## SciPost Submission Page

# Gauss Law, Minimal Coupling and Fermionic PEPS for Lattice Gauge Theories

### by Patrick Emonts, Erez Zohar

### Submission summary

As Contributors: | Patrick Emonts |

Arxiv Link: | https://arxiv.org/abs/1807.01294v3 |

Date submitted: | 2019-12-03 |

Submitted by: | Emonts, Patrick |

Submitted to: | SciPost Physics Lecture Notes |

Discipline: | Physics |

Subject area: | Quantum Physics |

Approaches: | Theoretical, Computational |

### Abstract

In these lecture notes, we review some recent works on Hamiltonian lattice gauge theories, that involve, in particular, tensor network methods. The results reviewed here are tailored together in a slightly different way from the one used in the contexts where they were first introduced, by looking at the Gauss law from two different points of view: for the gauge field it is a differential equation, while from the matter point of view, on the other hand, it is a simple, explicit algebraic equation. We will review and discuss what these two points of view allow and do not allow us to do, in terms of unitarily gauging a pure-matter theory and eliminating the matter from a gauge theory, and relate that to the construction of PEPS (Projected Entangled Pair States) for lattice gauge theories.

###### Current status:

### Author comments upon resubmission

We found the comments and suggestions for modifications very insightful and have modified the paper accordingly as summarized in the list of changes.

### List of changes

Below, we provide detailed answers to the referee's suggestions and explain the respective modifications.

1- Improve the introduction by adding the citations to the work described in the report where appropriate.

We have now added a few sentences on the topic, including the requested references in section 2.2 which addresses the structure of the Hilbert space.

Although it is not the introduction section, it is still an introductory part of the lecture notes, where we believe it fits best.

2- Define Dirac Gamma matrice after (2).

Agreed and done.

3- The notation in (12) is a bit unfortunate, consider replacing j by another letter (just a suggestion).

Agreed and done -- replaced by l.

4- After (13) in locally gauge invariant locally and gauge actually mean the same thing, chose either one or the other.

Indeed, the formulation locally gauge invariant is redundant, we modified the sentence to "[...] is gauge invariant, i.e. invariant under local transformations generated by the Gauss law operators [...]".

5- In section 2.3 state explicitly that there are many different ways of "fixing the gauge" via enforcing the Gauss law. Eq 41 could seem the more natural one but this is not the only one, please mention it.

We rephrased the passage next to the equation to clarify this issue.

6- I am a bit confused with 42 since I would have expected that the sum on the psi^dagger psi part, not on the phases.

Indeed, the sum over x was missing and we added it. Nevertheless, the phases must be summed as well in order to obtain the right transformation.

This is the manifestation of the non-locality of the Gauss law solution in the transformation.

7- Review the three last paragraphs of section 2.3 to accommodate the observations made in the Report section and better explain what is meant.

We modified those paragraphs accordingly, aiming to clarify the difference between a unitary transformation and the process of minimal coupling.

8- First line of section 2.4 specify what this refers to.

We substituted this by "unitary gauging of separate building blocks".

9- From the discussion in the text I do not understand why two fermionic modes per auxiliary leg are necessary. I understand this allows to avoid adding extra tensors. Is it equivalent to add an extra link tensor with two fermionic modes encoding the electric field as (72) that is then contracted with a fermionic peps tensor with just one fermionic mode per auxiliary link?

The construction with two fermionic modes per auxiliary leg is not necessary, but rather used as an example.

We made it clear in the text.

10- First line of 4.2 change this of for that of.

Done.

11- In section 4.3 possibly mention that the opposite of this idea is a good way to locally entangle some matter fields with gauge fields resulting in a usual gauge theory (in any dimension).

Here we did not make any change. As argued in the first sections of the lecture notes, disentangling gauge fields from matter is in general not possible unitarily (in more than 1+1d) because of the non-uniqueness of the Gauss law solution. For this reason, we do not see how the opposite process of entangling matter with gauge fields would be possible - unless some configurations of electric fields are arbitrarily chosen. For this reason we decided not to include this remark in the manuscript.