Valley splitting affects the energy dispersion of silicon quantum dot qubits, and occasionally manifests itself through some striking features. Here, the authors observe a strong correlation between unexpected ``sweet spots'' and ``hot spots'' in the coherence rates of a quantum-dot hybrid qubit and in some anomalous features in the energy dispersion. Through tight-binding simulations, they are able to attribute such effects to disorder at the quantum-well interface and speculate on the possibility of harnessing disorder to enhance qubit coherence times.
Achieving controllable coupling of dopants in silicon is crucial for operating donor-based qubit devices, but it is difficult because of the small size of donor-bound electron wavefunctions. Here we report the characterization of a quantum dot coupled to a localized electronic state, and we present evidence of controllable coupling between the quantum dot and the localized state. A set of measurements of transport through this device enable the determination of the most likely location of the localized state, consistent with an electronically active impurity in the quantum well near the edge of the quantum dot. The experiments we report are consistent with a gate-voltage controllable tunnel coupling, which is an important building block for hybrid donor and gate-defined quantum dot devices.
Identifying dominant sources of decoherence is an important step in understanding and improving quantum systems. Here we show that the free induction decay time ($T_{2}^{*}$) and the Rabi decay rate ($\Gamma_{\mathrm{Rabi}}$) of the quantum dot hybrid qubit are limited by charge noise for a large range of detunings. We show that by tuning the parameters of the qubit, and by operating the qubit at larger detunings, the coherence times can be increased by more than an order of magnitude. We achieve a Ramsey decay time $T_{2}^{*}$ of $127~\mathrm{ns}$ and a Rabi decay time, $1/\Gamma_{\mathrm{Rabi}}$, exceeding $1~\mathrm{\mu s}$. We show that the slowest $\Gamma_{\mathrm{Rabi}}$ is limited by fluctuations in the Rabi frequency induced by charge noise and not by fluctuations in the qubit energy itself.
Atomic-scale disorder at the top interface of a Si quantum well is known to suppress valley splitting. Such disorder may be inherited from the underlying substrate and relaxed buffer growth, but can also arise at the top quantum well interface due to the random SiGe alloy. Here, we perform activation energy (transport) measurements in the quantum Hall regime to determine the source of the disorder affecting the valley splitting. We consider three Si/SiGe heterostructures with nominally identical substrates but different barriers at the top of the quantum well, including two samples with pure-Ge interfaces. For all three samples, we observe a surprisingly strong and universal dependence of the valley splitting on the electron density (Ev ∼ n2.7) over the entire experimental range (Ev = 30–200 μeV). We interpret these results via tight binding theory, arguing that the underlying valley physics is determined mainly by disorder arising from the substrate and relaxed buffer growth.
TUNNEL DIODE CIRCUITRY incorporating transistors has been used to gain advanced digital speeds while improving circuit reliability and stability. Major disadvantages in the use of tunnel diodes are: (1) common input-output, (2) inability to obtain direct pulse inversion (3) bidirectional flow of information, and (4) the difficulty of stage-to-stage coupling caused by loading and insufficient signal transfer. Circuitry using transistors as emitter followers accomplishes pulse steering and impedance transformation. Nanosecond pulse transformers and transistor amplifiers provided signal inversion and amplification. A combination of these techniques makes possible advances in digital circuit operational speeds.
The Sierra Nevada Yellow-Legged Frog (Rana sierrae) has generally been viewed as a lake species, but it has increasingly been found in streams, including in the northern part of its range where it is particularly at risk. Developing effective conservation strategies has been hindered by a lack of knowledge of its basic ecological requirements in stream habitats. To address this information gap, we investigated the demography, habitat use, and movements of stream populations of this federally endangered species. We conducted capture–mark–recapture of adults, quantitatively described stream channel and riparian vegetation characteristics, and collected habitat use data at four northern Sierra Nevada mountain streams, counted egg masses at three central Sierra Nevada streams, and radio-tracked individuals at three central and southern Sierra Nevada streams. Stream populations in the northern range were very small with maximum abundances of <15 individuals, and apparent survival probability ranged from 0.57–0.81. In contrast, one southern Sierra Nevada stream had a large count of 547 adults. Egg mass counts ranged from 22–104 per stream. We found frogs in diverse headwater streams ranging from perennial to intermittent flow regimes, pool versus riffle dominated, and low to high channel gradient, and they used diverse microhabitats within these streams. In these stream habitats, frogs moved little over four-day survey periods but were capable of moving longer distances of up to 1248 m over the summer. Conservation and management of the at-risk R. sierrae are most likely to be effective when built on comprehensive quantitative information on basic ecological requirements in all habitats used by the species.
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