# Two-step pulse observation to improve resonance contrast for coherent population trapping atomic clock

## TL;DR

This paper introduces a two-step pulse observation method for CPT atomic clocks that enhances resonance contrast and frequency stability by optimizing light intensity during observation, supported by theoretical and experimental analysis.

## Contribution

The paper proposes a novel two-step Raman--Ramsey scheme that reduces light intensity variation and improves resonance contrast in CPT atomic clocks.

## Key findings

- The method maintains high signal-to-noise ratio during observation.
- Density matrix analysis confirms the effectiveness of the scheme.
- Experimental results demonstrate improved frequency stability.

## Abstract

We study resonance contrast by a two-step pulse observation method to enhance the frequency stability of coherent population trapping (CPT) atomic clocks. The proposed method is a two-step Raman--Ramsey scheme with low intensity during resonance observation and high intensity after the observation. This method reduces the frequency variation in the light intensity and maintains a high signal-to-noise ratio. The resonance characteristics were calculated by density matrix analysis of a $\Lambda$-type three-level system that was modeled on the $^{133}$Cs D1 line, and the characteristics were also measured using a vertical-cavity surface-emitting laser and a Cs vapor cell.