Phys. Rev. X (in the press); preprint at https://arxiv.org/abs/1707.07031

Newton’s cradle is a device that famously demonstrates the conservation of momentum and energy in classical mechanics. A row of metal balls swing back and forth, striking their neighbours without ever coming to rest. A similar non-equilibrium motion occurs in the quantum world if a large number of conserved quantities associated with the system’s integrability are available. The quantum version of Newton’s cradle has been observed previously in contact-interacting Bose gas confined in one dimension.

Now Yijun Tang and co-workers have taken a further step forward, studying the crossover from integrable to thermalizing behaviour in a quantum Newton’s cradle. The magnetic dysprosium atoms used in their experiment provide long-range and anisotropic dipole–dipole interactions, which can serve as an integrability-breaking perturbation in the system. By tuning the orientation of the dipoles — corresponding to different perturbation strength — a two-step thermalization process was identified: a rapid dephasing leading to the prethermal state followed by a nearly exponential approach to the final thermal distribution.