
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/(q2−1) = −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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