Progress Towards a Precision Measurement of the n = 2 Triplet P0-to-n = 2 Triplet P1 Transition in Atomic Helium

Significant progress towards a high-precision measurement of the 29.6 GHz 2^3P_1-to-2^3P_0 fine-structure interval in helium at an intended precision of 20 Hz is presented. The measurement is performed using a thermal beam of metastable helium atoms, and the transition is measured using the frequency-offset separated oscillatory fields (FOSOF) method, where the transition is driven by a pair of temporally separated microwave pulses. The two pulses are at slightly different frequencies, which can be viewed as a continuously advancing phase difference between the pulses. The advancing phase difference leads to a sinusoidally varying atomic signal due to quantum interference between atoms excited during the two pulses. The phase difference between the sinusoidal atomic signal and a reference signal generated by combining the two microwave frequencies is zero at resonance and approximately proportional to the difference between the applied microwave frequency and the centre frequency of the transition.

A large number of experiment parameters which could lead to systematic effects have been investigated and shown to be sufficiently well-managed at the intended precision level of the intended measurement. One effect which causes the measured linecentre to depend on the range of microwave frequencies at which data is taken has been thoroughly investigated in both experiments and in a numerical simulation developed to investigate systematic effects. This effect is still not sufficiently well-understood or controlled to allow completion of a measurement at the 20 Hz level of precision.

A completed measurement, at a precision of 20 Hz, would be able to be combined with our previous 25 Hz measurement of the 2.3 GHz 2^3P_2-to-2^3P_1 interval to obtain a part-per-billion (ppb) determination of the combined 31.9 GHz 2^3P_2-to-2^3P_0 interval. When compared to a sufficiently-precise theoretical calculation, the combined interval would allow a 0.5 ppb determination of the fine-structure constant, the most precise determination of the fine-structure constant in a two-electron system and within an order of magnitude of the most precise determinations of the fine-structure constant. This 0.5 ppb determination could be compared with other fine-structure constant determinations to test beyond-the-Standard-Model physics.