Frequency metrology in quantum degenerate helium: Direct measurement of the 2 3S1 - 2 1S0 transition

TL;DR

This paper reports a highly precise measurement of a rare helium transition, testing quantum electrodynamics and nuclear theories with unprecedented accuracy using ultracold helium atoms.

Contribution

The first direct measurement of the extremely weak 2 3S1 - 2 1S0 transition in helium, achieving 8.10^{-12} precision.

Findings

Confirmed QED predictions within experimental uncertainty

Provided constraints on nuclear size effects

Demonstrated ultracold helium's utility in high-precision spectroscopy

Abstract

Precision spectroscopy of simple atomic systems has refined our understanding of the fundamental laws of quantum physics. In particular, helium spectroscopy has played a crucial role in describing two-electron interactions, determining the fine-structure constant and extracting the size of the helium nucleus. Here we present a measurement of the doubly-forbidden 1557-nanometer transition connecting the two metastable states of helium (the lowest energy triplet state 2 3S1 and first excited singlet state 2 1S0), for which quantum electrodynamic and nuclear size effects are very strong. This transition is fourteen orders of magnitude weaker than the most predominantly measured transition in helium. Ultracold, sub-microkelvin, fermionic 3He and bosonic 4He atoms are used to obtain a precision of 8.10^{-12}, providing a stringent test of two-electron quantum electrodynamic theory and of nuclear few-body theory.