Ultrafast thermodynamics: Toward entropy, heat, and criticality on sub-nanosecond timescales

Recent experimental advancements have enabled the control and probing of a wide variety of material excitations using ultrafast laser pulses. This development lies at the core of the emerging field of ultrafast thermodynamics, which enables the definition of heat, work, and entropy production on picosecond timescales and along individual trajectories. In this framework, non-equilibrium material excitations are treated as effective systems coupled to a stochastic environment. Here, we outline the potential of ultrafast thermodynamics using spin and lattice excitations, non-equilibrium collective phenomena, and criticality. We conclude by presenting open problems and ideas for further progress in this promising research field.