A method of controlling an electromagnetic field detector, wherein the electromagnetic field detector is configured to: transmit a probe signal at a probe frequency in a first probe signal path and a second probe signal path to an optical receiver, wherein the first probe signal path passes through a transmission medium and the probe frequency is set to excite electrons of the transmission medium from a ground state to a first excited state; transmit a coupling signal at a coupling frequency in a first coupling signal path, wherein the first coupling signal path overlaps with the first probe signal path in the transmission medium in a first overlapping section, wherein the coupling frequency is set to excite electrons of the transmission medium to a predetermined excited state so as to induce an Electromagnetic Induced Transparency, EIT, effect in the transmission medium such that an incident electromagnetic field at the transmission medium causes a change in refractive index in the transmission medium at the first overlapping section such that there is a change in an optical path length difference between the first probe signal path and the second probe signal path; combine the first path of the probe signal, following its passage of the first overlapping section of the transmission medium, with the second path of the probe signal, such that the incident electromagnetic field at the first overlapping section is detectable, at the optical receiver, as a change in the intensity of the combined first and second paths of the probe signal caused by the change in the optical path length difference between the first probe signal path and the second probe signal path, the method characterised by: causing a first phase shift to be applied to the second probe signal path so as to cause a variation in the change in intensity of the combined first and second paths of the probe signal; obtaining data indicating a second phase shift to be applied to the second probe signal path so as to increase the change in intensity of the combined first and second paths of the probe signal, wherein the second phase shift is determined by monitoring the variation in the change in intensity of the combined first and second paths of the probe signal in response to application of the first phase shift; and causing the second phase shift to be applied to the second probe signal path. transmit a probe signal at a probe frequency in a first probe signal path and a second probe signal path to an optical receiver, wherein the first probe signal path passes through a transmission medium and the probe frequency is set to excite electrons of the transmission medium from a ground state to a first excited state; transmit a coupling signal at a coupling frequency in a first coupling signal path, wherein the first coupling signal path overlaps with the first probe signal path in the transmission medium in a first overlapping section, wherein the coupling frequency is set to excite electrons of the transmission medium to a predetermined excited state so as to induce an Electromagnetic Induced Transparency, EIT, effect in the transmission medium such that an incident electromagnetic field at the transmission medium causes a change in refractive index in the transmission medium at the first overlapping section such that there is a change in an optical path length difference between the first probe signal path and the second probe signal path; combine the first path of the probe signal, following its passage of the first overlapping section of the transmission medium, with the second path of the probe signal, such that the incident electromagnetic field at the first overlapping section is detectable, at the optical receiver, as a change in the intensity of the combined first and second paths of the probe signal caused by the change in the optical path length difference between the first probe signal path and the second probe signal path, the method characterised by: causing a first phase shift to be applied to the second probe signal path so as to cause a variation in the change in intensity of the combined first and second paths of the probe signal; obtaining data indicating a second phase shift to be applied to the second probe signal path so as to increase the change in intensity of the combined first and second paths of the probe signal, wherein the second phase shift is determined by monitoring the variation in the change in intensity of the combined first and second paths of the probe signal in response to application of the first phase shift; and causing the second phase shift to be applied to the second probe signal path. causing a first phase shift to be applied to the second probe signal path so as to cause a variation in the change in intensity of the combined first and second paths of the probe signal; obtaining data indicating a second phase shift to be applied to the second probe signal path so as to increase the change in intensity of the combined first and second paths of the probe signal, wherein the second phase shift is determined by monitoring the variation in the change in intensity of the combined first and second paths of the probe signal in response to application of the first phase shift; and causing the second phase shift to be applied to the second probe signal path.