  Why is thermalisation a universal phenomenon? How does a quantum system reach
thermodynamical equilibrium? These questions are not new, dating even from the
very birth of quantum theory and have been the subject of a renewed interest
over the two last decades (see for instance the review in Eisert et al, Nature
Physics 2015). In this work, we propose a universal model demonstrating that
thermalisation of a small quantum system is an emergent property of the unitary
evolution under a Schr\"{o}dinger equation of a larger composite system, whose
initial state can be arbitrary. We show that the origin of universality lies in
the phenomenon of 'measure concentration', which provides self-averaging
properties for the reduced density matrix characterizing the state of the small
subsystem. Using our framework, we focus on the asymptotic state at long times
and consider its stationary properties. In typical macroscopic conditions, we
recover the canonical state and the Boltzmann distribution well known from
statistical thermodynamics. This findings lead us to propose an alternative and
more general definition of the canonical partition function which also allow us
to describe non thermal stationary states.
