Recently, laser cooling methods have been extended from atoms to molecules. The complex rotational and vibrational energy level structure of molecules makes laser cooling difficult, but these difficulties have been overcome and molecules have now been cooled to a few microkelvin and trapped for several seconds. This opens many possibilities for applications in quantum science and technology, controlled chemistry, and tests of fundamental physics. This article explains how molecules can be decelerated, cooled and trapped using laser light, reviews the progress made in recent years, and outlines some future applications.
Recently, laser cooling methods have been extended from atoms to molecules. The complex rotational and vibrational energy level structure of molecules makes laser cooling difficult, but these difficulties have been overcome and molecules have now been cooled to a few microkelvin and trapped for several seconds. This opens many possibilities for applications in quantum science and technology, controlled chemistry, and tests of fundamental physics. This article explains how molecules can be decelerated, cooled and trapped using laser light, reviews the progress made in recent years, and outlines some future applications.
Different from atoms, molecules have unique properties, and play an important role in the research of atomic, molecular and optical physics. Cold molecules have important applications in science and have been studied for more than 20 years. But traditional methods, such as the Stark decelerator, have hit a bottleneck: it is hard to increase the phase space density of molecules. Extending the direct laser-cooling technique to new molecular species has recently been a hot topic and also a big challenge. In this review paper, on one hand, we make a brief review to recent progresses on the direct laser cooling of polar molecules. On the other hand, a demonstration on the feasibility of laser cooling BaF molecule has been experimentally illustrated, including the analysis on the molecular energy levels, measurements of the high-resolution spectroscopy, efficient pre-cooling and state preparation via buffer-gas cooling and detailed investigations on the molcule-light interactions. All these results not only pave the way for future laser-cooling and -trapping experiments, but also serve as a reference for the laser-cooling explorations on new molecular species.
Over the past decade, tremendous progress has been made to extend the tools of laser cooling and trapping to molecules. Those same tools have recently been applied to polyatomic molecules (molecules containing three or more atoms). In this review, we discuss the scientific drive to bring larger molecules to ultralow temperatures, the features of molecular structure that provide the most promising molecules for this pursuit, and some technical aspects of how lasers can be used to control the motion and quantum states of polyatomic molecules. We also present opportunities for and challenges to the use of polyatomic molecules for science and technology.
Abstract Many areas of physics—precision measurements, quantum information, and physical chemistry, to name a few—are starting to benefit from the enormous advantages offered by cold and ultracold polar molecules. Molecules have more states, more interactions, and more chemical properties compared to atoms, which make them exciting to study but difficult to tame. In particular, the powerful techniques of atomic laser cooling cannot be naïvely applied to molecules due to their complicated structure. Developments over the past few years have made directly laser cooled and trapped molecules a reality, and now much effort is focused on making these samples larger, denser, and colder—an important step to realizing many of their exciting applications. A careful experimental and numerical study by Truppe et al (2017 New J. Phys. 19 022001 ) demonstrates a significant improvement and advance in understanding of one of the most limiting steps in laser cooling and trapping of molecules—slowing them from a molecular beam to a near-standstill, with small enough kinetic energy that they can be loaded into a trap.
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.
Despite the prominent role of laser cooling in modern atomic physics, with applications ranging from quantum sensors to high-precision spectroscopy and ultracold chemistry, laser cooling techniques are still limited to atoms with simple energy level structure and closed transitions that are accessible by currently available continuous wave (cw) laser sources. Two main obstacles prevent the extension of laser cooling techniques to a variety of atomic species and molecules. The first is associated with technical difficulties in creating cw laser light in the vacuum ultraviolet (VUV) spectral range - the absence of those laser sources prevents laser cooling of simple atoms such as hydrogen, deuterium, and antihydrogen. The second obstacle is associated with complex energy level structure of many atoms and almost all molecules - it permits decay of an excited state into a number of lower-lying states that can be widely separated in energy, which makes laser cooling of such species inefficient and experimentally very challenging.
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.