Emergence of\nMultiphoton Quantum Coherence by Light\nPropagation

The modification of the quantum properties\nof coherence\nof photons\nthrough their interaction with matter lies at the heart of the quantum\ntheory of light. Indeed, the absorption and emission of photons by\natoms can lead to different kinds of light with characteristic quantum\nstatistical properties. As such, different types of light are typically\nassociated with distinct sources. Here, we report on the observation\nof the modification of quantum coherence of multiphoton systems in\nfree space. This surprising effect is produced by the scattering of\nthermal multiphoton wavepackets upon propagation. The modification\nof the excitation mode of a photonic system and its associated quantum\nfluctuations result in the formation of different light fields with\ndistinct quantum coherence properties. Remarkably, we show that these\nprocesses of scattering can lead to multiphoton systems with subshot-noise\nquantum properties. Our observations are validated through the nonclassical\nformulation of the emblematic van Cittert–Zernike theorem.\nWe believe that the possibility of producing quantum systems with\nmodified properties of coherence, through linear propagation, can\nhave dramatic implications for diverse quantum technologies.