Programmable non-Hermitian photonic quantum walks via dichroic metasurfaces

The evolution of a closed quantum system is described by a unitary operator generated by a Hermitian Hamiltonian. However, when certain degrees of freedom are coupled to an environment, the relevant dynamics can be captured by non-unitary evolution operators, arising from non-Hermitian Hamiltonians. Here we introduce a photonic platform that implements non-unitary quantum walks, commonly used to emulate open-system dynamics, in the synthetic space of light transverse momentum. These walks are realized by propagating light through a series of dichroic liquid-crystal metasurfaces that impart polarization-dependent momentum shifts. The non-unitary behavior stems from dichroic dye molecules with polarization-dependent absorption, whose orientation is coupled to that of the liquid crystals. We demonstrate multiple walks up to five time steps, with adjustable levels of dichroism set by the metasurface voltage, which is controlled remotely. This discrete-time process maps onto two-band tight-binding models with reciprocal yet non-Hermitian nearest-neighbor couplings, corresponding to a less-studied class of non-Hermitian systems. Our platform broadens the range of optical simulators for controlled investigations of non-Hermitian quantum dynamics.