Investigation of TLS-induced loss and decoherence in mechanical oscillators

Long-lived mechanical oscillators are promising bosonic elements for quantum computing, communication, and sensing applications [1]. At cryogenic temperatures, loss and decoherence in dielectric mechanical oscillators are primarily limited by coupling to two-level system (TLS) defects. In this work, we experimentally investigate the interaction between TLSs and phononic crystal oscillators, examining the influence of oscillator geometry and material composition. We find that in single-crystal silicon oscillators, decay and decoherence are dominated by losses associated with the metallic electrodes used to enable stronger electromechanical coupling. We further study the temperature dependence of energy relaxation and dephasing, comparing our observations with predictions from established TLS models. The frequency noise exhibits an anomalous temperature dependence consistent with other reports, which has been attributed to TLS–TLS interactions extending the standard TLS model. Finally, we discuss potential strategies to mitigate loss and decoherence induced by metallic electrodes while preserving strong electromechanical coupling.

[1] Bozkurt, A.B., Golami, O., Yu, Y. et al. Nat. Phys. 21, 1469–1474 (2025)