly symmetrical ring shape, with
a radius of about 7 nm. In the case of Rps. acidophila the ring has a ninefold
symmetry axis, and in LH-II from Rs. molischianum the ring has an eightfold
symmetry axis. These rings are found to exibit two bands of excitons. A
simplified mathematical description of the exciton states is given in Hu, X. &
Schulten, K. (1997) Physics Today 50, 28-34. Using this description, we will
show, by suitable labeling of the lowest energy (Qy) excited states of
individual BChls, that the resulting exciton states are the quantum Fourier
transform of the BChls excited states. For Rs. molischianum ring exciton states
will be modeled as the four qubit quantum Fourier transform and the explicit
circuit will be derived. Exciton states for Rps. acidophila ring cannot be
modeled with an integer number of qubits. Both quantum Fourier transforms are
instances of the hidden subgroup problem and this opens up a possibility that
both purple bacteria implement an efficient quantum circuit for light
harvesting.


title:
Accidental cloning of a single-photon qubit in two-channel
  continuous-variable quantum teleportation

abstract:
  The information encoded in the polarization of a single photon can be
transferred to a remote location by two-channel continuous-variable quantum
teleportation. However, the finite entanglement used in the teleportation
causes random changes in photon number. If more than one photon appears in the
output, the continuous-variable teleportation accidentally produces clones of
the original input photon. In this paper, we derive the polarization statistics
of the $N$-photon output components and show that they can be decomposed into
an optimal cloning term and completely unpolarized noise. We find that the
accidental cloning of the input photon is nearly optimal at experimentally
feasible squeezing levels, indicating that the loss of polarization information
is partially compensated by the availability of clones.


title: Non-Hermitian Hamiltonians with Real Spectra in Quantum Mechanics

abstract:
  A condition to have a real spectrum for a non-Hermitian Hamiltonian is given.
As special cases, it is shown that the condition is reduced to Hermiticity and
PT symmetric conditions.


title: Quantum information transfer for qutrits

abstract:
  We propose a scheme for the transfer of quantum information among distant
qutrits. We apply this scheme to the distribution of entanglement among distant
nodes and to the generation of multipartite antisymmetric states. We also
discuss applications to quantum secret sharing.


title: Resonance Theory of Decoherence and Thermalization

abstract:
  We present a rigorous analysis of the phenomenon of decoherence for general
$N-$level systems coupled to reservoirs. The latter are described by free
massless bosonic fields. We apply our general results to the specific cases of
the qubit and the quantum register. We compare our results with the explicitly
solvable case of systems whose interaction with the environment does not allow
for energy exchange (non-demolition, or energy conserving interactions). We
suggest a new approach which applies to a wide variety of systems which are not
explicitly solvable.


title: A proposed superluminal S-field mediating quantum entanglement

abstract:
  We propose a bimetric space-time consisting of two vierbein bundles, in which
a superluminal S-field mediates a causal exchange of quantum information
associated with quantum entanglement. The resulting theory leaves the usual
quantum field interactions, as well as the speed of light unchanged, but
introduces quantum information effects related to the S-field vierbein bundle.
We show that such S-field interactions with the Dirac field, affect its spin
density, as we would expect for a field that would impart entanglement related
information.


title: Efficient growth of complex graph states via imperfect path erasure

abstract:
  Given a suitably large and well connected (complex) graph state, any quantum
algorithm can be implemented purely through local measurements on the
individual qubits. Measurements can also be used to create the graph state:
Path erasure techniques allow one to entangle multiple qubits by determining
only global properties of the qubits. Here, this powerful approach is extended
by demonstrating that even imperfect path erasure can produce the required
graph states with high efficiency. By characterizing the degree of error in
each path erasure attempt, one can subsume the resulting imperfect entanglement
into an extended graph state formalism. The subsequent growth of the improper
graph state can be guided, through a series of strategic decisions, in such a
way as to bound the growth of the error and eventually yield a high-fidelity
graph state. As an implementation of these techniques, we develop an analytic
model for atom (or atom-like) qubits in mismatched cavities, under the
double-heralding entanglement procedure of Barrett and Kok [Phys. Rev. A 71,
060310 (2005)]. Compared to straightforward postselection techniques our
protocol offers a dramatic improvement in growing complex high-fidelity graph
states.


title: Why Kastner analysis does not apply to a modified Afshar experiment

abstract:
  In an analysis of the Afshar experiment R.E. Kastner points out that the
selection system used in this experiment randomly separates the photons that go
to the detectors, and therefore no which-way information is obtained. In this
paper we present a modified but equivalent version of the Afshar experiment
that does not contain a selection device. The double-slit is replaced by two
separate coherent laser beams that overlap under a small angle. At the
intersection of the beams an interference pattern can be inferred in a
non-perturbative manner, which confirms the existence of a superposition state.
In the far field the beams separate without the use of a lens system. Momentum
conservation warranties that which-way information is preserved. We also
propose an alternative sequence of Stern-Gerlach devices that represents a
close analogue to the Afshar experimental set up.


title: Parameter estimation for mixed states from a single copy

abstract:
  Given a single copy of a mixed state of the form
\rho=\lambda\rho_1+(1-\lambda)\rho_2, what is the optimal measurement to
estimate the parameter \lambda, if \rho_1 and \rho_2 are known? We present a
general strategy to obtain the optimal measurements employing a Bayesian
estimator. The measurements are chosen to minimize the deviation between the
estimated- and the true value of \lambda. We explicitly determine the optimal
measurements for a general two-dimensional system and for important higher
dimensional cases.


title:
Generalized Flow and Determinism in Measurement-based Quantum
  Computation

abstract:
  We extend the notion of quantum information flow defined by Danos and Kashefi
for the one-way model and present a necessary and sufficient condition for the
deterministic computation in this model. The generalized flow also applied in
the extended model with measurements in the X-Y, X-Z and Y-Z planes. We apply
both measurement calculus and the stabiliser formalism to derive our main
theorem which for the first time gives a full characterization of the
deterministic computation in the one-way model. We present several examples to
show how our result improves over the traditional notion of flow, such as
geometries (entanglement graph with input and output) with no flow but having
generalized flow and we discuss how they lead to an optimal implementation of
the unitaries. More importantly one can also obtain a better quantum
computation depth with the generalized flow rather than with flow. We believe
our characterization result is particularly essential for the study of the
algorithms and complexity in the one-way model.


title:
WKB formalism and a lower limit for the energy eigenstates of bound
  states for some potentials

abstract:
  In the present work the conditions appearing in the WKB approximation
formalism of quantum mechanics are analyzed. It is shown that, in general, a
careful definition of an approximation method requires the introduction of two
length parameters, one of them always considered in the text books on quantum
mechanics, whereas the second one is usually neglected. Afterwards we define a
particular family of potentials and prove, resorting to the aforementioned
length parameters, that we may find an energy which is a lower bound to the
ground energy of the system. The idea is applied to the case of a harmonic
oscillator and also to a particle freely falling in a homogeneous gravitational
field, and in both cases the consistency of our method is corroborated. This
approach, together with the Rayleigh--Ritz formalism, allows us to define an
energy interval in which the ground energy of any potential, belonging to our
family, must lie.


title:
Quantum annealing of the random-field Ising model by transverse
  ferromagnetic interactions

abstract:
  We introduce transverse ferromagnetic interactions, in addition to a simple
transverse field, to quantum annealing of the random-field Ising model to
accelerate convergence toward the target ground state. The conventional
approach using only the transverse-field term is known to be plagued by slow
convergence when the true ground state has strong ferromagnetic characteristics
for the random-field Ising model. The transverse ferromagnetic interactions are
shown to improve the performance significantly in such cases. This conclusion
is drawn from the analyses of the energy eigenvalues of instantaneous
stationary states as well as by the very fast algorithm of Bethe-type
mean-field annealing adopted to quantum systems. The present study highlights
the importance of a flexible choice of the type of quantum fluctuations to
achieve the best possible performance in quantum annealing. The existence of
such flexibility is an outstanding advantage of quantum annealing over
simulated annealing.


title:
Experimental Demonstration of Macroscopic Quantum Coherence in Gaussian
  States

abstract:
  We witness experimentally the presence of macroscopic coherence in Gaussian
quantum states using a recently proposed criterion (E.G. Cavalcanti and M.
Reid, Phys. Rev. Lett. 97, 170405 (2006)). The macroscopic coherence stems from
interference between macroscopically distinct states in phase space and we
prove experimentally that even the vacuum state contains these features with a
distance in phase space of $0.51\pm0.02$ shot noise units (SNU). For squeezed
states we found macroscopic superpositions with a distance of up to
$0.83\pm0.02$ SNU. The proof of macroscopic quantum coherence was investigated
with respect to squeezing and purity of the states.


title: Low Timing Jitter Detector for Gigahertz Quantum Key Distribution

abstract:
  A superconducting single-photon detector based on a niobium nitride nanowire
is demonstrated in an optical-fibre-based quantum key distribution test bed
operating at a clock rate of 3.3 GHz and a transmission wavelength of 850 nm.
The low jitter of the detector leads to significant reduction in the estimated
quantum bit error rate and a resultant improvement in the secrecy efficiency
compared to previous estimates made by use of silicon single-photon avalanche
detectors.


title: Entanglement-screening by nonlinear resonances

abstract:
  We show that nonlinear resonances in a classically mixed phase space allow to
define generic, strongly entangled multi-partite quantum states.
  The robustness of their multipartite entanglement increases with the particle
number, i.e. in the semiclassical limit, for those classes of diffusive noise
which assist the quantum-classical transition.


title:
Electron bound by a potential well in the presence of a constant uniform
  magnetic field

abstract:
  We study the effect of a constant uniform magnetic field on an electrically
charged massive particle (an electron) bound by a potential well, which is
described by means of a single attractive $\lambda\delta({\bf r})$ potential. A
transcendental equation that determines the electron energy spectrum is derived
and solved. The electron wave function in the ground (bound) state is
approximately constructed in a remarkable simple form. It is shown that there
arises the probability current in the bound state in the presence of a uniform
constant magnetic field. This (electric) current, being by the gauge invariant
quantity, must be observable and involve (and exercise influence on) the
electron scattering. The probability current density resembles a stack of
"pancake" vortices'' whose circulating "currents'' around the magnetic field
direction ($z$-axes) are mostly confined within the plane $z=0$. We also
compute the tunnelling probability of electron from the bound to free state
under a weak constant homogeneous electric field, which is parallel to the
magnetic field. The model under consideration is briefly discussed in two
spatial dimensions.


title:
Optimal spin squeezing inequalities detect bound entanglement in spin
  models

abstract:
  We determine the complete set of generalized spin squeezing inequalities.
These are entanglement criteria that can be used for the experimental detection
of entanglement in a system of spin-1/2 particles in which the spins cannot be
individually addressed. They can also be used to show the presence of bound
entanglement in the thermal states of several spin models.


title:
Generation of higher order nonclassical states via interaction of
  intense electromagnetic field with third order nonlinear medium

abstract:
  Interaction of intense laser beam with an inversion symmetric third order
nonlinear medium is modeled as a quartic anharmonic oscillator. A first order
operator solution of the model Hamiltonian is used to study the possibilities
of generation of higher order nonclassical states. It is found that the higher
order squeezed and higher order antibunched states can be produced by this
interaction. It is also shown that the higher order nonclassical states may
appear separately, i.e. a higher order antibunched state is not essentially
higher order squeezed state and vice versa.


title: Success Rate and Entanglement Evolution in Search Algorithm

abstract:
  Evolution of entanglement with the processing of quantum algorithms affects
the outcome of the algorithm. Particularly, the performance of Grover's search
algorithm gets worsened if the initial state of the algorithm is an entangled
one. Biham et al [14] have shown that the success probability of search
algorithm can be seen as an operational measure of entanglement. Following the
same line of thought, analytical expressions for entanglement measure for three
and five qubit system have been derived, which reveals that entanglement
measure based on Grover's search algorithm is valid for any state with real
coefficients for even number of qubits.


title: Evolution of Entanglement in Groverian Search Algorithm: n-qudit System

abstract:
  Entanglement plays a crucial role in quantum processes particularly those
pertaining to quantum information and computation. An analytical expression for
an entanglement measure defined in terms of success rate of Grover's search
algorithm has been obtained for qutrit systems with real coefficients and the
calculated results agree well with the conventional entropy based measure. The
entanglement measure is further generalised for qudit (d-level) systems with
real coefficients.


title: Entanglement Teleportation by Qutrits

abstract:
  Quantum entanglement, like othre resources, is now coonsidered to be a
resource which can be produced, concentrated if required, transported and
consumed. After its inception [1] in 1933, various schemes of quantum state
teleportation have been proposed using different types of channels. Not
restricting to qubit based systems, qutrit states and channels have also been
of considerable interest. In the present paper we investigate the teleportation
of an unknown single qutrit state as well as two qutrit state through a three
qutrit quantum channel along with the required operations to recover the state.
This is further generalized to the case of teleportation of n-qutrit system.


title: A causal look into the quantum Talbot effect

abstract:
  A well-known phenomenon in both optics and quantum mechanics is the so-called
Talbot effect. This near field interference effect arises when infinitely
periodic diffracting structures or gratings are illuminated by highly coherent
light or particle beams. Typical diffraction patterns known as quantum carpets
are then observed. Here the authors provide an insightful picture of this
nonlocal phenomenon as well as its classical limit in terms of Bohmian
mechanics, also showing the causal reasons and conditions that explain its
appearance. As an illustration, theoretical results obtained from diffraction
of thermal He atoms by both N-slit arrays and weak corrugated surfaces are
analyzed and discussed. Moreover, the authors also explain in terms of what
they call the Talbot-Beeby effect how realistic interaction potentials induce
shifts and distortions in the corresponding quantum carpets.


title: Quantum entanglement

abstract:
  All our former experience with application of quantum theory seems to say:
{\it what is predicted by quantum formalism must occur in laboratory}. But the
essence of quantum formalism - entanglement, recognized by Einstein, Podolsky,
Rosen and Schr\"odinger - waited over 70 years to enter to laboratories as a
new resource as real as energy.
  This holistic property of compound quantum systems, which involves
nonclassical correlations between subsystems, is a potential for many quantum
processes, including ``canonical'' ones: quantum cryptography, quantum
teleportation and dense coding. However, it appeared that this new resource is
very complex and difficult to detect. Being usually fragile to environment, it
is robust against conceptual and mathematical tools, the task of which is to
decipher its rich structure.
  This article reviews basic aspects of entanglement including its
characterization, detection, distillation and quantifying. In particular, the
authors discuss various manifestations of entanglement via Bell inequalities,
entropic inequalities, entanglement witnesses, quantum cryptography and point
out some interrelations. They also discuss a basic role of entanglement in
quantum communication within distant labs paradigm and stress some
peculiarities such as irreversibility of entanglement manipulations including
its extremal form - bound entanglement phenomenon. A basic role of entanglement
witnesses in detection of entanglement is emphasized.


title: Study of Phonon Modes in Germanium Nanowires

abstract:
  The observation of pure phonon confinement effect in germanium nanowires is
limited due to the illumination sensitivity of Raman spectra. In this paper we
measured Raman spectra for different size germanium nanowires with different
excitation laser powers and wavelengths. By eliminating the local heating
effect, the phonon confinement effect for small size nanowires was clearly
identified. We have also fitted the Raman feature changes to estimate the size
distribution of nanowires for the first time.


title: Adiabatically coupled systems and fractional monodromy

abstract:
  We present a 1-parameter family of systems with fractional monodromy and
adiabatic separation of motion. We relate the presence of monodromy to a
redistribution of states both in the quantum and semi-quantum spectrum. We show
how the fractional monodromy arises from the non diagonal action of the
dynamical symmetry of the system and manifests itself as a generic property of
an important subclass of adiabatically coupled systems.


title: Efficient multiqubit entanglement via a spin-bus

abstract:
  We propose an experimentally feasible architecture with controllable
long-range couplings built up from local exchange interactions. The scheme
consists of a spin-bus, with strong, always-on interactions, coupled
dynamically to external qubits of the Loss and DiVincenzo type. Long-range
correlations are enabled by a spectral gap occurring in a finite-size chain.
The bus can also form a hub for multiqubit entangling operations. We show how
multiqubit gates may be used to efficiently generate $W$-states (an important
entanglement resource). The spin-bus therefore provides a route for scalable
solid-state quantum computation, using currently available experimental
resources.


title: An extremal result for geometries in the one-way measurement model

abstract:
  We present an extremal result for the class of graphs G which (together with
some specified sets of input and output vertices, I and O) have a certain
"flow" property introduced by Danos and Kashefi for the one-way measurement
model of quantum computation. The existence of a flow for a triple (G,I,O)
allows a unitary embedding to be derived from any choice of measurement bases
allowed in the one-way measurement model. We prove an upper bound on the number
of edges that a graph G may have, in order for a triple (G,I,O) to have a flow
for some $I, O \subseteq V(G)$, in terms of the number of vertices in G and O.
This implies that finding a flow for a triple (G,I,O) when |I| = |O| = k
(corresponding to unitary transformations in the measurement model) and |V(G)|
= n can be performed in time O(k^2 n), improving the earlier known bound of
O(km) given in [quant-ph/0611284], where m = |E(G)|.


title: Quantum Computation and Bell-state Measurement with Double-Dot Molecules

abstract:
  We propose a quantum computation architecture of double-dot molecules, where
the qubit is encoded in the molecule two-electron spin states. By arranging the
two dots inside each molecule perpendicular to the qubit scaling line, the
interactions between neighboring qubits are largely simplified and the scaling
to multi-qubit system becomes straightforward. As an Ising-model effective
interaction can be expediently switched on and off between any two neighboring
molecules by adjusting the potential offset between the two dots, universal
two-qubit gates can be implemented without requiring time-dependent control of
the tunnel coupling between the dots. A Bell-state measurement scheme for qubit
encoded in double-dot singlet and triplet states is also proposed for quantum
molecules arranged in this way.


title: Pre- and post-selected ensembles and time-symmetry in quantum mechanics

abstract:
  An expression is proposed for the quantum mechanical state of a pre- and
post-selected ensemble, which is an ensemble determined by the final as well as
the initial state of the quantum systems involved. It is shown that the
probabilities calculated from the proposed state agree with previous
expressions, for cases where they both apply. The same probabilities are found
when they are calculated in the forward- or reverse-time directions. This work
was prompted by several problems raised by Shimony recently in relation to the
state, and time symmetry, of pre- and post-selected ensembles.


title: STIRAP-like transitions in a harmonically-modulated optical lattice

abstract:
  We introduce a method for the coherent acceleration of atoms trapped in an
optical lattice, using the well-known model for stimulated Raman adiabatic
passage (STIRAP). Specifically, we show that small harmonic modulations of the
optical lattice amplitude, with frequencies tuned to the eigenvalue spacings of
three "unperturbed" eigenstates, reveals a three-state STIRAP subsystem. We use
this model to realize an experimentally achievable method for transferring
trapped atoms from stationary to motional eigenstates.


title: Quantum Fermi's Golden Rule

abstract:
  We shall revisit the conventional adiabatic or Markov approximation, showing
its intrinsic failure in describing the proper quantum-mechanical evolution of
a generic subsystem interacting with its environment. In particular, we shall
show that -contrary to the semiclassical case- the Markov limit does not
preserve the positive-definite character of the corresponding density matrix,
thus leading to highly non-physical results. To overcome this problem, we shall
propose an alternative adiabatic procedure which (i) in the semiclassical limit
reduces to the standard Fermi's golden rule, and (ii) describes a genuine
Limblad evolution, thus providing a reliable/robust treatment of
energy-dissipation and dephasing processes.


title:
Tunable negative refraction without absorption via electromagnetically
  induced chirality

abstract:
  We show that negative refraction with minimal absorption can be obtained by
means of quantum interference effects similar to electromagnetically induced
transparency. Coupling a magnetic dipole transition coherently with an electric
dipole transition leads to electromagnetically induced chirality, which can
provide negative refraction without requiring negative permeability, and also
suppresses absorption. This technique allows negative refraction in the optical
regime at densities where the magnetic susceptibility is still small and with
refraction/absorption ratios that are orders of magnitude larger than those
achievable previously. Furthermore, the value of the refractive index can be
fine-tuned via external laser fields, which is essential for practical
realization of sub-diffraction-limit imaging.


title:
An alternative simple solution of the sextic anharmonic oscillator and
  perturbed Coulomb problems

abstract:
  Utilizing an appropriate ansatz to the wave function, we reproduce the exact
bound-state solutions of the radial Schrodinger equation to various exactly
solvable sextic anharmonic oscillator and confining perturbed Coulomb models in
$D$-dimensions. We show that the perturbed Coulomb problem with eigenvalue $E$
can be transformed to a sextic anharmonic oscillator problem with eigenvalue E.
We also check the explicit relevance of these two related problems in
higher-space dimensions. It is shown that exact solutions of these potentials
exist when their coupling parameters with $k=D+2\ell $ appearing in the wave
equation satisfy certain constraints.


title: The Maslov correction in the semiclassical Feynman integral

abstract:
  The Maslov correction to the wave function is to the jump of $-\pi/2$ in the
phase when the system passes through a caustic point. This phenomenon is
related to the second variation and to the geometry of paths, as conveniently
explained in Feynman's path integral framework. The results can be extended to
any system using the semiclassical approximation. The 1-dimensional harmonic
oscillator is used to illustrate the different derivations reviewed here.


title:
Dynamical diffusion as the approximation of one quantum particle
  dynamics

abstract:
  The paper contains the proof that the diffusion ensemble of point wise
particles with the intensity depending on the grain of spatial resolution
serves as the satisfactory approximation of one quantum particle dynamics.


title:
The Effect of Partial Coherence on the Statistics of Single-Photon
  Pulses Propagating in the Atmosphere

abstract:
  The photon density operator function is used to describe the propagation of
single-photon pulses through a turbulent atmosphere. The effects of statistical
properties of photon source and the effects of a random phase screen on the
variance of photon counting are studied. A procedure for reducing the total
noise is discussed. The physical mechanisms responsible for this reduction are
explained.


title: Deriving a Fault-Tolerant Threshold for a Global Control Scheme

abstract:
  In this paper, adapted from the author's PhD thesis, we present otherwise
unpublished results relating to global control schemes, culminating in the
calculation of a fault-tolerant threshold for one such scheme. As with early
fault-tolerant threshold results, the aim is to calculate a positive number,
not to optimise it. We also discuss how the results might affect other related
schemes, such as those based on cellular automata.


title: Optimal two-qubit gate for generation of random bipartite entanglement

abstract:
  We numerically study protocols consisting of repeated applications of two
qubit gates used for generating random pure states. A necessary number of steps
needed in order to generate states displaying bipartite entanglement typical of
random states is obtained. For generic two qubit entangling gate the decay rate
of purity is found to scale as $\sim n$ and therefore of order $\sim n^2$ steps
are necessary to reach random bipartite entanglement. We also numerically
identify the optimal two qubit gate for which the convergence is the fastest.
Perhaps surprisingly, applying the same good two qubit gate in addition to a
random single qubit rotations at each step leads to a faster generation of
entanglement than applying a random two qubit transformation at each step.


title: Limitations of some simple adiabatic quantum algorithms

abstract:
  Let $H(t)=(1-t/T)H_0 + (t/T)H_1$, $t\in [0,T]$, be the Hamiltonian governing
an adiabatic quantum algorithm, where $H_0$ is diagonal in the Hadamard basis
and $H_1$ is diagonal in the computational basis. We prove that $H_0$ and $H_1$
must each have at least two large mutually-orthogonal eigenspaces if the
algorithm's running time is to be subexponential in the number of qubits. We
also reproduce the optimality proof of Farhi and Gutmann's search algorithm in
the context of this adiabatic scheme; because we only consider initial
Hamiltonians that are diagonal in the Hadamard basis, our result is slightly
stronger than the original.


title: A critical analysis of Popper's experiment

abstract:
  An experiment which could decide against the Copenhagen interpretation of
quantum mechanics has been proposed by K. Popper and, subsequently, it has been
criticized by M.J. Collett and R. Loudon. Here we show that both the above
mentioned arguments are not correct because they are based on a misuse of basic
quantum rules.


title:
Criteria for dynamically stable decoherence-free subspaces and
  incoherently generated coherences

abstract:
  We present a detailed analysis of decoherence free subspaces and develop a
rigorous theory that provides necessary and sufficient conditions for
dynamically stable decoherence free subspaces. This allows us to identify a
special class of decoherence free states which rely on incoherent generation of
coherences. We provide examples of physical systems that support such states.
Our approach employs Markovian master equations and applies primarily to
finite-dimensional quantum systems.


title: Multi-particle decoherence free subspaces in extended systems

abstract:
  We develop a method to determine spatial configurations to realize
decoherence-free subspaces for spatially extended multi-particle systems. We
have assumed normal reservoir behavior including translational invariance of
the reservoir and preparation in stationary states or mixture thereof and weak
Markovian system-reservoir coupling that requires energy transfer. One
important outcome of our method is a proof that there does not exist a
multi-particle decoherence-free subspace in such systems except in the limit
that the spatial extent of the system becomes infinitesimal.


title: Unambiguous discrimination among oracle operators

abstract:
  We address the problem of unambiguous discrimination among oracle operators.
The general theory of unambiguous discrimination among unitary operators is
extended with this application in mind. We prove that entanglement with an
ancilla cannot assist any discrimination strategy for commuting unitary
operators. We also obtain a simple, practical test for the unambiguous
distinguishability of an arbitrary set of unitary operators on a given system.
Using this result, we prove that the unambiguous distinguishability criterion
is the same for both standard and minimal oracle operators. We then show that,
except in certain trivial cases, unambiguous discrimination among all standard
oracle operators corresponding to integer functions with fixed domain and range
is impossible. However, we find that it is possible to unambiguously
discriminate among the Grover oracle operators corresponding to an arbitrarily
large unsorted database. The unambiguous distinguishability of standard oracle
operators corresponding to totally indistinguishable functions, which possess a
strong form of classical indistinguishability, is analysed. We prove that these
operators are not unambiguously distinguishable for any finite set of totally
indistinguishable functions on a Boolean domain and with arbitrary fixed range.
Sets of such functions on a larger domain can have unambiguously
distinguishable standard oracle operators and we provide a complete analysis of
the simplest case, that of four functions. We also examine the possibility of
unambiguous oracle operator discrimination with multiple parallel calls and
investigate an intriguing unitary superoperator transformation between standard
and entanglement-assisted minimal oracle operators.


title: Abrupt Changes in the Dynamics of Quantum Disentanglement

abstract:
  Entanglement evolution in high dimensional bipartite systems under
dissipation is studied. Discontinuities for the time derivative of the lower
bound of entanglement of formation is found depending on the initial conditions
for entangled states. This abrupt changes along the evolution appears as
precursors of entanglement sudden death.


title:
Teleportation capability, distillability, and nonlocality on three-qubit
  states

abstract:
  In this paper, we consider teleportation capability, distillability, and
nonlocality on three-qubit states. In order to investigate some relations among
them, we first find the explicit formulas of the quantities about the maximal
teleportation fidelity on three-qubit states. We show that if any three-qubit
state is useful for three-qubit teleportation then the three-qubit state is
distillable into a Greenberger-Horne-Zeilinger state, and that if any
three-qubit state violates a specific form of Mermin inequality then the
three-qubit state is useful for three-qubit teleportation.


title: Reversible state transfer between light and a single trapped atom

abstract:
  We demonstrate the reversible mapping of a coherent state of light with mean
photon number n-bar ~= 1.1 to and from the hyperfine states of an atom trapped
within the mode of a high finesse optical cavity. The coherence of the basic
processes is verified by mapping the atomic state back onto a field state in a
way that depends on the phase of the original coherent state. Our experiment
represents an important step towards the realization of cavity QED-based
quantum networks, wherein coherent transfer of quantum states enables the
distribution of quantum information across the network.


title: Time-dependent density-functional theory for open systems

abstract:
  By introducing the self-energy density functionals for the dissipative
interactions between the reduced system and its environment, we develop a
time-dependent density-functional theory formalism based on an equation of
motion for the Kohn-Sham reduced single-electron density matrix of the reduced
system. Two approximate schemes are proposed for the self-energy density
functionals, the complete second order approximation and the wide-band limit
approximation. A numerical method based on the wide-band limit approximation is
subsequently developed and implemented to simulate the steady and transient
current through various realistic molecular devices. Simulation results are
presented and discussed.


title:
Upper bounds of eavesdropper's performances in finite-length code with
  decoy method

abstract:
  Security formulas of quantum key distribution (QKD) with imperfect resources
are obtained for finite-length code when the decoy method is applied. This
analysis is useful for guaranteeing the security of implemented QKD systems.
Our formulas take into account the effect of the vacuum state and dark counts
in the detector. We compare the asymptotic key generation rate in presence of
dark counts with that without.


title:
General theory for decoy-state quantum key distribution with arbitrary
  number of intensities

abstract:
  We develop a general theory for quantum key distribution (QKD) in both the
forward error correction and the reverse error correction cases when the QKD
system is equipped with phase-randomized coherent light with arbitrary number
of decoy intensities. For this purpose, generalizing Wang's expansion, we
derive a convex expansion of the phase-randomized coherent state. We also
numerically check that the asymptotic key generation rates are almost saturated
when the number of decoy intensities is three.


title: Convergence of Quantum Annealing with Real-Time Schrodinger Dynamics

abstract:
  Convergence conditions for quantum annealing are derived for optimization
problems represented by the Ising model of a general form. Quantum fluctuations
are introduced as a transverse field and/or transverse ferromagnetic
interactions, and the time evolution follows the real-time Schrodinger
equation. It is shown that the system stays arbitrarily close to the
instantaneous ground state, finally reaching the target optimal state, if the
strength of quantum fluctuations decreases sufficiently slowly, in particular
inversely proportionally to the power of time in the asymptotic region. This is
the same condition as the other implementations of quantum annealing, quantum
Monte Carlo and Green's function Monte Carlo simulations, in spite of the
essential difference in the type of dynamics. The method of analysis is an
application of the adiabatic theorem in conjunction with an estimate of a lower
bound of the energy gap based on the recently proposed idea of Somma et. al.
for the analysis of classical simulated annealing using a classical-quantum
correspondence.


title: Two-spin entanglement distribution near factorized states

abstract:
  We study the two-spin entanglement distribution along the infinite $S=1/2$
chain described by the XY model in a transverse field; closed analytical
expressions are derived for the one-tangle and the concurrences $C_r$, $r$
being the distance between the two possibly entangled spins, for values of the
Hamiltonian parameters close to those corresponding to factorized ground
states. The total amount of entanglement, the fraction of such entanglement
which is stored in pairwise entanglement, and the way such fraction distributes
along the chain is discussed, with attention focused on the dependence on the
anisotropy of the exchange interaction. Near factorization a characteristic
length-scale naturally emerges in the system, which is specifically related
with entanglement properties and diverges at the critical point of the fully
isotropic model. In general, we find that anisotropy rule a complex behavior of
the entanglement properties, which results in the fact that more isotropic
models, despite being characterized by a larger amount of total entanglement,
present a smaller fraction of pairwise entanglement: the latter, in turn, is
more evenly distributed along the chain, to the extent that, in the fully
isotropic model at the critical field, the concurrences do not depend on $r$.


title:
Measurement of Dicke Narrowing in Electromagnetically Induced
  Transparency

abstract:
  Dicke narrowing is a phenomena that dramatically reduces the Doppler width of
spectral lines, due to frequent velocity-changing collisions. A similar
phenomena occurs for electromagnetically induced transparency (EIT) resonances,
and facilitates ultra-narrow spectral features in room-temperature vapor. We
directly measure the Dicke-like narrowing by studying EIT line-shapes as a
function of the angle between the pump and the probe beams. The measurements
are in good agreement with an analytic theory with no fit parameters. The
results show that Dicke narrowing can increase substantially the tolerance of
hot-vapor EIT to angular deviations. We demonstrate the importance of this
effect for applications such as imaging and spatial solitons using a
single-shot imaging experiment, and discuss the implications on the feasibility
of storing images in atomic vapor.


title:
The Jamio{\l}kowski isomorphism and a conceptionally simple proof for
  the correspondence between vectors having Schmidt number $k$ and $k$-positive
  maps

abstract:
  Positive maps which are not completely positive are used in quantum
information theory as witnesses for convex sets of states, in particular as
entanglement witnesses and more generally as witnesses for states having
Schmidt number not greater than k. It is known that such witnesses are related
to k-positive maps. In this article we propose a new proof for the
correspondence between vectors having Schmidt number k and k-positive maps
using Jamiolkowski's criterion for positivity of linear maps; to this aim, we
also investigate the precise notion of the term "Jamiolkowski isomorphism". As
consequences of our proof we get the Jamiolkowski criterion for complete
positivity, and we find a special case of a result by Choi, namely that
k-positivity implies complete positivity, if k is the dimension of the smaller
one of the Hilbert spaces on which the operators act.


title: Spontaneous emission of graviton by a quantum bouncer

abstract:
  Spontaneous emission of graviton rates for the quantum bouncer states are
evaluated.


title: Classification and monogamy of three-qubit biseparable Bell correlations

abstract:
  We strengthen the set of Bell-type inequalities presented by Sun & Fei [Phys.
Rev. A 74, 032335 (2006)] that give a classification for biseparable
correlations and entanglement in tripartite quantum systems. We will
furthermore consider the restriction to local orthogonal spin observables and
show that this strengthens all previously known such tripartite inequalities.
The quadratic inequalities we find indicate a type of monogamy of maximal
biseparable three-particle quantum correlations, although the nonmaximal ones
can be shared. This is contrasted to recently found monogamy inequalities for
bipartite Bell correlations in tripartite systems.


title:
Entanglement of macroscopic test masses and the Standard Quantum Limit
  in laser interferometry

abstract:
  We show that the generation of entanglement of two heavily macroscopic
mirrors with masses of up to several kilograms are feasible with state of the
art techniques of high-precision laser interferometry. The basis of such a
demonstration would be a Michelson interferometer with suspended mirrors and
simultaneous homodyne detections at both interferometer output ports. We
present the connection between the generation of entanglement and the Standard
Quantum Limit (SQL) for a free mass. The SQL is a well-known reference limit in
operating interferometers for gravitational-wave detection and provides a
measure of when macroscopic entanglement can be observed in the presence of
realistic decoherence processes.


title: Microcavities Using Holey Fibers

abstract:
  Vacuum compatible microcavities consisting of microstructured holey fibers
and separate end mirrors have been constructed and tested. These devices
exhibit excellent transverse mode confinement and the ability to control the
percentage of power guided outside of the fiber core. As a result, these
devices may be a useful tool for enhancing the interaction between light and an
atomic medium.


title: Nodes of Wavefunctions

abstract:
  We give a simple argument to show that the $n$th wavefunction for the
one-dimensional Schr\"odinger equation has $n-1$ nodes. We also show that if
$n_1 < n_2$, then between two consecutive zeros of $\psi_{n_1}$, there is a
zero of $\psi_{n_2}$.


title:
On the Bandgap quantum coupler and the harmonic oscillator interacting
  with a reservoir: Defining the relative phase gate

abstract:
  In order to be able to study dissipation, the interaction between a single
system and their environment was introduced in quantum mechanics. Master and
quantum Langeving equations was derived and, also, decoherence was studied
using this approach. One of the most used model in this field is a single
harmonic oscillator interacting with a reservoir. In this work we solve
analytically this problem in the resonance case with the evolution operator
method. We use this result to study the conditional dynamics of a finite system
of coupling, a bandgap quantum coupler. We study the conditional dynamics of
the coupler on the computational basis by choosing a proper interaction time.
This conditional dynamics provides a distinct realization of a quantum phase
gate, which we name the relative phase gate.


title:
Faked states attack using detector efficiency mismatch on SARG04,
  phase-time, DPSK, and Ekert protocols

abstract:
  In quantum cryptosystems, variations in detector efficiency can be exploited
to stage a successful attack. This happens when the efficiencies of Bob's two
detectors are different functions of a control parameter accessible to Eve
(e.g., timing of the incoming pulses). It has previously been shown that the
Bennett-Brassard 1984 (BB84) protocol is vulnerable to this attack. In this
paper, we show that several other protocols and encodings may also be
vulnerable. We consider a faked states attack in the case of a partial
efficiency mismatch on the Scarani-Acin-Ribordy-Gisin 2004 (SARG04) protocol,
and derive the quantum bit error rate as a function of detector efficiencies.
Additionally, it is shown how faked states can in principle be constructed for
quantum cryptosystems that use a phase-time encoding, the differential phase
shift keying (DPSK) and the Ekert protocols.


title: Observables and unobservables in a non-associative quantum theory

abstract:
  It is shown that the non-associative operators in a non-associative quantum
theory are unobservables. The observable quantity may be presented only by the
elements of some associative subalgebra. It is shown that the elements of the
associative subalgebra are extended objects that can be similar to strings. It
is assumed that the non-associative quantum field theory can be applied to the
quantization of strongly interacting fields. The method for obtaining field
equations in a non-associative case is given.


title: Entanglement of thermal scalar fields in a compact space space

abstract:
  Using the thermal Green's function approach we propose a general method to
investigate entanglement of the vacuum state or thermal ground states in an
arbitrary dimensional space-time. As an application we show quantum
separability of the massive thermal scalar field vacuum in the 1+1 dimensional
cylindrical space-time. Separability is demonstrated using the positive partial
transpose criterion for effective two-mode Gaussian states of collective
operators. In this case, for all mass and temperature values entanglement is
absent between the collective operators.


title: Controlled Quantum State Transfer in a Spin Chain

abstract:
  Control of the transfer of quantum information encoded in quantum wavepackets
moving along a spin chain is demonstrated. Specifically, based on a
relationship with control in a paradigm of quantum chaos, it is shown that
wavepackets with slow dispersion can automatically emerge from a class of
initial superposition states involving only a few spins, and that arbitrary
unspecified travelling wavepackets can be nondestructively stopped and later
relaunched with perfection. The results establish an interesting application of
quantum chaos studies in quantum information science.


title: Optimal control of light pulse storage and retrieval

abstract:
  We demonstrate experimentally a procedure to obtain the maximum efficiency
for the storage and retrieval of light pulses in atomic media. The procedure
uses time reversal to obtain optimal input signal pulse-shapes. Experimental
results in warm Rb vapor are in good agreement with theoretical predictions and
demonstrate a substantial improvement of efficiency. This optimization
procedure is applicable to a wide range of systems.


title:
Measurement of EPR-type flavour entanglement in Upsilon(4S)->B0 B0bar
  decays

abstract:
  The neutral B-meson pair produced at the Upsilon(4S) should exhibit a
non-local correlation of the type discussed by Einstein, Podolski, and Rosen
(EPR). We measure this correlation using the time-dependent flavour asymmetry
of semileptonic B0 decays, which we compare with predictions from quantum
mechanics and two local realistic models. The data are consistent with quantum
mechanics, and inconsistent with the other models. Assuming that some B pairs
disentangle to produce B0 and B0bar with definite flavour, we find a decoherent
fraction of 0.029+-057, consistent with no decoherence.


title:
Quantum Critical Dynamics of A Qubit Coupled to An Isotropic
  Lipkin-Meshkov-Glick Bath

abstract:
  We explore a dynamic signature of quantum phase transition (QPT) in an
isotropic Lipkin-Meshkov-Glick (LMG) model by studying the time evolution of a
central qubit coupled to it. We evaluate exactly the time-dependent purity,
which can be used to measure quantum coherence, of the central qubit. It is
found that distinctly different behaviors of the purity as a function of the
parameter reveal clearly the QPT point in the system. It is also clarified that
the present model is equivalent to an anti Jaynes-Cummings model under certain
conditions.


title: Creating, distributing and freezing entanglement with spin chains

abstract:
  We show how branching spin chains can be used to both generate and distribute
entanglement from their natural dynamics. Such entanglement provides a useful
resource, for example for teleportation or distributed quantum processing. Once
distributed, this resource can be isolated through mapping or swapping the
entanglement into specific qubits at the ends of branches. Alternatively, as we
demonstrate for distributed bipartite entanglement, applying simple
single-qubit operations to the end spin of one or more branches can dynamically
freeze the entanglement at the branch ends.


title:
Tunnelling dynamics of Bose-Einstein condensate in a four wells loop
  shaped system

abstract:
  We investigated the tunnelling dynamics of a zero-temperature Bose-Einstein
condensate (BEC) in configuration of four potential wells arranged in a loop.
We found three interesting dynamic regimes: (a) flows of matter with small
amplitude, (b) steady flow and (c) forced flow of matter for large amplitudes.
The regime of quantum self-confinement has been studied and a new variant of it
has been found for this system.


title: Quantum states with less energy than the vacuum in Dirac Hole Theory

abstract:
  In Dirac's hole theory the vacuum state is generally believed to be the state
of minimum energy. However it has recently been shown that this is not the
case. In [1] it was shown that energy can be extracted from the hole theory
vacuum state through the application of an electric field so that the final
state has less energy than the vacuum state. In this paper we will confirm the
results of [1] by calculating the change in the energy of the vacuum state due
to its interaction with a specific electric field. It will be shown that the
final state has less energy than the original vacuum state.


title:
Generation of a stable low-frequency squeezed vacuum field with
  periodically-poled KTiOPO$_4$ at 1064 nm

abstract:
  We report on the generation of a stable continuous-wave low-frequency
squeezed vacuum field with a squeezing level of $3.8\pm0.1$ dB at 1064 nm, the
wavelength at which laser interferometers for gravitational wave (GW) detection
operate, using periodically poled KTiOPO$_4$ (PPKTP) in a sub-threshold optical
parametric oscillator. PPKTP has the advantages of higher nonlinearity, smaller
intra-crystal and pump-induced seed absorption, user-specified parametric
down-conversion temperature, wider temperature tuning range, and lower
susceptibility to thermal lensing over alternative nonlinear materials such as
MgO doped or periodically poled LiNbO$_3$, and is, therefore, an excellent
material for generation of squeezed vacuum fields for application to laser
interferometers for GW detection.


title: Macroscopic Reality in Quantum Mechanics; Origin and Dissipation

abstract:
  We study the connection between dissipation and reality in macroscopic
quantum systems. We present the following scenario; if we consider the dynamics
of a `partial' wave function, the dissipation is represented as a nonlocal term
and it causes destructive interference to suppress the quantum fluctuation.
Using the variational method, we confirm that this dissipation term is a
reasonable extension of the standard (Schr\"odinger) description for isolated
systems, from which we also derive the classical action. Consequently, in
macroscopic systems, the states whose time-integrated dissipation takes an
extreme value come true. This description, which is consistent with our sense
of reality, coexists with the usual linear-time-dependent description.


title: Entangling a nanomechanical resonator with a microwave field

abstract:
  We show how the coherent oscillations of a nanomechanical resonator can be
entangled with a microwave cavity in the form of a superconducting coplanar
resonator. Dissipation is included and realistic values for experimental
parameters are estimated.


title: Reply to "The three-box paradox revisited" by Ravon and Vaidman

abstract:
  I reply to Ravon and Vaidman's criticism (quant-ph/0606067) of my classical
implementation (quant-ph/0207124) of a three-box system as a card game.


title:
Toric varieties: Simple combinatorial and geometrical structure of
  multipartite quantum systems

abstract:
  We investigate the geometrical structure of multipartite states based on the
construction of toric varieties. We show that the toric variety represents the
space of general pure states and projective toric variety defines the space of
separable set of multi-qubits states. We also discuss in details the
construction of single-, two-, three-, and multi- qubits states. This
construction gives a very simple and elegant visual representation of the
geometrical structure of multipartite quantum systems.


title:
A necessary and sufficient condition to play games in quantum mechanical
  settings

abstract:
  Quantum game theory is a multidisciplinary field which combines quantum
mechanics with game theory by introducing non-classical resources such as
entanglement, quantum operations and quantum measurement. By transferring
two-player-two strategy (2x2) dilemma containing classical games into quantum
realm, dilemmas can be resolved in quantum pure strategies if entanglement is
distributed between the players who use quantum operations. Moreover, players
receive the highest sum of payoffs available in the game, which are otherwise
impossible in classical pure strategies. Encouraged by the observation of rich
dynamics of physical systems with many interacting parties and the power of
entanglement in quantum versions of 2x2 games, it became generally accepted
that quantum versions can be easily extended to N-player situations by simply
allowing N-partite entangled states. In this article, however, we show that
this is not generally true because the reproducibility of classical tasks in
quantum domain imposes limitations on the type of entanglement and quantum
operators. We propose a benchmark for the evaluation of quantum and classical
versions of games, and derive the necessary and sufficient conditions for a
physical realization. We give examples of entangled states that can and cannot
be used, and the characteristics of quantum operators used as strategies.


title: Remarks on Duality Transformations and Generalized Stabilizer States

abstract:
  We consider the transformation of Hamilton operators under various sets of
quantum operations acting simultaneously on all adjacent pairs of particles. We
find mappings between Hamilton operators analogous to duality transformations
as well as exact characterizations of ground states employing non-Hermitean
eigenvalue equations and use this to motivate a generalization of the
stabilizer formalism to non-Hermitean operators. The resulting class of states
is larger than that of standard stabilizer states and allows for example for
continuous variation of local entropies rather than the discrete values taken
on stabilizer states and the exact description of certain ground states of
Hamilton operators.


title:
Exact solutions of the modified Kratzer potential plus ring-shaped
  potential in the D-dimensional Schrodinger equation by the Nikiforov-Uvarov
  method

abstract:
  We present analytically the exact energy bound-states solutions of the
Schrodinger equation in D-dimensions for a recently proposed modified Kratzer
potential plus ring-shaped potential by means of the conventional
Nikiforov-Uvarov method. We give a clear recipe of how to obtain an explicit
solution to the wave functions in terms of orthogonal polynomials. The results
obtained in this work are more general and true for any dimension which can be
reduced to the standard forms in three-dimensions given by other works.


title: New Mathematical Tools for Quantum Technology

abstract:
  Progress in manufacturing technology has allowed us to probe the behavior of
devices on a smaller and faster scale than ever before. With increasing
miniaturization, quantum effects come to dominate the transport properties of
these devices, between collisions, carriers undergo ballistic motion under the
influence of local electric and magnetic fields. The often surprising
propertiesof quantum ballistic transport are currently elucidated in clean
atomic physics experiments. From a theoretical viewpoint, theelectron dynamics
is governed by ballistic propagators and Green functions, intriguing quantities
at the crossroads of classical and quantum mechanics. Here, we briefly describe
the propagator method, some ballistic Green functions, and their application in
a diverse range of problems in atomic and solid state physics, such as
photodetachment, atom lasers, scanning tunneling microscopy, and the quantum
Hall effect.


title: Entanglement in alternating open spin-1/2 chains with XY-Hamiltonian

abstract:
  We investigate entanglement of spin pairs in alternating open spin chains in
the equilibrium state in an external magnetic field. We calculate the reduced
density matrix of spin pairs and estimate the concurrence with Wootter's
criteria. The obtained results demonstrate the dependence of the entanglement
on the temperature, chain's length, the positions of the spins, and the ratio
of the spin-spin interaction constants.


title:
Stochastic particle annihilation: a model of state reduction in
  relativistic quantum field theory

abstract:
  A model of state reduction in relativistic quantum field theory involving a
nonlinear stochastic extension of Schr\"odinger's equation is outlined. The
eigenstates of the annihilation operator are chosen as the preferred basis onto
which reduction occurs. These are the coherent states which saturate the bound
of the Heisenberg uncertainty relation, exhibiting classical-like behavior. The
quantum harmonic oscillator is studied in detail before generalizing to
relativistic scalar quantum field theory. The infinite rates of increase in
energy density which have plagued recent relativistic proposals of dynamical
state reduction are absent in this model. This is because the state evolution
equation does not drive particle creation from the vacuum. The model requires
the specification of a preferred sequence of space-like hyper-surfaces
supporting the time-like state evolution. However, it is shown that the choice
of preferred surfaces has no effect on perturbative results to second order in
the coupling parameter. It is demonstrated how state reduction to a charge
density basis can be induced in fermionic matter via an appropriate coupling to
a bosonic field undergoing this mechanism.


title:
Problems of the CASCADE Protocol and Renyi Entropy Reduction in
  Classical and Quantum Key Generation

abstract:
  It is shown that the interactive error correction protocol `CASCADE' should
be analyzed taking the correlation between passes and finite length of sequence
into account. Furthermore we mention some problems in quantifying the reduction
of Renyi entropy by information announced during the error correction process.


title: Relativistic extension of the complex scaling method

abstract:
  We construct a tridiagonal matrix representation for the three dimensions
Dirac-Coulomb Hamiltonian that provides for a simple and straightforward
relativistic extension of the complex scaling method. Besides the Coulomb
interaction, additional vector, scalar, and pseudo-scalar coupling to
short-range potentials could also be included in the same representation. Using
that, we are able to obtain highly accurate values for the relativistic bound
states and resonance energies. A simple program code is developed to perform
the calculation for a given charge, angular momentum and potential
configuration. The resonance structure in the complex relativistic energy plane
is also shown graphically. Illustrative examples are given and we verify that
in the nonrelativistic limit one obtains known results. As an additional
advantage of this tridiagonal representation, we use it to obtain a highly
accurate evaluation of the relativistic bound states energies for the
Woods-Saxon potential (as a model of nuclear interaction) with the nucleus
treated as solid sphere of uniform charge distribution.


title: Quantum Capacity of a dephasing channel with memory

abstract:
  We show that the amount of coherent quantum information that can be reliably
transmitted down a dephasing channel with memory is maximized by separable
input states. In particular, we model the channel as a Markov chain or a
multimode environment of oscillators. While in the first model the maximization
is achieved for the maximally mixed input state, in the latter it is convenient
to exploit the presence of a decoherence-protected subspace generated by memory
effects. We explicitly compute the quantum channel capacity for the first model
while numerical simulations suggest a lower bound for the latter. In both cases
memory effects enhance the coherent information. We present results valid for
arbitrary size of the input.


title:
Every NAND formula of size N can be evaluated in time N^{1/2+o(1)} on a
  quantum computer

abstract:
  For every NAND formula of size N, there is a bounded-error N^{1/2+o(1)}-time
quantum algorithm, based on a coined quantum walk, that evaluates this formula
on a black-box input. Balanced, or ``approximately balanced,'' NAND formulas
can be evaluated in O(sqrt{N}) queries, which is optimal. It follows that the
(2-o(1))-th power of the quantum query complexity is a lower bound on the
formula size, almost solving in the positive an open problem posed by Laplante,
Lee and Szegedy.


title: Band theory in the context of the Hamilton-Jacobi formulation

abstract:
  In the one-dimensional periodic potential case, we formulate the condition of
Bloch periodicity for the reduced action by using the relation between the wave
function and the reduced action established in the context of the equivalence
postulate of quantum mechanics. Then, without appealing to the wave function
properties, we reproduce the well-known dispersion relations which predict the
band structure for the energy spectrum in the Kr\"onig-Penney model.


title:
A complete characterization of mixed state entanglement using
  probability density functions

abstract:
  We propose that the entanglement of mixed states is characterised properly in
terms of a probability density function $\mathcal{P}(\mathcal{E})$. There is a
need for such a measure since the prevalent measures (such as
\textit{concurrence} and \textit{negativity}) are rough benchmarks, and not
monotones of each other. Considering the specific case of two qubit mixed
states, we provide an explicit construction of $\mathcal{P}(\mathcal{E})$ and
show that it is characterised by a set of parameters, of which concurrence is
but one particular combination. $\mathcal{P}(\mathcal{E})$ is manifestly
invariant under $SU(2) \times SU(2)$ transformations. It can, in fact,
reconstruct the state up to local operations
  - with the specification of at most four additional parameters. Finally the
new measure resolves the controversy regarding the role of entanglement in
quantum computation in NMR systems.


title: Noise robustness of the nonlocality of entangled quantum states

abstract:
  We study the nonlocal properties of states resulting from the mixture of an
arbitrary entangled state rho of two d-dimensional systems and completely
depolarized noise, with respective weights p and 1-p. We first construct a
local model for the case in which rho is maximally entangled and p at or below
a certain bound. We then extend the model to arbitrary rho. Our results provide
bounds on the resistance to noise of the nonlocal correlations of entangled
states. For projective measurements, the critical value of the noise parameter
p for which the state becomes local is at least asymptotically log(d) larger
than the critical value for separability.


title:
Thermal entanglement and teleportation in a two-qubit Heisenberg chain
  with Dzyaloshinski-Moriya anisotropic antisymmetric interaction

abstract:
  Thermal entanglement of a two-qubit Heisenberg chain in presence of the
Dzyaloshinski-Moriya (DM) anisotropic antisymmetric interaction and
entanglement teleportation when using two independent Heisenberg chains as
quantum channel are investigated. It is found that the DM interaction can
excite the entanglement and teleportation fidelity. The output entanglement
increases linearly with increasing value of input one, its dependences on the
temperature, DM interaction and spin coupling constant are given in detail.
Entanglement teleportation will be better realized via antiferromagnetic spin
chain when the DM interaction is turned off and the temperature is low.
However, the introduction of DM interaction can cause the ferromagnetic spin
chain to be a better quantum channel for teleportation. A minimal entanglement
of the thermal state in the model is needed to realize the entanglement
teleportation regardless of antiferromagnetic or ferromagnetic spin chains.


title: Probability in modal interpretations of quantum mechanics

abstract:
  Modal interpretations have the ambition to construe quantum mechanics as an
objective, man-independent description of physical reality. Their second
leading idea is probabilism: quantum mechanics does not completely fix physical
reality but yields probabilities. In working out these ideas an important motif
is to stay close to the standard formalism of quantum mechanics and to refrain
from introducing new structure by hand. In this paper we explain how this
programme can be made concrete. In particular, we show that the Born
probability rule, and sets of definite-valued observables to which the Born
probabilities pertain, can be uniquely defined from the quantum state and
Hilbert space structure. We discuss the status of probability in modal
interpretations, and to this end we make a comparison with many-worlds
alternatives. An overall point that we stress is that the modal ideas define a
general framework and research programme rather than one definite and finished
interpretation.


title: Identical Quantum Particles and Weak Discernibility

abstract:
  We examine a recent argument that ``identical'' quantum particles with an
anti-symmetric state (fermions) are weakly discernible objects, just like
irreflexively related ordinary objects in situations with perfect symmetry
(Black's spheres, for example). We conclude that the argument uses a silent
premise that is not justified in the quantum case.


title: Completely Positive Maps and Classical Correlations

abstract:
  We expand the set of initial states of a system and its environment that are
known to guarantee completely positive reduced dynamics for the system when the
combined state evolves unitarily. We characterize the correlations in the
initial state in terms of its quantum discord [H. Ollivier and W. H. Zurek,
Phys. Rev. Lett. 88, 017901 (2001)]. We prove that initial states that have
only classical correlations lead to completely positive reduced dynamics. The
induced maps can be not completely positive when quantum correlations
including, but not limited to, entanglement are present. We outline the
implications of our results to quantum process tomography experiments.


title:
On the Exact Evaluation of Certain Instances of the Potts Partition
  Function by Quantum Computers

abstract:
  We present an efficient quantum algorithm for the exact evaluation of either
the fully ferromagnetic or anti-ferromagnetic q-state Potts partition function
Z for a family of graphs related to irreducible cyclic codes. This problem is
related to the evaluation of the Jones and Tutte polynomials. We consider the
connection between the weight enumerator polynomial from coding theory and Z
and exploit the fact that there exists a quantum algorithm for efficiently
estimating Gauss sums in order to obtain the weight enumerator for a certain
class of linear codes. In this way we demonstrate that for a certain class of
sparse graphs, which we call Irreducible Cyclic Cocycle Code (ICCC_\epsilon)
graphs, quantum computers provide a polynomial speed up in the difference
between the number of edges and vertices of the graph, and an exponential speed
up in q, over the best classical algorithms known to date.


title:
Entangled Causal Relation in Measurement by Two Measurement Devices
  Moving Mutually

abstract:
  I want to show situations in which causal relation of two events is entangled
and similar to time paradox, without employing time machines. Such situations
will be obtained in measurement by two measurement devices moving mutually.
According to the concept of the collapse of wave packet and the relativity of
simultaneity, the causal relation of two measurements can be entangled.


title:
Weighted complex projective 2-designs from bases: optimal state
  determination by orthogonal measurements

abstract:
  We introduce the problem of constructing weighted complex projective
2-designs from the union of a family of orthonormal bases. If the weight
remains constant across elements of the same basis, then such designs can be
interpreted as generalizations of complete sets of mutually unbiased bases,
being equivalent whenever the design is composed of d+1 bases in dimension d.
We show that, for the purpose of quantum state determination, these designs
specify an optimal collection of orthogonal measurements. Using highly
nonlinear functions on abelian groups, we construct explicit examples from d+2
orthonormal bases whenever d+1 is a prime power, covering dimensions d=6, 10,
and 12, for example, where no complete sets of mutually unbiased bases have
thus far been found.


title: Addendum to "Multipartite states under local unitary transformations"

abstract:
  In previous work the authors introduced a notion of generic states and
obtained criteria for local equivalence of them. Here they introduce the
concept of CHG states maintaining the criteria of local equivalence. This fact
allows the authors to halve the number of invariants necessary to characterize
the equivalence classes under local unitary transformations for the set of
tripartite states whose partial trace with respect to one of the subsystems
belongs to the class of CHG mixed states.


title: Are entropic Bell inequalities implied by the second law?

abstract:
  Bell's inequalities, in the form given by Cerf and Adami, are derived from
the combination of the second law of thermodynamics and the Markov postulate.
Violations of these inequalities are discussed in terms of the mixing
characteristics of the density operators. A quantum mechanical bound for a
system of three qubits, two of which are entangled, is found and is shown to
lead to strong subadditivity in one case. Finally, in interpreting the results,
the role the uncertainty relations play in the definition of entropy and the
second law is discussed, demonstrating a link to Bell's inequalities.
Additionally the second law is shown to be dependent on the type of measurement
in quantum systems. Specifically, a more general statement of the second law
appropriate to both quantum and classical systems is suggested.


title: Entanglement Entropy in Random Quantum Spin-S Chains

abstract:
  We discuss the scaling of entanglement entropy in the random singlet phase
(RSP) of disordered quantum magnetic chains of general spin-S. Through an
analysis of the general structure of the RSP, we show that the entanglement
entropy scales logarithmically with the size of a block and we provide a closed
expression for this scaling. This result is applicable for arbitrary quantum
spin chains in the RSP, being dependent only on the magnitude S of the spin.
Remarkably, the logarithmic scaling holds for the disordered chain even if the
pure chain with no disorder does not exhibit conformal invariance, as is the
case for Heisenberg integer spin chains. Our conclusions are supported by
explicit evaluations of the entanglement entropy for random spin-1 and spin-3/2
chains using an asymptotically exact real-space renormalization group approach.


title: Scalable Quantum Networks based on Few-Qubit Registers

abstract:
  We describe and analyze a hybrid approach to scalable quantum computation
based on an optically connected network of few-qubit quantum registers. We show
that probabilistically connected five-qubit quantum registers suffice for
deterministic, fault-tolerant quantum computation even when state preparation,
measurement, and entanglement generation all have substantial errors. We
discuss requirements for achieving fault-tolerant operation for two specific
implementations of our approach.


title:
Abrupt termination of a quantum channel and exactly solvable
  position-dependent mass models in three dimensions

abstract:
  We consider a particle with a position-dependent mass, moving in a
three-dimensional semi-infinite parallelepipedal or cylindrical channel under
the influence of some hyperbolic potential. We show that the lack of uniformity
in the environment generates an infinite number of bound states.


title:
Cold polar molecules in 2D traps: Tailoring interactions with external
  fields for novel quantum phases

abstract:
  We discuss techniques to engineer effective long-range interactions between
polar molecules using external static electric and microwave fields. We
consider a setup where molecules are trapped in a two-dimensional pancake
geometry by a far-off-resonance optical trap, which ensures the stability of
the dipolar collisions. We detail how to modify the shape and the strength of
the long-range part of interaction potentials, which can be utilized to realize
interesting quantum phases in the context of cold molecular gases.


title: State visibility in Q-bit space

abstract:
  We study by comparison the structure of singlet type states in Q-bit space in
the light of quantum and classical paradigms. It is shown that only the
classical paradigm implies a variation in the visibility of correlation
coefficients, that has been observed in fact in experiments. We conclude that
Q-bit space in not a appropriate venue for an EPR test of quantum completeness.


title: Multipartite unlockable bound entanglement in the stabilizer formalism

abstract:
  We find an interesting relationship between multipartite bound entangled
states and the stabilizer formalism. We prove that if a set of commuting
operators from the generalized Pauli group on $n$ qudits satisfy certain
constraints, then the maximally mixed state over the subspace stabilized by
them is an unlockable bound entangled state. Moreover, the properties of this
state, such as symmetry under permutations of parties, undistillability and
unlockability, can be easily explained from the stabilizer formalism without
tedious calculation. In particular, the four-qubit Smolin state and its recent
generalization to even number of qubits can be viewed as special examples of
our results. Finally, we extend our results to arbitrary multipartite systems
in which the dimensions of all parties may be different.


title: Exact Solutions of Schrodinger Equation for a Ring-Shaped Potential

abstract:
  The exact solutions of Schrodinger equation are obtained for a noncentral
potential which is a ring-shaped potential. The energy eigenvalues and
corresponding eigenfunctions are obtained for any angular momentum l.
Nikiforov-Uvarov method is used in the computations.


title: Objective probability and quantum fuzziness

abstract:
  This paper offers a critique of the Bayesian interpretation of quantum
mechanics with particular focus on a paper by Caves, Fuchs, and Schack
containing a critique of the "objective preparations view" or OPV. It also aims
to carry the discussion beyond the hardened positions of Bayesians and
proponents of the OPV. Several claims made by Caves et al. are rebutted,
including the claim that different pure states may legitimately be assigned to
the same system at the same time, and the claim that the quantum nature of a
preparation device cannot legitimately be ignored. Both Bayesians and
proponents of the OPV regard the time dependence of a quantum state as the
continuous dependence on time of an evolving state of some kind. This leads to
a false dilemma: quantum states are either objective states of nature or
subjective states of belief. In reality they are neither. The present paper
views the aforesaid dependence as a dependence on the time of the measurement
to whose possible outcomes the quantum state serves to assign probabilities.
This makes it possible to recognize the full implications of the only testable
feature of the theory, viz., the probabilities it assigns to measurement
outcomes...


title: Uncertainty Relations for Positive Operator Valued Measures

abstract:
  How much unavoidable randomness is generated by a Positive Operator Valued
Measure (POVM)? We address this question using two complementary approaches.
First we study the variance of a real variable associated to the POVM outcomes.
In this context we introduce an uncertainty operator which measures how much
additional noise is introduced by carrying out a POVM rather than a von Neumann
measurement. We illustrate this first approach by studying the variances of
joint estimates of \sigma_x and \sigma_z for spin 1/2 particles. We show that
for unbiased measurements the sum of these variances is lower bounded by 1. In
our second approach we study the entropy of the POVM outcomes. In particular we
try to establish lower bounds on the entropy of the POVM outcomes. We
illustrate this second approach by examples.