Thermal quantum field theory

In theoretical physics, thermal quantum field theory (thermal field theory for short) or finite temperature field theory is a set of methods to calculate expectation values of physical observables of a quantum field theory at finite temperature.

In the Matsubara formalism, the basic idea (due to Felix Bloch[1]) is that the expectation values of operators in a canonical ensemble

An alternative approach which is of interest to mathematical physics is to work with KMS states.