Universal logic with encoded spin qubits in silicon

Abstract

Qubits encoded in a decoherence-free subsystem and realized in exchange-coupled silicon quantum dots are promising candidates for fault-tolerant quantum computing. Benefits of this approach include excellent coherence, low control crosstalk, and configurable insensitivity to certain error sources. Key difficulties are that encoded entangling gates require a large number of control pulses and high-yielding quantum dot arrays. Here we show a device made using the single-layer etchdefined gate electrode architecture that achieves both the required functional yield needed for full control and the coherence necessary for thousands of calibrated exchange pulses to be applied. We measure an average two-qubit Clifford fidelity of 97.1 ± 0.2% with randomized benchmarking. We also use interleaved randomized benchmarking to demonstrate the controlled-NOT gate with 96.3 ± 0.7% fidelity, SWAP with 99.3 ± 0.5% fidelity, and a specialized entangling gate that limits spreading of leakage with 93.8 ± 0.7% fidelity.

Semiconductor spin qubits

“…Spin SWAPs can also be implemented in systems with a magnetic field gradient by periodically modulating the exchange coupling (Nichol et al, 2017). First demonstrations were achieved in GaAs, with more recent high fidelity demon-strations having been achieved in Si/SiGe QDs (Nichol et al, 2017;Sigillito et al, 2019b;Weinstein et al, 2022). Greentree et al, 2004 proposed using coherent transport via adiabatic passage (CTAP), in analogy to stimulated Raman adiabatic passage (STIRAP), commonly used in atomic physics (Vitanov et al, 2017), to achieve charge transfer in QD arrays (Ban et al, 2018;Cole et al, 2008).…”

“…Other constructions based on decoupling concepts have also been proposed (van Meter and Knill, 2019). The Fong-Wandzura sequence, as well as all two-qubit Clifford gates, variations of Fong-Wandzura related to leakage management, and logical swaps between encoded qubits were all demonstrated in a six-dot SLEDGE device via tomography and full two-qubit randomized benchmarking by Weinstein et al, 2022.…”

Noisy intermediate-scale quantum computers

“…Qubit construction.-Semiconductor qubits are defined on the charge and spin degrees of freedom of the carriers trapped in quantum dots or dopants. There are several types of qubits, such as spin qubits [10,11,509,510], charge qubits [511,512], exchange-only qubits [44,513,514], hybrid qubits [515], and singlettriplet qubits [516,517]. A spin qubit is usually defined by the spin states of a single electron or hole trapped in a semiconductor quantum dot or a dopant in silicon (see Fig.…”

“…A single-qubit operation could be performed utilizing a few different mechanisms for different qubits. For singlet-triplet qubits [517,543], exchangeonly qubits [44,513,514], and hybrid qubits [515], exchange coupling plays the key role. In comparison, electron spin resonance (ESR) [525,544,545] and electrical dipole spin resonance (EDSR) [524,538,546,547] are the main driving mechanisms for a spin qubit.…”

“…They control the spin rotation by either oscillating magnetic or electrical fields. After years of persistent quest for faulttolerant operations, single-qubit gate fidelities well beyond 99% have all been demonstrated in the donor [489], Si-MOS [485], and Si/SiGe [486,514,524,548] (see Table III).…”