Spin resonance linewidths of bismuth donors in silicon coupled to planar microresonators

TL;DR

This study investigates the linewidths of bismuth donor spins in silicon coupled to superconducting microresonators, identifying broadening mechanisms and pathways to optimize quantum memory performance.

Contribution

It provides detailed pulsed ESR measurements of bismuth donors in silicon with superconducting resonators, revealing linewidth broadening mechanisms and strategies to improve coupling.

Findings

Identified linewidth broadening mechanisms affecting spin-resonator coupling.

Demonstrated suppression of broadening at magnetic-field-insensitive clock transitions.

Discussed pathways to achieve unit cooperativity for quantum memory applications.

Abstract

Ensembles of bismuth donor spins in silicon are promising storage elements for microwave quantum memories due to their long coherence times which exceed seconds. Operating an efficient quantum memory requires achieving critical coupling between the spin ensemble and a suitable high-quality factor resonator -- this in turn requires a thorough understanding of the lineshapes for the relevant spin resonance transitions, particularly considering the influence of the resonator itself on line broadening. Here, we present pulsed electron spin resonance measurements of ensembles of bismuth donors in natural silicon, above which niobium superconducting resonators have been patterned. By studying spin transitions across a range of frequencies and fields we identify distinct line broadening mechanisms, and in particular those which can be suppressed by operating at magnetic-field-insensitive `clock transitions'. Given the donor concentrations and resonator used here, we measure a cooperativity $C\sim 0.2$ and based on our findings we discuss a route to achieve unit cooperativity, as required for a quantum memory.