In recent years, atomic clocks have been widely recognized as a key technology in global positioning system (GPS) and other sensing systems. State-of-the-art clocks such as optical lattice clocks showed outstanding stability and accuracy, which will possibly be used for the new definition of time in the near future. This special issue introduces recent research topics on laser cooling methods and optical lattice clocks. Evidently, all the contributing articles exhibit some association with our core research fields, that is, surface science and vacuum science. Therefore, we expect that fruitful communications and collaborations will be promoted by these articles, and hopefully a new research area would be discovered.

We demonstrate the cold emission of ytterbium atomic vapors from ytterbium oxide irradiated with a simple ultraviolet diode laser. Slow atoms are trapped by a magneto-optical trap using a dipole-allowed strong transition at 399 nm. Accompanying phenomenon of the emission of white light from the irradiated ytterbium oxide sample is preliminary investigated. This method will open the way towards a compact and transportable optical lattice clock.

Light-induced atom desorption (LIAD) of alkali-metal atoms in alkali-metal vapor cells can be used to quickly increase the vapor density. It is particularly useful as a source for laser-cooled atoms. We review the studies of LIAD for alkali-metal atoms, with particular attention to its application to laser cooling experiments. We show clear differences in LIAD properties for quartz and Pyrex substrates, which are widely used materials in laser-cooling cells.

The atom-ion hybrid system provides us with new platform to study ultracold collisions and chemical reactions, which play important role in quantum simulations, quantum information processing utilizing atom-ion interactions. To investigate chemical reactions in an ultracold regime, we start with neutral atoms trapped in an optical dipole trap and ions in a linear Paul trap, and combine them together to construct a hybrid system. Here we introduce our recent work on ultracold elastic/inelastic collisions at a single atom level with a state-by-state manner in our atom-ion hybrid system. From the quantitative discussion on the collision cross sections, the microscopic collisional mechanisms have been clarified.

Tapered optical fiber with subwavelength diameter waist, an optical nanofiber, provides a unique and versatile platform for manipulating atoms and photons. In the guided modes of the nanofibers, the optical field can be tightly confined in the transverse direction while enabling strong interaction with the surrounding medium in the evanescent region. Combining laser-cooled atoms with nanofibers has enabled surprising quantum optics experiments, e.g. the efficient channeling of emission from single atoms into the fiber-guided modes, spectroscopy of near-surface atoms, high optical depth with an array of atoms optically trapped around the nanofiber, atomic memories and Bragg reflectors for fiber-guided photons, chiral light-matter interaction etc. In addition, using moderate longitudinal confinement in nanofiber cavities has enabled strong coupling between a single atom and fiber-guided photons. In this article, I review some of the key experimental demonstrations on the “atom＋nanofiber” platform.

Several schemes to optically cool a micromechanical resonator are introduced. These schemes are classified into a cavity-assisted type and a cavity-free type, which both lead to damping of a micromechanical resonator with a laser light. The cavity-assisted schemes are based on the optomechanical coupling via the radiation pressure or bolometric effects, whereas the cavity-free scheme uses the optically excited carriers (electron-hole pairs) in a compound semiconductor. These laser cooling schemes allow us to reduce the thermal noise of a micromechanical resonator without using any refrigerant.

Argon ion sputter etching was applied to SUS420J2 stainless steel, oxygen free copper (OFC) and pure Ni plate specimens that were placed on a SUS304 stainless steel or an OFC target disk at a sputter power of 150–400 W for 2–180 min. Cone-shaped protrusions with bottom diameters ranging from 2–20 µm were formed at various intervals on the specimens. The field emission measurements showed that the SUS420J2 specimen has the best emission properties among the specimens, i.e., the lowest voltage to extract emission current, the largest upper limit current density of about 2 mA/cm², and the largest field enhancement factor of 810, attributed to its sharp protrusion tip, small vertical angle and relatively wide protrusion intervals.