Given that two initially independent systems A and à are in a pure state, we study the type of interaction between them that will lead an observer, limited to measuring system A only, to conclude that it is in a thermal bath. We show (under rather mild conditions) that there is always an interaction that does it, and study it for two particularly simple systems.

• Received 15 February 1989

DOI:https://doi.org/10.1103/PhysRevA.40.1674