Experimental Determination of the Helium 2 3 P 1 –1 1 S 0 Transition Rate R. G. Dall, K. G. H. Baldwin, * L. J. Byron, and A. G. Truscott ARC Centre of Excellence for Quantum-Atom Optics and Research School of Physical Sciences and Engineering, The Australian National University, Canberra, ACT 0200, Australia (Received 12 September 2007; published 15 January 2008; corrected 17 January 2008) We present the first experimental determination of the 2 3 P 1 –1 1 S 0 transition rate in helium and compare this measurement with theoretical quantum-electrodynamic predictions. The experiment exploits the very long (1 minute) confinement times obtained for atoms magneto-optically trapped in an apparatus used to create a Bose-Einstein condensate of metastable (2 3 S 1 ) helium. The 2 3 P 1 –1 1 S 0 transition rate is measured directly from the decay rate of the cold atomic cloud following 1083 nm laser excitation from the 2 3 S 1 to the 2 3 P 1 state, and from accurate knowledge of the 2 3 P 1 population. The value obtained is 177  8s 1 , which agrees very well with theoretical predictions, and has an accuracy that compares favorably with measurements for the same transition in heliumlike ions higher in the isoelectronic sequence. DOI: 10.1103/PhysRevLett.100.023001 PACS numbers: 32.70.Cs, 12.20.Fv, 37.10.Gh, 42.50.Xa Quantum electrodynamics (QED) is one of the most rigorously tested fundamental theories of modern physics, for which the atomic energy levels of helium and helium- like ions represent an important test bed. Helium is the simplest multielectron atom, enabling theoretical calcula- tions to be performed with greater accuracy than for more complex species. From a theoretical standpoint, the energy levels of helium can be determined via a power series expansion of the fine-structure constant to a high level of accuracy [1] which has been tested by modern experimen- tal advances in precision measurement. The ground-state energy has been determined using precision pulsed laser techniques to better than one part in 10 8 to derive the ground-state Lamb shift [2,3], while ultra-narrow-band cw lasers and optical frequency combs have enabled mea- surement of excited-state transitions with an accuracy that exceeds one part in 10 11 , challenging Lamb shift [4] and fine-structure [5] predictions. By contrast, other atomic parameters such as transition rates are much harder to determine —both experimentally and theoretically — with accuracies often at the percent level. The behavior of the transition rates of heliumlike atoms in an isoelectronic sequence is a case in point that has received considerable theoretical attention [6]. A num- ber of experimental determinations of the transition rates of highly ionized heliumlike species have tested these QED predictions, but there have been no published measure- ments of the decay of helium atoms from the 2 3 P states to the ground state. This Letter presents the first measure- ment of the helium 2 3 P 1 –1 1 S 0 transition rate, providing a benchmark for other isoelectronic measurements in this sequence. Helium has a simple electronic structure consisting of singlet (spin antiparallel) and triplet (spin parallel) excited states, of which the triplet manifold is connected by weak transitions that would otherwise be spin-forbidden to the 1 1 S 0 (singlet) ground state (Fig. 1). The first excited state (2 3 S 1 , some 20 eV above the ground state) is distinguished by the fact that it is also forbidden by parity selection rules from decaying via an electric-dipole transition to the ground state and, consequently, has a very long (meta- stable) lifetime of   8000 s [7], to our knowledge the longest of any atomic species yet measured. Thus meta- stable 2 3 S 1 helium can effectively be employed as a ‘‘ground-state’’ species for many atom optics and Bose- Einstein condensation experiments [8]. Excitation to the 2 3 P states is readily achieved using 1083 nm laser radia- tion, which is used to laser cool and trap metastable helium atoms via the 2 3 S 1 –2 3 P 2 transition. Transition rates from the 2 3 P manifold are dominated by rapid decay back to the 2 3 S 1 metastable level (inverse Energy 2 3 P 0 2 3 P 1 2 3 P 2 2 3 S 1 P 2 P 0 P 1 2.3 GHz 29.7 GHz (Metastable) 1 1 1 S 0 τ P 2 ~ 3.3 s τ P 1 ~ 5.7 ms τ S 1 ~ 8000 s FIG. 1. Energy level diagram for helium showing the inverse decay rates () for the helium n  2 triplet manifold to the ground state. PRL 100, 023001 (2008) PHYSICAL REVIEW LETTERS week ending 18 JANUARY 2008 0031-9007= 08=100(2)=023001(4) 023001-1  2008 The American Physical Society