  We consider thermalization of a microscopic quantum system by interaction
with a thermal bath. Our interest is the minimal size the bath can have while
still being able to thermalize the system. Within a specific thermalization
scheme we show that a single spin-1/2 can be fully thermalized by interaction
with a bath that consists of just two other spin-1/2. The two bath spins are
initially in a pure, entangled state, and the thermalizing interaction is a
Heisenberg exchange-interaction of the system-spin with one of the bath spins.
The time dependent coupling strength has to obey a single integral constraint.
We also present a simple generalization of this minimal model in which the bath
consists of an arbitrary number of spin-1/2 pairs.
