Single-Photon Atomic Cooling - Raizen Lab - The University of ...

the atomic ensemble. Not surprisingly and as shown in [34], the information
entropy content carried away by the scattered photons is equal to the reduction
in the entropy of the cooled atomic ensemble.
What is really striking is how extremely efficient the single-photon cool-
ing process is in using information entropy to cool. In the ideal case described
above it is perfectly efficient; the reduction in entropy of the atomic ensemble
equals the information entropy carried away by the spontaneously scattered
photons. When compared to other established atomic laser cooling techniques,
where the increase in photon entropy is typically several orders of magnitude
higher than the decrease in the entropy of the atomic ensemble cooled [35], we
see just how truly remarkable this process is. Additionally, single-photon cool-
ing is a passive technique in the sense that one need not monitor the scattered
photons at all for the process to work. In contrast, stochastic cooling methods,
which like single-photon techniques scatter photons which carry information
related the ensemble’s properties, must actively feedback on the data gathered
from scattered photons to operate [36, 37].
1.6 A Note on Units
A brief discussion of the terminology and units used throughout is
warranted. Of particular note is the loose manner in which the term “tem-
perature” is used in the laser cooling community and indeed in this text. The
concept of temperature is defined in thermodynamics as a property of a closed
system in thermal equilibrium with its surroundings [38, 39]. In typical atomic
19