Frequency metrology in quantum degenerate helium

We report the first direct observation and high precision measurement of the triplet to singlet transition between the metastable states of helium. We obtain a precision of 8×10
^-12
, providing a stringent test of two-electron quantum electrodynamic theory and of nuclear few-body theory. We use either the
⁴
He or the
³
He isotope, and from the isotope shift derive a new value for the
³
He charge radius.