I'd like some suggestions for good reading materials on the Keldysh formalism in a condensed matter context. I'm familiar with the imaginary time, coherent state, and path integral formalisms, but lately I've been seeing Keldysh more and more in papers. My understanding is that it is superior to the imaginary time formalism at least in that one can evaluate non-equilibrium expectations.
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I am somewhat biased towards condensed matter physics, even though the subject extends also to fields such as cosmology and QCD.
In the context of condensed matter physics I recommend the following books (even though various techniques also apply outside this regime):
There's also chapter 18 in Kleinert which I find a nice read. This book is huge though and treats a lot of other topics. Still, if you go through Rammer then this chapter by Kleinert nicely summarizes everything without dropping too many details. The newest edition of Altland and Simons has two chapters on classical and quantum systems out of equilibrium, but I was fairly disappointed with their treatment considering the rest of the book is fantastic.
As for quantum transport, where this formalism is frequently employed, I can recommend Di Ventra as an undergrad-level introductory book and this book by Datta for some other interesting topics. Weiss is excellent for dissipative (open) systems, although this field opens up a whole new can of worms so you might want to avoid at first.
Other sources not in book form:
For researchers who study condensed matter physics (i.e. low-energy physics), it might be helpful to read following books and articles.
H. Haug and A. P. Jauho: Quantum Kinetics in Transport and Optics of Semiconductors (Springer, New York, 2007).
J. Rammer, Quantum Field Theory of Non-equilibrium States, (Cambridge University Press, 2011).
The above articles will be reliable and readable. On top of them, one can learn important details from the sophisticated manuscripts by Alex Kamenev:
A. Kamenev: Field Theory of Non-Equilibrium Systems, (Cambridge University Press, 2011, arXiv:0412296).
Although (as far as I know) I have listed the relevant articles, I guess I have missed a lot of other important papers. Please forgive me if I have. I hope my contribution helps someone to learn Keldysh formalism.
Last, let me remark the points of Keldysh formalism which I have learned by the above articles; thanks to the Schwinger-Keldysh closed time path, the Schwinger-Keldysh formalism (i.e. closed time path formalism or the real-time formalism) is not based on the assumption usually called the Gell-Mann and Low theorem (i.e. the adiabatic theorem).
Therefore, within the perturbative theory via Schwinger-Keldysh (or contour-ordered) Green's functions, the formalism can deal with an arbitrary time-dependent Hamiltonian and treat the system out of the equilibrium. On top of this, this formalism is applicable to systems at finite temperature; the well-known Matsubara formalism (i.e. the imaginary-time formalism), which can also deal with thermodynamic average values, can be regarded as a simple corollary of the Schwinger-Keldysh formalism.
That is, the Schwinger-Keldysh formalism includes the Matsubara formalism and information about finite temperature is contained in the greater and lesser Green's functions. Consequently we can treat non-equilibrium phenomena at finite temperature thanks to the Schwinger-Keldysh formalism. This will be the strong point of the formalism.
This answer contains some additional resources that may be useful. Please note that answers which simply list resources but provide no details are strongly discouraged by the site's policy on resource recommendation questions. This answer is left here to contain additional links that do not yet have commentary.
J. Berges, Introduction to Nonequilibrium Quantum Field Theory, AIP Conf. Proc. 739 (2004), 3--62. hep-ph/0409233
W. Botermans and R. Malfliet, Quantum transport theory of nuclear matter, Physics Reports 198 (1990), 115--194.
E. Calzetta and B.L. Hu, Nonequilibrium quantum fields: Closed-time-path effective action, Wigner function, and Boltzmann equation, Phys. Dev. D 37 (1988), 2878--2900.
K. Chou, Z. Su, B. Hao, and L. Yu, Equilibrium and nonequilibrium formalisms made unified, Physics Reports 118 (1985), 1--131.
Yu. B. Ivanov, J. Knoll, and D. N. Voskresensky, Self-Consistent Approximations to Non-Equilibrium Many-Body Theory, Nucl. Phys. A 657 (1999), 413--445. hep-ph/9807351
Yu. B. Ivanov, J. Knoll, and D. N. Voskresensky, Resonance Transport and Kinetic Entropy, Nucl. Phys. A 672 (2000), 313--356. nucl-th/9905028
If it is still of interest, next to all the excellent suggestions above there is a book from 2013 which I found rather helpful as it makes some neat observations I could not find in other texts:
Nonequilibrium Many-Body Theory of Quantum Systems - Stefanucci and Leeuwen