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Subsystem NonInvertible Symmetry Operators and Defects
by Weiguang Cao, Linhao Li, Masahito Yamazaki, Yunqin Zheng
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Submission summary
Authors (as registered SciPost users):  Weiguang Cao 
Submission information  

Preprint Link:  https://arxiv.org/abs/2304.09886v2 (pdf) 
Date submitted:  20230501 03:43 
Submitted by:  Cao, Weiguang 
Submitted to:  SciPost Physics 
Ontological classification  

Academic field:  Physics 
Specialties: 

Approach:  Theoretical 
Abstract
We explore noninvertible symmetries in twodimensional lattice models with subsystem $\mathbb Z_2$ symmetry. We introduce a subsystem $\mathbb Z_2$gauging procedure, called the subsystem KramersWannier transformation, which generalizes the ordinary KramersWannier transformation. The corresponding duality operators and defects are constructed by gaugings on the whole or half of the Hilbert space. By gauging twice, we derive fusion rules of duality operators and defects, which enriches ordinary Ising fusion rules with subsystem features. Subsystem KramersWannier duality defects are mobile in both spatial directions, unlike the defects of invertible subsystem symmetries. We finally comment on the anomaly of the subsystem KramersWannier duality symmetry, and discuss its subtleties.
Current status:
Reports on this Submission
Anonymous Report 2 on 2023623 (Invited Report)
 Cite as: Anonymous, Report on arXiv:2304.09886v2, delivered 20230623, doi: 10.21468/SciPost.Report.7395
Report
The paper discusses and constructs noninvertible subsystem symmetries in 2+1 dimensions. This is the first example of such a generalized symmetry. It is wellwritten and contains concrete examples. I recommend the draft for publication.
Requested changes
In both section 2 and 3, the authors discuss KW duality operator/defect 'N' and find its fusion rule in generality. However, they should emphasize that what they actually discuss is the KW interface 'N' between a theory before and after gauging. This interface has to be fused with an isomorphism 'U' which identifies the theory before and after gauging. The KW defect/operator that lives in a single theory is 'NU' rather than just 'N'.
For instance, the authors find the fusion rule
N x N^\dagger
which makes sense when 'N' is an interface. But they do not consider the fusion
NU x NU
which can be different from example to example. Authors should add a short discussion emphasizing where they discuss the interface and when discuss a defect/operator that exists in a single theory.
Anonymous Report 1 on 2023614 (Invited Report)
 Cite as: Anonymous, Report on arXiv:2304.09886v2, delivered 20230614, doi: 10.21468/SciPost.Report.7341
Strengths
This paper makes a new generalization of the notion of symmetry.
Weaknesses
Relationships between different concepts are unclear.
Report
This paper presents a generalization of global symmetries, namely subsystem noninvertible symmetry. Especially, this paper studies the subsystem KramersWannier duality in (2+1)dimensional spin system and provides the detailed properties of duality operators (defects).
This paper points out a new important concept related to global symmetries. It provides excellent explanations and concrete examples. Therefore, I highly recommend publication of this paper in SciPost.
Requested changes
There are minor points that I would encourage the authors to address.
1  There seems to be no definition of (H_i^t)^\dagger in the sentence above (2.21). Should it be replaced with (N_i^t)^\dagger?
2  Even if the subsystem KW duality is a symmetry, subsystem KW duality operators (defects) are not recognized as subsystem symmetry operators (defects), opposed to their names. It is because the names of subsystem KW duality defects come from subsystem symmetry gauging, not from subsystem symmetry itself. If this understanding is correct, then I think that this point should be emphasized to avoid confusion.
3  There are possibly grammatical errors or typos (pages and lines are according to the arXiv version.).
page 2, line 5: "more than one properties" > "more than one property"
page 25, line 1: "partition function in" > "partition function is"
page 29 and 30: The sentence "The number of holonomy variables grows with system size" is repeated. The first one seems unnecessary.
Author: Weiguang Cao on 20230808 [id 3880]
(in reply to Report 1 on 20230614)
Dear referee,
We thank your effort and time on reading the draft. We fixed all typos you mentioned in comment 1 and 3.
Related to your second comment:
"2  Even if the subsystem KW duality is a symmetry, subsystem KW duality operators (defects) are not recognized as subsystem symmetry operators (defects), opposed to their names. It is because the names of subsystem KW duality defects come from subsystem symmetry gauging, not from subsystem symmetry itself. If this understanding is correct, then I think that this point should be emphasized to avoid confusion."
Our response: we added a comment in the end of section 1.2
"Here, the subsystem KW duality symmetry has codimension 1 noninvertible symmetry operator and defect, which is different from the codimension 2 invertible subsystem $\mathbb Z_2$ symmetry operators and defects. Furthermore, the noninvertible fusion rule will mix operators (defects) of different codimensions."
We hope this will clarify the confusions.
Best,
Weiguang, Linhao, Masahito and Yunqin
Author: Weiguang Cao on 20230808 [id 3881]
(in reply to Report 2 on 20230623)Dear referee,
We thank your effort and time on reading the draft. We include the following footnote in the introduction to clarify terminologies.
"We mainly use the terminology "operators" for mappings from one Hilbert space to another Hilbert space and "defects" for interfaces between two theories. However, in Sec.2.4 and 3.4, we also discussed about operators on one Hilbert space and defects within one theory."
We also include section 2.4 and 3.4 to discuss about operators and defects that exist in a single theory.
We hope this will clarify the confusions.
Best,
Weiguang, Linhao, Masahito and Yunqin