String Perturbation Theory on the Schwinger-Keldysh Time Contour

Abstract

We perform the large-$N$ expansion in the Schwinger-Keldysh formulation of non-equilibrium quantum systems with matrix degrees of freedom, and study universal features of the anticipated dual string theory. We find a rich refinement of the topological genus expansion: In the original formulation, the future time instant where the forward and backward branches of the Schwinger-Keldysh time contour meet is associated with its own worldsheet genus expansion. After the Keldysh rotation, the worldsheets decompose into a classical and quantum part.

Comments: 5 pages, a few figures; v2: references added; v3: references updated, small clarifying comments added. Final version accepted in Phys. Rev. Lett