The 2 3 P 1 -2 3 P 2 interval in helium is measured using microwave separated oscillatory fields. Our measured result is 2 291 177.53Ϯ 0.35 kHz . A disagreement with theory of over 15 kHz indicates a major problem with two-electron QED calculations. If this disagreement is resolved, measurements of both 2 3 P fine-structure intervals at this accuracy would lead to a six-parts-per-billion determination of the fine-structure constant.The present work continues the long history of precision measurements of the 2 3 P fine structure of helium ͓1-5͔. The main goal of these measurements is to determine the finestructure constant, ␣, from the larger ͑29.6 GHz͒ 2 3 P 1 -2 3 P 0 interval with the smaller ͑2.3 GHz͒ 2 3 P 1 -2 3 P 2 interval testing the two-electron QED theoretical calculations necessary to determine ␣. The present work differs by 36 standard deviations from theory ͓6,7͔. If this discrepancy is resolved, measurements of both 2 3 P finestructure intervals at the current accuracy would determine ␣ to 6 ppb. The most precise determination of ␣ ͑0.37 ppb͒ is from the magnetic moment of the electron ͑g e -2͒ ͓8͔. Independent determinations of ␣ ͓9͔ are needed to allow g e -2 to test QED to 0.37 ppb ͑the most precise test of QED theory͒ and to search for physics beyond the standard model ͓8͔.In this 2 3 P measurement separated oscillatory fields ͑SOFs͒ are used, compared to previous measurements which used continuous fields for excitation ͓1-5͔. SOFs allow for narrower linewidths ͑0.8-1.7 MHz͒ than the 3.25 MHz natural linewidth and also allow for a variety of line shapes. Figure 1 shows the experimental setup. A thermal beam of 2 3 S 1 metastable He is created in a dc discharge ͓3͔ and the 2 3 S 1 ͑m =0͒ state is depopulated by repeatedly driving 2 3 S 1 ͑m =0͒ -2 3 P 0 with a linearly polarized 1.08 m diode laser ͑A in Fig. 1͒. 2 3 S 1 ͑m = Ϯ 1͒ atoms are driven up to 2 3 P 1 ͑m =0͒ by a 15 ns laser pulse ͑B1 in Fig. 1, pulsed using two passes through an acousto-optic modulator͒, which is followed by the two 2.3 GHz SOF microwave pulses ͓P1 and P2 of Fig. 1͑c͒, of duration D = 50, 100, or 150 ns and separated in time by T = 300, 400, 500, or 600 ns͔ which drive the 2 3 P 1 ͑m =0͒ -2 3 P 2 ͑m =0͒ transition. Atoms can decay to 2 3 S 1 ͑m =0͒ only if the microwave transition is driven since the 2 3 P 1 ͑m =0͒ -2 3 S 1 ͑m =0͒ branching ratio is zero. This 2 3 S 1 ͑m =0͒ population is excited to 2 3 P 0 with laser C and the resulting fluorescence is collected on a 77 K InGaAs photodiode.The microwave transitions are driven inside of a 50 ⍀ coaxial transmission line ͓Fig. 1͑a͔͒. To double the signal size, another laser beam ͑B2͒ repeats the experiment on the other side of the 50 ⍀ line ͓inset of Fig. 1͑a͔͒. Microwaves ͑referenced to a Rb clock͒ are pulsed using fast switches and amplified to P = 25-125 W by a solid-state amplifier. The relative phase of P1 and P2 is controlled by additional switches which direct the pulses through paths L1 or L2 that differ in length by a half wavelength ͑ / 2͒. Every 79 ms, the rela...

The 2(3)P(1)-to- 2(3)P(0) interval in atomic helium is measured using a thermal beam of metastable helium atoms excited to the 2(3)P state using a 1.08-microm diode laser. The 2(3)P(1)-to- 2(3)P(0) transition is driven by 29.6-GHz microwaves in a rectangular waveguide cavity. Our result of 29,616,950.9+/-0.9 kHz is the most precise measurement of helium 2(3)P fine structure. When compared to precise theory for this interval, this measurement leads to a determination of the fine-structure constant of 1/137.0359864(31).