Schwinger-Keldysh formalism is an approach to the treatment of non-equilibrium quantum systems pioneered by Keldysh and in an archetypal case earlier by Julian Schwinger.

Literature

Historical papers:

L. V. Keldysh, Diagram technique for nonequilibrium processes, JETP 20:4 (1965) pdf

J.S. Schwinger, Brownian motion of a quantum oscillator, J. Math. Phys. 2 (1961) 407 doi

Recent papers

Mohammad F. Maghrebi, Alexey V. Gorshkov, Nonequilibrium many-body steady states via Keldysh formalism, Phys. Rev. B 93, 014307 doi

Paolo Glorioso, Michael Crossley, Hong Liu, A prescription for holographic Schwinger-Keldysh contour in non-equilibrium systems, arXiv:1812.08785

This formalism also involves the Feynman-Vernon influence functional also known as the Schwinger-Keldysh path integral.

Yoni BenTov, Schwinger-Keldysh path integral for the quantum harmonic oscillator, arXiv:2102.05029

Recent connection to a case of BRST complex and related equivariant cohomology

F.M. Haehl, R. Loganayagam, M. Rangamani, Schwinger-Keldysh formalism I: BRST symmetries and superspace, arXiv:1610.01940 J. High Energ. Phys. 69 (2017) doi; Schwinger-Keldysh formalism. Part II: thermal equivariant cohomology, J. High Energ. Phys. 2017, 70 (2017) doi