An atom interferometric measurement of the photon recoil frequency aboard the International Space Station*

The Cold Atom Laboratory on the International Space Station provides a platform for Bose-Einstein condensation and other cold-atom physics. A recent upgrade permits the implementation of light-pulse interferometry. We have used this facility to carry out a simple measurement of the photon recoil frequency, using a condensate source and a two-pulse Ramsey interferometer sequence. The duration of the measurement is limited to about 0.5 ms by the expansion velocity of the condensate. We observe coherent phase evolution over that time scale and obtain a measurement accuracy of 2\% for the recoil frequency and 4\% for the initial atom velocity. The interferometric measurement is consistent with estimates of the Bragg laser frequency and time-of-flight measurements of the atom cloud. This marks the first application of atom interferometry to a physical measurement in a space environment.