Measuring plasma fields in Rydberg based MW sensor

In this work, we report measurements of plasma-generated electric fields using an electromagnetically induced transparency (EIT) scheme in a four-level atomic ladder system. The scheme employs the electric-dipole-forbidden 41D_5/2 → 39G_9/2 microwave transition, which becomes allowed in the presence of a DC electric field. The atomic states are coupled as follows: a 780 nm probe beam connects the ground state 5S_1/2 to the intermediate state 5P_3/2; a 480 nm coupling beam connects 5P_3/2 to the Rydberg state 41D_5/2; and a microwave field at around 71.212 GHz couples 41D_5/2 to 39G_9/2. The forbidden nature of this microwave transition is overcome by the plasma electric field, which mixes opposite-parity states. We recorded the 41D_5/2 → 39G_9/2 spectra as a function of coupling beam intensity, observing a distinct resonance shift. This shift is attributed to the DC Stark effect on the high-ℓ 39G_9/2 state induced by the plasma electric field. A theoretical model to quantitatively reproduce experimental data is currently under development. Preliminary analysis, however, indicates a measurement sensitivity to electric fields as low as 10 mV/cm. This approach demonstrates potential for applications in microwave quantum metrology and precision electric field sensing.