Talk: Dogs and cats, living together! How approaches to Josephson junction and spin-based quantum computing can learn from each other

I will talk about two of our recent proposals for quantum computers based on superconducting JJ circuits [1] and spins in semiconductors [2]: what motivated them, why they are worth pursuing experimentally, and directions for future research.

[1] Semiconductor-inspired design principles for superconducting quantum computing, Nature Communications 7, 11059 (2016)

[2] Charge-noise-insensitive gate operations for always-on, exchange-only qubits, Phys. Rev. B 93, 121410(R) (2016)

Lincoln Laboratory, Boston, MA



PRB-Rapid: Charge-noise-insensitive gate operations for always-on, exchange-only qubits

We introduce an always-on, exchange-only (AEON) qubit made up of three localized semiconductor spins that offers a true “sweet spot” to fluctuations of the quantum dot energy levels. Both single- and two-qubit gate operations can be performed using only exchange pulses while maintaining this sweet spot. We show how to interconvert this qubit to other three-spin encoded qubits as a resource for quantum computation and communication.

Charge-noise-insensitive gate operations for always-on, exchange-only qubits (aps.org)



Talk: Silicon Quantum Information Technology

Information technology based on the fundamental nature of the universe, namely quantum physics, can in some cases dramatically outperform the best “classical” solution. In other words, a quantum computer will be important someday. But the challenges are still immense. Somewhat surprisingly, silicon may continue to be an exceptionally relevant material even into a future era of quantum-enhanced technology. Here I will discuss progress in silicon quantum computing and related semiconductor-based devices touching on my own research interests and highlighting experimental results across the community.

GOMACTech, Orlando, FL



Preprint: Entangling distant resonant exchange qubits via circuit quantum electrodynamics

We investigate a hybrid quantum system consisting of spatially separated resonant exchange qubits, defined in three-electron semiconductor triple quantum dots, that are coupled via a superconducting transmission line resonator. Drawing on methods from circuit quantum electrodynamics and Hartmann-Hahn double resonance techniques, we analyze three specific approaches for implementing resonator-mediated two-qubit entangling gates in both dispersive and resonant regimes of interaction. We calculate entangling gate fidelities as well as the rate of relaxation via phonons for resonant exchange qubits in silicon triple dots and show that such an implementation is particularly well-suited to achieving the strong coupling regime. Our approach combines the favorable coherence properties of encoded spin qubits in silicon with the rapid and robust long-range entanglement provided by circuit QED systems.

Entangling distant resonant exchange qubits via circuit quantum electrodynamics (arxiv.org)



Talk: A new look at encoded-qubit quantum dot quantum computing in silicon

Although the properties of spin-based qubits are specified by the material system they reside in, it’s possible to modify those properties by encoding a qubit into multiple physical spins. Here we consider new operating regimes for encoded spin qubits and discuss their relevance to spin-based quantum computing and qubit-qubit coupling, especially in silicon quantum dot systems. We will also briefly discuss recent developments in g-factor theory in silicon quantum dots and their possible implications.

We introduce an always-on, exchange-only qubit made up of three localized semiconductor spins that offers a true “sweet spot” to fluctuations of the quantum dot energy levels. Both single- and two-qubit gate operations can be performed using only exchange pulses while maintaining this sweet spot. We show how to interconvert this qubit to other three-spin encoded qubits as a new resource for quantum computation and communication.

APS March Meeting, Baltimore, MD

A new look at encoded-qubit quantum dot quantum computing in silicon




Preprint: Charge-noise-insensitive gate operations for always-on, exchange-only qubits

We introduce an always-on, exchange-only qubit made up of three localized semiconductor spins that offers a true “sweet spot” to fluctuations of the quantum dot energy levels. Both single- and two-qubit gate operations can be performed using only exchange pulses while maintaining this sweet spot. We show how to interconvert this qubit to other three-spin encoded qubits as a new resource for quantum computation and communication.

Charge-noise-insensitive gate operations for always-on, exchange-only qubits (arxiv.org)



PRB-Rapid: Spin-orbit coupling and operation of multivalley spin qubits

M. Veldhorst, R. Ruskov, C. H. Yang, J. C. C. Hwang, F. E. Hudson, M. E. Flatté, C. Tahan, K. M. Itoh, A. Morello, and A. S. Dzurak

Spin qubits composed of either one or three electrons are realized in a quantum dot formed at a Si/SiO2 interface in isotopically enriched silicon. Using pulsed electron-spin resonance, we perform coherent control of both types of qubits, addressing them via an electric field dependent g factor. We perform randomized benchmarking and find that both qubits can be operated with high fidelity. Surprisingly, we find that the g factors of the one-electron and three-electron qubits have an approximately linear but opposite dependence as a function of the applied dc electric field. We develop a theory to explain this g-factor behavior based on the spin-valley coupling that results from the sharp interface. The outer “shell” electron in the three-electron qubit exists in the higher of the two available conduction-band valley states, in contrast with the one-electron case, where the electron is in the lower valley. We formulate a modified effective mass theory and propose that intervalley spin-flip tunneling dominates over intravalley spin flips in this system, leading to a direct correlation between the spin-orbit coupling parameters and the g factors in the two valleys. In addition to offering all-electrical tuning for single-qubit gates, the g-factor physics revealed here for one-electron and three-electron qubits offers potential opportunities for different qubit control approaches.

Spin-orbit coupling and operation of multivalley spin qubits (aps.org)



Preprint: Spin-orbit coupling and operation of multi-valley spin qubits

M. Veldhorst, R. Ruskov, C.H. Yang, J.C.C. Hwang, F.E. Hudson, M.E. Flatté, C. Tahan, K.M. Itoh, A. Morello, A.S. Dzurak

Spin qubits composed of either one or three electrons are realized in a quantum dot formed at a Si/SiO_2-interface in isotopically enriched silicon. Using pulsed electron spin resonance, we perform coherent control of both types of qubits, addressing them via an electric field dependent g-factor. We perform randomized benchmarking and find that both qubits can be operated with high fidelity. Surprisingly, we find that the g-factors of the one-electron and three-electron qubits have an approximately linear but opposite dependence as a function of the applied dc electric field. We develop a theory to explain this g-factor behavior based on the spin-valley coupling that results from the sharp interface. The outer “shell” electron in the three-electron qubit exists in the higher of the two available conduction-band valley states, in contrast with the one-electron case, where the electron is in the lower valley. We formulate a modified effective mass theory and propose that inter-valley spin-flip tunneling dominates over intra-valley spin-flips in this system, leading to a direct correlation between the spin-orbit coupling parameters and the g-factors in the two valleys. In addition to offering all-electrical tuning for single-qubit gates, the g-factor physics revealed here for one-electron and three-electron qubits offers potential opportunities for new qubit control approaches.

Spin-orbit coupling and operation of multi-valley spin qubits (arxiv.org)



Nature Communications: Electron spin resonance and spin-valley physics in a silicon double quantum dot

Xiaojie Hao, Rusko Ruskov, Ming Xiao, Charles Tahan, HongWen Jiang

Silicon quantum dots are a leading approach for solid-state quantum bits. However, developing this technology is complicated by the multi-valley nature of silicon. Here we observe transport of individual electrons in a silicon CMOS-based double quantum dot under electron spin resonance. An anticrossing of the driven dot energy levels is observed when the Zeeman and valley splittings coincide. A detected anticrossing splitting of 60 MHz is interpreted as a direct measure of spin and valley mixing, facilitated by spin–orbit interaction in the presence of non-ideal interfaces. A lower bound of spin dephasing time of 63 ns is extracted. We also describe a possible experimental evidence of an unconventional spin–valley blockade, despite the assumption of non-ideal interfaces. This understanding of silicon spin–valley physics should enable better control and read-out techniques for the spin qubits in an all CMOS silicon approach.

Electron spin resonance and spin–valley physics in a silicon double quantum dot (nature.com)



Preprint: Electron spin resonance and spin-valley physics in a silicon double quantum dot

Xiaojie Hao, Rusko Ruskov, Ming Xiao, Charles Tahan, HongWen Jiang

Silicon quantum dots are a leading approach for solid-state quantum bits. However, developing this technology is complicated by the multi-valley nature of silicon. Here we observe transport of individual electrons in a silicon CMOS-based double quantum dot under electron spin resonance. An anticrossing of the driven dot energy levels is observed when the Zeeman and valley splittings coincide. A detected anticrossing splitting of 60 MHz is interpreted as a direct measure of spin and valley mixing, facilitated by spin-orbit interaction in the presence of non-ideal interfaces. A lower bound of spin dephasing time of 63 ns is extracted. We also describe a possible experimental evidence of an unconventional spin-valley blockade, despite the assumption of non-ideal interfaces. This understanding of silicon spin-valley physics should enable better control and read-out techniques for the spin qubits in an all CMOS silicon approach.

Electron spin resonance and spin-valley physics in a silicon double quantum dot (arxiv.org)




Charles Tahan
Physicist in Washington, D.C.