Correlated Dephasing in a Piezoelectrically Transduced Silicon Phononic Waveguide

TL;DR

This paper presents the design, fabrication, and characterization of a silicon phononic waveguide coupled with a piezoelectric element, revealing detailed decoherence dynamics and noise processes relevant for quantum acoustic systems.

Contribution

It introduces a hybrid silicon-piezoelectric waveguide with detailed measurements of coupling rates, energy relaxation, and dephasing, advancing understanding of noise in quantum acoustic devices.

Findings

Piezoelectric coupling rate of 1.1 MHz

Energy relaxation time of approximately 500 microseconds

Dephasing timescale around 60 microseconds

Abstract

Nanomechanical waveguides offer a multitude of applications in quantum and classical technologies. Here, we design, fabricate, and characterize a compact silicon single-mode phononic waveguide actuated by a thin-film lithium niobate piezoelectric element. Our device directly transduces between microwave frequency photons and phonons propagating in the silicon waveguide, providing a route for coupling to superconducting circuits. We probe the device at millikelvin temperatures through a superconducting microwave resonant matching cavity to reveal harmonics of the silicon waveguide and extract a piezoelectric coupling rate $g/2\pi= 1.1$ megahertz and a mechanical coupling rate $f/2\pi=5$ megahertz. Through time-domain measurements of the silicon mechanical modes, we observe energy relaxation timescales of $T_{1,\text{in}} \approx 500$ microseconds, pure dephasing timescales of $T_\phi \approx {60}$ microseconds and dephasing dynamics that indicate the presence of an underlying frequency noise process with a non-uniform spectral distribution. We measure phase noise cross-correlations between silicon mechanical modes and observe detuning-dependent positively-correlated frequency fluctuations. Our measurements provide valuable insights into the dynamics and decoherence characteristics of hybrid piezoelectric-silicon acoustic devices, and suggest approaches for mitigating and circumventing noise processes for emerging quantum acoustic systems.