The precision of interferometry is directly linked to the fringe spacing of the recorded interferogram. Whereas all interferometric devices show a fringe spacing equal to a wavelength of the laser light we present a novel scheme of a two-beam interferometer exhibiting a fringe spacing reduced by a factor of 2; the direct detection of the beat signal is replaced with the monitoring of the fluorescence of a twofold degenerate atomic system resonant with the laser. The fringe spacing in the fluorescence signal is demonstrated with a hot sodium vapor excited by a broadband laser tuned to the line. In the saturation regime, the dark fringes are expected to be extremely narrow, leading to the possibility of nanoscale displacement measurements or atom localization.
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