496 research outputs found
Interlaced Dynamical Decoupling and Coherent Operation of a Singlet-Triplet Qubit
We experimentally demonstrate coherence recovery of singlet-triplet
superpositions by interlacing qubit rotations between Carr-Purcell (CP) echo
sequences. We then compare performance of Hahn, CP, concatenated dynamical
decoupling (CDD) and Uhrig dynamical decoupling (UDD) for singlet recovery. In
the present case, where gate noise and drift combined with spatially varying
hyperfine coupling contribute significantly to dephasing, and pulses have
limited bandwidth, CP and CDD yield comparable results, with T2 ~ 80
microseconds.Comment: related papers at http://marcuslab.harvard.ed
The Resonant Exchange Qubit
We introduce a solid-state qubit in which exchange interactions among
confined electrons provide both the static longitudinal field and the
oscillatory transverse field, allowing rapid and full qubit control via rf
gate-voltage pulses. We demonstrate two-axis control at a detuning sweet-spot,
where leakage due to hyperfine coupling is suppressed by the large exchange
gap. A {\pi}/2-gate time of 2.5 ns and a coherence time of 19 {\mu}s, using
multi-pulse echo, are also demonstrated. Model calculations that include
effects of hyperfine noise are in excellent quantitative agreement with
experiment
Heterogeneity in susceptibility dictates the order of epidemiological models
The fundamental models of epidemiology describe the progression of an
infectious disease through a population using compartmentalized differential
equations, but do not incorporate population-level heterogeneity in infection
susceptibility. We show that variation strongly influences the rate of
infection, while the infection process simultaneously sculpts the
susceptibility distribution. These joint dynamics influence the force of
infection and are, in turn, influenced by the shape of the initial variability.
Intriguingly, we find that certain susceptibility distributions (the
exponential and the gamma) are unchanged through the course of the outbreak,
and lead naturally to power-law behavior in the force of infection; other
distributions often tend towards these "eigen-distributions" through the
process of contagion. The power-law behavior fundamentally alters predictions
of the long-term infection rate, and suggests that first-order epidemic models
that are parameterized in the exponential-like phase may systematically and
significantly over-estimate the final severity of the outbreak
Fast sensing of double-dot charge arrangement and spin state with an rf sensor quantum dot
Single-shot measurement of the charge arrangement and spin state of a double
quantum dot are reported, with measurement times down to ~ 100 ns. Sensing uses
radio-frequency reflectometry of a proximal quantum dot in the Coulomb blockade
regime. The sensor quantum dot is up to 30 times more sensitive than a
comparable quantum point contact sensor, and yields three times greater signal
to noise in rf single-shot measurements. Numerical modeling is qualitatively
consistent with experiment and shows that the improved sensitivity of the
sensor quantum dot results from reduced screening and lifetime broadening.Comment: related papers at http://marcuslab.harvard.ed
Conditional operation of a spin qubit
We report coherent operation of a singlet-triplet qubit controlled by the
arrangement of two electrons in an adjacent double quantum dot. The system we
investigate consists of two pairs of capacitively coupled double quantum dots
fabricated by electrostatic gates on the surface of a GaAs heterostructure. We
extract the strength of the capacitive coupling between qubit and double
quantum dot and show that the present geometry allows fast conditional gate
operation, opening pathways to multi-qubit control and implementation of
quantum algorithms with spin qubits.Comment: related papers here: http://marcuslab.harvard.ed
Self-Consistent Measurement and State Tomography of an Exchange-Only Spin Qubit
We report initialization, complete electrical control, and single-shot
readout of an exchange-only spin qubit. Full control via the exchange
interaction is fast, yielding a demonstrated 75 qubit rotations in under 2 ns.
Measurement and state tomography are performed using a maximum-likelihood
estimator method, allowing decoherence, leakage out of the qubit state space,
and measurement fidelity to be quantified. The methods developed here are
generally applicable to systems with state leakage, noisy measurements, and
non-orthogonal control axes.Comment: contains Supplementary Informatio
The phase stability of large-size nanoparticle alloy catalysts at ab initio quality using a nearsighted force-training approach
CoPt nanoparticle catalysts are integral to commercial fuel cells. Such
systems are prohibitive to fully characterize with electronic structure
calculations. Machine-learned potentials offer a scalable solution; however,
such potentials are only reliable if representative training data can be
employed, which typically requires large electronic structure calculations.
Here, we use the nearsighted-force training approach to make high-fidelity
machine-learned predictions on large nanoparticles with 5,000 atoms using
only systematically generated small structures ranging from 38-168 atoms. The
resulting ensemble model shows good accuracy and transferability in describing
relative energetics for CoPt nanoparticles with various shapes, sizes and Co
compositions. It is found that the fcc(100) surface is more likely to form a
L1 ordered structure than the fcc(111) surface. The energy convex hull of
the icosahedron shows the most stable particles have Pt-rich skins and Co-rich
underlayers. Although the truncated octahedron is the most stable shape across
all sizes of Pt nanoparticles, a crossover to icosahedron exists due to a large
downshift of surface energy for CoPt nanoparticle alloys. The downshift can be
attributed to strain release on the icosahedron surface due to Co alloying. We
introduced a simple empirical model to describe the role of Co alloying in the
crossover for CoPt nanoparticles. With Monte-Carlo simulations we additionally
searched for the most stable atomic arrangement for a truncated octahedron with
equal Pt and Co compositions, and also we studied its order-disorder phase
transition. We validated the most stable configurations with a new highly
scalable density functional theory code called SPARC. Lastly, the
order-disorder phase transition for a CoPt nanoparticle exhibits a lower
transition temperature and a smoother transition, compared to the bulk CoPt
alloy.Comment: 26 pages, 8 figure
Relaxation and Readout Visibility of a Singlet-Triplet Qubit in an Overhauser Field Gradient
Using single-shot charge detection in a GaAs double quantum dot, we
investigate spin relaxation time T_1 and readout visibility of a two-electron
singlet-triplet qubit following single-electron dynamic nuclear polarization
(DNP). For magnetic fields up to 2 T, the DNP cycle is in all cases found to
increase Overhauser field gradients, which in turn decrease T_1 and
consequently reduce readout visibility. This effect was previously attributed
to a suppression of singlet-triplet dephasing under a similar DNP cycle. A
model describing relaxation after singlet-triplet mixing agrees well with
experiment. Effects of pulse bandwidth on visibility are also investigated
Scaling of Dynamical Decoupling for Spin Qubits
We investigate scaling of coherence time, T2, with the number of
{\pi}-pulses, n_{\pi}, in a singlet- triplet spin qubit using
Carr-Purcell-Meiboom-Gill (CPMG) and concatenated dynamical decoupling (CDD)
pulse sequences. For an even numbers of CPMG pulses, we find a power law, T2 =
(n_{\pi})^({\gamma}_e), with {\gamma}_e = 0.72\pm0.01, essentially independent
of the envelope function used to extract T2. From this surprisingly robust
value, a power-law model of the noise spectrum of the environment, S({\omega})
~ {\omega}^(-{\beta}), yields {\beta} = {\gamma}_e/(1 - {\gamma}_e) = 2.6 \pm
0.1. Model values for T2(n_{\pi}) using {\beta} = 2.6 for CPMG with both even
and odd n_{\pi} up to 32 and CDD orders 3 through 6 compare very well with
experiment.Comment: related articles at http://marcuslab.harvard.ed
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