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Precision spectroscopy of simple atomic systems has refined our understandingof the fundamental laws of quantum physics. In particular, helium spectroscopyhas played a crucial role in describing two-electron interactions, determiningthe fine-structure constant and extracting the size of the helium nucleus. Herewe present a measurement of the doubly-forbidden 1557-nanometer transitionconnecting the two metastable states of helium (the lowest energy triplet state2 3S1 and first excited singlet state 2 1S0), for which quantum electrodynamicand nuclear size effects are very strong. This transition is fourteen orders ofmagnitude weaker than the most predominantly measured transition in helium.Ultracold, sub-microkelvin, fermionic 3He and bosonic 4He atoms are used toobtain a precision of 8.10^{-12}, providing a stringent test of two-electronquantum electrodynamic theory and of nuclear few-body theory.

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