Schematic daigram of four-dimensional time-bin HD-QKD system. (a) Representation of time-bin states (left) and phase states (right). (b) Experimental setup of time-bin HD-QKD system. 1550-nm LD means that Alice prepares signals with 1550-nm laser diode, ATT is the attenuator which is used to attenuate the light to single-photon level, BS is the beam splitter, DI₁, DI₂, and DI₃ are delay-line interferometers which are used to direct different phase states to the corresponding detectors, D t and D₀, D₁, D₂, D₃ are single-photon detectors which are used to detect time-bin states and phase states, respectively. See more details in Ref. [41].