Y. Liu et al., Phys. Rev. Lett. (2013)
(a) Conventional prepare-and-measure QKD setup, in which Alice sends qubits to Bob through an insecure quantum channel, controlled by Eve. (b) MDI-QKD setup: both Alice and Bob send quantum signals to a detection station for a Bell-state measurement. The two signals interfere in a beam splitter and are then sent to two detectors, which reveal publicly which qubit pairs are in the same Bell states (without revealing their values). Such qubits form the secret encryption key shared by Bob and Alice. Even if Eve controls the measurement station, she cannot gain information on the final key without being noticed.