Resonance fluorescence arises from the coherent interaction of laser light with a two-level system and constitutes a quantum light source exhibiting single-photon emission and antibunching, making it a fundamental building block for future quantum networks. Our recent work [Nat. Commun.16, 6453 (2025)10.1038/s41467-025-61884-x] demonstrates that under continuous-wave excitation and in the absence of pure dephasing, the single-time joint state of resonance fluorescence and the two-level system can be described by a pure entangled state. Here, we analyze the cross-correlation and auto-correlation properties of the quantum optical field generated by the interference of resonance fluorescence with laser light at the resonance excitation frequency. Simulation results indicate that by varying the intensity or phase of the interfering laser light, the higher-order coherence of the post-interference quantum optical field transitions from anti-bunching to super-bunching behavior, consistent with recent theoretical and experimental advances by mean-field engineering. This approach offers a complementary framework to characterize and manipulate the multiphoton quantum optical fields.
Resonance fluorescence arises from the coherent interaction of laser light with a two-level system and constitutes a quantum light source exhibiting single-photon emission and antibunching, making it a fundamental building block for future quantum networks. Our recent work [Nat. Commun.16, 6453 (2025)10.1038/s41467-025-61884-x] demonstrates that under continuous-wave excitation and in the absence of pure dephasing, the single-time joint state of resonance fluorescence and the two-level system can be described by a pure entangled state. Here, we analyze the cross-correlation and auto-correlation properties of the quantum optical field generated by the interference of resonance fluorescence with laser light at the resonance excitation frequency. Simulation results indicate that by varying the intensity or phase of the interfering laser light, the higher-order coherence of the post-interference quantum optical field transitions from anti-bunching to super-bunching behavior, consistent with recent theoretical and experimental advances by mean-field engineering. This approach offers a complementary framework to characterize and manipulate the multiphoton quantum optical fields.
Resonance fluorescence of a two-level emitter displays persistently anti-bunching irrespective of the excitation intensity, but inherits the driving laser’s linewidth under weak monochromatic excitation. These properties are commonly explained in terms of two disjoined pictures, i.e., the emitter’s single photon saturation or passively scattering light. Here, we propose a unified model that treats all fluorescence photons as spontaneous emission, one at a time, and can explain simultaneously both the spectral and correlation properties of the emission. We theoretically derive the excitation power dependencies, measurable at the single-photon incidence level, of the first-order coherence of the whole resonance fluorescence and super-bunching of the spectrally filtered, followed by experimental confirmation on a semiconductor quantum dot micro-pillar device. Furthermore, our model explains peculiar coincidence bunching observed in phase-dependent two-photon interference experiments. Our work provides an intuitive understanding of coherent light-matter interaction and may stimulate new applications. Standard descriptions of resonance fluorescence consider either absorption and re-emission in a single-photon picture or passive scattering. This study shows that resonance fluorescence can be described by a pure quantum state of the emitter and photon, revealing a coherent superposition that challenges the conventional view of spontaneous emission as an incoherent process.
Mixing the fields generated by different light sources has emerged as a powerful approach for engineering non-Gaussian quantum states. Understanding and controlling the resulting photon statistics is useful for emerging quantum technologies that are underpinned by interference. In this work, we investigate intensity correlation functions arising from the interference of resonance fluorescence from a quantum emitter with a coherent laser field. We show that the observed bunching behavior results from a subtle interplay between quantum interference and the normalization of the correlation functions. We show that by adjusting the mixing ratio and phase, one can achieve full tunability of the second-order correlation, ranging from anti-bunching to bunching. We further extend our analysis to third-order correlation functions, both experimentally and theoretically, to provide new insights into the interpretation of higher-order correlations and offer practical tools for shaping quantum optical fields.
Resonance fluorescence, the process in which a single two-level quantum emitter interacts with a near-resonant coherent light, is a cornerstone of quantum optics as a paradigmatic light-matter interaction. Quantum phenomena in resonance fluorescence have been accounted for in terms of one- and two-photon processes; however, the physical processes arising from the interaction of a single two-level atom with higher photon numbers remain unexplored experimentally. Here we experimentally reveal the multiphoton scattering nature of resonance fluorescence, which intrinsically gives rise to multiphoton entanglement. By accessing quantum fluctuations in the field emitted from a cavity-quantum electrodynamics system, we resolve third- and fourth-order photon scattering events from a single two-level atom. In the three-photon component, the scattered photons are shown to be genuinely energy-time entangled, as verified by a violation of Svetlichny's inequality. As a demonstration of an application, a quantum secret sharing protocol (QSS) is implemented using this entangled photon source. Our study refines the descriptive framework of resonance fluorescence and broadens the scope of its investigation. It also establishes a fundamentally simple route for generating multiphoton entangled states for quantum information processing and quantum metrology.
Resonance fluorescence from a two-level emitter is a standard route to single-photon generation, where the emitter produces only one photon at a time, and multiphoton emission or scattering is inherently suppressed. In this study, we challenge this paradigm in the weak-excitation limit, where coherent scattering dominates. Using a single quantum dot in a microcavity, we show that the incoherent component exhibits pronounced superbunching, revealing simultaneous multiphoton scattering from a single emitter. In contrast, the total reflected field—comprising the reflected driving field and incoherent scattering—displays perfect antibunching, arising from quantum interference that cancels two-photon probability amplitudes. By spectrally filtering the reflected field, we tune the relative contributions of coherent and incoherent components, observing transitions in photon statistics from antibunching to superbunching, or to Poissonian light. Supported by master-equation simulations and an interference-based model, these results uncover the hidden but fundamental role of multiphoton dynamics in resonance fluorescence, and open avenues for generating nonclassical light in cavity and waveguide quantum electrodynamics.
Abstract Frequency-resolved photon statistics of resonance fluorescence generated from a two-level system driven by a strong laser field and a weak laser field with equal frequencies are studied. The frequency resolution of fluorescent radiation is described by quantum filtering dynamics, which is simulated theoretically by two single-mode quantum optical cavities with tunable frequencies to scan the incident fluorescent radiation. By calculating the two-photon intensity–intensity correlation functions in terms of the cavity modes, we demonstrate that two-color strong correlations of resonance fluorescence can be generated not only between the opposite sidebands, but also between the central band and one of the sidebands: although both sidebands are broadened due to the perturbation of the weak laser field on the strong-field dressed atom. We emphasize that these properties are in contrast to the conventional case of the standard single-atom Mollow triplet. Moreover, if the resonance frequencies of the two filtering cavities are tuned appropriately, broadband two-color strong correlations are predicted, and the physical origin is revealed from the perspective of quantum interference of photon emission dynamics. This can be considered as a feasible scheme for the design of broadband non-classical light sources, and may be beneficial to the quantum precise detection of atomic and molecular dynamics via quantum optical spectroscopy.
Abstract Resonance fluorescence—the light emitted when exciting resonantly a two-level system—is a popular quantum source as it seems to inherit its spectral properties from the driving laser and its statistical properties from the two-level system, thus providing a subnatural-linewidth single-photon source (SPS). However, these two qualities do not actually coexist in resonance fluorescence, since an optical target detecting these antibunched photons will either be spectrally broad itself and not benefit from the spectrally narrow source, or match spectrally with the source but in this case the antibunching will be spoiled. We first explain this failure through a decomposition of the field-emission and how this gets affected by frequency resolution. We then show how to restore the sought joint subnatural linewidth and antibunched properties, by interfering the resonance fluorescence output with a coherent beam. We finally discuss how the signal that is eventually generated in this way features a new type of quantum correlations, with a plateau of antibunching which suppresses much more strongly close photon pairs. This introduces a new concept of perfect SPS.