Frequency Metrology in Quantum Degenerate Helium: Direct Measurement of the 2 ³ S ₁ → 2 ¹ S ₀ Transition

TLDR

Precision spectroscopy of simple atoms, especially helium, has refined quantum physics by elucidating two‑electron interactions, determining the fine‑structure constant, and measuring the helium nucleus size. The study measures the doubly‑forbidden 1557‑nm transition between helium’s 2³S₁ and 2¹S₀ metastable states to probe strong QED and nuclear size effects. The measurement employed ultracold, sub‑microkelvin fermionic ³He and bosonic ⁴He atoms to achieve an 8.10⁻¹² precision. The 8.10⁻¹² precision test provides a stringent validation of two‑electron QED and nuclear few‑body theory.

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.