Measurement of the1s−2sEnergy Interval in Muonium

The $1s-2s$ interval has been measured in the muonium $\left({\mu }^{+}{e}^{-}\right)$ 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 $-1\mathrm{}–1.1\left(2.1\right)\times {10}^{-9\mathrm{}}$. We expect significantly higher accuracy at future high flux muon sources and from cw laser technology.