8,108 research outputs found
Where are the Hedgehogs in Nematics?
In experiments which take a liquid crystal rapidly from the isotropic to the
nematic phase, a dense tangle of defects is formed. In nematics, there are in
principle both line and point defects (``hedgehogs''), but no point defects are
observed until the defect network has coarsened appreciably. In this letter the
expected density of point defects is shown to be extremely low, approximately
per initially correlated domain, as result of the topology
(specifically, the homology) of the order parameter space.Comment: 6 pages, latex, 1 figure (self-unpacking PostScript)
Anisotropic Energy Gaps of Iron-based Superconductivity from Intra-band Quasiparticle Interference in LiFeAs
If strong electron-electron interactions between neighboring Fe atoms mediate
the Cooper pairing in iron-pnictide superconductors, then specific and distinct
anisotropic superconducting energy gaps \Delta_i(k) should appear on the
different electronic bands i. Here we introduce intra-band Bogoliubov
quasiparticle scattering interference (QPI) techniques for determination of
\Delta_i(k) in such materials, focusing on LiFeAs. We identify the three
hole-like bands assigned previously as \gamma, \alpha_2 and \alpha_1, and we
determine the anisotropy, magnitude and relative orientations of their
\Delta_i(k). These measurements will advance quantitative theoretical analysis
of the mechanism of Cooper pairing in iron-based superconductivity
Quantum Bit Regeneration
Decoherence and loss will limit the practicality of quantum cryptography and
computing unless successful error correction techniques are developed. To this
end, we have discovered a new scheme for perfectly detecting and rejecting the
error caused by loss (amplitude damping to a reservoir at T=0), based on using
a dual-rail representation of a quantum bit. This is possible because (1)
balanced loss does not perform a ``which-path'' measurement in an
interferometer, and (2) balanced quantum nondemolition measurement of the
``total'' photon number can be used to detect loss-induced quantum jumps
without disturbing the quantum coherence essential to the quantum bit. Our
results are immediately applicable to optical quantum computers using single
photonics devices.Comment: 4 pages, postscript only, figures available at
http://feynman.stanford.edu/qcom
Factoring in a Dissipative Quantum Computer
We describe an array of quantum gates implementing Shor's algorithm for prime
factorization in a quantum computer. The array includes a circuit for modular
exponentiation with several subcomponents (such as controlled multipliers,
adders, etc) which are described in terms of elementary Toffoli gates. We
present a simple analysis of the impact of losses and decoherence on the
performance of this quantum factoring circuit. For that purpose, we simulate a
quantum computer which is running the program to factor N = 15 while
interacting with a dissipative environment. As a consequence of this
interaction randomly selected qubits may spontaneously decay. Using the results
of our numerical simulations we analyze the efficiency of some simple error
correction techniques.Comment: plain tex, 18 pages, 8 postscript figure
Avoiding Quantum Chaos in Quantum Computation
We study a one-dimensional chain of nuclear spins in an external
time-dependent magnetic field. This model is considered as a possible candidate
for experimental realization of quantum computation. According to the general
theory of interacting particles, one of the most dangerous effects is quantum
chaos which can destroy the stability of quantum operations. According to the
standard viewpoint, the threshold for the onset of quantum chaos due to an
interaction between spins (qubits) strongly decreases with an increase of the
number of qubits. Contrary to this opinion, we show that the presence of a
magnetic field gradient helps to avoid quantum chaos which turns out to
disappear with an increase of the number of qubits. We give analytical
estimates which explain this effect, together with numerical data supportingComment: RevTex, 5 pages including 3 eps-figure
Wall-crossing, open BPS counting and matrix models
We consider wall-crossing phenomena associated to the counting of D2-branes
attached to D4-branes wrapping lagrangian cycles in Calabi-Yau manifolds, both
from M-theory and matrix model perspective. Firstly, from M-theory viewpoint,
we review that open BPS generating functions in various chambers are given by a
restriction of the modulus square of the open topological string partition
functions. Secondly, we show that these BPS generating functions can be
identified with integrands of matrix models, which naturally arise in the free
fermion formulation of corresponding crystal models. A parameter specifying a
choice of an open BPS chamber has a natural, geometric interpretation in the
crystal model. These results extend previously known relations between open
topological string amplitudes and matrix models to include chamber dependence.Comment: 25 pages, 8 figures, published versio
Transforming quantum operations: quantum supermaps
We introduce the concept of quantum supermap, describing the most general
transformation that maps an input quantum operation into an output quantum
operation. Since quantum operations include as special cases quantum states,
effects, and measurements, quantum supermaps describe all possible
transformations between elementary quantum objects (quantum systems as well as
quantum devices). After giving the axiomatic definition of supermap, we prove a
realization theorem, which shows that any supermap can be physically
implemented as a simple quantum circuit. Applications to quantum programming,
cloning, discrimination, estimation, information-disturbance trade-off, and
tomography of channels are outlined.Comment: 6 pages, 1 figure, published versio
Quantum Clock Synchronization Based on Shared Prior Entanglement
We demonstrate that two spatially separated parties (Alice and Bob) can
utilize shared prior quantum entanglement, and classical communications, to
establish a synchronized pair of atomic clocks. In contrast to classical
synchronization schemes, the accuracy of our protocol is independent of Alice
or Bob's knowledge of their relative locations or of the properties of the
intervening medium.Comment: 4 page
Doubling of the bands in overdoped Bi2Sr2CaCu2O8-probable evidence for c-axis bilayer coupling
We present high resolution ARPES data of the bilayer superconductor
Bi2Sr2CaCu2O8 (Bi2212) showing a clear doubling of the near EF bands. This
splitting approaches zero along the (0,0)-(pi,pi) nodal line and is not
observed in single layer Bi2Sr2CuO6 (Bi2201), suggesting that the splitting is
due to the long sought after bilayer splitting effect. The splitting has a
magnitude of approximately 75 meV near the middle of the zone, extrapolating to
about 100 meV near the (pi,0) poin
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