
As a measure of the difference between |xi and |yi, it is convenient to use the normalized
value:
e
f(t) :=
hy|xi
p
hx|xihy|yi
(2.2)
Previous studies [2, 3, 7, 18] of such correlation function focused on the early time β <
t < t
scr
, i.e., between the dissipation time and the scrambling time, and found interesting
“Lyapunov behavior”
4
for those systems that can be holographically described by Einstein
gravity. However, for a generic system that does not have a large separation between
dissipation time t
d
∼ β and scrambling time t
scr
, the “Lyapunov behavior” is not well-
defined. Instead, one can focus on the later time regime t t
scr
, which characterized the
residue part of the system that “survived” under the butterfly effect.
For later convenience, we denote the inner product hx|xi = hV
†
W
†
(t)W (t)V i
β
by
g(t). In the regime t β, such a four point function generically factorizes to g(t) ∼
hV
†
V i
β
hW
†
W i
β
, which represents a normalization for operators W and V . The same
applies to hy|yi. In the following section, we will study the late time behavior of
e
f(t) in
the context of RCFTs.
2.2 An overview of existing results
As a starting point of our discussion, we briefly review the calculation of OTOC in (1+1)-d
CFT in ref. [18].
To setup the CFT computation, it is essential to use complexified time t
c
= t − iτ,
where t stands for the Minkowski time, and τ for the Euclidean time. The strategy is
to begin with a pure Euclidean computation with t = 0 and then analytically continue
it to the desired Minkowski time t. Such strategy enables us to manipulate the order
of operators by tuning the auxiliary small imaginary part τ = . After the conformal
mapping z = exp (2πw/β) , z = exp (2πw/β), with w = x − t
c
and w = x + t
c
,
5
the
thermal expectation value is mapped to the vacuum expectation value. In the end, it is
essential to consider the vacuum expectation value of the four point function which has a
general decomposition in terms of conformal blocks [18, 21, 22]
hW
†
(z
1
, z
1
)W (z
2
, z
2
)V
†
(z
3
, z
3
)V (z
4
, z
4
)i =
1
z
2h
w
12
z
2h
v
34
1
z
2h
w
12
z
2h
v
34
X
p,p
g
p,p
F
p
(η) F
p
(η) (2.3)
where z
ij
= z
i
−z
j
, and η =
z
12
z
34
z
13
z
24
, η =
z
12
z
34
z
13
z
24
are cross ratios, g
p,p
is the pairing coefficient
for holomorphic block p and anti-holomorphic block p. It is important to note that the
summation is only over conformal families labeled by {p, p} rather than all the primaries
and descendants individually. For the parameter regime t β that we are interested in,
4
Here Lyapunov behavior refers to the functional dependence of f (t) on t at early time:
f(t) ∼ 1 −
1
N
e
λ
L
t
+ . . .
where N is some big number in the model, and the exponent λ
L
is recognized as Lyapunov exponent.
Physically, the Lyapunov behavior characterizes how fast the chaos develops in the system.
5
We use a different sign convention comparing to ref. [18] for later convenience.
– 4 –