Anomalous quantum beats by coherent absorption of frequency-entangled photons

Quantum beats, the fact that periodic oscillations arise from coherent superposition of frequency-distinguishable states, have enabled various quantum information technologies beyond the possibilities of classical physics. Although the conventional quantum beats are based on unitary optical elements, we present that nonunitary material provides an alternative degree-of-freedom to control the quantum interference and even enables the observation of anomalous quantum beating signals. An elaborate lossy film is used to explore its tunability over the effective photon–photon interaction in frequency modes, which may introduce sophisticated nonunitary realization into integrated optical platforms. In addition, we confirm its ability of harnessing the suppression and enhancement of single- and two-photon absorption by manipulating the symmetric property of frequency entanglement, and this would provide significant benefits in entanglement-assisted absorption spectroscopy. These results show an alternative method to investigate the quantum light–matter interaction, facilitating the use of two-photon interference for quantum light-based spectroscopy and microscopy.