# Low-Frequency Communication Based on Rydberg-Atom Receiver
## 2 Experimental Setup  
The experimental setup is illustrated in Figure 1. The cylindrical cesium vapor cell used in the experiment had a length of 100 mm and a diameter of 40 mm. An 852 nm laser (DL pro, Toptica, Pittsburgh, NY, USA) with a diameter of 600 \(\upmu\)m was split into two beams: a probe beam (\(\lambda_{p}\)) with a power of 50 \(\upmu\)W, and an identical reference beam. Both beams propagated in parallel through the cell. A coupling laser (\(\lambda_{c}\) = 509 nm, DL pro, Toptica) with a power of 15 mW and a diameter of 1 mm counter-propagated and overlapped with the probe beam. To mitigate environmental sensitivity, the experimental setup was enclosed in a temperature-controlled chamber to minimize temperature fluctuations. Additionally, to enhance long-term stability, the laser system was equipped with active stabilization to correct frequency and intensity drifts. The probe laser was used to drive the transition \(|6S_{1/2},F=4>\rightarrow|6S_{3/2},Ft=5>\), while the coupling laser was scanned through the transition \(|6P_{3/2},Ft=5>\rightarrow|63D_{5/2}>\), establishing Rydberg-EIT spectroscopy to enhance the probe transmission at two-photon resonant condition. Additionally, the frequencies of the probe laser and coupling laser were both locked to an ultra-stable cavity (Pound-Drever-Hall, (PDH)) to minimize the laser frequency noise. The EIT signal was detected using a differential photodetector (Thorlabs PDA210A/M), which measured the transmission difference relative to the reference beam, thereby eliminating the probe laser's intensity noise. The EIT signal, after being displayed and captured by an oscilloscope (SDS5000X, SIGLENT, Shenzhen, China), was stored for demodulation and data recovery using a MATLAB (R2020b) program running on a personal computer. A pair of copper electrode plates (size of 80 mm \(\times\) 26 mm \(\times\) 1 mm) was integrated in parallel into the cesium cell, with a spacing of 18 mm between them. Each copper plate was connected to a copper pillar that passed through the vapor cell. One copper pillar was connected to a whip antenna to couple low-frequency signal into the vapor cell, while the other one was connected to ground.  
A 10 \(\Omega\) resistor, used to transmit the low-frequency signal, was connected to the signal generator (DG2000, RIGOL, Cleveland, OH, USA) via the OPA544 amplifier. The signal generator was capable of providing a variety of low-frequency modulated signals. The output voltage of the generator ranged from 0 to 20 volts. The OPA544 operational amplifier was used to augment the power radiated by the resistor into the surrounding space by increasing the output current. Consequently, the emission power increased linearly with the applied voltage. Although the radiative efficiency of the low-frequency signals in this study was relatively low, the emitted signals could still be detected by Rydberg-atom receiver within a defined distance and specific voltage threshold. To optimize signal reception, preliminary experiments were conducted to determine the optimal distance between the whip antenna and the resistor. It was observed that when the distance exceeded 1.5 m, the SNR of the received signal deteriorated significantly, even at maximum emission power. Therefore, the distance between the whip antenna and the resistor was maintained between 1.0 and 1.2 m in the experiment. Within this range, the SNR of the received signal remained relatively stable, with no significant changes observed when the emission power was held constantly.