Time-resolved spectral diffusion of a multimode mechanical memory

TL;DR

This study investigates spectral diffusion in high-frequency phonons used as quantum memories, revealing uncorrelated frequency fluctuations in adjacent modes and advancing understanding of dephasing mechanisms.

Contribution

It provides the first time-domain analysis showing uncorrelated spectral diffusion in neighboring mechanical modes, supported by theoretical and simulation models.

Findings

Frequency positions of adjacent modes are uncorrelated over time.

Spectral diffusion limits coherence times of confined phonons.

Theoretical and Monte-Carlo models agree with experimental results.

Abstract

High-frequency phonons hold great promise as carriers of quantum information on-chip and as quantum memories. Due to their coherent interaction with several systems, their compact mode volume, and slow group velocity, multiple experiments have recently demonstrated coherent transport of information on-chip using phonon modes, interconnecting distinct quantum devices. Strongly confined phonons in waveguide-like geometries are particularly interesting because of their long lifetime. However, spectral diffusion has been observed to substantially limit their coherence times. Coupling to two-level systems is suspected to be a major contributor to the diffusion; however, to date, the origin and underlying mechanisms are still not fully understood. Here, we perform a time-domain study on two adjacent mechanical modes (separated by around 5 MHz) and show that the frequency positions of the two modes are not correlated in time, in agreement with our theoretical model and Monte-Carlo simulations. This result is an important step in fully understanding the microscopic mechanisms of dephasing in mechanical quantum buses and memories.