Photon-Blockade Analogue Nonreciprocal Absorption in Spatiotemporal Metasurfaces

Controlling the flow of electromagnetic energy is essential for advancing quantum technologies. We introduce a spatiotemporally modulated superconducting metasurface that exhibits photon-blockade-analogue nonreciprocal absorption. In this system, the frequency of incident radiation is matched to the modulation frequency of the metasurface, enabling one-way directional absorption. Forward-traveling waves undergo resonant coupling to higher-order Floquet harmonics and are absorbed within the slab, while backward-traveling waves transmit freely without interaction. This behavior arises from classical wave interference and harmonic conversion in a space-time periodic medium---a classical analogue of quantum photon blockade. We present a design based on a superconductor-semiconductor metasurface incorporating cascaded Josephson field-effect transistors (JoFETs) for millikelvin-temperature operation. Starting from the microscopic Hamiltonian of a single gate-tunable JoFET cell, we derive the system's classical circuit relations, effective space-time-periodic permeability, Floquet band structure, and isofrequency diagrams from first principles, and validate the resulting nonreciprocal absorption with full-wave simulations. These findings establish a pathway toward compact, nonreciprocal superconducting devices for quantum information processing and microwave photonics.