Separated oscillatory-field microwave measurement of the2 P31–2 P3

Abstract

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...

Fine structure of helium and light helium-like ionsThis paper was presented at the International Conference on Precision Physics of Simple Atomic Systems, held at École de Physique, les Houches, France, 30 May – 4 June, 2010.

“…For the ν 12 interval, our theory is by about 2σ away from the values obtained in Refs. [7,26] but in agreement with the latest measurement by Hessels and coworkers [8]. Assuming the validity of the theory, we combine the theoretical prediction for the ν 01 interval in helium with the experimental result [7] and obtain the following value of the fine structure constant,…”

Cold and trapped metastable noble gases

“…Different high-precision spectroscopy approaches ranging from optically-pumped magnetic resonance microwave spectroscopy (Borbely et al, 2009;George et al, 2001) to heterodyne frequency differences of the 1083 nm transitions (Giusfredi et al, 2005;Smiciklas and Shiner, 2010;Zelevinsky et al, 2005) have produced sub-kHz accuracy 2 3 P fine structure measurements with a remarkable agreement among them. In particular the recently reported 9 ppb accurate value for the largest interval ∆ν 2 3 P0−2 = 31 908 131.25 (30) kHz, would lead to an uncertainty of 4.5 ppb in the inferred value of α if the theory were exact (Smiciklas and Shiner, 2010).…”

“…The readily accessible 2 3 P and 3 3 P manifolds have been the focus of much experimental activity, particularly following the introduction of optical frequency combs (see Maddaloni et al (2009) Among all triplet level transitions, that between the 2 3 S and 2 3 P states in 4 He at 1083 nm is by far the most experimentally studied. In a series of experiments (Borbely et al, 2009;Castillega et al, 2000;Frieze et al, 1981;Giusfredi et al, 2005;Minardi et al, 1999;Smiciklas and Shiner, 2010;Storry et al, 2000;Zelevinsky et al, 2005), 1083 nm light was used to access the 4 He 2 3 P fine structure, with the prospect of obtaining an accurate value for the fine structure constant α. Assuming the QED theory of fine structure energies of an atomic system to be correct, a determination of α is possible by frequency measurements of these fine structure splittings, which are proportional to α 2 .…”

Constraints on exotic spin-dependent interactions between electrons from helium fine-structure spectroscopy

“…Reference [49] summarizes the corrections and present status of measurements of the helium 2 3 P 1 → 2 3 P 2 interval, and we use the weighted average of the corrected results of Refs. [38,41,50,51] to determine the experimental value for the 2 3 P 1 → 2 3 P 2 transition frequency. On the other hand, the theoretical uncertainties of the 2 3 S 1 − 2 3 P 0,1,2 transition frequencies are so large that the interference effect can be neglected in these cases.…”