zeroth order, i.e. for AdS space itself. The fluctuation considered is dual to the energy-
momentum tensor on the field-theory side. The proposal (1.9) modifies the pure-state
volume expression (1.6) in a natural way such as to obtain a finite expression. C
d
is a
dimensionless constant which cannot be fixed from first principles on the gravity side.
3
We
will determine C
d
by comparison with results for the relative entropy [15, 16]. For metric
and marginal perturbations, this coefficient depends only on the spacetime dimension, while
for relevant scalar perturbations also the operator dimension enters.
We suggest that the holographic reduced fidelity susceptibility is obtained by taking
the second order variation of F with respect to m,
G
R,mm
= ∂
2
m
F. (1.10)
The definition (1.10), along with the proposal (1.9), ensures that the holographic fidelity
susceptibility is finite, as required for mixed states. G
R,mm
scales as G
R,mm
∝ R
2d
, with
R the radius of the spherical entangling region in the dual field theory in d spacetime
dimensions. This scaling behaviour is expected, as we discuss below. Moreover, the volume
difference in (1.9) entirely encodes the dependence on the shape of the entangling region.
The particular scaling behaviour G
R,mm
∝ R
2d
also follows from the alternative defi-
nition for Fisher information proposed in [11] in terms of the relative entropy ∆S, which
is a measure of entropic distance between two states. An example is the relative entropy
measuring this distance between a perturbed state and the ground state. The Fisher infor-
mation metric proposed in [11] is given by the second order variation of ∆S measuring the
entropic distance between the ground state of the boundary CFT and the state obtained
by perturbing this ground state by injecting energy. As holographically shown in [15] and
later confirmed in [17, 18] by a direct field theory computation, the relative entropy for this
perturbation scales precisely as R
2d
for a spherical subregion of radius R, at quadratic order
in energy fluctuations. In fact as discussed in [18], when taking into account a calculational
issue, both the expressions of relative entropy obtained holographically in [15] and from
the field theory computation of [18] match exactly including the prefactor. Consequently,
the Fisher information metric also scales as R
2d
. Now for the restricted class of states
introduced above, the reduced fidelity susceptibility coincides with the Fisher information
metric. Therefore, for this class of states, the reduced fidelity susceptibility G
R,mm
also
scales with R
2d
.
Our proposal of identifying the expression (1.10) with the holographic dual of the
fidelity susceptibility for mixed states, using the renormalized volume proposal (1.9), thus
provides a finite expression with the correct scaling behaviour R
2d
. This expression encodes
all information about the shape of the entangling region.
As a further example, we also consider the fluctuations caused by the insertion of
a scalar in AdS and compute the corresponding subtracted volume at the quadratic or-
der in the perturbation parameter. We obtain a scaling behaviour of the corresponding
contribution to the Fisher information metric of the form R
2∆
, where ∆ is the scaling
dimension of the operator dual to the scalar bulk AdS field. This behaviour is again con-
sistent with the quadratic variation of the relative entropy for such fluctuations [15], and
3
A similar undetermined coefficient is also present in the pure state proposal of [12], as is seen from
equations (2), (9) and (18) in that paper.
– 4 –