  The ER = EPR correspondence relates a superposition of entangled,
disconnected spacetimes to a connected spacetime with an Einstein-Rosen bridge.
We construct examples in which both sides may be described by weakly-coupled
string theory. The relation between them is given by a Lorentzian continuation
of the FZZ duality of the two-dimensional Euclidean black hole CFT in one
example, and in another example by continuation of a similar duality that we
propose for the asymptotic Euclidean AdS3 black hole. This gives a microscopic
understanding of ER = EPR: one has a worldsheet duality between string theory
in a connected, eternal black hole, and in a superposition of disconnected
geometries in an entangled state. The disconnected description includes a
condensate of entangled folded strings emanating from a strong-coupling region
in place of a horizon. Our construction relies on a Lorentzian interpretation
of Euclidean time winding operators via angular quantization, as well as some
lesser known worldsheet string theories, such as perturbation theory around a
thermofield-double state, which we define using Schwinger-Keldysh contours in
target space.
