Recently, the quantum dot quantum computing team led by Hu Chengyong at the Beijing Quantum Information Science Academy (hereinafter referred to as "the Academy") has realized a novel quantum light source-a laser-converted single-photon source-by utilizing the saturated nonlinear effect and single-photon switching effect of single quantum dots. This source exhibits an ultralong coherence time (258±2 microseconds) and robust photon homogeneity, with single-photon performance reaching the optimal level of conventional spontaneous emission-based single-photon sources. It holds promise as a standard quantum light source for quantum internet applications. On November 18, 2025, the research findings were published in Optica under the title "Converting laser light into single photons with ultralong coherence time."
Photons serve as ideal carriers for quantum information transmission and crucial vehicles for quantum information processing. Single-photon sources form the core components of quantum technologies such as optical quantum computing, distributed quantum computing, quantum communication, and quantum precision measurement. Currently, single-photon source preparation primarily relies on two technical approaches: one is probabilistic methods based on spontaneous parametric down-conversion (SPDC) or spontaneous four-wave mixing (SFWM); the other being deterministic methods based on spontaneous emission from single-quantum systems, such as cold atoms, ion traps, quantum dots, or color centers. In recent years, emission-type quantum dot single-photon sources have made significant progress toward achieving ideal single-photon sources, exhibiting nearly 100% single-photon purity and photon identity. However, emission-based single-photon sources still face limitations: constrained by twice the exciton lifetime, their first-order coherence time is extremely short (only tens to hundreds of picoseconds), and photon identity is susceptible to degradation from charge noise and spin noise. Future quantum internet development relies on coherent quantum communication based on two-photon or single-photon interference, demanding single-photon sources with excellent coherence and robust photon identity. Emission-based single-photon sources currently struggle to fully meet these requirements. Although lasers inherently possess outstanding coherence, they cannot be directly attenuated to single-photon states using linear optical elements.
To address these challenges, the research team collaborated with the Institute of Semiconductors, Chinese Academy of Sciences, proposing and realizing a third method for single-photon source preparation: the laser conversion-based single-photon source (LCSPS). Unlike the traditional single-sided optical microcavity structures commonly used in emission-type single-photon sources, the team designed a symmetric double-sided optical microcavity [see Figure 1(a)]. This structure effectively suppresses laser scattering within the cavity without relying on orthogonal polarization filters. After reflection within the quantum dot-microcavity coupling system, the laser is directly converted into a single photon [see Figure 1(a)], exhibiting the following outstanding properties: an ultra-long coherence time [258±2 μs, see Fig. 2(b)], robust photon indistinguishability [94.3±0.2%, see Fig. 2(c)], and perfect single-photon purity [g(2)(0)=0.030±0.002, see Fig. 1(e)]. All data represent raw measurement results.
The operating principle of the laser-converted single-photon source can be qualitatively explained based on the saturated nonlinearity and single-photon switching effects of single quantum dots: When a single photon interacts with and is reflected by the quantum dot, subsequent incident photons are transmitted within the exciton lifetime due to the quantum dot entering a saturated state. This process causes the reflected light to exhibit anti-coherence behavior, displaying single-photon characteristics, while the transmitted light exhibits coherence effects, possessing multi-photon properties. The underlying deep physical mechanism stems from quantum interference between coherent states (i.e., laser) and multi-photon states. This interference process effectively suppresses the probability of multi-photon components appearing in the reflected light field, transforming the reflected laser light field into single photons.
Inheriting the first-order coherence and robust photon identicity of lasers, laser-converted single-photon sources can be widely applied in various interference-based quantum communication protocols, single-photon phased-array quantum radars, and mode-locked single-photon sources. They hold promise as the standard quantum light source for future quantum internet.
Figure 1
(a) Schematic of the structure and operating principle of the laser-converted single-photon source; (b) Scanning electron microscope image of the device; (c) Coherent reflection spectra at different drive intensities, demonstrating a 50:1 single-photon switching ratio; (d) Zero value g(2)(0) of the second-order correlation function of the reflected light field as a function of laser detuning; (e) Second-order correlation function g(2)(t) of the reflected light field at low drive intensities.
Figure 2 (a) First-order coherence of the single-photon source characterized by Mach-Zehnder interferometry; (b) Demonstration that the laser-conversion-type single-photon source shares the same coherence time as the driving laser, achieved through delayed heterodyne interferometry and time-resolved coincidence measurements; (c) Evolution of two-photon interference visibility with emission time difference, proving the source's robust photon homogeneity.
The first authors of this paper are Wang Mannan and Li Yanfeng, doctoral students at the Institute of Quantum Information, with corresponding author Hu Chengyong, a researcher at the same institute. Co-authors include Zeng Chuanyu, a doctoral student at the Institute of Quantum Information; Huang Guoqi, a doctoral student at Beijing University of Posts and Telecommunications; engineers Liu Li, Wang Wenyan, and Ji Weijie from the Institute of Quantum Information; as well as postdoctoral researcher Liu Hanqing, researchers Ni Haiqiao and Niu Zhichuan from the Institute of Semiconductors, Chinese Academy of Sciences. This work was supported by the Beijing Natural Science Foundation and the National Key R&D Program of China.
