  The semiclassical electron trajectory, the so-called quantum orbits, is
employed to explain the terahertz wave generation (TWG) in dual-color strong
field, and the feasibility of the theory is validated by the measurement. We
find that TWG stems from quantum path interference of partial electron
wavepacket released at the neighbouring cycles of the dual-color electric
field, manifesting the temporal Young's double-slit interference of single
electron. The trajectories released from neighbouring cycles, creating TWG,
also account for intercycle interference fringes in the photoelectron momentum
distribution, whereas no signature of TWG is found when taking trajectories
only from a single cycle.
