Our ability to laser cool atoms depends on the availability of a closed-cycling transition that allows many successive photons to be scattered by the atom. By comparison, molecules have a richer internal structure that is an asset and a liability. It offers new degrees of freedom for exciting experiments but precludes a simple scheme for molecular laser cooling.
Our ability to laser cool atoms depends on the availability of a closed-cycling transition that allows many successive photons to be scattered by the atom. By comparison, molecules have a richer internal structure that is an asset and a liability. It offers new degrees of freedom for exciting experiments but precludes a simple scheme for molecular laser cooling.
When an atom or a molecule interacts with a light beam, the light emitted or absorbed carries valuable information about the atomic or molecular structure. This phenomenon underlies the whole field of spectroscopy. But the interaction of a photon with an atom can be used to manipulate the atom as well as to probe its structure. For example, in an approach called optical pumping, invented by Alfred Kastler, one can use the resonant exchange of angular momentum between atoms and polarized photons to align or orient the spins of atoms or to put them in nonequilibrium situations. In his original 1950 paper Kastler also proposed using optical pumping to cool and to heat the internal degrees of freedom, calling the phenomena the “effet luminofrigorique” and the “effet luminocalorique.” Another famous example of the use of photon-atom interaction to control atoms is laser cooling. This technique relies on resonant exchange of linear momentum between photons and atoms to control their external degrees of freedom and thus to reduce their kinetic energy. Laser cooling was suggested independently by Theodor Hänsch and Arthur Schawlow for neutral atoms and by David Wineland and Hans Dehmelt for trapped ions. In an article written three years ago for PHYSICS TODAY (June 1987, page 34), Wineland and Wayne Itano presented the principle of laser cooling and the potential applications of cold atoms to fields of physics such as ultrahigh resolution spectroscopy, atomic clocks, collisions, surface physics and collective quantum effects. At that time laser cooling had brought temperatures down to a few hundred microkelvin, but unexpected improvements during the last three years have dramatically lowered those temperatures to only a few microkelvin. We now feel we understand the new physical mechanisms responsible for these very low temperatures.
Abstract The extension of laser cooling and trapping techniques from atoms to molecules is currently the subject of intense and growing interest. Molecular laser cooling and trapping offers a general technique applicable to a variety of molecules with different properties and internal structures and is well-suited to the growing list of proposed applications for ultracold molecules. The recent development of optical cycling schemes for specific molecular species now allow each molecule to scatter up to ∼10 6 photons. This has proven sufficient for multiple groups to laser cool and slow molecular beams and, notably, capture molecules in magneto-optical traps (MOTs) and cool them to ultracold temperatures. MOTs are the workhorse technique for atomic physics in the ultracold regime and today’s molecular MOTs represent a powerful starting point for the study and manipulation of ultracold molecular gases. This Topical Review presents a summary of the progress made in this new and emerging field.
In a paper of 1950, Alfred Kastler proposed cooling of vapours by resonant light via optical pumping of the orientational levels of angular momentum, and successive collisions. Optical pumping of the degrees of freedom of the translational momentum is also feasible. This phenomenon has been demonstrated with ions, even with a single ion, in an electrodynamic or electromagnetic trap. Optical cooling by Raman-like two-photon transitions seems to be capable of generating ultra-low temperature levels.
Slowing, cooling, and trapping molecules One of the major achievements in physics during the last two decades of the twentieth century was the ability to cool and trap atoms [1]. Laser cooling can produce previously inconceivable atomic temperatures of less than 1 μK. Atoms with sufficiently low kinetic energy can be trapped in magnetic and optical traps and thus studied for a prolonged time, thereby improving the resolution of spectroscopy by many orders of magnitude as well as reducing Doppler broadening. Furthermore, besides the external degrees of freedom, the internal degrees of freedom of the atoms can also be manipulated, leading to full control of the atoms. One of the holy grails of physics, the achievement of Bose–Einstein condensation in a dilute gas, was achieved mainly because of laser cooling and trapping of atoms. One of the natural extensions of laser cooling and trapping of atoms is the application of the techniques to molecules. Molecules offer a much richer internal structure than atoms, and thus provide extended possibilities to experimentalists. However, atomic laser cooling is based on the repetitive scattering of light and requires a closed, or at least nearly closed, two-level system. Because of the complicated internal structure of molecules, a closed two-level system cannot usually be found because they have many different electronic, vibrational, and rotational states and in general can decay to many different states after absorption of light (see Chap. 16). This requires a complete rethinking of the schemes that are used to cool atoms. Still, because of the extended possibilities cold molecules can offer with respect to cold atoms, there have been a large number of techniques developed to slow, cool, and trap them. These techniques can be divided into two classes, namely direct and indirect cooling. In the first case, molecules at cryogenic or room temperature are directly slowed and cooled. Although many different slowing techniques are used, this chapter focuses on Stark slowing of molecules since it was the first technique developed. Apart from the Stark shift of the molecules, the Zeeman and light shift of molecules can also be used to slow them. The idea behind Stark slowing is to subject the moving molecules to a continuously increasing electric field so that their kinetic energy is converted into potential energy.
Optical pumping techniques using laser fields combined with photo-association of ultracold atoms leads to control of the vibrational and/or rotational population of molecules. In this study, we review the basic concepts and main steps that should be followed, including the excitation schemes and detection techniques used to achieve ro-vibrational cooling of Cs 2 molecules. We also discuss the extension of this technique to other molecules. In addition, we present a theoretical model used to support the experiment. These simulations can be widely used for the preparation of various experiments because they allow the optimization of several important experimental parameters.
Ultracold Physics
Experimental progress over the past few decades has led to the mastery of ultracold atomic gases. A major thrust of current research is to extend this success to ultracold molecules, which would open qualitatively new perspectives for quantum information science, precision measurement, quantum chemistry, and other fields. The internal degrees of freedom in molecules preclude immediate implementation of conventional methods. Using a specific combination of rovibronic optical transitions, Mitra et al. report direct Sisyphus laser cooling of the symmetric top molecule CaOCH3 to temperatures below 1 millikelvin (see the Perspective by Hudson). The proposed scheme for cooling is potentially applicable to a wide range of nonlinear polyatomic molecules.
Science , this issue p. [1366][1]; see also p. [1304][2]
[1]: /lookup/doi/10.1126/science.abc5357
[2]: /lookup/doi/10.1126/science.abd1657