# Si/SiGe Quantum Quantum Computing News

## Quantum Computing Postdocs Available

## 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.

"Mitigating the effects of charge noise and improving the
coherence of a quantum dot hybrid qubit." B. Thorgrimsson, Dohun
Kim, Yuan-Chi Yang, C. B. Simmons, D. R. Ward, R. H. Foote,
D. E. Savage, M. G. Lagally, Mark Friesen, S. N. Coppersmith, and
M. A. Eriksson, *submitted for publication*.
[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]

## 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]

## Record Figure of Merit in a Semiconductor Charge Qubit

We demonstrate a record figure of merit (number of oscillations per T2* time) in a semiconductor charge qubit. We perform rotations about two independent axes on the Bloch sphere, including both single pulse (Larmor precession, sometimes called Rabi oscillations in other contexts) and two-pulse (Ramsey fringe) measurements.

"Coherent quantum oscillations and echo measurements of a Si charge qubit." Zhan Shi, C. B. Simmons, D. R. Ward, J. R. Prance, R. T. Mohr, Teck Seng Koh, John King Gamble, X. Wu, D. E. Savage, M. G. Lagally, Mark Friesen, S. N. Coppersmith, and M. A. Eriksson, *Phys. Rev. B* **88**, 075416 (2013). [Journal Article | arXiv]

## Single-Shot Measurement of Singlet-Triplet Lifetimes

We demonstrate single-shot measurement of singlet and triplet 2-electron spin states in a Si/SiGe double quantum dot. We use this method to measure singlet and triplet lifetimes as a function of magnetic field all the way to zero field. High measurement fidleity enables the analysis of hundreds of thousands of measurements.

"Single-shot measurement of triplet-singlet relaxation in a Si/SiGe double quantum dot." J. R. Prance, Zhan Shi, C. B. Simmons, D. E. Savage, M. G. Lagally, L. R. Schreiber, L. M. K. Vandersypen, Mark Friesen, Robert Joynt, S. N. Coppersmith, and M. A. Eriksson, *Phys. Rev. Lett.* **108**, 046808 (2012). [Journal Article | arXiv]

## Single-Shot Spin Measurement in a Si/SiGe Quantum Dot

Single-shot spin measurement has been demonstrated in a Si/SiGe quantum dot spin qubit. The measurement approach also enabled the measurement of the fraction of spin-up electrons loaded as a function of the voltage on one of the gates, demonstrating that such loading can be tuned by adjusting gate voltages.

"Tunable Spin Loading and T_{1} of a Silicon Spin Qubit Measured by Single-Shot Readout." C. B. Simmons, J. R. Prance, B. J. Van Bael, Teck Seng Koh, Zhan Shi, D. E. Savage, M. G. Lagally, Robert Joynt, Mark Friesen, S. N. Coppersmith, and M. A. Eriksson, *Phys. Rev. Lett.* **106**, 156804 (2011). [Journal Article | arXiv]

## Measurement of Fast Tunnel Rates in a One-Electron Si/SiGe Double Quantum Dot

Fast tunnel rates in single electron dots are important for rapid readout of changes in the electron occupation in such dots.

"Fast tunnel rates in Si/SiGe one-electron single and double quantum dots." Madhu Thalakulam, C. B. Simmons, B. M. Rosemeyer, D. E. Savage, M. G. Lagally, Mark Friesen, S. N. Coppersmith, and M. A. Eriksson, *Appl. Phys. Lett.* **96**, 183104 (2010). [Journal Article | arXiv]

## Measurement of Tunable Tunnel Coupling in a Si/SiGe Double Quantum Dot

The tunnel coupling between two quantum dots is a key factor controlling the exchange interaction between the dots and thus several types of quantum operations for spin qubits. This tunnel coupling has now been measured and controlled in a Si/SiGe double quantum dot.

"Charge Sensing and Controllable Tunnel Coupling in a Si/SiGe Double Quantum Dot." C. B. Simmons, Madhu Thalakulam, B. M. Rosemeyer, B. J. Van Bael, E. K. Sackmann, D. E. Savage, M. G. Lagally, Robert Joynt, Mark Friesen, S. N. Coppersmith, and M. A. Eriksson, *Nano Lett.* **9**, 3234 (2009). [Journal Article | arXiv]

## Spin Blockade Observed in a Silicon/Silicon-Germanium Double Quantum Dot

Spin blockade is a strong reduction in current through a double quantum dot that occurs when the spin state of the system blocks a tunneling event that is the next, necessary step for charge transport through the system. Spin blockade is useful for spin readout in semiconductor quantum dots.

"Spin blockade and lifetime-enhanced transport in a few-electron Si/SiGe double quantum dot." Nakul Shaji, C. B. Simmons, Madhu Thalakulam, L. J. Klein, Hua Qin, H. Luo, D. E. Savage, M. G. Lagally, A. J. Rimberg, Robert Joynt, Mark Friesen, R. Blick, S. N. Coppersmith, and M. A. Eriksson, *Nat. Phys.* **4**, 540 (2008). [Journal Article | arXiv]

## One-Electron Occuption in a Silicon/Silicon-Germanium Single Quantum Dot

A single electron in a quantum dot forms the basis for one type of semicondcutor quantum dot spin qubit.

"Single-electron quantum dot in Si/SiGe with integrated charge sensing." C. B. Simmons, Madhu Thalakulam, Nakul Shaji, L. J. Klein, Hua Qin, R. Blick, D. E. Savage, M. G. Lagally, S. N. Coppersmith, and M. A. Eriksson, *Appl. Phys. Lett.* **91**, 213103 (2007). [Journal Article | arXiv]