220 research outputs found
Symmetric Operation of the Resonant Exchange Qubit
We operate a resonant exchange qubit in a highly symmetric triple-dot
configuration using IQ-modulated RF pulses. At the resulting three-dimensional
sweet spot the qubit splitting is an order of magnitude less sensitive to all
relevant control voltages, compared to the conventional operating point, but we
observe no significant improvement in the quality of Rabi oscillations. For
weak driving this is consistent with Overhauser field fluctuations modulating
the qubit splitting. For strong driving we infer that effective voltage noise
modulates the coupling strength between RF drive and the qubit, thereby
quickening Rabi decay. Application of CPMG dynamical decoupling sequences
consisting of up to n = 32 {\pi} pulses significantly prolongs qubit coherence,
leading to marginally longer dephasing times in the symmetric configuration.
This is consistent with dynamical decoupling from low frequency noise, but
quantitatively cannot be explained by effective gate voltage noise and
Overhauser field fluctuations alone. Our results inform recent strategies for
the utilization of partial sweet spots in the operation and long-distance
coupling of triple-dot qubits.Comment: 6 pages, 5 figure
Noise suppression using symmetric exchange gates in spin qubits
We demonstrate a substantial improvement in the spin-exchange gate using
symmetric control instead of conventional detuning in GaAs spin qubits, up to a
factor-of-six increase in the quality factor of the gate. For symmetric
operation, nanosecond voltage pulses are applied to the barrier that controls
the interdot potential between quantum dots, modulating the exchange
interaction while maintaining symmetry between the dots. Excellent agreement is
found with a model that separately includes electrical and nuclear noise
sources for both detuning and symmetric gating schemes. Unlike exchange control
via detuning, the decoherence of symmetric exchange rotations is dominated by
rotation-axis fluctuations due to nuclear field noise rather than direct
exchange noise.Comment: 5 pages main text (4 figures) plus 5 pages supplemental information
(3 figures
Spectrum of the Nuclear Environment for GaAs Spin Qubits
Using a singlet-triplet spin qubit as a sensitive spectrometer of the GaAs
nuclear spin bath, we demonstrate that the spectrum of Overhauser noise agrees
with a classical spin diffusion model over six orders of magnitude in
frequency, from 1 mHz to 1 kHz, is flat below 10 mHz, and falls as for
frequency Hz. Increasing the applied magnetic field from
0.1 T to 0.75 T suppresses electron-mediated spin diffusion, which decreases
spectral content in the region and lowers the saturation frequency,
each by an order of magnitude, consistent with a numerical model. Spectral
content at megahertz frequencies is accessed using dynamical decoupling, which
shows a crossover from the few-pulse regime ( -pulses),
where transverse Overhauser fluctuations dominate dephasing, to the many-pulse
regime ( -pulses), where longitudinal Overhauser
fluctuations with a spectrum dominate.Comment: 6 pages, 4 figures, 8 pages of supplementary material, 5
supplementary figure
Fast spin exchange between two distant quantum dots
The Heisenberg exchange interaction between neighboring quantum dots allows
precise voltage control over spin dynamics, due to the ability to precisely
control the overlap of orbital wavefunctions by gate electrodes. This allows
the study of fundamental electronic phenomena and finds applications in quantum
information processing. Although spin-based quantum circuits based on
short-range exchange interactions are possible, the development of scalable,
longer-range coupling schemes constitutes a critical challenge within the
spin-qubit community. Approaches based on capacitative coupling and
cavity-mediated interactions effectively couple spin qubits to the charge
degree of freedom, making them susceptible to electrically-induced decoherence.
The alternative is to extend the range of the Heisenberg exchange interaction
by means of a quantum mediator. Here, we show that a multielectron quantum dot
with 50-100 electrons serves as an excellent mediator, preserving speed and
coherence of the resulting spin-spin coupling while providing several
functionalities that are of practical importance. These include speed (mediated
two-qubit rates up to several gigahertz), distance (of order of a micrometer),
voltage control, possibility of sweet spot operation (reducing susceptibility
to charge noise), and reversal of the interaction sign (useful for dynamical
decoupling from noise).Comment: 6 pages including 4 figures, plus 8 supplementary pages including 5
supplementary figure
First Stellar Abundances in the Dwarf Irregular Galaxy Sextans A
We present the abundance analyses of three isolated A-type supergiant stars
in the dwarf irregular galaxy Sextans A from high-resolution spectra the UVES
spectrograph at the VLT. Detailed model atmosphere analyses have been used to
determine the stellar atmospheric parameters and the elemental abundances of
the stars. The mean iron group abundance was determined from these three stars
to be [(FeII,CrII)/H]=-0.99+/-0.04+/-0.06. This is the first determination of
the present-day iron group abundances in Sextans A. These three stars now
represent the most metal-poor massive stars for which detailed abundance
analyses have been carried out. The mean stellar alpha element abundance was
determined from the alpha element magnesium as
[alpha(MgI)/H]=-1.09+/-0.02+/-0.19. This is in excellent agreement with the
nebular alpha element abundances as determined from oxygen in the H II regions.
These results are consistent from star-to-star with no significant spatial
variations over a length of 0.8 kpc in Sextans A. This supports the nebular
abundance studies of dwarf irregular galaxies, where homogeneous oxygen
abundances are found throughout, and argues against in situ enrichment. The
alpha/Fe abundance ratio is [alpha(MgI)/FeII,CrII]=-0.11+/-0.02+/-0.10, which
is consistent with the solar ratio. This is consistent with the results from
A-supergiant analyses in other Local Group dwarf irregular galaxies but in
stark contrast with the high [alpha/Fe] results from metal-poor stars in the
Galaxy, and is most clearly seen from these three stars in Sextans A because of
their lower metallicities. The low [alpha/Fe] ratios are consistent with the
slow chemical evolution expected for dwarf galaxies from analyses of their
stellar populations.Comment: 40 pages, 8 figures, accepted for publication in A
Spin of a Multielectron Quantum Dot and Its Interaction with a Neighboring Electron
We investigate the spin of a multielectron GaAs quantum dot in a sequence of
nine charge occupancies, by exchange coupling the multielectron dot to a
neighboring two-electron double quantum dot. For all nine occupancies, we make
use of a leakage spectroscopy technique to reconstruct the spectrum of spin
states in the vicinity of the interdot charge transition between a single- and
a multielectron quantum dot. In the same regime we also perform time-resolved
measurements of coherent exchange oscillations between the single- and
multielectron quantum dot. With these measurements, we identify distinct
characteristics of the multielectron spin state, depending on whether the dot's
occupancy is even or odd. For three out of four even occupancies we do not
observe any exchange interaction with the single quantum dot, indicating a
spin-0 ground state. For the one remaining even occupancy, we observe an
exchange interaction that we associate with a spin-1 multielectron quantum dot
ground state. For all five of the odd occupancies, we observe an exchange
interaction associated with a spin-1/2 ground state. For three of these odd
occupancies, we clearly demonstrate that the exchange interaction changes sign
in the vicinity of the charge transition. For one of these, the exchange
interaction is negative (i.e. triplet-preferring) beyond the interdot charge
transition, consistent with the observed spin-1 for the next (even) occupancy.
Our experimental results are interpreted through the use of a Hubbard model
involving two orbitals of the multielectron quantum dot. Allowing for the spin
correlation energy (i.e. including a term favoring Hund's rules) and different
tunnel coupling to different orbitals, we qualitatively reproduce the measured
exchange profiles for all occupancies.Comment: 20 pages, 13 figures, 2 table
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