A method of operating an electromagnetic field detector in a wireless telecommunications network, the electromagnetic field detector including a first optical transmitter, a second optical transmitter, and a transmission medium, wherein the first optical transmitter is configured to transmit a probe signal at a probe frequency and the second optical transmitter is configured to transmit a coupling signal at a coupling frequency, wherein the probe frequency is set to excite electrons of the transmission medium from a ground state to a first excited state and 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 such that an incident electromagnetic field at the transmission medium causes a detectable change in the probe signal, characterized in that the method comprises the steps of: transmitting the probe signal and coupling signal within a plurality of interaction regions of the transmission medium such that each interaction region of the plurality of interaction regions switches between a first condition and a second condition, in which: in the first condition of each interaction region, the probe and coupling signals are transmitted within that interaction region, the second condition of each interaction region exists for a time period for which an effect of the coupling signal on the electrons of the transmission medium persists after a switch from the first condition to the second condition, and, and an intensity of the coupling signal is such that: Ω c 2 < Γ 1 Γ 2 in which Ω c is a Rabi frequency of the coupling signal, Γ 1 is the transition rate out of the first excited state of the transmission medium, and Γ 2 is the transition rate out of the predetermined excited state of the transmission medium. transmitting the probe signal and coupling signal within a plurality of interaction regions of the transmission medium such that each interaction region of the plurality of interaction regions switches between a first condition and a second condition, in which: in the first condition of each interaction region, the probe and coupling signals are transmitted within that interaction region, the second condition of each interaction region exists for a time period for which an effect of the coupling signal on the electrons of the transmission medium persists after a switch from the first condition to the second condition, and, and an intensity of the coupling signal is such that: Ω c 2 < Γ 1 Γ 2 in the first condition of each interaction region, the probe and coupling signals are transmitted within that interaction region, the second condition of each interaction region exists for a time period for which an effect of the coupling signal on the electrons of the transmission medium persists after a switch from the first condition to the second condition, and, and an intensity of the coupling signal is such that: Ω c 2 < Γ 1 Γ 2 in which Ω c is a Rabi frequency of the coupling signal, Γ 1 is the transition rate out of the first excited state of the transmission medium, and Γ 2 is the transition rate out of the predetermined excited state of the transmission medium.