Measurement of the 1s-2s energy interval in muonium

The 1s-2s interval has been measured in the muonium (mu(+)e(-)) atom by Doppler-free two-photon pulsed laser spectroscopy. The frequency separation of the states was determined to be 2 455 528 941.0(9.8) MHz, in good agreement with quantum electrodynamics. The result may be interpreted as a measurement of the muon-electron charge ratio as -1-1.1(2.1) x 10(-9). We expect significantly higher accuracy at future high flux muon sources and from cw laser technology.