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

ABSTRACT 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.