Foundation of quantum statistical mechanics has recently seen a resurgence of interest partly because ultracold atomic systems serve as ideal testbeds. These systems are at very low temperature and in ultra high vacuum, and, thus, can be regarded as isolated quantum systems, which are described by a single pure state evolving unitarily. In experiments, even under the unitary time evolution, effective stationary states have been observed, and they may or may not be thermal depending on the Hamiltonian that describes the system. Many active theoretical studies have been conducted to understand when and how thermalization emerges from quantum mechanics (see e.g., Ref. [1] for an overview).

The first half of this talk will be devoted to a brief introduction to the ultracold-atom systems and to the theoretical approach to the foundation of quantum statistical mechanics. In the second half, I will talk about our recent study on a non-thermal steady state realized in a coherent splitting of a one-dimensional Bose gas [2]. If time allows, I will talk about more pieces of work on this topic.

References:
[1] L. D’Alessio, Y. Kafri, A. Polkovnikov and M. Rigol, arXiv:1509.06411.
[2] E. Kaminishi, T. Mori, T. N. Ikeda, and M. Ueda, Nature Physics 11, 1050-1056 (2015).