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Crossed product algebras and generalized entropy for subregions
by Shadi Ali Ahmad, Ro Jefferson
Submission summary
Authors (as registered SciPost users):  Shadi Ali Ahmad · Ro Jefferson 
Submission information  

Preprint Link:  scipost_202308_00005v1 (pdf) 
Date submitted:  20230803 16:09 
Submitted by:  Jefferson, Ro 
Submitted to:  SciPost Physics 
Ontological classification  

Academic field:  Physics 
Specialties: 

Approach:  Theoretical 
Abstract
An early result of algebraic quantum field theory is that the algebra of any subregion in a QFT is a von Neumann factor of type $III_1$, in which entropy cannot be welldefined because such algebras do not admit a trace or density states. However, associated to the algebra is a modular group of automorphisms characterizing the local dynamics of degrees of freedom in the region, and the crossed product of the algebra with its modular group yields a type $II_\infty$ factor, in which traces and hence von Neumann entropy can be welldefined. In this work, we generalize recent constructions of the crossed product algebra for the TFD to, in principle, arbitrary spacetime regions in arbitrary QFTs, paving the way to the study of entanglement entropy without UV divergences. In this sense, the crossed product construction represents a refinement of Haag's assignment of nets of observable algebras to spacetime regions by providing a natural construction of a type $II$ factor. We present several concrete examples: a QFT in Rindler space, a CFT in an open ball of Minkowski space, and arbitrary boundary subregions in AdS/CFT. In the holographic setting, we provide a novel argument for why the bulk dual must be the entanglement wedge, and discuss the distinction arising from boundary modular flow between causal and entanglement wedges for excited states and disjoint regions.
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Strengths
1 The construction studied by the authors is very natural and general and has potential applications also outside of highenergy physics.
2 The authors consider many physically relevant scenarios.
Weaknesses
1 The paper is at time too terse, omitting some details which could be helpful to the nonexpert reader.
Report
In a spatial subregion, the algebra of observables does not possess neither a well defined trace nor a density matrix, leading to ultraviolet divergences in the entanglement entropy.
In this work, the authors review and extend a construction that alleviate this problem, the crossed product construction.
Every algebra of observables possesses a group of automorphisms generated by the modular Hamiltonian. By adjoining the action of this group to the algebra, the resulting extended algebra admits a trace and a density matrix. This allows the authors to define a generalised entanglement entropy which is in principle free from UV divergences.
The crossed product construction was already introduced in previous works, in particular [Witten 2021], where it was applied to the thermofield double state. The authors review this construction and extend it to many physically relevant examples. The main novelty with respect to the previous works is that the authors show that the construction is in principle independent of gravity, since it only requires the modular Hamiltonian.
After reviewing the general procedure, the authors apply it to several situations, in particular they consider the vacuum of a relativistic QFT in a Rindler wedge and the one of a CFT in a ball shaped region.
The authors consider also examples in holographic theories. They first study the vacuum state in the AdS/Rindler region, in which the action of the modular Hamiltonian is geometric, and then the case of excited states and of multiple boundary intervals.
The construction studied is very natural and general. While the procedure was already introduced in the literature, the authors demonstrate convincingly that it is independent of gravity. For this reason, this work has potentially vast applicability, even possibly in lowenergy physics.
The authors put the paper in the appropriate context, citing the previous works and remarking the main differences between their results and the state of the art.
The paper is well written, however at time the authors omit some details that could be helpful to the nonexpert reader. As an example, the modular Hamiltonian used in the construction depends on a specific state, however surprisingly the resulting extended algebra is independent of this state. This was well explained in [Witten 2021], but it is not very clear from this paper.
I recommend the publication of the paper after the authors answer some comments.
1 In Eq. (2.7), the authors show that the states belonging to the extended algebra are given by the tensor product of a state in the original algebra with a function $g$. This function $g$ enters the final result of the generalised entropy in, e.g., Eq. (3.14) for the Rindler wedge and Eq. (3.21) for the ballshaped region.
This is analogous to the result of [Witten 2021], where the function $g$ was determined to be a Gaussian for the thermofield double state.
In this paper, however, the authors do not discuss how to determine $g$ in the physical situations they consider. In particular, it is unclear if $g$ is fixed or it remains free. The authors could comment on this function.
2 In Eq. (3.22) the authors consider the known result for the entanglement entropy of the vacuum of a 2dCFT in an interval of length $\ell$ \[S = \frac{c}{3}\log\frac{\ell}{a}.\]The authors claim that the parameter $a$ plays the role of an infrared cutoff. This may simply be a typo, but $a$ is actually an ultraviolet cutoff on small lengths.
A way to see that this is the case is the construction of [Cardy, Tonni 2016]. In that paper, $a \ll \ell$ appears as the diameter of two small circles that have to be removed from the boundary of the interval. This regularisation is needed precisely because the entanglement entropy is UV divergent.
This is a minor point, but it is relevant to the stated purpose of comparing the known Eq. (3.22) with the authors' result in Eq. (3.21), since the generalised entropy (3.21) should be UV finite, differently from (3.22).
3 This last comment is optional, but i hope that the authors may clarify a doubt that arises with respect to excited states in AdS/CFT holography.
At pgs. 2425 the authors discuss the case of excited states in an holographic large$N$ CFT. They argue that by considering an excited state (keeping fixed the boundary region), the bulk dual region is modified in such a way as to include the bulk dual relative to the vacuum. As a consequence, the bulk algebra relative to the excited states is larger and contains the one relative to the vacuum.
My doubt regards the boundary algebra. Since the boundary subregion is kept fixed, one could naively expect that the boundary algebra does not change. Therefore it seems that the duality between boundary and bulk algebras does not hold anymore. I hope that the authors could clarify this point.
Requested changes
1 Comment on the role of the function $g$, by either briefly explaing how one could determine it or by making it explicit that it is left free.
2 Under Eq. (3.22), change "$a$ is an infrared cutoff" with "$a$ is an ultraviolet cutoff".