From Dual Unitarity to Generic Quantum Operator Spreading Michael A. Rampp Roderich Moessner and Pieter W. Claeys Max Planck Institute for the Physics of Complex Systems 01187 Dresden Germany

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From Dual Unitarity to Generic Quantum Operator Spreading
Michael A. Rampp, Roderich Moessner, and Pieter W. Claeys
Max Planck Institute for the Physics of Complex Systems, 01187 Dresden, Germany
(Dated: May 4, 2023)
Dual-unitary circuits are paradigmatic examples of exactly solvable yet chaotic quantum many-
body systems, but solvability naturally goes along with a degree of non-generic behaviour. By
investigating the effect of weakly broken dual unitarity on the spreading of local operators we study
whether, and how, small deviations from dual unitarity recover fully generic many-body dynamics.
We present a discrete path-integral formula for the out-of-time-order correlator and recover a but-
terfly velocity smaller than the light-cone velocity, vB< vLC , and a diffusively broadening operator
front, two generic features of ergodic quantum spin chains absent in dual-unitary circuit dynamics.
The butterfly velocity and diffusion constant are determined by a small set of microscopic quantities
and the operator entanglement of the gates has a crucial role.
The dynamical behaviour of strongly correlated quan-
tum many-body systems out of equilibrium is notoriously
hard to describe. Quantum many-body dynamics is in-
timately related to questions of thermalization, informa-
tion scrambling, quantum chaos, and the emergence of
hydrodynamics [18]. Simple model systems that can
be solved analytically are highly desirable since they of-
fer an invaluable window into these questions. In re-
cent years dual-unitary circuits (DUCs) have emerged
as paradigmatic examples of exactly solvable yet chaotic
many-body systems [927] in which a variety of dynam-
ical quantities are analytically accessible [1015,17,18].
However, solvability comes at a cost in genericity, and
in several respects DUCs display behaviour that differs
strikingly from the phenomenology observed numerically
in more generic models [48,28,29]. In particular, the
space-time duality present in DUCs enforces that corre-
lations and operators spread with the maximum possible
velocity. This nongeneric maximal spreading has also
been observed experimentally on Google’s quantum pro-
cessor [30].
It is natural to ask whether and in what sense DUCs
might serve as a starting point to understand the behav-
ior of more general systems. This question is especially
relevant given recent advances in noisy intermediate scale
quantum devices. While much is now known about DUCs
themselves, there are only few results about deviations
from dual unitarity [3133] and many questions remain
open. In Ref. [31] the behavior of local correlation func-
tions in circuits close to dual unitarity has been investi-
gated. It was found that in most instances local corre-
lators acquire a more generic spatio-temporal structure,
not being exclusively supported on the light cone any-
more.
In this paper we investigate operator dynamics by
considering out-of-time-order correlators (OTOCs) in a
broad class of chaotic quantum circuits in which dual uni-
tarity is weakly broken. We show that the OTOC can be
expressed as a sum over all possible paths resulting from
scattering on individual dual-unitarity-breaking gates,
acting as defects in an otherwise dual-unitary circuit. We
find that after an initial period in which the dual-unitary
form is approximately preserved, even a weakly broken
FIG. 1. (a) The unitary time evolution operator is con-
structed as a brickwork circuit composed out of identical
two-site gates. One row of gates corresponds to a time step
t= 1. (b) Graphical depiction of processes contributing to
the OTOC. When dual unitarity is broken, the edge of the
operator string can scatter into the light cone. The OTOC
is then given by a weighted sum of all paths. ’Folded’ dual-
unitary gates are depicted in red and dual-unitary-breaking
gates in purple.
duality leads the OTOC to recover a nonmaximal but-
terfly velocity and a diffusive broadening of the operator
front, hallmarks of generic one-dimensional chaotic quan-
tum many-body systems absent from pure DUCs. The
operator front at late times takes a universal form and
its parameters are microscopically determined by the en-
tangling properties of the gate. Our manipulations are
controlled in the limit where the dual-unitarity-breaking
gates are dilute along the direction of the light cone, but
we argue that they capture the relevant characteristics
of the OTOC on intermediate to long timescales even in
the case of a Floquet circuit with dense perturbations.
The developed framework is expected to be applicable to
different probes of operator spreading.
Dual-unitary circuits. We consider circuits composed
of unitary gates Uacting on two sites with local Hilbert
space dimension q, with matrix elements Uab,cd graphi-
cally expressed as
(1)
In this notation each leg corresponds to an index in the lo-
cal Hilbert space, and connecting two indices corresponds
arXiv:2210.13490v3 [quant-ph] 3 May 2023
2
to a tensor contraction (see e.g. [34]). Unitary gates ar-
ranged in a brickwork geometry [Fig. 1(a)] provide simple
models for local, unitary quantum many-body dynamics
on a one-dimensional lattice [36,35], with the number
of discrete time steps tcorresponding to the number of
rows in the circuit. A gate Uis called dual-unitary if the
associated dual gate ˜
Udefined by ˜
Uab,cd Udb,ca is also
unitary [10,13].
Let us review the computation of OTOCs in dual-
unitary circuits [17]. We consider a basis of local opera-
tors σαnormalized according to tr σ
ασβ=qδαβ. Set-
ting σ0= 1, the remaining operators are traceless, and we
take σα(x) to act as σαon site xand as the identity ev-
erywhere else. Denoting the time evolution operator as
U(t), we write σα(x, t) = U(t)σα(x)U(t) and consider
the OTOC
Cαβ(x, t) = hσα(0, t)σβ(x, 0)σα(0, t)σβ(x, 0)i,(2)
with respect to the maximally mixed state, hOi ≡
tr[O]/tr[ ]. This function quantifies the spreading of op-
erators and the scrambling of information into nonlocal
degrees of freedom [3638], and is experimentally accessi-
ble in (digital) quantum simulation platforms [30,3943].
The OTOC’s propagation speed is called butterfly veloc-
ity vBand sets the maximal speed at which information
can spread in the circuit [44,45].
The OTOC exhibits a strong parity dependence. Here,
we focus on C+(x, t) for which (tx)2Z, but the
derivation is analogous for C(x, t), for which (tx)
2Z+ 1. In general circuits, Eq. (2) can be graphically
represented as the contraction of a two-dimensional ten-
sor network, the size of which is set by the light-cone
coordinates n= (tx+ 2)/2, m = (t+x)/2,
C+
αβ(n, m) = 1
qn+m,
= (Ln(σα)|(Tn)m|R+
n(σβ)),(3)
where we have introduced the ‘folded’ gate acting on four
copies of the local Hilbert space,
= !2
,(4)
as well as the following vectors,
=1
q,=1
q,=1
q,=1
q.
(5)
The tensor network (3) can be understood as the con-
traction of powers of a transfer matrix
Tn=1
q
| {z }
n
.(6)
This transfer matrix is a contracting map, i.e. kTnvk ≤
kvk. Hence, all its eigenvalues lie inside or on the bound-
ary of the complex unit disk. In the limit m→ ∞ the
OTOC is completely determined by the eigenvectors of
Tnwith leading eigenvalue (i.e., modulus 1). If the gate
is dual-unitary there exist n+1 leading eigenvectors that
are independent of any further characteristics of the gate
and can be constructed explicitly [17]. If these vectors
exhaust the set of leading eigenvectors, the gate is called
maximally chaotic [15].
While the existence of a local conserved quantity im-
plies the existence of further leading eigenvectors, the set
of maximally chaotic gates is dense in the set of dual-
unitary gates [15]. We call the subspace spanned by
this generic set of vectors the maximally chaotic subspace
(MCS). Its construction and properties are elaborated in
the supplemental material [46].
For circuits composed of maximally chaotic dual-
unitary gates the following holds [17]: After an initial
transient regime, the OTOC for (tx)2Zis only non-
vanishing on the light cone edge, |x|=t, where it takes
the universal value 1/(q21), resulting in a maximal
butterfly velocity vB=vLC = 1. For (tx)2Z+ 1
the OTOC decays exponentially inside the light cone.
This behavior is to be contrasted with the OTOC in
generic unitary dynamics, where vB<1 and the ballis-
tic spreading is accompanied by a diffusively broadening
front [5,6].
Breaking dual unitarity. For concreteness we consider
gates of the form U=V eiεW where Vis a maximally
chaotic dual-unitary gate, Wis Hermitian, and εis taken
to be small (ε1).
How does the breaking of dual unitarity affect the
OTOC? In the absence of dual unitarity and with-
out further constraints, the transfer matrix has only
a single, ‘trivial’, leading eigenvector that leads to
limm→∞ C+
αβ(n, m) = 1 for finite n[17]. Signaling that
the butterfly velocity in such a circuit is nonmaximal,
vB<1, the butterfly velocity then needs to be deter-
mined from the subleading eigenvectors of the transfer
matrix.
The contribution of a subleading eigenvector with
eigenvalue |λ|<1 decays on a timescale τ(log|λ|)1.
For weak perturbations from dual unitarity, the manifold
of leading eigenvectors is split by an amount O(ε2). If in
the limit m→ ∞ a finite spectral gap to the remaining
spectrum exists, then for sufficiently small εthe largest
subleading eigenvectors are predominantly composed out
of vectors in the MCS, and the behavior of the OTOC on
long times t12is determined by those eigenvectors.
3
In the absence of symmetries that force a gap closing, we
expect this to be the generic scenario.
To proceed, we project the transfer matrix to the MCS,
similar in spirit to degenerate perturbation theory, and
compute the OTOC with the projected transfer matrix.
If the perturbed gates are dilute along the light-cone, this
approximation is controlled [46]. However, for perturba-
tions that are sufficiently small compared to the spectral
gap we expect this description to remain valid in the
dense limit.
Notably, the MCS only grows linearly with the size of
the transfer matrix compared to the full operator space,
which grows exponentially. Reducing the dynamics to
the MCS presents a significant computational advantage.
Moreover, the resulting transfer matrix can be efficiently
truncated, allowing for analytic evaluation (see below).
The matrix elements of the transfer matrix in the MCS,
(`|Tn|k), can be readily calculated [46]. These depend on
the properties of the gates through a set of quantities Bk,
k= 1, . . . , n. Graphically
Bk=1
qk+1 .(7)
Defining z1:= (B11)/(q21) and zk:= (Bk
Bk1)/(q21) for k > 1 we find that (0|Tn|0) = 1 and
(0|Tn|k) = pq21zk
qk1, k 1 (8a)
(`|Tn|k) =
q2zklzkl+1
qkl, k > l,
1z1, k =l,
0,otherwise.
(8b)
They have the following structure: (i) the matrix is upper
triangular, as a direct consequence of unitarity [46]. (ii)
Except in the first row, the k-th side diagonal has the
same entry everywhere. This is only the case if all gates
are identical and follows from translational invariance.
Taken together, these imply that the eigenvalues are λ0=
1 with algebraic multiplicity 1, and λ1= 1 z1<1 with
algebraic multiplicity n.
All matrix elements can be given a quantum-
information theoretic interpretation. Inserting the
Schmidt decomposition of the gate, U=PjσjXjYj,
into z1reveals that this quantity is equivalent to the lin-
ear operator entanglement of the gate E(U) [47], with
z1= 1 q2
q21E(U). The properties of the operator en-
tanglement imply 0 z11. As dual unitarity of U
is equivalent to Uhaving maximal operator entangle-
ment [48], z1= 0 iff Uis dual unitary, and z1hence
quantifies proximity to dual unitarity.
The subleading eigenvalues follow from Eq. (8b) as 1
z1, such that the timescale τfor deviations from dual
unitarity to become apparent follows as
τ1∼ −log (1 z1) = log q2
q21E(U).(9)
All Bkfor k > 1 can analogously be given the inter-
pretation as operator entanglements of k-fold diagonally
composed gates on enlarged Hilbert spaces [46]. Because
the diagonal composition preserves dual unitarity [24],
zk= 0 for all kiff Uis dual-unitary. The zkare bounded
by 0 zk(1 z1)k1, implying that zk0 for
k→ ∞ [46]. In general, knowledge of all higher-order op-
erator entanglements is necessary to compute the OTOC,
however they are increasingly less important.
While it is not possible to calculate arbitrary powers
of the transfer matrix exactly, the structure of Eq. (8)
allows for a systematic expansion of the OTOC using a
path-integral approach. We write,
Tn=D+
n
X
j=1|uj)(j|,(10)
D= diag (1,1z1,...,1z1),(11)
where the |uj) are vectors containing the off-diagonal ma-
trix elements, (`|uk)=(`|Tn|k) for k > ` and (`|uk)=0
otherwise.
The off-diagonal terms are of order zk2, and we
expand the OTOC in powers of (TnD). Each such
power can be expressed as
(TnD)ν=
n
X
j1,j2,...,jν=1
(|uj1)(j1|)(|uj2)(j2|). . . (|ujν)(jν|)
=
n
X
j1<j2···<jν|uj1)(j1|uj2). . . (jν1|ujν)(jν|(12)
The sum consists of products of off-diagonal matrix ele-
ments indexed by sets {j1, . . . , jν}. These indices can be
interpreted as nodes of a path and the off-diagonal ma-
trix elements (`|uk) act as propagators determining the
amplitude of jumping k`steps inside the light cone.
Crucially, these only depend on the difference k`and
the amplitudes for negative step sizes vanish, making sure
only causal paths contribute to the OTOC [Fig. 1(b)].
From Eq. (8) it follows that jumps of size kare controlled
by zk/qk, such that jumps with a large kare exponen-
tially suppressed.
We note that having maximal operator entanglement
implies that all off-diagonal matrix elements vanish.
Hence, in dual-unitary circuits the edges of an operator
string can only move along the edges of the light cone
and we recover the previous result.
Although the mathematical origin is different, the pic-
ture presented here is qualitatively similar to the one
presented in Ref. [31] for the two-point functions. The
skeleton diagrams appearing there resemble the scatter-
ing paths introduced above. The similarity of the two
results might merely be a reflection of the underlying
physics of dual-unitary circuits. In dual-unitary circuits
all excitations move with maximal velocity. Breaking
dual unitarity then allows processes that violate this rule,
suggesting expansions in orders of such processes.
4
0.0
0.5
1.0
OTOC
a) ε= 0.15
t= 450
t= 1000
t= 1800
40 20 0 20 40
xvBt
0.0
0.5
1.0
OTOC
b)
t= 1200, ε = 0.25
projection to MCS
error function fit
1-step approx.
2-step approx.
FIG. 2. (a) Broadening of the OTOC front as a function
of time. (b) Comparison of the asymptotic front for different
approximations (the shift in the center of the front is corrected
for better comparability).
Truncation of the path integral. To make analytical
progress, we restrict the sum over paths. In the large-
qlimit the path integral is dominated by those paths in
which single steps are at most of size 1, since higher-order
steps are suppressed by powers of q[see Eq. (8)].
We now approximate the OTOC for general qby only
considering these paths, and call this the one-step ap-
proximation. On the level of scattering amplitudes this
approximation corresponds to letting only z1be nonzero.
However, even for qubits (q= 2) the one-step approxi-
mation already produces the correct functional form of
the asymptotic profile close to the center of the front
[Fig. 2(c)].
Within this approximation the path integral can be
evaluated exactly, leading to
C+
(1) =q2Fz1(n, m)Fz1(n1, m)
q21,(13a)
Fz1(n, m)nm
nBz1(n, m n+ 1),(13b)
where Bz1(a, b) denotes the incomplete β-function. An
asymptotic expansion yields a butterfly velocity vB,1=
(1 z1)/(1 + z1) and a front that takes the form
C+
(1) 1
21 + erf xvB,1t
2D1t.(14)
This is the form expected from a diffusively broadening
front with diffusion constant D1=vB,1(1 v2
B,1). Both
the form of the front and the scaling of the diffusion con-
stant for vB,11 agree with results from Haar random
circuits [5,6].
To obtain a better quantitative agreement for q= 2,
we consider the two-step approximation. An exact cal-
culation yields [46]
C+
(2)(x, t)C+
(1) x+tx
2ξ, t tx
2ξ,(15)
100102104
t
0.5
0.0
0.5
1.0
C+(t, t)
ε= 0.00
ε= 0.01
ε= 0.03
ε= 0.10
FIG. 3. Early time relaxation of the dual-unitary form to
the generic shape on the geometric light cone for a range of
perturbation strengths. The timescale is indicated by vertical
dotted lines.
0.90
0.95
vB
a)
untruncated
1-step approximation
2-step approximation
0.10 0.15 0.20 0.25
perturbation strength ε
0.1
0.2
0.3
D
b)
FIG. 4. Plots of (a) butterfly velocity and (b) diffusion con-
stant as function of perturbation strength and comparison to
the analytical predictions of the one- and two-step approxi-
mations.
with ξ=z2/(q2z1). This result can be understood by
noting that the path integral is asymptotically domi-
nated by the typical path. If z2z1the typical fraction
of steps of size two is approximately given by the ratio
of scattering amplitudes ξ=z2/(q2z1). The steps of
size two therefore effectively shift the profile [Eq. (13b)]
deeper into the light cone, nn(1 ξ).
Close to the shifted front the shape of the profile is
preserved, with renormalized parameters
vB,2=vB,1δ
1δ, D2=D1
1(1 vB,2)ξ
2
(1 δ)2,(16)
where δ= (1 + vB,1)ξ/2. By taking into account longer
steps, operator strings can move into the light cone
faster, diminishing the butterfly velocity and, since this
increases the variance of the distribution of the endpoints
of operators, enhancing diffusion.
Numerical results. Restricting the dynamics to the
MCS (of linear size in n) presents an enormous simpli-
fication compared to the full exponentially large space
when probing the long time limit. This restriction allows
the efficient numerical evaluation of the OTOC under the
assumptions stated above.
5
First, we study the behavior of the OTOC on the geo-
metric light cone for a Floquet circuit consisting of iden-
tical perturbed dual-unitary gates with q= 2. We find
that the main features of dual unitarity persist up to the
timescale (9) [Fig. 3]. At this timescale, the deviation of
the OTOC on the geometric light cone C+(t, t) from the
dual-unitary prediction, 1/(q21) = 1/3, becomes of
order 1. This indicates that for earlier times most of the
operator strings still travel at maximal velocity.
In the late-time regime the operator front slows down,
moving ballistically with vB<1, and shows approximate
diffusive broadening [see Fig. 2(a,b)]. At late times the
shape of the operator front is well described by an error
function of the form described in Eq. (14), indicating
that higher k-steps only serve to further renormalize the
arguments of the obtained profile.
We extract the butterfly velocity and diffusion con-
stant and compare them to the analytic prediction ob-
tained by truncating the path integral [Fig. 4]. For a spe-
cific but randomly selected perturbation we find that the
two-step approximation is in good agreement with the
full path integral result for low to intermediate perturba-
tion strengths. For the diffusion constant the discrepancy
is larger, but the qualitative behavior is captured. We
attribute this discrepancy to paths which contain large
steps. For both quantities, the two-step approximation
significantly improves the one-step approximation, and
the accuracy of this approximation is expected to increase
for larger qor by including higher steps.
Discussion.
The observation of diffusively broadening fronts in
non-random systems far away from the dual-unitary
limit [8,29] hints at the presence of a more general mech-
anism. Numerical results indicate that the degeneracy of
the subleading eigenvalue can remain stable far from dual
unitarity, suggesting that a similar path-integral descrip-
tion remains possible beyond the perturbative regime.
Moreover, the non-Hermiticity of the transfer matrix
might play a central role – Ref. [49] previously observed
that non-Hermiticity can strongly influence the behavior
of OTOCs.
Our work shows that dual-unitary circuits can
serve as a starting point to investigate more generic
settings. We hope that this work opens up further
studies on perturbed dual-unitary cicuits, e.g., on
entanglement dynamics, the relation to transport, or
spectral properties. The developed framework can
be directly applied to more general probes of oper-
ator dynamics in perturbed dual-unitary dynamics
involving multiple replicas of the circuit, e.g. R´enyi
(operator) entanglement [15,32], spectral form factors
[16,50], or in studies of ‘deep thermalization’ [33,51,52].
ACKNOWLEDGMENTS
We are grateful to Dominik Hahn, Pavel Kos, Chris
R. Laumann, David M. Long, Frank Pollmann, Tomaˇz
Prosen, and Philippe Suchsland for useful discus-
sions. This work was in part funded by the Deutsche
Forschungsgemeinschaft (DFG) via the cluster of excel-
lence ct.qmat (EXC 2147, project-id 390858490).
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摘要:

FromDualUnitaritytoGenericQuantumOperatorSpreadingMichaelA.Rampp,RoderichMoessner,andPieterW.ClaeysMaxPlanckInstituteforthePhysicsofComplexSystems,01187Dresden,Germany(Dated:May4,2023)Dual-unitarycircuitsareparadigmaticexamplesofexactlysolvableyetchaoticquantummany-bodysystems,butsolvabilitynaturall...

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