68 research outputs found
Direct Measurement of Quantum Dot Spin Dynamics using Time-Resolved Resonance Fluorescence
We temporally resolve the resonance fluorescence from an electron spin
confined to a single self-assembled quantum dot to measure directly the spin's
optical initialization and natural relaxation timescales. Our measurements
demonstrate that spin initialization occurs on the order of microseconds in the
Faraday configuration when a laser resonantly drives the quantum dot
transition. We show that the mechanism mediating the optically induced
spin-flip changes from electron-nuclei interaction to hole-mixing interaction
at 0.6 Tesla external magnetic field. Spin relaxation measurements result in
times on the order of milliseconds and suggest that a magnetic field
dependence, due to spin-orbit coupling, is sustained all the way down to 2.2
Tesla.Comment: An additional EPAPS file in PDF format is available for download at
the publications section of our website
http://www.amop.phy.cam.ac.uk/amop-ma
A nanometer-scale optical electrometer
Self-assembled semiconductor quantum dots show remarkable optical and spin
coherence properties, which have lead to a concerted research effort examining
their potential as a quantum bit for quantum information science1-6. Here, we
present an alternative application for such devices, exploiting recent
achievements of charge occupation control and the spectral tunability of the
optical emission of quantum dots by electric fields7 to demonstrate
high-sensitivity electric field measurement. In contrast to existing
nanometer-scale electric field sensors, such as single electron transistors8-11
and mechanical resonators12,13, our approach relies on homodyning light
resonantly Rayleigh scattered from a quantum dot transition with the excitation
laser and phase sensitive lock-in detection. This offers both static and
transient field detection ability with high bandwidth operation and near unity
quantum efficiency. Our theoretical estimation of the static field sensitivity
for typical parameters, 0.5 V/m/ \surd Hz, compares favorably to the
theoretical limit for single electron transistor-based electrometers. The
sensitivity level of 5 V/m/ \surd Hz we report in this work, which corresponds
to 6.4 * 10-6 e/ \surd Hz at a distance of 12 nm, is worse than this
theoretical estimate, yet higher than any other result attained at 4.2 K or
higher operation temperature
Exciton mediated one phonon resonant Raman scattering from one-dimensional systems
We use the Kramers-Heisenberg approach to derive a general expression for the
resonant Raman scattering cross section from a one-dimensional (1D) system
explicitly accounting for excitonic effects. The result should prove useful for
analyzing the Raman resonance excitation profile lineshapes for a variety of 1D
systems including carbon nanotubes and semiconductor quantum wires. We apply
this formalism to a simple 1D model system to illustrate the similarities and
differences between the free electron and correlated electron-hole theories.Comment: 10 pages, 6 figure
Chirality dependence of the radial breathing phonon mode density in single wall carbon nanotubes
A mass and spring model is used to calculate the phonon mode dispersion for
single wall carbon nanotubes (SWNTs) of arbitrary chirality. The calculated
dispersions are used to determine the chirality dependence of the radial
breathing phonon mode (RBM) density. Van Hove singularities, usually discussed
in the context of the single particle electronic excitation spectrum, are found
in the RBM density of states with distinct qualitative differences for zig zag,
armchair and chiral SWNTs. The influence the phonon mode density has on the two
phonon resonant Raman scattering cross-section is discussed.Comment: 6 pages, 2 figures, submitted to Phys. Rev.
Supergrowth and sub-wavelength object imaging
We further develop the concept of supergrowth [Jordan, Quantum Stud.: Math.
Found. , 285-292 (2020)], a phenomenon complementary to
superoscillation, defined as the local amplitude growth rate of a function
being higher than its largest wavenumber. We identify the superoscillating and
supergrowing regions of a canonical oscillatory function and find the maximum
values of local growth rate and wavenumber. Next, we provide a quantitative
comparison of lengths and relevant intensities between the superoscillating and
the supergrowing regions of a canonical oscillatory function. Our analysis
shows that the supergrowing regions contain intensities that are exponentially
larger in terms of the highest local wavenumber compared to the
superoscillating regions. Finally, we prescribe methods to reconstruct a
sub-wavelength object from the imaging data using both superoscillatory and
supergrowing point spread functions. Our investigation provides an
experimentally preferable alternative to the superoscillation based
superresolution schemes and is relevant to cutting-edge research in far-field
sub-wavelength imaging.Comment: 9 pages, 3 figure
Quantum-limited estimation of the axial separation of two incoherent point sources
Improving axial resolution is crucial for three-dimensional optical imaging
systems. Here we present a scheme of axial superresolution for two incoherent
point sources based on spatial mode demultiplexing. A radial mode sorter is
used to losslessly decompose the optical fields into a radial mode basis set to
extract the phase information associated with the axial positions of the point
sources. We show theoretically and experimentally that, in the limit of a zero
axial separation, our scheme allows for reaching the quantum Cram\'er-Rao lower
bound and thus can be considered as one of the optimal measurement methods.
Unlike other superresolution schemes, this scheme does not require neither
activation of fluorophores nor sophisticated stabilization control. Moreover,
it is applicable to the localization of a single point source in the axial
direction. Our demonstration can be useful to a variety of applications such as
far-field fluorescence microscopy.Comment: Comments are welcom
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