METASTABLE HELIUM: LIFETIME MEASUREMENTS USING COLD ATOMS AS A TEST OF QED

Here we present the first complete measurement of the radiative decay rate to the ground state of the lowest four triplet states of helium i.e. the metastable 2³S₁ state and the 2³P manifold. We employ laser cooling and trapping in an ultrahigh vacuum chamber to enable direct measurement of the trap loss rate for decay from the 2³P₁ state to the ground state. We then use this rate to calibrate the XUV emission decay to the ground state for all the remaining transitions. The 2³P₁ and 2³P₂ transition rates are measured for the first time, an upper bound is placed on the 2³P₀ decay rate, and the 2³S₁ metastable lifetime is determined for only the second time with a five-fold improvement in accuracy. These results are in excellent agreement with theoretical QED predictions, and anchor the helium-like isoelectronic sequences for these transitions at low Z.