The experiment, published in Nature Photonics by Xiao-Long Zhou, Jian Wang, Chuan-Feng Li, Guang-Can Guo and their colleagues, builds on a theoretical picture that dates back to the work of Walter Heitler and was later refined by Jean Dalibard and Serge Reynaud in the 1980s. In this framework, resonance fluorescence is understood not as a stochastic sequence of independent emission events but as a scattering problem: an incoming coherent field interacts with a two-level system, and the outgoing light is a superposition of scattering amplitudes involving different photon numbers. When the driving laser is weak enough that the atom is almost always in its ground state, a regime known as the Heitler regime, the elastic component of the scattered light dominates, and the residual inelastic part consists of rare, well-separated multiphoton events. It is in these rare events that the quantum correlations live.

What makes the new result so striking is the sheer scale of the multiphoton processes involved. Previous experiments had observed the simultaneous scattering of two photons by a single two-level atom, and had even shown that the resulting photon pairs violate a Bell inequality. But the third and fourth orders of the scattering hierarchy had never been resolved experimentally. To reach them, the Hefei team employed a cavity quantum electrodynamics system in which a single atom of rubidium-87 is coupled to an optical cavity. The cavity serves two crucial purposes. First, the Purcell effect enhances the emission rate into well-defined spatial and temporal modes, making the rare multiphoton events easier to collect. Second, the cavity environment allows the researchers to access the quantum fluctuations of the emitted field with sufficient signal-to-noise ratio to resolve third- and fourth-order photon correlations, quantities known in the trade as third- and fourth-order correlation functions.

The technical challenge of extracting these correlations cannot be overstated. The scattered light from a single atom is extraordinarily faint, and it sits on top of an enormous coherent background: the elastically scattered component of the driving laser itself, which can exceed the inelastic signal by many orders of magnitude. The team tackled this with a combination of mean-field suppression and narrowband spectral filtering, using a notch filter to carve out the coherent peak while preserving the inelastic fluorescence. The researchers systematically studied how the zero-delay photon correlations evolved as they varied the mean-field suppression ratio and the linewidth of the notch filter, mapping out the experimental conditions under which the genuine multiphoton components become visible. This careful parameter-space exploration is what allowed them to separate the true three- and four-photon scattering events from artifacts of the filtering process.

With the three-photon component isolated, the team turned to the central question: are the three photons entangled, or merely correlated in some classical way? The answer came from a test of Svetlichny’s inequality, formulated in 1987 by George Svetlichny as a Bell-type inequality capable of distinguishing genuine tripartite nonlocality from scenarios in which only two of the three particles are entangled while the third is classically correlated. A violation of this inequality is a far stronger statement than an ordinary Bell test. It certifies that the three photons cannot be described even by a hybrid model in which one particle is allowed arbitrary classical correlations with a genuinely entangled pair. The scattered photons from the single rubidium atom passed this test, demonstrating genuine energy-time entanglement among three photons born from the interaction of a single quantum emitter with a classical driving field.

The energy-time entanglement itself is verified using Franson interferometry, an elegant technique proposed by John Franson in 1989 in which each photon is sent through an unbalanced interferometer whose path-length difference exceeds the coherence time of the photons but is much shorter than the entanglement’s correlation time. Because each photon’s detection time is ambiguous between the two interferometer paths, the joint detection probabilities acquire phase-dependent interference fringes whose visibility, when combined across different phase settings, yields the Svetlichny parameter. The construction and optimization of the Franson interferometer was a significant experimental undertaking, and the team’s ability to stabilize three such interferometers simultaneously while collecting threefold coincidence events from a single-atom source represents a considerable feat of quantum optical engineering.

To demonstrate that this exotic entanglement source is more than a laboratory curiosity, the researchers implemented a quantum secret sharing protocol, a cryptographic primitive first proposed theoretically by Mark Hillery, Vladimír Bužek and André Berthiaume in 1999 and first demonstrated experimentally by Wolfgang Tittel, Hugo Zbinden and Nicolas Gisin in 2001. In secret sharing, a dealer distributes a secret among several parties such that only authorized coalitions can reconstruct it. Using their triply entangled photons, the team showed that the measurement correlations could be used to distribute a secret among three parties in a way that no two of them alone could access, providing a concrete application example that connects the fundamental physics of single-atom scattering to the practical toolkit of quantum communication.

The conceptual implications extend well beyond the immediate demonstration. Resonance fluorescence has long been treated as a source of single photons and, more recently, of photon pairs, with its multiphoton content regarded as a negligible nuisance. The new work inverts that perspective: the multiphoton scattering processes are not only present but are the very origin of the entanglement, and they can be deliberately accessed and exploited. This establishes what the authors describe as a fundamentally simple route to multiphoton entangled states. Where conventional sources of tripartite entanglement rely on nonlinear crystals, parametric down-conversion cascades or complex multi-emitter arrangements, a single two-level atom driven by a laser now suffices, provided the experimenter knows how to filter and correlate the output correctly. The simplicity of the underlying physical system, combined with the intrinsic determinism of the scattering process, suggests potential advantages for quantum information processing and quantum metrology, where bright, stable and well-characterized entangled photon sources remain a bottleneck.

The work also refines the descriptive framework of resonance fluorescence itself, broadening the scope of a field that has been a cornerstone of quantum optics since the pioneering antibunching experiments of Kimble, Dagenais and Mandel in 1977. What began as a paradigmatic example of light-matter interaction, studied first with single atoms and later with artificial atoms such as quantum dots and superconducting circuits, is now revealed to contain a rich multiphoton structure that previous generations of experiments simply could not resolve. As cavity quantum electrodynamics technology matures and filtering techniques improve, the higher orders of the scattering hierarchy become accessible one by one, and each new order brings with it new forms of quantum correlation. The Hefei team’s demonstration that a single atom can serve as the birthplace of genuinely tripartite nonlocal entanglement is likely to inspire a new wave of experiments probing the multiphoton frontier, both in atomic systems and in the solid-state emitters that promise scalability. For a phenomenon once considered fully understood, resonance fluorescence has just shown that it still has profound surprises to offer.

Subject of Research: Multiphoton entanglement generated by resonance fluorescence of a single rubidium-87 atom in a cavity quantum electrodynamics system

Article Title: Observation of multiphoton entanglement in resonance fluorescence

Article References: Zhou, X.-L., Wang, J., Shen, Z.-M., Huang, D.-Y., He, S.-J., Huang, Q.-Y., Liu, Y.-J., Chen, Y.-S., Jiang, Q., Li, C.-F., & Guo, G.-C. (2026). Observation of multiphoton entanglement in resonance fluorescence. Nature Photonics. https://doi.org/10.1038/s41566-026-02022-x

Image Credits: AI Generated

DOI: 10.1038/s41566-026-02022-x

Keywords: resonance fluorescence, multiphoton entanglement, single atom, cavity quantum electrodynamics, Svetlichny inequality, energy-time entanglement, quantum optics, quantum secret sharing, rubidium-87, photon scattering, quantum information, Franson interferometry