6,657 research outputs found
QFlow lite dataset: A machine-learning approach to the charge states in quantum dot experiments
Over the past decade, machine learning techniques have revolutionized how
research is done, from designing new materials and predicting their properties
to assisting drug discovery to advancing cybersecurity. Recently, we added to
this list by showing how a machine learning algorithm (a so-called learner)
combined with an optimization routine can assist experimental efforts in the
realm of tuning semiconductor quantum dot (QD) devices. Among other
applications, semiconductor QDs are a candidate system for building quantum
computers. The present-day tuning techniques for bringing the QD devices into a
desirable configuration suitable for quantum computing that rely on heuristics
do not scale with the increasing size of the quantum dot arrays required for
even near-term quantum computing demonstrations. Establishing a reliable
protocol for tuning that does not rely on the gross-scale heuristics developed
by experimentalists is thus of great importance. To implement the machine
learning-based approach, we constructed a dataset of simulated QD device
characteristics, such as the conductance and the charge sensor response versus
the applied electrostatic gate voltages. Here, we describe the methodology for
generating the dataset, as well as its validation in training convolutional
neural networks. We show that the learner's accuracy in recognizing the state
of a device is ~96.5 % in both current- and charge-sensor-based training. We
also introduce a tool that enables other researchers to use this approach for
further research: QFlow lite - a Python-based mini-software suite that uses the
dataset to train neural networks to recognize the state of a device and
differentiate between states in experimental data. This work gives the
definitive reference for the new dataset that will help enable researchers to
use it in their experiments or to develop new machine learning approaches and
concepts.Comment: 18 pages, 6 figures, 3 table
Probing short-range magnetic order in a geometrically frustrated magnet by spin Seebeck effect
Competing magnetic interactions in geometrically frustrated magnets give rise
to new forms of correlated matter, such as spin liquids and spin ices.
Characterizing the magnetic structure of these states has been difficult due to
the absence of long-range order. Here, we demonstrate that the spin Seebeck
effect (SSE) is a sensitive probe of magnetic short-range order (SRO) in
geometrically frustrated magnets. In low temperature (2 - 5 K) SSE measurements
on a model frustrated magnet \mathrm{Gd_{3}Ga_{5}O_{12}}, we observe
modulations in the spin current on top of a smooth background. By comparing to
existing neutron diffraction data, we find that these modulations arise from
field-induced magnetic ordering that is short-range in nature. The observed SRO
is anisotropic with the direction of applied field, which is verified by
theoretical calculation.Comment: 5 pages, 4 figure
The phase diagram of twisted mass lattice QCD
We use the effective chiral Lagrangian to analyze the phase diagram of
two-flavor twisted mass lattice QCD as a function of the normal and twisted
masses, generalizing previous work for the untwisted theory. We first determine
the chiral Lagrangian including discretization effects up to next-to-leading
order (NLO) in a combined expansion in which m_\pi^2/(4\pi f_\pi)^2 ~ a \Lambda
(a being the lattice spacing, and \Lambda = \Lambda_{QCD}). We then focus on
the region where m_\pi^2/(4\pi f_\pi)^2 ~ (a \Lambda)^2, in which case
competition between leading and NLO terms can lead to phase transitions. As for
untwisted Wilson fermions, we find two possible phase diagrams, depending on
the sign of a coefficient in the chiral Lagrangian. For one sign, there is an
Aoki phase for pure Wilson fermions, with flavor and parity broken, but this is
washed out into a crossover if the twisted mass is non-vanishing. For the other
sign, there is a first order transition for pure Wilson fermions, and we find
that this transition extends into the twisted mass plane, ending with two
symmetrical second order points at which the mass of the neutral pion vanishes.
We provide graphs of the condensate and pion masses for both scenarios, and
note a simple mathematical relation between them. These results may be of
importance to numerical simulations.Comment: 13 pages, 5 figures, small clarifying comments added in introduction,
minor typos fixed. Version to be published in Phys. Rev.
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