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Laser & Photonics Review

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

Mark Dikopoltsev, Eliran Talker, Yefim Barash, Noa Mazurski, Uriel Levy

Published Jan 7, 2026• 1 citation

Abstract

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.

DOI: 10.1002/lpor.202502129

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Atomic beams are powerful tools for measuring spin coherence in hot vapors but require bulky setups, limiting device miniaturization. We demonstrate that micron-thin vapor cells can mimic atomic beam behavior by exploiting geometry-dependent velocity filtering. In a 5 μm rubidium cell, coherence is preserved for atoms moving parallel to the cell walls, enabling observation of the Faraday-Ramsey effect without buffer gas or anti-relaxation coatings. Using a spatially displaced pump-probe scheme and magnetic field scanning, we achieve clear Ramsey fringes and validate our model experimentally. This technique offers a compact alternative to atomic beam systems, supporting scalable sensors and frequency standards.

Atomic beams offer measurement capabilities for hot vapors but often require complex, bulky systems, limiting miniaturization. This study explores hot vapors in thin cells (L=30,5 μm) as a simplified analogy to atomic beams. Coherence is maintained only for atoms moving parallel to the cell walls, as wall collisions randomize spin. By filtering non-parallel atoms, we mimic atomic beam behavior and observe the Faraday-Ramsey effect. Rubidium atoms are optically pumped with linearly polarized resonance radiation, evolving in a perpendicular magnetic field and probed via Faraday rotation. We believe that thin cell technology is paving the way for the miniaturization of atomic beam-based sensors.

Atomic beams offer measurement capabilities for hot vapors but often require complex, bulky systems, limiting miniaturization. This study explores hot vapors in thin cells (L=30,5 μm) as a simplified analogy to atomic beams. Coherence is maintained only for atoms moving parallel to the cell walls, as wall collisions randomize spin. By filtering non-parallel atoms, we mimic atomic beam behavior and observe the Faraday-Ramsey effect. Rubidium atoms are optically pumped with linearly polarized resonance radiation, evolving in a perpendicular magnetic field and probed via Faraday rotation. We believe that thin cell technology is paving the way for the miniaturization of atomic beam-based sensors.

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In this paper we describe a few phenomena that may affect the linewidth and/or the line shape of Ramsey fringes observed in an alkali-metal vapor cell with buffer gas. Experiments are performed on a vapor-cell--microwave-cavity arrangement in which

{}^{87}\mathrm{Rb}

atoms successively experience laser pumping preparation (through a

{D}_{2}

line), microwave interrogation with a couple of coherent pulses, and clock signal detection by means of laser absorption. We show that the linewidth of the Ramsey central fringe is influenced by the Rabi pattern produced by a single microwave pulse. Moreover, cycle memory and atomic density effects can modify the shape of the Ramsey central fringe from the common sinusoidal behavior. A simple three-level model well reproduces the experimental observations.