Phys. Rev. Lett. 110, 093602 (2013)

Quantum mechanics and general relativity represent opposite length-scale limits of the laws of physics as we know them — and linking the two theories together has proved extremely difficult. But Hauke Müntinga and colleagues now demonstrate a free-falling matter-wave interferometer that could soon provide some clues.

Müntinga et al. packed all of the equipment required to trap and cool a Bose–Einstein condensate (BEC) of rubidium atoms into a capsule at the top of a 120-m tower — then they let it go. It took 2.7 seconds to create the BEC. A laser then separated the condensate into two wave packets that drifted slowly apart. Further laser pulses later reversed and then partially recombined the two. The team verified that the spacing of the interference fringes scales linearly with the time the BEC spent in the interferometer.

This proof-of-principle experiment demonstrates the potential of this approach for exploring atom interferometry with novel states of matter in extended free fall. The technique could verify Einstein's principle of equivalence with quantum objects, or even help in the search for gravitational waves.