title:
Security of Continuous-variable quantum cryptography using coherent
  states: Decline of postselection advantage

abstract:
  We investigate the security of continuous-variable (CV) quantum key
distribution (QKD) using coherent states in the presence of quadrature excess
noise. We consider an eavesdropping attack which uses a linear amplifier and
beam splitter. This attack makes a link between beam-splitting attack and
intercept-resend attack (classical teleportation attack). We also show how
postselection loses its efficiency in a realistic channel.


title:
Improving the security of multiparty quantum secret sharing against
  Trojan horse attack

abstract:
  We analyzed the security of the multiparty quantum secret sharing (MQSS)
protocol recently proposed by Zhang, Li and Man [Phys. Rev. A \textbf{71},
044301 (2005)] and found that this protocol is secure for any other
eavesdropper except for the agent Bob who prepares the quantum signals as he
can attack the quantum communication with a Trojan horse. That is, Bob replaces
the single-photon signal with a multi-photon one and the other agent Charlie
cannot find this cheating as she does not measure the photons before they runs
back from the boss Alice, which reveals that this MQSS protocol is not secure
for Bob. Finally, we present a possible improvement of the MQSS protocol
security with two single-photon measurements and six unitary operations.


title: Temporal shape manipulation of adiabatons

abstract:
  We describe how to control the temporal shape of adiabaton using
peculiarities of propagation dynamics under coherent population trapping.
Temporal compression is demonstrated as a special case of pulse shaping. The
general case of unequal oscillator strengths of two optical transitions in atom
is considered.


title:
Multiplicativity of completely bounded p-norms implies a new additivity
  result

abstract:
  We prove additivity of the minimal conditional entropy associated with a
quantum channel Phi, represented by a completely positive (CP),
trace-preserving map, when the infimum of S(gamma_{12}) - S(gamma_1) is
restricted to states of the form gamma_{12} = (I \ot Phi)(| psi >< psi |). We
show that this follows from multiplicativity of the completely bounded norm of
Phi considered as a map from L_1 -> L_p for L_p spaces defined by the Schatten
p-norm on matrices; we also give an independent proof based on entropy
inequalities. Several related multiplicativity results are discussed and
proved. In particular, we show that both the usual L_1 -> L_p norm of a CP map
and the corresponding completely bounded norm are achieved for positive
semi-definite matrices. Physical interpretations are considered, and a new
proof of strong subadditivity is presented.


title: Estimating the spectrum of a density matrix with LOCC

abstract:
  The problem of estimating the spectrum of a density matrix is considered.
Other problems, such as bipartite pure state entanglement, can be reduced to
spectrum estimation. A local operations and classical communication (LOCC)
measurement strategy is shown which is asymptotically optimal. This means that,
for a very large number of copies, it becomes unnecessary to perform collective
measurements which should be more difficult to implement in practice.


title:
Spin transport and quasi 2D architectures for donor-based quantum
  computing

abstract:
  Through the introduction of a new electron spin transport mechanism, a 2D
donor electron spin quantum computer architecture is proposed. This design
addresses major technical issues in the original Kane design, including spatial
oscillations in the exchange coupling strength and cross-talk in gate control.
It is also expected that the introduction of a degree of non-locality in qubit
gates will significantly improve the scaling fault-tolerant threshold over the
nearest-neighbour linear array.


title:
Experimental motivation and empirical consistency in minimal no-collapse
  quantum mechanics

abstract:
  We analyze three important experimental domains (SQUIDs, molecular
interferometry, and Bose-Einstein condensation) as well as quantum-biophysical
studies of the neuronal apparatus to argue that (i) the universal validity of
unitary dynamics and the superposition principle has been confirmed far into
the mesoscopic and macroscopic realm in all experiments conducted thus far;
(ii) all observed "restrictions" can be correctly and completely accounted for
by taking into account environmental decoherence effects; (iii) no positive
experimental evidence exists for physical state-vector collapse; (iv) the
perception of single "outcomes" is likely to be explainable through decoherence
effects in the neuronal apparatus. We also discuss recent progress in the
understanding of the emergence of quantum probabilities and the objectification
of observables. We conclude that it is not only viable, but moreover compelling
to regard a minimal no-collapse quantum theory as a leading candidate for a
physically motivated and empirically consistent interpretation of quantum
mechanics.


title: Solving Satisfiability Problems by the Ground-State Quantum Computer

abstract:
  A quantum algorithm is proposed to solve the Satisfiability problems by the
ground-state quantum computer. The scale of the energy gap of the ground-state
quantum computer is analyzed for the 3-bit Exact Cover problem. The time cost
of this algorithm on the general SAT problems is discussed.


title:
Internal Consistency of Fault-Tolerant Quantum Error Correction in Light
  of Rigorous Derivations of the Quantum Markovian Limit

abstract:
  We critically examine the internal consistency of a set of minimal
assumptions entering the theory of fault-tolerant quantum error correction for
Markovian noise. These assumptions are: fast gates, a constant supply of fresh
and cold ancillas, and a Markovian bath. We point out that these assumptions
may not be mutually consistent in light of rigorous formulations of the
Markovian approximation. Namely, Markovian dynamics requires either the
singular coupling limit (high temperature), or the weak coupling limit (weak
system-bath interaction). The former is incompatible with the assumption of a
constant and fresh supply of cold ancillas, while the latter is inconsistent
with fast gates. We discuss ways to resolve these inconsistencies. As part of
our discussion we derive, in the weak coupling limit, a new master equation for
a system subject to periodic driving.


title:
Spin-projection orientations in the plane square-lattice Ising model
  with periodic boundary conditions

abstract:
  The periodic boundary conditions changed the plane square-lattice Ising model
to the torus-lattice system which restricts the spin-projection orientations.
Only two of the three important spin-projection orientations, parallel to the
x-axis or to the y-axis, are suited to the torus-lattice system. The
infinitesimal difference of the free-energies of the systems between the two
systems mentioned above makes their critical temperatures infinitely close to
each other, but their topological fundamental groups are distinct.


title:
Low dimensional bound entanglement with one-way distillable
  cryptographic key

abstract:
  We provide a class of bound entangled states that have positive distillable
secure key rate. The smallest state of this kind is $4 \otimes 4$, which shows
that peculiar security contained in bound entangled states does not need high
dimensional systems. We show, that for these states a positive key rate can be
obtained by {\it one-way} Devetak-Winter protocol. Subsequently the volume of
bound entangled key-distillable states in arbitrary dimension is shown to be
nonzero. We provide a scheme of verification of cryptographic quality of
experimentally prepared state in terms of local observables. Proposed set of 7
collective settings is proven to be optimal in number of settings.


title:
Bound States and Band Structure - a Unified Treatment through the
  Quantum Hamilton - Jacobi Approach

abstract:
  We analyze the Scarf potential, which exhibits both discrete energy bound
states and energy bands, through the quantum Hamilton-Jacobi approach. The
singularity structure and the boundary conditions in the above approach,
naturally isolate the bound and periodic states, once the problem is mapped to
the zero energy sector of another quasi-exactly solvable quantum problem. The
energy eigenvalues are obtained without having to solve for the corresponding
eigenfunctions explicitly. We also demonstrate how to find the eigenfunctions
through this method.


title: Distinguishing sets of quantum states

abstract:
  Given two sets finite $S_0$ and $S_1$ of quantum states. We show necessary
and sufficient conditions for distinguishing them by a measurement.


title: Single-shot measurement of quantum optical phase

abstract:
  Although the canonical phase of light, which is defined as the complement of
photon number, has been described theoretically by a variety of distinct
approaches, there have been no methods proposed for its measurement. Indeed
doubts have been expressed about whether or not it is measurable. Here we show
how it is possible, at least in principle, to perform a single-shot measurement
of canonical phase using beam splitters, mirrors, phase shifters and
photodetectors.


title:
Comment on equivalence between quantum phase transition phenomena in
  radiation-matter and magnetic systems

abstract:
  In this Comment we show that the temperature-dependent effective Hamiltonian
derived by Reslen {\it et al} [Europhys. Lett., {\bf 69} (2005) 8] or that one
by Liberti and Zaffino [arXiv:cond-mat/0503742] for the Dicke model cannot be
correct for any temperature. They both violate a rigorous result. The former is
correct only in the quantum (zero-temperature) limit while the last one only in
the classical (infinite temperature) limit. The fact that the Dicke model
belongs to the universality class of the infinitely coordinated
transverse-field XY model is known for more then 30 years.


title: Spin gases as microscopic models for non-Markovian decoherence

abstract:
  We analyze a microscopic decoherence model in which the total system is
described as a spin gas. A spin gas consists of N classically moving particles
with additional, interacting quantum degrees of freedom (e.g. spins). For
various multipartite entangled probe states, we analyze the decoherence induced
by interactions between the probe- and environmental spins in such spin gases.
We can treat mesoscopic environments (10^5 particles). We present results for a
lattice gas, which could be realized by neutral atoms hopping in an optical
lattice, and show the effects of non-Markovian and correlated noise, as well as
finite size effects.


title: Entangling power of permutation invariant quantum states

abstract:
  We investigate the von Neumann entanglement entropy as function of the size
of a subsystem for permutation invariant ground states in models with finite
number of states per site, e.g., in quantum spin models. We demonstrate that
the entanglement entropy of $n$ sites in a system of length $L$ generically
grows as $\sigma\log_{2}[2\pi en(L-n)/L]+C$, where $\sigma$ is the on-site spin
and $C$ is a function depending only on magnetization.


title: On divergence, relative entropy and the substate property

abstract:
  In this article we study relationship between three measures of
distinguishability of quantum states called as divergence, relative entropy and
the substate property.


title:
Electromagnetically induced transparency for Lambda - like systems with
  a structured continuum

abstract:
  Electric susceptibility of a laser-dressed atomic medium is calculated for a
model Lambda - like system including two lower states and a continuum
structured by a presence of an autoionizing state or a continuum with a
laser-induced structure. Depending on the strength of a control field it is
possible to obtain a significant reduction of the light velocity in a narrow
frequency window in the conditions of a small absorption. A smooth transition
is shown between the case of a flat continuum and that of a discrete state
serving as the upper state of a Lambda system.


title: Dynamical Entanglement in Particle Scattering

abstract:
  This paper explores the connections between particle scattering and quantum
information theory in the context of the non-relativistic, elastic scattering
of two spin-1/2 particles. An untangled, pure, two-particle in-state is evolved
by an S-matrix that respects certain symmetries and the entanglement of the
pure out-state is measured. The analysis is phrased in terms of unitary,
irreducible representations (UIRs) of the symmetry group in question, either
the rotation group for the spin degrees of freedom or the Galilean group for
non-relativistic particles. Entanglement may occurs when multiple UIRs appear
in the direct sum decomposition of the direct product in-state, but it also
depends of the scattering phase shifts. \keywords{dynamical entanglement,
scattering, Clebsch-Gordan methods}


title: Entanglement sharing: from qubits to Gaussian states

abstract:
  It is a central trait of quantum information theory that there exist
limitations to the free sharing of quantum correlations among multiple parties.
Such 'monogamy constraints' have been introduced in a landmark paper by
Coffman, Kundu and Wootters, who derived a quantitative inequality expressing a
trade-off between the couplewise and the genuine tripartite entanglement for
states of three qubits. Since then, a lot of efforts have been devoted to the
investigation of distributed entanglement in multipartite quantum systems. In
these proceedings we report, in a unifying framework, a bird's eye view of the
most relevant results that have been established so far on entanglement sharing
in quantum systems. We will take off from the domain of N qubits, graze qudits,
and finally land in the almost unexplored territory of multimode Gaussian
states of continuous variable systems.

categories: quant-ph cond-mat.other math-ph math.MP physics.optics


title:
Adventures of the Coupled Yang-Mills Oscillators: I. Semiclassical
  Expansion

abstract:
  We study the quantum mechanical motion in the $x^2y^2$ potentials with
$n=2,3$, which arise in the spatially homogeneous limit of the Yang-Mills (YM)
equations. These systems show strong stochasticity in the classical limit
($\hbar = 0$) and exhibit a quantum mechanical confinement feature. We
calculate the partition function $Z(t)$ going beyond the Thomas-Fermi (TF)
approximation by means of the semiclassical expansion using the Wigner-Kirkwood
(WK) method. We derive a novel compact form of the differential equation for
the WK function. After separating the motion in the channels of the
equipotential surface from the motion in the central region, we show that the
leading higher-order corrections to the TF term vanish up to eighth order in
$\hbar$, if we treat the quantum motion in the hyperbolic channels correctly by
adiabatic separation of the degrees of freedom. Finally, we obtain an
asymptotic expansion of the partition function in terms of the parameter
$g^2\hbar^4t^3$.


title: Atomic Quantum Memory for Photonic Qubits via Scattering in Cavity QED

abstract:
  We investigate a scheme of atomic quantum memory to store photonic qubits in
cavity QED. This is motivated on the recent observation that the quantum-state
swapping between a single-photon pulse and a Lambda-type atom trapped in a
cavity is ideally realized via scattering for some specific case in the strong
coupling cavity regime [T. W. Chen, C. K. Law, and P. T. Leung, Phys. Rev. A
69, 063810 (2004)]. We derive a simple formula for calculating the fidelity of
this atom-photon swapping for quantum memory. We further propose a feasible
method which implements conditionally the quantum memory operation with the
fidelity of almost unity even if the atom-photon coupling is not so strong.
This method can also be applied to store a photonic entanglement in spatially
separated atomic quantum memories.


title: Probabilistic teleportation of unknown two-particle state via POVM

abstract:
  We propose a scheme for probabilistic teleportation of unknown two-particle
state with partly entangled four-particle state via POVM. In this scheme the
teleportation of unknown two-particle state can be realized with certain
probability by performing two Bell state measurements, a proper POVM and a
unitary transformation.


title: Entanglement of pair cat states and teleportation

abstract:
  The entanglement of pair cat states in the phase damping channel is studied
by employing the relative entropy of entanglement. It is shown that the pair
cat states can always be distillable in the phase damping channel. Furthermore,
we analyze the fidelity of teleportation for the pair cat states by using joint
measurements of the photon-number sum and phase difference.


title:
Influence of intrinsic decoherence on nonclassical properties of the
  output of a Bose-Einstein condensate

abstract:
  We investigate nonclassical properties of the output of a Bose-Einstein
condensate in Milburn's model of intrinsic decoherence. It is shown that the
squeezing property of the atom laser is suppressed due to decoherence.
Nevertheless, if some very special conditions were satisfied, the squeezing
properties of atom laser could be robust against the decoherence.


title: An Introduction to Quantum Game Theory

abstract:
  This essay gives a self-contained introduction to quantum game theory, and is
primarily oriented to economists with little or no acquaintance with quantum
mechanics. It assumes little more than a basic knowledge of vector algebra.
Quantum mechanical notation and results are introduced as needed. It is also
shown that some fundamental problems of quantum mechanics can be formulated as
games.


title: On recombination in strong laser fields: effect of a slow drift

abstract:
  The dynamics of the recombination in ultrastrong atomic fields is studied for
one-dimensional models by numerical simulations. A nonmonotonic behavior of the
bound state final population as a function of the laser field amplitude is
examined. An important role of a slow drift of an electron wave packet is
observed.


title: Quantum Random Walks without a Coin Toss

abstract:
  We construct a quantum random walk algorithm, based on the Dirac operator
instead of the Laplacian. The algorithm explores multiple evolutionary branches
by superposition of states, and does not require the coin toss instruction of
classical randomised algorithms. We use this algorithm to search for a marked
vertex on a hypercubic lattice in arbitrary dimensions. Our numerical and
analytical results match the scaling behaviour of earlier algorithms that use a
coin toss instruction.


title: Classicality in discrete Wigner functions

abstract:
  Gibbons et al. [Phys. Rev. A 70, 062101(2004)] have recently defined a class
of discrete Wigner functions W to represent quantum states in a Hilbert space
with finite dimension. We show that the only pure states having non-negative W
for all such functions are stabilizer states, as conjectured by one of us
[Phys. Rev. A 71, 042302 (2005)]. We also show that the unitaries preserving
non-negativity of W for all definitions of W form a subgroup of the Clifford
group. This means pure states with non-negative W and their associated unitary
dynamics are classical in the sense of admitting an efficient classical
simulation scheme using the stabilizer formalism.


title: Quantum estimation of relative information

abstract:
  We derive optimal schemes for preparation and estimation of relational
degrees of freedom between two quantum systems. We specifically analyze the
case of rotation parameters representing relative angles between elements of
the SU(2) symmetry group. Our estimation procedure does not assume prior
knowledge of the absolute spatial orientation of the systems and as such does
not require information on the underlying classical reference frame in which
the states are prepared.


title:
State space structure and entanglement of rotationally invariant spin
  systems

abstract:
  We investigate the structure of SO(3)-invariant quantum systems which are
composed of two particles with spins j_1 and j_2. The states of the composite
spin system are represented by means of two complete sets of rotationally
invariant operators, namely by the projections P_J onto the eigenspaces of the
total angular momentum J, and by certain invariant operators Q_K which are
built out of spherical tensor operators of rank K. It is shown that these
representations are connected by an orthogonal matrix whose elements are
expressible in terms of Wigner's 6-j symbols. The operation of the partial time
reversal of the combined spin system is demonstrated to be diagonal in the
Q_K-representation. These results are employed to obtain a complete
characterization of spin systems with j_1 = 1 and arbitrary j_2 > 1. We prove
that the Peres-Horodecki criterion of positive partial transposition (PPT) is
necessary and sufficient for separability if j_2 is an integer, while for
half-integer spins j_2 there always exist entangled PPT states (bound
entanglement). We construct an optimal entanglement witness for the case of
half-integer spins and design a protocol for the detection of entangled PPT
states through measurements of the total angular momentum.


title: Optimal Bell tests do not require maximally entangled states

abstract:
  Any Bell test consists of a sequence of measurements on a quantum state in
space-like separated regions. Thus, a state is better than others for a Bell
test when, for the optimal measurements and the same number of trials, the
probability of existence of a local model for the observed outcomes is smaller.
The maximization over states and measurements defines the optimal nonlocality
proof. Numerical results show that the required optimal state does not have to
be maximally entangled.


title:
Qualitative aspects of the entanglement in the three-level model with
  photonic crystals

abstract:
  This communication is an enquiry into the circumstances under which
concurrence and phase entropy methods can give an answer to the question of
quantum entanglement in the composite state when the photonic band gap is
exhibited by the presence of photonic crystals in a three-level system. An
analytic approach is proposed for any three-level system in the presence of
photonic band gap. Using this analytic solution, we conclusively calculate the
concurrence and phase entropy, focusing particularly on the entanglement
phenomena. Specifically, we use concurrence as a measure of entanglement for
dipole emitters situated in the thin slab region between two semi-infinite
one-dimensionally periodic photonic crystals, a situation reminiscent of planar
cavity laser structures. One feature of the regime considered here is that
closed-form evaluation of the time evolution may be carried out in the presence
of the detuning and the photonic band gap, which provides insight into the
difference in the nature of the concurrence function for atom-field coupling,
mode frequency and different cavity parameters. We demonstrate how fluctuations
in the phase and number entropies effected by the presence of the
photonic-band-gap. The outcomes are illustrated with numerical simulations
applied to GaAs. Finally, we relate the obtained results to instances of any
three-level system for which the entanglement cost can be calculated. Potential
experimental observations in solid-state systems are discussed and found to be
promising.


title: Massive Quantum Memories by Periodically Inverted Dynamic Evolutions

abstract:
  We introduce a general scheme to realize perfect quantum state reconstruction
and storage in systems of interacting qubits. This novel approach is based on
the idea of controlling the residual interactions by suitable external controls
that, acting on the inter-qubit couplings, yield time-periodic inversions in
the dynamical evolution, thus cancelling exactly the effects of quantum state
diffusion. We illustrate the method for spin systems on closed rings with XY
residual interactions, showing that it enables the massive storage of
arbitrarily large numbers of local states, and we demonstrate its robustness
against several realistic sources of noise and imperfections.


title: Quantum Relativity: Physical Laws Must be Invariant Over Quantum Systems

abstract:
  Decoherence may not solve all of the measurement problems of quantum
mechanics. It is proposed that a solution to these problems may be to allow
that superpositions describe physically real systems in the following sense.
Each quantum system "carries" around a local spacetime in whose terms other
quantum systems may take on nonlocal states. Each quantum system forms a
physically valid coordinate frame. The laws of physics should be formulated to
be invariant under the group of allowed transformations among such frames. A
transformation of relatively superposed spatial coordinates that allows an
electron system to preserve the de Broglie Relation in describing a double-slit
laboratory system-in analogy to a Minkowskian Transformation-is proposed. In
general, "quantum relativity" says is invariant under transformations among
quantum reference frames. Some conjectures on how this impacts gravity and
gauge invariance are made.


title: Mixed State Entanglement of Assistance and the Generalized Concurrence

abstract:
  We consider the maximum bipartite entanglement that can be distilled from a
single copy of a multipartite mixed entangled state, where we focus mostly on
$d\times d\times n$-dimensional tripartite mixed states. We show that this {\em
assisted entanglement}, when measured in terms of the generalized concurrence
(named G-concurrence) is (tightly) bounded by an entanglement monotone, which
we call the G-concurrence of assistance. The G-concurrence is one of the
possible generalizations of the concurrence to higher dimensions, and for pure
bipartite states it measures the {\em geometric mean} of the Schmidt numbers.
For a large (non-trivial) class of $d\times d$-dimensional mixed states, we are
able to generalize Wootters formula for the concurrence into lower and upper
bounds on the G-concurrence. Moreover, we have found an explicit formula for
the G-concurrence of assistance that generalizes the expression for the
concurrence of assistance for a large class of $d\times d\times n$ dimensional
tripartite pure states.


title: Bell inequalities for three particles

abstract:
  We present tight Bell inequalities expressed by probabilities for three four-
and five-dimensional systems. The tight structure of Bell inequalities for
three $d$-dimensional systems (qudits) is proposed. Some interesting Bell
inequalities of three qubits reduced from those of three qudits are also
studied.


title: Einstein's Photon Concept Quantified by the Bohr Model of the Photon

abstract:
  The photon is modeled as a monochromatic solution of Maxwell's equations
confined as a soliton wave by the principle of causality of special relativity.
The soliton travels rectilinearly at the speed of light. The solution can
represent any of the known polarization (spin) states of the photon. For
circularly polarized states the soliton's envelope is a circular ellipsoid
whose length is the observed wavelength ($\lambda$), and whose diameter is
$\lambda/\pi$; this envelope contains the electromagnetic energy of the wave
($h\nu=hc/\lambda$). The predicted size and shape is confirmed by experimental
measurements: of the sub-picosecond time delay of the photo-electric effect, of
the attenuation of undiffracted transmission through slits narrower than the
soliton's diameter of $\lambda/\pi$, and by the threshold intensity required
for the onset of multiphoton absorption in focussed laser beams. Inside the
envelope the wave's amplitude increases linearly with the radial distance from
the axis of propagation, being zero on the axis. Outside the envelope the wave
is evanescent with an amplitude that decreases inversely with the radial
distance from the axis. The evanescent wave is responsible for the observed
double-slit interference phenomenon.


title: Photon Echoes Produced by Switching Electric Fields

abstract:
  We demonstrate photon echoes in Eu$^{3+}$:Y$_{2}$SiO$_{5}$ by controlling the
inhomogeneous broadening of the Eu$^{3+}$
$^{7}$F$_{0}\leftrightarrow^{5}$D$_{0}$ optical transition. This transition has
a linear Stark shift and we induce inhomogeneous broadening by applying an
external electric field gradient. After optical excitation, reversing the
polarity of the field rephases the ensemble, resulting in a photon echo. This
is the first demonstration of such a photon echo and its application as a
quantum memory is discussed.


title:
Stopped light with storage times greater than one second using EIT in a
  solid

abstract:
  We report on the demonstration of light storage for times greater than a
second in praseodymium doped Y$_2$SiO$_5$ using electromagnetically induced
transparency. The long storage times were enabled by the long coherence times
possible for the hyperfine transitions in this material. The use of a solid
state system also enabled operation with the probe and coupling beam counter
propagating, allowing easy separation of the two beams. The efficiency of the
storage was low because of the low optical thickness of the sample, as is
discussed this deficiency should be easy to rectify.


title:
Microfabricated high-finesse optical cavity with open access and small
  volume

abstract:
  We present a novel microfabricated optical cavity, which combines a very
small mode volume with high finesse. In contrast to other micro-resonators,
such as microspheres, the structure we have built gives atoms and molecules
direct access to the high-intensity part of the field mode, enabling them to
interact strongly with photons in the cavity for the purposes of detection and
quantum-coherent manipulation. Light couples directly in and out of the
resonator through an optical fibre, avoiding the need for sensitive coupling
optics. This renders the cavity particularly attractive as a component of a
lab-on-a-chip, and as a node in a quantum network.


title: NMR analogues of the quantum Zeno effect

abstract:
  We describe Nuclear Magnetic Resonance (NMR) demonstrations of the quantum
Zeno effect, and discuss briefly how these are related to similar phenomena in
more conventional NMR experiments.


title: Entanglement between Collective Operators in a Linear Harmonic Chain

abstract:
  We investigate entanglement between collective operators of two blocks of
oscillators in an infinite linear harmonic chain. These operators are defined
as averages over local operators (individual oscillators) in the blocks. On the
one hand, this approach of "physical blocks" meets realistic experimental
conditions, where measurement apparatuses do not interact with single
oscillators but rather with a whole bunch of them, i.e., where in contrast to
usually studied "mathematical blocks" not every possible measurement is
allowed. On the other, this formalism naturally allows the generalization to
blocks which may consist of several non-contiguous regions. We quantify
entanglement between the collective operators by a measure based on the
Peres-Horodecki criterion and show how it can be extracted and transferred to
two qubits. Entanglement between two blocks is found even in the case where
none of the oscillators from one block is entangled with an oscillator from the
other, showing genuine bipartite entanglement between collective operators.
Allowing the blocks to consist of a periodic sequence of subblocks, we verify
that entanglement scales at most with the total boundary region. We also apply
the approach of collective operators to scalar quantum field theory.


title:
Microwave emission from a crystal of molecular magnets -- The role of a
  resonant cavity

abstract:
  We discuss the effects caused by a resonant cavity around a sample of a
magnetic molecular crystal (such as Mn${}_{12}$-Ac), when a time dependent
external magnetic field is applied parallel to the easy axis of the crystal. We
show that the back action of the cavity field on the sample significantly
increases the possibility of microwave emission. This radiation process can be
supperradiance or a maser-like effect, depending on the strength of the
dephasing. Our model provides further insight to the theoretical understanding
of the bursts of electromagnetic radiation observed in recent experiments
accompanying the resonant quantum tunneling of magnetization. The experimental
findings up to now can all be explained as being a maser effect rather than
superradiance. The results of our theory scale similarly to the experimental
findings, i.e., with increasing sweep rate of the external magnetic field, the
emission peaks are shifted towards higher field values.


title: Large-uncertainty intelligent states for angular momentum and angle

abstract:
  The equality in the uncertainty principle for linear momentum and position is
obtained for states which also minimize the uncertainty product. However, in
the uncertainty relation for angular momentum and angular position both sides
of the inequality are state dependent and therefore the intelligent states,
which satisfy the equality, do not necessarily give a minimum for the
uncertainty product. In this paper, we highlight the difference between
intelligent states and minimum uncertainty states by investigating a class of
intelligent states which obey the equality in the angular uncertainty relation
while having an arbitrarily large uncertainty product. To develop an
understanding for the uncertainties of angle and angular momentum for the
large-uncertainty intelligent states we compare exact solutions with analytical
approximations in two limiting cases.


title:
Adventures of the Coupled Yang-Mills Oscillators: II. YM-Higgs Quantum
  Mechanics

abstract:
  We continue our study of the quantum mechanical motion in the $x^2y^2$
potentials for $n=2,3$, which arise in the spatially homogeneous limit of the
Yang-Mills (YM) equations. In the present paper, we develop a new approach to
the calculation of the partition function $Z(t)$ beyond the Thomas-Fermi (TF)
approximation by adding a harmonic (Higgs) potential and taking the limit $v\to
0$, where $v$ is the vacuum expectation value of the Higgs field. Using the
Wigner-Kirkwood method to calculate higher-order corrections in $\hbar$, we
show that the limit $v\to 0$ leads to power-like singularities of the type
$v^{-n}$, which reflect the possibility of escape of the particle along the
channels in the classical limit. We show how these singularities can be
eliminated by taking into account the quantum fluctuations dictated by the form
of the potential.


title: Angular EPR paradox

abstract:
  The violation of local uncertainty relations is a valuable tool for detecting
entanglement, especially in multi-dimensional systems. The orbital angular
momentum of light provides such a multi-dimensional system. We study quantum
correlations for the conjugate variables of orbital angular momentum and
angular position. We determine an experimentally testable criterion for the
demonstration of an angular version of the EPR paradox. For the interpretation
of future experimental results from our proposed setup, we include a model for
the indeterminacies inherent to the angular position measurement. For this
measurement angular apertures are used to determine the probability density of
the angle. We show that for a class of aperture functions a demonstration of an
angular EPR paradox, according to our criterion, is to be expected.


title: Classification of n-qubit states with minimum orbit dimension

abstract:
  The group of local unitary transformations acts on the space of n-qubit pure
states, decomposing it into orbits. In a previous paper we proved that a
product of singlet states (together with an unentangled qubit for a system with
an odd number of qubits) achieves the smallest possible orbit dimension, equal
to 3n/2 for n even and (3n + 1)/2 for n odd, where n is the number of qubits.
In this paper we show that any state with minimum orbit dimension must be of
this form, and furthermore, such states are classified up to local unitary
equivalence by the sets of pairs of qubits entangled in singlets.


title:
Quantum error correction of systematic errors using a quantum search
  framework

abstract:
  Composite pulses are a quantum control technique for canceling out systematic
control errors. We present a new composite pulse sequence inspired by quantum
search. Our technique can correct a wider variety of systematic errors --
including, for example, nonlinear over-rotational errors -- than previous
techniques. Concatenation of the pulse sequence can reduce a systematic error
to an arbitrarily small level.


title: The de Broglie-Bohm pilot-wave interpretation of quantum theory

abstract:
  In this thesis we study the de Broglie-Bohm pilot-wave interpretation of
quantum theory. We consider the domain of non-relativistic quantum theory,
relativistic quantum theory and quantum field theory, and in each domain we
consider the possibility of formulating a pilot-wave interpretation. For
non-relativistic quantum theory a pilot-wave interpretation in terms of
particle beables can readily be formulated. But this interpretation can in
general not straightforwardly be generalized to relativistic wave equations.
The problems which prevent us from devising a pilot-wave interpretation for
relativistic wave equations also plague the standard quantum mechanical
interpretation, where these problems led to the conception of quantum field
theory. Therefore most of our attention is focussed on the construction of a
pilot-wave interpretation for quantum field theory. We thereby favour the field
beable approach, developed amongst others by Bohm, Hiley, Holland, Kaloyerou
and Valentini. Although the field beable approach can be successfully applied
to bosonic quantum field theory, it seems not straightforward to do so for
fermionic quantum field theory.


title: Type-II Quantum Algorithms

abstract:
  We review and analyze the hybrid quantum-classical NMR computing methodology
referred to as Type-II quantum computing. We show that all such algorithms
considered so far within this paradigm are equivalent to some classical
lattice-Boltzmann scheme. We derive a sufficient and necessary constraint on
the unitary operator representing the quantum mechanical part of the
computation which ensures that the model reproduces the Boltzmann approximation
of a lattice-gas model satisfying semi-detailed balance. Models which do not
satisfy this constraint represent new lattice-Boltzmann schemes which cannot be
formulated as the average over some underlying lattice gas. We close the paper
with some discussion of the strengths, weaknesses and possible future direction
of Type-II quantum computing.


title:
A simple scheme of teleportation of arbitrary multipartite qubit
  entanglement

abstract:
  In this paper, we define a cross product operator and construct the cross
Bell basis, by use this basis and Bell measurements we give a simple scheme of
the teleportation of arbitrary multipartite qubit entanglement.


title: Security proof of practical quantum key distribution schemes

abstract:
  This paper provides a security proof of the Bennett-Brassard (BB84) quantum
key distribution protocol in practical implementation. To prove the security,
it is not assumed that defects in the devices are absorbed into an adversary's
attack. In fact, the only assumption in the proof is that the source is
characterized. The proof is performed by lower-bounding adversary's Renyi
entropy about the key before privacy amplification. The bound reveals the
leading factors reducing the key generation rate.


title: Entanglement Effect on Off-diagonal Geometric Phase

abstract:
  The effect of entanglement on off-diagonal geometric phases is investigated
in the paper. Two spin-1/2 particles in magnetic fields along the $y$ direction
are taken as an example. Three parameters (the purity of state $r$, the mixing
angle $\theta$ and the relative phase $\beta$) are chosen to characterize the
initial states. The nodal points at which the usual geometric phases disappear
are calculated and illustrated as a function of the three parameters.


title: Quantum-classical correspondence in the oracle model of computation

abstract:
  The oracle model of computation is believed to allow a rigorous proof of
quantum over classical computational superiority. Since quantum and classical
oracles are essentially different, a correspondence principle is commonly
implicitly used as a platform for comparison of oracle complexity. Here, we
question the grounds on which this correspondence is based. Obviously, results
on quantum speed-up depend on the chosen correspondence. So, we introduce the
notion of genuine quantum speed-up which can serve as a tool for reliable
comparison of quantum vs classical complexity, independently of the chosen
correspondence principle.


title: Extended Jaynes-Cummings models and (quasi)-exact solvability

abstract:
  The original Jaynes-Cummings model is described by a Hamiltonian which is
exactly solvable. Here we extend this model by several types of interactions
leading to a nonhermitian operator which doesn't satisfy the physical condition
of space-time reflection symmetry (PT symmetry). However the new Hamiltonians
are either exactly solvable admitting an entirely real spectrum or quasi
exactly solvable with a real algebraic part of their spectrum.


title: Single-copy entanglement in critical spin chains

abstract:
  We introduce the single-copy entanglement as a quantity to assess quantum
correlations in the ground state in quantum many-body systems. We show for a
large class of models that already on the level of single specimens of spin
chains, criticality is accompanied with the possibility of distilling a
maximally entangled state of arbitrary dimension from a sufficiently large
block deterministically, with local operations and classical communication.
These analytical results -- which refine previous results on the divergence of
block entropy as the rate at which EPR pairs can be distilled from many
identically prepared chains, and which apply to single systems as encountered
in actual experimental situations -- are made quantitative for general
isotropic translationally invariant spin chains that can be mapped onto a
quasi-free fermionic system, and for the anisotropic XY model. For the XX
model, we provide the asymptotic scaling of ~(1/6) log_2(L), and contrast it
with the block entropy. The role of superselection rules on single-copy
entanglement in systems consisting of indistinguishable particles is
emphasized.


title: Superbroadcasting of mixed states

abstract:
  We derive the optimal universal broadcasting for mixed states of qubits. We
show that the nobroadcasting theorem cannot be generalized to more than a
single input copy. Moreover, for four or more input copies it is even possible
to purify the input states while broadcasting. We name such purifying
broadcasting superbroadcasting.


title: CHSH violation and entropy - concurrence plane

abstract:
  We characterize violation of CHSH inequalities for mixed two-qubit states by
their mixedness and entanglement. The class of states that have maximum degree
of CHSH violation for a given linear entropy is also constructed.


title:
Experimental demonstration of fractional orbital angular momentum
  entanglement of two photons

abstract:
  The singular nature of a non-integer spiral phase plate allows easy
manipulation of spatial degrees of freedom of photon states. Using two such
devices, we have observed very high dimensional (D > 3700) spatial entanglement
of twin photons generated by spontaneous parametric down-conversion.


title: Quantum dynamical entropies for discrete classical systems: a comparison

abstract:
  On a family of classical dynamical systems on the 2-torus, we perform a
discretization procedure similar to the Anti-Wick quantization. Such a
discretization is performed by using a particular class of states, fulfilling
an appropriate dynamical localization property, typical of quantum Coherent
States. The same set of states is involved in the construction of a quantum
entropy, that we test on the discrete approximants; a correspondence with the
classical metric entropy of Kolmogorov-Sinai is found only over time scales
that are logarithmic in the discretization parameter.


title: Quantum Passwords

abstract:
  A quantum password is a quantum mechanical analogue of the classical
password. Our proposal is completely quantum mechanical in nature, i.e. at no
point is information stored and manipulated classically. We show that, in
contrast to quantum protocols that encode classical information, we are able to
prevent the distribution of reusable passwords even when Alice actively
cooperates with Eve. This allows us to confront and address security issues
that are unavoidable in classical protocols.


title:
Numerical Analysis of Coherent Many-Body Currents in a Single Atom
  Transistor

abstract:
  We study the dynamics of many atoms in the recently proposed Single Atom
Transistor setup [A. Micheli, A. J. Daley, D. Jaksch, and P. Zoller, Phys. Rev.
Lett. 93, 140408 (2004)] using recently developed numerical methods. In this
setup, a localised spin 1/2 impurity is used to switch the transport of atoms
in a 1D optical lattice: in one state the impurity is transparent to probe
atoms, but in the other acts as a single atom mirror. We calculate
time-dependent currents for bosons passing the impurity atom, and find
interesting many body effects. These include substantially different transport
properties for bosons in the strongly interacting (Tonks) regime when compared
with fermions, and an unexpected decrease in the current when weakly
interacting probe atoms are initially accelerated to a non-zero mean momentum.
We also provide more insight into the application of our numerical methods to
this system, and discuss open questions about the currents approached by the
system on long timescales.


title:
Rapid optimization of working parameters of microwave-driven multi-level
  qubits for minimal gate leakage

abstract:
  We propose an effective method to optimize the working parameters (WPs) of
microwave-driven quantum logical gates implemented with multi-level physical
qubits. We show that by treating transitions between each pair of levels
independently, intrinsic gate errors due primarily to population leakage to
undesired states can be estimated accurately from spectroscopic properties of
the qubits and minimized by choosing appropriate WPs. The validity and
efficiency of the approach are demonstrated by applying it to optimize the WPs
of two coupled rf SQUID flux qubits for controlled-NOT (CNOT) operation. The
result of this independent transition approximation (ITA) is in good agreement
with that of dynamic method (DM). Furthermore, the ratio of the speed of ITA to
that of DM scales exponentially as 2^n when the number of qubits n increases.


title:
Non-Markov dynamics and phonon decoherence of a double quantum dot
  charge qubit

abstract:
  In this paper we investigate decoherence times of a double quantum dot (DQD)
charge qubit due to it coupling with acoustic phonon baths. We individually
consider the acoustic piezoelectric as well as deformation coupling phonon
baths in the qubit environment. The decoherence times are calculated with two
kinds of methods. One of them is based on the qusiadiabatic propagator path
integral (QUAPI) and the other is based on Bloch equations, and two kinds of
results are compared. It is shown that the theoretical decoherence times of the
DQD charge qubit are shorter than the experimental reported results. It implies
that the phonon couplings to the qubit play a subordinate role, resulting in
the decoherence of the qubit.


title: Relaxation of Pseudo pure states : The Role of Cross-Correlations

abstract:
  In Quantum Information Processing by NMR one of the major challenges is
relaxation or decoherence. Often it is found that the equilibrium mixed state
of a spin system is not suitable as an initial state for computation and a
definite initial state is required to be prepared prior to the computation. As
these preferred initial states are non-equilibrium states, they are not
stationary and are destroyed with time as the spin system relaxes toward its
equilibrium, introducing error in computation. Since it is not possible to cut
off the relaxation processes completely, attempts are going on to develop
alternate strategies like Quantum Error Correction Codes or Noiseless
Subsystems. Here we study the relaxation behavior of various Pseudo Pure States
and analyze the role of Cross terms between different relaxation processes,
known as Cross-correlation. It is found that while cross-correlations
accelerate the relaxation of certain pseudo pure states, they retard that of
others.


title:
Random subspaces for encryption based on a private shared Cartesian
  frame

abstract:
  A private shared Cartesian frame is a novel form of private shared
correlation that allows for both private classical and quantum communication.
Cryptography using a private shared Cartesian frame has the remarkable property
that asymptotically, if perfect privacy is demanded, the private classical
capacity is three times the private quantum capacity. We demonstrate that if
the requirement for perfect privacy is relaxed, then it is possible to use the
properties of random subspaces to nearly triple the private quantum capacity,
almost closing the gap between the private classical and quantum capacities.


title:
A unified approach to realize universal quantum gates in a coupled
  two-qubit system with fixed always-on coupling

abstract:
  We demonstrate that in a coupled two-qubit system any single-qubit gate can
be decomposed into two conditional two-qubit gates and that any conditional
two-qubit gate can be implemented by a manipulation analogous to that used for
a controlled two-qubit gate. Based on this we present a unified approach to
implement universal single-qubit and two-qubit gates in a coupled two-qubit
system with fixed always-on coupling. This approach requires neither
supplementary circuit or additional physical qubits to control the coupling nor
extra hardware to adjust the energy level structure. The feasibility of this
approach is demonstrated by numerical simulation of single-qubit gates and
creation of two-qubit Bell states in rf-driven inductively coupled two SQUID
flux qubits with realistic device parameters and constant always-on coupling.


title:
Demonstration of a simple entangling optical gate and its use in
  Bell-state analysis

abstract:
  We demonstrate a new architecture for an optical entangling gate that is
significantly simpler than previous realisations, using partially-polarising
beamsplitters so that only a single optical mode-matching condition is
required. We demonstrate operation of a controlled-Z gate in both
continuous-wave and pulsed regimes of operation, fully characterising it in
each case using quantum process tomography. We also demonstrate a
fully-resolving, nondeterministic optical Bell-state analyser based on this
controlled-Z gate. This new architecture is ideally suited to guided optics
implementations of optical gates.


title:
Demonstration of an optical quantum controlled-NOT gate without path
  interference

abstract:
  We report the first experimental demonstration of an optical quantum
controlled-NOT gate without any path interference, where the two interacting
path interferometers of the original proposals (Phys. Rev. A {\bf 66}, 024308
(2001), Phys. Rev. A {\bf 65}, 012314 (2002)) have been replaced by three
partially polarizing beam splitters with suitable polarization dependent
transmittances and reflectances. The performance of the device is evaluated
using a recently proposed method (Phys. Rev. Lett. {\bf 94}, 160504 (2005)), by
which the quantum process fidelity and the entanglement capability can be
estimated from the 32 measurement results of two classical truth tables,
significantly less than the 256 measurement results required for full quantum
tomography.


title: Quantum noise memory effect of multiple scattered light

abstract:
  We investigate frequency correlations in multiple scattered light that are
present in the quantum fluctuations. The memory effect for quantum and
classical noise is compared, and found to have markedly different frequency
scaling, which was confirmed in a recent experiment. Furthermore, novel
mesoscopic correlations are predicted that depend on the photon statistics of
the incoming light.


title: Quantum Complexity of Testing Group Commutativity

abstract:
  We consider the problem of testing the commutativity of a black-box group
specified by its k generators. The complexity (in terms of k) of this problem
was first considered by Pak, who gave a randomized algorithm involving O(k)
group operations. We construct a quite optimal quantum algorithm for this
problem whose complexity is in O (k^{2/3}). The algorithm uses and highlights
the power of the quantization method of Szegedy. For the lower bound of
Omega(k^{2/3}), we give a reduction from a special case of Element Distinctness
to our problem. Along the way, we prove the optimality of the algorithm of Pak
for the randomized model.


title: The Second Quantized Quantum Turing Machine and Kolmogorov Complexity

abstract:
  The Kolmogorov complexity of a physical state is the minimal physical
resources required to reproduce that state. We define a second quantized
quantum Turing machine and use it to define second quantized Kolmogorov
complexity. There are two advantages to our approach - our measure of second
quantized Kolmogorov complexity is closer to physical reality and unlike other
quantum Kolmogorov complexities it is continuous. We give examples where second
quantized and quantum Kolmogorov complexity differ.


title: Optimal estimation of group transformations using entanglement

abstract:
  We derive the optimal input states and the optimal quantum measurements for
estimating the unitary action of a given symmetry group, showing how the
optimal performance is obtained with a suitable use of entanglement. Optimality
is defined in a Bayesian sense, as minimization of the average value of a given
cost function. We introduce a class of cost functions that generalizes the
Holevo class for phase estimation, and show that for states of the optimal form
all functions in such a class lead to the same optimal measurement. A first
application of the main result is the complete proof of the optimal efficiency
in the transmission of a Cartesian reference frame. As a second application, we
derive the optimal estimation of a completely unknown two-qubit maximally
entangled state, provided that N copies of the state are available. In the
limit of large N, the fidelity of the optimal estimation is shown to be
1-3/(4N).


title:
Feedback stabilization of quantum ensembles: a global convergence
  analysis on complex flag manifolds

abstract:
  In an N-level quantum mechanical system, the problem of unitary feedback
stabilization of mixed density operators to periodic orbits admits a natural
Lyapunov-based time-varying feedback design. A global description of the domain
of attraction of the closed-loop system can be provided based on a
``root-space''-like structure of the space of density operators. This convex
set foliates as a complex flag manifold where each leaf is identified with the
coadjoint orbit of the eigenvalues of the density operator. The converging
conditions are time-independent but depend from the topology of the flag
manifold: it is shown that the closed loop must have a number of equilibria at
least equal to the Euler characteristic of the manifold, thus imposing
obstructions of topological nature to global stabilizability.


title: Linear Optics C-Phase gate made simple

abstract:
  Linear optics quantum logic gates are the best tool to generate multi-photon
entanglement. Simplifying a recent approach [Phys. Rev. A 65, 062324; Phys.
Rev. A 66, 024308] we were able to implement the conditional phase gate with
only one second order interference at a polarization dependent beam splitter,
thereby significantly increasing its stability. The improved quality of the
gate is evaluated by analysing its entangling capability and by performing full
process tomography. The achieved results ensure that this device is well suited
for implementation in various multi photon quantum information protocols.


title:
Spin-1/2 particles moving on a 2D lattice with nearest-neighbor
  interactions can realize an autonomous quantum computer

abstract:
  What is the simplest Hamiltonian which can implement quantum computation
without requiring any control operations during the computation process? In a
previous paper we have constructed a 10-local finite-range interaction among
qubits on a 2D lattice having this property. Here we show that
pair-interactions among qutrits on a 2D lattice are sufficient, too, and can
also implement an ergodic computer where the result can be read out from the
time average state after some post-selection with high success probability.
  Two of the 3 qutrit states are given by the two levels of a spin-1/2 particle
located at a specific lattice site, the third state is its absence. Usual
hopping terms together with an attractive force among adjacent particles induce
a coupled quantum walk where the particle spins are subjected to spatially
inhomogeneous interactions implementing holonomic quantum computing. The
holonomic method ensures that the implemented circuit does not depend on the
time needed for the walk.
  Even though the implementation of the required type of spin-spin interactions
is currently unclear, the model shows that quite simple Hamiltonians are
powerful enough to allow for universal quantum computing in a closed physical
system.


title: Some differences between Dirac's hole theory and quantum field theory

abstract:
  Diracs hole theory (HT) and quantum field theory (QFT) are generally
considered to be equivalent to each other. However, it has been recently shown
by several researchers that this is not necessarily the case. When the change
in the vacuum energy was calculated for a time independent perturbation HT and
QFT yielded different results. In this paper we extend this discussion to
include a time dependent perturbation for which the exact solution to the Dirac
equation is known. It will be shown that for this case, also, HT and QFT yield
different results. In addition, there will be some discussion of the problem of
anomalies in QFT.


title: Undecidable classical properties of observers

abstract:
  A property of a system is called actual, if the observation of the test that
pertains to that property, yields an affirmation with certainty. We formalize
the act of observation by assuming that the outcome correlates with the state
of the observed system and is codified as an actual property of the state of
the observer at the end of the measurement interaction. For an actual property,
the observed outcome has to affirm that property with certainty, hence in this
case the correlation needs to be perfect. A property is called classical if
either the property or its negation is actual. It is shown by a diagonal
argument that there exist classical properties of an observer that he cannot
observe perfectly. Because states are identified with the collection of
properties that are actual for that state, it follows that no observer can
perfectly observe his own state. Implications for the quantum measurement
problem are briefly discussed.


title:
Comment on "A Random Quantum Key Distribution Achieved by Using Bell
  States"

abstract:
  In a recent paper [J. Opt. B: Quantum Semiclass. Opt. 5 (2003) 155-157], a
quantum key distribution scheme based on entanglement swapping was proposed,
which exhibited two improvements over the previous protocols. In this Comment,
it is shown that the scheme has no properties as been discussed.


title: Monte Carlo techniques for real-time quantum dynamics

abstract:
  The stochastic-gauge representation is a method of mapping the equation of
motion for the quantum mechanical density operator onto a set of equivalent
stochastic differential equations. One of the stochastic variables is termed
the "weight", and its magnitude is related to the importance of the stochastic
trajectory. We investigate the use of Monte Carlo algorithms to improve the
sampling of the weighted trajectories and thus reduce sampling error in a
simulation of quantum dynamics. The method can be applied to calculations in
real time, as well as imaginary time for which Monte Carlo algorithms are
more-commonly used. The method is applicable when the weight is guaranteed to
be real, and we demonstrate how to ensure this is the case. Examples are given
for the anharmonic oscillator, where large improvements over stochastic
sampling are observed.


title: Large N Scaling Behavior of the Lipkin-Meshkov-Glick Model

abstract:
  We introduce a novel semiclassical approach to the Lipkin model. In this way
the well-known phase transition arising at the critical value of the coupling
is intuitively understood. New results -- showing for strong couplings the
existence of a threshold energy which separates deformed from undeformed states
as well as the divergence of the density of states at the threshold energy --
are explained straightforwardly and in quantitative terms by the appearance of
a double well structure in a classical system corresponding to the Lipkin
model. Previously unnoticed features of the eigenstates near the threshold
energy are also predicted and found to hold.


title: Surface plasmon modes and the Casimir energy

abstract:
  We show the influence of surface plasmons on the Casimir effect between two
plane parallel metallic mirrors at arbitrary distances. Using the plasma model
to describe the optical response of the metal, we express the Casimir energy as
a sum of contributions associated with evanescent surface plasmon modes and
propagative cavity modes. In contrast to naive expectations, the plasmonic
modes contribution is essential at all distances in order to ensure the correct
result for the Casimir energy. One of the two plasmonic modes gives rise to a
repulsive contribution, balancing out the attractive contributions from
propagating cavity modes, while both contributions taken separately are much
larger than the actual value of the Casimir energy. This also suggests
possibilities to tailor the sign of the Casimir force via surface plasmons.


title: Properties of Fermion Spherical Harmonics

abstract:
  The Fermion Spherical harmonics [$Y_\ell^{m}(\theta,\phi)$ for
half-odd-integer $\ell$ and $m$ - presented in a previous paper] are shown to
have the same eigenfunction properties as the well-known Boson Spherical
Harmonics [$Y_\ell^{m}(\theta,\phi)$ for integer $\ell$ and $m$]. The Fermion
functions are shown to differ from the Boson functions in so far as the ladder
operators $M_+$ ($M_-$) that ascend (descend) the sequence of harmonics over
the values of $m$ for a given value of $\ell$, do not produce the expected
result {\em in just one case}: when the value of $m$ changes from $\pm{1/2}$ to
$\mp{1/2}$; i.e. when $m$ changes sign; in all other cases the ladder operators
produce the usually expected result including anihilation when a ladder
operator attempts to take $m$ outside the range: $-\ell\le m\le +\ell$.
  The unexpected result in the one case does not invalidate this scalar
coordinate representation of spin angular momentum, because the eigenfunction
property is essential for a valid quantum mechanical state, whereas ladder
operators relating states with different eigenvalues are not essential, and are
in fact known only for a few physical systems; that this coordinate
representation of spin angular momentum differs from the abstract theory of
angular momentum in this respect, is simply an interesting curiosity. This new
representation of spin angular momentum is expected to find application in the
theoretical description of physical systems and experiments in which the
spin-angular momentum (and associated magnetic moment) of a particle is
oriented in space, since the orientation is specifiable by the spherical polar
angles, $\theta$ and $\phi$.


title: Maximum likelihood estimation for a group of physical transformations

abstract:
  The maximum likelihood strategy to the estimation of group parameters allows
to derive in a general fashion optimal measurements, optimal signal states, and
their relations with other information theoretical quantities. These results
provide a deep insight into the general structure underlying optimal quantum
estimation strategies. The entanglement between representation spaces and
multiplicity spaces of the group action appear to be the unique kind of
entanglement which is really useful for the optimal estimation of group
parameters.


title: Explicit Spin Coordinates

abstract:
  The recently established existence of spherical harmonic functions,
$Y_\ell^{m}(\theta,\phi)$ for half-odd-integer values of $\ell$ and $m$, allows
for the introduction into quantum chemistry of explicit electron
spin-coordinates; i.e. spherical polar angles $\theta_s, \phi_s$, that specify
the orientation of the spin angular momentum vector in space. In this
coordinate representation the spin angular momentum operators, $S^2, S_z$, are
represented by the usual differential operators in spherical polar coordinates
(commonly used for $L^2, L_z$), and their electron-spin eigenfunctions are
$\sqrt{\sin\theta_s} \exp(\pm\phi_s/2)$. This eigenfunction representation has
the pedagogical advantage over the abstract spin eigenfunctions, $\alpha,
\beta,$ that ``integration over spin coordinates'' is a true integration (over
the angles $\theta_s, \phi_s$). In addition they facilitate construction of
many electron wavefunctions in which the electron spins are neither parallel
nor antiparallel, but inclined at an intermediate angle. In particular this may
have application to the description of EPR correlation experiments.


title: Realism in the Realized Popper's Experiment

abstract:
  The realization of Karl Popper's experiment by Shih and Kim (published 1999)
produced the result that Popper hoped for: no ``action at a distance'' on one
photon of an entangled pair when a measurement is made on the other photon.
This experimental result is interpretable in local realistic terms: each photon
has a definite position and transverse momentum most of the time; the position
measurement on one photon (localization in a slit) disturbs the transverse
momentum of that photon in a non-predictable way in accordance with the
uncertainty principle; however, there is no effect on the other photon (the
photon that is not in a slit) no action at a distance. The position measurement
(localization within a slit) of the one photon destroys the coherence
(entanglement) between the photons; i.e. decoherence occurs. This can be
understood physically as an electromagnetic interaction between the photon in
the slit and the electrons of the atoms in the surface of the solid that forms
the slit.


title:
The quantum adiabatic search with decoherence in the instantaneous
  energy eigenbasis

abstract:
  In Phys. Rev. A {\bf 71}, 060312(R) (2005) the robustness of the local
adiabatic quantum search to decoherence in the instantaneous eigenbasis of the
search Hamiltonian was examined. We expand this analysis to include the case of
the global adiabatic quantum search. As in the case of the local search the
asymptotic time complexity for the global search is the same as for the ideal
closed case, as long as the Hamiltonian dynamics is present. In the case of
pure decoherence, where the environment monitors the search Hamiltonian, we
find that the time complexity of the global quantum adiabatic search scales
like $N^{3/2}$, where $N$ is the list length. We moreover extend the analysis
to include success probabilities $p<1$ and prove bounds on the run time with
the same scaling as in the conditions for the $p\to 1$ limit. We supplement the
analytical results by numerical simulations of the global and local search.


abstract:
  The realization of Karl Popper's EPR-like experiment by Shih and Kim
(published 1999) produced the result that Popper hoped for: no ``action at a
distance'' on one photon of an entangled pair when a measurement is made on the
other photon. This experimental result is interpretable in local realistic
terms: each photon has a definite position and transverse momentum most of the
time; the position measurement on one photon (localization within a slit)
disturbs the transverse momentum of that photon in a non-predictable way in
accordance with the uncertainty principle; however, there is no effect on the
other photon (the photon that is not in a slit) no action at a distance. The
position measurement (localization within a slit) of the one photon destroys
the entanglement between the photons; i.e. decoherence occurs.


title:
Abelian and non-Abelian geometric phases in adiabatic open quantum
  systems

abstract:
  We introduce a self-consistent framework for the analysis of both Abelian and
non-Abelian geometric phases associated with open quantum systems, undergoing
cyclic adiabatic evolution. We derive a general expression for geometric
phases, based on an adiabatic approximation developed within an inherently
open-systems approach. This expression provides a natural generalization of the
analogous one for closed quantum systems, and we prove that it satisfies all
the properties one might expect of a good definition of a geometric phase,
including gauge invariance. A striking consequence is the emergence of a finite
time interval for the observation of geometric phases. The formalism is
illustrated via the canonical example of a spin-1/2 particle in a
time-dependent magnetic field. Remarkably, the geometric phase in this case is
immune to dephasing and spontaneous emission in the renormalized Hamiltonian
eigenstate basis. This result positively impacts holonomic quantum computing.


title: Entanglement in thermal equilibrium states

abstract:
  We revisist the issue of entanglement of thermal equilibrium states in
composite quantum systems. The possible scenarios are exemplified in bipartite
qubit/qubit and qubit/qutrit systems.


title: Noninformative Quantum q-Priors

abstract:
  We find, in an analysis involving four prior probabilities (p's), that the
information-theoretic-based comparative noninformativity test devised by
Clarke, and applied by Slater in a quantum setting, yields a ranking
(p_{F_{q=1}} > p_{B} > p_{B_{q=1}trunc} >p_{F}) fully consistent with
Srednicki's recently-stated criterion for priors of ``biasedness to pure
states''. Two of the priors are formed by extending certain metrics of
quantum-theoretic interest from three- to four-dimensions -- by incorporating
the q-parameter (nonextensivity/Tsallis index/escort parameter). The
three-dimensional metrics are the Bures (minimal monotone) metric over the
two-level quantum systems and the Fisher information metric over the
corresponding family of Husimi distributions. The priors p_{B} and p_{F} are
the (independent-of-q) normalized volume elements of these metrics, and
p_{F_{q=1}} is the normalized volume element of the q-extended Fisher
information metric, with q set to 1. While originally intended to similarly be
the q-extension of the Bures metric, with q then set to 1, the prior
p_{B_{q=1}trunc}, actually entails the truncation of the only off-diagonal
entry of the extended Bures metric tensor. Without this truncation, the
q-extended Bures volume element is null, as is also the case in two other
quantum scenarios we examine.


title: Biorthogonal Quantum Systems

abstract:
  Models of PT symmetric quantum mechanics provide examples of biorthogonal
quantum systems. The latter incorporporate all the structure of PT symmetric
models, and allow for generalizations, especially in situations where the PT
construction of the dual space fails. The formalism is illustrated by a few
exact results for models of the form H=(p+\nu)^2+\sum_{k>0}\mu_{k}exp(ikx). In
some non-trivial cases, equivalent hermitian theories are obtained and shown to
be very simple: They are just free (chiral) particles. Field theory extensions
are briefly considered.


title: Deterministic cloning of an unknown Bell state

abstract:
  Recently Gupta and Panigrahi have shown how to deterministically identify an
unknown Bell state. The present paper extends their result to deterministic
cloning.


title:
Thermodynamic Limits, Non-commutative Probability, and Quantum
  Entanglement

abstract:
  We construct a rigourous model of quantum measurement. A two-state model of a
negative temperature amplifier, such as a laser, is taken to a classical
thermodynamic limit. In the limit, it becomes a classical measurement apparatus
obeying the stochastic axioms of quantum mechanics. Thus we derive the
probabilities from a deterministic Schroedinger's equation by procedures
analogous to those of classical statistical mechanics. This requires making
precise the notion of `macroscopic.'


title: Some Aspects of Minimal Length Quantum Mechanics

abstract:
  String theory, quantum geometry, loop quantum gravity and black hole physics
all indicate the existence of a minimal observable length on the order of
Planck length. This feature leads to a modification of Heisenberg uncertainty
principle. Such a modified Heisenberg uncertainty principle is referred as
gravitational uncertainty principle(GUP) in literatures. This proposal has some
novel implications on various domains of theoretical physics. Here, we study
some consequences of GUP in the spirit of Quantum mechanics. We consider two
problem: a particle in an one-dimensional box and momentum space wave function
for a "free particle". In each case we will solve corresponding perturbational
equations and compare the results with ordinary solutions.


title: Wave Packets Propagation in Quantum Gravity

abstract:
  Wave packet broadening in usual quantum mechanics is a consequence of
dispersion behavior of the medium which the wave propagates in it. In this
paper, we consider the problem of wave packet broadening in the framework of
Generalized Uncertainty Principle(GUP) of quantum gravity. New dispersion
relations are derived in the context of GUP and it has been shown that there
exists a gravitational induced dispersion which leads to more broadening of the
wave packets. As a result of these dispersion relations, a generalized
Klein-Gordon equation is obtained and its interpretation is given.


title: Reversible quantum measurement with arbitrary spins

abstract:
  We propose a physically reversible quantum measurement of an arbitrary spin-s
system using a spin-j probe via an Ising interaction. In the case of a spin-1/2
system (s=1/2), we explicitly construct a reversing measurement and evaluate
the degree of reversibility in terms of fidelity. The recovery of the measured
state is pronounced when the probe has a high spin (j>1/2), because the
fidelity changes drastically during the reversible measurement and the
reversing measurement. We also show that the reversing measurement scheme for a
spin-1/2 system can serve as an experimentally feasible approximate reversing
measurement for a high-spin system (s>1/2). If the interaction is sufficiently
weak, the reversing measurement can recover a cat state almost
deterministically in spite of there being a large fidelity change.


title: Stability of Pairwise Entanglement in a Decoherent Environment

abstract:
  Consider the dynamics of a two-qubit entangled system in the decoherence
environment, we investigate the stability of pairwise entanglement under
decoherence. We find that for different decoherence models, there exist some
special class of entangled states of which the pairwise entanglement is the
most stable. The lifetime of the entanglement in these states is larger than
other states with the same initial entanglement. In addition, we also
investigate the dynamics of pairwise entanglement in the ground state of spin
models such as Heisenberg and XXY models.


title: From Dirac to Diffusion: Decoherence in Quantum Lattice Gases

abstract:
  We describe a model for the interaction of the internal (spin) degree of
freedom of a quantum lattice-gas particle with an environmental bath. We impose
the constraints that the particle-bath interaction be fixed, while the state of
the bath is random, and that the effect of the particle-bath interaction be
parity invariant. The condition of parity invariance defines a subgroup of the
unitary group of actions on the spin degree of freedom and the bath. We derive
a general constraint on the Lie algebra of the unitary group which defines this
subgroup, and hence guarantees parity invariance of the particle-bath
interaction. We show that generalizing the quantum lattice gas in this way
produces a model having both classical and quantum discrete random walks as
different limits. We present preliminary simulation results illustrating the
intermediate behavior in the presence of weak quantum noise.


title: Architecture-Dependent Execution Time of Shor's Algorithm

abstract:
  We show how the execution time of algorithms on quantum computers depends on
the architecture of the quantum computer, the choice of algorithms (including
subroutines such as arithmetic), and the ``clock speed'' of the quantum
computer. The primary architectural features of interest are the ability to
execute multiple gates concurrently, the number of application-level qubits
available, and the interconnection network of qubits. We analyze Shor's
algorithm for factoring large numbers in this context. Our results show that,
if arbitrary interconnection of qubits is possible, a machine with an
application-level clock speed of as low as one-third of a (possibly encoded)
gate per second could factor a 576-bit number in under one month, potentially
outperforming a large network of classical computers. For nearest-neighbor-only
architectures, a clock speed of around twenty-seven gates per second is
required.


title: Normal covariant quantization maps

abstract:
  We consider questions related to quantizing complex valued functions defined
on a locally compact topological group. In the case of bounded functions, we
generalize R. Werner's approach to prove the characterization of the associated
normal covariant quantization maps.


title: Entanglement, chaos and atomic Wigner function of the Dicke model

abstract:
  We calculate the atomic (spin) Wigner function for the single mode Dicke
model in the regime of large number of two-level atoms. The dynamics of this
quasi-probability function on the Bloch sphere allows us to visualize the
consequences of the entanglement process between the boson and the spin
subsystems. Such investigation shows a distinct localization behavior of the
spin state with respect to the polar and azimuthal Bloch sphere angles. A
complete {\em breakdown of reflection symmetry} in the azimuthal angle is shown
in the non-integrable case, even at short evolution times. Also, in the
classically chaotic situation, the appearance of {\em sub-planck structures} in
the Wigner function is shown, and its evolution analyzed.


title: Quantum Entanglement and fixed point Hopf bifurcation

abstract:
  We present the qualitative differences in the phase transitions of the
mono-mode Dicke model in its integrable and chaotic versions. We show that a
first order phase transition occurs in the integrable case whereas a second
order in the chaotic one. This difference is also reflected in the classical
limit: for the integrable case the stable fixed point in phase space suffers a
bifurcation of Hopf type whereas for the second one a pitchfork type
bifurcation has been reported.


title: Complete disentanglement by partial pure dephasing

abstract:
  We study the effect of pure dephasing on the entanglement of a pair of
two-level subsystems (qubits). We show that partial dephasing induced by a
super-Ohmic reservoir, corresponding to well-established properties of confined
charge states and phonons in semiconductors, may lead to complete
disentanglement. We show also that the disentanglement effect increases with
growing distance between the two subsystems.


title: On Realism and Quantum Mechanics

abstract:
  A discussion of the quantum mechanical use of superposition or entangled
states shows that descriptions containing only statements about state vectors
and experiments outputs are the most suitable for Quantum Mechanics. In
particular, it is shown that statements about the undefined values of physical
quantities before measurement can be dropped without changing the predictions
of the theory. If we apply these ideas to EPR issues, we find that the concept
of non-locality with its 'instantaneous action at a distance' evaporates.
Finally, it is argued that usual treatments of philosophical realist positions
end up in the construction of theories whose major role is that of being
disproved by experiment. This confutation proves simply that the theories are
wrong; no conclusion about realism (or any other philosophical position) can be
drawn, since experiments deal always with theories and these are never logical
consequences of philosophical positions.


title: Entanglement measurement based on two-particle interference

abstract:
  We propose a simple and realizable method using a two-particle interferometer
for the experimental measurement of pairwise entanglement, assuming some prior
knowledge about the quantum state. The basic idea is that the properties of the
density matrix can be revealed by the single- and two-particle interference
patterns. The scheme can easily be implemented with polarized entangled
photons.


title:
On the Connection Between Entanglement and the Speed of Quantum
  Evolution

abstract:
  It has been recently pointed out [V. Giovanetti, S. Lloyd, and L. Maccone,
Europhys. Lett., {\bf 62} pp. 615-621 (2003)] that, for certain classes of
states, quantum entanglement enhances the "speed" of evolution of composite
quantum systems, as measured by the time a given initial state requires to
evolve to an orthogonal state. We provide here a systematic study of this
effect for pure states of bipartite systems of low dimensionality, considering
both distinguishable (two-qubits) subsystems, and systems constituted of two
indistinguishable particles.


title:
Analysis of localized Schmidt decomposition modes and of entanglement in
  atomic and optical quantum systems with continuous variables

abstract:
  We investigate the procedure of Schmidt modes extraction in systems with
continuous variables. An algorithm based on singular value matrix decomposition
is applied to the study of entanglement in an "atom-photon" system with
spontaneous radiation. Also, this algorithm is applied to the study of a
bi-photon system with spontaneous parametric down conversion with type-II phase
matching for broadband pump. We demonstrate that dynamic properties of
entangled states in an atom-photon system with spontaneous radiation are
defined by a parameter equal to the product of the fine structure constant and
the atom-electron mass ratio. We then consider the evolution of the system
during radiation and show that the atomic and photonic degrees of freedom are
entangling for the times of the same order of magnitude as the excited state
life-time. Then the degrees of freedom are de-entangling and asymptotically
approach to the level of small residual entanglement that is caused by momentum
dispersion of the initial atomic packet.Finally, we investigate the process of
coherence loss between modes in type-II parametric down conversion that is
caused by non-linear crystal properties.


title:
On a local hidden-variable model with `isolato' hypothesis of the
  EPR--Bohm Gedanken experiment

abstract:
  A local hidden-variable model based on `isolato' hypothesis of the EPR--Bohm
Gedanken experiment is presented. The `isolato' hypothesis states that one of a
pair of spin-${1}/{2}$ particles can be in `isolato' mode in which the
spin-${1}/{2}$ particle shuts itself from any external interactions, and hence
it can never be detected. As a result of this, data rejection is made; Bell's
inequality is violated, although the model is genuinely local. In this model,
$2/\pi$ ($\simeq$ 63.6%) of the initially prepared ensemble of pairs of
spin-${1}/{2}$ particles are detected as a pair of particles, and others are
detected as a single particle. This does not disagree with the results of the
experiments performed before, since these single spin-${1}/{2}$ particles were
regarded as noise.


title: Quantum breathers in a nonlinear Klein Gordon lattice

abstract:
  The quantum modes of a nonlinear Klein Gordon lattice have been computed
numerically [L. Proville, Phys. Rev. B, vol. 71, 104306 (2005)]. The on-site
nonlinearity has been found to lead to a phonon pairing and consequently some
phonon bound states. In the present paper, the time dependent Wannier transform
of these states is shown to exhibit a breather-like behavior, i.e., it is
spatially localized and time-periodic. The typical time the lattice may sustain
such breather states is studied as a function of the trapped energy and the
intersite lattice coupling.


title:
Local distinguishability of quantum states in infinite dimensional
  systems

abstract:
  We investigate local distinguishability of quantum states by use of the
convex analysis about joint numerical range of operators on a Hilbert space. We
show that any two orthogonal pure states are distinguishable by local
operations and classical communications, even for infinite dimensional systems.
An estimate of the local discrimination probability is also given for some
family of more than two pure states.


abstract:
  It has been shown that finding generic Bell states diagonal entanglement
witnesses (BDEW) for $d_{1}\otimes d_{2}\otimes ....\otimes d_{n}$ systems
exactly reduces to a linear programming if the feasible region be a polygon by
itself and approximately obtains via linear programming if the feasible region
is not a polygon. Since solving linear programming for generic case is
difficult, the multi-qubits, $2\otimes N$ and $3 \otimes 3$ systems for the
special case of generic BDEW for some particular choice of their parameters
have been considered. In the rest of this paper we obtain the optimal non
decomposable entanglement witness for $3 \otimes 3$ system for some particular
choice of its parameters. By proving the optimality of the well known reduction
map and combining it with the optimal and non-decomposable 3 $\otimes$ 3 BDEW
(named critical entanglement witnesses) the family of optimal and
non-decomposable 3 $\otimes$ 3 BDEW have also been obtained. Using the
approximately critical entanglement witnesses, some 3 $\otimes$ 3 bound
entangled states are so detected. So the well known Choi map as a particular
case of the positive map in connection with this witness via Jamiolkowski
isomorphism has been considered which approximately is obtained via linear
programming.


title:
Loss tolerance in one-way quantum computation via counterfactual error
  correction

abstract:
  We introduce a scheme for fault tolerantly dealing with losses (or other
"leakage" errors) in cluster state computation that can tolerate up to 50%
qubit loss. This is achieved passively using an adaptive strategy of
measurement - no coherent measurements or coherent correction is required.
Since the scheme relies on inferring information about what would have been the
outcome of a measurement had one been able to carry it out, we call this
"counterfactual" error correction.


title: Generation and Manipulation of Entanglement in Quantum Optical Systems

abstract:
  In this thesis, several new procedures for the generation and manipulation of
entangled states in quantum optical systems are introduced. Each is evaluated
in terms of realistic models of the imperfect apparatus of any real laboratory
implementation. The first half of the thesis considers entanglement generation
in Cavity QED, introducing several proposals for the creation of maximally
entangled two-qubit states. The second half of the thesis focuses on the
manipulation of entangled states of light pulses. In particular, an
entanglement distillation procedure for Gaussian states is introduced. This
combines a Procrustean protocol for the generation of highly entangled
non-Gaussian states, with a "Gaussification" procedure, which allows more
highly entangled approximately Gaussian states to be distilled from a
non-Gaussian supply.


title: On quantization of systems with second class constraints

abstract:
  It is shown that quantization of the dynamical systems with second class
constraints actually can be reduced to quantization of the systems with first
class constraints. The motion of the non-relativistic particle along the plane
curve and on a surface is considered. The results coincide with those of the
"thin layer method". Influence of the non-physical variables on the physical
sector is demonstrated.


title: Information Entropy, Information Distances and Complexity in Atoms

abstract:
  Shannon information entropies in position and momentum spaces and their sum
are calculated as functions of Z (Z=2-54) in atoms. Roothaan-Hartree-Fock
electron wave functions are used. The universal property S=a+b lnZ is verified.
In addition, we calculate the Kullback-Leibler relative entropy, the
Jensen-Shannon divergence, Onicescu's information energy and a complexity
measure recently proposed. Shell effects at closed shells atoms are observed.
The complexity measure shows local minima at the closed shells atoms indicating
that for the above atoms complexity decreases with respect to neighboring
atoms. It is seen that complexity fluctuates around an average value,
indicating that the atom cannot grow in complexity as Z increases. Onicescu's
information energy is correlated with the ionization potential. Kullback
distance and Jensen-Shannon distance are employed to compare
Roothaan-Hartree-Fock density distributions with other densities of previous
works.


title: Popper's test of quantum mechanics and two-photon "ghost" diffraction

abstract:
  A test on quantum mechanics proposed long ago by Karl Popper is reconsidered
with further detail and new insight. An ambiguity in the proposal, which turns
out to be essential in order to make the test conclusive, is identified and
taken into account for the first time. Its implications for recently performed
photon experiments [such as in D. V. Strekalov \textit{et al.}, Phys. Rev.\
Lett. {\bf 74}, 3600 (1995)] are briefly analyzed.


title: Classification of Tripartite Entanglement with one Qubit

abstract:
  We present a method to find the decompositions of tripartite entangled pure
states which are smaller than two successive Schmidt decompositions. The method
becomes very simple when one of the subsystems is a qubit. In this particular
case, we get a classification of states according to their decompositions.
Furthermore, we also use this method to classify the entangled states that can
be inter-converted through stochastic local operations and classical
communication (SLOCC). More general tripartite systems are briefly discussed.


title:
Casimir Effects: an Optical Approach II. Local Observables and Thermal
  Corrections

abstract:
  We recently proposed a new approach to the Casimir effect based on classical
ray optics (the "optical approximation"). In this paper we show how to use it
to calculate the local observables of the field theory. In particular we study
the energy-momentum tensor and the Casimir pressure. We work three examples in
detail: parallel plates, the Casimir pendulum and a sphere opposite a plate. We
also show how to calculate thermal corrections, proving that the high
temperature `classical limit' is indeed valid for any smooth geometry.


title:
Quantum stream cipher by Yuen 2000 protocol: Design and experiment by
  intensity modulation scheme

abstract:
  This paper shall investigate Yuen protocol, so called Y-00, which can realize
a randomized stream cipher with high bit rate(Gbps) for long distance(several
hundreds km). The randomized stream cipher with randomization by quantum noise
based on Y-00 is called quantum stream cipher in this paper, and it may have
security against known plaintext attacks which has no analog with any
conventional symmetric key ciphers. We present a simple cryptanalysis based on
an attacker's heterodyne measurement and the quantum unambiguous measurement to
make clear the strength of Y-00 in real communication. In addition, we give a
design for the implementation of an intensity modulation scheme and report the
experimental demonstration of 1 Gbps quantum stream cipher through 20 km long
transmission line.


title:
On the significance of a recent experiment demonstrating quantum
  interference in time

abstract:
  I comment on the interpretation of a recent experiment showing quantum
interference in time. It is pointed out that the standard nonrelativistic
quantum theory, used by the authors in their analysis, cannot account for the
results found, and therefore that this experiment has fundamental importance
beyond the technical advances it represents. Some theoretical structures which
consider the time as an observable, and thus could, in principle, have the
required coherence in time, are discussed briefly, and the application of
Floquet theory and the manifestly covariant quantum theory of Stueckelberg are
treated in some detail. In particular, the latter is shown to account for the
results in a simple and consistent way.

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


title: On environment-assisted capacities of quantum channels

abstract:
  Following initial work by Gregoratti and Werner [J. Mod. Optics 50, 913-933,
2003 and quant-ph/0403092] and Hayden and King [quant-ph/0409026], we study the
problem of the capacity of a quantum channel assisted by a "friendly (channel)
environment" that can locally measure and communicate classical messages to the
receiver.
  Previous work [quant-ph/0505038] has yielded a capacity formula for the
quantum capacity under this kind of help from the environment. Here we study
the problem of the environment-assisted classical capacity, which exhibits a
somewhat richer structure (at least, it seems to be the harder problem). There
are several, presumably inequivalent, models of the permitted local operations
and classical communications between receiver and environment: one-way,
arbitrary, separable and PPT POVMs. In all these models, the task of decoding a
message amounts to discriminating a set of possibly entangled states between
the two receivers, by a class of operations under some sort of locality
constraint.
  After introducing the operational capacities outlined above, we show that a
lower bound on the environment-assisted classical capacity is always half the
logarithm of the input space dimension. Then we develop a few techniques to
prove the existence of channels which meet this lower bound up to terms of much
smaller order, even when PPT decoding measurements are allowed (assuming a
certain superadditivity conjecture).


title:
Optimal focusing for maximal collection of entangled narrow-band photon
  pairs into single-mode fibers

abstract:
  We present a theoretical and experimental investigation of the emission
characteristics and the flux of photon pairs generated by spontaneous
parametric downconversion in quasi-phase matched bulk crystals for the use in
quantum communication sources. We show that, by careful design, one can attain
well defined modes close to the fundamental mode of optical fibers and obtain
high coupling efficiencies also for bulk crystals, these being more easily
aligned than crystal waveguides. We distinguish between singles coupling,
conditional coincidence, and pair coupling, and show how each of these
parameters can be maximized by varying the focusing of the pump mode and the
fiber-matched modes using standard optical elements. Specifically we analyze a
periodically poled KTP-crystal pumped by a 532 nm laser creating photon pairs
at 810 nm and 1550 nm. Numerical calculations lead to coupling efficiencies
above 94% at optimal focusing, which is found by the geometrical relation L/z_R
to be ~ 1 to 2 for the pump mode and ~ 2 to 3 for the fiber-modes, where L is
the crystal length and z_R is the Rayleigh-range of the mode-profile. These
results are independent on L. By showing that the single-mode bandwidth
decreases as 1/L, we can therefore design the source to produce and couple
narrow bandwidth photon pairs well into the fibers. Smaller bandwidth means
both less chromatic dispersion for long propagation distances in fibers, and
that telecom Bragg gratings can be utilized to compensate for broadened photon
packets--a vital problem for time-multiplexed qubits. Longer crystals also
yield an increase in fiber photon flux proportional to sqrt{L}, and so,
assuming correct focusing, we can only see advantages using long crystals.


title: Matter-wave interferometry in a double well on an atom chip

abstract:
  Matter-wave interference experiments enable us to study matter at its most
basic, quantum level and form the basis of high-precision sensors for
applications such as inertial and gravitational field sensing. Success in both
of these pursuits requires the development of atom-optical elements that can
manipulate matter waves at the same time as preserving their coherence and
phase. Here, we present an integrated interferometer based on a simple,
coherent matter-wave beam splitter constructed on an atom chip. Through the use
of radio-frequency-induced adiabatic double-well potentials, we demonstrate the
splitting of Bose-Einstein condensates into two clouds separated by distances
ranging from 3 to 80 microns, enabling access to both tunnelling and isolated
regimes. Moreover, by analysing the interference patterns formed by combining
two clouds of ultracold atoms originating from a single condensate, we measure
the deterministic phase evolution throughout the splitting process. We show
that we can control the relative phase between the two fully separated samples
and that our beam splitter is phase-preserving.