Discovery of Niobium Hydride Precipitates in Superconducting Qubits

TL;DR

This paper provides evidence of niobium hydride precipitates in superconducting qubits, revealing a new source of decoherence that impacts qubit performance and suggesting mitigation strategies.

Contribution

It introduces direct detection of niobium hydrides in qubits and links their formation to decoherence, offering insights for improving superconducting qubit reliability.

Findings

Niobium hydrides are present in superconducting qubits.

Hydride size and morphology vary with temperature.

Hydrides contribute to qubit decoherence and RF losses.

Abstract

We report the evidence of the formation of niobium hydride phase within niobium films on silicon substrates in superconducting qubits fabricated at Rigetti Computing. For this study, we combined complementary techniques, including room-temperature and cryogenic atomic force microscopy (AFM), synchrotron Xray diffraction, and time of flight secondary ion mass spectroscopy (ToF-SIMS), to directly reveal the existence of niobium hydride precipitates in the Rigetti chip area. Upon cryogenic cooling, we observed variation in the size and morphology of the hydrides, ranging from small (5 nm) irregular shapes to large (~10-100 nm) domain within the Nb grains, fully converted to niobium hydrides. Since niobium hydrides are non-superconducting and can easily change in size and location upon different cooldowns to cryogenic temperature, our finding highlights a new and previously unknown source of decoherence in superconducting qubits. This contributes to both quasiparticle and two level system (TLS) losses, offering a potential explanation for changes in qubit performance upon cooldowns. Finally, by leveraging the RF performance of a 3D bulk Nb resonator, we can quantify RF dissipation on a superconducting qubit, caused by hydrogen concentration variation, and are able to propose a practical engineering pathway to mitigate the formation of the Nb hydrides for superconducting qubit applications.