tokens: X.L. and J.Z. appreciate useful input from Maxim Lyutikov, in particular the suggestion to simplify the description of FFE eigenmodes by using the temporal gauge. A.M.B. is supported by NASA grant NNX17AK37G and a grant from the Simons Foundation No. 446228.
ner_tags: B-Person O B-Person O O O O B-Person I-Person O O O O O O O O O O O O O O O O B-Person O O O B-Organization O B-Grant O O O O O B-Organization I-Organization B-Grant I-Grant