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Phases of scrambling in eigenstates
by Tarek Anous, Julian Sonner
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Submission summary
As Contributors:  Tarek Anous · Julian Sonner 
Arxiv Link:  https://arxiv.org/abs/1903.03143v2 (pdf) 
Date submitted:  20190503 02:00 
Submitted by:  Anous, Tarek 
Submitted to:  SciPost Physics 
Academic field:  Physics 
Specialties: 

Approach:  Theoretical 
Abstract
We use the monodromy method to compute expectation values of an arbitrary number of light operators in finitely excited ("heavy") eigenstates of holographic 2D CFT. For eigenstates with scaling dimensions above the BTZ threshold, these behave thermally up to small corrections, with an effective temperature determined by the heavy state. Below the threshold we find oscillatory and not decaying behavior. As an application of these results we compute the expectation of the outoftime order arrangement of four light operators in a heavy eigenstate, i.e. a sixpoint function. Above the threshold we find maximally scrambling behavior with Lyapunov exponent $2\pi T_{\rm eff}$. Below threshold we find that the eigenstate OTOC shows persistent harmonic oscillations.
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Reports on this Submission
Anonymous Report 1 on 201964 (Invited Report)
 Cite as: Anonymous, Report on arXiv:1903.03143v2, delivered 20190604, doi: 10.21468/SciPost.Report.992
Strengths
1 Well written, transparent structure
2 Necessary background is explained in a useful fashion
3 Presentation of new results is intuitive, but at the same time precise
4 Content is interesting and timely
5 Referencing is good
Weaknesses
1 The content is interesting and useful, though perhaps a little bit limited in scope
Report
This paper investigates a version of the eigenstate thermalisation hypothesis in the context of 2d CFTs with large central charge and a sparse spectrum. The authors show that correlation functions of any even number of (simple) probe operators in heavy primary states look approximately like thermal expectation values. The relevant temperature is the ETH temperature associated with the heavy state. As an application, the authors compute the OTOC of four probe operators in a heavy primary state and show that under the above assumptions it exhibits maximal Lyapunov growth with the Lyapunov exponent given by the same microcanonical temperature associated with the heavy state. Interestingly, there is a sharp transition between exponential Lyapunov growth and oscillatory behaviour when the heavy primary creating the state approaches the BTZ threshold.
The paper is very well written, clearly structured, and shows appropriate awareness of previous literature. The authors explain all the necessary background in a pedagogical fashion. The paper definitively adds valuable information to the current state of the field. I will recommend the paper for publication.
One comment though, regarding identity block dominance: I understand that the full implications/justifications for this assumption are probably not entirely understood even for the HHLL block. But I wonder if the authors have some more to say about this assumption in the case of intermediate channels between light operators in the HHLL...L blocks. In particular, does the method presented here require a hierarchy of conformal weights of the string of light operators ($h_{Q_1} \gg h_{Q_2} \gg \ldots$)? If so, would one hope for the results about OTOCs to hold outside this regime?
typos:
 Caption of figure 2.
 First sentence on page 14.