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

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 (3)S(1) and first excited singlet state 2 (1)S(0)), for which quantum electrodynamic and nuclear size effects are very strong. This transition is weaker by 14 orders of magnitude than the most predominantly measured transition in helium. Ultracold, submicrokelvin, fermionic (3)He and bosonic (4)He 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.