We begin with perturbations around free theories in section 2. Here our classification
is performed simultaneously with the construction of examples exhibiting state-dependent
divergences. Section 3 then proceeds to classify possible state-dependent divergences at
leading order in large N for marginal or relevant deformations of d ≤ 6 holographic con-
formal theories; i.e., those with dual descriptions in terms of the classical gravitational
dynamics of asymptotically AdS bulk spacetimes. In this case we save the construction
of examples for separate treatment in section 3.1. These examples are constructed in a
bottom-up manner on the gravitational side of the duality. In section 4 we explain how
quantities such as the mutual information, generalized entropy, and relative entropy can
remain finite even when the entanglement entropy has state-dependent divergences. We
close with some final discussion in section 5.
2 Perturbatively renormalizable theories
Implementing the replica trick perturbatively involves evaluating Feynman diagrams on
the replica manifold M
(n)
. Assuming the spacetime metric to be smooth, obtaining a
state-dependent divergence from a path integral requires a Feynman diagram with three
properties:
a) it contains at least one loop (to make it divergent)
b) it has external legs ending on the entangling surface (to make it state-dependent)
c) it renormalizes a term in the action involving curvature (in order to contribute to the
replica trick)
In the case of free field theory, since there are no nontrivial vertices, a connected
Feynman diagram cannot have both loops and loose ends. So state-dependent divergences
are forbidden in regular states of a free theory.
6
On the other hand, it is easy to generate such diagrams in interacting theories. For
example, in a φ
4
theory with d = 4, heat kernel methods give a logarithmically divergent
counter-term in the action proportional to the integral of φ
2
R. Such curvature couplings
are well-known to contribute an entropy term proportional to the integral of φ
2
, here with
logarithmically divergent coefficient.
7
A similar result may also be obtained by noting the
mass-dependence of the logarithmic entropy divergence found in [47], and that linearizing
the φ
4
term about states with non-zero expectation values of φ
2
generally shifts the effective
mass by an amount that depends on the choice of such a state.
8
See figure 1 for an
explanation of this state-dependent divergence in terms of Feynman diagrams.
6
Cooperman and Luty [41] claim to have found states with state-dependent divergences in free field
theories. However, these states were constructed by a path integral on a Euclidean manifold M
0
which
differed from (the Wick rotation of) the manifold M on which the states were defined to live, and in
particular where M and M
0
do not match smoothly. States generated from this construction are in general
not guaranteed to be regular states, for example they need not obey the Hadamard condition [42, 43].
7
One may also use the results of [44] to renormalize the HRT entropy directly. In such calculations,
it is natural to expect such terms in analogy to similar holographic renormalizations of the action. See
e.g. [45, 46] for treatments of low dimension scalars in that context.
8
We thank Vladimir Rosenhaus for suggesting this point of view.
– 4 –