## Quantum Computing Postdoc Available

The Eriksson Group has an opening for a postdoc in the area of quantum dot qubits. If you are interested, please contact Mark Eriksson at maeriksson@wisc.edu.

## Eriksson Group Members Present at SQEW in San Sebastián, Spain

Several members of the Eriksson group presented a talk and 5 posters in October, 2019 at the Silicon Quantum Electronics Workshop (SQEW) in San Sebastián, Spain. The workshop science was outstanding, and so was the city of San Sebastián!

## Characterization of the capacitive coupling in a linear array of four quantum dots

The capacitive coupling strength *g* between two pairs of quantum dots is critical for performing capacitively-coupled two-qubit gates. Here we characterize both the strength and the tunability of this capacitive coupling in a highly-stable quadruple quantum dot array of gate-define, Si/SiGe quantum dots.

“Measurements of capacitive coupling within a quadruple quantum dot array.” S. F. Neyens, E. R. MacQuarrie, J. P. Dodson, J. Corrigan, Nathan Holman, B. Thorgrimsson, M. Palma, T. McJunkin, L. F. Edge, Mark Friesen, S. N. Coppersmith, and M. A. Eriksson, *submitted for publication*. [arXiv]

## Physics Today article describing quantum dot qubits

This article provides a broad overview of experimental developments in the area of quantum dot qubits.

“Quantum computing with semiconductor spins.” L. M. K. Vandersypen and M. A. Eriksson, *Physics Today ***72**, 38 (2019). [Journal Article]

## Congratulations, Brandur!

Brandur Thorgrimsson graduates with his Ph.D. in Summer, 2019. Brandur is off to UNSW, Australia for a postdoc in the group of Professor Michelle Simmons doing work on donor-based qubits in silicon.

## A two-qubit quantum processor in silicon

In collaboration with the group of Lieven Vandersypen, TU Delft, we demonstrate a two-fully programmable two-qubit quantum processor in a pair of tunnel-coupled Si/SiGe gate-defined quantum dots.

“A programmable two-qubit quantum processor in silicon.” T. F. Watson, S. G. J. Philips, E. Kawakami, D. R. Ward, P. Scarlino, M. Veldhorst, D. E. Savage, M. G. Lagally, Mark Friesen, S. N. Coppersmith, M. A. Eriksson, and L. M. K. Vandersypen, *Nature* **555**, 633 (2018). [Journal Article | arXiv]

## Extending the coherence of a quantum dot hybrid qubit

We demonstrate more than an order of magnitude increase in the coherence times of the quantum dot hybrid qubit (QDHQ). This increase is enabled by choosing internal parameters of the qubit, such as the tunnel couplings between the two quantum dots and the energy detuning between the dots, to make the qubit more robust against charge noise.

“Extending the coherence of a quantum dot hybrid qubit.” B. Thorgrimsson, Dohun Kim, Yuan-Chi Yang, L. W. Smith, C. B. Simmons, D. R. Ward, R. H. Foote, J. Corrigan, D. E. Savage, M. G. Lagally, Mark Friesen, S. N. Coppersmith, and M. A. Eriksson, *npj Quant. Inf.* **3**, 32 (2017). [Journal Article | arXiv]

## State-conditional coherent charge qubit oscillations in a Si/SiGe quadruple quantum dot.

Two-qubit gates require a qubit-qubit interaction, which for many semiconductor quantum dot qubits is the Coulomb interaction. Here we demonstrate such an interaction between a pair of double quantum dots in Si/SiGe. We use the classical charge state of one double dot to control coherent charge qubit oscillations in a neighboring double dot, enabling the extraction of the interaction energy available for driving two-qubit gates.

“State-conditional coherent charge qubit oscillations in a Si/SiGe quadruple quantum dot.” D. R. Ward, Dohun Kim, D. E. Savage, M. G. Lagally, R. H. Foote, Mark Friesen, S. N. Coppersmith, and M. A. Eriksson, *npj Quant. Inf.* **2**, 16032 (2016). [Journal Article | arXiv]

## AC Driving of the Quantum Dot Hybrid Qubit (QDHQ)

The quantum dot hybrid qubit is composed of 3 electrons in 2 quantum dots, and it offers an useful combination of fast manipulation speeds and relatively long coherence times. In this work we demonstrate that the QDHQ can be driven with microwave bursts, in close analogy with traditional single spin qubits manipulated by electron spin resonance, but in the QDHQ we can use an electric field drive rather than a magnetic field drive. Operating in this way, using AC gating, improves the coherence of the qubit by enabling operation in regions of the qubit phase space that are relatively well protected from charge noise.

“High-fidelity resonant gating of a silicon-based quantum dot hybrid qubit.” Dohun Kim, D. R. Ward, C. B. Simmons, D. E. Savage, M. G. Lagally, Mark Friesen, S. N. Coppersmith, and M. A. Eriksson, *npj Quant. Inf.* **1**, 15004 (2015). [Journal Article | arXiv]

## Experimental Demonstration of the Quantum Dot Hybrid Qubit (QDHQ)

We demonstrate experimentally the quantum hybrid qubit, which consists of three electrons in two quantum dots. This qubit is a hybrid, on the one hand, of spin and charge: in the far-detuned regime, it is purely a spin qubit, and in the near-detuned regime, it mixes in a charge component, doing so in a manner that can be made remarkably robust against charge noise. On the other hand, the QDHQ is a hybrid of a single-spin (Loss-DiVincenzo) qubit and a singlet-triplet (ST) qubit, and this combination enables high-speed quantum gates.

“Quantum control and process tomography of a semiconductor quantum dot hybrid qubit.” Dohun Kim, Zhan Shi, C. B. Simmons, D. R. Ward, J. R. Prance, Teck Seng Koh, John King Gamble, D. E. Savage, M. G. Lagally, Mark Friesen, S. N. Coppersmith, and M. A. Eriksson, *Nature ***511**, 70 (2014). [Journal Article | arXiv]

## Two-axis control of a singlet-triplet qubit with an integrated micromagnet

We demonstrate two-axis control of a singlet-triplet qubit in Si/SiGe through use of an integrated micromagnet. We also demonstrate use of gates directly over the tunnel barriers in a Si/SiGe gate-defined quantum dot to provide enhanced control over the tunnel rates into and out of the quantum dots.

“Two-axis control of a singlet-triplet qubit with an integrated micromagnet.” X. Wu, D. R. Ward, J. R. Prance, Dohun Kim, John King Gamble, R. T. Mohr, Zhan Shi, D. E. Savage, M. G. Lagally, Mark Friesen, S. N. Coppersmith, and M. A. Eriksson, *Proc. Natl. Acad. Sci.* **111**, 11938 (2014). [Journal Article | arXiv]