Preprint: Electron g-factor of valley states in realistic silicon quantum dots

We theoretically model the spin-orbit interaction in silicon quantum dot devices, relevant for quantum computation and spintronics. Our model is based on a modified effective mass approach with spin-valley boundary conditions, derived from the interface symmetry under presence of perpendicular to the interface electric field. The g-factor renormalization in the two lowest valley states is explained by the interface-induced spin-orbit 2D (3D) interaction, favoring intervalley spin-flip tunneling over intravalley processes. We show that the quantum dot level structure makes only negligible higher order effects to the g-factor. We calculate the g-factor as a function of the magnetic field direction, which is sensitive to the interface symmetry. We identify spin-qubit dephasing sweet spots at certain directions of the magnetic field, where the g-factor renormalization is zeroed: these include perpendicular to the interface magnetic field, and also in-plain directions, the latter being defined by the interface-induced spin-orbit constants. The g-factor dependence on electric field opens the possibility for fast all-electric manipulation of an encoded, few electron spin-qubit, without the need of a nanomagnet or a nuclear spin-background. Our approach of an almost fully analytic theory allows for a deeper physical understanding of the importance of spin-orbit coupling to silicon spin qubits.

Electron g-factor of valley states in realistic silicon quantum dots (arxiv.org)



Preprint: Quantum-limited measurement of spin qubits via curvature coupling to a cavity

We investigate coupling an encoded spin qubit to a microwave resonator via qubit energy level curvature versus gate voltage. This approach enables quantum non-demolition readout with strength of tens to hundred MHz all while the qubit stays at its full sweet-spot to charge noise, with zero dipole moment. A “dispersive-like” spin readout approach similar to circuit-QED but avoiding the Purcell effect is proposed. With the addition of gate voltage modulation, selective longitudinal readout and n-qubit entanglement-by-measurement are possible.

Quantum-limited measurement of spin qubits via curvature coupling to a cavity (arxiv.org)



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)



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)



Preprint: Semiconductor-inspired superconducting quantum computing

Yun-Pil Shim, Charles Tahan

Superconducting circuits offer tremendous design flexibility in the quantum regime culminating most recently in the demonstration of few qubit systems supposedly approaching the threshold for fault-tolerant quantum information processing. Competition in the solid-state comes from semiconductor qubits, where nature has bestowed some almost magical and very useful properties which can be utilized for spin qubit based quantum computing. Here we begin to explore how selective design principles deduced from spin-based systems could be used to advance superconducting qubit science. We take an initial step along this path proposing an encoded qubit approach realizable with state-of-the-art tunable Josephson junction qubits. Our results show that this design philosophy holds promise, enables microwave-free control with minimal overhead (zero overhead in 2-qubit gates), and offers a pathway to future qubit designs with new capabilities such as with higher fidelity or, perhaps, operation at higher temperature. The approach is especially suited to qubits based on variable super-semi junctions.

Semiconductor-inspired superconducting quantum computing (arxiv.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)



Preprint: Superconducting-semiconductor quantum devices: from qubits to particle detectors

Yun-Pil Shim, Charles Tahan

Recent improvements in materials growth and fabrication techniques may finally allow for superconducting semiconductors to realize their potential. Here we build on a recent proposal to construct superconducting devices such as wires, Josephson junctions, and qubits inside and out-of single crystal silicon or germanium. Using atomistic fabrication techniques such as STM hydrogen lithography, heavily-doped superconducting regions within a single crystal could be constructed. We describe the characteristic parameters of basic superconducting elements—-a 1D wire and a tunneling Josephson junction—-and estimate the values for boron-doped silicon. The epitaxial, single-crystal nature of these devices, along with the extreme flexibility in device design down to the single-atom scale, may enable lower-noise or new types of devices and physics. We consider applications for such super-silicon devices, showing that the state-of-the-art transmon qubit and the sought-after phase-slip qubit can both be realized. The latter qubit leverages the natural high kinetic inductance of these materials. Building on this, we explore how kinetic inductance based particle detectors (e.g., photon or phonon) could be realized with potential application in astronomy or nanomechanics. We discuss super-semi devices (such as in silicon, germanium, or diamond) which would not require atomistic fabrication approaches and could be realized today.

Superconducting-semiconductor quantum devices: from qubits to particle detectors (arxiv.org)



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)



Preprint: Bottom-up superconducting and Josephson junction devices inside a Group-IV semiconductor

Yun-Pil Shim, Charles Tahan

Superconducting circuits are exceptionally flexible, enabling many different devices from sensors to quantum computers. Separately, epitaxial semiconductor devices such as spin qubits in silicon offer more limited device variation but extraordinary quantum properties for a solid-state system. It might be possible to merge the two approaches, making single-crystal superconducting devices out of a semiconductor by utilizing the latest atomistic fabrication techniques. Here we propose superconducting devices made from precision hole-doped regions within a silicon (or germanium) single crystal. We analyze the properties of this superconducting semiconductor and show that practical superconducting wires, Josephson tunnel junctions or weak links, superconducting quantum interference devices (SQUIDs), and qubits are feasible. This work motivates the pursuit of “bottom-up” superconductivity for improved or fundamentally different technology and physics.

Bottom-up superconducting and Josephson junction devices inside a group-IV semiconductor (arxiv.org)



Preprint: Observation of Autler-Townes effect in a dispersively dressed Jaynes-Cummings system

B. Suri, Z. K. Keane, R. Ruskov, Lev S. Bishop, Charles Tahan, S. Novikov, J. E. Robinson, F. C. Wellstood, B. S. Palmer

Photon number splitting is observed in a transmon coupled to a superconducting quasi-lumped-element resonator in the strong dispersive limit. A thermal population of 5.474 GHz photons at an effective resonator temperature of T = 120mK results in a weak n = 1 photon peak along with the n = 0 photon peak in the qubit spectrum in the absence of a coherent drive on the resonator. Two-tone spectroscopy using independent coupler and probe tones reveals an Autler-Townes splitting in the thermal n = 1 photon peak. The observed effect is explained accurately using the four lowest levels of the dispersively dressed qubit-resonator system and compared to results from numerical simulations of the steady-state master equation for the coupled system.

Observation of Autler-Townes effect in a dispersively dressed Jaynes-Cummings system (arxiv.org)




Charles Tahan
Physicist in Washington, D.C.