Fig. 1. (a) Energy level diagram of Rydberg atoms; (b) Schematic diagram of the experimental setup

Fig. 2. EIT-AT splitting caused by increased microwave intensity^[42]

Fig. 3. (a) Basic principle of the Rydberg-atom superhet^[24]; (b) Linear versus nonlinear detections of three measurement schemes^[24]

Fig. 4. Rydberg atomic many-body systems with spatially ordered excitations. (a) Examples of false-colour fluorescence images; (b) Histograms of the spatial distribution of Rydberg atoms; (c) Theoretical prediction from numerical simulations^[38]

Fig. 5. The probe transmission withΔC under different coupling Rabi frequencies^[39]

Fig. 6. Optical transmission spectra with (red) and without (black) phase transition^[5], where the dased lines A and B show the maximum slopes near the half-transmission

Fig. 7. Principle of single-body (top) and many-body (bottom) Rydberg metrology^[5]. (a) Energy diagram for a four-level atom model; (b) The spectrum with and without the external MW field; (c) Many-body advantage corresponds to a metrological ruler with thinner tick marks than in a single-body case^[5]

Fig. 8. Change in transmission spectra by application of MW fields^[5]. (a) Single-body; (b) Many-body Rydberg system

Fig. 9. Experimental scheme^[41]. (a) Conventional linear sensors; (b) Stochastic resonance sensor