# The effect of atomic response time in the theory of Doppler cooling of trapped ions

## TL;DR

This paper introduces a simple method to incorporate atomic response time into Doppler cooling theory, explaining observed experimental results for trapped ions like calcium-40, especially when slow decay affects cooling efficiency.

## Contribution

It presents a new approach to include atomic response time in Doppler cooling models, improving accuracy for ions with complex internal states and slow decay processes.

## Key findings

- The theory explains why calcium-40 ions are colder than predicted by static models.
- Inclusion of response time accounts for observed cooling behavior in experiments.
- The model predicts temperature limits more accurately under realistic conditions.

## Abstract

We describe a simple approach to the problem of incorporating the response time of an atom or ion being Doppler-cooled into the theory of the cooling process. The system being cooled does not in general respond instantly to the changing laser frequencies it experiences in its rest frame, and this "dynamic effect" can affect significantly the temperatures attainable. It is particularly important for trapped ions when there is a slow decay out of the cooling cycle requiring the use of a repumping beam. We treat the cases of trapped ions with two and three internal states, then apply the theory to $^{40}{\rm Ca}^+$. For this ion experimental data exist showing the ion to be cold under conditions for which heating is predicted if the dynamic effect is neglected. The present theory accounts for the observed behaviour.