  Recently, a quantum key distribution (QKD) scheme based on entanglement
swapping, called measurement-device-independent QKD (mdiQKD), was proposed to
bypass all detector side-channel attacks. While mdiQKD is conceptually elegant
and offers a supreme level of security, the experimental complexity is
challenging for practical systems. For instance, it requires interference
between two widely separated independent single-photon sources, and the rates
are dependent on detecting two photons - one from each source. Here we
experimentally demonstrate a QKD scheme that removes the need for a two-photon
system and instead uses the idea of a two-qubit single-photon (TQSP) to
significantly simplify the implementation and improve the efficiency of mdiQKD
in several aspects.
