23,922 research outputs found
Controlling qubit transitions during non-adiabatic rapid passage through quantum interference
In adiabatic rapid passage, the Bloch vector of a qubit is inverted by slowly
inverting an external field to which it is coupled, and along which it is
initially aligned. In non-adiabatic twisted rapid passage, the external field
is allowed to twist around its initial direction with azimuthal angle \phi(t)
at the same time that it is non-adiabatically inverted. For polynomial twist,
\phi(t) \sim Bt^{n}. We show that for n \ge 3, multiple qubit resonances can
occur during a single inversion of the external field, producing strong
interference effects in the qubit transition probability. The character of the
interference is controllable through variation of the twist strength B.
Constructive and destructive interference are possible, greatly enhancing or
suppressing qubit transitions. Experimental confirmation of these controllable
interference effects has already occurred. Application of this interference
mechanism to the construction of fast fault-tolerant quantum CNOT and NOT gates
is discussed.Comment: 8 pages, 7 figures, 2 tables; submitted to J. Mod. Op
The POOL Data Storage, Cache and Conversion Mechanism
The POOL data storage mechanism is intended to satisfy the needs of the LHC
experiments to store and analyze the data from the detector response of
particle collisions at the LHC proton-proton collider. Both the data rate and
the data volumes will largely differ from the past experience. The POOL data
storage mechanism is intended to be able to cope with the experiment's
requirements applying a flexible multi technology data persistency mechanism.
The developed technology independent approach is flexible enough to adopt new
technologies, take advantage of existing schema evolution mechanisms and allows
users to access data in a technology independent way. The framework consists of
several components, which can be individually adopted and integrated into
existing experiment frameworks.Comment: Talk from the 2003 Computing in High Energy and Nuclear Physics
(CHEP03), La Jolla, Ca, USA, March 2003, 5 pages, PDF, 6 figures. PSN MOKT00
Phase Behavior of Colloidal Superballs: Shape Interpolation from Spheres to Cubes
The phase behavior of hard superballs is examined using molecular dynamics
within a deformable periodic simulation box. A superball's interior is defined
by the inequality , which provides a
versatile family of convex particles () with cube-like and
octahedron-like shapes as well as concave particles () with
octahedron-like shapes. Here, we consider the convex case with a deformation
parameter q between the sphere point (q = 1) and the cube (q = 1). We find that
the asphericity plays a significant role in the extent of cubatic ordering of
both the liquid and crystal phases. Calculation of the first few virial
coefficients shows that superballs that are visually similar to cubes can have
low-density equations of state closer to spheres than to cubes. Dense liquids
of superballs display cubatic orientational order that extends over several
particle lengths only for large q. Along the ordered, high-density equation of
state, superballs with 1 < q < 3 exhibit clear evidence of a phase transition
from a crystal state to a state with reduced long-ranged orientational order
upon the reduction of density. For , long-ranged orientational order
persists until the melting transition. The width of coexistence region between
the liquid and ordered, high-density phase decreases with q up to q = 4.0. The
structures of the high-density phases are examined using certain order
parameters, distribution functions, and orientational correlation functions. We
also find that a fixed simulation cell induces artificial phase transitions
that are out of equilibrium. Current fabrication techniques allow for the
synthesis of colloidal superballs, and thus the phase behavior of such systems
can be investigated experimentally.Comment: 33 pages, 14 figure
Squeezars: Tidally powered stars orbiting a massive black hole
We propose that there exists a class of transient sources, "squeezars", which
are stars caught in highly eccentric orbits around a massive (m<10^8 Mo) black
hole (MBH), whose atypically high luminosity (up to a significant fraction of
their Eddington luminosity) is powered by tidal interactions with the MBH.
Their existence follows from the presence of a mass sink, the MBH, in the
galactic center, which drives a flow of stars into nearly radial orbits to
replace those it has destroyed. We consider two limits for the stellar response
to tidal heating: surface heating with radiative cooling ("hot squeezars") and
bulk heating with adiabatic expansion ("cold squeezars"), and calculate the
evolution of the squeezar orbit, size, luminosity and effective temperature.
The squeezar formation rate is only ~0.05 that of tidal disruption flares, but
squeezar lifetimes are many orders of magnitude longer, and so future
observations of squeezars in nearby galaxies can probe the tidal process that
feeds MBHs and the effects of extreme tides on stars. The mean number of
squeezars orbiting the Galactic MBH is estimated at 0.1-1.Comment: ApJ Lett. accepted. 4 pp. 1 fi
Stability conditions and Fermi surface topologies in a superconductor
Candidate homogeneous, isotropic superfluid or superconducting states of
paired fermion species with different chemical potentials, can lead to
quasiparticle excitation energies that vanish at either zero, one, or two
spheres in momentum space. With no zeroes, we have a conventional BCS
superconductor. The other two cases, ``gapless'' superconductors, appear in
mean field theory for sufficiently large mismatches and/or sufficiently large
coupling strengths. Here we examine several stability criteria for those
candidate phases. Positivity of number susceptibility appears to provide the
most powerful constraint, and renders all the two-zero states that we have
examined mechanically unstable. Our results should apply directly to ultracold
fermionic atom systems.Comment: 18 pages, 7 figures; v2: some clarifications in Sec. IIC; references
added; version accepted for publication in Phys. Rev.
Cherenkov Radiation from Pairs and Its Effect on Induced Showers
We calculate the Cherenkov radiation from an pair at small
separations, as occurs shortly after a pair conversion. The radiation is
reduced (compared to that from two independent particles) when the pair
separation is smaller than the wavelength of the emitted light. We estimate the
reduction in light in large electromagnetic showers, and discuss the
implications for detectors that observe Cherenkov radiation from showers in the
Earth's atmosphere, as well as in oceans and Antarctic ice.Comment: Final version, with minor changes, to appear in PRD. 5 pages with 4
figure
X-ray induced persistent photoconductivity in Si-doped AlGaAs
We demonstrate that X-ray irradiation can be used to induce an
insulator-metal transition in Si-doped AlGaAs, a
semiconductor with {\it DX} centers. The excitation mechanism of the {\it DX}
centers into their shallow donor state was revealed by studying the
photoconductance along with fluorescence. The photoconductance as a function of
incident X-ray energy exhibits an edge both at the Ga and As K-edge, implying
that core-hole excitation of Ga and As are efficient primary steps for the
excitation of {\it DX} centers. A high quantum yield () suggests that
the excitation is indirect and nonlocal, due to secondary electrons, holes, and
fluorescence photons.Comment: 3 pages of text, 6 figures. An error in Fig.5 was detected, so we
corrected i
Classical no-cloning theorem under Liouville dynamics by non-Csisz\'ar f-divergence
The Csisz\'ar f-divergence, which is a class of information distances, is
known to offer a useful tool for analysing the classical counterpart of the
cloning operations that are quantum mechanically impossible for the factorized
and marginality classical probability distributions under Liouville dynamics.
We show that a class of information distances that does not belong to this
divergence class also allows for the formulation of a classical analogue of the
quantum no-cloning theorem. We address a family of nonlinear Liouville-like
equations, and generic distances, to obtain constraints on the corresponding
functional forms, associated with the formulation of classical analogue of the
no-cloning principle.Comment: 6 pages, revised, published versio
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