Fig. 1. (a) Energy level diagram. The level system for Rb⁸⁵ atoms used in the experiment consists of a ground state 5S_1/2, an intermediate state 5P_3/2, and the Rydberg states 58D_5/2, 57F_7/2, and 59P_3/2. A probe laser of 780 nm couples the transition 5S_1/2 to 5P_3/2, and a coupling laser drives the transition 5P_3/2 to 58D_5/2. The resonance frequencies of the Rydberg transition, 59P_3/2 ↔ 58D_5/2 and 58D_5/2 ↔ 57F_7/2, are 10.78 GHz and 11.83 GHz, respectively. The MFC field and the signal microwave field off-resonantly couple to Rydberg atoms with a large detuning Δ from the resonance frequencies of the microwave transitions (10.78 GHz and 11.83 GHz). (b) Diagram of the optical path. DM, dichroic mirror; PD, photodetector; RPD, resistance power divider; ESA, electric spectrum analyzer; MW, microwave; LO, local oscillator. (c) Schematic diagram of the off-resonant MFC spectrum based on meta-waveguide-coupled Rydberg atoms. LNA, low-noise amplifier; MFC, microwave frequency comb. The dashed line box shows the front end for the free-space microwave signal reception demonstration.

Fig. 2. Normalized wide-band response of the system, the dynamic range of the resonance, and the instantaneous bandwidth. (a) The resonant power sensitivity at 10.78 GHz of the meta-waveguide-coupled system is about −115 dBm/Hz. (b) Wide-band normalized response from near 0.5 GHz to 13.5 GHz. (c) Instantaneous bandwidth of the system at 10.78 GHz and 6 GHz.

Fig. 3. (a) Radio station signal near 107.5 MHz is measured with the low-noise amplifier both on and off. (b) When the frequency of the microwave LO is set to 2.44 GHz, the WiFi signal near the LO frequency is measured.

Fig. 4. Output beat signal spectrum under the driving of the off-resonant MFC field is measured near (a) 2 GHz, (b) 3 GHz, and (c) 5.8 GHz, respectively. Most data points are recorded with a step of 200 kHz except for the two maximum points near the resonance, which have a ±50 kHz offset from the nearest comb line. (d) Schematic diagram of the MFC spectrum. (e) Off-resonant Rydberg MFC spectrum with a covering range of 125 MHz using 25 comb lines at 2 GHz.

Fig. 5. At 2 GHz, the output beat signal amplitude for the off-resonant Rydberg MFC spectrum versus the MFC power and comb lines phases.