Helium 2?3S-2?1S metrology at 1.557??m

An experiment is proposed to excite the forbidden 1s2s 3S1 - 1s2s 1S0 magnetic dipole (M1) transition at 1.557 ?m in a collimated and slow atomic beam of metastable helium atoms. It is demonstrated that an excitation rate of 5000 ?1 can be realised with the beam of a 2 W narrow-band telecom fiber laser intersecting the atomic beam perpendicularly. A Doppler-limited sub-MHz spectroscopic linewidth is anticipated. Doppler-free excitation of 2% of trapped and cooled atoms may be realised in a one-dimensional optical-lattice geometry, using the 2 W laser both for trapping and spectroscopy. The very small (8 Hz) natural linewidth of this transition presents an opportunity for accurate tests of atomic structure calculations of the helium atom. A measurement of the 3He-4He isotope shift allows for accurate determination of the difference in nuclear charge radius of both isotopes.