16 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
Fundamental mechanisms of energy exchanges in autonomous measurements based on dispersive qubit-light interaction
Measuring an observable that does not commute with the system's Hamiltonian
usually leads to a variation of its energy. Unveiling the first link of the von
Neumann chain, the quantum meter has to account for this energy change. Here,
we consider an autonomous meter-system dynamics: a qubit interacting
dispersively with a light pulse propagating in a one-dimensional waveguide. The
light pulse (the meter) measures the qubit's state along the -axis while the
qubit's Hamiltonian is oriented along another direction. As the interaction is
dispersive, photon number is conserved so that energy balance has to be
attained by spectral deformations of the light pulse. An accurate and
repeatable measurement can be achieved only by employing short pulses, where
their spectral deformation is practically undetectable. Increasing the pulse's
duration, the measurement's quality drops and the spectral deformation of the
scattered field becomes visible. Building on analytical and numerical
solutions, we reveal the mechanism underlying this spectral deformation and
display how it compensates for the qubit's energy change. We explain the
formation of a three-peak structure of the output spectrum and we provide the
conditions under which this is observable.Comment: 9 pages plus appendices, 9 figure
Quantum energetics of a non-commuting measurement
When a measurement observable does not commute with a quantum system's
Hamiltonian, the energy of the measured system is typically not conserved
during the measurement. Instead, energy can be transferred between the measured
system and the meter. In this work, we experimentally investigate the
energetics of non-commuting measurements in a circuit quantum electrodynamics
system containing a transmon qubit embedded in a 3D microwave cavity. We show
through spectral analysis of the cavity photons that a frequency shift is
imparted on the probe, in balance with the associated energy changes of the
qubit. Our experiment provides new insights into foundations of quantum
measurement, as well as a better understanding of the key mechanisms at play in
quantum energetics.Comment: 9 pages, 4 figure
Van der Waals Engineering of Ferromagnetic Semiconductor Heterostructures for Spin and Valleytronics
The integration of magnetic material with semiconductors has been fertile
ground for fundamental science as well as of great practical interest toward
the seamless integration of information processing and storage. Here we create
van der Waals heterostructures formed by an ultrathin ferromagnetic
semiconductor CrI3 and a monolayer of WSe2. We observe unprecedented control of
the spin and valley pseudospin in WSe2, where we detect a large magnetic
exchange field of nearly 13 T and rapid switching of the WSe2 valley splitting
and polarization via flipping of the CrI3 magnetization. The WSe2
photoluminescence intensity strongly depends on the relative alignment between
photo-excited spins in WSe2 and the CrI3 magnetization, due to ultrafast
spin-dependent charge hopping across the heterostructure interface. The
photoluminescence detection of valley pseudospin provides a simple and
sensitive method to probe the intriguing domain dynamics in the ultrathin
magnet, as well as the rich spin interactions within the heterostructure.Comment: Supplementary Materials included. To appear in Science Advance
Ensemble spin relaxation of shallow donor qubits in ZnO
We present an experimental and theoretical study of the longitudinal electron
spin relaxation () of shallow donors in the direct band-gap semiconductor
ZnO. is measured via resonant excitation of the Ga donor-bound exciton.
exhibits an inverse-power dependence on magnetic field , with , over a field range of 1.75 T to 7 T. We derive
an analytic expression for the donor spin-relaxation rate due to spin-orbit
(admixture mechanism) and electron-phonon (piezoelectric) coupling for the
wurtzite crystal symmetry. Excellent quantitative agreement is found between
experiment and theory suggesting the admixture spin-orbit mechanism is the
dominant contribution to in the measured magnetic field range.
Temperature and excitation-energy dependent measurements indicate a donor
density dependent interaction may contribute to small deviations between
experiment and theory. The longest measured is 480 ms at 1.75 T with
increasing at smaller fields theoretically expected. This work highlights
the extremely long longitudinal spin-relaxation time for ZnO donors due to
their small spin-orbit coupling.Comment: 9 pages, 11 figure