6,669 research outputs found
On a Uniformly Accelerated Point Charge moving along a Cusp
A uniformly accelerated point charge which moves neither in a straight line
nor in a circle, but in a cusp, is investigated. We find the angular
distribution of the Larmor radiation, the constant power, and the intensity in
the maximal direction. It is found that the intensity of uniformly accelerated
cusp motion scales like non-uniformly accelerated synchrotron radiation. We
confirm the exact vacuum excitation spectra of quantized field detectors on the
world line.Comment: 13 pages, 6 figure
Polarization entangled photons from quantum dots embedded in nanowires
We present a first measurement of photon polarization entanglement from the
biexciton to ground state cascade of a single InAsP quantum dot embedded in an
InP nanowire. We observe a fidelity of 0.76(2) to a reference maximally
entangled state as well as a concurrence of 0.57(6)
A dip in the UHECR spectrum and the transition from galactic to extragalactic cosmic rays
The dip is a feature in the diffuse spectrum of ultra-high energy (UHE)
protons caused by electron-positron pair production on the cosmic microwave
background (CMB) radiation. For a power-law generation spectrum , the
calculated position and shape of the dip is confirmed with high accuracy by the
spectra observed by the Akeno-AGASA, HiRes, Yakutsk and Fly's Eye detectors.
When the particle energies, measured in these detectors, are calibrated by the
dip, their fluxes agree with a remarkable accuracy. The predicted shape of the
dip is quite robust. The dip is only modified strongly when the fraction of
nuclei heavier than protons is high at injection, which imposes some
restrictions on the mechanisms of acceleration operating in UHECR sources. The
existence of the dip, confirmed by observations, implies that the transition
from galactic to extragalactic cosmic rays occurs at E \lsim 1\times 10^{18}
eV. We show that at energies lower than a characteristic value eV, the spectrum of extragalactic cosmic rays
flattens in all cases of interest, and it provides a natural transition to a
steeper galactic cosmic ray spectrum. This transition occurs at some energy
below , corresponding to the position of the so-called second knee.
We discuss extensively the constraints on this model imposed by current
knowledge of acceleration processes and sources of UHECR and compare it with
the traditional model of transition at the ankle.Comment: Version Accepted for Publication in Astroparticle Physics (minor
changes
Determinisitic Writing and Control of the Dark Exciton Spin using Short Single Optical Pulses
We demonstrate that the quantum dot-confined dark exciton forms a long-lived
integer spin solid state qubit which can be deterministically on-demand
initiated in a pure state by one optical pulse. Moreover, we show that this
qubit can be fully controlled using short optical pulses, which are several
orders of magnitude shorter than the life and coherence times of the qubit. Our
demonstrations do not require an externally applied magnetic field and they
establish that the quantum dot-confined dark exciton forms an excellent solid
state matter qubit with some advantages over the half-integer spin qubits such
as the confined electron and hole, separately. Since quantum dots are
semiconductor nanostructures that allow integration of electronic and photonic
components, the dark exciton may have important implications on implementations
of quantum technologies consisting of semiconductor qubits.Comment: Added two authors, minor edits to figure captions, expanded
discussion of dark exciton eigenstate
Solid-state ensemble of highly entangled photon sources at rubidium atomic transitions
Semiconductor InAs/GaAs quantum dots grown by the Stranski-Krastanov method
are among the leading candidates for the deterministic generation of
polarization entangled photon pairs. Despite remarkable progress in the last
twenty years, many challenges still remain for this material, such as the
extremely low yield (<1% quantum dots can emit entangled photons), the low
degree of entanglement, and the large wavelength distribution. Here we show
that, with an emerging family of GaAs/AlGaAs quantum dots grown by droplet
etching and nanohole infilling, it is possible to obtain a large ensemble
(close to 100%) of polarization-entangled photon emitters on a wafer without
any post-growth tuning. Under pulsed resonant two-photon excitation, all
measured quantum dots emit single pairs of entangled photons with ultra-high
purity, high degree of entanglement (fidelity up to F=0.91, with a record high
concurrence C=0.90), and ultra-narrow wavelength distribution at rubidium
transitions. Therefore, a solid-state quantum repeater - among many other key
enabling quantum photonic elements - can be practically implemented with this
new material
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