A microwave-to-optical photon transducer for providing coupling between at least a microwave signal (S in2) and an optical signal (S pi_in1, S pi_out1), the transducer comprising: • a first input port for receiving the optical signal (S pi_in1), • a second input port for receiving the microwave signal (S in2); • a first output port for outputting the optical signal (S pi_out1) and one or more optical sideband signals (S out1, S out11, S out12); • a first waveguide disposed between the first input port and the first output port to allow the optical signal (S pi_in1) and the one or more optical sideband signals (S out1, S out11, S out12) to propagate in the first waveguide; • a second waveguide connected to the second input port, and extending in the transducer adjacent to the first waveguide to allow the microwave signal (S in2) to propagate in the second waveguide; • a phase-matching arrangement to cause at least the optical signal (S pi_in1) in the first waveguide and the microwave signal (S in2) in the second waveguide to be phase-matched or quasi-phase-matched, wherein the first waveguide is made of, or the first waveguide comprises non-linear material such that a first electromagnetic field generated by the optical signal (S pi_in1) in the first waveguide and a second electromagnetic field generated by the microwave signal (S in2) in the second waveguide are arranged to interact and overlap in the non-linear material, the microwave-to-optical photon transducer being characterised in that the microwave-to-optical photon transducer is configured to use travelling waves for mixing the microwave signal (S in2) and the optical signal (S pi_in1, S pi_out1) to allow the one or more optical sideband signals (S out1, S out11, S out12) to be sensed at the first output port to allow the power of the microwave signal (S in2) at the second input port to be determined based on the sensed one or more sideband signals (S out1, S out11, S out12). • a first input port for receiving the optical signal (S pi_in1), • a second input port for receiving the microwave signal (S in2); • a first output port for outputting the optical signal (S pi_out1) and one or more optical sideband signals (S out1, S out11, S out12); • a first waveguide disposed between the first input port and the first output port to allow the optical signal (S pi_in1) and the one or more optical sideband signals (S out1, S out11, S out12) to propagate in the first waveguide; • a second waveguide connected to the second input port, and extending in the transducer adjacent to the first waveguide to allow the microwave signal (S in2) to propagate in the second waveguide; • a phase-matching arrangement to cause at least the optical signal (S pi_in1) in the first waveguide and the microwave signal (S in2) in the second waveguide to be phase-matched or quasi-phase-matched, wherein the first waveguide is made of, or the first waveguide comprises non-linear material such that a first electromagnetic field generated by the optical signal (S pi_in1) in the first waveguide and a second electromagnetic field generated by the microwave signal (S in2) in the second waveguide are arranged to interact and overlap in the non-linear material, the microwave-to-optical photon transducer being characterised in that the microwave-to-optical photon transducer is configured to use travelling waves for mixing the microwave signal (S in2) and the optical signal (S pi_in1, S pi_out1) to allow the one or more optical sideband signals (S out1, S out11, S out12) to be sensed at the first output port to allow the power of the microwave signal (S in2) at the second input port to be determined based on the sensed one or more sideband signals (S out1, S out11, S out12).