摘要:

Resonance fluorescence from a coherently driven two-level emitter combines photon antibunching with laser-like spectral coherence - a seemingly paradoxical coexistence that has recently reignited debate. Using an InAs quantum dot embedded in a micropillar cavity [1], we revisit this phenomenon through a pure-state description [2] that treats the emitter and its radiative field as a single coherent quantum system. In this framework, resonance fluorescence emerges as a stream of phase-locked, lifetime-limited single photons whose mutual interference gives rise to sub-natural linewidths, with experimentally verified predictions for both first- and second-order coherence. The same pure-state framework also accounts for the recently observed tunable high-order coherence in the interference between resonance fluorescence and laser light [3], and provides a unified description of spontaneous emission, coherence, and quantum interference in detuned resonance fluorescence [4].We further show that these quantum correlations can be converted into energy-time entanglement using only passive linear optics, leading to violation of the CHSH Bell inequality [5,6]. Finally, by operating the same device platform with dark-state biexciton loading, we realise a genuine two-photon emitter with a two-photon fraction of 98.3% [7].

[1] B. Wu et al., “Mollow triplets under few-photon excitation,” Optica 10, 1018 (2023).

[2] X.-J. Wang et al., “Coherence in resonance fluorescence,” Nat. Commun. 16, 6453 (2025).

[3] H.-L. Yin et al., “Tunable high-order coherence in the interference of resonance fluorescence and laser light,” Opt. Lett. 51, 2616 (2026).

[4] G Huang et al, “Two-photon interference between mutually detuned resonance-fluorescence signals scattered off a semiconductor quantum dot,” Phys. Rev. A 113, 023718 (2026).

[5] Z.-Q. Zeng et al., “Bell inequality violation with vacuum-one-photon number superposition states,” arXiv:2511.15413.

[6] J. Wang et al., “Engineering energy-time entanglement from resonance fluorescence,” arXiv:2603.25341.

[7] B. Wu et al., “Purcell-enhanced two-photon emission from a quantum dot via dark-state biexciton loading,” Nat. Mater. 25, 595 (2026).