Quantum communication networks rely on quantum cryptographic protocols including quantum key distribution (QKD) based on single photons. A critical element regarding the security of QKD protocols is the photon number coherence (PNC), i.e., the phase relation between the vacuum and one-photon Fock state. To obtain single photons with the desired properties for QKD protocols, optimal excitation schemes for quantum emitters need to be selected. As emitters, we consider semiconductor quantum dots, that are known to generate on-demand single photons with high purity and indistinguishability. Exploiting two-photon excitation of a quantum dot combined with a stimulation pulse, we demonstrate the generation of high-quality single photons with a controllable degree of PNC. The main tuning knob is the pulse area giving full control from minimal to maximal PNC, while without the stimulating pulse the PNC is negligible in our setup for all pulse areas. Our approach provides a viable route toward secure communication in quantum networks.

量子通信网络依赖于量子加密协议，包括基于单光子的量子密钥分发（QKD）。QKD 协议安全性的一个关键因素是光子数相干性 (PNC)，即真空和单光子 Fock 态之间的相位关系。为了获得具有 QKD 协议所需特性的单光子，需要选择量子发射器的最佳激发方案。作为发射器，我们考虑半导体量子点，众所周知，半导体量子点可以按需产生高纯度和不可区分的单光子。利用量子点的双光子激发与刺激脉冲相结合，我们演示了具有可控 PNC 程度的高质量单光子的生成。主调谐旋钮是脉冲区域，可完全控制从最小到最大 PNC，而如果没有刺激脉冲，PNC 在我们所有脉冲区域的设置中可以忽略不计。我们的方法为量子网络中的安全通信提供了一条可行的途径。