Preface: special topic on cold atoms

For several decades, Atomic,Molecular, andOptical Physics has undergone and is still going through a renaissance. Research on cold atoms has played a key role in a profound transformation of this traditional field. Ultracold atoms allow us to reach down to the core of AMO physics where detailed knowledge of atomic andmolecular structure forms the foundation for understanding light–matter interactions. This understanding then fuels the development of increasingly sophisticated control of individual quantum systems. The increasing ability for quantum control has stimulated the simultaneous, rapid development of the state-of-the-art lasers. These activities have not only dramatically advanced the frontier of precision measurement, but also enabled explorations of increasingly complex quantumphenomenawhere fundamental connectionsbetween few-bodyand many-body physics are probed. The desire for improved control of atomic motions for spectroscopy applications motivated the early development of laser cooling in 1980s. In a personal interview for this special topic, W.Phillips, oneof the pioneers of laser cooling, gives an intimate accountof how thefield started alongwith thebroad rangeof scientific and technological innovations that have ensued. For example, one imaginative andpractical application of cold atoms is radio-krypton dating. As described in a perspective by Z.-T. Lu, the exquisite isotope selectivity and detection efficacy for single trapped atoms have provided a unique background-free–tracedetection capability for earth science applications. In another application where high-quality atomic transitions in the optical domain are used for the new-generation optical atomic clocks, preserving the maximum coherence between atomic states and anoptical field requires that the atomic center-of-massmotion is controlled at the scale of an optical wavelength and that the optical probe of internal atomic states do not introduce measurement uncertainty fromphoton recoils.Meeting these challenges demands preparing atoms at ultralow temperatures and confining them in traps that decouple the atomic internal and external degrees of freedom.The full control of both internal and external dynamics is critical for precisionmeasurement, frequency metrology (discussed in a review by X. Zhang and J. Ye), and quantum information science. It is now routine to prepare, manipulate, and measure cold atoms in the formof quantumdegenerate gases inwhich striking quantumbehavior dominates the observation of systemdynamics. Two important tools have been invented to enrich quantum physics in strongly interacting regimes: the first powerful tool is the control of interatomic scattering resonances, and thus interactions, via the Feshbach resonance. A perspective by C. Chin beautifully illustrates the transition from a weakly interacting quantum system to a strongly interacting one, where new universal principles can be explored that govern correlated quantum behavior. The second useful tool is the introduction of optical lattices to regulate atomicmotions, which provides an intimate interface betweenultracold atoms and condensed-matter systems. A perspective by S. Kuhr shows the powerful in-situ visualization of individual atoms confined in an optical lattice via a recently developed technique of a quantum gas microscope, which is opening a new approach to the study of many-body quantum systems. With these tools closely integratedwithwell prepared atomic quantum systems, we are ready to take on challenging and outstandingexperiments toprobe the connectionbetween twoand few-body physics and to understand the emergence of quantum many-body correlations. A research highlight by A. M. Rey discusses the importance of introducing synthetic gauge fields to ultracold atoms and turning them into a versatile playground for simulating and gaining insights to new classes of quantummaterials. A reviewbyM.Weidemuller explores recent developments of resonant three-body Efimov effects in ultracold heteronuclear atomic mixtures, helping illuminate universal three-body physics and atom-molecule interactions. Plus, polar molecules have been cooled to quantum gases. The rich internal structure of these molecules provides a unique experimental testing ground for the fundamental laws of nature. They also provide new opportunities for the study of novel many-body quantum systems with tunable, long-range, and anisotropic interactions. The strong interdisciplinary character of these new atomic andmolecular systems facilitates powerful connections to other scientific fields, including chemistry, quantum information, condensed-matter physics, and high-energy physics. Many new research opportunities are emerging. I sincerely thank the contributing authors who invested time and effort for this special topic, and I hope it will serve to inspire young researchers to develop more creative ideas and approaches.