Zero-index metamaterials show the unique feature of uniform spatial phase distributions, enabling the interaction of single electromagnetic mode with matter over an infinite area in an arbitrary shape. This feature brings various novel optical physics and devices, such as supercoupler, large-area single-mode laser, and extended superradiance. However, the state-of-the-art zero-index waveguide shows a propagation loss as high as 1000 dB/mm, hampering most potential applications of zero-index metamaterials. Although zero-index metamaterials based on bound state in the continuum can show a lower propagation loss of 45 dB/mm, the photonic crystal slab configuration which are boundless in the in-plane direction limits the devices’ footprint and flexibility drastically. Here we demonstrated a one-dimensional metawaveguide with zero refractive index along the propagation direction, featuring a high flexibility, a compact footprint, and a low propagation loss of 5.45 dB/mm near the zero-index wavelength. This metawaveguide could enable many zero index-based linear, nonlinear, and quantum photonic devices such as entangled photon pair sources based on spontaneous four-wave mixing.
Bohr’s complementarity is one central tenet of quantum physics. The paradoxical wave-particle duality of quantum matters and photons has been tested in Young’s double-slit (double-path) interferometers. The object exclusively exhibits wave and particle nature, depending measurement apparatus that can be delayed chosen to rule out too-naive interpretations of quantum complementarity. All experiments to date have been implemented in the double-path framework, while it is of fundamental interests to study complementarity in multipath interferometric systems. Here we demonstrate generalised multipath wave-particle duality in a quantum delayed-choice experiment, implemented by large-scale silicon-integrated multipath interferometers. Single-photon displays sophisticated transitions between wave and particle characters, determined by the choice of quantum-controlled generalised Hadamard operations. We characterise particle-nature by multimode which-path information and wave-nature by multipath coherence of interference, and demonstrate the generalisation of Bohr’s multipath duality relation. Our work provides deep insights into multidimensional quantum physics and benchmarks controllability of integrated photonic quantum technology.
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