# Engineering electron-phonon coupling of quantum defects to a semi-confocal acoustic resonator

## TL;DR

This paper presents a semi-confocal diamond-based acoustic resonator that significantly enhances electron-phonon coupling with NV centers, enabling efficient mechanical control of quantum states at GHz frequencies.

## Contribution

The authors design and fabricate a semi-confocal high-overtone bulk acoustic resonator on diamond, achieving a drastic reduction in modal volume and enhanced electron-phonon interaction for quantum defect control.

## Key findings

- Achieved high quality factor resonance modes with f·Q > 10^12.
- Demonstrated mechanically driven spin transitions with high Rabi frequency.
- Enhanced defect center electron-phonon coupling through resonator design.

## Abstract

Diamond-based microelectromechanical systems (MEMS) enable direct coupling between the quantum states of nitrogen-vacancy (NV) centers and the phonon modes of a mechanical resonator. One example, diamond high-overtone bulk acoustic resonators (HBARs), feature an integrated piezoelectric transducer and support high-quality factor resonance modes into the GHz frequency range. The acoustic modes allow mechanical manipulation of deeply embedded NV centers with long spin and orbital coherence times. Unfortunately, the spin-phonon coupling rate is limited by the large resonator size, $>100~\mu$m, and thus strongly-coupled NV electron-phonon interactions remain out of reach in current diamond BAR devices. Here, we report the design and fabrication of a semi-confocal HBAR (SCHBAR) device on diamond (silicon carbide) with $f\cdot Q>10^{12}$($>10^{13}$). The semi-confocal geometry confines the phonon mode laterally below 10~$\mu$m. This drastic reduction in modal volume enhances defect center electron-phonon coupling. For the native NV centers inside the diamond device, we demonstrate mechanically driven spin transitions and show a high strain-driving efficiency with a Rabi frequency of $(2\pi)2.19(14)$~MHz/V$_{p}$, which is comparable to a typical microwave antenna at the same microwave power.