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.
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.
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.
Resonant laser excitation of a two-level system with subsequent single-photon emission can be used to generate single photons with high indistinguishability or Hong-Ou-Mandel (HOM) visibility. However, spectral overlap between excitation laser and emitted photons generally poses significant challenges. Furthermore, emitter re-excitation intrinsically limits achievable single-photon purity. Established solutions mitigate these issues at significant cost to source efficiency and with increased source complexity. This motivates the use of few-level systems with spectral separation of excitation and emission pathways. One option is a three-level cascade. However, without targeted lifetime engineering of emitting states, the cascade naturally limits achievable photon indistinguishability. Here we study a semiconductor quantum dot with resonant and selective cavity-enhancement of biexciton-to-exciton transition. Following resonant two-photon excitation of the biexciton state, we collect the emitted single photon with the cavity. This approach circumvents emitter re-excitation and naturally introduces spectral separation of excitation laser and emitted single photon. Supported by first experimental results, we demonstrate theoretically that with selective Purcell enhancement, the observed quality quantifiers of single-photon emission (purity, equivalently g(2)(0), and HOM visibility V, equivalently indistinguishability) are competitive with respect to high-quality deterministic quantum-dot single-photon sources. This is already achieved without systematic optimization or targeted system engineering, which firmly places the reported approach as a viable route to the next generation of highest-quality quantum-dot based deterministic single-photon sources.
Photon blockade is vital for single-photon generation, but current schemes with conventional and unconventional photon blockade face critical limitations like the purity-brightness trade-off, hindering the generation of high-performance single-photons. To overcome these limitations, we introduce a fundamentally new photon blockade mechanism by utilizing three-body interactions between a single photonic mode and two qubits. This kind of interaction intrinsically cuts off the excitation path to the two-photon state, resulting in a perfect photon blockade effect. The mechanism operates across a broad parameter range, free from the constraints of strong coupling or weak driving. Remarkably, it breaks the purity-brightness trade-off, enabling the simultaneous achievement of extreme purity and high brightness, both significantly outperforming previous mechanisms. Furthermore, this approach demonstrates robustness against thermal noise and avoids unwanted oscillations in the time-delayed correlation function. This work provides a path for generating high-purity, high-brightness, and robust single-photon sources, a key resource for quantum technologies.
The generation of high-performance nonclassical light remains a cornerstone of quantum technologies, yet faces a fundamental trade-off between emission purity and brightness. Here, we demonstrate that cavity-mediated many-body spin-exchange interactions provide a route to overcome this constraint by collectively amplifying spectral anharmonicity. In a cavity-coupled atomic array with a programmable relative phase φ, the resulting interference-interaction mechanism reshapes the dressed-state manifold and enables deterministic switching between distinct quantum emission regimes. For φ=0, constructive interference yields high-purity single-photon emission with antibunching improved by four orders of magnitude while preserving strong photon flux. Conversely, for φ=π, destructive interference creates a dark single-photon manifold, resonantly activating two-photon processes to produce bright and pure photon-pair bundles. Our work establishes interference-engineered many-body interactions as a scalable mechanism for on-demand quantum light generation and open a new avenue for harnessing collective many-body physics in quantum photonics.
High-purity and bright single-photon sources are important for quantum information processing and precision measurement. We propose a collectively enhanced universal photon-blockade scheme in a multi-emitter two-photon Tavis--Cummings system. Independent coherent drives of the cavity and collective emitter enable destructive interference between two-photon excitation pathways, while collective coupling supplies level anharmonicity. Full-quantum master-equation simulations, the Holstein--Primakoff approximation, and a non-Hermitian probability-amplitude analysis yield the optimal conditions: cavity resonance together with phase and amplitude matching. Compared with a purely collective blockade, the proposed scheme lowers g(2)(0), and essentially preserves the single-photon population. As the emitter number N increases, the optimal point remains at Δc=0, while the minimum correlation follows gmin(2)(0)∝N−2. By contrast, unconventional photon blockade shifts away from resonance as N increases, limiting its purity improvement and brightness. In the weak-drive regime, universal blockade obeys g(2)(0)∝εa2, owing to a higher-order bypass through the three-excitation manifold. The drive strength therefore provides an additional purity--brightness control, although the interference mechanism makes the scheme more sensitive to dissipation-rate mismatch. Our results establish a scalable route to bright, high-purity, and tunable single-photon emission in multi-emitter cavity-QED systems.
Quantum emitters promise to emit exactly one photon when pumped by a laser pulse. However, even in ideal systems, re-excitation during the laser pulse duration causes the consecutive emission of two photons, thus limiting the single-photon purity. Although the probability and characteristics of re-excitation are largely determined by the excitation scheme, until now, only resonant driving has been studied. Here, we demonstrate qualitative differences for phonon-assisted excitation and resolve the distinct temporal and spectral profile of each of the photons. These uncover correlations between the emission time and wavelength, resulting in an asymmetric two-photon spectrum. On a fundamental level, we show that this spectrum provides direct access to the Rabi frequency of an incoherently driven quantum dot. On the application side, we leverage the asymmetry to selectively suppress multiphoton noise, ensuring high single-photon purity regardless of the laser pulse length and thus enhancing implementations across quantum cryptography and quantum computing.