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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 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.