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Quantum null energy condition, entanglement wedge nesting,
and quantum focusing
Chris Akers ,
*
Venkatesa Chandrasekaran,
†
Stefan Leichenauer,
‡
Adam Levine,
§
and Arvin Shahbazi Moghaddam
∥
Center for Theoretical Physics and Department of Physics,
University of California, Berkeley, California 94720, USA
and Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
(Received 14 November 2019; published 22 January 2020)
We study the consequences of entanglement wedge nesting for conformal field theories (CFTs) with
holographic duals. The CFT is formulated on an arbitrary curved background, and we include the effects of
curvature-squared couplings in the bulk. In this setup we find necessary and sufficient conditions for
entanglement wedge nesting to imply the quantum null energy condition in d ≤ 5, extending its earlier
holographic proofs. We also show that the quantum focusing conjecture yields the quantum null energy
condition as its nongravitational limit under these same conditions.
DOI: 10.1103/PhysRevD.101.025011
I. INTRODUCTION AND SUMMARY
The quantum focusing conjecture (QFC) is a new
principle of semiclassical quantum gravity proposed in
[1]. Its formulation is motivated by classical focusing,
which states that the expansion θ of a null congruence of
geodesics is nonincreasing. Classical focusing is at the
heart of several important results of classical gravity [2–5],
and likewise quantum focusing can be used to prove
quantum generalizations of many of these results [6–9].
One of the most important and surprising consequences of
the QFC is the quantum null energy condition (QNEC),
which was discovered as a particular nongravitational limit
of the QFC [1]. Subsequently the QNEC was proven for free
fields [10] and for holographic conformal field theories
(CFTs) on flat backgrounds [11] (and recently extended in
[12] in a similar way as we do here). The formulation of the
QNEC which naturally comes out of the proofs we provide
here is as follows.
Consider a codimension-two Cauchy-splitting surface Σ ,
which we will refer to as the entangling surface. The Von
Neumann entropy S½Σ of the interior (or exterior) or Σ is a
functional of Σ, and in particular is a functional of the
embedding functions X
i
ðyÞ that define Σ. Choose a one-
parameter family of deformed surfaces ΣðλÞ, with
Σð0Þ¼Σ, such that (i) ΣðλÞ is given by flowing along
null geodesics generated by the null vector field k
i
normal
to Σ for affine time λ, and (ii) ΣðλÞ is either “shrinking” or
“growing” as a function of λ, in the sense that the domain of
dependence of the interior of Σ is either shrinking or
growing. Then for any point on the entangling surface we
can define the combination
T
ij
ðyÞk
i
ðyÞk
j
ðyÞ −
1
2π
d
dλ
k
i
ðyÞ
ffiffiffiffiffiffiffiffiffi
hðyÞ
p
δS
ren
δX
i
ðyÞ
: ð1:1Þ
Here
ffiffiffiffiffiffiffiffiffi
hðyÞ
p
is the induced metric determinant on Σ.
Writing this down in a general curved background requires
a renormalization scheme both for the energy-momentum
tensor T
ij
and the renormalized entropy S
ren
. Assuming that
this quantity is scheme-independent (and hence well
defined), the QNEC states that it is positive. Our main
task is to determine the necessary and sufficient conditions
we need to impose on Σ and the background spacetime at
the point y in order that the QNEC hold.
In addition to a proof through the QFC, the holographic
proof method of [11] is easily adaptable to answering this
question in full generality. The backbone of that proof is
entanglement wedge nesting (EWN), which is a conse-
quence of subregion duality in AdS/CFT [9]. A given
region on the boundary of AdS is associated with a
particular region of the bulk, called the entanglement
wedge, which is defined as the bulk region spacelike-
related to the extremal surface [13–16] used to compute the
CFT entropy on the side toward the boundary region. This
bulk region is dual to the given boundary region, in the
sense that there is a correspondence between the algebras of
operators in the bulk region and the operators in the
boundary region which are good semiclassical gravity
*
cakers@berkeley.edu
†
ven_chandrasekaran@berkeley.edu
‡
sleichen@berkeley.edu
§
arlevine@berkeley.edu
∥
arvinshm@berkeley.edu
Published by the American Physical Society under the terms of
the Creative Commons Attribution 4.0 International license.
Further distribution of this work must maintain attribution to
the author(s) and the published article’s title, journal citation,
and DOI. Funded by SCOAP
3
.
PHYSICAL REVIEW D 101, 025011 (2020)
2470-0010=2020=101(2)=025011(21) 025011-1 Published by the American Physical Society
operators (i.e., they act within the subspace of semiclassical
states) [17–19]. EWN is the statement that nested boundary
regions must be dual to nested bulk regions, and clearly
follows from the consistency of subregion duality.
While the QNEC can be derived from both the QFC and
EWN, there has been no clear connection between these
derivations.
1
As it stands, there are apparently two QNECs,
the QNEC-from-QFC and the QNEC-from-EWN. We will
show in full generality that these two QNECs are in fact the
same, at least in d ≤ 5 dimensions.
Here is a summary of our results:
(i) The holographic proof of the QNEC from EWN is
extended to CFTs on arbitrary curved backgrounds.
In d ¼ 5 we find necessary that the necessary and
sufficient conditions for the ordinary QNEC to hold
at a point are that
2
θ
ðkÞ
¼ σ
ðkÞ
ab
¼ D
a
θ
ðkÞ
¼ D
a
σ
ðkÞ
bc
¼ R
ka
¼ 0 ð1:2Þ
at that point. For d<5 only a subset of these
conditions are necessary. This is the subject of
Sec. II C.
(ii) We also show holographically that under the weaker
set of conditions
σ
ðkÞ
ab
¼ D
a
θ
ðkÞ
þ R
ka
¼ D
a
σ
ðkÞ
bc
¼ 0 ð1:3Þ
the Conformal QNEC holds. The Conformal QNEC
was introduced in [11] as a conformally transformed
version of the QNEC. This is the strongest inequality
that we can get out of EWN. This is the subject of
Sec. II E
(iii) By taking the nongravitational limit of the QFC we
are able to derive the QNEC again under the same
set of conditions as we did for EWN. This is the
subject of § III B.
(iv) We argue in Sec. III C that the statement of the
QNEC is scheme-independent whenever the con-
ditions that allow us to prove it hold. This shows that
the two proofs of the QNEC are actually proving the
same, unambiguous field–theoretic bound.
We conclude in Sec. IV with a discussion and suggest
future directions. A number of technical Appendices are
included as part of our analysis.
A. Relation to other work
While this work was in preparation, [12] appeared
which has overlap with our discussion of EWN and the
scheme-independence of the QNEC. The results of [12]
relied on a number of assumptions about the background:
the null curvature condition and a positive energy con-
dition. From this they derive certain sufficient conditions
for the QNEC to hold. We do not assume anything about
our backgrounds a priori, and include all relevant higher
curvature corrections. This gives our results greater gen-
erality, as we are able to find both necessary and sufficient
conditions for the QNEC to hold.
II. ENTANGLEMENT WEDGE NESTING
A. Subregion duality
The statement of AdS/CFT includes a correspondence
between operators in the semiclassical bulk gravitational
theory and CFT operators on the boundary. Moreover, it has
been shown [19,20] that such a correspondence exists
between the operator algebras of subregions in the CFT and
certain associated subregions in the bulk as follows:
Consider a spatial subregion A in the boundary geometry.
The extremal surface anchored to ∂A, which is used to
compute the entropy of A [13,14], bounds the so-called
entanglement wedge of A, EðAÞ, in the bulk. More precisely
EðAÞ is the codimension-zero bulk region spacelike-related
to the extremal surface on the same side of the extremal
surface as A. Subregion duality is the statement that the
operator algebras of DðAÞ and EðAÞ are dual, where DðAÞ
denotes the domain of dependence of A
.
1. Entanglement wedge nesting
The results of this section follow from EWN, which we
now describe. Consider two boundary regions A
1
and A
2
such that DðA
1
Þ ⊆ DðA
2
Þ. Then consistency of subregion
duality implies that EðA
1
Þ ⊆ EðA
2
Þ as well, and this is the
statement of EWN. In particular, EWN implies that the
extremal surfaces associated to A
1
and A
2
cannot be
timelike-related.
We will mainly be applying EWN to the case of a one-
parameter family of boundary regions, AðλÞ, where
DðAðλ
1
ÞÞ ⊆ DðAðλ
2
ÞÞ whenever λ
1
≤ λ
2
. Then the union
of the one-parameter family of extremal surfaces associated
to AðλÞ forms a codimension-one surface in the bulk that is
nowhere timelike. We denote this codimension-one surface
by M. See Fig. 1 for a picture of the setup.
Since M is nowhere timelike, every one of its tangent
vectors must have non-negative norm. In particular, con-
sider the embedding functions
¯
X
μ
of the extremal surfaces
in some coordinate system. Then the vectors δ
¯
X
μ
≡∂
λ
¯
X
μ
is
tangent to M, and represents a vector that points from one
extremal surface to another. Hence we have ðδ
¯
XÞ
2
≥ 0 from
EWN, and this is the inequality that we will discuss for
most of the remainder of this section.
Before moving on, we will note that ðδ
¯
XÞ
2
≥ 0 is not
necessarily the strongest inequality we get from EWN. At
each point on M, the vectors which are tangent to the
1
In [9] it was shown that the QFC in the bulk implies EWN,
which in turn implies the QNEC. This is not the same as the
connection we are referencing here. The QFC which would imply
the boundary QNEC in the sense that we mea n is a boundary
QFC, obtained by coupling the boundary theory to gravity.
2
Here σ
ðkÞ
ab
and θ
ðkÞ
are the shear and expansion in the k
i
direction, respectively, and D
a
is a surface covariant derivative.
Our notation is further explained in Appendix A.
CHRIS AKERS et al. PHYS. REV. D 101, 025011 (2020)
025011-2
extremal surface passing through that point are known to be
spacelike. Therefore if δ
¯
X
μ
contains any components which
are tangent to the extremal surface, they will serve to make
the inequality ðδ
¯
XÞ
2
≥ 0 weaker. We define the vector s
μ
at
any point of M to be the part of δ
¯
X
μ
orthogonal to the
extremal surface passing through that point. Then
ðδ
¯
XÞ
2
≥ s
2
≥ 0. We will discuss the s
2
≥ 0 inequality in
Sec. II E after handling the ðδ
¯
XÞ
2
≥ 0 case.
B. Near-boundary EWN
In this section we explain how to calculate the vector δ
¯
X
μ
and s
μ
near the boundary explicitly in terms of CFT data.
Then the EWN inequalities ðδ
¯
XÞ
2
> 0 and s
2
> 0 can be
given a CFT meaning. The strategy is to use a Fefferman-
Graham expansion of both the metric and extremal surface,
leading to equations for δ
¯
X
μ
and s
μ
as power series in the
bulk coordinate z (including possible log terms). In the
following sections we will analyze the inequalities that are
derived in this section.
1. Bulk metric
We work with a bulk theory in AdS
dþ1
that consists
of Einstein gravity plus curvature-squared corrections.
For d ≤ 5 this is the complete set of higher curvature
corrections that have an impact on our analysis. The
Lagrangian is
3
L ¼
1
16πG
N
dðd − 1Þ
˜
L
2
þ R þ l
2
λ
1
R
2
þ l
2
λ
2
R
2
μν
þ l
2
λ
GB
L
GB
; ð2:1Þ
where L
GB
¼ R
2
μνρσ
− 4R
2
μν
þ R
2
is the Gauss–Bonnet
Lagrangian, l
2
is the cutoff scale, and
˜
L
2
is the scale
of the cosmological constant. The bulk metric has the
following near boundary expansion in Fefferman-Graham
gauge [21]:
ds
2
¼
L
2
z
2
ðdz
2
þ
¯
g
ij
ðx; zÞdx
i
dx
j
Þ; ð2:2Þ
¯
g
ij
ðx; zÞ¼g
ð0Þ
ij
ðxÞþz
2
g
ð2Þ
ij
ðxÞþz
4
g
ð4Þ
ij
ðxÞþ
þ z
d
log zg
ðd;logÞ
ij
ðxÞþz
d
g
ðdÞ
ij
ðxÞþoðz
d
Þ: ð2:3Þ
Note that the length scale L is different from
˜
L, but the
relationship between them will not be important for us.
Demanding that the above metric solve bulk gravitational
equations of motion gives expressions for all of the g
ðnÞ
ij
for
n<d, including g
ðd;logÞ
ij
ðxÞ, in terms of g
ð0Þ
ij
ðxÞ. This means,
in particular, that these terms are all state-independent. One
finds that g
ðd;logÞ
ij
ðxÞ vanishes unless d is even. We provide
explicit expressions for some of these terms in Appendix C.
The only state-dependent term we have displayed,
g
ðdÞ
ij
ðxÞ, contains information about the expectation value
of the energy-momentum tensor T
ij
of the field theory. In
odd dimensions we have the simple formula [22]
4
g
ðd¼oddÞ
ij
¼
16πG
N
ηdL
d−1
hT
ij
i; ð2:4Þ
with
η ¼ 1 − 2ðdðd þ 1Þλ
1
þ dλ
2
þðd − 2Þðd − 3Þλ
GB
Þ
l
2
L
2
ð2:5Þ
In even dimensions the formula is more complicated. For
d ¼ 4 we discuss the form of the metric in Appendix E.
2. Extremal surface
EWN is a statement about the causal relation between
entanglement wedges. To study this, we need to calculate
the position of the extremal surface. We parametrize our
extremal surface by the coordinate ðy
a
;zÞ, and the position
FIG. 1. Here we show the holographic setup which illustrates
entanglement wedge nesting. A spatial region A
1
on the boundary
is deformed into the spatial region A
2
by the null vector δX
i
. The
extremal surfaces of A
1
and A
2
are connected by a codimension-
one bulk surface M (shaded blue) that is nowhere timelike by
EWN. Then the vectors δ
¯
X
μ
and s
μ
, which lie in M, have non-
negative norm.
3
For simplicity we will not include matter fields explicitly in
the bulk, but their presence should not alter any of our
conclusions.
4
Even though [22] worked with a flat boundary theory, one can
check that this formula remains unc hanged when the boundary is
curved.
QUANTUM NULL ENERGY CONDITION, ENTANGLEMENT WEDGE … PHYS. REV. D 101, 025011 (2020)
025011-3
of the surface is determined by the embedding functions
¯
X
μ
ðy
a
;zÞ. The intrinsic metric of the extremal surface is
denoted by
¯
h
αβ
, where α ¼ða; zÞ. For convenience we will
impose the gauge conditions
¯
X
z
¼ z and
¯
h
az
¼ 0.
The functions
¯
Xðy
a
;zÞ are determined by extremizing
the generalized entropy [15,16] of the entanglement
wedge. This generalized entropy consists of geometric
terms integrated over the surface as well as bulk entropy
terms. We defer a discussion of the bulk entropy terms to
Sec. IVA and write only the geometric terms, which are
determined by the bulk action:
S
gen
¼
1
4G
N
Z
ffiffiffi
¯
h
p
1þ2λ
1
l
2
Rþλ
2
l
2
R
μν
N
μν
−
1
2
K
μ
K
μ
þ2λ
GB
l
2
¯
r
: ð2:6Þ
We discuss this entropy functional in more detail in
Appendix C2. The Euler-Lagrange equations for S
gen
are the equations of motion for
¯
X
μ
. Like the bulk metric,
the extremal surface equations can be solved at small-z with
a Fefferman–Graham-like expansion:
¯
X
i
ðy;zÞ¼X
i
ð0Þ
ðyÞþz
2
X
i
ð2Þ
ðyÞþz
4
X
i
ð4Þ
ðyÞþ
þz
d
logzX
i
ðd;logÞ
ðyÞþz
d
X
i
ðdÞ
ðyÞþoðz
d
Þ: ð2:7Þ
As with the metric, the coefficient functions X
i
ðnÞ
for n<d,
including the log term, can be solved for in terms of X
i
ð0Þ
and g
ð0Þ
ij
, and again the log term vanishes unless d is even.
The state-dependent term X
i
ðdÞ
contains information about
variations of the CFT entropy, as we explain below.
3. The z-expansion of EWN
By taking the derivative of (2.7) with respect to λ,we
find the z-expansion of δ
¯
X
i
. We will discuss how to take
those derivatives momentarily. But given the z-expansion
of δ
¯
X
i
, we can combine this with the z-expansion of
¯
g
ij
in
(2.3) to get the z-expansion of ðδ
¯
XÞ
2
:
z
2
L
2
ðδ
¯
XÞ
2
¼g
ð0Þ
ij
δX
i
ð0Þ
δX
j
ð0Þ
þz
2
ð2g
ð0Þ
ij
δX
i
ð0Þ
δX
j
ð2Þ
þg
ð2Þ
ij
δX
i
ð0Þ
δX
j
ð0Þ
þX
m
ð2Þ
∂
m
g
ð0Þ
ij
δX
i
ð0Þ
δX
j
ð0Þ
Þþ
ð2:8Þ
EWN implies that ðδ
¯
XÞ
2
≥ 0, and we will spend the next
few sections examining this inequality using the expansion
(2.8). From the general arguments given above, we can get
a stronger inequality by considering the vector s
μ
and its
norm rather than δ
¯
X
μ
. The construction of s
μ
is more
involved, but we would similarly construct an equation for
s
2
at small z. We defer further discussion of s
μ
to Sec. II E.
Now we return to the question of calculating δ
¯
X
i
. Since
all of the X
i
ðnÞ
for n<dare known explicitly from solving
the equation of motion, the λ-derivatives of those terms can
be taken and the results expressed in terms of the boundary
conditions for the extremal surface. The variation of the
state-dependent term, δX
i
ðdÞ
, is also determined by the
boundary conditions in principle, but in a horribly nonlocal
way. However, we will now show that X
i
ðdÞ
(and hence
δX
i
ðdÞ
) can be reexpressed in terms of variations of the CFT
entropy.
4. Variations of the entropy
The CFT entropy S
CFT
is equal to the generalized entropy
S
gen
of the entanglement wedge in the bulk. To be precise,
we need to introduce a cutoff at z ¼ ϵ and use holographic
renormalization to properly define the entropy. Then we
can use the calculus of variations to determine variations of
the entropy with respect to the boundary conditions at
z ¼ ϵ. There will be terms which diverge as ϵ → 0, as well
as a finite term, which is the only one we are interested in at
the moment. In odd dimensions, the finite term is given by a
simple integral over the entangling surface in the CFT:
δS
CFT
j
finite
¼ ηdL
d−1
Z
d
d−2
y
ffiffiffi
h
p
g
ij
X
i
ðdÞ
δX
j
: ð2:9Þ
This finite part of S
CFT
is the renormalized entropy, S
ren
,in
holographic renormalization. Eventually we will want to
assure ourselves that our results are scheme-independent.
This question was studied in [23], and we will discuss it
further in Sec. III C. For now, the important take-away from
(2.9) is
1
ffiffiffi
h
p
δS
ren
δX
i
ðyÞ
¼ −
ηdL
d−1
4G
N
X
i
ðd;oddÞ
: ð2:10Þ
The case of even d is more complicated, and we will cover
the d ¼ 4 case in Appendix E.
C. State-independent inequalities
The basic EWN inequality is ðδ
¯
XÞ
2
≥ 0. The challenge is
to write this in terms of boundary quantities. In this section
we will look at the state-independent terms in the expansion
of (2.8). The boundary conditions at z ¼ 0 are given by the
CFT entangling surface and background geometry, which
we denote by X
i
and g
ij
without a (0) subscript. The
variation vector of the entangling surface is the null vector
k
i
¼ δX
i
. We can use the formulas of Appendix D to
express the other X
i
ðnÞ
for n<din terms of X
i
and g
ij
. This
allows us to express the state-independent parts of ðδ
¯
XÞ
2
≥
0 in terms of CFT data. In this subsection we will look at
the leading and subleading state-independent parts. These
will be sufficient to fully cover the cases d ≤ 5.
CHRIS AKERS et al. PHYS. REV. D 101, 025011 (2020)
025011-4
1. Leading inequality
From (2.8), we see that the first term is actually k
i
k
i
¼ 0.
The next term is the one we call the leading term, which is
L
−2
ðδ
¯
XÞ
2
j
z
0
¼2k
i
δX
i
ð2Þ
þg
ð2Þ
ij
k
i
k
j
þX
m
ð2Þ
∂
m
g
ij
k
i
k
j
: ð2:11Þ
From (C10), we easily see that this is equivalent to
L
−2
ðδ
¯
X
i
Þ
2
j
z
0
¼
1
ðd − 2Þ
2
θ
2
ðkÞ
þ
1
d − 2
σ
2
ðkÞ
; ð2:12Þ
where σ
ðkÞ
ab
and θ
ðkÞ
are the shear and expansion of the null
congruence generated by k
i
, and are given by the trace and
trace-free parts of k
i
K
i
ab
, with K
i
ab
the extrinsic curvature of
the entangling surface. This leading inequality is always
non-negative, as required by EWN. Since we are in the
small-z limit, the subleading inequality is only relevant
when this leading inequality is saturated. So in our analysis
below we will focus on the θ
ðkÞ
¼ σ
ðkÞ
ab
¼ 0 case, which can
always be achieved by choosing the entangling surface
appropriately. Note that in d ¼ 3 this is the only state-
independent term in ðδ
¯
XÞ
2
, and furthermore we always
have σ
ðkÞ
ab
¼ 0 in d ¼ 3.
2. Subleading inequality
The subleading term in ðδ
¯
XÞ
2
is order z
2
in d ≥ 5, and
order z
2
log z in d ¼ 4. These two cases are similar, but it
will be easiest to focus first on d ≥ 5 and then explain what
changes in d ¼ 4. The terms we are looking for are
L
−2
ðδ
¯
XÞ
2
j
z
2
¼2k
i
δX
i
ð4Þ
þ2g
ð2Þ
ij
k
i
δX
j
ð2Þ
þg
ij
δX
i
ð2Þ
δX
j
ð2Þ
þg
ð4Þ
ij
k
i
k
j
þX
m
ð4Þ
∂
m
g
ij
k
i
k
j
þ2X
m
ð2Þ
∂
m
g
ij
k
i
δX
j
ð2Þ
þX
m
ð2Þ
∂
m
g
ð2Þ
ij
k
i
k
j
þ
1
2
X
m
ð2Þ
X
n
ð2Þ
∂
m
∂
n
g
ij
k
i
k
j
:
ð2:13Þ
This inequality is significantly more complicated than the
previous one. The details of its evaluation are left to
Appendix D. The result, assuming θ
ðkÞ
¼ σ
ðkÞ
ab
¼ 0,is
L
−2
ðδ
¯
XÞ
2
j
z
2 ¼
1
4ðd − 2Þ
2
ðD
a
θ
ðkÞ
þ 2R
ka
Þ
2
þ
1
ðd − 2Þ
2
ðd − 4Þ
ðD
a
θ
ðkÞ
þ R
ka
Þ
2
þ
1
2ðd − 2Þðd − 4Þ
ðD
a
σ
ðkÞ
bc
Þ
2
þ
κ
d − 4
ðC
kabc
C
abc
k
− 2C
c
kca
C
ba
kb
Þ: ð2:14Þ
where κ is proportional to λ
GB
l
2
=L
2
and is defined in
Appendix D. Aside from the Gauss–Bonnet term we have a
sum of squares, which is good because EWN requires this
to be positive when θ
ðkÞ
and σ
ðkÞ
vanish. Since κ ≪ 1,it
cannot possibly interfere with positivity unless the other
terms were zero. This would require D
a
θ
ðkÞ
¼ D
a
σ
ðkÞ
bc
¼
R
ka
¼ 0 in addition to our other conditions. But, following
the arguments of [24], this cannot happen unless the
components C
kabc
of the Weyl tensor also vanish at the
point in question. Thus EWN is always satisfied. Also
note, the last two terms in middle line of (2.14) are each
conformally invariant when θ
ðkÞ
¼ σ
ðkÞ
ab
¼ 0, which we have
assumed. This will become important later.
Finally, though we have assumed d ≥ 5 to arrive
at this result, we can use it to derive the expression
for L
−2
ðδ
¯
XÞ
2
j
z
2
log z
in d ¼ 4. The rule, explained in
Appendix E, is to multiply the right-hand side (RHS) by
4 − d and then set d ¼ 4. This has the effect of killing the
conformally noninvariant term, leaving us with
L
−2
ðδ
¯
XÞ
2
j
z
2
log z;d¼4
¼ −
1
4
ðD
a
θ
ðkÞ
þ R
ka
Þ
2
−
1
4
ðD
a
σ
ðkÞ
bc
Þ
2
:
ð2:15Þ
The Gauss–Bonnet term also disappears because of a special
Weyl tensor identity in d ¼ 4 [23]. The overall minus sign is
required since log z<0 in the small z limit. In addition, we
no longer require that R
ka
and D
a
θ
ðkÞ
vanish individually to
saturate the inequality: only their sum has to vanish. This still
requires that C
kabc
¼ 0, though.
D. The quantum null energy condition
The previous section dealt with the two leading state-
independent inequalities that EWN implies. Here we deal
with the leading state-dependent inequality, which turns out
to be the QNEC.
At all orders lower than z
d−2
, ðδ
¯
XÞ
2
is purely geometric.
At order z
d−2
, however, the CFT energy-momentum tensor
enters via the Fefferman–Graham expansion of the metric,
and variations of the entropy enter through X
i
ðdÞ
. In odd
dimensions the analysis is simple and we will present it
here, while in general even dimensions it is quite compli-
cated. Since our state-independent analysis is incomplete
for d>5 anyway, we will be content with analyzing only
d ¼ 4 for the even case. The d ¼ 4 calculation is presented
in Appendix E. Though is it more involved that the odd-
dimensional case, the final result is the same.
Consider first the case where d is odd. Then we have
L
−2
ðδ
¯
XÞ
2
j
z
d−2
¼ g
ðdÞ
ij
k
i
k
j
þ 2k
i
δX
i
ðdÞ
þ X
m
ðdÞ
∂
m
g
ij
k
i
k
j
¼ g
ðdÞ
ij
k
i
k
j
þ 2δðk
i
δX
i
ðdÞ
Þ: ð2:16Þ
From (2.4) and (2.10), we find that
QUANTUM NULL ENERGY CONDITION, ENTANGLEMENT WEDGE … PHYS. REV. D 101, 025011 (2020)
025011-5
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