Precision, Not Power

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Science  08 Jul 2011:
Vol. 333, Issue 6039, pp. 164-165
DOI: 10.1126/science.1208276

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The essence of precision measurement is to investigate elusive physical phenomena by achieving high accuracy, or observing very weak events, or both. This normally requires a system sufficiently simple that quantitative comparisons with theory are possible. On page 196 of this issue van Rooij et al. (1) present results of a study that encompasses all of these elements. They report on a precise measurement of laser-driven transitions between the singlet and triplet metastable excited states of atomic helium, 2 1S and 2 3S, a transition so strongly forbidden by spin and parity selection rules that even 10 years ago, its excitation would have been regarded as impossible. The transition is weaker by 14 orders of magnitude than a typical fully allowed electric dipole transition (see the figure) (2) and is one of the weakest optical transitions to be measured with accuracy. This feat is accomplished not by resorting to high laser powers but by using modest powers in conjunction with long interaction times and very narrow linewidths.