Renyi divergences from Euclidean quenches

TL;DR

This paper explores the generalization of relative entropy, the Renyi divergence, in 2D conformal field theories, analyzing its behavior under various Hamiltonian deformations and demonstrating its potential to impose stronger thermodynamic constraints.

Contribution

It provides the first calculation of Renyi divergences in 2D CFTs for scalar and current deformations, including inhomogeneous profiles like SSD, revealing universal features and thermodynamic implications.

Findings

Renyi divergence depends universally on the parameter for inhomogeneous deformations.

The second laws based on Renyi divergences impose stronger constraints than the traditional second law.

Leading contributions to the divergence match those of harmonic oscillators with linear potentials.

Abstract

We study the generalisation of relative entropy, the Renyi divergence $D_{\alpha} ( \rho||\rho_\beta) $ in 2$d$ CFTs between an excited state density matrix $\rho$, created by deforming the Hamiltonian, and the thermal density matrix $\rho_\beta$. Using the path integral representation of this quantity as a Euclidean quench, we obtain the leading contribution to the Renyi divergence for deformations by scalar primaries and by conserved holomorphic currents in conformal perturbation theory. Furthermore, we calculate the leading contribution to the Renyi divergence when the conserved current perturbations have inhomogeneous spatial profiles which are versions of the sine-square deformation (SSD). The dependence on the Renyi parameter ($\alpha$) of the leading contribution have a universal form for these inhomogeneous deformations and it is identical to that seen in the Renyi divergence of the simple harmonic oscillator perturbed by a linear potential. Our study of these Renyi divergences shows that the family of second laws of thermodynamics, which are equivalent to the monotonicity of Renyi divergences, do indeed provide stronger constraints for allowed transitions compared to the traditional second law.