Stability studies on subtractively-fabricated CMOS-compatible superconducting transmon qubits

TL;DR

This study examines the short- and long-term stability of CMOS-compatible superconducting transmon qubits, revealing fluctuations in coherence times and frequencies over hours and years, crucial for fault-tolerant quantum computing.

Contribution

It provides the first comprehensive analysis of the temporal stability of subtractively-fabricated superconducting qubits over extended periods and multiple thermal cycles.

Findings

Coherence times fluctuate due to TLS interactions.

Qubit frequencies shift by about 61 MHz over a year.

T1 times remain relatively stable despite fluctuations.

Abstract

Developing fault-tolerant quantum processors with error correction demands large arrays of physical qubits whose key performance metrics (coherence times, control fidelities) must remain within specifications over both short and long timescales. Here we investigated the temporal stability of subtractively fabricated CMOS-compatible superconducting transmon qubits. During a single cooldown and over a period of 95 hours, we monitored several parameters for 8 qubits, including coherence times $T_1$ and $T_2^*$, which exhibit fluctuations originating primarily from the interaction between two-level system (TLS) defects and the host qubit. We also demonstrate that subtractively-fabricated superconducting quantum devices align with the theoretical predictions that higher mean lifetimes $T_1$ correspond to larger fluctuations. To assess long-term stability, we tracked two representative qubits over 10 cooldown cycles spanning more than one year. We observed an average total downward shift in both qubit transition frequencies of approximately 61 MHz within the thermal cycles considered. In contrast, readout resonator frequencies decreased only marginally. Meanwhile, $T_1$ exhibits fluctuations from cycle to cycle, but maintains a stable baseline value.