  Quantum communication networks rely on quantum cryptographic protocols
including quantum key distribution (QKD) using single photons. A critical
element regarding the security of QKD protocols is the photon number coherence
(PNC), i.e. the phase relation between the zero and one-photon Fock state,
which critically depends on the excitation scheme. Thus, to obtain flying
qubits with the desired properties, optimal pumping schemes for quantum
emitters need to be selected. Semiconductor quantum dots 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. Our approach provides a viable route toward secure communication in
quantum networks.
