Mark Kasevich and Steven Chu demonstrated an atom interferometer using stimulated Raman transitions. A pair of laser beams changes an atom's internal state while transferring a controlled momentum, coherently splitting its matter wave into distinguishable paths.
Later pulses redirect and recombine the paths. The measured internal-state populations then depend on their relative phase, converting otherwise invisible wave evolution into a particle-counting signal. The 1991 experiment established a practical bridge between optical control and precision inertial measurements with freely moving chemical atoms.[ref]
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