Observation of quantum interference of optical transition pathways in Doppler-free two-photon spectroscopy and implications for precision measurements

TL;DR

This study demonstrates quantum interference effects in Doppler-free two-photon spectroscopy of cesium, revealing asymmetric line shapes that impact the accuracy of hyperfine splitting measurements and emphasizing the need to include interference effects in spectral analysis.

Contribution

It provides the first experimental observation of quantum interference in Doppler-free two-photon spectroscopy and develops a spectral line shape model that accounts for this interference.

Findings

Quantum interference causes asymmetric line shapes in hyperfine spectra.

Ignoring interference leads to systematic errors in transition frequency measurements.

Accounting for interference improves the accuracy of hyperfine splitting determination.

Abstract

Doppler-free two-photon spectroscopy is a standard technique for precision measurement of transition frequencies of dipole-forbidden transitions. The accuracy of such measurements depends critically on fitting of the spectrum to an appropriate line shape model, often a Voigt profile which neglects the effect of quantum interference of optical transitions. Here, we report the observation of quantum interference of optical transition pathways in Doppler-free two-photon spectroscopy of the cesium 6S-7D transitions. The quantum interference manifests itself as asymmetric line shapes of the hyperfine lines of the 7D states, observed through spontaneous emission following excitation by a narrow-linewidth cw laser. The interference persists despite the lines being spectrally well-resolved. Ignoring the effect of quantum interference causes large systematic shifts in the determination of the line-centers, while accounting for it resolves the apparent line shift and enables the precise determination of hyperfine splitting in the 7D states. We calculate the spectral line shape including the effect of quantum interference and show that it agrees with the experimental observations. Our results have implications for precision measurements of hyperfine splittings, isotope shifts and transition frequencies, including those of the hydrogen S-S and S-D transitions.