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Entanglement dynamics in Rule 54: exact results and quasiparticle picture
by Katja Klobas, Bruno Bertini
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Submission summary
Authors (as registered SciPost users):  Bruno Bertini · Katja Klobas 
Submission information  

Preprint Link:  scipost_202104_00017v1 (pdf) 
Date submitted:  20210414 10:30 
Submitted by:  Klobas, Katja 
Submitted to:  SciPost Physics 
Ontological classification  

Academic field:  Physics 
Specialties: 

Approach:  Theoretical 
Abstract
We study the entanglement dynamics generated by quantum quenches in the quantum cellular automaton Rule 54. We consider the evolution from a recently introduced class of solvable initial states. States in this class relax (locally) to a oneparameter family of Gibbs states and the thermalisation dynamics of local observables can be characterised exactly by means of an evolution in space. Here we show that the latter approach also gives access to the entanglement dynamics and derive exact formulas describing the asymptotic linear growth of all Rényi entropies in the thermodynamic limit and their eventual saturation for finite subsystems. While in the case of von Neumann entropy we recover exactly the predictions of the quasiparticle picture, we find no physically meaningful quasiparticle description for other Rényi entropies. Our results apply to both homogeneous and inhomogeneous quenches.
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Reports on this Submission
Report #2 by Anonymous (Referee 2) on 2021822 (Invited Report)
 Cite as: Anonymous, Report on arXiv:scipost_202104_00017v1, delivered 20210822, doi: 10.21468/SciPost.Report.3425
Strengths
1 First exact calculation of R\'enyi entropies in an outofequilibrium interacting system.
2 Clear exposition of technical details about the rule 54 and its solution.
3 Rigorous and clearly written.
Weaknesses
1No major weaknesses.
Report
The paper by Klobas and Bertini study the outofequilibrium dynamics of the R\'enyi entropies after a quantum quench in the rule 54 chain, which in recent years emerged as a valuable toy model for generic outofequilibrium interacting integrable systems.
This is an excellent paper that deserves publication in Scipost. The most important result of the paper is the calculation of the dynamics of the Renyi entropies in an interacting integrable system. This is a challenging problem. The authors find that while a hydrodynamic description of the dynamics of the Renyi entropies is possible, it is difficult to reconcile it with the standard quasiparticle picture for entanglement spreading, in agreement with previous observations in the literature. I believe that the exact results derived by Klobas and Bertini could be useful to shed light on this problem.
I have only some minor remarks:
1) In the introduction the authors discuss the applicability of the quasiparticle picture to describe the entanglement dynamics. They mention that it has not been applied in systems where the dynamics is not unitary. This is not entirely correct, as it has been extended recently for free fermions and free bosons models in
https://arxiv.org/ct?url=https%3A%2F%2Fdx.doi.org%2F10.1103%2FPhysRevB.103.L020302&v=1136ae07
and
https://arxiv.org/abs/2106.11997
and in a related setting in
https://scipost.org/10.21468/SciPostPhys.7.2.024
2) The authors observe that the dynamics of the Renyi entropies is seemingly not compatible with the quasiparticle picture. As they stress this is in accord with previous observations in the literature (Ref. 73).
However, they should mention that it is still possible to derive the steady
state R\'enyi entropies (as done in Ref. 73) for interacting integrable systems.
3) The the most important results of the paper is the calculation of the Renyi entropies. However, in the introduction they are barely discussed. I would suggest to move part of the discussion on the Renyi entropies in the introduction, for instance mentioning why they are important and what has been done already in the literature (i.e., the calculation of their steadystate value in integrable systems).
Report #1 by Anonymous (Referee 1) on 2021526 (Invited Report)
 Cite as: Anonymous, Report on arXiv:scipost_202104_00017v1, delivered 20210526, doi: 10.21468/SciPost.Report.2979
Report
In this work the authors study the quench dynamics in a quantum model for the discrete unitary evolution, the socalled Rule54. This is an interesting model, which has recently received renewed attention due to the fact that, in some sense, it represents one of the simplest, interacting integrable models. The authors expand the result recently presented in Ref. [31]. They certainly go much beyond what was discussed therein, and the manuscript contains many new and nontrivial results.
The research presented is timely, as it explores an approach to the quench dynamics based on a ``transverse evolution”, which is currently of interest for different groups. In fact, the authors provide a series of exact results which are expected to be appreciated even beyond the field of integrability.
In summary, the main results of the present manuscript are: 1) the derivation of a new family of initial states for which the quench dynamics can be solved exactly; 2) a rigorous derivation of a formula for the asymptotic growth of the Renyi entropies after the quench; 3) the first analytical test of a conjecture due to Alba and Calabrese for the growth of the Von Neumann entanglement entropy; 4) finally, the authors exhibited evidence of the impossibility of establishing a quasiparticle picture for the growth of Renyi entropies.
All these results are of physical significance. In addition, I believe that the paper is very clear and well written.
For the reasons above, I recommend publication.
I have, however, one question for the authors. Although I believe the answer could be of interest, I do not expect them to comment on this on the manuscript.
My question is the following. It is known that Rule 54 can be solved using the Bethe Ansatz method (as shown for instance in Ref. [67]). Therefore, in principle one could expect that the folded transfer matrix could also be diagonalized using Bethe Ansatz. Have the authors explored this direction? How does this relate to the tensornetwork approach presented by the authors? In fact, a similar approach could presumably be applied to more general (Floquet) integrable evolutions, such as the Heisenberg XXZ chain, where transfer matrices can be also diagonalized via Bethe Ansatz. Has this approach been explored or do the authors believe there is some fundamental obstacle?