Phys. Rev. Lett. 115, 043602 (2015)

Linear optical elements, such as beamsplitters, are commonly used for setting up separate optical paths and recombining light in interferometers. Bing Chen and colleagues from China and the USA have now reported an interferometer in which Raman amplification replaces the traditional beamsplitting elements. Optical and atomic correlation is established through the Raman process, which enables high-contrast interference fringes that are particularly sensitive to changes in optical path length and atomic phase. The atomic phase can be adjusted by magnetic field or Stark shifts. Previous work on similar ideas was mainly sensitive only to the optical phase but here the atomic phase is involved. In the set-up used by Chen et al. an amplified Stokes beam is combined with a write beam in an atomic cell. The recombination delay is adjusted by a Sagnac loop, which also plays an important role in stabilizing the optical phase. The interference fringes resulting from atom–photon correlations may be used to interrogate atomic states and could offer phase measurement precision exceeding the standard quantum limit.