2,327 research outputs found
A cryogenic amplifier for fast real-time detection of single-electron tunneling
We employ a cryogenic High Electron Mobility Transistor (HEMT) amplifier to
increase the bandwidth of a charge detection setup with a quantum point contact
(QPC) charge sensor. The HEMT is operating at 1K and the circuit has a
bandwidth of 1 MHz. The noise contribution of the HEMT at high frequencies is
only a few times higher than that of the QPC shot noise. We use this setup to
monitor single-electron tunneling to and from an adjacent quantum dot and we
measure fluctuations in the dot occupation as short as 400 nanoseconds, 20
times faster than in previous work.Comment: 4 pages, 3 figure
Detection of single electron spin resonance in a double quantum dot
Spin-dependent transport measurements through a double quantum dot are a
valuable tool for detecting both the coherent evolution of the spin state of a
single electron as well as the hybridization of two-electron spin states. In
this paper, we discuss a model that describes the transport cycle in this
regime, including the effects of an oscillating magnetic field (causing
electron spin resonance) and the effective nuclear fields on the spin states in
the two dots. We numerically calculate the current flow due to the induced spin
flips via electron spin resonance and we study the detector efficiency for a
range of parameters. The experimental data are compared with the model and we
find a reasonable agreement.Comment: 7 pages, 5 figures. To be published in Journal of Applied Physics,
proceedings ICPS 200
Single-shot readout of electron spin states in a quantum dot using spin-dependent tunnel rates
We present a method for reading out the spin state of electrons in a quantum
dot that is robust against charge noise and can be used even when the electron
temperature exceeds the energy splitting between the states. The spin states
are first correlated to different charge states using a spin dependence of the
tunnel rates. A subsequent fast measurement of the charge on the dot then
reveals the original spin state. We experimentally demonstrate the method by
performing read-out of the two-electron spin states, achieving a single-shot
visibility of more than 80%. We find very long triplet-to-singlet relaxation
times (up to several milliseconds), with a strong dependence on in-plane
magnetic field.Comment: 4 pages, 4 figure
Semiconductor few-electron quantum dot operated as a bipolar spin filter
We study the spin states of a few-electron quantum dot defined in a
two-dimensional electron gas, by applying a large in-plane magnetic field. We
observe the Zeeman splitting of the two-electron spin triplet states. Also, the
one-electron Zeeman splitting is clearly resolved at both the zero-to-one and
the one-to-two electron transition. Since the spin of the electrons transmitted
through the dot is opposite at these two transitions, this device can be
employed as an electrically tunable, bipolar spin filter. Calculations and
measurements show that higher-order tunnel processes and spin-orbit interaction
have a negligible effect on the polarization.Comment: 4 pages, 3 figure
The VLT-FLAMES Tarantula Survey X: Evidence for a bimodal distribution of rotational velocities for the single early B-type stars
Aims: Projected rotational velocities (\vsini) have been estimated for 334
targets in the VLT-FLAMES Tarantula survey that do not manifest significant
radial velocity variations and are not supergiants. They have spectral types
from approximately O9.5 to B3. The estimates have been analysed to infer the
underlying rotational velocity distribution, which is critical for
understanding the evolution of massive stars.
Methods: Projected rotational velocities were deduced from the Fourier
transforms of spectral lines, with upper limits also being obtained from
profile fitting. For the narrower lined stars, metal and non-diffuse helium
lines were adopted, and for the broader lined stars, both non-diffuse and
diffuse helium lines; the estimates obtained using the different sets of lines
are in good agreement. The uncertainty in the mean estimates is typically 4%
for most targets. The iterative deconvolution procedure of Lucy has been used
to deduce the probability density distribution of the rotational velocities.
Results: Projected rotational velocities range up to approximately 450 \kms
and show a bi-modal structure. This is also present in the inferred rotational
velocity distribution with 25% of the sample having \ve100\,\kms
and the high velocity component having \ve\,\kms. There is no
evidence from the spatial and radial velocity distributions of the two
components that they represent either field and cluster populations or
different episodes of star formation. Be-type stars have also been identified.
Conclusions: The bi-modal rotational velocity distribution in our sample
resembles that found for late-B and early-A type stars. While magnetic braking
appears to be a possible mechanism for producing the low-velocity component, we
can not rule out alternative explanations.Comment: to be publisged in A&
Control and Detection of Singlet-Triplet Mixing in a Random Nuclear Field
We observe mixing between two-electron singlet and triplet states in a double
quantum dot, caused by interactions with nuclear spins in the host
semiconductor. This mixing is suppressed by applying a small magnetic field, or
by increasing the interdot tunnel coupling and thereby the singlet-triplet
splitting. Electron transport involving transitions between triplets and
singlets in turn polarizes the nuclei, resulting in striking bistabilities. We
extract from the fluctuating nuclear field a limitation on the time-averaged
spin coherence time T2* of 25 ns. Control of the electron-nuclear interaction
will therefore be crucial for the coherent manipulation of individual electron
spins.Comment: 4 pages main text, 4 figure
Effect of metallicity on the gravitational-wave signal from the cosmological population of compact binary coalescences
Recent studies on stellar evolution have shown that the properties of compact
objects strongly depend on the metallicity of the environment in which they
were formed. Using some very simple assumptions on the metallicity of the
stellar populations, we explore how this property affects the unresolved
gravitational-wave background from extragalactic compact binaries. We obtained
a suit of models using population synthesis code, estimated the
gravitational-wave background they produce, and discuss its detectability with
second- (advanced LIGO, advanced Virgo) and third- (Einstein Telescope)
generation detectors. Our results show that the background is dominated by
binary black holes for all considered models in the frequency range of
terrestrial detectors, and that it could be detected in most cases by advanced
LIGO/Virgo, and with Einstein Telescope with a very high signal-to-noise ratio.
The observed peak in a gravitational wave spectrum depends on the metallicity
of the stellar population.Comment: 9 pages, 5 figures, accepted to A&
Spin filling of a quantum dot derived from excited-state spectroscopy
We study the spin filling of a semiconductor quantum dot using excited-state
spectroscopy in a strong magnetic field. The field is oriented in the plane of
the two-dimensional electron gas in which the dot is electrostatically defined.
By combining the observation of Zeeman splitting with our knowledge of the
absolute number of electrons, we are able to determine the ground state spin
configuration for one to five electrons occupying the dot. For four electrons,
we find a ground state spin configuration with total spin S=1, in agreement
with Hund's first rule. The electron g-factor is observed to be independent of
magnetic field and electron number.Comment: 11 pages, 7 figures, submitted to New Journal of Physics, focus issue
on Solid State Quantum Informatio
Quenched QCD at finite density
Simulations of quenched at relatively small but {\it nonzero} chemical
potential on lattices indicate that the nucleon
screening mass decreases linearly as increases predicting a critical
chemical potential of one third the nucleon mass, , by extrapolation.
The meson spectrum does not change as increases over the same range, from
zero to . Past studies of quenched lattice QCD have suggested that
there is phase transition at . We provide alternative
explanations for these results, and find a number of technical reasons why
standard lattice simulation techniques suffer from greatly enhanced
fluctuations and finite size effects for ranging from to
. We find evidence for such problems in our simulations, and suggest
that they can be surmounted by improved measurement techniques.Comment: 23 pages, Revte
Multiple Nuclear Polarization States in a Double Quantum Dot
We observe multiple stable states of nuclear polarization in a double quantum
dot under conditions of electron spin resonance. The stable states can be
understood within an elaborated theoretical rate equation model for the
polarization in each of the dots, in the limit of strong driving. This model
also captures unusual features of the data, such as fast switching and a
`wrong' sign of polarization. The results reported enable applications of this
polarization effect, including manipulation and control of nuclear fields.Comment: 5 pages, 4 figures, 7 pages supplementary materia
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