I am reading Prof. Sachdev's review on quantum phases of matter (link: https://arxiv.org/abs/1203.4565) and I am a bit confused about how the analogy between U(1) photon excitations in QED and vison (plaquette) defects in a $\mathbb{Z2}$ RVB state can be made precise. How is this analogous to propagating photons in gauge theories?
