High-dimensional frequency entanglement filter with anomalous Hong–Ou–Mandel interference

Quantum entanglement is essential for various quantum information technologies that are beyond the possibilities of classical physics. However, it is susceptible to decoherence that may arise from noisy channels and environmental noise. While this deleterious problem has been explored by using filtering techniques, such as photon ancillas and Rydberg atom blockade, a concise yet efficient approach to entanglement retrieval is still a formidable challenge. Here, we present that nonunitary material provides an alternative tool to flexibly control the anomalous Hong–Ou–Mandel (HOM) interference and enables the selective entanglement filter with coherent perfect absorption of anti-symmetric quantum states. An elaborate Cr–SiN–Cr film is prepared as a nonunitary beam splitter such that its tunability over the effective photon–photon interaction in frequency modes can be readily explored in the experiment, which may introduce sophisticated nonunitary realization into integrated optical platforms. In addition, we confirm its ability for selective filtering of high-dimensional frequency entanglement by observing the output quantum state of the conventional HOM interference with a unitary beam splitter and the anomalous HOM interference with a nonunitary beam splitter. These results may offer new insights into investigating the quantum light–matter interaction and facilitating the use of two-photon interference for addressing central challenges in quantum technologies.