  We study nonreciprocity in spatiotemporally modulated 1D resonator chains
from the perspective of equivalent 2D resonator arrays with a synthetic
dimension and transverse synthetic electric and magnetic fields. The synthetic
fields are respectively related to temporal and spatial modulation of the
resonator chain, and we show that their combination can break transmission
reciprocity without additional elements. This nonreciprocal effect is analogous
to the Hall effect for charged particles. We experimentally implement chains of
2 and 3 spatiotemporally modulated resonators and measure over 58 dB of
isolation contrast.
