A system for extracting data from a plurality of electromagnetic data signals encoding the data is provided. The system includes a phase modulator which receives an electromagnetic beam, and generates an electromagnetic primary reference beam with a defined phase. The system further includes one or more mixing units, each mixing unit being arranged to receive a respective one of the data signals and a reference beam, and to generate two mixed signals. The one or more mixing units include a first mixing unit for which the reference beam is the primary reference beam. The system also includes a plurality of detection units. Each detection unit is arranged to receive a respective one of the data signals and a respective mixed signal from one of the mixing units, and to obtain a difference measurement indicative of a difference between the respective data signal and the respective mixed signal. The difference measurements generated by the detection units are received by a summation unit, which obtains a summed difference value indicative of the sum of the difference measurements.