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ACS Photonics

Revealing the Origins of Two-Photon Components in Solid-State Single-Photon Filters

Jiajun Wang, Jian Wang, Bang Ye Wu, Xu-Jie Wang, Xinrui Mao, Zi-Qi Zeng, Li Liu, Hanqing Liu, Haiqiao Ni, Zhichuan Niu, Zhiliang L. Yuan

Published Dec 9, 2025• 0 citations
Publisher

Abstract

Solid-state single-photon filters are pivotal for quantum networks, yet multiphoton contamination fundamentally limits their fidelity. While prior works attributed degradation primarily to residual laser reflection in bare cavities, we employ low-reflectivity micropillar cavities to minimize this background without postselection. Crucially, through dynamic analysis of the two-photon processes, we reveal that multiexcitation events and stimulated emission─rather than the residual laser light─dominate the multiphoton generation. Specifically, when 68 ps coherent light wavepackets with an average photon number of 1 are incident, the output field exhibits a single-photon purity of 0.8. Further analysis identifies that 61.9% of the two-photon component originates from multiple excitations, 37.6% from stimulated emission, and only 0.5% from the residual laser background. Our findings provide critical insights into the atom–photon interface, laying the groundwork for new design strategies toward high-purity quantum light sources.

DOI
10.1021/acsphotonics.5c02014

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Solid-state single-photon filters are pivotal for quantum networks, yet multiphoton contamination fundamentally limits their fidelity. While prior works attributed degradation primarily to residual laser reflection in bare cavities, we employ low-reflectivity micropillar cavities to minimize this background without postselection. Crucially, through dynamic analysis of the two-photon processes, we reveal that multiexcitation events and stimulated emission─rather than the residual laser light─dominate the multiphoton generation. Specifically, when 68 ps coherent light wavepackets with an average photon number of 1 are incident, the output field exhibits a single-photon purity of 0.8. Further analysis identifies that 61.9% of the two-photon component originates from multiple excitations, 37.6% from stimulated emission, and only 0.5% from the residual laser background. Our findings provide critical insights into the atom–photon interface, laying the groundwork for new design strategies toward high-purity quantum light sources.
Scalable quantum networks and photonic quantum technologies require single-photon sources with extremely high purity. In resonantly driven two-level systems typically employed in realizing such sources, photon purity is generally limited by multiphoton emission and residual laser scattering, traditionally treated as independent noise sources. Here, we reveal that residual laser scatter can be coherently engineered to interfere destructively with unwanted multiphoton emission, dramatically enhancing single-photon purity. Through theoretical modeling, we identify optimal excitation conditions where phase-sensitive interference harnesses noise to boost emission purity by several orders of magnitude without sacrificing brightness. These results introduce a new design paradigm for quantum light sources, where scattered light becomes a powerful resource for advancing scalable, high-purity single-photon generation in integrated photonic platforms.
Single atoms or atom-like emitters are the purest source of single photons, they are intrinsically incapable of multi-photon emission. To demonstrate this degree of photon number-state purity we have realized a single-photon source using a single ion trapped at the common focus of high numerical aperture lenses. Our trapped-ion source produces single-photon pulses with without any background subtraction. After subtracting detector dark counts the residual is less than 3 × 10 −4 (95% confidence interval). The multi-photon component of the source light field is low enough that we measure violation of a quantum non-Gaussian state witness, by this characterization the source output is indistinguishable from ideal attenuated single photons. In combination with efforts to enhance collection efficiency from single emitters, our results suggest that single trapped ions are not only ideal stationary qubits for quantum information processing, but promising sources of light for scalable optical quantum networks.
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.
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Resonance fluorescence of natural or artificial atoms constitutes a prime method for generating non-classical light. While most efforts have focused on producing single photons, multi-photon emission is unavoidably present in the resonant driving of an atom. Here, we study the extent to which these processes occur: we quantify the multi-photon emission statistics in a resonantly driven two-level artificial atom—a semiconductor quantum dot in a micropillar cavity—when pumped by short optical pulses. By measuring auto-correlation functions up to the fourth order, we observe up to four photons emitted after a single pumping pulse and investigate the emission dynamics with finely resolved temporal measurements. Furthermore, we propose a method based on acquisition time gating to enhance the purity of a single-photon source while maintaining high efficiencies. Our results deepen the understanding of the photon emission processes in coherently driven atomic systems and suggest a simple but effective technique to reduce the multi-photon components of a single-photon source.
Solid-state sources capable of emitting single photons on demand are of great interest in quantum information applications. Ideally, such a source should emit exactly one photon into the collection optics per trigger, the emitted photons should be indistinguishable, and the source should be electrically driven. Several design strategies addressing these requirements have been proposed. In the cavity-based source, light emission is controlled using resonant cavity quantum electrodynamics effects, whereas in the waveguide-based source, broadband electric field screening effects are employed to direct the light emission into the optical mode of interest. For all the strategies, accurate modeling and careful optical engineering is required to achieve high performance. In this study, we discuss the models and numerical techniques used to analyze such structures. The physical effects governing the light emission profile and the possibilities of tailoring it as well as the mechanisms governing the coherence are elucidated. The major design strategies pursued to optimize the single-photon source performance and the remaining challenges are reviewed.