109 research outputs found
Nonequilibrium phases in hybrid arrays with flux qubits and NV centers
We propose a startling hybrid quantum architecture for simulating a
localization-delocalization transition. The concept is based on an array of
superconducting flux qubits which are coupled to a diamond crystal containing
nitrogen-vacancy (NV) centers. The underlying description is a
Jaynes-Cummings-lattice in the strong-coupling regime. However, in contrast to
well-studied coupled cavity arrays the interaction between lattice sites is
mediated here by the qubit rather than by the oscillator degrees of freedom.
Nevertheless, we point out that a transition between a localized and a
delocalized phase occurs in this system as well. We demonstrate the possibility
of monitoring this transition in a non-equilibrium scenario, including
decoherence effects. The proposed scheme allows the monitoring of
localization-delocalization transitions in Jaynes-Cummings-lattices by use of
currently available experimental technology. Contrary to cavity-coupled
lattices, our proposed recourse to stylized qubit networks facilitates (i) to
investigate localization-delocalization transitions in arbitrary dimensions and
(ii) to tune the inter-site coupling in-situ.Comment: Version to be published in Phys. Rev.
Cavity-enhanced Raman Microscopy of Individual Carbon Nanotubes
Raman spectroscopy reveals chemically specific information and provides
label-free insight into the molecular world. However, the signals are
intrinsically weak and call for enhancement techniques. Here, we demonstrate
Purcell enhancement of Raman scattering in a tunable high-finesse microcavity,
and utilize it for molecular diagnostics by combined Raman and absorption
imaging. Studying individual single-wall carbon nanotubes, we identify crucial
structural parameters such as nanotube radius, electronic structure and
extinction cross-section. We observe a 320-times enhanced Raman scattering
spectral density and an effective Purcell factor of 6.2, together with a
collection efficiency of 60%. Potential for significantly higher enhancement,
quantitative signals, inherent spectral filtering and absence of intrinsic
background in cavity-vacuum stimulated Raman scattering render the technique a
promising tool for molecular imaging. Furthermore, cavity-enhanced Raman
transitions involving localized excitons could potentially be used for gaining
quantum control over nanomechanical motion and open a route for molecular
cavity optomechanics
Simplified models for photohadronic interactions in cosmic accelerators
We discuss simplified models for photo-meson production in cosmic
accelerators, such as Active Galactic Nuclei and Gamma-Ray Bursts. Our
self-consistent models are directly based on the underlying physics used in the
SOPHIA software, and can be easily adapted if new data are included. They allow
for the efficient computation of neutrino and photon spectra (from pi^0
decays), as a major requirement of modern time-dependent simulations of the
astrophysical sources and parameter studies. In addition, the secondaries
(pions and muons) are explicitely generated, a necessity if cooling processes
are to be included. For the neutrino production, we include the helicity
dependence of the muon decays which in fact leads to larger corrections than
the details of the interaction model. The separate computation of the pi^0,
pi^+, and pi^- fluxes allows, for instance, for flavor ratio predictions of the
neutrinos at the source, which are a requirement of many tests of neutrino
properties using astrophysical sources. We confirm that for charged pion
generation, the often used production by the Delta(1232)-resonance is typically
not the dominant process in Active Galactic Nuclei and Gamma-Ray Bursts, and we
show, for arbitrary input spectra, that the number of neutrinos are
underestimated by at least a factor of two if they are obtained from the
neutral to charged pion ratio. We compare our results for several levels of
simplification using isotropic synchrotron and thermal spectra, and we
demonstrate that they are sufficiently close to the SOPHIA software.Comment: Treatment of high energy interactions refined, additional black body
benchmark added (v2), some references corrected (v3). A Mathematica notebook
which illustrates the implementation of one model can be found at
http://theorie.physik.uni-wuerzburg.de/~winter/Resources/AstroModel/Sim-B.html
. 46 pages, 14 (color) figures, 7 tables. Final version, accepted for
publication in Ap
Weak and Strong coupling regimes in plasmonic-QED
We present a quantum theory for the interaction of a two level emitter with
surface plasmon polaritons confined in single-mode waveguide resonators. Based
on the Green's function approach, we develop the conditions for the weak and
strong coupling regimes by taking into account the sources of dissipation and
decoherence: radiative and non-radiative decays, internal loss processes in the
emitter, as well as propagation and leakage losses of the plasmons in the
resonator. The theory is supported by numerical calculations for several
quantum emitters, GaAs and CdSe quantum dots and NV centers together with
different types of resonators constructed of hybrid, cylindrical or wedge
waveguides. We further study the role of temperature and resonator length.
Assuming realistic leakage rates, we find the existence of an optimal length at
which strong coupling is possible. Our calculations show that the strong
coupling regime in plasmonic resonators is accessible within current technology
when working at very low temperatures (<4K). In the weak coupling regime our
theory accounts for recent experimental results. By further optimization we
find highly enhanced spontaneous emission with Purcell factors over 1000 at
room temperature for NV-centers. We finally discuss more applications for
quantum nonlinear optics and plasmon-plasmon interactions.Comment: published as Phys. Rev. B 87, 115419 (2013
Neutrino Decays over Cosmological Distances and the Implications for Neutrino Telescopes
We discuss decays of ultra-relativistic neutrinos over cosmological distances
by solving the decay equation in terms of its redshift dependence. We
demonstrate that there are significant conceptual differences compared to more
simplified treatments of neutrino decay. For instance, the maximum distance the
neutrinos have traveled is limited by the Hubble length, which means that the
common belief that longer neutrino lifetimes can be probed by longer distances
does not apply. As a consequence, the neutrino lifetime limit from supernova
1987A cannot be exceeded by high-energy astrophysical neutrinos. We discuss the
implications for neutrino spectra and flavor ratios from gamma-ray bursts as
one example of extragalactic sources, using up-to-date neutrino flux
predictions. If the observation of SN 1987A implies that \nu_1 is stable and
the other mass eigenstates decay with rates much smaller than their current
bounds, the muon track rate can be substantially suppressed compared to the
cascade rate in the region IceCube is most sensitive to. In this scenario, no
gamma-ray burst neutrinos may be found using muon tracks even with the full
scale experiment, whereas reliable information on high-energy astrophysical
sources can only be obtained from cascade measurements. As another consequence,
the recently observed two cascade event candidates at PeV energies will not be
accompanied by corresponding muon tracks.Comment: 20 pages, 6 figures, 1 table. Matches published versio
Sources of UHECRs in view of the TUS and JEM-EUSO experiments
The origin of ultra-high-energy cosmic rays (UHECRs) is one of the most
intriguing problems of modern cosmic ray physics. We briefly review the main
astrophysical models of their origin and the forthcoming orbital experiments
TUS and JEM-EUSO, and discuss how the new data can help one solve the
long-standing puzzle.Comment: 4 pages; prepared for ECRS-2012 (http://ecrs2012.sinp.msu.ru/); v2: a
reference adde
Cavity-enhanced Raman microscopy of individual carbon nanotubes
Raman spectroscopy reveals chemically specific information and provides label-free insight into the molecular world. However, the signals are intrinsically weak and call for enhancement techniques. Here, we demonstrate Purcell enhancement of Raman scattering in a tunable high-finesse microcavity, and utilize it for molecular diagnostics by combined Raman and absorption imaging. Studying individual single-wall carbon nanotubes, we identify crucial structural parameters such as nanotube radius, electronic structure and extinction cross-section. We observe a 320-times enhanced Raman scattering spectral density and an effective Purcell factor of 6.2, together with a collection efficiency of 60%. Potential for significantly higher enhancement, quantitative signals, inherent spectral filtering and absence of intrinsic background in cavity-vacuum stimulated Raman scattering render the technique a promising tool for molecular imaging. Furthermore, cavity-enhanced Raman transitions involving localized excitons could potentially be used for gaining quantum control over nanomechanical motion and open a route for molecular cavity optomechanics
Beyond the Jaynes-Cummings model: circuit QED in the ultrastrong coupling regime
In cavity quantum electrodynamics (QED), light-matter interaction is probed
at its most fundamental level, where individual atoms are coupled to single
photons stored in three-dimensional cavities. This unique possibility to
experimentally explore the foundations of quantum physics has greatly evolved
with the advent of circuit QED, where on-chip superconducting qubits and
oscillators play the roles of two-level atoms and cavities, respectively. In
the strong coupling limit, atom and cavity can exchange a photon frequently
before coherence is lost. This important regime has been reached both in cavity
and circuit QED, but the design flexibility and engineering potential of the
latter allowed for increasing the ratio between the atom-cavity coupling rate
and the cavity transition frequency above the percent level. While these
experiments are well described by the renowned Jaynes-Cummings model, novel
physics is expected in the ultrastrong coupling limit. Here, we report on the
first experimental realization of a superconducting circuit QED system in the
ultrastrong coupling limit and present direct evidence for the breakdown of the
Jaynes-Cummings model.Comment: 5 pages, 3 figure
Theory and Applications of X-ray Standing Waves in Real Crystals
Theoretical aspects of x-ray standing wave method for investigation of the
real structure of crystals are considered in this review paper. Starting from
the general approach of the secondary radiation yield from deformed crystals
this theory is applied to different concreat cases. Various models of deformed
crystals like: bicrystal model, multilayer model, crystals with extended
deformation field are considered in detailes. Peculiarities of x-ray standing
wave behavior in different scattering geometries (Bragg, Laue) are analysed in
detailes. New possibilities to solve the phase problem with x-ray standing wave
method are discussed in the review. General theoretical approaches are
illustrated with a big number of experimental results.Comment: 101 pages, 43 figures, 3 table
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