Three-dimensional photonic quantum Hall effect of Fermi arcs

The recent discovery of three-dimensional (3D) quantum Hall effect (QHE) of Fermi arcs in topological semimetals has revolutionized our understanding of Hall physics in 3D systems. However, its most prominent hallmark, the one-sided chiral hinge states of Fermi arcs, has thus far never been experimentally observed in any physical system. Here, we report the first photonic realization of 3D QHE of Fermi arcs and directly observe the one-sided chiral hinge states of Fermi arcs in an inhomogeneous magnetic Weyl photonic crystal under a pseudomagnetic field (PMF) with time-reversal symmetry breaking. We experimentally demonstrate that the PMF quantizes both the bulk and Fermi arc surface states into Landau plateaus, giving rise to chiral Landau levels and robust one-sided chiral hinge states localized at only one edge on the front surface and at the opposite edge on the back surface, both of which are the signatures of 3D QHE of Fermi arcs. Moreover, we show that the one-sided chiral hinge states of Fermi arcs can be switched between the two pairs of diagonal hinges by reversing the PMF. Our work not only provides an ideal platform for exploring 3D quantum Hall physics but also opens new avenues for the design of robust photonic devices.