Yixiong Chen, Benjamin Doyon
Using the "Liouville space" (the space of operators) of the massive Ising model of quantum field theory, there is a natural definition for form factors in any mixed state. These generalize the usual form factors, and are building blocks for mixed-state correlation functions. We study the cases of mixed states that are diagonal in the asymptotic particle basis, and obtain exact expressions for all mixed-state form factors of order and disorder fields. We use novel techniques based on deriving and solving a system of nonlinear functional differential equations. We then write down the full form factor expansion for mixed-state correlation functions of these fields. Under weak analytic conditions on the eigenvalues of the density matrix, this is an exact large-distance expansion. The form factors agree with the known finite-temperature form factors when the mixed state is specialized to a thermal Gibbs ensemble. Our results can be used to analyze correlation functions in generalized Gibbs ensembles (which occur after quantum quenches). They can also be used for non-equilibrium steady states where a constant energy flow exists, using the recently derived density matrix. We observe that non-equilibrium form factors have branch cuts in rapidity space, we verify that this is in agreement with a non-equilibrium generalization of the KMS relations, and we show that the leading large-distance behavior of order and disorder non-equilibrium correlation functions contains oscillations in the log of the distance between the fields.
View original:
http://arxiv.org/abs/1305.0518
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