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arXiv · 2512.24539

TLS-induced thermal nonlinearity in a micro-mechanical resonator

Cyril Metzger·Alec L. Emser·Brendon C. Rose·Konrad W. Lehnert
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Abstract

We present experimental evidence of a thermally-driven amplitude-frequency nonlinearity in a thin-film quartz phononic crystal resonator at millikelvin temperatures. The nonlinear response arises from the coupling of the mechanical mode to an ensemble of microscopic two-level system defects driven out of equilibrium by a microwave drive. In contrast to the conventional Duffing oscillator, the observed nonlinearity exhibits a mixed reactive-dissipative character. Notably, the reactive effect can manifest as either a softening or hardening of the mechanical resonance, depending on the ratio of thermal to phonon energy. By combining the standard TLS theory with a thermal conductance model, the measured power-dependent response is quantitatively reproduced and readout-enhanced relaxation damping from off-resonant TLSs is identified as the primary mechanism limiting mechanical coherence. Within this framework, we delineate the conditions under which similar systems will realize this nonlinearity.

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Cyril Metzger, Alec L. Emser, Brendon C. Rose, Konrad W. Lehnert. 2025-12-31. TLS-induced thermal nonlinearity in a micro-mechanical resonator. https://arxiv.org/abs/2512.24539

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