Volume 4 Issue 3, March 2022

100 years ago, Otto Stern and Walther Gerlach demonstrated that silver atoms have a quantized magnetic moment, as predicted from the Bohr–Sommerfeld model of the atom. But the correct interpretation of the result proved to be far more subtle — and revolutionary.

Quantum machine learning may provide powerful tools for data analysis in high-energy physics. Sau Lan Wu and Shinjae Yoo describe how the potential of these tools is starting to be tested and what has been understood thus far.

Machine learning methods relying on synthetic data are starting to be used in mathematics and theoretical physics. Michael R. Douglas discusses recent advances and ponders on the impact these methods will have in science.

Francis Russell reflects on the history of nonlinear excitations.

A paper in Nature shows how reinforcement learning can solve the problem of optimizing the magnetic field in a tokamak to create a variety of plasma configurations, including ones that haven’t been achieved before.

A paper in Physical Review Letters reports how twisting yarn makes it stronger.

Spintronic properties of layered materials combining magnetism and strong spin–orbit coupling can be tailored by proper optimization of chemical interactions and structural material symmetries. This Review draws a route to achieving best performing material design for reaching the upper limit of spin–orbit torque efficiency in switching magnetization.

Much of the rich phenomenology of active matter can be traced back to violations of time-reversal symmetry and departure from Boltzmann statistics. This Review disentangles these nonequilibrium signatures for interacting and dilute systems, in the presence of obstacles and external potentials.

Topological insulators are unique materials giving rise to unconventional quantum phenomena. This Technical Review discusses how various physics effects can only be observed in devices carefully fabricated to address them, including topological superconductivity, quantum anomalous Hall states, spintronic functionalities and topological mesoscopic physics.

Photonics is one of the key platforms for emerging quantum technologies, but its full potential can only be harnessed by exploiting miniaturization via on-chip integration. This Roadmap charts new directions and discusses the challenges associated with the hybrid integration of a variety of materials, devices and components.