Perturbation Theory: Wick’s Theorem for Strings of Operators Ordered along a Contour
Abstract
This chapter considers a general form of the Wick’s theorem, which leads to a perturbation expansion of the (contour) single- and two-particle Green’s functions, which are expressed in terms of the contour time-ordering operator. The strategy for proving the Wick’s theorem is similar to that adopted within the Matsubara formalism for the Green’s functions at finite temperature and relies on the Gibbs form of the statistical operator in the interaction picture. An extension to superfluid Bose and Fermi systems is also considered.
Fields of Study & Subjects
Quantum many-body systems
Cold Atom Physics and Bose-Einstein Condensates
Theoretical and Computational Physics
Publication details
- Journal / source
- Cambridge University Press eBooks
- Pages
- 49–60
- Publication date
- 6 February 2026
- Document type
- Book chapter