Chiral entanglement in massive quantum field theories in 1+1 dimensions

Abstract

A bstract
We determine both analytically and numerically the entanglement between chiral degrees of freedom in the ground state of massive perturbations of 1+1 dimensional conformal field theories quantised on a cylinder. Analytic predictions are obtained from a variational Ansatz for the ground state in terms of smeared conformal boundary states recently proposed by J. Cardy, which is validated by numerical results from the Truncated Conformal Space Approach. We also extend the scope of the Ansatz by resolving ground state degeneracies exploiting the operator product expansion. The chiral entanglement entropy is computed both analytically and numerically as a function of the volume. The excellent agreement between the analytic and numerical results provides further validation for Cardy’s Ansatz. The chiral entanglement entropy contains a universal O(1) term γ for which an exact analytic result is obtained, and which can distinguish energetically degenerate ground states of gapped systems in 1+1 dimensions.

Full text

JHEP01(2019)177
Published for SISSA by Springer
Received:November 21, 2018
Accepted:January 20, 2019
Published:January 23, 2019
Chiral entanglement in massive quantum field theories
in 1+1 dimensions
M. Lencs´es,aJ. Vitia,b and G. Tak´acsc
aInternational Institute of Physics, UFRN,
Campos Universit´ario, Lagoa Nova 59078-970 Natal, Brazil
bEscola de Ciˆencia e Tecnologia, UFRN,
Campos Universit´ario, Lagoa Nova 59078-970 Natal, Brazil
cBME “Momentum” Statistical Physics Research Group,
Department of Theoretical Physics, Budapest University of Technology and Economics,
1111 Budapest, Budafoki ´ut 8, Hungary
E-mail: mate.lencses@gmail.com,viti.jacopo@gmail.com,
takacsg@eik.bme.hu
Abstract: We determine both analytically and numerically the entanglement between
chiral degrees of freedom in the ground state of massive perturbations of 1+1 dimensional
conformal field theories quantised on a cylinder. Analytic predictions are obtained from
a variational Ansatz for the ground state in terms of smeared conformal boundary states
recently proposed by J. Cardy, which is validated by numerical results from the Truncated
Conformal Space Approach. We also extend the scope of the Ansatz by resolving ground
state degeneracies exploiting the operator product expansion. The chiral entanglement
entropy is computed both analytically and numerically as a function of the volume. The
excellent agreement between the analytic and numerical results provides further validation
for Cardy’s Ansatz. The chiral entanglement entropy contains a universal O(1) term γ
for which an exact analytic result is obtained, and which can distinguish energetically
degenerate ground states of gapped systems in 1+1 dimensions.
Keywords: Conformal Field Theory, Field Theories in Lower Dimensions, Integrable
Field Theories
ArXiv ePrint: 1811.06500
Open Access,c
The Authors.
Article funded by SCOAP3.https://doi.org/10.1007/JHEP01(2019)177

JHEP01(2019)177
Contents
1 Introduction 1
2 Variational Ansatz for the ground state of the perturbed CFT 5
2.1 The Ansatz for the non-degenerate case 5
2.2 The degenerate case 8
3 Chiral entanglement spectrum and entropy 9
4 Examples 11
4.1 Ising field theory 12
4.2 Tricritical Ising field theory 13
5 Numerical results from TCSA 16
5.1 Verifying the Ansatz: energy densities and overlaps 16
5.2 Chiral entanglement entropy 18
5.2.1 Ising field theory 18
5.2.2 Tricritical Ising field theory 22
5.2.3 Detailed numerical comparison 23
6 Conclusions 24
A CFT and TCSA on a cylinder 25
B Cut-off extrapolation and finite volume deviations from the Ansatz 28
1 Introduction
Entanglement entropy plays a central role both in Quantum Field Theory (QFT) and con-
densed matter theory [1–4]. Universal behaviour of entanglement can be used to deduce
information about the conformal field theory underlying quantum phase transitions [5–8],
while studying the entanglement spectrum leads to deeper insights of topological proper-
ties [9,10] of quantum Hall states [11]. In non-equilibrium situations entanglement and en-
tropy production are deeply connected [12–14], and are also diagnostic of non-perturbative
effects such as confinement [15] and thresholds in the quasi-particle spectrum [16,17]. Black
hole entropy was identified with the entanglement of modes across the horizon [18–20], and
entanglement entropy was also connected to gravitational space-time geometry in a holo-
graphic context [21,22].
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JHEP01(2019)177
Most of the above developments concern entanglement between spatially separated
subsystems. In contrast, the present work proposes a characterisation of the Renormalisa-
tion Group (RG) flow in 1+1 dimensional quantum field theories through a quantification
of the entanglement between left and right moving excitations, which are decoupled in the
ultraviolet limit and become entangled during the flow to the infrared.
Non-trivial fixed points of RG [23] trajectories in QFT are characterised by scale
invariance and consequently a gapless spectrum in infinite volume. In relativistic quantum
field theories, scale invariance is generally promoted to conformal symmetry [24,25], which
is extended to an infinite dimensional symmetry in the 1+1 dimensional case [26]. 1+1
dimensional Conformal Field Theories (CFTs) obey holomorphic (chiral) factorisation:
the excitation spectrum consists of left (l) and right (r) movers which transform under a
separate chiral symmetry algebra and do not interact. When quantised on a cylinder of
circumference L, the eigenstates of the CFT Hamiltonian span irreducible representations
of the tensor product of two identical Virasoro algebras, one for each chirality [26]. The
Hilbert space of the CFT is further restricted by physical requirements such as modular
invariance on the torus [27]. In this paper we only consider the simplest case when only
left and right movers coming from the same representation are paired in the tensor product
(the so-called diagonal or more precisely A-type modular invariant theories [28]).
Coupling the CFT to a relevant scalar field φ(x, y) with coupling constant λaccording
to the formal Euclidean action
A=ACFT +λZR
dx ZL
0
dy φ(x, y),(1.1)
results in breaking conformal invariance. Here we consider the case when this leads to a
finite mass gap m, related to the coupling λas
m∝λ
1
2−∆φ(1.2)
where ∆φ<2 is the scaling dimension of the field φat the RG fixed point associated to
the CFT. The action (1.1) describes an RG flow from an ultraviolet (UV) massless fixed
point for mL 1 to a (trivial) infrared (IR) fixed point when mL 1. In general, the
theory is regularised using a high energy (equivalently short distance) cut-off Λ which is
sent to infinity to obtain physical predictions.
Properties of RG flows in 1+1 dimensions have been extensively studied with sophis-
ticated analytical and numerical techniques, inspired by the seminal contributions [29–31],
with most of the analysis focused on the evolution of the QFT spectrum. Real space en-
tanglement was also computed for integrable flows using exact form factors of branch point
twist fields [32–35] and was studied numerically in [36].
However, universal information in the IR limit can also be obtained determining over-
laps between eigenstates of the QFT Hamiltonian associated to (1.1) and states of the UV
conformal basis [37,38]. More explicitly, assume that |ΨΛ(L)iis the ground state of the
theory (1.1) with a finite cut-off, and define the left/right chiral Hilbert spaces
Hl=Hr=M
aVa(1.3)
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JHEP01(2019)177
as a direct sum of inequivalent irreducible representations Vaof the symmetry chiral alge-
bra. Then the Hilbert space of the CFT corresponding to a diagonal modular invariant is
the diagonal subspace
HCFT =M
aVa⊗ Va(1.4)
of Hl⊗Hr. The finite volume ground state of the perturbed conformal field theory can be
fully characterised by the overlaps
vab(L)≡lim
Λ→∞hΨΛ(L)| · (|ail⊗ |bir) (1.5)
where |ailand |birenumerate two orthonormal basis of Hland Hr. Note that the over-
laps (1.5) vanish when the left and right chiral vectors transform under different irreducible
representations of the chiral algebra.
Based on the observation that vab(L) can be considered as a density matrix on the
chiral space of states, the right chiral reduced density matrix ρrcan be constructed tracing
out the left chiral sector with matrix elements
[ρr(L)]bb0=X
a
v∗
ab(L)vab0(L) (1.6)
The spectrum of the right reduced density matrix ρr(or equivalently the left ρl) quantifies
the entanglement of the UV chiral degrees of freedom along the RG flow. We refer to the
corresponding von Neumann entropy
Sχ≡ −Tr[ρrlog(ρr)] = −Tr[ρllog(ρl)] (1.7)
as the chiral entanglement entropy.
The chiral entanglement entropy vanishes at the UV fixed point and remains finite
along the RG flow; i.e. the limit Λ → ∞ in eq. (1.5) always exists. We will demonstrate
that for mL 1 the chiral entanglement entropy grows linearly with Lwith a non-universal
(i.e. mass dependent) slope B
Sχ(L)∼ BL−γ+Oe−L.(1.8)
Moreover, we conjecture that the sub-leading term γ, which is O(1) in the system size, is
universal and provides an independent characterisation of the ground state of the (infinite
volume) massive theory, which is especially useful when it is degenerate. Such a term first
appeared in the analysis of chiral entanglement of regularised conformal boundary states
in [39,40] (see also [41–43]), and was suggested as a possible benchmark for topological
states in two spatial dimensions following [9,10]. These claims were tested numerically
using matrix product states in [44,45].
The idea of [9,10] regarding the universality of the O(1) term γcan be extended to
the ground state of massive perturbations of conformal field theories based on a variational
Ansatz in finite volume proposed recently by J. Cardy [46], which describes the exact
ground state
|Ψ(L)i ≡ lim
Λ→∞ |ΨΛ(L)i(1.9)
in terms of smeared conformal boundary states.
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JHEP01(2019)177
Here we use this Ansatz to determine analytically both the reduced chiral density
matrix and its chiral entanglement entropy. In cases where the variational Ansatz predicts
degenerate ground states, we resolved the degeneracies utilising the conformal OPEs. As
argued in [38] and proved there for free Ising Field Theory, a representation of the ground
state of the massive QFT as a conformal boundary state should be exact in the IR limit
mL → ∞. Based on this assumption, of which we provide numerical tests in the rest of the
paper, Cardy’s variational Ansatz should be exact (apart from a divergent normalisation)
in infinite volume. This implies that the O(1) term of the chiral entanglement entropy
obtained from such an Ansatz in the limit mL → ∞ is exact and indeed a universal feature
of the RG flow. In addition, the Ansatz and the results derived for the chiral entanglement
entropy do not depend on any special properties of the flow such as integrability at all. To
provide numerical support and verification of our claims, we use the Truncated Conformal
Space Approach (TCSA) introduced in [31] (cf. [47] for a recent review). First we validate
the variational Ansatz, using the scaling Ising Field Theory (IFT) and Tricritical Ising
Field Theory (TIFT) as examples, and then analyse the overlaps with the conformal states
and the large volume behaviour of the chiral entanglement entropy.
There are some important differences between the properties of the chiral and real
space entanglements. Strictly speaking, the real space ground state entanglement entropy
of 1+1 dimensional QFTs does not contain any universal term. Indeed, the leading term
is logarithmically divergent [5] in the limit Λ → ∞ and requires a regularisation, e.g.
through introduction of a relative entropy [48–51]. For a spatial interval of length R
embedded into an infinite system, the coefficient of the leading cut-off dependence of real
space entanglement entropy is universal for both mR →0 and mR → ∞, and related to the
central charge of the UV theory [7,32]. This feature has found important applications in
statistical mechanics models such as spin systems [6,52] where the lattice spacing provides
a natural regularisation. However, since the leading cut-off dependence is logarithmic, there
cannot be any universal O(1) term in contrast to γdefined from the chiral entanglement
entropy in eq. (1.8).
The real space entanglement spectrum was also analysed recently in [53]. For a bipar-
tition in two semi-infinite halves and in the limit of a large mass gap, the entanglement
Hamiltonian [54] is the generator of translation around an annulus with certain confor-
mal boundary conditions [55]. Similarly to the entanglement entropy, the entanglement
spectrum also exhibits universal logarithmic short-distance cut-off dependence, but it is
not finite in the limit Λ → ∞. In contrast, the chiral entanglement spectrum is finite for
Λ→ ∞; we return to comment on this issue in section 3.
We finally note that the chiral entanglement entropy can be calculated from TCSA in
a rather simple and straightforward way, in marked contrast to the real space entanglement
entropy [36].
The outline of the paper is as follows. In section 2we review and slightly extend Cardy’s
variational Ansatz for the QFT ground state in finite volume. In section 3the variational
ground state is used to compute the chiral entanglement entropy and in particular the
universal O(1) term characterising the RG flow. In section 4we discuss applications to
Ising field theory and the Tricritical Ising field theory. In section 5all our claims contained
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JHEP01(2019)177
in the previous sections are subjected to numerical tests based on TCSA. Our conclusions
are presented in section 6. In order to keep the main line of the argument focused, technical
details regarding the implementation of numerical computations were relegated to two
appendices.
2 Variational Ansatz for the ground state of the perturbed CFT
The QFT Hamiltonian on the cylinder associated to the Euclidean action in eq. (1.1) is
H=HCFT +λZL
0
dy φ(0, y),(2.1)
where the relevant scalar field φhas conformal dimension ∆φ≡2hφat the UV fixed
point (see also eq. (A.2)). The IR regime is reached for mL 1, which is equivalent to
either increasing the volume Lor the coupling constant λ. The UV fixed point is a CFT
with central charge cand a torus partition function which is assumed to correspond to
a diagonal modular invariant. In finite volume any eigenstate of the QFT Hamiltonian
can be expanded in the eigenstates of HCFT which form the so-called conformal basis
(cf. appendix A).
2.1 The Ansatz for the non-degenerate case
If we now imagine to turn on the relevant perturbation only for negative imaginary times
x < 0, in the limit mL → ∞, the perturbation flows in the far infrared to a conformal
invariant boundary condition for the theory in the upper half [38,40,56]. In the crossed
channel, the conformal boundary condition is represented by a boundary state, while evolu-
tion in imaginary time asymptotically projects any state of the QFT onto the ground state
of the Hamiltonian. Therefore for a large but finite mL the ground state of the massive
QFT is expected to correspond to a deformation of the asymptotic conformal boundary
state by boundary irrelevant operators [40]. The simplest such operator is given by the
stress tensor T00, and Cardy suggested in [46] that it should be possible to approximate
the exact ground state of (2.1) by a suitable linear combination of smeared conformal
boundary states
|{αa}, τ i=X
a
αae−τHCFT |ai.(2.2)
where the |aiare conformal boundary states (defined in detail later) and τis a smearing
parameter proportional to the inverse of the mass gap. The normalisation of the smeared
boundary states is given by
Zaa =ha|e−2τHCFT |ai,(2.3)
which is the conformal partition function on the annulus of figure 1with boundary con-
ditions aon both sides. In general we will denote by Zab the same conformal partition
function with boundary condition aand b.
The coefficients of the linear combination in eq. (2.2) and the smearing parameter are
determined by minimising the energy density in infinite volume
E[{αa}, τ ] = lim
L→∞
1
Lh{αa}, τ |H|{αa}, τi
h{αa}, τ |{αa}, τi.(2.4)
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JHEP01(2019)177
L
2τ
a
b
HCFT
Figure 1. Conformal partition function Zab on an annulus. The system is quantised on a cylinder
of length L, the smearing parameter τhas the dimension of the inverse of the mass gap (correlation
length). The conformal Hamiltonian HCFT generates translation along the annulus, between two
boundary states aand b.
For Lτ, the matrix elements of Hare diagonal on the smeared boundary states up to
exponentially small corrections [46]. In particular it turns out that
E[{αa}, τ] = Paα2
aea(τ)
Paα2
a
,(2.5)
where the functions eaare given by
ea(τ) = πc
24(2τ)2+λ˜
Aa
φπ
4τ∆φ.(2.6)
The coefficients ˜
Aa
φare fixed by the normalisation of the one-point function of the field φ
on the conformal upper half plane [57] with conformal boundary condition aon the real
axis (see also eq. (2.16))
hφ(z)iUHP =
˜
Aa
φ
[2=m(z)]∆φ.(2.7)
Eq. (2.5) is minimized keeping in eq. (2.2) only the boundary state |a∗isuch that ea∗(τ)
has a global minimum at τ=τ∗which is smaller than all the other minima (if any) of the
functions ea(τ) in eq. (2.6). As discussed in [46], in the case of a single relevant perturbation
with the sign of λpositive (negative), the variational Ansatz selects a∗such that ˜
Aa∗
φis
minimum (maximum). If there is a unique solution of the variational equations, the best
approximation of the QFT ground state |Ψvar(L)iis then obtained normalising the unique
smeared boundary state selected by the variational principle, i.e.
|Ψvar(L)i=e−τ∗HCFT
√Za∗a∗|a∗i.(2.8)
We return to the case of degenerate solutions at the end of the this section.
For perturbations with several relevant fields φjwith couplings λjand 0 <∆φj<2
one must find the minimum of
ea(τ) = πc
24(2τ)2+X
j
λj˜
Aa
φjπ
4τ∆φj; (2.9)
however we will not consider this case here.
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JHEP01(2019)177
For an explicit evaluation of the variational functions (2.6), we recall some basic prop-
erties of the conformal boundary states in CFTs with diagonal modular invariant partition
functions. They can be expressed as a linear combination of Ishibashi states
|ai=X
h
cah|Φhii,(2.10)
where the sum runs over all allowed irreducible representations of the Virasoro algebra.
The Ishibashi states |Φhii themselves are in one-to-one correspondence with irreducible
representations and have the form
|Φhii =X
NX
k|h, N, kil⊗ |h, N, kir,(2.11)
where the vectors |h, N, kiform an orthonormal basis of the irreducible Virasoro repre-
sentation of highest weight h, with Ndenoting their descendant level and ka further
index enumerating linearly independent vectors at a fixed level. The character of such a
representation is given by
χh(q) = X
N,k
qh+N−c/24 with q=e−4πτ
L.(2.12)
For a diagonal CFT, the physical conformal boundary states (also called Cardy states)
satisfy further constraints [55] and are in one-to-one correspondence with the scalar primary
fields Φhgenerating the operator content of the theory. Introducing the chiral weight of
the primary field Φhaas ha, the corresponding conformal boundary state is denoted as
|˜
hai. For a Cardy state |˜
haithe coefficients of the linear combination in eq. (2.10) are
cah =Sh
ha
[Sh
0]1/2(2.13)
where Sis the orthogonal symmetric modular Smatrix [58], which describes the linear
transformation of Virasoro characters
χh(q) = X
h0
Sh0
hχh0(˜q) (2.14)
under the modular transformation
q→˜q=e−Lπ
2τ.(2.15)
Then the normalisation coefficient in eq. (2.7) can also be expressed in terms of the modular
Smatrix as [57]
˜
Aa
φ=Shφ
ha
S0
ha S0
0
Shφ
0!1/2
,(2.16)
which can be inserted into the variational functions (2.6) to obtain them explicitly in terms
of the modular Smatrix.
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JHEP01(2019)177
2.2 The degenerate case
So far it was assumed that one of the variational functions (2.6) had a minimum lower
than all the others. Suppose that, on the contrary, the minimisation of eq. (2.6) leads to
two different Cardy states |˜
ha∗iand |˜
hb∗iwith a∗6=b∗such that ea∗(τ) and eb∗(τ) have
the the same minimum at τ=τ∗, which can happen e.g. when the two functions ea∗(τ)
and eb∗(τ) are identical.
To resolve this problem, note that while the functional (2.5) is obtained for L→ ∞,
the degeneracy between candidate smeared conformal boundary states is generally lifted
in finite volume. The way the degeneracy is lifted is constrained by the block diagonal
structure of the QFT Hamiltonian dictated by the OPE of the perturbing field with the
primaries belonging to the CFT. The Hilbert space HQFT spanned by the eigenstates of H
in eq. (2.1) (see also eq. (A.6)) then splits into a direct sum
HQFT =M
νHν
QFT.(2.17)
The vector spaces Hν
QFT are direct sums of tensor products of left/right irreducible Vi-
rasoro representations as in eq. (1.4). They are composed of modules corresponding to
sets of scalar primaries which form a closed set under operator product with the the per-
turbing field φin eq. (1.1). The decomposition (2.17) follows from the explicit expression
of the interaction matrix Bij in eq. (A.7) and reflects residual discrete symmetries of the
perturbed CFT.
In all the cases considered in this paper the index νcan have at most two values.
For the degenerate case we denote them as ±and the Hilbert space H+
QFT (resp. H−
QFT)
is identified to be the sector which contains (resp. does not contain) the highest weight
representation associated to the identity field. Provided there is no level crossing involving
the ground state in finite volume (which is generally the case) the exact QFT ground state
belongs to the subspace H+
QFT, whereas in general the degenerate variational states selected
by minimizing eq. (2.6) do not. Consistency requires that the best approximation of the
ground state must be the smeared boundary state constructed from a linear combination
of the degenerate Cardy states belonging to H+
QFT. It is also possible to construct a
linear combination of the degenerate Cardy states belonging to H−
QFT, which gives the best
approximation of a state with an energy gap that decays exponentially with the volume of
the system.
Finally we note that the linear combinations of smeared Cardy states obtained from
the argument above should be reproduced directly analysing finite size effects in the QFT
Hamiltonian restricted to the degenerate subspace spanned by the variational states. If
|˜
ha∗iand |˜
hb∗iare the degenerate Cardy states, one must examine the 2 ×2 matrix
Mab =1
√ZaaZbb h˜
ha|e−τ∗HCFT He−τ∗HCFT |˜
hbi,(2.18)
with Hgiven in (2.1) and a, b ={a∗, b∗}. The true ground state corresponds to the
eigenvector of (2.18) with the lower eigenvalue in the limit Lτ∗and belongs to H+
QFT,
while the other eigenstate of (2.18) is in H−
QFT. Constructing the above matrix is quite
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JHEP01(2019)177
involved due to the matrix elements of the perturbing field φwhich also require the bulk-
boundary and boundary OPE structure constants calculated in [59,60]. However, one can
consider a simplified version of this computation by neglecting the perturbing field and
replacing Hby HCFT, resulting in the matrix
M0
ab =1
√ZaaZbb h˜
ha|e−τ∗HCFT HCFTe−τ∗HCFT |˜
hbi,(2.19)
which has the leading large volume behaviour
M0
ab =Lπ(c−24hab)
24(2τ∗)2e−Lπhab /2τ∗,(2.20)
where hab is the minimum (chiral) conformal weight of the fields that occur in the OPE
Φha×Φhb. Note that haa = 0 while hab >0 for a6=b, i.e. the off-diagonal elements are
exponentially suppressed with the volume L[46], and therefore the same is true for the
level splitting generated by them.
We checked by explicit computation that in all cases considered in this work (2.20)
gives the same eigenvectors lying in the subspaces H±
QFT as the previous consideration,
with the lower level always lying in the subspace H+
QFT. In fact, this result is easy to
understand: replacing Hby HCFT corresponds to setting λ= 0, but once the appropriate
eigenstates are determined to lie in the subspaces H±
QFT, switching on the perturbation
cannot mix them. In addition, it cannot change their ordering in energy either due to the
absence of level crossings along the RG flow generated by the perturbation.
3 Chiral entanglement spectrum and entropy
The Ansatz (2.2) immediately implies an analytic expression for the chiral reduced den-
sity matrix and the chiral entanglement entropy introduced in section 1. To include the
possibility of degenerate solutions and linear combinations of physical boundary states, we
consider a variational state
|Ψvar(L)i=e−τ∗HCFT
√Nvar X
h
ca∗h|Φhii,(3.1)
which is a linear combination of Ishibashi states as in eq. (2.10), but with a priori arbitrary
coefficients ca∗h(i.e. not necessarily a conformal boundary state). The normalization Nvar is
fixed by hΨvar(L)|Ψvar(L)i= 1 and is a suitable linear combination of Virasoro characters,
cf. eq. (2.12). The overlaps of the variational state with the conformal basis are then
hΨvar(L)|h, N, kil⊗ |h, N 0, k0ir=δNN0δkk0
ca∗h
√Nvar
e−2πτ ∗
L(2h+2N−c/12) ,(3.2)
and we used eq. (A.2) for the conformal Hamiltonian. For a scalar perturbation the actual
ground state of the QFT Hamiltonian (2.1) has zero conformal spin and therefore can be
expanded in the conformal basis as
|Ψ(L)i=X
hX
NX
k,k0
ChN
kk0(L)|h, N, kil⊗ |h, N , k0ir; (3.3)
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JHEP01(2019)177
It is then clear from eq. (3.2) that the variational state can only provide a valid approx-
imation of the QFT ground state if off-diagonal elements of the matrices ChN are small
and diagonal ones satisfy eq. (3.2). This is verified explicitly in section 5using the TCSA.
The chiral reduced density matrix ρrfor the variational state defined in eq. (1.6) is
then diagonal on the orthonormal basis {|h, N, kir}:
ρr(L) = 1
Nvar X
h,N,k |ca∗h|2qh+N−c/24|h, N, kir rhh, N, k|.(3.4)
where q=e−8πτ ∗/L.
The modular property (2.14) of the conformal characters allows an analytic evaluation
of physical quantities in the limit Lτas we now demonstrate for the chiral entanglement
entropy. It is convenient to start with the n-th chiral R´enyi entropy for the variational
state defined as
Sn
χ=1
1−nlog [Trρn
r].(3.5)
Substituting the expression eq. (3.4) for the chiral reduced density matrix, (3.5) is ex-
pressed as
Sn
χ=1
1−nlog 



P
h|ca∗h|2nχh(qn)
P
h|ca∗h|2χh(q)n



.(3.6)
The limit Lτcan be determined applying the modular transformation (2.14), and
retaining only the leading order contribution which consists of only that of the conformal
vacuum state:
Sn
χ
Lτ∗
'n+1
nπcL
48τ∗+1
1−nlog "Ph|ca∗h|2nS0
h
Ph|ca∗h|2S0
hn#.(3.7)
The chiral entanglement entropy of the variational ground state (2.8) is then obtained in
the limit n→1:
Sχ≡lim
n→1Sn
χ
Lτ∗
'πcL
24τ∗−γa∗,(3.8)
which shows an extensive dependence on the volume as anticipated in eq. (1.8). The
O(1) term γa∗in the large volume expansion is independent on the smearing parameter τ∗
and reads
γa∗=PhSh
0|ca∗h|2log |ca∗h|2
PhSh
0|ca∗h|2−log X
h
Sh
0|ca∗h|2!.(3.9)
Since the variational Ansatz (2.8) is supposed to be exact in the limit L→ ∞, we conjecture
that γa∗in eq. (3.9) is a universal number characterising the ground state of a perturbed
CFT in infinite volume. In section 5TCSA is used to verify numerically the validity of
eqs. (3.8), (3.9) which were derived assuming the variational Ansatz.
We close this section with a few comments and remarks. First, analogous expressions
for the density matrix (3.4) and the universal term γa∗in eq. (3.9) contained in this
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JHEP01(2019)177
section already appeared in [39,40] (see also [42]), in the context of topological states of
matter [9,10]. In [39,40], eq. (3.4) was in particular conjectured to explain the conformal
nature of the entanglement spectrum [11] in a 2+ 1 d Quantum Hall fluid. Here we derived
these results in the context of 1 + 1 d massive QFT RG flows, as direct consequences of
the variational Ansatz (2.8). Secondly, it is important that in our context the validity of
these results is numerically testable with TCSA.
Finally, notice that γa∗is obtained in the opposite limit compared to the better-
known Affleck-Ludwig boundary entropy [61]. This observation allows establishing a partial
analogy with recent studies [53] concerning the real space entanglement spectrum in the
ground state of a perturbed CFT. In [53] the entanglement Hamiltonian is the generator of
translation around the annulus (i.e. in the periodic direction) in figure 1with aspect ratio
L
2τ'2π
log(ξ/a),(3.10)
where ξis the inverse of the mass gap and ais an UV cut-off. The boundary conditions
on the annulus are left free on one side and are fixed by a conformal boundary state
on the other, although the boundary states are not explicitly determined in [53]. The
entanglement spectrum in real space displays universal features in the limit ξawhich
corresponds to Lτ, and the O(1) term in the corresponding entanglement entropy is
the Affleck-Ludwig boundary entropy, however due to the logarithmic dependence on the
cut-off this result is not generally universal [7].
In contrast, the chiral entanglement Hamiltonian following from (3.4) is the chiral part
of the generator of translation along the annulus (i.e. in the open direction), with the same
(linear combination of) conformal boundary states on both sides. As a result, the chiral
entanglement spectrum displays universal features in the opposite limit Lτ, while the
O(1) term in the entropy is given by eq. (3.9) and is universal. Note that albeit the two
settings are related by a modular transformation corresponding to crossing between the
open and closed channels, the relevant boundary conditions are generally different.
4 Examples
We now present applications of the formalism we discussed in the previous two sections.
In the examples below the UV fixed point of the RG flow (1.1) is a Virasoro minimal
model [26] with diagonal partition function on the torus. The central charge of the minimal
model Mpis
c= 1 −6
p(p+ 1), p = 3,4,5, . . . (4.1)
while the primary fields Φhr,s have scaling dimensions 2hr,s with
hr,s =[r(p+ 1) −sp]2−1
4p(p+ 1) ,(4.2)
for 1 ≤r≤p−1 and 1 ≤s≤p. Taking into account the symmetry hr,s =hp−r,p+1−s, there
are exactly p(p−1)/2 primary fields. The CFT Mpdescribes the (p−1)th Ising multicritical
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JHEP01(2019)177
point, with discrete symmetry Z2[62], and explicit expressions for the modular Smatrices
can be found for instance in [63]:
χr,s(q) = X
r0,s0
Sr0s0
rs χr0,s0(˜q)
Sr0s0
rs = 2s2
p(p+ 1)(−1)1+sr0+s0rsin πprr0
p+ 1 sin π(p+ 1)ss0
p.(4.3)
4.1 Ising field theory
The simplest example is the perturbed Ising CFT with the fixed point M3. The non-trivial
relevant fields that could be considered as perturbations are
•Z2odd sector: spin field σ= Φ2,2(h2,2= 1/16),
•Z2even sector: energy field ε= Φ2,1(h2,1= 1/2)
The coupling to the spin field is denoted by hand the one to the energy field by t, where
one can always choose h≥0 since h < 0 can be obtained by Z2symmetry σ→ −σ. As
noticed in [46], for h= 0 the Ansatz does not reproduce the logarithmic singularity of
the ground state energy correctly. However this contribution is a universal shift of all the
energy eigenvalues and does not affect the state vectors themselves (cf. also section 5). The
Cardy boundary states are given in terms of Ishibashi states as [55]
|e
0i=1
√2|1ii +1
√2|εii +1
21/4|σii (4.4)
|g
1/2i=1
√2|1ii +1
√2|εii − 1
21/4|σii (4.5)
|g
1/16i=|1ii − |εii,(4.6)
and have a simple interpretation in terms of spins in the lattice model [55]: the first two
correspond to the boundary spins fixed in up/down position, while the third one describes
the free boundary condition. The variational analysis leads to the following results [46]:
•For h > 0 and t= 0 the variational solution is given by |a∗i=|g
1/2iin eq. (2.8).
Applying eq. (3.9), the universal O(1) term for the chiral entanglement entropy is
γg
1/2=−3
4log(2) .(4.7)
Including the thermal perturbation t6= 0 this result is unaffected as long as h/|t|15/8
is sufficiently large [46].
•For t > 0 and h= 0, the ground state is best approximated by |a∗i=|g
1/16i, for
which γg
1/16 = 0. In the infinite volume limit the variational state coincides with the
ground state in the Neveu-Schwarz sector of a free Majorana fermion [38].
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JHEP01(2019)177
•The case t < 0 and h= 0 corresponds to the Z2spontaneously broken phase and
the variational Ansatz is degenerate: the two solutions are given by |a∗i=|g
1/2i
and |b∗i=|e
0i. Their degeneracy is lifted in a finite volume and the corresponding
eigenvectors can be identified following the method discussed in subsection 2.2. The
OPEs of the three conformal modules with the perturbing field are [26]
1×ε=ε ,
ε×ε= 1 ,
σ×ε=σ . (4.8)
Therefore the Hilbert space splits into the Z2-even (Neveu-Schwarz) sector H+
QFT
built upon the primaries 1 and , and the Z2-odd (Ramond) sector H−
QFT built upon
the primary σ.
The finite volume ground state is the unique linear combination of the Cardy states
|e
0iand |g
1/2iwhich belongs to H+
QFT:
|NSi≡|e
0i+|g
1/2i=√2 (|1ii +|εii),(4.9)
for which the O(1) term of the chiral entanglement entropy vanishes:
γNS = 0 .(4.10)
The antisymmetric linear combination
|Ri≡|e
0i−|g
1/2i= 23/4|σii,(4.11)
belongs to H−
QFT and corresponds to the first excited state which has a level spac-
ing from the ground state |NSiwhich decays exponentially with the volume L. The
smeared boundary state obtained form eq. (4.11) has a different universal O(1) con-
stant in the chiral entanglement entropy:
γR= log(√2) .(4.12)
The same conclusions could be reached using the Kramers-Wannier duality [64] which
maps εinto −εand transforms the state |g
1/16iinto eq. (4.9), thereby mapping the
variational solution for t > 0 into the result for t < 0.
Note that the two degenerate ground states in the ferromagnetic phase of the free
Ising field theory are distinguished by a different O(1) term in the chiral entanglement
entropy, in contrast to their real space entanglement.
4.2 Tricritical Ising field theory
The minimal model M4describes the universality class of the tricritical Ising model [65,66]
and has a global Z2symmetry with six primary fields organized under the Z2symmetry
as [62]
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JHEP01(2019)177
•Z2odd sector: σ= Φh2,2(h2,2= 3/80), σ0= Φh2,1(h2,1= 7/16)
•Z2even sector: ε= Φh3,3(h3,3= 1/10), ε0= Φh3,2(h3,2= 3/5), ε00 = Φh3,2(h3,2=
3/2)
The couplings to the Z2odd fields σand σ0are denoted by hand h0, while couplings to
the Z2even fields εand ε0are denoted by tand t0, respectively (note that the field ε00 is
irrelevant). The microscopic interpretation of the different physical boundary states was
given in [67] in the context of the dilute Ising model.
For the sake of brevity we only consider a perturbations by a single field; the general-
ization to more than one perturbing fields is straightforward. The variational analysis leads
to the following results (by convention all couplings not explicitly specified are assumed to
be zero):
•h > 0: the variational solution is obtained by selecting |a∗i=|g
3/2iin eq. (2.8) which
corresponds to a boundary condition with all spins fixed.
•t > 0: the unique solution is |a∗i=|g
7/16i, corresponding to a Z2symmetric ground
state.
•t < 0: analogously to the Ising case there are two degenerate solutions of (2.5)
corresponding to the Cardy states |e
0iand |g
3/2i. The two sectors H±
QFT correspond
again to even/odd primaries. In a finite volume the true ground state is in the even
sector and is given by the smeared boundary state (2.8) obtained from
|tNSi≡|e
0i+|g
3/2i=N|1ii +|ε00ii +√ϕ|ε0ii +√ϕ|εii,(4.13)
where ϕ= (1 + √5)/2 is the golden ratio and Na numerical coefficient. The lowest
energy state in the odd sector is given by
|tRi ≡ |e
0i−|g
3/2i=N0|σ0ii +√ϕ|σii,(4.14)
where N0is another numerical coefficient. The two states are degenerate in the
thermodynamic limit and their splitting decays exponentially with the volume.
Note that the even resp. odd combinations only contain fields in the Neveu-Schwarz
resp. Ramond sectors of the super-Virasoro algebra of the minimal model M4[65],
respectively (indeed this explains our choice of labelling for them). The difference of
the chiral entanglement entropies is given by
γtR −γtNS = log(√2),(4.15)
which coincides with the analogous expression γR−γNS in the Ising case.
•h0>0: again there are two degenerate solutions to the variational equations cor-
responding to the Cardy states |e
0iand |g
3/5iwith associated primaries Φ0= 1 and
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JHEP01(2019)177
Φ3/5=ε0. The way the degeneracy is lifted in a finite volume can be obtained by
examining the relevant OPEs [63]:
1×σ0=σ0
ε00 ×σ0=σ0
σ0×σ0= 1 + ε00
,
σ×σ0=ε+ε0
ε×σ0=σ
ε0×σ0=σ
; (4.16)
from which we can infer that sector H+
QFT contains representations built from the
primaries {1, ε00, σ0}, while H−
QFT consists of {σ, ε, ε0}. The unique linear combination
of degenerate Cardy states belonging to H+
QFT is
|GS+i=|e
0i+ϕ|g
3/5i,(4.17)
while the one giving the lowest energy state in H−
QFT is
|GS−i=−ϕ|e
0i+|g
3/5i,(4.18)
or in terms of Ishibashi states as
|GS+i=N1
√2|1ii +1
√2|ε00ii +1
21/4|σ0ii(4.19)
|GS−i=N01
√2|εii +1
√2|ε0ii +1
21/4|σii.(4.20)
Moreover, using eq. (3.9), the difference between their chiral entanglement entropy
is given by
γGS−−γGS+=−log(ϕ).(4.21)
Comparing to eq. (4.4) one can note that |GS+iand |GS−iare effectively two Ising-
type boundary states; in particular, the fields {1, σ0, ε00 }realize an OPE isomorphic
to that of the Ising model. This observation confirms that the tricritical Ising model
perturbed by σ0describes a phase separation between two magnetized Ising pure
phases that are not distinguished by Z2symmetry alone [68].
From the QFT point of view this conclusion is quite remarkable since it implies a
double degeneracy of the ground state of different nature respect to the most familiar
one observed in Ising field theory, which is potentially interesting to the study of
topological phases of matter [69] (cf. also section 6).
•h0<0: the degenerate Cardy states are |g
3/2iand |g
1/10i. Repeating the previous
analysis gives the same states (4.19) and (4.20) but with a negative coefficient in
front of the Z2-odd terms |σii and |σ0ii. Similarly to the case h0>0, note again the
parallel with the Ising case, but this time the analogous state is given by (4.5).
So far we only gave the difference of the chiral entanglement entropy between degen-
erate ground states where applicable. To complete this information, the values of the O(1)
universal contributions for the true ground states themselves are summarised in table 1.
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JHEP01(2019)177
Perturbation Coupling γfor the true ground state
σ h > 0γg
3/2= 4[s2
1log(s1) + s2
2log(s2)] + 1
4log(2)
ε t > 0γg
7/16 = 4[s2
1log(s1) + s2
2log(s2)] + log(2)
ε t < 0γtNS =s1ϕlog(ϕ)
s2+s1ϕ−log[2(s2+s1ϕ)]
σ0h0>0γGS+=−log(s2)−7
4log(2)
Table 1. O(1) universal contributions to the chiral entanglement entropy for the true ground
state in the perturbed tricritical Ising model, with the notations s1= sin(2π/5)/√5 and s2=
sin(4π/5)/√5 (note that s1/s2=ϕ).
5 Numerical results from TCSA
Now we turn to a numerical computation of the chiral entanglement using the Truncated
Conformal Space Approach (TCSA), which is a variational method originally introduced
to construct the approximate low energy spectrum of field theories in finite volume in [31].
Since then it was applied for various models and its scope extended to extracting a range of
quantities as well as determining phase diagrams and simulating non-equilibrium time evo-
lution; for a recent review we refer the interested reader to [47]. The ingredients necessary
for the present computations are briefly reviewed in appendices Aand B.
The TCSA consists of considering the Hamiltonian (2.1) in finite volume with appropri-
ate boundary conditions, which are chosen to be periodic for the subsequent calculations.
The finite volume spectrum is discrete and introducing an upper energy cut-off reduces
the Hilbert space to a finite-dimensional subspace, in which all states and operators are
represented as finite-dimensional vectors and matrices, respectively. Units are chosen by
expressing the coupling in terms of the mass gap m(i.e. the mass of the lightest excitation),
and the volume is parameterized by the dimensionless combination mL, while energies are
measured in units of m. The determination of the spectrum is reduced to the diagonal-
ization of the finite truncated Hamiltonian matrix, with the components of eigenstates
numerically known in the conformal basis. As a result, the reduced density matrix and
the chiral entanglement entropy can be computed in a straightforward way. The results
obtained from TCSA depend on the cut-off: the more relevant the perturbing operator the
faster the convergence with the cut-off. The convergence can be improved further by elim-
inating the leading cut-off dependence using a renormalization group approach, explained
in appendix Bfor the case of the chiral entanglement entropy.
5.1 Verifying the Ansatz: energy densities and overlaps
Before considering the chiral entanglement entropy, we first test Cardy’s variational Ansatz.
The simplest test is to compare the predicted ground state energy density Epredicted obtained
by substituting the variational solution into (2.5) to exact results coming from integrabil-
ity [70], or from the one determined numerically from TCSA when the perturbation is
non-integrable. The results of this comparison are summarized in table 2. Note that for
the thermal perturbation of the Ising model the energy density predicted by the Ansatz
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JHEP01(2019)177
Model κ mτ ∗Epredicted/m2Etheory/m2
IFT+σ0.06203236 1.9970027 −0.0615441 −0.0617286
IFT+ε±0.159155 0.261799 – –
TIFT+σ∗0.1 2.0844592 −0.135319 −0.135496
TIFT+ε±0.0928344 1.484332 −0.0935744 −0.0942097
TIFT+σ0±0.166252 0.370412 −0.214659 −0.415603
Table 2. Summary of the results from Cardy’s Ansatz. The second column shows the value of κ
(cf. (A.8)), which for integrable models was chosen so that the mass unit mequals the mass gap
of the model. The model labeled by star is non-integrable, therefore the choice of κis arbitrary
resulting in some fixed scale mwhich is not equal to the mass gap, and for this case the theoretical
prediction is replaced by the numerical value obtained from the TCSA ground state by extrapolation
using cut-off levels from 5 to 12.
is not meaningful since there is a logarithmic divergence with the UV cut-off. The results
demonstrate that the Ansatz predicts the energy density for strongly relevant perturba-
tions quite precisely, however its accuracy gets progressively worse when the weight of the
perturbation increases.
One can also compare the overlaps vab,Λ(L) = hΨΛ(L)||ail⊗|bircalculated from TCSA
to the prediction from (3.2) using the ca∗hcoefficients obtained by the variational method.
The results are demonstrated in figures 2,3,4and 5. The exponential fall-off of the
coefficients with the conformal level is clearly visible and agrees with the predictions of
the Ansatz. However, it is also clear that there are some discrepancies since the boundary
states only have diagonal components in an orthonormal basis according to the structure
of Ishibashi states (2.10), which should be exactly the same for components with a given
highest weight hand level N. The TCSA shows that this is violated: there are non-zero
non-diagonal components, and the diagonal ones have slightly different values as well, in
some cases they clearly deviate from the predicted exponential curve. These deviations
indicate that the Ansatz is not perfect and is only strictly valid in the limit L→ ∞, and
for a more precise description in finite volume it is necessary to include more irrelevant
operators around the boundary fixed point as pointed out in [46]. However as we show in
appendix B(cf. figure 11) both the (relative) difference between the diagonal coefficients
and the non-diagonal coefficients converge to zero with increasing volume.
The variational Ansatz (2.4) and its extension in presence of degeneracy are quite poor
for Ising field theory perturbed by the energy operator; see figure 3. This is not surprising
since it was already pointed out in [46] that the Ansatz cannot account for the logarithmic
contribution to the ground state energy either. Exact calcuation performed in [38] shows
that the ratio of overlaps between the true QFT ground state and two states in the CFT
basis can converge to the IR value (obtained for mL → ∞) with a power law behaviour,
rather than exponentially as predicted instead by (3.2). It is worth noticing, however,
that the agreement obtained for the chiral entanglement entropy remains remarkable; see
figure 7.
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JHEP01(2019)177
0 2 4 6 8 10 12 14 16 18 20
−0.4
−0.2
0
0.2
0.4
ECF T
vab,20(mL = 80)
1 TCSA
σTCSA
εTCSA
1 pred.
σpred.
εpred.
0 2 4 6 8 10 12 14 16 18 20
−0.4
−0.2
0
0.2
0.4
ECF T
vab,20(mL = 80)
1 TCSA
σTCSA
εTCSA
1 pred.
σpred.
εpred.
Figure 2. Coefficients of the ground state eigenvector as a function of the conformal energy
(eigenvalue of L0+¯
L0−c/12) in Ising field theory perturbed by the spin field σ(t= 0, h > 0)
at the dimensionless volume mL = 80. Discrete dots are TCSA data at level 20 with 28624 states,
while the lines show the prediction of (3.2) with τ∗given in table 2(with an overall sign difference
which is due to the choice of the numerics). Different colours correspond to different modules: blue–
1, red–σand black–ε; the point lying on (or close to) the horizontal axis correspond to non-diagonal
contributions. Inset: zooming on the conformal energy region ≈14 −15 shows the deviations from
the diagonal form of the Ansatz discussed in the text.
5.2 Chiral entanglement entropy
Now we turn to the predictions for the chiral entanglement entropy, which for large enough
volume can be written in the form (1.8):
Sχ(L)∼ BL−γ+O(e−L).(5.1)
The prediction therefore consists of two quantities: the linear slope B, which according
to (3.8) can be obtained as
B=πc
24τ∗(5.2)
from the UV central charge cand the variational solution for τ∗listed in table 2, and the
O(1) contribution γwhich was obtained in section 4.
5.2.1 Ising field theory
In the case of the Ising field theory we studied numerically three different perturbations of
the fixed point CFT.
•t= 0, h > 0 (magnetic perturbation): as already demonstrated in figure 2the
variational state resulting from Cardy’s Ansatz provides an excellent approximation
to the ground state. The chiral entanglement entropy as a function of the volume
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JHEP01(2019)177
0 2 4 6 8 10 12 14 16 18 20
0
0.2
0.4
0.6
0.8
ECF T
vab,20(mL = 10)
1 TCSA (|NSi)
εTCSA (|NSi)
1 pred. (|NSi)
εpred. (|NSi)
σTCSA (|Ri)
σpred. (|Ri)
Figure 3. Coefficients of the ground state vector as a function of the conformal energy (eigenvalue
of L0+¯
L0−c/12) of the CFT state in the thermal perturbation of the Ising fixed point at mL = 10.
Dots are from TCSA at level 20 with 28624 states, the lines are predicted by using (3.2) the smearing
from Cardy’s Ansatz (see table 2) and ca∗hcorresponding to |NSiand |Ri. It is clear the overlaps
are not in full quantitative agreement. The reason is that the conformal weight of the perturbation
is high: the energy levels are logarithmically divergent, and the cut-off effects are relatively high.
However, it turns out that the cut-off extrapolated chiral entanglement entropy calculated from the
overlaps agrees very well with the theoretical prediction (cf. section 5).
0 2 4 6 8 10 12 14 16 18 20
−0.6
−0.4
−0.2
0
0.2
0.4
0.6
0.8
ECF T
vab,14(mL = 30)
1 TCSA (|tNSi)ε0pred. (|tNSi)
εTCSA (|tNSi)ε00 pred. (|tNSi)
ε0TCSA (|tNSi)σTCSA (|tRi)
ε00 TCSA (|tNSi)σ0TCSA (|tRi)
1 pred. (|tNSi)σpred. (|tRi)
εpred. (|tNSi)σ0pred. (|tRi)
Figure 4. Coefficients of the ground state vector as a function of the conformal energy (eigenvalue
of L0+¯
L0−c/12) of the CFT state in the t < 0 perturbation of the tricritical Ising fixed point at
mL = 30. Dots are from TCSA at level 14 with 22559 states in the NS sector and 18751 in the R
sector. The lines are predicted by using (3.2) the smearing from Cardy’s Ansatz (see table 2) and
ca∗hcorresponding to |tNSiand |tRi.
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JHEP01(2019)177
0 2 4 6 8 10 12 14 16 18 20
−1
−0.8
−0.6
−0.4
−0.2
0
0.2
0.4
0.6
0.8
ECF T
vab,14(mL = 5)
1 TCSA (|GS+i)σTCSA (|GS−i)
σ0TCSA (|GS+i)εTCSA (|GS−i)
ε00 TCSA (|GS+i)ε0TCSA (|GS−i)
1 pred. (|GS+i)σpred. (|GS−i)
σ0pred (|GS+i)εpred. (|GS−i)
ε00 pred. (|GS+i)ε0pred (|GS−i)
Figure 5. Coefficients of the ground state vector as a function of the conformal energy (eigenvalue
of L0+¯
L0−c/12) of the CFT state in the h0>0 perturbation of the tricritical Ising fixed point
at mL = 5. Dots are from TCSA at level 14 with 13373 states in the sector {1, σ0, ε00 }and 27937
in {σ, ε, ε0}. The lines are predicted by using (3.2) the smearing from Cardy’s Ansatz (see table 2)
and ca∗hcorresponding to |GS+iand |GS−i.
0 10 20 30 40 50 60 70 80 90 100
0
1
2
3
4
mL
Sχ
Λ = 15
Λ = 16
Λ = 17
Λ = 18
Λ = 19
Λ = 20
extrapolated
prediction
Figure 6. Chiral entanglement entropy in the magnetic perturbation of the Ising model. TCSA
(circles/dots) against eq. (3.8) with the corresponding value of τ∗(see table 2). TCSA data are
shown at different cut-offs and also after extrapolation.
is showed in figure 6, and it is clear that the predictions are in excellent agreement
with the extrapolated TCSA value.
•t6= 0, h = 0 (thermal perturbation): the positive/negative signs of the coupling
correspond to the paramagnetic/ferromagnetic phases, for which the chiral entangle-
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JHEP01(2019)177
0 1 2 3 4 5 6 7 8 9 10
0
1
2
3
mL
Sχ
Λ = 18
Λ = 19
Λ = 20
extrapolated
prediction
(a) Paramagnetic phase, ground state corresponding to |
g
1/16i.
0 1 2 3 4 5 6 7 8 9 10
0
1
2
3
mL
Sχ
Λ = 18 NS Λ = 18 R
Λ = 19 NS Λ = 19 R
Λ = 20 NS Λ = 20 R
extrap. NS extrap. R
pred. NS pred. R
(b) Ferromagnetic phase, Neveu-Schwarz and Ramond ground states.
Figure 7. Chiral entanglement entropy in the thermal Ising field theory.
Volume cut-off level 20 Extrapolated
6 0.33993 0.34105
7 0.34363 0.34441
8 0.34549 0.34577
9 0.34663 0.34628
10 0.34753 0.34641
Table 3. Difference between the chiral entanglement entropy of the ground state in the Neveu-
Schwarz and the Ramond sector, showing both the bare TCSA value obtained for the highest
available cut-off and the one obtained after cut-off extrapolation. The predicted value is log √2≈
0.346547.
ment entropy as a function of the volume is shown in figure 7. In the paramagnetic
phase (t > 0) the unique ground state |g
1/16iis in the Neveu-Schwarz sector, while
in the ferromagnetic case there are two degenerate ground states in large but finite
volume, namely |NSiin the Neveu-Schwarz and |Riin the Ramond sector. In all
cases the predictions from Cardy’s Ansatz combined with eq. (3.8) are in agreement
– 21 –

JHEP01(2019)177
0 5 10 15 20 25 30 35 40 45 50
0
1
2
3
4
mL
Sχ
TCSA |
g
7/16i
prediction |
g
7/16i
(a) t > 0
0 5 10 15 20 25 30 35 40 45 50
0
1
2
3
4
mL
Sχ
TCSA |tNSi
prediction
TCSA |tRi
prediction
(b) t < 0
Figure 8. Chiral entanglement entropy in TIFT+ε. TCSA data are extrapolated to infite cut-off.
with the extrapolated TCSA data. In table 3we present the difference between the
chiral entanglement in the two ground states in different volumes, which shows that
it converges to the predicted value with increasing volume.
5.2.2 Tricritical Ising field theory
•σperturbation: Cardy’s Ansatz predicts |a∗i=|g
3/2iwith the τ∗= 2.0844592 (cf.
table 2). The predicted chiral entanglement is compared to the TCSA reuslt in
figure 9which shows an excellent agreement. The universal constants γand the
slope Bobtained from a linear fit of the TCSA data agrees with the theoretical
prediction up to four and three digits respectively as shown in table 4.
•εperturbation: for t > 0 there is a unique ground state corresponding to |a∗i=
|g
7/16i, while for t < 0 the two degenerate ground states are |tNSiand |tRigiven
in (4.13) and (4.14). The theoretical prediction and the TCSA results for the chiral
entanglement entropy are shown in figure 8, and the extrapolated data are in good
agreement with the predictions for each case.
– 22 –

JHEP01(2019)177
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30
0
0.5
1
1.5
2
mL
Sχ
TCSA
prediction
Figure 9. Chiral entanglement entropy in TIFT+σ. TCSA (dots) against eq. (3.8) with the
corresponding value of τ∗(see table 2). TCSA data are extrapolated to infinite cut-off.
0 1 2 3 4 5 6 7 8 9 10
0
1
2
3
mL
Sχ
TCSA |GS+i
prediction |GS+i
TCSA |GS−i
prediction |GS−i
Figure 10. Chiral entanglement entropy in TIFT+σ0. TCSA data are extrapolated to infinite
cutoff.
•σ0perturbation: for h0>0 there are two ground states |GS+i(4.19) and |GS−i(4.20)
which are degenerate in infinite volume. The theoretical prediction and the TCSA
results for the chiral entanglement entropy are compared in figure 10. For h0<
0 there are again two ground states, but their expressions differ from the h0>0
result (4.19), (4.20) by flipping sign of the |σ0ii/|σii terms, respectively, as discussed
in section 4. This sign change has no effect neither on the slope Bnor the intercept γ,
the prediction is the same for both signs, while the overlaps obtained from TCSA are
also the same up to a sign flip that has no effect on the chiral entanglement entropy.
5.2.3 Detailed numerical comparison
Beyond the graphical comparison presented so far, we also extracted the values of Band γ
by fitting the volume dependence of the chiral entanglement entropy obtained from TCSA
with a linear function. For any quantity determined from TCSA there is a so-called scaling
regime where finite size effects (which decrease with the volume) are of the same order of
– 23 –

JHEP01(2019)177
Perturbed CFT with γtheory γTCSA Btheory BTCSA TCSA fit
ground state range in mL
IFT+σ,h > 0−0.51986 −0.5168(20) 0.032774 0.032653(49) 20 −60
IFT+ε,t > 0 0 0.0031(29) 0.25 0.24756(44) 5.5−8.5
IFT+ε,t < 0, |NSi0 0.0031(29) 0.25 0.24756(44) 5.5−8.5
IFT+ε,t < 0, |Ri0.346574 0.32196(35) 0.25 0.244009(48) 6 −8.5
TIFT+σ,h > 0−0.814618 −0.81476(22) 0.043959 0.043720(10) 18 −28
TIFT+ε,t > 0−0.294757 −0.3000(12) 0.061731 0.061546(35) 30 −40
TIFT+ε,t < 0, |tNSi −0.294757 −0.3000(12) 0.061731 0.061545(35) 30 −40
TIFT+ε,t < 0, |tRi0.051816 0.0577(14) 0.061731 0.06154(36) 32 −48
TIFT+σ0,h0>0, |GS+i0.123105 0.101(11) 0.247372 0.2491(30) 2 −5
TIFT+σ0,h0>0, |GS−i −0.358107 −0.3304(15) 0.247372 0.24330(43) 2 −5
Table 4. Summary of numerical results on the chiral entanglement entropies in different models.
Theoretical predictions for universal constants and slopes are calculated with the corresponding
τ∗and central charge. Results from TCSA are results of linear fit the extrapolated data in the
indicated volume region. The parentheses show the uncertainty of the last digits resulting from the
fits. Note that this does not account for the total error budget, since it does not include finite size
effects, and also residual truncation effects remaining after extrapolation.
magnitude as truncation effects (which increase with the volume). For the entanglement
entropy, since the predicted behaviour in large volume is linear, the scaling regime can be
found by examining the numerically computed second derivative and choosing an interval
in mL where it is sufficiently small. This cannot be done completely automatically, as
sometimes a blind search would find the minimum of the second derivative in a range that
is clearly at too small volume still dominated by finite size effects, and can be aided by the
graphical comparison between prediction and TCSA data discussed above. The detailed
numerical matching of the predicted to the results obtained from TCSA is presented in
table 4.
6 Conclusions
In this paper we analysed entanglement production among UV chiral degrees of freedom
along a massive renormalisation group flow in 1+1 dimensions for theories with Z2invariant
fixed points, belonging to the A-series of Minimal Models and perturbed by a relevant scalar
field. We studied the reduced density matrix, obtained from the finite volume ground state
projector after tracing out one chiral sector of the Hilbert space, and showed that contrary
to real space entanglement, chiral entanglement is finite when the short-distance cut-off
is sent to zero. Moreover, in the large volume limit, the chiral entanglement entropy
grows linearly with the system size and contains a subleading constant term γ, which
was argued to be a universal property of the RG flow and can characterise uniquely the
QFT ground state also in presence of degeneracies. We considered examples in Ising Field
Theory and Tricritical Ising Field Theory. In the latter theory, we also pointed out the
existence of a two-fold ground state degeneracy in presence of a Z2odd perturbation of
– 24 –

JHEP01(2019)177
weight 7/8. Interestingly, in such a case, the QFT Hilbert space splits into the same two
sectors discussed in [69] when describing lattice realisations of Fibonacci anyons.
For an analytic determination of chiral entanglement we employed a recently proposed
variational Ansatz [46] for the QFT ground state in terms of smeared conformal boundary
states. We tested the variational Ansatz extensively with TCSA, and our detailed numerical
analysis of the energy and the overlaps suggests that the variational Ansatz for the QFT
ground state is exact in the infinite volume limit. We then computed both the volume
density and the constant term of the chiral entanglement from the Ansatz and demonstrated
that it matches excellently the TCSA data, which also provides a further validation of
Cardy’s Ansatz.
There are several interesting directions for future investigation. Among them, we
could mention: the extension of the present investigation to non-diagonal theories, such
as e.g. the Z3invariant Potts Field Theory [30] or to massive deformations of free bosonic
theories, such as e.g. the Sine-Gordon model. The behaviour of the chiral entanglement
and its universal features along a massless RG flow [71] is an important challenge as well;
in this case a variational Ansatz for the ground state wave function is currently missing.
It would also be interesting to extract the coefficient γin eq. (1.8) (cf. also (3.9))
directly from lattice calculations, either analytically or numerically. This requires the
construction of the conformal basis [72] on the lattice, and also the determination of the
ground state overlaps. Both are challenging tasks; nevertheless, some progress has been
made recently in [73]. We hope our results could trigger more activity in this direction.
There is also a number of potential applications to quantum quenches in field theory
since Cardy’s Ansatz determines the ground state of a massive quantum field theory, and so
it can be used as a starting point to study the time evolution. There has been considerable
progress in both numerical [74–78] and analytical approaches [79–86]. However, finding the
overlaps of the initial state with the post-quench energy levels and characterising the time
evolution especially for long times is still largely an open problem. Chiral entanglement
also looks interesting as a potential tool to analyse and understand the temporal dynamics.
Acknowledgments
We are grateful to F. Essler for a useful discussion, and also to J. Dubail and H. Katsura for
discussions and for drawing our attention to the papers [40] and [39]. M.L. was supported
by the Brazilian Ministry of Education. G.T. was supported by the National Research
Development and Innovation Office (NKFIH) under a K-2016 grant no. 119204 by the
Quantum Technology National Excellence Program (Project No. 2017-1.2.1-NKP-2017-
00001), and also by the BME-Nanotechnology FIKP grant of EMMI (BME FIKP-NAT).
A CFT and TCSA on a cylinder
We briefly recall some relevant notions regarding diagonal CFTs on a cylinder to set up
our conventions. The Euclidean space-time cylinder of circumference Lis parameterised
by the Euclidean time xand the spatial coordinate ywith the identification y≡y+L,
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JHEP01(2019)177
which can be mapped onto the conformal plane by the exponential function
z= exp 2π
L(x+iy),¯z= exp 2π
L(x−iy).(A.1)
The conformal Hamiltonian on the cylinder can be writen in terms of the left/right moving
(l/r) Virasoro generators on the plane as
HCFT =2π
LLl
0+Lr
0−c
12,(A.2)
and the eigenvalues of Ll
0+Lr
0are called the scaling dimensions. For a diagonal CFT the
states in the conformal basis are of the form
|h, N, kil⊗ |h, N 0, k0ir,(A.3)
where his the highest weight of the left and right component, Nand N0are the left/right
descendant levels and the integers kand k0label an orthonormal basis at the given level of
left/right moving degrees of freedom which satisfy
Ll,r
0|h, N, kil,r = (h+N)|h, N, kil,r .(A.4)
The conserved (spatial) momentum operator on the cylinder Pis given by
P=2π
LLl
0−Lr
0.(A.5)
where the eigenvalues of Ll
0−Lr
0give the conformal spin. All states in the Hilbert space
have integer conformal spin given by the difference N−N0of the left and right descendant
levels. For the minimal models considered in the main text, the set of allowed values
for hand consequently for the conformal dimension at the UV fixed point is finite. The
perturbing operator is one of the spin-0 fields in the CFT spectrum, which is supposed to
be relevant and therefore it is a primary field.
Using the exponential mapping, the matrix elements of the Hamiltonian can be writ-
ten as
Hij =δsi,sj
2π
L2hi+Nl
i+Nr
i−c
12 +λL2−2h
(2π)1−2hBij,(A.6)
where si, sjare the conformal spin of the states, his the conformal weight of the perturbing
field φ(satisfying h < 1, with the scaling dimension of φgiven by ∆ = 2h), and
Bij =hi|φ(1,1)|jipl (A.7)
is the matrix element on the plane which can be evaluated in terms of the known structure
constants of the CFT [87] using the conformal Ward identities.
The coupling constant is a dimensionful quantity and can be represented in terms of
a mass scale mas
λ=κm2−2h(A.8)
– 26 –

JHEP01(2019)177
Ising Tricritical Ising
Λ 1 σ ε Λ 1 σ ε σ0ε0ε00
15 1037 2167 1272 9 156 877 449 428 606 389
16 1566 3191 1897 10 300 1553 810 752 1090 678
17 2242 4635 2738 11 496 2709 1386 1281 1819 1119
18 3331 6751 3963 12 896 4645 2410 2181 3115 1903
19 4700 9667 5644 13 1425 7781 4010 3625 5140 3059
20 6816 13763 8045 14 2449 12822 6611 5929 8504 4995
Table 5. Dimensions of the truncated modules at different cut-offs in the critical and in the
tricritical Ising field theory.
where κis a dimensionless number. The standard choice for mis provided by the massgap
i.e. the mass of the lowest lying particle, and for integrable models the values of κresulting
in this particular choice of units are exactly known; for the models used in this paper they
can be found in [70] and are listed in table 2. For the non-integrable perturbation of the
tricritical Ising model with the leading magnetization operator σ, we used κ= 0.1 to define
a mass scale mwhich for this particular case is just an arbitrary choice of units.
Substituting (A.8) into (A.6) shows that the finite volume spectrum can be considered
as a function of the dimensionless variable mL when the energies are measured in units
of m, and therefore a CFT perturbed by a single relevant field has no free parameters
apart from the choice of the perturbing field, and the overall sign of the coupling when the
perturbing field is even.
Due to translational invariance the Hilbert space can be split into different momentum
sectors. We restrict our attention to the zero-momentum sector since we are interested in
the ground state. To set up the TCSA we truncate the Hilbert space by introducing a cut-
off Λ in the descendant level Nand keep only states with N≤Λ, resulting in a restriction
of the Hamiltonian to a finite dimensional matrix. The dimensions of the truncated zero
momentum sectors in the Ising and tricritical Ising models are given in table 5.
States from the TCSA at cut-off Λ and in volume Lcan be written in the following way
|ΨΛ(L)i=X
a,b
vab,Λ(L)|ail⊗ |bir(A.9)
Since the ground state is in the zero momentum sector vab,Λ(L) = δha,hbδNa,Nbωab, therefore
the density matrix is block-diagonal formed by blocks from the same module and descendant
level. The partial trace necessary to obtain the reduced chiral density matrix can be carried
out separately in each block, and the chiral entanglement spectrum can be obtained by
diagonalizing the reduced blocks separately. From the entanglement spectrum one can
calculate the chiral R´enyi entropies Sn
χ,Λ(L) and the chiral entangement entropy Sχ,Λ(L)
at a given truncation level Λ as a function of the volume L.
– 27 –

JHEP01(2019)177
B Cut-off extrapolation and finite volume deviations from the Ansatz
In this appendix we describe the cut-off extrapolation procedure using the TCSA renor-
malization group approach developed in [88–93]. The full Hilbert space can be split as
H=Hl⊕ Hhwhere Hland Hhthe low and high energy subspaces respectively given in
terms. The full Hamiltonian can be written in a block diagonal form as
H= Hll Hhl
Hlh Hhh !.(B.1)
In the conformal basis the off-diagonal parts only contain the perturbing field and there
dependence of the coupling can be written as
Hhl =λVhl , Hlh =λVlh .(B.2)
Diagonalizing the truncated Hamiltonian Hll results in energy levels and eigenstates
{EΛ,|ΨΛi}. Since the perturbation is a relevant operator, the effect of the high-energy
subspace can be taken into account perturbatively, which results in
E∞=EΛ−λ2X
n∈Hh
hΨΛ|Vlh|nihn|Vhl |ΨΛi
En−EΛ
+O(λ3)
|Ψ∞i=EΛ+λ2X
n,m∈Hh
|mihm|Vhh|nihn|Vhl |ΨΛi
(En−EΛ)(Em−EΛ)
−λ2
2|ΨΛiX
n∈Hh
hΨΛ|Vlh|nihn|Vhl |ΨΛi
(En−EΛ)2+O(λ3) (B.3)
to the first nontrivial order, where Enscales linearly with Λ. The dependence of the
perturbing matrix elements on the cutoff can be evaluated with the result [90,91]
hΨΛ|Vlh|nihn|Vhl |ΨΛi=X
a∈φ×φ
Λ2ha−1Aa,0+Aa,1
Λ+Aa,2
Λ2+. . . (B.4)
where ha= 2hφ−haand the summation is over the fields which can be found in the OPE
of the perturbation with itself. Therefore to leading order the cut-off dependence of energy
levels can be extrapolated with a function of the form
EΛ=E∞+AΛe1+BΛe2(B.5)
where the leading exponent is always e1= 4h−2 corresponding to the identity operator
in the OPE, while the next-to-leading e2depends on the particular model and perturbing
operator under consideration. From (B.3) the overlaps with the low energy CFT states
can be extrapolated with the same functional form with exponents shifted by +1, therefore
the leading one is given by e1= 4h−3. The exponents necessary for our calculations are
summarized in table 6, while figure 11 shows the results of extrapolation in the E8scattering
theory for overlaps with conformal states at level 6 in the identity module using cut-offs
– 28 –

JHEP01(2019)177
Model OPE 1 2 3 4 To fit
IFT σ×σ= 1 + ε−11
4−15
4−11
4,−15
4
IFT ε×ε= 1 −1−1,−2
TIFT σ×σ= 1 + ε+ε0+ε00 −57
20 −61
20 −81
20 −117
20 −57
20 ,−61
20
TIFT ε×ε= 1 + ε0−13
5−19
5−13
5,−18
5
TIFT σ0×σ0= 1 + ε00 −5
4−17
4−5
4,−9
4
Table 6. Fitting exponents for overlaps and chiral entanglement entropy. The columns 1-4 give
the leading exponent coming from the corresponding term of the OPE. Note that each one is
accompanied by sub-leading exponents resulting from 1/Λ corrections, and in some cases such as
TIFT perturbed by or by σ0, a sub-leading exponent from the identity channel is more relevant
than a leading one from another channel. For the thermal perturbation of the Ising model the OPE
contains only the identity channel, so all exponents come from the identity contribution and are
given by negative integers. The last column lists the exponents we used for cut-off extrapolation.
0 10 20 30 40 50 60 70 80 90 100
0
2
4
6
·10−2
mL
vab
|1il⊗ |1ir|1il⊗ |2ir
|1il⊗ |3ir|2il⊗ |1ir
|2il⊗ |2ir|2il⊗ |3ir
|3il⊗ |1ir|3il⊗ |2ir
|3il⊗ |3irprediction
Figure 11. Behaviour of the overlaps in E8field theory with increasing volume. The data indicated
by circles are the extrapolated overlaps from TCSA at descendent level 6 in the identity module of
the critical Ising field theory. There are 3 states in each chiral sector (labelled by |1il,r ,|2il,r and
|3il,r), therefore there are 9 states in the full Hilbert space in the zero momentum sector at this
level. The Ansatz predicts a vanishing overlap when the left and right chiral components differ,
while the continuous line shows the prediction for the diagonal overlaps calculated using (3.2).
10 ≤Λ≤20. Besides demonstrating the extrapolation procedure it also demonstrates that
the deviations of the overlaps from prediction of eq. (3.2) and the overlaps converge to the
predicted value when the volume is increased.
For the chiral entanglement entropy we assumed that the TCSA cut-off is high enough
so that the overlaps only differ from their exact value by a small amount, therefore the cut-
off dependence of the chiral entanglement entropy can be considered as a linear function
of cut-off dependence of the overlaps. Therefore the chiral entanglement entropy was
extrapolated with the same exponents as the overlaps, and figure 12 demonstrates that the
proposed extrapolating functions indeed provide an excellent fit.
– 29 –

JHEP01(2019)177
10 11 12 13 14 15 16 17 18 19 20
2.6
2.8
3
3.2
3.4
3.6
Λ
Sχ
mL = 80 ”raw” mL = 80 fit
mL = 80 ”raw” mL = 90 fit
mL = 100 ”raw” mL = 100 fit
Figure 12. Cut-off extrapolation of the chiral entanglement in the E8field theory for different
volumes.
Open Access. This article is distributed under the terms of the Creative Commons
Attribution License (CC-BY 4.0), which permits any use, distribution and reproduction in
any medium, provided the original author(s) and source are credited.
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