This Research Topic is Volume II of a series. The previous volume can be found here:

Quantum Precision Measurement and Cold Atom PhysicsEver since the invention of the cesium atomic clock in 1955, quantum frequency standards have seen considerable development over the decades, as a representative of quantum precision measurement. The progress in frequency measurements achieved in the past allowed one to perform quantum precision measurements of other physical and technical quantities with unprecedented precision, whenever they could be traced back to a frequency measurement. Using atomic transitions as frequency reference, quantum frequency standards are far less susceptible to external perturbations, and the identity of microscopic particles allows easy replication of a quantum standard with the same frequency. With laser cooling and trapping, cold atomic ensembles eliminate Doppler shift broadening, and have become the go-to quantum reference when precision and new physics are pursued.

The advancement of laser cooling and cold atom physics, in addition to novel physical matter states such as Bose-Einstein Condensation, give rise to new experimental techniques in quantum precision measurement, especially quantum frequency standards, such as cesium fountain clocks dictating the SI second, as well as optical lattice clocks and single-ion optical clocks pushing the frontier of quantum metrology. Other areas of quantum metrology, such as gravitometers and magnetometers, also benefit greatly from cold atoms. For practical applications, quantum frequency standards are usually required to be compact and portable, and thermal atoms in the form of atomic beams or vapor cells are utilized. Commercially available quantum frequency standards such as cesium beam clocks or rubidium clocks have become the cornerstone of navigation and timekeeping. Compact optical clocks based on various laser spectroscopic techniques have also been developed. Challenges still remain, as researchers strive to push forward the limit in quantum precision measurement and search for novel physical phenomena in cold atomic systems.

In honor of Prof. Yiqiu Wang on the occasion of his 90th birthday, the main goal of this Research Topic is to provide a platform to exhibit the recent achievements and reveal the future challenges in quantum precision measurement, as well as studies of cold atom physics with quantum metrology, closely related to the long-term scientific research areas of Prof. Yiqiu Wang. Both Original Research and Review articles are encouraged. Topics of interest to this collection include, but are not limited to:

• Quantum precision measurements;

• Microwave atomic clocks and their applications;

• Optical frequency standards, laser spectroscopy, and their applications;

• Cold atom systems for quantum metrology and novel physics.