Leakage-protected idle operation of a triangular exchange-only spin qubit

TL;DR

This paper investigates a leakage-protected idle operation for a triangular exchange-only spin qubit, demonstrating enhanced coherence times and precise control of exchange interactions, which could improve qubit performance.

Contribution

The study introduces a method to operate a triangular EO spin qubit at a leakage-protected point with tunable exchange interactions, enhancing coherence and control.

Findings

Leakage-protected idle point suppresses leakage from the computational subspace.

Coherence time exceeds conventional EO qubits for certain gap energies.

Precise all-to-all exchange control enables improved qubit performance.

Abstract

We characterize the coherence of a triangular exchange-only (EO) spin qubit operated at a leakage-protected idle (LPI) point. The triangular geometry enables independent control of all three pairwise exchange interactions, and the LPI condition occurs when these couplings are turned on simultaneously and tuned to equal strength. In this configuration, the exchange interaction induces an energy gap $E_g = 3J/2$ that suppresses leakage from the computational subspace while leaving the qubit state unaffected. We develop procedures to calibrate the LPI point and measure $E_g$, and use these to characterize the qubit dephasing time over a broad range of gap energies. While operating with large always-on exchange couplings exposes the qubit to charge noise, we find that $\tilde{T}_2^*$ still exceeds that of conventional exchange-only spin qubits for $E_g/h < 60$ MHz. The precise control of simultaneous, all-to-all connected exchange demonstrated here presents a natural path towards improving the performance of EO qubits and also enables novel qubit encodings.