5,676 research outputs found
Observation of the single-electron regime in a highly tunable silicon quantum dot
We report on low-temperature electronic transport measurements of a silicon
metal-oxide-semiconductor quantum dot, with independent gate control of
electron densities in the leads and the quantum dot island. This architecture
allows the dot energy levels to be probed without affecting the electron
density in the leads, and vice versa. Appropriate gate biasing enables the dot
occupancy to be reduced to the single-electron level, as evidenced by
magnetospectroscopy measurements of the ground state of the first two charge
transitions. Independent gate control of the electron reservoirs also enables
discrimination between excited states of the dot and density of states
modulations in the leads.Comment: 4 pages, 3 figures, accepted for Applied Physics Letter
Applications of Machine-Learning Algorithms for Infrared Colour Selection of Galactic Wolf-Rayet Stars
We have investigated and applied machine-learning algorithms for Infrared
Colour Selection of Galactic Wolf-Rayet (WR) candidates. Objects taken from the
GLIMPSE catalogue of the infrared objects in the Galactic plane can be
classified into different stellar populations based on the colours inferred
from their broadband photometric magnitudes (, and from 2MASS, and
the four \textit{Spitzer}/IRAC bands). The algorithms tested in this pilot
study are variants of the -Nearest Neighbours (-NN) approach, which is
ideal for exploratory studies of classification problems where interrelations
between variables and classes are complicated. The aims of this study are (1)
to provide an automated tool to select reliable WR candidates and potentially
other classes of objects, (2) to measure the efficiency of infrared colour
selection at performing these tasks and, (3) to lay the groundwork for
statistically inferring the total number of WR stars in our Galaxy. We report
the performance results obtained over a set of known objects and selected
candidates for which we have carried out follow-up spectroscopic observations,
and confirm the discovery of 4 new WR stars.Comment: Authors' version of published paper, now at MNRAS, 473, 256
Orbital and valley state spectra of a few-electron silicon quantum dot
Understanding interactions between orbital and valley quantum states in
silicon nanodevices is crucial in assessing the prospects of spin-based qubits.
We study the energy spectra of a few-electron silicon metal-oxide-semiconductor
quantum dot using dynamic charge sensing and pulsed-voltage spectroscopy. The
occupancy of the quantum dot is probed down to the single-electron level using
a nearby single-electron transistor as a charge sensor. The energy of the first
orbital excited state is found to decrease rapidly as the electron occupancy
increases from N=1 to 4. By monitoring the sequential spin filling of the dot
we extract a valley splitting of ~230 {\mu}eV, irrespective of electron number.
This indicates that favorable conditions for qubit operation are in place in
the few-electron regime.Comment: 4 figure
IQRM: real-time adaptive RFI masking for radio transient and pulsar searches
In a search for short timescale astrophysical transients in time-domain data,
radio-frequency interference (RFI) causes both large quantities of false
positive candidates and a significant reduction in sensitivity if not correctly
mitigated. Here we propose an algorithm that infers a time-variable frequency
channel mask directly from short-duration (1 s) data blocks: the method
consists of computing a spectral statistic that correlates well with the
presence of RFI, and then finding high outliers among the resulting values. For
the latter task, we propose an outlier detection algorithm called
Inter-Quartile Range Mitigation (IQRM), that is both non-parametric and robust
to the presence of a trend in sequential data. The method requires no training
and can in principle adapt to any telescope and RFI environment; its efficiency
is shown on data from both the MeerKAT and Lovell 76-m radio telescopes. IQRM
is fast enough to be used in a streaming search and has been integrated into
the MeerTRAP real-time transient search pipeline. Open-source Python and C++
implementations are also provided.Comment: Accepted for publication in MNRAS, 11 pages, 9 figures. Minor
corrections, improved clarity, additional analysis of impact on single pulse
searches. IQRM implementations can be found at
https://github.com/v-morello/iqrm (Python) and
https://gitlab.com/kmrajwade/iqrm_apollo (C++
Impact of g-factors and valleys on spin qubits in a silicon double quantum dot
We define single electron spin qubits in a silicon MOS double quantum dot
system. By mapping the qubit resonance frequency as a function of gate-induced
electric field, the spectrum reveals an anticrossing that is consistent with an
inter-valley spin-orbit coupling. We fit the data from which we extract an
inter-valley coupling strength of 43 MHz. In addition, we observe a narrow
resonance near the primary qubit resonance when we operate the device in the
(1,1) charge configuration. The experimental data is consistent with a
simulation involving two weakly exchanged-coupled spins with a g-factor
difference of 1 MHz, of the same order as the Rabi frequency. We conclude that
the narrow resonance is the result of driven transitions between the T- and T+
triplet states, using an ESR signal of frequency located halfway between the
resonance frequencies of the two individual spins. The findings presented here
offer an alternative method of implementing two-qubit gates, of relevance to
the operation of larger scale spin qubit systems
An automated and versatile ultra-low temperature SQUID magnetometer
We present the design and construction of a SQUID-based magnetometer for
operation down to temperatures T = 10 mK, while retaining the compatibility
with the sample holders typically used in commercial SQUID magnetometers. The
system is based on a dc-SQUID coupled to a second-order gradiometer. The sample
is placed inside the plastic mixing chamber of a dilution refrigerator and is
thermalized directly by the 3He flow. The movement though the pickup coils is
obtained by lifting the whole dilution refrigerator insert. A home-developed
software provides full automation and an easy user interface.Comment: RevTex, 10 pages, 10 eps figures. High-resolution figures available
upon reques
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