Multiport quantum interferometry with universal nonunitary transformations

Quantum interferometry provides a pivotal method for investigating complex photon-photon interaction, enabling various quantum information technologies that beyond the possibilities of classical physics. While such an effective interaction is mostly studied in N × N unitary transformation, m × n nonunitary transformation that relates to lossy multiport quantum interferometry requires a different approach. Here, we experimentally demonstrate a concise yet powerful tool to unravel fundamental features of multiport quantum interferometry with universal nonunitary transformations. To confirm their intriguing unitary and nonunitary properties, Hong-Ou-Mandel (HOM) interference is observed to reveal its nontrivial particle exchange phase, wherein the conventional HOM interference based on unitary transformation and the anomalous HOM interference based on nonunitary transformation can be readily harnessed. These results provide an alternative approach to investigate the quantum light-matter interaction, facilitating the use of complex multiport quantum interference for large-scale quantum information processing tasks such as quantum simulation and quantum computation.