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 ). <EOS>