Ramp from replica trick

Abstract We compute the spectral form factor of the modular Hamiltonian K = −ln ρ A associated to the reduced density matrix of a Haar random state. A ramp is demonstrated and we find an analytic expression for its slope. Our method involves an application of the replica trick, where we first calculate the correlator tr ρ A n tr ρ A m $leftlangle textrm{tr}{ρ}_Aⁿ tr{ρ}_A^mrightrangle$ at large bond dimension and then analytically continue the indices n, m from integers to arbitrary complex numbers. We use steepest descent methods at large modular times to extract the ramp. The large bond dimension limit of the replicated partition function is dominated by a sum over annular non-crossing permutations. We explored the similarity between our results and calculations of the spectral form factor in low dimensional gravitational theories where the ramp is determined by the double trumpet geometry. We find there is an underlying resemblance in the two calculations, when we interpret the annular non-crossing permutations as representing a discretized version of the double trumpet. Similar results are found for an equilibrated pure state in place of the Haar random state.