527 research outputs found
Controlling spin-orbit interactions in silicon quantum dots using magnetic field direction
Silicon quantum dots are considered an excellent platform for spin qubits,
partly due to their weak spin-orbit interaction. However, the sharp interfaces
in the heterostructures induce a small but significant spin-orbit interaction
which degrade the performance of the qubits or, when understood and controlled,
could be used as a powerful resource. To understand how to control this
interaction we build a detailed profile of the spin-orbit interaction of a
silicon metal-oxide-semiconductor double quantum dot system. We probe the
derivative of the Stark shift, -factor and -factor difference for two
single-electron quantum dot qubits as a function of external magnetic field and
find that they are dominated by spin-orbit interactions originating from the
vector potential, consistent with recent theoretical predictions. Conversely,
by populating the double dot with two electrons we probe the mixing of singlet
and spin-polarized triplet states during electron tunneling, which we conclude
is dominated by momentum-term spin-orbit interactions that varies from 1.85 MHz
up to 27.5 MHz depending on the magnetic field orientation. Finally, we exploit
the tunability of the derivative of the Stark shift of one of the dots to
reduce its sensitivity to electric noise and observe an 80 % increase in
. We conclude that the tuning of the spin-orbit interaction will be
crucial for scalable quantum computing in silicon and that the optimal setting
will depend on the exact mode of qubit operations used
Measuring the degree of starshape in genealogies - summary statistics and demographic inference
High-fidelity adiabatic inversion of a electron spin qubit in natural silicon
The main limitation to the high-fidelity quantum control of spins in
semiconductors is the presence of strongly fluctuating fields arising from the
nuclear spin bath of the host material. We demonstrate here a substantial
improvement in single-qubit gate fidelities for an electron spin qubit bound to
a P atom in natural silicon, by applying adiabatic inversion instead of
narrow-band pulses. We achieve an inversion fidelity of 97%, and we observe
signatures in the spin resonance spectra and the spin coherence time that are
consistent with the presence of an additional exchange-coupled donor. This work
highlights the effectiveness of adiabatic inversion techniques for spin control
in fluctuating environments.Comment: 4 pages, 2 figure
Bell's inequality violation with spins in silicon
Bell's theorem sets a boundary between the classical and quantum realms, by
providing a strict proof of the existence of entangled quantum states with no
classical counterpart. An experimental violation of Bell's inequality demands
simultaneously high fidelities in the preparation, manipulation and measurement
of multipartite quantum entangled states. For this reason the Bell signal has
been tagged as a single-number benchmark for the performance of quantum
computing devices. Here we demonstrate deterministic, on-demand generation of
two-qubit entangled states of the electron and the nuclear spin of a single
phosphorus atom embedded in a silicon nanoelectronic device. By sequentially
reading the electron and the nucleus, we show that these entangled states
violate the Bell/CHSH inequality with a Bell signal of 2.50(10). An even higher
value of 2.70(9) is obtained by mapping the parity of the two-qubit state onto
the nuclear spin, which allows for high-fidelity quantum non-demolition
measurement (QND) of the parity. Furthermore, we complement the Bell inequality
entanglement witness with full two-qubit state tomography exploiting QND
measurement, which reveals that our prepared states match the target maximally
entangled Bell states with 96\% fidelity. These experiments demonstrate
complete control of the two-qubit Hilbert space of a phosphorus atom, and show
that this system is able to maintain its simultaneously high initialization,
manipulation and measurement fidelities past the single-qubit regime.Comment: 10 pages, 3 figures, 1 table, 4 extended data figure
Patterns of neutral diversity under general models of selective sweeps
Two major sources of stochasticity in the dynamics of neutral alleles result
from resampling of finite populations (genetic drift) and the random genetic
background of nearby selected alleles on which the neutral alleles are found
(linked selection). There is now good evidence that linked selection plays an
important role in shaping polymorphism levels in a number of species. One of
the best investigated models of linked selection is the recurrent full sweep
model, in which newly arisen selected alleles fix rapidly. However, the bulk of
selected alleles that sweep into the population may not be destined for rapid
fixation. Here we develop a general model of recurrent selective sweeps in a
coalescent framework, one that generalizes the recurrent full sweep model to
the case where selected alleles do not sweep to fixation. We show that in a
large population, only the initial rapid increase of a selected allele affects
the genealogy at partially linked sites, which under fairly general assumptions
are unaffected by the subsequent fate of the selected allele. We also apply the
theory to a simple model to investigate the impact of recurrent partial sweeps
on levels of neutral diversity, and find that for a given reduction in
diversity, the impact of recurrent partial sweeps on the frequency spectrum at
neutral sites is determined primarily by the frequencies achieved by the
selected alleles. Consequently, recurrent sweeps of selected alleles to low
frequencies can have a profound effect on levels of diversity but can leave the
frequency spectrum relatively unperturbed. In fact, the limiting coalescent
model under a high rate of sweeps to low frequency is identical to the standard
neutral model. The general model of selective sweeps we describe goes some way
towards providing a more flexible framework to describe genomic patterns of
diversity than is currently available.Comment: 44 pages. 5 figure
A complex pattern of post‐divergence expansion, contraction, introgression and asynchronous responses to Pleistocene climate changes in two Dipelta sister species from western China
The well-known vicariance and dispersal models dominate in understanding the allopatric pattern for related species and presume the simultaneous occurrence of speciation and biogeographic events. However, the formation of allopatry may postdate the species divergence. We examined this hypothesis using DNA sequence data from 3 chloroplast fragments and 5 nuclear loci of Dipelta floribunda and D. yunnanensis, two shrub species with the circum Sichuan Basin distribution, combining the climatic niche modeling approach. The best-fit model supported by the approximate Bayesian computation (ABC) analysis indicated that, D. floribunda and D. yunnanensis diverged during the mid-Pleistocene period, consistent with the largest glacial period in the Qinghai-Tibet Plateau (QTP). The historically inter-specific gene flow was identified but seemed to have ceased after the last interglacial period (LIG), when the range of D. floribunda moved northward from the south of the Sichuan Basin. Further, populations of D. floribunda had expanded obviously in the north of the Sichuan Basin after the last glacial maximum (LGM). Relatively, the range of D. yunnanensis expanded before the LGM, reduced during the post-LGM especially in the north of the Sichuan Basin, reflecting the asynchronous responses of related species to the contemporary climate changes. Our results suggested that complex topography should be considered in understanding the distributional patterns even for closely related species and their demographic responses
Electrically controlling single spin qubits in a continuous microwave field
Large-scale quantum computers must be built upon quantum bits that are both
highly coherent and locally controllable. We demonstrate the quantum control of
the electron and the nuclear spin of a single 31P atom in silicon, using a
continuous microwave magnetic field together with nanoscale electrostatic
gates. The qubits are tuned into resonance with the microwave field by a local
change in electric field, which induces a Stark shift of the qubit energies.
This method, known as A-gate control, preserves the excellent coherence times
and gate fidelities of isolated spins, and can be extended to arbitrarily many
qubits without requiring multiple microwave sources.Comment: Main paper: 13 pages, 4 figures. Supplementary information: 25 pages,
13 figure
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