801 research outputs found
Orbital magnetoelectric coupling in band insulators
Magnetoelectric responses are a fundamental characteristic of materials that
break time-reversal and inversion symmetries (notably multiferroics) and,
remarkably, of "topological insulators" in which those symmetries are unbroken.
Previous work has shown how to compute spin and lattice contributions to the
magnetoelectric tensor. Here we solve the problem of orbital contributions by
computing the frozen-lattice electronic polarization induced by a magnetic
field. One part of this response (the "Chern-Simons term") can appear even in
time-reversal-symmetric materials and has been previously shown to be quantized
in topological insulators. In general materials there are additional orbital
contributions to all parts of the magnetoelectric tensor; these vanish in
topological insulators by symmetry and also vanish in several simplified models
without time-reversal and inversion those magnetoelectric couplings were
studied before. We give two derivations of the response formula, one based on a
uniform magnetic field and one based on extrapolation of a long-wavelength
magnetic field, and discuss some of the consequences of this formula.Comment: 13 page
Semidefinite programming, multivariate orthogonal polynomials, and codes in spherical caps
We apply the semidefinite programming approach developed in
arxiv:math.MG/0608426 to obtain new upper bounds for codes in spherical caps.
We compute new upper bounds for the one-sided kissing number in several
dimensions where we in particular get a new tight bound in dimension 8.
Furthermore we show how to use the SDP framework to get analytic bounds.Comment: 15 pages, (v2) referee comments and suggestions incorporate
Measurement-Induced Entanglement for Excitation Stored in Remote Atomic Ensembles
A critical requirement for diverse applications in Quantum Information
Science is the capability to disseminate quantum resources over complex quantum
networks. For example, the coherent distribution of entangled quantum states
together with quantum memory to store these states can enable scalable
architectures for quantum computation, communication, and metrology. As a
significant step toward such possibilities, here we report observations of
entanglement between two atomic ensembles located in distinct apparatuses on
different tables. Quantum interference in the detection of a photon emitted by
one of the samples projects the otherwise independent ensembles into an
entangled state with one joint excitation stored remotely in 10^5 atoms at each
site. After a programmable delay, we confirm entanglement by mapping the state
of the atoms to optical fields and by measuring mutual coherences and photon
statistics for these fields. We thereby determine a quantitative lower bound
for the entanglement of the joint state of the ensembles. Our observations
provide a new capability for the distribution and storage of entangled quantum
states, including for scalable quantum communication networks .Comment: 13 pages, 4 figures Submitted for publication on August 31 200
Quantum repeaters based on entanglement purification
We study the use of entanglement purification for quantum communication over
long distances. For distances much longer than the coherence length of a
corresponding noisy quantum channel, the fidelity of transmission is usually so
low that standard purification methods are not applicable. It is however
possible to divide the channel into shorter segments that are purified
separately and then connected by the method of entanglement swapping. This
method can be much more efficient than schemes based on quantum error
correction, as it makes explicit use of two-way classical communication. An
important question is how the noise, introduced by imperfect local operations
(that constitute the protocols of purification and the entanglement swapping),
accumulates in such a compound channel, and how it can be kept below a certain
noise level. To treat this problem, we first study the applicability and the
efficiency of entanglement purification protocols in the situation of imperfect
local operations. We then present a scheme that allows entanglement
purification over arbitrary long channels and tolerates errors on the per-cent
level. It requires a polynomial overhead in time, and an overhead in local
resources that grows only logarithmically with the length of the channel.Comment: 19 pages, 16 figure
Structural and electronic transformation in low-angle twisted bilayer graphene
Experiments on bilayer graphene unveiled a fascinating realization of
stacking disorder where triangular domains with well-defined Bernal stacking
are delimited by a hexagonal network of strain solitons. Here we show by means
of numerical simulations that this is a consequence of a structural
transformation of the moir\'{e} pattern inherent of twisted bilayer graphene
taking place at twist angles below a crossover angle
. The transformation is governed by the interplay
between the interlayer van der Waals interaction and the in-plane strain field,
and is revealed by a change in the functional form of the twist energy density.
This transformation unveils an electronic regime characteristic of vanishing
twist angles in which the charge density converges, though not uniformly, to
that of ideal bilayer graphene with Bernal stacking. On the other hand, the
stacking domain boundaries form a distinct charge density pattern that provides
the STM signature of the hexagonal solitonic network.Comment: published version with supplementary materia
Continuous-variable optical quantum state tomography
This review covers latest developments in continuous-variable quantum-state
tomography of optical fields and photons, placing a special accent on its
practical aspects and applications in quantum information technology. Optical
homodyne tomography is reviewed as a method of reconstructing the state of
light in a given optical mode. A range of relevant practical topics are
discussed, such as state-reconstruction algorithms (with emphasis on the
maximum-likelihood technique), the technology of time-domain homodyne
detection, mode matching issues, and engineering of complex quantum states of
light. The paper also surveys quantum-state tomography for the transverse
spatial state (spatial mode) of the field in the special case of fields
containing precisely one photon.Comment: Finally, a revision! Comments to lvov(at)ucalgary.ca and
raymer(at)uoregon.edu are welcom
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