Drifting performance of an exchange-only qubit in Si/SiGe

Electron spins confined to silicon quantum dots are a promising qubit platform for quantum computing due to leveraging established silicon growth and processing technologies. These devices support a range of qubit encodings. The exchange-only (EO) encoding, using three electron spins, is of interest due to its robust decoherence free subspace (DFS) and fast baseband performance. Excellent blind randomized benchmarking (bRB) numbers have been reported using EO qubits with average error per gate near 1e-3 and leakage per gate near 1e-4. However, these results involve averaging over multiple bRB passes over long times to achieve high SNR, which masks qubit stability over this time.

Here, we perform high-fidelity bRB experiments with an EO qubit and study pass-to-pass variability to probe qubit stability. We observe variability with error rates, averaging 7e-3, jumping to over 2.5e-2 over a timescale of minutes. We characterize our EO qubit, gathering noise parameters through measurements of T2* at various exchange frequencies. We develop candidate noise models to understand the stability and limitations of our qubit. The combination of experiment and theoretical modeling provides insight into qubit error mechanisms and improves our understanding as we build to higher qubit numbers.