Brandur Thorgrimsson

UNSW Sydney

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S. N. Coppersmith

University of Wisconsin–Madison

Mark Friesen

University of Wisconsin–Madison

M. A. Eriksson

University of Wisconsin–Madison

Ryan H. Foote

University of Wisconsin–Madison

D. E. Savage

University of Wisconsin–Madison

Samuel F. Neyens

Intel (United States)

Thomas McJunkin

University of Wisconsin–Madison

E. R. MacQuarrie

Simon Fraser University

J. P. Dodson

University of Wisconsin–Madison

T. J. Knapp

University of Wisconsin–Madison

Cooperative Institutions

University of Wisconsin–Madison

UNSW Sydney

Centre for Quantum Computation and Communication Technology

Intel (United States)

Delft University of Technology

Quantum (Australia)

QuTech

Wisconsin Institutes for Discovery

Australian Research Council

University of Cambridge

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Signatures of atomic-scale structure in the energy dispersion and coherence of a Si quantum-dot qubit

Physical review. B./Physical review. B (2018)

J. C. Abadillo-Uriel Brandur Thorgrimsson Dohun Kim L. W. Smith C. B. Simmons

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.

Coherence time

10.1103/physrevb.98.165438

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Transport through an impurity tunnel coupled to a Si/SiGe quantum dot

Applied Physics Letters (2015)

Ryan H. Foote Daniel R. Ward J. R. Prance John King Gamble Erik Nielsen

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.

10.1063/1.4930909

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Mitigating the effects of charge noise and improving the coherence of a quantum dot hybrid qubit

Bulletin of the American Physical Society (2017)

Brandur Thorgrimsson Dohun Kim Yuan-Chi Yang C. B. Simmons Daniel R. Ward

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.

Rabi cycle

Charge qubit

Rabi frequency

Coherence time

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Coherence and Manipulation of Quantum Dot Hybrid Qubits

Brandur Thorgrimsson

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Progress towards two double-dot qubits in Si/SiGe: quadruple quantum dots

Bulletin of the American Physical Society (2017)

Ryan H. Foote Daniel R. Ward Dohun Kim Brandur Thorgrimsson L. W. Smith

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The critical role of substrate disorder in valley splitting in Si quantum wells

Applied Physics Letters (2018)

Samuel F. Neyens Ryan H. Foote Brandur Thorgrimsson T. J. Knapp Thomas McJunkin

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.

Shubnikov–de Haas effect

10.1063/1.5033447

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Citations (36)

Latched readout for the quantum dot hybrid qubit

Applied Physics Letters (2023)

J. Corrigan J. P. Dodson Brandur Thorgrimsson Samuel F. Neyens T. J. Knapp

A primary method of reading out a quantum dot hybrid qubit involves projection of the logical basis onto distinct charge states that are readily detected by an integrated charge sensing dot. However, in the simplest configuration, the excited charge state decays rapidly, making a single-shot readout challenging. Here, we demonstrate a readout procedure where the qubit excited state is latched to a metastable charge configuration whose lifetime is tunnel-rate limited, persisting here as long as 2.5 ms. Additionally, we show that working in the (4,1)–(3,2) charge configuration enables a latched readout window that is larger and more tunable than typical charge configurations, because the size of the readout window is determined by an orbital splitting rather than a valley splitting.

Metastability

Charge qubit

10.1063/5.0130865

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Tuning of Two-Stage HEMT Cryogenic Amplifier to Reduce Electron Temperature in a Nearby Quantum Dot

Bulletin of the American Physical Society (2018)

T. J. Knapp J. P. Dodson Brandur Thorgrimsson D. E. Savage M. G. Lagally

Cryogenic temperature

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Citations (1)