Measurement of a helium tune-out frequency: an independent test of quantum electrodynamics

TL;DR

This study precisely measures the helium `tune-out' frequency to test quantum electrodynamics, finding good agreement with theoretical predictions and resolving key QED and retardation effects.

Contribution

It provides an independent, high-precision measurement of helium's `tune-out' frequency, offering a stringent test of QED calculations beyond traditional energy level comparisons.

Findings

Measured tune-out frequency within 1.7 sigma of theory

Resolved QED contributions with high significance (~30 sigma)

Confirmed retardation corrections at ~2 sigma level

Abstract

Despite quantum electrodynamics (QED) being one of the most stringently tested theories underpinning modern physics, recent precision atomic spectroscopy measurements have uncovered several small discrepancies between experiment and theory. One particularly powerful experimental observable that tests QED independently of traditional energy level measurements is the `tune-out' frequency, where the dynamic polarizability vanishes and the atom does not interact with applied laser light. In this work, we measure the `tune-out' frequency for the $2^{3\!}S_1$ state of helium between transitions to the $2^{3\!}P$ and $3^{3\!}P$ manifolds and compare it to new theoretical QED calculations. The experimentally determined value of $725\,736\,700\,$$(40_{\mathrm{stat}},260_{\mathrm{syst}})$ MHz is within ${\sim} 1.7\sigma$ of theory ($725\,736\,252(9)$ MHz), and importantly resolves both the QED contributions (${\sim} 30 \sigma$) and novel retardation (${\sim} 2 \sigma$) corrections.