Unifying frequency metrology across microwave, optical, and free-electron domains

TL;DR

This paper demonstrates a novel method to unify frequency measurements across microwave, optical, and free-electron domains using optical frequency combs and laser modulation, enabling precise electron spectroscopy and frequency calibration.

Contribution

It introduces a new technique to coherently link microwave, optical, and free-electron frequencies, bridging different physical objects and extending frequency metrology.

Findings

Successfully transferred microwave standards to the optical domain.

Imprinted optical frequency information onto electron spectra.

Enabled precise calibration of electron spectrometers using this frequency link.

Abstract

Frequency metrology lies at the heart of precision measurement. Optical frequency combs provide a coherent link uniting the microwave and optical domains in the electromagnetic spectrum, with profound implications in timekeeping, sensing and spectroscopy, fundamental physics tests, exoplanet search, and light detection and ranging. Here, we extend this frequency link to free electrons by coherent modulation of the electron phase by a continuous-wave laser locked to a fully stabilized optical frequency comb. Microwave frequency standards are transferred to the optical domain via the frequency comb, and are further imprinted in the electron spectrum by optically modulating the electron phase with a photonic chip-based microresonator. As a proof-of-concept demonstration, we apply this frequency link in the calibration of an electron spectrometer, and use the electron spectrum to measure the optical frequency. Our work bridges frequency domains differed by a factor of $\sim10^{13}$ and carried by different physical objects, establishes a spectroscopic connection between electromagnetic waves and free-electron matter waves, and has direct ramifications in ultrahigh-precision electron spectroscopy.