Inhomogeneous light shifts of coherent population trapping resonances

Abstract

Coherent population trapping (CPT) in atomic vapors using all-optical interrogation has enabled the miniaturization of microwave atomic clocks. Light shifts induced by the CPT driving fields can impact the spectral profile of CPT resonances and are a common limit to the long-term stability of CPT clocks. Nonlinear light shifts have been observed in several CPT systems and have not been explored in detail. In this Letter, we demonstrate that nonlinear light shifts in CPT clocks can arise from spatially inhomogeneous CPT driving fields. We measure this effect using Gaussian laser beams in a buffer gas cell and show strong agreement with a four-level model describing the CPT Λ-system with a noninteracting “trap” state. We estimate the effect of this nonlinearity on recently developed light shift mitigation techniques and suggest improvements to existing techniques.

Level-crossing resonances on open atomic transitions in a buffered Cs vapor cell: Linewidth narrowing, high contrast and applications to atomic magnetometry

“…Otherwise, in open systems (β < 1), the linewidth demonstrates a square-root-like behavior, ∆ c ∝ √ I c , which is clearer at ξ ≫ 1. The similar behavior has been recently noted in [70] for two-frequency CPT resonances in a buffered vapor cell. We demonstrate this narrowing effect in Fig.…”