# Atomic Receiver by Utilizing Multiple Radio-Frequency Coupling at Rydberg States of Rubidium
## 2 Methods
### Performance Test  
To verify the actual performance of the Rydberg atom-based FDM receiver, it was necessary to study the SNR, which is the main technical indicator for measuring the quality of communication systems. Moreover, to detect time-varying modulated signals, both the frequency of the probe and the coupling laser must be locked [15]. In this study, two modulated RF carriers for dual-channel  
Figure 1: The experimental set-up. Frequency division multiplexing (FDM) receiver basing on Rydberg atoms is shown in the right part. Abbreviations: RM: reflector mirror; DM: dichroic mirror; PD: photodetector. Figure 2: (a) Experiment-related energy levels. (b) RF-induced symmetric or asymmetric Autler-Townes splitting. communication were investigated. Signals received from a photodetector were separated by a high-pass filter and a low-pass filter. The modulation signals of the two channels were set as single-tone signals. The probe laser was frequency-locked to the D2 transition of \({}^{87}\)Rb atoms, and the best SNR was obtained by scanning the frequency of the coupling laser. SNR values of receiving signals of channel 1 (CH1) and channel 2 (CH2) were separately determined by a spectrum analyzer. The frequency of the modulation signal of CH1 was 15 kHz with a peak-to-peak value of 3 V, whereas the frequency of the modulation signal of CH2 was 10 kHz with a peak-to-peak value of 4 V. The purpose of adjusting the amplitude of the modulation signal was to make the amplitude of actually receiving signal identical. As the resonant responses of Rydberg atoms were dependent on the amplitude and frequency of RF E-field [19], to achieve optimal communication, the performance of the atomic receiver needs to be optimized to the best experimental SNR. The coupling laser was frequency-swept from \(-80\) MHz to \(80\) MHz respect to a resonance. The experimental results are presented in Figure 3. From Figure 3a, it can be seen that to achieve a high SNR in both channels for FM communication, the frequency of coupling laser should be locked away from the resonance (with zero coupling laser detuning). This is because, at this point, the dynamic range of probe transmission change caused by FM signals was the largest. For receiving AM signals, the frequency of the coupling laser should be locked at the resonance. The working points of coupling laser are indicated in Figure 3.  
In practical FDM communication, it is also very important that different channels should concurrently work together without any mutual interference. Interchannel interference (ICI) refers to the interference caused by signals in one or more channels in a given transmission channel. To analyze the ICI, we studied the variation of SNR of receiving signals with \(\Delta_{RF}\), and the cross-talk between the actual signals obtained on the spectrum analyzer; here, \(\Delta_{RF}\) represents the frequency difference between two carriers. A 10.22 GHz carrier was applied to CH1, and a single-tone signal with an amplitude of 2.5 V and a frequency of 500 kHz was modulated on the carrier. In addition, another carrier modulated by a single-tone signal with an amplitude of 2.5 V and a frequency of 550 kHz was applied to CH2, and its frequency was scanned between 10.22 and 10.23 GHz (i.e., \(\Delta_{RF}\) was between 0 and 10 MHz). The corresponding results are shown in Figure 4. By observing the relative change in the SNR of receiving signals, the obtained SNR range of the two channels is from 26.4 to 28.2 dB when the \(\Delta_{RF}\) is less than 10 MHz. By observing the signal from the spectrum analyzer, it can be seen that the two signals have a relatively pure spectrum even at a very close \(\Delta_{RF}\) and there is no other spurious spectrum component, which can meet the requirements for practical  
Figure 3: Observed signal-to-noise ratio (SNR) and working points of coupling laser. Results for dual-channel AM and FM communication are shown in (**a**) and (**b**), respectively. communication and solve the problem of cross-talk.