# 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.