1,864 research outputs found
Optical Visualization of Radiative Recombination at Partial Dislocations in GaAs
Individual dislocations in an ultra-pure GaAs epilayer are investigated with
spatially and spectrally resolved photoluminescence imaging at 5~K. We find
that some dislocations act as strong non-radiative recombination centers, while
others are efficient radiative recombination centers. We characterize
luminescence bands in GaAs due to dislocations, stacking faults, and pairs of
stacking faults. These results indicate that low-temperature,
spatially-resolved photoluminescence imaging can be a powerful tool for
identifying luminescence bands of extended defects. This mapping could then be
used to identify extended defects in other GaAs samples solely based on
low-temperature photoluminescence spectra.Comment: 4 pages, 4 figure
Quantum computers based on electron spins controlled by ultra-fast, off-resonant, single optical pulses
We describe a fast quantum computer based on optically controlled electron
spins in charged quantum dots that are coupled to microcavities. This scheme
uses broad-band optical pulses to rotate electron spins and provide the clock
signal to the system. Non-local two-qubit gates are performed by phase shifts
induced by electron spins on laser pulses propagating along a shared waveguide.
Numerical simulations of this scheme demonstrate high-fidelity single-qubit and
two-qubit gates with operation times comparable to the inverse Zeeman
frequency.Comment: 4 pages, 4 figures, introduction is clarified, the section on
two-qubit gates was expanded and much more detail about gate fidelities is
given, figures were modified, one figure replaced with a figure showing gate
fidelities for relevant parameter
Ultrafast optical spin echo for electron spins in semiconductors
Spin-based quantum computing and magnetic resonance techniques rely on the
ability to measure the coherence time, T2, of a spin system. We report on the
experimental implementation of all-optical spin echo to determine the T2 time
of a semiconductor electron-spin system. We use three ultrafast optical pulses
to rotate spins an arbitrary angle and measure an echo signal as the time
between pulses is lengthened. Unlike previous spin-echo techniques using
microwaves, ultrafast optical pulses allow clean T2 measurements of systems
with dephasing times T2* fast in comparison to the timescale for microwave
control. This demonstration provides a step toward ultrafast optical dynamic
decoupling of spin-based qubits.Comment: 4 pages, 3 figure
Selective active resonance tuning for multi-mode nonlinear photonic cavities
Resonant enhancement of nonlinear photonic processes is critical for the
scalability of applications such as long-distance entanglement generation. To
implement nonlinear resonant enhancement, multiple resonator modes must be
individually tuned onto a precise set of process wavelengths, which requires
multiple linearly-independent tuning methods. Using coupled auxiliary
resonators to indirectly tune modes in a multi-resonant nonlinear cavity is
particularly attractive because it allows the extension of a single physical
tuning mechanism, such as thermal tuning, to provide the required independent
controls. Here we model and simulate the performance and tradeoffs of a
coupled-resonator tuning scheme which uses auxiliary resonators to tune
specific modes of a multi-resonant nonlinear process. Our analysis determines
the tuning bandwidth for steady-state mode field intensity can significantly
exceed the inter-cavity coupling rate if the total quality factor of the
auxiliary resonator is higher than the multi-mode main resonator. Consequently,
over-coupling a nonlinear resonator mode to improve the maximum efficiency of a
frequency conversion process will simultaneously expand the auxiliary resonator
tuning bandwidth for that mode, indicating a natural compatibility with this
tuning scheme. We apply the model to an existing small-diameter triply-resonant
ring resonator design and find that a tuning bandwidth of 136 GHz ~ 1.1 nm can
be attained for a mode in the telecom band while limiting excess scattering
losses to a quality factor of 10^6. Such range would span the distribution of
inhomogeneously broadened quantum emitter ensembles as well as resonator
fabrication variations, indicating the potential for the auxiliary resonators
to enable not only low-loss telecom conversion but also the generation of
indistinguishable photons in a quantum network.Comment: 16 pages, 7 figure
On the indistinguishability of Raman photons
We provide a theoretical framework to study the effect of dephasing on the
quantum indistinguishability of single photons emitted from a coherently driven
cavity QED -system. We show that with a large excited-state detuning,
the photon indistinguishability can be drastically improved provided that the
fluctuation rate of the noise source affecting the excited state is fast
compared with the photon emission rate. In some cases a spectral filter is
required to realize this improvement, but the cost in efficiency can be made
small.Comment: 18 pages, 3 figures, final versio
Astrophysical Ionizing Radiation and the Earth: A Brief Review and Census of Intermittent Intense Sources
Cosmic radiation backgrounds are a constraint on life, and their distribution
will affect the Galactic Habitable Zone. Life on Earth has developed in the
context of these backgrounds, and characterizing event rates will elaborate the
important influences. This in turn can be a base for comparison with other
potential life-bearing planets. In this review we estimate the intensities and
rates of occurrence of many kinds of strong radiation bursts by astrophysical
entities ranging from gamma-ray bursts at cosmological distances to the Sun
itself. Many of these present potential hazards to the biosphere: on timescales
long compared with human history, the probability of an event intense enough to
disrupt life on the land surface or in the oceans becomes large. We enumerate
the known sources of radiation and characterize their intensities at the Earth
and rates or upper limits on these quantities. When possible, we estimate a
"lethal interval", our best estimate of how often a major extinction-level
event is probable given the current state of knowledge; we base these estimates
on computed or expected depletion of stratospheric ozone. In general, moderate
level events are dominated by the Sun, but the far more severe infrequent
events are probably dominated by gamma-ray bursts and supernovae. We note for
the first time that so-called "short-hard" gamma-ray bursts are a substantial
threat, comparable in magnitude to supernovae and greater than that of the
higher-luminosity long bursts considered in most past work. Given their
precursors, short bursts may come with little or no warning.Comment: to be published in Astrobiolog
400%/W second harmonic conversion efficiency in -diameter gallium phosphide-on-oxide resonators
Second harmonic conversion from 1550~nm to 775~nm with an efficiency of 400%
W is demonstrated in a gallium phosphide (GaP) on oxide integrated
photonic platform. The platform consists of doubly-resonant, phase-matched ring
resonators with quality factors , low mode volumes , and high nonlinear mode overlaps. Measurements and simulations
indicate that conversion efficiencies can be increased by a factor of 20 by
improving the waveguide-cavity coupling to achieve critical coupling in current
devices.Comment: 13 pages, 6 figure
Population pulsation resonances of excitons in monolayer MoSe2 with sub 1 {\mu}eV linewidth
Monolayer transition metal dichalcogenides, a new class of atomically thin
semiconductors, possess optically coupled 2D valley excitons. The nature of
exciton relaxation in these systems is currently poorly understood. Here, we
investigate exciton relaxation in monolayer MoSe2 using polarization-resolved
coherent nonlinear optical spectroscopy with high spectral resolution. We
report strikingly narrow population pulsation resonances with two different
characteristic linewidths of 1 {\mu}eV and <0.2 {\mu}eV at low-temperature.
These linewidths are more than three orders of magnitude narrower than the
photoluminescence and absorption linewidth, and indicate that a component of
the exciton relaxation dynamics occurs on timescales longer than 1 ns. The
ultra-narrow resonance (<0.2 {\mu}eV) emerges with increasing excitation
intensity, and implies the existence of a long-lived state whose lifetime
exceeds 6 ns.Comment: (PRL, in press
Optical Detection of a Single Nuclear Spin
We propose a method to optically detect the spin state of a 31-P nucleus
embedded in a 28-Si matrix. The nuclear-electron hyperfine splitting of the
31-P neutral-donor ground state can be resolved via a direct frequency
discrimination measurement of the 31-P bound exciton photoluminescence using
single photon detectors. The measurement time is expected to be shorter than
the lifetime of the nuclear spin at 4 K and 10 T.Comment: 4 pages, 3 figure
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