Faraday–Ramsey Rotation in Ultrathin Alkali Vapor Cells as an Analogy to Atomic-Beam Systems

Atomic beams are powerful tools for measuring fundamental physical constants, probing atomic interactions, and developing precision metrology techniques such as atomic clocks and interferometry. However, most atomic-beam devices are bulky, which limits their miniaturization. Recent efforts toward the miniaturization of atomic beams have been reported (see, Ref. [10]). Here, we demonstrate an alternative approach, showing that micrometer-scale vapor cells can emulate atomic-beam behavior through geometry-dependent velocity filtering. Specifically, in a 5-µm-thick rubidium vapor cell, coherence is preserved for atoms moving parallel to the cell walls, enabling observation of the Faraday–Ramsey effect in the absence of buffer gas or anti-relaxation coatings. Using a spatially displaced pump–probe scheme and magnetic-field scanning, we observe distinct Ramsey fringes in excellent agreement with the theoretical model. This technique enables compact implementations of Faraday–Ramsey measurements in microfabricated alkali-vapor cells.