Operating two exchange-only qubits in parallel

Abstract

Semiconductors are among the most promising platforms to implement large-scale quantum computers, as advanced manufacturing techniques allow fabrication of large quantum dot arrays1. Various qubit encodings can be used to store and manipulate quantum information on these quantum dot arrays. Regardless of qubit encoding, precise control over the exchange interaction between electrons confined in quantum dots in the array is critical. Furthermore, it is necessary to execute high-fidelity quantum operations concurrently to make full use of the limited coherence of individual qubits. Here we demonstrate the parallel operation of two exchange-only qubits, consisting of six quantum dots in a linear arrangement. Using randomized benchmarking (RB) techniques, we show that issuing pulses on the five barrier gates to modulate exchange interactions in a maximally parallel way maintains the quality of qubit control relative to sequential operation. The techniques developed to perform parallel exchange pulses can be readily adapted to other quantum-dot-based encodings. Moreover, we show the first, to our knowledge, experimental demonstrations of an iSWAP gate for exchange-only qubits and of a charge-locking Pauli spin blockade (PSB) readout method. The results are validated using cross-entropy benchmarking (XEB)2, a technique useful for performance characterization of larger quantum computing systems; here it is used for the first time on a quantum system based on semiconductor technology. Parallel operation of two exchange-only qubits consisting of six quantum dots arranged linearly is shown to be achievable and maintains qubit control quality compared with sequential operation, with potential for use in scaled quantum computing.