Phase space density limitation in laser cooling without spontaneous emission

TL;DR

This paper investigates the fundamental limits of increasing phase space density in laser cooling without spontaneous emission, highlighting the bounds and potential methods to surpass them using advanced techniques.

Contribution

It establishes a fundamental bound on phase space density enhancement in non-interacting particles without spontaneous emission, emphasizing the importance of quantum descriptions.

Findings

Maximum phase space density gain is limited to a factor equal to the number of atomic levels.

Semi-classical models can lead to large errors if small structures are not smoothed.

Using non-coherent light, feedback, or collective states can help overcome the limit.

Abstract

We study the possibility to enhance the phase space density of non-interacting particles submitted to a classical laser field without spontaneous emission. We clearly state that, when no spontaneous emission is present, a quantum description of the atomic motion is more reliable than semi-classical description which can lead to large errors especially if no care is taken to smooth structures smaller than the Heisenberg uncertainty principle. Whatever the definition of position - momentum phase space density, its gain is severely bounded especially when started from a thermal sample. More precisely, the maximum phase space density, can only be improved by a factor M for M-level atoms. This bound comes from a transfer between the external and internal degrees of freedom. To circumvent this limit, one can use non-coherent light fields, informational feedback cooling schemes, involve collectives states between fields and atoms, or allow a single spontaneous emission even