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Optica

Generalized photon-bunching bounds in linear and non-linear quantum optical dynamics

Tommaso Francalanci, Giovanni Rodari, Eugenio Caruccio, Francesco Hoch, Gonzalo Carvacho, Taira Giordani, Nicolò Spagnolo, Riccardo Albiero, Niki Di Giano, Giacomo Corrielli, Francesco Ceccarelli, Roberto Osellame, Ulysse Chabaud, Fabio Sciarrino

Published Apr 15, 2026• 0 citations
Publisher

Abstract

To assess the capabilities of photonic quantum systems, it is essential to develop analytical and experimental methods for probing the underlying optical dynamics. Here, we investigate fundamental constraints arising when multi-photon states evolve through a quantum linear optical network, showing how such limitations can be overcome when introducing measurement-induced nonlinearities. Specifically, we analyze bounds related to photon-bunching behavior at the output of a linear interferometer and test them via a hybrid photonic platform realizing experiments with up to four photons. When the analysis is extended to a nonlinear regime, such bounds can be violated. We experimentally validate such behavior by employing an optical architecture featuring measurement-induced nonlinearity, thus introducing dynamics that go beyond standard linear optics. Our results highlight how the constraints implied by the linear optical formalism can be used as tools to test and characterize photonic models going beyond quantum linear optical dynamics.

DOI
10.1364/optica.579537

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This paper investigates a critical challenge at the intersection of quantum optics and fundamental physics: the strict physical constraints governing Bell state measurement (BSM) efficiency within linear-optical frameworks. Without the integration of ancilla photons, conventional setups relying on standard linear-optical components are bounded by a fundamental efficiency threshold that cannot be exceeded. We conduct an analytical analysis of the physical roots of this barrier using Hong-Ou-Mandel photon interference phenomena and multi-photon state space geometry. Furthermore, this study models advanced physical techniques to break past this threshold, focusing on non-linear optical mediums—such as parametric down-conversion setups—and polarization-entangled ancilla flying qubits. These analytical insights deliver foundational physical parameters necessary for engineering fault-tolerant optical quantum computing networks and scalable quantum internet repeaters.