In the embodiments of the present invention, a transmit optical signal includes a reference optical signal and a quantum optical signal, optical splitting processing and coherent coupling are performed on the transmit optical signal by using a local oscillator optical signal to obtain at least two coherently coupled optical signals, and then optical-to-electrical conversion and amplification are separately performed on a first coherently coupled optical signal that includes the reference optical signal and a second coherently coupled optical signal that includes the quantum optical signal, to obtain a first electrical signal and a second electrical signal. Then, phase frequency information between the local oscillator optical signal and the reference optical signal is obtained from the first electrical signal, and an original key is recovered from the second electrical signal based on the phase frequency information.