Charles Smith

Semiconductor spin qubits are an attractive quantum computing platform that offers long qubit coherence times and compatibility with existing semiconductor fabrication technology for scale-up. Here, we propose a spin-qubit architecture based on spinless multielectron quantum dots that act as low-crosstalk couplers between a two-dimensional arrangement of spin qubits in a hexagonal lattice. The multielectron couplers are controlled by voltage signals, which mediate fast Heisenberg exchange and thus enable coherent multiqubit operations. For the proposed architecture, we discuss the implementation of the rotated XZZX surface code and numerically study its performance for a circuit-level noise model. We predict a threshold of 0.18 % for the error rate of the entangling gates. We further evaluate the scalability of the proposed architecture and predict the need for 4480 physical qubits per logical qubit with logical error rates of 10 − 12 considering entangling gate fidelities of 99.99 % , resulting in a chip size of 2.6 cm 2 to host 10 000 logical qubits.

We demonstrate high-fidelity single qubit control in a natural Si-MOS quantum dot fabricated in an industrial 300 mm wafer process on a silicon on insulator wafer using electron spin resonance. A relatively high optimal Rabi frequency of 5 MHz is achieved, decoupling the electron spin from its 29Si environment. Tracking the qubit frequency reduces the impact of low-frequency noise in the qubit frequency and improves the TRabi from 7 to 11 μs at a Rabi frequency of 5 MHz, resulting in Q-factors exceeding 50. Randomized benchmarking returns an average single gate control fidelity of 99.5±0.3%. As a result of pulse-area calibration, this fidelity is limited by the Rabi Q-factor. These results show that a fast Rabi frequency and a feedback protocol enable high fidelity in these Si-MOS devices, despite the low-frequency magnetic noise.

Silicon offers the enticing opportunity to integrate hybrid quantum-classical computing systems on a single platform. For qubit control and readout, high-frequency signals are required. Therefore, devices that can facilitate its generation are needed. Here, we present a quantum dot-based radio-frequency multiplier operated at cryogenic temperatures. The device is based on the nonlinear capacitance-voltage characteristics of quantum dot systems arising from their low-dimensional density of states. We implement the multiplier in a multigate silicon-nanowire transistor using two complementary device configurations: a single quantum dot coupled to a charge reservoir and a coupled double quantum dot. We study the harmonic voltage conversion as a function of the energy detuning, the multiplication factor, and the harmonic phase noise and find near ideal performance up to a multiplication factor of 10. Our results demonstrate a method for high-frequency conversion that could be readily integrated into silicon-based quantum computing systems and be applied to other semiconductors.

Gated-charge force microscopy (GCFM) generates images of subsurface induced charge with single-electron charge resolution and zero sensitivity to background charge, for the noninvasive study of confined quantum electronic systems. By gating the radio-frequency signal applied to transducers, which generate surface-acoustic waves (SAWs), we use GCFM to generate images of a GaAs depleted one-dimensional channel and provide insight to SAW-induced transport. Unexpected SAW-induced subsurface charge was imaged at points of high potential gradient. This suggests that electrons depopulating a SAW minimum due to a high potential gradient rapidly lose energy and interact with following SAW minima.