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| Figure 1: Doyle and colleagues [2] have cooled SrOH molecules using Sisyphus cooling. In this type of cooling, the SrOH molecules are made to climb a potential energy hill, only to be transported back to the bottom, much like their Greek eponym who was doomed to roll a boulder up a hill over and over again. The energy lost in climbing the hill cools the SrOH molecules to ultracold temperatures. Show less |
Topics: Bose-Einstein Condensate, Laser, Modern Physics, Nobel Prize, Quantum Mechanics
Only because of the illustration and the myth, but the process of laser cooling is quite sound, as the article describes below.
Physicists considering a foray into the study of molecules are often warned that “a diatomic molecule is one atom too many!” [1]. Now John Doyle and colleagues [2] at Harvard University have thrown this caution to the wind and tackled laser cooling of a triatomic molecule with success, opening the door to the study of ultracold polyatomic molecules.
The technique of laser cooling [3], which uses the scattering of laser photons and the concomitant momentum transfer to bring atoms to a near halt, has revolutionized atomic, molecular, and optical (AMO) physics. Laser cooling and an important variant known as Sisyphus cooling [4] underpin three Nobel prizes in physics—for magneto-optical trapping (1997), Bose-Einstein condensation (2001), and the manipulation of individual quantum systems (2012)—and are crucial to a host of quantum-assisted technologies and fundamental physics measurements.
Since photons carry very little momentum and therefore reduce an atom’s velocity by just a small amount, a prerequisite for effective laser cooling is the ability to scatter thousands of photons. Thus laser cooling has predominantly been used only to cool simple atoms, whose electronic structure dictates that after a photon is absorbed, spontaneous emission places the atomic electron back into its original state, allowing the process to repeat nearly ad infinitum.
Spurred on by the possibility of another revolution in AMO physics when ultracold molecules become available [5], a brave group of researchers recently began work to achieve laser cooling of diatomic molecules, guided by a new proposal for how to deal with their complex structure [6]. Diatomic molecules, or “diatoms,” are challenging targets for laser cooling as their electronic structure is complicated by their rotational and vibrational degrees of freedom. When a diatom absorbs a photon from the laser, spontaneous emission can place it in any of a multitude of these rotational and vibrational states, whose transition frequencies no longer match that of the cooling laser. These so-called dark states are the bane of laser cooling, bringing the cooling process to a stop. Nonetheless, by carefully choosing molecules with unique properties—for example, those which contain an optically active electron that does not strongly participate in the molecular bonding—laser cooling of molecules has been successful, and it has culminated in the demonstration of magneto-optical trapping of SrF molecules [7].
APS Viewpoint: A Diatomic Molecule is One Atom too FewPaul Hamilton, Eric Hudson, University of California, Los Angeles
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