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

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 ${}^{40}{\mathrm{Ca}}^{+}$. 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.

Keywords:

• Laser cooling
• optical Bloch equations
• ion trap

Acknowledgements

We are very glad that this contribution can be included in a special issue for Prof Danny Segal. His good nature, irrepressible cheerfulness and ‘can do’ attitude were an inspiration to students and colleagues alike.

Notes

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was funded by EPSRC.